Positive Association between Autoantibodies Against Oxidized LDL and HDL-C: A Novel Mechanism for HDL Cardioprotection?

BACKGROUND

In the atherosclerotic plaque microenvironment, oxidized phospholipids expressed in the oxidized low-density lipoprotein (oxLDL) surface bind to scavenger receptors of macrophages eliciting foam cell formation and plaque progression. Auto-antibodies against oxLDL (oxLDL-Ab) interact with oxidative epitopes leading to the formation of immune complexes that are unable to interact with macrophage receptors, thus abrogating atherogenesis. Release of oxLDL-Ab by B cells involves interleukin 5 and Th2 response, which in turn are potentiated by HDL. Thereby, we hypothesized that individuals with higher levels of HDL-C may plausibly display elevated titers of oxLDL-Ab.

OBJECTIVE

To evaluate the relationship between HDL-C and oxLDL-Ab levels.

METHODS

Asymptomatic individuals (n = 193) were grouped according to their HDL-C concentration to one of three categories: low (< 68 mg/dL), intermediate (68 to 80 mg/dL) or high (> 80 mg/dL). P values < 0.05 were considered statistically significant.

RESULTS

Our analysis included 193 individuals (mean age: 47 years; male: 26.3%). Compared to individuals in the lowest HDL-C tertile, those in the highest tertile were older (36 versus 53 years; p = 0.001) and less frequently male (42.6% versus 20.9%; p = 0.001). Mean values of oxLDL-Ab increased as the HDL-C group escalated (0.31, 0.33 and 0.43 units, respectively; p = 0.001 for trend). Simple linear regression found a significant, positive relationship between the independent variable, HDL-C, and the dependent variable, oxLDL-Ab (R = 0.293; p = 0.009). This relation remained significant (R = 0.30; p = 0.044), after adjustment by covariates. Apolipoprotein AI levels were also related to oxLDL-Ab in both simple and adjusted linear regression models.

CONCLUSION

HDL-C and oxLDL-Ab are independently related.

Monocyte/High-Density Lipoprotein Ratio Predicts the Prognosis of Large Artery Atherosclerosis Ischemic Stroke

Objective: Monocyte to high-density lipoprotein ratio is considered as a new inflammatory marker and has been used to predict the severity of coronary heart disease and the incidence of adverse cardiovascular events (ACEs). However, there is a lack of data relative to large artery atherosclerosis (LAA) ischemic stroke. We investigated whether the monocyte to high-density lipoprotein (HDL) ratio (MHR) is related to the 3-month functional prognosis of LAA ischemic stroke.

Materials and Methods: A retrospective analysis was conducted on 316 LAA ischemic stroke patients. The 3-month functional outcome was divided into good and poor according to the modified Rankin Scale (mRS) score. Multivariate logistic regression analysis was performed to evaluate the correlation between MHR and prognosis of ischemic stroke.

Results: The MHR level of poor functional outcome group was higher than that of the good functional outcome group [0.44 (0.3, 0.55) vs. 0.38 (0.27, 0.5), P = 0.025]. Logistic stepwise multiple regression revealed that MHR [odds ratio (OR) 9.464, 95%CI 2.257–39.678, P = 0.002] was an independent risk factor for the 3-month poor outcome of LAA ischemic stroke. Compared to the lower MHR tertile, the upper MHR tertile had a 3.03-fold increase (95% CI 1.475–6.225, P = 0.003) in the odds of poor functional outcome after adjustment for potential confounders. Moreover, a multivariable-adjusted restricted cubic spline (RCS) showed a positive close to a linear pattern of this association.

Conclusion: Elevated MHR was independently associated with an increased risk of poor 3-month functional outcome of patients with LAA ischemic stroke.

Uptake of oxLDL and IL-10 production by macrophages requires PAFR and CD36 recruitment into the same lipid rafts

Macrophage interaction with oxidized low-density lipoprotein (oxLDL) leads to its differentiation into foam cells and cytokine production, contributing to atherosclerosis development. In a previous study, we showed that CD36 and the receptor for platelet-activating factor (PAFR) are required for oxLDL to activate gene transcription for cytokines and CD36. Here, we investigated the localization and physical interaction of CD36 and PAFR in macrophages stimulated with oxLDL. We found that blocking CD36 or PAFR decreases oxLDL uptake and IL-10 production. OxLDL induces IL-10 mRNA expression only in HEK293T expressing both receptors (PAFR and CD36). OxLDL does not induce IL-12 production. The lipid rafts disruption by treatment with βCD reduces the oxLDL uptake and IL-10 production. OxLDL induces co-immunoprecipitation of PAFR and CD36 with the constitutive raft protein flotillin-1, and colocalization with the lipid raft-marker GM1-ganglioside. Finally, we found colocalization of PAFR and CD36 in macrophages from human atherosclerotic plaques. Our results show that oxLDL induces the recruitment of PAFR and CD36 into the same lipid rafts, which is important for oxLDL uptake and IL-10 production. This study provided new insights into how oxLDL interact with macrophages and contributing to atherosclerosis development.

The role of Fcγ receptors in atherosclerosis

Atherosclerosis is widely considered to be an immune-mediated process. Fcγ receptors (Fcγ Rs) contribute to the regulation of a multitude of immune and inflammatory responses and are implicated in human atherosclerotic lesions. Major cell types involved in the pathogenesis of atherosclerosis express Fcγ Rs and their proatherogenic ligands such as immune complexes and C-reactive protein, which act to activate Fcγ R signaling pathways. This review summarizes recent significant progress addressing the multifaceted roles of Fcγ Rs in atherogenesis which comes from the studies of Fcγ R-deficient animal models, clinical investigations and in vitro molecular and cellular studies. These new findings help us appreciate the emerging role of Fcγ Rs in atherosclerosis, and suggest Fcγ Rs as a potential therapeutic target for atherosclerosis.

Co-stimulation of PAFR and CD36 is required for oxLDL-induced human macrophages activation

The oxidative process of LDL particles generates molecules which are structurally similar to platelet-activating factor (PAF), and some effects of oxidized LDL (oxLDL) have been shown to be dependent on PAF receptor (PAFR) activation. In a previous study, we showed that PAFR is required for upregulation of CD36 and oxLDL uptake. In the present study we analyzed the molecular mechanisms activated by oxLDL in human macrophages and the contribution of PAFR to this response. Human adherent monocytes/macrophages were stimulated with oxLDL. Uptake of oxLDL and CD36 expression were determined by flow cytometry; MAP kinases and Akt phosphorylation by Western blot; IL-8 and MCP-1 concentration by ELISA and mRNA expression by real-time PCR. To investigate the participation of the PI3K/Akt pathway, Gαi-coupled protein or PAFR, macrophages were treated with LY294002, pertussis toxin or with the PAFR antagonists WEB2170 and CV3988, respectively before addition of oxLDL. It was found that the addition of oxLDL to human monocytes/macrophages activates the PI3K/Akt pathway which in turn activates the MAPK (p38 and JNK). Phosphorylation of Akt requires the engagement of PAFR and a Gαi-coupled protein. The upregulation of CD36 protein and the uptake of oxLDL as well as the IL-8 production are dependent on PI3K/Akt pathway activation. The increased CD36 protein expression is dependent on PAFR and Gαi-coupled protein. Transfection studies using HEK 293t cells showed that oxLDL uptake occurs with either PAFR or CD36, but IL-8 production requires the co-transfection of both PAFR and CD36. These findings show that PAFR has a pivotal role in macrophages response to oxLDL and suggest that pharmacological intervention at the level of PAFR activation might be beneficial in atherosclerosis.

Pivotal role for platelet-activating factor receptor in CD36 expression and oxLDL uptake by human monocytes/macrophages

The uptake of oxLDL by CD36 is not regulated by intracellular levels of cholesterol, leading to macrophage differentiation into foam cells which play a major role in atherosclerosis. Furthermore, oxLDL competes with PAF in macrophages for binding to PAF receptors (PAFR). Here we investigated the involvement of PAFR in CD36 expression and uptake of oxLDL by human monocytes/macrophages. Adherent peripheral blood mononuclear cells were treated with PAFR-antagonists (WEB2170, CV3988); inhibitors of ERK1/2 (PD98059), p38 (SB203580), JNK (SP600125) or diluents, before stimulation with oxLDL or PAF. After 24 h, uptake of FITC-oxLDL and expression of CD36 was determined by flow cytometry and phosphorylation of MAP-kinases by Western blot. It was shown that the uptake of oxLDL was reduced by PAFR antagonists. CD36 expression was up-regulated by oxLDL, an effect reversed by PAFR antagonists. The up-regulation of CD36 and oxLDL uptake both required MAP-kinases activation. The oxLDL-induced ERK1/2 and JNK but not p38 phosphorylation was reversed by PAFR-antagonists suggesting that oxLDL signalling involves PAFR dependent and independent pathways. In macrophages from PAFR(-/-) mice, oxLDL was unable to up-regulate CD36 expression and the oxLDL uptake was reduced compared to wild type. These results suggest that oxLDL interacts with PAFR in macrophages to increase CD36 expression and oxLDL uptake. Whereas pharmacological intervention at the level of PAFR would be beneficial in atherosclerosis remains to be determined.