Proinsulin stimulates growth of small intestinal crypt-like cells acting via specific receptors

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 of 125I-proinsulin but not for 125I-IGF-I or 125I-insulin, suggesting a specific binding epitope of the proinsulin molecule within or close to the C-peptide region. In contrast, insulin showed approximately 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.