title: Lysosomal protease pathways to apoptosis. Cleavage of bid, not pro-caspases, is the most likely route.
abstract: We investigated the mechanism of lysosome-mediated cell death using purified recombinant pro-apoptotic proteins, and cell-free extracts from the human neuronal progenitor cell line NT2. Potential effectors were either isolated lysosomes or purified lysosomal proteases. Purified lysosomal cathepsins B, H, K, L, S, and X or an extract of mouse lysosomes did not directly activate either recombinant caspase zymogens or caspase zymogens present in an NT2 cytosolic extract to any significant extent. In contrast, a cathepsin L-related protease from the protozoan parasite Trypanosoma cruzi, cruzipain, showed a measurable caspase activation rate. This demonstrated that members of the papain family can directly activate caspases but that mammalian lysosomal members of this family may have been negatively selected for caspase activation to prevent inappropriate induction of apoptosis. Given the lack of evidence for a direct role in caspase activation by lysosomal proteases, we hypothesized that an indirect mode of caspase activation may involve the Bcl-2 family member Bid. In support of this, Bid was cleaved in the presence of lysosomal extracts, at a site six residues downstream from that seen for pathways involving capase 8. Incubation of mitochondria with Bid that had been cleaved by lysosomal extracts resulted in cytochrome c release. Thus, cleavage of Bid may represent a mechanism by which proteases that have leaked from the lysosomes can precipitate cytochrome c release and subsequent caspase activation. This is supported by the finding that cytosolic extracts from mice ablated in the bid gene are impaired in the ability to release cytochrome c in response to lysosome extracts. Together these data suggest that Bid represents a sensor that allows cells to initiate apoptosis in response to widespread adventitious proteolysis.
authors.affiliation: Programs in Apoptosis and Cell Death Research and Glycobiology, Burnham Institute, La Jolla, California 92037, USA.
authors.forename: V
authors.initials: V
authors.lastname: Stoka
authors.forename: B
authors.initials: B
authors.lastname: Turk
authors.forename: S L
authors.initials: SL
authors.lastname: Schendel
authors.forename: T H
authors.initials: TH
authors.lastname: Kim
authors.forename: T
authors.initials: T
authors.lastname: Cirman
authors.forename: S J
authors.initials: SJ
authors.lastname: Snipas
authors.forename: L M
authors.initials: LM
authors.lastname: Ellerby
authors.forename: D
authors.initials: D
authors.lastname: Bredesen
authors.forename: H
authors.initials: H
authors.lastname: Freeze
authors.forename: M
authors.initials: M
authors.lastname: Abrahamson
authors.forename: D
authors.initials: D
authors.lastname: Bromme
authors.forename: S
authors.initials: S
authors.lastname: Krajewski
authors.forename: J C
authors.initials: JC
authors.lastname: Reed
authors.forename: X M
authors.initials: XM
authors.lastname: Yin
authors.forename: V
authors.initials: V
authors.lastname: Turk
authors.forename: G S
authors.initials: GS
authors.lastname: Salvesen
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D051023:BH3 Interacting Domain Death Agonist Protein; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism; D053148:Caspase 3; D053179:Caspase 7; D020169:Caspases / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D010450:Endopeptidases / Q000502:physiology*; D006801:Humans; D008247:Lysosomes / Q000201:enzymology*; D051379:Mice; D008958:Models, Molecular; D051381:Rats; D014407:Tumor Cells, Cultured
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; C494814:BID protein, human; C494815:Bid protein, mouse; C494816:Bid protein, rat; D002352:Carrier Proteins; D010450:Endopeptidases; C505407:CASP3 protein, human; C505520:CASP7 protein, human; C505409:Casp3 protein, mouse; C505410:Casp3 protein, rat; C505524:Casp7 protein, mouse; D053148:Caspase 3; D053179:Caspase 7; D020169:Caspases