The
androgen receptor (AR) acts as a
ligand-dependent transcriptional factor controlling development or progression of
prostate cancer.
Androgen ablation by
castration is an effective
therapy for
prostate cancer, whereas eventually most of the
tumors convert from a
hormone-sensitive to a
hormone-refractory disease state and grow even in a low
androgen environment (e.g., 0.1nM 5α-
dihydrotestosterone (DHT)) like the
castration-resistant stage.
Androgen ablation results in
hypoxia, and solid
tumors possess hypoxic environments.
Hypoxia-inducible factor (HIF)-1, which is composed of HIF-1α and HIF-1β/ARNT subunits, functions as a master
transcription factor for
hypoxia-inducible genes. Here, we report that
hypoxia enhances AR transactivation in the presence of 0.05 and 0.1nM DHT in LNCaP
prostate cancer cells.
siRNA-mediated knockdown of HIF-1α inhibited
hypoxia-enhanced AR transactivation. Its inhibition by HIF-1α
siRNA was canceled by expression of a
siRNA-resistant form of HIF-1α. HIF-1α
siRNA repressed
hypoxia-stimulated expression of the
androgen-responsive NKX3.1 gene in the presence of 0.1nM DHT, but not in the absence of DHT. In
hypoxia, HIF-1α
siRNA-repressed AR transactivation was restored in mutants in which HIF-1α lacked
DNA-binding activity. Furthermore, a dominant negative form of HIF-1α canceled
hypoxia-enhanced AR transactivation, and HIF-1β/ARNT siRNAs had no influence on
hypoxia-enhanced AR transactivation. These results indicate that
hypoxia leads to HIF-1α-mediated AR transactivation independent of HIF-1 activity and that HIF-1β/ARNT is not necessarily required for the transactivation.