Cinnamaldehyde suppresses toll-like receptor 4 activation mediated through the inhibition of receptor oligomerization

Toll-like receptors (TLRs) play a critical role in induction of innate immune and inflammatory responses by recognizing invading pathogens or non-microbial endogenous molecules. TLRs have two major downstream signaling pathways, MyD88- and TRIF-dependent pathways leading to the activation of NFκB and IRF3 and the expression of inflammatory mediators. Deregulation of TLR activation is known to be closely linked to the increased risk of many chronic diseases. Cinnamaldehyde (3-phenyl-2-propenal) has been reported to inhibit NFκB activation induced by pro-inflammatory stimuli and to exert anti-inflammatory and anti-bacterial effects. However, the underlying mechanism has not been clearly identified. Our results showed that cinnamaldehyde suppressed the activation of NFκB and IRF3 induced by LPS, a TLR4 agonist, leading to the decreased expression of target genes such as COX-2 and IFNβ in macrophages (RAW264.7). Cinnamaldehyde did not inhibit the activation of NFκB or IRF3 induced by MyD88-dependent (MyD88, IKKβ) or TRIF-dependent (TRIF, TBK1) downstream signaling components. However, oligomerization of TLR4 induced by LPS was suppressed by cinnamaldehyde resulting in the downregulation of NFκB activation. Further, cinnamaldehyde inhibited ligand-independent NFκB activation induced by constitutively active TLR4 or wild-type TLR4. Our results demonstrated that the molecular target of cinnamaldehyde in TLR4 signaling is oligomerization process of receptor, but not downstream signaling molecules suggesting a novel mechanism for anti-inflammatory activity of cinnamaldehyde.