SGK1 Is Upregulated in Retained Placenta and Mediates Estradiol Effects in Bovine Endometrial Cells

Retained placenta (RP) is a significant postpartum complication in dairy cows. Although abnormal estradiol (E2) levels are implicated, the underlying cellular mechanisms remain poorly defined. Through RNA-seq analysis of postpartum blood from cows with or without RP, we identified Serum and Glucocorticoid-regulated Kinase 1 (SGK1) as a differentially expressed gene candidate. Analysis of fetal cotyledonary tissues revealed that SGK1 expression was significantly elevated in these tissues, concomitant with markers of suppressed apoptosis, increased levels of tight junction proteins, and an inhibited epithelial–mesenchymal transition (EMT) phenotype. To explore a potential mechanistic link between E2 and these cellular alterations, we investigated the E2-SGK1 axis in bovine endometrial epithelial cells in vitro. E2 treatment upregulated SGK1 expression, reduced apoptosis, increased tight junction protein levels, and suppressed EMT. Conversely, SGK1 knockdown induced apoptosis, disrupted tight junctions, and impaired EMT. Notably, E2 could not rescue the apoptosis and EMT alterations in SGK1-knockdown cells, indicating that SGK1 is a critical mediator of these E2 effects in this cellular model. Based on these initial correlative findings in tissues, combined with the subsequent mechanistic experiments in cells, we propose a novel model whereby dysregulation of the E2- SGK1 axis could contribute to RP pathogenesis by stabilizing the placental interface. Our findings provide the first experimental evidence linking SGK1 to RP and establish a foundation for future in vivo validation.