Mitochondrial Pathway Of Caspase Activation Research Articles

Fumonisin B1 Induces Immunotoxicity and Apoptosis of Chicken Splenic Lymphocytes

Fumonisin B1 (FB1), produced by Fusarium, is among the most abundant and toxic mycotoxin contaminations in feed, causing damages to the health of livestock. However, the mechanisms of FB1 toxicity in chickens are less understood. As splenic lymphocytes play important roles in the immune system, the aim of this study was to investigate the immunotoxic effects and mechanisms of FB1 on chicken splenic lymphocytes. In the present study, the chicken primary splenic lymphocytes were harvested and treated with 0, 2.5, 5, 10, 20 and 40 μg/mL FB1. Then, the cell proliferation, damage, ultrastructure, inflammation and apoptosis were evaluated. Results showed that the proliferation rate of splenic lymphocytes was decreased by FB1 treatments. The activity of lactate dehydrogenase (LDH) was increased by FB1 treatments in a dose-dependent manner, implying the induction of cell damage. Consistently, the ultrastructure of splenic lymphocytes showed that FB1 at all the tested concentrations caused cell structure alterations, including nuclear vacuolation, mitochondrial swelling and mitochondrial crest fracture. Besides, immunosuppressive effects of FB1 were observed by the decreased concentrations of interleukin-2 (IL-2), IL-4, IL-12 and interferon-γ (IFN-γ) in the cell culture supernatant. Furthermore, apoptosis was observed in FB1-treated cells by flow cytometry. The mRNA expressions of apoptosis-related genes showed that the expression of Bcl-2 was decreased, while the expressions of the P53, Bax, Bak-1, and Caspase-3 were increased with FB1 treatment. Similar results were found in the concentrations of apoptosis-related proteins in the cell supernatant by ELISA assay. Moreover, regression analysis indicated that increasing FB1 concentration increased LDH activity, concentrations of Bax, Bak-1 and mRNA expression of Bak-1 linearly, increased M1 area percentage quadratically, decreased concentration of IFN-γ, mRNA expression of Bcl-2 linearly, and decreased concentrations of IL-2 and IL-4 quadratically. Besides, regression analysis also showed reciprocal relationships between IL-12 concentration, Caspase-3 mRNA expression and increasing FB1 concentration. The increasing FB1 concentration could decrease IL-12 concentration and increase Caspase-3 mRNA expression. Altogether, this study reported that FB1 induced the immunotoxicity of chicken splenic lymphocytes and caused splenic lymphocytes apoptosis by the Bcl-2 family-mediated mitochondrial pathway of caspase activation.

Abstract 4891: Radiation enhances the efficacy of Bcl-2 and Bcl-XL inhibition in small cell lung cancer: A new concept of using radiation for targeted therapy through contextual oncogene addiction

Abstract Purpose: Radiotherapy has a critical role in the treatment of small cell lung cancer (SCLC). Effectiveness of radiation in SCLC remains limited as resistance results from defects in apoptosis. Experimental Design: We investigated whether using a new Bcl-2/Bcl-XL inhibitor, S44563, can enhance radiosensitivity of SCLC cells in vitro and in vivo. We also explored the mechanisms of the potential interaction. Results: In vitro studies confirmed that S44563 induce apoptosis in SCLC cells by inducing hallmarks of apoptosis (cleaved caspase-3, sub-G1 fraction induction and induction of the mitochondrial caspase activation pathway). S44563 markedly enhanced sensitivity of H146 and H69 cells to radiation in clonogenic assay. In vitro experiments demonstrated a greater induction of apoptosis (sub-G1 fraction and cleaved caspase-3) with the combination as well as in vivo caspase-3 immunostaining. This positive interaction between S44563 and radiation was greater when S44563 was given during and after the completion of the radiation, which was explained by the radiation-induced over expression of anti-apoptotic proteins (Bcl-2 and Bcl-XL). in vitro studies based on gene expression and immunoblotting indicated that radiation activates NF-κB pathways which results in anti-apoptotic expression. In addition, S44563 combined with cisplatin-based chemo-radiation showed a dramatic tumor growth delay and increased overall survival in mouse xenograft models.Conclusion: This study underlines the critical role of sequence administration of targeted therapies with conventional therapies such as radiation and chemotherapy. SCLC cells which survive to IR highly rely on the induction of anti apoptotic proteins for their survival therefore yielding to the concept of “contextual oncogene addiction” to the Bcl-2/Bcl-XL pathway. These results suggest that radioresistance results in part from resistance to apoptosis in SCLC mediated by up regulation of Bcl-2/Bcl-XL. The inhibition of Bcl-2 and Bcl-XL leads to the induction of radiation-induced apoptosis and enhances radiation therapy both in vitro and in SCLC xenograft models. Our data suggest that targeting radiation induced survival pathways may constitute an efficient strategy to potentiate IR. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4891. doi:1538-7445.AM2012-4891

Inhibition of the Sodium Potassium Adenosine Triphosphatase Pump Sensitizes Cancer Cells to Anoikis and Prevents Distant Tumor Formation

Normal epithelial cells undergo apoptosis upon detachment from the extracellular matrix, a process termed "anoikis." However, malignant epithelial cells with metastatic potential resist anoikis and can survive in an anchorage-independent fashion. Molecules that sensitize resistant cells to anoikis will be useful chemical probes to understand this pathway. To identify novel anoikis sensitizers in anoikis-resistant PPC-1 prostate adenocarcinoma cells, a library of 2,000 off-patent drugs and natural products was screened for their ability to preferentially induce cell death in suspension over adherent culture conditions. This screen identified five members of the family of cardiac glycosides as anoikis sensitizers, including ouabain, peruvoside, digoxin, digitoxin, and strophanthidin. We conducted further studies with ouabain to discern the mechanism of cardiac glycoside-induced anoikis sensitization. Ouabain initiated anoikis through the mitochondrial pathway of caspase activation. In addition, ouabain sensitized cells to anoikis by inhibiting its known target, the Na(+)/K(+) ATPase pump, and inducing hypoosmotic stress. Resistance to anoikis permits cancer cells to survive in the circulation and facilitates their metastasis to distant organs, so we tested the effects of Na(+)/K(+) ATPase inhibition on distant tumor formation in mouse models. In these mouse models, ouabain inhibited tumor metastases but did not alter the growth of subcutaneous tumors. Thus, we have identified a novel mechanism to sensitize resistant cells to anoikis and decrease tumor metastasis. These results suggest a potential mechanism for the observed clinical reduction in metastasis and relapse in breast cancer patients who have undergone treatments with cardiac glycosides.

Targeting Bcl-2 family proteins modulates the sensitivity of B-cell lymphoma to rituximab-induced apoptosis

AbstractThe chimeric monoclonal antibody rituximab is the standard of care for patients with B-cell non-Hodgkin lymphoma (B-NHL). Rituximab mediates complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity of CD20-positive human B cells. In addition, rituximab sensitizes B-NHL cells to cytotoxic chemotherapy and has direct apoptotic and antiproliferative effects. Whereas expression of the CD20 antigen is a natural prerequisite for rituximab sensitivity, cell-autonomous factors determining the response of B-NHL to rituximab are less defined. To this end, we have studied rituximab-induced apoptosis in human B-NHL models. We find that rituximab directly triggers apoptosis via the mitochondrial pathway of caspase activation. Expression of antiapoptotic Bcl-xL confers resistance against rituximab-induced apoptosis in vitro and rituximab treatment of xenografted B-NHL in vivo. B-NHL cells insensitive to rituximab-induced apoptosis exhibit increased endogenous expression of multiple antiapoptotic Bcl-2 family proteins, or activation of phosphatidylinositol-3-kinase signaling resulting in up-regulation of Mcl-1. The former resistance pattern is overcome by treatment with the BH3-mimetic ABT-737, the latter by combining rituximab with pharmacologic phosphatidylinositol-3-kinase inhibitors. In conclusion, sensitivity of B-NHL cells to rituximab-induced apoptosis is determined at the level of mitochondria. Pharmacologic modulation of Bcl-2 family proteins or their upstream regulators is a promising strategy to overcome rituximab resistance.

Cyproheptadine displays preclinical activity in myeloma and leukemia

AbstractD-cyclins are regulators of cell division that act in a complex with cyclin-dependent kinases to commit cells to a program of DNA replication. D-cyclins are overexpressed in many tumors, including multiple myeloma and leukemia, and contribute to disease progression and chemoresistance. To better understand the role and impact of D-cyclins in hematologic malignancies, we conducted a high throughput screen for inhibitors of the cyclin D2 promoter and identified the drug cyproheptadine. In myeloma and leukemia cells, cyproheptadine decreased expression of cyclins D1, D2, and D3 and arrested these cells in the G0/G1 phase. After D-cyclin suppression, cyproheptadine induced apoptosis in myeloma and leukemia cell lines and primary patient samples preferentially over normal hematopoietic cells. In mouse models of myeloma and leukemia, cyproheptadine inhibited tumor growth without significant toxicity. Cyproheptadine-induced apoptosis was preceded by activation of the mitochondrial pathway of caspase activation and was independent of the drug's known activity as an H1 histamine and serotonin receptor antagonist. Thus, cyproheptadine represents a lead for a novel therapeutic agent for the treatment of malignancy. Because the drug is well tolerated and already approved in multiple countries for clinical use as an antihistamine and appetite stimulant, it could be moved directly into clinical trials for cancer.

A High Throughput Screen Identified Cyproheptadine That Decreases D-Type Cyclins, Arrests Cells in the G1 Phase, and Induces Apoptosis in Multiple Myeloma and Leukemia Cells.

Cyclin Ds are key regulators of the cell cycle that are frequently over-expressed in multiple myeloma and leukemia and act to promote the phosphorylation of Rb, thereby facilitating the transition from G1 to S phase. Over-expression of cyclin Ds increases cell proliferation and chemoresistance. In contrast, reducing cyclin Ds levels directly or indirectly through G1 arrest can decrease cellular proliferation and induce apoptosis. To identify novel pharmaceutical inhibitors of cyclin D transactivation, we screened the LOPAC and Prestwick libraries of drugs and natural compounds (n = 2400) using NIH 3T3 cells stably expressing the Cyclin D2 promoter-driving a luciferase reporter gene. From this library screen and subsequent validation experiments we identified Cyproheptadine as a novel inhibitor of Cyclin D2 transactivation. Cyproheptadine has been used previously for the treatment of atopic dermatitis and anorexia, but its ability to inhibit cyclin D expression has not previously been reported. By immunoblotting, Cyproheptadine decreased expression of cyclin D1, D2 and D3 proteins in human myeloma and leukemia cell lines at low micromolar concentrations. Consistent with effects on the cyclin Ds, Cyproheptadine arrested cells in the G1 phase at concentrations associated with reduction in cyclin D expression. Decreased cyclin D expression and G1 arrest can induce apoptosis, so we tested the effects of Cyproheptadine on cell viability. Myeloma and leukemia cell lines were treated with increasing concentrations of Cyproheptadine and viability was measured by the MTS assay. Cyproheptadine reduced the viability of 7/10 myeloma and 7/8 AML cells lines with an IC50 ranging from 10–25μM. In contrast, it was less toxic to HeLa or NIH3T3 cells with IC20 > 50 μM. Cyproheptadine also reduced the viability of primary myeloma (8/8) and AML patient samples (7/9) with an IC50 <25 μM, but was less toxic to normal hematopoietic cells (IC20 > 50 μM). In a MDAY-D2 mouse model of leukemia, treatment with Cyproheptadine (50mg/kg/d) abolished formation of malignant leukemic ascites without untoward toxicity. Cyproheptadine-induced cell death was associated with reductions in mitochondrial membrane potential. Furthermore, reductions of pro-caspases -3 and -9 were observed prior to the reduction in pro-caspase-8, indicating that Cyproheptadine activates the mitochondrial pathway of caspase activation. Cyproheptadine is a known H1 histamine and serotonin receptor inhibitor, but pre-incubation with histamine, serotonin, or a combination of histamine and serotonin did not abrogate Cyproheptadine-induced cell death. Moreover, the structurally related H1 receptor inhibitor loratadine did not decrease cyclin D expression or reduce cell viability. Therefore, the pro-apoptotic activity of Cyproheptadine is not due to a competitive inhibition of the H1 and/or serotonin receptors, suggesting that Cyproheptadine has additional targets. In summary, Cyproheptadine arrests cells in G1, reduces cyclin D expression, and induces apoptosis via the mitochondrial pathway of caspase activation. Given the prior safety and toxicity record of Cyproheptadine, this drug could be rapidly advanced into clinical trial for the treatment of hematologic malignancies.

Assessment of the downstream portion of the mitochondrial pathway of caspase activation in patients with acute myeloid leukemia

Most chemotherapeutic agents used in the treatment of acute myeloid leukemia (AML) induce apoptosis by triggering the mitochondrial pathway of caspase activation. To investigate the downstream portion of the mitochondrial pathway of caspase activation in patients with AML, cytosolic lysates were stimulated with cytochrome c and dATP and hydrolysis of Ac-DEVD-AFC by effector caspases was measured. Defects in the distal mitochondrial pathway were more common in samples from patients with AML that relapsed rapidly after induction chemotherapy compared to samples from treatment naïve patients. The incidence of blocked pathways did not differ based on response to induction chemotherapy, as even nonresponders generally had an intact pathway. When the distal mitochondrial pathway was blocked, defects were usually at the level of the effector caspases. Thus, functional defects in the distal portion of the mitochondrial pathway of caspase activation may help explain the nature of response and relapse after treatment.

Induction of Apoptosis by the Hydrocarbon Oil Pristane: Implications for Pristane-Induced Lupus

Intraperitoneal injection of the hydrocarbon oil pristane into normal mice leads to a lupus-like autoimmune syndrome. Although advances in defining the roles of cellular and humoral mediators involved in this syndrome have been made, the mechanisms that initiate a break in tolerance leading to autoimmunity remain unknown. We describe in this study that pristane induces apoptosis both in vivo and in vitro. Pristane arrests cell growth and induces cell death by apoptosis via the mitochondrial pathway of caspase activation in a dose-dependent manner. Nuclear autoantigens created by pristane-induced apoptosis of lymphoid cells within the peritoneal cavity in the setting of a profoundly altered cytokine milieu may be the initiating event in the development of autoimmunity in this syndrome. These findings suggest that apoptosis may be a critical initial event in the pathogenesis of pristane-induced lupus and are of potential relevance for human systemic lupus erythematosus.

Functional Assessment of the Mitochondrial Pathway of Caspase Activation in Patients with Acute Myeloid Leukemia (AML).

Chemotherapeutic agents used in the treatment of AML such as cytarabine and etoposide can induce caspase activation through the mitochondrial pathway. Blocks in this pathway can render cells resistant to these agents and may contribute to poor patient outcomes. To explore the frequency of defects in this pathway, the functional integrity of the downstream portion of the mitochondrial pathway of caspase activation was assessed in cytosolic lysates from patients with newly diagnosed (n=29) or relapsed (n=11) AML who had >80% blasts in their peripheral blood. Lysates were stimulated with cytochrome c and dATP (cy c/dATP) and hydrolysis of Ac-DEVD-AFC by effector caspases was measured over time. The specificity of the assay was tested by pretreating lysates with the BIR3 domain of XIAP (a caspase-9 inhibitor) or Crm A (a caspase-8 inhibitor). Pretreatment with BIR3, but not CrmA protein, inhibited cy c/dATP-induced effector caspase activation. In contrast, Crm A, but not BIR3 inhibited recombinant caspase-8-induced effector caspase activation, demonstrating the specificity of the assay. Cy c/dATP-induced effector caspase activation correlated with etoposide-induced apoptosis. Samples with the highest increases in effector caspase activation had the greatest sensitivity to etoposide (R2=0.69, p=0.006). To relate the functional status of the mitochondrial pathway to patient outcome, mitochondrial pathways were divided into intact and blocked based on the increase in effector caspase activity in normal mobilized peripheral blood mononuclear stem cells (PBSC) stimulated with cy c/dATP. AML samples with increases in effector caspases ≥ PBSC were deemed to have intact pathways, while AML samples with increases in effector caspases < PBSC were deemed to have blocked pathways. With this division, 8/11 (73%) samples from patients with relapsed disease had a blocked mitochondrial pathway vs 7/29 (24%) samples from patients with newly diagnosed disease (p = 0.009 by Fisher Exact test). In the 22 patients who completed induction chemotherapy, functional defects in caspase activation did not predict response, as 3/13 (23%) responders and 2/9 (22%) nonresponders had a blocked pathway (p = 1.0 by Fisher Exact test). Defects in the mitochondrial pathway of caspase activation may be due to blocks above or below the level of effector caspases. To distinguish between these possibilities, effector caspases were directly activated by stimulating lysates with recombinant active caspase-8. Data were dichotomized based on caspase-8-induced activation of effector caspases in lysates of PBSC. Active caspase-8 failed to activate effector caspases in 13/15 (87%) samples with a blocked mitochondrial pathway. In contrast, active caspase-8 failed to activate effector caspase in 3/27 (11%) samples with an intact mitochondrial pathway (p<0.0001 by Fisher Exact test). These results suggest that most samples with a blocked mitochondrial pathway had a distal defect in caspase activation. In conclusion, functional defects in the distal portion of the mitochondrial pathway of caspase activation were observed more frequently in patients with relapsed AML than newly diagnosed disease. Future studies with longitudinal samples are required to determine whether defects in relapsed patients were present at diagnosis or emerged at relapse. Nonetheless, for patients with blocked pathways, therapeutic strategies that overcome these distal blocks or induce cell death through caspase-independent mechanisms could be useful.

S-Nitrosothiol-induced rapid cytochrome c release, caspase activation and mitochondrial permeability transition in perfused heart

Nitric oxide (NO) is a physiological signalling molecule, however, at high concentrations NO is cytotoxic, and has been implicated in a wide range of inflammatory, ischaemic and degenerative diseases, including heart failure. We investigated whether NO or S-nitrosothiols can induce apoptosis in perfused heart, and whether it is mediated via the mitochondrial pathway of caspase activation. We found that perfusion of rat hearts with a physiological S-nitrosothiol, S-nitrosoglutathione, at 0.4–1 mM concentrations for just 10 min caused the release of cytochrome c from mitochondria into the cytosol, inhibition of mitochondrial respiration and caspase activation. Inhibited mitochondrial respiration was restored when exogenous cytochrome c was added to mitochondria, indicating that respiratory inhibition was caused by lack of cytochrome c in mitochondria. Release of cytochrome c, respiratory inhibition and caspase activation were prevented when hearts were pre-perfused with cyclosporin A, suggesting that mitochondrial permeability transition pore was involved. In contrast, perfusion of the hearts with diethylenetriamine/NO adduct releasing similar levels of NO to the S-nitrosoglutathione had no measurable effect on the heart. These data suggest that S-nitrosothiols are potent inducers of apoptosis in the heart and that S-nitrosothiol-induced apoptosis is mediated by mitochondrial permeability transition but not via NO.

A Small Activator of Apoptosis

The mitochondrial caspase activation pathway regulates caspase-9, a protease that is required for programmed cell death. Various apoptotic stimuli can release cytochrome c from mitochondria and activate caspase-9 through the formation of a proapoptotic protein complex called the apoptosome. Jiang et al. (see the Perspective by Nicholson and Thornberry) discovered a proapoptotic compound through a high throughput chemical screen and then, using a combination of biochemical fractionation and biological tests, identified the molecules responsible for the drug's effect. An oncoprotein called prothymosin-α and a tumor suppressor protein called PHAP (for putative HLA-DR-associated proteins) inhibited or accelerated the apoptosome-caspase-9 pathway, respectively. The study points to the apoptosome as a potential target for drug development. X. Jiang, H.-E. Kim, H. Shu, Y. Zhao, H. Zhang, J. Kofron, J. Donnelly, D. Burns, S. Ng, S. Rosenberg, X. Wang, Distinctive roles of PHAP proteins and prothymosin-α in a death regulatory pathway. Science 299 , 223-226 (2003). [Abstract] [Full Text] D. W. Nicholson, N. A. Thornberry, Life and death decisions. Science 299 , 214-215 (2003). [Summary] [Full Text]

A novel Apaf-1-independent putative caspase-2 activation complex.

CVaspase activation is a key event in apoptosis execution. In stress-induced apoptosis, the mitochondrial pathway of caspase activation is believed to be of central importance. In this pathway, cytochrome c released from mitochondria facilitates the formation of an Apaf-1 apoptosome that recruits and activates caspase-9. Recent data indicate that in some cells caspase-9 may not be the initiator caspase in stress-mediated apoptosis because caspase-2 is required upstream of mitochondria for the release of cytochrome c and other apoptogenic factors. To determine how caspase-2 is activated, we have studied the formation of a complex that mediates caspase-2 activation. Using gel filtration analysis of cell lysates, we show that caspase-2 is spontaneously recruited to a large protein complex independent of cytochrome c and Apaf-1 and that recruitment of caspase-2 to this complex is sufficient to mediate its activation. Using substrate-binding assays, we also provide the first evidence that caspase-2 activation may occur without processing of the precursor molecule. Our data are consistent with a model where caspase-2 activation occurs by oligomerization, independent of the Apaf-1 apoptosome.

Overexpression of Copper/Zinc Superoxide Dismutase in Transgenic Rats Protects Vulnerable Neurons against Ischemic Damage by Blocking the Mitochondrial Pathway of Caspase Activation

Mitochondria are known to be involved in the early stage of apoptosis by releasing cytochrome c, caspase-9, and the second mitochondria-derived activator of caspases (Smac). We have reported that overexpression of copper/zinc superoxide dismutase (SOD1) reduced superoxide production and ameliorated neuronal injury in the hippocampal CA1 subregion after global ischemia. However, the role of oxygen free radicals produced after ischemia/reperfusion in the mitochondrial signaling pathway has not been clarified. Five minutes of global ischemia was induced in male SOD1-transgenic (Tg) and wild-type (Wt) littermate rats. Cytosolic expression of cytochrome c and Smac and activation of caspases were evaluated by immunohistochemistry, Western blot, and caspase activity assay. Apoptotic cell death was characterized by DNA nick end and single-stranded DNA labeling. In the Wt animals, early superoxide production, mitochondrial release of cytochrome c, Smac, and cleaved caspase-9 were observed after ischemia. Active caspase-3 was subsequently increased, and 85% of the hippocampal CA1 neurons showed apoptotic DNA damage 3 d after ischemia. Tg animals showed less superoxide production and cytochrome c and Smac release. Subsequent active caspase-3 expression was not evident, and only 45% of the neurons showed apoptotic DNA damage. A caspase-3 inhibitor (N-benzyloxycarbonyl-val-ala-asp-fluoromethyl ketone) reduced cell death only in Wt animals. These results suggest that overexpression of SOD1 reduced oxidative stress, thereby attenuating the mitochondrial release of cytochrome c and Smac, resulting in less caspase activation and apoptotic cell death. Oxygen free radicals may play a pivotal role in the mitochondrial signaling pathway of apoptotic cell death in hippocampal CA1 neurons after global ischemia.

Bcl-2 increases emptying of endoplasmic reticulum Ca2+ stores during photodynamic therapy-induced apoptosis

Photodynamic therapy (PDT) is clinically approved for the treatment of several types of cancer as well as age-related macular degeneration, the leading cause of blindness in the elderly. PDT using the photosensitizer verteporfin has been previously shown to induce rapid apoptosis via a mitochondrial-caspase activation pathway. The impact of PDT on other cell ular organelles such as the endoplasmic reticulum (ER) is undefined. The effect of PDT on intracellular Ca2+ ([Ca2+]i) in control and Bcl-2-overexpressing HeLa cells was assessed. A greater [Ca2+]i transient was observed for Bcl-2 overexpressing cells in response to PDT. The PDT-induced Ca2+ release was due to the emptying of Ca2+ from ER and possibly mitochondrial stores and was not due to an influx of Ca2+ from the medium. For Bcl-2-transfected cells, the release of Ca2+ was incomplete as determined by a further [Ca2+]i transient produced by the addition of the Ca2+ ionophore ionomycin after PDT. Furthermore, extrusion of Ca2+ was not hindered while ER-mediated sequestration of Ca2+ was impaired after PDT. Impairment of ER-mediated sequestration of Ca2+ may be due to the immediate caspase-independent depletion of sarco/endoplasmic reticulum Ca2+ ATPase-2 (SERCA2) that occurred in response to PDT in birth HeLa/Neo and Bcl-2 overexpressed HeLa cells. In summary, PDT induced the rapid degradation of SERCA2 and release of ER and mitochondrial Ca2+ stores. Although overexpression of Bcl-2 did not protect against SERCA2 degradation, it may influence the release of Ca2+ from ER and mitochondrial stores in PDT-treated cells.

Cascade of Caspase Activation in Potassium-Deprived Cerebellar Granule Neurons: Targets for Treatment with Peptide and Protein Inhibitors of Apoptosis

Cerebellar granule neurons (CGN) cultured in the presence of serum and depolarizing potassium concentrations undergo apoptosis when switched to serum-free medium containing physiological potassium concentrations. Here we show that processing of the key protease, caspase-3, depends on the activation of caspase-9, but not of caspase-8. Selective peptide inhibitors of caspase-9 block processing of caspase-3 and caspase-8 and inhibit apoptosis, whereas a selective inhibitor of caspase-8 blocks neither processing of caspase-3 nor cell death. The data obtained with peptide inhibitors were confirmed by adenovirally mediated ectopic expression of the cytokine response modifier A (crmA), the baculovirus protein p35, and the X chromosome-linked inhibitor of apoptosis (XIAP). Further, caspase-8-activating death receptors do not mediate apoptosis in CGN and potassium withdrawal-induced apoptosis evolves unaltered in gld or lpr mice, which harbor mutations in the CD95/CD95 ligand system. Thus, neuronal apoptosis triggered by potassium deprivation is death receptor-independent but involves the mitochondrial pathway of caspase activation.

Deficiency in caspase-9 or caspase-3 induces compensatory caspase activation.

Dysregulation of apoptosis contributes to the pathogenesis of many human diseases. As effectors of the apoptotic machinery, caspases are considered potential therapeutic targets. Using an established in vivo model of Fas-mediated apoptosis, we demonstrate here that elimination of certain caspases was compensated in vivo by the activation of other caspases. Hepatocyte apoptosis and mouse death induced by the Fas agonistic antibody Jo2 required proapoptotic Bcl-2 family member Bid and used a Bid-mediated mitochondrial pathway of caspase activation; deficiency in caspases essential for this pathway, caspase-9 or caspase-3, unexpectedly resulted in rapid activation of alternate caspases after injection of Jo2, and therefore failed to protect mice against Jo2 toxicity. Moreover, both ultraviolet and gamma irradiation, two established inducers of the mitochondrial caspase-activation pathway, also elicited compensatory activation of caspases in cultured caspase-3(-/-) hepatocytes, indicating that the compensatory caspase activation was mediated through the mitochondria. Our findings provide direct experimental evidence for compensatory pathways of caspase activation. This issue should therefore be considered in developing caspase inhibitors for therapeutic applications.