Overexpression of nPKC ? is permissive for myogenic differentiation

Protein kinase Cδ promotes proliferation and induces malignant transformation in skeletal muscle

In this paper, we investigated the isoform-specific roles of certain protein kinase C (PKC) isoforms in the regulation of skeletal muscle growth. Here, we provide the first intriguing functional evidence that nPKCδ (originally described as an inhibitor of proliferation in various cells types) is a key player in promoting both in vitro and in vivo skeletal muscle growth. Recombinant overexpression of a constitutively active nPKCδ in C2C12 myoblast increased proliferation and inhibited differentiation. Conversely, overexpression of kinase-negative mutant of nPKCδ (DN-nPKCδ) markedly inhibited cell growth. Moreover, overexpression of nPKCδ also stimulated in vivo tumour growth and induced malignant transformation in immunodeficient (SCID) mice whereas that of DN-nPKCδ suppressed tumour formation. The role of nPKCδ in the formation of rhabdomyosarcoma was also investigated where recombinant overexpression of nPKCδ in human rhabdomyosarcoma RD cells also increased cell proliferation and enhanced tumour formation in mouse xenografts. The other isoforms investigated (PKCα, β, ε) exerted only minor (mostly growth-inhibitory) effects in skeletal muscle cells. Collectively, our data introduce nPKCδ as a novel growth-promoting molecule in skeletal muscles and invite further trials to exploit its therapeutic potential in the treatment of skeletal muscle malignancies.

Suppression of protein kinase C theta contributes to enhanced myogenesis in vitro via IRS1 and ERK1/2 phosphorylation

Background

Differentiation and fusion of skeletal muscle myoblasts into multi-nucleated myotubes is required for neonatal development and regeneration in adult skeletal muscle. Herein, we report novel findings that protein kinase C theta (PKCθ) regulates myoblast differentiation via phosphorylation of insulin receptor substrate-1 and ERK1/2.

Results

In this study, PKCθ knockdown (PKCθ^shRNA) myotubes had reduced inhibitory insulin receptor substrate-1 ser1095 phosphorylation, enhanced myoblast differentiation and cell fusion, and increased rates of protein synthesis as determined by [³H] phenylalanine incorporation. Phosphorylation of insulin receptor substrate-1 ser632/635 and extracellular signal-regulated kinase1/2 (ERK1/2) was increased in PKCθ^shRNA cells, with no change in ERK5 phosphorylation, highlighting a PKCθ-regulated myogenic pathway. Inhibition of PI3-kinase prevented cell differentiation and fusion in control cells, which was attenuated in PKCθ^shRNA cells. Thus, with reduced PKCθ, differentiation and fusion occur in the absence of PI3-kinase activity. Inhibition of the ERK kinase, MEK1/2, impaired differentiation and cell fusion in control cells. Differentiation was preserved in PKCθ^shRNA cells treated with a MEK1/2 inhibitor, although cell fusion was blunted, indicating PKCθ regulates differentiation via IRS1 and ERK1/2, and this occurs independently of MEK1/2 activation.

Conclusion

Cellular signaling regulating the myogenic program and protein synthesis are complex and intertwined. These studies suggest that PKCθ regulates myogenic and protein synthetic signaling via the modulation of IRS1and ERK1/2 phosphorylation. Myotubes lacking PKCθ had increased rates of protein synthesis and enhanced myotube development despite reduced activation of the canonical anabolic-signaling pathway. Further investigation of PKCθ regulated signaling may reveal important interactions regulating skeletal muscle health in an insulin resistant state.

Dilinoleoyl-phosphatidic acid mediates reduced IRS-1 tyrosine phosphorylation in rat skeletal muscle cells and mouse muscle

AIMS/HYPOTHESIS

Insulin resistance in skeletal muscle is strongly associated with lipid oversupply, but the intracellular metabolites and underlying mechanisms are unclear. We therefore sought to identify the lipid intermediates through which the common unsaturated fatty acid linoleate causes defects in IRS-1 signalling in L6 myotubes and mouse skeletal muscle.

MATERIALS AND METHODS

Cells were pre-treated with 1 mmol/l linoleate for 24 h. Subsequent insulin-stimulated IRS-1 tyrosine phosphorylation and its association with the p85 subunit of phosphatidylinositol 3-kinase were determined by immunoblotting. Intracellular lipid species and protein kinase C activation were modulated by overexpression of diacylglycerol kinase epsilon, which preferentially converts unsaturated diacylglycerol into phosphatidic acid, or by inhibition of lysophosphatidic acid acyl transferase with lisofylline, which reduces phosphatidic acid synthesis. Phosphatidic acid species in linoleate-treated cells or muscle from insulin-resistant mice fed a safflower oil-based high-fat diet that was rich in linoleate were analysed by mass spectrometry.

RESULTS

Linoleate pretreatment reduced IRS-1 tyrosine phosphorylation and p85 association. Overexpression of diacylglycerol kinase epsilon reversed the activation of protein kinase C isoforms by linoleate, but paradoxically further diminished IRS-1 tyrosine phosphorylation. Conversely, lisofylline treatment restored IRS-1 phosphorylation. Mass spectrometry indicated that the dilinoleoyl-phosphatidic acid content increased from undetectable levels to almost 20% of total phosphatidic acid in L6 cells and to 8% of total in the muscle of mice fed a high-fat diet. Micelles containing dilinoleoyl-phosphatidic acid specifically inhibited IRS-1 tyrosine phosphorylation and glycogen synthesis in L6 cells.

CONCLUSIONS/INTERPRETATION

These data indicate that linoleate-derived phosphatidic acid is a novel lipid species that contributes independently of protein kinase C to IRS-1 signalling defects in muscle cells in response to lipid oversupply.

Akt Mediates Insulin-stimulated Phosphorylation of Ndrg2

The protein kinase Akt mediates several metabolic and mitogenic effects of insulin, whereas activation of protein kinase C (PKC) isoforms has been implicated in the inhibition of insulin action. We have previously shown that both PKC and PKCepsilon are activated in skeletal muscle of insulin-resistant high fat-fed rats, and to identify potential substrates for these kinases, we incubated recombinant PKC isoforms with rat muscle fractions in vitro. PKC specifically phosphorylated a 48-kDa protein that was subsequently identified by mass spectrometry as Ndrg2. Ndrg2 is highly related to N-Myc downstream-regulated protein 1, which has been linked to stress responses, cell proliferation, and differentiation, although Ndrg2 itself is not repressed by N-Myc. Ndrg2 contains several potential phosphorylation sites, including three Akt consensus sequences. Ndrg2 phosphorylation was enhanced in [32P]orthophosphate-labeled C2C12 muscle cells co-overexpressing either PKC or Akt. Phosphorylation of Ndrg2 was examined further using a phospho (Ser/Thr) Akt substrate antibody. Insulin increased Ndrg2 phosphorylation in C2C12 cells in a wortmannin- and palmitate-inhibitable manner, whereas rapamycin, PD98059, and bisindoylmaleimide I had no effect, supporting a direct role for Akt. Mutation of Ndrg2 indicated that Thr-348 is the major phosphorylation site detected by the antibody and that Akt stimulates phosphorylation of this site, whereas PKC phosphorylates Ser-332. PKC overexpression, however, diminished the effect of insulin on Thr-348 phosphorylation without reducing Akt activation, suggesting that this is mediated through phosphorylation of Ndrg2 at Ser-332. Our data identify Ndrg2 as a novel insulin-dependent phosphoprotein and suggest that PKC may inhibit insulin action in part by reducing its phosphorylation by Akt.

Overexpression of nPKC theta is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubes

Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin-induced nAChR clustering in C(2)C(12) cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two-fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC alpha, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC theta, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin-induced nAChR cluster formation we overexpressed an nPKC theta -green fluorescent protein (GFP) fusion protein in C(2)C(12) myotubes. The number of nAChR clusters was significantly reduced in nPKC theta-GFP compared to GFP overexpressing myotubes at less-than-maximal clustering concentrations of agrin. These data indicate that nPKC theta activity inhibits nAChR cluster formation. To examine whether nPKC theta activation by phorbol esters regulates agrin-induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC theta-GFP compared to GFP-overexpressing myotubes, suggesting that stimulating nPKC theta activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC theta activity inhibits agrin-induced nAChR cluster formation and disrupts preexisting agrin-induced nAChR clusters, suggest that nPKC theta activity is inhibitory for agrin function.

Inhibition of glycogen synthesis by fatty acid in C(2)C(12) muscle cells is independent of PKC-alpha, -epsilon, and -theta

We have previously shown that glycogen synthesis is reduced in lipid-treated C(2)C(12) skeletal muscle myotubes and that this is independent of changes in glucose uptake. Here, we tested whether mitochondrial metabolism of these lipids is necessary for this inhibition and whether the activation of specific protein kinase C (PKC) isoforms is involved. C(2)C(12) myotubes were pretreated with fatty acids and subsequently stimulated with insulin for the determination of glycogen synthesis. The carnitine palmitoyltransferase-1 inhibitor etomoxir, an inhibitor of beta-oxidation of acyl-CoA, did not protect against the inhibition of glycogen synthesis caused by the unsaturated fatty acid oleate. In addition, although oleate caused translocation, indicating activation, of individual PKC isoforms, inhibition of PKC by pharmacological agents or adenovirus-mediated overexpression of dominant negative PKC-alpha, -epsilon, or -theta mutants was unable to prevent the inhibitory effects of oleate on glycogen synthesis. We conclude that neither mitochondrial lipid metabolism nor activation of PKC-alpha, -epsilon, or -theta plays a role in the direct inhibition of glycogen synthesis by unsaturated fatty acids.