Rheinanthrone, a metabolite of sennoside A, triggers macrophage activation to decrease aquaporin-3 expression in the colon, causing the laxative effect of rhubarb extract

Ethnopharmacological relevance Aquaporin-3 (AQP3) is expressed in mucosal epithelial cells in the colon and is important for regulating fecal water content. We examined the role of AQP3 in the laxative effect of rhubarb extract. Methods After orally administering rhubarb extract or its major component (sennoside A) to rats, the fecal water content, AQP3 expression and prostaglandin E 2 (PGE 2 ) concentrations in the colon were examined. The mechanism by which sennoside A decreases the expression of AQP3 was examined using the human colon cancer HT-29 cells and macrophage-derived Raw264.7 cells. Results During diarrhea by rhubarb extract administration, the PGE 2 levels in the colon increased while the AQP3 expression significantly decreased. Similar changes were also observed when sennoside A was administered. When sennoside A or its metabolites, rheinanthrone and rhein were added to Raw264.7 cells, a significant increase in the PGE 2 concentration was observed only in cells treated with rheinanthrone. Fifteen minutes after adding PGE 2 to the HT-29 cells, the AQP3 expression decreased to approximately 40% of the control. When pretreated with indomethacin, sennoside A neither decreased the AQP3 expression nor induced diarrhea. Conclusions Sennoside A may decrease AQP3 expression in the colon to inhibit water transport from the luminal to the vascular side, leading to a laxative effect. The decreases in the levels of AQP3 are caused by rheinanthrone, which is a metabolite of sennoside A, this metabolite activates the macrophages in the colon and increases the secretion of PGE 2 ; PGE 2 acts as a paracrine factor and decreases AQP3 expression in colon mucosal epithelial cells.