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Mol. Cell. Biol., 07 1997, 3547-3555, Vol 17, No. 7
MB Ramocki, SE Johnson, MA White, CL Ashendel, SF Konieczny and EJ Taparowsky
The ability of basic helix-loop-helix muscle regulatory factors (MRFs),
such as MyoD, to convert nonmuscle cells to a myogenic lineage is regulated
by numerous growth factor and oncoprotein signaling pathways. Previous
studies have shown that H-Ras 12V inhibits differentiation to a skeletal
muscle lineage by disrupting MRF function via a mechanism that is
independent of the dimerization, DNA binding, and inherent transcriptional
activation properties of the proteins. To investigate the intracellular
signaling pathway(s) that mediates the inhibition of MRF-induced myogenesis
by oncogenic Ras, we tested two transformation- defective H-Ras 12V
effector domain variants for their ability to alter terminal
differentiation. H-Ras 12V,35S retains the ability to activate the
Raf/MEK/mitogen-activated protein (MAP) kinase cascade, whereas H- Ras
12V,40C is unable to interact directly with Raf-1 yet still influences
other signaling intermediates, including Rac and Rho. Expression of each
H-Ras 12V variant in C3H10T1/2 cells abrogates MyoD- induced activation of
the complete myogenic program, suggesting that MAP kinase-dependent and
-independent Ras signaling pathways individually block myogenesis in this
model system. However, additional studies with constitutively activated
Rac1 and RhoA proteins revealed no negative effects on MyoD-induced
myogenesis. Similarly, treatment of Ras-inhibited myoblasts with the MEK1
inhibitor PD98059 revealed that elevated MAP kinase activity is not a
significant contributor to the H- Ras 12V effect. These data suggest that
an additional Ras pathway, distinct from the well-characterized MAP kinase
and Rac/Rho pathways known to be important for the transforming function of
activated Ras, is primarily responsible for the inhibition of myogenesis by
H-Ras 12V.
Copyright © 1997, American Society for Microbiology
Signaling through mitogen-activated protein kinase and Rac/Rho does not duplicate the effects of activated Ras on skeletal myogenesis
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
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