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LXR-Dependent Gene Expression Is Important for Macrophage Survival and the Innate Immune Response

Cell, Vol 119, 299-309, 15 October 2004

Sean B. Joseph,^1,4 Michelle N. Bradley,^1,4 Antonio Castrillo,^1,4 Kevin W. Bruhn,² Puiying A. Mak,³ Liming Pei,¹ John Hogenesch,³ Ryan M. O'Connell,² Genhong Cheng,² Enrique Saez,³ Jeffery F. Miller,² and Peter Tontonoz¹

The nuclear receptors LXRα and LXRβ are important regulators of lipid metabolism in many cell types. LXRs bind to DNA as heterodimers with the retinoid X receptor （RXR）. Physiologic activators for the LXRs include oxysterols and intermediates in the cholesterol biosynthetic pathway （Janowski et al., 1996; Lehmann et al., 1997）. LXR activity controls bile acid synthesis in the liver and regulates cholesterol absorption in the intestine （Peet et al., 1998; Repa et al., 2000; Tangirala et al., 2002）. In macrophages, scavenger receptor-mediated uptake of oxidized lipoproteins leads to the transcriptional activation of LXRs and the induction of genes, such as ABCA1 and apoE, that facilitate cholesterol removal from the cell （Repa and Mangelsdorf, 2000; Tontonoz and Mangelsdorf, 2003）. Both LXRα and LXRβ are expressed at high levels in macrophages, and activation of either receptor promotes cholesterol efflux, indicating that the function of these receptors in macrophage lipid metabolism is largely overlapping.

Macrophages are essential for innate and adaptive immunity; however the function of LXRs in this context is not yet clear. Macrophages play a direct role in microbial killing and orchestrate inflammatory responses through the release of immune modulators such as chemokines and cytokines. The same scavenger receptors that mediate uptake of oxidized lipoproteins during atherosclerosis （e.g., SR-A, CD36） are also critical for the clearance of bacterial pathogens and apoptotic cells （Glass and Witztum, 2001; Terpstra et al., 2000）. Accordingly, defects in macrophage scavenger receptor function are associated with pathological susceptibility to microbial infection （Suzuki et al., 1997; van der Laan et al., 1999）. Previous studies have shown that expression of certain scavenger receptors can be modulated by nuclear receptors. For example, PPARγ positively regulates CD36 and negatively regulates SR-A （Ricote et al., 1998; Tontonoz et al., 1998）. However, the significance of this regulation in the setting of microbial infection is unknown.

Recent studies have also revealed the existence of crosstalk between macrophage inflammatory pathways and nuclear receptor signaling. Synthetic ligands for several different orphan nuclear receptors, including PPARγ, PPARα, PPARδ, and LXR, have been reported to inhibit inflammatory gene expression （Joseph et al., 2003; Lee et al., 2003; Ricote et al., 1998; Ziouzenkova et al., 2003）. For example, LXR ligands inhibit the LPS- or cytokine-induced expression of inflammatory genes such as iNOS and IL-6 by interfering with NF-kB signaling （Joseph et al., 2003）. Other studies have shown that activation of TLR3 or TLR4 by microbial ligands inhibits the expression of LXR-dependent cholesterol efflux genes through a mechanism involving the transcription factor IRF3 （Castrillo et al., 2003）. These observations have implications for the pharmaceutical control of inflammation and the pathogenesis of atherosclerosis; however, the role of such crosstalk in normal physiology remains unknown. The possibility that nuclear receptor signaling may be directly involved in antimicrobial responses has not been adequay explored. We demonstrate here that LXR-dependent gene expression impacts macrophage function in the setting of bacterial infection. These results outline an unexpected role for an orphan nuclear receptor pathway in innate immunity.

Mice Lacking LXRs Are Susceptible to Bacterial Infection
To investigate the potential involvement of LXRs in innate immunity, we challenged LXRαβ^−/− mice with the gram-positive intracellular bacteria Listeria monocytogenes （LM）. Infection with LM triggers an immediate innate response involving macrophages, neutrophils, and natural killer cells. This model organism has been used to define the importance of numerous proteins in immune function （Edelson and Unanue, 2000; North et al., 1997）. Mice lacking expression of LXRs were highly susceptible to infection with LM （Figure 1A） . Whereas LXRαβ^+/+ （WT） mice effectively cleared an i.v. dose of 1 × 10⁴ cfu, none of the background-matched LXRαβ^−/− mice survived （p < 0.0006）. At higher doses of LM （1 × 10⁵ cfu）, LXRαβ^−/− mice succumbed to the infection 2–3 days sooner than their WT controls （p < 0.0009）. LXR-null mice also showed increased susceptibility when compared to pure strain C57bl/6 or sv129 controls （not shown）. Surprisingly, susceptibility to LM was associated primarily with the loss of LXRα （Figure 1A）. This LXRα-selective phenotype was unexpected since the macrophage cholesterol efflux pathway is redundantly controlled by both LXRs （Laffitte et al., 2001b）.