Transcriptional regulation of the adipocyte fatty acid synthase gene by the agouti gene product: Interaction with insulin

Dominant mutations at the mouse agouti locus exhibit ectopic expression of agouti gene transcripts resulting in obesity and hyperinsulinemia through as yet unknown mechanisms. To gain insight into the role of agouti protein in adipocyte lipogenesis we investigated regulation of a key lipogenic gene, fatty acid synthase (FAS) by the agouti recombinant protein and its interaction with insulin in murine 3T3L-1 adipocytes. FAS plays a central role in de novo lipogenesis and is regulated nutritionally and hormonally. We demonstrated that FAS mRNA levels and enzyme activities are induced by insulin and by purified agouti recombinant protein in adipocytes. We further investigated the mechanism responsible for the agouti-induced FAS expression and demonstrated that both insulin (3-fold increase) and agouti (2-fold) increase FAS gene expression at the transcriptional level. Furthermore, these two hormones when added together have additive effects (5-fold increase) on FAS activity and gene transcription. Preliminary data from transfection assays of FAS promoter-luciferase fusion gene constructs indicated that agouti response element(s) is (are) located in the -500 -300 region upstream of the FAS gene transcription start site. Thus the agouti response sequences mapped to a region upstream of the insulin responsive element (which we previously reported). Further studies will define specific sequences within this region that mediate responsiveness to agouti and the trans-acting protein(s) binding to this regulatory region.