Systemic lupus erythematosus (SLE) is an autoimmune rheumatic disease characterized by systemic inflammation and increased production of a wide range of autoantibodies directed against a multiplicity of antigens (1). Premature atherosclerosis has recently been recognized as an important cause of morbidity and mortality in SLE (2), and coronary artery disease accounts for up to 30% of all deaths in some reported series (3). Classic risk factors for cardiovascular disease in SLE appear to be similar to those in the general population, but specific factors such as steroid treatment, chronic inflammation, and renal disease could account for enhanced atheroma formation (4).
Dyslipoproteinemia is a major factor in the development of atherosclerosis in SLE (5, 6). Low levels of high-density lipoproteins (HDL) and apolipoprotein A-I (Apo A-I) have been related to disease activity and the presence of anticardiolipin antibodies (aCL) (7). Anticardiolipin antibodies, a hallmark of the antiphospholipid antibody syndrome (APS), can also be found in a wide range of different conditions and are present in 30–40% of patients with SLE (8). Cross-reactivity between these autoantibodies and plasma lipoproteins, particularly when the latter are oxidized, has been described to occur in SLE and APS and could contribute to the increased risk of atherosclerosis found in both conditions (9). Oxidation is a major process in atherosclerosis (10), and oxidative stress has been described in both SLE and APS (11). Paraoxonase (PON) is an enzyme with antioxidant activity, which circulates in plasma attached to HDL. Its function is to prevent oxidation of low-density lipoprotein (LDL), accounting thus for the antioxidant effect of HDL and explaining why HDL has a protective effect against atherosclerosis (12, 13). PON activity has been shown to decrease with age (14) and increase with intake of lipid-lowering drugs (15).
This study was undertaken to explore whether the activity of PON is impaired in patients with SLE and primary APS (contributing to enhanced atherosclerotic progression in these conditions), and, if so, whether the impairment could be dependent on the presence of autoantibodies against cardiolipin, β2-glycoprotein I (anti-β2GPI), prothrombin, and HDL.
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- PATIENTS AND METHODS
Systemic lupus erythematosus and the antiphospholipid syndrome are both characterized by an enhanced risk of thrombosis and atherosclerosis. Even though some pathogenic pathways might be shared, there are different factors that ultimately account for the development of vascular damage in the 2 conditions.
In SLE, prolonged steroid treatment seems to be the major cause in that it induces an atherogenic lipid profile, characterized by increased levels of very low-density lipoprotein and LDL and decreased levels of HDL, together with hypertension and diabetes (25). In the present study, HDL was reduced in SLE patients compared with controls, as has been reported previously (26). This reduction was quite significant for the HDL2 subfraction. A decrease in HDL2 in patients with SLE has not been reported previously, and is particularly important because this subfraction contains the higher percentage of Apo A-I, which partly accounts for the protective effect of HDL against atherosclerosis (27). The importance of Apo A-I in the context of SLE is further enhanced by the recent observation that Apo A-I exerts antiinflammatory properties by blocking the contact-mediated activation of monocytes by T lymphocytes (28).
Other pro-atherogenic factors include chronic inflammation (29) and enhanced lipid peroxidation (30). Both conditions have been found in patients with SLE (11, 31), whereas a strong link between lipid peroxidation and IgG aCL titers has been demonstrated in APS (32). Furthermore, aCL antibodies have also been related to atherosclerosis (33, 34). This may occur either via direct activation of the vascular endothelium or by the increased uptake of anti-oxidized LDL–LDL immune complexes by macrophages (35). In this context, anti-β2GPI antibodies could also be of importance, because they hinder the protective role of β2GPI in preventing oxidized LDL uptake by macrophages (35). In our study, patients with SLE and primary APS had higher titers of aCL, anti-β2GPI, and IgG antiprothrombin antibodies than controls, as expected. Interestingly, we found higher titers of IgG anti-HDL in patients with SLE than in those with primary APS. This difference and the lack of correlation between IgG anti-HDL and aCL antibodies suggest that the former represent a specific antibody subset and are not the result of cross-reactivity with aCL antibodies.
Antibodies to lipoproteins have been detected both in patients with SLE (36) and in those with APS (37). Whether these antibodies are specifically directed against an antigen present in the lipoproteins or are simply cross-reactive aCL or anti-β2GPI antibodies is still unclear. Previous studies have shown that both situations may coexist (38). However, most studies to date have investigated antibodies to LDL, and few have explored HDL as a possible target (39). Dinu et al reported the presence of antibodies against Apo A-I in SLE patients and found that these antibodies were more frequent in aCL-positive patients (39), suggesting the possibility of cross-reactivity. More recently, anti–Apo A-I antibodies were reported by Abe et al, and a human anti–Apo A-I monoclonal antibody was described (40). However, there are no data regarding the impact of these antibodies on clinical and biologic markers of atherosclerosis. One of the most important of these markers is lipid peroxidation, and one of the main defense mechanisms against lipid peroxidation is PON. PON is an antioxidant enzyme found in the liver, arterial wall, and plasma, where it travels attached to HDL (41). It has an important action in preventing LDL oxidation by peroxynitrite, in turn preventing the generation of oxidized LDL in plasma (12). Apo A-I stabilizes the enzyme (41), hence, interference with Apo A-I or with the HDL molecule itself could cause a reduction in the activity of the enzyme.
PON activity has not previously been assessed in patients with SLE, but a decrease in its activity in patients with different autoimmune conditions and with aCL antibodies has been described (42). However, no correlation between clinical or biologic variables and the enzymatic activity was detected, nor was a mechanism suggested to explain this finding. Our data show that PON is reduced in patients with SLE and, to an even greater extent, in patients with primary APS. In the latter group, PON activity was inversely correlated with IgG anti-CL and IgG and IgM anti-β2GPI titers. No correlation with antiprothrombin antibodies was found. This is not unexpected since prothrombin is not associated with HDL and has no reported interaction with PON, while β2GPI and cardiolipin can be present in the HDL complex (43). That antibodies against HDL (or one of its components) may decrease PON activity is suggested by the strong inverse correlation between IgG anti-HDL and PON activity found in patients with SLE. Therefore, IgG anti-HDL may indirectly affect the oxidant/antioxidant balance in SLE via an inhibitory effect on PON.
Total antioxidant capacity quantifies the overall antioxidant defense of plasma. A higher TAC would suggest an increased resistance to oxidation. Recently, Nuttall et al demonstrated a decrease in TAC along with a pro-atherogenic lipid profile (elevated cholesterol, triglycerides, and lipid peroxides) in SLE (44). TAC levels in patients with SLE and primary APS in our cohort did not differ significantly from levels in the control group. However, in the SLE group, TAC was correlated positively with PON activity and inversely with anti-HDL, suggesting that IgG anti-HDL may decrease TAC by reducing PON activity.
Interestingly, no correlation between aCL, anti-β2GPI, anti-HDL, and TAC was found in the primary APS population. This suggests that other mechanisms might be affecting TAC in primary APS. A possible explanation could be that the decrease in PON activity found in primary APS is due to a higher prevalence of the RR polymorphism, known to be associated with decreased enzymatic activity (45), which could be less affected by the presence of these autoantibodies. In fact, Lambert et al described an increased incidence of this PON polymorphism in patients with aCL antibodies and arterial thrombosis (42).
In conclusion, our findings highlight the relevance of HDL, IgG anti-HDL, and PON in patients with SLE. Total HDL, and in particular HDL2, are decreased in SLE. This lipoprotein or some of its components may represent target (auto)antigens, since elevated IgG anti-HDL titers were noted in the patients investigated. More importantly, we have demonstrated that PON activity is markedly reduced in both SLE and primary APS and that this reduction correlates with the presence of IgG anti-HDL, IgG aCL, and IgG and IgM anti-β2GPI antibodies. However, only IgG anti-β2GPI was independently associated with decreased PON in the primary APS group. This complements the observation that β2GPI prevents the oxidation of LDL (46), and hence, antibodies against β2GPI may deprive β2GPI of its antioxidant properties. Because PON is important for the prevention of LDL oxidation, we suggest that IgG anti-HDL and IgG anti-β2GPI, via an inhibitory effect on PON activity, contribute to the oxidation of LDL and thus differential atherogenic routes in SLE and primary APS.