Mutation causing loss of cell cycle
- 1. CANCER BIOLOGY TOPIC: MUTATION CAUSING LOSS OF CELL CYCLE & INDUCTION OF CANCER T.ANANTHA THAMIZHAN BMS14312
- 2. The entry of cells into the cell cycle from a quiescent state and the progression of cells around the cycle are precisely controlled events. This assures that cellular growth and the coordination of DNA synthesis with cell-size increase and cytokinesis are monitored and do not fall out of regulated synchrony. Once a cell progresses past a certain point in late G1, called the restriction point It becomes irreversibly committed to entering the S phase and replicating its DNA INTRODUCTION
- 3. Cyclins, cyclin-dependent kinases (Cdks), and the Rb protein are all elements of the control system that regulate passage through the restriction point. The ability of these proteins to check cell-cycle progression, and hold cells in quiescence or even lead cells to commit suicide unless conditions are appropriate, means that they can prevent cells from becoming cancerous. Altered regulation of expression of at least one cyclin as well as mutation of several proteins that negatively regulate passage through the restriction point can be oncogenic .
- 5. Passage fromG1 to S Phase Is Controlled by Proto- Oncogenes andTumor-Suppressor Genes The expression of D-type cyclins in mammals is induced by many extracellular substances called growth factors or mitogens. These cyclins (D1, D2, and D3) assemble with their partners Cdk4 and Cdk6 to generate catalytically active kinases. Mitogen withdrawal prior to passage through the restriction point leads to accumulation of p27 or p16, which bind to and inhibit cyclin D – dependent kinase activity, thus causing G1 arrest.
- 6. Another key player in cell-cycle control is the Rb protein Its unphosphorylated form, Rb binds transcription factors collectively called E2F Thereby prevents E2F-mediated transcriptional activation of many genes whose products are required for DNA synthesis, such as DNA polymerase. Also, the Rb-E2F complex acts as a transcriptional repressor for many of these same genes.
- 7. Phosphorylation of Rb protein midway through G1 causes it to dissociate from E2F, allowing E2F to induce synthesis of these DNA replication enzymes, which irreversibly commits the cell to DNA synthesis. Rb phosphorylation is initiated by an active Cdk4 – cyclin D complex and is completed by other cyclin- dependent kinases. Most tumors contain an oncogenic mutation of one of the genes in this pathway such that the cells are propelled into the S phase in the absence of the proper extracellular growth signals that regulate Cdk activity.
- 8. Rb/E2F Stimulation of G0 cells with mitogens induces expression of Cdk4, Cdk6, D- type cyclins and E2F transcription factors (E2Fs), all encoded by delayed- response genes Interaction of E2Fs with hypophosphorylated Rb protein initially inhibits E2F activity. When signaling from mitogens is sustained, the resulting Cdk4 – cyclin D and Cdk6 – cyclin D complexes (Cdk4/6 – cyclin D) initiate the phosphorylation of Rb, converting some E2F to the active form. Active E2F then stimulates its own synthesis and the synthesis of Cdk2 and cyclin E. Cdk2 – cyclin E further stimulates Rb phosphorylation releasing more E2F activity. These processes result in positive feedback loops (blue arrows) leading to a rapid rise in both E2F and Cdk2 – cyclin E activity as the cell approaches the G1→S transition.
- 9. Overexpression of Cyclin D1 Gene amplification or a chromosome translocation that places cyclin D1 under control of an inappropriate promoter leads to overexpression of this cyclin in many human cancers, indicating that it can function as an oncoprotein. That cyclin D1 overexpression can directly cause cancer was shown by studies with a transgenic mouse in which the cyclin D gene was placed under control of an enhancer specific for mammary ductal cells Initially the ductal cells underwent hyperproliferation, and eventually breast tumors developed in these transgenic mice. Amplification of the cyclin D1 gene and concomitant overexpression of the cyclin D1 protein is common in human breast cancer.
- 10. Loss of p16 Function The group of proteins that function as cyclin kinase inhibitors (CKIs) play an important role in regulating the cell cycle Mutations,especially deletions of the p16 gene, that inactivate the ability of p16 to inhibit cyclin D – dependent kinase activity are common in several human cancers. Loss of p16 would mimic cyclin D1 overexpression, leading to Rb hyperphosphorylation and release of active E2F transcription factor Thus p16 normally acts as a tumor suppressor.
- 11. Loss of TGFβ SignalingContributes to Abnormal Cell Proliferation and Malignancy Until now we have considered growth factors that mainly stimulate proliferation of different types of cells Tumorderived growth factor β (TGFβ) is secreted by most body cells and has a diverse range of biological activities. Most relevant to the present discussion however,is the ability of TGFβ to inhibit the growth of many types of cells, including most epithelial and immune system cells.
- 12. Loss of TGFβ-mediated growth inhibition contributes to the development and progression of a variety of tumors. TGFβ signals through the sequential activation of two cell-surface receptors, termed type I and type II,both of which have intrinsic serine/threonine protein kinase activity. Binding of TGFβ induces formation of a complex of the type I and type II receptors and phosphorylation and activation of the type I receptor by the type II receptor kinase.
- 13. Smad proteins are the key intracellular signal transducers in the pathway downstream from the TGFβ receptors. The ligand-activated type I receptor phosphorylates conserved serines at the C-terminus of either Smad2 or Smad3, which enables them to bind to one or more molecules of Smad4, a common partner for all phosphorylated Smads involved in signaling by both TGFβ and bone morphogenic proteins.
- 14. These Smad complexes then enter the nucleus and activate transcription of a variety of genes. One important gene induced by TGFβ encodes p15. This G1 cyclin-kinase inhibitor displaces p27 from the Cdk4-cyclin D complex, freeing p27 to bind to and inhibit the Cdk2- cyclin E complex, which is required for entry into the S phase. Thus by inducing expression of p15, TGFβ causes the cell to arrest in G1.
- 16. Many tumors contain inactivating mutations in either the TGFβ receptors or the Smad proteins, and thus are resistant to growth inhibition by TGFβ. Most human pancreatic cancers contain a deletion in the gene encoding Smad4; this mutation-defined gene was originally called DPC (deleted in pancreatic cancer). Retinoblastoma, colon and gastric cancer, hepatoma, and some T- and B-cell malignancies are unresponsive to TGFβ growth inhibition.
- 17. This loss of responsiveness correlates with loss of type I or type II TGFβ receptors; responsiveness to TGFβ can be restored by recombinant expression of the “missing” protein Mutations in Smad2 also commonly occur in several types of human tumors. The TGFβ signaling pathway also induces expression of the genes encoding many extracellular matrix proteins, such as collagens, and plasminogen activator inhibitor-1 (PAI-1), an inhibitor of a serum protease that degrades many matrix proteins.
- 18. An inability to synthesize these proteins may contribute to metastasis, allowing tumor cells to “escape,” since less matrix will be made by the cells and matrix that is present may be degraded by inappropriately activated proteases.
- 19. SUMMARY 1. Overexpression of the protooncogene encoding cyclin D1 or loss of the tumor-suppressor genes encoding p16 and Rb can cause inappropriate, unregulated passage through the restriction point in late G1, a key element in cell-cycle control. Such abnormalities are common in human tumors. 2. TGFβ induces expression of p15, leading to arrest in G1, and synthesis of extracellular matrix proteins such as collagens and plasminogen activator inhibitor-1. 3. Loss of TGFβ receptors or Smad 4, a characteristic of many human tumors, abolishes TGFβ signaling. This promotes cell proliferation and development of malignancy.
- 20. QUESTIONS 1.The entry of cells into the cell cycle called a _______state (quiescent/G0) 2.The point in late G1 is called as_________ (restriction point) 3.The are two elements of the control system that regulate passage through the restriction point? (Cyclins, cyclin-dependent kinases (Cdks), and the Rb protein ) 4. Cyclins (D1, D2, and D3) assemble with their partners Cdk4 and Cdk6 to generate catalytically active kinases(True /false) True
- 21. 5. Rb phosphorylation is initiated by Active Cdk4 – cyclin D complex 6. Overexpression of cyclin D2 is responsiple for many human cancers True/false? (False-cyclin D1) 7.Abbreviation of CKIs is____ Cyclin-kinase inhibitors 8. p16 normally acts as a____ Tumor suppressor. 9. Abbreviation of TGFβ is______ Tumorderived growth factor β 10.Loss of TGFβ signaling contributes to_________ Abnormal cell proliferation and malignancy
- 22. 11. TGFβ signals pass through the ____and _____cell surface receptor Type I and Type II 12.Type I receptors is activated by _____ Type II receptors 13.Give a example of extracellular matrix protein Collagens and plasminogen activator inhibitor-1. 14. Which proteins are the key intracellular signal transducers in the pathway downstream from the TGFβ receptors Smad 15.Name any one mportant gene induced by TGFβ? p15
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