The cholesterol transporter ABCG1 modulates macrophage polarization in human monocyte-derived macrophages

Abstract

The ATP-binding Cassette Transporter G1 (ABCG1) promotes cholesterol efflux from cells and regulates intracellular cholesterol homeostasis. Recent studies have shown that in mice in the absence of ABCG1, macrophages shift from a tumor-promoting M2 phenotype to a tumor-fighting M1 phenotype within the tumor and suppress bladder cancer growth in vivo. Two important aspects of this effect are still unknown: (1) whether ABCG1-would also impact polarization of human macrophages, and (2) the underlying molecular mechanism of this effect. Here, we show that in human monocyte-derived macrophages the inhibition of ABCG1 by siRNA significantly increased the expression of the M1 markers IDO1, CXCL10, CD64 and TNF after LPS/IFNγ (M1) stimulation. In contrast, after IL-4 (M2) stimulation the expression of the M2 markers MRC1, TGM2 and CD163 was significantly reduced. These data indicate that ABCG1-deficiency promotes the polarization of macrophage to an M1 phenotype in human macrophages. To address the molecular mechanism, we analysed cell signalling in bone marrow-derived macrophages from Abcg1−/− mice. These macrophages displayed reduced levels of Akt activation after stimulation with LPS/IFNγ or IL4. In addition, while Abcg1−/− macrophages stimulated with LPS/IFNγ produced more TNF compared to WT macrophages, after treatment with an Akt activator the TNF-production of Abcg1−/− and WT macrophages was comparable. These data suggest that the M1 bias of Abcg1−/− macrophages is mediated through the Akt signalling pathway. In summary, these findings deepen our mechanistic understanding of the M1/M2 switch in macrophages and suggest that ABCG1 could be a potential new target to modulate macrophage polarization for cancer immunotherapy.