Protease inhibitors restore radiation-induced apoptosis to Bcl-2-expressing lymphoma cells

Effects of FOXM1 inhibition and ionizing radiation on melanoma cells

Metastatic melanoma can be highly refractory to conventional radiotherapy and chemotherapy but combinatorial-targeted therapeutics are showing greater promise on improving treatment efficacy. Previous studies have shown that knockdown of Forkhead box M1 (FOXM1) can sensitize various tumor types to radiation-induced cell death. The effect of combining radiation with a small molecule FOXM1 inhibitor, Siomycin A, on growth, death and migration of a metastatic melanoma cell line (SK-MEL-28) that overexpresses this pleiotropic cell cycle regulator was investigated. Siomycin A (SIOA) was found to be a strong inducer of apoptosis, and inhibitor of proliferation and migration in a scratch wound assay in this cell line. Induction of apoptosis occurred at concentrations >1 µM in association with reductions in the constitutive FOXM1 and anti-apoptotic B-cell lymphoma 2 protein levels found in these cells. Single doses of ionizing radiation (0-40 Gy) delivered by linear accelerator caused inhibition of growth and migration without significant induction of cell death. Pretreatment with SIOA did not increase the sensitivity of this melanoma cell line to radiation as observed in other tumor types. These data confirm that as a single agent, SIOA is an effective inducer of cell death and inhibitor of migration in metastatic melanoma cells expressing constitutive FOXM1. In combination with radiation, SIOA pre-treatment, however, may not be of added benefit.

90 Y-ibritumomab tiuxetan: a nearly forgotten opportunityr

Y-ibritumomab tiuxetan (90Y-IT) combines the benefits of a monoclonal antibody with the efficacy of radiation in the treatment of B-cell non-Hodgkin lymphoma (NHL), a remarkably radiosensitive hematologic malignancy. 90Y-IT activity has been well established in the indolent setting, being approved in front-line treatment of follicular lymphoma (FL) patients as well as salvage therapy. However, no advantage in OS was observed with respect to standard treatment. Promising data are available also for aggressive B-cell lymphoma. In particular, the addition of RIT to short-course first line chemotherapy enables reduction of chemotherapy while maintaining cure rates in elderly, untreated diffuse large B-cell lymphoma (DLBCL) patients. Furthermore, 90Y-IT improves response rate and outcomes of relapsed/refractory DLBCL patients, eligible and ineligible for autologous stem cell transplantation (ASCT). Clinical results have shown a role of 90Y-IT even in mantle cell lymphoma (MCL). RIT might improve responses and treat minimal residual disease when used as consolidation after first-line chemotherapy in MCL. Moreover, 90Y-IT has demonstrated its efficacy in combination with high-dose chemotherapies as conditioning regimen for ASCT, with evidence suggesting the ability to overcome chemotherapy resistance. Herein, we review the available evidence for this approved drug and examine the recently published and ongoing trials for potential novel indication in aggressive B-cell NHL.

Activity enhancement of the synthetic syrbactin proteasome inhibitor hybrid and biological evaluation in tumor cells

Syrbactins belong to a recently emergent class of bacterial natural product inhibitors that irreversibly inhibit the proteasome of eukaryotes by a novel mechanism. The total syntheses of the syrbactin molecules syringolin A, syringolin B, and glidobactin A have been achieved, which allowed the preparation of syrbactin-inspired derivatives, such as the syringolin A-glidobactin A hybrid molecule (SylA-GlbA). To determine the potency of SylA-GlbA, we employed both in vitro and cell culture-based proteasome assays that measure the subcatalytic chymotrypsin-like (CT-L), trypsin-like (T-L), and caspase-like (C-L) activities. We further studied the inhibitory effects of SylA-GlbA on tumor cell growth using a panel of multiple myeloma, neuroblastoma, and ovarian cancer cell lines and showed that SylA-GlbA strongly blocks the activity of NF-κB. To gain more insights into the structure-activity relationship, we cocrystallized SylA-GlbA in complex with the proteasome and determined the X-ray structure. The electron density map displays covalent binding of the Thr1O(γ) atoms of all active sites to the macrolactam ring of the ligand via ether bond formation, thus providing insights into the structure-activity relationship for the improved affinity of SylA-GlbA for the CT-L activity compared to those of the natural compounds SylA and GlbA. Our study revealed that the novel synthetic syrbactin compound represents one of the most potent proteasome inhibitors analyzed to date and therefore exhibits promising properties for improved drug development as an anticancer therapeutic.

Dysfunction of the TP53 tumor suppressor gene in lymphoid malignancies

Mutations of the TP53 gene and dysregulation of the TP53 pathway are important in the pathogenesis of many human cancers, including lymphomas. Tumor suppression by p53 occurs via both transcription-dependent activities in the nucleus by which p53 regulates transcription of genes involved in cell cycle, DNA repair, apoptosis, signaling, transcription, and metabolism; and transcription-independent activities that induces apoptosis and autophagy in the cytoplasm. In lymphoid malignancies, the frequency of TP53 deletions and mutations is lower than in other types of cancer. Nonetheless, the status of TP53 is an independent prognostic factor in most lymphoma types. Dysfunction of TP53 with wild-type coding sequence can result from deregulated gene expression, stability, and activity of p53. To overcome TP53 pathway inactivation, therapeutic delivery of wild-type p53, activation of mutant p53, inhibition of MDM2-mediated degradation of p53, and activation of p53-dependent and -independent apoptotic pathways have been explored experimentally and in clinical trials. We review the mechanisms of TP53 dysfunction, recent advances implicated in lymphomagenesis, and therapeutic approaches to overcoming p53 inactivation.

Proteasome and cancer

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

Bortezomib 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.

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.

P53-mediated cell cycle arrest and apoptosis through a caspase-3- independent, but caspase-9-dependent pathway in oridonin-treated MCF-7 human breast cancer cells

AIM

To study the caspase-3-independent mechanisms in oridonin-induced MCF-7 human breast cancer cell apoptosis in vitro.

METHODS

The viability of oridonin-treated MCF-7 cells was measured by MTT (thiazole blue) assay. Apoptotic cells with condensed nuclei were visualized by phase contrast microscopy. Nucleosomal DNA fragmentation was assayed by agarose gel electrophoresis. The apoptotic ratio was determined by lactate dehydrogenase assay. Cell cycle alternation and mitochondrial membrane potential were measured by flow cytometric analysis. Bax, Bcl-2, caspase-3, caspase-9, heat shock protein (Hsp)90, p53, p-p53, p21, Poly (ADP-ribose) polymerase (PARP), and the inhibitor of caspase-activated DNase (ICAD) protein expressions were detected by Western blot analysis.

RESULTS

Oridonin inhibited cell growth in a time- and dose-dependent manner. Cell cycle was altered through the upregulation of p53 and p21 protein expressions. Pancaspase inhibitor Z-VAD-fmk and calpain inhibitor II both decreased cell death ratio. Nucleosomal DNA fragmentation and the downregulation of DeltaPhimit were detected in oridonin-induced MCF-7 cell apoptosis, which was involved in a postmitochondrial caspase-9-dependent pathway. Decreased Bcl-2 and Hsp90 expression levels and increased Bax and p21 expression levels were positively correlated with elevated levels of phosphorylated p53 phosphorylation. Moreover, PARP was partially cleaved by calpain rather than by caspase-3.

CONCLUSION

DNA damage provoked alternations in the mitochondrial and caspase-9 pathways as well as p53-mediated cell cycle arrest, but was not related to caspase-3 activity in oridonin-induced MCF-7 cells.

Proteasome Inhibitors in the Clinical Setting

The majority of intracellular proteins undergo degradation through the ubiquitin-proteasome pathway. The proteasome pathway has a role in regulating cell proliferation, differentiation, survival and apoptosis. The naturally occurring proteasome inhibitor lactacystin was the first proteasome inhibitor noted to induce apoptosis in vitro. Compared with first-generation proteasome inhibitors, bortezomib (PS-341), a dipeptide boronic acid, has exhibited higher potency and specificity, and has been approved for the treatment of relapsed or refractory myeloma. However, there are some patients who do not respond to therapy or who respond briefly and then relapse. It is becoming increasingly clear that myeloma cells respond to the stress caused by proteasome inhibitors (bortezomib) via rapidly up-regulating pathways that suppress apoptosis, thus attenuating its antitumour activity. The delineation of these molecular pathways and mechanisms to circumvent them are needed to allow this important class of agents to remain vital in the armamentarium of the management of multiple myeloma and other malignancies.

Apoptosis induced by proteasome inhibition in cancer cells: predominant role of the p53/PUMA pathway

The proteasome has emerged as a novel target for antineoplastic treatment of hematological malignancies and solid tumors, including those of the central nervous system. To identify cell death pathways activated in response to inhibition of the proteasome system in cancer cells, we treated human SH-SY5Y neuroblastoma cells with the selective proteasome inhibitor (PI) epoxomicin (Epoxo). Prolonged exposure to Epoxo was associated with increased levels of poly-ubiquitinylated proteins and p53, release of cytochrome c from the mitochondria, and activation of caspases. Analysis of global gene expression using high-density oligonucleotide microarrays revealed that Epoxo triggered transcriptional activation of the two Bcl-2-homology domain-3-only (BH3-only) genes p53 upregulated modulator of apoptosis (PUMA) and Bim. Subsequent studies in PUMA- and Bim-deficient cells indicated that Epoxo-induced caspase activation and apoptosis was predominantly PUMA-dependent. Further characterization of the transcriptional response to Epoxo in HCT116 human colon cancer cells demonstrated that PUMA induction was p53-dependent; with deficiency in either p53 or PUMA significantly protected HCT116 cells against Epoxo-induced apoptosis. Our data suggest that p53 activation and the transcriptional induction of its target gene PUMA play an important role in the sensitivity of cancer cells to apoptosis induced by proteasome inhibition, and imply that antineoplastic therapies with PIs might be especially useful in cancers with functional p53.

THE PROTEASOME AND PROTEASOME INHIBITORS IN CANCER THERAPY

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

Chemosensitization and radiosensitization of tumors by plant polyphenols

The treatment of cancer with chemotherapeutic agents and radiation has two major problems: time-dependent development of tumor resistance to therapy (chemoresistance and radioresistance) and nonspecific toxicity toward normal cells. Many plant-derived polyphenols have been studied intently for their potential chemopreventive properties and are pharmacologically safe. These compounds include genistein, curcumin, resveratrol, silymarin, caffeic acid phenethyl ester, flavopiridol, emodin, green tea polyphenols, piperine, oleandrin, ursolic acid, and betulinic acid. Recent research has suggested that these plant polyphenols might be used to sensitize tumor cells to chemotherapeutic agents and radiation therapy by inhibiting pathways that lead to treatment resistance. These agents have also been found to be protective from therapy-associated toxicities. How these polyphenols protect normal cells and sensitize tumor cells to treatment is discussed in this review.

Differential roles of constitutively activated ERK1/2 and NF-kappa B in cytotoxicity and proliferation by human NK cell lines

Compared with freshly isolated peripheral blood natural killer (NK) cells, the YT and NK-92 cell lines are characterized by elevated cytolytic activity. The molecular mechanisms underlying the rapid proliferation and enhanced lytic activity of NK cell lines are poorly understood. Investigation of these cell lines revealed that ERK1/2 and NF-kappa B are constitutively activated, providing evidence that these two signaling pathways are differentially involved in cytolysis and proliferation. Furthermore, blocking ERK1/2 activation with the specific inhibitor, PD098059, inhibited cytolytic activity in both cell lines and reduced mRNA expression of cytolysis-related effector molecules such as Fas-L and IFN gamma, as measured by semi-quantitative RT-PCR. However, MTT colormetric analysis showed that treatment with the PD098059 inhibitor did not affect cell proliferation. Meanwhile, blockade of the NF-kappa B signaling pathway using MG132 inhibited cellular growth without impacting cytolytic capability. No synergistic interactions were observed between ERK1/2 and NF-kappa B after combination treatment with PD098059 and MG132, suggesting that these two signaling pathways likely affect cellular proliferation and cytotoxicity by NK cells differentially.

Differential gene expression in HIV/SIV-associated and spontaneous lymphomas

Diffuse large B-cell lymphoma (DLBCL) is more prevalent and more often fatal in HIV-infected patients and SIV-infected monkeys compared to immune-competent individuals. Molecular, biological, and immunological data indicate that virus-associated lymphomagenesis is similar in both infected hosts. To find genes specifically overexpressed in HIV/SIV-associated and non-HIV/SIV-associated DLBCL we compared gene expression profiles of HIV/SIV-related and non-HIV-related lymphomas using subtractive hybridization and Northern blot analysis. Our experimental approach allowed us to detect two genes (a-myb and pub) upregulated solely in HIV/SIV-associated DLBCLs potentially involved in virus-specific lymphomagenesis in human and monkey. Downregulation of the pub gene was observed in all non-HIV-associated lymphomas investigated. In addition, we have found genes upregulated in both non-HIV- and HIV-associated lymphomas. Among those were genes both with known (set, ND4, SMG-1) and unknown functions. In summary, we have demonstrated that simultaneous transcriptional upregulation of at least two genes (a-myb and pub) was specific for AIDS-associated lymphomas.

[Biomodulation of transcriptional factor NF-kappa B by ionizing radiation]

NF-kappaB (Nuclear Factor-kappaB) was described for the first time in 1986 as a nuclear protein binding to the kappa immunoglobulin-light chain enhancer. Since then, NF-kappaB has emerged as an ubiquitous factor involved in the regulation of numerous important processes as diverse as immune and inflammatory responses, apoptosis and cell proliferation. These last two properties explain the implication of NF-kappaB in the tumorigenic process as well as the promise of a targeted therapeutic intervention. This review focuses on the current knowledge on NF-kappaB regulation and discusses the therapeutic potential of targeting NF-kappaB in cancer in particular during radiotherapy.

Apoptosis-modulating agents in combination with radiotherapy-current status and outlook

PURPOSE

To increase the therapeutic efficacy of ionizing radiation or to reduce radiation-mediated side effects, diverse research centers for translational radiation oncology have headed for a specific modulation of defined cellular death pathways. In this regard, several signaling systems have proved to be of high potential value.

RESULTS

It has previously been shown that apoptotic pathways induced by ionizing radiation are distinct from death pathways triggered by death ligands such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). The combination of both radiation and TRAIL was highly efficient in vitro and in preclinical mouse models. However, several aspects of normal tissue toxicity have not been solved, and no Phase I data are available yet. A second approach tested in a Phase I trial is based on the observation that synthetic phospholipid derivatives (alkyllysophospholipids and alkylphosphocholines) strongly enhance apoptotic effects by modulating the balance among the mitogenic, anti-apoptotic MAPK, phosphatidylinositol 3'-kinase (PI3K)/Akt, and the pro-apoptotic SAPK/JNK signaling pathways. Furthermore, others have provided evidence that inhibition of anti-apoptotic signals generated by mitogenic stimuli may increase radiation responses. In this context, controversial data are available regarding the influence of a pharmacologic abrogation of MEK1, Erk1/2 signaling on apoptotic sensitivity but no Phase I trials of MEK inhibitors either alone or in combination with radiation have yet been published. However, inhibition of the PI3K/Akt survival pathway using compounds such as the protein kinase C (PKC) inhibitor PKC412 has been shown to induce apoptosis or to increase the apoptotic sensitivity of tumor cells. Therefore, these drugs may be used alone or in combination with radiation to increase tumor control; however, Phase I data are lacking. Several other drugs, including cyclooxygenase-2 inhibitors, betulinic acid, and proteasome inhibitors, have been shown to interact with apoptotic signal transduction. Again, most of the drugs have not been tested in combination with radiation in vivo or-in the case of cyclooxygenase-2 inhibitors-exert pleiotropic effects.

CONCLUSION

Although the examples do not reflect all available strategies, it is clear that several promising approaches targeting defined cell death pathways have been developed and entered into clinical trials. The use of synthetic phospholipid derivatives in a Phase I trial is an important example, proving that basic research in radiation biology finally guides the development of new treatment strategies. This, and other approaches, will hopefully increase tumor control rates and reduce side effects in the future.

Expression of ErbB2 enhances radiation-induced NF-kappaB activation

Her-2/neu (ErbB2) oncogene, the second member of the epidermal growth factor receptor (EGFR) family, encodes a transmembrane tyrosine kinase receptor in Her-2-positive tumors. Accumulating evidences demonstrate that signaling networks activated by EGFR and transcription factor NF-kappaB are associated with cell response to ionizing radiation (IR). The present study shows that overexpression of ErbB2 enhanced NF-kappaB activation induced by IR in human breast carcinoma MCF-7 cells transfected with ErbB2 genes (MCF-7/ErbB2). Stable transfection of dominant-negative mutant IkappaB (MCF-7/ErbB2/mIkappaB) or treatment with anti-ErbB2 antibody, Herceptin, inhibited NF-kappaB activation and radiosensitized MCF-7/ErbB2 cells. Consistent with NF-kappaB regulation, basal and IR-induced Akt, a kinase downstream of ErbB2, was activated in MCF-7/ErbB2 cells and inhibited by Herceptin. To identify specific genes affected by ErbB2-mediated NF-kappaB activation, a group of IR-responsive elements Cyclin B1, Cyclin D1, Bcl-2, Bcl/XL, BAD and BAX were evaluated. Basal levels of prosurvival elements Cyclin B1, Cyclin D1, Bcl-2 and Bcl/XL but not apoptotic BAD and BAX were upregulated in MCF-7/ErbB2 cells with striking enhancements in Bcl-2 and Bcl/XL. IR further induced Cyclin B1 and Cyclin D1 expression that was reduced by Herceptin. Bcl-2 kept a high steady level after Herceptin+IR treatment and, in contrast to control MCF-7/Vector cells, Bcl/XL was inhibited in MCF-7/ErbB2 cells by Herceptin+IR treatment. However, all four prosurvival proteins were downregulated by inhibition of NF-kappaB in MCF-7/ErbB2/mIkappaB cells. These results thus provide evidence suggesting that overexpression of ErbB2 is able to enhance NF-kappaB response to IR, and that a specific prosurvival network downstream of NF-kappaB is triggered by treatments using anti-ErbB2 antibody combined with radiation.

p53-Mediated Cell Cycle Arrest and Apoptosis Induced by Shikonin via a Caspase-9-Dependent Mechanism in Human Malignant Melanoma A375-S2 Cells

Natural products regulate cell growth in response to oncogene activation that induces cell cycle arrest and apoptosis in tumor cell lines. We investigated the mechanisms of caspase activation in human malignant melanoma, A375-S2 cells, by the natural product shikonin, which was isolated from the plant Lithospermum erythrorhizon SIEB. et ZUCC. Shikonin inhibited cell growth in a time- and dose-dependent manner, which might be mediated through up-regulation of p53 and down-regulation of cyclin-dependent protein kinase 4. Caspase activation was detected in shikonin-induced cell apoptosis, which involved in a post-mitochondrial caspase-9-dependent pathway. Decreased Bcl-2 protein levels and increased Bax protein levels were positively correlated with elevated expression of p53 protein. Apoptosis-inducing factor, another apoptotic protein of mitochondria, partially contributed to shikonin-induced release of cytochrome c. Taken together, shikonin-induced DNA damage activates p53 and caspase-9 pathways.

Nuclear factor-κB as a predictor of treatment response in breast cancer

PURPOSE OF REVIEW

To examine the links of nuclear factor-kappa B (NF-kappa B) to treatment-induced signaling in breast cancer and to propose further studies to elucidate the role of NF-kappa B in breast cancer response to chemotherapy and radiation.

RECENT FINDINGS

The authors' group and others have investigated the clinical relevance of ubiquitously expressed NF-kappa B in breast cancer. Possibly through its effects on apoptosis, NF-kappa B has been implicated in tumor resistance to chemotherapy and radiation in many types of tumors. Furthermore, both in vitro and in vivo studies have shown that targeted inhibition of NF-kappa B can sensitize tumor cells to chemotherapy and radiation.

SUMMARY

The molecular mechanisms involved in chemotherapy-induced and radiation-induced cell death in breast cancer are not fully known, nor are the mechanisms of treatment resistance. NF-kappa B is a transcription factor for a number of genes involved in tumor progression and resistance to systemic therapies and is a major regulator of the apoptotic pathway. Gaining further insights into molecular factors such as NF-kappa B as biomarkers for treatment response may help clinicians predict treatment outcome and lead to the development of targeted therapeutics.

p53-induced inhibition of protein synthesis is independent of apoptosis

Activation of a temperature-sensitive form of p53 in murine erythroleukaemia cells results in a rapid impairment of protein synthesis that precedes inhibition of cell proliferation and loss of cell viability by several hours. The inhibition of translation is associated with specific cleavages of polypeptide chain initiation factors eIF4GI and eIF4B, a phenomenon previously observed in cells induced to undergo apoptosis in response to other stimuli. Although caspase activity is enhanced in the cells in which p53 is activated, both the effects on translation and the cleavages of the initiation factors are completely resistant to inhibition of caspase activity. Moreover, exposure of the cells to a combination of the caspase inhibitor z-VAD.FMK and the survival factor erythropoietin prevents p53-induced cell death but does not reverse the inhibition of protein synthesis. We conclude that the p53-regulated cleavages of eIF4GI and eIF4B, as well as the overall inhibition of protein synthesis, are caspase-independent events that can be dissociated from the induction of apoptosis per se.

Proteasome inhibitors as new anticancer drugs

The targeted degradation of key regulatory proteins is an essential element of cell cycle control. The proteasome plays a central role in the degradation of such proteins and has therefore become an important therapeutic target for diseases involving cell proliferation, notably cancer. This review summarizes numerous studies demonstrating that proteasome inhibition induces apoptosis and sensitizes cancer cells to traditional tumoricidal agents both and The potent and selective proteasome inhibitor, PS-341, is particularly promising from a therapeutic perspective, and it is the only such inhibitor that has progressed to clinical trials. Preliminary data indicate that the drug is well tolerated by patients with cancer, and further trials are underway to assess the safety and efficacy of proteasome inhibition in hematologic and solid tumors, both as a monotherapy and in combination with other chemotherapeutics.

Proteasome inhibitors in the treatment of B-cell malignancies

The proteasome, which plays a pivotal role in the control of many cell cycle-regulatory processes, has become the focus of new approaches to the treatment of cancer, including B-cell malignancies, and the first proteasome inhibitor, bortezomib (VELCADE; formerly PS-341), has entered clinical trials. The proteasome controls the stability of numerous proteins that regulate progression through the cell cycle and apoptosis, such as cyclins, cyclin-dependent kinases, tumor suppressors, and the nuclear factor-kB. By altering the stability or activity of these proteins, proteasome inhibitors sensitize malignant cells to apoptosis. Bortezomib is a dipeptidyl boronic acid proteasome inhibitor that effectively and specifically inhibits proteasome activity. In preclinical studies, bortezomib and other proteasome inhibitors have shown activity against a variety of B-cell malignancies, including multiple myeloma, diffuse large B-cell lymphoma, mantle cell lymphoma, and Hodgkin's lymphoma. These agents can induce apoptosis and sensitize tumor cells to radiation or chemotherapy. Based on these findings, phase I clinical trials were conducted with bortezomib in various solid and hematologic malignancies. In these studies, bortezomib was generally well tolerated with manageable toxicities. Phase II trials have been initiated for relapsed and refractory multiple myeloma, refractory chronic lymphocytic leukemia, and non-Hodgkin's lymphoma. Preliminary data from the multiple myeloma phase II study indicate that a significant number of patients responded to therapy or exhibited stable disease and that the drug had manageable toxicities. These findings, along with extensive preclinical data, suggest that bortezomib and other proteasome inhibitors may have far-reaching potential in the treatment of various cancers, including B-cell malignancies.