RAS P21(VAL) INHIBITS MYOGENESIS WITHOUT ALTERING THE DNA-BINDING OR TRANSCRIPTIONAL ACTIVITIES OF THE MYOGENIC BASIC HELIX-LOOP-HELIX FACTORS

MRF4, MyoD, myogenin, and Myf-5 are muscle-specific basic helix-loop-h elix transcription factors that share the ability to activate the expr ession of skeletal muscle genes such as those encoding alpha-actin, my osin heavy chain, and the acetylcholine receptor subunits. The muscle regulatory factors (MRFs) also exhibit the unique capacity to initiate the myogenic program when ectopically expressed in a variety of nonmu scle cell types, most notably C3H10T1/2 fibroblasts (10T1/2 cells), Th e commitment of myoblasts to terminal differentiation, although positi vely regulated by the MRFs, also is controlled negatively by a variety of agents, including several growth factors and oncoproteins such as fibroblast growth factor (FGF-2), transforming growth factor beta 1 (T GF-beta 1), and Ras p21(Val), The molecular mechanisms by which these varied agents alter myogenic terminal differentiation events remain un clear, In an effort to establish whether Ras p21(Val) represses MRF ac tivity by directly targeting the MRF proteins, we examined the DNA bin ding and transcription activation potentials of MRF4 and MyoD when exp ressed in 10T1/2 cells or in 10T1/2 cells expressing Ras p21(Val). Our results demonstrate that Ras p21(Val) inhibits terminal differentiati on events by targeting the basic domain of the MRFs, and yet the mecha nism underlying this inhibition does not involve altering the DNA bind ing or the inherent transcriptional activity of these regulatory facto rs, In contrast, FGF-2 and TGF-beta 1 block terminal differentiation b y repressing the transcriptional activity of the MRFs, We conclude tha t the Ras p21(Val) block in differentiation operates via an intracellu lar signaling pathway that is distinct from the FGF-2 and TGF-beta 1 p athways.