PLK1 promotes cholesterol efflux and alleviates atherosclerosis by up-regulating ABCA1 and ABCG1 expression via the AMPK/PPARγ/LXRα pathway

Polo-like kinase 1 (PLK1) is a serine/threonine kinase involving lipid metabolism and cardiovascular disease. However, its role in atherogenesis has yet to be determined. The aim of this study was to observe the impact of PLK1 on macrophage lipid accumulation and atherosclerosis development and to explore the underlying mechanisms. We found a significant reduction of PLK1 expression in lipid-loaded macrophages and atherosclerosis model mice. Lentivirus-mediated overexpression of PLK1 promoted cholesterol efflux and inhibited lipid accumulation in THP-1 macrophage-derived foam cells. Mechanistic analysis revealed that PLK1 stimulated the phosphorylation of AMP-activated protein kinase (AMPK), leading to activation of the peroxisome proliferator-activated receptor γ (PPARγ)/liver X receptor α (LXRα) pathway and up-regulation of ATP binding cassette transporter A1 (ABCA1) and ABCG1 expression. Injection of lentiviral vector expressing PLK1 increased reverse cholesterol transport, improved plasma lipid profiles and decreased atherosclerotic lesion area in apoE-deficient mice fed a Western diet. PLK1 overexpression also facilitated AMPK and HSL phosphorylation and enhanced the expression of PPARγ, LXRα, ABCA1, ABCG1 and LPL in the aorta. In summary, these data suggest that PLK1 inhibits macrophage lipid accumulation and mitigates atherosclerosis by promoting ABCA1- and ABCG1-dependent cholesterol efflux via the AMPK/PPARγ/LXRα pathway.     

Neopterin negatively regulates expression of ABCA1 and ABCG1 by the LXRα signaling pathway in THP-1 macrophage-derived foam cells

To investigate the effects of neopterin on ABCA1 expression and cholesterol efflux in human THP-1 macrophage-derived foam cells, and to explore the role of the liver X receptor alpha (LXRα) involved. In the present study, THP-1 cells were pre-incubated with ox-LDL to become foam cells. The protein and mRNA expression were examined by Western blot assays and real-time quantitative PCR, respectively. Liquid scintillation counting and high performance liquid chromatography assays were used to test cellular cholesterol efflux and cholesterol content. Neopterin decreased ABCA1 expression and cholesterol efflux in a time- and concentration-dependent manner in THP-1 macrophage-derived foam cells, and the LXRα siRNA can reverse the inhibitory effects induced by neopterin. Neoterin has a negative regulation on ABCA1 expression via the LXRα signaling pathway, which suggests the aggravated effects of neopterin on atherosclerosis.     

Arctigenin promotes cholesterol efflux from THP-1 macrophages through PPAR-γ/LXR-α signaling pathway

Cholesterol efflux from macrophages is a critical mechanism to prevent the development of atherosclerosis. Here, we sought to investigate the effects of arctigenin, a bioactive component of Arctium lappa, on the cholesterol efflux in oxidized low-density lipoprotein (oxLDL)-loaded THP-1 macrophages. Our data showed that arctigenin significantly accelerated apolipoprotein A-I- and high-density lipoprotein-induced cholesterol efflux in both dose- and time-dependent manners. Moreover, arctigenin treatment enhanced the expression of ATP binding cassette transporter A1 (ABCA1), ABCG1, and apoE, all of which are key molecules in the initial step of cholesterol efflux, at both mRNA and protein levels. Arctigenin also caused a concentration-dependent elevation in the expression of peroxisome proliferator-activated receptor-gamma (PPAR-γ) and liver X receptor-alpha (LXR-α). The arctigenin-mediated induction of ABCA1, ABCG1, and apoE was abolished by specific inhibition of PPAR-γ or LXR-α using small interfering RNA technology. Our results collectively indicate that arctigenin promotes cholesterol efflux in oxLDL-loaded THP-1 macrophages through upregulation of ABCA1, ABCG1 and apoE, which is dependent on the enhanced expression of PPAR-γ and LXR-α.     

The marine-derived furanone reduces intracellular lipid accumulation in vitro by targeting LXRα and PPARα

Recent studies have demonstrated that commercially available lipid-lowering drugs cause various side effects; therefore, searching for anti-hyperlipidaemic compounds with lower toxicity is a research hotspot. This study was designed to investigate whether the marine-derived compound, 5-hydroxy-3-methoxy-5-methyl-4-butylfuran-2(5H)-one, has an anti-hyperlipidaemic activity, and the potential underlying mechanism in vitro. Results showed that the furanone had weaker cytotoxicity compared to positive control drugs. In RAW 264.7 cells, the furanone significantly lowered ox-LDL-induced lipid accumulation (~50%), and its triglyceride (TG)-lowering effect was greater than that of liver X receptor (LXR) agonist T0901317. In addition, it significantly elevated the protein levels of peroxisome proliferator-activated receptors (PPARα) and ATP-binding cassette (ABC) transporters, which could be partially inhibited by LXR antagonists, GSK2033 and SR9243. In HepG2 cells, it significantly decreased oleic acid-induced lipid accumulation, enhanced the protein levels of low-density lipoprotein receptor (LDLR), ABCG5, ABCG8 and PPARα, and reduced the expression of sterol regulatory element-binding protein 2 (~32%). PPARα antagonists, GW6471 and MK886, could significantly inhibit the furanone-induced lipid-lowering effect. Furthermore, the furanone showed a significantly lower activity on the activation of the expression of lipogenic genes compared to T0901317. Taken together, the furanone exhibited a weak cytotoxicity but had powerful TC- and TG-lowering effects most likely through targeting LXRα and PPARα, respectively. These findings indicate that the furanone has a potential application for the treatment of dyslipidaemia.     

Regulation of chemokine and chemokine receptor expression by PPARγ in adipocytes and macrophages

Background

PPARγ plays a key role in adipocyte biology, and Rosiglitazone (Rosi), a thiazolidinedione (TZD)/PPARγ agonist, is a potent insulin-sensitizing agent. Recent evidences demonstrate that adipose tissue inflammation links obesity with insulin resistance and that the insulin-sensitizing effects of TZDs result, in part, from their anti-inflammatory properties. However the underlying mechanisms are unclear.

Methodology and Principal Findings

In this study, we establish a link between free fatty acids (FFAs) and PPARγ in the context of obesity-associated inflammation. We show that treatment of adipocytes with FFAs, in particular Arachidonic Acid (ARA), downregulates PPARγ protein and mRNA levels. Furthermore, we demonstrate that the downregulation of PPARγ by ARA requires the activation the of Endoplamsic Reticulum (ER) stress by the TLR4 pathway. Knockdown of adipocyte PPARγ resulted in upregulation of MCP1 gene expression and secretion, leading to enhanced macrophage chemotaxis. Rosi inhibited these effects. In a high fat feeding mouse model, we show that Rosi treatment decreases recruitment of proinflammatory macrophages to epididymal fat. This correlates with decreased chemokine and decreased chemokine receptor expression in adipocytes and macrophages, respectively.

Conclusions and Significance

In summary, we describe a novel link between FAs, the TLR4/ER stress pathway and PPARγ, and adipocyte-driven recruitment of macrophages. We thus both describe an additional potential mechanism for the anti-inflammatory and insulin-sensitizing actions of TZDs and an additional detrimental property associated with the activation of the TLR4 pathway by FA.     

25-Hydroxycholesterol-3-sulfate attenuates inflammatory response via PPARγ signaling in human THP-1 macrophages

The nuclear receptor peroxisome proliferator-activated receptors (PPARs) are important in regulating lipid metabolism and inflammatory responses in macrophages. Activation of PPARγ represses key inflammatory response gene expressions. Recently, we identified a new cholesterol metabolite, 25-hydroxycholesterol-3-sulfate (25HC3S), as a potent regulatory molecule of lipid metabolism. In this paper, we report the effect of 25HC3S and its precursor 25-hydroxycholesterol (25HC) on PPARγ activity and on inflammatory responses. Addition of 25HC3S to human macrophages markedly increased nuclear PPARγ and cytosol IκB and decreased nuclear NF-κB protein levels. PPARγ response element reporter gene assays showed that 25HC3S significantly increased luciferase activities. PPARγ competitor assay showed that the K(i) for 25HC3S was ～1 μM, similar to those of other known natural ligands. NF-κB-dependent promoter reporter gene assays showed that 25HC3S suppressed TNFα-induced luciferase activities only when cotransfected with pcDNAI-PPARγ plasmid. In addition, 25HC3S decreased LPS-induced expression and release of IL-1β. In the PPARγ-specific siRNA transfected macrophages or in the presence of PPARγ-specific antagonist, 25HC3S failed to increase IκB and to suppress TNFα and IL-1β expression. In contrast to 25HC3S, its precursor 25HC, a known liver X receptor ligand, decreased nuclear PPARγ and cytosol IκB and increased nuclear NF-κB protein levels. We conclude that 25HC3S acts in macrophages as a PPARγ ligand and suppresses inflammatory responses via the PPARγ/IκB/NF-κB signaling pathway.     

Chlamydia pneumoniae disturbs cholesterol homeostasis in human THP-1 macrophages via JNK-PPARγ dependent signal transduction pathways

Chlamydia pneumoniae (C. pneumoniae) induces macrophage-derived foam cell formation, a hallmark of early atherosclerosis, in the presence of low density lipoprotein (LDL). However, its mechanisms have yet to be elucidated. In this study we examined the effects of live, heat-killed and UV-inactivated C. pneumoniae on cholesterol metabolism in THP-1-derived macrophages and the role of c-Jun NH₂ terminal kinase (JNK), which may participate in the C. pneumoniae-induced disruption of intracellular cholesterol homeostasis. We investigated whether SP600125, a special JNK inhibitor, affects the expression of peroxisome proliferator-activated receptor gamma (PPARγ), and also its downstream target genes Acyl-CoA cholesterol acyltransferase-1 (ACAT1), ATP-binding cassette transporter A1 and G1 (ABCA1/G1) in human THP-1 macrophages infected with C. pneumoniae. In this paper we found that both live and inactivated C. pneumoniae infection induce intracellular cholesterol accumulation and foam cell formation. C. pneumoniae infection increased the expression of ACAT1 and decreased the expression of ABCA1/G1, all of which facilitated cholesterol accumulation and promoted macrophage-derived foam cell formation. However, these responses were attenuated by SP600125 in a dose-dependent manner. These results demonstrate for the first time that both live and inactivated C. pneumoniae infections disturb cholesterol homeostasis in human THP-1 macrophages and C. pneumoniae infection disturbs cholesterol homeostasis via JNK-PPARγ dependent signal transduction pathways.     

PPARα and PPARγ regulate the nucleoside transporter hENT1

Human equilibrative nucleoside transporter 1 (hENT1) is an important determinant for nucleoside analog based chemotherapy success. Preliminary data suggest hENT1 regulation by PPARs. Using A2780 cells, we investigated the role of PPARs on hENT1 expression and activity. PPARα and PPARγ agonists, Wy14,643 and RGZ, increased hENT1 expression, but only PPARα activation or overexpression resulted in higher hENT1 transport activity. On the other hand, promoter analysis showed two putative PPRE in hENT1 promoter and luciferase-coupled promoter constructs were generated and analyzed. Our results suggest that PPARα-but not PPARγ-mediated expression regulation of hENT1 is PPRE-dependent. In conclusion, PPARα and PPARγ activation modulate hENT1 expression.     

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

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

PPARγ and LXR Signaling Inhibit Dendritic Cell-Mediated HIV-1 Capture and trans-Infection

Dendritic cells (DCs) contribute to human immunodeficiency virus type 1 (HIV-1) transmission and dissemination by capturing and transporting infectious virus from the mucosa to draining lymph nodes, and transferring these virus particles to CD4+ T cells with high efficiency. Toll-like receptor (TLR)-induced maturation of DCs enhances their ability to mediate trans-infection of T cells and their ability to migrate from the site of infection. Because TLR-induced maturation can be inhibited by nuclear receptor (NR) signaling, we hypothesized that ligand-activated NRs could repress DC-mediated HIV-1 transmission and dissemination. Here, we show that ligands for peroxisome proliferator-activated receptor gamma (PPARγ) and liver X receptor (LXR) prevented proinflammatory cytokine production by DCs and inhibited DC migration in response to the chemokine CCL21 by preventing the TLR-induced upregulation of CCR7. Importantly, PPARγ and LXR signaling inhibited both immature and mature DC-mediated trans-infection by preventing the capture of HIV-1 by DCs independent of the viral envelope glycoprotein. PPARγ and LXR signaling induced cholesterol efflux from DCs and led to a decrease in DC-associated cholesterol, which has previously been shown to be required for DC capture of HIV-1. Finally, both cholesterol repletion and the targeted knockdown of the cholesterol transport protein ATP-binding cassette A1 (ABCA1) restored the ability of NR ligand treated cells to capture HIV-1 and transfer it to T cells. Our results suggest that PPARγ and LXR signaling up-regulate ABCA1-mediated cholesterol efflux from DCs and that this accounts for the decreased ability of DCs to capture HIV-1. The ability of NR ligands to repress DC mediated trans-infection, inflammation, and DC migration underscores their potential therapeutic value in inhibiting HIV-1 mucosal transmission.     

Upregulation of miR-181a impairs lipid metabolism by targeting PPARα expression in nonalcoholic fatty liver disease

Recent studies have reported elevated expression of miR-181a in patients with non-alcoholic fatty liver disease (NAFLD), suggesting that it may play an important role in liver lipid metabolism and insulin resistance. We aimed to investigate the effect of miR-181a in lipid metabolism and find new treatments for NAFLD. The expression level of miR-181a in NAFLD patient serum and a palmitic acid (PA)-induced NAFLD cell model was examined by Q-PCR. Oil red O staining and triglyceride assays were used to assess lipid accumulation in hepatocytes. Western blotting was used to detect the protein expression levels of peroxisome proliferator-activated receptor-α (PPARα) and the fatty acid β-oxidation-related genes. Direct interactions were validated by dual-luciferase reporter gene assays. MiR-181a expression was significantly upregulated in the serum of NAFLD patients and PA-induced hepatocytes. Inhibition of miR-181a expression resulted in the increased expression of PPARα and its downstream genes, and PA-induced lipid accumulation in hepatocytes was also inhibited. Upregulation of miR-181a resulted in the downregulation of its direct target PPARα and downstream gene expression of PPARα as well as aggravated lipid accumulation in hepatocytes. At the same time, the increased expression of PPARα can offset lipid accumulation in hepatocytes induced by miR-181a mimics. This study demonstrates that reducing the expression of miR-181a may improve lipid metabolism in NAFLD. The downregulation of miR-181a expression can be a therapeutic strategy for NAFLD by modulating its target PPARα.     

Glucose regulates LXRα subcellular localization and function in rat pancreatic β-cells

Liver X receptors （LXRs） are members of the nuclear receptor superfamily, which have been implicated in lipid homeostasis and more recently in glucose metabolism. Here, we show that glucose does not change LXRα protein level, but affects its localization in pancreatic β-cells. LXRα is found in the nucleus at 8 mM glucose and in the cytoplasm at 4.2 mM. Addition of glucose translocates LXRα from the cytoplasm into the nucleus. Moreover, after the activation of LXR by its synthetic non-steroidal agonist （T0901317）, insulin secretion and glucose uptake are increased at 8 mM and decreased at 4.2 mM glucose in a dose-dependent manner. Furthermore, at low glucose condition, okadaic acid reversed LXRα effect on insulin secretion, suggesting the involvement of glucose signaling through a phosphorylation-dependent mechanism.     

microRNA-210 negatively regulates LPS-induced production of proinflammatory cytokines by targeting NF-κB1 in murine macrophages

Ligation of TLR4 with LPS in macrophages leads to the production of proinflammatory cytokines, which are central to eliminate viral and bacterial infection. However, uncontrolled TLR4 activation may contribute to pathogenesis of inflammatory diseases such as septic shock. In this study, we found microRNA-210 was induced in murine macrophages by LPS. Transfection of miR-210 mimics significantly inhibited LPS-induced production of inflammatory cytokines. In contrast, transfection of anti-miR-210 inhibitors increased LPS-induced expression of proinflammatory cytokines. Furthermore, we demonstrated that miR-210 targets NF-κB1. Therefore, our data identify miR-210 as a very important feedback negative regulator for LPS-induced production of proinflammatory cytokines.