Evidence that kidney function but not type 2 diabetes determines retinol-binding protein 4 serum levels

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.

Vitamin A and retinoid signaling in the kidneys

Vitamin A (VA, retinol) and its metabolites (commonly called retinoids) are required for the proper development of the kidney during embryogenesis, but retinoids also play key roles in the function and repair of the kidney in adults. Kidneys filter 180-200 liters of blood per day and each kidney contains approximately 1 million nephrons, which are often referred to as the 'functional units' of the kidney. Each nephron consists of a glomerulus and a series of tubules (proximal tubule, loop of Henle, distal tubule, and collecting duct) surrounded by a network of capillaries. VA is stored in the liver and converted to active metabolites, most notably retinoic acid (RA), which acts as an agonist for the retinoic acid receptors ((RARs α, β, and γ) to regulate gene transcription. In this review we discuss some of the actions of retinoids in the kidney after injury. For example, in an ischemia-reperfusion model in mice, injury-associated loss of proximal tubule (PT) differentiation markers occurs, followed by re-expression of these differentiation markers during PT repair. Notably, healthy proximal tubules express ALDH1a2, the enzyme that metabolizes retinaldehyde to RA, but transiently lose ALDH1a2 expression after injury, while nearby myofibroblasts transiently acquire RA-producing capabilities after injury. These results indicate that RA is important for renal tubular injury repair and that compensatory mechanisms exist for the generation of endogenous RA by other cell types upon proximal tubule injury. ALDH1a2 levels also increase in podocytes, epithelial cells of the glomeruli, after injury, and RA promotes podocyte differentiation. We also review the ability of exogenous, pharmacological doses of RA and receptor selective retinoids to treat numerous kidney diseases, including kidney cancer and diabetic kidney disease, and the emerging genetic evidence for the importance of retinoids and their receptors in maintaining or restoring kidney function after injury. In general, RA has a protective effect on the kidney after various types of injuries (eg. ischemia, cytotoxic actions of chemicals, hyperglycemia related to diabetes). As more research into the actions of each of the three RARs in the kidney is carried out, a greater understanding of the actions of vitamin A is likely to lead to new insights into the pathology of kidney disorders and the development of new therapies for kidney diseases.

Metabolic-Associated Fatty Liver Disease and Insulin Resistance: A Review of Complex Interlinks

Metabolic-associated fatty liver disease (MAFLD) has now surpassed alcohol excess as the most common cause of chronic liver disease globally, affecting one in four people. Given its prevalence, MAFLD is an important cause of cirrhosis, even though only a small proportion of patients with MAFLD ultimately progress to cirrhosis. MAFLD suffers as a clinical entity due to its insidious and often asymptomatic onset, lack of an accurate and reliable non-invasive diagnostic test, and lack of a bespoke therapy that has been designed and approved for use specifically in MAFLD. MAFLD sits at a crossroads between the gut and the periphery. The development of MAFLD (including activation of the inflammatory cascade) is influenced by gut-related factors that include the gut microbiota and intactness of the gut mucosal wall. The gut microbiota may interact directly with the liver parenchyma (through translocation via the portal vein), or indirectly through the release of metabolic metabolites that include secondary bile acids, trimethylamine, and short-chain fatty acids (such as propionate and acetate). In turn, the liver mediates the metabolic status of peripheral tissues (including insulin sensitivity) through a complex interplay of hepatokines, liver-secreted metabolites, and liver-derived micro RNAs. As such, the liver plays a key central role in influencing overall metabolic status. In this concise review, we provide an overview of the complex mechanisms whereby MAFLD influences the development of insulin resistance within the periphery, and gut-related factors impact on the development of MAFLD. We also discuss lifestyle strategies for optimising metabolic liver health.

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.

Altering retinol binding protein 4 levels in hepatitis C: Inflammation and steatosis matter

Both hepatitis C virus (HCV) infection and retinol-binding protein 4 (RBP4) might contribute to insulin resistance (IR), how RBP4 links to IR in HCV infection remain elusive. A joint study of a prospective cohort of 842 chronically HCV-infected (CHC) patients (with 842 controls) and a line of HCV core transgenic mice was conducted. Of 842 patients, 771 had completed anti-HCV therapy and 667 had sustained virological responses (SVRs). Compared with controls, CHC patients had lower RBP4 levels. At baseline, age (95% CI β: -0.87~-0.317), BMI (0.516~2.036), triglycerides (0.03~0.127), neutrophil-to-lymphocyte ratio (NLR) (1.561~7.327), and estimated glomerular filtration rate (eGFR) (-0.342~-0.149) levels were associated with RBP4 levels in CHC patients. At 24-week post-therapy, male sex (0.652~8.129), BMI (0.199~1.254), triglycerides (0.039~0.088), uric acid (0.599~3.067), eGFR (-0.247 ~-0.14) levels, and fibrosis-4 (-3.602~-0.039) scores were associated with RBP4 levels in SVR patients; compared with baseline, except genotype 3 HCV-infected patients, SVR patients had increased RBP4 levels, which were comparable with controls, while no HOMA-IR index alteration was noted after SVR. The HCV core transgenic mice exhibited nonobese hepatic steatosis, had higher hepatic RBP4 expression, higher serum levels of RBP4 and triglycerides, but comparable HOMA-IR levels than non-transgenic littermates. In conclusion, steatosis, sex, age, uric acid, NLR, and FIB-4 levels were associated with HCV-related RBP4 levels; BMI, triglycerides, and eGFR levels were associated with non-HCV-related RBP4 levels. Reversal of low RBP4 levels after SVR was evident in non-genotype 3 HCV-infected patients. Steatosis and inflammation linked with metabolic alteration other than IR, determined RBP4 levels in HCV-infected patients.

The weak correlation between serum vitamin levels and chronic kidney disease in hospitalized patients: a cross-sectional study

Background

Chronic kidney disease (CKD) has become a global public health problem. Accumulating evidence suggested that vitamins play important roles in the progression of CKD.

Methods

A cross-sectional study was conducted to investigate the vitamin status of patients with CKD at stage 1–5. The serum concentrations of 9 vitamins, vitamin A, B1, B2, B6, B9, B12, C, D, and E were measured by electroanalytical method with a Multi-Vitamin Analyzer. Pearson correlation and multiple linear regression between serum level of vitamins were analyzed.

Results

The median levels of vitamin A, B1, B2, B6, B9, B12, C and E were within the reference ranges or on the borderline. Vitamin D deficiency was found in all patients. Weak correlation was found between vitamin A or vitamin D and estimated glomerular filtration rate (eGFR). The Pearson correlation coefficient were − 0.21766 and 0.19752, respectively. Hypertension, diabetes mellitus, and atherosclerosis were the major comorbidities.

Conclusions

For the first time, the serum levels of 9 vitamins were measured simultaneously in patients with CKD at different stages. Vitamin D deficiency was found in all patients. Weak correlation between vitamin A or vitamin D and eGFR was found.

Alteration of circadian machinery in monocytes underlies chronic kidney disease-associated cardiac inflammation and fibrosis

Dysfunction of the circadian clock has been implicated in the pathogenesis of cardiovascular disease. The CLOCK protein is a core molecular component of the circadian oscillator, so that mice with a mutated Clock gene (Clk/Clk) exhibit abnormal rhythms in numerous physiological processes. However, here we report that chronic kidney disease (CKD)-induced cardiac inflammation and fibrosis are attenuated in Clk/Clk mice even though they have high blood pressure and increased serum angiotensin II levels. A search for the underlying cause of the attenuation of heart disorder in Clk/Clk mice with 5/6 nephrectomy (5/6Nx) led to identification of the monocytic expression of G protein-coupled receptor 68 (GPR68) as a risk factor of CKD-induced inflammation and fibrosis of heart. 5/6Nx induces the expression of GPR68 in circulating monocytes via altered CLOCK activation by increasing serum levels of retinol and its binding protein (RBP4). The high-GPR68-expressing monocytes have increased potential for producing inflammatory cytokines, and their cardiac infiltration under CKD conditions exacerbates inflammation and fibrosis of heart. Serum retinol and RBP4 levels in CKD patients are also sufficient to induce the expression of GPR68 in human monocytes. Our present study reveals an uncovered role of monocytic clock genes in CKD-induced heart failure.

Disparate roles of retinoid acid signaling molecules in kidney disease

Retinoid acid (RA) is synthesized mainly in the liver and has multiple functions in development, cell differentiation and proliferation, and regulation of inflammation. RA has been used to treat multiple diseases, such as cancer and skin disorders. The kidney is a major organ for RA metabolism, which is altered in the diseased condition. RA is known to have renal-protective effects in multiple animal models of kidney disease. RA has been shown to ameliorate podocyte injury through induction of expression of differentiation markers and regeneration of podocytes from its progenitor cells in animal models of kidney disease. The effects of RA in podocytes are mediated mainly by activation of the cAMP/PKA pathway via RA receptor-α (RARα) and activation of its downstream transcription factor, Kruppel-like factor 15. Screening of RA signaling molecules in human kidney disease has revealed RAR responder protein 1 (RARRES1) as a risk gene for glomerular disease progression. RARRES1, a podocyte-specific growth arrest gene, is regulated by high doses of both RA and TNF-α. Mechanistically, RARRES1 is cleaved by matrix metalloproteinases to generate soluble RARRES1, which then induces podocyte apoptosis through interaction with intracellular RIO kinase 1. Therefore, a high dose of RA may induce podocyte toxicity through upregulation of RARRES1. Based on the current findings, to avoid potential side effects, we propose three strategies to develop future therapies of RA for glomerular disease: 1) develop RARα- and Kruppel-like factor 15-specific agonists, 2) use the combination of a low dose of RAR-α agonist with phosphodiesterase 4 inhibitors, and 3) use a combination of RARα agonist with RARRES1 inhibitors.NEW & NOTEWORTHY Retinoic acid (RA) exerts pleotropic cellular effects, including induction of cell differentiation while inhibiting proliferation and inflammation. These effects are mediated by both RA responsive element-dependent or -independent pathways. In kidneys, RA confers renoprotection by signaling through podocyte RA receptor (RAR)α and activation of cAMP/PKA/Kruppel-like factor 15 pathway to promote podocyte differentiation. Nevertheless, in kidney disease settings, RA can also promote podocyte apoptosis and loss through downstream expression of RAR responder protein 1, a recently described risk factor for glomerular disease progression. These disparate roles of RA underscore the complexity of its effects in kidney homeostasis and disease, and a need to target specific RA-mediated pathways for effective therapeutic treatments against kidney disease progression.

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.

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.

The molecular aspects of absorption and metabolism of carotenoids and retinoids in vertebrates

Vitamin A is an essential nutrient necessary for numerous basic physiological functions, including reproduction and development, immune cell differentiation and communication, as well as the perception of light. To evade the dire consequences of vitamin A deficiency, vertebrates have evolved specialized metabolic pathways that enable the absorption, transport, and storage of vitamin A acquired from dietary sources as preformed retinoids or provitamin A carotenoids. This evolutionary advantage requires a complex interplay between numerous specialized retinoid-transport proteins, receptors, and enzymes. Recent advances in molecular and structural biology resulted in a rapid expansion of our understanding of these processes at the molecular level. This progress opened new avenues for the therapeutic manipulation of retinoid homeostasis. In this review, we summarize current research related to the biochemistry of carotenoid and retinoid-processing proteins with special emphasis on the structural aspects of their physiological actions. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Retinol, Retinoic Acid, and Retinol-Binding Protein 4 are Differentially Associated with Cardiovascular Disease, Type 2 Diabetes, and Obesity: An Overview of Human Studies

Vitamin A is a fat-soluble essential nutrient obtained from plant- and animal-based sources that has roles in growth, vision, and metabolism. Vitamin A circulates mainly as retinol bound to retinol-binding protein 4 (RBP4), and is delivered to tissues and converted to retinoic acid, which is a ligand for several nuclear receptors. In recent years, aspects of vitamin A metabolism have been under scrutiny with regards to the development of metabolic and lifestyle diseases including cardiovascular disease (CVD), type 2 diabetes mellitus (T2DM), and overweight and obesity in humans. Studies have mainly focused on RBP4 in this context, whereas the major circulating form, retinol, and the major bioactive form, retinoic acid, have been overlooked in this regard until recently. As one of the main roles of RBP4 is to deliver retinol to tissues for biological action, the associations of retinol and retinoic acid with these diseases must also be considered. In this review, we summarize and discuss recent and available evidence from human studies with focus on retinol, retinoic acid, and RBP4 and provide an overview of these crucial components of vitamin A metabolism in CVD, T2DM, and obesity. In summary, retinol was found to be both inversely and positively associated with CVD whereas the associations with T2DM and obesity were less clear. Although only a few studies have been published on retinoic acid, it was inversely associated with CVD. In contrast, serum RBP4 was mostly found to be positively associated with CVD, T2DM, and obesity. At present, it is difficult to ascertain why the reported associations differ depending on the compound under study, but there is a clear imbalance in the literature in disfavor of retinol and retinoic acid, which needs to be considered in future human studies.

Transcriptome profiling of lncRNA related to fat tissues of Qinchuan cattle

Qinchuan cattle is one of the five yellow cattle breeds in China with good performance of meat. The proliferation and differentiation level of muscle and fat are closely related to the growth and development of the organism and are the key factors affecting the quality of meat. In order to study the effect of lncRNA on the fat tissues of Qinchuan cattle, six calf and adult bovine adipose tissues were selected for high-throughput sequencing. We obtained 3,716 lncRNA candidates from calves and adult cattle fat samples, among them 789 lncRNA were annotated and 2,927 lncRNA were novel lncRNA. A number of lncRNAs were highly abundant, and 119 lncRNA were differentially expressed between two developmental stages. We further validated several differentially expressed lncRNAs using qPCR, and the results were consistent with the sequencing data. Therefore, we conclude that lncRNA may play an important role in adipose tissue in different age groups of cattle.

Retinoids in health and disease: A role for hepatic stellate cells in affecting retinoid levels

Vitamin A (retinol) is important for normal growth, vision and reproduction. It has a role in the immune response and the development of metabolic syndrome. Most of the retinol present in the body is stored as retinyl esters within lipid droplets in hepatic stellate cells (HSCs). In case of liver damage, HSCs release large amounts of stored retinol, which is partially converted to retinoic acid (RA). This surge of RA can mediate the immune response and enhance the regeneration of the liver. If the damage persists activated HSCs change into myofibroblast-like cells producing extracellular matrix, which increases the chance of tumorigenesis to occur. RA has been shown to decrease proliferation and metastasis of hepatocellular carcinoma. The levels of RA and RA signaling are influenced by the possibility to esterify retinol towards retinyl esters. This suggests a complex regulation between different retinoids, with an important regulatory role for HSCs.

Animal Models and Renal Biomarkers of Diabetic Nephropathy

Diabetes mellitus (DM) is the first cause of end stage chronic kidney disease (CKD). Animal models of the disease can shed light on the pathogenesis of the diabetic nephropathy (DN) and novel and earlier biomarkers of the condition may help to improve diagnosis and prognosis. This review summarizes the most important features of animal models used in the study of DN and updates the most recent progress in biomarker research.

Comparison of Kidney Transcriptomic Profiles of Early and Advanced Diabetic Nephropathy Reveals Potential New Mechanisms for Disease Progression

To identify the factors mediating the progression of diabetic nephropathy (DN), we performed RNA sequencing of kidney biopsy samples from patients with early DN, advanced DN, and normal kidney tissue from nephrectomy samples. A set of genes that were upregulated at early but downregulated in late DN were shown to be largely renoprotective, which included genes in the retinoic acid pathway and glucagon-like peptide 1 receptor. Another group of genes that were downregulated at early but highly upregulated in advanced DN consisted mostly of genes associated with kidney disease pathogenesis, such as those related to immune response and fibrosis. Correlation with estimated glomerular filtration rate (eGFR) identified genes in the pathways of iron transport and cell differentiation to be positively associated with eGFR, while those in the immune response and fibrosis pathways were negatively associated. Correlation with various histopathological features also identified the association with the distinct gene ontological pathways. Deconvolution analysis of the RNA sequencing data set indicated a significant increase in monocytes, fibroblasts, and myofibroblasts in advanced DN kidneys. Our study thus provides potential molecular mechanisms for DN progression and association of differential gene expression with the functional and structural changes observed in patients with early and advanced DN.

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).

Global Transcriptome Analysis During Adipogenic Differentiation and Involvement of Transthyretin Gene in Adipogenesis in Cattle

Adipose tissue plays central role in determining the gustatory quality of beef, but traditional Chinese beef cattle have low levels of fat content. We applied RNA-seq to study the molecular mechanisms underlying adipocyte differentiation in Qinchuan cattle. A total of 18,283 genes were found to be expressed in preadipocytes and mature adipocytes, respectively. 470 of which were significantly differentially expressed genes (DEGs) [false discovery rate (FDR) values < 0.05 and fold change ≥ 2]. In addition, 4534 alternative splicing (AS) events and 5153 AS events were detected in preadipocytes and adipocytes, respectively. We constructed a protein interaction network, which suggested that collagen plays an important role during bovine adipogenic differentiation. We characterized the function of the most down-regulated DEG (P < 0.001) among genes we have detected by qPCR, namely, the transthyretin (TTR) gene. Overexpression of TTR appears to promote the expression of the peroxisome proliferator activated receptor γ (PPARγ) (P < 0.05) and fatty acid binding Protein 4 (FABP4) (P < 0.05). Hence, TTR appears to be involved in the regulation of bovine adipogenic differentiation. Our study represents the comprehensive approach to explore bovine adipocyte differentiation using transcriptomic data and reports an involvement of TTR during bovine adipogenic differentiation. Our results provide novel insights into the molecular mechanisms underlying bovine adipogenic differentiation.

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.

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.

Liver-secreted RBP4 does not impair glucose homeostasis in mice

Retinol-binding protein 4 (RBP4) is the major transport protein for retinol in blood. Recent evidence from genetic mouse models shows that circulating RBP4 derives exclusively from hepatocytes. Because RBP4 is elevated in obesity and associates with the development of glucose intolerance and insulin resistance, we tested whether a liver-specific overexpression of RBP4 in mice impairs glucose homeostasis. We used adeno-associated viruses (AAV) that contain a highly liver-specific promoter to drive expression of murine RBP4 in livers of adult mice. The resulting increase in serum RBP4 levels in these mice was comparable with elevated levels that were reported in obesity. Surprisingly, we found that increasing circulating RBP4 had no effect on glucose homeostasis. Also during a high-fat diet challenge, elevated levels of RBP4 in the circulation failed to aggravate the worsening of systemic parameters of glucose and energy homeostasis. These findings show that liver-secreted RBP4 does not impair glucose homeostasis. We conclude that a modest increase of its circulating levels in mice, as observed in the obese, insulin-resistant state, is unlikely to be a causative factor for impaired glucose homeostasis.

Plasma Level of Retinol-Binding Protein 4, N-Terminal proBNP and Renal Function in Older Patients Hospitalized for Heart Failure

BACKGROUND/AIM

Elevated plasma concentration of retinol-binding protein 4 (RBP4) has recently emerged as a potential new risk factor for cardiovascular diseases, including hypertension (HT) and coronary artery disease (CAD). Limited data suggest that RBP4 promotes inflammatory damage to cardiomyocytes and participates in the development of heart failure (HF). This study aimed to analyze the relationship between concentrations of plasma RBP4 and serum N-terminal proBNP (NT-proBNP), a powerful biomarker of left ventricle dysfunction, in the older Polish population.

METHODS

The study sample consisted of 2,826 (1,487 men) participants of the PolSenior study, aged 65 years and older, including a subgroup hospitalized for HF (n = 282). In all subjects, plasma concentrations of RBP4, interleukin-6 (IL-6), serum level of NT-proBNP, and hs-CRP were measured. Additionally, BMI, estimated glomerular filtration rate (eGFR), and HOMA-IR were calculated. The prevalence of HT, CAD, atrial fibrillation (AF), and medication were considered as potential confounders.

RESULTS

Similar RBP4 levels were found in subjects with NT-proBNP < 125 and ≥125 ng/mL, with and without AF, and in the subgroups hospitalized for HF with and without AF. Regression analysis revealed no association between log10(NT-proBNP) and log10(RBP4). Plasma levels of RBP4 were increased by HT occurrence and diuretic therapy, while diminished with regard to female gender, age, eGFR values, AF, and IL-6 levels.

CONCLUSION

Our results show that RBP4 is affected by GFR but cannot be considered as an independent biomarker of heart muscle dysfunction.

Body fat mass is correlated with serum transthyretin levels in maintenance hemodialysis patients

Serum transthyretin (TTR), also known as prealbumin, is a reliable nutritional indicator and an independent prognostic factor for maintenance hemodialysis patients. However, we recently reported that serum TTR levels did not affect protein-energy wasting (PEW). In this study, we investigated factors affecting serum TTR levels in 60 maintenance hemodialysis patients. The patients were divided into High-TTR and Low-TTR groups according to the median serum TTR level. Albumin levels were significantly higher and C-reactive protein (CRP) levels were significantly lower in the High-TTR group than in the Low-TTR group. Although body fat mass was significantly higher in the High-TTR group than in the Low-TTR group, no significant difference in body fat ratio were observed. These findings suggest that body fat mass is related to serum TTR levels, apart from factors such as albumin and CRP levels, which showed correlations with serum TTR levels. Because body fat mass is related to better survival in maintenance hemodialysis patients, it may contribute to the prognostic value of serum TTR levels. In addition, in such patients, it may be important to evaluate body fat mass rather than body fat ratio and to maintain the minimum necessary body fat mass. J. Med. Invest. 64: 222-227, August, 2017.

Childhood retinol-binding protein 4 (RBP4) levels predicting the 10-year risk of insulin resistance and metabolic syndrome: the BCAMS study

Background

Elevated retinol-binding protein 4 (RBP4) levels may contribute to the development of metabolic abnormalities, but prospective studies evaluating the association between childhood RBP4 levels and metabolic syndrome (MS) in adulthood are lacking. We investigated whether RBP4 levels during childhood predict cardiometabolic risk at 10-year follow-up.

Methods

The relationships between RBP4 levels, the established adipokines (leptin and adiponectin) and the components of MS were examined in 3445 school-aged children recruited in 2004 for the Beijing Child and Adolescent Metabolic Syndrome study. In 2015, 352 of these individuals completed an in-depth follow-up examination.

Results

Participants with higher childhood RBP4 levels had adverse cardiometabolic profiles at follow-up. Those with incident or persistent MS had higher baseline RBP4 levels than those who never exhibited the elements of MS. Moreover, baseline RBP4 predicted hyperglycemia (OR per SD increase = 1.48, P = 0.009), elevated triglyceride (OR = 1.54, P < 0.001), elevated blood pressures (OR = 1.46, P = 0.015), MS (OR = 1.68, P = 0.002) and insulin resistance (OR = 1.44, P = 0.015) in the 10-year follow-up phase, independent of baseline BMI. Significant improvements were seen for the net reclassification improvement and integrated discrimination index after adding childhood RBP4 levels into the risk models using conventional cardiometabolic risk factors in predicting MS at follow-up (P < 0.05). Leptin and adiponectin demonstrated the expected associations with metabolic disorders.

Conclusions

Childhood RBP4 serves as a risk factor for subsequent development of MS and its components, independent of pediatric obesity. Incorporating childhood RBP4 into conventional cardiometabolic risk assessment models significantly improves the prediction of MS.

Electronic supplementary material

The online version of this article (10.1186/s12933-018-0707-y) contains supplementary material, which is available to authorized users.

The Promise of Systems Biology for Diabetic Kidney Disease

Diabetic kidney disease (DKD) has a complex and prolonged pathogenesis involving many cell types in the kidney as well as extrarenal factors. It is clinically silent for many years after the onset of diabetes and usually progresses over decades. Given this complexity, a comprehensive and unbiased molecular approach is best suited to help identify the most critical mechanisms responsible for progression of DKD and those most suited for targeted intervention. Systems biological investigations provide such an approach since they examine the entire network of molecular changes that occur in a disease process in a comprehensive way instead of focusing on a single abnormal molecule or pathway. Systems biological studies can also start with analysis of the disease in humans, not in animal or cell culture models that often poorly reproduce the changes in human DKD. Indeed, in the last decade, systems biological approaches have led to the identification of critical molecular abnormalities in DKD and have directly led to development of new biomarkers and potential treatments for DKD.

Disturbed Vitamin A Metabolism in Non-Alcoholic Fatty Liver Disease (NAFLD)

Vitamin A is required for important physiological processes, including embryogenesis, vision, cell proliferation and differentiation, immune regulation, and glucose and lipid metabolism. Many of vitamin A’s functions are executed through retinoic acids that activate transcriptional networks controlled by retinoic acid receptors (RARs) and retinoid X receptors (RXRs).The liver plays a central role in vitamin A metabolism: (1) it produces bile supporting efficient intestinal absorption of fat-soluble nutrients like vitamin A; (2) it produces retinol binding protein 4 (RBP4) that distributes vitamin A, as retinol, to peripheral tissues; and (3) it harbors the largest body supply of vitamin A, mostly as retinyl esters, in hepatic stellate cells (HSCs). In times of inadequate dietary intake, the liver maintains stable circulating retinol levels of approximately 2 μmol/L, sufficient to provide the body with this vitamin for months. Liver diseases, in particular those leading to fibrosis and cirrhosis, are associated with impaired vitamin A homeostasis and may lead to vitamin A deficiency. Liver injury triggers HSCs to transdifferentiate to myofibroblasts that produce excessive amounts of extracellular matrix, leading to fibrosis. HSCs lose the retinyl ester stores in this process, ultimately leading to vitamin A deficiency. Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome and is a spectrum of conditions ranging from benign hepatic steatosis to non-alcoholic steatohepatitis (NASH); it may progress to cirrhosis and liver cancer. NASH is projected to be the main cause of liver failure in the near future. Retinoic acids are key regulators of glucose and lipid metabolism in the liver and adipose tissue, but it is unknown whether impaired vitamin A homeostasis contributes to or suppresses the development of NAFLD. A genetic variant of patatin-like phospholipase domain-containing 3 (PNPLA3-I148M) is the most prominent heritable factor associated with NAFLD. Interestingly, PNPLA3 harbors retinyl ester hydrolase activity and PNPLA3-I148M is associated with low serum retinol level, but enhanced retinyl esters in the liver of NAFLD patients. Low circulating retinol in NAFLD may therefore not reflect true “vitamin A deficiency”, but rather disturbed vitamin A metabolism. Here, we summarize current knowledge about vitamin A metabolism in NAFLD and its putative role in the progression of liver disease, as well as the therapeutic potential of vitamin A metabolites.

Circulating retinol binding protein 4 levels in nonalcoholic fatty liver disease: a systematic review and meta-analysis

Background

Retinol binding protein 4 (RBP4) is implicated in obesity, insulin resistance and type 2 diabetes mellitus that are closely associated with nonalcoholic fatty liver disease (NAFLD). However, recent investigations regarding circulating RBP4 levels in NAFLD are conflicting. This meta-analysis is to determine whether NAFLD, non-alcoholic steatohepatitis (NASH) and simple steatosis (SS) patients have altered RBP4 levels.

Methods

We performed a systematic search in PubMed, EMBASE and The Cochrane Library up until 18 March 2017, and 12 studies comprising a total of 4247 participants (2271 NAFLD patients and 1976 controls) were included in the meta-analysis.

Results

There were no significant differences of circulating RBP4 levels in the following comparisons: (1) NAFLD patients vs controls (standardized mean differences [SMD]: 0.08; 95% CI: −0.21, 0.38); (2) NASH patients vs controls (SMD: −0.49; 95% CI: −1.09, 0.12); (3) SS patients vs controls (SMD: −0.72; 95% CI: −1.64, 0.20) and (4) NASH vs SS patients (SMD: −0.04; 95% CI: −0.32, 0.24). The results remained essentially unchanged in the comparisons between NAFLD patients and controls after excluding single individual study or bariatric studies (n = 2). No significant publication bias was detected. However, there was significant heterogeneity among studies and the subgroup and meta-regression analyses did not find the potential sources.

Conclusions

Circulating RBP4 levels may not be associated with NAFLD. Further prospective cohort studies are required to confirm these findings.

Electronic supplementary material

The online version of this article (10.1186/s12944-017-0566-7) contains supplementary material, which is available to authorized users.

Serum RBP4 and CKD: Association with insulin resistance and lipids

OBJECTIVE

Serum RBP4 is new adipokine and it has been related to insulin resistance and diabetes risk in animal and clinical studies. However, there is controversy on this relationship among CKD patients. In this study, we evaluated the association of serum RBP4 with insulin resistance and cardiovascular risk factors in CKD.

METHODS

Baseline data from the PROGREDIR Study (Sao Paulo, Brazil) comprising 454 participants (mainly stages 3 and 4) was analyzed.

RESULTS

In univariable analysis, RBP4 was inversely related to renal function, age and HDL, and positively related to other lipids, insulinemia, HOMA, glycemia, albumin, phosphorus and right hepatic lobe diameter. After adjustment for sex, age and eGFR, HOMA and lipids remained associated to RBP4. In multivariable analysis, eGFR and triglyceride remained significantly associated with RBP4, while HOMA showed no longer a significant positive association. An interaction term between RBP4 and eGFR was significantly related to HOMA.

CONCLUSIONS

Renal function is inversely related to serum RBP4. As GFR decreases, the relationship between RBP4 and HOMA is attenuated. On the other hand, triglycerides remained strongly related to RBP4 and this was not affected by eGFR, suggesting that in the CKD population triglycerides may be a better marker of RBP4-associated metabolic effects.

Retinol-binding protein-4 expression marks the short-term mortality of critically ill patients with underlying liver disease: Lipid, but not glucose, matters

The implications of retinol-binding protein-4 (RBP4) expression in critically ill patients with underlying liver diseases remain unclear. A prospective cohort study involving 200 liver intensive care unit (ICU) patients was conducted, with 274 blood donors as controls. Patient outcomes were assessed using Cox and Kaplan-Meier analyses. Of the 200 ICU patients (mean age: 56.0 yrs), 79.5% were male, 72.5% were cirrhotic, 62% were septic, 29.5% were diabetic, and 29% expired in the ICU (median admission: 7.5 days). ICU patients had lower baseline RBP4 (25.6+/−18.4 vs. 43.8+/−35.0 mg/L, p < 0.001) and total cholesterol (TC) levels than controls. The surviving ICU patients had lower baseline international normalized ratios (INRs) of prothrombin time, model for end-stage liver disease (MELD) scores and sepsis rates, but higher estimated glomerular filtration rates (eGFRs) and RBP4 levels than non-surviving patients. eGFRs, INRs and TC levels were independently associated with RBP4 levels. Only surviving patients exhibited significantly increased RBP4 levels after ICU discharge. Baseline RBP4 levels and MELD scores predicted 21-day (≤10 mg/L) and 1-year (≥25) mortality, respectively. In critically ill patients with underlying liver disease, with a link to eGFRs, INRs and TC levels, the baseline RBP4 may serve as a marker for short-term mortality.

Increased retinol-free RBP4 contributes to insulin resistance in gestational diabetes mellitus

PURPOSE

Retinol-binding protein 4 (RBP4) is a circulating retinol transporter that is strongly associated with insulin resistance. The aim of this study was to evaluate the RBP4 and retinol level in rat model of gestational diabetes mellitus and the relationship between retinol-free RBP4 (apo-RBP4), retinol-bound RBP4 (holo-RBP4) and insulin resistance.

METHODS

Pregnant rats were administered streptozotocin to induce diabetes. The RBP4 and retinol levels were evaluated in GDM and normal pregnant rats. After then, normal pregnant rats were divided into two groups to receive either apo-RBP4 or vehicle injection. The metabolic parameters and insulin signaling in adipose tissue, skeletal muscle and liver were determined in apo-RBP4 and control groups. Primary human adipocytes were cultured in vitro with different proportions of apo-RBP4 and holo-RBP4 for 24 h. The interaction between RBP4 and STRA6 was assessed by co-immunoprecipitation, and the expression of JAK-STAT pathway and insulin signaling were detected by Western blotting and immunofluorescence.

RESULTS

We found increases in serum RBP4 levels and the RBP4:retinol ratio but not in the retinol levels in GDM rats. Exogenous apo-RBP4 injection attenuated insulin sensitivity in pregnant rats. In vitro, a prolonged interaction between RBP4 and STRA6 was observed when apo-RBP4 was present. In response to increased apo-RBP4 levels, cells showed elevated activation of the JAK2/STAT5 cascade and SOCS3 expression, decreased phosphorylation of IR and IRS1, and attenuated GLUT4 translocation and glucose uptake upon insulin stimulation.

CONCLUSION

Apo-RBP4 is a ligand that activates the STRA6 signaling cascade, inducing insulin resistance in GDM.

Retinol Binding Protein-4 Levels and Non-alcoholic Fatty Liver Disease: A community-based cross-sectional study

Previous reports on the association between retinol binding protein 4 (RBP4) and nonalcoholic fatty liver disease (NAFLD) were controversial. This study aimed to investigate the association between the serum RBP4 levels and occurrence of NAFLD in Chinese population. In total, 2938 participants aged 40–75 years were involved in this community-based cross-sectional study. General information, lifestyle factors, serum levels of RBP4 and the presence of NAFLD were determined. Patients with NAFLD had significantly higher concentrations of RBP4 (37.9 ± 6.8 μg/ml) than did non-NAFLD controls (35.0 ± 6.7 μg/ml) (P < 0.001). The odds ratios (ORs) of NAFLD for the highest (vs. lowest) quartile of RBP4 were 1.884 (95% CI: 1.391, 2.551) for females (P < 0.001), and 2.107 (95% CI: 1.357, 3.273) for male participants (P < 0.01) after adjusting for related factors. The serum RBP4 levels were positively associated with the prevalence of NAFLD in middle-aged and elderly Chinese people, and Homeostatic model assessment-insulin resistance (HOMA-IR), trunk fat, the waist-to-hip ratio (WHR), systolic blood pressure (SBP), fasting insulin, high density lipoprotein cholesterol (HDL-C) and triglycerides (TG) might be implicated in the pathogenesis of RBP4 in NAFLD.

RBP4: A Culprit for Insulin Resistance in End Stage Renal Disease That Can Be Cleared by Hemodiafiltration

INTRODUCTION

Retinol Binding Protein 4 (RBP4) is mainly excreted by the kidney and plays a pivotal role in insulin resistance (IR). In our study, we evaluated the association between RBP4 and IR in hemodialysis subjects (HD). We also assessed how circulating RBP4 could be influenced by kidney transplant or different dialytic techniques.

METHODS

RBP4 serum levels were evaluated in HD (n = 16) and matched healthy controls (C; n = 16). RBP4 and glucose transporter type 4 (GLUT4) mRNA expressions were also determined in adipose tissue. Circulating RBP4 was evaluated after kidney transplant (n = 7) and in hemodialysis patients (n = 10) enrolled in a cross-over study treated with standard bicarbonate dialysis (BD) or hemodiafiltration (HDF).

RESULTS

HOMA index (P < 0.05) and serum RBP4 (P < 0.005) were higher in HD compared to C. RBP4 levels positively correlated with fasting serum glucose (P < 0.05). RBP4 mRNA was lower in HD compared to C (P < 0.05) and positively correlated with kidney function (P < 0.05) and GLUT4 mRNA (P < 0.001). Transplant or HDF reduced circulating RBP4 (P < 0.01 and P < 0.05, resp.). Our results demonstrate that IR is associated with high circulating RBP4 and that suppressed RBP4 adipose tissue expression is accompanied by reduced GLUT4 expression in HD. Renal transplantation or HDF are effective in lowering serum RBP4 levels.

Chronic Kidney Disease Alters Vitamin A Homeostasis via Effects on Hepatic RBP4 Protein Expression and Metabolic Enzymes

Vitamin A, via retinoic acid (RA), is a critical micronutrient. Normally, plasma concentrations are tightly regulated. Concentrations of vitamin A metabolites (13cis-RA, atRA) and relationships between RBP4 and retinoids have never been fully evaluated in adult patients with CKD. We measured retinoid and RBP4 concentrations in plasma and urine from 55 adult patients with CKD and 21 matched healthy subjects. RBP4 and retinol levels were increased approximately twofold in patients with CKD, with a negative correlation between plasma retinol and eGFR (p = 0.006) and plasma RBP4 and eGFR (p = 0.0007). RBP4 renal clearance was higher in patients with CKD than healthy subjects but not associated with eGFR. Circulating concentrations of atRA increased and concentrations of 13cis-RA decreased in subjects with CKD with no change in RA-to-retinol ratio. Increases in circulating retinol, RBP4, and atRA may be due to increased hepatic RBP4 synthesis, retinyl ester hydrolysis, and/or hepatic secretion of RBP4-retinol.

Is retinol binding protein 4 a link between adiposity and cancer?

Retinol binding protein 4 (RBP4) is synthesized in the liver where it binds vitamin A, retinol, and transports it to tissues throughout the body. It has been shown in some studies that the level of circulating RBP4 increases with body mass, and the protein has been implicated as a mediator in the development of insulin resistance and the metabolic disease. Adipose tissue serves as another site of RBP4 synthesis, accounting for its designation as an adipokine. In addition to its function as a transport protein, RBP4 serves as a signaling molecule which, by binding to the membrane receptor STRA6, triggers downstream activation of pro-oncogenic pathways including JAK2/STAT3/5. Taken together, available information suggests the possibility that RBP4 may be a link between obesity and cancer.

Assessment of Neutrophil Gelatinase-Associated Lipocalin (NGAL) and Retinol-Binding Protein 4 (RBP4) in Type 2 Diabetic Patients with Nephropathy

Diabetic nephropathy (DN) is one of the most serious microvascular complications of diabetes. The study aims to evaluate the diagnostic value of serum neutrophil gelatinase-associated lipocalin (NGAL) and retinol-binding protein 4 (RBP4) as biomarkers for early detection of nephropathy in type 2 diabetic patients. The current study was performed on 150 type 2 diabetic patients. These patients were classified into three equal groups according to their albumin/creatinine ratio (ACR), including patients with normoalbuminuria (ACR <30 mg/g creatinine), patients with microalbuminuria (ACR = 30-300 mg/g creatinine), and patients with macroalbuminuria (ACR >300 mg/g creatinine). Fifty apparently healthy subjects matching the same age and socioeconomic status with diabetic subjects were selected as a control group. The plasma glucose, insulin, glycosylated hemoglobin (HbA1c), homeostasis model assessment of insulin resistance (HOMA-IR), lipid profile, urea, creatinine, cystatin C, glomerular filtration rate (GFR), NGAL, and RBP4 were measured in the studied groups. Significantly elevated NGAL and RBP4 levels were observed in micro- and macroalbuminuric diabetic groups when compared to the control and normoalbuminuric diabetic groups. NGAL and RBP4 were found to correlate positively with duration of diabetes, systolic and diastolic blood pressure, glucose, HbA1c, HOMA-IR, triacylglycerol, and ACR, but correlate inversely with GFR in DN groups. Receiver operating characteristic curves revealed that for early detection of DN, the best cutoff values to discriminate DN and diabetic without nephropathy groups were 91.5 ng/mL for NGAL with 87% sensitivity, 74% specificity, and area under the curve (AUC) = 0.881; 24.5 ng/mL for RBP4 with 84% sensitivity, 90% specificity, and AUC = 0.912; and 37.5 mg/g creatinine for ACR with 89% sensitivity, 72% specificity, and AUC = 0.819. RBP4 is more specific (90% specificity) than NGAL (74% specificity) and ACR (72% specificity). Therefore, RBP4 marker may serve as a tool to follow-up clinical monitoring of the development and progression of DN.

Retinol binding protein 4 and its membrane receptors: a metabolic perspective

Nearly a decade of intense research has passed since the first report linking circulating retinol binding protein 4 (RBP4) to the development of insulin resistance. By now, a variety of underlying mechanisms have been identified; some of them are adherent to the canonical role of this circulating protein, which is to transport and deliver retinol to target tissues, and others that seem rather independent of retinol transport. Despite all these efforts, a consensus in the basic principles of RBP4's metabolic effects has not been reached and some controversy remains. Using this as an opportunity, we here review and discuss current data on RBP4's action on insulin sensitivity and its dependency on retinol homeostasis. We pay special attention to the involvement of RBP4 membrane receptors that were identified during these years, such as 'stimulated by retinoic acid 6' (STRA6), and whose identification added another layer of complexity to RBP4's diverse actions. A better understanding of RBP4's functions might allow its therapeutic exploitations, urgently needed in our period that is defined by an epidemic increase in metabolic diseases such as obesity and type 2 diabetes.

Reproducibility of Retinol Binding Protein 4 and Omentin-1 Measurements over a Four Months Period: A Reliability Study in a Cohort of 207 Apparently Healthy Participants

The reliability of single time point measurements of the novel adipokines retinol-binding protein 4 and omentin-1 in the blood has not been evaluated in large samples yet. The present study aimed to assess the amount of biological variation of these two adipokines within individuals. The study sample comprised 207 participants (124 women and 83 men) from Potsdam (Germany) and surrounding areas, with an average age of 56.5 years (SD 4.2). Blood samples were collected from each participant twice, approximately four months apart. Using enzyme linked immunosorbent assays, the concentrations of retinol-binding protein 4 and omentin-1 were determined in EDTA plasma. As indicators of reliability, intraclass correlation coefficients (ICCs) were calculated from the repeated biomarker measurements. The ICCs for repeated retinol-binding protein 4 and omentin-1 measurements were 0.77 (95% CI 0.71, 0.82) and 0.83 (95% CI 0.78, 0.87), respectively, indicating for both adipokines excellent reliability. ICCs were stable across strata according to sex, age, BMI, and blood pressure. Thus, for epidemiological studies it seems reasonable to rely on concentrations of retinol-binding protein 4 and omentin-1 in samples from a single time point if repeated measurements are not available.

Vitamin A: potential misclassification of vitamin A status among patients with type 2 diabetes and hypertension in urban Ghana

BACKGROUND

Sub-Saharan Africa is facing a double burden of malnutrition: vitamin A deficiency (VAD) prevails, whereas the nutrition-related chronic conditions type 2 diabetes (T2D) and hypertension are emerging. Serum retinol—a VAD marker—increases in kidney disease and decreases in inflammation, which can partly be attributed to alterations in the vitamin A-transport proteins retinol-binding protein 4 (RBP4) and prealbumin. Kidney dysfunction and inflammation commonly accompany T2D and hypertension.

OBJECTIVE

Among urban Ghanaians, we investigated the associations of T2D and hypertension with serum retinol as well as the importance of kidney function and inflammation in this regard.

DESIGN

A hospital-based, case-control study in individuals for risk factors of T2D, hypertension, or both was conducted in Kumasi, Ghana (328 controls, 197 with T2D, 354 with hypertension, and 340 with T2D plus hypertension). In 1219 blood samples, serum retinol, RBP4, and prealbumin were measured. Urinary albumin and estimated glomerular filtration rate (eGFR) defined kidney function. C-reactive protein (CRP) >5 mg/L indicated inflammation. We identified associations of T2D and hypertension with retinol by linear regression and calculated the contribution of RBP4, prealbumin, urinary albumin, eGFR, and CRP to these associations as the percentages of the explained variance of retinol.

RESULTS

VAD (retinol <1.05 μmol/L) was present in 10% of this predominantly female, middle-aged, overweight, and deprived population. Hypertension, but not T2D, was positively associated with retinol (β: 0.12; 95% CI: 0.08, 0.17), adjusted for age, sex, socioeconomic factors, anthropometric measurements, and lifestyle. In addition to RBP4 (72%) and prealbumin (22%), the effect of increased retinol on individuals with hypertension was mainly attributed to impaired kidney function (eGFR: 30%; urinary albumin: 5%) but not to inflammation.

CONCLUSIONS

In patients with hypertension, VAD might be underestimated because of increased serum retinol in the context of kidney dysfunction. Thus, the interpretation of serum retinol in sub-Saharan Africa should account for hypertension status.

Central obesity, type 2 diabetes and insulin: exploring a pathway full of thorns

The prevalence of type 2 diabetes (T2D) is rapidly increasing. This is strongly related to the contemporary lifestyle changes that have resulted in increased rates of overweight individuals and obesity. Central (intra-abdominal) obesity is observed in the majority of patients with T2D. It is associated with insulin resistance, mainly at the level of skeletal muscle, adipose tissue and liver. The discovery of macrophage infiltration in the abdominal adipose tissue and the unbalanced production of adipocyte cytokines (adipokines) was an essential step towards novel research perspectives for a better understanding of the molecular mechanisms governing the development of insulin resistance. Furthermore, in an obese state, the increased cellular uptake of non-esterified fatty acids is exacerbated without any subsequent β-oxidation. This in turn contributes to the accumulation of intermediate lipid metabolites that cause defects in the insulin signaling pathway. This paper examines the possible cellular mechanisms that connect central obesity with defects in the insulin pathway. It discusses the discrepancies observed from studies organized in cell cultures, animal models and humans. Finally, it emphasizes the need for therapeutic strategies in order to achieve weight reduction in overweight and obese patients with T2D.

Serum levels of retinol binding protein 4 in women with different levels of adiposity and glucose tolerance

OBJECTIVE

Retinol-binding protein 4 (RBP4) is an adipokine responsible for vitamin A (retinol) transportation. Studies associated RBP4 increased levels with severity of type 2 diabetes mellitus (T2DM) and insulin resistance (IR). The study aimed to quantify RBP4 serum standards in women with a wide range of body mass index (BMI) and glucose tolerance level.

SUBJECTS AND METHODS

Cross-sectional study was performed with 139 women divided into three groups: Group 1 (lean-control, n = 45) and Group 2 (obese, n = 53) with normal glucose tolerance and group 3 (obese with T2DM, n = 41), called G1, G2 and G3. Were assessed clinical, biochemical, anthropometric and body composition parameters.

RESULTS

According to data analysis, we obtained in G1 higher RBP4 levels (104.8 ± 76.8 ng/mL) when compared to G2 (87.9 ± 38 ng/mL) and G3 (72.2 ± 15.6 ng/mL) levels. Also, were found: in G1 positive correlations of RBP4 with BMI (r = 0.253), glycated hemoglobin (r = 0.378) and fasting insulin (r = 0.336); in G2 with glycated hemoglobin (r = 0.489); in G3 with glycated hemoglobin (r = 0.330), fasting glucose (r = 0.463), HOMA-IR (r = 0.481).

CONCLUSIONS

Although RBP4 have shown lower levels in diabetic and obese, a strong correlation with HOMA-IR index highlights that, in our study, there is growing IR when there is an increasing in RBP4 levels.

Drug management in the elderly adult with chronic kidney disease: a review for the primary care physician

With advancing age, the functional reserve of many organs tends to decrease. In particular, the lean body mass, the levels of serum albumin, the blood flow to the liver, and the glomerular filtration rate are reduced in elderly individuals and can be further impaired by the concomitant presence of acute or chronic kidney disease. Moreover, patients with kidney disease are often affected by comorbid processes and are prescribed multiple medications. The aging process also modifies some drug interactions, including the affinity of some drugs for their receptor, the number of receptors, and the cell responses upon receptor activation. Therefore, older patients with kidney disease are particularly susceptible to the risks of adverse drug reactions. Planning a pharmacological regimen in such patients is confounded by the paucity of information available on the pharmacokinetic and pharmacodynamic profiles of a large number of drugs commonly used in this group of patients. Finally, many aged patients suffer from unintentional poor compliance. In this review, the problems physicians face in designing safe and effective medication management in elderly individuals are discussed, paying attention to those more frequently used, which may be potentially harmful in patients with kidney disease. The risks of overdosing and underdosing are outlined, and some recommendations to reduce the risk of adverse drug reactions are provided. A review of the literature covering the field of drug management in older patients with kidney disease was performed by selecting those articles published between January 1, 1990, and December 1, 2014, using PubMed as a search engine with the keywords elderly, kidney disease, drugs, drug interaction, and renal function.

Association of urinary RBP4 with insulin resistance, inflammation, and microalbuminuria

OBJECTIVE

Serum concentrations of retinol-binding protein 4 (RBP4) are elevated in type 2 diabetes and associated with the severity of insulin resistance; however, there are few data about the relationship between urinary RBP4 levels and metabolic parameters. We assessed urinary RBP4 as a new biomarker by establishing its relationship with clinical parameters associated with insulin resistance and urinary albumin excretion.

DESIGN AND METHODS

We measured RBP4 in the serum and urine of 689 subjects with diverse glucose tolerance status. We also evaluated the relationship between urinary RBP4 and cardiometabolic risk factors, including insulin resistance, high-sensitivity C-reactive protein (hsCRP), arterial stiffness, and microalbuminuria.

RESULTS

Urinary RBP4 levels were higher in insulin-resistant subjects with prediabetes or type 2 diabetes than in subjects with normal glucose tolerance (NGT) (type 2 diabetes>prediabetes>NGT; all P<0.001). Urinary RBP4 correlated strongly with homeostasis model assessments of insulin resistance (HOMA-IR), fasting glucose, triglycerides, blood pressure, hsCRP, arterial stiffness, estimated glomerular filtration rate, and urinary albumin-to-creatinine ratio (all P<0.01). HOMA-IR and arterial stiffness were found to be independent determinants of urinary RBP4 concentration. Furthermore, urinary RBP4 was highly predictive of microalbuminuria (odds ratio 2.6, 95% CI 1.6-4.2), even after adjustment for other metabolic parameters. The area under the ROC curve for urinary RBP4 to detect the presence of microalbuminuria was 0.80±0.02 (95% CI 0.76-0.84) and the cut-off value was 157.01 μg/gCr.

CONCLUSIONS

Urinary RBP4 concentrations were elevated in patients with dysregulation of glucose metabolism and were related to various cardiometabolic risk factors including insulin resistance, inflammation, and microalbuminuria.

Ectopic expression of RBP4 impairs the insulin pathway and inguinal fat deposition in mice

A large body of evidence has linked retinol-binding protein 4 (RBP4) to systemic insulin resistance, but little is known about its function in fat deposition. This study aimed to confirm the involvement of RBP4 in inguinal fat deposition and insulin by intraperitoneal injection of adenovirus-mediated RBP4 to mice. Intraperitoneal injection of adenoviral vectors was validated as an efficient gene manipulation tool for over-expressing recombinant proteins in vivo. Ectopic expression of RBP4 decelerated inguinal fat deposition by decreasing the size of adipocytes. Moreover, the introduction of exogenous RBP4 blunted the response of inguinal adipocytes to insulin signals. These findings suggest that RBP4 impaired in vivo adipogenesis, partly through the repression of the insulin pathway.

Retinol-binding protein 4 and its membrane receptor STRA6 control adipogenesis by regulating cellular retinoid homeostasis and retinoic acid receptor α activity

Retinoids are vitamin A (retinol) derivatives and complex regulators of adipogenesis by activating specific nuclear receptors, including the retinoic acid receptor (RAR) and retinoid X receptor (RXR). Circulating retinol-binding protein 4 (RBP4) and its membrane receptor STRA6 coordinate cellular retinol uptake. It is unknown whether retinol levels and the activity of RAR and RXR in adipocyte precursors are linked via RBP4/STRA6. Here, we show that STRA6 is expressed in precursor cells and, dictated by the apo- and holo-RBP4 isoforms, mediates bidirectional retinol transport that controls RARα activity and subsequent adipocyte differentiation. Mobilization of retinoid stores in mice by inducing RBP4 secretion from the liver activated RARα signaling in the precursor cell containing the stromal-vascular fraction of adipose tissue. Retinol-loaded holo-RBP4 blocked adipocyte differentiation of cultured precursors by activating RARα. Remarkably, retinol-free apo-RBP4 triggered retinol efflux that reduced cellular retinoids, RARα activity, and target gene expression and enhanced adipogenesis synergistically with ectopic STRA6. Thus, STRA6 in adipocyte precursor cells links nuclear RARα activity to the circulating RBP4 isoforms, whose ratio in obese mice was shifted toward limiting the adipogenic potential of their precursors. This novel cross talk identifies a retinol-dependent metabolic function of RBP4 that may have important implications for the treatment of obesity.

Retinol binding protein 4 and incident diabetes--the Atherosclerosis Risk in Communities Study (ARIC Study)

BACKGROUND

Retinol binding protein 4 (RBP4) has been described as a link between impaired glucose uptake in adipocytes and systemic insulin sensitivity.

OBJECTIVE

To determine whether RBP4 fasting levels predict the development of type 2 diabetes.

METHODS

Using a case-cohort design, we followed 543 middle-aged individuals who developed diabetes and 537 who did not over ~9 years within the population-based Atherosclerosis Risk in Communities Study. Weighted Cox proportional hazards analyses permitted statistical inference of the RBP4 - incident diabetes associations to the entire cohort.

RESULTS

Women in the highest tertile of RBP4 presented greater risk of developing diabetes (HR = 1.74; 95%CI 1.03 - 2.94) in analyses adjusted for age, ethnicity, study center, parental history of diabetes, hypertension, glomerular filtration rate, body mass index, waist-hip ratio, nonesterified fatty acids, adiponectin, leptin, triglycerides and HDL-C. When additionally adjusted for fasting insulin, this association's significance became borderline (HR = 1.68; 95%CI 1.00 - 2.82). No association between RBP4 levels and incident diabetes was found in men.

CONCLUSION

These findings suggest that RBP4 levels may be directly involved in the pathogenesis of type 2 diabetes in women.

Molecular characterization and tissue distribution of feline retinol-binding protein 4

Retinol-binding protein 4 (RBP4) is a specific transporter of retinol and was recently identified as an adipokine potentially involved in type 2 diabetes in humans and rodents. However, the function and structure of feline RBP4 have not been reported. In this study, we describe the molecular cloning and expression analysis of feline RBP4. The complete feline RBP4 cDNA encodes a precursor protein comprising an 18 amino acid signal peptide and a 183 amino acid mature protein. Feline RBP4 was mapped to chromosome D2. Mature feline RBP4 is 83–94% homologous to the RBPs of humans, cows and rodents. RT-PCR analysis revealed feline RBP4 expression in liver and adipose tissues.

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.

Prealbumin is associated with visceral fat mass in patients receiving hemodialysis

OBJECTIVE

Albumin and prealbumin are associated with nutritional status and inflammatory status. Each has a residual effect on mortality outcomes when included in regression models that include the other. Prealbumin is increased in the obese mouse model as a consequence of stabilization of prealbumin by retinol binding protein 4 (RBP4) secreted by adipocytes. We carried out this study to establish the contribution of adiposity to prealbumin levels in prevalent patients receiving dialysis and the relationship of prealbumin to RBP4.

DESIGN AND METHODS

We determined whether prealbumin was associated with adiposity in patients receiving hemodialysis (HD), controlling for the effects of inflammation and nutrition. We evaluated body composition in 48 prevalent patients receiving HD by magnetic resonance imaging (MRI), measuring total skeletal muscle mass (SM), visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and serum albumin, prealbumin, RBP4, and interleukin-6 (IL-6) levels. We used normalized protein catabolic rate (nPCR) to report nutrition and separately analyzed the determinants of albumin and then of prealbumin by multiple stepwise regression.

RESULTS

Thirty-two patients were women, 16 patients were diabetic, and median age and body mass index were 54.5 and 27.3 kg/m(2), respectively. Median total adipose tissue (TAT) was 24.3 kg and VAT was 3.25 kg. Prealbumin was positively associated with VAT, nPCR, and RBP4 and was negatively associated with IL-6; r(2) for the model was 0.64. By contrast, albumin was positively associated with nPCR and negatively associated with IL-6 but not with any measure of adiposity (r(2) for the model = 0.2).

CONCLUSIONS

Prealbumin, like albumin, is associated with markers of nutrition (nPCR) and inflammation, but unlike albumin, prealbumin levels are positively associated with visceral adiposity.