Methadone Potentiates the Cytotoxicity of Temozolomide by Impairing Calcium Homeostasis and Dysregulation of PARP in Glioblastoma Cells

SIMPLE SUMMARY: Temozolomide is a widely used chemotherapeutic agent for the treatment of glioblastoma multiforme, but its efficacy is often severely limited by cell resistance to this treatment. Opioids have been found to increase the sensitivity of glioblastoma cells to temozolomide. However, the molecular mechanism underlying chemotherapeutic sensitization to temozolomide by methadone is unknown. Here, we observed that treatment of rat C6 glioblastoma cells and human U251 and T98G glioblastoma cells with a combination of these two drugs significantly reduced cell viability and increased apoptosis compared with temozolomide treatment alone. We found that the major factors involved in the observed effects include increased intracellular Ca(2+) concentration, oxidative stress, PARP-1 protein cleavage, and impaired DNA integrity. The present study provides evidence for the potential value of methadone in the treatment of glioblastoma with temozolomide and sheds light on the molecular mechanism of synergistic action of these two drugs. ABSTRACT: Methadone is commonly used as an alternative to morphine in patients with pain associated with glioblastoma and other cancers. Although concomitant administration of methadone and cytostatics is relatively common, the effect of methadone on the efficacy of cytostatic drugs has not been well studied until recently. Moreover, the mechanism behind the effect of methadone on temozolomide efficacy has not been investigated in previous studies, or this effect has been automatically attributed to opioid receptors. Our findings indicate that methadone potentiates the effect of temozolomide on rat C6 glioblastoma cells and on human U251 and T98G glioblastoma cells and increases cell mortality by approximately 50% via a mechanism of action independent of opioid receptors. Our data suggest that methadone acts by affecting mitochondrial potential, the level of oxidative stress, intracellular Ca(2+) concentration and possibly intracellular ATP levels. Significant effects were also observed on DNA integrity and on cleavage and expression of the DNA repair protein PARP-1. None of these effects were attributed to the activation of opioid receptors and Toll-like receptor 4. Our results provide an alternative perspective on the mechanism of action of methadone in combination with temozolomide and a potential strategy for the treatment of glioblastoma cell resistance to temozolomide.