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Free access

RH McCusker and J Novakofski

Zinc (Zn(2+)), a multifunctional micronutrient, was recently shown to lower the affinity of cell-associated insulin-like growth factor (IGF) binding protein (IGFBP)-3 and IGFBP-5 for both IGF-I and IGF-II, but to increase the affinity of the cell surface type 1 IGF receptor (IGF-1R) for the same two ligands. However, there is a need for data concerning the effects of Zn(2+) on soluble IGFBPs and the type 2 IGF receptor (IGF-2R). In the current work, we demonstrate that Zn(2+) affects the affinity of IGFBP-5 secreted by myoblasts but not IGFBP-4. Zn(2+), at physiological levels, depressed binding of both IGF-I and IGF-II to IGFBP-5, affecting (125)I-IGF-I more than (125)I-IGF-II. Both (125)I-IGF-I and (125)I-IGF-II bound to high and low affinity sites on IGFBP-5. Zn(2+) converted the high affinity binding sites of IGFBP-5 into low affinity binding sites. An IGF-I analog, (125)I-R(3)-IGF-I, did not bind to the soluble murine IGFBP-5. Zn(2+) also decreased the affinity of the IGF-2R on L6 myoblasts. In contrast, Zn(2+) increased IGF-I, IGF-II and R(3)-IGF-I binding to the IGF-1R by increasing ligand binding affinity on both P(2)A(2a)-LISN and L6 myoblasts. Soluble IGFBP-5 and IGFBP-4 depressed the binding of (125)I-IGF-I and (125)I-IGF-II to the IGF-1R, but did not affect binding of (125)I-R(3)-IGF-I. By depressing the association of the IGFs with soluble IGFBP-5, Zn(2+) partitioned (125)I-IGF-I and (125)I-IGF-II from soluble IGFBP-5 onto cell surface IGF-1Rs. This effect is not seen when soluble L6-derived IGFBP-4 is present in extracellular fluids. We introduce a novel mechanism by which the trace micronutrient Zn(2+) may alter IGF distribution, i.e. Zn(2+) acts to increase IGF-1R binding at the expense of IGF binding to soluble IGFBP-5 and the IGF-2R.

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M. Tepel, S. Bauer, S. Husseini, A. Raffelsiefer, and W. Zidek

ABSTRACT

Cytosolic free sodium concentrations ([Na+]i) in intact platelets from 32 type 2 (non-insulin-dependent) diabetic patients and from 27 age- and sex-matched non-diabetic control subjects were measured with the novel sodium-sensitive fluorescent dye sodium-binding-benzofuran-isophthalate. [Na+]i was significantly higher in platelets from type 2 diabetic patients compared with control subjects (40·6 ± 2·4 vs 32·0 ± 2·0 mmol/l, means ± s.e.m., P<0·03). Both systolic and diastolic blood pressure were significantly elevated in diabetic patients compared with control subjects. Analysis of diabetic patients showed a significant association between [Na+]i and diastolic blood pressure (P =0·026). Stimulation of Na/H exchange by thrombin increased [Na+]i in both groups. After inhibition of Na/K/ATPase by ouabain (1 mmol/l), [Na+]i was significantly increased both in diabetic patients and non-diabetic subjects in a similar way (by 40·2 ± 7·3 and 31·7 ± 5·3 mmol/l respectively). It is concluded that increased [Na+]i in cells from type 2 diabetic patients may be related to hypertension.

Journal of Endocrinology (1993) 138, 565–572

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J. M. H. M. Reul, F. R. van den Bosch, and E. R. de Kloet

ABSTRACT

The rat brain contains two receptor systems for corticosterone: the type-I corticosterone-preferring receptor and the classical type-II glucocorticoid receptor. The two receptor populations can be distinguished in binding studies with the 'pure' synthetic glucocorticoid 11β,17β-dihydroxy-6-methyl-17α (1-propynyl)-androsta-1,4,6-trione-3-one (RU 28362). In-vitro autoradiography and quantitative image analysis showed that the type-I receptor was localized almost exclusively in the hippocampus, whereas the type-II receptor extended throughout the brain, with the highest levels in the nucleus paraventricularis, nucleus supraopticus and in the thalamic, amygdaloid, hippocampal and septal regions. Unoccupied type-I and type-II receptor sites, as measured in vitro by cytosol binding of 3H-labelled steroids, displayed a large difference in the rate of appearance after adrenalectomy. The availability of type-I receptors exhibited a marked increase, reaching maximal levels within 4–7 h, and then remained constant until 2 weeks after adrenalectomy. The availability of type-II receptors did not change considerably during the first 24 h after adrenalectomy, but displayed a large increase in capacity during the subsequent 2 weeks. After adrenocortical activation as a consequence of exposure to a novel environment, plasma concentrations of corticosterone increased to reach a peak of 811 nmol/l after 30 min and attained the basal concentration (43 nmol/l) after 240 min. During this time, occupation of type-I receptors increased from 77·8% at 0 min to 97% at 30–60 min and then declined to 84·8% after 240 min. Occupation of the type-II receptors was 28·1% at 0 min, 74·5% after 30 min and 32·8% after 240 min. Injection of dexamethasone (25 μg/100 g body wt) at 08.00 h resulted in suppression of basal plasma concentrations of corticosterone and prevented the circadian-driven rise in circulating corticosterone. Occupation of type-I receptors did not change considerably as a result of injection of dexamethasone, but occupation of type-II receptors was markedly increased till 16.00 h compared with that after injection of vehicle.

It was concluded that the type-I and type-II receptors are not only localized differently in the rat brain, but also exhibit a striking difference in occupation after manipulation of the pituitary-adrenocortical system. The data further support the concept of a type-I receptor-mediated tonic activating influence and a type-II receptor-mediated feedback action of corticosterone on brain function.

J. Endocr. (1987) 115, 459–467

Free access

Weixia Han, Chen Wang, Zhifen Yang, Lin Mu, Ming Wu, Nan Chen, Chunyang Du, Huijun Duan, and Yonghong Shi

Renal fibrosis is the major pathological characteristic of diabetic nephropathy (DN). Reportedly, increased SIRT1 expression played a renal protective role in animal models of DN. This study was designed to elucidate the molecular mechanisms underlying the protective effects of SRT1720, an SIRT1 activator, against diabetes-induced renal fibrosis. Type 2 diabetic mice (db/db) were treated with SRT1720 (50 mg/kg/day) by gavage for 10 weeks. Renal proximal tubular epithelial cells (HK-2 cells) were treated with high glucose (HG, 30 mM) in the presence or absence of SRT1720 (2.5 µM) for 48 h. We observed that impaired SIRT1 expression and activity were restored by SRT1720 administration in db/db mice as well as in HG-treated HK-2 cells. Moreover, SRT1720 administration improved the renal function, attenuated glomerular hypertrophy, mesangial expansion, glomerulosclerosis and interstitial fibrosis and inhibited TGFB1 and CTGF expressions and nuclear factor κB (NF-KB) activation in db/db mice. Similarly, HG-induced epithelial-to-mesenchymal transformation (EMT) and collagen IV and fibronectin expressions were inhibited in SRT1720-treated HK-2 cells. Mechanistic studies demonstrated that SRT1720 suppressed HIF1A, GLUT1 and SNAIL expressions both in vivo and in vitro. Furthermore, HIF1A or GLUT1 knockdown effectively abrogated HG-induced EMT and collagen IV and fibronectin expressions in HK-2 cells. These findings suggest that SRT1720 prevented diabetes-induced renal fibrosis via the SIRT1/HIF1A/GLUT1/SNAIL pathway.

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T Matsumoto, S E Gargosky, Y Oh, and R G Rosenfeld

Abstract

The aim of this study was to assess the regulation of insulin-like growth factor-binding proteins (IGFBPs) by IGFs in primary cultures of rat articular chondrocytes (RAC). Employing Western ligand blotting, immunoprecipitation and Northern blot analysis, RAC were found to secrete IGFBP-5 (29 kDa) and IGFBP-4 (24 kDa) as the predominant IGFBPs, as well as IGFBP-2 (32–30 kDa) and IGFBP-3 (43–39 kDa) as the minor species. Treatment of cells with IGF-I and IGF-II resulted in a dose-dependent increase of IGFBP-5 and a small increase in IGFBP-4 in conditioned media (CM). Des(1–3) IGF-I and [Gln6, Ala7,Tyr18, Leu19] IGF-II ([QAYL] IGF-II), which bind to the type 1 IGF receptor but not to IGFBPs, also induced IGFBP-5 peptide, although the increase was less than with IGF-I or IGF-II treatment of RAC. [Leu27] IGF-II, which does not bind to the type 1 IGF receptor but binds to IGFBPs, resulted in little induction of IGFBP-5, while [QAYL-Leu27] IGF-II, which has reduced affinity for both the type 1 IGF receptor and IGFBPs, did not increase IGFBP-5. These data suggest that the increase in IGFBP-5 in CM is modulated by both the type 1 IGF receptor and the interaction between IGFs and IGFBPs. Northern blotting analysis showed that IGF-I, IGF-II and des(1–3) IGF-I treatment of RAC increased steady state levels of IGFBP-5 mRNA, suggesting that the IGF-mediated increase in IGFBP-5 is transcriptionally modulated. Interestingly, the increase in IGFBP-5 peptide levels and mRNA were not parallel, suggesting the possibility of post-translational modifications of IGFBP-5, such as those seen with IGFBP-5 protease. IGFBP-5 protease activity was detectable in untreated CM, whereas treatment with IGF-I and IGF-II partially protected IGFBP-5 from proteolysis. In summary, treatment of RAC with IGF-I and IGF-II results in dose-dependent increases in both IGFBP-5 peptide in the CM and mRNA levels. These changes are mediated by interactions via the type 1 IGF receptor as well as IGFBPs, both transcriptionally and post-translationally.

Journal of Endocrinology (1996) 148, 355–369

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P. J. Miettinen, T. Otonkoski, and R. Voutilainen

ABSTRACT

To understand the development of the human pancreas better, we studied the expression and regulation of insulin, insulin-like growth factor-II (IGF-II) and transforming growth factor-α (TGF-α) genes in the human fetal pancreas and islet-like cell clusters (ICC) from the second trimester human fetuses. Northern blot analysis revealed an abundant expression of IGF-II, insulin and TGF-α mRNAs in the intact pancreas and the cultured ICCs. Furthermore, transcripts for insulin receptor, type-1 and -2 IGF receptors, and GH receptor could be amplified by polymerase chain reaction analysis from the pancreas and the ICCs. With in-situ hybridization, IGF-II mRNA was found in abundance in both the exocrine and endocrine pancreas, exceeding the amount of insulin mRNA. In ICCs, insulin mRNA-containing cells were present as small clusters in the periphery and in the centre of the clusters corresponding to the immunolocation of insulin. The ICCs also contained many epidermal growth factor-, insulin- and type-1 IGF receptor- and TGF-α-positive cells.

When the ICCs were cultured in the presence of various secretagogues, only dibutyryl cyclic AMP was found to up-regulate insulin mRNA (39%; P < 0·05). IGF-II mRNA was also under cyclic AMP-dependent regulation (threefold increase; P = 0·025). Furthermore, blocking the type-1 IGF receptor with a monoclonal receptor antibody drastically reduced insulin expression (87%; P = 0·005) and additionally down-regulated IGF-II mRNA (49%; P = 0·005). IGF-1, IGF-II, TGF-α or epidermal growth factor-receptor antibody had no significant effect on either insulin or IGF-II mRNA. Exogenous TGF-α inhibited the release of insulin by the ICCs. It was concluded that IGF-II and TGF-α may be involved in the regulation of islet growth and differentiation.

Journal of Endocrinology (1993) 138, 127–136

Free access

E A Parker, A Hegde, M Buckley, K M Barnes, J Baron, and O Nilsson

Previous studies of the GH–IGF system gene expression in growth plate using immunohistochemistry and in situ hybridization have yielded conflicting results. We therefore studied the spatial and temporal patterns of mRNA expression of the GH–IGF system in the rat proximal tibial growth plate quantitatively. Growth plates were microdissected into individual zones. RNA was extracted, reverse transcribed and analyzed by real-time PCR. In 1-week-old animals, IGF-I mRNA expression was minimal in growth plate compared with perichondrium, metaphyseal bone, muscle, and liver (70-, 130-, 215-, and 400-fold less). In contrast, IGF-II mRNA was expressed at higher levels than in bone and liver (65- and 2-fold). IGF-II expression was higher in the proliferative and resting zones compared with the hypertrophic zone (P < 0.001). GH receptor and type 1 and 2 IGF receptors were expressed throughout the growth plate. Expression of IGF-binding proteins (IGFBPs)-1 through -6 mRNA was low throughout the growth plate compared with perichondrium and bone. With increasing age (3-, 6-, 9-, and 12-week castrated rats), IGF-I mRNA levels increased in the proliferative zone (PZ) but remained at least tenfold lower than levels in perichondrium and bone. IGF-II mRNA decreased dramatically in PZ (780-fold; P < 0.001) whereas, type 2 IGF receptor and IGFBP-1, IGFBP-2, IGFBP-3, and IGFBP-4 increased significantly with age in growth plate and/or surrounding perichondrium and bone. These data suggest that IGF-I protein in the growth plate is not produced primarily by the chondrocytes themselves. Instead, it derives from surrounding perichondrium and bone. In addition, the decrease in growth velocity that occurs with age may be caused, in part, by decreasing expression of IGF-II and increasing expression of type 2 IGF receptor and multiple IGFBPs.

Free access

Thangiah Geetha, Paul Langlais, Michael Caruso, and Zhengping Yi

Skeletal muscle insulin resistance is an early abnormality in individuals with metabolic syndrome and type 2 diabetes (T2D). Insulin receptor substrate-1 (IRS1) plays a key role in insulin signaling, the function of which is regulated by both phosphorylation and dephosphorylation of tyrosine and serine/threonine residues. Numerous studies have focused on kinases in IRS1 phosphorylation and insulin resistance; however, the mechanism for serine/threonine phosphatase action in insulin signaling is largely unknown. Recently, we identified protein phosphatase 1 (PP1) regulatory subunit 12A (PPP1R12A) as a novel endogenous insulin-stimulated interaction partner of IRS1 in L6 myotubes. The current study was undertaken to better understand PPP1R12A's role in insulin signaling. Insulin stimulation promoted an interaction between the IRS1/p85 complex and PPP1R12A; however, p85 and PPP1R12A did not interact independent of IRS1. Moreover, kinase inhibition experiments indicated that insulin-induced interaction between IRS1 and PPP1R12A was reduced by treatment with inhibitors of phosphatidylinositide 3 kinase, PDK1, Akt, and mTOR/raptor but not MAPK. Furthermore, a novel insulin-stimulated IRS1 interaction partner, PP1 catalytic subunit (PP1cδ), was identified, and its interaction with IRS1 was also disrupted by inhibitors of Akt and mTOR/raptor. These results indicate that PPP1R12A and PP1cδ are new members of the insulin-stimulated IRS1 signaling complex, and the interaction of PPP1R12A and PP1cδ with IRS1 is dependent on Akt and mTOR/raptor activation. These findings provide evidence for the involvement of a particular PP1 complex, PPP1R12A/PP1cδ, in insulin signaling and may lead to a better understanding of dysregulated IRS1 phosphorylation in insulin resistance and T2D.

Free access

Qinkai Li, Weidong Yin, Manbo Cai, Yi Liu, Hongjie Hou, Qingyun Shen, Chi Zhang, Junxia Xiao, Xiaobo Hu, Qishisan Wu, Makoto Funaki, and Yutaka Nakaya

Insulin resistance and dyslipidemia are both considered to be risk factors for metabolic syndrome. Low levels of IGF1 are associated with insulin resistance. Elevation of low-density lipoprotein cholesterol (LDL-C) concomitant with depression of high-density lipoprotein cholesterol (HDL-C) increase the risk of obesity and type 2 diabetes mellitus (T2DM). Liver secretes IGF1 and catabolizes cholesterol regulated by the rate-limiting enzyme of bile acid synthesis from cholesterol 7α-hydroxylase (CYP7A1). NO-1886, a chemically synthesized lipoprotein lipase activator, suppresses diet-induced insulin resistance with the improvement of HDL-C. The goal of the present study is to evaluate whether NO-1886 upregulates IGF1 and CYP7A1 to benefit glucose and cholesterol metabolism. By using human hepatoma cell lines (HepG2 cells) as an in vitro model, we found that NO-1886 promoted IGF1 secretion and CYP7A1 expression through the activation of signal transducer and activator of transcription 5 (STAT5). Pretreatment of cells with AG 490, the inhibitor of STAT pathway, completely abolished NO-1886-induced IGF1 secretion and CYP7A1 expression. Studies performed in Chinese Bama minipigs pointed out an augmentation of plasma IGF1 elicited by a single dose administration of NO-1886. Long-term supplementation with NO-1886 recovered hyperinsulinemia and low plasma levels of IGF1 suppressed LDL-C and facilitated reverse cholesterol transport by decreasing hepatic cholesterol accumulation through increasing CYP7A1 expression in high-fat/high-sucrose/high-cholesterol diet minipigs. These findings indicate that NO-1886 upregulates IGF1 secretion and CYP7A1 expression to improve insulin resistance and hepatic cholesterol accumulation, which may represent an alternative therapeutic avenue of NO-1886 for T2DM and metabolic syndrome.

Free access

B D Green, N Irwin, V A Gault, C J Bailey, F P M O’Harte, and P R Flatt

Glucagon-like peptide-1 (GLP-1) is a potent insulinotropic hormone proposed to play a role in both the pathophysiology and treatment of type 2 diabetes. This study has employed the GLP-1 receptor antagonist, exendin-4(9–39)amide (Ex(9–39)) to evaluate the role of endogenous GLP-1 in genetic obesity-related diabetes and related metabolic abnormalities using ob/ob and normal mice. Acute in vivo antagonistic potency of Ex(9–39) was confirmed in ob/ob mice by blockade of the insulin-releasing and anti-hyperglycaemic actions of intraperitoneal GLP-1. In longer term studies, ob/ob mice were given once daily injections of Ex(9–39) or vehicle for 11 days. Feeding activity, body weight, and both basal and glucose-stimulated insulin secretion were not significantly affected by chronic Ex(9–39) treatment. However, significantly elevated basal glucose concentrations and impaired glucose tolerance were evident at 11 days. These disturbances in glucose homeostasis were independent of changes of insulin sensitivity and reversed by discontinuation of the Ex(9–39) for 9 days. Similar treatment of normal mice did not affect any of the parameters measured. These findings illustrate the physiological extrapancreatic glucose-lowering actions of GLP-1 in ob/ob mice and suggest that the endogenous hormone plays a minor role in the metabolic abnormalities associated with obesity-related diabetes.