Bacteriophage-encoded toxins: The λ-holin protein causes caspase-independent non-apoptotic cell death of eukaryotic cells

The bacteriophage-encoded holin proteins are known to promote bacterial cell lysis by forming lesions within the cytoplasmic membrane. Recently, we have shown that the bacteriophage λ-holin protein exerts cytotoxic activity also in eukaryotic cells accounting for a reduced tumour growth in vivo. In order to elucidate the mechanisms of λ-holin-induced mammalian cell death, detailed biochemical and morphological analyses were performed. Colocalization analyses by subcellular fractionation and organelle-specific fluorescence immunocytochemistry indicated the presence of the λ-holin protein in the endoplasmic reticulum and in mitochondria. Functional studies using the mitochondria-specific fluorochrome JC-1 demonstrated a loss of mitochondrial transmembrane potential in response to λ-holin expression. Morphologically, these cells exhibited unfragmented nuclei but severe cytoplasmic vacuolization representing signs of oncosis/ necrosis rather than apoptosis. Consistently, Western blot analyses indicated neither an activation of effector caspases 3 and 7 nor cleavage of the respective substrate poly(ADP-ribose) polymerase (PARP) in an apoptosis-specific manner. These findings suggest that the λ-holin protein mediates a caspase-independent non-apoptotic mode of cell death. © 2007 The Authors; Journal compilation © 2007 Blackwell Publishing Ltd.