S179D-human PRL, a pseudophosphorylated human PRL analog, is an agonist and not an antagonist

For many years, our group has been involved in the development of human PRL antagonists. In two recent publications, S179D-human PRL, a human PRL anal og designed to mimic a putative S179-phosphorylated human PRL, was reported to be a highly potent antagonist of human PRL-induced proliferation and si gnaling in rat Nb2 cells. We prepared this analog with the aim of testing i t in various bioassays involving the homologous, human PRL receptor. In our hands, S179D-human PRL was able to stimulate 1) the proliferation of rat N b2 cells and of human mammary tumor epithelial cells (T47D), 2) transcripti onal activation of the lactogenic hormone response element-luciferase repor ter gene, and 3) activation of the Janus kinase/signal transducer and activ ator of transcription and MAPK pathways. Using the previously characterized antagonist G129R-human PRL as a control, we failed to observe any evidence for antagonism of S179D-human PRL toward any of the human PRL-induced effe cts analyzed, including cell proliferation, transcriptional activation, and signaling. In conclusion, our data argue that S179D-human PRL is an agonis t displaying slightly reduced affinity and activity due to local alteration of receptor binding site 1, and that the antagonistic properties previousl y attributed to S179D-human PRL cannot be confirmed in any of the assays an alyzed in this study.