Insulin-induced increase in insulin binding to cultured chondrosarcoma chondrocytes

125I-Insulin binding to rat chondrosarcoma chondrocytes increased, rather than decreased, when these transformed cells were maintained for 1-8 days in primary culture with incremental concentrations of insulin (3 to 1000 ng/ml). The increase in 125I-insulin binding was reversible and depended on the concentration of insulin in which the cells had been previously cultured. The chondrocytes did not change their phenotype in response to the incremental insulin concentrations in the culture medium. The variations in binding of radioligand were not due to different rates of internalization of the ligand-receptor complex, to protease degradation of the radioligand, to shedding of the insulin receptor, or to secretion of an inhibitory insulin binding component. Scatchard analyses of 125I-insulin binding data indicated the induction of a high affinity insulin receptor in the cells cultured in the presence of insulin. The insulin-induced increase in 125I-insulin binding was not associated with an increase in the binding of 125I-labeled multiplication-stimulating activity (125I-MSA), an insulin-like growth factor. Conversely, culturing the chondrocytes in 0.1-1.0 micrograms/ml of MSA did not alter the subsequent binding of either 125I-MSA or 125I-insulin. The insulin-induced increase in insulin binding to these transformed cells may be an abnormality in metabolic control that facilitates growth of the tumor in vivo.