Lipid products of phosphoinositide 3-kinase abrogate genistein-induced fusion inhibition in myoblasts

Bidirectional regulation of genistein on the proliferation and differentiation of C2C12 myoblasts

Genistein is a widely studied phytoestrogen. The effects of genistein on myoblasts were reported long ago, but the conclusions are controversial. In this study, we evaluated the effects of different concentrations of genistein on C2C12 myoblasts. Genistein treatment promoted myoblast proliferation in a dose-dependent manner in the concentration range of 0-2 µM/L, reaching its maximum effect at 2 µM/L. Proliferation then declined, and a concentration higher than 20 µM/L showed significant inhibition. In addition, genistein treatment promoted myoblast differentiation at a dose of 10 µM/L. However, at treatment concentrations higher than 10 µM/L, the effect on myoblast differentiation was rapidly inhibited as the concentration increased. Genistein treatment also down-regulated the expression of miR-222, resulting in increased expression of its target genes, MyoG, MyoD, and ERα and thereby promoting myoblast differentiation. Our results suggest that genistein has a dose-dependent and bidirectional regulation effect on myoblast proliferation and differentiation. We also found that genistein is a miRNA inducer, and it specifically affects the expression of miR-222 to regulate myoblast differentiation.

Deregulation of Nrf2/ARE signaling pathway causes susceptibility of dystrophin-deficient myotubes to menadione-induced oxidative stress

Duchenne muscular dystrophy (DMD) is an X chromosome-linked disorder caused by a mutation in the dystrophin gene. Many previous studies reported that the skeletal muscles of DMD patients were more susceptible to oxidative stress than those of healthy people. However, not much has been known about the responsible mechanism of the differential susceptibility. In this study, we established dystrophin knock-down (DysKD) cell lines by transfection of dystrophin shRNA lentiviral particles into C2 cells and found that DysKD myotubes are more vulnerable to menadione-induced oxidative stress than control myotubes. We focused on the nuclear erythroid 2-related factor 2 (Nrf2) which is a transcription factor that regulates the expression of phase II antioxidant enzymes by binding to the antioxidant response element (ARE). Under menadione-induced oxidative stress, the translocation of Nrf2 to the nucleus is significantly decreased in the DysKD myotubes. In addition, the binding of Nrf2 to ARE site of Bcl-2 gene as well as protein expression of Bcl-2 is decreased compared to the control cells. Interestingly, sulforaphane increased Akt activation and Nrf2 translocation to the nucleus in the DysKD myotubes. These results suggest that the Nrf2 pathway might be the responsible pathway to the oxidative stress-induced muscle damage in DMD.

N-terminal cleavage fragment of focal adhesion kinase is required to activate the survival signalling pathway in cultured myoblasts under oxidative stress

We have previously shown that the cultured L6 myoblasts are susceptible to menadione-induced oxidative stress. Damaged cells were detached from the culture dishes. In the present study, we focused on focal adhesion kinase (FAK), which plays pivotal roles in maintaining focal adhesion function and cell survival. FAK, normally localized at the focal adhesion regions of the myoblasts, was not observed at the regions under oxidative stress induced by menadione and H(2) O(2) . Two cleavage products, 80-kDa N-terminal FAK and 35-kDa C-terminal FAK fragments, as well as full-length FAK (125 kDa) were detected in myoblasts cultured under normal conditions by western blotting with anti-N-terminal FAK or anti-C-terminal FAK sera. Of interest was the finding that the cleavage products of FAK (but not full-length FAK) disappeared under oxidative stress. The cleavage of full-length FAK to N-terminal FAK and C-terminal FAK was inhibited by calpeptin, a specific calpain inhibitor. In addition, pre-incubation of cells with calpeptin resulted in a sharp decrease in survival signals, such as Akt phosphorylation and the ratio of Bcl-2/Bax, under stress conditions. By contrast, not only relative viability, but also Akt phosphorylation and the ratio of Bcl-2/Bax was significantly improved when cells were transfected with a DNA construct of N-terminal FAK-Myc. These results suggest that the N-terminal FAK positively regulates survival signalling in early phases of oxidative stress in the cultured myoblasts.

Dose-dependent effects of genistein and daidzein on protein metabolism in porcine myotube cultures

This study was conducted to investigate whether the isoflavones genistein and daidzein, which are components of soy-based diets, and the estrogen 17beta-estradiol affect differentiation and protein metabolism of porcine skeletal muscle cells in vitro. Serum-free porcine myotube cultures expressing the estrogen receptors ERalpha and ERbeta were treated with various concentrations of genistein, daidzein, or 17beta-estradiol for 26 h. The degree of differentiation by creatine phosphokinase activity was not altered by treatment. At 100 micromol/L both genistein and daidzein caused decreases in protein amount due to cell loss. In addition, 100 micromol/L genistein reduced protein synthesis rate of the surviving cells (P < 0.05) measured as [3H]-phenylalanine incorporation. Interestingly, genistein (0.1 micromol/L), daidzein (10, 100 micromol/L), and 17beta-estradiol (0.1, 1 nmol/L) slightly reduced protein degradation (P < 0.05). The results suggest that both genistein and daidzein affect protein metabolism in a dose-dependent manner and that estrogenic actions may play a role in decreasing protein degradation in porcine skeletal muscle.