The Receptor for Advanced Glycation Endproducts (RAGE) Contributes to Severe Inflammatory Liver Injury in Mice

Background: The receptor for advanced glycation end products (RAGE) is a multiligand receptor involved in a number of processes and disorders. While it is known that RAGE-signaling can contribute to toxic liver damage and fibrosis, its role in acute inflammatory liver injury and septic multiorgan failure is yet undefined. We examined RAGE in lipopolysaccharide (LPS) induced acute liver injury of D-galN sensitized mice as a classical model for tumor necrosis factor alpha (TNF-α) dependent inflammatory organ damage.

Methods: Mice (Rage–/– and C57BL/6) were intraperitoneally injected with D-galN (300 mg/kg) and LPS (10 μg/kg). Animals were monitored clinically, and cytokines, damage associated molecular pattern molecules (DAMPs) as well as liver enzymes were determined in serum. Liver histology, hepatic cytokines as well as RAGE mRNA expression were analyzed. Cellular activation and functionality were evaluated by flow cytometry both in bone marrow- and liver-derived cells.

Results: Genetic deficiency of RAGE significantly reduced the mortality of mice exposed to LPS/D-galN. Hepatocyte damage markers were reduced in Rage–/– mice, and liver histopathology was less severe. Rage–/– mice produced less pro-inflammatory cytokines and DAMPs in serum and liver. While immune cell functions appeared normal, TNF-α production by hepatocytes was reduced in Rage–/– mice.

Conclusions: We found that RAGE deletion attenuated the expression of pro-inflammatory cytokines and DAMPs in hepatocytes without affecting cellular immune functions in the LPS/D-galN model of murine liver injury. Our data highlight the importance of tissue-specific RAGE-signaling also in acute inflammatory liver stress contributing to sepsis and multiorgan failure.

H5N1 virus activates signaling pathways in human endothelial cells resulting in a specific imbalanced inflammatory response

H5N1 influenza virus infections in humans cause a characteristic systemic inflammatory response syndrome; however, the molecular mechanisms are largely unknown. Endothelial cells (ECs) play a pivotal role in hyperdynamic septic diseases. To unravel specific signaling networks activated by H5N1 we used a genome-wide comparative systems biology approach analyzing gene expression in human ECs infected with three different human and avian influenza strains of high and low pathogenicity. Blocking of specific signaling pathways revealed that H5N1 induces an exceptionally NF-κB-dependent gene response in human endothelia. Additionally, the IFN-driven antiviral program in ECs is shown to be dependent on IFN regulatory factor 3 but significantly impaired upon H5N1 infection compared with low pathogenic influenza virus. As additional modulators of this H5N1-specific imbalanced gene response pattern, we identified HMGA1 as a novel transcription factor specifically responsible for the overwhelming proinflammatory but not antiviral response, whereas NFATC4 was found to regulate transcription of specifically H5N1-induced genes. We describe for the first time, to our knowledge, defined signaling patterns specifically activated by H5N1, which, in contrast to low pathogenic influenza viruses, are responsible for an imbalance of an overwhelming proinflammatory and impaired antiviral gene program.

Expression and functional characterization of ABCG1 splice variant ABCG1(666)

The ATP-binding cassette transporter ABCG1 mediates the transport of excess cholesterol from macrophages and other cell types to high density lipoprotein (HDL) but not to lipid-depleted apolipoprotein AI. Several splice variants which may have different functions have been identified in mammals. In the current study, we characterized the human splice variant ABCG1(666), which differs from full-length ABCG1(678) by absence of an internal segment of 12 amino acids (VKQTKRLKGLRK). Accordingly spliced ABCG1 transcripts were detected in macrophages and liver in approximately twofold higher amounts than the alternatively spliced ABCG1 form encoding full-length ABCG1. We used transient and stable expression of ABCG1(666) fusion proteins to characterize glycosylation, subcellular localization, molecular interaction and functions of this ABCG1 variant. It could be demonstrated that ABCG1(666) is located at the cell surface and has the ability to form cholesterol transport competent homodimers which affect cellular cholesterol export in a similar manner as previously characterized forms of ABCG1. Our results support that ABCG1(666) may in fact be the most prominent form of functional ABCG1 expressed in the human.

Pharmacological regulation of cholesterol efflux in human monocyte-derived macrophages in the absence of exogenous cholesterol acceptors

Cholesterol efflux from human monocyte-derived macrophages in the absence of exogenous acceptors has been described, but is unclear in mechanism. We investigated this process in relation to the expression of relevant genes, intracellular cholesterol storage and apoE secretion using drugs affecting different aspects of cholesterol metabolism. Both natural (22R-hydroxycholesterol/9-cis-retinoic acid) and synthetic (T0901317 and RO264456) LXR/RXR ligands increased ABCA1 and ABCG1 mRNAs in native macrophages and in cells loaded with acetylated LDL (acLDL). The ACAT inhibitor avasimibe increased only ABCG1 mRNA, whereas no treatment affected apoE mRNA. Avasimibe, progesterone, and natural but not synthetic LXR/RXR ligands prevented cholesterol esterification after acLDL-loading. Cholesterol efflux into acceptor-free medium was increased only by synthetic LXR/RXR ligands and avasimibe in acLDL-loaded cells. ApoE secretion was reduced by drugs affecting cholesterol trafficking but enhanced by LXR/RXR ligands. Incubation with an anti-apoE antibody virtually removed immunodetectable apoE from the medium, significantly increasing cholesterol storage and decreasing efflux. These findings indicate that in human macrophages spontaneous cholesterol efflux: (i) is not necessarily promoted by increasing intracellular free cholesterol, (ii) is increased by compounds that activate ABCA1 and, to a greater extent, ABCG1 and (iii) is only partially correlated with secretion of endogenous apoE, which acted as a cholesterol acceptor.

Cholesterol absorption inhibitor Ezetimibe blocks uptake of oxidized LDL in human macrophages

Ezetimibe belongs to a group of selective and very effective 2-azetidione cholesterol absorption inhibitors which act on the level of cholesterol entry into enterocytes. Recent data indicated that the drug prevents the formation of a heterocomplex consisting of annexin-2 and caveolin-l and leads to specific inhibition of an NPCILI-dependent cholesterol uptake pathway required for uptake of micellar cholesterol into enterocytes. Earlier studies have shown that caveolin-l and annexin-2 are also expressed in human macro-phages and we show in this study that human macrophages express NPC1L1. Moreover in human macrophages, Ezetimibe(SCH58235) and its analogue, SCH354909, are bound to specific cell surface receptors followed by endocytosis via the classical endocytic pathway. SCH58235 had no effect on uptake and/or processing of acetylated LDL (Ac-LDL). In contrast, the compound inhibited uptake of oxidized LDL (Ox-LDL) by -50% in a dose-dependent manner. SCH58235 blocked the lipid-induced induction of LXR/RXR target genes ABCAI, ABCGI, and apolipoprotein E distinctively more effectively in macrophages loaded with Ox-LDL than in those loaded with Ac-LDL. Based on these findings, we presume that the caveolin-l-, annexin-2-, and NPClLI-dependent cholesterol uptake system that is operating in enterocytes may also contribute to class B scavenger receptor-dependent uptake of Ox-LDL in human monocyte-derived macrophages.

ADP-ribosylation factor (ARF)-like 7 (ARL7) is induced by cholesterol loading and participates in apolipoprotein AI-dependent cholesterol export

Here, we identify ADP-ribosylation factor (ARF)-like 7 (ARL7) as the only ARF- and ARL-family member whose mRNA-expression is induced by liver X-receptor/retinoid X-receptor agonists or cholesterol loading in human macrophages. Moreover, subcellular distribution of mutant and wild type ARL7-enhanced green fluorescent protein (EGFP) supports that ARL7 may be involved in a vesicular transport step between a perinuclear compartment and the plasma membrane. Therefore, we investigated the effect of ARL7 over-expression on the cholesterol secretory pathway. We found that expression of wild type and dominant active ARL7-EGFP stimulated the rate of apolipoprotein AI-specific cholesterol efflux 1.7- and 2.8-fold. In contrast, expression of the dominant negative form of ARL7-EGFP led to approximately 50% inhibition of cholesterol efflux. This data is consistent with a model in which ARL7 is involved in transport between a perinuclear compartment and the plasma membrane apparently linked to the ABCA1-mediated cholesterol secretion pathway.

Synergistic DNA damage by oxidative stress (induced by H2O2) and nongenotoxic environmental chemicals in human fibroblasts

Genotoxic combination effects of oxidative stress (induced by H2O2) and eight nongenotoxic environmental chemicals (4-chloroaniline, 2,3,4,6-tetrachlorophenol, lindane, 2,4-dichloroacetic acid (2,4-D), m-xylene, glyphosate, nitrilotriacetic acid and n-hexanol) were determined in human fibroblasts. Genotoxicity was measured quantitatively by the single cell gel electrophoresis assay. The nongenotoxic chemicals were used in non cytotoxic concentrations. H2O2 was used in concentrations producing low (50 microM) and no cytotoxicity (40 microM). All environmental chemicals acted in a synergistic way with H2O2 except DMSO which effectively inhibited H2O(2)-induced DNA damage. The most effective enhancers were 4-chloroaniline, 2,3,4,6-tetrachlorophenol, m-xylene, and n-hexanol. Synergistic effects of hexanol/H2O2 were still evident at a concentration of 0.09 noec (no observed effect concentration). In contrast to synergistic DNA damage in the cell antagonism was found measuring DNA breakage in isolated PM2 DNA. From the results we concluded that synergisms between H2O2 and nongenotoxic chemicals may be a general phenomenon which is not observed on the level of isolated DNA.

The human ABCG4 gene is regulated by oxysterols and retinoids in monocyte-derived macrophages

Here we report the induction of gene expression of ABCG4, a member of the ABC transporter subfamily G, from human macrophages by oxysterols and retinoids, agonists of the nuclear receptors LXR and RXR. The cloned ABCG4 transcript has a size of 3.5 kb and contains an open reading frame which encodes a polypeptide of 646 amino acids. Structurally, the putative ABC transporter protein consists of a nucleotide binding fold followed by a cluster of six transmembrane-spanning domains and thus conforms to the group of half-size ABC transporters. Among the human ABC transporter subfamily G members the novel transporter shows highest protein sequence homology and identity to ABCG1 (84 and 72%, respectively). Analysis of the genomic organization demonstrates that the ABCG4 gene is composed of at least 14 exons which extend across a region of 12.6 kb in size on chromosome 11q23.3. Based on its structural features and an LXR/RXR-responsive regulation similar to the cellular lipid export protein ABCA1, we conclude that ABCG4 may be involved in macrophage lipid homeostasis.

Protection by desferrioxamine and other hydroxamic acids against tetrachlorohydroquinone-induced cyto- and genotoxicity in human fibroblasts

Tetrachlorohydroquinone (TCHQ) has been identified as a major toxic metabolite of the widely used wood preservative pentachlorophenol and has also been implicated in its genotoxicity. We have recently demonstrated that protection by the trihydroxamate iron chelator desferrioxamine (DFO) on TCHQ-induced single-strand breaks in isolated DNA was not the result of its chelation of iron but rather of its efficient scavenging of the reactive tetrachlorosemiquinone (TCSQ) radical. In this study, we extended our research from isolated DNA to human fibroblasts. We found that DFO provided marked protection against both the cyto- and genotoxicity induced by TCHQ in human fibroblasts when it was incubated simultaneously with TCHQ. Pretreatment of the cells with DFO followed by washing also provided marked protection, although less efficiently compared with the simultaneous treatment. Similar patterns of protection were also observed for three other hydroxamic acids (HAs): aceto-, benzo-, and salicylhydroxamic acid. Dimethyl sulfoxide, an efficient hydroxyl radical scavenger, provided only partial protection even at high concentrations. In vitro studies showed that the HAs tested effectively scavenged the reactive TCSQ radical and enhanced the formation of the less reactive and less toxic 2,5-dichloro-3, 6-dihydroxy-1,4-benzoquinone (chloranilic acid). The results of this study demonstrated that the protection provided by DFO and other HAs against TCHQ-induced cyto- and genotoxicity in human fibroblasts is mainly through scavenging of the observed reactive TCSQ radical and not through prevention of the Fenton reaction by the binding of iron in a redox-inactive form.