The Maf transcription factors are involved in a variety of developmental and cellular differentiation processes, but their role in the differentiation of mesenchymal cells has not been described. Here, we have analyzed c-maf expression during the differentiation of adipocytes and muscle cells in cultured systems. The expression of c-maf mRNA was down-regulated during adipogenesis and up-regulated during myogenesis. In adipogenesis, the c-maf mRNA was down-regulated 58h after switching to the differentiation medium and just after PPARgamma2 mRNA was induced. A transient transfection analysis of a reporter gene containing the 5(')-flanking region of the c-maf gene showed that PPARgamma2 represses c-maf gene expression. We previously found that c-Maf, c-Jun, and Pax6 bind to and stimulate the c-maf gene. The PPARgamma2 repression of c-maf expression seems to be due, at least in part, to inhibition of the transactivation functions of c-Maf, c-Jun, and Pax6. The repression of c-maf was partly reversed by CBP, suggesting that these transcription factors compete for CBP or related transcription co-factors. In myogenesis, there was a differentiation-dependent stimulation of c-maf mRNA expression. The increased expression correlated with myoD expression. A transient transfection analysis showed that myoD stimulated a c-maf reporter gene through binding to two typical E-box elements located between 160 and 180 nucleotides upstream of the cap site. Binding of MyoD to the E-boxes was confirmed by a gel mobility shift assay and DNaseI footprinting analysis. Combined, these results suggest that the c-maf gene plays an important role during the differentiation of adipocyte and muscle cells from mesenchymal fibroblast cells.