SPECIFIC INHIBITORS OF P38 MITOGEN-ACTIVATED PROTEIN-KINASE BLOCK 3T3-L1 ADIPOGENESIS

SB203580 and SB202190, pyridinyl imidazoles that selectively inhibit p 38 mitogen-activated protein (MAP) kinase, are widely utilized to asse ss the physiological roles of p38, Here, we demonstrate that treatment of 3T3-L1 fibroblasts with these p38 MAP kinase inhibitors prevents t heir differentiation into adipocytes as judged by an absence of lipid accumulation, a lack of expression of adipocyte-specific genes, and a fibroblastic morphological appearance. In 3T3-L1 fibroblasts and devel oping adipocytes, p38 is active. p38 activity decreases dramatically d uring later stages of differentiation. In accordance with the time cou rse of p38 activity, p38 inhibitor treatment during only the early sta ges of differentiation is sufficient to block adipogenesis. In additio n, we constructed a 3T3-L1 cell line harboring an inducible dominant n egative p38 mutant. The induction of this dominant negative mutant of p38 prevents adipocyte differentiation. Thus, it is likely that the an tiadipogenic activity of SB203580 and SB202190 is indeed due to inhibi tion of p38 MAP kinase, This study points out that CCAAT/enhancer-bind ing protein beta (C/EBP beta), a transcription factor critical for the initial stages of 3T3-L1 adipogenesis, bears a consensus site for p38 phosphorylation and serves as a substrate for p38 MAP kinase in vitro . Although the induction of C/EBP beta is not significantly altered in the presence of the p38 inhibitor, the amount of in vivo phosphorylat ed C/EBP beta is reduced by SB203580, The transcriptional induction of PPAR gamma, a gene whose expression is induced by C/EBP beta, and a f actor critically involved in terminal differentiation of adipocytes, i s impaired in the presence of p38 inhibitors. Thus, transcription fact ors such as C/EBP beta that promote adipocyte differentiation may be p 38 targets during adipogenesis. Collectively, the data in this study s uggest that p38 MAP kinase activity is important for proper 3T3-L1 dif ferentiation.