Raf-1 is activated by the p38 mitogen-activated protein kinase inhibitor, SB203580

SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imi dazole) is widely used as a specific inhibitor of p38 mitogen-activated pro tein kinase (MAPK). Here, we report that SB203580 activates the serine/thre onine kinase Raf-1 in quiescent smooth muscle cells in a dose-dependent fas hion. The concentrations of SB203580 required lie above those necessary to inhibit p38 MAPK and me were unable to detect basal levels of active p38 MA PK. SB203580 does not directly activate Raf-1 in vitro, and fails to activa te Ras, MEK, and ERK in intact cells. In vitro, however, SB203580-stimulate d Raf-1 activates MEK1 in a coupled assay. We conclude that activation of R af-1 by SB203580 is not mediated by an inhibition of p38 MAPK, is Ras-indep endent, and is uncoupled from MEK/ERK signaling. (C) 1999 Federation of Eur opean Biochemical Societies.