Ras p21Val 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-helix transcription factors that share the ability to activate the expression of skeletal muscle genes such as those encoding alpha-actin, myosin 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 nonmuscle cell types, most notably C3H10T1/2 fibroblasts (10T1/2 cells). The commitment of myoblasts to terminal differentiation, although positively 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 (TGF-beta 1), and Ras p21Val. The molecular mechanisms by which these varied agents alter myogenic terminal differentiation events remain unclear. In an effort to establish whether Ras p21Val represses MRF activity by directly targeting the MRF proteins, we examined the DNA binding and transcription activation potentials of MRF4 and MyoD when expressed in 10T1/2 cells or in 10T1/2 cells expressing Ras p21Val. Our results demonstrate that Ras p21Val inhibits terminal differentiation events by targeting the basic domain of the MRFs, and yet the mechanism underlying this inhibition does not involve altering the DNA binding or the inherent transcriptional activity of these regulatory factors. In contrast, FGF-2 and TGF-beta 1 block terminal differentiation by repressing the transcriptional activity of the MRFs. We conclude that the Ras p21Val block in differentiation operates via an intracellular signaling pathway that is distinct from the FGF-2 and TGF-beta 1 pathways.
Kong
Y
Y
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
Johnson
S E
SE
Taparowsky
E J
EJ
Konieczny
S F
SF
D002454:Cell Differentiation; D002460:Cell Line; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D016222:Fibroblast Growth Factor 2 / Q000494:pharmacology; D005347:Fibroblasts; D019204:GTP-Binding Proteins / Q000502:physiology; D005786:Gene Expression Regulation / Q000502:physiology*; D018257:Helix-Loop-Helix Motifs; D009132:Muscles / Q000166:cytology* / Q000378:metabolism; D017570:MyoD Protein / Q000378:metabolism / Q000502:physiology; D018006:Myogenic Regulatory Factors / Q000378:metabolism / Q000502:physiology*; D015689:Oncogene Protein p21(ras) / Q000502:physiology*; D011401:Promoter Regions, Genetic / Q000235:genetics; D011950:Receptors, Cholinergic / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D015398:Signal Transduction / Q000235:genetics; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors / Q000378:metabolism / Q000502:physiology; D015533:Transcriptional Activation / Q000187:drug effects; D016212:Transforming Growth Factor beta / Q000494:pharmacology; D020668:rap GTP-Binding Proteins
D004268:DNA-Binding Proteins; D017570:MyoD Protein; D018006:Myogenic Regulatory Factors; D011950:Receptors, Cholinergic; D011993:Recombinant Fusion Proteins; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; C063403:myogenic factor 6; D016222:Fibroblast Growth Factor 2; D004247:DNA; D019204:GTP-Binding Proteins; D015689:Oncogene Protein p21(ras); D020668:rap GTP-Binding Proteins