Inhibition of thymidine kinase in cultured mammary tumor cells by the chemopreventive organoselenium compound, 1,4-phenylenebis(methylene)selenocyanate

To identify mechanisms by which the organoselenium compound 1,4-phenylenebis(methylene)selenocyanate (p-XSC) mediates its chemopreventive activities, we have examined its effects on cell lines derived from breast cancer of humans and rats. When log-phase cells were treated with a dose of 1 μM p-XSC, we observed a significant decrease in thymidine kinase (TK) activity within 4 h, and reduced thymidine incorporation after 24-48 h. When the dose of p-XSC was increased to 2 μM, the decrease in TK was accompanied by a modest, but significant, decrease in thymidine incorporation at 4 h, and a greater inhibition after 24-48 h. At a dose of ≥ 3 μM, we observed a large decrease in TK, accompanied by >70% reduction in thymidine incorporation, as well as decreases in mitochondrial activity and cell numbers, all within 4 h. Equal concentrations of selenium in the form of Na₂SeO₃ had no effect on the parameters described above. These data suggest that inhibition of thymidine kinase is an early effect of p-XSC in cultured breast tumor cells.