The mechanisms which regulate cell turnover in the intestinal epithelium are incompletely understood. The present study was performed to characterize the role of autocrine IGF system components in intestine epithelial cell proliferation and differentiation comparing rapidly growing crypt cells (IEC-6) with differentiating enterocytes (CaCo-2). The autocrine release of IGF-I, IGF-II and IGFBP-1 through -3 was determined by specific RIAs and western ligand blotting. In addition, binding and growth-promoting activity of insulin, IGF-I and IGF-II was investigated. Enterocytic differentiation was assessed by measuring the brush-border enzymes alkaline phosphatase and sucrase. During IEC-6 growth, the autocrine release of IGF-I and -II increased, whereas IGFBP-2 levels decreased. Specific receptors for IGF-I and IGF-II but not insulin could be detected. IGF-I was 100-fold more potent than insulin to stimulate IEC-6 cell proliferation. In contrast, CaCo-2 cells revealed higher binding of insulin than IGF-I/-II and no release of IGF-I. At switch from CaCo-2 cell proliferation to differentiation a marked increase in the secretion of IGF-II (10-fold), IGFBP-1 (2.5-fold), IGFBP-2 (3-fold), and IGFBP-3 (6-fold) was measured. Our data indicate that IGF system components differentially modulate enterocytic cell proliferation and differentiation.

The mechanisms that regulate cell turnover in the intestinal epithelium are incompletely understood. Here we tested the hypothesis that proinsulin, present in serum and pancreatic juice in picomolar concentrations, stimulates growth of the rat small intestinal crypt-like cell line IEC-6 under serum-free conditions. Proinsulin binding was assessed by competitive ligand binding studies. Proinsulin and insulin-like growth factor I (IGF-I) stimulated cell proliferation up to threefold above controls, with half-maximal action already in the picomolar range and with additive effects. In early confluent cell monolayers, proinsulin bound with higher affinity (IC50 1.3 ± 0.05 nM) and capacity (87,200 ± 2,500 receptors/cell) than IGF-I (4.0 ± 0.6; 23,700 ± 2,200, P < 0.05). C-peptide competed with 10-fold lower affinity for binding of125I-proinsulin but not for125I-IGF-I or125I-insulin, suggesting a specific binding epitope of the proinsulin molecule within or close to the C-peptide region. In contrast, insulin showed ∼100-fold lower binding affinity and growth-promoting potency than proinsulin or IGF-I. We conclude that proinsulin stimulates growth of small intestinal crypt cells through specific binding and may play a physiological role in the regulation of intestinal epithelial cell proliferation.