Abstract 589: Macrophage SR-BI Binds GST-Pi and Suppresses Cell Apoptosis in Atherosclerosis

Abstract

Background: Oxidative stress accelerates atherosclerosis by promoting cell death. We hypothesize that macrophage scavenger receptor class B I (SR-BI) inhibits apoptosis by reducing oxidative stress in atherosclerosis. Methods and Results: Macrophage SR-BI deficiency led to 2.1-, 1.9-, and 2.7-fold more apoptosis after treatment with OxLDL, H2O2, or isolevugladin-phosphatidylethanolamine (IsoLG-PE) versus WT cells (P<0.05), respectively. To examine the impact of SR-BI deficiency in atherosclerosis, bone marrow (BM) transplantation studies were performed using apoE-/- and LDLr-/- recipient mice fed a Western diet for 8 or 16 weeks, respectively. Apoptosis as measured by active Caspase 3 was increased 2.6-fold in atherosclerotic lesions of LDLr-/- recipients of SR-BI-/- versus WT BM (P<0.01). Consistent with SR-BI reducing oxidative stress, the lesions of ApoE-/- mice reconstituted with SR-BI-/-ApoE-/- versus ApoE-/- BM showed 5.1-fold higher immunostaining for 4-HNE (P<0.05) and 70.4% lower GSH/GSSG ratio as determined by an enzymatic assay of aortic tissue lysates (P<0.01). Similarly, OxLDL treatment resulted in a 64.5% decrease in GSH/GSSG ratio in SR-BI-/- versus WT macrophages. ER stress was significantly increased in SR-BI-/- versus WT macrophages treated with OxLDL as evaluated by CHOP and BiP (P<0.05). Similarly, ER stress was increased in atherosclerotic lesions of ApoE-/- recipients of SR-BI-/-ApoE-/- versus ApoE-/- BM. Importantly, an antibody-based proteomic assay revealed that macrophage SR-BI interacts with glutathione S-transferase pi (GST-pi). SR-BI deficiency caused a 48.7% reduction of GST-pi expression in atherosclerotic lesions (P<0.05) and a 51.3% decrease in macrophage GST activity (P<0.05). Forced expression of GST-pi rescued OxLDL-induced apoptosis in SR-BI-/- macrophages. A lipid-protein overlap assay showed SR-BI binds IsoLG-PE, but not native PE, suggesting that SR-BI recognizes oxidized lipid for clearance via a GST-pi detoxification pathway. Conclusion: Macrophage SR-BI interacts with GST-pi and suppresses oxidative stress-induced apoptosis in atherosclerosis. This novel finding of SR-BI GST-pi interaction provides a new target for prevention and treatment of atherosclerosis.