The insulin-like growth factors (IGFs) I and II are present in extracellular fluids associated with specific binding proteins (IGFBPs) that can modify their biologic actions. These studies were undertaken to determine which forms of IGFBP are secreted by endometrial carcinoma (HEC-1 B) and breast carcinoma (MDA-231) cells, to characterize variables that control IGFBP secretion, and to study the effect of IGFBP-1 and IGFBP-2 on IGF-I stimulated cell proliferation. Secreted IGFBPs were identified by ligand blotting and IGFBP-1 was quantified using a specific radioimmunoassay (RIA). MDA-231 cell conditioned media (CM) contained four (43,000, 39,000, 30,000 and 24,000 M,) forms of IGFBP, and HEC-1 B cell CM contained three forms (39,000, 34,000 and 30,000 MJ. lmmunoblotting showed that the 30,000 M, form secreted by both cell types was IGFBP-1. Likewise the 34,000 M, band in HEC-IB media reacted with IGFBP-2 antiserum and the 39,000 and 43,000 M, bands reacted with IGFBP-3 antiserum.
IGF-I stimulated the secretion of IGFBP-3 from both cell types and IGFBP-2 from HEC-1 B cells but either decreased or caused no change in secretion of IGFBP-1 and a 24,000 M, form. In contrast, insulin inhibited the secretion of IGFBP-1 but increased the secretion of the 24,000 M, form. Compounds that elevate intracellular CAMP levels increased the secretion of IGFBP-3, IGFBP-1, and the 24,000 M, form from both MDA-231 and HEC-1 B cells.
When sparse cultures of MDA-231 cells were used, addition of IGF-I caused a 24% increase in cell number after 48 hr. This mitogenic response was enhanced by the presence of recombinant human IGFBP-1 (45% increase in cell number, P < 0.001). Bovine IGFBP-2 did not potentiate IGF-I stimulated cell proliferation.
These findings show that two tumor cell lines secrete distinct forms of IGFBPs and that there is differential regulation of IGFBP secretion. At least one form secreted by both tumors may act as a positive autocrine modulator of IGF-1's growth stimulating actions.