From anomalous cell cycle regulation
to malignant tumors
• Regulation of cell proliferation
• Regulation of cell death
• Oncogenes and tumor suppressors
– viral oncogenes
• Telomerase activity
• Angiogenesis
• Immune modulation
• Metastasis
• Multi drug resistance
• Genetic instability
Anomalies of cell cycle regulation
- continuously proliferating cells
• Somatic cells divide only „by permission”: they require growth factors
(GF).
• Cell types, which divide permanently
exhibit very limited lifespan and have
a tendency to undergo apoptosis.
• Mutations are continuously repaired.
Irreparable mutations elicit cell cycle
arrest, cells with mutant genomes
undego apoptosis.
Regulation of the cell cycle
• There is a dynamic balance between lost/destroyed cells and cell divisions in
our tissues. Stem cells and regenerating cells replace cells lost through injuries
or wear. High degree of differentiation usually blocks cell division, while
proliferation inhibits differentiation.
• Cell surface receptors bind tissue specific growth factors and elicit cascades of
signaling events, eventually leading to DNA synthesis and cell division.
• Unambiguous growth signals or withdrowal of growth factors during the cell
cycle force cells into apoptosis. Continuously dividing cells are killed in the
absence of growth factors.
Anomalies of cell cycle regulation:
autocrine growth factors
• Production of autocrin growth factors can lead to uncontroled
proliferation of a certain cell type
• Usually the mutation affects the
enhancer-promoter region of
the growth factor gene
• Chromosome aberrations and
recombinational events might
replace the regulatory region
of the gene or
leading to gene multiplication
• EGF – hepatocellular cc
gastrin – colorectal cc
PDGF – embryonic cc
IGF-1 – esophageal cc
Anomalies of cell cycle regulation:
receptor mutations
• Mutations affecting the structure of the receptor („active” conformation is
stable in the absence of ligand)
• Mutation affecting the regulatory region of the gene, or gene amplification:
overproduction of the receptor molecule
• Chromosome aberrations and
recombinational events might
replace the regulatory region
of the gene
• erb B2/HER-2 over-expressed
in mammary carcinoma
Anomalies of cell cycle regulation:
G-protein mutations
• Mutations affecting the structure or GTP-ase activity of G-proteins (if
GTP can not be split, G-protein remains active)
• Ras is mutated in many human tumors. In Ras gene most mutations affect
aa 12 (61 or 13), the GTP-ase site
• Effects of Ras mutations:
Raf (MAPK) pathway,
PI-3K (Akt/PKB pathway)
RasGEF (in humas very important)
Anomalies of cell cycle regulation:
mutations affecting signal molecules
• Most mutations found in the coding region of the genes of signal molecules
• Mutations affecting the balance of kinases/phosphatases or
kinases/ubiquitin ligases
• Mutations altering the length of the signal
• Mutations affecting the compartmentalization of activated signal
molecules
• 33 human lung tumors (141 PK genes tested) - 41 PK mutations found
typically CG/AT mutations: caused by smoking
Anomalies of cell cycle regulation:
mutations affecting transcription factors
•
Mutations both in the control and coding region of the genes of transcription
factors are found in human tumors
•
TFs are master regulatory genes: affecting activity of many different genes
•
TFs have direct effect on cell cycle
•
Myc in AML, pancreatic cc
abnormal STAT3 activity
found in the majority of
human malignancies
Anomalies of cell cycle regulation
Mutations affecting regulatory
molecules: cyclins, cdk’s
and inhibitors of cell cycle
progression (p21, p27KIP,
etc)
Activity of these proteins is
normally regulated by
MAPKs, transcription
factors
Anomalies of cell cycle regulation
A large number of
genes participate in
cell cycle control
Their mutation might
cause deregulated
cell proliferation
Anomalies of cell cycle regulation
Two
keykey
control points: G1 / S phase transition
Two
governed by Rb
DNA synthesis is not allowed in the absence of
mitogenic signals
Non-phosphorylated Rb
binds E2F, S phase genes
are inactive
Anomalies of cell cycle regulation
G2 / M phase transition, controlled by p53
Mitosis can start only after repair of all mutation
If reparation is impossible, p53 induces apoptosis
Loss of the apoptotic tendency in continuously
dividing cells: a way to „immortalization”
•
Terminal differentiation of permanently proliferating cell types includes apoptosis.
Extended survival of these cell types tips the balance.
•
Abnormal level of survival factors (eg. autocrine survival factors) or
absence of death signals (mutations causing loss of response to death signals)
•
Mutations abrogating dependence on survival factors
•
In the absence of pro-apoptotic mediators the number of these cell types increases
to abnormal ranges causing severe symptoms (lpr/gld mutations in mice).
•
Increased production of anti-apoptotic factors leads to similar effects (bcl2
system).
The balance of pro- and anti-apoptotic
proteins in the mitochondria
The bcl2/bax family of proteins regulate death (release of cytochrome C, activation
of the caspase cascade)
or survival
In cells of human B-cell
lymphoma bcl2 production
is upregulated,
B cell accumulate
Pro-apoptotic ligands and their receptors belong
to the TNF family of proteins
Killing of (mutant) cells is triggered by TNF-family ligands
acting on death receptors.
Immune cells use a
membrane destructing
enzyme, Granzyme B
to kill tumor cells (and
parasite-infected cells)
Loss of the apoptotic tendency
Apoptosis is an active process,
dependent on ATP and coordinated action of many
proteins
Mutations leading to inactivation
of any of pro-apoptotic proteins might cause
immortalization of the cells
Telomers and telomerase
Unlimited proliferation of somatic cells is prevented, because chromosome replication is
accompanied by the shortening of telomers
Only cells with active telomerase (germinal cells, stem cells and progenitor cells) display
telomerase activity
Telomers and telomerase
Telomerase can be activated because of mutations affecting the regulatory region of the
gene
Telomerase „positivity” is a general property of malignant cells
Telomerase has an RNA
template and produces
telomeris sequences of
DNA by reverse
transcription
Oncogenes, protooncogenes, viral oncogenes
P. Rous isolated a „filtratable” agent causing sarcoma of chickens almost acentury ago
Mouse virus transmitter by mothers milk
Leukemia and sarcoma viruses of birds, rodents and primates
One single gene of Rous sarcoma virus is capable of inducing oncogenic transformation of
chicken cells. Src gene encodes a protein tyrosine kinase
Sarcoma viruses carry oncogenes. Retrovirus genomes encode processed oncogenes
The „normal”, cellular gene is a protooncogene
Retroviruses use reverse transcriptase to convert RNA into dsDNA
Retroviral oncogenes
Retroviral oncogenes are of host origin
Each type of these viruses cause a
special form of malignant disease
(a few carry two genes, causing
two kind of tumors)
Retroviruses without oncogenes can
cause tumor growth by enhancerinsertion (into regultory elements of
cellular protooncogenes)
B cell lymphoma
Viral oncogenes of DNA viruses
Papillomaviruses
DNA viruses carry viral oncogenes: these genes increase the accident of malignant
transformation
Each tumor is different – one virus strain can caause many types of malignancies
Papillomavirus proteins interfere with the action of p53 and Rb
Tumor suppressors are host genes preventing oncogenic transformation: both genes
must be inactivated in order to allow uncontroled proliferation!
Cervical cancer
Oral papilloma
Viral oncogenes of DNA viruses
Hepatitis B and C viruses
Hepatitis B and C viruses tend to cause persistant infection
which might convert into hepatocellular carcinoma
Both viruses are blood-borne viruses
do not (infrequently) infectious orally
A good HBV vaccine is available
Viral oncogenes of DNA viruses
Herpesviruses
Herpesvirus-8 – in collaboration with retroviruses (leukemia viruses or HIV) –
might cause Kaposi sarcoma, a pigmented skin tumor. It is it is not rare among
AIDS patients
Viral oncogenes of DNA viruses
Herpesviruses
Another herpesvirus, Epstein-Barr virus (EBV) is the
causative agent of Burkitt lymphoma.
Though EBV is infecting more than half of the population
everywhere, BL is much more frequented among starving
children in third world countries and people simultaneously
infected by parasites of malaria, sleeping sickness or other
devastating diseases.
Anti-tumor mechanisms
Cells proliferation is stricktly controlled. Mitogenic signals are needed.
Checkpoints prevent unscheduled cell division and mitosis of mutant cells
Continuously proliferating cells have a limited life span, dfferentiation is linked to
apoptosis
Senescence of telomers prevent long term proliferation of somatic cells
Tumor suppressors inhibit false mitotic signals or uncontrolled cell division
The proofreading of new DNA and repair of mutations is continuous.
Absence of blood vessels in neplastic tissue inhibit growth of cell mass
Immune cells produce death signals to induce apoptosis of mutant cells.
Complement activation lyses membranes of cell labelled by antibodies against tumor
antigens
Cytotoxic lymphocytes eliminate cells presenting tumor antigens
Natural killer cells destroy mutant cells non-presenting cells, resistant to apoptosis
TNF induces death in cells of new endothelium of tumor tissues
Cellular mechanisms leading to immortalization
and tumor cell formation
Mutations affecting mitotic factor production, receptor function, normal duration and
direction of signaling, activity of enzyme activities controlling signal events and
acivity of transcription factors might disturb control of proliferation
Dominant mutations of protooncogenes (converting them into oncogenes) and
concommittant loss of tumor suppressor functions (recessive mutations!) are also
needed. Telomerase must be activated.
Further mutation are necessary which cause loss of death receptors, induce resistance
to apoptosis, CTL, NK and complement-induced killing, capacity to induce growth
of new blood vessels, to block action of death signals on tumor cells and capillary
endothel. Local immunosuppression should be achieved by producing cytokines.
Still further mutations are necessary to upregulate many genes essential for
mobilization and metastatic growth, multi-drug resistance, radiation resistance,
etc.
How is it possible to collect so many mutations in relatively short time?
Genetic instability is cause,
not result of abnormal proliferation!
Increased mutation rate is essential for the emergence of malignant cells
Mutations of DNA polymerases, proofreading and repair enzymes
(or long term exposure to mutagenic agents, especially if
combined with repair-inhibitors) might only result in an
excessive number of mutation
Retroviruses are able to immortalize specific cell types causing well
defined malignancies (erythroleukemia, bone tumors, sarcoma,
myeloblastosis, etc.) in animal models and human patients.
Polyoma viruses induce many different kinds of tumors in immunesuppressed or –incompetent animals or individuals.
Several human viruses (HPV, EBV, CMV, HHV-8, HBV és HCV, HLTV)
and bacteria (Helicobacter, Chlamydia, stb.) are established
etiological agents of malignancies.
Virulence factors of pathogenic organisms can help in the development of
tumor cells and in their immune-escape
Mutation rate and tumor formation
Loss of repair enzyme activity leads to early death of experimental mice
Heterozygotes show very little effect in the absence of mutagenic agents, but
became vulnerable if exposed to DNAdamaging compounds or effects (X ray)
DNA repair is vital
The genome-protecting gene
network
A large number of gene products
are involved in keeping the
genome free of mutations,
genedefects and chromosome
aberrations.
Genetic and environmental factors in
oncogenesis
• Human genetic background: Gene defects affecting
– DNA synthesis
– or repair enzymes
– or proto-oncogenes, anti-oncogenes,
• Environmental factors
– mutagenic compounds, UV, X-ray, etc.,
– repair-inhibitors
– immunosuppressive compounds, effects,
• Living environment
– viruses, pathogenes
– immunosuppressive agents and treatments, starving, ageing
– social stress