Low Concentrations of Metformin Suppress Glucose Production in Hepatocytes through AMP-activated Protein Kinase (AMPK)*♦

Jia Cao,Shumei Meng,Evan Chang,Katherine Beckwith-Fickas,Lishou Xiong,Robert N Cole,Sally Radovick,Fredric E Wondisford,Ling He

doi:10.1074/jbc.m114.567271

Abstract

Metformin is a first-line antidiabetic agent taken by 150 million people across the world every year, yet its mechanism remains only partially understood and controversial. It was proposed that suppression of glucose production in hepatocytes by metformin is AMPK-independent; however, unachievably high concentrations of metformin were employed in these studies. In the current study, we find that metformin, via an AMP-activated protein kinase (AMPK)-dependent mechanism, suppresses glucose production and gluconeogenic gene expression in primary hepatocytes at concentrations found in the portal vein of animals (60-80 μM). Metformin also inhibits gluconeogenic gene expression in the liver of mice administered orally with metformin. Furthermore, the cAMP-PKA pathway negatively regulates AMPK activity through phosphorylation at Ser-485/497 on the α subunit, which in turn reduces net phosphorylation at Thr-172. Because diabetic patients often have hyperglucagonemia, AMPKα phosphorylation at Ser-485/497 is a therapeutic target to improve metformin efficacy.

Highlights

Whether suppression of glucose production by metformin is through AMPK-dependent inhibition of gluconeogenic gene expression remains controversial
Metformin Suppresses Gluconeogenic Gene Expression—To assess whether metformin is able to inhibit the expression of genes related to gluconeogenesis, we conducted glucose production assays and measured gluconeogenic gene expression in primary hepatocytes treated with metformin and the cAMP analog Bt-cAMP simultaneously as was done in a previous study [20]
We and others have proposed that metformin suppresses hepatic glucose production through activation of AMPK pathway to inhibit gluconeogenic gene expression [11, 13, 15]

Summary

Background

Whether suppression of glucose production by metformin is through AMPK-dependent inhibition of gluconeogenic gene expression remains controversial. We find that metformin, via an AMP-activated protein kinase (AMPK)-dependent mechanism, suppresses glucose production and gluconeogenic gene expression in primary hepatocytes at concentrations found in the portal vein of animals (60 – 80 ␮M). Metformin suppressed glucose production in primary hepatocytes lacking AMPK␣1 and AMPK␣2, and the authors proposed that inhibition of mitochondrial complex 1 by metformin results in a decrease in ATP levels together with an increase in the AMP/ATP ratio, which both directly suppress gluconeogenesis. We tested 1)whether metformin at therapeutic concentrations is able to suppress expression of genes related to gluconeogenesis and 2) whether metformin directly affects AMPK␣ subunit phosphorylation at Thr-172 to suppress glucose production in hepatocytes In studies proposing an AMPKindependent mechanism, the authors employed metformin concentrations (1–2 mM) that were 10 –100-fold higher than maximal concentrations achieved in the hepatic portal vein after standard therapeutic dosing, and the high concentration of metformin used in these studies has raised safety concerns. In the current study, we tested 1)whether metformin at therapeutic concentrations is able to suppress expression of genes related to gluconeogenesis and 2) whether metformin directly affects AMPK␣ subunit phosphorylation at Thr-172 to suppress glucose production in hepatocytes

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION