Expression of the high-mobility group protein HMGI(Y) in human trophoblast:: potential role in trophoblast invasion of maternal tissue

The high-mobility group protein HMGI(Y) is a member of a family of non-histone chromosomal proteins, which have been implicated in the regulation of inducible gene transcription, integration of retroviruses into chromosomes and induction of neoplastic transformation and metastatic progression in cancer cells. The human trophoblast is a tissue that shares proliferation capacity and invasiveness with neoplastic tissues, but in which these processes are tightly regulated. In the present study, we analyzed the expression of HMGI(Y) in the human placenta using immunohistochemistry. We found expression of HMGI(Y), with nuclear localization, in the villous cytotrophoblast (vCT), which is a highly proliferative cell type. In contrast, the majority of the nuclei of the villous syncytiotrophoblast, a terminally differentiated tissue, was negative. Interestingly, expression of HMGI(Y) was strongest in anchoring villi at the implantation site and in extravillous (intermediate) trophoblast (EVT) invading the maternal decidua. As vCT cells differentiate to become EVT, the HMGI(Y) protein appears to switch from a nuclear to a cytoplasmic localization. Expression of HMGI(Y) in isolated trophoblast populations in primary cell culture was also confirmed using Western-blot analysis. This study shows for the first time expression and localization of HMGI(Y) in the subpopulations of placental tissue.