Parathyroid hormone-related protein for the treatment of postmenopausal osteoporosis: defining the maximal tolerable dose

CONTEXT

PTH is the only approved skeletal anabolic agent for the treatment of human osteoporosis. Unlike PTH, which is a mixed anabolic and catabolic agent, PTHrP displays features suggesting that it may be a pure anabolic agent when intermittently administered. The full dose range of PTHrP is unknown.

OBJECTIVES

The primary objective of the study was to define the complete therapeutic window and dose-limiting toxicities of PTHrP. The secondary objective was to determine whether PTHrP retains a pure anabolic profile at the highest usable doses.

DESIGN

This was a single-blinded, two-part, dose-escalating clinical trial.

SETTING

The study was conducted in a university academic setting.

PATIENTS OR OTHER PARTICIPANTS

Participants included 41 healthy postmenopausal women between the ages of 45 and 75 yr.

INTERVENTION

INTERVENTIONs included PTHrP(1-36) or placebo in a dose-escalating design for 3 wk.

MAIN OUTCOME MEASURES

Safety measures (hypercalcemia, nausea, vomiting, hemodynamics, flushing, miscellaneous) and bone turnover markers were measured.

RESULTS

Intermittent PTHrP was administered safely and without serious adverse events in subjects receiving 500 and 625 microg/d for 3 wk. Subjects receiving 750 microg/d developed mild hypercalcemia. Bone turnover markers suggested that even at the highest doses, daily sc PTHrP may not activate bone resorption, i.e. may be purely anabolic. Interestingly, when hypercalcemia occurred, it may have resulted not from bone resorption but from activation of intestinal calcium absorption by 1,25 dihydroxyvitamin D.

CONCLUSIONS

In doses as high as 750 microg/d, in contrast to PTH, intermittently administered PTHrP appears to act as a pure skeletal anabolic agent. Surprisingly, PTHrP in the high doses studied activates 1,25 dihydroxyvitamin D production. Dosing information obtained herein can be used to design a longer term head-to-head comparative efficacy trial of PTHrP vs. PTH.

Endogenous parathyroid hormone-related protein regulates the expression of PTH type 1 receptor and proliferation of vascular smooth muscle cells

The PTH type 1 receptor (PTH1R) and PTHrP are expressed in vessels, where they contribute to regulating vascular smooth muscle cell (VSMC) function. Elevated PTHrP levels in VSMC are often associated with hyperplasia. In contrast, exogenous PTHrP, acting through the PTH1R, inhibits VSMC proliferation. In this study, we investigated the regulation of PTH1R expression by endogenous PTHrP and the associated effects on VSMC proliferation. Blocking binding of secreted PTHrP fragments to the PTH1R by treatment with either an antagonist or an antibody against PTHrP, and inhibition of PTHrP expression by small interfering RNA significantly increased PTH1R expression. Interestingly, treatment of the cells with a PTHrP analog (Bpa(1)-PTHrP) that activates the PTH1R without inducing its internalization had the same effect on receptor expression. To examine the association between receptor expression and the antiproliferative effect of N-terminal fragments of PTHrP, VSMC were treated with exogenous PTHrP (1-36) acutely and chronically to induce receptor down-regulation. Stimulation of VSMC with exogenous PTHrP (1-36) significantly reduced cell proliferation during the first 18 h of treatment but was no longer effective after 3 d, a time when PTH1R was down-regulated. In contrast, treatment with the noninternalizing agonist Bpa(1)-PTHrP strongly inhibited cell proliferation at all time points. In conclusion, our study show that PTHrP, after its intracellular processing and secretion, promotes down-regulation of the PTH1R in VSMC, thereby regulating cell proliferation in an auto/paracrine fashion. This regulatory mechanism may have important implication during vascular remodeling, in particular in the development of neointima after arterial injury, where PTHrP overexpression occurs.

Regulation of parathyroid hormone type 1 receptor dynamics, traffic, and signaling by the Na+/H+ exchanger regulatory factor-1 in rat osteosarcoma ROS 17/2.8 cells

The effects of the expression of the Na+/H+ exchanger regulatory factor-1 (NHERF1) on the distribution, dynamics, and signaling properties of the PTH type 1 receptor (PTH1R) were studied in rat osteosarcoma cells ROS 17/2.8. NHERF1 had a dramatic effect on the subcellular distribution of PTH1R, promoting a substantial relocation of the receptor to regions of the plasma membrane located in very close proximity to cytoskeletal fibers. Direct interactions of NHERF1 with the PTH1R and the cytoskeleton were required for these effects, because they were abolished by 1) PTH1R mutations that impair NHERF1 binding, and 2) NHERF1 mutations that impair binding to the PTH1R or the cytoskeleton. NHERF1 reduced significantly the diffusion of the PTH1R by a mechanism that was also dependent on a direct association of NHERF1 with the PTH1R and the cytoskeleton. NHERF1 increased ligand-dependent production of cAMP and induced ligand-dependent rises in intracellular calcium. These effects on calcium were due to increased calcium uptake, as they were blocked by calcium channel inhibitors and by the addition of EGTA to the medium. These calcium effects were abolished by protein kinase A inhibition but phospholipase C inhibition was without effect. Based on these analyses, we propose that, in ROS cells, the presence of NHERF1 induces PTH-dependent calcium signaling by a cAMP-mediated mechanism that involves local protein kinase A-dependent activation of calcium channels.

NHERF1 Regulates Parathyroid Hormone Receptor Membrane Retention without Affecting Recycling

Na/H exchange regulatory factor-1 (NHERF1) is a PDZ protein that regulates trafficking of several G protein-coupled receptors. The phenotype of NHERF1-null mice suggests that the parathyroid hormone (PTH) receptor (PTH1R) is the principal GPCR interacting with NHERF1. The effect of NHERF1 on receptor recycling is unknown. Here, we characterized NHERF1 effects on PTH1R membrane tethering and recycling by radio-ligand binding and recovery after maximal receptor endocytosis. Using Chinese hamster ovary cells expressing the PTH1R, where NHERF1 expression could be induced by tetracycline, NHERF1 inhibited PTH1R endocytosis and delayed PTH1R recycling. NHERF1 also inhibited PTH-induced receptor internalization in MC4 osteoblast cells. Reducing constitutive NHERF1 levels in HEK-293 cells with short hairpin RNA directed against NHERF1 augmented PTH1R endocytosis in response to PTH. Mutagenesis of the PDZ-binding domains or deletion of the MERM domain of NHERF1 demonstrated that both are required for inhibition of endocytosis and recycling. Likewise, an intact COOH-terminal PDZ recognition motif in PTH1R is needed. The effect of NHERF1 on receptor internalization and recycling was not associated with altered receptor expression or binding, activation, or phosphorylation but involved beta-arrestin and dynamin. We conclude that NHERF1 inhibits endocytosis without affecting PTH1R recycling in MC4 and PTH1R-expressing HEK-293 cells. Such an effect may protect against PTH resistance or PTH1R down-regulation in certain cells harboring NHERF1.

Cellular mechanism through which parathyroid hormone-related protein induces proliferation in arterial smooth muscle cells: definition of an arterial smooth muscle PTHrP/p27kip1 pathway

Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27(kip1). Adenoviral restoration of p27(kip1) fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27(kip1). In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27(kip1) expression and accelerates cell cycle progression. p27(kip1) is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27(kip1) pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury.

Caveolin-1 regulates cellular trafficking and function of the glucagon-like Peptide 1 receptor

The glucagon-like peptide 1 receptor (GLP-1R) mediates important effects on beta-cell function and glucose homeostasis and is one of the most promising therapeutic targets for type 2, and possibly type 1, diabetes. Yet, little is known regarding the molecular and cellular mechanisms that regulate its function. Therefore, we examined the cellular trafficking of the GLP-1R and the relation between receptor localization and signaling activity. In resting human embryonic kidney 293 and insulinoma MIN6 cells, a fully functional green fluorescent protein-tagged GLP-1R was localized both at the cell membrane and in highly mobile intracellular compartments. Real-time confocal fluorescence microscopy allowed direct visualization of constitutive cycling of the receptor. Overexpression of K44A-dynamin increased the number of functional receptors at the cell membrane. Immunoprecipitation, sucrose sedimentation, and microscopy observations demonstrated that the GLP-1R localizes in lipid rafts and interacts with caveolin-1. This interaction is necessary for membrane localization of the GLP-1R, because overexpression of a dominant-negative form of caveolin-1 (P132L-cav1) or specific mutations within the putative GLP-1R's caveolin-1 binding domain completely inhibited GLP-1 binding and activity. Upon agonist stimulation, the GLP-1R underwent rapid and extensive endocytosis independently from arrestins but in association with caveolin-1. Finally, GLP-1R-stimulated activation of ERK1/2, which involves transactivation of epidermal growth factor receptors, required lipid raft integrity. In summary, the interaction of the GLP-1R with caveolin-1 regulates subcellular localization, trafficking, and signaling activity. This study provides further evidence of the key role of accessory proteins in specifying the cellular behavior of G protein-coupled receptors.

Induction of leptin resistance through direct interaction of C-reactive protein with leptin

The mechanisms underlying leptin resistance are still being defined. We report here the presence in human blood of several serum leptin-interacting proteins (SLIPs), isolated by leptin-affinity chromatography and identified by mass spectrometry and immunochemical analysis. We confirmed that one of the major SLIPs is C-reactive protein (CRP). In vitro, human CRP directly inhibits the binding of leptin to its receptors and blocks its ability to signal in cultured cells. In vivo, infusion of human CRP into ob/ob mice blocked the effects of leptin upon satiety and weight reduction. In mice that express a transgene encoding human CRP, the actions of human leptin were completely blunted. We also found that physiological concentrations of leptin can stimulate expression of CRP in human primary hepatocytes. Recently, human CRP has been correlated with increased adiposity and plasma leptin. Thus, our results suggest a potential mechanism contributing to leptin resistance, by which circulating CRP binds to leptin and attenuates its physiological functions.

Safety and tolerability of subcutaneous PTHrP(1-36) in healthy human volunteers: a dose escalation study

Parathyroid hormone-related protein (PTHrP) is an anabolic skeletal agent in mice, rats and humans. In previous studies, we have demonstrated that PTHrP can be administered to osteoporotic postmenopausal women at a dose of 6.56 microg/kg/day (or approximately 400 microg/day) for 3 months to yield a 4.7% increase in lumbar spine BMD. This regimen was free of hypercalcemia or adverse effects. Moreover, PTHrP appeared to stimulate bone formation selectively, without stimulating bone resorption. This efficacy in the absence of adverse effects, as well as the apparent "pure anabolic" action of PTHrP, prompted us to attempt to define the complete therapeutic window for PTHrP. In this study, we gradually escalated the dose of PTHrP(1-36) from 9 to 28 microg/kg (or approximately 570 microg to 1,946 microg) administered as a single subcutaneous dose to 22 healthy young adult subjects. PTHrP(1-36) was well tolerated even at the highest dose, just under 2.0 mg, some five times higher than we have previously demonstrated to be effective in increasing bone mass, and some 100 times higher than the maximal approved dose of PTH(1-34). Despite the large dose of PTHrP, the highest serum calcium achieved was 10.6 mg/dl, and this was observed in only one subject at the highest dose. The mean serum calcium in subjects receiving the highest dose was 9.6 mg/dl. Only one subject experienced adverse symptoms/signs, and this was at the highest dose. We conclude that subcutaneous PTHrP(1-36) is safe when administered in single doses approaching 2.0 mg. These findings indicate that the therapeutic window for PTHrP(1-36) in humans is wide and permit the design and implementation of longer safety and efficacy trials.

Continuous PTH and PTHrP infusion causes suppression of bone formation and discordant effects on 1,25(OH)2 vitamin D

Osteoblast activity and plasma 1,25(OH)2 vitamin D are increased in HPT but suppressed in HHM. To model HPT and HHM, we directly compared multiday continuous infusions of PTH versus PTHrP in humans. Continuous infusion of both PTH and PTHrP results in marked and prolonged suppression of bone formation; renal 1,25(OH)2D synthesis was stimulated effectively by PTH but poorly by PTHrP.

INTRODUCTION

PTH and PTH-related protein (PTHrP) cause primary hyperparathyroidism (HPT) and humoral hypercalcemia of malignancy (HHM), respectively. Whereas HHM and HPT resemble one another in many respects, osteoblastic bone formation and plasma 1,25(OH)2 vitamin D are increased in HPT but reduced in HHM.

MATERIALS AND METHODS

We performed 2- to 4-day continuous infusions of escalating doses of PTH and PTHrP in 61 healthy young adults, comparing the effects on serum calcium and phosphorus, renal calcium and phosphorus handling, 1,25(OH)2 vitamin D, endogenous PTH(1-84) concentrations, and plasma IGF-1 and markers of bone turnover.

RESULTS

PTH and PTHrP induced comparable effects on renal calcium and phosphorus handling, and both stimulated IGF-1 and bone resorption similarly. Surprisingly, PTH was consistently more calcemic, reflecting a selectively greater increase in renal 1,25(OH)2 vitamin D production by PTH. Equally surprisingly, continuous infusion of both peptides markedly, continuously, and equivalently suppressed bone formation.

CONCLUSIONS

PTHrP and PTH produce markedly different effects on 1,25(OH)2 vitamin D homeostasis in humans, leading to different calcemic responses. Moreover, both peptides produce profound suppression of bone formation over multiple days, contrasting with events in HPT, but mimicking HHM. These findings underscore the facts that the mechanisms underlying the anabolic skeletal response to PTH and PTHrP in humans is poorly understood, as are the signal transduction mechanisms that link the renal PTH receptor to 1,25(OH)2 vitamin D synthesis. These studies emphasize that much remains to be learned regarding the normal regulation of vitamin D metabolism and bone formation in response to PTH and PTHrP in humans.

Parathyroid hormone receptor trafficking contributes to the activation of extracellular signal-regulated kinases but is not required for regulation of cAMP signaling

Agonist-mediated activation of the type 1 parathyroid hormone receptor (PTH1R) results in several signaling events and receptor endocytosis. It is well documented that arrestins contribute to desensitization of both G(s)- and G(q)-mediated signaling and mediate PTH1R internalization. However, whether PTH1R trafficking directly contributes to signaling remains unclear. To address this question, we investigated the role of PTH1R trafficking in cAMP signaling and activation of extracellular signal-regulated kinases ERK1/2 in HEK-293 cells. Dominant negative forms of dynamin (K44A-dynamin) and beta-arrestin1 (beta-arrestin1-(319-418)) abrogated PTH1R internalization but had no effect on cAMP signaling; neither acute cAMP production by PTH nor desensitization and resensitization of cAMP signaling were affected. Therefore, PTH1R trafficking is not necessary for regulation of cAMP signaling. PTH-(1-34) induced rapid and robust activation of ERK1/2. A PTHrP-based analog ([p-benzoylphenylalanine1, Ile5,Arg(11,13),Tyr36]PTHrP-(1-36)NH2), which selectively activates the G(s)/cAMP pathway without inducing PTH1R endocytosis, failed to stimulate ERK1/2 activity. Inhibition of PTH1R endocytosis by K44A-dynamin dampened ERK1/2 activation in response to PTH-(1-34) by 69%. Incubation with the epidermal growth factor receptor inhibitor AG1478 reduced ERK1/2 phosphorylation further. In addition, ERK1/2 phosphorylation occurred following internalization of a PTH1R mutant induced by PTH-(7-34) in the absence of G protein signaling. Collectively, these data indicate that PTH1R trafficking and G(q) (but not G(s)) signaling independently contribute to ERK1/2 activation, predominantly via transactivation of the epidermal growth factor receptor.

Ligand-selective dissociation of activation and internalization of the parathyroid hormone (PTH) receptor: conditional efficacy of PTH peptide fragments

G protein-coupled receptors (GPCRs) mediate the action of many hormones, cytokines, and sensory and chemical signals. It is generally thought that receptor desensitization and internalization require occupancy and activation of the GPCR. PTH and PTHrP receptor (PTH1R) belongs to GPCR class B and is the major regulator of extracellular calcium homeostasis. Using kidney distal convoluted tubule cells transfected with a human PTH1R/enhanced green fluorescent protein fusion protein, quantitative, real-time fluorescence microscopy was used to analyze receptor internalization. In these cells, which are the target of the calcium-sparing action of PTH, PTH(1-34) activated adenylyl cyclase (AC) and phospholipase C (PLC) and PTH1R endocytosis. PTH(1-31), however, stimulated AC and PLC but not PTH1R endocytosis. Conversely, PTH(7-34) rapidly stimulated PTH1R internalization without activating AC or PLC. PTH(2-34) and (3-34) caused PTH1R internalization intermediate between PTH(1-34) and (7-34). PTH1R sequestration occurred in a dynamin- and clathrin-dependent manner. Directly activating AC inhibited PTH1R internalization in response to PTH(7-34). PTH1R endocytosis was sensitive to protein kinase C inhibition. PTH(1-34), (7-34), and (1-31) evoked PTH1R phosphorylation. Removal of most of the C terminus of the PTH1R eliminated receptor phosphorylation and the cAMP/protein kinase C sensitivity of internalization. PTH(1-34) and (7-34) internalized the truncated PTH1R with identical kinetics, and the response was unaffected by forskolin. Thus, the PTH1R C terminus contains regulatory sequences that are involved in, but not required for, PTH1R internalization. The results demonstrate that receptor activation and internalization can be selectively dissociated.

Agonist-specific regulation of parathyroid hormone (PTH) receptor type 2 activity: structural and functional analysis of PTH- and tuberoinfundibular peptide (TIP) 39-stimulated desensitization and internalization

The human PTH receptor type 2 (PTH2R) is activated by PTH and tuberoinfundibular peptide of 39 residues (TIP39), resulting in cAMP and intracellular Ca signaling. We now report that, despite these similarities, PTH and TIP39 elicit distinct responses from PTH2R. First, TIP39 induced beta-arrestin and protein kinase Cbeta mobilization and receptor internalization, whereas PTH did not. However, PTH stimulated trafficking of these molecules for a chimeric PTH2R containing the N terminus and third extracellular loop of PTH receptor type 1 (PTH1R). Second, whereas PTH-stimulated cAMP activity was brief and rapidly resensitized, the response to TIP39 was sustained and partly desensitized for a prolonged period. PTH2R desensitization was mediated by beta-arrestin interaction with the C terminus (amino acids 426-457) of PTH2R, whereas beta-arrestin mobilization had a minor influence on PTH2R internalization in response to TIP39, as shown with C terminus deletion mutants and/or dominant negative forms of beta-arrestin and dynamin. These data contrast with PTH1R, at which these dominant negative mutants markedly inhibited receptor internalization. Collectively, these results further highlight how specific interactions within the ligand-receptor bimolecular complex mediate distinct postactivation responses of class II G protein- coupled receptors and provide novel insights into the physiological regulation of PTH2R activity.

Conformational and biological characterization of human parathyroid hormone hPTH(1-34) analogues containing beta-amino acid residues in positions 17-19

The N-terminal 1-34 fragment of parathyroid hormone (PTH) elicits the full spectrum of bone-related biological activities of the intact native sequences. It has been suggested that the structural elements essential for bioactivity are two helical segments located at the N-terminal and C-terminal sequences, connected by hinges or flexible points around positions 12 and 19. In order to assess the relevance of the local conformation around Gly(18) upon biological function, we synthesized and characterized the following human (h) PTH(1-34) analogues containing beta-amino acid residues: [analogues: see text]. Biological activity and binding affinity of analogue I are one order of magnitude lower than those of the parent compound. In analogue II, both binding affinity and biological activity are partially recovered. Analogues III and V have no binding affinity and very low biological activity. Both bioactivity and binding affinity are partially recovered in analogue IV. The conformational properties of the analogues in aqueous solution containing dodecylphosphocholine micelles were studied by CD, 2D-nuclear magnetic resonance and molecular dynamics calculations. The results confirmed the presence in all analogues of two helical segments located at the N-terminal and C-terminal sequences. The insertion of beta-amino acid residues around position 18 does not cause appreciable conformational differences in the five analogues. The differences in biological activity and binding affinity among the five analogues cannot be related to structural differences in the membrane mimetic environment reported in this study. Our results stress the importance of the side-chain functionalities in the sequence 17-19 for biological function.

Activation-independent parathyroid hormone receptor internalization is regulated by NHERF1 (EBP50)

Parathyroid hormone (PTH) regulates extracellular calcium homeostasis through the type 1 PTH receptor (PTH1R) expressed in kidney and bone. The PTH1R undergoes beta-arrestin/dynamin-mediated endocytosis in response to the biologically active forms of PTH, PTH-(1-34), and PTH-(1-84). We now show that amino-truncated forms of PTH that do not activate the PTH1R nonetheless induce PTH1R internalization in a cell-specific pattern. Activation-independent PTH1R endocytosis proceeds through a distinct arrestin-independent mechanism that is operative in cells lacking the adaptor protein Na/H exchange regulatory factor 1 (NHERF1) (ezrin-binding protein 50). Using a combination of radioligand binding experiments and quantitative, live cell confocal microscopy of fluorescently tagged PTH1Rs, we show that in kidney distal tubule cells and rat osteosarcoma cells, which lack NHERF1, the synthetic antagonist PTH-(7-34) and naturally circulating PTH-(7-84) induce internalization of PTH1R in a beta-arrestin-independent but dynamin-dependent manner. Expression of NHERF1 in these cells inhibited antagonist-induced endocytosis. Conversely, expression of dominant-negative forms of NHERF1 conferred internalization sensitivity to PTH-(7-34) in cells expressing NHERF1. Mutation of the PTH1R PDZ-binding motif abrogated interaction of the receptor with NHERF1. These mutated receptors were fully functional but were now internalized in response to PTH-(7-34) even in NHERF1-expressing cells. Removing the NHERF1 ERM domain or inhibiting actin polymerization allowed otherwise inactive ligands to internalize the PTH1R. These results demonstrate that NHERF1 acts as a molecular switch that legislates the conditional efficacy of PTH fragments. Distinct endocytic pathways are determined by NHERF1 that are operative for the PTH1R in kidney and bone cells.

Selective ligand-induced stabilization of active and desensitized parathyroid hormone type 1 receptor conformations

For many G protein-coupled receptors, agonist-induced activation is followed by desensitization, internalization, and resensitization. In most cases, these processes are dependent upon interaction of agonist-occupied receptor with cytoplasmic beta-arrestins. The ligand-induced intramolecular rearrangements of the receptor responsible for the desensitized versus active conformational states, which dictate both the pharmacological properties of ligands and the biological activity of G protein-coupled receptors, have not been fully elucidated. Here, we identify specific interactions between parathyroid hormone (PTH)-related protein and the human PTH type 1 receptor (PTH1Rc) and the related receptor conformational changes that lead to beta-arrestin-2-mediated desensitization. PTH-related protein analogs modified at position 1 induced selective stabilization of the active G protein-coupled state of the receptor, resulting in lack of beta-arrestin-2 recruitment to the cell membrane, sustained cAMP signaling, and absence of ligand-receptor complex internalization. Mechanistically, the ligands modified at position 1, interacting with the extracellular end of helix VI of PTH1Rc, produced a translocation of transmembrane helices V and VI that differed from that induced by the cognate agonist, resulting in significantly different conformations of the third intracellular loop. These results show how specific interactions between PTH1Rc and its ligands may stabilize distinct conformational states, representing either the active G protein-coupled or a desensitized beta-arrestin-coupled receptor state. In addition, they establish that sustained biological activity of PTH1Rc may be induced by appropriately designed agonist ligands.

Structure-function studies of analogues of parathyroid hormone (PTH)-1-34 containing beta-amino acid residues in positions 11-13

The 1-34 N-terminal fragments of human parathyroid hormone (PTH) and PTH-related protein (PTHrP) elicit the full spectrum of bone-relevant activities characteristic of the intact hormones. The structural elements believed to be required for receptor binding and biological activity are two helical segments, one N-terminal and one C-terminal, connected by hinges or flexible points located around positions 12 and 19. To test this hypothesis, we synthesized and characterized the following analogues of PTH-(1-34), each containing single or double substitutions with beta-amino acid residues around the putative hinge located at position 12: I. [Nle(8,18),beta-Ala(11,12),Nal(23),Tyr(34)]bPTH-(1-34)NH(2); II. [Nle(8,18),beta-Ala(12,13),Nal(23),Tyr(34)]bPTH-(1-34)NH(2); III. [Nle(8,18),beta-Ala(11),Nal(23),Tyr(34)]bPTH-(1-34)NH(2); IV. [Nle(8,18),beta-hLeu(11),Nal(23),Tyr(34)]bPTH-(1-34)NH(2); V. [Nle(8,18),beta-Ala(12), Nal(23),Tyr(34)]bPTH-(1-34)NH(2); VI. [Nle(8,18),beta-Ala(13), Nal(23),Tyr(34)]bPTH-(1-34)NH(2) (beta-hLeu = beta-homo-leucine; beta-Ala = beta-alanine; Nal = L-2-naphthyl-alanine; Nle = norleucine). Analogues I and III exhibit very low binding affinity and are devoid of adenylyl cyclase activity. Analogue II, despite its very low binding capacity is an agonist. Biological activity and binding capacity are partially restored in analogue IV, and completely restored in analogues V and VI. The conformational properties of the analogues were investigated in aqueous solution containing dodecylphosphocholine (DPC) micelles as a membrane-mimetic environment using CD, 2D-NMR, and molecular dynamics calculations. All peptides fold partially into the alpha-helical conformation in the presence of DPC micelles, with a maximum helix content in the range of 30-35%. NMR analysis reveals the presence of two helical segments, one N-terminal and one C-terminal, as a common structural motif in all analogues. Incorporation of beta-Ala dyads at positions 11,12 and 12,13 in analogues I and II, respectively, enhances the conformational disorder in this portion of the sequence but also destabilizes the N-terminal helix. This could be one of the possible reasons for the lack of biological activity in these analogues. The partial recovery of binding affinity and biological activity in analogue IV, compared to the structurally similar analogue III, is clearly the consequence of the reintroduction of Leu side-chain of the native sequence. In the fully active analogues V and VI, the helix stability at the N-terminus is further increased. Taken together, these results stress the functional importance of the conformational stability of the helical activation domain in PTH-(1-34). Contrary to expectation, insertion of a single beta-amino acid residue in positions 11, 12, or 13 in analogues III-VI does not favor a disordered structure in this portion of the sequence.

Aib-rich peptides containing lactam-bridged side chains as models of the 3(10)-helix

Aib-rich side chain lactam-bridged oligomers with n =1, 2, 3, were designed and synthesized as putative models of the 3(10)-helix. These peptides were conformationally characterized in aqueous solution containing SDS micelles by CD, NMR, and computer simulations. The lactam bridge between the side chains of L-Glu and L-Lys in (i) and (i+3) positions was introduced in order to enhance the conformational preference toward the right-handed 3(10)-helix. The NMR results clearly indicate that there is an increase of 3(10)-helix formation upon chain elongation. In the dimer and trimer (n = 2 and n = 3, respectively, in the structure reported above) the observed NOE connectivities are compatible with the 3(10)-helical arrangement, confirmed by the temperature coefficients of the amide proton resonances which suggest the presence of a hydrogen-bonded structure. The phi and psi dihedral angles of the structures obtained by molecular dynamics calculations are also compatible with the 3(10)-helix. Identification of the hydrogen-bond pattern indicate that C=O(i)- - -HN(i+3) hydrogen bonds, typical of the 3(10)-helical conformation, are highly probable in all low-energy structures. The CD spectra of these Aib-rich lactam-bridged oligopeptides, obtained in the same solvent system used for NMR experiments, provide important insight into the spectroscopic characteristics of the 3(10)-helix.

Cellular distribution of constitutively active mutant parathyroid hormone (PTH)/PTH-related protein receptors and regulation of cyclic adenosine 3',5'-monophosphate signaling by beta-arrestin2

PTH promotes endocytosis of human PTH receptor 1 (PTH1Rc) by activating protein kinase C and recruiting beta-arrestin2. We examined the role of beta-arrestin2 in regulating the cellular distribution and cAMP signaling of two constitutively active PTH1Rc mutants, H223R and T410P. Overexpression of a beta-arrestin2-green fluorescent protein (GFP) conjugate in COS-7 cells inhibited constitutive cAMP accumulation by H223R and T410P in a dose-dependent manner, as well as the response to PTH of both mutant and wild-type PTH1Rcs. The cellular distribution of PTH1Rc-GFP conjugates, fluorescent ligands, and ssarrestin2-GFP was analyzed by fluorescence microscopy in HEK-293T cells. In cells expressing either receptor mutant, a ligand-independent mobilization of beta-arrestin2 to the cell membrane was observed. In the absence of ligand, H223R and wild-type PTH1Rcs were mainly localized on the cell membrane, whereas intracellular trafficking of T410P was also observed. While agonists promoted beta-arrestin2-mediated endocytosis of bot PTH1Rc mutants, antagonists were rapidly internalized only with T410P. The protein kinases inhibitor, staurosporine, significantly decreased internalization of ligand-PTH1Rc mutant complexes, although the recruitment of beta-arrestin2 to the cell membrane was unaffected. Moreover, in cells expressing a truncated wild-type PTH1Rc lacking the C-terminal cytoplasmic domain, agonists stimulated translocation of beta-arrestin2 to the cell membrane followed by ligand-receptor complex internalization without associated beta-arrestin2. In conclusion, cAMP signaling by constitutively active mutant and wild-type PTH1Rcs is inhibited by a receptor interaction with beta-arrestin2 on the cell membrane, possibly leading to uncoupling from G(s)alpha. This phenomenon is independent from protein kinases activity and the receptor C-terminal cytoplasmic domain. In addition, there are differences in the cellular localization and internalization features of constitutively active PTH1Rc mutants H223R and T410P.

Mapping the bimolecular interface of the parathyroid hormone (PTH)-PTH1 receptor complex: spatial proximity between Lys(27) (of the hormone principal binding domain) and leu(261) (of the first extracellular loop) of the human PTH1 receptor

In an effort to characterize the bimolecular interface between parathyroid hormone (PTH) and its human receptor PTH1-Rc (hPTH1-Rc), we previously identified two contact sites in the receptor: one for position 1 and another for position 13 (located at the ends of the principal activation domain) in PTH(1-34). The present study reports a third, novel "contact site" between hPTH1-Rc and Lys(27) of PTH(1-34). Lys(27) is located in the principal binding domain of the hormone (residues 25-34). The photoreactive PTH(1-34) analogue K27 contains a benzophenone (BP) moiety on Lys(27). The analogue binds to stably transfected HEK 293/C-21 cells (which express a high level of recombinant hPTH1-Rc) and stimulates adenylyl cyclase activity with a potency similar to PTH(1-34). In addition, (125)I-K27 cross-links effectively and specifically to the hPTH1-Rc. Enzymatic (Glu-C and Lys-C) and chemical (CNBr and BNPS-skatole) digestions of the photoconjugate between (125)I-K27 and hPTH1-Rc were performed. In addition, photoconjugates involving the bioactive mutants [L261M]- and [R262K]-hPTH1-Rc, transiently expressed in COS-7 cells, were also digested. The data obtained clearly identify L(261) or R(262) of the first extracellular loop of hPTH1-Rc as the contact site for Lys(27) in the hormone. On the basis of (i) the similarity in molecular mass between the CNBr digest of the (125)I-K27-[L261M]hPTH1-Rc conjugate and free (125)I-K27 and (ii) the failure to cross-link (125)I-K27 to a bioactive mutant receptor [L261A]hPTH1-Rc, we conclude that L(261) is the cross-linking site. These results provide the first demonstration of an interaction between the principal binding domain of PTH and the first extracellular loop of hPTH1-Rc. Revealing proximity of Lys(27) (in PTH) to L(261) (in hPTH1-Rc) provides additional insight into the nature of the ligand-receptor bimolecular interface and clearly illustrates that the extracellular loops of the receptor contribute to the specificity of the PTH-PTH1-Rc interaction. Taken together with previous studies, the new findings add important constraints on the possible positioning of the C-terminal helix of PTH (which contains the principal binding domain) relative to the first extracellular loop and the distal C-terminal helix of the large extracellular amino terminal domain of the PTH1-Rc.

Characterization of parathyroid hormone/receptor interactions: structure of the first extracellular loop

The structural features of the first extracellular loop (ECL1) of the parathyroid hormone receptor (PTH1R) in the presence of dodecylphosphocholine micelles have been determined using high-resolution NMR techniques. The structure of the receptor fragment, PTH1R(241-285), includes three alpha-helices for residues 241-244, 256-264, and 275-284. The first and third correspond to the end and the beginning of transmembrane helices 2 and 3, respectively. Centrally located in the second helix is L(261), found to cross-link to Lys(27) of parathyroid hormone, PTH(1-34) [Greenberg, Z., Bisello, A., Mierke, D. F., Rosenblatt, M., and Chorev, M. (2000) Biochemistry 39, 8142-8152]. On the basis of nitroxide radical-induced relaxation studies, the central helix is found to associate with the surface of the membrane mimetic. These data, in conjunction with previous results indicating a preference of PTH for the lipid surface, suggest a membrane-associated pathway for the initial recognition and binding of PTH to its G-protein-coupled receptor. Using the structural features of ECL1 as determined here, along with the structure of the PTH(1-34), the intermolecular interactions consistent with the contact point between L(261)(receptor)-Lys(27)(ligand) are identified.

3-(3'-fluorenyl-9'-oxo)-L-alanine: a novel photoreactive conformationally constrained amino acid

Photoaffinity scanning of the ligand-G-protein-coupled receptor bimolecular interface is a direct approach to mapping the interactions of ligands and receptors. Such studies are an important first step toward generating an experimentally based model of the ligand-receptor complex. The synthesis and spectroscopic characterization of Boc-3-(3'-fluorenyl-9'-oxo)-L-alanine and 9-fluorenone-3-carboxylic acid are described. Incorporation of these two photophores into the parathyroid hormone (PTH) molecule yields potent agonists. These photoreactive analogs cross-link specifically with the recombinant human PTH1 receptor stably expressed in human embryonic kidney cells. The availability of the 9-fluorenone (a conformationally constrained derivative of benzophenone, the abundantly used photophore) for photoaffinity scanning provides an important tool to probe the effect of conformational flexibility of the photophore on the selection of the cross-linking site in the macromolecular acceptor.

A role for N-cadherin in the development of the differentiated osteoblastic phenotype

Cadherins are a family of cell surface adhesion molecules that play an important role in tissue differentiation. A limited repertoire of cadherins has been identified in osteoblasts, and the role of these molecules in osteoblast function remains to be elucidated. We recently cloned an osteoblast-derived N-cadherin gene from a rat osteoblast complementary DNA library. After in situ hybridization of rat bone and immunohistochemistry of human osteophytes, N-cadherin expression was localized prominently in well-differentiated (lining) osteoblasts. Northern blot hybridization in primary cultures of fetal rat calvaria and in human SaOS-2 and rat ROS osteoblast-like cells showed a relationship between N-cadherin messenger RNA expression and cell-to-cell adhesion, morphological differentiation, and alkaline phosphatase and osteocalcin gene expression. Treatment with a synthetic peptide containing the His-Ala-Val (HAV) adhesion motif of N-cadherin significantly decreased bone nodule formation in primary cultures of fetal rat calvaria and inhibited cell-to-cell contact in rat osteoblastic TRAB-11 cells. HAV peptide also regulated the expression of specific genes such as alkaline phosphatase and the immediate early gene zif268 in SaOS-2 cells. Transient transfection of SaOS-2 cells with a dominant-negative N-cadherin mutant (NCADdeltaC) significantly inhibited their morphological differentiation. In addition, aggregation of NCTC cells derived from mouse connective tissue stably transfected with osteoblast-derived N-cadherin was inhibited by either treatment with HAV or transfection with NCADdeltaC. Together, these results strongly support a role for N-cadherin, in concert with other previously identified osteoblast cadherins, in the late stages of osteoblast differentiation.

Photoaffinity cross-linking identifies differences in the interactions of an agonist and an antagonist with the parathyroid hormone/parathyroid hormone-related protein receptor

Analogs of parathyroid hormone (PTH)-related protein (PTHrP), singularly substituted with a photoreactive L-p-benzoylphenylalanine (Bpa) at each of the first 6 N-terminal positions, were pharmacologically evaluated in human embryonic kidney cells stably expressing the recombinant human PTH/PTHrP receptor. Two of these analogs, in which the photoreactive residue is either in position 1 or 2 (Bpa(1)- and Bpa(2)-PTHrP, respectively) displayed high affinity binding. Bpa(1)-PTHrP also displayed high efficacy for the stimulation of increased cAMP levels. Surprisingly, Bpa(2)-PTHrP was found to be a potent antagonist, despite the presence of the principal activation domain (sequence 1-6). Analysis of the digestion profiles of the ligand-receptor photoconjugates revealed that both the agonist and the antagonist cross-link to the S-CH(3) group of Met(425) in transmembrane domain 6 of the human PTH/PTHrP receptor. However, the antagonist Bpa(2)-PTHrP also cross-links to a proximal site within the receptor domain Pro(415)-Met(425). Unlike the antagonist Bpa(2)-PTHrP, the potent agonist Bpa(2)-PTH, also bearing the Bpa residue in position 2, cross-links only to the S-CH(3) group of Met(425) (similar to Bpa(1)-PTHrP and Bpa(1)-PTH). Taken together, these results suggest that the antagonist Bpa(2)-PTHrP is able to distinguish between two distinct conformations of the receptor. The comparison between PTHrP analogs substituted by Bpa at two consecutive positions and across PTH and PTHrP reveals insights into the PTH/PTHrP ligand-receptor bimolecular interaction at the level of a single amino acid.

Endocytosis of ligand-human parathyroid hormone receptor 1 complexes is protein kinase C-dependent and involves beta-arrestin2. Real-time monitoring by fluorescence microscopy

Endocytosis and intracellular trafficking of the human parathyroid hormone receptor subtype 1 (hPTH1-Rc) and its ligands was monitored independently by real-time fluorescence microscopy in stably transfected HEK-293 cells. Complexes of fluorescence-labeled parathyroid hormone (PTH)-(1-34) agonist bound to the hPTH1-Rc internalized rapidly at 37 degrees C via clathrin-coated vesicles, whereas fluorescent PTH-(7-34) antagonist-hPTH1Rc complexes did not. A functional C terminus epitope-tagged receptor (C-Tag-hPTH1-Rc) was immunolocalized to the cell membrane and, to a lesser extent, the cytoplasm. PTH and PTH-related protein agonists stimulated C-Tag-hPTH1-Rc internalization. Relocalization to the cell membrane occurred 1 h after removal of the ligand. Endocytosis of fluorescent PTH agonist-hPTH1-Rc complexes was blocked by the protein kinase C (PKC) inhibitor staurosporine but not by the specific protein kinase A inhibitor N-(2-(methylamino)ethyl)-5-isoquinoline-sulfonamide. Fluorescent PTH antagonist-hPTH1-Rc complexes were rapidly internalized after PKC activation by phorbol 12-myristate 13-acetate or thrombin, but not after stimulation of the cAMP/protein kinase A pathway by forskolin. In cells co-expressing the hPTH1-Rc and a green fluorescent protein-beta-arrestin2 fusion protein (beta-Arr2-GFP), PTH agonists stimulated beta-Arr2-GFP mobilization to the cell membrane. Subsequently, fluorescent PTH-(1-34)-hPTH1Rc complexes and beta-Arr2-GFP co-localized intracellularly. In conclusion, agonist-activated hPTH1-Rc internalization involves beta-arrestin mobilization and targeting to clathrin-coated vesicles. Our results also indicate that receptor occupancy, rather than receptor-mediated signaling, is necessary, although not sufficient, for endocytosis of the hPTH1-Rc. Activation of PKC, however, is absolutely required.