Characterization of insulin-like growth factor 1 receptors (IGF1-R) in human breast cancer cell lines.

The binding characteristics of IGF1 on membranes prepared from 5 human breast cancer cell lines were investigated in detail. The presence of one class of high affinity IGF1 binding sites was demonstrated (BT-20: n = 230 fmol/mg protein, Ka = 0.7 nM-1; MCF-7: n = 124 fmol/mg protein, Ka = 1 nM-1; T-47D: n = 61 fmol/mg protein, Ka = 1.1 nM-1; HBL-100: n = 18 fmol/mg protein, Ka = 3.2 nM-1; MDA-MB-231: n = 7 fmol/mg protein, Ka = 2.8 nM-1). Chemical cross-linking of 125I IGF1 to breast cancer cell membranes then sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed one major band of relative molecular weight 130000. The specificity of these receptors was studied: native or recombinant IGF1 had the same potency to inhibit 125I IGF1 binding; IGF2 was able to compete for binding, whereas insulin competed with a potency lower than 1/100 that of IGF1. These characteristics of IGF1 binding sites in breast cancer cell membranes correspond to the previously described binding unit of type I IGF receptors (IGF1-R). Finally we determined that for a routinely used standard assay, it was necessary to incubate for 5 h at 4 degrees C a high amount of membrane protein (400 micrograms) and 200,000 cpm of tracer. Considering the known effect of IGF1 on breast cancer cell multiplication, it is tempting to suggest that this factor might play a major role in the growth of breast cancer: the measurement of IGF1-R, using this standardized method, will give an assessment of these tumors IGF1 sensitivity; it can be performed on the membrane fraction obtained when preparing cytosol for steroid receptor assay.