Scavenger receptor SR-BI significantly contributes to HDL cholesterol metabolism and atherogenesis in mice. However, the role of SR-BI may not be as pronounced in humans due to cholesteryl ester transfer protein (CETP) activity. To address the impact of CETP expression on the adverse effects associated with SR-BI deficiency, we cross-bred our SR-BI conditional knock-out mouse model with CETP transgenic mice. CETP almost completely restored the abnormal HDL-C distribution in SR-BI-deficient mice. However, it did not normalize the elevated plasma free to total cholesterol ratio characteristic of hepatic SR-BI deficiency. Red blood cell and platelet count abnormalities observed in mice liver deficient for SR-BI were partially restored by CETP, but the elevated erythrocyte cholesterol to phopholipid ratio remained unchanged. Complete deletion of SR-BI was associated with diminished adrenal cholesterol stores, whereas hepatic SR-BI deficiency resulted in a significant increase in adrenal gland cholesterol content. In both mouse models, CETP had no impact on adrenal cholesterol metabolism. In diet-induced atherosclerosis studies, hepatic SR-BI deficiency accelerated aortic lipid lesion formation in both CETP-expressing (4-fold) and non-CETP-expressing (8-fold) mice when compared with controls. Impaired macrophage to feces reverse cholesterol transport in mice deficient for SR-BI in liver, which was not corrected by CETP, most likely contributed by such an increase in atherosclerosis susceptibility. Finally, comparison of the atherosclerosis burden in SR-BI liver-deficient and fully deficient mice demonstrated that SR-BI exerted an atheroprotective activity in extra-hepatic tissues whether CETP was present or not. These findings support the contention that the SR-BI pathway contributes in unique ways to cholesterol metabolism and atherosclerosis susceptibility even in the presence of CETP.Epidemiological studies have demonstrated that high density lipoprotein cholesterol is a strong, independent, inverse predictor of the risk of coronary heart disease. High plasma levels of the major apolipoprotein of HDL, APOA-I, have been also shown to predict decreased risk of coronary heart disease. One major atheroprotective mechanism by which HDL/APOA-I may protect against atherosclerosis involves the transport of excess cholesterol from peripheral tissues, including macrophages, to the liver, bile, and eventually feces for excretion, a process known as reverse cholesterol transport.The scavenger receptor SR-BI is an 82-kDa glycosylated plasma membrane protein that binds HDL/APOA-I with high affinity and stimulates the bi-directional flux of cholesterol between cells and extracellular HDL particles. In mice, SR-BI, encodeed by the Scarb1 gene, is a major determinant of HDL metabolism in mice and is protective against atherosclerosis. Indeed, hepatic overexpression of SR-BI markedly reduces plasma HDL-C levels, increases biliary secretion of cholesterol, and decreases atherosclerosis (1-3), whereas deficiency of this receptor resu...