FUNCTIONAL-ACTIVITY OF THE HUMAN PROLACTIN RECEPTOR AND ITS LIGANDS

Prolactin receptors (PRLR) have been identified in a number of human t issues and cell lines, although little is known about the human recept or protein. The cloning of the human PRLR cDNA has enabled further cha racterization of the receptor protein in transfected cells. Since the human cDNA is expressed at lower levels than the rat cDNA, we have con structed a hybrid cDNA (pECE r5'hPRLR) containing nucleotides of the 5 ' untranslated region and signal peptide of the rat PRLR and the prote in coding and 3' untranslated portion of the human receptor. Expressio n of the hybrid receptor was increased more than two-fold compared to the human receptor as detected by specific binding of I-125-human grow th hormone (GH) to transfected COS-7 cells. The relative molecular mas s of the receptor was 93 000 Da, as determined by chemical cross-linki ng studies. Transcriptional assays were used to show the human PRLR wa s able to activate two milk protein genes; ovine beta-lactoglobulin an d rat beta-casein. Transfected cells expressing the human PRLR recepto r, treated with human GH or prolactin (PRL), induced a dose-dependent increase in transcriptional activation of the beta-casein/luciferase f usion gene. Glycosylated, and non-glycosylated human PRL, and ovine PR L were equally effective in activating the beta-casein promoter. Human placental lactogen and bovine PRL could also induce a greater than 10 -fold induction, whereas insulin did not significantly stimulate the b eta-casein promoter. The results show that the human PRLR can activate both beta-lactoglobulin and beta-casein milk gene promoters and that these reporter genes can be used to evaluate the functional activity o f agonists and antagonists of the human PRLR.