The gene encoding the human pregnancy-specific glycoprotein (PSG) belongs to a gene subfamily, comprised of the carcinoembryonic antigen (CEA) and PSG subgroups, within the immunoglobulin superfamily. To study the functional roles of PSG during development in an animal model, we isolated and characterized a near full-length cDNA (rnCGM6) encoding a PSG-related protein from a rat placental cDNA library. rnCGM6 is 2,068 bp in length and contains an open reading frame that encodes a 475-amino-acid polypeptide with a predicted molecular mass of 53 kD. The 5' noncoding sequence is 173 nucleotides, and primer-extension experiments demonstrate that the transcriptional initiation site is located 22-24 nucleotides further upstream. The 3' noncoding sequence contains 470 nucleotides which is followed by a poly(A) tail. In contrast to human PSGs, which contain one immunoglobulin variable-like and two to three immunoglobulin constant-like protein domains, rnCGM6 contains three immunoglobulin variable-like domains and one immunoglobulin constant-like domain. rnCGM6 contains six potential N-linked glycosylation sites and, in its carboxyl-terminal domain, a tyrosine protein kinase phosphorylation site. The tyrosine phosphorylation site is conserved among all rat and human PSG members. rnCGM6 hybridized with a major 2.5-kb and two minor 3.0- and 3.5-kb mRNAs, all primarily expressed in the rat placenta. Ribonuclease protection analysis, using probes specific to the 5', middle, and 3' regions of rnCGM6, and the 5' region of a previously identified cDNA, rnCGM1, mainly yielded fully-protected fragments indicating relatively low sequence similarity among rat PSG-related proteins. Northern hybridization and ribonuclease protection assays also suggest that rnCGM6 may be the major PSG member in rat.

Pregnancy-specific glycoproteins (PSGs), which are the major placental proteins, and the carcinoembryonic antigens comprise a subfamily within the immunoglobulin superfamily. To understand the molecular mechanisms underlying the control of PSG expression, we characterized the promoter elements of a rodent PSG gene, rnCGM3, and showed that DNA elements at nucleotides -326 to -185 (PI) relative to the translation start site of rnCGM3 function as a promoter. The rnCGM3 PI promoter contains two placental factor binding sites, PISI and PISII. Both are transcription activation elements. In the present report, we screened a placental expression cDNA library with a rnCGM3-PISII probe (nucleotides -263 to -233) encompassing two overlapping palindromes (TGTTGCTCAACATGTTG) and demonstrated that the PISII-binding factor is C/EBP beta, a leucine zipper family of transcription factor. Gel mobility-shift and transient expression analyses showed that C/EBP beta and C/EBP isoforms, C/EBP alpha and C/EBP delta, bind to the PISII element and trans-activate rnCGM3 gene expression. Deletion of PISII from the rnCGM3 PI promoter greatly reduced the basal as well as the C/EBP-activated rnCGM3 expression. Gel supershift assays demonstrated that C/EBP beta is the placental isoform that binds to the PISII site rnCGM3. Moreover, C/EBP beta is expressed in high levels in the placenta, ovary, liver, lung, heart, and spleen, in contrast to C/EBP alpha, which is expressed primarily in the liver and only low levels in the placenta. Our results demonstrate that C/EBP beta is one of the transcription factors that positively regulate rnCGM3 expression during pregnancy.