Hypoxia-HIF-1α-C/EBPα/Runx1 signaling in leukemic cell differentiation

Acute myeloid leukemia (AML), a class of prevalent hematopoietic malignancies, is caused by the acquisition of gene mutations that confer deregulated proliferation, impaired differentiation and a survival advantage of hematopoietic progenitors. More recently, we reported that cobalt chloride (CoCl₂)/iron chelator desferrioxamine (DFO)-mimicked hypoxia or moderate hypoxia (2% and 3% O₂) can directly trigger differentiation of many subtypes of AML cells. Also, intermittent hypoxia significantly prolongs the survival of the transplanted leukemic mice with differentiation induction of leukemic cells. Additionally, these hypoxia-simulating agents selectively stimulate differentiation in acute promyelocytic leukemic cells induced by arsenic trioxide, an effective second-line drug for this unique type of leukemia. Based on this interesting evidence in vitro and in vivo, the ongoing investigations showed the role of hypoxia-inducible factor-1alpha (HIF-1α) protein through its non-transcriptional activity in myeloid cell differentiation, as evidenced by chemical interference, the conditional HIF-1α induction, the specific short hairpin RNAs (shRNAs) against HIF-1α and HIF-1β, an essential partner for transcription activity of HIF-1. Furthermore, HIF-1α and two hematopoietic transcription factors CCAAT/enhancer binding protein alpha (C/EBPα) and Runx1/AML1 interact directly with each other. Such interactions increase the transcriptional activities of C/EBPα and Runx1/AML1, while C/EBPα competes with HIF-1β for direct binding to HIF-1α protein, and significantly inhibits the DNA-binding ability of HIF-1. As a protein is rapidly responsive to all-trans retinoic acid (ATRA), a classical clinical differentiation-inducing drug for AML, HIF-1α also plays a role in ATRA-induced differentiation of leukemic cells.