C-reactive protein induces pro- and anti-inflammatory effects, including activation of the liver X receptor alpha, on human monocytes.

Non-specific markers of inflammation such as C-reactive protein (CRP) are associated statistically with an increased risk of atherosclerosis through mechanisms that have not yet been fully elucidated. We investigated the effects of CRP on several aspects of human monocyte biology, a cell type involved in the initiation and progression of atherosclerosis. Blood monocytes isolated from healthy men and premenopausal women (n = 9/group) were exposed to purified CRP (25 microg/ml) for 12 hours. Changes in gene expression were analyzed using a custom-made array containing oligonucleotide sequences of 250 genes expressed by activated monocytes and confirmed by quantitative PCR. CRP increased significantly the expression of the cytokines interleukin (IL)-1alpha, IL-1beta and IL-6, and the chemokines GRO-alpha, GRO-beta and IL-8. CRP also displayed anti-inflammatory effects through upregulation of liver X receptor (LXR) alpha and activin receptor expression, and down-regulation of alpha 2-macroglobulin expression. Increased LXRalpha mRNA expression in both monocytes and the monocytic cell lineTHP-1 was associated with increased LXRalpha protein expression and nuclear translocation, as well as increased ABCA1 mRNA expression, a target gene of LXRalpha. Western blot analysis revealed CRP-induced nuclear translocation of NF-kappaB and activation of p42/44, MAP and Akt kinases. CRP-induced LXRalpha mRNA expression was inhibited by anti-CD64 (FcgammaRI) antibodies and by p42/44 and PI3 kinase inhibitors. This hypothesis-generating study demonstrates that CRP modulates the expression of genes that contribute to both pro- and anti-inflammatory responses in human monocytes. Among these novel anti-inflammatory effects, we show clearly that CRP activates the LXRalpha pathway.