Inhibition of the 19 S proteasome by bAP-15 in lymphoma cell lines

• Inhibition of activity of proteasome by bortezomib has been shown to selectively kill cancer cells. Bortezomib is approved for the treatment of multiple myeloma and mantle cell lymphoma and the proteolytic 20S core particle of the proteasome, has also been clinically validated as a therapeutic target in oncology. However, despite its acceptable therapeutic index, patients treated with bortezomib show toxic side effects and eventually acquire resistance to the drug. A lot of efforts are currently been made to develop new proteasome inhibitors that perform through mechanisms different from that of bortezomib.We have studied the effects of b-AP15, a novel inhibitor of the deubiquitinase activity in the 19S regulatory subunit of the proteasome, on a panel of nine cell-lines from diffuse large B-cell lymphomas (DLBCL) and three from Hodgkin lymphoma (HL). All cell lines showed a dose dependent reduction of viability. The inhibition of the 19S subunit by b-AP15 resulted, as expected, in accumulation of ubiquitinylated proteins at concentrations close to cytotoxic IC50. Increases in polyubiquitinated proteins were paralleled by increases in the inducible form of heat shock protein 70 (Hsp70B´), and a strong association between Hsp70B´ induction and cleavage of PARP and caspase-3 was observed. These data suggest that proteotoxic stress mediates the sensitivity of lymphoma cells to the deubiquitinase inhibitor b-AP15.The findings in this study suggest that b-AP15 should further evaluated as a drug in lymphoma treatment.

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