Insulin‐and insulin‐like growth‐factor‐I receptor tyrosine‐kinase activities in human renal carcinoma

We studied expression and functional characteristics of the insulin- and insulin-like-growth-factor-I (IGF-I) receptors in human renal carcinoma. Ligand-binding properties and tyrosine-kinase activity of both receptors, as well as the expression of the 2 isoforms of the human insulin receptor (HIR-A and -B) were analyzed in renal carcinoma and normal adjacent kidney tissue of 8 adult patients. Partially purified insulin- and IGF-I receptors from normal and renal cell carcinoma tissue possessed identical affinities for their ligands. Renal cell carcinoma, however, contained 3- to 4-fold more specific insulin-binding sites and 2-fold more IGF-I binding sites than adjacent normal kidney tissue. In addition, we determined the relative content of insulin/IGF-I receptor hybrids in both tissues. Renal cell carcinoma and adjacent normal tissue revealed similar amounts of insulin/IGF-I receptor hybrids, i.e., 44 +/- 8.2% of tracer IGF-I binding in normal tissue and 46 +/- 12.0% in renal cell carcinoma. When equal amounts of insulin- and IGF-I receptor protein were studied, we found significantly increased receptor autophosphorylation and elevated substrate phosphorylation in carcinoma tissue. To assess whether the differences in insulin-receptor tyrosine-kinase activity were caused by an altered pattern of insulin receptor isoform expression, we determined mRNA levels for HIR-A and -B. The 2 insulin receptor isoforms were, however, expressed in highly variable ratios in both normal and tumor tissue. Our experiments show that renal carcinoma expresses an elevated amount of insulin- and IGF-I receptor protein with increased specific autophosphorylation and tyrosine-kinase activity each. The increase of insulin-receptor tyrosine-kinase activity in renal carcinoma cannot be explained by an altered expression pattern of insulin receptor isoforms.