Retinol-binding protein-4 in women with untreated essential hypertension

Are variants of the RBP4 gene associated with serum retinol-binding protein 4 concentrations and carotid intima-media thickness values in women with obesity?

INTRODUCTION

Several studies showed the correlation of retinol-binding protein 4 (RBP4) with increased cardiovascular risk - including higher values of carotid intima-media thickness (cIMT) - particularly in individuals with obesity.

OBJECTIVES

Our study aimed to investigate the impact of rs10882273; rs3758538; rs3758539, and rs7094671 RBP4 gene variants on RBP4 serum concentrations as well as cIMT values (a marker of subclinical atherosclerosis) among female patients with obesity.

PATIENTS AND METHODS

We recruited 74 women with obesity and 24 women without obesity as a study and control group, respectively. The genotypic and allelic frequencies of RBP4 gene variants were evaluated for associations with serum RBP4 and cIMT.

RESULTS

The median serum RBP4 concentrations were 20.30 µg/mL and 19.80 µg/mL in the patients and control group, respectively (p = 0.740). No significant differences were seen in cIMT values between the two studied groups (0.60 [0.50-1.00] vs. 0.60 ± 0.10 in the patient and control group, respectively); however, the results were close to reaching significance (p = 0.071), similar as in observed association of the minor haplotype AA for rs7084671 and rs375839 with female obesity (p = 0.0559). The correlation analysis showed no significant differences between RBP4 gene variants with serum RBP4 and cIMT.

CONCLUSIONS

According to our knowledge, this is the first study investigating the association between RBP4 gene variants and serum RBP4 and cIMT among Polish female patients with obesity. However, our results show that genetic variants rs10882273, rs3758538, rs3758539, and rs7094671 of the RBP4 gene are not associated with RBP4 serum concentrations or cIMT values among women with obesity.

The associations of adipokines with hypertension in youth with cardiometabolic risk and the mediation role of insulin resistance: The BCAMS study

The mechanisms link obesity and hypertension are not well understood. One possibility is the alterations in adipose-derived adipokines that modulate insulin resistance (IR) and cardiovascular homeostasis. We aimed to assess the associations between hypertension and four adipokine levels in Chinese youth, and to examine to what extent the associations are mediated by IR. We utilized cross-sectional data from the Beijing Children and Adolescents Metabolic Syndrome (BCAMS) Study Cohort (n = 559, mean age = 20.2 years). Plasma leptin, adiponectin, retinol binding protein 4 (RBP4) and fibroblast growth factor 21 (FGF21) levels were assayed. The relationships between adipokines and hypertension and the possible mediation effect of IR were evaluated. Youth with hypertension have lower adiponectin and higher leptin, FGF21 (all P < 0.001) and RBP4 levels (p = 0.06) compared to their counterparts. Moreover, the co-existence of these two or more adipokine abnormalities in youth leads to a 9-fold increased risk for hypertension (OR: 9.19; 95% CI, 4.01-21.08) compared with these without abnormalities. However, in the fully adjusted and BMI-adjusted analyses, only FGF21 was a significant predictor of hypertension (OR: 2.12; 95% CI, 1.34-3.36). Mediation analysis revealed that the associations between leptin, adiponectin, RBP4 and hypertension are totally mediated by IR (proportion: 63.9%, 65.4%, and 31.6%, respectively), while BMI and IR partly mediated the association between FGF21 and hypertension (proportion: 30.6%, 21.2%). Our findings suggest that dysregulation of adipokines might result in hypertension in youth. Leptin, adiponectin and RBP4 may exert their functions in hypertension through adiposity-related IR, whereas FGF21 might be used as an independent marker of hypertension in youth.

Lipocalin family proteins and their diverse roles in cardiovascular disease

The lipocalin (LCN) family members, a group of small extracellular proteins with 160-180 amino acids in length, can be detected in all kingdoms of life from bacteria to human beings. They are characterized by low similarity of amino acid sequence but highly conserved tertiary structures with an eight-stranded antiparallel β-barrel which forms a cup-shaped ligand binding pocket. In addition to bind small hydrophobic ligands (i.e., fatty acids, odorants, retinoids, and steroids) and transport them to specific cells, lipocalins (LCNs) can interact with specific cell membrane receptors to activate their downstream signaling pathways, and with soluble macromolecules to form the complex. Consequently, LCNs exhibit great functional diversity. Accumulating evidence has demonstrated that LCN family proteins exert multiple layers of function in the regulation of many physiological processes and human diseases (i.e., cancers, immune disorders, metabolic disease, neurological/psychiatric disorders, and cardiovascular disease). In this review, we firstly introduce the structural and sequence properties of LCNs. Next, six LCNs including apolipoprotein D (ApoD), ApoM, lipocalin 2 (LCN2), LCN10, retinol-binding protein 4 (RBP4), and Lipocalin-type prostaglandin D synthase (L-PGDS) which have been characterized so far are highlighted for their diagnostic/prognostic values and their potential effects on coronary artery disease and myocardial infarction injury. The roles of these 6 LCNs in cardiac hypertrophy, heart failure, diabetes-induced cardiac disorder, and septic cardiomyopathy are also summarized. Finally, their therapeutic potential for cardiovascular disease is discussed in each section.

Role of major adipokines in hypertension: A literature review

The incidence and prevalence of hypertension are increasing as a consequence of the obesity epidemic. Adipocytes and their variety of factors make contributions to the long-term regulation of blood pressure. The pathophysiologic states of hypertension, including obesity, are regulated by the production of adipocyte-derived factors. Increased body mass index was closely linked to elevated blood pressure. Mostly the hypertensive subjects were obese as well as overweight. There are numerous adipokines, however, this review article only focuses on the major adipokines including chemerin, visfatin, retinol-binding protein 4, plasminogen activator inhibitor-1, monocyte chemotactic protein-1, omentin-1, lipocalin-2, vaspin, progranulin, complement c1q tumor necrosis factor-related protein, and nesfatin-1 role in the pathogenesis of hypertension. This review article concludes the significant association of major adipokines in the pathogenesis of hypertensives. New research should be focused on other newly reported adipokine roles in hypertensive subjects and the management of these adipokines in hypertensive subjects. The discovery of this information could result in the creation of antihypertensive medications, particularly those that focus on obesity-related hypertension.

Correlation of serum RBP4 level with oxidative stress and unstable carotid plaque in patients with cerebral infarction

Objectives

This study aimed to investigate the changes in serum levels of retinol-binding protein 4 (RBP4) with cerebral infarction, relationship of RBP4 with oxidative stress and carotid atherosclerosis, and its possible role in cerebral infarction.

Materials and methods

According to the results of cervical vascular ultrasound, the experimental group was divided into three groups: intima thickening group (n = 31), stable plaque group (n = 51), and unstable plaque group (n = 54). Forty healthy subjects were selected as the control group. Their serum levels of RBP4, 8-iso-prostaglandin-F2alpha (8-iso-PGF2α), and catalase (CAT) were measured. Carotid vascular ultrasound was used to measure the plaque area and intima-media thickness (IMT).

Results

The serum RBP4 and 8-iso-PGF2α levels, IMT and plaque area in the control, intimal thickening, stable plaque, and unstable plaque groups increased, while the serum level of CAT decreased (P < 0.001). The serum levels of RBP4 positively correlated with 8-iso-PGF2α, IMT, and plaque area and negatively correlated with CAT level. The area under the receiver operating characteristic curve was 0.778 in predicting unstable plaques.

Conclusions

The serum levels of RBP4 were significantly elevated in elderly patients with cerebral infarction and correlated with oxidative stress injury and the degree of atherosclerosis. Serum RBP4 has diagnostic value for unstable plaques in carotid arteries.

Association Between Circulating Retinol-Binding Protein 4 and Adverse Cardiovascular Events in Stable Coronary Artery Disease

Background

The predictive role of retinol-binding protein 4 (RBP4) in the adverse prognosis of patients with stable coronary artery disease (CAD) has not been well-defined. We thus conducted this cohort study to investigate the association between circulating RBP4 level and major adverse cardiovascular events (MACEs) in Chinese patients with stable CAD.

Methods

Patients with stable CAD and serum RBP4 concentration measurement at admission between July 2012 and January 2015 were included. The primary outcome in this study was incident MACEs, which included acute coronary syndrome, heart failure, stroke, peripheral vascular disease, and cardiovascular death. Cox proportional hazards regression was adopted to investigate the association between RBP4 and the incidence of MACEs.

Results

A total of 840 patients with stable CAD were analyzed. The mean age of patients was 61.2 ± 15.9 years, and 56.1% of them were men. After a median follow-up of 2.3 years, 129 MACEs were observed. Compared to participants exposed to the first quartile of serum RBP4 level, those in the second, the third, and the fourth quartiles had associated hazard ratios (HRs) of 2.38 [95% confidence interval (CI): 1.33-4.26], 2.35 (95% CI: 1.31-4.21), and 2.27 (95% CI: 1.28-4.04) after adjusted for confounders, respectively. Every 5 μg/ml increment in serum RBP4 concentration was associated with an adjusted HR of 1.13 (95% CI: 1.05-1.22) for the occurrence of MACEs. Subgroup analyses suggested no significant modifying effects of baseline characteristics for the association between RBP4 and MACEs in patients with stable CAD.

Conclusion

Our finding suggested that the higher circulating RBP4 level was significantly associated with an increased risk of MACEs in patients with stable CAD.

Retinoid Homeostasis and Beyond: How Retinol Binding Protein 4 Contributes to Health and Disease

Retinol binding protein 4 (RBP4) is the specific transport protein of the lipophilic vitamin A, retinol, in blood. Circulating RBP4 originates from the liver. It is secreted by hepatocytes after it has been loaded with retinol and binding to transthyretin (TTR). TTR association prevents renal filtration due to the formation of a higher molecular weight complex. In the circulation, RBP4 binds to specific membrane receptors, thereby delivering retinol to target cells, rendering liver-secreted RBP4 the major mechanism to distribute hepatic vitamin A stores to extrahepatic tissues. In particular, binding of RBP4 to 'stimulated by retinoic acid 6' (STRA6) is required to balance tissue retinoid responses in a highly homeostatic manner. Consequently, defects/mutations in RBP4 can cause a variety of conditions and diseases due to dysregulated retinoid homeostasis and cover embryonic development, vision, metabolism, and cardiovascular diseases. Aside from the effects related to retinol transport, non-canonical functions of RBP4 have also been reported. In this review, we summarize the current knowledge on the regulation and function of RBP4 in health and disease derived from murine models and human mutations.

Adipokine Retinol Binding Protein 4 and Cardiovascular Diseases

The morbidity and mortality of cardiovascular diseases (CVDs) have been increasing year by year all over the world and expanding greatly to the younger population, which becomes the leading causes of death globally that threatens human life safety. Prediction of the occurrence of diseases by using risk related adverse events is crucial for screening and early detection of CVDs. Thus, the discovery of new biomarkers that related to risks of CVDs are of urgent in the field. Retinol-binding protein 4 (RBP4) is a 21-kDa adipokine, mainly secreted by adipocytes. Besides its well-established function in the induction of insulin resistance, it has also been found in recent years to be closely associated with CVDs and other risk factors, such as hypertension, coronary heart disease, heart failure, obesity, and hyperlipidemia. In this review, we mainly focus on the progress of research that establishes the correlation between RBP4 and CVDs and the corresponding major risk factors in recent years.

Retinol-binding protein 4 in obesity and metabolic dysfunctions

Excessive increased adipose tissue mass in obesity is associated with numerous co-morbid disorders including increased risk of type 2 diabetes, fatty liver disease, hypertension, dyslipidemia, cardiovascular diseases, dementia, airway disease and some cancers. The causal mechanisms explaining these associations are not fully understood. Adipose tissue is an active endocrine organ that secretes many adipokines, cytokines and releases metabolites. These biomolecules referred to as adipocytokines play a significant role in the regulation of whole-body energy homeostasis and metabolism by influencing and altering target tissues function. Understanding the mechanisms of adipocytokine actions represents a hot topic in obesity research. Among several secreted bioactive signalling molecules from adipose tissue and liver, retinol-binding protein 4 (RBP4) has been associated with systemic insulin resistance, dyslipidemia, type 2 diabetes and other metabolic diseases. Here, we aim to review and discuss the current knowledge on RBP4 with a focus on its role in the pathogenesis of obesity comorbid diseases.

Activation of the Complement System on Human Endothelial Cells by Urban Particulate Matter Triggers Inflammation-Related Protein Production

Exposure to particulate matter (PM) is becoming a major global health issue. The amount and time of exposure to PM are known to be closely associated with cardiovascular diseases. However, the mechanism through which PM affects the vascular system is still not clear. Endothelial cells line the interior surface of blood vessels and actively interact with plasma proteins, including the complement system. Unregulated complement activation caused by invaders, such as pollutants, may promote endothelial inflammation. In the present study, we sought to investigate whether urban PM (UPM) acts on the endothelial environment via the complement system. UPM-treated human endothelial cells with normal human serum showed the deposition of membrane attack complexes (MACs) on the cell surface via the alternative pathway of the complement system. Despite the formation of MACs, cell death was not observed, and cell proliferation was increased in UPM-mediated complement activation. Furthermore, complement activation on endothelial cells stimulated the production of inflammation-related proteins. Our results revealed that UPM could activate the complement system in human endothelial cells and that complement activation regulated inflammatory reaction in microenvironment. These findings provide clues with regard to the role of the complement system in pathophysiologic events of vascular disease elicited by air pollution.

Biological Functions of RBP4 and Its Relevance for Human Diseases

Retinol binding protein 4 (RBP4) is a member of the lipocalin family and the major transport protein of the hydrophobic molecule retinol, also known as vitamin A, in the circulation. Expression of RBP4 is highest in the liver, where most of the body’s vitamin A reserves are stored as retinyl esters. For the mobilization of vitamin A from the liver, retinyl esters are hydrolyzed to retinol, which then binds to RBP4 in the hepatocyte. After associating with transthyretin (TTR), the retinol/RBP4/TTR complex is released into the bloodstream and delivers retinol to tissues via binding to specific membrane receptors. So far, two distinct RBP4 receptors have been identified that mediate the uptake of retinol across the cell membrane and, under specific conditions, bi-directional retinol transport. Although most of RBP4’s actions depend on its role in retinoid homeostasis, functions independent of retinol transport have been described. In this review, we summarize and discuss the recent findings on the structure, regulation, and functions of RBP4 and lay out the biological relevance of this lipocalin for human diseases.

Potassium supplementation blunts the effects of high salt intake on serum retinol-binding protein 4 levels in healthy individuals

Aims/Introduction

Excessive dietary salt or low potassium intakes are strongly correlated with insulin resistance (IR) and type 2 diabetes mellitus. In epidemiological and experimental studies, increased serum retinol‐binding protein 4 (RBP4) contributes to the pathogenesis of type 2 diabetes mellitus. Herein, we hypothesized that RBP4 might be an adipocyte‐derived “signal” that plays the crucial role in salt‐related insulin resistance or type 2 diabetes mellitus. This study aimed to assess whether salt consumption and potassium supplementation influence serum RBP4 levels in healthy individuals.

Materials and Methods

A total of 42 participants (aged 25–50 years) in a rural area of Northern China were successively provided normal (3 days at baseline), low‐salt (7 days; 3 g/day NaCl) and high‐salt (7 days; 18 g/day) diets, and a high‐salt diet with potassium additive (7 days; 18 g/day NaCl and 4.5 g/day KCl). Urinary sodium and potassium were measured to ensure compliance to dietary intervention. Then, RBP4 levels were evaluated by enzyme‐linked immunosorbent assay.

Results

High salt intake significantly raised serum RBP4 levels in healthy participants (17.5 ± 0.68 vs 28.6 ± 1.02 µg/mL). This phenomenon was abrogated by potassium supplementation (28.6 ± 1.02 vs 17.6 ± 0.88 µg/mL). In addition, RBP4 levels presented positive (r = 0.528, P < 0.01) and negative (r = −0.506, P < 0.01) associations with 24‐h urinary sodium‐ and potassium excretion levels.

Conclusions

RBP4 synthesis is motivated by high salt intake and revoked by potassium supplementation. Our pioneer work has contributed to the present understanding of salt‐induced insulin resistance or type 2 diabetes mellitus.

Is the Retinol-Binding Protein 4 a Possible Risk Factor for Cardiovascular Diseases in Obesity?

Although many preventive and treatment approaches have been proposed, cardiovascular disease (CVD) remains one of the leading causes of deaths worldwide. Current epidemiological data require the specification of new causative factors, as well as the development of improved diagnostic tools to provide better cardiovascular management. Excessive accumulation of adipose tissue among patients suffering from obesity not only constitutes one of the main risk factors of CVD development but also alters adipokines. Increased attention is devoted to bioactive adipokines, which are also produced by the adipose tissue. The retinol-binding protein 4 (RBP4) has been associated with numerous CVDs and is presumably associated with an increased cardiovascular risk. With this in mind, exploring the role of RBP4, particularly among patients with obesity, could be a promising direction and could lead to better CVD prevention and management in this patient group. In our review, we summarized the current knowledge about RBP4 and its association with essential aspects of cardiovascular disease—lipid profile, intima-media thickness, atherosclerotic process, and diet. We also discussed the RBP4 gene polymorphisms essential from a cardiovascular perspective.

Retinol-binding protein 4, fetal overgrowth and fetal growth factors

BACKGROUND

Retinol-binding protein 4 (RBP-4) is an adipokine involved in regulating insulin sensitivity which would affect fetal growth. It is unclear whether RBP-4 is associated with fetal overgrowth, and unexplored which fetal growth factor(s) may mediate the association.

METHODS

In the Shanghai Birth Cohort, we studied 125 pairs of larger-for-gestational-age (LGA, birth weight >90th percentile, an indicator of fetal overgrowth) and optimal-for-gestational-age (OGA, 25-75th percentiles) control infants matched by sex and gestational age. We measured cord blood concentrations of RBP-4, insulin, proinsulin, insulin-like growth factor-I (IGF-I), and IGF-II.

RESULTS

Cord blood RBP-4 concentrations were elevated in LGA vs. OGA infants (21.9 ± 6.2 vs. 20.2 ± 5.1 µg/ml, P = 0.011), and positively correlated with birth weight z score (r = 0.19, P = 0.003), cord blood proinsulin (r = 0.21, P < 0.001), IGF-I (r = 0.24, P < 0.001), and IGF-II (r = 0.15, P = 0.016). Adjusting for maternal and neonatal characteristics, each SD increment in cord blood RBP-4 was associated with a 0.28 (0.12-0.45) increase in birth weight z score (P < 0.001). Mediation analyses showed that IGF-I could account for 31.7% of the variation in birth weight z score in association with RBP-4 (P = 0.01), while IGF-II was not an effect mediator.

CONCLUSIONS

RBP-4 was positively associated with fetal overgrowth. IGF-I (but not IGF-II) may mediate this association.

Expression of cholesterol packaging and transport genes in human and rat placenta: impact of obesity and a high-fat diet

Evidence suggests that sub-optimal maternal nutrition has implications for the developing offspring. We have previously shown that exposure to a low-protein diet during gestation was associated with upregulation of genes associated with cholesterol transport and packaging within the placenta. This study aimed to elucidate the effect of altering maternal dietary linoleic acid (LA; omega-6) to alpha-linolenic acid (ALA; omega-6) ratios as well as total fat content on placental expression of genes associated with cholesterol transport. The potential for maternal body mass index (BMI) to be associated with expression of these genes in human placental samples was also evaluated. Placentas were collected from 24 Wistar rats at 20-day gestation (term = 21-22-day gestation) that had been fed one of four diets containing varying fatty acid compositions during pregnancy, and from 62 women at the time of delivery. Expression of 14 placental genes associated with cholesterol packaging and transfer was assessed in rodent and human samples by quantitative real time polymerase chain reaction. In rats, placental mRNA expression of ApoA2, ApoC2, Cubn, Fgg, Mttp and Ttr was significantly elevated (3-30 fold) in animals fed a high LA (36% fat) diet, suggesting increased cholesterol transport across the placenta in this group. In women, maternal BMI was associated with fewer inconsistent alterations in gene expression. In summary, sub-optimal maternal nutrition is associated with alterations in the expression of genes associated with cholesterol transport in a rat model. This may contribute to altered fetal development and potentially programme disease risk in later life. Further investigation of human placenta in response to specific dietary interventions is required.

Components of metabolic syndrome in relation to plasma levels of retinol binding protein 4 (RBP4) in a cohort of people aged 65 years and older

PURPOSE

Elevated plasma concentration of retinol binding protein 4 (RBP4) has recently emerged as a potential risk factor as a component of developing metabolic syndrome (MS). Therefore, this study aimed to analyse the relationship between components of MS and concentrations of plasma RBP4 in a population of subjects 65 years and older.

METHODS

The study sample consisted of 3038 (1591 male) participants of the PolSenior study, aged 65 years and older. Serum lipid profile, concentrations of RBP4, glucose, insulin, C-reactive protein, IL-6, and activity of aminotransferases were measured. Nutritional status (BMI/waist circumference) and treatment with statins and fibrates were evaluated. Glomerular filtration rate (eGFR), de Ritis ratio, and fatty liver index (FLI), as well as HOMA-IR were calculated.

RESULTS

Our study revealed a strong relationship between components of MS and RBP4 in both sexes: plasma RBP4 levels were increased in men by at least 3×, and in women by at least 4×. Hypertriglyceridemia was most strongly associated with elevated plasma RBP4 levels. Multivariate, sex-adjusted regression analysis demonstrated that chronic kidney disease [OR 1.86 (95% CI 1.78-1.94)], hypertriglyceridemia [OR 1.52 (1.24-1.87)], hypertension [OR 1.15 (1.12-1.19)], low serum HDL cholesterol [OR 0.94 (0.92-0.97)], and age > 80 years [OR 0.86 (0.81-0.90)] were each independently associated with RBP4 concentration (all p < 0.001).

CONCLUSIONS

In Caucasians 65 years and older, RBP4 serum levels are associated with a number of components of MS, independent of sex and kidney function. Hypertriglyceridemia as a component of MS is most significantly related to RBP4 concentration.

RBP4 regulates trophoblastic cell proliferation and invasion via the PI3K/AKT signaling pathway

Insufficient trophoblast invasion is associated with preeclampsia (PE) development. Retinol-binding protein 4 (RBP4) is important for regulating cell differentiation, migration and invasion. The aim of the present study was to determine RBP4 expression and function in the human placenta and to examine the underlying mechanisms. In the present study, RBP4 expression was determined in serum samples from 35 pregnant women with PE and 30 healthy pregnant women using enzyme-linked immunosorbent assays. Cell proliferation was assessed by Cell Counting Kit-8 assays, and cell invasion was examined with transwell assays. RBP4 concentrations were significantly lower in the PE group when compared with the control group. RBP4 overexpression enhanced HTR8/SVneo cell proliferation and invasion, and the levels of phosphorylated (p-) phosphoinositide 3-kinase (PI3K) and p-protein kinase B (AKT) in HTR8/SVneo cells. RBP4 knockdown significantly inhibited HTR8/SVneo cell proliferation and invasion, and repressed the expression of matrix metalloproteinases. In addition, RBP4 knockdown significantly reduced the levels of p-PI3K and p-AKT in HTR8/SVneo cells. Taken together, the results of the present study demonstrated that RBP4 overexpression increased HTR8/SVneo cell proliferation and invasion by suppressing PI3K/AKT signaling and RBP4 knockdown induced the opposite effects.

Plasma Retinol-Binding Protein 4 Levels and the Risk of Ischemic Stroke among Women

BACKGROUND

Plasma retinol-binding protein 4 (RBP4) levels have been associated with cardiovascular risk factors and risk of coronary heart disease, but little is known about the association between RBP4 and the risk of ischemic stroke. We hypothesized that elevated RBP4 levels would be associated with an increased risk of ischemic stroke among women.

METHODS

We performed a nested case-control study among women enrolled in the Nurses' Health Study who provided blood samples between 1989 and 1990 and were free of prior stroke and cancer. We measured prediagnostic RBP4 levels in 471 ischemic stroke cases who were confirmed by medical record review and in 471 controls who were matched 1:1 to the cases on age, race, blood collection date, menopausal status, postmenopausal hormone use, and smoking status. We analyzed the association between RBP4 levels and ischemic stroke using multivariable conditional logistic regression conditional on the matching factors and adjusted for physical activity, body mass index, aspirin use, alcohol consumption, diet, history of diabetes, high cholesterol, high blood pressure, or heart disease, and cholesterol and hemoglobin A1C levels.

RESULTS

Median levels of RBP4 were similar in cases (31.1 µg/mL) and controls (31.0 µg/mL; P value from the Wilcoxon rank-sum test = .82). Quartiles of RBP4 were not associated with an increased risk of ischemic stroke (highest quartile compared to lowest quartile: multivariate-adjusted odds ratio, .75; 95% confidence interval, .48, 1.17). We also did not observe associations between RBP4 and ischemic stroke of thrombotic or embolic origin.

CONCLUSIONS

Elevated levels of RBP4 were not associated with an increased risk of ischemic stroke.

Retinol-Binding Protein 4 and Lipids Prospectively Measured During Early to Mid-Pregnancy in Relation to Preeclampsia and Preterm Birth Risk

BACKGROUND

Maternal retinol-binding protein 4 (RBP4) and lipids may relate to preeclampsia and preterm birth risk but longitudinal data are lacking. This study examines these biomarkers longitudinally during pregnancy in relation to preeclampsia and preterm birth risk.

METHODS

Maternal serum samples from the Calcium for Preeclampsia Prevention (CPEP) trial were analyzed at baseline: average 15 gestational weeks; mid-pregnancy: average 27 weeks; and at >34 weeks. We measured RBP4, total cholesterol, high-density lipoprotein, low-density lipoprotein, triglycerides and lipoprotein (a) (Lp(a)). Cross-sectional logistic regression analyses estimated the odds ratio (OR) and 95% confidence intervals (CI) for preterm preeclampsia (n = 63), term preeclampsia (n = 104), and preterm delivery (n = 160) associated with RBP4 and lipids at baseline and mid-pregnancy compared with controls (n = 136). Longitudinal trajectories across pregnancy were assessed using mixed linear models with fixed effects. Adjusted models included clinical and demographic factors.

RESULTS

RBP4 concentrations at baseline and mid-pregnancy were associated with a 4- to 8-fold increase in preterm preeclampsia risk but were not associated with term preeclampsia. RBP4 measured mid-pregnancy was also associated with preterm birth (OR = 6.67, 95% CI: 1.65, 26.84). Higher triglyceride concentrations in mid-pregnancy were associated with a 2- to 4-fold increased risk for both preeclampsia and preterm birth. Longitudinal models demonstrate that both preterm preeclampsia and preterm birth cases had elevated RBP4 throughout gestation.

CONCLUSIONS

Elevated RBP4 is detectable early in pregnancy and its strong relation with preterm preeclampsia merits further investigation and confirmation to evaluate its potential use as a predictor, particularly among high-risk women.

Identification of Tengfu Jiangya Tablet Target Biomarkers with Quantitative Proteomic Technique

Tengfu Jiangya Tablet (TJT) is a well accepted antihypertension drug in China and its major active components were Uncaria total alkaloids and Semen Raphani soluble alkaloid. To further explore treatment effects mechanism of TJT on essential hypertension, a serum proteomic study was performed. Potential biomarkers were quantified in serum of hypertension individuals before and after taking TJT with isobaric tags for relative and absolute quantitation (iTRAQ) coupled two-dimensional liquid chromatography followed electrospray ionization-tandem mass spectrometry (2D LC-MS/MS) proteomics technique. Among 391 identified proteins with high confidence, 70 proteins were differentially expressed (fold variation criteria, >1.2 or <0.83) between two groups (39 upregulated and 31 downregulated). Combining with Gene Ontology annotation, KEGG pathway analysis, and literature retrieval, 5 proteins were chosen as key target biomarkers during TJT therapeutic process. And the alteration profiles of these 5 proteins were verified by ELISA and Western Blot. Proteins Kininogen 1 and Keratin 1 are members of Kallikrein system, while Myeloperoxidase, Serum Amyloid protein A, and Retinol binding protein 4 had been reported closely related to vascular endothelial injury. Our study discovered 5 target biomarkers of the compound Chinese medicine TJT. Secondly, this research initially revealed the antihypertension therapeutic mechanism of this drug from a brand-new aspect.

Retinol-Binding Protein-Dependent Cholesterol Uptake Regulates Macrophage Foam Cell Formation and Promotes Atherosclerosis

BACKGROUND

Retinol-binding protein 4 (RBP4) is an adipokine that plays decisive roles in glucose metabolism and insulin sensitivity. Elevated circulating RBP4 levels were reported to be associated with increased risk for cardiovascular disease, but the precise role of RBP4 in atherosclerotic diseases and its mechanisms of action remain elusive.

METHODS

Serum RBP4 levels of 1683 participants from South China were evaluated and the occurrence of major adverse cardiovascular events was followed up for 5 years. Apolipoprotein E-deficient mice infected with RBP4-overexpressing/silencing adenovirus, J774A.1 macrophages, and primary peritoneal macrophages from RBP4 transgenic mice were used for investigating the function of RBP4 in foam cell formation.

RESULTS

Prospective cohort studies revealed that baseline serum RBP4 level was an independent predictor for incidence of adverse cardiovascular events after adjustment for traditional risk factors. Increased RBP4 expression was observed in atherosclerotic lesions of aortic specimens from both humans and apolipoprotein E-deficient mice, and RBP4 was localized to areas rich in macrophage foam cells. RBP4 inhibition attenuated whereas overexpression accelerated atherosclerosis progression in apolipoprotein E-deficient mice. Both treatment with exogenous recombinant RBP4 and overexpression of RBP4 gene promoted macrophage-derived foam cell formation through the activation of scavenger-receptor CD36-mediated cholesterol uptake, and RBP4 transcriptionally upregulated CD36 expression in a manner dependent on jun N-terminal kinase and signal transducer and activator of transcription 1. The tyrosine kinase c-Src was identified as the upstream regulator of jun N-terminal kinase-signal transducer and activator of transcription 1-mediated CD36-dependent cholesterol uptake, and RBP4 challenge was found to alter the membrane distribution of c-Src and cause c-Src to partition into lipid-raft membrane subdomains, where the kinase was activated. Lastly, Toll-like receptor 4, but not retinol or stimulated by retinoic acid 6, mediated the inductive effects of RBP4 in macrophages.

CONCLUSIONS

Inclusion of RBP4 levels in traditional models enhances the predictive ability for the incidence of atherosclerotic events. RBP4 promotes atherogenesis by inducing macrophage-derived foam cell formation.

Increased plasma RBP4 concentration in older hypertensives is related to the decreased kidney function and the number of antihypertensive drugs-results from the PolSenior substudy

Increased plasma retinol-binding protein 4 (RBP4), a novel adipokine, has been associated in previous studies with obesity, type 2 diabetes, dyslipidemia, hypertension (HT), atherosclerosis, and coronary artery disease. This study aimed to analyze the relationship between HT occurrence and its treatment, and plasma RBP4 concentrations in the older polish population. The study sample consisted of 1728 (890 men and 838 women) PolSenior study participants aged 65 years and older with available plasma samples and NT-proBNP values below 2000 pg/mL. The analysis included body mass index, waist circumference, blood pressure, antihypertensive medication, estimated glomerular filtration rate, serum glucose and insulin (and the homeostatic model assessment of insulin resistance), and plasma RBP4 levels. RBP4 plasma concentrations were higher in hypertensive (N = 645) than normotensive (N = 236) men (43.4 [30.4-64.8] vs. 38.1 [27.1-54.4] ng/mL, respectively; P < .01) but not in women (44.6 [29.6-63.5] vs. 40.7 [29.1-58.1] ng/mL, respectively; P = .21). In the subanalysis, higher plasma RBP4 levels were observed in women with treated than untreated HT and in subjects taking four of more antihypertensive drugs. The linear regression shown that estimated glomerular filtration rate (β = -0.015), thiazide diuretics (β = 0.041), and α-blockers (β = 0.049) were explaining log₁₀RBP4 plasma levels variability in the study group. Older male Caucasians with HT are characterized by elevated plasma RBP4 levels. This increase is proportional to the number of antihypertensive drugs and decreased glomerular filtration rate. Among the antihypertensive drugs, only thiazide diuretics and α-blockers had a significant influence on RBP4 levels.

Retinol-Binding Protein 4 Induces Cardiomyocyte Hypertrophy by Activating TLR4/MyD88 Pathway

Insulin resistance plays a major role in the development and progression of cardiac hypertrophy and heart failure. Heart failure in turn promotes insulin resistance and increases the risk for diabetes. The vicious cycle determines significant mortality in patients with heart failure and diabetes. However, the underlying mechanisms for the vicious cycle are not fully elucidated. Here we show that circulating levels and adipose expression of retinol-binding protein 4 (RBP4), an adipokine that contributes to systemic insulin resistance, were elevated in cardiac hypertrophy induced by transverse aortic constriction and angiotensin-II (Ang-II) infusion. Ang-II increased RBP4 expression in adipocytes, which was abolished by losartan, an Ang-II receptor blocker. The elevated RBP4 in cardiac hypertrophy may have pathophysiological consequences because RBP4 increased cell size, enhanced protein synthesis, and elevated the expression of hypertrophic markers including Anp, Bnp, and Myh7 in primary cardiomyocytes. Mechanistically, RBP4 induced the expression and activity of toll-like receptor 4 (TLR4) and myeloid differentiation primary response gene 88 (MyD88) in cardiomyocytes, resulting in enhanced inflammation and reactive oxygen species production. Inhibition or knockdown of the TLR4/MyD88 pathway attenuated inflammatory and hypertrophic responses to RBP4 stimulation. Importantly, RBP4 also reduced the expression of glucose transporter-4 and impaired insulin-stimulated glucose uptake in cardiomyocytes. This impairment was ameliorated in cardiomyocytes from TLR4 knockout mice. Therefore, RBP4 may be a critical modulator promoting the vicious cycle of insulin resistance and heart failure by activating TLR4/MyD88-mediated inflammatory pathways. Potentially, lowering RBP4 might break the vicious cycle and improve both insulin resistance and cardiac hypertrophy.

Retinol binding protein 4 in relation to diet, inflammation, immunity, and cardiovascular diseases

Retinol binding protein 4 (RBP4), previously called retinol binding protein (RBP), is considered a specific carrier of retinol in the blood. It is also an adipokine that has been implicated in the pathophysiology of insulin resistance. RBP4 seems to be correlated with cardiometabolic markers in inflammatory chronic diseases, including obesity, type 2 diabetes, metabolic syndrome, and cardiovascular diseases (CVDs). It has recently been suggested that inflammation produced by RBP4 induces insulin resistance and CVD. The clinical relevance of this hypothesis is discussed in this review. Knowledge concerning the association of RBP4 with inflammation markers, oxidative stress, and CVDs as well as concerning the role of diet and antioxidants in decreasing RBP4 concentrations are discussed. Special attention is given to methodologies used in previously published studies and covariates that should be controlled when planning new studies on this adipokine.

Transgenic Mice Overexpressing Serum Retinol-Binding Protein Develop Progressive Retinal Degeneration through a Retinoid-Independent Mechanism

Serum retinol-binding protein 4 (RBP4) is the sole specific transport protein for retinol in the blood, but it is also an adipokine with retinol-independent, proinflammatory activity associated with obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. Moreover, two separate studies reported that patients with proliferative diabetic retinopathy have increased serum RBP4 levels compared to patients with mild or no retinopathy, yet the effect of increased levels of RBP4 on the retina has not been studied. Here we show that transgenic mice overexpressing RBP4 (RBP4-Tg mice) develop progressive retinal degeneration, characterized by photoreceptor ribbon synapse deficiency and subsequent bipolar cell loss. Ocular retinoid and bisretinoid levels are normal in RBP4-Tg mice, demonstrating that a retinoid-independent mechanism underlies retinal degeneration. Increased expression of pro-interleukin-18 (pro-IL-18) mRNA and activated IL-18 protein and early-onset microglia activation in the retina suggest that retinal degeneration is driven by a proinflammatory mechanism. Neither chronic systemic metabolic disease nor other retinal insults are required for RBP4 elevation to promote retinal neurodegeneration, since RBP4-Tg mice do not have coincident retinal vascular pathology, obesity, dyslipidemia, or hyperglycemia. These findings suggest that elevation of serum RBP4 levels could be a risk factor for retinal damage and vision loss in nondiabetic as well as diabetic patients.

Transthyretin Antisense Oligonucleotides Lower Circulating RBP4 Levels and Improve Insulin Sensitivity in Obese Mice

Circulating transthyretin (TTR) is a critical determinant of plasma retinol-binding protein 4 (RBP4) levels. Elevated RBP4 levels cause insulin resistance, and the lowering of RBP4 levels improves glucose homeostasis. Since lowering TTR levels increases renal clearance of RBP4, we determined whether decreasing TTR levels with antisense oligonucleotides (ASOs) improves glucose metabolism and insulin sensitivity in obesity. TTR-ASO treatment of mice with genetic or diet-induced obesity resulted in an 80-95% decrease in circulating levels of TTR and RBP4. Treatment with TTR-ASOs, but not control ASOs, decreased insulin levels by 30-60% and improved insulin sensitivity in ob/ob mice and high-fat diet-fed mice as early as after 2 weeks of treatment. The reduced insulin levels were sustained for up to 9 weeks of treatment and were associated with reduced adipose tissue inflammation. Body weight was not changed. TTR-ASO treatment decreased LDL cholesterol in high-fat diet-fed mice. The glucose infusion rate during a hyperinsulinemic-euglycemic clamp was increased by 50% in high-fat diet-fed mice treated with TTR-ASOs, demonstrating improved insulin sensitivity. This was also demonstrated by 20% greater inhibition of hepatic glucose production, a 45-60% increase of glucose uptake into skeletal and cardiac muscle, and a twofold increase in insulin signaling in muscle. These data show that decreasing circulating TTR levels or altering TTR-RBP4 binding could be a potential therapeutic approach for the treatment of type 2 diabetes.

Novel role for retinol-binding protein 4 in the regulation of blood pressure

Elevated levels of serum retinol-binding protein 4 (RBP4) contribute to insulin resistance and correlate with increased prevalence of hypertension and myocardial infarction. We sought to determine whether lowering RBP4 would improve blood pressure (BP) and protect against obesity- or angiotensin (Ang)-II-induced hypertension. Systolic and diastolic BP were lower in the RBP4-knockout (RBP4-KO) mice and higher in the RBP4-overexpressing (RBP4-Tg) mice compared with BP in the wild-type (WT) littermates. Carbachol-induced vasodilatation was increased in arteries from the RBP4-KO compared with the WT mice and was impaired in the RBP4-Tg mice. Aortic eNOS(Ser1177) phosphorylation was enhanced ∼50% in the RBP4-KO mice, with no change in total eNOS protein. Feeding a high-fat diet increased BP in the RBP4-KO mice only to the level in the WT mice fed chow and had no effect on aortic eNOS(Ser1177) phosphorylation. Ang-II infusion resulted in 22 mmHg lower systolic BP in the RBP4-KO than in the WT mice, although the relative BP increase over saline infusion was ∼30% in both. Ang-II treatment decreased aortic eNOS(Ser1177) phosphorylation in the WT and RBP4-KO mice, but phosphorylation remained higher in the RBP4-KO mice. Cardiac hypertrophy with Ang-II treatment was diminished by 56% in the RBP4-KO mice. Thus, elevated serum RBP4 raises BP and lack of RBP4 reduces it, with commensurate changes in aortic eNOS(Ser1177) phosphorylation. Lowering RBP4 may reduce BP through enhanced eNOS-mediated vasodilatation and may be a novel therapeutic approach for hypertension.

Adipokines, Vascular Wall, and Cardiovascular Disease

Epidemiological evidence has shown that abdominal obesity is closely associated with the development of cardiovascular (CV) disease, suggesting that it might be considered as an independent CV risk factor. However, the pathophysiological mechanisms responsible for the association between these 2 clinical entities remain largely unknown. Adipocytes are considered able to produce and secrete chemical mediators known as “adipokines” that may exert several biological actions, including those on heart and vessels. Of interest, a different adipokine profile can be observed in the plasma of patients with obesity or metabolic syndrome compared with healthy controls. We consider the main adipokines, focusing on their effects on the vascular wall and analyzing their role in CV pathophysiology.

Serum retinol-binding protein 4 levels are elevated but do not contribute to insulin resistance in newly diagnosed Chinese hypertensive patients

BACKGROUND

Insulin resistance (IR) is closely correlated with cardiovascular disease (CVD). Retinol-binding protein 4 (RBP4) is a novel adipokine that modulates the action of insulin in various diseases. This study addressed the relationship between RBP4 and IR in newly diagnosed essential hypertension.

METHODS

Serum RBP4, anthropometric and metabolic parameters were determined in 267 newly diagnosed essential hypertensive patients not taking antihypertensive medications. The patients along with 64 control (NC) normotensive and lean subjects paired by age and sex were divided into two groups depending on body mass index (BMI), hypertension with obesity (HPO) and hypertension without obesity (HP).

RESULTS

A striking difference was observed in RBP4 levels between the HP and NC groups. Significantly higher levels were noted in the HP group compared with the NC group; slightly, but not significantly, lower levels were observed in the HPO group compared with the HP group. After adjusting for BMI, WC and WHR, a modestly linear relationship was observed between RBP4 levels and SBP (r = 0.377; p = 0.00), DBP (r = 0.288; p = 0.00) and HOMA-β(r = 0.121; p = 0.028). Multiple stepwise regression analysis showed that SBP, WHR and drinking were independently related with serum RBP4 levels.

CONCLUSIONS

The results of this study indicated that RBP4 levels were increased in naive hypertensive patients; however, no differences were observed in obese or non-obese hypertensive subjects. Our data suggest for the first time that RBP4 levels are significantly increased but do not contribute to the development of IR in newly diagnosed hypertensive Chinese patients.

Circulating lipocalin-2 and retinol-binding protein 4 are associated with intima-media thickness and subclinical atherosclerosis in patients with type 2 diabetes

BACKGROUND

The lipocalin family proteins, including lipocalin-2 and retinol-binding protein 4 (RBP4), are adipokines closely associated with obesity-related metabolic disorders. In this study, we evaluated the association of serum lipocalin-2 and RBP4 with intima-media thickness (IMT) and subclinical atherosclerosis in type 2 diabetic patients.

METHODS AND RESULTS

Serum levels of lipocalin-2 and RBP4 were measured in 284 type 2 diabetic patients. Subclinical atherosclerosis was assessed by IMT at carotid, femoral and iliac arteries with ultrasound. Patients with subclinical atherosclerosis showed significantly higher circulating concentrations of lipocalin-2 and RBP4 when compared to those without [112.9 (86.4 to 202.1) µg/L versus 77.2(55.0-150.4) µg/L, 37.1(32.3-40.8) mg/L versus 23.2(20.1-29.2) mg/L, respectively; P = 0.002, P<0.001, respectively]. Moreover, positive correlations were observed between carotid IMT and lipocalin-2 (r = 0.170, P = 0.018) or RBP4 (r = 0.132, P = 0.040), femoral IMT and lipocalin-2 (r = 0.160, P = 0.027), as well as between iliac IMT and RBP4 (r = 0.241, P<0.001). Multiple logistic regression analysis further demonstrated that these two adipokines were independent risk factors for subclinical atherosclerosis in type 2 diabetes.

CONCLUSION

Circulating levels of lipocalin-2 and RBP4 are positively correlated with carotid IMT and subclinical atherosclerosis in type 2 diabetes, which suggests a potential role of these two lipid-binding chaperones in the pathogenesis of vascular complications of diabetes.

Maternal serum retinol-binding protein-4 at 11-13 weeks' gestation in normal and pathological pregnancies

OBJECTIVE

To examine maternal serum levels of retinol-binding protein-4 (RBP4) at 11-13 weeks' gestation in normal and pathological pregnancies.

METHODS

Serum RBP4 at 11-13 weeks was measured in 480 singleton pregnancies, including 240 with normal outcome, 60 that subsequently developed preeclampsia (PE), 60 that developed gestational diabetes mellitus (GDM), 60 that delivered large for gestational age (LGA) neonates and 60 that delivered small (SGA) neonates. The values in each adverse pregnancy outcome group were compared to those of normal pregnancies.

RESULTS

Serum concentration of RBP4 was not significantly different in women who subsequently developed PE (p=0.925), or GDM (p=0.074), or had pregnancies that led to delivery to SGA (p=0.085), LGA (p=0.332) neonates.

CONCLUSION

Maternal serum RBP4 in the first trimester is not significantly altered in pathological pregnancies.

Plasma retinol-binding protein 4 (RBP4) levels and risk of coronary heart disease: a prospective analysis among women in the nurses' health study

BACKGROUND

Retinol-binding protein 4 (RBP4) may play an important role in the origin of insulin resistance and metabolic syndrome. Few prospective data are available on the relationship between RBP4 and coronary heart disease (CHD). Furthermore, previous studies did not distinguish among full-length and truncated forms of RBP4 that might have various biological activities.

METHODS AND RESULTS

We measured plasma levels of full-length and several C-terminally truncated subfractions of RBP4 among 468 women who developed CHD and 472 matched controls in the Nurses' Health Study cohort during 16 years of follow-up (1990-2006). We observed a temporal variation in the association of full-length RBP4 levels with CHD risk (P=0.04 for testing proportional hazard assumption). In the first 8 years of follow-up, after multivariate adjustment for covariates, the odds ratio of CHD risk comparing extreme quartiles of full-length RBP4 levels was 3.56 (95% confidence interval, 1.21-10.51; Ptrend=0.003), whereas this association was 0.77 (95% confidence interval, 0.38-1.56; Ptrend=0.44) in the follow-up period of 9 to 16 years. Results were similar for total RBP4 levels (summed levels of all RBP4 isoforms). Levels of the primary truncated isoform, RBP4-L, were not associated with CHD risk in any follow-up period; the odds ratios for extreme quartiles were 1.29 (95% confidence interval, 0.50-3.32) and 1.20 (95% confidence interval, 0.64-2.26) in the first and second 8 years of follow-up, respectively.

CONCLUSIONS

In this cohort of women, higher circulating full-length and total RBP4 levels were associated with increased risk of CHD in a time-dependent fashion. Additional data are warranted to confirm the present findings.

Adipocytokines mark insulin sensitivity in euthyroid Hashimoto's patients

The relationship between inflammation, Hashimoto's thyroiditis (HT) and insulin resistance is still controversial. In this regard, a pretty complete evaluation of adipocytokines levels in patients with HT has not been performed so far. We assessed retinol binding protein-4 (RBP4), adipocyte-fatty acid binding protein (A-FABP), neutrophil gelatinase-associated lipocalin (NGAL) and tumor necrosis factor-α (TNFα) levels in 93 euthyroid HT patients and 51 healthy controls (CTL), also evaluating the possible correlation between adipocytokines levels and markers of insulin resistance. No significant differences between HT patients and CTL in fasting plasma glucose and insulin levels, and HOMA index were observed. HT patients had significantly higher RBP4, NGAL and A-FABP levels than CTL, while TNFα levels did not differ between the two groups. In HT patients, RBP4 was significantly related with fT3 and fT4 levels, while A-FABP with fT4 only. Moreover, in HT patients, either RBP4 or A-FABP was directly associated with plasma insulin and HOMA index. Circulating levels of these adipocytokines were not influenced by the presence of antithyroid peroxidase or antithyroglobulin autoantibodies or only one of them, neither by autoantibodies titer. In conclusion, euthyroid HT patients are characterized by a peculiar inflammatory response of the adipose tissue, apparently related to an early reduction in insulin sensitivity and to serum thyroid hormone levels, although within the normal range. These results suggest that HT patients with high RBP4 and A-FABP levels might deserve a particular attention, being potentially more exposed to develop insulin resistance and increased cardiovascular risk.

Clinical implications of adipocytokines and newly emerging metabolic factors with relation to insulin resistance and cardiovascular health

Adipose tissue is known to secrete hormones actively and produces many biologically active proteins called adipocytokines. Typically, obesity is followed by low-grade inflammation, which is characterized by increased circulating levels of pro-inflammatory cytokines. Macrophages play a role in the inflammatory process by secreting many cytokines such as tumor necrosis factor alpha, interleukin-6, resistin, and retinol binding protein-4. These cytokines and chemokines participate in low-grade pro-inflammatory processes leading to insulin resistance, metabolic impairment, and cardiovascular diseases. More metabolic regulators, such as fibroblast growth factor (FGF)21, FGF19, FGF1, vaspin, and visfatin have now been discovered but their exact roles in human diseases are still unclear. This review focuses on recent research regarding the role of adipokines and new metabolic factors in metabolic derangement or cardiovascular disease.

Retinol-binding protein 4 induces inflammation in human endothelial cells by an NADPH oxidase- and nuclear factor kappa B-dependent and retinol-independent mechanism

Serum retinol-binding protein 4 (RBP4) is the sole specific vitamin A (retinol) transporter in blood. Elevation of serum RBP4 in patients has been linked to cardiovascular disease and diabetic retinopathy. However, the significance of RBP4 elevation in the pathogenesis of these vascular diseases is unknown. Here we show that RBP4 induces inflammation in primary human retinal capillary endothelial cells (HRCEC) and human umbilical vein endothelial cells (HUVEC) by stimulating expression of proinflammatory molecules involved in leukocyte recruitment and adherence to endothelium, including vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), E-selectin, monocyte chemoattractant protein 1 (MCP-1), and interleukin-6 (IL-6). We demonstrate that these novel effects of RBP4 are independent of retinol and the RBP4 membrane receptor STRA6 and occur in part via activation of NADPH oxidase and NF-κB. Importantly, retinol-free RBP4 (apo-RBP4) was as potent as retinol-bound RBP4 (holo-RBP4) in inducing proinflammatory molecules in both HRCEC and HUVEC. These studies reveal that RBP4 elevation can directly contribute to endothelial inflammation and therefore may play a causative role in the development or progression of vascular inflammation during cardiovascular disease and microvascular complications of diabetes.

Adipocytokine levels mark endothelial function in normotensive individuals

Background

Endothelial dysfunction is an independent risk factor for cardiovascular events. Inflammatory mediators released by the adipose tissue can lead to local insulin resistance and endothelial dysfunction. This study addressed the relationship of adipocytokines with endothelial function and blood pressure.

Methods

In 92 newly diagnosed, drug-naïve essential hypertensive patients (HT, mean age 49 yrs) without organ damage and 66 normotensive subjects (NT, mean age 47 yrs), by an automated system, we measured endothelium-dependent and -independent vasodilation as brachial artery flow-mediated dilation before and after administration of glyceryl-trinitrate. Retinol binding protein-4 (RBP4) and resistin levels were determined by ELISA and RIA, respectively. Oxidative stress was evaluated by measuring serum malondyaldehyde (MDA).

Results

Flow-mediated dilation was significantly (p = 0.03) lower in HT (5.3 ± 2.6%) than NT (6.1 ± 3.1%), while response to glyceryl-trinitrate (7.5 ± 3.7% vs 7.9 ± 3.4%) was similar. RBP4 (60.6 ± 25.1 vs 61.3 ± 25.9 μg/ml), resistin (18.8 ± 5.3 vs 19.9 ± 6.1 ng/ml) and MDA levels (2.39 ± 1.26 vs 2.08 ± 1.17 nmol/ml) were not different in HT and NT.

RBP4 (r = −0.25; p = 0.04) and resistin levels (r = −0.29; p = 0.03) were related to flow-mediated dilation in NT, but not in HT (r = −0.03 and r = −0.10, respectively). In NT, multivariate analysis including RBP4 and confounders showed that only BMI or waist circumference remained related to flow- mediated dilation. In the multivariate model including resistin and confounders, BMI, age and resistin were significantly related to flow-mediated dilation, while only age significant correlated with this parameter when BMI was replaced by waist circumference.

Conclusions

Adipocytokine levels may be independent predictors of endothelial dysfunction in the peripheral circulation of healthy subjects, providing a pathophysiological link between inflammation from adipose tissue and early vascular alterations.

Associations between retinol-binding protein 4 and cardiometabolic risk factors and subclinical atherosclerosis in recently postmenopausal women: cross-sectional analyses from the KEEPS study

Background

The published literature regarding the relationships between retinol-binding protein 4 (RBP4) and cardiometabolic risk factors and subclinical atherosclerosis is conflicting, likely due, in part, to limitations of frequently used RBP4 assays. Prior large studies have not utilized the gold-standard western blot analysis of RBP4 levels.

Methods

Full-length serum RBP4 levels were measured by western blot in 709 postmenopausal women screened for the Kronos Early Estrogen Prevention Study. Cross-sectional analyses related RBP4 levels to cardiometabolic risk factors, carotid artery intima-media thickness (CIMT), and coronary artery calcification (CAC).

Results

The mean age of women was 52.9 (± 2.6) years, and the median RBP4 level was 49.0 (interquartile range 36.9-61.5) μg/mL. Higher RBP4 levels were weakly associated with higher triglycerides (age, race, and smoking-adjusted partial Spearman correlation coefficient = 0.10; P = 0.01), but were unrelated to blood pressure, cholesterol, C-reactive protein, glucose, insulin, and CIMT levels (all partial Spearman correlation coefficients ≤0.06, P > 0.05). Results suggested a curvilinear association between RBP4 levels and CAC, with women in the bottom and upper quartiles of RBP4 having higher odds of CAC (odds ratio [95% confidence interval] 2.10 [1.07-4.09], 2.00 [1.02-3.92], 1.64 [0.82-3.27] for the 1^st, 3^rd, and 4^th RBP4 quartiles vs. the 2^nd quartile). However, a squared RBP4 term in regression modeling was non-significant (P = 0.10).

Conclusions

In these healthy, recently postmenopausal women, higher RBP4 levels were weakly associated with elevations in triglycerides and with CAC, but not with other risk factors or CIMT. These data using the gold standard of RBP4 methodology only weakly support the possibility that perturbations in RBP4 homeostasis may be an additional risk factor for subclinical coronary atherosclerosis.

Trial registration

ClinicalTrials.gov number NCT00154180

Retinol-binding protein 4 inhibits insulin signaling in adipocytes by inducing proinflammatory cytokines in macrophages through a c-Jun N-terminal kinase- and toll-like receptor 4-dependent and retinol-independent mechanism

Retinol-binding protein 4 (RBP4), the sole retinol transporter in blood, is secreted from adipocytes and liver. Serum RBP4 levels correlate highly with insulin resistance, other metabolic syndrome factors, and cardiovascular disease. Elevated serum RBP4 causes insulin resistance, but the molecular mechanisms are unknown. Here we show that RBP4 induces expression of proinflammatory cytokines in mouse and human macrophages and thereby indirectly inhibits insulin signaling in cocultured adipocytes. This occurs through activation of c-Jun N-terminal protein kinase (JNK) and Toll-like receptor 4 (TLR4) pathways independent of the RBP4 receptor, STRA6. RBP4 effects are markedly attenuated in JNK1-/- JNK2-/- macrophages and TLR4-/- macrophages. Because RBP4 is a retinol-binding protein, we investigated whether these effects are retinol dependent. Unexpectedly, retinol-free RBP4 (apo-RBP4) is as potent as retinol-bound RBP4 (holo-RBP4) in inducing proinflammatory cytokines in macrophages. Apo-RBP4 is likely to be physiologically significant since RBP4/retinol ratios are increased in serum of lean and obese insulin-resistant humans compared to ratios in insulin-sensitive humans, indicating that higher apo-RBP4 is associated with insulin resistance independent of obesity. Thus, RBP4 may cause insulin resistance by contributing to the development of an inflammatory state in adipose tissue through activation of proinflammatory cytokines in macrophages. This process reveals a novel JNK- and TLR4-dependent and retinol- and STRA6-independent mechanism of action for RBP4.

Retinol-binding protein 4 : a possible role in cardiovascular complications

BACKGROUND AND PURPOSE

Retinol-binding protein 4 (RBP4) is an adipocyte-secreted hormone proposed to link obesity with insulin resistance. However, the role of RBP4 in cardiovascular complications is yet to be fully understood. The present study is aimed to decipher the association between RBP4 with pro-inflammatory cytokines and low-density lipoprotein (LDL) cholesterol in diet-induced obese and hyperlipidaemic mice. To understand the correlation, rimonabant, an anti-obesity drug, has been used to relieve the atherosclerotic predisposition.

EXPERIMENTAL APPROACH

Adipose and/or aortic tissue expressions of RBP4, pro-inflammatory cytokine genes and circulating LDL levels were measured in high fat (HF)-fed female C57BL/6 and high cholesterol (HC)-fed apolipoprotein E3 (ApoE3) Leiden mice.

KEY RESULTS

Mice fed a HF diet had a significantly increased adipose expression of RBP4, TNF-α and monocyte chemoattractant protein 1 (MCP-1) and down-regulated adiponectin mRNA levels. A significant increase in aortic RBP4 and MCP-1 expression and circulating levels of LDL and C-reactive protein (CRP) was found in the ApoE3 mice fed a HC diet. Interestingly, rimonabant treatment lowered the elevated aortic RBP4, MCP-1 expressions and significantly reduced the serum levels of LDL, CRP, RBP4 and MCP-1.

CONCLUSION AND IMPLICATIONS

Our results indicate that RBP4 is positively associated with markers of inflammation in obese and pro-atherogenic conditions and could play a role in a predisposition to atherosclerosis. Furthermore, our results indicate that rimonabant may improve vascular function by modulating RBP4 along with pro-inflammatory cytokines.

Treatment with captopril abrogates the altered expression of alpha1 macroglobulin and alpha1 antiproteinase in sera of spontaneously hypertensive rats

Background

Proteins that are associated with hypertension may be identified by comparing the 2-dimensional gel electrophoresis (2-DE) profiles of the sera of spontaneously hypertensive rats (SHR) with those generated from normotensive Spraque-Dawley rats (SDR).

Results

Five proteins of high abundance were found to be significantly altered when the 2-DE serum profiles of the SHR were compared to those that were similarly generated from the SDR. Analysis by mass spectrometry and database search identified the proteins as retinol binding protein 4, complement C3, albumin (19.9 kDa fragment), alpha1 macroglobulin and alpha1 antiproteinase, which are all known to be associated with hypertension. The altered expression of the two latter proteins was found to be abrogated when similar analysis was performed on sera of the SHR that were treated with captopril.

Conclusion

Our data suggests that serum alpha1 macroglobulin and alpha1 antiproteinase are potentially useful complementary biomolecular indicators for monitoring of hypertension.

Association among retinol-binding protein 4, small dense LDL cholesterol and oxidized LDL levels in dyslipidemia subjects

OBJECTIVES

To investigate retinol-binding protein 4 (RBP4), small dense low-density lipoprotein cholesterol (sdLDL-C) and oxidized low-density lipoprotein (ox-LDL) levels and their associations in dyslipidemia subjects.

DESIGN AND METHODS

We determined RBP4, sdLDL-C, ox-LDL levels in 150 various dyslipidemia subjects and 50 controls. The correlation analysis and multiple linear regression analysis were performed.

RESULTS

The RBP4, sdLDL-C and ox-LDL levels were found increased in various dyslipidemia subjects. The sdLDL-C levels were positively correlated with RBP4 (r=0.273, P=0.001) and ox-LDL (r=0.273, P=0.001). RBP4 levels were also correlated with ox-LDL (r=0.167, P=0.043). The multiple regression analysis showed that only sdLDL-C was a significant independent predictor for RBP4 (β coefficient=0.219, P=0.009; adjusted R(2)=0.041) and ox-LDL (β coefficient=0.253, P=0.003; adjusted R(2)=0.057) levels, respectively.

CONCLUSIONS

The independent associations of sdLDL-C with RBP4 and ox-LDL were observed in dyslipidemia subjects. RBP4 may play an important role in lipid metabolism of atherosclerosis, particularly in formation of sdLDL.

Coronary artery disease is associated with higher epicardial retinol-binding protein 4 (RBP4) and lower glucose transporter (GLUT) 4 levels in epicardial and subcutaneous adipose tissue

OBJECTIVE

Retinol-binding protein 4 (RBP4), produced by adipocytes and hepatocytes, contributes to an unfavourable lipid profile and insulin resistance, which can contribute to the development of coronary artery disease (CAD). Recently, several studies have shown that epicardial adipose tissue (EAT) differs from subcutaneous adipose tissue (SAT) and plays a role on the physiopathology of CAD because of its proximity to the coronary arteries. We aimed to study the expression and secretion levels of RBP4 in both fat tissues and explore its possible association with CAD.

RESEARCH DESIGN AND METHODS

Fifty-eight patients undergoing heart surgery were included in the study. We analysed RBP4 mRNA expression by real-time PCR, protein expression by Western blot and immunohistochemistry, and secretion of EAT and SAT explants from CAD and non-CAD patients by Enzyme Immunoassay.

RESULTS

Retinol-binding protein 4 is expressed at similar levels in EAT and SAT, mainly from adipocytes. Protein levels were higher in EAT from CAD than non-CAD patients (0·63 ± 0·09 arbitrary units (a.u).; n = 10) vs (0·41 ± 0·04 a.u.; n = 13, P = 0·039). In contrast, GLUT4 mRNA levels were lower in EAT from CAD than non-CAD patients (6·55 ± 0·16 a.u.; n = 13) vs (7·21 ± 0·18 a.u.; n = 14, P = 0·012). We also found differential expression in SAT between samples from CAD and non-CAD patients [(6·63 ± 0·16 a.u.; n = 14) vs (7·21 ± 0·14 a.u.; n = 14, P = 0·009)]. Besides, EAT releases higher RBP4 levels than SAT after 3, 6, 24 and 48 h of culture. These levels were independent of CAD but significantly higher in diabetic than nondiabetic patients.

CONCLUSION

Retinol-binding protein 4 levels behave differently in EAT and SAT with respect to CAD. However, both adipose tissues have lower GLUT4 levels in patients with CAD. These findings suggest a differential regulation of RBP4 production in EAT and SAT that may be influenced by local factors.

Adipocytokines: The pied pipers

Even though there have been major advances in therapy, atherosclerosis and coronary artery disease retain their lead as one of the major causes of morbidity and mortality in the first decade of 21(st) century. To add to the woes, we have diabetes, obesity and insulin resistance as the other causes. The adipose tissue secretes several bioactive mediators that influence inflammation, insulin resistance, diabetes, atherosclerosis and several other pathologic states besides the regulation of body weight. These mediators are mostly proteins and are termed "adipocytokines". Adiponectin, resistin, visfatin, retinol binding protein-4 (RBP-4) and leptin are a few such proteins. Adiponectin is a multimeric protein, acting via its identified receptors, AdipoR1 and AdipoR2. It is a potential biomarker for metabolic syndrome and has several antiinflammatory actions. Adiponectin increases insulin sensitivity and ameliorates obesity. Resistin, another protein secreted by the adipose tissue, derived its name due to its involvement in the development of insulin resistance. It plays a role in the pathophysiology of several conditions because of its robust proinflammatory activity mediated through the activation of extracellular signal regulated kinases 1 and 2 (ERK 1/2). In 2007, resistin was reported to have protective effect in ischemia-reperfusion injury and myocyte-apoptosis in the setting of myocardial infarction (MI). RBP-4 is involved in the developmental pathology of type 2 diabetes mellitus and obesity. Visfatin has been described as an inflammatory cytokine. Increased expression of visfatin mRNA has been observed in inflammatory conditions like atherosclerosis and inflammatory bowel disease. Leptin mainly regulates the food intake and energy homeostasis. Leptin resistance has been associated with development of obesity and insulin resistance. Few drugs (thiazolidinediones, rimonabant, statins, etc.) and some lifestyle modifications have been found to improve the levels of adipocytokines. Their role in therapy has a lot in store to be explored upon.

The changes in plasma retinol-binding protein 4 levels are associated with those of the apolipoprotein B-containing lipoproteins during dietary and drug treatment

We investigated the association between retinol-binding protein 4 (RBP(4)) and apolipoprotein B (ApoB)-containing lipoproteins. Obese or overweight, hypertriglyceridemic patients underwent the following interventions for 3 months: (1) Diet (n = 20), (2) Diet + fenofibrate (n = 18), (3) Diet + rimonabant (n = 8). Circulating RBP4 decreased during dietary treatment. The percentage change in RBP(4) was positively correlated with the percentage changes in very-low density lipoprotein cholesterol (r = .570, P = .02), low-density lipoprotein cholesterol ([LDL-C]; r = .605, P = .01), ApoB (r = .705, P = .007), and small dense LDL-C ([sdLDL-C]; r = .872, P < .001). The percentage change in RBP4 was the best predictor of the percentage changes in sdLDL-C and ApoB. Rimonabant treatment reduced RBP4, whereas fenofibrate increased RBP4 during the first month of therapy followed by a subsequent decrease. In conclusion, RBP4 may significantly influence the metabolic pathways responsible for changes in ApoB lipoprotein subspecies, thus RBP4 may be associated with cardiovascular disease risk.