The human placenta is a tissue with a unique capacity for rapid, but--as opposed to malignant tumours--tightly controlled proliferation and invasion capacity. The members of the activating protein-1 (AP-1) family of transcription factors are key regulators of cellular proliferation, differentiation and invasion processes in many systems and could, thus, play an important role in regulating these processes in the human placenta as well. In the present study, we used immunohistochemistry with specific antibodies against all members of the AP-1 family (c-Jun, JunB, JunD and c-Fos, FosB, Fra-1, Fra-2) to investigate their expression pattern and tissue localization in the human placenta. With the exception of c-Jun, which was expressed in a small fraction of villous cytotrophoblast nuclei and JunD, expressed in some syncytiotrophoblast nuclei, all other members of the AP-1 family were completely absent from villous cyto- and syncytiotrophoblast. Interestingly, most AP-1 factors were expressed in the intermediate (extravillous) trophoblast, with expression being strongest for JunD and Fra2 (100% of nuclei showing strong expression), followed by c-Jun (80% positive nuclei), c-Fos and FosB (50% positive nuclei). This was true for samples of both first trimester and later pregnancy. These data show that, in the human placenta, the AP-1 transcription factors are specifically expressed in the intermediate (extravillous) trophoblast, were they could be implicated in regulating proliferation, differentiation and/or expression of invasion-specific molecules, such as matrix metalloproteinases, which have been shown to be regulated by AP-1 in vitro and are expressed by the invasive trophoblast.
