Control of cAMP-mediated glucagon response by bile acids

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Cholestatic liver disease with cirrhosis in particular, is the 9th leading cause of death in the USA. Prognosis is poor, with a generally irreversible condition marked by progressive destruction of liver cells. Around 50 percent of patients with liver disease and 80 percent of cirrhotic patients display glucose intolerance associated with decreased gluconeogenic response to glucagon. Regulation of helpatocellular metabolism by glucagon is in part through increased cAMP synthesis. The central hypothesis is that certain bile acids alter the glucagon receptor-stimulatory G protein (Gs) coupling through a phosphorylation/dephosphorylation mechanism and that these alterations are responsible for attenuation and delayed recovery of glucagon responsiveness in cholestasis. We have shown that bile acids inhibit hepatic glucagon-induced cAMP synthesis at physiologic concentrations. The effect was at the level of receptor-Gs coupling, most likely through phosphorylation, and was mediated by a calcium-dependent PKC. We have reported that hepatic glucagon-mediated cAMP production was attenuated in cholestasis in hamster induced by ligation of the common bile duct (BDL). Bile acids were either without or with reduced effects after BDL suggesting that the site of cAMP synthesis cascade altered in cholestasis is the same as that altered by bile acids. Specific aims will test the hypotheses: 1)that short-term incubation of hepatocytes with bile acids leads to decreased glucagon receptor-Gs coupling through a phosphorylation/dephosphorylation mechanism involving PKC; 2)that alteration of both glucagon receptor-Gs coupling and receptor dephosphorylation are responsible for the respective attenuation and delayed recovery of glucagon responsiveness in cholestasis. In HEK293 clones expressing glucagon receptor, and in hepatocytes from BDL hamsters we will study the respective effect of physiologic/pathophysiologic bile acid concentrations and cholestasis on receptor/Gs coupling and phosphorylation using a multifaceted approach designed to determine the protein phosphorylation target. We will study the role of protein phosphatases on the time course of glucagon response recovery in cholestasis. Knowledge gained from these studies will have bearing on both diagnosis and treatment of cholestatic hepatobiliary disorders.