Energy-Dependent Types of Cell Death in MCF-7 Breast Cancer Cell Tumors Implanted into Nude Mice

The present study aims at verifying whether aponecrosis, a novel type of cell demise sharing the features of both apoptosis and necrosis previously identified in in vitro experiments, could represent a natural phenomenon that also occurs in vivo. The pathways of cell death were analyzed morphologically in MCF-7 breast cancer cell tumors implanted into nude mice. These tumors grew as encapsulated masses devoid of blood vessels, thus allowing for the creation of a gradient of oxygen and nutrients from the periphery inwards. Expression of wild-type p53 and DNA fragmentation were investigated as apoptotic markers. Further experiments were carried out on MCF-7 cells cultured in vitro in the presence and absence of oxygen and nutrients. In these cultures, the intracellular ATP levels were measured and related to the types of cell death recognized morphologically. Apoptotic cells were observed in the outer portion of the MCF-7 cell tumors close to blood vessels. In the inner portion of the tumors which lacks blood supply, several cells simultaneously showed signs of both apoptosis and necrosis, being identifiable as aponecrotic cells. Similarly, aponecrotic cells were also observed in the starved cultures, in concomitance with intracellular ATP depletion. These findings suggest that MCF-7 cells may accomplish the apoptotic process only when sufficient oxygen and energy supply are available. As the energy level decreases, aponecrosis may ensue as the result of an incomplete execution of the apoptotic program.