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Open Access Highly Accessed Research article

Adaptation of leukemia cells to hypoxic condition through switching the energy metabolism or avoiding the oxidative stress

Mineaki Goto1, Hiroshi Miwa1*, Kazuto Suganuma1, Norikazu Tsunekawa-Imai1, Masato Shikami2, Motonori Mizutani1, Shohei Mizuno1, Ichiro Hanamura1 and Masakazu Nitta1

Author Affiliations

1 Department of Internal Medicine, Division of Hematology, Aichi Medical University School of Medicine, 1-1 Yazakokarimata, Nagakute, Aichi 480-1195, Japan

2 Department of Internal Medicine, Division of Hematology, Daiyukai General Hospital, Ichinomiya, Aichi 491-0113, Japan

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BMC Cancer 2014, 14:76  doi:10.1186/1471-2407-14-76

Published: 10 February 2014

Abstract

Background

Like normal hematopoietic stem cells, leukemia cells proliferate in bone marrow, where oxygen supply is limited. However, the growth and energy metabolism of leukemia cells under hypoxia have not been well understood. Although it has been known that reactive oxygen species (ROS) is generated under hypoxic conditions, normal and leukemia stem cells were characterized by relatively low levels of ROS. Roles of ROS on leukemia cells under hypoxia also have not been well understood.

Methods

Four Leukemia cell lines were cultured under normoxia (21% O2) or hypoxia (1% O2), where NB4 and THP-1 were most extensively studied. To evaluate energy metabolism, we estimated whole cell number or apoptotic cells with or without a glycolysis inhibitor or an oxidative phosphorylation (OXPHOS) inhibitor. Glucose consumption and lactate production were also measured. To evaluate oxidative stress in hypoxic condition, the ROS level and GSH (reduced glutathione) / GSSG (oxidized glutathione) ratio was measured. In addition, pyruvate dehydrogenase kinase 1 (PDK1) and cytochrome c oxidase subunit 4 (COX4) were examined by western blotting or RT-PCR.

Results

NB4, which grows well under normoxia depending on glycolysis, demonstrated prominent apoptosis and growth suppression after 48 hours culture under hypoxia. NB4 cells cultured under hypoxia showed significantly increased ROS. Culture with a ROS scavenger resulted in decrease of apoptotic cell death of NB4 under hypoxia. NB4 cells cultured for longer period (7 days) under hypoxia did not come to extinction, but grew slowly by upregulating GSH synthesis to protect from ROS generated in hypoxic condition. By contrast, THP-1, which largely depends on OXPHOS in mitochondria under normoxia, demonstrated more growth under hypoxia by changing metabolism from OXPHOS to glycolysis through upregulating PDK1. Moreover, THP-1 avoided ROS generation by substituting COX 4 subunit (from COX 4–1 to COX 4–2) through upregulation of LON, a mitochondrial protease under hypoxia.

Conclusions

We showed that leukemia cells survive and adapt to the hypoxic condition through various pathways. Our results will help understanding energy metabolism of leukemia cells and creating novel therapeutics.

Keywords:
Leukemia; Hypoxia; Energy metabolism; Reactive oxygen species; Pyruvate dehydrogenase kinase; Cytochrome c oxidase