1. ¹The Department of Medicine II, University of Tübingen, Tübingen, Germany
2. ²The Department of Medicine IV, University of Tübingen, Tübingen, Germany
3. ³ZBiT/Proteomics, University of Tübingen, Tübingen, Germany
4. ⁴Division of Cellular Biochemistry, The Netherlands Cancer Institute, Amsterdam, The Netherlands
5. ⁵Department of Chemistry, Leiden University, Leiden, The Netherlands

Correspondence: Dr C Driessen, Department of Hematology, Oncology and Immunology, University of Tübingen, Otfried Müller Strasse 10, D-72076 Tübingen, Germany. E-mail: christoph.driessen@med.uni-tuebingen.de

⁶These authors contributed equally to this work.

Received 11 April 2006; Revised 28 August 2006; Accepted 30 August 2006; Published online 5 October 2006.

Abstract

Proteasomal proteolysis relies on the activity of six catalytically active proteasomal subunits (1, 2, 5, 1i, 2i and 5i). Applying a functional proteomics approach, we used a recently developed activity-based, cell-permeable proteasome-specific probe that for the first time allows differential visualization of individual active proteasomal subunits in intact primary cells. In primary leukemia samples, we observed remarkable variability in the amounts of active 1/1i-, 2/2i- and 5/5i-type of subunits, contrasting with their constant protein expression. Bortezomib inhibited 5- and 1-type, but to a lesser extend 2-type of subunits in live primary cells in vitro and in vivo. When we adapted the bortezomib-sensitive human acute myeloid leukemia cell line HL-60 to bortezomib 40 nM (HL-60a), proteasomal activity profiling revealed an upregulation of active subunits, and residual 1/5-type of activity could be visualized in the presence of bortezomib 20 nM, in contrast to control cells. In a panel of cell lines from hematologic malignancies, the ratio between 2-type and (1+5)-type of active proteasomal polypeptides mirrored different degrees of bortezomib sensitivity. We thus conclude that the proteasomal activity profile varies in primary leukemia cells, and that the pattern of proteasomal subunit activity influences the sensitivity of hematologic malignancies toward bortezomib.