1. ¹Apoptosis Department, Institute of Cancer Biology, Danish Cancer Society, Copenhagen, Denmark
2. ²Institute of Molecular Pathology, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark
3. ³Cancer and Immunobiology, Novo Nordisk A/S, Måløv, Denmark

Correspondence: M Jäättelä, Apoptosis Department, Institute of Cancer Biology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen, Denmark. Tel: +45 35357318; Fax: +45 35257721; Email: mj@cancer.dk

Received 5 October 2004; Revised 9 March 2005; Accepted 23 May 2005; Published online 20 May 2005.

Abstract

A chemotherapeutic vitamin D analogue, EB1089, kills tumor cells via a caspase-independent pathway that results in chromatin condensation and DNA fragmentation. Employing transmission- and immunoelectronmicroscopy as well as detection of autophagosome-associated LC3- protein in the vacuolar structures, we show here that EB1089 also induces massive autophagy in MCF-7 cells. Interestingly, inhibition of autophagy effectively hindered apoptosis-like nuclear changes and cell death in response to EB1089. Furthermore, restoration of normal levels of beclin 1, an autophagy-inducing tumor suppressor gene that is monoallelically deleted in MCF-7 cells, greatly enhanced the EB1089-induced nuclear changes and cell death. Thus, EB1089 triggers nuclear apoptosis via a pathway involving Beclin 1-dependent autophagy. Surprisingly, tumor cells depleted for Beclin 1 failed to proliferate suggesting that even though the monoallelic depletion of beclin 1 in human cancer cells suppresses EB1089-induced autophagic death, one intact beclin 1 allele is essential for tumor cell proliferation.