Hypoxia enhances the stemness markers of cochlear stem/progenitor cells and expands sphere formation through activation of hypoxia-inducible factor-1alpha

Unlike neural stem cells that maintain populations in the adult brains of both rodents and humans, cochlear stem cells appear to diminish in number after birth and may become quiescent in adult mammalian cochleae. Hypoxia has been observed to promote an undifferentiated cell state in various stem cell populations; however, little is known about such an effect on cochlear stem/progenitor cells (SPCs). The aims of this study were to assess the effect of hypoxia on cochlear SPCs and to examine the impact of hypoxia-inducible factor-1alpha (Hif-1a) on regulating such an effect. Our data demonstrate that hypoxic culturing for 24 h significantly increased sphere formation and viability of cochlear SPCs compared with those cultured under normoxic conditions. Concurrent with these proliferation promotion effects are changes in the expression of multiple stemness and cell-cycle quiescent associated gene targets, including Abcg2, nestin, p27 ^Kip1and Vegf. Knockdown of Hif-1a expression by small-interfering RNA inhibited hypoxia-induced cochlear SPC expansion and resulted in downregulation of Vegf, Abcg2, and nestin and upregulation of p27 ^Kip1 gene expression. These results suggest that Hif-1a plays an important role in the stimulation of the proliferation of cochlear SPCs, which confers a great benefit of expanding cochlear SPCs via hypoxic conditions.