Understanding Cell Death Evasion Mechanisms in Neoplasia

1. Neoplasia lecture 8 DrHeyamAwad FRCPath

2. Fourth hallmark • Evasion of cell death by evading apoptosis or autophagy

3. apoptosis • Apoptosis: programmed cell death in which cells activate enzymes that degrade the cells’ own nuclear DNA and nuclear and cytoplasmic proteins • So the cells commit suicide! • The cells fragment and the fragments are phagocytosed without eliciting inflammatory response

4. extrinsic pathway • Trigger that starts apoptosis is outside the cells. • The pathway starts when Fas ligand binds to Fas receptor • Upon this the receptor is activated; it trimerizes and its cytoplasmic part (death domain) is activated. • Activation of the receptor attracts a cytoplasmic protein= FADD • FADD recruits procaspase 8 • Procaspase cleaved to active caspase 8 (initiation caspase) • Caspase 8 activates caspase 3 (executioner) which cleaves DNA and cellular protein

5. Extrinsic pathway • Fas ligand • Fas receptor • FADD • Caspase 8 • Caspase 3 • Decrease any of the above….. Evasion of cell death

6. Extrinsic pathway • FLIP is a caspase 8 antagonist • So if FLIP is increased cells can evade apoptosis • FLIP-similar proteins are produced by some viruses.. Helping them to keep infected cells alive.

7. Intrinsic pathway = mitochondrial pathway • This pathway is stimulated if there is DNA damage secondary to stress, radiation, chemicals or due to withdrawal of survival factors • This pathway is intrinsic.. So not initiated by membrane receptors… instead it is initiated by increased mitochondrial permeability • When mitochondrial permeability increases ..cytochrome c leaks out and initiates apoptosis • Now cytochrome c is in the cytosol.. So it binds APAF 1 • This binding activates caspase 9 • Caspase 9 activates caspase 3

8. Intrinsic pathway Internal stresses within cells Increase mitochondrial permeability Cytochrome c leaks outside the mitochondria Cytochrome c binds to APAF1 Caspase 9 activated Caspase 3 activated Again: decrease any of these and the cell can avoid apoptosis

9. note • IAP= inhibitor of apoptotic protein , inhibits caspase 9 • So increase IAP and apoptosis can be avoided.

10. apoptosis

11. Mitochondrial permeability • Mitochondrial permeability is controlled by BH 3 proteins (BAD, BID, PUMA) • When BH3 proteins sense internal stress.. Stimulate proapoptotic proteins and inhibit antiapoptotic ones • Proapoptotic: BAX, BAK • Antiapoptotic: BCL2, BCL- Xl • So decrease BAD, BID, PUMA, BAX, BAK… NO APOPTOSIS • Increase BCL2 AND BCL-Xl…. No apoptosis

12. bcl2 • Follicular lymphomas are slow growing (indolent) tumors that have a translocation causing increased bcl2 • T (14;18) …. Bcl2 translocated and overexpressed • In lymphocytes having this mutation… apoptosis is decreased • These lymphocytes live longer…. Rather than being transformed… that’s why this type of lymphoma ( follicular lymphoma) is indolent

13. Follicular lymphoma/ note the formation of follicles

14. P53 is important in regulating apoptosis • P53 induces apoptosis when there is DNA damage or increased expression of oncogenes • So if p53 is normal , cells that have increased oncogenes or have damaged DNA will die and no tumor will occur • But if p53 is inhibited.. Transformation can happen in such cells

15. Evasion of cell death by autophagy • Autophagy is a catabolic process that balances synthesis, degradation and recycling of cellular products • The recycling of the cell’s organelles can produce energy needed for the stressed cells. • This process can signal cell death if the cell cannot be rescued by the recycling process

16. Autophagy is regulated by mechanisms and proteins that overlap with apoptosis • The main stimulus for autophagy is Beclin 1 which is a member of the BH3 Family. • So: internal stresses can stimulate cell death by apoptosis or autophagy. • Decreased autophagy… helps in tumorogenesis

17. note • Although autophagy is an anti-tumor process….. Later on if there is a tumor mass formed, autophagy can help the tumor to survive if it’s used to recycle organelles to be used as an energy source . • Autophagy can help tumor cells to survive during unfriendly climates: for example during chemotherapy treatment.

18. autophagy

19. Fifth hallmark: sustained angiogenesis • Tumors cannot grow for more than 1-2mm without blood supply • This 1-2 mm zone is the maximum direct diffusion distance. • Angiogenesis important for tumors to: • 1. supply oxygen and nutrients • 2.Get rid of waste products • 3. gain access to host blood vessels which is important for invasion and metastasis. • 4.the endothelial cells in these vessels secrete growth factors that can help tumor growth

20. angiogenesis • Two processes: • 1. neoangiogensis: new vessels formed from preexisting host capillaries • 2.vasculogenesis: completely new vessels are formed by recruiting endothelial cells from bone marrow.

21. note • Tumor blood vessels are abnormal : they are leaky, dilated and have haphazard pattern of connections

22. angiogenesis • Angiogenesis is accomplished by factors secreted from the parenchymal tumor cells as well as the stroma. Also inflammatory cells surrounding the tumor can produce angiogenic factors. • the balance between pro-angiogenic and anti-angiogenic factors controls formation of new blood vessels • Main pro-angiogenic: VEGF= vascular endothelial growth factor • Main anti-angiogenic: TSP1= thrombospondin 1

23. Tumors usually stay in situ or small for several years… at this stage there is no angiogenesis • Angiogenesis switch happens when VEGF ( and other proangiogenic factors) increases and TSP 1 ( or other antiangiogenic factors) decreases.

24. Angiogenic switch • VEGF produced from tumor cells or macrophages • Protease (secreted from tumor cells or stromal cells) can release FGF (an angiogenicagnt) from ECM • TSP1 is produced from fibroblasts in response to tumor cells…. TSP is anti angiogenic • Normal P53 induces synthesis of TSP1.. So if p53 is deleted.. Decreased TSP1

25. What causes the angiogenic switch • Hypoxia is an important factor that favours angiogenesis • Hypoxia.. Stimulates production of hypoxia –inducible factor 1alpha (HIF 1 alpha) • HIF is a transcription factor which will stimulate production of VEGF • HIF is destructed by VHL (von Hipple- Lindau )protein • Hypoxia prevents VHL from recognizing HIF … no destruction ..more angiogenesis

26. VEGF.. Stimulates Notch pathway which regulates the branching and density of vessels.

27. Von Hippel- Lindau syndrome • VHL gene is a tumor suppressor gene ( because it decreases angiogenesis) • Rarely some pole inherit defective VHL gene… they develop tumors like renal cell carcinoma, pheochromocytoma..

28. Sixth hallmark: reprogramming of energy metabolism Normal cells obtain energy by: • Oxidative phosphorylation if oxygen is available. In this process each glucose molecule used produces 36 ATP molecules. • Anaerobic respiration if oxygen levels are low. In this process glucose is converted to lactic acid and for each glucose molecule used only 2 ATP molecules are produced.

29. Reprogramming of energy metabolism • Cancer cells have a third way! • They convert glucose to lactic acid even in the presence of adequate oxygen • This process is called : aerobic glycolysis or Warburg effect.

30. Warburg effect • Although less ATP is produced… the Warburg effect ensures that carbon atoms in glucose ( which is converted to Pyruvate) areused for synthesis of organic compounds like lipids and proteins which are important in building new cells in the highly proliferative tumor. • Oncogenes ( myc, ras) and tumor suppressor genes that favor cell growth ( TP53 can affect ) and upregulatethis process.

31. PET scan • Tumor cells are “glucose hungry” and this property is used in PET scans • PET: positron emission tomography • Patient is injected with a glucose derivative.. Tumor cells take this derivative more than normal cells and as such detected with the scan • The more proliferative the tumor is… more uptake and more positivity with PET scan

32. PET scan

33. PET scan

34. PET scan