Hypoxia-dependent accumulation of hypoxia-inducible factor-1 alpha induces transient cell cycle arrest in porcine trophectoderm cells

In the uterine environment, the pre-implantation embryo adapts to low oxygen concentrations through intracellular responses including modification of gene expression, progression via the cell cycle and metabolism. In this study, we determined mechanisms underlying the adaptation of pig embryos to oxygen deficiency in the maternal-conceptus microenvironment in in vitro experiments using our established porcine trophectoderm (pTr) cells in culture. The transition from G1 to S phase in pTr cells was reduced in response to 2% oxygen during a short period (<24 h), and the hypoxia-induced G1 arrest was reversible during prolonged hypoxia exposure. Acute hypoxia up-regulated expression of transcription factors p21 and p27 and down-regulated cell cycle regulators associated with the G1/S phase transition including cyclin D1, cyclin E1 and E2F1 mRNAs and proteins. Furthermore, hypoxia exposure for 24 h markedly increased the abundance of HIF-1α protein. Even under acute hypoxia, by HIF-1α silencing reduced the hypoxia-induced transient G1 arrest and expression of p21 and p27 genes was restored. Contrary to acute hypoxia, the accumulation of HIF-1α protein decreased as the length of the hypoxic period increased. Overall results of the present study suggest that increases in HIF-1α are responsible for initial response to hypoxia that results in a transient cell cycle arrest in pTr cells and cell cycle progression is restored by increasing degradation of HIF-1α during prolonged hypoxia. These findings advance understating of cellular adaptation of developing pre-implantation porcine conceptuses to hypoxic stress.