AUTOCATALYTIC INACTIVATION OF LYSOSOMAL CATHEPSINS IS ASSOCIATED WITHINHIBITION OF PROTEIN BREAKDOWN BY INSULIN-LIKE GROWTH-FACTOR-I (IGF-1) IN MYOTUBES

Protein breakdown was monitored in C2C12 myotubes as the rate of relea se of radioactivity after prelabeling cell protein with [H-3] tyrosine . IGF-1 (13 nM) and insulin (100 nM) prolonged the half-life of long-l ived proteins. Enzymatic activities of cathepsins B and B+L were inhib ited by the addition of IGF-1 or insulin. Immunoblotting of cathepsins B and L revealed extensive degradation of heavy chain forms by IGF-1. However, neither expression of cathepsins B and L genes nor expressio n of cystatin beta, an intrinsic inhibitor for cathepsins, was influen ced. The addition of E-64, trans-2,3-epoxypropionyl-leucylamide-(4-gua nidino) butane, a inhibitor of cathepsins B and L, increased protein c ontents of heavy chains of cathepsins B and L in the IGF-1 treated cel ls. Inhibition of protein breakdown by IGF-1 is mediated by autocataly tic inactivation of lysosomal cathepsins B and L. (C) 1995 Academic Pr ess. Inc.