The apoptogenic response of human myeloid leukaemia cell lines and of normal and malignant haematopoietic progenitor cells to the proteasome inhibitor PSI

Protein clearance strategies for disease intervention

Protein homeostasis, or proteostasis, is essential for cell function and viability. Unwanted, damaged, misfolded and aggregated proteins are degraded by the ubiquitin–proteasome system (UPS) and the autophagy-lysosome pathway. Growing evidence indicates that alterations in these major proteolytic mechanisms lead to a demise in proteostasis, contributing to the onset and development of distinct diseases. Indeed, dysregulation of the UPS or autophagy is linked to several neurodegenerative, infectious and inflammatory disorders as well as cancer. Thus, modulation of protein clearance pathways is a promising approach for therapeutics. In this review, we discuss recent findings and open questions on how targeting proteolytic mechanisms could be applied for disease intervention.

Copper(ii) l/d-valine-(1,10-phen) complexes target human telomeric G-quadruplex motifs and promote site-specific DNA cleavage and cellular cytotoxicity

Chiral l-/d-valine-(1,10-phen)-Cu(ii) complexes that target G-quadruplex DNA were synthesized and thoroughly characterized by UV-vis, IR, EPR, ESI-MS, elemental analysis and single crystal X-ray spectroscopy. Complexes 1a and 1b crystallized in the monoclinic P21/c and C2 space groups, respectively. On the basis of Wolfe-Shimer analyses, the binding affinities of 1a and 1b with G-quadruplex telomeric DNA were determined, and 1a exhibited significantly higher binding as compared to 1b. Site selective cleavage of G4-DNA was demonstrated by employing the time-dependent PAGE assay, with 1a exhibiting a significantly higher cleavage rate from A1 to G22 (4.32 (±0.13) μM h-1) than 1b (4.29 (±0.11) μM h-1). The DNA cleavage profile demonstrated that both complexes perform non-random double-strand cleavage by following first-order kinetics (kobs = 0.9432 min-1 for 1a and kobs = 0.6574 min-1 for 1b). Molecular docking simulations were performed with both parallel and anti-parallel topologies of the quadruplex to provide a clear insight on G-quadruplex-complex interactions. Complexes 1a and 1b were found to interact strongly at the minor groove cavity of the quadruplex with preferential selectivity for the parallel vs. anti-parallel quadruplex. The cytotoxic activities of complexes 1a and 1b were evaluated on a few notably important human cancer cell lines, viz, breast (MCF-7), pancreatic strains (BxPC3, AsPC1) and liver (Huh7) by an MTT assay. Both 1a and 1b exhibited pronounced cytotoxic activity with remarkably low IC50 values (1-3 μM) for all tested cancer strains.

Proteasome Inhibitor-Induced IκB/NF-κB Activation is Mediated by Nrf2-Dependent Light Chain 3B Induction in Lung Cancer Cells

IκB, a cytoplasmic inhibitor of nuclear factor-κB (NF-κB), is reportedly degraded via the proteasome. However, we recently found that long-term incubation with proteasome inhibitors (PIs) such as PS-341 or MG132 induces IκBα degradation via an alternative pathway, lysosome, which results in NF-κB activation and confers resistance to PI-induced lung cancer cell death. To enhance the anti-cancer efficacy of PIs, elucidation of the regulatory mechanism of PI-induced IκBα degradation is necessary. Here, we demonstrated that PI upregulates nuclear factor (erythroid-derived 2)-like 2 (Nrf2) via both de novo protein synthesis and Kelch-like ECH-associated protein 1 (KEAP1) degradation, which is responsible for IκBα degradation via macroautophagy activation. PIs increased the protein level of light chain 3B (LC3B, macroautophagy marker), but not lysosome-associated membrane protein 2a (Lamp2a, the receptor for chaperone-mediated autophagy) in NCI-H157 and A549 lung cancer cells. Pretreatment with macroautophagy inhibitor or knock-down of LC3B blocked PI-induced IκBα degradation. PIs up-regulated Nrf2 by increasing its transcription and mediating degradation of KEAP1 (cytoplasmic inhibitor of Nrf2). Overexpression of dominant-negative Nrf2, which lacks an N-terminal transactivating domain, or knock-down of Nrf2 suppressed PI-induced LC3B protein expression and subsequent IκBα degradation. Thus, blocking of the Nrf2 pathway enhanced PI-induced cell death. These findings suggest that Nrf2-driven induction of LC3B plays an essential role in PI-induced activation of the IκB/NF-κB pathway, which attenuates the anti-tumor efficacy of PIs.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

(Immuno)proteasomes as therapeutic target in acute leukemia

The clinical efficacy of proteasome inhibitors in the treatment of multiple myeloma has encouraged application of proteasome inhibitor containing therapeutic interventions in (pediatric) acute leukemia. Here, we summarize the positioning of bortezomib, as first-generation proteasome inhibitor, and second-generation proteasome inhibitors in leukemia treatment from a preclinical and clinical perspective. Potential markers for proteasome inhibitor sensitivity and/or resistance emerging from leukemia cell line models and clinical sample studies will be discussed focusing on the role of immunoproteasome and constitutive proteasome (subunit) expression, PSMB5 mutations, and alternative mechanisms of overcoming proteolytic stress.

Combination of bortezomib and daunorubicin in the induction of apoptosis in T-cell acute lymphoblastic leukemia

Despite advances in the treatment of T‑cell acute lymphoblastic leukemia (T‑ALL), the outcome of T‑ALL treatment remains unsatisfactory, therefore, more effective treatment is urgently required. The present study examined the cytotoxicities of bortezomib in combination with daunorubicin against human Jurkat and Molt‑4 T‑ALL cells and primary T‑ALL cells. Compared with treatment alone, co‑exposure of cells to bortezomib and daunorubicin resulted in a significant increase in cell death in the Jurkat cells, as evidenced by the increased percentage of Annexin V‑positive cells, the formation of apoptotic bodies. In addition, the administration sequence of bortezomib and daunorubicin had an effect on cell viability. Treatment with bortezomib followed by daunorubicin treatment was more effective, compared with treatment with daunorubicin followed by bortezomib. Co-treatment with bortezomib and daunorubicin markedly enhanced the activation of caspase‑3, ‑8 and ‑9, which was reversed by the pan‑caspase inhibitor, Z‑VAD‑FMK. In addition, cotreatment with bortezomib and daunorubicin enhanced the collapse of mitochondrial transmembrane potential and upregulated the proapoptotic protein, B‑cell lymphoma 2 (Bcl‑2)‑interacting mediator of cell death (Bim), but not Bcl‑2 or Bcl‑extra large. Consistent with this, it was demonstrated that cotreatment of bortezomib and daunorubicin efficiently induced apoptosis in primary T‑ALL cells, and cell death was associated with the collapse of mitochondrial transmembrane potential and the upregulation of Bim. Taken together, these findings indicated that the combination of bortezomib and daunorubicin significantly enhanced their apoptosis‑inducing effect in T‑ALL cells, which may warrant further investigation in preclinical and clinical investigations.

Bortezomib-mediated downregulation of S-phase kinase protein-2 (SKP2) causes apoptotic cell death in chronic myelogenous leukemia cells

Background

Proteasome inhibitors are attractive cancer therapeutic agents because they can regulate apoptosis-related proteins. Bortezomib also known as Velcade^®, a proteasome inhibitor that has been approved by the food and drug administration for treatment of patients with multiple myeloma, and many clinical trials are ongoing to examine to the efficacy of bortezomib for the treatment of other malignancies. Bortezomib has been shown to induce apoptosis and inhibit cell growth of many cancer cells. In current study, we determine whether bortezomib induces cell death/apoptosis in CML.

Methods

Cell viability was measured using MTT assays. Apoptosis was measured by annexin V/PI dual staining and DNA fragmentation assays. Immunoblotting was performed to examine the expression of proteins. Colony assays were performed using methylcellulose.

Results

Treatment of CML cells with bortezomib results in downregulation of S-phase kinase protein 2 (SKP2) and concomitant stabilization of the expression of p27Kip1. Furthermore, knockdown of SKP2 with small interference RNA specific for SKP2 caused accumulation of p27Kip1. CML cells exposed to bortezomib leads to conformational changes in Bax protein, resulting in loss of mitochondrial membrane potential and leakage of cytochrome c to the cytosol. In the cytosol, cytochrome c causes sequential activation of caspase-9, caspase-3, PARP cleavage and apoptosis. Pretreatment of CML cells with a universal inhibitor of caspases, z-VAD-fmk, prevents bortezomib-mediated apoptosis. Our data also demonstrated that bortezomib treatment of CML downregulates the expression of inhibitor of apoptosis proteins. Finally, inhibition of proteasome pathways by bortezomib suppresses colony formation ability of CML cells.

Conclusions

Altogether, these findings suggest that bortezomib suppresses the cell proliferation via induction of apoptosis in CML cells by downregulation of SKP2 with concomitant accumulation of p27Kip1, suggesting that proteasomal pathway may form novel therapeutic targets for better management of CML.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0823-y) contains supplementary material, which is available to authorized users.

Proteasome inhibitor inhibits proliferation and induces apoptosis in renal interstitial fibroblasts

BACKGROUND

The ubiquitin proteasome pathway plays a pivotal role in controlling cell proliferation, apoptosis and differentiation in a variety of normal and tumor cells. This study aimed to investigate the role of a proteasome inhibitor on proliferation, apoptosis and related proteins in renal interstitial fibroblasts (NRK-49F).

METHODS

NRK-49F cells were induced using transforming growth factor-β1 (TGF-β1) and pretreated with the proteasome inhibitor MG-132. Cell proliferation was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). The cell cycle and apoptosis were analyzed using flow cytometry. Apoptosis was also analyzed using a DNA ladder. The protein expression of p53, p27, p21, caspase-3, Bcl-2 and Bax was examined using western blots.

RESULTS

The results showed that TGF-β1 (5 ng/ml) can stimulate the proliferation of NRK-49F cells.MG-132 (0.25-5 μM) inhibited TGF-β1-induced proliferation in a dose-dependent manner through G1-arrest; TGF-β1 alone did not induce apoptosis (3.8 ± 0.4% vs. 4.7 ± 1.6%). However, pretreatment with MG-132 significantly induced apoptosis in TGF-b1-stimulated NRK-49F cells in a dose-dependent manner. A typical DNA ladder was also confirmed in these two groups. Western blot analysis showed that MG-132 activated p53, p21, caspase-3 and Bax, and inhibited Bcl-2 in a dose-dependent manner, while p27 expression remained unchanged.

CONCLUSIONS

A proteasome inhibitor inhibited proliferation and induced apoptosis in renal interstitial fibroblasts stimulated by TGF-β1. The mechanism may relate to the p53, p21, caspase-3, Bcl-2 and Bax pathways. Our results suggest that a proteasome inhibitor could be a new strategy to treat renal interstitial fibrosis.

Relapsed and refractory pediatric acute myeloid leukemia: current and emerging treatments

Survival rates for children with acute myeloid leukemia (AML) exceed 60 % when modern, intensified chemotherapeutic regimens and enhanced supportive care measures are employed. Despite well-recognized improvements in outcomes, primary refractory or relapsed pediatric AML yields significant morbidity and mortality, and improved understanding of this obstinate population along with refined treatment protocols are urgently needed. Although a significant number of patients with refractory or relapsed disease will achieve remission, long-term survival rates remain poor, and efforts to identify therapies which will improve OS are under continuous investigation. The current fundamental goal of such investigation is the achievement of as complete a remission as possible without dose-limiting toxicities, and the progression to hematopoietic stem cell transplantation thereafter. In this review the scope of the problem of relapsed and refractory AML as well as current and emerging chemotherapy options will be discussed.

The Mechanistic Links Between Proteasome Activity, Aging and Age-related Diseases

Damaged and misfolded proteins accumulate during the aging process, impairing cell function and tissue homeostasis. These perturbations to protein homeostasis (proteostasis) are hallmarks of age-related neurodegenerative disorders such as Alzheimer’s, Parkinson’s or Huntington’s disease. Damaged proteins are degraded by cellular clearance mechanisms such as the proteasome, a key component of the proteostasis network. Proteasome activity declines during aging, and proteasomal dysfunction is associated with late-onset disorders. Modulation of proteasome activity extends lifespan and protects organisms from symptoms associated with proteostasis disorders. Here we review the links between proteasome activity, aging and neurodegeneration. Additionally, strategies to modulate proteasome activity and delay the onset of diseases associated to proteasomal dysfunction are discussed herein.

Synthesis of micheliolide derivatives and their activities against AML progenitor cells

Micheliolide (MCL) derivatives with etherification or esterification of the hydroxyl group at the C4 position were synthesized and evaluated for their activities against different acute myelogenous leukemia (AML) cell lines. These derivatives demonstrated comparable activities against AML cell lines HL-60 and doxorubicin resistant cell line HL-60/A. As to multi-drug resistant AML progenitor cells KG-1a, MCL and some of its derivatives maintained significant activities, and only 1.1–2.7 fold activity reductions were observed when compared with the activities against HL-60, while doxorubicin showed 20-fold activity reduction. Our study demonstrated that the C4 hydroxyl group of MCL might not only be a suitable position for structural modifications, but also be a starting point for the design of appropriate molecular probes to explore the specific targets in the progenitor cell line KG-1a.

Relapsed/Refractory multiple myeloma: defining refractory disease and identifying strategies to overcome resistance

Despite the development of more effective therapies for multiple myeloma (MM) over the past decade, nearly all patients will eventually experience disease relapse and require further therapy. Designing the next generation of therapies for relapsed and refractory disease will depend on understanding the complex molecular pathogenesis of MM and mechanisms of resistance. Oncogenomic studies have identified many potential therapeutic targets and have led to emerging models of the multistep molecular pathogenesis of MM. The key to overcoming resistance may depend on interrupting the complex interactions between MM cells and the bone microenvironment. Direct interaction between MM cells and bone marrow cells activates pleiotropic signaling pathways that mediate growth, survival, and migration of MM cells as well as resistance to chemotherapy (known as cell adhesion-mediated drug resistance). The bone marrow also secretes growth factors and cytokines that maintain MM cells and inhibit apoptosis. Therefore, successful therapeutic strategies must target not only the MM plasma cell but also the bone microenvironment. The benefit of immunomodulatory drugs such as thalidomide and lenalidomide and the proteasome inhibitor bortezomib in relapsed/refractory MM is related to their ability to target both. Novel agents and combination strategies are building on the success of these agents and targeting synergistic pathways.

WITHDRAWN: Evidence for the critical roles of NF-κB p65 and specificity proteins in the apoptosis-inducing activity of proteasome inhibitors in leukemia cells

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Proteasome and cancer

Cancer is one of the most frightful diseases mostly resulting in mortality; it has recently become more possible to overcome with the help of new therapies. In this direction, carcinogenesis is defined as a complicated process that can include several different factors that contribute to its progress. Proteasome is implicated in cancer studies as it is the main degradation system for oxidatively damaged proteins and also for several proteins playing a role in the cell cycle and transcription, which are important for cancer improvement. Because of this crucial role of proteasome in cancer development, myriad in vitro and in vivo studies have focused on the proteasome in different cancer cases. In this chapter, the involvement of proteasome in the degradation of cancer-related proteins is explained with the results of representative studies. Related to these proteins, the use of proteasome inhibitors in cancer treatment is reviewed.

Bortezomib as the first proteasome inhibitor anticancer drug: current status and future perspectives

Targeting the ubiquitin-proteasome pathway has emerged as a rational approach in the treatment of human cancer. Based on positive preclinical and clinical studies, bortezomib was subsequently approved for the clinical use as a front-line treatment for newly diagnosed multiple myeloma patients and for the treatment of relapsed/refractory multiple myeloma and mantle cell lymphoma, for which this drug has become the staple of treatment. The approval of bortezomib by the US Food and Drug Administration (FDA) represented a significant milestone as the first proteasome inhibitor to be implemented in the treatment of malignant disease. Bortezomib has shown a positive clinical benefit either alone or as a part of combination therapy to induce chemo-/radio-sensitization or overcome drug resistance. One of the major mechanisms of bortezomib associated with its anticancer activity is through upregulation of NOXA, which is a proapoptotic protein, and NOXA may interact with the anti-apoptotic proteins of Bcl-2 subfamily Bcl-X(L) and Bcl-2, and result in apoptotic cell death in malignant cells. Another important mechanism of bortezomib is through suppression of the NF-κB signaling pathway resulting in the down-regulation of its anti-apoptotic target genes. Although the majority of success achieved with bortezomib has been in hematological malignancies, its effect toward solid tumors has been less than encouraging. Additionally, the widespread clinical use of bortezomib continues to be hampered by the appearance of dose-limiting toxicities, drug-resistance and interference by some natural compounds. These findings could help guide physicians in refining the clinical use of bortezomib, and encourage basic scientists to generate next generation proteasome inhibitors that broaden the spectrum of efficacy and produce a more durable clinical response in cancer patients. Other desirable applications for the use of proteasome inhibitors include the development of inhibitors against specific E3 ligases, which act at an early step in the ubiquitin-proteasome pathway, and the discovery of less toxic and novel proteasome inhibitors from natural products and traditional medicines, which may provide more viable drug candidates for cancer chemoprevention and the treatment of cancer patients in the future.

Resveratrol protects leukemic cells against cytotoxicity induced by proteasome inhibitors via induction of FOXO1 and p27Kip1

BACKGROUND

It was reported recently that resveratrol could sensitize a number of cancer cells to the antitumoral effects of some conventional chemotherapy drugs. The current study was designed to investigate whether resveratrol could sensitize leukemic cells to proteasome inhibitors.

METHODS

Leukemic cells were treated with MG132 alone or in combination with resveratrol. Cell viability was investigated using MTT assay, and induction of apoptosis and cell cycle distribution was measured using flow cytometry. Western blot and real-time RT-PCR were used to investigate the expression of FOXO1 and p27Kip1. CHIP was performed to investigate the binding of FOXO1 to the p27 Kip1 promoter.

RESULTS

Resveratrol strongly reduced cytotoxic activities of proteasome inhibitors against leukemic cells. MG132 in combination with resveratrol caused cell cycle blockade at G1/S transition via p27Kip1 accumulation. Knockdown of p27Kip1 using siRNA dramatically attenuated the protective effects of resveratrol on cytotoxic actions of proteasome inhibitors against leukemic cells. Resveratrol induced FOXO1 expression at the transcriptional level, while MG132 increased nuclear distribution of FOXO1. MG132 in combination with resveratrol caused synergistic induction of p27Kip1 through increased recruitment of FOXO1 on the p27Kip1 promoter.

CONCLUSIONS

Resveratrol may have the potential to negate the cytotoxic effects of proteasome inhibitors via regulation of FOXO1 transcriptional activity and accumulation of p27Kip1.

Inhibition of all-trans-retinoic acid-induced proteasome activation potentiates the differentiating effect of retinoid in acute myeloid leukemia cells

All-trans retinoic acid (ATRA) is nowadays considered to be the sole efficient agent for differentiation-based therapy in leukemia; however, the mechanisms of ATRA's biological effects remain largely unknown. Here we first reported that ATRA-induced myeloid leukemia differentiation was accompanied with the increased level of ubiquitin-protein conjugates and the upregulation of proteasome activity. To explore the functional role of the activated proteasome in retinoic acid (RA) signaling, the effects of proteasome inhibitors on RA-induced cell differentiation were determined. Our results demonstrated that inhibition of ATRA-elevated proteasome activity obviously promoted the myeloid maturation program triggered by ATRA, suggesting that the overactivated proteasome is not beneficial for ATRA's effects. Further studies demonstrated that the synergistic differentiating effects of ATRA and proteasome inhibitors might be associated with the protection of retinoic acid receptor alpha (RARα) from degradation by the ubiquitin-proteasome pathway (UPP). Moreover, the accumulated RARα was able to enhance the transcription of its target gene, which might also contribute to the enhanced differentiation of leukemia cells. Together, by linking the UPP to ATRA-dependent signaling, our data provide a novel insight into studying the mechanisms of ATRA-elicited cellular effects and imply the possibility of combination of ATRA and proteasome inhibitors in leukemia therapy.

Proteasome inhibitor-induced apoptosis in acute myeloid leukemia: a correlation with the proteasome status

The proteasome plays a critical role in the regulation of many cellular processes, including the cell cycle and tumor growth. The proteasome inhibitor bortezomib has recently been approved for the treatment of relapsed and refractory multiple myeloma. In this study, we investigated the induction of apoptosis by proteasome inhibitors in several human acute myeloid leukemia (AML) cell lines and in primary cells from patients. We demonstrate that these drugs induce a high level of apoptosis in the KG1a cell line, in which the therapeutic drug daunorubicin is poorly active, compared to other AML cell lines. In parallel, we found that significantly different levels of apoptosis were induced in primary cells from patients depending on the FAB-based differentiation status of these cells. Moreover, the level of 20S proteasome in KG1a cells was also high compared to other AML cell lines, suggesting a relationship between the high sensitivity to proteasome inhibitors and an elevated amount of 20S proteasome. In good accordance, we identified two groups of patient cells expressing high and low levels of 20S proteasome, with respective high and low sensitivity to proteasome inhibitors. Further comparison of the proteasome status in KG1a and U937 cells also suggests that a high proportion of the 19S regulatory complex in U937 cells compared to the 20S core complex may explain an increased proteasome activity. Altogether, our results suggest that various AML subtypes may present different responses to proteasome inhibitors, that these molecules can be potentially considered as interesting therapeutic alternatives for these pathologies, and that the amount of 20S proteasome in AML cells may be predictive of the cellular response to these inhibitors.

Bortezomib-mediated expression of p27Kip1 through S-phase kinase protein 2 degradation in epithelial ovarian cancer

S-phase kinase protein 2 (SKP2), an F-box protein, targets cell-cycle regulators including cyclin-dependent kinase inhibitor p27Kip1 through ubiquitin-mediated degradation. SKP2 is frequently overexpressed in variety of cancers. We investigated the function of SKP2 and its ubiquitin-proteasome pathway in a large series (156) of epithelial ovarian cancer (EOC) patient samples, using a panel of cell lines, and nude mouse model. Using immunohistochemistry, we detected SKP2 in 13.2% tumor samples and found that it was inversely associated with p27Kip1. EOC subset with high level of SKP2 and low level of p27Kip1 showed a strong association with proliferative marker Ki167 (P<0.0014). Treatment of EOC cell lines with bortezomib or expression of siRNA of SKP2 causes downregulation of SKP2 and accumulation of p27Kip1. In addition, co-treatment of EOC with bortezomib and cisplatin causes more pronounced effect on cell proliferation, apoptosis and downregulation of SKP2 leading to accumulation of p27kip1. Bortezomib treatment of EOC cells causes apoptosis by involving mitochondrial pathway, activation of caspases and downregulation of XIAP, and survivin. Finally, treatment of EOC cell line xenografts with bortezomib resulted in growth inhibition of tumors in nude mice through downregulation of SKP2 and accumulation of p27Kip1. Altogether, our results suggest that SKP2 and ubiquitin-proteasome pathway may be a potential target for therapeutic intervention for treatment of EOC.

Proteasome inhibitor MG-132 mediated expression of p27Kip1 via S-phase kinase protein 2 degradation induces cell cycle coupled apoptosis in primary effusion lymphoma cells

Primary effusion lymphoma (PEL) is an incurable, aggressive B-cell malignancy that develops rapid resistance to conventional chemotherapy. MG-132, a proteasome inhibitor, suppresses cell proliferation and induces apoptosis in several PEL cell lines. Treatment of PEL cells with MG-132 results in downregulation of S-phase kinase protein 2 (SKP2) and accumulation of p27Kip1. Furthermore, MG-132 treatment of PEL cells causes Bax conformational changes, leading to loss of mitochondrial membrane potential and release of cytochrome c to the cytosole. Such cytochrome c release results in sequential activation of caspases and apoptosis, while pretreatment of PEL cells with universal inhibitor of caspases, z-VAD-fmk prevents cell death induced by MG-132. Finally, our data demonstrated in PEL cells that MG-132 downregulates the expression of inhibitor of apoptosis proteins XIAP, cIAP1 and survivin. Altogether, these findings suggest that MG-132 is a potent inducer of apoptosis of PEL cells via downregulation of SKP2 leading to accumulation of p27Kip1, resulting in cell cycle arrest and apoptosis and strongly suggest that targeting the proteasomal pathway may provide a novel therapeutic approach for the treatment of PEL.

Enhanced delivery of cisplatin to intraperitoneal ovarian carcinomas mediated by the effects of bortezomib on the human copper transporter 1

PURPOSE

The copper transporter 1 (CTR1) is a major influx transporter for platinum drugs. However, the accumulation of cisplatin in human ovarian carcinoma cells is limited by the fact that cisplatin triggers the down-regulation and proteasomal degradation of CTR1, thereby limiting its own uptake. We sought to determine whether proteasome inhibition using bortezomib would prevent human CTR1 (hCTR1) degradation and increase platinum accumulation in ovarian cancer cells.

EXPERIMENTAL DESIGN

The effects of bortezomib on human hCTR1 expression and cisplatin accumulation were measured by Western blot, flow cytometric, and confocal digital imaging analyses. Platinum accumulation was measured by inductively coupled plasma mass spectrometry and bortezomib concentrations by liquid chromatography/mass spectrometry.

RESULTS

Bortezomib blocked the cisplatin-induced down-regulation of hCTR1 in a concentration-dependent manner and increased cisplatin uptake 1.6- to 2.4-fold. Median effect analysis showed a combination index of 0.37 at 50% cell kill, indicating a high level of synergy. The effect of bortezomib was muted in cells lacking both alleles of CTR1, showing that bortezomib was working primarily through its effect on blocking hCTR1 degradation. I.p. administration of bortezomib produced a peritoneal/plasma area under the curve ratio of 252 in a murine model. I.p. administration of bortezomib before i.p. cisplatin increased platinum accumulation in peritoneal tumors by 33% (P = 0.006).

CONCLUSIONS

Proteasomal inhibition prevented cisplatin-induced down-regulation of hCTR1 in ovarian cancer cells and enhanced drug uptake and cell killing in a synergistic manner. Bortezomib shows a large pharmacologic advantage when administered i.p. There is a strong rationale for the combined i.p. administration of bortezomib and cisplatin.

Synergy between proteasome inhibitors and imatinib mesylate in chronic myeloid leukemia

Background

Resistance developed by leukemic cells, unsatisfactory efficacy on patients with chronic myeloid leukemia (CML) at accelerated and blastic phases, and potential cardiotoxity, have been limitations for imatinib mesylate (IM) in treating CML. Whether low dose IM in combination with agents of distinct but related mechanisms could be one of the strategies to overcome these concerns warrants careful investigation.

Methods and Findings

We tested the therapeutic efficacies as well as adverse effects of low dose IM in combination with proteasome inhibitor, Bortezomib (BOR) or proteasome inhibitor I (PSI), in two CML murine models, and investigated possible mechanisms of action on CML cells. Our results demonstrated that low dose IM in combination with BOR exerted satisfactory efficacy in prolongation of life span and inhibition of tumor growth in mice, and did not cause cardiotoxicity or body weight loss. Consistently, BOR and PSI enhanced IM-induced inhibition of long-term clonogenic activity and short-term cell growth of CML stem/progenitor cells, and potentiated IM-caused inhibition of proliferation and induction of apoptosis of BCR-ABL+ cells. IM/BOR and IM/PSI inhibited Bcl-2, increased cytoplasmic cytochrome C, and activated caspases. While exerting suppressive effects on BCR-ABL, E2F1, and β-catenin, IM/BOR and IM/PSI inhibited proteasomal degradation of protein phosphatase 2A (PP2A), leading to a re-activation of this important negative regulator of BCR-ABL. In addition, both combination therapties inhibited Bruton's tyrosine kinase via suppression of NFκB.

Conclusion

These data suggest that combined use of tyrosine kinase inhibitor and proteasome inhibitor might be helpful for optimizing CML treatment.

Wheat sprout extract-induced apoptosis in human cancer cells by proteasomes modulation

Natural occurring modulators of proteasome functionality are extensively investigated for their implication in cancer therapy. On the basis of our previous evidences both on proteasomal inhibition by monomeric polyphenols, and on the characterization of wheat sprout hydroalcoholic extract, herein we thoroughly report on a comparative study of the effect of wheat sprout extract on both normal and tumour cells. Treatment of isolated 20S proteasomes with wheat sprout extracts induced a gradual inhibition of all proteasome activities. Next, two wheat sprout extract components were separated: a polyphenol and a protein fraction. Both components exerted an in vitro inhibitory effect on proteasome activity. HeLa tumour cells and FHs 74 Int normal cells were exposed to both fractions, resulting in different rates of proteasome inhibition, with tumour cells showing a significantly higher degree of proteasome impairment and apoptosis induction. Furthermore, a decrease in proteasome activities and in cell survival of the human plasmacytoma RPMI 8226 cell line, upon the same treatments, was observed. Collectively, our results provide additional evidences supporting the possible use of natural extracts as coadjuvants in cancer treatments.

The tumor proteasome as a novel target for gold(III) complexes: implications for breast cancer therapy

Although cisplatin plays a vital role in the treatment of several types of human cancer, its wide use is limited by the development of drug resistance and associated toxic side effects. Gold and gold complexes have been used to treat a wide range of ailments for many centuries. In recent years, the use of gold(III) complexes as an alternative to cisplatin treatment was proposed due to the similarities of gold and platinum. Gold(III) is isoelectronic with platinum(II) and gold(III) complexes have the same square-planar geometries as platinum(II) complexes, such as cisplatin. Although it was originally thought that gold(III) complexes might have the same molecular target as cisplatin, several lines of data indicated that proteins, rather than DNA, are targeted by gold complexes. We have recently evaluated cytotoxic and anti-cancer effects of several gold(III) dithiocarbamates against human breast cancer cells in vitro and in vivo. We have identified the tumor proteasome as an important target for gold(III) complexes and have shown that proteasome inhibition by gold(III) complexes is associated with apoptosis induction in breast cancer cells in vitro and in vivo. Furthermore, treatment of human breast tumor-bearing nude mice with a gold(III) dithiocarbamate complex was associated with tumor growth inhibition, supporting the significance of its potential development for breast cancer treatment.

The therapeutic potential of the proteasome in leukaemia

Many cellular processes converge on the proteasome, and its key regulatory role is increasingly being recognized. Proteasome inhibition allows the manipulation of many cellular pathways including apoptotic and cell cycle mechanisms. The proteasome inhibitor bortezomib has enhanced responses in newly diagnosed patients with myeloma and provides a new line of therapy in relapsed and refractory patients. Malignant cells are more sensitive to proteasome inhibition than normal haematopoietic cells. Proteasome inhibition enhances many conventional therapies and its role in leukaemia is promising.

Bortezomib (Velcade) induces p27Kip1 expression through S-phase kinase protein 2 degradation in colorectal cancer

S-phase kinase protein 2 (SKP2), an F-box protein, targets cell cycle regulators including cycle-dependent kinase inhibitor p27Kip1 via ubiquitin-mediated degradation. SKP2 is frequently overexpressed in a variety of cancers. We investigated the role of SKP2 and its ubiquitin-proteasome pathway in colorectal carcinoma using a panel of cell lines, clinical samples, and the NUDE mouse model. Using immunohistochemical analysis on a large tissue microarray of 448 samples, an inverse association of SKP2 expression with p27Kip1 protein levels was seen. A colorectal cancer (CRC) subset with high level of SKP2 and low level of p27Kip1 showed a decreased overall survival (P = 0.0057). Treatment of CRC cell lines with bortezomib or expression of small interfering RNA of SKP2 causes down-regulation of SKP2 and accumulation of p27Kip1. Furthermore, treatment of CRC cells with bortezomib causes apoptosis by involving the mitochondrial pathway and activation of caspases. In addition, treatment of CRC cells with bortezomib down-regulated the expression of XIAP, cIAP1, and survivin. Finally, treatment of CRC cell line xenografts with bortezomib resulted in growth inhibition of tumors in NUDE mice via down-regulation of SKP2 and accumulation of p27Kip1. Altogether, our results suggest that SKP2 and the ubiquitin-proteasome pathway may be potential targets for therapeutic intervention for treatment of CRC.

Natural compounds with proteasome inhibitory activity for cancer prevention and treatment

The proteasome is a multicatalytic protease complex that degrades most endogenous proteins including misfolded or damaged proteins to ensure normal cellular function. The ubiquitin-proteasome degradation pathway plays an essential role in multiple cellular processes, including cell cycle progression, proliferation, apoptosis and angiogenesis. It has been shown that human cancer cells are more sensitive to proteasome inhibition than normal cells, indicating that a proteasome inhibitor could be used as a novel anticancer drug. Indeed, this idea has been supported by the encouraging results of the clinical trials using the proteasome inhibitor Bortezomib (Velcade, PS-341), a drug approved by the US Food and Drug Administration (FDA). Several natural compounds, including the microbial metabolite lactacystin, green tea polyphenols, and traditional medicinal triterpenes, have been shown to be potent proteasome inhibitors. These findings suggest the potential use of natural proteasome inhibitors as not only chemopreventive and chemotherapeutic agents, but also tumor sensitizers to conventional radiotherapy and chemotherapy. In this review, we will summarize the structures and biological activities of the proteasome and several natural compounds with proteasome inhibitory activity, and will discuss the potential use of these compounds for the prevention and treatment of human cancers.

The protein degradation response of Saccharomyces cerevisiae to classical DNA-damaging agents

Genome wide experiments indicate both proteasome- and vacuole-mediated protein degradation modulate sensitivity to classical DNA-damaging agents. Here, we show that global protein degradation is significantly increased upon methyl methanesulfonate (MMS) exposure. In addition, global protein degradation is similarly increased upon exposure to 4-nitroquinoline-N-oxide (4NQO) and UV and, to a lesser extent, tert-butyl hydroperoxide. The proteasomal inhibitor MG132 decreases both MMS-induced and 4NQO-induced protein degradation, while addition of the vacuolar inhibitor phenylmethanesulfonyl fluoride does not. The addition of both inhibitors grossly inhibits cell growth upon MMS exposure over and above the growth inhibition induced by MMS alone. The MMS-induced protein degradation response remains unchanged in several ubiquitin-proteasome and vacuolar mutants, presumably because these mutants are not totally deficient in either essential pathway. Furthermore, MMS-induced protein degradation is independent of Mec1, Mag1, Rad23, and Rad6, suggesting that the protein degradation response is not transduced through the classical Mec1 DNA damage response pathway or through repair intermediates generated by the base excision, nucleotide excision, or postreplication-DNA repair pathways. These results identify the regulation of protein degradation as an important factor in the recovery of cells from toxicity induced by classical DNA-damaging agents.

FoxO tumor suppressors and BCR-ABL-induced leukemia: a matter of evasion of apoptosis

Numerous studies have revealed that the BCR-ABL oncoprotein abnormally engages a multitude of signaling pathways, some of which may be important for its leukemogenic properties. Central to this has been the determination that the tyrosine kinase function of BCR-ABL is mainly responsible for its transforming potential, and can be targeted with small molecule inhibitors, such as imatinib mesylate (Gleevec, STI-571). Despite this apparent success, the development of clinical resistance to imatinib therapy, and the inability of imatinib to eradicate BCR-ABL-positive malignant hematopoietic progenitors demand detailed investigations of additional effector pathways that can be targeted for CML treatment. The promotion of cellular survival via the suppression of apoptotic pathways is a fundamental characteristic of tumor cells that enables resistance to anti-cancer therapies. As substrates of survival kinases such as Akt, the FoxO family of transcription factors, particularly FoxO3a, has emerged as playing an important role in the cell cycle arrest and apoptosis of hematopoietic cells. This review will discuss our current understanding of BCR-ABL signaling with a focus on apoptotic suppressive mechanisms and alternative approaches to CML therapy, as well as the potential for FoxO transcription factors as novel therapeutic targets.

Apoptosis induction of 2′-hydroxycinnamaldehyde as a proteasome inhibitor is associated with ER stress and mitochondrial perturbation in cancer cells

2'-Hydroxycinnamaldehyde (HCA), isolated from the stem bark of Cinnamomum cassia, and 2'-benzoyloxycinnamaldehyde (BCA), one of HCA derivatives, have antiproliferative activities on several human cancer cell lines. Our previous study suggested that reactive oxygen species (ROS) and caspase-3 are the major regulators of HCA-induced apoptosis. In the present study, we demonstrated a novel molecular target using in vitro pull-down assay by biotin-labeled HCA (biotin-HCA) in SW620 cells. We analyzed 11 differential spots of 2-dimensional gel prepared with pull-downed proteins by biotin-HCA. Among them, five spots were identified as proteasome subunits. An in vitro 26S proteasome function assay using specific fluorogenic substrates showed that HCA potently inhibits L3-like activity of the proteasome. In addition, HCA showed inhibitory action against chymotrypsin-like, trypsin-like, and PGPH-like activities. DNA microarray showed that HCA induced heat shock family and ER stress-responsive genes, which reflects the accumulation of misfolded proteins by proteasome inhibition. On western blot analysis, it was confirmed that HCA induces glucose-regulated protein, 78 kDa (GRP78) and some representative endoplasmic reticulum (ER) stress-responsive proteins. Furthermore, HCA treatment decreased mitochondrial membrane potential. The effect of HCA on cytochrome c and Bax translocation between cytosol and mitochondrial membrane was clarified using western blot analysis. These results suggest that HCA-induced apoptosis is associated with the inhibition of the proteasome activity that leads in turn to the increase of ER stress and mitochondrial perturbation.

Therapeutic targets in chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) is a clonal disorder of the haemopoietic stem cell arising as a consequence of the formation of the bcr-abl oncogene. The particular molecular basis of this condition has enabled the development of therapies that selectively target diseased cells. The success of the rationally designed first-line therapy imatinib mesylate (IM) is tempered by the problems of disease persistence and resistance. Novel strategies have been identified to take forward therapy in CML and these will be discussed in this review. This work is generated from a review of published literature and contains particular insight into the work performed by our group in this field.

A phase 1 study of the proteasome inhibitor bortezomib in pediatric patients with refractory leukemia: a Children's Oncology Group study

PURPOSE

A phase 1 study to determine the maximum-tolerated dose, dose-limiting toxicity, pharmacokinetics, and biological effects of bortezomib in children with recurrent/refractory leukemia.

EXPERIMENTAL DESIGN

Bortezomib was administered twice weekly for 2 consecutive weeks at either 1.3 or 1.7 mg/m(2) dose followed by a 1-week rest. Bortezomib pharmacokinetics and nuclear factor kappaB (NF-kappaB) binding activity were evaluated during the first treatment cycle.

RESULTS

Twelve patients (nine with acute lymphoblastic leukemia, three with acute myelogenous leukemia), median age 11 years (range, 1-18 years), were enrolled between May 2004 and November 2005, of whom seven were not fully evaluable for toxicity due to rapidly progressive disease or uncontrolled infection. Dose-limiting toxicities occurred in two patients at the 1.7 mg/m(2) dose level. One patient experienced grade 3 confusion and the other patient had grade 4 febrile neutropenia associated with grade 4 hypotension and grade 3 creatinine. Pharmacokinetic analysis at 1.3 mg/m(2) revealed a clearance of 11 mL/h/m(2), a central volume of distribution of 6.7 L/m(2), and a terminal half-life of 12.6 h. NF-kappaB activity was examined in five patients and was noted to transiently increase and then decrease 4- to 6-fold by 24 h following bortezomib in two patients. There were no objective clinical responses.

CONCLUSIONS

For children with leukemia, the recommended phase 2 dose of bortezomib, administered twice weekly for 2 weeks followed by a 1-week rest, is 1.3 mg/m(2)/dose. Although bortezomib treatment inhibited NF-kappaB activity, bortezomib had little activity as a single agent in this population.

New molecular therapy targets in acute myeloid leukemia

Despite improvements to acute myelogenous leukemia (AML) therapy during the last 25 years, the majority of patients still succumb to the disease. Thus, there remains an urgent need for further improvements in this field. The present chapter focuses on exciting areas of research in the field of AML therapy, including promising results with regards to recent improvements in our understanding of angiogenesis, tyrosine kinase signaling, farnesylation, cell cycling, modulation of gene expression, protein degradation, modulation of intracellular proteins, apoptosis, metabolism, and the possible retargeting of oncogenic proteins.

Molecular and clinical aspects of proteasome inhibition in the treatment of cancer

The proteasome is a multicatalytic threonine protease responsible for intracellular protein turnover in eukaryotic cells, including the processing and degradation of several proteins involved in cell cycle control and the regulation of apoptosis. Preclinical studies have shown that the treatment with proteasome inhibitors results in decreased proliferation, induction of apoptosis, and sensitization of tumor cells against conventional chemotherapeutic agents and irradiation. The effects were conferred to stabilization of p21, p27, Bax, p53, I-KB, and the resulting inhibition of the nuclear factor-KB (NF-KB) activation. Bortezomib is the first proteasome inhibitor that has entered clinical trials. In multiple myeloma, both the FDA (United States Food and Drug Administration) and EMEA (European Medicine Evaluation Agency) granted an approval for the use of bortezomib (Velcade, Millennium Pharmaceuticals, Cambridge, MA, USA) for the treatment of relapsed multiple myeloma. At present, clinical trials are examining the activity in a variety of solid tumors and hematological malignancies.

Comparison of biochemical and biological effects of ML858 (salinosporamide A) and bortezomib

Strains within the genus Salinospora have been shown to produce complex natural products having antibiotic and antiproliferative activities. The biochemical basis for the cytotoxic effects of salinosporamide A has been linked to its ability to inhibit the proteasome. Synthetically accessible salinosporamide A (ML858) was used to determine its biochemical and biological activities and to compare its effects with those of bortezomib. ML858 and bortezomib show time- and concentration-dependent inhibition of the proteasome in vitro. However, unlike bortezomib, which is a reversible inhibitor, ML858 covalently binds to the proteasome, resulting in the irreversible inhibition of 20S proteasome activity. ML858 was equipotent to bortezomib in cell-based reporter stabilization assays, but due to intramolecular instability is less potent in long-term assays. ML858 failed to maintain levels of proteasome inhibition necessary to achieve efficacy in tumor models responsive to bortezomib. Our results show that ML858 and bortezomib exhibit different kinetic and pharmacologic profiles and suggest that additional characterization of ML858 is warranted before its therapeutic potential can be fully appreciated.

Biology and management of malignant pleural mesothelioma

Malignant mesothelioma is an aggressive tumour, with a poor prognosis and an increasing incidence as a result of widespread exposure to asbestos. The results of the treatments available are poor. Surgery and radiotherapy have a limited role in highly selected patients and systemic therapy is the only potential treatment option for the majority of patients. Despite some definite activity of the novel antifolates such as pemetrexed and raltitrexed, the results, even in combination with platinating agents, are still modest, with a median survival of approximately one year. The better understanding of the biology of mesothelioma makes the assessment of a number of targeted agents particularly interesting. Unfortunately, the targeted agents imatinib, gefitinib, erlotinib and thalidomide have been shown to be ineffective in unselected patients. Studies with anti-angiogenesis agents are ongoing. An improvement of the knowledge of major molecular pathways involved in malignant mesothelioma is needed in order to define proper targets for the systemic treatment of this disease.

The proteasome: a novel target for anticancer therapy

The proteasome is an ubiquituous enzyme complex that plays a critical role in the degradation of many proteins involved in cell cycle regulation, apoptosis and angiogenesis. Since these pathways are fundamental for cell survival and proliferation, particularly in cancer cells, the inhibition of proteasome is an attractive potential anticancer therapy. Bortezomib (Velcade, formerly PS-341) is an extremely potent and selective proteasome inhibitor that shows strong activity in in vitro and in vivo laboratory studies against many solid and hematologic tumor types. Moreover, bortezomib, mainly by inhibition of the NF-kappaB pathway, has a chemosensitizing effect when administered together with other antitumoral drugs. Clinical phase I trials, showed good tolerance of bortezomib at doses that achieved a desired degree of proteasome inhibition. Phase II studies showed high response rates in refractory multiple myeloma patients, which led to the accelerated approval of bortezomib by the Food and Drug Administration (FDA) and the European Medicines Agency (EMEA) for this indication. A phase III trial comparing bortezomib to dexamethasone in refractory/relapsed multiple myeloma patients had to be halted due to a survival advantage in the bortezomib arm. Additional studies are focusing in the potential benefit of bortezomib in newly diagnosed multiple myeloma patients. In other solid and hematological malignancies, phase II studies with bortezomib alone or in combination are ongoing with encouraging results, particularly in lung cancer and lymphoma.

Proteasome inhibition as novel treatment strategy in leukaemia

Following its success in multiple myeloma (MM), proteasome inhibition has become a topic of interest as novel treatment strategy of cancer. By simultaneously affecting multiple pathways in the cancer cell, such as deregulation of the programmed degradation of many cellular proteins, proteasome inhibition causes rapid apoptosis of these cells. Both in rapidly proliferating leukaemic cell lines and in primary leukaemic cells isolated from patients, proteasome inhibition results in antileukaemic activity. The normal counterparts of these cells are much more resistant to proteasome inhibitors (PI), thereby resulting in a favourable therapeutic index. Importantly, while leukaemic stem cells are sensitive to proteasome inhibition, normal haematopoietic stem cells are still viable after drug exposure. Nowadays, many PIs are being identified; bortezomib is the most well known since obtaining Food and Drug Administration approval for clinical use in MM. This review summarises the biological and clinical aspects of proteasome inhibition and discusses the potential role of these inhibitors in the treatment of leukaemia.

Ubiquitin-Proteasome System Stress Sensitizes Ovarian Cancer to Proteasome Inhibitor–Induced Apoptosis

The ubiquitin-proteasome system (UPS) mediates targeted protein degradation. Notably, the UPS determines levels of key checkpoint proteins controlling apoptosis and proliferation by controlling protein half-life. Herein, we show that ovarian carcinoma manifests an overstressed UPS by comparison with normal tissues by accumulation of ubiquitinated proteins despite elevated proteasome levels. Elevated levels of total ubiquitinated proteins and 19S and 20S proteasome subunits are evident in both low-grade and high-grade ovarian carcinoma tissues relative to benign ovarian tumors and in ovarian carcinoma cell lines relative to immortalized surface epithelium. We find that ovarian carcinoma cell lines exhibit greater sensitivity to apoptosis in response to proteasome inhibitors than immortalized ovarian surface epithelial cells. This sensitivity correlates with increased cellular proliferation rate and UPS stress rather than absolute proteasome levels. Proteasomal inhibition in vitro induces cell cycle arrest and the accumulation of p21 and p27 and triggers apoptosis via activation of caspase-3. Furthermore, treatment with the licensed proteasome inhibitor PS-341 slows the growth of ES-2 ovarian carcinoma xenograft in immunodeficient mice. In sum, elevated proliferation and metabolic rate resulting from malignant transformation of the epithelium stresses the UPS and renders ovarian carcinoma more sensitive to apoptosis in response to proteasomal inhibition.

Proteasome inhibitor PSI induces apoptosis in human mesothelioma cells

Malignant mesothelioma is an increasingly common tumor with an almost 100% mortality rate. It is refractory to conventional treatment. We have previously shown with SSH and microarray that the mRNA expression level of proteasome is higher in epithelioid mesothelioma cell lines than in sarcomatoid ones. This study evaluates the differential apoptotic effect of proteasome inhibitors on both of these mesothelioma sub-lines. Proteasome inhibitors show substantial anti-tumor activity in some tumor cells in vitro and in vivo, but the effects on mesothelioma cells has not been studied. The viability of mesothelioma cells was reduced in a dose- and time-dependent manner by the proteasome inhibitors tested; PSI was effective with a low dose, but higher concentrations were needed for calpain inhibitor I. The epithelioid mesothelioma cells are more sensitive to the inhibitors than the sarcomatoid ones, their IC50 after 24 h of treatment with PSI being 4 and 16 microm, respectively. Other mesothelioma cell lines show similar sensitivity. PSI seemed to decrease mesothelioma viability by inducing apoptosis, as verified by cell morphology, Western blotting analysis of caspase 3 cleavage, and flow-cytometric analysis. In conclusion, PSI, a representative agent that reduces viability and induces apoptosis of mesothelioma cells, might be useful in the treatment of patients with mesothelioma, especially of epithelioid phenotype.

THE PROTEASOME AND PROTEASOME INHIBITORS IN CANCER THERAPY

The proteasome, a multicatalytic proteinase complex, is responsible for the majority of intracellular protein degradation. Pharmacologic inhibitors of the proteasome possess in vitro and in vivo antitumor activity, and bortezomib, the first such agent to undergo clinical testing, has significant efficacy against multiple myeloma and non-Hodgkin lymphoma (NHL). Preclinical studies demonstrate that proteasome inhibition potentiates the activity of other cancer therapeutics, in part by downregulating chemoresistance pathways. Early clinical studies of bortezomib-based combinations, showing encouraging activity, support this observation. Molecular characterization of resistance to proteasome inhibitors has revealed novel therapeutic targets for sensitizing malignancies to these agents, such as the heat shock pathway. Below, we review the pharmacologic, preclinical, and clinical data that have paved the way for the use of proteasome inhibitors for cancer therapy; outline strategies aimed at enhancing the efficacy of proteasome inhibitors; and review other potential targets in the ubiquitin proteasome pathway for the treatment of cancer.

Sensitivity of human multiple myelomas and myeloid leukemias to the proteasome inhibitor I

The proteasome inhibitor PSI is potently cytotoxic in vitro against human chronic myeloid leukemia (CML) and acute myeloid leukemias (AML). Here, we have tested proteasome inhibitor I (PSI) in a panel of 11 human multiple myeloma (MM) cell lines and found that it has antiproliferative activity, with an IC50 between 4.5 and 557 nM at 48 h. PSI potentiated the toxicity of a number of chemotherapeutic agents in myeloid leukemia but not in MM cell lines, while in combination with therapeutic proteasome inhibitor PS-341 (Bortezomib) it had a synergistic effect. PSI suppressed the growth of AML cell lines more effectively than PS-341. CFU-GM colony assays revealed that CD34+ bone marrow progenitors from CML and AML patients were more sensitive to PSI than those from normal subjects (IC50: 5, 15 and 50 nM for AML, CML and normal, respectively). Moreover, the growth of normal primitive progenitors (LTC-IC) was unaffected by 15 nM PSI (P=0.576). PSI-induced cell death required RNA transcription and protein synthesis, but not DNA replication, was accompanied by the upregulation of Bcl-2 and modest reduction of Bax and Bcl-XL proteins, and involved the activation of caspases 2, 3, 7 and 8. These findings lend additional support to preclinical investigations with PSI.

Bortezomib interactions with chemotherapy agents in acute leukemia in vitro

Although there is effective chemotherapy for many patients with leukemia, 20% of children and up to 65% of adults relapse. Novel therapies are needed to treat these patients. Leukemia cells are very sensitive to the proteasome inhibitor bortezomib (VELCADE(R), PS-341), which enhances the in vitro cytotoxic effects of dexamethasone and doxorubicin in multiple myeloma. To determine if bortezomib enhances the cytotoxicity of agents used in leukemia, we employed an in vitro tetrazolium-based colorimetric assay (MTT) to evaluate the cytotoxic effects of bortezomib alone and in combination with dexamethasone, vincristine, doxorubicin, cytarabine, asparaginase, geldanamycin, trichostatin A, and the bcl-2 inhibitor HA14.1. We demonstrated that primary leukemia lymphoblasts and leukemia cell lines are sensitive to bortezomib, with an average IC(50) of 12 nM. Qualitative and quantitative bortezomib-drug interactions were evaluated using the universal response surface approach (URSA). Bortezomib was synergistic with dexamethasone in dexamethasone-sensitive leukemia cells, and additive with vincristine, asparaginase, cytarabine, and doxorubicin. The anti-leukemic activity of bortezomib was also additive with geldanamycin and HA14.1, and additive or synergistic with trichostatin A. These results were compared to analysis using the median-dose effect method, which generated complex drug interactions due to differences in dose-response curve sigmoidicities. These data suggest bortezomib could potentiate the cytotoxic effects of combination chemotherapy in patients with leukemia.

Distinct molecular phenotype of malignant CD34(+) hematopoietic stem and progenitor cells in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a malignant disorder of the hematopoietic stem cell characterized by the BCR-ABL oncogene. We examined gene expression profiles of highly enriched CD34(+) hematopoietic stem and progenitor cells from patients with CML in chronic phase using cDNA arrays covering 1.185 genes. Comparing CML CD34(+) cells with normal CD34(+) cells, we found 158 genes which were significantly differentially expressed. Gene expression patterns reflected BCR-ABL-induced functional alterations such as increased cell-cycle and proteasome activity. Detoxification enzymes and DNA repair proteins were downregulated in CML CD34(+) cells, which might contribute to genetic instability. Decreased expression of junction plakoglobulin and CXC chemokine receptor 4 (CXCR-4) might facilitate the release of immature precursors from bone marrow in CML. GATA-2 was upregulated in CML CD34(+) cells, suggesting an increased self-renewal in comparison with normal CD34(+) cells. Moreover, we found upregulation of the proto-oncogene SKI and of receptors for neuromediators such as opioid mu1 receptor, GABA B receptor, adenosine A1 receptor, orexin 1 and 2 receptors and corticotropine-releasing hormone receptor. Treatment of CML progenitor cells with the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) resulted in a dose-dependent significant inhibition of clonogenic growth by 40% at a concentration of 10(-5) M, which could be reversed by the equimolar addition of the receptor agonist 2-chloro-N6-cyclopentyladenosine (P<0.05). The incubation of normal progenitor cells with DPCPX resulted in an inhibition of clonogenic growth to a significantly lesser extent in comparison with CML cells (P<0.05), suggesting that the adenosine A1 receptor is of functional relevance in CML hematopoietic progenitor cells.

Effects of the proteasome inhibitor bortezomib on osteolytic human prostate cancer cell metastases

Prostate adenocarcinoma is the most common malignancy diagnosed in males, and bone metastases remain a significant source of morbidity and mortality in this population. The ubiquitin-proteasome cascade is responsible for the degradation of intracellular proteins, and this pathway is thought to play an essential role in the development of malignancies by altering the levels of various proteins involved in the regulation of cell division. Proteasome inhibitors represent a class of chemotherapeutic agents that have been shown to inhibit tumor growth by a number of different mechanisms. Using a murine intratibial injection model, we examined the effects of the proteasome inhibitor bortezomib on the establishment and progression of osteolytic bone lesions induced by human CaP cells (PC-3 cell line). In this study, the intravenous administration of bortezomib (1 mg/kg) did not prevent the initial formation of osteolytic lesions but did appear to inhibit their growth in a time-dependent fashion. In contrast, bortezomib therapy effectively inhibited the establishment and progression of subcutaneous PC-3 tumors, which served as a positive control. These results suggest that proteasome inhibitors such as bortezomib may represent a novel adjunctive therapy for the treatment of osteolytic skeletal metastases, especially when treatment is initiated early during the disease process.

Discovering significant and interpretable patterns from multifactorial DNA microarray data with poor replication

MOTIVATION

Multivariate analyses are advantageous for the simultaneous testing of the separate and combined effects of many variables and of their interactions. In factorial designs with many factors and/or levels, however, sufficient replication is often prohibitively costly. Furthermore, complicated statements are often required for the biological interpretation of the higher-order interactions determined by standard statistical techniques like analysis of variance.

RESULTS

Because we are usually interested in finding factor-specific effects or their interactions, we assumed that the observed expression profile of a gene is a manifestation of an underlying factor-specific generative pattern (FSGP) combined with noise. Thus, a genetic algorithm was created to find the nearest FSGP for each expression profile. We then measured the distance between each profile and the corresponding nearest FSGP. Permutation testing for the distance measures successfully identified those genes with statistically significant profiles, thus yielding straightforward biological interpretations. Association networks of genes, drugs, and cell lines were created as tripartite graphs, representing significant and interpretable relations, by using a microarray experiment of gastric-cancer cell lines with a factorial design and no replication. The proposed method may benefit the combined analysis of heterogeneous expression data from the growing public repositories.

The Hematopoietic Stem Cell in Myelodysplasia

The mechanisms underlying hematopoietic stem cell or progenitor cell abnormalities in myelodysplastic syndromes (MDSs) remain poorly characterized. Current evidence exists for multiple intrinsic and extrinsic influences upon the stem cell in these disorders. These influences are outlined in this review and include: stem cell characteristics in MDSs, as compared with those in acute myelogenous leukemia; the role of increased apoptosis; the role of signaling pathway abnormalities; the influences of immune modulation; and the effect of stromal cells and stromal cell cytokine production. Despite numerous studies that have examined these factors, how they converge to produce a situation in which accelerated proliferation and accelerated death occur simultaneously remains largely an unexplored area. It is anticipated that future studies that focus on well-characterized and purified progenitor populations in these disorders will elucidate the process by which ineffective hematopoiesis results from the influences of stem cell abnormalities versus abnormalities in the stem cell's microenvironmental and immunologic milieu.