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0023/0110: DNA & disease
2004-05
Cell cycle and cancer
Objectives: The material covered in this lecture should enable you to

Understand the general mechanisms involved in cell cycle control and its deregulation in cancer

Understand how cell cycle is regulated in yeast and multicellular organisms
Cancer as a result of deregulated cell proliferation:
Beneath the complexity of every cancer lies a limited number of 'critical' events that have propelled the
tumour cell and its progeny into uncontrolled expansion and invasion. One of these is deregulated cell
proliferation, which, together with the suppression of apoptosis needed to support it, provides a
minimal 'platform' necessary to support further
neoplastic progression. Pathways that restrict this
proliferative response in normal cells is perturbed
in most cancers. One such pathway targets the
principal late-G1 cell-cycle checkpoint regulated by
cyclin / cdk (cyclin dependent kinases) systems.
This checkpoint signalling pathway arrests the cell
cycle when genomic integrity is threatened,
preventing the transmission of genetic mutations
into subsequent cell generations. The restriction
point also regulates cell cycle progression based on
environmental signals (growth factors, extracellular
matrix attachment, cell–cell contacts etc). Most
malignant cancers possess mutations in one or more
checkpoint genes and are genetically unstable
Cell cycle control
The core components of the eukaryotic cell cycle engine are cyclin-dependent protein kinases (Cdks) and
their regulatory subunits (cyclin). Higher eukaryotes have more forms of both cyclins and Cdks compared
to lower eukaryotes. Cyclin–Cdk complexes fall into three general classes that have different substrate
specificities and therefore initiate different cell cycle events
3 main classes of cyclins
1) G1 cyclins: G1 cyclin–Cdk complexes are important for progression through G1 phase and
commitment to S phase;
2) S cyclin: S cyclin–Cdk complexes are responsible for initiating and completing DNA replication;
3) mitotic cyclins: M cyclin–Cdk complexes drive eukaryotes into mitosis and restrain reentry into
G1 phase.
Cyclin–Cdk activity is modulated by the following mechanisms
1. Cyclin availability. Association with a cyclin is absolutely
required for Cdk activity. Cyclin levels can be changed by
transcriptional regulation and/or by ubiquitin-dependent
proteolysis.
2. Stoichiometric inhibition. Assembled cyclin–Cdk complexes
can be kept inactive by stoichiometric inhibitors (cyclin kinase
inhibitors; CKIs), which form inactive trimers (CKI–cyclin–
Cdk). Lower eukaryotes possess a single CycB–Cdk1 specific
inhibitor, whereas higher eukaryotes utilize many inhibitors in
the INK4a(p15/16/17), Cip1(p21) and Kip1(p27) families.
3. Inhibitory phosphorylation. Cyclin–Cdk complexes can also be
inactivated by phosphorylation of tyrosine and threonine
residues close to the active site of the Cdk subunit. This
phosphorylation is mediated by Wee1-type protein kinases, and
the inhibitory phosphate groups are removed by Cdc25-type
phosphatases.
Dr. MV Hejmadi
0023/0110: DNA & disease
2004-05
Cell cycle control in yeast
Regulated by
 size : maintained by length of cell cycle at G1 &G2 checkpoints

shape: differences in phases of 2 types of yeast correlated with spindle structure
genes: 1) cdc gene family (~70 known) : allows passage through checkpoint
2) wee gene family: inhibits passage thru checkpoint, encodes regulatory proteins
Fission yeast (Schizosaccharomyces pombe)
Size checkpoint located at G2.
M-phase promoting factor (MPF) formed by cyclin B and cdc2
Cdc2 (protein kinase) drives cell into M
Wee1, cdc25 phosphatase & cdc13 (cyclins) regulate function of cdc2
MPF activity regulated by inhibitory phosphorylation
Cdc2 + cyclin B = inactive MPF
MPF kept inactive by wee1kinase, which phosphorylates cdc2 on Y15, and MO15 kinase which
phosphorylates T160. Mitosis is initiated when phosphate on Y15 is removed by cdc25 phosphatase and
the balance of nuclear cdc25 exceeds that of wee1.

In response to DNA damage, cells are prevented from entering
mitosis by increasing Y15 phosphorylation, keeping MPF
switched off leading to cell cycle arrest. cdc25 phosphatase is
removed from its substrate cdc2 and exported to the
cytoplasm, thereby stopping cells from entering mitosis. This export is mediated by Rad24 (attachable
nuclear export sequence) Reference: Nature (1999) 397: 104-105
Cell cycle control in multicellular organisms
Growth & division regulated independently unlike yeast. At least 9 cdk’s identified so far of which 5 are
active during the cell cycle viz: G1(DK2,4,6), S(CDK2) & M(CDK1) & multiple cyclins A,B,C, D, E, F
Uncontrolled proliferation regulated by
CDK1
1)
External factors: These include growth
factors like PDGF & EGF, which are involved
in activation of either early response genes
(myc, fos & jun) or late response genes (cdks &
most cyclins) or anchorage dependent factors
like integrins.
2)
Tumour suppressor genes that encode
proteins that normally inhibit cell division. e.g
retinoblastoma (Rb) gene, P53 gene,BRCA1 etc
3)
Cdk inhibitors 2 families identified
1) INK4 family (inhibitor kinase4) e.g. p15, p16, p18, p19 bind specifically to CDK4 & CDK6,
inhibiting cyclin D activity
2) KIP/CIP family (kinase inhibitor protein/Cdk interacting protein) e.g. p27, p57 and p21; bind & inhibit
most CDK-cyclin complexes (esp G1 cyclins)
4)
Oncogenes: Genes known as proto-oncogenes code for proteins that stimulate cell division;
mutated forms, called oncogenes, cause stimulatory proteins to be overactive, with the result that cells
proliferate excessively.e.g. genes for transcription factors like c-myc
General reading: MBoC by Alberts et al (4th ed): pgs 863-906 OR Cancer Biology by RJB King
Optional reading:1) Cell Proliferation (June 2003) 36(3)pp131 - The cell cycle: a review ……. targets in cancer
2) Cell cycle by Gary S Stein et al www.els.net
Dr. MV Hejmadi