White adipose tissue inflammation is linked with increased aromatase gene expression and estrogen production, a major risk factor for breast cancer in obese postmenopausal women. TNF-α, a proinflammatory cytokine, is a key driver of aromatase promoter I.4-mediated expression in adipose tissue. In this study, we have shown that IL-10, an anti-inflammatory cytokine, suppressed both TNF-α-stimulated human aromatase reporter-luciferase (hARO-Luc) expression in mouse bone marrow mesenchymal stromal cells and aromatase gene expression in human breast adipose stromal cells (ASCs). IL-10 blocked TNF-α-stimulated ERK1/2 activation in ASCs, suggesting an inhibitory effect through the MAPK signaling pathway. The links among obesity, IL-10, and aromatase were confirmed in ovariectomized (OVX) hARO-Luc mice, where increased adiposity was associated with upregulation of aromatase reporter activity and reduced IL-10 level in the mammary fat pad. OVX mice also exhibited changes in gut microbiota, similar to that in obese women, indicating altered immune function. In summary, our results suggest that increased adiposity, induced by the lack of ovarian hormones, results in enhanced expression of aromatase in mammary adipose tissue, mediated by reduction in local IL-10. These findings may bring new insights into the mechanisms involved in the development of postmenopausal breast cancer, as well as novel approaches for prevention.-Martínez-Chacón, G., Brown, K. A., Docanto, M. M., Kumar, H., Salminen, S., Saarinen, N., Mäkelä, S. IL-10 suppresses TNF-α-induced expression of human aromatase gene in mammary adipose tissue.