100 6 ART for Antiaging
that mitochondrial uncoupling-elicited increases of AMP/ATP and NAD+/NADH
ratios can separately activate AMPK and SIRT1 (Rodgers et al. 2005 ; Lee et al.
2006 ), we assumed that at least two NO-involved signaling pathways exist for
sensing such decreases of ATP and NADH levels in mitochondria: one is the
AMP-dependent and AMPK-sensed pathway, and another is the NAD+-dependent
and SIRT1-sensed pathway. It has been documented that both AMPK and SIRT1
can coordinately activate PGC-1α, a pivotal modulator for mitochondrial biogen-
esis (Lee et al. 2006 ; Suwa et al. 2006 ). The activated AMPK is known to phos-
phorylate PGC-1α at threonine and serine residues, thus enabling deacetylation by
SIRT1 and, therefore, activation (Canto et al. 2009 ). Furthermore, AMPK is also
informed to catalyze the phosphorylation of TSC2 and regulatory-associated pro-
tein of mTOR (Raptor), which lead to the rapid inhibition of mTORC1 activity
(Shaw 2009 ). A most recent finding has indicated that CR decreases ATP levels
in nematodes through the tricarboxylic acid cycle intermediate α-ketoglutarate, by
which the subunit β of ATP synthase (complex V) is targeted and inhibited (Chin
et al. 2014 ).
It is known that ART inhibits NOS activity and induces NOS overexpression
for enhanced NO production (Zeng and Zhang 2011 ), and SNP, ARG, and H 2 O 2
were confirmed to elevate the serum NO levels in treated mice, demonstrating that
those NO generators can mimic CR to trigger an elevated NO level. Therefore,
CR and mimetics enable the coordinated accumulations of some relevant kinases,
acetylases, and mitochondrial signatures/biomarkers. Importantly, we did detect
the remarkable elevation of ATP levels and synchronous increase of mitochondria
in mice treated by CR mimetics.
According to the earlier findings that CR activates eNOS via Akt (Cerqueira
et al. 2011 ), we suggested the following interactive NO signaling pathways:
(1) Akt-eNOS-AMPK-PGC-1α axis, in which AMPK senses the increase of AMP
levels; (2) Akt-eNOS-SIRT1-PGC-1α axis, in which SIRT1 senses the increase
of NAD+ levels. The activity of eNOS is known to be regulated by phospho-
rylation at multiple sites, in which the activation site Ser1177 and the inhibitory
site Thr495 are the most thoroughly studied sites (Chen et al. 1999 ). It should
be understandable for the above described scenario because AMPK and Akt are
responsible for the phosphorylation of Ser1177 of eNOS in response to various
stimuli (Dimmeler et al. 1999 ; Fulton et al. 1999 ). Therefore, it should be that a
dual signaling cascade may be operated step-by-step: CR activates Akt for phos-
phorylation of eNOS; Akt activates eNOS for NO production; NO interacts with
COX leading to decreases of ATP and NADH; and high-ratio AMP/ATP and
NAD+/NADH eventually activate AMPK and SIRT1. Most recently, the AMPK
activator metformin has been revealed to extend the lifespan of fruit flies for
30 %, during which Atg1 is activated, autophagy initiated, and lifespan prolonged
(Ulgherait et al. 2014 ).
Evidence is emerging to support a concept of mitochondrial hormesis (mito-
hormesis), which suggests that ROS triggers defense responses, thus leading to
increased stress resistance and extended lifespan (Schriner et al. 2005 ; Schulz
et al. 2007 ). As another supporting evidence, mitochondrial superoxide was found