Mutation of phenylalanine-34 of parathyroid hormone disrupts NHERF1 regulation of PTH type I receptor signaling

β-Arrestin-biased signaling of PTH analogs of the type 1 parathyroid hormone receptor

Parathyroid hormone (PTH) is an anabolic agent that mediates bone formation through activation of the Gα(s)-, Gα(q)- and β-arrestin-coupled parathyroid hormone receptor type 1 (PTH1R). Pharmacological evidence based on the effect of PTH(7-34), a PTH derivative that is said to preferentially activate β-arrestin signaling through PTH1R, suggests that PTH1R-activated β-arrestin signaling mediates anabolic effects on bone. Here, we performed a thorough evaluation of PTH(7-34) signaling behaviour using quantitative assays for β-arrestin recruitment, Gα(s)- and Gα(q)-signaling. We found that PTH(7-34) inhibited PTH-induced cAMP accumulation, but was unable to induce β-arrestin recruitment, PTH1R internalization and ERK1/2 phosphorylation in HEK293, CHO and U2OS cells. Thus, the β-arrestin bias of PTH(7-34) is not apparent in every cell type examined, suggesting that correlating in vivo effects of PTH(7-34) to in vitro pharmacology should be done with caution.

Arrestin scaffolds NHERF1 to the P2Y12 receptor to regulate receptor internalization

We have recently shown in a patient with mild bleeding that the PDZ-binding motif of the platelet G protein-coupled P2Y(12) receptor (P2Y(12)R) is required for effective receptor traffic in human platelets. In this study we show for the first time that the PDZ motif-binding protein NHERF1 exerts a major role in potentiating G protein-coupled receptor (GPCR) internalization. NHERF1 interacts with the C-tail of the P2Y(12)R and unlike many other GPCRs, NHERF1 interaction is required for effective P2Y(12)R internalization. In vitro and prior to agonist stimulation P2Y(12)R/NHERF1 interaction requires the intact PDZ binding motif of this receptor. Interestingly on receptor stimulation NHERF1 no longer interacts directly with the receptor but instead binds to the receptor via the endocytic scaffolding protein arrestin. These findings suggest a novel model by which arrestin can serve as an adaptor to promote NHERF1 interaction with a GPCR to facilitate effective NHERF1-dependent receptor internalization.

Beta-arrestin-dependent parathyroid hormone-stimulated extracellular signal-regulated kinase activation and parathyroid hormone type 1 receptor internalization

PTH regulates renal calcium homeostasis by actions on the distal nephron. PTH-induced calcium transport in mouse distal convoluted tubule (DCT) cells requires activation of ERK1/2. ERK activation by beta-adrenergic receptors occurs in a biphasic manner and involves receptor internalization. An early rapid phase is beta-arrestin (betaAr) independent, whereas prolonged activation is betaAr dependent. We characterized PTH-stimulated ERK activation and the involvement of receptor internalization and betaAr dependence. In DCT cells, PTH transiently activated ERK maximally at 5 min and then returned to baseline. betaAr dependence of PTH receptor (PTH1R)-mediated ERK stimulation was assessed using mouse embryonic fibroblasts (MEFs) from betaAr1- and -2-null mice. In wild-type MEFs, PTH(1-34)-stimulated ERK activation peaked after 5 min, was 50% maximal after 15 min, and then recovered to 80% of maximal stimulation by 30 min. In MEFs null for betaAr1 and -2, PTH-stimulated ERK activation peaked by 5 min and returned to baseline. The effect was identical in betaAr2-null MEFs. In betaAr1-null MEFs, ERK exhibited delayed activation and remained elevated. PTH-stimulated ERK activation and receptor endocytosis were not inhibited by the clathrin-binding domain of betaAr1 [Ar(319-418)]. Coexpression of the sodium proton exchanger regulatory factor 1 (NHERF1) with Ar(319-418) blocked PTH1R internalization. We conclude that PTH-stimulated ERK activation in DCT cells proceeds with a rapid but transient phase that may involve betaAr1. Furthermore, the betaAr-dependent late phase of ERK activation by PTH requires the participation of betaAr2 and PTH1R internalization.