Characterization of pregnancy-specific β1-glycoprotein synthesized by human placental fibroblasts

Abstract

We have previously demonstrated that human placental fibroblasts produce a pregnancy-specific β1-glycoprotein (PSβG) immunologically indistinguishable from placental PSβG This was confirmed by the immunocytochemical localization of PSβG in these fibroblasts. In addition, placental fibroblasts contain all three PSβG mRNAs of 2.3, 2.2, and 1.7 kilobases which hybridize with the three PSβG cDNAs (PSG16, PSG93, and PSG95) identified, although at 1.4-2.5% of the levels in human term placenta. The major PSβG species synthesized by placental fibroblats is a 62K glycopolypeptide formed from a 58K intracellular precursor polypeptide. However, the PSβG species found in human placenta are one major glycoprotein of 72K and two minor ones of 64K and 54K. Poly(A)+ RNA from placental fibroblasts directed the synthesis of two polypeptides of 48K and 46K (major), whereas, poly(A)+ RNA from human placenta directed the synthesis of higher levels of four polypeptides of 50 K, 48 K (major), 46 K, and 36 K. Thus, the major PSβG species found in fibroblasts and human placenta differ. The carbohydrate side-chains are essential for the stability of fibroblast PSβG, because PSβG synthesis in these fibroblasts could not be detected in the presence of tunicamycin, a protein glycosylation inhibitor which did not affect PSβG mRNA expression. Our finding that a variant PSβG species is produced in placental fibroblasts raises the possibility that the authentic placental PSβG species may have different functions. © 1989 by The Endocrine Society.