Hello Aubrey,
Thanks for you interest.
Here is my belated reply.
Hopefully, you are still following this 'ageing' thread.
Aubrey de Grey wrote in message <83tmd2$926$1 at pegasus.csx.cam.ac.uk>...
>>Lou Pagnucco wrote:
>>> (1) Apoptosis appears to be upregulated in CR lab animals
>> (as discussed in J. of Gerontology last year). So a natural
>> (speculative) deduction is that apoptosis may be responsible
>> for the life extending effects of CR. However, it has been
>> (supposedly) demonstrated in some papers that I sent Aubrey
>> last year that CR also greatly reduces cellular turnover, and hence
>> the mitotic rates in many tissues. This seems to suggest that
>> apoptosis in CR animals is far more selective than in AL animals.
>>Maybe. Another interpretation is that apoptosis is up-regulated only
>in rather unusual tissues in which cell attrition is very high anyway,
>i.e. it's up-regulated in order to keep these short-lived cells in the
>best of shape on average by not letting them deteriorate too far. (I
>know only of reports of CR up-regulating apoptosis in liver and gut.) In
>that model, apoptosis in most cell types (those with slow cell turnover)
>would be reduced simply because of the well-known lowering of oxidant
>production. I think that model is more economical (but I don't claim
>to have an objective measure of a theory's economy!) I recall your
>suggesting last year that one might be able to have high apoptosis
>and low mitosis in all tissues, but I don't think that situation can
>be sustained long-term: CR mice are small, sure, but their body size
>is pretty constant during adulthood and so is that of AL mice, so in
>both cases cell division must match cell death.
>
I found an interesting abstract on Medline (see (A) below), that shows
that pre-cancerous regions in the mouse bladder are eliminated via
apoptosis in CR animals, but that when IGF-I (insulin-like growth factor I)
is supplemented to maintain control levels, the CR anti-cancer effect
is eliminated.
My (possibly naive) speculation is that apoptosis commands are received
by wayward pre-cancerous cells from their neighbours through gap
junctions. Perhaps when these deviant clusters grow too fast they lose
contact with adjacent cells so quickly that the apoptotic agents do not
have time to initiate cell death (at which point the mass becomes an overt
cancer.) After all, gap junction inhibitors are cancer promoters.
Wouldn't this explain both the reduction in the mitotic rate and increase in
the mitotic rate effected by CR?
>> Could this be a factor in the results reported by the Milan lab?
>> (i.e., could they be inhibiting the less discriminating apoptosis in
>> AL animals that lead to proliferative exhaustion in certain tissues?)
>>I don't think so. In the model I just outlined, there's no proliferative
>exhaustion in the AL animals: the low apoptosis is due to low collateral
>damage to tissues which normally maintain homeostasis by getting rid of
>damaged cells and thus don't drive aging. However, in view of Sydney's
>comment that rates of cell turnover may be much higher than most of us
>believe, I will be very interested in his answer to this.
>>> (2) It seems to me questionable to assert that post-mitotic cells are
>> too sparsely distributed to cause any real damage.
>>I infer from what you say later that you mean replicative-senescent, as
>opposed to intrinsically post-mitotic like muscle. It is best to avoid
>using "post-mitotic" for senescent cells -- even though it's logically
>impeccable, the term is usually reserved for muscle etc. so readers will
>be confused.
>>> The recent results reported on gene upregulation/suppression in aged
>> tissues (using micro-arrays) appear to show a profound difference
>> between tissues derived from young and old animals. Specifically, a
>> diffuse inflammatory condition appears to pervade old tissues. Might
>> one reason be that post-mitotic fibroblasts generate approximately 40
>> times the collagenases (and possibly other extra-cellular matrix
>> destroying enzymes) as juvenile fibroblast do?
>>I think some people think this is so, but I don't think it stands up to
>quantitative scrutiny. Cristofalo says that only one dermal fibroblast
>per 10,000 shows senescent gene expression (such as high collagenase) in
>aged tissue. That means collagenase is only 0.4% higher in bulk tissue
>as a result of the 40-fold overproduction you cite. One could say that
>the overproduction affects only the region quite near the senescent cell,
>but then it would only affect a small proportion of the tissue. The big
>difference with the mitochondrial theory, in which again only one cell
>in 1000 or more is affected, is that those cells may generate reactive
>oxygen species at their surface that can initiate peroxidation chain
>reactions, which would amplify and spread the damage enormously. No
>analogous scenario has been proposed (yet!) for anything that senescent
>cells have yet been found to secrete.
>
Still my reading of recent lab findings compels me to think that
ageing tissues are subject to what Floyd refers to as "smouldering
inflammation" when he refers to the processes in the brain
responsible for age related cognitive decline. (See paper for which
I provide the Medline abstract at (B) below.)
This is corroborated by findings showing that serum inflammatory
cytokines are good predictors of future disability in the aged.
Micro-array results demonstrating a diffuse inflammatory process
are consistent with these clinical numbers (I think). Some recent
results showing that inhibitors of the proteolytic metallomatrix enzymes
appear to be effective against a large subset of the symptoms
of aging are also intriguing since these proteases (like the collagenases)
are end points of the inflammatory processes.
I believe that your 0.4% figure is much too conservative, but do not have
figures to contest it.
Also, is it certain that only Hayflick-limited post-motitic fibroblasts
are responsible for excess production of the matrix degrading enzymes?
Is it not more likely, that fibroblasts approaching the Hayflick limit are
already expressing some of these senescent traits?
>> (3) If it has not been ruled out that cell division itself may ratchet
>> cells further down the developmental pathways that lead to terminal
>> differentiated, post-mitotic states, shouldn't we be investigating
>> mitosis inhibitors as life extending agents? (Hasn't the Russian
>> gerontologist Frolkis published some encouraging results about anti-
>> mitotic drugs, e.g. olivomycins, on life extension?)
>>Even though I'm doubtful about your premise, I think mitosis inhibitors
>might conceivably have effects similar to p66, if they were similarly
>specific (i.e. to mitosis induced by cell death induced by oxidative
>damage). I don't think non-specific mitosis inhibitors would be good,
>for the same reason that I don't think non-specific apoptosis inhibitors
>would be good -- namely, mitosis and apoptosis are usually good things.
>Probably.
But if my speculation above is correct, then reducing the mitotic rate
would make escaping apoptosis more difficult for wayward cells by
keeping them in contact with their normal neighbours who would have
more time to eliminate their deviant siblings.
Cheers,
Lou Pagnucco
(A) Cancer Res 1997 Nov 1;57(21):4667-72
Dietary restriction reduces insulin-like growth factor I
levels, which modulates apoptosis, cell proliferation, and
tumor progression in p53-deficient mice.
Dunn SE, Kari FW, French J, Leininger JR, Travlos G, Wilson R, Barrett JC
Laboratory of Molecular Carcinogenesis, National Institute of
Environmental Health Sciences,
NIH, Research Triangle Park, North Carolina 27709, USA.
Diet contributes to over one-third of cancer deaths in the Western world,
yet the factors in the diet that influence cancer are not elucidated. A
reduction
in caloric intake dramatically slows cancer progression in rodents, and this
may be a major contribution to dietary effects on cancer.
Insulin-like growth factor I (IGF-I) is lowered during dietary restriction
(DR) in both humans and rats. Because IGF-I modulates cell proliferation,
apoptosis, and tumorigenesis, the mechanisms behind the protective effects
of DR may depend on the reduction of this multifaceted growth factor. To
test
this hypothesis, IGF-I was restored during DR to ascertain if lowering of
IGF-I
was central to slowing bladder cancer progression during DR. Heterozygous
p53-deficient mice received a bladder carcinogen, p-cresidine, to induce
preneoplasia. After confirmation of bladder urothelial preneoplasia, the
mice
were divided into three groups: (a) ad libitum; (b) 20% DR; and
(c) 20% DR plus IGF-I (IGF-I/DR). Serum IGF-I was lowered 24% by DR
but was completely restored in the IGF-I/DR-treated mice using recombinant
IGF-I administered via osmotic minipumps. Although tumor progression
was decreased by DR, restoration of IGF-I serum levels in DR-treated mice
increased the stage of the cancers. Furthermore, IGF-I modulated tumor
progression independent of changes in body weight. Rates of apoptosis in
the preneoplastic lesions were 10 times higher in DR-treated mice compared
to those in IGF/DR- and ad libitum-treated mice. Administration of IGF-I to
DR-treated mice also stimulated cell proliferation 6-fold in hyperplastic
foci.
In conclusion, DR lowered IGF-I levels, thereby favoring apoptosis over cell
proliferation and ultimately slowing tumor progression. This is the first
mechanistic study demonstrating that IGF-I supplementation abrogates the
protective effect of DR on neoplastic progression.
(B) Proc Soc Exp Biol Med 1999 Dec;222(3):236-45
Antioxidants, oxidative stress, and degenerative neurological disorders.
Floyd RA
Recently, clinical trials of several neurodegenerative diseases have
increasingly targeted the evaluation of the effectiveness of various
antioxidants. The results so far are encouraging but variable and thus
confusing. Rationale for the possible clinical effectiveness of antioxidants
in several degenerative conditions has arisen out of the many years of
basic science generally showing that reactive oxygen species (ROS)
and oxidative damage are important factors in the processes involved.
Aging is one of the most significant risk factors for degenerative
neurological disorders. Basic science efforts in our laboratory have
centered on exploring
the role of ROS and oxidative stress in neurodegenerative processes. The
present review brings together some of the basic concepts we have learned
by following this approach for the last 20 years and specifically the
results we
have obtained by following up on our serendipitous findings that a
nitrone-based
free radical trap, alpha-phenyl-tert-butylnitrone (PBN), has neuroprotective
activity in several experimental neurodegenerative models. The mechanistic
basis of the neuroprotective activity of PBN does not appear to rely on its
general
free radical trapping or antioxidant activity per se, but its activity in
mediating the suppression of genes induced by pro-inflammatory cytokines and
other mediators
associated with enhanced neuroinflammatory processes. Neuroinflammatory
processes, induced in part by pro-inflammatory cytokines, yield enhanced ROS
and reactive nitric oxide species (RNS) as well as other unknown components
that have neurotoxic properties. Neurotoxic amounts of RNS are formed by the
activity of inducible nitric oxide synthase (iNOS). The demonstration of
enhanced
3-nitro-tyrosine formation in affected regions of the Alzheimer's brain, in
comparison
to age-matched controls, reinforces the importance of neuroinflammatory
processes. iNOS induction involves activation by phosphorylation of the MAP
kinase p38 and
can be induced in cultured astrocytes by IL-1beta or H2O2. The action of PBN
and
N-acetyl cysteine to suppress the activation of p38 was demonstrated in
cultured astrocytes.
The demonstration of activated p38 in neurons surrounding amyloid plaques in
affected regions of the Alzheimer's brain attest to enhanced signal
transduction processes in this neurodegenerative condition. The major themes
of ROS and
RNS formation associated with neuroinflammation processes and the
suppression
of these processes by antioxidants and PBN continue to yield promising leads
for
new therapies. Outcomes of clinical trials on antioxidants will become less
confusing as more knowledge is amassed on the basic processes involved.
