Napoleon Busbuso

Introduction. Cardiovascular disease originating from atherosclerosis is the leading cause of death worldwide¹. In the setting of dyslipidemia, LDL accumulates within the intima of arteries, and subsequent oxidation of LDL by free radicals incites a persistent inflammatory reaction that ultimately produces an atherosclerotic plaque². Macrophages appear in the intima to clear the oxidized LDL and, with time, convert to lipid-laden foam cells that recruit more immune cells and further develop the plaque². Characterizing and ultimately preventing the persistent epigenetic changes that underlie this conversion represents a potential avenue for slowing the progression of atherosclerosis³. Additionally, the NLRP3 inflammasome of macrophages is hypothesized to sense oxidized LDL and induce inflammation through production of IL-1ß^4,5. Methods. To assess the ability of oxidized LDL to induce persistent changes in proinflammatory gene expression in macrophages, isolated human monocytes were incubated with oxidized LDL for 24 hours with subsequent 24-hour washout and were then stimulated to differentiate into macrophages with LPS³. Methylation status of promoters of proinflammatory genes were analyzed by chromatin immunoprecipitation³. In a similar protocol, expression of proinflammatory cytokines was quantified by mRNA³. To determine the role of the NLRP3 inflammasome in plaque formation, mice lacking both NLRP3 and the LDL receptor were generated⁴. Atherosclerotic plaques within the aorta were then assessed after 8-weeks of feeding with western diet, which is 21.2% fat and 48.5% carbohydrate in composition⁴. Results. Promoters of proinflammatory genes showed increased methylation in isolated human monocytes and were largely consistent with increased expression when exposed to oxidized LDL prior to stimulation with LPS³. mRNA of proinflammatory cytokines were elevated in a similar protocol³. Mice lacking both NLRP3 and the LDL receptor showed reduced atherogenesis within the aorta following 8-week western diet feeding compared to mice lacking only the LDL receptor⁴. Conclusions. The data suggests that blocking the epigenetic changes in macrophages that result in persistently elevated production of inflammatory cytokines would slow the development of a plaque³. In addition, the NLRP3 inflammasome and subsequent production of IL-1ß appears to be the bridge between oxidized LDL and the persistent inflammation seen in atherosclerosis⁴. Further work to target delivery of therapeutic agents (histone methyltransferase inhibitors and IL1-RAs) only to desired cells is needed to bring these approaches to slowing the course of atherosclerosis closer to use in the clinic^4,5.

1. Wolf D, Ley K. Immunity and Inflammation in Atherosclerosis. Circulation Research. 2019;124(2):315-327. doi:10.1161/circresaha.118.313591
2. Rhoads JP, Major AS. How Oxidized Low-Density Lipoprotein Activates Inflammatory Responses. Critical Reviews in Immunology. 2018;38(4):333-342. doi:10.1615/critrevimmunol.2018026483
3. Bekkering S, Quintin J, Joosten LA, Meer JWVD, Netea MG, Riksen NP. Oxidized Low-Density Lipoprotein Induces Long-Term Proinflammatory Cytokine Production and Foam Cell Formation via Epigenetic Reprogramming of Monocytes. Arteriosclerosis, Thrombosis, and Vascular Biology. 2014;34(8):1731-1738. doi:10.1161/atvbaha.114.303887.
4. Christ A, Günther P, Lauterbach MA, et al. Western Diet Triggers NLRP3-Dependent Innate Immune Reprogramming. Cell. 2018;172(1-2). doi:10.1016/j.cell.2017.12.013.
5. Sheedy FJ, Grebe A, Rayner KJ, et al. CD36 coordinates NLRP3 inflammasome activation by facilitating intracellular nucleation of soluble ligands into particulate ligands in sterile inflammation. Nature Immunology. 2013;14(8):812-820. doi:10.1038/ni.2639.