L8: Cell Cycle and Oncogenes Flashcards
G0
- quiescence
- induced by cell-cell contact or lack of growth factors
- cell isn’t dividing but is waiting around to start dividing
G1
- DNA is diploid
- 2N
- prepares for DNA replication
- cell grows an synthesizes compounds, including nucleotides
S phase
- DNA synthesis
- DNA replicates and chromosomes duplicated
- histones synthesized
- DNA becomes tetraploid
- 2N –> 4N
G2
- preparation for division
- cell grows and synthesizes microtubules and organelles
- DNA hasn’t divided yet
- Still 4N
Mitosis
- cells split back into 2N
- cell division occurs with sister chromatids partitioned into the 2 daughter cells
- 4n –> 2n
Why we have checkpoints?
- doesn’t want to commit to DNA synthesis unless everything is there and it can complete the cycle
G1 checkpoint
- growth factors sufficient?
- protein synthesis sufficient?
- No DNA damage?
G2 checkpoint
- has DNA been replicated?
- Don’t want the cell to divide if there is DNA that is still 2N instead of 4N
M checkpoint?
- mitotic spindles assembled?
- are the chromosomes aligned?
proteins that govern cell cycle transitions
- cyclins and cyclin-dependent kinases (CDKs)
Cdk
- a serine/threonine protein kinases that phosphorylate proteins that control cell cycle transitions.
- Cdks are constitutively expressed (present all the times of the cell cycle).
- takes binding of cyclin to make it active and activate the CDK serine-threonine kinase activity.
cyclin
- protein binding partner for Cdk that activates Cdk enzymatic activity
- gets degraded in reverse reaction to restore the inactive state of Cdk
- cyclin expression controls the timing of CDK activation
- cyclins are degraded by regulated proteolysis
Cdks and cyclin relevant to cell cycle
- Cdk4,6; Cdk2; Cdk1,2; Cdk1
- Cyclins D, E, A, B
CDKs throughout cell cycle
- CDKs are constantly present but require cyclin binding to be active
cyclin throughout cell cycle
- cyclins are made at cell cycle transitions
Cyclin D/Cdk4,6
G1 –> S transition
- cyclin D not initially expressed; begins to be expressed in G1 phase
- controls progression from G1 to S
- arrests systems that block S
- induces expression of Cyclin E and A
Cyclin E/Cdk2
S –> G2 (induces DNA synthesis)
- controls progression from S to G2
- induces DNA synthesis
- activates DNA helicase
Cyclin A/Cdk1,2
S –> G2/M
- controls progression through S and G2
- stabilizes microtubules at centrosome at G2
- induces DNA synthesis
Cyclin B/Cdk1
M phase progression
- controls progression through M (mitotic spindle assembly and nuclear breakdown)
cyclin A
- the only one that has a really long expression phase
cyclin D synthesis
- synthesized in response to extracellular growth factors
- very low in G0 and G1
- induced to be expressed
- CDK4,6 is constitutively expressed
How is Cyclin D transcription controlled?
Growth factor stimulation
- growth factors (ex: PDGF, EGF) bind their TKR membrane receptors. Receptor binding activates the Ras signaling pathway leading to the phosphorylation and activation of the Jun/Fos and Serum Response Factor (SRF) transcription factors. The TFs activate transcription of the cyclin D gene.
Cyclin D/Cdk4 in Retinoblastoma
- phosphorylates Rb protein
- Rb protein blocks G1 entry when unphosphorylated
- phosphorylation inactivates Rb and allows G1 entry
Rb/E2F complex
- represses gene expression
- Rb binds to E2F and blocks transcription and the transition of G1 to S
E2F
- transcription factor that activates expression of S phase genes
- DNA replication factors, cyclin E, and A
phosphorylation of Rb by Cdk4 and Cdk6
- releases Rb from E2F to reactivate gene expression and cell cycle progression
retinoblastoma
- 3% of all childhood cancers
- usually diagnosed before age 3
- can metastasize to the brain and spine
hereditary form of retinoblastoma
- affects both eyes
sporadic form of retinoblastoma
- affects one eye
loss of heterozygosity
- originally heterozygous but when you lose another allele, you are now homozygous
normal
- an occasional cell inactivates one of its two good Rb alleles
familial retinoblastoma
- individuals are both heterozygous for an Rb mutation
- An occasional cell inactivates one of only good Rb alleles
- excessive cell proliferation leads to retinoblastoma
- most individuals with inherited mutation develop retinal tumors
sporadic retinoblastoma
- an occasional cell inactivates one of its TWO good RB alleles
- rarely the second Rb allele is inactivated in the same line of cells
- excessive cell proliferation leads to retinoblastoma
- 1/30,000
normally
- an occasional cell inactivates one of its two good Rb alleles
- no tumor because you haven’t inactivated both copies
- not in germ line so won’t be transmitted
tumor suppressors
- Rb is one
- normal function is to inhibit cell cycle progression or promote genome stability
mutation of tumor suppressors
- inactivates tumor suppressor
- cell cycle progression occurs without regulation
- generally requires two hits
- both copies of tumor suppressor gene need to be mutated
tumor suppressor P53
- transcription factor that induces expression of cyclin inhibitor P21
P53/cyclin inhibitor P21 process
- DNA damage induces high levels of p53
- P53 expression induces cyclin inhibitor P21
- P21 inhibits cyclinD/Cdk4
- prevents phosphorylation of Rb
- Cell cycle stops in G1 and DNA is repaired
importance of loss of heterozygosity in human cancers
- occurs in 65% of colon cancer
- 30-50% of breast cancers
- 50% of lung cancers
proto-oncogenes
- present in normal DNA
- normal function to regulate cell proliferation and/or cell differentiation
mutation in coding or regulatory sequence of proto-oncogene
- converts it to an oncogene with unregulated activity or too much expression; chromosomal translocation can also result in misregulated expression
Normal process of Ras
- normal Ras is inactive in GDP bound form
- upstream stimulatory signal and Ras activity triggered by GEF
- downstream signaling to Raf then to MAPK - Cyclin/Cdk4 - proliferation
- GTP hydrolysis and RAS inactivation induced by GAP
Ras oncogenic mutation
- glycine 12 to valine (bulky side group)
- GTP cannot be effectively hydrolyzed to GDP even with GAP
c-myc transcription factor overexpression
- over expressed in leukemias and lymphomas
- main target of Ras MAPK pathway
- mutation causes over expression
- leads to unregulated protein synthesis
Tumor progression
- multiple mutations lead to colon cancer
- genetic change -> tumor changes
loss of tumor suppressor genetic APC
- leads to polyps
- leads to small benign growth
Ras mutation colon cancer
- activation of Ras oncogene
DCC deletion
- leads to larger benign carcinoma
loss of tumor suppression and additional mutations
- leads to malignant tumor
What are the possibilities for newly synthesized daughter cells?
New daughter cells can:
(1) reenter G1 for additional divisions
(2) exit the cell cycle, or
(3) enter G0 and remain quiescent until stimulated to divide (e.g. by a growth factor).
What genes control cell division?
There are a variety of genes that control cell division, including proto-oncogenes, tumor-suppressor genes, and cyclin-CDKs.
Rb Tumor-Suppressor Gene function
Retinoblastoma (Rb) gene normally inhibits proliferation
inheritance of mutations in Rb results n retinoblastoma (tumor of the eye)
What happens when there is a loss of Rb function?
Loss of Rb function by mutation results in E2F Transcription Factor free to transcribe proliferation-promoting genes –> uncontrolled proliferation
Explain how growth factors signal in the cell cycle to promote cell-cycle progression.
Growth factors that stimulate cell division bind to their receptors on the cells, initiate a signaling pathway that activates cyclin D, and ultimately leads to the expression of cell division genes and cell division.
For G1/S control, how do Cyclin D and Cdk4,6 signal to promote cell cycle progression?
A growth factor binds to its receptor (TKR) and initiates the Ras/Raf signal pathway, which induces Cyclin D expression.
Cyclin D binds and activates Cdk4 and Cdk6. The activated Cyclin-Cdk complexes phosphorylate Rb, which displaces E2F that had been bound up with the Rb. The freed E2F then enters the nucleus and activate transcription of genes involved in cell cycle progression.
Target genes of cyclin D include Cyclin A, Cyclin E, Cdc25 phosphatase, and proteins that initiate DNA synthesis (ex: DNA polymerase and DNA ligase).
Note: Cyclin E-Cdk2 hyperphosphorylates Rb to keep it inactive. Cyclin E is degraded but Cyclin A persists. Cyclin A-Cdk2 phosphorylates and inactivates E2F so that the cell cycle progresses.
Why is Rb a tumor-suppressor gene?
Because it inhibits proliferation when active.
Function of cMyc in controlling the cell cycle/cell growth.
cMyc is a downstream target of the Ras/Raf signaling cascade initiated when a growth factor binds to its TKR. cMyc becomes phosphorylated and induces ribososmal protein gene expression (G0 –> G1 phase)
The increase in ribosomes results in increased protein synthesis, which is required for growth.
In Leukemias and lymphomas, cMyc mutations result in unregulated overexpression and increased growth and proliferation.
