Differential Regulation of TLR-Dependent MyD88 and TRIF Signaling Pathways by Free Zinc Ions

Zinc signals are utilized by several immune cell receptors. One is TLR4, which causes an increase of free zinc ions (Zn2+) that is required for the MyD88-dependent expression of inflammatory cytokines. This study investigates the role of Zn2+ on Toll/IL-1R domain-containing adapter inducing IFN-beta (TRIF)-dependent signals, the other major intracellular pathway activated by TLR4. Chelation of Zn2+ with the membrane-permeable chelator N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine augmented TLR4-mediated production of IFN-beta and subsequent synthesis of inducible NO synthase and production of NO. The effect is based on Zn2+ acting as a negative regulator of the TRIF pathway via reducing IFN regulatory factor 3 activation. This was also observed with TLR3, the only TLR that signals exclusively via TRIF, but not MyD88, and does not trigger a zinc signal. In contrast, IFN-gamma-induced NO production was unaffected by N,N,N',N'-Tetrakis(2-pyridylmethyl) ethylenediamine. Taken together, Zn2+ is specifically involved in TLR signaling, where it differentially regulates MyD88 and TRIF signaling via a zinc signal or via basal Zn2+ levels, respectively.