Stimulation of protein kinase C activity in cells expressing human parathyroid hormone receptors by C- and N-terminally truncated fragments of parathyroid hormone 1-34

hPTH(3-34)(29-34) selectively activated PKC and mimicked osteoanabolic effects of hPTH(1-34)

Teriparatide (hPTH(1-34)) exhibits both osteoanabolic and osteocatabolic effects. We generated a novel PTH analog by duplicating the PTH(29-34) domain to hPTH(3-34) (named MY-1), which was identified to activate PKC but not PLC and cAMP/PKA signaling. It increased osteo-differentiation but did not affect osteoclastogenesis and RANKL expression in primary osteoblasts or bone marrow cells. MY-1 and hPTH(1-34) increased the synthesis and decreased the degradation οf β-catenin protein in osteoblasts, while PKC inhibitor blunted such effects. In vivo results indicated that intermittent MY-1 and hPTH(1-34) prevented bone loss in ovariectomized mice, and that MY-1 infusion increased bone volume in normal mice. Histological analysis observed more osteoclasts surrounding the cancellous bone surface in hPTH(1-34), but not MY-1 treated mice. We conclude that MY-1 mimicked the osteoanabolic but not the osteocatabolic effects of hPTH(1-34), which is related to PKC and β-catenin signaling. Such anabolic-only analog provides a new strategy to study PTH's versatile functions and design new medicines to treat osteoporosis and bone defects.

Parathyroid Hormone Activates Phospholipase C (PLC)-Independent Protein Kinase C Signaling Pathway via Protein Kinase A (PKA)-Dependent Mechanism: A New Defined Signaling Route Would Induce Alternative Consideration to Previous Conceptions

Background

Parathyroid hormone (PTH) is an effective anti-osteoporosis agent, after binding to its receptor PTHR1, several signaling pathways, including cAMP/protein kinase A (PKA) and phospholipase C (PLC)/protein kinase C (PKC), are initiated through G proteins; with the cAMP/PKA pathway as the major pathway. Earlier studies have reported that PTHR1 might also activate PKC via a PLC-independent mechanism, but this pathway remains unclear.

Material/Methods

In HEK293 cells, cAMP accumulation was measured with ELISA and PKC was measured with fluorescence resonance energy transfer (FRET) analysis using CKAR plasmid. In MC3T3-E1 cells, real-time PCR was performed to examine gene expressions. Then assays for cell apoptosis, cell differentiation, alkaline phosphatase activity, and mineralization were performed.

Results

The FRET analysis found that PTH(1–34), [G1,R19]PTH(1–34) (GR(1–34), and [G1,R19]PTH(1–28) (GR(1–28) were all activated by PKC. The PKC activation ability of GR(1–28) was blocked by cAMP inhibitor (Rp-cAMP) and rescued with the addition of active PKA-α and PKA-β. The PKC activation ability of GR(1–34) was partially inhibited by Rp-cAMP. In MC3T3-E1 cells, gene expressions of ALP, CITED1, NR4a2, and OSX that was regulated by GR(1–28) were significantly changed by the pan-PKC inhibitor Go6983. After pretreatment with Rp-cAMP, the gene expressions of ALP, CITED1, and OPG were differentially regulated by GR(1–28) or GR(1–34), and the difference was blunted by Go6983. PTH(1–34), GR(1–28), and GR(1–34) significantly decreased early apoptosis and augmented osteoblastic differentiation in accordance with the activities of PKA and PKC.

Conclusions

PLC-independent PKC activation induced by PTH could be divided into two potential mechanisms: one was PKA-dependent and associated with PTH(1–28); the other was PKA-independent and associated with PTH(29–34). We also found that PTH could activate PLC-independent PKC via PKA-dependent mechanisms.

Characterization of a PTH1R missense mutation responsible for Jansen type metaphyseal chondrodysplasia

Parathyroid hormone and parathyroid hormone-related peptide (PTHrP), and its receptor (PTH1R) play an important role in differentiation of bone and cartilage in the developing stages. Constitutive dimers of PTH1R are believed to be dissociated by ligand binding, and monomeric PTH1R is capable of activating G protein. Jansen type metaphyseal chondrodysplasia is caused by missense mutations in PTH1R, which are constitutively active even without the presence of the ligands. However, the underlying pathomechanisms remained largely unknown. In this study, we have attempted to further characterize a PTH1R missense mutation H223R responsible for Jansen type metaphyseal chondrodysplasia. cDNAs encoding wild-type (Wt)- and H223R mutant (Mut)-PTH1R were transfected into HEK293T cells, and as a consequence of western blots, both the Wt- and Mut-PTH1R proteins showed several fragments between 55 and 65 kDa in size, while the patterns of N-glycosylation were distinct between them. Then we hypothesized that the Mut-PTH1R might physically interact with the Wt-PTH1R, leading to affect the downstream cAMP accumulation. Co-immunoprecipitation assays clearly showed that interaction occurred not only between the Wt-PTH1R themselves, but also between the Wt- and Mut-PTH1R. Furthermore, we performed CRE reporter assays to investigate cAMP accumulation. Constitutive, ligand-independent cAMP accumulation was observed in HEK293T cells expressing the Mut-PTH1R. Interestingly, there was a statistically lower constitutive activity in HEK293T cells co-expressing the Wt- and Mut-PTH1R proteins. Summarizing, it seems likely that Mut-PTH1R may be, at least in part, co-localized with Wt-PTH1R by forming a heterodimer, leading to affect the function to each other.

Evaluation of the efficacy, safety and pharmacokinetic profile of oral recombinant human parathyroid hormone [rhPTH(1-31)NH(2)] in postmenopausal women with osteoporosis

CONTEXT

Treatment of osteoporosis with subcutaneous (SC) injections of rhPTH(1-34) or rhPTH(1-84) is associated with significant improvements in BMD and reductions in osteoporotic fractures. However, subcutaneous injections can be associated with discomfort and thus deteriorating compliance.

OBJECTIVE

The UGL-OR1001 trial aimed to establish the efficacy and safety parameters of a novel oral tablet formulation of rhPTH(1-31)NH(2) and matching placebo tablets and open-label teriparatide positive control in postmenopausal women with osteoporosis.

DESIGN

24 weeks of randomized, double-blind treatment with once daily doses of 5mg oral treatment or corresponding placebo, or open-label subcutaneous teriparatide.

PATIENTS OR OTHER PARTICIPANTS

Women diagnosed with postmenopausal osteoporosis as detected by lumbar spine DXA, with an exclusion of those with prior treatment with bone active agents.

INTERVENTION(S)

Orally formulated recombinant human PTH(1-31)NH(2) and placebo, or open-label subcutaneous teriparatide as a positive control.

MAIN OUTCOME MEASURE(S)

The primary endpoint was to characterize the percent change from baseline in bone mineral density (BMD) at L1-L4 axial lumbar spine after 24 weeks in the rhPTH(1-31)NH(2) arm. Secondary and exploratory endpoints included safety and tolerability of the oral formulation, measurement of biochemical markers of bone turnover, and evaluation of the PK profile at first and last dose. The study was registered at ClinicalTrials.gov with the identifier: NCT01321723.

RESULTS

The oral tablet formulation of rhPTH(1-31)NH(2) resulted in similar PK profiles at both timepoints with mean C(max) values similar to subcutaneous administration. In the rhPTH(1-31)NH(2) arm, a 2.2% increase in lumbar spine BMD was observed compared to baseline (p<0.001), while no change was observed in the placebo arm. Open-label teriparatide resulted in a 5.1% increase in LS BMD (p<0.001). In the oral PTH study arm, the bone formation marker osteocalcin was increased by 32%, 21% and 23% at Weeks 4, 12 and 24, respectively. There was no significant increase in the level of the bone resorption marker CTx-1.

CONCLUSIONS

In summary, these data demonstrate that enteric-coated oral tablet formulation technology consistently generated robust levels of exposure of rhPTH(1-31)NH(2) leading to induction of bone formation without inducing bone resorption resulting in significantly increased levels of LS BMD. Few adverse events were observed, recommending this orally delivered drug candidate for further development.

Parathyroid hormone related protein (PTHrP) in tumor progression

Parathyroid hormone-related protein (PTHrP) is widely expressed in fetal and adult tissues and is a key regulator for cellular calcium transport and smooth muscle cell contractility, as well as a crucial control factor in cell proliferation, development and differentiation. PTHrP stimulates or inhibits apoptosis in an autocrine/paracrine and intracrine fashion, and is particularly important for hair follicle and bone development, mammary epithelial development and tooth eruption. PTHrP's dysregulated expression has traditionally been associated with oncogenic pathologies as the major causative agent of malignancy-associated hypercalcemia, but recent evidence revealed a driving role in skeletal metastasis progression. Here, we demonstrate that PTHrP is also closely involved in breast cancer initiation, growth and metastasis through mechanisms separate from its bone turnover action, and we suggest that PTHrP as a facilitator of oncogenes would be a novel target for therapeutic purposes.

Refining efficacy: exploiting functional selectivity for drug discovery

Early models of G protein-coupled receptor (GPCR) activation envisioned the receptor in equilibrium between unique "off" and "on" states, wherein ligand binding affected signaling by increasing or decreasing the fraction of receptors in the active conformation. It is now apparent that GPCRs spontaneously sample multiple conformations, any number of which may couple to one or more downstream effectors. Such "multistate" models imply that the receptor-ligand complex, not the receptor alone, defines which active receptor conformations predominate. "Functional selectivity" refers to the ability of a ligand to activate only a subset of its receptor's signaling repertoire. There are now numerous examples of ligands that "bias" receptor coupling between different G protein pools and non-G protein effectors such as arrestins. The type 1 parathyroid hormone receptor (PTH(1)R) is a particularly informative example, not only because of the range of biased effects that have been produced, but also because the actions of many of these ligands have been characterized in vivo. Biased PTH(1)R ligands can selectively couple the PTH(1)R to G(s) or G(q/11) pathways, with or without activating arrestin-dependent receptor desensitization and signaling. These reagents have provided insight into the contribution of different signaling pathways to PTH action in vivo and suggest it may be possible to exploit ligand bias to uncouple the anabolic effects of PTH(1)R from its catabolic and calcitropic effects. Whereas conventional agonists and antagonists only modulate the quantity of efficacy, functionally selective ligands qualitatively change GPCR signaling, offering the prospect of drugs with improved therapeutic efficacy or reduced side effects.

Acute down-regulation of sodium-dependent phosphate transporter NPT2a involves predominantly the cAMP/PKA pathway as revealed by signaling-selective parathyroid hormone analogs

The parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTHR1) in cells of the renal proximal tubule mediates the reduction in membrane expression of the sodium-dependent P(i) co-transporters, NPT2a and NPT2c, and thus suppresses the re-uptake of P(i) from the filtrate. In most cell types, the liganded PTHR1 activates Gα(S)/adenylyl cyclase/cAMP/PKA (cAMP/PKA) and Gα(q/11)/phospholipase C/phosphatidylinositol 1,4,5-trisphosphate (IP(3))/Ca(2+)/PKC (IP(3)/PKC) signaling pathways, but the relative roles of each pathway in mediating renal regulation P(i) transport remain uncertain. We therefore explored the signaling mechanisms involved in PTH-dependent regulation of NPT2a function using potent, long-acting PTH analogs, M-PTH(1-28) (where M = Ala(1,12), Aib(3), Gln(10), Har(11), Trp(14), and Arg(19)) and its position 1-modified variant, Trp(1)-M-PTH(1-28), designed to be phospholipase C-deficient. In cell-based assays, both M-PTH(1-28) and Trp(1)-M-PTH(1-28) exhibited potent and prolonged cAMP responses, whereas only M-PTH(1-28) was effective in inducing IP(3) and intracellular calcium responses. In opossum kidney cells, a clonal cell line in which the PTHR1 and NPT2a are endogenously expressed, M-PTH(1-28) and Trp(1)-M-PTH(1-28) each induced reductions in (32)P uptake, and these responses persisted for more than 24 h after ligand wash-out, whereas that of PTH(1-34) was terminated by 4 h. When injected into wild-type mice, both M-modified PTH analogs induced prolonged reductions in blood P(i) levels and commensurate reductions in NPT2a expression in the renal brush border membrane. Our findings suggest that the acute down-regulation of NPT2a expression by PTH ligands involves mainly the cAMP/PKA signaling pathway and are thus consistent with the elevated blood P(i) levels seen in pseudohypoparathyroid patients, in whom Gα(s)-mediated signaling in renal proximal tubule cells is defective.

Parathyroid hormone inhibits phosphorylation of mitogen-activated protein kinase (MAPK) ERK1/2 through inhibition of c-Raf and activation of MKP-1 in osteoblastic cells

Parathyroid hormone (PTH) regulation of mitogen-activated protein kinases (MAPK) ERK1/2 contributes to PTH regulation of osteoblast growth and apoptosis. We investigated the mechanisms by which PTH inhibits ERK1/2 activity in osteoblastic UMR 106-01 cells. Treatment with PTH significantly inhibited phosphorylated ERK1/2 between 5 and 60 min. Transient transfection of cells with a cDNA encoding MAPK phosphatase-1 (MKP-1) resulted in 30-40% inhibition of pERK1/2; however MKP-1 protein levels were only significantly stimulated by PTH after 30 mins, suggesting another mechanism for the early phase of pERK1/2 inhibition. The active upstream kinase c-Raf phosphorylation at serine 338 (ser(338)) was significantly inhibited by PTH treatment within 5 min and transfection of the cells with constitutively-active c-Raf blocked PTH inhibition of pERK1/2. Inhibition of pERK1/2 and phosphor-c-Raf were seen when cells were treated with PTH(1-34) or PTH(1-31) analogues that stimulate cAMP, but not with PTH(3-34), PTH(7-34) or PTH(18-48) that do not stimulate cAMP. Stimulation of the cells with forskolin or 8BrcAMP also inhibited pERK1/2 and c-Raf.p338. Our results suggest that rapid PTH inhibition of ERK1/2 activity is mediated by PKA dependent inhibition of c-Raf activity and that stimulation of MKP-1 may contribute to maintaining pERK1/2 inhibition over prolonged time.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Parathyroid hormone synergizes with non-cyclic AMP pathways to activate the cyclic AMP response element

Parathyroid hormone (PTH) activates multiple signaling pathways following binding to the PTH1 receptor in osteoblasts. Previous work revealed a discrepancy between cAMP stimulation and CRE reporter activation of truncated PTH peptides, suggesting that additional signaling pathways contribute to activation of the CRE. Using a CRE-Luciferase reporter containing multiple copies of the CRE stably transfected into the osteoblastic cell line Saos-2, we tested the ability of modulators of alternative pathways to activate the CRE or block the PTH-induced activation of the CRE. Activators of non-cyclic AMP pathways, that is, EGF (Akt, MAPK, JAK/STAT pathways); thapsigargin (intracellular calcium pathway); phorbol myristate acetate (protein kinase C, PKC pathway) induced minor increases in CRE-luciferase activity alone but induced dramatic synergistic effects in combination with PTH. The protein kinase A (PKA) inhibitor H-89 (10 microM) almost completely blocked PTH-induced activation of the CRE-reporter. Adenylate cyclase inhibitors SQ 22536 and DDA had profound and time-dependent biphasic effects on the CRE response. The MAPK inhibitor PD 98059 partially inhibited basal and PTH-induced CRE activity to the same degree, while the PKC inhibitor bisindolylmaleimide (BIS) had variable effects. The calmodulin kinase II inhibitor KN-93 had no significant effect on the response to PTH. We conclude that non-cAMP pathways (EGF pathway, calcium pathway, PKC pathway) converge on, and have synergistic effects on, the response of a CRE reporter to PTH.

Expression of PTH1R constructs in LLC-PK1 cells: protein nuclear targeting is mediated by the PTH1R NLS

This study demonstrates that the PTH1R NLS can target a fusion protein to the nucleus, and that this is blocked by sequences downstream of the NLS. GFP fused to the NLS showed a significant increase in nuclear targeting compared to GFP alone or GFP fused to a peptide of the same length. In previous studies, we demonstrated that the type I PTH/PTHrP receptor (PTH1R) localizes to the nucleus of cells within rat liver, kidney, uterus, ovary and gut. Similarly, nuclear localization of the PTH1R was observed in the cultured osteoblast-like cells MC3T3-E1, UMR106, ROS 17/2.8 and SaOS-2. We have identified a putative bipartite nuclear localization signal (NLS), from residues 471-488 in the protein sequence of the PTH1R. In this study, several PTH1R constructs were made in the Enhanced Green Fluorescent Protein (EGFP) expression vector (Clontech), transiently transfected into LLC-PK1 Clone 46 cells, and the resultant fusion protein expression followed by fluorescence microscopy. This particular clone of LLC-PK1 shows no biochemical response in vitro to parathyroid hormone. Constructs included the entire PTH1R sequence (PTH1R-GFP), the putative NLS fused to the C-terminus of GFP (GFP-NLS) or the NLS through to the C-terminus of the PTH1R fused to GFP (GFP-NLSCT). Deconvolution fluorescence microscopy of cells transfected with PTH1R-GFP showed abundant fluorescent signal throughout the cells with distinctly fluorescing plasma membranes. These cells also exhibited an increase in cAMP production in response to (0-10(-8) M) hPTH(1-34), with an increase in cAMP from 11 fmol/mug of protein to 101 fmol/microg. In contrast, cells transfected with the GFP-NLS construct showed significant nuclear sequestration of fluorescence as compared to GFP alone, GFP-NLSCT, or a short amino acid sequence fused to GFP (GFP-FFVAIYCFCNGEVQAEI). These results indicate that the NLS at residues 471-488 of the mature rat PTH1R is functional and plays a role in targeting the PTH1R the nucleus, also the addition of GFP to the C-terminus of the PTH1R still allows cAMP generation which will be useful for further studies.

Parathyroid hormone activates PKC-delta and regulates osteoblastic differentiation via a PLC-independent pathway

PTH exerts major effects upon bone by activating PTH/PTHrP receptors (PTH1Rs) expressed on osteoblasts. The PTH1R is capable of engaging multiple signaling pathways in parallel, including Gs/adenylyl cyclase (AC), Gq/phospholipase C/protein kinase C (PLC/PKC) and a distinct mechanism, involving activation of PKC via a PLC-independent pathway, that depends upon ligand determinants within the PTH(29-34) sequence. The involvement of PLC-dependent vs. PLC-independent PKC activation in PTH action was studied in clonal PTH1R-expressing murine calvarial osteoblasts ("Wt9") using two signal-selective analogs, [G1,R19]hPTH(1-28) and [G1,R19]hPTH(1-34). Both analogs lack PLC signaling but differ in their capacity to activate the PLC-independent PKC pathway. Both hPTH(1-34) and [G1,R19]hPTH(1-34), but not [G1,R19]hPTH(1-28), increased differentiation of Wt9 cells during a 16-day alternate-daily treatment protocol. Wt9 cells expressed PKC-betaI, -delta, -epsilon and -zeta, none of which exhibited net translocation to membranes in response to hPTH(1-34) or either analog. hPTH(1-34) induced activation of membrane-associated PKC-delta, however, and a time- and concentration-dependent increase in cytosolic [phospho-Thr505]PKC-delta which was maximal within 40 s at 100 nM in both Wt9 cells and primary osteoblasts. This response was mimicked by [G1,R19]hPTH(1-34) but not by [G1,R19]hPTH(1-28). Increased expression of bone sialoprotein (BSP) and osteocalcin (OC) mRNAs induced by PTH(1-34) and [G1,R19]hPTH(1-34) in Wt9 cells was blocked by rottlerin, a PKC-delta inhibitor. We conclude that PTH1Rs activate PKC-delta by a PLC-independent, PTH(29-34)-dependent mechanism that promotes osteoblastic differentiation.

Backbone-methylated Analogues of the Principle Receptor Binding Region of Human Parathyroid Hormone

We have used backbone N-methylations of parathyroid hormone (PTH) to study the role of these NH groups in the C-terminal amphiphilic alpha-helix of PTH (1-31) in binding to and activating the PTH receptor (P1R). The circular dichroism (CD) spectra indicated the structure of the C-terminal alpha-helix was locally disrupted around the methylation site. The CD spectra differences were explained by assuming a helix disruption for four residues on each side of the site of methylation and taking into account the known dependence of CD on the length of an alpha-helix. Binding and adenylyl cyclase-stimulating data showed that outside of the alpha-helix, methylation of residues Asp30 and Val31 had little effect on structure or activities. Within the alpha-helix, disruption of the structure was associated with increased loss of activity, but for specific residues Val21, Leu24, Arg25, and Leu28 there was a dramatic loss of activities, thus suggesting a more direct role of these NH groups in correct P1R binding and activation. Activity analyses with P1R-delNT, a mutant with its long N-terminal region deleted, gave a different pattern of effects and implicated Ser17, Trp23, and Lys26 as important for its PTH activation. These two groups of residues are located on opposite sides of the helix. These results are compatible with the C-terminal helix binding to both the N-terminal segment and also to the looped-out extracellular region. These data thus provide direct evidence for important roles of the C-terminal domain of PTH in determining high affinity binding and activation of the P1R receptor.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Stimulation of cortisol release by the N terminus of teleost parathyroid hormone-related protein in interrenal cells in vitro

The mode of action of PTHrP in the regulation of sea bream (Sparus auratus) interrenal cortisol production was studied in vitro using a dynamic superfusion system. Piscine (1-34)PTHrP (10(-6)-10(-11) M) stimulated cortisol production in a dose-dependent manner. The ED50 of (1-34)PTHrP was 2.8 times higher than that of (1-39)ACTH, and maximum increase in cortisol production in response to 10(-8) M of (1-34)PTHrP was approximately 7-fold lower than for 10(-8) M of (1-39)ACTH. In contrast to (1-34)PTHrP, piscine (10-20)PTHrP, (79-93)PTHrP, and (100-125)PTHrP (10(-9)-10(-7) M) did not stimulate cortisol production. The effect of piscine (1-34)PTHrP on cortisol production was abolished by N-terminal peptides in which the first amino acid (Ser) was absent and by simultaneous addition of inhibitors of the adenylyl cyclase-protein kinase A and phospholipase C-protein kinase C intracellular pathways but not by each separately. The PTHrP-induced signal transduction was further investigated by measurements of cAMP production and [H3]myo-inositol incorporation in an interrenal cell suspension. Piscine (1-34)PTHrP increased cAMP and total inositol phosphate accumulation, which is indicative that the mechanism of action of PTHrP in interrenal tissue involves the activation of both the adenylyl cyclase-cAMP and phospholipase C-inositol phosphate signaling pathways. These results, together with the expression of mRNA for PTHrP and for PTH receptor (PTHR) type 1 and PTHR type 3 receptors in sea bream interrenal tissue, suggest a specific paracrine or autocrine steroidogenic action of PTHrP mediated by the PTHRs.

PTHrP, PTH, and the PTH/PTHrP receptor in endochondral bone development

Endochondral bone development is a fascinating story of proliferation, maturation, and death. An understanding of this process at the molecular level is emerging. In particular, significant advances have been made in understanding the role of parathyroid-hormone-related peptide (PTHrP), parathyroid hormone (PTH), and the PTH/PTHrP receptor in endochondral bone development. Mutations of the PTH/PTHrP receptor have been identified in Jansen metaphyseal chondrodysplasia, Blomstrand's lethal chondrodysplasia, and enchondromatosis. Furthermore, genetic manipulations of the PTHrP, PTH, and the PTH/PTHrP receptor genes, respectively, have demonstrated the critical role of these proteins in regulating both the switch between proliferation and differentiation of chondrocytes, and their replacement by bone cells. A future area of investigation will be the identification of downstream effectors of PTH, PTHrP, and PTH/PTHrP receptor activities. Furthermore, it will be of critical importance to study how these proteins cooperate and integrate with other molecules that are essential for growth plate development.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Cyclic adenosine monophosphate/protein kinase A mediates parathyroid hormone/parathyroid hormone-related protein receptor regulation of osteoclastogenesis and expression of RANKL and osteoprotegerin mRNAs by marrow stromal cells

Parathyroid hormone (PTH) is a major regulator of osteoclast formation and activation, effects that are associated with reciprocal up- and down-regulation of RANKL and osteoprotegerin (OPG), respectively. The roles of specific downstream signals generated by the activated PTH/PTH-related protein (PTHrP) receptor (PTH1R), such as cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) and phospholipase C/protein kinase C (PLC/PKC), in controlling RANKL and OPG expression and osteoclastogenesis remain uncertain. In MS1 conditionally transformed clonal murine marrow stromal cells, which support PTH-induced osteoclast formation from cocultured normal spleen cells, PTH(1-34) increased RANKL and macrophage colony-stimulating factor (M-CSF) mRNA expression and decreased that of OPG when present continuously for 7-20 days at 37 degrees C in the presence of dexamethasone (Dex). In cells precultured for 7 days and then treated with PTH(1-34), similar reciprocal regulation of RANKL and OPG occurred, maximally at 6-24 h, that was of greater amplitude than the changes induced by chronic (7-10 days) PTH exposure. These acute effects of PTH(1-34) were mimicked by PKA stimulators (8-bromoadenosine [8Br]-cAMP or forskolin [FSK]), blocked by the PKA inhibitor Rp-cAMPs but unaffected by the PKC inhibitor GF109203X. Amino-truncated PTH(1-34) analogs PTH(5-34) and PTH(7-34) neither increased cAMP production in MS1 cells nor regulated RANKL or OPG mRNA. Reciprocal RANKL/OPG mRNA regulation was induced in MS1 cells by PTH(3-34) but only at high concentrations that also increased cAMP. The highly PKA-selective PTH analog [Gly1,Arg19]human PTH(1-28) exerted effects similar to PTH(1-34) on RANKL and OPG mRNAs and on osteoclast formation, both in MS1/spleen cell cocultures and in normal murine bone marrow cultures. The direct PKC stimulator 12-O-tetradecanoylphorbol-13-acetate (PMA) did not induce RANKL mRNA in MS1 cells, but it did up-regulate OPG mRNA and also antagonized osteoclast formation induced by PTH(1-34) in both MS1/spleen cocultures and normal bone marrow cultures. Thus, cAMP/PKA signaling via the PTH1R is the primary mechanism for controlling RANKL-dependent osteoclastogenesis, although direct PKC activation may negatively regulate this effect of PTH by inducing expression of OPG.

Bioactivity of PTH/PTHrP analogs lacking the 1-14 N-terminal domain

The N-terminal regions of 1-34 parathyroid hormone (PTH) and 1-34 parathyroid hormone related protein (PTHrP) are thought to be required for full agonist activity of these molecules and for signal transduction by cyclic AMP (cAMP). The C-terminal regions are thought to be involved in receptor binding and protein kinase C activation. In this study, two analogs of PTH/PTHrP lacking the segment 1-14 exhibited agonist activity in opossum kidney (OK) 3B2 cells. Analogs cPTHrP(15-34) and ANA NPY(13-36), an analog of neuropeptide Y, which both have amphipathic alpha helices, inhibited phosphate uptake and stimulated cAMP production in a dose-dependent manner, with half maximal activity in the microM range, compared to the nM range for hPTHrP(1-34) and hPTH(1-34). They also exhibited proportionately lower receptor binding affinities. cAMP production by these analogs was suppressed by the antagonist hPTHrP(7-34). Inhibition of phosphate uptake in response to the analogs was partially suppressed by H-89, but not by bisindolylmaleimide. The analogs also inhibited phosphate uptake and stimulated cAMP in parent OK cells and stimulated cAMP production in UMR-106 cells. These studies present the novel finding that in these cell types, a C-terminal region encompassing PTH/PTHrP(24-31), with the alpha-helical structure maintained, is sufficient for full activity at reduced potency.

Structural requirements for conserved arginine of parathyroid hormone

Arg-20 is one of two residues conserved in all peptides known to activate the parathyroid hormone (PTH) receptor. Previous studies have failed to find any naturally encoded analogues of residue 20 that had any adenylyl cyclase (AC) stimulating activity. In this work we have studied substitutions of Arg-20 with nonencoded amino acids and conformationally constrained analogues with side chains mimicking that of Arg. No analogue had more than 20% of the AC-stimulating ability of the natural Arg-20-bearing peptide. In descending order of activity, the most active analogues had (S)-4-piperidyl-(N-amidino)glycine (PipGly), norleucine (Nle), citrulline (Cit), or ornithine (Orn) at residue 20. Analogues with Arg-20 substituted with L-4-piperidyl-(N-amidino)alanine, Lys, Glu, Ala, Gln, (S)-2-amino-4-[(2-amino)pyrimidinyl]butanoic acid, or L-(4-guanidino)phenylalanine had very low or negligible activity. Low or negligible activities of Lys or Orn analogues suggested ionic interactions play a minor role in the Arg interaction with the receptor. The conformational constraints imposed by the PipGly ring had a negative effect on its ability to substitute for Arg. The side-chain H-bonding potential of the Cit ureimido group was likely an important factor in its mimicry of Arg. The increase in amphiphilicity, as demonstrated by its greater high-performance liquid chromatographic retention, and increased alpha-helix, as shown by circular dichroic spectroscopy, likely contributed to the activity of the Nle-20 analogue. The data demonstrated that specific H-bonding, hydrophobicity of the side chain, stabilization of alpha-helix, and possibly specific cation positioning were all important in the interaction of Arg-20 with receptor groups.

Molecular properties of the PTH/PTHrP receptor

The receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) is a G protein-coupled receptor (GPCR) that plays a key role in controlling blood Ca(2+) concentration and endochondral bone formation. This review focuses on the molecular mechanisms by which the receptor recognizes the PTH and PTHrP peptide ligands and transmits their signal across the cell membrane. The available data suggest that there are two principal components to the ligand-receptor interaction. First, a docking interaction between the C-terminal portion of PTH(1-34) and the N-terminal extracellular domain of the receptor; and second, a weaker interaction between the N-terminal portion of the ligand and the juxtamembrane region of the receptor, which induces signal transduction. A full understanding of these processes could lead to new PTH/PTHrP receptor ligands that are effective in controlling diseases of bone and mineral metabolism, such as osteoporosis.