Vo1.158, No. 1,1989 Janua~ 16,1989 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages 38-44 THE INSULIN-LIKE GROWTH FACTOR BINDING PROTEIN BP-25 IS EXPRESSED BY HUMAN BREAST CANCER CELLS D. Yee*, R.E. Favoni*, R. Lupu*, K.J. Cullen*, G.S. Lebovic, K.K. Huff*, P.D.K. Lee#, Y.L. Lee+, D.R. Powell+, R.B. Dickson*, N. Rosen*, and M.E. Lippman* Medicine Branch, National Cancer Institute Bethesda, Maryland 20892 #Division of Endocrinology, The Children's Hospital Denver, Colorado 80218 +Department of Pediatrics, Baylor College of Medicine Houston, Texas 77054 Received September 27, 1988 Specific binding proteins are thought to modulate the effects of IGF-I. Previous work has demonstrated that media conditioned by human breast cancer cells contains IGF-I binding a Radiolabelled IGF-I incubated with serum-free conditioned media from the breast cancer cell MDA-MB 231 elutedwith an apparent M.W. of 35-40kDa when analyzed by gel filtration chromatography at pH 7.4. The M.W. of this binding activity corresponded to that of BP-25, a binding protein cloned from the hepatocellular carcinoma cell line HepG2. Two breast cancer cell lines, MDA-MB 231 and Hs578T, were found to expressBP-25 RNA. Specific BP-25 radioimmunoassay detected BP-25 production in the conditioned media of these two cell lines. Immunoprecipitation confirmed that metabolically labelled MDA-MB 231 released 30 kDa BP-25 into its medium. This study demonstrates that some breast cancer ceils express the IGF-I bin protein, BP-25. © 1 9 8 9 Academlc P r e s s , I n c . IGF-I is a mediator of the effects of growth hormone and a mitogen for both normal and malignant cell lines. These mitogenic effects are thought to be a result of the interaction between IGF-I and the type I IGF receptor, however IGF-I is also associated with specific serum and extracellular binding proteins that may also modulate the biological actions of IGF-I(1). We have previously shown that IGF-I is a potent mitogen for human breast cancer cell lines and these cells produce an IGF-I related protein that is associated with a bindin * Current address:Lombardi Cancer Research Center, GeorgetownUniversity Medical Center, Washington, D.C. 20007 Abbreviations used: Insulin-like growth factor I-IGF-I, Molecular weight-M.W., Radioimmunoassay-RIA, Sodium dodecyl sulfate-SDS, Polyacrylamide gel electrophoresis-PAGE, Improved minimal essential media-IMEM, Phosphate buffered saline-PBS 0006-291X/89 $1.50 Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved. 38