Metalloendoprotease inhibitors that block fusion also prevent biochemical differentiation in L6 myoblasts

The effect of metalloendoprotease inhibitors on the biochemical differentiation of the rat skeletal muscle line, L6, was investigated. Confluent unfused L6 cells exposed briefly to 1,10-phenanthroline, a chelator of divalent metal cations, or continuously to dipeptide amide metalloendoprotease substrates that are blocked at the NH2-terminals, N-carbobenzyloxyserylleucyl amide and N-carbobenzyloxyglycylleucyl amide, did not fuse or express creatine kinase, myosin heavy chain, or alpha-actin. These effects were reversible and dose-dependent. Exposure to N-carbobenzyloxylglycylglycyl amide, which is not a metalloendoprotease inhibitor, had no effect. As the differentiation in a culture progressed, 1,10-phenanthroline became less effective in blocking the accumulation of creatine kinase and myosin heavy chain. Exposure of partially fused cultures to N-carbobenzyloxyserylleucyl amide prevented any further accumulation of muscle-specific proteins. In confluent cultures where cell division was blocked before the onset of differentiation, N-carbobenzyloxyserylleucyl amide still prevented fusion and the induction of creatine kinase. This indicates that these inhibitors do not act by interfering with the cell cycle. Experiments that measured DNA synthesis rates, plating efficiencies, and the effects of sequential dipeptide and dimethyl sulfoxide treatments indicate that L6 myoblasts do not irreversibly withdraw from the cell cycle when exposed to N-carbobenzyloxyserylleucyl amide. These results are consistent with the role of a metalloendoprotease in initiating the terminal differentiation of cultured muscle cells.