Lack of Abcg1 results in decreased plasma HDL cholesterol levels and increased biliary cholesterol secretion in mice fed a high cholesterol diet
Introduction
Atherosclerotic cardiovascular disease (CVD) is the prime cause of mortality in developed countries [1], [2]. Plasma levels of high density lipoprotein (HDL) cholesterol are closely inversely correlated with the risk of atherosclerotic CVD [1], [2]. These protective effects of HDL are largely ascribed to the central role of the HDL particle in reverse cholesterol transport (RCT) [3], [4]. RCT comprises the transport of excess cholesterol from the periphery back to the liver for excretion into bile and final disposition into feces. In this respect, biliary cholesterol secretion is generally considered as an important step in the completion of RCT [3], [4].
Different ATP Binding Cassette (ABC) transporters play essential roles in HDL metabolism and atherosclerotic lesion formation (e.g. ABCA1) as well as biliary cholesterol excretion (e.g. ABCG5/ABCG8) [5]. Recently, ABCG1 was identified as a novel 75.6 kDa ABC transporter expressed in macrophages and hepatocytes, amongst other cell types, that is mediating cellular cholesterol efflux to HDL particles in vitro [6], [7], [8] and contributing to RCT in vivo [9]. However, the impact of Abcg1 on experimental atherosclerosis has not been unequivocally established. Whole body Abcg1 knockout mice fed a cholate-containing atherogenic diet showed accelerated atherogenesis [10]. Conversely, transgenic overexpression of murine Abcg1 on the Ldlr-deficient genetic background also increased atherosclerotic lesion formation in response to a Western-type diet [11]. In bone marrow transplant experiments, macrophage Abcg1 has been demonstrated to exert moderate effects on atherosclerotic lesion formation, although these reports were not consistent and the outcome conceivably depended on the plasma total cholesterol levels [12], [13], [14], [15]. The role of hepatocyte Abcg1 has not been experimentally addressed thus far. However, it has been demonstrated that Abcg1 is expressed in hepatocytes and that liver Abcg1 expression is significantly increased in response to pharmacological Lxr activation [8]. Interestingly, Abcg1 has been recognized to not only be expressed at the plasma membrane but also intracellularly and a role for Abcg1 in intracellular metabolism and routing of cholesterol has been suggested [16], [17], [18].
Therefore, the aim of the present study was to determine the effects of Abcg1 on plasma HDL cholesterol levels and biliary cholesterol secretion using Abcg1 knockout mice fed either chow or a high cholesterol diet (1%) or administered the synthetic LXR agonist T0901317. Our results demonstrate that (i) general Lxr activation is required to elicit a raise in plasma HDL cholesterol attributable to the expression of Abcg1 and (ii) lack of Abcg1 results in increased biliary cholesterol secretion in response to hepatic cholesterol loading suggesting that in hepatocytes Abcg1 is accessing a cholesterol pool available either for efflux towards plasma or secretion into bile.
Section snippets
Animals
Abcg1-null/LacZ knockin mice (Deltagene Inc., San Carlos, CA, USA) were backcrossed to the C57BL/6 background for 7 generations. The animals were caged in animal rooms with alternating 12-h periods of light and dark, with ad libitum access to water and chow diet. For the experiments described, 8 weeks old Abcg1−/− mice and wild-type littermates were either fed (i) mouse chow diet (Abdiets, Woerden, The Netherlands) as a control or (ii) a 1% cholesterol diet (Abdiets, Woerden, The Netherlands)
Hepatocyte expression of Abcg1 is induced by feeding a high cholesterol diet
Abcg1 expression has been reported to be responsive towards Lxr activation [8]. To increase hepatocyte Abcg1 in wild-type mice, we employed two different strategies: (i) a dietary approach by feeding a diet containing 1% cholesterol for 2 weeks and (ii) a pharmacological approach by administering the LXR agonist T0901317 for 7 days. Then hepatocytes were isolated and gene expression was determined by quantitative real-time PCR. Compared with chow-fed mice, Abcg1 expression specifically in
Discussion
The results of this study demonstrate that in mice (i) Lxr activation is required for Abcg1 to contribute to plasma HDL cholesterol levels and (ii) Abcg1 modulates hepatic cholesterol metabolism.
ABCG1 has been characterized by several groups as a cellular transporter mediating mass cholesterol efflux towards HDL particles [6], [7], [8], in contrast to ABCA1, which is mainly responsible for efflux towards apoA-I [1], [2], [3]. Thus far, the analysis of the function of ABCG1 has been largely
Conflict of interest
The authors have no conflicts of interest to disclose.
Acknowledgments
We are indebted to Rick Havinga for expert technical assistance. This work was supported by the Netherlands Organization for Scientific Research (VIDI Grant 917-56-358 to U.J.F.T.).
We are shocked and deeply grieved by the untimely death of the first author of this study.
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