260 Another important step in RCT is hepatic uptake of HDL cholesterol, and several lines of evidence indicate that HL stimulates this process (Table 1). HL increases the selective uptake of HDL cholesteryl ester (CE) in hepatic and non- hepatic cell lines [4]. Moreover, in vivo hepatic selective uptake of HDL-CE was impaired in HL knockout mice [6 ] and significantly higher in mice overexpressing human HL [5]. Both the hydrolytic activity of HL and its heparan sulfate proteoglycan-dependent bridging properties contrib- ute to the stimulating effect of HL on SR-BI—mediated selective uptake [5]. Although HL facilitates the uptake of HDL cholesterol into the liver, this does not appear to translate into increased biliary sterol secretion (Table 1), a third important step in RCT. The biliary secretion rates of cholesterol and bile acids were identical in HL-deficient mice and wild-type controls, even when fed cholesterol-enriched or lithogenic diets [10]. Consequently, HL deficiency did not change fecal neutral sterol or bile acid excretion (Table 1) [10]. HL and Macrophage-Specific RCT Even though RCT from macrophages to feces represents only a small fraction of the total cholesterol pool, this flux is nevertheless considered highly relevant for the develop- ment of atherosclerosis. When mice were injected with 3H cholesterol-labeled macrophages into the peritoneal cavity and the appearance of label in plasma and feces was followed for 48 h, plasma levels of 3H-sterol were higher in mice deficient in HL compared to controls. However, overall macrophage-to-feces RCT determined by excretion of macrophage-derived label into the feces over 48 h remained essentially unaffected in HL knockouts (Table 1) [6 ]. On the other hand, the effects of HL overexpression have not been addressed thus far. Also, HL expression by macrophages might have an impact on RCT that has not been experimentally explored yet. HL and Atherosclerosis Several studies have thus far addressed the role of HL in atherosclerosis development in different rodent models (Table 1) and humans but without reaching conclusive results. In LDL receptor–knockout mice, aortic atheroscle- rotic lesion formation was accelerated when endogenous HL was also absent [11], whereas transgenic overexpres- sion of human HL reduced atherosclerosis by 40% to 70% [12, 13]. Overexpressing the catalytically inactive variant of human HL was atheroprotective in chow-fed LDL recep- tor–knockout mice lacking endogenous HL [13, 14], whereas no effect on atherosclerosis development was seen in the same model on a Western diet [12]. In contrast, HL deficiency in apoE knockout mice on a regular chow diet Curr Atheroscler Rep (2011) 13:257–265 reduced aortic lesion size by 75% despite increased circulating levels of atherogenic apoB-containing lip- oproteins. Decreased atherosclerosis development in this mouse model was proposed to be due to higher HDL levels and an increased capacity of HDL to elicit cholesterol efflux [15]. Furthermore, HL deficiency delayed the onset of occlusive atherosclerosis, resulting in myocardial infarction and cardiac dysfunction in SR-BI/ apoE double-knockout mice [16]. However, the impact of HL on atherosclerotic lesion formation in an experimental animal model expressing CETP has not been reported. Bone marrow transplantation studies showed that restrict- ing HL expression to macrophages enhanced the forma- tion of early atherosclerotic lesions in both apoE knockout mice and LCAT transgenic mice without changing plasma lipoprotein levels [17]. On the other hand, a more recent publication demonstrated that deficiency of leukocyte- derived HL reduced circulating HDL cholesterol levels and increased the extent of aortic atherosclerosis in LDL receptor—knockout mice expressing human CETP [18]. Whether the experimental design or the mouse model used offers the explanation for these divergent results remains to be seen. Complete HL deficiency in humans is a rare condition; however, some of these patients develop premature coro- nary artery disease despite increased plasma HDL choles- terol levels [19]. Likewise, a lower post-heparin plasma HL activity has been related to a higher extent of coronary artery disease in 200 men undergoing coronary angiogra- phy [20]. In contrast, human studies investigating the relationship between polymorphisms in the HL gene, which supposedly resulted in reduced HL activity, and atheroscle- rotic cardiovascular disease did not provide a clear picture (summarized in Table 2). In an autopsy cohort of 700 middle-aged Finnish men, the HL C-480T promoter polymorphism was related to a larger coronary atheroscle- rotic plaque area [21], and in a large population-based Danish study homozygosity for three common single nucleotide polymorphisms in the HL promoter (−216A, −480T, and −729G) was associated with an increased risk of ischemic heart disease despite higher plasma HDL cholesterol [22]. Furthermore, the prevalence of the C to T substitution at position 514 of the HL gene was significantly higher in 490 middle-aged male Caucasian Australian patients with coronary heart disease compared with 330 controls [23]. Controversially, in the almost 9000 partic- ipants of the Copenhagen City Heart study, no association between risk for ischemic cardiovascular disease and six genetic variants of the HL gene was detectable [8]. Importantly, the frequency of the −514T allele of HL was identical between men with documented coronary artery disease and healthy controls [24]. Other researchers found in a multiethnic population that individuals with the CC