1. ¹Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, MN, USA
2. ²Department of Pediatric Oncology, Johns Hopkins University, Baltimore, MD, USA
3. ³Department of Ophthalmology, University of Minnesota, Minneapolis, MN, USA
4. ⁴Cancer Center, University of Minnesota, Minneapolis, MN, USA

Correspondence: A Kelekar, Department of Laboratory Medicine and Pathology, University of Minnesota, 312 Church Street SE, 7-122 Nils Hasselmo Hall, Minneapolis, MN 55455, USA. Tel: +1-612-625-3204; Fax: +1-612-625-1121; E-mail: ameeta@umn.edu

Received 18 January 2006; Revised 27 June 2006; Accepted 7 August 2006; Published online 22 September 2006.

Abstract

Early signaling in camptothecin-treated MCF-7 cells followed an intrinsic pathway, but death was delayed and late events exhibited few hallmarks of apoptosis. BH3-only proteins, such as Noxa, Puma and BimEL, were activated and localized to mitochondrial sites within 24 h following drug exposure. However, caspase activity was low and death was unaffected by caspase inhibition. Transmission electron micrographs showed the presence of large vacuoles in drug-treated cells. An autophagic survival response has been attributed to MCF-7 cells following nutrient starvation or exposure to tamoxifen. Here, we show that autophagy also plays an important role in the delayed DNA damage response. Confocal microscopy revealed colocalization of mitochondria with large autophagic vacuoles and inhibitors of autophagy increased mitochondrial depolarization and caspase-9 activity, and accelerated cell death. Furthermore, downregulation of autophagy proteins, Beclin 1 and Atg7, unmasked a caspase-dependent, apoptotic response to DNA damage. We propose that a post-mitochondrial caspase cascade is delayed as a result of early disposal of damaged mitochondria within autophagosomes. Our data also suggest that the use of autophagy as a means of delaying apoptosis or prolonging survival may be characteristic of noninvasive breast tumor cells. These studies underscore a potential role for autophagy inhibitors in combination with conventional chemotherapeutic drugs in early breast cancer therapy.