TGFβ1 Attenuates Expression of Prolactin and IGFBP-1 in Decidualized Endometrial Stromal Cells by Both SMAD-Dependent and SMAD-Independent Pathways

BACKGROUND: Decidualization (differentiation) of the endometrial stromal cells during the secretory phase of the menstrual cycle is essential for successful implantation. Transforming Growth Factor β1 (TGFβ1) canonically propagates its actions via SMAD signalling. A role for TGFβ1 in decidualization remains to be established and published data concerning effects of TGFβ1 on markers of endometrial decidualization are inconsistent. METHODOLOGY/PRINCIPAL FINDINGS: Non-pregnant endometrial stromal cells (ESC) and first trimester decidual stromal cells (DSC) were cultured in the presence or absence of a decidualizing stimulus. Incubation of ESCs with TGFβ1 (10 ng/ml) down-regulated the expression of transcripts encoding the decidual marker proteins prolactin (PRL), insulin-like growth factor binding protein-1 (IGFBP-1) and tissue factor (TF). TGFβ1 also inhibited secretion of PRL and IGFBP-1 proteins by ESCs and surprisingly this response preceded down-regulation of their mRNAs. In contrast, DSCs were more refractory to the actions of TGFβ1, characterized by blunted and delayed down-regulation of PRL, IGFBP-1, and TF transcripts, which was not associated with a significant reduction in secretion of PRL or IGFBP-1 proteins. Addition of an antibody directed against TGFβ1 increased expression of IGFBP-1 mRNA in decidualised cells. Knockdown of SMAD 4 using siRNAs abrogated the effect of TGFβ1 on expression of PRL in ESCs but did not fully restore expression of IGFBP-1 mRNA and protein. CONCLUSIONS/SIGNIFICANCE: TGFβ1 inhibits the expression and secretion of decidual marker proteins. The impact of TGFβ1 on PRL is SMAD-dependent but the impact on IGFBP1 is via an alternative mechanism. In early pregnancy, resistance of DSC to the impact of TGFβ1 may be important to ensure tissue homeostasis.