Liver X receptors as integrators of metabolic and inflammatory signaling (original) (raw)

In addition to their essential role in innate immunity, macrophages are central to the development of the atherosclerotic lesion because of their ability to take up modified lipoproteins and to release inflammatory mediators (45, 46). Within the lesion, macrophages are postulated to accumulate ligands of LXRs by several distinct pathways. Uptake of modified lipoproteins may provide the cell with preformed oxysterol activators of LXR. Ligands may also be generated intracellularly from accumulated cholesterol by action of the mitochondrial Cyp27 (47). Although Cyp27 is not a direct target of LXRs, its enzymatic product 27-hydroxycholesterol is an LXR ligand (48, 49), albeit a relatively weak one (50). Additionally, the intracellular production of 24-(S),25-epoxycholesterol, a potent naturally occurring LXR ligand, in rodent and human macrophages was reported recently (51). Increasing the levels of this metabolite by partially inhibiting the enzyme 2,3-oxidosqualene:lanosterol cyclase results in increased LXR transcriptional activity (52).

A primary function of LXRs in macrophages is to maintain cellular cholesterol homeostasis. Activation of LXRs in lipid-loaded macrophages leads to induction of genes involved in the cholesterol efflux pathway in an attempt to reduce the intracellular cholesterol burden. The ABC transporters discussed above are critical for the ability LXRs to enhance efflux to cholesterol acceptors. Expression of ABCA1 is strongly induced by natural and synthetic LXR ligands as well as by loading of cells with modified lipoproteins. This induction has been attributed to the presence of LXREs in the proximal promoter of the ABCA1 gene (15, 17, 26, 27). LXRs are in fact essential for lipid-inducible ABCA1 expression, as induction is lost in macrophages from Lxrab double-knockout mice (_Lxrab–/–_mice). Conversely, LXRs are unable to stimulate cholesterol efflux to lipid-poor lipoproteins in fibroblasts from Tangier disease patients, demonstrating that ABCA1 is essential for the LXR-mediated efflux pathway (27). The importance of ABCA1 for atherogenesis is underscored by the fact that macrophage-specific loss of this gene results in increased lesion formation in murine models (53, 54).

ABCG1, another member of the ABC transporter family, is also strongly induced by cholesterol loading of macrophages (22, 55) and was recently identified as a direct target of LXRs in mouse and human cells (20, 21). Induction of ABCG1 may provide an additional pathway for cholesterol efflux from macrophages or may act in concert with ABCA1. ABCG1 is thought to function as a homodimer (56), although a functional partnership with ABCG4 has been also suggested (57). In in vitro assays, ABCG1 has been demonstrated to facilitate cholesterol efflux to HDL-2 and -3 particles, but not to apoA-I, thus distinguishing it mechanistically from ABCA1 (55, 56, 58). At present, however, the cellular localization of ABCG1 is not defined, and it is therefore unclear whether ABCG1 directly mediates efflux to HDL particles or facilitates this process by influencing intracellular cholesterol trafficking. In line with the latter possibility is a recent study demonstrating that activation of LXRs in human macrophages boosts cholesterol trafficking to the plasma membrane at the expense of esterification (59).

The generation and initial characterization of _Abcg1–/–_mice has revealed striking phenotypes that point to a critical function for this transporter in whole-body lipid homeostasis (58). In support of in vitro experiments, macrophages lacking ABCG1 showed a diminished cholesterol efflux capacity to HDL. Cholesterol efflux to apoA-I, which is mainly mediated by ABCA1, was unchanged, however. In accordance with these findings, lipid-laden macrophages were detected in the lungs and liver of _Abcg1_-null mice after 9 weeks of a high-fat and -cholesterol diet. Remarkably, this phenotype was not accompanied by changes in the profile of plasma lipoproteins. It is tempting to speculate that ABCG1 activity, like ABCA1 activity, would be antiatherogenic, but this remains to be tested directly. The closely related protein ABCG4 is also modestly induced in macrophages by cholesterol loading and by LXR ligands and has been reported to promote cholesterol efflux to HDL particles when overexpressed in HEK293 cells (56, 60). Studies of the physiological roles of this transporter are eagerly awaited.

An additional mechanism that may contribute to the LXR-driven reverse cholesterol transport is the induction of a subset of apolipoproteins that may serve as cholesterol acceptors. Specifically, LXRs induce Apoe gene expression in macrophages and adipose tissue, but not in the liver (61). Additionally, the Apoc gene cluster (ApocI, ApocII, and ApocIV) is also induced by LXRs in macrophages (62), and Apod is a target for LXR in adipose tissue (63). The significance of the induction of the Apoc cluster and of Apod by LXR for lipoprotein metabolism is at present unknown. In contrast, the protective role of Apoe in atherogenesis is well established. Loss of macrophage apoE leads to increased lesions, whereas overexpression of apoE in these cells is protective (reviewed in ref. 64). More recently, LXR was shown to directly regulate hepatic, but not intestinal, Apoa4 in mouse liver, and APOA4 in the human HepG2 cell line (65). In humans, plasma levels of APOA4 are inversely correlated with cardiovascular disease. Whether APOA4 is regulated in macrophages by LXR is at present unknown. APOA5 is the only apolipoprotein known to be repressed by LXRs (66). Repression of hepatic APOA5 by LXR is not direct but appears to be secondary to induction of SREBP-1c. As increased levels of APOA5 are strongly correlated with reduced plasma triglycerides (67), repression by LXR may contribute to the hypertriglyceridemic effects of synthetic LXR agonists (32).

An additional level of LXR regulation, beyond ligand availability, is the level of LXRα receptor expression. In human and rodent macrophages, PPARγ agonists induce the expression of LXRα, suggesting a functional link between the uptake of oxidized LDL (oxLDL) and cholesterol removal (68). Furthermore, in human macrophages LXRα is able to induce its own transcript via an autoregulatory loop (69, 70). This autoregulation does not occur in rodent macrophages, however.

Collectively, the studies highlighted above point to the central role of LXRs in governing cholesterol efflux from macrophages. Under conditions of increased intracellular cholesterol levels, as is the case in lesion macrophages, these pathways would be expected to impact disease development. However, the presence of lipid-laden macrophages in various tissues of _Lxrab_–/– mice demonstrates that these same pathways are important for normal cholesterol homeostasis as well (71, 72).