The bone marrow microenvironment is physiologically hypoxic with areas being as low as 1% O2, e.g. the stem cell niche. Acute myeloid leukaemia (AML) blasts misuse these bone marrow niches for protection by the local microenvironment, but also might create their own microenvironment. Here we identify IL-8 as a hypoxia-regulated cytokine in both AML cell lines and primary AML samples that is induced within 48 hours of severe hypoxia (1% O2). IL-8 lacked effects on AML cells but induced migration in mesenchymal stromal cells (MSC), an integral part of the bone marrow. Accordingly, MSC were significantly increased in AML bone marrow as compared to healthy bone marrow. Interestingly, mononuclear cells obtained from healthy bone marrow displayed both significantly lower endogenous and hypoxia-induced production of IL-8. IL-8 mRNA expression in AML blasts from 533 patients differed between genetic subgroups with significantly lower expression of IL-8 in acute promyelocytic leukaemia (APL), while in non APL-AML patients with FLT ITD had the highest IL-8 expression. In this subgroup, high IL-8 expression was also prognostically unfavourable. In conclusion, hypoxia as encountered in the bone marrow specifically increases IL-8 expression of AML, which in turn impacts niche formation. High IL-8 expression might be correlated with poor prognosis in certain AML subsets.

Fms-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase constitutively expressed by acute myeloid leukaemia (AML) blasts. In addition, 25% of AML patients harbour a FLT3-ITD mutation, associated with inferior outcome due to increased relapse rate. Relapse might be propagated by interactions between AML blasts and the bone marrow microenvironment. Besides cellular elements of the microenvironment (e.g. mesenchymal stromal cells), bone marrow hypoxia has emerged as an additional crucial component. Hence, effects of hypoxia on FLT3 expression and biology could provide novel insight into AML biology. Here we show that 25% of AML patients down-regulate FLT3 expression on blasts in response to in vitro hypoxia (1% O2), which was independent of its mutational state. While virtually no AML cell lines regulate FLT3 in response to hypoxia, the down-regulation could be observed in Ba/F3 cells stably transfected with different FLT3 mutants. Hypoxia-mediated down-regulation was specific for FLT3, reversible and proteasome-dependent; with FLT3 half-life being significantly shorter at hypoxia. Also, PI-3K inhibition could partially abrogate down-regulation of FLT3. Hypoxia-mediated down-regulation of FLT3 conferred resistance against cytarabine in vitro. In conclusion, FLT3 expression in AML is dependent on the oxygen partial pressure, but response to hypoxia differs.