The association of HIF-2α expression with stemness and survival genes in human breast cancer stem cells (CD24-/CD44+) exposed to hypoxia

Similar to normal stem cells, breast cancer stem cells (BCSCs) exist in prolonged hypoxia in vivo and are cultured in normoxic conditions (20% O2) in vitro. Our previous study showed that the treatment of in vitro BCSCs with 1% O2 upregulates HIF-1α. This study aimed to investigate HIF-2α expression under prolonged hypoxia and the effects on its downstream gene expression (Oct-4, ALDH1, KLF4, and c-MYC) along with an associated survival gene, survivin. Human BCSCs (CD24-/CD44+) were exposed to hypoxia (1% O2, 5% CO2) at intervals of 0.5 h, 4 h, 6 h, and 24 h. HIF2α, Oct-4, ALDH-1, KLF4, c-MYC, and survivin mRNA expression levels were relatively quantified using qRT-PCR. Data were analyzed using the Livak's formula and one-way ANOVA. In contrast to HIF-1α levels, HIF-2α levels were gradually downregulated under 24 h of hypoxia treatment. In concert, Oct-4, ALDH1, KLF4, and c-MYC expression levels, as a measure of stemness, significantly decreased following hypoxia from 0.5 h to 24 h. The reduced expression of these transcription factors suggests diminished proliferation and a possibility of differentiation of the cells, thus sensitizing them to cell death. In support of this, survivin downregulation was also shown throughout the 0.5-24 h of hypoxia, which confirms our previous result that demonstrated enhanced apoptosis. Hypoxia treatment of in vitro BCSCs, unlike in vivo BCSCs, resulted in decreased HIF-2 levels, indicating acute hypoxia, which may lead to decreases in stemness and cell viability.