In neuroblastoma specimens, HIF-2alpha but not HIF-1alpha is strongly expressed in well-vascularized areas. In vitro, HIF-2alpha protein was stabilized at 5% O2 (resembling end capillary oxygen conditions) and, in contrast to the low HIF-1alpha activity at this oxygen level, actively transcribed genes like VEGF. Under hypoxia (1% O2), HIF-1alpha was transiently stabilized and primarily mediated acute responses, whereas HIF-2alpha protein gradually accumulated and governed prolonged hypoxic gene activation. Knockdown of HIF-2alpha reduced growth of neuroblastoma tumors in athymic mice. Furthermore, high HIF-2alpha protein levels were correlated with advanced clinical stage and high VEGF expression and predicted poor prognosis in a clinical neuroblastoma material. Our results demonstrate the relevance of HIF-2alpha in neuroblastoma progression and have general tumor biological implications.

Summary Stem-like glioma cells reside within a perivascular niche and display hallmark radiation resistance. Understanding of the mechanisms underlying these properties will be vital for the development of effective therapies. Here we show that the stem cell marker CD44 promotes cancer stem cell phenotypes and radiation resistance. In a mouse model of glioma, Cd44−/− and Cd44+/− animals showed improved survival compared to controls. The CD44 ligand Osteopontin shared a perivascular expression pattern with CD44 and promoted glioma stem cell-like phenotypes. These effects were mediated via the γ-secretase regulated intracellular domain of CD44, which promoted aggressive glioma growth in vivo and stem cell-like phenotypes via CBP/p300-dependent enhancement of HIF-2α activity. In human glioblastoma multiforme, expression of CD44 correlated with hypoxia-induced gene signatures and poor survival. Together, these data suggest that in the glioma perivascular niche, Osteopontin promotes stem cell-like properties and radiation resistance in adjacent tumor cells via activation of CD44 signaling.