Tethering of the platelet-derived growth factor ss receptor to G-protein-coupled receptors - A novel platform for integrative signaling by these receptor classes in mammalian cells

Here we provide evidence to show that the platelet-derived growth factor be ta receptor is tethered to endogenous G-protein-coupled receptor(s) in huma n embryonic kidney 293 cells. The tethered receptor complex provides a plat form on which receptor tyrosine kinase and G-protein-coupled receptor signa ls can be integrated to produce more efficient stimulation of the p42/p44 m itogen-activated protein kinase pathway. This was based on several lines of evidence. Fix st, we have shown that pertussis toxin (which uncouples G-pr otein-coupled receptors from inhibitory G-proteins) reduced the platelet-de rived growth factor stimulation of p42/p44 mitogen-activated protein kinase . Second, transfection of cells with inhibitory G-protein a subunit increas ed the activation of p42/p44 mitogen-activated protein kinase by platelet-d erived growth factor. Third, platelet-derived growth factor stimulated the tyrosine phosphorylation of the inhibitory G-protein a subunit, which was b locked by the platelet-derived growth factor kinase inhibitor, tyrphostin A G 1296. We have also shown that the platelet-derived growth factor beta rec eptor forms a tethered complex with Myc-tagged endothelial differentiation gene 1 (a G-protein-coupled receptor whose agonist is sphingosine I-phospha te) in cells co-transfected with these receptors. This facilitates platelet -derived growth factor-stimulated tyrosine phosphorylation of the inhibitor y G-protein a subunit and increases p42/p44 mitogen-activated protein kinas e activation. In addition, we found that G-protein-coupled receptor kinase 2 and beta -arrestin I can associate with the platelet-derived growth facto r beta receptor. These proteins play an important role in regulating endocy tosis of G-protein-coupled receptor signal complexes, which is required for activation of p42/p44 mitogen-activated protein kinase. Thus, platelet-der ived growth factor beta receptor signaling may be initiated by G-protein-co upled receptor kinase 2/beta -arrestin I that has been recruited to the pla telet-derived growth factor beta receptor by its tethering to a G-protein-c oupled receptor(s). These results provide a model that may account for the co-mitogenic effect of certain G-protein-coupled receptor agonists with pla telet-derived growth factor on DNA synthesis.