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

Mice carrying dominant mutations at the agouti locus exhibit ectopic expres sion of agouti gene transcripts, obesity, and type II diabetes through unkn own mechanisms. To gain insight into the role of agouti protein in modulati ng adiposity, we investigated regulation of a key lipogenic gene, fatty aci d synthase (FAS) by agouti alone and in combination with insulin. Both agou ti and insulin increase FAS activity in 3T3-L1 and in human adipocytes. Ago uti and insulin independently and additively increase FAS activity in 3T3-L 1 adipocytes. We further investigated the mechanism responsible for the ago uti-induced FAS expression in these cells and demonstrated that both insuli n (3-fold increase) and agouti (2-fold) increased FAS gene expression at th e transcriptional level. Furthermore, insulin and agouti together exerted a dditive effects (5-fold increase) on FAS gene transcription. Transfection a ssays of FAS promoter-luciferase fusion gene constructs into 3T3-L1 adipocy tes indicated that the agouti response element(s) is (are) located in the - 435 to -415 region (-435/-415) of the FAS promoter. Nuclear proteins bindin g to this novel sequence are adipocyte specific. Thus the agouti response s equences mapped to a region upstream of the insulin-responsive element (whi ch we previously reported to be located at -67/-52), consistent with additi ve effects of these two factors on FAS gene transcription.