Regulation of differentiation of the BC3H1 muscle cell line through cAMP-dependent and -independent pathways

Advances and Challenges in Targeting TGF-β Isoforms for Therapeutic Intervention of Cancer: A Mechanism-Based Perspective

The TGF-β family is a group of 25 kDa secretory cytokines, in mammals consisting of three dimeric isoforms (TGF-βs 1, 2, and 3), each encoded on a separate gene with unique regulatory elements. Each isoform plays unique, diverse, and pivotal roles in cell growth, survival, immune response, and differentiation. However, many researchers in the TGF-β field often mistakenly assume a uniform functionality among all three isoforms. Although TGF-βs are essential for normal development and many cellular and physiological processes, their dysregulated expression contributes significantly to various diseases. Notably, they drive conditions like fibrosis and tumor metastasis/progression. To counter these pathologies, extensive efforts have been directed towards targeting TGF-βs, resulting in the development of a range of TGF-β inhibitors. Despite some clinical success, these agents have yet to reach their full potential in the treatment of cancers. A significant challenge rests in effectively targeting TGF-βs' pathological functions while preserving their physiological roles. Many existing approaches collectively target all three isoforms, failing to target just the specific deregulated ones. Additionally, most strategies tackle the entire TGF-β signaling pathway instead of focusing on disease-specific components or preferentially targeting tumors. This review gives a unique historical overview of the TGF-β field often missed in other reviews and provides a current landscape of TGF-β research, emphasizing isoform-specific functions and disease implications. The review then delves into ongoing therapeutic strategies in cancer, stressing the need for more tools that target specific isoforms and disease-related pathway components, advocating mechanism-based and refined approaches to enhance the effectiveness of TGF-β-targeted cancer therapies.

Regulation of SIK1 abundance and stability is critical for myogenesis

cAMP signaling can both promote and inhibit myogenic differentiation, but little is known about the mechanisms mediating promyogenic effects of cAMP. We previously demonstrated that the cAMP response element-binding protein (CREB) transcriptional target salt-inducible kinase 1 (SIK1) promotes MEF2 activity in myocytes via phosphorylation of class II histone deacetylase proteins (HDACs). However, it was unknown whether SIK1 couples cAMP signaling to the HDAC-MEF2 pathway during myogenesis and how this response could specifically occur in differentiating muscle cells. To address these questions, we explored SIK1 regulation and function in muscle precursor cells before and during myogenic differentiation. We found that in primary myogenic progenitor cells exposed to cAMP-inducing agents, Sik1 transcription is induced, but the protein is rapidly degraded by the proteasome. By contrast, sustained cAMP signaling extends the half-life of SIK1 in part by phosphorylation of Thr475, a previously uncharacterized site that we show can be phosphorylated by PKA in cell-free assays. We also identified a functional PEST domain near Thr475 that contributes to SIK1 degradation. During differentiation of primary myogenic progenitor cells, when PKA activity has been shown to increase, we observe elevated Sik1 transcripts as well as marked accumulation and stabilization of SIK1 protein. Depletion of Sik1 in primary muscle precursor cells profoundly impairs MEF2 protein accumulation and myogenic differentiation. Our findings support an emerging model in which SIK1 integrates cAMP signaling with the myogenic program to support appropriate timing of differentiation.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Localized cyclic AMP-dependent protein kinase activity is required for myogenic cell fusion

Multinucleated myotubes are formed by fusion of mononucleated myogenic progenitor cells (myoblasts) during terminal skeletal muscle differentiation. In addition, myoblasts fuse with myotubes, but terminally differentiated myotubes have not been shown to fuse with each other. We show here that an adenylate cyclase activator, forskolin, and other reagents that elevate intracellular cyclic AMP (cAMP) levels induced cell fusion between small bipolar myotubes in vitro. Then an extra-large myotube, designated a "myosheet," was produced by both primary and established mouse myogenic cells. Myotube-to-myotube fusion always occurred between the leading edge of lamellipodia at the polar end of one myotube and the lateral plasma membrane of the other. Forskolin enhanced the formation of lamellipodia where cAMP-dependent protein kinase (PKA) was accumulated. Blocking enzymatic activity or anchoring of PKA suppressed forskolin-enhanced lamellipodium formation and prevented fusion of multinucleated myotubes. Localized PKA activity was also required for fusion of mononucleated myoblasts. The present results suggest that localized PKA plays a pivotal role in the early steps of myogenic cell fusion, such as cell-to-cell contact/recognition through lamellipodium formation. Furthermore, the localized cAMP-PKA pathway might be involved in the specification of the fusion-competent areas of the plasma membrane in lamellipodia of myogenic cells.

Functions and regulation of transforming growth factor-beta (TGF-β) in the prostate

The prostate is a highly androgen-dependent tissue that in humans exhibits marked susceptibility to carcinogenesis. The malignant epithelium generated from this tissue ultimately loses dependence on androgens despite retention or amplification of the androgen receptor. Accumulating evidence support that transforming growth factor-beta (TGF-beta) plays key roles in the control of androgen dependence and acquisition of resistance to such hormonal control. Although TGF-beta functions as a key tumour suppressor of the prostate, it can also promote malignant progression and metastasis of the advanced disease, through undefined mechanisms. In addition to giving an overview of the TGF-beta field as related to its function in prostate cancer, this Review focuses on novel findings that support the tumour suppressor function of TGF-beta is lost or altered by changes in the activity of the androgen receptor, insulin-like growth factor-I, Akt, and mTOR during malignant progression. Understanding the mechanisms of cross-talk between TGF-beta and such growth modulators has important implications for the rational therapeutics of prostate cancer.

Differentiation of H9c2 cardiomyoblasts: The role of adenylate cyclase system

The adenylate cyclase (AC)/cAMP/cAMP-dependent protein kinase pathway controls many biological phenomena. The molecular mechanisms by which cAMP induces alternative commitment towards differentiation or proliferation are not still completely known. The differentiation of myoblast cell lines into myocytes/myotubes represents a well-established model of skeletal muscle differentiation. We analyzed the AC/cAMP pathway during terminal differentiation of H9c2 myoblasts. When cultured in low-serum containing medium, H9c2 myoblasts exit the cell cycle and differentiate into myocytes/myotubes. A key step of this process is the expression of myogenin, an essential transcription factor for the terminal differentiation into myocytes. During this phenomenon we observed a decrease in both cAMP levels and AC activity, which suggests a functional negative role of cAMP on the differentiation process of H9c2 cells. 8-Br-cAMP and other cAMP-elevating agents, such as forskolin, IBMX, and isoproterenol, negatively affected skeletal muscle differentiation of H9c2 myoblasts. Both AC activity down-regulation and intracellular cAMP reduction were accompanied by significant variations in the levels of membrane proteins belonging to the AC system (AC catalytic subunit, G(alphai-1), G(alphas)). The functional relationship between intracellular cAMP content and protein levels of AC system is discussed.

A bimodal modulation of the cAMP pathway is involved in the control of myogenic differentiation in l6 cells

We have previously shown that myogenesis induction by Arg8-vasopressin (AVP) in L6 rat myoblasts involves a sustained stimulation of type 4 cAMP-phosphodiesterase. In this model, we observed that a transient cAMP generation occurs in the minutes following AVP addition. Evidence suggests that cAMP generation is due to the prostaglandins produced in response to AVP binding to V1a receptors and subsequent activation of phospholipase A2. The early cAMP increase was effective in activating cAMP-dependent protein kinase (PKA) and increasing phosphorylation of CREB transcription factor. Inhibition of PKA by compound H89 prior to AVP addition led to a significant reduction of expression of the differentiation marker creatine kinase, whereas H89 added 1-5 h after AVP had no significant effect. Furthermore, PKA inhibition 24 h after the beginning of AVP treatment potentiated differentiation. This shows that both an early activation and a later down-regulation of the cAMP pathway are required for AVP induction of myogenesis. Because phosphodiesterase PDE4D3 overexpressed in L6 cells lost its ability to potentiate AVP-induced differentiation when mutated and rendered insensitive to PKA phosphorylation and activation, we hypothesize that the early cAMP increase is required to trigger the down-regulation of cAMP pathway through stimulation of phosphodiesterase.

Astrocyte differentiation states and glioma formation

Gliomas are the most common primary malignancy in human central nervous system. Many similarities in cell morphology and expression of markers exist between cancerous cells and normal undifferentiated progenitor cells. At the molecular level, many important gene products are causally implicated in both the glial differentiation process and glial neoplasm formation. These observations raise the question of to what degree cell differentiation state influences glioma formation. In this review, we discuss new insights into the parallels between glial differentiation and glioma formation as well as the potential application of differentiation-inducing therapy.

A cyclic AMP-dependent pathway regulates the expression of acetylcholinesterase during myogenic differentiation of C2C12 cells

The expression of acetylcholinesterase (AChE) is markedly increased during myogenic differentiation of C2C12 myoblasts to myotubes; the expression is mediated by intrinsic factor(s) during muscle differentiation. In order to analyze the molecular mechanisms regulating AChE expression during myogenic differentiation, a approximately 2.2-kb human AChE promoter tagged with a luciferase reporter gene, namely pAChE-Luc, was stably transfected into C2C12 cells. The profile of promoter-driven luciferase activity during myogenic differentiation of C2C12 myotubes was found to be similar to that of endogenous expression of AChE catalytic subunit. The increase of AChE expression was reciprocally regulated by a cAMP-dependent signaling pathway. The level of intracellular cAMP, the activity of cAMP-dependent protein kinase, the phosphorylation of cAMP-responsive element binding protein and the activity of cAMP- responsive element (CRE) were down-regulated during the myotube formation. Mutating the CRE site of human AChE promoter altered the original myogenic profile of the promoter activity and its suppressive response to cAMP. In addition, the suppressive effect of the CRE site is dependent on its location on the promoter. Therefore, our results suggest that a cAMP-dependent signaling pathway serves as a suppressive element in regulating the expression of AChE during early myogenesis.

Involvement of type 4 cAMP-phosphodiesterase in the myogenic differentiation of L6 cells

Myogenic cell differentiation is induced by Arg(8)-vasopressin, whereas high cAMP levels and protein kinase A (PKA) activity inhibit myogenesis. We investigated the role of type 4 phosphodiesterase (PDE4) during L6-C5 myoblast differentiation. Selective PDE4 inhibition resulted in suppression of differentiation induced by vasopressin. PDE4 inhibition prevented vasopressin-induced nuclear translocation of the muscle-specific transcription factor myogenin without affecting its overall expression level. The effects of PDE4 inhibition could be attributed to an increase of cAMP levels and PKA activity. RNase protection, reverse transcriptase PCR, immunoprecipitation, Western blot, and enzyme activity assays demonstrated that the PDE4D3 isoform is the major PDE4 expressed in L6-C5 myoblasts and myotubes, accounting for 75% of total cAMP-hydrolyzing activity. Vasopressin cell stimulation caused a biphasic increase of PDE4 activity, which peaked at 2 and 15 min and remained elevated for 48 h. In the continuous presence of vasopressin, cAMP levels and PKA activity were lowered. PDE4D3 overexpression increased spontaneous and vasopressin-dependent differentiation of L6-C5 cells. These results show that PDE4D3 plays a key role in the control of cAMP levels and differentiation of L6-C5 cells. Through the modulation of PDE4 activity, vasopressin inhibits the cAMP signal transduction pathway, which regulates myogenesis possibly by controlling the subcellular localization of myogenin.

Immunocytochemical expression of human muscle cell p75 neurotrophin receptor is down-regulated by cyclic adenosine 3',5'-monophosphate

To investigate whether the immunocytochemical expression of low affinity neurotrophin receptor (p75) in human muscle is modulated by increased levels of intracellular cyclic adenosine 3',5'-monophosphate (cAMP), human cultured myogenic cells were treated with cAMP analogues dibutyryl cAMP (dbcAMP 0.5-1 mM) and 8-bromo cAMP (1 mM) or the adenylate cyclase activator forskolin (10-100 microM). Cultures were processed for indirect immunofluorescence microscopy using an anti-human p75 mAb. The treatment of cultured muscle cells with cAMP analogues or forskolin for two days induced a decrease of immunoreactivity for p75 and a reduction of both myotube formation and morphological cell differentiation. Removal of cAMP derivatives from the medium resulted in a return of immunoreactive cells to the levels of untreated controls. These data indicate that adenylate cyclase is involved in the regulation of human muscle p75.

bFGF induces BCK promoter-driven expression in muscle via increased binding of a nuclear protein

Changes in gene expression occurring during skeletal muscle differentiation are exemplified by downregulation of brain creatine kinase (BCK) and induction of muscle creatine kinase (MCK). Although both are transcriptionally regulated, there appears to be no transcription factor-element overlap, suggesting that their coordinate expression results from culture medium-related influences. Basic fibroblast growth factor (bFGF) prevents myogenesis and represses MCK expression by inhibiting transcriptional activation. It was hypothesized that bFGF similarly influenced BCK by inducing its expression. Accordingly, BCK promoter constructs were transiently transfected into C2C12 cells and, after a switch to differentiation medium, were treated with bFGF, bFGF plus herbimycin, adenosine 3',5'-cyclic monophosphate (cAMP), or phorbol 12-myristate 13-acetate (PMA). Analyses demonstrated that bFGF responsiveness was contained within a 33-base pair element. Electromobility shift assays showed that bFGF induction increased the abundance of the nuclear factor binding the element. Both effects were prevented by herbimycin. Neither cAMP nor PMA specifically induced the construct containing the bFGF-responsive element. The induced factor required phosphorylation to bind, implying that bFGF-mediated increases in binding may be due to transcription factor phosphorylation.

Defective myogenesis in NFB-s mutant associated with a saturable suppression of MYF5 activity

Myogenic cell lines have proved to be useful tools for investigating the molecular mechanisms that control cellular differentiation. NFB-s is a mutant myogenic cell line which fails to differentiate in vitro, and can repress differentiation in normal myogenic cells when fused to form heterokaryons. The NFB-s cell line was used here to study the molecular mechanisms underlying such myogenic repression. Using muscle-specific reporter genes, we show that NFB-s cells fail to activate fully the muscle differentiation program at a transcriptional level, although muscle-specific transcription can be enhanced by regulators of differentiation such as pertussis toxin. Paradoxically we find that the myogenic regulator myf5 is expressed at constitutively high levels in NFB-s cells, and retains DNA binding activity. Expression plasmids encoding NFB-derived myf5 cDNA can rescue the myogenic phenotype in NFB-s cells, demonstrating that a threshold level of positive regulators must be reached before the myogenic program is activated. Thus, the dominant negative phenotype does not appear to result from defective myf5, but is due to a dosage-dependent saturable mechanism that interferes with myf5 function. These studies demonstrate that the stoichiometric ratio of positive and negative regulators is critical for determining the myogenic differentiation state.

The kinase inhibitor iso-H7 stimulates rat satellite cell differentiation through a non-protein kinase C pathway by increasing myogenin expression level

We analysed the signaling pathways involved in myogenic differentiation of primary cultures of rat satellite cells using substances targeting the protein kinase C (PKC) and the cAMP protein kinase (PKA) pathways. We have previously shown that iso-H7, which putatively inhibits both PKC and PKA, strongly stimulates satellite cell differentiation, as well as the PKA inhibitor HA1004. In the study reported here, the effects of iso-H7 on satellite cell differentiation were compared to those observed in the presence of agents which reduce PKC activity. It was shown that treatments with the highly specific PKC inhibitor GF109203X or with 12-O-tetradecanoylphorbol 13-acetate (TPA) which induced a partial PKC downregulation, did not significantly alter myogenic differentiation. Northern blot analyses showed that iso-H7 activated the expression of myogenin but not that of MyoD mRNA. Concurrently, iso-H7 increased myosin light-chain mRNA expression. In contrast, TPA had no effect on these syntheses. Taken together, these results showed that iso-H7 did not act intracellularly as a PKC inhibitor but rather as a PKA inhibitor as previously suggested. Our results are compatible with the hypothesis that a reduction in PKA activity controls satellite cell myogenesis through an increased myogenin mRNA expression.

Non-histone protein 1 (NHP1) is a member of the Ku protein family which is upregulated in differentiating mouse myoblasts and human promyelocytes

We have previously purified and characterized a ubiquitous non-histone protein (NHP1) which has a high affinity (Kd 10(-11) M) for different avian vitellogenin gene sequences containing CpGs (Hughes et al. (1989) Biochemistry 28, 9137-9142; Hughes and Jost (1989) Nucleic Acids Res. 17, 8511-8520). Here we show by microsequencing that the peptides derived from the purified p75 and p85 subunits of NHP1 from HeLa cells have between 64 and 100% identity with the human Ku autoantigen. During the differentiation of human HL-60 promyelocytes there is an increase in the amount of p85 subunit protein whereas the level of the p75 subunit is unchanged. In differentiating mouse G8 myoblasts there is, however, an upregulation of both the p75 and p85 subunits and of the p85 mRNA. An inhibition of mouse myoblast differentiation by either cAMP, 3-aminobenzamide or sodium butyrate abolishes the upregulation of the p85 subunit. In G8 myoblasts chemical, or physical stress by UV light or X-rays does not upregulate the level of the p85 subunit. The possible involvement of NHP1 in the active demethylation of bifilarly methylated DNA will be discussed.

Differential regulation of primary response gene expression in skeletal muscle cells through multiple signal transduction pathways

One of the earliest cellular responses to growth factors is the rapid induction of primary response genes. One group of such genes was originally isolated as tetradecanoyl phorbol acetate (TPA) inducible sequences (TIS genes) from mouse 3T3 cells. Proteins encoded by the TIS genes include two transcription factors: TIS8 (also known as egr1/NGFIA/zif268) and TIS1 (also known as NGFIB/nur77/N10). We have examined the inducibility of these two genes in a skeletal muscle cell line in response to agents that have been reported to block muscle differentiation. We report here that basic fibroblast growth factor (bFGF) induced the expression of both TIS1 and TIS8 in mouse C2C12cells. Both genes were also inducible by TPA while forskolin which activates the cAMP-dependent pathway induced TIS1 but not TIS8. Down-regulation of protein kinase C (PKC) activity by TPA pretreatment repressed the bFGF induction of TIS1 but had little effect on the bFGF-stimulated expression of TIS8. Moreover, while both TPA and bFGF stimulated the hyperphosphorylation of c-RAF and the activity of MAP kinase, TPA pretreatment failed to block RAF phosphorylation or the stimulation of MAP kinase activity by bFGF. Induction of the two TIS genes in skeletal myoblasts therefore appeared to be dependent to different extents on the activation of protein kinase A (PKA), PKC and MAP kinase.

Regulation of differentiation and protein kinase C expression in 3T3 T proadipocytes: effects of TGF-beta and transformation

We are studying the mechanisms that regulate proliferation and differentiation of normal 3T3 T proadipocytes and neoplastically transformed clones which have lost the ability to differentiate. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and transforming growth factor beta (TGF-beta) are known inhibitors of the same step of the differentiation process in 3T3 T cells. Here, we examined the expression of the phorbol ester receptor/protein kinase C (PKC) during adipocytic differentiation of 3T3 T cells and its modulation by the differentiation inhibitor TGF-beta. PKC receptor assays were performed using a tritiated analogue of TPA and it was found that PKC receptor levels decreased approximately threefold during differentiation. Northern blot analyses revealed an even greater decrease of PKC transcripts during differentiation. TGF-beta inhibited not only differentiation, but the differentiation-dependent decrease in PKC levels as well. Transformed 3T3 T cells which have lost the ability to differentiate were found to express aberrant levels of PKC. The data suggest that TGF-beta may inhibit differentiation via a PKC-dependent pathway and that disruption of normal PKC levels or its regulation may be involved in the loss of differentiation control in transformed 3T3 T cells.

Nuclear import of the myogenic factor MyoD requires cAMP-dependent protein kinase activity but not the direct phosphorylation of MyoD

MyoD is a nuclear phosphoprotein that belongs to the family of myogenic regulatory factors and acts in the transcriptional activation of muscle-specific genes. We have investigated the role of cAMP-dependent protein kinase (A-kinase) in modulating the nuclear locale of MyoD. Purified MyoD protein microinjected into the cytoplasm of rat embryo fibroblasts is rapidly translocated into the nucleus. Inhibition of A-kinase activity through injection of the specific inhibitory peptide PKI prevents this nuclear localisation. This inhibition of nuclear location is specifically reversed by injection of purified A-kinase catalytic subunit, showing the requirement for A-kinase in the nuclear import of MyoD. Site-directed mutagenesis of all the putative sites for A-kinase-dependent phosphorylation on MyoD, substituting serine or threonine residues for the non-phosphorylatable amino acid alanine, had no effect on nuclear import of mutated MyoD. These data exclude the possibility that the effect of A-kinase on the nuclear translocation of MyoD is mediated by direct phosphorylation of MyoD and imply that A-kinase operates through phosphorylation of components involved in the nuclear transport of MyoD.

Stimulation of rat satellite cell myogenesis by inhibitors of ser/thr protein kinases

Satellite cells are involved in physiological growth and post-traumatic regeneration of adult skeletal muscle fibres. In this study, it is shown that differentiation of primary cultures of rat satellite cells is increased by inhibitors of ser/thr protein kinases such as iso-H7, which both inhibit cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) activities, and HA1004, a PKA inhibitor. These results, showing a preponderant effect of PKA inhibition on myogenesis in vitro, prompted the effects of iso-H7 on muscular regeneration in vivo to be tested. Preliminary results showed that regeneration of rat muscle EDL was improved by iso-H7 treatment.

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.

Electrical pacing induces adenylyl cyclase in skeletal muscle independent of the beta-adrenergic receptor

Continuous electrical pacing (EP) at 10 Hz of the peroneal nerve innervating fast-twitch muscles of the hindlimb in adult rabbits increases skeletal muscle concentrations of adenosine 3',5'-cyclic monophosphate (cAMP) by 3.1-fold at 10 days and increases beta-adrenergic receptor (beta-AR) density by 2.0-fold at 21 days. To determine whether beta-AR, the alpha-subunit of guanine nucleotide proteins (Gs alpha), or adenylyl cyclase is primarily responsible for pacing-induced increases in muscle cAMP, we measured adenylyl cyclase activity (ACA) in muscles that were electrically paced for 3 (n = 4), 10 (n = 8), and 21 (n = 8) days. EP resulted in a time-dependent increase in ACA that was 2.2 +/- 0.3-fold (P less than 0.005) at 21 days. EP significantly increased GTP-, 5'-guanylylimidodiphosphate-, isoproterenol-, NaF-, and forskolin-stimulated ACA, and propranolol administration to rabbits during EP did not alter pacing-induced changes in ACA. There were no changes in protein concentration, Na(+)-K(+)-adenosinetriphosphatase activity, or Gs alpha with EP. Based on these studies, we conclude that EP appears to increase cAMP through mechanisms independent of the beta-AR and through mechanisms that may involve alterations at the level of adenylyl cyclase.

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.

TGF-beta blocks early but not late differentiation-specific gene expression and morphologic differentiation of 3T3 T proadipocytes

Transforming growth factor-beta (TGF-beta) inhibits morphologic differentiation of BALB/c 3T3 T cells as well as other proadipocyte models. Our prior studies suggested that TGF-beta may act only during the early stages of differentiation induction. However, we did not determine whether TGF-beta was differentially effecting expression of any of the various differentiation-specific genes or if it could cause down-regulation of these genes in differentiated cells. Therefore, in the current study we tested the effects of exogenous TGF-beta (0.01-5.0 ng/ml) on morphologic differentiation and on differentiation-dependent gene expression (Northern and slot blot analyses) at various times during differentiation. When induced to differentiate, 3T3 T cells first undergo predifferentiation growth arrest and from this state molecular, biochemical, and morphological differentiation proceeds. Here it was found that when added prior to the onset of differentiation, TGF-beta was a potent inhibitor or morphologic differentiation as well as of the expression of differentiation-specific genes such as lipoprotein lipase (LPL) and glycerol-3-phosphate dehydrogenase (GPD). However, once morphologic differentiation began, TGF-beta was ineffective in blocking differentiation. In addition, exposure of fully differentiated cells to TGF-beta for up to 72 hours caused no decrease of differentiation-specific genes and even a 7-day treatment caused no morphologic dedifferentiation. Tumor necrosis factor also had no detectable effect on fully differentiated cells.

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.

Modulation of response to adenosine in vascular smooth muscle cells cultured in defined medium

Cultured pig aortic smooth muscle cells maintain a viable, quiescent state in a chemically defined medium that contains 10(-6) M insulin, 5 micrograms/ml transferrin, and 0.2 mM ascorbate. DNA synthesis and DNA content were determined by measuring tritiated thymidine incorporation and DNA-binding to the fluorescent probe 4',6-diamidino-2-phenylindole, respectively. The majority of the population of cells in defined medium cultures were diploid. Tritiated thymidine uptake in cells in defined medium was one-tenth that observed in cells in fetal bovine serum-containing medium. The study of cellular cyclic AMP level in response to extracellular adenosine stimulation in dividing cells and quiescent cells showed that cells in defined medium had a lower extent of response to adenosine compared to cells cultured in serum-containing medium. Both the cell growth index and the response to adenosine of cells cultured in defined medium were reversible after replacing the medium with 10% fetal bovine serum-containing medium, which suggests that the cells in defined medium were healthy and were capable of modulating cellular metabolism depending on culture conditions.

Requirement for G protein activity at a specific time during embryonic chick myogenesis

Signaling between embryonic myoblasts involves prostaglandin metabolism, the activation of a membrane receptor and changes in polyphosphatidyl inositol metabolism. Many of these membrane-localized events occur between 33 to 35 h of differentiation, concomitant with a dramatic change in membrane organization, in myoblast aggregates in culture. Since many receptors affect inositol phosphate metabolism by activating a GTP-binding protein (G protein), we asked if there was evidence for such a protein in myogenic signaling. We show that during the period of differentiation in culture when prostaglandin is needed to bind to a transient receptor, a pertussis toxin-sensitive but cholera toxin-insensitive G protein must act. If this activation is blocked, the characteristic change in myoblast cell adhesion and subsequent membrane fusion do not occur. We suggest that a G protein couples the activated prostaglandin receptor and the change in polyphosphatidyl inositol metabolism and that this membrane transduction step is necessary for subsequent membrane differentiation events during myogenesis.

Developmental regulation of expression of the alpha 1 and alpha 2 subunits mRNAs of the voltage-dependent calcium channel in a differentiating myogenic cell line

The voltage-dependent calcium channel (VDCC) in skeletal muscle probably plays a key role in transducing membrane charge movement to the calcium release channel. We report here that the expression of VDCC alpha 1 and alpha 2 mRNAs is developmentally regulated in differentiating C2C12 myogenic cells. The alpha 1 mRNA is not detectable in the myoblast form of C2C12 cells while its expression is induced 20-fold in differentiated myotubes. In contrast, the alpha 2 mRNA is weakly expressed in myoblasts but is also induced upon myogenic differentiation.

A gene with homology to the myc similarity region of MyoD1 is expressed during myogenesis and is sufficient to activate the muscle differentiation program

MyoD1 is a nuclear phosphoprotein that is expressed in skeletal muscle in vivo and in certain muscle cell lines in vitro; it has been shown to convert fibroblasts to myoblasts through a mechanism requiring a domain with homology to the myc family of proteins. The BC3H1 muscle cell line expresses skeletal muscle-specific genes upon exposure to mitogen-deficient medium, but does not express MyoD1 at detectable levels. To determine whether BC3H1 cells may express regulatory genes functionally related to MyoD1, a cDNA library prepared from differentiated BC3H1 myocytes, was screened at reduced stringency with the region of the MyoD1 cDNA that shares homology with c-myc. From this screen, a cDNA was identified that encodes a major open reading frame with 72% homology to the myc domain and basic region of MyoD1. The mRNA encoded by this MyoD1-related gene is expressed in skeletal muscle in vivo and in differentiated skeletal myocytes in vitro and is undetectable in cardiac or smooth muscle, nonmuscle tissues, or nonmyogenic cell types. During myogenesis, the MyoD1-related mRNA accumulates several hours prior to other muscle-specific mRNAs and therefore represents an early molecular marker for entry of myoblasts into the differentiation pathway. Transient transfection of 10T1/2 or 3T3 cells with the MyoD1-related cDNA is sufficient to induce myosin heavy-chain expression and to activate a reporter gene under transcriptional control of the muscle creatine kinase 5' enhancer, which functions only in differentiated myocytes. Expression of this cDNA in stably transfected 10T1/2 cells also leads to fusion and muscle-specific gene expression upon exposure to mitogen-deficient medium. Thus, the product of this MyoD1-related gene is sufficient to activate the muscle differentiation program and may substitute for MyoD1 in certain developmental situations. Together, these results suggest the existence of a family of myogenic regulatory genes that share a conserved motif with c-myc.