Differentiation of BC 3 H1 and primary skeletal muscle cells and the activity of their endogenous insulin‐degrading enzyme are inhibited by the same metalloendoprotease inhibitors

Upon reduction of serum in their media, mouse BC 3 H1 muscle cells withdraw from the cell cycle and begin to differentiate. In differentiating cells, the induction of muscle‐specific genes is accompanied by a distinct morphological chance. However, differentiated BC 3 H1 cells do not fuse with each other; they remain mononucleated. Metalloendoprotease inhibitors selectively block the differentiation of BC 3 H1 cells while inhibitors of other protease types are ineffective. In these cells, the degradation of the internalized insulin is initiated by a 110 kDa, non‐lysosomal protease known as the insulin‐degrading enzyme. The same metalloendoprotease inhibitors that block BC 3 H1 differentiation also inhibit, with a similar specificity and potency, the in vitro and the in vivo degradation of insulin by the insulin‐degrading enzyme. When the serum in the medium is reduced, the activity of the insulin‐degrading enzyme in the cell cytoplasm increases rapidly. This increase precedes any detectable change in the differentiation state of these cells by about 12 hours. These results, together with very similar ones obtained with primary rat skeletal muscle cells, support our earlier proposal that the insulin‐degrading enzyme is the metalloendoprotease involved in the initiation of the morphological and biochemical differentiation of muscle cells in culture.