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A key transcriptional regulator of cell metabolism, the peroxisome proliferator-activated receptor γ co-activator 1-α (PPARGC-1-α or PGC-1α), also regulates mitochondrial biogenesis, but its role in antioxidant gene regulation is not well understood. Here, we asked whether genetic heterozygosity of PGC-1α modulates gene expression for the mitochondrial antioxidant enzyme SOD-2 during hepatic inflammatory stress. Using Staphylococcus aureus peritonitis in mice, we found significant Sod2 gene induction in WT mice, whereas PGC-1α heterozygotes (PGC-1α(+/-)) failed to augment Sod2 mRNA and protein levels. Impaired Sod2 regulation in PGC-1α(+/-) mice was accompanied by oxidative stress shown by elevated mitochondrial GSSG/GSH and protein carbonyls. In silico analysis of the mouse proximal Sod2 promoter region revealed consensus binding sites for the Nfe2l2 (Nrf2) transcription factor. Chromatin immunoprecipitation demonstrated diminished Nfe2l2 protein binding to the antioxidant response element promoter site proximal to the Sod2 start site in PGC-1α heterozygous mice, implicating PGC-1α in facilitation of Nfe2l2 DNA binding. Nuclear protein co-immunoprecipitation demonstrated an interaction between hepatic Nfe2l2 and PGC-1α in WT mice that was greatly reduced in PGC-1α(+/-) mice. The data indicate that PGC-1α promotes mitochondrial antioxidant enzyme expression through Nfe2l2-mediated SOD-2 expression in sepsis. The presence of this new PGC-1α-dependent signaling axis indicates that PGC-1α opposes mitochondrial oxidative stress by means of selective induction of one or more antioxidant response element-driven genes. By implication, exploitation of this axis could lead to new pharmacological interventions to improve the antioxidant defenses during oxidative stress-induced mitochondrial damage.

Given that the physiology of heme oxygenase-1 (HO-1) encompasses mitochondrial biogenesis, we tested the hypothesis that the HO-1 product, carbon monoxide (CO), activates mitochondrial biogenesis in skeletal muscle and enhances maximal oxygen uptake (Vo(2max)) in humans. In 10 healthy subjects, we biopsied the vastus lateralis and performed Vo(2max) tests followed by blinded randomization to air or CO breathing (1 h/day at 100 parts/million for 5 days), a contralateral muscle biopsy on day 5, and repeat Vo(2max) testing on day 8. Six independent subjects underwent CO breathing and two muscle biopsies without exercise testing. Molecular studies were performed by real-time RT-PCR, Western blot analysis, and immunochemistry. After Vo(2max) testing plus CO breathing, significant increases were found in mRNA levels for nuclear respiratory factor-1, peroxisome proliferator-activated receptor-gamma coactivator-1alpha, mitochondrial transcription factor-A (Tfam), and DNA polymerase gamma (Polgamma) with no change in mitochondrial DNA (mtDNA) copy number or Vo(2max). Levels of myosin heavy chain I and nuclear-encoded HO-1, superoxide dismutase-2, citrate synthase, mitofusin-1 and -2, and mitochondrial-encoded cytochrome oxidase subunit-I (COX-I) and ATPase-6 proteins increased significantly. None of these responses were reproduced by Vo(2max) testing alone, whereas CO alone increased Tfam and Polgamma mRNA, and COX-I, ATPase-6, mitofusin-2, HO-1, and superoxide dismutase protein. These findings provide evidence linking the HO/CO response involved in mitochondrial biogenesis in rodents to skeletal muscle in humans through a set of responses involving regulation of the mtDNA transcriptosome and mitochondrial fusion proteins autonomously of changes in exercise capacity.