Effects Of TNF-α On The Expression Of ABCA1 In Macrophage Foam Cell

Artherosclerosis (AS), as a chronic inflammation, has become one of the most dangerous diseases all over the world. Macrophages play a critical role in the pathogenesis of atherosclerosis by triggering fatty streak formation. As an early injure, fatty streak is mainly composed of macrophage derived foam cells which are results from intracellular lipid metabolism and transport dysfunction. The control of macrophage cholesterol homeostasis involves serials of factors include scavenger receptors, enzymes undertake cholesterol metabolism, and cholesterol transporter at the plasma membrane such as ATP-binding cassette transporters (ABCA1, ABCG1, and ABCG4) and the scavenger receptor B1 (SR-B1). Macrophage scavenger receptors, including scavenger receptor A (SR-A) and CD36, mediate the uptake of modified lipoproteins, especially oxidized-LDL (ox-LDL). Lipid contained in ox-LDL is degraded into free cholesterol by lysosome or modified into inactivated ester. ABCA1 transporter facilitates efflux of cellular free cholesterol to acceptors, such as apoA-I and apoE, to prohibit the process of foam cell formation. Increase ABCA1 expression can help to prevent AS. Research over the last few years has revealed important roles for peroxisome proliferator activated receptorγ(PPARγ) and nuclear factor-κB (NF-κB) in macrophage inflammation and cholesterol homeostasis. As an important pro-inflammatory factor, TNF-αprobably has the function to control ABCA1 expression in foam cell.In this study, we will discuss the role of the pro-inflammatory factor TNF-αin the regulation of ABCA1 expression, with particular attention to the crosstalk between PPARγand NF-κB. Macrophages selected from TNF-α~-/- and WT C57BL/6J mice peritoneal liquid were cultured with 50μg/ml ox-LDL for 48h to induce foam cell formation. There were two compare groups in this study, one group of foam cells were treated with 10ng/ml TNF-αfor 24h, and the other were incubated in both ox-LDL (50μg/ml) and TNF-α(10ng/ml) to investigate the expression and activation of different factors involved in macrophage lipid homeostasis. Fluorescence immunohistochemistry was applied to detect the influence to the intracellular ABCA1 level by addition of TNF-α, as well as some other relative factors possibly occurred in this process.ABCA1 is low expressed in TNF-α~-/- macrophages before transforming into foam cell, as well as in WT cell. Treated with ox-LDL, macrophages have a significant increase in ABCA1 level. However, there has no markedly difference between the genotypes. Foam cells exhibit notable decrease of ABCA1 when incubated with exogenous TNF-α. The results suggest that the inhibition of ABCA1by TNF-αis dose-dependence. The expression of NF-κB was not raised markedly in foam cells, in contrast, the activation of NF-κB was notably enhanced by adding TNF-α. Actually, ox-LDL can stimulate NF-κB nuclear translocation to some extent. The expression and nuclear translocation activation of PPARγwere mainly increased by ox-LDL, and this effect is time-dependence. TNF-αprobably repressed PPARγactivation in foam cells in insignificant way.TNF-α, by activating NF-κB pathway, markedly decreased ABCA1 expression in foam cell, however, PPARγseems has no significant effect on the reversion of downregulating ABCA1 by NF-κB. The results suggest that NF-κB pathway play an important role in the downregulation of ABCA1 expression in foam cell and there seems no notable influence of PPARγon ABCA1 expression in this condition.