The proteasome is a mutlicatalytic protease with three main catalytic activities - chymotrypsin-like (CT-L), trypsin-like (T-L) and peptidylglutamyl peptide hydrolising (PGPH). Proteasome inhibition is an emerging therapy for many cancers and is a novel treatment for multiple myeloma (MM). The CT-L activity, considered to be the rate-limiting step in protein degradation, is the primary target of many proteasome inhibitors. We have compared the specificity and potency of the novel proteasome inhibitor BzLLLCOCHO to the previously characterised inhibitors PS-341 (Velcade, bortezomib) and MG-132. Specific fluorogenic substrates were used to measure proteasome proteolytic activity in the presence and absence of the inhibitory compounds. An active site directed probe with a dansyl-sulfonamidohexanoyl hapten tag was used in conjuction with immunoblotting to determine the subunit specificity of the proteasome inhibitors (

Nature Methods
2005
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2
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357
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). MM cell lines (U266, OPM-2, KMS-11, KMS-18) were incubated with 10 μM BzLLLCOCHO, 5 nM PS-341 or 1 μM MG-132 for 24 hrs and proteasome activity was measured. Addition of BzLLLCOCHO reduced CT-L activity by 83 ± 13 % in the fluorogenic assay, and T-L and PGPH activities were reduced by 93 ± 6 % and 92 ± 2 % respectively. Immunoblot results revealed a similar pattern, the T-L and PGPH subunits were completely inhibited by BzLLLCOCHO and there was only weak labeling of the CT-L subunit with the active site probe. In contrast, treatment with PS-341 completely inhibited the CT-L and PGPH activities and incubation with MG-132 resulted in weak inhibition of the CT-L and PGPH activities, neither inihibitor significantly affected T-L activity. The ability of the different inihibitors to induce apoptosis in MM cell lines was then evaluated. All three inhibitors were demonstrated to act through both the caspase-8 and caspase-9 signalling pathways. Using Mitosensor™ and Hoescht/Propidium Iodide staining we found that MM cells were more sensitive to the induction of apoptosis by PS-341 and MG-132 than BzLLLCOCHO (U266 cells treated for 72 hrs with BzLLLCOCHO 51 % apoptosis, PS-341 79 % apoptosis and MG-132 84 % apoptosis). BzLLLCOCHO is a cell permeable and potent inhibitor of all three proteolytic activities of the proteasome. PS-341 and MG-132 inhibited only two of the three proteasome activities but were more efficient than BzLLLCOCHO at inducing apoptosis in MM cell lines. MG-132 is known to inhibit non proteasomal proteases such as Cathepsin B and Calpain 1 which may contribute to its potency. Further investigation on the effects of these inhibitors on gene and protein expression in the cell may lead to the development of more specific and targeted inhibitors.

Topics:
cytotoxicity, proteasome inhibitors, bortezomib, endopeptidases, immunoblotting, peptide hydrolases, calpain, cancer, caspase-8, caspase-9

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2005, The American Society of Hematology
2005
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