title: Mitochondrially localized active caspase-9 and caspase-3 result mostly from translocation from the cytosol and partly from caspase-mediated activation in the organelle. Lack of evidence for Apaf-1-mediated procaspase-9 activation in the mitochondria.
abstract: Active caspase-9 and caspase-3 have been observed in the mitochondria, but their origins are unclear. Theoretically, procaspase-9 might be activated in the mitochondria in a cytochrome c/Apaf-1-dependent manner, or activated caspase-9 and -3 may translocate to the mitochondria, or the mitochondrially localized procaspases may be activated by the translocated active caspases. Here we present evidence that the mitochondrially localized active caspase-9 and -3 result mostly from translocation from the cytosol (into the intermembrane space) and partly from caspase-mediated activation in the organelle rather than from the Apaf-1-mediated activation. Apaf-1 localizes exclusively in the cytosol and, upon apoptotic stimulation, translocates to the perinuclear area but not to the mitochondria. In most cases, the mitochondrially localized procaspase-9 and -3 are released early during apoptosis and translocate to the cytosol and/or perinuclear area. Cytochrome c and the mitochondrial matrix protein Hsp60 are also rapidly released to the cytosol early during apoptosis. Both the early release of proteins like cytochrome c and Hsp60 from the mitochondria as well as the later translocation of the active caspase-9/-3 are partially inhibited by cyclosporin A, an inhibitor of mitochondrial membrane permeabilization. The mitochondrial active caspases may function as a positive feedback mechanism to further activate other or residual mitochondrial procaspases, degrade mitochondrial constituents, and disintegrate mitochondrial functions.
authors.affiliation: Department of Carcinogenesis, University of Texas MD Anderson Cancer Center, Science Park Research Division, Smithville, Texas 78957, USA.
authors.forename: Dhyan
authors.initials: D
authors.lastname: Chandra
authors.forename: Dean G
authors.initials: DG
authors.lastname: Tang
mesh_terms: D017209:Apoptosis; D053477:Apoptotic Protease-Activating Factor 1; D001692:Biological Transport; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism*; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000378:metabolism*; D006801:Humans; D008928:Mitochondria / Q000201:enzymology*; D011506:Proteins / Q000378:metabolism*
chemical_list: C506592:APAF1 protein, human; D053477:Apoptotic Protease-Activating Factor 1; D004792:Enzyme Precursors; D011506:Proteins; C505407:CASP3 protein, human; C506433:CASP9 protein, human; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases