Control of cAMP-mediated glucagon response by bile acids

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Chronic liver disease, and cirrhosis in particular, is the 12th leading cause of mortality in the United States and the 4th leading cause of death for individuals aged 45 to 54 years. The prognosis is poor, generally irreversible,and marked by progressive destruction of the liver cells. Around 50% of patients with liver disease and 80% of cirrhotic patients display glucose intolerance associated with a decreased gluconeogenic response to glucagon. This decreased glucagon-induced hepatocellular glucose production is due to a significant hepatic resistance to glucagon and attenuation of glucagon-induced cAMP production. Our early studies showed glucagon responsiveness to be significantly attenuated in hepatocytes isolated from cholestatic hamsters, and this effect was mimicked by submicellar concentrations of bile acids including chenodeoxycholic acid (CDCA) in hepatocytes isolated from control hamsters. Our data suggested the attenuated cAMP response induced by bile acids to be PKCalpha and/or PKCdelta-mediated. Our overarching hypothesis is that in hepatocytes,phosphorylation of PKC by CDCA leads to activation of this kinase, which in turn phosphorylates the glucagon receptor and attenuates glucagon responsiveness, which is one of the events associated to the progression of liver disease during cholest asis. The aims of the proposed study are: 1) Identification in vitro by a proteomic approach of the PKC residues that are phosphorylated in the presence of CDCA, and the consequences that the phosphorylation has on PKC activity. 2) Determination in vivo the sites and the mechanism responsible for the PKC phosphorylation and the consequences on the localization and activity of this kinase. 3) The impact that the activation of this kinase has on glucagon receptor function in cholestasis. Findings from our proposed studies will have direct clinical significance and add new insight to the understanding of the attenuation of glucagon responsiveness in cholestatic liver diseases. These studies will define molecular mechanisms regulating both PKC and glucagon receptor activation by bile acids. Furthermore, the delineation of the molecular balance between phosphorylation and activation of PKC may have the added benefit of identifying novel molecular targets for rational drug design in the treatment of cholestatic hepatobiliary disorders, as well as diabetes.