The effects of in vivo modulation of HDL phospholipid (PL) on scavenger receptor class BI (SR-BI)-and ATP binding cassette transporter 1 (ABCA1)-mediated efflux were examined by overexpressing either endothelial lipase (EL) or phosphatidylserine phospholipase (PS-PLA 1 ) in human apolipoprotein A-I (apoA-I) transgenic mice. Overexpression of EL led to large reductions in the serum PL/apoA-I ratio ( ؊ 60%), total cholesterol (TC; ؊ 89%), and HDL cholesterol ( ؊ 91%). Relative to the serum before overexpression of EL, the efflux potential of the serum via SR-BI decreased by 90% and ABCA1-mediated efflux increased by 63%. In contrast to overexpression of EL, overexpression of PS-PLA 1 led to increases in the PL/apoA-I ratio (88%), TC (78%), HDL cholesterol (57%), and HDL size. The efflux potential of the serum increased by 60% via SR-BI and decreased by 57% via ABCA1. There were significant positive correlations between SR-BI-mediated efflux and a number of serum parameters, including PL/apoA-I ratio, PL, TC, free cholesterol (FC), and HDL cholesterol. In striking contrast, the same correlations were seen with ABCA1-mediated efflux, but the relationships were inverse. In summary, in vivo modulation of HDL PL content affects ABCA1-and SR-BI-mediated efflux in a reciprocal manner. These findings indicate that the type of lipase acting on HDL in vivo will determine which FC efflux pathway the HDL serves. Additionally, the extent of lipolysis will determine the efficiency of FC removal via this pathway. HDL cholesterol levels are inversely correlated with the incidence of coronary artery disease (1-4). One mechanism by which HDL is thought to protect against atherosclerosis is by the removal of excess free cholesterol (FC) from peripheral cells and subsequent delivery to the liver for excretion (5-7). There are three known mechanisms by which HDL and/or its apolipoproteins can remove FC from cells. Aqueous diffusion is a relatively inefficient efflux mechanism that occurs with all cell types (8). In recent years, two proteins have been discovered that mediate efficient cholesterol efflux. The scavenger receptor class BI (SR-BI) facilitates the bidirectional flux of FC between cells and HDL (9, 10), and the ATP binding cassette transporter 1 (ABCA1) (11-13) mediates the unidirectional efflux of cellular FC and phospholipid (PL) to lipid-poor apolipoprotein A-I (apoA-I) and other exchangeable apolipoproteins. A number of studies suggest that both mechanisms of efflux may operate in atherosclerotic lesions (14-16).The goal of the present study was to determine the effects of in vivo modulation of HDL PL on both ABCA1-and SR-BI-mediated FC efflux. The importance of studying the role of HDL PL in the flux of FC between cells and HDL is supported by a number of previous observations. The efflux of cholesterol from cells to serum is correlated with HDL PL content (17, 18). Importantly, it has been demonstrated that patients with coronary artery disease have low HDL PL levels (19). Past studies have used in vitro mani...