Abstract
To clarify whether insulin-like growth factor I (IGF-I) is an autocrine growth factor of rat medullary thyroid carcinoma (MTC) cell line, 6 – 23 (clone 6), IGF-I binding to MTC cell membranes, IGF-I levels in the conditioned culture medium of MTC cells and the effects of IGF-I on methyl-[ 3H]thymidine incorporation to MTC cells were examined. Scatchard analysis of saturation binding studies revealed the association constant and the maximal binding capacity were 1.0 × 10 9 M −1 and 199 fmol/mg of membrane protein, respectively. The binding of [ 125I]IGF-I to MTC cell membranes was inhibited by unlabeled IGF-I, IGF-II and insulin; the relative potencies were IGF-I > IGF-II ⪢ insulin, suggesting the presence of type I IGF receptors in MTC cells. IGF-I levels in the conditioned culture medium of MTC cells were 120 ± 3 pM (mean + SE). IGF-I (10 −10 to 10 −8 M) dose-dependently stimulated methyl-[ 3H]thymidine incorporation to MTC cells. These findings suggest a possible role of IGF-I as an autocrine growth factor for MTC cells.
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