Effects of oxygen tension and IGF-I on HIF-1 alpha protein expression in mouse blastocysts

Abstract

Hypoxia inducible factors (HIFs) are key regulators of oxygen homeostasis in response to reduced oxygenation in somatic cells. In addition, HIF-1 alpha protein can be also induced by insulin-like growth factor I (IGF-I) treatment in various cell lines under normoxic condition. However, the expression and function of HIF-1 alpha in embryogenesis are still unclear. Therefore, the objectives of this study were to examine the expression of HIF-1 alpha in mouse blastocysts cultured under hypoxic and normoxic conditions, and to determine whether oxygen tension and IGF-I influence embryonic development through stimulation of HIF-1 alpha expression. Mouse embryos were cultured from the 1-cell to blastocyst stage under 5 % or 20 % O-2 in both the absence and presence of IGF-I. The embryonic development rates to the blastocyst stage were not affected by oxygen tension or IGF-I treatment. HIF-1 alpha protein was localized to the cytoplasm of blastocysts, and its levels were independent of oxygen concentration or IGF-I treatment. Blastocysts cultured under 5 % O-2 exhibited significantly higher total cell numbers (83.4 +/- 18.1) and lower apoptotic index (3.7 +/- 1.5) than those cultured under 20 % O-2 (67.4 +/- 15.6) (6.9 +/- 3.5) (P < 0.05). IGF-I reduced the apoptotic index in both oxygen conditions, but a significant decrease was detected in the 20 % O-2 group. HIF-1 alpha may not be a major mediator that responds to change in oxygen tension within blastocysts, inconsistent with that of somatic cells. Supplementation of culture media with IGF-I has been shown to promote embryo development by an anti-apoptotic effect, instead of increasing HIF-1 alpha protein expression.