91deficiency during CR. However, this is an apparent paradoxical phenomenon for
CR because glucose supply is restricted during CR.
As an urgent response to the suddenly happened nutritional stress, of course,
a transient respiratory burst just at the start of glucose restriction should be possi-
ble because the unexpected insufficiency of ATP and NADH must be compensated
by this emergent response. It has reported that yeast can respond robustly to the
decreasing glucose levels by shifting their metabolic state from one that favors fer-
mentation to one that favors respiration (Kaeberlein et al. 2007 ). It is demonstrated
that respiratory burst is by no means a prerequisite for longer life expectancy of
yeast because CR promotes the longevity in respiratory-deficient yeast strains
(Kaeberlein et al. 2005a, b). Considering previous results regarding CR-mediated
mitochondrial biogenesis, we suggested a dual-phase mode of CR-impacted mito-
chondrial functions: a ME phase representing the “short-term CR” or “acute CR”
and a PME phase characterizing the “long-term CR” or “chronic CR”. Indeed, we
observed an enhanced mitochondrial respiratory activity in the ME phase, whereas
an attenuated one in the PME phase.
To provide the experimental evidence supporting the suggestion of dual-phase
modes, we investigated the CR-mediated expression fashions and activity dynam-
ics of mitochondrial signatures, including those involved in oxidative phospho-
rylation and antioxidation. Consequently, CR-induced Sod2 overexpression was
found to correlate increased Mn-SOD activity in ME yeast cells. In contrast,
downregulated Sod2 expression and accordingly decreased Mn-SOD activity were
measured in PME yeast cells. The “dual-phase responses” of CR, therefore, can
be distinguished based on the “up-and-down” modes of mitochondrial signatures.
In particular, PME during chronic CR allows the downregulation of biosynthesis
pathway genes and the synchronous upregulation of degradation pathway genes
involved in major common metabolic pathways. In consistence with our findings,
it has been previously described that a shift of the carbon metabolism may account
for the extension of yeast lifespan responsive to CR (Lin et al. 2002 ).
Except for the upregulation of ubiquitylation genes responsible for selective
protein degradation and amino acid recycle, CR also allows the overexpression of
autophagy genes responsible for re-utilization of cellular constituents. It is known
that yeast can augment autophagy during entry into stationary phase, presumably
as an adaptive response to starvation (Kamada et al. 2004 ). Consistent with this
observation, it has been demonstrated that yeast mutants defective for autophagy do
exhibit short-lived phenotypes (Powers et al. 2006 ). We also found that autophagy-
involved Atg genes are upregulated by all treatments, thereby validating that CR
and mimetic treatments should involve autophagy. Interestingly, our study revealed
that ribosome biogenesis genes are induced to counteract the downregulation of
ribosome genes, hence highlighting that translation occurring in the ribosomes is
controlled by an interactive way. Besides, we also observed the upregulation of per-
oxisomal β-oxidation genes responsible for the oxidation of fatty acids in CR and
mimetic-exposed yeast. A most recently published report has indicated that CR-
activated peroxisomal β-oxidation systems contribute to energy regeneration by
stored lipids and recycled cellular components (Lefevre et al. 2013 ).
6.2 ART Extends Yeast Lifespan via NO Signaling