Regulation of the Cell Cycle and Cancer

Chapter 12: The Cell Cycle – Regulation and Cancer

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It is essential for growth, development, and tissue repair in multicellular organisms.

• Phases of the Cell Cycle:

  • G1 phase (First Gap): Cell grows and carries out normal functions.

  • S phase (Synthesis): DNA is replicated.

  • G2 phase (Second Gap): Cell prepares for division.

  • M phase (Mitosis and Cytokinesis): Division of the nucleus and cytoplasm to form two daughter cells.

• Interphase: Consists of G1, S, and G2 phases; the cell spends most of its time here.

• Division (M phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Control of the Cell Cycle

Cell cycle length and progression are tightly regulated and can vary among different cell types and conditions.

• Rapidly Dividing Cells: Have a short G1 phase (e.g., embryonic cells).

• Non-dividing Cells: Some cells exit the cycle and enter a quiescent state called G0 (e.g., neurons, muscle cells).

• Environmental Response: Cell cycle length can change in response to external signals or conditions.

Discovery of Cell Cycle Regulatory Molecules

Key discoveries in the 1970s and 2001 Nobel Prize-winning research identified molecules that regulate the cell cycle, especially the transition into mitosis.

• M phase-promoting factor (MPF): A complex that induces mitosis in all eukaryotic cells.

• Key Scientists: Leland Hartwell, Tim Hunt, and Paul Nurse were awarded the Nobel Prize for their discoveries of cell cycle regulators.

What is MPF?

MPF (M phase-promoting factor) is a dimeric protein complex crucial for the initiation of mitosis.

• Cyclin: A regulatory protein whose concentration fluctuates throughout the cell cycle.

• Protein kinase (Cdk): An enzyme that transfers phosphate groups from ATP to target proteins, regulating their activity.

Regulation of MPF Activity

The activity of MPF is controlled by the concentration of cyclin and the phosphorylation state of Cdk.

• Activation: Cyclin binds to Cdk, and after specific phosphorylation events, MPF becomes active, triggering mitosis.

• Deactivation: During anaphase, cyclin is tagged for destruction, leading to MPF inactivation. This is an example of negative feedback regulation.

Key Equation:

$\text{MPF Activity} \propto [\text{Cyclin}] \times [\text{Cdk}]_{\text{active}}$

Cell Cycle Checkpoints

Checkpoints are critical control points where the cell assesses whether to proceed with division.

• G1 Checkpoint: Determines if the cell will divide, delay, or enter G0.

• G2 Checkpoint: Ensures DNA is fully replicated and undamaged before mitosis.

• M Checkpoint: Confirms all chromosomes are properly attached to the spindle before anaphase.

Table: Major Cell Cycle Checkpoints

Checkpoint	Main Function	Key Regulators
G1	Cell size, nutrients, DNA damage, growth signals	p53, G1 cyclin-Cdk
G2	DNA replication completion, DNA damage	MPF (cyclin B-Cdk1)
M	Chromosome attachment to spindle	APC/C, spindle assembly checkpoint proteins

Role of p53 in the Cell Cycle

p53 is a tumor suppressor protein that acts as a brake on the cell cycle in response to DNA damage.

• Activation: Stabilized when DNA damage is detected.

• Functions:

  • Activates DNA repair proteins.

  • Pauses the cell cycle for repair.

  • Initiates apoptosis if damage is irreparable.

Consequences of Checkpoint Failure: Cancer

Cancer is a group of diseases characterized by uncontrolled cell division, invasion of nearby tissues, and sometimes metastasis (spread to other body sites).

• Benign Tumors: Noncancerous, noninvasive.

• Malignant Tumors: Cancerous, invasive, can metastasize.

• Common Cancer Mutations:

  • Overproduction of G1 cyclin: Leads to constant Cdk activation and uncontrolled cell cycle entry.

  • Defective Rb protein: Fails to inhibit E2F, allowing inappropriate S phase entry.

  • Defective p53: DNA damage is not repaired, leading to mutation accumulation.

Table: Cancer-Related Mutations at the G1 Checkpoint

Mutation	Effect	Consequence
G1 cyclin overproduction	Permanent Cdk activation	Uncontrolled cell division
Rb defect	Cannot bind E2F	Unregulated S phase entry
p53 defect	DNA damage not repaired	Mutation accumulation, tumorigenesis

Summary

• The cell cycle is tightly regulated by cyclins, Cdks, and checkpoint proteins.

• Disruption of these controls can lead to cancer.

• Understanding these mechanisms is crucial for developing cancer therapies and understanding cell biology.