Regulator of G Protein Signaling (RGS16) Inhibits Hepatic Fatty Acid Oxidation in a Carbohydrate Response Element-binding Protein (ChREBP)-dependent Manner

Victoria Esser,Victor Pashkov,Wojciech Kedzierski,Deborah M Kurrasch,Howard C Towle,Kosaku Uyeda,Jie Huang,Thomas M Wilkie,Robert D Gerard,Vinay K Parameswara,Gregory G Tall

doi:10.1074/jbc.m110.216234

Abstract

G protein-coupled receptor (GPCR) pathways control glucose and fatty acid metabolism and the onset of obesity and diabetes. Regulators of G protein signaling (RGS) are GTPase-activating proteins (GAPs) for G(i) and G(q) α-subunits that control the intensity and duration of GPCR signaling. Herein we determined the role of Rgs16 in GPCR regulation of liver metabolism. Rgs16 is expressed during the last few hours of the daily fast in periportal hepatocytes, the oxygen-rich zone of the liver where lipolysis and gluconeogenesis predominate. Rgs16 knock-out mice had elevated expression of fatty acid oxidation genes in liver, higher rates of fatty acid oxidation in liver extracts, and higher plasma β-ketone levels compared with wild type mice. By contrast, transgenic mice that overexpressed RGS16 protein specifically in liver exhibited reciprocal phenotypes as well as low blood glucose levels compared with wild type littermates and fatty liver after overnight fasting. The transcription factor carbohydrate response element-binding protein (ChREBP), which induces fatty acid synthesis genes in response to high carbohydrate feeding, was unexpectedly required during fasting for maximal Rgs16 transcription in liver and in cultured primary hepatocytes during gluconeogenesis. Thus, RGS16 provides a signaling mechanism for glucose production to inhibit GPCR-stimulated fatty acid oxidation in hepatocytes.

Highlights

Because the N-terminal amphipathic helix of R4 class Regulators of G protein signaling (RGS) proteins contributes to membrane localization, receptor-selective interactions, post-translational regulation of GTPase-activating proteins (GAPs) activity, and protein stability (4, 8, 28 –33), transgenic RGS16 protein was tagged at the C terminus with a triple repeat of the Myc epitope to distinguish it from endogenous RGS16 protein (Fig. 1A)
Combined analysis of gain-of-function and loss-of-function mutants indicates that RGS16 inhibits Gi/Gq-mediated fatty acid oxidation in hepatocytes
Consistent with this interpretation, Rgs16 is not expressed in liver during early fasting, when glucagon evokes Gs-coupled stimulation of adenylyl cyclase, cAMP production, activation of the transcription factor CRTC2, and glucose production from glycogen and gluconeogenic amino acids [12]

Summary

Introduction

The rate of fatty acid oxidation in liver extracts was correspondingly low in TgR16 mice and high from Rgs16 KO mice (Fig. 2, B and D) compared with wild type controls, consistent with plasma levels of ␤-hydroxybutyrate in TgR16 and Rgs16 KO mice. We characterized the expression of transcription factors and target genes that regulate fatty acid and glucose metabolism in TgR16 transgenic and Rgs16 KO mice (Fig. 2E).

Results

Conclusion