Proteomic and Transcriptomic Analyses of Macrophages with an Increased Resistance to Oxidized Low Density Lipoprotein (oxLDL)-induced Cytotoxicity Generated by Chronic Exposure to oxLDL*

The uptake of oxidized low density lipoprotein (oxLDL) by macrophages leads to foam cell formation and fatty streaks, which represent early sites of potential atheroma development. We developed a cell culture model of chronic oxLDL exposure to determine whether hallmark parameters of oxLDL uptake and cytotoxicity are altered during foam cell formation and to determine changes in protein and mRNA expression that distinguish acute and chronic oxLDL exposure. Although the extent of oxLDL uptake did not change, a resistance to oxLDL-induced cytotoxicity was observed in the chronically exposed cells. Macrophages that have been chronically exposed to oxLDL required a 40% higher concentration of oxLDL to achieve 50% survival in a 48-h treatment relative to macrophages subjected to a single oxLDL exposure. A main feature of the differentially expressed proteome was a series of significantly overexpressed antioxidant and antioxidant-related proteins in the oxLDL-exposed cells. A large proportion of these proteins (45%) was overexpressed in the chronically exposed cells prior to the oxLDL treatment, indicative of the unique phenotype produced by the chronic treatment. Analysis of the transcriptome also revealed a broad increase in the expression of antioxidant and antioxidant-related proteins. In addition, the transcriptome experiments found an increased inflammatory response under conditions of both acute and chronic oxLDL exposure. Overall the combined functional, proteomic, and transcriptomic experiments show that macrophages respond to oxLDL by developing an oxidative stress resistance that increases and stabilizes with chronic exposure. Furthermore this protective response and the increased foam cell survival that it supports amplifies their proatherogenic role by promoting a continued inflammatory state.