Embryo implantation is effected by a myriad of signaling cascades acting on the embryo-endometrium axis. Here we show, by using MALDI TOF analysis, far-western analysis and colocalization and co-transfection studies, that STAT3 and MCL-1 are interacting partners during embryo implantation. We show in vitro that the interaction between the two endogenous proteins is strongly regulated by estrogen and progesterone. Implantation, pregnancy and embryogenesis are distinct from any other process in the body, with extensive, but controlled, proliferation, cell migration, apoptosis, cell invasion and differentiation. Cellular plasticity is vital during the early stages of development for morphogenesis and organ homeostasis, effecting the epithelial to mesenchymal transition (EMT) and, the reverse process, mesenchymal to epithelial transition (MET). STAT3 functionally associates with MCL-1 in the mammalian breast cancer cell line MCF7 that overexpresses STAT3 and MCL-1, which leads to an increased rate of apoptosis and decreased cellular invasion, disrupting the EMT. Association of MCL-1 with STAT3 modulates the normal, antiapoptotic, activity of MCL-1, resulting in pro-apoptotic effects. Studying the impact of the association of STAT3 with MCL-1 on MET could lead to an enhanced understanding of pregnancy and infertility, and also metastatic tumors.

Signaling via estrogen receptor (ER) occurs by interacting with many proteins. Nuclear interactome analysis of ERα in an embryo implantation model revealed the association of chicken tumor virus no. 10 regulator of kinase like (CrkL) with ERα, which was further validated by mammalian two-hybrid assay as well as coimmunoprecipitation and colocalization. Mutation in LPALL motif of CrkL disrupts the ERα-CrkL interaction and its transactivation potential, thereby suggesting that the interaction is mediated via its single ER binding motif, Leu-Pro-Ala-Leu-Leu (LXXLL) motif in the sarcoma homology (SH)2 domain. CrkL deletion constructs of SH2 domain target to the nucleus due to presence of nuclear localization signal. Interestingly, the SH2-SH3 (N terminal) construct shows an increased transactivation potential like CrkI. Weak interaction capability of mutated ERα-Y538F with CrkL validates that CrkL interacts with ERα via its YDLL motif at Tyr 541. In an attempt to understand the physiological relevance of this association, we investigated the impact on cell proliferation using a cancer model, because events associated in the process of pregnancy and cancer are analogous. Also, overexpression of CrkL is frequently associated with tumorigenesis. However, its significance in hormone-regulated cancers still remains obscure. Here, we demonstrate that association of ERα and CrkL directly enhances the tumorigenic potential of CrkL, thus pointing to its role in cell proliferation. In human endometrial cancers, we observed a strong association between ERα and CrkL levels. Thus, the molecular signaling set off by ERα and CrkL association may have a central role in pregnancy and cancer, two events which share parallels in growth, invasion, and immune tolerance.

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