DIFFERENTIAL REGULATION BY TGF-BETA-1 AND INSULIN OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-2 IN IEC-6 CELLS

The purposes of this study were to determine the regulation of insulin -like growth factor binding protein-2 (IGFBP-2) in IEC-B cells by tran sforming growth factor-beta 1 (TGF-beta 1) and insulin and to determin e whether IGFBP-2 mediated the growth-inhibitory action on the cells. Utilizing Western ligand blot analysis, we found that TGF-beta 1 at co ncentrations of 0.5, 1.0, and 2 ng/ml significantly increased levels o f 32-kDa IGFBP in the conditioned medium (CM) of IEC-6 cells in a dose -dependent fashion and that low doses of insulin (1.0 and 5.0 mu g/ml) also increased IGFBP levels in the CM of IEC-6 cells, but a high dose of insulin (10 mu g/ml) depressed IGFBP release in the CM. Immunoblot ting has shown that the IGFBP of 32 kDa was IGFBP-2 and further confir med the above results. IGFBP-2 mRNA levels were stimulated by TGF-beta 1 (2.0 ng/ml) and suppressed by insulin (5.0 mu g/ml). In addition, d es (1-3) IGF-I (50 ng/ml) and insulin stimulated the proliferation of IEC-6 cells. Anti-IGFBP-2 antibodies partially blocked the inhibitory role in IEC-6 cell growth evoked by des (1-3) IGF-I. These findings su ggest that the upregulation of IGFBP-2 by TGF-beta 1 occurs, at least in part, at the level of mRNA, whereas the regulation by insulin appea rs to be at a posttranslational level, and that the TGF-beta 1-stimula ted production of IGFBP may contribute to the growth-inhibitory action in intestinal epithelial cells.