p38 Isoforms Have Opposite Effects on AP-1-dependent Transcription through Regulation of c-Jun THE DETERMINANT ROLE OF THE ISOFORMS IN THE p38 MAPK SIGNAL SPECIFICITY

Abstract p38 MAPK pathway signaling is known to participate in cell proliferation, apoptosis, and differentiation, in a manner dependent on the cellular context. The factors that determine the specific biological response in a given cell type, however, remain largely unknown. We report opposite effects of the p38 isoforms on regulation of AP-1-dependent activities by p38 activators MAPK kinase 6 (MKK6) and/or arsenite in human breast cancer cells. The p38β isoform increases the activation of AP-1 transcriptional activities by MKK6 and/or arsenite, whereas p38γ/p38δ inhibits or has no effect on the stimulation. The p38β does so by increasing the levels of phosphorylated c-Jun, whereas the p38γ and -δ isoforms may act by regulating the c-jun transcription. AP-1-dependent processes such as vitamin D receptor gene promoter activation and cellular proliferation were similarly activated by the p38β or inhibited by the p38γ and/or -δ isoforms. Whereas the human breast cancer cells express all four isoforms, mouse NIH 3T3 and EMT-6 cells express only some of the p38 family members, with p38β higher in 3T3 cells but p38δ only detected in the EMT-6 line. Consistent with the positive and negative roles of p38β and p38δ in AP-1 regulation, MKK6 stimulates AP-1-dependent transcription in NIH 3T3 but not EMT-6 cells. In support of a role of c-Jun regulation by p38 isoforms in determining AP-1 activity, the levels of endogenous c-Jun and its phosphorylated form on p38 activation are higher in NIH 3T3 cells. These results demonstrate the contrasting activities of the different p38 isoforms in transmitting the upstream signal to AP-1 and show that the expression profile of p38 isoforms determines whether the p38 signal pathway activates or inhibits AP-1-dependent processes.