Bid activates multiple mitochondrial apoptotic mechanisms in primary hepatocytes after death receptor engagement.
OBJECTIVE
Activation of Fas or tumor necrosis factor receptor 1 (TNF-R1) on hepatocytes leads to apoptosis, which requires mitochondria activation. The pro-death Bcl-2 family protein, Bid, mediates this pathway by inducing mitochondrial releases of cytochrome c and other apoptotic factors. How Bid activates mitochondria has been studied in vitro with isolated mitochondria. We intended to study the mechanisms in intact hepatocytes so that findings could be made in a proper cellular context and would be more physiologically relevant.


METHODS
Hepatocytes were isolated from wild-type and bid-deficient mice and treated with anti-Fas or TNF-alpha. Mechanisms of mitochondria activation were dissected with genetic, biochemical, and morphologic approaches.


RESULTS
bid-deficient hepatocytes were much more resistant to apoptosis. Bid was required for permeability transition and mitochondria depolarization in addition to the previously defined release of cytochrome c. Permeability transition inhibitors cyclosporin A and aristolochic acid could inhibit mitochondria activation effectively, but not as much as the deletion of the bid gene, and they could not inhibit Bak oligomerization. In addition, mitochondria depolarization also could be induced by caspases, whose activation was mainly dependent on Bid.


CONCLUSIONS
Bid may activate mitochondria by 2 mechanisms, one is related to permeability transition and the other is related to Bak oligomerization. Bid can further affect mitochondria potentials by indirectly regulating caspase activity. This in vivo study provides novel findings not previously disclosed by in vitro studies, and indicates the importance of several mechanisms in contributing Bid-mediated mitochondria dysfunction that could be potential cellular targets of intervention.
Zhao
Yongge
Y
Department of Pathology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA.
Ding
Wen-Xing
WX
Qian
Ting
T
Watkins
Simon
S
Lemasters
John J
JJ
Yin
Xiao-Ming
XM
D000818:Animals; D015703:Antigens, CD / Q000502:physiology*; D017209:Apoptosis; D051023:BH3 Interacting Domain Death Agonist Protein; D002352:Carrier Proteins / Q000502:physiology*; D020169:Caspases / Q000502:physiology; D002478:Cells, Cultured; D016572:Cyclosporine / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism; D022781:Hepatocytes / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008930:Mitochondria, Liver / Q000502:physiology*; D018124:Receptors, Tumor Necrosis Factor / Q000502:physiology*; D047888:Receptors, Tumor Necrosis Factor, Type I; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology; D019014:fas Receptor / Q000502:physiology*
D015703:Antigens, CD; D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; D002352:Carrier Proteins; D003574:Cytochrome c Group; D018124:Receptors, Tumor Necrosis Factor; D047888:Receptors, Tumor Necrosis Factor, Type I; D014409:Tumor Necrosis Factor-alpha; D019014:fas Receptor; D016572:Cyclosporine; D020169:Caspases