Hypoxia Up-regulates Estrogen Receptor β in Pulmonary Artery Endothelial Cells in a HIF-1α Dependent Manner.

17 beta-Estradiol (E2) attenuates hypoxia-induced pulmonary hypertension (HPH) through estrogen receptor (ER)-dependent effects, including inhibition of hypoxia-induced endothelial cell proliferation; however, the mechanisms responsible for this remain unknown. We hypothesized that the protective effects of E2 in HPH are mediated through hypoxia-inducible factor 1 alpha (HIF-1 alpha)-dependent increases in ER beta expression. Sprague-Dawley rats and ER alpha or ER beta knockout mice were exposed to hypobaric hypoxia for 2-3 weeks. The effects of hypoxia were also studied in primary rat or human pulmonary artery endothelial cells (PAECs). Hypoxia increased expression of ER beta, but not ER alpha, in lungs from HPH rats as well as in rat and human PAECs. ER beta mRNA time dependently increased in PAECs exposed to hypoxia. Normoxic HIF-1 alpha/HIF-2 alpha stabilization increased PAEC ER beta, whereas HIF-1 alpha knockdown decreased ER beta abundance in hypoxic PAECs. In turn, ER beta knockdown in hypoxic PAECs increased HIF-2 alpha expression, suggesting a hypoxia-sensitive feedback mechanism. ER beta knockdown in hypoxic PAECs also decreased expression of the HIF inhibitor prolyl hydroxylase 2 (PHD2), whereas ERb activation increased PHD2 and decreased both HIF-1 alpha and HIF-2 alpha, suggesting that ER beta regulates the PHD2/HIF-1 alpha/HIF-2 alpha axis during hypoxia. Whereas hypoxic wild-type or ERa knockout mice treated with E2 demonstrated less pulmonary vascular remodeling and decreased HIF-1 alpha after hypoxia compared with untreated hypoxic mice, ER beta knockout mice exhibited increased HIF-2 alpha and an attenuated response to E2 during hypoxia. Taken together, our results demonstrate a novel and potentially therapeutically targetable mechanismwhereby hypoxia, via HIF-1 alpha, increases ER beta expression and the E2-ER beta axis targets PHD2, HIF-1 alpha, and HIF-2 alpha to attenuate HPH development.