Potentiation of Mitomycin C Tumoricidal Activity for Transitional Cell Carcinoma by Histone Deacetylase Inhibitors In Vitro

Abstract

Histone deacetylase inhibitors represent promising cancer treatments since they offer improved access to target DNA/protein complexes by cytotoxic agents. We hypothesized that histone deacetylase inhibitors would be most effective when combined with DNA damaging agents such as mitomycin C. Valproic acid is a safe, affordable histone deacetylase inhibitor. We examined the effect of the combination of valproic acid and mitomycin C on human bladder cancer cells in vitro and compared this to the effect of valproic acid or mitomycin C alone on the cells. We used HTB5 and HTB9 cells derived from low and high grade bladder tumors, respectively. HTB5 and HTB9 cells were grown in modified Eagle's and RPMI medium, respectively. Cell growth and proliferation were measured by standard methods. Apoptosis was evaluated microscopically after dual staining of cells with annexin V-fluorescein isothiocyanate/propidium iodide. The change in protein expression was analyzed by Western blot. Treatment of HTB5 and HTB9 bladder cancer cells for 24 to 72 hours with valproic acid and mitomycin C resulted in concentration and time dependent decreases in viability and proliferation. HTB9 cells showed marked sensitivity to mitomycin C with a 48-hour 50% median inhibitory concentration of 1 μg. Cells were less sensitive to valproic acid alone with a 48-hour 50% median inhibitory concentration of 2.5 mM. The chromatin structure relaxation induced by valproic acid pretreatment sensitized the bladder cancer cell lines, augmenting the cytotoxic action of mitomycin C. Valproic acid potentiated the induction of cell death by mitomycin C in each cell line in synergistic fashion. The effect of combining the 2 drugs was greater than the sum effect of each drug alone. Results indicate that the valproic acid and mitomycin C combination is effective, likely due to synergistic mechanisms. Animal model validation is needed but early results suggest promising intravesical treatments for superficial bladder cancer.