Changes in the expression and cytological localization of betacellulin and its receptors (ErbB-1 and ErbB-4) in the trophoblasts in human placenta over the course of pregnancy

Objective: Betacellulin (BTC), purified and cloned from mouse beta cell tumor (BTC-JC10), is regarded as a new member of the epidermal growth factor family. The present study was conducted to clarify the expression of BTC and its receptors, ErbB-1 and ErbB-4, in the trophoblasts in the human placenta over the course of pregnancy. Design and Methods: Human placental tissues were obtained from 4 pregnant women at the 4th to 5th week of pregnancy (very early placentas), 10 women at the 6th to 12th week (early placentas), 5 women at the 18th to 21st week (mid placentas) and 8 women at the 38th and 40th week (term placentas). The mRNA expressions of BTC, erbB-1 and erbB-4 were evaluated by quantitative RTPCR with Southern blotting and the expression of the soluble form of BTC was determined by western immunoblot with a specific antibody to BTC protein. Immunohistochemical staining of BTC, ErbB-1 and ErbB-4 was also performed. Results: The levels of BTC mRNA expression in early and mid placentas were significantly higher than those in term placentas. The soluble form of BTC protein with an estimated molecular mass of 9.5 kDa was expressed in early and mid placentas, whereas the soluble form was not detected in term placentas. BTC from very early placentas until mid placentas was immunolocalized in syncytiotrophoblasts (S-cells), and was most abundant in early placentas. In contrast, BTC was immunolocalized in extravillous trophoblasts (EVTs), but not in villous trophoblasts in term placentas. The levels of erbB-1 mRNA in the early and mid placentas were significantly higher than those in term placentas, whereas the levels of erbB-4 mRNA in early placentas were significantly lower than those in mid and term placentas. ErbB-1 was immunolocalized in cytotrophoblasts in very early placentas, whereas it was immunolocalized in S-cells from early until term placentas. ErbB-4 from very early placentas until mid placentas was immunolocalized in S-cells, whereas ErbB-4 in the term placentas was detected in EVTs, but not in villous trophoblasts. Conclusions: These findings provide evidence for changes in expression and cytological localization of BTC and its receptors in the trophoblasts in human placenta over the course of pregnancy. BTC may play a pivotal role as a local growth factor in promoting the differentiated villous trophoblastic function via ErbB-1 in early placentas and in contributing to placental growth through the maintenance of EVT cell function via ErbB-4 in term placentas.