Regulation of the Cell Cycle

Regulation of the Cell Cycle

Topic Objectives

  • Review the stages/phases of the cell cycle.
  • Introduce cell cycle checkpoints and the key proteins involved with their regulation.
  • Introduce and describe the roles of oncogenes and tumor suppressor genes and their effect on the cell cycle.
  • Introduce the process of cell death and describe necrosis and apoptosis.

Cell Division Regulation

  • Cell division is tightly regulated, maintaining a balance between cell proliferation and cell death.
  • The ability of cells to divide depends on cell and tissue type.
  • Cell fate includes:
    • Apoptosis (programmed cell death)
    • Survival
    • Quiescence (Go phase)
    • Differentiation
    • Senescence (aging)
    • Cell division
  • Go/G1 phase can be induced to re-enter the active cell cycle.
  • Permanent Go: Cells that no longer divide.

Cell Cycle Refresher

  • Two major stages:
    • Mitosis (M phase): When the cell is actively dividing.
    • Interphase: The time between one M phase and the next M phase.
      • Growth – increase in cell size
      • Replication of DNA
      • Growth and assessment of success of replication
  • Interphase is further divided into 3 phases:
    • G1 (Gap 1):
      • The period just after mitosis and before the beginning of DNA synthesis.
    • S phase (synthesis):
      • The time when the cell’s DNA is replicated.
    • G2 (Gap 2):
      • The time after S and prior to mitosis.

Regulation of Cell Cycle Progression

  • Cells move from one phase to the next via a cell cycle control system.
  • This system checks that each step has completed successfully before allowing the cell to move on.
  • Ensures that all DNA and organelles have been replicated before actual division.
  • This control is achieved through a series of checkpoints.
  • Checkpoints are controlled by regulatory proteins, which act like biochemical on/off switches.

Cell Cycle Checkpoints

  • Regulated by activation and inactivation of key proteins or protein complexes.
  • Key proteins involved:
    • p53 (TP53)
    • p21 (CDKN1A)
    • p16 (CDKN2A)
    • CDK1
    • Cyclin B (CCNB-1/-2/-3)
    • CDK4/6
    • Cyclin D (CCND-1/-2/-3)
    • Cyclin A (CCNA-1/-2/-3)
    • CDK2
    • Cyclin E (CCNE-1/-2/-3)
    • E2F
    • Rb1

Protein Activity Regulation

  • Protein activity is switched on and off via phosphorylation followed by dephosphorylation.
  • Protein Kinase:
    • ATP is converted to ADP.
    • A phosphate group is added to a serine, threonine, or tyrosine side chain of a protein.
  • Protein Phosphatase:
    • A phosphate group is removed from a phosphorylated protein.

Cell Cycle Checkpoints - Phosphorylation

  • Phosphorylation is the addition of phosphate groups to a substrate.
  • This can change a protein’s three-dimensional structure.
  • Thereby altering its function (in many cases activating the protein).
  • Phosphorylation reactions that control the cell cycle are carried out by a specific set of protein kinases.

Cell Cycle Checkpoints - Dephosphorylation

  • Dephosphorylation is the removal of phosphate groups from a substrate.
  • This usually turns a protein off.
  • It is performed by a set of protein phosphatases.

Cyclin-Dependent Kinases (CDKs) and Cyclins

  • Cdks are a specific set of protein kinases that trigger the transitions from phase to phase in the cell cycle.
  • Present in a proliferating cell throughout the cell cycle.
  • Not always active though.
  • Have to be activated and deactivated at appropriate times.
  • Transitions from G1 to S and G2 to M depend on their activation.
  • Activity of Cdks are dependent on proteins called cyclins.
  • Cyclins are produced ONLY in response to extracellular signals (hormones and growth factors).

Cyclins Work by Activating Kinases

  • A Cdk is activated by binding to a Cyclin.
  • This alters its shape and exposes the active site.
  • The Cyclin–Cdk complex acts as a protein kinase
  • Transfers phosphates from ATP to amino acid residues of regulatory proteins.
  • This triggers phase transitions.
  • The Cyclin then breaks down and the Cdk becomes inactive.
  • These are the ‘gas pedals’ of the cell cycle, also known as Proto-oncogenes

Cyclins and CDKs

  • Distinct Cdks associate with different cyclins to trigger the different events of the cell cycle.

Types of Cyclin-CDKs

  • Cyclin D–Cdk4 and Cyclin E–Cdk2
    • Act during the G1-S transition
    • Phosphorylates Rb
    • This moves the cell past the restriction point (R).
      • Point of no return
  • Cyclin A–Cdk2
    • Acts during S
    • Stimulates DNA replication.
  • Cyclin B–Cdk1
    • Acts at the G2–M transition
    • Initiating mitosis.

Rb

  • Retinoblastoma protein (Rb)
  • Tumor Suppressor Gene
  • It is an inhibitor of the cell cycle
  • Acts as a Brake
  • The key to progressing past the restriction point in G1.

How Rb Works

  • Inhibitor of the Cell Cycle
  • Cyclins D and E activate Cdk 4 and 2
  • Which in turn inactivates Rb by phosphorylating it.
  • When Rb becomes inactivated, the cell can progress past G1 into S phase.
  • “Inhibiting the inhibitor”
  • If there is a mutation to both copies of the Rb gene, a rare childhood retinal cancer can result.
  • People with this mutation are also susceptible to other cancers as well.

Checkpoints

  • Other regulatory proteins act at checkpoints.
  • They allow or prevent the passage to the next cell cycle stage

CDK Inhibitor Proteins

  • Checkpoints are also controlled by Cdk inhibitors
  • Inhibitors arrest the cell cycle at specific checkpoints
  • Best studied example is p21, which blocks G1/S–Cdk
  • Serves to protect against DNA damage
  • If DNA is damaged by UV radiation, p21 is synthesized
  • It binds to G1 Cdk molecules, preventing their activation until damaged DNA is repaired.

P21 is Activated by P53

  • Damaged DNA causes phosphorylation of p53
  • Which stimulates the expression of p21
  • Like Rb, p53 and p21 are tumor suppressor genes.
  • If DNA cannot be repaired, p53 also stimulates apoptosis

P53 - The Guardian of the Genome

  • Checkpoint protein defects have been found in some cancer cells
  • A breast cancer cell line with too much cyclin D has been found.
  • p53 has been found to be defective in half of all human cancers.

Oncogenes and Tumor Suppressor Genes

  • Can be compared to the 'gas pedal' and 'brakes' of cell cycling.

Cell Death

  • Cells die in one of two ways: necrosis or apoptosis.
    • Necrosis
      • Occurs when cells either are damaged or are starved
    • Apoptosis
      • Genetically programmed series of events that results in cell death.

Necrosis

  • Pathological process caused by injury
  • Usually involves multiple cells
  • Cells swell and burst
  • Cell contents simply “spill” into the extracellular space
  • Inflammatory reaction causes additional damage
  • Process takes place over the course of several days

Apoptosis

  • Genetically programmed cell death
    • The cell is no longer needed by the organism.
      • Elimination of the cells of the weblike tissue between the fingers of a developing human fetus.
    • The cells are too old.
      • The longer cells live, the more prone they are to mutations that could lead to cancer.
    • Apoptosis can be stimulated by external or internal signals.

Apoptosis - Internal Signaling

  • Damage activates p53 which then actives Bax
  • Bax leads to cytochrome c release from the mitochondria.
  • This activates enzymes called caspases.
  • Caspases “cut up” chromatin into uniform pieces.
  • They also break down all the other proteins and membranes of the cell
  • Membranes form ‘blebs’

Apoptosis - External Signaling

  • Initiated by the Fas/FasL system
  • FasL bind to Fas receptor on cell surface and this activates caspases

Apoptosis - Completion

  • Cells and blebs are engulfed by phagocytes
  • Phagocytosis of membrane-enclosed components reduces risk of inflammation
  • Process is completed in a few hours