Activity ratio measurements reflect intracellular activation of adenosine 3',5'-monophosphate-dependent protein kinase in osteoblasts

PTH1R Actions on Bone Using the cAMP/Protein Kinase A Pathway

After the initial signaling action of parathyroid hormone (PTH) on bone was shown to be activation of adenylyl cyclase, its target was found to be cells of the osteoblast lineage, to the exclusion of osteoclasts and their precursors. This led to the view that the osteoblast lineage regulated osteoclast formation, a proposal that was established when the molecular mechanisms of osteoclast formation were discovered. This is in addition to the effect of PTH1Rv signaling throughout the osteoblast differentiation process to favour the formation of bone-forming osteoblasts. Initial signaling in the PTH target cells through cAMP and protein kinase A (PKA) activation is extremely rapid, and marked by an amplification process in which the later event, PKA activation, precedes cAMP accumulation in time and is achieved at lower concentrations. All of this is consistent with the existence of "spare receptors", as is the case with several other peptide hormones. PTH-related protein (PTHrP), that was discovered as a cancer product, shares structural similarity with PTH in the amino-terminal domain that allows the hormone, PTH, and the autocrine/paracrine agent, PTHrP, to share actions upon a common G protein coupled receptor, PTH1R, through which they activate adenylyl cyclase with equivalent potencies. Studies of ligand-receptor kinetics have revealed that the PTH/PTH1R ligand-receptor complex, after initial binding and adenylyl cyclase activation at the plasma membrane, is translocated to the endosome, where adenylyl cyclase activation persists for a further short period. This behavior of the PTH1R resembles that of a number of hormones and other agonists that undergo such endosomal translocation. It remains to be determined whether and to what extent the cellular effects through the PTH1R might be influenced when endosomal is added to plasma membrane activation.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Brief exposure to full length parathyroid hormone-related protein (PTHrP) causes persistent generation of cyclic AMP through an endocytosis-dependent mechanism

Parathyroid hormone (PTH)-related protein (PTHrP) (gene name Pthlh) was discovered as the factor responsible for the humoral hypercalcemia of malignancy. It shares such sequence similarity with PTH in the amino-terminal region that the two are equally able to act through a single G protein-coupled receptor, PTH1R. A number of biological activities are ascribed to domains of PTHrP beyond the amino-terminal domain. PTH functions as a circulating hormone, but PTHrP is generated locally in many tissues including bone, where it acts as a paracrine factor on osteoblasts and osteocytes. The present study compares how PTH and PTHrP influence cyclic AMP (cAMP) formation through adenylyl cyclase, the first event in cell activation through PTH1R. Brief exposure to full length PTHrP(1-141) in several osteoblastic cell culture systems was followed by sustained adenylyl cyclase activity for more than an hour after ligand washout. This effect was dose-dependent and was not found with shorter PTHrP or PTH peptides even though they were fully able to activate adenylyl cyclase with acute treatment. The persistent activation response to PTHrP(1-141) was seen also with later events in the cAMP/PKA pathway, including persistent activation of CRE-luciferase and sustained regulation of several CREB-responsive mRNAs, up to 24 h after the initial exposure. Pharmacologic blockade of endocytosis prevented the persistent activation of cAMP and gene responses. We conclude that full length PTHrP, the likely local physiological effector in bone, differs in intracellular action to PTH by undergoing endosomal translocation to induce a prolonged adenylyl cyclase activation in its target cells.

The role of Iloprost on bone edema and osteonecrosis: Safety and clinical results

INTRODUCTION

Iloprost is a commercially available prostaglandin I₂ (PGI₂) analogue that is shown to have antithrombotic, vasodilatative and antiproliferative effects. A number of clinical studies have shown that Iloprost can be effective in the management of bone marrow oedema and the treatment of avascular necrosis. The aim of this manuscript is to present our current understanding on the effect of Iloprost on the treatment of these conditions.

AREAS COVERED

The authors offer a comprehensive review of the existing literature on the experimental and clinical studies analysing the effect of Iloprost on bone, bone marrow oedema and avascular necrosis.

EXPERT OPINION

The available data from the clinical studies suggest that Iloprost has limited effect in advanced stages of avascular necrosis. However, literature suggests that Iloprost administration can be a viable option in the management of bone marrow oedema and early stages of osteonecrosis. Despite these promising results its effect on bone homeostasis needs further elucidation. Moreover, further data on its safety, dosage and efficiency through randomized multicenter studies are desirable in order to reach final conclusions.

Identification of an orally active small-molecule PTHR1 agonist for the treatment of hypoparathyroidism

Parathyroid hormone (PTH) is essential for calcium homeostasis and its action is mediated by the PTH type 1 receptor (PTHR1), a class B G-protein-coupled receptor. Hypoparathyroidism and osteoporosis can be treated with PTH injections; however, no orally effective PTH analogue is available. Here we show that PCO371 is a novel, orally active small molecule that acts as a full agonist of PTHR1. PCO371 does not affect the PTH type 2 receptor (PTHR2), and analysis using PTHR1–PTHR2 chimeric receptors indicated that Proline 415 of PTHR1 is critical for PCO371-mediated PTHR1 activation. Oral administration of PCO371 to osteopenic rats provokes a significant increase in bone turnover with limited increase in bone mass. In hypocalcemic rats, PCO371 restores serum calcium levels without increasing urinary calcium, and with stronger and longer-lasting effects than PTH injections. These results strongly suggest that PCO371 can provide a new treatment option for PTH-related disorders, including hypoparathyroidism.

Hypoparathyroidism and osteoporosis can be treated with parathyroid hormone, but frequent injections are required. Here the authors develop a small-molecule agonist for the parathyroid hormone type I receptor that can be administered orally, and demonstrate its efficacy in rats.

Loss of Gsα in the Postnatal Skeleton Leads to Low Bone Mass and a Blunted Response to Anabolic Parathyroid Hormone Therapy

Parathyroid hormone (PTH) is an important regulator of osteoblast function and is the only anabolic therapy currently approved for treatment of osteoporosis. The PTH receptor (PTH1R) is a G protein-coupled receptor that signals via multiple G proteins including Gsα. Mice expressing a constitutively active mutant PTH1R exhibited a dramatic increase in trabecular bone that was dependent upon expression of Gsα in the osteoblast lineage. Postnatal removal of Gsα in the osteoblast lineage (P-Gsα(OsxKO) mice) yielded markedly reduced trabecular and cortical bone mass. Treatment with anabolic PTH(1-34) (80 μg/kg/day) for 4 weeks failed to increase trabecular bone volume or cortical thickness in male and female P-Gsα(OsxKO) mice. Surprisingly, in both male and female mice, PTH administration significantly increased osteoblast numbers and bone formation rate in both control and P-Gsα(OsxKO) mice. In mice that express a mutated PTH1R that activates adenylyl cyclase and protein kinase A (PKA) via Gsα but not phospholipase C via Gq/11 (D/D mice), PTH significantly enhanced bone formation, indicating that phospholipase C activation is not required for increased bone turnover in response to PTH. Therefore, although the anabolic effect of intermittent PTH treatment on trabecular bone volume is blunted by deletion of Gsα in osteoblasts, PTH can stimulate osteoblast differentiation and bone formation. Together these findings suggest that alternative signaling pathways beyond Gsα and Gq/11 act downstream of PTH on osteoblast differentiation.

Historically significant events in the discovery of RANK/RANKL/OPG

After it was suggested 30 years ago that the osteoblast lineage controlled the formation of osteoclasts, methods were developed that established this to be the case, but the molecular controls were elusive. Over more than a decade much evidence was obtained for signaling mechanisms that regulated the production of a membrane - bound regulator of osteoclastogenesis, in the course of which intercellular communication in bone was revealed in its complexity. The discovery of regulation by tumor necrosis factor ligand and receptor families was made in the last few years of the twentieth century, leading since then to a new physiology of bone, and to exciting drug development.

Role of prostaglandin I2 in the gene expression induced by mechanical stress in spinal ligament cells derived from patients with ossification of the posterior longitudinal ligament

Ossification of the posterior longitudinal ligament of the spine (OPLL) is characterized by ectopic bone formation in the spinal ligaments, and mechanical stress has been suggested to play an important role in the progression of OPLL. To identify the genes that participate in OPLL, the differential display reverse transcription-polymerase chain reaction (RT-PCR) method was used. A 283-base pair cDNA fragment corresponding to prostaglandin I2 (PGI2) synthase was highly expressed in OPLL cells compared with non-OPLL cells. To examine the effect of mechanical stress on the expression of PGI2 synthase, cells were subjected to uniaxial cyclic stretch (0.5 Hz, 20% stretch), and PGI2 synthase mRNA expression was assessed by quantitative RT-PCR. Cyclic stretch induced an increase in PGI2 synthase in OPLL cells in a time-dependent manner, whereas no change was observed in non-OPLL cells. Cyclic stretch for 9 h also induced a 2.86x increase in PGI2 production. Beraprost (a stable PGI2 analog) and dibutyryl cAMP (a membrane-permeable cAMP analog) increased the mRNA expression of alkaline phosphatase (ALP) as a marker for osteogenic differentiation up to 240 and 200%, respectively, in OPLL cells, whereas no change was observed in non-OPLL cells. The increases in ALP mRNA induced by beraprost and cyclic stretch were both inhibited by SQ22536, a potent adenylate cyclase inhibitor. These data suggest that the increase in PGI2 synthase induced by mechanical stress plays a key role in the progression of OPLL, at least in part through the induction of osteogenic differentiation in spinal ligament cells via the PGI2/cAMP system.

Gene expression profiles and transcription factors involved in parathyroid hormone signaling in osteoblasts revealed by microarray and bioinformatics

Parathyroid hormone (PTH) binds to its receptor PTH1R (parathyroid hormone 1 receptor) in osteoblastic cells to regulate bone remodeling and calcium homeostasis. While prolonged exposure to PTH causes increased bone resorption, intermittent injections of PTH have an anabolic effect on bone. The molecular mechanisms regulating these processes are still largely unknown. Here, we present our results on gene expression profile changes in the PTH-treated osteoblastic cell line, UMR 106-01, using DNA microarray analysis. A total of 125 known genes and 30 unknown expressed sequence tags (ESTs) were found to have at least 2-fold expression changes after PTH treatment at 4, 12, and 24 h. 14 genes were previously known to be PTH-regulated but many were unknown to be regulated by PTH prior to our experiments. Real-time reverse transcriptase-PCR confirmed that 90 and 50% of the genes are regulated more than 2-fold by PTH in UMR 106-01 and rat primary osteoblastic cells, respectively. Most genes belong to the following protein families: hormones, growth factors, and receptors; signal transduction pathway proteins; transcription factors; proteases; metabolic enzymes; structural and matrix proteins; transporters; etc. These results provide a comprehensive and deeper knowledge about PTH regulation of osteoblastic gene expression. Next, we designed a computational method to extract information about transcription factors likely involved in regulating these genes. These factors include those previously known to be involved in PTH signaling (AP-1 and the cAMP response element-binding protein), those that were identified by microarray data (C/EBP), and some novel transcription factors (AP-2, AP-4, SP1, FoxD3, etc.). Our results suggest that a reliable bioinformatics approach can be easily applied for other systems.

Stimulation of extracellular signal-regulated kinases and proliferation in rat osteoblastic cells by parathyroid hormone is protein kinase C-dependent

Parathyroid hormone (PTH) is known to have both catabolic and anabolic effects on bone. The dual functionality of PTH may stem from its ability to activate two signal transduction mechanisms: adenylate cyclase and phospholipase C. Here, we demonstrate that continuous treatment of UMR 106-01 and primary osteoblasts with PTH peptides, which selectively activate protein kinase C, results in significant increases in DNA synthesis. Given that ERKs are involved in cellular proliferation, we examined the regulation of ERKs in UMR 106-01 and primary rat osteoblasts following PTH treatment. We demonstrate that treatment of osteoblastic cells with very low concentrations of PTH (10(-12) to 10(-11) m) is sufficient for substantial increases in ERK activity. Treatment with PTH-(1-34) (10(-8) m), PTH-(1-31), or 8-bromo-cAMP failed to stimulate ERKs, whereas treatment with phorbol 12-myristate 13-acetate, serum, or PTH peptides lacking the N-terminal amino acids stimulated activity. Furthermore, the activation of ERKs was prevented by pretreatment of osteoblastic cells with inhibitors of protein kinase C (GF 109203X) and MEK (PD 98059). Treatment of UMR cells with epidermal growth factor (EGF), but not PTH, promoted tyrosine phosphorylation of the EGF receptor. Transient transfection of UMR cells with p21(N17Ras) did not block activation of ERKs following treatment with low concentrations of PTH. Thus, activation of ERKs and proliferation by PTH is protein kinase C-dependent, but stimulation occurs independently of the EGF receptor and Ras activation.

Reversible suppression of in vitro biomineralization by activation of protein kinase A

Parathyroid hormone (PTH-(1-34)) potently suppresses apatite deposition in osteoblastic cultures. These inhibitory effects are mediated through signaling events following PTH receptor binding. Using both selective inhibitors and activators of protein kinase A (PKA), this study shows that a transient activation of PKA is sufficient to account for PTH's inhibition of apatite deposition. This inhibition is not a result of reduced cell proliferation, reduced alkaline phosphatase activity, increased collagenase production, or lowering medium pH. Rather, data suggest a functional relationship between matrix assembly and apatite deposition in vitro. Bone sialoprotein (BSP) and apatite co-localize in the extracellular matrix of mineralizing cultures, with matrix deposition of BSP temporally preceding that of apatite. Transient activation of PKA by either PTH-(1-34) or short term cAMP analog treatment blocks the deposition of BSP in the extracellular matrix without a significant reduction in the total amount of BSP synthesized and secreted. This effect is reversible after allowing the cultures to recover in the absence of PKA activators for several days. Thus, a transient activation of PKA may suppress mineral deposition in vitro as a consequence of altering the assembly of an extracellular matrix permissive for apatite formation.

Differential effect of TPA on PGE2 and cicaprost-induced cAMP synthesis in UMR-106 cells

PGE2 and prostacyclin each enhance cAMP synthesis in the osteoblast-like cell line UMR-106. The amount of cAMP induced by PGE2 was 5-7-fold greater than the amount induced by cicaprost or iloprost, stable prostacyclin analogues. Both PGE2 and the two prostacyclin analogues enhanced cAMP synthesis with similar time dependence. The EC50 values of PGE2 and cicaprost were 3 X 10(-6) and 5 x 10(-8) M, respectively. Short-term incubation of the cells with 12-o-tetradecanoylphorbol 13-acetate (TPA) markedly reduced the PGE2-induced cAMP synthesis. In contrast, cells that were incubated with the same concentrations of TPA in the presence of cicaprost or iloprost showed a 1.6-fold increase in cAMP formation. The marked disparity between the cAMP response to cicaprost and PGE2 in the presence of TPA suggests that the two prostanoids induce cAMP synthesis in the UMR-106 cells by interaction with different receptors. These observations support the idea that the osteoblastic UMR-106 cells may express specific prostacyclin receptors and suggest that prostacyclin may have a unique role in osteoblasts.

C-terminal parathyroid hormone-related protein inhibits proliferation and differentiation of human osteoblast-like cells

Parathyroid hormone-related protein (PTHrP) is synthesized by osteoblasts, although its local role in bone is not completely understood. The C-terminal (107-111) region of PTHrP seems to be a potent inhibitor of osteoblastic bone resorption. We studied the effect of this PTHrP domain on the proliferation and synthesis of osteoblastic markers in osteoblast-like cells from adult human bone. We found that the human (h)PTHrP(107-139) fragment, between 10 fM and 10 nM, inhibited 3H-thymidine incorporation into these cells. The antiproliferative effect of the latter fragment, or that of hPTHrP(107-111), was similar to that induced by [Tyr34] hPTHrP(1-34) amide, bovine PTH(1-34), and hPTHrP(1-141), while hPTHrP(38-64) amide was ineffective. Human PTHrP(7-34) amide, at 10 nM, and 1 microM phorbol-12-myristate-13-acetate also significantly decreased DNA synthesis in human osteoblast-like cells. Neither hPTHrP(7-34) amide nor hPTHrP(107-139), at 10 nM, stimulated protein kinase A (PKA) activity in these cells. Moreover, 100 nM H-89, a PKA inhibitor, did not eliminate the inhibitory effect of hPTHrP(107-139) on these cells' growth. However 100 nM calphostin C, a PKC inhibitor, blunted this effect of PTHrP(107-139). In addition to their antimitogenic effect, hPTHrP(107-139) and hPTHrP(107-111) inhibited basal and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-stimulated alkaline phosphatase activity in these cells. Both fragments, like 1,25(OH)2D3, decreased C-terminal type I procollagen secretion into the cell-conditioned medium, but osteocalcin secretion by these cells was unaffected by the C-terminal PTHrP fragments. These findings suggest that PTHrP may act as a local regulator of bone formation.

Antiproliferative effect of the C-terminal fragments of parathyroid hormone-related protein, PTHrP-(107-111) and (107-139), on osteoblastic osteosarcoma cells

The C-terminal region of parathyroid hormone-related protein (PTHrP) containing the sequence (107-111) appears to be a potent inhibitor of osteoclastic bone resorption. In the present study, we have investigated the effect of human (h)PTHrP (107-139) and hPTHrP (107-111)NH2 on the proliferation of osteoblastic rat osteosarcoma UMR 106 cells. We found that both C-terminal PTHrP peptides, like hPTHrP (1-141), were antimitogenic for these cells, between 1 pM and 10 nM. [Tyr34]hPTHrP (1-34)NH2 was as potent as these peptides but less effective as growth inhibitor in these cells. UMR 106 cells were found to produce and secrete immunoreactive PTHrP. Addition of anti-PTHrP neutralizing antibodies to C- and N-terminal epitopes of PTHrP increased the growth of these cells. Our data suggest that the antiproliferative effect of these C-terminal PTHrP analogs may be independent of cyclic adenosine 3':5'-monophosphate (cAMP) and mediated by protein kinase C. These findings support an autocrine role of PTHrP in bone metabolism.

Influence of high glucose on 1,25-dihydroxyvitamin D3-induced effect on human osteoblast-like MG-63 cells

Impaired bone formation due to defective osteoblast function, as reflected in a decreased serum osteocalcin (OC) concentration in the patients with diabetes, has been implicated in the development of diabetic osteopenia. The role of hyperglycemia in this decrease in serum OC concentration was investigated. 1,25-dihydroxyvitamin D3 (1,25[OH]2D3), an active form of vitamin D3, stimulated OC secretion from the human osteosarcoma cell line MG-63 in a dose-dependent manner. Exposure of the cells to high concentrations of glucose for 7 days significantly impaired 1,25(OH)2D3-induced OC secretion as compared with that observed with cells maintained under normal glucose (5.5 mM) or high mannitol conditions. The inhibitory effect of glucose was in a dose-dependent manner up to 55 mM. High glucose (55 mM) also attenuated the 1,25(OH)2D3-induced increase in OC mRNA abundance in MG-63 cells, suggesting that the inhibition of the 1,25(OH)2D3-induced increase in OC secretion by exposure to a high concentration of glucose was, at least in part, mediated at the transcriptional level. High glucose significantly decreased the number of 1,25(OH)2D3 receptors in MG-63 cells, without any change in the dissociation constant for 1,25(OH)2D3; this effect was not mimicked by high mannitol, indicating specificity for glucose. These observations suggest that a high glucose concentration significantly impairs the ability of osteoblastic cells to synthesize OC in response to 1,25(OH)2D3 by reducing 1,25(OH)2D3 receptor number, and that impaired cell function caused by sustained exposure to high glucose contributes to the defect in bone formation observed in the patients with diabetic osteopenia.

The regulation of calcitriol by parathyroid hormone and absorbed dietary phosphorus in subjects with moderate chronic renal failure

The two best-recognized stimuli for calcitriol production are parathyroid hormone (PTH) and dietary phosphorus deprivation. We studied the relative importance of these two stimuli in subjects with moderate chronic renal failure (MCRF). We recruited 10 subjects with MCRF aged 49 +/- 13 years (mean +/- SD) and having a creatinine clearance rate of 31 +/- 24 mL/min. After an overnight fast, they received 400 IU human PTH 1-34 subcutaneously, and blood and urine samples were collected over the subsequent 24 hours. They then took aluminum hydroxide 30 mmol/d for 2 months to reduce gut phosphorus absorption, at the end of which the PTH stimulation test was repeated. Responses were compared with those obtained in 11 normal subjects aged 44 +/- 16 years and having a creatinine clearance rate of 102 +/- 37 mL/min who were only studied while on their normal diet. Following PTH stimulation in the normal subjects but not the MCRF subjects, there was a significant increase in plasma calcium and calcitriol levels, with a significant decrease in renal phosphorus threshold. In both groups there was a similar and significant increase in urine cyclic adenosine monophosphate (cAMP) levels. Following restriction of absorption of dietary phosphorus in MCRF subjects, plasma calcitriol levels increased compared with baseline values. This study shows that these MCRF subjects were unable to respond with an increase in calcitriol to a PTH stimulus that produced a similar urine cAMP response in normal subjects. However, they were capable of responding to a reduction in phosphorus absorption with an increase in calcitriol.(ABSTRACT TRUNCATED AT 250 WORDS)

G-protein stimulation inhibits amiloride-sensitive Na/H exchange independently of cyclic AMP

G-proteins are heterotrimeric proteins involved in many transmembrane signaling events. Both the renal basolateral membrane and the renal brush border membrane contain large quantities of these proteins. G-proteins appear related to hormonal signaling in the basolateral membrane and presumably affect ion gating in the brush border. We investigated the influence of G-proteins on the amiloride-sensitive Na/H exchanger, the activity of which is regulated at least in part by cAMP-dependent protein kinase, by measuring the amiloride-sensitive component of [22Na+] uptake in rat renal brush border membrane vesicles (BBMV) in the presence of a pH gradient. Incubation of vesicles with AlF4- (10 microM Al3+, 10 mM F-) resulted in significant inhibition of amiloride-sensitive [22Na+] uptake at both 20 seconds and 5 minutes of incubation. Incorporation of GTP gamma S into BBMV by transient hypotonic lysis also resulted in significantly reduced amiloride-sensitive [22Na+] uptake compared to controls at both time points. This inhibition could be reversed by GDP beta S. Similar lysis in the presence of 10 microM GDP beta S alone had no significant effect. When Na(+)-dependent [14C]-D-glucose uptake into BBMV was studied no significant effect of these G-protein modulating agents was observed. Adenylate cyclase activity could not be stimulated in these BBMV preparations using standard techniques. Furthermore, cAMP-dependent protein kinase activity, strongly stimulated in these BBMV by exogenously added cAMP, was not stimulated by 10 microM GTP gamma S alone. These findings suggest that the amiloride-sensitive Na/H exchanger can be regulated by G-proteins independently of adenylate cyclase and cAMP-dependent protein kinase.

Effects of beta-adrenergic blockade in an osteoblast-like cell line

The beta-adrenergic blocking agent propranolol was shown in previous studies to increase orthotopic bone formation in rats. To understand the cellular mechanisms underlying this observation, propranolol was tested for its effects on osteoblastic cells, which possess adenylate cyclase-coupled beta-adrenergic receptors. The ability of propranolol to modulate parathyroid hormone (PTH) and isoproterenol effects on adenylate cyclase activity and on alkaline phosphatase expression was studied in the osteoblast-like rat osteosarcoma cell line ROS 17/2.8. At concentrations between 0.1 and 10 microM, DL-propranolol specifically inhibited adenylate cyclase stimulation by the beta-adrenergic agonist isoproterenol, but did not alter either basal or PTH-stimulated activity. At these concentrations, propranolol also blunted the inhibition of alkaline phosphatase activity by isoproterenol but not PTH. Propranolol alone had minimal effects on ROS alkaline phosphatase activity at low concentrations (0.1-1 microM), but became inhibitory at high concentrations (10-100 microM). Thus, the direct effects of physiologically relevant propranolol concentrations on osteoblastic cells can be attributed principally to beta-adrenergic blockade. These findings further suggest that propranolol may enhance bone formation by preserving osteoblastic activity in the face of inhibition by beta-adrenergic agonists.

Down-regulation of adenylate cyclase-coupled response to native bovine parathyroid hormone and fragments in the osteoblast-like cell line UMR-106

1. Parathyroid hormone-induced down-regulation was studied in the osteosarcoma cell line UMR-106. 2. A maximal priming does of bPTH (1-84) down-regulated PTH-responsiveness to 40% of its initial value; bPTH (1-41) was less effective than bPTH (1-84), whereas bPTH (42-84) had no effect, alone or in combination with bPTH (1-41). 3. A tentative model for the function of different domains of parathyroid hormone in down-regulation is suggested.

Increase in cyclic AMP levels by relaxin in newborn rhesus monkey uterus cell culture

A novel relaxin sensitive cell line of apparent smooth muscle origin has been established from a newborn rhesus monkey uterus (NRMU). NRMU cells respond to relaxin, in the presence of 1 microM forskolin, by producing intracellular adenosine 3', 5'-cyclic monophosphate (cAMP). The increase in cAMP levels is dose, time and cell density dependent, reaching peak levels at 10 min when cells are seeded at 1 X 10(5) cells/well. Specificity was demonstrated by neutralization of the relaxin activity with anti-relaxin monoclonal and polyclonal antibodies, degradation of cAMP in the presence of phosphodiesterase, and confirmation of the absence of cGMP. Three synthetic analogs of human relaxin generated a dose-related cAMP response as did synthetic native human relaxin. Natural relaxin purified from human corpora lutea tissue also generated a response similar to synthetic human relaxin. Porcine and rat relaxins also increased levels of cAMP. Insulin, but not IGF I or IGF II, was capable of increasing cAMP levels in NRMU cells, however, 200 ng/mL were required to achieve cAMP levels comparable to 6.25 ng/ml relaxin. Combinations of relaxin with insulin, IGF I or IGF II did not increase cAMP levels above levels obtained with relaxin alone. The effect on NRMU cells of other hormones, growth factors and drugs potentially present in cell culture systems or serum samples was evaluated. In combination with relaxin, oxytocin significantly decreased the cAMP production below the levels induced by relaxin alone, whereas progesterone and prostaglandin E2 resulted in additive increases in cAMP. These data suggest that the NRMU cell line is an appropriate target tissue for studying relaxin-mediated biological responses in vitro as well as functioning as the primary component of a relaxin in vitro bioassay.

Effect of isoproterenol and 3-isobutyl-1-methylxanthine on junctional conductance in heart cell pairs

The influence of isoproterenol (10(-6) M) and 3-isobutyl-1-methylxanthine (IBMX) (10(-6) M) on the junctional conductance (gj) of isolated rat ventricular cell pairs was investigated. It was found that both drugs increased gj within 25-30 s. To test the phosphorylation hypothesis, cAMP-dependent protein kinase inhibitor (20 micrograms/ml) was dialyzed into both cells of the pair. The protein kinase inhibitor suppressed the effect of both drugs on gj. Moreover, the protein kinase inhibitor by itself, reduced gj by 18% (S.E. +/- 9) (n = 10), suggesting that basal levels of cAMP in the cells contributes to modulation of gj. Dialysis of cAMP-protein kinase inhibitor (20 micrograms/ml) in just one cell of the pair induced rectification of the junctional membrane. These results indicate that the activation of cAMP-dependent protein kinase is necessary for the effect of cAMP on junctional conductance in heart.

Parathyroid hormone-induced ornithine decarboxylase activity in fetal rat osteoblasts

Induction of ornithine decarboxylase (ODC, E.C. 4.1.1.17) activity by parathyroid hormone (PTH) in cultured fetal rat osteoblasts was studied. PTH induced ODC activity and stimulated cAMP production in a dose-dependent manner, the ED50 for cAMP being five times as high as that for ODC. Induction of ODC activity by PTH was partly inhibited by actinomycin D and cycloheximide, with 40 and 55% inhibition, respectively. PTH increased the intracellular ionized calcium concentration ([Ca2+]i), which was absent in a Ca2+-free medium. Blocking calcium influx, lowering the extracellular calcium concentration, and adding trifluoperazine inhibited both induction of ODC activity and stimulation of cAMP production by PTH. A23187 (100 nM and 1 microM), combined with a low dose of PTH (4 nM), resulted in a synergistic induction of ODC activity and an inhibition of cAMP production. A23187 inhibited induction of ODC activity as well as stimulation of cAMP production by the dose of PTH (20 nM) maximally effective in inducing ODC activity. Forskolin together with this maximal dose of PTH resulted in an additive effect on ODC activity and a synergistic stimulation of cAMP production. The current results show similarities and differences with respect to results obtained with osteoblasts from other species and osteoblast cell lines. The present data indicate that (1) PTH stimulates ODC activity and this is partly due to new enzyme synthesis; (2) calcium is involved in induction of ODC activity and stimulation of cAMP production by PTH; furthermore, it is suggestive that calmodulin and/or protein kinase C are involved; and (3) stimulation of cAMP production by PTH depends on an optimal intracellular calcium concentration range.

Vasoactive intestinal peptide inhibits the respiratory burst in human monocytes by a cyclic AMP-mediated mechanism

The neuropeptide vasoactive intestinal peptide (VIP) was shown to inhibit the production of reactive oxygen compounds (respiratory burst) in monocytes activated by serum opsonized zymosan. Reactive oxygen compounds are of importance for host defence against micro-organisms and cancer, but normal tissues are also susceptible to damage from these reactive substances. Maximum inhibition of respiratory burst was 40% by 0.1 microM VIP (ID100), while ID50 for the VIP effect was 0.36 nM VIP. PHM-27, closely related to VIP on the basis of the amino acid sequence, inhibited the respiratory burst with much lower potency (ID50 = 60 nM, ID100 = 1 microM). Secretin, related to VIP and PHM-27, produced no effect on the respiratory burst in monocytes. VIP was also shown to stimulate the cyclic AMP production in monocytes in a dose dependent manner. IBMX and forskolin, as well as the cyclic AMP analogue butyryl cyclic AMP were shown to produce an inhibition of the respiratory burst. In conclusion, this study showed that VIP inhibited the respiratory burst in monocytes by a cyclic AMP-mediated mechanism, and serves to establish still another role for VIP as a mediator in the neuro-immune axis.

Thyroid hormone 5'-deiodinase activity, nuclear binding, and effects on mitogenesis in UMR-106 osteoblastic osteosarcoma cells

The hyperthyroid state in vivo is associated with an increase in osteoblast number and activity, suggesting that thyroid hormone may stimulate osteoblast replication and function. We therefore examined the effects of T3 (16-1170 pM) on replication rate as assessed by cell counts in UMR-106 osteoblastic osteosarcoma cells cultured for 5-10 days in medium supplemented with 10% hormone-stripped fetal calf serum (FCS). Despite the virtual absence of thyroid hormone in the control medium (total T3 concentration, 0.02 ng/ml), the addition of T3 in concentrations to 1000 pM did not increase the cell replication rate. At higher T3 concentrations, a slight decrease in growth rate was observed. No significant 5'-monodeiodinase activity was detected in UMR-106 cell homogenates. However, nuclear binding of T3 was demonstrated in intact cells. A high-affinity nuclear binding component was identified with a Ka of 2.6 x 10(10) M-1 and a maximum binding capacity of 7.7 pg T3 per mg DNA, equivalent to 51 binding sites per cell nucleus. A lower affinity nuclear T3 binding component with a Ka of 1.8 x 10(9) M-1 was also identified. Thus, despite the presence of nuclear T3 receptors, UMR-106 cells do not exhibit a mitogenic response to T3.

Forskolin sensitizes parathyroid hormone-induced cyclic AMP response, but not the bone resorptive effect, in mouse calvarial bones

The effect of forskolin on parathyroid hormone (PTH) stimulated bone resorption, as assessed in vitro by the release of 45Ca from prelabelled neonatal mouse calvarial bones, and cyclic AMP formation in mouse calvarial bones and osteoblast-like cells was investigated. Much higher concentrations (100-300-times) of PTH were required to stimulate cyclic AMP accumulation than to stimulate mineral mobilization in murine calvarial bones. PTH, in the absence of phosphodiesterase inhibitor, stimulated cyclic AMP formation in mouse calvarial bones at and above concentrations of 3-10 nmol/l with EC50 at 10-15 nmol/l. In the presence of forskolin (1 or 10 mumol/l) the minimal concentration required to obtain a cyclic AMP response to PTH was decreased by a factor of 30-100 and the EC50 value was decreased to 1-2 nmol/l. Similar results were seen in osteoblast-enriched cells. In addition, the magnitude of the PTH-induced cyclic AMP response was substantially potentiated by forskolin, both in calvarial bones and in isolated osteoblasts. Forskolin, in the absence of PTH, stimulated cyclic AMP levels in mouse calvaria at and above 1 mumol/l. In the presence of PTH, the response to forskolin was potentiated over the whole dose-response curve with apparent EC50 value at 1-2 mumol/l of forskolin. Forskolin (1 mumol/l) did not affect the magnitude of the 45Ca release response to PTH in 24 or 48 h cultures. In 96 h cultures, forskolin, in an additive manner, potentiated the effect of PTH on calcium mobilization. These results show that forskolin, in mouse calvarial bones and in isolated osteoblasts, in addition to directly stimulating cyclic AMP, can enhance receptor-mediated activation of adenylate cyclase. The finding that forskolin did not synergistically potentiate PTH-induced bone resorption suggests that there is no simple relationship between PTH-induced cyclic AMP formation and stimulation of bone resorption.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Human relaxin increases cyclic AMP levels in cultured anterior pituitary cells

Although relaxin acts at several abdominal sites and mammary tissue associated with pregnancy and parturition, the scope of target tissues and the signals conveying the relaxin message into the cell are poorly defined. We found that human relaxin rapidly elevates the cyclic AMP content of cultured rat anterior pituitary cells. This is a graded response (EC50 0.3 nM relaxin) that can be blocked by anti-relaxin antibodies or the hormones somatostatin and dopamine. Furthermore, other hormones with some sequence homology to relaxin, such as insulin and insulin-like growth factor-I, have no such action. We conclude that the anterior pituitary may be a target tissue for relaxin and that cyclic AMP may act as an intracellular messenger for relaxin in these cells.

Parathyroid hormone induction of creatine kinase activity and DNA synthesis is mimicked by phospholipase C, diacylglycerol and phorbol ester

Parathyroid hormone (PTH), which increases cAMP levels, also induces an increase in the activity of the brain isozyme of creatine kinase and in DNA synthesis in osteoblast-enriched bone cell cultures by a cAMP-independent mechanism. The following results lead us to the conclusion that PTH induction of brain isozyme of creatine kinase activity and DNA synthesis occurs by activation of membranal phospholipid metabolism leading to increased protein kinase C activity and Ca2+ mobilization, a mechanism demonstrated for several growth factors and other hormones. (1) Binding of membranal phospholipids by agents such as gentamycin or antiphospholipid antibodies abolishes the stimulation by PTH of creatine kinase activity and DNA synthesis but not of cAMP production. (2) Treatment of cell cultures with exogenous phospholipase C increases brain isozyme of creatine kinase activity and DNA synthesis, but not cAMP production; these stimulations are also blocked by serum containing anti-phospholipid antibodies. PTH has no additional effect on stimulation of creatine kinase activity by phospholipase C (and only a slight effect on DNA synthesis). (3) A synthetic diacylglycerol (1-oleyl-2-acetyl glycerol) or phorbol ester (phorbol 12-myristate 13-acetate) or Ca2+ ionophore, A23187 induces creatine kinase activity and DNA synthesis in the cultures. However, this effect is not blocked by antiphospholipid sera and PTH has no additional effect. (4) Inhibition of protein kinase C activity by drugs reported to inhibit the enzyme (retinoic acid, quercetin) abolishes the stimulation of brain isozyme of creatine kinase activity and of DNA synthesis by PTH.

Synergism between parathyroid hormone and interleukin 1 in stimulating bone resorption in organ culture

The interaction of interleukin 1 (IL-1), a locally produced factor, and parathyroid hormone (PTH), a systemic factor, in stimulating bone resorption was examined using fetal rat long bone organ culture. Concentrations of IL-1 and PTH, which stimulated little bone resorption when present singly, produced marked resorption when present simultaneously. This synergistic interaction of IL-1 and PTH was not affected by the presence of the prostaglandin synthetase inhibitor indomethacin. Both interleukin 1 alpha and interleukin 1 beta were capable of producing synergy. Synergy was not produced by sequential exposure of bone to IL-1 and PTH, but required the simultaneous presence of both mediators. The leftward shift in the dose response curve of PTH produced by IL-1 may be an important mechanism controlling localized bone resorption. A role for IL-1 in stimulating bone resorption in pathologic conditions, such as arthritis and periodontal disease, is strengthened by the finding that even low concentrations of IL-1 can produce resorptive effects by synergistic interaction with PTH.

Hormonal influences on bone cells

The methods for establishing osteoblast-rich rat calvarial cell cultures have been described, together with methods for the use of clonal osteogenic sarcoma cells of osteoblast phenotype. The latter clonal lines are useful for several purposes, but all the precautions and quality control measures necessary in the study of clonal lines must be observed. Some of the techniques for studying biochemical responses to hormones in these cells have also been detailed, but clearly others are applicable, including studies of the synthesis of matrix constituents. Osteoclast-like cells have not been considered in this chapter, because osteoclast culture methods have not yet been developed to the degree of purity and reproducibility necessary for this type of biochemical approach.

Comparative study of the effects of cyclic nucleotide phosphodiesterase inhibitors on bone resorption and cyclic AMP formation in vitro

The relation between the level of cyclic AMP and bone resorption was studied in a bone organ culture system, using calvaria from newborn mice. Two methylxanthines, iso-butyl-methylxanthine and theophylline and two non-xanthine inhibitors of cyclic AMP phosphodiesterase, Ro 20-1724 and rolipram, stimulated the release of [45Ca] and [3H] from bones prelabelled in vivo with [45Ca]- and [3H]proline, respectively. The release occurred after a delay of more than 24 hr. In 120-hr cultures, theophylline, IBMX, rolipram and Ro 20-1724, all stimulated the release of stable calcium, inorganic phosphate and the lysosomal enzymes, beta-glucuronidase and beta-N-acetylglucosaminidase from mouse calvarial bones. In addition, all four phosphodiesterase inhibitors decreased the amount of hydroxyproline in the bones at the end of the culture period. The release of minerals and the decrease of hydroxyproline was abolished by indomethacin. In short-term cultures (24 hr), rolipram and Ro 20-1724 did not reduce PTH-stimulated mineral mobilization, whereas the two methylxanthines, and dibutyryl cyclic AMP and 8-bromo cyclic AMP, did cause a reduction of PTH-stimulated mineral release during the first 24 hr. All four phosphodiesterase inhibitors increased the accumulation of cyclic AMP in the calvaria and inhibited cyclic AMP hydrolysis in extracts of calvarial bone. There was a correlation between the magnitude of the initial rise in cyclic AMP and the delayed stimulation of bone resorption. However, much lower concentrations of the PDE inhibitors were sufficient to produce a delayed increase in bone resorption than to block phosphodiesterase and significantly raise cyclic AMP levels. It is suggested that the elevation of cyclic AMP in a subset of bone cells results in an acute reduction of bone mobilization and the cAMP elevation in another subset to a delayed rise in bone resorption.

Inhibitory effects of parathyroid hormone on growth of osteogenic sarcoma cells

The effects of the bone resorbing hormone, parathyroid hormone (PTH), on the growth of malignant osteoblastic cells have been examined. The malignant osteoblastic cells were a clonal line (UMR 106) derived from a transplantable rat osteogenic sarcoma. The predominant effect of PTH at doses above 10(-10) M was an inhibition of replication and DNA synthesis. Replication was decreased by PTH in both the presence or absence of serum and at various cell seeding densities. Both bovine PTH (1-84) and the synthetic hormone, human PTH (1-34), inhibited replication, but with bovine hormone being an order of magnitude more potent. The effects could be observed in as short a time as 6 hours with DNA synthesis and 24 hours with replication.

The effects of calcium regulating hormones on bone resorption by isolated human osteoclastoma cells

Cells were disaggregated from osteoclastomas, and the response of the giant cells to calcium-regulating hormones, prostaglandin (PG)E1 and dibutyryl cyclic AMP (dbcAMP) was observed by phase-contrast time-lapse video microscopy. The pattern and nature of their response was very similar to that previously found to be characteristic of osteoclasts: calcitonin (CT), PGE1 and dbcAMP induced cytoplasmic quiescence, while parathyroid hormone (PTH) showed no influence on cytoplasmic motility or behaviour. The cells were also cultured on slices of devitalized cortical bone for 5 or 18 h. After this time the giant cells were associated with the appearance in the scanning electron microscope of characteristic resorption pits, the volume of which was calculated by computer-assisted morphometric and stereophotogrammetric techniques after removal of cells. Calcitonin caused a dramatic reduction in the volume of bone resorbed by these isolated cells compared with control cultures, while PTH was without significant effect. This result supports the view that PTH does not increase bone resorption in intact bone through a direct effect on osteoclasts. PGE1, which stimulates bone resorption when added to intact bone, paradoxically reduced resorption in our cultures. It thus appears possible that PGE1 acts as a direct inhibitor of osteoclastic bone resorption but has an additional effect on other cells in bone, which are induced by PGE1 to cause osteoclastic stimulation.

The pathobiology of the osteoclast

This article reviews recent information concerning the origin of osteoclasts and the local and systemic regulation of their activity. It appears that much of the environmental responsiveness of osteoclasts is mediated by cells of the osteoblastic lineage, which exert a major influence on the localisation, induction, stimulation, and inhibition of osteoclastic bone resorption. Some of the mechanisms by which osteoclast function may be disturbed by inflammatory and neoplastic diseases are discussed, and it is suggested that many pathological disturbances of osteoclastic bone resorption may be explicable as mimicry of physiological regulatory mechanisms by local hormones introduced into bone as the local regulators of the diseased tissue.

Stimulation of creatine kinase BB activity by parathyroid hormone and by prostaglandin E2 in cultured bone cells

Bone cells in culture responded to parathyroid hormone (PTH) and prostaglandin E2 (PGE2) by a 2-fold increase in creatine kinase (CK) activity. Combined treatment resulted in a higher response than with PTH alone. Calcitonin (CT) failed to stimulate CK activity, did not affect the response of CK to PTH, but inhibited slightly the increase in CK activity by PGE2. Bone-cell cultures grown in low [Ca2+] (0.125 mM), enriched in PTH-responsive osteoblast-like cells, responded to PTH, but not to PGE2 or CT, by increased CK activity. In both normal and low-[Ca2+] cultures, 8-bromo cyclic AMP did not affect CK activity, nor did it change the response of the cells to PTH, PGE2 or CT. The increase in CK activity was time- and dose-dependent and inhibited both by cycloheximide and by actinomycin D. The isoenzyme of CK stimulated was the CKBB form, the isoenzyme induced by other hormones. This appears to be the first report of the stimulation of CK activity by a polypeptide hormone or a prostaglandin. We suggest that stimulation of CKBB can serve as a marker for the action of a variety of hormones and growth promoters.

Stimulation of calcium uptake in rat calvaria by prostacyclin

Treatment of newborn rat calvaria discs with a variety of unsaturated fatty acids led to a 50% enhancement of calcium uptake. Arachidonic acid was effective at lower concentrations than cis-vaccenic or oleic acid, while trans-vaccenic acid and saturated fatty acids did not enhance calcium uptake. Cyclooxygenase inhibitors indomethacin and acetylsalicylic acid abolished the enhancement of calcium uptake seen in response to cis-vaccenic acid and inhibited calcium uptake by otherwise untreated bones. Prostacyclin was found to produce up to 2 fold stimulation of calcium uptake with an EC50 of approximately 0.1 microM. No statistically significant stimulation of calcium uptake was seen in response to PGE2 or PGE1 alpha up to 25 microM, while slight stimulation was produced by 6-keto PGE1 alpha but only at concentrations of 10 microM. Prostacyclin production by calvaria was demonstrated and was stimulated over 50% by cis-vaccenic acid. These results suggest that not only is enhanced prostacyclin production responsible for elevation of calcium uptake in response to unsaturated fatty acids, but also that prostacyclin may be an important regulator of bone calcium homeostasis.

Characterization of an osteoblast-like clonal cell line which responds to both parathyroid hormone and calcitonin

The clonal cell line UMR 106, which was originally derived from a rat transplantable osteogenic sarcoma with an osteoblastic phenotype, was subcloned after the emergence of a calcitonin-responsive adenylate cyclase was noted in late passages. Detailed studies on the stimulation of adenylate cyclase and activation profile of the cyclic AMP-dependent protein kinase isoenzymes in response to parathyroid hormone (PTH) and salmon calcitonin (SCT) were conducted on two subclones (UMR 106-01 and UMR 106-06). Both subclones responded in an identical manner to PTH, which stimulated adenylate cyclase and activated both isoenzyme I and isoenzyme II of cyclic AMP-dependent protein kinase. In contrast, only UMR 106-06 cells responded to calcitonin. At 3 X 10(-8)M SCT, there was a sevenfold stimulation of adenylate cyclase, 84% activation of isoenzyme I, and 44% activation of isoenzyme II. The activation profiles of the isoenzymes to PTH and SCT in UMR 106-06 were similar. Furthermore, their response to SCT correlates with the presence of specific, saturable binding of 125I-labeled SCT. Binding parameters indicate apparent Kd = 0.8 nM and 6,000 receptors/cell. These data point to a significant phenotypic change having taken place in this clonal cell line with prolonged maintenance in culture, with the emergence of a calcitonin receptor linked to adenylate cyclase and protein kinase activation.

Parathormone promotes glycogen formation from [14C]glucose in cultured osteoblast-like cells

Parathyroid hormone stimulates [U-14C]glucose incorporation into glycogen of cultured osteoblast-like calvaria cells. This effect is detectable only several hours after the addition of PTH and it is mimicked by dibutyryl cyclic AMP. In contrast to insulin (in pharmacological concentrations), PTH enhances glycogen formation only in calvaria cells, but not in fibroblasts. Insulin-like growth factor I in physiological concentrations promotes glycogen-synthesis shortly after addition.