Presence of alpha 2-macroglobulin in normal but not in malignant human syncytiotrophoblasts.

alpha 2-Macroglobulin (alpha 2M) was demonstrated in normal syncytiotrophoblasts of both early and full-term human placentas using immunocytological staining. alpha 2M was also detected in hydatidiform moles, the benign tumors of proliferating syncytiotrophoblasts. In contrast, no alpha 2M was detected in invasive moles or choriocarcinomas. In culture conditions, both normal syncytiotrophoblasts and choriocarcinoma cells, identified by production of human chorionic gonadotropin, were negative when stained for alpha 2M or when studied using metabolic labeling and immunoprecipitation or radioimmunoassay. However, alpha 2M was taken up from added human serum by the cultured syncytiotrophoblasts, whereas choriocarcinoma cells remained negative also under these conditions. The possible role of alpha 2M in the regulation of proteolysis in cell invasion is considered.