IDENTIFICATION AND CHARACTERIZATION OF INSULIN-LIKE GROWTH-FACTOR-I (TGF-I) AND IGF-II/MANNOSE-6-PHOSPHATE (IGF-II/M6P) RECEPTORS IN BOVINE ADRENAL-CELLS

We have identified and characterized insulin-like growth factor I (IGF-I) and IGF-II/mannose-6-phosphate (IGF-II/M6P) receptors in bovine adrenal cells. Iodine-125-labeled IGF-I ([I-125]IGF-I) binding was characteristic of the IGF-I receptor, and binding kinetics as well as receptor densities were similar in cortical and medullary membranes. Scatchard analysis of [I-125]IGF-I binding to cultured adrenocortical cells showed a single class of high-affinity binding sites with a K-d of 1.4 nmol/l and an average of 150 000 binding sites/cell. Affinity cross-linking experiments displayed a band at an apparent molecular weight of 135 kD, corresponding to the size of the alpha-subunit of the IGF-I receptor. In analogy, the binding of [I-125]IGF-II to bovine adrenal membranes was characteristic of the IGF-II/ M6P receptor and no differences between cortical and medullary membrane fractions were found. Scatchard analysis revealed a single class of high-affinity binding sites in adrenocortical cells with a K-d of 1.1 nmol/l and an average of 280 000 binding sites/cell. The identity of the IGF-II/M6P receptor was confirmed by western blotting of adrenocortical membranes with an anti-IGF-II/M6P receptor antibody and by affinity cross-linking of adrenocortical cells with labeled IGF-II. In conclusion, we have identified and characterized IGF-I and IGF-II/M6P receptors in bovine adrenocortical as well as medullary cells. In both regions of the bovine adrenal gland the IGF-II/M6P receptor is much more abundant than the IGF-I receptor.