A novel HIF-2α targeted inhibitor suppresses hypoxia-induced breast cancer stemness via SOD2-mtROS-PDI/GPR78-UPR ER axis.

Hypoxic tumor microenvironment (TME) plays critical roles in induction of cancer stem cell-like phenotype in breast cancer and contribute to chemoresistance. However, the mechanism underlying stemness reprogramming of breast cancer cells (BCs) by hypoxic TME remains largely unknown. In the present study, we illustrated that HIF-2α, but not HIF-1α, induces stemness in BCs under hypoxia through SOD2-mtROS-PDI/GRP78-UPR ER pathway, linking mitochondrial metabolic state to endoplasmic reticulum (ER) response via mitochondrial reactive oxygen species (mtROS) level. HIF-2α activates endoplasmic reticulum unfolded protein response (UPR ER ) in drug-sensitive MCF7 and T47D cells to induce drug-resistant stem-like phenotype. Genetic depletion or pharmacological inhibition (YQ-0629) of HIF-2α abolished hypoxia-induced stem-like phenotype in vitro and in vivo. Mechanistically, HIF-2α activates transcription of superoxide dismutase 2 (SOD2) under hypoxia and thereby decreases mtROS level. With less mtROS transported to endoplasmic reticulum, the expression and activity of protein disulfide isomerase (PDI) is suppressed, allowing glucose-regulated protein 78 (GRP78) to dissociate from receptor proteins of UPR ER and bind misfolded protein to activate UPR ER , which eventually confer chemoresistance and stem-like properties to BCs. Moreover, the increase in mtROS and PDI levels caused by HIF-2α knockdown and the subsequent UPR ER inhibition could be substantially rescued by mitoTEMPOL (a mtROS scavenger), 16F16 (a PDI inhibitor), or GRP78 overexpression. Overall, we reported the critical roles of HIF-2α-SOD2-mtROS-PDI/GRP78-UPR ER axis in mediating hypoxia-induced stemness in BCs, highlighting the interaction between organelles and providing evidence for further development of targeted HIF-2α inhibitor as a promising therapeutic strategy for chemoresistant breast cancer.