Effects of Bafilomycin A1: an inhibitor of vacuolar H (+)-ATPases on endocytosis and apoptosis in RAW cells and RAW cell-derived osteoclasts

Omeprazole antagonizes virulence and inflammation in Salmonella enterica-infected RAW264.7 cells

The proton pump inhibitor omeprazole reduced the intracellular replication of Salmonella enterica serovar Typhimurium in RAW264.7 cells without affecting bacterial growth in vitro or the viability of the host cells. The mechanism was bacteriostatic and interfered with replication mediated by the virulence-associated SPI2 type III secretion system. The proton pump inhibitor bafilomycin A(1), in contrast, mediated killing of intracellular bacteria and imposed a marked cytotoxicity on RAW264.7 cells. The two compounds also differentially affected the proinflammatory responses of the infected cells. Bafilomycin A(1) enhanced nitric oxide production, whereas omeprazole delayed IkappaB degradation and blocked nitric oxide production and the secretion of proinflammatory cytokines. These results imply that omeprazole can be used to block the virulence factor-mediated intracellular replication of S. Typhimurium, and that its mechanism of growth inhibition is different from that mediated by bafilomycin A(1).

Trabecular bone deterioration in col9a1+/- mice associated with enlarged osteoclasts adhered to collagen IX-deficient bone

INTRODUCTION

Short collagen IX, the exclusive isoform expressed by osteoblasts, is synthesized through alternative transcription of the col9a1 gene. The function of short collagen IX in bone was characterized in col9a1-null mutant mice.

MATERIALS AND METHODS

Trabecular bone morphometry of lumbar bones and tibias was evaluated by muCT and nondecalcified histology. Osteoblastic and osteoclastic activities were evaluated by PCR- and microarray-based gene expression assays and TRACP-5b and C-terminal telopeptide (CTX) assays, as well as in vitro using bone marrow stromal cells and splenocytes. The effect of col9a1(+/-) mutation on osteoclast morphology was evaluated using RAW264.7-derived osteoclastic cells cultured on the mutant or wildtype calvarial bone substrates.

RESULTS

Col9a1 knockout mutation caused little effects on the skeletal development; however, young adult female col9a1(-/-) and col9a1(+/-) mice exhibited significant loss of trabecular bone. The trabecular bone architecture was progressively deteriorated in both male and female heterozygous col9a1(+/-) mice while aging. The aged mutant mice also exhibited signs of thoracic kyphosis and weight loss, resembling the clinical signs of osteoporosis. The col9a1(+/-) osteoblasts synthesized short col9a1 transcripts at decreased rates. Whereas bone formation activities in vitro and in vivo were not affected, the mutant osteoblast expressed the elevated ratio of RANKL/osteoprotegerin. Increased serum TRACP-5b and CTX levels were found in col9a1(+/-) mice, whose bone surface was associated with osteoclastic cells that were abnormally flattened and enlarged. The mutant and wildtype splenocytes underwent similar osteoclastogenesis in vitro; however, RAW264.7-derived osteoclastic cells, when cultured on the col9a1(+/-) calvaria, widely spread over the bone surface and formed large resorption pits. The surface of col9a1(+/-) calvaria was found to lack the typical nanotopography.

CONCLUSIONS

The mineralized bone matrix deficient of short collagen IX may become susceptible to osteoclastic bone resorption, possibly through a novel non-cell-autonomous mechanism. The data suggest the involvement of bone collagen IX in the pathogenesis of osteoporosis.

Cytoplasmic terminus of vacuolar type proton pump accessory subunit Ac45 is required for proper interaction with V(0) domain subunits and efficient osteoclastic bone resorption

Solubilization of mineralized bone by osteoclasts is largely dependent on the acidification of the extracellular resorption lacuna driven by the vacuolar (H+)-ATPases (V-ATPases) polarized within the ruffled border membranes. V-ATPases consist of two functionally and structurally distinct domains, V(1) and V(0). The peripheral cytoplasmically oriented V(1) domain drives ATP hydrolysis, which necessitates the translocation of protons across the integral membrane bound V(0) domain. Here, we demonstrate that an accessory subunit, Ac45, interacts with the V(0) domain and contributes to the vacuolar type proton pump-mediated function in osteoclasts. Consistent with its role in intracellular acidification, Ac45 was found to be localized to the ruffled border region of polarized resorbing osteoclasts and enriched in pH-dependent endosomal compartments that polarized to the ruffled border region of actively resorbing osteoclasts. Interestingly, truncation of the 26-amino acid residue cytoplasmic tail of Ac45, which encodes an autonomous internalization signal, was found to impair bone resorption in vitro. Furthermore, biochemical analysis revealed that although both wild type Ac45 and mutant were capable of associating with subunits a3, c, c'', and d, deletion of the cytoplasmic tail altered its binding proximity with a3, c'', and d. In all, our data suggest that the cytoplasmic terminus of Ac45 contains elements necessary for its proper interaction with V(0) domain and efficient osteoclastic bone resorption.

Diphyllin, a novel and naturally potent V-ATPase inhibitor, abrogates acidification of the osteoclastic resorption lacunae and bone resorption

Dissolution of the inorganic phase of bone by the osteoclasts mediated by V-ATPase and ClC-7 is a prerequisite for bone resorption. Inhibitors of osteoclastic V-ATPase or ClC-7 are novel approaches for inhibition of osteoclastic bone resorption. By testing natural compounds in acidification assays, diphyllin was identified. We characterized diphyllin with respect to the pharmacological effects on osteoclasts.

INTRODUCTION

Osteoclastic acidification of the resorption lacuna and bone resorption requires activity of both V-ATPase and the chloride channel ClC-7. Inhibition of these processes represents a novel approach for treatment of bone metabolic disorders. We identified diphyllin, a novel inhibitor of V-ATPase, and characterized this natural compound with respect to activity in human osteoclasts.

MATERIALS AND METHODS

Diphyllin was tested in the acid influx assay and V-ATPase assay using bovine chromaffin granules. Human osteoclasts were generated from CD14+ monocytes cultured with macrophage-colony stimulating factor (M-CSF) and RANKL. The effect of diphyllin on lysosomal acidification in human osteoclasts was studied using acridine orange. The effect of diphyllin on bone resorption by osteoclasts was measured as release of C-terminal cross-linked telopeptide of type I collagen (CTX-I) and calcium into the supernatants and by scoring pit area. Osteoclast number, TRACP activity, and cell viability were measured. Furthermore, the effect of diphyllin on bone nodule formation was tested using the mouse osteoblast cell line MC3T3-E1.

RESULTS

In the acid influx assay, diphyllin potently inhibited the acid influx (IC50 = 0.6 nM). We found that diphyllin inhibited V-ATPase with an IC50 value of 17 nM, compared with 4 nM for bafilomycin A1. Moreover, diphyllin dose-dependently inhibited lysosomal acidification in human osteoclasts. Furthermore, we found that diphyllin inhibited human osteoclastic bone resorption measured by CTX-I (IC50 = 14 nM), calcium release, and pit area, despite increasing TRACP activity, numbers of osteoclasts, and cell viability. Finally, diphyllin showed no effect on bone formation in vitro, whereas bafilomycin A1 was toxic.

CONCLUSIONS

We identified a natural compound that potently inhibits V-ATPase and thereby lysosomal acidification in osteoclasts, which leads to abrogation of bone resorption. Because recent studies indicate that inhibition of the osteoclastic acidification leads to inhibition of resorption without inhibiting formation, we speculate that diphyllin is a potential novel treatment for bone disorders involving excessive resorption.

Characterization of vacuolar-ATPase and selective inhibition of vacuolar-H(+)-ATPase in osteoclasts

V-ATPase plays important roles in controlling the extra- and intra-cellular pH in eukaryotic cell, which is most crucial for cellular processes. V-ATPases are composed of a peripheral V(1) domain responsible for ATP hydrolysis and integral V(0) domain responsible for proton translocation. Osteoclasts are multinucleated cells responsible for bone resorption and relate to many common lytic bone disorders such as osteoporosis, bone aseptic loosening, and tumor-induced bone loss. This review summarizes the structure and function of V-ATPase and its subunit, the role of V-ATPase subunits in osteoclast function, V-ATPase inhibitors for osteoclast function, and highlights the importance of V-ATPase as a potential prime target for anti-resorptive agents.

Calcium/calmodulin-dependent kinase activity is required for efficient induction of osteoclast differentiation and bone resorption by receptor activator of nuclear factor kappa B ligand (RANKL)

Calcium/calmodulin-dependent protein kinase (CaMK) is a major down stream mediator of Ca(2+) signaling in a wide range of cellular functions, including ion channel and cell cycle regulation and neurotransmitter synthesis and release. Here we have investigated the role of the CaMK signaling pathway in osteoclast differentiation and bone resorption. We observed that the CaMKI, CaMKII gamma isoforms were present in both bone-marrow derived macrophages and RAW264.7 murine macrophage cell line, and that expression persisted during osteoclast differentiation in the presence of receptor activator of nuclear factor kappa B (NF-kappaB) ligand (RANKL). RANKL-induced differentiation was accompanied by increased cyclic AMP response element transcriptional activity, and ERK phosphorylation, which are both downstream targets of CaMK. Two selective inhibitors of CaMKs, KN-93 and KN-62, inhibited osteoclastogenesis in a time and concentration-dependent manner. This was accompanied by suppression of cathepsin K expression and osteoclastic bone resorption, which are markers for differentiated osteoclast function. KN-93 and KN-62 both inhibited RANKL-induced ERK phosphorylation and CREB transcriptional activity. These findings imply a role for CaMK in osteoclast differentiation and bone resorption.

Nitric Oxide Production by the Vacuolar-Type (H+)-ATPase Inhibitors Bafilomycin A1 and Concanamycin A and Its Possible Role in Apoptosis in RAW 264.7 Cells

In the mouse leukemic monocyte cell line RAW 264.7, the vacuolar-type (H(+))-ATPase (V-ATPase) inhibitors bafilomycin A1 and concanamycin A induced nitric oxide (NO) production through the expression of inducible nitric-oxide synthase mRNA and its protein and decreased cell growth and survival as determined by 3-(4,5-dimethyl(thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Bafilomycin A1 and concanamycin A activated nuclear factor (NF)-kappaB and activator protein-1 and decreased the level of IkappaB-alpha and increased that of phosphorylated c-Jun N-terminal kinase (JNK). NO production induced by these V-ATPase inhibitors was suppressed by the NF-kappaB inhibitor Bay 11-7082 [(E)3-[(4-methylphenyl)sulfonyl])-2-propenenitrile] and the JNK inhibitor SP600125 [anthra[1,9-cd]pyrazol-6(2H)-one] in parallel with the partial alleviation of the V-ATPase inhibitor-induced decrease in MTT response. The Na(+),K(+)-ATPase inhibitor dibucaine and the F-ATPase inhibitor oligomycin did not induce NO production at which concentrations the MTT response was decreased. The NO donor S-nitroso-N-acetyl-dl-penicillamine further lowered the V-ATPase inhibitor-induced decrease in the MTT response, and the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, sodium salt (carboxy-PTIO) alleviated it partially. Mitochondrial depolarization, an index of apoptosis, was induced by bafilomycin A1 and concanamycin A. On treatment with the nitric-oxide synthase inhibitor N(G)-monomethyl-l-arginine acetate, the disruption of mitochondrial membrane potential induced by bafilomycin A1 and concanamycin A was alleviated partially in parallel with the decrease in NO production. Carboxy-PTIO also alleviated it partially. Our findings suggest that the V-ATPase inhibitors bafilomycin A1 and concanamycin A similarly induce NO production and the newly produced NO participates partially in the V-ATPase inhibitor-induced apoptosis in RAW 264.7 cells.

Proton pump inhibitors, histamine H2 receptor antagonists, and other antacid medications and the risk of fracture

We studied the effect of proton pump inhibitors, histamine H(2) receptor antagonists, and other types of antacid drugs on fracture risk. All cases were subjects with any fracture sustained during the year 2000 (n = 124,655). For each case, three controls (n = 373,962) matched on age and gender were randomly drawn from the background population. The primary exposure variables were use of proton pump inhibitors, histamine H(2) antagonists, and other antacid drugs. Adjustments were made for several confounders, including diagnosis of an ulcer, nonsteroidal anti-inflammatory drug use, use of histamine H(1) antagonists, stomach resection, previous fracture, and use of corticosteroids. The effect of dose was examined by stratifying for cumulated dose (defined daily dose). Use of proton pump inhibitors was associated with an increase in fracture risk for use within the last year [odds ratio (OR) = 1.18, 95% confidence interval (CI) 1.12-1.43 for overall fracture risk; OR = 1.45, 95% CI 1.28-1.65 for hip fractures; and OR = 1.60, 95% CI 1.25-2.04 for spine fractures). Histamine H(2) antagonists were associated with a decreased fracture risk if they had been used within the last year (OR = 0.88, 95% CI 0.82-0.95 for any fracture, OR = 0.69, 95% CI 0.57-0.84 for hip fractures). Other antacids were not associated with overall fracture risk but were associated with hip and spine fractures. Proton pump inhibitors appeared to be associated with a limited increase in fracture risk, in contrast to histamine H( 2 ) antagonists, which seemed to be associated with a small decrease in fracture risk. In all cases, the changes in risk estimates were small and the clinical significance was limited.

p62 ubiquitin binding-associated domain mediated the receptor activator of nuclear factor-kappaB ligand-induced osteoclast formation: a new insight into the pathogenesis of Paget's disease of bone

Paget's disease of bone (PDB) is a debilitating bone disorder characterized by giant osteoclasts, enhanced bone destruction, and irregular bone formation. Recently, mutations in SQSTM1 (also known as p62) have been detected in PDB sufferers, with all mutations resulting in either loss of function or truncation/deletion of the ubiquitin binding-associated (UBA) domain. We hypothesized that mutation in the p62 gene resulting in either deletion or premature termination of the UBA domain accounts for the elevated osteoclastic formation and bone resorption associated with PDB. Remarkably, overexpression of the p62 UBA domain deletion mutant (p62DeltaUBA) significantly enhanced osteoclastogenesis in vitro compared to cells expressing either wild-type p62 (p62WT) or a control vector in a RAW264.7 osteoclastogenic system. Overexpression of p62DeltaUBA potentiated the formation of abnormally large multinucleated osteoclasts and resorption of bone, reminiscent of PDB. Consistent with the enhancement of osteoclastogenesis, overexpression of p62DeltaUBA potentiated receptor activator of nuclear factor-kappaB ligand-induced activation of nuclear factor-kappaB, NFAT, and ERK phosphorylation. Furthermore, as determined by confocal microscopy, deletion of the p62 UBA domain impaired the association of p62 with TRAF6 in the proteasomal compartment. These results suggest that the UBA domain encodes essential regulatory elements required for receptor activator of nuclear factor-kappaB ligand-induced osteoclast formation and bone resorption that may be directly associated with the progression of PDB.

Inhibition of vacuolar-type (H+)-ATPase by the cytostatic macrolide apicularen A and its role in apicularen A-induced apoptosis in RAW 264.7 cells

Apicularen A and the known vacuolar-type (H(+))-ATPase (V-ATPase) inhibitor bafilomycin A(1) induced apoptosis of RAW 264.7 cells, while apicularen B, an N-acetyl-glucosamine glycoside of apicularen A, was far less effective. Apicularen A inhibited vital staining with acridine orange of the intracellular organelles of RAW 264.7 cells, inhibited the ATP-dependent proton transport into inside-out microsome vesicles, and inhibited the bafilomycin A(1)-sensitive ATP hydrolysis. The IC(50) values of the proton transport were 0.58 nM for apicularen A, 13 nM for apicularen B, and 0.95 nM for bafilomycin A(1). Furthermore, apicularen A inhibited the bafilomycin A(1)-sensitive ATP hydrolysis more potently than apicularen B. F-ATPase and P-ATPase were not inhibited by apicularen A. We concluded that apicularen A inhibits V-ATPase, and thus induces apoptosis in RAW 264.7 cells.

Degradation of the organic phase of bone by osteoclasts: a secondary role for lysosomal acidification

Osteoclasts degrade bone matrix by secretion of hydrochloric acid and proteases. We studied the processes involved in the degradation of the organic matrix of bone in detail and found that lysosomal acidification is involved in this process and that MMPs are capable of degrading the organic matrix in the absence of cathepsin K.

INTRODUCTION

Osteoclasts resorb bone by secretion of acid by the vacuolar H+-adenosine triphosphatase (V-ATPase) and the chloride channel ClC-7, followed by degradation of the matrix, mainly collagen type I, by cathepsin K and possibly by matrix metalloproteinases (MMPs). However, the switch from acidification to proteolysis and the exact roles of both the ion transporters and the proteinases still remain to be studied.

MATERIALS AND METHODS

We isolated CD14+ monocytes from human peripheral blood from either controls or patients with autosomal dominant osteopetrosis type II (ADOII) caused by defective ClC-7 function and cultured them in the presence of RANKL and macrophage-colony stimulating factor (M-CSF) to generate osteoclasts. We decalcified cortical bovine bone slices and studied the osteoclasts with respect to morphology, markers, and degradation of the decalcified matrix in the presence of various inhibitors of osteoclast acidification and proteolysis, using normal calcified bone as a reference.

RESULTS

We found that ADOII osteoclasts not only have reduced resorption of the calcified matrix, but also 40% reduced degradation of the organic phase of bone. We found that both acidification inhibitors and cathepsin K inhibitors reduced degradation of the organic matrix by 40% in normal osteoclasts, but had no effect in the ADOII osteoclasts. Furthermore, we showed that inhibition of MMPs leads to a 70% reduction in the degradation of the organic bone matrix and that MMPs and cathepsin K have additive effects. Finally, we show that osteoclastic MMPs mediate release of the carboxyterminal telopeptide of type I collagen (ICTP) fragment in the absence of cathepsin K activity, and therefore, to some extent, are able to compensate for the loss of cathepsin K activity.

CONCLUSIONS

These data clearly show that osteoclastic acidification of the lysosomes plays a hitherto nonrecognized role in degradation of the organic matrix. Furthermore, these data shed light on the complicated interplay between acidification dependent and independent proteolytic processes, mediated by cathepsin K and the MMPs, respectively.

Transcytosis of calcium from bone by osteoclast-like cells evidenced by direct visualization of calcium in cells

'Transcytosis' of calcium (Ca) from bone by osteoclasts was identified by using a newly developed method that uses fixed or living osteoclast-like cells previously differentiated in vitro, a Ca-specific cell-membrane-impermeable fluorescent dye, and confocal laser scanning microscopy. This method, called the cell-membrane-impermeable dye method, revealed that in fixed osteoclast-like cells, a large quantity of Ca was confined within vacuoles and transported toward the apical cell membrane in the cells. These Ca-confined vacuoles were co-localized with marker proteins of both ruffled border and lysosome. The vacuoles were disrupted when treated with an inhibitor of ruffled border ATPase. In living osteoclast-like cells, Ca-confined vacuoles were again preferentially located at the central region and near the apical cell membrane. These results suggest actual transcytosis of Ca from bone by osteoclasts, and are the first direct evidence of the significant role of osteoclasts in the entire process of Ca metabolism in bone.

Binding interaction of SARS coronavirus 3CL(pro) protease with vacuolar-H+ ATPase G1 subunit

The pathogenesis of severe acute respiratory syndrome coronavirus (SARS-CoV) is an important issue for treatment and prevention of SARS. Recently, SARS-CoV 3CL(pro) protease has been implied to be possible relevance to SARS-CoV pathogenesis. In this study, we intended to identify potential 3CL(pro)-interacting cellular protein(s) using the phage-displayed human lung cDNA library. The vacuolar-H+ ATPase (V-ATPase) G1 subunit that contained a 3CL(pro) cleavage site-like motif was identified as a 3CL(pro)-interacting protein, as confirmed using the co-immunoprecipitation assay and the relative affinity assay. In addition, our result also demonstrated the cleavage of the V-ATPase G1 fusion protein and the immunoprecipitation of cellular V-ATPase G1 by the 3CL(pro). Moreover, loading cells with SNARF-1 pH-sensitive dye showed that the intracellular pH in 3CL(pro)-expressing cells was significantly lower as compared to mock cells.

CAP-1A is a novel linker that binds clathrin and the voltage-gated sodium channel Na(v)1.8

The voltage-gated sodium channel Na(v)1.8 produces a tetrodotoxin-resistant current and plays a key role in nociception. Annexin II/p11 binds to Na(v)1.8 and facilitates insertion of the channel within the cell membrane. However, the mechanisms responsible for removal of specific channels from the cell membrane have not been studied. We have identified a novel protein, clathrin-associated protein-1A (CAP-1A), which contains distinct domains that bind Na(v)1.8 and clathrin. CAP-1A is abundantly expressed in DRG neurons and colocalizes with Na(v)1.8 and can form a multiprotein complex with Na(v)1.8 and clathrin. Coexpression of CAP-1A and Na(v)1.8 in DRG neurons reduces Na(v)1.8 current density by approximately 50% without affecting the endogenous or recombinant tetrodotoxin-sensitive currents. This effect of CAP-1A is blocked by bafilomycin A1 treatment of transfected DRG neurons. CAP-1A thus is the first example of an adapter protein that links clathrin and a sodium channel and may regulate Na(v)1.8 channel density at the cell surface.

Thapsigargin modulates osteoclastogenesis through the regulation of RANKL-induced signaling pathways and reactive oxygen species production

The mechanism by which TG modulates osteoclast formation and apoptosis is not clear. In this study, we showed a biphasic effect of TG on osteoclast formation and apoptosis through the regulation of ROS production, caspase-3 activity, cytosolic Ca2+, and RANKL-induced activation of NF-kappaB and AP-1 activities.

INTRODUCTION

Apoptosis and differentiation are among the consequences of changes in intracellular Ca2+ levels. In this study, we investigated the effects of the endoplasmic reticular Ca2+-ATPase inhibitor, thapsigargin (TG), on osteoclast apoptosis and differentiation.

MATERIALS AND METHODS

Both RAW264.7 cells and primary spleen cells were used to examine the effect of TG on RANKL-induced osteoclastogenesis. To determine the action of TG on signaling pathways, we used reporter gene assays for NF-kappaB and activator protein-1 (AP-1) activity, Western blotting for phospho-extracellular signal-related kinase (ERK), and fluorescent probes to measure changes in levels of intracellular calcium and reactive oxygen species (ROS). To assess rates of apoptosis, we measured changes in annexin staining, caspase-3 activity, and chromatin and F-actin microfilament structure.

RESULTS

At concentrations that caused a rapid rise in intracellular Ca2+, TG increased caspase-3 activity and promoted apoptosis in osteoclast-like cells (OLCs). Low concentrations of TG, which were insufficient to measurably alter intracellular Ca2+, unexpectedly suppressed caspase-3 activity and enhanced RANKL-induced osteoclastogenesis. At these lower concentrations, TG potentiated ROS production and RANKL-induced NF-kappaB activity, but suppressed RANKL-induced AP-1 activity and had little effect on ERK phosphorylation.

CONCLUSION

Our novel findings of a biphasic effect of TG are incompletely explained by our current understanding of TG action, but raise the possibility that low intensity or local changes in subcellular Ca2+ levels may regulate intracellular differentiation signaling. The extent of cross-talk between Ca2+ and RANKL-mediated intracellular signaling pathways might be important in determining whether cells undergo apoptosis or differentiate into OLCs.

Evidence of reciprocal regulation between the high extracellular calcium and RANKL signal transduction pathways in RAW cell derived osteoclasts

During bone resorption, osteoclasts are exposed to high Ca2+ concentrations (up to 40 mM). The role of high extracellular Ca2+ in receptor activator of NF-kappaB ligand (RANKL)-mediated osteoclast survival and their functional interrelationship is unclear. In this study, we show that RANKL enhances osteoclast tolerance to high extracellular Ca2+ by protecting the cell from cell death in a dose dependent manner. We have provided evidence that RANKL does this by attenuating high extracellular Ca2+-induced Ca2+ elevations. Moreover, we have found that high extracellular Ca2+-induced cell death was partially inhibited by a caspase-3 inhibitor, suggesting caspase-3-mediated apoptosis is involved. Conversely, using reporter gene assays and Western blot analysis, we have demonstrated that high extracellular Ca2+ desensitizes the RANKL-induced activation of NF-kappaB and c-Jun N-terminal kinase (JNK), and inhibits constitutive and RANKL-stimulated ERK phosphorylation, indicating a negative feed-back mechanism via specific RANKL signaling pathways. Taken together, this study provides evidence for a reciprocal regulation between high extracellular Ca2+ and RANKL signaling in RAW cell derived osteoclasts. Our data imply a cross talk mechanism of extracellular Ca2+ on osteoclast survival through the regulation of RANKL.

Sesquiterpene lactone parthenolide blocks lipopolysaccharide-induced osteolysis through the suppression of NF-kappaB activity

Effective treatment for bacteria-induced bone lytic diseases is not yet available. In this study, we showed that PAR, an NF-kappaB inhibitor found in medicinal herbs, can block LPS-induced osteolysis. PAR does this by inhibiting osteoclastogenesis and bone resorption and promoting apoptosis of osteoclasts through the suppression of NF-kappaB activity.

INTRODUCTION

Osteolysis induced by chronic gram-negative bacterial infection underlies many bone diseases such as osteomyelitis, septic arthritis, and periodontitis. Drugs that inhibit lipopolysaccharide (LPS)-induced osteolysis are critically needed for the prevention of bone destruction in infective bone diseases. In this study, we investigated the effect of parthenolide (PAR) on LPS-induced osteolysis in vivo and studied its role in osteoclastogenesis, bone resorption, apoptosis, and NF-kappaB activity.

MATERIALS AND METHODS

The LPS-induced osteolysis in the mouse calvarium model was used to examine the effect of PAR in vivo. RANKL-induced osteoclast differentiation from RAW264.7 cells and bone resorption assays were used to assess the effect of PAR in vitro. Assays for NF-kappaB activation, p65 translocation, and IkappaB-alpha degradation were used to determine the mechanism of action of PAR in osteoclasts and their precursors. Flow cytometry and confocal microscopic analysis were used to examine cell apoptosis. Semiquantitative RT-PCR was performed to examine the effect of PAR on gene expression of RANK and TRAF6.

RESULTS

We found that PAR (0.5 and 1 mg/kg), injected simultaneously with LPS (25 mg/kg) or 3 days later, blocked the LPS-induced osteolysis in the mouse calvarium model. In vitro studies showed that low concentrations of PAR (<1 microM) inhibited in vitro osteoclastogenesis and osteoclastic bone resorption, whereas higher concentrations (>5 microM) triggered apoptotic cell death of osteoclasts and their precursor cells in a dose-dependent manner. Furthermore, PAR inhibited LPS-induced NF-kappaB activation, p65 translocation, and IkappaB-alpha degradation both in mature osteoclasts and their precursors in a time- and dose-dependent manner. In addition, PAR inhibited NF-kappaB activation induced by osteoclastogenic factors RANKL, interleukin (IL)-1beta, or TNF-alpha to varying degrees and reduced the gene expression of RANK and TRAF6.

CONCLUSION

The NF-kappaB pathway is known to mediate both osteoclast differentiation and survival. These findings indicate that PAR blocks LPS-induced osteolysis through the suppression of NF-kappaB activity and suggest that it might have therapeutic value in bacteria-induced bone destruction.

A novel inhibitor of vacuolar ATPase, FR167356, which can discriminate between osteoclast vacuolar ATPase and lysosomal vacuolar ATPase

1 Vacuolar ATPase (V-ATPase) has been proposed as a drug target in lytic bone diseases. Studies of bafilomycin derivatives suggest that the key issue regarding the therapeutic usefulness of V-ATPase inhibitors is selective inhibition of osteoclast V-ATPase. Previous efforts to develop therapeutic inhibitors of osteoclast V-ATPase have been frustrated by a lack of synthetically tractable and biologically selective leads. Therefore, we tried to find novel potent and specific V-ATPase inhibitors, which have new structural features and inhibition selectivity, from random screening using osteoclast microsomes. Finally, a novel V-ATPase inhibitor, FR167356, was obtained through chemical modification of a parental hit compound. 2 FR167356 inhibited not only H+ transport activity of osteoclast V-ATPase but also H+ extrusion from cytoplasm of osteoclasts, which depends on the V-ATPase activity. As expected, FR167356 remarkably inhibited bone resorption in vitro. 3 FR167356 also showed inhibitory effects on other V-ATPases, renal brush border V-ATPase, macrophage microsome V-ATPase and lysosomal V-ATPase. However, FR167356 was approximately seven-fold less potent in inhibiting lysosomal V-ATPase compared to osteoclast V-ATPase. Moreover, LDL metabolism in cells, which depends on acidification of lysosome, was blocked merely at higher concentration than bone resorption, suggesting that FR167356 inhibits V-ATPase of osteoclast ruffled border membrane still more selectively than lysosome at the cellular level. 4 These results from the experiments seem to indicate that osteoclast V-ATPase may be different from lysosomal V-ATPase in respect of their structure. 5 FR167356 had a novel chemical structural feature as well as inhibitory characteristics distinctly different from any previously known V-ATPase inhibitor family. Therefore, FR167356 is thought to be a useful tool for estimating the essential characteristics of V-ATPase inhibitors for drug development.

Vacuolar H+-ATPase in human breast cancer cells with distinct metastatic potential: distribution and functional activity

Tumor cells thrive in a hypoxic microenvironment with an acidic extracellular pH. To survive in this harsh environment, tumor cells must exhibit a dynamic cytosolic pH regulatory system. We hypothesize that vacuolar H(+)-ATPases (V-ATPases) that normally reside in acidic organelles are also located at the cell surface, thus regulating cytosolic pH and exacerbating the migratory ability of metastatic cells. Immunocytochemical data revealed for the first time that V-ATPase is located at the plasma membrane of human breast cancer cells: prominent in the highly metastatic and inconspicuous in the lowly metastatic cells. The V-ATPase activities in isolated plasma membranes were greater in highly than in lowly metastatic cells. The proton fluxes via V-ATPase evaluated by fluorescence spectroscopy in living cells were greater in highly than in lowly metastatic cells. Interestingly, lowly metastatic cells preferentially used the ubiquitous Na(+)/H(+) exchanger and HCO(3)(-)-based H(+)-transporting mechanisms, whereas highly metastatic cells used plasma membrane V-ATPases. The highly metastatic cells were more invasive and migratory than the lowly metastatic cells. V-ATPase inhibitors decreased the invasion and migration in the highly metastatic cells. Altogether, these data indicate that V-ATPases located at the plasma membrane are involved in the acquisition of a more metastatic phenotype.

Endocytic trafficking in actively resorbing osteoclasts

Endocytosis and the subsequent intracellular trafficking of the endocytosed material are important determinants of cellular function. Osteoclasts, cells of the monocyte/macrophage family, are specialized for the internalization and processing of bone matrix. Transcytosis of endocytosed material has been observed in osteoclasts but the precise mechanism controlling this process is unclear. Here, we investigate the regulation of these trafficking events. To establish the directionality and kinetics of trafficking events in resorbing osteoclasts, we devised a system using fluorescent low-molecular-weight markers as probes to follow the route taken by the digested bone matrix. We demonstrate that this route is largely distinct from the pathway followed by proteins taken up by receptor-mediated endocytosis at the basolateral plasma membrane. Endocytosis and transcytosis from the ruffled border are fast processes, with a half-life of the endocytosed material inside the cells of 22 minutes. We demonstrate the crucial role of the microtubule network in transport from the ruffled-border area and provide evidence for a role of the cytoskeleton in the overall efficacy of trafficking. Moreover, we analyse the effect of the V-ATPase inhibitor bafilomycin A1 on endocytic uptake, which gives insight into the pH-dependent regulation of membrane trafficking and resorption in osteoclasts.