Synthetic LXR ligand inhibits the development of atherosclerosis in mice

Promiscuous activity of the LXR antagonist GSK2033 in a mouse model of fatty liver disease

The liver X receptor (LXR) functions as a receptor for oxysterols and plays a critical role in the regulation of glucose and lipid metabolism. We recently described a synthetic LXR inverse agonist that displayed efficacy in treatment of hepatic steatosis in a mouse model of non-alcoholic fatty liver disease (NAFLD). This compound, SR9238, was designed to display liver specificity so as to avoid potential detrimental effects on reverse cholesterol transport in peripheral tissues. Here, we examined the effects of a LXR antagonist/inverse agonist, GSK2033, which displays systemic exposure. Although GSK2033 performed as expected in cell-based models as a LXR inverse agonist, it displayed unexpected activity in the mouse NAFLD model. The expression of lipogenic enzyme genes such as fatty acid synthase and sterol regulatory binding protein 1c were induced rather than suppressed and no effect on hepatic steatosis was found. Further characterization of the specificity of GSK2033 revealed that it displayed a significant degree of promiscuity, targeting a number of other nuclear receptors that could clearly alter hepatic gene expression.

Betulin attenuates atherosclerosis in apoE-/- mice by up-regulating ABCA1 and ABCG1

AIM

Betulin is a pentacyclic triterpenoid isolated from the bark of yellow and white birch trees with anti-cancer and anti-malaria activities. In this study we examined the effects of betulin on atherosclerosis in apoE^-/- mice and the underlying mechanisms.

METHODS

Murine macrophage RAW264.7 cells and human monocyte-derived THP-1 cells were tested. Foam cell formation was detected with Oil Red O staining. Cholesterol efflux was assessed using [³H]-cholesterol efflux assay. The expression of ATP-binding cassette transporter A1 and G1 (ABCA1 and ABCG1) was examined using RT-PCR and Western-blotting. The ABCA1 promoter activity was evaluated using luciferase activity assay. Male apoE^-/- mice fed on a high-fat-diet (HFD), and received betulin (20 and 40 mg·kg^-1·d^-1, ig) for 12 weeks. The macrophage content and ABCA1 expression in the aortic sinuses were evaluated with immunofluorescence staining. The hepatic, intestinal and fecal cholesterol were also analyzed in the mice.

RESULTS

In RAW264.7 cells, betulin (0.1-2.5 μg/mL) dose-dependently ameliorated oxLDL-induced cholesterol accumulation and enhanced cholesterol efflux. In both RAW264.7 and THP-1 cells, betulin increased the expression of ABCA1 and ABCG1 via suppressing the transcriptional repressors sterol-responsive element-binding proteins (SREBPs) that bound to E-box motifs in ABCA1 promoter, whereas E-box binding site mutation markedly attenuated betulin-induced ABCA1 promoter activity. In HFD-fed apoE^-/- mice, betulin administration significantly reduced lesions in en face aortas and aortic sinuses. Furthermore, betulin administration significantly increased ABCA1 expression and suppressed macrophage positive areas in the aortic sinuses. Moreover, betulin administration improved plasma lipid profiles and enhanced fecal cholesterol excretion in the mice.

CONCLUSION

Betulin attenuates atherosclerosis in apoE^-/- mice by promoting cholesterol efflux in macrophages.

KLF15 Enables Rapid Switching between Lipogenesis and Gluconeogenesis during Fasting

Hepatic lipogenesis is nutritionally regulated (i.e., downregulated during fasting and upregulated during the postprandial state) as an adaptation to the nutritional environment. While alterations in the expression level of the transcription factor SREBP-1c are known to be critical for nutritionally regulated lipogenesis, upstream mechanisms governing Srebf1 expression remain unclear. Here, we show that the fasting-induced transcription factor KLF15, a key regulator of gluconeogenesis, forms a complex with LXR/RXR, specifically on the Srebf1 promoter. This complex recruits the corepressor RIP140 instead of the coactivator SRC1, resulting in reduced Srebf1 and thus downstream lipogenic enzyme expression during the early and euglycemic period of fasting prior to hypoglycemia and PKA activation. Through this mechanism, KLF15 overexpression specifically ameliorates hypertriglyceridemia without affecting LXR-mediated cholesterol metabolism. These findings reveal a key molecular link between glucose and lipid metabolism and have therapeutic implications for the treatment of hyperlipidemia.

Liver X receptor reduces proliferation of human oral cancer cells by promoting cholesterol efflux via up-regulation of ABCA1 expression

Liver X receptors (LXRs) contribute not only to maintain cholesterol homeostasis but also to control cell growth. However, the molecular mechanisms behind the LXR-mediated anti-proliferative effects are largely unknown. Here we show, by immunohistochemistry, that LXRα and LXRβ are differentially distributed in oral stratified squamous epithelia. By immunohistochemical and Western blot analyses, we also reveal that LXRα is abundantly expressed in human oral squamous cell carcinoma (HOSCC) tissues and cell lines. Cell counting, BrdU labeling and cell cycle assay indicated that LXR stimulation led to significant reduction of proliferation in HOSCC cells. Importantly, our study highlights, by using RNA interference, that the ATP-binding cassette transporter A1 (ABCA1)-accelerated cholesterol efflux is critical for the growth inhibitory action of LXRs in HOSCC cells. Moreover, we demonstrate that LXR activation reduces the growth of xenograft tumour of HOSCC cells in mice accompanied by the upregulation of ABCA1 expression and the decline of cholesterol levels in the tumour. These findings strongly suggested that targeting the LXR-regulated cholesterol transport, yielding in lowering intracellular cholesterol levels, could be a promising therapeutic option for certain types of cancers.

Beneficial and Adverse Effects of an LXR Agonist on Human Lipid and Lipoprotein Metabolism and Circulating Neutrophils

The development of LXR agonists for the treatment of coronary artery disease has been challenged by undesirable properties in animal models. Here we show the effects of an LXR agonist on lipid and lipoprotein metabolism and neutrophils in human subjects. BMS-852927, a novel LXRβ-selective compound, had favorable profiles in animal models with a wide therapeutic index in cynomolgus monkeys and mice. In healthy subjects and hypercholesterolemic patients, reverse cholesterol transport pathways were induced similarly to that in animal models. However, increased plasma and hepatic TG, plasma LDL-C, apoB, apoE, and CETP and decreased circulating neutrophils were also evident. Furthermore, similar increases in LDL-C were observed in normocholesterolemic subjects and statin-treated patients. The primate model markedly underestimated human lipogenic responses and did not predict human neutrophil effects. These studies demonstrate both beneficial and adverse LXR agonist clinical responses and emphasize the importance of further translational research in this area.

Activation of liver X receptor attenuates lysophosphatidylcholine-induced IL-8 expression in endothelial cells via the NF-κB pathway and SUMOylation

The liver X receptor (LXR) is a cholesterol‐sensing nuclear receptor that has an established function in lipid metabolism; however, its role in inflammation is elusive. In this study, we showed that the LXR agonist GW3965 exhibited potent anti‐inflammatory activity by suppressing the firm adhesion of monocytes to endothelial cells. To further address the mechanisms underlying the inhibition of inflammatory cell infiltration, we evaluated the effects of LXR agonist on interleukin‐8 (IL‐8) secretion and nuclear factor‐kappa B (NF‐κB) activation in human umbilical vein endothelial cells (HUVECs). The LXR agonist significantly inhibited lysophosphatidylcholine (LPC)‐induced IL‐8 production in a dose‐dependent manner without appreciable cytotoxicity. Western blotting and the NF‐κB transcription activity assay showed that the LXR agonist inhibited p65 binding to the IL‐8 promoter in LPC‐stimulated HUVECs. Interestingly, knockdown of the indispensable small ubiquitin‐like modifier (SUMO) ligases Ubc9 and Histone deacetylase 4 (HDAC4) reversed the increase in IL‐8 induced by LPC. Furthermore, the LPC‐induced degradation of inhibitory κBα was delayed under the conditions of deficient SUMOylation or the treatment of LXR agonist. After enhancing SUMOylation by knockdown SUMO‐specific protease Sentrin‐specific protease 1 (SENP1), the inhibition of GW3965 was rescued on LPC‐mediated IL‐8 expression. These findings indicate that LXR‐mediated inflammatory gene repression correlates to the suppression of NF‐κB pathway and SUMOylation. Our results suggest that LXR agonist exerts the anti‐atherosclerotic role by attenuation of the NF‐κB pathway in endothelial cells.

Cholesterol-sensing liver X receptors stimulate Th2-driven allergic eosinophilic asthma in mice

INTRODUCTION

Liver X receptors (LXRs) are nuclear receptors that function as cholesterol sensors and regulate cholesterol homeostasis. High cholesterol has been recognized as a risk factor in asthma; however, the mechanism of this linkage is not known.

METHODS

To explore the importance of cholesterol homeostasis for asthma, we investigated the contribution of LXR activity in an ovalbumin- and a house dust mite-driven eosinophilic asthma mouse model.

RESULTS

In both models, airway inflammation, airway hyper-reactivity, and goblet cell hyperplasia were reduced in mice deficient for both LXRα and LXRβ isoforms (LXRα(-/-)β(-/-)) as compared to wild-type mice. Inversely, treatment with the LXR agonist GW3965 showed increased eosinophilic airway inflammation. LXR activity contributed to airway inflammation through promotion of type 2 cytokine production as LXRα(-/-)β(-/-) mice showed strongly reduced protein levels of IL-5 and IL-13 in the lungs as well as reduced expression of these cytokines by CD4(+) lung cells and lung-draining lymph node cells. In line herewith, LXR activation resulted in increased type 2 cytokine production by the lung-draining lymph node cells.

CONCLUSIONS

In conclusion, our study demonstrates that the cholesterol regulator LXR acts as a positive regulator of eosinophilic asthma in mice, contributing to airway inflammation through regulation of type 2 cytokine production.

Distinct gene regulatory programs define the inhibitory effects of liver X receptors and PPARG on cancer cell proliferation

Background

The liver X receptors (LXRs, NR1H2 and NR1H3) and peroxisome proliferator-activated receptor gamma (PPARG, NR1C3) nuclear receptor transcription factors (TFs) are master regulators of energy homeostasis. Intriguingly, recent studies suggest that these metabolic regulators also impact tumor cell proliferation. However, a comprehensive temporal molecular characterization of the LXR and PPARG gene regulatory responses in tumor cells is still lacking.

Methods

To better define the underlying molecular processes governing the genetic control of cellular growth in response to extracellular metabolic signals, we performed a comprehensive, genome-wide characterization of the temporal regulatory cascades mediated by LXR and PPARG signaling in HT29 colorectal cancer cells. For this analysis, we applied a multi-tiered approach that incorporated cellular phenotypic assays, gene expression profiles, chromatin state dynamics, and nuclear receptor binding patterns.

Results

Our results illustrate that the activation of both nuclear receptors inhibited cell proliferation and further decreased glutathione levels, consistent with increased cellular oxidative stress. Despite a common metabolic reprogramming, the gene regulatory network programs initiated by these nuclear receptors were widely distinct. PPARG generated a rapid and short-term response while maintaining a gene activator role. By contrast, LXR signaling was prolonged, with initial, predominantly activating functions that transitioned to repressive gene regulatory activities at late time points.

Conclusions

Through the use of a multi-tiered strategy that integrated various genomic datasets, our data illustrate that distinct gene regulatory programs elicit common phenotypic effects, highlighting the complexity of the genome. These results further provide a detailed molecular map of metabolic reprogramming in cancer cells through LXR and PPARG activation. As ligand-inducible TFs, these nuclear receptors can potentially serve as attractive therapeutic targets for the treatment of various cancers.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0328-6) contains supplementary material, which is available to authorized users.

Compound K Attenuates the Development of Atherosclerosis in ApoE(-/-) Mice via LXRα Activation

Background: Atherosclerosis is a fundamental pathological process responded to some serious cardiovascular events. Although the cholesterol-lowering drugs are widely prescribed for atherosclerosis therapy, it is still the leading cause of death in the developed world. Here we measured the effects of compound K in atherosclerosis formation and investigated the probably mechanisms of the anti-antherosclerosis roles of compound K. Methods: We treated the atherosclerotic model animals (apoE^−/− mice on western diet) with compound K and measured the size of atherosclerotic lesions, inflammatory cytokine levels and serum lipid profile. Peritoneal macrophages were collected in vitro for the foam cell and inflammasome experiments. Results: Our results show that treatment with compound K dose-dependently attenuates the formation of atherosclerotic plaques by 55% through activation of reverse cholesterol transport pathway, reduction of systemic inflammatory cytokines and inhibition of local inflammasome activity. Compound K increases the cholesterol efflux of macrophage-derived foam cells, and reduces the inflammasome activity in cholesterol crystal stimulated macrophages. The activation of LXRα may contribute to the athero-protective effects of compound K. Conclusion: These observations provide evidence for an athero-protective effect of compound K via LXRα activation, and support its further evaluation as a potential effective modulator for the prevention and treatment of atherosclerosis.

MEK1/2 inhibitors activate macrophage ABCG1 expression and reverse cholesterol transport-An anti-atherogenic function of ERK1/2 inhibition

Expression of ATP-binding cassette transporter G1 (ABCG1), a molecule facilitating cholesterol efflux to HDL, is activated by liver X receptor (LXR). In this study, we investigated if inhibition of ERK1/2 can activate macrophage ABCG1 expression and functions. MEK1/2 inhibitors, PD98059 and U0126, increased ABCG1 mRNA and protein expression, and activated the natural ABCG1 promoter but not the promoter with the LXR responsive element (LXRE) deletion. Inhibition of ABCG1 expression by ABCG1 siRNA did enhance the formation of macrophage/foam cells and it attenuated the inhibitory effect of MEK1/2 inhibitors on foam cell formation. MEK1/2 inhibitors activated macrophage cholesterol efflux to HDL in vitro, and they enhanced reverse cholesterol transport (RCT) in vivo. ApoE deficient (apoE(-/-)) mice receiving U0126 treatment had reduced sinus lesions in the aortic root which was associated with activated macrophage ABCG1 expression in the lesion areas. MEK1/2 inhibitors coordinated the RXR agonist, but not the LXR agonist, to induce ABCG1 expression. Furthermore, induction of ABCG1 expression by MEK1/2 inhibitors was associated with activation of SIRT1, a positive regulator of LXR activity, and inactivation of SULT2B1 and RIP140, two negative regulators of LXR activity. Taken together, our study suggests that MEK1/2 inhibitors activate macrophage ABCG1 expression/RCT, and inhibit foam cell formation and lesion development by multiple mechanisms, supporting the concept that ERK1/2 inhibition is anti-atherogenic.

Activation of liver X receptor/retinoid X receptor pathway ameliorates liver disease in Atp7B(-/-) (Wilson disease) mice

Wilson disease (WD) is a hepatoneurological disorder caused by mutations in the copper-transporter, ATP7B. Copper accumulation in the liver is a hallmark of WD. Current therapy is based on copper chelation, which decreases the manifestations of liver disease, but often worsens neurological symptoms. We demonstrate that in Atp7b(-/-) mice, an animal model of WD, liver function can be significantly improved without copper chelation. Analysis of transcriptional and metabolic changes in samples from WD patients and Atp7b(-/-) mice identified dysregulation of nuclear receptors (NRs), especially the liver X receptor (LXR)/retinoid X receptor heterodimer, as an important event in WD pathogenesis. Treating Atp7b(-/-) mice with the LXR agonist, T0901317, ameliorated disease manifestations despite significant copper overload. Genetic markers of liver fibrosis and inflammatory cytokines were significantly decreased, lipid profiles normalized, and liver function and histology were improved.

CONCLUSIONS

The results demonstrate the major role of an altered NR function in the pathogenesis of WD and suggest that modulation of NR activity should be explored as a supplementary approach to improving liver function in WD. (Hepatology 2016;63:1828-1841).

The Flavone Luteolin Inhibits Liver X Receptor Activation

Luteolin is a dietary flavonoid with medicinal properties including antioxidant, antimicrobial, anticancer, antiallergic, and anti-inflammatory. However, the effect of luteolin on liver X receptors (LXRs), oxysterol sensors that regulate cholesterol homeostasis, lipogenesis, and inflammation, has yet to be studied. To unveil the potential of luteolin as an LXRα/β modulator, we investigated by real-time RT-PCR the expression of LXR-target genes, namely, sterol regulatory element binding protein 1c (SREBP-1c) in hepatocytes and ATP-binding cassette transporter (ABC)A1 in macrophages. The lipid content of hepatocytes was evaluated by Oil Red staining. The results demonstrated, for the first time, that luteolin abrogated the LXRα/β agonist-induced LXRα/β transcriptional activity and, consequently, inhibited SREBP-1c expression, lipid accumulation, and ABCA1 expression. Therefore, luteolin could abrogate hypertriglyceridemia associated with LXR activation, thus presenting putative therapeutic effects in diseases associated with deregulated lipid metabolism, such as hepatic steatosis, cardiovascular diseases, and diabetes.

HDL-cholesterol and cardiovascular disease: rethinking our approach

PURPOSE OF REVIEW

A low level of plasma high density lipoprotein cholesterol (HDL-C) is a strong and independent risk factor for atherosclerotic cardiovascular disease (ASCVD). However, several large studies recently revealed that pharmacologic interventions that increase HDL-C concentration have not improved cardiovascular outcomes when added to standard therapy. In addition, specific genetic variants that raise HDL-C levels are not clearly associated with reduced risk of coronary heart disease. These observations have challenged the 'HDL hypothesis' that HDL-C is causally related to ASCVD and that intervention to raise HDL-C will reduce ASCVD events. This article will present the current data on the HDL hypothesis and provide a revised paradigm of considering HDL in the atherosclerotic pathway.

RECENT FINDINGS

Recent evidence has shed light on the complex nature of HDL-C metabolism and function. There are compelling data that the ability of HDL to promote cholesterol efflux from macrophages, the first step in the 'reverse cholesterol transport' (RCT) pathway, is inversely associated with risk for ASCVD even after controlling for HDL-C. This has led to the 'HDL flux hypothesis' that therapeutic intervention that targets macrophage cholesterol efflux and RCT may reduce risk. Preclinical studies of such interventions show promise and early phase clinical studies, though small, are encouraging.

SUMMARY

The role of HDL-C in modulating atherosclerotic disease is as yet uncertain. However, new findings and therapies targeting HDL-C show early promise and may provide an important intervention in attenuating the burden of ASCVD in the future.

E17110 promotes reverse cholesterol transport with liver X receptor β agonist activity in vitro

Liver X receptor (LXR) plays an important role in reverse cholesterol transport (RCT), and activation of LXR could reduce atherosclerosis. In the present study we used a cell-based screening method to identify new potential LXRβ agonists. A novel benzofuran-2-carboxylate derivative was identified with LXRβ agonist activity: E17110 showed a significant activation effect on LXRβ with an EC₅₀ value of 0.72 μmol/L. E17110 also increased the expression of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) in RAW264.7 macrophages. Moreover, E17110 significantly reduced cellular lipid accumulation and promoted cholesterol efflux in RAW264.7 macrophages. Interestingly, we found that the key amino acids in the LXRβ ligand-binding domain had distinct interactions with E17110 as compared to TO901317. These results suggest that E17110 was identified as a novel compound with LXRβ agonist activity in vitro via screening, and could be developed as a potential anti-atherosclerotic lead compound.

LXRα represses LPS-induced inflammatory responses by competing with IRF3 for GRIP1 in Kupffer cells

Liver X receptors (LXRs) in the nucleus play important roles in lipid metabolism and inflammation. The mechanism of LXR regulation of the LPS-induced Toll-like receptor 4 (TLR4) inflammatory signaling pathway remains to be elucidated. C57/BL6 mice were randomly divided into four groups: control, T0901317 (a LXRs agonist), LPS and T0901317+LPS. Additionally, Kupffer cells isolated from male C57/BL6 mice were divided into the same four groups. A decreased amount of inflammatory cells infiltrated the portal areas and the hepatic sinusoids in the livers of mice in the T0901317+LPS group than in those of mice in the LPS group. In the T0901317+LPS group, the serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and tumor necrosis factor alpha (TNF-α) were lower, while the serum level of interleukin-10 (IL-10) was higher. In vitro, Kupffer cells pretreated with T0901317 for 24h presented reduced TNF-α, interferon-beta (IFN-β) and interleukin-1 beta (IL-1β) levels, while the IL-10 level increased; however, the mRNA and protein expression levels of interferon regulatory factor 3 (IRF3) and glucocorticoid receptor-interacting protein 1 (GRIP1) were not significantly reduced. The co-IP data illustrated that LXRα bound to GRIP1 specifically in the T0901317+LPS group, while less IRF3 was bound to GRIP1 in the T0901317+LPS group than in the LPS group. Furthermore, the DNA-binding activity of NF-κB was decreased by pretreating Kupffer cells with T0901317 for 24h. These results suggest that activated LXRα competes with IRF3 for GRIP1 binding, thus repressing IRF3 and NF-κB transcriptional activity and inhibiting the inflammatory response initiated by LPS in Kupffer cells.

Phosphatidylcholine: Greasing the Cholesterol Transport Machinery

Negative feedback regulation of cholesterol metabolism in mammalian cells ensures a proper balance of cholesterol with other membrane lipids, principal among these being the major phospholipid phosphatidylcholine (PC). Processes such as cholesterol biosynthesis and efflux, cholesteryl ester storage in lipid droplets, and uptake of plasma lipoproteins are tuned to the cholesterol/PC ratio. Cholesterol-loaded macrophages in atherosclerotic lesions display increased PC biosynthesis that buffers against elevated cholesterol levels and may also facilitate cholesterol trafficking to enhance cholesterol sensing and efflux. These same mechanisms could play a generic role in homeostatic responses to acute changes in membrane free cholesterol levels. Here, I discuss the established and emerging roles of PC metabolism in promoting intracellular cholesterol trafficking and membrane lipid homeostasis.

MEK1/2 inhibitors induce interleukin-5 expression in mouse macrophages and lymphocytes

Uptake of oxidized low-density lipoprotein (oxLDL) by macrophages facilitates the formation of foam cells, the prominent part of atherosclerotic lesions. Interleukin-5 (IL-5) is a cytokine regulating interactions between immune cells. It also activates the production of T15/EO6 IgM antibodies in B-1 cells, which can bind oxLDL thereby demonstrating anti-atherogenic properties. We previously reported that inhibition of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by mitogen-activated protein kinase kinases 1/2 (MEK1/2) inhibitors can reduce atherosclerosis. In this study, we determined the effects of MEK1/2 inhibitors on IL-5 production both in vitro and in vivo. In vitro, MEK1/2 inhibitors (PD98059 and U0126) substantially inhibited phosphorylation of MEK1/2 and ERK1/2. Associated with inhibition of ERK1/2 phosphorylation both in vitro and in vivo, MEK1/2 inhibitors induced IL-5 protein expression in macrophages (RAW macrophages and peritoneal macrophages) and lymphocytes (EL-4 cells). In vivo, administration of mice with MEK1/2 inhibitors increased serum IL-5 levels, and IL-5 protein expression in mouse spleen and liver. At the mechanistic level, we determined that MEK1/2 inhibitors activated IL-5 mRNA expression and IL-5 promoter activity in the liver X receptor (LXR) dependent manner indicating the induction of IL-5 transcription. In addition, we determined that MEK1/2 inhibitors enhanced IL-5 protein stability. Taken together, our study demonstrates that MEK1/2 inhibitors induce IL-5 production which suggests another anti-atherogenic mechanism of MEK1/2 inhibitors.

Poly(ADP-ribose) Polymerase 1 Represses Liver X Receptor-mediated ABCA1 Expression and Cholesterol Efflux in Macrophages

Liver X receptors (LXR) are oxysterol-activated nuclear receptors that play a central role in reverse cholesterol transport through up-regulation of ATP-binding cassette transporters (ABCA1 and ABCG1) that mediate cellular cholesterol efflux. Mouse models of atherosclerosis exhibit reduced atherosclerosis and enhanced regression of established plaques upon LXR activation. However, the coregulatory factors that affect LXR-dependent gene activation in macrophages remain to be elucidated. To identify novel regulators of LXR that modulate its activity, we used affinity purification and mass spectrometry to analyze nuclear LXRα complexes and identified poly(ADP-ribose) polymerase-1 (PARP-1) as an LXR-associated factor. In fact, PARP-1 interacted with both LXRα and LXRβ. Both depletion of PARP-1 and inhibition of PARP-1 activity augmented LXR ligand-induced ABCA1 expression in the RAW 264.7 macrophage line and primary bone marrow-derived macrophages but did not affect LXR-dependent expression of other target genes, ABCG1 and SREBP-1c. Chromatin immunoprecipitation experiments confirmed PARP-1 recruitment at the LXR response element in the promoter of the ABCA1 gene. Further, we demonstrated that LXR is poly(ADP-ribosyl)ated by PARP-1, a potential mechanism by which PARP-1 influences LXR function. Importantly, the PARP inhibitor 3-aminobenzamide enhanced macrophage ABCA1-mediated cholesterol efflux to the lipid-poor apolipoprotein AI. These findings shed light on the important role of PARP-1 on LXR-regulated lipid homeostasis. Understanding the interplay between PARP-1 and LXR may provide insights into developing novel therapeutics for treating atherosclerosis.

The clearance of dying cells: table for two

Phagocytic cells of the immune system must constantly survey for, recognize, and efficiently clear the billions of cellular corpses that arise as a result of development, stress, infection, or normal homeostasis. This process, termed efferocytosis, is critical for the prevention of autoimmune and inflammatory disorders, and persistence of dead cells in tissue is characteristic of many human autoimmune diseases, notably systemic lupus erythematosus. The most notable characteristic of the efferocytosis of apoptotic cells is its ‘immunologically silent' response. Although the mechanisms by which phagocytes facilitate engulfment of dead cells has been a well-studied area, the pathways that coordinate to process the ingested corpse and direct the subsequent immune response is an area of growing interest. The recently described pathway of LC3 (microtubule-associated protein 1A/1B-light chain 3)-associated phagocytosis (LAP) has shed some light on this issue. LAP is triggered when an extracellular particle, such as a dead cell, engages an extracellular receptor during phagocytosis, induces the translocation of autophagy machinery, and ultimately LC3 to the cargo-containing phagosome, termed the LAPosome. In this review, we will examine efferocytosis and the impact of LAP on efferocytosis, allowing us to reimagine the impact of the autophagy machinery on innate host defense mechanisms.

Revisiting cobalt chloride preconditioning to prevent hypobaric hypoxia-induced damage: identification of global proteomic alteration and key networks

Several studies have supported the hypoxia mimetic roles and cytoprotective properties of cobalt chloride in vitro and in vivo. However, a clear understanding of biological process-based mechanism that integrates the available information remains unknown. This study was aimed to explore the potential mechanism of cobalt chloride deciphering its benefits and well-known physiological challenge caused by hypobaric hypoxia that reportedly affects nearly 24 % of the global population. In order to explore the mechanism of CoCl2, we used global proteomic and systems biology approach in rat model to provide a deeper insight into molecular mechanisms of preconditioning. Furthermore, key conclusions were drawn based on biological network analysis and their enrichment with ontological overlaps. The study was further strengthened by consistent identification of validation of proteins using immunoblotting. CoCl2-pretreated animals exposed to hypoxia showed two significant networks, one lipid metabolism and other cell cycle associated, with a total score of 23 and eight focus molecules. In this study, we delineated two primary routes: one, by direct modulation of reactive oxygen species metabolism and, second, by regulation of lipid metabolism which was not known until now. The previously known benefits of cobalt chloride during physiological challenge by hypobaric hypoxia are convincing and could be explained by some basic set of metabolic and molecular reorganization within the hypoxia model. Interestingly, we also observed some of the completely unknown roles of cobalt chloride such as regulation of lipid that could undulate the translational roles of cobalt chloride supplementation beyond hypoxia preconditioning.

miRNAs and High-Density Lipoprotein metabolism

Altered lipoprotein metabolism plays a key role during atherogenesis. For over 50years, epidemiological data have fueled the proposal that HDL-cholesterol (HDL-c) in circulation is inversely correlated to cardiovascular risk. However, the atheroprotective role of HDL is currently the focus of much debate and remains an active field of research. The emerging picture from research in the past decade suggests that HDL function, rather than HDL-c content, is important in disease. Recent developments demonstrate that miRNAs play an important role in fine-tuning the expression of key genes involved in HDL biogenesis, lipidation, and clearance, as well as in determining the amounts of HDL-c in circulation. Thus, it has been proposed that miRNAs that affect HDL metabolism might be exploited therapeutically in patients. Whether HDL-based therapies, alone or in combination with LDL-based treatments (e.g. statins), provide superior outcomes in patients has been recently questioned by human genetics studies and clinical trials. The switch in focus from "HDL-cholesterol" to "HDL function" opens a new paradigm to understand the physiology and therapeutic potential of HDL, and to find novel modulators of cardiovascular risk. In this review we summarize the current knowledge on the regulation of HDL metabolism and function by miRNAs. This article is part of a Special Issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

Liver X receptors as regulators of metabolism

The liver X receptors (LXR) are crucial regulators of metabolism. After ligand binding, they regulate gene transcription and thereby mediate changes in metabolic pathways. Modulation of LXR and their downstream targets has appeared to be a promising treatment for metabolic diseases especially atherosclerosis and cholesterol metabolism. However, the complexity of LXR action in various metabolic tissues and the liver side effect of LXR activation have slowed down the interest for LXR drugs. In this review, we summarized the role of LXR in the main metabolically active tissues with a special focus on obesity and associated diseases in mammals. We will also discuss the dual interplay between the two LXR isoforms suggesting that they may collaborate to establish a fine and efficient system for the maintenance of metabolism homeostasis.

Thermoneutral Housing Accelerates Metabolic Inflammation to Potentiate Atherosclerosis but Not Insulin Resistance

Chronic, low-grade inflammation triggered by excess intake of dietary lipids has been proposed to contribute to the pathogenesis of metabolic disorders, such as obesity, insulin resistance, type 2 diabetes, and atherosclerosis. Although considerable evidence supports a causal association between inflammation and metabolic diseases, most tests of this link have been performed in cold-stressed mice that are housed below their thermoneutral zone. We report here that thermoneutral housing of mice has a profound effect on the development of metabolic inflammation, insulin resistance, and atherosclerosis. Mice housed at thermoneutrality develop metabolic inflammation in adipose tissue and in the vasculature at an accelerated rate. Unexpectedly, this increased inflammatory response contributes to the progression of atherosclerosis but not insulin resistance. These findings not only suggest that metabolic inflammation can be uncoupled from obesity-associated insulin resistance, but also point to how thermal stress might limit our ability to faithfully model human diseases in mice.

Excess Nitric Oxide Activates TRPV1-Ca(2+)-Calpain Signaling and Promotes PEST-dependent Degradation of Liver X Receptor α

Excess nitric oxide (NO) deregulates cholesterol metabolism in macrophage foam cells, yet the underlying molecular mechanism is incompletely understood. To investigate the mechanism, we found that in macrophages, treatment with NO donors S-nitroso-N-acetyl-D,L-penicillamine (SNAP) or diethylenetriamine/nitric oxide induced LXRα degradation and reduced the expression of the downstream target of LXRα, ATP-binding cassette transporter A1 (ABCA1), and cholesterol efflux. In addition, SNAP induced calcium (Ca²⁺) influx into cells, increased calpain activity and promoted the formation of calpain-LXRα complex. Pharmacological inhibition of calpain activity reversed the SNAP-induced degradation of LXRα, down-regulation of ABCA1 and impairment of cholesterol efflux in macrophages. SNAP increased the formation of calpain-LXRα complex in a Pro-Glu-Ser-Thr (PEST) motif-dependent manner. Truncation of the PEST motif in LXRα abolished the calpain-dependent proteolysis. Removal of extracellular Ca²⁺ by EGTA or pharmacological inhibition of TRPV1 channel activity diminished SNAP-induced increase in intracellular Ca²⁺, calpain activation, LXRα degradation, ABCA1 down-regulation and impaired cholesterol efflux. In conclusion, excess NO may activate calpain via TRPV1-Ca²⁺ signaling and promote the recognition of calpain in the PEST motif of LXRα, thereby leading to degradation of LXRα and, ultimately, downregulated ABCA1 expression and impaired ABCA1-dependent cholesterol efflux in macrophages.

Pathophysiology and Mechanisms of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver disorders characterized by abnormal hepatic fat accumulation, inflammation, and hepatocyte dysfunction. Importantly, it is also closely linked to obesity and the metabolic syndrome. NAFLD predisposes susceptible individuals to cirrhosis, hepatocellular carcinoma, and cardiovascular disease. Although the precise signals remain poorly understood, NAFLD pathogenesis likely involves actions of the different hepatic cell types and multiple extrahepatic signals. The complexity of this disease has been a major impediment to the development of appropriate metrics of its progression and effective therapies. Recent clinical data place increasing importance on identifying fibrosis, as it is a strong indicator of hepatic disease-related mortality. Preclinical modeling of the fibrotic process remains challenging, particularly in the contexts of obesity and the metabolic syndrome. Future studies are needed to define the molecular pathways determining the natural progression of NAFLD, including key determinants of fibrosis and disease-related outcomes. This review covers the evolving concepts of NAFLD from both human and animal studies. We discuss recent clinical and diagnostic methods assessing NAFLD diagnosis, progression, and outcomes; compare the features of genetic and dietary animal models of NAFLD; and highlight pharmacological approaches for disease treatment.

Emerging role of liver X receptors in cardiac pathophysiology and heart failure

Liver X receptors (LXRs) are master regulators of metabolism and have been studied for their pharmacological potential in vascular and metabolic disease. Besides their established role in metabolic homeostasis and disease, there is mounting evidence to suggest that LXRs may exert direct beneficial effects in the heart. Here, we aim to provide a conceptual framework to explain the broad mode of action of LXRs and how LXR signaling may be an important local and systemic target for the treatment of heart failure. We discuss the potential role of LXRs in systemic conditions associated with heart failure, such as hypertension, diabetes, and renal and vascular disease. Further, we expound on recent data that implicate a direct role for LXR activation in the heart, for its impact on cardiomyocyte damage and loss due to ischemia, and effects on cardiac hypertrophy, fibrosis, and myocardial metabolism. Taken together, the accumulating evidence supports the notion that LXRs may represent a novel therapeutic target for the treatment of heart failure.

New aspects of 24(S)-hydroxycholesterol in modulating neuronal cell death

24(S)-Hydroxycholesterol (24S-OHC), which is enzymatically produced in the brain, has been known to play an important role in maintaining cholesterol homeostasis in the brain and has been proposed as a possible biomarker of neurodegenerative disease. Recent studies have revealed diverse functions of 24S-OHC and gained increased attention. For example, 24S-OHC at sublethal concentrations has been found to induce an adaptive response via activation of the liver X receptor signaling pathway, thereby protecting neuronal cells against subsequent oxidative stress. It has also been found that physiological concentrations of 24S-OHC suppress amyloid-β production via downregulation of amyloid precursor protein trafficking in neuronal cells. On the other hand, high concentrations of 24S-OHC have been found to induce a type of nonapoptotic programmed cell death in neuronal cells expressing little caspase-8. Because neuronal cell death induced by 24S-OHC has been found to proceed by a unique mechanism, which is different from but in some ways similar to necroptosis-necroptosis being a type of programmed necrosis induced by tumor necrosis factor α-neuronal cell death induced by 24S-OHC has been called "necroptosis-like" cell death. 24S-OHC-induced cell death is dependent on the formation of 24S-OHC esters but not on oxidative stress. This review article discusses newly reported aspects of 24S-OHC in neuronal cell death and sheds light on the possible importance of controlling 24S-OHC levels in the brain for preventing neurodegenerative disease.

The emerging roles of liver X receptors and their ligands in cancer

INTRODUCTION

Liver X receptors (LXRs) are nuclear receptors with well-known functions in cholesterol transport, fatty acid and glucose metabolism, and modulation of immune responses. Natural and synthetic ligands have been identified and are under development for the treatment of metabolic and inflammatory conditions and diseases. There is mounting evidence pointing to functional roles for LXRs in a variety of malignancies and the potential therapeutic efficacy of their ligands.

AREAS COVERED

This review summarizes the discovery and characterization of LXRs and their ligands, surveys their effects and mechanisms of action in cell-based and animal models of cancer, and proposes the future direction of basic and translational studies of LXRs and their ligands in cancer research and therapeutics.

EXPERT OPINION

Targeting LXRs is a promising strategy for cancer treatment, particularly for those cancers which do not have effective treatment options. Key questions remain, however, regarding the specific mechanisms of action, effects on other target cells within the tumor microenvironment, and receptor status in patient populations. Moreover, LXR ligands optimized for disease-specific functions and cancer-related endpoints are currently not available. These issues represent both challenges and significant opportunities for future research and development efforts.

Environmental pollutants directly affect the liver X receptor alpha activity: Kinetic and thermodynamic characterization of binding

Liver X receptor is a ligand-activated transcription factor, which is mainly involved in cholesterol homeostasis, bile acid and triglycerides metabolism, and, as recently discovered, in the glucose metabolism by direct regulation of liver glucokinase. Its modulation by exogenous factors, such as drugs, industrial by-products, and chemicals is documented. Owing to the abundance of these synthetic molecules in the environment, and to the established target role of this receptor, a number of representative compounds of phthalate, organophosphate and fibrate classes were tested as ligands/modulators of human liver X receptor, using an integrated approach, combining an in silico molecular docking technique with an optical SPR biosensor binding study. The compounds of interest were predicted and proved to target the oxysterols-binding site of human LXRα with measurable binding kinetic constants and with affinities ranging between 4.3 × 10(-7) and 4.3 × 10(-8)M. Additionally, non-cytotoxic concentration of these chemicals induced relevant changes in the LXRα gene expression levels and other target genes (SREBP-1c and LGK) in human liver hepatocellular carcinoma cell line (HepG2), as demonstrated by q-RT-PCR.

Classical nuclear hormone receptor activity as a mediator of complex biological responses: a look at health and disease

Nuclear hormone receptors are a large family of receptors that bind a wide range of lipolic hormones and intracellular ligands. They act as ligand-inducible transcription factors to regulate the expression of target genes and play important roles in normal development, reproduction, and metabolism. NRs bind to hormones steroids, thyroid hormone, and vitamin D as well as metabolites of fatty acids, cholesterol, and bild acids. Orphan receptors are another group of NRs for which no known ligands have been identified yet but appear to have major roles in regulating intracellular metabolism. Targeting NRs has been a major source for the development of new drugs, particularly selective agonists and antagonists for cancer and metabolic diseases. Additionally, hormone resistance syndromes in man have enlarged our understanding of the functions of specific NRs and their isoforms as well as genetic mechanisms for phenotype expression.

Discovery and structure-guided optimization of tert-butyl 6-(phenoxymethyl)-3-(trifluoromethyl)benzoates as liver X receptor agonists

To obtain potent liver X receptor (LXR) agonists, a structure-activity relationship study was performed on a series of tert-butyl benzoate analogs. As the crystal structure analysis suggested applicable interactions between the LXR ligand-binding domain and the ligands, two key functional groups were introduced. The introduction of the hydroxyl group on the C6-position of the benzoate part enhanced the agonistic activity in a cell-based assay, and the carboxyl group in terminal improved the pharmacokinetic profile in mice, respectively. The obtained compound 32b increased blood ABCA1 mRNA expression without plasma TG elevation in both mice and cynomolgus monkeys.

Stimulation of Liver X Receptor Has Potent Anti-HIV Effects in a Humanized Mouse Model of HIV Infection

Previous studies demonstrated that liver X receptor (LXR) agonists inhibit human immunodeficiency virus (HIV) replication by upregulating cholesterol transporter ATP-binding cassette A1 (ABCA1), suppressing HIV production, and reducing infectivity of produced virions. In this study, we extended these observations by analyzing the effect of the LXR agonist T0901317 [N-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenyl]-N-(2,2,2-trifluoroethyl)benzenesulfonamide] on the ongoing HIV infection and investigating the possibility of using LXR agonist for pre-exposure prophylaxis of HIV infection in a humanized mouse model. Pre-exposure of monocyte-derived macrophages to T0901317 reduced susceptibility of these cells to HIV infection in vitro. This protective effect lasted for up to 4 days after treatment termination and correlated with upregulated expression of ABCA1, reduced abundance of lipid rafts, and reduced fusion of the cells with HIV. Pre-exposure of peripheral blood leukocytes to T0901317 provided only a short-term protection against HIV infection. Treatment of HIV-exposed humanized mice with LXR agonist starting 2 weeks postinfection substantially reduced viral load. When eight humanized mice were pretreated with LXR agonist prior to HIV infection, five animals were protected from infection, two had viral load at the limit of detection, and one had viral load significantly reduced relative to mock-treated controls. T0901317 pretreatment also reduced HIV-induced dyslipidemia in infected mice. In conclusion, these results reveal a novel link between LXR stimulation and cell resistance to HIV infection and suggest that LXR agonists may be good candidates for development as anti-HIV agents, in particular for pre-exposure prophylaxis of HIV infection.

Liver X receptors contribute to periodontal pathogen-elicited inflammation and oral bone loss

Periodontal diseases are chronic oral inflammatory diseases that are polymicrobial in nature. The presence of specific bacteria in subgingival plaque such as Porphyromonas gingivalis is associated with microbial dysbiosis and the modulation of host immune response. Bacterially elicited innate immune activation and inflammation are key elements implicated in the destruction of soft and hard tissues supporting the teeth. Liver X receptors (LXRs) are nuclear hormone receptors with important function in lipid homeostasis, inflammation, and host response to infection; however, their contribution to chronic inflammatory diseases such as periodontal disease is not understood. The aim of this study was to define the contribution of LXRs in the development of immune response to P. gingivalis and to assess the roles that LXRs play in infection-elicited oral bone loss. Employing macrophages, we observed that P. gingivalis challenge led to reduced LXRα and LXRβ gene expression compared with that observed with unchallenged wild-type cells. Myeloid differentiation primary response gene 88 (MyD88)-independent, Toll/interleukin-1 receptor-domain-containing adapter-inducing interferon-β (TRIF)-dependent signaling affected P. gingivalis-mediated reduction in LXRα expression, whereas neither pathway influenced the P. gingivalis effect on LXRβ expression. Employing LXR agonist and mice deficient in LXRs, we observed functional effects of LXRs in the development of a P. gingivalis-elicited cytokine response at the level of the macrophage, and participation of LXRs in P. gingivalis-elicited oral bone loss. These findings identify novel importance for LXRs in the pathogenesis of P. gingivalis infection-elicited inflammation and oral bone loss.

The liver X receptor agonist TO901317 protects mice against cisplatin-induced kidney injury

Liver X receptors are in the nuclear receptor superfamily and are contained in the regulation of lipid and cholesterol metabolism. Besides, liver X receptors are considered crucial regulators of the inflammatory response and innate immunity. The current study evaluates the in vivo effects that the synthetic liver X receptor agonist TO901317 protects against cisplatin-induced kidney injury in mice. Mice received cisplatin administration through a single intraperitoneal injection (20 mg/kg in saline). And then the mice were treated with the TO901317 by daily gavage (10 mg/kg/day) 12 h postcisplatin administration, and cisplatin nephrotoxicity was evaluated. At 72 h after cisplatin treatment, elevated plasma urea and creatinine levels (P < 0.05) were evidenced which indicates the renal dysfunction of the vehicle-treated mice, consistent with tubular necrosis, protein cast, dilation of renal tubules, and desquamation of epithelial cells in renal tubules. In contrast, the severity of renal dysfunction and histological damage was reduced in TO901317 treated mice (P < 0.05). In accordance, circulating tumor necrosis factor alpha levels, renal tumor necrosis factor alpha, p47(phox), gp91(phox), and protein expression levels and COX-2 mRNA, renal monocyte chemoattractant protein 1, VACAM-1 mRNA and intercellular adhesion molecule-1 contents, and renal prostaglandin E2 amounts, were higher in samples from cisplatin-treated mice in comparison with controls (P < 0.05) but attenuated in the TO901317 treatment group (P < 0.05). Taken together, treatment with the liver X receptor agonist TO901317 ameliorated the inflammatory response and oxidative stress in cisplatin-induced kidney injury in mice.

High-density lipoprotein-based drug discovery for treatment of atherosclerosis

INTRODUCTION

Although there has been great progress achieved by the use of intensive statin therapy, the burden of atherosclerotic cardiovascular disease (CVD) remains high. This has initiated the search for novel high-density lipoprotein (HDL)-based therapeutics. Recent years have witnessed a shift from traditional raising HDL-C levels to enhancing HDL functionality, in which the process of reverse cholesterol transport (RCT) has acquired much attention.

AREAS COVERED

In this review, the authors describe the key factors involved in RCT process for potential drug targets to reduce the CVD risk. Furthermore, the review provides a summary of the effective screening methods that have been developed to target RCT and their applications. This review also introduces some new strategies currently being clinically developed, which have the potential to improve HDL function in the RCT process.

EXPERT OPINION

It is rational that the functionality of HDL is more important than the plasma HDL-C level in the evaluation of pharmacological treatment in atherosclerosis. HDL-based strategies designed to promote macrophage RCT are a major area of current drug discovery and development for atherosclerotic diseases. A better understanding of the functionality of HDL and its relationship with atherosclerosis will expand our knowledge of the role of HDL in lipid metabolism, holding promise for a future successful HDL-based therapy.

Liver X receptor activation enhances CVB3 viral replication during myocarditis by stimulating lipogenesis

AIMS

Viral myocarditis (VM) is severe cardiac inflammation that can result in sudden death or congestive heart failure in previously healthy adults, with no effective therapy. Liver X receptor (LXR) agonists have both anti-inflammatory and lipid-lowering properties. This study investigates whether LXR agonist T0901317 may modulate viral replication and cardiac inflammation during VM.

METHODS AND RESULTS

(i) Adult mice were administered T0901317 or vehicle with the onset of inflammation during CVB3 virus myocarditis or (ii) treated 2 days prior to CVB3 infection. Against what we expected, T0901317 treatment did not alter leucocyte infiltration after CVB3 infection; yet pre-administration with T0901317 resulted in increased mortality upon CVB3 infection, higher cardiac viral presence, and increased cardiomyocyte damage when compared with the vehicle. Furthermore, we show a correlation of fatty acid synthase (FAS) and sterol regulatory element-binding protein 1c (SREBP-1c) with CVB3 viral load in the heart and that T0901317 is able to enhance the cardiac expression of FAS and SREBP-1c. Finally, we show in vitro that T0901317 is able to exaggerate CVB3-mediated damage of Vero cells, whereas inhibitors of FAS and the SREBP-1c reduce the viral presence of CVB3 in neonatal cardiomyocytes.

CONCLUSION

LXR agonism does not modulate cardiac inflammation, but exacerbates virus-mediated myocardial damage during VM by stimulating lipid biosynthesis and enhancing CVB3 replication.

The Hexane Fraction of Cyperus rotundus Prevents Non-Alcoholic Fatty Liver Disease Through the Inhibition of Liver X Receptor α-Mediated Activation of Sterol Regulatory Element Binding Protein-1c

The goals of this study were (1) to examine the effects of Cyperus rotundus (CR) rhizome on cellular lipogenesis and non-alcoholic/diet-induced fatty liver disease, and (2) to elucidate the molecular mechanism behind its actions. The present investigation showed that the hexane fraction of CR rhizome (CRHF) reduced the elevated transcription levels of sterol regulatory element binding protein-1c (SREBP-1c) in primary hepatocytes following exposure to the liver X receptor α (LXRα) agonist. The SREBP-1c gene is a master regulator of lipogenesis and a key target of LXRα. CRHF inhibited not only the LXRα-dependent activation of the synthetic LXR response element (LXRE) promoter, but also the activation of the natural SREBP-1c promoter. Moreover, CRHF decreased (a) the recruitment of RNA polymerase II to the LXRE of the SREBP-1c gene; (b) the LXRα-dependent up-regulation of various lipogenic genes; and (c) the LXRα-mediated accumulation of triglycerides in primary hepatocytes. Furthermore, CRHF ameliorated fatty liver disease and reduced the expression levels of hepatic lipogenic genes in high sucrose diet (HSD)-fed mice. Interestingly, CRHF did not affect the expression of ATP-binding cassette transporter A1, another important LXR target gene that is required for reverse cholesterol transport (RCT) and protects against atherosclerosis. Taken together, these results suggest that CRHF might be a novel therapeutic remedy for fatty liver disease through the selective inhibition of the lipogenic pathway.

Low folate and selenium in the mouse maternal diet alters liver gene expression patterns in the offspring after weaning

During pregnancy, selenium (Se) and folate requirements increase, with deficiencies linked to neural tube defects (folate) and DNA oxidation (Se). This study investigated the effect of a high-fat diet either supplemented with (diet H), or marginally deficient in (diet L), Se and folate. Pregnant female mice and their male offspring were assigned to one of four treatments: diet H during gestation, lactation and post-weaning; diet L during gestation, lactation and post-weaning; diet H during gestation and lactation but diet L fed to offspring post-weaning; or diet L during gestation and lactation followed by diet H fed to offspring post-weaning. Microarray and pathway analyses were performed using RNA from colon and liver of 12-week-old male offspring. Gene set enrichment analysis of liver gene expression showed that diet L affected several pathways including regulation of translation (protein biosynthesis), methyl group metabolism, and fatty acid metabolism; this effect was stronger when the diet was fed to mothers, rather than to offspring. No significant differences in individual gene expression were observed in colon but there were significant differences in cell cycle control pathways. In conclusion, a maternal low Se/folate diet during gestation and lactation has more effects on gene expression in offspring than the same diet fed to offspring post-weaning; low Se and folate in utero and during lactation thus has persistent metabolic effects in the offspring.

Direct intracerebral delivery of a miR-33 antisense oligonucleotide into mouse brain increases brain ABCA1 expression. [Corrected]

The ATP-binding cassette transporter A1 (ABCA1) is a membrane bound protein that serves to efflux cholesterol and phospholipids onto lipid poor apolipoproteins during HDL biogenesis. Increasing the expression and activity of ABCA1 have beneficial effects in experimental models of various neurologic and cardiovascular diseases including Alzheimer's disease. Despite the beneficial effects of liver X receptor (LXR) agonists--compounds that increase ABCA1 expression--in preclinical studies, their therapeutic utility is limited by systemic adverse effects on lipid metabolism. Interestingly, microRNA-33 (miR-33) inhibition increases ABCA1 expression and activity in rodents and non-human primates without severe metabolic adverse effects. Herein, we demonstrate that treatment of cultured mouse neurons, astrocytes and microglia with an antisense oligonucleotide (ASO) targeting miR-33 increased ABCA1 expression, which was accompanied by increased cholesterol efflux and apoE secretion in astrocytic cultures. We also show that intracerebral delivery of an ASO targeting miR-33 leads to increased ABCA1 expression in cerebral cortex or subcortical structures such as hippocampus. These findings highlight an effective strategy for increasing brain ABCA1 expression/activity for relevant mechanistic studies. [Corrected]

The PPAR-γ antagonist GW9662 elicits differentiation of M2c-like cells and upregulation of the MerTK/Gas6 axis: a key role for PPAR-γ in human macrophage polarization

Background

The nuclear receptors PPAR-γ and LXRs regulate macrophage lipid metabolism and macrophage mediated inflammation. We examined the influence of these molecules on macrophage alternative activation, with particular focus on differentiation of “M2c” anti-inflammatory cells.

Methods

We cultured human monocytes in M0, M1, M2a or M2c macrophage differentiating conditions, in the presence or absence of PPAR-γ and LXR ligands. Flow cytometry was used to analyze membrane expression of phenotypic markers. Basal and LPS-stimulated production of soluble mediators was measured by ELISA. Efferocytosis assays were performed by coincubating monocytes/macrophages with apoptotic neutrophils.

Results

We found that PPAR-γ inhibition, using the PPAR-γ antagonist GW9662, elicits differentiation of M2c-like (CD206⁺ CD163⁺ CD16⁺) cells and upregulation of the MerTK/Gas6 axis. Exposure of differentiating macrophages to IFN-γ, GM-CSF or LPS (M1 conditions), however, hampers GW9662 induction of MerTK and Gas6. When macrophages are differentiated with IL-4 (M2a conditions), addition of GW9662 results into an M2a (CD206⁺ CD209⁺ CD163⁻ MerTK⁻) to M2c (CD206^high CD209⁻ CD163⁺ MerTK⁺) polarization shift. Conversely, in the presence of dexamethasone (M2c conditions), the PPAR-γ agonist rosiglitazone attenuates CD163 and MerTK upregulation. The LXR agonist T0901317 induces MerTK independently of M2c polarization; indeed, CD206, CD163 and CD16 are downregulated. GW9662-differentiated M2c-like cells secrete high levels of Gas6 and low amounts of TNF-α and IL-10, mimicking dexamethasone effects in vitro. However, unlike conventional M2c cells, GW9662-differentiated cells do not show enhanced efferocytic ability.

Conclusions

Our results provide new insights into the role of PPAR-γ and LXR receptors in human macrophage activation and reveal the existence of different patterns regulating MerTK expression. Unexpectedly, PPAR-γ appears to negatively control the expansion of a discrete subset of M2c-like anti-inflammatory macrophages.

Inhibition of lipopolysaccharide-induced gene expression by liver X receptor ligands in macrophages involves interference with early growth response factor 1

Liver X receptors (LXRs) are nuclear receptors that act as ligand-dependent transcription factors forming permissive heterodimers with retinoid X receptors (RXRs). In this study we aimed to assess the effect of LXR/RXR activation on the transcriptional induction of pro-inflammatory genes including cyclooxygenase-2 (COX-2) and microsomal prostaglandin E2 synthase-1 (mPGES-1) in activated macrophages. Our study shows that LXR ligands such as oxysterols, GW3965 or TO901317, as well as RXR ligands like 9cis retinoic acid or SR11237, decreased LPS-induced expression of COX-2 and mPGES-1. Consequently, LPS-dependent PGE2 production was substantially reduced in macrophages treated with LXR/RXR ligands. The inhibitory effects of LXR/RXR activation on LPS-induced expression of COX-2 and mPGES-1 in macrophages, occurred by a mechanism involving interference with transcriptional activation of these genes. LXR/RXR activation interfered with the activity of transcription factors essential in the up-regulation of the expression of pro-inflammatory genes in these cells, such as NFκB, but also Egr-1, which had not been previously associated with LXR-mediated gene repression. As this transcription factor is involved in the regulation of a variety of genes involved in inflammatory processes, LXR and RXR-mediated interference with Egr-1 signaling could represent an important event mediating the anti-inflammatory effects of these receptors in macrophages.

Regulation of Hepatic Cholesteryl Ester Transfer Protein Expression and Reverse Cholesterol Transport by Inhibition of DNA Topoisomerase II

Cholesteryl ester transfer protein (CETP) transfers cholesteryl esters from high density lipoprotein to triglyceride-rich lipoproteins. CETP expression can be transcriptionally activated by liver X receptor (LXR). Etoposide and teniposide are DNA topoisomerase II (Topo II) inhibitors. Etoposide has been reported to inhibit atherosclerosis in rabbits with un-fully elucidated mechanisms. In this study we determined if Topo II activity can influence cholesterol metabolism by regulating hepatic CETP expression. Inhibition of Topo II by etoposide, teniposide, or Topo II siRNA increased CETP expression in human hepatic cell line, HepG2 cells, which was associated with increased CETP secretion and mRNA expression. Meanwhile, inhibition of LXR expression by LXR siRNA attenuated induction of CETP expression by etoposide and teniposide. Etoposide and teniposide induced LXRα expression and LXRα/β nuclear translocation while inhibiting expression of receptor interacting protein 140 (RIP140), an LXR co-repressor. In vivo, administration of teniposide moderately reduced serum lipid profiles, induced CETP expression in the liver, and activated reverse cholesterol transport in CETP transgenic mice. Our study demonstrates a novel function of Topo II inhibitors in cholesterol metabolism by activating hepatic CETP expression and reverse cholesterol transport.