Fatty acids and hypolipidemic drugs regulate peroxisome proliferator-activated receptors alpha - and gamma-mediated gene expression via liver fatty acid binding protein: a signaling path to the nucleus.
Peroxisome proliferator-activated receptor alpha (PPARalpha) is a key regulator of lipid homeostasis in hepatocytes and target for fatty acids and hypolipidemic drugs. How these signaling molecules reach the nuclear receptor is not known; however, similarities in ligand specificity suggest the liver fatty acid binding protein (L-FABP) as a possible candidate. In localization studies using laser-scanning microscopy, we show that L-FABP and PPARalpha colocalize in the nucleus of mouse primary hepatocytes. Furthermore, we demonstrate by pull-down assay and immunocoprecipitation that L-FABP interacts directly with PPARalpha. In a cell biological approach with the aid of a mammalian two-hybrid system, we provide evidence that L-FABP interacts with PPARalpha and PPARgamma but not with PPARbeta and retinoid X receptor-alpha by protein-protein contacts. In addition, we demonstrate that the observed interaction of both proteins is independent of ligand binding. Final and quantitative proof for L-FABP mediation was obtained in transactivation assays upon incubation of transiently and stably transfected HepG2 cells with saturated, monounsaturated, and polyunsaturated fatty acids as well as with hypolipidemic drugs. With all ligands applied, we observed strict correlation of PPARalpha and PPARgamma transactivation with intracellular concentrations of L-FABP. This correlation constitutes a nucleus-directed signaling by fatty acids and hypolipidemic drugs where L-FABP acts as a cytosolic gateway for these PPARalpha and PPARgamma agonists. Thus, L-FABP and the respective PPARs could serve as targets for nutrients and drugs to affect expression of PPAR-sensitive genes.
Department of Biochemistry, University of Münster, Wilhelm-Klemm-Strasse 2, 48149 Münster, Germany.
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Wolfrum
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Borrmann
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Borchers
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Spener
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D000071438:Fatty Acid-Binding Protein 7; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids / Q000502:physiology*; D005786:Gene Expression Regulation; D006801:Humans; D000960:Hypolipidemic Agents / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism; D051379:Mice; D009363:Neoplasm Proteins; D009419:Nerve Tissue Proteins; D011485:Protein Binding; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics*; D015398:Signal Transduction; D014157:Transcription Factors / Q000235:genetics*; D015533:Transcriptional Activation / Q000187:drug effects / Q000502:physiology; D025521:Tumor Suppressor Proteins; D020798:Two-Hybrid System Techniques
D002352:Carrier Proteins; C493319:FABP1 protein, human; C107608:FABP7 protein, human; C493320:Fabp1 protein, mouse; C082667:Fabp5 protein, mouse; C086936:Fabp7 protein, mouse; D000071438:Fatty Acid-Binding Protein 7; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D000960:Hypolipidemic Agents; D009363:Neoplasm Proteins; D009419:Nerve Tissue Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins