Hypoxia-Inducible Factors in Tumor Cell Differentiation and Tumor Vascularization

Mammalian cells, including tumor cells, adapt to hypoxia via stabilization of two transcription factors - Hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha. Among other consequences, hypoxia is a main driving force behind angiogenesis. The differentiation stage of tumor cells is recognized as an important clinical factor in several malignancies including neuroblastoma. Tumors with an immature phenotype are more aggressive than differentiated tumors. Moreover, recent hypotheses on tumor progression stipulate that maintenance of tumor growth is dependent on immature cells called tumor-initiating cells (TICs). Here, we identify HIF-2alpha as a marker of immature neuroblastoma cells and TICs isolated from patient bone marrows irrespective of oxygen levels. We further demonstrate that HIF-2alpha is required to maintain the undifferentiated state, as knockdown of HIF-2alpha induced differentiation. Knockdown of HIF-2alpha revealed a role for HIF-2alpha in recruitment of new blood vessels to experimental neuroblastomas, as the resulting xenograft tumors were highly necrotic. In addition and in agreement with in vitro data, we show that HIF-1alpha and HIF-2alpha proteins are differentially regulated in clinical neuroblastomas. While both proteins correlated with expression of VEGF, only HIF-2alpha was associated with aggressive disease and only HIF-1alpha correlated with tumor hypoxia. Finally, we demonstrate a novel mechanism of HIF regulation of endothelial tube formation via Notch regulation. HIF-dependent JAG2 induction in hypoxic tumor cells alters Notch activity in tumor cells and adjacent endothelial cells and thus represents a novel layer of complexity of hypoxia/Notch interplay. Together, our findings reveal novel roles of HIFs in regulation of tumor cell differentiation and tumor vascularization