Lower serum apelin levels in women with polycystic ovary syndrome

Apelin receptor modulation mitigates letrozole-induced polycystic ovarian pathogenesis in mice

AIMS

Polycystic ovarian syndrome (PCOS) is one of the most common (about 5-20%) reproductive disorders in women of reproductive age; it is characterized by polycystic ovaries, hyperandrogenism, and oligo/ anovulation. The levels and expression of ovarian adipokines are deregulated in the PCOS. Apelin is an adipokine that acts through its receptor (APJ) and is known to express in the various tissues including the ovary. It has also been suggested that apelin and APJ could be targeted as therapeutic adjuncts for the management of PCOS. However, no study has been conducted on the management of PCOS by targeting the apelin system. Thus, we aimed to evaluate its impact on combating PCOS-associated ovarian pathogenesis.

METHODS

The current work employed a letrozole-induced-hyperandrogenism PCOS-like mice model to investigate the effects of apelin13 and APJ, antagonist ML221. The PCOS model was induced by oral administration of letrozole (1 mg/kg) for 21 days. A total of four experimental groups were made, control, PCOS control, PCOS + aplein13, and PCOS + ML221. The treatment of apelin13 and ML221 was given from day 22 for two weeks.

KEY FINDINGS

The letrozole-induced PCOS-like features such as hyperandrogenism, cystic follicle, decreased corpus luteum, elevated levels of LH/FSH ratio, and up-regulation of ovarian AR expression were ameliorated by apelin13 and ML221 treatment. However, the PCOS-augmented oxidative stress and apoptosis were suppressed by apelin 13 treatments only. ML221 treatment still showed elevated oxidative stress and stimulated apoptosis as reflected by decreased antioxidant enzymes and increased active caspase3 and Bax expression. The expression of ERs was elevated in all groups except control. Furthermore, the PCOS model showed elevated expression of APJ and apelin13 treatment down-regulated its own receptor. Overall, observing the ovarian histology, corpus luteum formation, and decreased androgen levels by both apelin13 and ML221 showed ameliorative effects on the cystic ovary.

SIGNIFICANCE

Despite the similar morphological observation of ovarian histology, apelin13 and ML221 exhibited opposite effects on oxidative stress and apoptosis. Therefore, apelin13 (which down-regulates APJ) and ML221 (an APJ antagonist) may have suppressed APJ signalling, which would account for our findings on the mitigation of polycystic ovarian syndrome. In conclusion, both apelin13 and ML221 mediated mitigation have different mechanisms, which need further investigation.

The role of apelinergic system in metabolism and reproductive system in normal and pathological conditions: an overview

Lifestyle changes have made metabolic disorders as one of the major threats to life. Growing evidence demonstrates that obesity and diabetes disrupt the reproductive system by affecting the gonads and the hypothalamus-pituitary-gonadal (HPG) axis. Apelin, an adipocytokine, and its receptor (APJ) are broadly expressed in the hypothalamus nuclei, such as paraventricular and supraoptic, where gonadotropin-releasing hormone (GnRH) is released, and all three lobes of the pituitary, indicating that apelin is involved in the control of reproductive function. Moreover, apelin affects food intake, insulin sensitivity, fluid homeostasis, and glucose and lipid metabolisms. This review outlined the physiological effects of the apelinergic system, the relationship between apelin and metabolic disorders such as diabetes and obesity, as well as the effect of apelin on the reproductive system in both gender. The apelin-APJ system can be considered a potential therapeutic target in the management of obesity-associated metabolic dysfunction and reproductive disorders.

The Complex Roles of Adipokines in Polycystic Ovary Syndrome and Endometriosis

Polycystic ovary syndrome (PCOS) and endometriosis are frequent diseases of the female reproductive tract causing high morbidity as they can significantly affect fertility and quality of life. Adipokines are pleiotropic signaling molecules secreted by white or brown adipose tissues with a central role in energy metabolism. More recently, their involvement in PCOS and endometriosis has been demonstrated. In this review article, we provide an update on the role of adipokines in both diseases and summarize previous findings. We also address the results of multi-omics approaches in adipokine research to examine the role of single nucleotide polymorphisms (SNPs) in genes coding for adipokines and their receptors, the secretome of adipocytes and to identify epigenetic alterations of adipokine genes that might be conferred from mother to child. Finally, we address novel data on the role of brown adipose tissue (BAT), which seems to have notable effects on PCOS. For this review, original research articles on adipokine actions in PCOS and endometriosis are considered, which are listed in the PubMed database.

Circulating apelin and chemerin levels in patients with polycystic ovary syndrome: A meta-analysis

BACKGROUND

Polycystic ovary syndrome (PCOS) is one of the most common gynecological endocrine disorders. Apelin and chemerin are newly identified adipokines, which are higher in obesity and diabetes. Studies have found that the serum apelin and chemerin levels in patients with PCOS are significantly increased. However, other studies showed the opposite results. Therefore, the relationship between those two adipokines and PCOS is still controversial.

AIM

This meta-analysis was conducted to statistically evaluate the apelin and chemerin levels of patients with PCOS.

METHODS

We searched the Web of Science, Embase, PubMed, and Google Scholar databases for potential studies. "Polycystic ovary syndrome" or "PCOS" in combination with the terms "apelin" or "chemerin" were used as keywords search titles or abstracts. The publication period examined was between 1990 and 2021. Standardized mean differences (SMD) with corresponding 95% confidence intervals (CIs) were determined as the results of the meta-analysis.

RESULTS

A total of 148 articles were initially retrieved, and 18 qualified articles were finally obtained through preliminary screening and quality evaluation. The publications together contain 1,265 cases and 894 controls. The results of the meta-analysis showed that the circulating chemerin levels in patients with PCOS were significantly higher than those in the controls (SMD: 0.79, 95% CI [0.36, 1.23]), and there was no significant difference in circulating apelin between patients with PCOS and controls (SMD: 0.57, 95% CI [-0.21, 1.35]).

CONCLUSIONS

This meta-analysis is the first to evaluate circulating apelin and chemerin levels in patients with PCOS. Our findings suggest that circulating chemerin levels of patients with PCOS are significantly higher than those of healthy controls.

SYSTEMATIC REVIEW REGISTRATION

https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=218316, identifier CRD42020218316.

Is Apelin a new biomarker in patients with polycystic ovary syndrome?

Polycystic ovary syndrome (PCOS) is associated with multiple risk factors for cardiovascular diseases, including insulin resistance, diabetes mellitus type 2, obesity, hypertension, and dyslipidaemia. Many studies have assessed the role of adipokines in the etiopathogenesis of PCOS, however, no single biomarker has been recognized to be in causal relation to the syndrome. Apelin has been identified as a new adipokine linked to obesity and insulin resistance. Some studies demonstrated that the apelin / apelin receptor could play a pivotal role in the pathogenesis of polycystic ovary syndrome, however the other yielded controversial results. Underlying mechanisms of possible involvement of apelin/apelin receptor complex are discussed.

Adipokine levels and carbohydrate metabolism in patients diagnosed de novo with polycystic ovary syndrome

INTRODUCTION

Central obesity appears to play a major role in the pathogenesis of metabolic disorders in polycystic ovary syndrome. Insulin resistance and carbohydrate disorders are associated with dysfunctional secretion of various adipokines by the adipose tissue.

OBJECTIVES

This study aimed to evaluate leptin, apelin, and visfatin against a background of carbohydrate metabolism parameters in patients diagnosed de novo with polycystic ovary syndrome (PCOS).

MATERIAL AND METHODS

The study group consisted of 40 patients with PCOS (mean age, 29 years) diagnosed in accordance with the American Society for Reproductive Medicine criteria from 2003. The control group consisted of 37 clinically healthy women (mean age, 26 years). All controls had regular menses and no clinical or biochemical signs of hyperandrogenism. Concentrations of leptin, apelin, visfatin, and insulin were measured by immunoenzymatic methods. Glucose concentrations were determined using spectrophotometry.

RESULTS

Significantly higher concentrations of leptin, insulin, homeostatic model assessment for insulin resistance (HOMA-IR) index, and the immunoreactive insulin (IRI)/glucose index were found in the PCOS group than in the control group. Notably, the concentration of apelin was over five times lower in the PCOS group than in the control group. In patients with PCOS, a positive correlation was found between the concentrations of insulin and leptin and concentrations of leptin and IRI/glucose. Patients of the PCOS group with body mass index (BMI) ≥  25 had significantly higher values of leptin, insulin, HOMA-IR index, and IRI/glucose index than patients of the PCOS group with normal BMI. In the PCOS group, a positive correlation was found between BMI and leptin concentration (r = 0.7176; p < 0.0001) and carbohydrate metabolism, such as insulin (r = 0.5524; p = 0.0003), glucose (r = 0.3843; p = 0.0157), HOMA-IR (r = 0.5895; p < 0.0001), and IRI/glucose (r = 0.3872; p = 0.0163). These findings were not observed in the control group.

CONCLUSIONS

(1) Increased leptin concentration observed in women diagnosed de novo with PCOS as well as positive correlations between leptin and HOMA-IR, and IRI/glucose and BMI may indicate a potential role of leptin in the reduction of tissue sensitivity to insulin. (2) Significantly lower apelin concentration in the PCOS group (>5 fold) than in the control group, associated with a concomitant increase in leptin, may also contribute to carbohydrate metabolism disorders occurring in the course of PCOS.

Blood Circulating Levels of Adipokines in Polycystic Ovary Syndrome Patients: A Systematic Review and Meta-analysis

A body of studies has examined the circulating concentration of adipokines including apelin, vapin, resistin, and chemerin in polycystic ovary syndrome (PCOS) patients. However, their findings have been inconclusive. Therefore, we systematically reviewed available studies to illuminate the overall circulating concentration of adipokines in PCOS subjects. Cochrane's Library, PubMed, Scopus, and ISI Web of Science databases were searched from the earliest available date up to April 2021 for relevant articles. The quality of each study was assessed by the Newcastle-Ottawa Quality Assessment Scale. The pooled effect size was estimated based on the random effects model, and the standard mean differences (SMD) with a 95% confidence interval (CI) were reported. A total of 88 studies met the inclusion criteria and were included in the current systematic review and meta-analysis. The results of the analysis showed that serum levels of vaspin (SMD 0.69; 95% CI, 0.22 to 1.17; P = 0.004; I² = 90.6%), chemerin (SMD 1.87; 95% CI, 1.35 to 2.40; P < 0.001; I² = 94.4%), and resistin (SMD 0.66; 95% CI, 0.41 to 0.91; P < 0.001; I² = 92.6%) were significantly higher in the PCOS group compared to controls. However, there was no significant difference between the PCOS and control groups in relation to apelin levels (SMD - 0.17; 95% CI, - 1.06 to 0.73; P = 0.714; I² = 97.8%). We found that serum levels of vaspin, chemerin, and resistin were significantly higher in PCOS subjects compared with controls. It seems that these adipokines can be measured as a useful marker to predict the development of PCOS.

Role of adipokines in the ovarian function: Oogenesis and steroidogenesis

Adipokines are mainly produced by adipose tissue; however, their expression has been reported in other organs including female reproductive tissues. Therefore, adipokines have opened new avenues of research in female fertility. In this regard, studies reported different roles for certain adipokines in ovarian function, although the role of other recently identified adipokines is still controversial. It seems that adipokines are essential for normal ovarian function and their abnormal levels could be associated with ovarian-related disorders. The objective of this study is to review the available information regarding the role of adipokines in ovarian functions including follicular development, oogenesis and steroidogenesis and also their involvement in ovary-related disorders.

Apelin/Apelin receptor: A new therapeutic target in Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is an endocrinopathy, and it accounts for 75% of non-ovulatory infertile in women of childbearing age. It is clear that obesity, insulin resistance, dyslipidaemia coexist in PCOS. Apelin, as an endogenous ligand of the previously orphan receptor, is an adipokine that secreted by adipose tissue. Apelin and apelin receptors are expressed in many tissues and organ to regulate their physiological functions. Studies have shown that Apelin/apelin-receptor also expressed in ovary such as follicles, granulosa cells. Furthermore, Apelin/apelin-receptor play roles in vascular establishment and hormone metabolism in ovary. These indicate that the Apelin/apelin-receptor play an important role in the development of follicle. Apelin/apelin-receptor are increased in ovary of PCOS, which are associated with abnormal ovarian hormones and function. These are important causes of menstrual cycle disorders and anovulation. Moreover, apelin now appears clearly as a new player in energy metabolism. Apelin can regulate glucose and lipid metabolism but also modulate insulin secretion. And plasma apelin concentrations are elevated in obesity and type 2 diabetes patients. Interestedly, obesity and type 2 diabetes are also companied with polycystic ovary syndrome patients. We speculate apelin/apelin-receptor may play a vital role in pathogenesis of polycystic ovary syndrome, but the underlying mechanisms remain under exploration. Here, we review apelin/apelin-receptor, as a new therapeutic target, have effects on ovarian function and energy metabolism in polycystic ovary syndrome.

Involvement of Novel Adipokines, Chemerin, Visfatin, Resistin and Apelin in Reproductive Functions in Normal and Pathological Conditions in Humans and Animal Models

It is well known that adipokines are endocrine factors that are mainly secreted by white adipose tissue. Their central role in energy metabolism is currently accepted. More recently, their involvement in fertility regulation and the development of some reproductive disorders has been suggested. Data concerning the role of leptin and adiponectin, the two most studied adipokines, in the control of the reproductive axis are consistent. In recent years, interest has grown about some novel adipokines, chemerin, visfatin, resistin and apelin, which have been found to be strongly associated with obesity and insulin-resistance. Here, we will review their expression and role in male and female reproduction in humans and animal models. According to accumulating evidence, they could regulate the secretion of GnRH (Gonadotropin-Releasing Hormone), gonadotropins and steroids. Furthermore, their expression and that of their receptors (if known), has been demonstrated in the human and animal hypothalamo-pituitary-gonadal axis. Like leptin and adiponectin, these novel adipokines could thus represent metabolic sensors that are able to regulate reproductive functions according to energy balance changes. Therefore, after investigating their role in normal fertility, we will also discuss their possible involvement in some reproductive troubles known to be associated with features of metabolic syndrome, such as polycystic ovary syndrome, gestational diabetes mellitus, preeclampsia and intra-uterine growth retardation in women, and sperm abnormalities and testicular pathologies in men.

The Role of Apelin in the Functioning of the Reproductive System

Adipokine apelin through the apelin receptors activates a wide range of signaling cascades in the target cells and controls their growth, differentiation, apoptosis, and energy metabolism. In the recent years, the evidence has been obtained that all components of the hypothalamic-pituitary-gonad axis, in which apelin and its receptor are expressed, are targets of apelin. In the hypothalamus, apelin modulates the activity of the melanocortin and ghrelin systems and indirectly affects the production of gonadoliberin. In the ovaries, it controls the growth and maturation of the follicles, stimulates the angiogenesis, and affects the basal and stimulated by the other factors steroidogenic activity in follicular cells. The changes in the apelin signaling system are closely associated with dysfunctions of the female reproductive system, such as polycystic ovary syndrome, endometriosis, and cancer. Information on the regulation of the male reproductive system by apelin is limited to animal studies showing the effect of apelin on the hypothalamic components of the gonad axis. The participation of apelin in the regulation of the reproductive system opens up the broad opportunities for the development of new approaches for the correction of abnormalities in this system and for the treatment of infertility.

Effect of troxerutin on apelin-13, apelin receptors (APJ), and ovarian histological changes in the offspring of high-fat diet fed rats

OBJECTIVES

Maternal high-fat diet (HFD) consumption has been linked to metabolic disorders and reproductive dysfunctions in offspring. Troxerutin (TRO) has anti-hyperlipidemic, anti-oxidant, and anti-inflammatory effects. This study examined the effects of TRO on apelin-13, its receptors mRNA and ovarian histological changes in the offspring of HFD fed rats.

MATERIALS AND METHODS

Female Wistar rats were randomly divided into control diet (CD) or HFD groups and received these diets for eight weeks. After mating, dams were assigned into four subgroups: CD, CD + TRO, HFD, and HFD + TRO, and received their respective diets until the end of lactation. Troxerutin (150 mg/kg/day) was gavaged in the CD + TRO and HFD + TRO groups during pregnancy. On the postnatal day (PND) 21 all female offspring were separated and fed CD until PND 90. On PND 90 animals were sacrificed and ovarian tissue samples were collected for further evaluation.

RESULTS

Results showed that HFD significantly decreased serum apelin-13 in the female offspring of the HFD dams, which was significantly reversed by TRO. Moreover, real-time polymerase chain reaction (PCR) analysis revealed that TRO treatment significantly decreased the ovarian mRNA expression of the apelin-13 receptor in the troxerutin-received offspring. Furthermore, histological examination revealed that TRO increased the number of atretic follicles in the ovaries of HFD+TRO offspring.

CONCLUSION

Maternal high fat feeding compromises ovarian health including follicular growth and development in the adult offspring and troxerutin treatment improved negative effects of maternal HFD on the apelin-13 level and ovarian development of offspring.

Apelin in Reproductive Physiology and Pathology of Different Species: A Critical Review

Apelin has been isolated from the bovine stomach extracts as an endogenous ligand of the previously orphan receptor APJ. Expression of the apelinergic system (apelin and APJ) was described in many organs where pleiotropic effects like regulation of food intake, body weight, or cardiovascular and immune function were described. Recent studies have shown that apelin also plays an important role in the regulation of female and male reproduction. Some data showed that the gene and protein of apelin/APJ are expressed in the hypothalamic-pituitary-gonad (HPG) axis tissue. Thus, apelin is synthesized locally in the hypothalamus, pituitary, ovaries, and testis of many species and has autocrine and/or paracrine effects. Most research indicates that apelin has an inhibitory effect on gonadotropin secretion and participates in the direct regulation of steroidogenesis, cell proliferation, and apoptosis in gonads. The article summarizes also results of a series of recent studies on the effect of apelin on reproduction pathology, like polycystic ovarian syndrome, endometriosis, and ovarian cancer. Many of these pathologies are still in critical need of therapeutic intervention, and recent studies have found that apelin can be targets in reproductive pathological states.

The Effect of Estrogen Replacement Therapy on Visceral Fat, Serum Glucose, Lipid Profiles and Apelin Level in Ovariectomized Rats

Objectives

Ovarian hormones have been shown to regulate body weight, intra-abdominal fat accumulation and plasma level of cytokines. The aim of this study was to investigate the effect of estrogen replacement therapy on visceral adipose tissue, plasma level of apelin, lipid profiles, and glucose in ovariectomized (OVX) rats.

Methods

Thirty female Wistar rats were divided into OVX (n = 20) and sham (n = 10) groups. OVX rats were subdivided into estrogen replacement therapy (OVX+est; n = 10) receiving 17 β-estradiol valerates (30 µg/kg, s.c., 5 day/week, for eight weeks), and vehicle control group receiving sesame oil same as experiment group (OVX+ses oil; n = 10). After the treatments, all groups were sacrificed and blood samples were collected, visceral fats were taken from the abdominal cavity and weighed immediately. Apelin were measured using enzyme-linked immunosorbent assay kits. Lipid profiles and glucose were measured using the enzymatic colorimetric method. Data were analyzed with one-way analysis of variance and (P < 0.05) determined as the statistical significance level.

Results

After eight weeks, body weight, body mass index (BMI), visceral fat, apelin and lipid profiles (P < 0.01) were increased significantly in OVX rats compared to sham group. Treatment with estrogen leads to significant reduction in body weight and BMI (P < 0.05), there was no significant change in serum apelin level in OVX+est rats compared to OVX+ses.

Conclusions

These results suggest that estradiol replacement therapy successfully attenuated some of the metabolic syndrome components, and apelin does not probably stand as a mediator of these physiological functions.

Apelin (APLN) regulates progesterone secretion and oocyte maturation in bovine ovarian cells

APLN and its G-protein coupled receptor APLNR are expressed in the bovine ovary. However their role in granulosa cells and oocytes is unknown. Here, we studied their expression in bovine ovarian cells and investigated their regulation in cultured luteinizing granulosa cells in response to IGF1 and FSH. We determined the effect and the molecular mechanism of APLN (isoforms 17 and 13) on bovine granulosa cell progesterone secretion and on oocyte maturation. By RT-qPCR and immunoblot, we showed that the expression of both APLN and APLNR in granulosa and oocytes significantly increased with ovarian follicles size whereas it was similar in theca interstitial cells. In vitro, in unstimulated luteinizing bovine granulosa cells and in response to IGF1 (10^-8 M) but not to FSH (10^-8 M), we observed that APLN (-17 and -13) (10^-9 M) increased progesterone production; this was abolished in response to the APLNR antagonist ML221. These latter effects were dependent on the MAPK ERK1/2 kinase. Furthermore, we showed that APLN (-17 and -13) (10^-9 M) increased cell proliferation through AKT signaling. Conversely, the addition of APLN-13 and APLN-17 to in vitro maturation medium containing IGF1 (10^-8 M) but not FSH (10^-8 M) arrested most oocytes at the germinal vesicle stage, which was associated with a decrease in progesterone secretion, an inhibition in MAPK ERK1/2 phosphorylation and an increase in PRKA phosphorylation in oocytes. Thus, APLN can increase progesterone secretion and cell proliferation in bovine luteinizing granulosa cells in vitro, while it blocks meiotic progression at the germinal vesicle stage during bovine oocyte in vitro maturation.

Evaluation of Apelin and Insulin Resistance in Patients with PCOS and Therapeutic Effect of Drospirenone-Ethinylestradiol Plus Metformin

BACKGROUND

The aim of this study was to determine the relevance of apelin and insulin resistance (IR) with polycystic ovary syndrome (PCOS) and to assess the possible therapeutic effect of the combined therapy of drospirenone-ethinylestradiol (DRSP-EE) combined with metformin.

MATERIAL AND METHODS

Sixty-three PCOS patients and 40 non-PCOS infertile patients were recruited. The fasting serum levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), testosterone (T), prolactin (PRL), estradiol (E2), glucose (FBG), insulin (FINS), and apelin at the early follicular phase were measured. To further investigate the relation between apelin and IR, we treated the PCOS patients with DRSP-EE (1 tablet daily, 21 d/month) plus metformin (500 mg tid) for 3 months. All of the above indices were measured again after treatment.

RESULTS

1) Levels of apelin, LH, LH/FSH, T, and FINS, as well as homeostatic model assessment of IR (HOMA-IR) in PCOS patients, were significantly higher than in the control group before treatment. 2) These indices significantly decreased after treatment with DRSP-EE plus metformin. 3) Correlation analysis showed that apelin level was positively correlated with body mass index (BMI), FINS level, and HOMA-IR.

CONCLUSIONS

Apelin level significantly increased in PCOS patients. The combined therapy of DRSP-EE plus metformin not only decreases IR, but also improves apelin level. This combination is a superior approach for PCOS treatment.

Does polycystic ovary syndrome itself have additional effect on apelin levels?

Objective. The present study was designed to compare serum levels of apelin between lean PCOS women and healthy women with regular menses. Study Design. A total of 30 lean patients with PCOS and 30 healthy subjects were included in this study. Serum apelin levels were compared between groups. Results. Serum apelin levels in lean PCOS patients were not significantly different from the control subjects. Conclusion. Our findings indicate that PCOS itself does not seem to change apelin levels. Further investigation on a large number of subjects will need to be conducted to prove the consistent or variable association in PCOS.

Apelin levels in relation with hormonal and metabolic profile in patients with polycystic ovary syndrome

OBJECTIVE

To evaluate serum concentrations of apelin in relation to the hormonal and metabolic profile in patients with and without polycystic ovary syndrome (PCOS).

STUDY DESIGN

A total of 45 women with PCOS and 45 age-matched controls were eligible for the study. Hirsutism scores, hormonal and metabolic profile as well as apelin levels were evaluated in each subject.

RESULTS

Women with PCOS exhibited lower serum concentrations of apelin than controls (194.1±50.7pg/ml versus 292.1±85.6pg/ml, p<0.001). Apelin levels were significantly correlated positively with body mass index (BMI), insulin, triglyceride (TG), free testosterone (fT) levels, modified Ferriman Gallwey scores and Homeostasis Model Assessment (HOMA) index; but negatively correlated with high density lipoprotein (HDL) levels in patients with PCOS (p<0.05). When overweight or obese (BMI≥25) and normal weight (BMI<25) women with PCOS were compared to BMI-matched controls, lower apelin levels were found in the PCOS group (226.7±45.4pg/ml versus 378.6±80.9pg/ml and 165.5±36.1 versus 249.0±46.6pg/ml, p<0.001, respectively).

CONCLUSION

The data suggest that serum apelin levels are lower in women with PCOS than in controls. Furthermore, apelin can be used as a specific marker for insulin sensitivity and lipid profile and this adipokine might play a role in the pathogenesis of PCOS.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

The apelin receptor (APJ; gene symbol APLNR) is a member of the G protein-coupled receptor gene family. Neural gene expression patterns of APJ, and its cognate ligand apelin, in the brain implicate the apelinergic system in the regulation of a number of physiological processes. APJ and apelin are highly expressed in the hypothalamo-neurohypophysial system, which regulates fluid homeostasis, in the hypothalamic-pituitary-adrenal axis, which controls the neuroendocrine response to stress, and in the forebrain and lower brainstem regions, which are involved in cardiovascular function. Recently, apelin, synthesised and secreted by adipocytes, has been described as a beneficial adipokine related to obesity, and there is growing awareness of a potential role for apelin and APJ in glucose and energy metabolism. In this review we provide a comprehensive overview of the structure, expression pattern and regulation of apelin and its receptor, as well as the main second messengers and signalling proteins activated by apelin. We also highlight the physiological and pathological roles that support this system as a novel therapeutic target for pharmacological intervention in treating conditions related to altered water balance, stress-induced disorders such as anxiety and depression, and cardiovascular and metabolic disorders.

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.