Aging and Cancer: the Double-edged sword of cellular senescence

And

How to teach an old cell new tricks!

Lawrence Berkeley National Laboratory and Buck Institute for Age Research

Cancer Rises Exponentially with Age

AGE

INC

IDE

NC

E

Age is largest single risk factorIncidence vs mortality

What Causes Cancer?

Mutations, mutations, mutations …

AND

A permissive tissue

Organisms with renewable tissueshad to evolve mechanisms

to prevent cancer

Tumor Suppressor Mechanisms

Tumor Suppressor Mechanisms/Genes

(eliminates or arrests potential cancer cells)

Apoptosis (programmed cell death)Cellular senescence

CARETAKERS Prevent or repair genomic damage

(prevent mutations)

GATEKEEPERS Control cellular responses to damage

Caretaker tumor suppressor genes arelongevity assurance genes

Gatekeeper tumor suppressor genes areantagonistically 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 ……..

100%

SU

RV

IVO

RS

AGE

"Natural" Environment(hazards, predators, infection, etc.)

80 and >3-4

"Protected" Environment (climate control, biomedical intervention etc.)

40 yrsHUMANS:4 mos MICE:

Aging before cell phones ……..

100%S

UR

VIV

OR

S

AGE

"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 suppressorsinactivated in most tumors

Mouse models/human cancer-prone syndromes

Short/dysfunctionaltelomeres

(REPLICATIVE SENESCENCE)

DNADamage Oncogenes

ChromatinInstability

Stress/Signals

Cellular Senescence: Induced by Potentially Cancer-Causing Events

Irreversiblearrest of

cell proliferation

The senescence response

is not simply an arrest of

cell growth

The Senescent Phenotype

IrreversibleGrowth Arrest

Resistanceto

Apoptosis

AlteredDifferentiated

Functions

Selected/Unselected (deleterious) Traits

IrreversibleGrowth Arrest

Resistanceto

Apoptosis

AlteredDifferentiated

Functions

Cellular Senescence and Antagonistic Pleiotropy

Unselected traits of little consequence, unless senescent cells accumulate to appreciable levels

Senescent Cells Accumulate In Vivo

With Increasing AgeSkin

RetinaLiver

At Sites of Age-Related PathologyVenous ulcers

Atherosclerotic plaquesBenign prostatic hyperplasiaPreneoplastic hepatic lesions

Senescent Cells May Contribute to Aging Phenotypes/Diseases …….

Including Cancer

Ana KrtolicaSimona Parrinello

Steve Lockett - LBNL Imaging Group

Pierre Desprez - CPMC

Proc. Natl Acad. Sci USA 98:12072-12077 (2001)

Ep

ith

elia

l F

luo

resc

ence

HaCAT SCp2 S1

Senescent Fibroblasts Stimulate the Proliferation of Premalignant Epithelial Cells

Premalignant Epithelial Cells

Fibroblasts:Presenescent

Senescent

Ep

ith

elia

l F

luo

resc

ence

AdultNHEK

Neonatal NHEK

Adult HMEC

Senescent Fibroblasts Do NOT Stimulate Normal Epithelial Cells

Fibroblasts:Presenescent

Senescent

Genetically Normal Human Epithelial Cells

Senescent Fibroblasts Stimulate Tumorigenesis of Premalignant Epithelial Cells In Vivo

Days40 80 120

Tu

mo

r si

ze (

mm

3 x

10)

100

0

100

0

200

100

0

200

SCp2 cells alone

+ Presenescent Fibroblasts

+ Senescent Fibroblasts

Christian BeausejourAna 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

p53 pRB

Growth Arrest + Senescent Gene Expression

Tx Changes(downstream effectors)

ARF

MDM2

CDK4

p16

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)

p53 pRB

ARF

MDM2

CDK4

p16

SenescentBJ

(foreskin fb)

SenescentWI-38

(fetal lung fb)

+ Lenti-GSE (inactivate p53)

20 DoublingsNo proliferation

0

20

40

60

80

100

0 20 40 60 80 1000

5

10

15

20

25

GSE

LgT

LgTK1

CDK4

GF

Ph

TE

RT

GS

E

Lg

T

Lg

TK

1

GS

E+

Lg

T

% L

N

S-BJ

% GROWTH: 0 0 >90 60 40 >90 20*

Po

pu

lati

on

do

ub

lin

g

Days post infection

S-BJ rescuedC

DK

4

0

20

40

60

80

100

GF

P

hT

ER

T

GS

EL

gT

Lg

TK

1G

SE

+L

gT

% L

N

S-WI

CD

K4

CD

K4

+G

SE

% GROWTH: 0 0 <1 <1 <1 <1 0 <1

GSE

LgT, LgT[K1]

CDK4m

p53 inactivation can reverse the senescent growth arrest of BJ, but not WI-38, cells

What distinguishes reversiblyfrom irreversibly senescent

cells??

Fibroblasts differ in expressionof p16 at senescence:

BJ = low p16WI38 = high p16

BJ WI38 P S P S

actin

p16

pRBCDK4

p16

Do differences in p16 expression

explain differences in

reversibility of the senescence arrest?

Presenescent BJ fibroblasts (low p16):

DNA synthesis, but no proliferation

0

40

80

100

% g

row

th

GFP p16 p16+

GSE

p16+

LgT

LgT +GFP

LgT +p16

1) + lenti-p16

2) + lenti-GSE

Presenescent WI-38 fibroblasts (high p16):

DNA synthesis + proliferation

0

40

80

100%

gro

wth

GFP p16RNAi + Sn

p16RNAi

+Sn +

GSE

Sn +GFP +LgT

Sn +p16RNA +LgT

p16RNAi

+Sn+

LgT

1) + lenti-p16-RNAi ----> Senescence

2) + lenti-GSE

p53 maintains the senescent state; p16 maintains a dominant barrier to reversal

The senescent phenotype is reversibleupon p53 inactivation ……

Providing the p16/pRB pathway hasnot been engaged

Why does p16 render the senescencegrowth arrest irreversible?

HYPOTHESIS: p16 enables pRB toestablish an “irreversibly” repressive

chromatin state that, once established,is independent of p16 or pRB

Senescent cells form RB-dependent heterochromaticdomains that repress positive acting cell cycle genes

Lowe and colleagues, Cell, 2003

Once cells express high levels of p16, they nolonger require p16 or active pRB to maintain the

senescence growth arrest

p16 renders senescence irreversible

0

40

80

100

% g

row

thGFPp16 p16

+GSE

p16+

LgT

LgT +GFP

LgT +p16

PBJ +p16

0

40

80

100

% g

row

th

GFPp16RNAi + Sn

p16RNAi

+Sn +

GSE

Sn +GFP +LgT

Sn +p16RNAi +LgT

p16RNAi

+Sn+

LgT

PWI +P16-RNAi

Senescence is not necessarily irreversiblein 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 thep16/pRB-initiated chromatin?

Aging and Tumor Suppression

Tumor suppressors

Aging Phenotypes Cancer

Can tumor suppression and agingbe uncoupled??