The 10th and 11th residues of short length N-terminal PTH(1-12) analogue are important for its optimum potency

Inactivation of parathyroid hormone: perspectives of drug discovery to combating hyperparathyroidism

Hormonal coordination is tightly regulated within the human body and thus regulates human physiology. The parathyroid hormone (PTH), a member of the endocrine system, regulates the calcium and phosphate level within the human body. Under non-physiological conditions, PTH levels get upregulated (hyperparathyroidism) or downregulated (hypoparathyroidism) due to external or internal factors. In the case of hyperparathyroidism, elevated PTH stimulates cellular receptors present in the bones, kidneys, and intestines to increase the blood calcium level, leading to calcium deposition. This eventually causes various symptoms including kidney stones. Currently, there is no known medication that directly targets PTH in order to suppress its function. Therefore, it is of great interest to find novel small molecules or any other means that can modulate PTH function. The molecular signaling of PTH starts by binding of its N-terminus to the G-protein coupled PTH1/2 receptor. Therefore, any intervention that affects the N-terminus of PTH could be a lead candidate for treating hyperparathyroidism. As a proof-of-concept, there are various possibilities to inhibit molecular PTH function by (i) a small molecule, (ii) N-terminal PTH phosphorylation, (iii) fibril formation and (iv) residue-specific mutations. These modifications put PTH into an inactive state, which will be discussed in detail in this review article. We anticipate that exploring small molecules or other means that affect the N-terminus of PTH could be lead candidates in combating hyperparathyroidism.

Small Molecule Inhibited Parathyroid Hormone Mediated cAMP Response by N-Terminal Peptide Binding

Ligand binding to certain classes of G protein coupled receptors (GPCRs) stimulates the rapid synthesis of cAMP through G protein. Human parathyroid hormone (PTH), a member of class B GPCRs, binds to its receptor via its N–terminal domain, thereby activating the pathway to this secondary messenger inside cells. Presently, GPCRs are the target of many pharmaceuticals however, these drugs target only a small fraction of structurally known GPCRs (about 10%). Coordination complexes are gaining interest due to their wide applications in the medicinal field. In the present studies we explored the potential of a coordination complex of Zn(II) and anthracenyl–terpyridine as a modulator of the parathyroid hormone response. Preferential interactions at the N–terminal domain of the peptide hormone were manifested by suppressed cAMP generation inside the cells. These observations contribute a regulatory component to the current GPCR–cAMP paradigm, where not the receptor itself, but the activating hormone is a target. To our knowledge, this is the first report about a coordination complex modulating GPCR activity at the level of deactivating its agonist. Developing such molecules might help in the control of pathogenic PTH function such as hyperparathyroidism, where control of excess hormonal activity is essentially required.

N-terminal phosphorylation of parathyroid hormone (PTH) abolishes its receptor activity

The parathyroid hormone (PTH) is an 84-residue peptide, which regulates the blood Ca(2+) level via GPCR binding and subsequent activation of intracellular signaling cascades. PTH is posttranslationally phosphorylated in the parathyroid glands; however, the functional significance of this processes is not well characterized. In the present study, mass spectrometric analysis revealed three sites of phosphorylation, and NMR spectroscopy assigned Ser1, Ser3, and Ser17 as modified sites. These sites are located at the N-terminus of the hormone, which is important for receptor recognition and activation. NMR shows further that the three phosphate groups remotely disturb the α-helical propensity up to Ala36. An intracellular cAMP accumulation assay elucidated the biological significance of this phosphorylation because it ablated the PTH-mediated signaling. Our studies thus shed light on functional implications of phosphorylation at native PTH as an additional level of regulation.

Peptide-peptoid hybrids based on (1-11)-parathyroid hormone analogs

A series of peptide-peptoid hybrids, containing N-substituted glycines, were synthesized based on the H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH(2) (Har = Homoarginine) as the parent parathyroid hormone (1-11) analog. The compounds were pharmacologically characterized in their agonistic activity at the parathyroid hormone 1 receptor.

Spectroscopic techniques applied to the study of laminin fragments inserted into model membranes

The influence of four laminin-derived peptides on bilayer organization is studied. Spectroscopic methods applied were based on pyrene fluorescence properties (quenching, I1/I3, and monomer/excimer equilibrium), asymmetric membrane fluorescence (NBD-PE/dithionite), and polarization fluorescence (TMA-DPH). Also, the ability of these peptides to release carboxyfluorescein entrapped in vesicles was determined. Results suggest that these peptides do not noticeably modify the packing and motion of lipids (in the gel state), but coat its surface, preventing penetration of quenchers and chemical reactants. Nevertheless, their presence promotes a soft release of entrapped CF after incubation at 37 degrees C.

Novel bioactive parathyroid hormone and related peptides in teleost fish

We report the identification, gene expression and biological activity of two parathyroid hormones (PTH; PTHA and PTHB), two PTH-related peptides (PTHrP; PTHrPA and PTHrPB) and a PTH-like ligand (PTH-L) with hybrid characteristics in puffer fishes (Takifugu rubripes and Tetraodon fluviatilis). Experimental data are consistent with PTH-L and PTHrPA having calciotropic activities equivalent, respectively, to tetrapod PTH and PTHrP. We hypothesise on the basis of phylogenetic and functional analysis that PTH-L could be a fish relic of an ancestral PTH/PTHrP gene.

Bioactive N-terminal undecapeptides derived from parathyroid hormone: the role of alpha-helicity

The N-terminal 1-34 segment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses in bone characteristic of the native intact PTH. Recent studies have demonstrated that N-terminal fragments presenting the principal activating domain such as PTH(1-11) and PTH(1-14) with helicity-enhancing substitutions yield potent analogues with PTH(1-34)-like activity. To further investigate the role of alpha-helicity on biological potency, we designed and synthesized by solid-phase methodology the following hPTH(1-11) analogues substituted at positions 1 and/or 3 by the sterically hindered and helix-promoting C(alpha)-tetrasubstituted alpha-amino acids alpha-amino isobutyric acid (Aib), 1-aminocyclopentane-1-carboxylic acid (Ac(5)c) and 1-aminocyclohexane-1-carboxylic acid (Ac(6)c): Ac(5)c-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (I); Aib-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (II); Ac(6)c-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (III); Aib-V-Ac(6)c-E-I-Q-L-M-H-Q-R-NH(2) (IV); Aib-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (V); S-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (VI), S-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (VII); Ac(5)c-V-S-E-I-Q-L-M-H-Q-R-NH(2) (VIII); Ac(6)c-V-S-E-I-Q-L-M-H-Q-R-NH(2) (IX); Ac(5)c-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (X); Ac(6)c-V-Ac(6)c-E-I-Q-L-M-H-Q-R-NH(2) (XI). All analogues were biologically evaluated and conformationally characterized in 2,2,2-trifluoroethanol (TFE) solution by circular dichroism (CD). Analogues I-V, which cover the full range of biological activity observed in the present study, were further conformationally characterized in detail by nuclear magnetic resonance (NMR) and computer simulations studies. The results of ligand-stimulated cAMP accumulation experiments indicated that analogues I and II are active, analogues III, VI and VII are very weakly active and analogues IV, V, VIII-XI are inactive. The most potent analogue, I exhibits biological activity 3500-fold higher than that of the native PTH(1-11) and only 15-fold weaker than that of the native sequence hPTH(1-34). Remarkably, the two most potent analogues, I and II, and the very weakly active analogues, VI and VII, exhibit similar helix contents. These results indicate that the presence of a stable N-terminal helical sequence is an important but not sufficient condition for biological activity.