Mechanisms of proteasome inhibitor action and resistance in cancer

Rad knockdown induces mitochondrial apoptosis in bortezomib resistant leukemia and lymphoma cells

To understand the molecular mechanism(s) underlying bortezomib resistance, we sought to identify potential target genes that were differentially expressed in bortezomib-resistant leukemia cells versus parental controls. Microarray analysis revealed that the mRNA levels of Rad (Ras associated with diabetes) were higher in the bortezomib-resistant Jurkat (Jurkat-R) cells than in the parental control cells. The importance of Rad for bortezomib resistance was supported by three observations. First, Rad knockdown overcame bortezomib resistance and induced mitochondrial apoptosis via Noxa/Bcl-2 modulation. Second, Rad decreased cell death in response to bortezomib. Third, leukemia and lymphoma cell lines (K-562, Raji, IM-9 and Jurkat-R) with elevated Rad expression levels showed higher degrees of bortezomib resistance versus those (Sup-B15, JVM-2, U266 and Jurkat) with low Rad expression levels (r=0.48, P=0.0004). Thus, Rad over expression could be a molecular target to improve bortezomib sensitivity in human leukemia and lymphoma.

Novel therapy for therapy-resistant mantle cell lymphoma: multipronged approach with targeting of hedgehog signaling

Mantle cell lymphoma (MCL) is one of the most aggressive B-cell lymphomas with a median patient survival of only 5-7 years. The failure of existing therapies is mainly due to disease relapse when therapy-resistant tumor cells remain after chemotherapy. Therefore, development and testing of novel therapeutic strategies to target these therapy-resistant MCL are needed. Here, we developed an in vivo model of therapy-resistant MCL by transplanting a patient-derived MCL cell line (Granta 519) into NOD/SCID mice followed by treatment with combination chemotherapy. Cytomorphologic, immunophenotypic, in vitro and in vivo growth analyses of these therapy-resistant MCL cells confirm their MCL origin and resistance to chemotherapy. Moreover, quantitative real-time PCR revealed the upregulation of GLI transcription factors, which are mediators of the hedgehog signaling pathway, in these therapy-resistant MCL cells. Therefore, we developed an effective therapeutic strategy for resistant MCL by treating the NOD/SCID mice bearing Granta 519 MCL with CHOP chemotherapy to reduce tumor burden combined with GLI-antisense oligonucleotides or bortezomib, a proteosome inhibitor, to target therapy-resistant MCL cells that remained after chemotherapy. This regimen was followed by treatment with MCL-specific cytotoxic T lymphocytes to eliminate all detectable leftover minimal residual disease. Mice treated with this strategy showed a significantly increased survival and decreased tumor burden compared to the mice in all other groups. Such therapeutic strategies that combine chemotherapy with targeted therapy followed by tumor-specific immunotherapy are effective and have excellent potential for clinical application to provide long-term, disease-free survival in MCL patients.

CIP2A is a target of bortezomib in human triple negative breast cancer cells

Introduction

Triple negative breast cancer (TNBC) is very aggressive and currently has no specific therapeutic targets, such as hormone receptors or human epidermal growth factor receptor type 2 (HER2); therefore, prognosis is poor. Bortezomib, a proteasome inhibitor, may exert efficacy in TNBC through its multiple cellular effects. Here, we tested the efficacy of bortezomib and examined the drug mechanism in breast cancer cells.

Methods

Five breast cancer cell lines: TNBC HCC-1937, MDA-MB-231, and MDA-MB-468; HER2-overexpressing MDA-MB-453; and estrogen receptor positive MCF-7 were used for in vitro studies. Apoptosis was examined by both flow cytometry and Western Blot. Signal transduction pathways in cells were assessed by Western Blot. Gene silencing was done by small interfering RNA (siRNA). In vivo efficacy of bortezomib was tested in nude mice with breast cancer xenografts. Immunohistochemical study was performed on tumor tissues from patients with TNBC.

Results

Bortezomib induced significant apoptosis, which was independent of its proteasome inhibition, in the three TNBC cell lines, but not in MDA-MB-453 or MCF-7 cells. Furthermore, cancerous inhibitor of protein phosphatase 2A (CIP2A), a cellular inhibitor of protein phosphatase 2A (PP2A), mediated the apoptotic effect of bortezomib. We showed that bortezomib inhibited CIP2A in association with p-Akt downregulation in a dose- and time-dependent manner in all sensitive TNBC cells, whereas no alterations in CIP2A expression and p-Akt were noted in bortezomib-resistant cells. Overexpression of CIP2A upregulated p-Akt and protected MDA-MB-231 and MDA-MB-468 cells from bortezomib-induced apoptosis, whereas silencing CIP2A by siRNA overcame the resistance to bortezomib-induced apoptosis in MCF-7 cells. In addition, bortezomib downregulated CIP2A mRNA but did not affect the degradation of CIP2A protein. Furthermore, bortezomib exerted in vivo antitumor activity in HCC-1937 xenografted tumors, but not in MCF-7 tumors. Bortezomib downregulated CIP2A expression in the HCC-1937 tumors but not in the MCF-7 tumors. Importantly, CIP2A expression is readily detectable in tumor samples from TNBC patients.

Conclusions

CIP2A is a major determinant mediating bortezomib-induced apoptosis in TNBC cells. CIP2A may thus be a potential therapeutic target in TNBC.

Transcriptome analysis in tardigrade species reveals specific molecular pathways for stress adaptations

Tardigrades have unique stress-adaptations that allow them to survive extremes of cold, heat, radiation and vacuum. To study this, encoded protein clusters and pathways from an ongoing transcriptome study on the tardigrade Milnesium tardigradum were analyzed using bioinformatics tools and compared to expressed sequence tags (ESTs) from Hypsibius dujardini, revealing major pathways involved in resistance against extreme environmental conditions. ESTs are available on the Tardigrade Workbench along with software and databank updates. Our analysis reveals that RNA stability motifs for M. tardigradum are different from typical motifs known from higher animals. M. tardigradum and H. dujardini protein clusters and conserved domains imply metabolic storage pathways for glycogen, glycolipids and specific secondary metabolism as well as stress response pathways (including heat shock proteins, bmh2, and specific repair pathways). Redox-, DNA-, stress- and protein protection pathways complement specific repair capabilities to achieve the strong robustness of M. tardigradum. These pathways are partly conserved in other animals and their manipulation could boost stress adaptation even in human cells. However, the unique combination of resistance and repair pathways make tardigrades and M. tardigradum in particular so highly stress resistant.

Physiological and pathological role of the ubiquitin-proteasome system in the vascular smooth muscle cell

Vascular smooth muscle cell (VSMC) plasticity implies a capacity for rapid change and adaptability through processes requiring protein turnover. The ubiquitin-proteasome system (UPS) is at the core of protein turnover as the main pathway for the degradation of proteins related to cell-cycle regulation, signalling, apoptosis, and differentiation. This review briefly addresses some structural UPS aspects under the perspective of VSMC (patho)biology. The UPS loss-of-function promotes direct cell effects and many indirect effects related to the adaptation to apoptosis/survival signalling, oxidative stress, and endoplasmic reticulum stress. The UPS regulates redox homeostasis and is redox-regulated. Also, the UPS closely interacts with endoplasmic reticulum (ER) homeostasis as the effector of un/misfolded protein degradation, and ER stress is strongly involved in atherosclerosis. Inhibition of cell cycle-controlling ubiquitin ligases or the proteasome reduces VSMC proliferation and prevents modulation of their synthetic phenotype. Proteasome inhibition also strongly promotes VSMC apoptosis and reduces neointima. In atherosclerosis models, proteasome inhibitors display vasculoprotective effects and reduce inflammation. However, worsening of atherosclerosis or vascular dysfunction has also been reported. Proteasome inhibitors sensitize VSMC to increased ER stress-mediated cell death and suppress unfolded protein response signalling. Taken together, these observations show that the UPS has powerful effects in the control of VSMC phenotype and survival signalling. However, more profound knowledge of mechanisms is needed in order to render the UPS an operational therapeutic target.

Antitumor effect of non-steroid glucocorticoid receptor ligand CpdA on leukemia cell lines CEM and K562

Glucocorticoids (GCs) are widely used in chemotherapy of hematological malignancies, particularly leukemia. Their effect is mediated by glucocorticoid receptor (GR), a well-known transcription factor. Besides their therapeutic impact, GCs may cause a number of side effects leading to various metabolic complications. The goal of immediate interest is testing glucocorticoid analogs capable of induction/enhancement of GR transrepression, but preventing GR dimerization and transactivation leading to side effects. In this work we have investigated effects of a promising new selective GR agonist, 2-(4-acetoxyphenyl)-2-chloro-N-methylethylammonium chloride (CpdA), on CEM and K562 leukemia cells. Both cell lines express functional GR. CpdA compared with the glucocorticoid fluocinolone acetonide (FA) exerted more prominent cytostatic and apoptotic effects on the cells. Both cell lines exhibited sensitivity to CpdA, demonstrating a good correlation with the effects of FA on cell growth and viability. In contrast to FA, CpdA did not induce GR transactivation evaluated by no obvious increase in expression of GR target (and dependent) gene FKBP51. At the same time, luciferase assay showed that CpdA efficiently activated transrepression of NF-κB and AP-1 factors. We also evaluated the effect of combined action of CpdA and the proteasome inhibitor Bortezomib. The latter induced a caspase-dependent apoptosis in both T-cell leukemia cell lines. By treatment of CEM cells with different CpdA/GC and Bortezomib doses, we have designed a protocol where CpdA shows potentiating effect on Bortezomib cytotoxic activity. Generally, the present work characterizes a novel non-steroid GR ligand, CpdA, as a promising compound for possible application in leukemia chemotherapy.

Differential targeting of androgen and glucocorticoid receptors induces ER stress and apoptosis in prostate cancer cells: a novel therapeutic modality

Androgen (AR) and glucocorticoid (GR) receptor signaling play opposing roles in prostate tumorigenesis: in prostate, AR acts as an oncogene, and GR is a tumor suppressor. Recently, we found that non-steroidal phyto-chemical Compound A (CpdA) is AR/GR modulator acting as anti-inflammatory anti-androgen. CpdA inhibits AR and prevents GR transactivation while enhancing GR transrepression. GR and AR are controlled by proteasomal degradation. We found that prolonged exposure of LNCaP, LNCaP-GR, DU145 and PC3 prostate carcinoma (PCa) cells to proteasome inhibitor Bortezomib (BZ) caused AR degradation and GR accumulation. BZ enhanced CpdA ability to inhibit AR and to augment GR transrepression. We also found that CpdA+BZ differentially regulated GR/AR to cooperatively suppress PCa cell growth and survival and to induce endoplasmic reticulum stress (ERS). Importantly, CpdA+BZ differentially regulated GR-responsive genes. CpdA+BZ blocked activation of glucocorticoid-responsive pro-survival genes, including SGK1, but activated BZ-induced ERS-related genes BIP/HSPA5 and CHOP /GADD153. Using ChIP, we showed that SGK1, BIP/HSPA5 and CHOP regulation was due to effects of CpdA and CpdA+BZ on GR loading on their promoters. We also found that AR and GR are abundant in advanced PCa from patients treated by androgen ablation and/or chemotherapy: 56% of carcinomas from treated patients expressed both receptors, and the other 27% expressed either GR or AR. Overall, our data validate the concept of dual AR/GR targeting in prostate cancer (PC) and suggest that BZ combination with dual-target steroid receptor modulator CpdA has high potential for PC therapy.

Phase 1 trial of gemcitabine with bortezomib in elderly patients with advanced solid tumors

INTRODUCTION

Bortezomib, a proteasome inhibitor, has synergistic antitumor activity with gemcitabine, an antimetabolite, in preclinical and clinical studies. The safety of this combination has not yet been established in elderly patients; therefore, this dose-escalation study was designed to assess the maximum-tolerated dose of bortezomib and gemcitabine in patients aged 70 years or older with advanced-stage solid tumors.

PATIENTS AND METHODS

Gemcitabine was administered intravenously (800 to 1000 mg/m) over 30 minutes on days 1 and 8, followed 60 minutes later by bortezomib administered as an intravenous push over 3 to 5 seconds (1.0 to 1.8 mg/m) on a 21-day cycle. This study used a standard phase 1 dose-escalation design with 3 or 6 patients per dose level.

RESULTS

Seventeen patients with stage IV solid tumors were treated. Median age was 73 years (range: 70 to 87 y). All patients had an Eastern Cooperative Oncology Group (ECOG) performance status less than 2. Median number of earlier chemotherapy regimens was 2 (range: 0 to 6). Dose-limiting toxicities were seen in 2 of 8 patients enrolled at the second dose level of gemcitabine (1000 mg/m) and bortezomib (1.0 mg/m), which consisted of grade ≥3 lower extremity edema, thrombocytopenia, fatigue, and dehydration. The most common grade ≥3 toxicities included thrombocytopenia (n=9), neutropenia (n=6), and anemia (n=5). Partial response (n=3) or disease stabilization (n=3) was seen in 6 of 14 evaluable patients.

CONCLUSIONS

Concurrent weekly gemcitabine (800 mg/m) and bortezomib (1 mg/m) is the recommended schedule for future phase 2 trials in elderly patients with stage IV solid tumors.

HDAC inhibitor modulation of proteotoxicity as a therapeutic approach in cancer

The strong clinical activity of the proteasome inhibitor bortezomib (Velcade) in multiple myeloma and other hematological malignancies has focused considerable attention on its mechanisms of action. Although NFκB inhibition was initially the mechanism in focus, accumulating evidence indicates that misfolded protein accumulation leading to proteotoxicity plays an even more important role in cell killing. Proteotoxicity that occurs as a consequence of protein aggregate accumulation has long been associated with the development of neurodegenerative diseases, and a large and growing body of literature has documented how protein aggregates are handled and disposed of via evolutionarily conserved mechanisms involving cross talk between the proteasome and autophagy in normal cells. The type II histone deacetylase HDAC6 plays important roles in these processes and HDAC6 inhibition enhances proteotoxicity. These observations served as the basis for the development of HDAC6-specific chemical inhibitors that are now being evaluated in combination with proteasome inhibitors in preclinical models. Nonetheless, there is also strong evidence that the more classical, chromatin-associated (type I) HDACs are also involved in the regulation of proteotoxicity, although the biochemical mechanisms underlying their effects are not well defined. Importantly, emerging evidence indicates that subsets of tumor cells contain defects in these protein quality control pathways, which may underlie their vulnerability to proteasome inhibitor-induced death. In addition, our clearer understanding of cytoprotective protein quality control responses is identifying novel candidate targets for therapeutic intervention. In this chapter, we present an overview of protein quality control mechanisms in normal tissues and describe how this information is informing our development of proteasome inhibitors and other agents that impact upon these pathways for cancer therapy.

The 26S proteasome complex: an attractive target for cancer therapy

The 26S proteasome complex engages in an ATP-dependent proteolytic degradation of a variety of oncoproteins, transcription factors, cell cycle specific cyclins, cyclin-dependent kinase inhibitors, ornithine decarboxylase, and other key regulatory cellular proteins. Thus, the proteasome regulates either directly or indirectly many important cellular processes. Altered regulation of these cellular events is linked to the development of cancer. Therefore, the proteasome has become an attractive target for the treatment of numerous cancers. Several proteasome inhibitors that target the proteolytic active sites of the 26S proteasome complex have been developed and tested for anti-tumor activities. These proteasome inhibitors have displayed impressive anti-tumor functions by inducing apoptosis in different tumor types. Further, the proteasome inhibitors have been shown to induce cell cycle arrest, and inhibit angiogenesis, cell-cell adhesion, cell migration, immune and inflammatory responses, and DNA repair response. A number of proteasome inhibitors are now in clinical trials to treat multiple myeloma and solid tumors. Many other proteasome inhibitors with different efficiencies are being developed and tested for anti-tumor activities. Several proteasome inhibitors currently in clinical trials have shown significantly improved anti-tumor activities when combined with other drugs such as histone deacetylase (HDAC) inhibitors, Akt (protein kinase B) inhibitors, DNA damaging agents, Hsp90 (heat shock protein 90) inhibitors, and lenalidomide. The proteasome inhibitor bortezomib is now in the clinic to treat multiple myeloma and mantle cell lymphoma. Here, we discuss the 26S proteasome complex in carcinogenesis and different proteasome inhibitors with their potential therapeutic applications in treatment of numerous cancers.

Electrophoretic mobility shift assay analysis of NFκB transcriptional regulation by nuclear IκBα

Transcription factor NFκB is a key regulator of genes involved in immune and inflammatory responses, as well as genes regulating cell proliferation and survival. In addition to many inflammatory disorders, NFκB is constitutively activated in a variety of human cancers and leukemia. Thus, inhibition of NFκB DNA binding activity represents an important therapeutic approach for disorders characterized by high levels of constitutive NFκB activity. We have previously shown that NFκB DNA binding activity is suppressed by the nuclear translocation and accumulation of IκBα, which is induced by inhibition of the 26S proteasome. In this chapter, we describe a protocol that uses small inhibitory RNA (si RNA) interference followed by electrophoretic mobility shift assay (EMSA) to analyze the regulation of NFκB DNA binding by nuclear IκBα induced by the proteasome inhibitor MG132. Using this protocol, we show that in human leukemia Hut-78 cells that exhibit high levels of NFκB DNA binding activity, MG132 induces nuclear translocation and accumulation of IκBα, which then specifically inhibits NFκB DNA binding. This protocol uses human leukemia Hut-78 cells; however, it can be easily adapted for other cells exhibiting high levels of constitutive NFκB DNA binding.

Molecular mechanisms of bortezomib resistant adenocarcinoma cells

Bortezomib (Velcade™) is a reversible proteasome inhibitor that is approved for the treatment of multiple myeloma (MM). Despite its demonstrated clinical success, some patients are deprived of treatment due to primary refractoriness or development of resistance during therapy. To investigate the role of the duration of proteasome inhibition in the anti-tumor response of bortezomib, we established clonal isolates of HT-29 adenocarcinoma cells adapted to continuous exposure of bortezomib. These cells were ∼30-fold resistant to bortezomib. Two novel and distinct mutations in the β5 subunit, Cys63Phe, located distal to the binding site in a helix critical for drug binding, and Arg24Cys, found in the propeptide region were found in all resistant clones. The latter mutation is a natural variant found to be elevated in frequency in patients with MM. Proteasome activity and levels of both the constitutive and immunoproteasome were increased in resistant cells, which correlated to an increase in subunit gene expression. These changes correlated with a more rapid recovery of proteasome activity following brief exposure to bortezomib. Increased recovery rate was not due to increased proteasome turnover as similar findings were seen in cells co-treated with cycloheximide. When we exposed resistant cells to the irreversible proteasome inhibitor carfilzomib we noted a slower rate of recovery of proteasome activity as compared to bortezomib in both parental and resistant cells. Importantly, carfilzomib maintained its cytotoxic potential in the bortezomib resistant cell lines. Therefore, resistance to bortezomib, can be overcome with irreversible inhibitors, suggesting prolonged proteasome inhibition induces a more potent anti-tumor response.

Sorafenib: complexities of Raf-dependent and Raf-independent signaling are now unveiled

Hepatocellular carcinoma (HCC) is the most common primary cancer worldwide. The only current drug available for clinical treatment of HCC is sorafenib, which inhibits multiple signaling kinases including Raf family members, platelet-derived growth factor receptor, vascular endothelial growth factor receptors 1 and 2, c-Kit, and Fms-like tyrosine kinase 3. Many studies have revealed that the mechanism underlying the antitumor effect of sorafenib is complex. Because sorafenib inhibits C-Raf more potently than B-Raf, the therapeutic efficacy of sorafenib is strongly influenced by the relative expression and activity of B-Raf and C-Raf and the complex interactions between these factors. Moreover, Rafindependent signaling mechanisms have recently emerged as important pathways of sorafenib-induced cell death. Basic research studies have suggested that using sorafenib as part of a combination therapy may improve its effect, although this has yet to be confirmed by clinical evidence. Further studies of the functional mechanism of sorafenib are required to advance the development of targeted therapy for HCC. To aid future work on sorafenib, we here review the current literature pertaining to sorafenib signaling and its clinical efficacy in both monotherapy and combination therapy.

Redox control of the ubiquitin-proteasome system: from molecular mechanisms to functional significance

In their natural environments, cells are regularly exposed to oxidizing conditions that may lead to protein misfolding. If such misfolded proteins are allowed to linger, they may form insoluble aggregates and pose a serious threat to the cell. Accumulation of misfolded, oxidatively damaged proteins is characteristic of many diseases and during aging. To counter the adverse effects of oxidative stress, cells can initiate an antioxidative response in an attempt to repair the damage, or rapidly channel the damaged proteins for degradation by the ubiquitin-proteasome system (UPS). Recent studies have shown that elements of the oxidative stress response and the UPS are linked on many levels. To manage the extra burden of misfolded proteins, the UPS is induced by oxidative stress, and special proteasome subtypes protect cells against oxidative damage. In addition, the proteasome is directly associated with a thioredoxin and other cofactors that may adjust the particle's response during an oxidative challenge. Here, we give an overview of the UPS and a detailed description of the degradation of oxidized proteins and of the crosstalk between oxidative stress and protein degradation in health and disease.

Targeted molecular therapy in peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) are rare neoplasms constituting a heterogeneous group of diseases. At present, available chemotherapy regimens that have improved outcomes in B-cell lymphomas appear to be less efficacious in the context of PTCLs and, thus, alternative strategies are warranted. In the last few years, based on the recent, deeper understanding of PTCL biology, several molecules and/or pathways have been proposed for targeted therapy in this setting, including surface antigens, tyrosine kinases, the NF-κB pathway, folate metabolism, histone modification and others. Of particular interest, histone deacetylase and proteasome inhibitors, as well as novel chemotherapeutic agents such as pralatrexate, have already demonstrated efficacy in PTCL therapy. In addition, a strong biological rationale and early clinical evidence supports the future study of tyrosine kinase inhibitors in this setting. In this article, the authors review the available literature on targeted therapy in PTCLs and also, based on their own experience, discuss potential opportunities in this intriguing area.

Proteomic changes and molecular effects associated with Cr(III) and Cr(VI) treatments on germinating kiwifruit pollen

The present study is aimed at identifying molecular changes elicited by Cr(III) and Cr(VI) on germinating kiwifruit pollen. To address this question, comparative proteomic and DNA laddering analyses were performed. While no genotoxic effect was detected, a number of proteins whose accumulation levels were altered by treatments were identified. In particular, the upregulation of some proteins involved in the scavenging response, cell redox homeostasis and lipid synthesis could be interpreted as an oxidative stress response induced by Cr treatment. The strong reduction of two proteins involved in mitochondrial oxidative phosphorylation and a decline in ATP levels were also observed. The decrease of pollen energy availability could be one of the causes of the severe inhibition of the pollen germination observed upon exposure to both Cr(III) and Cr(VI). Finally, proteomic and biochemical data indicate proteasome impairment: the consequential accumulation of misfolded/damaged proteins could be an important molecular mechanism of Cr(III) toxicity in pollen.

Bortezomib induces autophagy in head and neck squamous cell carcinoma cells via JNK activation

The molecular mechanism of autophagy induction following proteasome inhibition is not fully understood. We report that the proteasome inhibitor bortezomib potently induces autophagy in head and neck squamous cell carcinoma (HNSCC) cells, as demonstrated by autophagosome formation and induction of complete autophagic flux. Bortezomib treatment led to phosphorylation/activation of jun N-terminal kinase (JNK) enzymes and JNK-dependent phosphorylation of Bcl-2 on serine 70. Pharmacologic inhibition of JNK, but not p38 MAPK, dramatically inhibited bortezomib induction of autophagy regulatory proteins and autophagosome formation. These results demonstrate a key requirement for JNK signaling in the activation of autophagy by bortezomib.

Identification of genes affecting the toxicity of anti-cancer drug bortezomib by genome-wide screening in S. pombe

Bortezomib/PS-341/Velcade, a proteasome inhibitor, is widely used to treat multiple myeloma. While several mechanisms of the cytotoxicity of the drug were proposed, the actual mechanism remains elusive. We aimed to identify genes affecting the cytotoxicity of Bortezomib in the fission yeast S.pombe as the drug inhibits this organism's cell division cycle like proteasome mutants. Among the 2815 genes screened (covering 56% of total ORFs), 19 genes, whose deletions induce strong synthetic lethality with Bortezomib, were identified. The products of the 19 genes included four ubiquitin enzymes and one nuclear proteasome factor, and 13 of them are conserved in humans. Our results will provide useful information for understanding the actions of Bortezomib within cells.

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.

Ubiquitylation in apoptosis: a post-translational modification at the edge of life and death

The proper regulation of apoptosis is essential for the survival of multicellular organisms. Furthermore, excessive apoptosis can contribute to neurodegenerative diseases, anaemia and graft rejection, and diminished apoptosis can lead to autoimmune diseases and cancer. It has become clear that the post-translational modification of apoptotic proteins by ubiquitylation regulates key components in cell death signalling cascades. For example, ubiquitin E3 ligases, such as MDM2 (which ubiquitylates p53) and inhibitor of apoptosis (IAP) proteins, and deubiquitinases, such as A20 and ubiquitin-specific protease 9X (USP9X) (which regulate the ubiquitylation and degradation of receptor-interacting protein 1 (RIP1) and myeloid leukaemia cell differentiation 1 (MCL1), respectively), have important roles in apoptosis. Therapeutic agents that target apoptotic regulatory proteins, including those that are part of the ubiquitin-proteasome system, might afford clinical benefits.

Proteasome inhibitors in cancer therapy

The ubiquitin proteasome pathway plays a critical role in regulating many processes in the cell which are important for tumour cell growth and survival. Inhibition of proteasome function has emerged as a powerful strategy for anti-cancer therapy. Clinical validation of the proteasome as a therapeutic target was achieved with bortezomib and has prompted the development of a second generation of proteasome inhibitors with improved pharmacological properties. This review summarises the main mechanisms of action of proteasome inhibitors in cancer, the development of proteasome inhibitors as therapeutic agents and the properties and progress of next generation proteasome inhibitors in the clinic.

p21(WAF1/CIP1) upregulation through the stress granule-associated protein CUGBP1 confers resistance to bortezomib-mediated apoptosis

Background

p21^WAF1/CIP1 is a well known cyclin-dependent kinase inhibitor induced by various stress stimuli. Depending on the stress applied, p21 upregulation can either promote apoptosis or prevent against apoptotic injury. The stress-mediated induction of p21 involves not only its transcriptional activation but also its posttranscriptional regulation, mainly through stabilization of p21 mRNA levels. We have previously reported that the proteasome inhibitor MG132 induces the stabilization of p21 mRNA, which correlates with the formation of cytoplasmic RNA stress granules. The mechanism underlying p21 mRNA stabilization, however, remains unknown.

Methodology/Principal Findings

We identified the stress granules component CUGBP1 as a factor required for p21 mRNA stabilization following treatment with bortezomib ( =  PS-341/Velcade). This peptide boronate inhibitor of the 26S proteasome is very efficient for the treatment of myelomas and other hematological tumors. However, solid tumors are sometimes refractory to bortezomib treatment. We found that depleting CUGBP1 in cancer cells prevents bortezomib-mediated p21 upregulation. FISH experiments combined to mRNA stability assays show that this effect is largely due to a mistargeting of p21 mRNA in stress granules leading to its degradation. Altering the expression of p21 itself, either by depleting CUGBP1 or p21, promotes bortezomib-mediated apoptosis.

Conclusions/Significance

We propose that one key mechanism by which apoptosis is inhibited upon treatment with chemotherapeutic drugs might involve upregulation of the p21 protein through CUGBP1.

A critical role for the NFkB pathway in multiple myeloma

NFkB transcription factors play a key role in the survival and proliferation of many kinds of B-cell tumors, including multiple myeloma (MM). It was shown that NFkB activation in MM tumors results mainly from extrinsic signaling by APRIL and BAFF ligands that stimulate receptors on normal plasma cells as well as on pre-malignant monoclonal gammopathy of undetermined significance (MGUS) and MM tumors. However, the mutations that occur during MM progression and that constitutively activate NFkB would be expected to decrease dependence of tumor cells on the bone marrow microenvironment. These mutations can activate the classical or alternative NFkB pathways selectively, but usually both pathways are activated in MM. Significantly, activation of either NFkB pathway leads to a similar response of MM cell lines. This frequent activation of the alternative pathway distinguishes MM from other B-cell tumors, which more frequently have mutations that are predicted to activate only the classical NFkB pathway. Given the strong dependence of MGUS and MM tumors on NFkB pathway activation, inhibition by a combination of targeting extrinsic signaling plus both NFkB pathways appears to be an attractive therapeutic approach in MM tumors.

In vitro antitumor effect of the proteasome inhibitor bortezomib on human gastric cancer SGC7901 cells

AIM: To investigate the anti-tumor effect of bortezomib, a proteasome inhibitor, on human gastric cancer SGC7901 cells and to explore possible mechanism involved.

METHODS: Human gastric cancer SGC7901 cells were cultured and treated with different concentrations of bortezomib (1-500 nmol/L) for 24-48 h. Cell viability was determined by MTT assay. Apoptosis was detected by flow cytometry. The cleavage of PARP and caspase-3 and level of phosphor-Akt were determined by Western blot.

RESULTS: Bortezomib inhibited the viability of SGC7901 cells in a dose- and time-dependent manner. The IC₅₀ value at 48 h was 67.39 nmol/L. Treatment with 60 or 180 nmol/L of bortezomib induced cell cycle arrest at G₂/M phase at both 24 and 48 h but induced apoptosis only at 48 h. The cleavage of caspase-3 and PARP was observed in cells treated with 60 or 180 nmol/L of bortezomib for 48 h. Treatment with bortezomib for 48 h down-regulated the level of phosphor-Akt in SGC7901 cells.

CONCLUSION: Bortezomib induced apoptosis and cell cycle arrest at G₂/M phase by inhibiting the activity of the PI3K/Akt signaling pathway in human gastric cancer SGC7901 cells.

Emerging role of carfilzomib in treatment of relapsed and refractory lymphoid neoplasms and multiple myeloma

Proteasome inhibition forms the cornerstone of antimyeloma therapy. The first-in-class proteasome inhibitor, bortezomib, either alone or in combination with other chemotherapeutic agents, induces high overall response rates and response qualities in patients with clinically and molecularly defined high-risk disease. However, resistance to bortezomib and neurotoxicity associated with the treatment remain challenging issues. Carfilzomib is a novel, well tolerated, irreversible proteasome inhibitor with minimal neurotoxicity. Carfilzomib demonstrates promising activity in myeloma patients who are refractory to bortezomib and immunomodulatory agents. This review focuses on the pharmacology, safety, and efficacy of carfilzomib for the treatment of multiple myeloma in bortezomib-naïve and bortezomib-exposed populations.

Novel agents for diffuse large B-cell lymphoma

INTRODUCTION

Although diffuse large B-cell lymphoma (DLBCL) is commonly considered as a cancer with a high cure rate with conventional therapies recent studies demonstrate that different biological variants of DBLCL exist, and that patients with one DLBCL variant and DLBCL patients who relapse early following rituximab-based therapy have markedly poorer outcomes with conventional management strategies. Over the last decade, there has been an increasing exploration of novel therapies derived from improved understanding of DLBCL biology and tumor-host interactions.

AREAS COVERED

This review examines the biological basis for novel therapeutic approaches in DLBCL and the early clinical data on compounds derived from this research. A description of the expanding options of novel agents and combination therapies for patients with poor risk DLBCL is provided.

EXPERT OPINION

Several promising novel agents and novel therapeutic combinations are under development for patients with poor risk DLBCL. Carefully designed clinical trials that build on our improved understanding of DLBCL biology and utilize tissue samples to examine the activity of novel combination therapies should expand treatment options for DLBCL patients in the future.

Constitutive activation of metalloproteinase ADAM10 in mantle cell lymphoma promotes cell growth and activates the TNFα/NFκB pathway

One of the main functions of A Disintegrin and Metalloproteinase 10 (ADAM10) is to regulate the bioavailability of adhesion molecules and ligands to various cellular-signaling receptors. Constitutive activation of ADAM10 has been implicated in the pathogenesis of several types of solid tumors. In this study, we found that mantle cell lymphoma (MCL) cell lines and all 12 patient samples examined expressed the active/mature form of ADAM10. In contrast, PBMCs from healthy donors (n = 5) were negative. Using immunohistochemistry, ADAM10 was readily detectable in 20 of 23 (87%) MCL tumors, but absent in 5 reactive tonsils. Knockdown of ADAM10 using short interfering RNA (siRNA) in MCL cells significantly induced growth inhibition and cell-cycle arrest, and these changes were correlated with down-regulation of cyclin D1, up-regulation of p21(waf1), and significant reductions in the TNFα production/transcriptional activity of NFκBp65. The addition of recombinant ADAM10 to MCL cells led to the opposite biologic effects. Lastly, down-regulation of ADAM10 using siRNA enhanced the growth-suppressing effects mediated by the proteasome inhibitors MG132 and bortezomib. We conclude that constitutive activation of ADAM10 contributes to the growth of MCL and therefore inhibition of ADAM10 may be a useful strategy to enhance the response of MCL to other therapeutic agents.

Noxa/Bcl-2 protein interactions contribute to bortezomib resistance in human lymphoid cells

Previous studies have suggested that the BH3 domain of the proapoptotic Bcl-2 family member Noxa only interacts with the anti-apoptotic proteins Mcl-1 and A1 but not Bcl-2. In view of the similarity of the BH3 binding domains of these anti-apoptotic proteins as well as recent evidence that studies of isolated BH3 domains can potentially underestimate the binding between full-length Bcl-2 family members, we examined the interaction of full-length human Noxa with anti-apoptotic human Bcl-2 family members. Surface plasmon resonance using bacterially expressed proteins demonstrated that Noxa binds with mean dissociation constants (K(D)) of 3.4 nm for Mcl-1, 70 nm for Bcl-x(L), and 250 nm for wild type human Bcl-2, demonstrating selectivity but not absolute specificity of Noxa for Mcl-1. Further analysis showed that the Noxa/Bcl-2 interaction reflected binding between the Noxa BH3 domain and the Bcl-2 BH3 binding groove. Analysis of proteins expressed in vivo demonstrated that Noxa and Bcl-2 can be pulled down together from a variety of cells. Moreover, when compared with wild type Bcl-2, certain lymphoma-derived Bcl-2 mutants bound Noxa up to 20-fold more tightly in vitro, pulled down more Noxa from cells, and protected cells against killing by transfected Noxa to a greater extent. When killing by bortezomib (an agent whose cytotoxicity in Jurkat T-cell leukemia cells is dependent on Noxa) was examined, apoptosis was enhanced by the Bcl-2/Bcl-x(L) antagonist ABT-737 or by Bcl-2 down-regulation and diminished by Bcl-2 overexpression. Collectively, these observations not only establish the ability of Noxa and Bcl-2 to interact but also identify Bcl-2 overexpression as a potential mechanism of bortezomib resistance.

Proteome mapping of overexpressed membrane-enriched and cytosolic proteins in sodium antimony gluconate (SAG) resistant clinical isolate of Leishmania donovani

AIMS

This study aimed to identify differentially overexpressed membrane-enriched as well as cytosolic proteins in SAG sensitive and resistant clinical strains of L. donovani isolated from VL patients which are involved in the drug resistance mechanism.

METHODS

The proteins in the membrane-enriched as well as cytosolic fractions of drug-sensitive as well as drug-resistant clinical isolates were separated using two-dimensional gel electrophoresis and overexpressed identified protein spots of interest were excised and analysed using MALDI-TOF/TOF.

RESULTS

Six out of 12 overexpressed proteins were identified in the membrane-enriched fraction of the SAG resistant strain of L. donovani whereas 14 out of 18 spots were identified in the cytosolic fraction as compared with the SAG sensitive strain. The major proteins in the membrane-enriched fraction were ABC transporter, HSP-83, GPI protein transamidase, cysteine-leucine rich protein and 60S ribosomal protein L23a whereas in the cytosolic fraction proliferative cell nuclear antigen (PCNA), proteasome alpha 5 subunit, carboxypeptidase, HSP-70, enolase, fructose-1,6-bisphosphate aldolase, tubulin-beta chain have been identified. Most of these proteins have been reported as potential drug targets, except 60S ribosomal protein L23a and PCNA which have not been reported to date for their possible involvement in drug resistance against VL.

CONCLUSION

This study for the first time provided a cumulative proteomic analysis of proteins overexpressed in drug resistant clinical isolates of L. donovani indicating their possible role in antimony resistance of the parasite. Identified proteins provide a vast field to be exploited for novel treatment strategies against VL such as cloning and overexpression of these targets to produce recombinant therapeutic/prophylactic proteins.

Bortezomib enhances the efficacy of fulvestrant by amplifying the aggregation of the estrogen receptor, which leads to a proapoptotic unfolded protein response

PURPOSE

Fulvestrant is known to promote the degradation of the estrogen receptor (ER) in the nucleus. However, fulvestrant also promotes the aggregation of the newly synthesized ER in the cytoplasm. Accumulation of protein aggregates leads to cell death but this effect is limited as a result of their elimination by the proteasome. We tested whether combining fulvestrant with the proteasome inhibitor, bortezomib, could enhance the accumulation of ER aggregates and cause apoptotic cell death.

EXPERIMENTAL DESIGN

The rate of aggregation of the ER was monitored in ER(+) breast cancer cells lines, T47D, ZR-75.1, BT474, MDA-MB-361, MCF-7, fulvestrant resistance MCF-7, and tamoxifen-resistant T47D-cyclin D1 cells. Activation of the unfolded protein response, apoptosis, and metabolic rate were also monitored in these cell lines following treatment with fulvestrant, bortezomib, or bortezomib in combination with fulvestrant.

RESULTS

We found that bortezomib enhances the fulvestrant-mediated aggregation of the ER in the cytoplasm without blocking the degradation of the ER in the nucleus. Further, these aggregates activate a sustained unfolded protein response leading to apoptotic cell death. Further, we show that the combination induced tumor regression in a breast cancer mouse model of tamoxifen resistance.

CONCLUSIONS

Adding bortezomib to fulvestrant enhances its efficacy by taking advantage of the unique ability of fulvestrant to promote cytoplasmic aggregates of the ER. As this effect of fulvestrant is independent of the transcriptional activity of the ER, these results suggest that this novel combination may be effective in breast cancers that are ER(+) but estrogen independent.

Proteasome inhibition represses unfolded protein response and Nox4, sensitizing vascular cells to endoplasmic reticulum stress-induced death

Background

Endoplasmic reticulum (ER) stress has pathophysiological relevance in vascular diseases and merges with proteasome function. Proteasome inhibition induces cell stress and may have therapeutic implications. However, whether proteasome inhibition potentiates ER stress-induced apoptosis and the possible mechanisms involved in this process are unclear.

Methodology/Principal Findings

Here we show that proteasome inhibition with MG132, per se at non-lethal levels, sensitized vascular smooth muscle cells to caspase-3 activation and cell death during ER stress induced by tunicamycin (Tn). This effect was accompanied by suppression of both proadaptive (KDEL chaperones) and proapoptotic (CHOP/GADD153) unfolded protein response markers, although, intriguingly, the splicing of XBP1 was markedly enhanced and sustained. In parallel, proteasome inhibition completely prevented ER stress-induced increase in NADPH oxidase activity, as well as increases in Nox4 isoform and protein disulfide isomerase mRNA expression. Increased Akt phosphorylation due to proteasome inhibition partially offset the proapoptotic effect of Tn or MG132. Although proteasome inhibition enhanced oxidative stress, reactive oxygen species scavenging had no net effect on sensitization to Tn or MG132-induced cell death.

Conclusion/Relevance

These data indicate unfolded protein response-independent pathways whereby proteasome inhibition sensitizes vascular smooth muscle to ER stress-mediated cell death. This may be relevant to understand the therapeutic potential of such compounds in vascular disease associated with increased neointimal hyperplasia.

Bortezomib induces nuclear translocation of IκBα resulting in gene-specific suppression of NF-κB--dependent transcription and induction of apoptosis in CTCL

Cutaneous T-cell lymphoma (CTCL) is characterized by constitutive activation of nuclear factor κB (NF-κB), which plays a crucial role in the survival of CTCL cells and their resistance to apoptosis. NF-κB activity in CTCL is inhibited by the proteasome inhibitor bortezomib; however, the mechanisms remained unknown. In this study, we investigated mechanisms by which bortezomib suppresses NF-κB activity in CTCL Hut-78 cells. We demonstrate that bortezomib and MG132 suppress NF-κB activity in Hut-78 cells by a novel mechanism that consists of inducing nuclear translocation and accumulation of IκBα (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha), which then associates with NF-κB p65 and p50 in the nucleus and inhibits NF-κB DNA binding activity. Surprisingly, however, while expression of NF-κB-dependent antiapoptotic genes cIAP1 and cIAP2 is inhibited by bortezomib, expression of Bcl-2 is not suppressed. Chromatin immunoprecipitation indicated that cIAP1 and cIAP2 promoters are occupied by NF-κB p65/50 heterodimers, whereas Bcl-2 promoter is occupied predominantly by p50/50 homodimers. Collectively, our data reveal a novel mechanism of bortezomib function in CTCL and suggest that the inhibition of NF-κB-dependent gene expression by bortezomib is gene specific and depends on the subunit composition of NF-κB dimers recruited to NF-κB-responsive promoters.

Rapid Remission in Peripheral T-Cell Lymphoma of the Nasal Type by the Bortezomib plus CHOP Therapy

Peripheral T-cell lymphoma (PTCL) is rare and difficult to treat for its high relapse rate. The authors report a case of PTCL of the skin, regarding which clinical and pathological features, treatment, and prognosis were discussed. A 66-year-old woman was admitted with complaints of enlarging erythematous noduloplaques on the right anterior tibial skin for one year and similar lesions on the left for 6 months. Surgical resection of right leg lesion and biopsy of enlarged inguinal lymph nodes histologically indicated a PTCL of the nasal type. The patient was treated by CHOP plus bortezomib, reached complete remission just after two courses of chemotherapy and then received another two as consolidation. The patient remained in remission for 11 months until local relapse. As for cutaneous lesions, detailed lymph node examination and prompt tissue biopsy are judicious choices prior to any medical management. The chemotherapy consisting of bortezomib and CHOP is safe and efficient in PTCL of the skin.

Inhibition of Bcl-2 and Bcl-X enhances chemotherapy sensitivity in hepatoblastoma cells

BACKGROUND

An increased expression of anti-apoptotic proteins is regularly found in malignant cells, contributing to their clonal expansion by conferring an improved survival ability. In Hepatoblastoma (HB) apoptosis regulation contributes to resistance and therapy failure, therefore we modulated apoptosis sensitivity of HB cells for an improved cytotoxic activity of commonly used drugs.

PROCEDURE

Apoptosis-related proteins were quantified in HB cells (HuH6 and HepT1) using protein assays. Interaction of ABT-737, a small-molecule inhibitor of Bcl-2, Bcl-xL, and Bcl-W with cytotoxic drugs was monitored in a proliferation assay. Apoptosis induction was measured by caspase-3 activity.

RESULTS

We found high levels of the anti-apoptotic protein Bcl-2 and Bcl-X as well as low levels of pro-apoptotic protein Bax and Bad in both HB cell lines. ABT-737 induced apoptosis in HuH6 and HepT1 cells at concentrations higher than 1 µM. ABT-737 also enhanced the cytotoxic effect of cisplatin (CDDP), doxorubicin (DOXO), etoposide and paclitaxel when used as combination therapy. HuH6 expressed slightly higher pro-apoptotic and lower anti-apoptotic protein levels than HepT1, which may explain the stronger enhancement of cytostatic drug effects in HuH6 cells when treated in combination with ABT-737.

CONCLUSION

The observed anti-apoptotic phenotype in HB cell lines may contribute to resistance to cytotoxic drugs used in the standard treatment protocol of HB. These pre-clinical results suggest that apoptosis sensitizers with BH-3 mimicry, such as ABT-737, should be further evaluated in preclinical models of HB.

Obtusilactone A and (-)-sesamin induce apoptosis in human lung cancer cells by inhibiting mitochondrial Lon protease and activating DNA damage checkpoints

Several compounds from Cinnamomum kotoense show anticancer activities. However, the detailed mechanisms of most compounds from C. kotoense remain unknown. In this study, we investigated the anticancer activity of obtusilactone A (OA) and (-)-sesamin in lung cancer. Our results show that human Lon is upregulated in non-small-cell lung cancer (NSCLC) cell lines, and downregulation of Lon triggers caspase-3 mediated apoptosis. Through enzyme-based screening, we identified two small-molecule compounds, obtusilactone A (OA) and (-)-sesamin from C. kotoense, as potent Lon protease inhibitors. Obtusilactone A and (-)-sesamin interact with Ser855 and Lys898 residues in the active site of the Lon protease according to molecular docking analysis. Thus, we suggest that cancer cytotoxicity of the compounds is partly due to the inhibitory effects on Lon protease. In addition, the compounds are able to cause DNA double-strand breaks and activate checkpoints. Treatment with OA and (-)-sesamin induced p53-independent DNA damage responses in NSCLC cells, including G(1) /S checkpoint activation and apoptosis, as evidenced by phosphorylation of checkpoint proteins (H2AX, Nbs1, and Chk2), caspase-3 cleavage, and sub-G(1) accumulation. In conclusion, OA and (-)-sesamin act as both inhibitors of human mitochondrial Lon protease and DNA damage agents to activate the DNA damage checkpoints as well induce apoptosis in NSCLC cells. These dual functions open a bright avenue to develop more selective chemotherapy agents to overcome chemoresistance and sensitize cancer cells to other chemotherapeutics.

The Hsp90 inhibitor IPI-504 overcomes bortezomib resistance in mantle cell lymphoma in vitro and in vivo by down-regulation of the prosurvival ER chaperone BiP/Grp78

Despite the promising introduction of the proteasome inhibitor bortezomib in the treatment of mantle cell lymphoma (MCL), not all patients respond, and resistance often appears after initial treatment. By analyzing a set of 18 MCL samples, including cell lines with constitutive or induced resistance to bortezomib, we found a high correlation between loss of sensitivity to the proteasome inhibitor and up-regulation of the prosurvival chaperone BiP/Grp78. BiP/Grp78 stabilization was ensured at a posttranscriptional level by an increase in the chaperoning activity of heat shock protein of 90 kDa (Hsp90). In bortezomib-resistant cells, both BiP/Grp78 knockdown and cell pretreatment with the Hsp90 inhibitor of the ansamycin class, IPI-504, led to synergistic induction of apoptotic cell death when combined with bortezomib. Cell exposure to the IPI-504-bortezomib combination provoked the dissociation of Hsp90/BiP complexes, leading to BiP/Grp78 depletion, inhibition of unfolded protein response, and promotion of NOXA-mediated mitochondrial depolarization. The IPI-504-bortezomib combination also prevented BiP/Grp78 accumulation, thereby promoting apoptosis and inhibiting the growth of bortezomib-resistant tumors in a mouse model of MCL xenotransplantation. These results suggest that targeting unfolded protein response activation by the inhibition of Hsp90 may be an attractive model for the design of a new bortezomib-based combination therapy for MCL.

Targeting apoptosis pathways in pancreatic cancer

Pancreatic cancer - here in particular pancreatic ductal adenocarcinoma (PDAC) - is still a highly therapy refractory disease. Amongst the mechanisms by which PDAC cells could escape any non-surgical therapy, anti-apoptotic protection seems to be the most relevant one. PDAC cells have acquired resistance to apoptotic stimuli such as death ligands (FasL, TRAIL) or anti-cancer drugs (gemcitabine) by a great number of molecular alterations either disrupting an apoptosis inducing signal or counteracting the execution of apoptosis. Thus, PDAC cells exhibit alterations in the EGFR/MAPK/Ras/raf1-, PI3K/Akt-, TRAIL/TRAF2-, or IKK/NF-κB pathway accompanied by deregulations in the expression of apoptosis regulators such as cIAP, Bcl2, XIAP or survivin. Along with protection against apoptosis, PDAC cells also overexpress histone deacetylases (HDACs) giving rise to epigenetic patterns of chemoresistance and to acetylation of other regulatory proteins, as well. With respect to the multitude of anti-apoptotic pathways, a great number of molecular targets might be of high potential in novel therapy strategies. Thus, natural compounds as well as novel synthetic drugs are considered to be used in single or combined therapy of PDAC. A number of proteasome and HDAC inhibitors or selective inhibitors of IKK, EGFR, Akt and mTOR have been widely explored in preclinical settings and clinical studies. Even though these early studies encouraged an application in a clinical setting, most of the trials have been rather disappointing yet. Thus, new molecular targets and novel concepts of combination therapies need to get access into clinical trials - either in neoadjuvant/adjuvant or in palliative treatments.

Dual role of NO donors in the reversal of tumor cell resistance and EMT: Downregulation of the NF-κB/Snail/YY1/RKIP circuitry

Several studies have implicated the role of Nitric Oxide (NO) in the regulation of tumor cell behavior and have shown that NO either promotes or inhibits tumorigenesis. These conflicting findings have been resolved, in part, by the levels of NO used such that low levels promote tumor growth and high levels inhibit tumor growth. Our studies have focused on the use of high levels of NO provided primarily by the NO donor, DETANONOate. We have shown that treatment of resistant tumor cells with DETANONOate sensitizes them to apoptosis by both chemotherapeutic drugs and cytotoxic immunotherapeutic ligands. The underlying mechanisms by which NO sensitizes tumor cells to apoptosis were shown to be regulated, in part, by NO-mediated inhibition of the NF-κB survival/anti-apoptotic pathways and downstream of NF-κB by inhibition of the transcription factor Yin Yang 1 (YY1). In addition to NO-induced sensitization to apoptosis, we have also shown that NO induced the expression of the metastasis-suppressor/immunosurveillance cancer gene product, Raf-1 kinase inhibitor protein (RKIP). Overexpression of RKIP mimics NO in tumor cells-induced sensitization to apoptosis. The induction of RKIP by NO was the result of the inhibition of the RKIP repressor, Snail, downstream of NF-κB. These findings established the presence of a dysregulated NF-κB/Snail/YY1/ RKIP circuitry in resistance and that treatment with NO modifies this loop in tumor cells in favor of the inhibition of tumor cell survival and the response to cytotoxic drugs. Noteworthy, the NF-κB/Snail/YY1/RKIP loop consists of gene products that regulate the epithelial to mesenchymal transition (EMT) and, thus, tumor metastasis. Hence, we have found that treatment of metastatic cancer cell lines with DETANONOate inhibited the EMT phenotype, through both the inhibition of the metastasis-inducers, NF-κB and Snail and the induction of the metastasis-suppressor, RKIP. Altogether, the above findings establish, for the first time, the dual role of high levels of NO in the sensitization of tumor cells to apoptotic stimuli as well as inhibition of EMT. Hence, NO donors may be considered as novel potential therapeutic agents with dual roles in the treatment of patients with refractory cancer and in the prevention of the initiation of the metastatic cascade via EMT.

Prolongation of myocardial viability by proteasome inhibition during hypothermic organ preservation

Recently, we provided evidence for a possible role of the cardiac proteasome during ischemia, suggesting that a subset of 26S proteasomes is a cell-destructive protease, which is activated as the cellular energy supply declines. Although proteasome inhibition during cold ischemia (CI) reduced injury of ischemic hearts, it remains unknown whether these beneficial effects are maintained throughout reperfusion, and thus, may have pathophysiological relevance. Therefore, we evaluated the effects of epoxomicin (specific proteasome inhibitor) in a rat heterotopic heart transplantation model. Donor hearts were arrested with University of Wisconsin solution (UW) and stored for 12 h/24 h in 4 °C UW±epoxomicin, followed by transplantation. Efficacy of epoxomicin was confirmed by proteasome peptidase activity measurements and analyses of myocardial ubiquitin pools. After 12hCI, troponin I content of UW was lower with epoxomicin. Although all hearts after 12hCI started beating spontaneously, addition of epoxomicin to UW during CI reduced cardiac edema and preserved the ultrastructural integrity of the post-ischemic cardiomyocyte. After 24hCI in UW±epoxomicin, hearts did not regain contractility. When hearts were perfused with epoxomicin during cardioplegia, the cardiac proteasome was inhibited immediately, all of these hearts started beating after 24hCI in UW plus epoxomicin and cardiac edema and myocardial ultrastructure were comparable to hearts after 12hCI. Epoxomicin did not affect markers of lipid peroxidation or neutrophil infiltration in post-ischemic hearts. These data further support the concept that proteasome activation during ischemia is of pathophysiological relevance and suggest proteasome inhibition as a promising approach to improve organ preservation strategies.

Proteasome inhibition in vivo promotes survival in a lethal murine model of severe acute respiratory syndrome

Ubiquitination is a critical regulator of the host immune response to viral infection, and many viruses, including coronaviruses, encode proteins that target the ubiquitination system. To explore the link between coronavirus infection and the ubiquitin system, we asked whether protein degradation by the 26S proteasome plays a role in severe coronavirus infections using a murine model of SARS-like pneumonitis induced by murine hepatitis virus strain 1 (MHV-1). In vitro, the pretreatment of peritoneal macrophages with inhibitors of the proteasome (pyrrolidine dithiocarbamate [PDTC], MG132, and PS-341) markedly inhibited MHV-1 replication at an early step in its replication cycle, as evidenced by inhibition of viral RNA production. Proteasome inhibition also blocked viral cytotoxicity in macrophages, as well as the induction of inflammatory mediators such as IP-10, gamma interferon (IFN-γ), and monocyte chemoattractant protein 1 (MCP-1). In vivo, intranasal inoculation of MHV-1 results in a lethal pneumonitis in A/J mice. Treatment of A/J mice with the proteasome inhibitor PDTC, MG132, or PS-341 led to 40% survival (P < 0.01), with a concomitant improvement of lung histology, reduced pulmonary viral replication, decreased pulmonary STAT phosphorylation, and reduced pulmonary inflammatory cytokine expression. These data demonstrate that inhibition of the cellular proteasome attenuates pneumonitis and cytokine gene expression in vivo by reducing MHV-1 replication and the resulting inflammatory response. The results further suggest that targeting the proteasome may be an effective new treatment for severe coronavirus infections.

CIP2A mediates effects of bortezomib on phospho-Akt and apoptosis in hepatocellular carcinoma cells

Previously, we reported that Akt inactivation determines the sensitivity of hepatocellular carcinoma (HCC) cells to bortezomib. In this study, we report that cancerous inhibitor of protein phosphatase 2A (CIP2A), a cellular inhibitor of protein phosphatase 2A (PP2A), mediates the apoptotic effect of bortezomib in HCC. Silencing PP2A by small interference RNA (siRNA) abolishes bortezomib-induced down-regulation of phospho-Akt and apoptosis. Bortezomib increases PP2A activity in sensitive HCC cells, including Sk-Hep1, Hep3B and Huh-7, but not in resistant PLC5 cells. Bortezomib down-regulates CIP2A in a dose- and time-dependent manner in all sensitive HCC cells, whereas no alterations in CIP2A were found in resistant PLC5 cells. Knockdown of CIP2A by siRNA restored bortezomib's effects on apoptosis and PP2A activity in PLC5 cells. Moreover, over-expression of CIP2A up-regulated phospho-Akt and protected Sk-Hep1 cells from bortezomib-induced apoptosis. It is significant that, ectopic expression of CIP2A decreased Akt-related PP2A activity, whereas silencing CIP2A increased this activity, indicating that CIP2A negatively regulates Akt-related PP2A activity in HCC cells, furthermore, our in vivo data showed that bortezomib down-regulates CIP2A and up-regulates PP2A activity in Huh-7 tumors, but not in PLC5 tumors. In conclusion, inhibition of CIP2A determines the effects of bortezomib on apoptosis and PP2A-dependent Akt inactivation in HCC.

Targeting NF-kappaB for colorectal cancer

IMPORTANCE OF THE FIELD

Colorectal cancer (CRC) is the second leading cause of cancer death. Progress has been made in the development of chemotherapy for advanced CRC. Targeted therapies against VEGF or EGFR are now commonly used. Many cases show that tolerance develops to such treatments and thus new strategies are required to replace or complement current therapies. The NF-kappaB signaling pathway plays critical roles in physiological and pathological processes, and the relationship between colon cancer development and NF-kappaB is becoming clear.

AREAS COVERED IN THIS REVIEW

We discuss evidence for the participation of activated NF-kappaB in carcinogenesis and consider the possibility of NF-kappaB being a target for CRC treatment.

WHAT THE READER WILL GAIN

NF-kappaB activation might be involved in development of not only colitis-associated cancer, but also sporadic CRC. NF-kappaB activation is associated with hallmarks of cancer. Constitutive NF-kappaB activation is frequently observed in CRC and is associated with angiogenesis and resistance to chemotherapy. Several NF-kappaB inhibitors have proven to be useful.

TAKE HOME MESSAGE

Induction of NF-kappaB activation leads to resistance to chemotherapy and constitutively activated NF-kappaB can often be seen in CRC. Anti-NF-kappaB therapy may rescue many cases of CRC and should be examined further for use as a therapy target.

Role of CAAT/enhancer binding protein homologous protein in panobinostat-mediated potentiation of bortezomib-induced lethal endoplasmic reticulum stress in mantle cell lymphoma cells

PURPOSE

Bortezomib induces unfolded protein response (UPR) and endoplasmic reticulum stress, as well as exhibits clinical activity in patients with relapsed and refractory mantle cell lymphoma (MCL). Here, we determined the molecular basis of the improved in vitro and in vivo activity of the combination of the pan-histone deacetylase inhibitor panobinostat and bortezomib against human, cultured, and primary MCL cells.

EXPERIMENTAL DESIGN

Immunoblot analyses, reverse transcription-PCR, and immunofluorescent and electron microscopy were used to determine the effects of panobinostat on bortezomib-induced aggresome formation and endoplasmic reticulum stress in MCL cells.

RESULTS

Treatment with panobinostat induced heat shock protein 90 acetylation; depleted the levels of heat shock protein 90 client proteins, cyclin-dependent kinase 4, c-RAF, and AKT; and abrogated bortezomib-induced aggresome formation in MCL cells. Panobinostat also induced lethal UPR, associated with induction of CAAT/enhancer binding protein homologous protein (CHOP). Conversely, knockdown of CHOP attenuated panobinostat-induced cell death of MCL cells. Compared with each agent alone, cotreatment with panobinostat increased bortezomib-induced expression of CHOP and NOXA, as well as increased bortezomib-induced UPR and apoptosis of cultured and primary MCL cells. Cotreatment with panobinostat also increased bortezomib-mediated in vivo tumor growth inhibition and improved survival of mice bearing human Z138C MCL cell xenograft.

CONCLUSION

These findings suggest that increased UPR and induction of CHOP are involved in enhanced anti-MCL activity of the combination of panobinostat and bortezomib.