Expression of the Pregnancy-Specific β1-Glycoprotein Gene in Cultured Human Trophoblasts

Abstract

Human pregnancy-specific beta 1-glycoprotein (PS beta G), the major placental glycoprotein, shares strong sequence similarity with carcinoembryonic antigen and is a member of the immunoglobulin superfamily. To understand the role of PS beta G in placental ontogeny during pregnancy, we examined its synthesis and regulation in primary cultures of trophoblast cells. Freshly plated (12 h) cytotrophoblasts expressed little PS beta G transcripts; however, within 24 h of culture, PS beta G mRNAs became detectable. PS beta G synthesis and mRNA expression increased with time in culture, and maximal synthesis was achieved at 4 days, indicating that primary trophoblasts continue differentiating in vitro. Molecular cloning revealed that PS beta G is an extremely polymorphic protein. Most PS beta G cDNAs identified to date, including the three cDNAs (PSG16, PSG93, and PSG95) isolated in this laboratory, share strong sequence similarity in the 5' (designated PSG-5') and coding regions, but differ in sequences at the 3' region. The PSG-5', PSG93-specific, PSG16/PSG93-3', and PSG95-3' probes, which identify the majority of PS beta G mRNAs, hybridized with three mRNAs of 2.3, 2.2, and 1.7 kilobases in primary trophoblasts and human term placental tissue. Ribonuclease protection analysis demonstrated that primary trophoblasts expressed most of the placental PS beta G transcripts. However, culturing in vitro altered PS beta G gene expression, and the level of PS beta G transcripts containing a PSG95-3' sequence was preferentially increased in primary trophoblasts. Moreover, primary trophoblasts synthesized a 64K PS beta G polypeptide in variable amounts and three PS beta Gs of 72K, 62K, and 54K in roughly equal amounts, whereas purified human term placental PS beta G consists of a major polypeptide of 72K and two minor ones of 64K and 54K. PS beta G gene expression in primary trophoblasts was slightly reduced by 8-bromo-cAMP, but was markedly inhibited by sodium butyrate.