(760g) Integrated Transcriptomic and Lipidomic Study of Macrophage Response to Liver X Receptor Ligand 25-Hydroxy-Cholesterol

Macrophage inflammation and resulting increase in the levels of oxysterols plays an important role in the progression of cardiovascular diseases such as atherosclerosis. Upon stimulation with pathogen markers such as lipopolysaccharide (LPS), primary macrophages such as bone-marrow derived macrophages (BMDMs) as well as RAW 264.7 cells (a leukemia-virus transformed macrophage cell line) show substantial lipidomic and transcriptomic changes. While lipids from several categories such as eicosanoids, other fatty acids, sphingolipids and sterols show changes, our focus here is on the changes in sterol levels. We have found that 25-hydroxy-cholesterol (25-OH-Ch) accumulates inside the cell even though the levels of its precursor, cholesterol, do not change substantially. Since 25-OH-Ch is a ligand for liver X receptors (LXRs), its increase results in up-regulation of its target genes such as ATP-binding cassette transporter (ABC) A1 (Abca1) and Abcg1 and down-regulation of some of the cholesterol biosynthesis genes. Abca1 and Abcg1 contribute to cholesterol-efflux. This results in negative feedback regulation of cholesterol homeostasis. Motivated by this observation, new experiments carried out where BMDMs are treated with 25-OH-Ch show substantial up-regulation of Abca1 and Abcg1 and down-regulation of lanosterol synthase (Lss). Sterol (de novo) biosynthetic genes such as HMG-CoA reductase (Hmgcr) and Squalene synthase (Sqle) are also down-regulated, albeit through sterol regulatory element (SRE; 10bp sequence) -binding protein (SREBP) 2, a LXR-independent mechanism. The overall negative feedback regulation of sterol biosynthesis and increased efflux of cholesterol results in relatively reduced levels of cholesterol in the cells. Further, cholesterol efflux results in reverse-cholesterol transport to the liver. Given that high-levels of oxysterols, often resulting from inflammation, contribute to atherosclerosis, LXR and SREBP2 and associated molecules are being investigated as putative drug targets. In this work, we will present a combined analysis of lipidomic and transcriptomic data from 25-OH-Ch stimulated BMDMs as it relates to the regulation of various targets of LXRs and SREBP and remodeling and homeostasis of various types of lipid and other pathways.

Acknowledgement: The experimental data used in this work was provided by the Lipid Metabolite and Pathway Strategy (LIPID MAPS) Consortium experimental laboratories and is available on the LIPID MAPS web site (www.lipidmaps.org). LIPID MAPS is funded by the National Institute of General Medical Sciences.