Decidualization and Syndecan-1 Knock Down Sensitize Endometrial Stromal Cells to Apoptosis Induced by Embryonic Stimuli

Human embryo invasion and implantation into the inner wall of the maternal uterus, the endometrium, is the pivotal process for a successful pregnancy. Whereas disruption of the endometrial epithelial layer was already correlated with the programmed cell death, the role of apoptosis of the subjacent endometrial stromal cells during implantation is indistinct. The aim was to clarify whether apoptosis plays a role in the stromal invasion and to characterize if the apoptotic susceptibility of endometrial stromal cells to embryonic stimuli is influenced by decidualization and Syndecan-1. Therefore, the immortalized human endometrial stromal cell line St-T1 was used to first generate a new cell line with a stable Syndecan-1 knock down (KdS1), and second to further decidualize the cells with progesterone. As a replacement for the ethically inapplicable embryo all cells were treated with the embryonic factors and secretion products interleukin-1β, interferon-γ, tumor necrosis factor-α, transforming growth factor-β1 and anti-Fas antibody to mimic the embryo contact. Detection of apoptosis was verified via Caspase ELISAs, PARP cleavage and Annexin V staining. Apoptosis-related proteins were investigated via antibody arrays and underlying signaling pathways were analyzed by Western blot. Non-decidualized endometrial stromal cells showed a resistance towards apoptosis which was rescinded by decidualization and Syndecan-1 knock down independent of decidualization. This was correlated with an altered expression of several pro- and anti-apoptotic proteins and connected to a higher activation of pro-survival Akt in non-differentiated St-T1 as an upstream mediator of apoptotis-related proteins. This study provides insight into the largely elusive process of implantation, proposing an important role for stromal cell apoptosis to successfully establish a pregnancy. The impact of Syndecan-1 in attenuating the apoptotic signal is particularly interesting in the light of an already described influence on pregnancy disorders and therefore might provide a useful clinical tool in the future to prevent pregnancy complications provoked by inadequate implantation.