Zinc is an essential trace element required for enzymatic activity and for maintaining the conformation of many transcription factors; thus, zinc homeostasis is tightly regulated. Although zinc affects several signaling molecules and may act as a neurotransmitter, it remains unknown whether zinc acts as an intracellular second messenger capable of transducing extracellular stimuli into intracellular signaling events. In this study, we report that the cross-linking of the high affinity immunoglobin E receptor (Fcɛ receptor I [FcɛRI]) induced a release of free zinc from the perinuclear area, including the endoplasmic reticulum in mast cells, a phenomenon we call the zinc wave. The zinc wave was dependent on calcium influx and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation. The results suggest that the zinc wave is involved in intracellular signaling events, at least in part by modulating the duration and strength of FcɛRI-mediated signaling. Collectively, our findings indicate that zinc is a novel intracellular second messenger.
Zinc is a structural constituent of a great number of proteins, including enzymes belonging to cellular signaling pathways and transcription factors, and it is essential for their biological activity (Vallee and Auld, 1993; Prasad, 1995). Zinc has a variety of effects on the immune and nervous systems in vivo and vitro, and these effects mainly depend on the zinc concentration (Rink and Gabriel, 2000; Frederickson et al., 2005). Many researchers have reported that immune function decreases after zinc depletion. Zinc-deficient mice exhibit reduced natural killer cell–mediated cytotoxic activity, antibody-mediated responses, and host defense against pathogens and tumors (Fernandes et al., 1979; Fraker et al., 1982; Keen and Gershwin, 1990). The requirement for zinc is most likely because of its essential constitutive role in maintaining the conformation or enzymatic activity of many important components of these processes, including enzymes, transcription factors, and signaling molecules. On the other hand, zinc itself is cytotoxic: zinc induces apoptosis in T and B cells (Telford and Fraker, 1995; Ibs and Rink, 2003) and neuronal death (Koh et al., 1996; Sensi and Jeng, 2004). Therefore, the intracellular zinc concentration is tightly controlled by zinc importers (ZIPs/SLA39s; Eide, 2004), exporters (zinc transporters/SLC30s; Palmiter and Huang, 2004), and binding proteins such as metallothioneins (Vallee, 1995). In addition, zinc-sensing molecules such as metal response element–binding transcription factor-1 respond to free zinc levels by regulating gene expression to maintain zinc homeostasis (Andrews, 2001).
Zinc has been shown to act as a neurotransmitter (Colvin et al., 2003; Frederickson, 2003). In neurons, exocytotic stimuli induce zinc release into the surrounding milieu and its uptake into the cytoplasm through gated zinc channels on neighboring cells. Synaptically released zinc probably travels to adjacent cells such as postsynaptic neurons and glial cells and functions as a modulator and mediator of cell-to-cell signaling (Xie and Smart, 1994; Hershfinkel et al., 2001; Li et al., 2001). In this role, zinc acts as an autocrine or paracrine, transcellular, transmembrane signaling factor, like a neurotransmitter.
Zinc mimics the actions of hormones, growth factors, and cytokines, which suggests that zinc may act on intracellular signaling molecules (Beyersmann and Haase, 2001). In fact, zinc is a well-known inhibitor of protein tyrosine phosphatases (Brautigan et al., 1981). The inhibition constant is reported to be in the nanomolar range (Maret et al., 1999). In addition, zinc affects the regulation of transcription factors. Zinc can induce the expression of some genes, including those coding for molecules involved in zinc homeostasis, like zinc transporters and metallothioneins (Palmiter, 2004). The gene expression of metallothioneins by zinc is regulated by metal response element–binding transcription factor-1 (Lichtlen and Schaffner, 2001). We previously reported that the nuclear localization of the transcription factor Snail is dependent on the zinc transporter Zip6, suggesting that zinc plays a role in the nuclear localization of Snail and may act as an intracellular signaling molecule (Yamashita et al., 2004). This notion was further supported by the finding that toll-like receptor 4–mediated dendritic cell maturation is, at least in part, dependent on a toll-like receptor 4–induced decrease in intracellular free zinc (Kitamura et al., 2006). Collectively, this evidence suggests that zinc may act as an intracellular signaling molecule. However, the toll-like receptor 4–mediated decrease in intracellular free zinc is dependent on the change in the expression profile of zinc transporters. Therefore, it remains unknown whether zinc acts as an intracellular second messenger like calcium and cAMP. A second messenger is defined as a molecule whose intracellular status is directly altered by extracellular stimuli and that can transduce the extracellular stimuli into intracellular signaling events.
ZnD’s multifaceted effect on the immune system results in a high susceptibility to a variety of infections. Zn supplementation effectively improves immunity on the one hand and efficiently ameliorates chronic dysfunctional inflammatory responses on the other. These findings strongly suggest that Zn is essential for normal immune-cell homeostasis and function. There is already an excellent body of literature about Zn’s roles in specific innate cell types, such as monocytes/macrophages and natural killer cells [84, 85], and we here focus on the roles of Zn and Zn transporters specifically in DCs, T cells, and B cells, which are bridging populations that enable crosstalk between the innate and adaptive immune systems.