Proteasome inhibitors potentiate etoposide-induced cell death in human astrocytoma cells bearing a mutated p53 isoform

RAD54L2-mediated DNA damage avoidance pathway specifically preserves genome integrity in response to topoisomerase 2 poisons

Type II topoisomerases (TOP2) form transient TOP2 cleavage complexes (TOP2ccs) during their catalytic cycle to relieve topological stress. TOP2ccs are covalently linked TOP2-DNA intermediates that are reversible but can be trapped by TOP2 poisons. Trapped TOP2ccs block transactions on DNA and generate genotoxic stress, which are the mechanisms of action of TOP2 poisons. How cells avoid TOP2cc accumulation remains largely unknown. In this study, we uncovered RAD54 like 2 (RAD54L2) as a key factor that mediates a TOP2-specific DNA damage avoidance pathway. RAD54L2 deficiency conferred unique sensitivity to treatment with TOP2 poisons. RAD54L2 interacted with TOP2A/TOP2B and ZATT/ZNF451 and promoted the turnover of TOP2 from DNA with or without TOP2 poisons. Additionally, inhibition of proteasome activity enhanced the chromatin binding of RAD54L2, which in turn led to the removal of TOP2 from chromatin. In conclusion, we propose that RAD54L2-mediated TOP2 turnover is critically important for the avoidance of potential TOP2-linked DNA damage under physiological conditions and in response to TOP2 poisons.

Suppressing proteasome mediated processing of topoisomerase II DNA-protein complexes preserves genome integrity

Topoisomerase II (TOP2) relieves topological stress in DNA by introducing double-strand breaks (DSBs) via a transient, covalently linked TOP2 DNA-protein intermediate, termed TOP2 cleavage complex (TOP2cc). TOP2ccs are normally rapidly reversible, but can be stabilized by TOP2 poisons, such as the chemotherapeutic agent etoposide (ETO). TOP2 poisons have shown significant variability in their therapeutic effectiveness across different cancers for reasons that remain to be determined. One potential explanation for the differential cellular response to these drugs is in the manner by which cells process TOP2ccs. Cells are thought to remove TOP2ccs primarily by proteolytic degradation followed by DNA DSB repair. Here, we show that proteasome-mediated repair of TOP2cc is highly error-prone. Pre-treating primary splenic mouse B-cells with proteasome inhibitors prevented the proteolytic processing of trapped TOP2ccs, suppressed the DNA damage response (DDR) and completely protected cells from ETO-induced genome instability, thereby preserving cellular viability. When degradation of TOP2cc was suppressed, the TOP2 enzyme uncoupled itself from the DNA following ETO washout, in an error-free manner. This suggests a potential mechanism of developing resistance to topoisomerase poisons by ensuring rapid TOP2cc reversal.

Adenosine Signaling in Glioma Cells

Purines and pyrimidines are fundamental signaling molecules in controlling the survival and proliferation of astrocytes, as well as in mediating cell-to-cell communication between glial cells and neurons in the healthy brain. The malignant transformation of astrocytes towards progressively more aggressive brain tumours (from astrocytoma to anaplastic glioblastoma) leads to modifications in both the survival and cell death pathways which overall confer a growth advantage to malignant cells and resistance to many cytotoxic stimuli. It has been demonstrated, however, that, in astrocytomas, several purinergic (in particular adenosinergic) pathways controlling cell survival and death are still effective and, in some cases, even enhanced, providing invaluable targets for purine-based chemotherapy, that still represents an appropriate pharmacological approach to brain tumours. In this chapter, the current knowledge on both receptor-mediated and receptor-independent adenosine pathways in astrocytomas will be reviewed, with a particular emphasis on the most promising targets which could be translated from in vitro studies to in vivo pharmacology. Additionally, we have included new original data from our laboratory demonstrating a key involvement of MAP kinases in the cytostastic and cytotoxic effects exerted by an adenosine analogue, 2-CdA, which with the name of Cladribine is already clinically utilized in haematological malignancies. Here we show that 2-CdA can activate multiple intracellular pathways leading to cell cycle block and cell death by apoptosis of a human astrocytoma cell line that bears several pro-survival genetic mutations. Although in vivo data are still lacking, our results suggest that adenosine analogues could therefore be exploited to overcome resistance to chemotherapy of brain tumours.

Targeting the Proteasome in Refractory Pediatric Leukemia Cells: Characterization of Effective Cytotoxicity of Carfilzomib

BACKGROUND

Leukemia accounts for 30% of all childhood cancers and although the survival rate for pediatric leukemia has greatly improved, relapse is a major cause of treatment failure. Therefore, the development and introduction of novel therapeutics to treat relapsed pediatric leukemia is urgently needed. The proteasome inhibitor bortezomib has been shown to be effective against adult hematological malignancies such as multiple myeloma and lymphoma, but is frequently associated with the development of resistance. Carfilzomib is a next-generation proteasome inhibitor that has shown promising results against refractory adult hematological malignancies.

OBJECTIVE

Carfilzomib has been extensively studied in adult hematological malignancies, providing the rationale for evaluating proof-of-concept activity of carfilzomib in pediatric leukemia.

METHODS

The effects of carfilzomib on pediatric leukemia cell lines and primary pediatric leukemia patient samples were investigated in vitro using the alamar blue cytotoxicity assay, western blotting, and a proteasome activity assay. Synergy with commonly used anticancer drugs was determined by calculation of combination indices.

RESULTS

In vitro preclinical data show pharmacologically relevant concentrations of carfilzomib are cytotoxic to pediatric leukemia cell lines and primary pediatric leukemia cells. Target modulation studies validate the effective inhibition of the proteasome and induction of apoptosis. We also identify agents that have effective synergy with carfilzomib in these cells.

CONCLUSIONS

Our data provide pre-clinical information that can be incorporated into future early-phase clinical trials for the assessment of carfilzomib as a treatment for children with refractory hematological malignancies.

Cytosolic 5'-Nucleotidase II Silencing in a Human Lung Carcinoma Cell Line Opposes Cancer Phenotype with a Concomitant Increase in p53 Phosphorylation

Purine homeostasis is maintained by a purine cycle in which the regulated member is a cytosolic 5'-nucleotidase II (cN-II) hydrolyzing IMP and GMP. Its expression is particularly high in proliferating cells, indeed high cN-II activity or expression in hematological malignancy has been associated to poor prognosis and chemoresistance. Therefore, a strong interest has grown in developing cN-II inhibitors, as potential drugs alone or in combination with other compounds. As a model to study the effect of cN-II inhibition we utilized a lung carcinoma cell line (A549) in which the enzyme was partially silenced and its low activity conformation was stabilized through incubation with 2-deoxyglucose. We measured nucleotide content, reduced glutathione, activities of enzymes involved in glycolysis and Krebs cycle, protein synthesis, mitochondrial function, cellular proliferation, migration and viability. Our results demonstrate that high cN-II expression is associated with a glycolytic, highly proliferating phenotype, while silencing causes a reduction of proliferation, protein synthesis and migration ability, and an increase of oxidative performances. Similar results were obtained in a human astrocytoma cell line. Moreover, we demonstrate that cN-II silencing is concomitant with p53 phosphorylation, suggesting a possible involvement of this pathway in mediating some of cN-II roles in cancer cell biology.

Mechanisms of proteasome inhibitor-induced cytotoxicity in malignant glioma

The 26S proteasome constitutes an essential degradation apparatus involved in the consistent recycling of misfolded and damaged proteins inside cells. The aberrant activation of the proteasome has been widely observed in various types of cancers and implicated in the development and progression of carcinogenesis. In the era of targeted therapies, the clinical use of proteasome inhibitors necessitates a better understanding of the molecular mechanisms of cell death responsible for their cytotoxic action, which are reviewed here in the context of sensitization of malignant gliomas, a tumor type particularly refractory to conventional treatments.

Interferon-gamma bolsters CD95/Fas-mediated apoptosis of astroglioma cells

In the present study, we investigated the mechanisms of the resistance to CD95-mediated cell death and the effects of interferon-gamma in modulating the susceptibility to CD95-induced apoptosis of human astroglioma cells. We found that interferon-gamma administration sensitized cancer cells to CD95-mediated apoptosis. The mechanism underlying this sensitization appeared to be associated with a framework of cell changes, including up-regulation of death receptor at the cell surface, pro-apoptotic molecule Bax and Bak over-expression and mitochondria hyperpolarization, as is known to be associated with cell sensitization to apoptosis. An involvement of the proteasome activity in the mechanism of sensitization by interferon-gamma was also detected, probably as a result of the differing expression of catalytic proteasome subunits. Taken together, these findings suggest that interferon-gamma could represent a promising candidate for modulating astroglioma cell apoptotic susceptibility.

Myc down-regulation affects cyclin D1/cdk4 activity and induces apoptosis via Smac/Diablo pathway in an astrocytoma cell line

OBJECTIVES

We investigated the antiproliferative effect of Myc down-regulation via cell proliferation inhibition, cell cycle perturbation and apoptosis in two human astrocytoma models (T98G and ADF) steadily expressing an inducible c-myc Anti-sense RNA.

MATERIALS AND METHODS

Cell growth experiments were performed using the trypan blue dye exclusion test and cell cycle analysis was evaluated by flow cytometry. Cell cycle molecules were detected by Western blot analysis, co-immunoprecipitation and reverse transcription-polymerase chain reaction assays.

RESULTS

We showed that Myc down-regulation in astrocytoma cells led to G1 accumulation and an inhibition of cell proliferation characterized by S phase delay. Co-immunoprecipitation experiments detected formation of inactive cyclin D1/cdk4 complexes as evaluated by presence of an active unphosphorylated form of retinoblastoma protein, the best characterized target substrate for cyclin D1/cdk4 complex, in ADF pINDc-myc anti-sense 7 cells. We also found that either p57Kip2 "apice" or p27Kip1 "apice" inhibitors bound to cyclin D1/cdk4 complex, thus, suggesting that they cooperated to inhibit the activity of cyclin D1/cdk4. Moreover, c-Myc down-regulation led to activation of the apoptotic mitochondrial pathway, characterized by release of cytochrome c and Smac/Diablo proteins and by reduction of c-IAP levels through activation of proteasome-mediated protein degradation system.

CONCLUSIONS

Our results suggest that c-Myc could be considered as a good target for the study of new approaches in anticancer astrocytoma treatment.

Pharmacotherapy for adults with tumors of the central nervous system

Tumors of the adult central nervous system are among the most common and most chemoresistant neoplasms. Malignant tumors of the brain and spinal cord collectively account for approximately 1.3% of all cancers and 2.2% of all cancer-related deaths. Novel pharmacological approaches to nervous system tumors are urgently needed. This review presents the current approaches and challenges to successful pharmacotherapy of adults with malignant tumors of the central nervous system and discusses novel approaches aimed at overcoming these challenges.

The role of mitochondria in pharmacotoxicology: a reevaluation of an old, newly emerging topic

In addition to their well-known critical role in energy metabolism, mitochondria are now recognized as the location where various catabolic and anabolic processes, calcium fluxes, various oxygen-nitrogen reactive species, and other signal transduction pathways interact to maintain cell homeostasis and to mediate cellular responses to different stimuli. It is important to consider how pharmacological agents affect mitochondrial biochemistry, not only because of toxicological concerns but also because of potential therapeutic applications. Several potential targets could be envisaged at the mitochondrial level that may underlie the toxic effects of some drugs. Recently, antiviral nucleoside analogs have displayed mitochondrial toxicity through the inhibition of DNA polymerase-γ (pol-γ). Other drugs that target different components of mitochondrial channels can disrupt ion homeostasis or interfere with the mitochondrial permeability transition pore. Many known inhibitors of the mitochondrial electron transfer chain act by interfering with one or more of the respiratory chain complexes. Nonsteroidal anti-inflammatory drugs (NSAIDs), for example, may behave as oxidative phosphorylation uncouplers. The mitochondrial toxicity of other drugs seems to depend on free radical production, although the mechanisms have not yet been clarified. Meanwhile, drugs targeting mitochondria have been used to treat mitochondrial dysfunctions. Importantly, drugs that target the mitochondria of cancer cells have been developed recently; such drugs can trigger apoptosis or necrosis of the cancer cells. Thus the aim of this review is to highlight the role of mitochondria in pharmacotoxicology, and to describe whenever possible the main molecular mechanisms underlying unwanted and/or therapeutic effects.