Hepatocytes contribute to residual glucose production in a mouse model for glycogen storage disease type Ia

Abstract

It is a longstanding enigma how glycogen storage disease (GSD) type I patients retain a limited capacity for endogenous glucose production (EGP) despite the loss of glucose-6-phosphatase (G6Pase) activity. Insight into the source of residual EGP is of clinical importance given the risk of sudden death in these patients, but so far contradictory mechanisms have been proposed.

We investigated G6Pase-independent EGP in hepatocytes isolated from a liver-specific GSD Ia mouse model (L-G6pc^-/- mice), and performed real-time analysis of hepatic glucose fluxes and glycogen metabolism in L-G6pc^-/- mice using state-of-the-art stable isotope methodologies.

Here we show that G6pc-deficient hepatocytes are capable of producing glucose. In vivo analysis of hepatic glucose metabolism revealed that the hepatic glucokinase (GCK) flux was decreased by 95% in L-G6pc^-/- mice. It also showed increased glycogen phosphorylase flux in L-G6pc^-/- mice, which is coupled to the release of free glucose via glycogen debranching. Although the ex vivo activities of debranching enzyme and lysosomal acid maltase, two major hepatic α-glucosidases, were unaltered in L-G6pc^-/- mice, pharmacological inhibition of α-glucosidase activity almost completely abolished residual glucose production by G6pc-deficient hepatocytes.

Conclusion. Our data indicate that hepatocytes contribute to residual glucose production in GSD Ia. We show that α-glucosidase activity, i.e. glycogen debranching and/or lysosomal glycogen breakdown, contributes to residual glucose production by GSD Ia hepatocytes. A strong reduction in hepatic GCK flux in L-G6pc^-/- mice furthermore limits the phosphorylation of free glucose synthesized by G6pc-deficient hepatocytes, allowing the release of glucose into the circulation. The almost complete abrogation of GCK flux in G6pc-deficient liver also explains the contradictory reports on residual glucose production in GSD Ia patients. This article is protected by copyright. All rights reserved.

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