Lisa Crawford

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.

Comparative Selectivity and Specificity of the Proteasome Inhibitors BzLLLCOCHO, PS-341, and MG-132

The 26S proteasome is a multicatalytic protease responsible for regulated intracellular protein degradation. Its function is mediated by three main catalytic activities: (a) chymotrypsin-like (CT-L), (b) trypsin-like, and (c) peptidylglutamyl peptide hydrolysing (PGPH). Proteasome inhibition is an emerging therapy for many cancers and is a novel treatment for multiple myeloma. Here, we profile the contributions of the three catalytic activities in multiple myeloma cell lines and compare the specificity and cytotoxicity of the novel proteasome inhibitor BzLLLCOCHO and inhibitors PS-341 (Velcade, bortezomib) and MG-132. Using fluorogenic substrates and an active site-directed probe specific for proteasome catalytic subunits, we show differential subunit specificity for each of the inhibitors. Addition of BzLLLCOCHO strongly inhibited all three catalytic activities, treatment with PS-341 completely inhibited CT-L and PGPH activities, and treatment with MG-132 resulted in weak inhibition of the CT-L and PGPH activities. Multiple myeloma cells were more sensitive to induction of apoptosis by PS-341 and MG-132 than BzLLLCOCHO. This study emphasizes the need for further investigation of the effects of these compounds on gene and protein expression in the cell to allow for the development of more specific and targeted inhibitors.

Bortezomib induces apoptosis in primitive chronic myeloid leukemia cells including LTC-IC and NOD/SCID repopulating cells.

Chronic myeloid leukemia (CML) is treated effectively with tyrosine kinase inhibitors (TKIs); however, 2 key problems remain-the insensitivity of CML stem and progenitor cells to TKIs and the emergence of TKI-resistant BCR-ABL mutations. BCR-ABL activity is associated with increased proteasome activity and proteasome inhibitors (PIs) are cytotoxic against CML cell lines. We demonstrate that bortezomib is antiproliferative and induces apoptosis in chronic phase (CP) CD34+ CML cells at clinically achievable concentrations. We also show that bortezomib targets primitive CML cells, with effects on CD34+38(-), long-term culture-initiating (LTC-IC) and nonobese diabetic/severe combined immunodeficient (NOD/SCID) repopulating cells. Bortezomib is not selective for CML cells and induces apoptosis in normal CD34+38(-) cells. The effects against CML cells are seen when bortezomib is used alone and in combination with dasatinib. Bortezomib causes proteasome but not BCR-ABL inhibition and is also effective in inhibiting proteasome activity and inducing apoptosis in cell lines expressing BCR-ABL mutations, including T315I. By targeting both TKI-insensitive stem and progenitor cells and TKI-resistant BCR-ABL mutations, we believe that bortezomib offers a potential therapeutic option in CML. Because of known toxicities, including myelosuppression, the likely initial clinical application of bortezomib in CML would be in resistant and advanced disease.

Targeting the ubiquitin proteasome system in haematological malignancies

The ubiquitin proteasome system (UPS) plays a central role in cellular protein homeostasis through the targeted destruction of damaged/misfolded proteins and regulatory proteins that control critical cellular functions. The UPS comprises a sequential series of enzymatic activities to covalently attach ubiquitin to proteins to target them for degradation through the proteasome. Aberrancies within this system have been associated with transformation and tumourigenesis and thus, the UPS represents an attractive target for the development of anti-cancer therapies. The use of the first-in-class proteasome inhibitor, bortezomib, in the treatment of Plasma Cell Myeloma and Mantle Cell Lymphoma has validated the UPS as a therapeutic target. Following on its success, efforts are focused on the development of second-generation proteasome inhibitors and small molecule inhibitors of other components of the UPS. This review will provide an overview of the UPS and discuss current and novel therapies targeting the UPS.

Adiponectin is produced by lymphocytes and is a negative regulator of granulopoiesis

Lymphocytes have long been established to play an important role in the regulation of hematopoiesis and produce many cytokines that act on hematopoietic progenitor cells. Previous studies by our group have shown that normal, unstimulated lymphocytes produce a protein that inhibits normal bone marrow GM colony formation. Adiponectin is an adipokine that has been demonstrated to act as a negative regulator of hematopoiesis and immune function. This study aimed to determine if the inhibitory molecule that we described previously was adiponectin. Here, we show transcription, translation, and secretion of adiponectin from lymphocytes and demonstrate that its receptors, AdipoR1 and AdipoR2, are expressed by bone marrow MNCs. We show that although the adiponectin expression is low in lymphocytes, it is sufficient to induce a significant inhibitory effect on GM precursors (CFU‐GM) and activate the AMPK pathway in these cells. The regulation of adiponectin production by lymphocytes and its detailed function in suppressing GM colony formation need to be elucidated now. Our findings suggest a functional role for adiponectin as a negative regulator of granulopoiesis.

Methods for measuring proteasome activity: Current limitations and future developments

The proteasome has been validated as a therapeutic target, with proteasome inhibitors showing particular efficacy in the treatment of Multiple Myeloma. A wide range of methods have been developed to profile proteasome activity. These include the current method of choice fluorogenic peptide substrates, as well as bioluminescent imaging, immunological methods, and more recently, site-specific fluorescent probes. The aim of this review is to evaluate the currently available methods for profiling proteasome activity and their suitability for use in translational studies. Ongoing development of techniques for profiling proteasome activity will facilitate future research into proteasome-related pathologies, thus accelerating the development of more specific drug regimes.

Proteasome proteolytic profile is linked to Bcr-Abl expression

OBJECTIVE We have previously demonstrated that proteasome activity is higher in bone marrow from patients with chronic myeloid leukemia (CML) than normal controls. This study investigates whether there is any relationship between Bcr-Abl expression and proteasome activity. MATERIALS AND METHODS Fluorogenic substrate assays and an activity-based probe were used to profile proteasome activity in CML cell-line models and the effect of the proteasome inhibitor BzLLLCOCHO on these cell-line models and primary CML cells was investigated. RESULTS We have demonstrated that oncogenic transformation by BCR-ABL is associated with an increase in proteasome proteolytic activity. Furthermore, small interfering RNA targeted against BCR-ABL reduces proteasome activity. In addition, we have found that Bcr-Abl-positive cells are more sensitive than Bcr-Abl-negative cells to induction of apoptosis by the proteasome inhibitor BzLLLCOCHO, and that sequential addition of imatinib followed by BzLLLCOCHO has an additive effect on the induction of apoptosis in Bcr-Abl-positive cells. Finally, we demonstrate that cell lines that become resistant to imatinib remain sensitive to proteasome inhibition. CONCLUSION This is the first time that a direct relationship has been demonstrated between BCR-ABL transformation and the enzymatic activity of the proteasome. Our results suggest that the proteasome might provide a useful therapeutic target in CML, particularly in those patients who have developed resistance to conventional treatment.

Proteasome inhibitors: a therapeutic strategy for haematological malignancy.

The proteasome is a multicatalytic enzyme complex responsible for the regulated degradation of intracellular proteins. In recent years, inhibition of proteasome function has emerged as a novel anti-cancer therapy. Proteasome inhibition is now established as an effective treatment for relapsed and refractory multiple myeloma and offers great promise for the treatment of other haematological malignancies, when used in combination with conventional therapeutic agents. Bortezomib is the first proteasome inhibitor to be used clinically and a second generation of proteasome inhibitors with differential pharmacological properties are currently in early clinical trials. This review summarises the development of proteasome inhibitors as therapeutic agents and describes how novel assays for measuring proteasome activity and inhibition may help to further delineate the mechanisms of action of different proteasome inhibitors. This will allow for the optimized use of proteasome inhibitors in combination therapies and provide the opportunity to design more potent and therapeutically efficacious proteasome inhibitors.

The matricellular protein CCN3 regulates NOTCH1 signalling in chronic myeloid leukaemia.

Deregulated NOTCH1 has been reported in lymphoid leukaemia, although its role in chronic myeloid leukaemia (CML) is not well established. We previously reported BCR‐ABL down‐regulation of a novel haematopoietic regulator, CCN3, in CML; CCN3 is a non‐canonical NOTCH1 ligand. This study characterizes the NOTCH1–CCN3 signalling axis in CML. In K562 cells, BCR‐ABL silencing reduced full‐length NOTCH1 (NOTCH1‐FL) and inhibited the cleavage of NOTCH1 intracellular domain (NOTCH1‐ICD), resulting in decreased expression of the NOTCH1 targets c‐MYC and HES1. K562 cells stably overexpressing CCN3 (K562/CCN3) or treated with recombinant CCN3 (rCCN3) showed a significant reduction in NOTCH1 signalling (> 50% reduction in NOTCH1‐ICD, p < 0.05). Gamma secretase inhibitor (GSI), which blocks NOTCH1 signalling, reduced K562/CCN3 colony formation but increased that of K562/control cells. GSI combined with either rCCN3 or imatinib reduced K562 colony formation with enhanced reduction of NOTCH1 signalling observed with combination treatments. We demonstrate an oncogenic role for NOTCH1 in CML and suggest that BCR‐ABL disruption of NOTCH1–CCN3 signalling contributes to the pathogenesis of CML.

Proteasome Inhibitors in the Treatment of Multiple Myeloma

The ubiquitin proteasome system is responsible for the degradation of proteins involved in a wide range of cellular processes such as the cell cycle, apoptosis, transcription, cell signalling, immune response and antigen presentation. Protein homeostasis is essential for normal cell growth and inhibition of proteasome function has emerged as a viable strategy for anti-cancer treatment. The first proteasome inhibitor to enter clinical practice, bortezomib, was approved by the Food and Drug Administration as a single agent to treat relapsed/refractory Multiple Myeloma in 2003 and expanded to first-line treatment in combination with melphalan and prednisone in 2008. It is now a routine component of Multiple Myeloma therapy and has had a major impact on expanding treatment options in the last few years. Bortezomib exhibits novel action against Multiple Myeloma by targeting both intracellular mechanisms and interactions within the bone marrow environment. Although it demonstrates significant anti-Myeloma activity when used alone, it has been shown to have even greater benefits when used in combination with conventional and novel chemotherapeutic agents. There are currently over 200 clinical trials ongoing or recently completed examining bortezomib alone and in combination in various stages of disease and treatment. The clinical success of bortezomib has prompted the development of a number of second generation proteasome inhibitors with improved pharmacological properties. In this chapter, we review the development of bortezomib as a novel therapeutic agent in Multiple Myeloma and summarize the key observations from recently completed and ongoing studies on the effect of bortezomib both as a single agent and in combination therapies in the setting of newly diagnosed Multiple Myeloma and for relapsed disease. We also discuss the progress of next generation proteasome inhibitors in the clinic.