Insulin-like growth factor binding protein-3 is secreted as a phosphoprotein by human breast cancer cells

The growth regulatory activity of the insulin-like growth factor binding pr oteins (IGFBPs) may be modulated by post-translational 0modifications such as glycosylation, limited proteolysis and phosphorylation. In this study, w e have examined phosphorylation of IGFBP-3 in two breast cancer cell lines: the estrogen receptor negative (ER-ve) Hs578T cell line in which IGFBP-3 i s normally expressed, and ER+ve T47D breast cancer cells transfected with I GFBP-3 cDNA (T47D(BP-3)) and therefore expressing IGFBP-3 constitutively. M etabolic labelling with [P-32] orthophosphate revealed that both cell lines secreted phosphorylated IGFBP-3 similar in size to plasma IGFBP-3 phosphor ylated in vitro with casein kinase II, and that IGFBP-3 phosphorylation was differentially modulated in the two cell lines. In Hs578T cells, retinoic acid (10-100 nM) increased IGFBP-3 phosphorylation to a maximum of 150% of control. IGF-I, but not [LR3]IGF-I, reduced the: proportion of phosphorylat ed IGFBP-3 in Hs578T conditioned medium, consistent with increased release of non-phosphorylated, cell-associated IGFBP-3. By contrast, IGFBP-3 phosph orylation in T47D(BP-3) cells was not affected by retinoic acid or TGF-I, b ut appeared slightly increased by estradiol. Together these data indicate t hat phosphorylation of IGFBP-3 in breast cancer cells may be regulated by a gents known to affect breast cancer cell proliferation. (C) 1999 Elsevier S cience Ireland Ltd. All rights reserved.