Proteasome inhibition leads to significant reduction of Bcr-Abl expressionand subsequent induction of apoptosis in K562 human chronic myelogenous leukemia cells

The chimeric oncogene bcr-abl is detected in virtually every case of chroni c myelogenous leukemia, It has been shown that cells (such as K562) express ing Bcr-Abl/p210, a protein tyrosine kinase, not only undergo cellular tran sformation but also demonstrate multiple drug resistance. Recent studies al so demonstrate that the proteasome is involved in the survival signaling pa thway(s). In the current study, we tested the hypothesis that the proteasom e might play a role in regulating Bcr-Abl function. We have demonstrated by using a variety of inhibitors that inhibition of the proteasome, but not o f the cysteine protease, activity is able to activate the apoptotic cell de ath program in K562 cells. Proteasome inhibition-induced apoptosis is demon strated by condensation and fragmentation of nuclei, appearance of an apopt otic population with sub-G(1) DNA content, the internucleosomal fragmentati on of DNA, and cleavage of poly(ADP-ribose) polymerase, and can be blocked by a specific caspase-3-like tetrapeptide inhibitor. Western blot analysis with specific antibodies to c-Abl and Bcr proteins show that treatment of K 562 cells with a proteasome inhibitor results in significant reduction of B cr-Abl protein expression, which occurs several hours before the onset of a poptotic execution. Levels of c-Abl/p145 and Bcr/p160 proteins, however, re main essentially unaltered at that time. Furthermore, reduced Bcr-Abl expre ssion is reflected in significantly attenuated Bcr-Abl-mediated protein tyr osine phosphorylation. Taken together, these results indicate that proteaso me inhibition is sufficient to inactivate Bcr-Abl function and subsequently activate the apoptotic death program in cells that are resistant to apopto sis induced by chemotherapy.