Oxidized Low-Density Lipoprotein Loading of Macrophages Downregulates TLR-Induced Proinflammatory Responses in a Gene-Specific and Temporal Manner through Transcriptional Control

Hypercholesterolemia is a key risk factor for atherosclerosis and leads to the uptake of native and oxidized low-density lipoprotein (oxLDL) by macrophages (M phi s) and foam cell formation. Inflammatory processes accompany M phi foam cell formation in the artery wall, yet the relationship between Mf lipid loading and their response to inflammatory stimuli remains elusive. We investigated proinflammatory gene expression in thioglycollate-elicited peritoneal M phi s, bone marrow-derived Mfs and dendritic cells, and RAW264.7 cells. Loading with oxLDL did not induce peritoneal M phi apoptosis or modulate basal-level expression of proinflammatory genes. Upon stimulation of TLR4, the rapid induction of IFN-beta was inhibited in cells loaded with oxLDL, whereas the induction of other proinflammatory genes by TLR4 (LPS), TLR3 (polyriboinosinic-polyribocytidylic acid), TLR2 (Pam(3)CSK(4)), and TLR9 (CpG) remained comparable within the first 2 h. Subsequently, the expression of a subset of proinflammatory genes (e.g., IL-1 beta, IL-6, CCL5) was reduced in oxLDL-loaded cells at the level of transcription. This phenomenon was partially dependent on NF erythroid 2-related factor 2 (NRF2) but not on nuclear liver X receptors alpha and beta (LXR alpha,beta), peroxisome proliferator-activated receptor-gamma (PPAR gamma), and activating transcription factor 3 (ATF3). LPS-induced NF-kappa B reporter activity and intracellular signaling by NF-kappa B and MAPK pathways were comparable in oxLDL-loaded M phi s, yet the binding of p65/RelA (the prototypic NF-kappa B family member) was reduced at IL-6 and CCL5 promoters. This study revealed that oxLDL loading of M phi s negatively regulates transcription at late stages of TLR-induced proinflammatory gene expression and implicates epigenetic mechanisms such as histone deacetylase activity.