Activated Raf inhibits myogenesis through a mechanism independent of activator protein 1-mediated myoblast transformation

Skeletal myogenesis is acutely affected by growth factors and subsequent ac tivation of their respective intracellular signaling cascades. Components o f the mitogenic Ras/Raf/mitogen-activated protein kinase (MAPK) signaling m odule are potent inhibitors of myoblast differentiation. However, the means by which these kinases prevent myocyte formation and activation of the mus cle gene program is unknown. Activator protein 1 (AP-1) is a transcriptiona l regulator the actions of which are up-regulated by signaling events, incl uding elevated MAPK, Because activated Raf inhibits avian myogenesis in a M APK-dependent fashion, we investigated the role of AP-1 as a mediator of th e Raf-imposed block to myogenesis. Avian myoblasts overexpressing activated Raf contain elevated levels of AP 1 DNA binding and transcriptional activi ty, Introduction of an AP-1 dominant inhibitory protein (AFOS) into Raf-exp ressing myoblasts prevented Requisition of a transformed morphology. Intere stingly, these cells remained differentiation-defective. Myogenic cells cot ransduced with RCAS(A)-Raf BXB and RCAS(B)-AFOS remained mononuclear and my osin-negative and did not activate significantly muscle-specific reporter g enes. These results argue that Raf inhibits muscle differentiation independ ent of AP-1-mediated cell transformation. Our results provide evidence for AP-1 as a critical component of the transforming capacity of activated Raf and evidence that AP-1 is not involved in the myogenic inhibitory effects o f the kinase.