CELL-DIFFERENTIATION AND ENDOGENOUS CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE REGULATE OSTEOPONTIN EXPRESSION IN HUMAN TROPHOBLASTS

Integrin receptors and their extracellular matrix ligands have been im plicated in the molecular and cellular mechanisms of trophoblast adhes ion and migration. In the present series of experiments, the regulatio n of expression of osteopontin (OPN), a secretory extracellular matrix protein that mediates cell adhesion by binding to members of the alph a(v) family of integrins was investigated. Human chorionic villi were obtained during the mid and late first trimester, early second trimest er, and late third trimester of pregnancy. In addition, cytotrophoblas ts were isolated from chorionic villi, and the effects of exposure of cultured cytotrophoblasts to cAMP agonists or antagonists were assesse d. OPN messenger RNA (mRNA) was expressed by human trophoblasts in the first trimester and throughout pregnancy. Using immunolocalization in prepared tissue sections, cytotrophoblasts stained intensely for OPN, but syncytial trophoblasts did not. This differentiation-dependent ex pression was confirmed in vitro by demonstrating that freshly isolated mononuclear cytotrophoblasts exhibited a high level of OPN mRNA, but as the cells aggregated and fused to form multinucleated syncytia in, vitro, mRNA levels decreased dramatically. 8-Bromo-cAMP inhibited the expression of OPN mRNA, whereas the cAMP antagonist Rp-cAMP inhibited the OPN mRNA decrease during the in, vitro differentiation of the cell s. This study demonstrates the regulated expression of OPN by human tr ophoblasts. In addition, our data suggest that this expression is depe ndent upon the state of cellular differentiation of the trophoblasts a nd is regulated by endogenous cAMP. We speculate that binding of OPN t o its alpha(v) beta(3)-integrin receptor may be a critical signaling p athway contributing to the integrity of the chorionic villus and may a lso play a role in maternal-embryonic communication during the process of placentation.