Hyperactivation of Notch signaling and the cellular hypoxic response are
frequently observed in cancers, with increasing reports of connections
to tumor initiation and progression. The two signaling mechanisms are
known to intersect, but while it is well established that hypoxia
regulates Notch signaling, less is known about whether Notch can
regulate the cellular hypoxic response. We now report that Notch
signaling specifically controls expression of HIF2α, a key mediator of
the cellular hypoxic response. Transcriptional upregulation of HIF2α by
Notch under normoxic conditions leads to elevated HIF2α protein levels
in primary breast cancer cells as well as in human breast cancer,
medulloblastoma, and renal cell carcinoma cell lines. The elevated level
of HIF2α protein was in certain tumor cell types accompanied by
downregulation of HIF1α protein levels, indicating that high Notch
signaling may drive a HIF1α-to-HIF2α switch. At the transcriptome level,
the presence of HIF2α was required for approximately 21% of all
Notch-induced genes: among the 1062 genes that were upregulated by Notch
in medulloblastoma cells during normoxia, upregulation was abrogated in
227 genes when HIF2α expression was knocked down by HIF2α siRNA. In
conclusion, our data show that Notch signaling affects the hypoxic
response via regulation of HIF2α, which may be important for future
cancer therapies.