Editorial: Retinol binding protein 4 and pathogenesis of diabetes

The effect of dietary carbohydrate restriction and aerobic exercise on retinol binding protein 4 (RBP4) and fatty acid binding protein 5 (FABP5) in middle-aged men with metabolic syndrome

Exercise and dietary interventions have been described to positively affect metabolic syndrome (MetS) via molecular-induced changes. The purpose of this study was to investigate the effects of dietary carbohydrate restriction and aerobic exercise (AE) on retinol binding protein 4 (RBP4) and fatty acid binding protein 5 (FABP5) in middle-aged men with MetS. The study had a randomised, double-blinded, parallel-controlled design. Forty middle-aged men with MetS (age: 53·97 ± 2·85 years, BMI = 31·09 ± 1·04 kg/m2) were randomly assigned to four groups, AE (n 10), ketogenic diet (KD; n 10), AE combined with KD (AE + KD; n 10) or control (C; n 10). RBP4, FABP5, body composition (body mass, BMI and body fat), insulin resistance, insulin sensitivity and MetS factors were evaluated prior to and after the 12-week intervention. AE + KD significantly decreased the body fat percentage (P = 0·006), BMI (P = 0·001), Zmets (P = 0·017), RBP4 (P = 0·017) and the homeostasis model of insulin resistance (HOMA-IR) (P = 0·001) as compared with control group and marginally significantly decreased the Zmets as compared with exercise group (P = 0·086). KD significantly decreased RBP4 levels as compared with control group (P = 0·041). Only the AE intervention (P = 0·045) significantly decreased FABP5 levels. Combining intervention of carbohydrate restriction with AE compared with carbohydrate restriction and AE alone improved RBP4, HOMA-IR as well as different body composition and MetS factors in middle-aged men with MetS.

Association of serum uromodulin with adipokines in dependence of type 2 diabetes

BACKGROUND

The renal tubular glycoprotein uromodulin is associated with obesity and type 2 diabetes, but the underlying mechanisms are elusive. We investigated the association of serum uromodulin with adipokines and tested the effect modification by diabetes status.

METHODS

The associations of serum uromodulin with eight adipokines were assessed in 795-1080 participants of the KORA F4 study aged 62-81 years using linear regression models adjusted for sex, age, BMI, estimated glomerular filtration rate and diabetes. Significant associations were assessed for effect modification by diabetes status. We further tested using logistic regression whether adjustment for the significant adipokines affected the association of uromodulin with type 2 diabetes.

RESULTS

Serum uromodulin was inversely associated with chemerin and retinol-binding protein-4 after multivariable adjustment (p < 0.001) and Bonferroni correction for multiple testing. No significant association was observed between uromodulin and the other adipokines (leptin, adiponectin, secreted frizzled-related protein 5, progranulin, omentin-1 and vaspin) after correcting for multiple testing. The association of uromodulin with chemerin and retinol-binding protein-4 was stronger in participants with type 2 diabetes than in participants without diabetes (p for interaction < 0.05). However, inclusion of chemerin and retinol-binding protein-4 in logistic regression models did not attenuate the association of serum uromodulin with diabetes.

CONCLUSIONS

Serum uromodulin was inversely associated with the predominantly pro-inflammatory adipokines chemerin and retinol-binding protein-4. The associations were stronger in participants with type 2 diabetes compared to participants without diabetes. However, the association of serum uromodulin with type 2 diabetes was independent of chemerin and retinol-binding protein-4.

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 as a cofactor for PKCδ-mediated impairment of insulin sensitivity in a mouse model of diet-induced obesity

We previously defined that the mitochondria-localized PKCδ signaling complex stimulates the conversion of pyruvate to acetyl-coenzyme A by the pyruvate dehydrogenase complex. We demonstrated in vitro and ex vivo that retinol supplementation enhances ATP synthesis in the presence of the PKCδ signalosome. Here, we tested in vivo if a persistent oversupply of retinol would further impair glucose metabolism in a mouse model of diet-induced insulin resistance. We crossed mice overexpressing human retinol-binding protein (hRBP) under the muscle creatine kinase (MCK) promoter (MCKhRBP) with the PKCδ(-/-) strain to generate mice with a different status of the PKCδ signalosome and retinoid levels. Mice with a functional PKCδ signalosome and elevated retinoid levels (PKCδ(+/+)hRBP) developed the most advanced stage of insulin resistance. In contrast, elevation of retinoid levels in mice with inactive PKCδ did not affect remarkably their metabolism, resulting in phenotypic similarity between PKCδ(-/-)hRBP and PKCδ(-/-) mice. Therefore, in addition to the well-defined role of PKCδ in the etiology of metabolic syndrome, we present a novel PKCδ signaling pathway that requires retinol as a metabolic cofactor and is involved in the regulation of fuel utilization in mitochondria. The distinct role in whole-body energy homeostasis establishes the PKCδ signalosome as a promising target for therapeutic intervention in metabolic disorders.

Fenretinide prevents obesity in aged female mice in association with increased retinoid and estrogen signaling

OBJECTIVE

The synthetic retinoid fenretinide (FEN) inhibits adiposity in male mice fed a high-fat diet (HFD) in association with alterations in retinoic acid (RA) signaling. Young female mice are protected from obesity via estrogen signaling. We, therefore, investigated whether FEN also influences adiposity in aged female mice differing in parity and whether such effects are mediated by retinoid and estrogen signaling.

METHODS

Aged nulliparous and parous female mice were maintained on HFD ± FEN, and adiposity was assessed. Quantitative polymerase chain reaction was performed on white adipose tissue (WAT), liver, and 3T3-L1 adipocytes treated with RA or FEN ± estrogen.

RESULTS

Parous females were more obese than nulliparous mice independent of age. FEN-HFD prevented the HFD-induced increase in adiposity and leptin levels independently of parity. FEN-HFD induced retinoid-responsive genes in WAT and liver. Parous females had reduced expression of hepatic estrogen-responsive genes, but FEN-HFD up-regulated WAT Cyp19a1 and Esr2 in parous mice. Estrogen and RA acted synergistically to increase RA receptor-mediated gene expression in 3T3-L1 adipocytes. FEN increased Cyp19a1 and Esr2, similar to our findings in vivo.

CONCLUSIONS

The prevention of adiposity by FEN in response to HFD in female mice seems to involve increased retinoid signaling in association with induction of local estrogen production and estrogen signaling in WAT.

Metabolic syndrome in pediatrics

The ever growing prevalence of childhood obesity is being accompanied by an increase in the pediatric population of diseases once believed to be exclusive of the adulthood such as the metabolic syndrome (MS). The MS has been defined as the link between insulin resistance, hypertension, dyslipidemia, impaired glucose tolerance, and other metabolic abnormalities associated with an increased risk of atherosclerotic cardiovascular diseases in adults. In this review, we will discuss the peculiar aspects of the pediatric MS and the role of novel molecules and biomarkers in its pathogenesis.

Association between retinol-binding protein 4 and polycystic ovary syndrome: a meta-analysis

Studies have examined the association between retinol-binding protein 4 (RBP4) and polycystic ovary syndrome (PCOS). However, the results have been inconsistent. To investigate the association between RBP4 and PCOS, we performed a meta-analysis. The Cochrane Library, MEDLINE, the ISI Web of Science, and EMBASE were searched to identify all of the studies that examined the relationship between circulating RBP4 levels and PCOS. Standard mean difference (SMD) values and 95% confidence interval (CI) were estimated and pooled using meta-analysis methodology. A total of seven studies were involved in the meta-analysis, which included a total of 636 subjects (260 controls and 376 patients with PCOS). The RBP4 level was higher in PCOS patients than in non-PCOS patients (random effects MD (95% CI)=0.69, [0.20, 1.18], P=0.006). However, the RBP4 level was not higher in nonobese PCOS patients than in nonobese controls (random effects MD (95% CI)=0.38, [-0.21, 0.98], P=0.20). The effect size revealed that the RBP4 level was higher in overweight or obese PCOS patients than weight-matched controls (fixed effects MD (95% CI)=7.95, [5.96, 9.93], P<0.05). In the subgroup analysis by region, the RBP4 level was higher in PCOS patients in Asia than controls (random effects MD (95% CI)=0.85, [0.54, 1.15], P<0.05), but not in European PCOS patients compared with controls (random effects MD (95% CI)=0.34, [-1.12, 1.80], P=0.65). This subgroup analysis also showed that nonobese PCOS patients have higher RBP4 levels than controls in Asia. Our meta-analysis results indicated that RBP4 might be a useful tool for identifying PCOS women.

Higher serum retinol binding protein 4 may be a predictor of weak metabolic control in chinese patients with type 2 diabetes mellitus

OBJECTIVE

To investigate relationships between serum levels of retinol binding protein 4 (RBP4) and clinical and metabolic variables in Chinese patients with type 2 diabetes mellitus.

METHODS

A total of 513 patients (286 males/227 females) provided clinical and lifestyle data and blood and urine samples for analysis. Patients were stratified into four quartile groups according to serum RBP4 concentrations.

RESULTS

RBP4 concentration was independently associated with gender, systolic blood pressure, serum triglyceride and creatinine levels, and estimated glomerular filtration rate (eGFR). Hypertension, dyslipidaemia, micro albuminuria and impaired eGFR (<60 ml/min per 1.73 m2) were significantly more prevalent in patients with the highest RBP4 levels than in those with the lowest levels. Increased serum RBP4 was associated with increased risk of hypertension, dyslipidaemia, micro albuminuria and impaired eGFR after adjusting for gender, age, body mass index and duration of diabetes.

CONCLUSION

Serum RBP4 may be a useful marker of overall metabolic control in Chinese patients with type 2 diabetes mellitus.

Obesity, systemic inflammation, and increased risk for cardiovascular disease and diabetes among adolescents: a need for screening tools to target interventions

Cardiovascular disease (CVD) and type 2 diabetes mellitus have their roots in childhood, particularly in obese children and adolescents, raising important opportunities for early lifestyle intervention in at-risk individuals. However, not all obese individuals are at the same risk for disease progression. Accurate screening of obese adolescents may identify those in greatest need for intensive intervention to prevent or delay future disease. One potential screening target is obesity-related inflammation, which contributes to insulin resistance, metabolic syndrome, and CVD. In adults, the inflammatory marker high-sensitivity C-reactive protein (hsCRP) has utility for risk stratification and treatment initiation in individuals of intermediate CVD risk. In adolescents, hsCRP shares many of the associations of hsCRP in adults regarding the degree of insulin resistance, metabolic syndrome, and carotid artery media thickness. However, long-term data linking increased hsCRP levels-and increased insulin or decreased adiponectin-in childhood to adult disease outcomes are lacking at this time. Future efforts continue to be needed to identify childhood clinical and laboratory characteristics that could be used as screening tests to predict adult disease progression. Such tests may have utility in motivating physicians and patients' families toward lifestyle changes, ultimately improving prevention efforts.

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

Serum levels of RBP4 and adipose tissue levels of PTP1B are increased in obese men resident in northeast Scotland without associated changes in ER stress response genes

Background

Retinol-binding protein 4 (RBP4) is an adipokine identified as a marker of insulin resistance in mice and humans. Protein tyrosine phosphatase 1B (PTP1B) expression levels as well as other genes involved in the endoplasmic reticulum (ER) stress response are increased in adipose tissue of obese, high-fat-diet-fed mice. In this study we investigated if serum and/or adipose tissue RBP4 protein levels and expression levels of PTP1B and other ER stress-response genes are altered in obese and obese/diabetic men resident in northeast Scotland.

Methods

We studied three groups of male volunteers: (1) normal/overweight (body mass index [BMI] < 30), (2) obese (BMI > 30), and (3) obese/diabetic (BMI > 30) controlling their diabetes either by diet or the antidiabetic drug metformin. We analyzed their serum and adipose tissue RBP4 protein levels as well as adipose tissue mRNA expression of PTP1B, binding immunoglobulin protein (BIP), activated transcription factor 4 (ATF4), and glucose-regulated protein 94 (GRP94) alongside other markers of adiposity (percentage body fat, leptin, cholesterol, triglycerides) and insulin resistance (oral glucose tolerance tests, insulin, homeostatic model assessment–insulin resistance, C-reactive protein, and adiponectin).

Results

We found that obese Scottish subjects had significantly higher serum RBP4 protein levels in comparison to the normal/overweight subjects (P < 0.01). Serum RBP4 levels were normalized in obese/diabetic subjects treated with diet or metformin (P < 0.05). Adipose tissue RBP4 protein levels were comparable between all three groups of subjects as were serum and adipose transthyretin levels. Adipose tissue PTP1B mRNA levels were increased in obese subjects in comparison to normal/overweight subjects (P < 0.05); however diet and/or metformin treatment did not reverse this effect. Adipose tissue BIP, ATF4, and GRP94 expression levels were unchanged in obese and obese/diabetic subjects.

Conclusions

Human obesity results in an increase in serum but not adipose tissue RBP4 protein levels, and these are normalized in obese/diabetic subjects, which exhibit improvements in insulin sensitivity through diet or metformin treatment. However, while adipose tissue PTP1B mRNA levels increase in obese Scottish subjects, these remain high in obese/diabetics on diet or metformin treatment.

Susceptibility to diet-induced obesity and glucose intolerance in the APP (SWE)/PSEN1 (A246E) mouse model of Alzheimer's disease is associated with increased brain levels of protein tyrosine phosphatase 1B (PTP1B) and retinol-binding protein 4 (RBP4), and basal phosphorylation of S6 ribosomal protein

AIMS/HYPOTHESIS

Obesity is a major risk factor for development of insulin resistance, a proximal cause of type 2 diabetes and is also associated with an increased relative risk of Alzheimer's disease. We therefore investigated the susceptibility of transgenic mice carrying human mutated transgenes for amyloid precursor protein (APP (SWE)) and presenilin 1 (PSEN1 (A246E)) (APP/PSEN1), or PSEN1 (A246E) alone, which are well-characterised animal models of Alzheimer's disease, to develop obesity, glucose intolerance and insulin resistance, and whether this was age- and/or diet-dependent.

METHODS

We analysed the effects of age and/or diet on body weight of wild-type, PSEN1 and APP/PSEN1 mice. We also analysed the effects of diet on glucose homeostasis and insulin signalling in these mice.

RESULTS

While there were no body weight differences between 16-17- and 20-21-month-old PSEN1 mice, APP/PSEN1 mice and their wild-type controls on standard, low-fat, chow diet, the APP/PSEN1 mice still exhibited impaired glucose homeostasis, as investigated by glucose tolerance tests. This was associated with increased brain protein tyrosine phosphatase 1B protein levels in APP/PSEN1 mice. Interestingly, short-term high-fat diet (HFD) feeding of wild-type, PSEN1 and APP/PSEN1 mice for a period of 8 weeks led to higher body weight gain in APP/PSEN1 than in PSEN1 mice and wild-type controls. In addition, HFD-feeding caused fasting hyperglycaemia and worsening of glucose maintenance in PSEN1 mice, the former being further exacerbated in APP/PSEN1 mice. The mechanism(s) behind this glucose intolerance in PSEN1 and APP/PSEN1 mice appeared to involve increased levels of brain retinol-binding protein 4 and basal phosphorylation of S6 ribosomal protein, and decreased insulin-stimulated phosphorylation of Akt/protein kinase B and extracellular signal-regulated kinase 1/2 in the brain.

CONCLUSIONS/INTERPRETATION

Our results indicate that Alzheimer's disease increases susceptibility to body weight gain induced by HFD, and to the associated glucose intolerance and insulin resistance.

Retinol-binding protein 4 is associated with prediabetes in adults from the general population: the Cooperative Health Research in the Region of Augsburg (KORA) F4 Study

OBJECTIVE

We examined the association between retinol-binding protein 4 (RBP4), a novel adipokine, and prediabetes (isolated impaired fasting glucose [i-IFG], isolated impaired glucose tolerance [i-IGT], and combined IFG and IGT) in men and women aged 32-81 years.

RESEARCH DESIGN AND METHODS

The analysis was based on 2,614 participants without previously diagnosed diabetes and those with newly diagnosed diabetes of the Cooperative Health Research in the Region of Augsburg (KORA) F4 Study, conducted from 2006 to 2008 in southern Germany. Plasma RBP4 was analyzed by immunonephelometry.

RESULTS

In logistic regression analysis, RBP4 levels in the fourth quartile versus the first quartile were significantly associated with prediabetes (i-IGT, i-IFG, and IFG/IGT; reference normal glucose tolerance) independent of known metabolic risk factors and lifestyle variables (odds ratio 1.63 [95% CI 1.17-2.27] after multivariable adjustment). Stratification by sex showed generally similar results.

CONCLUSIONS

RBP4 levels were associated with prediabetes in individuals from the general population. Prospective studies investigating the impact of RBP4 on the development of glucose intolerance are needed.

Long-term Fenretinide treatment prevents high-fat diet-induced obesity, insulin resistance, and hepatic steatosis

The synthetic retinoid Fenretinide (FEN) increases insulin sensitivity in obese rodents and is in early clinical trials for treatment of insulin resistance in obese humans with hepatic steatosis (46). We aimed to determine the physiological mechanisms for the insulin-sensitizing effects of FEN. Wild-type mice were fed a high-fat diet (HFD) with or without FEN from 4-5 wk to 36-37 wk of age (preventive study) or following 22 wk of HF diet-induced obesity (12 wk intervention study). Retinol-binding protein-4 (RBP4) knockout mice were also fed the HFD with or without FEN in a preventive study. FEN had minimal effects on HFD-induced body weight gain but markedly reduced HFD-induced adiposity and hyperleptinemia in both studies. FEN-HFD mice gained epididymal fat but not subcutaneous or visceral fat mass in contrast to HFD mice without FEN. FEN did not have a measurable effect on energy expenditure, food intake, physical activity, or stool lipid content. Glucose infusion rate during hyperinsulinemic-euglycemic clamp was reduced 86% in HFD mice compared with controls and was improved 3.6-fold in FEN-HFD compared with HFD mice. FEN improved insulin action on glucose uptake and glycogen levels in muscle, insulin-stimulated suppression of hepatic glucose production, and suppression of serum FFA levels in HFD mice. Remarkably, FEN also reduced hepatic steatosis. In RBP4 knockout mice, FEN reduced the HFD-induced increase in adiposity and hyperleptinemia. In conclusion, long-term therapy with FEN partially prevents or reverses obesity, insulin resistance, and hepatic steatosis in mice on HFD. The anti-adiposity effects are independent of the RBP4 lowering effect.

Retinol-Binding Protein 4 and Lipocalin-2 in Childhood and Adolescent Obesity: When Children Are Not Just “Small Adults”

Background: Although there is much evidence regarding the physiologic and pathogenic roles of the newly described adipokines retinol-binding protein 4 (RBP4) and lipocalin-2 as potential promoters of insulin resistance in obese adults, relatively little information exists regarding their roles in obese children.

Methods: We investigated the circulating concentrations of RBP4 and lipocalin-2 in 80 obese girls (ages 9– 15 years) and their relationships with high-sensitivity C-reactive protein (hs-CRP) and the adipokines leptin and adiponectin. We divided participants by their body mass index standard deviation scores (BMI SDSs) into 4 groups of 20 girls each: overweight [mean BMI SDS (SD), 1.8 (0.4)], obese [2.2 (0.4)], morbidly obese [3.6 (0.4)], and lean controls [−0.11 (0.4)]. We measured plasma-soluble RBP4, the RBP4-binding protein transthyretin, lipocalin-2, hs-CRP, leptin, and adiponectin and calculated the homeostatic assessment model (HOMA) index from fasting glucose and insulin concentrations.

Results: Unexpectedly, plasma RBP4 and lipocalin-2 concentrations were correlated negatively with BMI SDS values (P = 0.005, and P &lt; 0.03, respectively). These results were different from those of adults and were not correlated with the HOMA index. In contrast, hs-CRP and leptin concentrations were positively correlated with BMI SDS values (P &lt; 0.0001, and P &lt; 0.00001, respectively), as expected, whereas the adiponectin concentration was negatively correlated (P = 0.008).

Conclusions: Although the correlations of leptin, adiponectin, and hs-CRP concentrations with BMI in children are similar to those of adults, the correlations of RBP4 and lipocalin-2 with BMI in children are the inverse of those observed in adults. Thus, although systemic inflammation and mild insulin resistance are present in childhood obesity, RBP4 and lipocalin-2 concentrations are not increased in children as they are in obese adults with long-standing severe insulin resistance and type 2 diabetes.