Regulation of Metallothionein Gene Transcription

Abstract

Metallothioneins are small, cysteine-rich proteins that function in metal detoxification and homeostasis. Metallothionein transcription is controlled by cell-specific factors, as well as developmentally modulated and metal-responsive pathways. By using the nematode Caenorhabditis elegans as a model system, the mechanism that controls cell-specific metallothionein transcription in vivo was investigated. The inducible expression of the C. elegans metallothionein genes, mtl-1 and mtl-2, occurs exclusively in intestinal cells. Sequence comparisons of these genes with other C. elegans intestinal cell-specific genes identified multiple repeats of GATA transcription factor-binding sites (i.e. GATA elements). In vivo deletion and site-directed mutation analyses confirm that one GATA element in mtl-1 and two in mtl-2 are required for transcription. Electrophoretic mobility shift assays show that the C. elegans GATA transcription factor ELT-2 specifically binds to these elements. Ectopic expression of ELT-2 in non-intestinal cells of C. elegans activates mtl-2 transcription in these cells. Likewise, mtl-2 is not expressed in nematodes in which elt-2 has been disrupted. These results indicate that cell-specific transcription of the C. elegans metallothionein genes is regulated by the binding of ELT-2 to GATA elements in these promoters. Furthermore, a model is proposed where ELT-2 constitutively activates metallothionein expression; however, a second metal-responsive factor prevents transcription in the absence of metals. Metallothioneins (MT)1 are a family of structurally related, low molecular weight, cysteine-rich proteins (1). The precise physiological role of MT has not been elucidated. However, evolutionary conservation across many phyla suggests that it serves important roles in cell function. Proposed roles include the following: (a) participation in maintaining the homeostasis of essential trace metals; (b) sequestration of toxic metals, such as cadmium and mercury; (c) acting as a reservoir of essential metals that can be donated to other metalloproteins; and (d) protection against intracellular oxidative damage (2).Exposure of cultured cells or whole organisms to transition metals, ionizing radiation, heat-shock, or oxidative stress induces MT transcription (1, 3-5). Metallothioneins typically occur in multigene families. The mammalian MT family consists of four members designated MT-I to MT-IV (6). It has been commented that all organisms express MT or MT-like proteins. However, not all tissues within an organism will express all MT isoforms. Numerous studies indicate that individual MT family members display specific cellular patterns of expression. Typically, MT-I and MT-II genes are coordinately expressed. It has been shown, however, that MT-2A mRNA levels do not increase in response to cadmium exposure in human proximal tubule cells (7). Expression of the human MT-I B , MT-I E , MT-I F , and MT-I G genes varies in a cell line-specific manner in response to transition metals (8 -12). Mouse MT-I is weakly exp...

Combination of Two Regulatory Elements in the Tetrahymena thermophila HSP70-1 Gene Controls Heat Shock Activation

“…In the human ␤-globin locus control region, the NF-E2 and GATA transcription factors, when bound to their respective regulatory elements, control the position-independent expression of ␤-globin genes (16,32). In C. elegans the GATA transcription factor (ELT-2) constitutively activates the metallothionein gene expression by binding specific GATA elements, whereas a second metal-responsive factor prevents transcription in the absence of metals (28). Our results constitute the second report that GATA elements regulate HSP70 genes, the first being the description of their role in the regulation of the HSP70 gene of chronic myelogenous leukemia cells (33).…”

“…We found that in addition to having a strictly regulated pattern of activation, it possesses a promoter region containing two conventional HSEs and an unexpected cluster of GATA motifs. GATA motifs have already been described in stress gene promoters, for example, in the Caenorhabditis elegans metallothionein genes mtl-1 and mtl-2 (28) and in the T. thermophila metallothionein gene MTT1 (10); however, these GATA motifs have never been shown to interact with the heat shock response machinery.…”

Use of metallothioneins as biomarkers for environmental quality assessment in the Gulf of Gabès (Tunisia)

“…Binz and Kagi (1999) did not allocate annelid MTs to a family. Metallothionein genes are organized as multigene families displaying developmentally regulated expression patterns, as well as cell-type-specific regulation (Jahroudi et al 1990;Freedman et al 1993;Moilanen et al 1999). Expression of MT is controlled mainly at the transcriptional level by several agents, including metals, hormones, UV irradiation, and free radicals (Imbert et al 1990;Thiele 1992;Andrews 2000).…”

Identification of Mutations in Caenorhabditis elegans That Cause Resistance to High Levels of Dietary Zinc and Analysis Using a Genomewide Map of Single Nucleotide Polymorphisms Scored by Pyrosequencing

“…The transcription of these genes is induced by cadmium exposure and other stresses that do not involve metals, but not by high levels of zinc exposure. Loss-of-function mutants of mtl-1 and mtl-2 display enhanced cadmium toxicity, but a role in zinc metabolism has not been detected (Freedman et al 1993;Moilanen et al 1999;Swain et al 2004). Phytochelatins are peptides that can bind metals and contribute to cadmium tolerance.…”