Nitric oxide regulates stretch-induced proliferation in C2C12 myoblasts.

Mechanical stretch of skeletal muscle activates nitric oxide (NO) production and is an important stimulator of satellite cell proliferation. Further, cyclooxygenase (COX) activity has been shown to promote satellite cell proliferation in response to stretch. Since COX-2 expression in skeletal muscle can be regulated by NO we sought to determine if NO is required for stretch-induced myoblast proliferation and whether supplemental NO can counter the effects of COX-2 and NF-kappaB inhibitors. C2C12 myoblasts were cultured for 24 h, then switched to medium containing either the NOS inhibitor, L-NAME (200 microM), the COX-2 specific inhibitor NS-398 (100 microM), the NF-kappaB inhibiting antioxidant, PDTC (5 mM), the nitric oxide donor, DETA-NONOate (10-100 microM) or no supplement (control) for 24 h. Subgroups of each treatment were exposed to 1 h of 15% cyclic stretch (1 Hz), and were then allowed to proliferate for 24 h before fixing. Proliferation was measured by BrdU incorporation during the last hour before fixing, and DAPI stain. Stretch induced a twofold increase in nuclear number compared to control, and this effect was completely inhibited by L-NAME, NS-398 or PDTC (P < 0.05). Although DETA-NONOate (10 microM) did not affect basal proliferation, the NO-donor augmented the stretch-induced increase in proliferation and rescued stretch-induced proliferation in NS-398-treated cells, but not in PDTC-treated cells. In conclusion, NO, COX-2, and NF-kappaB are necessary for stretch-induced proliferation of myoblasts. Although COX-2 and NF-kappaB are both involved in basal proliferation, NO does not affect basal growth. Thus, NO requires the synergistic effect of stretch in order to induce muscle cell proliferation.

Nitric oxide reverses prednisolone-induced inactivation of muscle satellite cells.

Long-term corticosteroid therapy causes myopathy and can inhibit regeneration of skeletal muscle. Therefore, we hypothesized that corticosteroid exposure reduces satellite cell activity in skeletal myofibers. Male Swiss-Webster mice were injected daily for 8 weeks with prednisolone (GC) or vehicle (control). Single myofibers were isolated from the gastrocnemius, centrifuged to mechanically activate satellite cells, and maintained in culture for 48 h. Both constitutive nitric oxide synthase (NOS) isoforms were reduced in muscle by GC treatment (nNOS: -30%, eNOS: -34%). Fewer myogenic (myoD+) cells emanated from GC myofibers compared to control (-61%, P < 0.05). Supplementation of culture media with the nitric oxide donor, diethylenetriamine NONOate (DETA-NO; 5-50 microM), caused a dose-dependent increase in the number of myoD+ cells arising from both control and GC myofibers (P < 0.05), and 10 and 50 microM DETA-NO eliminated the GC-induced deficit in myogenic cells (P > 0.05). Therefore, supplementation of GC myofibers with DETA-NO restores satellite cell activity to control levels. Nitric oxide production could be an important therapeutic target for the prevention of corticosteroid myopathy.

Nitric oxide increases GLUT4 expression and regulates AMPK signaling in skeletal muscle.

Nitric oxide (NO) and 5'-AMP-activated protein kinase (AMPK) are involved in glucose transport and mitochondrial biogenesis in skeletal muscle. Here, we examined whether NO regulates the expression of the major glucose transporter in muscle (GLUT4) and whether it influences AMPK-induced upregulation of GLUT4. At low levels, the NO donor S-nitroso-N-penicillamine (SNAP, 1 and 10 microM) significantly increased GLUT4 mRNA ( approximately 3-fold; P < 0.05) in L6 myotubes, and cotreatment with the AMPK inhibitor compound C ablated this effect. The cGMP analog 8-bromo-cGMP (8-Br-cGMP, 2 mM) increased GLUT4 mRNA by approximately 50% (P < 0.05). GLUT4 protein expression was elevated 40% by 2 days treatment with 8-Br-cGMP, whereas 6 days treatment with 10 microM SNAP increased GLUT4 expression by 65%. Cotreatment of cultures with the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one prevented the SNAP-induced increase in GLUT4 protein. SNAP (10 microM) also induced significant phosphorylation of alpha-AMPK and acetyl-CoA carboxylase and translocation of phosphorylated alpha-AMPK to the nucleus. Furthermore, L6 myotubes exposed to 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) for 16 h presented an approximately ninefold increase in GLUT4 mRNA, whereas cotreatment with the non-isoform-specific NOS inhibitor N(G)-nitro-l-arginine methyl ester, prevented approximately 70% of this effect. In vivo, GLUT4 mRNA was increased 1.8-fold in the rat plantaris muscle 12 h after AICAR injection, and this induction was reduced by approximately 50% in animals cotreated with the neuronal and inducible nitric oxide synthases selective inhibitor 1-(2-trifluoromethyl-phenyl)-imidazole. We conclude that, in skeletal muscle, NO increases GLUT4 expression via a cGMP- and AMPK-dependent mechanism. The data are consistent with a role for NO in the regulation of AMPK, possibly via control of cellular activity of AMPK kinases and/or AMPK phosphatases.

Arginine supplementation induces myoblast fusion via augmentation of nitric oxide production.

The semi-essential amino acid, L-arginine (L-Arg), is the substrate for endogenous synthesis of nitric oxide, a molecule that is involved in myoblast proliferation and fusion. Since L-Arg supply may limit nitric oxide synthase (NOS) activity in endothelial cells, we examined L-Arg supplementation in differentiating mouse myoblasts and tested the hypothesis that L-Arg exerts direct effects on myoblast fusion via augmentation of endogenous nitric oxide production. C(2)C(12) myoblasts in differentiation media received one of the following treatments for 120 h: 1 mM L-Arg, 0.1 mM N-nitro-L-arginine methyl ester (L-NAME), L-Arg + L-NAME, 10 mM L-Lysine, or no supplement (Control). Cultures were fixed and stained with hematoxylin and eosin for microphotometric image analysis of myotube density, nuclear density, and fusion index (% of total nuclei in myotubes). Endogenous production of nitric oxide during the treatment period peaked between 24 and 48 h. L-Arg amplified nitric oxide production between 0 and 24 h and increased myotube density, total nuclei number, and nuclear fusion index. These L-Arg effects were prevented by the NOS inhibitor, L-NAME. Further, L-Lysine, a competitive inhibitor of L-Arg uptake, repressed nitric oxide production and reduced myotube density and fusion index. In summary, L-Arg augments myotube formation and increases nitric oxide production in a process limited by cellular L-Arg uptake.

Ibuprofen inhibits skeletal muscle hypertrophy in rats.

PURPOSE: We sought to determine whether cyclooxygenase (COX) activity is necessary for overload-induced growth of adult rat skeletal muscle, and whether nitric oxide synthase (NOS) activity is involved in upregulation of COX messenger RNA (mRNA) expression in skeletal muscle. METHODS: Unilateral surgical removal of the gastrocnemius and soleus was performed on the right hindlimb of 16 female Sprague-Dawley rats (approximately 230 g) to induce chronic overload (OL) of the plantaris for 14 d, with sham surgeries performed on the contralateral leg as a normally loaded (NL) control. Half of the rats were treated with the nonspecific COX inhibitor, ibuprofen (0.2 mg.mL(-1) in drinking water; approximately 20 mg.kg(-1).d(-1)). In a second experiment, the plantaris was unilaterally overloaded for 5 or 14 d in male rats (approximately 350 g; N = 16 rats per time point) and half of the animals were treated with the NOS inhibitor, L-NAME (0.75 mg.mL(-1) in drinking water; approximately 90 mg.kg(-1).d(-1)). RESULTS: Ibuprofen treatment inhibited plantaris hypertrophy by approximately 50% (P < 0.05) following 14 d of OL, as did L-NAME treatment (P < 0.05). COX-1 and COX-2 mRNA did not differ between any groups at 5 d. At 14 d, however, L-NAME caused a 30-fold increase in plantaris COX-1 mRNA expression independent of loading condition. Additionally, OL induced a 20-fold increase in COX-2 mRNA expression compared with NL (P < 0.05) at 14 d, without affecting COX-1 mRNA level. L-NAME treatment significantly inhibited OL-induced expression of COX-2 mRNA. CONCLUSION: COX activity is important for in vivo muscle hypertrophy, and plantaris overload is associated with NOS activity-dependent COX-2 expression.