Progastrin_1-80 stimulates growth of intestinal epithelial cells in vitro via high-affinity binding sites

Proliferation and carcinogenesis of the large intestinal epithelial cells (IEC) cells is significantly increased in transgenic mice that overexpress the precursor progastrin (PG) peptide. It is not known if the in vivo growth effects of PG on IEC cells are mediated directly or indirectly. Full-length recombinant human PG (rhPG_1-80) was generated to examine possible direct effects of PG on IEC cells. Surprisingly, rhPG (0.1-1.0 nM) was more effective than the completely processed gastrin 17 (G17) peptide as a growth factor. Even though IEC cells did not express CCK₁ and CCK2 receptors (-R), fluorescently labeled G17 and Gly-extended G17 (G-Gly) were specifically bound to the cells, suggesting the presence of binding proteins other than CCK₁-R and CCK₂-R on IEC cells. High-affinity (K_d = 0.5-1.0 nM) binding sites for ¹²⁵I-rhPG were discovered on IEC cells that demonstrated relative binding affinity for gastrin-like peptides in the order PG ≧ COOH-terminally extended G17 ≧ G-Gly > G17 > *CCK-8 (*significant difference; P < 0.05). In conclusion, our studies demonstrate for the first time direct growth effects of the full-length precursor peptide on IEC cells in vitro that are apparently mediated by the high-affinity PG binding sites that were discovered on these cells.