1Department of Nucelar Medicine, Chonbuk National University Medical School, Korea. 2Department of Internal Medicine, Chonbuk National University Medical School, Korea. 3Institute for Medical Science, Chonbuk National University Medical School, Korea.
ABSTRACT
PURPOSE: Metallothionein is an intracellular cystein-rich thiol-containing protein. Increased metallothionein content in tumor cells has been suggested to be a mechanism of resistance to cisplatin. In most of previous studies evaluating the role of metallothionein in cisplatin resistance, tumor cells were usually exposed to cadmium to increase metallothionein content. Therefore, cisplatin resistance of the cells may be related to cadmium exposure itself, which induces various changes in cell characteristics, but not to increased metallothionein content. The purpose of this study is to evaluate the role of metallothionein content alone in cellular resistance to cisplatin without exposure of cells to cadmium.
MATERIALS AND METHOD: We measured the toxicity of cisplatin in mouse NIH/3T3 cells that vary in their content of metallothionein as a consequence of transfection with a plasmid that result in the constitutive expression of metallothionein. MT cells were derived from NIH/3T3 cells by transfection with a plasmid containing the genome of bovine papilloma virus and the mouse metallothionein-I, derived by the promoter for the glucose-regulated protein of 78kD.
Control cells were similary transfected with bovine papilloma virus-based plasmids with the gene for metallothionein inverted and thus separated from the promoter (TM), or deleted, along with promoter (BPA). The number of copies of the plasmid were similar in each kind of transfected cells. Expression of metallothionein required neither selection nor maintenance of cells in the presence of heavy metals.
RESULTS: Synthesis of metallothionein was 15-fold greater in the MT cells than in the TM or BPA cells. The concentration of cisplatin sufficient to reduce the cells per well by one-half (IC-50) was 0.40+/-0.075 uM in MT cells. In TM and BPA cells, it was 0.36 0.035 uM and 0.423+/-0.032 uM. There were no significant differences in IC-50 between three cell lines.
CONCLUSION: In spite of large differences between MT and control cells in their cellular content of metallothionein, no differences in resistance to cisplatin were observed.
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