MT-III, a brain-specific member of the metallothionein gene family.

Abstract

A third member of the metallothionein (MT) gene family, designated MT-HI, was cloned by virtue of its homology to a human protein that was shown previously to inhibit neuronal survival in culture and to be deficient in the brains of people with Alzheimer disease. Human and mouse MT-Ms have two insertions relative to all other known mammalian MTs: a threonine after the fourth amino acid and a block of six amino acids near the carboxyl terminus. The genes encoding MT-r resemble all other mammalian MT genes in their small size and exon/intron organization. The MT-Il genes are closely linked to the other functional MT genes on human chromosome 16 and mouse chromosome 8. Mouse MT-HI gene expression appears to be restricted to brain; in addition, it fails to respond to zinc, cadmium, dexamethasone, or bacterial endotoxin in vivo, thereby distinguishing MT-rn from other known MTs.Neurofibrillary tangles, which are paired helical arrays of modified X proteins, are one of the characteristic features observed in brains of individuals with Alzheimer disease (AD). Uchida et al.(1) hypothesized that these tangles, which occur in neurites, might result from inappropriate control of a neuronal growth factor. In pursuing this hypothesis, they identified, and then purified and sequenced, a protein that inhibits survival of cultured rat neurons. That protein, which they called growth inhibitory factor (GIF), resembles metallothioneins (MTs) in that it is short (68 amino acids), contains 20 cysteine residues, and binds zinc and copper. Immunocytochemistry revealed that GIF is present in glial (astrocyte) cells of normal aged individuals but is depleted in individuals with AD (1). These observations suggested that this MT-like protein normally plays a role in preventing neuronal sprouting and development of neurofibrillary tangles, but they raise the tantalizing question of how this might occur.The MT-like protein that Uchida et al. (1) sequenced differs from all other mammalian MTs in having two insertions of amino acids. The structure of the traditional MTs has been solved by NMR and x-ray crystallographic methods, which confirmed that 20 cysteines coordinate 7 metal atoms in a tetrahedral configuration (reviewed in ref. 2). Thus, one can view the structure as a metal-cysteine core with several short loops of amino acids connecting the cysteines. In this view, one ofthe loops ofGIF would be extended by one amino acid, while another would be extended by six amino acids. Considering that traditional MTs do not substitute for GIF in their bioassay, the modified loops may be responsible for its particular function.To gain additional insight into the relationship of this MT-like protein to traditional MTs, we cloned the mouse and human genes encoding these proteinst and compared them with traditional MT genes. This analysis revealed that these genes retain all the characteristics of the traditional MT genes, suggesting that they be designated MT-III rather than GIF. MATERIALS AND METHODSIsolation of Mouse MT-rn (mMT-rn) cDNA C...

Role of oxidative stress in the induction of metallothionein-2A and heme oxygenase-1 gene expression by the antineoplastic agent gallium nitrate in human lymphoma cells

“…Although an increase in metallothionein-3 (MT-3) in gallium-treated cells was also found on microarray analysis ( Figure 1B and Table 1), this actually represents an increase in MT2A and not MT-3 since PCR analysis using stringent primer pairs previously revealed that only MT2A is expressed in CCRF-CEM cells [21]. MT2A and MT-3 share 65% nucleotide sequence homology which may explain the increased hybridization signal at the MT-3 position on the membrane [30]. Considering that MT-3 expression in gallium-treated cells in the microarray actually reflects MT2A, it can be concluded that MT2A expression in gallium-treated cells is increased by approximately 9-fold rather than by 2.8-fold (Table 1).…”

Low-Dose Cadmium Exposure Reduces Human Prostate Cell Transformation in Culture and Up-Regulates Metallothionein and MT-1G mRNA

“…Mammalian MTs belong to Class I and are composed of four subfamilies MT-l, MT-2, MT-3, and MT-4. A total of 10 functional and 6 nonfunctional MT isoforms have been identified in humans (Quaife et al 1994;Palmiter et al 1992;West et al 1990;and Stennard et al 1994). The role of these subfamilies and isoforms in metal homeostasis and detoxification are not completely clear.…”

Metallothioneins Are Upregulated in Symptomatic Mice with Astrocyte-Targeted Expression of Tumor Necrosis Factor-α

“…MTs are cysteine-rich, heavy metal binding proteins, which in rodents are composed of four isoforms (MT-I to MT-IV) (48,53). Only three isoforms are expressed in the brain, namely MT-I ϩ II (which are also widely expressed and regulated coordinately) and MT-III (also known as growth inhibitory factor).…”