REGULATION OF INSULIN-LIKE GROWTH-FACTOR (IGF) BINDING PROTEIN-5 IN THE T47D HUMAN BREAST-CARCINOMA CELL-LINE BY IGF-I AND RETINOIC ACID

The T47D human breast carcinoma cell line has been shown to synthesize insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs) and IG F-I receptors, and to exhibit a mitogenic response to exogenous IGF-I. We have used T47D cells to investigate the regulation of IGFBPs by IG F-I and retinoic acid (RA), agents that affect cell proliferation and have been shown to regulate IGFBP levels in other cell types. Exposure of T47D cells to IGF-I resulted in the appearance of IGFBP-2, -4, and -5 in conditioned medium but had no effect on the levels of IGFBPs in Triton X-100-extracted cells. This effect was most pronounced for IGF BP-5 and was also elicited by an IGF-I analog that retains affinity fo r IGFBPs but not by insulin or IGF analogs that have decreased affinit y for IGFBPs. Additionally, this effect was not associated with a chan ge in IGFBP-5 messenger RNA (mRNA) levels; however, the appearance of IGFBP-5 in the conditioned medium was inhibited by an anti-IGF-I recep tor antibody (alpha IR-3). RA decreased IGFBP-5 mRNA levels and cell-a ssociated IGFBP-5 in both the presence and absence of IGF-I and inhibi ted the IGF-I-stimulated secretion of IGFBP-5 into T47D cell condition ed medium. These results suggest that IGF-I increases IGFBP-5 levels i n the T47D cell line both through direct interaction with IGFBP-5 as w ell as through a receptor-mediated process that does not require direc t interaction with IGFBPs. The latter results are consistent with an e ffect of IGF-I on a factor that may modulate an IGFBP protease activit y. The inhibitory effect of RA, on the other hand, appears to be due p rimarily to regulation of IGFBP-5 mRNA levels. Thus, IGFBP-5 accumulat ion appears to be positively regulated by IGF-I, potentially at the le vel of susceptibility to proteolysis, and negatively regulated at the level of gene expression by RA.