Role of cyclooxygenase-2 in tumor progression and immune

Indian Journal of Biochemistry & Biophysics
Vol. 44, December 2007, pp. 419-428
Role of Cyclooxygenase-2 in Tumor Progression and Immune Regulation in
Lung Cancer
Swati Patel and Shubhada Chiplunkar
Chiplunkar Lab, Advanced Centre for Treatment, Research and Education in Cancer (ACTREC),
Tata Memorial Centre, Kharghar, Navi Mumbai 410 210, India
Received 29 August 2007; revised 08 October 2007
Lung cancer is the leading cause of cancer death all over the world. The low 5-year survival rate (under 15%) has
changed minimally in the last 25 years. Amongst different types of lung cancers, non-small cell lung carcinoma (NSCLC)
types account 25-40%. To improve the survival of lung cancer patients, new therapeutic strategies are needed. The search
for improved therapies has led to the investigation of agents that target novel pathways involved in tumor proliferation,
invasion, survival and immune regulation. Cyclooxygenase-2 (COX-2) is one of the novel targets under evaluation for
NSCLC therapy and chemoprevention. Although multiple genetic alterations are necessary for lung cancer invasion and
metastasis, COX-2 may act as central element in orchestring these processes. COX-2 plays an important role in all aspects
of tumor development and growth. It also plays a pivotal role in regulation of cytokines and immune responses in NSCLC
patients. In this article, we review the experimental and clinical evidences on the possible link between COX and NSCLC.
Keywords: Cyclooxygenase (COX), Non-small cell lung carcinoma (NSCLC), Non-steroidal anti-inflammatory drugs
Lung cancer results in an enormous clinical burden
for health-care providers with very high mortality
rates. Despite extensive research, the overall 5-year
survival rate is only 8-14% and has improved only
marginally during the past 25 years. Amongst
different types of lung cancers, non-small cell lung
carcinoma (NSCLC) types account 25-40%. To
improve the survival, advances are needed in early
For correspondence
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Abbreviations: AA, arachidonic acid; APC, antigen presenting
cell; Bcl-2, B-cell lymphoma leukemia-2; C/EBP, CCAAT
enhancer-binding protein; DCs, dendritic cells; ECM,
extracellular matrix; EGF, epidermal growth factor; EGFR, EGF
receptor; EP, PGE2 receptor; ERK, extracellular signal-regulated
kinase; GSK-3b, glycogen synthase kinase-3b; IP3, inositol
trisphosphate; JNK, c-Jun N-terminal kinase; LAT, linker for
activation of T cells; LEF, lymphoid enhancer factor LPS,
lippopolysaccharide; MAPK, mitogen-activated protein kinase;
MEK1/2, mitogen-activated extracellular signal-regulated kinase;
NSCLC, non-small cell lung carcinoma; 15-PDGH, 15-hydroxyprostaglandin dehydrogenase; PGs, prostaglandins; mPGES,
microsomal prostaglandin E synthase; PI3K, phosphatidylinositol
3-kinase; PKC, protein kinase C; PLA2, phospholipase A2; PLC,
phospholipase C; PPAR, peroxisome-activated receptor; Src, nonreceptor tyrosine kinase; TCF, T cell factor; TNM, tumors, nodes
and metastases.
diagnosis and in development of new therapeutic
strategies. These require a better understanding of the
molecular mechanisms underlying cell transformation
and tumor progression. New therapies could target
molecular pathways essential for tumor proliferation,
angiogenesis, apoptosis and immune suppression.
Cyclooxygenase (COX)-2 is one of the novel
targets under evaluation for NSCLC therapy and
chemoprevention. It plays an important role in all
aspects of tumor development and growth. Evidences
suggest that it is involved in multiple pathways,
conferring the malignant and metastatic phenotype in
lung cancer. Although multiple genetic alterations are
necessary for lung cancer invasion and metastasis,
COX-2 may act as central element in orchestring
these processes1-3. Various studies have shown
association of overexpressed COX-2 with apoptosis
resistance4,5, angiogenesis6, decreased host immunity7-9
and enhanced invasion and metastasis10,11. In this
article, we have reviewed experimental and clinical
evidences on the possible link between COX and
NSCLC with respect to tumor progression and
Biochemistry of COX-1 and COX-2
Receptor activation or cell injury signals causes
release of arachidonic acid (AA) from lipid membrane
stores by activity of lipases, such as cytosolic
phospholipase A212. COX catalyzes the cyclization of
AA to PGH2, hence also known as prostaglandin H
synthase (PGHS)13. It accomplishes this by first
generating prostaglandin (PG) G2, by conversion of
polyunsaturated 20-carbon AA to varying degree of
unsaturation by oxidative cyclization of central 5
carbon atoms. COXs also have peroxidase activity
and catalyze hydroxylation of cyclopentens through
endoperoxidation at position 15C converting PGG2 to
PGH2. Isomerase enzymes that are expressed in both
tissue and cell type selective manner govern the next
step, namely conversion of PGH2 to cell type specific
production of individual prostanoids. Five primary
prostanoids PGE2, PGD2, PGF2, PGI2 and
tromboxane A2 (TXA2) are produced by different cell
types which are involved in different physiological
functions (Fig. 1).
Two isoforms of COX, COX-1 and -2 are known
COX-1 is constitutively expressed in most cells
and tissues and regulates normal house-keeping
functions14, while COX-2 is inducible in response to
various stimuli like cytokines and growth factors in
most cells, except in neuronal and renal cells in which
it is constitutively expressed14. Despite overall
similarities, two enzymes show subtle differences in
the amino acid composition at the active sites. COX-2
has valine at positions 89 and 523, while COX-1 has
isoleucine, resulting in large space availability in the
former. Further, the presence of valine at position 434
in COX-2 as against isoleucine in COX-1 allows a
gate mechanism to operate in favor of the former15.
These differences in active site structures provide
specificity to COX inhibitors as selective COX-1 or
COX-2 inhibitors. Also, despite overall similarities in
structure, substrate and catalytic activities, both the
isoforms display differences in their expression
profiles and in the panel of cellular responses evoked.
These activities of COX are likely to be dictated by a
localization and coupling to specific prostaglandin
synthases, as well as by the specific cellular
pattern of expression of prostanoid receptors, which
are known to trigger different intracellular signaling
pathways. The major metabolite of COX
pathway is PGE216, which regulates key responses in
humans including reproductive, gastrointestinal,
neuroendocrine and immune system. PGE2 exerts
its effect through G protein coupled 7 span
transmembrane receptors EP1-4.
Fig. 1 — PGE2 biosynthesis pathway [Phospholipase A2 (PLA2)
catalyzes conversion of phospholipids to arachidonic acid which
is converted to prostaglandin G2 (PGG2) by COX-1/2, which in
turn gets converted to prostaglandin H2 (PGH2) by peroxidase
activity of COX isoforms. PGH2 is then converted to
prostaglandin I2 (PGI2), thromboxane A2 (TxA2), prostaglandin
E2 (PGE2), prostaglandin D2 (PGD2) and prostaglandin F2
(PGF2) by specific isomerases and synthases depending upon
type of tissue or cells involved. PG, prostaglandin, Tx,
COX-2 in NSCLC Patients
COX-2 overexpression and high production of
PGE2 is reported in variety of malignancies including
NSCLC1,3,7. COX-2 activity has been detected
throughout the progression of a premalignant lesion to
the metastatic phenotype. Higher COX-2 expression
has been observed in lung cancer lymph node
metastasis, compared to primary adenocarcinoma1.
COX-2 over expression appears to portend a shorter
survival among patients with early stage NSCLC and
the extent of its expression is found to be associated
with both decreased overall and diminished diseasefree survival rates17. In another study, linear relationship between COX-2 expression and aggressive
behavior of tumors in NSCLC and increased mortality
rate has been found18. Association of COX-2
expression with poor prognosis, independent of TNM
staging in surgically-resected NSCLC patients has
also been reported19. A polymorphism in the COX-2
gene with an increased risk of lung cancer has been
observed20. Thus, these reports suggest the involvement of COX-2 in the pathogenesis of NSCLC.
COX-2 in Tumor Progression
Smoking and other mutagenic environmental
factors capable of inducing cell transformation play
an important role in tumor progression in the
respiratory tract. COX-2 activates tobacco smoke
carcinogens such as benzoapyrene (BaP)21. It
catalyzes the conversion of BaP-7, 8-dihydrodiol to
BaP-diol epoxide, which binds to DNA22. Also,
BaP has been demonstrated to induce epithelial
COX-2 expression and PGE2 production23. COX-2
expression is induced by the various inflammatory
stimuli such as TGF-β, IL-1β, hypoxia and epidermal
growth factor (EGF) generated in pulmonary
microenvironment of those at high risk of lung
cancer24. In addition, various types of mutations and
oncogenes expressions are also associated with high
COX-2 expression and progression of lung tumors.
Wild-type tumor suppressor p53 suppresses
COX-2 transcription, but not the mutant p53,
suggesting that p53 mutation also has a role
in COX-2 overexpression25. Also, K ras and β catenin
mutations are associated with elevated COX-2
CCAAT/enhancer binding protein (C/EBP) and
activating transcription factor/cAMP response
element binding protein (ATF/CREB) also play a
critical role in the regulation of basal COX-2
expression in murine lung carcinoma27. These
molecules can be potential target for down regulating
COX-2 expression in lung cancer. Transcription
regulation of COX-2 also involves nuclear factor of k
chain in B cell (NF-kB), activator protein-1 (AP-1)
and nuclear factor of activated T cells (NF-AT).
COX-2-/- mice have shown attenuation of Lewis lung
carcinoma (LLC) tumor growth, but not the wild type
and COX-1-/- mice, confirming the role of COX-2 in
tumor progression28. Microtubule-interfering agents
such as taxol can stimulate COX-2 transcription via
ERK and p38 MAP kinase pathways and promotes
carcinogenesis by overexpressing COX-229.
COX-2 in Angiogenesis
Angiogenesis is an important factor in tumor
development. Tumor associated angiogenesis is
mediated by the migration and proliferation of host
endothelial cells and is a requisite for tumor growth30.
The mechanisms for promotion of tumor-associated
angiogenesis are suggested to be activated in the early
stages of tumor development31. Vascular endothelial
growth factor (VEGF), transforming growth factor
(TGF)  and β, basic fibroblast growth factors (FGF)
and chemokines like IL-8 are implicated in tumorrelated angiogenesis in lung cancer32.
Because COX-2 has been reported to be localized
in both tumor cells and the adjacent stromal cells, PGs
generated through the presence of COX-2 might act
on the tumor cells (a cell autonomous effect) or
surrounding stroma cells (a cell non autonomous or
landscaping effect) to facilitate tumor development33.
Thus, the products of COX pathway in tumor milieu
may promote tumor growth. PGE2 not only activates
G coupled proteins, but also indirectly activates Wnt
signaling, peroxisome poliferator activated receptors
(PPAR)  and epidermal growth factor receptor
(EGFR) pathway34. It also up regulates Wnt target
genes, such as cyclin D1 and VEGF35. It is a potent
vasodilator and can act synergistically with other
mediators and chemotaxins to increase microvascular
permeability35. In conjunction with cytokines, PGE2
is a central mediator of febrile responses36. Genetic
mutations of various oncogenes and tumor suppressor
genes and dysregulated immune responses are
associated with regulation of these angiogenic
Host-derived COX-2 products are shown to be
instrumental for sustained tumor growth, as lung
cancer cells injected into syngeneic mice bearing a
genetic background of COX-2 (not COX-1)
deficiency have shown a markedly reduced growth
rate28. Expression of COX-2 and nitric oxide synthase
(NOS)-2 is correlated with the VEGF expression level
as well as microvascular density in tumors of NSCLC
patients38. COX-2-dependent expression of ELR+
angiogenic chemokines CXCL5 and CXCL8 has been
demonstrated in vitro and in vivo in NSCLC. In
addition, CXCL5 and CXCL8 are decreased with
inhibition of COX-239. Treatment with anti-CXCL5 or
anti-CXCL8 neutralizing antibodies results in a
significant decrease in tumor growth. Moreover,
neutralization of PGE2 leads to a decrease in CXCL5
and CXCL8 production39. This implies specific role of
PGE2 in angiogenesis and neovasculararization
during tumor development process.
COX-2 in Tumor Invasion and Metastasis
COX-2 can also affect cell invasion, which is vital
for the dissemination of metastatic cells across
extracellular matrix (ECM) and spread to distant
organ sites. Its elevated expression has been shown to
increase tumor invasiveness and enhance metastatic
potential28,40. Lung cancer cells with elevated COX-2
also display concomitant increase in expression of
CD44, a cell surface receptor for hyaluronate, a major
glycosaminoglycan component of ECM41. In fact,
antibody-mediated blockade of COX-2-derived PGE2
is sufficient to decrease CD44 and matrix
metalloproteinase-2 (MMP-2) expression as well as
invasion in an EP4-dependent manner9. Exposure of
NSCLC cells to PGE2 up-regulates CD44, EP4, and
MMP-2 expression and enhances cell invasion. CD44
induces co-clustering with MMPs and can, therefore,
promote MMP activity, tumor invasion and
angiogenesis42. In monocytes, PGE2 is found to
activate membrane type-1 MMP (MT-1 MMP) and
thereby promotes activation of MMP-243. This study
in NSCLC indicates an important autocrine/paracrine
role for PGE2 in regulation of CD44 and MMP-2dependent invasion in human NSCLC.
Laminin 5, an ECM protein involved in cell
migration and invasion is frequently expressed in
several different malignancies10. It is also coexpressed with COX-2 in early stage lung
adenocarcinomas10. EGFR signaling pathway has
been shown to induce both COX-244 as well as
laminin 510. Also, RAS mutations in multiple
myeloma cells cause selective induction of COX-2,
resulting in high binding to ECM protein and
chemotherapeutic drug resistance, conferring high
metastatic properties to these cells45. Another
mechanism of metastasis and invasion in lung cancer
includes lymphangiogenesis or formation of new
demonstrated that human lung adenocarcinoma cells
that overexpress COX-2 significantly increases vegf-c
gene expression, associated commonly with
lymphangiogenesis in animal models11. Induction of
vegf-c occurs through PGE2 activation of the
Her2/neu tyrosine kinase signaling pathway via the
EP1 receptor. Thus, this study provides further
evidence for interactions between COX-2 and human
EGF signaling in the development and progression of
lung cancer.
COX-2 in Apoptosis
Early tumor growth is dependent on the balance
between the rates at which cells are produced through
cell division and die through a natural cell death
process known as programmed cell death or
apoptosis. Defects in apoptosis pathway allow tumor
cells to survive for prolonged periods of time,
accumulate genetic errors and live in a suspended
state that permits metastatic spread. Also defects in
apoptosis contribute to resistance to radiation or
chemotherapy4,5,48. In hostile and often hypoxic tumor
environment, genetic mutations may allow cells to
develop resistance to apoptosis and increased
metastatic potential4,5. Studies have demonstrated the
role of COX-2 in this process48. Increased cell survival
has been observed in lung cancer cells expressing
high COX-248. Selective COX-2 inhibitors have been
shown to induce apoptosis in lung carcinoma cells48.
This ability to induce apoptosis is of particular
interest, considering their potential with other
treatment modalities such as radiation therapy. Preclinical studies have indicated improved response on
combined treatment with celecoxib (a COX-2
inhibitor) and radiation therapy.
Studies have shown that the Ras/Raf/MAPK
signaling pathway is necessary for transcriptional
induction of COX-2 by several kinds of stimuli and
COX-2 is linked to epithelial tumor cell survival29,49.
Apoptosis induction has been widely investigated to
define the potential anti-neoplastic mechanisms of
COX-2 inhibition. In the landmark study, forced
expression of COX-2 has been found to increase Bcl2 expression and resistance to apoptosis4. Subsequent
studies in a variety of tumors suggest that both Bcl-2dependent50 and independent5 pathways may be
operative. Selective COX-2 inhibitors also induce
apoptosis in several different types of tumors like
prostate, colorectal and colon50,51, including lung
The overexpression of COX-2 or exposure to
PGE2 can increase apoptosis threshold in lung
adenocarcinoma cells by up-regulation of the mcl-1
gene in a PI3K/Akt-dependent manner18. A strong
correlation has also been found between expressions
of anti-apoptotic protein survivin and COX-253. High
tumor COX-2 expression leads to decreased
ubiquitination and stabilization of survivin, an effect
replicated with exogenous PGE2 treatment and
inhibited by COX-2 inhibitor. Also, PGE2 rapidly
stimulates ERK phosphorylation and proliferation in a
subset of NSCLC cell lines, contributing to increased
migration and resistance to EGFR inhibitors53.
COX-1 and 2 in Immune Regulation
The immune system is able to recognize tumor
cells and eliminate them54. Paradoxically, tumor cells
acquire various characteristics, which allow them to
evade this immunological surveillance through
‘‘tumor escape mechanisms’’. The spreading of
cancerous growth is also dependent on successful
evasion of the immune system. Chronic inflammation
is one such process, which helps in tumor growth and
progression55. A key role for COX-2 in inflammation
has been established56. Various studies have also
shown involvement of COXs in immune regulation
and surveillance56-60.
COX-1 and 2 in T cell Development
COX-1 and 2 are expressed in T lymphocytes and
appear to play a role in T cell development, activation,
chemotaxis, polarization and apoptosis57-60. PGE2 is
also found to modulate cellular proliferation of T cells
in thymus59. In thymus, COX-1 is expressed in
developing (CD4-CD8- and CD4+CD8+) thymocytes,
whereas COX-2 in a subset of medullary stromal
cells59. Both isoforms play a distinct role in thymic
development. COX-1 (not COX-2) is present in
immature thymocytes, whereas COX-2 expression
changes as the thymus matures. COX-1-dependent
formation of PGE2 acting through EP2 receptors
plays an important role in double-negative to doublepositive transition. COX-2 expression in the stroma
influences positive selection of thymocytes through
EP1 receptors in CD4 single positive formation59.
Mice deficient for COX-1 show an early block in
thymic development, resulting in a significant
reduction in double-positive thymocytes, whereas
COX-2 deficiency selectively affects maturation of
single-positive CD4+ thymocytes59. Phenocopies of
COX deficiency can be obtained by pharmacologic
inhibition of specific COX isozymes in the fetal
thymus, supporting a crucial physiologic role of COX
in T-cell development59.
COX-1 and 2 in Activation and Functions of T Cells
Activation of T cells through the T cell antigen
receptor (TCR)/CD3 complex triggers a program of
gene expression that can be divided into three phases
— immediate events (independent on new protein
synthesis), early events (dependent on protein
synthesis and preceding cell division) and late events
(occurring after cell proliferation)61. COX-1 and 2
play an important role in all these phases of T cell
activation and cytokine production (Fig. 2). COX-1 is
constitutively expressed in T cells, where as COX-2 is
expressed as an early response gene, following TCR
engagement through two NFAT motifs present in its
promoter region, suggesting its role in T-cell
activation62. This is further supported by the
inhibitory activity of selective COX-2 NSAIDs on
T-cell activation and cytokine production, which
correlates with the inhibition of key transcription
COX-1 is required for TCR-dependent activation of
p38 kinase, a member of mitogen-activated protein
(MAP) kinase family activated in response to TCR
engagement and required for T-cell activation and
differentiation. It plays a pivotal role in Fyndependent activation of Rac and stress kinases. Thus,
identifies COX-1 as an important early modulator of
TCR signaling upon TCR engagement. Furthermore,
COX-1 inhibition by selective NSAIDs results in
impaired COX-2 expression, suggesting a sequential
role of COX-1 and 2 in T-cell activation63. Selective
COX-2 inhibitors have been found to negatively
regulate proliferation, cell surface expression of
activation markers (CD69, CD71, CD25) and
cytokine production (TNF-, IL-2, and IFN-) by
activated T cells62. These effects could be attributed to
down-regulation of the transcription of genes such as
NF-B and NF-AT by inhibition of COX-2,
suggesting a role of COX-2 in tyrosine kinase
signaling involved in activation of NF-B and
COX-2 in Maturation of Dendritic Cells
Dendritic cells (DCs) are professional antigen
presenting cells (APC) that are pivotal participants in
the initiation of T-cell responses64. Activation of naïve
T cells and their polarization into effector cells
require interactions with mature DCs in the lymphoid
organs. DCs acquire antigen in the periphery and
subsequently transport it to lymphoid organs, where
they prime specific immune responses64. Induction of
COX-2 and PGE2 production via p38 MAPK is an
early event observed even in APC upon
lipopolysaccharide (LPS) stimulation65. PGD2,
another product of COX-2 also partially inhibits the
maturation of human monocyte-derived DCs through
DP (receptor for PGD2) dependent or independent
manner to favor the development of Th2 cells66.
Collectively, it can be concluded that COX-1 and 2
are important immune modulators in development and
functional regulation of immune responses, hence
particularly playing a major role in pathogenesis of
various diseases including NSCLC.
COX-2 in Regulation of Immunity
Tissue trauma normally causes infiltration of
inflammatory cells into tissue and production of
variety of cytokines or growth factors, suppressing or
promoting cellular proliferation. If inflammation
becomes persistent, the cytokine cascade that evolves
could mediate either augmentation or suppression of
Fig. 2 — Role of COX-1 and COX-2 in T cell activation [Upon TCR activation, induction of proximal signaling events in lymphocytes
requires constitutive activity of COX-1 to activate Fyn and ROS. Induction of these events along with other molecular pathways
stimulates complex TCR activation cascade, transactivating bunch of transcription factors involved in COX-2 induction, proliferation and
cytokine production. The whole cascade is critically regulated at each step. High PGE2 due to overexpression of COX-2, prolonged
activation of G protein coupled receptor causes sustain ROS and Ras activation. This may lead to alteration of cytokine balance and
lymphocyte response playing important role in pathogenesis of various diseases including malignancies]
local immune responses, depending on the cellular
make-up of the microenvironment. When transformed
cells, interacting with inflammatory cells and growth
factors (e.g., TNF-α) at sites of chronic inflammation
continue to proliferate, the persistent inflammatory
process may become a crucial step in carcinogenesis.
Many cancers have been associated with persistent
inflammation in lung carcinomas with asbestosis or
silicosis67. These tumors directly interfere with the
host immune system and release factors that modulate
functions of immune cells or induce apoptosis
of these cells. Lung cancer cells elaborate
immunosuppressive mediators including type-2
cytokines, PGE2 and transforming growth factor-β
(TGF-β) that may interfere directly with cellmediated, anti-tumor immune responses7. In addition
to producing their own suppressive factors, tumor
cells may also direct surrounding inflammatory cells
to release suppressive cytokines in the tumor milieu68.
Tumor-derived PGE2 is one mediator that
orchestrates an imbalance in the production of
suppressive and immune potentiating cytokines by
lymphocytes and macrophages in the tumor
COX-2 in Cytokine Regulation in NSCLC
Expression of cytokines by T lymphocytes is a
highly balanced process, involving stimulatory and
inhibitory signaling pathway. The cytokine profile
determines the effector functions of T cells. 3’,5’cAMP has long been recognized as a secondary
messenger in the control of cellular proliferation69.
cAMP elevating agents are known to induce IL-1070
and inhibit IL-2 and IFN-71. IL-10, IL-12 and IFN-
are critical regulatory elements of cell-mediated
immunity. IL-10 inhibits cellular immunity, while
IL-12 and IFN- induce type-1 cytokine production
and effective cell-mediated immunity72. PGE2 inhibits
IFN-, IL-12 and induces high production of IL-4 and
IL-1072. IL-10 overproduction at the tumor site is
implicated in tumor-mediated immunosuppression73,74,
enhanced angiogenesis75 and appears to be an
indicator of poor prognosis in NSCLC76. A correlation
of increased serum IL-10 levels with clinical
prognosis of NSCLC has been found76.
COX-2 overexpression has been found in human
lung cancer and tumor-derived high-level PGE2
production is reported to mediate dysregulation of
host immunity by altering the balance of IL-10 and
IL-12. Importantly, lung cancer-derived PGE2
induces a 10-100-fold increase in lymphocyte IL-10
production7. IL-10 augmentation by PGE2 in tumor
bearing host mediates via phosphotyrosine kinase
pathway7, but the specific down-stream molecular
mechanism is not understood. Studies from our
laboratory have shown involvement of p38 MAPK
and ERK pathway in PGE2-mediated IL-10
production in NSCLC (Unpublished data). Specific
inhibition of these pathways and COX-2 are
accompanied by a significant decrement in IL-10 and
a concomitant restoration of IFN- production,
following TCR stimulation in these patients
(Unpublished data).
In normal cells, COX-2 expression is regulated by
a negative feedback loop and elevated levels of IL-10
inhibit COX-2 expression, thereby limiting PGE2
production77. However, in NSCLC cells, COX-2
expression and subsequent PGE2 production is not
down-regulated by IL-1078. This unresponsiveness of
COX-2 to IL-10 may be due to the lack of surface
expression of IL-10Rα, the IL-10 binding subunit78. In
the lung tumor microenvironment, deficiency of an
IL-10-mediated COX-2 regulatory feedback loop may
contribute to COX-2 overexpression and maintenance
of high levels of PGE2, thereby promoting the
malignant phenotype. In contrast to NSCLC, normal
lung epithelial cells display cell surface IL-10Rα,
suggesting that NSCLC responsiveness to IL-10 may
be regulated through modulation of cell surface
IL-10Rα expression occurring during malignant
COX-2 in Regulation of Antigen Presentation
DCs serve as professional APCs and are pivotal in
the host immune response to tumor antigens.
Antitumor immune responses require the coordinate
activities of professional APCs and lymphocyte
effectors. DCs acquire antigen in the periphery and
subsequently transport it to lymphoid organs, where
they prime specific immune responses64. The tumor
microenvironment can adversely affect DC
maturation and function74. Tumor-derived cytokines
that mediate DC dysfunction include IL-10, IL-6,
vascular endothelial growth factor and macrophage
colony stimulating factor74,79.
COX-2 metabolites can play a major role in tumorinduced inhibition of DC differentiation80 and
prostanoids mediate these effects by both IL-10dependent81 and independent pathways80. Tumorderived PGE2 has been shown to inhibit DC
differentiation and function in an EP2 receptordependent manner82. Significant reduction in cell
surface expression of CD11c, DEC205, MHC class I
and II, CD80, CD86 on DCs cultured in the presence
of tumor supernatant as well as a decrease in
transporter associated proteins TAP1 and TAP2 have
been reported by tumor secreted soluble mediators74.
However these changes are not evident, when DCs are
cultured in tumor supernatant from COX-2-inhibited
tumors. Thus, COX-2 expression or activity of tumor
induces suppression of DC activities and hence
antigen presentation.
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)
Unlike corticosteroids, which inhibit phospholipase
A2 activity, NSAIDs inhibit COX-1 and 2 activity or
both and ultimately inhibit the formation of proinflammatory PGs. NSAIDs are widely used in
pharmacology as anti-inflammatory, antipyretic,
antithrombotic and analgesic agents. They can be
selective COX-1 (SC-560, dexketoprofene, and
mofezolac), COX-2 (celecoxib, rofecoxib, nimesulide, NS-398 and valdecoxib) and non-selective
COX inhibitors (aspirin, indomethacin, diclofenac,
piroxicam, diflunisal, and ibuprofen). All NSAIDs in
clinical use inhibit COX reversibly by competing with
the substrate AA for the active site of the enzyme,
leading to a marked reduction in PG synthesis.
However, aspirin causes irreversible acetylation of the
cyclooxygenase component of COX, without
affecting peroxidase activity83.
The selective COX-2 inhibitors are preferred over
non-selective COX inhibitors because of the
ulcerogenic and renal side effects. The non-selective
NSAIDs, such as aspirin and ibuprofen inhibit
production of prostacyclin, which has a cytoprotective
effect on the gastric mucosa and regulation of kidney
function83. However, despite side effects are present,
pharmacological properties of NSAIDs have been
extensively exploited in numerous fields of clinical
medicine since 19th century. In recent years, due to
pivotal role of COX in development and growth of
malignancies, interest has been aroused in a possible
role for NSAIDs in prevention and treatment of
malignancies. Use of NSAIDs has shown to reduce
the risk of colorectal, breast, cervical, esophageal,
prostate and lung cancers48,84-87.
Clinical Intervention of NSAIDs in NSCLC
COX-2 inhibition has shown beneficial effect in
attenuation of tumor development and activation of
host immune responses in various in vitro and animal
studies. Nimesulide, a COX-2 inhibitor can induce
apoptosis in NSCLC cell lines. The IC50 values of
various anticancer agents including etoposide and
cisplatin are significantly reduced in vitro using
nimesulide as an adjunct to chemotherapy in
NSCLC48. This has also been validated in vivo. COX-2
inhibitors can significantly enhanced radiationinduced apoptosis in fibrosarcoma84 and such increase
is also demonstrated in lung cancer models48,84,85. In
NSCLC, COX-2 inhibitors potentiate radiation
therapy in model systems by enhancing radiationinduced apoptosis86,87.
Clinical trials are currently evaluating COX-2
inhibition in the context of treatment and
chemoprevention of lung cancer. In clinical trial, with
celecoxib a COX-2 inhibitor in combination with
chemotherapy has shown significant decrease in
PGE2 level within tumor tissues, suggesting that
COX-2-dependent expression of genes, deleterious to
the anti-tumor response, may also be decreased.
Celecoxib combined with radiation therapy may also
lead to changes in serum/plasma VEGF in gastric
cancer88. The phase I and II trials are also in progress
to evaluate the role of COX-2 inhibition at various
stages of prostate, pancreatic, glioma and lung
cancers86,87,89-92. Evidences, that COX-2 and EGFR
have related signaling pathways that can interact to
regulate cellular proliferation, migration and invasion
in colon cancer and NSCLC44,93 has triggered interest
in evaluating the combination of COX-2 and EGFR
inhibition in NSCLC. There are various clinical trials
in phase II or III in progress in NSCLC using COX-2
Risk against Benefit of NSAIDs
The non-selective COX inhibitors are found to be
associated with ulcerogenic and renal side effects,
whereas COX-2 selective NSAIDs are associated with
myocardial infarction and cardiac attack, but no
ulcers. A leading hypothesis to explain the
cardiovascular side effects related to COX-2
inhibition involve an imbalance between COX-2
derived prostacyclin (PGI2) and thromboxane A2
(TxA2). PGI2 and TxA2 both have opposing effects
on platelets and smooth muscle cells in blood vessels.
PGI2 inhibits platelet aggregation and causes
vasodilation, conferring cardioprotective effects,
whereas TxA2 induces platelet aggregation and
causes vasoconstriction. Thus their balance is thought
to be important for maintaining vascular homeostasis
(i.e., preventing thrombosis and vasospasm, while
performing efficient hemostasis). Inhibition of COX-2
derived PGI2 could result in a more thrombogenic
state, leading to cardiovascular events.
However, due to involvement of COX in various
physiological and pathological conditions including
various malignancies, there is considerable interest in
the further potential clinical use of COX-2 inhibitors.
The various clinical trials are going on with different
NSAIDs alone or in combination to conventional
treatment of cancer, in order to make tumors more
sensitive to treatment and easy to eliminate. The
phase II trials in NSCLC, prostate cancer, pancreatic
and malignant glioma reveal that NSAIDs can be
highly beneficial, when combined with radiotherapy
or chemotherapy, with no or minimal cardiovascular
side effects. In phase II trial, it has been demonstrated
that celecoxib in combination with CPT-11 can be
safely administered concurrently at full dose levels in
patients with malignant glioma without any adverse
effects91. Also, other phase II trials with celecoxib in
combination with gemcitabine and radiation therapy
in advanced pancreatic and prostate cancer
respectively have shown low toxicity and good
clinical response90,92. The future clinical trials would
focus on establishing the relatively low risk
therapeutic dose or combinations of NSAIDs that may
be used in lung cancer or other malignancies.
Another approach to minimize side effects is
shifting the therapeutic target selection down-stream
in the PG biosynthetic/response pathway towards
specific PG synthase and receptors. These novel
potential down-stream targets could be receptors for
PGE2, PPAR, Wnt signaling molecules, microsomal
prostaglandin E synthase (mPGES) and 15-hydroxyprostaglandin dehydrogenase (15-PDGH).
COX-2 is implicated in NSCLC in each step of
multi-factorial process of tumor development, such as
tumor progression, angiogenesis, metastasis and
immune dysfunction. It has also shown an important
role in regulation of cytokine balance, functions of
APCs and immune activation. Pre-clinical and clinical
studies have demonstrated the benefits of NSAIDs in
prevention and treatment of NSCLC. Early human
trials are in progress to investigate COX-2 inhibition
in chemoprevention and in combination with standard
chemotherapy, radiation therapy and new biologic
agents in regulation of NSCLC. Thus, involvement of
COX in diverse regulatory mechanisms gives
potential to COX inhibitors as therapeutic tools in
management of lung cancer.
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