Transcript No Slide Title

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!
INCIDENCE
Cancer Rises Exponentially with Age
AGE
Age is largest single risk factor
Incidence vs mortality
What Causes Cancer?
Mutations, mutations, mutations …
AND
A permissive tissue
Organisms with renewable tissues
had to evolve mechanisms
to prevent cancer
Tumor Suppressor Mechanisms
Tumor Suppressor Mechanisms/Genes
CARETAKERS
Prevent or repair genomic damage
(prevent mutations)
GATEKEEPERS
Control cellular responses to damage
(eliminates or arrests potential cancer cells)
Apoptosis (programmed cell death)
Cellular senescence
Caretaker tumor suppressor genes are
longevity assurance genes
Gatekeeper tumor suppressor genes are
antagonistically pleiotropic
Antagonistic Pleiotropy
What’s good for you when you are young,
can be bad for you when you are old.
Aging before cell phones ……..
"Protected"
Environment
(climate control,
biomedical intervention
etc.)
100%
"Natural"
Environment
(hazards, predators,
infection, etc.)
HUMANS: 40 yrs
MICE:
4 mos
80 and >
3-4
Aging before cell phones ……..
100%
"Natural"
Environment
(hazards, predators,
infection, etc.)
"Protected"
Environment
(climate control,
biomedical intervention
etc.)
Mutation Accumulation
Antagonistic Pleiotropy
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
Cellular Senescence: Induced by Potentially
Cancer-Causing Events
Chromatin
Instability
Irreversible
arrest of
cell
proliferation
DNA
Damage
Stress/
Signals
Oncogenes
Short/dysfunctional
telomeres
(REPLICATIVE SENESCENCE)
The senescence response
is not simply an arrest of
cell growth
The Senescent Phenotype
Irreversible
Growth
Arrest
Resistance
to
Apoptosis
Altered
Differentiated
Functions
Cellular Senescence and Antagonistic Pleiotropy
Selected/Unselected (deleterious) Traits
Irreversible
Growth
Arrest
Resistance
to
Apoptosis
Altered
Differentiated
Functions
Unselected traits of little consequence, unless
senescent cells accumulate to appreciable levels
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
Senescent Cells May Contribute to
Aging Phenotypes/Diseases …….
Including Cancer
Ana Krtolica
Simona Parrinello
Steve Lockett - LBNL Imaging Group
Pierre Desprez - CPMC
Proc. Natl Acad. Sci USA 98:12072-12077 (2001)
Senescent Fibroblasts Stimulate the
Proliferation of Premalignant Epithelial Cells
Epithelial Fluorescence
Fibroblasts:
Presenescent
Senescent
HaCAT
SCp2
S1
Premalignant Epithelial Cells
Epithelial Fluorescence
Senescent Fibroblasts Do NOT
Stimulate Normal Epithelial Cells
Fibroblasts:
Presenescent
Senescent
Adult
NHEK
Neonatal
NHEK
Adult
HMEC
Genetically Normal Human Epithelial Cells
Senescent Fibroblasts Stimulate Tumorigenesis
of Premalignant Epithelial Cells In Vivo
100
SCp2 cells alone
Tumor size (mm3 x 10)
0
200
100
+ Presenescent
Fibroblasts
0
200
+ Senescent
Fibroblasts
100
0
40
80
120
Days
Christian Beausejour
Ana Krtolica
Francesco Galimi (Verma lab, Salk Institute)
Masasha Narita, Scott Lowe (CSH)
Paul Yaswen (LBNL)
EMBO J 22:4212-4222 (2003)
Senescence Response,
Controlled by p53 and pRB Pathways
p16
ARF
MDM2
p53
CDK4
pRB
Tx Changes
(downstream effectors)
Growth Arrest + Senescent Gene Expression
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)
ARF
p16
MDM2
p53
CDK4
pRB
Senescent
WI-38
Senescent
BJ
(fetal lung fb)
(foreskin fb)
+ Lenti-GSE (inactivate p53)
No proliferation
20 Doublings
% LN
100
S-WI
80
60
40
20
% GROWTH: 0
0 <1 <1 <1 <1
CDK4+GSE
CDK4
LgTK1
GSE+LgT
GSE
LgT
GFP
hTERT
0
0 <1
S-BJ rescued
60
40
20
LgTK1
GSE+LgT
CDK4
0
GFP
hTERT
GSE
LgT
S-BJ
80
Population doubling
% LN
100
25
LgT
20
LgTK1
CDK4
15
LgT, LgT[K1]
10
5
CDK4m
0
0
% GROWTH:
0
0 >90 60 40 >90 20*
GSE
GSE
20
40
60
80
100
Days post infection
p53 inactivation can reverse the
senescent growth arrest
of BJ, but not WI-38, cells
What distinguishes reversibly
from irreversibly senescent
cells??
Fibroblasts differ in expression
of p16 at senescence:
BJ = low p16
WI38 = high p16
BJ
P
S
p16
WI38
P
S
p16
actin
CDK4
pRB
Do differences in p16 expression
explain differences in
reversibility of the senescence arrest?
Presenescent BJ fibroblasts (low p16):
1) + lenti-p16
2) + lenti-GSE
DNA synthesis, but no proliferation
% growth
100
80
40
0
GFP p16 p16
+
GSE
p16
+
LgT
LgT LgT
+
+
GFP p16
Presenescent WI-38 fibroblasts (high p16):
1) + lenti-p16-RNAi ----> Senescence
2) + lenti-GSE
DNA synthesis + proliferation
% growth
100
80
40
0
GFP p16
p16
RNAi RNAi
+
Sn
+
Sn
+
GSE
Sn
RNAi +
+
GFP
Sn
+
+
LgT
LgT
p16
Sn
+
p16
RNA
+
LgT
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?
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
PWI +
P16-RNAi
100
80
40
0
GFPp16 p16 p16
+
+
GSE LgT
% growth
PBJ +
p16
% growth
p16 renders senescence irreversible
LgT LgT
+
+
GFP p16
100
80
40
0
GFPp16 p16
p16 Sn Sn
RNAi +
+
+
+
+
GFP p16
Sn Sn
Sn + RNAi
+
+
LgT +
GSE LgT
LgT
RNAi RNAi
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?
Aging and Tumor Suppression
Aging Phenotypes
Cancer
Tumor suppressors
Can tumor suppression and aging
be uncoupled??