Retinol-binding protein 4 levels are elevated in polycystic ovary syndrome women with obesity and impaired glucose metabolism

Nutritional Supplements and Complementary Therapies in Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) affects 1 in 5 women of reproductive age, and is characterized by menstrual irregularities, clinical or biochemical hyperandrogenism, and the presence of polycystic ovary morphology. One of the recommended treatment strategies in the international evidence-based guidelines is lifestyle modification, which includes diet and exercise, with the aim of improving a range of health outcomes. The incurable nature of PCOS reinforces the importance of developing novel and innovative symptomatic relief strategies, which are currently the only available approaches for improving quality of life for these women. Women with PCOS tend to be nutrient deficient in many common vitamins and minerals, thought to be associated with the psychological (depression, anxiety, etc.) and physiological (insulin resistance, diabetes, infertility, etc.) sequelae of the condition. Nutrient supplementation and the integration of complementary medicine as adjuncts to traditional lifestyle-based therapies in PCOS could therefore provide additional benefits to these women. In this review, we synthesize the evidence regarding nutrient supplementation and complementary therapies in PCOS, predominantly from randomized controlled trials, systematic reviews, and meta-analyses, to provide an overview of the state of knowledge in this field. The evidence to date suggests that specific vitamins (B-12, inositols, folate, vitamins D, E, and K), vitamin-like nutrients (bioflavonoids and α-lipoic acid), minerals (calcium, zinc, selenium, and chromium picolinate), and other formulations (melatonin, ω-3 fatty acids, probiotics, and cinnamon), as well as some complementary approaches such as acupuncture and yoga may be beneficial in PCOS. However, there remain areas of uncertainty and key limitations in the literature that must be overcome before these therapies can be integrated into routine clinical practice.

The Effect of RBP4 on microRNA Expression Profiles in Porcine Granulosa Cells

Simple Summary

Retinol binding protein 4 (RBP4), mainly secreted by the liver and adipocytes, is a transporter of vitamin A. RBP4 has been shown to be involved in several pathophysiological processes, such as polycystic ovary syndrome (PCOS), obesity, insulin resistance, and cardiovascular risk. However, the role of RBP4 in mammalian follicular granulosa cells (GCs) remains largely unknown. To characterize the molecular pathways associated with the effects of RBP4 on GCs, we used sRNA deep sequencing to detect differential microRNA (miRNA) expression in GCs overexpressing RBP4. A total of 17 miRNAs were significantly different between the experimental and control groups. Our results support the notion that several miRNAs are involved in important biological processes associated with folliculogenesis and pathogenesis. These results will be useful for further studies investigating the role of RBP4 in porcine GCs.

Abstract

Retinol binding protein 4 (RBP4) is a transporter of vitamin A that is secreted mainly by hepatocytes and adipocytes. It affects diverse pathophysiological processes, such as obesity, insulin resistance, and cardiovascular diseases. MicroRNAs (miRNAs) have been reported to play indispensable roles in regulating various developmental processes via the post-transcriptional repression of target genes in mammals. However, the functional link between RBP4 and changes in miRNA expression in porcine granulosa cells (GCs) remains to be investigated. To examine how increased expression of RBP4 affects miRNA expression, porcine GCs were infected with RBP4-targeted lentivirus for 72 h, and whole-genome miRNA profiling (miRNA sequencing) was performed. The sequencing data were validated using real-time quantitative polymerase chain reaction (RT-qPCR) analysis. As a result, we obtained 2783 known and 776 novel miRNAs. In the experimental group, 10 and seven miRNAs were significantly downregulated and upregulated, respectively, compared with the control group. Ontology analysis of the biological processes of these miRNAs indicated their involvement in a variety of biological functions. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses indicated that these miRNAs were involved mainly in the chemokine signaling pathway, peroxisome proliferators-activated receptors (PPAR) signaling pathway, insulin resistance pathway, nuclear factor-kappa B(NF-kappa B) signaling pathway, and steroid hormone biosynthesis. Our results indicate that RBP4 can regulate the expression of miRNAs in porcine GCs, with consequent physiological effects. In summary, this study profiling miRNA expression in RBP4-overexpressing porcine GCs provides an important reference point for future studies on the regulatory roles of miRNAs in the porcine reproductive system.

The Complementary and Alternative Medicine for Polycystic Ovary Syndrome: A Review of Clinical Application and Mechanism

As a reproductive endocrine disease, polycystic ovary syndrome (PCOS) has influenced billions of women during childbearing age worldwide. Owing to its complex etiology and ambiguous pathogenesis, there is still not a specific method to cure it. Clinical treatments, such as hormone therapy and surgical treatment, have side effects. Therefore, it is essential and urgent to seek alternative treatment to solve these problems. The satisfactory efficacy of complementary and alternative medicine (CAM), such as traditional Chinese medicine (TCM), immunotherapy, medicinal foods, vitamin therapy, diet therapy, psychotherapy, spa, and oxygen therapy, in treating PCOS, has aroused an increasing number of medical workers' concern and gradually become the mainstream. This paper reviews the application of CAM in the treatment of PCOS, especially from the perspective of TCM. Meanwhile, the limitations of the literature about CAM in the treatment of PCOS are mentioned and analyzed as well.

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.

Serum retinol-binding protein 4 levels in polycystic ovary syndrome

OBJECTIVE

Serum levels of retinol-binding protein 4 (RBP4), an adipokine thought to affect systemic insulin sensitivity, were compared between women with polycystic ovary syndrome (PCOS) and non-PCOS controls to evaluate the association of RBP4 with clinical, hormonal and metabolic parameters of PCOS.

SUBJECTS AND METHODS

Serum RBP4 levels were analysed in 278 women with PCOS (age range 18-57 years) and 191 non-PCOS controls (age 20-53 years) by enzyme-linked immunosorbent assay.

RESULTS

Serum levels of RBP4 were increased in women with PCOS compared with control women in the whole population (45.1 ± 24.0 (s.d.) vs 33.5 ± 18.3 mg/L, P < 0.001). Age-stratified analysis showed that serum RBP4 levels were increased in women with PCOS aged ≤30 years compared with controls (47.7 ± 23.5 vs 27.1 ± 10.4 mg/L, P < 0.001), whereas no significant differences were seen in the other age groups. No significant correlations of RBP4 were seen with either steroids or indices of insulin resistance.

CONCLUSIONS

Although serum RBP4 levels were increased in younger women with PCOS compared with age-matched non-PCOS controls, RBP4 does not seem to be a good marker of insulin resistance or other metabolic derangements in women with PCOS.

The effect of nutrient supplementation in the management of polycystic ovary syndrome-associated metabolic dysfunctions: A critical review

Polycystic ovary syndrome (PCOS) is complex heterogeneous disorder that has several aspects in terms of pathology such as metabolic, endocrine, reproductive, and psychological. However, the etiology of PCOS remains poorly understood. Several studies suggest that insulin resistance and hyperandrogenism play a central role in the progression of PCOS pathophysiology. Therefore, common treatment strategies of PCOS are based on lifestyle modification, which include exercise, diet, and nutrient supplementation therapy. Recent studies have recommended some nutrients such as vitamins, minerals, and vitamin-like nutrients for the therapy of PCOS because each has at least one functional property in PCOS-induced pathways. Therefore, it is claimed that the cause of PCOS could be vitamin or mineral deficiency. This review aims to provide a critical literature survey on nutritional supplementation for the treatment of PCOS-associated endocrine and metabolic dysfunctions and discuss the role of nutrients in the management of PCOS in view of the clinical trials and experimental studies.

Adipose tissue and reproductive health

The understanding of adipose tissue role has evolved from that of a depot energy storage organ to a dynamic endocrine organ. While genetics, sexual phenotype and sex steroids can impact the mass and distribution of adipose tissue, there is a counter-influence of white adipocytes on reproduction. This primarily occurs via the secretion of adipokines, the most studied of which- leptin and adiponectin- are highlighted in this article. Leptin, the "satiety hormone" primarily acts on the hypothalamus via pro-opiomelanocortin (POMC), neuropeptide Y (NPY), and agouti-related peptide (AgRP) neurons to translate acute changes in nutrition and energy expenditure, as well as chronic adipose accumulation into changes in appetite and potentially mediate insulin resistance via shared pathway and notably impacting reproductive health via influence on GnRH secreting neurons. Meanwhile, adiponectin is notable for its action in mediating insulin sensitivity, with receptors found at every level of the reproductive axis. Both have been examined in the context of physiologic and pathologic reproductive conditions. Leptin has been shown to influence puberty, pregnancy, hypothalamic amenorrhea, and lipodystrophy, and with a potential therapeutic role for both metabolic and reproductive health. Adiponectin mediates the relative state of insulin resistance in pregnancy, and has been implicated in conditions such as polycystic ovary syndrome and reproductive malignancies. There are numerous other adipokines, including resistin, visfatin, chemerin and retinol binding protein-4, which may also play roles in reproductive health and disease states. The continued examination of these and other adipokines in both normal reproduction and reproductive pathologies represents an important avenue for continued study. Here, we seek to provide a broad, yet comprehensive overview of many facets of these relationships and highlight areas of consideration for clinicians and future study.

New markers of insulin resistance in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrine-metabolic disorder in women of reproductive age. The diagnostic criteria include two out of three features: hyperandrogenism, polycystic ovaries on ultrasound and menstrual irregularities (Rotterdam Criteria 2003). PCOS patients are more vulnerable to develop diabetes, cardiovascular diseases and metabolic syndrome. Insulin resistance (IR) is prevalent in women with PCOS independently of obesity and is critically involved in reproductive and metabolic complications of the syndrome. Several tests have been developed to measure IR, some very reliable but complex like the hyperinsulinemic euglycemic glucose clamp and others less precise but easier and less invasive like HOMA-IR. New markers are needed to reach a more reliable assessment of insulin resistance. To date, several surrogate markers have been proposed in the literature to facilitate and improve the determination of IR. Many new proteins are strongly involved with PCOS physiopathology and IR, such as some adipocytokines (adiponectin, visfatin, vaspin and apelin), copeptin, irisin, PAI-1 and zonulin. Many other proteins have been proposed as potential new markers of IR in PCOS, such as resistin, leptin, RBP4, kisspetin and ghrelin, but their role is still controversial. In this review, we provide a short characterization of these new markers, recently studied as indicators of metabolic state.

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.

Retinol-binding protein 4 is elevated and is associated with free testosterone and TSH in postmenopausal women

The aim of this study was to understand the relationship of retinol-binding protein 4 (RBP4) with hormonal and biochemical parameters in pre- and postmenopausal women. We included 69 postmenopausal women and 27 regularly menstruating premenopausal women. Postmenopausal women had statistically significantly higher RBP4 levels when compared to premenopausal women. RBP4 levels were negatively associated with free testosterone and positively associated with thyroid stimulating hormone in postmenopausal women. In premenopausal women RBP4 was positively associated with body mass index. RBP4 levels were increased in postmenopausal women. Although the mechanism is not clear, these findings suggest that RBP4 has a role in the regulation of hormonal and metabolic parameters.

Kisspeptin, leptin, and retinol-binding protein 4 in women with polycystic ovary syndrome

OBJECTIVES

To compare plasma kisspeptin, serum leptin, and serum retinol-binding protein 4 (RBP4) levels in women with and without polycystic ovary syndrome (PCOS) and to correlate these among each other and with clinical, hormonal, and metabolic parameters.

METHODS

Ninety women, including 54 women with PCOS and 36 without PCOS, participated in this study. For all patients, history and physical examinations were performed and blood samples were collected between days 3 and 8 of a spontaneous bleeding episode in the PCOS group and during normal menses of controls. Plasma kisspeptin, serum leptin, and serum RBP4 levels were measured using specific commercial assays.

RESULTS

Kisspeptin, leptin, and RBP4 levels were significantly higher in the PCOS group than in controls. Kisspeptin and RBP4 levels were significantly higher among obese PCOS patients than controls. Leptin levels were higher among obese PCOS patients than non-obese PCOS patients or controls. Kisspeptin and leptin levels of PCOS patients were significantly correlated with RBP4 levels. When only obese PCOS patients were analyzed, kisspeptin levels correlated with only the free androgen index.

CONCLUSIONS

These findings suggest that kisspeptin, leptin, and RBP4 are associated with metabolic disturbances in women with PCOS.

Liver retinol transporter and receptor for serum retinol-binding protein (RBP4)

Vitamin A (retinol) is absorbed in the small intestine, stored in liver, and secreted into circulation bound to serum retinol-binding protein (RBP4). Circulating retinol may be taken up by extrahepatic tissues or recycled back to liver multiple times before it is finally metabolized or degraded. Liver exhibits high affinity binding sites for RBP4, but specific receptors have not been identified. The only known high affinity receptor for RBP4, Stra6, is not expressed in the liver. Here we report discovery of RBP4 receptor-2 (RBPR2), a novel retinol transporter expressed primarily in liver and intestine and induced in adipose tissue of obese mice. RBPR2 is structurally related to Stra6 and highly conserved in vertebrates, including humans. Expression of RBPR2 in cultured cells confers high affinity RBP4 binding and retinol transport, and RBPR2 knockdown reduces RBP4 binding/retinol transport. RBPR2 expression is suppressed by retinol and retinoic acid and correlates inversely with liver retinol stores in vivo. We conclude that RBPR2 is a novel retinol transporter that potentially regulates retinol homeostasis in liver and other tissues. In addition, expression of RBPR2 in liver and fat suggests a possible role in mediating established metabolic actions of RBP4 in those tissues.

Study of Omentin1 and Other Adipokines and Hormones in PCOS Patients

OBJECTIVES

Polycystic ovary syndrome (PCOS) is associated with insulin resistance and obesity. Recent studies have shown that plasma omentin-1 levels decrease with obesity. Currently, no data exists on the relative correlation between omentin-1 with other adipokines or the expression and regulation of omentin-1 in the serum of women with PCOS. The objective of this study is to evaluate the role of omentin-1 levels or omentin-1 /adipokines ratio in the serum of women with PCOS compared with matched control subjects.

METHODS

The study involved 60 patients with PCOS and 30 women without PCOS who were used as controls. To examine the relationship between fasting serum omentin-1 and serum interleukin-6 (IL-6), interleukin-8 (IL-8), resistin, ghrelin, leptin RBP-4 and tumor necrosis factor-a (TNF-a) levels in infertile PCOS and non-PCOS subjects. Also, insulin and other hormones were measured in both groups. All these factors were measured by enzyme-linked immunosorbent assays.

RESULTS

From the total of 60 cases, there was a significant increase (p<0.001) in PCOS patients when compared to the control group in fasting serum, serum interleukin-6 (IL-6), interleukin-8 (IL-8), resistin, leptin RBP-4, tumor necrosis factor-a (TNF-a) levels and insulin. A significant decrease in omentin-1 and ghrelin (p<0.001) was observed. The results also showed that 93.33% and 98.30% in PCOS patients had abnormal omentin1: Insulin ratio and omentin1: Resistin ratio respectively according to the cut off values ≤27.42 and ≤31.35. Moreover, 81.67% of PCOS patients had abnormal omentin1:IL-6 ratio according to the cut of value (≤66.09).

CONCLUSION

This is the first time the role of plasma omentin1 has been investigated with respect to its implication in PCOS. Usually, LH/FSH ratio and FAI (ratio of total testosterone to SHBG) are the important factors used for the diagnosis of PCOS in all previous literature, but none of them referred to the parameters discussed in this report. Also, the percentage of sensitivity and the difference between range of these parameters in PCOS patients and the controls may give a different perspective in trying to understand the etiology of PCOS. Therefore, these parameters may be used for future diagnosis of PCOS. This study also suggested that omentin/resistin ratio may play a crucial paracrine or endocrine role in modulating insulin sensitivity.

Metabolic manifestations of polycystic ovary syndrome in nonobese adolescents: retinol-binding protein 4 and ectopic fat deposition

OBJECTIVE

To determine whether nonobese adolescents with polycystic ovary syndrome (PCOS) have higher levels of retinol-binding protein 4 (RBP4) and ectopic fat than controls and whether RBP4 and ectopic fat correlate with comorbidities of metabolic disease.

DESIGN

Cross-sectional case-control study.

SETTING

Pediatric clinical research center based in a quaternary care medical center.

PATIENT(S)

Twenty-four nonobese adolescents between the ages of 13 and 21 years, 13 with PCOS and 11 controls.

INTERVENTION(S)

Measurement of RBP4, insulin resistance, lipids, and body composition.

MAIN OUTCOME MEASURE(S)

Retinol-binding protein 4, reproductive and adrenal hormones, insulin resistance, intrahepatic and intramyocellular lipid levels, and visceral adipose tissue.

RESULT(S)

Adolescents with PCOS had higher intrahepatic lipid content and a statistical trend for higher RBP4 compared with controls. Retinol-binding protein 4 correlated with body fat, triglycerides, insulin resistance, and androgens but not intrahepatic lipid content; however, when adjusted for body fat, the correlation between RBP4 and triglycerides weakened to a statistical trend and was no longer statistically significant for the other measures.

CONCLUSION(S)

This small preliminary study of nonobese adolescent girls suggests that RBP4 may be involved in the dyslipidemia associated with PCOS and that there may be an independent relationship between RBP4 and triglycerides but not between RBP4 and insulin resistance. Although intrahepatic lipid content was higher in PCOS, it did not correlate with RBP4, triglycerides, or insulin resistance.

Consensus on women's health aspects of polycystic ovary syndrome (PCOS): the Amsterdam ESHRE/ASRM-Sponsored 3rd PCOS Consensus Workshop Group

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder in females, with a high prevalence. The etiology of this heterogeneous condition remains obscure, and its phenotype expression varies. Two widely cited previous ESHRE/ASRM sponsored PCOS consensus workshops focused on diagnosis (published in 2004) and infertility management (published in 2008), respectively. The present third PCOS consensus report summarizes current knowledge and identifies knowledge gaps regarding various women's health aspects of PCOS. Relevant topics addressed-all dealt with in a systematic fashion-include adolescence, hirsutism and acne, contraception, menstrual cycle abnormalities, quality of life, ethnicity, pregnancy complications, long-term metabolic and cardiovascular health, and finally cancer risk. Additional, comprehensive background information is provided separately in an extended online publication.

The relationship between retinol-binding protein 4 levels, insulin resistance, androgen hormones and polycystic ovary syndrome

BACKGROUND

Retinol-binding protein 4 (RPB4), a 21-kDa peptide, is a recently identified adipokine that may contribute to the pathogenesis of polycystic ovary syndrome (PCOS). The aim of this study was to explore the association between serum RBP4 levels, androgen hormones and insulin resistance (IR) in women with PCOS.

METHODS

In this case-control study, 75 PCOS patients and 53 age- and body mass index (BMI)-matched control subjects referred to the Zanjan Metabolic Disease Research Center were enrolled. Serum RBP4 was measured using an enzyme-linked immunosorbent assay. BMI, waist circumference (WC), fasting levels of glucose, lipid profiles and insulin were also measured. A homeostatic model assessment of insulin resistance (HOMA-IR) value was used to determine the level of insulin resistance.

RESULTS

PCOS cases had significantly higher serum RBP4 and insulin levels than control subjects (44130 ± 12760 vs. 32980 ± 9560 μg/L, p < 0.001, and 11790 ± 11480 vs. 7890 ± 4300 μU/L, p < 0.05, respectively), in univariable analysis. RBP4 showed a positive correlation with serum testosterone (r = 0.62, p < 0.0001), dehydroepiandrosterone sulfate (r = 0.45, p < 0.0001) and the waist circumference (r = 0.37, p < 0.001) of PCOS patients but not with other measured clinical and biochemical variables. However, no correlation was observed between serum RBP4 levels and HOMA-IR in all studied subjects. A final logistic regression analysis demonstrated that testosterone and dehydroepiandrosterone sulfate are independently associated with PCOS.

CONCLUSION

These findings indicate that RBP4 is not independently associated with PCOS. The elevation of RBP4 levels in PCOS women might be influenced by androgen hormones. Further prospective studies are needed to clarify molecular mechanisms.

Serum lipocalin-2 as an insulin resistance marker in patients with polycystic ovary syndrome

BACKGROUND

Our aim was to investigate levels of lipocalin-2 and its relationship with metabolic factors in women with polycystic ovary syndrome (PCOS).

MATERIALS/SUBJECTS AND METHODS

In this cross-sectional study, two groups of women were studied: group 1 consisted of women with PCOS (no.=30), and group 2 consisted of control women with normal ovulatory function (no.=30). The circulating levels of free testosterone (T), DHEAS, glucose, insulin, triglycerides (TG), HDL, LDL and lipocalin were measured. Insulin resistance was assessed using the homeostasis model assessment (HOMA-IR). In order to determine a lipocalin value indicating insulin resistance, receiver operating characteristic (ROC) curves were established.

RESULTS

Serum lipocalin was significantly higher in PCOS subjects (54.26 ± 15.58 vs 26.09 ± 7.47 ng/ml, p=0.0001).We found a close correlation between lipocalin and insulin, lipocalin and HOMA-IR, lipocalin and T, and lipocalin and DHEAS. A cut-off level of >39.54 ng/ml for serum lipocalin has a predictive value for insulin resistance of 81% sensitivity and 82.1% specificity.

CONCLUSION

In our study, lipocalin-2 levels were found to be significantly higher in women with PCOS compared to body mass index-matched controls. Serum lipocalin-2 may prove to be a useful marker for insulin resistance in patients with PCOS.

Retinol-binding protein 4: a novel adipokine implicated in the genesis of LGA in the absence of gestational diabetes mellitus

OBJECTIVE

Adipokines (cytokines produced by adipose tissue) play a major role in the control of body weight and energy distribution. Retinol-binding protein 4 (RBP4), only recently recognized as an adipokine, has been proposed to modulate systemic insulin sensitivity. The goal of this study was to determine whether there is an association between maternal plasma RBP4 concentration and the birth of a large-for-gestational-age (LGA) newborn in women with and without gestational diabetes mellitus (GDM).

STUDY DESIGN

This cross-sectional study included pregnant women at term in the following groups: 1) normal pregnancy with an appropriate-for-gestational-age (AGA) neonate (n=64); 2) normal pregnancy with an LGA neonate (n=44); 3) GDM with an AGA neonate (n=55); and 4) GDM with an LGA neonate (n=42). Maternal plasma RBP4 concentration was determined by ELISA. Parametric and non-parametric statistics were used for analyses.

RESULTS

1) Patients with GDM, either with AGA or LGA neonates, had a higher median plasma concentration of RBP4 than normal pregnant women who delivered an AGA neonate (P=0.01 and P=0.008, respectively); 2) mothers without GDM but with LGA neonates had a higher median plasma concentration of RBP4 than those with normal pregnancy and AGA newborns (P=0.001); 3) these findings remained significant after adjusting for maternal age, body mass index and gestational age at blood sampling.

CONCLUSION

GDM is characterized by alterations in maternal circulating RBP4 concentrations akin to those of Type 2 diabetes mellitus. RBP4 concentrations in maternal plasma may play a role in accelerated fetal growth in the absence of overt carbohydrate intolerance.

Visfatin and retinol-binding protein 4 concentrations in lean, glucose-tolerant women with PCOS

Since insulin resistance is accepted to be a common feature of polycystic ovary syndrome (PCOS), the exact molecular mechanism(s) involved in glucose and lipid metabolism have been under investigation in the syndrome. Recently, two novel adipokines, namely visfatin and retinol-binding protein 4 (RBP4), have been suggested to play a role in insulin resistance and diabetes. This study sought to determine whether plasma concentrations of visfatin and RBP4 are altered in PCOS by comparing a total of 27 lean, normal glucose-tolerant PCOS patients with 19 age- and body mass index-matched healthy controls. The mean plasma visfatin concentrations were higher in PCOS patients than those in healthy subjects (37.9+/-18.2 versus 19.8+/-17.5, P<0.01), while RBP4 concentrations were similar between the two. Both adipokines were correlated with each other in the whole (r=0.50, P<0.01) and in PCOS (r=0.52, P<0.01) groups but not in controls. The results suggest that lean, glucose-tolerant women with PCOS have increased circulating visfatin and unaltered RBP4 concentrations compared with healthy lean women. In order to clarify overlapping effects and their potential contribution to the pathophysiology of PCOS, further studies are needed.

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

BACKGROUND

Retinol-binding protein-4 (RBP4) is a novel adipokine able to modulate the action of insulin in several tissues. A variable degree of insulin resistance characterizes the vast majority of hypertensive (HYP) patients. The aim of this study was to evaluate the relationship between RBP4 and essential hypertension, exploring potential links between RBP4 and other adipokines with some proxies of early vascular damage in female naive HYP patients.

METHODS

Serum RBP4, leptin, adiponectin, and resistin levels were determined in 35 HYP and 35 normotensive lean women with normal glucose tolerance paired by age and body mass index (BMI) served as controls (CTL); carotid intima-media thickness (IMT) was also measured.

RESULTS

A striking difference was observed in RBP4 levels between HYP and CTL with significantly higher levels in the former than in the latter. No relationship was observed between glomerular filtration rate (GFR) and RBP4. Adiponectin levels were slightly but significantly lower in HYP than in CTL, whereas no differences were observed in resistin and leptin concentrations between the two groups of women. In the whole study group, a strong linear relationship was observed between IMT value and both RBP4 (rho = 0.321, P = 0.0076) and resistin (rho = 0.340, P = 0.0048); these two adipocytokines, together with cholesterol, were the only variables independently related to IMT (r(2) = 0.24; P = 0.004) by a stepwise analysis.

CONCLUSIONS

RBP4 levels are increased in naive HYP women and correlated with the degree of IMT suggesting a participation of this adipocytokine in the modulation of the atherosclerotic process exerted by the adipose tissue as endocrine organ.

Serum retinol-binding protein 4 is associated with insulin secretion in Chinese people with normal glucose tolerance

BACKGROUND

Serum levels of retinol-binding protein 4 (RBP4) are associated with insulin resistance and type 2 diabetes mellitus (T2DM) and may impact on β-cell function. Thus, the present study investigated the relationship between serum RBP4 and insulin secretion in Chinese people with and without T2DM.

METHODS

A 75 g oral glucose tolerance test was administered to all 867 subjects and serum RBP4 concentrations were determined. Insulin secretion was assessed by ΔI/ΔG (increment in plasma insulin concentration/plasma glucose concentration 30 min after the oral administration of 75 g glucose) and the total area under the curve for insulin over 180 min (AUC-I). Magnetic resonance imaging was used to measure visceral fat area (VFA) at L4-L5; subjects with VFA ≥80 cm(2) were considered to have visceral obesity (VO).

RESULTS

Serum RBP4 concentrations were significantly higher in subjects with VO than without, regardless of the presence of T2DM. In addition, in the entire group with normal glucose tolerance (NGT), serum RBP4 was positively correlated with ΔI/ΔG (r = 0.152; P < 0.01) and AUC-I (r = 0.218; P < 0.01) after adjustment for gender. The correlation between RBP4 and ΔI/ΔG (r =  0.162; P < 0.05) and AUC-I (r = 0.195; P < 0.01) remained in NGT non-VO subjects. No correlation was found between serum RBP4 and ΔI/ΔG or AUC-I in T2DM patients. Stepwise multiple regression analysis showed that serum RBP4 is an independent factor that contributes to ΔI/ΔG (β = 0.176) and AUC-I (β = 0.204) in NGT non-VO subjects.

CONCLUSIONS

Serum RBP4 is correlated with glucose-stimulated insulin secretion in NGT non-VO subjects, but not in NGT VO subjects and T2DM patients.

Retinol-binding protein 4 in polycystic ovary syndrome--association with steroid hormones and response to pioglitazone treatment

CONTEXT

Polycystic ovary syndrome (PCOS) is frequently associated with insulin resistance.

OBJECTIVE

The aim of the study was to investigate a putative role of the adipokines retinol-binding protein 4 (RBP4), adiponectin, and visfatin in a cohort of patients with PCOS and their response to treatment with pioglitazone.

DESIGN AND SETTING

We conducted a randomized, controlled, double-blind study at a tertiary referral center.

PATIENTS AND INTERVENTIONS

Forty premenopausal women with PCOS were allocated to receive treatment with either pioglitazone (30 mg/d) or a placebo for a period of 3 months.

MAIN OUTCOME MEASURES

Serum concentrations of RBP4, adiponectin, and visfatin were determined along with metabolic and hormonal parameters before and after treatment.

RESULTS

Serum adiponectin concentrations were higher after treatment with pioglitazone (P = 0.003), whereas RBP4 levels tended to decrease (P = 0.06), and visfatin concentrations remained unchanged. We found RBP4 serum concentrations at baseline to be positively correlated with serum levels of testosterone (R = 0.446; P = 0.005), 17-OH progesterone (R = 0.345, P = 0.037), and dehydroepiandrosterone sulfate (R = 0.347; P = 0.041). However, these correlations were abolished after treatment with pioglitazone. Patients with high RBP4 levels had significantly higher hirsutism scores (P = 0.038 before and P = 0.034 after treatment). In contrast, serum adiponectin concentrations were related to parameters of impaired glucose metabolism, and no significant associations were detected for visfatin.

CONCLUSIONS

Our results suggest that RBP4 may contribute to endocrine changes and to the phenotypic manifestation of patients with PCOS because higher RBP4 concentrations are associated with higher androgen levels and higher clinical hirsutism scores independently of pioglitazone treatment. The molecular involvement of RBP4 in human steroid metabolism requires further clarification.

Serum retinol-binding protein 4 levels in nonobese women with polycystic ovary syndrome

OBJECTIVE

To test whether there was a difference in serum retinol-binding protein 4 (RBP4) levels between subjects with polycystic ovary syndrome (PCOS) and those with a healthy regular menstrual cycle and, in addition, to correlate serum RBP4 levels with a variety of parameters.

DESIGN

Clinical study.

SETTING

University hospital.

PATIENT(S)

A total of 74 nonobese women were evaluated. Thirty-seven had PCOS, whereas the remaining 37 served as control subjects.

INTERVENTION(S)

Serum RBP4 levels were analyzed using ELISA.

MAIN OUTCOME MEASURE(S)

Serum levels of FSH, LH, TSH, E(2), T, insulin, glucose, cholesterol, triglycerides, and RBP4.

RESULT(S)

The women with PCOS had higher levels of serum RBP4, waist-to-hip ratio, LH, T, insulin, homeostatic model assessment of insulin resistance, cholesterol, and triglycerides. Logistic regression analyses revealed a significant association between odds ratio (OR) values of PCOS and both T (OR = 1.125; 95% confidence interval [CI] 1.050-1.205), and cholesterol levels (OR = 1.029; 95% CI 1.004-1.056). Age and triglycerides were significantly correlated to serum RBP4 levels by multiple linear regression analysis.

CONCLUSION(S)

Our study has shown that [1] elevated RBP4 levels might arise from triglyceride metabolism, and that RBP4 levels might not be influenced by PCOS itself. [2] RBP4 might not be a useful marker of insulin resistance in subjects with PCOS.

Low free plasma levels of retinol-binding protein 4 in insulin-resistant subjects with polycystic ovary syndrome

BACKGROUND

It has been shown in animals and in humans that retinol-binding protein 4 (RBP4) production from adipose tissue leads to generalized insulin resistance (IR). A more sensitive marker of circulating RBP4 is free plasma RBP4 expressed by RBP4 to transthyretin (TTR) ratio, since in circulation RBP4 is bound to TTR.

AIM

To investigate RBP4 levels in insulin-resistant women with polycystic ovary syndrome (PCOS) and to estimate free plasma RBP4 expressed by RBP4/TTR ratio.

SUBJECTS AND METHODS

Thirty-five PCOS subjects were compared with 45 controls matched for age and body mass index (BMI). In each subject, fasting values of glucose, insulin, gonadotropins, estradiol, androgens, C-reactive protein (CRP), RBP4, and TTR were determined.

RESULTS

PCOS subjects in comparison to controls were more insulin-resistant [homeostasis model assessment for IR (HOMA-IR): 2.6+/-0.3 vs 1.9+/-0.1, p=0.043], and presented lower RBP4 levels (28.3+/-1.1 vs 32.4+/-1.2 microg/ml, p=0.021) and RBP4/TTR ratio (0.26+/-0.01 vs 0.31+/-0.01, p=0.0014). When RBP4 and RBP4/TTR values where stratified according to BMI status (obese, overweight, and lean subjects), it was noticed that both RBP4 and RBP4/TTR values in lean PCOS were significantly lower than in controls (RBP4: 25.0+/-5.5 vs 34.1+/-9.0 microg/ml, p=0.0063, RBP4/TTR: 0.25+/-0.3 vs 0.35+/-0.1, p=0.016). No correlation was observed between RBP4 and RBP4/TTR with any hormonal or metabolic parameter including BMI.

CONCLUSIONS

RBP4 and free plasma RBP4 expressed as RBP4/TTR ratio are statistically and significantly lower in insulin-resistant PCOS subjects in comparison to controls. Therefore, our findings do not confirm a link between IR, neither with RBP4 nor with free plasma RBP4 levels. The significance of these findings remains to be elucidated, since RBP4 might prove to have different actions, like other adipokines, from humans and rodents.

Adipocytokines and the metabolic complications of obesity

CONTEXT

Adipose tissue is increasingly recognized as an active endocrine organ with many secretory products and part of the innate immune system. With obesity, macrophages infiltrate adipose tissue, and numerous adipocytokines are released by both macrophages and adipocytes. Adipocytokines play important roles in the pathogenesis of insulin resistance and associated metabolic complications such as dyslipidemia, hypertension, and premature heart disease.

EVIDENCE ACQUISITION

Published literature was analyzed with the intent of addressing the role of the major adipose secretory proteins in human obesity, insulin resistance, and type 2 diabetes.

EVIDENCE SYNTHESIS

This review analyzes the characteristics of different adipocytokines, including leptin, adiponectin, pro-inflammatory cytokines, resistin, retinol binding protein 4, visfatin, and others, and their roles in the pathogenesis of insulin resistance.

CONCLUSIONS

Inflamed fat in obesity secretes an array of proteins implicated in the impairment of insulin signaling. Further studies are needed to understand the triggers that initiate inflammation in adipose tissue and the role of each adipokine in the pathogenesis of insulin resistance.

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

OBJECTIVE

Controversy exists regarding the elevation of serum retinol-binding protein 4 (RBP4) in human obesity and type 2 diabetes mellitus (T2DM). In the present study, we have compared serum RBP4 in lean and obese patients with or without T2DM, and analysed the effect of weight loss on serum RBP4.

DESIGN

Forty-two Caucasian subjects were included in the study. Serum RBP4 was measured by ELISA and Western blot. In addition, serum RBP4 was measured in 21 morbidly obese patients before and after 4, 8 and 15 months of weight loss following Roux-en-Y gastric bypass (RYGBP).

RESULTS

No significant effect of either obesity or diabetes on serum RBP4 was observed. Serum RBP4 concentrations (measured by either ELISA or Western blot) did not correlate with body mass index (BMI), body fat or any indicator of glucose metabolism or insulin resistance. Weight loss following RYGBP did not modify serum RBP4 at 15 months (P = 0.472). However, the variations in serum RBP4 were significantly associated with the reduction in body fat (r = 0.48; P = 0.026). Patients loosing over 20% of fat mass (n = 11) showed significantly different RBP4 concentrations compared to those individuals exhibiting smaller adiposity reductions (n = 10) (-11.0 +/- 6.4 vs.+5.8 +/- 3.6 mg/l; P = 0.036). Furthermore, RBP4 levels were significantly reduced at 4 (P = 0.006) and 8 (P = 0.015) months only in those patients loosing over 20% of fat mass.

CONCLUSION

Serum RBP4 concentrations are not increased in obese patients with or without T2DM. A decrease in RBP4 levels was only observed after surgically induced weight loss accompanied by relevant reductions in body fat. RBP4 might be considered as a dynamic marker of negative energy balance being reduced during weight loss when a negative energy balance threshold is reached. Furthermore, RBP4 variation in the first month after RYGBP may be a predictor of weight loss success.

Retinol to retinol-binding protein (RBP) is low in obese adults due to elevated apo-RBP

Elevated serum retinol-binding protein (RBP) concentration has been associated with obesity and insulin resistance, but accompanying retinol values have not been reported. Assessment of retinol is required to discriminate between apo-RBP, which may act as an adipokine, and holo-RBP, which transports vitamin A. The relations between serum RBP, retinol, retinyl esters, BMI, and measures of insulin resistance were determined in obese adults. Fasting blood (> or =8 h) was collected from obese men and women (n = 76) and blood chemistries were obtained. Retinol and retinyl esters were quantified by HPLC and RBP by ELISA. RBP and retinol were determined in age and sex-matched, nonobese individuals (n = 41) for comparison. Serum apo-RBP was two-fold higher in obese (0.90 +/- 0.62 microM) than nonobese subjects (0.44 +/- 0.56 microM) (P < 0.001). The retinol to RBP ratio (retinol:RBP) was significantly lower in obese (0.73 +/- 0.13) than nonobese subjects (0.90 +/- 0.22) (P < 0.001) and RBP was strongly associated with retinol in both groups (r = 0.71 and 0.90, respectively, P < 0.0001). In obese subjects, RBP was associated with insulin (r = 0.26, P < 0.05), homeostatic model assessment of insulin resistance (r = 0.29, P < 0.05), and quantitative insulin sensitivity check index (r = -0.27, P < 0.05). RBP was associated with BMI only when obese and nonobese subjects were combined (r = 0.25, P < 0.01). Elevated serum RBP, derived in part from apo-RBP, was more strongly associated with retinol than with BMI or measures of insulin resistance in obese adults. Investigations into the role of RBP in obesity and insulin resistance should include retinol to facilitate the measurement of apo-RBP and retinol:RBP. When evaluating the therapeutic potential of lowering serum RBP, consideration of the consequences of vitamin A metabolism is paramount.

Retinol-binding protein 4 and insulin resistance in polycystic ovary syndrome

OBJECTIVE

Polycystic ovary syndrome (PCOS) is an insulin-resistant state with insulin resistance being an established therapeutic target; however, measurement of insulin resistance remains challenging. We aimed to 1) determine serum retinol-binding protein 4 (RBP4) levels (purported to reflect insulin resistance) in women with PCOS and control subjects, 2) examine the relationship of RBP4 to conventional markers of insulin resistance, and 3) examine RBP4 changes with interventions modulating insulin resistance in overweight women with PCOS.

RESEARCH DESIGN AND METHODS

At baseline, 38 overweight women (BMI >27 kg/m(2)) with PCOS and 17 weight-matched control subjects were compared. Women with PCOS were then randomly assigned to 6 months of a higher-dose oral contraceptive pill (OCP) (35 microg ethinyl estradiol/2 mg cyproterone acetate) or metformin (1 g b.i.d.). Outcome measures were insulin resistance (total insulin area under the curve) on an oral glucose tolerance test, RBP4, and metabolic/inflammatory markers.

RESULTS

Overweight women with PCOS were more insulin resistant than control subjects, yet RBP4 levels were not different in women with PCOS versus those in control subjects (35.4 +/- 4.3 vs. 28.9 +/- 3.1 microg/ml, P = 0.36). RBP4 correlated with cholesterol and triglycerides but not with insulin resistance. Metformin improved insulin resistance by 35%, whereas the OCP worsened insulin resistance by 33%. However, RBP4 increased nonsignificantly in both groups (43.7 +/- 6.3 vs. 42.6 +/- 5.5 microg/ml, P = 0.92).

CONCLUSIONS

Overweight women with PCOS were more insulin resistant than control subjects, but this finding was not reflected by RBP4 levels. RBP4 correlated with lipid levels but not with insulin resistance markers. RBP4 levels did not change when insulin resistance was reduced by metformin or increased by the OCP. These data suggest that RBP4 is not a useful marker of insulin resistance in PCOS but may reflect other metabolic features of this condition.

Retinol-binding protein 4 and its relation to insulin resistance in obese children before and after weight loss

CONTEXT

There are limited and controversial data concerning the relationships between retinol-binding protein 4 (RBP4), weight status, and insulin resistance in obese humans and especially in children.

OBJECTIVE

Our objective was to study the longitudinal relationships among RBP4, insulin resistance and weight status in obese children.

DESIGN, SETTING, AND PATIENTS

We conducted a 1-yr longitudinal follow-up study in a primary-care setting with 43 obese children (median age 10.8 yr) and 19 lean children of same the age and gender.

INTERVENTION

Our outpatient 1-yr intervention program was based on exercise, behavior, and nutrition therapy.

MAIN OUTCOMES MEASURES

Changes of weight status (body mass index sd score), RBP4, molar RBP4/serum retinol (SR) ratio, insulin resistance index homeostasis model assessment (HOMA), and quantitative insulin sensitivity check index (QUICKI).

RESULTS

Obese children had significantly (P < 0.01) higher RBP4 concentrations and a higher RBP4/SR ratio compared with lean children. In multiple linear regression analyses adjusted to age, gender, and pubertal stage, RBP4 was significantly correlated to insulin and body mass index. Pubertal children demonstrated significantly decreased QUICKI and significantly increased HOMA index, insulin, and RBP4 concentrations compared with prepubertal children. Changes of RBP4 correlated significantly to changes of insulin (r = 0.29), HOMA index (r = 0.29), QUICKI (r = 0.22), and weight status (r = 0.31). Substantial weight loss in 25 children led to a significant (P < 0.001) decrease of RBP4, RBP4/SR, blood pressure, triglycerides, insulin, and HOMA index and an increase in QUICKI in contrast to the 18 children without substantial weight loss.

CONCLUSION

RBP4 levels were related to weight status and insulin resistance in both cross-sectional and longitudinal analyses, suggesting a relationship between RBP4, obesity, and insulin resistance in children.

Retinol-binding protein 4 is associated with insulin resistance, but appears unsuited for metabolic screening in women with polycystic ovary syndrome

OBJECTIVE

Adiposity, insulin resistance (IR), and hyperandrogenism are features of polycystic ovary syndrome (PCOS). Retinol-binding protein 4 (RBP4) secreted from adipose and liver tissues has been linked to IR. The impact of RBP4 on IR in PCOS and its usability to identify women with metabolic syndrome (MS) or impaired glucose tolerance ((IGT) or diabetes) were investigated.

DESIGN

Plasma RBP4 was determined in 115 consecutive PCOS women. Associations with IR, body composition, and hyperandrogenemia were investigated by correlation and multiple linear regression analyses in 110 non-diabetics. Receiver operating characteristic curve analysis was used to evaluate RBP4 as a parameter for identifying MS and IGT or diabetes.

RESULTS

RBP4 increased over tertiles of IR (P=0.009). RBP4 correlated with HOMA %S (R=-0.286, P= 0.002), waist-to-hip ratio (WHR) (R=0.233, P=0.034), and dual energy X-ray absorptiometry (DEXA)-lean body mass (R=0.282, P=0.016) but not with body mass index (BMI), DEXA-total or -trunk fat mass, hsCRP, free testosterone, DHEAS, androstenedione, and 17beta-estradiol. Adjusted for age, BMI, smoking, and IGT, the association between RBP4 and HOMA %S remained significant (P=0.032). RBP4 explained 4.6% of the variation in HOMA %S. RBP4 was higher in MS and IGT or diabetes, but its ability to identify these women was low (area under the curve, AUC=0.631, P=0.041 or AUC=0.660, P=0.016).

CONCLUSIONS

In PCOS, RBP4 has a small independent impact on IR. It is not correlated with hyperandrogenemia, 17beta-estradiol, other adrenal steroids, or with markers of adiposity in general. Furthermore, RBP4 does not appear suitable for screening MS or impaired glucose metabolism (IGT or diabetes).

Retinol binding protein 4, low birth weight-related insulin resistance and hormonal contraception

It has been recently reported that increased serum levels of retinol binding protein 4 (RBP4), a molecule secreted by adipocytes and liver, could be an early marker of insulin resistance (IR). We determined whether serum RBP4 was increased in low birth weight (LBW)-young women as a model of early-onset IR, through a historical prospective study. The study-population included 35 LBW and 35 born at term appropriate for gestational age (term AGA) young women. Metabolic evaluations included the composite-insulin sensitivity index (composite ISI). Serum RBP4 was measured with a competitive enzyme-linked immunosorbent assay (ELISA). RBP4 levels were similar in LBW and term AGA women, while composite ISI was significantly lower in the former group. With multivariate logistic regression analysis hormonal contraception (HC) use but not birth weight, diabetes in either parents and body mass index was significantly associated with higher RBP4 levels: odds ratio = 10.6; 95% confidence interval (CI) = 2.4-76.6. In spite of higher RBP4 levels in women under HC, composite ISI was similar in women with or without HC. Women under HC also exhibited significantly higher levels of sex hormone binding globulin (SHBG), triglycerides, cholesterol, and C-reactive protein (CRP), and all of them, but not composite ISI, were significantly correlated with RBP4 levels. In conclusion, RBP4 serum level was not a marker of IR but, for the first time, it is documented a sustained increase of serum RBP4 under HC. Pathophysiological and clinical significance of this novel finding requires further investigations.