WITHAFERIN A: A NOVEL THERAPEUTIC APPROACH FOR MALIGANT BRAIN TUMORS

Abstract

High-grade gliomas, including the astrocytoma glioblastoma multiforme (GBM), are the most common adult primary malignant brain tumor. The mean post-diagnosis survival time of patients with GBM is approximately 14 months and has improved only minimally over the last several decades given a lack of novel and effective therapeutic strategies or interventions. Similar issues persist for other forms of brain cancer, notably medulloblastomas (MB) in the pediatric patient population. Given our inability to extend survival and enhance quality of life adequately in these brain tumor patients, there is a critical need for novel chemotherapeutic agents in the treatment of GBM and MB that may work as monotherapy agents or in synergistic combinations with current interventions. In this work, the role of the natural product withaferin A (WA), a steroidal lactone with intriguing cytotoxic properties, was studied alone or in combination with currently approved anti-cancer agents (temozolomide, radiation therapy, and proteosome inhibitors) against GBM and MB brain tumors. It was shown that WA could produce G2/M cell cycle arrest and apoptosis with inhibitory modulation of the Akt/mammalian target of rapamycin (mTOR) pathway in GBM. Similarly, WA inhibited Wnt/β-catenin signaling through degradation of transcription factor (TCF)/lymphoid enhancer-binding factor (LEF) family members in MB. Overall, exposure to WA was associated with generalized N-acetyl-L-cysteine-repressible cellular oxidation, thiol reactivity, and alterations in the heat shock protein (HSP) 90 chaperone axis. WA failed to alter intrinsic HSP90 activity but reduced the association between HSP90 and co-chaperone Cdc37. These findings were expanded to demonstrate WA-mediated potentiation of cytotoxicity with concurrent proteasomal inhibition through an accumulation of aberrant proteins. WA also increased tumor cell radiosensitivity through disruption of normal DNA damage recognition and repair. While WA failed to significantly enhance the cytotoxicity of temozolomide (TMZ), it demonstrated the ability to re-sensitize TMZ-resistant GBM through reduction in O6-methylguanine-DNA methyltransferase (MGMT). This study identifies novel utility for the cytotoxic steroid lactone WA in the treatment of the malignant brain tumors GBM and MB through its alterations of oncogenic cellular signaling pathways, protein homeostasis, and the DNA-damage response mechanism. As such, WA represents a promising experimental therapeutic that warrants further translational exploration.