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 IGF-binding proteins (IGFB Ps) which sequester and modulate IGF action. The primary circulatory I GFBP is IGFBP-3. In the present study, cultured MCF-7 breast cancer ce lls regulated clearance of IGFBP-3 via both cell association and prote olysis. Exogenously added IGFBP-3 was significantly cleared from the m edium over time yielding the formation of smaller sized immunodetected fragments. Clearance was inhibited by IGF-I and -II. In contrast, cle arance was not affected by growth factors and an IGF-analog having mit ogenic activity but not binding to IGFBPs. In fact, activity of the IG Fs and analogs paralleled their degree of binding to the IGFBP, sugges ting that the TGF-binding altered IGFBP-3 making it less susceptible t o clearance. Qualitatively similar results were obtained when these ex periments were conducted using cell-free conditioned medium, thus sugg esting the presence of secreted protease(s). However, level of proteol ytic activity was much less than that found in the presence of cells. Clearance of rhIGFBP-3 also involved binding to the cell. Disappearanc e of rhIGFBP-3 was shown to be attenuated by heparin, which blocks cel l surface binding sites. In contrast, compounds which block internaliz ation did not inhibit IGFBP-3 clearance.