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.
Schiemann
W P
WP
Department of Pharmacology, University of Washington, Seattle 98195-7750, USA.
Nathanson
N M
NM
D016475:3T3 Cells; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D005576:Colforsin / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D006131:Growth Inhibitors / Q000494:pharmacology*; D015850:Interleukin-6; D053605:Leukemia Inhibitory Factor; D008222:Lymphokines / Q000494:pharmacology*; D051379:Mice; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D015398:Signal Transduction; D013997:Time Factors
D006131:Growth Inhibitors; D015850:Interleukin-6; D053605:Leukemia Inhibitory Factor; C507838:Lif protein, mouse; D008222:Lymphokines; D005576:Colforsin; D019908:Proto-Oncogene Proteins c-raf; D017871:Calcium-Calmodulin-Dependent Protein Kinases