CONCERTED REGULATION OF EARLY ENTEROCYTE DIFFERENTIATION BY INSULIN-LIKE-GROWTH-FACTOR-I, INSULIN, AND TRANSFORMING GROWTH-FACTOR-BETA-1

To clarify the roles of insulin on the proliferation and differentiati on of intestinal epithelial cells (IECs), we examined the effect of in sulin-like growth factor-I (IGF-I) and insulin for the growth and diff erentiation of IEC-6 cells, a crypt cell line derived from rat small i ntestine. IGF-I (100 nM) stimulated the proliferation of IEC-6 cells, and insulin (1-100 nM) antagonized the IGF-I effect and caused the cel ls' G(1)-arrest, resulting in differentiated characteristics of IECs, such as increased general protein synthesis and the formation of micro villi. To clarify the mechanisms of these phenomena, cell surface [I-1 25]insulin binding and the content of immunoreactive insulin receptors were analyzed by Western blotting. Insulin receptors transiently appe ared on the cell surface during the early G(1) phase after the IGF-I s timulation. Under those conditions, the concomitant presence of insuli n stimulated the appearance of active transforming growth factor-beta 1 (TGF-beta 1) in the media, and then TGF-beta 1 antagonized the IGF-I -induced cell proliferation. Such a TGF-beta 1 effect was blunted by a neutral izing antibody against TGF-beta 1, indicating that the insuli n effect was in part mediated through the autocrine-paracrine secretio n of TGF-beta 1. These results suggest that the regulation of the prol iferation of IECs are an early step in those cells' differentiation th at may accompany hormonal changes during nutrient intake and may be ca used by the sequential effects of IGF-I, insulin, and TGF-beta 1.