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* This work was supported in part by National Institutes of Health Grants GM61712 and AG15393.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. § Supported by a stipend from the Boehringer Ingelheim Fonds. ¶ Both authors contributed equally to this work. ‖ Supported by Molecular and Cell Biology Training Grant 5T32GM07183. ** Supported by Cancer Biology Training Program 5T32CA09594. §§ Supported by United States Army Medical Research and Materiel Command Grant DAMD17-99-1-9511.
Apoptosis induction through CD95 (APO-1/Fas) critically depends on generation of active caspase-8 at the death-inducing signaling complex (DISC). Depending on the cell type, active caspase-8 either directly activates caspase-3 (type I cells) or relies on mitochondrial signal amplification (type II cells). In MCF7-Fas cells that are deficient for pro-caspase-3, even high amounts of caspase-8 produced at the DISC cannot directly activate downstream effector caspases without mitochondrial help. Overexpression of Bcl-xL in these cells renders them resistant to CD95-mediated apoptosis. However, activation of caspase-8 in control (vector) and Bcl-xL transfectants of MCF7-Fas cells proceeds with similar kinetics, resulting in a complete processing of cellular caspase-8. Most of the cytosolic caspase-8 substrates are not cleaved in the Bcl-xL protected cells, raising the question of how Bcl-xL-expressing MCF7-Fas cells survive large amounts of potentially cytotoxic caspase-8. We now demonstrate that active caspase-8 is initially generated at the DISC of both MCF7-Fas-Vec and MCF7-Fas-Bcl-xL cells and that the early steps of CD95 signaling such as caspase-8-dependent cleavage of DISC bound c-FLIPL, caspase-8-dependent clustering, and internalization of CD95, as well as processing of pro-caspase-8 bound to mitochondria are very similar in both transfectants. However, events downstream of mitochondria, such as release of cytochrome c, only occur in the vector-transfected MCF7-Fas cells, and no in vivocaspase-8 activity can be detected in the Bcl-xL-expressing cells. Our data suggest that, in Bcl-xL-expressing MCF7-Fas cells, active caspase-8 is sequestered on the outer mitochondrial surface presumably by association with the protein “bifunctional apoptosis regulator” in a way that does not allow substrates to be cleaved, identifying a novel mechanism of regulation of apoptosis sensitivity by mitochondrial Bcl-xL.
death-inducing signaling complex
Fas-associated death domain
cellular FLICE-inhibitory protein
bifunctional apoptosis regulator
amino trifluoromethyl coumarin
death effector domain-like
inhibitor of apoptosis protein
CD95 (APO-1/Fas) is a member of a family of death receptors that regulates apoptosis in many different tissues. Other members of this family include TNF-R1, DR3 (TRAMP, wsl-1, APO-3, LARD), TRAIL-R1 (DR4, APO-2), TRAIL-R2 (DR5, TRICK, KILLER), and DR6 (for review, see Ref.
). Induction of apoptosis through CD95 involves activation of caspase-8 at the death-inducing signaling complex (DISC).1 The DISC comprises oligomerized CD95, the adapter FADD/Mort1, and the cysteine protease caspase-8, activation of which initiates the caspase cascade for apoptosis (
). We have previously shown that transient expression of pro-caspase-3 in MCF7-Fas cells converted them from type II to type I cells, in which Bcl-xL could not protect them from CD95-mediated apoptosis (
). We now show that all of the initial CD95 signaling events such as cleavage of DISC-bound c-FLIPL and pro-caspase-8 and clustering and internalization of CD95 are found in both vector-transfected and Bcl-xL-expressing MCF7-Fas cells. However, shortly after generation at the DISC, active caspase-8 subunits become inactivated in the Bcl-xL-expressing cells, at a time when this protease can be detected on mitochondria. We also demonstrate that the mitochondrial protein BAR, which has been shown to simultaneously bind caspase-8 and Bcl-2 (
), also binds active caspase-8 in a manner similar to heavy membrane fractions isolated from Bcl-xL-expressing cells. The data point to a model in which Bcl-xL gives mitochondria of MCF7 cells the ability to sequester and neutralize active caspase-8 through binding to BAR.
The breast carcinoma cell line MCF7-Fas behaves as a type II cell in that overexpression of Bcl-xL protects these cells from CD95-induced apoptosis (
), decline of mitochondrial transmembrane potential (ΔΨm), and release of cytochrome c from mitochondria (Fig.1A). In contrast, processing of caspase-8 was not reduced in the presence of Bcl-xL(Fig. 1B). In fact, after 16 h, most of the cellular pro-caspase-8 had been converted to active caspase-8, with no resulting loss of cell viability. Unexpectedly, however, cleavage of PARP and Bid was inhibited by Bcl-xL (Fig. 1, B andC). Furthermore, degradation of plectin, another directin vivo substrate of caspase-8 (
), was also inhibited (Fig. 1C). These data demonstrate the ability of MCF7-Fas-Bcl-xL cells to tolerate large quantities of active caspase-8. It therefore appears that, in these cells, active caspase-8 subunits are either inactivated and/or prevented from reaching their cytosolic targets.
We have recently shown that, in MCF7 cells, pro-caspase-8 is mainly localized on mitochondria (
). In contrast, using confocal microscopy analysis of cells costained for caspase-8 and phosphodiesterase, an enzyme of the endoplasmic reticulum, we did not observe a significant localization of caspase-8 to the endoplasmic reticulum2(
). The localization of caspase-8 to mitochondria in these cells is not the result of the absence of caspase-3, because caspase-8 is still localized mainly on mitochondria in MCF7 cells reconstituted with pro-caspase-3 (Fig. 2A). MCF7 cells therefore express caspase-8 at two intracellular locations, in the cytosol and on mitochondria. This raises the question of how mitochondrial pro-caspase-8 is processed. Active caspase-8 may move from the DISC to mitochondria and directly process mitochondrial pro-caspase-8, or mitochondrial pro-caspase-8 may be recruited to the cell surface and become activated at the DISC.
Because no significant cleavage of Bid could be detected in the Bcl-xL-expressing MCF7-Fas cells, we tested whether caspase-8 itself might directly process mitochondrial pro-caspase-8. To test this hypothesis we incubated isolated heavy membranes (HM) from MCF7-Fas-Bcl-xL cells with various recombinant caspases and examined which of these proteases are capable of processing HM-associated pro-caspase-8 (Fig. 2B). Among the caspases tested, caspase-8 most efficiently cleaved pro-caspase-8 in this assay. Because all of the pro-caspase-8 in these cells becomes activated after triggering CD95, we postulate that a small quantity of active caspase-8 generated at the DISC might directly activate mitochondrial pro-caspase-8 independently of the apoptogenic activity of mitochondria. Because this activity is blocked by Bcl-xLin vivo (Fig. 1A) but processing of HM-bound pro-caspase-8 occurs, this suggests that caspase-8 is neutralized at the level of mitochondria, assuming that active caspase-8 is generated at the DISC in similar amounts in both transfectants. To directly test this assumption, we immunoprecipitated the DISC and determined its IETD-cleaving activity in a fluorogenic assay (Fig. 2C). Early after receptor triggering, the activity of caspase-8 in the DISC was the same in both transfectants, confirming that the block in the apoptosis pathway in MCF7-Fas-Bcl-xL cells must be located downstream of the DISC.
To identify the step in the CD95 pathway where caspase-8 is inactivated in the MCF7-Fas-Bcl-xL cells, we directly compared the time course of processing of pro-caspase-8 at the DISC and on HMs in both vector control and Bcl-xL transfectants (Fig.3A). Stimulation of CD95 in both transfectants resulted in rapid recruitment of caspase-8 and its processing at the DISC (Fig. 3A). The activity of the DISC is reflected by the early appearance of the caspase-8 cleavage intermediates p43/p41. We further found evidence that caspase-8 is initially active when converted at the DISC in both transfectants, as the cleaved p43 form of c-FLIPL is present in the DISC of both vector control and Bcl-xL-expressing cells. Pretreating cells with the caspase-8-selective inhibitor zIETD-fmk inhibited processing of c-FLIPL. Interestingly, most of the cytosolic c-FLIPL was cleaved very early after triggering CD95.2 Thus c-FLIPL and caspase-8 are among the few caspase-8 substrates in MCF7-Fas cells we detected whose cleavage is not blocked by Bcl-xL. The appearance of p43/p41 partially processed pro-caspase-8 on isolated HM fractions seemed to be delayed when compared with its appearance in the DISC, consistent with the initiation of the death signal at the plasma membrane. However, no difference was observed in the rate of loss of full-length pro-caspase-8 in HM fractions when comparing vector and Bcl-xL transfectants, despite the fact that release of cytochrome c from mitochondria could only be detected in the MCF7-Fas-Vec cells (Fig. 3A). The data therefore suggest that the block in the CD95 apoptosis pathway in the Bcl-xL-expressing MCF7-Fas cells resides at a step between proteolytic processing of mitochondria bound pro-caspase-8 and mitochondrial dysfunction (as measured by cytochrome crelease and drop in ΔΨm).
Our data suggested that caspase-8 was inhibited in Bcl-xL-expressing cells either by a soluble cytosolic caspase inhibitor or by being bound and sequestered at intracellular membranes. To distinguish between these two possibilities, we permeabilized MCF7-Fas cells using lysophosphatidylcholine, separated supernatant (cytosol) from pellet (membranes), and determined by immunoblot analysis whether active caspase-8 was released or retained in cells (Fig. 3B). Tim23, a protein of the inner mitochondrial membrane, was analyzed to demonstrate that the supernatant was not contaminated with mitochondrial membranes. In addition, we determined the caspase-8 protease activity of supernatants and pellets using a fluorogenic substrate (Fig. 3B,bottom). When MCF7-Fas-Vec cells were tested, the caspase-8 p18 subunit was detected in the supernatant in an active form (Fig.3B, lane 2). In contrast, the caspase-8 cleavage intermediates p41 and p43 were found in the pellet fraction of MCF7-Fas-Vec cells, indicating that they are bound to intracellular membranes and that activation of caspase-8 takes place in a membrane context. In contrast to MCF7-Fas-Vec cells, no fully processed active caspase-8 was released into the supernatant of MCF7-Fas-Bcl-xL cells, suggesting that active caspase-8 is not freely soluble in these cells. Additionally, we could not reproducibly detect the caspase-8 p18 subunits associated with the membrane fraction of the Bcl-xL-transfected cells, likely because of secondary degradation of the protein after detergent lysis.2 When taken together, these data predict that after generation at the DISC, active caspase-8 is neutralized by binding to intracellular membranes in a Bcl-xL-dependent fashion in MCF7-Fas cells. The finding that the rate of processing of pro-caspase-8 was very similar in both the vector and the Bcl-xL transfectants whereas cytochrome crelease was blocked by Bcl-xL strongly suggested that the block in the apoptosis pathway in MCF7-Fas-Bcl-xL cells was located downstream of caspase-8 bound to intracellular membranes and upstream of mitochondrial cytochrome c release.
To further explore at which step the apoptosis pathway was blocked by expression of Bcl-xL, we tested the activity of caspase-8 in these cells in vivo during the course of CD95-mediated apoptosis. We used the membrane-permeable fluorescein-conjugated peptide-based caspase substrate CaspaTag 8, which becomes covalently coupled to caspase-8 when it is cleaved by the protease. This compound labels active caspase subunits directly in living cells. Cells were incubated for different times with anti-CD95, and, 1 h before stopping the reaction, the fluorescent compound was added. CaspaTag 8 specifically labeled apoptosing MCF7-Fas-Vec cells (Fig.4A). Three independent experiments are shown. Caspase-8 activity in MCF7-Fas-Vec cells increased over the entire time course of anti-CD95 treatment. In contrast, we only detected a small but reproducible caspase-8 activity in the apoptosis-resistant Bcl-xL-expressing cells 4 h after stimulation (Fig. 4A). This activity was not detectable 24 h after stimulation, despite full processing of pro-caspase-8, suggesting that processed caspase-8 was inactivated in these cells at a relatively early time after CD95 stimulation.
We wished to probe in more detail the issue of when caspase-8 becomes neutralized in Bcl-xL-overexpressing cells. In this regard, we have recently found that one of the earliest events during CD95-mediated apoptosis, occurring shortly after formation of the DISC, is CD95 surface clustering. Furthermore, we have shown that this CD95 clustering and subsequent internalization of the receptor are caspase-8-dependent (
). To test whether Bcl-xL affects this early event associated with caspase-8 activity, we incubated MCF7-Fas-Vec and Bcl-xL-expressing cells with anti-CD95 mAb and a FITC-conjugated goat anti-mouse IgG antibody and monitored clustering of CD95 by fluorescence microscopy. For this experiment detached cells were used, because the clustering effect was more pronounced than in attached cells.2 In contrast to the diffuse staining of unstimulated cells, a 60-min stimulation with anti-CD95 caused clustering of the receptor in both vector and Bcl-xL transfectants. Consistent with recent data, this effect was inhibited by the caspase-8-selective inhibitor zIETD (
). We found that the small amount of caspase-8 formed in the apoptosis-resistant Bcl-xL-expressing cells was sufficient to efficiently internalize CD95 (Fig. 4C), consistent with our conclusion that caspase-8 generated at the DISC of Bcl-xL-expressing cells is initially active, thus allowing early caspase-8-regulated steps in CD95 signaling to occur. However, caspase-8 presumably becomes inactivated shortly thereafter.
To directly detect caspase-8 active subunits in cells treated with anti-CD95, we compared the localization of active caspase-8 in the MCF7-Fas-Vec cells with the MCF7-Fas-Bcl-xL cells, using the p10-specific anti-caspase-8 antibody C5 for laser scanning microscopy analysis. Pro-caspase-8 was localized at mitochondria in nonapoptotic MCF7-Fas-Vec and MCF7-Fas-Bcl-xL cells (Fig.5, A and B). Co-staining for caspase-8 and Bcl-xL showed that both Bcl-xL and caspase-8 colocalized almost entirely on mitochondria of the MCF7-Fas-Bcl-xL cells (Fig.5B). Consistent with our previous report (
), upon induction of apoptosis through CD95, the staining for caspase-8 in most MCF7-Fas control cells changed from a mitochondrial pattern to a more diffuse meshwork-like pattern, which we have recently identified as the cytoskeletal protein plectin (
) (Fig. 5A). In contrast, following treatment of MCF7-Fas-Bcl-xL cells with an anti-CD95 antibody for 16 h, we still detected only a mitochondrial staining of caspase-8 active subunits (Fig.5B). The C5 antibody recognizes both full-length and the active subunit p10 of caspase-8, and it therefore also stains pro-caspase-8 in unstimulated cells. We have recently established that, in both transfectants, the prodomain of caspase-8 stays bound to mitochondria following cleavage (
). Because the processing of caspase-8 in this experiment showed a very similar degree of conversion of caspase-8 between vector and Bcl-xL-transfected cells (Fig. 5C), the difference between the staining patterns seen in cells stimulated with anti-CD95 indicates that the p10 subunits in MCF7-Fas-Vec and MCF7-Fas-Bcl-xL cells were at different intracellular locations. The data gave rise to the hypothesis that mitochondria of MCF7-Fas cells have the ability to bind and inactivate caspase-8 when they express Bcl-xL.
To directly demonstrate that endogenous caspase-8 bound to Bcl-xL-expressing HMs, we isolated cytoplasmic and HM fractions from MCF7-Fas-Vec and MCF7-Fas-Bcl-xL cells and subjected them to a caspase-8 activity assay (Fig. 5D). After stimulation of MCF7-Fas control cells for 3 h, caspase-8 activity in the cytoplasm was high, whereas no caspase-8 activity was detected in the HM fraction. In contrast, in Bcl-xL-overexpressing MCF7-Fas cells, significant caspase-8 activity was found associated with HM, whereas the caspase-8 activity in the cytosolic fraction was reduced when compared with control cells. Extensive work demonstrated that, during the incubation of 1 h at 37 °C for fluorogenic assays, inactive caspase-8 regains activity by either dissociating from HM fractions or by undergoing a conformational change.2 The data suggest that Bcl-xL-expressing intracellular membranes have the capacity to actively recruit caspase-8 from the cytoplasm when Bcl-xL is overexpressed in MCF7-Fas cells.
We predicted a binding partner for active caspase-8 on intracellular membranes that would bind and neutralize active caspase-8 in a Bcl-xL-dependent fashion. Recently, a novel protein, BAR, was described that binds both pro-caspase-8 and Bcl-2 and is localized to mitochondria (
). Thus, we initiated experiments to explain the role of BAR in the caspase-8 sequestration phenomenon. Our first task was to deal with the issue of where BAR, Bcl-xL, and caspase-8 reside when associated with mitochondria because if BAR is involved it must have the same topological orientation on intracellular membranes as caspase-8 and Bcl-xL. To determine the localization of caspase-8, Bcl-xL, and BAR on HMs of MCF7-Fas-Bcl-xL cells, we treated HM fractions of these cells with trypsin (Fig. 6A). Membrane-associated pro-caspase-8, Bcl-xL, and BAR were all determined to be protease-sensitive, whereas an integral transmembrane protein of the inner mitochondrial membrane, Tim23, was resistant to the trypsin treatment. Addition of Triton X-100 detergent to solubilize membranes resulted in sensitivity of Tim23 to tryptic degradation. These experiments suggest that Bcl-xL, BAR, and caspase-8 are located on the outer mitochondrial membrane, oriented toward the cytosol.
We have recently shown that isolated HM fractions from MCF7-Fas-Bcl-xL cells can bind pro-caspase-8 in vitro (
). Because BAR was exquisitely sensitive to trypsin treatment, we treated HMs with trypsin to test whether HMs lose their activity to bind caspase-8 in the absence of BAR (Fig. 6B).35S-Labeled pro-caspase-8 bound very efficiently to isolated HMs before trypsin treatment (Fig. 6B, lane 2). In contrast, after trypsin treatment of HMs, caspase-8 did not bind. Trypsin treatment also resulted in a complete loss of BAR correlating with the complete loss of caspase-8 binding to these membranes (Fig. 6B, lane 4). In comparison, trypsin treatment only partially reduced the amount of Bcl-xL on these HM fractions, and it had no effect on the inner membrane protein Tim23 (Fig. 6B, lane 7).
To directly test which domain of caspase-8 binds to HMs from MCF7 cells, we performed an in vitro binding assay in whichin vitro translated pro-caspase-8 was added to isolated HM fractions from Bcl-xL-expressing MCF7-Fas cells, essentially as described (
). When pro-caspase-3 and pro-caspase-8 were mixed and applied to HM fractions, only pro-caspase-8 was quantitatively bound (Fig. 6C, lane 2), demonstrating the specificity of this binding. Comparisons of full-length caspase-8 were then made with various fragments of this protein to map the relevant domains. The prodomain of caspase-8 (N-CASP-8; corresponding to amino acids 1–216) bound to HMs (Fig.6C, lane 5) but not as effectively as full-length caspase-8 (based on comparisons of the relative amounts of material in the “bound” versus “unbound” fractions). The C-terminal domain of caspase-8 (C-CASP-8, corresponding to the unprocessed catalytic domain, residues 224–479) also bound to HM fractions (Fig. 6C, lane 8). Note that an aberrant C-CASP-8 translation product (C-CASP-8*) failed to bind HMs (Fig. 6C, lane 9). To test whether fully processed active caspase-8 would also bind to HMs, we generated active caspase-8 p18/p10 subunits by treating in vitro translated C-CASP-8 with recombinant caspase-8 (Fig. 6C, lane 10). Although processed caspase-8 is notoriously difficult to detect by SDS-PAGE/immunoblotting, making signals weak, these binding experiments suggest that the processed caspase-8 p18 (large) and p10 (small) catalytic subunits also are capable of associating with HM fractions (Fig. 6C, lane 11).
If BAR is the protein that binds active caspase-8 on the surface of mitochondria, then we expected its binding properties to be similar to those of HMs isolated from BAR-expressing MCF7-Fas-Bcl-xLcells and therefore performed in vitro protein binding assays using a GST fusion protein containing the DED-L protein from BAR (
) and testing its ability to interact with in vitrotranslated N-CASP-8, C-CASP-8, and processed p18/p10 caspase-8 (Fig.6D). Similar to HM fractions, GST-BAR-DED-L bound pro-caspase-8 but did not bind pro-caspase-3 (Fig. 6D,lanes 2 and 3), thus confirming the specificity of this result. The prodomain of caspase-8 (N-CASP-8) as well as the unprocessed catalytic domain (C-CASP-8) bound to GST-BAR-DED-L (Fig.6D, lanes 6 and 9) but not to GST control protein (Fig. 6D, lanes 5 and8) and not to GST-FADD.2 Similar to the binding of caspase-8 to HM fractions, GST-BAR-DED-L also bound specifically processed p18/p10 caspase-8 subunits (Fig. 6D, lane 12). Neither control GST (Fig. 6D, lane 11) nor GST-FADD2 bound processed caspase-8, thus confirming the specificity of this result. These experiments identify BAR as a protein that binds both unprocessed and active caspase-8, making it a potential candidate for explaining the sequestration of active caspase-8 on the surface of Bcl-xL-expressing mitochondria.
We next tested whether BAR in combination with Bcl-xL could protect cells from caspase-8-induced apoptosis. To mimic the situation in MCF7 cells, we induced apoptosis in 293T cells by transiently transfecting them with a plasmid encoding a CD8-caspase-8 fusion protein (
). When 293T cells were transfected with this plasmid, DNA fragmentation could be detected, which was somewhat reduced by expression of either BAR or Bcl-xL (Fig. 7A). However, when BAR was co-expressed with Bcl-xL, DNA fragmentation was reduced almost to background levels. Because both BAR (Fig. 7B) and Bcl-xL (Fig. 7C) are present on HMs when expressed in 293T cells, the data are consistent with a model in which mitochondrial BAR can collaborate with Bcl-xL to bind active caspase-8. Consistent with previous data, we found that BAR lacking the N-terminal RING domain was much more efficiently expressed than wild-type BAR (Fig. 7B) (
). Hence, BAR protein content appears to be regulated at a post-translational level, consistent with the observation that the N-terminal RING domain of BAR has an ubiquitin-protein isopeptide ligase activity that might regulate the turnover of this protein (
To directly test whether mitochondrial BAR has the ability to inhibit caspase-8 in a Bcl-xL-regulated fashion, we isolated HM fractions from 293T cells transiently transfected with BARΔR and/or Bcl-xL and incubated these HM fractions with cellular lysates prepared from MCF7-Fas cells stimulated through CD95 for 3 h. We then determined the activity of caspase-8 in the supernatant after removal of HMs by centrifugation (Fig. 7C). In this experiment, only HMs expressing BARΔR in combination with Bcl-xL were able to significantly deplete cellular lysates of active caspase-8. The data suggest that active caspase-8 in MCF7-Fas-Bcl-xL cells is bound and inactivated on the outer mitochondrial membrane with the help of BAR and Bcl-xL.
Our original description of the two CD95 signaling pathways (
), one mitochondrial-dependent (type II) and the other mitochondrial-independent (type I), is consistent with data from a variety of cell culture models and has been supported by transgenic and gene knock-out experiments in mice. For example, studies of Bcl-2 transgenic mice (
) have shown that T lymphocytes and especially thymocytes are type I cells, whereas hepatocytes represent type II cells. The main difference between type I and type II cells lies in the amount of caspase-8 generated at the DISC after CD95 activation. We have demonstrated that the amount of active caspase-8 generated at the DISC in type II cells is so small that it cannot be detected by Western blotting (
). It has been demonstrated in a cell-free system that the amount of active caspase-8 required to induce apoptotic changes in isolated nuclei through direct cleavage of pro-caspase-3 in the absence of mitochondria is orders of magnitudes higher than that required to induce these nuclear changes in the presence of mitochondria (
). These changes can be inhibited by expression of Bcl-2 or Bcl-xL on mitochondria.
Because Bcl-2 and Bcl-xL inhibit the apoptogenic activity of mitochondria without interfering with the activity of the DISC, a confounding question has been the fate of active caspase-8 generated at the level of the death receptors in Bcl-2/Bcl-xL protected cells. To study the biochemical mechanisms underlying this phenomenon, we chose to use MCF7-Fas cells as a model because they produce large type I-like amounts of caspase-8 at the DISC upon addition of an agonistic anti-CD95 antibody but behave like type II cells because of the absence of pro-caspase-3.
Most of the pro-caspase-8 in MCF7 cells is localized to mitochondria. Although it has been suggested that a number of caspases, including caspase-8, reside in the intermembrane space of mitochondria (
), our data favor a model in which caspase-8 is bound to the outer mitochondrial membrane. This model is also consistent with the finding that isolated HM fractions efficiently bind in vitrotranslated caspase-8 (Ref.
, and present study). We did not succeed in isolating mitochondria from MCF7 cells. However, we repeated thisin vitro binding experiment with mitochondria isolated from CEM cells stably transfected with Bcl-xL (
), and these purified mitochondria showed the same binding specificity for caspase-8 and its cleavage products as the HM fractions used in this study.2 These data indicate that mitochondria have a capacity to bind caspase-8. This finding, however, does not exclude the possibility that other intracellular compartments such as the endoplasmic reticulum might also have this capacity, particularly because the intracellular localization of caspases seems to be tissue-specific.
During CD95-mediated apoptosis in MCF7-Fas cells, all caspase-8, both cytosolic and mitochondrial, is activated. Because the signal is initiated at the cell surface, it is conceivable that a small amount of caspase-8 generated at the DISC travels into the cytoplasm and cleaves mitochondria-bound pro-caspase-8. This hypothesis is supported by our observation that addition of recombinant caspase-8 (but not other caspases) resulted in proteolytic processing of pro-caspase-8 when added to HM fractions. It occurs in both the vector-transfected as well as the Bcl-xL-expressing MCF7-Fas cells consistent with our finding that both DISC-bound and cytosolic caspase-8 (and c-FLIPL) are completely cleaved in both transfectants.
Recently, it was demonstrated that inhibition of TRAIL-induced apoptosis of MCF7 cells by transient expression of Bcl-xLcould be overcome by co-expression of Smac/Diablo (
). The authors concluded that overexpression of Smac/Diablo converted MCF7 cells from type II into type I cells. It is unlikely that Smac/Diablo is responsible for the differences between MCF7-Fas-Vec and MCF7-Fas-Bcl-xL cells used in our study. If Smac/Diablo were to regulate the activity of caspase-8 in these cells, it would likely antagonize a caspase-8-specific member of the IAP family. However, no examples of caspase-8 suppression by IAP proteins have been reported (
) should render even the MCF7-Fas-Vec cells apoptosis-resistant. Our finding that active caspase-8 could be found in the supernatant of permeabilized MCF7-Fas-Vec but not MCF7-Fas-Bcl-xL cells argues for a different mechanism. Altogether, our data suggest the existence of an inhibitory mechanism that takes place on the outer mitochondrial membrane. Interestingly, we found that caspase-8 is not irreversibly inactivated when bound to HMs, because we could detect significant amounts of caspase-8 activity on HM fractions of Bcl-xL-expressing MCF7-Fas cells using an in vitro caspase assay. This is likely the result of the conditions of this assay (incubation with the substrate for 1 h at 37 °C, during which sufficient amounts of caspase-8 became released from-inhibitory proteins to reveal caspase-8 enzymatic activity). These data are consistent with a binding of caspase-8 to intracellular membranes in a form in which its access to substrate is restricted.
Neither pro-caspase-8 nor active caspase-8 can bind to Bcl-xL directly (
), suggesting the presence of a protein domain on the surface of mitochondria that specifically binds active caspase-8 and that is regulated by Bcl-2 and Bcl-xL. Recently it was suggested that BAR could mediate binding of pro-caspase-8 to mitochondria in a Bcl-2-regulated fashion (
). Prior to our report, however, binding of active caspase-8 to BAR had not been tested. We now demonstrate that BAR binds active caspase-8, pro-caspase-8, and the caspase-8 prodomain (containing the two tandem death effector domains) in vitro. The binding characteristics of BAR are similar to those of HM fractions of MCF7-Fas-Bcl-xL cells, implicating BAR as a candidate for binding of active caspase-8 by Bcl-xL-expressing mitochondria. It should be noted, however, that recent data suggest that, depending on the cell type, BAR together with Bcl-2/Bcl-xL can also be found in the endoplasmic reticulum.
To test BAR's role in the CD95 apoptosis resistance of MCF7-Fas-Bcl-xLcells, we recently down-regulated BAR in these cells using specific antisense oligonucleotides.3 Reduction of BAR expression resulted in a significant sensitization of MCF7-Fas-Bcl-xLcells to anti-CD95-induced apoptosis.3
Our data also support a model in which binding of active caspase-8 to mitochondrial BAR in the presence of Bcl-xL results in its inactivation. MCF7 cells express large amounts of the mitochondrial protein BAR and lack expression of caspase-3. They therefore have a high capacity to buffer active caspase-8. We observed that, after an initial detection of the active p18 subunits of caspase-8, which peaks between 2 and 4 h after CD95 stimulation in both the vector-transfected and the Bcl-xL-expressing cells, all caspase-8 fragments are apparently degraded as they are no longer detectable by immunoblotting. Degradation of active caspase subunits is a commonly observed phenomenon, although it has never adequately been explained. Late during apoptosis, when cells undergo secondary necrosis, many cellular substrates are degraded because of the destruction of the cells.2 However, secondary necrosis cannot explain our results because active caspase-8 was also rapidly degraded in MCF7-Fas-Bcl-xL cells and these cells did not die. Given that the BAR protein binds caspase-8 and possesses a RING domain with probable ubiquitin-protein isopeptide ligase-like activity, we speculate that active caspase-8 might be targeted for ubiquitination and proteasome dependent degradation. However, we failed to prevent the degradation of caspase-8 by pre-treating cells with proteasome inhibitors or by adding zVAD-fmk to the cells.2 This finding suggests that the apparent degradation of caspase-8 as seen by immune blotting may reflect an intrinsic instability of the active caspase-8 enzyme molecule. Consistent with that idea, degradation of caspase-8 in the surviving MCF7-Fas-Bcl-xL cells occurred with the same kinetics as in cells undergoing apoptosis, despite the complete lack of PARP cleavage.
Recently, we have demonstrated that shortly after binding of anti-CD95 antibody or CD95 ligand, the CD95 receptor forms clusters on the cell surface followed by receptor internalization, a process dependent on polymerization of actin and one that requires caspase-8 activity (
). Interestingly, we now show that even non-apoptosing MCF7-Fas-Bcl-xL cells cluster and internalize activated CD95, and this process can be inhibited by a caspase-8-selective inhibitor. Our data suggest that initially MCF7-Fas-Bcl-xLcells activate caspase-8 at the DISC, resulting in generation of small amounts of active caspase-8. This activity is then so effectively sequestered as a result of BAR and Bcl-xL co-expression that even Bid and plectin are not cleaved. Our data therefore provide an explanation for the striking finding that even microinjection of recombinant active caspase-8 did not induce apoptosis in these cells (
). Late after the triggering of CD95, caspase-8 activity is specifically lost in the Bcl-xL-expressing MCF7-Fas cells at time points when we detect caspase-8 active subunits at mitochondria. At this time, the bound caspase-8 subunits are not active, because we detected no caspase activity in these cells in vivo. We therefore conclude that binding of caspase-8 to mitochondria results in its inactivation or at least its sequestration in a manner that effectively neutralizes the protease. This mechanism may also be important in other type II cells in which less active caspase-8 is produced at the DISC. In these cells, a much lower expression of BAR may be sufficient to neutralize the smaller amounts of active caspase-8 generated at the DISC. However, this hypothesis could be very difficult to test, because of the detection limits of the assays currently available. Details of the mechanism of sequestration and inactivation of caspase-8 at mitochondria of Bcl-xL-expressing type II cells await further investigation.
The MCF7-Fas and MCF7-Fas-Bcl-xLcells, the MCF7-pro-caspase-3 cells, the anti-APO-1 mAb, the plasmid to express the CD8:caspase-8 fusion protein, and the pro-caspase-3 plasmid were generous gifts from Drs. M. Jäättelä, A. Porter, P. Krammer, M. Lenardo, and V. Dixit, respectively. We are grateful to Dr. G. Salvesen for providing active recombinant caspases and to S. Lampel and A. Murmann for help with the confocal microscopy.