Inhibition of myogenesis by the H-ras oncogene: implication of a role for protein kinase C

Inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling mediates delayed myogenesis in Duchenne muscular dystrophy fetal muscle

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder characterized by muscle wasting and premature death. The defective gene is dystrophin, a structural protein, absence of which causes membrane fragility and myofiber necrosis. Several lines of evidence showed that in adult DMD patients dystrophin is involved in signaling pathways that regulate calcium homeostasis and differentiation programs. However, secondary aspects of the disease, such as inflammation and fibrosis development, might represent a bias in the analysis. Because fetal muscle is not influenced by gravity and does not suffer from mechanical load and/or inflammation, we investigated 12-week-old fetal DMD skeletal muscles, highlighting for the first time early alterations in signaling pathways mediated by the absence of dystrophin itself. We found that PLC/IP3/IP3R/Ryr1/Ca2+ signaling is widely active in fetal DMD skeletal muscles and, through the calcium-dependent PKCα protein, exerts a fundamental regulatory role in delaying myogenesis and in myofiber commitment. These data provide new insights into the origin of DMD pathology during muscle development.

Mechanisms of nuclear vitamin D receptor resistance in Harvey-ras-transfected cells

The hormone 1,25 dihydroxyvitamin D (1,25(OH)(2)D) binds to the nuclear vitamin D receptor (nVDR), which heterodimerizes with retinoid X receptor alpha (RXRalpha), and this complex interacts with specific response elements [vitamin D response elements (VDREs)] to regulate gene transcription. Previous results show a significant reduction in 1,25(OH)(2)D-induced nVDR transcriptional activity in fibroblast (C3H10T1/2) cells transfected with the Harvey ras gene (ras cells) compared with parental cells. The purpose of this study was to investigate the mechanisms by which the H-ras gene interferes with nVDR transcriptional activity. Similar to the ras cells, transcriptional activity of the nVDR was reduced following induction of the H-ras gene for 9 days. The ras cells expressed similar protein levels of RXRalpha with the parent cells, and overexpression of the wild-type RXRalpha plasmid did not restore 1,25(OH)(2)D-mediated nVDR activity in ras cells. Inhibiting activation of extracellular signal-regulated kinase (ERK1/2) had no effect on nVDR activity in ras cells. Furthermore, the binding of nVDR to VDREs was reduced in 1,25(OH)(2)D-treated ras cells. In addition, neither treatment of ras cells with an inhibitor (ketoconazole) of the 1,25(OH)(2)D degradative enzyme, 24-hydroxylase, nor the protein kinase C inhibitors, bisindoylmaleimide I and Gö 6976, had an effect on nVDR activity. In contrast, inhibition of phosphatidylinositol 3-kinase (PI3K) with LY294002 resulted in a 1.6-fold significant increase in the nVDR activity in the ras cells. Taken together, these results indicate that PI3K may, at least in part, mediate the suppression of the 1,25(OH)(2)D regulation of nVDR transcriptional activity by the H-ras gene, leading to reduced ability to associate with response elements.

Increased expression of the pro-apoptotic Bcl2 family member PUMA is required for mitochondrial release of cytochrome C and the apoptosis associated with skeletal myoblast differentiation

We have previously shown that when skeletal myoblasts are cultured in differentiation medium (DM), roughly 30% undergo caspase 3-dependent apoptosis rather than differentiation. Herein, we investigate the molecular mechanism responsible for the activation of caspase 3 and the ensuing apoptosis. When 23A2 myoblasts are cultured in DM, caspase 9 activity is increased and pharmacological abrogation of caspase 9 activation impairs caspase 3 activation and apoptosis. Further, we detect a time dependent release of mitochondrial cytochrome C into the cytosol in roughly 30% of myoblasts. Inclusion of cycloheximide inhibits the release of cytochrome C, the activation of caspase 9 and apoptosis. These data indicate that the mitochondrial pathway plays a role in this apoptotic process and that engagement of this pathway relies on de novo protein synthesis. Through RT-PCR and immunoblot analysis, we have determined that the expression level of the pro-apoptotic Bcl2 family member PUMA is elevated when 23A2 myoblasts are cultured in DM. Further, silencing of PUMA inhibits the release of cytochrome C and apoptosis. Signaling by the transcription factor p53 is not responsible for the increased level of PUMA. Finally, myoblasts rescued from apoptosis by either inhibition of elevated caspase 9 activity or silencing of PUMA are competent for differentiation. These results indicate a critical role for PUMA in the apoptosis associated with skeletal myoblast differentiation and that a p53-independent mechanism is responsible for the increased expression of PUMA in these cells.

Ran binding protein 9 interacts with Raf kinase but does not contribute to downstream ERK1/2 activation in skeletal myoblasts

Raf kinase is the upstream activator of MEK1/2 leading to phosphorylation and activation of ERK1/2. Sustained activation of Raf represses skeletal muscle-specific reporter gene transcription and formation of multinucleated myofibers. Inhibition of myogenesis by activated Raf involves downstream ERK1/2 as well as undefined mediators. To identify Raf-interacting proteins that may influence repression of muscle formation, a yeast two-hybrid screen was performed using a MEK1-binding defective Raf (RafBXB-T481A) as bait. Twenty cDNAs coding for Raf-interacting proteins were identified including Ran binding protein 9 (RanBP9), a protein previously reported to interact with receptor tyrosine kinases. Forced expression of RanBP9 in myogenic cells did not alter myogenesis. Co-expression of RanBP9 with constitutively active RafBXB, but not RafBXB-T481A, synergistically inhibited MyoD-directed muscle reporter gene transcription. Knockdown of RanBP9 expression did not restore the differentiation program to Raf-expressing myoblasts. Thus, RanBP9 physically associates with Raf but does not substantially contribute to the inhibitory actions of the kinase.

Sphingosine-1-Phosphate Inhibition of Apoptosis Requires Mitogen-Activated Protein Kinase Phosphatase-1 in Mouse Fibroblast C3H10T½ Cells

The roles of extracellular regulated kinase (ERK) activation and mitogen-activated protein kinase phosphatase-1 (MKP-1) were examined in sphingosine-1-phosphate (S1P)-mediated inhibition of apoptosis in C3H10T 1/2 fibroblast cells. Apoptosis induced by the ceramide analog, C8-ceramine, was inhibited by S1P (ceramine/S1P). Stress activated protein kinase or c-Jun N-terminal kinase (SAPK/JNK) activation was significantly higher after ceramine and ceramine/S1P treatments. Ceramine/S1P treatment also significantly increased ERK activation and MKP-1 protein levels. ERK activation was required for the inhibition of apoptosis by S1P as shown using the mitogen-activated protein kinase kinase inhibitor, PD98059. Transfection with a dominant negative mutant construct of the MKP-1 gene prevented S1P inhibition of apoptosis and resulted in sustained SAPK/JNK activity. The MKP-1 mutant did not affect ERK activity, indicating that MKP-1 preferentially down-regulates SAPK/JNK in C3H10T 1/2 cells. Finally, the S1P activation of ERK and inhibition of apoptosis were reduced by pertussis toxin treatment, suggesting that G-protein-coupled receptors, such as the endothelial differentiation gene (EDG) receptor, play a role. Thus, both ERK activation and MKP-1, which down-regulates SAPK/JNK, are required for S1P-mediated inhibition of apoptosis.

PKC-regulated myogenesis is associated with increased tyrosine phosphorylation of FAK, Cas, and paxillin, formation of Cas-CRK complex, and JNK activation

Previous reports suggest that PKC plays an important role in regulating myogenesis. However, the regulatory signaling pathways are not fully understood. We examined the effects of PKC downregulation on signaling events during skeletal muscle differentiation. We found that downregulation of PKC results in increased myogenesis in C2C12 cells as measured by creatine kinase activity and myogenin expression. We showed that, during differentiation, downregulation of PKC expression results in increased tyrosine phosphorylation of FAK, Cas, and paxillin, concomitant with enhanced Cas-CrkII complex formation, which leads to activation of JNK2. But in proliferated muscle cells, PKC inhibition results in FAK and Cas tyrosine dephosphorylation. Further, disruption of actin cytoskeleton by cytochalasin D prevents the activation of FAK and Cas as well as the formation of Cas-CrkII complex stimulated by PKC downregulation during muscle cell differentiation. Finally, we observed that PKC downregulation increases the tyrosine phosphorylation of focal adhesion associated proteins. Based on the above data, we propose that PKC downregulation results in enhanced tyrosine phosphorylation of FAK, Cas, and paxillin, thus promoting the establishment of Cas-CrkII complex, leading to activation of JNK and that these interactions are dependent upon the integrity of actin cytoskeleton during muscle cell differentiation. Data presented here significantly contribute to elucidating the regulatory role of PKC in myogenesis possibly through integrin signaling pathway.

Activation of Ras and the mitogen-activated protein kinase pathway promotes protein degradation in muscle cells of Caenorhabditis elegans

To discover and study intracellular signals that regulate proteolysis in muscle, we have employed transgenic strains of Caenorhabditis elegans that produce a soluble LacZ reporter protein limited to body-wall and vulval muscles. This reporter protein is stable in well-fed wild-type animals, but its degradation is triggered upon a shift to 25 degrees C in a strain carrying a temperature-sensitive activating mutation in the Ras oncogene homologue let-60. These mutants are not physiologically starved, inasmuch as growth rates are normal at 25 degrees C. Ras-induced degradation is not prevented by the presence of cycloheximide added at or before the temperature shift and thus uses preexisting proteolytic systems and signaling components. Furthermore, degradation is triggered when adult animals are shifted to conditions of 25 degrees C, confirming that Ras acutely promotes protein degradation in muscles whose developmental history is normal. Reduction-of-function mutations in the downstream protein kinase Raf (lin-45), MEK (mek-2), or mitogen-activated protein kinase (MAPK) (mpk-1) prevent Ras-induced protein degradation, whereas activated MPK-1 is sufficient to trigger degradation, indicating that this kinase cascade is the principal route by which Ras signaling triggers protein degradation in muscle. This pathway is activated in hypodermal cells by the LET-23 epidermal growth factor receptor homologue, but an activating mutation in let-23 does not promote proteolysis in muscle. Starvation-induced LacZ reporter degradation is unaffected by reduction-of-function mutations in Ras, Raf, MEK, or MAPK, implying that Ras activation and starvation trigger proteolysis by mechanisms that are at least partially independent. This is the first evidence that Ras-Raf-MEK-MAPK signaling activates protein degradation in differentiated muscle.

Antisense oligonucleotides targeted against protein kinase c alpha inhibit proliferation of cultured avian myoblasts

Protein kinase C (PKC) has been implicated in the control of proliferation and differentiation of many cell types. There is evidence indicating that it plays a role in signal transduction mechanisms related to myogenesis, but little is known about the individual functions of PKC isoforms in muscle cell development. Data obtained in previous studies using cultured chick embryo skeletal muscle cells suggested that PKC alpha is linked to the regulation of myoblast proliferation. However, this causal relationship could not be definitively established as no experiments based on selective inhibition of this isoform were carried out. In the present work, specific inhibition of the expression of PKC alpha in cultured myoblasts by using antisense oligonucleotide technology resulted in a significant decrease of culture cell density and DNA synthesis, clearly showing that this isoenzyme is involved in signalling pathways which promote muscle cell proliferation.

Differential effects of Ras signaling through NFkappaB on skeletal myogenesis

Oncogenic Ras (H-Ras G12V) inhibits skeletal myogenesis through multiple signaling pathways. Previously, we demonstrated that the major downstream effectors of Ras (i.e., MEK/MAPK, RalGDS and Rac/Rho) play a minor, if any, role in the differentiation-defective phenotype of Ras myoblasts. Recently, NFkappaB, another Ras signaling target, has been shown to inhibit myogenesis presumably by stimulating cyclin D1 accumulation and cell cycle progression. In this study, we address the involvement of NFkappaB activation in the Ras-induced inhibition of myogenesis. Using H-Ras G12V and three G12V effector-loop variants, we detect high levels of NFkappaB transcriptional activity in C3H10T1/2-MyoD cells treated with differentiation medium. Myogenesis is blocked by all Ras proteins tested, yet only in the case of H-Ras G12V are cyclin D1 levels increased and cell cycle progression maintained. Expression of IkappaBalpha SR, an inhibitor of NFkappaB, does not reverse the differentiation-defective phenotype of Ras expressing cultures, but does induce differentiation in cultures treated with tumor necrosis factor (TNFalpha) or in cultures expressing the RelA/p65 subunit of NFkappaB. These data confirm that NFkappaB is a target of Ras and suggest that the cellular actions of NFkappaB require additional signals that are discriminated by the Ras effector-loop variants. Results with IkappaBalpha SR convincingly demonstrate that H-Ras G12V does not rely on NFkappaB activity to block myogenesis, an observation that continues to implicate another unidentified signaling pathway(s) in the inhibition of skeletal myogenesis by Ras.

Activated raf kinase inhibits muscle cell differentiation through a MEF2-dependent mechanism

Muscle cell development is dependent on the activity of cell type-specific basic-helix-loop-helix transcription factors, MyoD, Myf-5, myogenin, and MRF4 which collaborate with myocyte enhancer factor 2 proteins to activate muscle-specific gene expression. Growth factors and activated Ras prevent differentiation of myoblasts in culture but the downstream signalling pathways are not well understood. Here, we demonstrate that active Raf kinase (Raf-BxB) completely inhibits myogenic conversion of 10T1/2 cells mediated by Myf-5 and differentiation of L6 myoblasts as indicated by the absence of myotubes, lack of myogenin expression, and markedly reduced expression of myosin heavy chain. However, activated Raf inhibits transcriptional activation by Myf-5 only partially suggesting that other potential targets of Ras/Raf signalling may be involved. Significantly, we observed that elevated Raf kinase activity in L6 muscle cells suppresses the accumulation of MEF2 protein in nuclei, while MEF2 transcription appears unaffected. Moreover, forced expression of MEF2A in 10T1/2 cells rescues MyoD dependent myogenic conversion in the presence of constitutively active Raf kinase and partially restores transactivation of a myogenin promoter-dependent reporter gene in L6 muscle cells containing activated Raf kinase. From these observations we conclude that persistent activation of Raf signalling affects nuclear MEF2 functions which may explain why myogenin expression and myoblast differentiation are inhibited.

Expression of a PKR dominant-negative mutant in myogenic cells interferes with the myogenic process

In this study, we explored the possibility that PKR, a dsRNA-activated regulatory protein, is an essential component in the differentiation program of myogenic cells in vitro. For this purpose, we used a retroviral expression vector pMV7, harboring the PKR dominant-negative mutant PKRDelta6 (pMV7-p68Delta6). Murine C2C12 myogenic cells were transfected either with pMV7 or with pMV7-p68Delta6. Neomycin-resistant clones from both types were isolated and expanded and the results obtained with the representative clones C2-NEO (transfected with pMV7) and clone 17 and clone 22 (both transfected with pMV7-p68Delta6) are presented. In clone 17 and 22 cells, regardless of IFN treatment, a similar level of the transfected human p68 PKR mutant was detected. This protein was absent in C2-NEO cells. In parallel, in all types of cells, a low basal level of the endogenous murine p65 PKR protein was observed, which was further induced by IFN. However, PKR enzymatic activity was significantly induced by IFN only in C2-NEO cells, while it was hardly detected in both clones 17 and 22, even after IFN treatment. Furthermore, in contrast to C2-NEO cells, only a slight to moderate increase in enzymatic activity was observed in clone 17 and 22 differentiating cells. Next, cells were grown either in growth medium (GM) or differentiation medium (DM), and the progression of the myogenic program was studied. An inhibition in myotube formation in clone 17 versus C2-NEO cells cultivated in DM was clearly observed. Furthermore, while the growth rate and thymidine incorporation were reduced in C2-NEO cells grown in DM, both clone 17 and 22 cells were less affected under the same conditions. Similarly, a delay in the accumulation of the transcription factors MyoD and myogenin, as well as in creatine kinase activity and accumulation of troponin T, was detected in DM-cultivated clone 17 and clone 22 cells. Moreover, a delay in the induction of p21 (WAF1), in down-regulation of cyclin D1 and c-myc, and in the accumulation of the underphosphorylated form of pRb was also observed in clone 17 cells. We conclude that inhibition of endogenous PKR activity by a PKR dominant-negative mutant interferes with the myogenic program of murine C2C12 myogenic cells.

Activated Raf inhibits avian myogenesis through a MAPK-dependent mechanism

Chronic overexpression of the oncogenic form of Ras is a potent inhibitor of skeletal myogenesis. However, the intracellular signaling pathways that mediate the repressive actions of Ras on myogenic differentiation have yet to be identified. We examined the role of Raf-mediated signaling as a modulator of avian myogenesis. Raf overexpression elicited pronounced effects on both myoblasts and mature myocytes. Most notably, the embryonic chick myoblasts overexpressing a constitutively active form of Raf (RCAS-Raf CAAX or RCAS-Raf BXB) fail to form the large multinucleated myofibers characteristic of myogenic cultures. While residual myofibers were apparent in the RCAS-Raf BXB and RCAS-Raf CAAX infected cultures, these fibers had an atrophic phenotype. The altered morphology is not a result of reinitiation of the myonuclei cell cycle nor is it due to apoptosis. Furthermore, the mononucleated myoblasts misexpressing Raf BXB are differentiation-defective due to overt MAPK activity. Supplementation of the culture media with the MAPK kinase (MEK) inhibitor, PD98059, caused a reversal of the phenotype and allowed the formation of multinucleated myofibers at levels comparable to controls. Our results indicate that the Raf/MEK/MAPK axis is intact in chick myoblasts and that persistent activation of this signaling cascade is inhibitory to myogenesis.

An essential role of phosphatidylinositol 3-kinase in myogenic differentiation

The oncogene p3k, coding for a constitutively active form of phosphatidylinositol 3-kinase (PI 3-kinase; EC 2.7.1.137), strongly enhances myogenic differentiation in cultures of chicken-embryo myoblasts. It increases the size of the myotubes and induces elevated levels of the muscle-specific proteins MyoD, myosin heavy chain, creatine kinase, and desmin. Inhibition of PI 3-kinase activity with LY294002 or with dominant-negative mutants of PI 3-kinase interferes with myogenic differentiation and with the induction of muscle-specific genes. PI 3-kinase is therefore an upstream mediator for the expression of the muscle-specific genes and is both necessary and rate-limiting for the process of myogenesis.

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.

Neural influence on protein kinase C isoform expression in skeletal muscle

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC alpha and novel PKC theta, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta protein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the increase in nPKC theta expression that occurred during differentiation into myotubes resulted in levels of nPKC theta significantly below adult skeletal muscle. cPKC alpha expression in myoblastic increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes within neuroblastoma X glioma hybrid clonal cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC theta expression.

Neural influence on protein kinase C isoform expression in skeletal muscle

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC alpha and novel PKC theta, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta protein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the increase in nPKC theta expression that occurred during differentiation into myotubes resulted in levels of nPKC theta significantly below adult skeletal muscle. cPKC alpha expression in myoblastic increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes within neuroblastoma X glioma hybrid clonal cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC theta expression.

Tumor cell complementation groups based on myogenic potential: evidence for inactivation of loci required for basic helix-loop-helix protein activity

Basic helix-loop-helix (bHLH) proteins mediate terminal differentiation in many lineages. By using the bHLH protein MyoD, which can dominantly activate the myogenic differentiation program in numerous cell types, we demonstrated that recessive defects in bHLH protein function are present in human tumor lines. In contrast to prior work with primary cell cultures, MyoD did not activate the myogenic program in six of the eight tumor lines we tested. Cell fusions between the MyoD-defective lines and fibroblasts restored MyoD activity, indicating that the deficiency of a gene or factor prevents bHLH protein function in the tumor lines. Fusions between certain pairings of the MyoD-defective lines also restored MyoD activity, allowing the tumor lines to be assigned to complementation groups on the basis of their ability to execute the myogenic program and indicating that multiple mechanisms exist for abrogation of bHLH protein activity. These groups provide a basis for identifying genes critical for bHLH-mediated differentiation and tumor progression by using genetic complementation.

Activation of the epidermal growth factor receptor signal transduction pathway stimulates tyrosine phosphorylation of protein kinase C delta

The expression of an oncogenic rasHa gene in epidermal keratinocytes stimulates the tyrosine phosphorylation of protein kinase C delta and inhibits its enzymatic activity (Denning, M. F., Dlugosz, A. A., Howett, M. K., and Yuspa, S. H. (1993) J. Biol. Chem. 268, 26079-26081). Keratinocytes expressing an activated rasHa gene secrete transforming growth factor alpha (TGFalpha) and have an altered response to differentiation signals involving protein kinase C (PKC). Because the neoplastic phenotype of v-rasHa expressing keratinocytes can be partially mimicked in vitro by chronic treatment with TGF alpha and the G protein activator aluminum fluoride (AlF4-), we determined if TGF alpha or AlF4- could induce tyrosine phosphorylation of PKCdelta. Treatment of primary keratinocyte cultures for 4 days with TGFalpha induced tyrosine phosphorylation of PKCdelta, whereas AlF4- only slightly stimulated PKCdelta tyrosine phosphorylation. The PKCdelta that was tyrosine-phosphorylated in response to TGFalpha had reduced activity compared with the nontyrosine-phosphorylated PKCdelta. Treatment of keratinocytes expressing a normal epidermal growth factor receptor (EGFR) with TGFalpha or epidermal growth factor for 5 min induced PKCdelta tyrosine phosphorylation. This acute epidermal growth factor treatment did not induce tyrosine phosphorylation of PKCdelta in keratinocytes isolated from waved-2 mice that have a defective epidermal growth factor receptor. In addition, the level of PKCdelta tyrosine phosphorylation in v-rasHa-transduced keratinocytes from EGFR null mice was substantially lower than in v-rasHa transduced wild type cells, suggesting that activation of the EGFR is important for PKC delta tyrosine phosphorylation in ras transformation. However, purified EGFR did not phosphorylate recombinant PKC delta in vitro, whereas members of the Src family (c-Src, c-Fyn) and membrane preparations from keratinocytes did. Furthermore, clearing c-Src or c-Fyn from keratinocyte membrane lysates decreased PKCdelta tyrosine phosphorylation, and c-Src and c-Fyn isolated from keratinocytes treated with TGFalpha had increased kinase activity. Acute or chronic treatment with TGFalpha did not induce significant PKCdelta translocation in contrast to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, which induced both translocation and tyrosine phosphorylation of PKCdelta. This suggests that TGFalpha-induced tyrosine phosphorylation of PKC delta results from the activation of a tyrosine kinase rather than physical association of PKCdelta with a membrane-anchored tyrosine kinase. Taken together, these results indicate that PKCdelta activity is inhibited by tyrosine phosphorylation in response to EGFR-mediated signaling and activation of a member of the Src kinase family may be the proximal tyrosine kinase acting on PKCdelta in keratinocytes.

Both v-Ha-Ras and v-Raf stimulate expression of the vascular endothelial growth factor in NIH 3T3 cells

Stimulation of NIH 3T3 cells with platelet-derived growth factor (PDGF)-BB and 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances vascular endothelial growth factor (VEGF) gene expression. To address the question of whether Ras and Raf are involved in the induction of VEGF gene expression by PDGF and TPA, we examined the effects of both factors on NIH 3T3 cells stably transfected with v-Ha-ras or v-raf. In serum-starved NIH 3T3 cells, only low levels of mRNA expression can be detected, whereas both ras and raf transformed cell lines express enhanced levels of a 4.3-kilobase VEGF transcript. Stimulation with PDGF or TPA resulted in increased VEGF mRNA in all cell lines, with highest levels found in the transformed cells. Immunofluorescence studies confirmed that the elevated VEGF mRNA expression correlated with enhanced protein levels. Positive immunofluorescence signals could be detected in v-Ha-ras or v-raf transformed cell lines but not in unstimulated NIH 3T3 cells. VEGF from conditioned medium of v-raf transformed NIH 3T3 cells was partially purified by chromatography on heparin-Sepharose. Biological activity of this VEGF protein was demonstrated by competition with binding of recombinant 125I-VEGF165 to human umbilical vein endothelial cells and by its ability to stimulate proliferation of these cells.

Increased expression of a high molecular weight (130 KD) protein kinase C isoform in a differentiation-defective ras-transfected keratinocyte line

The role of ras on protein kinase C (PKC) signaling was examined in two keratinocyte cell lines. Increasing the level of extracellular calcium from 0.15 mM to 1.0 mM induces some features of differentiation in the spontaneously immortalized HaCaT line, but fails to do so in a c-H-ras-transfected subline (ras-HaCaT). Raising extracellular calcium also induced a transient increase in membrane-associated PKC activity 5 min after calcium addition, in HaCaT, but not in the ras-HaCaT cells. Partial purification of PKC from the membrane/particulate fraction revealed the major isoform expressed in HaCaT to be an 80 KD species recognized by the anti-PKC alpha antibody. In ras-HaCaT, the major expressed isoform is a 130 KD species recognized by the PKC beta antibody. The kinase activity of the partially purified high molecular weight PKC is phospholipid dependent but calcium independent. Further evaluation of PKC in the HaCaT and ras-HaCaT membrane/particulate cell fraction by immunoblotting using affinity-purified antibodies against PKC alpha, beta, delta, epsilon and zeta revealed a 130 KD band reacting with the PKC delta antibody. Increased expression of this high molecular weight protein was observed in ras-HaCaT. Immunoprecipitation of PKC in ras-HaCaT using the PKC delta antibody also revealed a 130 KD species. Analysis of the PKC delta immunoprecipitate demonstrated a phospholipid, but not calcium-dependent kinase which autophosphorylated. These results suggest that the 130 KD protein may be a novel (calcium-independent) PKC (nPKC) isoform and increased expression in the ras-transfected HaCaT may be a consequence of oncogenic ras expression. This 130 KD species may also play a role in the ras-associated inhibition of differentiation in HaCaT.

Rapid and long-term effects on protein kinase C on receptor tyrosine kinase phosphorylation and degradation

Rapid and long term effects of protein kinase C alpha activation on receptor tyrosine kinase signaling parameters were investigated in human 293 embryonic fibroblasts and mouse NIH 3T3 cells. Within minutes of phorbol 12-myristate 13-acetate treatment, epidermal growth factor receptor and HER2 tyrosine phosphorylation was decreased, while platelet-derived growth factor receptor and insulin receptor autophosphorylation was upregulated. These effects are not mediated by protein kinase C-dependent receptor tyrosine kinase phosphorylation but apparently by activation or inactivation of receptor tyrosine kinase-specific phosphatases, as indicated by neutralization of these phenomena upon treatment of cells with sodium orthovanadate. In contrast to these short term effects, sustained activation of protein kinase C alpha by phorbol 12-myristate 13-acetate results in translocation of protein kinase C from the cytosol to the membrane fraction where it forms stable complexes with all receptor tyrosine kinases investigated. Ligand-induced receptor tyrosine kinase/protein kinase C association in NIH 3T3 fibroblasts is accompanied by a mobility shift of the receptor, indicating phosphorylation by activated protein kinase C. This phenomenon correlates with the disappearance of receptor tyrosine kinases from the cell surface, implying that this interaction plays a role in the process of receptor internalization and degradation. Interestingly, ligand-stimulated receptor down-regulation is also enhanced by overexpression of phospholipase C gamma, which strongly indicates a role for this common receptor tyrosine kinase substrate in negative regulation of growth factor signals.

Influence of cell type on the steroidogenic potential and basal cyclic AMP levels of ras-oncogene-transformed rat cells

Transformation with ras oncogenes causes loss, maintenance or modulation of differentiation, depending on the developmental history of the target cells. In the present study, we examined steps in signal transduction that may underlie some of this variation, using steroidogenic cells of adult rats as the model system. Steroidogenesis in normal cells is regulated by cyclic AMP and protein kinase A (the cAMP/PKA pathway). We showed previously that transformation with v-Ki-ras induces constitutive progesterone secretion in ovarian and adrenocortical cells that are normally steroidogenic (ovarian granulosa and adrenal glomerulosa cells) and also in developmentally related cells that are normally nonsteroidogenic (ovarian surface epithelium and adrenal capsular fibroblasts), but not in unrelated nonsteroidogenic cells, such as muscle fascia fibroblasts and peritoneal mesothelium. In the present study, basal cAMP levels in all transformed ovarian and adrenal cell-lines were increased over basal levels in normal cells, and of transformed muscle fascia and mesothelial cell-lines. As in normal cells, transformation-induced steroidogenesis was stimulated by cAMP and was PKA dependent. A comparison of malignancy-related characteristics showed that transformed cells from nonsteroidogenic organs were more tumorigenic in vivo and less sensitive to growth inhibition by cAMP in vitro than transformed ovarian and adrenocortical cells. The results show that the abnormal, constitutive steroidogenesis induced by the viral form of the Kirsten ras oncogene (v-Ki-ras) in certain cell types is associated with tissue-specific increases in basal cAMP levels. Thus, although the ras oncogenes function primarily through other signal transduction pathways, transformation with ras oncogenes alters PKA-mediated signal transduction in a manner that is developmentally determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of tyrosine kinase in the regulation of myogenin expression

Using an affinity-purified anti-myogenin antibody, three stages of mouse myoblast C2C12 cells during myogenesis could be identified: proliferating myoblasts as myogenin-negative mononucleated cells, differentiating myoblasts as myogenin-positive mononucleated cells, and myotubes as myogenin-positive multinucleated cells. We found differential effects of genistein, an inhibitor of protein-tyrosine kinase, on myogenic cells during these three stages. Genistein severely inhibited myotube formation and myogenin production in differentiating myoblasts by inhibiting the transcription of the myogenin gene in a dose-dependent manner. We also found that genistein inactivated mitogen-activated protein kinase (MAP kinase) accompanied by suppression of myogenin expression. In contrast, genistein failed to inactivate MAP kinase and eliminate myogenin from myotubes. The results suggest that protein-tyrosine kinase plays a role in the transcriptional regulation of myogenin through the MAP kinase cascade during myogenesis. Furthermore, genistein inhibited the transactivation of the myosin heavy chain gene by constitutively expressed myogenin. Therefore, it is suggested that protein-tyrosine kinase is involved in the post-translational regulation of myogenin as well as in transcriptional regulation during myogenesis.

Phosphorylation of a proline-directed kinase motif is responsible for structural changes in myogenin

Myogenin, a member of the MyoD family which governs skeletal muscle differentiation, was identified as a pair of phosphorylated bands on SDS-PAGE during myogenesis. The slow migrating form was found to be hyperphosphorylated myogenin. In vitro phosphorylation by CDC2 kinase caused a prominent reduction in electrophoretic mobility of myogenin. Furthermore, we demonstrated that phosphorylation of the serine residue at position 43 contributes to the modification of myogenin in vivo and in vitro resulting in the reduction in electrophoretic mobility. We propose here that a CDC2-like proline-directed kinase regulates myogenin activity through its phosphorylation.

Sequential appearance of muscle-specific proteins in myoblasts as a function of time after cell division: evidence for a conserved myoblast differentiation program in skeletal muscle

Based on the assumption that a conserved differentiation program governs the assembly of sarcomeres in skeletal muscle in a manner analogous to programs for viral capsid assembly, we have defined the temporal and spatial distribution of 10 muscle-specific proteins in mononucleated myoblasts as a function of the time after terminal cell division. Single cells in mitosis were identified in monolayer cultures of embryonic chicken pectoralis, followed for selected time points (0-24 h postmitosis) by video time-lapse microscopy, and then fixed for immunofluorescence staining. For convenience, the myoblasts were termed x-h-old to define their age relative to their mitotic "birthdate." All 6 h myoblasts that emerged in a mitogen-rich medium were desmin+ but only 50% were positive for a alpha-actin, troponin-I, alpha-actinin, MyHC, zeugmatin, titin, or nebulin. By 15 h postmitosis, approximately 80% were positive for all of the above proteins. The up-regulation of these 7 myofibrillar proteins appears to be stochastic, in that many myoblasts were alpha-actinin+ or zeugmatin+ but MyHC- or titin- whereas others were troponin-I+ or MyHC+ but alpha-actinin- or alpha-actin-. In 15-h-old myoblasts, these contractile proteins were organized into nonstriated myofibrils (NSMFs). In contrast to striated myofibrils (SMFs), the NSMFs exhibited variable stoichiometries of the sarcomeric proteins and these were not organized into any consistent pattern. In this phase of maturation, two other changes occurred: (1) the microtubule network was reorganized into parallel bundles, driving the myoblasts into polarized, needle-shaped cells; and (2) the sarcolemma became fusion-competent. A transition from NSMFs to SMFs took place between 15 and 24 h (or later) postmitosis and was correlated with the late appearance of myomesin, and particularly, MyBP-C (C protein). The emergence of one, or a string of approximately 2 mu long sarcomeres, was invariably characterized by the localization of myomesin and MyBP-C to their mature positions in the developing A-bands. The latter group of A-band proteins may be rate-limiting in the assembly program. The great majority of myoblasts stained positively for desmin and myofibrillar proteins prior to, rather than after, fusing to form myotubes. This sequential appearance of muscle-specific proteins in vitro fully recapitulates myofibrillar assembly steps in myoblasts of the myotome and limb bud in vivo, as well as in nonmuscle cells converted to myoblasts by MyoD. We suggest that this cell-autonomous myoblast differentiation program may be blocked at different control points in immortalized myogenic cell lines.

Are fibroblast growth factors regulators of myogenesis in vivo?

Recent advances in understanding of skeletal muscle differentiation implicate fibroblast growth factors (FGFs) as regulators of myogenesis; however, the identity and actions of factors that repress myogenesis in vivo remain to be established. This review will focus on the fibroblast growth factor family and the evidence for its role in regulating myogenesis in culture and in vivo.

Fibroblast growth factor inhibits MRF4 activity independently of the phosphorylation status of a conserved threonine residue within the DNA-binding domain

MRF4 is a member of the muscle-specific basic helix-loop-helix transcription factor family that also includes MyoD, myogenin, and Myf-5. Each of these proteins, when overexpressed in fibroblasts, converts the cells to differentiated muscle fibers that express several skeletal muscle genes, such as those for alpha-actin, muscle creatine kinase, and troponin I. Despite the fact that MRF4 functions as a positive transcriptional regulator, the MRF4 protein is subject to negative regulation by a variety of agents, most notably by exposure of cells to purified growth factors, such as basic fibroblast growth factor (bFGF). In an effort to establish whether bFGF inhibits MRF4 activity through specific posttranslational modifications, we examined whether MRF4 exists in vivo as a phosphoprotein and whether the phosphorylation status of the protein regulates its activity. Our results indicate that MRF4 is phosphorylated predominantly on serine residues, with weak phosphorylation occurring on threonine residues. Both cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC) phosphorylate MRF4 in vitro as well as in vivo, and the overexpression of each kinase inhibits MRF4 activity and thus blocks terminal differentiation. PKC-directed phosphorylation of a conserved threonine residue (T-99) situated within the DNA-binding domain inhibits MRF4 from binding in vitro to specific DNA targets. However, although T-99 itself is essential for myogenic activity, our studies demonstrate that the phosphorylation status of T-99 does not play a major role in regulating MRF4 activity in vivo, since PKA, PKC, and bFGF inhibit the activity of MRF4 proteins in which the identified PKA and PKC sites have been mutated. We suggest that the negative regulation of MRF4 imposed by bFGF does not involve a direct modification of the protein at the identified PKA and PKC sites but instead may involve the modification of specific coregulators that interact with this muscle regulatory factor.

Fibroblast growth factor inhibits MRF4 activity independently of the phosphorylation status of a conserved threonine residue within the DNA-binding domain

MRF4 is a member of the muscle-specific basic helix-loop-helix transcription factor family that also includes MyoD, myogenin, and Myf-5. Each of these proteins, when overexpressed in fibroblasts, converts the cells to differentiated muscle fibers that express several skeletal muscle genes, such as those for alpha-actin, muscle creatine kinase, and troponin I. Despite the fact that MRF4 functions as a positive transcriptional regulator, the MRF4 protein is subject to negative regulation by a variety of agents, most notably by exposure of cells to purified growth factors, such as basic fibroblast growth factor (bFGF). In an effort to establish whether bFGF inhibits MRF4 activity through specific posttranslational modifications, we examined whether MRF4 exists in vivo as a phosphoprotein and whether the phosphorylation status of the protein regulates its activity. Our results indicate that MRF4 is phosphorylated predominantly on serine residues, with weak phosphorylation occurring on threonine residues. Both cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC) phosphorylate MRF4 in vitro as well as in vivo, and the overexpression of each kinase inhibits MRF4 activity and thus blocks terminal differentiation. PKC-directed phosphorylation of a conserved threonine residue (T-99) situated within the DNA-binding domain inhibits MRF4 from binding in vitro to specific DNA targets. However, although T-99 itself is essential for myogenic activity, our studies demonstrate that the phosphorylation status of T-99 does not play a major role in regulating MRF4 activity in vivo, since PKA, PKC, and bFGF inhibit the activity of MRF4 proteins in which the identified PKA and PKC sites have been mutated. We suggest that the negative regulation of MRF4 imposed by bFGF does not involve a direct modification of the protein at the identified PKA and PKC sites but instead may involve the modification of specific coregulators that interact with this muscle regulatory factor.

Fibroblast contraction of collagen gels requires activation of protein kinase C

Fibroblasts stimulated to contract collagen gels with serum were completely inhibited by staurosporine, a broad spectrum kinase inhibitor. Further analysis demonstrated that staurosporine is potent (IC50 17 nM), rapid in onset, and completely reversible. Complete inhibition of gel contraction was also observed with calphostin C (IC50 48 nM), an inhibitor specific for protein kinase C (PKC). Similar effects were not observed with KT5926 or KT5720, inhibitors for myosin light chain kinase and cAMP-dependent kinases, respectively. These data suggested that serum-stimulated fibroblast contraction is dependent upon activation of PKC. This was also observed with fibroblast contraction stimulated with endothelin-1, platelet-derived growth factor, and transforming growth factor beta. PKC activated directly with low concentrations of phorbol ester was observed to stimulate fibroblast contraction of collagen gels, in some cases within 30 minutes of exposure.

FGF inactivates myogenic helix-loop-helix proteins through phosphorylation of a conserved protein kinase C site in their DNA-binding domains

Myogenin belongs to a family of myogenic helix-loop-helix (HLH) proteins that activate muscle transcription through binding to a conserved DNA sequence associated with numerous muscle-specific genes. Fibroblast growth factor (FGF) inhibits myogenesis by inactivating myogenic HLH proteins. We show that activated protein kinase C (PKC) can substitute for FGF and inhibit transcriptional activity of myogenic HLH proteins. In transfected cells, FGF induces phosphorylation of a conserved site in the DNA-binding domain of myogenin. This site is phosphorylated by PKC in vivo and in vitro and mediates repression of the myogenic program through a loss in DNA binding activity. A myogenin mutant lacking the PKC phosphorylation site is not repressed by FGF, confirming this site as a molecular target for FGF-dependent repression of muscle transcription. These results establish a direct link between the signal transduction pathways that inhibit myogenesis and the transcription factors directly activating muscle-specific genes.

Interplay between proliferation and differentiation within the myogenic lineage

In muscle cells, as in a variety of cell types, proliferation and differentiation are mutually exclusive events controlled by a balance of opposing cellular signals. Members of the MyoD family of muscle-specific helix-loop-helix proteins which, in collaboration with ubiquitous factors, activate muscle differentiation and inhibit cell proliferation function at the nexus of the cellular circuits that control proliferation and differentiation of muscle cells. The activities of these myogenic regulators are negatively regulated by peptide growth factors and activated oncogenes whose products transmit growth signals from the membrane to the nucleus. Recent studies have revealed multiple mechanisms through which intracellular growth factor signals may interfere with the functions of the myogenic regulators. When expressed at high levels, members of the MyoD family can override mitogenic signals and can cause growth arrest independent of their effects on differentiation. The ability of these myogenic regulators to inhibit proliferation of normal as well as transformed cells from multiple lineages suggests that they interact with conserved components of the cellular machinery involved in cell cycle progression and that similar types of regulatory factors participate in differentiation and cell cycle control in diverse cell types.

A new member of the protein kinase C family, nPKC theta, predominantly expressed in skeletal muscle

A new protein kinase C (PKC)-related cDNA with unique tissue distribution has been isolated and characterized. This cDNA encodes a protein, nPKC theta, which consists of 707 amino acid residues and showed the highest sequence similarity to nPKC delta (67.0% in total). nPKC theta has a zinc-finger-like cysteine-rich sequence (C1 region) and a protein kinase domain sequence (C3 region), both of which are common in all PKC family members. However, nPKC theta lacks a putative Ca2+ binding region (C2 region) that is seen only in the conventional PKC subfamily (cPKC alpha, -beta I, -beta II, and -gamma) but not in the novel PKC subfamily (nPKC delta, -epsilon, -zeta, and -eta). Northern (RNA) blot analyses revealed that the mRNA for nPKC theta is expressed predominantly in skeletal muscle. Furthermore, nPKC theta mRNA is the most abundantly expressed PKC isoform in skeletal muscle among the nine PKC family members. nPKC theta expressed in COS1 cells serves as a phorbol ester receptor. By the use of an antipeptide antibody specific to the D2-D3 region of the nPKC theta sequence, nPKC theta was recognized as a 79-kDa protein upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis in mouse skeletal muscle extract and also in an extract from COS1 cells transfected with an nPKC theta cDNA expression plasmid. Autophosphorylation of immunoprecipitated nPKC theta was observed; it was enhanced by phosphatidylserine and 12-O-tetradecanoylphorbol-13-acetate but attenuated by the addition of Ca2+. These results clearly demonstrate that nPKC theta should be considered a member of the PKC family of proteins that play crucial roles in the signal transduction pathway.

A new member of the protein kinase C family, nPKC theta, predominantly expressed in skeletal muscle

A new protein kinase C (PKC)-related cDNA with unique tissue distribution has been isolated and characterized. This cDNA encodes a protein, nPKC theta, which consists of 707 amino acid residues and showed the highest sequence similarity to nPKC delta (67.0% in total). nPKC theta has a zinc-finger-like cysteine-rich sequence (C1 region) and a protein kinase domain sequence (C3 region), both of which are common in all PKC family members. However, nPKC theta lacks a putative Ca2+ binding region (C2 region) that is seen only in the conventional PKC subfamily (cPKC alpha, -beta I, -beta II, and -gamma) but not in the novel PKC subfamily (nPKC delta, -epsilon, -zeta, and -eta). Northern (RNA) blot analyses revealed that the mRNA for nPKC theta is expressed predominantly in skeletal muscle. Furthermore, nPKC theta mRNA is the most abundantly expressed PKC isoform in skeletal muscle among the nine PKC family members. nPKC theta expressed in COS1 cells serves as a phorbol ester receptor. By the use of an antipeptide antibody specific to the D2-D3 region of the nPKC theta sequence, nPKC theta was recognized as a 79-kDa protein upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis in mouse skeletal muscle extract and also in an extract from COS1 cells transfected with an nPKC theta cDNA expression plasmid. Autophosphorylation of immunoprecipitated nPKC theta was observed; it was enhanced by phosphatidylserine and 12-O-tetradecanoylphorbol-13-acetate but attenuated by the addition of Ca2+. These results clearly demonstrate that nPKC theta should be considered a member of the PKC family of proteins that play crucial roles in the signal transduction pathway.

Regulation of tetradecanoyl phorbol acetate-induced responses in NIH 3T3 cells by GAP, the GTPase-activating protein associated with p21c-ras

Proteins of the ras family of oncogenes have been implicated in signal transduction pathways initiated by protein kinase C (PKC) and by tyrosine kinase oncogenes and receptors, but the role that ras plays in these diverse signalling systems is poorly defined. The activity of ras proteins has been shown to be controlled in part by a cellular protein, GAP (GTPase-activating protein), that negatively regulates p21c-ras by enhancing its intrinsic GTPase activity. Thus, overexpression of GAP provides a tool for determining the step(s) in signal transduction dependent on p21c-ras activity. In this paper, we report that overexpression of GAP blocks the phorbol ester (tetradecanoyl phorbol acetate [TPA])-induced activation of p42 mitogen-activated protein kinase (p42mapk), c-fos expression, and DNA synthesis. GAP overexpression did not block responses to serum or fluoroaluminate. Moreover, not all biochemical events elicited by TPA were affected by GAP overexpression, as increased glucose uptake and phosphorylation of MARCKS, a major PKC substrate, occurred normally. Reduction of GAP expression to near normal levels restored the ability of the cells to activate p42mapk in response to TPA. These findings suggest that ras and GAP together play a key role in a PKC-dependent signal transduction pathway which leads to p42mapk activation and cell proliferation.

Fos and Jun repress transcriptional activation by myogenin and MyoD: the amino terminus of Jun can mediate repression

Myogenin and MyoD belong to a family of muscle-specific helix-loop-helix (HLH) proteins that have the potential to activate muscle-specific genes in nonmyogenic cells. Peptide growth factors can block the ability of myogenin and MyoD to activate their target genes. Here, we show that the growth factor-inducible proto-oncogenes c-fos, c-jun, and junB mimic the effects of exogenous growth factors and suppress trans-activation of the muscle creatine kinase (MCK) enhancer by myogenin and MyoD. In contrast, JunD, which shares DNA-binding specificity with JunB and c-Jun but is expressed constitutively in muscle cells, is an inefficient inhibitor of the trans-activating capacity of myogenin and MyoD. Transcriptional repression by Fos and Jun is specific to myogenic HLH proteins and is not observed with the widely expressed HLH protein E47, which recognizes the same DNA sequence. Repression of the MCK enhancer by Fos and Jun is targeted at the myogenin and MyoD DNA recognition sequence and can be mediated by the amino terminus of c-Jun. Comparison of several myogenin mutants for their responsiveness to Fos and Jun shows that repression is directed at the basic-HLH region. These results indicate that members of the Jun family can be distinguished on the basis of their effects on muscle-specific transcription and suggest there is cross talk between transcription factors that control myogenesis and those involved in cell proliferation.

Regulation of tetradecanoyl phorbol acetate-induced responses in NIH 3T3 cells by GAP, the GTPase-activating protein associated with p21c-ras

Proteins of the ras family of oncogenes have been implicated in signal transduction pathways initiated by protein kinase C (PKC) and by tyrosine kinase oncogenes and receptors, but the role that ras plays in these diverse signalling systems is poorly defined. The activity of ras proteins has been shown to be controlled in part by a cellular protein, GAP (GTPase-activating protein), that negatively regulates p21c-ras by enhancing its intrinsic GTPase activity. Thus, overexpression of GAP provides a tool for determining the step(s) in signal transduction dependent on p21c-ras activity. In this paper, we report that overexpression of GAP blocks the phorbol ester (tetradecanoyl phorbol acetate [TPA])-induced activation of p42 mitogen-activated protein kinase (p42mapk), c-fos expression, and DNA synthesis. GAP overexpression did not block responses to serum or fluoroaluminate. Moreover, not all biochemical events elicited by TPA were affected by GAP overexpression, as increased glucose uptake and phosphorylation of MARCKS, a major PKC substrate, occurred normally. Reduction of GAP expression to near normal levels restored the ability of the cells to activate p42mapk in response to TPA. These findings suggest that ras and GAP together play a key role in a PKC-dependent signal transduction pathway which leads to p42mapk activation and cell proliferation.