Loss-of-function of EBP50 is a new cause of hereditary peripheral neuropathy: EBP50 functions in peripheral nerve system

Finding causative genetic mutations is important in the diagnosis and treatment of hereditary peripheral neuropathies. This study was conducted to find new genes involved in the pathophysiology of hereditary peripheral neuropathy. We identified a new mutation in the EBP50 gene, which is co-segregated with neuropathic phenotypes, including motor and sensory deficit in a family with Charcot-Marie-Tooth disease. EBP50 is known to be important for the formation of microvilli in epithelial cells, and the discovery of this gene mutation allowed us to study the function of EBP50 in the nervous system. EBP50 was strongly expressed in the nodal and paranodal regions of sciatic nerve fibers, where Schwann cell microvilli contact the axolemma, and at the growth tips of primary Schwann cells. In addition, EBP50 expression was decreased in mouse models of peripheral neuropathy. Knockout mice were used to study EBP50 function in the peripheral nervous system. Interestingly motor function deficit and abnormal histology of nerve fibers were observed in EBP50^+/- heterozygous mice at 12 months of age, but not 3 months. in vitro studies using Schwann cells showed that NRG1-induced AKT activation and migration were significantly reduced in cells overexpressing the I325V mutant of EBP50 or cells with knocked-down EBP50 expression. In conclusion, we show for the first time that loss of function due to EBP50 gene deficiency or mutation can cause peripheral neuropathy.

Site-specific polyubiquitination differentially regulates parathyroid hormone receptor-initiated MAPK signaling and cell proliferation

G protein-coupled receptor (GPCR) signaling and trafficking are essential for cellular function and regulated by phosphorylation, β-arrestin, and ubiquitination. The GPCR parathyroid hormone receptor (PTHR) exhibits time-dependent reversible ubiquitination. The exact ubiquitination sites in PTHR are unknown, but they extend upstream of its intracellular tail. Here, using tandem MS, we identified Lys³⁸⁸ in the third loop and Lys⁴⁸⁴ in the C-terminal tail as primary ubiquitination sites in PTHR. We found that PTHR ubiquitination requires β-arrestin and does not display a preference for β-arrestin1 or -2. PTH stimulated PTHR phosphorylation at Thr³⁸⁷/Thr³⁹² and within the Ser⁴⁸⁹-Ser⁴⁹³ region. Such phosphorylation events may recruit β-arrestin, and we observed that chemically or genetically blocking PTHR phosphorylation inhibits its ubiquitination. Specifically, Ala replacement at Thr³⁸⁷/Thr³⁹² suppressed β-arrestin binding and inhibited PTHR ubiquitination, suggesting that PTHR phosphorylation and ubiquitination are interdependent. Of note, Lys-deficient PTHR mutants promoted normal cAMP formation, but exhibited differential mitogen-activated protein kinase (MAPK) signaling. Lys-deficient PTHR triggered early onset and delayed ERK1/2 signaling compared with wildtype PTHR. Moreover, ubiquitination of Lys³⁸⁸ and Lys⁴⁸⁴ in wildtype PTHR strongly decreased p38 signaling, whereas Lys-deficient PTHR retained signaling comparable to unstimulated wildtype PTHR. Lys-deficient, ubiquitination-refractory PTHR reduced cell proliferation and increased apoptosis. However, elimination of all 11 Lys residues in PTHR did not affect its internalization and recycling. These results pinpoint the ubiquitinated Lys residues in PTHR controlling MAPK signaling and cell proliferation and survival. Our findings suggest new opportunities for targeting PTHR ubiquitination to regulate MAPK signaling or manage PTHR-related disorders.

Abstract 261: EBP50/NHERF-1 Regulates Basal and TNFα-induced Mitochondrial Dynamics in Vascular Smooth Muscle Cells

Mitochondrial dysfunction has been associated with the phenotypic switch of VSMC and vascular disease. Changes in mitochondrial dynamics (fission and fusion) are linked to VSMC proliferation and metabolism during vascular remodeling. Mitochondrial fission and fusion are regulated by several key molecules, including dynamin-related protein 1 (Drp1) and mitofusin2 (Mfn2). We previously showed that scaffolding Ezrin-radixin-moesin binding phosphoprotein of 50 kDa (EBP50/NHERF1) increases inflammatory responses and proliferation of VSMC. These actions are mediated by the activation of PKCζ under inflammatory stimuli and the stabilization of the S-phase kinase associated protein 2 (Skp2), a component of an E3 ligase that promotes proliferation. Thus, EBP50 knockout (EBP50 -/- ) mice are protected neointimal hyperplasia following arterial injury. Here we test the hypothesis that EBP50 regulates mitochondrial dynamics and responses to inflammatory stimuli in vascular smooth muscle cells (VSMC). We found that EBP50 knockdown, by decreasing Skp2 levels, increased FoxO1 stability and nuclear localization leading to higher mitofusin-2 (Mfn-2) expression. In contrast, inhibition of FoxO1 reduced Mfn2 levels. High resolution morphological analysis with both TEM and confocal microscopy revealed that mitochondria were more elongated in EBP50 -/- than WT VSMC. In WT VSMC, TNFα induced PKCζ-mediated phosphorylation of Drp1. In contrast, EBP50 -/- VSMC exhibited significantly reduced Drp1 phosphorylation following TNFα treatment. Live-cell 3D imaging followed by morphological analysis showed that TNFα elicited rapid and significantly greater mitochondria fragmentation in WT compared to EBP50 -/- VSMC. Finally, EBP50 -/- VSMC exhibited lower extracellular acidification rate (ECAR) than WT VSMC consistent with their lower proliferation. Collectively, these findings delineate a new mechanism of regulation of mitochondrial dynamics by the scaffolding protein EBP50 in response to inflammatory stimuli. Therefore EBP50 can be viewed as a potential therapeutic target for vascular proliferative diseases.

Origins of PDZ Binding Specificity. A Computational and Experimental Study Using NHERF1 and the Parathyroid Hormone Receptor

Na⁺/H⁺ exchanger regulatory factor-1 (NHERF1) is a scaffolding protein containing two PSD95/discs large protein/ZO1 (PDZ) domains that modifies the signaling, trafficking, and function of the parathyroid hormone receptor (PTHR), a family B G-protein-coupled receptor. PTHR and NHERF1 bind through a PDZ-ligand-recognition mechanism. We show that PTH elicits phosphorylation of Thr591 in the canonical -ETVM binding motif of PTHR. Conservative substitution of Thr591 with Cys does not affect PTH(1-34)-induced cAMP production or binding of PTHR to NHERF1. The findings suggested the presence of additional sites upstream of the PDZ-ligand motif through which the two proteins interact. Structural determinants outside the canonical NHERF1 PDZ-PTHR interface that influence binding have not been characterized. We used molecular dynamics (MD) simulation to predict residues involved in these interactions. Simulation data demonstrate that the negatively charged Glu side chains at positions -3, -5, and -6 upstream of the PDZ binding motif are involved in PDZ-PTHR recognition. Engineered mutant peptides representing the PTHR C-terminal region were used to measure the binding affinity with NHERF1 PDZ domains. Comparable micromolar affinities for peptides of different length were confirmed by fluorescence polarization, isothermal titration calorimetry, and surface plasmon resonance. Binding affinities measured for Ala variants validate MD simulations. The linear relation between the change in enthalpy and entropy following Ala substitutions at upstream positions -3, -5, and -6 of the PTHR peptide provides a clear example of the thermodynamic compensation rule. Overall, our data highlight sequences in PTHR that contribute to NHERF1 interaction and can be altered to prevent phosphorylation-mediated inhibition.

Actin-Sorting Nexin 27 (SNX27)-Retromer Complex Mediates Rapid Parathyroid Hormone Receptor Recycling

The G protein-coupled parathyroid hormone receptor (PTHR) regulates mineral-ion homeostasis and bone remodeling. Upon parathyroid hormone (PTH) stimulation, the PTHR internalizes into early endosomes and subsequently traffics to the retromer complex, a sorting platform on early endosomes that promotes recycling of surface receptors. The C terminus of the PTHR contains a type I PDZ ligand that binds PDZ domain-containing proteins. Mass spectrometry identified sorting nexin 27 (SNX27) in isolated endosomes as a PTHR binding partner. PTH treatment enriched endosomal PTHR. SNX27 contains a PDZ domain and serves as a cargo selector for the retromer complex. VPS26, VPS29, and VPS35 retromer subunits were isolated with PTHR in endosomes from cells stimulated with PTH. Molecular dynamics and protein binding studies establish that PTHR and SNX27 interactions depend on the PDZ recognition motif in PTHR and the PDZ domain of SNX27. Depletion of either SNX27 or VPS35 or actin depolymerization decreased the rate of PTHR recycling following agonist stimulation. Mutating the PDZ ligand of PTHR abolished the interaction with SNX27 but did not affect the overall rate of recycling, suggesting that PTHR may directly engage the retromer complex. Coimmunoprecipitation and overlay experiments show that both intact and mutated PTHR bind retromer through the VPS26 protomer and sequentially assemble a ternary complex with PTHR and SNX27. SNX27-independent recycling may involve N-ethylmaleimide-sensitive factor, which binds both PDZ intact and mutant PTHRs. We conclude that PTHR recycles rapidly through at least two pathways, one involving the ASRT complex of actin, SNX27, and retromer and another possibly involving N-ethylmaleimide-sensitive factor.

Phosphorylation of ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) by Akt promotes stability and mitogenic function of S-phase kinase-associated protein-2 (Skp2)

The regulation of the cell cycle by the ubiquitin-proteasome system is dependent on the activity of E3 ligases. Skp2 (S-phase kinase associated protein-2) is the substrate recognition subunit of the E3 ligase that ubiquitylates the cell cycle inhibitors p21(cip1) and p27(kip1) thus promoting cell cycle progression. Increased expression of Skp2 is frequently observed in diseases characterized by excessive cell proliferation, such as cancer and neointima hyperplasia. The stability and cellular localization of Skp2 are regulated by Akt, but the molecular mechanisms underlying these effects remain only partly understood. The scaffolding protein Ezrin-Binding Phosphoprotein of 50 kDa (EBP50) contains two PDZ domains and plays a critical role in the development of neointimal hyperplasia. Here we report that EBP50 directly binds Skp2 via its first PDZ domain. Moreover, EBP50 is phosphorylated by Akt on Thr-156 within the second PDZ domain, an event that allosterically promotes binding to Skp2. The interaction with EBP50 causes cytoplasmic localization of Skp2, increases Skp2 stability and promotes proliferation of primary vascular smooth muscle cells. Collectively, these studies define a novel regulatory mechanism contributing to aberrant cell growth and highlight the importance of scaffolding function of EBP50 in Akt-dependent cell proliferation.

A potent α/β-peptide analogue of GLP-1 with prolonged action in vivo

Glucagon-like peptide-1 (GLP-1) is a natural agonist for GLP-1R, a G protein-coupled receptor (GPCR) on the surface of pancreatic β cells. GLP-1R agoinsts are attractive for treatment of type 2 diabetes, but GLP-1 itself is rapidly degraded by peptidases in vivo. We describe a design strategy for retaining GLP-1-like activity while engendering prolonged activity in vivo, based on strategic replacement of native α residues with conformationally constrained β-amino acid residues. This backbone-modification approach may be useful for developing stabilized analogues of other peptide hormones.

Abstract 338: Akt-Mediated Phosphorylation of EBP50 Regulates Skp2 Stability and Vascular Smooth Muscle Cell Proliferation

The PDZ domain-containing scaffolding protein, Ezrin-Radixin-Moesin-Binding Phosphoprotein 50 (EBP50) increases vascular smooth muscle cells (VSMC) proliferation and vascular remodeling following endoluminal vessel injury. Indeed, neointima formation is significantly reduced in EBP50 knockout mice. The mitogenic effect of EBP50 derives from the Akt-dependent stabilization of the S-phase kinase associating protein Skp2. However, the mechanisms by which Akt regulates EBP50-mediated cell proliferation are not known. We hypothesize that phosphorylation of EBP50 by Akt promotes its interaction with Skp2. This causes cytoplasmic re-localization and stabilization of Skp2 and VSMC proliferation.

Using Akt inhibitors and siRNA inhibition of Skp2, we show that the mitogenic effect of EBP50 in VSMC is dependent on both Akt and Skp2. In vitro and in vivo phosphorylation assays show that Akt phosphorylates EBP50 at T156 in the second PDZ domain. Interestingly, NMR, fluorescence polarization and co-immunoprecipitation experiments show that Skp2 interacts with the first PDZ domain of EBP50, suggesting that T156 phosphorylation exerts allosteric effects on the interaction with Skp2. Expression of EBP50 in primary VSMC induces a re-localization of Skp2 from the nucleus to the cytoplasm, leading to its stabilization and VSMC proliferation. These effects were abrogated by mutations within the first PDZ domain or of T156 to Alanine. Notably, the interaction with EBP50 does not directly affect the function of Skp2 because mutation of the C-terminal Leu to Ala, which abrogates the interaction with EBP50, does not decrease the ability of Skp2 to degrade p21cip1 and p27kip1. Consistent with the observations in primary cells, the expression of Skp2 in injured femoral arteries was reduced in EBP50 knockout mice compared to WT mice.

In conclusion, our studies identify a novel mechanism for the Akt-dependent regulation of cell proliferation by EBP50 during vascular remodeling. They indicate that inhibition of EBP50 may be useful in preventing neointima formation following angioplasty.

Ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) and nuclear factor-κB (NF-κB): a feed-forward loop for systemic and vascular inflammation

The interaction between vascular cells and macrophages is critical during vascular remodeling. Here we report that the scaffolding protein, ezrin-binding phosphoprotein 50 (EBP50), is a central regulator of macrophage and vascular smooth muscle cells (VSMC) function. EBP50 is up-regulated in intimal VSMC following endoluminal injury and promotes neointima formation. However, the mechanisms underlying these effects are not fully understood. Because of the fundamental role that inflammation plays in vascular diseases, we hypothesized that EBP50 mediates macrophage activation and the response of vessels to inflammation. Indeed, EBP50 expression increased in primary macrophages and VSMC, and in the aorta of mice, upon treatment with LPS or TNFα. This increase was nuclear factor-κB (NF-κB)-dependent. Conversely, activation of NF-κB was impaired in EBP50-null VSMC and macrophages. We found that inflammatory stimuli promote the formation of an EBP50-PKCζ complex at the cell membrane that induces NF-κB signaling. Macrophage activation and vascular inflammation after acute LPS treatment were reduced in EBP50-null cells and mice as compared with WT. Furthermore, macrophage recruitment to vascular lesions was significantly reduced in EBP50 knock-out mice. Thus, EBP50 and NF-κB participate in a feed-forward loop leading to increased macrophage activation and enhanced response of vascular cells to inflammation.

A comparison of parathyroid hormone-related protein (1-36) and parathyroid hormone (1-34) on markers of bone turnover and bone density in postmenopausal women: the PrOP study

Parathyroid hormone-related protein (PTHrP)(1-36) increases lumbar spine (LS) bone mineral density (BMD), acting as an anabolic agent when injected intermittently, but it has not been directly compared with parathyroid hormone (PTH)(1-34). We performed a 3-month randomized, prospective study in 105 postmenopausal women with low bone density or osteoporosis, comparing daily subcutaneous injections of PTHrP(1-36) to PTH(1-34). Thirty-five women were randomized to each of three groups: PTHrP(1-36) 400 µg/day; PTHrP(1-36) 600 µg/day; and PTH(1-34) 20 µg/day. The primary outcome measures were changes in amino-terminal telopeptides of procollagen 1 (PINP) and carboxy-terminal telopeptides of collagen 1 (CTX). Secondary measures included safety parameters, 1,25(OH)2 vitamin D, and BMD. The increase in bone resorption (CTX) by PTH(1-34) (92%) (p < 0.005) was greater than for PTHrP(1-36) (30%) (p < 0.05). PTH(1-34) also increased bone formation (PINP) (171%) (p < 0.0005) more than either dose of PTHrP(1-36) (46% and 87%). The increase in PINP was earlier (day 15) and greater than the increase in CTX for all three groups. LS BMD increased equivalently in each group (p < 0.05 for all). Total hip (TH) and femoral neck (FN) BMD increased equivalently in each group but were only significant for the two doses of PTHrP(1-36) (p < 0.05) at the TH and for PTHrP(1-36) 400 (p < 0.05) at the FN. PTHrP(1-36) 400 induced mild, transient (day 15) hypercalcemia. PTHrP(1-36) 600 required a dose reduction for hypercalcemia in three subjects. PTH(1-34) was not associated with hypercalcemia. Each peptide induced a marked biphasic increase in 1,25(OH)2 D. Adverse events (AE) were similar among the three groups. This study demonstrates that PTHrP(1-36) and PTH(1-34) cause similar increases in LS BMD. PTHrP(1-36) also increased hip BMD. PTH(1-34) induced greater changes in bone turnover than PTHrP(1-36). PTHrP(1-36) was associated with mild transient hypercalcemia. Longer-term studies using lower doses of PTHrP(1-36) are needed to define both the optimal dose and full clinical benefits of PTHrP. © 2013 American Society for Bone and Mineral Research.

Aquaporin 1, Nox1, and Ask1 mediate oxidant-induced smooth muscle cell hypertrophy

AIMS

Reactive oxygen species (ROS)-mediated intracellular signalling is well described in the vasculature, yet the precise roles of ROS in paracrine signalling are not known. Studies implicate interstitial ROS hydrogen peroxide (H(2)O(2)) in vascular disease, and plasma H(2)O(2) levels in the micromolar range are detectable in animal models and humans with hypertension. Recently, H(2)O(2) was shown to cross biological membranes of non-vascular cells via aquaporin (Aqp) water channels. Previous findings suggest that H(2)O(2) activates NADPH oxidase (Nox) enzymes in vascular cells and apoptosis signal-regulating kinase 1 (Ask1) in non-vascular cells. We hypothesized that extracellular H(2)O(2) induces smooth muscle cell (SMC) hypertrophy by a mechanism involving Aqp1, Nox1, and Ask1.

METHODS AND RESULTS

Treatment of rat aortic SMCs (rASMC) with exogenous H(2)O(2) resulted in a concentration-dependent increase in Nox-derived superoxide (O(2)(•-)), determined by L-012 chemiluminescence, cytochrome c and electron paramagnetic resonance. Nox1 was verified as the source of O(2)(·-) by siRNA. Aqp1 siRNA attenuated H(2)O(2) cellular entry and H(2)O(2)-induced O(2)(•-) production. H(2)O(2) treatment increased Ask1 activation and induced rASMC hypertrophy in a Nox1-dependent mechanism. Adenoviral-dominant-negative Ask1 attenuated H(2)O(2)-induced rASMC hypertrophy and adenoviral overexpression of Ask1 augmented it.

CONCLUSION

Our results demonstrate for the first time that extracellular H(2)O(2), at pathophysiological concentrations, stimulates rASMC Nox1-derived O(2)(•-), subsequent Ask1 activation and SMC hypertrophy. The data demonstrate a novel pathway by which H(2)O(2) enters vascular cells via aquaporins and activates Nox, leading to hypertrophy, and provide multiple novel targets for combinatorial therapeutics development targeting hypertrophy and vascular disease.

EBP50 promotes focal adhesion turnover and vascular smooth muscle cells migration

The ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) is a PDZ-containing scaffolding protein that regulates a variety of physiological functions. In the vasculature, EBP50 promotes neointima formation following arterial injury. In this study the role of EBP50 on vascular smooth muscle cell (VSMC) migration was characterized. The spreading and motility of primary VSMC isolated from EBP50 knockout (KO) mice were significantly reduced compared to wild-type (WT) cells. EBP50-null VSMC had fewer and larger focal adhesions than wild-type cells. Assembly and disassembly of focal adhesion-assessed by live-cell total internal reflection fluorescence imaging-in response to epidermal growth factor (EGF) were significantly reduced in KO cells. Immunoprecipitation experiments showed that EBP50 interacts with EGF receptor via the PDZ2 domain and with focal adhesion kinase (FAK) via the C-terminal ERM domain. EBP50 promoted the formation of a complex containing both EGF receptor and FAK. Phosphorylation of Tyr-925 of FAK in response to EGF was significantly reduced in KO cell compared to WT cells. The residence time of FAK in focal adhesions-determined by fluorescence recovery after photobleaching-was increased in WT cells. Collectively, these studies indicate that EBP50, by scaffolding EGF receptor and FAK, facilitates activation of FAK, focal adhesion turnover, and migration of VSMC.

Abstract 94: Scaffolding Protein EBP50 Regulates Vascular Smooth Muscle Nox1 Activity by Binding p47 phox

The NADPH oxidase (Nox) enzyme family is implicated in the pathogenesis of cardiovascular diseases (CVDs) including hypertension. In large vessel smooth muscle cells (SMCs), a unique hybrid Nox1 oxidase exists which propagates SMC dysfunction and hypertrophy. We recently demonstrated that these actions involve activation of Nox1 by hydrogen peroxide (H 2 O 2 ) leading to feed-forward production of reactive oxygen species and activation of apoptosis signal regulating kinase 1 (Ask1). Here we postulate that Ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50), a PDZ-domain scaffolding protein, is required for H 2 O 2 -mediated activation of SMC Nox1. In SMCs, knockdown of EBP50 by shRNA abolished superoxide (O 2 •- ) production in 50 μM H 2 O 2 -treated cells (37.7 ± 4.5, 79.9 ± 2.3 and 29.0 ± 3.5 x10 4 O 2 •- -mediated RLU for GFP plasmid vehicle-treated, GFP plasmid H 2 O 2 -treated and EBP50 shRNA plasmid H 2 O 2 -treated groups, respectively). Furthermore, H 2 O 2 -induced O 2 •- production was absent in SMCs isolated from EBP50 knockout (KO) mice (29.0 ± 2.7 vs 79.1 ± 2.4 x10 4 RLU for EBP50 KO vs wildtype after H 2 O 2 treatment). Intriguingly, p47 phox , an essential cytosolic subunit of the hybrid SMC Nox1 system, upon phosphorylation co-immunoprecipitated with EBP50 in COS-7 cells transfected with both proteins (Fig 1). Taken together, our data suggest that EBP50 promotes Nox activity by binding active p47 phox . These studies identify a novel partner in Nox1 activation and potentially provide new insights into the broader regulation of this enzyme family. Moreover, this added level of complexity could yield new therapeutic strategies to disrupt Nox in the treatment of hypertension and other CVDs.

Ezrin-anchored protein kinase A coordinates phosphorylation-dependent disassembly of a NHERF1 ternary complex to regulate hormone-sensitive phosphate transport

Congenital defects in the Na/H exchanger regulatory factor-1 (NHERF1) are linked to disordered phosphate homeostasis and skeletal abnormalities in humans. In the kidney, these mutations interrupt parathyroid hormone (PTH)-responsive sequestration of the renal phosphate transporter, Npt2a, with ensuing urinary phosphate wasting. We now report that NHERF1, a modular PDZ domain scaffolding protein, coordinates the assembly of an obligate ternary complex with Npt2a and the PKA-anchoring protein ezrin to facilitate PTH-responsive cAMP signaling events. Activation of ezrin-anchored PKA initiates NHERF1 phosphorylation to disassemble the ternary complex, release Npt2a, and thereby inhibit phosphate transport. Loss-of-function mutations stabilize an inactive NHERF1 conformation that we show is refractory to PKA phosphorylation and impairs assembly of the ternary complex. Compensatory mutations introduced in mutant NHERF1 re-establish the integrity of the ternary complex to permit phosphorylation of NHERF1 and rescue PTH action. These findings offer new insights into a novel macromolecular mechanism for the physiological action of a critical ternary complex, where anchored PKA coordinates the assembly and turnover of the Npt2a-NHERF1-ezrin complex.

Abstract 487: EBP50 Promotes Epidermal Growth Factor-Dependent Activation of Focal Adhesion Kinase and Vascular Smooth Muscle Cells Migration

The Ezrin-Radixin-Moesin-Binding Phosphoprotein 50 (EBP50) is a scaffolding protein that regulates a variety of physiological functions. Previous studies showed that EBP50 promotes vascular smooth muscle cells (VSMC) proliferation and neointima formation following arterial injury. In this study the role of EBP50 on VSMC migration was characterized. VSMC migration was determined by wound-healing assays. The motility of primary VSMC isolated from EBP50 KO mice was significantly reduced compared to WT cells (25 um/h WT vs. 19 um/h KO, p<0.05) resulting in a 50% reduction in wound coverage. Conversely, EBP50 expression increased VSMC migration. EBP50-null VSMC had fewer and larger focal adhesions than EBP50-expressing cells. Both assembly and disassembly of focal adhesion in response to epidermal growth factor (EGF) (determined by real time TIRF microscopy) were significantly reduced in KO cells. Immunoprecipitation experiments showed that EBP50 interacts with both focal adhesion kinase (FAK) and EGF receptor (EGFR) and the formation of a complex containing both EGFR and FAK was increased by EBP50. Stimulation of VSMC with EGF induced FAK phosphorylation on Tyr925 in WT, but not in KO cell, whereas phosphorylation of Tyr397 was unaffected. Collectively, these observations indicate that EBP50 facilitates growth factors-dependent activation of FAK, and consequently, migration of VSMC. Therefore, in addition to increasing VSMC proliferation, EBP50 promotes neointima formation following arterial injury by increasing VSMC motility.

Abstract 32: EBP50 and NF-κB: A Feed-Forward Regulation of Vascular Inflammation

Inflammation plays a fundamental role in the development of cardiovascular diseases such as atherosclerosis and restenosis. The vascular response to inflammation in these diseases requires communication between multiple cell types such as vascular smooth muscle cells (VSMC) and macrophages. We have identified a scaffolding protein, Ezrin Binding Protein 50 (EBP50, also known as NHERF-1), which is expressed at very low levels in normal vessels but is up-regulated following vascular injury. EBP50 promotes VSMC proliferation and neointima formation after wire injury in mouse femoral arteries. In the current study, we hypothesized that EBP50 functions as a central mediator linking macrophage activation and the response of vessels to inflammation. Similar to the observations in vessels, treatment of primary VSMC and macrophages with LPS or TNFα increased EBP50 expression. This increase was dependent on NF-κB signaling because pharmacological inhibition of NF-κB and expression of a dominant negative IκBα abolished the induction of EBP50. Interestingly, LPS-induced activation of NF-κB (determined by phosphorylation of IKKα/β, degradation of IκBα, and p65 phosphorylation) was impaired in EBP50 null VSMC and macrophages. Moreover, nuclear translocation of the p65 subunit of NF-κB was also reduced in EBP50 -/- VSMC. Consistent with the role of EBP50 on NF-κB, LPS- and TNFα-induced macrophage activation (determined by the expression of the inflammatory markers iNOS, TNFα, and IL-1β) was reduced in EBP50 -/- macrophages and mice. Similarly, the expression of iNOS, VCAM-1, and ICAM-1 in VSMC was decreased in EBP50 -/- VSMC and mice. Collectively, these observations indicate that EBP50 and NF-κB participate in a positive feedback loop leading to increased macrophage activation and enhanced response of VSMC to inflammation, suggesting its involvement in vascular remodeling.

Abstract 362: Akt-Dependent Interaction with EBP50 Regulates Skp2 Localization and Stability

The PDZ domain-containing scaffolding protein, Ezrin-Radixin-Moesin-Binding Phosphoprotein 50 (EBP50) regulates vascular stenosis following endoluminal vessel injury. Its expression in vascular smooth muscle cells (VSMC) increases after wire injury, and neointima formation is significantly reduced in EBP50 knockout mice. We previously reported that the mitogenic effect of EBP50 is Akt-dependent and mediated by the S-phase kinase-associated protein 2 (Skp2), a component of the E3 ligase complex that degrades the cell cycle inhibitor p21cip1. The mechanisms by which Akt directly increases Skp2 stability are controversial. We hypothesize that EBP50 regulates Akt-dependent stabilization of Skp2. The C-terminal four amino acids of Skp2 (PSCL) are a canonical PDZ-binding sequence. Co-immunoprecipitation and in-gel overlay assays demonstrated the direct interaction between EBP50 and Skp2. Mutation of the C-terminal Leu to Ala (L424A-Skp2) abrogated the interaction with EBP50. Akt potentiated the association between EBP50 and Skp2. Sequence analysis indicated the presence of an Akt phosphorylation motif (RPRLCT) in EBP50 between residues 151 and 156. Indeed, in vitro assays using purified EBP50 and constitutively active Akt showed that Akt phosphorylates EBP50. This effect was abrogated by a naturally occurring mutation R153Q in EBP50 that disrupts the Akt consensus sequence. EBP50 was expressed exclusively in the cytoplasm of VSMC, where it associated with the cytoskeleton. Cell fractionation and fluorescence microscopy studies showed that expression of EBP50, but not of R153Q, induced relocalization of Skp2 from the nucleus to the cytoplasm. EBP50 expression increased Skp2 stability and, consequently, VSMC proliferation. Collectively, these experiments show that Akt-mediated EBP50 phosphorylation increases the interaction with Skp2, regulates Skp2 cellular localization and increases Skp2 stability. These studies identify a novel function for EBP50 in the direct regulation of the cell cycle and provide the mechanistic basis for the effect of this scaffolding protein on VSMC proliferation and neointima formation.

Abstract P110: EBP50 Regulates Vascular Smooth Muscle Cell Growth by Skp2-Mediated Degradation of p21/cip1

The PDZ domain-containing scaffolding protein, Ezrin-Radixin-Moesin-binding phosphoprotein 50 (EBP50) regulates vascular stenosis following endoluminal vessel injury. Its expression in vascular smooth muscle cells (VSMC) increases after wire injury, and neointima formation is significantly reduced in EBP50 knockout (KO) mice. The molecular mechanisms underlying EBP50 actions in VSMC are unknown.

Genetic ablation of EBP50 reduced VSMC proliferation and was associated with increased (5-fold) expression of the cell cycle inhibitor p21cip1 both in vessels and in primary cells. No differences in mRNA levels of p21cip1 were observed in WT and KO cells. However, the half-life of p21cip1 in KO VSMC was significantly longer than in WT VSMC (80 min vs. 45 min) and p21cip1 levels were similar in WT and KO VSMC treated with the proteasome inhibitor MG132. These observations suggest that EBP50 regulates post-translational degradation of p21cip1. The S-phase kinase-associated protein 2 (skp2) is a component of the E3 ligase complex that degrades p21cip1. The C-terminal four amino acids of skp2 (ProSerCysLeu) are a canonical PDZ-binding sequence. Indeed, co-immunoprecipitation and in-gel overlay assays demonstrated the direct interaction between EBP50 and skp2. Mutation of the C-terminal Leu to Ala (L424A-skp2) abrogated the interaction with EBP50. Skp2 expression was significantly lower in KO than in WT cells and inhibition of EBP50 expression by an shRNA lentivirus decreased skp2 expression in WT cells. Moreover, expression of skp2, but not of the mutant L424A-skp2, in WT cells reduced p21cip1 levels. Therefore, EBP50 regulates both expression and activity of skp2 with attendant effects on p21cip1 and VSMC proliferation.

Collectively, these experiments show that EBP50, by regulating skp2 and p21cip1 expression, controls VSMC proliferation and the progression of neointima formation. These studies identify a novel function for EBP50 in the direct regulation of the cell cycle and provide a mechanistic basis for the remarkable effect of this scaffolding protein on vascular remodeling.

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

Systemic and acute administration of parathyroid hormone-related peptide(1-36) stimulates endogenous beta cell proliferation while preserving function in adult mice

AIMS/HYPOTHESIS

A major focus in the treatment of diabetes is to identify factors that stimulate endogenous beta cell growth while preserving function. The first 36 amino acids of parathyroid hormone-related protein (PTHrP) are sufficient to enhance proliferation and function in rodent and human beta cells in vitro. This study examined whether acute and systemic administration of the amino-terminal PTHrP(1-36) peptide can achieve similar effects in rodent beta cells in vivo.

METHODS

Adult male mice were injected with 40, 80 or 160 μg of PTHrP(1-36) per kg body weight or with vehicle for 25 days. Glucose and beta cell homeostasis, as well as expression of differentiation markers and cell cycle genes were analysed.

RESULTS

All three doses of PTHrP(1-36) significantly enhanced beta cell proliferation in vivo at day 25, with 160 μg/kg PTHrP(1-36) increasing proliferation as early as day 5. Importantly, the two higher doses of PTHrP(1-36) caused a significant 30% expansion of beta cell mass, with a short-term improvement in glucose tolerance. PTHrP(1-36) did not cause hypercalcaemia, or change islet number, beta cell size, beta cell death or expression of differentiation markers. Analysis of islet G1/S cell cycle proteins revealed that chronic overabundance of PTHrP(1-139) in the beta cell significantly increased the cell cycle activator cyclin D2 and decreased levels of cyclin-dependent kinase 4 inhibitor (p16( Ink4a ) [Ink4a also known as Cdkn2a]), but acute treatment with PTHrP(1-36) did not.

CONCLUSIONS/INTERPRETATION

Acute and systemic administration of PTHrP(1-36) increases rodent beta cell proliferation and mass without negatively affecting function or survival. These findings highlight the future potential therapeutic effectiveness of this peptide under diabetes-related pathophysiological conditions.

The scaffolding protein EBP50 promotes vascular smooth muscle cell proliferation and neointima formation by regulating Skp2 and p21(cip1)

OBJECTIVE

The Ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) is a scaffolding protein known to regulate ion homeostasis in the kidney and intestine. Previous work showed that EBP50 expression increases after balloon injury in rat carotids. This study was designed to determine the role of EBP50 on vascular smooth muscle cells (VSMC) proliferation and the development of neointimal hyperplasia.

METHODS AND RESULTS

Wire injury was performed in wild type (WT) and EBP50 knockout (KO) mice. Two weeks after injury, neointima formation was 80% lower in KO than in WT mice. Proliferation of KO VSMC was significantly lower than WT cells and overexpression of EBP50 increased VSMC proliferation. Akt activity and expression of S-phase kinase protein2 decreased in KO cells resulting in the stabilization of the cyclin-dependent kinase inhibitor, p21(cip1). Consequently, KO cells were arrested in G(0)/G(1) phase. Consistent with these observations, p21(cip1) was detected in injured femoral arteries of KO but not WT mice. No differences in apoptosis between WT and KO were observed.

CONCLUSIONS

EBP50 is critical for neointima formation and induces VSMC proliferation by decreasing S-phase kinase protein2 stability, thereby accelerating the degradation of the cell cycle inhibitor p21(cip1).

A 7-day continuous infusion of PTH or PTHrP suppresses bone formation and uncouples bone turnover

Human in vivo models of primary hyperparathyroidism (HPT), humoral hypercalcemia of malignancy (HHM), or lactational bone mobilization for more than 48 hours have not been described previously. We therefore developed 7-day continuous-infusion models using human parathyroid hormone(1-34) [hPTH(1-34)] and human parathyroid hormone-related protein(1-36) [hPTHrP(1-36)] in healthy human adult volunteers. Study subjects developed sustained mild increases in serum calcium (10.0 mg/dL), with marked suppression of endogenous PTH(1-84). The maximal tolerated infused doses over a 7-day period (2 and 4 pmol/kg/h for PTH and PTHrP, respectively) were far lower than in prior, briefer human studies (8 to 28 pmol/kg/h). In contrast to prior reports using higher PTH and PTHrP doses, both 1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ] and tubular maximum for phosphorus (TmP/GFR) remained unaltered with these low doses despite achievement of hypercalcemia and hypercalciuria. As expected, bone resorption increased rapidly and reversed promptly with cessation of the infusion. However, in contrast to events in primary HPT, bone formation was suppressed by 30% to 40% for the 7 days of the infusions. With cessation of PTH and PTHrP infusion, bone-formation markers abruptly rebounded upward, confirming that bone formation is suppressed by continuous PTH or PTHrP infusion. These studies demonstrate that continuous exposure of the human skeleton to PTH or PTHrP in vivo recruits and activates the bone-resorption program but causes sustained arrest in the osteoblast maturation program. These events would most closely mimic and model events in HHM. Although not a perfect model for lactation, the increase in resorption and the rebound increase in formation with cessation of the infusions are reminiscent of the maternal skeletal calcium mobilization and reversal that occur following lactation. The findings also highlight similarities and differences between the model and HPT.

Development of potent glucagon-like peptide-1 agonists with high enzyme stability via introduction of multiple lactam bridges

Glucagon-like peptide-1 (GLP-1) has the ability to lower the blood glucose level, and its regulatory functions make it an attractive therapeutic agent for the treatment of type 2 diabetes. However, its rapid degradation by enzymes like dipeptidyl peptidase-IV (DPP-IV) and neutral endopeptidase (NEP) 24.11 severely compromises its effective clinical use. Whereas specific DPP-IV inhibitors have been developed, NEP 24.11 targets multiple sites in the GLP-1 sequence, which makes it difficult to block. To address this drawback, we have designed and synthesized conformationally constrained GLP-1 analogues by introducing multiple lactam bridges that stabilized both alpha-helices in the N- and C-terminal regions simultaneously. In addition to improving the receptor activation capability (up to 5-fold) by fixing the alpha-helical conformations required for optimal receptor interaction, the introduced lactam bridges provided outstanding shielding over NEP 24.11 (half-life of >96 h). These highly constrained peptides are the first examples of NEP 24.11-resistant GLP-1 analogues.

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.