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MODULATION OF THE RESPONSE TO CISPLATIN BY NITRIC OXIDE AND
REACTIVE OXYGEN SPECIES IN MELANOMA CELLS
by
ARCHNES
MASSACHUSETTS INSTItME
OF TECHNOLOGY
CHASE THADDEUS MACEO ANDERSON
JUN 2 6 2013
Submitted to the Division of Biological Engineering Lin Partial Fulfillment of the Requirements for the Degree of
LIBRA RIES
LIBRARIES
MASTER OF SCIENCE
in Biological Engineering
at the
THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June, 2013
Copyright @
Massachusetts Institute of Technology 2013
All rights reserved
Signature of Author
2011 S.B. from TrlVfassachusetts Instituteof Technology in Chemistry & Graduate
Student in the Division of Biological Engineering
May 8, 2013
Certified by
Dr. Gerald N. Wogan
Thesis Supervisor
Certified by
Dr. Luiz C. Godoy
Co-Advisor
Accepted by
l
-
-
v
Professor Forest M. White
Chairman, Department Committee on Graduate Students
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MODULATION OF THE RESPONSE TO CISPLATIN BY NITRIC OXIDE AND
REACTIVE OXYGEN SPECIES IN MELANOMA CELLS
by
CHASE THADDEUS MACEO ANDERSON
Submitted to the Division of Biological Engineering on May 10, 2013
in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Biological Engineering
ABSTRACT
Malignant melanoma causes the highest mortality rate in skin cancers. Although cisplatin has
proved efficacious in the treatment of various solid tumors, melanoma seems particularly
resistant to this chemotherapeutic drug. Reports show that melanoma patients whose tumors
express nitric oxide (NO) synthase and/or nitrotyrosine are often faced with poor prognosis.
Moreover, it has been shown that NO produced by melanoma cells sustains lower sensitivity to
cisplatin toxicity in vitro. Because inflammatory products such as NO and reactive oxygen
species (ROS) are associated with the genesis and evolution of cancer, we hypothesized that
these oxidative species may regulate key components of the response of melanoma to cisplatin.
Using a system for controlled delivery of NO to simulate the NO levels believed to occur during
inflammation, we showed that human melanoma (A375) cells pre-exposed to submicromolar NO
concentrations were protected from a subsequent challenge with cisplatin. This protection was
strongly associated with increased activity of the MAP-kinase cascade leading to activation of
ERK1/2, as well as with downstream modulation of the apoptotic factors Bax and Bcl-2, and the
transcription factors p53 and MiTF. Although NO favored increased expression and
phosphorylation of p53, it also increased the expression of the p53 inhibitor MDM2, which may
have counteracted p53-induced apoptosis upon cisplatin treatment. Also, likely via ERK1/2
activation, NO favored phosphorylation of MiTF, which is associated with survival signals.
Furthermore, NO displayed the remarkable ability to overcome the effect of U0126, a MEK
inhibitor, and promoted continuous phosphorylation of ERK1/2 (and hence cell survival), in
contrast to cells not exposed to NO. Results also demonstrated that cisplatin-induced apoptosis
was substantially decreased by the antioxidant precursor N-acetylcysteine (NAC). Unlike
exogenous NO, cisplatin-induced ROS were linked to lower activation of ERK1/2, which was
reversed by NAC. During the treatment with cisplatin, NAC led to lower levels of p53, which
may have partially contributed to increased cell survival. However, in contrast to NO, NAC did
not significantly alter the effects of cisplatin upon MiTF and apoptotic proteins studied.
Altogether, our findings illustrate the complexity of the regulation of signaling components by
oxidative species of distinct natures.
Thesis Supervisor: Gerald N. Wogan
Title: Underwood-Prescott Professor Emeritus of Toxicology and Chemistry
Thesis Co-Advisor: Luiz C. Godoy
Title: Research Associate
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BIOGRAPHICAL NOTE
I began my collegiate career in the fall of 2007 at The Massachusetts Institute of
Technology and started working in Wogan Laboratories in the summer of 2008. During that time,
I worked with Dr. Min-Young Kim on protein expression analysis as a chemistry major. In the
spring of 2009, I began studying the effects of nitric oxide upon chemoresistance with Dr. Luiz
Godoy, with the help of Laura Trudel. I graduated in June 2011 with a Bachelor of Science
degree in Chemistry, and was honored with the Chemistry Research Award by the department
for my work in Wogan Laboratories.
During the summer of 2011, I immediately began my thesis project, switching
departments to become a graduate student in Biological Engineering pursuing a master's degree
in Toxicology. Under the guidance of Dr. Gerald Wogan, Dr. Luiz Godoy, and Laura Trudel, I
studied the effects of nitric oxide and reactive oxygen species upon the chemosensitivity of
melanoma cells. In the fall of 2012, Dr. Luiz Godoy published his paper, "Endogenously
produced nitric oxide mitigates sensitivity of melanoma cells to cisplatin," in the journal,
"Proceedings of the National Academy of Sciences of the United States of America," where I
was graciously listed as second author. This spring, 2013, I will graduate with a master's degree
in Biological Engineering with a thesis, and a paper is in progress summarizing my results.
This fall, I will be continuing my educational career a first-year medical student at The
Northwestern Feinberg School of Medicine.
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ACKNOWLEDGEMENTS
First and foremost, I would like to thank every member of Wogan Laboratories for
welcoming me into their lab and lives with open arms. Never have I found a more accepting
environment of people. Each of them is not only trying to make the students in their lab better
researchers, they are trying to make better people. From Dr. Min-Young Kim, who took me on as
a student when I barely knew the right end of a pipette, to Dr. Apinya Thiantanawat, Dr. Crystal
Rogers, Dr. Nicole Iverson, Dr. Ming-Wei Chao, Ms. Denise MacPhail, and Dr. Rajdeep
Chowdhury, each person mentioned has been a mentor at some point in my academic career.
Being a part of Wogan Laboratories helped me regain my confidence in my abilities as a student
after I had lost it at the beginning of my MIT career, and everyone here pushed me to excel.
In particular, I would like to express my sincerest and most heart-felt thanks to Dr.
Gerald Wogan for allowing me to join Wogan Laboratories all those years ago. His constant
guidance and profound knowledge have continued to be a source of inspiration, something to
aspire towards in terms of my scientific aims. His ability to push a person to dig deeper, to
understand the why and the how of something, and to not simply be satisfied that an experiment
worked, propelled me to even greater heights as a researcher and as a person. From lab meetings
to lunches with our group, his leadership is what makes Wogan Laboratories such a welcoming
family. Dr. Wogan is the heart of the laboratory, and he makes it beat so strongly and fuller
every day. I count myself extremely lucky to have learned from and been a part of a laboratory
led by someone as wonderful as Dr. Wogan.
I would also like to add my deepest appreciation for Laura Trudel. From the very first
moment that I stepped into lab, she has guided my actions, from learning how to culture cells to
helping me learn the ropes around lab. I hope she knows how much of an inspiration she is to
others. The amount of projects that she keeps track of, while still making space in her day to help
a U.R.O.P. student just getting his feet wet in the waters of research, is a staggering feat. Her
love for every member of lab pushed me to become someone who became a contributor in our
lab instead of just a bystander who was there to do a task and not think more deeply about what I
was contributing.
To my mentee and friend, Will Watkins, I would like to put in writing how much of an
influence he has had upon my life. Although I introduced him to Wogan Laboratories, he helped
me learn about how to truly be a good mentor to someone - in research and in life. Without his
experiment to springboard off of a year ago, this project would have turned out very differently.
His charisma and selflessness are something that is rare in our society today - I shall remember
to keep his care for others in mind for all my interactions with future colleagues and friends.
My family has played an enormous role in my development, and I would like to thank
them all here. Dr. Corrie Anderson, Mrs. Virginia Elizabeth Williams II, Ms. Virginia Elizabeth
Williams III Anderson, Dr. Theodore Hall, Mrs. Rosalind Lee, and Dr. Victor Ambrose, each of
you has played an enormous role in shaping who I am - you all have taught me to be proud of
who I am, but always aspire to more, while also ensuring that I make things better for those
coming after. You all were the spark that first ignited my intellectual curiosity, and have been
there as pillars of support throughout my life. Thank you all for your compassion, your wisdom,
and your love.
I could not write this final chapter of my life at MIT without mentioning the one person
who truly showed me over five years of working in Wogan Laboratories what it means to be a
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good person and to be a mentor - Dr. Luiz Godoy. Even though I am a person of many words, I
am at a loss to describe what an integral part of my life Dr. Godoy has been. To put it as
eloquently as possible, he has been the perfect role model for me at M.I.T. When I began
working under his tutelage, I still was extremely unsure of my place at M.I.T. and in my abilities
as a student. He took me under his wing, always making time to help me and ensure that I
succeeded, even if it was in a manner I did not expect. Even though we formed an immediate
friendship, over the years I came to realize that he was not only teaching me about how to work
and be confident, he was teaching me about life and how to embrace all my gifts. From the dayto-day in lab until even the end with the writing of this thesis, he has guided me but also let me
craft my own path - he is everything one could hope for in a mentor and friend. Luiz, thank you
for all that you have done. I can never say thank you enough.
This work was supported by the Program Project Grant 5 P01 CA26731 from the National
Cancer Institute and Grant ESO2109 from NIEHS
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TABLE OF CONTENTS
Title page
1
Abstract
2
Biographical Note
3
Acknowledgements
4
Table of Contents
6
Section 1: Introduction
7
Section 2: Materials and Methods
17
Section 3: Results
21
Section 4: Figures
28
Section 5: Discussion
38
Section 6: Pathway Construction
48
Section 7: Conclusions
49
Bibliography
51
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INTRODUCTION
Melanoma
Malignant melanoma, the abnormal growth of melanocyte cells, causes the highest rate of
mortality in skin cancers (1, 2). Though melanocytes supposedly develop into melanoma through
accumulation of genetic and molecular alterations, the exact development of this deadly disease
still remains unclear in many ways (3-5). Approximately 20-25% of patients afflicted with
malignant melanoma die of metastatic disease (1). As one of the most common forms of cancer,
more than 2 million cases are diagnosed annually, with an estimated 66,000 deaths per year (6).
Of the three major types of skin cancer, namely basal cell carcinoma, squamous cell
carcinoma, and malignant melanoma, the latter is the most aggressive. Along with being resistant
to chemotherapeutic agents, most therapies have limited cure and low efficacy rates. Surgical
removal of the tumor has shown favorable outcomes only if diagnosed in early stages. Without
early detection, at most patients have 6-10 months on average to live. In addition, single-agent
chemotherapeutics have proved mostly ineffective, having a success rate of less than 15% in
terms of curing patients with metastatic disease (7-9). Melanoma resistance to treatment comes
about through many factors, including altered drug uptake, intracellular and extracellular drug
transport, redundant pathways for proliferation of the cancer, and alteration of drug targets (1013). More cases are reported annually, far outstripping the methods of treatment. As of this time,
there is no standardized therapy that has proved completely efficacious (14).
Cisplatin
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From the time of its first use in patients, cisplatin (Cis-diammine-dichloro-platinum")
revolutionized therapy for different tumor types, such as cancers of the neck, ovaries, and testes.
Although studies are ongoing to elucidate the exact mode of action, DNA is believed to be the
main target of cisplatin (15). Cisplatin cytotoxicity is believed to mainly occur through formation
of inter- and intra-strand adducts with DNA, hindering RNA transcription and DNA replication.
This leads to cell cycle arrest and eventual death by apoptosis mediated by p5 3 (16, 17).
Despite its efficacy, the chronic use of cisplatin often leads to drug-resistance. Through
mechanisms such as DNA mismatch repair and nucleotide excision repair (NER), cancer cells
may evolve to evade apoptosis. NER is an important mode of drug resistance, for cisplatin is
known to form DNA adducts, and an increase in NER will confer higher resistance to the
chemotherapeutic DNA-damaging compounds. NER occurs mainly through upregulation of
ERCC 1, as discussed below, as well as repairing the DNA adducts by removing or altering the
said adduct (16).
The cisplatin molecule, converted into its mono- and bi-aquated cisplatin forms in the
cytoplasm, may interact with several different cytoplasmic substrates before reaching DNA
strands, including endogenous nucleophiles such as reduced glutathione (GSH). GSH
conjugation to cisplatin leads to high rates of drug export, decreasing the drug's efficacy. As a
mechanism of action against cisplatin, cells may upregulate levels of GSH S-transferase or yglutamylcysteine synthetase. GSH S-conjugates are readily extruded from cells through the use
of Multiple Drug Resistance Protein (MRP)-1 or -2 (17, 18).
There are several other routes through which resistance to cisplatin may occur, such as
increased production of nitric oxide (NO) and its protective role against apoptosis (19, 20). As a
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whole, each pathway or mechanism of action presents a challenge to utilizing cisplatin as a
chemotherapeutic agent (17, 18).
Cisplatin toxicity in and of itself is also an issue, of which several reasons exist as to why
it occurs. One reason is thought to be the formation of intrastrand crosslinks, for similar toxicity
was not observed with the trans-platinum form of the compound (21). Toxicity ranges from
severe to mild, with nephrotoxicity and peripheral neurotoxicity being the most serious. The first
occurs due to cisplatin uptake by proximal tubule cells of the nephron, yet diuretics and prehydration, which have turned damage into a dose-limiting effect, may control this. Overall,
nephrotoxicity is due to the production of reactive oxygen species (ROS), which may mediate
apoptotic pathways and have not been directly associated with necrotic cell death. A human
organic cation transporter has also been implicated in potentiating cisplatin nephrotoxicity (17).
Other toxic effects are attributed to cisplatin. Ototoxicity occurs in approximately 2354% of patients, and is caused by damage to cochlear outer hair cells. Again, the mechanism
causing this side-effect is believed to involve ROS and cell death. This may involve activation of
the NADPH oxidase isoform NOX3, which, upon treatment with cisplatin, generates superoxide
radicals. These radicals are converted by cellular enzymes into hydrogen peroxide and hydroxyl
radicals, causing ototoxicity through release of cytochrome c and caspase activation (17). Finally,
neurotoxicity, characterized by peripheral neuropathies, occurs due to cisplatin damage of the
dorsal root ganglia of the spinal cord, while ocular toxicity, hepatotoxicity, and other such events
are due to cisplatin-mediated apoptosis (17, 22).
Although hailed as a revolutionary drug, cellular resistance to cisplatin's effects
compromises the overall effectiveness of this anticancer agent. Presently, cisplatin is often used
as a combinatorial drug, for when used alone cells will often develop resistance in many cases
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(21, 23). Therefore, other methods are needed in order to rid a patient of their disease (24). To
overcome the problem of drug resistance, one must consider multiple pathways of drugresistance, in order to find a common thread that will synergistically provide better solutions.
Inflammation and Cancer
Inflammation has been shown to effect a tumorigenic environment that allows for cancer
growth, angiogenesis, and metastasis. Such an environment also provides a platform through
which cancers initiate, occurring through the secretion of growth and survival factors,
extracellular proteases, chemokines, and pro-angiogenic factors by macrophages, neutrophils,
and other inflammatory cells (25-29). Most of these inflammatory responses come about due to
injuries, where the response of cells is to secrete these factors, such as reactive oxidants and
radicals, to clear the body of infectious agents. This scenario sometimes results in damage by
such factors, causing DNA adducts and somatic mutations, eventually leading to the
development of cancer (30-36).
Reactive Oxygen Species (ROS)
The stress caused by inflammation leads to oxidative stress and provides mechanisms for
cancer survival, such as resistance to apoptosis, hyper-proliferation, and molecular system
changes that contribute to tumor growth (37-41). When transformed into a tumor cell, the usual
signaling that may have led to eventual senescence will instead lead to proliferation. The
inflammatory factors are recruited to suppress cytotoxic responses and enhance proliferative and
pro-angiogenic activities (42-45). ROS play a large role in melanogenesis, being generated in
response to UVA by inflammation. After their generation, ROS damage DNA through the
mutation of guanine bases to thymidine, leading to the accumulation of mutations and eventual
cancer (5, 46). One reason for the increase in ROS is the hydrogen peroxide generated by
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autooxidation of eumelanin precursors during melanogenesis. In addition, malformed
melanosomes in melanoma cells leak oxidative quinone and semiquinone intermediates, leading
to normal melanin becoming oxidized and generating prooxidative ROS. Modulation of factors
induced by ROS can be controlled through many factors, such as the microphtalmia-associated
transcription factor (MiTF), but this regulation often leads to overactivation in cancer cells (4749), (50), (51).
Nitric Oxide (NO)
NO is a hydrophobic molecule generated by inflammatory tissues that leads to damage of
DNA, RNA, lipids and proteins, and can cause increased mutations, along with altering protein
function (52-55). As well, NO has been associated with the development of resistance to
chemotherapy. As a cellular signaling mediator synthesized from L-arginine in a reaction
catalyzed by NO synthases (NOS), NO has both pro- and anti-apoptotic functions, which are
controlled by the cellular redox state, flux of NO, as well as other factors, including its overall
concentration in cells (19). Endogenous NO, as sometimes produced by macrophages and other
cell types during inflammation, or in response to stimuli such as cisplatin, has been shown to
inhibit apoptosis (20, 56, 57). NO is enhanced in patients with malignancy, and can regulate and
post-translationally modify proteins and pathways to produce anti-apoptotic outcomes in
cisplatin-induced cell death (19).
Whereas at first the effects of NO in relation to cancer and chemotherapy were not well
understood, NO is currently seen as both protective and destructive. In murine models of
mammary carcinoma, it has been shown that NO production by exogenous sources within the
host promotes tumor growth, while NO produced by the tumor itself inhibited its progression (58,
59). On the other hand, others have demonstrated the opposite, with endogenously generated NO
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as a promoter of cancer growth (60), (61), (62). Because NO is an important part of
physiological responses, affecting respiration, cell migration, apoptosis, and post-translational
protein modifications (20) (63), it is not surprising that it also has an effect upon cancer growth.
Inducible NOS (iNOS) is one of the enzymes known to generate NO. This isoform in
particular has been commonly associated with malignant disease, depending on the level of
expression and activity of iNOS within the tumor microenvironment. In patients with Stage III
melanoma, expression of iNOS was associated with shortened survival (64, 65). In many cell
types, iNOS generates high amounts of NO. In the case of tumors, though, constitutive
expression of iNOS and low amounts of NO seem to contribute to cell survival (64). In particular,
NO may modify key apoptosis-regulatory proteins through S-nitrosation (19, 20). This enhances
resistance to cisplatin, for example, via S-nitrosation of Bcl-2 and Caspase-3, which promotes
increased resistance to degradation of the first and lower activity of the latter, resulting in overall
resistance to cell death (20, 66).
While the aforementioned information dealt with endogenous NO, exogenous nitric oxide
has proved troublesome in terms of melanoma resistance to chemotherapeutics as well. In many
cases, NO is used as a donor to increase the sensitivity of cells, such as the case with glioma cells
becoming more sensitive to chemotherapeutics through high levels of NO (67, 68). On the other
hand, the NO donor (Z)- 1-[2-(2-aminoethyl)-N-(2-ammonioethyl)aminoldiazen- 1-ium- 1,2diolate (DETANONOate) was shown to exhibit effects upon growth inhibition as well, where
higher concentrations of NO led to proliferation, and low levels of NO caused inhibition in
growth. NO donors and scavengers, such as 2-(4-Carboxyphenyl)-4,4,5,5tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), were also shown to have an effect upon the
tumor suppressor p53, which resulted in conformational changes in the protein and changed the
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survival signal p2 1 (12, 69, 70). The use of NO in terms of protection or induction of cell death,
whether endogenous or exogenous, appears to vary greatly depending on the level of nitric oxide
introduced (68).
The NO Delivery System, a method of delivering a steady-state concentration of NO to
cells, has been particularly useful in studying the effects of exogenous NO (71). Silastic tubing
allows for diffusion of different gases, such as oxygen and various concentrations of NO into
reactor vessels containing dishes with cells (71). Unlike other methods of delivering exogenous
NO, such as NO-donor drugs, the system is able to continuously introduce the same
concentration of NO or other gases throughout a given time period, providing well controlled
possibilities for simulating physiological conditions. The mimicking of the chemical
environment produced by inflamed tissues in the NO Delivery System, for example, may be
attained through the delivery of 0.72 iM NO and 180
tM
02
(52, 71-73). Developed by Wang &
Deen and further described by Dendroulakis et al., as one of only two models of this nature in
the world, the NO system has been invaluable in providing a controlled way in which to study
the effects of NO upon cells (71, 74).
Protein S-nitrosylation
S-nitrosation, the modification of critical cysteine residues by NO (SNO), can cause posttranslational effects in proteins, leading, for instance, to increased cancer survival through
inhibitory changes in caspases and other factors. S-nitrosylation has also been implicated in
protein processing, vesicle-mediated insulin release, and vectorial membrane trafficking (37, 7585). Of note, Foster et al. pointed out that exogenous mediators of protein S-nitrosylation can
alter the development of disease, evidencing that targeted therapy of S-nitrosylation could be
exploited for cancer therapy (86). S-nitrosylation that occurs inappropriately leads to
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dysregulation of the cell cycle, and plays an important role in the development of the cancer
phenotype (86). It is suggested that cancer cells may use NO to nitrosylate certain caspases at
their active site to inhibit enzymatic activity, blocking apoptosis and leading to cell survival.
Reports provide evidence that many other key proteins could be nitrosylated to ensure cell
survival. (20, 76, 80, 87).
Signaling pathways in melanoma
Signaling pathway analysis is key to understanding melanogenesis and possible therapeutic
targets in melanoma. Most strategies for elucidating new methods of treatment focus on critical
signaling pathways involved in cancer proliferation, survival, and migration. In nucleotide
excision repair (NER), for instance, the cancerous cells evolve to evade apoptosis. In response to
adduct formations nearly 20 proteins involved in NER initiate repair of the lesion. NER is an
important mode of drug resistance, for cisplatin is known to form DNA adducts, and an increase
in NER confers higher resistance to chemotherapeutic DNA-damaging compounds. NER occurs
mainly through upregulation of ERCC 1, a core protein required for NER, as well as repairing the
DNA adducts by removing or altering said adduct. ERCC 1 also plays an important role in
intrastrand crosslink repair (ICR), which is augmented in cisplatin-resistant tumors (16, 17, 88).
Among the various pathways used by melanoma to increase survival and proliferation,
from the NF-KB pathway to the modulation of the apoptotic mitochondrial pathway (20, 75, 89),
the mitogen-activated protein kinases (MAPKs) hold much promise in terms of increasing cancer
sensitivity to chemotherapeutics (90). Although there are four different kinase routes that can
lead to proliferation, perhaps one of the most studied has been the MAPK-ERK pathway, which
involves N-ras, B-raf and Mekl/2. MAPK induction is initiated by factors binding to the receptor
tyrosine kinase binding ligand, or through integrin adhesion to the extracellular matrix, leading
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to Ras activation and downstream effects, culminating with transcriptional upregulation and
proliferation. Involved in controlling cell growth and proliferation, this pathway has widereaching effects on the ability of cancerous cells to survive and progress to metastasis (5, 91-94)
B-raf, which is constitutively activated in melanoma through a mutation of thymidine to adenine
(V600E), leads to higher amounts of kinase activity and a higher amount of phosphorylation of
downstream targets (95-97).
The extracellular signal-regulated kinases (ERK1/2) have been the subject of intense
study in terms of the progression and proliferation of melanoma. ERK1/2 may either
phosphorylate multiple cytoplasmic targets or migrate to the nucleus and activate, through
phosphorylation, transcription factors. One such factor, MiTF, is a gene that regulates
melanocyte development and differentiation, and has been shown to interact with the antiapoptotic factor Bcl-2, while being controlled by ERK1/2 (2, 89, 98-101). In addition, MiTF can
regulate several other transcriptional factors, including HIF-la and MARTI (100). It is
important to note that ERK1/2 phosphorylation of MiTF leads to proteasome-mediated
degradation of the latter, which is linked to temporary G 1 cell cycle arrest through p21, a factor
that can inhibit cell cycle, but also bind to proliferating cell nuclear components for replication
initiation. However, during UVC stress, MEK1/2 kinase activity is inhibited, leading to MiTF
stabilization. Moreover, degradation after UV insult is thought to cause G1 cell cycle arrest and
allow for DNA repair (102-105). If true, the same could hold for cancerous cells when affected
by cisplatin, and thus the ERK pathway presents a potential chemotherapeutic target.
A last component that is key to melanoma survival is the tumor suppressor p53, which
has been shown to help mediate the anti-apoptotic effects of NO (19, 80, 81, 87, 106-108). As a
sensor of DNA damage, oxidative stress, and other damaging factors, activation of p53 leads to
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cell cycle arrest and apoptosis. However, this tumor suppressor is often mutated in various
cancers, except there is very little evidence that p53 is altered in melanoma, and its increased
activation is associated with progression of tumors (12, 109, 110). Another piece of evidence
shows that the p53 and B-Raf pathways interact and induce melanomagenesis (5, 111). In
addition to activating factors such as p21 and promoting survival in cancers against
chemotherapy, evidence shows that p53 is often dysregulated or its function is altered in
melanoma. More importantly, Tang et al established a link showing that endogenous NO
controls the expression and effects of p53 signaling and that depletion of NO led to increased
sensitivity to cisplatin (12, 69, 70, 112, 113).
Even with targeted therapies against specific pathways, due to redundancy and multiple
pathways for proliferation, cancer cells evade inhibition of one pathway by using another. Such
evasion leads to higher resistance to further chemotherapeutic insults (14). To address this issue,
a feasible strategy seems to be the focus on multiple pathways and factors, including specific
proteins and free radical species, such as NO. We provide evidence here that exogenous NO,
delivered to cells at levels believed to occur during inflammation, modulates the ERK1/2
pathway, leading to upregulation of p53, regulation of MiTF, and other factors that enhance
survival of melanoma. Most importantly, we provide evidence that exogenous NO confers
resistance to cisplatin in a similar fashion to endogenously produced NO in melanoma cells, as
previously reported by our group (20).
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MATERIALS AND METHODS
Reagents
Cisplatin [cis-diamminedichloroplatinum(II)] was purchased from Sigma-Aldrich. NMA (nmethylarginine) was purchased from EMD-Calbiochem and also provided by Dr. Robert G. Croy
from MIT. N-acetylcysteine (NAC) and lipopolysaccharide (LPS) were from Sigma Aldrich.
Recombinant mouse IFN-y was from R&D Systems. Antibodies were purchased as follows:
PUMA, p-actin, MiTF and Bcl-2 from Santa Cruz Biotechnology; p-ERK, ERK, p-MEK, MEK,
p-RAF, and Survivin were from Cell Signaling Technologies; Heme-Oxygenase-1 from
Stressgene Biotechnologies; phospho-MiTF was from Assay Biotech.
Cell culture
A375 and RAW264.7: The human malignant melanoma A375 cell line was purchased
from ATCC and grown at 37'C and 5% CO2 in DMEM and supplemented with 10% FBS
(Atlanta Biologicals), 0.1 mM L-Glutamine, 10 U/mL penicillin, 10 ptg/mL streptomycin, and 1
mM sodium pyruvate (Lonza). The Raw 264.7 cell line was grown in the same conditions and
procured from The American Type Culture Collection.
NO delivery system
The NO delivery system is comprised of a vessel for exposing cells to NO, a system for
delivering NO (besides other necessary gases for cell growth, namely 02, CO 2 and N2 ) to the
exposure vessel, and accessories for temperature control and culture medium stirring (71, 74).
A375 cells were maintained in DMEM at 37'C in a humidified, 5% CO 2 atmosphere. Cells (1 x
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106 cells in 5 mL) were seeded on 60 mm dishes for 24h and then placed in reactor vessels as
previously described (71). Briefly, the medium from culture dishes was aspirated and 105 mL
DMEM was added, followed by sealing the chamber with the cap containing an attached magnet
for stirring and ports for gas delivery. Cells were exposed, at 37'C, to 1%NO (balanced with
argon) for the indicated times. Gases are delivered by diffusion through permeable Silastic
tubing (length of 7 cm) at a steady-state concentration of 0.72 pM. A mixture of 50% 02 and 5%
CO 2 was delivered through a second tubing to maintain medium oxygen concentration at a level
that matched air saturation. Cells exposed to argon gas under the same condition served as
controls.
Co-culture system
We exposed A375 cells to activated macrophages using a co-culture system described
before (114). Transwell Permeable Supports (Corning, NY) are comprised of 100 mm culture
dishes, which contain thin, porous 75 mm diameter inserts. The polycarbonate membrane
contains pore diameters of 0.4 pm, a porosity of 0.13, and a thickness of 10 pm. The membrane
allows rapid diffusion between the upper and lower parts of the system, except no direct contact
between target cells and macrophages can occur. Membranes were pretreated with 3 mL DMEM
for 1 h at 37'C and placed in 150 mm dishes to maintain sterility. The medium was discarded
and RAW264.7 cells (1 mL containing 1 x 107 cells) were seeded onto the membrane side facing
the bottom chamber of the dish, for 4h to allow adherence at 37'C while still in 150 mm dishes.
Next, the membrane was inverted onto the original 100 mm culture dish so now macrophages
faced the bottom of system. The lower chamber was filled with 10 mL culture medium (DMEM)
containing 10 units/ml IFN-y and 10 ng/mL LPS. A375 cells in 10 mL DMEM were seeded onto
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the membrane in the upper compartment, with the ratio of A375 cells to macrophages being 2:5
(4 x 106 and 1 x 107 , respectively). The co-cultures were incubated for 24, 48, or 72h at 37'C in a
5% CO 2 atmosphere. After incubation, trypsinized A375 cells were collected for western blot
analysis. Various combinations of 5 mM NMA and 5 mM NAC were used in the lower chamber,
while 12.5 pM cisplatin were used in the lower and upper chambers. NMA inhibits NO synthesis.
NAC is a precursor for the synthesis of antioxidants, used to minimize the effects of ROS.
Cell extracts and Western blotting: Cells were harvested at indicated times from either
reactors or dishes in incubators as specified in the results, and lysates were made. All procedures
were carried out at 4C unless otherwise stated. Briefly, cells were washed with PBS twice and
then treated with trypsin, then scraped and placed in centrifuge tubes. These tubes were then
spun in a centrifuge at 2,000 r.p.m. and then treated with RI[PA buffer supplemented with
cocktails of phosphatase inhibitors (Thermo Scientific) and protease inhibitors (Sigma Aldrich)
(both at 1:100 dilution when based off of the amount of RIPA buffer). Cells homogenates were
then frozen at -80'C for 15 minutes and defrosted at 37'C for 1 minute. Next, lysates were
passed through a 29-gauge needle eight times to reduce viscosity and then spun at 4,000 x g for
10 minutes. Protein concentration was analyzed colorimetrically with Coomassie Brilliant Blue
dye and a standard curve generated with bovine serum albumin. For western blotting, samples
were mixed with Laemmli reducing electrophoresis buffer (BIORAD) and boiled for 10 minutes.
Samples (40 [tg) were submitted to SDS-PAGE in 10% acrylamide gels (Lonza) at 25 mA per
gel for approximately 1 hour and then transferred to a nitrocellulose membrane using an iBlot
rapid-transfer system (BIORAD). Membranes were blocked for 1 hour with 5% nonfat milk in
TBS containing 0.1% Tween 20 (TB S-T) at room temperature and then probed with primary
antibodies (1:1,000 dilution) overnight at 4'C, rocking slowly on a platform. The following day,
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membranes were washed with TBS-T three times then incubated with secondary goat anti-rabbit
or anti-mouse IgG conjugated to horseradish peroxidase (1:2,000-1:10,000 dilution). Proteins
were visualized by chemiluminescence by exposure to ECL Prime (G.E. Healthcare Life
Sciences). Digital images were generated using a GEL Logic 2200 equipment. To control for
protein loading, we also probed for
@-actin. For total protein after looking
at the phospho-results,
membranes were stripped and then reprobed as described before (72).
Cell viability
Cell Proliferation and Cytotoxicity: WST- 1 reagent (Roche) was used to analyze viability
and cell growth. Ten microliters of reagent was added per 100 ptL of culture medium in the 96well plates. The absorbance was read at 450 nm after incubation for 30 min at 37'C (20).
Cell viability was ascertained by using trypan blue exclusion dye under light microscopy.
Briefly, cells were homogenized and 20 itL of cell suspension were mixed with 20 1 iL of 0.4 %
trypan blue (Sigma Aldrich), and then 10 itL were analyzed in a hemocytometer chamber.
Statistical analysis
Every experiment was performed at least three times, with similar results obtained.
Figures show results representative of each experiment. Paired data were analyzed with the
unpaired t test. Multiple treatments were analyzed by one-way ANOVA and complemented by
the Student-Newman-Keuls multiple comparisons test.
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RESULTS
Exogenous NO confers chemoresistance in melanoma cells
Due to evidence showing that low levels of endogenous NO provided melanoma cells a higher
resistance to cisplatin-induced death (20), we sought to investigate whether low levels of
exogenously provided NO mimicking levels found during inflammation would effect the same
results. In order to ensure that the cells were being affected at levels of exogenous NO that they
could tolerate indefinitely, we used 0.72 1 M NO, which has been shown to not kill A375 cells
(William G. Watkins and Luiz C. Godoy, unpublished results). Using the NO delivery system
(Figure 1A), we looked at the survival of cells treated with or without 12.5 [tM cisplatin, a level
that is known to kill 20-40% of A375 cells based on titration assessments (Figure IB). Cells
treated with NO alone did not show signs of toxic effects of NO (Figure 2). More importantly,
cells treated with NO for 24 hours and then cisplatin for another 24 hours survived at a much
higher rate than cells treated with argon and then cisplatin (Figure 2).
Identifying the signaling components affected by NO during the response to cisplatin
The discovery that exogenous NO delivered to melanoma cells in our system provided for
protection against cisplatin-induced cell death led us to seek out which signaling components are
affected by NO during the response to cisplatin. A result from Godoy et al. (20), which showed
that ERK1/2 - a MAPK pathway linked to melanoma survival - was upregulated in the response
of cells to cisplatin while studying the role of endogenous NO led us to wonder if the pathway
was affected in a similar manner during treatment with exogenous NO. First, we sought to
elucidate the effects of NO alone upon the ERK1/2 pathway. What we observed was the fact that
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the addition of NO to melanoma cells caused an increase in the phosphorylation of ERK (Figure
3A). As well, an increase in the phosphorylation of MEK, the upstream protein that
phosphorylates and activates ERK, was seen in these cells (Figure 3A).
After this result, we also wondered whether p53, a tumor suppressor that is mutated in
many cancer cells and is often dysregulated in A375 cells, perhaps was affected by the addition
of exogenous nitric oxide. We observed that total p53 protein was upregulated when exogenous
NO was added to the system for 24 hours (Figure 3B). Higher levels of phosphorylation of p53 at
serine 15, which is involved in cellular response to DNA damage, was also seen to be increased
in the response to exogenous NO (Figure 3B). MDM2, a negative regulator and part of a
feedback system relating to p53, was increased in response to exogenous NO (Figure 3B).
To find downstream factors that could be part of the ERK1/2 pathway through which
melanoma cells were possibly exhibiting increased survival signals, we looked at the response of
MiTF to exogenous NO. Results demonstrated that during the response to NO there was an
increase in the phosphorylation of MiTF, which leads to MiTF degradation at a higher rate
(Figure 3C).
Modulation of the ERK1/2 pathway
We then hypothesized that modulation of the ERK1/2 pathway directly through specific
inhibitors could lead to changes in downstream factors related to melanoma survival upon a
subsequent challenge with cisplatin. In order to test the validity of this theory, we first showed
that the inhibitor of MEK of choice in fact lead to decreased phosphorylation of ERK1/2. We
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treated cells with increasing concentrations of the MEK inhibitor U0126 for 24 hours. Starting at
0.1 tM of U0126, we showed that increasing the concentration of U0126 led to decreased
phospho-(p-)ERK detection, meaning that the activity of MEK was in fact declining (Figure 4A).
The amount of total ERK remained the same, even with the increasing concentrations of U0126,
and even at the highest level tested, 10 1iM. Moreover, the relative survival of cells decreased
gradually with increasing doses of U0126 after a 24-hour treatment, as shown in Figure 4B.
NO-modulation of the ERK1/2 pathway affects apoptosis
After observing that U0126 did indeed inhibit the activity of the ERK1/2 pathway in our model,
we decided to look at cell survival signaling in cells treated with NO, cisplatin, and U0126,
believing that each one of them could play a role in the downstream signaling via ERK1/2. We
hypothesized - based on the literature and our own results - that the combination of U0126 and
cisplatin might prove synergistic in nature and lead to decreased cell survival versus cells treated
with cisplatin alone. Thus, cells were exposed to NO for 24 hours as described above and then
we added cisplatin, either in combination with 5 tM UO 126 or alone. As a control, each cell
reactor treatment also had a reactor done in parallel that was flushed with argon and the
aforementioned treatments. Exposure to NO and to these compounds was then continued for an
extra 24 hours. Lysates of the samples were then made and studied by western blot. As shown in
Figure 4C, addition of NO alone increased the phosphorylation of ERK1/2 when compared to the
cells treated with argon alone, while the addition of cisplatin to NO-treated cells also increased
the phosphorylation of ERK1/2 when compared to the cells with argon and cisplatin. U0126 led
to inhibition of ERK1/2 phosphorylation in argon/cisplatin-treated cells. Interestingly, though,
when looking at cells treated with NO, cisplatin, and UO 126 in combination, we observed an
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increase in the phosphorylation of the ERK1/2 pathway, suggesting that NO could overcome the
inhibitory effects of U0126 over MEK1/2.
Under the same conditions, we looked at other factors downstream of the ERK1/2
pathway, such as Bcl-2, Bax, and PARP, for it has been shown that endogenous NO modulates
these apoptotic factors in our cells (20). When first looking at Bcl-2, an anti-apoptotic factor, we
saw that in response to the treatment of NO and cisplatin, expression went down in contrast to
argon and cisplatin (Figure 4C). However, when U0126 plus cisplatin was studied in cells treated
with NO, we saw a marked increase in the level of Bcl-2 expression when compared with argon,
cisplatin, and U0126 in combination. Bax, the aforesaid pro-apoptotic protein that can be
inhibited by Bcl-2, showed decreased levels under NO treatment versus argon. In addition, the
same decrease in Bax levels was seen when cells under argon were treated with cisplatin as
opposed to cells treated with NO and cisplatin (Figure 4C). Of note, inhibition of MEK with
U0126 in combination with cisplatin and NO did not lead to changes in Bax levels when
compared to cells exposed to argon, UG 126 and cisplatin. Furthermore, a somewhat
contradictory results was seen when analyzing PARP cleavage, which is induced during
apoptosis upon DNA damage by cisplatin, for we observed that higher cleavage occurred in cells
treated with NO than with argon, even though cell death was more pronounced in cells treated
with argon (Figure 1C).
NO-modulation of p53 and MiTF via the ERK1/2 pathway
After the results that showed NO can in fact modulate the ERK1/2 pathway and its downstream
effectors, we also hypothesized that exogenous NO could modulate the extensively studied
tumor suppressor p53. At the same time, we sought to elucidate the phosphorylation status of
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MiTF, which we had seen before was a signaling component affected by the introduction of
exogenous NO to the system. Each of these factors this time was studied in order to see the
effects that cisplatin and U0126 might have upon cell survival signaling.
Regarding p53, we looked at phosphorylation of serine 46 in order to study the effects of
cisplatin, NO, and U0126 upon the cells. p53 phosphorylation on serine 46 is involved with
induction of pro-apoptotic behavior of p53 (115). Most curiously, every instance where
exogenous NO was introduced into the cells produced a striking increase in the amount of
phosphorylation of p53(S46) when compared against cells treated with argon (Figure 5). This
includes cells treated with NO, cisplatin, and U0126, versus cells exposed to argon, cisplatin, and
U0126. In contrast, when looking at the total amount of p53, there was a decrease in the level
measured.
When observing the effects of said treatments upon the phosphorylation of MiTF, which
has been shown to be phosphorylated by ERK1I/2 then degraded (105), we observed an increase
in the level of phosphorylation when cells were treated with exogenous NO. The same increase
in phosphorylation was observed across all treatments when compared with argon treatments of
the same type, suggesting NO may favor degradation of MiTF (Figure 5B).
Cisplatin-induced ROS is a major inducer of cell death
Due to evidence in literature showing that ROS is involved in the response of cells to cisplatin
(116, 117), we sought to evaluate the role of cisplatin-induced ROS - which, like NO, are
oxidative species - in our model. For that purpose, we treated cells with the antioxidant
precursor NAC during the response to cisplatin. Treatment of cells with NAC and cisplatin
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showed a statistically significant increase in relative cell survival when compared with cells
treated with cisplatin alone (Figure 6A).
Cisplatin-induced ROS-modulation of the ERK1/2 pathway affects apoptosis
We next sought to define the effects that the combinations of ROS and cisplatin were having
upon the signaling components in our model. Leading from the previous result that showed ROS
were inducing cells death, we sought to establish if the ERK1/2 pathway was somehow
modulated by ROS. We also decided to look at the same apoptotic factors that were modulated in
the response to NO in earlier experiments. Briefly, cells were grown overnight and then cisplatin
or cisplatin and NAC were added to the melanoma cells for 24 hours. Modulation of the ERK1/2
pathway and downstream factors were studied by western blot. In terms of ERK1/2, decreased
phosphorylation was seen in cells treated with cisplatin alone versus the control, while the
addition of cisplatin and NAC in combination restored levels to those observed in the control,
indicating that ROS dramatically decrease MEKl/2 activity (Figure 6B).
When looking at PARP cleavage - an indicator of undergoing apoptosis -, there was
evidence of more cleavage when cisplatin was introduced alone when compared with the
combinatorial effects of cisplatin and NAC (Figure 6B). The anti-apoptotic protein Bcl-2 was
observed to have decreased levels in the presence of cisplatin alone, whereas the addition of
cisplatin and NAC to the cells restored Bcl-2 amounts to near control levels. Finally, the proapoptotic protein Bax was highly upregulated with the addition of cisplatin. In contrast, the
introduction of cisplatin and NAC to the cells led to a decrease in the activity of Bax. These
results show that lower levels of ROS promote an anti-apoptotic phenotype during the response
to cisplatin (Figure 6B).
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Cisplatin-induced ROS modulation of p53 and MiTF via the ERK1/2 pathway
After noticing the striking effects that ROS appeared to have upon the cells and the ERKl/2
pathway, we hypothesized that other factors seen in our experiments with NO could be
modulated by ROS as well. Knowing that the p53 pathway is often involved in cell cycle
inhibition and death, but can also be dysregulated in melanoma, we used cisplatin and NAC to
test this theory. Upon the addition of cisplatin we observed an increase in S46 phosphorylated
p53, and a similar increase when cells were treated with cisplatin and NAC and compared
against control cells (Figure 6C). In addition, when considering the effects of cisplatin and NAC
upon total p53 levels, we noticed that total p53 and MDM2 were induced more strongly when
treated with cisplatin alone.
Seeking to understand other factors that could be affecting downstream survival through
the ERK1/2 pathway, we decided to look at the transcription factor MiTF in the context of ROS
induced by cisplatin. As a transcription factor that can activate many other pro- and antiapoptotic factors, MiTF has a diverse array of functions that lead to cell survival or apoptosis,
and has been shown to regulate cellular responses to ROS (51). When phosphorylated by
ERK1/2, this leads to degradation of MiTF in melanoma cells. Our results showed that the
addition of cisplatin to melanoma cells alone caused an increase in the amount of phosphorylated
MiTF, a result that was also seen when cisplatin and NAC were added together to the cells
(Figure 6C). The same result was detected in the context of total MiTF levels, suggesting that,
unlike NO, ROS do not regulate MiTF in this model.
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Figures
outlets
NO
inlets
NO 02
02
I
U
propeller
I
t
U mU0
Silastic tubing
(1.5 mm i.d.)
stir bar
suspension cell culture
NOX 02
plate cell culture
NOX 02
Int t
stir bar
cover-slip reactor
FIGURE 1A: NO delivery system. Top: the original delivery system developed Wang & Deen
in 2003. The Silastic tubing allows for diffusion of controlled amounts of NO and 02 to the cell
culture medium in the reactor. Using 1%NO and 7.0 cm tubing, a steady-state concentration of
0.72 M NO is attained. Stir-bars stir the medium and other components introduced to the
system. In control reactors, the system is flushed with argon instead of NO. Ports on the lid allow
access to the system for injection of drugs such as cisplatin and U0126. In this study, this system
was used to expose cells to NO and several compounds for further analysis of cell homogenates
by western blotting. Bottom: the setup for the NO delivery system used to expose cells adhered
to coverslips for further analysis of cell survival. Right: detail of the rack used to place coverslips
with cells.
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1.21.5625
1+1
E
3.125
6.25
0.812.5
0.60.4-
li0
a0
0.2-
25
50
0
I
10
100
pM cisplatin
FIGURE 1B: Cisplatin toxicity in A375 cells. Cells (5 x 103 per well) were grown in 96-well
plates and treated with increasing concentrations of cisplatin. Cell viability was estimated with
the WST- 1 assay 24 hours later. Numbers along the curve refer to cisplatin concentration in pM.
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1.60 -_ns
1.40 -
cn 1.20 +1
e1.00 -
E 0.80
>0.60
S0.40o 0.20
I
tI
C 0.00
W
Argon
NO
Argon+cis
NO+cis
FIGURE 2: NO protects cells from cisplatin toxicity. Cells pretreated with 0.72 sM of NO for
24 hours were protected against a subsequent treatment with 25 [M cisplatin. Cells were tested
using the coverslip reactor described in Figure 1. Viability was tested using the WST- 1 reagent.
ns: not significant; **P<0.01; ***P<0.001.
30
1
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Of9
11
C
B
A
0
P-MEK
Oq
P-p53 (Ser15)
MEK
won"
P-ERK1/2
p53
ERK1/2
MDM2
P
P-MitF
MitF
P-actin
JP-actin
P-actin
FIGURE 3: NO modulates cell signaling during the response to cisplatin.
(A) ERK1/2 is regulated by exogenous NO, as is MEK. A375 cells were placed in reactors and
treated with 0.72 FM NO for 24 hours. The cells were then lysed and proteins analyzed by
western blot. Total ERK and total MEK were analyzed by reprobing the membranes after they
were probed for phosphor-proteins and stripped. (B) NO affects the activation of p53 (S 15) and
its inhibitor, MDM2 in cells treated as in (A). Total p53 was analyzed by stripping the membrane
after analysis of phospho-p53. (C) NO modulates MiTF. Cells treated as in (A) were processed
for analysis of phosphor-MiTF and total MiTF by western blot. Total MiTF was analyzed by
reprobing a stripped membrane previously used for phosphor-protein analysis. Cells exposed to
argon were used as controls. p-actin is shown as a loading control.
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0.1
0
A
0.5
5
1
10
pM U0126
P-ERK
ERK
s-Actin
B
1.0
.5
60.8
to
0
I'
0
EO.6
0.4
1
0.1
10
pM U0126
FIGURE 4: U0126 inhibition of ERK1/2 phosphorylation. (A) Increasing concentrations of
the MEK inhibitor U0126 led to a corresponding inhibition of the phosphorylation of ERK1/2.
Cells grown in 10 cm dishes were treated with differing amounts of U0126 for 24 hours. Cell
extracts were made and then analyzed by western blot with antibodies for phosphor- and total
ERK1/2. P-actin is shown as a loading control. (B) Dose-dependent cell death upon increasing
concentrations of U0126. Cells were grown for 24 hours in 96-well plates and then treated with
the indicated concentrations of U0126. Relative survival of cells was analyzed 24 hours later
with the WST- 1 reagent.
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C
x
XU- -6\co
0
ERK1/2
x
'C
x
(
P-ERK
ERK
BcI-2
Apoptotic
Bax
factors
PARP
P-actin
FIGURE 4C: NO modulates the ERK1/2 pathway and apoptotic factors during the
response to cisplatin. (C) Cells were grown overnight in 10 cm dishes and 24 hours later placed
within reactors and exposed to 0.72 [tM NO for 24 hours. Cisplatin (12.5 tM) and U0126 (5
1 M) were then added and exposure to NO continued for an additional 24 hours. Cell extracts
were prepared and analyzed by western blot. p-actin is shown as a loading control.
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4)
X
C ;14
p
C
.S.2
P
P-p53 (S46)
p53
P53
~"WON"m
P-MitF
MITF
MitF
0
j P-actin
FIGURE 5: NO modulates p53 and MiTF during the response to cisplatin. Cells were
treated as described in Figure 4C and protein expression analyzed by western blot. P-actin is
shown as a loading control.
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A
1.2 cc
+ 1.0 -
T~
"r
E0
0.8 -r
0.6 .0
0
+
0.4 0.2 0.0 -
--- T-
Control
--
Cisplatin
r--
-I
Cisplatin+NAC
FIGURE 6A: Cisplatin-induced ROS is a major inducer of melanoma cell death.
(A) Removal of ROS by treatment with the antioxidant precursor (NAC) leads to increased cell
survival upon challenge with cisplatin. Cells grown in 96-well microplates were treated with
12.5 sM cisplatin and 5 mM NAC. Relative survival was estimated 24 hours later by WST-l
analysis. **P<0.01; ***P<0.001.
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B
#'
\
G
INV
C)",
Phospho-ERK
ERK1/2
ERK
PARP
Apoptotic
factors
BcI-2
Bax
P-Actin
FIGURE 6B: Cisplatin-induced ROS modulates the ERK1/2 pathway but does not affect
apoptotic proteins. (B) Cells were grown overnight in 10 cm dishes then treated with 12.5 pLM
cisplatin or 5mM NAC. After 24 hours, cell extracts were made and analyzed by western blot.
Immunoblot analysis of total ERK protein was performed on the same membranes used for
phospho-protein analysis after stripping. p-actin is shown as a loading control.
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C,
C
.\t
NISO
C;
Phospho p53 (S46)
p53
p53
MDM2
Phospho-MitF
MITF
MitF
P-Actin
FIGURE 6C: Cisplatin-induced ROS partially modulate p53 but not MiTF. (C) Protein
expression and modification by phosphorylation was studied in cells treated as described in
Figure 6B.
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DISCUSSION
Melanoma treatment has proved problematic over the years, especially because the exact
development of the disease remains unclear in many ways. Due to this fact, elucidating effective
therapies that deal with cellular resistance to chemotherapeutics remains a problem (3, 5, 67).
Although cisplatin has proved more efficacious in other cell types, such as ovarian cancer, it is
not uncommon that even in those cases the cancers have later become resistant to the
chemotherapeutic and patients often suffer a relapse. In those instances, the treatment of cisplatin
will no longer be efficacious, owing to tumor acquired resistance (118). From nucleotide
excision repair (NER) to glutathione (GSH) conjugation, cancer cells have continued to evade
death by cisplatin and its large-helix distorting DNA damage (17, 18, 119). In addition, cisplatin
resistance has been shown to arise through the upregulation of several signal transduction
pathways, including MAPK and p53 (15), causing major limitations to cisplatin-based
chemotherapy. Our present findings evidence the dysregulation that can happen in the tumor
suppressor p53 upon exposure to exogenous NO and to endogenously generated ROS. As well,
we provide a framework through which melanoma cells provide for cell survival even in the
context of cisplatin treatments, and we establish a pathway through which resistance to apoptosis
occurs in A375 cells. Overall, our results bring together key components of the cell regulation
that result from the addition of NO at physiologically relevant concentrations and the generation
of ROS by cisplatin-stimulation through the ERK1/2 pathway and p53.
Two years ago, we first began the project by researching NO and how it might be
affecting the nuclear factor-kB (NF-KB), which is a ubiquitously expressed family of Rel-related
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transcription factors. In response to stimuli such as inflammatory cytokines, oncogenes, and
viruses, the proteasome-dependent degradation of IKB allows the translocation of NF-KB to the
nucleus where it binds to the promoter region of target genes involved in cellular responses such
as apoptosis. It had been noticed that in many cancer cells, constitutive activation of NF-KB
activity lowered cell sensitivity to apoptotic stimuli and favored neoplastic cell survival (120).
NF-KB consists of a family of structurally related proteins, including Rel A (p65), Rel B, c-Rel,
NF-KB 1 (P50/p105), and NF-KB2 (p52/p100). In human melanoma cell lines it had been shown
that the MAPK pathway regulates NF-icB and that this in turn is a regulator or iNOS expression
(121). Activation of NF-KB is implicated as an important mechanism for the development of
anti-apoptotic signals and drug resistance in multiple myeloma (122).
Within this structure, we looked specifically at the subunit of NF-KB, the p50 homodimer,
which binds DNA in vitro but lacked a transcriptional activation domain and only weakly
initiated transcription, if it initiated transcription at all (121). BCL3, which is a putative
oncogene encoding for a protein that belongs to the inhibitory KB-family and is a proto-oncogene,
has been seen to either activate or inhibit NF-KB-dependent gene transcription through
interactions with p50 or p52 homodimers. High expression of BCL-3 contributes to increased
proliferation of cells and contributes to the regulation of NF-KB-dependent gene transcription
(122). There is also evidence that either p50 is required for transcription of the BCL-3 gene or
association with p50 stabilizes the BCL-3 protein and may also promote its nuclear localization.
The p50*BCL-3 complexes recruit HDAC- 1 to specific KB sites in the TNF-a promoter and
thereby repress transcription (123).
Upon further research, we also tied in the component of CYLD perhaps being involved.
CYLD is also important in this pathway of NF-KB, for it encodes a 107 kDa polypeptide that is
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ubiquitously expressed and contains a deubiquitinating domain at the C terminus, which removes
lysine 63 linked polyubiquitin chains from TNF receptor-bound TRAF2. This removal leads to
inhibition of IKB kinase complex and stabilization of the NF-KB inhibitor IKB-a, resulting in
retention of NF-KB heterodimer in the cytoplasm. Mutations in the CYLD gene cause
hyperactivation of p65/p5O NF-KB, leading to tumor cell survival. It had been evidenced that
nuclear accumulation of BCL-3 increased in tumors lacking CYLD and this also led to increased
expression of cyclin D1. Cancerous cells often have severely downregulated levels of CYLD
(124).
We raised a question about this potential pathway based on the fact that RAF enhances
NF-KB transcriptional activity through an autocrine feedback loop using the MEK/ERK pathway
(125). This result shown in the MEK/ERK pathway would later lead us back to this MAP kinase.
Pertaining to NF-KB, it had not been deduced if NO, which is known to post-translationally
modify proteins and pathways, was involved in said feedback loop. Through the use of inhibitors
and western blots looking at endogenous NO already within four cell lines, namely MeWo, Mel
100, Mel 28, and A375, we sought to establish a novel NO-based modulatory component in the
NF-KB pathway. Cells were treated with cisplatin and the NOS inhibitor 1400W for various time
points, and the proteins phospho-p65, phospho-p50, BCL-3, CYLD, p65, and p50 were analyzed
by western blotting. Total p50 and p65 were observed, but not CYLD and BCL-3. The
phosphorylated forms of p50 and p65, which indicate their activation, were not observed under
any conditions, though. After months of using different parameters, we were unable to see any
appreciable results or modulations in the system. This indicated that, under the conditions
investigated, the NF-KB pathway does not play a significant role in the response to cisplatin.
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Remembering that Godoy et al. (20) had seen that ERK1/2 was more highly
phosphorylated in cells that were treated with NMA and cisplatin and resulted in enhanced death,
we turned our attention to the ERK1/2 pathway. That work had provided evidence that
endogenous NO activated ERKlI/2 and that removal of said NO by NMA led to decreased levels
of BCL-2, and a resultant increase in the pro-apoptotic protein Bax. The work provided a
springboard for further research into the whole of the ERKl/2 pathway, for there was a lack of
information at the time about how endogenous NO, cisplatin, and the ERK1/2 pathway were
interrelated, if they were at all.
ERK1/2 is one of the most studied proteins in terms of its relation to cancer progression
and resistance to cell death. However, the protein can play several different roles in different
contexts and with different perturbations. In addition to its role in upregulating survival signaling
and downstream effectors of proliferation, it has also been seen to mediate apoptosis through
acting upstream of mitochondrial cytochrome c and caspase-3, both of which lead to cell death.
Not only that, but ERK1/2 has been linked to neuronal death signaling, calling into question
what signals are acting upon ERK1/2 and its resultant regulation of cells (20, 126). Although
ERKl/2 has been observed to play a role in cell death, much research has been focused on the
pro-survival aspects elicited during and after the activation of ERK1/2 when phosphorylated by
MEK. It has been evidenced that various growth factors can work together in order to activate
ERK1/2 in primary human melanocytes, and, more importantly, mutations in BRAF (which is
mutated in about 70% of melanomas) can lead to growth-factor independent activation of
ERK1/2, and mutations have also been observed in N-Ras, another upstream factor linked to the
MAPK pathway involving ERK (127, 128). Yet, drugs have only proven so effective, for the
cancers often find a way around single therapies, such as using the inhibitor PLX4032 to inhibit
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RAF (2, 129). In particular, though, our studies were guided by findings that ERK1/2 could drive
iNOS expression in human melanoma (130), and caused us to wonder whether NO generation
was leading to the modulation of the MAPK pathway.
Another piece of information that directed our experiments came from looking for other
signaling pathways that could be relevant to the response to drugs in melanoma. We found no
evidence of Akt activation upon cisplatin treatment. On the other hand, several components of
the MAP kinase family of signaling proteins were found to be expressed and modulated.
Phospho-ERK1/2 was expressed rather strongly under cisplatin treatment for 24 hours, with
indications that the concomitant addition of NOS inhibitors led to alteration in the level of
expression and activation of ERK1/2. For quite some time, our efforts focused on the ERK 1/2
pathway, and each experiment revolved around this pathway in relation to NO and cisplatin. In
previous experiments, our lab had shown that NMA and cisplatin both had an effect in
upregulating activation of ERK in A375 cells. Through various experiments, starting with the
dosing of A375 and MeWo cell lines with cisplatin, the expression of phospho-ERK was
characterized.
Gradually, results became more promising, for there was an effect on the expression of
phospho-ERK when cisplatin and NMA were utilized against the two different cell lines. Cells
were treated with 5 mM NMA for 96 hours and then 12.5 or 25 tM cisplatin for 24 hours in
order to see the effects NO decrease on ERK during the response to cisplatin. Bcl-2, which is an
anti-apoptotic protein of relevance to melanoma cells, was added to the study as well. With the
use of NMA, Bcl-2 expression decreased significantly, and the change was even more drastic
with the combination of NMA and cisplatin. However, producing these results using
endogenously generated NO gave varying results that were often not consistent or reproducible.
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Our lab had also been looking at the response of melanoma cells to cisplatin after the
delivery of exogenous NO. It had been shown by Oliveira et al. that low levels of S-nitroso-Nacetylpenicillamine (SNAP) led to upregulation of ERK1/2 and downstream ELK1, leading to
upregulation of ERK1/2 signaling (131). In preliminary experiments, we saw that low levels of
exogenous NO could also provide protection against cisplatin, which led to our hypothesis that
exogenous NO in low levels that mimicked inflammation could protect melanoma cells against
anti-cancer agents such as cisplatin. As mentioned in the introduction, NO and iNOS in
particular can have a whole host of effects, which lead to survival or apoptosis depending on NO
concentration (20, 59-61). Here we have provided evidence that, not only does exogenous NO
lead to the upregulation of anti-apoptotic signals while also downregulating signaling from proapoptotic sources, but we have also shown that cisplatin-induced ROS is a major inducer of cell
death, both factors modulating cell proliferation and death through p53 and the ERK1/2 pathway.
This work adds another component to the complex network that is tumor resistance to
chemotherapeutic agents.
The lack of literature dealing with systems that mimic inflammation except for in the
context of high levels of NO leading to cell death (132) led to our use of the NO delivery system
(71, 74) and later macrophages in a co-culture system (114) in order to deliver a prescribed
amount of NO to melanoma cells. Our results showed that not only does exogenous NO in
submicromolar levels protect against cisplatin, but the results also provide a stark contrast
between the effects of endogenous and exogenous NO. Whereas Godoy et al. showed that
ERK1/2 was being upregulated and leading to cell death when endogenous NO was depleted, we
observed that in the context of exogenously delivered NO cells not only upregulated ERKl/2,
but this also led to the upregulation of p53 in response to NO alone, NO and cisplatin in
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combination, and NO, cisplatin, and U0126 all together when compared to control (i.e. not
treated with NO) cells.
As well, and just as importantly, MiTF was affected by the delivery of NO. MiTF
controls a variety of factors that can lead to angiogenesis, apoptosis, block DNA damage, stop
proliferation, and lead to migration/invasion, giving it a central role in melanoma progression
and death. From controlling Bcl-2 to leading to p21 inhibition of proliferation, MiTF appears to
be at the center of a tangled web of interactions that could lead to apoptosis for melanoma cells,
or allow them to survive chemotherapeutic insults. It has already been established that ERK1/2
interacts with Bcl-2 through MiTF, and other studies done looked at a moderate increase of ROS
in the framework of MiTF, showing that such cells were more resistant to H20 2-induced cell
death, but our results served to heighten aforementioned analyses and broaden their context
through the use of cisplatin and ROS inhibition (51, 89, 100). In addition, we hoped to enhance
the groundwork laid by other in terms of how ERK1/2, p53, MiTF, and Bcl-2 were interacting in
melanoma cells in response to NO, ROS, and cisplatin.
We here provide evidence that modulation of the ERK1/2 pathway through inhibitors
indeed leads to changes in downstream factors related to melanoma survival. By pre-exposing
cells to non-toxic doses of NO, and then treating them with the MEK inhibitor U0126 and with
cisplatin, we showed that the phosphorylation of ERK1/2 increased dramatically when compared
with cells not exposed to NO. Importantly, we also showed the upregulation of downstream
factors such as anti-apoptotic Bcl-2, while the opposite was seen for pro-apoptotic Bax. Yet, the
most exciting results came from p53 and MiTF analyses. p53 has been extensively studied in the
context of cancer and is often mutated in other cancers - although not in most melanoma, where
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it is instead more frequently dysregulated and its accumulation seems to be a sign of tumor
progression (12).
It is known that MAP-kinases can lead to phosphorylation of p53 (12, 133-136). We here
showed upon the addition of 5 itM U0126, plus NO and cisplatin, there was an increase in the
phosphorylation of p53. This could indicate that p53 is dysregulated in our system, and is in fact
halting the cell cycle to allow melanoma cells to fix the damage induced by cisplatin. In addition,
another intriguing result came from the fact that it appears as though exogenous NO can counter
the effects of U0126 upon the phosphorylation of ERK1/2 by MEK. Perhaps this is occurring
through another molecule, such as PKC or KSR (Kinase Suppressor of Ras) or MP 1 (MEK- 1
Partner 1) interacting with ERK1/2 (25, 137), or through a yet unknown mechanism, but the
simple fact that ERK1/2 is still phosphorylated in the presence of U0126 and NO while cells
treated with U0126 and argon are not hints at a much larger role for NO and ERK1/2 in cell
signaling.
Adding another layer to the evidence that halting the cell cycle may be a strategy for cell
survival in response to cisplatin in the present model is the increased phosphorylation of MiTF in
presence of NO, regardless of other concomitant treatments. MiTF participates in Gl cell cycle
arrest after exposure to UVC via ERK1/2 and p21, and, although the transcription factor is
degraded when phosphorylated, others have shown that the gene is amplified in 15-20% of
metastatic melanoma. A possible explanation for this increased MiTF activity in spite of its
degradation comes from studies that suggest that the transactivation activity of MiTF is increased
while its half-life is decreased (5, 138). We speculate that the fact that both p53 and MiTF were
affected in a similar fashion by exogenous NO in our experimental model suggests that both
could be part of an NO-dependent mechanism of survival of melanoma cells in response to
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cisplatin. Moreover, as both MiTF and p53 are substrates of the ERK1/2 kinase activity, these
two transcription factors could integrate a novel NO-controlled mechanism interconnecting the
ERK1/2 pathway and chemoresistance.
ROS have been associated with ERK activation and death in renal cells. In addition, ROS
modulate several cellular events and to cause cell death when induced by cisplatin in renal and
neuronal cells (21, 126, 139-144). We hypothesized that a similar ROS-mediated effect might be
happening with melanoma cells challenged with cisplatin. In fact, we saw a statistically
significant increase in relative cell survival in cell treated with the antioxidant NAC in
combination with cisplatin when compared with cells treated with cisplatin alone. This provided
evidence that ROS in A375 cells is indeed acting upon the cells and causing a higher amount of
cell death induced by cisplatin.
Considering that, like NO, ROS are not only oxidative species but also cell-signaling
modulators, we investigated whether the above described components modulated by NO were
affected by ROS. Treating cells once again with either cisplatin or cisplatin and NAC, we looked
at resultant protein expression and modification by western blotting analysis. While decreased
phosphorylation of ERK1/2 was seen upon cisplatin treatment, the addition of NAC to the
cisplatin challenge restored levels of ERK1/2 phosphorylation to the observed in control,
untreated cells. This result provided evidence that ROS was indeed strongly modulating the
ERK1/2 pathway. As an additional proof of the capability of NAC to reverse the inactivation of
ERK1/2 by cisplatin and rescue cells from apoptosis, was the lower amount of PARP cleavage in
cells treated with cisplatin plus NAC in contrast with cisplatin alone. Furthermore, the antiapoptotic protein Bcl-2 had decreased levels of activity in cisplatin alone when compared with
cisplatin and NAC, while the pro-apoptotic Bax showed the reverse result.
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Finally, in an approach similar to the used in our NO studies, we evaluated the effects of
ROS on cisplatin-challenged cells regarding the possible modulation of p53 and the transcription
factor MiTF. We observed an increase in S46 phosphorylation of p53 upon addition of cisplatin
alone; however, the addition of NAC did not change this profile. Results with MiTF were
similar, in the sense that NAC-mediated decrease in ROS did not change cisplatin induced MiTF
phosphorylation. Interestingly, MiTF has also been shown to modulate cell responses to ROS
(51).
In light of the above results, we conclude that while ERK1/2 activity is stimulated by
NO, ROS exerts the opposite effect on this MAP-kinase pathway, since NAC is able to restore
MEK activity in presence of cisplatin. Intriguingly, the removal of ROS by treatment with NAC
did not promote increased cell survival through modulation of MiTF and p53 through the same
mechanisms as NO. Also noteworthy, MDM2 - which is produced in response to p53 and also
inhibits the transcriptional activity of p53 - was decreased in presence of NAC and cisplatin
when compared to cisplatin alone. While Wang et al. showed that NO stabilizes p53 via MDM2
(145), our results show that ROS do not promote the same phenotype. These findings
demonstrate the complexity involved in the regulation of these signaling components by
oxidative species of distinct natures such as NO and ROS, as illustrated on Figure 7.
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Pathway Construction
NO!
U0126
MEK
NO
NAC
Cisplatin
ERKP
ROS
ROS
NO
DNA
Damage
IV
MiTFP
* Degredation
MiTF
p53P
NO Y
Bel-2
MDM2
Survival
Apoptosis
FIGURE 7: The possible pathways regulated by ERK1/2 through NO and ROS during the
response to cisplatin. Depending on the extent of cell damage induced by cisplatin, cells will
activate DNA-repair and cell-cycle arrest mechanisms that may be successful, leading to cell
survival, or insufficient to allow the progress of cell cycle - in which case apoptotic cell death
ensues. This study addressed the effects of cell exposure to exogenously generated NO, as well
as to cisplatin-induced ROS, on signaling components that regulate the cell fate during the
challenge with cisplatin. NO and ROS showed opposite effects on the promotion of cell
resistance to cisplatin toxicity, with NO being revealed as a strong activator of the survival
signals provided by the ERKl/2 signaling cascade. In this experimental approach, ERK1/2
activity modulation with the MEK inhibitor U0126 suggested that ERK1/2 be the kinase leading
to phosphor-regulation of downstream transcription factors p53 and MiTF. NO was shown not
only to favor the activity of ERKl/2 itself, as evidenced by ERK increased phosphorylation, but
also downstream components such as p53, MiTF and pro- (Bax) and anti-apoptotic (Bcl-2)
proteins. On the other hand, ROS was associated with lower activation of ERK1/2, limited
regulation of p53, and no clear effect on MiTF or apoptotic proteins.
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CONCLUSIONS
We here have established new mechanisms through which ERK1/2 activity modulates
melanoma response to cisplatin through ROS and NO. Activation of ERK1/2 by NO leads to
subsequent increase in phosphorylation of p53. This tumor suppressor, which is often
dysregulated in melanoma, subsequently leads to higher activation of MiTF, p21, and Bcl-2,
halting the cell cycle and perhaps allowing for DNA repair in response to cisplatin. Another
possible pathway could be that p53 and MiTF are working independently through separate
pathways that are both activated by ERK1/2.
Even though the present results have broadened our understanding of melanoma and
chemoresistance, there is much work that can be done to not only strengthen these results, but
also provide a larger picture of how NO and ROS modulate the fate of melanoma cells during the
treatment with cisplatin. For example, an obvious direction would be to study S-nitrosylation of
the proteins approached in this context, and how their activity may change as a result of such
post-translational modification during the cell response to cisplatin. Work following this thesis
will focus on characterizing the levels of S-nitrosylation of specific proteins, namely Bcl-2, p53,
and MiTF, and perhaps even ERK1/2 itself. Using immunoprecipitation coupled to an NO
Analyzer, we would be able to identify a link between the amount of S-nitrosylation of given
targets and the phenotype cells exhibit when challenged with cisplatin, which will add to the
overall picture of the pathways that we have presented. It would be particularly interesting to see
whether NO and ROS are somehow interacting through both the use of NAC and exogenously
delivered NO measured in the NOA. In addition, through the use of the co-culture system to
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expose melanoma cells to activated macrophages, we could establish another model with which
to measure and study levels of NO that mimic inflammation and their modulatory effects upon
target cells. Finally, through combinatorial therapy, perhaps we may overcome the resistance to
cisplatin while also knocking out key pathways that lend themselves to said resistance. All this
information could serve to bring into focus methods through which more permanent recession of
cancer can be attained.
Although there is much work still to be done, this study provides groundwork for the
future, bringing to light novel interactions among once seemingly different pathway components,
broadening the knowledge about chemoresistance.
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