Abstract 310 Poster Session I, Saturday, 5/1 (poster 135)

Glypican-3 (GPC3), a heparan sulfate proteoglycan, is expressed primarily by embryonic tissues and has been shown to be associated with the Simpson-Golabi-Behmel fetal overgrowth syndrome. The mechanism(s) by which it regulates cell growth have not been determined but recent studies have demonstrated its ability to associate with IGF-II, suggesting that it's action at the cellular level may be related, in part, to its interaction with members of the IGF peptide family. IGFs are expressed by the human placenta and have been shown to regulate its growth in a cell-specific manner. Specifically, type I IGF receptor expression increases during trophoblast differentiation consistent with important roles for IGFs in differentiated trophoblast function. The expression of GPC3 was therefore studied in normal, human placental tissue and cell lines derived from human placenta to determine its site of synthesis and it's spatial relationship to the expression of IGF-related peptides. First, poly A enriched mRNA was obtained from human placenta and placenta-derived cells and analyzed with a cDNA probe specific for the GPC3 gene. GPC3 mRNA expression in term placenta was confirmed by the identification of its characteristic 2.5 kb transcript. Next, cytotrophoblasts derived from term placentae were obtained and maintained in culture. Undifferentiated cytotrophoblasts expressed small amounts of GPC3 mRNA. Expression of GPC3 mRNA increased significantly over 48-72 hours in culture during trophoblast differentiation. By contrast, fibroblast cell lines derived from normal placentae did not express any GPC3 mRNA in culture. Unlike normal trophoblasts, undifferentiated choriocarcinoma cell lines derived from human placentae (BeWo, JAR, JEG) did not express GPC3 mRNA. Likewise, the b30 clone of the BeWo choriocarcinoma cell line, which differentiates upon exposure to forskolin in a manner analogous to normal trophoblasts, failed to express GPC3 mRNA at any stage of differentiation. Finally, polyclonal antibodies generated against the peptide epitope were affinity purified and used to localize placental GPC3, in situ. Immunohistochemical analysis of paraffin imbedded tissue demonstrated clear staining of the syncytiotrophoblast cell layer but no staining of mesenchymal elements. These data confirm the presence of GPC3 in human placenta and localize it to the syncytiotrophoblast. They suggest that GPC3 functions in the placenta to regulate trophoblast growth, specifically. We speculate that GPC3 may function, in part, by interacting with the IGF axis at the trophoblast/decidual interface.