Thallium induces metallothionein gene expression in Huh-7 human hepatoma cells

Thallium (Tl) is one of the most toxic heavy metals and is found ubiquitously in the earth's crust. To investigate the cellular responses to and against Tl cytotoxicity, we conducted DNA microarray analysis using three human cell lines of different origins: SH-SY5Y (neuroblast-derived), HEK293T (embryonic kidney-derived), and Huh-7 (hepatoma-derived) cells. All of the ten genes that showed the highest inductions in Huh-7 cells treated with 60 µM Tl₂SO₄ for 72 hours are metallothionein (MT) genes. The induction of the MT genes appears specific to Huh-7 cells; increases of 50-140-fold in the ten MT genes were observed in Huh-7 cells, while the increases were less than 4-fold in HEK293T and SH-SY5Y cells by microarray analysis. Investigation of the pathway responsible for Tl₂SO₄-induced MT expression in Huh-7 cells revealed that the RNA interference-mediated forced downregulation of MTF1 transcription factor resulted in the suppression of Tl₂SO₄-induced MT gene expressions, but not Tl₂SO₄-induced cell death, suggesting that MTF1-mediated MT gene expression is insufficient to protect Huh-7 cells against death by Tl₂SO₄. In contrast, the knockdown of nrf1 worsened Tl₂SO₄-induced cell death without suppressing MT gene expressions. These results indicate that MT gene induction in response to Tl₂SO₄ is mediated at least in part by MTF1 in Huh-7 cells. Nevertheless, MT gene induction through MTF1 seems insufficient to prevent the cell death caused by Tl₂SO₄. Nrf1 appears to be involved in protection against Tl₂SO₄ toxicity through mechanisms other than MT gene induction.