The human placenta is a unique organ in terms of oxygenation as it undergoes a transition from a low to a more oxygenated environment. This physiological switch in oxygen tension is a prerequisite for proper placental development and involves the hypoxia inducible factor (HIF). HIF is stable and initiates gene transcription under hypoxia, whereas in normoxia, interaction with the von Hippel-Lindau tumor suppressor protein (VHL) leads to rapid degradation of the HIF1A subunit. The degradation requires formation of a multiprotein complex (VHLCBC) and hydroxylation of HIF1A proline residues via members of the egg-laying-defective nine (EGLN) family. Herein, we have investigated the regulatory mechanisms of HIF1A expression during human placental development. Expression of HIF1A and VHL was high at 7-9 wk of gestation, when oxygen tension is low, and decreased when placental oxygen tension increases (10-12 wk of gestation). During early placentation, HIF1A localized in cytotrophoblasts, while VHL was present in syncytiotrophoblasts. At 10-12 wk, VHL appeared in cytotrophoblast cells, which coincided with the disappearance of HIF1A. At the same time the association of VHL and Cullin 2 as well as ubiquitination of HIF1A was maximal. EGLN1, EGLN2, and EGLN3 were also temporally expressed in an oxygen-dependent fashion, with greatest mRNA expression at 10-12 wk of gestation. Inhibition of EGLN activity increased HIF1A stability in villous explants and stimulated transforming growth factor beta 3 (TGFB3) expression consistent with promoter analyses showing that HIF1A transactivates TGFB3. These data demonstrate that during placental development, HIF1A is regulated by temporal and spatial changes in expression and association of molecules forming the multi-protein VHLCBC complex as well as prolyl hydroxylase activities.
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