Signal transducer and activator of transcription 1 is required for optimal foam cell formation and atherosclerotic lesion development

Suppressor of cytokine signaling 1-derived peptide inhibits Janus kinase/signal transducers and activators of transcription pathway and improves inflammation and atherosclerosis in diabetic mice

OBJECTIVE

Activation of Janus kinase/signal transducers and activators of transcription (STAT) pathway by hyperglycemia and dislypidemia contributes to the progression of diabetic complications, including atherosclerosis. Suppressor of cytokine signaling (SOCS) proteins negatively regulate Janus kinase/STAT and have emerged as promising target for anti-inflammatory therapies. We investigated whether a cell-permeable lipopeptide corresponding to the kinase inhibitory region of SOCS1 could reduce atherosclerosis in diabetic mice and identified the mechanisms involved.

APPROACH AND RESULTS

Streptozotocin-induced diabetic apolipoprotein E-deficient mice (aged 8 and 22 weeks) were given intraperitoneal injections of vehicle, SOCS1-derived peptide, or control mutant peptide for 6 to 10 weeks. SOCS1 therapy suppressed STAT1/STAT3 activation in atherosclerotic plaques of diabetic mice and significantly reduced lesion size at both early and advanced stages of lesion development compared with vehicle group. Plaque characterization demonstrated that SOCS1 peptide decreased the accumulation of lipids, macrophages, and T lymphocytes, whereas increasing collagen and smooth muscle cell content. This atheroprotective effect was accompanied by systemic (reduced proinflammatory Ly6C(high) monocytes and splenic cytokine expression) and local (reduced aortic expression of chemokines and cytokines) mechanisms, without impact on metabolic parameters. In vitro, SOCS1 peptide dose dependently inhibited STAT1/STAT3 activation and target gene expression in vascular smooth muscle cells and macrophages and also suppressed cytokine-induced cell migration and adhesion processes.

CONCLUSIONS

SOCS1-based targeting Janus kinase/STAT restrains key mechanisms of atherogenesis in diabetic mice, thereby preventing plaque formation and increasing plaque stability. Approaches to mimic native SOCS1 functions may have a therapeutic potential to retard the progression of diabetic complications.

Adaptive (T and B cells) immunity and control by dendritic cells in atherosclerosis

Chronic inflammation in response to lipoprotein accumulation in the arterial wall is central in the development of atherosclerosis. Both innate and adaptive immunity are involved in this process. Adaptive immune responses develop against an array of potential antigens presented to effector T lymphocytes by antigen-presenting cells, especially dendritic cells. Functional analysis of the role of different T-cell subsets identified the Th1 responses as proatherogenic, whereas regulatory T-cell responses exert antiatherogenic activities. The effect of Th2 and Th17 responses is still debated. Atherosclerosis is also associated with B-cell activation. Recent evidence established that conventional B-2 cells promote atherosclerosis. In contrast, innate B-1 B cells offer protection through secretion of natural IgM antibodies. This review discusses the recent development in our understanding of the role of T- and B-cell subsets in atherosclerosis and addresses the role of dendritic cell subpopulations in the control of adaptive immunity.

Comparative screening and validation as a novel tool to identify STAT-specific inhibitors

Signal transducers and activators of transcription (STATs) facilitate action of cytokines, growth factors and pathogens. STAT activation is mediated by a highly conserved SH2 domain, which interacts with phosphotyrosine (pTyr) motifs for specific STAT-receptor contacts and STAT dimerization. The active dimers induce gene transcription in the nucleus by binding to specific DNA-response elements of target genes. Abnormal activation of STAT signaling pathways is implicated in many human diseases, like cancer, inflammation and auto-immunity. STAT inhibitory strategies mostly focus on inhibiting STAT dimerization using small molecules identified by molecular modeling, virtual or library screening, or natural products. Searches for STAT-targeting compounds, exploring the pTyr-SH2 interaction area, yielded many small molecules for STAT3 but sparsely for other STATs. So far, no STAT-targeting drug is approved by the FDA. Moreover, many of these inhibitors do not seem STAT-specific, thereby questioning the present selection strategies of SH2 domain-based STAT inhibitors. This illustrates the need for better models, and screening and validation tools for more druggable STAT inhibitors with high specificity, potency and excellent bioavailability. Based on newly developed 3D structure models for all human (h)STATs, we propose a pipeline approach that combines comparative in silico docking of STAT-SH2 models with an in vitro STAT phosphorylation assay, as a novel tool to screen multi-million compound libraries and identify specific inhibitors for different STATs. Identification of specific and effective STAT inhibitory compounds could provide a tool to increase our understanding of their functional role in different diseases, and serve as therapeutic strategies in cancer, inflammation and auto-immunity.

Therapeutic modulators of STAT signalling for human diseases

The signal transducer and activator of transcription (STAT) proteins have important roles in biological processes. The abnormal activation of STAT signalling pathways is also implicated in many human diseases, including cancer, autoimmune diseases, rheumatoid arthritis, asthma and diabetes. Over a decade has passed since the first inhibitor of a STAT protein was reported and efforts to discover modulators of STAT signalling as therapeutics continue. This Review discusses the outcomes of the ongoing drug discovery research endeavours against STAT proteins, provides perspectives on new directions for accelerating the discovery of drug candidates, and highlights the noteworthy candidate therapeutics that have progressed to clinical trials.

Pathology caused by persistent murine norovirus infection

Subclinical infection of murine norovirus (MNV) was detected in a mixed breeding group of WT and Stat1(-/-) mice with no outward evidence of morbidity or mortality. Investigations revealed the presence of an attenuated MNV variant that did not cause cytopathic effects in RAW264.7 cells or death in Stat1(-/-) mice. Histopathological analysis of tissues from WT, heterozygous and Stat1(-/-) mice revealed a surprising spectrum of lesions. An infectious molecular clone was derived directly from faeces (MNV-O7) and the sequence analysis confirmed it was a member of norovirus genogroup V. Experimental infection with MNV-O7 induced a subclinical infection with no weight loss in Stat1(-/-) or WT mice, and recapitulated the clinical and pathological picture of the naturally infected colony. Unexpectedly, by day 54 post-infection, 50 % of Stat1(-/-) mice had cleared MNV-O7. In contrast, all WT mice remained infected persistently. Most significantly, this was associated with liver lesions in all the subclinically infected WT mice. These data confirmed that long-term persistence in WT mice is established with specific variants of MNV and that despite a subclinical presentation, active foci of acute inflammation persist within the liver. The data also showed that STAT1-dependent responses are not required to protect mice from lethal infection with all strains of MNV.

Role of renin-angiotensin system in activation of macrophages by modified lipoproteins

Angiotensin II favors the development of atherosclerosis. Our goal was to determine if foam cell formation increases angiotensin II generation by the endogenous renin-angiotensin system (RAS) and if endogenously produced angiotensin II promotes lipid accumulation in macrophages. Differentiated THP-1 cells were treated with acetylated low-density lipoproteins (ac-LDL), native LDL (n-LDL), or no LDL. Expression of RAS genes was assessed and angiotensin I/II levels were quantified in media and cell lysate. Ac-LDL increased angiotensin I/II levels and the angiotensin II/I ratio in cells and media after foam cell formation. Renin mRNA or activity did not change, but renin blockade completely inhibited the increase in angiotensin II. Angiotensinogen mRNA but not protein level was increased. Angiotensin-converting enzyme (ACE) and cathepsin G mRNA and activities were enhanced by ac-LDL. Inhibition of renin, ACE, or the angiotensin II receptor 1 (AT1-receptor) largely abolished cholesteryl ester formation in cells exposed to ac-LDL and decreased scavenger receptor A (SR-A) and acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT-1) protein levels. Inhibition of renin or the AT1-receptor in cells treated with oxidized LDL also decreased SR-A and ACAT-1 protein and foam cell formation. ac-LDL also increased angiotensin II by human peripheral blood monocyte-derived macrophages, whereas blockade of renin decreased cholesterol ester formation in these macrophages. These findings indicate that, during foam cell formation, angiotensin II generation by the endogenous RAS is stimulated and that endogenously generated angiotensin II is crucial for cholesterol ester accumulation in macrophages exposed to modified LDL.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Interference with akt signaling protects against myocardial infarction and death by limiting the consequences of oxidative stress

The intricacy of multiple feedback loops in the pathways downstream of Akt allows this kinase to control multiple cellular processes in the cardiovascular system and precludes inferring consequences of its activation in specific pathological conditions. Akt1, the major Akt isoform in the heart and vasculature, has a protective role in the endothelium during atherosclerosis. However, Akt1 activation may also have detrimental consequences in the cardiovascular system. Mice lacking both the high-density lipoprotein receptor SR-BI (scavenger receptor class B type I) and ApoE (apolipoprotein E), which promotes clearance of remnant lipoproteins, are a model of severe dyslipidemia and spontaneous myocardial infarction. We found that Akt1 was activated in these mice, and this activation correlated with cardiac dysfunction, hypertrophy, and fibrosis; increased infarct area; cholesterol accumulation in macrophages and atherosclerosis; and reduced life span. Akt1 activation was associated with inflammation, oxidative stress, accumulation of oxidized lipids, and increased abundance of CD36, a major sensor of oxidative stress, and these events created a positive feedback loop that exacerbated the consequences of oxidative stress. Genetic deletion of Akt1 in this mouse model resulted in decreased mortality, alleviation of multiple complications of heart disease, and reduced occurrence of spontaneous myocardial infarction. Thus, interference with Akt1 signaling in vivo could be protective and improve survival under dyslipidemic conditions by reducing oxidative stress and responses to oxidized lipids.

Macrophage heterogeneity and cholesterol homeostasis: classically-activated macrophages are associated with reduced cholesterol accumulation following treatment with oxidized LDL

Macrophages are centrally involved during atherosclerosis development and are the predominant cell type that accumulates cholesterol in the plaque. Macrophages however, are heterogeneous in nature reflecting a variety of microenvironments and different phenotypes may be more prone to contribute towards atherosclerosis progression. Using primary human monocyte-derived macrophages, we sought to evaluate one aspect of atherogenic potential of different macrophage phenotypes by determining their propensity to associate with and accumulate oxidized low density lipoprotein (oxLDL). Classically-activated macrophages treated simultaneously with interferon γ (IFNγ) and tumor necrosis factor α (TNFα) associated with less oxLDL and accumulated less cholesterol compared to untreated controls. The combined treatment of IFNγ and TNFα reduced the mRNA expression of CD36 and the expression of both cell surface CD36 and macrophage scavenger receptor 1 (MSR1) protein. Under oxLDL loaded conditions, IFNγ and TNFα did not reduce macrophage protein expression of the transcription factor peroxisome proliferator-actived receptor γ (PPARγ) which is known to positively regulate CD36 expression. However, macrophages treated with IFNγ attenuated the ability of the PPARγ-specific agonist rosiglitazone from upregulating cell surface CD36 protein expression. Our results demonstrate that the observed reduction of cholesterol accumulation in macrophages treated with IFNγ and TNFα following oxLDL treatment was due at least in part to reduced cell surface CD36 and MSR1 protein expression.

Molecular pathways regulating macrophage polarization: implications for atherosclerosis

Recent years have seen a tremendous development of our insight into the biology of atherosclerosis and its acute thrombotic manifestations. Inflammation now takes center stage among traditional risk factors as a decisive factor in cardiovascular risk. Consequently, its assessment and modulation have become key to clinical care and fundamental research alike. Plaque macrophages orchestrate many of the inflammatory processes that occur throughout atherogenesis. These cells are characteristically heterogeneous and adopt diverse activation states in response to micro-environmental triggers. In this review, macrophage-mediated inflammation in atherosclerosis sets the scene for a discussion of the gene regulatory mechanisms that facilitate and shape polarized macrophage phenotypes. When applicable, we consider these factors within the context of atherosclerosis and reflect on opportunities for future application.

Role of macrophage scavenger receptors in atherosclerosis

Accumulating evidence indicates that atherosclerosis is a chronic inflammatory disease. The key innate immune cells that are involved in the pathogenesis of atherosclerosis are circulating monocytes and plaque macrophages. Complex interplay between immune and metabolic processes results in pathological activity of these cells. The best understood pathological process mediated by macrophages is their inability to process modified lipoproteins properly resulting in the formation of foamy cells, which are a dangerous component of atherosclerotic plaques. Key molecules involved in the recognition and processing of modified lipoproteins are scavenger receptors (SR). This is a large family of surface expressed structurally heterogeneous receptors with a broad spectrum of endogenous and exogenous ligands. The common functional feature of SR is internalisation of extracellular components and targeting them for lysosomal degradation. However, these relatively simple functions can have complex consequences, since they are linked to diverse specific signalling pathways and to other membrane transport pathways. Moreover, scavenger receptors can co-operate with other types of receptors increasing the variability of the macrophage response to multiple extracellular ligands. At least some SRs respond to modified lipoproteins by amplification of inflammation and accumulation of macrophages in the plaque, while some SRs may support tolerogenic reactions. Outcome of different SR activities will be the decision of monocytes and macrophage to guard homeostatic balance, support atherosclerosis progression and plaque instability by inflammatory reactions, or support rapid fibrotic processes in the plaque that stabilise it. Despite the accumulating knowledge about the molecular mechanisms of scavenger receptor action, their role in the progression of atherosclerosis remains controversial. The activities of scavenger receptors that can contribute to each of these processes are a subject of current review.

STAT1 as a novel therapeutical target in pro-atherogenic signal integration of IFNγ, TLR4 and IL-6 in vascular disease

Inflammation participates importantly in host defenses against infectious agents and injury, but it also contributes to the pathophysiology of atherosclerosis. Recruitment of blood leukocytes to the injured vascular endothelium characterizes the initiation and progression of atherosclerosis and involves many inflammatory mediators, modulated by cells of both innate and adaptive immunity. The pro-inflammatory cytokine, interferon (IFN)-γ derived from T cells, is vital for both innate and adaptive immunity and is also expressed at high levels in atherosclerotic lesions. As such IFN-γ plays a crucial role in the pathology of atherosclerosis through activation of signal transducer and activator of transcription (STAT) 1. Toll-like receptors (TLRs) are innate immune pattern recognition receptors (PRRs) expressed on a variety of cells, and thus initiate and sustain the inflammatory response in atherosclerosis. More recent studies have revealed that STAT1 is involved in the signaling events mediated by TLR4, leading to increased expression of several pro-inflammatory and pro-atherogenic mediators. By upregulating members of the Suppressors Of Cytokine Signaling (SOCS) family that regulate cellular responsiveness to immune signals, IFNγ and TLR4-activated pathways have also shown to inhibit IL-6 STAT3-dependent anti-inflammatory signaling and potentially shift IL-6 to a STAT1 activating pro-inflammatory cytokine. Consequently, STAT1 has been identified as a point of convergence for the cross-talk between the pro-atherogenic IFN-γ, TLR4 and IL-6 activated pathways in immune as well as vascular cells, as such amplifying pro-inflammatory signals. This results in augmented smooth muscle cell (SMC) and leukocyte migration, leukocyte to endothelial cell (EC) adhesion and foam cell formation, and could encompass a novel mechanism involved in the initiation and progression of atherosclerosis. Therefore, application of small inhibitory compounds that specifically interact with the SH2-phosphotyrosine pocket of STAT1, proposed here as a novel working mechanism for the known STAT1 inhibitor fludarabine, could be a promising tool in the development of a therapeutical strategy for atherosclerosis.

Adaptive immunity in atherosclerosis: mechanisms and future therapeutic targets

Chronic inflammation drives the development of atherosclerosis, and adaptive immunity is deeply involved in this process. Initial studies attributed a pathogenic role to T cells in atherosclerosis, mainly owing to the proatherogenic role of the T-helper (T(H))-1 cell subset, whereas the influence of T(H)2 and T(H)17 subsets is still debated. Today we know that T regulatory cells play a critical role in the protection against atherosclerotic lesion development and inflammation. In contrast to T cells, B cells were initially considered to be protective in atherosclerosis, assumingly through the production of protective antibodies against oxidized LDL. This concept has now been refined and proatherogenic roles of certain mature B cell subsets have been identified. We review the current knowledge about the role of various lymphocyte subsets in the development and progression of atherosclerosis and highlight future targets for immunomodulatory therapy.

ERK is integral to the IFN-γ-mediated activation of STAT1, the expression of key genes implicated in atherosclerosis, and the uptake of modified lipoproteins by human macrophages

The proinflammatory cytokine IFN-gamma is a master regulator of atherosclerosis and mediates its cellular actions mainly through STAT1. Unfortunately, the impact of other IFN-gamma inducible pathways on STAT1 activation and the regulation of downstream responses associated with atherosclerosis in human macrophages are poorly understood and were therefore investigated. In this study, we demonstrate that the IFN-gamma-mediated phosphorylation of STAT1 on Ser(727), crucial for its maximal activity, was attenuated in human macrophages by pharmacological inhibition of ERK. In these cells, IFN-gamma induced changes in the expression of several key genes implicated in atherosclerosis, such as MCP-1, through an ERK-dependent mechanism. Additionally, the IFN-gamma-induced activity of STAT1-responsive promoters was attenuated by transfection of dominant-negative forms of ERK and other key components of this pathway. Furthermore, the IFN-gamma-induced uptake of acetylated and oxidized low-density lipoprotein by human macrophages was attenuated by pharmacological inhibition or RNA interference-mediated knockdown of ERK. These studies suggest a critical role for ERK signaling in the IFN-gamma-mediated changes in macrophage cholesterol homeostasis and gene expression during atherosclerosis.

Loss of STAT1 is associated with increased aortic rupture in an experimental model of aortic dissection and aneurysm formation

BACKGROUND

Transcription factor signal transducer and activator of transcription (STAT) 1 has been linked to a variety of pathologic states involved with matrix remodeling, but its role in aortic pathology has not been previously described. The current study hypothesized that STAT1 regulates aneurysmal degeneration and its role was evaluated in human abdominal aortic aneurysm (AAA) and in a mouse model of aortic dissection.

METHODS

Apolipoprotein E knockout mice (ApoE-/-) or ApoE/STAT1 double knockout mice (ApoE/STAT1-/-) were infused with 1000 ng/kg/min of angiotensin II. Systolic blood pressure (SBP) was measured in the rodent tail. At sacrifice, aortic diameters and extent of aneurysm formation were measured by digital microscopy. STAT1 and phosphorylated-STAT1 protein levels were assessed in ApoE-/- mice at 0, 7, 14, and 28 days (n = 8/time point) by enzyme-linked immunosorbent assay. Histology was performed using hematoxylin and eosin (H&E) and Movat stains. Statistical analyses included chi(2) test, t test, and analysis of variance.

RESULTS

STAT1 messenger RNA and total protein were greater in human AAA vs non-AAA controls. In addition, aneurysms occurred in 8%, 50%, and 80% of ApoE-/- mice at 7, 14, and 28 days, respectively. Total STAT1 levels were not altered during the course of angiotensin II infusion. Phosphorylated STAT1 levels peaked at 7 days with a 1.4-fold increase over baseline (P < .05). Aneurysms occurred in 0%, 100%, and 100% of ApoE/STAT1-/- mice at 3, 5, and 28 days. In mice infused with angiotensin II for >3 days, aortic rupture occurred more frequently in ApoE/STAT-/- mice (53% vs 19%, P < .05) and at earlier time points (4.0 +/- 0.5 vs 9.2 +/- 0.77 days, P < .05) vs ApoE-/- mice. SBP did not differ between the groups during angiotensin II infusion. By 28 days, aneurysms were larger in ApoE/STAT1-/- mice compared with ApoE-/- mice (2.7 +/- 0.4 vs 1.9 +/- 0.1 mm, P < .05) and were more extensive. H&E and Movat stain did not reveal differences in aortic wall structural content at baseline between ApoE-/- and ApoE/STAT1-/- mice. Both groups demonstrated equal disorganization in the aneurysmal state.

CONCLUSIONS

Phosphorylated STAT1 is elevated during aneurysmal degeneration. Its loss is associated with a higher rate of acute aortic rupture and more extensive aneurysms in a mouse model of aortic dissection. Further investigation is necessary to determine whether these observations are secondary to an underlying aortic wall abnormality or alterations in vessel wall matrix remodeling.

Nitro-fatty acids reduce atherosclerosis in apolipoprotein E-deficient mice

OBJECTIVE

Inflammatory processes and foam cell formation are key determinants in the initiation and progression of atherosclerosis. Electrophilic nitro-fatty acids, byproducts of nitric oxide- and nitrite-dependent redox reactions of unsaturated fatty acids, exhibit antiinflammatory signaling actions in inflammatory and vascular cell model systems. The in vivo action of nitro-fatty acids in chronic inflammatory processes such as atherosclerosis remains to be elucidated.

METHODS AND RESULTS

Herein, we demonstrate that subcutaneously administered 9- and 10-nitro-octadecenoic acid (nitro-oleic acid) potently reduced atherosclerotic lesion formation in apolipoprotein E-deficient mice. Nitro-fatty acids did not modulate serum lipoprotein profiles. Immunostaining and gene expression analyses revealed that nitro-oleic acid attenuated lesion formation by suppressing tissue oxidant generation, inhibiting adhesion molecule expression, and decreasing vessel wall infiltration of inflammatory cells. In addition, nitro-oleic acid reduced foam cell formation by attenuating oxidized low-density lipoprotein-induced phosphorylation of signal transducer and activator of transcription-1, a transcription factor linked to foam cell formation in atherosclerotic plaques. Atherosclerotic lesions of nitro-oleic acid-treated animals also showed an increased content of collagen and alpha-smooth muscle actin, suggesting conferral of higher plaque stability.

CONCLUSION

These results reveal the antiatherogenic actions of electrophilic nitro-fatty acids in a murine model of atherosclerosis.

Stat1 nuclear translocation by nucleolin upon monocyte differentiation

BACKGROUND

Members of the signal transducer and activator of transcription (Stat) family of transcription factors traverse the nuclear membrane through a specialized structure, called the nuclear pore complex (NPC), which represents a selective filter for the import of proteins. Karyophilic molecules can bind directly to a subset of proteins of the NPC, collectively called nucleoporins. Alternatively, the transport is mediated via a carrier molecule belonging to the importin/karyopherin superfamily, which transmits the import into the nucleus through the NPC.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we provide evidence for an alternative Stat1 nuclear import mechanism, which is mediated by the shuttle protein nucleolin. We observed Stat1-nucleolin association, nuclear translocation and specific binding to the regulatory DNA element GAS. Using expression of nucleolin transgenes, we found that the nuclear localization signal (NLS) of nucleolin is responsible for Stat1 nuclear translocation. We show that this mechanism is utilized upon differentiation of myeloid cells and is specific for the differentiation step from monocytes to macrophages.

CONCLUSIONS/SIGNIFICANCE

Our data add the nucleolin-Stat1 complex as a novel functional partner for the cell differentiation program, which is uniquely poised to regulate the transcription machinery via Stat1 and nuclear metabolism via nucleolin.

Adenosine blocks IFN-gamma-induced phosphorylation of STAT1 on serine 727 to reduce macrophage activation

Macrophages are activated by IFN-gamma, a proinflammatory and proatherogenic cytokine that mediates its downstream effects primarily through STAT1. IFN-gamma signaling induces phosphorylation of two STAT1 residues: Tyr(701) (Y701), which facilitates dimerization, nuclear translocation, and DNA binding; and Ser(727) (S727), which enables maximal STAT1 transcription activity. Immunosuppressive molecules such as adenosine in the cellular microenvironment can reduce macrophage inflammatory and atherogenic functions through receptor-mediated signaling pathways. We hypothesized that adenosine achieves these protective effects by interrupting IFN-gamma signaling in activated macrophages. This investigation demonstrates that adding adenosine to IFN-gamma-stimulated murine RAW 264.7 and human THP-1 macrophages results in unique modulation of STAT1 serine and tyrosine phosphorylation events. We show that adenosine inhibits IFN-gamma-induced STAT1 S727 phosphorylation by >30% and phosphoserine-mediated transcriptional activity by 58% but has no effect on phosphorylation of Y701 or receptor-associated JAK tyrosine kinases. Inhibition of the adenosine A(3) receptor with a subtype-specific antagonist (MRS 1191 in RAW 264.7 cells and MRS 1220 in THP-1 cells) reverses this adenosine suppressive effect on STAT1 phosphoserine status by 25-50%. Further, RAW 264.7 A(3) receptor stimulation with Cl-IB-MECA reduces IFN-gamma-induced STAT1 transcriptional activity by 45% and STAT1-dependent gene expression by up to 80%. These data suggest that A(3) receptor signaling is key to adenosine-mediated STAT1 modulation and anti-inflammatory action in IFN-gamma-activated mouse and human macrophages. Because STAT1 plays a key role in IFN-gamma-induced inflammation and foam cell transformation, a better understanding of the mechanisms underlying STAT1 deactivation by adenosine may improve preventative and therapeutic approaches to vascular disease.

4-Hydroxynonenal enhances CD36 expression on murine macrophages via p38 MAPK-mediated activation of 5-lipoxygenase

Increased levels of 4-hydroxynonenal (HNE) and 5-lipoxygenase (5-LO) coexist in atherosclerotic lesions but their relationship in atherogenesis is unclear. This study investigated the role of 5-LO in HNE-induced CD36 expression and macrophage foam cell formation, and the link between HNE and 5-LO. In J774A.1 murine macrophages, HNE (10 microM) enhanced CD36 expression in association with an increased uptake of oxLDL, which was blunted by inhibition of 5-LO with MK886, a 5-LO inhibitor, or with 5-LO siRNA. In peritoneal macrophages from 5-LO-deficient mice, HNE-induced CD36 expression was markedly attenuated, confirming a pivotal role of 5-LO in HNE-induced CD36 expression. In an assay for 5-LO activity, stimulation of macrophages with HNE led to increased leukotriene B(4) production in the presence of exogenous arachidonic acid in association with an increased association of 5-LO to the nuclear membrane. Among the mitogen-activated protein kinase (MAPK) pathways involved in 5-LO phosphorylation, HNE predominantly activated p38 MAPK in macrophages, and the p38 MAPK inhibitor SB203580, but not an extracellular signal-regulated kinase inhibitor, suppressed HNE-induced LTB(4) production. Collectively, these data suggest that p38 MAPK-mediated activation of 5-LO by HNE might enhance CD36 expression, consequently leading to the formation of macrophage foam cells.

Interferon gamma: a master regulator of atherosclerosis

Atherosclerosis is a chronic inflammatory disease that is characterized by the development of fibrotic plaques in the arterial wall. The disease exhibits a complex aetiology and its progression is influenced by a number of environmental and genetic risk factors. The cytokine interferon-gamma (IFN-gamma), a key regulator of immune function, is highly expressed in atherosclerotic lesions and has emerged as a significant factor in atherogenesis. Evidence from both mouse models of atherosclerosis and in vitro cell culture has suggested that the role of IFN-gamma is complex since both pro- and anti-atherogenic actions have been affiliated to it. This review will focus on evaluating the contribution of IFN-gamma to atherosclerosis and, in particular, how it regulates immune responses to the disease.

Pravastatin attenuates interferon-gamma action via modulation of STAT1 to prevent aortic atherosclerosis in apolipoprotein E-knockout mice

1. The beneficial effects of pravastatin, beyond that of lowering cholesterol in atherosclerosis, include reducing the action of interferon (IFN)-gamma. Interferon-gamma activates the signal transducer and activator of transcription 1 (STAT1), but it is unclear whether the inhibitory effect of pravastatin in atherosclerosis is via modulation of the IFN-gamma/STAT1 pathway. Thus, the aim of the present study was to determine whether the action of pravastatin in preventing aortic atherosclerosis by attenuation of IFN-gamma action is dependent on STAT1. 2. Male apolipoprotein E-knockout (apoE(-/-)) mice were fed a diet containing 1.25% cholesterol (w/w). Mice were divided into two groups, one of which was supplemented with pravastatin (80 mg/kg per day). Male C57BL/6J mice were fed a normal diet and served as the control group (n = 12 per group). 3. Atherosclerotic lesions in the aortic root were assessed by staining sections haematoxylin and eosin. Serum concentrations of IFN-gamma and IFN-gamma mRNA expression in the thoracoabdominal aorta were determined by ELISA and real-time quantitative polymerase chain reaction methods, respectively. Expression of phosphorylated STAT1 (pSTAT1), interferon regulating factor (IRF)-1 and suppressors of cytokine signalling 1 (SOCS1) was determined in the thoracoabdominal aorta using Western blot analysis. 4. After 8 weeks, pravastatin treatment significantly prevented the formation of atherosclerotic lesions (P < 0.05) and reduced serum IFN-gamma concentrations (P < 0.05) and levels of IFN-gamma mRNA within the aorta (P < 0.01). Pravastatin significantly decreased the expressions of pSTAT1 and IRF-1 within the aorta and significantly increased expression of SOCS1. 5. These results suggest that the actions of pravastatin in attenuating the action of IFN-gamma and subsequently preventing aortic atherosclerosis may depend, at least in part, on modulation of STAT1 activity. This providing us with a new therapeutic approach and a clearer insight into the clinical benefits of pravastatin.

4-hydroxynonenal contributes to macrophage foam cell formation through increased expression of class A scavenger receptor at the level of translation

4-Hydroxynonenal (HNE) is known to be atherogenic, but its mechanism of action in atherogenesis is not clear. Therefore, this study investigated the role of HNE in macrophage foam cell formation and the underlying mechanism involved in HNE-induced expression of scavenger receptors (SRs). In the aortic sinus of ApoE-deficient mice fed a high-fat diet, multiple plaque lesions were accompanied by increased accumulation of HNE adducts in the enhanced Mac-2 stained area. In an in vitro study, HNE exposure to J774A.1 macrophages led to increased expression of class A SR (SR-A) and CD36 at the protein level with a concomitant increase in endocytic uptake of oxLDL. In contrast to CD36 protein expression, which was associated with an increase in mRNA expression, the HNE-enhanced SR-A protein expression was neither accompanied by its mRNA expression nor affected by actinomycin D. HNE enhanced the incorporation rates of (35)S-Met/Cys into SR-A, and HNE-induced SR-A protein expression was effectively attenuated by translation inhibitors such as cycloheximide and rapamycin. Taken together, these data suggest that HNE contributes to macrophage foam cell formation through increased synthesis of SR-A at the level of mRNA translation, consequently leading to the progression of atherosclerosis.

Protein kinase CK2, an important regulator of the inflammatory response?

Casein kinase 2 (CK2) is a highly conserved serine-threonine kinase that uses both adenosine triphosphate and guanosine triphosphate as phosphate donors. This constitutively active and ubiquitously expressed enzyme is often present as a tetrameric holoenzyme complex of two catalytic subunits (alpha and/or alpha') and two regulatory beta subunits. The enzyme is known to phosphorylate more than 300 substrates and controls a wide range of processes, including the regulation of cell cycle, apoptosis, transformation, and circadian rhythm. Several lines of recent evidence also suggest a potentially important role for CK2 in the control of the inflammatory response. This review will give an overview of CK2 and its regulation and describe the evidence implicating its role in inflammation.

Signal transducer and activator of transcription-1 is critical for apoptosis in macrophages subjected to endoplasmic reticulum stress in vitro and in advanced atherosclerotic lesions in vivo

BACKGROUND

Macrophage apoptosis is a critical process in the formation of necrotic cores in vulnerable atherosclerotic plaques. In vitro and in vivo data suggest that macrophage apoptosis in advanced atheromata may be triggered by a combination of endoplasmic reticulum stress and engagement of the type A scavenger receptor, which together induce death through a rise in cytosolic calcium and activation of toll-like receptor-4.

METHODS AND RESULTS

Using both primary peritoneal macrophages and studies in advanced atheromata in vivo, we introduce signal transducer and activator of transcription-1 (STAT1) as a critical and necessary component of endoplasmic reticulum stress/type A scavenger receptor-induced macrophage apoptosis. We show that STAT1 is serine phosphorylated in macrophages subjected to type A scavenger receptor ligands and endoplasmic reticulum stress in a manner requiring cytosolic calcium, calcium/calmodulin-dependent protein kinase II, and toll-like receptor-4. Remarkably, apoptosis was inhibited by approximately 80% to 90% (P<0.05) by STAT1 deficiency or calcium/calmodulin-dependent protein kinase II inhibition. In vivo, nuclear Ser-P-STAT1 was found in macrophage-rich regions of advanced murine and human atheromata. Most important, macrophage apoptosis was decreased by 61% (P=0.034) and plaque necrosis by 34% (P=0.02) in the plaques of fat-fed low density lipoprotein receptor null Ldlr-/- mice transplanted with Stat1-/- bone marrow.

CONCLUSIONS

STAT1 is critical for endoplasmic reticulum stress/type A scavenger receptor-induced apoptosis in primary tissue macrophages and in macrophage apoptosis in advanced atheromata. These findings suggest a potentially important role for STAT1-mediated macrophage apoptosis in atherosclerotic plaque progression.

Absence of CD36 protects against atherosclerosis in ApoE knock-out mice with no additional protection provided by absence of scavenger receptor A I/II

AIMS

The role of scavenger receptors in atherogenesis is controversial as a result of conflicting reports and a recent hypothesis suggesting that scavenger receptor absence would enhance the pro-inflammatory, pro-atherogenic milieu. This study addresses the effect of combined absence of scavenger receptors CD36 and SRA I/II on atherosclerosis lesion development in the apolipoprotein E knock-out (apoE degrees ) model.

METHODS

We created background-related strains of apoE degrees , scavenger receptor A I/II knock-out (SRA degrees )/apoE degrees , CD36 knock-out (CD36 degrees )/apoE degrees , and CD36 degrees /SRA degrees /apoE degrees mice that were >99% C57Bl/6. Four-week-old mice were fed a Western diet for 12 weeks and were assessed for lesion burden/morphology, risk factors for atherosclerosis, inflammatory mediators, and macrophage function.

RESULTS

There was a 61 and 74% decrease in total aortic lesion area in CD36 degrees /apoE degrees males and females, respectively, compared with apoE degrees controls. The absence of SRA was protective (32% decrease in lesion) in female mice. The combined absence of CD36 and SRA provided no further protection in either gender. Macrophages from mice lacking CD36 had decreased pro-inflammatory characteristics and less migration to a pro-inflammatory stimulus. Plasma levels of cytokines/chemokines showed that CD36 degrees /apoE degrees and CD36 degrees /SRA degrees /apoE degrees mice had a less pro-inflammatory phenotype compared with apoE degrees and SRA degrees /apoE degrees mice. Oblivious mice in the apoE degrees background ruled out potential 'passenger gene' effects in the case of CD36.

CONCLUSION

These results provide new insights into the pro-atherogenic mechanisms of CD36 by implicating processes other than modified lipoprotein uptake.