Regulation of insulin-like growth factor-binding-protein-1, 2, 3, 4, 5, and 6: synthesis, secretion, and gene expression in estrogen receptor-negative human breast carcinoma cells.

Insulin-like growth-factors I and II (IGF-I, II) are potent mitogens for breast carcinoma proliferation. In extracellular fluids, most of the IGF-I and II is associated with specific IGF-binding proteins (IGFBPs). The role of these IGFBPs in IGF action is still not clear, but it has been demonstrated that these proteins may either enhance or inhibit IGF-mediated cellular effects. Synthesis and secretion of IGFBPs have been demonstrated in breast carcinoma cells. In this study, we examined retinoic acid (RA) and IGF-I modulation of IGFBP mRNA and IGFBP levels in two ER-negative human breast carcinoma cell lines. Treatment of MDA-MB-231 and MDA-MB-468 cells with RA increased the levels in conditioned media of a M(r) 42-46-kDa IGFBP, which was immunoprecipitated by an IGFBP-3 antibody. IGF-I also increased the accumulated levels of IGFBP-3 in the conditioned media of both cell lines. Both cell lines expressed high basal levels of IGFBP-3 mRNA; the addition of RA increased IGFBP-3 mRNA levels by 1.5-fold, whereas the addition of IGF-I had no effect on IGFBP-3 mRNA levels in either cell line. The difference in the magnitude of the RA enhancement of IGFBP-3 mRNA levels (1.5-fold) and RA stimulation of IGFBP-3 levels in conditioned media (3.5-4-fold) suggests that some of the effect of RA is at a posttranscriptional level. IGF-I increased the levels of IGFBP-2 and IGFBP-5 in conditioned media by greater than tenfold but had no effect on IGFBP-2 and IGFBP-5 mRNA levels, again suggesting the involvement of posttranscriptional controls. Pretreatment of MDA-MB-468 and MDA-MB-231 cells with IGF-I receptor antibody (alpha IR3) blocked the IGF-I effect on IGFBP-3 levels in the media in both cell lines and IGFBP-2 and IGFBP-5 secreted levels in MDA-MB-468 cell conditioned media. The addition of RA also blocked IGF-I stimulation of IGFBP-2 and IGFBP-5 levels. Cycloheximide treatment completely blocked the RA and/or IGF-I-mediated modulation of these binding proteins, suggesting that these agents enhance IGFBP-3, IGFBP-2, and IGFBP-5 synthesis and consequent secretion. MDA-MB-468 cells expressed IGFBP-5 mRNA, whereas both MDA-MB-231 and MDA-MB-468 expressed IGFBP-6 mRNA. RA enhanced IGFBP-6 gene expression by threefold in MDA-MB-231 cells, whereas IGF-1 had no effect on IGFBP-6 gene expression in either cell line.(ABSTRACT TRUNCATED AT 400 WORDS)