Cell Proliferation, Death, and Epigenetics: Determination of Cancer Cell Fate

Outline

  • Review of the cell cycle and apoptosis
  • Discussion on dysregulation of these processes in cancer
  • Functions of oncogenes and tumor suppressor genes
  • Role of epigenetics in cancer

The Mammalian Cell Cycle

  • Mitosis (M): Actual cell division
  • Interphase: Stages between mitosis, includes:
    • G1: First gap phase
    • S: Synthesis phase (DNA replication)
    • G2: Second gap phase
  • Quiescent state (G0): Cells that are not progressing through the cycle.

Cell Cycle Checkpoints

  • Mechanisms to prevent progression: Checkpoints ensure integrity before proceeding through the cycle.
  • Key checkpoints:
    • G1 Restriction point: Cell evaluates external signals (to enter G0 or continue)
    • Other checkpoints respond to:
    • DNA damage
    • Incomplete DNA synthesis
    • Incomplete spindle assembly

Molecules Involved in Checkpoints

  • Many checkpoint molecules are also involved in DNA damage response/repair
  • Spindle Assembly Checkpoint: Senses DNA damage via spindle proteins.

Cell Cycle Control

  • Cyclin-dependent kinases (CDKs):
    • Regulate the cell cycle, active when bound to Cyclins.
    • Cycles of expression vary throughout the cell cycle.
  • Ubiquitin E3 ligase complexes degrade Cyclins/CDKs to mediate this oscillation.

Inhibitory Molecules Regulating CDKs

  • Examples:
    • P16INK4A
    • P15INK4B
    • P21 Cip1
    • P27Kip1
    • P57Kip2

Oscillation of CDK/Cyclin Levels

  • Positive feedback loop: Cyclin/CDK complexes feedback to regulate their own degradation and activities, involved in the oscillation of activity.

External Influence on the Cell Cycle

  • TGF-β: Inhibitory signal for cell cycle
  • Growth factors (mitogens): Promote cell cycle progression.

Role of pRb in the Cell Cycle

  • pRb Regulation:
    • Phosphorylated by CDKs, influences binding to E2F transcription factors.
    • Phosphorylation states:
    • Hypophosphorylated in G1 until the restriction point.
    • Hyperphosphorylated through S/G2/M phases.

E2F Transcription Factors

  • Released from pRb at the Restriction point.
  • Governs gene expression to promote entry into S phase.

MYC in the Cell Cycle

  • MYC:
    • Transcription factor activated by growth signals.
    • Drives expression of Cyclins and CDKs necessary for progression through the cell cycle, especially from G1 to S phase.

Overview of Cell Cycle Regulation in Cancer

  • Cancer often sees:
    • Upregulation of oncogenes like MYC.
    • Loss of tumor suppressor genes (e.g., Rb, p16INK4A).
    • Mutations in genes that control the progression of the cell cycle.

Overview of Apoptosis

  • Different forms of cell death:
    • Apoptosis: Programmed cell death, critical for development.
    • Necrosis: Uncontrolled cell death.
    • Autophagy: A survival mechanism that can lead to cell death.

The Role of Apoptosis in Development

  • Studies with C. elegans show the importance of apoptosis in normal development.

Genes Controlling Apoptosis

  • Exposing C. elegans to mutagens aids in identifying genes affecting apoptosis.
  • Genes identified show conservation across species.

Major Apoptosis Pathways

  • Extrinsic Pathway: Triggered by extracellular death signals.
  • Intrinsic Pathway: Triggered by cellular stress, key molecule involved: p53.

p53 Functionality in Cancer

  • Role of p53 mutations: Cooperates with oncogenes like RAS, affecting transformation capacity.
  • Inactive p53 can block normal p53 function, acting as a dominant-negative mutant.

p53 Expression Regulation by Stress

  • Stressors increase p53 levels; post-translational modifications stabilize it.
  • Gene expression program by p53 leads to either cell cycle arrest or apoptosis.

p53 Mutations in Cancers

  • p53 mutations: Often point mutations, leading to dominant-negative phenotypes without needing loss of heterozygosity (LOH).

Dysregulation of Apoptosis in Cancer

  • Bcl-2 upregulation and other anti-apoptotic factors often over-expressed in malignancies.
  • Potential silencing of pro-apoptotic genes via epigenetic mechanisms.

Understanding Epigenetics

  • Epigenetics involves modifications that regulate gene expression without altering DNA sequence.
  • Key mechanisms:
    • Histone modifications
    • DNA methylation

Histones and Gene Regulation

  • Histone modifications affect gene transcription and other cellular processes.
  • Histone Code: Interactions of writer, reader, and eraser proteins govern chromatin states.

DNA Methylation in Cancer

  • Methylation of cytosines can silence tumor suppressor genes.
  • Hyper- and hypomethylation patterns observed in various tumors, influencing gene expression and stability.