Toll-like receptor 9 is involved in the induction of galectin-9 protein by dietary anti-allergic compound fucoidan

Dietary intervention of fucoidan extracted from Saccharina japonica brown seaweed has been ascertained to favor an increase of galectin-9 protein in the intestine of allergic mice, resulting in the attenuation of the food allergy symptoms. The molecular mechanism underpinning that galectin-9 secretion remains unclear. Recently, some evidence has suggested an implication of Toll-like receptor 9 (TLR9) in galectin-9 secretion. However, no investigation has been done. For this study, we aimed to understand the relationship between galectin-9 production and fucoidan intake, which will improve the therapeutic use of fucoidan in allergy treatment. Intestinal epithelial cells (IECs) were cultured in solid or transwell plates and apically exposed to fucoidan solutions and/or synthetic TLR9 agonist (CpG-ODN). The transcriptional response of the cells to galectin-9 (lgals9) and the TLR9 gene was evaluated by using q-RTPCR, and the protein expression of galectin-9 was analyzed by conducting an ELISA test. Knockdown of TLR9 in IECs was performed by targeting TLR9 siRNA, and its effect on galectin-9 release was assessed. We found that the interaction of fucoidan and IECs resulted in the upregulation of galectin-9 released in a dose- and time-dependent manner. The increase was further potentiated in combination with the TLR9 agonist. Fucoidan exposure to IECs tended to increase the mRNA expression of TLR9 in a way similar to that of the TLR9 agonist effect, and knockdown of TLR9 in IECs resulted in a decreased tendency of fucoidan-induced galectin-9 protein. TLR9 activation is therefore involved in the increased release of galectin-9 protein observed in IECs upon fucoidan exposure.