211. Proteomic identification of caldesmon as one of the physiological substrates of proprotein convertase 6 during decidualisation

We have previously demonstrated that proprotein convertase 5/6 (PC6), a member of the proprotein convertase (PC) family, is a critical endometrial factor for implantation. PC6 is upregulated in the endometrium specifically at implantation in association with epithelial differentiation (in human and monkey) and stromal cell decidualisation (in the mouse, human and monkey). Knockdown of endometrial PC6 during early pregnancy in mice in vivo led to complete failure of implantation, while blocking of PC6 production in human endometrial stromal cells in vitro inhibited decidualisation. PCs convert a range of precursor proteins of important functions into their bioactive forms; they are thus regarded as critical ‘master switch’ molecules. We hypothesise that PC6 exerts its roles in the endometrium by regulating proteins of diverse functions essential for implantation. In this study, we utilised proteomic technology and aimed to identify proteins that are specifically cleaved by PC6 in human endometrial stromal cells (HESC) during decidualisation. HESC were decidualised with cyclic AMP, the cell lysates were treated with and without recombinant human PC6-A (rPC6-A), and the 2D Differential in Gel Electrophoresis (2D DiGE) protein profiles were compared between the two treatments. We identified several proteins which were differentially cleaved following the addition of rPC6-A. Mass spectrometric analysis confirmed that the most abundant of these were caldesmon, tropomyosin-2, tropomyosin-4, hypoxia Inducible factor-1 and chloride intracellular channel-1. These proteins showed spot shifts in hPC6-A treated HESC lysates consistent with hPC6-A cleavage. western blot analysis confirmed the specific cleavage of caldesmon by PC6 in HESCs, and immunohistochemical analysis showed co-localisation of caldesmon and PC6 in decidual cells in human endometrial tissue. Given that caldesmon is a structural protein previously found to be involved in actin filament reorganisation, our results strongly suggest that PC6 is a mediator of structural remodelling of stromal cells during decidualisation in the endometrium.

Pro-protein convertases (PCs) other than PC6 are not tightly regulated for implantation in the human endometrium

Pro-protein convertases (PCs) are a family of serine proteases (furin, PC1/3, PC2, PACE4, PC4, PC5/6, PC7/8) responsible for post-translational processing and activation of inactive precursors of many regulatory proteins. Endometrial PC6 is critical for implantation in mice and for decidualization of human endometrial stromal cells (ESCs). This study investigated the endometrial expression of other PCs during the menstrual cycle and early pregnancy to elucidate potential redundancies. Furin, PC4, PACE4, and PC7 along with PC6 transcripts were detected in total endometrial RNA, whereas PC1 and PC2 transcription levels were negligible. Quantitative RT-PCR demonstrated highest levels of furin mRNA during menstruation and lowest levels during the proliferative phase. Furin protein was immunolocalized in endometrial luminal and glandular epithelia, stromal fibroblasts, endothelia, and leukocytes. PACE4 and PC7 proteins were also immunodetected in endometrial stroma and glands. Total furin, PC7, and PACE4 proteins were constitutive in both stromal and glandular compartments throughout the cycle and during first trimester pregnancy. Furthermore, Furin and PC7 transcription was unaltered during decidualization of ESCsin vitroin contrast to PC6 which is significantly up-regulated during decidualization. Thus, whereas PC6 is tightly regulated during endometrial preparation for implantation, furin, PACE4, and PC7 are constitutively expressed in human endometrium, but must be considered if PC6 is to be targeted for manipulation of fertility.

Transcription factors Ets1, Ets2, and Elf1 exhibit differential localization in human endometrium across the menstrual cycle and alternate isoforms in cultured endometrial cells

To better understand the transcriptional regulation of human endometrial remodeling, the localization of three members of the Ets family of transcription factors was examined at different stages of the menstrual cycle. Elf1 was found by immunohistochemistry to be predominantly localized to the glandular epithelium. In contrast, Ets1 and Ets2 were found at lower intensities in both glandular epithelial and stromal cells. Low expression during the menstrual phase of the cycle, and high expression and intensity of staining in decidualized stromal cells of the late secretory phase were common to Ets1, Ets2, and Elf1. These localization patterns were confirmed in cultured human endometrial stromal and epithelial cells by Western blotting, which also demonstrated different isoforms and phosphorylation products of Ets1 and Ets2 in the two cell types. This study has shown for the first time that members of the Ets family of transcription factors, previously found predominantly during development and in hematopoietic cells, are expressed in the human endometrium and display cell and cycle-stage specificity. Expression of Elf1 predominantly in the glandular epithelium may indicate that Elf1 plays a unique role in epithelium-specific gene regulation in the endometrium.

Homeobox genes DLX4 and HB24 are expressed in regions of epithelial-mesenchymal cell interaction in the adult human endometrium

The adult human endometrium rapidly cycles through stages of cell proliferation, differentiation and degeneration. Inappropriate endometrial cell differentiation is a contributing factor in diseases such as endometrial carcinoma and endometriosis. We have identified two homeobox genes that may play a role in the control of endometrial cell differentiation and development. In-situ mRNA hybridization experiments were used to show differential expression of DLX4 at different phases of the endometrial cycle. Higher levels of DLX4 expression were observed in proliferative phase endometrial epithelium compared with secretory phase endometrial epithelium. The HB24 homeobox gene was shown to be expressed in both the proliferative and secretory phase endometrial epithelium. We predict that DLX4 and HB24 will be required for the transcriptional control of genes important for endometrial cell differentiation. Furthermore, we propose that DLX4 and HB24 are part of a conserved combinatorial code of homeobox genes that are required for controlling epithelial-mesenchymal cell interactions in the endometrium.