Abstract 1262: Bif-1 is required for keeping mitochondrial and lysosomal positive feedback loops in check to prevent apoptosis induced by death receptors in type II cells

Abstract

Abstract Bif-1, also known as endophilin B1 and SH3GLB1, is involved in the regulation of apoptosis, mitochondrial morphogenesis, and autophagy. We have previously shown that increasing the levels of Bif-1 accelerates Bax conformational change, caspase-3 activation, and apoptotic cell death in response to a variety of intrinsic apoptotic stimuli, whereas the loss of Bif-1 delays all these processes. Here, we report the role of Bif-1 as a negative regulator in the extrinsic pathway of apoptosis by controlling mitochondrial and lysosomal amplification loops in type II cells. There are two different types of cells distinguished by their distinct extrinsic apoptotic signaling pathways. In type I cells, the activation of caspase-8 by death receptor ligation is sufficient to activate downstream effector caspases (e.g. caspaspe-3) for cell killing, whereas in type II cells, caspase cascade amplification through mitochondria and/or lysosome is required. We found that depletion of Bif-1 dramatically enhances not only caspase-3 but also caspase-8 activation and cell death in type II cell lines including MEF, HeLa, and Jurkat, but not in type I H9 cells, following treatment by the extrinsic apoptotic stimuli TNFα/cycloheximide, TRAIL or Fas ligand. Overexpression of Bcl-XL or XIAP in Bif-1 knockout MEFs significantly diminishes the enhanced caspase activation, indicating that a mitochondrial feedback loop is elevated by the loss of Bif-1. Moreover, while treatment with cathepsin inhibitors does not suppress TNFα/cycloheximide-induced caspase-3 activation in Bif-1 knockout MEFs, it completely blocks the activation of caspase-3 in Bif-1 knockout MEFs overexpressing Bcl-XL or XIAP, implying that lysosomal pathway is also involved. Furthermore, inhibition of caspase-8 is able to completely block the activation of caspase-3 induced by TNFα/cycloheximide, indicating that caspase-8 initiates the mitochondrial and lysosomal amplification loops observed in Bif-1 deficient type II cells. Taken together, these studies suggest that Bif-1 plays an important role in the regulation of caspase-8-initiated mitochondrial and lysosomal positive feedback loops for killing type II cells by death receptors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1262.