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Summary
Cell Senescence
Importance
What is it?
How identified in a cell (markers)?
Senescence defective in cancer cells
Mechanisms of normal cell senescence; Telomere shortening and telomerase
Cell senescence and ageing
Why?
Cell senescence in neoplasia: benign lesions and tumour suppression
Cancer, crisis and cell immortalisation
Cell senescence - key points
A form of permanent arrest of cell proliferation
A major defence against cancer
Probably the basis of ageing

Replica(ve*senescence
*
e
...
Human melanocytes
(neonatal)

Permanent cell growth arrest following:
Extended proliferation
Activation of an oncogene (OIS, oncogene-induced
senescence)
Overexpression of an oncogene can fast forward to
senescence
or other cellular “stress”
This type called replicative senescence

Population doublings

Cell senescence - what is it?

Total days in culture

Identifying cell senescence
Morphological changes
E
...
In senescent human fibroblasts

S
Features:
Large, flat cells (S)
Prominent
nucleoli (

S

dividing
cells

)

“Stress fibres”
(actin) (
)
Edge of nucleus
hard to see (lamin
loss) (
)

S
S

Senescent cells: molecular properties/markers
Not all necessarily present - varies with context
Expression of effectors (cell cycle inhibitors): like p16, p53, p21, ARF
CDK inhibitors
DNA damage signalling – possibly universal
...

Increased lysosomal content (high β-galactosidase)
Increased ROS (reactive oxygen species) levels
“SASP”- Senescence-associated secretory phenotype
Secretion of inflammatory and related factors:
Cytokines & receptors (eg IL6, IL8, CCLs, CXCRs, CXCLs)
Proteases (eg MMP’s)
Angiogenic factors (eg VEGFs)
Other growth factors (eg IGF2, PDGF, FGFs)
Aids body in clearing senescent cells --> inflammation recruits macrophages to
clear cells
Mechanisms of normal cell senescence
Telomeres and why we have them
DNA 'end replication problem'
One$
double$
helix
$

The$DNA$at$the$3’$end$of$each$strand,$where$the$RNA$primer$binds,$is$not$
itself$replicated$by$DNA$polymerase
...
$

Gap
Can't replicate last bit of 3' end - so shortened after each division
Telomeres solve this

Chromosomes(with(
labelled(telomeres(

Telomeres
Telomeres: structures at the ends of chromosomes
Made of ~0
...
%
%TERC%(or%TR):%telomerase%RNA%
component%

TERT - important in cancer
Telomeres and cell senescence
In humans, most somatic cells lack telomerase activity, and therefore telomeres shorten
as cells divide
Replicative senescence is triggered in normal cells when telomere(s) get quite short
(Possibly 1-2 such short telomeres may be enough
...
'
p53'signals'growth'
arrest'

Phosphorylates and activates p53
ARF & p16 --> made from the same locus
So in cancer can be whole locus deletion --> taking out both

Main effector pathways of cell senescence
Stresses,'e
...
'

Telomere'
shortening/
'
damage
'

Oncogene'
ac3va3on
'

ROS,'radia3on
'

?'

DNA'damage'
signalling
'

p53
'

p16'
'

ARF'

(esp
...
g
...
)
Cells with p16, β-galactosidase and Telomere-Induced Foci of DNA damage (TIFs)
found increasingly in ageing tissues
Werner syndrome (see later)
Fibroblasts*in*biopsied*baboon*skin*show*senescence*markers**
(p16,*TIFs*)*increasingly*with*age!
*TIF"="Telomere*Induced"Focus"(of"DNA"
damage"signalling"–"ATM,"CHEK2"etc)
"

Single*TIFs*?
ac@vated*ATM*
(DNA*damage)
*

nuclei
*

p16

The*same*cells*that*have*nuclear*p16*(=senescent)*also*have*TIFs
*

%*fibroblasts*with*
senescence*markers*
rises*with*age
*

Werner syndrome
Premature ageing and premature cell senescence
Werner fibroblasts undergo only about 20 population doublings in culture (normal =
about 60)
Mutations in the Werner gene, RECQL2, a DNA helicase, lead to accelerated telomere
shortening
Cell immortality (cells don't senesce)
Naturally immortal cells
Immortalisation
The germ line
Normal germ cells express telomerase so they maintain full-length telomeres
Therefore germ line is naturally immortal - can divide forever

+
oocyte

sperm
diploid
progenitors*
oogonia etc

zygote
Soma
...
g
...

So in children too
...

All these three commonest changes (p53, p16 defects, telomerase activation) are those
required to immortalise cells
Immortalisation may thus be a necessary step in producing an advanced cancer

Viral oncogenes also target (inactivate) both p53 & p16/Rb pathway

More evidence for importance of senescence in cancer
Viruses have tiny genomes and retain only essential genes
But cancer viruses need the host cells to proliferate, not senesce
Call senescence in benign lesions
Cell senescence is now known to occur in a number of types of benign, static tumour,
including in animal models, and human moles

E
...
cell senescence markers in moles (naevi)
It is thought that oncogene activation in one cell (BRAF, NRAS) leads to proliferation
to form a clone, which however then arrests growth through cell senescence
To form a growing cancer, one cell would have to mutate to evade senescence
Naevi are about 3,000x more common than melanomas
So senescence has successfully suppressed melanoma, 2,999 in 3,000 times

Disruption of normal cell senescence (in cell culture)

If%disrupt%senescence,%2nd%arrest:%telomeric%crisis%

Telomeres&geCng&
shorter
&

&Senescence&1&short&

telomeres
...
&

Time&in&culture&(months)&

large,&flat,&
arrested&
cells&

Cell%popula4on%doublings%

Cell&popula
...
c&cells
...
%%
Crisis:%cells%divide%&%die
...
g
...
%%p53M%and%RBM
dependent,%or%RB%mainly%in%
some%cells
...
))

Telomerase)
ac+vated,)or)
expressed)
ar+ficially)

)))Telomere(s))lost/)fuse
...
)

)Senescence)E)short)
Deficiency)of)p53)
&)RB,)e
...
)HPV)
oncogenes,)or)
muta+ons)in)a))
prolifera+ve)lesion
)

telomeres
...
)

Time)in)culture)(months))

<-- Puts back TERT (normally lost)

Disruption of normal cell senescence (in cancer development)
The sequence of events is thought to be similar, but through spontaneous genetic or
epigenetic changes

Watching)cancer)cells)become)immortal)
Growth)curve)of)explanted)cells)from)an)early)human)melanoma)(at)St)George’s)
)
50

Cells)didn’t)
senesce)aBer)
this)–)now)
immortal
...
g
...
g
...

Rarely a gene rearrangement/mutation in a cell may permit telomere extension (e
...

TERT promoter mutation)
This gives a clone that can proliferate again, thus cancer progression, leading to
escape from crisis, or immortalisation



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