Differential effects of Ras signaling through NF kappa B on skeletal myogenesis

Oncogenic Ras (H-Ras G12V) inhibits skeletal myogenesis through multiple si gnaling pathways. Previously, we demonstrated that the major downstream eff ecters of Ras (i.e., MEK/MAPK, RalGDS and Rac/Rho) play a minor, if any, ro le in the differentiation-defective phenotype of Ras myoblasts, Recently, N F kappaB, another Ras signaling target, has been shown to inhibit myogenesi s presumably by stimulating cyclin D1 accumulation and cell cycle progressi on. In this study, we address the involvement of NF kappaB activation in th e Ras-induced inhibition of myogenesis, Using H-Ras G12V and three G12V eff ector-loop variants, we detect high levels of NF kappaB transcriptional act ivity in C3H10T1/2-MyoD cells treated with differentiation medium. Myogenes is is blocked by all Ras proteins tested, yet only in the case of H-Ras G12 V are cyclin D1 levels increased and cell cycle progression maintained, Exp ression of I kappaB alpha SR, an inhibitor of NF kappaB, does not reverse t he differentiation-defective phenotype of Ras expressing cultures, but does induce differentiation in cultures treated with tumor necrosis factor (TNF alpha) or in cultures expressing the RelA/p65 subunit of NF kappaB, These data confirm that NF kappaB is a target of Ras and suggest that the cellula r actions of NF kappaB require additional signals that are discriminated by the Ras effector-loop variants. Results with I kappaB alpha SR convincingl y demonstrate that H-Ras G12V does not rely on NF kappaB activity to block myogenesis, an observation that continues to implicate another unidentified signaling pathway(s) in the inhibition of skeletal myogenesis by Ras.