1. Molecular basis of Cancer
Prepared by Dr. Ashish Adhikari
1st year resident, NAIHS
Moderator: Maj. Dr. Suman Gurung
2. Cellular and molecular hallmarks of cancer
a) Self-sufficiency in growth signals.
b) Insensitivity to growth inhibitory signals
c) Altered cellular metabolism.
d) Evasion of apoptosis.
e) Limitless replicative potential (Immortality)
f) Sustained angiogenesis.
g) Ability to invade and metastasize.
h) Ability to evade the host immune response.
3. c) Altered cellular metabolism: The Warburg
effect
• Two main pathways for ATP generation.
a) Aerobic glycolysis:
- Generates 2 molecules of ATP/ Glucose molecule.(Inefficient)
- Generates metabolic intermediates needed for synthesizing DNA, RNA, Protein,
lipids required for cell division.
b) Oxidative phosphorylation:
- Efficient, Generates 36 ATP/Glucose molecule.
4. Contd….
• Normal cells: Go predominantly for oxidative phosphorylation.(Efficient)
• Rapidly dividing normal cells/ Cancer cells: Go for Aerobic glycolysis.(Inefficient)
• Why???
- Aerobic glycolysis provides metabolic intermediates(DNA, RNA, Protein, Lipid)
required by rapidly dividing normal cells(embryonal cells) or cancer cells.
5. Contd…
- Oxidative phosphorylation on other hand don’t provide metabolic
intermediates/ carbon moieties required for cell growth.
- Thus, in presence of ample amount of oxygen, cancer cells
demonstrate distinctive form of cellular metabolism characterized by
high levels of glucose uptake and increased conversion of glucose to
lactose via glycolytic pathway.
- This phenomenon is known as “ Warburg effect”.
6. Clinical implications
• As cancer cells are hungry for glucose (Useful to visualize tumor via
PET scan): 18-Fluoro-deoxy-glucose (Non-metabolizable derivative of
glucose) injected.
• Preferentially taken up by tumor cells (As well as normal actively
dividing tissues such as Bone marrow).
7. • Normally
Tissue Growth required ( Increased Aerobic fermentation)
When growth is achieved
In Normal cells Cancer cells
Aerobic fermentation stops Aerobic fermentation continues
because of unopposed action of oncogenes and
loss of function of tumor suppressor genes.
8. Quiescent cell
• Quiescent cells rely mainly in
Kreb’s cycle for ATP
production.
• If starved, Autophagy(self
eating) is induced.
10. a) PI3K-AKT pathway:
- Upregulates activity of
glucose transporters and
glycolytic enzymes. Increased
glycolysis.
- Promotes shunting of
mitochondrial intermediates
to pathways leading to lipid
biosynthesis.
- Stimulates factors required
for protein synthesis.
11. b) Receptor tyrosine kinase
- Inhibits enzyme pyruvate
kinase which catalyses last step
of glycolysis.
- Build up glycolytic
intermediates.
- Used for DNA, RNA, Protein
and Lipid synthesis.
12. c) MYC
- Activates multiple glycolytic
enzymes and glutaminase (
Increased glutamine
utilization by mitochondria)
- Biosynthesis of cellular
components.
13. Onco-metabolism
• Surprising group of genetic alteration discovered through tumor
genome sequencing studies.
• Consists of mutation in enzymes that participate in Kreb’s cycle.
• Of these, mutation in Isocitrate dehydrogenase (IDH) is of most
interest.
• Revealed a new mechanism of oncogenesis.
14. • IDH acquires mutation via
amino acid substitution.
• Mutated IDH loses its
function.
• Instead catalyzes formation
of 2HG(2- Hydroxy glutarate)
• Inhibits several enzymes
including TET family.
• Loss of TET2 activity leads to
abnormal patterns of DNA
methylation.
15. Contd…
• Oncogenic IDH mutations occur in a diverse collection of cancers
including Cholangiocarcinomas, gliomas, acute myeloid leukemias and
sarcomas.
16. d) Evasion of cell death
• Tumor cells in order to survive contain mutations in genes that result
in resistance to apoptotic cell death.
• Most disabled / dysregulated in cancer cells. (Intrinsic / Mitochondrial
pathway)
• WHY??
- Cancer cells are subject to intrinsic stresses(DNA damage, Hypoxia,
metabolic alterations, Increased misfolded protein,
chemoradiotherapy)that initiates Apoptosis via Intrinsic pathway.
17. • Mechanism by which apoptosis is
avoided by cancer:
- Loss of p53 response.
- Overexpression of anti-apoptotic
members of BCL-2 family. (BCL-2,
BCL-XL, MCL-1) [ Also associated
with drug resistance]
Eg: Follicular lymphoma t (14;18)
Increased BCL-2 which protects
transformed lymphocytes from
apoptosis.
Tumor grows by evading
apoptosis.(Not by increased
growth)
Therefore, Follicular lymphoma is
slow growing (Indolent tumor)
- Might upregulate members of
inhibitor of apoptosis family (IAP).
18. e) Limitless replicative potential:
- The stem cell like property of tumor cells.
- How?
• Evasion of senescene.
• Evasion of mitotic crisis.
• Capacity for self renewal.
19. • Evasion of Senescence
Normally human cells
Divide 60-70 times
Permanent cell cycle arrest (Senescence)
Due to increased p53 and INK4A/ p16 which maintains Rb gene
(Hypophosporylated – Active state) which enforces G1/S checkpoint to
cell cycle arrest.
20. Contd…
• This G1/ S check point is disrupted in virtually every cancer.
(Mutation/ Epigenetic changes)
21. • Evasion of mitotic crisis
- Telomeres: Present at the chromosomal ends. Normally prevents
senescence(ageing).
- Normally length of this telomeres maintained by enzyme telomerase.
- Very low level of this enzyme is present in most somatic cells.
22. Contd…
• Normal cells(Somatic) undergoes senescence after 50-60 divisions.
• If they escape senescense by chance, then they undergo mitotic crisis
ultimately leading to death.
23.
24. • Capacity of self renewal:
• Tissue stem cells and germ cells
- Has telomerase activity.
- Resistant to mitotic crisis.
- Resistant to genetic and epigenetic changes that trigger senescence.
- Capacity of self renewal:
Each time a stem cell divides at least one of the two daughter cells
remain stem cells.
25. Contd…
• Types of self renewal:
- Symmetric: Both daughter cells remain stem cells. Eg: During
embryogenesis/ Stress.
- Asymmetric: Only one daughter cell remain stem cell.
Non stem cell undergoes differentiation pathway with highly
proliferative cells.
But eventually differentiate and stop dividing because of (Apoptosis
and senescence)
26. Contd…
• Cancer cells too must contain cells that self renew. (Known as cancer
stem cells)
27.
28. f) Angiogenesis
• Solid tumors with all genetic aberrations required for malignant
transformation can’t enlarge beyond 1-2mm in diameter unless it has
capacity to induce angiogenesis.
• Growing tumor also needs O2 and nutrients for growth and need to
remove waste products.
• Growing tumor cells induce new blood vessels formation. Relaese of
growth factors (IGFs and PDGF) which stimulate growth of adjacent
cells.
• Vessels are leaky and dilated and have a haphazard pattern of
connection.
29. Contd…
• Angiogenesis is controlled by a balance between angiogenesis
promoters and inhibitors.
• In angiogenic tumors, this balance is skewed in favour of Promotors.
• Angiogenic switch : Tumor cells, Inflammatory cells, Stromal cells,
ECM release factors which is responsible for switch.
• Promotors: VEGF, bFGF, TNFα, Platelets activating factors, Angiogenin,
IL-8.
• Inhibitors: Thrombospondin-1, Angiostatin/Endostatin.
30. Factors responsible:
Regulation of balance between pro and anti- angiogenic factors:
a) Hypoxia: Most important driving force for angiogenesis.
- Stabilises HIF-1α transcription factor.
- Stimulates transcription of VEGF and bFGF leading to angiogenesis.
31. Contd…
b) Mutation in tumor suppressor genes and oncogenes:
• Normally, p53 + Antiangiogenic factor (Thrombospondin-1)
- Proangiogenic factor (VEGF)
• Loss of p53 activity favor angiogenesis.
• Gain of function mutation of RAS/ MYC upregulate the production of
VEGF.
32. Contd…
c) Proteases from tumor cells /stromal cells :
Many proteases release bFGF from ECM favouring angiogenesis.
33. g) Invasion and metastasis:
• Hallmark of malignancy.
• Associated with cancer mortality and morbidity.
• Metastasis:
- Tumor cells must undergo series of steps.
- Avoid immune defense.
- Adapt to new microenvironment.
34. Contd….
• Metastatic cascade has 2 phases:
i) Invasion of ECM.
ii) Vascular dissemination, Tissue homing; Colonization.
35. i) Invasion of ECM:
Steps:
- “Lossening up” of tumor cell- tumor cell interactions.
- Degradation of ECM.
- Attachment to “remodeled” ECM components.
- Migration and invasion of tumor cells.
36. Loosening of
intercellular
junctions
• First step in process of invasion
/ metastasis.
• Downregulation of E-cadherin
due to mutation or silencing
(Epithelial to mesenchymal
transition) EMT.
• Metastatic oncogenes encode
Transcription factors SNAIL and
TWIST control EMT.
• EMT characterized by:
- Downregulation of Epithelial
markers( E-cadherin)
- Upregulation of Mesenchymal
markers( eg; Vimentin, Smooth
muscle actin)
- EMT is integral in breast and
prostate carcinoma mets.
37. Degradation
of ECM
• Second step in invasion.
• Accomplished by secretion of
proteolytic enzymes:
- MMPs (Matrix
Metalloproteinases) (2,9)
- Cathepsin D
- Urokinase plasminogen
activator
38. Attachment to remodelled ECM components
• Fibronectin and Laminin receptors called as Integrins.
- Bind to basement membrane collagen and laminin.
- Helps maintain cell in resting polarized state.
• If adhesion between integrins and (collagen/laminin) break, leads to
Apoptosis.
• In case of tumor cells,
Although shows loss of adhesion, doesn’t trigger apoptosis. Escapes
cell death. Anoikis (Meaning ‘Without Home”)
39. Migration and
invasion of
tumor cells
• Locomotion : Final step of
invasion
• Propelling tumor cells through
degradation of Basement
membrane and zones of matrix
proteolysis.
• Cells attach to matrix at their
leading edge , detach from
matrix at trailing edge, contract
the actin cytoskeleton to rachet
forward.
40. Contd…
• Such movement is stimulated by multiple factors:
- Tumor cell derived cytokines, chemokines and growth factors(IGF)[
Autocrine]
- Cleavage product of matrix components (Collagen and Laminin)
- Stromal cell derived factor. (Hepatocyte growth factor) [Paracrine]
41. ii) Vascular dissemination,Tissue homing;
Colonization
• Once tumor cells are at circulation, vulnerable to destruction.
• So, How do tumor cells survive in circulation?
- Metastatic tumor cells are much more likely to establish metastasis as
tumor aggregates than as single cells.
42. Contd…
• Tumor aggregates:
- Platelet activation: Enhance tumor survival
- Fibrin deposition due to activation of Factor XII by
polyphosphates(anions) on surface of tumor cells: Further stabilise
the tumor emboli.
- Stem cells giving plasticity to tumor cells: Helps tumor to adapt new
microenvironment.
43. Contd…
• Metastatic deposits appear to relate with 3 factors:
- Location and vascular drainage of primary tumor:
Eg: Colon Ca metastasis to Liver.
- Tropism of particular kind of tumor cell for specific tissues: (Organ
tropism)
Eg: Carcinoma breast/ Prostate metastatis to bone.
44.
45. h) Evasion of immune surveillance
• Four factors supports role of immune cells in destroying tumor cells:
- Presence of lymphocytic infiltrates around tumor and reactive changes in lymph
node draining sites of cancer.
- Increased cancer incidence in immunodeficiency individual.
- Demonstration of tumor specific T-cells and Ab in patients.
- Response of cancer cells to agents that act by stimulating host T-cell responses.
• Therefore, in order to survive, tumor cells must develop certain mechanisms to
escape host immune system.
46.
47. Antitumor effect
mechanism:
• The principal immune
mechanism of tumor
eradication is killing of tumor
cells by CTLs specific for
tumor antigens.
49. 1) Selective outgrowth of Antigen
negative variants:
-During tumor progression, strongly
immunogenic antigen-expressing
subclones may be eliminated.
- Only those tumor cells that have lost
their Antigen survive.
2) Loss or reduced expression of MHC
molecules
- Tumor cells may fail to express
normal levels of HLA Class-I
molecule.
- Therefore, they may escape CD8+
CTLs.
50. 3) Engagement of pathways that
inhibit T-cell activation.
- CTLA-4 downregulates B7 in APCs.
Leads to decreased Tcell activation.
- PD-1 , like CTLA-4 inhbits Tcell
production.
51. Reference
• Robbins S, Cotran RS, Kumar V, Abbas AK, Aster JC. Pathologic
Basis of Disease. 10th ed. South Asia Edition. 2021;p300-12.
