ROLE OF INSULIN-LIKE GROWTH-FACTORS IN AUTOCRINE GROWTH OF HUMAN RETINOBLASTOMA Y79 CELLS

In this study, we have demonstrated that human retinoblastoma Y79 cell s produce insulin-like growth factors (IGFs) type I and type Il and re lease them into the medium. We have also ascertained, by means of comp etitive studies and cross-linking procedure, that Y79 cells contain th e type-I IGF receptor (IGF-IR). Furthermore, surface-bound IGF-I is in ternalised by the receptor, then degraded to amino acids. Insulin, IGF -I and IGF-II caused down-regulation of IGF-IR; the effect is concentr ation and time dependant. Scatchard analysis demonstrated that incubat ion with insulin markedly decreased the binding capacity measured for IGF-I while the apparent K-d value calculated for IGF-I binding was no t significantly modified. IGF-I, IGF-II and insulin induced tyrosine p hosphorylation of IGF-IR. Tyrosine phosphorylation of this receptor wi th, however, a less strong signal, was detectable even in cells cultur ed in serum-free medium without the addition of any exogenous growth f actor. Similar results have been found concerning the tyrosine phospho rylation of insulin receptor substrate-1 (IRS 1). Tyrosine phosphoryla tion of both IGF-IR and IRS 1, either under basal conditions or after stimulation with growth factors, was strongly inhibited when alpha-IR3 , a monoclonal antibody to IGF-IR, was added to the culture. IGF-I was capable of inducing Y79 cell proliferation and its effect was entirel y inhibited by the addition of alpha-IR3. This antibody also markedly reduced the proliferation of Y79 cells cultured in serum-free medium n ot supplemented with stimulatory factors. Our results indicate that IG F-I and IGF-IR mediate an autocrine growth mechanism in Y79 cells.