Abstract
BackgroundThe ultra-low redox potential and zinc binding properties of the intracellular pool of mammalian metallothioneins (MT) suggest a role for MT in the transduction of redox signals into intracellular zinc signals. Increased expression of MT after exposure to heavy metals, oxidative stress, or inflammatory cytokines leads to an increased intracellular redox-mobilizable zinc pool that can affect downstream zinc-sensitive signaling pathways. CD4+ T helper cells are poised to be influenced by MT transduced zinc signaling because they produce intracellular reactive oxygen species following activation through the T cell receptor and are sensitive to small changes in intracellular [Zn2+].ResultsMT expression and intracellular [Zn2+] are both increased during primary activation and expansion of naïve CD4+ T cells into the Tr1 phenotype in vitro. When Tr1 cells from wildtype mice are compared with congenic mice lacking functional Mt1 and Mt2 genes, the expression of intracellular MT is associated with a greater increase in intracellular [Zn2+] immediately following exposure to reactive oxygen species or upon restimulation through the T cell receptor. The release of Zn2+ from MT is associated with a greater increase in p38 MAPK activation following restimulation and decreased p38 MAPK activation in MT knockout Tr1 cells can be rescued by increasing intracellular [Zn2+]. Additionally, IL-10 secretion is increased in MT knockout Tr1 cells compared with wildtype controls and this increase is prevented when the intracellular [Zn2+] is increased experimentally.ConclusionsDifferences in zinc signaling associated with MT expression appear to be a result of preferential oxidation of MT and concomitant release of Zn2+. Although zinc is released from many proteins following oxidation, release is greater when the cell contains an intracellular pool of MT. By expressing MT in response to certain environmental conditions, CD4+ T cells are able to more efficiently release intracellular zinc and regulate signaling pathways following stimulation. The link between MT expression and increased zinc signaling following activation represents an important immunomodulatory mechanism of MT and illuminates the complex role MT plays in shaping immune responses.Electronic supplementary materialThe online version of this article (doi:10.1186/s12865-016-0151-2) contains supplementary material, which is available to authorized users.
Highlights
The ultra-low redox potential and zinc binding properties of the intracellular pool of mammalian metallothioneins (MT) suggest a role for MT in the transduction of redox signals into intracellular zinc signals
The release of zinc ions by MT in response to redox signaling may play a role in immune responses by directly influencing zinc signaling pathways during CD4+ T helper cell activation
We show that expression of MT provides CD4+ T cells with a redoxsensitive pool of intracellular zinc that can be mobilized under conditions of oxidative stress or in response to intracellular reactive oxygen species (ROS) generation following signaling through the T cell receptor (TcR)
Summary
The ultra-low redox potential and zinc binding properties of the intracellular pool of mammalian metallothioneins (MT) suggest a role for MT in the transduction of redox signals into intracellular zinc signals. Increased expression of MT after exposure to heavy metals, oxidative stress, or inflammatory cytokines leads to an increased intracellular redox-mobilizable zinc pool that can affect downstream zinc-sensitive signaling pathways. Metallothioneins (MT) are low molecular weight, high cysteine content proteins that are expressed in most mammalian cells. Their upregulation in response to increases in intracellular zinc ion concentration ([Zn2+]i), reactive oxygen species (ROS), pro-inflammatory cytokines, and as part of proliferation and differentiation [1] suggests that MT may play an important role during the development of immune responses. The release of zinc ions by MT in response to redox signaling may play a role in immune responses by directly influencing zinc signaling pathways during CD4+ T helper cell activation.
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