Characterization of insulin-like growth factor receptors in the insulin-responsive R3230AC mammary adenocarcinoma.
Academic Article
Overview
abstract
To ascertain whether insulin-like growth factors (IGF-1 and IGF-2) affect the estrogen- and insulin-responsive R3230AC mammary carcinoma, studies of IGF-1 and IGF-2 receptors were conducted on primary-culture tumor cells and on membranes purified from whole tumors. By saturation binding analysis, cells in culture displayed one class of IGF-1 binding sites with an affinity constant, Kd, of 5.5 +/- 0.7 x 10(-9) M, whereas in membrane preparations, high and low affinity IGF-1 binding sites, with Kd's of 5 +/- 1.7 x 10(-9) M and 1 +/- 0.4 x 10(-7) M, respectively, were detected. Specificity of binding was demonstrated by 85% displacement of 125I-IGF-1 with 1,000-fold excess unlabeled IGF-1 and 50% displacement with 6.5 x 10(-9) M IGF-1 or 3 microM insulin. Binding sites for IGF-2 were also demonstrable in cultured cells, having a Kd of 7 +/- 0.8 x 10(-10) M, and 50% displacement was obtained with 1.5 x 10(-9) M IGF-2 or 1.5 x 10(-8) M IGF-1. Cross-linking experiments on cultured cells confirmed the presence of IGF-1 and IGF-2 receptors. In purified tumor membranes, IGF binding proteins (M(r) 28,000-32,000) were also detected; their labeling was not displaced by 10(-5) M insulin. In vitro, the tumor cells secrete one or more IGF binding proteins into the medium. Despite the fact that these cells expressed specific IGF receptors, their growth was apparently independent of these growth factors, since neither IGF-1 nor IGF-2 was mitogenic for R3230AC cells in vitro. Nevertheless, IGF-1 caused concentration-related significant increases in the plating efficiency of these cells. Further studies are necessary to determine the functional role of these growth factors, their receptors, and their binding proteins in the biology of this rodent mammary tumor.
