Raf-1 independent stimulation of mitogen-activated protein kinase by leukemia inhibitory factor in 3T3-L1 cells

We show here that treatment of 3T3-L1 cells with leukemia inhibitory factor (LIF) stimulated Raf-1 activity in a time- and dose-dependent manner. Although phorbol ester failed to activate Raf-1 directly, a protein kinase C-stimulated signal was found to be necessary, but not sufficient, for LIF-mediated activation of Raf-1. Elevation of intracellular cAMP levels completely blocked Raf-1 activation by LIF, but was without effect on the magnitude of mitogen-activated protein kinase (MAPK) stimulation by the cytokine, suggesting the presence of a Raf-1-independent, cAMP-insensitive MAPK kinase kinase (MAPKKK) pathway in 3T3-L1 cells. Mono Q-fractionation of LIF-stimulated 3T3-L1 extracts identified a single peak of MAPKKK activity that was largely insensitive to elevated intracellular levels of cAMP, and that failed to correlate with stimulation of either Raf-1 or MEKK1 protein kinases. Our results demonstrate that LIF-mediated activation of the MAP kinase cascade in 3T3-L1 cells proceeds through both Raf-1-dependent and -independent pathways which differ in their sensitivity to inhibition by intracellular cAMP.