The In Vitro effect of myeloperoxidase modified LDL on THP-1 derived macrophages

Abstract

Background: Atherosclerosis is best described as a chronic inflammatory state resulting in the aggregation of lipids, inflammatory cells, and fibrous elements inside the arterial intimal layer. The accumulation of modified LDL particles within macrophages, leading to foam cell formation, hallmarks atherosclerosis development. Myeloperoxidase-modified LDL (Mox-LDL) is thought to be the most physiologically relevant form of post translational modification of LDL in the context of atherosclerosis, playing a key role in the initiation and exacerbation of atherosclerosis. Classically activated, proinflammatory (M1) macrophages, and alternatively activated, anti-inflammatory (M2) macrophages greatly influence the progression of atherosclerotic lesions; and a shift from the M2 towards the pro-inflammatory M1 phenotype seems to accompany the progression of atherosclerotic plaques. Mox-LDL was shown to possess some pro-inflammatory effects on macrophages. However, the potential mechanisms through which Mox-LDL might induce the observed phenotypic shift towards a more inflammatory state within atherosclerotic lesions remain poorly understood. Aim: Our study was designed in an effort to test for a potential differential effect of Mox-LDL at the level of reactive oxygen species production, TNF-α secretion, apoptosis, and lipid uptake within the different macrophage phenotypes in vitro, by making use of the THP-1 cell line model. Methods: THP-1 monocytes were cultured and differentiated into macrophages via PMA stimulation in vitro. Once the process of differentiation was completed, resting macrophages (M0) were either kept unpolarized, or were polarized towards M1 and M2 macrophages (MΦs). Following the polarization process, both unpolarized and polarized macrophages were treated with Mox-LDL. ROS generation assay, TNF-α-specific ELISA, Annexin V/PI apoptosis assay, and Oil Red O staining were all employed in order to assess overall oxidative stress, inflammation, cytotoxicity, and lipid uptake within THP-1 derived M0, M1, and M2-MΦs, respectively, following Mox-LDL treatment. Results: Mox-LDL treated M0, M1, and M2-MΦs did not display any statistically significant difference at the level of ROS generation, TNF-α secretion, and apoptosis, when compared to their respective untreated controls. As for the Oil Red O experiment, the different macrophage subtypes were treated with either native-LDL, or Mox-LDL. In this experiment, only Mox-LDL was significantly taken up by M0, M1, and M2-MΦs, when compared to the non-treated control M0, M1, and M2-MΦs and native-LDL-treated M0/LDL, M1/LDL and M2/LDL MΦs (p<0.05). Conclusion: Our data suggest that Mox-LDL is taken up by macrophages, when native-LDL is not. Most definitely, additional studies, perhaps employing different Mox-LDL concentrations as well as different treatment time periods, need to be directed in order to further investigate the various mechanisms driving the process of atherosclerosis. Our study will hopefully reinforce the understanding of the interaction of Mox-LDL with macrophages, at the base of atherosclerotic development.