Toll-like Receptors as a Target of Food-derived Anti-Inflammatory Compounds [Plant Biology]

October 7th, 2014 by Shibata, T., Nakashima, F., Honda, K., Lu, Y.-J., Kondo, T., Ushida, Y., Aizawa, K., Suganuma, H., Oe, S., Tanaka, H., Takahashi, T., Uchida, K.

Toll-like receptors (TLRs) play a key role in linking pathogen recognition with the induction of innate immunity. They have been implicated in the pathogenesis of chronic inflammatory diseases, representing potential targets for prevention/treatment. Vegetable-rich diets are associated with the reduced risk of several inflammatory disorders. In the present study, based on an extensive screening of vegetable extracts for the TLR-inhibiting activity in the HEK293 cells co-expressing TLR together with the NF-κB reporter gene, we found the cabbage and onion extracts as the richest sources of a TLR signaling inhibitor. To identify the active substances, we performed the activity-guiding separation of the principal inhibitors and identified 3-methylsulfinylpropyl isothiocyanate (iberin) from the cabbage and quercetin and quercetin-4'-O-β-glucoside from the onion, among which iberin showed the most potent inhibitory effect. It was revealed that iberin specifically acted on the dimerization step of TLRs in the TLR signaling pathway. To gain an insight into the inhibitory mechanism of the TLR dimerization, we developed a novel probe, combining an isothiocyanate-reactive group and an alkyne functionality, for click chemistry and detected the probe bound to the TLRs in living cells, suggesting that iberin disrupts dimerization of the TLRs via covalent binding. Furthermore, we designed a variety of iberin analogues and found that the inhibition potency was influenced by the oxidation state of the sulfur. Modeling studies of the iberin analogues showed that the oxidation state of sulfur might influence the global shape of the isothiocyanates. These findings establish the TLR dimerization step as a target of food-derived anti-inflammatory compounds.