Differential effect of cross-linking the CD98 heavy chain on fusion and amino acid transport in the human placental trophoblast (BeWo) cell line

Methylmercury Uptake into BeWo Cells Depends on LAT2-4F2hc, a System L Amino Acid Transporter

The organic mercury compound methylmercury (MeHg) is able to target the fetal brain. However, the uptake of the toxicant into placental cells is incompletely understood. MeHg strongly binds to thiol-S containing molecules such as cysteine. This MeHg-l-cysteine exhibits some structural similarity to methionine. System L plays a crucial role in placental transport of essential amino acids such as leucine and methionine and thus has been assumed to also transport MeHg-l-cysteine across the placenta. The uptake of methylmercury and tritiated leucine and methionine into the choriocarcinoma cell line BeWo was examined using transwell assay and small interfering (si)RNA mediated gene knockdown. Upon the downregulation of large neutral amino acids transporter (LAT)2 and 4F2 cell-surface antigen heavy chain (4F2hc), respectively, the levels of [³H]leucine in BeWo cells are significantly reduced compared to controls treated with non-targeting siRNA (p < 0.05). The uptake of [³H]methionine was reduced upon LAT2 down-regulation as well as methylmercury uptake after 4F2hc silencing (p < 0.05, respectively). These findings suggest an important role of system L in the placental uptake of the metal. Comparing the cellular accumulation of mercury, leucine, and methionine, it can be assumed that (1) MeHg is transported through system L amino acid transporters and (2) system L is responsible for the uptake of amino acids and MeHg primarily at the apical membrane of the trophoblast. The findings together can explain why mercury in contrast to other heavy metals such as lead or cadmium is efficiently transported to fetal blood.

Deep RNA sequencing analysis of syncytialization-related genes during BeWo cell fusion

The syncytiotrophoblast (STB) plays a key role in maintaining the function of the placenta during human pregnancy. However, the molecular network that orchestrates STB development remains elusive. The aim of this study was to obtain broad and deep insight into human STB formation via transcriptomics. We adopted RNA sequencing (RNA-Seq) to investigate genes and isoforms involved in forskolin (FSK)-induced fusion of BeWo cells. BeWo cells were treated with 50 μM FSK or dimethylsulfoxide (DMSO) as a vehicle control for 24 and 48 h, and the mRNAs at 0, 24 and 48 h was sequenced. We detected 28,633 expressed genes and identified 1,902 differentially expressed genes (DEGs) after FSK treatment for 24 and 48 h. Among the 1,902 DEGs, 461 were increased and 395 were decreased at 24 h, while 879 were up-regulated and 763 were down-regulated at 48 h. When the 856 DEGs identified at 24 h were traced individually at 48 h, they separated into 6 dynamic patterns via a K-means algorithm, and most were enriched in down-even and up-even patterns. Moreover, the Gene Ontology (GO) terms syncytium formation, cell junction assembly, cell fate commitment, calcium ion transport, regulation of epithelial cell differentiation and cell morphogenesis involved in differentiation were clustered, and the MAPK pathway was most significantly regulated. Analyses of alternative splicing isoforms detected 123,200 isoforms, of which 1,376 were differentially expressed. The present deep analysis of the RNA-Seq data of BeWo cell fusion provides important clues for understanding the mechanisms underlying human STB formation.

CD99 inhibits CD98-mediated β1 integrin signaling through SHP2-mediated FAK dephosphorylation

The human CD99 protein is a 32-kDa type I transmembrane glycoprotein, while CD98 is a disulfide-linked 125-kDa heterodimeric type II transmembrane glycoprotein. It has been previously shown that CD99 and CD98 oppositely regulate β1 integrin signaling, though the mechanisms by which this regulation occurs are not known. Our results revealed that antibody-mediated crosslinking of CD98 induced FAK phosphorylation at Y397 and facilitated the formation of the protein kinase Cα (PKCα)-syntenin-focal adhesion kinase (FAK), focal adhesions (FAs), and IPP-Akt1-syntenin complex, which mediates β1 integrin signaling. In contrast, crosslinking of CD99 disrupted the formation of the PKCα-syntenin-FAK complex as well as FA via FAK dephosphorylation. The CD99-induced dephosphorylation of FAK was apparently mediated by the recruitment of Src homology region 2 domain-containing phosphatase-2 (SHP2) to the plasma membrane and subsequent activation of its phosphatase activity. Further consequences of the activation of SHP2 included the disruption of FAK-talin and talin-β1 integrin interactions and attenuation in the formation of the IPP-Akt1-syntenin complex at the plasma membrane, which resulted in reduced cell-ECM adhesion. This report uncovers the molecular mechanisms underlying the inverse regulation of β1 integrin signaling by CD99 and CD98 and may provide a novel therapeutic approach to treat inflammation and cancer.

The contribution of apoptosis-inducing factor (AIF) to villous trophoblast differentiation

Apoptosis is postulated to be a delayed but important part of the differentiation of placental villous cytotrophoblasts (CT) into functional syncytiotrophoblast (ST). This hypothesis is based on the observation that the externalization of phosphatidylserine and the activation of caspase 8 are required for trophoblast differentiation. In contradiction to this hypothesis we have previously found that differentiation occurs in the presence of both broad spectrum and caspase 8 specific inhibitors. Apoptosis-inducing factor (AIF) is a mitochondria-associated protein which is known to translocate to the nucleus and induce caspase-independent nuclear condensation, phosphatidylserine externalization and cell death. Thus AIF nuclear translocation may result in the apoptotic-like features associated with trophoblast differentiation and may be an obligatory event for differentiation to proceed. AIF translocation was assessed in isolated primary trophoblasts by optical section microscopy of antibody stained cells. We found AIF to be strongly expressed in the villous trophoblast and that small amounts of AIF were localized to the nucleus of the cells. Significantly, inhibitors of AIF translocation (calpeptin and zFA-fmk) blocked translocation but not differentiation of the cells. We conclude that AIF translocation is not involved in CT differentiation in isolated cell culture.

Genipa americana (Rubiaceae) fruit extract affects mitogen-activated protein kinase cell pathways in human trophoblast-derived BeWo cells: implications for placental development

Genipa americana L. (Rubiaceae) is a fruit tree and a traditional medicine used to treat anemia, icterus, asthma, and liver and spleen problems. The aim of the present study was to verify the effect of G. americana fruit ethanolic extract on the mechanism for proliferation and differentiation of trophoblast-like cells. Qualitative analysis of G. americana fruit extract was performed, and BeWo cells, a well-established placental choriocarcinoma cell line that can undergo differentiation, were used to analyze cell viability and proliferation. Methods consisted of cytotoxic and proliferation measurement, detection of release of human chorionic gonadotrophins, cell fusion observation, and evaluation of cell-signaling pathways (production of cyclic adenosine monophosphate and phosphorylation of mitogen-activated protein kinases [MAPKs]). A stock solution of the extract was diluted in Ham's F-12 medium with 10% fetal bovine serum at concentrations ranging from 50 to 1000 μg/mL. Cells treated with dimethylsulfoxide, forskoline, and MAPK inhibitors (PD98059 or SB203580) were used as a control. Forskoline was used to induce the differentiation state in BeWo cells. Phytoanalysis indicated the presence of steroids only. Results showed that the G. americana fruit extract did not cause any cytotoxicity or interference in cell differentiation. However, a significant antiproliferative state related to inhibition and reactivation of ERK1/2 and p38 MAPK in BeWo cells was seen. These results suggest that steroids from G. americana may affect placental cell regulation.

The monocarboxylate transporter 8 and L-type amino acid transporters 1 and 2 are expressed in mouse skeletons and in osteoblastic MC3T3-E1 cells

BACKGROUND

Several plasma membrane transporters have been shown to mediate the cellular influx and/or efflux of iodothyronines, including the sodium-independent organic anion co-transporting polypeptide 1 (OATP1), the sodium taurocholate co-transporting polypeptide (NTCP), the L-type amino acid transporter 1 (LAT1) and 2 (LAT2), and the monocarboxylate transporter 8 (MCT8). The aim of this study was to investigate if the mRNAs of these transporters were expressed and regulated by thyroid hormone (TH) in mouse calvaria-derived osteoblastic MC3T3-E1 cells and in the fetal and postnatal bones of mice.

METHODS

The mRNA expression of the iodothyronine transporters was investigated with real-time polymerase chain reaction analysis in euthyroid and hypothyroid fetuses and litters of mice and in MC3T3-E1 cells treated with increasing doses of triiodothyronine (T(3); 10(-10) to 10(-6) M) or with 10(-8) M T(3) for 1-9 days.

RESULTS

MCT8, LAT1, and LAT2 mRNAs were detected in fetal and postnatal femurs and in MC3T3-E1 cells, while OATP1 and NTCP mRNAs were not. LAT1 and LAT2 mRNAs were not affected by TH status in vivo or in vitro or by the stage of bone development or osteoblast maturation (analyzed by the expression of osteocalcin and alkaline phosphatase, which are key markers of osteoblastic differentiation). In contrast, the femoral mRNA expression of MCT8 decreased significantly during post-natal development, whereas MCT8 mRNA expression increased as MC3T3-E1 cells differentiated. We also showed that MCT8 mRNA was up-regulated in the femur of hypothyroid animals, and that it was down-regulated by treatment with T(3) in MC3T3-E1 cells.

CONCLUSIONS

This is the first study to demonstrate the mRNA expression of LAT1, LAT2, and MCT8 in the bone tissue of mice and in osteoblast-like cells. In addition, the pattern of MCT8 expression observed in vivo and in vitro suggests that MCT8 may be important to modulate TH effects on osteoblast differentiation and on bone development and metabolism.

CD98 activation increases surface expression and clusteringof beta1 integrins in MCF-7 cells through FAK/Src- and cytoskeleton-independent mechanisms

CD98, a disulfide-linked 125-kDa heterodimeric type II transmembrane glycoprotein, regulates beta1 integrin- mediated cell adhesion. However, the molecular mechanisms underlying CD98-mediated activation of beta1 integrin are presently unclear. In this study, the effects of CD98 signaling on the expression and clustering of beta1 integrin were investigated. Activation of CD98 augmented surface expression of beta1 integrin on MCF-7 cells. Cross-linking CD98 induced clustering of beta1 integrins. Inhibition of phosphorylation of focal adhesion kimase (FAK) by PP2, an inhibitor of Src family kinase, reduced cell-extracellular matrix adhesion, but not surface expression and clustering of beta1 integrin on MCF-7 cells. This result was confirmed by over-expression of dominant negative forms of FAK. In addition, phalloidin or cytochalasin D inhibited CD98-mediated induction of cell-ECM adhesion, but not surface expression and clustering of beta1 integrins. The inhibitory effects of PP2, cytochalasin D or phalloidin on CD98-stimulated cell adhesion were diminished by pretreatment of cells with Mn2+, which is shown to induce conformational change of integrins. These results provide the first evidence that CD98 activation increases not only beta1 integrin affinity but also its surface expression and clustering and the latter is independent of FAK/Src and cytoskeleton.

Membrane trafficking of CD98 and its ligand galectin 3 in BeWo cells − implication for placental cell fusion

CD98 heavy chain (CD98hc), expressed at high levels in developing human trophoblasts, is an integral membrane protein with multiple N-linked glycosylation sites and known to be important for cell fusion, adhesion, and amino acid transport. Western blotting and flow cytometry were used to study the effect of brefeldin A, an inhibitor of protein translocation through the Golgi, on CD98hc in the human placental trophoblast cell line BeWo. Although brefeldin A treatment caused increased cell surface expression of CD98hc, a novel partially glycosylated form of the protein was found and, concomitantly, cell fusion was reduced. Western blotting showed that CD98 and galectin 3, a proposed ligand for the glycosylated extracellular domain of CD98hc, co-immunoprecipitated, and double-label immuno-electron microscopy confirmed that CD98hc associated with galectin 3. Furthermore, cell fusion was reduced (specifically) by the disaccharide lactose, a known ligand for the carbohydrate recognition domain of galectin 3, suggesting that the association was functional. Taken together, the data suggest that N-glycosylation of CD98 and subsequent interaction with galectin 3 is critical for aspects of placental cell biology, and provides a rationale for the observation that, in the mouse, truncation of the CD98hc extracellular domain leads to early embryonic lethality [Tsumura H, Suzuki N, Saito H, Kawano M, Otake S, Kozuka Y, Komada H, Tsurudome M & Ito Y (2003) Biochem Biophys Res Commun 308, 847-851].