Aging and Cancer: the

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Lawrence Berkeley National Laboratory and Buck
Institute for Age Research
Aging and Cancer the Double-edged sword of
cellular senescence And How to teach an old
cell new tricks!
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Cancer Rises Exponentially with Age
Age is largest single risk factor Incidence vs
mortality
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What Causes Cancer?
Mutations, mutations, mutations
AND
A permissive tissue
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Organisms with renewable tissues had to evolve
mechanisms to prevent cancer
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Tumor Suppressor Mechanisms/Genes
(prevent mutations)
(eliminates or arrests potential cancer
cells) Apoptosis (programmed cell
death) Cellular senescence
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Caretaker tumor suppressor genes are longevity
assurance genes
Gatekeeper tumor suppressor genes
are antagonistically pleiotropic
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Antagonistic Pleiotropy
Whats good for you when you are young, can be
bad for you when you are old.
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Mutation Accumulation
Antagonistic Pleiotropy
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Cellular Senescence A Gatekeeper Tumor
Suppressor Induced by potentially oncogenic
stimuli Most tumor cells acquire mutations that
abrogate the senescence response Controlled by
p53 and pRB -- tumor suppressors inactivated in
most tumors Mouse models/human cancer-prone
syndromes
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The senescence response is not simply an arrest
of cell growth
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Cellular Senescence and Antagonistic Pleiotropy
Selected/Unselected (deleterious) Traits
Irreversible Growth Arrest
Unselected traits of little consequence, unless
senescent cells accumulate to appreciable levels
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Senescent Cells Accumulate In Vivo
With Increasing Age Skin Retina Liver
At Sites of Age-Related Pathology Venous
ulcers Atherosclerotic plaques Benign prostatic
hyperplasia Preneoplastic hepatic lesions
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Senescent Cells May Contribute to Aging
Phenotypes/Diseases . Including Cancer
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Ana Krtolica Simona Parrinello Steve Lockett -
LBNL Imaging Group Pierre Desprez - CPMC
Proc. Natl Acad. Sci USA 9812072-12077 (2001)
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Senescent Fibroblasts Stimulate the
Proliferation of Premalignant Epithelial Cells
Fibroblasts Presenescent Senescent
Epithelial Fluorescence
HaCAT
SCp2
S1
Premalignant Epithelial Cells
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Senescent Fibroblasts Do NOT Stimulate Normal
Epithelial Cells
Fibroblasts Presenescent Senescent
Epithelial Fluorescence
Adult HMEC
Adult NHEK
Neonatal NHEK
Genetically Normal Human Epithelial Cells
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Senescent Fibroblasts Stimulate Tumorigenesis of
Premalignant Epithelial Cells In Vivo
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SCp2 cells alone
0
200
Presenescent Fibroblasts
100
Tumor size (mm3 x 10)
0
200
Senescent Fibroblasts
100
0
Days
40
80
120
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Christian Beausejour Ana Krtolica Francesco
Galimi (Verma lab, Salk Institute) Masasha
Narita, Scott Lowe (CSH) Paul Yaswen (LBNL)
EMBO J 224212-4222 (2003)
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Senescence Response, Controlled by p53 and pRB
Pathways
p53
pRB
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Lentiviruses for high-efficiency expression of
genes in senescent cells
Lenti-GSE (inactivates p53) Lenti-CDK4m
(inactivates pRB) Lenti-p16 (activates
pRB) Lenti-p16(RNAi) (inactivates pRB)
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Senescent BJ (foreskin fb)
Senescent WI-38 (fetal lung fb)
Lenti-GSE (inactivate p53)
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LN
S-WI
LgT
GFP
GSE
hTERT
CDK4
LgTK1
GSELgT
CDK4GSE
GROWTH 0 0 lt1 lt1 lt1 lt1 0 lt1
S-BJ rescued
GSE
LN
S-BJ
Population doubling
LgT, LgTK1
LgT
GFP
GSE
LgTK1
CDK4
hTERT
GSELgT
CDK4m
GROWTH 0 0 gt90 60 40 gt90 20
Days post infection
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p53 inactivation can reverse the senescent
growth arrest of BJ, but not WI-38, cells
What distinguishes reversibly from irreversibly
senescent cells??
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Fibroblasts differ in expression of p16 at
senescence BJ low p16 WI38 high p16
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Do differences in p16 expression explain
differences in reversibility of the senescence
arrest?
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Presenescent BJ fibroblasts (low p16)
1) lenti-p16
2) lenti-GSE
DNA synthesis, but no proliferation
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Presenescent WI-38 fibroblasts (high p16)
1) lenti-p16-RNAi ----gt Senescence
2) lenti-GSE
DNA synthesis proliferation
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p53 maintains the senescent state p16 maintains
a dominant barrier to reversal The senescent
phenotype is reversible upon p53 inactivation
Providing the p16/pRB pathway has not been
engaged Why does p16 render the
senescence growth arrest irreversible?
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HYPOTHESIS p16 enables pRB to establish an
irreversibly repressive chromatin state that,
once established, is independent of p16 or pRB
Senescent cells form RB-dependent
heterochromatic domains that repress positive
acting cell cycle genes Lowe and colleagues,
Cell, 2003 Once cells express high levels of
p16, they no longer require p16 or active pRB to
maintain the senescence growth arrest
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p16 renders senescence irreversible
PBJ p16
PWI P16-RNAi
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Senescence is not necessarily irreversible in
human cells Hint ask about mouse cells! p53
inactivation is not a recommended therapy (but
p53 modifiers, such as SIR2, may be!) What
determines the extent to which cells express
p16? How can we reverse the p16/pRB-initiated
chromatin?
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