ROLES OF INSULIN-RECEPTOR SUBSTRATE-1 AND SHC ON INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR SIGNALING IN EARLY PASSAGES OF CULTURED HUMAN FIBROBLASTS

Insulin-like growth factor-I (IGF-I) improves glucose metabolism and g rowth in patients with leprechaunism. We investigated signal transduct ion through IGF-I receptor in comparison with epidermal growth factor (EGF) receptor in early passages of cultured skin fibroblasts from a n ormal subject and a patient with leprechaunism whose insulin receptor tyrosine kinase was almost nonexistent. Insulin receptor substrate-1 ( IRS-1) became tyro sine-phosphorylated and bound growth factor recepto r-bound protein 2 (GRB2) quickly by IGF-I. The association of She with GRB2 by IGF-I was detected by immunoblot with anti-She antibody but w as hardly visible with antiphosphotyrosine antibody, which was in mark ed contrast to efficient tyrosine phosphorylation of She by EGF. Howev er, the potency of IGF-I for DNA synthesis was far stronger than EGF, which was not parallel with the potency of these growth factors to act ivate She or MAP kinase. Rather, phosphatidylinositol (PI) S-kinase ac tivity, which was activated by IGF-I about 5- to 10-fold more strongly than EGF, appeared to correlate with mitogenesis. Signal transduction pathways following IGF-I receptor or EGF receptor activation were ind istinguishable between the normal subject and the patient. Our results strongly suggest that in human skin fibroblasts, which rep, resent a more physiological cell culture: 1) IRS-I, rather than She, is the maj or tyrosine-phosphorylated protein binding GRB2 in initial phase of IG F-I signaling; 2) mitogenic potency of receptor tyrosine kinases such as IGF-I receptor and EGF receptor may not be determined solely by the amount of Shc-GRB2 complex or the activity of MAP kinase; and 3) in c ontrast to previous reports, IGF-I and EGF receptor signalings are not defective in leprechaunism.