HDL particles are substrates for EL in in vitro assays (7, 28, 29). In vivo, EL overexpression has been shown to increase the catabolic rate of HDL apolipoproteins as the underlying metabolic sellekchem mechanism of decreased HDL cholesterol plasma levels (12). Analogous to other lipases, also for EL a nonlipolytic ligand function has been demonstrated that might represent an alternative mechanistic basis contributing to the results obtained in our study. However, the liganding function of EL might be less relevant for the in vivo effect of EL on HDL metabolism compared with the lipolytic activity of EL (30). Therefore, as a working model, EL-mediated hydrolysis of HDL phospholipids has been proposed to result in destabilization of the HDL particle, followed by shedding of poorly lipidated apoA-I molecules that are then more rapidly cleared by the kidneys (9).

Our present study confirms and extends these observations by showing that, besides uniformly mediating decreased plasma HDL cholesterol levels in all models used, hepatic EL expression also results in a net increase in hepatic cholesterol content by enhancing HDL selective as well as holoparticle uptake. A likely candidate system to mediate HDL holoparticle uptake in the absence of SR-BI is the recently described complex containing the ectopic �� -chain of ATP synthase (31). This enzyme generates extracellular ADP upon HDL binding, which then activates the nucleotide receptor P2Y13 resulting in clathrin-dependent HDL holoparticle endocytosis (32).

However, independent of the underlying mechanism, biliary cholesterol secretion is apparently unaffected by an acute influx of HDL-derived cholesterol. Interestingly, in the three models with different hepatic SR-BI expression used, the EL-mediated increase in hepatic cholesterol content did not affect the gene expression levels of the heterodimer ABCG5/G8. ABCG5/G8 are LXR target genes and were recently identified to play a key role in biliary cholesterol secretion (33). In ABCG5/G8 knockout mice, biliary cholesterol secretion is severely reduced (34, 35), whereas it is significantly increased in response to ABCG5/G8 overexpression in hepatocytes (36). The lack of an increase in mRNA levels of these key proteins mediating biliary cholesterol secretion in our models is consistent with the physiological data we obtained.

However, ABCG5/G8�Cindependent biliary cholesterol secretion pathways have been suggested to occur. While ABCG5 knockout mice have residual cholesterol AV-951 secretion that is subject to stimulation (37), correlation studies in different mouse models (38) as well as in humans (39) indicated that biliary cholesterol secretion might be independent from the expression of ABCG5/G8 within liver. In addition, a recent study demonstrated that in ATP8B1-deficient mice increased biliary cholesterol secretion is independent from the expression of ABCG8 (40).