Hypoxia induces inflammatory microenvironment by priming specific macrophage polarization and modifies LSC behaviour via VEGF-HIF1α signalling

BACKGROUND: Leukemia stem cells (LSCs) play pivotal roles in leukemogenesis, and are closely implicated in leukemia relapse and chemoresistance. LSCs are tightly modulated by hypoxic exposure and macrophage-conditioned microenvironment. Nevertheless, the impacts on the biology of LSCs imposed by the interaction of hypoxia and macrophage polarization remain elusive. METHODS: In the study, LSCs characterized by CD34+CD38− immunophenotype sorted from KG1α and primary AML cells were employed as in vitro and ex vivo cell models. mRNA and protein expressions of cytokine/chemokine of cells under normoxic and hypoxic conditions were determined by RT-PCR and western blot. Macrophage polarization, cell cycle and apoptosis were determined by flowcytometry. Cell viability was assayed by CCK-8. RESULTS: Macrophages preferentially presented with M2 polarization phenotype characterized by upregulated VEGF and CCL17 cytokine/chemokine profile, when stimulated by specific set of cytokines under hypoxic exposure, and induced an anti-inflammatory microenvironment. LSCs exhibited significantly increased cell viability, colony-forming capacity and chemoresistance when co-incubated in hypoxic conditioned medium (H-CM) primed by polarized M1 macrophages. VEGF expression was upregulated in LSCs which in turn activated survivin expression. VEGF-mediated upregulation of survivin could be abolished by inhibition of VEGF receptor, but not blocked by survivin-targeting siRNA. In addition, survivin upregulation exerted antiapoptotic effects and was associated with increased chemoresistance. Finally, VEGF mediated transcriptional induction of HIF-1α of LSCs coincubated in H-CM, and HIF-1α induction in turn enhanced chemoresistance and reduced cell apoptosis. CONCLUSIONS: To our best knowledge, this is the first study that focus to explore molecule players and interacting signal pathways regulating LSC biology under hypoxic exposure. It reveals that hypoxia preferentially skew macrophage M2 polarization with specific cytokine profile and proinflammatory microenvironment, which impacts malignant behavior of LSCs. VEGF-HIF-1α interaction is closely implicated in sustaining LSCs survival under hypoxic exposure and might be of potential target of novel therapy.