Lipids and autoimmunity in atherosclerosis and cardiovascular diseases

Abstract

Atherosclerosis is an inflammatory disease. The rupture of athrerosclerotic plaques is the major cause of severe cardiovascular disease (CVD), such as myocardial infarction (MI) and stroke. The stability of the plaque depends on its cellular and extracellular contents. Plaques with large lipid cores, thin fibrous caps, and active inflammation are most likely to rupture. Oxidized low density lipoprotein (oxLDL) is proinflammatory and can promote both apoptosis and necrosis at higher concentrations and could play important roles in promoting plaque rupture. We and others have demonstrated that inflammatory phospholipids such as platelet activating factor (PAF)-like lipids and lysophosphatidylcholine (LPC) could be of major importance in oxLDL. Antibodies against phosphorylcholine exposed on inflammatory phospholipids in oxLDL, are risk factors for CVD at low levels, though the mechanisms by which anti-PC protect have not been clear. Another factor that we have proposed to interfere with inflammatory phospholipids is the protein Annexin A5. Incidence of CVD in systemic lupus erythematosus (SLE) patients is exceedingly high, and is an important clinical problem. Further, this renders SLE an interesting 'human model' for CVD and especially for the study of the role of immune reactions and autoimmunity in CVD.

Aim: The objective of this study is to investigate by what mechanisms of anti-PC may protect against CVD; the role of inflammatory phospholipids like LPC and others including cardiolipin (CL) in atherosclerosis and CVD and if Annexin A5 (ANXA5), has any potential therapeutic role. In addition, the roles of dyslipidemia and anti-PC in SLE are also investigated.

In Paper I, the underlying mechanisms of anti-PC and SLE-related CVD were studied. The study group consisted of 26 women with SLE with a history of CVD (myocardial infarction, angina pectoris, thromboembolic stroke or intermittent claudication), 26 age-matched women with SLE but without clinical manifestations of CVD and 26 age-matched control women. IgG anti-PC were decreased among the SLE-cases and SLE-controls as compared to the controls, respectively. SLE cases were more prevalent in the lowest 25th percentile of anti-PC-IgM (and IgG) as compared to the controls. Among the SLE cases, anti-PC were associated negatively with disease severity. We could extract IgG anti-PC from human immunoglobulin (Ig) and the specificity of aPC-IgG was confirmed by a competition ELISA. PAF-induced expression of adhesion molecules by HUVECs could be inhibited by these IgG anti-PC which suggests a mechanism by which anti-PC could be atheroprotective. In conclusion, we report that there were low levels of anti-PC in SLE-patients (where antibody production is typically very high) and demonstrate that anti-PC can be extracted from human Ig. Also, these anti-PC have anti-inflammatory properties with direct relevance to CVD.

In Paper II, we focused on CL and could demonstrate that this phospholipid could be oxidized in vitro with results confirmed by mass spectrophotometry. In contrast to native CL, oxCL can significantly induce ICAM-1 and VCAM-1 expression on HUVECs; activate intracellular calcium mobilization and induce production of leukotrienes (LTB4). These effects of oxCL were inhibited by ANXA5, which bound oxCL. In conclusion, we demonstrate for the first time that oxCL but not native CL can cause inflammatory effects that could contribute to human chronic inflammatory disease in general and CVD. ANXA5 could have potential therapeutic role.

In Paper III, we studied the pro-inflammatory effects of oxLDL and LPC and how ANXA5 interfere with these, with relevance to CVD and plaque rupture. Both oxLDL and LPC stimulated the expression of MMP-9 in co-cultures of HUVECs and macrophages and LPC induced LTB4 production in macrophages, effects which could be inhibited by ANXA5. Furthermore, ANXA5 was capable of inhibiting macrophage uptake of oxLDL in a concentration dependent manner. These results support the notion that oxLDL and LPC are involved in CVD and plaque rupture. ANXA5 could play a role as a novel anti-inflammatory agent in atherosclerosis and CVD and inhibit foam cell formation.

In Paper IV, the focus was placed on dyslipidemia in SLE. Small dense LDL (sdLDL), usually considered proatherogenic, were not raised among SLE and SLE-related CVD. The lipoprotein pattern in SLE and SLE-related CVD was not typically proatherogenic. Small HDL often assumed to be less atheroprotective than larger HDL-particles, were also found less common among both SLE cases and SLE controls. LDL from SLE patients did not bind to proteoglycans more than LDL from controls. The lipid profile in SLE-related CVD does not appear to be typically atherogenic.

Conclusions: oxLDL, LPC and oxCL can promote the production of factors implicated in inflammation and plaque rupture, and these effects can be inhibited by ANXA5. Both SLE cases and SLE controls exhibited low anti-PC levels and this might contribute both to CVD in SLE and to SLE per se. Anti-PC extracted from IVIG can neutralize PAF-induced inflammatory effects, suggesting a potential atheroprotective role for anti-PC.