O-linked oligosaccharides on insulin receptor

A novel role for GalNAc-T2 dependent glycosylation in energy homeostasis

OBJECTIVE

GALNT2, encoding polypeptide N-acetylgalactosaminyltransferase 2 (GalNAc-T2), was initially discovered as a regulator of high-density lipoprotein metabolism. GalNAc-T2 is known to exert these effects through post-translational modification, i.e., O-linked glycosylation of secreted proteins with established roles in plasma lipid metabolism. It has recently become clear that loss of GALNT2 in rodents, cattle, nonhuman primates, and humans should be regarded as a novel congenital disorder of glycosylation that affects development and body weight. The role of GALNT2 in metabolic abnormalities other than plasma lipids, including insulin sensitivity and energy homeostasis, is poorly understood.

METHODS

GWAS data from the UK Biobank was used to study variation in the GALNT2 locus beyond changes in high-density lipoprotein metabolism. Experimental data were obtained through studies in Galnt2-/- mice and wild-type littermates on both control and high-fat diet.

RESULTS

First, we uncovered associations between GALNT2 gene variation, adiposity, and body mass index in humans. In mice, we identify the insulin receptor as a novel substrate of GalNAc-T2 and demonstrate that Galnt2-/- mice exhibit decreased adiposity, alterations in insulin signaling and a shift in energy substrate utilization in the inactive phase.

CONCLUSIONS

This study identifies a novel role for GALNT2 in energy homeostasis, and our findings suggest that the local effects of GalNAc-T2 are mediated through posttranslational modification of the insulin receptor.

Evaluation of guidelines on the screening and diagnosis of gestational diabetes mellitus: systematic review

OBJECTIVE

Guidelines for screening and diagnosis of gestational diabetes mellitus (GDM) have been updated in the past several years, and various inconsistencies exist across these guidelines. Moreover, the quality of these updated guidelines has not been clarified. We thus conducted this systematic review to evaluate the relationship between the quality and detailed recommendations of these guidelines.

DATA SOURCES

The Guidelines International Network Library, the National Institute for Health and Clinical Excellence (NICE) database, the Medline database, the Embase and the National Guidelines Clearinghouse were searched for guidelines containing recommendations on screening and diagnosis strategies for GDM between 2009 and November 2018.

METHODS

Guidelines included a target group of women with GDM, and contained recommendations for screening and diagnostic strategies for GDM were included in the present systematic review. Reviewers summarised recommendations on screening and diagnosis strategies from each guideline and rated the quality of guidelines by using the Appraisal of Guidelines Research and Evaluation (AGREE) criteria.

RESULTS

A total of 459 citations were collected by the preliminary literature selection, and 16 guidelines that met the inclusion criteria were assessed. The inconsistencies of the guidelines mainly focus on the screening process (one step vs two step) and criteria of oral glucose tolerance test (OGTT) (International Association of Diabetes and Pregnancy Study Groups [IADPSG] vs CarpenterandCoustan). Guidelines with higher AGREE scores usually recommend a one-step OGTT strategy with IADPSG criteria between 24 and 28 gestational weeks, and the majority of these guidelines likely to select evidence by Grading of Recommendations Assessment, Development and Evaluation criteria.

CONCLUSIONS

The guidelines of WHO-2013, NICE-2015, American Diabetes Association-2018, Endocrine Society-2013, Society of Obstetricians and Gynaecologists of Canada-2016, International Federation of Gynecology and Obstetrics-2015, American College of Obstetricians and Gynecologists-2018, United States Preventive Services Task Force-2014 and IADPSG-2015 are strongly recommended in the present evaluation, according to the AGREE II criteria. Guidelines with higher quality tend to recommend a one-step 75 g OGTT strategy with IADPSG criteria between 24 and 28 gestational weeks.

Association of risk variants for type 2 diabetes and hyperglycemia with gestational diabetes

OBJECTIVE

The aim of this study was to investigate the association of risk variants for type 2 diabetes (T2D) and hyperglycemia with gestational diabetes (GDM).

DESIGN AND METHODS

Five hundred and thirty-three Finnish women who were diagnosed with GDM and 407 controls with normal glucose tolerance during the pregnancy were genotyped for 69 single-nucleotide polymorphisms (SNPs) which have been previously verified as susceptibility risk variants for T2D and hyperglycemia. All participants underwent an oral glucose tolerance test at the follow-up study after the index pregnancy.

RESULTS

Risk variants rs10830963 and rs1387153 of MTNR1B were significantly associated with GDM (odds ratio (OR)=1.62 (95% CI 1.34-1.96), P=4.5 × 10⁻⁷ and 1.38 (1.14-1.66), P=7.6 × 10⁻⁴ respectively). Both SNPs of MTNR1B were also significantly associated with elevated fasting glucose level and reduced insulin secretion at follow-up. Additionally, risk variants rs9939609 of FTO, rs2796441 of TLE1, rs560887 of G6PC2, rs780094 of GCKR, rs7903146 of TCF7L2 and rs11708067 of ADCY5 showed nominally significant associations with GDM (OR range from 1.25 to 1.30).

CONCLUSIONS

Our study suggests that GDM and T2D share a similar genetic background. Our findings also provide further evidence that risk variants of MTNR1B are associated with GDM by increasing fasting plasma glucose and decreasing insulin secretion.

Identification of Potential Glycoprotein Biomarkers in Estrogen Receptor Positive (ER+) and Negative (ER-) Human Breast Cancer Tissues by LC-LTQ/FT-ICR Mass Spectrometry

Breast cancer is the second most fatal cancer in American women. To increase the life expectancy of patients with breast cancer new diagnostic and prognostic biomarkers and drug targets must be identified. A change in the glycosylation on a glycoprotein often causes a change in the function of that glycoprotein; such a phenomenon is correlated with cancerous transformation. Thus, glycoproteins in human breast cancer estrogen receptor positive (ER+) tissues and those in the more advanced stage of breast cancer, estrogen receptor negative (ER-) tissues, were compared. Glycoproteins showing differences in glycosylation were examined by 2-dimensional gel electrophoresis with double staining (glyco- and total protein staining) and identified by reversed-phase nano-liquid chromatography coupled with a hybrid linear quadrupole ion trap/ Fourier transform ion cyclotron resonance mass spectrometer. Among the identified glycosylated proteins are alpha 1 acid glycoprotein, alpha-1-antitrypsin, calmodulin, and superoxide dismutase mitochondrial precursor that were further verified by Western blotting for both ER+ and ER- human breast tissues. Results show the presence of a possible glycosylation difference in alpha-1-antitrypsin, a potential tumor-derived biomarker for breast cancer progression, which was expressed highest in the ER- samples.

Neurofunctional evaluation of young male offspring of rat dams with diabetes induced by streptozotocin

Diabetes mellitus (DM) is a complex disease, being one of the most prevalent diseases worldwide. As a consequence, pregnancy-associated diabetes is increasingly common. Given the numerous studies about the influence of diabetes on offspring of diabetic rat dams, the neurological outcome is of outmost importance. This paper aimed at evaluating the neurofunctional performance of young male offspring of rat dams with diabetes induced by streptozotocin. Diabetes was induced in Wistar female rats by streptozotocin administration, while control groups received vehicle injection. At two-month survival period, male offspring from each group were randomized to the water maze Morris test, in order to assess their neurofunctional status. There was no significant difference between the groups as assessed by the Morris water maze test for spatial reference task. Our results point to the need of further investigation on the offspring neurofunctional performance.

The location and characterisation of the O-linked glycans of the human insulin receptor

O-linked glycosylation is a post-translational and post-folding event involving exposed S/T residues at beta-turns or in regions with extended conformation. O-linked sites are difficult to predict from sequence analyses compared to N-linked sites. Here we compare the results of chemical analyses of isolated glycopeptides with the prediction using the neural network prediction method NetOGlyc3.1, a procedure that has been reported to correctly predict 76% of O-glycosylated residues in proteins. Using the heavily glycosylated human insulin receptor as the test protein six sites of mucin-type O-glycosylation were found at residues T744, T749, S757, S758, T759, and T763 compared to the three sites (T759 and T763- correctly, T756- incorrectly) predicted by the neural network method. These six sites occur in a 20 residue segment that begins nine residues downstream from the start of the insulin receptor beta-chain. This region which also includes N-linked glycosylation sites at N742 and N755, is predicted to lack secondary structure and is followed by residues 765-770, the known linear epitope for the monoclonal antibody 18-44.

Increased levels of insulin and insulin-like growth factor-1 hybrid receptors and decreased glycosylation of the insulin receptor alpha- and beta-subunits in scrapie-infected neuroblastoma N2a cells

We have previously shown that ScN2a cells (scrapie-infected neuroblastoma N2a cells) express 2-fold- and 4-fold-increased levels of IR (insulin receptor) and IGF-1R (insulin-like growth factor-1 receptor) respectively. In addition, the IR alpha- and beta-subunits are aberrantly processed, with apparent molecular masses of 128 and 85 kDa respectively, as compared with 136 and 95 kDa in uninfected N2a cells. Despite the 2-fold increase in IR protein, the number of (125)I-insulin-binding sites was slightly decreased in ScN2a cells [Ostlund, Lindegren, Pettersson and Bedecs (2001) Brain Res. 97, 161-170]. In order to determine the cellular localization of IR in ScN2a cells, surface biotinylation was performed, showing a correct IR trafficking and localization to the cell surface. The present study shows for the first time that neuroblastoma N2a cells express significant levels of IR-IGF-1R hybrid receptors, and in ScN2a cells the number of hybrid receptors was 2-fold higher than that found in N2a cells, potentially explaining the apparent loss of insulin-binding sites due to a lower affinity for insulin compared with the homotypic IR. Furthermore, the decreased molecular mass of IR subunits in ScN2a cells is not caused by altered phosphorylation or proteolytic processing, but rather by altered glycosylation. Enzymic deglycosylation of immunoprecipitated IR from N2a and ScN2a cells with endoglycosidase H, peptide N-glycosidase F and neuraminidase all resulted in subunits with increased electrophoretic mobility; however, the 8-10 kDa shift remained. Combined enzymic or chemical deglycosylation using anhydrous trifluoromethane sulphonic acid treatment ultimately showed that the IR alpha- and beta-subunits from ScN2a cells are aberrantly glycosylated. The increased formation of IR-IGF-1R hybrids in ScN2a cells may be part of a neuroprotective response to prion infection. The degree and functional significance of aberrantly glycosylated proteins in ScN2a cells remain to be determined.

Purification and characterization of a feline hepatic insulin receptor

OBJECTIVE

To elucidate the functional characteristics of a highly purified soluble liver insulin receptor in cats.

SAMPLE POPULATION

Frozen livers from domestic cats were obtained commercially.

PROCEDURES

The feline hepatic insulin receptor was purified from Triton X-100 solubilized plasma membranes by the use of several chromatography matrices, including affinity chromatography on an insulin-Sepharose matrix.

RESULTS

The receptor, although not homogeneous, was purified 3,000-fold. Two silver-stained protein bands were identified following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with molecular weight of 134,000 and 97,000, which are similar to insulin receptors isolated from other animals. This isolated receptor had steady-state insulin binding by 40 minutes at 24 C. Optimal insulin binding occurred at pH 7.8 and with 150 mM NaCl. Under these conditions, a curvilinear Scatchard plot was obtained with the isolated receptor. Using a 2 binding-site model, the feline insulin receptor had a high-affinity low-capacity site with a dissociation constant (KD; nM) of 3 and a low-affinity high-capacity site with a K(D) of 1,180. The receptor also had tyrosine kinase activity toward an exogenous substrate that was stimulated by insulin and protamine.

CONCLUSIONS AND CLINICAL RELEVANCE

Many of the reported characteristics of the liver insulin receptor in cats are similar to those for the receptor isolated from other animals and tissues, although some differences exist. These similarities suggest that characterization of the feline insulin receptor is important to understanding insulin resistance in cats with diabetes as well as in humans with diabetes.

Is mild gestational hyperglycaemia associated with maternal and neonatal complications? The Diagest Study

AIMS

To evaluate the maternal and neonatal complications rates of mild gestational hyperglycaemia (MGH) compared to a control group in France.

METHODS

A systematic screening by a 50-g glucose challenge test was offered to all women between 24 and 28 weeks of gestation in 15 maternity units. If the 50-g glucose challenge test was > or = 7.2 mmol/l, a 100-g 3-h oral glucose tolerance test (OGTT) was performed. MGH (n = 131) was defined by one abnormal value on the 3-h OGTT (Carpenter and Coustan criteria). The control group (n = 108) was defined by a 50-g glucose challenge test below 7.2 mmol/l. Women with MGH received no treatment or specific advice during the pregnancy. Large for gestational age (LGA) was defined by a birth weight of at least the 90th percentile on French standard growth curves.

RESULTS

Women with MGH were older than the controls (28.8 (5.8) vs. 27.0 (5.2); P < 0.05) and had a higher body mass index (24.8 (4.8) vs. 23.0 (3.9); P < 0.01). The rate of pregnancy-induced hypertension and Caesarean section were not different between the MGH and control group. The rate of LGA was significantly higher in the MGH group than the control group (22.1% vs. 11.4%; P < 0.05). After adjustment for confounding factors of macrosomia (pre-pregnancy body mass index > 27, maternal age > 35, multiparity and educational level), there was a persistent relationship between LGA and MGH (odds ratio 2.50; 95% confidence interval (1.16-5.40); P < 0.05). MGH was more frequently associated with adverse maternal and fetal outcome than in the controls (53.4% vs. 28.7%; P < 0.01).

CONCLUSIONS

This study suggested that the increased rate of adverse maternal and fetal outcome, especially LGA, was associated with untreated mild gestational hyperglycaemia women compared to a control group. This link to lower degrees of hyperglycaemia during pregnancy is independent of confounding factors.

Effect of alternative glycosylation on insulin receptor processing

The mature insulin receptor is a cell surface heterotetrameric glycoprotein composed of two alpha- and two beta-subunits. In 3T3-L1 adipocytes as in other cell types, the receptor is synthesized as a single polypeptide consisting of uncleaved alpha- and beta-subunits, migrating as a 190-kDa glycoprotein. To examine the importance of N-linked glycosylation on insulin receptor processing, we have used glucose deprivation as a tool to alter protein glycosylation. Western blot analysis shows that glucose deprivation led to a time-dependent accumulation of an alternative proreceptor of 170 kDa in a subcellular fraction consistent with endoplasmic reticulum localization. Co-precipitation assays provide evidence that the alternative proreceptor bound GRP78, an endoplasmic reticulum molecular chaperone. N-Glycosidase F treatment shows that the alternative proreceptor contained N-linked oligosaccharides. Yet, endoglycosidase H insensitivity indicates an aberrant oligosaccharide structure. Using pulse-chase methodology, we show that the synthetic rate was similar between the normal and alternative proreceptor. However, the normal proreceptor was processed into alpha- and beta-subunits (t((1)/(2)) = 1.3 +/- 0.6 h), while the alternative proreceptor was degraded (t((1)/(2)) = 5.1 +/- 0.6 h). Upon refeeding cells that were initially deprived of glucose, the alternative proreceptor was processed to a higher molecular weight form and gained sensitivity to endoglycosidase H. This "intermediate" form of the proreceptor was also degraded, although a small fraction escaped degradation, resulting in cleavage to the alpha- and beta-subunits. These data provide evidence for the first time that glucose deprivation leads to the accumulation of an alternative proreceptor, which can be post-translationally glycosylated with the readdition of glucose inducing both accelerated degradation and maturation.

Etiology, Detection, Diagnosis, and Clinical Management of Gestational Diabetes Mellitus

Healthy outcomes for both mother and child are expected and fortunately seen in most pregnancies. In some cases, serious or potentially serious problems arise during the pregnancy that mandate a need for both close monitoring and treatment interventions by health care providers. Gestational diabetes mellitus (GDM) is such a condition that may evolve during pregnancy. Women who experienced gestational diabetes during pregnancy are at increased risk of developing Type 2 diabetes as are their offspring. As defined, GDM is a type of diabetes restricted to pregnant women in whom the recognition of glucose intolerance first occurs during pregnancy.1 Physicians and pharmacists who are trained in the management of diabetes can help guide the patient with GDM through the pregnancy and after delivery, monitor her and the infant. This article reviews the current concepts pertaining to the basic pathophysiology, detection, diagnosis, and management of gestational diabetes mellitus.

Substrates for protein kinase CK2 in insulin receptor preparations from rat liver membranes: identification of a 210-kDa protein substrate as the dimeric form of endoplasmin

Chromatography of extracts from rat liver membranes on wheat-germ lectin-Sepharose resulted in a partial resolution of the insulin receptor from other phosphorylatable proteins. Among the latter, a protein (p210, with an apparent M(r) of 210 kDa on SDS/PAGE under nonreducing conditions) was found to be phosphorylated by protein kinase CK2 on Thr and Ser residues. Under reducing conditions p210 was resolved into two phosphopolypeptides with apparent M(r) of 95 and 105 kDa. Neither the 95-kDa nor the 105-kDa polypeptides were recognized by antibodies against the beta-subunit of the insulin receptor. Both polypeptides gave identical phosphopeptide maps after protease V8 digestion and contained the same N-terminal amino acid sequence. This sequence coincided with that of endoplasmin, and both polypeptides as well as p210 were recognized by antibodies against this protein. This shows that p210 corresponds to the dimeric form of rat liver endoplasmin. DEAE-Sepharose chromatography of p210 preparations removed most other contaminating proteins and revealed the presence of a protein kinase activity that coeluted with p210. This protein kinase possessed the properties (substrate specificity and inhibition by heparin) that are characteristic of the protein kinase CK2 enzymes. Furthermore, phosphoamino acid analysis and phosphopeptide maps of the 95/105-kDa polypeptides phosphorylated either by the endogenous protein kinase or by exogenous protein kinase CK2 gave similar results. The phosphorylation of p210/endoplasmin by protein kinase CK2 and its coelution gives support to the involvement of this protein kinase in membrane-associated processes.

Cleavage of the transferrin receptor is influenced by the composition of the O-linked carbohydrate at position 104

A soluble form of the human transferrin receptor (TfR) resulting from proteolytic cleavage at Arg 100 has been measured in human blood. In tissue culture cells elimination of the O-linked carbohydrate at Thr 104, four amino acids from the cleavage site, results in enhanced cleavage of the TfR (Rutledge et al., 1994, Blood, 83:580-586). In the present set of studies, the influence of amino acid substitution and the composition of the oligosaccharide at amino acid 104 on the cleavage of the TfR was examined. Site-directed mutagenesis was used to generate six different amino acids at position 104 which varied in size and charge. Measurement of the soluble TfR in the conditioned medium of the transfected cells of each mutant TfR showed that the large and charged side chains inhibited TfR cleavage the most. Otherwise the properties of the mutant TfRs were indistinguishable from the wild-type TfR in that the affinity of transferrin for these receptors, the extent of disulfide bond formation of the TfRs, and the proportion of TfRs at the cell surface were similar to that of the wild-type TfR. Removal of the sialic acid component of the carbohydrate from wild-type TfR by treatment of live cells with neuraminidase enhances TfR cleavage. Expression of wild-type TfR in CHO IdlD cells (a glycosylation defective cell line) also shows enhanced cleavage under conditions that produce truncated or no O-linked carbohydrates. Treatment of IdlD cells with neuraminidase reveals that the sialic acid of the O-linked carbohydrate protects against TfR cleavage, whereas the core sugars Gal-NAc and Gal do not protect as much. These results show that the terminal charged sialic acid residues are important for protection from proteolytic cleavage and suggest that cleavage could be regulated in the cell by removal of all or part of the carbohydrate.

Prevalence and predictive value of islet cell antibodies and insulin autoantibodies in women with gestational diabetes

The objective of the present study was to investigate the predictive value of islet cell antibodies (ICA) and insulin autoantibodies (IAA) for development of diabetes in women with previous gestational diabetes (GDM). Two hundred and forty-one previous diet-treated GDM patients and 57 women without previous GDM were examined 2-11 years after the index pregnancy. In subgroups, plasma from the diagnostic OGTT during index pregnancy was analysed for ICA and IAA. Among the previous GDM patients, 3.7% had developed Type 1 diabetes and 13.7% Type 2 diabetes. Four (2.9%) of the 139 GDM patients tested for ICA were ICA-positive and three of these had Type 1 diabetes at follow-up, as well as three ICA-negative patients. The sensitivity, specificity, and predictive value of ICA-positivity for later development of diabetes were 50%, 99%, and 75%, respectively. None of the women was IAA-positive during pregnancy. In conclusion, the majority of the patients with GDM did not show evidence of ongoing autoimmune destruction of the beta cells during the index pregnancy. However, ICA-positive GDM patients had a high risk of developing Type 1 diabetes later in life.

Specific glycosylation site mutations of the insulin receptor alpha subunit impair intracellular transport

The insulin receptor is a transmembrane protein found on multiple cell types. This receptor is synthesized as a 190-kDa proreceptor which is cleaved to produce mature alpha and beta subunits. The proreceptor contains 18 potential sites for N-linked glycosylation: 14 on the alpha subunit and 4 on the beta subunit. The codons for asparagine in the first four sites at the amino terminus of the alpha subunit were mutated to code for glutamine. This mutant receptor cDNA was stably transfected into NIH 3T3 cells. The insulin receptor produced in these cells remained in the proreceptor form; no mature alpha and beta subunits were produced. The proreceptor was slightly smaller on SDS-PAGE gels than the wild-type proreceptor and contained four less oligosaccharide chains by tryptic peptide mapping. The carbohydrate chains on the mutant proreceptor remained endoglycosidase H sensitive. However, in the presence of brefeldin A, these oligosaccharide chains could be processed to endoglycosidase H resistant chains. By immunofluorescence, the mutant proreceptor was shown to be localized to the endoplasmic reticulum. No insulin receptors could be found on the cell-surface either with cell surface labeling with biotin or with 125I-insulin binding. Thus, glycosylation of the first four N-linked glycosylation sites of the insulin receptor is necessary for the proper processing and intracellular transport of the receptor. This is in contrast to glycosylation at the four sites on the beta subunit which appear not to be important for processing but necessary for signal transduction. Therefore, N-linked glycosylation of the insulin receptor at specific sites has multiple distinctive roles.