Induction of miR-29a by saturated fatty acids impairs insulin signaling and glucose uptake through translational repression of IRS-1 in myocytes

Identification of microRNA that represses IRS-1 expression in liver

MicroRNAs (miRNAs) are short, non-coding RNAs that post-transcriptionally regulate gene expression and have been shown to participate in almost every cellular process. Several miRNAs have recently been implicated in glucose metabolism, but the roles of miRNAs in insulin-resistant conditions, such as obesity or type 2 diabetes, are largely unknown. Herein, we focused on miR-222, the expression of which was increased in the livers of high fat/high sucrose diet-fed mice injected with gold thioglucose (G+HFHSD). Overexpression of miR-222 in primary mouse hepatocytes attenuated Akt phosphorylation induced by insulin, indicating that miR-222 negatively regulates insulin signaling. As per in silico analysis, miR-222 potentially binds to the 3′ untranslated region (3′ UTR) of the IRS-1 gene, a key insulin signaling molecule. In fact, IRS-1 protein expression was decreased in the livers of G+HFHSD-fed mice. We further confirmed a direct interaction between miR-222 and the 3′ UTR of IRS-1 via luciferase assays. Our findings suggest that up-regulation of miR-222 followed by reduction in IRS-1 expression may be a viable mechanism of insulin resistance in the liver.

The Destiny of Glucose from a MicroRNA Perspective

Glucose serves as a primary, and for some tissues the unique, fuel source in order to generate and maintain the biological functions. Hyperglycemia is a hallmark of type 2 diabetes and is the direct consequence of perturbations in the glucose homeostasis. Insulin resistance, referred to as a reduced response of target tissues to the hormone, contributes to the development of hyperglycemia. The molecular mechanisms responsible for the altered glucose homeostasis are numerous and not completely understood. MicroRNAs (miRNAs) are now recognized as regulators of the lipid and glucose metabolism and are involved in the onset of metabolic diseases. Indeed, these small non-coding RNA molecules operate in the RNA silencing and posttranscriptional regulation of gene expression and may modulate the levels of kinases and enzymes in the glucose metabolism. Therefore, a better characterization of the function of miRNAs and a deeper understanding of their role in disease may represent a fundamental step toward innovative treatments addressing the causes, not only the symptoms, of hyperglycemia, using approaches aimed at restoring either miRNAs or their specific targets. In this review, we outline the current understanding regarding the impact of miRNAs in the glucose metabolism and highlight the need for further research focused on altered key kinases and enzymes in metabolic diseases.

Micromanaging Glucose Tolerance and Diabetes

MicroRNAs (miRNAs) are endogenous non-coding RNAs that have significant roles in biological processes such as glucose homoeostasis. MiRNAs fine-tune target genes expression via sequence-specific binding of their seed sequence to the untranslated region of mRNAs and degrade target mRNAs. MicroRNAs in islet β-cells regulate β-cell differentiation, proliferation, insulin transcription and glucose-stimulated insulin secretion. Furthermore, miRNAs play key roles in the regulation of glucose and lipid metabolisms and modify insulin sensitivity by controlling metabolic functions in main target organs of insulin such as skeletal muscle, liver and adipose tissue. Moreover, since circulating miRNAs are detectable and stable in serum, levels of certain miRNAs seem to be novel biomarkers for prediction of diabetes mellitus. In this article, due to the prominent impact of miRNAs on diabetes, we overviewed the microRNAs regulatory functions in organs related to insulin resistance and diabetes and shed light on their potential as diagnostic and therapeutic markers for diabetes.

Metabolic Circuit Involving Free Fatty Acids, microRNA 122, and Triglyceride Synthesis in Liver and Muscle Tissues

BACKGROUND & AIMS

Effective treatments are needed for hepatic steatosis characterized by accumulation of triglycerides in hepatocytes, which leads to hepatocellular carcinoma. MicroRNA 122 (MIR122) is expressed only in the liver, where it regulates lipid metabolism. We investigated the mechanism by which free fatty acids (FFAs) regulate MIR122 expression and the effect of MIR122 on triglyceride synthesis.

METHODS

We analyzed MIR122 promoter activity and validated its target mRNAs by transfection of Luciferase reporter plasmids into Huh7, BNL-1ME, and HEK293 cultured cell lines. We measured levels of microRNAs and mRNAs by quantitative real-time PCR analysis of RNA extracted from plasma, liver, muscle, and adipose tissues of C57BL/6 mice given the FFA-inducer CL316243. MIR122 was inhibited using an inhibitor of MIR122. Metabolic profiles of mice were determined using metabolic chambers and by histologic analyses of liver tissues. We performed RNA sequence analyses to identify metabolic pathways involving MIR122.

RESULTS

We validated human Agpat1 and Dgat1 mRNAs, involved in triglyceride synthesis, as targets of MIR122. FFAs increased MIR122 expression in livers of mice by activating the retinoic acid-related orphan receptor alpha, and induced secretion of MIR122 from liver to blood. Circulating MIR122 entered muscle and adipose tissues of mice, reducing mRNA levels of genes involved in triglyceride synthesis. Mice injected with an inhibitor of MIR122 and then given CL316243, accumulated triglycerides in liver and muscle tissues, and had reduced rates of β-oxidation. There was a positive correlation between level of FFAs and level of MIR122 in plasma samples from 6 healthy individuals, collected before and during fasting.

CONCLUSIONS

In biochemical and histologic studies of plasma, liver, muscle, and adipose tissues from mice, we found that FFAs increase hepatic expression and secretion of MIR122, which regulates energy storage vs expenditure in liver and peripheral tissues. Strategies to reduce triglyceride levels, by increasing MIR122, might be developed for treatment of metabolic syndrome.

Nutrimiromics: Role of microRNAs and Nutrition in Modulating Inflammation and Chronic Diseases

Nutrimiromics studies the influence of the diet on the modification of gene expression due to epigenetic processes related to microRNAs (miRNAs), which may affect the risk for the development of chronic diseases. miRNAs are a class of non-coding endogenous RNA molecules that are usually involved in post-transcriptional gene silencing by inducing mRNA degradation or translational repression by binding to a target messenger RNA. They can be controlled by environmental and dietary factors, particularly by isolated nutrients or bioactive compounds, indicating that diet manipulation may hold promise as a therapeutic approach in modulating the risk of chronic diseases. This review summarizes the evidence regarding the influence of nutrients and bioactive compounds on the expression of miRNAs related to inflammation and chronic disease in several models (cell culture, animal models, and human trials).

Identification and characterization of lipid metabolism-related microRNAs in the liver of genetically improved farmed tilapia (GIFT, Oreochromis niloticus) by deep sequencing

MicroRNAs (miRNAs) play vital roles in modulating diverse metabolic processes in the liver, including lipid metabolism. Genetically improved farmed tilapia (GIFT, Oreochromis niloticus), an important aquaculture species in China, is susceptible to hepatic steatosis when reared in intensive culture systems. To investigate the miRNAs involved in GIFT lipid metabolism, two hepatic small RNA libraries from high-fat diet-fed and normal-fat diet-fed GIFT were constructed and sequenced using high-throughput sequencing technology. A total of 204 known and 56 novel miRNAs were identified by aligning the sequencing data with known Danio rerio miRNAs listed in miRBase 21.0. Six known miRNAs (miR-30a-5p, miR-34a, miR-145-5p, miR-29a, miR-205-5p, and miR-23a-3p) that were differentially expressed between the high-fat diet and normal-fat diet groups were validated by quantitative real-time PCR. Bioinformatics tools were used to predict the potential target genes of these differentially expressed miRNAs, and Gene Ontology enrichment analysis indicated that these miRNAs may play important roles in diet-induced hepatic steatosis in GIFT. Our results provide a foundation for further studies of the role of miRNAs in tilapia lipid homeostasis regulation, and may help to identify novel targets for therapeutic interventions to reduce the occurrence of fatty liver disease in farmed tilapia.

Role of microRNAs in obesity and obesity-related diseases

In recent years, the link between regulatory microRNAs (miRNAs) and diseases has been the object of intensive research. miRNAs have emerged as key mediators of metabolic processes, playing crucial roles in maintaining/altering physiological processes, including energy balance and metabolic homeostasis. Altered miRNAs expression has been reported in association with obesity, both in animal and human studies. Dysregulation of miRNAs may affect the status and functions of different tissues and organs, including the adipose tissue, pancreas, liver, and muscle, possibly contributing to metabolic abnormalities associated with obesity and obesity-related diseases. More recently, the discovery of circulating miRNAs easily detectable in plasma and other body fluids has emphasized their potential as both endocrine signaling molecules and disease indicators. In this review, the status of current research on the role of miRNAs in obesity and related metabolic abnormalities is summarized and discussed.

Altered miR-29 Expression in Type 2 Diabetes Influences Glucose and Lipid Metabolism in Skeletal Muscle

MicroRNAs have emerged as important regulators of glucose and lipid metabolism in several tissues; however, their role in skeletal muscle remains poorly characterized. We determined the effects of the miR-29 family on glucose metabolism, lipid metabolism, and insulin responsiveness in skeletal muscle. We provide evidence that miR-29a and miR-29c are increased in skeletal muscle from patients with type 2 diabetes and are decreased following endurance training in healthy young men and in rats. In primary human skeletal muscle cells, inhibition and overexpression strategies demonstrate that miR-29a and miR-29c regulate glucose uptake and insulin-stimulated glucose metabolism. We identified that miR-29 overexpression attenuates insulin signaling and expression of insulin receptor substrate 1 and phosphoinositide 3-kinase. Moreover, miR-29 overexpression reduces hexokinase 2 expression and activity. Conversely, overexpression of miR-29 by electroporation of mouse tibialis anterior muscle decreased glucose uptake and glycogen content in vivo, concomitant with decreased abundance of GLUT4. We also provide evidence that fatty acid oxidation is negatively regulated by miR-29 overexpression, potentially through the regulation of peroxisome proliferator-activated receptor γ coactivator-1α expression. Collectively, we reveal that miR-29 acts as an important regulator of insulin-stimulated glucose metabolism and lipid oxidation, with relevance to human physiology and type 2 diabetes.

microRNA in Cardiovascular Aging and Age-Related Cardiovascular Diseases

Over the last decades, life expectancy has significantly increased although several chronic diseases persist in the population, with aging as the leading risk factor. Despite improvements in diagnosis and treatment, many elderlies suffer from cardiovascular problems that are much more frequent in an older, more fragile organism. In the long term, age-related cardiovascular diseases (CVDs) contribute to the decline of quality of life and ability to perform normal activities of daily living. microRNAs (miRNAs) are a class of small non-coding RNAs that regulate gene expression at the posttranscriptional level in both physiological and pathological conditions. In this review, we will focus on the role of miRNAs in aging and age-related CVDs as heart failure, hypertension, atherosclerosis, atrial fibrillation, and diabetes mellitus. miRNAs are key regulators of complex biological mechanisms, representing an exciting potential therapeutic target in CVDs. Moreover, one major challenge in geriatric medicine is to find reliable biomarkers for diagnosis, prognosis, and prediction of the response to specific drugs. miRNAs represent a very promising tool due to their stability in the circulation and unique signature in CVDs. However, further studies are needed to investigate their translational potential in the real clinical practice.

Emerging Roles for MicroRNAs in Diabetic Microvascular Disease: Novel Targets for Therapy

Chronic, low-grade systemic inflammation and impaired microvascular function are critical hallmarks in the development of insulin resistance. Accordingly, insulin resistance is a major risk factor for type 2 diabetes and cardiovascular disease. Accumulating studies demonstrate that restoration of impaired function of the diabetic macro- and microvasculature may ameliorate a range of cardiovascular disease states and diabetes-associated complications. In this review, we focus on the emerging role of microRNAs (miRNAs), noncoding RNAs that fine-tune target gene expression and signaling pathways, in insulin-responsive tissues and cell types important for maintaining optimal vascular homeostasis and preventing the sequelae of diabetes-induced end organ injury. We highlight current pathophysiological paradigms of miRNAs and their targets involved in regulating the diabetic microvasculature in a range of diabetes-associated complications such as retinopathy, nephropathy, wound healing, and myocardial injury. We provide an update of the potential use of circulating miRNAs diagnostically in type I or type II diabetes. Finally, we discuss emerging delivery platforms for manipulating miRNA expression or function as the next frontier in therapeutic intervention to improve diabetes-associated microvascular dysfunction and its attendant clinical consequences.

miR-29a modulates SCD expression and is regulated in response to a saturated fatty acid diet in juvenile genetically improved farmed tilapia (Oreochromis niloticus)

MicroRNAs (miRNAs) are small non-coding RNAs that regulate target gene expression by binding to the 3′ untranslated region (3′ UTR) of the target mRNA. MiRNAs regulate a large variety of genes, including those involved in liver biology and disease. Here, we report for the first time that miR-29a post-transcriptionally regulates stearoyl-CoA desaturase (SCD) by binding to its 3′ UTR in genetically improved farmed tilapia (GIFT), Oreochromis niloticus, as shown by a 3′ UTR luciferase reporter assay. miR-29a antagomir treatment in vivo resulted in significant upregulation of SCD expression. We found that miR-29a expression was negatively correlated with SCD expression in GIFT liver. Inhibition of miR-29a led to a significant increase in SCD expression on day 60 induced by a saturated fatty acid diet, thereby increasing conversion of 16:0 and 18:0 to 16:1 and 18:1, respectively, and activating serum insulin, which would favor glucose and lipid uptake by the liver. These results indicate that miR-29a regulates SCD levels by binding to its 3′ UTR, and this interaction affects saturated fatty acid stress induction and insulin and lipid accumulation in serum. Our results suggest that miR-29a is critical in regulating lipid metabolism homeostasis in GIFT liver, and this might provide a basis for understanding the biological processes and therapeutic intervention encountered in fatty liver.

Summary: miR-29a targets SCD 3′ UTR directly. Inhibition of miR-29a could mediate conversion of C16:0 and C18:0 to C16:1 and C18:1, respectively, and activate serum insulin and glucose uptake in GIFT by increasing SCD.

MicroRNAs-Mediated Regulation of Skeletal Muscle GLUT4 Expression and Translocation in Insulin Resistance

The solute carrier family 2 facilitated glucose transporter member 4 (GLUT4) plays a key role in the insulin-induced glucose uptake by muscle and adipose tissues. In prediabetes and diabetes, GLUT4 expression/translocation has been detected as reduced, participating in mechanisms that impair glycemic control. Recently, a class of short endogenous noncoding RNAs named microRNAs (miRNAs) has been increasingly described as involved in the posttranscriptional epigenetic regulation of gene expression. The present review focuses on miRNAs potentially involved in the expression of GLUT4 expression, and proteins related to GLUT4 and translocation in skeletal muscle, seeking to correlate them with insulin resistance and diabetes. So far, miR-21a-5p, miR-29a-3p, miR-29c-3p, miR-93-5p, miR-106b-5p, miR-133a-3p, miR-133b-3p, miR-222-3p, and miR-223-3p have been reported to directly and/or indirectly regulate the GLUT4 expression; and their expression is altered under diabetes-related conditions. Besides, some miRNAs that have been linked to the expression of proteins involved in GLUT4 translocation machinery in muscle could also impact glucose uptake. That makes these miRNAs promising targets for preventive and/or therapeutic approaches, which could improve glycemic control, thus deserving future new investigations.

Regulatory Roles of MicroRNAs in Diabetes

MicroRNAs (miRNAs), a class of endogenous small noncoding RNAs in eukaryotes, have been recognized as significant regulators of gene expression through post-transcriptional mechanisms. To date, >2000 miRNAs have been identified in the human genome, and they orchestrate a variety of biological and pathological processes. Disruption of miRNA levels correlates with many diseases, including diabetes mellitus, a complex multifactorial metabolic disorder affecting >400 million people worldwide. miRNAs are involved in the pathogenesis of diabetes mellitus by affecting pancreatic β-cell functions, insulin resistance, or both. In this review, we summarize the investigations of the regulatory roles of important miRNAs in diabetes, as well as the potential of circulating miRNAs as diagnostic markers for diabetes mellitus.

Nutrigenomics, the Microbiome, and Gene-Environment Interactions: New Directions in Cardiovascular Disease Research, Prevention, and Treatment: A Scientific Statement From the American Heart Association

Cardiometabolic diseases are the leading cause of death worldwide and are strongly linked to both genetic and nutritional factors. The field of nutrigenomics encompasses multiple approaches aimed at understanding the effects of diet on health or disease development, including nutrigenetic studies investigating the relationship between genetic variants and diet in modulating cardiometabolic risk, as well as the effects of dietary components on multiple "omic" measures, including transcriptomics, metabolomics, proteomics, lipidomics, epigenetic modifications, and the microbiome. Here, we describe the current state of the field of nutrigenomics with respect to cardiometabolic disease research and outline a direction for the integration of multiple omics techniques in future nutrigenomic studies aimed at understanding mechanisms and developing new therapeutic options for cardiometabolic disease treatment and prevention.

Could gestational diabetes mellitus be managed through dietary bioactive compounds? Current knowledge and future perspectives

Gestational diabetes mellitus (GDM) is a serious problem growing worldwide that needs to be addressed with urgency in consideration of the resulting severe complications for both mother and fetus. Growing evidence indicates that a healthy diet rich in fruit, vegetables, nuts, extra-virgin olive oil and fish has beneficial effects in both the prevention and management of several human diseases and metabolic disorders. In this review, we discuss the latest data concerning the effects of dietary bioactive compounds such as polyphenols and PUFA on the molecular mechanisms regulating glucose homoeostasis. Several studies, mostly based on in vitro and animal models, indicate that dietary polyphenols, mainly flavonoids, positively modulate the insulin signalling pathway by attenuating hyperglycaemia and insulin resistance, reducing inflammatory adipokines, and modifying microRNA (miRNA) profiles. Very few data about the influence of dietary exposure on GDM outcomes are available, although this approach deserves careful consideration. Further investigation, which includes exploring the ‘omics’ world, is needed to better understand the complex interaction between dietary compounds and GDM.

MicroRNA-29a mitigates glucocorticoid induction of bone loss and fatty marrow by rescuing Runx2 acetylation

Glucocorticoid treatment reportedly increases the morbidity of osteoporotic or osteonecrotic disorders. Exacerbated bone acquisition and escalated marrow adipogenesis are prominent pathological features of glucocorticoid-mediated skeletal disorders. MicroRNAs reportedly modulate tissue metabolism and remodeling. This study was undertaken to investigate the biological roles of microRNA-29a (miR-29a) in skeletal and fat metabolism in the pathogenesis of glucocorticoid-induced osteoporosis. Transgenic mice overexpressing miR-29a precursor or wild-type mice were given methylprednisolone. Bone mass, microarchitecture and histology were assessed by dual energy X-ray absorptiometry, μCT and histomorphometry. Differential gene expression and signaling components were delineated by quantitative RT-PCR and immunoblotting. Glucocorticoid treatment accelerated bone loss and marrow fat accumulation in association with decreased miR-29a expression. The miR-29a transgenic mice had high bone mineral density, trabecular microarchitecture and cortical thickness. miR-29a overexpression mitigated the glucocorticoid-induced impediment of bone mass, skeletal microstructure integrity and mineralization reaction and attenuated fatty marrow histopathology. Ex vivo, miR-29a increased osteogenic differentiation capacity and alleviated the glucocorticoid-induced promotion of adipocyte formation in primary bone-marrow mesenchymal progenitor cell cultures. Through inhibition of histone deacetylase 4 (HDAC4) expression, miR-29a restored acetylated Runx2 and β-catenin abundances and reduced RANKL, leptin and glucocorticoid receptor expression in glucocorticoid-mediated osteoporosis bone tissues. Taken together, glucocorticoid suppression of miR-29a signaling disturbed the balances between osteogenic and adipogenic activities, and thereby interrupted bone formation and skeletal homeostasis. miR-29a inhibition of HDAC4 stabilized the acetylation state of Runx2 and β-catenin that ameliorated the detrimental effects of glucocorticoid on mineralization and lipogenesis reactions in bone tissue microenvironments. This study highlighted emerging skeletal-anabolic actions of miR-29a signaling in the progression of glucocorticoid-induced bone tissue destruction. Sustaining miR-29a actions is beneficial in protecting against glucocorticoid-mediated osteoporosis.

Novel Mediators of Adipose Tissue and Muscle Crosstalk

The crosstalk between adipose tissue and skeletal muscle has gained considerable interest, since this process, specifically in obesity, substantially drives the pathogenesis of muscle insulin resistance. In this review, we discuss novel concepts and targets of this bidirectional organ communication system. This includes adipo-myokines like apelin and FGF21, inflammasomes, autophagy, and microRNAs (miRNAs). Literature analysis shows that the crosstalk between fat and muscle involves both extracellular molecules and intracellular organelles. We conclude that integration of these multiple crosstalk elements into one network will be required to better understand this process.

Obesity-induced miR-15b is linked causally to the development of insulin resistance through the repression of the insulin receptor in hepatocytes

SCOPE

Obesity increases intracellular lipid accumulation in key tissues or organs, which often leads to metabolic dysfunction and insulin resistance. Diets rich in saturated fatty acid (SFA) exacerbate obesity and hepatic steatosis, which accentuate the risk of insulin resistance and type 2 diabetes (T2DM). Although microRNAs (miRNAs) play a critical role in the regulation of gene expression, the implication of obesity-induced miRNAs in metabolic disorders particularly in the development of insulin resistance is largely unknown. Here, we investigated the implication of miR-15b, which is induced by SFA palmitate or obesity, in hepatic insulin resistance.

METHODS AND RESULTS

Diet-induced obesity (DIO) in mice developed hyperglycemia and insulin resistance, accompanying with a reduction of insulin receptor (INSR) expression. Palmitate impaired insulin signaling as well as a decrease of INSR in hepatocytes. The expression of miR-15b was upregulated by DIO or palmitate in hepatocytes. Furthermore, the overexpression of miR-15b suppressed the protein expression of INSR through targeting INSR 3' untranslated region directly, resulting in an impairment of the insulin signaling and glycogen synthesis in hepatocytes.

CONCLUSION

These results unveil a novel mechanism whereby miR-15b is linked causally to the pathogenesis of hepatic insulin resistance in SFA-induced obesity.

Growth hormone replacement therapy regulates microRNA-29a and targets involved in insulin resistance

Abstract

Replacement of growth hormone (GH) in patients suffering from GH deficiency (GHD) offers clinical benefits on body composition, exercise capacity, and skeletal integrity. However, GH replacement therapy (GHRT) is also associated with insulin resistance, but the mechanisms are incompletely understood. We demonstrate that in GH-deficient mice (growth hormone-releasing hormone receptor (Ghrhr)^lit/lit), insulin resistance after GHRT involves the upregulation of the extracellular matrix (ECM) and the downregulation of microRNA miR-29a in skeletal muscle. Based on RNA deep sequencing of skeletal muscle from GH-treated Ghrhr^lit/lit mice, we identified several upregulated genes as predicted miR-29a targets that are negative regulators of insulin signaling or profibrotic/proinflammatory components of the ECM. Using gain- and loss-of-function studies, five of these genes were confirmed as endogenous targets of miR-29a in human myotubes (PTEN, COL3A1, FSTL1, SERPINH1, SPARC). In addition, in human myotubes, IGF1, but not GH, downregulated miR-29a expression and upregulated COL3A1. These results were confirmed in a group of GH-deficient patients after 4 months of GHRT. Serum IGF1 increased, skeletal muscle miR-29a decreased, and miR-29a targets were upregulated in patients with a reduced insulin response (homeostatic model assessment of insulin resistance (HOMA-IR)) after GHRT. We conclude that miR-29a could contribute to the metabolic response of muscle tissue to GHRT by regulating ECM components and PTEN. miR-29a and its targets might be valuable biomarkers for muscle metabolism following GH replacement.

Key messages

GHRT most significantly affects the ECM cluster in skeletal muscle from mice.

GHRT downregulates miR-29a and upregulates miR-29a targets in skeletal muscle from mice.

PTEN, COL3A1, FSTL1, SERPINH1, and SPARC are endogenous miR-29a targets in human myotubes.

IGF1 decreases miR-29a levels in human myotubes.

miR-29a and its targets are regulated during GHRT in skeletal muscle from humans.

Electronic supplementary material

The online version of this article (doi:10.1007/s00109-015-1322-y) contains supplementary material, which is available to authorized users.

PLIN2 inhibits insulin-induced glucose uptake in myoblasts through the activation of the NLRP3 inflammasome

Impaired lipid metabolism and inflammatory pathways have individually been implicated in the development of insulin resistance in skeletal muscle; however, little evidence is available to date linking the two in this context. In this study, we explored a potential molecular mechanism underlying insulin resistance in myoblasts mediated by the crosstalk between lipid accumulation and inflammatory pathways. We examined the influence of perilipin 2 (PLIN2), one of the most highly expressed lipid droplet-associated proteins in skeletal muscle, on glucose uptake and on the nucleotide‑binding domain, leucine‑rich repeat containing protein 3 (NLRP3) inflammasome in vitro. PLIN2 overexpression in C2C12 cells led to an increased expression of NLRP3, caspase‑1 and interleukin (IL)‑1β, along with an impaired insulin‑induced glucose uptake. This defect was remedied by the RNAi‑mediated knockdown of NLRP3 expression. We also found that insulin receptor substrate‑1 (IRS‑1), a component of insulin signaling, was negatively regulated by NLRP3 and IL‑1β, and that IL‑1β inhibited insulin‑induced glucose uptake in myoblasts. These results suggest that PLIN2 inhibits insulin‑induced glucose uptake by activating NLRP3, caspase‑1 and IL‑1β, leading to a decreased IRS‑1 expression. This study provides in vitro evidence supporting an association between lipid metabolism and inflammatory pathways in the pathogenesis of insulin resistance in skeletal muscle, and suggests potential therapeutic targets that warrant further investigation.

The pathogenic role of persistent milk signaling in mTORC1- and milk-microRNA-driven type 2 diabetes mellitus

Milk, the secretory product of the lactation genome, promotes growth of the newborn mammal. Milk delivers insulinotropic amino acids, thus maintains a molecular crosstalk with the pancreatic β-cell of the milk recipient. Homeostasis of β-cells and insulin production depend on the appropriate magnitude of mTORC1 signaling. mTORC1 is activated by branched-chain amino acids (BCAAs), glutamine, and palmitic acid, abundant nutrient signals of cow´s milk. Furthermore, milk delivers bioactive exosomal microRNAs. After milk consumption, bovine microRNA-29b, a member of the diabetogenic microRNA-29- family, reaches the systemic circulation and the cells of the milk consumer. MicroRNA-29b downregulates branchedchain α-ketoacid dehydrogenase, a potential explanation for increased BCAA serum levels, the metabolic signature of insulin resistance and type 2 diabetes mellitus (T2DM). In non-obese diabetic mice, microRNA-29b downregulates the antiapoptotic protein Mcl-1, which leads to early β-cell death. In all mammals except Neolithic humans, milk-driven mTORC1 signaling is physiologically restricted to the postnatal period. In contrast, chronic hyperactivated mTORC1 signaling has been associated with the development of age-related diseases of civilization including T2DM. Notably, chronic hyperactivation of mTORC1 enhances endoplasmic reticulum stress that promotes apoptosis. In fact, hyperactivated β-cell mTORC1 signaling induced early β-cell apoptosis in a mouse model. The EPIC-InterAct Study demonstrated an association between milk consumption and T2DM in France, Italy, United Kingdom, Germany, and Sweden. In contrast, fermented milk products and cheese exhibit an inverse correlation. Since the early 1950´s, refrigeration technology allowed widespread consumption of fresh pasteurized milk, which facilitates daily intake of bioactive bovine microRNAs. Persistent uptake of cow´s milk-derived microRNAs apparently transfers an overlooked epigenetic diabetogenic program that should not reach the human food chain.

MicroRNA-185 regulates expression of lipid metabolism genes and improves insulin sensitivity in mice with non-alcoholic fatty liver disease

AIM: To assess the regulatory effect of microRNA-185 (miR-185) on lipid metabolism and the insulin signalling pathway in human HepG2 hepatocytes and a high-fat diet mouse model.

METHODS: Quantitative reverse transcription-polymerase chain reaction was used to assess the mRNA levels of lipogenic genes after loss or gain of miR-185. In addition, the amounts of insulin signalling intermediates were determined after transfection of HepG2 cells with pre-miR-185.

RESULTS: MiR-185 levels decreased in a time- and dose-dependent manner in response to palmitic acid in human HepG2 hepatocytes. Transfection of HepG2 cells with miR-185 significantly decreased the mRNA levels of fatty acid synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, sterol-regulatory element binding protein-2, and sterol-regulatory element binding protein-1c, whereas inhibition of miR-185 using an anti-miR-185 oligonucleotide produced the opposite effect in HepG2 cells. In a high-fat diet mouse model, the accumulation of lipids was significantly improved after treatment with miR-185, compared with control animals. Induction of miR-185 enhanced the insulin signalling pathway by up-regulating the insulin-receptor substrate-2.

CONCLUSION: These findings suggest that miR-185 plays an important role in regulating fatty-acid metabolism and cholesterol homeostasis in hepatocytes, as well as in improving insulin sensitivity, both in vitro and in vivo.