Abstract 1277: Group X Secretory Phospholipase A2 Negatively Regulates Activation of Liver X Receptors
Background: GX sPLA2 potently hydrolyzes plasma membranes to liberate lysophospholipids and free fatty acids and has been implicated in inflammatory diseases including atherosclerosis. Polyunsaturated fatty acids antagonize ligand-dependent activation of liver X receptors (LXR), nuclear receptors known to reciprocally regulate genes involved in lipid metabolism and inflammation. We tested the hypothesis that lipolytic products generated by GX sPLA2 suppress LXR target gene expression by blocking LXR activation.
Methods and Results: Three in vitro systems were utilized:
J774 cells stably transfected with mGX sPLA2 (J774-GX cells);
J774 cells treated with recombinant GX sPLA2; and
Peritoneal macrophages isolated from GX sPLA2-deficient (GX KO) mice.
Increased GX sPLA2 significantly reduced LXR target gene expression (ABCA1, ABCG1 and SREBP1-c) in macrophages, whereas GX sPLA2 deficiency was associated with enhanced expression. The effect of GX sPLA2 was dependent on its hydrolytic activity, as evidenced by experiments using a catalytically inactive mutant or an inhibitor ofsPLA2 activity. Cholesterol efflux was reduced in J774-GX cells compared to control cells, indicating that down-regulation of ABCA1 and ABCG1 had functional consequences. Addition of arachidonic acid (AA), but not lysophosphatidylcholine, mimicked the effects of GX sPLA2 on LXR target gene expression. As expected, J774-GX cells exhibited significantly increased AA release and prostaglandin E2 generation compared to control cells. Cyclooxygenase inhibition did not normalize ABCA1 or ABCG1 expression in J774-GX cells indicating that GX sPLA2 does not regulate LXR target genes by amplifying prostanoid synthesis. LXR-α promoter luciferase reporter gene assays indicated that GX sPLA2 antagonizes the activation of LXR. ChIP assays confirmed that overexpression of GX sPLA2 inhibits LXRα/β binding to both the ABCA1 and SREBP1-c promoters.
Conclusions: We conclude that fatty acids released by GX sPLA2 repress transcription of LXR target genes by preventing the ability of LXR to trans-activate their promoters. Our findings identify a previously unrecognized mechanism by which GX sPLA2 modulates macrophage lipid metabolism and inflammatory responses.