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Division of Nephrology, Departments of Internal Medicine I and II, University of Ulm, 89070 Ulm, Germany
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 ~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.
cell growth; C-peptide; insulin-like growth factors
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