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 ß-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.

Proteasome inhibitors activate autophagy as a cytoprotective response in human prostate cancer cells.

The ubiquitin-proteasome and lysosome-autophagy pathways are the two major intracellular protein degradation systems that work cooperatively to maintain homeostasis. Proteasome inhibitors (PIs) have clinical activity in hematological tumors, and inhibitors of autophagy are also being evaluated as potential antitumor therapies. In this study, we found that chemical PIs and small interfering RNA-mediated knockdown of the proteasome's enzymatic subunits promoted autophagosome formation, stimulated autophagic flux, and upregulated expression of the autophagy-specific genes (ATGs) (ATG5 and ATG7) in some human prostate cancer cells and immortalized mouse embryonic fibroblasts (MEFs). Upregulation of ATG5 and ATG7 only occurred in cells displaying PI-induced phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2alpha), an important component of the unfolded protein responses. Furthermore, PIs did not induce autophagy or upregulate ATG5 in MEFs expressing a phosphorylation-deficient mutant form of eIF2alpha. Combined inhibition of autophagy and the proteasome induced an accumulation of intracellular protein aggregates reminiscent of neuronal inclusion bodies and caused more cancer cell death than blocking either degradation pathway alone. Overall, our data show that proteasome inhibition activates autophagy through a phospho-eIF2alpha-dependent mechanism to eliminate protein aggregates and alleviate proteotoxic stress.

Measuring soluble forms of extracellular cytokeratin 18 identifies both apoptotic and necrotic mechanisms of cell death produced by adenoviral-mediated interferon alpha: possible use as a surrogate marker.

Adenoviral transduction of human bladder cancer cells with human interferon alpha-2b (Ad-IFN) produces cancer-specific cell death through direct and indirect mechanisms. The indirect mechanisms involve the secreted IFN produced, which kill, IFN protein-sensitive cancer cells, as well as yet unidentified bystander factors, which are cytotoxic to neighboring cancer cells. The direct cell kill results from transfection and expression of Ad-IFN in the cancer cells. As the molecular forms of cytokeratin 18, either caspase cleaved or not, have been associated with apoptotic or necrotic cell death, respectively, we determined if increases in either or both cytokeratin 18 forms could be observed following IFNalpha protein or Ad-IFN treatment of bladder carcinoma cells. Quantification of M30 and M65 enzyme-linked immunosorbent assays (assays for cytokeratin 18 associated apoptotic and necrotic cell death, respectively) were used as surrogate markers of the cell death produced. In the IFN protein-sensitive RT4 bladder cancer cells, IFN produced primarily M30-related cell death, whereas Ad-IFN treatment resulted in high levels of both M30 and M65. In contrast, conditioned medium from Ad-IFN-treated cells whether from normal human urothelial cells or bladder cancer cells caused increases mainly in M30 levels when added to IFN protein resistant KU7 or UC9 bladder cancer cells, suggesting that the bystander factors present in the conditioned medium produced primarily apoptotic cell death. In addition, a significant increase in M65 levels above that observed for M30 was seen when the IFN protein resistant KU7 and UC9 cells were treated with Ad-IFN, again indicating there is additional necrotic-related cell death produced by Ad-IFN as well. Normal urothelial cells showed no cytotoxicity nor increases in M30 or M65 after Ad-IFN treatment. As intravesical Ad-IFN treatment is presently being evaluated for its efficacy in superficial bladder cancer measurement of M30 and M65 levels in the urine at various time points before and after Ad-IFN treatment may provide not only a biomarker of efficacy but also evidence for the different types and proportion of cell kill produced by the various mechanisms of cell kill in the tumors of individual patients.

Early RB94-produced cytotoxicity in cancer cells is independent of caspase activation or 50 kb DNA fragmentation.

RB94, which lacks the N-terminal 112 amino-acid residues of the full-length retinoblastoma protein (RB110) is a more potent inhibitor of cancer cell growth than RB110, being cytotoxic to all cancer cell lines studied, independent of their genetic abnormalities. Although we initially thought RB94-induced cell death was caspase-dependent, such caspase activation now appears to be a late event. Cells that remained attached 48 h after transduction with Ad-RB94 showed, among other changes, nuclear enlargement, peripheral nuclear chromatin condensation and often micronucleation. In addition, the cells were TdT-mediated dUTP nick end labeling (TUNEL) positive but showed no cleavage of caspase 3 or 9. Only after the cells detached was cleavage of both caspase 3 and 9 observed. These TUNEL-positive cells showed neither cytochrome c mitochondrial translocation usually found in typical apoptotic cells nor DNA laddering indicative of oligonucleosomal DNA fragmentation. In addition, although 50 kb DNA fragmentation was produced in these TUNEL-positive cells, which was dependent on apoptosis-inducing factor (AIF), inhibiting this fragmentation by siAIF did not inhibit TUNEL formation or cytotoxicity. As RB94 will soon be used for gene therapy further understanding the molecular basis of these early changes in killing cancer cells is one of our particularly important present goals.

Phase II trial of imatinib mesylate in patients with metastatic melanoma.

Metastatic melanoma cells express a number of protein tyrosine kinases (PTKs) that are considered to be targets for imatinib. We conducted a phase II trial of imatinib in patients with metastatic melanoma expressing at least one of these PTKs. Twenty-one patients whose tumours expressed at least one PTK (c-kit, platelet-derived growth factor receptors, c-abl, or abl-related gene) were treated with 400 mg of imatinib twice daily. One patient with metastatic acral lentiginous melanoma, containing the highest c-kit expression among all patients, had dramatic improvement on positron emission tomographic scan at 6 weeks and had a partial response lasting 12.8 months. The responder had a substantial increase in tumour and endothelial cell apoptosis at 2 weeks of treatment. Imatinib was fairly well tolerated: no patient required treatment discontinuation because of toxicity. Fatigue and oedema were the only grade 3 or 4 toxicities that occurred in more than 10% of the patients. Imatinib at the studied dose had minimal clinical efficacy as a single-agent therapy for metastatic melanoma. However, based on the characteristics of the responding tumour in our study, clinical activity of imatinib, specifically in patients with melanoma with certain c-kit aberrations, should be examined.

Adenoviral-mediated interferon alpha overcomes resistance to the interferon protein in various cancer types and has marked bystander effects.

We have previously shown that intravesical administration of adenovirus encoding human interferon alpha-2b (Ad-IFN) induced a marked regression of superficial human bladder tumors derived from cells that are resistant to over 1 million units/ml of IFNalpha protein in vitro. In addition, Ad-IFN appeared to produce strong bystander effects. In this study, we show that Ad-IFN causes marked inhibition of cell growth and apoptosis in cells of various tumor types, all of which are resistant to IFNalpha protein. In addition, strong perinuclear IFN staining was seen in all cell lines following Ad-IFN transfection and was never observed after exposure to the IFN protein. Ad-IFN induced proteolytic processing of caspases 3, 8 and 9, indicative of enzymatic activation. However, the caspase-8-selective inhibitor, IETDfmk, blocked apoptosis only in the cell lines that were sensitive to the IFNalpha protein and had minimal effect on Ad-IFN-induced caspase-3 or -9 processing and cell death, indicating that death receptor-independent mechanism(s) were involved in the cytotoxic effects observed for cancer cell lines resistant to the IFNalpha protein. Moreover, we document that a yet to be identified soluble factor(s) is responsible for causing the bystander effect observed following Ad-IFN treatment in IFN protein-resistant cancer cells.

Increased mitochondrial biogenesis in primary leukemia cells: the role of endogenous nitric oxide and impact on sensitivity to fludarabine.

B cell chronic lymphocytic leukemia (CLL) is the most prevalent adult leukemia in the Western hemisphere, yet many biological and molecular features of the disease remain undefined. CLL cells generate increased levels of radical species such as superoxide and nitric oxide (NO), which is associated with mitochondrial DNA mutations. Considering that NO levels can affect mitochondrial biogenesis, we hypothesized that the inherent nitrosative stress in CLL cells may lead to hyperactive mitochondrial biogenesis. Here we report that primary CLL cells contained significantly more mitochondria than normal lymphocytes and that their mitochondrial mass was significantly related to endogenous NO levels. Expression of the mitochondrial biogenesis factors nuclear respiratory factor-1 and mitochondrial transcription factor A was elevated in most CLL specimens examined and appeared to be related to cellular NO levels. Treatment of B cells with exogenous NO caused a substantial increase in mitochondrial mass. In vitro sensitivity of CLL cells to fludarabine was highly related to mitochondrial mass in that cells with greater mitochondrial mass were less sensitive to the drug. Taken together, our results suggest that NO is a key mediator of mitochondrial biogenesis in CLL and that modulation of mitochondrial biogenesis by NO may alter cellular sensitivity to fludarabine.

Surrogate markers in antiangiogenesis clinical trials.

Novel antiangiogenic agents currently being developed may ultimately be more effective against solid tumours and less toxic than cytotoxic chemotherapy. As a result of the early clinical trials of angiogenesis inhibitors, investigators are beginning to appreciate the complexity of targeting angiogenesis and the realisation that developing clinically useful antiangiogenic therapy will be more challenging than originally thought. It is now apparent that new methods and surrogate markers to assess these agents' biological activity are crucial for their successful development. This review summarises the currently available clinical data on the development of surrogate markers of angiogenesis inhibitors.

p53 and Fas ligand are required for psoralen and UVA-induced apoptosis in mouse epidermal cells.

A combination of 8-methoxypsoralen (8-MOP) and ultraviolet-A (UVA) radiation (320-400 nm) (PUVA) is widely used in the treatment of psoriasis and other skin diseases. PUVA is highly effective in eliminating hyperproliferative cells in the epidermis, but its mechanism of action has not been fully elucidated. In this study, we used immortalized JB6 mouse epidermal cells, p53(-/-), and Fas ligand deficient (gld) mice to investigate the molecular mechanism by which PUVA induces cell death. The results indicate that PUVA treatment induces apoptosis in JB6 cells. In addition, PUVA treatment of JB6 cells results in p53 stabilization, phosphorylation, and nuclear localization as well as induction of p21(Waf/Cip1) and caspase-3 activity. In vivo studies reveal that PUVA treatment induces significantly less apoptosis in the epidermis of p53(-/-) mice compared to p53(+/+) mice. Furthermore, FasL-deficient (gld) mice are completely resistant to PUVA-induced apoptosis compared to wild-type mice. These results indicate that PUVA treatment induces apoptosis in mouse epidermal cells in vitro and in vivo and that p53 and Fas/Fas ligand interactions are required for this process, at least in vivo. This implies that similar mechanisms may be involved in the elimination of psoriatic keratinocytes from human skin following PUVA therapy.

Histone deacetylase inhibitor downregulation of bcl-xl gene expression leads to apoptotic cell death in mesothelioma.

It has been shown that mesothelioma expresses the antiapoptotic protein BCL-XL, but not BCL-2, rendering bcl-xl gene expression a potential therapeutic target. Sodium butyrate (NaB) is a histone deacetylase inhibitor capable of alteration of bcl-2 family protein expression in other tumor types. Mesothelioma cell lines (REN, I-45) were exposed to NaB, and viability (colorimetric assay) and apoptosis (TUNEL, Hoescht staining, flow cytometry) were evaluated. Effects on bcl-2 family protein, fas-fas ligand, and caspases were examined by Western blot analysis and functional assay. An RNase assay evaluated bcl-2 family messenger RNA (mRNA) expression. Overexpressing BCL-XL mesothelioma clones were created by plasmid transfer. Cells were sensitive to NaB at low IC(50) (REN, 0.3 mM; I-45, 1 mM) and demonstrated apoptosis (percentage of cells below G1 phase by flow cytometry [sub-G1]: REN, 38.5%; I-45, 30.9%). A significant decrease in BCL-XL protein expression was noted with BAK, BAX, and BCL-2 unchanged, and this was corroborated at the transcriptional level with selectively decreased bcl-xl mRNA production after sodium butyrate exposure. Fas expression and fas-fas ligand sensitivity were unchanged. Caspases demonstrated low-level activation. Stable overexpressing BCL-XL clones were proportionally resistant to the NaB effect. This study suggests that mesothelioma cells are sensitive to the induction of apoptosis related to the attenuation of antiapoptotic bcl-xl gene and protein expression. Additional study of the therapeutic benefit of targeting bcl-xl gene expression in mesothelioma is warranted.