Postsynthetic regulation of insulin-like growth factor-binding protein-3 by MCF-7 human breast cancer cells in culture.
Breast cancer cells are exposed to insulin-like growth factors (IGFs) which stimulate their proliferation, and to IFG-binding proteins (IGFBPs) which sequester and modulate IGF action. The primary circulatory IGFBP is IGFBP-3. In the present study, cultured MCF-7 breast cancer regulated clearance of IGFBP-3 via both cell association and proteolysis. Exogenously added IGFBP-3 was significantly cleared from the medium over time yielding the formation of smaller sized immunodetected fragments. Clearance was inhibited by IGF-I and -II. In contrast, clearance was not affected by growth factors and an IGF-analog having mitogenic activity but not binding to IGFBPs. In fact, activity of the IGFs and analogs paralleled their degree of binding to the IGFBP, suggesting that the IGF-binding altered IGFBP-3 making it less susceptible to clearance. Qualitatively similar results were obtained when these experiments were conducted using cell-free conditioned medium, thus suggesting the presence of secreted protease(s). However, level of proteolytic activity was much less than that found in the presence of cells. Clearance of rhIGFBP-3 also involved binding to the cell. Disappearance of rhIGFBP-3 was shown to be attenuated by heparin, which blocks cell surface binding sites. In contrast, compounds which block internalization did not inhibit IGFBP-3 clearance.
Grimes
R W
RW
Department of Medicine, Pennsylvania State University College of Medicine, Hershey, 17033, USA.
Manni
A
A
Hammond
J M
JM
D001943:Breast Neoplasms / Q000378:metabolism*; D017077:Culture Media, Conditioned / Q000494:pharmacology; D005260:Female; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000096:biosynthesis* / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000502:physiology*; D011480:Protease Inhibitors / Q000494:pharmacology; D011994:Recombinant Proteins / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D017077:Culture Media, Conditioned; D018972:Insulin-Like Growth Factor Binding Protein 3; D011480:Protease Inhibitors; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D006493:Heparin