Influence of low oxygen tensions on expression of pluripotency genes in stem cells.

The stem cells are characterized by self-renewal ability and potential to differentiate into other cell types of the body. They are residuing in defined microenvironments - "stem cell niches". The embryonic stem cells (ESC) are derived from embryos which exist in 3-5 percent oxygen condition. This environment is physiologically normal not only for ES cells but also for many other types of stem cells including neural stem cells (NSC). These observations suggest that low oxygen condition plays a very important role in the maintenance of cell stemness. Pluripotency is regulated by the family of hypoxia inducible factors (HIFs), which are dependent on oxygen tensions. HIF-2α is an upstream regulator of Oct4, which is one of the main transcription factors used to generate the first induced pluripotent stem cells (iPSCs). It has been shown that knock-down of HIF-2α but not HIF-1α, leads to a decrease in the expression of Oct4, Nanog and Sox2, which are important stem cells markers. The structure of hypoxia inducible factors as well as their behavior in hypoxia and normoxia was described. Therefore optimization of oxygen concentration seems to be crucial from the stem cell transplantation as well as iPS transplantation standpoint. Although many experiments with cell culture under low oxygen condition were performed, there is still much that is unknown. This short review presents some aspects on important issue of hypoxia induced regulation of stemness.