Differential Effects of Phenethyl Isothiocyanate and D,L-Sulforaphane on TLR3 Signaling

Naturally occurring isothiocyanates (ITCs) from cruciferous vegetables are widely studied for their cancer chemopreventive effects. In this study, we investigated the effects of ITCs on TLR signaling, and found that the two most promising ITCs, phenethyl ITCs (PEITC) and D,L-sulforaphane (SFN), have differential effects on dsRNA-mediated innate immune signaling through TLR3. PEITC preferentially inhibited TLR3-mediated IFN regulatory factor 3 (IRF3) signaling and downstream gene expression in vivo and in vitro, whereas SFN caused inhibition of TLR3-mediated NF-κB signaling and downstream gene expression. Mechanistically, PEITC inhibited ligand (dsRNA)-dependent dimerization of TLR3, resulting in inhibition of signaling through IFN regulatory factor 3. In contrast, SFN did not disrupt TLR3 dimerization, indicating that it affects further downstream pathway resulting in NF-κB inhibition. To examine the biological significance of these findings in the context of antitumor activities of these compounds, we used two approaches: first, we showed that dsRNA-mediated apoptosis of tumor cells via TLR3 was inhibited in the presence of PEITC, whereas this response was augmented by SFN treatment; second, in a separate assay measuring anchorage-independent growth and colony formation by immortalized fibroblasts, we made similar observations. Again in this study, PEITC antagonized dsRNA-mediated inhibition of colony formation, whereas SFN enhanced the inhibition. These results indicate biologically relevant functional differences between two structurally similar ITCs and may provide important insights in therapeutic development of these compounds targeted to specific cancer.