Tissues normally consist of epithelial or mesenchymal cells. Epithelial cells may be induced to change to a mesenchymal phenotype through EMT, an organized process first recognized in developmental biology as a means of achieving morphogenetic changes. In the instances where EMT is not controlled, pathologies arise whereby cell growth, proliferation, migration, and invasion are altered. A key example of this is carcinoma progression, whereby cells, which normally show resting epithelial morphologies, acquire a mesenchymal migratory potential and translocate to distant sites before reverting to an epithelial phenotype [17, 18]. The expression of epithelial markers is reduced, while mesenchymal marker expression is increased. OSM has been identified as an EMT factor in lung and pancreatic tumor models . It has also recently been reported that oncostatin-M promotes EMT, including E-cadherin loss in breast cancer . In human renal tubular cells, it has been shown that OSM induces EMT through the Jak/Stat pathway and ERK signaling [21, 22].
E-cadherin is usually expressed in epithelial cells and is involved in calcium-dependent cell-cell adhesion. In the placenta, E-cadherin mediates a strong intercellular interaction between adjacent trophoblast cells. During the first trimester of pregnancy, trophoblastic E-cadherin expression is temporarily down-regulated so that the EVTs acquire invasiveness . Recent studies support the important role of E-cadherin in the regulation of the invasive behavior of human trophoblast cells [10, 24].
In the present study, we used real-time PCR analysis, western blotting, and indirect immunofluorescence staining to demonstrate that the expression of the epithelial marker E-cadherin was significantly decreased by OSM. We also demonstrated that OSM stimulated the migration of HTR8/SVneo cells and that the addition of an anti-gp130 antibody decreased the stimulatory effects of OSM on migration. OSM belongs to the IL-6 family of cytokines and acts on target cells by binding to a heterodimeric membrane receptor composed of LIF- or OSM-specific receptor and the gp130 receptor chain . In addition, OSM stimulated the proliferation of HTR8/SVneo cells at 48 h assay, not at 12 h assay. It is considered that significant increase in cell migration distance by OSM (12 h) represents an increased migration by OSM, because proliferation has not been changed significantly at 12 h assay. It has been shown that phosphorylated STAT3 enhances the invasiveness of tumors and trophoblast cells, where it is mainly activated by LIF . We demonstrated that the migration and proliferation of trophoblasts are stimulated, E-cadherin is suppressed by OSM, and that these events are related to STAT3 phosphorylation. The down-regulation of E-cadherin by OSM was restored following treatment with a STAT3 inhibitor. In addition, OSM-stimulated migration and proliferation were significantly suppressed by STAT3 inhibition. Because it has been recently reported that a STAT3 inhibitor, stattic, has limitations to inhibit STAT3, selectively , we investigated the STAT3 pathway with STAT3 siRNA. The down-regulation of E-cadherin by OSM was restored following treatment with a STAT3 siRNA, with the same pattern. These results suggest that OSM stimulates the migration and proliferation of trophoblasts through STAT3 signaling, although the other pathway could be engaged by OSM, with or without STAT3 signaling.
No data regarding the effects of OSM on EMT in EVTs have yet been published. It has been reported that a significantly higher expression of OSM was identified in the cytotrophoblasts, syncytotorophoblasts and endothelium of the preeclamptic placenta compared with the normal placenta . On the basis of the present study, OSM was found to induce the migration and proliferation of EVTs, through the down-regulation of E-cadherin. The effects of OSM on E-cadherin observed and the migration and proliferation of EVTs were contrary to observations that the invasion of EVT is shallow and that expression of OSM is elevated in the preeclamptic placenta [10, 28–30]. The elevated expression of OSM in the preeclamptic placenta could be an adaptive phenomenon to rescue the shallow invasion of EVT. Another possibility is that the increased expression of OSM in preeclampsia may not be related to the effects of OSM on migration, proliferation, and invasion of EVTs, but instead could be related to the other effects of OSM. However, we do not yet know the effects of OSM on trophoblast migration, proliferation, and the invasion of EVTs in hypoxic environments. Recently, it was reported that recombinant interleukin-6 (IL-6) and TNFα were capable of activating endothelial cells, which is a hallmark of preeclampsia . Another study demonstrated that IL-6 stimulates cell migration and invasion accompanied by the increased expression of related integrin subunits on the HTR8/SVneo cell line , although the former study only suggested the effects of IL-6 on EVT invasion cellular cascades . LIF, a member of the IL-6 family, has been suggested to increase the invasiveness of trophoblastic cells through the activation of STAT1 or STAT3 [26, 34]. Because OSM is a cytokine in the IL-6 family, its role in activating endothelial cells should be investigated to evaluate the role of OSM in the preeclamptic placenta [31, 32]. The functional role of OSM in the human placenta has not yet been clarified. Because OSM has cell-type specific effects, the effects and mechanisms of OSM related to normal and pathologic pregnancies should be evaluated both in vitro and in vivo.