Tipifarnib-induced apoptosis in acute myeloid leukemia and multiple myeloma cells depends on Ca2+ influx through plasma membrane Ca2+ channels.

A major contributing factor to the high mortality rate associated with acute myeloid leukemia and multiple myeloma is the development of resistance to chemotherapy. We have shown that the combination of tipifarnib, a nonpeptidomimetic farnesyltransferase inhibitor (FTI), with bortezomib, a proteosome inhibitor, promotes synergistic death and overcomes de novo drug resistance in acute myeloid leukemia cell lines. Experiments were undertaken to identify the molecular mechanisms by which tipifarnib produces cell death in acute myeloid leukemia and multiple myeloma cell lines (U937 and 8226, respectively). Tipifarnib, but not other FTIs tested [N-[4-[2(R)-amino-3-mercaptopropyl]amino-2-phenylbenzoyl]methionine methyl ester trifluoroacetate salt (FTI-277) and 2'-methyl-5-((((1-trityl-1H-imidazol-4-yl)methyl)amino)methyl)-[1,1'-biphenyl]-2-carboxylic acid (FTI-2153), promotes elevations in intracellular free-calcium concentrations ([Ca(2+)](i)) in both cell lines. These elevations in [Ca(2+)](i) were accompanied by highly dynamic plasmalemmal blebbing and frequently resulted in membrane lysis. The tipifarnib-induced elevations in [Ca(2+)](i) were not blocked by thapsigargin or ruthenium red, but were inhibited by application of Ca(2+)-free extracellular solution and by the Ca(2+) channel blockers Gd(3+) and La(3+). Conversely, 2-aminoethoxydiphenyl borate (2-APB) potentiated the tipifarnib-evoked [Ca(2+)](i) overload. Preventing Ca(2+) influx diminished tipifarnib-evoked cell death, whereas 2-APB potentiated this effect, demonstrating a link between tipifarnib-induced Ca(2+) influx and apoptosis. These data suggest that tipifarnib exerts its effects by acting on a membrane channel with pharmacological properties consistent with store-operated channels containing the Orai3 subunit. It is noteworthy that Orai3 transcripts were found to be expressed at lower levels in tipifarnib-resistant 8226/R5 cells. Our results indicate tipifarnib causes cell death via a novel mechanism involving activation of a plasma membrane Ca(2+) channel and intracellular Ca(2+) overload.