Human prolactin gene expression. The use of an alternative noncoding exon in decidua and the IM-9-P3 lymphoblast cell line

The prolactin (PRL) gene is normally expressed in the anterior pituitary lactotrope and the decidualized stromal cell of the human endometrium. We have recently described the ectopic expression of the human PRL gene in a B-lymphoblastoid cell line, IM-9-P. The IM-9-P and decidual PRL mRNAs are approximately 150 nucleotides longer than the pituitary transcript. The protein coding sequence of the IM-9-P and decidual PRL cDNAs is identical to the pituitary PRL message, and the 3'-untranslated region (UTR) exhibited a 7-14-nucleotide elongation. The 5'-UTR of IM-9-P and decidual PRL cDNAs revealed an extension of 41 base pairs upstream of the normal transcription start site for the human pituitary PRL gene. This is preceded by an additional sequence that is not homologous to the DNA found 5' to the pituitary PRL cap site. Decidual and IM-9-P PRL mRNAs possess identical 5' ends having 5'-UTRs of 83-263 nucleotides longer than that of the pituitary mRNA. The decidual/IM-9-P-specific segment of the 5'-UTR is located as a new 5'-noncoding exon 5-7 kilobase pairs upstream of the human pituitary PRL gene, exon 1. The clonal IM-9-P3 cell line provides a unique and easily manageable resource with which to study decidual specific PRL gene transcription.