Differentiation-dependent expression of the BCL-2 proto-oncogene in the human trophoblast lineage

We explored the role of the BCL-2 proto-oncogene in the life cycle of trophoblast cells by examining: 1) the patterns of BCL-2 expression in normal placenta at various gestational ages and in specimens of hydatidiform moles and choriocarcinomas, and 2) the effects of cyclic adenosine monophosphate (cAMP) treatment of JEG-3 choriocarcinoma cells, which induces differentiated functions, on BCL-2.

BCL-2 protein was localized by indirect immunofluorescence and immunoperoxidase staining of tissue sections and cells using monoclonal and polyclonal antibodies. Western and Northern blotting were used to assess BCL-2 and p53 protein and mRNA levels, respectively. JEG-3 cells were transfected with a BCL-2 expression plasmid to establish that BCL-2 protein could be expressed at high levels in this cell type.

BCL-2 immunostaining was most prominent in the syncytiotrophoblast of normal placenta. It was found in syncytiotrophoblast of complete and partial hydatidiform moles, whereas cytotrophoblast staining was weak. BCL-2 immunostaining was also barely detectable in choriocarcinoma cells (JEG-3 cells) and a primary choriocarcinoma. However, BCL-2 protein could be transiently overexpressed in JEG-3 cells by transfection with an expression plasmid. Western blot analysis revealed low levels of BCL-2 in JEG-3 cells and a rise in BCL-2 protein in placental extracts from 10 weeks' gestation to term. In contrast, p53 protein was abundant in JEG-3 cells and normal placenta at 10 weeks' gestation, but low at term, BCL-2 transcripts were substantially more abundant in term placenta than in JEG-3 cells. Treatment of JEG-3 cells with 8-Br-cAMP, which induces genes characteristic of the syncytiotrophoblast, raised BCL-2 protein approximately twofold, whereas p53 mRNA declined.

We conclude that: 1) There is a differentiation-dependent pattern of BCL-2 expression in the placenta, with the protein being most abundant in terminally differentiated trophoblast cells; 2) there appears to be an inverse relation between BCL-2 and p53 expression in trophoblast; and 3) cAMP regulates BCL-2 protein in trophoblast cells. We speculate that the expression of BCL-2 in terminally differentiated trophoblast cells, and hence resistance to apoptotic cell death, may be one mechanism by which trophoblast mass is preserved during pregnancy. Conversely, the relatively low expression of BCL-2 in choriocarcinoma cells may render them more susceptible to apoptosis.