Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles

Effects of cold stress on juvenile Piaractus mesopotamicus and the mitigation by β-carotene

This study investigated the effects of cold stress on morphometrical and hematological biomarkers, energy metabolism, and oxidative stress in different tissues of P. mesopotamicus, and the protective role of β-carotene. Fish were fed with a control diet (CD) and the same diet supplemented with 105 mg/kg β-carotene (BD) for 60 days. After the feeding trial, fish fed CD or BD diets were exposed to control (24 °C) and low temperature (14 °C) for 24 h. Fish (CD and BD) exposed to thermal stress showed lower hepatosomatic index. The hemoglobin increased only in CD-fed fish exposed to 14 °C. Increased glycemia, plasmatic protein depletion, and decreased hepatic glycogen were observed in fish fed the CD, while only the lipid levels in liver were augmented in BD-fed fish exposed at 14 °C. Regarding the oxidative stress, increased antioxidant enzymes activity and lipid peroxidation were observed in CD-fed fish exposed to cold. The two-way ANOVA showed an interaction between dietary treatment and temperature for glucose and oxidative stress biomarkers, with the highest values recorded in 14 °C-exposed fish fed with the CD. Our study demonstrated that cold stress had the greatest impact on fish oxidative status, and β-carotene reduces harmful effects induced by cold in P. mesopotamicus.

LXR-inverse agonism stimulates immune-mediated tumor destruction by enhancing CD8 T-cell activity in triple negative breast cancer

Triple-negative breast cancer (TNBC) is a highly aggressive subtype that is untreatable with hormonal or HER2-targeted therapies and is also typically unresponsive to checkpoint-blockade immunotherapy. Within the tumor microenvironment dysregulated immune cell metabolism has emerged as a key mechanism of tumor immune-evasion. We have discovered that the Liver-X-Receptors (LXRα and LXRβ), nuclear receptors known to regulate lipid metabolism and tumor-immune interaction, are highly activated in TNBC tumor associated myeloid cells. We therefore theorized that inhibiting LXR would induce immune-mediated TNBC-tumor clearance. Here we show that pharmacological inhibition of LXR activity induces tumor destruction primarily through stimulation of CD8+ T-cell cytotoxic activity and mitochondrial metabolism. Our results imply that LXR inverse agonists may be a promising new class of TNBC immunotherapies.

Stigmasterol stimulates transintestinal cholesterol excretion independent of liver X receptor activation in the small intestine

Despite advances in healthcare, cardiovascular disease (CVD) remains the leading cause of death in the United States. Elevated levels of plasma cholesterol are highly predictive of CVD and stroke and are the principal driver of atherosclerosis. Unfortunately, current cholesterol lowering agents, such as statins, are not known to reverse atherosclerotic disease once it has been established. In preclinical models, agonists of nuclear receptor, LXR, have been shown to reduce and reverse atherosclerosis. Phytosterols are bioactive non-cholesterol sterols that act as LXR agonists and regulate cholesterol metabolism and transport. We hypothesized that stigmasterol would act as an LXR agonist and alter intestinal cholesterol secretion to promote cholesterol elimination. Mice were fed a control diet, or a diet supplemented with stigmasterol (0.3% w/w) or T0901317 (0.015% w/w), a known LXR agonist. In this experiment we analyzed the sterol content of bile, intestinal perfusate, plasma, and feces. Additionally, the liver and small intestine were analyzed for relative levels of transcripts known to be regulated by LXR. We observed that T0901317 robustly promoted cholesterol elimination and acted as a strong LXR agonist. Stigmasterol promoted transintestinal cholesterol secretion through an LXR-independent pathway.

Programmed increases in LXRα induced by paternal alcohol use enhance offspring metabolic adaptation to high-fat diet induced obesity

Objectives

Paternally inherited alterations in epigenetic programming are emerging as relevant factors in numerous disease states, including the growth and metabolic defects observed in fetal alcohol spectrum disorders. In rodents, chronic paternal alcohol use induces fetal growth restriction, as well as sex-specific alterations in insulin signaling and lipid homeostasis in the offspring. Based on previous studies, we hypothesized that the observed metabolic irregularities are the consequence of paternally inherited alterations liver x receptor (LXR) activity.

Methods

Male offspring of alcohol-exposed sires were challenged with a high-fat diet and the molecular pathways controlling glucose and lipid homeostasis assayed for LXR-induced alterations.

Results

Similar to findings in studies employing LXR agonists we found that the male offspring of alcohol-exposed sires display resistance to diet-induced obesity and improved glucose homeostasis when challenged with a high-fat diet. This improved metabolic adaptation is mediated by LXRα trans-repression of inflammatory cytokines, releasing IKKβ inhibition of the insulin signaling pathway. Interestingly, paternally programmed increases in LXRα expression are liver-specific and do not manifest in the pancreas or visceral fat.

Conclusions

These studies identify LXRα as a key mediator of the long-term metabolic alterations induced by preconception paternal alcohol use.

•

Chronic paternal alcohol use induces up-regulation of LXRα in the male offspring.

•

Male offspring of alcohol-exposed fathers are protected from diet-induced obesity.

•

Paternally-inherited up-regulation of LXRα only manifests in the liver.

•

Improved metabolic adaptation is linked to LXRα suppression of cytokine production.

•

Male offspring exhibit the same phenotypes observed in studies of LXR agonists.

Antitumor effects of curcumin: A lipid perspective

Lipid metabolism plays an important role in cancer development due to the necessities of rapidly dividing cells to increase structural, energetic, and biosynthetic demands for cell proliferation. Basically, obesity, type 2 diabetes, and other related diseases, and cancer are associated with a common hyperactivated "lipogenic state." Recent evidence suggests that metabolic reprogramming and overproduction of enzymes involved in the synthesis of fatty acids are the new hallmarks of cancer, which occur in an early phase of tumorigenesis. As the first evidence to confirm dysregulated lipid metabolism in cancer cells, the overexpression of fatty acid synthase (FAS) was observed in breast cancer patients and demonstrated the role of FAS in cancer. Other enzymes of fatty acid synthesis have recently been found to be dysregulated in cancer, including ATP-dependent citrate lyase and acetyl-CoA carboxylase, which further underscores the connection of these metabolic pathways with cancer cell survival and proliferation. The degree of overexpression of lipogenic enzymes and elevated lipid utilization in tumors is closely associated with cancer progression. The question that arises is whether the progression of cancer can be suppressed, or at least decelerated, by modulating gene expression related to fatty acid metabolism. Curcumin, due to its effects on the regulation of lipogenic enzymes, might be able to suppress, or even cause regression of tumor growth. This review discusses recent evidence concerning the important role of lipogenic enzymes in the metabolism of cancer cells and whether the inhibitory effects of curcumin on lipogenic enzymes is therapeutically efficacious.

Therapeutic targeting of nuclear receptors, liver X and retinoid X receptors, for Alzheimer's disease

After 15 years of research into Alzheimer's disease (AD) therapeutics, including billions of US dollars provided by federal agencies, pharmaceutical companies, and private foundations, there are still no meaningful therapies that can delay the onset or slow the progression of AD. An understanding of the proteolytic processing of amyloid precursor protein (APP) and the hypothesis that pathogenic mechanisms in familial and sporadic forms of AD are very similar led to the assumption that pharmacological inhibition of secretases or immunological approaches to clear amyloid depositions in the brain would have been the core to drug discovery strategies and successful therapies. However, there are other understudied approaches including targeting genes, gene networks, and metabolic pathways outside the proteolytic processing of APP. The advancement of newly developed sequencing technologies and mass spectrometry, as well as the availability of animal models expressing human apolipoprotein E isoforms, has been critical in rationalizing additional AD therapeutics. The purpose of this review is to present one of those approaches, based on the role of ligand-activated nuclear liver X and retinoid X receptors in the brain. This therapeutic approach was initially proposed utilizing in vitro models 15 years ago and has since been examined in numerous studies using AD-like mouse models. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

The hepatokine Tsukushi is released in response to NAFLD and impacts cholesterol homeostasis

Nonalcoholic fatty liver disease (NAFLD) prevails in obesity and is linked to several health complications including dyslipidemia and atherosclerosis. How exactly NAFLD induces atherogenic dyslipidemia to promote cardiovascular diseases is still elusive. Here, we identify Tsukushi (TSK) as a hepatokine induced in response to NAFLD. We show that both endoplasmic reticulum stress and inflammation promote the expression and release of TSK in mice. In humans, hepatic TSK expression is also associated with steatosis, and its circulating levels are markedly increased in patients suffering from acetaminophen-induced acute liver failure (ALF), a condition linked to severe hepatic inflammation. In these patients, elevated blood TSK levels were associated with decreased transplant-free survival at hospital discharge, suggesting that TSK could have a prognostic significance. Gain- and loss-of-function studies in mice revealed that TSK impacts systemic cholesterol homeostasis. TSK reduces circulating HDL cholesterol, lowers cholesterol efflux capacity, and decreases cholesterol-to-bile acid conversion in the liver. Our data identify the hepatokine TSK as a blood biomarker of liver stress that could link NAFLD to the development of atherogenic dyslipidemia and atherosclerosis.

Revisiting the Role of LXRs in PUFA Metabolism and Phospholipid Homeostasis

Liver X receptors (LXRs) play a pivotal role in fatty acid (FA) metabolism. So far, the lipogenic consequences of in vivo LXR activation, as characterized by a major hepatic steatosis, has constituted a limitation to the clinical development of pharmacological LXR agonists. However, recent studies provided a different perspective. Beyond the quantitative accumulation of FA, it appears that LXRs induce qualitative changes in the FA profile and in the distribution of FAs among cellular lipid species. Thus, LXRs activate the production of polyunsaturated fatty acids (PUFAs) and their distribution into phospholipids via the control of FA desaturases, FA elongases, lysophosphatidylcholine acyltransferase (LPCAT3), and phospholipid transfer protein (PLTP). Therefore, LXRs control, in a dynamic manner, the PUFA composition and the physicochemical properties of cell membranes as well as the release of PUFA-derived lipid mediators. Recent studies suggest that modulation of PUFA and phospholipid metabolism by LXRs are involved in the control of lipogenesis and lipoprotein secretion by the liver. In myeloid cells, the interplay between LXR and PUFA metabolism affects the inflammatory response. Revisiting the complex role of LXRs in FA metabolism may open new opportunities for the development of LXR modulators in the field of cardiometabolic diseases.

Identification of the fructose transporter GLUT5 (SLC2A5) as a novel target of nuclear receptor LXR

Fructose has become a major constituent of our modern diet and is implicated as an underlying cause in the development of metabolic diseases. The fructose transporter GLUT5 (SLC2A5) is required for intestinal fructose absorption. GLUT5 expression is induced in the intestine and skeletal muscle of type 2 diabetes (T2D) patients and in certain cancers that are dependent on fructose metabolism, indicating that modulation of GLUT5 levels could have potential in the treatment of these diseases. Using an unbiased screen for transcriptional control of the human GLUT5 promoter we identified a strong and specific regulation by liver X receptor α (LXRα, NR1H3). Using promoter truncations and site-directed mutagenesis we identified a functional LXR response element (LXRE) in the human GLUT5 promoter, located at −385 bp relative to the transcriptional start site (TSS). Finally, mice treated with LXR agonist T0901317 showed an increase in Glut5 mRNA and protein levels in duodenum and adipose tissue, underscoring the in vivo relevance of its regulation by LXR. Together, our findings show that LXRα regulates GLUT5 in mice and humans. As a ligand-activated transcription factor, LXRα might provide novel pharmacologic strategies for the selective modulation of GLUT5 activity in the treatment of metabolic disease as well as cancer.

Peptide Amphiphile Supramolecular Nanostructures as a Targeted Therapy for Atherosclerosis

The rising prevalence of cardiovascular disease worldwide necessitates novel therapeutic approaches to manage atherosclerosis. Intravenously administered nanostructures are a promising noninvasive approach to deliver therapeutics that reduce plaque burden. The drug liver X receptor agonist GW3965 (LXR) can reduce atherosclerosis by promoting cholesterol efflux from plaque but causes liver toxicity when administered systemically at effective doses, thus preventing its clinical use. The ability of peptide amphiphile nanofibers containing apolipoprotein A1-derived targeting peptide 4F to serve as nanocarriers for LXR delivery (ApoA1-LXR PA) in vivo is investigated here. These nanostructures are found to successfully target atherosclerotic lesions in a mouse model within 24 h of injection. After 8 weeks of intravenous administration, the nanostructures significantly reduce plaque burden in both male and female mice to a similar extent as LXR alone in comparison to saline-treated controls. Furthermore, they do not cause increased liver toxicity in comparison to LXR treatments, which may be related to more controlled release by the nanostructure. These findings demonstrate the potential of supramolecular nanostructures as safe, effective drug nanocarriers to manage atherosclerosis.

Role of Myeloid-Epithelial-Reproductive Tyrosine Kinase and Macrophage Polarization in the Progression of Atherosclerotic Lesions Associated With Nonalcoholic Fatty Liver Disease

Recent lines of evidence highlight the involvement of myeloid-epithelial-reproductive tyrosine kinase (MerTK) in metabolic disease associated with liver damage. MerTK is mainly expressed in anti-inflammatory M2 macrophages where it mediates transcriptional changes including suppression of proinflammatory cytokines and enhancement of inflammatory repressors. MerTK is regulated by metabolic pathways through nuclear sensors including LXRs, PPARs, and RXRs, in response to apoptotic bodies or to other sources of cholesterol. Nonalcoholic fatty liver disease (NAFLD) is one of the most serious public health problems worldwide. It is a clinicopathological syndrome closely related to obesity, insulin resistance, and oxidative stress. It includes a spectrum of conditions ranging from simple steatosis, characterized by hepatic fat accumulation with or without inflammation, to nonalcoholic steatohepatitis (NASH), defined by hepatic fat deposition with hepatocellular damage, inflammation, and accumulating fibrosis. Several studies support an association between NAFLD and the incidence of cardiovascular diseases including atherosclerosis, a major cause of death worldwide. This pathological condition consists in a chronic and progressive inflammatory process in the intimal layer of large- and medium-sized arteries. The complications of advanced atherosclerosis include chronic or acute ischemic damage in the tissue perfused by the affected artery, leading to cellular death. By identifying specific targets influencing lipid metabolism and cardiovascular-related diseases, the present review highlights the role of MerTK in NAFLD-associated atherosclerotic lesions as a potential innovative therapeutic target. Therapeutic advantages might derive from the use of compounds selective for nuclear receptors targeting PPARs rather than LXRs regulating macrophage lipid metabolism and macrophage mediated inflammation, by favoring the expression of MerTK, which mediates an immunoregulatory action with a reduction in inflammation and in atherosclerosis.

LXRα-mediated downregulation of EGFR suppress colorectal cancer cell proliferation

Liver X receptors (LXRs) are members of the nuclear receptor family, including the LXRα (NR1H3) and LXRβ (NR1H2) subtypes, which are related to the metabolism of glucose and cholesterol and possess anti-inflammatory functions. Mounting evidence has linked LXRs to the inhibition of cell proliferation in a variety of cancers. We revealed a differential distribution for NR1H3, but not for NR1H2, in colorectal cancer and adjacent normal tissues. We found that NR1H3 enhanced the inhibitory action of GW3965, an agonist of LXRs, on the proliferation of colorectal cancer cells. Upregulation of NR1H3 enhanced the inhibition of cell proliferation by GW3965 while silencing of NR1H3 attenuated the inhibitory effect of GW3965 on cell proliferation. Bioinformatic prediction and luciferase assays showed that NR1H3 was able to inhibit the activity of the epidermal growth factor receptor (EGFR) promoter. Moreover, we demonstrated that activation of NR1H3 inhibited the growth of transplanted tumors in an animal experiment, with the inhibition accompanied by downregulation of EGFR. Our findings suggest that NR1H3 controls cell proliferation by affecting EGFR promoter activity. The high expression of EGFR was due to the downregulation of NR1H3 which is a novel molecular mechanism in the development of colorectal cancer.

Salvianolic acids improve liver lipid metabolism in ovariectomized rats via blocking STAT-3/SREBP1 signaling

Postmenopausal women, who have reduced circulating estrogen levels, are more prone to develop obesity and related metabolic diseases than premenopausal women. The absence of safe and effective treatments for postmenopausal obesity has changed the focus to natural products as alternative remedies. Total salvianolic acids (TSA) are the major water-soluble ingredients of Danshen. Salvianolic acid (SA) is the major constituent of the TSA. Salvianolic acids, including TSA and SA, are widely used in traditional Chinese medicine. In the present study, ovariectomized rats and LO2 cells were used to study the effects of salvianolic acids on body weight gain and hepatic steatosis. Salvianolic acids reduced ovariectomy (OVX)-induced body weight gain, attenuated the expressions of hepatic lipogenic genes, such as sterol regulatory element binding protein (SREBP)1, fatty acid synthase (FAS), and stearoyl-CoA desaturase (SCD)1, and decreased the liver triglyceride (TG) and total cholesterol (TC). For the molecular mechanisms, OVX and high glucose-induced phosphorylation of signal transducer and activator of transcription (STAT)-3 was inhibited by salvianolic acids treatment. In LO2 cells, inhibition of STAT-3 by siRNA attenuated the increased expression of SREBP1 and TG induced by high glucose. Salvianolic acids reduced the upregulation of SREBP1 and TG induced by high glucose in LO2 cells. In conclusion, these findings illustrated that salvianolic acids markedly alleviated the lipid metabolism disorders and protected against the postmenopausal obesity. The underlying mechanism was probably associated with the regulation of STAT-3 signaling.

Dietary Sargassum fusiforme improves memory and reduces amyloid plaque load in an Alzheimer's disease mouse model

Activation of liver X receptors (LXRs) by synthetic agonists was found to improve cognition in Alzheimer's disease (AD) mice. However, these LXR agonists induce hypertriglyceridemia and hepatic steatosis, hampering their use in the clinic. We hypothesized that phytosterols as LXR agonists enhance cognition in AD without affecting plasma and hepatic triglycerides. Phytosterols previously reported to activate LXRs were tested in a luciferase-based LXR reporter assay. Using this assay, we found that phytosterols commonly present in a Western type diet in physiological concentrations do not activate LXRs. However, a lipid extract of the 24(S)-Saringosterol-containing seaweed Sargassum fusiforme did potently activate LXRβ. Dietary supplementation of crude Sargassum fusiforme or a Sargassum fusiforme-derived lipid extract to AD mice significantly improved short-term memory and reduced hippocampal Aβ plaque load by 81%. Notably, none of the side effects typically induced by full synthetic LXR agonists were observed. In contrast, administration of the synthetic LXRα activator, AZ876, did not improve cognition and resulted in the accumulation of lipid droplets in the liver. Administration of Sargassum fusiforme-derived 24(S)-Saringosterol to cultured neurons reduced the secretion of Aβ₄₂. Moreover, conditioned medium from 24(S)-Saringosterol-treated astrocytes added to microglia increased phagocytosis of Aβ. Our data show that Sargassum fusiforme improves cognition and alleviates AD pathology. This may be explained at least partly by 24(S)-Saringosterol-mediated LXRβ activation.

Inhibition of PRMT3 activity reduces hepatic steatosis without altering atherosclerosis susceptibility in apoE knockout mice

The nuclear receptor liver X receptor (LXR) impacts on cholesterol metabolism as well as hepatic lipogenesis via transcriptional regulation. It is proposed that inhibition of the protein arginine methyltransferase 3 (PRMT3) uncouples these two transcriptional pathways in vivo by acting as a specific lipogenic coactivator of LXR. Here we validated the hypothesis that treatment with the allosteric PRMT3 inhibitor SGC707 will diminish the hepatic steatosis extent, while leaving global cholesterol metabolism, important in cholesterol-driven pathologies like atherosclerosis, untouched. For this purpose, 12-week old hyperlipidemic apolipoprotein E knockout mice were fed a Western-type diet for six weeks to induce both hepatic steatosis and atherosclerosis. The mice received 3 intraperitoneal injections with SGC707 or solvent control per week. Mice chronically treated with SGC707 developed less severe hepatic steatosis as exemplified by the 51% reduced (P < 0.05) liver triglyceride levels. In contrast, the extent of in vivo macrophage foam cell formation and aortic root atherosclerosis was not affected by SGC707 treatment. Interestingly, SGC707-treated mice gained 94% less body weight (P < 0.05), which was paralleled by changes in white adipose tissue morphology, i.e. reduction in adipocyte size and browning. In conclusion, we have shown that through PRMT3 inhibitor treatment specific functions of LXR involved in respectively the development of fatty liver disease and atherosclerosis can be uncoupled, resulting in an overall diminished hepatic steatosis extent without a negative impact on atherosclerosis susceptibility. As such, our studies highlight that PRMT3 inhibition may constitute a novel therapeutic approach to limit the development of fatty liver disease in humans.

SIRT1 overexpression attenuates offspring metabolic and liver disorders as a result of maternal high-fat feeding

KEY POINTS

Maternal high-fat diet (MHF) consumption led to metabolic and liver disorders in male offspring, which are associated with reduced sirtuin (SIRT)1 expression and activity in the offspring liver SIRT1 overexpression in MHF offspring reduced their body weight and adiposity and normalized lipid metabolic markers in epididymal and retroperitoneal adipose tissues SIRT1 overexpression in MHF offspring improved glucose tolerance, as well as systemic and hepatic insulin sensitivity SIRT1 overexpression ameliorated MHF-induced lipogenesis, oxidative stress and fibrogenesis in the liver of offspring.

ABSTRACT

Maternal obesity can increase the risk of metabolic disorders in the offspring. However, the underlying mechanism responsible for this is not clearly understood. Previous evidence implied that sirtuin (SIRT)1, a potent regulator of energy metabolism and stress responses, may play an important role. In the present study, we have shown, in C57BL/6 mice, that maternal high-fat diet (HFD) consumption can induce a pre-diabetic and non-alcoholic fatty liver disease phenotype in the offspring, associated with reduced SIRT1 expression in the hypothalamus, white adipose tissues (WAT) and liver. Importantly, the overexpression of SIRT1 in these offspring significantly attenuated the excessive accumulation of epididymal (Epi) white adipose tissue (WAT) and retroperitoneal (Rp)WAT (P < 0.001), glucose intolerance and insulin resistance (both P < 0.05) at weaning age. These changes were associated with the suppression of peroxisome proliferator-activated receptor gamma (PPAR)γ (P < 0.01), PPARγ-coactivator 1-alpha (P < 0.05) and sterol regulatory element-binding protein-1c in EpiWAT (P < 0.01), whereas there was increased expression of PPARγ in RpWAT (P < 0.05). In the liver, PPARγ mRNA expression, as well as Akt protein expression and activity, were increased (P < 0.05), whereas fatty acid synthase and carbohydrate response element binding protein were downregulated (P < 0.05), supporting increased insulin sensitivity and reduced lipogenesis in the liver. In addition, hepatic expression of endogenous anti-oxidants, including glutathione peroxidase 1 and catalase, was increased (P < 0.01 and P < 0.05 respectively), whereas collagen and fibronectin deposition was suppressed (P < 0.01). Collectively, the present study provides direct evidence of the mechanistic significance of SIRT1 in maternal HFD-induced metabolic dysfunction in offspring and suggests that SIRT1 is a promising target for fetal reprogramming.

Mitochondrial dysfunction in metabolism and ageing: shared mechanisms and outcomes?

Mitochondria are key in the metabolism of aerobic organisms and in ageing progression and age-related diseases. Mitochondria are essential for obtaining ATP from glucose and fatty acids but also in many other essential functions in cells including aminoacids metabolism, pyridine synthesis, phospholipid modifications and calcium regulation. On the other hand, the activity of mitochondria is also the principal source of reactive oxygen species in cells. Ageing and chronic age-related diseases are associated with the deregulation of cell metabolism and dysfunction of mitochondria. Cell metabolism is controlled by three major nutritional sensors: mTOR, AMPK and Sirtuins. These factors control mitochondrial biogenesis and dynamics by regulating fusion, fission and turnover through mito- and autophagy. A complex interaction between the activity of these nutritional sensors, mitochondrial biogenesis rate and dynamics exists and affect ageing, age-related diseases including metabolic disease. Further, mitochondria maintain a constant communication with nucleus modulating gene expression and modifying epigenetics. In this review we highlight the importance of mitochondria in ageing and the repercussion in the progression of age-related diseases and metabolic disease.

Multiple effects of cold exposure on livers of male mice

Cold exposure of mice is a common method to stimulate brown adipose tissue (BAT) activity and induce browning of white adipose tissue (WAT) that has beneficial effects on whole-body lipid metabolism, including reduced plasma triglyceride (TG) concentrations. The liver is a key regulatory organ in lipid metabolism as it can take up as well as oxidize fatty acids. The liver can also synthesize, store and secrete TGs in VLDL particles. The effects of cold exposure on murine hepatic lipid metabolism have not been addressed. Here, we report the effects of 24-h exposure to 4°C on parameters of hepatic lipid metabolism of male C57BL/6J mice. Cold exposure increased hepatic TG concentrations by 2-fold (P < 0.05) but reduced hepatic lipogenic gene expression. Hepatic expression of genes encoding proteins involved in cholesterol synthesis and uptake such as the LDL receptor (LDLR) was significantly increased upon cold exposure. Hepatic expression of Cyp7a1 encoding the rate-limiting enzyme in the classical bile acid (BA) synthesis pathway was increased by 4.3-fold (P < 0.05). Hepatic BA concentrations and fecal BA excretion were increased by 2.8- and 1.3-fold, respectively (P < 0.05 for both). VLDL-TG secretion was reduced by approximately 50% after 24 h of cold exposure (P < 0.05). In conclusion, cold exposure has various, likely intertwined effects on the liver that should be taken into account when studying the effects of cold exposure on whole-body metabolism.

Inhibition of protein arginine methyltransferase 3 activity selectively impairs liver X receptor-driven transcription of hepatic lipogenic genes in vivo

BACKGROUND AND PURPOSE

Agonists for the liver X receptor (LXR) are considered promising therapeutic moieties in cholesterol-driven diseases by promoting cellular cholesterol efflux pathways. However, current clinical application of these agents is hampered by concomitant LXR-induced activation of a lipogenic transcriptional network, leading to hepatic steatosis. Recent studies have suggested that protein arginine methyltransferase 3 (PRMT3) may act as a selective co-activator of LXR activity. Here, we verified the hypothesis that PRMT3 inhibition selectively disrupts the ability of LXR to stimulate lipogenesis while maintaining its capacity to modulate macrophage cholesterol homeostasis.

EXPERIMENTAL APPROACH

A combination of the LXR agonist T0901317 and palm oil was administered to C57BL/6 mice to maximally stimulate LXR and PRMT3 activity. PRMT3 activity was inhibited using the allosteric inhibitor SGC707.

KEY RESULTS

Treatment with SGC707 did not negatively influence the T0901317/palm oil-induced up-regulation of the cholesterol efflux ATP-binding cassette transporter genes, ABCA1 and ABCG1, in peritoneal cells. In contrast, SGC707 treatment was associated with a significant decrease in the hepatic expression of the lipogenic gene fatty acid synthase (-64%). A similar trend was observed for stearoyl-coenzyme A desaturase and acetyl CoA carboxylase expression (-43%; -56%). This obstruction of lipogenic gene transcription coincided with a significant 2.3-fold decrease in liver triglyceride content as compared with the T0901317 and palm oil-treated control group.

CONCLUSION AND IMPLICATIONS

We showed that inhibition of PRMT3 activity by SGC707 treatment selectively impairs LXR-driven transcription of hepatic lipogenic genes, while the positive effect of LXR stimulation on macrophage cholesterol efflux pathways is maintained.

Inhibition of Hepatotoxicity by a LXR Inverse Agonist in a Model of Alcoholic Liver Disease

Alcohol abuse is a major cause of liver disease and mortality worldwide and is a significant public health issue. Patients with alcoholic liver disease (ALD) have severe hepatic lipid accumulation, inflammation, and fibrosis. Therapies for ALD are very limited and even abstinence from alcohol consumption does not necessarily protect patients from progression of the disease. We sought to evaluate the efficacy of a liver X receptor (LXR) inverse agonist, SR9238, in an animal model of ALD. SR9238 suppresses hepatic lipogenesis, a pathological hallmark of ALD, and we hypothesized that targeting suppression of hepatic metabolic pathways that are activated in ALD may be an effective treatment for the disease. A chronic ethanol diet with or without a final ethanol binge treatment was used to induce ALD in mice. Mice were administered the liver specific LXR inverse agonist SR9238 for 4 weeks after the mice had been maintained on the ethanol diet for 14 days. Mice developed all the hallmarks of advanced ALD demonstrating significant pathophysiology and hepatotoxicity. SR9238 significantly attenuated liver injury and hepatic steatosis and fibrosis was nearly eliminated in SR9238 treated mice. SR9238 treatment reversed the damage associated with chronic ethanol use returning the liver to near normal morphology. These results indicate that inhibiting LXR activity using the inverse agonist has a hepatoprotective effect in rodent models of ALD; thus, this pharmacological approach may be efficacious for treatment of ALD in humans.

Blocking of STAT-3/SREBP1-mediated glucose-lipid metabolism is involved in dietary phytoestrogen-inhibited ovariectomized-induced body weight gain in rats

Postmenopausal women have a decline in circulating estrogen levels and are more prone to obesity and its related metabolic diseases than premenopausal women are. The absence of safe and effective conventional treatments for postmenopausal obesity has changed the focus to natural products as alternative remedies. Here, ovariectomized rats and LO2 cells were used to study the molecular basis of the effect of dietary phytoestrogens on body weight gain and hepatic steatosis. Dietary phytoestrogens can inhibit ovariectomy (OVX)-induced body weight gain, blood glucose concentration, expression of hepatic lipogenic genes, such as sterol regulatory element binding protein (SREBP)1, acetyl-CoA carboxylase (ACC)1, fatty acid synthase (FAS), and stearoyl-CoA desaturase (SCD)1, and decrease liver triglyceride (TG) content, but later estradiol withdrawal increased expression of SREBP1. Histological analysis of liver showed that dietary phytoestrogens improved OVX-induced morphological abnormalities. OVX and high glucose-induced phosphorylation of signal transducer and activator of transcription (STAT)-3 were inhibited by phytoestrogens treatment. In LO2 cells, inhibition of STAT-3 by siRNA attenuated the increased TG content and expression of SREBP1 induced by high glucose. Phytoestrogens reduced the upregulation of SREBP1 and TG induced by high glucose in LO2 cells. In conclusion, these findings illustrated that dietary phytoestrogens markedly alleviated the derangement of lipid metabolism. The underlying mechanism is probably associated with regulating STAT-3/SREBP1 signaling.

Essential Dietary Bioactive Lipids in Neuroinflammatory Diseases

SIGNIFICANCE

Under physiological conditions, neurons and glia are in a healthy, redox-balanced environment; when injury perturbs this equilibrium, a neuroinflammatory state is established by activated microglia that triggers pro-inflammatory responses and alters the oxidant/antioxidant balance, thus leading to neuronal loss and neurodegeneration. In neurodegenerative diseases (such as Alzheimer's disease, Parkinson's disease, amyothrophic lateral sclerosis, and multiple sclerosis), the brain is in a constitutively self-sustaining cycle of inflammation and oxidative stress that prompts and amplifies brain damage. Recent Advances: Recently, an increasing amount of scientific data highlight the ability of specific nutrients to cross the blood-brain barrier, and to modulate inflammatory and oxidative pathways. Therefore, nutritional approaches may contribute to restore the lost equilibrium among factors accounting for neurodegeneration.

CRITICAL ISSUES

Herein, we critically examine how essential lipids (including fatty acids, liposoluble vitamins and phytosterols) might contribute to accelerate or prevent the onset and progression of such pathologies. In particular, we highlight that experimental and clinical findings, although promising, are still inadequate to draw definitive conclusions.

FUTURE DIRECTIONS

More research is warranted in order to establish the real impact of lipid intake on brain health, especially when redox balance and inflammatory responses have been already compromised. In the future, it would be hoped to gain a detailed knowledge of chemical modifications and dynamic properties of such nutrients, before planning to exploit them as potential therapeutics. Antioxid. Redox Signal. 29, 37-60.

Editor's Highlight: Mechanistic Toxicity Tests Based on an Adverse Outcome Pathway Network for Hepatic Steatosis

Risk assessors use liver endpoints in rodent toxicology studies to assess the safety of chemical exposures. Yet, rodent endpoints may not accurately reflect human responses. For this reason and others, human-based invitro models are being developed and anchored to adverse outcome pathways to better predict adverse human health outcomes. Here, a networked adverse outcome pathway-guided selection of biology-based assays for lipid uptake, lipid efflux, fatty acid oxidation, and lipid accumulation were developed. These assays were evaluated in a metabolically competent human hepatocyte cell model (HepaRG) exposed to compounds known to cause steatosis (amiodarone, cyclosporine A, and T0901317) or activate lipid metabolism pathways (troglitazone, Wyeth-14,643, and 22(R)-hydroxycholesterol). All of the chemicals activated at least one assay, however, only T0901317 and cyclosporin A dose-dependently increased lipid accumulation. T0901317 and cyclosporin A increased fatty acid uptake, decreased lipid efflux (inferred from apolipoprotein B100 levels), and increased fatty acid synthase protein levels. Using this biologically-based evaluation of key events regulating hepatic lipid levels, we demonstrated dysregulation of compensatory pathways that normally balance hepatic lipid levels. This approach may provide biological plausibility and data needed to increase confidence in linking invitro-based measurements to chemical effects on adverse human health outcomes.

Transcriptional control of intestinal cholesterol absorption, adipose energy expenditure and lipid handling by Sortilin

The sorting receptor Sortilin functions in the regulation of glucose and lipid metabolism. Dysfunctional lipid uptake, storage, and metabolism contribute to several major human diseases including atherosclerosis and obesity. Sortilin associates with cardiovascular disease; however, the role of Sortilin in adipose tissue and lipid metabolism remains unclear. Here we show that in the low-density lipoprotein receptor-deficient (Ldlr^−/−) atherosclerosis model, Sortilin deficiency (Sort1^−/−) in female mice suppresses Niemann-Pick type C1-Like 1 (Npc1l1) mRNA levels, reduces body and white adipose tissue weight, and improves brown adipose tissue function partially via transcriptional downregulation of Krüppel-like factor 4 and Liver X receptor. Female Ldlr^−/−Sort1^−/− mice on a high-fat/cholesterol diet had elevated plasma Fibroblast growth factor 21 and Adiponectin, an adipokine that when reduced is associated with obesity and cardiovascular disease-related factors. Additionally, Sort1 deficiency suppressed cholesterol absorption in both female mice ex vivo intestinal tissue and human colon Caco-2 cells in a similar manner to treatment with the NPC1L1 inhibitor ezetimibe. Together our findings support a novel role of Sortilin in energy regulation and lipid homeostasis in female mice, which may be a potential therapeutic target for obesity and cardiovascular disease.

Liver X receptor β in the hippocampus: A potential novel target for the treatment of major depressive disorder?

Liver X receptors (LXRs), including LXRα and LXRβ isoforms, have been implicated in multiple physiological functions including promoting neurogenesis, improving synaptic plasticity, preventing neurodegeneration, inhibiting inflammation as well as regulating cholesterol metabolism. However, a potential role of LXRs in the treatment of major depressive disorder (MDD) has never been investigated previously. Our present results demonstrated that levels of hippocampal LXRβ but not LXRα were down-regulated in rats exposed to chronic unpredictable stress (CUS) and were negatively correlated with the severity of CUS-induced depressive-like behaviors. Furthermore, rats with LXRβ knockdown by short hairpin RNA (shRNA) in hippocampus displayed depressive-like behaviors and impaired hippocampal neurogenesis similar to those observed after CUS exposure. Conversely, LXRs activation by GW3965 (GW), a synthetic dual agonist for both LXRα and LXRβ isoforms, could improve depression-like behaviors and reverse the impaired hippocampal neurogenesis in rats exposed to CUS. LXRβ knockdown by shRNA completely abrogated the antidepressant and hippocampal neurogenesis-promoting effects of GW, suggesting that LXRβ isoform mediated the antidepressant and hippocampal neurogenesis-promoting effects of the LXRα/β dual agonist. However, ablation of hippocampal neurogenesis with x-irradiation only partly but not completely abolished the antidepressant effects of GW in the behavioral tests, implying that the antidepressant effects mediated by LXRβ isoform are likely through both neurogenesis-dependent and -independent pathways. Thus, our findings suggest that LXRβ activation may represent a potential novel target for the treatment of MDD and also provide a novel insight into the underlying mechanisms of MDD.

Cell-specific discrimination of desmosterol and desmosterol mimetics confers selective regulation of LXR and SREBP in macrophages

The beneficial effects of LXR-pathway activation have long been appreciated, but clinical application of synthetic LXR ligands has been limited by coactivation of SREBP1c and consequent hypertriglyceridemia. Natural LXR ligands such as desmosterol do not promote hypertriglyceridemia because of coordinate down-regulation of the SREBP pathway. Here we demonstrate that synthetic desmosterol mimetics activate LXR in macrophages both in vitro and in vivo while suppressing SREBP target genes. Unexpectedly, desmosterol and synthetic desmosterol mimetics have almost no effect on LXR activity in hepatocytes in comparison with conventional synthetic LXR ligands. These findings reveal cell-specific differences in LXR responses to natural and synthetic ligands in macrophages and liver cells that provide a conceptually new basis for future drug development.

Activation of liver X receptors (LXRs) with synthetic agonists promotes reverse cholesterol transport and protects against atherosclerosis in mouse models. Most synthetic LXR agonists also cause marked hypertriglyceridemia by inducing the expression of sterol regulatory element-binding protein (SREBP)1c and downstream genes that drive fatty acid biosynthesis. Recent studies demonstrated that desmosterol, an intermediate in the cholesterol biosynthetic pathway that suppresses SREBP processing by binding to SCAP, also binds and activates LXRs and is the most abundant LXR ligand in macrophage foam cells. Here we explore the potential of increasing endogenous desmosterol production or mimicking its activity as a means of inducing LXR activity while simultaneously suppressing SREBP1c-induced hypertriglyceridemia. Unexpectedly, while desmosterol strongly activated LXR target genes and suppressed SREBP pathways in mouse and human macrophages, it had almost no activity in mouse or human hepatocytes in vitro. We further demonstrate that sterol-based selective modulators of LXRs have biochemical and transcriptional properties predicted of desmosterol mimetics and selectively regulate LXR function in macrophages in vitro and in vivo. These studies thereby reveal cell-specific discrimination of endogenous and synthetic regulators of LXRs and SREBPs, providing a molecular basis for dissociation of LXR functions in macrophages from those in the liver that lead to hypertriglyceridemia.

Tangshen Formula Attenuates Diabetic Nephropathy by Promoting ABCA1-Mediated Renal Cholesterol Efflux in db/db Mice

The commonly prescribed Tangshen Formula (TSF) is a traditional Chinese formulation that has been shown to reduce plasma lipid metabolism and proteinuria and improve the estimated glomerular filtration rate (eGFR) in patients with diabetic kidney disease. This study investigated the underlying mechanism whereby TSF regulates renal lipid accumulation and ameliorates diabetic renal injuries in spontaneous diabetic db/db mice and in vitro in sodium palmitate (PA)-stimulated and Abca1-SiRNA-transfected mouse tubular epithelial cells (mTECs). The results revealed that TSF treatment significantly ameliorated the renal injuries by lowering urinary albumin excretion and improving renal tissue injuries in diabetic (db/db) mice. Interestingly, the treatment with TSF also resulted in decreased cholesterol levels in the renal tissues of db/db mice, which was associated with increased expression of the peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), the Liver X receptors (LXR), and ATP-binding cassette subfamily A member 1 (ABCA1), suggesting that TSF might attenuate diabetic kidney injury via a mechanism associated with improving cholesterol efflux in the diabetic kidney. This was investigated in vitro in mTECs, and the results showed that TSF reduced the PA-stimulated cholesterol accumulation in mTECs. Mechanistically, the addition of TSF was capable of reversing PA-induced downregulation of PGC-1α, LXR, and ABCA1 expression and cholesterol accumulation in mTECs, suggesting that TSF might act the protection via the PGC-1α-LXR-ABCA1 pathway to improve the cholesterol efflux in the renal tissues of db/db mice. This was further confirmed by silencing ABCA1 to block the promotive effect of TSF on cholesterol efflux in vitro. In conclusion, TSF might ameliorate diabetic kidney injuries by promoting ABCA1-mediated renal cholesterol efflux.

Ouabagenin is a naturally occurring LXR ligand without causing hepatic steatosis as a side effect

Ouabagenin (OBG) is an aglycone of the cardiotonic steroid ouabain and until now was considered a biologically inactive biosynthetic precursor. Herein, we revealed that OBG functions as a novel class of ligand for the liver X receptor (LXR). Luciferase reporter assays and in silico docking studies suggested that OBG has LXR-selective agonistic activity. In addition, OBG repressed the expression of epithelial sodium channel (ENaC), a LXR target gene, without causing hepatic steatosis, a typical side effect of conventional LXR ligands. This remarkable biological activity can be attributed to a unique mode of action; the LXR agonist activity mainly proceeds through the LXRβ subtype without affecting LXRα, unlike conventional LXR ligands. Thus, OBG is a novel class of LXR ligand that does not cause severe side effects, with potential for use as an antihypertensive diuretic or a tool compound for exploring LXR subtype-specific biological functions.

25-Hydroxycholesterol activates the expression of cholesterol 25-hydroxylase in an LXR-dependent mechanism

Cholesterol 25-hydroxylase (CH25H) catalyzes the production of 25-hydroxycholesterol (25-HC), an oxysterol that can play an important role in different biological processes. However, the mechanisms regulating CH25H expression have not been fully elucidated. In this study, we determined that CH25H is highly expressed in mouse liver and peritoneal macrophages. We identified several liver X receptor (LXR) response elements (LXREs) in the human CH25H promoter. In HepG2 cells, activation of LXR by 25-HC or other oxysterols and synthetic ligands [T0901317 (T317) and GW3965] induced CH25H protein expression, which was associated with increased CH25H mRNA expression. 25-HC or T317 activated CH25H transcription in an LXRE-dependent manner. Thus, high-expressing LXRα or LXRβ activated CH25H expression, and the activation was further enhanced by LXR ligands. In contrast, inhibition of LXRα/β expression attenuated 25-HC or T317-induced CH25H expression. Deficiency of interferon γ expression reduced, but did not block, LXR ligand-induced hepatic CH25H expression. Activation of LXR also substantially induced macrophage CH25H expression. In vivo, administration of GW3965 to mice increased CH25H expression in both liver and peritoneal macrophages. Taken together, our study demonstrates that 25-HC can activate CH25H expression in an LXR-dependent manner, which may be an important mechanism to exert the biological actions of 25-HC.

Current and future therapies for addressing the effects of inflammation on HDL cholesterol metabolism

Cardiovascular disease (CVD) is a major cause of morbidity and mortality worldwide. Inflammatory processes arising from metabolic abnormalities are known to precipitate the development of CVD. Several metabolic and inflammatory markers have been proposed for predicting the progression of CVD, including high density lipoprotein cholesterol (HDL-C). For ~50 years, HDL-C has been considered as the atheroprotective 'good' cholesterol because of its strong inverse association with the progression of CVD. Thus, interventions to increase the concentration of HDL-C have been successfully tested in animals; however, clinical trials were unable to confirm the cardiovascular benefits of pharmaceutical interventions aimed at increasing HDL-C levels. Based on these data, the significance of HDL-C in the prevention of CVD has been called into question. Fundamental in vitro and animal studies suggest that HDL-C functionality, rather than HDL-C concentration, is important for the CVD-preventive qualities of HDL-C. Our current review of the literature positively demonstrates the negative impact of systemic and tissue (i.e. adipose tissue) inflammation in the healthy metabolism and function of HDL-C. Our survey indicates that HDL-C may be a good marker of adipose tissue health, independently of its atheroprotective associations. We summarize the current findings on the use of anti-inflammatory drugs to either prevent HDL-C clearance or improve the function and production of HDL-C particles. It is evident that the therapeutic agents currently available may not provide the optimal strategy for altering HDL-C metabolism and function, and thus, further research is required to supplement this mechanistic approach for preventing the progression of CVD.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Nutritional regulation of renal lipogenic factor expression in mice: comparison to regulation in the liver and skeletal muscle

Regulation of lipogenesis by pathophysiological factors in the liver and skeletal muscle is well understood; however, regulation in the kidney is still unclear. To elucidate nutritional regulation of lipogenic factors in the kidney, we measured the renal expression of lipogenic transcriptional factors and enzymes during fasting and refeeding in chow-fed and high-fat-fed mice. We also examined the regulatory effect of the liver X receptor (LXR) on the expression of lipogenic factors. The renal gene expression of sterol regulatory element-binding protein (SREBP)-1c and fatty acid synthase (FAS) was reduced by fasting for 48 h and restored by refeeding, whereas the mRNA levels of forkhead box O (FOXO)1/3 were increased by fasting and restored by refeeding. Accordingly, protein levels of SREBP-1, FAS, and phosphorylated FOXO1/3 were reduced by fasting and restored by refeeding. The patterns of lipogenic factors expression in the kidney were similar to those in the liver and skeletal muscle. However, this phasic regulation of renal lipogenic gene expression was blunted in diet-induced obese mice. LXR agonist TO901317 increased the lipogenic gene expression and the protein levels of SREBP-1 precursor and FAS but not nuclear SREBP-1. Moreover, increases in insulin-induced gene mRNA and nuclear carbohydrate-responsive element binding protein (ChREBP) levels were observed in the TO901317-treated mice. These results suggest that the kidney shows flexible suppression and restoration of lipogenic factors following fasting and refeeding in lean mice, but this is blunted in obese mice. LXR is involved in the renal expression of lipogenic enzymes, and ChREBP may mediate the response.

Targeted Nanotherapeutics Encapsulating Liver X Receptor Agonist GW3965 Enhance Antiatherogenic Effects without Adverse Effects on Hepatic Lipid Metabolism in Ldlr-/- Mice

The pharmacological manipulation of liver X receptors (LXRs) has been an attractive therapeutic strategy for atherosclerosis treatment as they control reverse cholesterol transport and inflammatory response. This study presents the development and efficacy of nanoparticles (NPs) incorporating the synthetic LXR agonist GW3965 (GW) in targeting atherosclerotic lesions. Collagen IV (Col IV) targeting ligands are employed to functionalize the NPs to improve targeting to the atherosclerotic plaque, and formulation parameters such as the length of the polyethylene glycol (PEG) coating molecules are systematically optimized. In vitro studies indicate that the GW-encapsulated NPs upregulate the LXR target genes and downregulate proinflammatory mediator in macrophages. The Col IV-targeted NPs encapsulating GW (Col IV-GW-NPs) successfully reaches atherosclerotic lesions when administered for 5 weeks to mice with preexisting lesions, substantially reducing macrophage content (≈30%) compared to the PBS group, which is with greater efficacy versus nontargeting NPs encapsulating GW (GW-NPs) (≈18%). In addition, mice administered the Col IV-GW-NPs do not demonstrate increased hepatic lipid biosynthesis or hyperlipidemia during the treatment period, unlike mice injected with the free GW. These findings suggest a new form of LXR-based therapeutics capable of enhanced delivery of the LXR agonist to atherosclerotic lesions without altering hepatic lipid metabolism.

Morin, a novel liver X receptor α/β dual antagonist, has potent therapeutic efficacy for nonalcoholic fatty liver diseases

BACKGROUND AND PURPOSE

Morin is a natural occurring flavonoid in many dietary plants and has a wide range of beneficial effects on metabolism; however, the mechanism underlying its action remains elusive.

EXPERIMENTAL APPROACH

A reporter assay and the time-resolved FRET assay were used to identify morin as a dual antagonist of liver X receptor (LXR)-α and -β. Morin (100 mg^. 100 g^-1 diet) was administered to high-fat diet-induced obese or LXRβ^-/- mice. The pharmacological effects and mechanism of action of morin were evaluated by Western blot and RT-PCR analyses.

KEY RESULTS

From the in vitro assays, morin was shown to be a dual antagonist of LXRα and LXRβ. In vivo, morin blunted the development of liver hepatic steatosis, reduced body weight gains, lowered triglyceride levels and improved glucose and insulin tolerance in mice fed a high-fat diet. Mechanistically, morin inhibited 3T3-L1 adipocyte differentiation and lipid formation in human hepatic HepG2 cells and suppressed the mRNA expression of genes downstream of LXR. Consistently, the effects of morin on metabolic disorders were attenuated in LXRβ^-/- mice.

CONCLUSION AND IMPLICATIONS

Our data reveal that morin is a dual antagonist of LXRα and LXRβ and suggest that morin may alleviate hepatic steatosis and other associated metabolic disorders via the suppression of LXR signalling and, therefore, shows promise as a novel therapy or nutraceutical for nonalcoholic fatty liver disease.

Dietary oleic acid regulates hepatic lipogenesis through a liver X receptor-dependent signaling

Olive oil consumption is beneficial for health as it is associated with a decreased prevalence of cancer and cardiovascular diseases. Oleic acid is, by far, the most abundant component of olive oil. Since it can be made through de novo synthesis in animals, it is not an essential fatty acid. While it has become clear that dietary oleic acid regulates many biological processes, the signaling pathway involved in these regulations remains poorly defined. In this work we tested the impact of an oleic acid-rich diet on hepatic gene expression. We were particularly interested in addressing the contribution of Liver X Receptors (LXR) in the control of genes involved in hepatic lipogenesis, an essential process in whole body energy homeostasis. We used wild-type mice and transgenic mice deficient for both α and β Liver X Receptor isoforms (LXR-/-) fed a control or an oleate enriched diet. We observed that hepatic-lipid accumulation was enhanced as well as the expression of lipogenic genes in the liver of wild-type mice fed the oleate enriched diet. In contrast, none of these changes occurred in the liver of LXR-/- mice. Strikingly, oleate-rich diet reduced cholesterolemia in wild-type mice and induced signs of liver inflammation and damage in LXR-/- mice but not in wild-type mice. This work suggests that dietary oleic acid reduces cholesterolemia while promoting LXR-dependent hepatic lipogenesis without detrimental effects to the liver.

Role of Intestinal LXRα in Regulating Post-prandial Lipid Excursion and Diet-Induced Hypercholesterolemia and Hepatic Lipid Accumulation

Post-prandial hyperlipidemia has emerged as a cardiovascular risk factor with limited therapeutic options. The Liver X receptors (Lxrs) are nuclear hormone receptors that regulate cholesterol elimination. Knowledge of their role in regulating the absorption and handling of dietary fats is incomplete. The purpose of this study was to determine the role of intestinal Lxrα in post-prandial intestinal lipid transport. Using Lxrα knockout (nr1h3^−/−) and intestine-limited Lxrα over-expressing [Tg(fabp2a:EGFP-nr1h3)] zebrafish strains, we measured post-prandial lipid excursion with live imaging in larvae and physiological methods in adults. We also conducted a long-term high-cholesterol dietary challenge in adults to examine the chronic effect of modulating nr1h3 gene dose on the development of hypercholesterolemia and hepatic lipid accumulation. Over-expression of Lxrα in the intestine delays the transport of ingested lipids in larvae, while deletion of Lxrα increases the rate of lipid transport. Pre-treating wildtype larvae with the liver-sparing Lxr agonist hyodeoxycholic acid also delayed the rate of intestinal lipid transport in larvae. In adult males, deletion of Lxrα accelerates intestinal transport of ingested lipids. Adult females showed higher plasma Lipoprotein lipase (Lpl) activity compared to males, and lower post-gavage blood triacylglycerol (TAG) excursion. Despite the sexually dimorphic effect on acute intestinal lipid handling, Tg(fabp2a:EGFP-nr1h3) adults of both sexes are protected from high cholesterol diet (HCD)-induced hepatic lipid accumulation, while nr1h3^−/− mutants are sensitive to the effects of HCD challenge. These data indicate that intestinal Lxr activity dampens the pace of intestinal lipid transport cell-autonomously. Selective activation of intestinal Lxrα holds therapeutic promise.

Water Extract of Dolichos lablab Attenuates Hepatic Lipid Accumulation in a Cellular Nonalcoholic Fatty Liver Disease Model

Nonalcoholic fatty liver disease (NAFLD) is a common chronic liver disease that is rising in prevalence worldwide. Therapeutic strategies for patients with NAFLD are limited by a lack of effective drugs. In this report, we show that Dolichos lablab water extract (DLL-Ex) protects against free fatty acid (FFA)-induced lipid accumulation and attenuates expression of genes involved in lipid droplet accumulation in cellular NAFLD models. The hepatoprotective effects and underlying mechanism of DLL-Ex were assessed using an in vitro cellular model in which NAFLD was simulated by inducing excessive FFA influx into hepatocytes. HepG2 cells were treated with DLL-Ex and FFAs for 24 h, after which intracellular lipid content was observed by using Nile Red and Oil Red O staining. Quantitative real-time polymerase chain reaction was used to measure expression levels of genes related to FFA-mediated cellular energy depletion. Western blotting was used to measure protein levels of phosphorylated c-Jun N-terminal kinase, AMP-activated protein kinase alpha (AMPKα), and peroxisome proliferator-activated receptor γ coactivator 1 alpha. In HepG2 cells, DLL-Ex inhibited expression of CD36, which regulates fatty acid uptake, as well as BODIPY-labeled fatty acid uptake. Additionally, DLL-Ex significantly attenuated FFA-mediated cellular energy depletion and mitochondrial membrane depolarization. Furthermore, DLL-Ex enhanced phosphorylation of AMPK, indicating that AMPK is a critical regulator of DLL-Ex-mediated inhibition of hepatic lipid accumulation, possibly through its antioxidative effect. These results demonstrate that DLL-Ex exerts potent anti-NAFLD activity, suggesting that it could be a potential adjuvant treatment for patients with NAFLD.

Hyodeoxycholic acid derivatives as liver X receptor α and G-protein-coupled bile acid receptor agonists

Bile acids are extensively investigated for their potential in the treatment of human disorders. The liver X receptors (LXRs), activated by oxysterols and by a secondary bile acid named hyodeoxycholic acid (HDCA), have been found essential in the regulation of lipid homeostasis in mammals. Unfortunately, LXRα activates lipogenic enzymes causing accumulation of lipid in the liver. In addition to LXRs, HDCA has been also shown to function as ligand for GPBAR1, a G protein coupled receptor for secondary bile acids whose activation represents a promising approach to liver steatosis. In the present study, we report a library of HDCA derivatives endowed with modulatory activity on the two receptors. The lead optimization of HDCA moiety was rationally driven by the structural information on the binding site of the two targets and results from pharmacological characterization allowed the identification of hyodeoxycholane derivatives with selective agonistic activity toward LXRα and GPBAR1 and notably to the identification of the first example of potent dual LXRα/GPBAR1 agonists. The new chemical entities might hold utility in the treatment of dyslipidemic disorders.

ABCA1/ApoE/HDL Pathway Mediates GW3965-Induced Neurorestoration After Stroke

BACKGROUND AND PURPOSE

ATP-binding cassette transporter A1 (ABCA1) is a major reverse cholesterol transporter and plays critical role in the formation of brain high-density lipoprotein (HDL) cholesterol. Apolipoprotein E (ApoE) is the most abundant apolipoprotein and transports cholesterol into cells in brain. ABCA1 and ApoE are upregulated by liver-X receptors. Activation of liver-X receptors has neurorestorative benefit for stroke. The current study investigates whether ABCA1/ApoE/HDL pathway mediates GW3965, a synthetic dual liver-X receptor agonist, induced neurorestoration after stroke.

METHODS

Middle-aged male specific brain ABCA1-deficient (ABCA1^-B/-B) and floxed-control (ABCA1^fl/fl) mice were subjected to distal middle-cerebral artery occlusion (dMCAo) and gavaged with saline or GW3965 (10 mg/kg) or intracerebral infusion of artificial cerebrospinal fluid or human plasma HDL3 in ABCA1^-B/-B stroke mice, starting 24 hours after dMCAo and daily until euthanization 14 days after dMCAo.

RESULTS

No differences in the blood level of total cholesterol and triglyceride and lesion volume were found among the groups. Compared with ABCA1^fl/fl ischemic mice, ABCA1^-B/-B ischemic mice exhibited impairment functional outcome and decreased ABCA1/ApoE expression and decreased gray/white matter densities in the ischemic boundary zone 14 days after dMCAo. GW3965 treatment of ABCA1^fl/fl ischemic mice led to increased brain ABCA1/ApoE expression, concomitantly to increased blood HDL, gray/white matter densities and oligodendrocyte progenitor cell numbers in the ischemic boundary zone, as well as improved functional outcome 14 days after dMCAo. GW3965 treatment had negligible beneficial effects in ABCA1^-B/-B ischemic mice. However, intracerebral infusion of human plasma HDL3 significantly attenuated ABCA1^-B/-B-induced deficits. In vitro, GW3965 treatment (5 μM) increased ABCA1/synaptophysin level and neurite/axonal outgrowth in primary cortical neurons derived from ABCA1^fl/fl embryos, but not in neurons derived from ABCA1^-B/-B embryos. HDL treatment (80 μg/mL) attenuated the reduction of neurite/axonal outgrowth in neurons derived from ABCA1^-B/-B embryos.

CONCLUSIONS

ABCA1/ApoE/HDL pathway, at least partially, contributes to GW3965-induced neurorestoration after stroke.

Curcumin inhibits hyperlipidemia and hepatic fat accumulation in high-fructose-fed male Wistar rats

CONTEXT

Curcumin, an active principal of Curcuma longa Linn. (Zingiberaceae), has potent antioxidant and anti-inflammatory properties.

OBJECTIVES

This study investigated the effects of curcumin on hyperlipidemia and hepatic steatosis in high-fructose-fed Wistar rats.

MATERIALS AND METHODS

Forty male Wistar rats were divided into four groups with 10 rats in each. Two groups were fed with standard rodent diet and the other two with 60% high-fructose diet for 10 weeks. Curcumin (200 mg/kg body weight) was administered along with the diets simultaneously to each of the aforementioned diet groups. After 10 weeks of experiment, blood samples were collected from tail vein. Liver, adipose and epididymal tissues were collected after sacrifice of the animals and stored for further analyses.

RESULTS

Administration of curcumin reduced body weight (280.6 ± 7.4 g), liver weight (2.5 ± 0.2 g/100 g BW), adipose weight (1.4 ± 0.3 g/100 g BW), plasma levels of TAG (86.1 ± 13.5 mg/dL), VLDL-C (17.2 ± 2.7 mg/dL), lipid ratios and increased HDL-C (28.4 ± 4.5 mg/dL) in fructose-fed rats. Curcumin supplementation significantly lowered TAG content and decreased the protein expression of LXR-α (43%) and SREBP1c (59%) in the liver. Furthermore, curcumin suppressed the expression of lipogenic enzymes, ACLY (95%), ACC (50%) and FAS (77%) in rats fed with high-fructose diet. No significant change was found in the expression of PPAR-α.

DISCUSSION AND CONCLUSION

Curcumin prevented the high-fructose induced hyperlipidemia and hepatic steatosis.

Tangshen formula attenuates hepatic steatosis by inhibiting hepatic lipogenesis and augmenting fatty acid oxidation in db/db mice

Tangshen formula (TSF), a well-prescribed traditional Chinese formula, has been used in the treatment of diabetic nephropathy. However, whether TSF ameliorates dyslipidemia and liver injury associated with diabetes remains unclear. In this study, we examined the effects of TSF on lipid profiles and hepatic steatosis in db/db mice. For this purpose, 8‑week-old db/db mice were treated with TSF or saline for 12 weeks via gavage and db/m mice were used as controls. Body weight and blood glucose levels were monitored weekly and bi-weekly, respectively. Blood samples were obtained for the analysis of lipids and enzymes related to hepatic function, and liver tissues were analyzed by histology, immunohistochemistry and molecular examination. The results revealed that TSF markedly reduced body weight, liver index [liver/body weight (LW/BW)] and improved lipid profiles, hepatic function and steatosis in db/db mice. TSF induced the phosphoralation of AMP-activated protein kinase and inhibited the activity of sterol regulatory element-binding protein 1 together with the inhibition of the expression of genes involved in de novo lipogenesis (DNL) and gluconeogenesis, such as fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC), stearoyl CoA desaturase 1 (SCD1), glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase 1 (Pck1). Additionally, the silent mating type information regulation 2 homolog 1 (Sirt1)/peroxisome proliferator-activated receptor α (PPARα)/malonyl-CoA decarboxylase (MLYCD) cascade was potently activated by TSF in the liver and skeletal muscle of db/db mice, which led to enhanced fatty acid oxidation. These findings demonstrated that TSF attenuated hepatic fat accumulation and steatosis in db/db mice by inhibiting lipogenesis and augmenting fatty acid oxidation.

Nogo-B receptor deficiency increases liver X receptor alpha nuclear translocation and hepatic lipogenesis through an adenosine monophosphate-activated protein kinase alpha-dependent pathway

Nogo-B receptor (NgBR) was identified as a specific receptor for binding Nogo-B and is essential for the stability of Niemann-Pick type C2 protein (NPC2) and NPC2-dependent cholesterol trafficking. Here, we report that NgBR expression levels decrease in the fatty liver and that NgBR plays previously unrecognized roles in regulating hepatic lipogenesis through NPC2-independent pathways. To further elucidate the pathophysiological role of NgBR in mammals, we generated NgBR liver-specific knockout mice and investigated the roles of NgBR in hepatic lipid homeostasis. The results showed that NgBR knockout in mouse liver did not decrease NPC2 levels or increase NPC2-dependent intracellular cholesterol levels. However, NgBR deficiency still resulted in remarkable cellular lipid accumulation that was associated with increased free fatty acids and triglycerides in hepatocytes in vitro and in mouse livers in vivo. Mechanistically, NgBR deficiency specifically promotes the nuclear translocation of the liver X receptor alpha (LXRα) and increases the expression of LXRα-targeted lipogenic genes. LXRα knockout attenuates the accumulation of free fatty acids and triglycerides caused by NgBR deficiency. In addition, we elucidated the mechanisms by which NgBR bridges the adenosine monophosphate-activated protein kinase alpha signaling pathway with LXRα nuclear translocation and LXRα-mediated lipogenesis.

CONCLUSION

NgBR is a specific negative regulator for LXRα-dependent hepatic lipogenesis. Loss of NgBR may be a potential trigger for inducing hepatic steatosis. (Hepatology 2016;64:1559-1576).