Oxidized Low Density Lipoprotein Blocks Lipopolysaccharide-induced Interferon β Synthesis in Human Macrophages by Interfering with IRF3 Activation

Oxidized Low-Density Lipoprotein Loading of Macrophages Downregulates TLR-Induced Proinflammatory Responses in a Gene-Specific and Temporal Manner through Transcriptional Control

Hypercholesterolemia is a key risk factor for atherosclerosis and leads to the uptake of native and oxidized low-density lipoprotein (oxLDL) by macrophages (Mϕs) and foam cell formation. Inflammatory processes accompany Mϕ foam cell formation in the artery wall, yet the relationship between Mϕ lipid loading and their response to inflammatory stimuli remains elusive. We investigated proinflammatory gene expression in thioglycollate-elicited peritoneal Mϕs, bone marrow-derived Mϕs and dendritic cells, and RAW264.7 cells. Loading with oxLDL did not induce peritoneal Mϕ apoptosis or modulate basal-level expression of proinflammatory genes. Upon stimulation of TLR4, the rapid induction of IFN-β was inhibited in cells loaded with oxLDL, whereas the induction of other proinflammatory genes by TLR4 (LPS), TLR3 (polyriboinosinic-polyribocytidylic acid), TLR2 (Pam₃CSK₄), and TLR9 (CpG) remained comparable within the first 2 h. Subsequently, the expression of a subset of proinflammatory genes (e.g., IL-1β, IL-6, CCL5) was reduced in oxLDL-loaded cells at the level of transcription. This phenomenon was partially dependent on NF erythroid 2-related factor 2 (NRF2) but not on nuclear liver X receptors α and β (LXRα,β), peroxisome proliferator-activated receptor-γ (PPARγ), and activating transcription factor 3 (ATF3). LPS-induced NF-κB reporter activity and intracellular signaling by NF-κB and MAPK pathways were comparable in oxLDL-loaded Mϕs, yet the binding of p65/RelA (the prototypic NF-κB family member) was reduced at IL-6 and CCL5 promoters. This study revealed that oxLDL loading of Mϕs negatively regulates transcription at late stages of TLR-induced proinflammatory gene expression and implicates epigenetic mechanisms such as histone deacetylase activity.

Augmented osteogenic responses in human aortic valve cells exposed to oxLDL and TLR4 agonist: a mechanistic role of Notch1 and NF-κB interaction

Aortic valve calcification causes the progression of calcific aortic valve disease (CAVD). Stimulation of aortic valve interstitial cells (AVICs) with lipopolysaccharide (LPS) up-regulates the expression of osteogenic mediators, and NF-κB plays a central role in mediating AVIC osteogenic responses to Toll-like receptor 4 (TLR4) stimulation. Diseased aortic valves exhibit greater levels of oxidized low-density lipoprotein (oxLDL). This study tested the hypothesis that oxLDL augments the osteogenic responses in human AVICs through modulation of NF-κB and Notch1 activation. AVICs isolated from normal human aortic valves were treated with LPS (0.1 µg/ml), oxLDL (20 µg/ml) or LPS plus oxLDL for 48 h. OxLDL alone increased cellular bone morphogenetic protein-2 (BMP-2) levels while it had no effect on alkaline phosphatase (ALP) levels. Cells exposed to LPS plus oxLDL produced higher levels of BMP-2 and ALP than cells exposed to LPS alone. Further, LPS plus oxLDL induced greater NF-κB activation, and inhibition of NF-κB markedly reduced the expression of BMP-2 and ALP in cells treated with LPS plus oxLDL. OxLDL also induced Notch1 activation and resulted in augmented Notch1 activation when it was combined with LPS. Inhibition of Notch1 cleavage attenuated NF-κB activation induced by LPS plus oxLDL, and inhibition of NF-κB suppressed the expression of BMP-2 and ALP induced by the synergistic effect of Jagged1 and LPS. These findings demonstrate that oxLDL up-regulates BMP-2 expression in human AVICs and synergizes with LPS to elicit augmented AVIC osteogenic responses. OxLDL exerts its effect through modulation of the Notch1-NF-κB signaling cascade. Thus, oxLDL may play a role in the mechanism underlying CAVD progression.

Modelling the interactions between TLR4 and IFNβ pathways

Bacterial lipopolysaccharide (LPS) association with their connate receptor TLR4 triggers Type I interferon signaling cascade through its MyD88 independent downstream. Compared to plethora of reported empirical data on both TLR4 and Type I interferon pathways, there is no known model to decipher crosstalk mechanisms between these two crucial innate immune pathogen activated pathways regulating vital transcriptional factors such as nuclear factor-κB (NFκB), IFNβ, the interferon-stimulated gene factor-3 (ISGF3) and an important cancer drug target protein kinase-R (PKR). Innate immune system is based on a sensitive balance of intricate interactions. In elucidating these interactions, in silico integration of pathways has great potential. Attempts confined to single pathway may not be effective in truly addressing source of real systems behavior. This is the first report combining toll-like receptor-4 (TLR4) and interferon beta (IFNβ) pathways in a single in silico model, analyzing their interactions, pinpointing the source of delay in PKR late phase activity and limiting the transcription of IFN and PKR by using a method including an statistical physics technique in reaction equations. The model quite successfully recapitulates published interferon regulatory factor-3 (IRF3) and IFNβ data from mouse macrophages and PKR data from mouse embryonic fibroblast cell lines. The simulations end up with an estimate of IRF3, IFNβ, ISGF3 dose dependent profiles mimicking nonlinear dose response characteristic of the system. Involvement of concomitant PKR downstream can unravel elusive mechanisms in specific profiles like NFκB regulation.

Protective effects of (-)-epigallocatechin-3-gallate against TNF-α-induced lung inflammation via ROS-dependent ICAM-1 inhibition

Oxidative stresses are considered to play an important role in the induction of cell adhesion molecules and proinflammatory cytokines implicated in inflammatory processes. Heme oxygenase (HO)-1 and suppressors of cytokine signaling (SOCS)-3 exert several biological functions, including antiapoptotic and anti-inflammatory effects. Here, we report that HO-1 and SOCS-3 were induced in A549 cells and human pulmonary alveolar epithelial cells (HPAEpiCs) treated with (-)-epigallocatechin-3-gallate (EGCG). EGCG protected against tumor necrosis factor (TNF)-α-mediated lung inflammation by down-regulation of oxidative stress and intercellular adhesion molecule (ICAM)-1 expression in A549 cells or HPAEpiCs and the lungs of mice. EGCG inhibited TNF-α-induced ICAM-1 expression, THP-1 cells adherence, pulmonary hematoma and leukocyte (eosinophils and neutrophils) count in bronchoalveolar lavage fluid in mice. In addition, EGCG also attenuated TNF-α-induced oxidative stress, p47(phox) translocation, MAPKs activation, and STAT-3 and activating transcription factor (ATF)2 phosphorylation. EGCG also reduced the formation of a TNFR1/TRAF2/Rac1/p47(phox) complex. Moreover, in this study, the observed suppression of TNF-α-stimulated ICAM-1 expression and reactive oxygen species (ROS) generation by EGCG was abrogated by transfection with siRNA of SOCS-3 or HO-1. These results suggested that HO-1 or SOCS-3 functions as a suppressor of TNF-α signaling, not only by inhibiting adhesion molecules expression but also by diminishing intracellular ROS production and STAT-3 and ATF2 activation in A549 cells or HPAEpiCs and the lungs of mice.

The MyD88-independent pathway is not mobilized in human neutrophils stimulated via TLR4

LPS activates both MyD88-dependent and -independent signaling via TLR4, but the extent to which each cascade is operative in different cell types remains unclear. This prompted us to revisit the intriguing issue of CXCL10 production, which we previously showed to be inducible in neutrophils stimulated with LPS and IFN-gamma but not with either stimulus alone, contrary to other myeloid cells. We now report that in neutrophils the MyD88-independent pathway is not activated by LPS. Indeed, microarray and real-time PCR experiments showed that neither IFNbeta nor IFNbeta-dependent genes (including CXCL10) are inducible in LPS-treated neutrophils, in contrast to monocytes. Further investigation into the inability of LPS to promote IFNbeta expression in neutrophils revealed that the transcription factors regulating the IFNbeta enhanceosome, such as IFN-regulatory factor-3 and AP-1, are not activated in LPS-treated neutrophils as revealed by lack of dimerization, nuclear translocation, confocal microscopy, and inducible binding to DNA. Moreover, we show that the upstream TANK-binding kinase-1 is not activated by LPS in neutrophils. A lack of IFNbeta/CXCL10 mRNA expression and IFN-regulatory factor 3 activation was also observed in myeloid leukemia HL60 cells differentiated to granulocytes and then stimulated with LPS, indicating that the inability of neutrophils to activate the MyD88-independent pathway represents a feature of their terminal maturation. These results identify a disconnected activation of the two signaling pathways downstream of TLR4 in key cellular components of the inflammatory and immune responses and help us to better understand the primordial role of neutrophils in host defense against nonviral infections.

Combinatorial pattern recognition receptor signaling alters the balance of life and death in macrophages

Macrophage pattern recognition receptors (PRRs) play key roles in innate immunity, but they also may contribute to disease processes under certain pathological conditions. We recently showed that engagement of the type A scavenger receptor (SRA), a PRR, triggers JNK-dependent apoptosis in endoplasmic reticulum (ER)-stressed macrophages. In advanced atherosclerotic lesions, the SRA, activated JNK, and ER stress are observed in macrophages, and macrophage death in advanced atheromata leads to plaque necrosis. Herein, we show that SRA ligands trigger apoptosis in ER-stressed macrophages by cooperating with another PRR, Toll-like receptor 4 (TLR4), to redirect TLR4 signaling from prosurvival to proapoptotic. Common SRA ligands activate both TLR4 signaling and engage the SRA. The TLR4 effect results in activation of the proapoptotic MyD88-JNK branch of TLR4, whereas the SRA effect silences the prosurvival IRF-3-IFN-beta branch of TLR4. The normal cell-survival effect of LPS-induced TLR4 activation is converted into an apoptosis response by immunoneutralization of IFN-beta, and the apoptosis effect of SRA ligands is converted into a cell-survival response by reconstitution with IFN-beta. Thus, combinatorial signaling between two distinct PRRs results in a functional outcome-macrophage apoptosis that does not occur with either PRR alone. PRR-induced macrophage death may play important roles in advanced atherosclerosis and in other innate immunity-related processes in which the balance between macrophage survival and death is critical.

Toll-like receptor signaling and atherosclerosis

PURPOSE OF REVIEW

Chronic inflammation and disordered lipid metabolism represent hallmarks of atherosclerosis. Considerable evidence has accumulated to suggest that innate immune defense mechanisms might interact with proinflammatory pathways and exacerbate or perhaps even initiate development of arterial plaques. Until recently the preponderance of such evidence has been indirectly emerging from clinical and epidemiologic studies, with some support from experimental animal models of atherosclerosis.

RECENT FINDINGS

Recent data now directly implicate signaling by toll-like receptor 4 and the common adaptor molecule MyD88 in the pathogenesis of atherosclerosis, establishing a key link between atherosclerosis and defense against both foreign pathogens and endogenously generated inflammatory ligands.

SUMMARY

Here we briefly review these and closely related studies, highlighting areas that should provide fertile ground for future studies aimed at a more comprehensive understanding of the interplay between innate immune defense mechanisms, atherosclerosis and related vascular disorders.

Lipopolysaccharide-induced gene expression in murine macrophages is enhanced by prior exposure to oxLDL

Uptake of modified lipoproteins by macrophages results in the formation of foam cells. We investigated how foam cell formation affects the inflammatory response of macrophages. Murine bone marrow-derived macrophages were treated with oxidized LDL (oxLDL) to induce foam cell formation. Subsequently, the foam cells were activated with lipopolysaccharide (LPS), and the expression of lipid metabolism and inflammatory genes was analyzed. Furthermore, gene expression profiles of foam cells were analyzed using a microarray. We found that prior exposure to oxLDL resulted in enhanced LPS-induced tumor necrosis factor (TNF) and interleukin-6 (IL-6) gene expression, whereas the expression of the anti-inflammatory cytokine IL-10 and interferon-beta was decreased in foam cells. Also, LPS-induced cytokine secretion of TNF, IL-6, and IL-12 was enhanced, whereas secretion of IL-10 was strongly reduced after oxLDL preincubation. Microarray experiments showed that the overall inflammatory response induced by LPS was enhanced by oxLDL loading of the macrophages. Moreover, oxLDL loading was shown to result in increased nuclear factor-kappaB activation. In conclusion, our experiments show that the inflammatory response to LPS is enhanced by loading of macrophages with oxLDL. These data demonstrate that foam cell formation may augment the inflammatory response of macrophages during atherogenesis, possibly in an IL-10-dependent manner.

Histone deacetylase inhibitor, trichostatin A induces ubiquitin-dependent cyclin D1 degradation in MCF-7 breast cancer cells

BACKGROUND

Cyclin D1 is an important regulator of G1-S phase cell cycle transition and has been shown to be important for breast cancer development. GSK3beta phosphorylates cyclin D1 on Thr-286, resulting in enhanced ubiquitylation, nuclear export and degradation of the cyclin in the cytoplasm. Recent findings suggest that the development of small-molecule cyclin D1 ablative agents is of clinical relevance. We have previously shown that the histone deacetylase inhibitor trichostatin A (TSA) induces the rapid ubiquitin-dependent degradation of cyclin D1 in MCF-7 breast cancer cells prior to repression of cyclin D1 gene (CCND1) transcription. TSA treatment also resulted in accumulation of polyubiquitylated GFP-cyclin D1 species and reduced levels of the recombinant protein within the nucleus.

RESULTS

Here we provide further evidence for TSA-induced ubiquitin-dependent degradation of cyclin D1 and demonstrate that GSK3beta-mediated nuclear export facilitates this activity. Our observations suggest that TSA treatment results in enhanced cyclin D1 degradation via the GSK3beta/CRM1-dependent nuclear export/26S proteasomal degradation pathway in MCF-7 cells.

CONCLUSION

We have demonstrated that rapid TSA-induced cyclin D1 degradation in MCF-7 cells requires GSK3beta-mediated Thr-286 phosphorylation and the ubiquitin-dependent 26S proteasome pathway. Drug induced cyclin D1 repression contributes to the inhibition of breast cancer cell proliferation and can sensitize cells to CDK and Akt inhibitors. In addition, anti-cyclin D1 therapy may be highly specific for treating human breast cancer. The development of potent and effective cyclin D1 ablative agents is therefore of clinical relevance. Our findings suggest that HDAC inhibitors may have therapeutic potential as small-molecule cyclin D1 ablative agents.

Inhibition of histone deacetylase suppresses osteoclastogenesis and bone destruction by inducing IFN-beta production

Osteoclasts are bone-resorptive multinucleated cells that are differentiated from hemopoietic cell lineages of monocyte/macrophages in the presence of receptor activator of NF-kappaB ligand (RANKL) and M-CSF. Downstream signaling molecules of the receptor of RANKL, RANK, modulate the differentiation and the activation of osteoclasts. We recently found that histone deacetylase inhibitors (HDIs), known as anticancer agents, selectively suppressed osteoclastogenesis in vitro. However, the molecular mechanism underlying inhibitory action of HDIs in osteoclastogenesis and the effect of HDIs on pathological bone destruction are still not remained to be elucidated. In this study, we show that a depsipeptide, FR901228, inhibited osteoclast differentiation by not only suppressing RANKL-induced nuclear translocation of NFATc1 but also increasing the mRNA level of IFN-beta, an inhibitor of osteoclastogenesis. The inhibition of osteoclast formation by FR901228 was abrogated by the addition of IFN-beta-neutralizing Ab. In addition, treatment of adjuvant-induced arthritis in rats revealed that FR901228 inhibited not only disease development in a prophylactic model but also bone destruction in a therapeutic model. Furthermore, immunostaining of the joints of therapeutically treated rats revealed significant production of IFN-beta in synovial cells. Taken together, these data suggest that a HDI inhibits osteoclastogenesis and bone destruction by a novel action to induce the expression of osteoclast inhibitory protein, IFN-beta.

FcepsilonR1 and toll-like receptors mediate synergistic signals to markedly augment production of inflammatory cytokines in murine mast cells

Mast cells mediate both IgE-dependent allergic reactions and protective responses against acute infections, possibly through the activation of Toll-like receptors (TLRs). We find that antigen interacts synergistically with TLR2 and TLR4 ligands to markedly enhance production of cytokines in murine mast cell lines. However, the TLR ligands neither stimulated degranulation and release of arachidonic acid nor influenced such responses to antigen, probably because these ligands failed to generate a necessary calcium signal. The enhanced cytokine production could be attributed to synergistic activation of mitogen-activated protein kinases in addition to the engagement of a more effective repertoire of transcription factors for cytokine gene transcription. The synergistic interactions of TLR ligands and antigen might have relevance to the exacerbation of IgE-mediated allergic diseases by infectious agents.