Expression of Two Insulin-Like Growth Factor-Binding Proteins in a Human Endometrial Cancer Cell Line: Structural, Immunological, and Genetic Characterization*

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Abstract
Insulin-like growth factors (IGFs) bind with high affinity to specific proteins in human serum, cerebrospinal fluid (CSF), and amniotic fluid. In serum, IGFs are bound to a complex with an apparent mol wt of 150K in which an acid-stable binding protein of 53K, termed BP-53, is found. In amniotic fluid, a different binding protein with an apparent mol wt of 25K, termed hBP-25, has been identified. This binding protein is secreted by endometrial cells and has been shown to block the binding of IGFs to their membrane receptors and inhibit the mitogenic action of the IGFs on human choriocarcinoma cells. It has been proposed that hBP-25 modulates the action of IGFs on endometrial tissue, especially during pregnancy. We have identified two binding proteins produced by an endometrial adenocarcinoma, HEC 1A, neither of which is related structurally or genetically to hBP-25. One is present in two glycosylated forms with apparent mol wt of 37K and 40K, both of which are immunoprecipitated with an antiserum made to BP-53. Both forms are reduced to a core protein of 30K upon digestion with endoglycosidase-F. Furthermore, BP-53-specific RNA was detected in HEC 1A cells. A second binding protein with an apparent mol wt of 32K was also detected and did not increase in size upon treatment with endo glycosidase F. These two binding proteins were partially purified by a cominbation of wheat germ agglutinin and IGF affinity chromatography, and polyclonal antibodies were generated. The polyclonal antiserum specifically recognizes a 32K protein in human CSF, suggesting that the HEC 1A cells produce the same binding protein that is predominant in CSF. The findings suggest that IGF action in the endometrium may be modulated by more than a single binding protein, and that at least three structurally distinct human IGF binding proteins exist.

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