Ginsenoside Rg5 attenuates hepatic glucagon response via suppression of succinate-associated HIF-1α induction in HFD-fed mice.

AIMS/HYPOTHESIS: Ginsenosides regulate glucose homeostasis. This study investigated the effect of ginsenoside Rg5 (Rg5) on the hepatic glucagon response, focusing on the regulation of metabolism. METHODS: Mice fed a high-fat diet (HFD) showed increased hepatic glucose production (HGP). We observed the effects of Rg5 on hepatic fatty acid oxidation and glucagon response. The regulation of phosphodiesterase (PDE) 4B by succinate was also investigated in hepatocytes. RESULTS: Rg5 inhibited endogenous glucose production in HFD-fed mice. Rg5 reduced cyclic AMP (cAMP) accumulation and inhibited transcriptional regulation of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) by dephosphorylation of the cAMP response element-binding transcription factor in the liver, demonstrating the inhibitory effect on hepatic glucagon response. HFD feeding increased succinate accumulation in the liver due to the reversal of succinate dehydrogenase activation and triggered hypoxia-inducible factor-1α (HIF-1α) induction. Succinate prevented cAMP degradation by inactivating PDE4B, thereby increasing cAMP accumulation in response to glucagon. Knockdown of HIF-1α with small interfering RNA diminished the effect of succinate, indicating that HIF-1α was essential for succinate to inactivate PDE4B. Rg5 inhibited succinate accumulation in hepatocytes by combating fatty acid oxidation, and thus reduced cAMP accumulation by blocking succinate/HIF-1α induction. Rg5 reduced HGP as a consequence of the inhibition of the glucagon response. CONCLUSIONS/INTERPRETATION: Succinate acted as a metabolic signal to enhance the hepatic glucagon response. Rg5 reduced hepatic succinate accumulation by combating fatty acid oxidation and attenuated the hepatic glucagon response by suppressing succinate/HIF-1α induction, suggesting that succinate-associated HIF-1α induction in hepatocytes might be a therapeutic target in the treatment of diabetes.

Glucagon up-regulates hepatic mitochondrial pyruvate carrier 1 through cAMP-responsive element-binding protein; inhibition of hepatic gluconeogenesis by ginsenoside Rb1.

BACKGROUND AND PURPOSE: Hepatic mitochondrial pyruvate carrier (MPC) transports pyruvate into mitochondria. This study investigated the involvement of MPC1 in hepatic glucagon response, in order to identify a possible pharmacological intervention. EXPERIMENTAL APPROACH: The correlation between hepatic glucagon response and MPC1 induction was investigated in fasted mice and primary hepatocytes. The effects of ginsenoside Rb1 on MPC1 function were observed. KEY RESULTS: Glucagon challenge raised blood glucose with hepatic MPC1 induction, and inhibition of MPC induction coincided with a reduced rise in blood glucose. cAMP-responsive element-binding protein (CREB) knockdown blocked glucagon-induced MPC1 expression, while CREB overexpression increased MPC1 expression. Luciferase reporter, chromatin immunoprecipitation assay, and promoter mutation confirmed that CREB increased MPC1 transcription through gene promoter induction. CREB regulated transcription co-activator 2 nuclear translocation was also required for CREB to promote MPC1 induction. Glucagon shifted mitochondrial pyruvate towards carboxylation for gluconeogenesis via the opposite regulation of pyruvate dehydrogenase and carboxylase with respect to MPC1 induction. MPC1 induction was necessary for glucagon to promote pyruvate-driven hepatic glucose production (HGP), but glucagon failed to influence HGP from other gluconeogenic substrates routed into the tricarboxylic acid cycle, independent of MPC. Rb1 blocked cAMP signalling by inhibiting AC activity and deactivated CREB by dephosphorylation, possibly contributing to inhibiting MPC1 induction to reduce HGP. CONCLUSIONS AND IMPLICATIONS: CREB transcriptionally up-regulates MPC1 to provide pyruvate for gluconeogenesis. Rb1 reduced cAMP formation which consequently reduced CREB-mediated MPC1 induction and thereby might contribute to limiting pyruvate-dependent HGP. These results suggest a therapeutic strategy to reduce hyperglycaemia in diabetes.