Question

B. (2 pts) From the data, propose a simple molecular explanation for how MI575 acts to block the cell cycle. 28. Examine the following pathway: TGFB - TGFB-R — P15(Ink4B) — cyclin D-CDK4/6 A. (3 pts) What will happen to Rb under these circumstances? B. (3 pts) Will the cell progress through the R point? Briefly explain your answer. C. (3 pts) What will happen to Rb and the cell cycle when TGFB is removed? 29. (3 pts) Based on the following pathway: Ras - PI3K - Akt - GSK-3B - B-catenin What effect will a mitogen have on the level of cyclin D?

Answer 1

B. MI575 acts by some molecular mechanism that keeps the M cyclin level high; any reasonable mechanism would be a correct answer. For example, MI575 may inhibit the destruction of M cyclin, creating an excess amount of M cyclin for prolonged periods in the cell. It could do so by either inhibiting some reasonable aspect of the Proteasome, the E3 Ligase or block some post-translational modification that acts as a signal for degradation (i.e. a kinase or phosphatase that works to signal the breakdown of M cyclin.

28. A. A.Cycl D-CDK4/6 complexes in turn mono-phosphorylates the retinoblastoma susceptibility protein (Rb) to pRb. The un-phosphorylated Rb tumour suppressor functions in inducing cell cycle exit and maintaining G0 arrest (senescence).

the pRB proteins are inactivated by cyclin D-Cdk4/6-mediated phosphorylation. Rb has 14+ potential phosphorylation sites. Cyclin D-Cdk 4/6 progressively phosphorylates Rb to hyperphosphorylated state, which triggers dissociation of pRB–E2F complexes, thereby inducing G1/S cell cycle gene expression and progression into S phase.

Deregulation of Cyclin D - Cdk 4/6 phosphorylates un-phosphorylated Rb in senescent cells to mono-phosphorylated Rb, causing them to enter G1. The mechanism of the switch for Cyclin E activation is not known, but one hypothesis is that it is a metabolic sensor. Mono-phosphorylated Rb induces an increase in metabolism, so the accumulation of mono-phosphorylated Rb in previously G0 cells then causes hyper-phosphorylation and mitotic entry. Since any un-phosphorylated Rb is immediately phosphorylated, the cell is then unable to exit the cell cycle, resulting in continuous division.

DNA damage to G0 cells activates Cyclin D - Cdk 4/6, resulting in mono-phosphorylation of un-phosphorylated Rb.....

B. No... , as Anti-mitogens like the cytokine TGF-β inhibit progression through the restriction point, causing a G1 arrest. TGF-β signaling activates Smads, which complex with E2F4/5 to repress Myc expression and also associate with Miz1 to activate expression of the Cdk inhibitor p15INK4b to block cyclin D-Cdk complex formation and activity. Cells arrested with TGF-β also accumulate p21 and p27.

Physiologically, TGF-β is a potent inhibitor of cell cycle; it induces expression of p15INK4B and represses expression of c-Myc. p15INK4B is able to prevent cyclin D-CDK4/6 complex formation; moreover, it displaces p21CIP1 and p27KIP1 from cyclin D-CDK4/6 complexes. These CIP/KIP inhibitors are subsequently able to inactivate other complexes of G1 and S phase and thereby inhibit cell cycle. Moreover, low levels of c-Myc allows for TGF-β induced p15INK4B and p21CIP1 transcription....

Once a cell is committed to enter replication, it will continue to double its DNA, divide and then arrest when entering the following G1 phase. At this point, TGF-β mediates cell cycle arrest by suppressing expression and function of c-Myc, members of the Id family inhibitors and CDKs and enhancing expression of CDK inhibitors, such as p15INK4B, p21CIP1 and p27KIP1...

C. I] It is well known that perturbations of these activating factors can lead to unregulated TGF-β signaling levels that may cause several complications including inflammation, autoimmune disorders, fibrosis, cancer and cataracts.

Growth inhibition by TGF-beta linked to suppression of retinoblastoma protein phosphorylation is lost if it is removed..