Luminal closure and embryo apposition are essential for blastocyst attachment during early pregnancy. In our preliminary microarray results (unpublished data), sodium-potassium adenosine triphosphatase (Na/K-ATPase) β1 (Atp1b1) was highly expressed in mouse uterus on Days 3 and 4 of pregnancy. However, expression and regulation of Atp1b1 in the mammalian uterus during early pregnancy are unknown. Using in situ hybridization, a strong level of Atp1b1 mRNA was detected in luminal epithelial cells on Days 3 and 4 of pregnancy (Day 1 = day of vaginal plug). The expression pattern of FXYD domain-containing ion transport regulator 4 (Fxyd4) was similar to that of Atp1b1. Real-time reverse transcription polymerase chain reaction confirmed the high expression level of Atp1b1 mRNA. Compared with Day 1, the mRNA level of Atp1b1 on Days 3 and 4 increased by 3.5 ± 0.5 and 4.5 ± 0.5 fold, respectively. When the embryo invaded through epithelial cells into the maternal stromal compartment on day 5, Atp1b1 expression decreased to a basal level. Progesterone stimulated Atp1b1 expression by 2.8 ± 1 fold compared with oil in ovariectomized mice at 24 hours after treatment. Expression of Atp1b1 was further upregulated to 4 ± 0.4 fold by estrogen and progesterone. Based on time-course study, progesterone rapidly induced Atp1b1 expression at 6 and 12 hours (13.7 ± 0.5 and 16.6 ± 1.4, respectively); furthermore, this upregulation was blocked by RU486 (progesterone receptor antagonist). Transcription activity of the Atp1b1 promoter was (Day 1 = day of vaginal plug) stimulated by CCAAT/enhancer binding protein beta (Cebpb). In conclusion, Atp1b1 was highly expressed in luminal epithelium during peri-implantation and upregulated by progesterone.
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