Hypoxia-inducible Factor-1 (HIF-1) Is a Transcriptional Activator of the TrkB Neurotrophin Receptor Gene*

Neurotrophins and their cognate receptors play a pivotal role in the development and function of the nervous system. High expression levels of the neurotrophin receptor TrkB and its ligands in neuroblastomas are associated with an unfavorable outcome. We report here that NTRK2, which encodes the TrkB receptor tyrosine kinase, is an oxygen-regulated gene, whose expression is stimulated by the hypoxia-inducible factor-1 (HIF-1). TrkB mRNA and protein levels were elevated nearly 30-fold in neuroblastoma-derived Kelly cells in hypoxia (1% O₂) versus normoxia (21% O₂). A luciferase reporter construct containing ≈2.1 kilobases of the human TrkB promoter was activated about 6-fold both in hypoxia and after stimulation with the hypoxia mimetic 2,2′-dipyridyl (100 μm) at 21% O₂. Luciferase activity in the presence of 2,2′-dipyridyl was reduced significantly upon small interfering RNA knockdown of HIF-1α but not of HIF-2α. Accordingly, hypoxia failed to stimulate the TrkB promoter in mouse embryonic fibroblasts that lacked HIF-1α. The hypoxia-responsive promoter region could be mapped to three HIF-1 binding elements that were located between -923 and -879 bp relative to the transcription start site. The migration of cultured neuroblastoma cells was increased ∼2-fold upon incubation at 1 versus 21% O₂. This effect of hypoxia was abrogated with the tyrosine kinase inhibitor K252a (200 nm). Our findings indicate that transcription of the NTRK2 gene is stimulated at low oxygen tension through a HIF-1-dependent mechanism. In conclusion, enhanced expression of TrkB could represent a critical switch for the previously reported dedifferentiation of neuroblastoma cells under hypoxic conditions.