1. ¹Division of Immunology, The Netherlands Cancer Institute, Amsterdam, The Netherlands
2. ²Division of Apoptosis Regulation, German Cancer Research Center (DFKZ), Heidelberg, Germany
3. ³Division of Radiotherapy, The Netherlands Cancer Institute, Amsterdam, The Netherlands

Correspondence: Professor J Borst, Division of Immunology, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. E-mail: j.borst@nki.nl

⁴Current address: Department of Immunology, St Jude Children's Research Hospital, Memphis, TN 38105-2794, USA.

⁵Current address: Division of Virology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.

Received 28 February 2007; Revised 25 June 2007; Accepted 28 June 2007; Published online 6 August 2007.

Abstract

In many tumor cell types, ionizing radiation (IR) or DNA-damaging anticancer drugs enhance sensitivity to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, which is of great clinical interest. We have investigated the molecular mechanism underlying the response to combined modality treatment in p53-mutant Jurkat T leukemic cells overexpressing Bcl-2. These cells are largely resistant to individual treatment with TRAIL or IR, but sensitive to combined treatment, in vitro as well as in vivo. We demonstrate that IR and DNA-damaging anticancer drugs enable TRAIL receptor-2 and CD95/Fas to bypass the mitochondrial pathway for effector caspase activation. This was validated by RNA interference for Bax and Bak and by overexpression of dominant-negative Caspase-9. Improved effector caspase activation was neither caused by altered expression of proapoptotic components nor by impaired activity of inhibitor of apoptosis proteins or nuclear factor-B signaling. Rather, we found that pretreatment of cells with IR caused quantitative and qualitative changes in death receptor signaling. It strongly improved the capacity of ligand-bound receptors to recruit FADD and activate Caspase-8 and -10 in the death-inducing signaling complex, while c-FLIP_L levels were unaffected.