biology of cancer - kocwcontents.kocw.net/kocw/document/2014/pusan/kimnamdeuk/19.pdf · robert a....

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The Biology of Cancer First Edition

pRb and Control of the

Cell Cycle Clock

Nam Deuk Kim, Ph.D.

Copyright © Garland Science 2007

Robert A. Weinberg

Figure 8.1 The Biology of Cancer (© Garland Science 2007)

The central

governor of

growth and

proliferation

Summary

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Figure 8.3a The Biology of Cancer (© Garland Science 2007)

The mammalian

cell cycle

1. External signals influence a cell’s decision to enter into the

active cell cycle

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Examples of checkpoints in the cell cycle

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Responsiveness to extracellular signals during the cell cycle

2. Cells make decisions about growth and quiescence during a

specific period in the G1 phase

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Cyclin-CDK complexes

3. Cyclins and cyclin-dependent kinases constitute the core

components of the cell cycle clock

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Pairing of cyclins with

cyclin-dependent kinases

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*CDC2 = CDK1


Cell cycle-dependent fluctuations in cyclin B levels

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Fluctuation of cyclin levels during the cell cycle

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Control of cyclin D1 levels: Cyclin D1 was

discovered as a protein whose levels are strongly

induced by exposure of macrophages to the

mitogen CSF-1 (colony-stimulating factor-1).

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Figure 8.11b The Biology of Cancer (© Garland Science 2007)

Control of cyclin D1

levels: The control of

cyclin D1 levels by

extracellular mitogens

can be explained, in part,

by a signal-transduction

cascade that leads from

growth factor receptors,

one of a number of

factors that modulate the

transcription of the cyclin D1 gene in the nucleus.

As indicated, a number of

other signaling cascades

converge on the

promoted of this gene.

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Table 8.1 The Biology of Cancer (© Garland Science 2007)

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Control of cyclin levels during the cell cycle

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4. Cyclin-CDK complexes are also regulated by CDK

inhibitors

Actions of CDK inhibitors 15


The complex

between p27Kip1 and

cyclin A-CDK2,

derived by X-ray

crystallography.

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Figure 8.13c The Biology of Cancer (© Garland Science 2007)

Inhibitors of the

INK4 class, such as

p16INK4A, bind to

CDK6. This CDK

inhibitor distort the

cyclin-binding site of

CDK6, reducing its

affinity for D-type

cyclins.

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TGF-β

Control of cell cycle

progression by TGF-β

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When TGF-β is applied to human keratinocytes, it evokes a

dramatic 30-fold induction of p15 mRNA synthesis as

demonstrated here by RNA (Northern) blotting analysis. These

cells were exposed to TGF-β for the time periods (in hours)

indicated.

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Control of cell cycle

advance by

extracellular signals.

receptor

tyrosine kinase

pathway

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p15INK4B


Suppression of p27 function by Akt/PKB in human breast cancers.

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• At low grade (less advanced) tumor A: p27 in the nucleus (>50%

nuclear) with less pAkt/PKB increased survival

• At low grade tumor B (less advanced) tumor B: p27 in the

nucleus (≤ N + C) with higher pAkt/PKB decreased

survival


5. Viral oncoproteins reveal how pRb blocks advance

through the cell cycle

Cell cycle-

dependent

phosphorylation

of pRb

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Control of the restriction-point transition by mitogens.

6. pRb is deployed by the cell cycle clock to serve as

a guardian of the restriction-point gate.

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7. E2F transcription factors enable pRb to implement

growth-versus-quiescence decisions.

E2Fs and their

interactions.

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Figure 8.23c The Biology of Cancer (© Garland Science 2007)

The E2F transcription factors bind DNA as heterodimeric

complexes with DP partners: E2F4-DP2 complex to the DNA

double helix.

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Figure 8.23d The Biology of Cancer (© Garland Science 2007)

The E2Fs constitute a family of at least seven distinct proteins.

• E2Fs 1, 2 and 3a: transcriptional activators

• E2F3b, E2F4, E2F5: transcriptional repressors

• E2F6, E2F7: poorly resolved

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Modification of chromatin by pocket proteins

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When the E2Fs are not complexed with pRb (or its p107 and

p130 cousins), they attract histone acetylases (red), which place

chromatin in a configuration that I conductive for transcription.

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Positive-feedback loops and the

irreversibility of cell cycle advance.

The irreversibility of certain key steps in

cell cycle progression and the rapidity of

their execution is ensured, in part, by the

activation of certain positive-feedback

loops.

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Positive-feedback loops and the irreversibility of cell cycle

advance.

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8. A variety of mitogenic signaling pathways control

the phosphorylation state of pRb

• Growth factors

growth factor receptors

Ras

cyclin D1 and E

inactivation of pRb

activation of E2Fs

S-phase entrance

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Countervailing controls

on cyclin D1 levels.

The Ras signaling

pathway influences

cyclin D1 levels in at

least three major ways.

1. AP-1 (Fos-Jun)

pathway

2. PI3K pathway

3. Ras pathway

A fourth minor

pathway:

1. GSK-3β pathway

(dashed line)

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9. The Myc oncoprotein perturbs the decision to phosphorylate

pRb and thereby deregulates control of cell cycle progression.

The Myc transcription factor:

• Myc belongs to a family of

bHLH (basic helix-loop-helix)

transcription factors.

• Myc-Max: promote

transcription active

proliferation

• Mad-Max: repress

transcription of most target

genes increased

differentiation

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Actions of Myc on the cell cycle

clock:

• Myc-Max: induce expression of the

growth-promoting proteins cyclin D2

and CDK4 promote advance

through early G1.

• By increasing the expression of

Cul1 (which is responsible for

degrading the p27 CDK inhibitor) as

well as E2Fs 1, 2, and 3, Myc

favors advance into S phase.

• In addition, Myc, acting with its Miz-

1 partner, is able to repress

expression of the p15, p21, and p27

CDK inhibitors; once again, effects

are felt both in early/mid and in

late G1.

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Powers of the Myc oncoprotein: The wide-ranging effects of the Myc

protein are illustrated by an experiment

in which the Myc protein has been

fused to the estrogen receptor (ER)

protein (blue). In the absence of ER

ligands, such as estrogen or tamoxifen,

the Myc-ER protein is trapped in the

cytoplasm (through association with

heat shock proteins, not shown). When

estrogen or tamoxifen ligands of the ER

(small purple ball) are added to cells,

the Myc-ER protein migrates into the

nucleus, associates with Max, and

activates Myc target genes within

minutes. Such activation, when induced

in serum-starved cells in the G0 phase,

enables them to enter the active cell

cycle and advance all the way through

G1 into the S phase.

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10. TGF-β prevents

phosphorylation of pRb and

thereby blocks cell cycle

progression.

Countervailing actions of

TGF-β and Myc.

• TGF-β: a major growth-inhibitory signal

that normal cells, especially epithelial

cells, must learn to evade in order to

become cancer cells.

• Many types of cancer cells must evade

TGF-β-imposed growth inhibition if they

are to thrive.

• TGF-β phosphorylation of Smad3

complex with Smad4 and Miz1 induce

expression of p15 and p21 (weakly)

• TGF-β dispaches Smad3 to form a

complex with E2F4/5/p107 represses

expression of Myc gene

• TGF-β succeeds in inducing expression

of the two CDK inhibitors -p15 and p21-

and thereby shuts down cell cycle

progression in the early/mid G1 phase of

the cell cycle. 36


11. Control of pRb function is perturbed in most if not all human cancer.

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Perturbation of the R-point transition in human tumors.

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• The decision to advance through the R-point transition (yellow) can be

perturbed in a variety of ways in human tumors.

• [Brown color]: favor advance through the R point

• [Blue color]: block this advance


Amplification of the cyclin D1 gene.

• Cyclin D1 gene (CCND1) in the cells

of a head-and-neck squamous cell

carcinoma (HNSCC).

• CCND1 is amplified to various

extents.

• About one-third of all of these

tumors, leading to corresponding

increases in cyclin D1 expression

and resulting loss of proper control

of pRb phosphorylation.

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12. Synopsis and prospects

Cyclin E and breast

cancer progression. • This Kaplan-Meier plot

presents the clinical

progression of disease

of women with stage III

breast cancer.

• Plotted is the fraction of

patients (ordinate) who

are still alive at the

indicated times after

initial diagnosis

(abscissa).

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biology of cancer - kocwcontents.kocw.net/kocw/document/2014/pusan/kimnamdeuk/19.pdf · robert a....

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