Oxidized LDL enhances pro-inflammatory responses of alternatively activated M2 macrophages: a crucial role for Kruppel-like factor 2

Abstract

OBJECTIVE: Macrophages are key players in atherogenesis because of their properties to form foam cells that produce a large variety of pro-inflammatory mediators. We addressed the potency of phenotypic different macrophages to accumulate oxidized LDL. METHODS AND RESULTS: Surprisingly, anti-inflammatory M2 macrophages but not pro-inflammatory M1 macrophages rapidly accumulated oxidized LDL. Simultaneously, expression of Kruppel-like factor 2, a nuclear transcription factor known to suppress inflammation in endothelial cells and monocytes, decreased and the functional phenotype of M2 macrophages shifted towards a pro-inflammatory profile, characterized by higher production of IL-6, IL-8 and MCP-1 and lower expression of IL-10 upon stimulation with LPS. In contrast, Kruppel-like factor 2 expression and the phenotype of M1 macrophages remained largely unchanged upon oxidized LDL exposure. Downregulation of Kruppel-like factor 2 expression of M2 macrophages using siRNA technology led to a significant increase of LPS-induced MCP-1 secretion. CONCLUSIONS: We show that (1) anti-inflammatory M2 macrophages are more susceptible to foam cell formation than pro-inflammatory M1 macrophages, (2) exposure to oxidized LDL renders M2 macrophages pro-inflammatory, and (3) Kruppel-like factor 2 is involved in the enhanced secretion of MCP-1 by M2 macrophages loaded with oxidized LDL. The phenotype switch of M2 macrophages from an anti- to a pro-inflammatory profile may play an important role in pathogenesis of atherosclerosis, and could represent a novel therapeutic target.