Atherogenic Role of LDL(-) on Macrophages

This chapter assesses the induction of an inflammatory profile and the accumulation of intracellular lipids in macrophages by electronegative LDL (LDL(-)). LDL(-), a modified LDL that is present in blood, exerts atherogenic effects on endothelial cells and monocytes. Numerous studies focusing on LDL(-) have since been performed, and the most widely accepted idea is that LDL(-) is a pool of LDL particles modified by several mechanisms. In the study shown in this chapter, LDL(-) and in vitro-modified LDLs (oxidized, aggregated, acetylated) were added to macrophages derived from THP1 monocytes over-expressing CD14 (THP1-CD14). Then, cytokine release, cell differentiation, lipid accumulation, and gene expression were measured by ELISA, flow cytometry, thin-layer chromatography, and real-time PCR, respectively. Compared to other modified LDLs, LDL(-) caused THP1-CD14 macrophages to produce more cytokines. Additionally, LDL(-) stimulated morphological alterations linked to mature macrophages. HDL and anti-TLR4 were added to offset these effects. Macrophages ingested large amounts of LDL(-), which was the main initiator of intracellular lipid buildup in lipid droplets enriched in triglycerides. In contrast to inflammation, the addition of anti-TLR4 had no effect on lipid accumulation, thus suggesting an uptake pathway alternative to TLR4. In our study, compared to other in vitro-modified LDLs, LDL(-) was the main inductor of GM-CSF, IL6, and IL10 release, and it had a similar effect on IL1
than oxLDL. This occurred in the absence of changes in cell proliferation or mortality. In this regard, LDL(-) upregulated the expression of the scavenger receptors CD36 and LOX-1, as well as several genes involved in TG accumulation. In summary, LDL(-) promoted macrophage differentiation, inflammation, and triglyceride-enriched lipid droplets formation in THP1-CD14 macrophages, probably through different receptors. The complex interaction between these pathways should be addressed in future studies. Taken together, our findings highlight new significant actions of LDL(-) on macrophage activation in the context of the development of atherosclerosis.