Melatonin-mediated insulin synthesis during endoplasmic reticulum stress involves HuD expression in rat insulinoma INS-1E cells

Melatonin regulates endoplasmic reticulum stress in diverse pathophysiological contexts: A comprehensive mechanistic review

The endoplasmic reticulum (ER) is crucial for protein quality control, and disruptions in its function can lead to various diseases. ER stress triggers an adaptive response called the unfolded protein response (UPR), which can either restore cellular homeostasis or induce cell death. Melatonin, a safe and multifunctional compound, shows promise in controlling ER stress and could be a valuable therapeutic agent for managing the UPR. By regulating ER and mitochondrial functions, melatonin helps maintain cellular homeostasis via reduction of oxidative stress, inflammation, and apoptosis. Melatonin can directly or indirectly interfere with ER-associated sensors and downstream targets of the UPR, impacting cell death, autophagy, inflammation, molecular repair, among others. Crucially, this review explores the mechanistic role of melatonin on ER stress in various diseases including liver damage, neurodegeneration, reproductive disorders, pulmonary disease, cardiomyopathy, insulin resistance, renal dysfunction, and cancer. Interestingly, while it alleviates the burden of ER stress in most pathological contexts, it can paradoxically stimulate ER stress in cancer cells, highlighting its intricate involvement in cellular homeostasis. With numerous successful studies using in vivo and in vitro models, the continuation of clinical trials is imperative to fully explore melatonin's therapeutic potential in these conditions.

Advances in The Study of RNA-binding Proteins in Diabetic Complications

Background

It has been reported that diabetes mellitus affects 435 million people globally as a primary health care problem. Despite many therapies available, many diabetes remains uncontrolled, giving rise to irreversible diabetic complications that pose significant risks to patients’ wellbeing and survival.

Scope of Review

In recent years, as much effort is put into elucidating the posttranscriptional gene regulation network of diabetes and diabetic complications; RNA binding proteins (RBPs) are found to be vital. RBPs regulate gene expression through various post-transcriptional mechanisms, including alternative splicing, RNA export, messenger RNA translation, RNA degradation, and RNA stabilization.

Major Conclusions

Here, we summarized recent studies on the roles and mechanisms of RBPs in mediating abnormal gene expression in diabetes and its complications. Moreover, we discussed the potential and theoretical basis of RBPs to treat diabetes and its complications.

• Mechanisms of action of RBPs involved in diabetic complications are summarized and elucidated.

• We discuss the theoretical basis and potential of RBPs for the treatment of diabetes and its complications.

• We summarize the possible effective drugs for diabetes based on RBPs promoting the development of future therapeutic drugs.

An updated review of mechanistic potentials of melatonin against cancer: pivotal roles in angiogenesis, apoptosis, autophagy, endoplasmic reticulum stress and oxidative stress

Cancers are serious life-threatening diseases which annually are responsible for millions of deaths across the world. Despite many developments in therapeutic approaches for affected individuals, the rate of morbidity and mortality is high. The survival rate and life quality of cancer patients is still low. In addition, the poor prognosis of patients and side effects of the present treatments underscores that finding novel and effective complementary and alternative therapies is a critical issue. Melatonin is a powerful anticancer agent and its efficiency has been widely documented up to now. Melatonin applies its anticancer abilities through affecting various mechanisms including angiogenesis, apoptosis, autophagy, endoplasmic reticulum stress and oxidative stress. Regarding the implication of mentioned cellular processes in cancer pathogenesis, we aimed to further evaluate the anticancer effects of melatonin via these mechanisms.

Melatonin protects mouse testes from palmitic acid-induced lipotoxicity by attenuating oxidative stress and DNA damage in a SIRT1-dependent manner

Palmitic acid (PA), the main component of dietary saturated fat, has been known to increase in patients with obesity, and PA-induced lipotoxicity may contribute to obesity-related male infertility. Melatonin has beneficial effects on reproductive processes; however, the effect and the underlying molecular mechanism of melatonin's involvement in PA-induced cytotoxicity in the testes are poorly understood. Our findings showed that lipotoxicity was observed in mouse testes after long-term PA treatment and that melatonin therapy restored spermatogenesis and fertility in these males. Moreover, melatonin therapy suppressed PA-induced apoptosis by modulating apoptosis-associated proteins such as Bcl2, Bax, C-Caspase3, C-Caspase12, and CHOP in type B spermatogonial stem cells. Changes in the expression of endoplasmic reticulum (ER) stress markers (p-IRE1, p-PERK, ATF4) and intracellular Ca²⁺ levels showed that melatonin relieved PA-induced ER stress. Mechanistically, melatonin stimulated the expression and nuclear translocation of SIRT1 through its receptors and prevented PA-induced ROS production and mitochondrial dysfunction via SIRT1 signaling pathway. Furthermore, melatonin promoted SIRT1-mediated p53 deacetylation, thereby relieving G2/M arrest in response to PA-stimulated DNA damage. Collectively, these findings indicate that melatonin protects the testes from PA-induced lipotoxicity through the activation of SIRT1, which alleviates oxidative stress, ER stress, mitochondrial dysfunction, and DNA damage.

mRNA Processing: An Emerging Frontier in the Regulation of Pancreatic β Cell Function

Robust endocrine cell function, particularly β cell function, is required to maintain blood glucose homeostasis. Diabetes can result from the loss or dysfunction of β cells. Despite decades of clinical and basic research, the precise regulation of β cell function and pathogenesis in diabetes remains incompletely understood. In this review, we highlight RNA processing of mRNAs as a rapidly emerging mechanism regulating β cell function and survival. RNA-binding proteins (RBPs) and RNA modifications are primed to be the next frontier to explain many of the poorly understood molecular processes that regulate β cell formation and function, and provide an exciting potential for the development of novel therapeutics. Here we outline the current understanding of β cell specific functions of several characterized RBPs, alternative splicing events, and transcriptome wide changes in RNA methylation. We also highlight several RBPs that are dysregulated in both Type 1 and Type 2 diabetes, and discuss remaining knowledge gaps in the field.

The role of melatonin, a multitasking molecule, in retarding the processes of ageing

Biological ageing is generally accompanied by a gradual loss of cellular functions and physiological integrity of organ systems, the consequential enhancement of vulnerability, senescence and finally death. Mechanisms which underlie ageing are primarily attributed to an array of diverse but related factors including free radical-induced damage, dysfunction of mitochondria, disruption of circadian rhythms, inflammaging, genomic instability, telomere attrition, loss of proteostasis, deregulated sensing of nutrients, epigenetic alterations, altered intercellular communication, and decreased capacity for tissue repair. Melatonin, a prime regulator of human chronobiological and endocrine physiology, is highly reputed as an antioxidant, immunomodulatory, antiproliferative, oncostatic, and endocrine-modulatory molecule. Interestingly, several recent reports support melatonin as an anti-ageing agent whose multifaceted functions may lessen the consequences of ageing. This review depicts four categories of melatonin's protective effects on ageing-induced molecular and structural alterations. We also summarize recent findings related to the function of melatonin during ageing in various tissues and organs.

Melatonin attenuates detrimental effects of diabetes on the niche of mouse spermatogonial stem cells by maintaining Leydig cells

Diabetes mellitus affects a large number of men of reproductive age and it usually leads to serious reproductive disorders. However, the underlying mechanisms and specific therapies still remain largely unknown. We observed Leydig cell loss in the testes of diabetic mice. Continuous high glycemic status of testes stimulated expression of Caspase12, Grp78, and Chop, the three ERS response factors; this might induce cell cycle arrest and apoptosis of Leydig cells in response to ERS. In these diabetic mouse models, melatonin alleviated apoptosis of testicular stromal cell induced by ERS, and promoted SSCs self-renewal by recovering Leydig cells secretion of CSF1 after 8 weeks of treatment. To explore the relationship between CSF-1 and ERS in Leydig cells, we treated Leydig tumor cell line with an activator Tuniamycin and an inhibitor 4-Phenylbutyrate of ERS. Our data showed that the CSF-1 expression in mouse Leydig cell lines decreased six-fold while reversely increasing five-fold in the 4-Phenylbutyrate-treated group. Thus, melatonin likely alleviates the loss of Leydig cells in diabetic testes and provides a healthier niche for SSCs to self-renew and continually provide healthy sperm for male fertility.

Interaction of HuDA and PABP at 5'UTR of mouse insulin2 regulates insulin biosynthesis

Understanding the regulation of insulin biosynthesis is important as it plays a central role in glucose metabolism. The mouse insulin gene2 (Ins2) has two splice variants; long (Ins2L) and short (Ins2S), that differ only in their 5’UTR sequence and Ins2S is the major transcript which translate more efficiently as compared to Ins2L. Here, we show that cellular factors bind preferentially to the Ins2L 5’UTR, and that PABP and HuD can bind to Ins2 splice variants and regulate its translation. In vitro binding assay with insulin 5’UTR and different HuD isoforms indicate that the ‘N’ terminal region of HuD is important for RNA binding and insulin translation repression. Using reporter assay we showed that specifically full-length HuD A isoform represses translation of reporter containing insulin 5’UTR. We further show that PABP and HuD interact with each other in RNA-dependent manner and this interaction is affected by glucose and PDI (5’UTR associated translation activator). These results suggest that PABP interacts with HuD in basal glucose conditions making translation inhibitory complex, however upon glucose stimulation this association is affected and PABP is acted upon by PDI resulting in stimulation of insulin translation. Together, our findings snapshot the mechanism of post-transcriptional regulation of insulin biosynthesis.

Streptozotocin-Induced Autophagy Reduces Intracellular Insulin in Insulinoma INS-1E Cells

Streptozotocin (STZ), a glucose analog, induces diabetes in experimental animals by inducing preferential cytotoxicity in pancreatic beta cells. We investigated whether STZ reduced the production of intracellular insulin through autophagy in insulinoma INS-1E cells. Typically, 2 mM STZ treatment for 24 h significantly decreased cell survival. STZ treatment led to significant decrease in phospho-AMP-activated protein kinase (p-AMPK) level; reduction in levels of phospho-protein kinase R-like endoplasmic reticulum kinase (PERK) and inositol-requiring enzyme 1α (IRE1α); significant reduction in levels of p85α, p110, phospho-serine and threonine kinase/protein kinase B (p-Akt/PKB) (Ser473), phospho-extracellular-regulated kinase (p-ERK), and phospho-mammalian target of rapamycin (p-mTOR); increase in levels of Cu/Zn-superoxide dismutase (SOD), Mn-SOD, and catalase; decrease in B-cell lymphoma 2 (Bcl-2) expression; increase in Bcl-2-associated X protein (Bax) expression; increase in levels of microtubule-associated protein 1 light chain 3 (LC3) and Beclin 1; and reduction in production of intracellular insulin. These results suggest that insulin synthesis during STZ treatment involves autophagy in INS-1E cells and, subsequently, results in a decrease in intracellular production of insulin.

Snapshot: implications for melatonin in endoplasmic reticulum homeostasis

The endoplasmic reticulum (ER) is an important intracellular membranous organelle. Previous studies have demonstrated that the ER is responsible for protein folding and trafficking, lipid synthesis and the maintenance of calcium homeostasis. Interestingly, the morphology and structure of the ER were recently found to be important. Melatonin is a hormone that anticipates the daily onset of darkness in mammals, and it is well known that melatonin acts as an antioxidant by scavenging free radicals and increasing the activity of antioxidant enzymes in the body. Notably, the existing evidence demonstrates that melatonin is involved in ER homeostasis, particularly in the morphology of the ER, indicating a potential protective role of melatonin. This review discusses the existing knowledge regarding the implications for the involvement of melatonin in ER homeostasis.

Thapsigargin sensitizes human esophageal cancer to TRAIL-induced apoptosis via AMPK activation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent for esophageal squamous cell carcinoma (ESCC). Forced expression of CHOP, one of the key downstream transcription factors during endoplasmic reticulum (ER) stress, upregulates the death receptor 5 (DR5) levels and promotes oxidative stress and cell death. In this study, we show that ER stress mediated by thapsigargin promoted CHOP and DR5 synthesis thus sensitizing TRAIL treatment, which induced ESCC cells apoptosis. These effects were reversed by DR5 siRNA in vitro and CHOP siRNA both in vitro and in vivo. Besides, chemically inhibition of AMPK by Compound C and AMPK siRNA weakened the anti-cancer effect of thapsigargin and TRAIL co-treatment. Therefore, our findings suggest ER stress effectively sensitizes human ESCC to TRAIL-mediated apoptosis via the TRAIL-DR5-AMPK signaling pathway, and that activation of ER stress may be beneficial for improving the efficacy of TRAIL-based anti-cancer therapy.

Melatonin reduces endoplasmic reticulum stress and autophagy in liver of leptin-deficient mice

The sedentary lifestyle of modern society along with the high intake of energetic food has made obesity a current worldwide health problem. Despite great efforts to study the obesity and its related diseases, the mechanisms underlying the development of these diseases are not well understood. Therefore, identifying novel strategies to slow the progression of these diseases is urgently needed. Experimental observations indicate that melatonin has an important role in energy metabolism and cell signalling; thus, the use of this molecule may counteract the pathologies of obesity. In this study, wild-type and obese (ob/ob) mice received daily intraperitoneal injections of melatonin at a dose of 500 μg/kg body weight for 4 weeks, and the livers of these mice were used to evaluate the oxidative stress status, proteolytic (autophagy and proteasome) activity, unfolded protein response, inflammation and insulin signalling. Our results show, for the first time, that melatonin could significantly reduce endoplasmic reticulum stress in leptin-deficient obese animals and ameliorate several symptoms that characterize this disease. Our study supports the potential of melatonin as a therapeutic treatment for the most common type of obesity and its liver-associated disorders.

Melatonin-Mediated Intracellular Insulin during 2-Deoxy-d-glucose Treatment Is Reduced through Autophagy and EDC3 Protein in Insulinoma INS-1E Cells

2-DG triggers glucose deprivation without altering other nutrients or metabolic pathways and then activates autophagy via activation of AMPK and endoplasmic reticulum (ER) stress. We investigated whether 2-DG reduced intracellular insulin increased by melatonin via autophagy/EDC3 in insulinoma INS-1E cells. p-AMPK and GRP78/BiP level were significantly increased by 2-DG in the presence/absence of melatonin, but IRE1α level was reduced in 2-DG treatment. Levels of p85α, p110, p-Akt (Ser473, Thr308), and p-mTOR (Ser2481) were also significantly reduced by 2-DG in the presence/absence of melatonin. Mn-SOD increased with 2-DG plus melatonin compared to groups treated with/without melatonin alone. Bcl-2 was decreased and Bax increased with 2-DG plus melatonin. LC3II level increased with 2-DG treatment in the presence/absence of melatonin. Intracellular insulin production increased in melatonin plus 2-DG but reduced in treatment with 2-DG with/without melatonin. EDC3 was increased by 2-DG in the presence/absence of melatonin. Rapamycin, an mTOR inhibitor, increased GRP78/BiP and EDC3 levels in a dose-dependent manner and subsequently resulted in a decrease in intracellular production of insulin. These results suggest that melatonin-mediated insulin synthesis during 2-DG treatment involves autophagy and EDC3 protein in rat insulinoma INS-1E cells and subsequently results in a decrease in intracellular production of insulin.

Data of intracellular insulin protein reduced by autophagy in INS-1E cells

Autophagy appears to be involved in maintaining normal intracellular insulin content by accelerating the insulin degradation rate in β-cells (Marsh et al., 2007) [1]. 2-deoxy-d-glucose (2-DG) is metabolized by hexokinase, and acts as an inhibitor of glycolysis. 2-DG triggers glucose deprivation without altering other nutrients or metabolic pathways (Aghaee et al., 2012) [2], and appears to be an ideal tool for studying autophagy. Rapamycin induced upregulation of autophagy in both cultured isolated islets and pancreatic β-cells (Tanemura et al., 2012) [3]. Here, we examined that 2-DG or rapamycin-induced autophagy may decrease the production of intracellular insulin in INS-1E insulinoma cells. Data showed that autophagy was increased by 2-DG or rapamycin by Western blotting and Immunofluorescence staining analyses. Also, intracellular insulin decreased by 2-DG or rapamycin. Furthermore, the autophagy inhibitors, bafilomycin A1 and/or 3-methyladenine, in the presence or absence of 2-DG or rapamycin increased intracellular insulin in INS-1E insulinoma cells.

Melatonin and endoplasmic reticulum stress: relation to autophagy and apoptosis

Endoplasmic reticulum (ER) is a dynamic organelle that participates in a number of cellular functions by controlling lipid metabolism, calcium stores, and proteostasis. Under stressful situations, the ER environment is compromised, and protein maturation is impaired; this causes misfolded proteins to accumulate and a characteristic stress response named unfolded protein response (UPR). UPR protects cells from stress and contributes to cellular homeostasis re-establishment; however, during prolonged ER stress, UPR activation promotes cell death. ER stressors can modulate autophagy which in turn, depending of the situation, induces cell survival or death. Interactions of different autophagy- and apoptosis-related proteins and also common signaling pathways have been found, suggesting an interplay between these cellular processes, although their dynamic features are still unknown. A number of pathologies including metabolic, neurodegenerative and cardiovascular diseases, cancer, inflammation, and viral infections are associated with ER stress, leading to a growing interest in targeting components of the UPR as a therapeutic strategy. Melatonin has a variety of antioxidant, anti-inflammatory, and antitumor effects. As such, it modulates apoptosis and autophagy in cancer cells, neurodegeneration and the development of liver diseases as well as other pathologies. Here, we review the effects of melatonin on the main ER stress mechanisms, focusing on its ability to regulate the autophagic and apoptotic processes. As the number of studies that have analyzed ER stress modulation by this indole remains limited, further research is necessary for a better understanding of the crosstalk between ER stress, autophagy, and apoptosis and to clearly delineate the mechanisms by which melatonin modulates these responses.

Melatonin prevents pancreatic β-cell loss due to glucotoxicity: the relationship between oxidative stress and endoplasmic reticulum stress

Prolonged hyperglycemia results in pancreatic β-cell dysfunction and apoptosis, referred to as glucotoxicity. Although both oxidative and endoplasmic reticulum (ER) stresses have been implicated as major causative mechanisms of β-cell glucotoxicity, the reciprocal importance between the two remains to be elucidated. The aim of this study was to evaluate the differential effect of oxidative stress and ER stress on β-cell glucotoxicity, by employing melatonin which has free radical-scavenging and antioxidant properties. As expected, in β-cells exposed to prolonged high glucose levels, cell viability and glucose-stimulated insulin secretion (GSIS) were significantly impaired. Melatonin treatment markedly attenuated cellular apoptosis by scavenging reactive oxygen species via its plasmalemmal receptor-independent increase in antioxidant enzyme activity. However, treatments with antioxidants alone were insufficient to recover the impaired GSIS. Interestingly, 4-phenylbutyric acid (4-PBA), a chemical chaperone that attenuate ER stress by stabilizing protein structure, alleviated the impaired GSIS, but not apoptosis, suggesting that glucotoxicity induces oxidative and ER stress independently. We found that cotreatment of glucotoxic β-cells with melatonin and 4-PBA dramatically improved both their survival and insulin secretion. Taken together, these results suggest that ER stress may be the more critical mechanism for prolonged high-glucose-induced GSIS impairment, whereas oxidative stress appears to be more critical for the impaired β-cell viability. Therefore, combinatorial therapy of melatonin with an ER stress modifier may help recover pancreatic β-cells under glucotoxic conditions in type 2 diabetes.

Melatonin and the theories of aging: a critical appraisal of melatonin's role in antiaging mechanisms

The classic theories of aging such as the free radical theory, including its mitochondria-related versions, have largely focused on a few specific processes of senescence. Meanwhile, numerous interconnections have become apparent between age-dependent changes previously thought to proceed more or less independently. Increased damage by free radicals is not only linked to impairments of mitochondrial function, but also to inflammaging as it occurs during immune remodeling and by release of proinflammatory cytokines from mitotically arrested, DNA-damaged cells that exhibit the senescence-associated secretory phenotype (SASP). Among other effects, SASP can cause mutations in stem cells that reduce the capacity for tissue regeneration or, in worst case, lead to cancer stem cells. Oxidative stress has also been shown to promote telomere attrition. Moreover, damage by free radicals is connected to impaired circadian rhythmicity. Another nexus exists between cellular oscillators and metabolic sensing, in particular to the aging-suppressor SIRT1, which acts as an accessory clock protein. Melatonin, being a highly pleiotropic regulator molecule, interacts directly or indirectly with all the processes mentioned. These influences are critically reviewed, with emphasis on data from aged organisms and senescence-accelerated animals. The sometimes-controversial findings obtained either in a nongerontological context or in comparisons of tumor with nontumor cells are discussed in light of evidence obtained in senescent organisms. Although, in mammals, lifetime extension by melatonin has been rarely documented in a fully conclusive way, a support of healthy aging has been observed in rodents and is highly likely in humans.

Regulation of β-cell function by RNA-binding proteins

β-cells of the pancreatic islets are highly specialized and high-throughput units for the production of insulin, the key hormone for maintenance of glucose homeostasis. Elevation of extracellular glucose and/or GLP-1 levels triggers a rapid upregulation of insulin biosynthesis through the activation of post-transcriptional mechanisms. RNA-binding proteins are emerging as key factors in the regulation of these mechanisms as well as in other aspects of β-cell function and glucose homeostasis at large, and thus may be implicated in the pathogenesis of diabetes. Here we review current research in the field, with a major emphasis on RNA-binding proteins that control biosynthesis of insulin and other components of the insulin secretory granules by modulating the stability and translation of their mRNAs.