Brassinin, a phytoalexin in cruciferous vegetables, suppresses obesity-induced inflammatory responses through the Nrf2-HO-1 signaling pathway in an adipocyte-macrophage co-culture system

The aim of this study was to investigate the effect of brassinin (BR), a phytoalexin found in plants belonging to the Brassicaceae family, on the obesity-induced inflammatory response and its molecular mechanism in co-culture of 3T3-L1 adipocytes and RAW264.7 macrophages. BR effectively suppressed lipid accumulation by down-regulating the expression of adipogenic factors, which in turn, were regulated by early adipogenic factors such as CCAAT-enhancer-binding protein-beta and Kruppel-like factor 2. Production of inflammatory cytokines and reactive oxygen species, induced by adipocyte-conditioned medium, was significantly decreased in BR-treated cells. This effect of BR was more prominent in contact co-culture of adipocytes and macrophages with a 90% and 34% reduction in IL-6 and MCP-1 levels, respectively. BR also restored adiponectin expression, which was significantly reduced by culturing adipocytes in macrophage-conditioned medium. In the transwell system, BR increased the protein levels of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and its target molecule, hemoxygenase-1 (HO-1), by 55%-93% and 45%-48%, respectively, and also increased Nrf2 translocation into the nucleus. However, knockdown of Nrf2 or HO-1 in RAW264.7 cells restored this BR-mediated inhibition of IL-6 and MCP-1 production. These results indicated that BR inhibited obesity-induced inflammation via the Nrf2-HO-1 pathway.