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)

Growth Factor‐Dependent Proliferation of the Pancreatic β‐cell Line βTC‐tet: An Assay for β‐cell Mitogenic Factors

“…Although the heterologous human growth hormone can stimulate rat islet cell replication [16], most likely through the PRL receptor, it has only a modest effect on mouse islet cells [17]. The relatively lower mitogenic effect of IGF I in this assay, as compared with that of IGF II, is inconsistent with previous results that showed similar effects of both factors on fetal rat islets [7], and with the action of both factors through the IGF I receptor. It is possible that βTC-tet cells express IGF binding proteins that preferentially neutralize IGF I activity.…”

“…Significant mitogenic effects of hepatocyte growth factor (HGF) have been observed on human fetal and adult islets [4] and mouse islets [5]. Insulin-like growth factors (IGF) I and II, and platelet-derived growth factor, affect fetal rat islet cell growth [6,7]. IGF I has also been shown to stimulate replication of cultured rat insulinoma cells [8], and IGF II activates the growth of both normal [9] and transformed [10] mouse β cells in vivo.…”

Early Influences of Nutrition on Postnatal Growth

“…In addition, dietary factors and especially the intakes of energy and protein are very important regulators of serum IGF-I concentrations and its biological activity, especially during the first months of infancy [6]. Amino acids were also reported to be more potent stimulators of IGF-I release than glucose [7]. For example, studies in 4-week-old rats showed that feeding a diet with 15 instead of 5% protein for only one week increased serum IGF-I more than 4-fold [8].…”

IGF2: an endocrine hormone to improve islet transplant survival

“…Interestingly, IGF2 expression declines in mice and rats postnatally and this correlates with increased b-cell apoptosis in vivo (Petrik et al 1998(Petrik et al , 1999, suggesting that endogenous IGF2 exerts an early anti-apoptotic effect within native pancreatic islets. In contrast to this, in humans, IGF2 remains the most abundant IGF in circulation throughout life, the expression of which is detected in many tissues including the CNS, adrenal medulla, pancreas and purified b-cells (Han et al 1987, Bryson et al 1989, Hill 1990, Hogg et al 1993, Miettinen et al 1993, Asfari et al 1995, Bergmann et al 1996, Katz et al 1997. The post-natal decline in IGF2 expression may be a result of differences in the IGF2 gene promoter structure between humans and rodents (Foulstone et al 2005).…”