Use of forskolin to study the relationship between cyclic AMP formation and bone resorption in vitro

Circulating Retinol-Binding Protein 4 as a Possible Biomarker of Treatment Response for Ankylosing Spondylitis: An Array-Based Comparative Study

Objective

To explore proteins associated with ankylosing spondylitis (AS) and to investigate potential proteins that may predict treatment response of adalimumab (ADA) in AS patients.

Methods

In the discovery cohort, 39 AS patients and 20 healthy controls (HCs) were included, and 16 AS patients received ADA treatment for 24 weeks after included. In the validation cohort, 43 AS patients and 39 HCs were enrolled, and all 43 patients received ADA treatment after enrollment. Blood samples and clinical information were collected from two cohorts at baseline from all participants and week 24 from patients received ADA treatment. A human antibody array containing 1,000 proteins was used in the discovery phase, and Elisa kits were used for protein validation.

Results

Compared with HCs, we identified 53 differentially expressed proteins (DEPs) in AS patients. Bioinformatics analysis revealed they were mostly enriched in coagulation function-related pathways, acute response signaling, and LXR/RXR activation. Bone metabolism pathways were also associated. Comparison between samples of pre- and post-ADA treatment revealed 42 DEPs. They were mostly associated with bone metabolism and inflammation response pathways. Significant enrichment was also found in LXR/RXR activation but not the coagulation function-related pathways. Upstream regulator analysis suggested that most regulators also significantly functioned under usage of ADA. Precisely, seven proteins were abnormally expressed in AS and restored after ADA treatment. Retinol-binding protein 4 (RBP4), one of the seven proteins, was validated that its baseline levels were inversely correlated with improvements in Ankylosing Spondylitis Disease Activity Score-C-reactive protein (ASDAS-CRP). Likewise, percentage changes in RBP4 levels were inversely correlated with changes in ASDAS-CRP score.

Conclusion

A dysregulated serum protein profile existed in AS. ADA exerted a considerable but not entire alteration toward the dysregulation. RBP4 could be a biomarker for predicting and monitoring ADA treatment response.

Study on site-specific expression of bone formation and resorption factors in human dental follicles

We sought to investigate site‐specific expression of bone‐regulatory factors expressed by human dental follicles and to compare the stimulated expression of tumour necrosis factor (ligand) superfamily, member 11/tumour necrosis factor receptor superfamily, member 11b (RANKL/OPG) in human dental follicle cells (HDFCs) from different patients. Analysis of bone‐regulatory markers in follicles from 12 different study participants was performed using RT‐qPCR and immunofluorescence; apical and coronal segments from each dental follicle were processed independently. Four additional dental follicles were used for cell cultures; HDFCs were precultured in osteogenic medium to initiate differentiation and thereafter cultured with 10⁻⁶ M forskolin (FSK) to activate the protein kinase cAMP (PKA/cAMP) signalling pathway and induce RANKL/OPG expression. We demonstrate that RANKL expression is significantly higher in the coronal part of follicles than in the apical part. High levels of collagen type 1 (COL1), alkaline phosphatase (ALP) and Gap‐junction protein, alpha 1, 43 kDa (CX43) were expressed, whereas expression of Sp7 transcription factor (OSX), bone morphogenetic protein 2 (BMP2), colony‐stimulating factor 1 (CSF‐1), chemokine (C‐C motif) ligand 2 (MCP1), and OPG was low in all samples. The immunofluorescence localization of CSF‐1, MCP1, osteocalcin (OCN), RANKL, and BMP2 was not specific for either part of the follicles. In conclusion, a consistently high expression of CX43 suggests that gap‐junction communication in HDFCs is essential for the eruption process. Furthermore, the induced expression of RANKL in HDFCs varies significantly between individuals and may relate to clinical variations in tooth eruption.

Effects of catecholamines on calvarial bone resorption in vitro

Many important diseases in otolaryngology manifest through abnormal bone remodeling or destruction. The mechanisms for such pathological remodeling remain poorly understood. Bone is known to be innervated by norepinephrine-containing sympathetic nerves, and sympathectomy is known to induce bone resorption. The role, however, of norepinephrine as a potential bone-modulatory substance is unknown. Using the calvarial calcium release assay, we conducted the following experiment to evaluate the bone-modulatory activity of norepinephrine, the alpha-agonist octopamine, and the beta-agonist isoproterenol. Each agent was tested at 2 concentrations with and without parathyroid hormone. Norepinephrine was found to have no effect on calcium release. In contrast, octopamine at 10(-8) mol/L exerted a significant stimulatory effect on calcium release, and isoproterenol at 10(-6) mol/L exerted a significant inhibitory effect on parathyroid hormone-induced calcium release. The investigation suggests that a bimodal, concentration-dependent, receptor-specific model for catecholamine-mediated modulation of bone resorption may operate in calvarial bone.

Mechanisms involved in prostaglandin-induced increase in bone resorption in neonatal mouse calvaria

Prostaglandins (PG) E1, E2 and F2alpha induce bone resorption in isolated neonatal parietal bone cultures, and an associated increase in interleukin-6 (IL-6) production. Indomethacin had little effect on the response to PGE2, or the relatively non-selective EP receptor agonists 11-deoxy PGE1 and misoprostol, but blocked the effects of PGF2alpha and the F receptor agonist fluprostenol, indicating an indirect action via release of other prostaglandins. It is more likely that there is positive autoregulation of prostaglandins production in this preparation mediated via stimulation of F receptors. The effects of selective EP receptor agonists sulprostone (EP1,3) and 17-phenyl trinor PGE2(EP1), indicated the involvement of EP2 and/or EP4 receptors, which signal via cAMP. The relatively weak increase in IL-6 production by misoprostol (with respect to resorption) suggests that these responses are controlled by different combination of EP2 and EP4 receptors. The PKA activator, forskolin, induced small increases in bone resorption at lower concentrations (50-500 ng/ml) but a reversal of this effect, and inhibition of resorption induced by other stimuli (PTH, PGE2), at higher concentrations (0.5-5 microg/ml). IL-6 production was markedly increased only at the higher concentrations. The inhibitory effect of forskolin may be a calcitonin-mimetic effect. PMA induced both resorption and IL-6 production which were both blocked by indomethacin, indicating a role for PKC in the control of prostaglandin production.

Neuro-hormonal control of bone metabolism: vasoactive intestinal peptide stimulates alkaline phosphatase activity and mRNA expression in mouse calvarial osteoblasts as well as calcium accumulation mineralized bone nodules

Based upon the immunohistochemical demonstration of neuropeptides in the skeleton, including vasoactive intestinal peptide (VIP), we have addressed the question of whether neuropeptides may exert regulatory roles on bone tissue metabolism or not. In the present communication, we have investigated if VIP can affect anabolic processes in osteoblasts. Osteoblasts were isolated from neonatal mouse calvariae by time sequential enzyme-digestion and subsequently cultured for 2-28 days in the presence of VIP and other modulators of cyclic AMP formation. VIP (10(-6) M) stimulated ALP activity and calcium content. The cyclic AMP phosphodiesterase inhibitors ZK 62 711 (10(-4) M) and isobutyl-methylxanthine (10(-4) M) stimulated ALP activity and synergistically potentiated the effect of VIP. Neither VIP, nor isobutyl-methylxanthine or ZK 62 711, in the absence or presence of VIP, affected cell number. The stimulatory effect of VIP on ALP activity, in the presence of ZK 62 711, was dependent on time and concentration of VIP. The stimulatory effects of VIP and ZK 62 711 on ALP activity was seen also in cells stained for ALP. VIP (10(-6) M), in the presence of ZK 62 711 (10(-6) M), significantly enhanced mRNA for tissue non-specific ALP. VIP (10(-6) M), in the presence of ZK 62 711, stimulated cyclic AMP production. Forskolin and choleratoxin stimulated ALP activity and cyclic AMP formation in a concentration-dependent manner, without affecting cell number. VIP (10(-6) M) and ZK 62 711 (10(-5) M) stimulated, and their combination synergistically enhanced, calcium content in bone noduli. These data show that VIP, without affecting cell proliferation, can stimulate osteoblastic ALP biosynthesis and bone noduli formation by a mechanism mediated by cyclic AMP. Our observations suggest a possibility that anabolic processes in bone are under neurohormonal control.

Bone resorption induced by A23187 is abolished by indomethacin: implications for second messenger utilised by parathyroid hormone

Parathyroid hormone acts on the osteoblast to induce osteoclastic bone resorption. Parathyroid hormone utilises cyclic AMP as a second messenger in osteoblasts, but may also cause an increase in cytoplasmatic free calcium ions ([Ca2+]i) in the same cell. To investigate the role of osteoblastic [Ca2+]i in the induction of bone resorption, we have compared the effects of parathyroid hormone and the Ca2+-ionophore, A23187, as well as the adenylate cyclase stimulating agent, forskolin, and the phorbol ester, phorbole 12,13 dibutyrate (PDB), on bone resorption in neonatal mouse calvarial bones. Parathyroid hormone (0.1 and 1 nM) dose dependently stimulated the release of prelabelled 45Ca2+ in 72 h culture. Parathyroid hormone-induced bone resorption was not affected by the addition of 1 microM indomethacin to the incubation media, and was therefore, not mediated by local prostaglandin formation. A23187 stimulated the release of 45Ca2+ at 1-10 nM. Above 100 nM, A23187 inhibited bone resorption. The A23187 (3 and 10 nM)-induced bone resorption was abolished by the cyclooxygenase inhibitor, indomethacin (1 microM), indicating that the stimulatory effect was mediated via prostaglandin formation. The adenylate cyclase stimulating agent, forskolin, dose dependently stimulated bone resorption at and above 1 microM. There was no additive or synergistic effect of forskolin and A23187 on 45Ca2+ release. Forskolin-induced bone resorption was, as with parathyroid hormone but in contrast to ionophore-induced bone resorption, not abolished by indomethacin (1 microM). The protein kinase C activator, PDB, at 10 and 1000 nM stimulated the release of prelabelled 45Ca2+. The stimulatory effect of the protein kinase C stimulating phorbol ester, PDB, on bone resorption was abolished by the addition of indomethacin. In summary, bone resorption induced by a Ca2+-ionophore is abolished by indomethacin. This indicates that bone resorbing agents known to increase [Ca2+]i subsequently enhance local prostaglandin formation. Bone resorption induced by the protein kinase C activator, PDB, was also abolished by indomethacin, whereas, forskolin and parathyroid hormone-induced bone resorption was unaffected. These data indicate that cyclic AMP, but not [Ca2+]i, is involved as a second messenger in parathyroid-induced bone resorption.

PGE2 induces the transition from non-adherent to adherent bone marrow mesenchymal precursor cells via a cAMP/EP2-mediated mechanism

When mesenchymal precursor cells from bone marrow are cultured in the presence of dexamethasone, the existence of distinct non-adherent and adherent populations can be demonstrated. The addition of PGE2, forskolin, or dibutyryl-cAMP can induce a transition from the former to the latter and this may be an important mechanism in the bone anabolic effects of PGE2. On the other hand, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, and sulprostone, an agonist for the PGE2 receptor EP1/EP3 subtypes, had no effect. The phosphodiesterase inhibitor, isobutylmethylxanthine (IBMX), had a synergistic effect in combination with PGE2, whereas neomycin, an inhibitor of inositol phosphate activity, had no effect, and LiC1, an inhibitor of inositol triphosphate metabolism, had an inhibitory effect on the PGE2-induced transition. Consistent with this, the addition of PGE2 to non-adherent bone marrow cells caused a 100% increase in cAMP synthesis. These results suggest that the induction of the transition from non-adherent to adherent osteoblast precursor is mediated by the EP2-PGE2 receptor subtype via an increase in intracellular cAMP synthesis.

Cholera toxin-stimulated bone resorption in cultured mouse calvarial bones not inhibited by calcitonin: a possible interaction at the stimulatory G protein

We examined the effect of calcitonin in cultured mouse calvarial bones after prestimulation with different activators of adenylyl cyclase. Calcitonin (100 ng/ml), added after 48 h of culture, inhibited bone resorption (assessed as release of 45Ca from prelabeled bones cultured for 96-144 h) stimulated with parathyroid hormone (PTH, 10 nM; 0-144 h) or the adenylyl cyclase stimulator forskolin (2 microM; 0-144 h). However, no effect of calcitonin was demonstrated when bone resorption was prestimulated with the adenylyl cyclase stimulator cholera toxin, at and above 1 ng/ml, at any time point studied. In contrast, two other types of inhibitors of bone resorption in vitro, the carbonic anhydrase inhibitor acetazolamide (10 microM) and the aminobisphosphonate AHPrBP (10 microM), significantly inhibited cholera toxin-stimulated bone resorption. No cyclic AMP response to calcitonin was seen after preculture for 48 h with cholera toxin (0.1-100 ng/ml), although bones precultured in basic medium, in the absence or presence of forskolin, were still able to respond to calcitonin with elevation of cyclic AMP. Binding studies with [125I]calcitonin demonstrated that the preculture with cholera toxin did not affect the binding of calcitonin to the receptor. In summary, our data show that cholera toxin pretreatment makes calvarial bones insensitive to calcitonin-induced inhibition of bone resorption as a result of an interaction with cholera toxin at the level of calcitonin receptor-linked signal transduction. We suggest that the interaction, distal to the calcitonin receptor, is caused by the irreversible activation of Gs produced by cholera toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-1 beta induces cyclic AMP formation in isolated human osteoblasts: a signalling mechanism that is not related to enhanced prostaglandin formation

Interleukin-1 (IL-1) is a potent stimulator of bone resorption. Induction of osteoclastic bone resorption by various endocrine or paracrine factors is mediated via the osteoblasts. We have therefore investigated the effects of IL-1 beta on cell signalling in isolated human osteoblasts. Special interest was focused on prostaglandin synthesis, since indomethacin, an inhibitor of prostaglandin synthesis, partly inhibits IL-1-induced bone resorption. IL-1 beta, at and above 0.3 pM, dose dependently stimulated PGE2 formation in isolated human osteoblasts, with half maximal stimulation, EC50, at 3 pM. Treatment with the calcium ionophore A23187 (1 microM), or with forskolin (30 microM), also stimulated PGE2 formation in human osteoblasts. The time-course for IL-1 beta-induced PGE2 formation was similar to that of forskolin, with a significant increase in the formation of PGE2 seen after 1 h. In contrast, A23187-induced PGE2 formation was seen within minutes. IL-1 beta stimulated the accumulation of cyclic AMP in isolated human osteoblasts incubated for 15 min. This increase in cyclic AMP formation was not secondary to PGE2 formation since it was not blocked by the addition of indomethacin (1 microM). Pretreatment with the phosphodiesterase inhibitor IBMX did not augment IL-1 beta-induced PGE2 formation, nor did the protein kinase A inhibitor Rp-cAMPs inhibit IL-1 beta-induced PGE2 formation, suggesting that cyclic AMP does not mediate the stimulatory effect of IL-1 on PGE2 formation. We conclude that IL-1 beta enhances the formation of cyclic AMP as well as PGE2 in primary cultures of isolated human osteoblasts. The IL-1 beta-induced cyclic AMP formation is, however, not related to the enhanced prostaglandin formation. The findings implicate that both cyclic AMP- and PGE2-formation in osteoblasts might be involved as independent mediators of IL-1 beta-induced bone resorption.

Pertussis toxin-sensitive activation of phospholipase A2 can be resolved from phosphoinositidase C in primary cultures of mouse osteoblasts using indomethacin

Recent work has established that various bone-resorbing hormones are able to activate phosphoinositide metabolism as well as eicosanoid production in osteoblast-like cells, although the relationship between these pathways is unclear. We used pertussis toxin and indomethacin to inhibit the stimulation of [3H]arachidonic acid release and [3H]phosphoinositide turnover caused by treating primary cultures of mouse osteoblasts with fetal calf serum. We found (1) that pertussis toxin and indomethacin each inhibited both pathways and (2) that although pertussis toxin inhibited [3H]arachidonic acid release to a greater extent than indomethacin, [3H]inositol phosphate accumulation was inhibited rather more effectively by indomethacin. These data suggest that whereas ligands in fetal calf serum activate [3H]arachidonic acid release largely directly via the action of a pertussis-sensitive G protein, activation of phosphoinositidase C is indirect, being substantially dependent upon eicosanoid production. These experiments suggest that serial activation of phospholipase A2 and phosphoinositidase C may occur in osteoblasts and that only the former enzyme is regulated by a pertussis toxin-sensitive G protein.

Recent advances in understanding mechanically induced bone remodeling and their relevance to orthodontic theory and practice

This review highlights recent developments in bone cell biology, evaluates previous research, and offers future direction toward improving our understanding of events that mediate orthodontic tooth movement. The in vivo and in vitro models that have been developed to examine the responses of connective tissues and how they have contributed to our understanding of the mechanisms involved in mechanically induced bone remodeling are discussed in detail. Osteoblasts are now recognized as the cells that control both the resorptive and the formative phases of the remodeling cycle, and receptor studies have shown them to be the target cells for resorptive agents in bone. The osteoblast is perceived as a pivotal cell, controlling many of the responses of bone to stimulation with hormones and mechanical forces. It is apparent that not all the cellular responses induced by mechanically deformed tissues can be explained by the current paradigm emphasizing the importance of prostaglandin production and cAMP elevation; the mobilization of membrane phospholipids giving rise to inositol phosphates offers an alternative second messenger pathway. It is also argued from circumstantial evidence that changes in cell shape produce a range of effects mediated by membrane integral proteins (integrins) and the cytoskeleton, which may be important in transducing mechanical deformation into a meaningful biologic response.

Caffeine has the capacity to stimulate calcium release in organ culture of neonatal mouse calvaria

In view of the possible association between ingestion of caffeine (a constituent of coffee, tea, and several beverages) and osteoporosis, we have studied the effect of caffeine on bone resorption in vitro. Caffeine caused a dose-dependent increase of the spontaneous release of 45Ca from neonatal mouse calvarial bones. The effect of caffeine was less pronounced than that of parathyroid hormone (PTH), but of the same magnitude as that of theophylline, a structurally related methylxanthine. The enhancement of 45Ca release induced by caffeine and PTH was observed in 5 days culture. In 2 days culture, however, only PTH stimulated mineral mobilization. The delayed stimulatory effect of caffeine in long-term cultures was abolished by indomethacin and flurbiprofen. In indomethacin-treated bones, however, caffeine potentiated the stimulatory effect on 45Ca release induced by choleratoxin and forskolin. In contrast, caffeine did not potentiate 45Ca release stimulated by PTH. These data show that caffeine can stimulate calcium release from bone in vitro and that this effect is due to potentiation of a stimulatory action of a bone resorptive agonist acting via the adenylate cyclase-cyclic AMP system.

Effects of parathyroid hormone on cyclic AMP-formation and cytoplasmic free Ca2+ in the osteosarcoma cell line UMR 106-01

The effects of parathyroid hormone (PTH) on cytoplasmic free Ca2+ (Cai2+) and cAMP-formation were investigated in the rat osteosarcoma cell line UMR 106-01. In fura-2 loaded adherent single cells bPTH 1-34 (10 nM - 1 microM) induced a rapid transient increase in Cai2+ in 11% of the studied cells. In fura-2 tracings from UMR 106-01 cells in suspension, bPTH 1-34 (0.1 microM) induced a transient increase in Cai2+ in 20% of the experiments. The transient increase in Cai2+ seen in suspensions of cells was not abolished by addition of EGTA (2.5 mM) prior to challenge with PTH, suggesting that the increase in Cai2+ was derived from intracellular stores. A marked rapid increase in cAMP-formation was observed in all experiments with cells in suspension, also in the experiments where PTH did not affect Cai2+. These data show that PTH causes a release of Ca2+ from intracellular stores in a small percentage of osteosarcoma UMR 106-01 cells, and that PTH is capable of inducing an increase in cAMP-formation without affecting Cai2+ in osteoblasts.

Protein kinase C activating phorbolesters enhance the cyclic AMP response to parathyroid hormone, forskolin and choleratoxin in mouse calvarial bones and rat osteosarcoma cells

The protein kinase C-(PKC) activating phorbol esters 12-O-tetradecanoylphorbol-13-acetate (TPA; 100 nmol/l) and phorbol 12,13-dibutyrate (PDBU; 100 nmol/l) enhanced basal cyclin AMP accumulation in cultured neonatal mouse calvaria. The cyclic AMP response to parathyroid hormone (PTH; 10 nmol/l) and the adenylate cyclase activators forskolin (1-3 mumol/l) and choleratoxin (0.1 mumg/ml) was potentiated in a more than additive manner by TPA and PDBU. In contrast, phorbol 13-monoacetate (phorb-13; 100 nmol/l), a related compound but inactive on PKC, had no effect on basal or stimulated cyclic AMP accumulation. In the presence of indomethacin (1 mumol/l), TPA and PDBU had no effect on cyclic AMP accumulation in calvarial bones per se, but were still able to cause a significant enhancement of the response to PTH, forskolin and choleratoxin. PTH-, forskolin- and choleratoxin-stimulated cyclic AMP accumulation in rat osteosarcoma cells UMR 106-01 was synergistically potentiated by TPA and PDBU, but not by phorb.-13. These data indicate that PKC enhances cyclic AMP formation and that the level of interaction may be at, or distal to, adenylate cyclase.

Purinergic regulation of cytosolic calcium and phosphoinositide metabolism in rat osteoblast-like osteosarcoma cells

We have shown that ATP increases cytosolic Ca2+ in UMR-106 cells through P2-purinergic receptor stimulation (Calcif Tissue Int 45:251-254). This response was further characterized using cells loaded with indo-1/AM or prelabeled with [3H]inositol. ATP elicited a rapid transient increase in Ca2+ from 148 to 540 nM, followed by a biphasic decline (first rapid and then slower) to basal within 1 minute and then a late slow rise to 200 nM by 4 minutes. ADP also elicited a rapid transient increase, but this was followed by a second smaller transient and a later, slow increase above basal Ca2+. These transient increases in Ca2+ induced by ATP and ADP were dose dependent, detected at 10(-6)M ATP and 10(-7)M ADP, and saturated at 10(-4)M with both nucleotides. The maximum increase in Ca2+ was 20% greater with ATP than ADP. EGTA chelation of extracellular Ca2+ abolished the biphasicity of the ATP-induced Ca2+ transient, the second ADP-induced transient, and all late slower increases in Ca2+. Desmethoxyverapamil pretreatment attenuated the biphasicity of the ATP-induced transient and the second peak elicited by ADP. Elevated extracellular Ca2+ (5 mM) prevented the return to the basal level that normally follows the ATP-induced Ca2+ transient and amplified the sustained increase in Ca2+ but had little effect on the response to ADP. IP3 and IP4 increased rapidly after addition of ATP, with I(1,4,5)P3 increasing before I(1,3,4)P3. These data indicate that P2-purinergic stimulation of UMR-106 cells causes three consecutive responses in cytosolic Ca2+: (1) a transient increase due to IP3-mediated mobilization of intracellular Ca2+; (2) a transient increase due in part to influx, probably associated with a Ca2+ channel; and (3) a later sustained increase that requires extracellular calcium.

On the role of cyclic AMP as a mediator of bone resorption: gamma-interferon completely inhibits cholera toxin- and forskolin-induced but only partially inhibits parathyroid hormone-stimulated 45Ca release from mouse calvarial bones

The effects of gamma-interferon (gamma-IFN) on bone resorption and cyclic AMP formation stimulated by parathyroid hormone (PTH), forskolin, and cholera toxin have been studied in cultured neonatal mouse calvarial bones. Bone resorption was assessed by the release of 45Ca from prelabeled mouse calvarial bone fragments. Cyclic AMP formation was quantified by analyzing the amount of the nucleotide in calvarial bone tissue. gamma-IFN completely blocked the 45Ca release response to forskolin and cholera toxin in 96 h cultures. In contrast, the 45Ca release response to PTH was only partially inhibited, an effect that was seen over a wide range of PTH concentrations. The inhibitory effect of gamma-IFN was dose dependent, with a threshold for action at 10 U/ml. Forskolin-stimulated 45Ca release could only be inhibited when gamma-IFN was added simultaneously with forskolin; gamma-IFN added to bones prestimulated with forskolin had no effect. The inhibitory effect of gamma-IFN on PTH-stimulated 45Ca release was seen first after a time lag of 48 h. In contrast calcitonin caused an inhibition after only 3 h. PTH and cholera toxin stimulation of radioactive calcium release was also inhibited by gamma-IFN in bones treated with indomethacin. gamma-IFN inhibited forskolin-induced 45Ca release in bones treated with the mitotic inhibitor hydroxyurea. No effect of gamma-IFN on cyclic AMP formation induced by PTH, cholera toxin, or forskolin could be seen. These data show that gamma-IFN inhibits forskolin- and cholera toxin-induced bone resorption by a mechanism unrelated to prostaglandin production or mitotic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin 1 interacts synergistically with forskolin and isobutylmethylxanthine in stimulating bone resorption in organ culture

This study examined the interaction of interleukin 1 (IL-1) with forskolin and isobutyl-methylxanthine (3-isobutyl-1-methyl-xanthine) (IBMX) in stimulating bone resorption in 5-day fetal rat long bone organ culture. Forskolin and IBMX are pharmacologic agents that elevate cyclic adenosine monophosphate (AMP) levels in many cell types, including osteoblasts and osteoclasts. The interaction of IL-1 with forskolin and IBMX are synergistic when submaximal resorptive concentrations of agonists were examined. Stimulated resorption was 2 to 5 times that expected for an additive response. When maximally resorptive concentrations of agonists were examined, the interaction between IL-1 and the other agents was, at most, additive. We have previously reported that parathyroid hormone and prostaglandin E, agents that also activate the cyclic AMP pathway in bone cells, interact synergistically with IL-1 in stimulating bone resorption. The results of this study, together with our previous studies, suggest that activation of the cyclic AMP pathway is a sufficient signal for an agent to interact synergistically with IL-1 in stimulating bone resorption.

Adenylate cyclase blockers dissociate PTH-stimulated bone resorption from cAMP production

It is uncertain whether adenosine 3',5'-cyclic monophosphate (cAMP) or the inositol-calcium pathway mediates the stimulation of bone resorption by parathyroid hormone (PTH). Incubation of bone organ cultures with cAMP analogues and forskolin has not resolved this question because of the cellular inhomogeneity of bone and the consequent presence of adenylate cyclase-linked receptors for both PTH and calcitonin, hormones with opposite effects on bone resorption. We have used two new inhibitors of adenylate cyclase, 9-(tetrahydro-2-furyl)adenine (SQ 22536) and 2',5'-dideoxyadenosine (DDA), to directly reassess the role of cAMP in PTH-stimulated osteolysis. SQ 22536 (0.01-1.0 mM) and DDA (0.01-1.0 mM) completely blocked PTH stimulation of cAMP production measured in the absence of a phosphodiesterase blocker. In the presence of 1 mM 3-isobutyl-1-methylxanthine, half-maximal inhibition of PTH-induced cAMP production occurred with 0.2 mM SQ and 0.1 mM DDA, respectively. These concentrations of SQ and DDA had no effect on PTH-stimulated 45Ca release from calvaria, although both agents inhibited bone resorption when present at concentrations of 1-2 mM. At these levels, SQ and DDA caused equivalent inhibition of 45Ca release stimulated by 1,25-dihydroxyvitamin D3 but did not affect basal 45Ca release or [3H]-phenylalanine incorporation. It is concluded that substantial blockade of PTH-induced cAMP production does not affect this hormone's stimulation of bone resorption, which is therefore likely to be mediated by another intracellular messenger system, possibly calcium. In millimolar concentrations, SQ and DDA appear to be nonspecific blockers of osteoclastic bone resorption.

Fluid shear stress as a mediator of osteoblast cyclic adenosine monophosphate production

Effects of interstitial fluid flow on osteoblasts were investigated. Intracellular cyclic adenosine monophosphate (cAMP) levels were monitored in cultured osteoblasts subjected to shear rates ranging from 10 to 3,500 sec-1. Cyclic AMP levels were significantly increased at all shear rates from 1 pmole/mg protein to 10-16 pmole/mg protein. Osteoblasts subjected to a shear rate of 430 sec-1 for 0.5-15 minutes exhibited elevated levels (12-fold) of intracellular cAMP, which were sustained throughout the perfusion period. Osteoblasts were three times more sensitive to flow stimulation than human umbilical vein endothelial cells and baby hamster kidney fibroblasts, which also displayed higher cAMP levels (4-fold) after exposure to flow. To distinguish streaming potential effects from shear stress effects, viscosity was increased 5-fold by addition of neutral dextran to the perfusing medium. Shear stress is a function of viscosity, and streaming potentials are not for a given shear rate. The mechanism of this cellular response to flow was shown to be shear stress dependent. Inhibition of cyclooxygenase by 20 microM ibuprofen completely inhibited the flow-dependent cAMP response, indicating the cAMP response is mediated by prostaglandins. Our results suggest that fluid flow induced by mechanical stress may be an important mediator of bone remodeling.

The effects of stimulators of intracellular cyclic AMP on rat and chick osteoclasts in vitro: validation of a simplified light microscope assay of bone resorption

The aim of this study was to investigate whether a cyclic AMP-mediated inhibitory mechanism is present in embryonic chick osteoclasts and to extend data implicating cyclic AMP in the inhibition of neonatal rat osteoclasts. Dibutyryl cyclic AMP ((Bu)2cAMP) (5 x 10(-4) M and above) and isobutylmethylxanthine (IBMX) (10(-4) M and above) reduced the number of pits made in slices of devitalized bovine cortical bone by chick osteoclasts over 24 h. The effect of forskolin (FSK) on chick osteoclasts was biphasic, 10(-5) M producing a weak and variable reduction in pit number while 10(-6) M and 10(-7) M stimulated resorption. Doses of FSK (10(-5) M) and (Bu)2cAMP (3 x 10(-4) M), which individually produced no consistent significant effect, produced a synergistic and highly significant reduction in pit number when used in combination, implying that these agents were acting through a common mechanism, presumably cyclic AMP. Stimulatory doses of FSK were associated with increased osteoclast numbers, implicating cyclic AMP in the formation of osteoclasts. In comparative experiments using neonatal rat osteoclasts, (Bu)2cAMP (10(-4) M and above), IBMX (10(-3) M) and FSK (10(-7) M and above) all reduced the number of pits excavated. Strongly inhibitory doses of these agents caused contraction of chick osteoclasts into a hemispherical shape; contraction of rat osteoclasts into a stellate shape occurred with (Bu)2cAMP and FSK, but not with IBMX. Our results implicate cyclic AMP in the inhibition of both rat and chick osteoclasts, and show that pit counting in the light microscope is a valid method of analyzing the disaggregated osteoclast resorption assay.

Forskolin has biphasic effects on osteoprogenitor cell differentiation in vitro

Cells isolated from fetal rat calvaria (RC) and maintained in vitro in medium containing ascorbic acid and B-glycerophosphate form three-dimensional, mineralized nodules having the histological, immunohistological, and ultrastructural characteristics of woven bone. We have studied the effects of forskolin (FSK), a diterpene that activates adenylate cyclase, in this system. While 10(-7)-10(-5) M FSK significantly stimulated cAMP levels in RC cells, lower concentrations did not. cAMP levels with 10(-5) M FSK reached a maximum by 30 min at 37 degrees C and returned to basal level in 2-3 hr. Changes in cAMP levels correlated with changes in cellular shape: cells treated with 10(-5) M FSK assumed a stellate morphology, lost microfilament bundles, and reduced their substrate adhesiveness, while cells treated with 10(-9) M were not affected. Exponential growth and saturation densities of FSK-treated cultures were similar to untreated cultures, indicating that FSK was neither toxic nor stimulatory to the population. The effect on bone nodule formation of FSK present continuously depended on concentration: 10(-5) M FSK significantly inhibited the number of nodules formed, while 10(-9) M FSK significantly stimulated bone nodule formation. Single short treatments with either 10(-5) M or 10(-9) M FSK had no effect on nodule formation, but repeated short duration treatments (1 hr every 2 days for 21 days) gave results similar to continuous exposure. These results indicate that intermittent elevations in intracellular cAMP have an inhibitory effect on bone formation. In addition, our work indicates that low concentrations of FSK stimulate differentiation of osteoprogenitor cells possibly through a non-cAMP-dependent process.

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.

Bone remodeling signaled by a dihydropyridine- and phenylalkylamine-sensitive calcium channel

An osteoblast calcium channel demonstrated by single channel recordings is associated with calcium antagonist receptor binding sites in osteoblast-like osteosarcoma cells. By using whole cell current recordings we now show that this channel is stimulated by the dihydropyridine calcium agonist drug BAY K 8644. A physiological relevance of these channels is apparent from the stereoselective, potent inhibition of parathyroid hormone-stimulated calcium uptake into osteoblast-like cells in culture by desmethoxyverapamil, a phenylalkylamine calcium antagonist. Secretion by these cells of the bone matrix protein osteocalcin is stimulated by BAY K 8644 and blocked by desmethoxyverapamil and nitrendipine. Evidence for a role of this channel in bone remodeling in intact animals comes from enhanced bone resorption in fetal rat bones observed with BAY K 8644 and stereoselective, potent blockade of resorption by desmethoxyverapamil.

Dual elevation of cyclic AMP and inositol phosphates in response to mechanical deformation of murine osteoblasts

Mechanical deformation of bone cells was thought to be mediated via prostaglandin production and the cyclic AMP pathway. We present evidence that the phosphoinositide pathway is also activated by mechanical stress. We find that inositol phosphate production, but not glycerophosphoinositol production, is elevated, and the activation of adenylate cyclase is relatively small. These results are not compatible with the proposal that mechanical deformation of bone cells acts solely via prostaglandin synthesis.

Comparison between the effects of forskolin and calcitonin on bone resorption and osteoclast morphology in vitro

The adenylate cyclase activator forskolin (1-10 mumol/L) inhibited 45Ca release from parathyroid hormone (PTH; 10 nmol/L) stimulated prelabeled neonatal mouse calvaria in short term culture (24 h). This effect of forskolin was potentiated by rolipram, Ro 20-1724, and isobutyl-methylxanthine, three structurally different inhibitors of cyclic AMP phosphodiesterase. Forskolin (10 mumol/L) and calcitonin (30 mU/mL) inhibited the mobilization of stable calcium and inorganic phosphate as well as the release of the lysomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase from PTH-stimulated unlabeled bones. Osteoclasts in PTH-stimulated calvaria showed active ruffled borders with numerous membrane infoldings. Treatment of PTH-stimulated bones with forskolin and calcitonin resulted in a rapid (2 h) loss of the active ruffled border. In addition, forskolin and calcitonin induced similar changes with respect to the number and size distribution of cytoplasmic vesicles in PTH-activated osteoclasts. After 24 h, all signs of osteoclast inactivation were still prominent, whereas after 48 h of treatment with forskolin or calcitonin, the reappearance of a ruffled border on a number of osteoclasts signaled an escape from the inhibitory action of both calcitonin or forskolin. These data indicate that forskolin inhibits bone resorption by a cyclic AMP dependent mechanism and that the effect of forskolin and calcitonin on bone resorption and osteoclast morphology are comparable. These observations lend further support to the view that cyclic AMP may be an intracellular mediator of the inhibitory action of calcitonin on multinucleated osteoclasts.

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.

Calcitonin-like effects of forskolin and choleratoxin on surface area and motility of isolated rabbit osteoclasts

We have previously found that the adenylate cyclase stimulators forskolin and choleratoxin increase cyclic AMP and transiently inhibit bone resorption in cultured mouse calvaria, suggesting that the compounds, directly or indirectly, may inhibit osteoclast activity. In the present study, forskolin and choleratoxin were investigated for their direct effects on surface area and motility of isolated rabbit osteoclasts, and the effects were compared to those of calcitonin (CT). Osteoclasts were cultured on coverslips for different times in the absence or presence of the compounds. The effect on osteoclast mean area was quantified on fixed and stained osteoclasts, and in addition effects were recorded with time-lapse cinemicrography. The effects of CT (100 mU/ml) on mean area and motility were seen within minutes and were maximal after 10-60 minutes. Forskolin (10-30 mumol/liter) produced a rapid (15-60 minutes) inhibition of motility and decrease in area (contraction) of osteoclasts. Choleratoxin (1 microgram/ml) treatment also resulted in cell contraction and inhibition of motility; however, the response was not seen before 45-60 minutes. The difference in the kinetics of the osteoclast response between forskolin, CT, and choleratoxin is similar to differences in time course for the effect on cyclic AMP in calvarial bones, which we reported earlier. Although cells were incubated continuously with forskolin, choleratoxin, or CT, the effects were transient. Thus, after 7-8 h incubation with CT, 3-4 h treatment with forskolin, or 4-6 h with choleratoxin, the osteoclasts started to recover from contraction and immotility. The effect of forskolin and choleratoxin on the mean surface area of osteoclasts was dose dependent. The present study shows that forskolin and choleratoxin have a direct inhibitory action on osteoclast activity and thus provide further evidence that cyclic AMP is a mediator of the action of CT on bone resorption.

Parathyroid hormone and prostaglandin E2 stimulate both inositol phosphates and cyclic AMP accumulation in mouse osteoblast cultures

Parathyroid hormone (PTH) and prostaglandin E2 (PGE2) are physiological agonists which stimulate bone cells to resorb bone, a process by which the mineralized extracellular bone matrix is dissolved. Bone resorption has a key role in the maintenance of plasma calcium levels. It has been established that both PTH and PGE2 activate adenylate cyclase in osteoblasts, but it is apparent that (1) the two agents have qualitatively different effects on osteoblasts, and (2) the generation of cyclic AMP cannot account for all the effects of PTH on bone cell metabolism. Others have demonstrated that PTH and PGE2 may also elevate intracellular calcium levels, but the mechanism by which this is achieved has not been fully defined. Here we have investigated the effects of PTH on neonatal mouse osteoblasts in culture and shown that physiological concentrations of the hormone (50 nM) caused a small increase (22%) in total inositol phosphates accumulation, with a larger increase (40%) in inositol trisphosphate. We found that this activation occurred at lower concentration than was necessary to activate adenylate cyclase. PGE2 was a more effective activator of inositol phosphates accumulation than PTH, causing up to 300% increase in the total inositol phosphates after 30 min. Both PTH and PGE2 stimulated cyclic AMP accumulation, but the activation of adenylate cyclase by forskolin did not enhance inositol phosphates production. We conclude that both PTH and PGE2 stimulate phosphoinositide turnover in mouse osteoblasts and suggest that this mechanism may contribute to their elevation of intracellular calcium in bone cells.

Characterization of adenosine receptors in bone. Studies on the effect of adenosine analogues on cyclic AMP formation and bone resorption in cultured mouse calvaria

The effect of different adenosine analogues on cyclic AMP (cAMP) formation and bone resorption in cultured mouse calvarial bones was investigated. 5'-N-ethylcarboxamidoadenosine (NECA), R-N6-phenylisopropyl-adenosine (PIA), N6-cyclohexyl-adenosine (CHA) and 2-chloroadenosine all stimulated cyclic AMP formation with a threshold close to 1 mumol l-1); NECA was the most potent agonist. Theophylline (10, 100 mumol l-1) inhibited the cAMP accumulation induced by NECA and 2-chloroadenosine (30 and 300 mumol l-1), dose dependently. There was no inhibition of cAMP formation by PIA and CHA in forskolin-treated bone tissue. SQ 22, 536 and 2',5'-dideoxyadenosine (100 mumol l-1) both inhibited rolipram-stimulated cAMP formation. Cyclic AMP accumulation in isolated osteoblast-like cells from neonatal mouse calvarial bones was stimulated by NECA (10 and 100 mumol l-1) and 2-chloroadenosine (100 mumol l-1). 2-chloroadenosine (10 and 30 mumol l-1), but not NECA, PIA nor CHA, caused a dose-dependent stimulation of 45Ca release in both 48- and 120-h culture. The effect of 2-chloroadenosine on 45Ca release could not be antagonized by theophylline. Neither NECA, PIA, CHA nor 2-chloroadenosine could affect PTH-stimulated 45Ca release in short term cultures (6, 24 h). By contrast, stimulation of cAMP formation by forskolin or dibutyryl cAMP caused a rapid (6 h) inhibition of PTH-stimulated bone resorption. The results demonstrate functional A2 and P-site receptors in mouse calvaria and osteoblast-like cells, but no A1-receptor was detected. These adenosine receptors regulate cAMP, but are not intimately linked to bone resorption. The calcium mobilization induced by 2-chloroadenosine appears to be unrelated to adenosine receptors.

Effects of cholera toxin on cyclic AMP accumulation and bone resorption in cultured mouse calvaria

We have utilized the adenylate cyclase stimulator, cholera toxin, as a tool to test the role of cyclic AMP as a mediator of the effects on bone resorption by the calcium-regulating hormones, parathyroid hormone (PTH) and calcitonin. The effects on bone resorption were studied in an organ culture system using calvarial bones from newborn mice. Cyclic AMP response was assayed in calvarial bone explants and isolated osteoblasts from neonatal mouse calvaria. Cholera toxin caused a dose-dependent cAMP response in calvarial bones, seen at and above approx. 1-3 ng/ml and calculated half-maximal stimulation (EC50) at 18 ng/ml. The stimulatory effect of cholera toxin could be potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 0.2 mmol/l). Cyclic AMP accumulation in the bones was maximal after 4-6 h, and thereafter declined. However, activation of the adenylate cyclase was irreversible and the total amount (bone + medium) of cAMP produced, in the presence of IBMX (0.2 mmol/l), increased with time, for at least 48 h. In osteoblast-like cells cholera toxin (1 microgram/ml) stimulated the cellular levels of cAMP with a peak after 60-120 min, which could be potentiated with IBMX. The total cAMP accumulation indicated an irreversible response. In short-term bone organ cultures (at most, 24 h) cholera toxin, at and above 3 ng/ml, inhibited the stimulatory effect of PTH (10 nmol/l) on 45Ca release from prelabelled calvarial bones. The inhibitory effect of cholera toxin (0.1 microgram/ml) on 45Ca release was significant after 6 h and the calculated IC50 value at 24 h was 11.2 ng/ml. Cholera toxin (0.1 microgram/ml) also inhibited PTH-stimulated (10 nmol/l) release of Ca2+, inorganic phosphate (Pi), beta-glucuronidase, beta-N-acetylglucosaminidase and degradation of organic matrix (release of 3H from [3H]proline-labelled bones) in 24 h cultures. 45Ca release from bones stimulated by prostaglandin E2 (1 mumol/l) and 1 alpha-hydroxyvitamin D3 (0.1 mumol/l) was also inhibited by cholera toxin (0.3 microgram/ml) in 24-h cultures. The inhibitory effect of cholera toxin on bone resorption was transient, and in long-term cultures (120 h) cholera toxin caused a dose-dependent, delayed stimulation of mineral mobilization (Ca2+, 45Ca, Pi), degradation of matrix and release of the lysosomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase.(ABSTRACT TRUNCATED AT 400 WORDS)