Bone growth during daily or intermittent calcitriol treatment during renal failure with advanced secondary hyperparathyroidism

Calcitriol is a standard therapy for secondary hyperparathyroidism in chronic renal failure. We evaluated whether the effect of daily or intermittent calcitriol administration is more efficient in enhancing bone growth in renal failure with advanced secondary hyperparathyroidism in weanling 5/6 nephrectomized rats loaded with phosphorus to induce severe secondary hyperparathyroidism. The animals were treated daily or three times weekly with calcitriol for 4 weeks but the total weekly dose of calcitriol was the same. Although calcitriol increased the serum calcium, it did not lower parathyroid hormone (PTH) or improve tibia and body length. Animals with renal failure and advanced secondary hyperparathyroidism had decreased PTH/PTHrP, which was accompanied by an increase in the cyclin kinase inhibitor p57(Kip2). Calcitriol treatment upregulated the PTH/PTHrP receptor but also increased inhibitors of cell proliferation such as p21(Waf1/Cip1), IGFBP3, and FGFR3. Calcitriol also enhanced markers of chondrocyte differentiation, such as IGF1, Vitamin D receptor, FGF23, and bone morphogenetic protein-7. Receptor activator of nuclear factor-kappabeta ligand levels improved with calcitriol treatment but without changes in osteoprotegerin suggesting an enhancement of osteo/chondroclastogenesis and mineralization. Overall, both daily and intermittent calcitriol had similar effects on endochondral bone growth in phosphorus-loaded rats with renal failure.

The assay of the hypocalcemic PTH fragment inhibitor with PTH provides a more accurate assessment of renal osteodystrophy compared to the intact PTH assay

As the chronic kidney disease patient is being managed for PTH, calcium, phosphate, vitamin D, calcium x phosphate product and bone quality an accurate PTH measurement is essential. Over and under PTH suppressive therapies pose significant risks of mineral metabolism disturbances, osteodystrophies and soft tissue calcifications. Until recently it was thought that there was only one hormone secreted by the parathyroid gland, 1-84 PTH (or CAP). It is now known that there is another hormone secreted by the parathyroid gland (CIP) which is most likely 7-84 PTH. 7-84 PTH has been demonstrated to be an antagonist of 1-84 PTH with inverse biological activities. 7-84 PTH has been demonstrated to be hypocalcemic and able to lower bone turnover through an inhibition of osteoclast formation resulting in an overall inhibition of bone resorption. Whereas, 1-84 PTH operates through the PTH/PTHrp receptor the 7-84 PTH appears to operate through a C terminal PTH receptor. The CAP/CIP ratio decreases in the dialysis patient when calcium increases and vice versa. The 2nd generation "intact" PTH assays measure the sum of CAP plus CIP which render them ineffective at predicting bone turnover (72% predictive) and monitoring PTH suppressive treatments. By contrast the CAP/CIP ratio predicts bone turnover in the dialysis patient with a histologically determined 93% predictability. An elevated CAP/CIP ratio indicates high bone turnover and a decreased CAP/CIP ratio indicates adynamic low bone turnover.

Tuberoinfundibular peptide of 39 residues (TIP39): molecular structure and activity for parathyroid hormone 2 receptor

The neuropeptide TIP39 was recently purified from bovine hypothalamus based on the ability of the peptide to activate the parathyroid hormone 2 receptor (PTH2R) ( Nat. Neurosci. 2 (1999) 941). PTH2R is abundantly expressed in the nervous system, and its expression pattern suggests that it may play a role in modulation of pituitary function and in nociception. Towards understanding the physiological role of TIP39 and PTH2R, we cloned human, mouse and rat TIP39 gene. Our results revealed that: (1) the mature peptide is processed from a precursor; (2) TIP39 peptide is highly conserved among species; and (3) TIP39 from all species activates adenylyl cyclase and elevates intracellular calcium levels through PTH2R. We also defined and compared the structure-activity relationship of TIP39 on both activation of adenylyl cyclase and calcium mobilization pathways through PTH2R, finding common and differential determinants of TIP39 that are required for these pathways. Furthermore, we observed that TIP39 elevates intracellular calcium levels in primary dorsal root ganglion neurons whereas the peptide inactive on PTH2R do not, suggesting that TIP39 may activate these neurons important for nociception in vivo through PTH2R-dependent mechanisms.

Interleukin-11 induces osteoblast differentiation and acts synergistically with bone morphogenetic protein-2 in C3H10T1/2 cells

Interleukin-11 (IL-11) is a pleiotropic cytokine that supports various types of hematopoietic cell growth and is involved in bone resorption. We report here the involvement of recombinant human IL-11 (rHuIL-11) in osteoblast differentiation in mouse mesenchymal progenitor cells, C3H10T1/2. rHuIL-11 alone increased alkaline phosphatase (ALP) activity and upregulated expression levels of osteocalcin (OC), bone sialo protein (BSP), and parathyroid hormone receptor (PTHR) mRNA. rHuIL-11 had no effect on expression of type II collagen, peroxisome proliferator-activated receptor-gamma2 (PPAR-gamma2), adipocyte fatty acid-binding protein P2 (aP2), and myogenic MyoD protein (MyoD). Recombinant human bone morphogenetic protein (rHuBMP)-2 increased ALP activity and mRNA expression of these genes except for MyoD. The expression patterns of ALP activity and osteoblast-specific or chondrocyte-specific genes suggest that rHuIL-11 may be involved in early differentiation of osteoblasts at a step earlier than that which is affected by rHuBMP-2. In support of this hypothesis, combined treatment with rHuIL-11 and rHuBMP-2 synergistically increased ALP activity and mRNA expression of OC and type II collagen, rHuIL-11 also abrogated the increased levels of PPAR-gamma2, aP2 mRNA caused by rHuBMP-2. Our results suggest that rHuIL-11 alone and in combination with rHuBMP-2 can induce osteoblastic differentiation of progenitor cells and plays an important role in osteogenesis.

Enhanced activity in parathyroid hormone-(1-14) and -(1-11): novel peptides for probing ligand-receptor interactions

The amino-terminal portion of PTH is critical for PTH-1 receptor (P1Rc) activation. In exploring this component of the ligand receptor interaction, we recently showed that the agonist potency of the weakly active PTH-(1-14)NH(2) peptide can be enhanced by natural amino acid substitutions at several positions, including position 11 (normally leucine). Here we show that the potency of PTH-(1-14)NH(2) can be enhanced by using nonnatural amino acids that increase the length and polarizability of the position 11 side-chain. Thus, in LLC-PK(1) cells stably expressing high levels of the human P1Rc, [homoarginine([Har)(11)]PTH-(1-14)NH(2) was 30-fold more potent for cAMP production than was native PTH-(1-14)NH(2). Combining the homoarginine-11 substitution with other recently identified activity-enhancing substitutions yielded [Ala(3,12),Gln(10),Har(11),Trp(14)]PTH-(1-14)NH(2), which was 1500-fold more potent than PTH-(1-14)NH(2) (EC(50) = 0.12 +/- 0.04 and 190 +/- 20 microM, respectively) and only 63-fold less potent than PTH-(1-34) (EC(50) = 1.9 +/- 0.5 nM). The even shorter analog [Ala(3),Gln(10),Har(11)]PTH-(1-11)NH(2) was also a full cAMP agonist (EC(50) = 3.1 +/- 1.5 microM). Receptor mutations at Phe(184) and Leu(187) located near the boundary of the amino-terminal domain and transmembrane domain-1 severely impaired responsiveness to the PTH-(1-11) analog. Overall, these studies demonstrate that PTH analogs of only 11 amino acids are sufficient for activation of the PTH-1 receptor through interaction with its juxtamembrane region.

Multiple sites of contact between the carboxyl-terminal binding domain of PTHrP-(1--36) analogs and the amino-terminal extracellular domain of the PTH/PTHrP receptor identified by photoaffinity cross-linking

The carboxyl-terminal portions of parathyroid hormone (PTH)-(1--34) and PTH-related peptide (PTHrP)-(1-36) are critical for high affinity binding to the PTH/PTHrP receptor (P1R), but the mechanism of receptor interaction for this domain is largely unknown. To identify interaction sites between the carboxyl-terminal region of PTHrP-(1--36) and the P1R, we prepared analogs of [I(5),W(23),Y(36)]PTHrP-(1--36)-amide with individual p-benzoyl-l-phenylalanine (Bpa) substitutions at positions 22--35. When tested with LLC-PK(1) cells stably transfected with human P1R (hP1R), the apparent binding affinity and the EC(50) of agonist-stimulated cAMP accumulation for each analog was, with the exception of the Bpa(24)-substituted analog, similar to that of the parent compound. The radiolabeled Bpa(23)-, Bpa(27)-, Bpa(28)-, and Bpa(33)-substituted compounds affinity-labeled the hP1R sufficiently well to permit subsequent mapping of the cross-linked receptor region. Each of these peptides cross-linked to the amino-terminal extracellular domain of the P1R: [I(5),Bpa(23),Y(36)]PTHrP-(1-36)-amide cross-linked to the extreme end of this domain (residues 33-63); [I(5),W(23),Bpa(27),Y(36)]PTHrP-(1--36)-amide cross-linked to residues 96--102; [I(5),W(23),Bpa(28),Y(36)]PTHrP-(1--36)- amide cross-linked to residues 64--95; and [I(5),W(23), Bpa(33),Y(36)]PTHrP-(1--36)-amide cross-linked to residues 151-172. These data thus predict that residues 23, 27, 28, and 33 of native PTHrP are each near to different regions of the amino-terminal extracellular receptor domain of the P1R. This information helps define sites of proximity between several ligand residues and this large receptor domain, which so far has been largely excluded from models of the hormone-receptor complex.

Synthetic carboxyl-terminal fragments of parathyroid hormone (PTH) decrease ionized calcium concentration in rats by acting on a receptor different from the PTH/PTH-related peptide receptor

Even if the carboxyl-terminal (C-) fragments/intact (I-) PTH ratio is tightly regulated by the ionized calcium (Ca(2+)) concentration in humans and animals, in health and in disease, the physiological roles of C-PTH fragments and of the C-PTH receptor remain elusive. To explore these issues, we studied the influence of synthetic C-PTH peptides of various lengths on Ca(2+) concentration and on the calcemic response to human (h) PTH-(1-34) and hPTH-(1-84) in anesthetized thyroparathyroidectomized (TPTX) rats. We also looked at the capacity of these PTH preparations to react with the PTH/PTHrP receptor and with a receptor for the carboxyl (C)-terminal portion of PTH (C-PTH receptor) in rat osteosarcoma cells, ROS 17/2.8. The Ca(2+) concentration was reduced by 0.19 +/- 0.03 mmol/liter over 2 h in all TPTX groups. Infusion of solvent over 2 more h had no further effect on the Ca(2+) concentration (-0.01 +/- 0.01 mmol/liter), whereas infusion of hPTH-(7-84) or a fragment mixture [10% hPTH-(7-84) and 45% each of hPTH-(39-84) and hPTH-(53-84)] 10 nmol/h further decreased the Ca(2+) concentration by 0.18 +/- 0.02 (P<0.001) and 0.07+/-0.04 mmol/liter (P< 0.001), respectively. Infusion of hPTH-(1-84) or hPTH-(1-34) (1 nmol/h) increased the Ca(2+) concentration by 0.16 +/- 0.03 (P < 0.001) and 0.19 +/- 0.03 mmol/liter (P < 0.001), respectively. Adding hPTH-(7-84) (10 nmol/h) to these preparations prevented the calcemic response and maintained Ca(2+) concentrations equal to or below levels observed in TPTX animals infused with solvent alone. Adding the fragment mixture (10 nmol/h) to hPTH-(1-84) did not prevent a normal calcemic response, but partially blocked the response to hPTH-(1-34), and more than 3 nmol/h hPTH-(7-84) prevented it. Both hPTH-(1-84) and hPTH-(1-34) stimulated cAMP production in ROS 17/2.8 clonal cells, whereas hPTH-(7-84) was ineffective in this respect. Both hPTH-(1-84) and hPTH-(1-34) displaced (125)I-[Nle(8,18),Tyr(34)]hPTH-(1-34) amide from the PTH/PTHrP receptor, whereas hPTH-(7-84) had no such influence. Both hPTH-(1-84) and hPTH-(7-84) displaced (125)I-[Tyr(34)]hPTH-(19-84) from the C-PTH receptor, the former preparation being more potent on a molar basis, whereas hPTH-(1-34) had no effect. These results suggest that C-PTH fragments, particularly hPTH-(7-84), can influence the Ca(2+) concentration negatively in vivo and limit in such a way the calcemic responses to hPTH-(1-84) and hPTH-(1-34) by interacting with a receptor different from the PTH/PTHrP receptor, possibly a C-PTH receptor.

Endogenous prostaglandin E2 and insulin-like growth factor 1 can modulate the levels of parathyroid hormone receptor in human osteoarthritic osteoblasts

Subchondral bone sclerosis may be important for the onset and/or progression of cartilage loss/damage in human osteoarthritis (OA). OA osteoblasts are resistant to parathyroid hormone (PTH) stimulation, which could explain bone sclerosis via the inhibition of PTH-dependent catabolism. Here, we investigated the molecular mechanism(s) responsible for reduced PTH-dependent cyclic adenosine monophosphate (cAMP) synthesis in OA subchondral osteoblasts. Although cholera toxin (CTX) increased basal cAMP formation in these cells, it failed to stimulate PTH-dependent cAMP synthesis, whereas pertussis toxin (PTX) did not inhibit basal cAMP, yet diminished PTH-dependent cAMP production. Binding of 125I-PTH indicated lower PTH receptor levels in OA than in normal osteoblasts (-50.5 +/- 9.5%). This could be attributed to either reduced expression of the PTH receptor (PTH-R) or altered recycling of existing pools of receptors. Reverse-transcription polymerase chain reaction (RT-PCR) analysis indicated decreased PTH-R messenger RNA (mRNA) levels in OA cells that were highly variable (ranging from -10% to -60%), a situation that reflects disease severity. Interestingly, OA osteoblasts produced more prostaglandin E2 (PGE2) than normal osteoblasts, and using naproxen, a cyclo-oxygenase inhibitor, increased PTH-dependent cAMP formation to a level similar to normal osteoblasts. Because heterologous desensitization can explain a decrease in PTH binding but cannot account for reduced PTH-R expression, we looked at the possible effect of insulin-like growth factor 1 (IGF-1) on this parameter. Blocking IGF-1 signaling with a neutralizing receptor antibody increased 125I-PTH binding in both normal and OA osteoblasts. Conversely, treatments with IGF-1 receptor (IGF-1R) antibody only slightly increased the levels of PTH-R mRNA whereas the addition of IGF-1 significantly reduced PTH-R mRNA levels (-24.1 +/- 7.1%), yet neither PGE2 nor naproxen modified PTH-R levels. These results suggest that both IGF-1 signaling and PGE2 formation repress PTH-dependent response in OA osteoblasts, a situation that can contribute to abnormal bone remodeling and bone sclerosis in OA.

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

The parathyroid hormone (PTH) fragment PTH(1-34) stimulates adenylyl cyclase, phospholipase C (PLC), and protein kinase C's (PKCs) in cells that express human, opossum, or rodent type 1 PTH/PTH-related protein (PTHrP) receptors (PTHR1s). Certain carboxyl (C)-terminally truncated fragments of PTH(1-34), such as human PTH(1-31) [hPTH-(1-31)NH2], stimulate adenylyl cyclase but not PKCs in rat osteoblasts or PLC and PKCs in mouse kidney cells. The hPTH(1-31)NH2 peptide does fully stimulate PLC in HKRK B7 porcine renal epithelial cells that express 950,000 transfected hPTHR1s per cell. Amino (N)-terminally truncated fragments, such as bovine PTH(3-34) [bPTH(3-34)], hPTH(3-34)NH2, and hPTH(13-34), stimulate PKCs in Chinese hamster ovary (CHO) cells expressing transfected rat receptors, opossum kidney cells, and rat osteoblasts, but an intact N terminus is needed to stimulate PLC via human PTHR1s in HKRK B7 cells. We now report that the N-terminally truncated analogs bPTH(3-34)NH2 and hPTH(13-34)OH do activate PKC via human PTHR1s in HKRK B7 cells, although less effectively than hPTH(1-34)NH2 and hPTH(1-31)NH2. Moreover, in a homologous human cell system (normal foreskin fibroblasts), these N-terminally truncated fragments stimulate PKC activity as strongly as hPTH(1-34)NH2 and hPTH(1-31)NH2. Thus, it appears that unlike their opossum and rodent equivalents, hPTHR1s can stimulate both PLC and PKCs when activated by C-terminally truncated fragments of PTH(1-34). Furthermore, hPTHR1s, like the PTHR1s in rat osteoblasts, opossum kidney cells, and rat PTHR1-transfected CHO cells also can stimulate PKC activity by a mechanism that is independent of PLC. The efficiency with which the N-terminally truncated PTH peptides stimulate PKC activity depends on the cellular context in which the PTHR1s are expressed.

A N-terminal PTHrP peptide fragment void of a PTH/PTHrP-receptor binding domain activates cardiac ET(A) receptors

1. Adult ventricular cardiomyocytes show an unusual structure-function relationship for cyclic AMP-dependent effects of PTHrP. We investigated whether PTHrP(1 - 16), void of biological activity on classical PTHrP target cells, is able to mimic the positive contractile effect of PTHrP(1 - 34), a fully biological agonist on cardiomyocytes. 2. Adult ventricular cardiomyocytes were paced at a constant frequency of 0.5 Hz and cell contraction was monitored using a cell-edge-detection system. Twitch amplitudes, expressed as per cent cell shortening of the diastolic cell length, and rate constants for maximal contraction and relaxation velocity were analysed. 3. PTHrP(1 - 16) (1 micromol l(-1)) mimicked the contractile effects of PTHrP(1 - 34) (1 micromol l(-1)). It increased the twitch amplitude from 5.33+/-0.72 to 8.95+/-1.10 (% dl l(-1)) without changing the kinetic of contraction. 4. PTH(1 - 34) (10 micromol l(-1)) affected the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 5. RpcAMPS (10 micromol l(-1)) inhibited the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 6. The positive contractile effect of PTHrP(1 - 16) was antagonized by the ET(A) receptor antagonist BQ123. 7. Sarafotoxin 6b and PTHrP(1 - 16), but not PTHrP(1 - 34), replaced (3)H-BQ123 from cardiac binding sites. 8. We conclude that N-terminal PTHrP peptides void of a PTH/PTHrP-receptor binding domain are able to bind to, and activate cardiac ET(A) receptors.

Chronic phosphorus supplementation decreases the expression of renal PTH/PTHrP receptor mRNA in rats

Dietary intake of high levels of phosphorus is known to increase serum levels of parathyroid hormone (PTH); however, how this increased serum PTH affects the action of PTH in major target tissues, particularly by kidney, remains unknown. In the present study, we therefore undertook to clarify this point in intact animals fed a high-P diet by examining various parameters of PTH action. Twelve weanling Wistar male rats were assigned randomly to two groups: a control group with dietary Ca:P = 1:1 and a high-P group (Ca:P = 1:3) fed the standard AIN-76 diet supplemented with P (0.5 and 1.5 g/100 g of diet). After 3 weeks of feeding, in the high-P diet group, we observed that serum Ca was lowered, without a difference in serum P, when compared to the control group. Excretion of urinary cAMP, an index of renal PTH action, was also decreased, with higher excretion of urinary P in those rats fed the high-P diet. In agreement with the decreased cAMP excretion, a clear reduction in PTH/PTH-related protein (PTHrP) receptor gene expression as estimated by Northern blotting was observed in the kidney, despite increased levels of serum PTH. Thus, the present study indicated that a high-P diet reduces PTH action in the kidney, though the serum PTH is increased.

Evaluating the ligand specificity of zebrafish parathyroid hormone (PTH) receptors: comparison of PTH, PTH-related protein, and tuberoinfundibular peptide of 39 residues

Homologs of mammalian PTH1 and PTH2 receptors, and a novel PTH3 receptor have been identified in zebrafish (zPTH1, zPTH2, and zPTH3). zPTH1 receptor ligand specificity is similar to that of mammalian PTH1 receptors. The zPTH2 receptor is selective for PTH over PTH-related protein (PTHrP); however, PTH produces only modest cAMP accumulation. A PTH2 receptor-selective peptide, tuberoinfundibular peptide of 39 residues (TIP39), has recently been purified from bovine hypothalamus. The effect of TIP39 has not previously been examined on zebrafish receptors. The zPTH3 receptor was initially described as PTHrP selective based on comparison with the effects of human PTH. We have now examined the ligand specificity of the zebrafish PTH-recognizing receptors expressed in COS-7 cells using a wide range of ligands. TIP39 is a potent agonist for stimulation of cAMP accumulation at two putative splice variants of the zPTH2 receptor (EC50, 2.6 and 5.2 nM); in comparison, PTH is a partial agonist [maximal effect (Emax) of PTH peptides ranges from 28-49% of the TIP39 Emax]. As TIP39 is much more efficacious than any known PTH-like peptide, a homolog of TIP39 may be the zPTH2 receptor's endogenous ligand. At the zPTH3 receptor, rat PTH-(1-34) and rat PTH-(1-84) (EC50, 0.22 and 0.45 nM) are more potent than PTHrP (EC50, 1.5 nM), and rPTH-(1-34) binds with high affinity (3.2 nM). PTH has not been isolated from fish. PTHrP-like peptides, which have been identified in fish, may be the natural ligands for zPTH1 and zPTH3 receptors.

Role of asparagine-linked oligosaccharides in the function of the rat PTH/PTHrP receptor

The receptor for parathyroid hormone (PTH) and PTH-related peptide (PTHrP) is a G-protein-coupled receptor with four potential sites for N-linked glycosylation. The contribution of the oligosaccharide moieties to cell surface expression, ligand binding, and signal transduction was investigated. Site-directed mutagenesis of the rat PTH/PTHrP receptor cDNA was performed at single or combination of the four potential glycosylation sites to determine the effect of the putative carbohydrate chains on the activities of the receptor. The results revealed that all four potential N-glycosylation sites in the PTH/PTHrP receptor are glycosylated. Receptors missing a single or multiple glycosylation consensus but with at least one intact glycosylation site expressed sufficiently and functioned normally. In contrast, the nonglycosylated receptor, in which all four glycosylation sites were mutated, is deficient in these functions. These data indicate important roles for N-linked glycosylation in PTH/PTHrP receptor functions.

Expression-level dependent activation of recombinant human parathyroid hormone/parathyroid hormone-related peptide receptor: effect of human parathyroid hormone (1-34), (1-31), and (28-48)

A stable recombinant chinese hamster ovary (CHO) cell model system expressing the human type-1 receptor for parathyroid hormone and parathyroid hormone-related peptide (hPTH-R) was established for the analysis of human PTH (hPTH) variants. The cell lines showed receptor expression in the range from 10(5) to I.9 x 10(6) receptors per cell. The affinity of the receptors for hPTH-(1-34) was independent of the receptor number per cell (Kd approximately = 8 nmol/1). The induction of cAMP by hPTH-(1-34) is maximal in clones expressing >2x10(5) receptors per cell and Ca++ signals were maximal in cell lines expressing >1.4x10(6) receptors per cell. Second messenger specific inhibitors demonstrated that PTH-induced increases in intracellular cAMP and Ca++ are independent and Ca++ ions are derived from intracellular stores. The cAMP-specific receptor activator hPTH-(1-31) showed also an increase in intracellular Ca++. Even in cell lines expressing more than 10(6) receptors per cell the Ca++/PKC specific activator hPTH-(28-48) did not activate hPTH-Rs. Based on these results, synthesis of further derivatives of PTH is required to identify pathway-specific ligands for the type-1 hPTH-R.

Response of immortalized murine cementoblasts/periodontal ligament cells to parathyroid hormone and parathyroid hormone-related protein in vitro

Cementum is an essential component of the periodontium, but the mechanisms involved in regulating the activity of this tissue are poorly understood. As one approach to better defining the cellular and molecular properties of cementum and the associated ligament, immortalized murine cell populations expressing gene markers associated with both cementoblasts (CM) and periodontal ligament cells (PDL), termed CM/PDL cells, were established. To further characterize these cells, their responsiveness to parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) was examined. CM/PDL cells were tested for the presence of steady state PTH-1 receptor mRNA using Northern blot analysis. In addition, the ability of PTH and PTHrP to stimulate cAMP production and c-fos mRNA expression in CM/PDL cells was determined, using a cAMP-binding assay and northern blot hybridization, respectively. Rat osteosarcoma cells (ROS 17/2.8) were used as a positive control and human periodontal ligament cells as a negative control. Northern blot analysis demonstrated that cells within the CM/PDL cell population expressed PTH-1 receptor mRNA. Both PTH (1-34) and PTHrP (1-34) increased cAMP and c-fos mRNA in CM/PDL cells. Furthermore, PTHrP treatment for either 24 or 48 h downregulated expression of transcripts for bone sialoprotein, osteocalcin and PTH-1 receptor by CM/PDL cells and abolished CM/PDL cell-mediated mineralization in vitro. These results indicate that cells within the CM/PDL population are targets for PTH and PTHrP action and that PTHrP may play an important part in regulating the biomineralization of cementum.

Regulation of PTHrP and PTH/PTHrP receptor by extracellular Ca2+ concentration and hormones in the breast cancer cell line 8701-BC

It was previously reported that 8701-BC breast tumour cells express the gene for parathyroid hormone-related peptide (PTHrP) and PTH/PTHrP receptor (PTHrP-R) and release immunoreactive PTHrP (iPTHrP) into the extracellular medium. Since the regulation of PTHrP and PTHrP-R by breast cancer cells has been poorly investigated so far, we have chosen the 8701-BC cell line as a model system to investigate whether alterations in the extracellular Ca2+ concentration ([Ca2+]e) and treatment with some well-known differentiation agents for breast cells, such as dimethyl sulfoxide, hydrocortisone, progesterone, prolactin, all-trans retinoic acid and transforming growth factor-beta1 might (i) modulate quantitatively the release of iPTHrP, (ii) affect the PTHrP promoter usage and mRNA splicing patterns, and (iii) modify the expression of PTHrP-R. The data obtained indicate that 8701-BC cells are potentially able to utilise different start sites and mRNA splicing patterns for PTHrP transcription, and respond to variations of [Ca2+]e and to the addition of two hormones, hydrocortisone and progesterone, with modifications in the extracellular amount of iPTHrP. Moreover, expression of PTHrP-R is also modulated by changes of [Ca2+]e or treatment with hydrocortisone. This indicates that the 8701 -BC cell line is a suitable in vitro model for further studies on the complex molecular regulation of the PTHrP/PTHrP-R pair in breast cancer.

Adverse effects of an active fragment of parathyroid hormone on rat hippocampal organotypic cultures

Adverse effects of an active fragment of parathyroid hormone (PTH(1 - 34)), a blood Ca(2+) level-regulating hormone, were examined using rat hippocampal slices in organotypic culture. Exposure of cultured slice preparations to 0.1 microM PTH(1 - 34) for 60 min resulted in a gradual increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)); this effect was most obvious in the apical dendritic region of CA1 subfield. When PTH(1 - 34) at a lower concentration (1 nM) was added to the culture medium and its toxic effects examined using a propidium iodide intercalation method, significant toxicity was seen 3 days after exposure and increased with time. Cells in the CA1 region seemed more vulnerable to the hormone than cells in other regions. At 1 week of exposure, the toxic effects were dose-dependent over the range of 0.1 pM to 0.1 microM, the minimum effective dose being 10 pM. The adverse effects were not induced either by the inactive fragment, PTH(39 - 84), or by an active fragment of PTH-related peptide (PTHrP(1 - 34)), an intrinsic ligand of the brain PTH receptor. The PTH(1 - 34)-induced adverse effects were significantly inhibited by co-administration of 10 microM nifedipine, an L-type Ca(2+) channel blocker, but not by co-administration of blockers of the other types of Ca(2+) channel. The present study demonstrates that sustained high levels of PTH in the brain might cause degeneration of specific brain regions due to Ca(2+) overloading via activation of dihydropyridine-sensitive Ca(2+) channels, and suggests that PTH may be a risk factor for senile dementia. British Journal of Pharmacology (2000) 129, 21 - 28

Design and applications of parathyroid hormone analogues

The endocrine parathyroid hormone (PTH) is the major regulator of serum calcium levels. In contrast, the autocrine/paracrine parathyroid hormone-related peptide (PTHrP) has been associated with organism development. Both are secreted as much larger molecules but have their major functions associated with their N-terminal 34 residues. They share a common receptor expressed in organs critical to PTH function - bone, kidney, and intestine. PTH and PTHrP receptor activation stimulates adenylyl cyclase (AC), phospholipase C (PLC), and phospholipase D (PLD) in target cells. It has been possible to separate the AC-stimulation from that of PLC. AC-stimulation requires at least the N-terminal 28 residues of PTH and PLC-stimulation requires a minimum of residues 29-32-NH2. Intermittent administration of PTH stimulates bone growth and requires AC-stimulation. The shortest linear sequence of hPTH with essentially full anabolic activity for bone growth-stimulation is hPTH(1-31)NH2. Two applications are postulated for PTH and PTHrP-based pharmaceuticals - treatment of bone loss due to osteoporosis and reversal of the hypercalcemic effect of malignancy. PTHrP analogues which strongly inhibit PTHrP AC-stimulation showed promise for the treatment of malignancy-associated hypercalcemia in animal trials but failed in human ones. However, both animal and human trials of hPTH have shown significant bone growth-stimulating effects. New deletion, substitution and cyclized analogues of PTH show great promise both for greater in vitro activity and possibly for improved delivery and greater specificity as agents for restoration of bone loss in osteoporosis.

Similar structures and shared switch mechanisms of the beta2-adrenoceptor and the parathyroid hormone receptor. Zn(II) bridges between helices III and VI block activation

The seven transmembrane helices of serpentine receptors comprise a conserved switch that relays signals from extracellular stimuli to heterotrimeric G proteins on the cytoplasmic face of the membrane. By substituting histidines for residues at the cytoplasmic ends of helices III and VI in retinal rhodopsin, we engineered a metal-binding site whose occupancy by Zn(II) prevented the receptor from activating a retinal G protein, Gt (Sheikh, S. P., Zvyaga, T. A. , Lichtarge, O., Sakmar, T. P., and Bourne, H. R. (1996) Nature 383, 347-350). Now we report engineering of metal-binding sites bridging the cytoplasmic ends of these two helices in two other serpentine receptors, the beta2-adrenoreceptor and the parathyroid hormone receptor; occupancy of the metal-binding site by Zn(II) markedly impairs the ability of each receptor to mediate ligand-dependent activation of Gs, the stimulatory regulator of adenylyl cyclase. We infer that these two receptors share with rhodopsin a common three-dimensional architecture and an activation switch that requires movement, relative to one another, of helices III and VI; these inferences are surprising in the case of the parathyroid hormone receptor, a receptor that contains seven stretches of hydrophobic sequence but whose amino acid sequence otherwise shows no apparent similarity to those of receptors in the rhodopsin family. These findings highlight the evolutionary conservation of the switch mechanism of serpentine receptors and help to constrain models of how the switch works.

Sequential progression of the differentiation program by bone morphogenetic protein-2 in chondrogenic cell line ATDC5

During embryonic development of long bones, chondroprogenitor cells exhibit the transitions of phenotype, i.e., from type I collagen-expressing cells to type II collagen-expressing chondrocytes through cellular condensation (early-phase differentiation) and then to type X collagen-expressing mineralizing chondrocytes (late-phase differentiation). The chondrogenic cell line ATDC5 displays the sequential transitions of phenotype in a synchronous manner in vitro. Taking advantage of the sequential differentiation, the effects of growth factors were evaluated at each differentiation step of ATDC5 cells. Among the factors examined, bone morphogenetic protein-2 (BMP-2) specifically stimulated a progression of the early-phase differentiation. Rounded chondrocytic cells were formed all over the culture plates by skipping out a cellular condensation stage. Fibroblast growth factor-2 stimulated growth of undifferentiated ATDC5 cells, but failed to stimulate overt chondrogenesis. The proliferation of differentiated cells ceased as cartilage nodules became maturated. At this stage, BMP-2 markedly up-regulated expression of type X collagen mRNA (a 9.1-fold increase) and alkaline phosphatase mRNA (a 7.5-fold increase) within 48 h. On the other hand, it down-regulated expression of type II collagen and parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor mRNAs, markers of the early differentiation. BMP-2 stimulated the formation of calcified matrix, an end product of terminally differentiated chondrocytes. These results indicated that BMP stimulated the sequential progression of early- and late-phase differentiation of ATDC5 cells.

Effects of excess PTH on nonclassical target organs

The classical target organs for parathyroid hormone (PTH) are the bone and kidneys. In uremia, however, numerous studies have shown that PTH may also affect the function of a number of nonclassical organs and tissues besides the bone and kidney, including the brain, heart, smooth muscles, lungs, erythrocytes, lymphocytes, pancreas, adrenal glands, and testes. Most of these effects do not apply to the generally accepted actions or normal regulatory mechanisms of PTH. Thus, the potential role of PTH as one of the possibly many toxins in uremia is of current interest. The molecular basis for the actions of elevated PTH levels on various nonrenal and nonskeletal organs or tissues might be mediated via the widespread distribution of the classical PTH/PTH-related peptide (PTHrP) receptors and via the novel PTH2 receptors. The present survey deals with an evaluation of the nonrenal and nonskeletal effects of excess PTH in uremia, taking into consideration the presently available information on the organ-specific expression of the classical and novel PTH receptors, and of the expression and function of PTHrP.

Control of hair growth with parathyroid hormone (7-34)

Parathyroid hormone (PTH) related peptide (PTHrP) is thought to influence the proliferation and differentiation of the epidermis and hair follicle. As a means of elucidating the biologic function of PTHrP on the hair follicle, a PTHrP analog PTH (7-34), which is a PTH/PTHrP receptor antagonist, was given intraperitoneally twice daily to C57 BL/6 mice at different stages of the hair cycle. PTH (7-34) induced 99 +/- 4.5% (mean +/- SEM) of resting telogen hair follicles into a proliferative (anagen) state, whereas 100% of the hair follicles in the control group remained in telogen. To determine whether this peptide influenced the progression of the hair follicles from anagen to catagen (hair follicle maturation and regression), groups of mice that were either spontaneously in or induced to anagen received either PTH (7-34) or placebo. Morphometric analysis of the hair follicles from the middle back region of the spontaneous anagen mice that received PTH (7-34) revealed that 19 +/- 4% (mean +/- SEM) of the follicles were in anagen VI, whereas none (0%) were in anagen in the control group. Similarly, in induced anagen mice treated with PTH (7-34), 22.3 +/- 1.4 (mean +/- SEM) of the follicles were in anagen VI compared to only 1.3 +/- 0.7% in the control mice. Together these observations suggest that PTHrP is a hair follicle morphogen that may be a major factor responsible for controlling the hair cycle. These studies provide a new insight for development of PTHrP analogs for a wide variety of disorders related to disturbances of hair cycling.

Evidence for a parathyroid hormone-2 receptor selective ligand in the hypothalamus

The PTH2 receptor is expressed in several brain nuclei but we have been unable to detect mRNA encoding PTH, which is the only known ligand for the PTH2 receptor, in the brain. We now have evidence for a PTH2 receptor selective ligand in an acid-acetone extract made from bovine hypothalamus. The partially purified extract activates the PTH2 receptor more effectively than it activates the PTH/PTHrP receptor, while PTH activates these two receptors at similar concentration. The activity appears immunologically distinct from PTH and its effect is potently antagonized by [D-Trp12]bPTH(7-34). These data provide evidence for a biologically active peptide, which may be related to PTH, and which is a potential new neurotransmitter or hormone.

Coordinate regulation of PTH/PTHrP receptors by PTH and calcitriol in UMR 106-01 osteoblast-like cells

High levels of PTH result in desensitization of target cells to further stimulation with PTH in vivo and in vitro. While studies in vitro demonstrate that the effect of PTH can be direct, it is also possible that studies in vivo may be complicated by the fact that other potential regulators of PTH action, such as increased levels of calcitriol, may play a role. Thus, we examined the actions of calcitriol and PTH on PTH/PTHrP-receptor expression in confluent cultures of UMR 106-01 osteoblast-like cells treated with calcitriol, PTH or both hormones for one to three days. Following these treatments, studies of PTH receptor binding, cAMP generation, and steady-state levels of PTH/PTHrP receptor mRNA were performed. Exposure to PTH resulted in a decrease in PTH stimulated cAMP generation by 88 +/- 2%, and PTH binding by 63 +/- 3%. Levels of PTH/PTHrP-receptor mRNA decreased progressively reaching 20% of control values after three days of PTH (100 nM) treatment. Calcitriol also resulted in a dose and time-dependent decrease in PTH/PTHrP-receptor mRNA, decreasing by 72 +/- 4% after 48 hours. PTH receptor binding and cAMP generation were diminished by 42 +/- 3% and 42 +/- 4%, respectively. Co-incubation of UMR 106-01 cells with submaximal doses of calcitriol and PTH together revealed that the levels of PTH/PTHrP-receptor mRNA were decreased by both hormones together to a greater extent than with either alone. These studies show that both calciotropic hormones, PTH and calcitriol, are potent regulators of PTH/PTHrP-receptor gene expression in UMR 106-01 osteoblast-like cells. Thus, stimulation of calcitriol production by PTH may result in a coordinated down-regulation of PTH receptor expression by these hormones.

Inositol 1-,4-,5-trisphosphate-dependent Ca2+ signaling by the recombinant human PTH/PTHrP receptor stably expressed in a human kidney cell line

We previously reported the preparation and partial characterization of a series of human embryonic kidney cell lines (HEK-293) stably expressing various numbers of the recombinant human (h) parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (Rc). Using this expression system we examined ligand (PTH or PTHrP) binding characteristics and cyclic AMP responsiveness. We have now extended these studies to investigate the calcium signal transduction pathways activated by the hPTH/PTHrP Rc. In parental HEK-293 cells, which lack endogenous PTH/PTHrP Rc, incubation with hPTH(1-34) had no effect on cytosolic free Ca2+ concentration [Ca2+]i. In HEK-293 clone C-21, stably expressing approximately 400,000 Rc/cell, PTH stimulated an increase in [Ca2+]i by Ca2+ release from intracellular stores; PTH released Ca2+ exclusively from the IP3 sensitive Ca2+ pool. Unlike previous studies, the ability of PTH to elicit both cAMP responses and [Ca2+]i transients occurred over a wide range of Rc numbers (between 400,000 and 3000 Rc/cell); both responses were always observed at PTH concentrations in the same dose range although the magnitude of the responses decrease with Rc number. Pretreatment of C-21 cells with pertussis toxin for 24 h, which significantly enhanced PTH-stimulated cAMP accumulation, did not modulate PTH-stimulated [Ca2+]i transients. At each PTH concentration tested which resulted in increased cAMP levels, there was also an increase in [Ca2+]i transients. Treatment of C-21 cells with a battery of midregion and C-terminal PTH or PTHrP peptides showed no effect on either [Ca2+]i transients or cAMP accumulation, indicating a lack of functional interactions between these peptides and the form of the hPTH/PTHrP Rc stably expressed in these cells. Immunological analysis of G-protein expression demonstrated the presence of Gs, Gi, and Gq in all parental and transfected cells lines examined. Taken together, these data demonstrate that the hPTH/PTHrP Rc, stably expressed in HEK-293 cells, elicits responses in both the cAMP and IP3-dependent [Ca2+]i pathways and is responsive only to N-terminal PTH/PTHrP peptides.

Downregulation of the PTH/PTHrP receptor by vitamin D3 in the osteoblast-like ROS 17/2.8 cells

Effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on the expression of the parathyroid hormone (PTH)/PTH-related peptide (rP) receptor protein and mRNA in ROS 17/2.8 cells were studied. Treatment of ROS 17/2.8 cells with 1,25(OH)2D3 caused time- and dose-dependent suppression of PTH/PTHrP receptor number and immunoreactivity. The effects required more than 24 h incubation with 1,25(OH)2D3 and were maximal by 72 h. The cells did not recover their PTH/PTHrP receptors even after 4 days of treatment with control medium. Treatment with low concentrations of 1,25(OH)2D3 (0.1 M) dramatically decreased the PTH/PTHrP receptor mRNA levels, which were maximal after 24 h of incubation. The half-life of the PTH/PTHrP receptor transcript, 6-8 h, was similar in control and 1,25(OH)2D3-treated cells, suggesting that 1,25(OH)2D3 acts in controlling transcription of the PTH/PTHrP receptor gene but does not change the degradation rate of the PTH/PTHrP receptor transcripts. These data indicate that 1,25(OH)2D3 has a potent inhibitory effect on the expression of the PTH/PTHrP receptor protein and mRNA in ROS 17/2.8 cells.

Coexpression and stimulation of parathyroid hormone receptor positively regulates slowly activating IsK channels expressed in Xenopus oocytes

Expression of the IsK protein in Xenopus oocytes induced the characteristically slow, voltage-dependent outward currents. Superfusion with the parathyroid hormone (PTH) peptide 1-34 had no effect on IsK when expressed alone, but increased IsK when IsK was coexpressed with the PTH-receptor. PTH receptor stimulation caused a shift of IsK conductance-voltage relationship to more negative potentials, and a decrease of both the rate of IsK activation and deactivation. IsK regulation by PTH was independent of extracellular Ca2+, and was also present IsK protein mutants lacking the protein kinase C consensus site. However, regulation of IsK by PTH was mimicked by activators of protein kinase A (PKA) and greatly reduced in the presence of the kinase inhibitors staurosporine and H89. These results suggest that PTH regulates IsK by a mechanism involving phosphorylation independent of protein kinase C (PKC). Such regulation may play a role in proximal tubule cells of the kidney, where both PTH receptor and the IsK protein are expressed.

Molecular basis for the management of secondary hyperparathyroidism in chronic renal failure

Recent clinical and experimental data suggest that the resistance of parathyroid cells to the physiological concentration of calcitriol plays an important role in the pathogenesis and the progression of secondary hyperparathyroidism in chronic renal failure. This resistance is due to the decreased density of the calcitriol receptor in parathyroid cells, which may result from impaired upregulation of calcitriol receptor. Since patients with larger parathyroid glands were more resistance to calcitriol pulse therapy than those with smaller glands and calcitriol receptor density inversely correlated with gland weight, the size of the parathyroid gland may serve as a marker for the degree of resistance to calcitriol. Furthermore, the possible role of phosphorus in the control of parathyroid function has been suggested recently. Thus, it is most important to prevent the progression of parathyroid hyperplasia in chronic renal failure by the early use of active vitamin D, calcitriol pulse therapy, and dietary phosphorus restriction.

Signaling by N- and C-terminal sequences of parathyroid hormone-related protein in hippocampal neurons

Parathyroid hormone-related protein (PTHrP) is synthesized in the brain, and a single type of cloned receptor for the N-terminal portion of PTHrP and PTH is present in the central nervous system. Nothing is known about the physiological actions or signaling pathways used by PTHrP in the brain. Using cultured rat hippocampal neurons, we demonstrate that N-terminal PTHrP[1-34] and PTH[1-34] signal via cAMP and cytosolic calcium transients. The cAMP response showed strong acute (< or = 6 h) homologous and heterologous desensitization after preincubation with PTHrP or PTH. In contrast, the acute calcium response did not desensitize after preincubation with PTHrP; in fact, preincubation dramatically recruited additional responsive neurons. Unexpectedly, C-terminal PTHrP[107-139], which does not bind or activate the cloned PTH/PTHrP receptor, signaled in neurons via cytosolic calcium but not cAMP. Although some neurons responded to both PTHrP[1-34] and PTHrP[107-139], others responded only to PTHrP[1-34]. We conclude that certain hippocampal neurons exhibit dual signaling in response to PTHrP[1-34] and that some neurons have a receptor for C-terminal PTHrP that signals only via cytosolic calcium.

Functional properties of the PTH/PTHrP receptor

The PTH/PTHrP receptor belongs to a novel family of G-protein-coupled receptors which also includes an insect receptor for a diuretic hormone and the protein encoded by a genomic DNA clone from Caenorhabditis elegans. Despite significant structural conservation, rat, opossum, and human PTH/PTHrP receptor homologs display distinct functional characteristics when tested with either [Arg2, Tyr34]hPTH(1-34)amide or [Nle8.18, Tyr34]bPTH(7-34)-amide. These PTH analogs, and chimeras between rat/opossum and between rat/human PTH/PTHrP receptors, led to the identification of receptor residues that appear to be involved in ligand/receptor interaction and receptor activation, respectively. The search for mutations in the PTH/PTHrP receptor gene in genomic DNA of patients with pseudohypoparathyroidism type Ib (PHP-Ib) revealed several silent polymorphisms and a missense mutation in the receptor's tail region which did not affect receptor function. Mutations in the PTH/PTHrP receptor are therefore rarely, if at all, responsible for PHP-Ib. A mutation in the PTH/PTHrP receptor is, however, the most likely cause of Jansen-type metaphyseal chondrodysplasia, a rare form of short-limbed dwarfism which is associated with severe hypercalcemia despite normal or low levels of circulating PTH and PTHrP. A missense mutation was identified which causes constitutive, ligand-independent receptor activation, and thus explains the laboratory and the growth-plate abnormalities in affected individuals.

Effects of differentiation and transforming growth factor beta 1 on PTH/PTHrP receptor mRNA levels in MC3T3-E1 cells

TGF beta has opposing effects on osteoblasts which are thought to be differentiation stage dependent; however, little is known concerning the effects of TGF beta on osteoblastic characteristics at different stages of maturation. The purpose of this study was to characterize the pattern of mRNA expression for the PTH/PTHrP receptor during normal osteoblastic differentiation in vitro, and evaluate the effects of TGF beta 1 on PTH/PTHrP receptor and osteocalcin (OCN) steady-state mRNA at different stages of osteoblastic differentiation. MC3T3-E1 preosteoblasts were plated at low density and induced to differentiate with ascorbic acid and beta-glycerophosphate. The first group served as a vehicle control and the remaining five groups received a single 48 h TGF beta 1 (3.0 ng/ml)-pulse staggered on a weekly basis for 30 days. Cell cultures were harvested weekly and evaluated for: steady-state PTH/PTHrP receptor and OCN mRNA levels via northern analysis, calcium and phosphorous levels, bone nodules via Von Kossa staining, alkaline phosphatase enzyme levels, and hydroxyproline levels. Group 1 (control) samples followed a normal pattern of proliferation, extracellular matrix deposition, and mineralization. PTH/PTHrP receptor and OCN mRNA expression increased 8-fold and 10-fold respectively, over the collection periods. When TGF beta 1 was administered during the first 48 h period (group 2) while cells were rapidly proliferating, there was a persistent inhibition of PTH/PTHrP receptor expression and a striking reduction in OCN mRNA expression at all time points. There was also a down-regulation of PTH/PTHrP receptor and OCN expression when TGF beta 1 was administered later during osteoblast differentiation (groups 3-6); however, these effects were not persistent. In addition there was a total lack of bone nodule formation in group two cultures, whereas groups 3-6 had increasing bone nodule formation because the TGF beta 1 was administered later in the culture period. These studies indicate that expression of the PTH/PTHrP receptor increases with osteoblastic differentiation and suggest that TGF beta 1 inhibits osteoblastic maturation with more persistent effects found in less differentiated osteoblastic cells.

Demonstration of parathyroid hormone-related protein in meninges and its receptor in astrocytes: evidence for a paracrine meningo-astrocytic loop

In contrast to the nervous and glial tissue of the adult rat brain the meninges are immunoreactive for parathyroid hormone-related protein (PTHrP), a hormone that binds with high affinity to the recently cloned PTH/PTHrP receptor. Accordingly immunoreactivity is found in cultured meningeal cells but not in astrocytes. In contrast, astrocytes but not meningeal cells synthesize the mRNA for the PTHrP receptor shown by reverse transcription of total RNA preparations and subsequent polymerase chain reaction with primers specific for the PTHrP receptor. The expression of the PTH/PTHrP receptor was confirmed by the dose-dependent activation of the adenylate cyclase in astrocytes and the rapid development of cellular processes following on incubation with PTHrP. We conclude that PTHrP secreted by meninges forms a paracrine meningo-astrocytic loop and may cause astrocytic differentiation, possibly involved in the formation of the glial limiting membrane.

Down-regulation of PTH-PTHrP receptor of heart in CRF: role of [Ca2+]i

The mRNA of PTH-PTHrP receptor in the kidney and liver of CRF rats is down-regulated. It is not known whether this is a generalized phenomenon and the signals that mediate such down-regulation are not evident. Excess PTH in CRF induces a rise in basal levels of cytosolic calcium ([Ca2+]i), and the high [Ca2+]i is implicated in the genesis of cell dysfunction in CRF. Therefore, it is reasonable to propose that the PTH-induced rise in [Ca2+]i provides a negative feedback control system for the down-regulation of the mRNA of the PTH-PTHrP in order to protect the cells from the harmful effects of progressive rise in blood levels of PTH in CRF. The heart contains the mRNA for this receptor and it is a target organ for PTH action. We examined whether the message of the PTH-PTHrP receptor is down-regulated in the heart of CRF animals and evaluated the role of [Ca2+]i in this process. The expression of the mRNA of the PTH-PTHrP was significantly reduced in the heart of CRF rats as compared to normal animals. Also, the CRF rats had elevated blood levels of PTH and high [Ca2+]i of cardiac myocytes. The parathyroidectomy of CRF rats prevented the rise in blood PTH levels and normalized [Ca2+]i of cardiac myocytes and returned the mRNA of their PTH-PTHrP receptor towards normal levels. The treatment of CRF rats with verapamil normalized [Ca2+]i of cardiac myocytes and returned the mRNA of their PTH-PTHrP receptor towards normal levels, despite marked elevation of blood levels of PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin-insensitive calcium release from intracellular stores triggered by the recombinant human parathyroid hormone receptor

1. In the present study we have characterized the parathyroid hormone (PTH)-induced calcium signalling in 293 cells stably transfected with the human PTH receptor cDNA. In these cells, human PTH-1(1-38) strongly stimulates adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation (EC50 = 0.39 nM) but fails to activate phosphoinositide (PI) turnover. The latter pathway is strongly activated, however, by carbachol (CCh) acting through endogenous M3-muscarinic receptors. 2. Despite the lack of detectable inositol phosphate (IP) formation, hPTH-(1-38) elicited calcium transients (EC50 = 11.2 nM) which were comparable to the signals evoked by CCh. These signals are independent of cyclic AMP generation as cyclic AMP elevating agents did not mimic or modify the PTH response. 3. The PTH-stimulated calcium signal still occurred in calcium-free medium but was absent in cells pretreated with thapsigargin, an inhibitor of the calcium pump of the endoplasmic reticulum (ER). hPTH-(1-38) did not accelerate Mn(2+)-influx through the plasma membrane. These data indicate that PTH releases calcium from intracellular stores. 4. Using heparin, an inhibitor of the IP3-activated calcium release channel of the ER, we tested whether the formation of a low amount of IP3, escaping detection by our biochemical assay, might be the origin of the PTH-induced calcium response. However, intracellular infusion of heparin through patch pipettes in voltage clamp experiments failed to block hPTH-(1-38)-induced calcium signals, whereas it abolished the CCh response. 5. The PTH response, like the CCh response, was insensitive to micromolar concentrations of ryanodine and ruthenium red, eliminating the possibility that hPTH-(1-38) stimulates calcium-induced calcium release through ryanodine receptors.6. We conclude that the recombinant human PTH receptor stimulates calcium release from intracellular stores through a novel pathway not involving IP3- or ryanodine receptors.

Expression and characterization of a recombinant human parathyroid hormone partial agonist with antagonistic properties: Gly-hPTH(-1-->+84)

We have produced and characterized a hPTH analogue with an amino-terminal extension of glycine, Gly-hPTH(-1-->+84) (denoted Gly-hPTH). The hormone analogue was synthesized in E. coli strain BJ5183 transformed with the expression plasmid pKKPTH, extracted from the bacterial pellet and purified by reverse-phase high performance liquid chromatography. Its chemical nature, as determined by amino acid composition analysis, N-terminal amino acid analysis, and mass spectrometry, showed the 9480-Da Gly-hPTH as the predominant species. Because f-Met-Gly-hPTH was the expected form encoded by the plasmid construct, the results indicate that the f-Met residue was efficiently removed from the precurser form. The following functional characteristics of Gly-hPTH were demonstrated. 1) In cells transfected with the human PTH/PTHrP receptor, the receptor binding affinity was reduced threefold compared to the authentic hPTH(1-84) produced by Saccharomyces cerevisiae (apparent Kds: 8.4 and 2.7 nM, respectively). 2) Using the same cells, Gly-hPTH showed 27-fold reduced potency compared to hPTH(1-84) in stimulating intracellular cAMP production (EC50: 32 and 1.2 nM, respectively). 3) Gly-hPTH demonstrated antagonist activity by reducing hPTH-induced cAMP production by 33 +/- 5% (mean +/- SD) when tested at a 1:1 molar ratio. In these studies the recombinant authentic hPTH(1-84) was used as standard for comparisons, and it showed an equal receptor binding affinity and cAMP production as the chemically synthesized peptide [Nle8,18,Tyr34]bovinePTH(1-34)-NH2.

Determinants of [Arg2]PTH-(1-34) binding and signaling in the transmembrane region of the parathyroid hormone receptor

Previously, we reported that [Arg2]PTH-(1-34) bound to the rat osteosarcoma cell line, ROS 17/2.8, with 2-fold higher apparent affinity than it did to the opossum kidney cell line, OK, yet the analog was only a weak partial agonist for cAMP stimulation with ROS 17/2.8 cells, whereas it was a full cAMP agonist with OK cells. These results suggested that the rat and opossum PTH receptors differ in a region recognized by the hormone's amino-terminus. In this report we show that the cloned PTH receptors derived from ROS 17/2.8 and OK cells, expressed in COS-7 cells, also displayed altered responses to [Arg2]PTH-(1-34). Thus, [Arg2]PTH-(1-34) bound to the cloned rat PTH receptor with 7-fold higher affinity than it did to the cloned opossum PTH receptor, and in cAMP stimulation assays, it was a much weaker agonist with the rat receptor than it was with the opossum receptor. Studies with rat/opossum PTH receptor chimeras suggested that the membrane-spanning region of the receptor contributed to the different binding and signaling responses to [Arg2]PTH-(1-34). Point mutation analysis identified three sites in or near the extracellular ends of transmembrane domains V and VI, which specifically affected [Arg2]PTH-(1-34) binding and signaling.

Rapid desensitization of parathyroid hormone dependent adenylate cyclase in perifused human osteosarcoma cells (SaOS-2)

The pulsatile but not the continuous application of parathyroid hormone (PTH) increase bone mass in vivo. To study the effects of intermittent hormonal administration on bone-derived cells in vitro, we established a perifusion system using the human osteosarcoma cell line SaOS-2. Cells were grown in suspension culture attached to collagen beads and were then loaded into a 3 ml syringe for perifusion experiments. The application of PTH(1-34) resulted in a dose-dependent increase of cAMP release by SaOS-2 cells into the effluent medium. Cyclic AMP accumulation was rapidly desensitized by approx. 80% after 30 min of continuous exposure to PTH(1-34) (10(-7) M), while cells remained responsive to forskolin. The recovery of PTH responsiveness required at least 2 h of hormone-free perifusion. Desensitization in the experimental setting was dose-dependent (EC50 = 1 x 10(-10) M PTH(1-34)). Neither 8Br-cAMP (2 x 10(-4) M) nor PMA(1 x 10(-7) M) had an effect on the PTH(1-34)-induced desensitization of the adenylate cyclase. Radioreceptor assays showed that [125I]-[Tyr36]hPTHrP(1-36)amide binding to SaOS-2 cells was decreased by 60-70% by PTH(1-34) (1 x 10(-6) M), bPTH(1-84) (1.8 x 10(-6) M) and bPTH(3-34) (2 x 10(-6) M), whereas 8Br-cAMP (2 x 10(-4) M) had no effect on radioligand binding. PMA (1 x 10(-7) M) appeared to slightly increase [125I]PTHrP binding. This observation is consistent with a small (3-fold) increase in PTH-induced cAMP release as a result of PMA pre-treatment. Receptor internalization was dose-dependent EC50 = 3 x 10(-7) M PTH(1-34)). The maximal effect occurred after 10-30 min and was largely reversible within 2 h. Monensin (3 x 10(-5) M) inhibited the recovery from receptor internalization. We conclude that a perifusion system using SaOS-2 cells is a suitable model to study the effect of discontinuous application of PTH on cAMP release. A rapid, homologous desensitization of PTH(1-34) stimulated cAMP accumulation has been observed that does not appear to involve protein kinase A or C.

Effect of parathyroid hormone and calcitonin on acetylcholine release in rat sympathetic superior cervical ganglion

The effects of parathyroid hormone (PTH) and calcitonin on acetylcholine release by rat superior cervical ganglion (SCG) were evaluated in vitro. SCG labeled with [3H]choline were exposed to four 5 min-long pulses of 40 mM K+, 35 min apart. PTH increased, and calcitonin inhibited, in a dose-dependent way, K(+)-elicited [3H]acetylcholine release, with apparent effective doses 50 of about 10(-9) M. The effect of PTH was inhibited by co-incubation with the PTH receptor antagonist NLe [8-18]-PTH (3-34) amide. Incubation of SCG for 120 min with PTH or calcitonin resulted in dose-dependent augmentation or inhibition of K(+)-induced increase of high affinity [3H]choline uptake, respectively, with a maximal effect at 10(-8) M concentration (PTH) and 10(-9) M concentration (calcitonin) and declining at higher concentrations. The increase in SCG [3H]choline uptake induced by PTH was blunted by preincubation with the PTH antagonist NLe [8-18]-PTH (3-34) amide. At 10(-7) M concentrations, PTH increased significantly the in vitro conversion of [3H]choline to [3H]acetylcholine, an effect inhibited by PTH receptor antagonist. Calcitonin did not modify SCG [3H]acetylcholine synthesis by rat SCG. The results indicate that, in vitro, PTH increases, and calcitonin inhibits, acetylcholine release in rat SCG.

Studies on the mechanism of desensitization of the parathyroid hormone-stimulated calcium signal in UMR-106 cells: reversal of desensitization by alkaline phosphatase but not by protein kinase C downregulation

The involvement of protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and other phosphorylation mechanisms in the rapid desensitization of the [Ca2+]i response to parathyroid hormone (PTH) stimulation was investigated in osteoblast-like UMR-106 cells. A 5 minute preincubation of the cell suspension with phorbol 12,13-dibutyrate (PDB) decreased the response to PTH in a concentration-dependent manner. 1-Oleoyl-2-acetyl-r-glycerol (OAG) pretreatment likewise decreased the PTH response. Staurosporine, a potent protein kinase inhibitor, completely prevented the desensitization caused by PDB. These PDB and staurosporine effects were also observed in 3 mM EGTA-containing medium ([Ca2+]free < 10(-8) M). A 5 minute pretreatment of cells with 1 microM forskolin had no effect on the calcium response to PTH. Homologous and PDB-induced desensitizations differed in several respects. Staurosporine pretreatment resulted in only a slight restoration of the PTH response under conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acute phorbol treatment but not the homologous desensitization. Both homologous and PDB-induced desensitization were relieved by alkaline phosphatase treatment, consistent with the involvement of phosphorylation in the desensitization. This alkaline phosphatase effect on desensitization was inhibited by L-phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(es) other than or in addition to those of PKC.

Identification and characterization of parathyroid hormone/parathyroid hormone-related peptide receptor in cultured astrocytes

Parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor expressed in rat cultured cortical type I astrocytes was identified and characterized. Northern blot analysis demonstrated that the PTH/PTHrP receptor mRNA is expressed in astrocytes. A human PTH fragment, PTH [1-34], stimulated adenylate cyclase with the EC50 value of 3 nM, but not phospholipase C. PTH [1-34] changed the morphology of protoplasmic type astrocytes into process-bearing ones. Thus, type I astrocytes possess PTH/PTHrP receptor that is coupled to an adenylate cyclase-activating system but not to phospholipase C.