IFNT-induced IFI6 sustains bovine endometrial epithelial cell proliferation by activating AP-1 via the JAK–STAT signaling pathway

Endometrial epithelial cell proliferation is essential for establishing endometrial receptivity during embryo implantation. In ruminants, embryonic interferon-tau (IFNT) mediates receptivity establishment by activating endometrial interferon-stimulated genes (ISGs). This study investigates how IFNT-induced interferon alpha-inducible protein 6 (IFI6) regulates bovine endometrial epithelial cells (bEECs) proliferation. We identified the JAK2-STAT3 (rather than STAT1) as the primary signaling axis driving IFNT-mediated IFI6 expression. Functional experiments revealed that IFI6 knockdown markedly impaired bEECs proliferation and suppressed ERK1/2 phosphorylation, while IFNT supplementation reversed these defects. Notably, pharmacological activation using RO8191 (an IFN receptor agonist) restored STAT1/3 signaling and ERK1/2 activation in IFI6-knockdown bEECs, normalizing their proliferation rates. Mechanistically, IFI6 functions upstream of c-Jun/c-Fos (major functional form of AP-1), with its knockdown disrupted ERK1/2-dependent regulation by preventing c-Jun/c-Fos heterodimerization and nuclear translocation. Our findings reveal that IFI6 maintains proliferative balance in bEECs by functioning as a positive feedback modulator of the IFNT-activated JAK–STAT pathway, which subsequently triggers the ERK1/2-c-Jun/c-Fos signaling axis. These results provide novel insights into IFNT-mediated receptivity regulation and highlight IFI6 as a potential diagnostic marker for early pregnancy loss, as well as a therapeutic target for enhancing implantation success.