Abstract
BackgroundThe nuclear receptors PPAR-γ and LXRs regulate macrophage lipid metabolism and macrophage mediated inflammation. We examined the influence of these molecules on macrophage alternative activation, with particular focus on differentiation of “M2c” anti-inflammatory cells.MethodsWe cultured human monocytes in M0, M1, M2a or M2c macrophage differentiating conditions, in the presence or absence of PPAR-γ and LXR ligands. Flow cytometry was used to analyze membrane expression of phenotypic markers. Basal and LPS-stimulated production of soluble mediators was measured by ELISA. Efferocytosis assays were performed by coincubating monocytes/macrophages with apoptotic neutrophils.ResultsWe found that PPAR-γ inhibition, using the PPAR-γ antagonist GW9662, elicits differentiation of M2c-like (CD206+ CD163+ CD16+) cells and upregulation of the MerTK/Gas6 axis. Exposure of differentiating macrophages to IFN-γ, GM-CSF or LPS (M1 conditions), however, hampers GW9662 induction of MerTK and Gas6. When macrophages are differentiated with IL-4 (M2a conditions), addition of GW9662 results into an M2a (CD206+ CD209+ CD163− MerTK−) to M2c (CD206high CD209− CD163+ MerTK+) polarization shift. Conversely, in the presence of dexamethasone (M2c conditions), the PPAR-γ agonist rosiglitazone attenuates CD163 and MerTK upregulation. The LXR agonist T0901317 induces MerTK independently of M2c polarization; indeed, CD206, CD163 and CD16 are downregulated. GW9662-differentiated M2c-like cells secrete high levels of Gas6 and low amounts of TNF-α and IL-10, mimicking dexamethasone effects in vitro. However, unlike conventional M2c cells, GW9662-differentiated cells do not show enhanced efferocytic ability.ConclusionsOur results provide new insights into the role of PPAR-γ and LXR receptors in human macrophage activation and reveal the existence of different patterns regulating MerTK expression. Unexpectedly, PPAR-γ appears to negatively control the expansion of a discrete subset of M2c-like anti-inflammatory macrophages.
Highlights
The nuclear receptors PPAR-γ and Liver X Receptor (LXR) regulate macrophage lipid metabolism and macrophage mediated inflammation
IL-4 induces the “M2a” phenotype, which is characterized by expression of the universal M2 marker CD206, high levels of CD209 and low membrane expression of CD163 and Mer receptor Tyrosine Kinase (MerTK) [4]
Differentiation of monocytes/macrophages in the presence of IL-4 and the PPAR-γ antagonist GW9662 resulted in brighter expression of CD206, inhibition of CD209 induction, and upregulation of CD163 and MerTK
Summary
The nuclear receptors PPAR-γ and LXRs regulate macrophage lipid metabolism and macrophage mediated inflammation. Lipid handling and metabolism are associated with immune regulatory macrophage responses, finely controlled by the nuclear receptor superfamily members peroxisome proliferator activated receptor-gamma (PPAR-γ) and liver X receptors (LXRs). These transcription factors are tightly interlinked, and act as heterodimers with the same partner, the retinoid X receptor RXR-α [9]. PPAR-γ activation results in lipid uptake through the scavenger receptor CD36 and β-oxidation of fatty acids [7,9], modulation of the phospholipase A2/cyclooxygenase-2 axis [18], and macrophage differentiation via STAT-6 into M2a cells [7,8,12,17,19]. LXR activation results in a positive feedback loop driving further uptake of ACs through the induction of MerTK [20], inhibition of lipoprotein uptake [21] and reverse cholesterol transport from macrophages to high density lipoproteins [9]
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