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[MR]

Human Sirtuin (Yeast CR Gene) DEPRESSES Human Foxos
(Worm CR Gene)??

This interpretation of recent experiments that the very promising explanation of the mechanism of aging in lower organisms (yeast, worms and flies) may not apply to mammals, where cancer is a big factor limiting longevity.

In this pop press piece from 2004-02-05:

http://www.wired.com/news/business/0,1367,62169,00.html

... it says, "Researchers at the Massachusetts Institute of Technology
have identified key genes that regulate aging in humans."

Now, a casual reading might make one think that this was "merely" a
hopeful sign of the conservation of some of the signaling pathways
involved in longevity in lower organisms into human cells. Instead, this
study is really a bit of a shocker, whose implications are quite unclear
to me (at least).

What they're reporting (1): **in vitro,** SIRT1 (one human version of
Sir2, a gene thru' which CR works in yeast, flies, and flatworms & the
same one activated by trans-resveratrol) "REPRESSES the activity of the
forkhead transcription factor Foxo3a and other mammalian forkhead
factors." [my screaming emphasis].

Now the pop press is quoting Guarente and also a woman from Cynthia
Kenyon's lab as indicating that SIRT1 does indeed regulate aging in
humans. But aside from still needing to be proven, this is a very
ambiguous finding. Because the work that Cynthia Kenyon has been doing
in flatworms ahs been extending flatworm lifespan by inhibiting daf-2
(an homolog for the insulin-IGF1 receptor), which works precisely
because doing so PREVENTS the "normal" REPRESSION, by daf-2, of daf-16 - whose mammalian homologs are exactly the forkhead transcription factors (FOXOs or FKHRLs)!

That is: Kenyon's WORM longevity gene, daf-16, is ACTIVATED by CR or by inhibiting the insulin/IGF1 pathway; the human homolog is here being
reported to be INHIBITED by SIRT1 -- the homolog of the YEAST CR-like
longevity gene!

Indeed, part of the past optimism about the development of anti-aging
pills derived from Kenyon's work has been precisely from the seeming
continuity of this insulin/IGF1 pathway to forkhead factor pathway thru'
the phylae. Eg, "The life-span extension caused by daf-2 mutations
requires the activity of the gene daf-16. daf-16 . encodes a member of
the hepatocyte nuclear factor 3 (HNF-3)/forkhead family of
transcriptional regulators. In humans, insulin . antagoniz[es] the
activity of HNF-3, raising the possibility that aspects of this
regulatory system have been conserved." (2)

See, for some optimism on my part on all this, the (admittedly) somewhat
difficult slog here:

CR Society List Archives (#R13298)

Likewise,

-----------
"In C. elegans, the only FOXO transcription factor is encoded by daf-16.
Loss-of-function mutations in daf-16 completely suppress the
dauer-constitutive ["hibernation"] and longevity phenotypes associated
with reduced function of insulin-signaling components".

"[F]orkhead transcription factors belonging to the FOXO subfamily have
been identified as direct targets of insulin/IGF signaling in mammals
[refs]. The mammalian DAF-16 homologs comprise the proteins FOXO1
(FKHR), **FOXO3a (FKHRL1)** and FOXO4 (AFX)."

See: http://jbiol.com/content/2/3/20/figure/F1

" Their phosphorylation by the insulin-activated kinases . leads to
inactivation of FOXO proteins .... The result of this process is an
insulin-induced transcriptional repression of FOXO target genes, which
are involved in the response to DNA damage [ref ] and oxidative stress
[ref], apoptosis [ref], cell-cycle control [ref ] and metabolism [ref ]. "

"In addition to their transcriptional activation capabilities, FOXO
proteins have recently been shown to induce cell-cycle arrest by
repressing transcription of genes encoding D-type cyclins [ref]. FOXO
transcription factors mediate insulin resistance in diabetic mice [ref
], and have been proposed to be tumor suppressors, as several
chromosomal translocations disrupting FOXO genes are found in cancers
[ref], and overexpressed FOXO proteins can inhibit tumor growth [ref ]." (3)

-------------

Yet now we have Guarente et al reporting that SIRT1 INHIBITS FOXO3a -
the exact OPPOSITE of what happens in flatworms, where inhibiting
insulin/IGF1 signaling (which extends longevity) takes the brakes OFF of
daf-16 (the FOXO homolog)!

The pop press article doesn't even HINT about this inconsistency.
Guarente isn't blissfully, however: indeed, I'm assuming that he
originally set out with the expecation of finding that SIRT1 would
ACTIVATE FOXO3a. The abstract is up front about the paradox: "This
regulation appears to be in the opposite direction from the genetic
interaction of SIR2 with forkhead in C. elegans." (1)

Their proposal: "By restraining mammalian forkhead proteins, SIRT1 also
reduces forkhead-dependent apoptosis[, which] . parallels the effect of
this deacetylase [ie, SIRT1 and the yeast Sir2] on the tumor suppressor
p53. We speculate how downregulating these two classes of
damage-responsive mammalian factors may favor long lifespan under
certain environmental conditions ."(1)

This is the same argument being advanced by Sinclair, in his advocacy of
SIRT1 as an human longevity gene: that it "buys time" for cells stressed
by assorted challenges to repair themselves, rather than forcing them
into senescence or apoptosis. That makes sense in a yeast (unicellular
organims that don't get cancer), but it's a pretty damned curious way to
encourage longevity in a mammal! AND, again, it's working in seemingly
the opposite direction to the Kenyon pathway - whose conservation seems
established by the various GH/IGF1 signaling-inhibited rodents (Ames and
Snell dwarves, etc), which show enhanced longevity.

Just to add to the confusion, where light seemed to be emerging: recall
that animals with a knockout of p66shc (p66shc-/- mice) SEEM to exhibit
enhanced longevity relative to wild type(the background strain was
short-lived, so we can't definitively conclude that). Well, what does
p66shc do? It triggers apoptosis in response to oxidative stress -- &
aparently downstream of p53! So KNOCKING OUT **this** gene -- which
triggers apoptosis in response oxidative stressors -- APPEARS to extend
lifespan.

This is not JUST poetic, however: there IS a link between p66shc, FOXO3a
(FKHRL1), the tumor suppressor/apoptosis p53, and ROS (5,6,7)-- tho' at
at the moment, from surfing abstracts, I'm having having a hard time
teasing it out.

One obvious, pessimistic interpretation: in mammals, unlike in yeast,
there is an evolutionary antagonism between cellular senescence and
cancer (already widely accepted (see (8)) -- tho' we should obviously be
careful about equating CELLULAR with ORGANISMIC senescence. Mammals have conserved both of these critical pathways (sirtuins and insulin --> Forkhead) -- and they constitute opposing arms of this balancing act. Boosting either inhibits the other. Hence, ultimately, NEITHER of these 2 pathways can explain the effects of CR in mammals -- and neither will be exploitable as anti-aging drugs.

If anyone can FURTHER clarify any of this, please speak up!

-MR

1. Mammalian SIRT1 Represses Forkhead Transcription Factors
Cell, Vol 116, Issue 4, 5 February 2004
Maria Carla Motta , Nullin Divecha , Madeleine Lemieux , Christopher
Kamel , Delin Chen , Wei Gu , Yvette Bultsma , Michael McBurney , and
Leonard Guarente

... Here we show that the mammalian SIR2 ortholog SIRT1 deacetylates and represses the activity of the forkhead transcription factor Foxo3a and
other mammalian forkhead factors. This regulation appears to be in the
opposite direction from the genetic interaction of SIR2 with forkhead in
C. elegans. By restraining mammalian forkhead proteins, SIRT1 also
reduces forkhead-dependent apoptosis. The inhibition of forkhead
activity by SIRT1 parallels the effect of this deacetylase on the tumor
suppressor p53. We speculate how downregulating these two classes of
damage-responsive mammalian factors may favor long lifespan under
certain environmental conditions, such as calorie restriction.

http://www.cell.com/content/article/abstract?uid=PIIS0092867404001266


2. Lin K, Dorman JB, Rodan A, Kenyon C.
daf-16: An HNF-3/forkhead family member that can function to double
the life-span of Caenorhabditis elegans.
Science. 1997 Nov 14;278(5341):1319-22.
PMID: 9360933 [PubMed - indexed for MEDLINE]

3. Junger MA, Rintelen F, Stocker H, Wasserman JD, Vegh M, Radimerski T,
Greenberg ME, Hafen E.
The Drosophila Forkhead transcription factor FOXO mediates the
reduction in cell number associated with reduced insulin signaling.
J Biol. 2003;2(3):20. Epub 2003 Aug 07.
PMID: 12908874 [PubMed - as supplied by publisher]
http://jbiol.com/content/2/3/20

4. Migliaccio E, Giorgio M, Mele S, Pelicci G, Reboldi P, Pandolfi PP,
Lanfrancone L, Pelicci PG.
The p66shc adaptor protein controls oxidative stress response and life
span in mammals.
Nature. 1999 Nov 18;402(6759):309-13.
PMID: 10580504 [PubMed - indexed for MEDLINE

5. Nemoto S, Finkel T.
Redox regulation of forkhead proteins through a p66shc-dependent
signaling pathway.
Science. 2002 Mar 29;295(5564):2450-2. Epub 2002 Mar 07.
PMID: 11884717 [PubMed - indexed for MEDLINE]

6. Kops GJ, Dansen TB, Polderman PE, Saarloos I, Wirtz KW, Coffer PJ,
Huang TT, Bos JL, Medema RH, Burgering BM.
Forkhead transcription factor FOXO3a protects quiescent cells from
oxidative stress.
Nature. 2002 Sep 19;419(6904):316-21.
PMID: 12239572 [PubMed - indexed for MEDLINE]

7. Trinei M, Giorgio M, Cicalese A, Barozzi S, Ventura A, Migliaccio E,
Milia E, Padura IM, Raker VA, Maccarana M, Petronilli V, Minucci S,
Bernardi P, Lanfrancone L, Pelicci PG.
A p53-p66Shc signalling pathway controls intracellular redox status,
levels of oxidation-damaged DNA and oxidative stress-induced apoptosis.
Oncogene. 2002 May 30;21(24):3872-8.
PMID: 12032825 [PubMed - indexed for MEDLINE]

8. Campisi J.
Cancer and ageing: rival demons?
Nat Rev Cancer. 2003 May;3(5):339-49. Review.
PMID: 12724732 [PubMed - indexed for MEDLINE]