Valproic acid overcomes hypoxia-induced resistance to apoptosis

Valproic acid (VPA), a histone deacetylase inhibitor (HDACi), has been shownto be an effective tool in cancer treatment. Although its ability to induce apoptosishas been described in many cancer types, the data come from experiments performedin normoxic (21% O2) conditions only. Therefore, we questioned whether VPA wouldbe equally effective under hypoxic conditions (1% O2), which is known to induceresistance to apoptosis. Four neuroblastoma cell lines were used: UKF-NB-3, SK-N-AS,plus one cisplatin-resistant subline derived from each of the two original sensitivelines. All were treated with VPA and incubated under hypoxic conditions. Measurementof apoptosis and viability using TUNEL assay and Annexin V/propidium iodide labelingrevealed that VPA was even more effective under hypoxic conditions. We show herethat hypoxia-induced resistance to chemotherapeutic agents such as cisplatin couldbe overcome using VPA. We also demonstrated that apoptosis pathways induced byVPA do not differ between normoxic and hypoxic conditions. VPA-induced apoptosisproceeds through the mitochondrial pathway, not the extrinsic pathway (under bothnormoxia and hypoxia), since inhibition of caspase-8 failed to decrease apoptosisor influence bid cleavage. Our data demonstrated that VPA is more efficient intriggering apoptosis under hypoxic conditions and overcomes hypoxia-induced resistanceto cisplatin. The results provide additional evidence for the use of VPA in neuroblastoma(NBL) treatment.