Mitochondrial-mediated disregulation of Ca²⁺ is a critical determinant of velcade (PS-341/Bortezomib) cytotoxicity in myeloma cell lines

The proteasome inhibitor bortezomib (also known as Ps-341/Velcade) is a dipeptidyl boronic acid that has recently been approved for use in patients with multiple myeloma. Bortezomib inhibits the activity of the 26S proteasome and induces cell death in a variety of tumor cells; however, the mechanism of cytotoxicity is not well understood. In this report, oligonucleotide microarray analysis of the 8226 multiple myeloma cell line showed a predominant induction of gene products associated with the endoplasmic reticulum secretory pathway following short-term, high-dose exposure to bortezomib. Examination of mediators of endoplasmic reticulum stress-induced cell death showed specific activation of caspase 12, as well as of caspases 8, 9, 7, and 3, and cleavage of bid. Treatment of myeloma cells with bortezomib also showed disregulation of intracellular Ca²⁺ as a mechanism of caspase activation. Cotreatment with a panel of Ca²⁺-modulating agents identified the mitochondrial uniporter as a critical regulatory factor in bortezomib cytotoxicity. The uniporter inhibitors ruthenium red and Ru360 prevented caspase activation and bid cleavage, and almost entirely inhibited bortezomib-induced cell death, but had no effect on any other chemotherapeutic drug examined. Additional Ca ²⁺-modulating agents, including 2-amino-ethoxydiphenylborate, 1,2-bis (o-aminophenoxy) ethane-tretraacetic acid (acetoxymethyl) ester, and dantrolene, did not alter bortezomib cytotoxicity. Analysis of intracellular Ca²⁺ showed that the ruthenium-containing compounds inhibited Ca ²⁺ store loading and abrogated the desensitized capacitative calcium influx associated with bortezomib treatment. These data support the hypothesis that intracellular Ca²⁺ disregulation is a critical determinant of bortezomib cytotoxicity.