Regulation of interferon-gamma signal transduction in human and mouse trophoblast cells
Choi, Jason Cheol
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Trophoblast cells are extraembryonic cells that form the blastocyst-derived component of the placenta, and play essential roles in fetal maintenance. The proinflammatory cytokine IFN-γ plays a central role in activating genes involved in humoral and cell-mediated immunity, controlling cell proliferation and inducing apoptosis. IFN-γ is secreted by uterine NK cells in the placenta during pregnancy and in mice, IFN-γ is required for proper formation of the implantation site and the decidual layer and remodeling of the uterine vasculature. Despite the presence of IFN-γ in the placenta, human and mouse trophoblast cells do not express MHC class II molecules and are less susceptible to IFN-γ-induced apoptosis. To determine whether human and mouse trophoblast cells respond to IFN-γ, the expression of IFN-γ-inducible genes and the factors that mediate IFN-γ signaling, namely phosphorylated STAT-1 and IRF-1, were assessed. The expression of IFN-γ-inducible genes was significantly reduced in human and mouse trophoblast cells exposed to various concentrations of IFN-γ relative to their respective positive controls. In human trophoblast-derived choriocarcinoma cells, the reduced expression of IFN-γ-inducible genes was due to compromised tyrosine phosphorylation of JAK-2 following exposure to IFN-γ. Subsequently, phosphorylation of STAT-1 at tyrosine-701 was substantially reduced in both IFN-γ-treated human choriocarcinoma and primary trophoblast cells compared to HeLa, 2fTGH or primary foreskin fibroblast cells. A corresponding reduction of both IRF-1 mRNA and protein expression was observed in IFN-γ-treated trophoblast cells. Treatment of choriocarcinoma cells with the tyrosine phosphatase inhibitor pervanadate significantly enhanced IFN-γ-inducible JAK and STAT-1 tyrosine phosphorylation and select IFN-γ-inducible gene expression. In mouse trophoblast cell lines SM9 and M-11, as well as trophoblast stem cells and stem cell-derived giant cells, the basal and IFN-γ-induced expression of several IFN-γ-inducible genes was significantly reduced relative to mouse embryonic fibroblast cells. In contrast to human trophoblast cells, no impairment was observed in STAT-1 phosphorylation or DNA binding capacity in IFN-γ-treated mouse trophoblast cells. Despite comparable STAT-1 phosphorylation, the reduced IRF-1 mRNA expression in M-11 cells relative to NIH-3T3 cells was due to a lower rate of IRF-1 transcription. SM9 and M-11 cells simultaneously treated with HDAC inhibitors and IFN-γ significantly enhanced the expression of IRF-1, GBP, and subunits of the immunoproteasomes. Our data collectively demonstrate that responses to IFN-γ in trophoblast cells are dampened, a phenotype that appears to be evolutionarily conserved between human and mouse. Furthermore, the dampened response to IFN-γ in human and mouse trophoblast cells may be a necessary trait to prevent reactions that may interfere with trophoblast survival/function and ultimately, fetal survival.