Molecular basis of cancer Flashcards
Principles of basis cancers- factors that cause it
Tissue homeostasis
- This is key in cancer
Normal cell proliferation process
What do we mean by cell growth and proliferation
Mitogen - instruct cell to divide
Growth factors.- activation needed of nutrient uptake and utilisation
Signalling pathway example to membrane biosynthesis required for cell growth, and increased protein mTOR
Cell cycle block
- The term: cell cycle block” dneoates a molecular circuity operating in cell nucleus that processes and itnegrates a variety of afferent (incoming) signals originating from outside and isnide the cells and decides whether or not the cell should enter into active cell cycle or retreat into a non proliferating state
- In the event that acitve proliferation is decidded upon, this circuitry proceeds to program the complex sequence of biochemical change sin a cell that enabled it to doubles its contents and to divide into 2 daughter cells.
Checkpoint control system in cell cycle
- The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
- The cell cycle control system is regulated by both internal and external controls
- The clock has specific checkpoints where the cell cycle stops until a go ahead signal is received
- Signals indicate if key cellular processes have been completed correctly
- 2 types of regulatory proteins are invovled in cell cycle control cyclins and cyclin-dependent kinases (Cdks)
- The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner.
The 4 classes of Cyclins
Pairing of cyclins with cyclin-dependent kinases
Each type of cyclin pairs with a specific cyclin- dependent kinase (CDK) or set of CDKs
- D-type cyclins (D1, D2, and D3) bind CDK4 or CDK6
- E-type (E1 and E2) bind CDK2
- A-type cyclins (A1 and A2) bind CDK2 or CDC2
- B-type cyclins (B1 and B2) bind CDC2
Fluctuation of cyclin levels during the cell cycle
Induction of D-type cyclin expression by extracellular signals: source signal and intermediates and type of Cyclin D
Cell cycle dependent phosphorylation of Rb
- The phosphorylation state of Rb (red circle) is closely coordinated with cell cycle advance.
- As cells pass through the M/G1 transition, virtually all of the existing phosphate groups are stripped off Rb, leaving it in anunphosphorylated configuration.
- As cells progress through G1, a single phosphate group is attached as any one of 14 different phosphorylation sites (by cyclin D-CDK4/6 complexes), yielding hypophosphorylated Rb
- However, when cells pass through the restriction (R) point, cyclin E– CDK2 complexes phosphorylate Rb on at least 12 more sites, placing it in a hyperphosphorylated state.
- Throughout the remainder of the cell cycle, the extent of Rb phosphorylation remains constant until cells enter into M phase.
The functional consequences of phosphorylation on Rb
Non-phosphorylated Rb:
- Binds transcription factors collectively called E2Fs
- Prevents E2F-mediated transcriptional activation of many genes whose products (e.g., DNA polymerase) are required for DNA synthesis
Phosphorylated Rb:
- Cyclin D–CDK4/6 kinase activity phosphorylates Rb starting in mid- G1.
- Complete phosphorylation inactivates Rb and disassociates E2Fs to turn on genes required for transition to S and for DNA synthesis, irreversibly committing the cell to DNA synthesis.
- Deregulation of the cell cycle and genome maintenance pathways can cause cancer.
- Mutations that promote unregulated passage from G to S phase are oncogenic in ~80 percent of human cancers.
Rb loss-of-function mutations contribute to cancer:
- Childhood retinoblastoma, a relatively rare type of cancer
- More common cancers that arise later in life (e.g., carcinomas of lung, breast, and bladder)
CDK inhibiitors
- p27Kip1 blocks cyclin A–CDK2 function by obstructing the ATP-binding site in the catalytic cleft of the CDK.
- Inhibitors of the INK4 class, such as p16INK4A, bind to CDK6 and CDK4. These CDK inhibitors distort the cyclin- binding site of CDK6, reducing its affinity for D-type cyclins. At the same time, they distort the ATP-binding site and thereby compromise catalytic activity.
INK4b-ARF-INK4a locus encodes 3 TSG
P53 “guardian of the genome)
- The p53 (TP53) gene is located on chromosome 17p13.1, and it is the most common target for genetic alteration in human tumours
- p53 acts as a “molecular policeman” that prevents the propagation of genetically damaged cells
- G1/S and G2/M checkpoints
- p53 inhibits neoplastic transformation by three
- interlocking mechanisms:
- activation of temporary cell cycle arrest (quiescence)
- induction of permanent cell cycle arrest(senescence)
- triggering of programmed cell death(apoptosis)
- Homozygous loss of p53 occurs in virtually every type of cancer, including carcinomas of the lung, colon, and breast — the three leading causes of cancer death
Tumour viruses
- The World Health Organization estimates that 15.4% of all cancers are attributable to infections and 9.9% are linked to viruses
- Cancers that are attributable to infections have a greater incidence than any individual type of cancer worldwide
- Eleven pathogens have been classified as carcinogenic agents in humans by the International Agency for Research on
- Cancer (IARC)
- After Helicobacter pylori (associated with 770,000 cases worldwide), the four most prominent infection-related causes of cancer are estimated to be viral:
- human papillomavirus HPV (associated with 640,000 cases)
- hepatitis B virus (HBV) (420,000 cases)
- hepatitis C virus (HCV) (170,000 cases)
- oEpstein-Barr virus EBV (120,000 cases
- It has been shown that viruses can contribute to the biology of multistep oncogenesis and are implicated in many of the hallmarks of cancer
- Notably, the discovery of links between infection and cancer types has provided actionable opportunities, such as the use of HPV vaccines as a preventive measure, to reduce the global impact of cancer
Tumour viruses and how they often seek to inactivate Rb and p53 tumour supressors
- Diverse group of viruses specify oncoproteins that are designed to inactivate Rb and usually p53.
- These viruses have evolved to optimize only one outcome – their efficient multiplication in tissues of
- infected hosts.
- Most viruses parasitize the host-cell DNA replication machinery in order to replicate their own genomes; this machinery is available only in the late G1 and S phases of the cell cycle. Consequently, these viruses need initially to inactivate Rb as well as p107 and p130, thereby causing infected, initially quiescent cells to advance into S phase.
- Cells infected by these various viruses respond to Rb inactivation by activating their p53 alarm systems.
- Direct response to the excessive activity of E2Fs that results from the functional inactivation of Rb.
- Human papillomaviruses (HPVs) function differently.
- Infect replicating cells in the cervical epithelium and block the normally occurring exit from the active cell cycle that takes place as these cells differentiate.