Somatotropin (ST) antagonizes insulin stimulation of fatty acid synthase (FAS) enzyme activity and gene transcription in adipocytes. Previous studies have shown that an insulin response element (IRE) is located in the proximal region of the FAS promoter (-71 to -50) and upstream stimulatory factor (USF) 1 binds to this IRE. The present study was conducted to initially evaluate whether there is a somatotropin response element (STRE) in the 5'flanking region of the FAS gene and to determine whether USF1 mediates the effect of ST on FAS gene transcription in 3T3-F442A adipocytes. Two 5' deletion FAS promoter constructs (pFAS-CATS4 and pFAS-CAT5), which contain the 5' flanking sequences of the rat FAS gene at -112 to +65 and -2195 to +65, respectively, were stably transfected into 3T3-F442A preadipocytes. Insulin stimulated chloramphenicol acetyltransferase (CAT) activity 1.7- and 4.7-fold (P < 0.05) in 3T3-F442A adipocytes transfected with pFAS-CATS4 and pFAS-CAT5, respectively. In contrast, bovine somatotropin (bST) attenuated the stimulatory effect of insulin on CAT activity by approximately 60% (P < 0.05) in both constructs. When 3T3-F442A adipocytes were treated with insulin (10 ng/mL) or insulin (10 ng/mL) plus bST (100 ng/mL) for 24, 48, or 72 h, neither insulin nor bST significantly affected USF1 mRNA levels. When human USF1 (hUSF1) cDNA probe was used, however, insulin increased the abundance of an unidentified transcript (named hUSF1-like mRNA) 11- to 25-fold (P < 0.05) and ST decreased the stimulatory effect of insulin on hUSF1-like mRNA levels by 50 to 90% (P < 0.05). Western blot analyses of nuclear extracts from cells treated with insulin (10 ng/mL) or insulin (10 ng/mL) plus bST (100 ng/mL) for 48 h demonstrated that the abundance of USF1 was not affected by insulin or ST. Furthermore, electrophoretic mobility shift analyses (EMSA) of nuclear extracts revealed that neither insulin nor ST had an effect on the binding of USF1 to the IRE. These results suggest that a STRE may be located within the first 112 bp of the FAS promoter and that USF1 does not directly mediate the effect of ST on transcription of the FAS gene in 3T3-F442A adipocytes.