PGC mediated mitochondrial biogenesis in brown fat and endot

PGC-1α-mediated mitochondrial biogenesis in brown fat and endothelial ZD 7288 is in part regulated by eNOS and NO [28], [29]. Recent studies indicate that H2S not only augments NO bioavailability and signaling [11], [17], but provides protection against cardiac injury in an eNOS-dependent manner [15], [18]. Based on this evidence, we speculated that H2S induced mitochondrial biogenesis by activating eNOS and increasing NO levels. Indeed, we found that H2S therapy increased the phosphorylation of eNOS (activation site) and increased cardiac NO levels. However, H2S therapy did not induce mitochondrial biogenesis in an eNOS-dependent manner, as evidenced by a significant increase in mitochondrial content in the hearts of eNOS KO mice given SG-1002. This suggests that although eNOS/NO is important for H2S signaling in certain situations, it is not necessary for the induction of mitochondrial biogenesis in the naïve heart. With that being said, given the complexity by which PGC-1α signaling is regulated, we cannot rule out the possibility that under certain conditions or in different tissues eNOS/NO contributes to H2S-mediated mitochondrial biogenesis in some way. For instance, in the setting of ischemic injury, the induction of eNOS/NO could contribute to a pro-survival environment, which could indirectly aid in the promotion of mitochondrial biogenesis. Additionally, NO also modifies cysteine residues in a manner similar to sulfhydration; a process termed nitrosylation. While similar in some aspects to sulfhydration, nitrosylation appears to diminish cysteine reactivity, whereas sulfhydration seems to enhance it [43]. There also is some overlap in the proteins targeted by NO and H2S. However, it is not clear if NO and H2S target the same cysteine residues in these proteins or if they target distinct cysteine residues. Further it is not clear if they work together or oppose each other to regulate the function of proteins. Finally, it is not clear if there is a balance of sulfhydration and nitrosylation needed for proper function. Therefore, we cannot rule out the possibility that under certain conditions both NO and H2S target PP2A at the same or different cysteine residues to regulate its activity. Further, under certain conditions it is also possible that H2S induces the nitrosylation of PP2A via its actions on eNOS. Therefore, future studies aimed at identifying the specific residues in PP2A modified by H2S and NO are needed to fully understand if/how these signaling molecules work together to regulate of this protein. The AMPK-PGC-1α signaling cascade has been extensively studied in the context of cardiac metabolism [32]. So, the novel aspects of this study rest not in the determination that AMPK-PGC-1α signaling leads to mitochondrial biogenesis, but in the evidence that endogenous and exogenous H2S influence this pathway. More so, the finding that a decrease in endogenous H2S levels led to an impairment in AMPK-PGC-1α signaling becomes important when considering the evidence that H2S levels are decreased in pathological conditions – i.e. heart failure (Fig. 7 and [37]) – that also present with reduced mitochondrial content [7], [36]. Based on this evidence it can be suggested that endogenous H2S levels not only play an important role in maintaining the mitochondrial content of the heart, but that a reduction in endogenous H2S levels contributes to the pathophysiology of heart failure through a disruption in mitochondrial biogenesis. This idea is supported by our findings that restoring H2S levels with SG-1002 increased mitochondrial content, improved ATP production, and attenuated LV dysfunction in a murine model of ischemia-reperfusion injury. Our study has, therefore, identified an important regulatory mechanism in the mitochondrial biogenesis pathway that if corrected by restoring H2S levels could protect the failing heart and ultimately reduce mortality and morbidity associated with heart failure. Future studies aimed at determining if H2S therapy coupled with other pharmacological agents that target the AMPK-PGC-1α signaling pathway are warranted to fully test this postulate.