Interactions of nutrients, insulin-like growth factors (IGFs) and IGF-binding proteins in the regulation of DNA synthesis by isolated fetal rat islets of Langerhans

Abstract

Insulin is a major regulatory hormone for optimal tissue growth and function in utero. Its continued availability to the growing fetus depends on increasing islet cell mass. The purpose of the study was to examine the interactions between nutrient availability and insulin-like growth factor (IGF) release and action during DNA synthesis by isolated fetal rat islets of Langerhans. Specifically, we wished to determine (a) whether the availability of glucose or total amino acids altered the release of endogenous IGF-I or -II, (b) if both IGF-I and -II were effective mitogens for pancreatic beta-cells, (c) whether islets released IGF-binding proteins (IGFBPs) and their possible regulation by nutrient availability and (d) how IGFBPs might regulate the ability of IGFs to alter islet DNA synthesis. Islets of Langerhans were isolated from fetal rat pancreata on day 22 of gestation by collagenase digestion. Islets enriched in beta-cells following a 5-day preincubation regime were maintained in various concentrations of glucose (1.4-16.7 mmol/l) or amino acids (x1- x3 total concentrations), with or without exogenous IGF-I, -II, IGFBP-1 or IGFBP-2. The release of insulin and endogenous IGF-I and -II were each determined by radioimmunoassay, and IGFBP release characterized by Western ligand blot analysis. DNA synthesis was measured by the incorporation of [3H]thymidine. Isolated islets demonstrated an increased release of insulin in response to increasing amounts of both glucose and amino acids, demonstrating functional viability. Both classes of nutrients also increased the DNA synthetic rate of islets. Islets released almost twice as much IGF-II (0.22 +/- 0.08 nmol/l, mean +/- S.E.M., n = 4) as IGF-I (0.14 +/- 0.03 nmol/l) in cultures containing 8.7 mmol glucose/l and x1 amino acids. Lesser or greater concentrations of glucose did not alter the release of either IGF, but the release of IGF-II was significantly increased (0.53 +/- 0.08 nmol/l, P < 0.01) in the presence of x2 amino acids. Exogenous IGF-I was fivefold more active in stimulating DNA synthesis by islets (half maximal concentration (ED50) 1.6 +/- 0.4 nmol/l, n = 3) than was IGF-II (ED50 8.1 +/- 0.6 nmol/l), regardless of glucose concentration. Isolated islets released four species of IGFBP with molecular sizes of approximately 19, 25, 35 and 46 kDa respectively. The 35 kDa form was identified by Western immunoblot as IGFBP-2. Increasing the glucose concentration between 1.4 mmol/l and 16.7 mmol/l caused a dose-related increase in the release of the 19, 25 and 35 kDa IGFBP species.(ABSTRACT TRUNCATED AT 400 WORDS)