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% O(2)) versus normoxia (21% O(2)). A
luciferase reporter construct containing approximately 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 microm) at 21% O(2).
Luciferase activity in the presence of
2,2'-dipyridyl was reduced significantly upon
small interfering RNA knockdown of HIF-1alpha but not of HIF-2alpha. Accordingly,
hypoxia failed to stimulate the TrkB promoter in mouse embryonic fibroblasts that lacked HIF-1alpha. 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 approximately 2-fold upon incubation at 1 versus 21% O(2). 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.