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

Halogenated boroxine increases propensity to apoptosis in leukemia (UT-7) but not non-tumor cells in vitro

A hallmark of the development of solid and hematological malignancies is the dysregulation of apoptosis, which leads to an imbalance between cell proliferation, cell survival and death. Halogenated boroxine [K₂ (B₃ O₃ F₄ OH)] (HB) is a derivative of cyclic anhydride of boronic acid, with reproducible anti-tumor and anti-proliferative effects in different cell models. Notably, these changes are observed to be more profound in tumor cells than in normal cells. Here, we investigated the underlying mechanisms through an extensive evaluation of (a) deregulated target genes and (b) their interactions and links with main apoptotic pathway genes upon treatment with an optimized concentration of HB. To provide deeper insights into the mechanism of action of HB, we performed identification, visualization, and pathway association of differentially expressed genes (DEGs) involved in regulation of apoptosis among tumor and non-tumor cells upon HB treatment. We report that HB at a concentration of 0.2 mg·mL^-1 drives tumor cells to apoptosis, whereas non-tumor cells are not affected. Comparison of DEG profiles, gene interactions and pathway associations suggests that the HB effect and tumor-'selectivity' can be explained by Bax/Bak-independent mitochondrial depolarization by ROS generation and TRAIL-like activation, followed by permanent inhibition of NFκB signaling pathway specifically in tumor cells.

Novel boron-containing compound, halogenated boroxine, induces selective cytotoxicity through apoptosis triggering in UT-7 leukemia

Apoptosis induction is a promising approach in targeting tumor cells. As halogenated boroxine (HB) shows antitumor activity, but its mechanism of action in hematological tumors remains unclear, in this study, we aimed to analyze apoptosis triggering in normal and UT-7 leukemia cells by HB. Methods for assessing cell viability and cytotoxicity, apoptosis detection, relative expression of 84 apoptosis-associated genes and BCL-2, and functional analysis were applied. Pronounced HB activities in inhibition of cell viability, cytotoxicity, and apoptosis induction with measurable differences between tumor and normal cells were found. HB modulated the expression of 21 genes, predominantly downregulated the antiapoptotic genes in leukemia. The functional association revealed HB's impact on inhibition of NF-κB signaling pathway. BCL-2 expression decreasing was found only in UT-7 leukemia. This study identified HB as an apoptosis inducer affecting leukemia but not normal cells considering mechanisms of selective activity that may be a great advantage of HB applications.

Progress on the Application of Bortezomib and Bortezomib-Based Nanoformulations

Bortezomib (BTZ) is the first proteasome inhibitor approved by the Food and Drug Administration. It can bind to the amino acid residues of the 26S proteasome, thereby causing the death of tumor cells. BTZ plays an irreplaceable role in the treatment of mantle cell lymphoma and multiple myeloma. Moreover, its use in the treatment of other hematological cancers and solid tumors has been investigated in numerous clinical trials and preclinical studies. Nevertheless, the applications of BTZ are limited due to its insufficient specificity, poor permeability, and low bioavailability. Therefore, in recent years, different BTZ-based drug delivery systems have been evaluated. In this review, we firstly discussed the functions of proteasome inhibitors and their mechanisms of action. Secondly, the properties of BTZ, as well as recent advances in both clinical and preclinical research, were reviewed. Finally, progress in research regarding BTZ-based nanoformulations was summarized.

Carfilzomib: A Promising Proteasome Inhibitor for the Treatment of Relapsed and Refractory Multiple Myeloma

The proteasome is crucial for the degradation of intracellular proteins and plays an important role in mediating a number of cell survival and progression events by controlling the levels of key regulatory proteins such as cyclins and caspases in both normal and tumor cells. However, compared to normal cells, cancer cells are more dependent on the ubiquitin proteasome pathway (UPP) due to the accumulation of proteins in response to uncontrolled gene transcription, allowing proteasome to become a potent therapeutic target for human cancers such as multiple myeloma (MM). Up to date, three proteasome inhibitors namely bortezomib (2003), carfilzomib (2012) and ixazomib (2015) have been approved by the US Food and Drug Administration (FDA) for the treatment of patients with relapsed and/or refractory MM. This review mainly focuses on the biochemical properties, mechanism of action, toxicity profile and pivotal clinical trials related to carfilzomib, a second-generation proteasome inhibitor that binds irreversibly with proteasome to overcome the major toxicities and resistance associated with bortezomib.

Overcoming drug resistance by targeting protein homeostasis in multiple myeloma

Multiple myeloma (MM) is a plasma cell disorder typically characterized by abundant synthesis of clonal immunoglobulin or free light chains. Although incurable, a deeper understanding of MM pathobiology has fueled major therapeutical advances over the past two decades, significantly improving patient outcomes. Proteasome inhibitors, immunomodulatory drugs, and monoclonal antibodies are among the most effective anti-MM drugs, targeting not only the cancerous cells, but also the bone marrow microenvironment. However, de novo resistance has been reported, and acquired resistance is inevitable for most patients over time, leading to relapsed/refractory disease and poor outcomes. Sustained protein synthesis coupled with impaired/insufficient proteolytic mechanisms makes MM cells exquisitely sensitive to perturbations in protein homeostasis, offering us the opportunity to target this intrinsic vulnerability for therapeutic purposes. This review highlights the scientific rationale for the clinical use of FDA-approved and investigational agents targeting protein homeostasis in MM.

Towards Drug Repurposing in Cancer Cachexia: Potential Targets and Candidates

As a multifactorial and multiorgan syndrome, cancer cachexia is associated with decreased tolerance to antitumor treatments and increased morbidity and mortality rates. The current approaches for the treatment of this syndrome are not always effective and well established. Drug repurposing or repositioning consists of the investigation of pharmacological components that are already available or in clinical trials for certain diseases and explores if they can be used for new indications. Its advantages comparing to de novo drugs development are the reduced amount of time spent and costs. In this paper, we selected drugs already available or in clinical trials for non-cachexia indications and that are related to the pathways and molecular components involved in the different phenotypes of cancer cachexia syndrome. Thus, we introduce known drugs as possible candidates for drug repurposing in the treatment of cancer-induced cachexia.

Extrinsic interactions in the microenvironment in vivo activate an antiapoptotic multidrug-resistant phenotype in CLL

The Bcl-2 inhibitor venetoclax has yielded exceptional clinical responses in chronic lymphocytic leukemia (CLL). However, de novo resistance can result in failure to achieve negative minimal residual disease and predicts poor treatment outcomes. Consequently, additional proapoptotic drugs, such as inhibitors of Mcl-1 and Bcl-xL, are in development. By profiling antiapoptotic proteins using flow cytometry, we find that leukemic B cells that recently emigrated from the lymph node (CD69+/CXCR4Low) in vivo are enriched for cell clusters simultaneously overexpressing multiple antiapoptotic proteins (Mcl-1High/Bcl-xLHigh/Bcl-2High) in both treated and treatment-naive CLL patients. These cells exhibited antiapoptotic resistance to multiple BH-domain antagonists, including inhibitors of Bcl-2, Mcl-1, and Bcl-xL, when tested as single agents in a flow cytometry-based functional assay. Antiapoptotic multidrug resistance declines ex vivo, consistent with resistance being generated in vivo by extrinsic microenvironmental interactions. Surviving "persister" cells in patients undergoing venetoclax treatment are enriched for CLL cells displaying the functional and molecular properties of microenvironmentally induced multidrug resistance. Overcoming this resistance required simultaneous inhibition of multiple antiapoptotic proteins, with potential for unwanted toxicities. Using a drug screen performed using patient peripheral blood mononuclear cells cultured in an ex vivo microenvironment model, we identify novel venetoclax drug combinations that induce selective cytotoxicity in multidrug-resistant CLL cells. Thus, we demonstrate that antiapoptotic multidrug-resistant CLL cells exist in patients de novo and show that these cells persist during proapoptotic treatment, such as venetoclax. We validate clinically actionable approaches to selectively deplete this reservoir in patients.

Proteasome inhibition triggers the formation of TRAIL receptor 2 platforms for caspase-8 activation that accumulate in the cytosol

Cancer cells that are resistant to Bax/Bak-dependent intrinsic apoptosis can be eliminated by proteasome inhibition. Here, we show that proteasome inhibition induces the formation of high molecular weight platforms in the cytosol that serve to activate caspase-8. The activation complexes contain Fas-associated death domain (FADD) and receptor-interacting serine/threonine-protein kinase 1 (RIPK1). Furthermore, the complexes contain TRAIL-receptor 2 (TRAIL-R2) but not TRAIL-receptor 1 (TRAIL-R1). While RIPK1 inhibition or depletion did not affect proteasome inhibitor-induced cell death, TRAIL-R2 was found essential for efficient caspase-8 activation, since the loss of TRAIL-R2 expression abrogated caspase processing, significantly reduced cell death, and promoted cell re-growth after drug washout. Overall, our study provides novel insight into the mechanisms by which proteasome inhibition eliminates otherwise apoptosis-resistant cells, and highlights the crucial role of a ligand-independent but TRAIL-R2-dependent activation mechanism for caspase-8 in this scenario.

Proteasome Complexes and Their Heterogeneity in Colorectal, Breast and Pancreatic Cancers

Targeting the ubiquitin-proteasome system (UPS) - in particular, the proteasome complex - has emerged as an attractive novel cancer therapy. While several proteasome inhibitors have been successfully approved by the Food and Drug Administration for the treatment of hematological malignancies, the clinical efficacy of these inhibitors is unexpectedly lower in the treatment of solid tumors due to the functional and structural heterogeneity of proteasomes in solid tumors. There are ongoing trials to examine the effectiveness of compound and novel proteasome inhibitors that can target solid tumors either alone or in combination with conventional chemotherapeutic agents. The modest therapeutic efficacy of proteasome inhibitors such as bortezomib in solid malignancies demands further research to clarify the exact effects of these proteasome inhibitors on different proteasomes present in cancer cells. The structural, cellular localization and functional analysis of the proteasome complexes in solid tumors originated from different tissues provides new insights into the diversity of proteasomes' responses to inhibitors. In this study, we used an optimized iodixanol gradient ultracentrifugation to purify a native form of proteasome complexes with their intact associated protein partners enriched within distinct cellular compartments. It is therefore possible to isolate proteasome subcomplexes with far greater resolution than sucrose or glycerol fractionations. We have identified differences in the catalytic activities, subcellular distribution, and inhibitor sensitivity of cytoplasmic proteasomes isolated from human colon, breast, and pancreatic cancer cell lines. Our developed techniques and generated results will serve as a valuable guideline for investigators developing a new generation of proteasome inhibitors as an effective targeted therapy for solid tumors.

HCMV-Mediated Interference of Bortezomib-Induced Apoptosis in Colon Carcinoma Cell Line Caco-2

Human cytomegalovirus (HCMV) has been implicated in the development of human malignancies, for instance in colon cancer. Proteasome inhibitors were developed for cancer therapy and have also been shown to influence HCMV infection. The aim of this study was to investigate if proteasome inhibitors have therapeutic potential for colon carcinoma and how this is influenced by HCMV infection. We show by immunofluorescence and flow cytometry that the colon carcinoma cell line Caco-2 is susceptible to HCMV infection. Growth curve analysis as well as protein expression kinetics and quantitative genome analysis further confirm these results. HCMV has an anti-apoptotic effect on Caco-2 cells by inhibiting very early events of the apoptosis cascade. Further investigations showed that HCMV stabilizes the membrane potential of the mitochondria, which is typically lost very early during apoptosis. This stabilization is resistant to proteasome inhibitor Bortezomib treatment, allowing HCMV-infected cells to survive apoptotic signals. Our findings indicate a possible role of proteasome inhibitors in colon carcinoma therapy.

Targeting the ubiquitin-proteasome system in a pancreatic cancer subtype with hyperactive MYC

The myelocytomatosis oncogene (MYC) is an important driver in a subtype of pancreatic ductal adenocarcinoma (PDAC). However, MYC remains a challenging therapeutic target; therefore, identifying druggable synthetic lethal interactions in MYC‐active PDAC may lead to novel precise therapies. First, to identify networks with hyperactive MYC, we profiled transcriptomes of established human cell lines, murine primary PDAC cell lines, and accessed publicly available repositories to analyze transcriptomes of primary human PDAC. Networks active in MYC‐hyperactive subtypes were analyzed by gene set enrichment analysis. Next, we performed an unbiased pharmacological screen to define MYC‐associated vulnerabilities. Hits were validated by analysis of drug response repositories and genetic gain‐ and loss‐of‐function experiments. In these experiments, we discovered that the proteasome inhibitor bortezomib triggers a MYC‐associated vulnerability. In addition, by integrating publicly available data, we found the unfolded protein response as a signature connected to MYC. Furthermore, increased sensitivity of MYC‐hyperactive PDACs to bortezomib was validated in genetically modified PDAC cells. In sum, we provide evidence that perturbing the ubiquitin–proteasome system (UPS) might be an option to target MYC‐hyperactive PDAC cells. Our data provide the rationale to further develop precise targeting of the UPS as a subtype‐specific therapeutic approach.

Identification and Target-Modification of SL-BBI: A Novel Bowman-Birk Type Trypsin Inhibitor from Sylvirana latouchii

The peptides from the ranacyclin family share similar active disulphide loop with plant-derived Bowman-Birk type inhibitors, some of which have the dual activities of trypsin inhibition and antimicrobial. Herein, a novel Bowman-Birk type trypsin inhibitor of the ranacyclin family was identified from the skin secretion of broad-folded frog (Sylvirana latouchii) by molecular cloning method and named as SL-BBI. After chemical synthesis, it was proved to be a potent inhibitor of trypsin with a Ki value of 230.5 nM and showed weak antimicrobial activity against tested microorganisms. Modified analogue K-SL maintains the original inhibitory activity with a Ki value of 77.27 nM while enhancing the antimicrobial activity. After the substitution of active P1 site to phenylalanine and P2' site to isoleucine, F-SL regenerated its inhibitory activity on chymotrypsin with a Ki value of 309.3 nM and exhibited antiproliferative effects on PC-3, MCF-7 and a series of non-small cell lung cancer cell lines without cell membrane damage. The affinity of F-SL for the β subunits in the yeast 20S proteasome showed by molecular docking simulations enriched the understanding of the possible action mode of Bowman-Birk type inhibitors. Further mechanistic studies have shown that F-SL can activate caspase 3/7 in H157 cells and induce apoptosis, which means it has the potential to become an anticancer agent.

The BCL-2 selective inhibitor ABT-199 sensitizes soft tissue sarcomas to proteasome inhibition by a concerted mechanism requiring BAX and NOXA

Soft tissue sarcomas (STS) are a heterogeneous group of malignancies predominantly affecting children and young adults. Despite improvements in multimodal therapies, 5-year survival rates are only 50% and new treatment options in STS are urgently needed. To develop a rational combination therapy for the treatment of STS we focused on ABT-199 (Venetoclax), a BCL-2 specific BH3-mimetic, in combination with the proteasome inhibitor bortezomib (BZB). Simultaneous inhibition of BCL-2 and the proteasome resulted in strongly synergistic apoptosis induction. Mechanistically, ABT-199 mainly affected the multidomain effector BAX by liberating it from BCL-2 inhibition. The combination with BZB additionally resulted in the accumulation of BOK, a BAX/BAK homologue, and of the BH3-only protein NOXA, which inhibits the anti-apoptotic protein MCL-1. Thus, the combination of ABT-199 and BZB sensitizes STS cells to apoptosis by simultaneously releasing several defined apoptotic restraints. This synergistic mechanism of action was verified by CRISPR/Cas9 knock-out, showing that both BAX and NOXA are crucial for ABT-199/BZB-induced apoptosis. Noteworthy, efficient induction of apoptosis by ABT-199/BZB was not affected by the p53 status and invariably detected in cell lines and patient-derived tumor cells of several sarcoma types, including rhabdomyo-, leiomyo-, lipo-, chondro-, osteo-, or synovial sarcomas. Hence, we propose the combination of ABT-199 and BZB as a promising strategy for the treatment of STS, which should warrant further clinical investigation.

Multiple myeloma cells are exceptionally sensitive to heat shock, which overwhelms their proteostasis network and induces apoptosis

Proteasome inhibitors, such as bortezomib (BTZ), are highly effective and widely used treatments for multiple myeloma. One proposed reason for myeloma cells' exceptional sensitivity to proteasome inhibition is that they produce and continually degrade unusually large amounts of abnormal immunoglobulins. We, therefore, hypothesized that, heat shock may also be especially toxic to myeloma cells by causing protein unfolding, increasing further the substrate load on proteasomes, and, thus, putting further stress on their capacity for protein homeostasis. After a shift from 37 to 43 °C, all four myeloma lines studied underwent extensive apoptosis in 4 h, unlike 13 nonmyeloma cell lines, even though the myeloma cells induced heat-shock proteins and increased protein degradation similar to other cells. Furthermore, two myeloma lines resistant to proteasome inhibitors were also more resistant to 43 °C. Shifting myeloma cells to 43, 41, or 39 °C (which was not cytotoxic) dramatically increased their killing by proteasome inhibitors and inhibitors of ubiquitination or p97/VCP. Combining increased temperature with BTZ increased the accumulation of misfolded proteins and substrate load on the 26S proteasome. The apoptosis seen at 43 °C and at 39 °C with BTZ was mediated by caspase-9 and was linked to an accumulation of the proapoptotic Bcl-2-family member Noxa. Thus, myeloma cells are exceptionally sensitive to increased temperatures, which greatly increase substrate load on the ubiquitin-proteasome system and eventually activate the intrinsic apoptotic pathway. Consequently, for myeloma, mild hyperthermia may be a beneficial approach to enhance the therapeutic efficacy of proteasome inhibitors.

An Investigation Into the Prognostic Significance of High Proteasome PSB7 Protein Expression in Colorectal Cancer

Using unbiased proteomics, we had previously discovered that the catalytic proteasome subunit β type 7 (PSB7) protein is frequently overexpressed in colorectal adenocarcinomas. In this paper, we validate this finding and derive a prognostic significance for PSB7 by examining an expanded, well-annotated clinical cohort of 318 colorectal cancer patients. We found PSB7 protein levels to be similarly increased in both advanced stage primary disease and metastatic lesions. We then examined the prognostic value of PSB7 protein expression. Elevated PSB7 protein as well as PSMB7 mRNA levels showed associations with lower overall survival, particularly in female patients. The prognostic value of elevated PSB7 protein levels was highest for female patients who were older (>60 years of age at diagnosis) or who had received adjuvant chemotherapy. While high PSB7 did not retain its prognostic significance on multivariate analysis, we discuss the potential significance of PSB7 as a biomarker, considering its differential prognostic strength in different colorectal cancer patient groups and given its role as a subunit of the immunoproteasome for antigen presentation.

Lessons Learned from Proteasome Inhibitors, the Paradigm for Targeting Protein Homeostasis in Cancer

Targeting aberrant protein homeostasis (proteostasis) in cancer is an attractive therapeutic strategy. However, this approach has thus far proven difficult to bring to clinical practice, with one major exception: proteasome inhibition. These small molecules have dramatically transformed outcomes for patients with the blood cancer multiple myeloma. However, these agents have failed to make an impact in more common solid tumors. Major questions remain about whether this therapeutic strategy can be extended to benefit even more patients. Here we discuss the role of the proteasome in normal and tumor cells, the basic, preclinical, and clinical development of proteasome inhibitors, and mechanisms proposed to govern both intrinsic and acquired resistance to these drugs. Years of study of both the mechanism of action and modes of resistance to proteasome inhibitors reveal these processes to be surprisingly complex. Here, we attempt to draw lessons from experience with proteasome inhibitors that may be relevant for other compounds targeting proteostasis in cancer, as well as extending the reach of proteasome inhibitors beyond blood cancers.

Proteasome inhibition in multiple myeloma: lessons for other cancers

Cellular protein homeostasis (proteostasis) depends on the controlled degradation of proteins that are damaged or no longer required by the ubiquitin-proteasome system (UPS). The 26S proteasome is the principal executer of substrate-specific proteolysis in eukaryotic cells and regulates a myriad of cellular functions. Proteasome inhibitors were initially developed as chemical tools to study proteasomal function but rapidly became widely used anticancer drugs that are now used at all stages of treatment for the bone marrow cancer multiple myeloma (MM). Here, we review the mechanisms of action of proteasome inhibitors that underlie their preferential toxicity to MM cells, focusing on endoplasmic reticulum stress, depletion of amino acids, and effects on glucose and lipid metabolism. We also discuss mechanisms of resistance to proteasome inhibition such as autophagy and metabolic rewiring and what lessons we may learn from the success and failure of proteasome inhibition in MM for treating other cancers with proteostasis-targeting drugs.

The RNA polymerase III repressor MAF1 is regulated by ubiquitin-dependent proteasome degradation and modulates cancer drug resistance and apoptosis

MAF1 homolog, negative regulator of RNA polymerase III (MAF1) is a key repressor of RNA polymerase (pol) III-dependent transcription and functions as a tumor suppressor. Its expression is frequently down-regulated in primary human hepatocellular carcinomas (HCCs). However, this reduction in MAF1 protein levels does not correlate with its transcript levels, indicating that MAF1 is regulated post-transcriptionally. Here, we demonstrate that MAF1 is a labile protein whose levels are regulated through the ubiquitin-dependent proteasome pathway. We found that MAF1 ubiquitination is enhanced upon mTOR complex 1 (TORC1)-mediated phosphorylation at Ser-75. Moreover, we observed that the E3 ubiquitin ligase cullin 2 (CUL2) critically regulates MAF1 ubiquitination and controls its stability and subsequent RNA pol III-dependent transcription. Analysis of the phenotypic consequences of modulating either CUL2 or MAF1 protein expression revealed changes in actin cytoskeleton reorganization and altered sensitivity to doxorubicin-induced apoptosis. Repression of RNA pol III-dependent transcription by chemical inhibition or knockdown of BRF1 RNA pol III transcription initiation factor subunit (BRF1) enhanced HCC cell sensitivity to doxorubicin, suggesting that MAF1 regulates doxorubicin resistance in HCC by controlling RNA pol III-dependent transcription. Together, our results identify the ubiquitin proteasome pathway and CUL2 as important regulators of MAF1 levels. They suggest that decreases in MAF1 protein underlie chemoresistance in HCC and perhaps other cancers and point to an important role for MAF1 and RNA pol III-mediated transcription in chemosensitivity and apoptosis.

[Bortezomib and obatoclax for dual blockade of protein degradation pathways show synergistic anti-tumor effect in human acute T lymphoblastic leukemia cells]

OBJECTIVE

To explore whether bortezomib and a Bcl-2 inhibitor exhibit synergistic anti-tumor effect in human acute T lymphoblastic leukemia cells.

METHODS

MTT assay was used to determine the cytotoxicity of bortezomib in the absence or presence of Bcl-2 inhibitors (obatoclax, AT-101 and ABT-199) in Jurkat cells. The effects of drug treatment on the expression of Bcl-2 family proteins, LC3B, p62, ubiquitin, BiP/Grp78, p-JNK, p-p38 and CHOP proteins were examined by Western blotting. Flow cytometry was used to determine the effects of bortezomib and Bcl-2 inhibitors (obatoclax, AT-101 and ABT-199) on cell apoptosis. Quantitative real-time PCR was used to measure the mRNA expression levels of the key regulatory factors of unfolded protein reaction (UPR). A zebrafish xenograft model was used to study the anti-tumor effect of bortezomib, obatoclax and their combination in vivo.

RESULTS

Bortezomib or Bcl-2 inhibitors alone inhibited the cell viability of Jurkat cells, but only obatoclax and bortezomib showed synergistic cytotoxicity and pro-apoptotic effect. Obatoclax, rather than AT-101 and ABT- 199, blocked autophagic flux in the cells evidenced by concomitant accumulation of LC3B-Ⅱ and p62. Both bortezomib and obatoclax alone caused accumulation of polyubiquinated proteins, and their combination showed a synergistic effect, which was consistent with their synergistic cytotoxicity. The dual blockade of proteasome and autophagy by the combination of bortezomib and obatoclax triggered unfolded protein response followed by cell apoptosis. Preventing UPS dysfunction by tauroursodeoxycholic acid (TUDCA) significantly attenuated the cytotoxicity and pro-apoptotic effect of bortezomib in combination with obatoclax. In zebrafish xenograft models, bortezomib combined with obatoclax significantly decreased tumor foci formation.

CONCLUSIONS

Bortezomib and obatoclax for dual blockade of protein degradation pathways show synergistic anti-tumor effect in human acute T lymphoblastic leukemia cells.

[Effects of artesunate combined with bortezomib on apoptosis and autophagy of acute myeloid leukemia cells in vitro and its mechanism]

Objective: To investigate the effects of artesunate combined with bortezomib on the proliferation, apoptosis and autophagy of human acute myeloid leukemia cell lines MV4-11, and its mechanisms. Methods: MTT method was used to determine the anti-proliferation effect of different concentrations of artesunate, bortezomib and their combination on MV4-11 cells. The cell apoptosis were analyzed by flow cytometry. The expression of cleaved-Caspase-3, Bcl-2 family protein (Bcl-2, Mcl-1, Bim, Bax) and autophagy-related protein LC3B were assayed by Western blot. Results: Artesunate displayed a proliferation inhibition effect on MV4-11 with dose- and time-dependent manner, the IC(50) of artesunate on MV4-11 after 48 hours was 1.44 μg/ml. Bortezomib displayed a proliferation inhibition effect on MV4-11 with dose-dependent manner, the IC(50) of bortezomib on MV4-11 after 48 hours was 8.97 nmol/L. The combination of artesunate (0.75, 1.0 μg/ml) and Bortezomib (6, 8 nmol/L) showed higher inhibition on MV4-11 than artesunate or bortezomib alone in the same concentration gradient after 48 hours (P<0.05) . The cooperation index of the two drugs were all less than 1. The 48 h apoptotic rate of artesunate (1.5 μg/ml) on MV4-11 was (15.27±2.18) %, (19.85±3.23) % of bortezomib (8 nmol/L) , (81.67±5.96) % of combination of the two drugs, significantly higher than the single group (P<0.05) . When combination of the two drugs on MV4-11 after 24 hours, the levels of pro-apoptotic protein Bim and the cleaved activation of Caspase-3 and autophagy-related protein LC3B were up-regulated and the anti-apoptotic protein Bcl-2 expressions was down-regulated. Conclusion: Combination of artesunate with bortezomib shows a significant synergistic effects on proliferation, apoptosis and autophagy of MV4-11 cell lines, which may be associated with Bcl-2 family proteins expression.

MG-132 treatment promotes TRAIL-mediated apoptosis in SEB-1 sebocytes

AIMS

This study aimed to identify the mechanism of how MG-132 stimulates cell death in SEB-1 sebocytes.

MATERIALS AND METHODS

TUNEL staining and annexin-FITC/PI flow cytometry were utilized to examine the apoptotic cell number of SEB-1 sebocytes and HaCaT keratinocytes upon MG-132 and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) treatment. MTT assay and CCK-8 assay monitored the proliferative rate and viability of both cell lines with different treatment. Western blotting (WB) and qPCR were performed to detect the expression of TRAIL and members of Bcl-2 family at protein and gene level. Additionally, RNA interfering was used to knockdown the mRNA transcription of TRAIL and BIK gene.

KEY FINDINGS

MG-132 treatment enhanced cell death in SEB-1 sebocytes but not in HaCaT keratinocytes. Meanwhile, TRAIL concentrations in SEB-1 sebocytes treated with MG-132 were markedly elevated. Furthermore, treatment with TRAIL or the TRAIL receptor-specific monoclonal antibody AY4 at various doses stimulated cell death in SEB-1 sebocytes in a time- and dose-dependent manner. Silencing of TRAIL restored the cell viability of SEB-1 cells to a normal level after MG-132 treatment. Combined treatment of SEB-1 sebocytes with TRAIL and MG-132 synergistically triggered cell death, suppressed cell proliferation and survival, and promoted BIK expression. Furthermore, BCL2 Interacting Killer (BIK) knockdown via RNA interference participated in the recovery of cell survival reduced by treatment with TRAIL and MG-132.

SIGNIFICANCE

These findings suggest that treatment with the selective proteasome suppressor MG-132 and TRAIL induces cell death in sebocytes through upregulation of BIK, a member of the Bcl-2 family.

Targeting the overexpressed USP7 inhibits esophageal squamous cell carcinoma cell growth by inducing NOXA-mediated apoptosis

Increasing evidence suggests that deubiquitinase USP7 participates in tumor progression by various mechanisms and serves as a potential therapeutic target. However, its expression and role in esophageal cancer remains elusive; the anti-cancer effect by targeting USP7 still needs to be investigated. Here, we reported that USP7 was overexpressed in esophageal squamous cell carcinoma (ESCC) tissues compared with adjacent tissues, implying that USP7 was an attractive anticancer target of ESCC. Pharmaceutical or genetic inactivation of USP7 inhibited esophageal cancer cells growth in vitro and in vivo and induced apoptosis. Mechanistically, inhibition of USP7 accumulated poly-ubiquitinated proteins, activated endoplasmic reticulum stress, and increased expression of ATF4, which transcriptionally upregulated expression of NOXA and induced NOXA-mediated apoptosis. These results provide an evidence for clinical investigation of USP7 inhibitors for the treatment of ESCC.

The proton pump inhibitor pantoprazole disrupts protein degradation systems and sensitizes cancer cells to death under various stresses

Proton pump inhibitors (PPIs) play a role in antitumor activity, with studies showing specialized impacts of PPIs on cancer cell apoptosis, metastasis, and autophagy. In this study, we demonstrated that pantoprazole (PPI) increased autophagosomes formation and affected autophagic flux depending on the pH conditions. PPI specifically elevated SQSTM1 protein levels by increasing SQSTM1 transcription via NFE2L2 activation independent of the specific effect of PPI on autophagic flux. Via decreasing proteasome subunits expression, PPI significantly impaired the function of the proteasome, accompanied by the accumulation of undegraded poly-ubiquitinated proteins. Notably, PPI-induced autophagy functioned as a downstream response of proteasome inhibition by PPI, while suppressing protein synthesis abrogated autophagy. Blocking autophagic flux in neutral pH condition or further impairing proteasome function with proteasome inhibitors, significantly aggravated PPI cytotoxicity by worsening protein degradation ability. Interestingly, under conditions of mitochondrial stress, PPI showed significant synergism when combined with Bcl-2 inhibitors. Taken together, these findings provide a new understanding of the impact of PPIs on cancer cells’ biological processes and highlight the potential to develop more efficient and effective combination therapies.

The combination of ionizing radiation and proteasomal inhibition by bortezomib enhances the expression of NKG2D ligands in multiple myeloma cells

Bortezomib, which is a potent proteasome inhibitor, has been used as a first-line drugs to treat multiple myeloma for a few decades, and radiotherapy has frequently been applied to manage acute bone lesions in the patients. Therefore, it was necessary to investigate what the benefits might be if the two therapies were applied simultaneously in the treatment of multiple myeloma. Since it was known that radiotherapy and proteasome inhibitors could increase the expression of NKG2D ligands through induction of protein synthesis and suppression of protein degradation of NKG2D ligands, respectively, we supposed that the combined treatment might further enhance the expression of NKG2D ligands. In this study, we analyzed the expression level of NKG2D ligands using multiplex PCR and flow cytometry after treatment of IM-9 and RPMI-8226 myeloma cells with bortezomib and ionizing radiation; we then assayed the susceptibility to NK-92 cells. Although the expression of only some kinds of NKG2D ligands were increased by treatment with bortezomib alone, five kinds of NKG2D ligands that we assayed were further induced at the surface protein level after combined treatment with ionizing radiation and bortezomib. Furthermore, combined treatment made myeloma cells more susceptible to NK-92 cells, compared with treatment with bortezomib alone. In conclusion, the combination therapy of ionizing radiation plus the proteasome inhibitor bortezomib is a promising therapeutical strategy for enhancing NK cell-mediated anticancer immune responses.

Positioning of proteasome inhibitors in therapy of solid malignancies

Targeting of the protein degradation pathway, in particular, the ubiquitin-proteasome system, has emerged as an attractive novel cancer chemotherapeutic modality. Although proteasome inhibitors have been most successfully applied in the treatment of hematological malignancies, they also received continuing interest for the treatment of solid tumors. In this review, we summarize the current positioning of proteasome inhibitors in the treatment of common solid malignancies (e.g., lung, colon, pancreas, breast, and head and neck cancer), addressing topics of their mechanism(s) of action, predictive factors and molecular mechanisms of resistance.

Dual inhibition of Mcl-1 by the combination of carfilzomib and TG02 in multiple myeloma

Carfilzomib (Kyprolis®), a second generation proteasome inhibitor, is FDA approved for single-agent use among relapsed/refractory multiple myeloma (MM). To enhance the therapeutic efficacy of carfilzomib, we sought to combine carfilzomib with other novel agents. TG02, a multi-kinase inhibitor, targets JAK2 and CDK9. The rationale for co-treatment with carfilzomib and TG02 is that both independently target Mcl-1 and most myeloma cells are dependent on this anti-apoptotic protein for survival. We observed at least additive effects using the combination treatment in MM cell lines and patient samples. To determine how the bone marrow environment affects the efficacy of the combination we conducted co-culture experiments with Hs-5 stromal cells. We also examined the mechanism of increased apoptosis by determining the affect on expression of the Bcl-2 family of proteins. We found that carfilzomib increases NOXA mRNA expression, as expected, and TG02 treatment caused a decrease in Mcl-1 protein but not mRNA levels. Consistent with this possibility, we find silencing CDK9 does not change carfilzomib sensitivity in the same manner as addition of TG02. Since changes in Mcl-1 protein occur in the presence of a proteasome inhibitor we hypothesize that regulation of Mcl-1 translation is the most likely mechanism. Taken together our data suggest that dual inhibition of Mcl-1 via decreased expression and the induction of its antagonist NOXA by the combination of carfilzomib and TG02 is active in myeloma and warrants further testing preclinically and in clinical trials. Moreover, regulation of Mcl-1 by TG02 is more complex than initially appreciated.

Proteasome Stress Triggers Death of SH-SY5Y and T98G Cells via Different Cellular Mechanisms

Overload or dysfunction of ubiquitin-proteasome system (UPS) is implicated in mechanisms of neurodegeneration associated with neurodegenerative diseases, e.g. Parkinson and Alzheimer disease, and ischemia-reperfusion injury. The aim of this study was to investigate the possible association between viability of neuroblastoma SH-SY5Y and glioblastoma T98G cells treated with bortezomib, inhibitor of 26S proteasome, and accumulation of ubiquitin-conjugated proteins with respect to direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. Bortezomib-induced death of SH-SY5Y cells was documented after 24 h of treatment while death of T98G cells was delayed up to 48 h. Already after 4 h of treatment of both SH-SY5Y and T98G cells with bortezomib, increased levels of both ubiquitin-conjugated proteins with molecular mass more than 150 kDa and Hsp70 were observed whereas Hsp90 was elevated in T98G cells and decreased in SH-SY5Y cells. With respect to the cell death mechanism, we have documented bortezomib-induced activation of caspase 3 in SH-SY5Y cells that was probably a result of increased expression of pro-apoptotic proteins, PUMA and Noxa. In T98G cells, bortezomib-induced expression of caspase 4, documented after 24 h of treatment, with further activation of caspase 3, observed after 48 h of treatment. The delay in activation of caspase 3 correlated well with the delay of death of T98G cells. Our results do not support the possibility about direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. They are more consistent with a view that proteasome inhibition is associated with both transcription-dependent and -independent changes in expression of pro-apoptotic proteins and consequent cell death initiation associated with caspase 3 activation.

ABT-737 Synergizes with Cisplatin Bypassing Aberration of Apoptotic Pathway in Non-small Cell Lung Cancer

A subset of non-small cell lung cancer (NSCLC), which does not have a druggable driver mutation, is treated with platinum-based cytotoxic chemotherapy, but it develops resistance triggered by DNA damage responses. Here, we investigated the effect of activation of STAT3 by cisplatin on anti-apoptotic proteins and the effectiveness of a co-treatment with cisplatin and a BH3 mimetic, ABT-737. We analyzed the relationship between cisplatin and STAT3 pathway and effect of ABT-737, when combined with cisplatin in NSCLC cells and K-ras mutant mouse models. The synergism of this combination was evaluated by the Chou-Talalay Combination Index method. In vivo activity was evaluated by micro-CT. In NSCLC cells, there was a time and dose-dependent phosphorylation of SRC-JAK2-STAT3 by cisplatin, followed by increased expression of anti-apoptotic molecules. When the expression of the BCL-2 protein family members was evaluated in clinical samples, BCL-xL was most frequently overexpressed. Dominant negative STAT3 suppressed their expression, suggesting that STAT3 mediates cisplatin mediated overexpression of the anti-apoptotic molecules. ABT-737 displaced BCL-xL from mitochondria and induced oligomerization of BAK. ABT-737 itself showed cytotoxic effects and a combination of ABT-737 with cisplatin showed strong synergistic cytotoxicity. In a murine lung cancer model, co-treatment with ABT-737 and cisplatin induced significant tumor regression. These findings reveal a synergistic cytotoxic and anti-tumor activity of ABT-737 and cisplatin co-treatment in preclinical models, and suggest that clinical trials using this strategy may be beneficial in advanced NSCLC.

Selective targeting of BCL6 induces oncogene addiction switching to BCL2 in B-cell lymphoma

The BCL6 oncogene plays a crucial role in sustaining diffuse large B-cell lymphomas (DLBCL) through transcriptional repression of key checkpoint genes. BCL6-targeted therapy kills lymphoma cells by releasing these checkpoints. However BCL6 also directly represses several DLBCL oncogenes such as BCL2 and BCL-XL that promote lymphoma survival. Herein we show that DLBCL cells that survive BCL6-targeted therapy induce a phenomenon of “oncogene-addiction switching” by reactivating BCL2-family dependent anti-apoptotic pathways. Thus, most DLBCL cells require concomitant inhibition of BCL6 and BCL2-family members for effective lymphoma killing. Moreover, in DLBCL cells initially resistant to BH3 mimetic drugs, BCL6 inhibition induces a newly developed reliance on anti-apoptotic BCL2-family members for survival that translates in acquired susceptibility to BH3 mimetic drugs ABT-737 and obatoclax. In germinal center B cell-like (GCB)-DLBCL cells, the proteasome inhibitor bortezomib and the NEDD inhibitor MLN4924 post-transcriptionally activated the BH3-only sensitizer NOXA thus counteracting the oncogenic switch to BCL2 induced by BCL6-targeting. Hence our study indicates that BCL6 inhibition induces an on-target feedback mechanism based on the activation of anti-apoptotic BH3 members. This oncogene-addition switching mechanism was harnessed to develop rational combinatorial therapies for GCB-DLBCL.

Genistein suppresses the mitochondrial apoptotic pathway in hippocampal neurons in rats with Alzheimer's disease

Genistein is effective against amyloid-β toxicity, but the underlying mechanisms are unclear. We hypothesized that genistein may protect neurons by inhibiting the mitochondrial apoptotic pathway, and thereby play a role in the prevention of Alzheimer’s disease. A rat model of Alzheimer’s disease was established by intraperitoneal injection of D-galactose and intracerebral injection of amyloid-β peptide (25–35). In the genistein treatment groups, a 7-day pretreatment with genistein (10, 30, 90 mg/kg) was given prior to establishing Alzheimer’s disease model, for 49 consecutive days. Terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay demonstrated a reduction in apoptosis in the hippocampus of rats treated with genistein. Western blot analysis showed that expression levels of capase-3, Bax and cytochrome c were decreased compared with the model group. Furthermore, immunohistochemical staining revealed reductions in cytochrome c and Bax immunoreactivity in these rats. Morris water maze revealed a substantial shortening of escape latency by genistein in Alzheimer’s disease rats. These findings suggest that genistein decreases neuronal loss in the hippocampus, and improves learning and memory ability. The neuroprotective effects of genistein are associated with the inhibition of the mitochondrial apoptotic pathway, as shown by its ability to reduce levels of caspase-3, Bax and cytochrome c.

Oral ixazomib maintenance therapy in multiple myeloma

Continuous therapy has proven to be an effective therapeutic strategy to improve the outcome of both young and elderly multiple myeloma patients. Remarkably, lenalidomide and bortezomib showed to play a crucial role in this setting due to their safety profile allowing long-term exposure. Ixazomib, the first oral proteasome inhibitor to be evaluated in multiple myeloma, exerts substantial anti-myeloma activity as a single agent and particularly in combination with immunomodulatory drugs and it may be an attractive option for maintenance therapy. Here we address the issue of maintenance therapy as part of a therapeutic approach of multiple myeloma patients focusing on the potential role of ixazomib.

The resistance related to targeted therapy in malignant pleural mesothelioma: Why has not the target been hit yet?

Malignant pleural mesothelioma (MPM) is an aggressive tumor of the pleura with a poor prognosis. The most active first-line regimens are platinum compounds and pemetrexed. There is no standard second-line treatment in MPM. Advances in the understanding of tumor molecular biology have led to the development of several targeted treatments, which have been evaluated in clinical trials. Unfortunately none of the explored targeted treatments can currently be recommended as routine treatment in MPM. We reviewed the biological pathways involved in MPM, the clinical trials about targeted therapy, and possible related mechanisms of resistance. We suggest that specific genetic markers are needed as targets of selective therapy. By this way the selection of patients based on the molecular profile may facilitate a therapeutic strategy that allows the use of the most appropriate drug for each patient.

Dexamethasone treatment promotes Bcl-2 dependence in multiple myeloma resulting in sensitivity to venetoclax

Venetoclax (ABT-199), a specific inhibitor of the anti-apoptotic protein Bcl-2, is currently in phase I clinical trials for multiple myeloma. Results suggest that venetoclax is only active in a small cohort of patients therefore we wanted to determine its efficacy when used in combination. Combining venetoclax with melphalan or carfilzomib produced additive or better cell death in 4 of the 5 cell lines tested. The most striking results were seen with dexamethasone. Co-treatment of human myeloma cell lines and primary patient samples, with dexamethasone and venetoclax significantly increased cell death over venetoclax alone in 4 of the 5 cell lines, and in all patient samples tested. The mechanism by which this occurs is an increase in the expression of both Bcl-2 and Bim upon addition of dexamethasone. This results in alterations in Bim binding to anti-apoptotic proteins. Dexamethasone shifts Bim binding towards Bcl-2 resulting in increased sensitivity to venetoclax. These data suggest that knowledge of drug-induced alterations of Bim binding patterns may help inform better combination drug regimens. Furthermore, the data indicate combining this novel therapeutic with dexamethasone could be an effective therapy for a broader range of patients than would be predicted by single agent activity.

Signaling mechanisms of bortezomib in TRAF3-deficient mouse B lymphoma and human multiple myeloma cells

Bortezomib, a clinical drug for multiple myeloma (MM) and mantle cell lymphoma, exhibits complex mechanisms of action, which vary depending on the cancer type and the critical genetic alterations of each cancer. Here we investigated the signaling mechanisms of bortezomib in mouse B lymphoma and human MM cells deficient in a new tumor suppressor gene, TRAF3. We found that bortezomib consistently induced up-regulation of the cell cycle inhibitor p21(WAF1) and the pro-apoptotic protein Noxa as well as cleavage of the anti-apoptotic protein Mcl-1. Interestingly, bortezomib induced the activation of NF-κB1 and the accumulation of the oncoprotein c-Myc, but inhibited the activation of NF-κB2. Furthermore, we demonstrated that oridonin (an inhibitor of NF-κB1 and NF-κB2) or AD 198 (a drug targeting c-Myc) drastically potentiated the anti-cancer effects of bortezomib in TRAF3-deficient malignant B cells. Taken together, our findings increase the understanding of the mechanisms of action of bortezomib, which would aid the design of novel bortezomib-based combination therapies. Our results also provide a rationale for clinical evaluation of the combinations of bortezomib and oridonin (or other inhibitors of NF-κB1/2) or AD 198 (or other drugs targeting c-Myc) in the treatment of lymphoma and MM, especially in patients containing TRAF3 deletions or relevant mutations.

Lower expression of activating transcription factors 3 and 4 correlates with shorter progression-free survival in multiple myeloma patients receiving bortezomib plus dexamethasone therapy

Bortezomib (BTZ), a proteasome inhibitor, is widely used in the treatment of multiple myeloma (MM), but a fraction of patients respond poorly to this agent. To identify factors predicting the duration of progression-free survival (PFS) of MM patients on BTZ treatment, the expression of proteasome and endoplasmic reticulum (ER) stress-related genes was quantified in primary samples from patients receiving a combination of BTZ and dexamethasone (BD). Fifty-six MM patients were stratified into a group with PFS<6 months (n=33) and a second group with PFS⩾6 months (n=23). Of the 15 genes analyzed, the expression of activating transcription factor 3 (ATF3) and ATF4 was significantly lower in patients with shorter PFS (P=0.0157 and P=0.0085, respectively). Chromatin immunoprecipitation analysis showed that these ATFs bind each other and transactivate genes encoding the pro-apoptotic transcription factors, CHOP and Noxa, which promote ER stress-associated apoptosis. When either ATF3 or ATF4 expression was silenced, MM cells partially lost sensitivity to BTZ treatment. This was accompanied by lower levels of Noxa, CHOP and DR5. Thus low basal expression of ATF3 and ATF4 may attenuate BTZ-induced apoptosis. Hence, ATF3 and ATF4 could potentially be used as biomarkers to predict efficacy of BD therapy in patients with MM.

The Emerging Role of Extracellular Vesicle-Mediated Drug Resistance in Cancers: Implications in Advanced Prostate Cancer

Emerging evidence has shown that the extracellular vesicles (EVs) regulate various biological processes and can control cell proliferation and survival, as well as being involved in normal cell development and diseases such as cancers. In cancer treatment, development of acquired drug resistance phenotype is a serious issue. Recently it has been shown that the presence of multidrug resistance proteins such as Pgp-1 and enrichment of the lipid ceramide in EVs could have a role in mediating drug resistance. EVs could also mediate multidrug resistance through uptake of drugs in vesicles and thus limit the bioavailability of drugs to treat cancer cells. In this review, we discussed the emerging evidence of the role EVs play in mediating drug resistance in cancers and in particular the role of EVs mediating drug resistance in advanced prostate cancer. The role of EV-associated multidrug resistance proteins, miRNA, mRNA, and lipid as well as the potential interaction(s) among these factors was probed. Lastly, we provide an overview of the current available treatments for advanced prostate cancer, considering where EVs may mediate the development of resistance against these drugs.

Synergistic effects of cisplatin and proteasome inhibitor bortezomib on human bladder cancer cells

The proteasome inhibitor bortezomib is a promising novel agent in bladder cancer therapy; however, inducible cytoprotective mechanisms may limit its potential efficacy. To date, the cellular and molecular effects of proteasome inhibitors on bladder cancer cells have been poorly characterized. Despite the consistent rate of initial responses, cisplatin treatment typically results in the development of chemoresistance, leading to therapeutic failure. Therefore, the present study aimed to characterize the molecular mechanisms underlying the anti-proliferative effects of cisplatin and bortezomib combination therapy on the human T24 bladder cancer cell line, by analyzing the protein expression levels of apoptotic genes. Cytotoxic effects were measured using a water-soluble tetrazolium salt-1 assay, and the apoptosis-associated molecules were examined using western blot analysis and ELISA. It was observed that combined administration of cisplatin and bortezomib induced upregulation of caspase-3, -8 and -9, B-cell lymphoma-2 (Bcl-2)-like 11 and Bcl-2-interacting killer, but downregulated Bcl-2 and Bcl-extra large protein expression levels in T24 cells in a dose-dependent manner. Furthermore, enhanced protein expression of caspase-8 and -9, in line with the significantly increased caspase-3 activation, was detected when the cells were treated with a combination of cisplatin and bortezomib, compared with that of either agent alone. Bortezomib appeared to synergize with cisplatin to promote apoptosis via the extrinsic and intrinsic apoptotic pathways. Taken together, the results of the current study provide the preclinical framework for additional evaluation of the effects of combining bortezomib with other agents to induce apoptosis in bladder cancer cells.

Mesothelioma treatment: Are we on target? A review

Targeted treatment is a therapy directed at a specific molecular target close to a hallmark of cancer. The target should be measurable with a biomarker and measurement of the target should correlate with clinical outcome when targeted treatment is administered. Current clinical guidelines do not recommend targeted or biological therapy in MPM. However, since these recommendations came out, new agents have been investigated in MPM. This review updates the use of targeted and biological treatment in patients with mesothelioma.

Molecular Pathways: Leveraging the BCL-2 Interactome to Kill Cancer Cells--Mitochondrial Outer Membrane Permeabilization and Beyond

The inhibition of apoptosis enables the survival and proliferation of tumors and contributes to resistance to conventional chemotherapy agents and is therefore a very promising avenue for the development of new agents that will enhance current cancer therapies. The BCL-2 family proteins orchestrate apoptosis at the mitochondria and endoplasmic reticulum and are involved in other processes such as autophagy and unfolded protein response (UPR) that lead to different types of cell death. Over the past decade, significant efforts have been made to restore apoptosis using small molecules that modulate the activity of BCL-2 family proteins. The small molecule ABT-199, which antagonizes the activity of BCL-2, is currently the furthest in clinical trials and shows promising activity in many lymphoid malignancies as a single agent and in combination with conventional chemotherapy agents. Here, we discuss strategies to improve the specificity of pharmacologically modulating various antiapoptotic BCL-2 family proteins, review additional BCL-2 family protein interactions that can be exploited for the improvement of conventional anticancer therapies, and highlight important points of consideration for assessing the activity of small-molecule BCL-2 family protein modulators.

Apoptosis inducers in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a typical defect in apoptosis and is still an incurable disease. Numerous apoptosis inducers have been described. These synthetic compounds and natural products (mainly derived from plants) display antileukemic properties in vitro and in vivo and some have even been tested in the clinic in CLL. They act through several different mechanisms. Most of them involve proteins of the Bcl-2 family, which are the key regulators in triggering the mitochondrial pathway of caspase-dependent apoptosis. Thus, the Mcl-1/Noxa axis appeared as a target. Here I overview natural and synthetic apoptosis inducers and their mechanisms of action in CLL cells. Opportunities for developing novel, apoptosis-based therapeutics are presented.

Recent advances and future directions in targeting the secretory apparatus in multiple myeloma

Multiple myeloma is a genetically heterogeneous tumour of transformed plasma cells, terminally differentiated effectors of the B cell lineage specialized in producing large amounts of immunoglobulins. The uniquely well-developed secretory apparatus that equips normal and transformed plasma cells with the capacity for high-level protein secretion constitutes a distinctive therapeutic target. In this review we discuss how fundamental cellular processes, such as the unfolded protein response (UPR), endoplasmic reticulum (ER)-associated degradation and autophagy, maintain intracellular protein homeostasis (proteostasis) and regulate plasma cell ontogeny and malignancy. We summarize our current understanding of the cellular effects of proteasome inhibitors and the molecular bases of resistance to them. Furthermore, we discuss how improvements in our understanding of the secretory apparatus and of the complex interactions between intracellular protein synthesis and degradation pathways can disclose novel drug targets for multiple myeloma, defining a paradigm of general interest for cancer biology and disorders of altered proteostasis.

Role of AMP-activated protein kinase in cross-talk between apoptosis and autophagy in human colon cancer

Unresectable colorectal liver metastases remain a major unresolved issue and more effective novel regimens are urgently needed. While screening synergistic drug combinations for colon cancer therapy, we identified a novel multidrug treatment for colon cancer: chemotherapeutic agent melphalan in combination with proteasome inhibitor bortezomib and mTOR (mammalian target of rapamycin) inhibitor rapamycin. We investigated the mechanisms of synergistic antitumor efficacy during the multidrug treatment. All experiments were performed with highly metastatic human colon cancer CX-1 and HCT116 cells, and selected critical experiments were repeated with human colon cancer stem Tu-22 cells and mouse embryo fibroblast (MEF) cells. We used immunochemical techniques to investigate a cross-talk between apoptosis and autophagy during the multidrug treatment. We observed that melphalan triggered apoptosis, bortezomib induced apoptosis and autophagy, rapamycin caused autophagy and the combinatorial treatment-induced synergistic apoptosis, which was mediated through an increase in caspase activation. We also observed that mitochondrial dysfunction induced by the combination was linked with altered cellular metabolism, which induced adenosine monophosphate-activated protein kinase (AMPK) activation, resulting in Beclin-1 phosphorylated at Ser 93/96. Interestingly, Beclin-1 phosphorylated at Ser 93/96 is sufficient to induce Beclin-1 cleavage by caspase-8, which switches off autophagy to achieve the synergistic induction of apoptosis. Similar results were observed with the essential autophagy gene, autophagy-related protein 7, -deficient MEF cells. The multidrug treatment-induced Beclin-1 cleavage was abolished in Beclin-1 double-mutant (D133A/D146A) knock-in HCT116 cells, restoring the autophagy-promoting function of Beclin-1 and suppressing the apoptosis induced by the combination therapy. These observations identify a novel mechanism for AMPK-induced apoptosis through interplay between autophagy and apoptosis.

Efficient apoptosis and necrosis induction by proteasome inhibitor: bortezomib in the DLD-1 human colon cancer cell line

The inhibition of the 26S proteasome evokes endoplasmic reticulum stress, which has been shown to be implicated in the antitumoral effects of proteasome inhibitors. The cellular and molecular effects of the proteasome inhibitor—bortezomib—on human colon cancer cells are as yet poorly characterized. Bortezomib selectively induces apoptosis in some cancer cells. However, the nature of its selectivity remains unknown. Previously, we demonstrated that, in contrast to normal fibroblasts, bortezomib treatment evoked strong effect on apoptosis of breast cancer cells incubated in hypoxic and normoxic conditions. The study presented here provides novel information on the cellular effects of bortezomib in DLD-1 colon cancer cells line. We observe twofold higher percentage of apoptotic cells incubated for 48 h with 25 and 50 nmol/l of bortezomib in hypoxic conditions and four-, fivefold increase in normoxic conditions in comparison to control cells, incubated without bortezomib. It is of interest that bortezomib evokes strong effect on necrosis of DLD-1 colon cancer cell line. We observe the sixfold increase in necrosis of DLD-1 cells incubated with 25 or 50 nmol/l of bortezomib for 48 h in hypoxia and fourfold increase in normoxic conditions in comparison to adequate controls. We suggest that bortezomib may be candidates for further evaluation as chemotherapeutic agents for human colon cancer.

Systemic treatment of malignant pleural mesothelioma: new agents in clinical trials raise hope of relevant improvements

PURPOSE OF REVIEW

Malignant pleural mesothelioma (MPM) is a rare malignancy with limited therapeutic options and its incidence is still increasing in both Europe and the developing nations. Prognosis of MPM patients is poor even if the median survival durations have been slightly improved after the introduction of the up-to-date chemotherapy combination with pemetrexed and cisplatin. There is a continuing unmet need to develop better systemic treatment for this disease, but the rarity of the tumor type creates formidable challenges in clinical trial research.

RECENT FINDINGS

Better understanding of the molecular machinery of MPM leads to the design and synthesis of novel compounds targeted against pathways identified as crucial for MPM cell proliferation and metastasis. Most efforts aim at improving standard first-line therapy, or developing effective second-line treatments. Several classes of drugs are currently being explored either in combination with cisplatin and pemetrexed or as single agent for relapsed or progressive MPM.

SUMMARY

This review focuses on several ongoing or recently completed clinical trials investigating novel, promising agents as first-line or second-line therapy for advanced MPM.

MYC and EGR1 synergize to trigger tumor cell death by controlling NOXA and BIM transcription upon treatment with the proteasome inhibitor bortezomib

The c-MYC (MYC afterward) oncogene is well known for driving numerous oncogenic programs. However, MYC can also induce apoptosis and this function of MYC warrants further clarification. We report here that a clinically relevant proteasome inhibitor significantly increases MYC protein levels and that endogenous MYC is necessary for the induction of apoptosis. This kind of MYC-induced cell death is mediated by enhanced expression of the pro-apoptotic BCL2 family members NOXA and BIM. Quantitative promoter-scanning chromatin immunoprecipitations (qChIP) further revealed binding of MYC to the promoters of NOXA and BIM upon proteasome inhibition, correlating with increased transcription. Both promoters are further characterized by the presence of tri-methylated lysine 4 of histone H3, marking active chromatin. We provide evidence that in our apoptosis models cell death occurs independently of p53 or ARF. Furthermore, we demonstrate that recruitment of MYC to the NOXA as well as to the BIM gene promoters depends on MYC's interaction with the zinc finger transcription factor EGR1 and an EGR1-binding site in both promoters. Our study uncovers a novel molecular mechanism by showing that the functional cooperation of MYC with EGR1 is required for bortezomib-induced cell death. This observation may be important for novel therapeutic strategies engaging the inherent pro-death function of MYC.