Toxicity of 7-ketocholesterol as a mechanism of oxidised low density lipoprotein-induced cellular death

Abstract

Atherosclerosis is a complex inflammatory disease involving the deposition of cholesterolloaded macrophage cells within the artery wall. Progression of the initial fatty streak to an advanced atherosclerotic plaque is characterised by the development of a necrotic core region containing cholesterol and dead cells. It is well established that the formation of oxidised low-density lipoprotein (oxLDL) and the resulting toxicity to macrophage cells is a key driver in the development of the necrotic core. OxLDL contains the oxysterol, 7-ketocholesterol, which is the predominant oxysterol found within advanced atherosclerotic plaques. Numerous research groups have demonstrated the toxicity of 7- ketocholesterol to various cell types, but the route of 7-ketocholesterol delivery is important to its cytotoxicity. 7-Ketocholesterol is almost entirely lipoprotein-associated in vivo. The aim of this study was to use a more physiologically relevant model to assess the toxicity of 7-ketocholesterol to U937 human monocyte cells. U937 cells were found to be very sensitive to both oxLDL and 7-ketocholesterol. Yet incorporation of 7-ketocholesterol into high-uptake acetylated LDL greatly reduced the oxysterol cytotoxicity, when compared to an equivalent amount of 7-ketocholesterol added directly to U937 cell culture medium. While low intracellular concentrations of 7-ketocholesterol correlated with very high oxLDL toxicity, comparatively large intracellular 7-ketocholesterol content from the uptake of 7KC-acLDL caused only a small viability loss. Enrichment of oxLDL with 7- ketocholesterol did not significantly enhance lipoprotein toxicity. Collectively, these findings indicate that 7-ketocholesterol is not the toxic agent within oxLDL and have implications for the mechanism of oxLDL-induced cytotoxicity involved in atherosclerotic plaque necrotic core development.