The role of the cholesterol transporter ABCG1 in human macrophages

ABCG1 mediates the influx of cholesterol in the cell to HDL particles via reverse cholesterol transport. Tumor-associated macrophages with suppressed ABCG1 expression in mice have been shown to have an intrinsic trend from the tumor-promoting M2 to the tumor-fighting M1 phenotype and suppress bladder cancer growth in vivo. The effect of ABCG1 expression on macrophage polarization makes it an attractive target in cancer therapy. Hence, our aim is to investigate the effects of suppressed ABCG1 expression on human macrophage polarization. It has been shown that macrophages have low ABCG1 expression in patients with type-2 diabetes. Therefore, we considered macrophages obtained from diabetic patients as models in which ABCG1 expression is naturally silenced. The application time of the stimulants used in macrophage polarization in-vitro dramatically changes the expressions of polarization markers, and the lack of consensus in the literature negatively affects the results and accuracy of the studies. To determine the stimulation time used in this study and to determine the macrophage phenotypes; primary macrophages were polarized at six different times and the change in expression of M1/M2 polarization markers was determined by RT-qPCR, flow cytometry, and ELISA. Then, primary macrophages with suppressed ABCG1 expression using siRNA, macrophages with naturally low expression of ABCG1 obtained from type-2-diabetes patients and all control groups were polarized for four hours. The change in gene expressions of polarization markers were determined by RT-qPCR. Our study provides a guide for researchers in determining the appropriate stimulation time and selection of markers used in macrophage polarization and fills a large gap in the literature. While the expression of CXCL9, CXCL10, TNF and IDO1 was increased in ABCG1 suppressed M1 macrophages, MRC1 and CCL22 expression were decreased in ABCG1 suppressed M2 macrophages. Furthermore, while the expression of CXCL10 and IDO1 is increased in M1 macrophages obtained from diabetic patients, the expressions of MRC1, TGM2, CCL17, CCL22, IL-10 and CD163 are decreased in M2 macrophages. In addition, we showed that there is a negative correlation between ABCG1 expression and the expression of M1 markers in human macrophages. Therefore, according to our results, ABCG1 deficiency predisposes human macrophages to M1 polarization. Our findings suggest that ABCG1 may be a novel target for modulating macrophage polarization for the treatment of various diseases in which macrophages play an important role.