Campisi. "The Biology of
Replicative Senescence." Eur.J.Cancer, Vol. 33, No 5, pp. 703-709,
Campisi. "Aging and Cancer: The Double-Edged
Sword of Replicative Senescence" J Am Geriatr Soc 45:482-488,
Campisi. "The Role of Cellular
Senescence in Skin Aging" Journal of Investigative Dermatology
Symposium Proceedings 3:1-5, 1998
Senescence and Immortalization" Chapter 8 of The Molecular
Basis of Cell Cycle and Growth, G.S. Stein, R. Baserga, A. Giordano,
and D.T. Denhardt, ed. NY: Wiley. 1999. pp. 348-373
Campisi. "Cancer, Aging and Cellular
Senescence" In vivo 14:183-188 (2000)
Huang, Baomin Li, Matthew D. Gray, Junko Oshima , I. Saira Mian &
Judith Campisi. "The premature ageing syndrome
protein, WRN, is a 3' -> 5' exonuclease" Nature Genetics.
Oct. 1998, 20:114-116.
Campisi. "Replicative Senescence:
An Old Lives' Tale?" Cell, Vol. 84, 497-500, February 23,1996
Kim, Patrick Kaminker & Judith Campisi. "TIN2,
a new regulator of telomere length in human cells" Nature Genetics.
Dec.1999, 23: 405-412
Campisi. "The Biology of Replicative Senescence." Eur.J.Cancer, Vol.
33, No 5, pp. 703-709, 1997.
cells cannot divide indefinitely due to a process termed cellular or
replicative senescence. Replicative senescence appears to be a fundamental
feature of somatic cells, with the exception of most tumour cells and
possibly certain stem cells. How do cells sense the number of divisions
they have completed? Although it has not yet been critically tested,
the telomere shortening hypothesis is currently perhaps the best explanation
for a cell division 'counting' mechanism: Why do cells irreversibly
cease proliferation after completing a finite number of divisions? It
is now known that replicative senescence alters the expression of a
few crucial growth-regulatory genes. It is not known how these changes
in growth-regulatory gene expression are related to telomere shortening
in higher eukaryotes. However, lower eukaryotes have provided several
plausible mechanisms. Finally, what are the physiological consequences
of replicative senescence? Several lines of evidence suggest that, at
least in human cells, replicative senescence is a powerful tumour suppressive
mechanism. There is also indirect evidence that replicative senescence
contributes to ageing. Taken together, current findings suggest that,
at least in mammals, replicative senescence may have evolved to curtail
tumorigenesis, but may also have the unsdected effect of contributing
to age-related pathologies, induding cancer.
Campisi. "Aging and Cancer: The Double-Edged Sword of Replicative Senescence"
J Am Geriatr Soc 45:482-488, 1997.
cells do not divide indefinitely. This trait, termed the finite replicative
life span of cells, limits the capacity for cell division by a process
termed cellular or replicative senescence. Replicative senescence is
thought to be a tumor suppression mechanism and also a contributor to
organismic aging. This article reviews what is known about the genetics
and molecular biology of cell senescence. It discusses the evidence
that replicative senescence suppresses tumorigenesis, at least in young
organisms, and that it also contributes to the aging of mitotic tissues.
Finally, it puts forth the somewhat unorthodox view that, in older organisms,
senescent cells may actually contribute to carcinogenesis.
Campisi. The Role of Cellular Senescence in Skin Aging" Journal of Investigative
Dermatology Symposium Proceedings 3:1-5, 1998
organisms contain two types of cells: postmitotic cells, which never
divide, and mitotic (or mitotically competent) cells, which can divide.
Postmitotic cells include mature nerve, muscle, and fat cells, some
of which persist for life. Mitotic cells include epithelial and stromal
cells of organs such as the skin. Because postmitotic and mitotic cells
differ in their proliferative capacity, they may age by different mechanisms.
Normal somatic mitotically competent cells do not divide indefinitely.
The process that limits the cell division number is termed cellular
or replicative senescence. Replicative senescence is thought to be a
powerful, albeit imperfect, tumor suppressive mechanism. It is also
thought to contribute to organismic aging. Senescent cells undergo three
phenotypic changes: they irreversib1y arrest growth, they acquire resistance
to apoptotic death, and they acquire altered differentiated functions.
arrest is very likely critical for the role of replicative senescence
in tumor suppression, but may be less important for the aging of organs
such as the skin. On the other hand, the altered differentiation may
be critical for compromising the function and integrity of organs like
the skin during aging. Senescent keratinocytes and fibroblasts appear
to accumulate with age in human skin. Moreover, senescent cells express
genes that have long-range, pleiotropic effects - degradative enzymes,
growth factors, and inflammatory cytokines. Thus, relatively few senescent
cells might compromise skin function and integrity. Moreover, by altering
the tissue microenvironment, senescent cells may also contribute to
the rise in cancer that occurs with age.
Campisi. "Replicative Senescence and Immortalization" Chapter 8 of The
Molecular Basis of Cell Cycle and Growth, G.S. Stein, R. Baserga, A.
Giordano, and D.T. Denhardt, ed. NY: Wiley. 1999. pp. 348-373
somatic cells do not divide indefinitely. This property is termed the
finite replicative life span of cells, and it restricts cell division
by a process known as replicative or cellular senescence. Senescent
cells irreversibly arrest proliferation with a G, DNA content. Senescent
cells also become resistant to apoptotic death and show selective changes
in differentiated cell functions. Several lines of evidence suggest
that replicative senescence is a powerful albeit imperfect, tumor suppressive
mechanism and that it also contributes to organismic aging. Replicative
senescence appears to be controlled by multiple, dominant-acting genetic
loci. Cells escape senescence and acquire an infinite replicative life
span, or an immortal phenotype, due to a loss of gene function or genetic
loci, some of which have been mapped to specific human chromosomes.
determines replicative life span? What prevents cell division and alters
differentiation once replicative capacity is exhausted? Cell division
may be limited by progressive telomere shortening. A critically short
telomere may induce the expression of dominant growth inhibitors, which
institute and maintain the growth arrest. Presenescent and senescent
cells express many genes in common. However, some genes whose activities
are needed for cell cycle progression are repressed in senescent cells,
whereas at least tv~o inhibitors of cell cycle progression are overexpressed
by senescent cells. Whether and how these changes in the expression
of cell cycle regulatory genes participate in the senescence-associated
changes in differentiation is not yet known. Nonetheless, both the changes
in cell proliferation and the changes in differentiation that are the
hallmark of replicative senescence are likely to have physiological
consequences, particularly for mammalian organisms.
Campisi. "Cancer, Aging and Cellular Senescence" In vivo 14:183-188
Normal cells do not divide indefinitely due to a process termed cellular
or replicative senescence. Several lines of evidence suggest that replicative
senescence evolved to protect higher eukaryotes, particularly mammals,
from developing cancer. Senescent cells differ film their presenescent
counterparts in three way: 1) they arrest growth and cannot be stimulated
to reenter the cell cycle by physiological mitogens; 2) they become
resistant to apoptotic cell death; 3) they acquire altered differentiated
functions. Replicative senescence occurs because, owing to the biochemistry
of DNA replication, cells acquire one or more critically short telomere.
The mechanism by which a short telomere induces the senescent phenotype
is unknown. Recent findings suggest that certain types of DNA damage
and inappropriate mitogenic signals can also cause cells to adopt a
senescent phenotype. Thus, cells respond to a number of potentially
oncogenic stimuli by adopting a senescent phenotype. These findings
suggest that the senescence response is a failsafe mechanism that protects
cells from tumorigenic transformation. Despite the protection from cancer
conveyed by cellular senescence and other mechanisms that suppress tumorigenesis,
the development of cancer is almost inevitable as mammalian organisms
age. Why is this the case? Certainly, aging predisposes cells to accumulate
mutations, several of which are necessary before malignant transformation
occurs, particularly in humans. However, many benign or relatively well-controlled
tumors may also harbor many potentially oncogenic mutations, suggesting
that the tissue microenvironment can suppress the expression of many
malignant phenotypes. Although the idea remains controversial, cellular
senescence has also been proposed to contribute to organismal aging.
Senescent cells have recently been shown to accumulate with age in human
tissues. One possibility is that the tissue microenvironment is disrupted
by the accumulation of dysfunctional senescent cells. Thus, mutation
accumulation may synergize with the accumulation of senescent cells,
leading to increasing risk for developing cancer that is a hallmark
of mammalian aging.
Huang, Baomin Li, Matthew D. Gray, Junko Oshima , I. Saira Mian &
Judith Campisi. " The premature ageing syndrome protein, WRN, is a 3'
-> 5' exonuclease" Nature Genetics. Oct. 1998, 20:114-116.
syndrome (WS) is a human autosomal recessive disorder that causes the
premature appearance of a partial array of disorders characteristic
of old age(1,2). These disorders include atherosclerosis, cancer, type
2 diabetes, osteoporosis, cataracts, wrinkled skin and gray hair, among
other ailments. Cells cultured from WS subjects have a shortened replicative
life span(3,4) and elevated rates of chromosome translocations, large
deletions and homologous recombination(5,6). The gene defective in WS,
WRN, encodes a large RecQ-like DNA helicase(7) of 1432 ea. Defects in
another human RecQ-like helicase, BLM, result in Bloom's syndrome (8)
(BS), a genetic disorder that is quite different from WS. BS is manifested
by short stature, neoplasia, immunodeficiency and high risk of cancer.
Cells from BS subjects show an increase in sister chromatid exchanges.
helicases can function in replication, repair, recombination, transcription
or RNA processing. As WRN and BLM share no obvious homology outside
the helicase domain, the non-helicase domains probably determine in
which process each RecQ- like helicase participates, which provides
a basis for the disparate cellular and organismal phenotypes that result
from defects in these proteins.
sequence analyses showed subtle but significant similarities between
WRN and several 3' -> 5' exonucleases(9, 10). To test the prediction
that WRN is an exonuclease, we produced tagged recombinant wild-type
and mutant WRN proteins. Two mutants had amino-acid substitutions at
either position 82 (D82A) or 84 (E84A), two of the five residues predicted
to be critical for exonuclease activity (9,l0). A third mutant had a
substitution at position 577 (K577M), which abolished WRN helicase activity
(11). The fourth mutant was an N-terminal fragment (aa 1-333; N333)
containing the putative exonuclease domain, but lacking the helicase
domain. A tagged 36-aa vector derived peptide served as a negative control
WRN and mock proteins were incubated with doubled-stranded DNA substrates.
Wild-type WRN degraded a 5' labelled substrate to a series of smaller,
labelled products (Fig. 1a), and a 3' labelled substrate to a single
labelled product that migrated as a mononucleotide (Fig. 1b). Thus,
WRN degraded DNA with 3' -> 5' directionality. Although mock and
full-length WRN preparations contained low levels of a contaminating
5 -> 3' exonuclease, as shown by release of the 5' label as a mononucleotide
(Figs 1a,2b), 3' -> 5' degradation was entirely dependent on WRN.
exonuclease activity resided in the N terminus. N333, which was essentially
free of contaminating 5' -> 3' exonuclease, degraded 5' and 3' labelled
substrates similarly to full-length WRN (Fig. 1c,d). When incubated
with a 374-bp DNA fragment labelled at the 3' end with 32p, and internally
with 3H, N333 released most of both labels (Fig. 1e). Thus, the WRN
exonuclease is capable of substantial DNA degradation. Consistent with
3 -> 5' directionality, N333 released 32p from 3' ends more rapidly
than 3H from internal residues. In addition, gel-purified N333, which
lacked contaminating nuclease activities, efficiently removed the 3',
but not the 5', label when incubated with DNA substrates labelled at
either the 3' or the 5' end (Fig. 1p.
evidence for WRN exonuclease activity was obtained by introducing point
mutations at critical amino acids in the exonuclease domain (D82A and
E84A). These mutants retained the wild-type level of helicase activity
(Fig. 2a), but had little or no 3 -> 5' exonuclease activity, using
either a 5' (Fig. 2b) or 3' (Fig. 2c) 32p labelled substrate,
and were indistinguishable from mock protein in this regard (Fig. 2d).
The K577M mutant, in contrast, was devoid of helicase activity (Fig.
2a), as expected, but had 3 -> 5' exonuclease activity comparable
to that of wild-type WRN (Fig. 2b-d).
data indicate that WRN is indeed a 3' -> 5 exonuclease. This activity
resides in the N terminus, and is physically and functionally separable
from the helicase activity The identification of an exonuclease activity
in WRN clearly distinguishes it from other human RecQ-like helicases,
and may help explain the differences between WS and BS.
are the functions of the WRN exonuclease in vivo? It may participate
in recombination and DNA repair. Exonucleases are integral components
of some recombination pathways (12), and WRN appears to have a role
in recombination (5,6,13). The finding that WS cells are hypersensitive
to the DNA damaging agent 4-nitroquinoline-1-oxide (14) suggests a role
for WRN in DNA repair. Finally, WRN is homologous to FFA-1 (replication
focus-forming activity l) in Xenopus laevis (15), raising the
possibility that WRN may also be involved in DNA replication. In this
context, the WRN exonuclease may provide 3 -> 5' proofreading function
to DNA polymerases that lack such activity. Whatever the case, an understanding
of the functions of WRN exonuclease and their relationships to the other
functions of WRN will lead to new insights into the molecular and cellular
basis for WS and a subset of age-associated pathologies.
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to an eventual arrest of cell division by a process termed cellular
or replicative senescence.
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Kim, Patrick Kaminker & Judith Campisi. "TIN2, a new regulator of
telomere length in human cells" Nature Genetics. Dec.1999, 23: 405-412
are DNA-protein structures that cap linear chromosomes and are essential
for maintaining genomic stability and cell phenotype. We identified
a novel human telomere-associated protein, TIN2, by interaction cloning
using the telomeric DNA-binding-protein TRF1 as a bait. TIN2 interacted
with TRF1 in vitro and in cells, and colocalized with TRF1 in
nuclei and metaphase chromosomes. A mutant TIN2 that lacks amino-terminal
sequences effects elongated human telomeres in a telomerase-dependent
manner. Our findings suggest that TRF1 is insufficient for control of
telomere length in human cells, and that TIN2 is an essential mediator
of TRF1 function.
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