excelife at earthlink.net
Sat Aug 29 22:05:00 EST 1998
I downloaded the following paper from the net and when I got around to
reading it I couldn't identify its source. Does any one know who wrote it,
its cite or its URL?
It's an excellent article and I would like to credit its creator.
Senescence, Telomeres and Aging
(The loss of generalized growth factors and subsequent appearance of
senescent cells resulting in the outward manifestation of aging and
increasedrisk of cancer)
During cell division, the ends of linear chromosomes are not fully
duplicated resulting in slightly shorter daughter chromosomes. This is known
as the "end replication problem". All normal chromosomes have noncoding
repetitive DNA at the ends called telomeres. The "coding" portion of the DNA
is where genetic information is stored. One of the proposed functions of
telomeres is to act as a buffer for the coding region. "Normal" cells lose
DNA from the telomeres each time the cell divides and thus the telomeres get
shorter. Proliferating invitro cell populations also develop progressively
shorter telomeres as the population approaches its Hayflick limit also called
senescence. As cells approach senescence they begin to express P53, a
protein which stops the cell cycle in G1 or S phase. This stop mechanism is
different than the one which places
the cell in the quiescent Go state when it is serum deprived.
The P53 stop mechanism is considered to be associated with damaged
DNA. Double stranded breaks in the chromosomes are known to evoke P53
activation. What P53 does is "pause" the cell cycle, presumably so that DNA
repair can take place before replication proceeds. Cells modified to be
able to bypass the P53 block can continue to replicate beyond their Hayflick
limit. These cells eventually reach a state called crisis where most will
die. Cells approaching or at crisis have a high frequency of chromosomal
abnormalities. It is believed that the chromosomes become "unstable" causing
lethal mutations or rearrangements, this is what kills off most of the crisis
cells. The fusion-breakage-fusion cycle is thought to be at work as indicated
by the high number of karyotype abnormalities found in crisis and post crisis
cells. A small percentage of the cells will escape crisis and continue to
divide. It is believed that these cells have stabilized their chromosomes by
reactivating telomerase, a ribonuclear enzyme which adds telomeric repeats to
the telomeres essentially maintaining their length. These cells are usually
immortal and often cancerous or precancerous. Telomerase activity has been
detected in almost all cancer cell lines. (Some cancers have been found which
are able to maintain their telomeres without detectable telomerase. It is
believed that these cells use some form of recombination which enables a long
telomere to act as a template that can be used to extend a short one.)
Some normal cells also activate telomerase. These cells are found in
tissue which has a high turnover rate such as epithelial linings, epidermal
basal layers, blood presumptive-stem cell populations and embryonic tissue.
Why is this important to the study of aging? Cell Function As an individual
ages the number of cells stopped by P53, with short telomeres in G1 (called
"senescent cells") is found to increase in most replicative tissue.
Senescent cells often develop their own phenotype which is different
than that of earlier passages. Senescent cells adhere less rigorously to
their differentiated phenotype then do dividing or quiescent cells. As the
percentage of these senescent cells increase, it can be predicted that the
functionality of the tissue they are part will deteriorate or at least
change. Many age associated changes in tissue structure can be attributed
to this phenomena of cellular senescence.
Free Radical Production
Another characteristic of senescent cells is the increased rate of free
radical production which is also a hallmark of old tissue. Whether the free
radicals are the product of senescent cells or some other change that occurs
during aging is not known. It is interesting to note that in cells undergoing
apoptosis the rate of free radical production increases and the cells lose
contact with their extra cellular
environment. Could senescence be the result of failed or stalled
Replicative tissue architecture (where do senescent cells come from?)
We know replicative tissue has a common organizational scheme. Slowly
proliferating cells adherent to a basal layer which appear to be
immortal. These cells have been shown to have telomerase activity.
Associated with the basal cells, is a proliferation layer which populates the
rest of the tissue. The proliferation layer generally does not have
telomerase activity. Blood, epidermal, chondral and epithelial tissues all
adhere to this scheme.
In most replicative tissue, cells gradually take on a different
phenotype as they proliferate. This may be due to a genetic trigger based on
the number of cell divisions the cell has undergone or due to changes in the
cell's location in the tissue as it divides causing changes in the signals
or amount of nutrients reaching the cell which triggers the phenotype change.
In either case, there is exists a natural means to remove cells before
they are senescent. When the proliferation rate of the basal cells and the
growth layer is maintained within a certain window, cells are either
sloughed off or programmed to self destruct before they have reached their
telomeric proliferation limit.
Problems arise when the rate of cell division is disrupted. For
example; if the the telomerase positive basal cells divide too slow, the
burden of proliferation shifts more to the proliferation layer which may not
have active telomerase. This means each proliferative, telomerase negative
cell will have to divide more times to maintain tissue integrity. This
increases the likelihood that a cell will reach senescence before it is
removed. Likewise if the basal cells are stimulated to divide
more frequently than they are intended to, they may lose telomeric DNA
faster than telomerase can restore it, thus over time shortening the
telomeres of the proliferative daughters. This will shorten the number of
times a proliferative daughter can divide before reaching senescence, again
increasing the chance of a cell becoming senescent before removal..
Function of Senescent cells
The senescent phenotype has no obvious purpose in the organization of
tissue. The senescent phenotype is generally characteristic of the tissue
type but less differentiated. For example; senescent fibroblasts produce
different more embryonic isotypes of collagen than proliferating cells.
Senescent cells also produce more free radicals than nonsenescent cells.
This leads one to assume that senescence represents an aberrant condition
which a cell is not intended to remain in for any length of time.
How do senescent cells build up?
1. Stem cells produce daughters with shorter telomeres so that they
reach senescence before they are sloughed off.(This assumes that stem
cells are losing telomeric DNA)
2. Stem cells produce fewer daughters to proliferate so that each
daughtes must divide more times to maintain the population in the
tissue..thus cells are more prone to reach senescence before being removed.
3. Removal of old (more prone to senescence) cells is somehow
4. Stem cells are removed or inactivated by some undiscovered
means, perhaps autoimmune attack? (leading to condition #1)
Relationship to aging
Is the rate of appearance of senescent cells constant with age so that
we gradually build up a sizable population of senescent cells? I am
conducting a literature search to determine if any research has been done on
the subject. Is the rate of appearance subject to central control or is it
the result of accumulation of random wear and tear damage? Since we know
that the absolute number of senescent cells increases as
we age and we know that there are factors which influence stem cell
proliferation, is there a correlation between the appearance of senescent
cells and the amount of the various poetic factors. Is there a cofactor which
could influence stem cell proliferation such as a generalized growth factor.
Studies have revealed that the concentration of many circulating hormones
which may affect cell proliferation, change as we age. Hormones like, growth
hormone, many of the steroids, melatonin, cytokines, cortico-steroids and
peptide hormones. In order to determine whether these factors have any
effect on senescent cell appearance I propose to conduct several experiments
which will determine:
1 If telomerase is modulated to any extent by factors which are
known to change over time in correlation to the appearance of senescent
cells. Materials like melatonin, steroid hormones, GH ,IGF, hematopoietic
factors and other growth related materials .
2. If the senescent phenotype can be induced by changes other than
short telomeres. Changes in the availability of energy in the cell as
modulated by hormones (thyroid for example.)
3. Test protein turnover and overall profile in senescent cells as
compared to dividing cells.
4. Assay senescent cells for indications of an attempt to initiate
apoptosis, like relocation of cytC from mitochondria, amount of bcl-2, DNA
laddering and acridine orange staining.
Generalized Growth Factors
Since aging appears to occur throughout the body I have been thinking
of a global factor that could affect the aging process. The most obvious
source of global factors is the endocrine system. It also appears that large
changes do take place in the aging endocrine system, but changes in the
endocrine system cannot explain all the phenomena of aging. Up to this point
I hadn't considered the ECM as a global growth factor because it is so
heterogeneous. Heterogeneous as it may be, the effect of aging is the same
on essentially all ECM, the reduction in the number or
density of binding sites for cells. While studying developmental processes
I became aware of the importance of the ECM to cell viability. Many cells
must remain tightly adherent to the ECM to maintain their phenotype and or
viability. This is also true of stem cells. If a stem population is
dissociated from its native ECM it loses the stem characteristic, telomerase
is down regulated and some cells will differentiate. One of the effects of
aging is the modification of protein by glycosilation and free radicals. For
a long time I thought of this phenomena as secondary to the aging process,
but in light of recent findings I feel that protein modification may play a
more central role than I realized.
1 Glucose pretreatment of ECM reduces cell binding to ECM
2 Reduced cell binding "reduces" stem cell characteristics like
3 The only known method of increasing the mean and maximum life span is
Caloric Restriction, which also chronically reduces the concentration
of serum glucose.
4 Many cells lose their ability to perform specialized functions when
separated from their native ECM.
5 modified proteins are resistant to enzymatic degradation.
It is easy to see that how a glucose mediated process could initiate
aging-like processes leading to whole body disregulation and decline. Like
the accelerated aging of diabetics.
Testosterone and prostate telomerase Feb 1998
An interesting paper I read recently described the upregulation of
telomerase in the prostate of rats. Normally the prostate gland is telomerase
negative (or very low level
positive). The prostate gland of rats castrated surgically or chemically
will atrophy. As it atrophies the telomerase activity increases. When
testosterone is restored the prostate redevelops to its original size and
telomerase activity diminishes. Could a similar regimen be used in aging in
human males? Chemically lower testosterone levels, allow the prostate to
atrophy, then restore testosterone. Effectively regenerating
a younger prostate. From a purely aging interest, it testosterone a
telomerase suppresser? It would be interesting to assay non-sex tissue for
telomerase during this type of experiment.
On further investigation it appears that what these researchers are
seeing is the accumulation or concentration of prostate stem cells. This
makes it seem as though testosterone withdrawl activated telomerase. Most
likely testosterone withdrawl caused telomerase-negative
testosterone-dependant cells to undergo apoptosis leaving the atrophing
prostate with an increasing population of telomerase-positive stem cells.
Melatonin and Telomerase May 1998
While trying to find out how the body regulates telomerase activity I
hit on some generalizations;
1. telomerase is present in tel+ cells throughout the cell cycle.
Various reports have detected both enzyme and RNA template at all points in
the cell cycle.
2. Telomeres have specific binding proteins which cover and cap the
telomere during most of the cell cycle (including Go). It is believed that
these proteins keep telomeres from fusing. They may also have other
functions associated with the nucleolus.
3. Melatonin and DHEA both slow the cell cycle (G1-M).
Hypothosis: Melatonin slows the cell cycle extending the window of
accessability of the telomeres to telomerase. This alows the telomeres to be
extended more each cycle. Changes in melatonin during aging may shorten the
window of opportunity for telomerase allowing the telomeres to
shorten slowly over time.
Thomas Mahoney, Pres.
Lifeline Laboratories, Inc.
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