Overexpression of the insulin-like growth factor I receptor in human pheochromocytomas

Abstract

In order to determine the role of the IGF-I receptor (IGF-IR) in human pheochromocytomas we have compared the expression of the IGF-IR in normal tissues and in pheochromocytomas with regard to the IGF-IR mRNA levels and ligand binding. By semiquantitative reverse transcription polymerase chain reaction (RT-PCR), the mRNA of the IGF-IR could be detected in all samples of normal adrenomedullary cells (n = 13) and pheochromocytomas (n = 16). However, pheochromocytomas exhibited 2.8-fold higher mean IGF-IR mRNA levels than normal adrenomedullary cells (2.8 +/- 0.5 x 10(5) molecules/mug RNA vs 7.8 +/- 1.2 x 10(5) molecules/mug RNA; P < 0.001). This overexpression of the IGF-IR in pheochromocytomas could be confirmed at the protein level by binding studies. Radioligand assays and Scatchard analysis revealed a single class of high affinity IGF-IR binding sites with a similar dissociation constant (Kd: 0.32 +/- 0.1 nmol/l vs 0.22 +/- 0.08 nmol/l) for both normal adrenomedullary cells and pheochromocytomas. However, specific 125I-labeled IGF-I binding and the calculated receptor concentration were significantly elevated in pheochromocytomas as compared with normal adrenomedullary cells (58.3 +/- 5 vs 24.3 +/- 12 nmol/kg protein; P < 0.05). In summary, our results demonstrate significant overexpression of the IGF-IR in human pheochromocytomas. This suggests a possible role of the IGF system in the pathogenesis of adrenal neoplasia and thus IGF-IR may be a target for future therapeutic approaches.