We examined the role of PPARgamma2 and C/EBPalpha for adiponectin and aP2 gene activation in C/EBPalpha(-/-) fibroblasts by stably expressing PPARgamma2 or C/EBPalpha. PPARgamma2, but not PPARgamma1, mRNA markedly increased during the differentiation to adipocytes in cells expressing C/EBPalpha. Both infected cell lines differentiated to an adipocyte phenotype and the mRNA for both aP2 and adiponectin increased in parallel. However, adiponectin mRNA was considerably higher when C/EBPalpha was present, suggesting that this transcription factor is important for full gene activation. Thiazolidinediones markedly activated the gene in PPARgamma2-expressing cells in the absence of C/EBPalpha, suggesting that the adiponectin promoter may have functional PPARgamma-response elements. Several observations showed that the adiponectin and aP2 genes can be differentially regulated in adipocytes: (1) Topiramate, an anti-epileptic agent with weight-reducing properties, increased adiponectin mRNA levels and secretion, but did not, like the thiazolidinediones, increase aP2 expression; (2) IL-6 reduced adiponectin, but significantly increased, aP2 expression; and (3) TNFalpha inhibited adiponectin, but paradoxically increased, aP2 expression in PPARgamma2-infected C/EBPalpha null cells. These data show that activation of the adiponectin gene can be separated from effects on adipogenic genes. (C) 2003 Elsevier Inc. All rights reserved.