REGULATION OF INSULIN-LIKE GROWTH FACTOR-BINDING-PROTEIN-1, FACTOR-BINDING-PROTEIN-2, FACTOR-BINDING-PROTEIN-3, FACTOR-BINDING-PROTEIN-4, FACTOR-BINDING-PROTEIN-5, AND FACTOR-BINDING-PROTEIN-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 f or breast carcinoma proliferation. In extracellular fluids, most of th e IGF-I and II is associated with specific IGF-binding proteins (IGFBP s). The role of these IGFBPs in IGF action is still not clear, but it has been demonstrated that these proteins may either enhance or inhibi t IGF-mediated cellular effects. Synthesis and secretion of IGFBPs hav e been demonstrated in breast carcinoma cells. In this study, we exami ned retinoic acid (RA) and IGF-I modulation of IGFBP mRNA and IGFBP le vels in two ER-negative human breast carcinoma cell lines. Treatment o f MDA-MB-231 and MDA-MB-468 cells with RA increased the levels in cond itioned media of a Mr 42-46-kDa IGFBP, which was immunoprecipitated by an IGFBP-3 antibody. IGF-I also increased the accumulated levels of I GFBP-3 in the conditioned media of both cell lines. Both cell lines ex pressed high basal levels of IGFBP-3 mRNA; the addition of RA increase d IGFBP-3 mRNA levels by 1.5-fold, whereas the addition of IGF-I had n o 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 (3.54-fold) suggests that some of the effect of RA is at a posttranscriptional le vel. 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 r eceptor antibody (alphaIR3) blocked the IGF-I effect on IGFBP-3 levels in the media in both cell lines and IGFBP-2 and IGFBP-5 secreted leve ls in MDA-MB-468 cell conditioned media. The addition of RA also block ed IGF-I stimulation of IGFBP-2 and IGFBP-5 levels. Cycloheximide trea tment completely blocked the RA and/or IGF-I-mediated modulation of th ese binding proteins, suggesting that these agents enhance IGFBP-3, IG FBP-2, and IGFBP-5 synthesis and consequent secretion. MDA-MB-468 cell s expressed IGFBP-5 mRNA, whereas both MDA-MB-231 and MDA-MB-468 expre ssed 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 express ion in either cell line. These results demonstrate that RA and IGF-I m odulate IGFBP levels in a number of breast carcinoma cell lines. Such modulation of IGFBPs may in turn affect IGF mediated cellular response s and thus the biologic behavior of these malignant cells.