Evidence for a protective role of Mcl-1 in proteasome inhibitor-induced apoptosis

Small interfering RNA targeting mcl-1 enhances proteasome inhibitor-induced apoptosis in various solid malignant tumors

Background

Targeting the ubiquitin-proteasome pathway is a promising approach for anticancer strategies. Recently, we found Bik accumulation in cancer cell lines after they were treated with bortezomib. However, recent evidence indicates that proteasome inhibitors may also induce the accumulation of anti-apoptotic Bcl-2 family members. The current study was designed to analyze the levels of several anti-apoptotic members of Bcl-2 family in different human cancer cell lines after they were treated with proteasome inhibitors.

Methods

Different human cancer cell lines were treated with proteasome inhibitors. Western blot were used to investigate the expression of Mcl-1 and activation of mitochondrial apoptotic signaling. Cell viability was investigated using SRB assay, and induction of apoptosis was measured using flow cytometry.

Results

We found elevated Mcl-1 level in human colon cancer cell lines DLD1, LOVO, SW620, and HCT116; human ovarian cancer cell line SKOV3; and human lung cancer cell line H1299, but not in human breast cancer cell line MCF7 after they were treated with bortezomib. This dramatic Mcl-1 accumulation was also observed when cells were treated with other two proteasome inhibitors, MG132 and calpain inhibitor I (ALLN). Moreover, our results showed Mcl-1 accumulation was caused by stabilization of the protein against degradation. Reducing Mcl-1 accumulation by Mcl-1 siRNA reduced Mcl-1 accumulation and enhanced proteasome inhibitor-induced cell death and apoptosis, as evidenced by the increased cleavage of caspase-9, caspase-3, and poly (ADP-ribose) polymerase.

Conclusions

Our results showed that it was not only Bik but also Mcl-1 accumulation during the treatment of proteasome inhibitors, and combining proteasome inhibitors with Mcl-1 siRNA would enhance the ultimate anticancer effect suggesting this combination might be a more effective strategy for cancer therapy.

Noxa controls Mule-dependent Mcl-1 ubiquitination through the regulation of the Mcl-1/USP9X interaction

The level of the Mcl-1 pro-survival protein is highly regulated, and the down-regulation of Mcl-1 expression favors the apoptotic process. Mcl-1 physically interacts with different BH3-only proteins; particularly, Noxa is involved in the modulation of Mcl-1 expression. In this study, we demonstrated that Noxa triggers the degradation of Mcl-1 at the mitochondria according to the exclusive location of Noxa at this compartment. The Noxa-induced degradation of Mcl-1 required the E3 ligase Mule, which is responsible for the polyubiquitination of Mcl-1. Because the USP9X deubiquitinase was recently demonstrated to be involved in Mcl-1 protein turnover by preventing its degradation through the removal of conjugated ubiquitin, we investigated whether Noxa affected the deubiquitination process. Interestingly, Noxa over-expression caused a decrease in the USP9X/Mcl-1 interaction associated with an increase in the Mcl-1 polyubiquitinated forms. Additionally, Noxa over-expression triggered an increase in the Mule/Mcl-1 interaction in parallel with the decrease in Mule/USP9X complex formation. Taken together, these modifications result in the degradation of Mcl-1 by the proteasome machinery. The implication of Noxa in the regulation of Mcl-1 proteasomal degradation adds complexity to this process, which is governed by multiple interactions.

Novel proteasome-inhibitory syrbactin analogs inducing endoplasmic reticulum stress and apoptosis in hematological tumor cell lines

The proteasome has been recognized as a druggable target in cancer cells, and this has led to searches for pharmacologic agents that target this cellular organelle for cancer therapeutic purposes. Syrbactins are a group of microbial metabolites consisting of two related families, the glidobactins and the syringolins. Some members of this group have revealed cytotoxic efficacy in tumor cells, and more recently it was discovered that they exert proteasome-inhibitory function. Based on this therapeutic promise and to gain further understanding of their molecular modes of action, we chemically synthesized de-novo three novel syrbactin analogs and characterized their proteasome-inhibitory and in vitro anti-neoplastic activity in human cell lines representing multiple myeloma, Waldenström's macroglobulinemia, and lymphocytic leukemia. Our results show that two of these novel compounds are able to inhibit proteasome activity in the nanomolar range, reduce the expression of anti-apoptotic proteins survivin and Mcl-1, and cause severe endoplasmic reticulum (ER) stress, resulting in pronounced tumor cell death. These anticancer effects can be synergistically enhanced when the agents are combined with thapsigargin, which further aggravates ER stress by a different mechanism. Taken together, our findings support the notion that syrbactin analogs may provide a structural platform for the development of novel cancer therapeutics, and that their efficacy may be further increased when complemented with other agents that trigger ER stress.

Natural proteasome inhibitor celastrol suppresses androgen-independent prostate cancer progression by modulating apoptotic proteins and NF-kappaB

Background

Celastrol is a natural proteasome inhibitor that exhibits promising anti-tumor effects in human malignancies, especially the androgen-independent prostate cancer (AIPC) with constitutive NF-κB activation. Celastrol induces apoptosis by means of proteasome inhibition and suppresses prostate tumor growth. However, the detailed mechanism of action remains elusive. In the current study, we aim to test the hypothesis that celastrol suppresses AIPC progression via inhibiting the constitutive NF-κB activity as well as modulating the Bcl-2 family proteins.

Methodology/Principal Findings

We examined the efficacy of celastrol both in vitro and in vivo, and evaluated the role of NF-κB in celastrol-mediated AIPC regression. We found that celastrol inhibited cell proliferation in all three AIPC cell lines (PC-3, DU145 and CL1), with IC₅₀ in the range of 1–2 µM. Celastrol also suppressed cell migration and invasion. Celastrol significantly induced apoptosis as evidenced by increased sub-G1 population, caspase activation and PARP cleavage. Moreover, celastrol promoted cleavage of the anti-apoptotic protein Mcl-1 and activated the pro-apoptotic protein Noxa. In addition, celastrol rapidly blocked cytosolic IκBα degradation and nuclear translocation of RelA. Likewise, celastrol inhibited the expression of multiple NF-κB target genes that are involved in proliferation, invasion and anti-apoptosis. Celastrol suppressed AIPC tumor progression by inhibiting proliferation, increasing apoptosis and decreasing angiogenesis, in PC-3 xenograft model in nude mouse. Furthermore, increased cellular IκBα and inhibited expression of various NF-κB target genes were observed in tumor tissues.

Conclusions/Significance

Our data suggest that, via targeting the proteasome, celastrol suppresses proliferation, invasion and angiogenesis by inducing the apoptotic machinery and attenuating constitutive NF-κB activity in AIPC both in vitro and in vivo. Celastrol as an active ingredient of traditional herbal medicine could thus be developed as a new therapeutic agent for hormone-refractory prostate cancer.

VSV oncolysis in combination with the BCL-2 inhibitor obatoclax overcomes apoptosis resistance in chronic lymphocytic leukemia

In chronic lymphocytic leukemia (CLL), overexpression of antiapoptotic B-cell leukemia/lymphoma 2 (BCL-2) family members contributes to leukemogenesis by interfering with apoptosis; BCL-2 expression also impairs vesicular stomatitis virus (VSV)-mediated oncolysis of primary CLL cells. In the effort to reverse resistance to VSV-mediated oncolysis, we combined VSV with obatoclax (GX15-070)-a small-molecule BCL-2 inhibitor currently in phase 2 clinical trials-and examined the molecular mechanisms governing the in vitro and in vivo antitumor efficiency of combining the two agents. In combination with VSV, obatoclax synergistically induced cell death in primary CLL samples and reduced tumor growth in severe combined immunodeficient (SCID) mice-bearing A20 lymphoma tumors. Mechanistically, the combination stimulated the mitochondrial apoptotic pathway, as reflected by caspase-3 and -9 cleavage, cytochrome c release and BAX translocation. Combination treatment triggered the release of BAX from BCL-2 and myeloid cell leukemia-1 (MCL-1) from BAK, whereas VSV infection induced NOXA expression and increased the formation of a novel BAX-NOXA heterodimer. Finally, NOXA was identified as an important inducer of VSV-obatoclax driven apoptosis via knockdown and overexpression of NOXA. These studies offer insight into the synergy between small-molecule BCL-2 inhibitors such as obatoclax and VSV as a combination strategy to overcome apoptosis resistance in CLL.

Potent synergistic interaction between the Nampt inhibitor APO866 and the apoptosis activator TRAIL in human leukemia cells

OBJECTIVE

The nicotinamide phosphoribosyltransferase (Nampt) inhibitor APO866 depletes intracellular nicotinamide adenine dinucleotide (NAD(+)) and shows promising anticancer activity in preclinical studies. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) binds to plasma membrane receptors DR4 and DR5 and induces apoptosis via caspase-8 and -10. Here we have explored the interaction between APO866 and TRAIL in leukemia cell lines and in primary B-cell chronic lymphocytic leukemia cells.

MATERIALS AND METHODS

Cells were treated with APO866, TRAIL, or their combination. Viability and mitochondrial transmembrane potential (ΔΨ(m)) were determined by cell staining with propidium iodide and tetramethylrhodamine ethyl ester, respectively, and flow cytometry. Nampt and γ-tubulin levels, as well as caspase-3 cleavage were detected by immunoblotting. DR4 and DR5 expression were assessed by immunostaining and flow cytometry. Caspases were inhibited with zVAD-FMK and zDEVD-FMK; autophagy with 3-methyladenine, LY294002, and wortmannin. Intracellular NAD(+) and adenosine triphosphate (ATP) were measured by cycling assays and high-performance liquid chromatography (HPLC), respectively.

RESULTS

APO866 induced NAD(+) depletion, ΔΨ(m) dissipation, and ATP shortage in leukemia cells, thereby leading to autophagic cell death. TRAIL induced caspase-dependent apoptosis. TRAIL addition to APO866 synergistically increased its activity in leukemia cells by enhancing NAD(+) depletion, ΔΨ(m) dissipation, and ATP shortage. No DR5 upregulation at the cell surface in response to APO866 was observed. Remarkably, in healthy leukocytes APO866 and TRAIL were poorly active and failed to show any cooperation.

CONCLUSIONS

Activation of the extrinsic apoptotic cascade with TRAIL selectively amplifies the sequelae of Nampt inhibition in leukemia cells, and appears as a promising strategy to enhance APO866 activity in hematological malignancies.

ATM-dependent expression of IEX-1 controls nuclear accumulation of Mcl-1 and the DNA damage response

The early-response gene product IEX-1 (also known as IER3) was recently found to interact with the anti-apoptotic Bcl-2 family member, myeloid cell leukemia-1 (Mcl-1). In this study we show that this interaction specifically and timely controls the accumulation of Mcl-1 in the nucleus in response to DNA damage. The IEX-1 protein is rapidly induced by γ-irradiation, genotoxic agents or replication inhibitors, in a way dependent on ataxia telangiectasia mutated (ATM) activity and is necessary for Mcl-1 nuclear translocation. Conversely, IEX-1 protein proteasomal degradation triggers the return of Mcl-1 to the cytosol. IEX-1 and Mcl-1 are integral components of the DNA damage response. Loss of IEX-1 or Mcl-1 leads to genomic instability and increased sensitivity to genotoxic and replicative stresses. The two proteins cooperate to maintain Chk1 activation and G2 checkpoint arrest. Mcl-1 nuclear translocation may foster checkpoint and improve the tumor resistance to DNA damage-based cancer therapies. Deciphering the pathways involved in IEX-1 degradation should lead to the discovery of new therapeutic targets to increase sensitivity of tumor cells to chemotherapy.

Aspirin induces apoptosis in human leukemia cells independently of NF-kappaB and MAPKs through alteration of the Mcl-1/Noxa balance

Aspirin and other non-steroidal anti-inflammatory drugs induce apoptosis in most cell types. In this study we examined the mechanism of aspirin-induced apoptosis in human leukemia cells. We analyzed the role of nuclear factor-kappaB (NF-kappaB) and mitogen-activated protein kinases (MAPKs) pathways. Furthermore, we studied the changes induced by aspirin in some genes involved in the control of apoptosis at mRNA level, by performing reverse transcriptase multiplex ligation-dependent probe amplification (RT-MLPA), and at protein level by Western blot. Our results show that aspirin induced apoptosis in leukemia Jurkat T cells independently of NF-kappaB. Although aspirin induced p38 MAPK and c-Jun N-terminal kinase activation, selective inhibitors of these kinases did not inhibit aspirin-induced apoptosis. We studied the regulation of Bcl-2 family members in aspirin-induced apoptosis. Aspirin increased the mRNA levels of some pro-apoptotic members, such as BIM, NOXA, BMF or PUMA, but their protein levels did not change. In contrast, aspirin decreased the protein levels of Mcl-1. Interestingly, in the presence of aspirin the protein levels of Noxa remained high. This alteration of the Mcl-1/Noxa balance was also found in other leukemia cell lines and primary chronic lymphocytic leukemia cells (CLL). Furthermore, in CLL cells aspirin induced an increase in the protein levels of Noxa. Knockdown of Noxa or Puma significantly attenuated aspirin-induced apoptosis. These results indicate that aspirin induces apoptosis through alteration of the Mcl-1/ Noxa balance.

Bortezomib sensitizes human renal cell carcinomas to TRAIL apoptosis through increased activation of caspase-8 in the death-inducing signaling complex

Bortezomib (VELCADE) could sensitize certain human renal cell carcinoma (RCC) lines to the apoptotic effects of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Analysis of seven human RCC showed a clear increase in the sensitivity of four of the RCC to TRAIL cytotoxicity following bortezomib (5-20 nmol/L) treatment, whereas the remaining three remained resistant. Tumor cell death following sensitization had all the features of apoptosis. The enhanced antitumor activity of the bortezomib and TRAIL combination was confirmed in long-term (6 days) cancer cell outgrowth assays. The extent of proteasome inhibition by bortezomib in the various RCC was equivalent. Following bortezomib treatment, neither changes in the intracellular protein levels of various Bcl-2 and IAP family members, nor minor changes in expression of TRAIL receptors (DR4, DR5), correlated well with the sensitization or resistance of RCC to TRAIL-mediated apoptosis. However, enhanced procaspase-8 activation following bortezomib pretreatment and subsequent TRAIL exposure was only observed in the sensitized RCC in both cell extracts and death-inducing signaling complex immunoprecipitates. These data suggest that the molecular basis for bortezomib sensitization of RCC to TRAIL primarily involves early amplification of caspase-8 activity. In the absence of this increased caspase-8 activation, other bortezomib-induced changes are not sufficient to sensitize RCC to TRAIL-mediated apoptosis.

Combination bortezomib and rituximab treatment affects multiple survival and death pathways to promote apoptosis in mantle cell lymphoma

Mantle cell lymphoma (MCL) is a distinct histologic subtype of B cell non-Hodgkins lymphoma (NHL) associated with an aggressive clinical course. Inhibition of the ubiquitin-proteasome pathway modulates survival and proliferation signals in MCL and has shown clinical benefit in this disease. This has provided rationale for exploring combination regimens with B-cell selective immunotherapies such as rituximab. In this study, we examined the effects of combined treatment with bortezomib and rituximab on patient-derived MCL cell lines (Jeko, Mino, SP53) and tumor samples from patients with MCL where we validate reversible proteasome inhibition concurrent with cell cycle arrest and additive induction of apoptosis. When MCL cells were exposed to single agent bortezomib or combination bortezomib/rituximab, caspase dependent and independent apoptosis was observed. Single agent bortezomib or rituximab treatment of Mino and Jeko cell lines and patient samples resulted in decreased levels of nuclear NFkappaB complex(es) capable of binding p65 consensus oligonucleotides, and this decrease was enhanced by the combination. Constitutive activation of the Akt pathway was also diminished with bortezomib alone or in combination with rituximab. On the basis of in vitro data demonstrating additive apoptosis and enhanced NFkappaB and phosphorylated Akt depletion in MCL with combination bortezomib plus rituximab, a phase II trial of bortezomib-rituximab in patients with relapsed/refractory MCL is underway.

The anti-apoptotic protein BCL2L1/Bcl-xL is neutralized by pro-apoptotic PMAIP1/Noxa in neuroblastoma, thereby determining bortezomib sensitivity independent of prosurvival MCL1 expression

Neuroblastoma is the most frequent extracranial solid tumor in children. Here, we report that the proteasome inhibitor bortezomib (PS-341, Velcade) activated the pro-apoptotic BH3-only proteins PMAIP1/Noxa and BBC3/Puma and induced accumulation of anti-apoptotic MCL1 as well as repression of anti-apoptotic BCL2L1/Bcl-xL. Retroviral expression of Bcl-xL, but not of MCL1, prevented apoptosis by bortezomib. Gene knockdown of Noxa by shRNA technology significantly reduced apoptosis, whereas Puma knockdown did not affect cell death kinetics. Immunoprecipitation revealed that endogenous Noxa associated with both, Bcl-xL and MCL1, suggesting that in neuronal cells Noxa can neutralize Bcl-xL, explaining the pronounced protective effect of Bcl-xL. Tetracycline-regulated Noxa expression did not trigger cell death per se but sensitized to bortezomib treatment in a dose-dependent manner. This implies that the induction of Noxa is necessary but not sufficient for bortezomib-induced apoptosis. We conclude that MCL1 steady-state expression levels do not affect sensitivity to proteasome-inhibitor treatment in neuronal tumor cells, and that both the repression of Bcl-xL and the activation of Noxa are necessary for bortezomib-induced cell death.

A phase I trial of bortezomib with temozolomide in patients with advanced melanoma: toxicities, antitumor effects, and modulation of therapeutic targets

PURPOSE

Preclinical studies show that bortezomib, a proteasome inhibitor, blocks NF-kappaB activation and, combined with temozolomide, enhances activity against human melanoma xenografts and modulates other critical tumor targets. We initiated a phase I trial of temozolomide plus bortezomib in advanced melanoma. Objectives included defining a maximum tolerated dose for the combination, characterizing biomarker changes reflecting inhibition of both proteasome and NF-kappaB activity in blood (if possible tumor), and characterizing antitumor activity.

EXPERIMENTAL DESIGN

Cohorts were enrolled onto escalating dose levels of temozolomide (50-75 mg/m(2)) daily, orally, for 6 of 9 weeks and bortezomib (0.75-1.5 mg/m(2)) by i.v. push on days 1, 4, 8, and 11 every 21 days. Peripheral blood mononuclear cells were assayed at specified time points for proteasome inhibition and NF-kappaB biomarker activity.

RESULTS

Bortezomib (1.3 mg/m(2)) and temozolomide (75 mg/m(2)) proved to be the maximum tolerated dose. Dose-limiting toxicities included neurotoxicity, fatigue, diarrhea, and rash. Nineteen melanoma patients were enrolled onto four dose levels. This melanoma population (17 M1c, 10 elevated lactate dehydrogenase, 12 performance status 1-2) showed only one partial response (8 months) and three with stable disease >or=4 months. A significant reduction in proteasome-specific activity was observed 1 hour after infusion at all bortezomib doses. Changes in NF-kappaB electrophoretic mobility shift assay and circulating chemokines in blood failed to correlate with the schedule/dose of bortezomib, inhibition of proteasome activity, or clinical outcome.

CONCLUSIONS

We have defined phase II doses for this schedule of temozolomide with bortezomib. Although proteasome activity was inhibited for a limited time in peripheral blood mononuclear cells, we were unable to show consistent effects on NF-kappaB activation.

Glycolysis inhibition sensitizes tumor cells to death receptors-induced apoptosis by AMP kinase activation leading to Mcl-1 block in translation

Most cancer cells exhibit increased glycolysis for generation of their energy supply. This specificity could be used to preferentially kill these cells. In this study, we identified the signaling pathway initiated by glycolysis inhibition that results in sensitization to death receptor (DR)-induced apoptosis. We showed, in several human cancer cell lines (such as Jurkat, HeLa, U937), that glucose removal or the use of nonmetabolizable form of glucose (2-deoxyglucose) dramatically enhances apoptosis induced by Fas or by tumor necrosis factor-related apoptosis-inducing ligand. This sensitization is controlled through the adenosine monophosphate (AMP)-activated protein kinase (AMPK), which is the central energy-sensing system of the cell. We established the fact that AMPK is activated upon glycolysis block resulting in mammalian target of rapamycin (mTOR) inhibition leading to Mcl-1 decrease, but no other Bcl-2 anti-apoptotic members. Interestingly, we determined that, upon glycolysis inhibition, the AMPK-mTOR pathway controlled Mcl-1 levels neither through transcriptional nor through posttranslational mechanism but rather by controlling its translation. Therefore, our results show a novel mechanism for the sensitization to DR-induced apoptosis linking glucose metabolism to Mcl-1 downexpression. In addition, this study provides a rationale for the combined use of DR ligands with AMPK activators or mTOR inhibitors in the treatment of human cancers.

Simultaneous targeting of MCL1 and ABCB1 as a novel strategy to overcome drug resistance in human leukaemia

Drug resistance is a major obstacle to chemotherapy success in leukaemia. Although ABCB1 (MDR1) overexpression represents a critical mechanism of drug resistance, modulation of ABCB1 shows unsatisfactory clinical outcome. Recent studies showed that MCL1 was upregulated in numerous haematological and solid tumour malignancies. The present study found that patients with newly diagnosed or relapsed/refractory leukaemia expressed higher MCL1 levels than patients that were in complete remission. We demonstrated that overexpression of MCL1 decreased sensitivity of human leukaemia cell lines to cytotoxic drugs and inhibited drug-induced apoptosis. Specific downregulation of MCL1 via RNA interference sensitized multidrug resistant leukaemia cells towards chemotherapy and induced apoptosis. Our study also demonstrated that MCL1 and ABCB1 mediated drug resistance through different mechanisms and the depletion of both MCL1 and ABCB1 showed an additive effect in reversing drug resistance and promoting drug-induced apoptosis. Thus, this study documented an important role of MCL1 in drug resistance and apoptosis. Simultaneous targeting of MCL1 and ABCB1 could be a novel approach to overcome drug resistance in leukaemia.

Mcl-1 expression predicts progression-free survival in chronic lymphocytic leukemia patients treated with pentostatin, cyclophosphamide, and rituximab

Myeloid cell leukemia-1 (Mcl-1) is an antiapoptotic member of the Bcl-2 protein family. Increased Mcl-1 expression is associated with failure to achieve remission after treatment with fludarabine and chlorambucil in patients with chronic lymphocytic leukemia (CLL). However, the influence of Mcl-1 expression has not been examined in CLL trials using chemoimmunotherapy. We investigated Mcl-1 protein expression prospectively as part of a phase 2 study evaluating the efficacy of pentostatin, cyclophosphamide, and rituximab in patients with untreated CLL. No significant difference by Mcl-1 expression was noted in pretreatment or response parameters. However, in patients with higher Mcl-1 expression, both minimal residual disease-negative status and progression-free survival was found to be significantly reduced (57% vs 19%, P = .01; 50.8 vs 18.7 months; P = .02; respectively). Mcl-1 expression may therefore be useful in predicting poor response to chemoimmunotherapy. These findings further support pursuing treatment strategies targeting this important antiapoptotic protein. (Because the trials described were conducted before the requirement to register them was implemented, they are not registered in a clinical trial database.).

Cooperation between Apo2L/TRAIL and bortezomib in multiple myeloma apoptosis

The proteasome inhibitor bortezomib is currently an important drug for treatment of relapsed and refractory multiple myeloma (MM) and for elderly patients. However, cells from some patients show resistance to bortezomib. We have evaluated the possibility of improving bortezomib therapy with Apo2L/TRAIL, a death ligand that induces apoptosis in MM but not in normal cells. Results indicate that cotreatment with low doses of bortezomib significantly increased apoptosis of MM cells showing partial sensitivity to Apo2L/TRAIL. Bortezomib treatment did not significantly alter plasma membrane amount of DR4 and DR5 but increased Apo2L/TRAIL-induced caspase-8 and caspase-3 activation. Apo2L/TRAIL reverted bortezomib-induced up-regulation of beta-catenin, Mcl-1 and FLIP, associated with the enhanced cytotoxicity of combined treatment. More important, some cell lines displaying resistance to bortezomib were sensitive to Apo2L/TRAIL-induced apoptosis. A cell line made resistant by continuous culture of RPMI 8226 cells in the presence of bortezomib (8226/7B) was highly sensitive to Apo2L/TRAIL-induced apoptosis. Moreover, RPMI 8226 cells overexpressing Mcl-1 (8226/Mcl-1) or Bcl-x(L) (8226/Bcl-x(L)) also showed enhanced resistance to bortezomib, but co-treatment with Apo2L/TRAIL reverted this resistance. These results indicate that Apo2L/TRAIL can cooperate with bortezomib to induce apoptosis in myeloma cells and can be an useful adjunct for MM therapy.

Treating metastatic solid tumors with bortezomib and a tumor necrosis factor-related apoptosis-inducing ligand receptor agonist antibody

BACKGROUND

Resistance of tumors to cell death signals poses a complex clinical problem. We explored the therapeutic potential and in vivo toxicity of a combination of bortezomib, a proteasome inhibitor, and MD5-1, a tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor (DR5) agonist monoclonal antibody, in mouse carcinomas. METHODS; Mice bearing Renca-FLAG (renal) or 4T1 (mammary) tumors were treated with bortezomib and/or MD5-1 and examined for lung metastases (Renca-FLAG: n = 93; 4T1: n = 40) or monitored for survival (Renca-FLAG: n = 143). Toxicity was assessed by histopathology and hematology. Viability and apoptotic signaling in Renca-FLAG and 4T1 cells treated with bortezomib alone or in combination with TRAIL were analyzed using 3-[4,5-dimethyiazol-2-yl-5]-[3-carboxymethyloxyphenyl]-2-[4-sulfophenyl]-2H tetrazolium assay and by measuring mitochondrial membrane depolarization and caspase-8 and caspase-3 activation. All statistical tests were two-sided.

RESULTS

Bortezomib (20 nM) sensitized Renca-FLAG and 4T1 cells to TRAIL-mediated apoptosis (mean percent decrease in numbers of viable cells, bortezomib + TRAIL vs TRAIL: Renca-FLAG, 95% vs 34%, difference = 61%, 95% confidence interval [CI] = 52% to 69%, P < .001; 4T1, 85% vs 20%, difference = 65%, 95% CI = 62% to 69%, P < .001). Sensitization involved activation of caspase-8 and caspase-3 but not mitochondrial membrane depolarization, suggesting an amplified signaling of the extrinsic cell death pathway. Treatment with bortezomib and MD5-1 reduced lung metastases in mice carrying Renca and 4T1 tumors (mean number of metastases, bortezomib + MD5-1 vs MD5-1: Renca-FLAG, 1 vs 8, difference = 7, 95% CI = 5 to 9, P < .001; 4T1, 1 vs 12, difference = 11, 95% CI = 9 to 12, P < .001) and increased median survival of mice bearing Renca-FLAG tumors (bortezomib + MD5-1 vs bortezomib + control isotype antibody: 22 of 30 [73%] were still alive at day 180 vs median survival of 42 days [95% CI = 41 to 44 days, P < .001]) in the absence of obvious toxicity.

CONCLUSION

Bortezomib combined with DR5 agonist monoclonal antibody may be a useful treatment for metastatic solid tumors.

Down-regulation of mcl-1 by small interfering RNA sensitizes resistant melanoma cells to fas-mediated apoptosis

Resistance of malignant melanoma cells to Fas-mediated apoptosis is among the mechanisms by which they escape immune surveillance. However, the mechanisms contributing to their resistance are not completely understood, and it is still unclear whether antiapoptotic Bcl-2-related family proteins play a role in this resistance. In this study, we report that treatment of Fas-resistant melanoma cell lines with cycloheximide, a general inhibitor of de novo protein synthesis, sensitizes them to anti-Fas monoclonal antibody (mAb)-induced apoptosis. The cycloheximide-induced sensitization to Fas-induced apoptosis is associated with a rapid down-regulation of Mcl-1 protein levels, but not that of Bcl-2 or Bcl-xL. Targeting Mcl-1 in these melanoma cell lines with specific small interfering RNA was sufficient to sensitize them to both anti-Fas mAb-induced apoptosis and activation of caspase-9. Furthermore, ectopic expression of Mcl-1 in a Fas-sensitive melanoma cell line rescues the cells from Fas-mediated apoptosis. Our results further show that the expression of Mcl-1 in melanoma cells is regulated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) and not by phosphatidylinositol 3-kinase/AKT signaling pathway. Inhibition of ERK signaling with the mitogen-activated protein/ERK kinase-1 inhibitor or by expressing a dominant negative form of mitogen-activated protein/ERK kinase-1 also sensitizes resistant melanoma cells to anti-Fas mAb-induced apoptosis. Thus, our study identifies mitogen-activated protein kinase/ERK/Mcl-1 as an important survival signaling pathway in the resistance of melanoma cells to Fas-mediated apoptosis and suggests that its targeting may contribute to the elimination of melanoma tumors by the immune system.

Caspase-dependent cleavage of BAG3 in proteasome inhibitors-induced apoptosis in thyroid cancer cells

Proteasome inhibitors are emerging as effective drugs for the treatment of relapsed/refractory multiple myeloma and possibly some solid tumors. Bcl-2-associated athanogene 3 (BAG3) is a survival protein that has been shown to be stimulated during cell response to stressful conditions, such as exposure to high temperature, heavy metals. We have recently demonstrated that BAG3 is also induced by proteasome inhibitors at the transcriptional level and the induction of BAG3 by proteasome inhibition is antiapoptotic. Here, we demonstrated that although proteasome inhibitors triggered similar upregulation of BAG3 transcript in sensitive and insensitive thyroid cancer cells, persistent increase of BAG3 protein was detected in insensitive cells, whereas less increase or even decrease was observed in sensitive cells. Notably, decrease of BAG3 protein was associated with the appearance of a BAG3 fragment of approximately 40kDa, which appeared to be caspase-dependent. Therefore, caspase-dependent cleavage of BAG3 might facilitate apoptosis in sensitive cells.

Inhibition of interleukin-6 signaling with CNTO 328 enhances the activity of bortezomib in preclinical models of multiple myeloma

PURPOSE

Inhibition of the proteasome leads to the activation of survival pathways in addition to those that promote cell death. We hypothesized that down-regulation of interleukin-6 (IL-6) signaling using the monoclonal antibody CNTO 328 would enhance the antitumor activity of the proteasome inhibitor bortezomib in multiple myeloma by attenuating inducible chemoresistance.

EXPERIMENTAL DESIGN

The cytotoxicity of bortezomib, CNTO 328, and the combination, along with the associated molecular changes, was assessed in IL-6-dependent and IL-6-independent multiple myeloma cell lines, both in suspension and in the presence of bone marrow stromal cells and in patient-derived myeloma samples.

RESULTS

Treatment of IL-6-dependent and IL-6-independent multiple myeloma cell lines with CNTO 328 enhanced the cytotoxicity of bortezomib in a sequence-dependent fashion. This effect was additive to synergistic and was preserved in the presence of bone marrow stromal cells and in CD138(+) myeloma samples derived from patients with relative clinical resistance to bortezomib. CNTO 328 potentiated bortezomib-mediated activation of caspase-8 and caspase-9 and the common downstream effector caspase-3; attenuated bortezomib-mediated induction of antiapoptotic heat shock protein-70, which correlated with down-regulation of phosphorylated signal transducer and activator of transcription-1; and inhibited bortezomib-mediated accumulation of myeloid cell leukemia-1, an effect that was associated with down-regulation of phosphorylated signal transducer and activator of transcription-3.

CONCLUSIONS

Taken together, our results provide a strong preclinical rationale for the clinical development of the bortezomib/CNTO 328 combination for patients with myeloma.

Interactions between bortezomib and romidepsin and belinostat in chronic lymphocytic leukemia cells

PURPOSE

The goal of this study was to characterize interactions between the proteasome inhibitor bortezomib and the histone deacetylase (HDAC) inhibitors (HDACI) romidepsin or belinostat in chronic lymphocytic leukemia (CLL) cells.

EXPERIMENTAL DESIGN

Primary and cultured (JVM-3 and MEC-2) CLL cells were exposed to agents alone or in combination, after which cell death was determined by 7-aminoactinomycin D staining/flow cytometry. Acetylation of target proteins, activation of caspase cascades, and expression of apoptosis-regulatory proteins were monitored by Western blot analysis. Nuclear factor-kappaB (NF-kappaB) activity was determined by luciferase reporter assay. Cells were transiently transfected with wild-type and acetylation site-mutated (inactive) RelA(p65) (e.g., K221R, K310R, or K281/221/310R) and assessed for HDACI sensitivity.

RESULTS

Combined exposure to very low concentrations of romidepsin or belinostat (i.e., low nanomolar and submicromolar, respectively) in combination with low nanomolar concentrations of bortezomib synergistically induced cell death in primary and cultured CLL cells. These events were likely associated with prevention of HDACI-mediated RelA acetylation and NF-kappaB activation by bortezomib, down-regulation of antiapoptotic proteins (i.e., Bcl-xL, Mcl-1, and XIAP), as well as up-regulation of the proapoptotic protein Bim, resulting in activation of caspase cascade. Finally, CLL cells transfected with inactive RelA displayed a significant increase in HDACI lethality.

CONCLUSIONS

Coadministration of the clinically relevant HDACIs romidepsin or belinostat with bortezomib synergistically induces cell death in CLL cells, likely through mechanisms involving, among other factors, NF-kappaB inactivation and perturbation in the expression of proapoptotic and antiapoptotic proteins. A strategy combining HDAC with proteasome inhibition warrants further attention in CLL.

Transcriptional upregulation of BAG3 upon proteasome inhibition

Proteasome inhibitors exhibit antitumoral activity against malignancies of different histology. Emerging evidence indicates that antiapoptotic factors may also accumulate as a consequence of exposure to these drugs, thus it seems plausible that activation of survival signaling cascades might compromise their antitumoral effects. Bcl-2-associated athanogene (BAG) family proteins are characterized by their property of interaction with a variety of partners involved in modulating the proliferation/death balance, including heat shock proteins (HSP), Bcl-2, Raf-1. In this report, we demonstrated that BAG3 is a novel antiapoptotic molecule induced by proteasome inhibitors in various cancer cells at the transcriptional level. Moreover, we demonstrated that BAG3 knockdown by siRNA sensitized cancer cells to MG132-induced apoptosis. Taken together, our results suggest that BAG3 induction might represents as an unwanted molecular consequence of utilizing proteasome inhibitors to combat tumors.

Ubiquitination and proteasomal degradation of interferon regulatory factor-3 induced by Npro from a cytopathic bovine viral diarrhea virus

The pathogenesis of bovine viral diarrhea virus (BVDV) infections is complex and only partly understood. It remains controversial whether interferon is produced in cells infected with cytopathic(cp) BVDVs which do not persist in vivo. We show here that a cpBVDV (NADL strain) does not induce interferon responses in cell culture and blocks induction of interferon-stimulated genes by a super-infecting paramyxovirus. cpBVDV infection causes a marked loss of interferon regulatory factor 3 (IRF-3), a cellular transcription factor that controls interferon synthesis. This is attributed to expression of Npro, but not its protease activity. Npro interacts with IRF-3, prior to its activation by virus-induced phosphorylation, resulting in polyubiquitination and subsequent proteasomal degradation of IRF-3. Thermal inactivation of the E1 ubiquitin-activating enzyme prevents Npro-induced IRF-3 loss. These data suggest that inhibition of interferon production is a shared feature of both ncp and cpBVDVs and provide new insights regarding IRF-3 regulation in pestivirus pathogenesis.

BCL-2 family regulation by the 20S proteasome inhibitor bortezomib

Bortezomib (Velcade, PS341) was licensed in 2003 as a first-in-class 20S proteasome inhibitor indicated for treatment of multiple myeloma, and is currently being evaluated clinically in a range of solid tumours. The mechanisms underlying its cancer cell toxicity are complex. A growing body of evidence suggests proteasome inhibition-dependent regulation of the BCL-2 family is a critical requirement. In particular, the stabilization of BH3-only proteins BIK, NOXA and BIM, appear to be essential for effecting BAX- and BAK-dependent cell death. These mechanisms are reviewed and the implications for favourable novel drug interactions are highlighted.

Reciprocal protection of Mcl-1 and Bim from ubiquitin-proteasome degradation

Survival of multiple myeloma cells is essentially dependent on Mcl-1 protein that neutralizes the pro-apoptotic function of Bim and prevents activation of death effectors. To clarify the relationship between Mcl-1 and Bim, we generated cell lines silenced for Mcl-1 (shMcl-1) or Bim (shBim). We demonstrate that Mcl-1 and Bim proteins are concomitantly down-regulated in either shBim or shMcl-1 cells. We show that the down-regulation of either Mcl-1 in shBim or Bim in shMcl-1 cells is not due to a transcriptional event, but results from post-translational regulation. Indeed, the multi-ubiquitinated forms of Mcl-1 or Bim are increased in shBim and shMcl-1 cells, respectively, indicating proteasome degradation. Since Mcl-1/Bim complexes are predominant in myeloma cells the down-regulation of Mcl-1 by shRNA leads to unliganded Bim sensitive to degradation and reciprocally for unliganded Mcl-1 in shBim cells. Finally, our results support that the interaction between Mcl-1 and Bim confers to themselves mutual protection.

Mcl-1 is a relevant therapeutic target in acute and chronic lymphoid malignancies: down-regulation enhances rituximab-mediated apoptosis and complement-dependent cytotoxicity

PURPOSE

The antiapoptotic Bcl-2 family member protein Mcl-1 is dynamically regulated in transformed B-cells, has a short mRNA and protein half-life, and is rapidly processed during apoptosis. Multiple therapies cause down-regulation of Mcl-1 in chronic and acute lymphoid leukemia (CLL and ALL) cells. Mcl-1 has also been reported to mediate resistance to rituximab in CLL. We therefore investigated whether direct reduction of Mcl-1 was sufficient to induce apoptosis and increase sensitivity to rituximab.

EXPERIMENTAL DESIGN

We used Mcl-1-specific small interfering RNA in ALL cell lines and tumor cells from CLL patients to block transcription of Mcl-1.

RESULTS

We show that Mcl-1 down-regulation alone is sufficient to promote mitochondrial membrane depolarization and apoptosis in ALL and CLL cells. Given the importance of rituximab in B-cell malignancies, we next assessed the influence of Mcl-1 down-regulation on antibody-mediated killing. Mcl-1 down-regulation by small interfering RNA increased sensitivity to rituximab-mediated killing both by direct apoptosis and complement-dependent cytotoxicity, but did not enhance antibody-dependent cellular cytotoxicity.

CONCLUSIONS

These results show that Mcl-1 is a relevant therapeutic target for ALL and CLL, and its down-regulation has the potential to enhance the therapeutic effect of rituximab in CD20-bearing lymphoid cells.

Noxa Up-regulation and Mcl-1 Cleavage Are Associated to Apoptosis Induction by Bortezomib in Multiple Myeloma

Targeting the ubiquitin-proteasome pathway has emerged as a potent anticancer strategy. Bortezomib, a specific proteasome inhibitor, has been approved for the treatment of relapsed or refractory multiple myeloma. Multiple myeloma cell survival is highly dependent on Mcl-1 antiapoptotic molecules. In a recent study, proteasome inhibitors induced Mcl-1 accumulation that slowed down their proapoptotic effects. Consequently, we investigated the role of Bcl-2 family members in bortezomib-induced apoptosis. We found that bortezomib induced apoptosis in five of seven human myeloma cell lines (HMCL). Bortezomib-induced apoptosis was associated with Mcl-1 cleavage regardless of Mcl-1L accumulation. Furthermore, RNA interference mediated Mcl-1 decrease and sensitized RPMI-8226 HMCL to bortezomib, highlighting the contribution of Mcl-1 in bortezomib-induced apoptosis. Interestingly, an important induction of Noxa was found in all sensitive HMCL both at protein and mRNA level. Concomitant to Mcl-1 cleavage and Noxa induction, we also found caspase-3, caspase-8, and caspase-9 activation. Under bortezomib treatment, Mcl-1L/Noxa complexes were highly increased, Mcl-1/Bak complexes were disrupted, and there was an accumulation of free Noxa. Finally, we observed a dissociation of Mcl-1/Bim complexes that may be due to a displacement of Bim induced by Noxa. Thus, in myeloma cells, the mechanistic basis for bortezomib sensitivity can be explained mainly by the model in which the sensitizer Noxa can displace Bim, a BH3-only activator, from Mcl-1, thus leading to Bax/Bak activation.

Bortezomib induces different apoptotic rates in B-CLL cells according to IgVH and BCL-6 mutations

BACKGROUND

B-cell chronic lymphocytic leukemia (B-CLL) is a remarkably heterogeneous disorder. Some patients have an indolent disease whereas others undergo a more agressive presentation needing treatment. New therapeutics approaches are necessary for the treatment of B-CLL. Bortezomib (Btz), is a proteasome inhibitor, currently undergoing clinical trials whose function, at least in part, by stabilizing the IkappaBalpha protein and inhibiting NFkappaB activation.

OBJECTIVE

The objective of this work was to study the effects of Btz on isolated human B-CLL cells, in vitro, and to correlate the differential rates of apoptosis induction with biological variables.

MATERIAL AND METHODS

31 B-CLL samples, from patients in stage A of Binet were used for this study, and the apoptotic effect of Btz on these cells was measured.

RESULTS

Our data show that Btz treatment of B-CLL cells induces apoptosis in a time and dose-dependent manner. The apoptosis induction is mediated in part by inhibition of NFkappaB and is dependent on caspases activation. Interesting, in IgVH mutated cells, Btz have statistically significant differences in their in vitro activity on B-CLL cells according to their BCL-6 mutational status.

CONCLUSIONS

Btz is a promising pharmacologic agent for the treatment of B-CLL, but its efficacy seems to be related to IgVH and BCL-6 mutational status, therefore, it could be interesting to further investigate the mechanisms involved in the different behavior of the cells in response to apoptosis induction by this drug.

A coordinated action of Bax, PUMA, and p53 promotes MG132-induced mitochondria activation and apoptosis in colon cancer cells

Targeting the ubiquitin-proteasome degradation pathway has become a promising approach for cancer therapy. Previous studies have shown that proteasome inhibition leads to apoptosis in various cancer cells. The mechanism by which apoptosis occurs are not fully understood and can be cell type and/or inhibitor specific. In this study, we investigated the mechanism of mitochondrial activation by proteasome inhibitors in colon cancer cells. We found that Bax activation and mitochondria translocation were required for apoptosis induced by multiple proteasome inhibitors. In contrast, reactive oxygen species did not seem to be induced by MG132 or bortezomib and antioxidants had no effects on MG132-induced apoptosis. In contrast, treatment with MG132 or bortezomib induced a significant accumulation of p53 and PUMA. Genetic deletion of either p53 or PUMA led to a marked suppression of apoptosis induced by these inhibitors, accompanied with reduced Bax activation and cytochrome c release. Consistently, inhibition of translation by cycloheximide could also effectively abolish the accumulation of p53 and PUMA and suppress MG132-induced Bax activation and apoptosis. These findings thus strongly indicate the critical involvement of p53-, PUMA-, and Bax-mediated mitochondrial activation in proteasome inhibitor–induced apoptosis in colon cancer cells. [Mol Cancer Ther 2007;6(3):1062–9]

The BH3-mimetic GX15-070 synergizes with bortezomib in mantle cell lymphoma by enhancing Noxa-mediated activation of Bak

Mantle cell lymphoma (MCL) is an aggressive B-cell lymphoma resistant to conventional chemotherapy. The Bcl-2 pathway is deregulated in these tumors and may represent an interesting target for new therapeutic strategies. The new small-molecule pan-Bcl-2 inhibitor GX15-070 mimics BH3-only proteins by binding to multiple antiapoptotic Bcl-2 members. Here we show that GX15-070 induced apoptosis in vitro in MCL cell lines and primary cells from patients with MCL by releasing Bak from Mcl-1 and Bcl-X(L) at short incubation times and low micromolar doses. GX15-070 was effective in cells bearing defective DNA damage-sensor genes or cell-cycle regulators, inducing Bax and Bak conformational changes, mitochondrial depolarization, phosphatidylserine exposure, and caspase-3 activation. Furthermore, GX15-070 synergized with bortezomib, sensitizing MCL cells to low doses of this proteasome inhibitor, by neutralizing bortezomib-induced Mcl-1 accumulation and cooperating with Noxa to induce Bak displacement from this protein. These events led to an increased activation of the mitochondrial apoptotic pathway. Importantly, GX15-070 alone or in combination with bortezomib showed no significant cytotoxic effect in peripheral blood mononuclear cells from healthy donors. All these findings suggest that GX15-070 alone or in combination with bortezomib represents a new attractive therapeutic approach for MCL treatment.

Dendritic cell maturation stage determines susceptibility to the proteasome inhibitor bortezomib

The proteasome inhibitor bortezomib has been used successfully in the treatment of non-Hodgkin lymphomas in humans, and in the treatment of graft versus host disease (GVHD) and autoimmune diseases in animal models. The mechanism of growth inhibition and immunosuppression is only partly understood. Here, we have evaluated the differential effect of bortezomib on human monocyte derived immature and mature dendritic cells (DCs) as the maturation stage of DCs determines their function. We found bortezomib to induce apoptotic cell death in immature DCs and to a much lesser extent, in mature DCs. Furthermore, cytokine-induced maturation of immature DCs was inhibited by bortezomib, whereas already matured DCs remained unaffected as seen by phenotype and allo-stimulatory capacity. This corresponded to a decreased NF-kappaB activity in immature DCs, whereas NF-kappaB activity of mature DCs was not affected. In conclusion, our data expand on previous reports on the effects of proteasome inhibitors on human monocyte-derived DCs by demonstrating a differential effect of bortezomib on immature versus mature DCs. Our findings suggest a potential role of bortezomib in modulating immune responses in humans through inhibition of DC maturation.

Proteasome inhibitors: antitumor effects and beyond

Proteasome inhibitors are emerging as effective drugs for the treatment of multiple myeloma and possibly certain subtypes of non-Hodgkin's lymphoma. Bortezomib (Velcade) is the first proteasome inhibitor proven to be clinically useful and will soon be followed by a second generation of small molecule inhibitors with improved pharmacological properties. Although it is now understood that certain types of malignancies have an exquisite dependence on a functional proteasome for their survival, the underlying reason(s) remain unclear as of now. In this context, addiction to nuclear factor-kappaB (NF-kappaB)-induced survival signals, activation of the unfolded protein response as well as a reduced proteasomal activity in differentiated plasma cells have all been proposed to justify proteasome inhibitors' activity in susceptible tissues. In addition to their anticancer properties, bortezomib and related drugs modulate inflammatory and immune responses by affecting function and survival of immune cells such as lymphocytes and dendritic cells. The present review offers an overview of the biological effects that have been involved in proteasome inhibitors' antitumor activity and suggests prospective future applications for these drugs based on their recently characterized anti-inflammatory and immunomodulatory effects.

Mitochondria in hematopoiesis and hematological diseases

Mitochondria are involved in hematopoietic cell homeostasis through multiple ways such as oxidative phosphorylation, various metabolic processes and the release of cytochrome c in the cytosol to trigger caspase activation and cell death. In erythroid cells, the mitochondrial steps in heme synthesis, iron (Fe) metabolism and Fe-sulfur (Fe-S) cluster biogenesis are of particular importance. Mutations in the specific delta-aminolevulinic acid synthase (ALAS) 2 isoform that catalyses the first and rate-limiting step in heme synthesis pathway in the mitochondrial matrix, lead to ineffective erythropoiesis that characterizes X-linked sideroblastic anemia (XLSA), the most common inherited sideroblastic anemia. Mutations in the adenosine triphosphate-binding cassette protein ABCB7, identified in XLSA with ataxia (XLSA-A), disrupt the maturation of cytosolic (Fe-S) clusters, leading to mitochondrial Fe accumulation. In addition, large deletions in mitochondrial DNA, whose integrity depends on a specific DNA polymerase, are the hallmark of Pearson's syndrome, a rare congenital disorder with sideroblastic anemia. In acquired myelodysplastic syndromes at early stage, exacerbation of physiological pathways involving caspases and the mitochondria in erythroid differentiation leads to abnormal activation of a mitochondria-mediated apoptotic cell death pathway. In contrast, oncogenesis-associated changes at the mitochondrial level can alter the apoptotic response of transformed hematopoietic cells to chemotherapeutic agents. Recent findings in mitochondria metabolism and functions open new perspectives in treating hematopoietic cell diseases, for example various compounds currently developed to trigger tumor cell death by directly targeting the mitochondria could prove efficient as either cytotoxic drugs or chemosensitizing agents in treating hematological malignancies.

The three-substituted indolinone cyclin-dependent kinase 2 inhibitor 3-[1-(3H-imidazol-4-yl)-meth-(Z)-ylidene]-5-methoxy-1,3-dihydro-indol-2-one (SU9516) kills human leukemia cells via down-regulation of Mcl-1 through a transcriptional mechanism

Mechanisms of lethality of the three-substituted indolinone and putatively selective cyclin-dependent kinase (CDK)2 inhibitor 3-[1-(3H-imidazol-4-yl)-meth-(Z)-ylidene]-5-methoxy-1,3-dihydro-indol-2-one (SU9516) were examined in human leukemia cells. Exposure of U937 and other leukemia cells to SU9516 concentrations > or =5 microM rapidly (i.e., within 4 h) induced cytochrome c release, Bax mitochondrial translocation, and apoptosis in association with pronounced down-regulation of the antiapoptotic protein Mcl-1. These effects were associated with inhibition of phosphorylation of the carboxyl-terminal domain (CTD) of RNA polymerase (Pol) II on serine 2 but not serine 5. Reverse transcription-polymerase chain reaction analysis revealed pronounced down-regulation of Mcl-1 mRNA levels in SU9516-treated cells. Similar results were obtained in Jurkat and HL-60 leukemia cells. Furthermore, cotreatment with the proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132) blocked SU9516-mediated Mcl-1 down-regulation, implicating proteasomal degradation in diminished expression of this protein. Ectopic expression of Mcl-1 largely blocked SU9516-induced cytochrome c release, Bax translocation, and apoptosis, whereas knockdown of Mcl-1 by small interfering RNA potentiated SU9516 lethality, confirming the functional contribution of Mcl-1 down-regulation to SU9516-induced cell death. It is noteworthy that SU9516 treatment resulted in a marked increase in reactive oxygen species production, which was diminished, along with cell death, by the free radical scavenger N-acetylcysteine (NAC). We were surprised to find that NAC blocked SU9516-mediated inhibition of RNA Pol II CTD phosphorylation on serine 2, reductions in Mcl-1 mRNA levels, and Mcl-1 down-regulation. Together, these findings suggest that SU9516 kills leukemic cells through inhibition of RNA Pol II CTD phosphorylation in association with oxidative damage and down-regulation of Mcl-1 at the transcriptional level, culminating in mitochondrial injury and cell death.

Herceptin sensitizes ErbB2-overexpressing cells to apoptosis by reducing antiapoptotic Mcl-1 expression

PURPOSE

Monoclonal antibodies, such as herceptin and trastuzumab, against the epidermal growth factor receptor ErbB2 (also known as HER2/neu) are an effective therapy for breast cancer patients with overexpression of ErbB2. Herceptin, in combination with standard chemotherapy, such as taxol or etoposide, gives a synergistically apoptotic response in breast tumors.

EXPERIMENTAL DESIGN

The mechanism underlying this synergy between chemotherapy and herceptin treatment is not well understood. Herein, we have determined that addition of herceptin, sensitized breast cancer cell lines MDA-MB-231 and MCF-7 to etoposide- or taxol-induced apoptosis.

RESULTS

This treatment resulted in reduced expression of ErbB2 and the antiapoptotic Bcl-2 family member Mcl-1 in MDA-MB-231 cells. Using antisense oligonucleotides against Mcl-1, MDA-MB-231 cells were rendered sensitive to etoposide-induced apoptosis similar to herceptin, but combined treatment of antisense against Mcl-1 and herceptin failed to give a significant increase in apoptosis. In 29 human breast tumors immunostained for ErbB2 and Mcl-1, we found that when ErbB2 was overexpressed, there was a corresponding increase in Mcl-1 expression.

DISCUSSION

Using murine fibroblasts that express human ErbB2, but no other ErbB family member (NE2), these cells showed resistance to both taxol- and etoposide-induced apoptosis compared with parental cells. In addition, NE2 cells preferentially express the antiapoptotic Bcl-2 family member Mcl-1 compared with parental cells, and treatment with herceptin reduces Mcl-1 expression. Taken together, these results suggest that herceptin sensitizes ErbB2-overexpressing cells to apoptosis by reducing antiapoptotic Mcl-1 protein levels.

Molecular targets in melanoma from angiogenesis to apoptosis

Angiogenesis is a hallmark of melanoma progression. Antiangiogenic agents have been infrequently tested in patients with advanced melanoma. Experience with most other cancers suggests that single-agent application of angiogenic inhibitors is unlikely to have substantial clinical antitumor activity in melanoma. It is more likely that combinations of antiangiogenic agents with either chemotherapy or other targeted therapy will be needed to produce significant clinical benefit. In melanoma, numerous cellular pathways important to cell proliferation, apoptosis, or metastases have recently been shown to be activated. Activation occurs through specific mutations (B-RAF, N-RAS, and PTEN) or changes in expression levels of various proteins (PTEN, BCL-2, NF-kappaB, CDK2, and cyclin D1). Agents that block these pathways are rapidly entering the clinical setting, including RAF inhibitors (sorafenib), mitogen-activated protein kinase inhibitors (PD0325901), mammalian target of rapamycin inhibitors (CCI-779), and farnesyl transferase inhibitors (R115777) that inhibit N-RAS and proteasome inhibitors (PS-341) that block activation of nuclear factor-kappaB (NF-kappaB). It will be a challenge to evaluate these agents alone, in combination with each other, or with chemotherapy in patients with melanoma. Trials with large populations of biologically ill-defined tumors run the risk of missing clinical antitumor activity that is important for a particular yet-to-be-defined subset of patients. To rationally and optimally develop these targeted agents, it will be critical to adequately test for the presence of the presumed cellular target in tumor specimens and the effect of therapy on the proposed target (biological response). Investigators in this field will need to carefully plan these trials so that at the end of the day, we learn from both the failures and successes of targeted therapy.

The Noxa/Mcl-1 Axis Regulates Susceptibility to Apoptosis under Glucose Limitation in Dividing T Cells

Throughout lymphocyte development, cellular persistence and expansion are tightly regulated by survival and apoptosis. Within the Bcl-2 family, distinct apoptogenic BH3-only members like Bid, Bim, and Puma appear to function in specific cell death pathways. We found that naive human T cells after mitogenic activation, apart from expected protective Bcl-2 members, also rapidly upregulate the BH3-only protein Noxa in a p53-independent fashion. The specific role of Noxa became apparent during glucose limitation and involves interaction with the labile Bcl-2 homolog Mcl-1. Knockdown of Noxa or Mcl-1 results in protection or susceptibility, respectively, to apoptosis induced by glucose deprivation. Declining Mcl-1 levels and apoptosis induction are inversely correlated to Noxa levels and prevented by readdition of glucose. We propose that the Noxa/Mcl-1 axis is an apoptosis rheostat in dividing cells, in a selective pathway that functions to restrain lymphocyte expansion and can be triggered by glucose deprivation.

Poly(A) binding protein (PABP) homeostasis is mediated by the stability of its inhibitor, Paip2

The poly(A)-binding protein (PABP) is a unique translation initiation factor in that it binds to the mRNA 3' poly(A) tail and stimulates recruitment of the ribosome to the mRNA at the 5' end. PABP activity is tightly controlled by the PABP-interacting protein 2 (Paip2), which inhibits translation by displacing PABP from the mRNA. Here, we describe a close interplay between PABP and Paip2 protein levels in the cell. We demonstrate a mechanism for this co-regulation that involves an E3 ubiquitin ligase, EDD, which targets Paip2 for degradation. PABP depletion by RNA interference (RNAi) causes co-depletion of Paip2 protein without affecting Paip2 mRNA levels. Upon PABP knockdown, Paip2 interacts with EDD, which leads to Paip2 ubiquitination. Supporting a critical role for EDD in Paip2 degradation, knockdown of EDD expression by siRNA leads to an increase in Paip2 protein stability. Thus, we demonstrate that the turnover of Paip2 in the cell is mediated by EDD and is regulated by PABP. This mechanism serves as a homeostatic feedback to control the activity of PABP in cells.

Proteasome inhibitor-induced apoptosis in human monocyte-derived dendritic cells

Proteasome inhibitors possess potent antitumor activity against a broad spectrum of human malignancies. However, the effects of these compounds on the immune system still have to be clearly determined. In the present study, we have investigated the effects of proteasome inhibitors on dendritic cells (DC), antigen-presenting cells playing a key role in the initiation of immune responses. Exposure to the proteasome inhibitors bortezomib, MG132 or epoxomicin was found to promote apoptosis of human monocyte-derived DC and to reduce the yield of viable DC when given to monocytes early during differentiation to DC. DC apoptosis via proteasome inhibition was accompanied by mitochondria disruption and subsequent activation of the caspase cascade. Up-regulation and intracellular redistribution of Bcl-2-associated X protein (Bax), a pro-apoptotic Bcl-2 family protein, were observed in DC treated with these compounds and represent a suitable mechanism leading to activation of the intrinsic apoptotic pathway. Finally, active protein synthesis was found to represent an upstream prerequisite for DC apoptosis induced by proteasome inhibitors, since the translation inhibitor cycloheximide blocked all of the steps of the observed apoptotic response. In conclusion, induction of apoptosis in DC may represent a novel mechanism by which proteasome inhibitors affect the immune response at the antigen-presenting cell level.

Influence of proteasome inhibitors on apoptosis

PURPOSE OF REVIEW

Proteasome inhibitors are a novel class of drugs that alter normal cellular control of apoptosis. As such, they are being investigated as novel therapies to alter uncontrolled cellular proliferation and treat cancers. This review explores new information about how the proteasome regulates apoptosis and how proteasome inhibitors can be exploited as anti-tumor drugs.

RECENT FINDINGS

Proteasome inhibitors block the activation of nuclear factor kappa B in a number of cell systems, as well as altering apoptotic regulatory proteins and intracellular signals that influence the fate of the cell. These effects are true for many tumor cell lines. The US Food and Drug Administration-approved proteasome inhibitor bortezomib blocks erroneous cell proliferation and induces apoptosis in many tumor models.

SUMMARY

Proteasome inhibitors have demonstrated promise in vitro, and as a result clinical trials have begun to investigate these agents as therapy for numerous human cancers. Furthermore, newer agents are being designed to inhibit the proteasome system and exert further anti-tumor activity.

Altering protein turnover in tumor cells: New opportunities for anti-cancer therapies

The promising effects of the proteasome inhibitor bortezomib (Velcade, PS-341) in the treatment of certain types of cancer have fired up the interest on this multicatalytic proteolytic machinery. A number of recent reviews thoroughly describe various aspects of the ubiquitin-proteasome system and its importance in the control of cell growth and tumorigenesis. Here, we will focus on recent data unveiling a link between the proteasome and some elements of the apoptotic machinery including Bcl-2 members, caspases, IAPs and IAP antagonists. Perturbing their turnover significantly contributes to the apoptotic response and the anti-neoplastic activity of proteasome inhibitors.

Mitochondrial cell death suppressors carried by human and murine cytomegalovirus confer resistance to proteasome inhibitor-induced apoptosis

Human cytomegalovirus carries a mitochondria-localized inhibitor of apoptosis (vMIA) that is conserved in primate cytomegaloviruses. We find that inactivating mutations within UL37x1, which encodes vMIA, do not substantially affect replication in TownevarATCC (Towne-BAC), a virus that carries a functional copy of the betaherpesvirus-conserved viral inhibitor of caspase 8 activation, the UL36 gene product. In Towne-BAC infection, vMIA reduces susceptibility of infected cells to intrinsic death induced by proteasome inhibition. vMIA is sufficient to confer resistance to proteasome inhibition when expressed independent of viral infection. Murine cytomegalovirus m38.5, whose position in the viral genome is analogous to UL37x1, exhibits mitochondrial association and functions in much the same manner as vMIA in inhibiting intrinsic cell death. This work suggests a common role for vMIA in rodent and primate cytomegaloviruses, modulating the threshold of virus-infected cells to intrinsic cell death.

The proteasome inhibitor bortezomib induces apoptosis in mantle-cell lymphoma through generation of ROS and Noxa activation independent of p53 status

Mantle-cell lymphoma (MCL) is a mature B-cell lymphoma with an aggressive course and generally poor prognosis. Conventional chemotherapy has little efficacy. Bortezomib is a novel, reversible, and highly specific proteasome inhibitor that appears as a new hope for MCL treatment. We have analyzed the in vitro sensitivity to bortezomib in 4 MCL cell lines and in primary tumor cells from 10 MCL patients. Bortezomib induced phosphatidylserine exposure, mitochondrial depolarization, ROS generation, Bax and Bak conformational changes, and caspase activation. In addition, ROS scavengers, but not pancaspase inhibitors, blocked all apoptosis hallmarks. Protein and mRNA-expression analysis, revealed marked up-regulation of the BH3-only protein Noxa, between 4 to 6 hours after bortezomib addition, independent of p53 status. However, this up-regulation was faster and higher in cells with functional p53. Noxa RNA interference markedly decreased sensitivity to bortezomib, pointing to this protein as a key mediator between proteasome inhibition and mitochondrial depolarization in MCL cells. Noxa interacts with the antiapoptotic protein Mcl-1 and promotes Bak release from Mcl-1, suggesting that up-regulation of Noxa might counteract Mcl-1 accumulation after bortezomib treatment. These findings should be useful to extend the therapeutic strategies in MCL patients and to improve their prognosis.

Mule/ARF-BP1, a BH3-Only E3 Ubiquitin Ligase, Catalyzes the Polyubiquitination of Mcl-1 and Regulates Apoptosis

The elimination of Mcl-1, an anti-apoptotic Bcl-2 family member, is an early and required step for DNA damage-induced apoptosis. The degradation of Mcl-1 can be blocked by proteasome inhibitors, suggesting a role for the ubiquitin proteasome pathway in apoptosis. Here, we demonstrate that Mcl-1 is ubiquinated at five lysines. Biochemical fractionation of cell extracts allowed us to identify a 482 kDa HECT-domain-containing ubiquitin ligase named Mule (Mcl-1 ubiquitin ligase E3) that is both required and sufficient for the polyubiquitination of Mcl-1. Mule also contains a region similar to the Bcl-2 homology region 3 (BH3) domain that allows Mule to specifically interact with Mcl-1. Elimination of Mule expression by RNA interference stabilizes Mcl-1 protein, resulting in an attenuation of the apoptosis induced by DNA-damage agents. Thus, Mule is a unique BH3-containing E3 ubiquitin ligase apical to Bcl-2 family proteins during DNA damage-induced apoptosis.

Cooperative Cytotoxicity of Proteasome Inhibitors and Tumor Necrosis Factor–Related Apoptosis-Inducing Ligand in Chemoresistant Bcl-2-Overexpressing Cells

PURPOSE

Bcl-2 overexpression is frequently detected in lymphoid malignancies, being associated with poor prognosis and reduced response to therapy. Here, we evaluated whether Bcl-2 overexpression affects the cytotoxic activity of proteasome inhibitors taken alone or in association with conventional anticancer drugs or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL).

EXPERIMENTAL DESIGN

Jurkat cells engineered to overexpress Bcl-2 were treated with proteasome inhibitors (MG132, epoxomicin, and bortezomib), anticancer drugs (etoposide and doxorubicin), TRAIL, or combinations of these compounds. Cell death and loss of mitochondrial transmembrane potential were detected by flow cytometry. Cytosolic relocalization of cytochrome c and SMAC/Diablo, caspase cleavage, and Bcl-2 and Mcl-1 levels were determined by immunoblotting. Nuclear factor-kappaB inhibition was done by retroviral transduction with a dominant-negative mutant of IkappaBalpha.

RESULTS

Bcl-2 overexpression results in significant inhibition of apoptosis in response to proteasome inhibitors, antiblastics, and TRAIL. Addition of TRAIL to proteasome inhibitors results in a synergistic cytotoxic effect in Bcl-2-overexpressing cells, whereas this result is not reproduced by the combination of proteasome inhibitors with antiblastic drugs. Importantly, proteasome inhibitors plus TRAIL induce mitochondrial dysfunction irrespective of up-regulated Bcl-2. Bcl-2 cleavage to a fragment with putative proapoptotic activity and elimination of antiapoptotic Mcl-1 may both play a role in proteasome inhibitors-TRAIL cooperation. Conversely, nuclear factor-kappaB inhibition by proteasome inhibitors is per se insufficient to explain the observed synergy.

CONCLUSIONS

Combined proteasome inhibitors and TRAIL overcome the apoptotic threshold raised by Bcl-2 and may prove useful in the treatment of chemoresistant malignancies with up-regulated Bcl-2.