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Chapter 15
Regulation of Cell Number:
Normal and Cancer Cells
Regulated and unregulated
cell proliferation
21 and 23 March, 2005
Overview
• In higher eukaryotes, cell survival and proliferation are highly regulated
by integrated controls that continually evaluate the state of the cell and
its environment.
• Normal cell proliferation is modulated by regulation of the cell cycle.
• Apoptosis eliminates damaged cells and cells needed only temporarily
during development.
• The accumulation of mutations in cancer cells allows them to escape
apoptosis and proliferation controls.
Cell cycle regulation
• Cell cycle phases
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G1: period between mitosis and S
S: period of DNA replication
G2: period between S and mitosis
M: mitosis
• Cyclin
– transcribed in specific phase of cell cycle
– unstable, resulting in transient activity
• Cyclin-dependent protein kinase (CDK)
– substrate specificity and phosphorylation activity
controlled by bound cyclin
– phosphorylate serine or threonine of target protein
• Sequential activation of different CDK-cyclin
complexes controls cell cycle progression
Apoptosis
• Sometimes referred to as programmed cell death
• Triggered by variety of signals
• Sequential destruction of cell
– fragmentation of chromosomes
– organelle disruption
– fragmentation of cell
• Driven by activity of caspases
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cysteine-containing aspartate-specific proteases
normally inactive as zymogen (harmless to cell)
zymogen form activated by proteolysis
active caspases target other proteins for destruction
Cell-cell communication
• Signal-receptor systems
• Signal molecules (ligands)
– bind to receptor causing allosteric change
– endocrine signals: ligands secreted into
circulatory system
– paracrine signals: secreted ligands act locally
• Receptors
– usually membrane-bound
– conformational change upon binding of ligand
transduces signal to cytoplasm, activating
enzyme or enzyme cascade
Extracellular signals
• Cell cycle positive signals: mitogens (growth
factors) secreted by paracrine source
• Cell cycle negative controls: ligands initiate
signal cascade that inhibits cell cycle
progression
• Apoptosis positive controls: signal from
neighboring cell
• Apoptosis negative controls: survival factors
Cancer
• Aberrant cell cycle control
• Results from accumulation of mutations in
somatic cell clone
• Differ in many ways from neighboring cells
• Genetic basis for all cancer cells
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most carcinogens are also mutagenic
some highly penetrant inherited cancer genes
many less penetrant susceptibility genes
oncogenes transmitted by tumor viruses
Mutations in cancer cells
• Two categories
– Oncogenes, typically dominant
– Mutated tumor-suppressor genes, typically
recessive
• Sometimes associated with chromosomal
abnormalities, e.g., translocation that brings
gene under control of another gene’s strong
enhancer
• Cells that loose ability to undergo apoptosis
have longer time to accumulate
proliferation-promoting mutations
Tumor-suppressor genes
• May encode either negative regulators of cell
cycle or positive regulators of apoptosis
• Retinoblastoma caused by mutated RB gene
• p53 (refers to protein with mass of 53 kDa)
– ~50% of all tumors have mutated form
– normal p53 is transcription factor that is activated
in response to DNA damage
• prevents cell cycle progression to allow repair
• causes severely damaged cell to undergo apoptosis
– mutated form eliminates apoptotic response,
allowing damaged cells to survive, elevating
mutation level
Cancer complexities
•Different cancers have different phenotypes
with respect to rate of proliferation, ability to
metastasize, etc.
•Differences caused by:
–differences in somatic cell progenitor
–differences in types and severity of mutations
•Research from wide variety of areas
applicable to search for cures
Assignment: Concept map, Solved
Problem, All Basic and Challenging
Problems.