Insulin-like growth factor receptors in normal and tumorous adult human adrenocortical glands

We have identified and characterized insulin-like growth factor (IGF)-I and IGF-II/mannose-6-phosphate (IGF-II/M6P) receptors in normal adult human adrenocortical tissue. Furthermore, we investigated the IGF-I receptor concentration and binding characteristics in benign and carcinomatous adrenocortical tumors. Membrane preparations of 14 normal adrenocortical glands showed a mean specific I-125-IGF-I binding (SE) of 5.0+/-0.5% and a competition by unlabeled ligands which is characteristic of the IGF-I receptor. The Scatchard analysis revealed a single class of high affinity binding sites with a dissociation constant (K-d) of 0.16+/-0.03 nmol/l, and a receptor concentration (RC) of 19.2+/-2.5 nmol/kg protein. Affinity cross-linking experiments with normal and tumorous adrenocortical tissue displayed a band at an apparent molecular mass of 135 kDa, corresponding to the size of the normal alpha-subunit of the IGF-I receptor, In agreement, I-125-IGF-II binding to normal adult human adrenocortical membranes was characteristic for the IGF-II/M6P receptor, and the Scatchard analysis revealed the presence of a single class of high affinity binding sites (SB 7.5+/-0.5%, RC 1137+/-265 nmol/kg protein, K-d 2.20+/-0.46 nmol/l, n=6). The identity of the IGF-II/M6P receptor in adrenocortical tissue was further confirmed by Western blotting showing a specific band at 220kDa. When I-125-IGF-I binding in adrenocortical hyperplasias (SB 4.1+/-0.4%, RC 19.6+/-2.0 nmol/kg protein, K-d 0.19+/-0.04 nmol/l, n=4) and adenomas (SB 4.0+/-1.1%, RC 17.5+/-3.1 nmol/kg protein, K-d 0.21+/-0.04 nmol/l, n=4) was compared with the I-125-IGF-I binding in normal adrenocortical tissue, similar IGF-I receptor concentration and binding kinetics were found. In contrast, three out of four hormonally active adrenocortical carcinomas showed a strongly elevated specific I-125-IGF-I binding with a 3- to 4-fold increase in IGF-I receptor concentration, as compared with normal adrenocortical tissue. This resulted in a significantly higher mean specific binding and receptor concentration in adrenocortical carcinomas, while the binding kinetics and the size of the alpha-subunit of the IGF-I receptor remained unaltered (n=4, SB 13.8+/-4.2%, RC 72.2+/-21.3 nmol/kg protein, K-d 0.17+/-0.02 nmol/l). In summary, we show that intact IGF-I and IGF-II receptors are present in normal adult human adrenocortical tissue. While the abundance of the IGF-I receptor in adrenocortical hyperplasias and adenomas was similar to normal tissue, a strong overexpression of the intact TGF-T receptor was found in three out of four adrenocortical carcinomas.