Studies of b-AP15 : a novel inhibitor of proteasome deubiquitinase activity

Abstract: Bortezomib was the first FDA approved proteasome inhibitor that was initially very successful in treatment of multiple myeloma patients but acquired resistance and adverse side-effect highly decreased patients’ quality of life. Development of 2nd generation proteasome inhibitors that could overcome these shortcomings is thus of prime medical importance. Our group has developed b-AP15 as such a candidate, which targets a different subunit of the proteasome than does bortezomib. In study I, we determined that use of CpdA as a co- translational translocation inhibitor in a co-treatment protocol greatly enhanced proteasome inhibition by b-AP15. Aggresome formation is a resistance mechanism evident after bortezomib treatment. In study II, we demonstrated that b-AP15 did not induce aggresome formation under the same conditions and interestingly we observed less aggresome formation with co-treatment of b- AP15 and bortezomib compared to single treatment with bortezomib. In study III, we demonstrated that lymphoma cell lines were as sensitive to b-AP15 as other cancer cell lines previously reported. The apoptosis induced by b-AP15 correlated to accumulation of polyubiquitination and the ER stress response. In study IV, we observed that the gene expression patterns and apoptosis induction mechanisms of b-AP15 and bortezomib were similar, but not identical. Both induced the expression of Hmox-1 but only b-AP15 could induce ER stress. This study also revealed that ROS scavengers could reduce the apoptosis induced by b-AP15, which was due to activation of AP-1 In study V, the gene expression pattern following Piperlongumine treatment was similar to that of other proteasome inhibitors and the drug could block the ubiquitin–proteasome system in cancer cells. However, Piperlongumine was determined not to be a classic but instead interfered upstream of UPS system. The overall conclusion is that further development of proteasome inhibitors such as b-AP15 should be continued, as increased efficacy is expected following clinical translation.