Session: OR18-Diabetes-Associated Genes & Pathways
Room 304 (Moscone Center)
Ling He, Jia Cao, Shumei Meng, Lishou Xiong, Sally Radovick, Fredric E. Wondisford
Biguanides, such as metformin and phenformin, are the most commonly prescribed oral anti-diabetic drugs worldwide, and their major effect on the control of blood glucose levels in diabetes is through suppression of hepatic glucose production (HGP). However, the mechanism by which biguanides suppress HGP remains controversial. Recently, an AMPK-independent pathway of biguanide action on inhibition of HGP was proposed, where elevated intracellular AMP levels lead to the reduction of hepatic glucose production. In this and other studies, high concentrations of metformin were used (from several hundred uM to 100 mM), yet the hepatic portal vein concentration of metformin is reported to be typically 40-80 uM. Our data demonstrate that pre-treatment of primary hepatocytes with 80 uM metformin for 24 h inhibits cAMP- mediated glucose production along with the suppression of gluconeogenic enzyme gene expression. At this concentration, neither AMP levels nor the AMP/ATP ratio is altered. In addition, depletion of AMPK by adenoviral-shRNA against AMPK α1 and 2 blocks inhibition by metformin. We do find evidence that metformin elevates cellular AMP levels but only at very high concentrations (> 500 uM), which are not pharmacologically relevant. Furthermore, we find that cAMP antagonizes metformin’s activation of AMPK through phosphorylation of AMPK at S485, and this phosphorylation event is inversely correlated with AMPK activity. In contrast, mutation of AMPK at S485A increased phosphorylation of AMPK at T172, a critical phosphorylation site linked to its activation and action to suppress HGP. We find that pre-treatment with metformin greatly increases the suppression of HGP and gluconeogenic enzyme gene expression observed after cAMP treatment versus that seen with after simultaneous treatment with the same concentrations of metformin and cAMP. Overall, our studies indicate that typical hepatic portal vein concentrations of metformin inhibit gluconeogenic gene expression and glucose production via AMPK. Moreover, high serum glucagon and intracellular cAMP levels may decrease the efficacy of metformin through the phosphorylation of AMPK at S485 by PKA.
Nothing to Disclose: LH
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