INSULIN-LIKE GROWTH FACTOR (IGF) BINDING PROTEINS INHIBIT ICF-I-INDUCED DIFFERENTIATION IN L6E9 RAT SKELETAL MUSCLE CELLS

Abstract

The insulin-like growth factors (IGFs) stimulate the differentiation of muscle cells. IGF binding proteins (BPs), which are expressed by skeletal muscle cells, may enhance or inhibit IGF actions. To explore the role of muscle IGF-BPs In IGF-induced myogenesis, we compared the effects of IGF-I and des(1-3)IGF-I (gift of Genentech, Inc.), an IGF-I analog with markedly reduced affinity for IGF-BPs but normal affinity for the IGF-I receptor, on creatine phosphokinase (CK) activity in rat L6E9 skeletal muscle cells. CK colorimetric assay. Conditioned media were analyzed by 125I-IGF-I ligand blot. Myoblasts grown in medium supplemented with 20% fetal bovine serum were transferred to medium supplemented with 2% heat inactivated horse serum and studied for up to 5 days in the absence or presence of 3 nM IGF-I or des-IGF-I. In comparison to untreated cells, cells treated with ICF-I demonstrated an 8-9 fold increase in CK activity. In contrast, cells treated with des-IGF-I demonstrated a 19-20 fold increase in CK activity versus untreated cells. In dose-response studies (0.7nM-7nH), des-IGF-I was 3-6 times more potent than IGF-I in inducing muscle CK activity. IGF-BP production was greatest at the time at which the largest difference in CK activity induced by des-IGF-I vs. IGF-I was observed. Since an IGF-I analog with reduced affinity for IGF-BPs is more potent than native IGF-I in stimulating CK activity, these data suggest that IGF-BPs expressed by skeletal muscle cells inhibit differentiation induced by IGF-I.