Parathyroid Hormone (PTH) and PTH-Related Peptide Domains Contributing to Activation of Different PTH Receptor–Mediated Signaling Pathways

Abstract

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP), acting through the osteoblast PTH1 receptor (PTH1R), play important roles in bone remodeling. Intermittent administration of PTH(1-34) (teriparatide) leads to bone formation, whereas continuous administration paradoxically leads to bone resorption. Activation of PTH1R promotes regulation of multiple signaling pathways, including G s /cAMP/protein kinase A, G q /calcium/protein kinase C, b-arrestin recruitment, and extracellular signal-related kinase (ERK)1/2 phosphorylation, as well as receptor internalization, but their role in promoting anabolic and catabolic actions of PTH(1-34) are unclear. In the present investigation, a collection of PTH(1-34) and PTHrP(1-34) peptide analogs were evaluated in orthogonal human PTH1R (hPTH1R) functional assays capturing G s -and G q -signaling, b-arrestin recruitment, ERK1/2 phosphorylation, and receptor internalization to further define the patterns of PTH1R signaling that they stimulate and further establish peptide domains contributing to agonist activity. Results indicate that both Nand C-terminal domains of PTH and PTHrP are critical for activation of signaling pathways. However, modifications of both regions lead to more substantial decreases in agonist potency and efficacy to stimulate G q -signaling, b-arrestin recruitment, ERK1/2 phosphorylation, and receptor internalization than to stimulate G s -signaling. The substantial contribution of the peptide C-terminal domain in activation of hPTH1R signaling suggests a role in positioning of the peptide N-terminal region into the receptor J-domain. Several PTH and PTHrP peptides evaluated in this study promote different patterns of biased agonist signaling and may serve as useful tools to further elucidate therapeutically relevant PTH1R signaling in osteoblasts. With a better understanding of therapeutically relevant signaling, novel biased peptides with desired signaling could be designed for safer and more effective treatment of osteoporosis.

International Union of Basic and Clinical Pharmacology. XCIII. The Parathyroid Hormone Receptors—Family B G Protein–Coupled Receptors

“…In a previous study, a PTH(7-34) peptide failed to stimulate bone formation when tested in animals (Sebastian et al, 2008). More recent studies using plate reader technologies to assess PTHR1 signaling via multiple pathways including ERK-1/2 and b-arrestin recruitment did not confirm that ,Tyr 34 ]bovine PTH(7-34)NH 2 behaves as an ERK-1/2-selective biased agonist in several types including HEK-293, CHO, and the human osteoblastic cell line U2OS (Cupp et al, 2013a;van der Lee et al, 2013). Whether the differences in the findings involve differences in the assay formats or animal or cell systems used remain to be sorted out and thus seem uncertain whether or not the PTH(7-34) scaffold will be a key to the development of therapeutically useful biased agonists for the PTHR1.…”

Ligand Residence Time at G-protein–Coupled Receptors—Why We Should Take Our Time To Study It

“…In this context, different receptor conformations resulting from the interaction of structurally different ligands might account for ligand functional bias, but it is not clear to what extent ligandreceptor binding/unbinding kinetics also might play a role. Some examples of functional bias on GPCRs include that of the PTH1R, for which peptide ligands with different patterns of biased signaling have been described (Gesty-Palmer et al, 2009;Cupp et al, 2013). Human parathyroid hormone (PTH) and PTH-related protein (PTHrP), the two endogenous agonists of PTH1R, elicit different effects on the renal synthesis of 1,25(OH) 2 vitamin D and, therefore, on hypercalcemia in humans in continuous infusion (Horwitz et al, 2005), a dose regimen that discards differences in pharmacokinetics as the only explanation for the discordant effects.…”

Parathyroid Hormone-Related Protein Analogs as Osteoporosis Therapies

“…On the other hand, it normally acts in an autocrine, paracrine, and even intracrine manner (e.g., to modulate cell growth and cell differentiation) in many normal tissues [2,4]. Thus, considering the PTHrP homology with PTH in its N-terminal region, it is not surprising that PTHrP is a relevant ligand of the common PTH type 1 receptor (PTH1R) in PTH target cells [4][5][6]. Current evidence, though, points to the existence of PTH1R-unrelated receptors specific for PTHrP domains distal to its N-terminus in osteoblasts and other cell types, although their identification remains to be established [7][8][9].…”

“…The PTH1R is a member of the family B class of G protein-coupled receptors signaling through multiple pathways, including classical cAMP/protein kinase A (PKA) and phospholipase C (PLC) b/intracellular calcium/protein kinase C (PKC) activation cascades [5,6]. The domain responsible in PTH and PTHrP for targeting both adenylate cyclase and PLC b resides in the first two amino acids, while another PLC-independent PKC-activating domain covers amino acids 28-34.…”