Lecture 12 - Introduction to cell division and cancers Flashcards
What are the different phases in the cell division process?
+ G1 - gap phase 1 when growth occurs (e.g., protein synthesis)
+ S - synthesis phase when DNA synthesis occurs
+ G2 - second gap phase
+ M - mitotic phase when duplicated chromosomes segregate
Cell division is regulated tightly at different levels
There are also checkpoints to check that processes in the different phases are executed correctly
What regulates the cell cycle?
Different cyclin-CDK complexes drive the different cell cycle phases
Note that there are different cyclins and CDKs in the mammalian cells
There are also overlapping time-frames when the different complexes function
How are cyclin-CDKs regulated?
Cyclin-CDKs themselves are regulated:
+ Cyclin levels
+ CDK inhibitors (CDKI)
+ phosphorylation at T14 and Y15 (inactivated)
What is the effect of cyclin synthesis and destruction?
Cyclin abundance is largely cyclical.
Different phase-specific cyclins are synthesised and destroyed at specific times during cell division.
The explains the sequential occurrence of the phases such that G1 precedes S that occurs before G2 that is executed prior to M.
e.g., Cyclin B1 level is kept low, increasing until metaphase then falls after
Do all cells enter the cell cycle?
Not all cells are dividing all the time, especially in our adult bodies!
Differentiated cells normally exit the cell division cycle
e.g., neurons, we cannot regenerate certain damaged tissues
EXCEPTIONS include liver cells that can re-enter the cell division cycle upon stimulation
What are quiescent cells?
Quiescent cells (G0 phase) - cells that have exited the cell cycle but have the potential to enter upon the correct stimulation
Liver resection - patients can regenerate their liver
Cells at the site of the resection put out signals that trigger entry into the cell division cycle among
the neighbouring quiescent cells
Functional interaction between signalling pathways and cell cycle regulation
What are the features of cells in a quiescent state?
low levels of cyclin D, CDK4, CDK6
low levels of cyclin A, CDK2
high levels of CDK inhibitors, p15, p16, p21, p27
E2F, a transcription factor for cell cycle genes, is bound and inhibited by pRB
What is a key post-translational modification in an epidermal growth factor signalling cascade?
Phosphorylation as a key post-translational modification that occurs sequentially from one signalling component to the next.
So many of the components along the signalling pathway are kinases.
Mitogenic signals cause receptor tyrosine kinase to dimerise and trans-phosphorylate each other
Grb is recruited to the receptor
Which then recruits Sos, Ras, Raf, then Mek, MAPK, substrate etc
Signalling pathway leads to increase in transcription factor, c-myc
Increasing transcription and thus synthesis of cyclin D, CDK4, CDK6
Phosphorylation of pRB and release of E2F
Transcription of cyclin E, CDK2
How do cells pass the restriction point?
The level of mitogenic signals determine if cells can cross the R point
Low levels of mitogenic signals lead to insufficient increase in c-myc and other cell cycle gene products
-> cells fail to cross the restriction point and commit to the division cycle
Before cells pass the R point, withdrawal of serum will lead to failure to enter the cell division cycle.
Threshold levels of cyclin-CDK complexes needed to cross restriction point.
High levels of mitogenic signals lead to sufficient increase in c-myc and other cell cycle gene products
-> cells cross the restriction point and commit to the division cycle.
Once past the R point, withdrawal of serum (containing growth factors) will have no effect on the cell cycle progression.
How does progression through G1 to S occur?
cells exit G0 and enter G1
E2F transcribes cyclin E and CDK2, leading to increased levels and subsequent entry into S phase
CDKIs overcome by
cyclin-CDKs
pRB is hyper-phosphorylated and kept inactive throughout the cell division cycle
What are the complex roles of p27 (a CDKI) in cell division?
Presence of mitogens increases levels of cyclin D-CDK4 and cyclin D-CDK6 complexes.
Assume there are two major populations of p27:
- one population binds to and assembles cyclin D-CDK4 and cyclin D-CDK6 complexes
- one population binds to and inhibits cyclin E-CDK2
The cyclin-CDK complexes are found in the cytoplasm and nucleus
In G0 and early G1, p27 translation and protein stability are maximal
- p27 binds and inhibits the basal levels of cyclin E-CDK2, preventing cells from progressing through G1 to S phase.
What is a consequence of CDKI mis-regulation?
Lower p27 localisation in the nucleus was correlated to higher-grade renal cell carcinoma
What are karyopherins and importins?
Karyopherins are proteins involved in nuclear transport - both import and export
* Importins are an example of karyopherins that bind to their NLS-containing cargo in the cytoplasm.
* Importin is shown here as importin-α and importin-ß.
What are nucleoporins?
Nucleoporins are proteins that make up the complex lining nuclear pores.
* Importins are able to interact with the NPC via nucleoporins that have FG repeats
* By this interaction, the importins with their cargoes pass through the central transport channel.
After importin binds to the NLS of the cargo and pass through the central transport channel, how does importin release the cargo and translocate back into the cytoplasm?
- Once inside the nucleus importin binds to RAN.GTP, which causes a conformational change in importin that allows importin to release the cargo.
- The resulting complex of importin and RAN.GTP translocates back to the cytoplasm.
- Note thal importin-α is shown here to be transported back to the cytoplasm by associating with another protein CAS and RanGTP.
- CAS is a nuclear export factor.
- In the cytoplasm, the RAN.GTP hydrolyses GTP to GDP, thereby releasing the importin.
- hydrolysis is due to the GTPase activity in RAN. This is activated by the RANGAP, or the GTPase-activating protein.
- The importin then interacts with another cargo and the cycle repeats, resulting in more cargoes translocating into the nucleus.
