FINAL -- pRb control and the Cell Cycle Clock cont.. #13

Question 1

Cell cycle regulation pathways…

Answer 1

converge on the pRb and the R checkpoint

Question 2

Signaling through TGF-beta

Answer 2

An R-Smad (receptor-regulated, e.g. Smad2/3) is phosphorylated by an activated TGF-􏰖 receptor promoting its association with Smad4 (the human Co-Smad). The Smad complex translocates to the nucleus where it binds to its target genes and partner transcription factors.

Question 3

Smad

Answer 3

signal transducers and transcriptional modulators

Question 4

Signaling through AKT/PKB

Answer 4

• Two pathways
• AKT/PKB inhibits inhibitors of cyclin-CDK complex activity
• Cancer and the localization of inhibitor p21 kip1 (two types)
  – Nuclear localization
  – Cytoplasmic localization

Question 5

Nuclear Localization

Answer 5

• p21kip1can interact with cyclin-CDK and carry out anti-proliferative function
• Situation found in low-grade human mammary carcinomas

Question 6

pRb represses the actvity of

Answer 6

E2F Transcription factors

• pRb interferes with the transactivation domain of E2Fs preventing
  E2F interaction with general TFs

Question 7

RB1

Answer 7

• gene encodes a protein called pRb
• plays an integral role in the development of retinoblastoma

Question 7

Cytoplasmic localization

Answer 7

• High levels of AKT/PKB result in p21Klp1
  translocation to cytoplasm
• Situation found in high-grade human mammary carcinomas

Question 7

The most understood role of pRb is

Answer 7

to block a cell’s passage through the R checkpoint into the S phase
- pRb decides whether it should proliferate, become quiescent, and whether or not it will differentiate.

Question 8

E2F transcription factors

Answer 8

• a family of TFs involved in the cell cycle regulation and DNA synthesis in mammalian cells E2F1, 2, and E2F3a are activators
• E2F3b, E2F4-8 are suppressors
• All E2F TFs target genes especially those needed for the S phase

Question 9

pRb is a pocket protein which means…

Answer 9

• proteins have a pocket for the functional binding of other proteins
• pRb has A and B domains joined by a linker region. The confirmation forms a pocket

Question 10

How is pRb regulated by its phosphorylation state?

Answer 10

-there are many phosphorylation sites within pRb
- The activity of pRb is regulated by sequential phosphorylation events by cyclin-CDKs
- pRb holds the R-point gate shut unless and until it becomes hyper phosphorylated.

Question 11

How is E2F regulated by pRb?

Answer 11

• In the absence of growth signals pRb is hypophosphorylated
• pRb binds to a histone deacetylase and the E2F TF

Question 12

What happens when pRb binds to (HDAC)

Answer 12

• HDAC activity increases chromatin compaction
• E2F is inactive ; its target genes are not transcribed

Question 13

What is initiated in G1 response to growth signals (mitogens)?

Answer 13

Cyclin D

Question 14

Cyclin D-CDK 4/6 complex

Answer 14

• Binds CDK inhibitors that prevent interaction with E-CDK2 complexes wich leader to the Activation of E-CDK2 complexes
• Monophosphorylates pRb which caused the release od HDAC relieving the repression of some E2F-target genes, importantly the cyclin E gene

Question 15

Cyclin E-CDK2

Answer 15

phosphorylates pRb to generate the hyperphosphorylated state resulting in the complete functional inactivation of pRb

Question 16

Hyperphosphoylated pRb causes…

Answer 16

a conformation change resulting in the release of E2F and subsequent expression pf target genes

Question 17

MYC (c-Myc) Transcription Factor

Answer 17

• a multi-functional transcription factor affecting cellular activities such as the cell cycle, apoptosis, DNA damage response, and hematopoiesis
• May regulate expression of 15% of all genes

Question 18

MYC expression regulated by concentration:

Answer 18

+ levels of Myc mRNA heighten quickly following a growth factor addition
- Mcy activity decreases quickly with the removal of GF due to Myc proteins short half-life

Question 19

Oncogenic Switch (Myc

Answer 19

• When covered into an oncogene live becomes deregulated
• Enhanced expression of MYC proteins is estimated to occur in up to 70% of human malignancies

Question 20

What happens when Myc forms heterodimers with another transcription factor?

Answer 20

• When a heterodimer is formed with Max (or Miz), to activate the transcription of target genes which facilitates cell cycle progression

Question 21

What happens in the absence of Myc

Answer 21

• Max forms a complex with Mxd/Mxi/Mnt proteins and acts as a repressor of Myc transcriptional targets.