Cytochrome c oxidase mediates labile iron level and radioresistance in glioblastoma.

Radiotherapy is an important treatment modality for glioblastoma (GBM), yet the initial effectiveness of radiotherapy is eventually lost due to the development of adaptive radioresistance during fractionated radiation therapy. Defining the molecular mechanism(s) responsible for the adaptive radioresistance in GBM is necessary for the development of effective treatment options. The cellular labile iron pool (LIP) is very important for determining the cellular response to radiation, as it contributes to radiation-induced production of reactive oxygen species (ROS) such as lipid radicals through Fenton reactions. Recently, cytochrome c oxidase (CcO), a mitochondrial heme-containing enzyme also involved in regulating ROS production, was found to be involved in GBM chemoresistance. However, the role of LIP and CcO in GBM radioresistance is not known. Herein, we tested the hypothesis that CcO-mediated alterations in the level of labile iron contribute to adaptive radioresistance. Using an in vitro model of GBM adaptive radioresistance, we found an increase in CcO activity in radioresistant cells that associated with a decrease in the cellular LIP, decrease in lipid peroxidation, and a switch in the CcO subunit 4 (COX4) isoform expressed, from COX4-2 to COX4-1. Furthermore, knockdown of COX4-1 in radioresistant GBM cells decreased CcO activity and restored radiosensitivity, whereas overexpression of COX4-1 in radiosensitive cells increased CcO activity and rendered the cells radioresistant. Overexpression of COX4-1 in radiosensitive cells also significantly reduced the cellular LIP and lipid peroxidation. Pharmacological manipulation of the cellular labile iron level using iron chelators altered CcO activity and the radiation response. Overall, these results demonstrate a mechanistic link between CcO activity and LIP in GBM radioresistance and identify the CcO subunit isoform switch from COX4-2 to COX4-1 as a novel biochemical node for adaptive radioresistance of GBM. Manipulation of CcO and the LIP may restore the sensitivity to radiation in radioresistant GBM cells and thereby provide a strategy to improve therapeutic outcome in patients with GBM.