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A4747 - Very Low Density (VLDL) and Low Density Lipoproteins (LDL) Mediate the Uptake of Pathogen Lipids and Stimulate the Production of IL-1β in THP-1 Macrophages
Author Block: A. K. Leung, J. A. Russell, J. H. Boyd, K. R. Walley; Centre for Heart Lung Innovation, St. Paul's Hospital, The University of British Columbia, Vancouver, BC, Canada.
Rationale: Previously, we have discovered pathogen lipids carried in low density lipoproteins (LDL) are cleared from the circulation via hepatic LDL receptors (LDLR) and Proprotein convertase subtilisin-kexin type 9 (PCSK9) negatively correlates with this hepatic clearance by down-regulating LDLR expression on the surface of hepatocytes. In addition to hepatocytes, macrophages also express targets of PCSK9, including LDLR and VLDLR, and intracellular localization of these pathogen lipids has been linked to inflammasome activation and IL-1β production in macrophages. Therefore we hypothesize that lipoproteins, in particular VLDL and LDL, facilitate the internalization of pathogen lipids into macrophages, leading to inflammasome activation and IL-1β production.
Methods: We used the human leukemia monocytic cell line THP-1 as an in vitro model of pathogen lipid uptake and inflammatory response in macrophages. THP-1 cells were differentiated into macrophages with phorbol 12-myristate 13-acetate (PMA). Lipoproteins were isolated from the blood of pooled healthy donors. Lipopolysaccharide (LPS) and Lipoteichoic Acid (LTA) were incubated with isolated lipoproteins for 24 hours prior to addition to cells. Inflammatory IL-1β levels in cell-free media were measured using enzyme-linked immunosorbent assay (ELISA) to monitor the inflammatory response in THP-1 following treatment with lipoprotein-incubated pathogen lipids. Experiments were repeated in the presence of PCSK9. Flow cytometry was used to measure the cellular uptake of pathogen lipids.
Results: IL-1β levels were higher in cells treated with either VLDL- or LDL-bound LPS. Surprisingly, HDL-bound LPS elicited an IL-1β response that was one quarter of that observed in VLDL and LDL. Results were similar in cells treated with LTA. Uptake of pathogen lipids as monitored by flow cytometry were similar among the 3 classes of lipoproteins tested. Consistent with its ability to down-regulate the expression of VLDLR and LDLR, PCSK9 treatment greatly reduced the production of IL-1β in cells treated with VLDL- and LDL-bound LPS, while it had no effect on cells treated with HDL-bound LPS.
Conclusions: The ability of pathogen lipids to induce an IL-1β response in macrophages is dependent on the class of lipoproteins to which pathogen lipids are bound to. Our results suggest the possibility of modulating the inflammatory response of macrophages through strategies to influence the binding of pathogen lipids to different classes of lipoproteins.
This work is funded by the Canadian Institutes of Health Research (CIHR) and the Michael Smith Foundation for Health Research (MSFHR).