DIFFERENTIATION-DEPENDENT EXPRESSION OF THE BCL-2 PROTOONCOGENE IN THE HUMAN TROPHOBLAST LINEAGE

OBJECTIVE: We explored the role of the BCL-2 proto-oncogene in the lif e cycle of trophoblast cells by examining: 1) the patterns of BCL-2 ex pression in normal placenta at various gestational ages and in specime ns of hydatidiform moles and choriocarcinomas, and 2) the effects of c yclic adenosine monophosphate (cAMP) treatment of JEG-3 choriocarcinom a cells, which induces differentiated functions, on BCL-2. METHODS: BC L-2 protein was localized by indirect immunofluorescence and immunoper oxidase staining of tissue sections and cells using monoclonal and pol yclonal 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 pr otein could be expressed at high levels in this cell type. RESULTS: BC L-2 immunostaining was most prominent in the syncytiotrophoblast of no rmal placenta. It was found in syncytiotrophoblast of complete and par tial hydatidiform moles, whereas cytotrophoblast staining was weak. BC L-2 immunostaining was also barely detectable in choriocarcinoma cells (JEG-3 cells) and a primary choriocarcinoma. However, BCL-2 protein c ould be transiently overexpressed in JEG-3 cells by transfection with an expression plasmid. Western blot analysis revealed low levels of BC L-2 in JEG-3 cells and a rise in BCL-2 protein in placental extracts f rom 10 weeks' gestation, but at low term. BCL-2 transcripts were subst antially 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, w hereas p53 mRNA declined. CONCLUSIONS: 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 tr ophoblast cells; 2) there appears to be an inverse relation between BC L-2 and p53 expression in trophoblast; and 3) cAMP regulates BCL-2 pro tein in trophoblast cells. We speculate that the expression of BCL-2 i n 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 expressi on of BCL-2 in choriocarcinoma cells may render them more susceptible to apoptosis.