The apoptotic signal triggered by ligation of members of the death receptor family is promoted by sequential activation of caspase zymogens. We show here that in a purified system, the initiator caspases-8 and -10 directly process the executioner pro-caspase-3 with activation rates (k cat/K m) of 8.7 × 105 and 2.8 × 105m−1 s−1, respectively. These rates are of sufficient magnitude to indicate direct processingin vivo. Differentially processed forms of caspase-3 that accumulate during its activation have similar rates of activation, activities, and specificities. The pattern and rate of caspase-8 induced activation of pro-caspase-3 in cytosolic extracts was the same as in a purified system. Moreover, immunodepletion of a putative intermediary in the pathway to activation, pro-caspase-9, was without consequence. Taken together these data demonstrate that the initiator caspase-8 can directly activate pro-caspase-3 without the requirement for an accelerator. The in vitro data thus help to deconvolute previous in vivo transfection studies which have debated the role of a direct versus indirect transmission of the apoptotic signal generated by ligation of death receptors. The apoptotic signal triggered by ligation of members of the death receptor family is promoted by sequential activation of caspase zymogens. We show here that in a purified system, the initiator caspases-8 and -10 directly process the executioner pro-caspase-3 with activation rates (k cat/K m) of 8.7 × 105 and 2.8 × 105m−1 s−1, respectively. These rates are of sufficient magnitude to indicate direct processingin vivo. Differentially processed forms of caspase-3 that accumulate during its activation have similar rates of activation, activities, and specificities. The pattern and rate of caspase-8 induced activation of pro-caspase-3 in cytosolic extracts was the same as in a purified system. Moreover, immunodepletion of a putative intermediary in the pathway to activation, pro-caspase-9, was without consequence. Taken together these data demonstrate that the initiator caspase-8 can directly activate pro-caspase-3 without the requirement for an accelerator. The in vitro data thus help to deconvolute previous in vivo transfection studies which have debated the role of a direct versus indirect transmission of the apoptotic signal generated by ligation of death receptors. tumor necrosis factor receptor 1 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate 7-amino-4-trifluoromethyl coumarin p-nitroanilide polyacrylamide gel electrophoresis bovine poly(ADP-ribose)polymerase tumor necrosis factor carbobenzoxy-Asp-Glu-Val-Asp-7-amino-4-trifluoromethyl coumarin carbobenzoxy-Ile-Glu-Thr-Asp-7-amino-4-trifluoromethyl coumarin acetyl 1,4-piperazinediethanesulfonic acid polymerase chain reaction. Regulation of apoptosis is vital to the development and long term survival of metazoan animals. Apoptosis is required to maintain the balance between cell proliferation and cell death and, therefore, disruptions in the apoptotic program are associated with pathologies such as cancer, where there is too little cell death, and degenerative diseases, where there is too much cell death. Apoptosis can be initiated by at least three types of signals: (i) specific ligation of members on the tumor necrosis factor receptor (TNFR-1)1family, which includes Fas/Apo-1/CD95; (ii) cellular stress, which includes genotoxic damage and anti-neoplastic drugs; and (iii) delivery of granule-associated serine proteases from cytotoxic lymphocytes into target cells. Key mediators that initiate and execute the apoptotic program are members of the caspase family of cysteine proteases whose activation is believed to be essential for virtually all forms of apoptosis (1Salvesen G.S. Dixit V.M. Cell. 1997; 91: 443-446Abstract Full Text Full Text PDF PubMed Scopus (1932) Google Scholar, 2Cohen G.M. Biochem. J. 1997; 326: 1-16Crossref PubMed Scopus (4105) Google Scholar, 3Nicholson D.W. Thornberry N.A. Trends Biochem. Sci. 1997; 22: 299-306Abstract Full Text PDF PubMed Scopus (2176) Google Scholar). Caspases-3, -6, and -7 are involved in the execution of cells in response to many apoptotic stimuli including ligation of death receptors of the TNFR-1 receptor family, resulting in cleavage of a number of proteins whose limited proteolysis is definitive of apoptosis. However, these executioner caspases are not directly activated by receptor ligation, but rely on the proteolytic activity of upstream initiator caspases-8 and -10 (4Muzio M. Chinnaiyan A.M. Kischkel F.C. O'Rourke K. Shevchenko A. Ni J. Scaffidi C. Bretz J.D. Zhang M. Gentz R. Mann M. Krammer P.H. Peter M.E. Dixit V.M. Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2723) Google Scholar, 5Boldin M.P. Goncharov T.M. Goltsev Y.V. Wallach D. Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2100) Google Scholar, 6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar). In the case of caspase-8 the activation occurs by recruitment of the zymogen to the cytosolic face of the death receptor, such that the initial proteolytic signal originates by autoprocessing of the clustered zymogen (7Muzio M. Stockwell B.R. Stennicke H.R. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1998; 273: 2926-2930Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar, 8Yang X. Chang H.Y. Baltimore D. Mol Cell. 1998; 1: 319-325Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar).At the execution phase, caspase-3 seems to be upstream of caspases-6 and -7 and, therefore, its activation represents a key point in transmission of the proteolytic signal (9Hirata H. Takahashi A. Kobayashi S. Yonehara S. Sawai H. Okazaki T. Yamamoto K. Sasada M. J. Exp. Med. 1998; 187: 587-600Crossref PubMed Scopus (398) Google Scholar). However, the exact mechanism of how the death signal is conveyed from caspase-8 to caspase-3 remains unresolved. Is the apoptotic signal transmitted by direct activation of the executioners by the initiators, thus constituting a minimal two-step cascade that serves to mediate the apoptotic signals, or is the signal further amplified by the presence of additional factors as suggested by Scaffidi et al.(10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar).To address these issues we have performed a detailed kinetic study of the activation of pro-caspase-3 by caspases-8 and -10 using recombinant zymogens and active proteases in a defined system, and compared this to the activation of the zymogen by caspases-8 and -10 in cytosolic extracts. This allows us to predict the sequence of events that results in transmission of the proteolytic death signal originating from the initiator caspases to the executioners, and on the basis of in vitro observations, test the hypothesis that additional amplifiers and regulators of the apoptotic signals are required in vivo.DISCUSSIONCaspases are commonly divided into apical and executioner subsets (1Salvesen G.S. Dixit V.M. Cell. 1997; 91: 443-446Abstract Full Text Full Text PDF PubMed Scopus (1932) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar). There exist two well characterized points at which apical caspases initiate apoptotic signals. One is at the cell surface where members of the TNFR-1 family of death receptors transmit a signal across the cell membrane following receptor clustering. The second point of initiation follows the release of mitochondrial factors (34Liu X. Kim C.N. Yang J. Jemmerson R. Wang X. Cell. 1996; 86: 147-157Abstract Full Text Full Text PDF PubMed Scopus (4433) Google Scholar,35Kroemer G. Zamzami N. Susin S.A. Immunol. Today. 1997; 18: 44-51Abstract Full Text PDF PubMed Scopus (1379) Google Scholar), and although this post-mitochondrial pathway is well documented it is unclear how the mitochondrion perceives the apoptotic signal. Nevertheless, anti-neoplastic drugs, genotoxic damage, and inhibition of cellular signal transduction pathways all seem to converge on the mitochondrial route (36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). At each point of initiation the first recognizable biochemical event is specific caspase activation, and each initiation point utilizes distinct caspases. In the death receptor pathway(s) the apical caspase-8 (and possibly 10) transmits a proteolytic signal following autoactivation at the cytosolic face of the receptor (4Muzio M. Chinnaiyan A.M. Kischkel F.C. O'Rourke K. Shevchenko A. Ni J. Scaffidi C. Bretz J.D. Zhang M. Gentz R. Mann M. Krammer P.H. Peter M.E. Dixit V.M. Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2723) Google Scholar, 5Boldin M.P. Goncharov T.M. Goltsev Y.V. Wallach D. Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2100) Google Scholar, 6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 38Vincenz C. Dixit V.M. J. Biol. Chem. 1997; 272: 6578-6583Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar). The signal for activation appears to be local clustering of pro-caspase-8 that possesses enough activity in its zymogen to achieve autolytic proteolytic maturation (7Muzio M. Stockwell B.R. Stennicke H.R. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1998; 273: 2926-2930Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar, 8Yang X. Chang H.Y. Baltimore D. Mol Cell. 1998; 1: 319-325Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar, 39Martin D.A. Siegel R.M. Zheng L. Lenardo M.J. J. Biol. Chem. 1998; 273: 4345-4349Abstract Full Text Full Text PDF PubMed Scopus (329) Google Scholar). In the mitochondrial route specific or nonspecific delivery of cytochromec (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar, 40Zamzami N. Susin S.A. Marchetti P. Hirsch T. Gomez-Monterrey I.M.C. Kroemer G. J. Exp. Med. 1996; 183: 1533-1544Crossref PubMed Scopus (1262) Google Scholar, 41Kluck R.M. Bossy-Wetzel E. Green D.R. Newmeyer D.D. Science. 1997; 275: 1132-1136Crossref PubMed Scopus (4254) Google Scholar, 42Zou H. Henzel W.J. Liu X. Lutschg A. Wang X. Cell. 1997; 90: 405-413Abstract Full Text Full Text PDF PubMed Scopus (2727) Google Scholar) to the protein Apaf-1 results in recruitment and activation of caspase-9 (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar). Both caspase-8 and caspase-9 have been demonstrated to act on in vitro translated pro-caspases-3 and -7, the executioner caspases whose activation correlates with apoptosis. Thus there are two potential routes to activate the executioner caspases, and in this context both caspases-8 and -9 can be thought of as initiators whose pathways converge at the execution phase of apoptosis.Although caspases-8 and -10 can activate pro-caspase-3 in vitro, it has proven difficult to determine whether the apical caspases perform this function directly or indirectly, because previous studies have relied on in vitro translated zymogens or cytosolic extracts (6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 31Muzio M. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1997; 272: 2952-2956Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 43Orth K. Chinnaiyan A.M. Garg M. Froelich C.J. Dixit V.M. J. Biol. Chem. 1996; 271: 16443-16446Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar). We here demonstrate that caspase-8 and -10 can rapidly activate caspase-3. More importantly, focusing on caspase-8 we observe that activation proceeds with the same rate in a 293 cytosolic extract, eliminating a requirement for an intermediary component. Additionally, depletion of pro-caspase-9 from the extract has no impact on caspase activation by caspase-8, and MCF-7 cells deficient in caspase-3 failed to support processing of pro-caspase-9. Therefore, processing of pro-caspase-9 in death receptor-mediated apoptosis requires the presence and activation of caspase-3 which makes it a downstream event unlikely to play a major role in caspase activation.It has been proposed that mitochondria are required to transmit the apoptotic signal generated by treatment of cells by agonistic Fas antibodies, but only in a small selection of cell lines (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar). This would imply that caspase-8, the apical caspase of the Fas pathway, initiates a mitochondrial signal. In support of this, the generation of caspase activity initiated by addition of human caspase-8 to cytosolic extracts of Xenopus eggs is accelerated in the presence of mitochondria (44Kuwana T. Smith J.J. Muzio M. Dixit V. Newmeyer D.D. Kornbluth S. J. Biol. Chem. 1998; 273: 16589-16594Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar). However, the relevance of this to homologous systems is unclear, since it is not known whether Xenopus has a caspase-8, and the kinetics of activation of a putativeXenopus caspase-3 ortholog, or whether the caspase activity in Xenopus extracts is due to such an ortholog, have not been determined. The data presented above, in contrast, do not suggest any requirement for an intermediary between caspase-8 and caspase-3. Therefore, the question is whether the mitochondrial acceleration occurs in vivo, and whether caspase-8 must transmit its signal via mitochondria to the executioners in vivo. Currently the most valuable evidence for a role of mitochondria in apoptosis triggered by death receptor ligation comes from several studies investigating expression of ectopic or transgenic Bcl-2, which is hypothesized to operate by blocking mitochondrial-dependent apoptosis (reviewed in Ref. 36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Most investigators agree that Bcl-2 prevents apoptosis triggered by genotoxic damage, glucocorticoids, and chemotherapeutic drugs, but there are inconsistencies in the data describing the protective effect of Bcl-2 against apoptosis induced by death receptors (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar, 45Srinivasan A. Li F. Wong A. Kodandapani L. Smidt Jr., R. Krebs J.F. Fritz L.C. Wu J.C. Tomaselli K.J. J. Biol. Chem. 1998; 273: 4523-4529Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, 46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In a survey of several cell lines, Scaffidi et al. (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar) noted that most are not protected by Bcl-2 from apoptosis triggered by agonist Fas antibodies. However, the use of agonist antibodies and immortalized cell lines may not be the best way to determine a role for Bcl-2. More significantly, T-cell apoptosis in vivo, which is dependent on physiologic Fas ligation, is unaffected in Bcl-2 transgenic mice (46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In contrast, death following injection of agonist Fas antibodies in whole mice was significantly retarded in Bcl-2 transgenic mice (47Lacronique V. Mignon A. Fabre M. Viollet B. Rouquet N. Molina T. Porteu A. Henrion A. Bouscary D. Varlet P. Joulin V. Kahn A. Nat. Med. 1996; 2: 80-86Crossref PubMed Scopus (348) Google Scholar). These somewhat contradictory studies can be reconciled if some cells support direct transmission of caspase-8 to caspase-3, while others require a mitochondrial accelerator (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar).Since pro-caspase-9 is not processed by caspase-8, the mitochondrial accelerator must act upstream of the caspase-9 activator complex and, therefore, presumably upstream of the mitochondrion. Possibly the pro-apoptotic mitochondrial signal is activated by the action of caspase-8 on mitochondria, or a latent protein that activates mitochondria for apoptosis. The importance of the mitochondrial route in Fas death is not clear, since evidence suggests that only a minority of cell lines require mitochondria to transmit the caspase-8 signal. If direct transmission of the signal from caspase-8 to the executioner caspase-3 occurs in most cell lines, what is the advantage of a mitochondrial intermediate? Usually, levels of complexity are added to allow additional regulation points, as clearly evidenced by the evolution of the vertebrate blood coagulation cascade. It is not immediately clear what advantage cells would achieve by adding a level of regulation to the Fas-triggered death signal, but future studies will doubtlessly focus on this issue. Regardless, it would appear that those cells in the body that are primed to undergo apoptosis during education of the immune system have evolved death receptors, caspase-8, and the caspase-8 activator complex to allow rapid direct transmission of the death signal, bypassing any mitochondrial requirement, and that pro-caspase-3 is a major physiologic substrate of caspase-8 (Fig. 7). Regulation of apoptosis is vital to the development and long term survival of metazoan animals. Apoptosis is required to maintain the balance between cell proliferation and cell death and, therefore, disruptions in the apoptotic program are associated with pathologies such as cancer, where there is too little cell death, and degenerative diseases, where there is too much cell death. Apoptosis can be initiated by at least three types of signals: (i) specific ligation of members on the tumor necrosis factor receptor (TNFR-1)1family, which includes Fas/Apo-1/CD95; (ii) cellular stress, which includes genotoxic damage and anti-neoplastic drugs; and (iii) delivery of granule-associated serine proteases from cytotoxic lymphocytes into target cells. Key mediators that initiate and execute the apoptotic program are members of the caspase family of cysteine proteases whose activation is believed to be essential for virtually all forms of apoptosis (1Salvesen G.S. Dixit V.M. Cell. 1997; 91: 443-446Abstract Full Text Full Text PDF PubMed Scopus (1932) Google Scholar, 2Cohen G.M. Biochem. J. 1997; 326: 1-16Crossref PubMed Scopus (4105) Google Scholar, 3Nicholson D.W. Thornberry N.A. Trends Biochem. Sci. 1997; 22: 299-306Abstract Full Text PDF PubMed Scopus (2176) Google Scholar). Caspases-3, -6, and -7 are involved in the execution of cells in response to many apoptotic stimuli including ligation of death receptors of the TNFR-1 receptor family, resulting in cleavage of a number of proteins whose limited proteolysis is definitive of apoptosis. However, these executioner caspases are not directly activated by receptor ligation, but rely on the proteolytic activity of upstream initiator caspases-8 and -10 (4Muzio M. Chinnaiyan A.M. Kischkel F.C. O'Rourke K. Shevchenko A. Ni J. Scaffidi C. Bretz J.D. Zhang M. Gentz R. Mann M. Krammer P.H. Peter M.E. Dixit V.M. Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2723) Google Scholar, 5Boldin M.P. Goncharov T.M. Goltsev Y.V. Wallach D. Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2100) Google Scholar, 6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar). In the case of caspase-8 the activation occurs by recruitment of the zymogen to the cytosolic face of the death receptor, such that the initial proteolytic signal originates by autoprocessing of the clustered zymogen (7Muzio M. Stockwell B.R. Stennicke H.R. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1998; 273: 2926-2930Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar, 8Yang X. Chang H.Y. Baltimore D. Mol Cell. 1998; 1: 319-325Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar). At the execution phase, caspase-3 seems to be upstream of caspases-6 and -7 and, therefore, its activation represents a key point in transmission of the proteolytic signal (9Hirata H. Takahashi A. Kobayashi S. Yonehara S. Sawai H. Okazaki T. Yamamoto K. Sasada M. J. Exp. Med. 1998; 187: 587-600Crossref PubMed Scopus (398) Google Scholar). However, the exact mechanism of how the death signal is conveyed from caspase-8 to caspase-3 remains unresolved. Is the apoptotic signal transmitted by direct activation of the executioners by the initiators, thus constituting a minimal two-step cascade that serves to mediate the apoptotic signals, or is the signal further amplified by the presence of additional factors as suggested by Scaffidi et al.(10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar). To address these issues we have performed a detailed kinetic study of the activation of pro-caspase-3 by caspases-8 and -10 using recombinant zymogens and active proteases in a defined system, and compared this to the activation of the zymogen by caspases-8 and -10 in cytosolic extracts. This allows us to predict the sequence of events that results in transmission of the proteolytic death signal originating from the initiator caspases to the executioners, and on the basis of in vitro observations, test the hypothesis that additional amplifiers and regulators of the apoptotic signals are required in vivo. DISCUSSIONCaspases are commonly divided into apical and executioner subsets (1Salvesen G.S. Dixit V.M. Cell. 1997; 91: 443-446Abstract Full Text Full Text PDF PubMed Scopus (1932) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar). There exist two well characterized points at which apical caspases initiate apoptotic signals. One is at the cell surface where members of the TNFR-1 family of death receptors transmit a signal across the cell membrane following receptor clustering. The second point of initiation follows the release of mitochondrial factors (34Liu X. Kim C.N. Yang J. Jemmerson R. Wang X. Cell. 1996; 86: 147-157Abstract Full Text Full Text PDF PubMed Scopus (4433) Google Scholar,35Kroemer G. Zamzami N. Susin S.A. Immunol. Today. 1997; 18: 44-51Abstract Full Text PDF PubMed Scopus (1379) Google Scholar), and although this post-mitochondrial pathway is well documented it is unclear how the mitochondrion perceives the apoptotic signal. Nevertheless, anti-neoplastic drugs, genotoxic damage, and inhibition of cellular signal transduction pathways all seem to converge on the mitochondrial route (36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). At each point of initiation the first recognizable biochemical event is specific caspase activation, and each initiation point utilizes distinct caspases. In the death receptor pathway(s) the apical caspase-8 (and possibly 10) transmits a proteolytic signal following autoactivation at the cytosolic face of the receptor (4Muzio M. Chinnaiyan A.M. Kischkel F.C. O'Rourke K. Shevchenko A. Ni J. Scaffidi C. Bretz J.D. Zhang M. Gentz R. Mann M. Krammer P.H. Peter M.E. Dixit V.M. Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2723) Google Scholar, 5Boldin M.P. Goncharov T.M. Goltsev Y.V. Wallach D. Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2100) Google Scholar, 6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 38Vincenz C. Dixit V.M. J. Biol. Chem. 1997; 272: 6578-6583Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar). The signal for activation appears to be local clustering of pro-caspase-8 that possesses enough activity in its zymogen to achieve autolytic proteolytic maturation (7Muzio M. Stockwell B.R. Stennicke H.R. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1998; 273: 2926-2930Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar, 8Yang X. Chang H.Y. Baltimore D. Mol Cell. 1998; 1: 319-325Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar, 39Martin D.A. Siegel R.M. Zheng L. Lenardo M.J. J. Biol. Chem. 1998; 273: 4345-4349Abstract Full Text Full Text PDF PubMed Scopus (329) Google Scholar). In the mitochondrial route specific or nonspecific delivery of cytochromec (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar, 40Zamzami N. Susin S.A. Marchetti P. Hirsch T. Gomez-Monterrey I.M.C. Kroemer G. J. Exp. Med. 1996; 183: 1533-1544Crossref PubMed Scopus (1262) Google Scholar, 41Kluck R.M. Bossy-Wetzel E. Green D.R. Newmeyer D.D. Science. 1997; 275: 1132-1136Crossref PubMed Scopus (4254) Google Scholar, 42Zou H. Henzel W.J. Liu X. Lutschg A. Wang X. Cell. 1997; 90: 405-413Abstract Full Text Full Text PDF PubMed Scopus (2727) Google Scholar) to the protein Apaf-1 results in recruitment and activation of caspase-9 (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar). Both caspase-8 and caspase-9 have been demonstrated to act on in vitro translated pro-caspases-3 and -7, the executioner caspases whose activation correlates with apoptosis. Thus there are two potential routes to activate the executioner caspases, and in this context both caspases-8 and -9 can be thought of as initiators whose pathways converge at the execution phase of apoptosis.Although caspases-8 and -10 can activate pro-caspase-3 in vitro, it has proven difficult to determine whether the apical caspases perform this function directly or indirectly, because previous studies have relied on in vitro translated zymogens or cytosolic extracts (6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 31Muzio M. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1997; 272: 2952-2956Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 43Orth K. Chinnaiyan A.M. Garg M. Froelich C.J. Dixit V.M. J. Biol. Chem. 1996; 271: 16443-16446Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar). We here demonstrate that caspase-8 and -10 can rapidly activate caspase-3. More importantly, focusing on caspase-8 we observe that activation proceeds with the same rate in a 293 cytosolic extract, eliminating a requirement for an intermediary component. Additionally, depletion of pro-caspase-9 from the extract has no impact on caspase activation by caspase-8, and MCF-7 cells deficient in caspase-3 failed to support processing of pro-caspase-9. Therefore, processing of pro-caspase-9 in death receptor-mediated apoptosis requires the presence and activation of caspase-3 which makes it a downstream event unlikely to play a major role in caspase activation.It has been proposed that mitochondria are required to transmit the apoptotic signal generated by treatment of cells by agonistic Fas antibodies, but only in a small selection of cell lines (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar). This would imply that caspase-8, the apical caspase of the Fas pathway, initiates a mitochondrial signal. In support of this, the generation of caspase activity initiated by addition of human caspase-8 to cytosolic extracts of Xenopus eggs is accelerated in the presence of mitochondria (44Kuwana T. Smith J.J. Muzio M. Dixit V. Newmeyer D.D. Kornbluth S. J. Biol. Chem. 1998; 273: 16589-16594Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar). However, the relevance of this to homologous systems is unclear, since it is not known whether Xenopus has a caspase-8, and the kinetics of activation of a putativeXenopus caspase-3 ortholog, or whether the caspase activity in Xenopus extracts is due to such an ortholog, have not been determined. The data presented above, in contrast, do not suggest any requirement for an intermediary between caspase-8 and caspase-3. Therefore, the question is whether the mitochondrial acceleration occurs in vivo, and whether caspase-8 must transmit its signal via mitochondria to the executioners in vivo. Currently the most valuable evidence for a role of mitochondria in apoptosis triggered by death receptor ligation comes from several studies investigating expression of ectopic or transgenic Bcl-2, which is hypothesized to operate by blocking mitochondrial-dependent apoptosis (reviewed in Ref. 36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Most investigators agree that Bcl-2 prevents apoptosis triggered by genotoxic damage, glucocorticoids, and chemotherapeutic drugs, but there are inconsistencies in the data describing the protective effect of Bcl-2 against apoptosis induced by death receptors (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar, 45Srinivasan A. Li F. Wong A. Kodandapani L. Smidt Jr., R. Krebs J.F. Fritz L.C. Wu J.C. Tomaselli K.J. J. Biol. Chem. 1998; 273: 4523-4529Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, 46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In a survey of several cell lines, Scaffidi et al. (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar) noted that most are not protected by Bcl-2 from apoptosis triggered by agonist Fas antibodies. However, the use of agonist antibodies and immortalized cell lines may not be the best way to determine a role for Bcl-2. More significantly, T-cell apoptosis in vivo, which is dependent on physiologic Fas ligation, is unaffected in Bcl-2 transgenic mice (46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In contrast, death following injection of agonist Fas antibodies in whole mice was significantly retarded in Bcl-2 transgenic mice (47Lacronique V. Mignon A. Fabre M. Viollet B. Rouquet N. Molina T. Porteu A. Henrion A. Bouscary D. Varlet P. Joulin V. Kahn A. Nat. Med. 1996; 2: 80-86Crossref PubMed Scopus (348) Google Scholar). These somewhat contradictory studies can be reconciled if some cells support direct transmission of caspase-8 to caspase-3, while others require a mitochondrial accelerator (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar).Since pro-caspase-9 is not processed by caspase-8, the mitochondrial accelerator must act upstream of the caspase-9 activator complex and, therefore, presumably upstream of the mitochondrion. Possibly the pro-apoptotic mitochondrial signal is activated by the action of caspase-8 on mitochondria, or a latent protein that activates mitochondria for apoptosis. The importance of the mitochondrial route in Fas death is not clear, since evidence suggests that only a minority of cell lines require mitochondria to transmit the caspase-8 signal. If direct transmission of the signal from caspase-8 to the executioner caspase-3 occurs in most cell lines, what is the advantage of a mitochondrial intermediate? Usually, levels of complexity are added to allow additional regulation points, as clearly evidenced by the evolution of the vertebrate blood coagulation cascade. It is not immediately clear what advantage cells would achieve by adding a level of regulation to the Fas-triggered death signal, but future studies will doubtlessly focus on this issue. Regardless, it would appear that those cells in the body that are primed to undergo apoptosis during education of the immune system have evolved death receptors, caspase-8, and the caspase-8 activator complex to allow rapid direct transmission of the death signal, bypassing any mitochondrial requirement, and that pro-caspase-3 is a major physiologic substrate of caspase-8 (Fig. 7). Caspases are commonly divided into apical and executioner subsets (1Salvesen G.S. Dixit V.M. Cell. 1997; 91: 443-446Abstract Full Text Full Text PDF PubMed Scopus (1932) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar). There exist two well characterized points at which apical caspases initiate apoptotic signals. One is at the cell surface where members of the TNFR-1 family of death receptors transmit a signal across the cell membrane following receptor clustering. The second point of initiation follows the release of mitochondrial factors (34Liu X. Kim C.N. Yang J. Jemmerson R. Wang X. Cell. 1996; 86: 147-157Abstract Full Text Full Text PDF PubMed Scopus (4433) Google Scholar,35Kroemer G. Zamzami N. Susin S.A. Immunol. Today. 1997; 18: 44-51Abstract Full Text PDF PubMed Scopus (1379) Google Scholar), and although this post-mitochondrial pathway is well documented it is unclear how the mitochondrion perceives the apoptotic signal. Nevertheless, anti-neoplastic drugs, genotoxic damage, and inhibition of cellular signal transduction pathways all seem to converge on the mitochondrial route (36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). At each point of initiation the first recognizable biochemical event is specific caspase activation, and each initiation point utilizes distinct caspases. In the death receptor pathway(s) the apical caspase-8 (and possibly 10) transmits a proteolytic signal following autoactivation at the cytosolic face of the receptor (4Muzio M. Chinnaiyan A.M. Kischkel F.C. O'Rourke K. Shevchenko A. Ni J. Scaffidi C. Bretz J.D. Zhang M. Gentz R. Mann M. Krammer P.H. Peter M.E. Dixit V.M. Cell. 1996; 85: 817-827Abstract Full Text Full Text PDF PubMed Scopus (2723) Google Scholar, 5Boldin M.P. Goncharov T.M. Goltsev Y.V. Wallach D. Cell. 1996; 85: 803-815Abstract Full Text Full Text PDF PubMed Scopus (2100) Google Scholar, 6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 38Vincenz C. Dixit V.M. J. Biol. Chem. 1997; 272: 6578-6583Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar). The signal for activation appears to be local clustering of pro-caspase-8 that possesses enough activity in its zymogen to achieve autolytic proteolytic maturation (7Muzio M. Stockwell B.R. Stennicke H.R. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1998; 273: 2926-2930Abstract Full Text Full Text PDF PubMed Scopus (882) Google Scholar, 8Yang X. Chang H.Y. Baltimore D. Mol Cell. 1998; 1: 319-325Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar, 39Martin D.A. Siegel R.M. Zheng L. Lenardo M.J. J. Biol. Chem. 1998; 273: 4345-4349Abstract Full Text Full Text PDF PubMed Scopus (329) Google Scholar). In the mitochondrial route specific or nonspecific delivery of cytochromec (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar, 40Zamzami N. Susin S.A. Marchetti P. Hirsch T. Gomez-Monterrey I.M.C. Kroemer G. J. Exp. Med. 1996; 183: 1533-1544Crossref PubMed Scopus (1262) Google Scholar, 41Kluck R.M. Bossy-Wetzel E. Green D.R. Newmeyer D.D. Science. 1997; 275: 1132-1136Crossref PubMed Scopus (4254) Google Scholar, 42Zou H. Henzel W.J. Liu X. Lutschg A. Wang X. Cell. 1997; 90: 405-413Abstract Full Text Full Text PDF PubMed Scopus (2727) Google Scholar) to the protein Apaf-1 results in recruitment and activation of caspase-9 (29Li P. Nijhawan D. Budihardjo I. Srinivasula S.M. Ahmad M. Alnemri E.S. Wang X. Cell. 1997; 91: 479-489Abstract Full Text Full Text PDF PubMed Scopus (6183) Google Scholar). Both caspase-8 and caspase-9 have been demonstrated to act on in vitro translated pro-caspases-3 and -7, the executioner caspases whose activation correlates with apoptosis. Thus there are two potential routes to activate the executioner caspases, and in this context both caspases-8 and -9 can be thought of as initiators whose pathways converge at the execution phase of apoptosis. Although caspases-8 and -10 can activate pro-caspase-3 in vitro, it has proven difficult to determine whether the apical caspases perform this function directly or indirectly, because previous studies have relied on in vitro translated zymogens or cytosolic extracts (6Srinivasula S.M. Ahmad M. Fernandes-Alnemri T. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14486-14491Crossref PubMed Scopus (480) Google Scholar, 31Muzio M. Salvesen G.S. Dixit V.M. J. Biol. Chem. 1997; 272: 2952-2956Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar, 37Fernandes-Alnemri T. Armstrong R. Krebs J. Srinivasula S.M. Wang L. Bullrich F. Fritz L. Trapani J.A. Croce C.M. Tomaselli K.J. Litwack G. Alnemri E.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 7464-7469Crossref PubMed Scopus (692) Google Scholar, 43Orth K. Chinnaiyan A.M. Garg M. Froelich C.J. Dixit V.M. J. Biol. Chem. 1996; 271: 16443-16446Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar). We here demonstrate that caspase-8 and -10 can rapidly activate caspase-3. More importantly, focusing on caspase-8 we observe that activation proceeds with the same rate in a 293 cytosolic extract, eliminating a requirement for an intermediary component. Additionally, depletion of pro-caspase-9 from the extract has no impact on caspase activation by caspase-8, and MCF-7 cells deficient in caspase-3 failed to support processing of pro-caspase-9. Therefore, processing of pro-caspase-9 in death receptor-mediated apoptosis requires the presence and activation of caspase-3 which makes it a downstream event unlikely to play a major role in caspase activation. It has been proposed that mitochondria are required to transmit the apoptotic signal generated by treatment of cells by agonistic Fas antibodies, but only in a small selection of cell lines (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar). This would imply that caspase-8, the apical caspase of the Fas pathway, initiates a mitochondrial signal. In support of this, the generation of caspase activity initiated by addition of human caspase-8 to cytosolic extracts of Xenopus eggs is accelerated in the presence of mitochondria (44Kuwana T. Smith J.J. Muzio M. Dixit V. Newmeyer D.D. Kornbluth S. J. Biol. Chem. 1998; 273: 16589-16594Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar). However, the relevance of this to homologous systems is unclear, since it is not known whether Xenopus has a caspase-8, and the kinetics of activation of a putativeXenopus caspase-3 ortholog, or whether the caspase activity in Xenopus extracts is due to such an ortholog, have not been determined. The data presented above, in contrast, do not suggest any requirement for an intermediary between caspase-8 and caspase-3. Therefore, the question is whether the mitochondrial acceleration occurs in vivo, and whether caspase-8 must transmit its signal via mitochondria to the executioners in vivo. Currently the most valuable evidence for a role of mitochondria in apoptosis triggered by death receptor ligation comes from several studies investigating expression of ectopic or transgenic Bcl-2, which is hypothesized to operate by blocking mitochondrial-dependent apoptosis (reviewed in Ref. 36Reed J.C. Cell. 1997; 91: 559-562Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Most investigators agree that Bcl-2 prevents apoptosis triggered by genotoxic damage, glucocorticoids, and chemotherapeutic drugs, but there are inconsistencies in the data describing the protective effect of Bcl-2 against apoptosis induced by death receptors (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar, 33Chinnaiyan A.M. Dixit V.M. Curr. Biol. 1996; 6: 555-562Abstract Full Text Full Text PDF PubMed Google Scholar, 45Srinivasan A. Li F. Wong A. Kodandapani L. Smidt Jr., R. Krebs J.F. Fritz L.C. Wu J.C. Tomaselli K.J. J. Biol. Chem. 1998; 273: 4523-4529Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, 46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In a survey of several cell lines, Scaffidi et al. (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar) noted that most are not protected by Bcl-2 from apoptosis triggered by agonist Fas antibodies. However, the use of agonist antibodies and immortalized cell lines may not be the best way to determine a role for Bcl-2. More significantly, T-cell apoptosis in vivo, which is dependent on physiologic Fas ligation, is unaffected in Bcl-2 transgenic mice (46Strasser A. Harris A.W. Huang D.C. Krammer P.H. Cory S. EMBO J. 1995; 14: 6136-6147Crossref PubMed Scopus (662) Google Scholar). In contrast, death following injection of agonist Fas antibodies in whole mice was significantly retarded in Bcl-2 transgenic mice (47Lacronique V. Mignon A. Fabre M. Viollet B. Rouquet N. Molina T. Porteu A. Henrion A. Bouscary D. Varlet P. Joulin V. Kahn A. Nat. Med. 1996; 2: 80-86Crossref PubMed Scopus (348) Google Scholar). These somewhat contradictory studies can be reconciled if some cells support direct transmission of caspase-8 to caspase-3, while others require a mitochondrial accelerator (10Scaffidi C. Fulda S. Srinivasan A. Friesen C. Li F. Tomaselli K.J. Debatin K.M. Krammer P.H. Peter M.E. EMBO J. 1998; 17: 1675-1687Crossref PubMed Scopus (2617) Google Scholar). Since pro-caspase-9 is not processed by caspase-8, the mitochondrial accelerator must act upstream of the caspase-9 activator complex and, therefore, presumably upstream of the mitochondrion. Possibly the pro-apoptotic mitochondrial signal is activated by the action of caspase-8 on mitochondria, or a latent protein that activates mitochondria for apoptosis. The importance of the mitochondrial route in Fas death is not clear, since evidence suggests that only a minority of cell lines require mitochondria to transmit the caspase-8 signal. If direct transmission of the signal from caspase-8 to the executioner caspase-3 occurs in most cell lines, what is the advantage of a mitochondrial intermediate? Usually, levels of complexity are added to allow additional regulation points, as clearly evidenced by the evolution of the vertebrate blood coagulation cascade. It is not immediately clear what advantage cells would achieve by adding a level of regulation to the Fas-triggered death signal, but future studies will doubtlessly focus on this issue. Regardless, it would appear that those cells in the body that are primed to undergo apoptosis during education of the immune system have evolved death receptors, caspase-8, and the caspase-8 activator complex to allow rapid direct transmission of the death signal, bypassing any mitochondrial requirement, and that pro-caspase-3 is a major physiologic substrate of caspase-8 (Fig. 7). We thank Dr. Vishva Dixit for helpful discussion, and Scott Snipas and Annamarie Price for technical assistance.