Hepatocyte growth factor activator (HGF-A) and its zymogen in human placenta

HGF-activator (HGF-A) is a circulating serine protease known to be responsible for activation of hepatocyte growth factor (HGF). Active HGF is thought to be an important regulator of trophoblast growth. In vitro, HGF-A is produced via proteolytic cleavage of its zymogen by thrombin. Immunocytochemistry and Western immunoblotting were performed using human placental tissue from all three trimesters with an antibody that recognizes both HGF-A and its zymogen. Western immunoblotting revealed a 97 kDa band equivalent to the zymogen in placenta from all three trimesters. A smaller 34 kDa band equivalent to HGF-A was only seen in first and second trimester placenta. The anti-HGF-A/zymogen antibody demonstrated immunostaining in placental villi and membranes throughout gestation. Within first trimester villi immunostaining was strongest within the syncytio- and cytotrophoblast layers, but was also seen within stromal and endothelial cells. Likewise, in third trimester placenta the syncytio-cytotrophoblast layer showed the strongest immunoreactivity. In vitro, HGF can induce trophoblast DNA synthesis and the localization of HGF-A to the peri-villous trophoblast layer (which expresses c-met, the HGF receptor) suggests that it may be responsible for activation of pro-HGF at this site. This adds further weight to the hypothesis that HGF in vivo is an important regulator of trophoblast growth.