Enhanced proapoptotic gene expression of XAF1, CASP8 and TNFSF10 in the bovine endometrium during early pregnancy is not correlated with augmented apoptosis

Bovine trophoblast cells release interferon-τ (IFNT), a type I IFN, as the pregnancy recognition signal. Since type I IFNs exert growth inhibitory and proapoptotic actions, the effect of the conceptus on components of the apoptosis pathways was determined in the bovine endometrium during the periimplantation period. Uteri of Simmental heifers were flushed post mortem at days 12, 15, and 18 of cycle or pregnancy for the recovery of conceptuses and the sampling of ipsilateral endometrial tissue at slaughter for quantitative RT-PCR, immunohistochemistry, caspase activity and TUNEL assays. Endometrium samples of pregnant animals revealed increased transcript levels for the proapoptotic genes XAF1 (day 15: 2.9-fold; day 18: 15.1-fold; p = 0.005) and CASP8 (day 18: 2.4-fold; p = 0.007). The mRNA expression increased significantly with the day of the cycle for the proapoptotic genes FASLG, TNFSF10, TNF and TNFSF1A (p = 0.004, p = 0.006, p = 0.001 and p = 0.007) and the antiapoptotic gene BIRC4 (p = 0.03). We detected high amounts of FASLG transcripts in day 18 conceptuses (16-fold higher than day 18 endometria). This finding was validated at the protein level by immunohistochemistry. To further analyse the endometrial activation of the caspase cascade, the activities of initiator caspase 8 and effector caspases 3/7 were determined luminometrically. No difference between pregnant and cyclic animals was found for either caspase activity. Additionally, a TUNEL assay showed no increase of apoptotic cells in the pregnant endometrium. In conclusion, although the bovine conceptus induces the expression of proapoptotic genes, neither an activation of a caspase cascade nor an increase of apoptotic cells was noticed. These results suggest inhibitory mechanisms preventing endometrial cells from programmed cell death.