Serum retinol-binding protein 4 is not increased in obesity or obesity-associated type 2 diabetes mellitus, but is reduced after relevant reductions in body fat following gastric bypass

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.

Assessing the Impact of Bariatric Surgery on Retinol-Binding Protein 4 (RBP4): A Systematic Review and Meta-Analysis

This study aimed to analyze changes in retinol-binding protein 4 (RBP4) levels before and after bariatric surgery in obese individuals. Bariatric surgery is a safe and effective treatment for morbid obesity, impacting molecules like RBP4. A systematic review and meta-analysis of 12 relevant studies were conducted, utilizing databases such as PubMed, Cochrane Central, Web of Science, and Scopus. Significant differences in RBP4, glucose, and BMI levels pre- and post-surgery were observed. Meta-regression analysis explored associations with age, pre-BMI, triglycerides, glucose, and post-insulin levels. Findings suggest RBP4 may improve insulin sensitivity after bariatric surgery, warranting further investigation as a potential pharmacotherapeutic target. These results highlight the importance of understanding RBP4's role in the context of bariatric surgery and its implications for improving metabolic health in obese individuals.

Effects of bariatric surgery and dietary intervention on insulin resistance and appetite hormones over a 3 year period

To examine an impact of three types of bariatric surgery compared with dietary intervention (DIET), on concurrent changes in Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) and appetite hormones over 3 years. Fifty-five adults were studied during phase of weight loss (0-12 months) and during weight stability (12-36 months) post intervention. Measurements of HOMA-IR, fasting and postprandial PYY and GLP1, adiponectin, CRP, RBP4, FGF21 hormones and dual-Xray absorptiometry were performed throughout the study. All surgical groups achieved significant reductions in HOMA-IR with greatest difference between Roux-en-Y gastric bypass and DIET (- 3.7; 95% CI - 5.4, - 2.1; p = 0.001) at 12-36 months. Initial (0-12 months) HOMA-IR values were no different to DIET after adjustment for the lost weight. During 12-36 months, after controlling for treatment procedure and weight, for every twofold increase in postprandial PYY and adiponectin, HOMA-IR decreased by 0.91 (95% CI - 1.71, - 0.11; p = 0.030) and by 0.59 (95% CI - 1.10, - 0.10; p = 0.023) respectively. Initial, non-sustained changes in RBP4 and FGF21 were not associated with HOMA-IR values. While initial rapid weight loss reduces insulin resistance, the enhanced secretions of PYY and adiponectin may contribute to weight-independent improvements in HOMA-IR during weight stability.Clinical trial registration: Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12613000188730.

The prognostic value of retinol binding protein in geriatric hip fractures after surgeries: A propensity score matching and 1-year follow-up study

BACKGROUND

We aimed to explore the predictive value of retinol binding protein (RBP) for outcomes of hip fractures.

METHODS

Patients with hip fractures who underwent hip surgeries between December 2017 and February 2021 and met the inclusion criteria were analyzed. Propensity score matching was used to reduce the bias of co-factors and ROC curves based on matched populations were created to determine the optimal cutoff point of RBP. The outcomes between patients with low levels of RBP and high levels of RBP were compared.

RESULTS

Four hundred eighty patients were enrolled in this study and 69 patients died within one year. After a 1:1 PSM, patients with more than 1-year survival had significantly higher RBP (p = 0.013) than those who died within one year, as well as patients divided by 6-months survival (p = 0.012). Logistics analysis showed that low RBP may be an independent risk factor for 3-month survival, 6-month survival, 1-year survival, and 3-month free walking ability.

CONCLUSION

RBP may be associated with the survival and 3-month walking abilities of patients with hip fractures.

Retinol-binding protein-4 and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is becoming increasingly common as the global economy grows and living standards improve. Timely and effective preventions and treatments for NAFLD are urgently needed. Retinol-binding protein-4 (RBP4), the protein that transports retinol through the circulation, was found to be positively related to diabetes, obesity, cardiovascular disease, and other metabolic diseases. Observational studies on the association between serum RBP4 level and the prevalence of NAFLD found contradictory results. Some of the underlying mechanisms responsible for this association have been revealed, and the possible clinical implications of treating NAFLD by targeting RBP4 have been demonstrated. Future studies should focus on the predictive value of RBP4 on NAFLD development and its potential as a therapeutic target in NAFLD.

Small volume retinol binding protein measurement by liquid chromatography-tandem mass spectrometry

BACKGROUND

The measurement of plasma concentrations of retinol binding protein is a component of nutritional assessment in neonatal intensive care. However, serial testing in newborns is hampered by the limited amount of blood that can be sampled. Limitations are most severe with preterm infants, for whom close monitoring may be most important.

METHODS

We developed an assay to quantify retinol binding protein using trypsin digestion and liquid chromatography-tandem mass spectrometry, which requires a serum or plasma volume of 5 µl. Additionally, we validated the method according to current recommendations and performed comparison with a standard nephelometry platform in clinical use.

RESULTS

The assay demonstrated linearity from below 1 mg/dL (0.48 µM) to more than 20 mg/dL (9.7 µM), and an imprecision of 11.8% at 0.43 mg/dL (0.21 µM). The distribution of results observed with the new method was different when compared with nephelometry.

CONCLUSION

Liquid chromatography-tandem mass spectrometry facilitated testing a smaller sample volume, thereby increasing the ability to monitor key nutritional markers in premature infants. The differences in results compared with a commercially-available nephelometric assay revealed questionable results for lower concentrations by immunoassay.

Sexual Dimorphism in the Association of Serum Retinol-Binding Protein-4 With Long-Term Dynamic Metabolic Profiles in Non-Diabetes

OBJECTIVES

We aimed to investigate the association of circulating retinol-binding protein-4 (RBP4) levels with long-term cardiometabolic risk profiles and whether sex disparity mattered.

METHODS

We included 784 non-diabetic participants aged 40 years and above from a well-defined community-based cohort at baseline in 2005 and they were invited to attend the on-site follow-up examination for two consecutive times with 3-year intervals in 2008 and 2011, respectively. Serum RBP4 was measured at baseline, and the anthropometry and biochemical measurements were performed at each visit. Generalized estimating equation models were used to assess the association of serum RBP4 levels with the dynamic changes in adiposity and glucolipid profile.

RESULTS

Based on all the baseline and the 3- and 6-year follow-up data, baseline serum RBP4 levels (each 1-unit of log₁₀RBP4) were significantly associated with waist circumference [β=3.12, 95% confidence interval (CI) (0.77, 5.47), P=0.01], fasting, and 2-h post-loading glucose [β=0.26 (0.05, 0.47), P=0.02, and 1.70 (1.29, 2.12), P< 0.0001], serum triglycerides [β=0.75, 95% CI (0.54, 0.96), P< 0.0001], total cholesterol [β=0.47, 95% CI [0.23 0.70], P<0.0001), and marginally with body mass index (β=0.97, 95% CI (0.02, 1.93), P=0.046], in total participants, after adjusting potential confounders. The association of RBP4 with 2-h post-loading glucose was stronger in women than that in men [β=1.99, 95% CI (1.49, 2.50) vs. 0.61 (-0.14, 1.36), P for interaction=0.001]. The analysis of change in Z-score of cardiometabolic profiles corresponding to each 1-unit increment in log₁₀RBP4 showed consistent results.

CONCLUSIONS

Higher RBP4 levels are associated with longitudinal increase in adiposity and deteriorated glucolipid profile defined by repeated measurements. The associations differ in sex regarding to the 2-h post-loading glucose.

Ileal transposition helps to regulate plasma hepatokine levels in obese Zucker (Crl:ZUC(ORL)-Leprfa) rats

We studied the long-term effect of ileal transposition (IT) metabolic surgery on the hepatokines: retinol-binding protein-4 (RBP4), α-2-HS-glycoprotein (aHSG/fetuin-A), and fibroblast growth factor 21 (FGF21), C-reactive protein (CRP) plasma levels, glucose metabolism, body weight, liver histology, as well as total lipids concentration in muscle, liver, and fat tissue of obese Zucker (Crl:ZUC(ORL)-Lepr^fa) rats. 14 adult males were randomly submitted either to IT or SHAM (control) surgery. Pre-operative hepatokines plasma levels were not significantly different in rats submitted to IT or SHAM protocol. Three months after the procedures the plasma levels of RBP4, aHSG, FGF21, and CRP were significantly lower in IT-operated animals when compared to SHAM-operated group. Three and 12 weeks after the IT and SHAM surgery, the AUC_OGTT were significantly lower than AUC_OGTT before the surgery. HOMA-IR was lower in rats after IT surgery in comparison to the SHAM-operated rats. Muscle and liver total lipids concentration was reduced after the IT procedure when compared to pre-IT conditions. IT had a significant reductive impact on the body weight in comparison to SHAM surgery in the 4th, 6th, 8th, and 10th week after the surgery. We conclude that IT reduces hepatokines' plasma concentrations, muscle and liver total lipids concentration but not the inflammatory processes in the liver of Zucker (Crl:ZUC(ORL)-Lepr^fa) rats.

Resting Energy Expenditure Is Not Altered in Children and Adolescents with Obesity. Effect of Age and Gender and Association with Serum Leptin Levels

In children and adolescents, obesity does not seem to depend on a reduction of resting energy expenditure (REE). Moreover, in this young population, the interactions between either age and obesity or between age and gender, or the role of leptin on REE are not clearly understood. To compare the levels of REE in children and adolescents we studied 181 Caucasian individuals (62% girls) classified on the basis of age- and sex-specific body mass index (BMI) percentile as healthy weight (n = 50), with overweight (n = 34), or with obesity (n = 97) and in different age groups: 8-10 (n = 38), 11-13 (n = 50), and 14-17 years (n = 93). REE was measured by indirect calorimetry and body composition by air displacement plethysmography. Statistically significant differences in REE/fat-free mass (FFM) regarding obesity or gender were not observed. Absolute REE increases with age (p < 0.001), but REE/FFM decreases (p < 0.001) and there is an interaction between gender and age (p < 0.001) on absolute REE showing that the age-related increase is more marked in boys than in girls, in line with a higher FFM. Interestingly, the effect of obesity on absolute REE is not observed in the 8-10 year-old group, in which serum leptin concentrations correlate with the REE/FFM (r = 0.48; p = 0.011). In conclusion, REE/FFM is not affected by obesity or gender, while the effect of age on absolute REE is gender-dependent and leptin may influence the REE/FFM in 8-10 year-olds.

Adipokine role in physiopathology of inflammatory and degenerative musculoskeletal diseases

We performed a systematic literature review to summarize the underlying pathogenic mechanisms by which adipokines influence rheumatological diseases and the resulting clinical manifestations. Increasing evidence display that numerous adipokines may significantly influence the development or clinical course of various rheumatological diseases. Despite the normal anti- or pro-inflammatory role of the cytokines, the serum level varies enormously in various rheumatological diseases. The expression of high levels of pro-inflammatory cytokines such as leptin or visfatin, respectively in systemic lupus erythematosus and in rheumatoid arthritis, represents a negative prognostic factor; other adipokines such as adiponectin, broadly known for their anti-inflammatory effects, showed a correlation with disease activity in rheumatoid arthritis. In the near future pro-inflammatory cytokines may represent a potential therapeutic target to restrain the severity of rheumatological diseases. Further studies on adipokines may provide important information on the pathogenesis of these diseases, which are not yet fully understood. The mechanisms by which adipokines induce, worsen, or suppress inflammatory and degenerative musculoskeletal pathologies and their clinical significance will be discussed in this review.

Impact of Gastric Sleeve Surgery on Plasma Retinol Binding Protein 4 and Adipsin Levels in Healthy Male Population

Objectives

Bariatric surgery provides most substantial and sustainable weight loss measures in individuals with obesity. Caloric restriction is not only intervention, changes in hormonal secretions are also leading contributory mechanisms to reduce body weight and improve the glycaemic control. The aim of this study was to evaluate the impact of gastric sleeve surgery on plasma retinol binding protein 4 (RBP4) and adipsin levels among Saudi male obese population.

Methods

This prospective study was conducted in the Departments of Physiology and Surgery, College of Medicine, King Saud University. Thirty-three obese (BMI>38.3) male patients age ranged from 25 to 50 years were recruited. RBP4 and adipsin levels were analyzed before and 6-12 months after gastric sleeve surgery by ELISA along with plasma glucose, insulin, HOMA-IR and lipid profile.

Results

Circulating RBP4 levels were not significantly changed by bariatric surgery (4382.85±40.35 ng before, and 4393.28±33.13 ng after surgery, p=0.842), neither did adipsin (2949.68±46.86 pg before, and 2917.90±41.90 pg after surgery, p=0.535). Segregation of study participants into two age groups, 25-35 and 35-50 years of age, revealed that before surgery older age group (35-50) had higher RBP4 levels compared to younger group (25-35) (p=0.016). However, after surgery RBP4 levels were decreased in older group but not to a significant level (p=0.174). In younger age group after surgery, there was a near significant increase in RBP4 levels (p=0.052). There were no significant changes in RBP4 levels in both age groups after surgery (p=0.461). For adipsin, there were no significant differences before and after surgery in both age groups. Insulin, BMI and HOMA-IR index were decreased after surgery, however there was no correlation with RBP4 and adipsin levels.

Conclusions

The present study findings do not suggest a role for RBP4 and adipsin in the improvement of insulin sensitivity in Saudi male obese population after gastric sleeve surgery. However, a decrease in RBP4 levels in older individuals after surgery needs further investigations to understand its effect on weight and glycemic control.

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.

Dermatopontin, A Novel Adipokine Promoting Adipose Tissue Extracellular Matrix Remodelling and Inflammation in Obesity

Compelling evidence suggests that dermatopontin (DPT) regulates collagen and fibronectin fibril formation, the induction of cell adhesion and the prompting of wound healing. We aimed to evaluate the role of DPT on obesity and its associated metabolic alterations as well as its impact in visceral adipose tissue (VAT) inflammation and extracellular matrix (ECM) remodelling. Samples obtained from 54 subjects were used in a case-control study. Circulating and VAT expression levels of DPT as well as key ECM remodelling- and inflammation-related genes were analysed. The effect of pro- and anti-inflammatory mediators on the transcript levels of DPT in visceral adipocytes was explored. The impact of DPT on ECM remodelling and inflammation pathways was also evaluated in cultured adipocytes. We show that obesity and obesity-associated type 2 diabetes (T2D) increased (p < 0.05) circulating levels of DPT. In this line, DPT mRNA in VAT was increased (p < 0.05) in obese patients with and without T2D. Gene expression levels of DPT were enhanced (p < 0.05) in human visceral adipocytes after the treatment with lipopolysaccharide, tumour growth factor (TGF)-β and palmitic acid, whereas a downregulation (p < 0.05) was detected after the stimulation with interleukin (IL)-4 and IL-13, critical cytokines mediating anti-inflammatory pathways. Additionally, we revealed that DPT increased (p < 0.05) the expression of ECM- (COL6A3, ELN, MMP9, TNMD) and inflammation-related factors (IL6, IL8, TNF) in human visceral adipocytes. These findings provide, for the first time, evidence of a novel role of DPT in obesity and its associated comorbidities by influencing AT remodelling and inflammation.

Novel adipokine associated with OA: retinol binding protein 4 (RBP4) is produced by cartilage and is correlated with MMPs in osteoarthritis patients

Objective

Retinol binding protein 4 (RBP4) is a member of the lipocalin family and a vitamin A carrier in the blood. More recently, RBP4 has been described as an adipokine that is involved in insulin resistance and metabolic syndrome (MetS). As obesity, MetS and some adipokines contribute to the pathogenesis of osteoarthritis (OA), we investigated RBP4 in patients with OA.

Materials and methods

Cartilage, synovial fluid and blood samples were collected from 100 OA patients undergoing total knee replacement surgery. Primary chondrocytes and cartilage tissue were cultured to measure the RBP4 expression. The concentrations of RBP4, other adipokines (adipsin, adiponectin, leptin and resistin) and biomarkers of OA (COMP, MMP-1, MMP-3 and YKL-40) were measured by immunoassay, and gene expression was measured by next-generation RNA sequencing.

Results

The OA cartilage samples released RBP4 into the culture medium, and the levels correlated positively with the expression of the adipokines adipsin, adiponectin, leptin and resistin. RBP4 was the most prominently expressed of these adipokines in the OA chondrocytes, and the expression of the RBP4 receptors STRA6 (stimulated by retinoic acid gene homologue 6) and TLR4 (Toll-like receptor 4) was also detected. Within the cartilage culture medium, RBP4 showed a positive correlation with MMP-1, MMP-3 and YKL-40. RBP4 was also present in the synovial fluid from the OA patients and correlated positively with the concentrations of RBP4 found in the plasma and the cartilage culture medium. Plasma RBP4 concentrations also showed a positive correlation with MMP-3 and adipsin.

Conclusions

We show here, for the first time, that RBP4 is produced within OA joints and that it is associated with increased levels of adipokines and MMPs. The results suggest a role for RBP4 in the pathogenesis of OA and as a possible target for the disease-modifying drugs for the treatment of OA.

Plasma Levels of Retinol Binding Protein 4 Relate to Large VLDL and Small LDL Particles in Subjects with and without Type 2 Diabetes

Background: Retinol binding protein 4 (RBP4) carries retinol in plasma, but is also considered an adipokine, as it is implicated in insulin resistance in mice. Plasma RBP4 correlates with total cholesterol, low density lipoprotein (LDL)-cholesterol and triglycerides, and may confer increased cardiovascular risk. However, controversy exists about circulating RPB4 levels in type 2 diabetes mellitus (T2DM) and obesity. Here, we analyzed the relationships of RBP4 and retinol with lipoprotein subfractions in subjects with and without T2DM. Methods: Fasting plasma RBP4 (enzyme-linked immunosorbent assay) and retinol (high performance liquid chromatography) were assayed in 41 T2DM subjects and 37 non-diabetic subjects. Lipoprotein subfractions (NMR spectroscopy) were measured in 36 T2DM subjects and 27 non-diabetic subjects. Physical interaction of RBP4 with lipoproteins was assessed by fast protein liquid chromatography (FPLC). Results: Plasma RBP4 and retinol were strongly correlated (r = 0.881, p < 0.001). RBP4, retinol and the RBP4/retinol ratio were not different between T2DM and non-diabetic subjects (all p > 0.12), and were unrelated to body mass index. Notably, RBP4 and retinol were elevated in subjects with metabolic syndrome (p < 0.05), which was attributable to an association with elevated triglycerides (p = 0.013). Large VLDL, total LDL and small LDL were increased in T2DM subjects (p = 0.035 to 0.003). Taking all subjects together, RBP4 correlated with total cholesterol, non-HDL cholesterol, LDL cholesterol, triglycerides and apolipoprotein B in univariate analysis (p < 0.001 for each). Age-, sex- and diabetes status-adjusted multivariable linear regression analysis revealed that RBP4 was independently associated with large VLDL (β = 0.444, p = 0.005) and small LDL particles (β = 0.539, p < 0.001). Its relationship with large VLDL remained after further adjustment for retinol. RBP4 did not co-elute with VLDL nor LDL particles in FPLC analyses. Conclusions: Plasma RBP4 levels are related to but do not physically interact with large VLDL and small LDL particles. Elevated RBP4 may contribute to a proatherogenic plasma lipoprotein profile.

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.

Does Bariatric Surgery Improve Obesity Associated Comorbid Conditions

Obesity is a constantly growing health problem which reduces quality of life and life expectancy. Bariatric surgery for obesity is taken into account when all other conservative treatment modalities have failed. Comparison of the multidisciplinary programs with bariatric surgery regarding to weight loss showed that substantial and durable weight reduction have been achieved only with bariatric surgical treatments. However, the benefits of weight loss following bariatric procedures are still debated regarding the pro-inflammatory and metabolic profile of obesity.

Role of human lipocalin proteins in abdominal obesity after acute pancreatitis

Lipocalin proteins are small regulatory peptides implicated in metabolism, inflammation, and immunity. Although lipocalin proteins have been linked to various clinical conditions, their role in the acute inflammatory setting, such as acute pancreatitis (AP), has only been sparsely investigated. Two members of the lipocalin family, lipocalin-2 (LCN-2) and retinol binding protein -4 (RBP-4), play an important role in obesity and insulin resistance. In this study, we analysed circulating levels of LCN-2 and RBP-4 in 92 individuals after AP, of whom 41 individuals had abdominal obesity and 51 did not. Binary logistic regression analyses were performed to determine whether abdominal obesity was associated with the two lipocalin proteins. Lipocalin-2 was significantly associated with abdominal obesity in the unadjusted model (Odds ratio (OR)=1.014 [95% confidence interval (CI): 1.000, 1.028], P=0.05) and after adjusting for patient related (age, ethnicity, and diabetes mellitus) and pancreatitis related (aetiology, severity, recurrence, and duration of AP) characteristics (OR=1.018 [95% CI: 1.001, 1.036], p=0.04). Further, the association of LCN-2 with waist circumference was significant in individuals with alcohol aetiology of AP (β=1.082 [95% CI: 1.011, 1.158], p=0.02]. The association between RBP-4 and abdominal obesity was not significant in both unadjusted and adjusted models. These findings indicate that circulating levels of LCN-2 in patients after AP may play a role in chronic low grade inflammation associated with abdominal adiposity and that alcohol consumption may further exacerbate adipose tissue dysfunction.

Adipocyte-specific overexpression of retinol-binding protein 4 causes hepatic steatosis in mice

There is considerable evidence that both retinoids and retinol-binding protein 4 (RBP4) contribute to the development of liver disease. To understand the basis for this, we generated and studied transgenic mice that express human RBP4 (hRBP4) specifically in adipocytes. When fed a chow diet, these mice show an elevation in adipose total RBP4 (mouse RBP4 + hRBP4) protein levels. However, no significant differences in plasma RBP4 or retinol levels or in hepatic or adipose retinoid (retinol, retinyl ester, and all-trans-retinoic acid) levels were observed. Strikingly, male adipocyte-specific hRBP4 mice fed a standard chow diet display significantly elevated hepatic triglyceride levels at 3-4 months of age compared to matched littermate controls. When mice were fed a high-fat diet, this hepatic phenotype, as well as other metabolic phenotypes (obesity and glucose intolerance), worsened. Because adipocyte-specific hRBP4 mice have increased tumor necrosis factor-α and leptin expression and crown-like structures in adipose tissue, our data are consistent with the notion that adipose tissue is experiencing RBP4-induced inflammation that stimulates increased lipolysis within adipocytes. Our data further establish that elevated hepatic triglyceride levels result from increased hepatic uptake of adipose-derived circulating free fatty acids. We obtained no evidence that elevated hepatic triglyceride levels arise from increased hepatic de novo lipogenesis, decreased hepatic free fatty acid oxidation, or decreased very-low-density lipoprotein secretion.

CONCLUSION

Our investigations establish that RBP4 expressed in adipocytes induces hepatic steatosis arising from primary effects occurring in adipose tissue. (Hepatology 2016;64:1534-1546).

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.

FGF19 and FGF21 serum concentrations in human obesity and type 2 diabetes behave differently after diet- or surgically-induced weight loss

BACKGROUND & AIMS

Fibroblast growth factor 19 (FGF19) and 21 (FGF21) have emerged as key regulators of energy homeostasis. Our aim was to analyze the impact of weight loss (WL) induced either by conventional dietary treatment (CDT) or bariatric surgery on FGF19 and FGF21 concentrations. Furthermore, the diverse effect of sleeve gastrectomy (SG) versus RYGB (Roux-en-Y gastric bypass) as two surgical procedures that affect the gastrointestinal anatomy and physiology differently was also analyzed.

METHODS

Serum concentrations of FGF19 and FGF21 were measured in 137 obese patients with different degrees of insulin resistance matched by sex, age and body adiposity and compared to 33 lean individuals. Furthermore, FGF19 and FGF21 were measured in 114 subjects before and one-year after WL induced either by CDT (n = 28), SG (n = 20) or RYGB (n = 66).

RESULTS

Circulating serum FGF19 concentrations were decreased (P < 0.01) similarly in obese patients regardless of their degree of insulin resistance, while FGF21 levels were increased in obesity (P < 0.01), being further increased in obesity-associated T2D (P < 0.01). FGF19 concentrations were increased in obese subjects after surgically-induced WL (P < 0.01), but not after WL achieved by CDT, while FGF21 levels were reduced after CDT- (P < 0.05) or SG-induced WL (P < 0.05), but not after RYGB. The change in FGF21 concentrations emerged as a significant predictor of the change in insulin resistance (HOMA) after WL.

CONCLUSIONS

Based on the circulating concentrations and their subsequent pattern of response following WL, we conclude that FGF19 levels are mainly related to body adiposity, in particular visceral adiposity, while FGF21 is mainly related to glucose homeostasis. CLINICALTRIALS.

GOV IDENTIFIER

NCT01572090.

Circulating Adipokines and Vascular Function: Cross-Sectional Associations in a Community-Based Cohort

Adipokines may be potential mediators of the association between excess adiposity and vascular dysfunction. We assessed the cross-sectional associations of circulating adipokines with vascular stiffness in a community-based cohort of younger adults. We related circulating concentrations of leptin and leptin receptor, adiponectin, retinol-binding protein 4, and fatty acid-binding protein 4 to vascular stiffness measured by arterial tonometry in 3505 Framingham Third Generation cohort participants free of cardiovascular disease (mean age 40 years, 53% women). Separate regression models estimated the relations of each adipokine to mean arterial pressure and aortic stiffness, as carotid femoral pulse wave velocity, adjusting for age, sex, smoking, heart rate, height, antihypertensive treatment, total and high-density lipoprotein cholesterol, diabetes mellitus, alcohol consumption, estimated glomerular filtration rate, glucose, and C-reactive protein. Models evaluating aortic stiffness also were adjusted for mean arterial pressure. Mean arterial pressure was positively associated with blood retinol-binding protein 4, fatty acid-binding protein 4, and leptin concentrations (all P<0.001) and inversely with adiponectin (P=0.002). In fully adjusted models, mean arterial pressure was positively associated with retinol-binding protein 4 and leptin receptor levels (P<0.002 both). In fully adjusted models, aortic stiffness was positively associated with fatty acid-binding protein 4 concentrations (P=0.02), but inversely with leptin and leptin receptor levels (P≤0.03 both). In our large community-based sample, circulating concentrations of select adipokines were associated with vascular stiffness measures, consistent with the hypothesis that adipokines may influence vascular function and may contribute to the relation between obesity and hypertension.

Bariatric surgery and nonalcoholic fatty liver disease

The rising prevalence of nonalcoholic fatty liver disease (NAFLD) is associated with the increasing global pandemic of obesity. These conditions cluster with type II diabetes mellitus and the metabolic syndrome to result in obesity-associated liver disease. The benefits of bariatric procedures on diabetes and the metabolic syndrome have been recognized for some time, and there is now mounting evidence to suggest that bariatric procedures improve liver histology and contribute to the beneficial resolution of NAFLD in obese patients. These beneficial effects derive from a number of weight-dependent and weight-independent mechanisms including surgical BRAVE actions (bile flow changes, restriction of stomach size, anatomical gastrointestinal rearrangement, vagal manipulation, enteric hormonal modulation) and subsequent effects such as reduced lipid intake, adipocytokine secretion, modulation of gut flora, improvements in insulin resistance and reduced inflammation. Here, we review the clinical investigations on bariatric procedures for NAFLD, in addition to the mounting mechanistic data supporting these findings. Elucidating the mechanisms by which bariatric procedures may resolve NAFLD can help enhance surgical approaches for metabolic hepatic dysfunction and also contribute toward developing the next generation of therapies aimed at reducing the burden of obesity-associated liver disease.

Circulating retinol-binding protein 4 levels in gestational diabetes mellitus: a meta-analysis of observational studies

Retinol-binding protein 4 (RBP4) is a novel adipocyte-derived cytokine playing an important role in the regulation of energy metabolism and insulin sensitivity. Although the association between RBP4 and metabolic dysfunction is well established, studies on the relationship between circulating RBP4 levels and the risk of gestational diabetes mellitus (GDM) have yielded inconclusive results. We performed a meta-analysis to investigate whether women with GDM had higher circulating RBP4 levels than the normglycemic pregnant women. PubMed, Web of Science and EMBASE were searched up to 1 August 2014. A total of 14 studies comprised of 884 women with GDM and 1251 normglycemic pregnant women were included. The overall results suggested that maternal circulating RBP4 levels were significantly higher in GDM than their normal controls (SMD: 0.49 μg/ml, 95% CI: 0.23-0.75 μg/ml, p < 0.001, random effect model). However, stratified results indicated that this significant difference only existed in the second/third trimester and was limited to Asian populations. Furthermore, subgroup analysis according to matched maternal age and BMI still demonstrated that GDM had higher circulating RBP4 levels than the normal controls. Our findings suggested that Asian women with GDM had increased circulating RBP4 levels in their second/third trimester.

The multifaceted nature of retinoid transport and metabolism

Since their discovery over a century ago, retinoids have been the most studied of the fat-soluble vitamins. Unlike most vitamins, retinoids are stored at relatively high concentrations in the body to buffer against nutritional insufficiency. Until recently, it was thought that the sole important retinoid delivery pathway to tissues involved retinol bound to retinol-binding protein (RBP4). More recent findings, however, indicate that retinoids can be delivered to tissues through multiple overlapping delivery pathways, involving chylomicrons, very low density lipoprotein (VLDL) and low density lipoprotein (LDL), retinoic acid bound to albumin, water soluble β-glucuronides of retinol and retinoic acid, and provitamin A carotenoids. This review will focus on explaining this evolving understanding of retinoid metabolism and transport within the body.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

Lower fetuin-A, retinol binding protein 4 and several metabolites after gastric bypass compared to sleeve gastrectomy in patients with type 2 diabetes

Background

Bypass of foregut secreted factors promoting insulin resistance is hypothesized to be one of the mechanisms by which resolution of type 2 diabetes (T2D) follows roux-en-y gastric bypass (GBP) surgery.

Aim

To identify insulin resistance-associated proteins and metabolites which decrease more after GBP than after sleeve gastrectomy (SG) prior to diabetes remission.

Methods

Fasting plasma from 15 subjects with T2D undergoing GBP or SG was analyzed by proteomic and metabolomic methods 3 days before and 3 days after surgery. Subjects were matched for age, BMI, metformin therapy and glycemic control. Insulin resistance was calculated using homeostasis model assessment (HOMA-IR). For proteomics, samples were depleted of abundant plasma proteins, digested with trypsin and labeled with iTRAQ isobaric tags prior to liquid chromatography-tandem mass spectrometry analysis. Metabolomic analysis was performed using gas chromatography-mass spectrometry. The effect of the respective bariatric surgery on identified proteins and metabolites was evaluated using two-way analysis of variance and appropriate post-hoc tests.

Results

HOMA-IR improved, albeit not significantly, in both groups after surgery. Proteomic analysis yielded seven proteins which decreased significantly after GBP only, including Fetuin-A and Retinol binding protein 4, both previously linked to insulin resistance. Significant decrease in Fetuin-A and Retinol binding protein 4 after GBP was confirmed using ELISA and immunoassay. Metabolomic analysis identified significant decrease of citrate, proline, histidine and decanoic acid specifically after GBP.

Conclusion

Greater early decrease was seen for Fetuin-A, Retinol binding protein 4, and several metabolites after GBP compared to SG, preceding significant weight loss. This may contribute to enhanced T2D remission observed following foregut bypass procedures.

Retinol and retinyl esters: biochemistry and physiology

By definition, a vitamin is a substance that must be obtained regularly from the diet. Vitamin A must be acquired from the diet, but unlike most vitamins, it can also be stored within the body in relatively high levels. For humans living in developed nations or animals living in present-day vivariums, stored vitamin A concentrations can become relatively high, reaching levels that can protect against the adverse effects of insufficient vitamin A dietary intake for six months, or even much longer. The ability to accumulate vitamin A stores lessens the need for routinely consuming vitamin A in the diet, and this provides a selective advantage to the organism. The molecular processes that underlie this selective advantage include efficient mechanisms to acquire vitamin A from the diet, efficient and overlapping mechanisms for the transport of vitamin A in the circulation, a specific mechanism allowing for vitamin A storage, and a mechanism for mobilizing vitamin A from these stores in response to tissue needs. These processes are considered in this review.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Retinol-binding protein 4 in patients with gestational diabetes mellitus

OBJECTIVE

It is still unknown if serum retinol-binding protein 4 (RBP4) relates to metabolic control and the occurrence of gestational diabetes mellitus (GDM). We studied RBP4 levels in patients with insulin-treated GDM and healthy pregnant women twice during pregnancy.

METHODS

Sixty-three women with GDM and 38 healthy pregnant women were included. Serum RBP4 levels were measured at 24-28 weeks of gestation and 8 weeks later. Patients with GDM were treated with insulin.

RESULTS

Serum RBP4 levels increased significantly between the two measurements in patients with GDM (p = 0.03). In patients with GDM, RBP4 concentrations at 33 weeks of gestation correlated positively with mean blood glucose, hemoglobin A1c values, and cord blood insulin values right after delivery. Regression analyses showed that the diagnosis of GDM (p = 0.04) and hemoglobin A1c levels (p < 0.001) were related to RBP4 levels at 33 weeks of gestation.

CONCLUSIONS

Serum RBP4 levels increase during pregnancy in patients with insulin-treated GDM.

Effects of bariatric surgery on adipokine-induced inflammation and insulin resistance

Over a third of the US population is obese and at high risk for developing type 2 diabetes, insulin resistance, and other metabolic disorders. Obesity is considered a chronic low-grade inflammatory condition that is primarily attributed to expansion and inflammation of adipose tissues. Indeed, adipocytes produce and secrete numerous proinflammatory and anti-inflammatory cytokines known as adipokines. When the balance of these adipokines is shifted toward higher production of proinflammatory factors, local inflammation within adipose tissues and subsequently systemic inflammation occur. These adipokines including leptin, visfatin, resistin, apelin, vaspin, and retinol binding protein-4 can regulate inflammatory responses and contribute to the pathogenesis of diabetes. These effects are mediated by key inflammatory signaling molecules including activated serine kinases such as c-Jun N-terminal kinase and serine kinases inhibitor κB kinase and insulin signaling molecules including insulin receptor substrates, protein kinase B (PKB, also known as Akt), and nuclear factor kappa B. Bariatric surgery can decrease body weight and improve insulin resistance in morbidly obese subjects. However, despite reports suggesting reduced inflammation and weight-independent effects of bariatric surgery on glucose metabolism, mechanisms behind such improvements are not yet well understood. This review article focuses on some of these novel adipokines and discusses their changes after bariatric surgery and their relationship to insulin resistance, fat mass, inflammation, and glucose homeostasis.

The association of baseline adipocytokine levels with glycemic progression in nondiabetic Korean adults in 4 years of follow-up

AIMS

Low-grade inflammation and lipotoxicity contribute to insulin resistance and islet secretory dysfunction that lead to insulin deficiency. We analyzed the associations of several adipocytokines measured at baseline with glycemic progression in non-diabetic Korean subjects after a 4-year follow-up.

METHODS

In 479 non-diabetic Korean subjects who underwent medical screening in 2003, serum tumor necrosis factor (TNF)-α, interleukin (IL)-6, retinol-binding protein (RBP)-4, monocyte chemoattractant protein (MCP)-1, visfatin and fatty acid-binding protein (FABP)-4 were measured at baseline. After 4 years, changes in glycemia were assessed.

RESULTS

Among the subjects, 79.2% maintained their baseline glycemic status, 14.6% progressed to worse glycemic status (impaired fasting glucose (IFG) to diabetes, normoglycemia to IFG or normoglycemia to diabetes) and 5.8% regressed to normoglycemia after 4 years. Baseline TNF-α and FABP4 showed the highest values in the progression group. In the logistic regression analyses with glycemic progression as the dependent variable and TNF-α and FABP4 as independent variables in separate models, TNF-α and FABP4 individually predicted glycemic progression after adjustment for confounding variables. When both adipocytokines were included in the same model, only FABP4 significantly predicted glycemic progression after 4 years.

CONCLUSIONS

TNF-α and FABP4 were significant predictors for glycemic progression in 4 years, with FABP4 being the stronger predictor.

Identification of liver proteins altered by type 2 diabetes mellitus in obese subjects

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a well-known factor risk for non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) in obese patients.

AIMS

To better understand the association between T2DM and NAFLD, global changes in protein expression in diabetic and non-diabetic obese subjects were assessed by a proteomic approach.

METHODS

Liver samples were obtained from diabetic and non-diabetic morbid obese subjects (BMI>40 kg/m(2) ). Histological analysis was used to evaluate hepatic steatosis and the degree of anatomopathological alteration. Changes in protein expression were analysed by two-dimentional electrophoresis combined with MALDI-TOF mass spectrometry. Levels of glutathione, carbonyl and 4-HNE protein adducts were used to assess oxidative stress status.

RESULTS

Of 850 proteins analysed, 33 were differentially expressed in T2DM obese subjects. Of these, 27 were unequivocally identified by mass spectrometry. Analysis of protein sets revealed patterns of decreased abundance in mitochondrial enzymes, proteins involved in methione metabolism, and oxidative stress response. Accordingly, T2DM subjects showed decreased levels of glutathione, the antioxidant byproduct of methionine metabolism via the transsulfuration pathway, and higher levels of protein and lipid oxidative damage. Changes in detoxyfing enzymes, carbohydrate metabolism, proteasome subunits and retinoic acid synthesis were also found.

CONCLUSIONS

The results suggest alterations in mitochondrial function and methionine metabolism as potential contributing factors to increased oxidative stress in liver of obese diabetic patients which may be influencing the development of NAFLD and NASH.

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

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

Association of serum retinol-binding protein 4 with insulin resistance and metabolic parameters during olanzapine therapy

OBJECTIVE

Retinol-binding protein 4 (RBP4) has been shown to be associated with insulin resistance (IR), metabolic indices and metabolic syndrome (MetS) in various patient populations and in obesity. We investigated the association between metabolic parameters, IR and RBP4 during olanzapine therapy.

DESIGN

A prospective study.

PATIENTS

Thirty-seven participants with psychiatric disorder who were atypical antipsychotic naive and newly initiated on olanzapine were assessed.

MEASUREMENTS

Fasting RBP4, anthropometric and metabolic variables were measured before and after 3 months of olanzapine therapy.

RESULTS

Participants who developed MetS showed higher RBP4 levels compared with those without MetS, although not significant (P = 0·053). The variation in RBP4 level was correlated with changes in systolic blood pressure (r = 0·423, P = 0·009), diastolic blood pressure (r = 0·390, P = 0·017), total cholesterol (r = 0·446, P = 0·006) and low-density lipoprotein (LDL) (r = 0·407, P = 0·012). Multiple linear regression analysis illustrated that end-point log insulin level was the most significant independent predictor of final log RBP4 levels (standardized ß = 0·353, P = 0·024).

CONCLUSIONS

Our results suggest that RBP4 levels might be associated with at least some olanzapine-induced metabolic abnormalities and cardiovascular disease risk factors.

Effects of exercise training on circulating retinol-binding protein 4 and cardiovascular disease risk factors in obese men

INTRODUCTION

The purpose of this study was to investigate the effects of 12-week exercise training on circulating retinol-binding protein 4 (RBP4) concentration and cardiovascular disease (CVD) risk factors in obese men.

METHODS

29 obese men (age 48 ± 2 years; BMI 29.6 ± 0.7 kg/m(2)) participated in a 12-week exercise program (50-70% maximal heart rates; 3 times/week) without calorie restriction. Anthropometric parameters, CVD risk factors (total cholesterol, high-density lipoprotein cholesterol (HDLC), low-density lipoprotein cholesterol, triglycerides (TG), and quantitative insulin sensitivity check index), and circulating RBP4 and adipokine (adiponectin, leptin, and interleukin-6 (IL-6)) concentrations were measured.

RESULTS

Exercise training significantly improved in absolute peak oxygen consumption (p < 0.05), and CVD risk factors (p < 0.05), with the exception of HDLC. There was a significant decrease in circulating RBP4, leptin, and IL-6 concentrations (p < 0.05). Stepwise regression analysis revealed that changes in RBP4 concentration were independently related to the changes in TG concentration (β = 0.46, p < 0.05).

CONCLUSION

These results demonstrate that exercise training without calorie restriction improves several CVD risk factors and circulating RBP4 and adipokine concentrations. Moreover, the change in circulating RBP4 may be more associated with the improvements in TG concentration in obese men.

Suppression of retinol-binding protein 4 with RNA oligonucleotide prevents high-fat diet-induced metabolic syndrome and non-alcoholic fatty liver disease in mice

Conflicting data have been reported regarding the role of retinol-binding protein (RBP4) in insulin resistance, obesity, type 2 diabetes and non-alcoholic fatty liver disease (NAFLD). In this study, we used pharmacological methods to investigate the role of RBP4. RNA oligonucleotide against RBP4 (anti-RBP4 oligo) was transfected into 3T3-L1 adipocytes. RT-PCR analysis showed that RBP4 mRNA expression decreased by 55% (p<0.01) compared with control cells. Validated RNA oligo was used in an in vivo study with high fat diet (HFD) fed - mice. 14 weeks of HFD feeding increased RBP4 expression (associated with elevated serum levels measured with immunoblotting and ELISA) by 56% in adipose tissue (p<0.05) and 68% in the liver (p<0.01). Adipose RBP4 levels were significantly reduced after 4 weeks treatment with anti-RBP4 oligo (25mg/kg, p<0.01) and rosiglitazone (RSG, 10mg/kg, p<0.05) compared with scrambled RNA oligo (25mg/kg) treated mice. Only anti-RBP4 oligo significantly inhibited RBP4 protein (p<0.01) and mRNA expression (p<0.01) in the liver and reduced serum RBP4 levels. Anti-RBP4 oligo and RSG showed comparable effects on impaired glucose tolerance, hyperinsulinaemia and hyperglycaemia. Anti-RBP4 oligo significantly enhanced adipose-GLUT4 expression (p<0.01) but did not increase muscle-GLUT4. Both RSG and anti-RBP4 oligo significantly reduced hepatic phosphoenolpyruvate carboxykinase expression (both p<0.05). Histological analysis revealed that anti-RBP4 oligo ameliorated hepatic steatosis and reduced lipid droplets associated with normalized liver function. Histological and pharmacological results of this study indicate that RBP4 is not only an adipocytokine, but also a hepatic cytokine leading to metabolic syndrome, NAFLD and type 2 diabetes.

Circulating retinol-binding protein 4 and metabolic syndrome in the elderly

BACKGROUND

Retinol-binding protein (RBP) 4, a human adipokine that specifically binds to retinol, has been reported to provide a link between obesity and insulin resistance. Plasma RBP4 concentration may be under the influence of age and obesity, but only a few studies has investigated this link in elderly individuals. Consequently, we tested the correlation between RBP4 concentrations and type 2 diabetes/metabolic syndrome (MetS) components in a large population based cohort study (VITA) of elderly [1, 2]. Using a single birth cohort, this investigation could exclude the influence of age.

METHODS

We evaluated the correlation of RBP4 with type 2 diabetes and MetS components including Body Mass Index (BMI), blood pressure, lipid parameters, fasting glucose insulin, homeostasis model assessment insulin resistance (HOMA-IR), and smoking in exclusively 75-76 year old participants (N = 232).

RESULTS

In the present study, RBP4 concentrations were associated with type 2 diabetes and metabolic syndrome (MetS) components. Of all the individual components of metabolic syndrome that were associated with RBP4 concentrations, the correlations of RBP4 with serum triglycerides and a negative correlation with HDL were the strongest ones observed in our study cohort (p<0.0001).

CONCLUSIONS

RBP4 plays a role in biological mechanisms that are responsible for insulin resistance and development of type 2 diabetes.

Association of retinol-binding protein-4 (RBP4) with lipid parameters in obese women

BACKGROUND

Although the adipokine retinol-binding protein-4 (RBP4) has been implicated in the development of obesity-related insulin resistance, its role in human obesity is still unclear. Our objectives were to find out the effect on RBP4 systemic levels of a weight loss induced by gastric bypass surgery and to analyze RBP4 relationships with insulin resistance, parameters of body composition, lipid metabolism, and inflammation.

METHODS

Sixty-three obese women were analyzed before and 12 months after surgery of systemic concentrations of RBP4, fasting glucose, insulin, lipid profile molecules, and inflammation-related proteins (C-reactive protein, tumor necrosis factor-alpha receptors 1 and 2, interleukin-18, and adiponectin), and waist and hip circumference measurements, body mass index calculation, and insulin resistance index by homeostasis model assessment were also made.

RESULTS

We found that RBP4 levels were lower after weight reduction by gastric bypass surgery (p < 0.0001). We found RBP4 associated with triglycerides before (beta = 0.37, p = 0.02) and after surgery (beta = 0.59, p < 0.0001) and negatively with weight loss after surgery (beta = -0.37, p = 0.003). When expressed as a percentage of change, the decrease of RBP4 was related to the reduction in the levels of triglycerides and with the increase in HDL-cholesterol (beta = 0.73, p = 0.02 and beta = 0.62, p = 0.04, respectively). Others parameters analyzed, including inflammatory markers, were not related to RBP4.

CONCLUSIONS

This study shows that, in obese women and after a substantial weight loss due to bariatric surgery, RBP4 was related to weight status and lipid parameters rather than to insulin sensitivity or inflammatory markers.

Retinol binding protein 4 (RBP4) is primarily associated with adipose tissue mass in children

OBJECTIVE

Retinol binding protein 4 (RBP4) is a novel adipocytokine that may link obesity and insulin resistance. We aimed to discriminate between primary and secondary associations of RBP4 with obesity and related disease.

DESIGN

We applied clinical and experimental approaches to investigate the association of RBP4 levels with normal development, obesity, metabolic and cardiovascular parameters in 68 lean and 61 obese children.

RESULTS

RBP4 significantly increased with age and pubertal development in healthy lean children. Obese children had significantly higher RBP4 levels compared with lean controls (30.5±1.4 vs. 26.3±1.1 mg/L, P<0.05) and there was a clear association with BMI independent of age (r=0.33, P<0.0001). RBP4 levels correlated significantly with parameters of lipid and glucose metabolism, as well as cardiovascular parameters in univariate analyses. Multiple regression analyses confirmed the strong association of RBP4 with BMI z-score and age, while the association with most metabolic and cardiovascular parameters was abolished. To assess whether the association of RBP4 with obesity may be attributable to adipogenesis, we evaluated RBP4 expression and secretion during adipocyte differentiation using the human SGBS cell line. In preadipocytes, RBP4 mRNA expression was nearly undetectable but increased during differentiation up to approximately 1600-fold (P<0.05). Likewise, RBP4 secretion was restricted to mature adipocytes, further indicating that RBP4 is strongly related to differentiation of adipocytes.

CONCLUSION

RBP4 is a marker of adipose tissue mass and obesity already evident in children. The association of RBP4 with metabolic and cardiovascular sequelae of obesity appears to be secondary to the underlying relationship wtih body fat.

Comparative analysis of serum proteomes of patients with cardiovascular disease

OBJECTIVES

To monitor increases or decreases in cardiovascular disease (CVD)-related proteins that will be released from the deposits or plaque on the inner wall of blood vessels.

DESIGN AND METHODS

Protein profiles of sera from healthy subjects and CVD patients were determined via 2-DE. Differentially expressed spots in CVD patients were identified by ESI-Q-TOF MS/MS. Retinol binding protein 4 (RBP4), ceruloplasmin, and hemopexin were confirmed by Western blotting and RBP4 was further verified by ELISA.

RESULTS

Approximately, 400 spots were detected in each gel via comparisons of the serum proteome. Among these spots, 19 spots were selected and identified by ESI-Q-TOF MS/MS (P<0.05). The expression levels of RBP4 and ceruloplasmin were higher in CVD patients by Western blotting. The level of immunoreactive RBP4 in CVD patients was higher than that in healthy subjects.

CONCLUSIONS

The three proteins identified in the present study may constitute potential biomarkers for the diagnosis of CVD in patients.

Association of serum levels of retinol-binding protein 4 with male sex but not with insulin resistance in obese patients

BACKGROUND

Retinol-binding protein 4 (RBP4) is secreted by adipocytes and by the liver and modulates insulin sensitivity in animal models. However, controversial data exist regarding the association of serum levels of RBP4 with insulin resistance in humans. Obesity confers a major risk to develop insulin resistance.

AIM

Therefore, we investigated RBP4 levels in non-diabetic obese patients and analysed the association with insulin sensitivity and other metabolic markers.

SUBJECTS AND METHODS

Glucose tolerance was assessed by oral glucose tolerance tests and 70 normal glucose tolerant patients (36 women, 34 men; body mass index >30 kg/m(2)) were included in our study. We compared the serum level of RBP4 (measured by ELISA) with clinical features (age, sex, BMI, waist-to-hip-ratio, blood pressure) and laboratory findings (total cholesterol, triglycerides, fasting glucose, 2-hour glucose, fasting insulin, HOMA-IR and HOMA-B). The associations between RBP4 and the above mentioned variables were assessed using multiple linear regression models.

RESULTS

The mean age (+/-SD) of the subjects included was 48.1 (+/- 12.3) years and the mean BMI 41.6 (+/- 7.4) kg/m(2). We found significantly higher RBP4 levels in men (53.0 +/- 20.8 microg/ml) than in women (39.7 +/- 12.3 microg/ml) (p = 0.0013). However, age and sex-adjusted multiple linear regression models showed no significant association of serum RBP4 levels with BMI, waist-to-hip-ratio, blood pressure, cholesterol, triglycerides, fasting glucose, 2-hour glucose, insulin resistance (as assessed by HOMA-IR), or insulin secretion (as assessed by HOMA-B).

CONCLUSION

Our data show higher RBP4 levels in obese men than in obese women. However, there was no association of RBP4 levels with insulin resistance or other components of the metabolic syndrome. We conclude that obesity might already be associated with elevated RBP4 levels which then show no additional correlation with metabolic markers.

Impact of micronutrient deficiencies on obesity

Micronutrient deficiencies have been found in obese individuals across age groups worldwide. While the effects of micronutrient deficiencies on human functions have been studied widely in different populations, there is limited information on how these micronutrient deficiencies affect obese populations. An examination of the available literature suggests associations exist between micronutrient deficiencies and obesity in different populations. These associations and possible mechanisms of the deficiencies' metabolic effects, such as their influence on leptin and insulin metabolism, are discussed here. Further studies are needed to clarify the roles of the different micronutrient deficiencies with respect to obesity and its comorbid conditions.

Increased adipose tissue expression of lipocalin-2 in obesity is related to inflammation and matrix metalloproteinase-2 and metalloproteinase-9 activities in humans

Lipocalin-2 (LCN2) is a novel adipokine with potential roles in obesity, insulin resistance, and inflammation. The aim of the present work was to evaluate the effect of obesity on circulating concentrations and gene and protein expression levels of LCN2 in human visceral adipose tissue (VAT) as well as its involvement in inflammation. VAT biopsies from 47 subjects were used in the study. Real-time PCR and Western-blot analyses were performed to quantify levels of LCN2 in VAT as well as the association with other genes implicated in inflammatory pathways. Forty-four serum samples were used to analyze the circulating concentrations of LCN2. Zymography analysis was used to determine the activity of matrix metalloproteinase (MMP) in VAT. Obese patients exhibited increased mRNA (p < 0.0001) and protein (p = 0.017) expression levels of LCN2 compared to lean subjects. Although no differences in plasma LCN2 concentrations were observed, increased circulating LCN2/MMP-9 complex levels were found (p = 0.038) in the obese group. Moreover, obese individuals showed increased (p < 0.01) activity of MMP-2 and MMP-9/LCN2 complex, while a positive correlation (p < 0.01) between MMP-2 and MMP-9 activities and BMI was observed. Gene and protein expression levels of LCN2 in VAT were positively associated with inflammatory markers (p < 0.01). These findings represent the first observation that mRNA and protein levels of LCN2 are increased in human VAT of obese subjects. Furthermore, LCN2 is associated with MMP-2 and MMP-9 activities as well as with pro-inflammatory markers suggesting its potential involvement in the low-grade chronic inflammation accompanying obesity.