PROLIFERATION AND INVASIVENESS AS WELL AS EXPRESSION OF GAP JUNCTION CONNEXlNS OF TROPHOBLAST CELLS CHANGE WITH DIFFERENTIATION IN VIVO AND IN VITRO. R. Gr0mmer, B. Reu8, P. Hellmann, O. Traub, E. Winterhager Cell-cell communication via gap junctions plays an important role with regard to cell proliferation and differentiation. About 15 different gap junction connexins (cx) are known, but it is still not understood how this diversity is related to their function. We have investigated the pattern of connexin expression during trophoblast invasion and placental differentiation in the rat by immunohistochemistry and northern blot analysis. The invasive trophoblast cells of the ectoplacental cone express cx31, while during differentiation they switch to cx26 and cx43. This may point to an important role of those connexins in regulating the differentiation process of trophoblast lineages. To investigate the role of connexin expression for invasion and differentiation, we used two established cell lines, one corresponding to differentiated trophoblast cells (HRP; Hunt et al., Placenta 10: 161-177, 1989), and one rat choriocarcinoma cell line (RCHO; Faria and Soares, Endocrinol. 129: 2895-2906, 1991) which represents a malignant variant of the trophoblast. In parallel to our findings in the rat placenta, we could show that the undifferentiated RCHO cells are characterized by large amounts of cx31, while the differentiated HRP cells express cx43. With induction of differentiation by retinoic acid (RA; 10 -6 M) expression of cx31 is suppressed in RCHO cells, and in some cases, expression of cx43 was induced. Cell proliferation is greatly reduced and invasiveness is suppressed in a Matrigel invasion assay. In HRP cells application of RA did not alter connexin expression and only lead to a slight decrease of proliferation rate, but invasion was reduced compared to untreated cells. Thus the different connexin expression appears more likely to play a role in regulating the proliferation and differentiation than invasiveness of trophoblast cells.