p38α, but not other p38 isoforms or other kinases in the MAPK family, enhances STAT4 transcriptional activity.

(A), MKK6 and p38α selectively augment IL-12–dependent, STAT4-mediated transactivation. NIH3T3 cells were transiently transfected with the p3xGAS-luc and the pCMV-β-galactosidase reporters together with expression vectors encoding the human IL-12R subunits. Where indicated, cells were cotransfected with control vector (EV), wild-type STAT4, MKK6, p38α, MEKEE, ERK2, MEKK1, JNK1, MEK5DD, or ERK5. The experiment was performed as described in Figure 1C. Unstimulated, ■; +IL-12, ▪. (B), p38α, but not p38γ, enhances IL-12–dependent, STAT4-mediated transactivation. NIH3T3 cells were transiently transfected with the p3xGAS-luc and the pCMV-β-galactosidase reporters, together with expression vectors encoding the human IL-12R subunits. Where indicated, cells were cotransfected with control vector (EV), wild-type STAT4, MKK6, p38α, or p38γ. The experiment was performed as described in Figure 1C. In the lower panel, 100 μg of cell lysates, analyzed in the luciferase assay, were resolved by SDS/PAGE, transferred to membrane and blotted with anti-HA Ab to assure that the kinases were expressed at equal levels. Unstimulated, ■; +IL-12, ▪.