Role of Apolipoprotein E, Scavenger Receptor class B type I and ATP-Binding Cassette Transporter G1 in Reverse Cholesterol Transport and atherosclerosis

Several epidemiological studies established that HDL cholesterol inversely correlates with the incidence of cardiovascular events. HDL mainly exerts this atheroprotective function by promoting Reverse Cholesterol Transport (RCT) from cholesterol-loaded macrophages of the arterial wall to the liver. Many proteins are involved in RCT, including lipid transporters expressed on cell membrane and circulating lipoporteins. In particular, the apolipoprotein E (ApoE) expressed in macrophages modulates intracellular lipid metabolism, while systemic apoE regulates the plasma lipoprotein profile. Recently our group demonstrated that only the former contributes to the macrophage RCT in vivo, thus showing an antiatherosclerotic effect. Both ATP-Binding Cassette Transporter G1 (ABCG1) and Scavenger Receptor class B type I (SR-BI) promote cell cholesterol efflux to mature HDL, while SR-BI is also essential for the selective uptake of cholesteryl esters from HDL by the liver, the last step of RCT. SR-BI deficiency is associated with the accumulation of abnormally large HDL particles and increased susceptibility to atherosclerosis. ABCG1 deficiency does not affect plasma levels of HDL cholesterol on chow diet, but upon challenge with a high cholesterol diet ABCG1 knockout mice specifically lack the larger HDL particles. The impact of ABCG1 on atherosclerosis is not clear since contrasting effects have been reported of ABCG1 deletion on atherosclerotic lesion development. The general aim of this study is to further investigate the contribution of Apolipoprotein E (ApoE), ABC-transporter G1 (ABCG1), and scavenger receptor class B type I (SR-BI) on RCT and atherosclerosis. The present thesis is divided into two main parts. The first is focused on the role of systemic and macrophage ApoE in the first step of RCT, cellular cholesterol efflux. The study demonstrated that plasma from ApoE Knockout mice, despite showing an altered lipid profile, exerts a similar capacity to promote cholesterol efflux from lipid-loaded macrophages as compared to plasma from WT mice. Conversely, the absence of ApoE in macrophages reduces the capacity of cells to promote an efficient release of cholesterol to extracellular acceptors. Indeed, results obtained confirm that the expression of ApoE in macrophages affects RCT in vivo because it modulates cholesterol efflux from cells, whereas systemic ApoE, despite the impact on plasma lipoprotein composition, does not affect their capacity to act as cholesterol acceptors. The aim of the second part of this study was to verify the effects of combined ABCG1 and SR-BI deletion on HDL cholesterol metabolism and the development of atherosclerosis in murine models. The ABCG1 deficiency slightly reduces the hyperlipidemia associated with SR-BI deficiency in Double Knockout (DKO) mice. Despite this, DKO mice show no significant differences in the extent of atherosclerotic lesion as compared to SR-BI KO mice. Interestingly, an increased leukocytosis has been detected in DKO mice but not in single ABCG1 and SR-BI KO mice. We showed for the first time that, despite the lower serum cholesterol levels, deletion of ABCG1 did not affect the atherosclerosis susceptibility of SR-BI KO mice, probably as a result of increased leukocytosis.