Selective and Nonselective Inverse Agonists for Constitutively Active Type-1 Parathyroid Hormone Receptors: Evidence for Altered Receptor Conformations

Abstract The spontaneous signaling activity of some G protein-coupled receptors and the capacity of certain ligands (inverse agonists) to inhibit such constitutive activity are poorly understood phenomena. We investigated these processes for several analogs of PTH-related peptide (PTHrP) and the constitutively active human PTH/PTHrP receptors (hP1Rcs) hP1Rc-H223R and hP1Rc-T410P. The N-terminally truncated antagonist PTHrP(5-36) functioned as a weak partial/neutral agonist with both mutant receptors but was converted to an inverse agonist for both receptors by the combined substitution of Leu11 and d-Trp12. The N-terminally intact analog[ Bpa2]PTHrP(1–36)—a partial agonist with the wild-type hP1Rc—was a selective inverse agonist, in that it depressed basal cAMP signaling by hP1Rc-H223R but enhanced signaling by hP1Rc-T410P. The ability of [Bpa2]PTHrP(1–36) to discriminate between the two receptor mutants suggested that H223R and T410P confer constitutive receptor activity by inducing distinct conformational changes. This hypothesis was confirmed by the observations that: 1) the double mutant receptor hP1Rc-H223R/T410P exhibited basal cAMP levels that were 2-fold higher than those of either single mutant; and 2) hP1Rc-H223R and hP1Rc-T410P internalized 125I-PTHrP(5–36) to markedly different extents. The overall results thus reveal that two different types of inverse agonists are possible for PTHrP ligands (nonselective and selective) and that constitutively active PTH-1 receptors can access different conformational states.