Stimulation of protein synthesis and phospholipase D activity by vasopressin and phorbol ester in L6 myoblasts

V1a vasopressin receptor expression is modulated during myogenic differentiation

Neurohypophyseal peptides potently stimulate myogenic differentiation by acting through different receptors of the same family. Here, we show that L6C5 myogenic cells express, at a high density, a single class of V1a Arg8-vasopressin (AVP) receptor. The expression of the vasopressin receptor of type 1a (V1aR) is significantly higher in proliferating myoblasts than in differentiated myotubes. The differentiation-related decrease of V1aR expression was evident both at the mRNA and at the protein level as shown by the reduction of [(3)H]-AVP binding. However, in L6C5 cells transfected with a synthetic construct containing the luciferase gene driven by the 2 kb upstream region of V1aR, we observed a stimulation of the activity of the promoter when the cells were cultured in differentiative medium. The down-regulation of the V1aR correlated with a decreased half-life of its mRNA (half-life 5.86+/-0.74 hr in 10% fetal bovine serum [FBS] versus 3.53+/-0.72 hr in 1% FBS). Cyclosporine A and dexamethasone, but not 5'-azacytidine, treatments of cells in differentiation medium restored the V1aR level to that measured in proliferating L6C5 cells, thus confirming the role of post-transcriptional mechanisms in the modulation of V1aR expression. Taken together, these data show that mRNA stability plays a role in modulating protein expression during the myogenic differentiation process.

Hyperammonaemia in V1a vasopressin receptor knockout mice caused by the promoted proteolysis and reduced intrahepatic blood volume

An analysis of arginine-vasopressin (AVP) V1a receptor-deficient (V1aR-/-) mice revealed that glucose homeostasis and lipid metabolism were altered in the mutant mice. Here, we used V1aR-/- mice to investigate whether the deficiency of the V1a receptor, which led to altered insulin sensitivity, affected protein metabolism. The serum 3-methylhistidine levels were increased in V1aR-/- mice under feeding conditions, indicating that proteolysis was enhanced in muscle tissue from V1aR-/- mice. Furthermore, serum amino acid profiling revealed that the amino acid levels, including glycogenic and branched-chain amino acids, were reduced in V1aR-/- mice. In addition, an alanine-loading test showed that gluconeogenesis was enhanced in V1aR-/- mice. Blood ammonia, which is a by-product of amino acid catabolism, was two times higher in V1aR-/- mice without hepatopathy under the feeding and fasting conditions than in wild-type mice. Amino acid profiling also revealed that the amino acid pattern was not typical of a urea-cycle enzymatic disorder. An ammonia tolerance test and an indocyanine green elimination test showed that V1aR-/- mice had lower ammonia clearance due to a decreased intrahepatic circulating blood volume. Metabolic acidosis, including lactic- and keto-acidosis, was not observed in V1aR-/- mice. These results provide evidence that proteolysis promotes the production of glucose in the muscles of V1aR-/- mice and that hyperammonaemia is caused by promoted protein catabolism and reduced intrahepatic blood volume. Thus, our study with V1aR-/- mice indicates that AVP plays a physiological role via the V1a receptor in regulating both protein catabolism and glucose homeostasis.

Phospholipase D and extracellular signal-regulated kinase in hepatic stellate cells: effects of platelet-derived growth factor and extracellular nucleotides

We have previously provided evidence suggesting that phosphatidic acid, possibly derived from the hydrolysis of phosphatidylcholine by phospholipase D (PLD), is involved in platelet-derived growth factor (PDGF)-mediated increases in extracellular signal-regulated kinase (ERK) activity and DNA synthesis in rat hepatic stellate cells (HSC), the primary fibrogenic cells of the liver. A recent study has shown the presence of P2Y nucleotide receptors on HSC that are coupled to contraction and synthesis of the matrix component, alpha1-procollagen, leading to the suggestion that they may represent a new therapeutic target in the treatment of liver fibrosis. However, although extracellular nucleotides have been shown to stimulate both PLD and ERK, and to elicit proliferation of fibrogenic cells outside the liver, their effect on these parameters in HSC have not yet been investigated. PLD activity was determined by [3H]choline release and [3H]phosphatidylbutanol production, ERK activity by Western blotting, and DNA synthesis by [3H]thymidine incorporation. We report here, for the first time in HSC, that extracellular nucleotides stimulate PLD activity and a sustained activation of ERK. However, in contrast to PDGF, nucleotides had negligible effects on DNA synthesis. Moreover, the effects of PDGF and nucleotides on PLD and ERK were not additive, suggesting activation of the same PLD isoform and pool of ERK. The data demonstrate that nucleotide-stimulated PLD and ERK activities are not coupled to DNA synthesis in HSC. Instead, these responses may be linked to other phenotypic changes associated with activated HSC such as increases in contraction, motility, or extracellular matrix deposition.

Phorbol ester-induced differentiation of L6 myogenic cells involves phospholipase D activation

TPA, a potent PKC activator, inhibits myogenic differentiation and activates phospholipase D (PLD). We evaluated the involvement of PLD in the TPA effects on L6 myoblasts differentiation. TPA, at concentrations inhibiting differentiation of L6 cells, induced a strong, though transient, PLD activation. Surprisingly, at nanomolar concentration, TPA induced both myogenic differentiation and sustained activation of PLD. Differential effect of TPA can be ascribed to PKC downregulation induced by highest TPA concentrations. TPA-induced differentiation was inhibited by 1-butanol, confirming the involvement of PLD in this effect. These data suggest that prolonged elevation of PLD activity is required for myogenic differentiation.

Phospholipase D is involved in myogenic differentiation through remodeling of actin cytoskeleton

We investigated the role of phospholipase D (PLD) and its product phosphatidic acid (PA) in myogenic differentiation of cultured L6 rat skeletal myoblasts. Arginine-vasopressin (AVP), a differentiation inducer, rapidly activated PLD in a Rho-dependent way, as shown by almost total suppression of activation by C3 exotoxin pretreatment. Addition of 1-butanol, which selectively inhibits PA production by PLD, markedly decreased AVP-induced myogenesis. Conversely, myogenesis was potentiated by PLD1b isoform overexpression but not by PLD2 overexpression, establishing that PLD1 is involved in this process. The expression of the PLD isoforms was differentially regulated during differentiation. AVP stimulation of myoblasts induced the rapid formation of stress fiber-like actin structures (SFLSs). 1-Butanol selectively inhibited this response, whereas PLD1b overexpression induced SFLS formation, showing that it was PLD dependent. Endogenous PLD1 was located at the level of SFLSs, and by means of an intracellularly expressed fluorescent probe, PA was shown to be accumulated along these structures in response to AVP. In addition, AVP induced a PLD-dependent neosynthesis of phosphatidylinositol 4,5-bisphosphate (PIP2), which also was accumulated along actin fibers. These data support the hypothesis that PLD participates in myogenesis through PA- and PIP2-dependent actin fiber formation.

A role for phospholipase D activation in the lipid signalling cascade generated by bradykinin and thrombin in C2C12 myoblasts

In the present study evidence is provided for a rapid activation of lipid signalling pathways induced by thrombin and bradykinin (BK) in C2C12 myoblasts. Both agonists were able to increase [3H]inositol phosphates (InsP), 1,2-[3H]diacylglycerol (DAG) and [3H]phosphatidic acid (PtdOH) levels. In particular [3H]PtdOH values were rapidly increased and maintained at significantly high values at prolonged times of incubation. BK and thrombin were able to activate phospholipase D (PLD) in vivo as demonstrated by the accumulation of [3H]phosphatidylethanol (PtdEtOH) through the transphoshatidylation reaction catalyzed by the enzyme in the presence of ethanol. The observation that ethanol could significantly reduce [3H]PtdOH formation in myoblasts stimulated with BK and thrombin indicates that stimulation of PLD has a major role. The two agonists appear to stimulate PLD activity through a common molecular mechanism, involving the activation of protein kinase C (PKC). In addition, BK and thrombin appear able to activate DAG kinase at early times of incubation and also this pathway may contribute to determine the increase in [3H]PtdOH levels. This is the first report which describes activation of lipid signalling pathways by BK and thrombin in myoblast cells and it is possible that these early signals may have an important role in mediating the biological effects of the two agonists.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.

Regulation of phospholipase D in L6 skeletal muscle myoblasts. Role of protein kinase c and relationship to protein synthesis

The addition of vasopressin or 12-O-tetradecanoylphorbol-13-acetate (TPA) to prelabeled L6 myoblasts elicited increases in [14C]ethanolamine release, suggesting the activation of phospholipase D activity or activities. While the effects of both agonists on intracellular release were rapid and transient, when extracellular release of [14C]ethanolamine was measured, the effect of vasopressin was again rapid and transient, whereas that of TPA was delayed but sustained. Effects of both agonists on intra- and extracellular release were inhibited by the protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC down-regulation by preincubation with TPA. The formation of phosphatidylbutanol elicited by vasopressin and TPA mirrored their effects on extracellular [14C]ethanolamine release in that the former was transient, whereas the latter was sustained. Responses to both agonists were abolished by PKC down-regulation. When protein synthesis was examined, the stimulation of translation by TPA and transcription by vasopressin were inhibited by Ro-31-8220. In contrast, down-regulation of PKC inhibited the synthesis response to TPA but not vasopressin. Furthermore, following down-regulation, the effect of vasopressin was still blocked by the PKC inhibitors, Ro-31-8220 and bisindolylmaleimide. Analysis of PKC isoforms in L6 cells showed the presence of alpha, epsilon, delta, mu, iota, and zeta. Down-regulation removed both cytosolic (alpha) and membrane-bound (epsilon and delta) isoforms. Thus, the elevation of phospholipase D activity or activities induced by both TPA and vasopressin and the stimulation of translation by TPA involves PKC-alpha, -epsilon, and/or -delta. In contrast, the increase in transcription elicited by vasopressin involves mu, iota, and/or zeta. Hence, although phospholipase D may be linked to increases in translation elicited by TPA, it is not involved in the stimulation of transcription by vasopressin.

Role of phospholipase C and D signalling pathways in vasopressin-dependent myogenic differentiation

Arg8-vasopressin (AVP) is a potent inducer of myogenic differentiation stimulating the expression of myogenic regulatory factors. To understand the mechanism of its effect on myogenesis, we investigated the early signals induced by AVP in myogenic target cells. In the rat skeletal muscle cell line L6, AVP selectively stimulates phosphatidylinositol (PtdIns) and phosphatidylcholine (PtdCho) breakdown, through the activation of phospholipases C and D (PLC, PLD), as shown by the generation of Ins(1,4,5)P3 and phosphatidylethanol (PtdEtOH), respectively. AVP induces the biphasic increase of sn-1,2-diacylglycerol (DAG) consisting in a rapid peak followed by a sustained phase, and the monophasic generation of phosphatidic acid (PA). Propranolol (a PA phosphatase inhibitor) and Zn2+ (a PLD inhibitor), abolish the sustained phase of DAG generation. Our data indicate that PtdIns-PLC activity is mainly responsible for the rapid phase of AVP-dependent DAG generation, whereas the sustained phase is dependent upon PtdCho-PLD activity and PA dephosphorylation, ruling out any significant role of DAG kinase. Modifications of PA level correlate with parallel changes of PLC activity, indicating a possible cross-talk between the two signal transduction pathways in the intact cell. PLD activation is elicited at AVP concentrations two orders of magnitude lower than those required for PLC activation. The differentiation of L6 myoblasts into multinucleated fibers is stimulated significantly by AVP at concentrations at which PLD, but not PLC, is activated. These data provide the first evidence for an important role of PLD in the mechanism of AVP-induced muscle differentiation.

Insulin and insulin-like growth factor-I responsiveness and signalling mechanisms in C2C12 satellite cells: effect of differentiation and fusion

In proliferating C2C12 myoblasts, serum and physiological concentrations of insulin and IGF-I stimulated protein synthesis and RNA accretion. After fusion, the multinucleated myotubes remained responsive to serum but not to insulin or IGF-I, even though both insulin and type-I IGF receptor mRNAs increased in abundance. Protein synthetic responses to insulin and IGF-I in myoblasts were not inhibited by dexamethasone, ibuprofen or Ro-31-8220, thus phospholipase A2, cyclo-oxygenase and protein kinase C did not appear to be involved in the signalling mechanisms. Neither apparently were polyphosphoinositide-specific phospholipase C or phospholipase D since neither hormone increased inositol phosphate, phosphatidic acid, choline or phosphatidylbutanol production. Only the phosphatidylinositol-3-kinase inhibitor, wortmannin, and the 70 kDa S6-kinase inhibitor, rapamycin, wholly or partially blocked the effects of insulin and IGF-I on protein synthesis. 2-deoxyglucose uptake remained responsive to insulin and IGF-I after fusion and was also inhibited by wortmannin. The results suggest that the loss of responsiveness after fusion is not due to loss of receptors, but to the uncoupling of a post-receptor pathway, occurring after the divergence of the glucose transport and protein synthesis signalling systems, and that, if wortmannin acts at a single site, this is prior to that point of divergence.

N tau-methylhistidine turnover in skeletal muscle cells measured by GC-MS

A method that employs gas chromatography-mass spectrometry has been developed to measure N tau-methylhistidine (3-methylhistidine; 3-MH) synthesis and release from skeletal muscle myotubes in vitro. It shows excellent linearity (0.9999) over the range studied (0-4 nmol), high recovery (92.6%), and low coefficient of variation (1.6%). 3-MH release from myotubes was essentially linear over a 96-h incubation, whereas the loss of 3-MH from cell protein accelerated with increasing time, an effect due, at lest in part, to decreasing rates of total protein synthesis. When incubated in either glutamine-free or methionine-free medium for 48 h, 3-MH in cell protein and appearing in the medium were greatly reduced compared with the 48-h controls, suggesting that hypertrophy was greatly reduced. Similar but lesser trends were observed with adenosine 3',5' -cyclic monophosphate. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA) appeared to both stimulate 3-MII synthesis and inhibit its release during a 48-h incubation. The development of this method facilitates detailed investigation into the mechanisms through which agents such as TPA regulate myofibrillar protein degradation.

Cyclic AMP stimulates protein synthesis in L6 myoblasts and its effects are additive to those of insulin, vasopressin and 12-0-tetradecanoylphorbol-13-acetate. Possible involvement of mitogen activated protein kinase

The role of cyclic AMP as a second messenger in the stimulation of protein synthesis and the potential involvement of mitogen activated protein (MAP) kinase in this response was studied in L6 myoblasts. Dibutyryl-cAMP (dbt-cAMP) increased protein synthesis at 90 min and 6 h in a concentration-dependent manner. The responses at 90 min were probably mediated by increased translation as they were not blocked by actinomycin D; effects at 6 h were accompanied by increases in RNA content implying a transcriptional component. 100 nM 12-0-tetradecanoylphorbol-13-acetate (TPA), 1 nM Insulin (90 min incubations) and 100 nM vasopressin (6 h incubation) also increased protein synthesis and these responses were additive with those of 500 micron dbt-cAMP. Responses to forskolin were similar to dbt-cAMP whilst 1,9-dideoxyforskolin had no effect. Cell extracts immunoblotted with MAP kinase antibody showed bands corresponding to approx. 42, 44, 54 and 83 kDa. 500 micron dbt-cAMP elicited an increase in activity of both the 42 and 44 kDa bands when assayed by the 'in gel' method and a similar response was also observed with forskolin. TPA and vasopressin also stimulated the activity of these two isoforms, but had no significant additive or inhibitory effects when added in combination with 500 micron dbt-cAMP. In contrast, although 1 nM insulin alone had no effect, a synergistic response in terms of MAP kinase activation was observed in the presence of dbt-cAMP. The data demonstrate that cAMP stimulates protein synthesis in L6 cells and suggest a role for MAP kinase in this event.

Stimulation of protein and DNA synthesis in mouse C2C12 satellite cells: evidence for phospholipase D-dependent and -independent pathways

In C2C12 myoblasts, 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated a phospholipase D (PLD) to degrade phosphatidylcholine (PC) as measured by the release of choline and an increase in the formation of phosphatidic acid (PA) (or phosphatidylbutanol [PtdBuOH] in the presence of 0.5% butanol). Exogenous PLD also stimulated choline release, PA and PtdBuOH formation. The protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC downregulation significantly inhibited the effects of TPA but Ro-31-8220 had no effect on PLD action. Neither basic Fibroblast Growth Factor (bFGF) or Epidermal Growth Factor (EGF) increased PLD activity. All agonists stimulated protein synthesis during both a 90 min and a 6 hr incubation and increased RNA accretion after 6 hr. The response at 90 min was not inhibited by the transcription inhibitor, actinomycin D. Ro-31-8220 and PKC downregulation significantly inhibited all the effects of TPA. In contrast, Ro-31-8220 significantly inhibited the increase in RNA accretion elicited by PLD but had no effect on the ability of agonists other than TPA to enhance protein synthesis. All agonists also stimulated thymidine incorporation into DNA. The effects of EGF, bFGF, and PLD were rapid and transient whereas that of TPA was delayed and sustained. Ro-31-8220 and PKC downregulation significantly inhibited the response due to TPA. Furthermore, Ro-31-8220 also significantly inhibited the effects elicited by EGF and PLD but not that induced by bFGF. In differentiated myotubes, TPA and PLD, but not bFGF or EGF, again stimulated choline release and PtdBuOH formation. However, all agents failed to stimulate protein synthesis and RNA accretion. The data demonstrate the presence in C2C12 myoblasts, but not differentiated myotubes, of both a PLD-dependent and PLD-independent pathway(s) leading to the stimulation of protein synthesis, RNA accretion, and DNA synthesis.

Evidence that protein kinase C and mitogen activated protein kinase are not involved in the mechanism by which insulin stimulates translation in L6 myoblasts

Insulin stimulated a concentration-dependent increase in protein synthesis in L6 myoblasts which was significant at 1 nM. This response was not prevented by the transcription inhibitor, actinomycin D. The protein kinase C (PKC) inhibitor, Ro-31-8220, and downregulation of PKC by prolonged incubation of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA), had no effect on the ability of insulin to stimulate protein synthesis whilst completely blocking the response to TPA. In contrast, insulin failed to enhance protein synthesis significantly in the presence of either ibuprofen, a selective cyclooxygenase inhibitor or rapamycin, an inhibitor of the 70 kDa S6 kinase. When cell extracts were prepared and assayed for total myelin basic protein kinase activity, a stimulatory effect of insulin was not observed until the concentration approached 100-fold (i.e. 100 nM) that required to elicit increases in protein synthesis. Upon fractionation on a Mono-Q column, 100 nM insulin increased the activity of 3 peaks which phosphorylated myelin basic protein. Two of these peaks were identified as the 42 and 44 kDa forms of Mitogen Activated Protein (MAP) kinase by immunoblotting. In contrast, 1 nM insulin had no effect on the activity of these peaks. The data suggest that physiologically relevant concentrations of insulin do not stimulate translation in L6 cells through either PKC or the 42/44 kDa isoforms of MAP kinase and that this response is, at least in part, mediated through the activation of the 70 kDa S6 kinase by cyclooxygenase metabolites.