MAC Function Triggers a Bax/Bak Dependent Bystander Effect

Abstract

Collateral spread of apoptosis to nearby cells is referred to as the bystander effect - a process that is integral to tissue homeostasis and a challenge in anticancer therapies. In many systems, apoptosis relies on permeabilization of the mitochondrial outer membrane to factors like cytochrome c and Smac/DIABLO. This permeabilization occurs via formation of a mitochondrial apoptosis-induced channel, MAC, and was mimicked here by single-cell microinjection of cytochrome c in Xenopus embryos. Waves of apoptosis were observed in vivo from the injected to the neighboring cells. This finding indicates that a death signal generated downstream of cytochrome c release diffused to neighboring cells and ultimately killed the animals. The role of MAC in bystander effects was then assessed in mouse embryonic fibroblasts that did or did not express its main components Bax and/or Bak. Exogenous expression of GFP-Bax triggered permeabilization of the outer membrane and apoptosis in these cells. Time-lapse videos showed neighboring cells also underwent apoptosis, but expression of Bax and/or Bak was essential to this effect as no bystanders were observed in cells lacking both of these MAC components. In osteosarcoma cell lines, this effect relied upon gap junction intercellular communication, as bystander cell death was abrogated either by pharmacological or molecular inhibition of connexin 43. In contrast, an extracellular pathway seemed to underlie bystander effects in breast cancer cell lines. These results may impact development of novel therapeutic strategies to selectively eliminate tumors or minimize the size of tissue injury in degenerative or traumatic cell death.