Abstract 1457: Valproic acid enhances radiation therapy by protecting normal hippocampal neurons and sensitizing malignant glioblastoma cells

Neurocognitive deficits are devastating sequelae that follow cranial irradiation in patients with medulloblastoma and leukemia. Valproic acid (VPA) has been used effectively as a neuroprotector in various neurological insults. In the present study we demonstrate that VPA could be used to protect hippocampal neurons from the damaging effects of radiation. The effect of VPA pretreatment on cell death was examined using clonogenic survival assays. Mouse hippocampal neurons, HT-22 were treated with 0.6 mM VPA for 7 days prior to irradiation. A significant increase in survival was seen in HT-22 cells treated with VPA compared to cells treated with radiation alone. In contrast, VPA treatment decreased the survival of irradiated cancer cells, D54 and GL-261 compared to radiation alone. To elucidate the mechanism of this selective increased survival in normal hippocampal cells we monitored for program cell death by Annexin V-FITC or DAPI staining. Irradiated HT-22 cells which were pretreated with VPA demonstrated 2 fold decrease in apoptosis than irradiated cells. Similar results were obtained using irradiated mice. Seven-days old C57BL6J pups were either irradiated with the single dose of 7 Gy or treated with VPA (200mg/Kg) followed by 7Gy irradiation. VPA pretreatment protected hippocampal neurons from radiation-induced apoptosis as demonstrated by counting the pyknotic nuclei in the subgranular zone of the hippocampal region. Western immunoblotting of HT-22 cells revealed that VPA not only inhibited HDAC, but also inhibited glycogen synthase kinase-3β (GSK-3β), an enzyme required for apoptosis. A heterotopic glioma model of GL-261 was used to evaluate the tumor growth delay following treatment with VPA (200 mM) and radiation (five fractions of 2Gy). VPA alone delayed the tumor growth; however VPA in combination with radiation significantly delayed tumor growth. In conclusion, VPA protected hippocampal neurons from radiation-induced damage in vitro and in vivo. This protection is specific to hippocampal neuronal cells and not to cancer cells. The molecular mechanism could involve enzymatic inhibition of both HDAC and GSK-3β. VPA could be used as a novel therapy for the prevention of neurocognitive deficits resulting from cranial irradiation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1457. doi:1538-7445.AM2012-1457