Modulation of phosphatidylserine epitope expression by BeWo cells during forskolin treatment

The adenylcyclase activator forskolin induces the human choriocarcinoma line, BeWo, to undergo differentiation and fusion within 48 to 72 h. Using three monoclonal antibodies that differentiate between the anionic phospholipids cardiolipin (CL) and phosphatidylserine (PS) and immunoperoxidase techniques we investigated the expression of PS by BeWo during 48 h of forskolin treatment. We observed that BeWo cells not exposed to forskolin express an epitope of pS that reacts strongly with monoclonal antibody BA3B5C4 (CL+/PS+), whereas following treatment with forskolin there is a decrease in reactivity with BA3B5C4 and a concurrent increased activity with a second PS-reactive monoclonal antibody, 3SB9b (CL-/PS+). A third monoclonal antibody, D11A4 (CL+/PS-), that reacted with all anionic phospholipids except PS did not bind to BeWo cells, whether forskolin treated or not. These observations support previous interpretations using human placenta that during cytotrophoblast differentiation two antigenic forms of PS are expressed. Based on the described relationship of PS with cellular fusion events in other systems and the association of naturally occurring antibodies against PS with pregnancy loss and intrauterine growth retardation in humans, we propose that altered expression of PS during normal placental development and in BeWo after exposure to forskolin may be critical in the cytotrophoblast differentiation process.