Mol. Cell. Biol., Oct 1995, 5205-5213, Vol 15, No. 10
Y Kong, SE Johnson, EJ Taparowsky and SF Konieczny
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.
Copyright © 1995, American Society for Microbiology
Ras p21Val inhibits myogenesis without altering the DNA binding or transcriptional activities of the myogenic basic helix-loop-helix factors
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
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