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

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.

Association between transthyretin concentrations and gestational diabetes mellitus in Chinese women

BACKGROUND

Transthyretin (TTR) is considered to be associated with insulin resistance in humans. This study aimed to investigate TTR level in gestational diabetes mellitus (GDM) and its association with glucose metabolism.

METHODS

Fifty pregnant women with GDM and 47 pregnant women with normal glucose tolerance matched for body mass index and age were enrolled in this study. Their blood samples were collected to detect TTR, retinol-binding protein 4 (RBP4), and their association with glucose and lipid metabolism.

RESULTS

Serum TTR levels in the GDM group were significantly higher than those in the control group (median, 93.44 [interquartile range, 73.81, 117.79] μg/ml vs. 80.83 [74.19, 89.38] μg/ml; P = 0.006). GDM subjects had a lower RBP4/TTR ratio than the control subjects (median, 517.57 [interquartile range, 348.38, 685.27] vs. 602.56 [460.28, 730.62]; P = 0.02). The serum TTR concentrations were positively associated with neonatal weight (r = 0.223, P = 0.028), homeostatic model assessment of insulin resistance (r = 0.246, P = 0.015), and fasting blood glucose (FBG) (r = 0.363, P < 0.001). In stepwise multivariate linear regression analysis, FBG (standardized beta = 0.27, P = 0.004) and neonatal weight (standardized beta = 0.345, P < 0.001) were independent predictors of serum TTR concentrations. Additionally, FBG (standardized beta = - 0.306, P = 0.002) and triglyceride (TG) (beta = 0.219, P = 0.025) were independently associated with RBP4/TTR ratio.

CONCLUSIONS

Serum TTR concentrations were significantly higher in women with GDM than that in women without GDM, suggesting that elevated TTR level may play a role in the pathogenesis of GDM. Meanwhile, TTR was positively and independently associated with FBG and neonatal weight, while FBG and TG were independent predictors of RBP4/TTR ratio. Moreover, serum TTR levels and RBP4/TTR ratio were considered valuable markers of insulin resistance and GDM.

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.

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.

Circulating apelin level in relation to nutritional status in polycystic ovary syndrome and its association with metabolic and hormonal disturbances

OBJECTIVE

The aim of the study was to analyse relationships between plasma apelin-36 and apelin-12 levels, nutritional status, insulin resistance and hormonal disturbances, as well as plasma adiponectin, leptin and resistin concentrations in PCOS women.

STUDY DESIGN PATIENTS AND MEASUREMENTS

A cross-sectional study involving 87 PCOS (48 obese) and 67 non-PCOS women (36 obese). Anthropometric parameters and body composition were determined. Serum glucose, androgens, FSH, LH, SHBG, insulin, apelin-36, apelin-12, adiponectin, leptin and resistin were measured in the fasting state.

RESULTS

Plasma apelin-36 and apelin-12 levels were significantly higher in normal weight women than in the obese women with PCOS (3·1 ± 2·2 vs 1·2 ± 0·7 μg/l, P < 0·001; 2·9 ± 2·4 vs 0·5 ± 0·7 μg/l; P < 0·001 respectively). Both plasma apelin-36 and -12 levels correlated positively with adiponectin levels, and inversely with leptin or resistin levels. There was a negative correlation between plasma apelin-36, apelin-12 and serum LH levels. In addition, an inverse correlation between apelin-12 level and LH to FSH ratio was found. In multiple regression analysis 9% of LH variability was explained by apelin-12 levels (β = -0·14; P < 0·001).

CONCLUSIONS

Nutritional status seems to have different effects on apelin release, particularly, its active isoform, in women with PCOS compared with women without PCOS. This may be partially caused by changes in leptin and resistin secretion and may enhance pituitary-ovarian axis disturbances. The association between both isoforms of apelin and insulin resistance seems to be bidirectional.

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.

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.

Meta-analysis of cardiovascular disease risk markers in women with polycystic ovary syndrome

BACKGROUND

The relation between polycystic ovary syndrome (PCOS) and cardiovascular disease (CVD) remains unclear. In an attempt to provide high-quality evidence on the relation between PCOS and CVD, relevant literature for CVD risk markers [C-reactive protein (CRP), homocysteine (Hcy), tumor necrosis factor-alpha (TNF-α), plasminogen activator inhibitor-1 (PAI-1), lipoprotein (a) [Lp(a)], advanced glycation end-products (AGEs), vascular endothelial growth factor (VEGF), interleukin-6 (IL-6), asymmetric dimethylarginine (ADMA), endothelin-1 (ET-1) and fibrinogen] in women with PCOS was reviewed and analyzed.

METHODS

A systematic search was conducted electronically using specific eligibility criteria. Weighted mean differences (WMDs) and 95% confidence intervals (CIs) were calculated and combined appropriately. To ensure synthesis of the best available evidence, sensitivity analyses were performed. RESULTS A total of 130 data sets were included in 11 different outcomes, involving 7174 and 5076 CVD markers in women with PCOS and controls, respectively. Women with PCOS demonstrated significantly elevated CRP [WMD (95% CI) 0.99 (0.77-1.21)], Hcy [2.25 (1.46-3.03)], PAI-1 antigen [16.96 (7.25-26.28)], PAI-1 activity [0.71 (0.18-1.23)], VEGF [1.72 (0.96-2.48)], ADMA [0.19 (0.08-0.3)], AGEs [3.91 (2.36-5.45)] and Lp(a) [0.81 (0.58-1.04)] concentrations compared with controls, yet with significant between-study heterogeneity. Borderline significance (not robust in the sensitivity analyses) was detected for TNF-α [0.75 (0.07-1.44)], ET-1 [1.06 (0.52-1.59)] and fibrinogen [0.20 (0.01-0.39)], whereas no difference was detected for IL-6 [0.71 (-0.16 to 1.59)]. CONCLUSIONS Women with PCOS have increased serum concentrations of CVD risk markers compared with controls. Whether this apparent risk is translated into increased incidence of CVD in later life remains to be elucidated.

Serum concentrations of carboxylated osteocalcin are increased and associated with several components of the polycystic ovarian syndrome

Intriguing studies suggest that osteocalcin (OC) and its carboxylated (Gla)/uncarboxylated form are involved in the regulation of insulin secretion and action. Additionally, advanced glycated end products (AGEs) directly regulate the secretion of these osteoblast-derived molecules. In polycystic ovarian syndrome (PCOS), among the pathophysiological aberrations, deregulation of insulin secretion and action as well as elevated AGEs levels have been demonstrated. In this study, we evaluated the serum levels of osteocalcin and Gla osteocalcin and their possible associations with metabolic, hormonal, and ultrasonographic components of PSOS: 97 women were studied, 50 PCOS patients and 47 controls, matched for age and body mass index (BMI). In each subject, the levels of bone metabolism markers have been evaluated, and metabolic and hormonal profiles as well as ovarian ultrasound were carried out. Osteocalcin (4.30 ± 1.74 vs. 6.20 ± 1.78 ng/ml, P < 0.0005) values were significantly lower, whereas Gla osteocalcin (37.93 ± 6.87 vs. 9.64 ± 8.21 ng/ml, P < 0.0005) and receptor activator for nuclear factor-κB ligand (0.54 ± 0.26 vs. 0.16 ± 0.15 pmol/l, P < 0.0005) values were significantly higher in PCOS subjects compared to the control group, independently of obesity. A significant association was disclosed between osteocalcin and Gla osteocalcin with androgens, insulin resistance, AGEs, and ovarian morphology. Receiver operating curve analysis revealed that Gla osteocalcin [AUC, 0.975 (95% CI, 0.93-1.00)] as well as AGEs are significant prognostic factors of PCOS [AUC, 0.986 (95% CI, 0.97-1.00)]. Lower osteocalcin and elevated serum levels of its carboxylated form are displayed in PCOS subjects and are associated with several PCOS components. These findings suggest a potential interaction between bone-derived markers and the metabolic/hormonal abnormalities observed in PCOS. However, the pathophysiological mechanisms and moreover the possible clinical implications require further investigation.

Circulating inflammatory markers in polycystic ovary syndrome: a systematic review and metaanalysis

OBJECTIVE

To perform a review and metaanalysis of the studies evaluating the status of serum inflammatory markers in women with polycystic ovary syndrome (PCOS).

DESIGN

Systematic review and metaanalysis of articles published in English before January 2010 and identified using the PubMed search engine.

SETTING

Academic hospital.

PATIENT(S)

Women with PCOS and appropriate controls.

INTERVENTION(S)

Measurement of serum concentrations of inflammatory markers by high-sensitivity techniques.

MAIN OUTCOME MEASURE(S)

Metaanalyses of the mean difference in serum C-reactive protein (CRP), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) concentrations among patients with PCOS and appropriate controls, applying random-effects models to limit interstudy variability, and using appropriate estimates of evidence dissemination bias.

RESULT(S)

Metaanalysis of the 31 articles meeting inclusion criteria showed that circulating CRP was 96% higher in women with PCOS compared to controls (95% confidence interval, 71%-122%; z = 7.32) without evidence of dissemination bias (Egger's regression intercept, 0.45; 95% confidence interval, -2.30 to 3.21). These findings persisted after excluding five studies with mismatches in body mass, frequency of obesity, or both, between women with PCOS and controls. Metaanalyses involving 10 studies of IL-6, and nine studies of TNF-α revealed no statistically significant differences between PCOS and controls.

CONCLUSION(S)

Women with PCOS exhibit an elevation in circulating CRP that is independent of obesity. This finding corroborates existing molecular evidence of the chronic low-grade inflammation that may underpin the pathogenesis of this disorder.