Selective Induction of Apoptosis in Acute Myelogenous Leukemia Stem Cells by the Novel Agent Parthenolide.

Acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs), which are thought to perpetuate both de novo and relapsed disease. Because LSCs are often refractory to standard chemotherapy agents, it is crucial to investigate the ability of current and novel drugs to target this rare population of cells. Previously, we demonstrated that combined treatment with proteasome inhibitors and anthracyclines induced apoptosis in LSCs while sparing the normal hematopoietic stem cells (HSCs). Using this approach, death of leukemic cells was characterized by NF-kB inhibition and p53 activation. In the present study, we have extended this work by investigating parthenolide (PTL), a sesquiterpene lactone found in the herbal drug feverfew and that has been reported to inhibit NF-kB activation. Primary AML cells treated with increasing concentrations of PTL (1-10 microM) showed a rapid (18h) and dose dependent decrease in cell viability. The PTL concentration and its effect on induction of apoptosis directly correlated with decreased NF-kB binding activity and increased phosphorylation of p53. Importantly, apoptosis induction was also evident in the primitive LSC population, defined by the immunophenotypic profile CD34+, CD38− and CD123+. In contrast, PTL had little to no effect in normal HSCs obtained from either cord blood (CB) or bone marrow (BM). Interestingly, apoptosis induction upon PTL treatment was completely abolished upon treatment with the anti-oxidant, N-acetyl-cysteine (NAC), suggesting that PTL effects involve reactive oxygen species (ROS) induction. This was further corroborated using the fluorescent dyes DCFH-DA and mBBr to measure ROS and thiol content, respectively. When PTL effects were compared to the common chemotherapeutic drug cytosine arabinoside (Ara-C), we observed that PTL selectively targeted leukemic cells whereas Ara-C did not. Functional analyses using in vitro colony assays and the NOD/SCID mouse xenotransplant model were then employed to address whether the drugs used in this study affected stem cell potential. PTL treatment did not affect engraftment of normal cells while leukemic cell engraftment was dramatically impaired. Similarly, methylcellulose cultures showed a severe decrease on colony formation in PTL treated AML cells, while normal colonies were not impaired. Together, these data demonstrates that PTL is able to selectively target LSCs while sparing normal hematopoietic cells. The mechanism of leukemia-specific cell death appears to involve inhibition of NF-kB, activation of p53 and induction of oxidative stress.