Cell Cycle

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					Medical Cell and Tissue Biology, BMS 6110C

            Cell Cycle

            John P. Aris, Ph.D.
   Rm B1-8, 392-1873, johnaris@ufl.edu

        Ross & Pawlina, 5th Edition
                Chapter 3
                  Cancer Mortality Statistics
                   2007 Estimated US Cancer Deaths
Lung & bronchus        31%    Men       Women
                                                  26%     Lung & bronchus
                             289,550    270,100
Prostate               9%                         15%     Breast
Colon & rectum         9%
                                                  10%     Colon & rectum
Pancreas               6%
                                                   6%     Pancreas
Leukemia               4%
                                                   6%     Ovary
Liver & intrahepatic   4%
    bile duct                                      4%     Leukemia
Esophagus              4%                          3%     Non-Hodgkin
Urinary bladder        3%                                  lymphoma
Non-Hodgkin            3%                          3%     Uterine corpus
                                                   2%     Brain/Nervous system
Kidney                 3%
All other sites        24%                         2%     Liver & bile duct
                                                  23%     All other sites

                        Source: American Cancer Society
                      Skin Cancer
• Skin cancer - most common form of cancer in the US
• Major types - basal cell carcinoma (BCC), squamous cell
  carcinoma (SCC), malignant melanoma (MM)
• Prognosis - BCC and SCC are highly curable (>90%), MM is
  not (MM accounts for 75% of deaths from skin cancer)
• Estimated new cases - ~1 million BCC & SCC, ~75,000 MM
• Mortality - ~10,000 Americans
• Risk factors - exposure to UV light is most important factor
• Prevention - consistent use of sun-protective practices
  (a survey sponsored by the CDC found that ~43% of white
  children under age 12 had at least one sunburn during 2004)
        Characteristics of Cancers
Self-sufficiency in                     Insensitivity
Growth Signals                          to Anti-growth

Evading                                 Limitless
Apoptosis             Cancer            Replicative

Sustained                               Invasion and
Angiogenesis                            Metastasis

   Hanahan and Weinberg proposed that normal cells
   must acquire six phenotypes to become malignant
   (Hanahan and Weinberg, 2000, Cell 100:57).
  Cancer and Cell Cycle Regulation

Many of the genes found to be mutated in cancer
cells encode proteins that regulate the cell cycle.

Mutations that increase cell proliferation exert their
effects through regulation of the cell cycle.

Mutations in cell cycle regulatory genes are not
necessarily sufficient to cause cancer.

Additional mutations are usually required for
invasiveness, metastasis, and tumor formation.
         Cell Cycle Overview

2n, 4C

                               “Gap” or

                          2n, 2C
           Cell Cycle Definitions
G1 phase - first growth (or gap) period
S phase - synthesis of DNA
G2 phase - second growth (or gap) period
Interphase - G1 + S + G2
M phase - mitosis and cytokinesis
G0 phase - exit from cell cycle (e.g., differentiated cells)

n - number of each chromosome
C - DNA Content (= Number of each DNA molecule)
G1 or G0 - 2n, 2C
G2 - 2n, 4C
                  Cell Cycle Times

• Many human cell types have cell cycle times of ~1 day
• Mitosis is usually the shortest phase (usually ~1 hour)
• S phase is usually 1/3 to 2/3 of cell cycle (can't be too short)
           Cell Cycle Regulation

                       2          3


Key cell cycle transitions:
1. Restriction point (in late G1; regulates S phase onset)
2. G2 - M transition (in late G2, regulates M phase onset)
3. Metaphase-anaphase transition (regulates exit from M)
    Regulatory Mechanisms

Restriction point or G1/S Transition
Regulated by protein phosphorylation,
protein degradation, and inhibitory proteins

G2/M Transition - Entry into M phase
Regulated by protein phosphorylation

Anaphase onset - Exit from M phase
Regulated by protein degradation
             CDKs and Cyclins

Cyclin Dependent Kinase (CDK) - must associate
with a cyclin protein to have kinase activity
Cyclin - protein whose level "cycles” (increases
and decreases) during the cell cycle

Cdc2 - first CDK gene/protein discovered
MPF (M-phase promoting factor) - first cell cycle
regulatory factor discovered
MPF = 1 mitotic CDK + 1 mitotic cyclin
        Mitotic CDK (MPF) Activation
                          Cyclin-Cdk kinase activity

            Cyclin protein level

                Cyclin-dependent kinase (CDK) protein level

                          G2                            M

1. CDK protein level changes little during the cell cycle
2. Mitotic cyclin levels rise in G2 and reach a critical threshold
3. Active CDK/cyclin complex yields a peak of kinase activity
4. CDK complex phosphorylates proteins and drives G2 to M
5. Cyclin is degraded to inactivate CDK and permit exit from M
           Mitotic CDK Substrates

Histones and Condensins
Phosphorylation of linker histone H1 and condensin proteins
is necessary for chromosome condensation during

Mitotic Spindle Proteins
Phosphorylation of spindle proteins is required for assembly
of the mitotic spindle at the beginning of prophase.

Phosphorylation of lamin proteins causes depolymerization
of lamin filaments. Disassembly of nuclear lamina is
required for nuclear envelope disassembly during mitosis.
                    CDK Activation
         Inactive                          Active

CDK                         CDK                             Cyclin


                            Activating              Substrate
T-loop                      phosphate               recognition

Inactive - no cyclin present, T-loop blocks active site with ATP
Active - cyclin bound, phosphorylated T-loop does not block
CDK substrate recognition - requires both CDK and cyclin
Anaphase Promoting Complex -
protein complex required to pass
from metaphase into anaphase

APC is the main regulator of protein
degradation in cell cycle

APC triggers degradation of specific
target proteins during anaphase

APC activity is stimulated by active
mitotic CDK (active MPF)
                 APC Function

• APC is a ubiquitin ligase that adds ubiquitin to cyclin
• Ubiquitinated proteins are degraded by the proteasome
  Restriction Point and G1/S Transition

• Restriction point - main point of regulation of cell proliferation;
  cells make a “commitment” to completion of a new cell cycle
• Exit to G0 - regulated by growth factors, cell-cell interactions,
  and nutrients (differentiated, non-dividing cells are in G0)
• Certain cell types can re-enter the cell cycle in response to
  growth factors or other signals (e.g., in wound healing)
             CDKs and Cell Cycle

                                              Diagram shows
                                              CDK activity
                                              levels during the
                                              human cell cycle

• Human have multiple cyclins and multiple CDKs
• Specific cyclin-CDK complexes are active during specific
  cell cycle stages (CDK complexes are not active in G0)
       CDK Inhibitors (CKIs)

                                      Cyclin A

CKIs bind to and inactivate CDK/cyclin complexes
             Tumor Suppressor p53

p53 gene encodes a 53 kDa protein

p53 protein functions as a transcription factor

p53 responds to DNA damage - “guardian of the genome”

p53 can arrest cell cycle progression or promote apoptosis

About 50% of human tumors contain mutations in p53 gene

Most point mutants map to DNA-binding regions in p53
(inability to bind DNA causes loss of tumor suppressor activity)

Some DNA tumor virus oncoproteins bind to and inactivate p53
  p53 Function
• Mdm2 binds to p53,
  causing its degradation
• DNA damage triggers
  phosphorylation of p53
  (stable and active)
• p53 induces expression
  of p21, a CDK inhibitor
• p21 arrests the cell
  cycle, which allows
  repair of DNA damage
• p53 can also trigger
  apoptosis rather than
  cell cycle arrest
Stages in Carcinogenesis

      p16 - a CDK inhibitor
      p53 - tumor suppressor
           Cell Cycle Checkpoints
• Checkpoints are “quality control” steps in the cell cycle
  that determine if the cell is prepared to progress to the
  next stage of the cell cycle and ensure that one cell cycle
  phase has been completed before moving to the next

• Questions asked at checkpoints:
  Environment good? (G1)
  Growth factors present? (G1)
  DNA damage? (G1, S, G2)
  DNA replicated? (S)
  DNA replication errors? (G2-M)
  Mitotic spindle properly formed? (G2-M)
  Chromosomes attached to spindle? (anaphase transition)

• Checkpoints are enforced by regulation of CDK activities
        Checkpoint and Core Layers

Checkpoint layer - collects information about environment,
cell (e.g, DNA) damage, and completion of cellular processes
Core layer - CDK and APC activities drive progression of cell
cycle (thick arrows) and are regulated by checkpoints

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