Modulation of Macrophage Gene Expression via Liver X Receptor α Serine 198 Phosphorylation

Abstract

e In mouse models of atherosclerosis, normalization of hyperlipidemia promotes macrophage emigration and regression of atherosclerotic plaques in part by liver X receptor (LXR)-mediated induction of the chemokine receptor CCR7. Here we report that LXR␣ serine 198 (S198) phosphorylation modulates CCR7 expression. Low levels of S198 phosphorylation are observed in plaque macrophages in the regression environment where high levels of CCR7 expression are observed. Consistent with these findings, CCR7 gene expression in human and mouse macrophages cell lines is induced when LXR␣ at S198 is nonphosphorylated. In bone marrow-derived macrophages (BMDMs), we also observed induction of CCR7 by ligands that promote nonphosphorylated LXR␣ S198, and this was lost in LXR-deficient BMDMs. LXR␣ occupancy at the CCR7 promoter is enhanced and histone modifications associated with gene repression are reduced in RAW264.7 cells expressing nonphosphorylated LXR␣ (RAW-LXR␣ S198A) compared to RAW264.7 cells expressing wild-type (WT) phosphorylated LXR␣ (RAW-LXR␣ WT). Expression profiling of ligand-treated RAW-LXR␣ S198A cells compared to RAW-LXR␣ WT cells revealed induction of cell migratory and anti-inflammatory genes and repression of proinflammatory genes. Modeling of LXR␣ S198 in the nonphosphorylated and phosphorylated states identified phosphorylation-dependent conformational changes in the hinge region commensurate with the presence of sites for protein interaction. Therefore, gene transcription is regulated by LXR␣ S198 phosphorylation, including that of antiatherogenic genes such as CCR7.

Inhibiting LXRαPhosphorylation in Hematopoietic Cells Reduces Inflammation and Attenuates Atherosclerosis and Obesity

“…Whereas LXRa pS196 promotes an inflammatory gene signature, unphosphorylated LXRa stimulates an anti-inflammatory gene expression profile. Consistent with this idea, in mouse models we observed an increase in LXRa pS196 in plaque macrophages under inflammatory conditions associated with atherosclerosis progression, and a decrease in LXRa pS196 during the resolution of inflammation in regressing plaques (16,18). Cholesterol loaded macrophages and hepatocytes also showed increased LXRa pS196 (15,19).…”

“…We and others have shown that LXRa's ability to activate transcription is modulated by phosphorylation at serine 196 (S196 in mouse LXRa and S198 in human LXRa), which significantly modifies its target gene repertoire (14)(15)(16)(17). Whereas LXRa pS196 promotes an inflammatory gene signature, unphosphorylated LXRa stimulates an anti-inflammatory gene expression profile.…”

Macrophages and Dendritic Cells

“…119 On reversal of hypercholesterolemia or treatment with statins, the induction of CCR7 in plaque macrophages may promote plaque regression. [120][121][122][123][124][125] Collectively these data suggest that monocyte recruitment and macrophage proliferation, but not macrophage egress, are the key cellular events regulating foam cell lesion progression.…”

Liver X receptors in immune cell function in humans

“…Instead of directly interacting with its target genes, ligand-induced SUMOyLation of LXR allows it to bind to co-repressor complexes, preventing their release and the subsequent transcription of pro-inflammatory target genes of the transcription factors NF-K B and activator protein 1 (AP-1) [17]. Interestingly, we also reported that LXR phosphorylation affects corepressor interactions and chromatin modifications in regulatory sites of target genes [18,19].…”