Pro-apoptotic Protein Noxa Research Articles

Attacking Oncogene Addiction in B-CLL: Seliciclib First Engages the Mcl-1/Noxa Axis, after Which Gradual Exhaustion of Bcl-2 Protection Leads to Bax Activation.

Background : Seliciclib (R-roscovitine) is a cyclin-dependent kinase inhibitor in clinical development. It triggers apoptosis by inhibiting de novo transcription of the short-lived anti-apoptotic Mcl-1 protein, but it is unclear if and how this leads to Bax or Bak activation that is required for most forms of cell death.Aim: To study the effects of seliciclib on apoptosis gene expression and Mcl-1 and Bcl-2 protein interactions in B cell chronic lymphocytic leukemia (B-CLL), a malignancy with known aberrant apoptosis regulation.Methods: Purified B-CLL cells (PBMC consisting of >90% B-CLL cells; n=20) and Ramos cell lines overexpressing different apoptosis regulators were used in this study. The effect of seliciclib on viability, apoptosis gene expression pattern, and protein associations was investigated via RT-Multiplex-Ligation-dependent Probe Amplification (RT-MLPA), Western blotting and co-immunoprecipitation assays. Ramos cells were transduced with retroviral vectors expressing either Noxa siRNA, Bim siRNA or control-GFP virus, and tested for different apoptosis stimuli.Results: We found that although seliciclib resulted in proteasome-dependent Mcl-1 degradation within 4 hrs in B-CLL cells, Bax and Bak activation and apoptosis occurred with a considerable delay, i.e. at 16 hrs. During this period, there was no evidence of transcriptional changes in p53-responsive or apoptosis-related genes. In freshly isolated, viable B-CLL cells, pro-survival Mcl-1 was engaged by the pro-apoptotic proteins Noxa and Bim but not by Bak. The contribution of Noxa [Figure1] and Bim (liberated from McL-1 within 4 hours) as specific mediators of seliciclib-induced apoptosis was demonstrated via RNAi in two model systems. Interestingly, 16 hrs after seliciclib treatment, there was a clear accumulation of Bcl-2, Bim, and Bax in the detergent insoluble (mitochondria containing) fraction of B-CLL cells. This suggests that after Mcl-1 degradation, the remaining apoptosis neutralizing capacity of Bcl-2 is gradually overwhelmed, probably resulting in Bax multimerisation and pore formation in the mitochondria.Conclusions: These data support the 'oncogene-addiction' model in which malignant cells depend on increased Bcl-2 levels, and extend it to include Mcl-1. Furthermore, since Noxa is elevated in B-CLL, its involvement in p53-independent apoptosis suggests this BH3-only protein may be a therapeutic target. [Display omitted]

Proteasome Inhibitor PS-341 Induces Apoptosis in Cisplatin-resistant Squamous Cell Carcinoma Cells by Induction of Noxa

Proteasome Inhibitor PS-341 Induces Apoptosis in Cisplatin-resistant Squamous Cell Carcinoma Cells by Induction of Noxa

PARP-2 deficiency affects the survival of CD4+CD8+ double-positive thymocytes

Poly-(ADP-ribose) polymerase-2 (PARP-2) belongs to a large family of enzymes that synthesize and transfer ADP-ribose polymers to acceptor proteins, modifying their functional properties. PARP-2-deficient (Parp-2-/-) cells, similar to Parp-1-/- cells, are sensitive to both ionizing radiation and alkylating agents. Here we show that inactivation of mouse Parp-2, but not Parp-1, produced a two-fold reduction in CD4+CD8+ double-positive (DP) thymocytes associated with decreased DP cell survival. Microarray analyses revealed increased expression of the proapoptotic Bcl-2 family member Noxa in Parp-2-/- DP thymocytes compared to littermate controls. In addition, DP thymocytes from Parp-2-/- have a reduced expression of T-cell receptor (TCR)alpha and a skewed repertoire of TCRalpha toward the 5' Jalpha segments. Our results show that in the absence of PARP-2, the survival of DP thymocytes undergoing TCRalpha recombination is compromised despite normal amounts of Bcl-xL. These data suggest a novel role for PARP-2 as an important mediator of T-cell survival during thymopoiesis by preventing the activation of DNA damage-dependent apoptotic response during the multiple rounds of TCRalpha rearrangements preceding a positively selected TCR.

MG132 induced apoptosis is associated with p53-independent induction of pro-apoptotic Noxa and transcriptional activity of β-catenin

Noxa is a pro-apoptotic BH3-only member of the Bcl-2 family of proteins that is up-regulated at a transcriptional level by the nuclear protein p53 in response to cellular stresses such as DNA damage or growth factor deprivation. Noxa is able to interact with anti-apoptotic members of the Bcl-2 family and causes release of cytochrome c into the cytosol, leading to the activation of caspases and induction of apoptosis. Here we demonstrate that MG132, a proteasomal inhibitor, rapidly induces Noxa mRNA and protein in two human cell lines, T/C28a and Saos2. The induction of Noxa is associated with a significant reduction in the number of metabolically active cells over the first 24 h of exposure to MG132 and progressive activation of caspase-3, a hallmark of caspase-dependent apoptosis. Partial rescue of the phenotype is observed when cells are transfected with Noxa siRNA prior to treatment with MG132, indicating functional significance of the induction of Noxa. p53 has previously been shown to be non-functional in the T/C28a cell line and is absent by Western blotting in Saos2 cells, suggesting that the induction of Noxa is through a p53 independent mechanism. Western blotting and confocal microscopy showed that total beta-catenin protein is increased in both cell lines at the time of Noxa induction, with the bulk of the beta-catenin present in the nucleus. Transfection with the Tcf reporter vector pTOPFLASH confirms that treatment with MG132 leads to early increased transcriptional activity of beta-catenin in both T/C28a and Saos2 cells. However, although over-expression of transcriptionally active beta-catenin in T/C28a cells also induced apoptosis through a p53-independent mechanism, the levels of Noxa protein were unchanged, suggesting that beta-catenin mediated signaling and Noxa may play independent roles in MG132 induced apoptosis. In summary, our results demonstrate that MG132 induces the pro-apoptotic protein Noxa via a p53-independent mechanism that leads to caspase-dependent apoptosis. This is the first report showing that treatment with MG132 induces Noxa. This study also provides further evidence for a link between beta-catenin mediated signaling and the induction of apoptosis.

Chemical Blockage of the Proteasome Inhibitory Function of Bortezomib: IMPACT ON TUMOR CELL DEATH

This work was supported by a Dermatology Foundation Career Development Award and National Institutes of Health Grant R01 CA107237 (to M. S. S). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The Small Molecule Pan-Bcl-2 Inhibitor GX15-070 Induces Apoptosis In Vitro in Mantle Cell Lymphoma (MCL) Cells and Exhibits a Synergistic Effect in Combination with the Proteasome Inhibitor Bortezomib.

Mantle cell lymphoma (MCL) is a lymphoid malignancy derived from a subset of mature B-cells with coexpression of CD5 and the chromosomal translocation t(11;14)(q13;q32). MCL patients present an aggressive clinical course and poor response to conventional chemotherapy due to either rapid relapse after an initial response or primary resistance to drugs. Thus, there is currently a strong effort to develop compounds that target novel biological pathways. In addition, MCL cells overexpress the antiapoptotic proteins Bcl-2, Bcl-XL and Mcl-1. The dysregulated Bcl-2 pathway represents an important target for a new-mechanism therapeutic approach. In this context, the pan-Bcl-2 inhibitor GX15-070 that belongs to a new family of compounds which mimic BH3-only proteins by binding to multiple antiapoptotic Bcl-2 members and consequently eliciting Bax and Bak dependent cytotoxicity. GX15-070 is currently being evaluated in Phase I clinical trials. GX15-070 induced apoptosis in vitro in primary cells from MCL patients as well in MCL cells lines, at doses ranging from 0,5μM to 5μM. GX15-070 induced phosphatidylserine exposure, mitochondrial depolarization, ROS generation, Bax and Bak conformational changes and caspase activation. As expected, pan-caspase inhibitors did not block apoptosis in accordance with the mitochondria-targeted nature of this new compound. The susceptibility of MCL cells correlates with the expression of Bcl-2, Mcl-1 and Bcl-XL, both at protein and mRNA levels. Although GX15-070 exhibited significant in vitro cytotoxicity in MCL as a single agent, we tested also this compound in combination with proteasome inhibitors. Bortezomib (PS-341, Velcade®) has shown promising results in MCL treatment. One of the drawbacks of proteasome inhibitors is the undesirable accumulation, due to absence of degradation, of antiapoptotic factors which could impede or slow apoptosis, such as Mcl-1 protein. With this rationale we evaluated a combination of bortezomib with the pan-Bcl-2 inhibitor GX15-070 in vitro in MCL cells. Preincubation of MCL cells with GX15-070 at doses ranging from 0,1μM to 1μM before bortezomib (1–10nM) addition induced a synergistic cytotoxic effect (Chou Talalay CI =[0,26–0,575]). In vitro, this new combination allows to reduce bortezomib dose 5 to 20 times, depending on the individuals. Western blot analysis revealed that this combination reduced Mcl-1 levels and displaced Bak from Mcl-1 sequestration. Moreover, upregulation of the proapoptotic BH3-only protein Noxa by bortezomib, was further increased when combined with GX15-070. All these findings suggest that GX15-070 and bortezomib combination could be a new therapeutic approach for MCL treatment.

Chronic lymphocytic leukemia cells display p53-dependent drug-induced Puma upregulation

We investigated the apoptosis gene expression profile of chronic lymphocytic leukemia (CLL) cells in relation to (1) normal peripheral and tonsillar B-cell subsets, (2) IgV(H) mutation status, and (3) effects of cytotoxic drugs. In accord with their noncycling, antiapoptotic status in vivo, CLL cells displayed high constitutive expression of Bcl-2 and Flip mRNA, while Survivin, Bid and Bik were absent. Paradoxically, along with these antiapoptotic genes CLL cells had high-level expression of proapoptotic BH3-only proteins Bmf and Noxa. Treatment of CLL cells with fludarabine induced only the proapoptotic genes Bax and Puma in a p53-dependent manner. Interestingly, the degree of Puma induction was more pronounced in cells with mutated IgVH genes. Thus, disturbed apoptosis in CLL is the net result of both protective and sensitizing aberrations. This delicate balance can be tipped via induction of Puma in a p53-dependent matter, the level of which may vary between groups of patients with a different tendency for disease progression.

Elimination of Hepatic Metastases of Colon Cancer Cells via p53-Independent Cross-Talk between Irinotecan and Apo2 Ligand/TRAIL

The majority of colorectal cancers have lost/inactivated the p53 tumor suppressor gene. Using isogenic human colon cancer cells that differ only in their p53 status, we demonstrate that loss of p53 renders tumor cells relatively resistant to the topoisomerase I inhibitor, irinotecan. Whereas irinotecan-induced up-regulation of the proapoptotic proteins PUMA and Noxa requires p53, we find that irinotecan inhibits Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 and 5 (STAT3/5) signaling in both p53-proficient and p53-deficient tumor cells. We show that irinotecan inhibits JAK2-STAT3/5-dependent expression of survival proteins (Bcl-x(L) and XIAP) and cooperates with Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) to facilitate p53-independent apoptosis of colon cancer cells. Whereas xenografts of p53-deficient colon cancer cells are relatively resistant to irinotecan compared with their p53-proficient counterparts, combined treatment with irinotecan and Apo2L/TRAIL eliminates hepatic metastases of both p53-proficient and p53-deficient cancer cells in vivo and significantly improves the survival of animals relative to treatment with either agent alone. Although the synergy between chemotherapy and Apo2L/TRAIL has been ascribed to p53, our data demonstrate that irinotecan enhances Apo2L/TRAIL-induced apoptosis of tumor cells via a distinct p53-independent mechanism involving inhibition of JAK2-STAT3/5 signaling. These findings identify a novel p53-independent channel of cross-talk between topoisomerase I inhibitors and Apo2L/TRAIL and suggest that the addition of Apo2L/TRAIL can improve the therapeutic index of irinotecan against both p53-proficient and p53-deficient colorectal cancers, including those that have metastasized to the liver.