Valproic acid induces non-apoptotic cell death mechanisms in multiple myeloma cell lines

Multiple myeloma (MM) is an incurable hematological disorder characterized by dysregulated proliferation of terminally differentiated plasma cells. Aberrant histone acetylation has been observed in the development of numerous malignancies. Histone deacetylase inhibitors such as valproic acid (VPA) are promising drugs for cancer therapy since they have been reported to have antiproliferative effects and to induce differentiation in carcinoma and leukemic cells. Considering the advantage of being already in clinical use for epilepsy treatment, valproic acid might be a promising therapeutic candidate drug in the management of multiple myeloma. In this study, we show that the short fatty acid VPA has a time and dose-dependent cytotoxic effect on the MM cell lines OPM2, RPMI and U266. The influence of VPA on cell cycle and apoptosis have been evaluated by flow cytometry. Our results show that the three cell lines are blocked in G₀/G₁ phase. The observed sensitivity to VPA can be partially explained by late apoptosis. Since caspase 3 is activated in all tested cell lines after VPA treatment, a caspase-dependent pathway seems to be involved but not activated by the classic apoptotic pathways. We have also studied another mechanism of cell death, the senescence-like phenotype, but did not find any evidence for its implication. Thus, treatment with VPA may imply other alternative cell death mechanisms.