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Characterization of a Novel and Functional Human Prolactin Receptor Isoform (S1PRLr) Containing Only One Extracellular Fibronectin-Like Domain

Department of Pathology and Laboratory Medicine, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania 19104

Address all correspondence and requests for reprints to: Charles V. Clevenger, M. D., Ph.D. Department of Pathology & Laboratory Medicine, University of Pennsylvania Medical Center, 513 Stellar-Chance Laboratories, 422 Curie Boulevard, Philadelphia, Pennsylvania 19104. E-mail: clevengc{at}mail.med.upenn.edu.

Prolactin (PRL)-dependent signaling occurs as the result of ligand-induced homodimerization of the PRL receptor (PRLr). To date, short, intermediate, and long human PRLr isoforms have been characterized. To investigate the expression of other possible human PRLr isoforms, RT-PCR was performed on mRNA isolated from the breast carcinoma cell line T47D. A 1.5-kb PCR fragment was isolated, subcloned, and sequenced. The PCR product exhibited a nucleotide sequence 100% homologous to the human long isoform except bp 71–373 were deleted, which code for the S1 motif of the extracellular domain. Therefore, this isoform was designated the S1 PRLr. Northern analysis revealed variable S1 PRLr mRNA expression in a variety of tissues. Transfection of Chinese hamster ovary cells with S1 cDNA showed the isoform is expressed at the protein level on the cell surface with a molecular mass of approximately 70 kDa. Kinetic studies indicated the S1 isoform bound ligand at a lower affinity than wild-type receptor. The S1 PRLr was also shown to activate the proximal signaling molecule Jak2 upon addition of ligand to transfected cells, and, unlike the long PRLr, high concentrations of ligand did not function as a self-antagonist to signaling during intervals of PRL serum elevation, i.e. stress and pregnancy. Given its apparent widespread expression, this PRLr isoform may contribute to PRL action. Furthermore, the functionality of this receptor raises interesting questions regarding the minimal extracellular domain necessary for ligand-induced receptor signaling.

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