ALTERED CIRCULATING LEVELS OF RETINOL BINDING PROTEIN 4 AND TRANSTHYRETIN IN RELATION TO INSULIN RESISTANCE, OBESITY, AND GLUCOSE INTOLERANCE IN ASIAN INDIANS

Retinol binding protein 4 and type 2 diabetes: from insulin resistance to pancreatic β-cell function

BACKGROUND AND AIM

Retinol binding protein 4 (RBP4) is an adipokine that has been explored as a key biomarker of type 2 diabetes mellitus (T2DM) in recent years. Researchers have conducted a series of experiments to understand the interplay between RBP4 and T2DM, including its role in insulin resistance and pancreatic β-cell function. The results of these studies indicate that RBP4 has a significant influence on T2DM and is considered a potential biomarker of T2DM. However, there have also been some controversies about the relationship between RBP4 levels and T2DM. In this review, we update and summarize recent studies focused on the relationship between RBP4 and T2DM and its role in insulin resistance and pancreatic β-cell function to clarify the existing controversy and provide evidence for future studies. We also assessed the potential therapeutic applications of RBP4 in treating T2DM.

METHODS

A narrative review.

RESULTS

Overall, there were significant associations between RBP4 levels, insulin resistance, pancreatic β-cell function, and T2DM.

CONCLUSIONS

More mechanistic studies are needed to determine the role of RBP4 in the onset of T2DM, especially in terms of pancreatic β-cell function. In addition, further studies are required to evaluate the effects of drug intervention, lifestyle intervention, and bariatric surgery on RBP4 levels to control T2DM and the role of reducing RBP4 levels in improving insulin sensitivity and pancreatic β-cell function.

The association between retinol-binding protein 4 and risk of type 2 diabetes: A systematic review and meta-analysis

Retinol-binding protein 4 (RBP4) was controversially associated with type 2 diabetes mellitus (T2DM). This meta-analysis aimed at evaluating the association between RBP4 level and T2DM risk. MEDLINE and EMBASE were searched to identify relevant studies up to 3 December 2022. Random effects model was used to pool multivariate-adjusted odds ratios (ORs) and 95% confidence intervals (CIs). Publication bias was estimated by Funnel plot and Egger's test, it was considered to be significant when P < 0.05. Eight studies including 8087 participants were finally included. Compared to those with the lowest level, subjects with the highest level of RBP4 have a higher risk of T2DM (OR = 1.47, 95% CI: 1.16-1.78, P < 0.001, I2 = 86.9%). No publication bias among the included studies was found (t = 0.94, P = 0.377). This meta-analysis indicated that high RBP4 level was associated with increasing risk of T2DM.

Assessing the multitargeted antidiabetic potential of three pomegranate peel-specific metabolites: An in silico and pharmacokinetics study

Diabetes is a chronic metabolic disorder that occurs due to impaired secretion of insulin, insulin resistance, or both. Recent studies show that the antidiabetic drugs used to control hyperglycemic levels are associated with undesirable adverse effects. Therefore, developing a safe and effective medicine with antidiabetic potential is needed. In this context, in silico studies are considered a rapid, effectual, and cost-effective method in drug discovery procedures. It is evident from the literature that plant-based natural components have shown promising outcomes in drug development to alleviate various diseases and hence have diversified the screening of potential antidiabetic agents. Purposely, in the present study, an in silico approach was performed on three Punica granatum peel metabolites (punicalin, punicalagin, and ellagic acid). All these three compounds were docked against nine protein targets involved in glucose metabolism (GFAT, PTP1β, PPAR-ᵞ, TKIR, RBP4, α-amylase, α-glucosidase, GCK, and AQP-2). These three pomegranate-specific compounds demonstrated significant interactions with GFAT, PTP1β, PPAR-ᵞ, TKIR, RBP4, α-amylase, α-glucosidase, GCK, and AQP-2 protein targets. Specifically, punicalin, punicalagin, and ellagic acid revealed significant binding scores (-9.2, -9.3, -8.1, -9.1, -8.5, -11.3, -9.2, -9.5, -10.1 kcal/mol; -10, -9.9, -8.5, -8.9, -10.4, -9.0, -10.2, -9.4, -9.0 kcal/mol; and -8.1, -8.0, -8.0, -6.8, -8.7, -7.8, -8.3, -8.1, -8.1 kcal/mol, respectively), with nine protein targets mentioned above. Hence, punicalin, punicalagin, and ellagic acid can be promising candidates in drug discovery to manage diabetes. Furthermore, in vivo and clinical trials must be conducted to validate the outcomes of the current study.

BAL Proteomic Signature of Lung Adenocarcinoma in IPF Patients and Its Transposition in Serum Samples for Less Invasive Diagnostic Procedures

Idiopathic pulmonary fibrosis (IPF) is a form of chronic and irreversible fibrosing interstitial pneumonia of unknown etiology. Although antifibrotic treatments have shown a reduction of lung function decline and a slow disease progression, IPF is characterize by a very high mortality. Emerging evidence suggests that IPF increases the risk of lung carcinogenesis. Both diseases show similarities in terms of risk factors, such as history of smoking, concomitant emphysema, and viral infections, besides sharing similar pathogenic pathways. Lung cancer (LC) diagnosis is often difficult in IPF patients because of the diffuse lung injuries and abnormalities due to the underlying fibrosis. This is reflected in the lack of optimal therapeutic strategies for patients with both diseases. For this purpose, we performed a proteomic study on bronchoalveolar lavage fluid (BALF) samples from IPF, LC associated with IPF (LC-IPF) patients, and healthy controls (CTRL). Molecular pathways involved in inflammation, immune response, lipid metabolism, and cell adhesion were found for the dysregulated proteins in LC-IPF, such as TTHY, APOA1, S10A9, RET4, GDIR1, and PROF1. The correlation test revealed a relationship between inflammation- and lipid metabolism-related proteins. PROF1 and S10A9, related to inflammation, were up-regulated in LC-IPF BAL and serum, while APOA1 and APOE linked to lipid metabolism, were highly abundant in IPF BAL and low abundant in IPF serum. Given the properties of cytokine/adipokine of the nicotinamide phosphoribosyltransferase, we also evaluated its serum abundance, highlighting its down-regulation in LC-IPF. Our retrospective analyses of BAL samples extrapolated some potential biomarkers of LC-IPF useful to improve the management of these contemporary pathologies. Their differential abundance in serum samples permits the measurement of these potential biomarkers with a less invasive procedure.

Plasma Transthyretin Levels and Risk of Type 2 Diabetes Mellitus and Impaired Glucose Regulation in a Chinese Population

Plasma transthyretin may be engaged in glucose regulation. We aimed to investigate the association between plasma transthyretin levels and the risk of newly diagnosed T2DM and impaired glucose regulation (IGR) in a Chinese population. We conducted a case-control study including 1244 newly diagnosed T2DM patients, 837 newly diagnosed IGR patients, and 1244 individuals with normal glucose tolerance (NGT) matched by sex and age. Multivariate logistic regression analysis was utilized to estimate the independent association of plasma transthyretin concentrations with the risk of T2DM and IGR. Plasma transthyretin concentrations were significantly higher in T2DM and IGR patients compared with control subjects (p < 0.005). After multiple adjustment and comparison with the lowest quartile of plasma transthyretin concentrations, the odds ratios (95% confidence intervals) of T2DM and IGR in the highest quartile were 2.22 (1.66, 2.98) and 2.29 (1.72, 3.05), respectively. Plasma transthyretin concentrations also showed a great performance in predicting the risk of T2DM (AUC: 0.76). Moreover, a potential nonlinear trend was observed. Our results demonstrated that higher plasma transthyretin concentrations, especially more than 290 mg/L, were associated with an increased risk of T2DM and IGR. Further studies are warranted to confirm our findings and elucidate the potential mechanisms.

Interplay of retinol binding protein 4 with obesity and associated chronic alterations (Review)

Obesity is a multifactorial disease, defined as excessive fat deposition in adipose tissue. Adipose tissue is responsible for the production and secretion of numerous adipokines that induce metabolic disorders. Retinol-binding protein 4 (RBP4) is an adipokine that transports vitamin A or retinol in the blood. High levels of RBP4 are associated with development of metabolic disease, including obesity, insulin resistance (IR), metabolic syndrome, and type 2 diabetes (T2D). The present review summarizes the role of RBP4 in obesity and associated chronic alterations. Excessive synthesis of RBP4 contributes to inflammatory characteristic of obesity by activation of immune cells and release of proinflammatory cytokines, such as TNFα and ILs, via the Toll-like receptor/JNK pathway. The retinol-RBP4 complex inhibits insulin signaling directly in adipocytes by activating Janus kinase 2 (JAK2)/STAT5/suppressor of cytokine signaling 3 signaling. This mechanism is retinol-dependent and requires vitamin A receptor stimulation by retinoic acid 6 (STRA6). In muscle, RBP4 is associated with increased serine 307 phosphorylation of insulin receptor substrate-1, which decreases its affinity to PI3K and promotes IR. In the liver, RBP4 increases hepatic expression of phosphoenolpyruvate carboxykinase, which increases production of glucose. Elevated serum RBP4 levels are associated with β-cell dysfunction in T2D via the STRA6/JAK2/STAT1/insulin gene enhancer protein 1 pathway. By contrast, RBP4 induces endothelial inflammation via the NF-κB/nicotinamide adenine dinucleotide phosphate oxidase pathway independently of retinol and STRA6, which stimulates expression of proinflammatory molecules, such as vascular cell adhesion molecule 1, E-selectin, intercellular adhesion molecule 1, monocyte chemoattractant protein 1 and TNFα. RBP4 promotes oxidative stress by decreasing endothelial mitochondrial function; overall, it may serve as a useful biomarker in the diagnosis of obesity and prognosis of associated disease, as well as a potential therapeutic target for treatment of these diseases.

Interplay between Fatty Acid Binding Protein 4, Fetuin-A, Retinol Binding Protein 4 and Thyroid Function in Metabolic Dysregulation

Signalling between the tissues integrating synthesis, transformation and utilization of energy substrates and their regulatory hormonal axes play a substantial role in the development of metabolic disorders. Interactions between cytokines, particularly liver derived hepatokines and adipokines, secreted from adipose tissue, constitute one of major areas of current research devoted to metabolic dysregulation. The thyroid exerts crucial influence on the maintenance of basal metabolic rate, thermogenesis, carbohydrate and lipid metabolism, while its dysfunction promotes the development of metabolic disorders. In this review, we discuss the interplay between three adipokines: fatty acid binding protein type 4, fetuin-A, retinol binding protein type 4 and thyroid hormones, that shed a new light onto mechanisms underlying atherosclerosis, cardiovascular complications, obesity, insulin resistance and diabetes accompanying thyroid dysfunction. Furthermore, we summarize clinical findings on those cytokines in the course of thyroid disorders.

Serum Retinol-Binding Protein Levels Are Associated with Nonalcoholic Fatty Liver Disease in Chinese Patients with Type 2 Diabetes Mellitus: A Real-World Study

BACKGROUND

The association of serum retinol-binding protein (RBP) levels with nonalcoholic fatty liver disease (NAFLD) remains controversial. Furthermore, few studies have investigated their relationship in type 2 diabetes mellitus (T2DM) patients. Therefore, the aim of the present study was to explore the association between serum RBP levels and NAFLD in Chinese inpatients with T2DM.

METHODS

This cross-sectional, real-world study included 2,263 Chinese T2DM inpatients. NAFLD was diagnosed by abdominal ultrasonography. The subjects were divided into four groups based on RBP quartiles, and clinical characteristics were compared among the four groups. The associations of both RBP levels and quartiles with the presence of NAFLD were also analyzed.

RESULTS

After adjustment for sex, age, and diabetes duration, there was a significant increase in the prevalence of NAFLD from the lowest to the highest RBP quartiles (30.4%, 40.0%, 42.4%, and 44.7% for the first, second, third, and fourth quartiles, respectively, P<0.001 for trend). Fully adjusted multiple logistic regression analysis revealed that both increased RBP levels (odds ratio, 1.155; 95% confidence interval, 1.012 to 1.318; P=0.033) and quartiles (P=0.014 for trend) were independently associated with the presence of NAFLD in T2DM patients.

CONCLUSION

Increased serum RBP levels were independently associated with the presence of NAFLD in Chinese T2DM inpatients. Serum RBP levels may be used as one of the indicators to assess the risk of NAFLD in T2DM patients.

Associations of serum transthyretin with triglyceride in non-obese elderly Japanese women independently of insulin resistance, HDL cholesterol, and adiponectin

Objective

Studies are limited on the association between serum transthyretin (TTR), a negative acute phase reactant, and triglyceride (TG).

Research design and methods

TG, TTR, and insulin resistance-related cardiometabolic variables were measured in 159 fasting and 185 nonfasting community-dwelling elderly women aged 50-96 years. Pearson correlation analysis and then stepwise multiple regression analyses were performed to further identify the most significant variables contributing to the variation of fasting and nonfasting TG.

Results

Multiple regression analysis for fasting TG as a dependent variable revealed that TTR (standardized β: 0.299) and HDL cholesterol (standardized β: -0.545) emerged as determinants of TG independently of percentage of body fat, homeostasis model assessment insulin resistance, serum leptin and adiponectin, and plasminogen activator inhibitor-1 (PAI-1) (R ² = 0.36). For nonfasting TG, HDL cholesterol (standardized β: - 0.461), TTR (standardized β: 0.231), nonfasting insulin, a marker of insulin resistance, (standardized β: 0.202), and PAI-1 (standardized β: 0.187) emerged as determinants independently of percentage of body fat, nonfasting glucose, serum leptin and adiponectin, and high-sensitivity C-reactive protein (R ² = 0.45).

Conclusions

Fasting and nonfasting TG showed positive association with TTR in community-dwelling elderly non-obese women independently of insulin resistance, HDL cholesterol, and adiponectin. These findings may provide a clue as to a physiological function of circulating TTR in human: an influence factor of TG-rich lipoproteins in the circulation.

Augmentation of RBP4/STRA6 signaling leads to insulin resistance and inflammation and the plausible therapeutic role of vildagliptin and metformin

A role of Retinol Binding Protein-4 (RBP4) in insulin resistance is widely studied. However, there is paucity of information on its receptor viz., Stimulated by Retinoic Acid-6 (STRA6) with insulin resistance. To address this, we investigated the regulation of RBP4/STRA6 expression in 3T3-L1 adipocytes exposed to glucolipotoxicity (GLT) and in visceral adipose tissue (VAT) from high fat diet (HFD) fed insulin-resistant rats. 3T3-L1 adipocytes were subjected to GLT and other experimental maneuvers with and without vildagliptin or metformin. Real-time PCR and western-blot experiments were performed to analyze RBP4, STRA6, PPARγ gene and protein expression. Adipored staining and glucose uptake assay were performed to evaluate lipid and glucose metabolism. Oral glucose tolerance test (OGTT) and Insulin Tolerance Test (ITT) were performed to determine the extent of insulin resistance in HFD fed male Wistar rats. Total serum RBP4 was measured by quantitative sandwich enzyme-linked immunosorbent assay kit. Adipocytes under GLT exhibited significantly increased RBP4/STRA6 expressions and decreased insulin sensitivity/glucose uptake. Vildagliptin and metformin not only restored the above but also decreased the expression of IL-6, NFκB, SOCS-3 along with lipid accumulation. Furthermore, HFD fed rats exhibited significantly increased serum levels of RBP4 along with VAT expression of RBP4, STRA6, PPARγ, IL-6. These molecules were significantly altered by the vildagliptin/ metformin treatment. We conclude that RBP4/STRA6 pathway is primarily involved in mediating inflammation and insulin resistance in adipocytes and visceral adipose tissues under glucolipotoxicity and in insulin resistant rats.

Disruption of retinoid homeostasis induces RBP4 overproduction in diabetes: O-GlcNAcylation involved

BACKGROUND

Retinol-binding protein 4 (RBP4) is elevated and associated with inflammation in metabolic diseases. Disruption of the retinol cascade and O-GlcNAcylation of the RBP4 receptor (STRA6) are found in diabetic kidneys.

OBJECTIVES

We investigated whether the disruption of the retinol cascade induces RBP4 overproduction and if O-linked GlcNAc modification targets RBPR2 and contributes to the disruption of retinol cascades in diabetic livers.

METHODS

Western blot or immunohistochemistry for RBPR2, CRBP1, LRAT, RALDH, RARα, RARγ, RXRα, RBP4, GFAT, OGT, OGA and inflammatory markers, as well as ELISA for RBP4, were performed in livers of db/db and ob/ob mice and high glucose-cultured hepatocytes. Immunoprecipitation and dual fluorescence staining were used to explore O-GlcNAc-modified RBPR2 and RBP4 binding activity on RBPR2. Transfection of the CRBP1 gene was done to verify whether a disrupted retinol cascade induces RBP4 overproduction. OGT silencing was done to investigate the association of O-GlcNAcylation with the disruption of retinol cascade.

RESULTS

Disruption of retinol cascade, RBP4 overproduction, O-GlcNAcylation of RBPR2, decreased RBP4 binding activity on RBPR2 and inflammation were found in livers of db/db and ob/ob mice and high glucose-cultured hepatocytes. CRBP1 gene transfection reversed the suppression of the cellular retinol cascade and simultaneously attenuated the RBP4 overproduction and inflammation in high glucose-treated hepatocytes. The silencing of OGT reversed the disruption of the cellular retinol cascade, RBP4 overproduction and inflammation induced by high glucose in hepatocytes.

CONCLUSIONS

This study indicates that the disruption of cellular retinol cascade is strongly associated with RBP4 overproduction and inflammation in diabetic livers. RBPR2 is one target for high glucose-mediated O-linked GlcNAc modification, which causes liver retinol dyshomeostasis.

Retinol binding protein 4 primes the NLRP3 inflammasome by signaling through Toll-like receptors 2 and 4

Adipose tissue (AT) inflammation contributes to systemic insulin resistance. In obesity and type 2 diabetes (T2D), retinol binding protein 4 (RBP4), the major retinol carrier in serum, is elevated in AT and has proinflammatory effects which are mediated partially through Toll-like receptor 4 (TLR4). We now show that RBP4 primes the NLRP3 inflammasome for interleukin-1β (IL1β) release, in a glucose-dependent manner, through the TLR4/MD2 receptor complex and TLR2. This impairs insulin signaling in adipocytes. IL1β is elevated in perigonadal white AT (PGWAT) of chow-fed RBP4-overexpressing mice and in serum and PGWAT of high-fat diet-fed RBP4-overexpressing mice vs. wild-type mice. Holo- or apo-RBP4 injection in wild-type mice causes insulin resistance and elevates PGWAT inflammatory markers, including IL1β. TLR4 inhibition in RBP4-overexpressing mice reduces PGWAT inflammation, including IL1β levels and improves insulin sensitivity. Thus, the proinflammatory effects of RBP4 require NLRP3-inflammasome priming. These studies may provide approaches to reduce AT inflammation and insulin resistance in obesity and diabetes.

Alzheimer's disease and type 2 diabetes mellitus: A systematic review of proteomic studies

Similar to dementia, the risk for developing type 2 diabetes mellitus (T2DM) increases with age, and T2DM also increases the risk for dementia, particularly Alzheimer's disease (AD). Although T2DM is primarily a peripheral disorder and AD is a central nervous system disease, both share some common features as they are chronic and complex diseases, and both show involvement of oxidative stress and inflammation in their progression. These characteristics suggest that T2DM may be associated with AD, which gave rise to a new term, type 3 diabetes (T3DM). In this study, we searched for matching peripheral proteomic biomarkers of AD and T2DM based in a systematic review of the available literature. We identified 17 common biomarkers that were differentially expressed in both patients with AD or T2DM when compared with healthy controls. These biomarkers could provide a useful workflow for screening T2DM patients at risk to develop AD.

Inflammatory and angiogenic biomarkers in diabetic retinopathy

Diabetic retinopathy (DR) is one of the most common microvascular complications of diabetes mellitus (DM) and a leading cause of blindness in working-age adults in developed countries. Numerous investigations have recognised inflammation and angiogenesis as important factors in the development of this complication of diabetes. Current methods of DR treatment are predominantly used at advanced stages of the disease and could be associated with serious side effects. Therefore, new diagnostic methods are needed in order to identify the initial stages of DR as well as monitoring the effects of applied therapy. Biochemical biomarkers are molecules found in blood or other biological fluid and tissue that indicate the existence of an abnormal condition or disease. They could be a valuable tool in detecting early stages of DR, identifying patients most susceptible to retinopathy progression and monitoring treatment outcomes. Biomarkers related to DR can be measured in the blood, retina, vitreous, aqueous humour and recently in tears. As the retina represents a small part of total body mass, a circulating biomarker for DR needs to be highly specific. Local biomarkers are more reliable as indicators of the retinal pathology; however, obtaining a sample of aqueous humour, vitreous or retina is an invasive procedure with potential serious complications. As a non-invasive novel method, tear analysis offers a promising direction in further research for DR biomarker detection. The aim of this paper is to review systemic and local inflammatory and angiogenic biomarkers relevant to this sight threatening diabetic complication.

Vitamin A signaling and homeostasis in obesity, diabetes, and metabolic disorders

Much evidence has accumulated in the literature over the last fifteen years that indicates vitamin A has a role in metabolic disease prevention and causation. This literature proposes that vitamin A can affect obesity development and the development of obesity-related diseases including insulin resistance, type 2 diabetes, hepatic steatosis and steatohepatitis, and cardiovascular disease. Retinoic acid, the transcriptionally active form of vitamin A, accounts for many of the reported associations. However, a number of proteins involved in vitamin A metabolism, including retinol-binding protein 4 (RBP4) and aldehyde dehydrogenase 1A1 (ALDH1A1, alternatively known as retinaldehyde dehydrogenase 1 or RALDH1), have also been identified as being associated with metabolic disease. Some of the reported effects of these vitamin A-related proteins are proposed to be independent of their roles in assuring normal retinoic acid homeostasis. This review will consider both human observational data as well as published data from molecular studies undertaken in rodent models and in cells in culture. The primary focus of the review will be on the effects that vitamin A per se and proteins involved in vitamin A metabolism have on adipocytes, adipose tissue biology, and adipose-related disease, as well as on early stage liver disease, including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).

Protein signatures linking history of miscarriages and metabolic syndrome: a proteomic study among North Indian women

Background

Metabolic syndrome (MeS), a constellation of metabolic adversities, and history of miscarriage make women at a higher risk for cardiovascular diseases (CVDs). However, molecular evidence indicating a link between the two phenotypes (history of miscarriage and MeS) among women would offer an opportunity to predict the risk factor for CVDs at an early stage. Thus, the present retrospective study attempts to identify the proteins signatures (if any) to understand the connection between the history of miscarriage and MeS.

Methods

Age-matched 80 pre-menopausal women who were not on any medical intervention or drugs were recruited from a Mendelian population of the same gene pool. Recruited women were classified into four groups—(a) Group A—absolute cases with history of miscarriage and MeS, (b) Group B—absolute controls without any history of miscarriage and MeS, (c) Group C—cases with MeS but lack any history of miscarriage, (d) Group D—cases with history of miscarriage but lack MeS. Differentially expressed proteins in plasma samples of women from four groups were identified using 2-D gel electrophoresis and mass spectrometry.

Results

Three case groups (A, C, and D) showed 18 differentially expressed proteins. Nearly 60% of proteins (11/18) were commonly dysregulated in Group C (only with MeS) and Group D (only with miscarriage history). Nearly 40% of proteins (7/18) were commonly dysregulated in the three case groups (Groups A, C, and D), indicating a shared pathophysiology. Four proteins were exclusive but shared by case groups C and D indicating the independent routes for CVDs through MeS or miscarriages. In absolute cases, transthyretin (TTR) showed exclusive upregulation, which was further validated by Western blotting and ELISA. Networking analyses showed the strong association of TTR with haptoglobin, transferrin and ApoA1 hinting toward a cross-talk among these proteins which could be a cause or an effect of TTR upregulation.

Conclusion

The study provides evidence for molecular link between the history of miscarriage and MeS through a putative role of TTR. However, longitudinal follow-up studies with larger sample size would further help to demonstrate the significance of TTR and other targeted proteins in risk stratification and the onset of CVDs.

Correlates of resistin and retinol-binding protein 4 in metabolic syndrome patients with and without prediabetes

Background Resistin and retinol-binding protein 4 (RBP4) can work in an intricate in metabolic syndrome (MetS) and prediabetes (PreDM) molecular crosstalk. Materials and methods Resistin and RBP4 were evaluated using colorimetric enzyme-linked immunosorbent assays (ELISAs) in 29 normoglycemic MetS, 30 newly diagnosed drug naïve MetS-preDM patients and 29 lean and normoglycemic controls. Results In this cross-sectional design; the gradual increase in resistin levels (ng/mL), though not ascribed any statistically marked variation, was appreciable in both normoglycemic and preDM MetS groups vs. controls. RBP4 mean circulating levels (ng/mL) in both MetS groups (non-diabetic and preDM) invariably lacked discrepancy vs. controls. Except for fasting plasma glucose (FPG) and A1C; no further intergroup discrepancy could be identified between MetS arms. Adiposity indices: body mass index (BMI), body adiposity index (BAI) and lipid accumulation product (LAP) (but not conicity index) were substantially higher in both MetS (non- and preDM) groups vs. those of controls. Likewise, the atherogenicity index of plasma [but not non-high-density lipoprotein-cholesterol (nonHDL-C)/HDL-C ratio, or triglyceride (TG)/HDL-C ratio] or any of the hematological indices [red cell distribution width (RDW-CV %), monocyte to lymphocyte ratio (MLR), neutrophil to lymphocyte ratio (NLR) and platelet (PLT) to lymphocyte ratios (PLR)] had any marked variations as compared to controls. Low-density lipoprotein-cholesterol (LDL-C)/HDL-C ratio,visceral adiposity index, and waist circumference (WC)/hip circumference (HC) ratio were noticeably greater in MetS-preDM vs. normoglycemic MetS recruits. Neither biomarker could relate to each other, or any of the atherogenecity indices in 59 MetS participants (non- and preDM). Unlike RBP4; resistin associated proportionally with each of HC, BAI, MLR and NLR. Conclusions Both biomarkers can be putative indicator/surrogate prognostic tools for the prediction/prevention and pharmacotherapy of MetS anomalies.

Elevated RBP4 plasma levels were associated with diabetic retinopathy in type 2 diabetes

The retinol-binding protein 4 (RBP4) has been postulated to play a role in glucose homeostasis, insulin resistance, and diabetes mellitus in human and animal studies. The aim of the present study was to evaluate the role of RBP4 in Chinese patients with type 2 diabetes mellitus with and without diabetic retinopathy (DR). Plasma RBP4 concentrations were tested in 287 patients with type 2 diabetes. At baseline, demographic and clinical information including presence of DR and vision-threatening DR (VTDR) was collected. The relationship between RBP4 and DR (VTDR) was investigated using logistic regression. Patients with DR or VTDR had significantly higher plasma levels of RBP4 on admission (P<0.0001). Receiver operating characteristics (ROCs) to predict DR and VDTR demonstrated areas under the curve for RBP4 of 0.79 (95% confidence interval (CI): 0.73–0.85) and 0.90 (95% CI: 0.85–0.94), respectively, which were superior to other factors. For each 1 μg/ml increase in plasma level of RBP4, the unadjusted and adjusted risk of DR would be increased by 8% (with the odds ratio (OR) of 1.08 (95% CI: 1.05–1.13), P<0.001) and 5% (1.05 (1.02–1.11), P=0.001), respectively. It was 12% (with the OR of 1.12 (95% CI: 1.07–1.18), P<0.001) and 9% (1.09 (1.05–1.15), P<0.001) for VTDR. The present study shows that elevated plasma levels of RBP4 were associated with DR and VDTR in Chinese patients with type 2 diabetes, suggesting a possible role of RBP4 in the pathogenesis of DR complications. Lowering RBP4 could be a new strategy for treating type 2 diabetes with DR.

Association of retinol binding protein 4 and transthyretin with triglyceride levels and insulin resistance in rural thais with high type 2 diabetes risk

BACKGROUND

Retinol binding protein 4 (RBP4), a protein secreted by adipocytes and bound in plasma to transthyretin (TTR), has been associated with obesity, the early phase of insulin resistance, metabolic syndrome, and type 2 diabetes mellitus. The objective of this study was to elucidate the relationship between RBP4, TTR, triglyceride (TG) and type 2 diabetes risk in rural Thailand.

METHODS

We measured the serum RBP4, TTR, glucose, triglyceride and insulin levels, and glucose tolerance of 167 volunteers from Sung Noen District, Nakhon Ratchasima Province, Thailand. Student's t-test, Pearson's correlation and logistic regression analysis were used to evaluate the relationships between RBP4, TTR and type 2 diabetes markers.

RESULTS

RBP4 and TTR levels, as well as homeostatic model assessment of insulin resistance (HOMA-IR) values, were significantly elevated among subjects with high triglyceride levels (p < 0.01, p < 0.05, p < 0.05, respectively). Triglyceride levels correlated with RBP4 (r = 0.34, p < 0.001) and TTR (r = 0.26, p < 0.01) levels, as well as HOMA-IR values (r = 0.16, p < 0.05). After adjustment for age and gender, the risk of hypertriglyceridemia was 3.7 times greater (95% CI =1.42-9.73, p = 0.008) in the highest RBP4 tertile as compared to the lowest tertile. Similarly, the highest TTR and HOMA-IR tertiles had greater risk of hypertriglyceridemia at 3.5 (95% CI = 1.30-9.20, p = 0.01) and 3.6 (95% CI = 1.33-9.58, p = 0.01) times higher than the respective lowest tertiles. The correlation between TTR and blood glucose was statistically significant (r = 0.18, p < 0.05), but not found this relationship in RBP4.

CONCLUSIONS

The associations of RBP4 and TTR with hypertriglyceridemia and insulin resistance may have important implications for the risk of heart disease and stroke.

High-fat diet-induced plasma protein and liver changes in obese rats can be attenuated by melatonin supplementation

Obesity triggers changes in protein expression in various organs that might participate in the pathogenesis of obesity. Melatonin has been reported to prevent or attenuate such pathological protein changes in several chronic diseases. However, such melatonin effects on plasma proteins have not yet been studied in an obesity model. Using a proteomic approach, we investigated the effect of melatonin on plasma protein profiles after rats were fed a high-fat diet (HFD) to induce obesity. We hypothesized that melatonin would attenuate abnormal protein expression in obese rats. After 10weeks of the HFD, animals displayed increased body weight and fat accumulation as well as increased glucose levels, indicating an obesity-induced prediabetes mellitus-like state. Two-dimensional gel electrophoresis and liquid chromatography-mass spectrometry/mass spectrometry revealed 12 proteins whose expression was altered in response to the HFD and the melatonin treatment. The altered proteins are related to the development of liver pathology, such as cirrhosis (α1-antiproteinase), thrombosis (fibrinogen, plasminogen), and inflammation (mannose-binding protein A, complement C4, complement factor B), contributing to liver steatosis or hepatic cell death. Melatonin treatment most probably reduced the severity of the HFD-induced obesity by reducing the amplitude of HFD-induced plasma protein changes. In conclusion, we identified several potential biomarkers associated with the progression of obesity and its complications, such as liver damage. Furthermore, our findings reveal melatonin's beneficial effect of attenuating plasma protein changes and liver pathogenesis in obese rats.