Abstract
The redox environment in cells and organisms is set by low-molecular mass and protein-bound thiols, with glutathione (GSH) representing a major intracellular redox buffer. Subtle thiol oxidation elicits signal transduction processes and adaptive responses to cope with stressors, whereas highly oxidizing conditions may provoke cell death. We here tested how thiol depletion affects life span, stress resistance and stress signaling in the model organism Caenorhabditis elegans. Diethyl maleate (DEM), an α,β-unsaturated carbonyl compound that conjugates to GSH and other thiols, decreased C. elegans life span at a concentration of 1mM. In contrast, low and moderate doses of DEM (10–100µM) increased mean and maximum life span and improved resistance against oxidative stress. DEM-induced life span extension was not detectable in worms deficient in either the FoxO orthologue, DAF-16, or the Nrf2 orthologue, SKN-1, pointing to a collaborative role of the two transcription factors in life span extension induced by thiol depletion. Cytoprotective target genes of DAF-16 and SKN-1 were upregulated after at least 3 days of exposure to 100µM DEM, but not 1mM DEM, whereas only 1mM DEM caused upregulation of egl-1, a gene controlled by a p53-orthologue, CEP-1. In order to test whether depletion of GSH may elicit effects similar to DEM, we suppressed GSH biosynthesis in worms by attenuating γ-glutamylcysteine synthetase (gcs-1) expression through RNAi. The decline in GSH levels elicited by gcs-1 knockdown starting at young adult stage did not impair viability, but increased both stress resistance and life expectancy of the worms. In contrast, gcs-1 knockdown commencing right after hatching impaired nematode stress resistance and rendered young adult worms prone to vulval ruptures during egg-laying. Thus, modest decrease in GSH levels in young adult worms may promote stress resistance and life span, whereas depletion of GSH is detrimental to freshly hatched and developing worms.
Highlights
Glutathione (γ-L-glutamyl-L-cysteinylglycine; GSH) is considered the major intracellular low-molecular-mass thiol in eukaryotes, present in millimolar concentrations in cells
An increase in GSH levels was observed in C. elegans exposed to moderate concentrations (40–250 μM) of the reactive oxygen species (ROS) generator juglone; this was associated with nuclear translocation of the transcription factor abnormal dauer formation 16 (DAF-16), but only the lowest concentration of juglone resulted in lifespan extension [22,42]
We here demonstrate that moderate depletion of GSH – both chemically and through RNAiinduced attenuation of GSH biosynthesis – may enhance C. elegans stress resistance and life span
Summary
Glutathione (γ-L-glutamyl-L-cysteinylglycine; GSH) is considered the major intracellular low-molecular-mass thiol in eukaryotes, present in millimolar concentrations in cells. GSH has a pivotal role in antioxidant defense, serving as cosubstrate for glutathione peroxidase (GPX)-catalyzed reductions of H2O2 and lipid hydroperoxides. GPX-mediated removal of hydroperoxides is accompanied by oxidation of GSH to glutathione disulfide (GSSG), whose reduction back to GSH is catalyzed by glutathione reductase and occurs at the expense of NADPH [1]. Under conditions of oxidative stress, GSH forms mixed disulfides with cysteinyl residues in proteins. This reversible S-glutathiolation may protect proteins against cysteine oxidation beyond the disulfide stage, thereby preventing irreversible protein inactivation and degradation during stress [2]. GSH forms conjugates with xenobiotic and endogenous electrophilic compounds as part of phase II xenobiotic metabolism and assists in detoxification and excretion of said compounds [3]
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