FEEDBACK-REGULATION OF RAF-1 AND MITOGEN-ACTIVATED PROTEIN-KINASE (MAP) KINASE KINASE-1 AND KINASE-2 BY MAP KINASE PHOSPHATASE-1 (MKP-1)

Inactivation of growth factor-regulated mitogen-activated protein (MAP ) kinases (ERK1 and ERK2) has been proposed to occur in part through d ephosphorylation by the dual specificity MAP kinase phosphatase-1 (MKP -1), an immediate early gene that is induced by mitogenic signaling. I n this study, we examined the effect of MKP-1 on signaling components upstream of ERK1 and ERK2. Coexpression of MKK1 or MKK2 with MKP-1 res ulted in 7-10-fold activation of mitogen-activated protein kinase kina se (MKK), which required the presence of regulatory serine phosphoryla tion sites, Endogenous MKK1 and MKK2 were also activated upon MKP-1 ex pression. Raf-1, a direct regulator of MKK1 and MKK2, was activated un der these conditions, and a synergistic activation of MKK was observed upon coexpression of Raf-1 and MKP-1. This effect did not appear to i nvolve synthesis of autocrine growth factors or the inhibition of basa l extracellular signal-regulated kinase (ERK) activity but was inhibit ed by a dominant negative Ras mutant, indicating that MKP-1 enhances R as-dependent activation of Raf-1 in a cell autonomous manner. This stu dy demonstrates positive feedback regulation of Raf-1 and MMK by the M KP-1 immediate early gene and a potential mechanism for activating Raf -1/MKK signaling pathways alternative to those involving ERK.