CONVERTING PARATHYROID HORMONE-RELATED PEPTIDE (PTHRP) INTO A POTENT PTH-8 RECEPTOR AGONIST

Most of the bone and kidney-related functions of parathyroid hormone ( PTH) and parathyroid hormone-related peptide (PTHrP) are thought to be mediated by the PTH/PTHrP receptor. Recently, a homologous receptor, the PTH-2 receptor, was obtained from rat and human brain cDNA librari es. This receptor displayed the remarkable property of responding pote ntly to PTH, but not to PTHrP. To begin to define residues involved in the ligand specificity of the PTH-2 receptor, we studied the interact ion of several PTH/PTHrP hybrid ligands and other related peptide anal ogs with the human PTH-2 receptor The results showed that two sites in PTH and PTHrP fully account for the different potencies that the tu o ligands exhibited with PTH-2 receptors; residue 5 (His in PTHrP and I le in PTH) determined signaling capability, while residue 23 (Phe in P THrP and Trp in PTH) determined binding affinity. By changing these tw o residues of PTHrP to the corresponding residues of PTH, we were able to convert PTHrP into a ligand that avidly bound to the PTH-2 recepto r and fully and potently stimulated cAMP formation, Changing residue 2 3 alone yielded [Trp(23)]hPTHrP-(1-36), which was an antagonist for th e PTH-2 receptor, but a fall agonist for the PTH/PTHrP receptor. Resid ues 5 and 23 in PTH and PTHrP thus play keg roles in signaling and bin ding interactions, respectively, with the PTH-2 receptor. Receptor-sel ective agonists and antagonists derived hom these studies could help t o identify the biological role of the PTH-2 receptor and to map specif ic sites of ligand-receptor interaction.