FoxO1 deacetylation regulates thyroid hormone-induced transcription of key hepatic gluconeogenic genes.

Hepatic gluconeogenesis is a concerted process that integrates transcriptional regulation with hormonal signals. A major regulator is thyroid hormone (TH), which acts through its nuclear receptor (TR) to induce the expression of the hepatic gluconeogenic genes, phosphoenolpyruvate carboxykinase (PCK1) and glucose-6-phosphatase (G6PC). Forkhead transcription factor FoxO1 also is an important regulator of these genes; however, its functional interactions with TR are not known. Here, we report that TR-mediated transcriptional activation of PCK1 and G6PC in human hepatic cells and mouse liver was FoxO1-dependent and furthermore required FoxO1 deacetylation by the NAD(+)-dependent deacetylase, SirT1. siRNA knockdown of FoxO1 decreased, whereas overexpression of FoxO1 increased, TH-dependent transcriptional activation of PCK1 and G6PC in cultured hepatic cells. FoxO1 siRNA knockdown also decreased TH-mediated transcription in vivo. Additionally, TH was unable to induce FoxO1 deacetylation or hepatic PCK1 gene expression in TH receptor ß-null (TRß(-/-)) mice. Moreover, TH stimulated FoxO1 recruitment to the PCK1 and G6PC gene promoters in a SirT1-dependent manner. In summary, our results show that TH-dependent deacetylation of a second metabolically regulated transcription factor represents a novel mechanism for transcriptional integration of nuclear hormone action with cellular energy status.

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.