AMP‐activated protein kinase suppresses LXR‐dependent sterol regulatory element‐binding protein‐1c transcription in rat hepatoma McA‐RH7777 cells

Abstract

AMP-activated protein kinase (AMPK) is an intracellular fuel sensor that plays a key role in regulating fatty acid synthesis in liver. Sterol regulatory element-binding protein (SREBP)-1c is a master regulator of hepatic lipogenic gene expression. It has long been documented that AMPK activation suppresses hepatic SREBP-1 mRNA and nuclear SREBP-1 protein. But the mechanism remains undefined. In this study we investigated the molecular mechanisms by which AMPK downregulates hepatic SREBP-1c mRNA using a novel model cell line McA-RH7777. We found that AMPK is robustly activated in rat hepatoma McA-RH7777 cells treated with two widely used AMPK activators, AICAR and metformin, and AMPK activation sharply suppresses SREBP-1c mRNA and nuclear SREBP-1c protein, but not SREBP-1a mRNA derived from the same gene. These inhibitory effects are reversed by the AMPK inhibitor Compound C or 8-BrAMP, demonstrating the requirement of AMPK in the suppression of SREBP-1c mRNA and nuclear SREBP-1c protein by AICAR and metformin. AMPK does not enhance SREBP-1c mRNA degradation in the presence of the general transcription inhibitor actinomycin D; instead it inhibits SREBP-1c promoter activity in a luciferase reporter assay. AMPK-mediated inhibition of SREBP-1c promoter activity can also be abrogated by the AMPK inhibitor Compound C. Furthermore AMPK activation significantly attenuates the synthetic liver X receptor (LXR) ligand T0901317-induced SREBP-1c promoter activity. AMPK also inhibits cleavage of LXR ligand-induced SREBP-1c precursor. We conclude that AMPK suppresses hepatic SREBP-1c mRNA expression by inhibiting LXR-dependent SREBP-1c transcription via inhibition of endogenous LXR ligand production and by inhibiting SREBP-1c processing in McA-RH7777 cells.