Ras/MEK/ERK up-regulation of the fibroblast K-Ca channel FIK is a common mechanism for basic fibroblast growth factor and transforming growth factor-beta suppression of myogenesis

The 10T1/2-MRF4 fibroblast/myogenic cell system was used to address the fol lowing interrelated questions: whether distinct signaling pathways underlie myogenic inhibition by basic fibroblast growth factor (bFGF) and transform ing growth factor (TGF)-beta; which of these pathways also up-regulates the fibroblast intermediate conductance calcium-activated potassium channel, F IK, a positive regulator of cell proliferation; and whether FIK up-regulati on underlies some or all myogenic inhibitory signaling events. The results show that myogenic inhibition in 10T1/2-MRF4 cells, by both bFGF and TGF-be ta, requires activation of the Ras/mitogen-activated protein (MAP) kinase/M AP kinase-ERK kinase (MEK)/ extracellular signal-regulated kinase (ERK) pat hway, and resultant FIK up-regulation. We show that FIK is instrumental in MEK-dependent suppression of acetylcholine receptor channel expression but that MEK activation and FIK up-regulation are not essential to suppression of myosin heavy chain and myotube formation. These data indicate that Ras/M EK/ERK induction of FIK is pivotal to regulation of certain myogenic events by both receptor tyrosine kinases and TGF-beta receptor, and this is also the first demonstration of chronic FIK upregulation by the TGF-beta recepto r family. Furthermore, the results define the physiologic signaling require ments for growth factor-stimulated FIK up-regulation, whereas previous work has concentrated on constitutive FIK up-regulation in cells stably transfe cted with oncoprotein signaling molecules. This study, together with earlie r work showing that FIK positively regulates cell proliferation, establishe s this member of the IK channel family as a multifunctional, growth factor- regulated signaling molecule.