Complex interactions between hypoxia inducible factors, insulin-like growth Factor-II and oxygen in early murine trophoblasts

The human first trimester placenta experiences a low oxygen environment. The hypoxia inducible factors (HIFs) mediate the response to low oxygen, inducing genes such as insulin-like growth factor (IGF)-II. Interestingly, IGF-II has been shown to promote placenta] growth and function. Currently, the interaction between oxygen, IGF-II and HIFs in the regulation of trophoblast behaviour are unclear. Mudne implantation sites from days 5.5-10.5 were collected for immunohistochemical analyses. Use of the hypoxia marker pimonidazole indicated that the early mouse implantation site is exposed to low oxygen levels similar to those seen in the early human placenta. HIF-1 alpha protein immunostaining was also observed in the implantation site. Culturing murine ectoplacental cones in decreasing oxygen concentrations (20%, 5% and 1% O-2), either with or without the addition of IGF-II, induced complex responses by trophoblasts in terms of their migration and differentiation. Following 3 days exposure to low oxygen there was reduced EPC outgrowth, reduced Igf2 and increased Tpbp mRNA levels, suggesting commitment to the spongiotrophoblast lineage. In addition, Hif-1 alpha mRNA levels were decreased, whilst Hif-2 alpha mRNA was unchanged. This decrease in Hif-1 alpha may be due to the observed increase in antisense (as) Hif-1 alpha mRNA levels in 1% cultures. Furthermore, expression of Hif-2 alpha and the HIF target genes: asHif-1 alpha, Vegf and Slc2al were reduced under low oxygen with the addition of IGF-II. In conclusion, Hif-1 alpha and Hif-2 alpha are differentially regulated by oxygen and IGF-II in cultured trophoblast cells and asHif-1 alpha may mediate the response to prolonged hypoxia in murine trophoblasts. (c) 2007 Elsevier Ltd. All rights reserved.