Abstract 177: Glucagon Receptor Signaling-Mediated Regulation of PCSK9 and Cholesterol Metabolism

Abstract

Glucagon is one of the key hormones important in hepatic glucose homeostasis, and the pathophysiological role of hyperglucagonemia and unopposed hepatic glucagon action in type 2 diabetes (T2D) is now well-established. Accordingly, there has been great interest in developing glucagon receptor antagonists (GRAs) as a treatment for T2D. Although phase 2 clinical trials have shown that GRAs effectively lower blood glucose in T2D subjects, they increase plasma low density lipoprotein cholesterol (LDL-C), which has presented a significant block to their development. Consistent with a role of glucagon in cholesterol metabolism, recent studies suggested that proprotein convertase subtilisin/kexin type 9 (PCSK9), which increases plasma LDL-C through targeting LDL receptor (LDLR) for degradation, can be regulated by fasting, however, in-depth mechanistic information is lacking. In order to test the functional importance of hepatic glucagon action on cholesterol metabolism, we silenced hepatic glucagon receptor (GcgR) in obese mice using AAV8-mediated shRNA treatment. Consistent with previous reports, this treatment effectively lowered blood glucose in obese mice without a change in body weight. Moreover, GcgR silencing, like GRAs in humans, significantly increased plasma LDL-C. In search for the mechanism, we found that inhibition of GcgR significantly lowered hepatic LDLR protein levels and increased both hepatic and circulating PCSK9, without an effect on cholesterol synthesis. To determine causation, we silenced hepatic GcgR together with PCSK9 using AAV8 and found that this intervention restored hepatic LDLR and prevented the increase in plasma LDL-C. Further mechanistic studies showed that GcgR silencing in hepatocytes did not increase Pcsk9 mRNA. Rather, blocking GcgR increased the half-life of PCSK9 protein by suppressing signalling through exchange protein activated by cAMP-2 (Epac2). In particular, the ability of GcgR silencing to increase PCSK9 and suppress LDLR protein levels was mimicked by hepatocytes lacking Epac2. Thus, GcgR signalling through Epac2 appears to have critical effects on processes that regulate cholesterol metabolism through PCSK9.