|dc.description.abstract||Among many endogenous substances that regulate hepatic energy production is the gasotransmitter hydrogen sulfide (H2S). In the liver, H2S production is largely catalyzed by cystathionine γ-lyase (CSE) and, to a lesser degree, by cystathionine β-synthase. We previously showed that H2S stimulates glucose production in an immortalized carcinoma liver cell line (HepG2 cells) as well as induce ATP generation in isolated vascular smooth muscle cells (VSMCs). Furthermore, we found that H2S upregulates peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α expression in rat VSMCs. PGC-1α is a crucial regulator of hepatic gluconeogenesis and mitochondrial biogenesis. Both of these PGC-1α-mediated energy processes are pivotal to maintain whole-body energy homeostasis, whereby their sustained disturbance may lead to the development of type 2 diabetes and metabolic syndrome. Therefore, we investigated the regulation of gluconeogenesis and mitochondrial biogenesis by CSE-generated H2S under physiological conditions in isolated mouse hepatocytes. We found that CSE-knockout (KO) mice had a reduced rate of gluconeogenesis, which was reversed by administration of NaHS (an H2S donor) (i.p.). Interestingly, isolated CSE-KO hepatocytes exhibited a reduced glycemic response to chemical-induced activation of the cAMP/PKA and glucocorticoid pathways compared to wild-type (WT) hepatocytes. Treatment with the inhibitors for PKA (KT5720) or glucocorticoid receptor (RU-486) significantly reduced H2S-stimulated glucose production from both WT and CSE-KO mouse hepatocytes. NaHS treatment upregulated the protein levels of key gluconeogenic transcription factors, such as PGC-1α and CCAAT-enhancer-binding proteins-β (C/EBP-β). Moreover, exogenous H2S augmented the S-sulfhydration of the rate-limiting gluconeogenic enzymes and PGC-1α and increased their activities, which were lower in untreated CSE-KO hepatocytes. Finally, knockdown of PGC-1α, but not C/EBP-β, significantly decreased NaHS-induced glucose production from the primary hepatocytes.
After determining that H2S stimulates hepatic glucose production through the PGC-1α signaling pathway, we focused on whether or not H2S induces hepatic mitochondrial biogenesis. We found that CSE-KO hepatocytes produced less mtDNA compared to WT hepatocytes. Mitochondrial content was decreased in CSE-KO hepatocytes compared to normal hepatocytes, which was restored with NaHS treatment. CSE-KO hepatocytes exhibited lower levels of mitochondrial transcription factors and the mitochondrial transcription coactivator, peroxisome proliferator-activated receptor-γ coactivator-related protein (PPRC) compared to WT hepatocytes. Interestingly, NaHS administration upregulated PPRC, yet downregulated PGC-1β protein level in mouse hepatocytes. Moreover, exogenous H2S induced the S-sulfhydration of PPRC, which was lower in untreated CSE-KO hepatocytes, but not that of PGC-β. Finally, knockdown of either PGC-1α or PPRC significantly decreased NaHS-stimulated mitochondrial biogenesis in hepatocytes, where knockdown of both genes were required to completely abolish NaHS-induced mitochondrial biogenesis.
Overall this thesis demonstrates the stimulatory effect of endogenous H2S on liver glucose production and reveals four underlying mechanisms. 1) H2S upregulates the expression levels of PGC-1α and PEPCK via glucocorticoid receptor pathway. 2) H2S upregulates the expression level of PGC-1α through the activation of the cAMP/PKA pathway, as well as PGC-1α activity via S-sulfhydration. 3) H2S upregulates the expression and the activities (by S-sulfhydration) of G6Pase and FBPase. 4) H2S augments the protein expression level and activity (via S-sulfhydration) of PPRC. By stimulating the combined activities of PPRC and PGC-1α, iv
H2S induces mitochondrial biogenesis, thereby supplying energy to support its induction of hepatic glucose production.
This study may offer clues to the regulation of hepatic energy homeostasis under physiological conditions and its dysregulation in insulin-resistance diseases.||en_US