Role of ketone signaling in the hepatic response to fasting

Ketosis is a metabolic adaptation to fasting, nonalcoholic fatty liver disease (NAFLD), and prolonged exercise. beta-OH butyrate acts as a transcriptional regulator and at G protein-coupled receptors to modulate cellular signaling pathways in a hormone-like manner. While physiological ketosis is often adaptive, chronic hyperketonemia may contribute to the metabolic dysfunction of NAFLD. To understand how beta-OH butyrate signaling affects hepatic metabolism, we compared the hepatic fasting response in control and 3-hydroxy-3-methylglutaryl-CoA synthase II (HMGCS2) knockdown mice that are unable to elevate beta-OH butyrate production. To establish that rescue of ketone metabolic/endocrine signaling would restore the normal hepatic fasting response, we gave intraperitoneal injections of beta-OH butyrate (5.7 mmol/kg) to HMGCS2 knockdown and control mice every 2 h for the final 9 h of a 16-h fast. In hypoketonemic, HMGCS2 knockdown mice, fasting more robustly increased mRNA expression of uncoupling protein 2 (UCP2), a protein critical for supporting fatty acid oxidation and ketogenesis. In turn, exogenous beta-OH butyrate administration to HMGCS2 knockdown mice decreased fasting UCP2 mRNA expression to that observed in control mice. Also supporting feedback at the transcriptional level,beta-OH butyrate lowered the fasting-induced expression of HMGCS2 mRNA in control mice. beta-OH butyrate also regulates the glycemic response to fasting. The fast-induced fall in serum glucose was absent in HMGCS2 knockdown mice but was restored by beta-OH butyrate administration. These data propose that endogenous beta-OH butyrate signaling transcriptionally regulates hepatic fatty acid oxidation and ketogenesis, while modulating glucose tolerance. NEW & NOTEWORTHY Ketogenesis regulates whole body glucose metabolism and beta-OH butyrate produced by the liver feeds back to inhibit hepatic beta-oxidation and ketogenesis during fasting.