Heterologous desensitization by 1,25-dihydroxyvitamin D-3 of cyclic AMP response to parathyroid hormone in osteoblast-like cells and the role of the stimulatory guanine nucleotide regulatory protein

In vivo administration of 1,25-dihydroxyvitamin D3 suppresses the expression of RANKL mRNA in bone of thyroparathyroidectomized rats constantly infused with PTH

It is known that pharmacological or toxic doses of vitamin D induce bone resorption both in vivo and in vitro, whereas physiological doses of the vitamin have a protective effect on bone in vivo. To investigate the discrepancies of the dose-dependent effect of vitamin D on bone resorption, we examined the in vivo effect of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] on the expression of the receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG) mRNAs in bone of thyroparathyroidectomized (TPTX) rats infused with or without parathyroid hormone (PTH). Continuous infusion of 50 ng/h of PTH greatly increased the expression of RANKL mRNA in bone of TPTX rats. Expression of OPG mRNA was not altered by PTH infusion. When graded doses of 1,25(OH)(2)D(3) was daily administered orally for 14 days to normocalcemic TPTX rats constantly infused with PTH, 0.01 and 0.1 microg/kg of 1,25(OH)(2)D(3) inhibited the PTH-induced RANKL mRNA expression, but 0.5 microg/kg of the vitamin did not inhibit it. Regulator of G protein signaling-2 (RGS-2) gene expression was suppressed by 1,25(OH)(2)D(3) dose-dependently, but PTH/PTHrP receptor mRNA expression was not altered. Bone morphometric analyses revealed that 1,25(OH)(2)D(3) suppressed PTH-induced osteoclast number in vivo. These results suggest that pharmacological or toxic doses of 1,25(OH)(2)D(3) stimulate bone resorption by inducing RANKL, but a certain range of physiological doses of the vitamin inhibit PTH-induced bone resorption, the latter mechanism appeared to be mediated, at least in part, by the suppression of the PTH/PTHrP receptor-mediated signaling.

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.

Serpentine receptors for parathyroid hormone, calcitonin and extracellular calcium ions

The cloning of the receptors for PTH, CT and extracellular calcium ions represents a significant advance in the elucidation of the mechanisms through which extracellular calcium ions are regulated. All are members of the superfamily of GPCR, and the inclusion of the Ca2+o-sensing receptor in this superfamily documents that extracellular calcium ions can serve as an extracellular first messenger, in addition to subserving their better known role as a key intracellular second messenger. Furthermore, it has proved possible to identify several human diseases that result from inactivating or activating mutations in the PTH or Ca2+o-sensing receptor. Finally, the availability of these cloned receptors will enable many more studies on structure-function relationships for these receptors as well as clarifying their tissue distribution, regulation and roles in health and disease. It may also be possible to design novel therapeutic agents that permit manipulation of the receptors when their function is abnormal.

Regulation of parathyroid hormone/parathyroid hormone-related protein receptor expression by osteoblast-deposited extracellular matrix in a human osteoblast-like cell line

Parathyroid hormone (PTH) receptors and the biological response to PTH in osteoblasts have been shown to be influenced by glucocorticoids, growth factors, cytokines or PTH itself. Furthermore, components of extracellular matrix (ECM) appear to regulate the response to PTH as well. We investigated the effects of osteoblast-deposited ECM on PTH-related protein (PTHrP)-stimulated cAMP production, PTHrP binding and PTH/PTHrP receptor mRNA in the human osteoblast-like cell line SaOS-2. ECM was laid down by the human osteoblastic cell line MG-63. At confluence, maximal cAMP stimulation induced by 100 nmol/l PTHrP (1-34) was decreased in SaOS-2 cells grown on ECM as compared with cultures on plastic dishes, without any change in PTHrP concentration producing half-maximal stimulation. In contrast, cAMP production stimulated by PGE2 was increased in cells on ECM. Saturable 125I-PTHrP binding (as evaluated by Scatchard plot analysis) was markedly diminished in cells grown on ECM (5,600 +/- 2,010 vs. 20,700 +/- 1,710 binding sites/cell, x +/- S.E.M., P < 0.01, n = 4 experiments), without any significant change in affinity (1.3 +/- 0.4 vs. 2.5 +/- 0.5 nmol/l (NS), in cells on ECM and plastic, respectively). This apparent decrease in membrane receptor density was associated with markedly lower steady state PTH/PTHrP receptor mRNA levels as assessed by Northern blot analysis (ECM/control: 0.4 +/- 0.1). A difference in PTH/PTHrP receptor mRNA levels between cells on ECM or on plastic dishes was detectable by 8 hours but not by 4 hours, after seeding the cells at high density. By 24 hours after plating, PTH/PTHrP receptor mRNA levels were maximally decreased in cells on ECM. These results in the human osteoblast-like cell line SaOS-2 indicate that PTH/PTHrP receptors are down-regulated by growth on ECM. Thus, attachment of bone cells to bone surface could influence differentiation and function of osteoblasts.

Modulation of parathyroid hormone-sensitive adenylate cyclase in ROS 17/2.8 cells by dexamethasone 1,25-dihydroxyvitamin D3 and protein kinase C

We tested whether the protein kinase C (PKC) modulation of PTH-sensitive adenylate cyclase in ROS 17/2.8 cells is affected by the glucocorticoid dexamethasone and the vitamin D hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Basal and PTH- and forskolin-stimulated adenylate cyclase activities were determined in the presence or absence of 100 nM phorbol 12-myristate 13-acetate (PMA), the activator of PKC, in ROS 17/2.8 cells that had been previously cultured with or without dexamethasone or 1,25(OH)2D3. Dexamethasone treatment increased the basal, PMA-, PTH-, (PTH + PMA)- and (forskolin + PMA)-sensitive adenylate cyclase while 1,25(OH)2D3 decreased these effects. The stimulatory and inhibitory effects were dose-dependent with respect to dexamethasone and 1,25(OH)2D3, respectively. Dexamethasone increased, while 1,25(OH)2D3 decreased the maximal activity of both PTH-sensitive and PKC-modulated PTH-sensitive adenylate cyclase without affecting the half-maximal concentration (ED50) of PTH required for the activation of the enzyme. Additionally, dexamethasone, 1,25(OH)2D3 and PKC did not affect each other's ED50. Our results suggest that the effects of dexamethasone, 1,25(OH)2D3 and PKC on PTH-sensitive adenylate cyclase in ROS 17/2.8 cells are independent of each other.

Parathyroid hormone and glucagon compete for binding to low affinity sites on human skin fibroblasts

We have studied the binding sites for parathyroid hormone (PTH) present on normal human fibroblasts by studying these receptors with respect to (1) affinity and specificity towards various peptide hormones including human PTH(1-34) and porcine glucagon(1-29); (2) ability to mediate stimulation of cAMP production in response to these hormones; and (3) molecular size. Binding assays using 125I-labelled human PTH(1-34) and 125I-labelled porcine glucagon(1-29), and hormone stimulations of cAMP production were performed on confluent fibroblasts grown in 24-well dishes (passage 4-10). The molecular sizes of the binding sites for PTH and glucagon were assessed after cross-linking to the corresponding 125I-labelled ligand using the heterobifunctional reagent 1,4-difluoro-2,5-dinitrobenzene (DFDNB), by sodium dodecyl sulphate gel electrophoresis and autoradiography. The results demonstrate: (1) Biologically active PTH and glucagon, but not other peptide hormones tested, are equipotent competitors for binding on human fibroblasts to sites which have a relatively low affinity for these ligands (Kd approximately 0.8-2.4 x 10(-7) M); these sites have an apparent molecular weight of 95 kDa and are not linked to stimulation of cAMP production by PTH. (2) A distinct class of receptors for PTH with an apparent molecular weight of 60 kDa and which probably are linked to stimulation of cAMP production by PTH is also expressed by these cells; glucagon cannot compete with PTH for binding to these sites, and does not interfere with the stimulation of cAMP production by PTH. (3) Glucagon does not stimulate cAMP production in human fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25-dihydroxyvitamin D-3 and 24,25-dihydroxyvitamin D-3 affect parathormone (PTH) -sensitive adenylate cyclase activity and alkaline phosphatase secretion of osteoblastic cells through different mechanisms of action

In UMR 106 rat osteosarcoma cells, parathormone (1-34hPTH) and calcitonin (sCT) stimulated adenylate cyclase (AC) activity 5.5-and 2.8-fold, respectively. AC in osteoblasts (OB) from collagenase-treated calvaria of 3-day-old rats responded similarly to 1-34hPTH. In contrast, fibroblasts (mouse fibroblastomas) displayed a marginal 1-34hPTH sensitive AC. Osteoclasts (OC) of collagenase-treated rat calvariae, rat monocytes and mouse macrophages did not demonstrate 1-34hPTH inducable AC activity. Physiological concentrations of 24,25-dihydroxyvitamin D-3 attenuated PTH-sensitive AC in OB and UMR 106 cells within 20 min, while 1,25-dihydroxyvitamin D-3 showed no such immediate effect. In contrast, the AC response to Gpp(NH)p was unaffected by 24,25-(OH)2D3, indicating that 24,25-(OH)2D3 interrupts the coupling of the PTH receptor to the GTP binding protein Gs. OB and UMR 106 cells were also subjected to long-term (48 h) incubation with vitamin D-3 metabolites, 1-34hPTH or 20% serum from patients with secondary hyperparathyroidism (sHBT-serum), respectively. PTH-sensitive AC was markedly attenuated by pre-exposure to both 1-34hPTH and 1,25-(OH)2D3, while minimally affected by corresponding 24,25-(OH)2D3 and 20% sHPT-serum treatment. The secretion of alkaline phosphatase (Alphos) from the two cell types was strongly increased by 1-34hPTH, the effect being abolished by the presence of 24,25-(OH)2D3. Iliac crest biopsies of normal individuals exhibited a clear negative correlation between PTH-sensitive AC and corresponding serum 24,25-(OH)2D3 levels. Basal AC activity was, however, negatively correlated to serum 1,25-(OH)2D3 concentrations. In summary, the results show that 24,25-(OH)2D3 reduces PTH-stimulated AC activity in and Alphos secretion from osteoblastic bone cells by rapidly and directly interfering with the plasma membrane. These data reinforce the probable in vivo significance of 24,25-(OH)2D3. Moreover, the negative correlation between basal AC activity and serum 1,25-(OH)2D3 levels indicates a possible role for 1,25-(OH)2D3 in regulating bone cell synthesis of AC components in vivo.

Calcium modulation of the parathyroid hormone-sensitive adenylate cyclase in ROS 17/2.8 cells: effects of N-(6-aminohexyl-5-Cl-naphthalene sulfonamide) (W-7) and trifluoperazine (TFP)

The calcium modulation of the cyclic 3',5'-adenosine monophosphate (cAMP) response to parathyroid hormone (PTH) was studied in a clonal osteosarcoma cell line ROS 17/2.8. CaCl2 was found to stimulate the PTH-sensitive cAMP response of intact cells. At the maximal concentration of 1 mM CaCl2, the maximum response to PTH was increased, but the ED50 for PTH and the time course of maximal cAMP production were not affected. Verapamil blunted, while the cation ionophore A23187 enhanced, the stimulatory effect of CaCl2. Trifluoperazine (TFP) and N-(6-aminohexyl-5-Cl-naphthalene sulfonamide) (W-7) inhibited the stimulatory effect of CaCl2. In membranes prepared in the presence of 0.1 mM CaCl2, a biphasic effect of CaCl2 was demonstrated: stimulation at concentrations of 60-100 microM, and an inhibition above 200 microM, when adenylate cyclase was assayed in the presence of 200 microM EGTA. Addition of exogenous calmodulin to membranes prepared in the presence of EGTA did not have any effect on the PTH-sensitive adenylate cyclase activity, suggesting that endogenous calmodulin was not effectively stripped from the membranes by EGTA treatment. It is concluded that Ca2+ has both a stimulatory and an inhibitory role in modulating PTH-sensitive adenylate cyclase in ROS 17/2.8 cells by as yet unknown mechanisms, and that the involvement of endogenous calmodulin is implicated.

Stimulation of inositol phosphate formation in ROS 17/2.8 cell membranes by guanine nucleotide, calcium, and parathyroid hormone

In addition to stimulation of cyclic AMP, parathyroid hormone (PTH) may influence cellular events by utilizing other pathways of hormone action, such as the generation of inositol phosphates (IPs). We sought to examine this potential action of PTH by assessing the formation of inositol phosphates in PTH-sensitive ROS 17/2.8 cells. The polyphosphoinositides were labeled by growing the cells with [3H]inositol following which cell homogenates were prepared. The nonhydrolyzable guanine nucleotide, GTP gamma S, and calcium ion, alone and together, stimulated all three IPs, IP1, IP2, and IP3. IP1 formation was linear over 30 minutes but IP2 and IP3 accumulated more rapidly peaking by 5 minutes for all agonist conditions. The proportion of total P as IP3 was enhanced when the cells were grown with retinoic acid (1 microM) or when the assay was conducted at pH 4.5. In addition, the lower pH was associated with much more enzyme activity. PTH agonists, bPTH-(1-84) and bPTH-(1-34), both caused a small but significant stimulation of IP3 formation. When bPTH-(1-84), and the analog bPTH-(3-34)amide, that inhibits PTH-mediated adenylate cyclase activity were present together, there was additive stimulation of IP3 formation compared with that with either agent alone. The results demonstrate that inositol phosphate formation can be stimulated directly in a membrane preparation of ROS cells by GTP gamma S, calcium ion, and PTH and that the enzyme mediating this activity, phospholipase C, is regulated by a guanine nucleotide binding protein.

Corticosteroid-induced changes in the responsiveness of human osteoblast-like cells to parathyroid hormone

We investigated the in vitro effect of corticosteroids on the responsiveness of human cells of osteoblast lineage to parathyroid hormone (PTH). Prior to corticosteroid treatment, the cells demonstrated only a small increase in cAMP production and no measurable change in transmembrane potential in response to PTH. Exposure of cells to dexamethasone resulted in a 5-fold increase in PTH-induced cAMP production and in measurable PTH-induced membrane depolarization in all cells studied. The effect of corticosteroids on cAMP production was specific for PTH (not seen with PGE1 or forskolin), occurred in a time- and dose-dependent fashion and in the absence of cell proliferation. Most of the cells were of osteoblast lineage as determined by the presence of alkaline phosphatase activity and BGP secretion. These findings further support the idea that corticosteroids increase the sensitivity of cells of osteoblast lineage to PTH, perhaps by transforming cells which initially have a low responsiveness to PTH to a state of high responsiveness.

Effect of dexamethasone on parathyroid hormone stimulation of cyclic AMP in an opossum kidney cell line

The influence of the glucocorticoid dexamethasone on the cAMP response to parathyroid hormone (PTH) and various agonists was studied in epithelial monolayers of opossum kidney (OK) cells. The incubation with dexamethasone for 72 hours led to a dose-dependent higher cAMP response to PTH or forskolin in intact cells as well as in digitonin-permeabilized cells. This effect did not appear to result from changes in phosphodiesterase (PDE) activity nor from alterations in cAMP efflux from the cells. Moreover, dexamethasone increased the formation of domes by OK cell epithelium. Thus, dexamethasone seems to promote a more differentiated renal epithelial phenotype as suggested by enhanced hormonal response.