AMP-activated protein kinase suppresses LXR-dependent sterol regulatory element-binding protein-1c transcription in rat hepatoma McA-RH7777 cells.
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.
Yang
Jian
J
Department of Physiology, University of South Alabama College of Medicine, Mobile, Alabama 36688, USA. jyang@usouthal.edu
Craddock
Lauren
L
Hong
Sharon
S
Liu
Zhi-Mei
ZM
D055372:AMP-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism; D021581:Active Transport, Cell Nucleus; D000249:Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D008024:Ligands; D008074:Lipoproteins / Q000235:genetics / Q000378:metabolism; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D051780:Sterol Regulatory Element Binding Protein 1 / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000235:genetics*
D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters; C480692:Abcg5 protein, rat; C480693:Abcg8 protein, rat; D004268:DNA-Binding Proteins; D008024:Ligands; D008074:Lipoproteins; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear; D051780:Sterol Regulatory Element Binding Protein 1; D000249:Adenosine Monophosphate; D055372:AMP-Activated Protein Kinases