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Effects of PS-341 on the Activity and Composition of Proteasomes in Multiple Myeloma Cells

¹ Department of Pathology, Harvard Medical School; ² Program in Hematology/Oncology, Children's Hospital Boston and Dana-Farber Cancer Institute; ³ Jerome Lipper Multiple Myeloma Center, Department of Adult Oncology, Dana-Farber Cancer Institute, Boston, Massachusetts; and ⁴ Centre for Cellular and Molecular Physiology, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom

Requests for reprints: Benedikt Kessler, Centre for Cellular and Molecular Physiology, Nuffield Department of Clinical Medicine, University of Oxford, Roosevelt Drive, Oxford OX3 7BN, United Kingdom. Phone: 44-1865-287-799; Fax: 44-1865-287-787; E-mail: bmk{at}ccmp.ox.ac.uk.

Multiple myeloma is a B-cell malignancy for which no curative therapies exist to date, despite enormous research efforts. The remarkable activity of the proteasome inhibitor bortezomib (PS-341, Velcade) observed in clinical trials of patients with relapsed refractory myeloma has led to investigations of the role of the ubiquitin-proteasome pathway in the pathogenesis of myeloma. Here we report a biochemical analysis of proteasome activity and composition in myeloma cells exposed to PS-341 in the presence or absence of cytokines present in the bone marrow milieu. We observed that the myeloma cell lines MM1.S, RPMI8226, and U266 contain active immunoproteasomes, the amount of which is enhanced by IFN- and tumor necrosis factor-. Using a radiolabeled active site–directed probe specific for proteasome catalytic subunits, we show that PS-341 targets the ß5 and ß1 subunits in a concentration-dependent manner. Furthermore, PS-341 also targeted the corresponding catalytic subunits of the immunoproteasome, ß5i and ß1i, respectively. These data suggest that PS-341 targets both normal and immunoproteasome species to a similar extent in myeloma cells.

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