Human Cytotrophoblast Expression of the von Hippel–Lindau Protein Is Downregulated during Uterine Invasion in Situ and Upregulated by Hypoxia in Vitro

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The von Hippel-Lindau tumor-suppressor protein (pVHL) regulates the stability of HIF1 alpha and HIF2 alpha and thus is pivotal in cellular responses to changes in oxygen tension. Paradoxically, human cytotrophoblasts proliferate under hypoxic conditions comparable to those measured in the early gestation placenta (2% O(2)), but differentiate into tumorlike invasive cells under well-oxygenated conditions such as those found in the uterus. We sought to explain this phenomenon in terms of pVHL expression. In situ, pVHL immunolocalized to villous cytotrophoblast stem cells, and expression was enhanced at sites of cell column initiation; in both of these relatively hypoxic locations, cytoplasmic staining for HIF2 alpha was also detected. As cytotrophoblasts attached to and invaded the uterus, which results in their increased exposure to oxygen, pVHL staining was abruptly downregulated concordant with localization of HIF2 alpha to the nucleus. In vitro, hypoxia (2% O(2)) upregulated cytotrophoblast pVHL expression together with HIF2 alpha, which localized to the cytoplasm; culture under well-oxygenated conditions greatly reduced levels of both molecules. These results, together with the placental defects previously observed in VHL(-/-) mice, suggest that pVHL is a component of the mechanism that transduces local differences in oxygen tension at the maternal-fetal interface to changes in the biological behavior of cytotrophoblasts. Furthermore, these data provide the first example of oxygen-dependent changes in pVHL abundance.