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¹ Department of Animal Biology and Mari Lowe Center for Comparative Oncology Research² Cell and Molecular Biology Program, Biomedical Graduate School, University of Pennsylvania, 380 S University Avenue, Philadelphia, Pennsylvania 19104, USA³ Department of Pathology, Northwestern University, Chicago, Illinois 60611, USA⁴ Inserm, U845, Centre de Recherche Croissance et Signalisation, Equipe ‘PRL, GH and tumours’ and Université Paris Descartes, Faculté de Médecine, Paris F-75015, France⁵ Department of Cell Biology, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA

(Correspondence should be addressed to S Y Fuchs; Email: syfuchs{at}vet.upenn.edu)

^* (G Swaminathan, B Varghese, C Thangavel contributed equally to this work)

Prolactin (PRL) activates its receptor to initiate signal transduction pathways (including activation of Janus kinases, Jak) but also stimulates downregulation of this receptor to limit the magnitude and duration of signaling. Degradation of the long form of PRL receptor (PRLr) depends on its phosphorylation on Ser349 that is required to facilitate PRLr ubiquitination. Signaling events that mediate PRL-induced degradation of PRLr remain to be elucidated. Here, we investigated the role of Jak2 activity in ligand-triggered increase of PRLr phosphorylation on Ser349, PRLr ubiquitination, endocytosis, and degradation. Using Jak2 reconstitution in Jak2-null cells as well as pharmacologic approaches, we found that treatment with PRL (but not with PRLr antagonist) promotes phosphorylation of PRLr on Ser349 and accelerates endocytosis of PRLr. Furthermore, PRL-stimulated PRLr phosphorylation, endocytosis, and degradation in Jak2-null cells reconstituted with wild type but not with catalytically inactive Jak2. We discuss how Jak2-mediated signaling might be transduced into Ser349 phosphorylation of PRLr as well as its ubiquitination and endocytosis.

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