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Search for other papers by Simon J Dunmore in
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Search for other papers by Simon J Dunmore in
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Search for other papers by James E P Brown in
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β-Cell failure coupled with insulin resistance is a key factor in the development of type 2 diabetes. Changes in circulating levels of adipokines, factors released from adipose tissue, form a significant link between excessive adiposity in obesity and both aforementioned factors. In this review, we consider the published evidence for the role of individual adipokines on the function, proliferation, death and failure of β-cells, focusing on those reported to have the most significant effects (leptin, adiponectin, tumour necrosis factor α, resistin, visfatin, dipeptidyl peptidase IV and apelin). It is apparent that some adipokines have beneficial effects whereas others have detrimental properties; the overall contribution to β-cell failure of changed concentrations of adipokines in the blood of obese pre-diabetic subjects will be highly dependent on the balance between these effects and the interactions between the adipokines, which act on the β-cell via a number of intersecting intracellular signalling pathways. We emphasise the importance, and comparative dearth, of studies into the combined effects of adipokines on β-cells.
Search for other papers by I. M. SIMONS in
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Search for other papers by G. J. KING in
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Individual chorion taken from cows in early pregnancy (days 27, 44, 45 and 49) was incubated with [14C]androstenedione in short-term incubation in vitro. After preliminary extraction, separation and purification, metabolites were identified by recrystallization with authentic unlabelled steroids to constant specific activity. Major metabolites identified were 5β-androstane-3,17-dione (5β-androstanedione) and 5β-androstan-3α-ol-17-one (aetiocholanolone) while minor metabolites were identified as 5β-androstane-3α,17β-diol and 5β-androstan-17α-ol-3-one. There was no 14C-labelled oestradiol or oestrone detectable in the medium either as free oestrogens or as sulphates.
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Search for other papers by M Derouet in
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Search for other papers by M Taouis in
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Chronic treatment with corticosterone evokes insulin resistance in chickens, a species which is already resistant to insulin compared with mammals. The in vivo effects of corticosterone on insulin signaling were investigated in chicken liver and thigh muscle in two nutritional states: basal (overnight fasted) and stimulated (30 min refeeding). Corticosterone significantly decreased specific insulin binding in liver and the amount of insulin receptor substrate-1 (IRS-1) and p85 (regulatory subunit of phosphatidylinositol (PI) 3'-kinase) in both tissues. Insulin receptor (IR) and IRS-1 mRNAs generally varied accordingly. Src homology and collagen protein (Shc) and messenger were not altered. In liver, in the basal state, the tyrosine phosphorylation of IR, IRS-1 and Shc, and the IR-associated PI 3'-kinase activity were largely decreased by corticosterone. Following refeeding the cascade was activated in control but totally inhibited in treated chickens. In muscle, as previously observed, IR and IRS-1 phosphorylation and PI 3'-kinase were not stimulated by refeeding in controls. Only the phosphorylation of Shc was increased. On this background, corticosterone decreased the basal PI 3'-kinase activity and prevented the phosphorylation of Shc in response to refeeding. In conclusion, corticosterone largely impaired insulin signaling in liver and to some extent in muscle. This should contribute to the large impairment of growth. In addition, the present studies further emphasize the peculiarities of insulin signaling in chicken muscle, which needs further investigation.
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Search for other papers by E. MORAVA in
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Search for other papers by G. SIMON in
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Search for other papers by J. SÓS in
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SUMMARY
The absorption of calcium from duodenal and jejunal segments of the small intestine was studied in rats using an in-vivo loop technique. Previous parathyroidectomy decreased calcium absorption from both segments in rats fed a normal diet. Reduced calcium transport was greater in rats fed a calcium-deficient diet after parathyroidectomy. The slower clearance of radioactive calcium from the lumen of the intestine was not due to increased endogenous calcium excretion. Thyroidectomy, either alone or combined with parathyroidectomy, decreased calcium absorption but the effect of thyroidectomy alone requires further study. The decrease in calcium absorption after removal of the parathyroids was minimal or absent when the animals were fed a high calcium, low phosphorus, vitamin D-deficient diet or fasted 48 hr. before the experiment.
Search for other papers by M Taouis in
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Search for other papers by J P Caffin in
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Search for other papers by J Simon in
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Abstract
Insulin receptor number and insulin responsiveness were compared in a chicken hepatoma cell line (LMH) and in normal chicken hepatocyte (cHep) cells cultured in the same conditions. LMH cells expressed two- to threefold more insulin receptors than cHep cells, without significant changes in affinity. The tyrosine kinase activity of solubilized and lectin (lentil+wheat germ agglutinin; WGA)-purified LMH receptors was higher than that of cHep receptors. The ATP hydrolytic activity previously observed in WGA-purified receptors from chicken liver membranes was also present in WGA-purified receptors from cultured cHep cells. This unidentified membrane-associated ATPase was absent from LMH membrane-solubilized material and therefore from WGA-purified LMH insulin receptors. Finally, LMH cells incorporated at least tenfold more amino isobutyric acid than cHep cells in the absence of insulin and were more responsive to insulin. The enhanced basal amino acid transport of LMH cells was most probably the consequence of their proliferative activity. The enhanced insulin responsiveness of LMH cells can be accounted for, at least in part, by one or several of the modifications presently demonstrated in LMH cells when compared with normal cultured hepatocytes: increased insulin receptor number and tyrosine kinase activity and possibly the loss of the membrane-associated ATPase.
Journal of Endocrinology (1994) 140, 119–124
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Search for other papers by J. Simon in
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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
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Search for other papers by E. SIMON in
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Studies in laboratory animals (mice, rats and rabbits) have shown that parenteral administration of d-mannoheptulose (MH), a seven-carbon sugar occurring in the avocado, evokes a diabetic syndrome, caused by blockade of insulin secretion from the pancreas (Simon & Kraicer, 1966). Paulsen (1968) observed a lowering of plasma insulin after oral administration of MH in a leucine-sensitive hypoglycaemic child, with no impairment of glucose utilization. Viktora, Johnson, Penhos, Rosenberg & Wolff (1969) fed avocado, containing 2–13 g. MH, to healthy men. In five out of eight subjects plasma insulin decreased without significant elevation of blood glucose. The same group (Johnson, Viktora & Wolff, 1969) administered pure MH orally to healthy men; a small elevation of blood sugar with a significant drop of insulin was observed. The insulin peak and glucose utilization was significantly reduced when glucose, 0·5 g./kg. body weight, was injected 4 hr. after MH ingestion.
The present
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Search for other papers by J Simon in
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Insulin-like growth factors (IGFs) stimulate growth rate in a number of animal species and are likely to contribute to genetic variations of growth potential. The present study was designed to link levels of IGF-I and IGF-II mRNA and peptides with growth rate in divergently selected genotypes of chickens with high (HG) or low (LG) growth rates. Circulating IGF-I and -II and hepatic mRNA levels were measured under ad libitum feeding conditions from 1 to 12 weeks of age, and at 6 weeks of age under three different nutritional conditions (fed, fasted for 16 or 48 h, re-fed for 4 or 24 h after a 48-h fast). IGF binding proteins (IGFBPs) were also measured. Circulating IGFs increased with age and were higher in HG chickens from 1 to 6 weeks. They decreased with fasting and only IGF-II was fully restored after 24 h of re-feeding, while IGF-I remained low. A significant decrease in steady state IGF-I mRNA levels was also observed with fasting. Across the nutritional study, hepatic IGF-I mRNAs were significantly higher in HG chickens. Variations of IGF-II mRNA levels with nutritional state or genotype exhibited a similar trend. IGFBP (28, 34 and 40 kDa) levels increased with age, while only faint differences were observed between genotypes. IGFBP-28 transiently increased with fasting and was inversely related to blood glucose and insulin levels, suggesting that it is equivalent to mammalian IGFBP-1. In HG chickens, IGFBP-28 and IGFBP-34 levels decreased markedly following re-feeding. Therefore, high and low growth rates were respectively associated with high and low IGF-I and -II levels, supporting the hypothesis of a stimulatory role for both IGFs during post-hatching growth of chickens.
Search for other papers by Bethany R L Aykroyd in
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Search for other papers by Amanda N Sferruzzi-Perri in
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The placenta regulates materno-fetal nutrient transfer and secretes hormones that enable maternal physiological support of the pregnancy. In mice, these functions are performed by the labyrinth (Lz) and junctional (Jz) zones, respectively. Insulin-like growth factor 2 (Igf2) is an imprinted gene expressed by the conceptus that is important for promoting fetal growth and placenta formation. However, the specific role of Igf2 in the Jz in regulating placental endocrine function and fetal development is unknown. This study used a novel model to investigate the effect of conditional loss of Igf2 in the Jz (Jz-Igf2UE) on placental endocrine cell formation and the expression of hormones and IGF signaling components in placentas from female and male fetuses. Jz-Igf2UE altered gross placental structure and expression of key endocrine and signaling genes in a sexually dimorphic manner. The volumes of spongiotrophoblast and glycogen trophoblast in the Jz were decreased in placentas from female but not male fetuses. Expression of insulin receptor was increased and expression of the MAPK pathway genes (Mek1, P38α) decreased in the placental Jz of female but not male fetuses. In contrast, expression of the type-1 and -2 IGF receptors and the MAPK pathway genes (H-ras, N-ras, K-ras) was decreased in the placental Jz from male but not female fetuses. Expression of the steroidogenic gene, Cyp17a1, was increased and placental lactogen-2 was decreased in the placenta of both sexes. In summary, we report that Jz-Igf2UE alters the cellular composition, IGF signaling components and hormone expression of the placental Jz in a manner largely dependent on fetal sex.
