Caspase-3 triggers a TPCK-sensitive protease pathway leading to degradation of the BH3-only protein puma

Enhanced Apoptosis and Loss of Cell Viability in Melanoma Cells by Combined Inhibition of ERK and Mcl-1 Is Related to Loss of Mitochondrial Membrane Potential, Caspase Activation and Upregulation of Proapoptotic Bcl-2 Proteins

Targeting of MAP kinase pathways by BRAF inhibitors has evolved as a key therapy for BRAF-mutated melanoma. However, it cannot be applied for BRAF-WT melanoma, and also, in BRAF-mutated melanoma, tumor relapse often follows after an initial phase of tumor regression. Inhibition of MAP kinase pathways downstream at ERK1/2, or inhibitors of antiapoptotic Bcl-2 proteins, such as Mcl-1, may serve as alternative strategies. As shown here, the BRAF inhibitor vemurafenib and the ERK inhibitor SCH772984 showed only limited efficacy in melanoma cell lines, when applied alone. However, in combination with the Mcl-1 inhibitor S63845, the effects of vemurafenib were strongly enhanced in BRAF-mutated cell lines, and the effects of SCH772984 were enhanced in both BRAF-mutated and BRAF-WT cells. This resulted in up to 90% loss of cell viability and cell proliferation, as well as in induction of apoptosis in up to 60% of cells. The combination of SCH772984/S63845 resulted in caspase activation, processing of poly (ADP-ribose) polymerase (PARP), phosphorylation of histone H2AX, loss of mitochondrial membrane potential, and cytochrome c release. Proving the critical role of caspases, a pan-caspase inhibitor suppressed apoptosis induction, as well as loss of cell viability. As concerning Bcl-2 family proteins, SCH772984 enhanced expression of the proapoptotic Bim and Puma, as well as decreased phosphorylation of Bad. The combination finally resulted in downregulation of antiapoptotic Bcl-2 and enhanced expression of the proapoptotic Noxa. In conclusion, combined inhibition of ERK and Mcl-1 revealed an impressive efficacy both in BRAF-mutated and WT melanoma cells, and may thus represent a new strategy for overcoming drug resistance.

Lithocholic acid induces endoplasmic reticulum stress, autophagy and mitochondrial dysfunction in human prostate cancer cells

Lithocholic acid (LCA) is a secondary bile acid that is selectively toxic to human neuroblastoma, breast and prostate cancer cells, whilst sparing normal cells. We previously reported that LCA inhibited cell viability and proliferation and induced apoptosis and necrosis of androgen-dependent LNCaP and androgen-independent PC-3 human prostate cancer cells. In the present study, we investigated the roles of endoplasmic reticulum (ER) stress, autophagy and mitochondrial dysfunction in the toxicity of LCA in PC-3 and autophagy deficient, androgen-independent DU-145 cells. LCA induced ER stress-related proteins, such as CCAAT-enhancer-binding protein homologous protein (CHOP), and the phosphorylation of eukaryotic initiation factor 2-alpha (p-eIF2α) and c-Jun N-terminal kinases (p-JNK) in both cancer cell-types. The p53 upregulated modulator of apoptosis (PUMA) and B cell lymphoma-like protein 11 (BIM) levels were decreased at overtly toxic LCA concentrations, although PUMA levels increased at lower LCA concentrations in both cell lines. LCA induced autophagy-related conversion of microtubule-associated proteins 1A/1B light chain 3B (LC3BI-LC3BII), and autophagy-related protein ATG5 in PC-3 cells, but not in autophagy-deficient DU-145 cells. LCA (>10 µM) increased levels of reactive oxygen species (ROS) concentration-dependently in PC-3 cells, whereas ROS levels were not affected in DU-145 cells. Salubrinal, an inhibitor of eIF2α dephosphorylation and ER stress, reduced LCA-induced CHOP levels slightly in PC-3, but not DU-145 cells. Salubrinal pre-treatment increased the cytotoxicity of LCA in PC-3 and DU-145 cells and resulted in a statistically significant loss of cell viability at normally non-toxic concentrations of LCA. The late-stage autophagy inhibitor bafilomycin A1 exacerbated LCA toxicity at subtoxic LCA concentrations in PC-3 cells. The antioxidant α-tocotrienol strongly inhibited the toxicity of LCA in PC-3 cells, but not in DU-145 cells. Collectively, although LCA induces autophagy and ER stress in PC-3 cells, these processes appear to be initially of protective nature and subsequently consequential to, but not critical for the ROS-mediated mitochondrial dysfunction and cytotoxicity of LCA. The full mechanism of LCA-induced mitochondrial dysfunction and cytotoxicity in the similarly sensitive DU-145 cells remains to be elucidated.

Subcellular localization of PUMA regulates its pro-apoptotic activity in Burkitt's lymphoma B cells

The BH3-only protein PUMA (p53-upregulated modulator of apoptosis) is a major regulator of apoptosis. It belongs to the Bcl-2 family of proteins responsible for maintaining mitochondrial outer membrane integrity by controlling the intrinsic (mitochondrial) apoptotic pathway. We describe here a new pathway regulating PUMA activation through the control of its subcellular distribution. Surprisingly, neither PUMA upregulation in normal activated human B lymphocytes nor high levels of PUMA in Burkitt's lymphoma (BL) were associated with cell death. We show that PUMA is localized to the cytosol in these cells. By contrast, various apoptosis-triggering signals were found to promote the translocation of PUMA to the mitochondria in these cells, leading to their death by apoptosis. This apoptosis was associated with the binding of mitochondrial PUMA to anti-apoptotic members of the Bcl-2 family, such as Bcl-2 and Mcl-1. This translocation was caspase-independent but was prevented by inhibiting or knocking down the expression of the MAPK kinase p38. Our data suggest that the accumulation of PUMA in the cytosol may be important for the participation of this protein in apoptosis without the need for prior transcription. This regulatory pathway may be an important feature of differentiation and tumorigenic processes.

APR-246 potently inhibits tumour growth and overcomes chemoresistance in preclinical models of oesophageal adenocarcinoma

OBJECTIVES

p53 is a critical tumour suppressor and is mutated in 70% of oesophageal adenocarcinomas (OACs), resulting in chemoresistance and poor survival. APR-246 is a first-in-class reactivator of mutant p53 and is currently in clinical trials. In this study, we characterised the activity of APR-246 and its effect on p53 signalling in a large panel of cell line xenograft (CLX) and patient-derived xenograft (PDX) models of OAC.

DESIGN

In vitro response to APR-246 was assessed using clonogenic survival, cell cycle and apoptosis assays. Ectopic expression, gene knockdown and CRISPR/Cas9-mediated knockout studies of mutant p53 were performed to investigate p53-dependent drug effects. p53 signalling was examined using quantitative RT-PCR and western blot. Synergistic interactions between APR-246 and conventional chemotherapies were evaluated in vitro and in vivo using CLX and PDX models.

RESULTS

APR-246 upregulated p53 target genes, inhibited clonogenic survival and induced cell cycle arrest as well as apoptosis in OAC cells harbouring p53 mutations. Sensitivity to APR-246 correlated with cellular levels of mutant p53 protein. Ectopic expression of mutant p53 sensitised p53-null cells to APR-246, while p53 gene knockdown and knockout diminished drug activity. Importantly, APR-246 synergistically enhanced the inhibitory effects of cisplatin and 5-fluorouracil through p53 accumulation. Finally, APR-246 demonstrated potent antitumour activity in CLX and PDX models, and restored chemosensitivity to a cisplatin/5-fluorouracil-resistant xenograft model.

CONCLUSIONS

APR-246 has significant antitumour activity in OAC. Given that APR-246 is safe at therapeutic levels our study strongly suggests that APR-246 can be translated into improving the clinical outcomes for OAC patients.

EDD enhances cell survival and cisplatin resistance and is a therapeutic target for epithelial ovarian cancer

The E3 ubiquitin ligase EDD is overexpressed in recurrent, platinum-resistant ovarian cancers, suggesting a role in tumor survival and/or platinum resistance. EDD knockdown by small interfering RNA (siRNA) induced apoptosis in A2780ip2, OVCAR5 and ES-2 ovarian cancer cells, correlating with loss of the prosurvival protein myeloid cell leukemia sequence 1 (Mcl-1) through a glycogen synthase kinase 3 beta-independent mechanism. SiRNA to EDD or Mcl-1 induced comparable levels of apoptosis in A2780ip2 and ES-2 cells. Stable overexpression of Mcl-1 protected cells from apoptosis following EDD knockdown, accompanied by a loss of endogenous, but not exogenous, Mcl-1 protein, suggesting that EDD regulated Mcl-1 synthesis. Indeed, EDD knockdown induced a 1.87-fold decrease in Mcl-1 messenger RNA and EDD transfection enhanced murine Mcl-1 promoter-driven luciferase expression 5-fold. To separate EDD survival and potential cisplatin resistance functions, we generated EDD shRNA stable cell lines that could survive initial EDD knockdown and showed that these cells were 4- to 21-fold more sensitive to cisplatin. Moreover, transient EDD overexpression in COS-7 cells was sufficient to promote cisplatin resistance 2.4-fold, dependent upon its E3 ligase activity. In vivo, mouse intraperitoneal ES-2 and A2780ip2 xenograft experiments showed that mice treated with EDD siRNA by nanoliposomal delivery [1,2-dioleoyl-sn-glycero-3-phophatidylcholine (DOPC)] and cisplatin had significantly less tumor burden than those treated with control siRNA/DOPC alone (ES-2, 77.9% reduction, P = 0.004; A2780ip2, 75.9% reduction, P = 0.042) or control siRNA/DOPC with cisplatin in ES-2 (64.4% reduction, P = 0.035), with a trend in A2780ip2 (60.3% reduction, P = 0.168). These results identify EDD as a dual regulator of cell survival and cisplatin resistance and suggest that EDD is a therapeutic target for ovarian cancer.

Apoptosis induction by SAHA in cutaneous T-cell lymphoma cells is related to downregulation of c-FLIP and enhanced TRAIL signaling

Suberoylanilide hydroxamic acid (SAHA) has been approved for the treatment of cutaneous T-cell lymphoma (CTCL), but its mode of action remained largely elusive. As shown here in four CTCL cell lines, loss of cell viability correlated with significant time- and dose-dependent induction of apoptosis, whereas cytotoxicity was less pronounced. Both extrinsic and intrinsic apoptosis pathways were activated, as seen by processing of initiator caspases 8 and 9, loss of mitochondrial membrane potential, and cytochrome c release. Characteristically, antiapoptotic mediators such as Mcl-1, XIAP, survivin, and c-FLIP were downregulated. Consistent with its critical function, c-FLIP overexpression resulted in a significant decrease of SAHA-mediated apoptosis. Enhanced sensitivity to TRAIL (TNF-related apoptosis-inducing ligand) and enhanced TRAIL signaling was seen in CTCL cell lines with high sensitivity, whereas cell lines with moderate response were characterized by downregulation of TRAIL-R2 and weaker TRAIL expression. Comparable proapoptotic responses to SAHA and to the combination with TRAIL were seen in ex vivo tumor T cells of CTCL patients. Thus, activation of extrinsic apoptosis pathways, related to c-FLIP downregulation and enhanced TRAIL signaling, appeared as characteristic for CTCL cell responsiveness to SAHA. An improved understanding of the pathways may facilitate its targeted use and the selection of suitable combinations.

Regulation of memory B-cell survival by the BH3-only protein Puma

Apoptosis is crucial for immune system homeostasis, including selection and survival of long-lived antibody-forming cells and memory cells. The interactions between proapoptotic and pro-survival proteins of the Bcl-2 family are critical for this process. In this report, we show that expression of the proapoptotic BH3-only Bcl-2 family member Puma was selectively up-regulated on in vitro activation with antigens or mitogens of both human and mouse B cells. Puma expression coincided in vivo, with the prosurvival Bcl-2 family member Mcl-1 within the germinal centers and its expression correlates with the germinal center like phenotype of Burkitt lymphoma. Experiments performed in Puma-deficient mice revealed that Puma is essential for apoptosis of mitogen-activated B cells in vitro and for the control of memory B-cell survival. In conclusion, using both human and murine models, our data show that Puma has a major role in the T cell- dependent B-cell immune response. These data demonstrate that Puma is a major regulator of memory B lymphocyte survival and therefore a key molecule in the control of the immune response.

Synergistic killing of colorectal cancer cells by oxaliplatin and ABT-737

BACKGROUND

Oxaliplatin is frequently used in the treatment of metastatic colorectal cancer (CRC). Our previous work shows that oxaliplatin induces the pro-apoptotic protein Noxa in CRC cells. The Bcl2-inhibitor ABT-737 is particularly effective in cells with high Noxa levels. Therefore, we tested whether oxaliplatin and ABT-737 display synergy in killing CRC cells.

METHODS

A panel of CRC cell lines was treated with oxaliplatin and ABT-737, either alone or in combination. Apoptosis was measured by FACS analysis of sub-G1 DNA content and by Western blot analysis of caspase-3 processing. Noxa expression was suppressed by lentiviral RNA interference.

RESULTS

Oxaliplatin and ABT-737 displayed a strong synergistic apoptotic response, which was dependent on wildtype TP53 and oncogenic KRAS. TP53 and KRAS were required for drug-induced Noxa expression and this was essential for tumor cell apoptosis. Oxaliplatin, but not ABT-737, induced p53 accumulation, but both drugs stimulated Noxa expression. Combination treatment of mice with subcutaneous tumor xenografts drastically reduced tumor volume, while single drug treatment had no effect.

CONCLUSION

ABT-737 synergizes with oxaliplatin to kill colorectal cancer cells. This requires induction of Noxa by wildtype TP53 and oncogenic KRAS. Future studies should explore the anti-tumor efficacy of this drug combination in mouse models for spontaneous CRC development and in patient-derived tumor cell cultures and xenografts.