Electrical stimulation induces calcium-dependent up-regulation of neuregulin-1β in dystrophic skeletal muscle cell lines

Duchenne muscular dystrophy (DMD) is a neuromuscular disease originated by reduced or no expression of dystrophin, a cytoskeletal protein that provides structural integrity to muscle fibres. A promising pharmacological treatment for DMD aims to increase the level of a structural dystrophin homolog called utrophin. Neuregulin-1 (NRG-1), a growth factor that potentiates myogenesis, induces utrophin expression in skeletal muscle cells. Microarray analysis of total gene expression allowed us to determine that neuregulin-1β (NRG-1β) is one of 150 differentially expressed genes in electrically stimulated (400 pulses, 1 ms, 45 Hz) dystrophic human skeletal muscle cells (RCDMD). We investigated the effect of depolarization, and the involvement of intracellular Ca2+ and PKC isoforms on NRG-1β expression in dystrophic myotubes. Electrical stimulation of RCDMD increased NRG-1β mRNA and protein levels, and mRNA enhancement was abolished by actinomycin D. NRG-1β transcription was inhibited by BAPTA-AM, an intracellular Ca2+ chelator, and by inhibitors of IP3-dependent slow Ca2+ transients, like 2-APB, Ly 294002 and Xestospongin B. Ryanodine, a fast Ca2+ signal inhibitor, had no effect on electrical stimulation-induced expression. BIM VI (general inhibitor of PKC isoforms) and Gö 6976 (specific inhibitor of Ca2+-dependent PKC isoforms) abolished NRG-1β mRNA induction. Our results suggest that depolarization induced slow Ca2+ signals stimulate NRG-1β transcription in RCDMD cells, and that Ca2+-dependent PKC isoforms are involved in this process. Based on utrophin´s ability to partially compensate dystrophin disfunction, knowledge on the mechanism involved on NRG-1 up-regulation could be important for new therapeutic strategies design

Insulin-Dependent H2O2 Production Is Higher in Muscle Fibers of Mice Fed with a High-Fat Diet

Insulin resistance is defined as a reduced ability of insulin to stimulate glucose utilization. C57BL/6 mice fed with a high-fat diet (HFD) are a model of insulin resistance. In skeletal muscle, hydrogen peroxide (H2O2) produced by NADPH oxidase 2 (NOX2) is involved in signaling pathways triggered by insulin. We evaluated oxidative status in skeletal muscle fibers from insulin-resistant and control mice by determining H2O2 generation (HyPer probe), reduced-to-oxidized glutathione ratio and NOX2 expression. After eight weeks of HFD, insulin-dependent glucose uptake was impaired in skeletal muscle fibers when compared with control muscle fibers. Insulin-resistant mice showed increased insulin-stimulated H2O2 release and decreased reduced-to-oxidized glutathione ratio (GSH/GSSG). In addition, p47phox and gp91phox (NOX2 subunits) mRNA levels were also high (~3-fold in HFD mice compared to controls), while protein levels were 6.8- and 1.6-fold higher, respectively. Using apocynin (NOX2 inhibitor) during the HFD feeding period, the oxidative intracellular environment was diminished and skeletal muscle insulin-dependent glucose uptake restored. Our results indicate that insulin-resistant mice have increased H2O2 release upon insulin stimulation when compared with control animals, which appears to be mediated by an increase in NOX2 expression