Ras p21Val inhibits myogenesis without altering the DNA binding or transcriptional activities of the myogenic basic helix-loop-helix factors

Muscle LIM protein promotes myogenesis by enhancing the activity of MyoD

The muscle LIM protein (MLP) is a muscle-specific LIM-only factor that exhibits a dual subcellular localization, being present in both the nucleus and in the cytoplasm. Overexpression of MLP in C2C12 myoblasts enhances skeletal myogenesis, whereas inhibition of MLP activity blocks terminal differentiation. Thus, MLP functions as a positive developmental regulator, although the mechanism through which MLP promotes terminal differentiation events remains unknown. While examining the distinct roles associated with the nuclear and cytoplasmic forms of MLP, we found that nuclear MLP functions through a physical interaction with the muscle basic helix-loop-helix (bHLH) transcription factors MyoD, MRF4, and myogenin. This interaction is highly specific since MLP does not associate with nonmuscle bHLH proteins E12 or E47 or with the myocyte enhancer factor-2 (MEF2) protein, which acts cooperatively with the myogenic bHLH proteins to promote myogenesis. The first LIM motif in MLP and the highly conserved bHLH region of MyoD are responsible for mediating the association between these muscle-specific factors. MLP also interacts with MyoD-E47 heterodimers, leading to an increase in the DNA-binding activity associated with this active bHLH complex. Although MLP lacks a functional transcription activation domain, we propose that it serves as a cofactor for the myogenic bHLH proteins by increasing their interaction with specific DNA regulatory elements. Thus, the functional complex of MLP-MyoD-E protein reveals a novel mechanism for both initiating and maintaining the myogenic program and suggests a global strategy for how LIM-only proteins may control a variety of developmental pathways.

Signaling through mitogen-activated protein kinase and Rac/Rho does not duplicate the effects of activated Ras on skeletal myogenesis

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.

The anti-cancer agent distamycin A displaces essential transcription factors and selectively inhibits myogenic differentiation

The anticancer drug, distamycin A, alters DNA conformation by binding to A/T-rich domains. We propose that binding of the drug to DNA alters transcription factor interactions and that this may alter genetic regulation. We have analyzed the effects of distamycin A upon expression of the muscle-specific cardiac and skeletal alpha-actin genes which have A/T-rich regulatory elements in their promoters. Distamycin A specifically inhibited endogenous muscle genes in the myogenic C2 cell line and effectively eliminated the myogenic program. Conversely, when 10T1/2C18 derived pleuripotential TA1 cells were induced to differentiate in the presence of distamycin A, adipocyte differentiation was enhanced whereas the numbers of cells committing to the myogenic program decreased dramatically. Using the mobility shift assay distamycin A selectively inhibited binding of two important transcription factors, SRF and MEF2, to their respective A/T-rich elements. The binding of factors Sp1 and MyoD were not affected. The inhibition of factor binding correlated with a repression of muscle-specific promoter activity as assayed by transient transfection assays. Co-expression of the myoD gene, driven by a distamycin A-insensitive promoter, failed to relieve the inhibition of these muscle-specific promoters by distamycin A. Additionally, SRF and MEF2 dependent promoters were selectively down regulated by distamycin A. These results suggest that distamycin A may inhibit muscle-specific gene expression by selectively interfering with transcription factor interactions and demonstrate the importance of these A/T-rich elements in regulating differentiation of this specific cell type.

Selection for retroviral insertions into regulated genes

Gene traps can be used to monitor faithfully the changes in gene expression accompanying several cellular processes. Here, we present a strategy that combines retroviral gene trap vectors, efficient selection schemes based on fluorescence-activated cell sorting or dominant positive and negative drug selection, and appropriately responsive cell lines in order to enrich for retroviral insertions into regulated genes (i.e., genes participating in cellular differentiation processes and genes induced by growth factors, drugs, or neurotransmitters, etc.). As an example, we applied this approach to the identification of insertions into genes activated by a MyoD protein, using a MyoD-responsive fibroblast line. In a single experiment designed to demonstrate the feasibility of this approach, we have been able to screen thousands of gene trap integrations and to select those that represent direct or indirect targets of MyoD. Distinct patterns of regulation were observed during myogenic determination. Sequences flanking the integrations can be rescued with several approaches, and they can be used to isolate the host genes or can serve as entry points for genome-wide sequencing projects.

Cyclin-mediated inhibition of muscle gene expression via a mechanism that is independent of pRB hyperphosphorylation

It was recently demonstrated that ectopic expression of cyclin D1 inhibits skeletal muscle differentiation and, conversely, that expression of cyclin-dependent kinase (cdk) inhibitors facilitates activation of this differentiation program (S. S. Rao, C. Chu, and D. S. Kohtz, Mol. Cell. Biol. 14:5259-5267, 1994; S. S. Rao and D. S. Kohtz, J. Biol. Chem. 270:4093-4100, 1995; S. X. Skapek, J. Rhee, D. B. Spicer, and A. B. Lassar, Science 267:1022-1024, 1995). Here we demonstrate that cyclin D1 inhibits muscle gene expression without affecting MyoD DNA binding activity. Ectopic expression of cyclin D1 inhibits muscle gene activation by both MyoD and myogenin, including a mutated form of myogenin in which two potential inhibitory cdk phosphorylation sites are absent. Because the retinoblastoma gene product, pRB, is a known target for cyclin D1-cdk phosphorylation, we determined whether cyclin D1-mediated inhibition of myogenesis was due to hyperphosphorylation of pRB. In pRB-deficient fibroblasts, the ability of MyoD to activate the expression of muscle-specific genes requires coexpression of ectopic pRB (B. G. Novitch, G. J. Mulligan, T. Jacks, and A. B. Lassar, J. Cell Biol., 135:441-456, 1996). In these cells, the expression of cyclins A and E can lead to pRB hyperphosphorylation and can inhibit muscle gene expression. The negative effects of cyclins A or E on muscle gene expression are, however, reversed by the presence of a mutated form of pRB which cannot be hyperphosphorylated. In contrast, cyclin D1 can inhibit muscle gene expression in the presence of the nonhyperphosphorylatable form of pRB. On the basis of these results we propose that G1 cyclin-cdk activity blocks the initiation of skeletal muscle differentiation by two distinct mechanisms: one that is dependent on pRB hyperphosphorylation and one that is independent of pRB hyperphosphorylation.

Interaction and functional collaboration of p300/CBP and bHLH proteins in muscle and B-cell differentiation

Differentiation of skeletal muscle cells and B lymphocytes is regulated by basic helix-loop-helix (bHLH) proteins. Both differentiation programs are inhibited by the adenovirus E1A oncoprotein. Analysis of E1A mutants has implicated two of its cellular-binding proteins, p300 and CBP, in controlling certain aspects of differentiation. We find that p300 can cooperate with tissue-specific bHLH proteins in activating target genes and requires only the bHLH domain of such proteins to stimulate E box-directed transcription. Importantly, the ability of bHLH proteins to activate transcription correlates with the presence of p300/CBP in E box-dependent DNA-binding complexes, because both phenomena require at least two adjacent E-box motifs. Microinjection of p300/CBP antibodies into myoblasts blocks terminal differentiation, cell fusion, and transcriptional activity of myogenic bHLH proteins. These results suggest that the function of p300/CBP is essential for the execution of key aspects of cellular differentiation.

Regulation of differentiation by TGF-beta

Recent experiments in neural, skeletal, endothelial, and hematopoietic tissues have provided new insights into the way members of the transforming growth factor-beta (TGF-beta) superfamily regulate cellular differentiation. TGF-betas regulate the fate of multipotential stem cells instructively (in the neural crest) by regulating the expression or function of tissue-specific transcription factors, as well as selectively (in the mesenchyme) by regulating the expression of required growth factors and their receptors. During skeletal development, TGF-betas have unique functions and act sequentially to modulate chondrocyte and osteoblast differentiation. Responsiveness to TGF-betas changes as cells differentiate and evidence now suggests that changes in TGF-beta receptor profile may account for some of these differences. Drosophila and transgenic mouse models are now providing useful insights into mechanisms of TGF-beta action in vivo.

Phosphatidylinositol 3-kinase inhibitors block differentiation of skeletal muscle cells

Skeletal muscle differentiation involves myoblast alignment, elongation, and fusion into multinucleate myotubes, together with the induction of regulatory and structural muscle-specific genes. Here we show that two phosphatidylinositol 3-kinase inhibitors, LY294002 and wortmannin, blocked an essential step in the differentiation of two skeletal muscle cell models. Both inhibitors abolished the capacity of L6E9 myoblasts to form myotubes, without affecting myoblast proliferation, elongation, or alignment. Myogenic events like the induction of myogenin and of glucose carrier GLUT4 were also blocked and myoblasts could not exit the cell cycle, as measured by the lack of mRNA induction of p21 cyclin-dependent kinase inhibitor. Overexpresssion of MyoD in 10T1/2 cells was not sufficient to bypass the myogenic differentiation blockade by LY294002. Upon serum withdrawal, 10T1/2-MyoD cells formed myotubes and showed increased levels of myogenin and p21. In contrast, LY294002-treated cells exhibited none of these myogenic characteristics and maintained high levels of Id, a negative regulator of myogenesis. These data indicate that whereas phosphatidylinositol 3-kinase is not indispensable for cell proliferation or in the initial events of myoblast differentiation, i.e. elongation and alignment, it appears to be essential for terminal differentiation of muscle cells.

Casein kinase II increases the transcriptional activities of MRF4 and MyoD independently of their direct phosphorylation

The myogenic regulatory factors (MRFs) are a subclass of a much larger group of basic helix-loop-helix transcription factors which includes members of the E protein such as E47, E2-2, and HEB. Although the MRFs are unique in their ability to confer a myogenic phenotype on nonmuscle cells, they require E protein partners to form a MRF-E protein heterodimer, which represents the functional myogenesis-inducing complex. The mechanisms controlling homodimer and heterodimer formation in vivo remain largely unknown, although it is likely that posttranslational modification of one or both basic helix-loop-helix partners is critical to this regulatory event. In this respect, MyoD and MRF4, both members of the MRF family, exist in vivo as phosphoproteins and contains multiple consensus phosphorylation sites, including sites for casein kinase II (CKII) phosphorylation. In this study, we demonstrate that overexpression of CKII increases the transcriptional activities of MRF4 and MyoD in vivo. Interestingly, mutation of the individual CKII sites within MRF4 and MyoF does not alter the ability of CKII to enhance MRF transcriptional activity, suggesting that the effect of CKII expression on the MRFs is indirect. Given that the MRFs require dimerization with E protein partners to activate muscle-specific transcription, the effects of CKII expression on E protein function also were examined. Our studies show that E47 serves as an in vitro substrate for CKII and that CKII-phosphorylated E-47 proteins no longer bind to DNA. These observations were confirmed by in vivo experiments showing that overexpressing of CKII produces a dramatic reduction in E47 homodimer-directed transcription. We conclude from these studies that CKII may act as a positive regulator of myogenesis by preventing E protein homodimers from binding to muscle gene regulatory elements.