Impact of histone deacetylase inhibitor valproic acid on the anticancer effect of etoposide on neuroblastoma cells
Abstract
Etoposide (VP-16), an inhibitor of topoisomerase II, is a chemotherapeutic drug. By inhibiting topoisomerase II activity in cancer cells, this drug leads to DNA damage and subsequently to cell death.
We investigated the effect of VP-16 and in combination with a histone deacetylase (HDAC) inhibitor, valproic acid (VPA), on a human UKF-NB-4 neuroblastoma cells. The effects of etoposide and VPA on UKF-NB-4 cells were tested under normoxic and hypoxic (1% O2) conditions.
The cytotoxicity of VP-16 and VPA to a UKF-NB-4 cells was evaluated by MTT assay. Apoptosis of the cells was analyzed by flow cytometry using an Annexin V and propidium iodide binding.
The effect of VP-16 and VPA on the cell cycle was determined by flow cytometry using propidium iodide staining. The results of the study demonstrate that UKF-NB-4 cells are sensitive both to VP-16 and to VPA.
They also indicate that the impact of VPA on cytotoxicity of VP-16 in these tumor cells varies depending on the sequence of cultivation with the drugs. As a suitable sequence of cultivation, with a high rate of suppression of neuroblastoma cell growth was found the preincubation of the cells with VP-16, which was followed by their cultivation with VPA.
In contrast, the reversed combination did not give any increase in VP-16 cytotoxicity. The effect of such combined treatment can be explained by measuring the cell cycle distribution, which shows that both etoposide and VPA change the cell cycle phase distribution.
VP-16 and VPA are cycle phase specific drugs that are cytotoxic to human UKF-NB-4 neuroblastoma cells used either single or both together. However, whereas VPA might sensitize the cells to VP-16, inappropriate sequence of cultivation of the cells with VPA can decrease VP-16 cytotoxic efficacy.
The results found here warrant further studies of combined treatment of neuroblastoma cells with VP-16 with HDAC inhibitors and may help in the design of therapeutic protocols for high risk neuroblastoma.