Association of Di(2-ethylhexyl) Terephthalate and Its Metabolites with Nonalcoholic Fatty Liver Disease: An Epidemiology and Toxicology Study

As an alternative plasticizer to conventional phthalates, di(2-ethylhexyl) terephthalate (DEHTP) has attracted considerable concerns, given its widespread detection in the environment and humans. However, the potential toxicity, especially liver toxicity, posed by DEHTP remains unclear. In this study, based on the 2017-2018 National Health and Nutrition Examination Survey, two metabolites of DEHTP, i.e., mono(2-ethyl-5-hydroxyhexyl) terephthalate (MEHHTP) and mono(2-ethyl-5-carboxypentyl) terephthalate (MECPTP), were found to be present in the urine samples of nearly all representative U.S. adults. Moreover, a positive linear correlation was observed between the concentrations of the two metabolites and the risk of nonalcoholic fatty liver disease (NAFLD) in the population. Results of weighted quantile sum and Bayesian kernel machine regression indicated that MEHHTP contributed a greater weight to the risk of NAFLD in comparison with 12 conventional phthalate metabolites. In vitro experiments with hepatocyte HepG2 revealed that MEHHTP exposure could increase lipogenic gene programs, thereby promoting a dose-dependent hepatic lipid accumulation. Activation of liver X receptor α may be an important regulator of MEHHTP-induced hepatic lipid disorders. These findings provide new insights into the liver lipid metabolism toxicity potential of DEHTP exposure in the population.

Kimchi attenuates endoplasmic reticulum stress-induced hepatic steatosis in HepG2 cells and C57BL/6N mice

Kimchi is a traditional fermented food that contains abundant nutrients and functional ingredients with various health benefits. We previously reported that kimchi active components suppress hepatic steatosis caused by endoplasmic reticulum (ER) stress in vitro and in vivo. Therefore, we assessed the effect of kimchi on the inhibition of hepatic steatosis caused by ER stress in HepG2 cells and C57BL/6N mice to verify the hypothesis that kimchi may potentially inhibit nonalcoholic fatty liver disease. We investigated the effect of kimchi on cell viability and triglyceride concentrations in cells and on lipid profile, lipid accumulation, and expression of related genes in cells and mice with hepatic steatosis. A mechanistic study was also performed using the liver X receptor α agonist T0901317 and the AMP-activated protein kinase agonist AICAR. Kimchi was noncytotoxic and effectively reduced triglyceride concentrations and suppressed hepatic steatosis-related gene expression in cells and mice. Additionally, kimchi recovered weight loss, lowered the serum and liver tissue lipid profiles, suppressed lipid accumulation, and reduced the effects of T0901317 and AICAR on lipogenic gene expression in tunicamycin-treated mice. Our results highlight that kimchi could prevent hepatic steatosis caused by ER stress in cells and mice.

Compound K induces neurogenesis of neural stem cells in thrombin induced nerve injury through LXRα signaling in mice

Intracerebral hemorrhage (ICH) causes neurological function deficit due to the loss of neurons surrounding the hematoma. Increased neurogenesis of endogenous neural stem cells (EnNSCs) is believed to increase cell proliferation and differentiation, thereby improving the neurological deficit. However, there are still limited drugs that are effective for treating neurological deficit. So, the effects of compound K (CK) in EnNSCs were measured after thrombin-induced mice models both in vivo and in vitro, and investigated the probable mechanisms of CK during pro-neurogenesis. The results revealed that 10 μM CK promotes neurogenesis, proliferation and reduces apoptosis of EnNSCs after induction by thrombin. After that, CK treatment increased the neurogenesis of EnNSCs through liver X receptor α (LXRα) signaling pathway using adeno-associated virus knockdown and knocked out mice of LXRα gene. Finally, intraperitoneal injection of 10 mg/kg CK improved the neurogenesis of subventricular zone (SVZ), myelin repair and behavioral deficit after stereotaxic injection of thrombin in the basal ganglia of mice, and this process involved LXRα. These observations provided evidence regarding the effect of CK in pro-neurogenesis via LXRα activation, and suggested further evaluation of it due to its potential role as an effective modulator in the treatment of ICH.

The detrimental effect of asymmetric dimethylarginine on cholesterol efflux of macrophage foam cells: Role of the NOX/ROS signaling

BACKGROUND

Asymmetric dimethylarginine (ADMA) is an endogenous nitric oxide synthase inhibitor and has been proposed to be an independent risk factor for cardiovascular diseases. However, little is known about its role in the regulation of lipid metabolism. In this study, we investigated the effect of ADMA on cholesterol metabolism and its underlying molecular mechanism.

METHODS

Oxidized low-density lipoprotein (oxLDL)-induced macrophage foam cells were used as an in vitro model. Apolipoprotein E-deficient (apoE^-/-) hyperlipidemic mice were used as an in vivo model. Western blot analysis was used to evaluate protein expression. Luciferase reporter assays were used to assess the activity of promoters and transcription factors. Conventional assay kits were used to measure the levels of ADMA, cholesterol, triglycerides, and cytokines.

RESULTS

Treatment with oxLDL decreased the protein expression of dimethylarginine dimethylaminohydrolase-2 (DDAH-2) but not DDAH-1. Incubation with ADMA markedly increased oxLDL-induced lipid accumulation in macrophages. ADMA impaired cholesterol efflux following oxLDL challenge and downregulated the expression of ATP-binding cassette transporter A1 (ABCA1) and ABCG1 by interfering with liver X receptor α (LXRα) expression and activity. Additionally, this inhibitory effect of ADMA on cholesterol metabolism was mediated through the activation of the NADPH oxidase/reactive oxygen species pathway. In vivo experiments revealed that chronic administration of ADMA for 4 weeks exacerbated systemic inflammation, decreased the aortic protein levels of ABCA1 and ABCG1, and impaired the capacity of reverse cholesterol transport, ultimately, leading to the progression of atherosclerosis in apoE^-/- mice.

CONCLUSION

Our findings suggest that the ADMA/DDAH-2 axis plays a crucial role in regulating cholesterol metabolism in macrophage foam cells and atherosclerotic progression.

Thioredoxin-1 promotes macrophage reverse cholesterol transport and protects liver from steatosis

Atherosclerosis is characterized by the accumulation of excess cholesterol in plaques. Reverse cholesterol transport (RCT) plays a key role in the removal of cholesterol. In the present study, we examined the effect of thioredoxin-1 (Trx-1) on RCT and explored the underlying mechanism. We found that Trx-1 promoted RCT in vivo, as did T0901317, a known liver X receptor (LXR) ligand. T0901317 also inhibited the development of atherosclerotic plaques but promoted liver steatosis. Furthermore, Trx-1 promoted macrophage cholesterol efflux to apoAI in vitro. Mechanistically, Trx-1 promoted nuclear translocation of LXRα and induced the expression of ATP-binding cassette transporter A1 (ABCA1). Apolipoprotein E knockout (apoE-/-) mice fed an atherogenic diet were daily injected intraperitoneally with saline or Trx-1 (0.33 mg/kg). Trx-1 treatment significantly inhibited the development of atherosclerosis and induced the expression of ABCA1 in macrophages retrieved from apoE-/- mice. Moreover, the liver steatosis was attenuated by Trx-1. Overall, we demonstrated that Trx-1 promotes RCT by upregulating ABCA1 expression through induction of nuclear translocation of LXRα, and protects liver from steatosis.

Dihydromyricetin ameliorates foam cell formation via LXRα-ABCA1/ABCG1-dependent cholesterol efflux in macrophages

As the most abundant flavonoid in Ampelopsis grossedentata, the protective effects of dihydromyricetin on atherosclerosis have been well established, yet the detailed mechanisms are not fully understood. The aim of the present study was to examine the effect of dihydromyricetin on lipid accumulation and the underlying molecular mechanisms in macrophages and ApoE^-/- mice. Incubation with dihydromyricetin significantly attenuated oxidized low-density lipoprotein (ox-LDL)-mediated cholesterol and lipid accumulation in THP-1-derived macrophages, which was due to increased cholesterol efflux. In addition, dihydromyricetin increased mRNA and protein expressions of ATP-binding cassette transporter A1 (ABCA1) and ABCG1 but had no effect on the mRNA and protein expressions of SR-A, CD36, or SR-BI involved in cholesterol homeostasis. Furthermore, the upregulation of ABCA1 and ABCG1 by dihydromyricetin depended on liver X receptor α (LXRα), as evidenced by an increase in the nuclear level of LXRα and its prevention of the expression of ABCA1 and ABCG1 after inhibition of LXRα activity by knockdown of LXRα expression with small interfering RNA (siRNA). Accordingly, dihydromyricetin-mediated suppression of cholesterol and lipid accumulation in macrophages was also abrogated by LXRα siRNA. Moreover, the lesion size of atherosclerosis was smaller in dihydromyricetin-treated ApoE^-/- mice compared with the vehicle-treated mice, and the protein expression of CD36, SR-A, ABCA1, ABCG1 and LXRα in aortas was modulated similar to that observed in THP-1-derived macrophages. These data suggest that promotion of LXRα-ABCA1/ABCG1-dependent cholesterol efflux is crucial event in suppression of lipid accumulation by dihydromyricetin in the transformation of macrophage foam cells.

Prevention of cholesterol gallstone disease by schaftoside in lithogenic diet-induced C57BL/6 mouse model

Schaftoside (SS) is a bioactive compound present in the Herba Desmodii Styracifolii (DS), a herb that has been used to treat cholelithiasis and urolithiasis in Chinese medicine. Whether SS inhibits cholesterol (Ch) gallstone formation has not been investigated. This study examined the effects of oral intake of SS on Ch gallstone formation in C57BL/6 mice fed a lithogenic diet. The rate of gallstone formation was recorded. Levels of Ch, triglycerides (TG) and bile salts (BS) were measured in the bile and serum. Liver histopathology was examined microscopically, and mRNA expression levels of key genes involved in cholesterol and bile metabolism were determined by qPCR. Mice fed SS were protected against gallstone formation, had increased biliary levels of BS, and reduced biliary Ch levels, resulting in a lower Ch saturation index (CSI). In addition, mice fed SS had lower serum TG and Ch levels, increased mRNA expression of liver X receptor α, ATP binding cassette transporter 5/8 (ABCG5/8), and ileal bile acid binding protein (IBABP) in the ileum, and of farnesoid X receptor and bile salt export protein (BSEP) in the liver and ileum. SS also protected against histologically determined liver damage. Overall, these data indicate that SS protects against Ch gallstone formation in mice, and that the effect is mediated by activation of ileal liver X receptor α and hepatic farnesoid X receptor.

Deficiency of liver-X-receptor-α reduces glucose uptake and worsens post-myocardial infarction remodeling

Liver X receptor α (LXRα) is an endogenous protective receptor against ischemic heart diseases. However, whether LXRα regulated glucose metabolism in ischemic heart diseases has not been investigated. In this study we investigated the involvement of LXRα on glucose metabolism in cardiac remodeling after myocardial infarction (MI). MI was induced in mice by permanent ligation of the left anterior descending coronary artery (LCA). Genetic LXRα deletion significantly worsened cardiac remodeling and impaired cardiac function at 4 weeks after MI. Cardiac 18F-fluorodeoxyglucose (FDG) uptake by positron emission tomography (PET) demonstrated that the FDG standardized uptake value (SUV) was significantly lower in LXRα^-/- mice as compared to WT mice. Mechanistically, GLUT1/4 and AMPK phosphorylation were significantly downregulated while CD36 expression was markedly upregulated in LXRα^-/- mice. This study demonstrated that deficiency of LXRα decreased glucose uptake after MI, resulting in a metabolic shift that suppressed glucose metabolism, which was in association with adverse cardiac remodeling.

Liver X receptor α (LXRα) promoted invasion and EMT of gastric cancer cells by regulation of NF-κB activity

Aberrant expression of Liver X receptor α (LXRα) has been frequently reported in various types of cancers excluding gastric cancer (GC). Moreover, the role of LXRα in human GC has not been previously reported. In this study, we investigated the effect of LXRα down-regulation on invasion and EMT of GC. The expression of LXRα in GC cell lines was detected by real-time PCR. The LXRα siRNA was transiently transfected into GC cells using Lipofectamine™ 2000 reagent. Subsequently, cell invasive ability was evaluated by Transwell assays. Western blot and real-time PCR were used to determined the expressions of matrix metalloproteinase-2 and -9 (MMP-2 and -9), E-cadherin, N-cadherin, Vimentin, Snail, Slug, and Twist in GC cells. In addition, the effect of LXRα down-regulation on the phosphoinositide 3-kinase (PI3K)/Akt/nuclear factor (NF)-κB signaling pathway was explored by Western blot. From our results, we found that the expression of LXRα was significantly increased in GC tissues and cell lines. Knockdown of LXRα suppressed the invasive ability of GC cells. The levels of MMP-2 and -9 were dramatically decreased by down-regulating LXRα. In addition, we found a decrease of N-cadherin, Twist, and Slug expressions and an increase of E-cadherin expression, but no influence on the expression levels of Vimentin and Snail. We also found that LXRα down-regulation might suppress the phosphorylation of Akt, NF-κB, and IκB. Collectively, our results indicated that down-regulation of LXRα was shown to suppress invasion and EMT of GC cells by decreasing the expressions of related proteins through inhibiting the PI3K/Akt/NF-κB signaling pathway.

Tumor suppressor Menin acts as a corepressor of LXRα to inhibit hepatic lipogenesis

Menin, encoded by the MEN1 gene, was initially identified as a tumor suppressor for endocrine neoplasia. Our previous report showed that Menin enhances PPARα transactivity preventing triglyceride accumulation in the liver. Here, we further explore the role of Menin in liver steatosis. Transient transfection assays demonstrate that Menin inhibits the transcriptional activity of nuclear receptor liver X receptor α (LXRα). Accordingly, Menin overexpression results in reduced expression of LXRα target genes, such as lipogenic enzymes including SREBP-1c, FASN and SCD-1. Co-immunoprecipitation assays revealed physical interaction between Menin and LXRα. Collectively, our data suggest that Menin acts as a novel corepressor of LXRα and functions as a negative regulator of hepatic lipogenesis.

Hepatic PPARγ and LXRα independently regulate lipid accumulation in the livers of genetically obese mice

The nuclear hormone receptors liver X receptor α (LXRα) and peroxisome proliferator-activated receptor γ (PPARγ) play key roles in the development of fatty liver. To determine the link between hepatic PPARγ and LXRα signaling and the development of fatty liver, a LXRα-specific ligand, T0901317, was administered to normal OB/OB and genetically obese (ob/ob) mice lacking hepatic PPARγ (Pparγ(ΔH)). In ob/ob-Pparγ(ΔH) and OB/OB-Pparγ(ΔH) mice, as well as ob/ob-Pparγ(WT) and OB/OB-Pparγ(WT) mice, the liver weights and hepatic triglyceride levels were markedly increased in response to T0901317 treatment. These results suggest that hepatic PPARγ and LXRα signals independently contribute to the development of fatty liver.

The effect of LXRα, ChREBP and Elovl6 in liver and white adipose tissue on medium- and long-chain fatty acid diet-induced insulin resistance

AIMS

We aimed to investigate the effects of LXRα, ChREBP and Elovl6 in the development of insulin resistance-induced by medium- and long-chain fatty acids.

METHODS

Sprague Dawley rats were fed a standard chow diet (Control group) or a high-fat, high sucrose diet with different fat sources (coconut oil, lard, sunflower and fish oil) for 8 weeks. These oils were rich in medium-chain saturated fatty acids (MCFA group), long-chain saturated fatty acids (LCFA group), n-6 and n-3 long-chain polyunsaturated fatty acids (n-6 PUFA and n-3 PUFA groups), respectively, which had different chain lengths and degrees of unsaturation. Hyperinsulinemic-euglycemic clamp with [6-(3)H] glucose infusion was performed in conscious rats to assess hepatic insulin sensitivity.

RESULTS

LCFA and n-6 PUFA groups induced hepatic insulin resistance and increased liver X receptor α (LXRα), carbohydrate response element binding protein (ChREBP) and long-chain fatty acid elongase 6 (Elovl6) expression in liver and white adipose tissue (WAT). Furthermore, LCFA and n-6 PUFA groups suppressed Akt serine 473 phosphorylation in liver and WAT. By contrast, in liver and WAT, MCFA and n-3 PUFA groups decreased LXRα, ChREBP and Elovl6 expression and improved insulin signaling and insulin resistance, but Akt serine 473 phosphorylation was not restored by MCFA group in WAT.

CONCLUSIONS

This study demonstrated that the mechanism of the different effects of medium- and long-chain fatty acids on hepatic insulin resistance involves LXRα, ChREBP and Elovl6 alternations in liver and WAT. It points to a new strategy for ameliorating insulin resistance and diabetes through intervention on Elovl6 or its control genes.

Regulation of Zhenqing Recipe on expression of hepatic LXRα in type 2 diabetic rats complicated with non-alcoholic fatty liver disease

AIM: To investigate prevention and functional mechanism of Zhengqing Recipe (ZQR) in the models of type 2 diabetic rats complicated with non-alcoholic fatty liver disease.

METHODS: The model of type 2 diabetic rats complicated with non-alcoholic fatty liver was established by feeding high-glucose and high-fat diet, and injection of low dose streptozotocin. The model rats were randomly divided into four groups: model group, ZQR group and fructus ligustri lucidi group (n = 8), and normal control group (n = 10). Intragastric administration lasted for 8 wk. On 4 wk, 8 wk and 16 wk, the levels of fasting blood glucose (FBG), fasting serum insulin (FINs), serum triglyceride (TG) and serum total cholesterol (TC) in each group were tested and the level of insulin sensitivity index (ISI) was calculated. On 16 wk, serum alanine aminotransferase (ALT), index of liver and liver TG content in each group were examined as well. Meanwhile, pathological changes of liver, the expression of liver X receptor α (LXRα) mRNA, sterol regulatory element-binding protein-1c (SREBP-1c) mRNA and LXRα protein of liver tissues in each group were detected.

RESULTS: After treatment for 8 wk, the levels of FBG, serum TG and TC, index of liver and liver TG content were significantly higher (all P < 0.01), ISI was significantly lower (P < 0.01), liver cirrhosis was significantly exacerbated, and the expressions of LXRα mRNA,SREBP-1c mRNA and LXRα protein were significantly increased (all P < 0.01) in model group compared with control group. Compared with model rats, the levels of FBG, serum TG, index of liver and liver TG content were significantly lower (10.94 ± 3.33 mmol/L vs 16.67 ± 4.33 mmol/L; 0.79 ± 0.27 mmol/L vs 1.33 ± 0.33 mmol/L; 5.72 ± 0.81 vs 7.61 ± 1.24; 0.041 ± 0.0110 mmol/g vs 0.059 ± 0.0160 mmol/g, all P < 0.01). Liver cirrhosis was significantly improved, and the expressions of LXRα mRNA,SREBP-1c mRNA and LXRα protein were significantly decreased (0.75 ± 0.11 vs 1.23 ± 0.17, 0.68 ± 0.16 vs 1.07 ± 0.14, 0.220 ± 0.071 vs 0.334 ± 0.037, all P < 0.01) in ZQR group.

CONCLUSION: ZQR could possess favorable efficacy on non-alcoholic fatty liver in the model of type 2 diabetic rats and the mechanism may be related to the down-regulated expression of LXRα in non-alcoholic fatty liver.