Intestinal anti-inflammatory activity of luteolin: role of the aglycone in NF-κB inactivation in macrophages co-cultured with intestinal epithelial cells.

The flavonoid luteolin is reported to exert anti-inflammatory properties. In this study, we investigated whether luteolin inhibits gut inflammation, using in vivo and in vitro inflammation models. In a dextran sulfate sodium (DSS)-induced colitis mouse model, luteolin (20 and 50 mg/kg) significantly ameliorated shortening of colon length and histological score. Immunohistochemical analysis showed that luteolin also significantly inhibited infiltration of macrophages and interferon (IFN)-γ-producing CD4⁺ T cells into the colonic mucosa. Treatment with luteolin also improved IFN-γ mRNA expression in the colon. At the cellular level, a co-culture consisting of intestinal epithelial Caco-2 and macrophage RAW264.7 cells, stimulated with lipopolysaccharide, the addition of luteolin (100 μM) suppressed interleukin (IL)-8 mRNA expression in Caco-2 cells without epithelial monolayer disruption. Expression of tumor necrosis factor (TNF)-α protein and proinflammatory cytokines mRNA (TNF-α, IL-6, and IL-1β) in RAW264.7 cells were also suppressed. HPLC analysis and subsequent cellular assay revealed that aglycone of luteolin was present in the basolateral supernatant of this system at a sufficient concentration to suppress TNF-α production and nuclear factor (NF)-κB activation of RAW264.7 cells. These results suggest that the luteolin aglycones released from the Caco-2 epithelium inhibits NF-κB nuclear translocation in RAW264.7 cells, followed by reduction of TNF-α mRNA expression, which results in downregulation of IL-8 mRNA expression in Caco-2 cells. The mechanism by which aglycone inhibits inflammation is important for understanding the roles of luteolin in diet.