Trophoblast origin of hCG isoforms: cytotrophoblasts are the primary source of choriocarcinoma-like hCG

Abstract

We have previously demonstrated that a hyperglycosylated isoform of chorionic gonadotropin (hCG) (B152 hCG) is detected in the blood and urine in early pregnancy and is subsequently rapidly replaced by the hCG isoform (B109 hCG) characteristic of later pregnancy. In the current study we have extended our work on the origin of these isoforms. We have used a combination of in situ and in vitro approaches. Localization studies in placental tissues showed that monoclonal antibody B109 stained very specifically syncytiotrophoblast (STBs) from first and second trimester tissues. At term, STBs exhibited no B109 staining at all. Immunostaining with B152 antibody, that recognize the hyperglycosylated isoform of hCG, revealed only punctate staining of STBs in most villi of first trimester tissue. Both antibodies B109 and B152 failed to stain cytotrophoblasts (CTBs). To assess the functional relevance of these observations we analyzed conditioned media from purified CTBs using two immunometric assays, one of which (B152-B207*) has primary specificity for the hyperglycosylated, choriocarcinoma-like hCG and the other (B109-B108*) having primary specificity for the later pregnancy hCG isoform. Regardless of gestational age, isolated CTBs secreted predominantly B152 hCG isoform in contrast to placental villi (predominantly STBs), which released primarily the B109 hCG isoform. Isolated CTBs, however, failed to immunostain with both B109 and B152 antibodies. To resolve this contradiction, we cultured CTBs in the presence of brefeldin A, a drug known to block secretion by inhibiting protein translocation from the endoplasmic reticulum to the Golgi vesicles. Brefeldin A treated CTBs stained strongly with B109 and did not stain or stained weakly with B152 antibody. We assume that treatment with brefeldin A impaired glycosylation of beta subunit and consequently inhibited the production of hyperglycosylated form of hCG recognized by B152. In summary, our in vitro experiments indicate that both isoforms of hCG are produced by villus CTBs and that the dominant isoform is the one recognized by antibody B152. STBs produce primarily the less glycosylated B109 hCG isoform. This data suggests that at the beginning of pregnancy villus CTBs are the major source of the B152 hCG isoform. This finding is supported by our clinical data that show that the dominant hCG isoform in the blood and urine of pregnant women in the first 6 weeks of pregnancy is recognized by B152 ( Kovalevskaya et al., 1999, 2002). The inversion of the B152/B109 ratio observed after 6–7 weeks of pregnancy can be explained by the reduction of number of villus CTBs and/or by maturation of STBs.