Liver X receptor activation potentiates the lipopolysaccharide response in human macrophages

Macrophages play a central role in host defense against pathogen microbes by recognizing bacterial components, resulting in the activation of an arsenal of anti- microbial effectors. Toll-like receptor (TLR)-4 mediates the recognition of lipopolysaccharide, a pathogen-associated molecular pattern from Gram- negative bacteria. Activation of the TLR-4 signaling pathway by lipopolysaccharide increases antibacterial effects by inducing secretion of cytokines that activate an immune inflammatory response and by generating bactericidal reactive oxygen species via the NADPH oxidase system. Liver X Receptors (LXRs) are nuclear receptors controlling cholesterol homeostasis and inflammation in macrophages. In addition, LXRs are critical for macrophage survival and play a role in the innate immune response in the mouse. In this study, we investigated whether LXR activation also regulates host defense mechanisms in human macrophages. In primary human macrophages, oxidized LDL and synthetic LXR ligands increased TLR- 4 gene expression. Transient transfection assays, gel shift and chromatin immunoprecipitation analysis indicated that LXRs induce human TLR- 4 promoter activity by binding to a DR4-type LXR response element. LXR induction of TLR-4 mRNA was followed by an induction of TLR-4 protein expression. Moreover, although short- term pretreatment with LXR agonists significantly reduced the inflammatory response induced by lipopolysaccharide, pretreatment of macrophages for 48 hours with LXR agonists resulted in an enhanced lipopolysaccharide response. Finally, LXR activation increased reactive oxygen species generation by enhancing the expression of NADPH oxidase subunits. These data provide evidence for an immunomodulatory function of LXRs in human macrophages via mechanisms distinct from those previously identified in mouse macrophages.