Serine residues 994 and 1023/25 are important for insulin receptor kinase inhibition by protein kinase C isoforms beta2 and theta

AIMS/HYPOTHESIS: Inhibition of the signalling function of the human insulin receptor (HIR) is one of the principle mechanisms which induce cellular insulin resistance. It is speculated that serine residues in the insulin receptor beta-subunit are involved in receptor inhibition either as inhibitory phosphorylation sites or as part of receptor domains which bind inhibitory proteins or tyrosine phosphatases. As reported earlier we prepared 16 serine to alanine point mutations of the HIR and found that serine to alanine mutants HIR-994 and HIR-1023/25 showed increased tyrosine autophosphorylation when expressed in human embryonic kidney (HEK) 293 cells. In this study we examined whether these mutant receptors have a different susceptibility to inhibition by serine kinases or an altered tyrosine kinase activity. METHODS: Tyrosine kinase assay and transfection studies. RESULTS: In an in vitro kinase assay using IRS-1 as a substrate we could detect a higher intrinsic tyrosine kinase activity of both receptor constructs. Additionally, a higher capacity to phosphorylate the adapter protein Shc in intact cells was seen. To test the inhibition by serine kinases, the receptor constructs were expressed in HEK 293 cells together with IRS-1 and protein kinase C isoforms beta2 and theta. Phorbol ester stimulation of these cells reduced wild-type receptor autophosphorylation to 58 % or 55 % of the insulin simulated state, respectively. This inhibitory effect was not observed with HIR-994 and HIR-1023/25, although all other tested HIR mutants showed similar inhibition induced by protein kinase C. CONCLUSION/INTERPRETATION: The data suggest that the HIR-domain which contains the serine residues 994 and 1023/25 is important for the inhibitory effect of protein kinase C isoforms beta2 and theta on insulin receptor autophosphorylation.

Grb-IR: a SH2-domain-containing protein that binds to the insulin receptor and inhibits its function.

To identify potential signaling molecules involved in mediating insulin-induced biological responses, a yeast two-hybrid screen was performed with the cytoplasmic domain of the human insulin receptor (IR) as bait to trap high-affinity interacting proteins encoded by human liver or HeLa cDNA libraries. A SH2-domain-containing protein was identified that binds with high affinity in vitro to the autophosphorylated IR. The mRNA for this protein was found by Northern blot analyses to be highest in skeletal muscle and was also detected in fat by PCR. To study the role of this protein in insulin signaling, a full-length cDNA encoding this protein (called Grb-IR) was isolated and stably expressed in Chinese hamster ovary cells overexpressing the human IR. Insulin treatment of these cells resulted in the in situ formation of a complex of the IR and the 60-kDa Grb-IR. Although almost 75% of the Grb-IR protein was bound to the IR, it was only weakly tyrosine-phosphorylated. The formation of this complex appeared to inhibit the insulin-induced increase in tyrosine phosphorylation of two endogenous substrates, a 60-kDa GTPase-activating-protein-associated protein and, to a lesser extent, IR substrate 1. The subsequent association of this latter protein with phosphatidylinositol 3-kinase also appeared to be inhibited. These findings raise the possibility that Grb-IR is a SH2-domain-containing protein that directly complexes with the IR and serves to inhibit signaling or redirect the IR signaling pathway.

Independent, marked associations of alleles of the insulin receptor and dipeptidyl carboxypeptidase-I genes with essential hypertension

1. There is evidence to suggest that essential hypertension is a polygenic disorder and that it arises from yet-to-be-identified predisposing variants of certain genes that influence blood pressure. The cloning of various hormone, enzyme, adrenoceptor and hormone receptor genes whose products are involved in blood pressure control and the identification of polymorphisms of these has permitted us to test their genetic association with hypertension. 2. Cross-sectional analyses of a number of candidate gene markers were performed in hypertensive and normotensive subjects who were selected on the basis of both parents being either hypertensive or normotensive, respectively, and the difference in total alleles on all chromosomes for each polymorphism between the hypertensive and normotensive groups was test by χ analysis with one degree of freedom. 3. A marked association was observed between hypertension and insertion alleles of polymorphisms of the insulin receptor gene (INSR) (P<0.0040) and the dipeptidyl carboxypeptidase-1 (angiotensin I-converting enzyme; kininase II) gene (DCP1) (P<0.0018). No association with hypertension was evident, however, for polymorphisms of the growth hormone, low-density lipoprotein receptor, renal kallikrein, α2- and β1-adrenoreceptor, atrial natriuretic factor and insulin genes. 4. All but one of the hypertensive subjects had at least one of the hypertension-associated alleles, and although subjects homozygous for both were three times more frequent in the hypertensive group, examination of the nine possible genotypes suggested that the INSR and DCP1 alleles are independent markers for hypertension. 5. The present results suggest that genetic variant(s) in close linkage disequilibrium with polymorphisms at INSR and DCP1 may be involved in part in the aetiology of essential hypertension.

Association of a RFLP for the insulin receptor gene, but not insulin, with essential hypertension

Insulin has cardiovascular actions and patients with essential hypertension display insulin resistance. A cross-sectional study of the R1 RFLP of the insulin receptor gene (INSR) was carried out in 67 hypertensive (HT) and 75 normotensive (NT) subjects whose parents had a similar blood pressure status at age ≥50. The frequency of the minor (+) allele was 0.31 in HTs and 0.44 in NTs, and the difference between observed alleles in all subjects in each group was significant (χ2 = 4.8, P<0.05). Allele frequencies of a BglI RFLP of the insulin gene, however, did not differ between the HT and NT groups. The data thus provide evidence in favour of an association of HT with a polymorphism at the INSR locus (19p 13.3-13.2), so implicating this locus, and possibly a genetic variant of the insulin receptor itself, in HT.

Non-linkage of insulin receptor locus with essential hypertension in an affected pedigree

A recent cross-sectional study has demonstrated a significant association of the R1 RsaI restriction fragment length polymorphism of the insulin receptor gene (INSR) with human essential hypertension. In the present study, an alternative approach, involving linkage analysis, was carried out using 8 hypertensive families with 5 or more affected members. Five of the families were found to be informative and in one of these pedigrees a conclusion of non-linkage of INSR and hypertension could be made on the basis of an obligate recombinant in one generation which yielded a Lod score of - ∞ at a recombination fraction (θ) of zero. In another family, the largest studied, a positive Lod score was obtained at θ = 0, but this was below the level required for a conclusion of linkage. Lod score at θ = 0 for a marker at the insulin locus in this family was negative. The present study has thus demonstrated one pedigree in which hypertension is not linked to the insulin receptor locus.

Suggestive linkage of the parathyroid receptor type 1 to osteoporosis

We have investigated the role of 23 candidate genes in the control of bone mineral density (BMD) by linkage studies in families of probands with osteoporosis (lumbar spine [LS] or femoral neck [FN] BMD T score < -2.5) and low BMD relative to an age- and gender-matched cohort (Z score < -2.0). One hundred and fifteen probands (35 male, 80 female) and 499 of their first- or second-degree relatives (223 males and 276 females) were recruited for the study. BMD was measured at the LS and FN using dual-energy X-ray absorptiometry and expressed as age- and gender-matched Z scores corrected for body mass index. The candidate genes studied were the androgen receptor, type I collagen A1 (COLIA1), COLIA2, COLIIA1, vitamin D receptor (VDR), colony-stimulating factor 1, calcium-sensing receptor, epidermal growth factor (EGF), estrogen receptor 1 (ESR1), fibrillin type 1, insulin-like growth factor 1, interleukin-1 alpha (IL-1α), interleukin-4 (IL-4), interleukin-6 (IL-6), interleukin-11 (IL-11), osteopontin, parathyroid hormone (PTH), PTH-related peptide, PTH receptor type 1 (PTHR1), transforming growth factor-beta 1, and tumor necrosis factors alpha and beta. Sixty-four microsatellites lying close to or within these genes were investigated for linkage with BMD. Using the program MapMaker/Sibs there was suggestive evidence of linkage between BMD and PTHR1 (maximum LOD score obtained [MLS] 2.7-3.5). Moderate evidence of linkage was also observed with EGF (MLS 1.8), COLIA1 (MLS 1.7), COLIIA1/VDR (MLS 1.7), ESR1 (MLS 1.4), IL-1α (MLS 1.4), IL-4 (MLS 1.2), and IL-6 (MLS 1.2). Variance components analysis using the program ACT, correcting for proband-wise ascertainment, also showed evidence of linkage (p ≤0.05) at markers close to or within the candidate genes IL- 1α, PTHR1, IL-6, and COLIIA1/VDR. Further studies will be required to confirm these findings, to refine the location of gene responsible for the observed linkage, and to screen the candidate genes targeted at these loci for mutations.

A comparative structural bioinformatics analysis of the insulin receptor family ectodomain based on phylogenetic information

The insulin receptor (IR), the insulin-like growth factor 1 receptor (IGF1R) and the insulin receptor-related receptor (IRR) are covalently-linked homodimers made up of several structural domains. The molecular mechanism of ligand binding to the ectodomain of these receptors and the resulting activation of their tyrosine kinase domain is still not well understood. We have carried out an amino acid residue conservation analysis in order to reconstruct the phylogeny of the IR Family. We have confirmed the location of ligand binding site 1 of the IGF1R and IR. Importantly, we have also predicted the likely location of the insulin binding site 2 on the surface of the fibronectin type III domains of the IR. An evolutionary conserved surface on the second leucine-rich domain that may interact with the ligand could not be detected. We suggest a possible mechanical trigger of the activation of the IR that involves a slight ‘twist’ rotation of the last two fibronectin type III domains in order to face the likely location of insulin. Finally, a strong selective pressure was found amongst the IRR orthologous sequences, suggesting that this orphan receptor has a yet unknown physiological role which may be conserved from amphibians to mammals.

Expression of glucose transporter isoforms and the insulin receptor during hamster preimplantation embryo development

During preimplantation development, embryos of many species are known to express up to five isoforms of the facilitative glucose transporter proteins (GLUT). Development of hamster blastocysts is inhibited by glucose. We therefore investigated GLUT isoform and insulin receptor (IR) expression in hamster preimplantation embryos cultured in glucose-free medium from the 8-cell stage onwards. We show that GLUT1, 3 and 8 mRNA are constitutively expressed from the 8-cell to the blastocyst stage. The IR is expressed from the morula stage onwards. Messenger RNA of the insulin-responsive GLUT4 was not detected at any stage. GLUT1 and 3 were localised by immunocytochemistry. GLUT1 was expressed in both embryoblast and trophoblast, in the latter, mainly in basal and lateral membranes directed towards the blastocoel. and embryoblast. GLUT3 was exclusively localised in the apical. membrane of trophoblast cells. We show that hamster preimplantation embryos express several GLUT isoforms thus closely resembling embryos of other mammalian species. Despite endogenous IR expression, the insulin-sensitive isoform GLUT4 was not expressed, indicating that the insulin-mediated glucose uptake known from classical insulin target cells may not be relevant for hamster blastocysts.

The orphan nuclear receptor, liver receptor homolog-1 (LRH-1, NR5A2) regulates decidualization

La période de réceptivité endométriale chez l’humain coïncide avec la différentiation des cellules stromales de l’endomètre en cellules hautement spécifiques, les cellules déciduales, durant le processus dit de décidualisation. Or, on sait qu’une transformation anormale des cellules endométriales peut être à l’origine de pertes récurrentes de grossesses. LRH-1 est un récepteur nucléaire orphelin et un facteur de transcription régulant de nombreux évènements relatif à la reproduction et comme tout récepteur, son activation promouvoit l’activité transcriptionnelle de ses gènes cibles. Nous avons déjà montré que LRH-1 et son activité sont essentiels pour la décidualisation au niveau de l’utérus chez la souris et nous savons qu’il est présent dans l’utérus chez l’humain au moment de la phase de prolifération mais aussi de sécrétion du cycle menstruel, et que son expression augmente dans des conditions de décidualisation in vitro. Notre hypothèse est alors la suivante : LRH-1 est indispensable à la décidualisation du stroma endométrial, agissant par le biais de la régulation transcriptionnelle de gènes requis pour la transformation de cellules stromales en cellules déciduales. Afin d’explorer le mécanisme moléculaire impliqué dans la régulation transcriptionnelle effectuée par l’intermédiaire de ce récepteur, nous avons mis en place un modèle de décidualisation in vitro utilisant une lignée de cellules stromales de l’endomètre, cellules humaines et immortelles (hESC). Notre modèle de surexpression développé en transfectant les dites cellules avec un plasmide exprimant LRH-1, résulte en l’augmentation, d’un facteur 5, de l’abondance du transcriptome de gènes marqueurs de la décidualisation que sont la prolactine (PRL) et l’insulin-like growth factor binding protein-1 (IGFBP-1). En outre, la sous-régulation de ce récepteur par l’intermédiaire de petits ARN interférents (shRNA) abolit la réaction déciduale, d’un point de vue morphologique mais aussi en terme d’expression des deux gènes marqueurs cités ci-dessus. Une analyse par Chromatin ImmunoPrécipitation (ou ChIP) a démontré que LRH-1 se lie à des régions génomiques se trouvant en aval de certains gènes importants pour la décidualisation comme PRL, WNT 4, WNT 5, CDKN1A ou encore IL-24, et dans chacun de ces cas cités, cette capacité de liaison augmente dans le cadre de la décidualisation in vitro. Par ailleurs, des études structurelles ont identifié les phospholipides comme des ligands potentiels pour LRH-1. Nous avons donc choisi d’orienter notre travail de façon à explorer les effets sur les ligands liés à LRH-1 de traitements impliquant des agonistes et antagonistes à notre récepteur nucléaire. Les analyses par q-PCR et Western blot ont montré que la modulation de l’activité de LRH-1 par ses ligands influait aussi sur la réaction déciduale. Enfin, des études récentes de Salker et al (Salker, Teklenburg et al. 2010) ont mis en évidence que les cellules stromales humaines décidualisées sont de véritables biocapteurs de la qualité embryonnaire et qu’elles ont la capacité de migrer en direction de l’embryon. La série d’expériences que nous avons réalisée à l’aide de cellules hESC placées en co-culture avec des embryons de souris confirme que la migration cellulaire est bien dirigée vers les embryons. Cette propriété quant à l’orientation de la migration cellulaire est notoirement diminuée dans le cas où l’expression de LRH-1 est déplétée par shRNA dans les hESC. Nos données prouvent donc que LRH-1 régule non seulement la transcription d’un ensemble de gènes impliqués dans le processus de décidualisation mais agit aussi sur la motilité directionnelle de ces cellules hESC décidualisées in vitro.

Potential Role of Growth Hormone in Impairment of Insulin Signaling in Skeletal Muscle, Adipose Tissue, and Liver of Rats Chronically Treated with Arginine

We have shown that rats chronically treated with Arginine (Arg), although normoglycemic, exhibit hyperinsulinemia and decreased blood glucose disappearance rate after an insulin challenge. Attempting to investigate the processes underlying these alterations, male Wistar rats were treated with Arg (35 mg/d), in drinking water, for 4 wk. Rats were then acutely stimulated with insulin, and the soleus and extensorum digitalis longus muscles, white adipose tissue (WAT), and liver were excised for total and/or phosphorylated insulin receptor (IR), IR substrate 1/2, Akt, Janus kinase 2, signal transducer and activator of transcription (STAT) 1/3/5, and p85 alpha/55 alpha determination. Muscles and WAT were also used for plasma membrane (PM) and microsome evaluation of glucose transporter (GLUT) 4 content. Pituitary GH mRNA, GH, and liver IGF-I mRNA expression were estimated. It was shown that Arg treatment: 1) did not affect phosphotyrosine-IR, whereas it decreased phosphotyrosine-IR substrate 1/2 and phosphoserine-Akt content in all tissues studied, indicating that insulin signaling is impaired at post-receptor level; 2) decreased PM GLUT4 content in both muscles and WAT; 3) increased the pituitary GH mRNA, GH, and liver IGF-I mRNA expression, the levels of phosphotyrosine-STAT5 in both muscles, phosphotyrosine-Janus kinase 2 in extensorum digitalis longus, phosphotyrosine-STAT3 in liver, and WAT as well as total p85 alpha in soleus, indicating that GH signaling is enhanced in these tissues; and 4) increased p55 alpha total content in muscles, WAT, and liver. The present findings provide the molecular mechanisms by which insulin resistance and, by extension, reduced GLUT4 content in PM of muscles and WAT take place after chronic administration of Arg, and further suggest a putative role for GH in its genesis, considering its diabetogenic effect. (Endocrinology 150: 2080-2086, 2009)

Involvement of Phosphatidylinositol-3 Kinase/AKT/PKC zeta/lambda Pathway in the Effect of Palmitate on Glucose-Induced Insulin Secretion

Objectives: In the present study, a novel pathway by which palmilate potentiates glucose-induced insulin secretion by pancreatic beta cells was investigated. Methods: Groups of freshly isolated islets were incubated in 10 mM glucose with palmitate, LY294002, wortmannin, and fumonism B I for measurement of insulin secretion by radioimmunoassay (RIA). Also, phosphorylation and content of AKT and PKC proteins were evaluated by immunoblotting. Results: Glucose plus palmitate and glucose plus LY294002 or wortmannin (PI3K inhibitors) increased glucose-induced insulin secretion by isolated pancreatic islets. Glucose at 10 mM induced AKT and PKC zeta/lambda phosphorylation. Palmitate (0.1 mM) abolished glucose stimulation of AKT and PKC zeta/lambda phosphorylation possibly through PI3K inhibition because both LY294002 (50 mu M) and wortmannin (100 nM) caused the same effect. The inhibitory effect of palmitate on glucose-induced AKT and PKC zeta/lambda phosphorylation and the stimulatory effect of palmitate on glucose-induced insulin secretion were not observed in the presence of fumonisin B1, all inhibitor of ceramide synthesis. Conclusions: These findings support the proposition that palmilate increases insulin release in the presence of 10 mM glucose by inhibiting PI3K activity through a mechanism that involves ceramide synthesis.

Augmentation of insulin secretion by leucine supplementation in malnourished rats: possible involvement of the phosphatidylinositol 3-phosphate kinase/mammalian target protein of rapamycin pathway

A regimen of low-protein diet induces a reduction of pancreatic islet function that is associated with development of metabolic disorders including diabetes and obesity afterward. In the present study, the influence of leucine supplementation on metabolic parameters, insulin secretion to glucose and to amino acids, as well as the levels of proteins that participate in the phosphatidylinositol 3-phosphate kinase (PI3K) pathway was investigated in malnourished rats. Four groups were fed with different diets for 12 weeks: a normal protein diet (17%) without (NP) or with leucine supplementation (NPL) or a low (6%)-protein diet without (LP) or with leucine supplementation (LPL). Leucine was given in the drinking water during the last 4 weeks. As indicated by the intraperitoneal glucose tolerance test, LPL rats exhibited increased glucose tolerance as compared with NPL group. Both NPL and LPL rats had higher circulating insulin levels than controls. The LPL rats also showed increased insulin secretion by pancreatic islets in response to glucose or arginine compared with those observed in islets from LP animals. Glucose oxidation was significantly reduced in NPL, LP, and LPL isolated islets as compared with NP; but no alteration was observed for leucine and glutamate oxidation among the 4 groups. Western blotting analysis demonstrated increased PI3K and mammalian target protein of rapamycin protein contents in LPL compared with LP islets. A significant increase in insulin-induced insulin receptor substrate I associated PI3K activation was also observed in LPL compared with LP islets. These findings indicate that leucine supplementation can augment islet function in malnourished rats and that activation of the PI3K/maminalian target protein of rapamycin pathway may play a role in this process. (C) 2010 Elsevier Inc. All rights reserved.

Obesity induced by high-fat diet promotes insulin resistance in the ovary

Besides the effects on peripheral energy homeostasis, insulin also has an important role in ovarian function. Obesity has a negative effect on fertility, and may play a role in the development of the polycystic ovary syndrome in susceptible women. Since insulin resistance in the ovary could contribute to the impairment of reproductive function in obese women, we evaluated insulin signaling in the ovary of high-fat diet-induced obese rats. Female Wistar rats were submitted to a high-fat diet for 120 or 180 days, and the insulin signaling pathway in the ovary was evaluated by immunoprecipitation and immunoblotting. At the end of the diet period, we observed insulin resistance, hyperinsulinemia, an increase in progesterone serum levels, an extended estrus cycle, and altered ovarian morphology in obese female rats. Moreover, in female obese rats treated for 120 days with the high-fat diet, the increase in progesterone levels occurred together with enhancement of LH levels. The ovary from high-fat-fed female rats showed a reduction in the insulin receptor substrate/phosphatidylinositol 3-kinase/AKT intracellular pathway, associated with an increase in FOXO3a, IL1B, and TNF alpha protein expression. These changes in the insulin signaling pathway may have a role in the infertile state associated with obesity. Journal of Endocrinology (2010) 206, 65-74

NaF treatment increases TNF-alpha and resistin concentrations and reduces insulin signal in rats

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effects of fluoride intake on insulin sensitivity and insulin signal transduction

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)