PPARgamma is a member of the nuclear hormone receptor superfamily, and functions as a transcriptional regulator of genes linked to adipogenesis and lipid metabolism. The regulation of PPARgamma activity by insulin signaling molecules in adipocytes has yet to be clarified. Therefore, it is important to measure endogenous PPARgamma transcriptional activities in response to various stimuli in adipocytes. Herein, with a transcription reporter assay using recombinant adenovirus vectors expressing PPRE (PPAR responsive elements)-reporter genes, we established a novel system for measuring endogenous PPARgamma transcriptional activity in 3T3-LI adipocytes. By means of this system, a marked increase (8.5-fold) in PPARgamma transcriptional activity was detected after treatment with 10(-6) M pioglitazone, a thiazolidinedione (TZD), indicating that this system can measure PPARgamma activity accurately. Furthermore, MAPK activation, achieved by overexpressing constitutively activated MEK1, inhibited PPARgamma transcriptional activity. In contrast, treatment with PKA stimulators markedly increased PPARgamma activity. Interestingly, PI 3-kinase overexpression resulted in a marked decrease in PPARgamma activity. These observations have important implications for understanding the regulation of PPARgamma transcriptional activity. (C) 2002 Elsevier Science (USA). All rights reserved.