Chromium(VI) Down-regulates Heavy Metal-induced Metallothionein Gene Transcription by Modifying Transactivation Potential of the Key Transcription Factor, Metal-responsive Transcription Factor 1

Abstract

The robust induction of metallothionein-I and II (MT-I and MT-II) genes by several heavy metals such as zinc and cadmium requires the specific transcription factor metal-responsive transcription factor 1 (MTF1). Chromium (VI), a major environmental carcinogen, not only failed to activate these genes but also inhibited their induction by Zn 2؉ or Cd 2؉ . The heavy metal-induced expression of another MTF1 target gene, zinc transporter 1 (ZnT-1), was also down-regulated by Cr 6؉ . By contrast, the expression of two MTF1-independent Cd 2؉ -inducible genes, heme oxygenase 1 (HO-1) that was augmented in presence Cr 6؉ , whereas the onset of apoptosis was significantly delayed in cells overexpressing MTF1.Chromium, a potential human mutagen and carcinogen (1, 2), exists in many different oxidation states in the environment, Cr 6ϩ and Cr 3ϩ being the most stable forms. In contrast to Cr 6ϩ , Cr 3ϩ is generally benign and is considered an essential nutrient required for normal sugar and fat metabolism (3, 4). Although the predominant natural source of Cr 3ϩ is present in the environment, the majority of Cr 6ϩ originates from industrial activities. Hexavalent chromium, a powerful oxidant, is actively internalized by the anion transporter located on the plasma membrane (5). Subsequently, Cr 6ϩ is reduced through the reactive Cr 5ϩ and Cr 4ϩ intermediates to trivalent chromium, (6, 7), which are highly impermeable to cell membrane.The molecular mechanism(s) for the Cr 6ϩ -mediated toxicity has not been fully elucidated. The chromium intermediates appear to interact directly with cellular constituents that lead to generation of reactive oxygen species (8, 9). Chromium treatment results in DNA strand breakage (10, 11) and DNA-protein cross-linking in vivo (12, 13). Reactive oxygen species generated during intracellular reduction of Cr 6ϩ affects almost every aspect of cellular function. Damage to cellular macromolecules and aberrations in gene expression ultimately lead to apoptosis or necrosis in the majority of the cells and uncontrolled cell proliferation in a few, causing cancer. Apoptosis induced by Cr 6ϩ treatment in lung epithelial cells involves both p53-dependent and p53-independent pathways (14, 15). Cr 6ϩ also activates a stress response protein (NF-B), which in turn activates anti-apoptotic proteins (16), thus protecting cells from apoptotic cell death.Our laboratory has been studying the molecular mechanisms of heavy metal-induced expression of metallothionein genes (17). Metallothionein I and II (MT-I 1 and MT-II) belong to a family of low molecular weight, cysteine-rich, and high metalcontaining (as metal-thiolate clusters) stress response proteins that protect cells not only from heavy metals but also from reactive oxygen species (18,19). Metallothioneins induced in response to heavy metals like zinc, cadmium, copper, mercury, gold, silver, cobalt, nickel, and bismuth act as scavengers of these toxic metals and help maintain zinc and copper homeostasis (17,20). It was of interest to examine the induction of ...

Molecular Basis for Zinc Transporter 1 Action as an Endogenous Inhibitor of L-type Calcium Channels

“…ZnT-1 has been shown to function primarily as a zinc transporter (35,36,47), and hence it has been studied mainly in the context of zinc metabolism in the brain (39,47,55,56). It is believed to act as a metal ion extruder that, by facilitating efflux of divalent ions such as zinc, lowers their concentrations in the cytoplasm and consequently protects cells from zinc toxicity (32).…”

“…One day before transfection, the cells were subcultured into 60-mm culture dishes and seeded to reach 60 -70% confluency. HEK293T cells were transfected utilizing a calcium phosphate transfection protocol as previously described (47). CHO cells were transfected utilizing Lipofectamine 2000 (Invitrogen, Carlsbad, CA), according to the manufacturer's instructions.…”

Zinc ions upregulate the hormone gastrin via an E-box motif in the proximal gastrin promoter

“…factor MTF1 which induces expression of genes involved in metal homeostasis including metallothioneins in response to heavy metals such as Zn 2C and Cd 2C (Majumder et al 2003), the possibility of a direct involvement of MTF1 in gastrin induction was ruled out. However, to test for an indirect effect we showed that downregulation of MTF1 expression by transient transfection of AGS cells with MTF1 shRNA did not inhibit activation of gastrin promoter activity by Zn 2C ions (data not shown).…”

Transcriptional Regulation of the Metallothionein Genes

“…20) Majumder et al have shown that such effects of Cr 6+ are mediated via modulation of the MTF-1 function without affecting its DNA binding capacity. 20) They also suggested that Cr 6+ interferes with the function of three transactivation domains of MTF-1. These results indicate that MTF-1 can be a target of toxic chemicals as well as the activator of genes for cell protection.…”