1. ¹UPRES-EA2710, Institut Gustave Roussy, Villejuif, France
2. ²Centre National de la Recherche Scientifique, UMR8125, Institut Gustave Roussy, Villejuif, France

Correspondence: Dr G Kroemer, Centre National de la Recherche Scientifique, UMR8125, Institut Gustave Roussy, Pavillon de Recherche 1, 38 rue Camille Desmoulins, F-94805 Villejuif, France. E-mail: kroemer@igr.fr

³These authors contributed equally to this work.

⁴These authors are equal co-senior authors.

Received 1 February 2006; Revised 18 April 2006; Accepted 10 May 2006; Published online 24 July 2006.

Abstract

Colon carcinoma cells subjected to -irradiation (4 Gy) manifest signs of apoptosis (caspase activation, chromatin condensation, phosphatidylserine (PS) exposure on the cell surface, sub-diploid DNA content), correlating with their radiosensitivity, which is increased in cells lacking the 14-3-3 protein as compared to wild-type controls. Inhibition of caspases by addition of Z-Val-Ala-DL-Asp (OMe)-fluoromethylketone, by stable transfection with the Baculovirus gene coding for p35, or by Bax knockout reduced all signs of apoptosis, yet failed to suppress radio-induced micro- and multinucleation. Moreover, pharmacological caspase inhibition, p35 expression or Bax knockout had no effect on the clonogenic survival that was reduced by -irradiation and caspase inhibition failed to abolish the increased radiosensitivity of 14-3-3-deficient cells. Micro- and multinucleation was detectable among non-apoptotic cells lacking PS exposure, as well as among cells undergoing apoptosis. Moreover, a fraction of micro- or multinucleated cells manifested caspase activation, and videomicroscopic analyses revealed that such cells could succumb to caspase-dependent apoptosis. Altogether, these results suggest that genomic instability induced by -irradiation can trigger apoptosis, although apoptosis is dispensable for radio-induced clonogenic death.