Soybean isoflavones, genistein and genistin, inhibit rat myoblast proliferation, fusion and myotube protein synthesis.

The isoflavones, genistein and genistin, are cytotoxic in vitro (e.g. , inhibition of cell proliferation), due in part to inhibition of protein tyrosine kinase and DNA topoisomerase activities. Normal cell functions associated with these enzymatic activities could potentially be impaired in animals through ingestion of soybean products. In this study, cultured rat myogenic cells (L8) were used to determine whether genistein or genistin influences myoblast proliferation and fusion, and myotube protein synthesis and degradation. Genistein or genistin was dissolved in dimethylsulfoxide and included in the culture medium at 0, 1, 10 or 100 micromol/L. Myoblast proliferation was measured by methyl-3H-thymidine incorporation over 48 h. Myoblast differentiation was evaluated by the number of nuclei in multinucleated myotubes. Myotube protein synthesis was measured by 2-h 3H-amino acid incorporation into the myosin and total protein pools after acute (2 h) or chronic (24 h) exposure to similar treatments; protein degradation was measured by measuring radioactivity in protein pools following a time course of protein breakdown after myotube proteins were prelabeled with 3H-amino acids. Genistein or genistin strongly inhibited in vitro myoblast proliferation (P < 0.001) and fusion (P < 0.001) in a dose-dependent manner with effective genistein concentration as low as 1 micromol/L. Genistein or genistin inhibited protein accretion in myotubes (P < 0.001). Decreased protein accretion is largely a result of inhibition on cellular (myofibrillar) protein synthesis rate. No adverse effect on protein degradation was observed. Results suggest that if sufficient circulating concentrations are reached in tissues of animals consuming soy products, genistein/genistin can potentially affect normal muscle growth and development.