Multiple receptor tyrosine kinases regulate HIF-1α and HIF-2α in normoxia and hypoxia in neuroblastoma: Implications for antiangiogenic mechanisms of multikinase inhibitors

Novel treatment approaches are needed for children with advanced neuroblastoma. Studies with neuroblastoma cells have indicated the presence of a hypoxia-driven vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR)-1 autocrine loop modulating hypoxia-inducible factor-1alpha (HIF-1α). Whether other receptor tyrosine kinases (RTKs) are capable of modulating HIF-1α levels and whether RTKs can regulate HIF-2α as well is largely unknown. We evaluated neuroblastoma cell lines for expression of various RTKs. Although cell lines were heterogeneous in the expression of VEGFR-1,-3, c-Kit and RET, most cells expressed PDGFR-α and-Β. Ligand-induced activation of multiple RTKs upregulated HIF-1α levels, whereas activation of VEGFR-1 alone upregulated HIF-2α. Multitargeted tyrosine kinase inhibitor sunitinib reduced hypoxia-induced rises in HIF-1α and HIF-2α through mechanisms involving effects on both mRNA levels and protein stability. In addition, sunitinib and sorafenib had direct effects on tumor cell viability in vitro. In a neuroblastoma xenograft model, tumor growth inhibition by sunitinib was associated with inhibition of angiogenesis and reduced HIF-1α levels. These findings show that multiple RTKs may regulate the HIF axis in normoxia and hypoxia and suggest that multikinase inhibitors may exert antiangiogenic effects not only by direct effects on endothelial cells, but also by blocking compensatory hypoxia-and ligand-induced changes in HIF-1α and HIF-2α.