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Cell differentiation and endogenous cyclic adenosine 3',5'-monophosphate regulate osteopontin expression in human trophoblasts.

Integrin receptors and their extracellular matrix ligands have been implicated in the molecular and cellular mechanisms of trophoblast adhesion and migration. In the present series of experiments, the regulation of expression of osteopontin (OPN), a secretory extracellular matrix protein that mediates cell adhesion by binding to members of the alpha V family of integrins was investigated. Human chorionic villi were obtained during the mid and late first trimester, early second trimester, and late third trimester of pregnancy. In addition, cytotrophoblasts were isolated from chorionic villi, and the effects of exposure of cultured cytotrophoblasts to cAMP agonists or antagonists were assessed. 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 expression 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 cells. This study demonstrates the regulated expression of OPN by human trophoblasts. In addition, our data suggest that this expression is dependent upon the state of cellular differentiation of the trophoblasts and is regulated by endogenous cAMP. We speculate that binding of OPN to its alpha V beta 3-integrin receptor may be a critical signaling pathway contributing to the integrity of the chorionic villus and may also play a role in maternal-embryonic communication during the process of placentation.