Demonstration of receptors for insulin and insulin-like growth factors on Swarm rat chondrosarcoma chondrocytes. Evidence that insulin stimulates proteoglycan synthesis through the insulin receptor.

Abstract

The insulin-like growth factor, multiplication stimulating activity (MSA), and insulin were recently shown to stimulate proteoglycan synthesis in monolayer cultures of chondrocytes derived from the Swarm rat chondrosarcoma. Insulin produced significant stimulation at a concentration of less than 1 ng/ml, suggesting that insulin was acting through the insulin receptor rather than through a somatomedin receptor. In this paper we have shown that the insulin-like growth factors IGF-I and IGF-II also are potent stimulators of [35S]sulfate incorporation into macromolecules recovered from the medium and cell layer matrix of the chondrosarcoma chondrocytes. Proinsulin was 3% as potent as insulin in stimulating [35S]sulfate incorporation. We identified receptors for insulin and the insulin-like growth factors, MSA, IGF-I, and IGF-II. Insulin, at concentrations 1000 times the concentration required to produce the biologic response, did not compete for binding of 125I-MSA-II-1 or of 125I-IGF-II and only partially competed for 125I-IGF-I binding. Anti-insulin receptor IgG stimulated proteoglycan synthesis and competed for 125I-insulin binding. Fab fragment prepared from anti-insulin receptor IgG completely blocked the stimulation of [35S]sulfate incorporation into macromolecules by insulin while only partially inhibiting the biologic response to insulin-like growth factors, MSA, IGF-I, and IGF-II. Similarly, the anti-insulin receptor IgG only partially inhibited the binding of 125I-IGF-I and 125I-IGF-II while completely blocking the binding of 125I-insulin. We conclude that insulin stimulates proteoglycan synthesis in the chondrosarcoma chondrocytes by acting through the insulin receptor whereas the insulin-like growth factors probably act through their own receptors.