Nitric oxide regulation of cGMP production in osteoclasts

S-nitrosoglutathione reductase-dependent p65 denitrosation promotes osteoclastogenesis by facilitating recruitment of p65 to NFATc1 promoter

Osteoclasts, the exclusive bone resorptive cells, are indispensable for bone remodeling. Hence, understanding novel signaling modulators regulating osteoclastogenesis is clinically important. Nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) is a master transcription factor in osteoclastogenesis, and binding of NF-κB p65 subunit to NFATc1 promoter is required for its expression. It is well-established that DNA binding activity of p65 can be regulated by various post-translational modifications, including S-nitrosation. Recent studies have demonstrated that S-nitrosoglutathione reductase (GSNOR)-mediated protein denitrosation participated in cell fate commitment by regulating gene transcription. However, the role of GSNOR in osteoclastogenesis remains unexplored and enigmatic. Here, we investigated the effect of GSNOR-mediated denitrosation of p65 on osteoclastogenesis. Our results revealed that GSNOR was up-regulated during osteoclastogenesis in vitro. Moreover, GSNOR inhibition with a chemical inhibitor impaired osteoclast differentiation, podosome belt formation, and bone resorption activity. Furthermore, GSNOR inhibition enhanced the S-nitrosation level of p65, precluded the binding of p65 to NFATc1 promoter, and suppressed NFATc1 expression. In addition, mouse model of lipopolysaccharides (LPS)-induced calvarial osteolysis was employed to evaluate the therapeutic effect of GSNOR inhibitor in vivo. Our results indicated that GSNOR inhibitor treatment alleviated the inflammatory bone loss by impairing osteoclast formation in mice. Taken together, these data have shown that GSNOR activity is required for osteoclastogenesis by facilitating binding of p65 to NFATc1 promoter via promoting p65 denitrosation, suggesting that GSNOR may be a potential therapeutic target in the treatment of osteolytic diseases.

Guanylyl Cyclase-B Dependent Bone Formation in Mice is Associated with Youth, Increased Osteoblasts, and Decreased Osteoclasts

C-type natriuretic peptide (CNP) activation of guanylyl cyclase-B (GC-B) catalyzes the synthesis of cGMP in chondrocytes and osteoblasts. Elevated cGMP stimulates long bone growth, and inactivating mutations in CNP or GC-B reduce cGMP, which causes dwarfism. GC-B^7E/7E mice that express a GC-B mutant that cannot be inactivated by dephosphorylation exhibit increased CNP-dependent GC-B activity, which increases bone length, as well as bone mass and strength. Importantly, how GC-B increases bone mass is not known. Here, we injected 12-week-old, wild type mice once daily for 28 days with or without BMN-111 (Vosoritide), a proteolytically resistant CNP analog. We found that BMN-111 treated mice had elevated levels of osteocalcin and collagen 1 C-terminal telopeptide (CTX) as well as increased osteoblasts and osteoclasts. In BMN-111 injected mice, tibial mRNAs for Rank ligand and osteoprotegrin were increased and decreased, respectively, whereas sclerostin mRNA was elevated 400-fold, consistent with increased osteoclast activity and decreased osteoblast activity. Mineral apposition rates and trabecular bone mass were not elevated in response to BMN-111. Because 9-week-old male GC-B^7E/7E mice have increased bone mass but do not exhibit increased mineral apposition rates, we examined 4-week-old male GC-B^7E/7E mice and found that these animals had increased serum osteocalcin, but not CTX. Importantly, tibias from these mice had 37% more osteoblasts, 26% fewer osteoclasts as well as 36% and 40% higher mineral apposition and bone formation rates, respectively. We conclude that GC-B-dependent bone formation is coupled to an early juvenile process that requires both increased osteoblasts and decreased osteoclasts.

The Efficacy of Nitrates for Bone Health: A Systematic Review and Meta-Analysis of Observational and Randomized Controlled Studies

BACKGROUND

Although some studies have found that nitrates were beneficial for bone health, the findings are inconsistent. To assess the efficacy of nitrates for bone health, we conducted a meta-analysis.

METHODS

PubMed, EMBASE databases, Cochrane Library for relevant articles published before December 2021 were searched. All observational and randomized controlled studies that reporting bone mineral density (BMD), fractures with nitrates use were included. A meta-analysis was performed to calculate risk ratios (RRs) for fractures, change differences for bone mineral density.

RESULTS

Four cohort studies and two case-control studies examining the association between nitrates use and fractures were identified. The nitrates use was not associated with any fracture risk (RR = 0.97; 95% CI, 0.94-1.01; I² = 31.5%) and hip fracture (RR = 0.88; 95% CI, 0.76-1.02; I² = 74.5%). Subgroup analyses revealed no differences in fracture risk, whereas two cohort studies revealed a reduced risk of hip fracture (RR = 0.71, 95% CI, 0.58-0.86, I² = 0.0%). There were no statistically significant differences in BMD percent changes at lumbar spine (WMD = -0.07, 95% CI,-0.78-0.65; I² = 0.0%), total hip (WMD = -0.42, 95% CI,-0.88-0.04; I² = 0.0%), femoral neck (WMD = -0.38, 95% CI,-1.02-0.25; I² = 0.0%), or total body (WMD = -0.17, 95% CI,-0.51-0.17; I² = 0.0%) in two randomized controlled trials (RCTs) compared with a placebo. Another two RCTs compared nitrates with alendronate. Nitrates were comparable to alendronate in increasing bone mineral density at lumbar spine (WMD = 0.00, 95% CI,-0.01-0.02; I² = 0.0%). Besides, the most common adverse effect was headache, contributing to low adherence to therapy.

CONCLUSION

Our meta-analysis showed no association between nitrates use and fractures in observational studies. The results of RCTs on the usage of nitrates and their effects on BMD were inconsistent. High-quality, long-term studies are needed to clarify the efficacy of nitrates for bone health.

Inflammation in the Human Periodontium Induces Downregulation of the α1- and β1-Subunits of the sGC in Cementoclasts

Nitric oxide (NO) binds to soluble guanylyl cyclase (sGC), activates it in a reduced oxidized heme iron state, and generates cyclic Guanosine Monophosphate (cGMP), which results in vasodilatation and inhibition of osteoclast activity. In inflammation, sGC is oxidized and becomes insensitive to NO. NO- and heme-independent activation of sGC requires protein expression of the α₁- and β₁-subunits. Inflammation of the periodontium induces the resorption of cementum by cementoclasts and the resorption of the alveolar bone by osteoclasts, which can lead to tooth loss. As the presence of sGC in cementoclasts is unknown, we investigated the α₁- and β₁-subunits of sGC in cementoclasts of healthy and inflamed human periodontium using double immunostaining for CD68 and cathepsin K and compared the findings with those of osteoclasts from the same sections. In comparison to cementoclasts in the healthy periodontium, cementoclasts under inflammatory conditions showed a decreased staining intensity for both α₁- and β₁-subunits of sGC, indicating reduced protein expression of these subunits. Therefore, pharmacological activation of sGC in inflamed periodontal tissues in an NO- and heme-independent manner could be considered as a new treatment strategy to inhibit cementum resorption.

Nitric oxide and cyclic GMP functions in bone

Nitric oxide plays a central role in the regulation of skeletal homeostasis. In cells of the osteoblastic lineage, NO is generated in response to mechanical stimulation and estrogen exposure. Via activation of soluble guanylyl cyclase (sGC) and cGMP-dependent protein kinases (PKGs), NO enhances proliferation, differentiation, and survival of bone-forming cells in the osteoblastic lineage. NO also regulates the differentiation and activity of bone-resorbing osteoclasts; here the effects are largely inhibitory and partly cGMP-independent. We review the skeletal phenotypes of mice deficient in NO synthases and PKGs, and the effects of NO and cGMP on bone formation and resorption. We examine the roles of NO and cGMP in bone adaptation to mechanical stimulation. Finally, we discuss preclinical and clinical data showing that NO donors and NO-independent sGC activators may protect against estrogen deficiency-induced bone loss. sGC represents an attractive target for the treatment of osteoporosis.

8-Nitro-cGMP is a promoter of osteoclast differentiation induced by RANKL

Osteoclasts are multinucleated giant cells differentiated from monocyte-macrophage-lineage cells under stimulation of receptor activator of nuclear factor κ-B (RANK) ligand (RANKL) produced by osteoblasts and osteocytes. Although it has been reported that nitric oxide (NO) and reactive oxygen species (ROS) are involved in this process, the mechanism by which these labile molecules promote osteoclast differentiation are not fully understood. In this study, we investigated the formation and function of 8-nitro-cGMP, a downstream molecule of NO and ROS, in the process of osteoclast differentiation in vitro. 8-Nitro-cGMP was detected in mouse bone marrow macrophages and osteoclasts differentiated from macrophages in the presence of RANKL. Inhibition of NO synthase suppressed the formation of 8-nitro-cGMP as well as RANKL-induced osteoclast differentiation from macrophages. On the other hand, RANKL-induced osteoclast differentiation was promoted by addition of 8-nitro-cGMP to the cultures. In addition, 8-nitro-cGMP enhanced the mRNA expression of RANK, the receptor for RANKL. However, 8-bromo-cGMP, a membrane-permeable derivative of cGMP, did not have an effect on either RANKL-induced osteoclast differentiation or expression of the RANK gene. These results suggest that 8-nitro-cGMP is a novel positive regulator of osteoclast differentiation, which might help to explain the roles of NO and ROS in osteoclast differentiation.

A Novel, Direct NO Donor Regulates Osteoblast and Osteoclast Functions and Increases Bone Mass in Ovariectomized Mice

Most US Food and Drug Administration (FDA)-approved treatments for osteoporosis target osteoclastic bone resorption. Only PTH derivatives improve bone formation, but they have drawbacks, and novel bone-anabolic agents are needed. Nitrates, which generate NO, improved BMD in estrogen-deficient rats and may improve bone formation markers and BMD in postmenopausal women. However, nitrates are limited by induction of oxidative stress and development of tolerance, and may increase cardiovascular mortality after long-term use. Here we studied nitrosyl-cobinamide (NO-Cbi), a novel, direct NO-releasing agent, in a mouse model of estrogen deficiency-induced osteoporosis. In murine primary osteoblasts, NO-Cbi increased intracellular cGMP, Wnt/β-catenin signaling, proliferation, and osteoblastic gene expression, and protected cells from apoptosis. Correspondingly, in intact and ovariectomized (OVX) female C57Bl/6 mice, NO-Cbi increased serum cGMP concentrations, bone formation, and osteoblastic gene expression, and in OVX mice, it prevented osteocyte apoptosis. NO-Cbi reduced osteoclasts in intact mice and prevented the known increase in osteoclasts in OVX mice, partially through a reduction in the RANKL/osteoprotegerin gene expression ratio, which regulates osteoclast differentiation, and partially through direct inhibition of osteoclast differentiation, observed in vitro in the presence of excess RANKL. The positive NO effects in osteoblasts were mediated by cGMP/protein kinase G (PKG), but some of the osteoclast-inhibitory effects appeared to be cGMP-independent. NO-Cbi increased trabecular bone mass in both intact and OVX mice, consistent with its in vitro effects on osteoblasts and osteoclasts. NO-Cbi is a novel direct NO-releasing agent that, in contrast to nitrates, does not generate oxygen radicals, and combines anabolic and antiresorptive effects in bone, making it an excellent candidate for treating osteoporosis. © 2016 American Society for Bone and Mineral Research.

Possible Association between Erectile Dysfunction and Osteoporosis in Men

Sexual dysfunction in general and erectile dysfunction (ED) in particular significantly affect men’s quality of life. Some patients who have ED, also develop osteoporosis. So, in this study we investigated the relationship between erectile dysfunction and osteoporosis in men. 95 men with erectile dysfunction and 82 men with normal sexual function were included in the study. The men’s sexual functions were evaluated by International Index of Erectile Function-5 items (IIEF-5). All men received a Dual Energy X-ray Absorptiometry (DEXA; Hologic) scan to measure bone mineral density (BMD) for osteoporosis. Chi-square test was used for statistical analysis. Mean age was 53.5 (38–69) in ED group and 50.1 (31–69) in control group. In ED group the men have lower T score levels than those of the control group. In conclusion, the men who have erectile dysfunction were at more risk for osteoporosis. The results of the present study demonstrate that the men with erectile dysfunction have low bone mineral density and they are at higher risk for osteoporosis. Because of easy and noninvasive evaluation of osteoporosis, patients with ED should be checked for bone mineral density and osteoporotic male subjects should be evaluated for ED.

Nitric oxide as a mediator of estrogen effects in osteocytes

Postmenopausal osteoporosis due to estrogen deficiency is a major health problem, and available therapies rely largely on the inhibition of bone resorption, because estrogen replacement is associated with risks. Estrogen promotes bone health in large part by increasing osteocyte survival, but the molecular mechanisms involved are only partly understood. We showed that estradiol stimulates nitric oxide (NO) production in osteocytes, leading to increased cGMP synthesis and activation of cGMP-dependent protein kinases (PKGs). Moreover, we found that 17β-estradiol protects osteocytes against apoptosis via the NO/cGMP signaling pathway: type II PKG mediates estradiol-induced activation of the prosurvival kinases Erk and Akt, whereas type I PKG contributes to prosurvival signaling by directly phosphorylating and inactivating the cell death protein BAD. Preclinical data support an important role of NO in bone biology, and clinical trials suggest that NO donors may prevent bone loss in postmenopausal women. Our data provide novel insights into estrogen signaling through the NO/cGMP/PKG pathway and a rationale for using NO donors and other cGMP-elevating agents for treating postmenopausal osteoporosis.

S-nitroso-N-acetylpenicillamine (SNAP) derivatization of peptide primary amines to create inducible nitric oxide donor biomaterials

An S-nitroso-N-acetylpenicillamine (SNAP) derivatization approach was used to modify existing free primary amines found in fibrin (a natural protein-based biomaterial) to generate a controlled nitric oxide (NO) releasing scaffold material. The duration of the derivatization reaction affects the NO release kinetics, the induction of controlled NO-release, hydrophobicity, swelling behavior, elastic moduli, rheometric character, and degradation behavior. These properties were quantified to determine changes in fibrin hydrogels following covalent attachment of SNAP. NO-releasing materials exhibited minimal cytotoxicity when cultured with fibroblasts or osteoblasts. Cells maintained viability and proliferative character on derivatized materials as demonstrated by Live/Dead cell staining and counting. In addition, SNAP-derivatized hydrogels exhibited an antimicrobial character indicative of NO-releasing materials. SNAP derivatization of natural polymeric biomaterials containing free primary amines offers a means to generate inducible NO-releasing biomaterials for use as an antimicrobial and regenerative support for tissue engineering.

ABCC5 supports osteoclast formation and promotes breast cancer metastasis to bone

Introduction

Bone is the most common site of breast cancer metastasis, and complications associated with bone metastases can lead to a significantly decreased patient quality of life. Thus, it is essential to gain a better understanding of the molecular mechanisms that underlie the emergence and growth of breast cancer skeletal metastases.

Methods

To search for novel molecular mediators that influence breast cancer bone metastasis, we generated gene-expression profiles from laser-capture microdissected trephine biopsies of both breast cancer bone metastases and independent primary breast tumors that metastasized to bone. Bioinformatics analysis identified genes that are differentially expressed in breast cancer bone metastases compared with primary, bone-metastatic breast tumors.

Results

ABCC5, an ATP-dependent transporter, was found to be overexpressed in breast cancer osseous metastases relative to primary breast tumors. In addition, ABCC5 was significantly upregulated in human and mouse breast cancer cell lines with high bone-metastatic potential. Stable knockdown of ABCC5 substantially reduced bone metastatic burden and osteolytic bone destruction in mice. The decrease in osteolysis was further associated with diminished osteoclast numbers in vivo. Finally, conditioned media from breast cancer cells with reduced ABCC5 expression failed to induce in vitro osteoclastogenesis to the same extent as conditioned media from breast cancer cells expressing ABCC5.

Conclusions

Our data suggest that ABCC5 functions as a mediator of breast cancer skeletal metastasis. ABCC5 expression in breast cancer cells is important for efficient osteoclast-mediated bone resorption. Hence, ABCC5 may be a potential therapeutic target for breast cancer bone metastasis.

Functional osteoclast attachment requires inositol-1,4,5-trisphosphate receptor-associated cGMP-dependent kinase substrate

Osteoclast activity is central to balanced bone turnover to maintain normal bone mass. A specialized osteoclast attachment to bone localizes acid secretion to remove bone mineral; in some cases, attachment is functionally impaired despite normal attachment proteins. The inositol-1,4,5-trisphosphate receptor-1 (IP3R1) is an intracellular calcium channel required for regulation of reversible osteoclast attachment by nitric oxide (NO), an important regulator of both normal and pathological bone degradation. In studies using human osteoclasts produced in vitro, we found that IP3R1 binds an endosomal isoform of the IP3R-associated cGMP-dependent kinase substrate (IRAG). IRAG is a substrate of cGMP-dependent kinase-1 (PKG1) and binds the PKG1 isoform PKG1β, which was the predominant form of PKG1 in human osteoclasts. Western blots of IRAG were consistent with NO-dependent serine phosphorylation of IRAG. An additional effect of PKG1β activity in osteoclasts was disassociation of IP3R1-IRAG complexes, as shown by analysis of IP3R1 complexes and by localization of the proteins within cells. IP3R1-IRAG complexes were stabilized by PKG or Src antagonists, Src activity being a requirement for IP3R1 calcium release downstream of PKG. IP3R1-mediated calcium release regulates cellular detachment in part through the calcium-dependent proteinase μ-calpain. In osteoclasts with IRAG suppressed by siRNA, activity of μ-calpain was increased relative to cells with normal IRAG, and regulation of μ-calpain by NO was lost. Furthermore, cells deficient in IRAG detached easily from substrate and had smaller attached diameters and randomly distributed podosomes, although IRAG knockdown did not affect cell viability. Our results indicate that IRAG is required for PKG1β-regulated cyclic calcium release during motility, and that disruption of the IP3R1-IRAG calcium regulation system is a novel cause of dysfunctional osteoclasts unrelated to defects in attachment proteins or acid secretion.

Nitric oxide enhances osteoclastogenesis possibly by mediating cell fusion

Osteoclasts are multinucleated bone resorbing cells which form by fusion of pre-osteoclasts. Here, we investigate how nitric oxide (NO) affects osteoclastogenesis. Time lapse photomicrography, using the fluorescent NO indicator dye, 4,5-diaminofluorescein diacetate, revealed an intense NO signal in pre-osteoclasts preceding cell fusion. Osteoclastogenesis in RAW264.7 cells increased when exposed to the NO synthase inhibitor, L-NMMA (0.25 microM), for the initial 48 h. In contrast, pre-osteoclast fusion decreased when RAW264.7 cells were exposed to L-NMMA from 48 to 96 h. Both NO synthase inhibitors, L-NMMA and L-NAME, decreased osteoclast formation during this time period. The inhibitory effect of L-NMMA on osteoclast formation was abolished with increasing concentrations (25-200 ng/ml) of sRANKL suggesting signaling cross talk. NO donors increased osteoclast formation in a dose-dependent manner, with greatest stimulation at 15 microM NOC-12 (2.3 fold) and 5 microM NOC-18 (2.4 fold). Measuring nitrite (NO end product) daily from culture media of RAW264.7 cells undergoing osteoclastogenesis revealed that an increase in NO production coincided with the fusion of pre-osteoclasts (day 4). Inhibiting fusion by plating cells on polystyrene dishes pre-coated with poly-(L-lysine) decreased both osteoclast formation and NO production. To address if NO mediates fusion through the actin cytoskeleton, actin free barbed ends were measured. 0.25 microM L-NMMA decreased, while 15 microM NOC-12 and 5 microM NOC-18 increased actin free barbed ends. We hypothesize that while NO initially negatively regulates pre-osteoclast differentiation; it later facilitates the fusion of mononuclear pre-osteoclasts, possibly by up regulating actin remodeling.

Glucose is a key metabolic regulator of osteoclasts; glucose stimulated increases in ATP/ADP ratio and calmodulin kinase II activity

Glucose-stimulated increases in osteoclast activity are mediated, at least in part, by transcriptional regulation of H+-ATPase expression through a mechanism involving p38 mitogen-activated protein kinase. We hypothesized that early events in the glucose-dependent signaling pathway would be similar to those identified in other glucose-sensitive cells, such as islet beta-cells, including rapid changes in the cellular ATP/ADP ratio and mobilization of intracellular Ca2+. We demonstrate that glucose stimulates a prolonged 50% increase in the ATP/ADP ratio that was maximal 30 s after glucose concentrations were increased. Glucose stimulated a transient 30% increase in calcium/calmodulin-dependent kinase II (CaMK II) activity that was maximal 3 min after the glucose concentration was increased. CaMK II was activated maximally by 3 mmol D-glucose/L in 3-min assays. Activation of CaMK II in the presence of the nonmetabolizable glucose analog 2-deoxyglucose was 2-fold greater than with D-glucose but was unchanged by glucosamine. Pretreatment of osteoclasts with the intracellular Ca2+ chelator BAPTA-AM inhibited glucose transport by 75%. BAPTA-AM treatment also prevented glucose-dependent stimulation of CaMK II. The data indicate that osteoclasts utilize a glucose-sensing mechanism similar to that of beta-cells and that glucose-stimulated signaling in osteoclasts involves changes in the ATP/ADP ratio and mobilization of intracellular Ca2+, resulting in activation of CaMK II.

cGMP may have trophic effects on beta cell function comparable to those of cAMP, implying a role for high-dose biotin in prevention/treatment of diabetes

Incretin hormones have trophic effects on beta cell function that can aid prevention and treatment of diabetes. cAMP is the primary mediator of these effects, and has been shown to potentiate glucose-stimulated insulin secretion, promote proper beta cells differentiation by increasing expression of the crucial transcription factor PDX-1, and prevent beta cell apoptosis. cGMP's role in beta cell function has received far less scrutiny, but there is emerging evidence that it may have a trophic impact on beta cell function analogous to that of cAMP. An increase in plasma glucose boosts beta cell production of cGMP, which acts as a feed-forward mediator to enhance glucose-stimulated insulin secretion. cGMP also has an anti-apoptotic effect in beta cells, and there is now indirect evidence that it promotes expression of PDX-1. Supraphysiological concentrations of biotin can directly activate guanylate cyclase, and there is limited evidence that high intakes of this vitamin can be therapeutically beneficial in diabetics and in rodent models of diabetes. Beneficial effects of cGMP on muscle insulin sensitivity and on control of hepatic glucose output may contribute to biotin's utility in diabetes. The fact that nitric oxide/cGMP exert a range of favorable effects on vascular health should further encourage exploration of biotin's preventive and therapeutic potential. If an appropriate high-dose biotin regimen could achieve a modest systemic increase in guanylate cyclase activity, without entailing unacceptable side effects or risks, such a regimen might have considerable potential for promoting vascular health and preventing or managing diabetes.

NO-dependent osteoclast motility: reliance on cGMP-dependent protein kinase I and VASP

The osteoclast degrades bone in cycles; between cycles, the cell is motile. Resorption occurs by acid transport into an extracellular compartment defined by an alphavbeta3 integrin ring. NO has been implicated in the regulation of bone turnover due to stretch or via estrogen signals, but a specific mechanism linking NO to osteoclastic activity has not been described. NO stimulates osteoclast motility, and at high concentrations NO causes detachment and terminates resorption. Here we demonstrate that NO regulates attachment through the cGMP-dependent protein kinase I (PKG I) via phosphorylation of the intermediate protein VASP. VASP colocalized with the alphavbeta3 ring in stationary cells, but alternating bands of VASP and alphavbeta3 occurred when motility was induced by NO donors or cGMP. Redistribution of VASP correlated with its phosphorylation. Dependency of NO-induced motility on PKG I and on VASP was shown by siRNA knockdown of each protein. VASP knockdown also altered distribution of alphavbeta3 at the attachment site. We conclude that PKG I and VASP are essential for reorganization of attachment and cytoplasmic proteins in motility induced by NO or by cGMP.

Autocrine and paracrine nitric oxide regulate attachment of human osteoclasts

Nitric oxide (NO) can reduce bone loss in chronic bone diseases. NO inhibits or kills osteoclasts, but the mechanism of action of NO in human bone turnover is not clear. To address this, we studied effects of NO on attachment and motility of human osteoclasts on mineralized and tissue culture substrates under defined conditions. Osteoclasts were differentiated in vitro from CD14 selected monocytes in RANKL and CSF-1, and characterized by cathepsin K expression, tartrate-resistant acid phosphatase (TRAP) activity, acid secretion, and lacunar resorption. Cell attachment was labeled with monoclonal antibody 23C6, specific for a binding domain of a key osteoclast attachment protein, the CD51/CD61 integrin dimer (alpha(v)beta(3)), with or without cell permeabilization. A ring of integrin attachment during bone degradation delimits an extracellular acid compartment, while alpha(v)beta(3) forms focal attachments on non-resorbable substrates. On resorbable substrate but not non-resorbable substrate, alpha(v)beta(3) labeling required cell permeabilization, in keeping with the membrane-matrix apposition that excludes large molecules and allows extracellular acidification. Acid secretion was labeled with the fluorescent weak base indicator lysotracker. NO donors, S-nitroso-N-acetyl penicillamine (SNAP) or sodium nitroprusside (SNP), downmodulated acid secretion simultaneously with cytoskeletal rearrangement, with alpha(v)beta(3) redistributed to a discontinuous pattern that labeled, on bone substrate, without membrane permeabilization. These effects were reversible, and an inhibitor of NO synthesis, N(G)-monomethyl-L-arginine (l-NMMA), increased acid secretion and decreased heterogeneity of attachment structures, showing that NO is an autocrine regulator of attachment. A hydrolysis-resistant activating cGMP analog 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphate replicated effects of NO donors, while an inhibiting analog, 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, Rp-isomer, opposed them. On tissue culture or mineralized substrates, NO or cGMP analogs directly regulated motility; after washout cells reattached and survived for days. We conclude that NO is produced by human osteoclasts and regulates acid secretion and cellular motility, in keeping with autocrine and paracrine NO regulation of the resorption cycle.

Localization of the NO-cGMP signaling pathway molecules, NOS III-phosphorylation sites, ERK1/2, and Akt/PKB in osteoclasts

BACKGROUND

Nitric oxide (NO) mediates different cellular functions by activating soluble guanylate cyclase (sGC) that converts guanosine-5'-triphosphate (GTP) to cyclic guanosine-3',5'-monophosphate (cGMP). Membrane-bound GCs produce cGMP in response to natriuretic peptides in osteoblasts, but neither the NO-target enzyme sGC, nor the phosphorylation sites of NOS III, nor their regulation by extracellular signal-regulated kinases 1 and 2 (ERK1/2) and Akt/protein kinase B (Akt/PKB) in osteoclasts have been established.

METHODS

Rat molars with periodontium were perfusion- and post-fixed, decalcified, and frozen-sectioned. Free-floating sections were stained using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and tartrate-resistant acid phosphatase (TRAP) histochemical techniques and immunoreacted with antisera against NO-synthase (NOS) I-III, NOS III phoshorylated at Thr495, NOS III phoshorylated at Serine1177 (Ser1177), ERK1/2, phosphorylated ERK1/2, Akt/PKB, phosphorylated Akt/PKB, sGC (alpha2/beta1), and cGMP.

RESULTS

NADPH-d staining and immunostaining of NOS I-III, NOS III phosphorylated at Ser1177, ERK1/2, Akt/PKB, phosphorylated Akt/PKB, sGC (alpha2 and beta1-subunits), and cGMP were detected in osteoclasts. Immunohistochemical reaction products for NOS III phosphorylated at threonine495 (Thr495) and phosphorylated ERK1/2 could not be identified in osteoclasts. Comparison of TRAP activity and immunostaining for sGC beta1-subunit revealed that sGC beta1-subunit is only expressed in a sub-population of osteoclasts.

CONCLUSIONS

NO is likely to be generated by NOS I and NOS III in osteoclasts. The inconstant expression of NOS II in some osteoclasts may be explained with inducible expression of NOS II upon physiological cell activation. Localization of the sGC alpha2- and beta1-subunits and cGMP in osteoclasts is compatible with an involvement of NO-sGC signaling in the homeostasis of osteoclasts. The phosphorylation site of NOS III at Ser1177 and phosphorylated Akt/PKB are involved in regulation of NO production by NOS III in osteoclasts under basal conditions.

Differential effects of calmodulin and protein kinase C antagonists on bone resorption and acid transport activity

Tamoxifen inhibits bone resorption by disrupting calmodulin-dependent processes. Since tamoxifen inhibits protein kinase C in other cells, we compared the effects of tamoxifen and the PKC inhibitor, bis indolylmaleimide II (bIM), on bone resorption and acid transport activity in isolated membrane vesicles. Bis indolylmaleimide inhibited bone resorption 50% with an IC50 approximately 3 microM, as well as acid transport activity in a concentration -dependent manner with an IC50 of approximately 0.4 IM. The IC50 of bIM for inhibiting acid transport activity was similar to that of calmodulin antagonists. The potassium ionophore, valinomycin, failed to restore bIM or tamoxifen-dependent inhibition of acid transport, suggesting that bIM and tamoxifen both inhibit H(+)-ATPase activity. Half maximal inhibitory concentrations of tamoxifen and bIM were not additive in acid transport assays, suggesting different sites of action. Furthermore, exogenous calmodulin blocked tamoxifen, but not bIM, -dependent inhibition of acid transport. We also compared the effects of tamoxifen and bIM on phosphorylation of proteins in isolated membrane fractions as determined by 32P incorporation and autoradiography. Tamoxifen had no effect on protein phosphorylation in contrast to bIM, which inhibited phosphorylation of eight proteins with different apparent kinetics. The data suggest that, while tamoxifen and bIM both affect H(+)-ATPase activity, the mechanisms of action are different.

Glucose-dependent regulation of osteoclast H(+)-ATPase expression: potential role of p38 MAP-kinase

Bone resorption is glucose concentration dependent. Mechanisms regulating glucose-dependent increases in bone resorption have not been identified. Glucose activates p38 MAP-kinase in other cells and since MAP kinases activate transcription factors, we hypothesized that glucose-stimulated bone resorption may be modulated by increased expression of the vacuolar H(+)-ATPase. Glucose activates osteoclast p38 MAP-kinase in a time and concentration-dependent manner as determined by Western analysis with phospho-specific p38 antibody while total p38 levels are unchanged. The K0.5 for glucose-dependent activation of p38 MAP-kinase is approximately 7 mM, activation is maximal at 30 min and is elevated but returning to basal levels by 60 min. The concentration-dependent increase in H(+)-ATPase expression was confirmed by Northern analysis. The specific inhibitor of p38 MAP-kinase, SB203580, inhibited glucose transport in osteoclasts, as well as glucose concentration-dependent increases in bone resorption and expression of H(+)-ATPase A and B subunits. Glucose had no effect on calmodulin expression levels that are regulated in response to other environmental changes. The glucose-stimulated increase in H(+)-ATPase mRNA expression is a specific response to glucose since glucose has little effect on G3PDH mRNA levels. We conclude that glucose regulates osteoclast H(+)-ATPase expression by a mechanism likely to involve p38 MAP-kinase.