Characterization and Regulations of the Receptor for Insulin-Like Growth Factor-I in the FRTL-5 Rat Thyroid Follicular Cell Line*

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Abstract
In previous studies we have shown that insulin-like growth factor I (IGF-I) has a mitogenic effect in a line of rat thyroid follicular cells, the FRTL-5. In view of this effect, we undertook studies to identify and characterize some physicochemical and binding properties of the receptor for IGF-I in these cells and to determine what role it plays in the mitogenic activity of insulin and insulin-like growth factors in the FRTL-5 cell. Binding of 125I-labeled IGF-I (biosynthetic Thr59-IGF-I) to FRTL-5 was a function of time, temperature, and pH and was completely inhibited by high concentrations of unlabeled IGF-I. Scatchard plots of four saturation studies revealed a single apparent binding site with an average Ka of 4.2 .+-. 0.6 .times. 109 M-1 (mean .+-. SD) and an average maximum binding capacity of 20 .+-. 2 ptm/100 .mu.g cellular protein. Rat IGF-II (rIGF-II) and insulin were far less potent that IGF-I in inhibiting the binding of [125I] IGF-I and bovine TSH was without effect. 125I-Labeled IGF-II also bound to FRTL-5 cells. Binding was completely inhibited by unlabeled rIGF-II and, with lesser potency, by IGF-I. Even at high concentrations, insulin failed to inhibit the binding of [125I]IGF-II. Disuccinimidyl suberate cross-linked [125I]IGF-I to a moiety in FRTL-5 that had an apparent mol wt of approximately 135,000, as judged from sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Cross-linking of [125I]GF-I was inhibited in a dose-dependent manner by unlabeled IGF-I and, with far lower potency, by rIGF-II and insulin. All three peptides stimulated the incorporation of [3H]thymidine into the DNA of FRTL-5 cells, IGF-I being the most potent, followed in decreasing order of potency of rIGF-II and insulin. The mitogenic activities of these polypeptides correlated well with their abilities to inhibit the binding of [125I]IGF-I. These data indicate that the FRTL-5 cell possesses a receptor for IGF-I that resembles in its binding and physicochemical properties the receptor for IGF-I in other tissues (type I IGF receptor) and that mediates the mitogenic response to IGF-I and insulin in these cells. FRTL-5 cells also contain a receptor for IGF-II (type II IGF receptor), but its role vis-a-vis that of the type IIGF receptor in relation to the mitogenic effect of IGF-II in these cells is uncertain.

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