A role for JAK2 in mediating cell surface GHR-PRLR interaction

Growth hormone (GH) receptor (GHR) and (full-length) prolactin (PRL) receptor (PRLR) are transmembrane class I cytokine receptors that co-exist in various normal and cancerous cells. Both receptors respond to their associated ligands predominantly by activating the Janus Kinase 2 (JAK2)-signal transducer and activator of transcription (STAT) signaling pathways, and both are also known to initiate receptor-specific JAK2-independent signaling. Together with their cognate ligands, these receptors have been associated with pro-tumorigenic effects in various cancers, including breast cancer (BC). Human GH is known to bind GHR and PRLR, while PRL can only bind PRLR. A growing body of work suggests that GHR and PRLR can form heteromers in BC cells, modulating GH signal transduction. However, the dynamics of PRLR and GHR on the plasma membrane and how these could affect their respective signaling still need to be understood. To this end, we set out to unravel the spatiotemporal dynamics of GHR and PRLR on the surface of human T47D breast cancer cells and {gamma}2A-JAK2 cells. We applied direct stochastic optical reconstruction microscopy (dSTORM) and quantified the colocalization and availability of both receptors on the plasma membrane at the nanometer scale at different time points following treatment with GH and PRL. In cells co-expressing GHR and PRLR, we surprisingly observed that not only GH but also PRL treatment induces a significant loss of surface GHR. In cells lacking PRLR or expressing a mutant PRLR deficient in JAK2 binding, we observed that GH induces downregulation of cell surface membrane-bound GHR, but PRL no longer induces loss of surface GHR. Colocalizations of GHR and PRLR were confirmed by proximity ligation (PL) assay. Our results suggest that PRLR-GHR interaction, direct or indirect, is indispensable for PRL- but not GH- induced loss of surface GHR and for both GH-induced and PRL-induced increase of surface PRLR, with potential consequences for downstream signaling. Furthermore, our results suggest that JAK2 binding via the receptor intracellular domains Box1 element is crucial for the observed regulation of one class I cytokine receptors cell surface availability via ligand-induced activation of another class I cytokine receptor. Our findings shed new light on the reciprocal and collective role that PRLR and GHR play in regulating cell signaling.