Differential AMPK phosphorylation by glucagon and metformin regulates insulin signaling in human hepatic cells

Insulin and glucagon signaling in the liver are major contributors to glucose homeostasis. Patients with Type 1 and Type 2 diabetes have impaired glycemic control due, in part, to dysregulation of the opposing actions of these hormones. While hyperglucagonemia is a common feature in diabetes, its precise role in insulin resistance is not well understood. Recently, metformin, an AMPK activator, was shown to regulate hepatic glucose output via inhibition of glucagon-induced cAMP/PKA signaling; however, the mechanism for how PKA inhibition leads to AMPK activation in human hepatic cells is not known. Here we show that glucagon impairs insulin-mediated AKT phosphorylation in human hepatic cell line Huh7. This impairment of AKT activation by glucagon is due to PKA-mediated inhibition of AMPK via increased inhibitory phosphorylation of AMPK(Ser173) and reduced activating phosphorylation of AMPK(Thr172). In contrast, metformin decreases PKA activity, leading to decreased pAMPK(Ser173) and increased pAMPK(Thr172). These data support a novel mechanism involving PKA-dependent AMPK phosphorylation that provides new insight into how glucagon and metformin modulate hepatic insulin resistance.