CHARACTERIZATION OF INSULIN-LIKE GROWTH-FACTOR RECEPTORS IN HUMAN THYROID-TISSUE

We have characterized the binding of I-125-IGF-I and I-125-IGF-II to plasma membranes purified from human thyroid tissue. IGF binding was time- and temperature-dependent. At 4-degrees-C, maximal specific binding of I-125-IGF-I was 17.3 +/- 2.5% and of I-125-IGF-II was 8.8 +/- 2.0% (mean +/- SD /60 mug membrane protein). I-125-IGF-I binding was inhibited completely by unlabeled IGF-I, IGF-II, insulin, and the type-I IGF receptor monoclonal antibody, alphaIR-3. I-125-IGF-II was inhibited completely by unlabeled IGF-II and nearly completely by IGF-I. I-125-IGF-II binding also was inhibited significantly by insulin, suggesting that much or all of the IGF-II was bound to the type-I IGF receptor. Scatchard analysis revealed a single class of binding sites with a K(d) of 6.0 +/- 4.2 x 10(-10)M for IGF-I binding and 5.7 +/- 1.3 x 10(-10)M for IGF-II binding. IGF-I binding was inhibited by a variety of salts in a dose-dependent manner, calcium and magnesium salts being more effective than sodium or potassium salts. Affinity crosslinking of I-125-IGF-I and -II showed clear evidence only for type-I IGF receptors. Thus, a crude plasma membrane fraction of human thyroid tissue expresses predominantly type-I IGF receptors.