Melatonin improves insulin resistance and hepatic steatosis through attenuation of alpha‐2‐HS‐glycoprotein

Melatonin plays an important role in regulating circadian rhythms. It also acts as a potent antioxidant and regulates glucose and lipid metabolism, although the exact action mechanism is not clear. The α2‐HS‐glycoprotein gene (AHSG) and its protein, fetuin‐A (FETUA), are one of the hepatokines and are known to be associated with insulin resistance and type 2 diabetes. The aim of this study was to determine whether melatonin improves hepatic insulin resistance and hepatic steatosis in a FETUA‐dependent manner. In HepG2 cells treated with 300 μmol/L of palmitic acid, phosphorylated AKT expression decreased, and FETUA expression increased, but this effect was inhibited by treatment with 10 μmol/L of melatonin. However, melatonin did not improve insulin resistance in FETUA‐overexpressing cells, indicating that improvement in insulin resistance by melatonin was dependent on downregulation of FETUA. Moreover, melatonin decreased palmitic acid‐induced ER stress markers, CHOP, Bip, ATF‐6, XBP‐1, ATF‐4, and PERK. In addition, in the high‐fat diet (HFD) mice, oral treatment with 100 mg/kg/day melatonin for 10 weeks reduced body weight gain to one‐third of that of the HFD group and hepatic steatosis. Insulin sensitivity and glucose intolerance improved with the upregulation of muscle p‐AKT protein expression. FETUA expression and ER stress markers in the liver and serum of HFD mice were decreased by melatonin treatment. In conclusion, melatonin can improve hepatic insulin resistance and hepatic steatosis through reduction in ER stress and the resultant AHSG expression.     

Melatonin attenuates diabetic cardiomyopathy and reduces myocardial vulnerability to ischemia‐reperfusion injury by improving mitochondrial quality control: Role of SIRT6

Targeting mitochondrial quality control with melatonin has been found promising for attenuating diabetic cardiomyopathy (DCM), although the underlying mechanisms remain largely undefined. Activation of SIRT6 and melatonin membrane receptors exerts cardioprotective effects while little is known about their roles during DCM. Using high‐fat diet‐streptozotocin‐induced diabetic rat model, we found that prolonged diabetes significantly decreased nocturnal circulatory melatonin and heart melatonin levels, reduced the expressions of cardiac melatonin membrane receptors, and decreased myocardial SIRT6 and AMPK‐PGC‐1α‐AKT signaling. 16 weeks of melatonin treatment inhibited the progression of DCM and the following myocardial ischemia‐reperfusion (MI/R) injury by reducing mitochondrial fission, enhancing mitochondrial biogenesis and mitophagy via re‐activating SIRT6 and AMPK‐PGC‐1α‐AKT signaling. After the induction of diabetes, adeno‐associated virus carrying SIRT6‐specific small hairpin RNA or luzindole was delivered to the animals. We showed that SIRT6 knockdown or antagonizing melatonin receptors abolished the protective effects of melatonin against mitochondrial dysfunction as evidenced by aggravated mitochondrial fission and reduced mitochondrial biogenesis and mitophagy. Additionally, SIRT6 shRNA or luzindole inhibited melatonin‐induced AMPK‐PGC‐1α‐AKT activation as well as its cardioprotective actions. Collectively, we demonstrated that long‐term melatonin treatment attenuated the progression of DCM and reduced myocardial vulnerability to MI/R injury through preserving mitochondrial quality control. Melatonin membrane receptor‐mediated SIRT6‐AMPK‐PGC‐1α‐AKT axis played a key role in this process. Targeting SIRT6 with melatonin treatment may be a promising strategy for attenuating DCM and reducing myocardial vulnerability to ischemia‐reperfusion injury in diabetic patients.     

Melatonin prevents Drp1‐mediated mitochondrial fission in diabetic hearts through SIRT1‐PGC1α pathway

Myocardial contractile dysfunction is associated with an increase in mitochondrial fission in patients with diabetes. However, whether mitochondrial fission directly promotes diabetes‐induced cardiac dysfunction is still unknown. Melatonin exerts a substantial influence on the regulation of mitochondrial fission/fusion. This study investigated whether melatonin protects against diabetes‐induced cardiac dysfunction via regulation of mitochondrial fission/fusion and explored its underlying mechanisms. Here, we show that melatonin prevented diabetes‐induced cardiac dysfunction by inhibiting dynamin‐related protein 1 (Drp1)‐mediated mitochondrial fission. Melatonin treatment decreased Drp1 expression, inhibited mitochondrial fragmentation, suppressed oxidative stress, reduced cardiomyocyte apoptosis, improved mitochondrial function and cardiac function in streptozotocin (STZ )‐induced diabetic mice, but not in SIRT 1^?/? diabetic mice. In high glucose‐exposed H9c2 cells, melatonin treatment increased the expression of SIRT 1 and PGC ‐1α and inhibited Drp1‐mediated mitochondrial fission and mitochondria‐derived superoxide production. In contrast, SIRT 1 or PGC ‐1α siRNA knockdown blunted the inhibitory effects of melatonin on Drp1 expression and mitochondrial fission. These data indicated that melatonin exerted its cardioprotective effects by reducing Drp1‐mediated mitochondrial fission in a SIRT 1/PGC ‐1α‐dependent manner. Moreover, chromatin immunoprecipitation analysis revealed that PGC ‐1α directly regulated the expression of Drp1 by binding to its promoter. Inhibition of mitochondrial fission with Drp1 inhibitor mdivi‐1 suppressed oxidative stress, alleviated mitochondrial dysfunction and cardiac dysfunction in diabetic mice. These findings show that melatonin attenuates the development of diabetes‐induced cardiac dysfunction by preventing mitochondrial fission through SIRT 1‐PGC 1α pathway, which negatively regulates the expression of Drp1 directly. Inhibition of mitochondrial fission may be a potential target for delaying cardiac complications in patients with diabetes.     

Intracellular fatty acid metabolism in skeletal muscle and insulin resistance

Triglyceride accumulation in skeletal muscle is increased in subjects with insulin resistance. Increased intracellular lipolysis from stored triglyceride may induce insulin resistance in skeletal muscle by activating the glucose-fatty acid cycle. However, inconsistent with this hypothesis, intracellular lipolysis from skeletal muscle is decreased in high fat-fed, insulin resistant rats. Therefore, it is suggested that an increase in triglyceride accumulation is the result of decreased mitochondrial fatty acid oxidation in the cells. As evidence, fenofibrate (a PPARalpha activator), rosiglitazone (a PPARgamma activator) and alpha-lipoic acid completely prevented the development of diabetes in obese diabetes-prone rats. All three drugs increased fatty acid oxidation and decreased triglyceride accumulation in skeletal muscle. Administration of ALA activated AMPK and increased fatty acid oxidation. It is suggested that decreased fatty acid oxidation in skeletal muscle is one of the major factors leading to an accumulation of lipid metabolites and insulin resistance.     

Melatonin reverses tunicamycin‐induced endoplasmic reticulum stress in human hepatocellular carcinoma cells and improves cytotoxic response to doxorubicin by increasing CHOP and decreasing Survivin

Chemoresistance in hepatocellular carcinoma (HCC) is associated with multiple cellular responses to environmental stresses, such as nutrient deprivation and hypoxia. Nevertheless, whether ER stress resulting from nutrient deprivation and tumor hypoxia contributes to drug resistance remains unclear. Melatonin increased the efficacy of chemotherapeutic drugs in hepatocellular carcinoma in our previous studies. However, the effects of melatonin on endoplasmic reticulum (ER) stress‐induced resistance to chemotherapeutic agents in HCC have not been tested. The effect of the endoplasmic reticulum (ER) stress response during resistance of human hepatocellular carcinoma cells against doxorubicin was investigated in this study. Pretreatment of HepG2 and SMMC‐7721 cells (two human hepatocellular carcinoma cell lines) with tunicamycin, an ER stress inducer, drastically decreased the rate of apoptosis generated by doxorubicin. Interestingly, co‐pretreatment with tunicamycin and melatonin significantly increased apoptosis induced by doxorubicin. Simultaneously, the expression of phosphorylated AKT (p‐AKT) was elevated in HepG2 and SMMC‐7721 cells given tunicamycin but reduced in the presence of melatonin. Furthermore, consistent with inhibition of AKT activation by using the PI3K inhibitor LY294002, melatonin elevated the levels of CHOP (C/EBP‐homologous protein) and reduced the levels of Survivin (a member of the inhibitor of apoptosis protein family)suggesting that inhibition of the PI3K/AKT pathway by melatonin‐reversed ER stress‐induced resistance to doxorubicin in human hepatocellular carcinoma cells. These results demonstrate that melatonin attenuates ER stress‐induced resistance to doxorubicin in human hepatocellular carcinoma cells by down‐regulating the PI3K/AKT pathway, increasing the levels of CHOP and decreasing the levels of Survivin.     

NIA0004Unsaturated fatty acids diminish the detrimental effects of saturated fatty acids on insulin signalling in human muscle by decreasing de novo ceramide production

Insulin resistance in skeletal muscle is associated with impaired oxidative capacity and reduced size, number and function of mitochondria. Insulin‐resistant individuals have lower adiponectin concentrations, a characteristic predating the development of type‐2 diabetes (T2D). The aim of this study was to test the potential role of adiponectin in mitochondrial bioenergetics in individuals predisposed to develop T2D, in adiponectin KO mice and in primary muscle cell culture. Individuals with a family history of T2D displayed lower plasma adiponectin concentration (P = 0.03), reduced PGC1β (P = 0.04) and mtFAM (P = 0.03) mRNA, lower mitochondrial content (P = 0.006), citrate synthase (P = 0.02) and (‐HAD (P = 0.03) activity, suggesting defective mitochondrial bioenergetics in skeletal muscle. In addition, AdipoR1 was the principle correlate of mitochondrial content (r² = 0.81), suggesting an important role in mitochondrial biogenesis. Knock out of adiponectin in mice was associated with low PGC1α and PPARδ mRNA (both, P < 0.05), reduced mitochondrial content (P < 0.05) and COX II activity (P < 0.05) and markers for type‐1 oxidative fibers in skeletal muscle. This suggests that mitochondrial function is dependent on circulating adiponectin. In primary cultures of human myotubes, treatment with adiponectin induced AMPKK/ AMPK activity, PGC1α protein, mitochondrial biogenesis (P < 0.05), palmitate oxidation (P < 0.05), citrate synthase (P < 0.05) and SOD activity (P < 0.05), and reduced mitochondrial membrane potential and the production of ROS (P < 0.05). The inhibition of adiponectin receptor expression by siRNA or of AMPK by a pharmacological agent blunted the induction in mitochondrial function. Our findings indicate a novel pathway in skeletal muscle by which adiponectin increase mitochondrial number and function and exerts an insulin sensitizing effect.     

Melatonin improves insulin sensitivity independently of weight loss in old obese rats

In aged rats, insulin signaling pathway (ISP) is impaired in tissues that play a pivotal role in glucose homeostasis, such as liver, skeletal muscle, and adipose tissue. Moreover, the aging process is also associated with obesity and reduction in melatonin synthesis from the pineal gland and other organs. The aim of the present work was to evaluate, in male old obese Wistar rats, the effect of melatonin supplementation in the ISP, analyzing the total protein amount and the phosphorylated status (immunoprecipitation and immunoblotting) of the insulin cascade components in the rat hypothalamus, liver, skeletal muscle, and periepididymal adipose tissue. Melatonin was administered in the drinking water for 8‐ and 12 wk during the night period. Food and water intake and fasting blood glucose remained unchanged. The insulin sensitivity presented a 2.1‐fold increase both after 8‐ and 12 wk of melatonin supplementation. Animals supplemented with melatonin for 12 wk also presented a reduction in body mass. The acute insulin‐induced phosphorylation of the analyzed ISP proteins increased 1.3‐ and 2.3‐fold after 8‐ and 12 wk of melatonin supplementation. The total protein content of the insulin receptor (IR) and the IR substrates (IRS‐1, 2) remained unchanged in all investigated tissues, except for the 2‐fold increase in the total amount of IRS‐1 in the periepididymal adipose tissue. Therefore, the known age‐related melatonin synthesis reduction may also be involved in the development of insulin resistance and the adequate supplementation could be an important alternative for the prevention of insulin signaling impairment in aged organisms.     

Paradoxical effects of increased expression of PGC-1α on muscle mitochondrial function and insulin-stimulated muscle glucose metabolism

Peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α has been shown to play critical roles in regulating mitochondria biogenesis, respiration, and muscle oxidative phenotype. Furthermore, reductions in the expression of PGC-1α in muscle have been implicated in the pathogenesis of type 2 diabetes. To determine the effect of increased muscle-specific PGC-1α expression on muscle mitochondrial function and glucose and lipid metabolism in vivo, we examined body composition, energy balance, and liver and muscle insulin sensitivity by hyperinsulinemic-euglycemic clamp studies and muscle energetics by using ³¹P magnetic resonance spectroscopy in transgenic mice. Increased expression of PGC-1α in muscle resulted in a 2.4-fold increase in mitochondrial density, which was associated with an ≈60% increase in the unidirectional rate of ATP synthesis. Surprisingly, there was no effect of increased muscle PGC-1α expression on whole-body energy expenditure, and PGC-1α transgenic mice were more prone to fat-induced insulin resistance because of decreased insulin-stimulated muscle glucose uptake. The reduced insulin-stimulated muscle glucose uptake could most likely be attributed to a relative increase in fatty acid delivery/triglyceride reesterfication, as reflected by increased expression of CD36, acyl-CoA:diacylglycerol acyltransferase1, and mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase, that may have exceeded mitochondrial fatty acid oxidation, resulting in increased intracellular lipid accumulation and an increase in the membrane to cytosol diacylglycerol content. This, in turn, caused activation of PKCθ, decreased insulin signaling at the level of insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and skeletal muscle insulin resistance.     

Melatonin alleviates cognition impairment by antagonizing brain insulin resistance in aged rats fed a high‐fat diet

Brain insulin resistance, induced by neuroinflammation and oxidative stress, contributes to neurodegeneration, that is, processes that are associated with Aβ accumulation and TAU hyperphosphorylation. Here, we tested the effect of chronic administration of melatonin (MLT) on brain insulin resistance and cognition deficits caused by a high‐fat diet (HFD) in aged rats. Results showed that MLT supplementation attenuated peripheral insulin resistance and lowered hippocampal oxidative stress levels. Activated microglia and astrocytes and hippocampal levels of TNF‐α in HFD‐fed rats were reduced by MLT treatment. Melatonin also prevented HFD‐induced increases in beta‐amyloid (Aβ) accumulation and TAU phosphorylation in the hippocampus. In addition, impairments of brain insulin signaling elicited by long‐term HFD were restored by MLT treatment, as confirmed by ex vivo insulin stimulation. Importantly, MLT reversed HFD‐induced cognitive decline as measured by a water maze test, normalized hippocampal LTP and restored CREB activity and BDNF levels as well as cholinergic neuronal activity in the hippocampus. Collectively, these findings indicate that MLT may exhibit substantial protective effects on cognition, via restoration of brain insulin signaling.     

Chronic melatonin consumption prevents obesity-related metabolic abnormalities and protects the heart against myocardial ischemia and reperfusion injury in a prediabetic model of diet-induced obesity

Abstract: Obesity, a major risk factor for ischemic heart disease, is associated with increased oxidative stress and reduced antioxidant status. Melatonin, a potent free radical scavenger and antioxidant, has powerful cardioprotective effects in lean animals but its efficacy in obesity is unknown. We investigated the effects of chronic melatonin administration on the development of the metabolic syndrome as well as ischemia–reperfusion injury in a rat model of diet‐induced obesity (DIO). Male Wistar rats received a control diet, a control diet with melatonin, a high‐calorie diet, or a high‐calorie diet with melatonin (DM). Melatonin (4 mg/kg/day) was administered in the drinking water. After 16 wk, biometric and blood metabolic parameters were measured. Hearts were perfused ex vivo for the evaluation of myocardial function, infarct size (IFS) and biochemical changes [activation of PKB/Akt, ERK, p38 MAPK, AMPK, and glucose transporter (GLUT)‐4 expression). The high‐calorie diet caused increases in body weight (BW), visceral adiposity, serum insulin and triglycerides (TRIG), with no change in glucose levels. Melatonin treatment reduced the BW gain, visceral adiposity, blood TRIG, serum insulin, homeostatic model assessment index and thiobarbituric acid reactive substances in the DIO group. Melatonin reduced IFS in DIO and control groups and increased percentage recovery of functional performance of DIO hearts. During reperfusion, hearts from melatonin‐treated rats had increased activation of PKB/Akt, ERK42/44 and reduced p38 MAPK activation. Chronic melatonin treatment prevented the metabolic abnormalities induced by DIO and protected the heart against ischemia–reperfusion injury. These beneficial effects were associated with activation of the reperfusion injury salvage kinases pathway.     

Mitochondrial dysfunction in type 2 diabetes and obesity

Insulin resistance in skeletal muscle is a major hallmark of type 2 diabetes mellitus (T2D) and obesity that is characterized by impaired insulin-mediated glucose transport and glycogen synthesis and by increased intramyocellular content of lipid metabolites. Several studies have provided evidence for mitochondrial dysfunction in skeletal muscle of type 2 diabetic and prediabetic subjects, primarily due to a lower content of mitochondria (mitochondrial biogenesis) and possibly to a reduced functional capacity per mitochondrion. This article discusses the latest advances in the understanding of the molecular mechanisms underlying insulin resistance in human skeletal muscle in T2D and obesity, with a focus on possible links between insulin resistance and mitochondrial dysfunction.     

Central leptin action improves skeletal muscle AKT,AMPK, and PGC1α activation by hypothalamic PI3K-dependent mechanism

Central leptin action requires PI3K activity to modulate glucose homeostasis and peripheral metabolism. However, the mechanism behind this phenomenon is not clearly understood. We hypothesize that hypothalamic PI3K activity is important for the modulation of the AMP-activated protein kinase (AMPK)/acetyl-CoA carboxylase (ACC) pathway, PGC1α, and AKT in skeletal muscle (SM). To address this issue, we injected leptin into the lateral ventricle of rats. Hypothalamic JAK2 and AKT were activated by intracerebroventricular (ICV) injection of leptin in a time-dependent manner. Central leptin improved tolerance to glucose (GTT), increased PGC1α expression, and AKT, AMPK, ACC and JAK2 phosphorylation in the soleus muscle. Previous ICV administration of either LY294002 or propranolol (IP) blocked these effects. We concluded that the activation of the hypothalamic PI3K pathway is important for leptin-induced AKT phosphorylation, as well as for active catabolic pathway through AMPK and PGC1α in SM. Thus, a defective leptin signalling PI3K pathway in the hypothalamus may contribute to peripheral resistance to insulin associated to diet-induced obesity.     

Melatonin overcomes apoptosis resistance in human hepatocellular carcinoma by targeting Survivin and XIAP

Apoptosis resistance in hepatocellular carcinoma (HCC) is one of the most significant factors for hepatocarcinogenesis and tumor progression, and leads to resistance to conventional chemotherapy. It is well known that inhibitor of apoptosis proteins (IAPs) play key roles in apoptosis resistance, it has become an important target for antitumor therapy. In this study, we examined if melatonin, the main secretory product of the pineal gland, targeted IAPs, leading to the inhibition of apoptosis resistance. To accomplish this, we first observed that four members of IAPs (cIAP‐1, cIAP‐2, Survivin, and XIAP) were overexpressed in human HCC tissue. Interestingly, melatonin significantly inhibited the growth of HepG2 and SMMC‐7721 cells and promoted apoptosis along with the downregulation of Survivin and XIAP, but had no effect on the expression of cIAP‐1 and cIAP‐2. These data suggest that the inhibition of Survivin and XIAP by melatonin may play an important part in reversing apoptosis resistance. Notably, cIAP‐1, Survivin and XIAP were significantly associated with the coexpression of COX‐2 in human HCC specimens. Melatonin also reduced the expression of COX‐2 and inhibited AKT activation in HepG2 and SMMC‐7721 cells. Inhibition of COX‐2 activity with the selective inhibitor, NS398, and inhibition of AKT activation using the PI3K inhibitor, LY294002, in tumor cells confirmed that melatonin‐induced apoptosis was COX‐2/PI3K/AKT‐dependent, suggesting that the COX‐2/PI3K/AKT pathway plays a role in melatonin inhibition of IAPs. Taken together, these results suggest that melatonin overcomes apoptosis resistance by the suppressing Survivin and XIAP via the COX‐2/PI3K/AKT pathway in HCC cells.     

Mitochondrial dysfunction,insulin resistance,and type 2 diabetes mellitus

Insulin resistance is a characteristic feature of type 2 diabetes mellitus, obesity, and the metabolic syndrome. Increased intracellular fat content in skeletal muscle and liver associated with insulin resistance has led to the hypothesis that a mitochondrial defect in substrate oxidation exists in disorders of insulin resistance. In vivo measurements of metabolic fluxes through the tricarboxylic acid and oxidative phosphorylation with magnetic resonance spectroscopy have demonstrated multiple defects in mitochondrial function in skeletal muscle. A decrease in mitochondrial density and mitochondrial copy number has been reported in insulin-resistant individuals. However, these findings have not been a consistent observation in all studies. Similarly, an intrinsic functional defect in mitochondrial adenosine triphosphate production synthesis has been reported in some but not all studies. This review summarizes evidence that implicates a defect in mitochondrial oxidative phosphorylation and its relationship to insulin resistance in common metabolic diseases characterized by impaired insulin action.     

Opposite effects of pioglitazone and rosiglitazone on mitochondrial respiration in skeletal muscle of patients with type 2 diabetes

Aim: Skeletal muscle insulin resistance has been linked to mitochondrial dysfunction. We examined how improvements in muscular insulin sensitivity following rosiglitazone (ROSI) or pioglitazone (PIO) treatment would affect muscle mitochondrial function in patients with type 2 diabetes mellitus (T2DM). Methods: Muscle biopsies were obtained from 21 patients with T2DM before and after 12 weeks on either ROSI (4 mg once daily) [n = 12; age, 59.2 ± 2.2 years; body mass index (BMI), 29.6 ± 0.7 kg/m²] or PIO (30 mg once daily) (n = 9; age, 56.3 ± 2.4 years; BMI, 29.5 ± 1.5 kg/m²). An age‐ and BMI‐matched control group was also included (n = 8; age, 61.8 ± 2.3 years; BMI, 28.4 ± 0.6 kg/m²). Insulin sensitivity, citrate synthase‐ and β‐hydroxyacyl‐CoA‐dehydrogenase (HAD) activity, intramuscular triglyceride (IMTG) and protein content of complexes I–IV were measured, while mitochondrial respiration per milligram muscle was measured in saponin‐treated skinned muscle fibres using high‐resolution respirometry. Results: Mitochondrial respiration per milligram muscle was lower in T2DM compared to controls at baseline and decreased during ROSI treatment but increased during PIO treatment. Citrate synthase activity and average protein content of complexes I–IV were unchanged in the ROSI group, but protein content of complexes II and III increased during PIO treatment. Insulin sensitivity improved in all patients, but IMTG levels were unchanged. Conclusions: We show opposite effects of ROSI and PIO on mitochondrial respiration, and also show that insulin sensitivity can be improved independently of changes in mitochondrial respiration. We confirm that mitochondrial respiration is reduced in T2DM compared to age‐ and BMI‐matched control subjects.     

Long‐term enteral administration of melatonin reduces plasma insulin and increases expression of pineal insulin receptors in both Wistar and type 2‐diabetic Goto‐Kakizaki rats

Abstract: This paper represents an essential aspect of recent investigations into the functional and clinical implications of insulin–melatonin interrelationships. The aim of the study was to analyze whether melatonin reduces insulin secretion in an animal in a manner comparable to the pattern observed in previous in vitro experiments; to this end, we used two models: Wistar and type 2‐diabetic Goto‐Kakizaki (GK) rats. Thirty‐two Wistar and 32 GK rats were divided into two subgroups of 16 rats each; each subgroup was treated either with or without melatonin. The daily administration of melatonin, starting in 8‐ wk‐old rats, was adjusted to 2.5 mg/kg body weight. Melatonin was given daily during the dark period for 12 hr. After 9 wk of treatment, the rats were sacrificed in the middle of the dark period. Melatonin administration strongly enhanced the plasma melatonin level and diminished the expression of pancreatic melatonin receptor‐mRNA, whereas the expression of pineal AA‐NAT and HIOMT was unchanged. Furthermore, the experiments showed in agreement with recent in vitro results of pancreatic islets that plasma insulin levels were diminished after melatonin treatment. However, the pineal insulin receptor expression was increased after melatonin administration. The pancreatic expression of glucagon, GLUT2, and glucokinase was decreased in GK rats, whereas the glucose levels, as well as the parameters of glucose sensing, GLUT2‐mRNA, and glucokinase‐mRNA, were unchanged after melatonin administration in both Wistar and GK rats. In summary, the results show that melatonin administration decreases plasma insulin levels in vivo and, furthermore, that an insulin–melatonin antagonism exists.     

Melatonin stimulates glucose transport via insulin receptor substrate-1/phosphatidylinositol 3-kinase pathway in C2C12 murine skeletal muscle cells

The prevalence of diabetes has exponentially increased in recent decades due to environmental factors such as nocturnal lifestyle and aging, both of which influence the amount of melatonin produced in the pineal gland. The present study investigated the effect of melatonin on signaling pathways of glucose transport in C2C12 mouse skeletal muscle cells. Intriguingly, treatment of C2C12 cells with melatonin (1 nm) stimulated glucose uptake twofold increase. Melatonin-stimulated glucose transport was inhibited with co-treatment with the melatonin receptor antagonist luzindole. Furthermore, treatment of stably over-expressed melatonin receptor type 2B containing C2C12 myotubes with melatonin amplified glucose transport c. 13-fold. Melatonin also increased the phosphorylation level of insulin receptor substrate-1 (IRS-1) and the activity of phosphoinositide 3-kinase (PI-3-kinase). However, 3',5'-cyclic adenosine monophosphate-activated protein kinase (AMPK), another important glucose transport stimulatory mediator via an insulin-independent pathway, was not influenced by melatonin treatment. Activity of p38 mitogen-activated protein kinase (MAPK), a downstream mediator of AMPK, was also not changed by melatonin. In addition, melatonin increased the expression level of forkhead box A2, which was recently discovered to regulate fatty acid oxidation and to be inhibited by insulin. In summary, melatonin stimulates glucose transport to skeletal muscle cells via IRS-1/PI-3-kinase pathway, which implies, at the molecular level, its role in glucose homeostasis and possibly in diabetes. Additionally, exposure to light at night and aging, both of which lower endogenous melatonin levels may contribute to the incidence and/or development of diabetes.