Percentage of body fat and plasma glucose predict plasma sialic acid concentration in type 2 diabetes mellitus

The connections of sialic acids and diabetes mellitus: therapeutic or diagnostic value?

Modulation of sialic acids is one of the important pathological consequences of both type 1 and type 2 diabetes mellitus with or without the micro- and macrovascular complications. However, the mechanistic, therapeutic and/or diagnostic implications of these observations are uncoordinated and possibly conflicting. This review critically analyses the scientific investigations connecting sialic acids with diabetes mellitus. Generally, variations in the levels and patterns of sialylation, fucosylation and galactosylation were predominant across various tissues and body systems of diabetic patients, but the immune system seemed to be most affected. These might be explored as a basis for differential diagnosis of various diabetic complications. Sialic acids are predominantly elevated in nearly all forms of diabetic conditions, particularly nephropathy and retinopathy, which suggests some diagnostic value but the mechanistic details were not unequivocal from the available data. The plausible mechanistic explanations for the elevated sialic acids are increased desialylation by sialidases, stimulation of hexosamine pathway and synthesis of acute phase proteins as well as oxidative stress. Additionally, sialic acids are also profoundly associated with glucose transport and insulin resistance in human-based studies while animal-based studies revealed that the increased desialylation of insulin receptors by sialidases, especially NEU1, might be the causal link. Interestingly, inhibition of the diabetes-associated NEU1 desialylation was beneficial in diabetes management and might be considered as a therapeutic target. It is hoped that the article will provide an informed basis for future research activities on the exploitation of sialic acids and glycobiology for therapeutic and/or diagnostic purposes against diabetes mellitus.

Possible involvement of sialidase and sialyltransferase activities in a stage-dependent recycling of sialic acid in some organs of type 1 and type 2 diabetic rats

Background

Type 1 (T1D) and type 2 (T2D) diabetes lead to an aberrant metabolism of sialoglycoconjugates and elevated free serum sialic acid (FSSA) level. The present study evaluated sialidase and sialyltranferase activities in serum and some organs relevant to diabetes at early and late stages of T1D and T2D.

Methods

Sialic acid level with sialidase and sialyltransferase activities were monitored in the serum, liver, pancreas, skeletal muscle and kidney of diabetic animals at early and late stages of the diseases.

Results

The FSSA and activity of sialidase in the serum were significantly increased at late stage of both T1D and T2D while sialic acid level in the liver was significantly decreased in the early and late stages of T1D and T2D, respectively. Furthermore, the activity of sialidase was significantly elevated in most of the diabetes-relevant organs while the activity of sialyltransferase remained largely unchanged. A multiple regression analysis revealed the contribution of the liver to the FSSA while pancreas and kidney contributed to the activity of sialidase in the serum.

Conclusions

We concluded that the release of hepatic sialic acid in addition to pancreatic and renal sialidase might (in)directly contribute to the increased FSSA during both types of diabetes mellitus.

Hepatic sialic acid synthesis modulates glucose homeostasis in both liver and skeletal muscle

OBJECTIVE

Sialic acid is a terminal monosaccharide of glycans in glycoproteins and glycolipids, and its derivation from glucose is regulated by the rate-limiting enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE). Although the glycans on key endogenous hepatic proteins governing glucose metabolism are sialylated, how sialic acid synthesis and sialylation in the liver influence glucose homeostasis is unknown. Studies were designed to fill this knowledge gap.

METHODS

To decrease the production of sialic acid and sialylation in hepatocytes, a hepatocyte-specific GNE knockdown mouse model was generated, and systemic glucose metabolism, hepatic insulin signaling and glucagon signaling were evaluated in vivo or in primary hepatocytes. Peripheral insulin sensitivity was also assessed. Furthermore, the mechanisms by which sialylation in the liver influences hepatic insulin signaling and glucagon signaling and peripheral insulin sensitivity were identified.

RESULTS

Liver GNE deletion in mice caused an impairment of insulin suppression of hepatic glucose production. This was due to a decrease in the sialylation of hepatic insulin receptors (IR) and a decline in IR abundance due to exaggerated degradation through the Eph receptor B4. Hepatic GNE deficiency also caused a blunting of hepatic glucagon receptor (GCGR) function which was related to a decline in its sialylation and affinity for glucagon. An accompanying upregulation of hepatic FGF21 production caused an enhancement of skeletal muscle glucose disposal that led to an overall increase in glucose tolerance and insulin sensitivity.

CONCLUSION

These collective observations reveal that hepatic sialic acid synthesis and sialylation modulate glucose homeostasis in both the liver and skeletal muscle. By interrogating how hepatic sialic acid synthesis influences glucose control mechanisms in the liver, a new metabolic cycle has been identified in which a key constituent of glycans generated from glucose modulates the systemic control of its precursor.

Sialic acids Neu5Ac and KDN in adipose tissue samples from individuals following habitual vegetarian or non-vegetarian dietary patterns

Sialic acids (Sias) are a class of sugar molecules with a parent nine-carbon neuraminic acid, generally present at the ends of carbohydrate chains, either attached to cellular surfaces or as secreted glycoconjugates. Given their position and structural diversity, Sias modulate a wide variety of biological processes. However, little is known about the role of Sias in human adipose tissue, or their implications for health and disease, particularly among individuals following different dietary patterns. The goal of this study was to measure N-Acetylneuraminic acid (Neu5Ac), N-Glycolylneuraminic acid (Neu5Gc), and 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) concentrations in adipose tissue samples from participants in the Adventist Health Study-2 (AHS-2) and to compare the abundance of these Sias in individuals following habitual, long-term vegetarian or non-vegetarian dietary patterns. A method was successfully developed for the extraction and detection of Sias in adipose tissue. Sias levels were quantified in 52 vegans, 56 lacto-vegetarians, and 48 non-vegetarians using LC-MS/MS with Neu5Ac-D-1,2,3-13C3 as an internal standard. Dietary groups were compared using linear regression. Vegans and lacto-ovo-vegetarians had significantly higher concentrations of Neu5Ac relative to non-vegetarians. While KDN levels tended to be higher in vegans and lacto-ovo-vegetarians, these differences were not statistically significant. However, KDN levels were significantly inversely associated with body mass index. In contrast, Neu5Gc was not detected in human adipose samples. It is plausible that different Neu5Ac concentrations in adipose tissues of vegetarians, compared to those of non-vegetarians, reflect a difference in the baseline inflammatory status between the two groups. Epidemiologic studies examining levels of Sias in human adipose tissue and other biospecimens will help to further explore their roles in development and progression of inflammatory conditions and chronic diseases.

Elevated serum sialic acids, a potent biomarker of alloxan-induced type 1 diabetes in dogs by ethanolic extract of Anogeissus leiocarpus

Objectives

This study investigated serum sialic acids for a predictive and diagnostic biomarker of diabetes mellitus (DM) in dogs and its prognostic value with ethanolic extract of Anogeissus leiocarpus.

Design

Four groups of 3 dogs were used; non-diabetic controls (ND), diabetic-untreated (DU), diabetic insulin-treated (DI) and diabetic extract-treated (DE). Free serum sialic acids (FSSA) and erythrocyte surface sialic acids (ESSA) were assayed in all groups, pre-and post-induction of hyperglycaemia and results were presented as means ± standard error of means (SEM) and subjected to ANOVA using Tukey's post-hoc tests with GraphPad Prism® statistical package.

Results

FSSA increased in DU and plateaued at third week (61.8 ± 0.41 μg/ml), (P < 0.002) with additional 38.2 μg/ml (62%) generated, coinciding with hyperglycaemia. FSSA of DI increased but declined to 22.3 ± 1.55 μg/ml. Extract of Anogeissus leiocarpus effectively modulated FSSA in DE as increased value declined to 21.4 ± 0.78 μg/ml. Pre-induction DU ESSA (8.27 ± 0.11 μg/ml) significantly (P < 0.002) decreased by third week (2.33 ± 1.49 μg/ml), coinciding with hyperglycaemia. Strong negative correlation coefficient (r = -0.92) occurred between DU's FSSA and ESSA and ND (P < 0.03). Sialic acid expression in dog's insulin dependent diabetes mellitus (IDDM) is 18% lower than normal. Extract of A. leiocarpus restored ESSA completely. ESSA cleaved in DU, 5.94 μg/ml (72%), could not account for the extra FSSA (32.26 μg/ml); liver and kidneys are contributors.

Conclusion

FSSA predicts canine DM.

The association of total sialic acid and malondialdehyde levels with metabolic and anthropometric variables in obesity

Serum sialic acid levels are abnormally high in pathological states that exhibit tissue destruction, tissue proliferation or inflammation. We measured total serum sialic acid levels in 139 women and 125 men. Subjects were divided into quartiles according to their body mass index (BMI): Q1 (18-24.9 kg/m(2)), Q2 (25-29.9 kg/m(2)), Q3 (30-39.9 kg/m(2)) and Q4 (> 40 kg/m(2)). The patients in Q1 constituted the control group. Serum sialic acid levels of subjects in Q2, Q3 and Q4 were significantly higher than those in Q1. Higher BMI quartiles also were associated with higher levels of serum glucose, insulin, total cholesterol, LDL-cholesterol, triglycerides, high-sensitivity C-reactive protein, malondialdehyde levels, waist circumference, blood pressure and homeostasis model assessment of insulin resistance in both women and men. Lower BMI quartiles were associated with higher levels of serum HDL-cholesterol levels in both women and men. We found positive associations among serum sialic acid levels, BMI and oxidative stress. Serum sialic acid also is related to some conventional cardiovascular risk factors including elevated lipid profile, increased blood pressure, increased serum glucose and insulin levels, and insulin resistance in obese people.

Estimates of body composition with dual-energy X-ray absorptiometry in adults

BACKGROUND

Little is known about the distributions of percentage body fat (PBF), total body fat (TBF), and fat-free mass (FFM) in the adult population in the United States.

OBJECTIVES

We sought to estimate the means and percentile cutoffs of PBF, TBF, and FFM and to assess the differences by sex, age, race-ethnicity, and body mass index in US adults.

DESIGN

Data from the National Health and Nutrition Examination Survey (NHANES), which were collected during the 6-y period from 1999 to 2004 and comprise a large nationally representative sample of the US population, were analyzed (n = 6559 men and 6507 nonpregnant women). TBF and FFM were measured by using dual-energy X-ray absorptiometry. PBF was calculated as TBF divided by total mass multiplied by 100.

RESULTS

There were large differences between men and women in unadjusted mean PBF (28.1% compared with 40.0%, P < 0.001), TBF (25.4 compared with 30.8 kg, P < 0.001), and FFM (62.3 compared with 44.0 kg, P < 0.001); the sex differences persisted across all body mass index categories after adjustment for age and race-ethnicity (all P < 0.001). The common percentile cutoffs of PBF, TBF, and FFM were estimated by sex, race-ethnicity, and age groups. Equations for the estimation of PBF (R(2) = 0.85), TBF (R(2) = 0.94), and FFM (R(2) = 0.94) according to demographic characteristics and simple anthropometric measures were generated.

CONCLUSION

The estimates of means and percentile cutoffs for PBF, TBF, and FFM, on the basis of NHANES 1999-2004 dual-energy X-ray absorptiometry data, provide a reference in the US adult population.

Proinflammatory cytokines in obesity: impact of type 2 diabetes mellitus and gastric bypass

BACKGROUND

Obesity and obesity-associated type 2 diabetes mellitus (T2DM) are frequently related to a low-grade chronic inflammatory state, which increases the risk of developing cardiovascular diseases. The aim of the present work was to evaluate the effect of obesity and T2DM on the concentrations of pro-inflammatory factors and to study the effect of weight loss after Roux-en-Y gastric bypass (RYGBP).

METHODS

Plasma concentrations of monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-alpha), serum amyloid A (SAA) and sialic acid (SA) were measured in 25 female volunteers. The concentrations of these cytokines were determined in 14 female obese patients before and after weight loss following RYGBP. Additionally, visceral adipose tissue (VAT) obtained from 15 females was used to quantify expression levels of MCP-1 and CD68 by Real-Time PCR.

RESULTS

Both obese normoglycemic (NG) and T2DM groups exhibited significantly higher MCP-1 (P < 0.05), TNF-alpha (P < 0.01), SAA (P < 0.05) and SA (P < 0.05) concentrations, compared to the lean group. No differences were found between obese NG and obese T2DM subjects. A significant positive correlation was found between body fat percentage (BF) and all inflammatory markers (P < 0.05) studied. MCP-1 expression levels in VAT were upregulated in obese NG (P = 0.008) and obese T2DM (P = 0.032) patients compared to lean subjects, but no additional detrimental effect of T2DM was observed between both obese groups. After weight loss, SAA (P < 0.001) and SA (P < 0.05) concentrations diminished, whereas circulating levels of MCP-1 showed a tendency to decrease (P = 0.093) and TNF-alpha did not change.

CONCLUSION

The present findings suggest that elevated pro-inflammatory cytokine levels found in obese patients relate mainly to obesity rather than to T2DM. Moreover, surgery-induced weight loss reduces circulating concentrations of key pro-inflammatory factors, which contribute to the improvement in the cardiovascular co-morbidity following excess weight loss.