Progastrin-Induced Secretion of Insulin-Like Growth Factor 2 From Colonic Myofibroblasts Stimulates Colonic Epithelial Proliferation in Mice

Abstract

Many colon cancers produce the hormone progastrin, which signals via autocrine and paracrine pathways to promote tumor growth. Transgenic mice that produce high circulating levels of progastrin (hGAS) have increased proliferation of colonic epithelial cells and are more susceptible to colon carcinogenesis than control mice. We investigated whether progastrin affects signaling between colonic epithelial and myofibroblast compartments to regulate tissue homeostasis and cancer susceptibility. Colonic myofibroblast numbers were assessed in hGAS and C57BL/6 mice by immunohistochemistry. Human CCD18Co myofibroblasts were incubated with recombinant human progastrin (rhPG)(1-80) for 18 hours, and proliferation was assessed in the presence of pharmacologic inhibitors. The proliferation of human HT29 colonic epithelial cells was assessed after addition of conditioned media from CCD18Co cells incubated with progastrin. The effects of the insulin-like growth factor (IGF)-I receptor antagonist AG1024 were investigated in cultured HT29 cells and on the colonic epithelium of hGAS mice compared with mice that did not express transgenic progastrin (controls). The colonic mucosa of hGAS mice contained greater numbers of myofibroblasts that expressed α-smooth muscle actin and vimentin than controls. Incubation of CCD18Co myofibroblasts with 0.1 nmol/L rhPG(1-80) increased their proliferation, which required activation of protein kinase C and phosphatidylinositol-3 kinase. CCD18Co cells secreted IGF-II in response to rhPG(1-80), and conditioned media from CCD18Co cells that had been incubated with rhPG(1-80) increased the proliferation of HT29 cells. The colonic epithelial phenotype of hGAS mice (crypt hyperplasia, increased proliferation, and altered proportions of goblet and enteroendocrine cells) was inhibited by AG1024. Progastrin stimulates colonic myofibroblasts to release IGF-II, which increases proliferation of colonic epithelial cells. Progastrin might therefore alter colonic epithelial cells via indirect mechanisms to promote neoplasia.