Hypoxia Suppresses Arsenic Trioxide-Induced Apoptosis in Human Acute Myeloid Leukemia Cells.

Abstract 3109

Poster Board III-46

Acute myeloid leukemia (AML) is a class of prevalent hematopoietic malignancies, which often remains incurable because of the development of drug resistance. Leukemic cells originated from hypoxic condition in bone marrow that gives a benefit to leukemic cells by protecting them from anti-cancer drugs through physical barrier and by induction of some survival signal mediators. As the development of drug resistance is a key element in the failure of chemotherapy for leukemia, it has been studied about many factors make leukemic cells resist to chemotherapeutic drugs. However, effect of hypoxia on drug induced apoptosis is still poorly understood. Here, we investigated whether hypoxia affects the resistance to drugs in leukemic cells and whether hypoxia has some role in drug-induced apoptosis. To demonstrate whether hypoxia influences the apoptosis of human leukemic cells induced by chemotherapeutic drugs, the human AML cell lines (U937, HL-60, CCRF-CEM and K562) were treated with arsenic trioxide (ATO), actinomycin-D, 17-AAG, valproic acid and cytrabine under hypoxic condition and hypoxiamimetic agent cobalt chloride (CoCl2). Cellular proliferation was evaluated by methyl thiazolyl tetrazolium (MTT) assay. Subsequently, FACS analysis and western blot were performed to investigate the apoptosis related proteins. Among AML cell lines, HL-60 cells became resistant to apoptosis induced by chemotherapeutic drug, especially ATO, under the hypoxic condition. It was demonstrated through FACS analysis that level of Annexin-V staining in hypoxia (13.58%) was lower than in normoxia (29.07%). Also, among many apoptosis related molecules, activation of HIF-1 alpha under hypoxic condition was associated with the cell survival (HSP70) and apoptosis (BAX). Expression of HSP70 was increased and the level of BAX was dramatically down-regulated by HSP70 in ATO treated HL-60 cells. These data show that the hypoxia increases resistance to ATO induced apoptosis and the effect was mediated by HSP70/BAX dependent pathway. Collectively, this results provided several lines of direct evidence for the role of hypoxia on apoptosis of AML cells, in which HSP 70 and BAX elicit a role as an effector downstream to HIF-1alpha. These discoveries would shed new insights for understanding underlying mechanisms of hypoxia and designing new therapeutic strategy for AML.