Chapter 8: pRB and Control of the Cell Cycle Clock

Figure 8.1 - The cell cycle clock determines the fate of the cell, whether it will proliferate or not

post-mitotic
cell cycle clock

Section 8.1: External signals influence a cell's decision to enter the active cell cycle

figure 8.2 - Cell growth refers to the cell increasing in size and reproducing its genome while cell division refers to the acts of mitosis and cytokinesis
Figure 8.3 - Eukaryotic cells prolong the replication of their genome after a mitotic dvision, a phase known as G1.  During this time, it can choose to enter the non-dividing G0 state.
Figure 8.4 - Checkpoints help to ensure that the cell properly completes each step involved in division before moving onto the next
Figure 8.5 - Loss of checkpoint mechanisms leads to visible problems in the cells karyotype
Tumor progression --> Chapter 11
Breakdown of control --> Chapter 12

cell cycle
mitosiis
cytokinesis
prophase
metaphase
anaphase
telophase
chromatids
checkpoints
checkpoint controls
decatenation

Section 8.2: Cells make decisions about growth and quiescence during a specific period in the G1 phase

Sidebar 8.1 - Embryonic Stem Cells do not require any true growth factors in order to proliferate, rather, all they need is a chemical to prevent them from differentiating.  They are the only wild-type cell that is tumorigenic
Figure 8.6 - Somewhere between 80 and 90% of the way through the G1 phase, the cell makes a decision to divide, after which growth factors and anti-mitogenic factors have no effect on the current division

restriction point
R point

Section 8.3: Cyclins and cyclin dependent kinases constitute the core components of the cell cycle clock

Figure 8.7 - CDKs must bind cyclins along their PSTAIRE helix and can be phosphorylated on Ser/Thr residues in the activation loops in order to be activated
Figure 8.8 - CDKs can bind different cyclins in order to change their substrate specificity depending on which stage of the cell cycle the cell is in
Figure 8.9 - Cyclin B levels increase slowly after a mitotic division until they are broken down immediately after the next, this behavior gave cyclins their name
Figure 8.10 - Each of the cyclins follows a pattern similar to cyclin B, but differ in which part of the cell cycle they are linked to.  B is linked to the G2-M phase, A to the S-G2 phase, and E to the G1-S phase
Figure 8.11 - Cyclin D1 is not controlled by the cell's current position in the cell cycle, but rather by extracellular signals such as growth factors, Wnts, and cytokines.
Table 8.1 - The different D cyclins are controlled by various etracellular signals and different signalling pathways
Sidebar 8.2 - D cyclins can bind other receptors CDKs including estrogen receptors and and the transcription factor C/EBPbeta
Figure 8.12 - Cyclins that are utilized after the D cyclins in the cell cycle serve as a transcriptional activator for the following cyclin and a deactivator of the previous cyclin.  Thus, D cyclins serve as the impetus for cell division

centrosome
cyclin-dependent kinases (CDKs)
cyclin
synchronous

Section 8.4: Cyclin-CDK complexes are also regulated by CDK inhibitors

Figure 8.13 - Inhibitors of CDKs exist which operate by preventing binding of the CDK to either the appropriate cyclin or ATP.  Two classes exist, the INK4 proteins which inhibit the action of the D-CDK4/6 complex and the Kip/Cip proteins which inhibit the other cyclin-CDK complexes
Figure 8.14 - TGF-beta works as an inhibitor of cell division by eliciting an increase in the cellular concentration of INK4B, an inhibitor of the D-CDK4/6 complex.  p21^Cip1 is functional when the DNA is damaged and inhibits the actions of all other cyclin-CDK complexes.
More on p21^Cip1 --> Section 9.9
Figure 8.15 - The PI3K signaling route, stimulated by mitogens, leads to an enzyme known as Akt/PKB whose function is to phosphorylate p21^Cip1 and p27^Kip1, preventing them from entering the nucleus and acting as inhibitors of cell cycle progression
Figure 8.16 - Presence of p27^Kip1 in the cytoplasm is a sign of a more serious tumor, because the machinery the cell uses to prevent growth has been knocked out
Figure 8.17 - p21^Cip1/Kip1 act in an inhibitory manner wwhen bound to an E-CDK2 complex, but in a stimulatory manner when bound to a D-CDK4/6 complex
Figure 8.18 - p27^Kip1 in high enough concentrations serves to keep a cell in a post-mitotic, quiescent state
Sidebar 8.3 - CDK molecules are also inhibited by the lack of phosphorylation in some areas and the presence of phosphorylation in others.  The enzymes that add and remove these phosphate groups have not been linked to any cancers.

Section 8.5: Viral oncoproteins reveal how pRb blocks advance through the cell cycle

Figure 8.19 - pRb is hypophosphorylated in the G1 stage of the cell cycle.  Cyclin D-CDK4/6 complexes continue to phosphorylate pRb in this stage until it is hyperphosphorylated, after which the cell moves into the S phase.  pRb remains hyperphosphorylated until the completion of mitosis.
Figure 8.20 - The viral oncoprotein E1A binds to pRb and a few related proteins

hypophosphorylated
hyperphosphorylated

Section 8.6: pRb is developed by the cell cycle clock to serve as a guardian of the restriction-point gate

Figure 8.22 - Mitogens stimulate the production of Cyclin D1 which forms a complex with CDK4/6 which phosphorylates pRb and stimulates proudction of Cyclin E which when in a complex with CDK2 also phosphorylates pRb which can then no longer inhibit cell growth

Section 8.7: The E2F transcription factors enable pRb to implement growth-versus-quiescence decisions

Figure 8.23 - Hyperphosphorylation of pRb results in the progression of the cell cycle because it prevents binding of pRb to E2F transcription factors which are responsible for many of the genes associated with growth and division.  Oncogenes can form a protein that binds pRb in such a way as to prevent its binding to E2F.
Figure 8.24 - E2F remains bound to DNA, whether or not it is bound to pRb.  The difference in transactivation is provided by the pRb's blocking of domains associated with transactivation on E2F and recruiting a histone deacetylase
Figure 8.25 - E2F activation is an example of a postive feedback.

Section 8.8: A varety of mitogenic signaling pathways control the phosphorylation state of pRb

Figure 8.26 - Ras activation leads to an increased level of Cyclin D1 through AP1 transcription factors and prevention of ubiquitylation of beta-catenin.

dominant-negative

Section 8.9: The Myc oncoprotein perturbs the decision to phosphorylate pRb and thereby deregulates control of the cell cycle clock

Figure 8.27 - Myc forms a heterodimer with Max to encourage transcription while Mad forms a heterodimer with Max to discourage transcription.  Mad/Max dimers are more common in differentiated cells.
Oncogene collaboration --> Chapter 11
Figure 8.28 - Myc leads to the activation of genes promoting growth which include direct participants in the cell cycle clock and inhibitors of inhibitors
Sidebar 8.5 - Myc can cause a cell to escapse the normal limit on the amount of times it may divide
Figure 8.29 - Estrogen Receptor-Myc fusion proteins serve to display the hold Myc has over the cell's fate.  Stimulation of these proteins by estrogen/tamoxifen is enough to drive them over the R point
Figure 8.30 - Myc has a pleiotropic effect on cells, elliciting a wide range of effects

Section 8.10: TGF-beta prevents phosphorylations of pRb and thereby blocks cell cycle progression

Figure 8.31 - The overall effect of TFG-beta activatoin is repression of the cell cycle through expression of the CDK inhibitors p15^INK4B and p21^Cip 21.  It further ensures transcription of these two genes through repression of the myc gene by bonding to E2F4/5, p107, and the myc gene.
microsatellite instability --> Section 12.9

Section 8.11: pRb function and the controls of differentiation are closely linked

Figure 8.33 - Removal of growth factors can induce differentiation while overexpressed Cyclin D1 can prevent it
Figure 8.34 - Myc can block differentiation of muscle cells by promoting the expression of the Id2 protein, an inhibitor of the MyoD transcription factor

myoblasts
myocytes

Section 8.12: Control of pRb function is perturbed in most if not all human cancers

Table 8.3 + 8.4 - various aspects of the pRb regulatory system are disturbed in different cancers in different manners
Figure 8.35 - The pRb regulatory system is a complex machine
Gifure 8.36 - Having multiple copies of the Cyclin D1 gene is one manner of inducing cancer