INSULIN-LIKE GROWTH-FACTOR-I AND EPIDERMAL GROWTH-FACTOR INTERACT TO REGULATE GROWTH AND GENE-EXPRESSION IN IEC-6 INTESTINAL EPITHELIAL-CELLS

Epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I) exert trophic effects on bowel mucosa. Each growth factor uses a distinct tyrosine kinase receptor but the receptors share some common signal transduction pathways. In other systems, regulation of cell growth involves interactions among multiple growth factors. We used IEC-6 cells, an epithelial cell line established from rat small intestine, to test whether EGF and IGF-I interact to regulate intestinal epithelial cell growth. EGF and IGF-I alone each stimulated DNA synthesis in IEC-6 cells. EGF was more potent than IGF-I, and effects of the two growth factors in combination were synergistic. Characterization of the IGF system [IGF-I, IGF-II, type 1 IGF receptor, and six ICE binding proteins (IGFBPs) 1-6] revealed that IEC-6 cells express high levels of type 1 IGF receptor mRNA, low or undetectable levels of IGF-I and IGF-II mRNAs, and mRNA for only one of the six IGFBPs, IGFBP2. IGF-I decreases expression of type 1 IGF receptor mRNA in IEC-6 cells and EGF attenuates this effect. EGF and IGF-I both reduce IGFBP2 mRNA expression, and inhibitory effects of EGF and IGF-I in combination are additive. EGF reduces IGFBP2 accumulated in conditioned medium relative to levels observed with IGF-I alone. These effects of EGF on type 1 IGF receptor expression and on levels of IGFBP2 mRNA and IGFBP2 in medium may contribute to synergistic mitogenic effects with IGF-I by promoting IGF-I responsiveness. EGF potently and rapidly induces c-fos and c-jun mRNAs whereas IGF-I modestly induces c-fos, but not c-jun, mRNA. EGF, but not IGF-I, induces total AP-1 transcriptional activity and effects of EGF and IGF-I in combination are greater than additive. IGF-I, but not EGF, induces c-Jun phosphorylation and transactivation. These distinct effects of the two growth factors on intracellular signal transduction mechanisms leading to transcriptional activation of AP-1 may contribute to their synergistic effects on DNA synthesis when added in combination to IEC-6 cells.