ABSTRACT

Two distinct receptors for the insulin-like growth factors (IGF-I and IGF-II) have been identified in mammalian tissues, but so far only a receptor structurally related to the type I receptor has been identified in chicken embryonic tissues. This study was designed to characterize binding sites for IGF peptides in chicken liver microsomal membranes prepared from hatch to 10 weeks of age which is the period of most rapid growth.

Binding of both human (h) IGF-I and hIGF-II was displaceable by either peptide and exhibited similar pH, time and temperature dependency. Human IGF-II was more potent than hIGF-I in competing for the binding of the iodinated ligands with half-maximum effective concentrations of 3–5 μg/l and 7–13 μg/l respectively. Porcine insulin was also a potent competitor. Affinity cross-linking studies, followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis under reducing conditions demonstrated that both IGF peptides were linked to a protein with a molecular weight of about 130 000 Da characteristic of the α-subunit of the type I receptor. There was no evidence for the presence of a type II receptor similar to that found in mammals.

Specific binding of both peptides was low on the day of hatch, increased about threefold by day 3 of age and remained high for the first 3 weeks of life before returning to a lower steady state level up to 10 weeks of age. This was the result of variation in receptor number, with no change in the affinity of the binding site for either ligand. The affinity constant for IGF-II (4·5 ± 0·5 (s.e.m.) litres/nmol) was higher than for IGF-I (1·4 ± 0·3 litres/nmol).

Insulin-like growth factors have previously been reported to stimulate the metabolism and growth of hepatocytes in vitro and are produced by these cells. The occurrence of a higher number of receptors at the period of most rapid growth of the liver suggests that they may have a role in regulating normal liver growth in an autocrine or paracrine manner. Furthermore, present evidence suggests that this is through a single receptor related to the type I receptor.

Journal of Endocrinology (1990) 125, 199–206

ABSTRACT

Insulin-like growth factors-I and -II (IGF-I and IGF-II) stimulate proliferation, differentiation, nutrient uptake and protein accretion in muscle cells. These effects are thought to be mediated through the type-I IGF receptor although a role for the type-II IGF receptor cannot be ruled out, since it has been found in most cells studied so far. Current evidence suggests that the chicken does not have a type-II IGF receptor and therefore provides a good model to study the function of IGF peptides. We have compared the effects of insulin and insulin-like growth factors on DNA synthesis with the binding of these peptides to receptors in primary chicken muscle satellite cells.

Human IGF-I (hIGF-I), hIGF-II and porcine insulin increased thymidine incorporation into DNA by threefold in muscle satellite cells prepared from neonatal chickens. IGF-I and -II were almost equipotent, with half-maximum effective concentrations of 10 μg/l, and were 1000-fold more potent than insulin. A combination of maximum effective concentrations of all three peptides was not additive, suggesting that their effect was mediated by the same receptor.

Receptor binding studies on satellite cells demonstrated the presence of specific IGF receptors. Human IGF-I inhibited the binding of ¹²⁵I-labelled hIGF-I with a much higher potency than insulin, as usually observed for a type-I IGF receptor. However, unlabelled hIGF-II exhibited a higher potency than hIGF-I in displacing ¹²⁵I-labelled hIGF-I. Affinity cross-linking of ¹²⁵I-labelled hIGF-I and -II, followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, showed that hIGF-I and -II bound to a receptor with the structural characteristics of a type-I IGF receptor and confirmed the lack of a type-II IGF receptor in these cells. The concentrations of IGF-I, -II and insulin required for biological action and to displace ¹²⁵I-labelled hIGF-I binding were similar, and support the hypothesis that their effects on proliferation were mediated exclusively through a type-I IGF receptor.

Journal of Endocrinology (1991) 128, 35–42

ABSTRACT

The effects of insulin and insulin-like growth factor-I (IGF-I) on glucose transport were compared in myotubes derived from chicken breast muscle satellite cells in vitro. Myotubes were incubated (for 0·5 or 4 h) with or without glucose in the presence or absence of insulin or IGF-I. Glucose uptake was subsequently measured by the incorporation of 2-[1,2-³H(N)] deoxy-d-glucose ([³H]2DG) in glucose-free medium (10 min at 20 °C). Glucose uptake was almost completely abolished by the addition of cytochalasin B or phloretin. It was increased by a decrease in glucose concentration in the incubation medium. Insulin (5 mg/l) stimulated [³H]2DG uptake to a maximum of 43 ± 10% above basal after 30-min incubation and 101 ± 15% after 4-h incubation. IGF-I and insulin at equimolar concentrations (25 μg/l and 20 μg/l respectively) were almost equipotent after 0·5 h but after 4-h incubation IGF-I was 17-fold more potent, suggesting that this 'late' effect was mediated through the IGF-I receptor. Incubation with cycloheximide suggested that the effect of IGF-I involved increased protein synthesis. The results suggest that chicken myotubes express a glucose transporter which is regulated by IGF-I and glucose concentration. However, they do not appear to express a typical insulin-responsive transport system.

Journal of Endocrinology (1993) 137, 465–472