Hepatic SR-BI, not endothelial lipase, expression determines biliary cholesterol secretion in mice

Abstract

vascular disease ( 1,2 ). This protective effect of HDL is largely ascribed to the role of this lipoprotein in reverse cholesterol transport (RCT), a process comprising the movement of excess cholesterol from the periphery back to the liver for subsequent secretion into the bile ( 3,4 ). Within the plasma compartment, substantial remodeling of HDL particles occurs. A factor exerting a major impact in this regard is endothelial lipase (EL).EL has recently been identifi ed as a member of the triacylglycerol lipase gene family. It is expressed in endothelial cells as well as in macrophages and hepatocytes ( 5, 6 ). Remarkably, EL possesses merely phospholipase activity ( 7 ). EL expression is upregulated in vitro by proinfl ammatory stimuli ( 8, 9 ), and EL plasma levels correlate with the levels of proinfl ammatory cytokines in human populations ( 10, 11 ). In experimental animals, both overexpression ( 5, 12 ) as well as loss-of-function models ( 13-15 ) have established EL to be a negative regulator of plasma HDL cholesterol levels by increasing HDL catabolism. Moreover, accumulating evidence points to a comparable role of EL in human HDL metabolism (15)(16)(17).Consistent with the role of HDL in RCT, HDL is thought to represent a preferred source of sterols that are subsequently secreted into the bile ( 3,4,18,19 ). Currently, no data are available regarding the effect of an acute decrease of plasma HDL cholesterol levels on biliary sterol excretion caused by a single physiologically relevant stimulus. This study aimed to test the hypothesis that an acute, substantial decrease of plasma HDL cholesterol levels by EL overexpression impacts liver cholesterol metabolism and biliary cholesterol secretion. Our data demonstrate that in wild-type mice, virtual elimination of HDL cholesterol by Abstract High density lipoprotein cholesterol is thought to represent a preferred source of sterols secreted into bile following hepatic uptake by scavenger receptor class B type I (SR-BI). The present study aimed to determine the metabolic effects of an endothelial lipase (EL)-mediated stimulation of HDL cholesterol uptake on liver lipid metabolism and biliary cholesterol secretion in wild-type, SR-BI knockout, and SR-BI overexpressing mice. In each model, injection of an EL expressing adenovirus decreased plasma HDL cholesterol ( P < 0.001) whereas hepatic cholesterol content increased ( P < 0.05), translating into decreased expression of sterol-regulatory element binding protein 2 (SREBP2) and its target genes HMG-CoA reductase and LDL receptor (each P < 0.01). Biliary cholesterol secretion was dependent on hepatic SR-BI expression, being decreased in SR-BI knockouts ( P < 0.001) and increased following hepatic SR-BI overexpression ( P < 0.001). However, in each model, biliary secretion of cholesterol, bile acids, and phospholipids as well as fecal bile acid and neutral sterol content, remained unchanged in response to EL overexpression. Importantly, hepatic ABCG5/G8 expression did not correlate with biliary cholesterol secret...

ApoE promotes hepatic selective uptake but not RCT due to increased ABCA1-mediated cholesterol efflux to plasma

“…We therefore speculate that increased cholesterol uptake into the liver via SR-BI results in transport to a specifi c intrahepatic compartment also accessible for ABCA1-mediated resecretion back into the plasma compartment to generate new HDL particles. Increasing hepatic SR-BI expression levels ( 24,34 ) or decreasing the activity of ABCA1 might shift the balance toward biliary secretion (present report and Reference 31 ). However, the nature of these intrahepatic cholesterol pools is unknown.…”

“…Anti-infl ammatory activities of apoE also add to its atheroprotective properties. In vitro studies have shown that apoE can suppress proliferation of cultured peripheral blood T lymphocytes ( 42 ) and shifts polarization of or endothelial lipase (EL) ( 21,34 ). Transgenic overexpression of secretory phospholipase A 2 or adenovirusmediated overexpression of EL also resulted in an increased fl ux of cholesterol into the liver, whereas biliary cholesterol secretion and mass fecal excretion of sterols remained unaffected ( 21,23,34 ).…”

“…In vitro studies have shown that apoE can suppress proliferation of cultured peripheral blood T lymphocytes ( 42 ) and shifts polarization of or endothelial lipase (EL) ( 21,34 ). Transgenic overexpression of secretory phospholipase A 2 or adenovirusmediated overexpression of EL also resulted in an increased fl ux of cholesterol into the liver, whereas biliary cholesterol secretion and mass fecal excretion of sterols remained unaffected ( 21,23,34 ). On the other hand, altering the hepatic expression level of SR-BI has clear effects on biliary cholesterol secretion as well as RCT with overexpression resulting in an increase and knockdown in a decrease of both of these parameters ( 24,34,35 ).…”

Update on the Role of Endothelial Lipase in High-Density Lipoprotein Metabolism, Reverse Cholesterol Transport, and Atherosclerosis

“…Interestingly, EL overexpression has been reported to markedly reduce plasma HDL-C levels and increases hepatic cholesterol content without affecting biliary sterol secretion. 51 Furthermore, Brown et al have recently reported that EL-and/or HL-deficient mice exhibited the same degree of in vivo RCT as wild-type mice, despite increased plasma HDL-C levels. 49 These findings suggest that, when the complete RCT is defined as cholesterol flux from macrophages to feces, the net effect of EL on RCT remains speculative (Figure 2).…”

Intestinal SR-BI does not impact cholesterol absorption or transintestinal cholesterol efflux in mice

“…For SR-BI knockout studies, fractional cholesterol absorption and fecal neutral and acidic sterol loss were measured using slightly different methodology as previously described ( 63 mice, feces of individually housed mice were collected over a period of 24 h. Fecal samples were dried, weighed, and thoroughly ground. Aliquots were used for determination of bile acids and neutral sterols by gas-liquid chromatography as described ( 62 ).…”