Note: Suppl. 18C Reference UPRAD-ARTICLE-1994-003 URL: ://A1994MY17500184 Record created on 2006-12-05, modified on 2017-05-12

The Transcription Factor Mtf-1 Is Essential for Basal and Heavy Metal-Induced Metallothionein-I Gene-Expression

Immune gene discovery by expressed sequence tags generated from hemocytes of the bacteria-challenged oyster, Crassostrea gigas.

For more than 50 years, it has been known that zinc deficiency compromises immune function During this time, knowledge about the biochemistry of zinc has continued to grow, but only recent years have provided in-depth molecular insights into the multiple aspects of zinc as a regulator of immunity A network based on ZnT and ZIP proteins for transport and metallothionein for storage tightly regulates zinc availability, and virtually all aspects of innate and adaptive immunity are affected by zinc In vivo, zinc deficiency alters the number and function of neutrophil granulocytes, monocytes, natural killer (NK)-, T-, and B-cells T cell functions and balance between the different subsets are particularly susceptible to changes in zinc status This article focuses in particular on the main mechanisms by which zinc ions exert essential functions in the immune system On the one hand, this includes tightly protein bound zinc ions serving catalytic or structural functions in a multitude of different proteins, in particular enzymes and transcription factors On the other hand, increasing evidence arises for a regulatory role of free zinc ions in signal transduction, especially in cells of the immune system Identification of several molecular targets, including phosphatases, phosphodiesterases, caspases, and kinases suggest that zinc ions are a second messenger regulating signal transduction in various kinds of immune cells

Zinc signals and immune function

Metallothioneins (MTs) are a family of metal-binding proteins virtually expressed in all organisms including prokaryotes, lower eukaryotes, invertebrates and mammals. These proteins regulate homeostasis of zinc (Zn) and copper (Cu), mitigate heavy metal poisoning, and alleviate superoxide stress. In recent years, MTs have emerged as an important, yet largely underappreciated, component of the immune system. Innate and adaptive immune cells regulate MTs in response to stress stimuli, cytokine signals and microbial challenge. Modulation of MTs in these cells in turn regulates metal ion release, transport and distribution, cellular redox status, enzyme function and cell signaling. While it is well established that the host strictly regulates availability of metal ions during microbial pathogenesis, we are only recently beginning to unravel the interplay between metal-regulatory pathways and immunological defenses. In this perspective, investigation of mechanisms that leverage the potential of MTs to orchestrate inflammatory responses and antimicrobial defenses has gained momentum. The purpose of this review, therefore, is to illumine the role of MTs in immune regulation. We discuss the mechanisms of MT induction and signaling in immune cells and explore the therapeutic potential of the MT-Zn axis in bolstering immune defenses against pathogens.

Metallothioneins: Emerging Modulators in Immunity and Infection.

Metallothionein (MT) is a generic name for certain families of structurally rather variable metal-binding proteins. While purely chemical or biological approaches failed to establish a single physiologic function for MTs in any species, a combination of chemical and biological approaches and recent progress in defining the low but significant concentrations of cytosolic free zinc(II) ions have demonstrated that mammalian MTs function in cellular zinc metabolism in specific ways that differ from conventional knowledge about any other metalloprotein. Their thiolate coordination environments make MTs redox-active zinc proteins that exist in different molecular states depending on the availability of cellular zinc and the redox poise. The zinc affinities of MTs cover a range of physiologic zinc(II) ion concentrations and are modulated. Oxidative conditions make more zinc available, while reductive conditions make less zinc available. MTs move from the cytosol to cellular compartments, are secreted from cells, and are taken up by cells. They provide cellular zinc ions in a chemically available form and participate in cellular metal muffling: the combination of physiologic buffering in the steady state and the cellular redistribution and compartmentalization of transiently elevated zinc(II) ion concentrations in the pre-steady state. Cumulative evidence indicates that MTs primarily have a redox-dependent function in zinc metabolism, rather than a zinc-dependent function in redox metabolism.

Redox biochemistry of mammalian metallothioneins

Metallothionein (MT) is a low-molecular-mass, cysteine-rich metal binding protein thought to be involved in the detoxification of heavy metals and scavenging of free radicals. MT is directly induced not only by heavy metals, but also by hormones and cytokines. The present study, which uses mice with genetic deletions of the MT proteins (MT(-/-) mice), was designed to evaluate the effects of MT on the expression of pro-inflammatory cytokines in macrophages. We found that the production of tumour necrosis factor (TNF) induced by lipopolysaccharide (LPS) in peritoneal macrophages is up-regulated by MT via the modulation of nuclear factor kappaB (NF-kappaB) activity. This conclusion is supported by the following observations: (1) LPS stimulated the secretion of less TNF activity from MT(-/-) peritoneal exudate macrophages (PEMs) than from wild-type controls (MT(+/+) mice) without a difference in the pattern of kinetics; (2) LPS-stimulated expression of TNF-alpha mRNA was decreased in MT(-/-) PEMs; (3) LPS-stimulated activation of NF-kappaB was decreased in MT(-/-) PEMs; and (4) production of TNF in PEMs of MT(-/-) mice after LPS treatment in vivo was decreased (compared with MT(+/+) PEMs). Expression of other inflammatory cytokines, interleukin (IL)-1alpha and IL-6 mRNA, which were modulated by NF-kappaB, were also down-regulated in MT(-/-) PEMs. Thus MT plays a key role in the LPS-induced activation of PEMs via the modulation of NF-kappaB activity.

Metallothionein modulates lipopolysaccharide-stimulated tumour necrosis factor expression in mouse peritoneal macrophages

Macrophage colony stimulating factor (M-CSF) plays an important role in the proliferation and differentiation of mononuclear phagocytes The present study investigates the effect of zinc on M-CSF expression in MC3T3-E1 and L929 cells Zinc dose-dependently increased M-CSF mRNA levels The time-course of zinc-induced M-CSF mRNA expression peaked at 6 h Stability studies of mRNA using actinomycin D revealed that zinc does not affect M-CSF mRNA stability We examined the function of the M-CSF gene promoter using a luciferase reporter assay A construct containing the -467/+39 region of the promoter was upregulated by zinc In the presence of cycloheximide, zinc did not induce a greater increase in the M-CSF mRNA than cycloheximide alone To confirm the effect of MT on M-CSF mRNA expression, mouse lung fibroblasts (MLFs) were prepared from MT+/+ and MT-/- mice Zinc induced an increase in the expression of M-CSF in MT+/+ MLFs, but this response was not evident in MT-/- MLFs Moreover, overexpression of MT upregulated M-CSF mRNA expression as well as M-CSF secretion Our findings suggest that MT expression mediates zinc regulation of M-CSF gene expression at the transcriptional level

Kimihiro Matsuda

Papers

Metallothionein modulates lipopolysaccharide-stimulated tumour necrosis factor expression in mouse peritoneal macrophages

Metallothionein is required for zinc-induced expression of the macrophage colony stimulating factor gene.