Mitochondrial modulation of oxygen-dependent radiosensitivity in some human tumour cell lines

¹ Department of Pharmacology and Therapeutics, University College Cork, Cork, Ireland, 2Department of Medical Physics, Cork University Hospital, Wilton, Cork, Ireland and 3School of Pharmacy, Gold Coast Campus, Griffith University, Queensland 4222, Australia

	Abstract

Oxygen-dependent radiosensitivity of tumour cells reflects direct oxidative damage to DNA, but non-nuclear mechanisms including signalling pathways may also contribute. Mitochondria are likely candidates because not only do they integrate signals from each of the main kinase pathways but mitochondrial kinases responsive to oxidative stress communicate to the rest of the cell. Using pharmacological and immunochemical methods, we tested the role of mitochondrial permeability transition (MPT) and the Bcl-2 proteins in oxygen-dependent radiosensitivity. Drug-treated or untreated cervical cancer HeLa, breast cancer MCF-7 and melanoma MeWo cell lines were irradiated at 6.2 Gy under normoxic and hypoxic conditions then allowed to proliferate for 7 days. The results were that MPT blocker cyclosporin A (2 µM) strongly protected HeLa but not the other two lines against oxygen-dependent radiosensitivity. By contrast, bongkrekic acid (50 µm), which blocks MPT by targeting the ANT, had only marginal effect and calcineurin inhibitor FK-506 (0.1 µM) had none. Nor was evidence found for the modulation of oxygen-dependent radiosensitivity by Bax/Bcl-2 signalling, mitoK_ATP channels or mitochondrial Ca²⁺ uptake. In conclusion, calcineurin-independent protection by cyclosporin A suggests that MPT but not mitoK_ATP or the mitochondrial apoptosis pathway plays a causal role in oxygen-dependent radiosensitivity of HeLa cells. Targeting MPT may therefore improve the effectiveness of radiotherapy in some solid tumours.