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

Abstract

Abstract We have identified and characterized insulin-like growth factor (IGF)-I and IGF-II/mannose-6phosphate (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 125I-IGF-I binding (SB) 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 (Kd) 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 α-subunit of the IGF-I receptor. In agreement, 125I-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, Kd 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 220 kDa. When 125I-IGF-I binding in adrenocortical hyperplasias (SB 4·1±0·4% RC 19·6± 2·0 nmol/kg protein, Kd 0·19±0·04 nmol/l, n=4) and adenomas (SB 4·0±1·1% RC 17·5± 3·1 nmol/kg protein, Kd 0·21±0·nmol/l,04 n=4) was compared with the 125I-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 125I-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 α-subunit of the IGF-I receptor remained unaltered (n =4, SB 13·8±4·2%, RC 72·2 ± 21·3 nmol/kg protein, Kd 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 IGF-I receptor was found in three out of four adrenocortical carcinomas. European Journal of Endocrinology 136 296–303