HOX-1 and COX-2: Two differentially regulated key mediators of skeletal myoblast tolerance under oxidative stress

Verbascoside Elicits Its Beneficial Effects by Enhancing Mitochondrial Spare Respiratory Capacity and the Nrf2/HO-1 Mediated Antioxidant System in a Murine Skeletal Muscle Cell Line

Muscle weakness and muscle loss characterize many physio-pathological conditions, including sarcopenia and many forms of muscular dystrophy, which are often also associated with mitochondrial dysfunction. Verbascoside, a phenylethanoid glycoside of plant origin, also named acteoside, has shown strong antioxidant and anti-fatigue activity in different animal models, but the molecular mechanisms underlying these effects are not completely understood. This study aimed to investigate the influence of verbascoside on mitochondrial function and its protective role against H2O2-induced oxidative damage in murine C2C12 myoblasts and myotubes pre-treated with verbascoside for 24 h and exposed to H2O2. We examined the effects of verbascoside on cell viability, intracellular reactive oxygen species (ROS) production and mitochondrial function through high-resolution respirometry. Moreover, we verified whether verbascoside was able to stimulate nuclear factor erythroid 2-related factor (Nrf2) activity through Western blotting and confocal fluorescence microscopy, and to modulate the transcription of its target genes, such as heme oxygenase-1 (HO-1) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), by Real Time PCR. We found that verbascoside (1) improved mitochondrial function by increasing mitochondrial spare respiratory capacity; (2) mitigated the decrease in cell viability induced by H2O2 and reduced ROS levels; (3) promoted the phosphorylation of Nrf2 and its nuclear translocation; (4) increased the transcription levels of HO-1 and, in myoblasts but not in myotubes, those of PGC-1α. These findings contribute to explaining verbascoside's ability to relieve muscular fatigue and could have positive repercussions for the development of therapies aimed at counteracting muscle weakness and mitochondrial dysfunction.

NRF2 Regulates Viability, Proliferation, Resistance to Oxidative Stress, and Differentiation of Murine Myoblasts and Muscle Satellite Cells

Increased oxidative stress can slow down the regeneration of skeletal muscle and affect the activity of muscle satellite cells (mSCs). Therefore, we evaluated the role of the NRF2 transcription factor (encoded by the Nfe2l2 gene), the main regulator of the antioxidant response, in muscle cell biology. We used (i) an immortalized murine myoblast cell line (C2C12) with stable overexpression of NRF2 and (ii) primary mSCs isolated from wild-type and Nfe2l2 (transcriptionally)-deficient mice (Nfe2l2^tKO). NRF2 promoted myoblast proliferation and viability under oxidative stress conditions and decreased the production of reactive oxygen species. Furthermore, NRF2 overexpression inhibited C2C12 cell differentiation by down-regulating the expression of myogenic regulatory factors (MRFs) and muscle-specific microRNAs. We also showed that NRF2 is indispensable for the viability of mSCs since the lack of its transcriptional activity caused high mortality of cells cultured in vitro under normoxic conditions. Concomitantly, Nfe2l2^tKO mSCs grown and differentiated under hypoxic conditions were viable and much more differentiated compared to cells isolated from wild-type mice. Taken together, NRF2 significantly influences the properties of myoblasts and muscle satellite cells. This effect might be modulated by the muscle microenvironment.

Altered mitogen-activated protein kinase signaling in dystrophic (mdx) muscle

INTRODUCTION

Duchenne muscular dystrophy (DMD) results from a deficiency in the protein, dystrophin. Dystrophic myotubes are susceptible to stressful stimuli. This may be partly due to altered regulation of pro-survival signaling pathways, but a role for mitogen-activated protein (MAP) kinases has not been investigated.

METHODS

We examined patterns of phosphorylation of key MAP kinase proteins in cultured myotubes responding to oxidative stress, and in muscle tissue in vivo.

RESULTS

Dystrophic (mdx) myotubes have an increased susceptibility to oxidant-induced death compared with wild-type (C57Bl/10ScSn) myotubes. This correlates with late phosphorylation of c-Jun N-terminal kinase (JNK), and persistently high p38 MAP kinase phosphorylation in mdx myotubes. JNK and extracellular signal-regulated kinase 1/2 (ERK1/2) also showed altered phosphorylation levels in mdx muscle tissue.

CONCLUSIONS

We show altered patterns of MAP kinase protein phosphorylation in dystrophic muscle in vitro and in vivo. These pathways may be novel pharmacological targets for treating DMD.

Insulin-induced oxidative stress up-regulates heme oxygenase-1 via diverse signaling cascades in the C2 skeletal myoblast cell line

Impaired insulin sensitivity (insulin resistance) is a common denominator in many metabolic disorders, exerting pleiotropic effects on skeletal muscle, liver, and adipose tissue function. Heme oxygenase-1 (HOX-1), the rate-limiting enzyme in heme catabolism, has recently been shown to confer an antidiabetic effect while regulating cellular redox-buffering capacity. Therefore, in the present study, we probed into the mechanisms underlying the effect of insulin on HOX-1 in C2 skeletal myoblasts. Hence, insulin was found to suppress C2 myoblasts viability via stimulation of oxidative stress, with HOX-1 counteracting this action. Insulin induced HOX-1 expression in a time- and dose-dependent manner, an effect attenuated by selective inhibitors of ERK1/2 (PD98059), Src (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine), and c-Jun terminal kinases 1 and 2 (SP600125) pathways. Furthermore, nuclear factor-κB role in insulin-induced HOX-1 up-regulation was verified, with ERK1/2, Src, and c-Jun terminal kinases 1 and 2 mediating p65-nuclear factor-κB subunit phosphorylation. Overall, our novel findings highlight for the first time the transduction mechanisms mediating HOX-1 induction in insulin-treated C2 myoblasts. This effect was established to be cell type specific because insulin failed to promote HOX-1 expression in HepG2 hepatoma cells. Deciphering the signaling networks involved in insulin-stimulated HOX-1 up-regulation is of prominent significance because it may potentially contribute to elucidation of the mechanisms involved in associated metabolic pathologies.