An Innate Immunity Signaling Process Suppresses Macrophage ABCA1 Expression through IRAK-1-Mediated Downregulation of Retinoic Acid Receptor α and NFATc2

ABCA1 is widely expressed in diverse cells and tissues and mediates cholesterol and phospholipid trafficking from cells to plasma lipid poor apolipoproteins, such as apoA1, resulting in the formation of nascent high-density lipoprotein particles (HDL) (19, 50). Humans harboring a mutated ABCA1 gene develop the rare Tangier disease, characterized by low plasma HDL levels, accumulation of cholesterol in macrophages, and increased risk of cardiovascular diseases (6, 32, 39). Targeted disruption of the ABCA1 gene in mice produces a phenotype similar to that of Tangier disease in humans (40). Conversely, overexpression of ABCA1 in the transgenic mouse model increased plasma HDL levels and protected against atherosclerosis (53). In addition to exporting cholesterol and preventing foam cell formation, ABCA1 expressed in macrophages is also implicated in decreasing inflammatory responses (59, 60). ABCA1 expression in macrophages can also facilitate the uptake and clearance of apoptotic cells, by a process termed efferocytosis (16, 18). Collectively, proper function of ABCA1 in macrophages is essential to prevent both metabolic and inflammatory alterations contributing to the pathogenesis of atherosclerosis. The expression of ABCA1 is regulated by the family of nuclear receptors, including retinoic acid receptor (RAR), retinoid X receptor (RXR), and liver X receptor (LXR) (11, 42, 46, 48, 55). All-trans-retinoic acid (ATRA) and other agonists can induce the binding of RAR and related transcription factors to the direct repeat sequences spaced by 1 to 5 nucleotides (DR1 to -5) on the proximal promoter of ABCA1 (1). Using murine models, LXR agonists have been shown to promote cholesterol efflux and cause substantial regression of atherosclerosis (25, 51). Earlier studies have also demonstrated roles for RARα and RARγ in the upregulation of ABCA1 expression and cholesterol efflux (10). On the other hand, inflammatory mediators and conditions can suppress ABCA1 expression in macrophages and exacerbate the pathogenesis of atherosclerosis (9, 37), likely through decreasing the nuclear levels of RAR-related transcription factors. However, the underlying molecular mechanism is not yet clearly understood. IRAK-1 is an intracellular kinase utilized by multiple inflammatory signaling pathways and triggered by a diverse array of ligands, including interleukin-1 and Toll-like-receptor (TLR) agonists (13, 15, 31). The activation of IRAK-1 in macrophages and monocytes contributes to elevated expression of proinflammatory mediators (17, 49). In addition, IRAK-1 has been linked to the pathogenesis of atherosclerosis in both mice and humans (29, 52). Elevated expression of IRAK-1 has been observed in atherosclerotic plaques of human patients (29). In addition, genetic variants of the human IRAK-1 gene are associated with a higher risk of atherosclerosis and related cardiovascular complications (29). Furthermore, IRAK-1 is constitutively active in blood mononuclear cells isolated from patients with atherosclerosis (23, 29). Our recent study using ApoE−/−/IRAK-1−/− mice also demonstrated that IRAK-1 deletion renders protection from high-fat diet-induced atherosclerosis (56). Despite compelling evidence linking IRAK-1 with the pathogenesis of atherosclerosis, the underlying molecular mechanism is not fully defined. In the present study, we examined the regulation of ABCA1 expression as well as cholesterol efflux using bone marrow-derived macrophages (BMDMs) from wild-type (WT) and IRAK-1−/− mice. We demonstrate that ATRA induces significantly higher levels of ABCA1 and elevated cholesterol efflux in macrophages harvested from IRAK-1−/− mice compared to the WT mice. Nuclear RARα protein levels were also significantly higher in IRAK-1−/− macrophages. Additionally, lipopolysaccharide (LPS) pretreatment caused a significant reduction of nuclear RARα levels, ABCA1 expression, and cholesterol efflux in WT but not in IRAK-1−/− macrophages. Mechanistically, we demonstrate that the elevated induction of ABCA1 by ATRA in IRAK-1−/− cells is mediated by the synergistic actions of NFATc2 and RARα. This study defines IRAK-1 as a key inflammatory signaling component modulating the expression of ABCA1. Therefore, IRAK-1 may serve as a potential therapeutic target in the prevention of foam cell formation and thus the progression of atherosclerosis.