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

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 Zn²⁺ or Cd²⁺. The heavy metal-induced expression of another MTF1 target gene, zinc transporter 1 (ZnT-1), was also down-regulated by Cr⁶⁺. By contrast, the expression of two MTF1-independent Cd²⁺-inducible genes, heme oxygenase 1 (HO-1) and HSP-70, was not sensitive to Cr⁶⁺. Cr⁶⁺ did not also affect the expression of housekeeping genes such as GAPDH or β-actin. Stable cell lines overexpressing variable levels of MTF1, the key transactivator of the MT genes, demonstrated differential resistance toward the inhibitory effect of Cr⁶⁺, indicating MTF1 as a target of chromium toxicity. The basal and inducible binding of MTF1 to metal response elements was not affected by treatment of cells with Cr⁶⁺. Transient transfection studies showed that the ability of MTF1 to transactivate the MT-I promoter was significantly compromised by Cr⁶⁺. The fusion protein consisting of a Gal-4 DNA binding domain and one or more of the three transactivation domains of MTF1, namely the acidic domain, proline-rich domain, and serine-threonine rich domain, activated the GAL-4-driven luciferase gene to different degrees, but all were sensitive to Cr⁶⁺. MTF1 null cells were prone to apoptosis after exposure to Zn²⁺ or Cd²⁺ that was augmented in presence Cr⁶⁺, whereas the onset of apoptosis was significantly delayed in cells overexpressing MTF1.