title: Tuberoinfundibular peptide (7-39) [TIP(7-39)], a novel, selective, high-affinity antagonist for the parathyroid hormone-1 receptor with no detectable agonist activity.
abstract: The parathyroid hormone (PTH)-1 receptor mediates the pathophysiological effects of PTH in hyperparathyroidism and PTH-related protein (PTHrP) in humoral hypercalcemia of malignancy. A PTH1 receptor antagonist may be of therapeutic utility in these disorders. We recently identified a novel antagonist, tuberoinfundibular peptide (7-39) [TIP(7-39)], derived from the likely endogenous ligand for the PTH2 receptor TIP39. In this study its in vitro profile is evaluated and compared with that of [D-Trp(12),Tyr(34)]bPTH(7-34) and PTHrP(7-34), representing the two previously known structural classes of PTH1 receptor antagonists. TIP(7-39) binds with higher affinity (6.2 nM) to the PTH1 receptor than [D-Trp(12),Tyr(34)]bPTH(7-34) (45 nM) and PTHrP(7-34) (65 nM) and displays a 5.5-fold greater PTH1/PTH2 receptor selectivity. TIP(7-39) does not stimulate cAMP accumulation via the PTH1 receptor [in a sensitive assay that detects the activity of the weak partial agonist [Nle(8,18),Tyr(34)]bPTH(3-34)] and does not increase intracellular calcium. Schild analysis for TIP(7-39) was consistent with purely competitive antagonism of PTH(1-34)'s stimulation of cAMP accumulation (slope = 0.99 +/- 0.24). The pK(B) for TIP(7-39) (7.1 +/- 0.3) was higher than that for [D-Trp(12),Tyr(34)]bPTH(7-34) (6.5 +/- 0.0) and PTHrP(7-34) (6.0 +/- 0.1). Binding of (125)I-TIP(7-39) to the PTH1 receptor could be measured (K(D) = 1.3 +/- 0.1 nM, B(max) = 1.3 +/- 0.1 pmol/mg), whereas binding of (125)I-[Nle(8,18),D-Trp(12),Tyr(34)]bPTH(7-34) could not be detected. Kinetic analysis indicated that (125)I-TIP(7-39) dissociates much more slowly (t(1/2) = 14 min) than [D-Trp(12),Tyr(34)]bPTH(7-34) (13 s) and PTHrP(7-34) (9 s). The novel antagonist TIP(7-39) therefore displays a more favorable in vitro pharmacological profile than antagonists derived from PTH and PTHrP and may be useful for demonstrating the utility of PTH1 receptor antagonism in the treatment of hypercalcemia.
authors.affiliation: Unit on Cell Biology, Laboratory of Genetics, National Institute of Mental Health, Bethesda, Maryland, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Hoare
authors.forename: T B
authors.initials: TB
authors.lastname: Usdin
mesh_terms: D000818:Animals; D019556:COS Cells / Q000187:drug effects / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002417:Cattle; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D009479:Neuropeptides / Q000378:metabolism / Q000494:pharmacology*; D010281:Parathyroid Hormone / Q000494:pharmacology; D044162:Parathyroid Hormone-Related Protein; D010446:Peptide Fragments / Q000378:metabolism / Q000494:pharmacology*; D011506:Proteins / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D044183:Receptor, Parathyroid Hormone, Type 2; D018016:Receptors, Parathyroid Hormone / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000378:metabolism; D014162:Transfection
chemical_list: D007457:Iodine Radioisotopes; D009479:Neuropeptides; C470121:PTHLH protein, human; D010281:Parathyroid Hormone; D044162:Parathyroid Hormone-Related Protein; D010446:Peptide Fragments; D011506:Proteins; D044183:Receptor, Parathyroid Hormone, Type 2; D018016:Receptors, Parathyroid Hormone; C400136:tuberoinfundibular peptide 39; D000242:Cyclic AMP; D002118:Calcium