Dietary soy protein isolate ameliorates atherosclerotic lesions in apolipoprotein E-deficient mice potentially by inhibiting monocyte chemoattractant protein-1 expression.

Soy-based diets reportedly protect against the development of atherosclerosis; however, the underlying mechanism(s) for this protection remains unknown. In this report, the mechanism(s) contributing to the atheroprotective effects of a soy-based diet was addressed using the apolipoprotein E knockout (apoE-/-) mice fed soy protein isolate (SPI) associated with or without phytochemicals (SPI+ and SPI-, respectively) or casein (CAS). Reduced atherosclerotic lesions were observed in aortic sinus and enface analyses of the descending aorta in SPI+- or SPI(-)-fed apoE-/- mice compared with CAS-fed mice. SPI+-fed mice showed 20% fewer lesions compared with SPI(-)-fed mice. Plasma lipid profiles did not differ among the 3 groups, suggesting alternative mechanism(s) could have contributed to the atheroprotective effect of soy-based diets. Real-time quantitative PCR analyses of proximal aorta showed reduced expression of monocyte chemoattractant protein-1 (MCP-1), a monocyte chemokine, in mice fed both soy-based diets compared with the CAS-fed mice. These findings paralleled the reduced number of macrophages observed in the lesion site in the aorta of SPI+- or SPI(-)-fed mice compared with CAS-fed mice. In an in vitro LPS-induced inflammation model, soy isoflavones (genistein, daidzein, and equol alone or in combination) dose dependently inhibited LPS-induced MCP-1 secretion by macrophages, suggesting a role for soy isoflavones for the protective in vivo effects. Collectively, these findings suggest that the reduction in atherosclerotic lesions observed in mice fed the soy-based diet is mediated in part by inhibition of MCP-1 that could result in reduced monocyte migration, an early event during atherogenesis.

Milk thistle extracts inhibit the oxidation of low-density lipoprotein (LDL) and subsequent scavenger receptor-dependent monocyte adhesion.

Silymarin encompasses a group of flavonolignans that are extracted from Silybum marianum (Asteraceae) fruits. The silymarins have previously been reported to lower low-density lipoprotein (LDL) levels associated with high-fat diets. The present study reports the efficacy of the silymarins in inhibiting oxidized low-density lipoprotein (oxLDL) generation and subsequent scavenger receptor (SR) mediated monocyte adherence to oxLDL. The flavonolignans that comprise silymarin include silichristin (SC), silidianin (SD), silibinin (SBN), and isosilibinin (IS). These flavonolignans (300 microM) lowered oxLDL generation, measured by the thiobarbituric acid-reacting substances (TBARS) assay, by 60.0, 28.1, 60.0, and 30.1%, respectively. SBN treatment of LDL in the presence of copper sulfate (CuSO 4) resulted in a significant dose-dependent inhibition of monocyte adhesion. Inhibition was paralleled by a decrease in binding of anti-oxLDL antibodies recognized by U937 monocyte Fc gamma receptors (FcgammaR). These results showed that silymarin and SBN, likely through antioxidant and free radical scavenging mechanisms of action, inhibit the generation of oxLDL and oxidation-specific neoepitopes recognized by SR and FcgammaR expressed on monocytes/macrophages.

Recombinant CD36 inhibits oxLDL-induced ICAM-1-dependent monocyte adhesion.

A key event in atherosclerosis is the interaction between monocytes and endothelial cells. Binding of oxidized low-density lipoprotein (oxLDL) to CD36 on endothelial cells results in activation and subsequent monocyte adhesion. In this study, a recombinant soluble CD36 molecule was expressed to delineate its ability to block the adhesion of monocytes. To construct soluble CD36, the extra-cellular domain of CD36 was fused to the Fc domain of human IgG1. The N-terminal sequence of CD36 was replaced with N-terminal signal peptide sequence of CD59, a type I membrane protein. The resulting chimeric sCD36-Ig cDNA (sCD36-Ig) was transfected into COS-7 and CHOK1 cells and supernatants were analyzed for secretion of this molecule. Sandwich ELISA and oxLDL binding analyses showed that recombinant sCD36-Ig is secreted in a functionally active form. Western blot analysis of the purified sCD36-Ig using three different anti-CD36 monoclonal antibodies and anti-human IgG showed that the chimeric sCD36-Ig is a dimer of 220kDa. Further, the sCD36-Ig inhibited the adhesion of monocytes to oxLDL. Interestingly, sCD36-Ig blocked the oxLDL-induced adhesion of monocytes to the endothelial cell specific protein, ICAM-1. Our results indicate that the chimeric sCD36-Ig protein is folded correctly and can effectively compete for the binding of oxLDL to membrane-expressed CD36. These results suggest that oxLDL-induced monocyte adhesion can be blocked using sCD36-Ig and this may be useful in blocking the cell-cell interaction leading to atherogenesis.

Dietary homocysteine promotes atherosclerosis in apoE-deficient mice by inducing scavenger receptors expression.

Elevated plasma homocysteine (Hcy) levels have been recognized as an independent risk factor for atherosclerosis leading to cardiovascular diseases. However, the mechanisms contributing to atherosclerosis have not been delineated. Since, scavenger receptors mediated uptake of oxidized-LDL (oxLDL) by macrophages resulting in foam cell formation is an early event in atherosclerosis, we hypothesized that atherogenic effects of Hcy may be mediated via regulating expression of scavenger receptor(s). We have tested this hypothesis using apoE-/- female mice fed normal rodent chow (NC) diet or NC supplemented with Hcy in drinking water (9 g/L). Hcy-fed mice showed increased fatty streak lesions in aortic sinus/root compared to NC group without alterations in plasma lipid profiles. Similar findings were observed in the enface analysis of the descending aorta. To determine the molecular mechanisms underlying Hcy-mediated progression of fatty streak lesions, expression of scavenger receptors such as CD36 and lectin-like oxidized LDL binding protein-1 (LOX-1) in the aortic lesions were analyzed. Interestingly, Hcy-fed mice had increased immuno-positive staining for CD36 and LOX-1 in the atherosclerotic lesions compared to NC-fed mice. In vitro analyses showed neither Hcy nor HcyLDL directly affect the expression of CD36 and LOX-1 on mouse macrophages. However, Hcy supplementation in apoE-/- mice resulted in elevated oxLDL levels in plasma. Since oxLDL has been shown to upregulate the expression of CD36 and LOX-1, these findings suggest that Hcy may exert its atherogenic effect in part by elevating the levels of oxLDL. Interestingly, interaction of monocytes with Hcy-activated endothelial cells resulted in upregulation of CD36 expression on monocytes, suggesting a possible mechanism by which Hcy may upregulate CD36 expression at the lesion site. Further, these findings suggest a novel mechanism by which Hcy may promote atherogenesis.

Soy isoflavones attenuate human monocyte adhesion to endothelial cell-specific CD54 by inhibiting monocyte CD11a.

Soy-based diets have been shown to protect against the development of atherosclerosis; however, the underlying mechanism(s) remain unknown. Interaction between activated monocytes and inflamed endothelial cells is an early event in atherogenesis. Therefore, we examined whether treatment of monocytes with soy phytochemicals could inhibit their adhesion to the endothelial cell-specific protein, CD54, a key factor in monocyte adhesion. Female Sprague-Dawley rats were fed AIN-93G diets containing soy protein isolate or casein. Sera from soy-fed rats inhibited CD54-dependent monocyte adhesion, whereas sera from casein-fed rats did not. To determine whether isoflavones in the sera of soy-fed rats were involved in this inhibition, monocytes were preincubated with soy isoflavones. Isoflavone treatment inhibited monocyte adhesion to CD54 protein, as well as to endothelial cells expressing CD54. Monocyte expression of CD11a, the cognate receptor for CD54, was unaffected by isoflavones. However, binding of the activation epitope-specific antibody mAb24, which binds specifically to the active form of CD11a, was significantly lower in soy isoflavone-treated monocytes than in media-treated cells. These findings suggest that inhibition of CD54-dependent monocyte adhesion by soy isoflavones is mediated in part by affinity regulation of CD11a. Inhibition of monocyte adhesion to endothelial cells by isoflavones resulted in reduced expression of the inflammatory cytokines IL-6 and IL-8. Collectively, these data suggest that the athero-protective effect of soy diets may be mediated by blocking monocyte-endothelial cell interaction.