Characterization of Pregnancy-Specific β₁-Glycoprotein Synthesized by Human Placental Fibroblasts

Abstract

We have previously demonstrated that human placental fibroblasts produce a pregnancy-specific beta 1-glycoprotein (PS beta G) immunologically indistinguishable from placental PS beta G. This was confirmed by the immunocytochemical localization of PS beta G in these fibroblasts. In addition, placental fibroblasts contain all three PS beta G mRNAs of 2.3, 2.2, and 1.7 kilobases which hybridize with the three PS beta G cDNAs (PSG16, PSG93, and PSG95) identified, although at 1.4-2.5% of the levels in human term placenta. The major PS beta G species synthesized by placental fibroblasts is a 62K glycopolypeptide formed from a 58K intracellular precursor polypeptide. However, the PS beta G 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 PS beta G species found in fibroblasts and human placenta differ. The carbohydrate side-chains are essential for the stability of fibroblast PS beta G, because PS beta G synthesis in these fibroblasts could not be detected in the presence of tunicamycin, a protein glycosylation inhibitor which did not affect PS beta G mRNA expression. Our finding that a variant PS beta G species is produced in placental fibroblasts raises the possibility that the authentic placental PS beta G species may have different functions.