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The parathyroid hormone receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Parathyroid Hormone Receptors [49]) are class B G protein-coupled receptors. The parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor (PTH1 receptor) is activated by precursor-derived peptides: PTH (84 amino acids), and PTHrP (141 amino-acids) and related peptides (PTH-(1-34), PTHrP-(1-36)). The parathyroid hormone 2 receptor (PTH2 receptor) is activated by the precursor-derived peptide TIP39 (39 amino acids). [125I]PTH may be used to label both PTH1 and PTH2 receptors. The structure of a long-active PTH analogue (LA-PTH, an hybrid of PTH-(1-13) and PTHrP-(14-36)) bound to the PTH1 receptor-Gs complex has been resolved by cryo-electron microscopy [147]. Another structure of a PTH-(1-34) analog bound to a thermostabilized inactive PTH1 receptor has been obtained with X-ray crytallography [34].

The parathyroid hormone receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Parathyroid Hormone Receptors [49]) are class B G protein-coupled receptors. The parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor (PTH1 receptor) is activated by precursor-derived peptides: PTH (84 amino acids), and PTHrP (141 amino-acids) and related peptides (PTH-(1-34), PTHrP-(1-36)). The parathyroid hormone 2 receptor (PTH2 receptor) is activated by the precursor-derived peptide TIP39 (39 amino acids). [125I]PTH may be used to label both PTH1 and PTH2 receptors. The structure of a long-active PTH analog (LA-PTH, an hybrid of PTH-(1-13) and PTHrP-(14-36)) bound to the PTH1 receptor-Gs complex has been resolved by cryo-electron microscopy [147]. Another structure of a PTH-(1-34) analog bound to a thermostabilized inactive PTH1 receptor has been obtained with X-ray crytallography [34].

Prolonged signaling at the parathyroid hormone receptor 1 (PTHR1) correlates with the capacity of a ligand to bind to a G protein‐independent receptor conformation (R0). As long‐acting PTH (LA‐PTH) ligands hold interest as potential treatments for hypoparathyroidism (HP), we explored the structural basis in the ligand for stable R0 binding and prolonged cAMP signaling. A series of PTH/PTHrP hybrid analogs were synthesized and tested for actions in vitro and in vivo. Of the series, [Ala1,3,12,Gln10,Arg11,Trp14]‐PTH(1‐14)/PTHrP(15–36) (M‐PTH/PTHrP) bound with high affinity to R0, induced prolonged cAMP responses in UMR106 rat osteoblast‐derived cells, and induced the most prolonged increases in serum calcium (sCa) in normal rats. Daily s.c. injection of M‐PTH/PTHrP into thyroparathyroidectomized (TPTX) rats, a model of HP, normalized sCa without raising urine Ca. In contrast, oral alfacalcidol, a widely used treatment for HP, normalized sCa, but induced frank hypercalciuria. M‐PTH/PTHrP exhibited low solubility in aqueous solutions of neutral pH; however, replacement of Leu18, Phe22, and His26 with the less hydrophobic residues, Ala, Ala, and Lys, at those respective positions markedly improved solubility while maintaining bioactivity. Indeed, we recently showed that the resultant analog [Ala18,22,Lys26]‐M‐PTH/PTHrP or LA‐PTH, effectively normalizes sCa in TPTX rats and mediates prolonged actions in monkeys. These studies provide useful information for optimizing PTH and PTHrP ligand analogs for therapeutic development. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

The parathyroid hormone receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Parathyroid Hormone Receptors [47]) are family B G protein-coupled receptors. The parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor (PTH1 receptor) is activated by precursor-derived peptides: PTH (84 amino acids), and PTHrP (141 amino-acids) and related peptides (PTH-(1-34), PTHrP-(1-36)). The parathyroid hormone 2 receptor (PTH2 receptor) is activated by the precursor-derived peptide TIP39 (39 amino acids). [125I]PTH may be used to label both PTH1 and PTH2 receptors.

We examined mechanisms of down-regulation of PTH receptors and desensitization of the PTH-stimulated increase in intracellular cAMP in clonal rat osteosarcoma cells, ROS 17/2.8. ROS cells treated with 10 nM [Nle8,Nle18,Tyr34] bovine (b) PTH-(1-34) amide (NlePTH) for 3 days showed loss of specific PTH binding and PTH-stimulated cAMP accumulation to 10% of that in vehicle-treated control cells. Treatment of these cells with both 0.5 mM 8-bromo-cAMP (8-Br-cAMP) and 1 mM methylisobutylxanthine or 100 ng/ml cholera toxin for 3 days elicited no change in either of these responses. Treatment with 10 nM NlePTH for 3 days did not modify the cAMP accumulation stimulated by 30 microM forskolin or 1 micrograms/ml cholera toxin, indicating that agonist-specific desensitization of PTH-stimulated cAMP accumulation is not due to diminished activity of either the stimulatory guanyl nucleotide regulatory subunit (Gs) or the catalytic subunit of the adenylate cyclase. Treatment of ROS cells with pertussis toxin (PT; 10 ng/ml) for 12, 24, 48, and 72 h increased specific PTH binding by 21%, 28%, 35%, and 39%. The increase in PTH binding was associated with a parallel increase in PTH-stimulated cAMP accumulation and was due to an increase in the number of PTH receptors. PTH receptor affinity remained constant (apparent Kd = 0.3 nM). PT treatment of the cells partially blocked agonist-specific PTH receptor down-regulation. PT catalyzed ADP ribosylation of 41K and 39K membrane proteins, consistent with the alpha-subunits of Gi and Go, respectively. In conclusion, agonist-induced PTH receptor down-regulation in ROS 17/2.8 cells is cAMP independent and can be reversed by PT treatment. PTH receptor expression in these cells appears to be under tonic inhibitory control by mechanisms involving a PT-sensitive G protein(s).

PTH binds to specific receptors that are coupled to adenylate cyclase and activate cAMP-dependent protein kinase. Since it has been shown that PTH activates phospholipid inositol metabolism, we investigated whether PTH influences protein kinase-C (PKC) activity in rat osteosarcoma (ROS) cells 17/2.8 that contain a large number of PTH receptor. Incubation of ROS cells with PTH or phorbol 12-myristate 13-acetate (PMA) for 1-30 min caused a rapid and transient decrease in PKC activity in the cytosol, which was associated with a transient increase in PKC activity in the membrane fraction. After 1, 5, 15, and 30 min of incubation with PTH, cytosolic PKC activity decreased to 57%, 74%, 84%, and 93% of the control value, whereas membrane PKC activity increased to 156%, 122%, 111%, and 106% of the control value, respectively. After PMA treatment for 1, 5, 15, and 30 min, cytosolic PKC activity decreased by 81%, 74%, 63%, and 44%, whereas membrane-bound PKC activity increased by 83%, 44%, 28%, and 17%, respectively. The effects of PTH and PMA on PKC were dose dependent, with ED50 values of 0.3 nM PTH and 4 nM PMA. Chronic treatment of ROS cells for 3 days with PMA caused depletion of total PKC activity in cytosolic and membrane fractions to less than 10% of that in control cells. Conversely, chronic treatment of ROS cells with PTH did not deplete PKC. In addition, chronic treatment of ROS cells with PTH inhibited the responsiveness of PKC activity to subsequent acute PTH challenge, but not to acute PMA challenge, suggesting specific desensitization of this response by PTH. Activation of cytosolic PKC by diolein, phosphatidylserine, and calcium caused phosphorylation of many cytosolic proteins, including those having apparent mol wt of 39K, 35K, 33K, 25K, 19K, and 16K. Pretreatment of ROS cells with PTH resulted in a transient decrease in the phosphorylation of these cytosolic proteins by PKC. This decrease in cytosolic protein phosphorylation by treatment with PTH is temporally associated with PTH-stimulated translocation of PKC activity from the cytosol to the membranes. These data suggest a potential role for PKC in the mechanism of action of PTH in ROS cells.