Developmental regulation of hypoxia-inducible factor 1 and prolyl-hydroxylases in pulmonary vascular smooth muscle cells

The transcriptional machinery involved in the transition of an infant from intrauterine to air-breathing life is developmentally regulated, as the fetus and adult manifest differential genetic expression. The low oxygen (O₂) environment of the mammalian fetus and the increase in O₂ tension that occurs at birth may account for the developmentally regulated alterations in gene expression. We tested the hypothesis that hypoxia-inducible factor 1 (HIF-1) expression, an O₂-sensitive transcription factor, is developmentally regulated. We found that in fetal pulmonary artery (PA) smooth muscle cells (SMC), fetal HIF-1 protein levels were O₂-insensitive, whereas in adult PA SMC, hypoxia increased HIF-1 protein expression. Surprisingly, hypoxia increased HIF-1 mRNA expression in fetal, but not in adult, PA SMC. HIF-1 degradation and transcriptional activity is contingent on prolyl- and asparagyl-hydroxylases. To determine whether developmental differences in O₂ sensitivity or expression of these enzymes accounts for the divergence of HIF-1 sensitivity between fetus and adult, we studied the expression of the three most well characterized prolyl-hydroxylases, PHD1, PHD2, and PHD3, and the expression of regulators of HIF-1 transcriptional activity, asparagyl-hydroxylase, factor inhibiting HIF, and the oncogenic factor, CITED2 (CREB-binding protein/p300 interacting transactivator with ED-rich tail). We found that, as in the case of HIF-1, these genes are differentially regulated in the fetus, enabling the mammalian fetus to thrive in the low O₂ tension intrauterine environment even while rendering a newborn infant uniquely well adapted to respond to the acute increase in O₂ tension that occurs at birth.