Caspase-8 can be activated by interchain proteolysis without receptor-triggered dimerization during drug-induced apoptosis

Proteases of the caspase family are thought to be activated by proteolytic processing of their inactive zymogens. However, although proteolytic cleavage is sufficient for executioner caspases, a different mechanism has been recently proposed for initiator caspases, such as caspase-8, which are believed to be activated by proximity-induced dimerization. According to this model, dimerization rather than proteolytic processing is considered as the critical event for caspase-8 activation. Such a mechanism would suggest that in the absence of a dimerization platform such as the death-inducing signaling complex, caspase-8 proteolytic cleavage would result in an inactive enzyme. As several studies have described caspase-8 cleavage during mitochondrial apoptosis, we now investigated whether caspase-8 becomes indeed catalytically active in this pathway. Using an in vivo affinity labeling approach, we demonstrate that caspase-8 is activated in etoposide-treated cells in vivo in the absence of the receptor-induced death-inducing signaling complex formation. Furthermore, we show that both caspase-3 and -6 are required for the efficient activation of caspase-8. Our data therefore indicate that interchain cleavage of caspase-8 in the mitochondrial pathway is sufficient to produce an active enzyme even in the absence of receptor-driven procaspase-8 dimerization.     

Resistance to caspase-8 and -9 fragments in a malignant pleural mesothelioma cell line with acquired cisplatin-resistance

Apoptotic cysteine–aspartate proteases (caspases) are essential for the progression and execution of apoptosis, and detection of caspase fragmentation or activity is often used as markers of apoptosis. Cisplatin (cis-diamminedichloroplatinum (II)) is a chemotherapeutic drug that is clinically used for the treatment of solid tumours. We compared a cisplatin-resistant pleural malignant mesothelioma cell line (P31res1.2) with its parental cell line (P31) regarding the consequences of in vitro acquired cisplatin-resistance on basal and cisplatin-induced (equitoxic and equiapoptotic cisplatin concentrations) caspase-3, -8 and -9 fragmentation and proteolytic activity. Acquisition of cisplatin-resistance resulted in basal fragmentation of caspase-8 and -9 without a concomitant increase in proteolytic activity, and there was an increased basal caspase-3/7 activity. Similarly, cisplatin-resistant non-small-cell lung cancer cells, H1299res, had increased caspase-3 and -9 content compared with the parental H1299 cells. In P31 cells, cisplatin exposure resulted in caspase-9-mediated caspase-3/7 activation, but in P31res1.2 cells the cisplatin-induced caspase-3/7 activation occurred before caspase-8 or -9 activation. We therefore concluded that in vitro acquisition of cisplatin-resistance rendered P31res1.2 cells resistant to caspase-8 and caspase-9 fragments and that cisplatin-induced, initiator-caspase independent caspase-3/7 activation was necessary to overcome this resistance. Finally, the results demonstrated that detection of cleaved caspase fragments alone might be insufficient as a marker of caspase activity and ensuing apoptosis induction.     

Caspase-8 cleaves its substrates from the plasma membrane upon CD95-induced apoptosis

Apoptosis occurs through a tightly regulated cascade of caspase activation. In the context of extrinsic apoptosis, caspase-8 is activated by dimerization inside a death receptor complex, cleaved by auto-proteolysis and subsequently released into the cytosol. This fully processed form of caspase-8 is thought to cleave its substrates BID and caspase-3. To test if the release is required for substrate cleavage, we developed a novel approach based on localization probes to quantitatively characterize the spatial-temporal activity of caspases in living single cells. Our study reveals that caspase-8 is significantly more active at the plasma membrane than within the cytosol upon CD95 activation. This differential activity is controlled by the cleavage of caspase-8 prodomain. As a consequence, targeting of caspase-8 substrates to the plasma membrane can significantly accelerate cell death. Subcellular compartmentalization of caspase-8 activity may serve to restrict enzymatic activity before mitochondrial pathway activation and offers new possibilities to interfere with apoptotic sensitivity of the cells.     

The proteolytic procaspase activation network: an in vitro analysis

In general, apoptotic stimuli lead to activation of caspases. Once activated, a caspase can induce intracellular signaling pathways involving proteolytic activation of other caspase family members. We report the in vitro processing of eight murine procaspases by their enzymatically active counterparts. Caspase-8 processed all procaspases examined. Caspase-1 and -11 processed the effector caspases procaspase-3 and -7, and to a lesser extent procaspase-6. However, vice versa, none of the caspase-1-like procaspases was activated by the effector caspases. This suggests that the caspase-1 subfamily members either act upstream of the apoptosis effector caspases or else are part of a totally separate activation pathway. Procaspase-2 was maturated by caspase-8 and -3, and to a lesser extent by caspase-7, while the active caspase-2 did not process any of the procaspases examined, except its own precursor. Hence, caspase-2 might not be able to initiate a wide proteolytic signaling cascade. Additionally, cleavage data reveal not only proteolytic amplification between caspase-3 and -8, caspase-6 and -3, and caspase-6 and -7, but also positive feedback loops involving multiple activated caspases. Our results suggest the existence of a hierarchic proteolytic procaspase activation network, which would lead to a dramatic increase in multiple caspase activities once key caspases are activated. The proteolytic procaspase activation network might allow that different apoptotic stimuli result in specific cleavage of substrates responsible for typical processes at the cell membrane, the cytosol, the organelles, and the nucleus, which characterize a cell dying by apoptosis.     

Proteolytic activation of protein kinase C-epsilon by caspase-mediated processing and transduction of antiapoptotic signals

Several novel protein kinase C (PKC) isozymes have been identified as substrates for caspase-3. We have previously shown that novel PKCepsilon is cleaved during apoptosis in MCF-7 cells that lack any functional caspase-3. In the present study, we show that in vitro-translated PKCepsilon is processed by human recombinant caspase-3, -7, and -9. Tumor necrosis factor-alpha (TNF) triggered processing of PKCepsilon to a 43-kDa carboxyl-terminal fragment, and cell-permeable caspase inhibitors prevented TNF-induced processing of PKCepsilon in MCF-7 cells. PKCepsilon was cleaved primarily at the SSPD downward arrow G site to generate two fragments with an approximate molecular mass of 43 kDa. It was also cleaved at the DDVD downward arrow C site to generate two fragments with molecular masses of 52 and 35 kDa. Treatment of MCF-7 cells with TNF resulted in the activation of PKCepsilon, and mutation at the SSPD downward arrow G (D383A) site inhibited proteolytic activation of PKCepsilon. Overexpression of wild-type but not dominant-negative PKCepsilon in MCF-7 cells delayed TNF-induced apoptosis, and mutation at the D383A site prevented antiapoptotic activity of PKCepsilon. These results suggest that cleavage of PKCepsilon by caspase-7 at the SSPD downward arrow G site results in the activation of PKCepsilon. Furthermore, activation of PKCepsilon was associated with its antiapoptotic function.     

Active caspases and cleaved cytokeratins are sequestered into cytoplasmic inclusions in TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis- inducing ligand (TRAIL) -induced apoptosis, in transformed human breast epithelial MCF-7 cells, resulted in a time-dependent activation of the initiator caspases-8 and -9 and the effector caspase-7. Cleavage of caspase-8 and its preferred substrate, Bid, preceded processing of caspases-7 and -9, indicating that caspase-8 is the apical initiator caspase in TRAIL-induced apoptosis. Using transient transfection of COOH-terminal-tagged green fluorescent protein fusion constructs, caspases-3, -7, and -8 were localized throughout the cytoplasm of MCF-7 cells. TRAIL-induced apoptosis resulted in activation of caspases-3 and -7, and the redistribution of most of their detectable catalytically active small subunits into large spheroidal cytoplasmic inclusions, which lacked a limiting membrane. These inclusions, which were also induced in untransfected cells, contained cytokeratins 8, 18, and 19, together with both a phosphorylated form and a caspase-cleavage fragment of cytokeratin 18. Similarly, in untransfected breast HBL100 and lung A549 epithelial cells, TRAIL induced the formation of cytoplasmic inclusions that contained cleaved cytokeratin 18 and colocalized with active endogenous caspase-3. We propose that effector caspase-mediated cleavage of cytokeratins, resulting in disassembly of the cytoskeleton and formation of cytoplasmic inclusions, may be a characteristic feature of epithelial cell apoptosis.     

Phosphorylation of murine caspase-9 by the protein kinase casein kinase 2 regulates its cleavage by caspase-8

Previous studies from our laboratory had indicated that cytochrome c-independent processing and activation of caspase-9 by caspase-8 contributed to early amplification of the caspase cascade in tumor necrosis factor (TNF)-alpha-treated murine cells. Here we show that murine caspase-9 is phosphorylated by casein kinase 2 (CK2) on a serine near the site of caspase-8 cleavage. CK2 has been shown to regulate cleavage of the pro-apoptotic Bid protein by phosphorylating serine residues near its caspase-8 cleavage site. Similarly, CK2 modification of Ser(348) on caspase-9 appears to render the protease refractory to cleavage by active caspase-8. This phosphorylation did not affect the ability of caspase-9 to autoprocess. Substitution of Ser(348) abolished phosphorylation but not cleavage, and a phospho-site mutant promoted apoptosis in TNF-alpha-treated caspase-9 knock-out mouse embryo fibroblasts. Furthermore, inhibition of CK2 activity and RNA interference-mediated knockdown of the kinase accelerated caspase-9 activation, whereas phosphatase inhibition delayed both caspase-9 activation and death in response to TNF receptor occupation. Taken together, these studies show that TNF receptor cross-linking promotes dephosphorylation of caspase-9, rendering it susceptible to processing by activated caspase-8 protein. Thus, our data suggest that modification of procaspase-9 to protect it from inappropriate cleavage and activation is yet another mechanism by which the oncogenic kinase CK2 promotes survival.     

Nitric oxide suppresses apoptosis via interrupting caspase activation and mitochondrial dysfunction in cultured hepatocytes.

Nitric oxide (NO) is a potent inhibitor of apoptosis in many cell types, including hepatocytes. We and others have described NO-dependent decreases in caspase activity in cells undergoing apoptosis. However, previous work has not determined whether NO disrupts the proteolytic processing and thus the activation of pro-caspases. Here we report that NO suppresses proteolytic processing and activation of multiple pro-caspases in intact cells, including caspase-3 and caspase-8. We found that both exogenous NO as well as endogenously produced NO via adenoviral inducible NO synthase gene transfer protected hepatocytes from tumor necrosid factor (TNF) alpha plus actinomycin D (TNFalpha/ActD)-induced apoptosis. Affinity labeling with biotin-VAD-fmk of all active caspase species in TNFalpha-mediated apoptosis identified four newly labeled spots (activated caspases) present exclusively in TNFalpha/ActD-treated cells. Both NO and the caspase inhibitor, Ac-DEVD-CHO, prevented the appearance of the four newly labeled spots or active caspases. Immunoanalysis of affinity labeled caspases demonstrated that caspase-3 was the major effector caspase. Western blot analysis also identified the activation of caspase-8 in the TNFalpha/ActD-treated cells, and the activation was suppressed by NO. Furthermore, NO inhibited several other events associated with caspase activation in cells, including release of cytochrome c from mitochondria, decrease in mitochondrial transmembrane potential, and cleavage of poly(ADP-ribose) polymerase in TNFalpha/ActD-treated cells. These findings indicate the involvement of multiple caspases in TNFalpha-mediated apoptosis in hepatocytes and establish the capacity of NO to inhibit not only active caspases but also caspase activation.     

Type 1 inositol-1,4,5-trisphosphate receptor is a late substrate of caspases during apoptosis

Apoptosis is characterized by the proteolytic cleavage of hundreds of proteins. One of them, the type 1 inositol-1,4,5-trisphosphate receptor (IP(3) R-1), a multimeric receptor located on the endoplasmic reticulum (ER) membrane that is critical to calcium homeostasis, was reported to be cleaved during staurosporine (STS) induced-apoptosis in Jurkat cells. Because the reported cleavage site separates the IP(3) binding site from the channel moiety, its cleavage would shut down a critical signaling pathway that is common to several cellular processes. Here we show that IP(3) R-1 is not cleaved in 293 cells treated with STS, TNFα, Trail, or ultra-violet (UV) irradiation. Further, it is not cleaved in Hela or Jurkat cells induced to undergo apoptosis with Trail, TNFα, or UV. In accordance with previous reports, we demonstrate that it is cleaved in a Jurkat cell line treated with STS. However its cleavage occurs only after poly(ADP-ribose) polymerase (PARP), which cleavage is a hallmark of apoptosis, and p23, a poor caspase-7 substrate, are completely cleaved, suggesting that IP(3) R-1 is a relatively late substrate of caspases. Nevertheless, the receptor is fully accessible to proteolysis in cellulo by ectopically overexpressed caspase-7 or by the tobacco etch virus (TEV) protease. Finally, using recombinant caspase-3 and microsomal fractions enriched in IP(3) R-1, we show that the receptor is a poor caspase-3 substrate. Consequently, we conclude that IP(3) R-1 is not a key death substrate.     

Inhibition of ERK attenuates autophagy and potentiates tumour necrosis factor-α-induced cell death in MCF-7 cells

The role of autophagy in cell death is under considerable debate. The process of autophagy has been shown to lead to either cell survival or cell death depending on cell type and stimulus. In the present study, we determined the contribution of ERK1/2 signalling to autophagy and cell death induced by tumour necrosis factor-α (TNF) in MCF-7 breast cancer cells. Treatment of MCF-7 cells with TNF caused a time-dependent increase in ERK1/2 activity. There was an induction of autophagy and cleavage of caspase-7, -8, -9 and PARP. Pharmacological inhibition of ERK1/2 phosphorylation with U0126 or PD98059 resulted in a decrease in TNF-induced autophagy that was accompanied by an increase in cleavage of caspase-7, -8, -9 and PARP Furthermore, inhibition of ERK1/2 signalling resulted in decreased clonogenic capacity of MCF-7 cells. These data suggest that TNF-induces autophagy through ERK1/2 and that inhibition of autophagy increases cellular sensitivity to TNF.     

Fas death receptor signalling: roles of Bid and XIAP

Fas (also called CD95 or APO-1), a member of a subgroup of the tumour necrosis factor receptor superfamily that contain an intracellular death domain, can initiate apoptosis signalling and has a critical role in the regulation of the immune system. Fas-induced apoptosis requires recruitment and activation of the initiator caspase, caspase-8 (in humans also caspase-10), within the death-inducing signalling complex. In so-called type 1 cells, proteolytic activation of effector caspases (-3 and -7) by caspase-8 suffices for efficient apoptosis induction. In so-called type 2 cells, however, killing requires amplification of the caspase cascade. This can be achieved through caspase-8-mediated proteolytic activation of the pro-apoptotic Bcl-2 homology domain (BH)3-only protein BH3-interacting domain death agonist (Bid), which then causes mitochondrial outer membrane permeabilisation. This in turn leads to mitochondrial release of apoptogenic proteins, such as cytochrome c and, pertinent for Fas death receptor (DR)-induced apoptosis, Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP binding protein with low Pi), an antagonist of X-linked inhibitor of apoptosis (XIAP), which imposes a brake on effector caspases. In this review, written in honour of Juerg Tschopp who contributed so much to research on cell death and immunology, we discuss the functions of Bid and XIAP in the control of Fas DR-induced apoptosis signalling, and we speculate on how this knowledge could be exploited to develop novel regimes for treatment of cancer.     

A potent reporter applicable to the monitoring of caspase-3-dependent proteolytic cleavage

In this study, we developed a chimeric caspase-3 substrate (GST:DEVD:EGFP) comprised of glutathione-S transferase (GST) and enhanced green fluorescent protein (EGFP) with a specialized linker peptide harboring the caspase-3 cleavage sequence, DEVD. Using this reporter, we assessed the proteolytic cleavage of the artificial caspase-3 substrate for caspase-3. The common feature of this approach is that the presence of the DEVD sequence between GST and EGFP allows for caspase-3-dependent cleavage after the Asp (D) residue, resulting in the elimination of EGFP from the GST:DEVD:EGFP reporter. To the best of our knowledge, this study reports the first application employing a chimeric protein substrate, with the similar accuracy level compared to the conventional methods such as fluorometric assays. As a result, using this GST:DEVD:EGFP reporter, caspase-3 activation based on proteolytic properties could be monitored via a variety of bioanalytical techniques such as immunoblot analysis, glutathione-agarose bead assay, and on-chip visualization, providing both technical and economical advantages over the extensively utilized fluorogenic peptide assay. Our results convincingly showed that this versatile reporter (GST:DEVD:EGFP) constitutes a useful system for the monitoring of caspase-3 activation, potentially enabling the monitoring of the proteolytic activities of different intra-cellular proteases via the substitution of the cleavage sequence within the same schematic construct.     

Cathepsin D links TNF-induced acid sphingomyelinase to Bid-mediated caspase-9 and -3 activation

Acidic noncaspase proteases-like cathepsins have been introduced as novel mediators of apoptosis. A clear role for these proteases and the acidic endolysosomal compartment in apoptotic signalling is not yet defined. To understand the role and significance of noncaspases in promoting and mediating cell death, it is important to determine whether an intersection of these proteases and the caspase pathway exists. We recently identified the endolysosomal aspartate protease cathepsin D (CTSD) as a target for the proapoptotic lipid ceramide. Here, we show that tumor necrosis factor (TNF)-induced CTSD activation depends on functional acid sphingomyelinase (A-SMase) expression. Ectopic expression of CTSD in CTSD-deficient fibroblasts results in an enhanced TNF-mediated apoptotic response. Intracellular colocalization of CTSD with the proapoptotic bcl-2 protein family member Bid in HeLa cells, and the ability of CTSD to cleave directly Bid in vitro as well as the lack of Bid activation in cathepsin-deficient fibroblasts indicate that Bid represents a direct downstream target of CTSD. Costaining of CTSD and Bid with Rab5 suggests that the endosomal compartments are the common 'meeting point'. Caspase-9 and -3 activation also was in part dependent on A-SMase and CTSD expression as revealed in the respective deficiency models. Our results link as novel endosomal intermediates the A-SMase and the acid aspartate protease CTSD to the mitochondrial apoptotic TNF pathway.     

Granzyme B activates procaspase-3 which signals a mitochondrial amplification loop for maximal apoptosis

Granzyme B (GrB), acting similar to an apical caspase, efficiently activates a proteolytic cascade after intracellular delivery by perforin. Studies here were designed to learn whether the physiologic effector, GrB-serglycin, initiates apoptosis primarily through caspase-3 or through BH3-only proteins with subsequent mitochondrial permeabilization and apoptosis. Using four separate cell lines that were either genetically lacking the zymogen or rendered deficient in active caspase-3, we measured apoptotic indices within whole cells (active caspase-3, mitochondrial depolarization [DeltaPsim] and TUNEL). Adhering to these conditions, the following were observed in targets after GrB delivery: (a) procaspase-3-deficient cells fail to display a reduced DeltaPsim and DNA fragmentation; (b) Bax/Bak is required for optimal DeltaPsim reduction, caspase-3 activation, and DNA fragmentation, whereas BID cleavage is undetected by immunoblot; (c) Bcl-2 inhibits GrB-mediated apoptosis (reduced DeltaPsim and TUNEL reactivity) by blocking oligomerization of caspase-3; and (d) in procaspase-3-deficient cells a mitochondrial-independent pathway was identified which involved procaspase-7 activation, PARP cleavage, and nuclear condensation. The data therefore support the existence of a fully implemented apoptotic pathway initiated by GrB, propagated by caspase-3, and perpetuated by a mitochondrial amplification loop but also emphasize the presence of an ancillary caspase-dependent, mitochondria-independent pathway.     

Caspase-3 mediated feedback activation of apical caspases in doxorubicin and TNF-α induced apoptosis

Aberrant apoptosis has been associated with the development and therapeutic resistance of cancer. Recent studies suggest that caspase deficiency/downregulation is frequently detected in different cancers. We have previously shown that caspase-3 reconstitution significantly sensitized MCF-7 cells to doxorubicin and etoposide. In contrast to the well established role of caspase-3 as an effector caspase, the focus of this study is to delineate caspase-3 induced feedback activation of the apical caspases-2, -8, -9 and -10A in doxorubicin and TNF-α induced apoptosis. Using cell-free systems we show that caspases-9 and 2 are the most sensitive, caspase-8 is less sensitive and caspase-10A is the least sensitive to caspase-3 mediated-cleavage. When apoptosis is induced by doxorubicin or TNF-α in an intact cell model, cleavage of caspases-8 and -9, but not caspase-2, was markedly enhanced by caspase-3. Caspase-3 mediated-feedback and activation of caspase-8 and -9 in MCF-7/C3 cells is further supported by an increase in the cleavage of caspase-8 and 9 substrates and cytochrome c release. These data indicate that, in addition to its function as an effector caspase, caspase-3 plays an important role in maximizing the activation of apical caspases and crosstalk between the two major apoptotic pathways. The significant impact of caspase-3 on both effector and apical caspases suggests that modulation of caspase-3 activity would be a useful approach to overcome drug resistance in clinical oncology. XiaoHe Yang: This work was supported in part by the Career Development Award DAMD17-99-1-9180 from Department of Defense to X.H.Y.     

Blockade of tumor necrosis factor-induced Bid cleavage by caspase-resistant Rb

Tumor necrosis factor-alpha (TNF) activates caspase-8 to cleave effector caspases or Bid, resulting in type-1 or type-2 apoptosis, respectively. We show here that TNF also induces caspase-8-dependent C-terminal cleavage of the retinoblastoma protein (Rb). Interestingly, fibroblasts from Rb(MI/MI) mice, in which the C-terminal caspase cleavage site is mutated, exhibit a defect in Bid cleavage despite caspase-8 activation. Recent results suggest that TNF receptor endocytosis is required for the activation of caspase-8. Consistent with this notion, inhibition of V-ATPase, which plays an essential role in acidification and degradation of endosomes, specifically restores Bid cleavage in Rb(MI/MI) cells. Inhibition of V-ATPase sensitizes Rb(MI/MI) but not wild-type fibroblasts to TNF-induced apoptosis and stimulates inflammation-associated colonic apoptosis in Rb(MI/MI) but not wild-type mice. These results suggest that Rb cleavage is required for Bid cleavage in TNF-induced type-2 apoptosis, and this requirement can be supplanted by the inhibition of V-ATPase.     

Galectin-1 induced activation of the mitochondrial apoptotic pathway: evidence for a connection between death-receptor and mitochondrial pathways in human Jurkat T lymphocytes

Galectin-1 (gal-1) triggers T cell death by several distinct intracellular pathways including the activation of the death-receptor pathway. The aim of this study was to investigate whether gal-1 induced activation of the death-receptor pathway in Jurkat T lymphocytes mediates apoptosis via the mitochondrial pathway linked by truncated Bid (tBid). We demonstrate that gal-1 induced proteolytic cleavage of the death agonist Bid, a member of the Bcl-2/Bcl-xL family and a substrate of activated caspase-8, was inhibited by caspase-8 inhibitor II (Z-IETD-FMK). Downstream of Bid, gal-1 stimulated mitochondrial cytochrome c release as well as the activation and proteolytic processing of initiator procaspase-9 were effectively decreased by caspase-8 inhibitor II. Blocking of gal-1 induced cleavage of effector procaspase-3 by caspase-8 inhibitor II as well as by caspase-9 inhibitors I (Z-LEHD-FMK) and III (Ac-LEHD-CMK) indicates that receptor and mitochondrial pathways converged in procaspase-3 activation and contribute to proteolytic processing of effector procaspase-6 and -7. Western blot analyses and immunofluorescence staining revealed that exposure of Jurkat T cells to gal-1 resulted in the cleavage of the DNA-repair enzyme poly (ADP-ribose) polymerase, cytoskeletal α-fodrin, and nuclear lamin A as substrates of activated caspases. Our data demonstrate that Bid provides a connection between the death receptor and the mitochondrial pathway of gal-1 induced apoptosis in human Jurkat T lymphocytes.     

Activation of caspase-3 alone is insufficient for apoptotic morphological changes in human neuroblastoma cells

Activated caspase-3 is considered an important enzyme in the cell death pathway. To study the specific role of caspase-3 activation in neuronal cells, we generated a stable tetracycline-regulated SK-N-MC neuroblastoma cell line, which expressed a highly efficient self-activating chimeric caspase-3, consisting of the caspase-1 prodomain fused to the caspase-3 catalytic domain. Under expression-inducing conditions, we observed a time-dependent increase of processed caspase-3 by immunostaining for the active form of the enzyme, intracellular caspase-3 enzyme activity, as well as poly(ADP-ribose) polymerase (PARP) cleavage. Induced expression of the caspase fusion protein showed predominantly caspase-3 activity without any apoptotic morphological changes. In contrast, staurosporine treatment of the same cells resulted in activation of multiple caspases and profound apoptotic morphology. Our work provides evidence that auto-activation of caspase-3 can be efficiently achieved with a longer prodomain and that neuronal cell apoptosis may require another caspase or activation of multiple caspase enzymes.     

Inhibition of ERK attenuates autophagy and potentiates tumour necrosis factor-alpha-induced cell death in MCF-7 cells.

The role of autophagy in cell death is under considerable debate. The process of autophagy has been shown to lead to either cell survival or cell death depending on cell type and stimulus. In the present study, we determined the contribution of ERK1/2 signalling to autophagy and cell death induced by tumour necrosis factor-alpha (TNF) in MCF-7 breast cancer cells. Treatment of MCF-7 cells with TNF caused a time-dependent increase in ERK1/2 activity. There was an induction of autophagy and cleavage of caspase-7, -8, -9 and PARP. Pharmacological inhibition of ERK1/2 phosphorylation with U0126 or PD98059 resulted in a decrease in TNF-induced autophagy that was accompanied by an increase in cleavage of caspase-7, -8, -9 and PARP Furthermore, inhibition of ERK1/2 signalling resulted in decreased clonogenic capacity of MCF-7 cells. These data suggest that TNF-induces autophagy through ERK1/2 and that inhibition of autophagy increases cellular sensitivity to TNF.