Role of the Transcription Factor C/EBPβ in Expression of a Rat Pregnancy-Specific Glycoprotein Gene

Abstract

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/EBPβ, a leucine zipper family of transcription factor. Gel mobility-shift and transient expression analyses showed that C/EBPβ and C/EBP isoforms, C/EBPα and C/EBPδ, 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/EBPβ is the placental isoform that binds to the PISII site rnCGM3. Moreover, C/EBPβ is expressed in high levels in the placenta, ovary, liver, lung, heart, and spleen, in contrast to C/EBPα, which is expressed primarily in the liver and only low levels in the placenta. Our results demonstrate that C/EBPβ is one of the transcription factors that positively regulate rnCGM3 expression during pregnancy.