Proteasome inhibitor PS-341 causes cell growth arrest and apoptosis in human glioblastoma multiforme (GBM)

Bortezomib sensitizes malignant human glioma cells to TRAIL, mediated by inhibition of the NF-{kappa}B signaling pathway

Previous studies have shown that the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has significant apoptosis-inducing activity in some glioma cell lines, although many lines are either moderately or completely resistant, which has limited the therapeutic applicability of this agent. Because our recent studies showed that inhibition of proteasomal function may be independently active as an apoptosis-inducing stimulus in these tumors, we investigated the sensitivity of a panel of glioma cell lines (U87, T98G, U373, A172, LN18, LN229, LNZ308, and LNZ428) to TRAIL alone and in combination with the proteasome inhibitor bortezomib. Analysis of these cell lines revealed marked differences in their sensitivity to these treatments, with two (LNZ308 and U373) of the eight cell lines revealing no significant induction of cell death in response to TRAIL alone. No correlation was found between sensitivity of cells to TRAIL and expression of TRAIL receptors DR4, DR5, and decoy receptor DcR1, caspase 8, apoptosis inhibitory proteins XIAP, survivin, Mcl-1, Bcl-2, Bcl-Xl, and cFLIP. However, TRAIL-resistant cell lines exhibited a high level of basal NF-κB activity. Bortezomib was capable of potentiating TRAIL-induced apoptosis in TRAIL-resistant cells in a caspase-dependent fashion. Bortezomib abolished p65/NF-κB DNA-binding activity, supporting the hypothesis that inhibition of the NF-κB pathway is critical for the enhancement of TRAIL sensitization in glioma cells. Moreover, knockdown of p65/NF-κB by shRNA also enhanced TRAIL-induced apoptosis, indicating that p65/NF-κB may be important in mediating TRAIL sensitivity and the effect of bortezomib in promoting TRAIL sensitization and apoptosis induction.

Marizomib, a proteasome inhibitor for all seasons: preclinical profile and a framework for clinical trials

The proteasome has emerged as an important clinically relevant target for the treatment of hematologic malignancies. Since the Food and Drug Administration approved the first-in-class proteasome inhibitor bortezomib (Velcade) for the treatment of relapsed/refractory multiple myeloma (MM) and mantle cell lymphoma, it has become clear that new inhibitors are needed that have a better therapeutic ratio, can overcome inherent and acquired bortezomib resistance and exhibit broader anti-cancer activities. Marizomib (NPI-0052; salinosporamide A) is a structurally and pharmacologically unique β-lactone-γ-lactam proteasome inhibitor that may fulfill these unmet needs. The potent and sustained inhibition of all three proteolytic activities of the proteasome by marizomib has inspired extensive preclinical evaluation in a variety of hematologic and solid tumor models, where it is efficacious as a single agent and in combination with biologics, chemotherapeutics and targeted therapeutic agents. Specifically, marizomib has been evaluated in models for multiple myeloma, mantle cell lymphoma, Waldenstrom's macroglobulinemia, chronic and acute lymphocytic leukemia, as well as glioma, colorectal and pancreatic cancer models, and has exhibited synergistic activities in tumor models in combination with bortezomib, the immunomodulatory agent lenalidomide (Revlimid), and various histone deacetylase inhibitors. These and other studies provided the framework for ongoing clinical trials in patients with MM, lymphomas, leukemias and solid tumors, including those who have failed bortezomib treatment, as well as in patients with diagnoses where other proteasome inhibitors have not demonstrated significant efficacy. This review captures the remarkable translational studies and contributions from many collaborators that have advanced marizomib from seabed to bench to bedside.

Inhibition of heat shock protein response enhances PS-341-mediated glioma cell death

BACKGROUND

Previous study indicated that PS-341 induces cell death via JNK pathway in vitro in glioma. However, suppressing proteasome complex by PS-341 may induce expression of heat shock proteins (HSPs), which confer potential protection against cellular stress. In this study, we explored whether induction of HSPs could impair PS-341-induced cell death and whether inhibition of HSPs could enhance cell damage induced by PS-341 in glioma cells.

METHODS

HSP expression in glioma cells was modulated by HSP inhibitor, sublethal heat, or knockdown of heat shock factor1 (HSF1), then PS-341-induced cell damage was examined by different methods. Similar experiments were also performed in HSF1+/+ and HSF1-/- cells. HSP70 expression and HSF1 nuclear localization were compared between glioma and normal brain tissues.

RESULTS

HSP level was upregulated mediated by HSF1 when glioma cells were treated with PS-341. PS-341-mediated cell damage could be significantly augmented by HSP inhibition. Furthermore, HSP70 expression and HSF1 nuclear localization were much more abundant in gliomas than in normal brain tissues.

CONCLUSIONS

Our results demonstrated that HSP70 impaired cell death induced by PS-341 in glioma cells. Administration of PS-341 in combination with either HSP70 inhibitor or HSF1 knockdown may act as a new approach to treatment of glioma.

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.

Antiviral inhibition targeting the HCMV kinase pUL97 requires pUL27-dependent degradation of Tip60 acetyltransferase and cell-cycle arrest

Infection with the β-herpesvirus human cytomegalovirus (HCMV) is lifelong, causing limited disease in healthy adults, but life threatening in immunocompromised individuals. The viral kinase pUL97, a functional ortholog of cellular cyclin-dependent kinases (CDKs), is critical for HCMV replication and a target for antiviral drug development. Upon kinase inhibition, drug-resistant strains emerge with mutations in UL27, an HCMV gene of unknown function. Using a proteomics approach, we discovered that pUL27 is necessary and sufficient to degrade Tip60, a host acetyltransferase and interacting partner of HIV Tat. Consistent with this, the expression of Tat restored antiviral inhibition of an otherwise resistant HCMV strain. The functional consequence of Tip60 degradation was the induction of the CDK inhibitor p21(Waf1/Cip1) and cell-cycle arrest, representing changes necessary for the antiviral effects of pUL97 inhibition. Consequently, either increasing p21(Waf1/Cip1) expression or decreasing Tip60 levels improved the antiviral activity of the HCMV kinase inhibitor maribavir.

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.

Proteasome inhibitor MG-132 induces C6 glioma cell apoptosis via oxidative stress

AIM

Proteasome inhibitors have been found to suppress glioma cell proliferation and induce apoptosis, but the mechanisms are not fully elucidated. In this study we investigated the mechanisms underlying the apoptosis induced by the proteasome inhibitor MG-132 in glioma cells.

METHODS

C6 glioma cells were used. MTT assay was used to analyze cell proliferation. Proteasome activity was assayed using Succinyl-LLVY-AMC, and intracellular ROS level was evaluated with the redox-sensitive dye DCFH-DA. Apoptosis was detected using fluorescence and transmission electron microscopy as well as flow cytometry. The expression of apoptosis-related proteins was investigated using Western blot analysis.

RESULTS

MG-132 inhibited C6 glioma cell proliferation in a time- and dose-dependent manner (the IC(50) value at 24 h was 18.5 μmol/L). MG-132 (18.5 μmol/L) suppressed the proteasome activity by about 70% at 3 h. It induced apoptosis via down-regulation of antiapoptotic proteins Bcl-2 and XIAP, up-regulation of pro-apoptotic protein Bax and caspase-3, and production of cleaved C-terminal 85 kDa PARP). It also caused a more than 5-fold increase of reactive oxygen species. Tiron (1 mmol/L) effectively blocked oxidative stress induced by MG-132 (18.5 μmol/L), attenuated proliferation inhibition and apoptosis in C6 glioma cells, and reversed the expression pattern of apoptosis-related proteins.

CONCLUSION

MG-132 induced apoptosis of C6 glioma cells via the oxidative stress.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Combined modality therapy with TRAIL or agonistic death receptor antibodies

Molecularly targeted therapies, such as antibodies and small molecule inhibitors have emerged as an important breakthrough in the treatment of many human cancers. One targeted therapy under development is tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) due to its ability to induce apoptosis in a variety of human cancer cell lines and xenografts, while lacking toxicity in most normal cells. TRAIL and apoptosis-inducing agonistic antibodies to the TRAIL death receptors have been the subject of many preclinical and clinical studies in the past decade. However, the sensitivity of individual cancer cell lines of a particular tumor type to these agents varies from highly sensitive to resistant. Various chemotherapy agents have been shown to enhance the apoptosis-inducing capacity of TRAIL receptor-targeted therapies and induce sensitization of TRAIL-resistant cells. This review provides an overview of the mechanisms associated with chemotherapy enhancement of TRAIL receptor-targeted therapies including modulation of the apoptotic (death receptor expression, FLIP, and Bcl-2 or inhibitors of apoptosis (IAP) families) as well as cell signaling (NFκB, Akt, p53) pathways. These mechanisms will be important in establishing effective combinations to pursue clinically and in determining relevant targets for future cancer therapies.

Neural stem/progenitors and glioma stem-like cells have differential sensitivity to chemotherapy

OBJECTIVES

New data suggest that glioma stem-like cells (GSCs) and neural stem/progenitor cells (NSCs) may share common origins. GSCs drive tumor proliferation and appear to be resistant to classic chemotherapy, while the effects of chemotherapy on NSCs are not well studied. As the role of NSCs in learning and memory is increasingly recognized, we need to identify drugs that reduce neurotoxicity but are still effective against glial tumors.

METHODS

We treated 3 human NSC cultures and multiple low- and high-grade GSC cultures with the commonly used agents temozolomide (TMZ) and cisplatin (CIS), and with 2 newer, promising drugs: the proteasome inhibitor bortezomib (BTZ) and the epidermal growth factor receptor tyrosine kinase inhibitor erlotinib (ERL). We measured cell survival, proliferation, cell death induction, and drug resistance markers.

RESULTS

TMZ decreased NSC viability, while minimally affecting GSCs. TMZ induced NSC death, which was partially compensated for by increased proliferation. CIS had similar effects. The NSC's sensitivity to TMZ and CIS correlated with low expression of the multidrug resistance gene ABCG2, but not of MGMT or MSH1/MLH2. BTZ caused an 80%decrease in GSCs, while minimally affecting NSCs. GSCs had lower proteasome levels and activity after BTZ treatment. ERL treatment also decreased GSC numbers, but not NSC viability, which correlated with low EGFR expression in NSCs compared to GSCs.

CONCLUSIONS

Newer chemotherapy agents ERL and BTZ are effective against GSCs yet produce minimal effects on NSCs, while the older drugs TMZ and CIS are more toxic for NSCs than for GSCs. The identification and testing of more selective drugs is clearly warranted.

BAG3 expression in glioblastoma cells promotes accumulation of ubiquitinated clients in an Hsp70-dependent manner

Disposal of damaged proteins and protein aggregates is a prerequisite for the maintenance of cellular homeostasis and impairment of this disposal can lead to a broad range of pathological conditions, most notably in brain-associated disorders including Parkinson and Alzheimer diseases, and cancer. In this respect, the Protein Quality Control (PQC) pathway plays a central role in the clearance of damaged proteins. The Hsc/Hsp70-co-chaperone BAG3 has been described as a new and critical component of the PQC in several cellular contexts. For example, the expression of BAG3 in the rodent brain correlates with the engagement of protein degradation machineries in response to proteotoxic stress. Nevertheless, little is known about the molecular events assisted by BAG3. Here we show that ectopic expression of BAG3 in glioblastoma cells leads to the activation of an HSF1-driven stress response, as attested by transcriptional activation of BAG3 and Hsp70. BAG3 overexpression determines an accumulation of ubiquitinated proteins and this event requires the N-terminal region, WW domain of BAG3 and the association of BAG3 with Hsp70. The ubiquitination mainly occurs on BAG3-client proteins and the inhibition of proteasomal activity results in a further accumulation of ubiquitinated clients. At the cellular level, overexpression of BAG3 in glioblastoma cell lines, but not in non-glial cells, results in a remarkable decrease in colony formation capacity and this effect is reverted when the binding of BAG3 to Hsp70 is impaired. These observations provide the first evidence for an involvement of BAG3 in the ubiquitination and turnover of its partners.

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

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

Molecular tools: biology, prognosis, and therapeutic triage

Diffuse gliomas in adults continue to have a dismal prognosis with the current standard therapeutic methods, including maximal surgical resection, radiation, and chemotherapy. The pathogenesis of adult glioma is complex, involving the loss of function of tumor suppressor genes and activation of oncogenes, which are involved in a network of interconnected signaling pathways. Through activation of these pathways, characteristics of malignant gliomas, including uncontrolled proliferation and growth, invasion, and angiogenesis, are driven. Evolving therapeutic approaches are focused on specifically targeting these genetic lesions. This content gives an overview of the current knowledge about the pathogenesis of adult diffuse gliomas, emphasizing new targeted treatment approaches.

Novel medical therapeutics in glioblastomas, including targeted molecular therapies, current and future clinical trials

The prognosis for glioblastoma is poor despite optimal therapy with surgery, radiation, and chemotherapy. New therapies that improve survival and quality of life are needed. Research has increased our understanding of the molecular pathways important for gliomagenesis and disease progression. Novel agents have been developed against these targets, including receptor tyrosine kinases, intracellular signaling molecules, epigenetic abnormalities, and tumor vasculature and microenvironment. This article reviews novel therapies for glioblastoma, with an emphasis on targeted agents.

Connective tissue growth factor associated with oncogenic activities and drug resistance in glioblastoma multiforme

Connective tissue growth factor (CTGF or CCN2) is a secreted protein that belongs to the CCN [cysteine-rich CYR61/CTGF/nephroblastoma-overexpressed gene] family. These proteins have been implicated in various biological processes, including stimulation of cell proliferation, migration, angiogenesis and tumorigenesis. In a previous study, we found that CTGF mRNA was elevated in primary gliomas, and a significant correlation existed between CTGF mRNA levels versus tumor grade, histology and patient survival. In this study, the role of CTGF in glioma tumorigenesis was explored. Forced expression of CTGF in glioblastoma multiforme (GBM) cells accelerated their growth in liquid culture and soft agar, stimulated cells migration in Boyden chamber assays and significantly increased their ability to form large, vascularized tumors in nude mice. CTGF induced the expression of the antiapoptotic proteins, Bcl-xl, Survivin and Flip. Overexpression of CTGF caused the U343 GBM cells to survive for longer than 40 days in serum-free medium and resist antitumor drugs including tumor necrosis factor (TNF), TNF-related apoptosis-inducing ligand, VELCADE (bortezomib, proteasome inhibitor) and temozolomide. Our data suggest that CTGF plays an important role in glioma progression, by supporting tumor cells survival and drug resistance.

Essential role of TRPC6 channels in G2/M phase transition and development of human glioma

BACKGROUND

Patients with glioblastoma multiforme, the most aggressive form of glioma, have a median survival of approximately 12 months. Calcium (Ca(2+)) signaling plays an important role in cell proliferation, and some members of the Ca(2+)-permeable transient receptor potential canonical (TRPC) family of channel proteins have demonstrated a role in the proliferation of many types of cancer cells. In this study, we investigated the role of TRPC6 in cell cycle progression and in the development of human glioma.

METHODS

TRPC6 protein and mRNA expression were assessed in glioma (n = 33) and normal (n = 17) brain tissues from patients and in human glioma cell lines U251, U87, and T98G. Activation of TRPC6 channels was tested by platelet-derived growth factor-induced Ca(2+) imaging. The effect of inhibiting TRPC6 activity or expression using the dominant-negative mutant TRPC6 (DNC6) or RNA interference, respectively, was tested on cell growth, cell cycle progression, radiosensitization of glioma cells, and development of xenografted human gliomas in a mouse model. The green fluorescent protein (GFP) and wild-type TRPC6 (WTC6) were used as controls. Survival of mice bearing xenografted tumors in the GFP, DNC6, and WTC6 groups (n = 13, 15, and 13, respectively) was compared using Kaplan-Meier analysis. All statistical tests were two-sided.

RESULTS

Functional TRPC6 was overexpressed in human glioma cells. Inhibition of TRPC6 activity or expression attenuated the increase in intracellular Ca(2+) by platelet-derived growth factor, suppressed cell growth and clonogenic ability, induced cell cycle arrest at the G2/M phase, and enhanced the antiproliferative effect of ionizing radiation. Cyclin-dependent kinase 1 activation and cell division cycle 25 homolog C expression regulated the cell cycle arrest. Inhibition of TRPC6 activity also reduced tumor volume in a subcutaneous mouse model of xenografted human tumors (P = .014 vs GFP; P < .001 vs WTC6) and increased mean survival in mice in an intracranial model (P < .001 vs GFP or WTC6).

CONCLUSIONS

In this preclinical model, TRPC6 channels were essential for glioma development via regulation of G2/M phase transition. This study suggests that TRPC6 might be a new target for therapeutic intervention of human glioma.

Proteasome inhibitor PS-341 (bortezomib) induces calpain-dependent IkappaB(alpha) degradation

The proteasome, a key component of the ubiquitin-proteasome pathway, has emerged as an important cancer therapeutic target. PS-341 (also called Bortezomib or Velcade) is the first proteasome inhibitor approved for newly diagnosed and relapsed multiple myeloma and is currently being tested in many clinical trials against other types of cancers. One proposed mechanism by which PS-341 exerts its anticancer effect is inactivation of nuclear factor-kappaB (NF-kappaB) through prevention of IkappaB(alpha) degradation. In this study, we show that PS-341 at concentrations that effectively inhibited the growth of human cancer cells, instead of increasing IkappaB(alpha) stability, paradoxically induced IkappaB(alpha) degradation. As a result, PS-341 facilitated p65 nuclear translocation and increased NF-kappaB activity. Moreover, IkappaB(alpha) degradation by PS-341 occurred early before induction of apoptosis and could not be inhibited by a pan-caspase inhibitor or caspase-8 silencing; however, it could be prevented with calpain inhibitors, calcium-chelating agents, calpain knockdown, or calpastatin overexpression. In agreement, PS-341 increased calpain activity. These data together indicate that PS-341 induces a calpain-mediated IkappaB(alpha) degradation independent of caspases. In the presence of a calpain inhibitor, the apoptosis-inducing activity of PS-341 was dramatically enhanced. Collectively, these unexpected findings suggest not only a novel paradigm regarding the relationship between proteasome inhibition and NF-kappaB activity but also a strategy to enhance the anticancer efficacy of PS-341.

Proteasome inhibition: a new therapeutic strategy to cancer treatment

The ubiquitin-proteasome system is a major pathway for protein degradation. Targeting this pathway using proteasome inhibitors represents a novel approach for the treatment of cancer. Proteasome inhibitors lower cell proliferation and induce apoptosis in solid and hematologic malignancies through multiple mechanisms, including stabilization of cell cycle regulators and pro-apoptotic factors, stimulation of bone morphogenetic protein signaling, inhibition of protein translation, and sensitization to ligand-induced apoptosis. In this connection, proteasome inhibition activates macroautophagy, a compensatory protein degradation system, as well as other pro-survival signaling pathways. Inhibition of these auto-protective responses sensitizes cancer cells to the anti-proliferative effects of proteasome inhibitors.

Differential Effects of the Proteasome Inhibitor NPI-0052 against Glioma Cells

Proteasome inhibitors are emerging as a new class of cancer therapeutics, and bortezomib has shown promise in the treatment of multiple myeloma and mantle cell lymphoma. However, bortezomib has failed to have an effect in preclinical models of glioma. NPI-0052 is a new generation of proteasome inhibitors with increased potency and strong inhibition of all three catalytic activities of the 26S proteasome. In this article, we test the antitumor efficacy of NPI-0052 against glioma, as a single agent and in combination with temozolomide and radiation using five different glioma lines. The intrinsic radiation sensitivities differed for all the lines and correlated with their PTEN expression status. In vitro, NPI-0052 showed a dose-dependent toxicity, and its combination with temozolomide resulted in radiosensitization of only the cell lines with a mutated p53. The effect of NPI-0052 as a single agent on glioma xenografts in vivo was only modest in controlling tumor growth, and it failed to radiosensitize the glioma xenografts to fractionated radiation. We conclude that NPI-0052 is not a suitable drug for the treatment of malignant gliomas despite its efficacy in other cancer types.

Screening a panel of drugs with diverse mechanisms of action yields potential therapeutic agents against neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in children. Despite current aggressive therapy, the survival rate for high risk NB remains less than 40%. To identify novel effective chemo-agents against NB, we screened a panel of 96 drugs against two NB cell lines, SK-N-AS and SH-SY5Y. We found 30 compounds that were active against NB cell lines at < or =10 microM concentration. More interestingly, 17 compounds are active at < or =1 microM concentration, and they act through a wide spectrum of diverse mechanisms such as mitotic inhibition, topoisomerase inhibition, targeting various biological pathways, and unknown mechanisms. The majority of these active compounds also induced caspase 3/7 by more than 2-fold. Of these 17 active compounds against NB cell lines at sub-micromolar concentration, eleven compounds are not currently used to treat NB. Among them, nine are FDA approved compounds, and three agents are undergoing clinical trials for various malignancies. Furthermore, we identified four agents active against these NB cell lines that have not yet been tested in the clinical setting. Finally we demonstrated that Cucurbitacin I inhibits neuroblastoma cell growth through inhibition of STAT3 pathway. These drugs thus represent potential novel therapeutic agents for patients with NB, and further validation studies are needed to translate them to the clinic.

NF-kappaB as a prognostic marker and therapeutic target in chronic lymphocytic leukemia

Chronic lymphocytic leukemia is the most common adult leukemia and is currently incurable with conventional chemotherapeutic agents. Over the last few years, significant discoveries have been made regarding the biology that underpins this disease. These new insights have allowed us to develop more rational prognostic tools and identify promising novel therapeutic targets. In this review, we highlight the importance of both constitutive and inducible DNA binding of the transcription factor NF-kappaB in chronic lymphocytic leukemia. We describe the current knowledge regarding the activity and function of specific NF-kappaB subunits in this disease, and discuss the complex mechanisms that regulate NF-kappaB activation in vivo. In addition, we provide compelling evidence for the utility of the NF-kappaB subunit, Rel A, as a prognostic marker and as a therapeutic target in this disease, and we also describe how this protein may contribute to the drug resistance commonly encountered with this condition.

Molecularly targeted therapies for malignant glioma: rationale for combinatorial strategies

Median survival of patients with malignant glioma (MG) from time of diagnosis is approximately 1 year, despite surgery, irradiation and conventional chemotherapy. Improving patient outcome relies on our ability to develop more effective therapies that are directed against the unique molecular aberrations within a patient's tumor. Such molecularly targeted therapies may provide novel treatments that are more effective than conventional chemotherapeutics. Recently developed therapeutic strategies have focused on targeting several core glioma signaling pathways, including pathways mediated by growth-factors, PI3K/Akt/PTEN/mTOR, Ras/Raf/MEK/MAPK and other vital pathways. However, given the molecular diversity, heterogeneity and diverging and converging signaling pathways associated with MG, it is unlikely that any single agent will have efficacy in more than a subset of tumors. Overcoming these therapeutic barriers will require multiple agents that can simultaneously inhibit these processes, providing a rationale for combination therapies. This review summarizes the currently implemented single-agent and combination molecularly targeted therapies for MG.

Celastrol causes apoptosis and cell cycle arrest in rat glioma cells

Glioma still remains a major health problem in the world. Celastrol has been proved to be an effective natural proteasome inhibitor and was used for treatment of autoimmune disease, chronic inflammation and neurodegenerative disease. However, its effect on glioma is unclear. In this study, we investigated the therapeutic effects of celastrol on C6 glioma cells. The results demonstrated that celastrol inhibited cell proliferation in a time- and dose-dependent manner, suppressed proteasome chymotrypsin-like activity and induced apoptosis and cell cycle arrest at G2/M phase in C6 cells. Proapoptosis proteins bax and caspase-3 were up-regulated, as well as cell cycle G2/M-related proteins cyclin B(1), p21 and p27. Conversely, anti-apoptosis proteins bcl-2 and XIAP and cell cycle regulator cyclin-dependent kinase 2 were down-regulated. Taken together, our data suggest that celastrol can suppress proteasome activity and induce apoptosis and cell cycle arrest in C6 glioma cells, which make it be a potential drug for glioma.

Characterization of caspase-dependent and caspase-independent deaths in glioblastoma cells treated with inhibitors of the ubiquitin-proteasome system

The regulation of the necrotic death and its relevance in anticancer therapy are largely unknown. Here, we have investigated the proapoptotic and pronecrotic activities of two ubiquitin-proteasome system inhibitors: bortezomib and G5. The present study points out that the glioblastoma cell lines U87MG and T98G are useful models to study the susceptibility to apoptosis and necrosis in response to ubiquitin-proteasome system inhibitors. U87MG cells show resistance to apoptosis induced by bortezomib and G5, but they are more susceptible to necrosis induced by G5. Conversely, T98G cells are more susceptible to apoptosis induced by both inhibitors but show some resistance to G5-induced necrosis. No overt differences in the induction of Noxa and Mcl-1 or in the expression levels of other components of the apoptotic machinery were observed between U87MG and T98G cells. Instead, by comparing the transcriptional profiles of the two cell lines, we have found that the resistance to G5-induced necrosis could arise from differences in glutathione synthesis/utilization and in the microenvironment. In particular, collagen IV, which is highly expressed in T98G cells, and fibronectin, whose adhesive function is counteracted by tenascin-C in U87MG cells, can restrain the necrotic response to G5. Collectively, our results provide an initial characterization of the molecular signals governing cell death by necrosis in glioblastoma cell lines.

SC68896, a novel small molecule proteasome inhibitor, exerts antiglioma activity in vitro and in vivo

PURPOSE

Glioblastomas are among the most lethal neoplasms, with a median survival of <1 year. Modulation of the proteasome function has emerged as a novel approach to cancer pharmacotherapy. Here, we characterized the antitumor properties of SC68896, a novel small molecule proteasome inhibitor.

EXPERIMENTAL DESIGN

Different tumor cell lines were tested by crystal violet staining for sensitivity to SC68896, given alone or in combination with death ligands. The molecular mechanisms mediating SC68896-induced cell death and changes in cell cycle progression were assessed by immunoblot and flow cytometry. An orthotopic human glioma xenograft model in nude mice was used to examine the in vivo activity of SC68896.

RESULTS

SC68896 inhibits the proliferation of cell lines of different types of cancer, including malignant glioma. Exposure of LNT-229 glioma cells to SC68896 results in a concentration- and time-dependent inhibition of the proteasome, with a consequent accumulation of p21 and p27 proteins, cell cycle arrest, caspase cleavage, and induction of apoptosis. Using RNA interference, we show that the effect of SC68896 on glioma cells is facilitated by wild-type p53. SC68896 sensitizes glioma cells to tumor necrosis factor-related apoptosis-inducing ligand and CD95 ligand and up-regulates the cell surface expression of the tumor necrosis factor-related apoptosis-inducing ligand receptor cell death receptors 4 and 5, which may contribute to this sensitization. Intracerebral glioma-bearing nude mice treated either i.p. or intratumorally with SC68896 experience prolonged survival.

CONCLUSIONS

SC68896 is the first proteasome inhibitor that exerts antiglioma activity in vivo. It may represent a novel prototype agent for the treatment of malignant gliomas and warrants clinical evaluation.

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

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

Identification of survival genes in human glioblastoma cells by small interfering RNA screening

Target identification and validation remain difficult steps in the drug discovery process, and uncovering the core genes and pathways that are fundamental for cancer cell survival may facilitate this process. Glioblastoma represents a challenging form of cancer for chemotherapy. Therefore, we assayed 16,560 short interfering RNA (siRNA) aimed at identifying which of the 5520 unique therapeutically targetable gene products were important for the survival of human glioblastoma. We analyzed the viability of T98G glioma cells 96 h after siRNA transfection with two orthogonal statistical methods and identified 55 survival genes that encoded proteases, kinases, and transferases. It is noteworthy that 22% (12/55) of the survival genes were constituents of the 20S and 26S proteasome subunits. An expression survey of a panel of glioma cell lines demonstrated expression of the proteasome component PSMB4, and the validity of the proteasome complex as a target for survival inhibition was confirmed in a series of glioma and nonglioma cell lines by pharmacological inhibition and RNA interference. Biological networks were built with the other survival genes using a protein-protein interaction network, which identified clusters of cellular processes, including protein ubiquitination, purine and pyrimidine metabolism, nucleotide excision repair, and NF-kappaB signaling. The results of this study should broaden our understanding of the core genes and pathways that regulate cell survival; through either small molecule inhibition or RNA interference, we highlight the potential significance of proteasome inhibition.

The cytotoxicity of gamma-secretase inhibitor I to breast cancer cells is mediated by proteasome inhibition, not by gamma-secretase inhibition

Introduction

Notch is a family of transmembrane protein receptors whose activation requires proteolytic cleavage by γ-secretase. Since aberrant Notch signaling can induce mammary carcinomas in transgenic mice and high expression levels of Notch receptors and ligands correlates with overall poor clinical outcomes, inhibiting γ-secretase with small molecules may be a promising approach for breast cancer treatment. Consistent with this hypothesis, two recent papers reported that γ-secretase inhibitor I (GSI I), Z-LLNle-CHO, is toxic to breast cancer cells both in vitro and in vivo. In this study, we compared the activity and cytotoxicity of Z-LLNle-CHO to that of two highly specific GSIs, DAPT and L-685,458 and three structurally unrelated proteasome inhibitors, MG132, lactacystin, and bortezomib in order to study the mechanism underlying the cytotoxicity of Z-LLNle-CHO in breast cancer cells.

Methods

Three estrogen receptor (ER) positive cell lines, MCF-7, BT474, and T47D, and three ER negative cell lines, SKBR3, MDA-MB-231, and MDA-MB-468, were used in this study. Both SKBR3 and BT474 cells also overexpress HER2/neu. Cytotoxicity was measured by using an MTS cell viability/proliferation assay. Inhibition of γ-secretase activity was measured by both immunoblotting and immunofluorescent microscopy in order to detect active Notch1 intracellular domain. Proteasome inhibition was determined by using a cell-based proteasome activity assay kit, by immunoblotting to detect accumulation of polyubiquitylated protein, and by immunofluorescent microscopy to detect redistribution of cellular ubiquitin.

Results

We found that blocking γ-secretase activity by DAPT and L-685,458 had no effect on the survival and proliferation of a panel of six breast cancer cell lines while Z-LLNle-CHO could cause cell death even at concentrations that inhibited γ-secretase activity less efficiently. Furthermore, we observed that Z-LLNle-CHO could inhibit proteasome activity and the relative cellular sensitivity of these six breast cancer cell lines to Z-LLNle-CHO was the same as observed for three proteasome inhibitors. Finally, we found that the cell killing effect of Z-LLNle-CHO could be reversed by a chemical that restored the proteasome activity.

Conclusions

We conclude that the cytotoxicity of Z-LLNle-CHO in breast cancer cells is mediated by proteasome inhibition, not by γ-secretase inhibition.

Bortezomib reverses a post-translational mechanism of tumorigenesis for patched1 haploinsufficiency in medulloblastoma

BACKGROUND

Tumor initiation has been attributed to haploinsufficiency at a single locus for a large number of cancers. Patched1 (Ptc1) was one of the first such loci, and Ptc1 haploinsufficiency has been asserted to lead to medulloblastoma and rhabdomyosarcoma in mice.

PROCEDURE

To study the role of Ptc1 in cerebellar tumor development and to create a preclinical therapeutic platform, we have generated a conditional Ptc1 haploinsufficiency model of medulloblastoma by inactivating Ptc1 in Pax7-expressing cells of the cerebellum.

RESULTS

These mice developed exclusively medulloblastoma. We show that despite the presence of transcription of Ptc1, Ptc1 protein is nearly undetectable or absent in tumors. Our results suggest that Ptc1 loss of function is complete, but achieved at the protein level rather than by the classic genetic two-hit mechanism or a strict half-dosage genetic haploinsufficiency mechanism. Furthermore, we found that bortezomib, a 26S proteasome inhibitor, had a significant anti-tumor activity in vitro and in vivo, which was accompanied by restoration of Ptc1 protein and downregulation of the hedgehog signaling pathway. The same effect was seen for both human and mouse medulloblastoma tumor cell growth.

CONCLUSIONS

These results suggest that proteasome inhibition is a potential new therapeutic approach in medulloblastoma.

Inhibition of autophagy induced by proteasome inhibition increases cell death in human SHG-44 glioma cells

AIM

The ubiquitin-proteasome system (UPS) and lysosome-dependent macroautophagy (autophagy) are two major intracellular pathways for protein degradation. Recent studies suggest that proteasome inhibitors may reduce tumor growth and activate autophagy. Due to the dual roles of autophagy in tumor cell survival and death, the effect of autophagy on the destiny of glioma cells remains unclear. In this study, we sought to investigate whether inhibition of the proteasome can induce autophagy and the effects of autophagy on the fate of human SHG-44 glioma cells.

METHODS

The proteasome inhibitor MG-132 was used to induce autophagy in SHG-44 glioma cells, and the effect of autophagy on the survival of SHG-44 glioma cells was investigated using an autophagy inhibitor 3-MA. Cell viability was measured by MTT assay. Apoptosis and cell cycle were detected by flow cytometry. The expression of autophagy related proteins was determined by Western blot.

RESULTS

MG-132 inhibited cell proliferation, induced cell death and cell cycle arrest at G(2)/M phase, and activated autophagy in SHG-44 glioma cells. The expression of autophagy-related Beclin-1 and LC3-I was significantly up-regulated and part of LC3-I was converted into LC3-II. However, when SHG-44 glioma cells were co-treated with MG-132 and 3-MA, the cells became less viable, but cell death and cell numbers at G(2)/M phase increased. Moreover, the accumulation of acidic vesicular organelles was decreased, the expression of Beclin-1 and LC3 was significantly down-regulated and the conversion of LC3-II from LC3-I was also inhibited.

CONCLUSION

Inhibition of the proteasome can induce autophagy in human SHG-44 glioma cells, and inhibition of autophagy increases cell death. This discovery may shed new light on the effect of autophagy on modulating the fate of SHG-44 glioma cells.Acta Pharmacologica Sinica (2009) 30: 1046-1052; doi: 10.1038/aps.2009.71.

High-resolution genomic copy number profiling of glioblastoma multiforme by single nucleotide polymorphism DNA microarray

Glioblastoma multiforme (GBM) is an extremely malignant brain tumor. To identify new genomic alterations in GBM, genomic DNA of tumor tissue/explants from 55 individuals and 6 GBM cell lines were examined using single nucleotide polymorphism DNA microarray (SNP-Chip). Further gene expression analysis relied on an additional 56 GBM samples. SNP-Chip results were validated using several techniques, including quantitative PCR (Q-PCR), nucleotide sequencing, and a combination of Q-PCR and detection of microsatellite markers for loss of heterozygosity with normal copy number [acquired uniparental disomy (AUPD)]. Whole genomic DNA copy number in each GBM sample was profiled by SNP-Chip. Several signaling pathways were frequently abnormal. Either the p16(INK4A)/p15(INK4B)-CDK4/6-pRb or p14(ARF)-MDM2/4-p53 pathways were abnormal in 89% (49 of 55) of cases. Simultaneous abnormalities of both pathways occurred in 84% (46 of 55) samples. The phosphoinositide 3-kinase pathway was altered in 71% (39 of 55) GBMs either by deletion of PTEN or amplification of epidermal growth factor receptor and/or vascular endothelial growth factor receptor/platelet-derived growth factor receptor alpha. Deletion of chromosome 6q26-27 often occurred (16 of 55 samples). The minimum common deleted region included PARK2, PACRG, QKI, and PDE10A genes. Further reverse transcription Q-PCR studies showed that PARK2 expression was decreased in another collection of GBMs at a frequency of 61% (34 of 56) of samples. The 1p36.23 region was deleted in 35% (19 of 55) of samples. Notably, three samples had homozygous deletion encompassing this site. Also, a novel internal deletion of a putative tumor suppressor gene, LRP1B, was discovered causing an aberrant protein. AUPDs occurred in 58% (32 of 55) of the GBM samples and five of six GBM cell lines. A common AUPD was found at chromosome 17p13.3-12 (included p53 gene) in 13 of 61 samples and cell lines. Single-strand conformational polymorphism and nucleotide sequencing showed that 9 of 13 of these samples had homozygous p53 mutations, suggesting that mitotic recombination duplicated the abnormal p53 gene, probably providing a growth advantage to these cells. A significantly shortened survival time was found in patients with 13q14 (RB) deletion or 17p13.1 (p53) deletion/AUPD. Taken together, these results suggest that this technique is a rapid, robust, and inexpensive method to profile genome-wide abnormalities in GBM.

Combining the endoplasmic reticulum stress-inducing agents bortezomib and fenretinide as a novel therapeutic strategy for metastatic melanoma

PURPOSE

Single-agent chemotherapy is largely the treatment of choice for systemic therapy of metastatic melanoma, but survival rates are low, and novel adjuvant and systemic therapies are urgently required. Endoplasmic reticulum (ER) stress is a potential therapeutic target, and two relatively new drugs, fenretinide and bortezomib (Velcade), each acting via different cellular mechanisms, induce ER stress leading to apoptosis in melanoma cells. The aim of this study was to test the hypothesis that apoptosis of melanoma cells may be increased by combining clinically achievable concentrations of fenretinide and bortezomib.

EXPERIMENTAL DESIGN

Three human melanoma cell lines were used to assess changes in viability and the induction of apoptosis in response to fenretinide, bortezomib, or both drugs together. A s.c. xenograft model was used to test responses in vivo.

RESULTS

Fenretinide and bortezomib synergistically decreased viability and increased apoptosis in all three melanoma lines at clinically achievable concentrations. This was also reflected by increased expression of GADD153, a marker of ER stress-induced apoptosis. In vivo, fenretinide in combination with bortezomib gave a marked reduction in xenograft tumor volume and an increase in apoptosis compared with fenretinide or bortezomib alone. The cell cycle stage of tumor cells in vivo were similar to that predicted from the effects of each drug or the combination in vitro.

CONCLUSIONS

These results suggest that fenretinide and bortezomib, both of which are available in clinical formulation, warrant clinical evaluation as a combination therapy for metastatic melanoma.

Bmf is upregulated by PS-341-mediated cell death of glioma cells through JNK phosphorylation

Malignant glioma is resistant to the induction of apoptosis, resulting in a subsequent failure of chemotherapy in clinical treatment strategies. Downregulation of bcl-2 and bcl-xl expression in glioblastoma cells can induce apoptosis. BH3-only proteins, which include Bmf, are essential initiators of stress-induced cell death and apoptosis. Whether PS-341 regulates expression of BH3-only proteins in glioblastoma cells during the procedure of apoptosis is unclear. This study was designed to investigate the effects of PS-341 on glioma cell death and its possible signaling pathway. Our results demonstrate that Bmf is upregulated by PS-341 in A172 and T98G cells, and Bmf has a crucial role in PS-341-mediated cell death. In addition, we found that expression of Bmf is regulated by JNK phosphorylation.

Inhibition of eIF2alpha dephosphorylation maximizes bortezomib efficiency and eliminates quiescent multiple myeloma cells surviving proteasome inhibitor therapy

The proteasome inhibitor bortezomib (Velcade) effectively eradicates multiple myeloma (MM) cells, partly by activating endoplasmic reticulum (ER) stress apoptotic signaling. However, MM recurrences in bortezomib-treated patients are invariable. We have shown that ER stress signaling can also induce growth arrest and survival in cancer cells. Thus, we hypothesized that bortezomib therapy could induce quiescence and survival of residual MM cells, contributing to disease recurrence. Here, we report that in MM cells, proteasome inhibition with MG-132 or bortezomib results in a surviving cell fraction that enters a prolonged quiescent state (G(0)-G(1) arrest). Mechanism analysis revealed that bortezomib-surviving quiescent cells attenuate eIF2alpha phosphorylation and induction of the ER stress proapoptotic gene GADD153. This occurs independently of the eIF2alpha upstream kinases PERK, GCN2, and PKR. In contrast, the prosurvival ER-chaperone BiP/Grp78 was persistently induced. The bortezomib-surviving quiescent fraction could be eradicated by a simultaneous or sequential combination therapy with salubrinal, an inhibitor of GADD34-PP1C phosphatase complex, and, in consequence, eIF2alpha dephosphorylation. This effect was mimicked by expression of a phosphorylated mimetic eIF2alpha-S51D mutant. Our data indicate that bortezomib can induce growth arrest in therapy-surviving MM cells and that attenuation of eIF2alpha phosphorylation contributes to this survival. Most importantly, this survival mechanism can be blocked by inhibiting eIF2alpha dephosphorylation. Thus, strategies that maintain eIF2alpha in a hyperphosphorylated state may be a novel therapeutic approach to maximize bortezomib-induced apoptosis and reduce residual disease and recurrences in this type of cancer.

Bortezomib-mediated inhibition of steroid receptor coactivator-3 degradation leads to activated Akt

PURPOSE

To assess the safety of administering bortezomib to patients undergoing a radical prostatectomy, to assess pathologic changes induced by bortezomib in prostate cancer specimen, and to verify alterations by the drug in proteasome protein targets.

EXPERIMENTAL DESIGN

Bortezomib is a proteasome inhibitor that has shown activity in vitro and in vivo in prostate cancer. We performed a neoadjuvant clinical trial of bortezomib in men with prostate cancer at high risk of recurrence. The primary endpoints were to evaluate safety and biological activity.

RESULTS

Bortezomib is generally safe in the preoperative setting. Antitumor activity was manifested by tumor cytopathic effect, drops in serum prostate-specific antigen in some patients, and increases in tumor apoptosis. This was associated with cytoplasmic entrapment of nuclear factor-kappaB. We found an unexpected increase in proliferation in treated tissues and in vitro. Bortezomib also increased SRC-3 levels and phosphorylated Akt, both in vitro and in treated prostate cancer tissues. Knockdown of SRC-3 blocked the increase in activated Akt in vitro. Combined treatment with bortezomib and the Akt inhibitor perifosine was more effective than either agent alone in vitro.

CONCLUSION

These data suggest that combined therapies targeting the proteasome and the Akt pathway may have increased efficacy.

Designer therapies for glioblastoma multiforme

Primary brain tumors account for less than 2% of all cancers in adults; however, they are often associated with neurologic morbidity and high mortality. Glioblastoma multiforme (GBM) has been a focus of new therapy development in neurooncology because it is the most common primary brain tumor in adults. Standard-of-care therapy for newly diagnosed GBM includes surgical resection, radiotherapy, and temozolomide, administered both during and after radiotherapy. However, most patients develop tumor recurrence or progression after this multimodality treatment. Repeat resection and stereotactic radiosurgery upon recurrence may improve outcome only in selected patients. Most salvage chemotherapies offer only palliation. Recent advances in our understanding of the molecular abnormalities of GBM have generated new therapeutic venues of molecularly targeted agents (designer drugs) against key components of cellular pathways critical for cancer initiation and maintenance. Such drugs may offer the potential advantage to increase therapeutic efficacy and decrease systemic toxicity compared with traditional cytotoxic agents. Nonetheless, first-generation targeted agents have failed to demonstrate survival benefits in unselected GBM patient populations. Several mechanisms of treatment failure of the first-generation designer drugs have been proposed, whereas new strategies have been developed to increase effectiveness of these agents. Here we will discuss the recent development and the strategies to optimize the effectiveness of designer therapy for GBM.

Therapeutic application of noncytotoxic molecular targeted therapy in gliomas: growth factor receptors and angiogenesis inhibitors

Growth factor receptors and angiogenesis play major roles in the oncogenesis of gliomas. Over the last several years, several noncytotoxic molecular targeted therapies have been developed against growth factor receptors and tumor angiogenesis. In gliomas, two main anti-growth factor receptor strategies have been evaluated in phase I/II clinical trials: (a) small molecule tyrosine kinase inhibitors (TKIs) and (b) monoclonal antibodies that target growth factors or growth factor receptors other than vascular endothelial growth factor (VEGF). Up to now, few glioma patients have responded to small TKIs (0%-14%) or monoclonal antibodies (three case reports) delivered as a single agent. Greater doses, combined therapies, as well as the identification of molecular biomarkers predictive of response and resistance are important in order to optimize drug delivery and improve efficacy. Antiangiogenic therapies are promising for the treatment of gliomas. Thalidomide and metronomic chemotherapy were the first antiangiogenic strategies evaluated, but they have shown only modest activity. Recent studies of bevacizumab, an anti-VEGF antibody, and irinotecan, a topoisomerase I inhibitor, have demonstrated a high response rate, suggesting that targeted antiangiogenic therapies may play a significant role in the management of high-grade gliomas in the future. However, the toxicity profiles of these agents are not fully defined and the radiological evaluation of possible tumor response is challenging. Clinical evaluation of several VEGF receptor TKIs is currently ongoing; one of these inhibitors, cediranib, has already demonstrated interesting activity as a single agent. The integrin inhibitor cilengitide represents another promising strategy.

Cucurbitacin B markedly inhibits growth and rapidly affects the cytoskeleton in glioblastoma multiforme

Glioblastoma Multiforme (GBM) is almost inevitably a fatal tumor of the brain with most individuals dying within 1 year of diagnosis. It is the most frequent brain tumor in adults. Dose-response studies showed that Cucurbitacin B inhibited 50% growth (ED(50)) of 5 human GBM cell lines in liquid culture at approximately 10(-7) M. Soft-gel assays demonstrated that nearly all of the GBM clonogenic cells were inhibited at 10(-8) M of Cucurbitacin B. FACS analysis found that the compound (10(-7) M, 24 hr) caused G2/M arrest. The GBM cells underwent profound morphologic changes within 15-30 min after exposure to Cucurbitacin B (10(-7) M), rounding up and losing their pseudopodia associated with disruption of actin and microtubules, as observed by immunoflourescence. Cucurbitacin B (10(-7) M) caused prominent multinucleation of the cells after they were pulse-exposed (48 hr) to the drug, washed and cultured in normal medium for an additional 2 days. The drug (10(-7) M, 3-24 hr) increased levels of p-p38, p-JNK and p-JUN in U87 and T98G GBM cell lines as seen by Western blot. Interestingly, alterations in cell morphology caused by Cucurbitacin B (10(-7) M) were blocked by the JNK inhibitor SP600125. In summary, Cucurbitacin B has a prominent anti-proliferative activity on GBM cells; and at least in part, the mode of action is by affecting the cytoskeleton, as well as, the JNK pathway. Clinical trails of this drug should be pursued in GBM.

Dietary flavonoids inhibit the anticancer effects of the proteasome inhibitor bortezomib

Dietary flavonoids have many health-promoting actions, including anticancer activity via proteasome inhibition. Bor-tezomib is a dipeptide boronate proteasome inhibitor that has activity in the treatment of multiple myeloma but is not effective in chronic lymphocytic leukemia (CLL). Although CLL cells are sensitive in vitro to bortezomib-induced apoptosis when cultured in medium, the killing activity was blocked when cultured in 50% fresh autologous plasma. Dietary flavonoids, quercetin and myricetin, which are abundant in plasma, inhibited bortezomib-induced apoptosis of primary CLL and malignant B-cell lines in a dose-dependent manner. This inhibitory effect was associated with chemical reactions between quercetin and the boronic acid group, -RB(OH)2, in bortezomib. The addition of boric acid diminished the inhibitory effect of both quercetin and plasma on bortezomib-induced apoptosis. The protective effect was also reduced when myeloma cell lines, but not B-cell lines, were preincubated with quercetin, indicating a direct effect of quercetin on myeloma cells. At high doses, quercetin itself induced tumor cell death. These data indicate that dietary flavonoids limit the efficacy of bortezomib, whereas supplemental inorganic boric acid is able to reverse this. The complex interactions between quercetin, tumor cells, and bortezomib mean caution is required when giving dietary advice to patients.

A phase I study of bortezomib, etoposide and carboplatin in patients with advanced solid tumors refractory to standard therapy

PURPOSE

To evaluate the toxicity, pharmacological, and biological properties of the combination of bortezomib, etoposide, and carboplatin in adults with advanced solid malignancies.

PATIENTS AND METHODS

Patients received escalating doses of bortezomib, etoposide, and carboplatin every 21 days. Surrogate markers of angiogenesis were evaluated.

RESULTS

Twenty-four patients received 64 courses of therapy. The most common treatment-related adverse events were myelosuppression. Dose-limiting grade 3 and 4 neutropenia and thrombocytopenia were observed when bortezomib was given on days 1, 4, 8, 11. With revised dosing, the maximum tolerated dose (MTD) of bortezomib 0.75 mg/m(2) (days 1, 8), etoposide 75 mg/m(2) (days 1-3), and carboplatin AUC 5 (day 1) was well tolerated, and are the recommended doses for further studies with this combination. No objective responses were observed, however stable disease was noted for greater or equal to four cycles in nine highly refractory patients.

Mitochondrial Bax translocation partially mediates synergistic cytotoxicity between histone deacetylase inhibitors and proteasome inhibitors in glioma cells

The effects of combining histone deacetylase (HDAC) inhibitors and proteasome inhibitors were evaluated in both established glioblastoma multiforme (GBM) cell lines and short-term cultures derived from the Mayo Clinic xenograft GBM panel. Coexposure of LBH589 and bortezomib at minimally toxic doses of either drug alone resulted in a striking induction of apoptosis in established U251, U87, and D37 GBM cell lines, as well as in GBM8, GBM10, GBM12, GBM14, and GBM56 short-term cultured cell lines. Synergism of apoptosis induction was also observed in U251 cells when coexposing cells to other HDAC inhibitors, including LAQ824 and trichostatin A, with the proteasome inhibitor MG132, thus demonstrating a class effect. In U251 cells, bortezomib alone or in combination with LBH589 decreased Raf-1 levels and suppressed Akt and Erk activation. LBH589 or bortezomib alone increased expression of the cell cycle regulators p21 and p27. Additionally, the combination, but not the individual agents, markedly enhanced JNK activation. Synergistic induction of apoptosis after exposure to LBH589 and bortezomib was partially mediated by Bax translocation from the cytosol to the mitochondria resulting from Bax conformational changes. Bax translocation precedes cytochrome c release and apoptosis, and selective down-regulation of Bax using siRNA significantly mitigates the cytotoxicity of LBH589 and bortezomib. This combination regimen warrants further preclinical and possible clinical study for glioma patients.

Upregulation of DR5 by proteasome inhibitors potently sensitizes glioma cells to TRAIL-induced apoptosis

This study was undertaken to explore the potential of new therapeutic approaches designed to reactivate cell death pathways in apoptosis-refractory gliomas and to characterize the underlying molecular mechanisms of this reactivation. Here we investigated the sensitivity of a panel of glioma cell lines (U87, U251, U343, U373, MZ-54, and MZ-18) to apoptosis induced by the death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), TRAIL in combination with gamma irradiation, and TRAIL in combination with proteasome inhibitors (MG132 and epoxomicin). Analysis of these six glioma cell lines revealed drastic differences in their sensitivity to these treatments, with two of the six cell lines revealing no significant induction of cell death in response to TRAIL alone. Interestingly, the proteasome inhibitors MG132 and epoxomicin were capable of potentiating TRAIL-induced apoptosis in TRAIL-sensitive U87 and U251 cells and of reactivating apoptosis in TRAIL-resistant U343 and U373 cells. In contrast, gamma irradiation had no synergistic effects with TRAIL in the two TRAIL-resistant cell lines. RNA interference against death receptor 5 (DR5) revealed that reactivation of TRAIL-induced apoptosis by proteasome inhibitors depended on enhanced transcription and surface expression of DR5. Transient knockdown of the transcription factor GADD153/C/EBP homologous protein and application of the synthetic c-Jun N-terminal kinase inhibitor SP600125 indicated that enhanced DR5 expression occurred independently of GADD153/C/EBP homologous protein, but required activation of the c-Jun N-terminal kinase/c-Jun signaling pathway. Novel therapeutic approaches using TRAIL or agonistic TRAIL receptor antibodies in combination with proteasome inhibitors may represent a promising approach to reactivate apoptosis in therapy-resistant high-grade gliomas.

Increased expression and altered subunit composition of proteasomes induced by continuous proteasome inhibition establish apoptosis resistance and hyperproliferation of Burkitt lymphoma cells

The proteasome is the main protease for extralysosomal protein degradation in eukaryotic cells, and constitutes a sophisticated high molecular mass proteinase complex underlying a tightly coordinated expression and assembly of multiple subunits and subcomplexes. Here we show that continuous inhibition of proteasomal chymotrypsin-like peptidase activity by the proteasome inhibitor bortezomib induces in human Namalwa Burkitt lymphoma cells increased de novo biogenesis of proteasomes accompanied by increased expression of the proteasome maturation protein POMP, increased expression of 19S-20S-19S proteasomes, and abrogation of expression of beta 1i, beta 2i and beta 5i immunosubunits and PA28 in favor of increased expression of constitutive proteolytic beta1, beta2 and beta 5 subunits and 19S regulatory complexes. These alterations of proteasome expression and subunit composition are accompanied by an increase in proteasomal caspase-like, trypsin-like and chymotrypsin-like peptidase activities, not inhibitable by high doses of bortezomib. Cells harboring these proteasomal alterations display rapid proliferation and cell cycle progression, and acquire resistance to apoptosis induced by proteasome inhibitors, gamma-irradiation and staurosporine. This acquired apoptosis resistance is accompanied by de novo expression of anti-apoptotic Hsp27 protein and the loss of ability to accumulate and stabilize pro-apoptotic p53 protein. Thus, increased expression, altered subunit composition and increased activity of proteasomes constitute a hitherto unknown adaptive and autoregulatory feedback mechanism to allow cells to survive the lethal challenge of proteasome inhibition and to establish a hyperproliferative and apoptosis-resistant phenotype.

Aggravated endoplasmic reticulum stress as a basis for enhanced glioblastoma cell killing by bortezomib in combination with celecoxib or its non-coxib analogue, 2,5-dimethyl-celecoxib

The proteasome inhibitor bortezomib (Velcade) is known to trigger endoplasmic reticulum (ER) stress via the accumulation of obsolete and damaged proteins. The selective cyclooxygenase-2 (COX-2) inhibitor celecoxib (Celebrex) causes ER stress through a different mechanism (i.e., by causing leakage of calcium from the ER into the cytosol). Each of these two mechanisms has been implicated in the anticancer effects of the respective drug. We therefore investigated whether the combination of these two drugs would lead to further increased ER stress and would enhance their antitumor efficacy. With the use of human glioblastoma cell lines, we show that this is indeed the case. When combined, bortezomib and celecoxib triggered elevated expression of the ER stress markers GRP78/BiP and CHOP/GADD153, caused activation of c-Jun NH(2)-terminal kinase and ER stress-associated caspase-4, and greatly increased apoptotic cell death. Small interfering RNA-mediated knockdown of the protective ER chaperone GRP78/BiP further sensitized the tumor cells to killing by the drug combination. The contribution of celecoxib was independent of the inhibition of COX-2 because a non-coxib analogue of this drug, 2,5-dimethyl-celecoxib (DMC), faithfully and more potently mimicked these combination effects in vitro and in vivo. Taken together, our results show that combining bortezomib with celecoxib or DMC very potently triggers the ER stress response and results in greatly increased glioblastoma cytotoxicity. We propose that this novel drug combination should receive further evaluation as a potentially effective anticancer therapy.

Preclinical studies of Apogossypolone: a new nonpeptidic pan small-molecule inhibitor of Bcl-2, Bcl-XL and Mcl-1 proteins in Follicular Small Cleaved Cell Lymphoma model

Elevated expression of anti-apoptotic Bcl-2 family proteins have been linked to a poor survival rate of patients with Follicular Lymphoma (FL). This prompted us to evaluate a very potent non-peptidic Small-Molecule Inhibitor (SMI) targeting Bcl-2 family proteins, Apogossypolone (ApoG2) using follicular small cleaved cell lymphoma cell line (WSU-FSCCL) and cell isolated from lymphoma patients. ApoG2 inhibited the growth of WSU-FSCCL significantly with a 50% growth inhibition of cells (IC₅₀) of 109 nM and decreased cell number of fresh lymphoma cells. ApoG2 activated caspases-9, -3, and -8, and the cleavage of Poly (ADP-ribose) polymerase (PARP) and Apoptosis Inducing Factor (AIF). In the WSU-FSCCL-SCID xenograft model, ApoG2 showed a significant anti-lymphoma effect, with %ILS of 84% in the intravenous and 63% in intraperitoneal treated mice. These studies suggest that ApoG2 can be an effective therapeutic agent against FL.