Acrolein with an α,β-unsaturated carbonyl group inhibits LPS-induced homodimerization of toll-like receptor 4

Acrolein is a highly electrophilic α,β-unsaturated aldehyde present in a number of environmental sources, especially cigarette smoke. It reacts strongly with the thiol groups of cysteine residues by Michael addition and has been reported to inhibit nuclear factor-κB (NF-κB) activation by lipopolysaccharide (LPS). The mechanism by which it inhibits NF-κB is not clear. Toll-like receptors (TLRs) play a key role in sensing microbial components and inducing innate immune responses, and LPS-induced dimerization of TLR4 is required for activation of downstream signaling pathways. Thus, dimerization of TLR4 may be one of the first events involved in activating TLR4-mediated signaling pathways. Stimulation of TLR4 by LPS activates both myeloid differential factor 88 (MyD88)- and TIR domain-containing adapter inducing IFNβ (TRIF)-dependent signaling pathways leading to activation of NF-κB and IFN-regulatory factor 3 (IRF3). Acrolein inhibited NF-κB and IRF3 activation by LPS, but it did not inhibit NF-κB or IRF3 activation by MyD88, inhibitor κB kinase (IKK)β, TRIF, or TNF-receptor-associated factor family member-associated NF-κB activator (TANK)-binding kinase 1 (TBK1). Acrolein inhibited LPS-induced dimerization of TLR4, which resulted in the down-regulation of NF-κB and IRF3 activation. These results suggest that activation of TLRs and subsequent immune/ inflammatory responses induced by endogenous molecules or chronic infection can be modulated by certain chemicals with a structural motif that enables Michael addition.