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

Abstract

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. 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. 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. 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.