Low testosterone levels and unimpaired melatonin secretion in young males with metabolic syndrome

Agomelatine protects against obesity-induced renal injury by inhibiting endoplasmic reticulum stress/apoptosis pathway in rats

Obesity is recognized as a risk for the development of chronic kidney disease. Excessive fat accumulation in obesity is associated with the overproduction of reactive oxygen species with the underproduction of antioxidant mechanisms generating oxidative stress together with chronic low-grade inflammation which subsequently leads to the development of several obesity-related complications. It has been suggested that the abnormal lipid accumulation can induce endoplasmic reticulum (ER) stress and cellular apoptosis in several tissue types. Agomelatine is a relatively new antidepressant which is a synthetic agonist of melatonin. Previous study reported the antioxidant and anti-inflammatory effects of agomelatine. In this study, we investigated the therapeutic effects of agomelatine in obesity-related renal injury. Male Wistar rats were fed with normal diet or high-fat diet (HF) for 16 weeks. After that, vehicle or agomelatine or vildagliptin was orally administered to HF rats for 4 weeks. Our results indicated that HF rats demonstrated insulin resistance which was accompanied by an impairment of renal function and renal organic anion transporter 3 (Oat3) function as well as renal oxidative stress, ER stress, and apoptosis. Interestingly, agomelatine treatment not only improved the metabolic parameters, renal function and renal Oat3 function but also attenuated renal oxidative stress, ER stress and subsequent apoptosis. Therefore, agomelatine exerted renoprotective effects in obese insulin-resistant condition. These results suggested that agomelatine could be used as a drug to improve metabolic disturbance and prevent kidney dysfunction in obese condition.

The Prevalence and Components of Metabolic Syndrome in Men from Infertile Couples and Its Relation on Semen Analysis

PURPOSE

Metabolic syndrome (MetS) has been reported as a deleterious factor in male fertility potential, associated with hypogonadism, impaired spermatogenesis, decreased sperm concentration and motility, and increased sperm DNA damage. This study aimed to determine the prevalence of MetS in men from infertile couples and evaluate its effect on semen analysis (SA).

PARTICIPANTS AND METHODS

A cross-sectional descriptive study was performed in men from infertile couples diagnosed based on the World Health Organization 2010 criteria and treated at the Hue Center for Reproductive Endocrinology and Infertility, Vietnam. General information included medical history, lifestyle, MetS factors, SA, and sperm DNA fragmentation test were collected. Based on the diagnostic criteria of the American Heart Association and the National Heart, Lung, and Blood Institute for Asian men, the study population was divided into two groups: MetS and non-MetS groups. The outcomes were analyzed for any relationship between MetS and the SA index and the DNA fragmentation index (DFI).

RESULTS

A total of 534 men from infertile couples were included in this study. The prevalence of MetS was 23.4%, and abnormal semen analysis accounted for 93.8%. Age, body mass index (BMI), waist-to-hip ratio (WHR), waist-to-height ratio (WHtR), hepatitis B and total cholesterol were related to the occurrence of MetS in infertile men (p <0.05). MetS did not reveal any impact on the parameters of SA. There was a positive correlation between waist circumference (WC), WHR, WHtR, and systolic blood pressure (BP) with abnormal sperm head and DFI (p <0.05).

CONCLUSION

Although the prevalence of MetS was remarkable in men from infertile couples, there was no association between MetS and semen quality. However, WC, WHR, WHtR, and systolic BP were found to be significantly associated with abnormal sperm head and DFI.

Melatonin and cryptochrome 2 in metabolic syndrome patients with or without diabetes: a cross-sectional study

Background Metabolic syndrome (MetS) is a cluster of metabolic risk factors which increases the chances for future cardiovascular diseases, as well as diabetes. The underlying causes of MetS include overweight and obesity, physical inactivity and genetic factors. Our intension here was to focus in this study on the importance of the chronobiology, represented by melatonin (MT) and cryptochrome 2 (CRY2), in developing MetS and type 2 diabetes mellitus (T2DM). Thus, we aimed to compare MT and CRY2 plasma levels and correlate both biomarkers with adiposity, atherogenicity and hematological indices in MetS and T2DM cohorts. Methods In a cross-sectional study, 28 normoglycemic lean subjects (controls), 29 normoglycemic MetS subjects and 30 MetS (pre-diabetic/diabetic) were recruited. Results MT (pg/mL) was elevated significantly in MetS arm p-value < 0.05, whereas CRY2 levels (ng/mL) were markedly higher in both MetS groups (non-diabetic and pre-diabetic/diabetic) (all with p-value < 0.001). A reciprocal MT-CRY2 relationship was observed in the MetS (non-diabetic) group (p-value = 0.003). Of note in the total study population, both MT and CRY2 proportionally correlated with each of the following: atherogenicity index of plasma (AIP), waist circumference (WC) and systolic blood pressure (SBP) (all with p-value < 0.05) for MT and CRY2, respectively). Whereas MT correlated inversely with high-density lipoprotein-cholesterol (HDL-C) (p-value < 0.05). Additionally, CRY2 correlated directly with each of the following: diastolic blood pressure (DBP), total cholesterol (TC), low-density lipoprotein (LDL-C), hip circumference (HC), body adiposity index (BAI), weight-to-height (WHtR) ratio, mean platelet volume (MPV) and platelet/lymphocyte ratio (PLR) (p-value < 0.05). Conclusion These findings substantiate that both metabolic risk biomarkers can be prognostic tools and pharmacotherapeutic targets to slowdown the accelerated nature of T2DM.

Risk of prostate cancer-specific death in men with baseline metabolic aberrations treated with androgen deprivation therapy for biochemical recurrence

OBJECTIVES

To investigate the associations of host metabolic factors and metabolic syndrome on prostate cancer-specific death (PCSD) and overall survival (OS) in patients treated with androgen deprivation therapy (ADT) for biochemically recurrent disease.

PATIENTS AND METHODS

The analysis included 273 patients with prostate cancer treated with ADT for rising prostate-specific antigen level after surgery or radiotherapy. Patients were assessed for the presence of diabetes, hypertension, dyslipidaemia and obesity before commencing ADT, and Adult Treatment Panel III criteria were used to assess the presence of the composite diagnosis of metabolic syndrome. A competing risks regression model was used to assess associations of time to PCSD with the metabolic conditions, while a multivariable Cox regression model was used to assess associations of OS with metabolic syndrome and metabolic conditions.

RESULTS

During a median follow-up of 11.6 years, 157 patients (58%) died, of whom 58 (21%) died from prostate cancer. At the start of ADT the median (range) patient age was 74 (46-92) years and the median PSA level was 3.0 ng/mL. Metabolic syndrome was observed in 31% of patients; hypertension (68%) and dyslipidaemia (47%) were the most common metabolic conditions. No association of PCSD and metabolic syndrome status was observed. Patients with hypertension tended to have a higher cumulative incidence of PCSD than those without hypertension (sub-distribution hazard ratio [HR] 1.59, 95% confidence interval [CI] 0.89, 2.84; P = 0.11) although the difference was not statistically significant. Patients with metabolic syndrome had an increased risk of death from all causes (HR 1.56, 95% CI 1.07, 2.29; P = 0.02) when compared with patients without metabolic syndrome, as did patients with hypertension (HR 1.72, 95% CI 1.18, 2.49; P = 0.004).

CONCLUSIONS

No association of PCSD and metabolic syndrome was observed in this cohort of men receiving ADT for biochemically recurrent prostate cancer. Metabolic syndrome was associated with an increased risk of death from all causes and a similar effect was also observed for patients with prostate cancer with hypertension alone.

Experimental and clinical aspects of melatonin and clock genes in diabetes

The pineal hormone melatonin influences insulin secretion, as well as glucagon and somatostatin secretion, both in vivo and in vitro. These effects are mediated by two specific, high-affinity, seven transmembrane, pertussis toxin-sensitive, Gi-protein-coupled melatonin receptors, MT1 and MT2. Both isoforms are expressed in the β-cells, α-cells as well as δ-cells of the pancreatic islets of Langerhans and are involved in the modulation of insulin secretion, leading to inhibition of the adenylate cyclase-dependent cyclic adenosine monophosphate as well as cyclic guanosine monophosphate formation in pancreatic β-cells by inhibiting the soluble guanylate cyclase, probably via MT2 receptors. In this way, melatonin also likely inhibits insulin secretion, whereas using the inositol triphosphate pathway after previous blocking of Gi-proteins by pertussis toxin, melatonin increases insulin secretion. Desynchrony of receptor signaling may lead to the development of type 2 diabetes. This notion has recently been supported by genomewide association studies pinpointing variances of the MT2 receptor as a risk factor for this rapidly spreading metabolic disturbance. As melatonin is secreted in a clearly diurnal fashion, it is safe to assume that it also has a diurnal impact on the blood-glucose-regulating function of the islet. Observations of the circadian expression of clock genes (Clock, Bmal1, Per1,2,3, and Cry1,2) in pancreatic islets, as well as in INS1 rat insulinoma cells, may indicate that circadian rhythms are generated in the β-cells themselves. The circadian secretion of insulin from pancreatic islets is clock-driven. Disruption of circadian rhythms and clock function leads to metabolic disturbances, for example, type 2 diabetes. The study of melatonin-insulin interactions in diabetic rat models has revealed an inverse relationship between these two hormones. Both type 2 diabetic rats and patients exhibit decreased melatonin levels and slightly increased insulin levels, whereas type 1 diabetic rats show extremely reduced levels or the absence of insulin, but statistically significant increases in melatonin levels. Briefly, an increase in melatonin levels leads to a decrease in stimulated insulin secretion and vice versa. Melatonin levels in blood plasma, as well as the activity of the key enzyme of melatonin synthesis, AA-NAT (arylalkylamine-N-acetyltransferase) in pineal, are lower in type 2 diabetic rats compared to controls. In contrast, melatonin and pineal AA-NAT mRNA are increased and insulin receptor mRNA is decreased in type 1 diabetic rats, which also indicates a close relationship between insulin and melatonin. As an explanation, it was hypothesized that catecholamines, which reduce insulin levels and stimulate melatonin synthesis, control insulin-melatonin interactions. This conviction stems from the observation that catecholamines are increased in type 1 but are diminished in type 2 diabetes. In this context, another important line of inquiry involves the fact that melatonin protects β-cells against functional overcharge and, consequently, hinders the development of type 2 diabetes.

Testosterone, sex hormone-binding globulin and the metabolic syndrome in men: an individual participant data meta-analysis of observational studies

Background

Low total testosterone (TT) and sex hormone-binding globulin (SHBG) concentrations have been associated with the metabolic syndrome (MetS) in men, but the reported strength of association varies considerably.

Objectives

We aimed to investigate whether associations differ across specific subgroups (according to age and body mass index (BMI)) and individual MetS components.

Data sources

Two previously published meta-analyses including an updated systematic search in PubMed and EMBASE.

Study Eligibility Criteria

Cross-sectional or prospective observational studies with data on TT and/or SHBG concentrations in combination with MetS in men.

Methods

We conducted an individual participant data meta-analysis of 20 observational studies. Mixed effects models were used to assess cross-sectional and prospective associations of TT, SHBG and free testosterone (FT) with MetS and its individual components. Multivariable adjusted odds ratios (ORs) and hazard ratios (HRs) were calculated and effect modification by age and BMI was studied.

Results

Men with low concentrations of TT, SHBG or FT were more likely to have prevalent MetS (ORs per quartile decrease were 1.69 (95% CI 1.60-1.77), 1.73 (95% CI 1.62-1.85) and 1.46 (95% CI 1.36-1.57) for TT, SHBG and FT, respectively) and incident MetS (HRs per quartile decrease were 1.25 (95% CI 1.16-1.36), 1.44 (95% 1.30-1.60) and 1.14 (95% 1.01-1.28) for TT, SHBG and FT, respectively). Overall, the magnitude of associations was largest in non-overweight men and varied across individual components: stronger associations were observed with hypertriglyceridemia, abdominal obesity and hyperglycaemia and associations were weakest for hypertension.

Conclusions

Associations of testosterone and SHBG with MetS vary according to BMI and individual MetS components. These findings provide further insights into the pathophysiological mechanisms linking low testosterone and SHBG concentrations to cardiometabolic risk.

Male hypogonadism and metabolic syndrome

The role of androgens in cardiovascular disease is still controversial in men. In this study, we investigated metabolic disorders in Tunisian hypogonadal men compared with healthy controls. Forty hypogonadal men and 80 control subjects were enrolled. Patients with a history of pre-existing panhypopituitarism, thyroid dysfunction or inflammatory disease were excluded. Glycaemia, glycated haemoglobin (HbA1c), high-sensitive C-reactive protein (hsCRP), lipid profile, insulin, testosterone and gonadotrophins were measured. Insulin resistance was assessed by homoeostasis model assessment of insulin resistance (Homa IR). Waist circumference, body mass index and blood pressure were significantly higher in patients compared with controls. Glycemia, HbA1c, fasting serum insulin and Homa IR were significantly increased among hypogonadal men. In univariate analysis, testosterone levels were inversely correlated with body mass index, waist circumference, blood pressure, glycaemia, HbA1C, insulin, Homa IR and hsCRP. In multivariate analysis including all significant variables, initial testosterone level was the only independent risk factor for developing dyslipidaemia. With logistic regression, male hypogonadism was an independent risk factor for MS (P < 0.001). We conclude that low testosterone level plays a central role in the development of metabolic syndrome. Further prospective data are required to establish the causative link.

Sperm parameters: paradigmatic index of good health and longevity

Since the discovery of spermatozoon by Anton van Leeuwenhoek in 1677, there has been an ever increasing understanding of its role in reproduction. Many factors adversely affect sperm quality, including varicocele, accessory gland infection, immunological factors, congenital abnormalities, and iatrogenic systemic and endocrine causes, such as diabetes mellitus, obesity, metabolic syndrome, and smoking. The mechanisms responsible for the association between poor sperm parameters and ill health may include oxidative stress, low-grade inflammation, low testosterone, and low sex-hormone-binding globulin. Oxidative stress in the testicular microenvironment may result in decreased spermatogenesis and sperm DNA damage, loss of sperm motility, and abnormal sperm morphology. Low testosterone caused by advanced age, visceral obesity, and inflammation is associated with the development of cardiovascular disease. Hence, semen analysis has an important role in the routine evaluation of idiopathic male infertility, usually manifested as low sperm counts, impaired sperm motility, or absence of sperm, and remains the most common single diagnostic tool. Several studies have shown an inverse relationship between semen quality and medical disorders. This review elucidates the effect of medical disorders and social habits on sperm quality, the mechanisms that are involved in the impairment of sperm quality, and whether or not sperm quality can be used as an index of good health and longevity in a man.

Melatonin secretion and the incidence of type 2 diabetes

IMPORTANCE

Loss-of-function mutations in the melatonin receptor are associated with insulin resistance and type 2 diabetes. Additionally, in a cross-sectional analysis of persons without diabetes, lower nocturnal melatonin secretion was associated with increased insulin resistance.

OBJECTIVE

To study the association between melatonin secretion and the risk of developing type 2 diabetes.

DESIGN, SETTING, AND PARTICIPANTS

Case-control study nested within the Nurses' Health Study cohort. Among participants without diabetes who provided urine and blood samples at baseline in 2000, we identified 370 women who developed type 2 diabetes from 2000-2012 and matched 370 controls using risk-set sampling.

MAIN OUTCOME MEASURES

Associations between melatonin secretion at baseline and incidence of type 2 diabetes were evaluated with multivariable conditional logistic regression controlling for demographic characteristics, lifestyle habits, measures of sleep quality, and biomarkers of inflammation and endothelial dysfunction.

RESULTS

The median urinary ratios of 6-sulfatoxymelatonin to creatinine were 28.2 ng/mg (5%-95% range, 5.5-84.2 ng/mg) among cases and 36.3 ng/mg (5%-95% range, 6.9-110.8 ng/mg) among controls. Women with lower ratios of 6-sulfatoxymelatonin to creatinine had increased risk of diabetes (multivariable odds ratio, 1.48 [95% CI, 1.11-1.98] per unit decrease in the estimated log ratio of 6-sulfatoxymelatonin to creatinine). Compared with women in the highest ratio category of 6-sulfatoxymelatonin to creatinine, those in the lowest category had a multivariable odds ratio of 2.17 (95% CI, 1.18-3.98) of developing type 2 diabetes. Women in the highest category of melatonin secretion had an estimated diabetes incidence rate of 4.27 cases/1000 person-years compared with 9.27 cases/1000 person-years in the lowest category.

CONCLUSIONS AND RELEVANCE

Lower melatonin secretion was independently associated with a higher risk of developing type 2 diabetes. Further research is warranted to assess if melatonin secretion is a modifiable risk factor for diabetes within the general population.

Melatonin and pancreatic islets: interrelationships between melatonin, insulin and glucagon

The pineal hormone melatonin exerts its influence in the periphery through activation of two specific trans-membrane receptors: MT1 and MT2. Both isoforms are expressed in the islet of Langerhans and are involved in the modulation of insulin secretion from β-cells and in glucagon secretion from α-cells. De-synchrony of receptor signaling may lead to the development of type 2 diabetes. This notion has recently been supported by genome-wide association studies identifying particularly the MT2 as a risk factor for this rapidly spreading metabolic disturbance. Since melatonin is secreted in a clearly diurnal fashion, it is safe to assume that it also has a diurnal impact on the blood-glucose-regulating function of the islet. This factor has hitherto been underestimated; the disruption of diurnal signaling within the islet may be one of the most important mechanisms leading to metabolic disturbances. The study of melatonin–insulin interactions in diabetic rat models has revealed an inverse relationship: an increase in melatonin levels leads to a down-regulation of insulin secretion and vice versa. Elucidation of the possible inverse interrelationship in man may open new avenues in the therapy of diabetes.

Melatonin and the metabolic syndrome: a tool for effective therapy in obesity-associated abnormalities?

The metabolic syndrome (MetS) is a cluster of metabolic abnormalities associated with increased risk for cardiovascular diseases. Apart from its powerful antioxidant properties, the pineal gland hormone melatonin has recently attracted the interest of various investigators as a multifunctional molecule. Melatonin has been shown to have beneficial effects in cardiovascular disorders including ischaemic heart disease and hypertension. However, its role in cardiovascular risk factors including obesity and other related metabolic abnormalities is not yet established, particularly in humans. New emerging data show that melatonin may play an important role in body weight regulation and energy metabolism. This review will address the role of melatonin in the MetS focusing on its effects in obesity, insulin resistance and leptin resistance. The overall findings suggest that melatonin should be exploited as a therapeutic tool to prevent or reverse the harmful effects of obesity and its related metabolic disorders.

Presence of the metabolic syndrome is associated with shorter time to castration-resistant prostate cancer

BACKGROUND

Metabolic syndrome (MS) is a set of risk factors that includes obesity and insulin resistance and has been implicated in the development of prostate cancer. Its impact on androgen deprivation therapy (ADT) efficacy has not been studied.

PATIENTS AND METHODS

Retrospective study of prostate cancer patients seen from 1998 to 2005 in a medical oncology clinic. MS, as defined by modified Adult Treatment Panel III criteria, was assessed at the time of initiation of ADT. The study end points were time to prostate-specific antigen (PSA) progression and overall survival (OS) from time of starting ADT.

RESULTS

Eighty-two patients treated with ADT and data to assess for presence of MS were identified. Median age in men with and without MS was 70 years and 49% of the patients evaluated met criteria for MS. Median time to PSA progression for patients with MS was 16 versus 36 months without MS (P=0.003). The median OS for patients with MS was 36.5 months after commencing ADT compared with 46.7 months for those patients without MS (P=0.061).

CONCLUSIONS

This preliminary data suggest that MS is a risk factor for earlier development of castration-resistant prostate cancer and support the need for a prospective evaluation of this finding.

Testosterone and metabolic syndrome: a meta-analysis study

INTRODUCTION

Metabolic syndrome (MetS) is often associated with male hypogonadism. Despite the well-known link, the role of testosterone replacement therapy (TRT) in MetS has not been completely clarified.

AIM

To systematically analyse the relationship between androgen levels and MetS we performed a review and meta-analyses of available prospective and cross-sectional studies. In addition, a specific meta-analysis on the metabolic effects of TRT in available randomized clinical trials (RCTs) was also performed.

METHODS

An extensive Medline search was performed including the following words "testosterone,""metabolic syndrome," and "males".

MAIN OUTCOME MEASURES

Out of 323 retrieved articles, 302 articles were excluded for different reasons. Among the 20 published studies included, 13, 3, and 4 were cross-sectional, longitudinal, and RCTs, respectively. Another unpublished RCT was retrieved on http://www.clinicaltrials.gov.

RESULTS

MetS patients showed significantly lower T plasma levels, as compared with healthy individuals. Similar results were obtained when MetS subjects with and without erectile dysfunction were analyzed separately or when NCEP-ATPIII MetS criteria were compared with other definitions. Meta-regression analysis demonstrated that type 2 diabetes (T2DM) increased the MetS-associated T fall. In a multiple regression model, after adjusting for age and BMI, both T2DM and MetS independently predicted low testosterone (adj. r = -0.752; P < 0.001 and -0.271; P < 0.05, respectively). Analysis of longitudinal studies demonstrated that baseline testosterone was significantly lower among patients with incident MetS in comparison with controls (2.17 [-2.41;-1.94] nmol/L; P < 0.0001). Combining the results of RCTs, TRT was associated with a significant reduction of fasting plasma glucose, homeostatic model assessment index, triglycerides, and waist circumference. In addition, an increase of high-density lipoprotein cholesterol was also observed.

CONCLUSIONS

The meta-analysis of the available cross-sectional data suggests that MetS can be considered an independent association of male hypogonadism. Although only few RCTs have been reported, TRT seems to improve metabolic control, as well as central obesity.

Measuring serum melatonin in epidemiologic studies

BACKGROUND

Epidemiologic data on serum melatonin, a marker of circadian rhythms, and cancer are sparse due largely to the lack of reliable assays with high sensitivity to detect relatively low melatonin levels in serum collected during daylight, as commonly available in most epidemiologic studies.

METHODS

To help expand epidemiologic research on melatonin, we assessed the reproducibility and refined a currently available melatonin RIA, and evaluated its application to epidemiologic investigations by characterizing melatonin levels in serum, urine, and/or plasma in 135 men from several ethnic groups.

RESULTS

Reproducibility was high for the standard 1.0-mL serum [mean coefficient of variation (CV), 6.9%; intraclass correlation coefficient (ICC), 97.4%; n = 2 serum pools in triplicate] and urine-based (mean CV, 3.5%; ICC, 99.9%) assays. Reproducibility for the 0.5-mL refined-serum assay was equally good (mean CV, 6.6%; ICC, 99.0%). There was a positive correlation between morning serum melatonin and 6-sulfatoxymelatonin in 24-hour urine (r = 0.46; P = 0.008; n = 49 subjects). Melatonin levels in serum-plasma pairs had a high correlation (r = 0.97; P < 1x10(-4); n = 20 pairs). Morning serum melatonin levels were five times higher than those from the afternoon (before 9 a.m. mean, 11.0 pg/mL, versus after 11 a.m. mean, 2.0 pg/mL). Chinese men had lower melatonin levels (mean, 3.4 pg/mL), whereas Caucasian, African-American, and Ghanaian men had similar levels (mean, 6.7-8.6 pg/mL).

CONCLUSIONS

These results suggest that melatonin can be detected reliably in serum samples collected in epidemiologic studies in various racial groups.

IMPACT

With improved assays, it may be possible to investigate the role of melatonin and the emerging circadian rhythm hypothesis in cancer etiology in epidemiologic studies.

Melatonin-insulin interactions in patients with metabolic syndrome

Metabolic syndrome (MS) as a group of risk factors is strongly associated with diabetes type 2 and cardiovascular disease. Insulin resistance plays a key role in the pathogenesis of MS. Recent studies have shown that melatonin may influence insulin secretion and glucose homeostasis. Therefore, the present study analyzed the relationships between the melatonin and the insulin in patients with MS and controls. The melatonin rhythm, insulin and lipid levels were studied in 40 subjects (21 patients and 19 controls) in reproductive age. The night melatonin-insulin ratio was correlated negatively with low-density lipoprotein cholesterol (r = -0.370, p = 0.024) and total cholesterol (r = -0.348, p = 0.030), and positively with high-density lipoprotein cholesterol levels (r = +0.414, p = 0.010). Night-time melatonin levels were related to night-time insulin concentrations (r = +0.313, p = 0.049). The correlation was pronounced in patients with MS (r = +0.640, p = 0.002), but did not reach statistical significance in controls (P > 0.05). In the patients with MS unlike the controls the night-day melatonin difference (%) correlated negatively with the fasting glucose (r = -0.494, p = 0.023) and positively to daily insulin (r = +0.536, p = 0.012). Our results show that melatonin-insulin interactions may exist in patients with MS, as well as relationships between melatonin-insulin ratio and the lipid profile. Pineal disturbances could influence the pathogenesis and the phenotype variations of the MS. Larger studies are needed to confirm or reject this hypothesis and to clarify the role of the melatonin in the metabolic disturbances.

Melatonin, endocrine pancreas and diabetes

Melatonin influences insulin secretion both in vivo and in vitro. (i) The effects are MT(1)-and MT(2)-receptor-mediated. (ii) They are specific, high-affinity, pertussis-toxin-sensitive, G(i)-protein-coupled, leading to inhibition of the cAMP-pathway and decrease of insulin release. [Correction added after online publication 4 December 2007: in the preceding sentence, 'increase of insulin release' was changed to 'decrease of insulin release'.] Furthermore, melatonin inhibits the cGMP-pathway, possibly mediated by MT(2) receptors. In this way, melatonin likely inhibits insulin release. A third system, the IP(3)-pathway, is mediated by G(q)-proteins, phospholipase C and IP(3), which mobilize Ca(2+) from intracellular stores, with a resultant increase in insulin. (iii) Insulin secretion in vivo, as well as from isolated islets, exhibits a circadian rhythm. This rhythm, which is apparently generated within the islets, is influenced by melatonin, which induces a phase shift in insulin secretion. (iv) Observation of the circadian expression of clock genes in the pancreas could possibly be an indication of the generation of circadian rhythms in the pancreatic islets themselves. (v) Melatonin influences diabetes and associated metabolic disturbances. The diabetogens, alloxan and streptozotocin, lead to selective destruction of beta-cells through their accumulation in these cells, where they induce the generation of ROS. Beta-cells are very susceptible to oxidative stress because they possess only low-antioxidative capacity. Results suggest that melatonin in pharmacological doses provides protection against ROS. (vi) Finally, melatonin levels in plasma, as well as the arylalkylamine-N-acetyltransferase (AANAT) activity, are lower in diabetic than in nondiabetic rats and humans. In contrast, in the pineal gland, the AANAT mRNA is increased and the insulin receptor mRNA is decreased, which indicates a close interrelationship between insulin and melatonin.