Systemic lupus erythematosus (SLE) is a complex multisystem autoimmune disease that involves multiple organs as a result of autoimmune-mediated tissue damage. In recent years, it has been established that the incidence of premature atherosclerosis (and hence cardiovascular morbidity and mortality) is increased in patients with SLE (1–3). Although traditional risk factors such as hypertension, hypercholesterolemia, and diabetes mellitus, which can be promoted by immune dysregulation and glucocorticoid use, are thought to be important in mediating this increased risk of atherosclerosis in SLE, they fail to adequately explain the increased incidence of atherosclerotic diseases in patients with SLE (4, 5). Indeed, SLE itself is an independent risk factor for atherosclerosis, as reported in the past few years (6, 7). Thus, the increasing prevalence of atherosclerosis in SLE is likely attributable to a complex interaction involving traditional risk factors, disease-related factors such as medications and disease activity, and inflammatory and immunogenic factors (8, 9). It was recently reported that the level of proinflammatory high-density lipoprotein (HDL) was elevated and correlated with subclinical atherosclerosis in patients with SLE (10). More studies are certainly needed to define the exact mechanisms leading to this complication.
The expression of proinflammatory cytokines and chemokines is increased in SLE (11, 12). Among these cytokines and chemokines, type I interferon (IFN) has been recognized to play a pathogenic role in human SLE (13, 14). Serum levels of type I IFNs, predominantly IFNα, are elevated in ∼50% of patients with SLE (15), and gene expression profiling has revealed that the expression of IFN-inducible gene (IFIG) transcripts is also up-regulated (16). The presence of this “interferon signature” is positively associated with serologic and clinical manifestations, disease activity, and disease severity in SLE (17, 18). In animal experiments, a deficiency of type I IFN receptor significantly reduced lupus-like disease in NZB mice (19). More importantly, a murine model of pristane-induced lupus further confirmed the key role of the type I IFN pathway in lupus (20, 21).
IFNα has been reported to be involved in atherosclerosis through several different mechanisms. IFNα promoted endothelial progenitor cell deletion and endothelial dysfunction in lupus, leading to abnormal vascular repair (22, 23). Plaque-residing plasmacytoid dendritic cell–produced IFNα combined with lipopolysaccharide increased the expression of Toll-like receptor 4 and enhanced the production of tumor necrosis factor α (TNFα), interleukin-12 (IL-12), and matrix metalloproteinase 9 (MMP-9), threatening the stability of atherosclerotic plaques (24). Additionally, IFNα enhanced cytotoxic T cell activities that may also trigger plaque disruption in atherosclerosis (25). Interestingly, low-density lipoprotein (LDL) receptor–deficient mice showed significantly accelerated atherosclerosis accompanied by increased plasma levels of cholesterol and triglycerides after receiving an injection of IFNα (26).
The initiating force for the occurrence of atherosclerosis is the accumulation of cholesterol-laden foam cells in the arterial wall. The role of IFNα in this aspect of atherosclerosis remains unknown. In the early stage of atherosclerosis, circulating monocytes infiltrate into the subintima where they differentiate into macrophages. Upon exposure and uptake of modified lipoproteins, especially oxidized LDL (ox-LDL), the macrophages are transformed into foam cells, which are the primary components of the earliest atherosclerotic lesion. Macrophage scavenger receptor (SR) family proteins can internalize substantial quantities of cholesteryl ester from ox-LDL and play a leading role in lipid accumulation and foam cell formation (27). In this study, we focused on investigating the effects of IFNα on lipid uptake and foam cell formation, especially on the expression and activities of macrophage SRs.
We demonstrated that IFNα priming was able to promote ox-LDL engulfment and foam cell formation by up-regulating the expression of macrophage SR class A (SR-A). Enhanced SR-A promoter activities and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway appeared to be involved in this process. In addition, we observed that the expression of SR-A was significantly increased in the peripheral blood mononuclear cells (PBMCs) of patients with SLE and was positively correlated with IFN signaling activity.
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It is a well-known clinical phenomenon that patients with SLE have an increased risk of atherosclerosis, but the mechanisms of this occurrence have not been clarified. We proposed this study based on the fact that IFNα activities are prominently enhanced in SLE, and the presence of IFNα activities is required for the development of SLE in animal models (20, 21). IFNα has been reported to be correlated with atherosclerosis via different mechanisms (14, 22, 23, 41). Activation of the IFN pathway was significantly associated with carotid intima-media thickness and with carotid plaque in a recent lupus cohort study (42). Therefore, increasing attention is being paid to the roles of IFNα in premature atherosclerosis in patients with lupus (43). However, no studies on the role of IFNα in foam cell formation have been performed.
In the present study, we demonstrated for the first time that IFNα priming promoted lipid uptake and macrophage-derived foam cell formation in vitro. Mechanistically, up-regulated SR-A expression by IFNα was associated with enhanced uptake of modified lipids and an increased number of foam cells. Moreover, the expression of SR-A mRNA was significantly increased in PBMCs from patients with SLE and was positively correlated with activation of the type I IFN pathway. Besides, monocytes from patients with SLE showed a tendency toward augmented lipid uptake. Therefore, we believe that the premature atherosclerosis observed in patients with SLE may be a consequence of increased activities of type I IFN.
Macrophage-derived foam cells have been identified as one of the main components of early atherosclerotic lesions (44). The balance of lipid influx and cholesterol efflux in macrophages is strictly controlled in normal organisms by modulating the expression of SRs in macrophages, which is essential for preventing atherosclerosis. Previous studies demonstrated that the expression of SR-A was increased in macrophage-rich areas of human atherosclerotic lesions and played a proatherogenic role in plaque formation (34, 45). It has been reported that targeted disruption of SR-A mainly decreased modified LDL uptake. Moreover, SR-A–deficient mice showed a decreased tendency for the development of atherosclerosis (33, 45, 46). These results support the contribution of SR-A to the generation of atherosclerotic plaques and the development of atherosclerosis. Furthermore, a recent study demonstrated that SR-A polymorphisms were associated with the incidence of atherosclerosis (47). Consistent with these studies, we observed that IFNα activated the SR-A gene and up-regulated its expression but did not have that effect on CD36, although CD36 appears to be another important SR responsible for the uptake of modified LDL (33).
Enhanced SR-A promoter activities appeared to be involved in IFNα-induced SR-A expression. However, no definite ISRE motifs were identified in the SR-A promoter by bioinformatics analysis. Therefore, it is not clear presently which transcription factor or factors or unknown ISRE sequences are involved in activation of the SR-A promoter by IFNα treatment.
IFNα can activate the MAPK and PI3K signaling pathways in a STAT-independent manner; these cascades are known to be responsible for IFNα-induced biologic responses such as IFN-driven gene transcription (35). Our data suggested that the PI3K/Akt pathway but not the MAPK pathway was necessary for IFNα-induced SR-A expression. In contrast to a previous report that LY294002 had no effect on basal SR-A expression in RAW 264.7 cells (37), our data showed that LY294002 inhibited not only IFNα-induced SR-A expression but also basal SR-A expression in human macrophages. One of the possible reasons for these different results may be the different cell lines used. However, because it remains controversial whether STAT-1 can modulate SR-A expression (48, 49), more data will be needed to clarify this issue.
High SR-A gene expression in PBMCs provided a predictive marker for cardiovascular events (50). SR-A was expressed at low levels in circulating monocytes and was remarkably up-regulated during the process of monocyte differentiation into macrophages (36). It has been reported that SR-A gene expression was specifically increased in PBMCs from patients with acute coronary syndrome (50). In our investigation, SR-A mRNA expression was significantly increased in PBMCs from patients with SLE and positively correlated with activation of the type I IFN pathway. To our knowledge, this is the first study to reveal this point. Elevated expression of SR-A could promote cellular adhesion and therefore increase monocyte recruitment, facilitating their entry into the subendothelial space, as well as enhance modified LDL uptake (40). Meanwhile, preliminary results indicated that monocytes from patients with SLE had a tendency toward the uptake of more lipids compared with monocytes from healthy subjects, even though the difference was not statistically significant, which may be attributable to the small sample size and the variation in primary cells. This result was consistent with the observation that IFNα priming was able to facilitate cholesterol uptake.
All these activities together would likely be responsible for the increased incidence of atherosclerosis in patients with SLE. However, factors other than IFNα cannot be excluded in the explanation of increased SR-A expression in patients with SLE. We noticed that other types of cytokines, such as TNFα, IL-6, IFNγ, and monocyte chemotactic protein 1, may also have promoting effects on atherosclerosis (41). Considering the disorder of cytokine and chemokine production in SLE, cytokines other than type I IFN may also be involved in the initiation and progression of atherosclerosis in SLE (11, 12). In this study, we examined the effect of IFNα on lipid uptake and SR-A expression, but further studies are certainly needed to explore the detailed mechanisms underlying the role of IFNα in atherosclerosis. An animal model and a longitudinal multiple-cohort study will be beneficial to further clarify these points.
In summary, our study provides new evidence that IFNα, as the pathogenic factor in SLE, promoted lipid uptake and macrophage-derived foam cell formation by up-regulating SR-A expression, which was essential in the process of plaque formation and progress of atherosclerosis. These findings should be helpful in enhancing our understanding of the mechanisms of atherogenesis, especially in the setting of autoimmune disease. Furthermore, our findings may provide potential therapeutic targets for the prevention and treatment of premature atherosclerosis in patients with SLE.
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All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Shen had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study conception and design. Shen, Bao.
Acquisition of data. Li, Fu, Cui, Qu, Pan.
Analysis and interpretation of data. Li, Fu, Shen, Bao.