SR-BI: a new player in an old game

Rutaecarpine suppresses atherosclerosis in ApoE-/- mice through upregulating ABCA1 and SR-BI within RCT

ABCA1 and scavenger receptor class B type I (SR-BI)/CD36 and lysosomal integral membrane protein II analogous 1 (CLA-1) are the key transporter and receptor in reverse cholesterol transport (RCT). Increasing the expression level of ABCA1 and SR-BI/CLA-1 is antiatherogenic. The aim of the study was to find novel antiatherosclerotic agents upregulating expression of ABCA1 and SR-BI/CLA-1 from natural compounds. Using the ABCA1p-LUC and CLA-1p-LUC HepG2 cell lines, we found that rutaecarpine (RUT) triggered promoters of ABCA1 and CLA-1 genes. RUT increased ABCA1 and SR-BI/CLA-1 expression in vitro related to liver X receptor alpha and liver X receptor beta. RUT induced cholesterol efflux in RAW264.7 cells. ApoE-deficient (ApoE(-/-)) mice treated with RUT for 8 weeks showed ∼68.43, 70.23, and 85.56% less en face lesions for RUT (L), RUT (M), and RUT (H) groups, respectively, compared with the model group. Mouse macrophage-specific antibody and filipin staining indicated that RUT attenuated macrophages and cholesterol accumulations in atherosclerotic lesions, respectively. Additionally, ABCA1 and SR-BI expression was highly induced by RUT in livers of ApoE(-/-) mice. Meanwhile, RUT treatment significantly increased the fecal (3)H-cholesterol excretion, which demonstrated that RUT could promote RCT in vivo. RUT was identified to be a candidate that protected ApoE(-/-) mice from developing atherosclerosis through preferentially promoting activities of ABCA1 and SR-BI within RCT.

Paraoxonase 1 (PON1) enhances HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter in association with increased HDL binding to the cells: a possible role for lysophosphatidylcholine

We investigated the role of HDL-associated paraoxonase 1 (PON1) in HDL-mediated macrophage cholesterol efflux by using HDL derived from wild type mice (Control-HDL), from human PON1-transgenic mice (HDL-PON1Tg) or from PON1-knockout mice (HDL-PON1(0)). Cholesterol efflux from mouse peritoneal macrophages (MPM) or from J774 A.1 macrophage cell line by HDL-PON1Tg, was significantly increased (by 60%) compared to HDL-PON1(0). We demonstrated that this PON1 effect was associated with an increased HDL binding to the cells, as the binding of HDL-PON1Tg (or HDL-PON1(0) that was enriched with PON1) was increased by 50% compared to that of HDL-PON1(0). Using either a cAMP analogue, to increase ABCA1 receptor expression, or rabbit anti-mouse SR-BI specific antibody to block the SR-BI receptor, PON1 stimulation of HDL binding and of HDL-mediated macrophage cholesterol efflux, were both found to involve the ABCA1 transporter. Studies with PON1 specific inhibitors revealed that PON1 activity was required for its stimulation of HDL-mediated macrophage cholesterol efflux. Upon incubation of macrophages with Control-HDL or with HDL-PON1Tg, macrophage lysophosphatidylcholine (LPC) content was increased by 3.7- and 7.5-fold, respectively. Such an LPC enrichment of macrophages resulted in up to 60% increased HDL binding to the cells, and a 41% increased HDL-mediated cholesterol efflux. Similarly, macrophage loading with LPC (by either adding LPC, or PON1 or phospholipase A(2)) significantly increased apolipoprotein A-I (apoA-I) mediated cholesterol efflux by 104, 65 and 56%, respectively, in ABCA1 overexpressing macrophages. We conclude that HDL-associated PON1 may contribute to the attenuation of atherosclerosis development by its ability to act on macrophage phospholipids, to form LPC, in turn, stimulates HDL binding and HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter.

Thematic review series: the immune system and atherogenesis. Recent insights into the biology of macrophage scavenger receptors

Scavenger receptors were originally defined by their ability to bind and internalize modified lipoproteins. Macrophages express at least six structurally different cell surface receptors for modified forms of LDL that contribute to foam cell formation in atherosclerosis. In addition to their role in the pathology of atherosclerosis, macrophage scavenger receptors, especially SR-A, play critical roles in innate immunity, apoptotic cell clearance, and tissue homeostasis. In this review, we highlight recent advances in understanding the biology of macrophage scavenger receptors as pattern recognition receptors for both infectious nonself (pathogens) and modified self (apoptotic cells and modified LDL). We critically evaluate the potential of scavenger receptors and their ligands as targets for therapeutic intervention in human disease.

Paraoxonases 1, 2, and 3, oxidative stress, and macrophage foam cell formation during atherosclerosis development

Paraoxonases PON1 and PON3, which are both associated in serum with HDL, protect the serum lipids from oxidation, probably as a result of their ability to hydrolyze specific oxidized lipids. The activity of HDL-associated PON1 seems to involve an activity (phospholipase A2-like activity, peroxidase-like activity, lactonase activity) which produces LPC. To study the possible role of PON1 in macrophage foam cell formation and atherogenesis we used macrophages from control mice, from PON1 knockout mice, and from PON1 transgenic mice. Furthermore, we analyzed PON1-treated macrophages and PON1-transfected cells to demonstrate the contribution of PON1 to the attenuation of macrophage cholesterol and oxidized lipid accumulation and foam cell formation. PON1 was shown to inhibit cholesterol influx [by reducing the formation of oxidized LDL (Ox-LDL), increasing the breakdown of specific oxidized lipids in Ox-LDL, and decreasing macrophage uptake of Ox-LDL]. PON1 also inhibits cholesterol biosynthesis and stimulates HDL-mediated cholesterol efflux from macrophages. PON2 and PON3 protect against oxidative stress, with PON2 acting mainly at the cellular level. Whereas serum PON1 and PON3 were inactivated under oxidative stress, macrophage PON2 expression and activity were increased under oxidative stress, probably as a compensatory mechanism against oxidative stress. Intervention to increase the paraoxonases (cellular and humoral) by dietary or pharmacological means can reduce macrophage foam cell formation and attenuate atherosclerosis development.