The small GTPase protein Rac1 is involved in a wide range of biological processes, yet its role in cell differentiation is mostly unknown. Here we show that Rac1 activity is high in proliferating myoblasts and decreases during the differentiation process. To analyze the involvement of Rac1 in muscle differentiation, different forms of the protein were expressed in muscle cells. A constitutively activated form of Rac1 (Rac1Q61L) inhibited the activity of MyoD in promoting muscle differentiation, whereas a dominant negative form of Rac1 (Rac1T17N) induced the activity of MyoD in promoting muscle differentiation. Expression of Rac1T17N imposed myogenic differentiation on myoblasts growing under mitogenic conditions. In inquiring whether Rac1 affected the withdrawal of myoblasts from the cell cycle, we analyzed the expression of cyclin D1 and p21 The Rho family of small GTP-binding proteins, which includes Rho, Rac, and CDC42, is involved in a wide range of biological processes, including cell motility, cell adhesion, cell morphology, cytokinesis, and cell proliferation (for reviews, see Refs. 1 and 2).In mammalian cells, Rac functions by generating the actinrich lamellipodial protrusions and membrane ruffling that are thought to be a major part of the driving force for cell movement (3). Independent of this effect of cytoskeletal rearrangements, Rac1 and CDC42 induce the activation of mitogenactivated protein kinase cascades. Some reports suggest that Rac1 and CDC42 activate the c-Jun N-terminal kinase (JNK) 1 and the p38 MAPK (4 -7), whereas other studies indicate that Rac1 cross-talks with the ERK MAPK pathway. Rac and CDC42 can synergize with Raf1 or MEK1 to promote ERK activation (8). It turns out that p21-activated kinase (PAK), which is a CDC42 and Rac target, positively regulates Raf1 and MEK1 activities through phosphorylation of specific serine residues (9, 10). These functions of Rac and CDC42 in cytoskeletal reorganization and the induction of MAPK signaling pathways may explain their role in the regulation of a variety of cellular processes in development and morphogenesis (11).The differentiation of skeletal muscle cells involves two major stages: (a) withdrawal of myoblasts from the cell cycle, and (b) subsequent expression of myotube-specific genes. Proliferating myoblasts express two myogenic transcription factors from the basic helix-loop-helix family, MyoD and Myf5, before the onset of muscle differentiation (12)(13)(14). Once activated, MyoD and Myf5 induce the withdrawal of myoblasts from the cell cycle and the expression of another myogenic basic helixloop-helix factor, myogenin, as well as transcription factors from the MEF2 family. Together, myogenin and MEF2 family members cooperate in the activation of many muscle structural genes (15,16).Although the molecular mechanisms controlling myogenesis at the transcriptional level are well characterized, the signaling molecules that mediate the transduction of extracellular cues and affect the muscle regulatory factors are only partially uncovered....
The small GTPase RhoA regulates the expression of the myogenic transcription factor, MyoD, and the transcription of muscle-specific genes. We report that RhoA also affects the survival of differentiating myoblasts. Two signaling pathways, extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K)-Akt, are involved in myoblast survival. Here, we show that inhibition of RhoA prevents the phosphorylation of Akt, but does not affect the phosphorylation of ERK. Constitutive expression of an active form of Akt prevents apoptosis in myoblasts treated with the Rho inhibitor C3-transferase. These results indicate that RhoA functions to prevent myoblast death by inducing the PI3-K-Akt pathway.
The myogenic determination factor MyoD activates the transcription of muscle-specific genes by binding to consensus DNA sites found in the regulatory sequences of these genes. The interaction of MyoD with the basal transcription machinery is not known. Several activators induce transcription by recruiting TFIID and/or TFIIB to the promoter. We asked whether MyoD interacted functionally with TFIID and TFIIB in transcription. We reconstituted in vitro DNA binding and transcription systems of MyoD and basal transcription factors, and found that MyoD function in transcription occurred during the assembly of the preinitiation complex. Interestingly, MyoD activated transcription without affecting the binding of TFIID to the promoter. However, TFIID or TBP dramatically stabilized the binding of MyoD to its recognition site. MyoD and TBP interacted in solution. Deletion analysis of MyoD suggested that interaction of MyoD with TBP is needed for its activity in transcription. At a later stage of assembly, MyoD stabilized the binding of TFIIB to the preinitiation complex. These findings suggest that MyoD is involved in two steps of preinitiation; first, TFIID stabilizes MyoD binding to its DNA recognition site and at a later stage MyoD facilitates the association of TFIIB with the preinitiation complex.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.