Palmitate induces myocardial lipotoxic injury via the endoplasmic reticulum stress-mediated apoptosis pathway

Hypertriglyceridemia: Molecular and Genetic Landscapes

Lipid disorders represent one of the most worrisome cardiovascular risk factors. The focus on the impact of lipids on cardiac and vascular health usually concerns low-density lipoprotein cholesterol, while the role of triglycerides (TGs) is given poor attention. The literature provides data on the impact of higher plasma concentrations in TGs on the cardiovascular system and, therefore, on the outcomes and comorbidities of patients. The risk for coronary heart diseases varies from 57 to 76% in patients with hypertriglyceridemia. Specifically, the higher the plasma concentrations in TGs, the higher the incidence and prevalence of death, myocardial infarction, and stroke. Nevertheless, the metabolism of TGs and the exact physiopathologic mechanisms which try to explain the relationship between TGs and cardiovascular outcomes are not completely understood. The aims of this narrative review were as follows: to provide a comprehensive evaluation of the metabolism of triglycerides and a possible suggestion for understanding the targets for counteracting hypertriglyceridemia; to describe the inner physiopathological background for the relationship between vascular and cardiac damages derived from higher plasma concentrations in TGs; and to outline the need for promoting further insights in therapies for reducing TGs plasma levels.

Dietary palm oil enhances Sterol regulatory element-binding protein 2-mediated cholesterol biosynthesis through inducing endoplasmic reticulum stress in muscle of large yellow croaker (Larimichthys crocea)

Sterol regulatory element-binding protein 2 (SREBP2) is considered to be a major regulator to control cholesterol homoeostasis in mammals. However, the role of SREBP2 in teleost remains poorly understand. Here, we explored the molecular characterisation of SREBP2 and identified SREBP2 as a key modulator for 3-hydroxy-3-methylglutaryl-coenzyme A reductase and 7-dehydrocholesterol reductase, which were rate-limiting enzymes of cholesterol biosynthesis. Moreover, dietary palm oil in vivo or palmitic acid (PA) treatment in vitro elevated cholesterol content through triggering SREBP2-mediated cholesterol biosynthesis in large yellow croaker. Furthermore, our results also found that PA-induced activation of SREBP2 was dependent on the stimulating of endoplasmic reticulum stress (ERS) in croaker myocytes and inhibition of ERS by 4-Phenylbutyric acid alleviated PA-induced SREBP2 activation and cholesterol biosynthesis. In summary, our findings reveal a novel insight for understanding the role of SREBP2 in the regulation of cholesterol metabolism in fish and may deepen the link between dietary fatty acid and cholesterol biosynthesis.

Modeling and Phenotyping Acute and Chronic Type 2 Diabetes Mellitus In Vitro in Rodent Heart and Skeletal Muscle Cells

Type 2 diabetes (T2D) has a complex pathophysiology which makes modeling the disease difficult. We aimed to develop a novel model for simulating T2D in vitro, including hyperglycemia, hyperlipidemia, and variably elevated insulin levels targeting muscle cells. We investigated insulin resistance (IR), cellular respiration, mitochondrial morphometry, and the associated function in different T2D-mimicking conditions in rodent skeletal (C2C12) and cardiac (H9C2) myotubes. The physiological controls included 5 mM of glucose with 20 mM of mannitol as osmotic controls. To mimic hyperglycemia, cells were exposed to 25 mM of glucose. Further treatments included insulin, palmitate, or both. After short-term (24 h) or long-term (96 h) exposure, we performed radioactive glucose uptake and mitochondrial function assays. The mitochondrial size and relative frequencies were assessed with morphometric analyses using electron micrographs. C2C12 and H9C2 cells that were treated short- or long-term with insulin and/or palmitate and HG showed IR. C2C12 myotubes exposed to T2D-mimicking conditions showed significantly decreased ATP-linked respiration and spare respiratory capacity and less cytoplasmic area occupied by mitochondria, implying mitochondrial dysfunction. In contrast, the H9C2 myotubes showed elevated ATP-linked and maximal respiration and increased cytoplasmic area occupied by mitochondria, indicating a better adaptation to stress and compensatory lipid oxidation in a T2D environment. Both cell lines displayed elevated fractions of swollen/vacuolated mitochondria after T2D-mimicking treatments. Our stable and reproducible in vitro model of T2D rapidly induced IR, changes in the ATP-linked respiration, shifts in energetic phenotypes, and mitochondrial morphology, which are comparable to the muscles of patients suffering from T2D. Thus, our model should allow for the study of disease mechanisms and potential new targets and allow for the screening of candidate therapeutic compounds.

Down-regulation of OIP5-AS1 inhibits obesity-induced myocardial pyroptosis and miR-22/NLRP3 inflammasome axis

BACKGROUND

Obesity can induce myocardial pyroptosis, but the exact mechanism is still unknown. A recent study reported the association of opa-interacting protein 5-antisense transcript 1 (OIP5-AS1), an evolutionarily conserved long noncoding RNA, with pyroptosis. Therefore, this study aimed to investigate the role of OIP5-AS1 in obesity-induced myocardial pyroptosis.

METHODS

OIP5-AS1 was downregulated in H9c2 cells, followed by treatment with 400 μM palmitic acid (PA). Propidium iodide (PI) staining, lactic dehydrogenase (LDH) release assay, caspase-1 activity assay, IL-1β, and IL-18 activity assay were performed to detect pyroptotic phenotype. The interaction between OIP5-AS1 and microRNAs (miRNAs) was analyzed using RNA pull-down and luciferase assay. The effect of OIP5-AS1 knockdown in high-fat diet (HFD)-induced obesity rat on cardiac function, myocardial hypertrophy, fibrosis, and remodeling was evaluated.

RESULTS

Fat deposition was observed in cardiomyocytes 24 h after PA treatment; moreover, PA-treated cardiomyocytes showed significant increase in the rate of pyroptotic cells, release of LDH, protein expressions of NLRP3 and cleaved caspase-1, and the activity of caspase-1, IL-1β, and IL-18 as well as OIP5-AS1 expression. These findings suggested that PA activated pyroptosis and induced OIP5-AS1 expression in cardiomyocytes. Moreover, OIP5-AS1 knockdown inhibited PA-induced pyroptosis. Mechanistically, OIP5-AS1 was found to specifically bind to miR-22 and to regulate NLRP3 inflammasome-mediated pyroptosis via miR-22. Furthermore, OIP5-AS1 knockdown ameliorated HFD-induced cardiac dysfunction, myocardial hypertrophy, fibrosis, remodeling, and pyroptosis.

CONCLUSION

Our results revealed that downregulation of OIP5-AS1 can inhibit obesity-induced myocardial pyroptosis via miR-22/NLRP3 inflammasome axis. This finding lays a foundation of gene therapy for heart disease targeting OIP5-AS1.

Nicotinamide N-methyltransferase upregulation contributes to palmitate-elicited peroxisome proliferator-activated receptor transactivation in hepatocytes

Peroxisome proliferator-activated receptor γ (PPARγ) plays a pivotal role in regulating lipid metabolism and hepatic PPARγ transactivation contributes to fatty liver development. Fatty acids (FAs) are well-known endogenous ligands for PPARγ. Palmitate, a 16-C saturated FA (SFA) and the most abundant SFA in human circulation, is a strong inducer of hepatic lipotoxicity, a central pathogenic factor for various fatty liver diseases. In this study, using both alpha mouse liver 12 (AML12) and primary mouse hepatocytes, we investigated the effects of palmitate on hepatic PPARγ transactivation and underlying mechanisms, as well as the role of PPARγ transactivation in palmitate-induced hepatic lipotoxicity, all of which remain ambiguous currently. Our data revealed that palmitate exposure was concomitant with both PPARγ transactivation and upregulation of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing the degradation of nicotinamide, the predominant precursor for cellular NAD+ biosynthesis. Importantly, we discovered that PPARγ transactivation by palmitate was blunted by NNMT inhibition, suggesting that NNMT upregulation plays a mechanistic role in PPARγ transactivation. Further investigations uncovered that palmitate exposure is associated with intracellular NAD+ decline and NAD+ replenishment with NAD+-enhancing agents, nicotinamide and nicotinamide riboside, obstructed palmitate-induced PPARγ transactivation, implying that cellular NAD+ decline resulted from NNMT upregulation represents a potential mechanism behind palmitate-elicited PPARγ transactivation. At last, our data showed that the PPARγ transactivation marginally ameliorated palmitate-induced intracellular triacylglycerol accumulation and cell death. Collectively, our data provided the first-line evidence supporting that NNMT upregulation plays a mechanistic role in palmitate-elicited PPARγ transactivation, potentially through reducing cellular NAD+ contents.NEW & NOTEWORTHY Hepatic PPARγ transactivation contributes to fatty liver development. Saturated fatty acids (SFAs) induce hepatic lipotoxicity. Here, we investigated whether and how palmitate, the most abundant SFA in the human blood, affects PPARγ transactivation in hepatocytes. We reported for the first time that upregulation of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing the degradation of nicotinamide, the predominant precursor for cellular NAD+ biosynthesis, plays a mechanistic role in regulating palmitate-elicited PPARγ transactivation through reducing intracellular NAD+ contents.

Palmitate-Induced Cardiac Lipotoxicity Is Relieved by the Redox-Active Motif of SELENOT through Improving Mitochondrial Function and Regulating Metabolic State

Cardiac lipotoxicity is an important contributor to cardiovascular complications during obesity. Given the fundamental role of the endoplasmic reticulum (ER)-resident Selenoprotein T (SELENOT) for cardiomyocyte differentiation and protection and for the regulation of glucose metabolism, we took advantage of a small peptide (PSELT), derived from the SELENOT redox-active motif, to uncover the mechanisms through which PSELT could protect cardiomyocytes against lipotoxicity. To this aim, we modeled cardiac lipotoxicity by exposing H9c2 cardiomyocytes to palmitate (PA). The results showed that PSELT counteracted PA-induced cell death, lactate dehydrogenase release, and the accumulation of intracellular lipid droplets, while an inert form of the peptide (I-PSELT) lacking selenocysteine was not active against PA-induced cardiomyocyte death. Mechanistically, PSELT counteracted PA-induced cytosolic and mitochondrial oxidative stress and rescued SELENOT expression that was downregulated by PA through FAT/CD36 (cluster of differentiation 36/fatty acid translocase), the main transporter of fatty acids in the heart. Immunofluorescence analysis indicated that PSELT also relieved the PA-dependent increase in CD36 expression, while in SELENOT-deficient cardiomyocytes, PA exacerbated cell death, which was not mitigated by exogenous PSELT. On the other hand, PSELT improved mitochondrial respiration during PA treatment and regulated mitochondrial biogenesis and dynamics, preventing the PA-provoked decrease in PGC1-α and increase in DRP-1 and OPA-1. These findings were corroborated by transmission electron microscopy (TEM), revealing that PSELT improved the cardiomyocyte and mitochondrial ultrastructures and restored the ER network. Spectroscopic characterization indicated that PSELT significantly attenuated infrared spectral-related macromolecular changes (i.e., content of lipids, proteins, nucleic acids, and carbohydrates) and also prevented the decrease in membrane fluidity induced by PA. Our findings further delineate the biological significance of SELENOT in cardiomyocytes and indicate the potential of its mimetic PSELT as a protective agent for counteracting cardiac lipotoxicity.

Quercetin and Its Derivative Counteract Palmitate-Dependent Lipotoxicity by Inhibiting Oxidative Stress and Inflammation in Cardiomyocytes

Cardiac lipotoxicity plays an important role in the pathogenesis of obesity-related cardiovascular disease. The flavonoid quercetin (QUE), a nutraceutical compound that is abundant in the "Mediterranean diet", has been shown to be a potential therapeutic agent in cardiac and metabolic diseases. Here, we investigated the beneficial role of QUE and its derivative Q2, which demonstrates improved bioavailability and chemical stability, in cardiac lipotoxicity. To this end, H9c2 cardiomyocytes were pre-treated with QUE or Q2 and then exposed to palmitate (PA) to recapitulate the cardiac lipotoxicity occurring in obesity. Our results showed that both QUE and Q2 significantly attenuated PA-dependent cell death, although QUE was effective at a lower concentration (50 nM) when compared with Q2 (250 nM). QUE decreased the release of lactate dehydrogenase (LDH), an important indicator of cytotoxicity, and the accumulation of intracellular lipid droplets triggered by PA. On the other hand, QUE protected cardiomyocytes from PA-induced oxidative stress by counteracting the formation of malondialdehyde (MDA) and protein carbonyl groups (which are indicators of lipid peroxidation and protein oxidation, respectively) and intracellular ROS generation, and by improving the enzymatic activities of catalase and superoxide dismutase (SOD). Pre-treatment with QUE also significantly attenuated the inflammatory response induced by PA by reducing the release of key proinflammatory cytokines (IL-1β and TNF-α). Similar to QUE, Q2 (250 nM) also significantly counteracted the PA-provoked increase in intracellular lipid droplets, LDH, and MDA, improving SOD activity and decreasing the release of IL-1β and TNF-α. These results suggest that QUE and Q2 could be considered potential therapeutics for the treatment of the cardiac lipotoxicity that occurs in obesity and metabolic diseases.

The multifaceted roles of ER and Golgi in metabolic cardiomyopathy

Metabolic cardiomyopathy is a significant global financial and health challenge; however, pathophysiological mechanisms governing this entity remain poorly understood. Among the main features of metabolic cardiomyopathy, the changes to cellular lipid metabolism have been studied and targeted for the discovery of novel treatment strategies obtaining contrasting results. The endoplasmic reticulum (ER) and Golgi apparatus (GA) carry out protein modification, sorting, and secretion activities that are more commonly studied from the perspective of protein quality control; however, they also drive the maintenance of lipid homeostasis. In response to metabolic stress, ER and GA regulate the expression of genes involved in cardiac lipid biogenesis and participate in lipid droplet formation and degradation. Due to the varied roles these organelles play, this review will focus on recapitulating the alterations and crosstalk between ER, GA, and lipid metabolism in cardiac metabolic syndrome.

miR-141-3p regulates saturated fatty acid-induced cardiomyocyte apoptosis through Notch1/PTEN/AKT pathway via targeting PSEN1

It has been reported that miR-141-3p levels are markedly upregulated in the cardiomyocytes of obese rats induced by a high-fat diet. However, the role of miR-141-3p in myocardial lipotoxicity remains elusive. In the present study, the role of miR-141-3p in lipotoxic injury of H9c2 cells induced by palmitic acid (PA) and its possible mechanisms were assessed. The results indicated that miR-141-3p was significantly upregulated in PA-induced cardiomyocytes. miR-141-3p inhibitor enhanced the cell viability, reduced the release of lactate dehydrogenase (LDH), creatine kinase-MB (CK-MB), and troponin I (CTN-I), decreased cell apoptosis rate, and repressed the activation of mitochondrial apoptosis pathway in PA-treated H9c2, whereas treatment with miR-141-3p mimics resulted in the opposite effects. Mechanistically, it was further revealed that miR-141-3p could specifically bind to presenilin 1 (PSEN1) 3'UTR, and upregulating miR-141-3p levels reduced the expression of PSEN1, thereby inhibiting the activation of the Notch1/PTEN/AKT pathway. Additionally, inhibition of Notch1/AKT signaling pathway by its inhibitor could abrogate the effect of miR-141-3p on mitochondrial-mediated apoptosis induced by PA. In conclusion, the present study demonstrates that miR-141-3p regulates saturated fatty acid-induced cardiomyocyte apoptosis through Notch1/PTEN/AKT pathway via targeting PSEN1, which gains a new insight into the mechanisms of myocardial lipotoxic injury.

Molecular Mechanism of Lipotoxicity as an Interesting Aspect in the Development of Pathological States-Current View of Knowledge

Free fatty acids (FFAs) play numerous vital roles in the organism, such as contribution to energy generation and reserve, serving as an essential component of the cell membrane, or as ligands for nuclear receptors. However, the disturbance in fatty acid homeostasis, such as inefficient metabolism or intensified release from the site of storage, may result in increased serum FFA levels and eventually result in ectopic fat deposition, which is unfavorable for the organism. The cells are adjusted for the accumulation of FFA to a limited extent and so prolonged exposure to elevated FFA levels results in deleterious effects referred to as lipotoxicity. Lipotoxicity contributes to the development of diseases such as insulin resistance, diabetes, cardiovascular diseases, metabolic syndrome, and inflammation. The nonobvious organs recognized as the main lipotoxic goal of action are the pancreas, liver, skeletal muscles, cardiac muscle, and kidneys. However, lipotoxic effects to a significant extent are not organ-specific but affect fundamental cellular processes occurring in most cells. Therefore, the wider perception of cellular lipotoxic mechanisms and their interrelation may be beneficial for a better understanding of various diseases’ pathogenesis and seeking new pharmacological treatment approaches.

Curcumin Nanoparticles Attenuate Lipotoxic Injury in Cardiomyocytes Through Autophagy and Endoplasmic Reticulum Stress Signaling Pathways

Although curcumin (CUR) has many advantages, its hydrophobicity and instability limit its application. In this study, the anti-lipotoxic injury activity of CUR-loaded nanoparticles (CUR-NPs) and the corresponding mechanism were examined in palmitate (PA)-treated cardiomyocytes. An amphiphilic copolymer was selected as the vehicle material, and CUR-NPs with suitable sizes were prepared under optimized conditions. Cellular uptake was examined by confocal laser scanning microscopy, and cell proliferation inhibition rate was measured using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra bromide (MTT) assay. The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay was used to detect cell apoptosis. The protein expression was detected by western blot. Exposure to PA reduces the proliferation of cardiomyocytes, but this effect was strongly reversed by CUR-NPs. In addition, our data showed that CUR-NPs strongly inhibited cell apoptosis in PA-treated cardiomyocytes. Furthermore, CUR-NPs remarkably increased the expression of LC3-II, as well as inhibited the expression of p-PERK, p-eIF2α, and ATF4 in PA-treated cardiomyocytes. Salubrinal (an eIF2α inhibitor) blocked the protective effect of CUR-NPs against PA-induced cardiomyocyte injury. Our results suggested that CUR-NPs can activated the autophagy pathway and protect myocardial cells from apoptosis, and these effects may be mediated by the eIF2α-related endoplasmic reticulum stress signaling pathway.

Palmitic Acid Lipotoxicity in Microglia Cells Is Ameliorated by Unsaturated Fatty Acids

Obesity and metabolic syndrome are associated with cognitive decline and dementia. Palmitic acid (PA) is increased in the cerebrospinal fluid of obese patients with cognitive impairment. This study was therefore designed to examine fatty acid (FA) lipotoxicity in BV2 microglia cells. We found that PA induced time- and dose-dependent decrease in cell viability and increase in cell death without affecting the cell cycle profile and that PA lipotoxicity did not depend on cell surface free fatty acid receptors but rather on FA uptake. Treatment with sulfosuccinimidyl oleate (SSO), an irreversible inhibitor of fatty acid translocase CD36, significantly inhibited FA uptake in BSA- and PA-treated cells and blocked PA-induced decrease in cell viability. Inhibition of ER stress or treatment with N-acetylcysteine was not able to rescue PA lipotoxicity. Our study also showed that unsaturated fatty acids (UFAs), such as linoleic acid (LA), oleic acid (OA), α-linolenic acid (ALA), and docosahexaenoic acid (DHA), were not lipotoxic but instead protected microglia against PA-induced decrease in cell viability. Co-treatment of PA with LA, OA, and DHA significantly inhibited FA uptake in PA-treated cells. All UFAs tested induced the incorporation of FAs into and the amount of neutral lipids, while PA did not significantly affect the amount of neutral lipids compared with BSA control.

Icarrin prevents cardiomyocyte apoptosis in spontaneously hypertensive rats by inhibiting endoplasmic reticulum stress pathways

OBJECTIVES

This study aimed to explore whether icarrin (ICA) can protect cardiomyocytes from hypertension-induced damage by inhibiting endoplasmic reticulum stress (ERS).

METHODS

Spontaneously hypertensive rats (SHRs) were orally administered water or ICA at 10, 20 and 40 mg/kg once daily for 12 weeks, and Wistar-Kyoto (WKY) rats were used as control. Changes in the growth and blood pressure of rats were assessed. Cardiac function was determined by ultrasound and the left ventricle mass was calculated. Myocardial tissue structure was assessed by haematoxylin and eosin staining, cardiomyocyte apoptosis was observed by TUNEL staining and the expression of ERS-related proteins was determined by western blotting.

RESULTS

In the SHR group, blood pressure was significantly high, left ventricular function decreased and left ventricular mass index increased. Additionally, left ventricular cardiomyocyte hypertrophy, disordered myofilament arrangement and increased cardiomyocyte apoptosis were observed by histological staining. ERS-induced proteins associated with apoptosis, including GRP78, PERK, ATF-6, ATF-4, CHOP, DR5, Caspase 12, c-JUN and ASK-1 were found to be highly expressed. ICA treatment reduced blood pressure and regulated the expression of proteins induced by ERS. Cardiomyocyte apoptosis decreased and left ventricular function improved.

CONCLUSIONS

ICA can inhibit ERS-induced apoptosis of cardiomyocytes and protect ventricular function in SHR.

Cardiomyoblast caveolin expression: effects of simulated diabetes, α-linolenic acid, and cell signaling pathways

Caveolins regulate myocardial substrate handling, survival signaling, and stress resistance; however, control of expression is incompletely defined. We test how metabolic features of type 2 diabetes (T2D), and modulation of cell signaling, influence caveolins in H9c2 cardiomyoblasts. Cells were exposed to glucose (25 vs. 5 mM), insulin (100 nM), or palmitate (0.1 mM), individually or combined, and the effects of adenylate cyclase (AC) activation (50 μM forskolin), focal adhesion kinase (FAK) or protein kinase C β2 (PKCβ2) inhibition (1 μM FAK inhibitor 14 or CGP-53353, respectively) or the polyunsaturated fatty acid (PUFA) α-linolenic acid (ALA; 10 μM) were tested. Simulated T2D (elevated glucose + insulin + palmitate) depressed caveolin-1 and -3 without modifying caveolin-2. Caveolin-3 repression was primarily palmitate dependent, whereas high glucose (HG) and insulin independently increased caveolin-3 (while reducing expression when combined). Differential control was evident: baseline caveolin-3 was suppressed by FAK/PKCβ2 and insensitive to AC activities, with baseline caveolin-1 and -2 suppressed by AC and insensitive to FAK/PKCβ2. Forskolin and ALA selectively preserved caveolin-3 in T2D cells, whereas PKCβ2 and FAK inhibition increased caveolin-3 under all conditions. Despite preservation of caveolin-3, ALA did not modify nucleosome content (apoptosis marker) or transcription of proinflammatory mediators in T2D cells. In summary, caveolin-1 and -3 are strongly repressed with simulated T2D, with caveolin-3 particularly sensitive to palmitate; intrinsic PKCβ2 and FAK activities depress caveolin-3 in healthy and stressed cells; ALA and AC activation and PKCβ2 inhibition preserve caveolin-3 under T2D conditions; and caveolin-3 changes with T2D and ALA appear unrelated to inflammatory signaling or extent of apoptosis.

Long-term exposure to copper induces autophagy and apoptosis through oxidative stress in rat kidneys

Copper (Cu) is an essential trace element for most organisms. However, excessive Cu can be highly toxic. The purpose of this study was to elucidate the mechanism underlying Cu toxicity in the kidneys of rats after treatment with CuCl₂ (15 [control], 30, 60, or 120 mg/kg in the diet) for 180 days. Histological and ultrastructural changes, antioxidant enzyme activity, and the mRNA and protein levels of apoptosis and autophagy-related genes were measured. The results showed that Cu exposure led to significant accumulation of copper in kidneys and disorganized kidney morphology. The activities of total anti-oxidation capacity (T-AOC) and superoxide dismutase (SOD) in the kidneys decreased significantly, while the malondialdehyde (MDA) content increased. Furthermore, excessive Cu markedly upregulated the expression of autophagy and apoptosis-related genes (LC3A, LC3B, ATG-5, Beclin-1, Caspase3, CytC, P53, Bax), but downregulated the expression of P62, mTOR and BCL-2. Moreover, the LC3B/LC3A, ATG-5, Beclin-1, P53, Caspase3 proteins were up-regulated while P62 was down-regulated in the kidney tissues of the treatment groups. Overall, these findings provide strong evidence that excess Cu can trigger autophagy and apoptosis via the mitochondrial pathway by inducing oxidative stress in rat kidneys.

Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is considered the hepatic manifestation of the metabolic syndrome (MetS) and comprises one of the largest health threats of the twenty-first century. In this chapter, we review the current state of knowledge of NAFLD and underline the striking similarities with atherosclerosis. We first describe current epidemiological data showing the staggering increase of NAFLD numbers and its related clinical and economic costs. We then provide an overview of pathophysiological hepatic processes in NAFLD and highlight the systemic aspects of NAFLD that point toward metabolic crosstalk between organs as an important cause of metabolic disease. Finally, we end by highlighting the currently investigated therapeutic approaches for NAFLD, which also show strong similarities with a range of treatment options for atherosclerosis.

Lipotoxicity in Kidney, Heart, and Skeletal Muscle Dysfunction

Dyslipidemia is a common nutritional and metabolic disorder in patients with chronic kidney disease. Accumulating evidence supports the hypothesis that prolonged metabolic imbalance of lipids leads to ectopic fat distribution in the peripheral organs (lipotoxicity), including the kidney, heart, and skeletal muscle, which accelerates peripheral inflammation and afflictions. Thus, lipotoxicity may partly explain progression of renal dysfunction and even extrarenal complications, including renal anemia, heart failure, and sarcopenia. Additionally, endoplasmic reticulum stress activated by the unfolded protein response pathway plays a pivotal role in lipotoxicity by modulating the expression of key enzymes in lipid synthesis and oxidation. Here, we review the molecular mechanisms underlying lipid deposition and resultant tissue damage in the kidney, heart, and skeletal muscle, with the goal of illuminating the nutritional aspects of these pathologies.

Oxidative and endoplasmic reticulum stresses are involved in palmitic acid-induced H9c2 cell apoptosis

Palmitic acid (PA) is the most common saturated long-chain fatty acid that causes damage to heart muscle cells. However, the molecular mechanism of PA toxicity in myocardial cells is not fully understood. In the present study, we explored the effects of PA on proliferation and apoptosis of H9c2 cardiomyocytes, and uncovered the signaling pathways involved in PA toxicity. Our study revealed induction of both oxidative and endoplasmic reticulum (ER) stresses and exacerbation of apoptosis in PA-treated H9c2 cells. Inhibition of oxidative stress by N-acetylcysteine (NAC) reduced apoptosis and decreased ER stress in PA-treated H9c2 cells. Moreover, inhibition of ER stress by 4-phenyl butyric acid decreased apoptosis and attenuated oxidative stress. In summary, the present study demonstrated that oxidative stress coordinates with ER stress to play important roles in PA-induced H9c2 cell apoptosis.

Curcumin attenuates palmitic acid-induced cell apoptosis by inhibiting endoplasmic reticulum stress in H9C2 cardiomyocytes

Diabetic cardiomyopathy is mediated by multiple molecular mechanisms including endoplasmic reticulum (ER) stress. Curcumin, a phenolic compound, has cytoprotective properties, but its potential protective action against diabetic cardiomyopathy and the related molecular mechanisms are not fully elucidated. In this study, we evaluated the effects of curcumin on cell viability and apoptosis in palmitic acid (PA)-treated H9C2 cardiomyocytes and investigated the signaling pathways involved. Treatment with PA reduced cell viability, induced apoptosis, enhanced apoptosis-related protein expression (Caspase 3 and BCL-2 associated X protein (BAX)), and activated ER stress marker protein expression (glucose-regulated protein 78 (GRP78) and CCAAT/enhancer binding protein homologous protein (CHOP)). Curcumin attenuated PA-induced reduction in cell viability and activation of apoptosis, Caspase 3 activity, BAX, CHOP, and GRP78 expression. 4-Phenylbutyric acid (4-PBA) attenuated the PA-induced effects on cell viability and apoptosis, similar to curcumin. Both curcumin and 4-PBA also attenuated PA-induced increase in ER stress protein (CHOP and GRP78) expression. Curcumin also protected against cytotoxicity, apoptosis, and ER stress induced by thapsigargin. These findings indicate that PA triggers apoptosis in H9C2 cells via ER stress pathways and curcumin protects against this phenomenon.

Downregulation of MALAT1 alleviates saturated fatty acid-induced myocardial inflammatory injury via the miR-26a/HMGB1/TLR4/NF-κB axis

Purpose: The increased level of saturated fatty acids (SFAs) is found in patients with diabetes, obesity, and other metabolic disorders. SFAs can induce lipotoxic damage to cardiomyocytes, but the mechanism is unclear. The long noncoding RNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) acts as a key regulator in palmitic acid (PA)-induced hepatic steatosis, but its role in PA-induced myocardial lipotoxic injury is still unknown. The aim of this study was to explore the role and underlying mechanism of MALAT1 in PA-induced myocardial lipotoxic injury. Methods: MALAT1 expression in PA-treated human cardiomyocytes (AC16 cells) was detected by RT-qPCR. The effect of MALAT1 on PA-induced myocardial injury was measured by Cell Counting Kit-8, lactate dehydrogenase (LDH), and creatine kinase-MB (CK-MB) assays. Apoptosis was detected by flow cytometry. The activities of cytokines and nuclear factor (NF)-κB were detected by enzyme-linked immunosorbent assay. The interaction between MALAT1 and miR-26a was evaluated by a luciferase reporter assay and RT-qPCR. The regulatory effects of MALAT1 on high mobility group box 1 (HMGB1) expression were evaluated by RT-qPCR and western blotting. Results: MALAT1 was significantly upregulated in cardiomyocytes after PA treatment. Knockdown of MALAT1 increased the viability of PA-treated cardiomyocytes, decreased apoptosis, and reduced the levels of LDH, CK-MB, TNF-α, and IL-1β. Moreover, we found that MALAT1 specifically binds to miR-26a and observed a reciprocal negative regulatory relationship between these factors. We further found that the downregulation of MALAT1 represses HMGB1 expression, thereby inhibiting the activation of the Toll-like receptor 4 (TLR4)/NF-κB-mediated inflammatory response. These repressive effects were rescued by an miR-26a inhibitor. Conclusion: We demonstrate that MALAT1 is induced by SFAs and its downregulation alleviates SFA-induced myocardial inflammatory injury via the miR-26a/HMGB1/TLR4/NF-κB axis. Our findings provide new insight into the mechanism underlying myocardial lipotoxic injury.

SLC3A2 is a novel endoplasmic reticulum stress-related signaling protein that regulates the unfolded protein response and apoptosis

Endoplasmic reticulum (ER) stress results from imbalances in unfolded/misfolded proteins, contributing to a wide variety of human diseases. To better understand the mechanisms involved in the cellular response to ER stress in cardiomyocytes, we previously conducted a genome-wide screening in an in vitro ER stress model of rat cardiomyocytes, which highlighted amino acid transporter heavy chain, member 2 (SLC3A2) as an important factor in ER stress. In the present study, we characterized the role of SLC3A2 during the unfolded protein response (UPR), as one of the primary pathways activated during ER stress. First, we confirmed the induction of Slc3a2 mRNA expression following treatment with various ER stress inducers in rat cardiomyocytes (H9C2) and neural cells (PC12). Knockdown of Slc3a2 expression with small interfering RNA (siRNA) revealed that the encoded protein functions upstream of three important UPR proteins: ATF4, ATF6, and XBP1. siRNA-mediated knockdown of both SLC3A2 and mammalian target of rapamycin 1 (mTOR1) revealed that mTOR1 acts as a mediator between SLC3A2 and the UPR. RNA sequencing was then performed to gain a more thorough understanding of the function of SLC3A2, which identified 23 highly differentially regulated genes between the control and knockdown cell lines, which were related to the UPR and amino acid transport. Notably, flow cytometry further showed that SLC3A2 inhibition also enhanced the apoptosis of rat cardiomyocytes. Taken together, these results highlight SLC3A2 as a complex, multifunctional signaling protein that acts upstream of well-known UPR proteins with anti-apoptotic properties, suggesting its potential as a therapeutic target for ER stress-related diseases.

Downregulation of growth arrest‑specific transcript 5 alleviates palmitic acid‑induced myocardial inflammatory injury through the miR‑26a/HMGB1/NF‑κB axis

Palmitic acid (PA) can induce lipotoxic damage to cardiomyocytes, although its precise mechanism of action has not been completely elucidated. Growth arrest‑specific transcript 5 (GAS5) is a long noncoding RNA that serves a regulatory role in several pathological processes, including tumorigenesis, stroke, cardiac fibrosis and osteoarthritis; however, its role in PA‑induced myocardial injury remains elusive. The present study aimed to explore the role and underlying mechanism of GAS5 on PA‑induced myocardial injury. The expression of GAS5 in PA‑treated cardiomyocytes (H9c2 cells) was detected by reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR), and its effects on PA‑induced myocardial injury were measured by Cell Counting Kit‑8 and lactate dehydrogenase (LDH) assays. The activities of cytokines and nuclear factor (NF)‑κB were also detected by enzyme‑linked immunosorbent assay, while interactions between GAS5 and microRNA (miR)‑26a were evaluated by luciferase reporter assay and RT‑qPCR. The regulation of GAS5 on high mobility group box 1 (HMGB1) expression was detected by RT‑qPCR and western blotting. The results demonstrated that GAS5 was significantly upregulated in cardiomyocytes following treatment with PA. GAS5‑knockdown increased the viability of PA‑treated cardiomyocytes and reduced the activity of LDH, tumor necrosis factor‑α and interleukin‑1β. Furthermore, the present study identified that GAS5 specifically binds to miR‑26a, and a reciprocal negative regulation exists between the two. The present study also demonstrated that GAS5 downregulation inhibited HMGB1 expression and NF‑κB activation, while these suppressive effects were mediated by miR‑26a. In conclusion, the present study demonstrated that PA can induce GAS5 expression and that the downregulation of GAS5 alleviated PA‑induced myocardial inflammatory injury through the miR‑26a/HMGB1/NF‑κB axis. These data may provide a novel insight into the mechanism of myocardial lipotoxic injury.