Affinity For IGF-I Research Articles

The Actions of Insulin-Like Growth Factors I and II on Cultured Sertoli Cells*

Cultured Sertoli cells prepared from young rats (13 days old) showed increased incorporation of [3H]thymidine into DNA, increased production of lactate, and increased incorporation of [3H]leucine into protein in response to micromolar concentrations of insulin and nanomolar concentrations of insulin-like growth factor II (IGF-II). The first of these responses was also seen with nanomolar concentrations of IGF-I. Receptor affinity labeling studies using Sertoli cell membranes and whole Sertoli cells showed that these cells possess abundant growth factor receptors of type I (mol wt, 350,000) that show high affinity for IGF-I, moderate affinity for IGF-II, and low affinity for insulin. Sertoli cell membranes also show abundant growth factor receptors of type II (mol wt, 230,000) that show high affinity for IGF-II, moderate affinity for IGF-I, and no detectable affinity for insulin. Moreover, the responses of the Sertoli cell to insulin were observed at concentrations of 100 nM or higher, whereas insulin receptors are known to be saturated by insulin at concentrations of 10 nM or less. It is, therefore, concluded that Sertoli cells possess receptors for IGF-I and that the responses observed to insulin may result from binding of these hormones to receptors for IGF-I.

Structure of the insulin-like growth factor receptor in chicken embryo fibroblasts.

The insulin-like growth factors (IGFs) and insulin stimulate DNA synthesis and cell multiplication in chicken embryo fibroblasts in culture. This response appears to be mediated by interaction with a single type of IGF receptor. The present study examines the subunit structure of this receptor by covalently crosslinking two 125I-labeled IGFs, IGF-I and multiplication-stimulating activity (MSA), to chicken embryo fibroblasts by using disuccinimidyl suberate. After solubilization, NaDodSO4/polyacrylamide gel electrophoresis, and autoradiography, IGF receptor complexes of appropriate specificity were identified; they had Mr approximately 130,000 (major band) and approximately 260,000 (minor band) under reducing conditions and Mr greater than 300,000 without disulfide reduction. The proportion of the Mr 260,000 component increased with increasing concentration of crosslinking agent, suggesting that it was formed from smaller proteins during the crosslinking procedure. The IGF receptor in chicken embryo fibroblasts resembles the insulin receptor in size and structure but can be distinguished by a higher affinity for IGF-I and MSA than for insulin. Although IGF receptors with different structure and specificity have been recognized in other tissues, the function of these binding sites is unknown. The present study demonstrates that the IGF receptor of chicken embryo fibroblasts that appears to mediate the growth-promoting effects of the IGFs contains a Mr approximately 130,000 binding subunit and exists as a native receptor complex of Mr greater than 300,000.