Evidence for the presence of a proton pump of the vacuolar H(+)-ATPase type in the ruffled borders of osteoclasts

Expression of 16 kDa proteolipid of vacuolar-type H(+)-ATPase in human pancreatic cancer

Recent studies have shown that bafilomycin A1-sensitive vacuolar-type H(+)-ATPase (V-ATPase) plays important roles in cell growth and differentiation. However, there is no published study that has focused on the expression of V-ATPase in human tumour tissues. This study was designed to examine the mRNA and protein levels for the 16 kilodalton (kDa) proteolipid of V-ATPase in human pancreatic carcinoma tissues. We first investigated the mRNA level for V-ATPase in six cases of invasive pancreatic cancers and two normal pancreases, using reverse transcription-polymerase chain reaction technique. Then, we examined immunohistochemically the level of V-ATPase protein in 49 pancreatic cancers and ten benign cystic neoplasms of the pancreas, using antisera raised against the 16 kDa proteolipid. There was a notable difference in the level of V-ATPase mRNA between normal and pancreatic carcinoma tissues, with no evident difference in the expression of the beta-actin gene. Immunohistochemically, 42 out of 46 invasive ductal cancers (92%) displayed a mild to marked immunoreactivity for V-ATPase in the cytoplasm, whereas neither non-invasive ductal cancers nor benign cystic neoplasms expressed detectable immunoreactive proteins. These findings suggest that the overexpression of V-ATPase protein is characteristic of invasive pancreatic tumours. V-ATPase may play some crucial roles in tumour progression.

Immunocytochemical localization of vacuolar H+-ATPase and Cl--HCO3- anion exchanger (erythrocyte band-3 protein) in avian osteoclasts: effect of calcium-deficient diet on polar expression of the H+-ATPase pump

Osteoclasts attach to the bone surface and resorb bone by secreting protons into an isolated subosteoclastic compartment. Previous studies have shown the presence of a vacuolar type H+-ATPase, and a functional Cl--HCO3- anion exchanger in the osteoclast. In the present studies, using a monoclonal antibody to the 31-kDa subunit of H+-ATPase and a rabbit antiserum to the erythrocyte band-3 protein (Cl--HCO3- anion exchanger) we have immunocytochemically localized the respective pumps in bone sections obtained from chickens fed a normal or a calcium-deficient diet for 4 weeks. Our results indicate that although H+-ATPase is either evenly distributed throughout the osteoclast or is more polarized at its ruffled membrane juxtaposed to the bone surface, the band-3 protein immunoreactivity is always localized to the plasma membrane which is not attached to the bone surface (basolateral membrane). Four weeks of a calcium-deficient diet resulted in a significant increase in the percentage of osteoclasts that were polarized for the H+-ATPase pump at their ruffled membrane, and a trend toward increased total number of osteoclasts, although the latter did not reach statistical significance (P = 0.09). These changes were not accompanied by a significant increase in the intensity of staining for H+-ATPase. Band-3 protein immunoreactivity was always prominent, limited to the basolateral membrane, and did not alter with calcium-deficient diet or with changes in the degree of H+-ATPase polarization.

Vanadate inhibits vacuolar H(+)-ATPase-mediated proton transport in chicken kidney microsomes by an ADP-dependent mechanism

Recent reports indicate that vacuolar-type proton ATPases from chicken osteoclasts (Chatterjee et al. (1992) Proc. Natl. Acad. Sci. USA 89, 6257-6261), yeast vacuoles and chromaffin granules (Beltran and Nelson (1992) Acta Physiol. Scand. Suppl. 607, 41-47) can be inhibited by vanadate, albeit at a concentration much higher than that required to inhibit P-type ATPases. We have characterized the mechanism by which vanadate inhibits vacuolar-type ATPase-mediated proton transport by chicken kidney microsomes. The initial rate of proton transport is somewhat less sensitive to vanadate than the total acidification, with IC50 values of 1.58 mM and 0.78 mM vanadate, respectively. The inhibition of both the initial rate and total acidification is noncompetitive with respect to ATP. The inhibition is abolished when ADP is removed by an ATP-regenerating system, and the addition of exogenous ADP increases the vanadate inhibition of proton transport in a synergistic manner, thus demonstrating that inhibition by vanadate is dependent on the presence of ADP and explaining the lower effect of vanadate on the initial rate of acidification. Phosphate protects proton transport activity from inhibition by vanadate. These effects of ADP and phosphate suggest that inhibition by vanadate may involve the formation of a complex with ADP at a nucleotide binding site, possibly at the catalytic site of the enzyme.

[3H]Bafilomycin as a probe for the transmembrane proton channel of the osteoclast vacuolar H(+)-ATPase

Bone resorption by the osteoclast is dependent on acidification of the bone surface by a vacuolar type H+-ATPase (V-ATPase) present in the ruffled membrane of the actively resorbing cell. V-ATPases are a highly conserved family of proton pumps consisting of two functional complexes: a cytoplasmic catalytic sector (VC) and a membrane bound proton channel (VB). Bafilomycin A1, a macrolide antibiotic, is a highly potent inhibitor of V-ATPases, and inhibits bone resorption in vitro in isolated rat calvariae. In order to investigate the binding of bafilomycin to the osteoclast V-ATPase, we used a tritiated bafilomycin which had been prepared by acetylating the 21-hydroxyl group of bafilomycin A1. Osteoclast ruffled membrane vesicles were prepared from purified chicken osteoclasts by differential centrifugation and proton transport in these vesicles was shown to be inhibited by [3H]bafilomycin (IC50 approximately 2 nM). Control membrane vesicles or membrane vesicles partially inhibited with [3H]bafilomycin were solubilized and separated by centrifugation on 15-30% glycerol gradients. V-ATPase activity and reconstitutable proton transport activity could be recovered in high density fractions of the gradient. However, the peak of [3H]bafilomycin radioactivity (>70% of total radioactivity in the gradient) was present in a single peak at lower density. Antibodies against subunits of VC (70, 56 and 40 kDa) reacted only in fractions containing the peak V-ATPase activity whereas an antibody to the 39 kDa subunit of VB reacted both with fractions containing the peak V-ATPase activity but also, and more strongly, in fractions containing the peak [3H]bafilomycin. The fractions in the control gradient corresponding to the peak of [3H]bafilomycin were reconstituted into liposomes and shown to mediate passive bafilomycin A1-inhibitable proton conductance. SDS-PAGE followed by autoradiography, indicated that the bafilomycin was not covalently bound to the V-ATPase or the proton channel. Quantification of VB by [3H]bafilomycin binding or by antibody staining suggested an excess of the free proton channel to that of the intact holoenzyme. A corresponding amount of free catalytic sector could not be found in any fraction throughout the isolation procedure of the V-ATPase from the initial homogenate. Thus, in conclusion, bafilomycin inhibits the V-ATPase by binding tightly but non-covalently to the proton channel region of the V-ATPase which appears to be present in excess over the intact holoenzyme in the osteoclast. The possible role of an excess of the proton channel subcomplex in the osteoclast is discussed.

Recent advances in the ultrastructural assessment of osteoclastic resorptive functions

Ultrastructural enzyme and immunocytochemical studies have made great contributions to clarifying intriguing questions as to the actual role of osteoclastic ruffled borders in bone resorption. In the present study, vacuolar-type H(+)-ATPase and cysteine-proteinase (cathepsin) were localized in osteoclasts by means of light and electron microscopic immunocytochemistry. The specific immunoreactivity of vacuolar-type H(+)-ATPase was detected along the ruffled border membranes, associated pale vacuoles, and cisterns of the rough-surfaced endoplasmic reticulum of osteoclasts. Anti-cathepsin B immunoreaction occurred in Golgi vesicles, lysosomes, pale vesicles and vacuoles, and the extracellular canals of ruffled borders of osteoclasts. The resorbing bone surfaces were also immunoreactive for anti-cathepsin B. In a coculture system of osteoclasts with devitalized dentine slices, a specific H(+)-ATPase inhibitor (bafilomycin A1) markedly reduced both demineralized areas and resorption lacuna formation on the dentine slices. On the other hand, the cathepsin inhibitor, E-64, inhibited only resorption lacuna formation but had no effect on demineralization of the dentine slices. These results suggest that H(+)-ATPase and cathepsins in osteoclasts are involved, respectively, in the extracellular solubilization of apatite crystals and subsequent degradation of bone matrix and that the ruffled border-clear zone complex of osteoclasts is the main site of cell-matrix interactions during bone resorption processes.

Cytoskeletal changes in osteoclasts during the resorption cycle

Osteoclasts are large, multinucleated cells which change their shape and polarity according to their resorptive activity. At least in vitro, nonresorbing osteoclasts move on the bone surface and do not show clear evidence of apical-basolateral polarity. When stimulated for resorption, osteoclasts undergo a rapid reorganization of the cytoskeleton and appear clearly polarized. The detailed nature of different membrane domains in polarized osteoclasts is still far from clear, but a remarkable feature is the formation of a tight sealing zone between the ruffled border and the rest of the cell membrane. Characteristic organization of F-actin into a belt or ring-like structure with a double circle of vinculin around it is needed for the formation of the sealing zone. This type of microfilament organization is typical only for resorbing osteoclasts and can thus be used as a marker for resorbing cells. These characteristic changes in the molecular organization of the cytoskeleton in osteoclasts during the resorption cycle offer several potential targets to inhibit bone resorption, perhaps cell-specific.

Bone-resorbing osteoclasts reveal a dynamic division of basal plasma membrane into two different domains

Bone-resorbing multinucleate osteoclasts exhibit a ruffled border membrane apposing the bone and a basal membrane contacting the circulation. A junctional complex called the sealing zone separates these two membrane domains, but the defined nature of these membrane domains has remained obscure. We now show, using enveloped viral glycoproteins and lectins as tools, that osteoclasts exhibit a novel membrane domain in the basal surface when they are polarized for resorption. Influenza haemagglutinin, which is apically targeted in epithelial cells, is targeted to a restricted area at the top of the basal surface, while vesicular stomatitis virus G-protein which is basolaterally targeted in epithelia, occupies the rest of the basal surface. Neither of these viral glycoproteins is gathered to the ruffled border nor sealing zone area, but they share in a specific way the basal surface. To show that the division of basal membrane into two different domains also occurs in non-infected cells, we have analyzed the distribution of receptors for these viruses and binding sites of some lectins. Both of these methods show that also some endogenous proteins are located in different domains in the basal surface in active osteoclasts. We also show that these two different membrane domains can be distinguished in scanning electron microscopy level due to the villus appearance of the central basal domain.

Role of microscopy in elucidating the mechanism and regulation of the osteoclast resorptive apparatus

Microscopic studies have assisted in revealing some of the components of the resorptive apparatus of osteoclasts, specifically carbonic anhydrase and the proton-translocating ATPase. Further, microscopy has helped substantiate the types of proteolytic enzymes secreted into the resorption lacuna. Regulatory agents affecting the resorptive process in vitro include parathyroid hormone, 17 beta-estradiol, calcitonin, and 1,25-dihydroxyvitamin D3. Studies showing the specific binding of parathyroid hormone, estradiol, and calcitonin to osteoclast plasma membrane are discussed. While specific binding suggests that direct effects may occur, further investigation is needed to substantiate this possibility.

Resorption-cycle-dependent polarization of mRNAs for different subunits of V-ATPase in bone-resorbing osteoclasts

Protein sorting in eukaryotic cells is mainly done by specific targeting of polypeptides. The present evidence from oocytes, neurons, and some other polarized cells suggests that protein sorting can be further facilitated by concentrating mRNAs to their corresponding subcellular areas. However, very little is known about the mechanism(s) involved in mRNA targeting, or how widespread and dynamic such mRNA sorting might be. In this study, we have used an in vitro cell culture system, where large multinucleated osteoclasts undergo continuous structural and functional changes from polarized (resorbing) to a nonpolarized (resting) stage. We demonstrate here, using a nonradioactive in situ hybridization technique and confocal microscopy, that mRNAs for several vacuolar H(+)-ATPase subunits change their localization and polarity in osteoclasts according to the resorption cycle, whereas mRNA for cytoplasmic carbonic anhydrase II is found diffusely located throughout the osteoclast during the whole resorption cycle. Antisense RNA against the 16-kDa or 60-kDa V-ATPase subunit inhibits polarization of the osteoclasts, as determined by cytoskeleton staining. Antisense RNA against carbonic anhydrase II, however, has no such effect.

Molecular aspects of osteoclast function

In this article we have overviewed recent important advances in understanding the molecular mechanisms involved in osteoclastic bone resorption. Specifically, new findings relating to osteoclast activation and the process of bone resorption are reviewed and a current overall model of how osteoclasts resorb bone is presented. Controversial research topics concerning the regulation of osteoclast activity are also critically discussed.

The first putative transmembrane helix of the 16 kDa proteolipid lines a pore in the Vo sector of the vacuolar H(+)-ATPase

The 16 kDa proteolipid is the major component of the vacuolar H(+)-ATPase membrane sector, responsible for proton translocation. Expression of a related proteolipid from the arythropod Nephrops norvegicus in a Saccharomyces strain in which the VMA3 gene for the endogenous proteolipid has been disrupted results in restored vacuolar H(+)-ATPase function. We have used this complementation system, coupled to cysteine substitution mutagenesis and protein chemistry, to investigate structural features of the proteolipid. Consecutive cysteines were introduced individually into putative transmembrane segment 1 of the proteolipid, and at selected sites in extramembranous regions and in segment 3 and 4. Analysis of restored vacuolar H(+)-ATPase function showed that segment 1 residues sensitive to mutation to cysteine were clustered on a single face, but only if the segment was helical. Only residues insensitive to mutation could be covalently modified by the cysteine-specific reagent fluorescein 5-maleimide. A cysteine introduced into segment 3 was the only residue accessible to a relatively hydrophobic reagent, suggesting accessibility to the lipid phase. Analysis of disulphide bond formation between introduced cysteines indicates that the first transmembrane alpha-helices of each monomer are adjacent to each other at the centre of the proteolipid multimeric complex. The data are consistent with a model in which the fluorescein maleimide-accessible face of helix I lines a pore at the centre of a hexameric complex formed by the proteolipid, with the mutationally sensitive face oriented into the protein core. The implications for ion-transport function in this family of proteins are discussed in the context of this structural model.

Acid extrusion is induced by osteoclast attachment to bone. Inhibition by alendronate and calcitonin

Acid extrusion is essential for osteoclast (OC) activity. We examined Na+ and HCO3(-)-independent H+ extrusion in rat- and mouse OCs by measuring intracellular pH (pHi) changes, with the pHi indicator BCECF (biscarboxyethyl-5-(6) carboxyfluorescein) after H+ loading with an ammonium pulse. 90% of OCs attached to glass do not possess HCO3- and Na(+)-independent H(+)-extrusion (rate of pHi recovery = 0.043 +/- 0.007 (SEM) pH U/min, n = 26). In contrast, in OCs attached to bone, the pHi recovery rate is 0.228 +/- 0.011 pHi U/min, n = 25. OCs on bone also possess a NH(4+)-permeable pathway not seen on glass. The bone-induced H+ extrusion was inhibited by salmon calcitonin (10(-8) M, for 2 h), and was not present after pretreating the bone slices with the aminobisphosphonate alendronate (ALN). At ALN levels of 0.22 nmol/mm2 bone, H+ extrusion was virtually absent 12 h after cell seeding (0.004 +/- 0.002 pH U/min) and approximately 50% inhibition was observed at 0.022 pmol ALN/mm2 bone. The Na(+)-independent H+ extrusion was not inhibited by bafilomycin A1 (up to 10(-7) M), although a bafilomycin A1 (10(-8) M)-sensitive H+ pump was present in membrane vesicles isolated from these osteoclasts. These findings indicate that Na(+)-independent acid extrusion is stimulated by osteoclast attachment to bone and is virtually absent when bone is preincubated with ALN, or when osteoclasts are treated with salmon calcitonin.

Osteoclast ATP receptor activation leads to a transient decrease in intracellular pH

Application of extracellular adenosine triphosphate (ATP) induces a pulsed decrease in osteoclast intracellular pH (pHi), as measured with seminaphthofluorescein (SNAFL)-calcein on a laser scanning confocal microscope. Adenosine diphosphate also produces a pHi decrease, but adenosine monophosphate, uridine triphosphate, 2-methylthio-ATP, and beta, gamma-methylene-ATP have little effect on pHi. The ATP-induced pHi decrease is largely inhibited by suramin, a P2 purinergic receptor blocker. Clamping intracellular free [Ca2+] ([Ca2+]i) with BAPTA/AM does not affect the ATP-induced pHi change, showing that this pHi decrease is not caused by the increased intracellular [Ca2+]i that is produced by activation of osteoclast purinergic receptors. We show that an increase in [Ca2+]i by itself will produce a pHi increase. The ATP effect is not blocked by inhibition of Na+/H+ exchange by either Na(+)-free bathing medium or amiloride. Two inhibitors of the osteoclast cell membrane proton pump, N-ethylmaleimide and vanadate, produce partial inhibition of the ATP-induced pHi decrease. Two other proton pump inhibitors, bafilomycin and N,N'-dicyclohexylcarbodiimide, have no influence on the ATP effect. None of the proton pump inhibitors but vanadate has a direct effect on pHi. Vanadate produces a transient pHi increase upon application to the bathing medium, possibly as a result of its known effect of stimulating the Na+/H+ exchanger. Inhibition of Cl-/HCO3- exchange by decreasing extracellular Cl- gives a pronounced long-term pHi increase, supporting the hypothesis that this exchange has an important role in osteoclast pHi homeostasis. In Cl(-)-free extracellular medium, there is a greatly reduced effect of extracellular ATP on pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Bafilomycin A1 in bone resorption and tooth eruption in dogs

Tooth eruption depends on bone resorption to form an eruption pathway. We have previously shown that a 2-wk local infusion of bafilomycin A1, an inhibitor of vacuolar H(+)-ATPases in osteoclasts, into the crypts of erupting mandibular premolars in dogs blocks bone resorption during this period and eruption of these teeth is delayed for 8 wk. Here we report the limits of inhibition of resorption that still permit eruption of these teeth. In 3 dogs 10(-6) M bafilomycin was delivered by osmotic minipumps early (18 wk) in eruption to the fourth premolar for 1, 3 or 4 wk. Radiographs taken at weekly intervals thereafter showed that bafilomycin delivery for 1 wk delayed eruption for 3 wk, delivery for 3 wk delayed eruption 9 wk and delivery for 4 wk prevented eruption. These data show that tooth eruption is delayed in direct proportion to the time resorption is blocked, and that this process for dog premolars cannot be blocked for more than 3 wk with 10(-6) M bafilomycin without blocking eruption itself.

Characterization of the osteoclast vacuolar H(+)-ATPase B-subunit

During bone resorption, osteoclasts acidify the extracellular bone resorbing compartment via a vacuolar H(+)-ATPase (V-ATPase), which resides in the ruffled-border membrane. In an effort to characterize the composition of the osteoclast V-ATPase catalytic domain, we have isolated a cDNA clone that encodes the V-ATPase B-subunit from a cDNA library constructed from highly purified chicken osteoclasts. Comparison of the predicted amino-acid sequence with the published sequences of isoforms of V-ATPase B-subunits from other sources revealed that the chicken osteoclast B-subunit is brain type and not kidney type. Furthermore, only clones encoding the brain type isoform of subunit B could be generated by PCR from a cDNA library prepared from human osteoclastoma osteoclast-like cells. Northern blot analysis revealed that two B-subunit mRNAs, approx. 1.7 and 3.5 kb in length, are expressed in chicken bone marrow mono-nuclear cells, brain and kidney, although the relative amounts of these two transcripts were different in each tissue. In brain, the 3.5-kb mRNA was predominantly expressed. In bone marrow cells, the levels of the 1.7-kb mRNA were higher than in other tissues and expression of this message was increased by 1,25-dihydroxyvitamin D-3, suggesting that this mRNA is specifically upregulated during osteoclast differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific immunocytochemical localization of cathepsin E at the ruffled border membrane of active osteoclasts

The immunocytochemical localization of cathepsin E, a non-lysosomal aspartic proteinase, was investigated in rat osteoclasts using the monospecific antibody to this protein. At the light-microscopic level, the preferential immunoreactivity for cathepsin E was found at high levels in active osteoclasts in the physiological bone modeling process. Neighboring osteoblastic cells were devoid of its immunoreactivity. At the electron-microscopic level, cathepsin E was exclusively confined to the apical plasma membrane at the ruffled border of active osteoclasts and the eroded bone surface. Cathepsin E was also concentrated in some endocytotic vacuoles of various sizes in the vicinity of the ruffled border membrane, some of which appeared to be secondary lysosomes containing the phagocytosed materials. These results strongly suggest that this enzyme is involved both in the extracellular degradation of the bone organic matrix and in the intracellular breakdown of the ingested substances in osteoclasts.

Alternative splicing generates a second isoform of the catalytic A subunit of the vacuolar H(+)-ATPase

We have identified a second isoform of the catalytic A subunit of the vacuolar H+ pump in chicken osteoclasts. In this isoform (A2) a 72-bp cassette replaces a 90-bp cassette present in the classical A1 isoform. The A1-specific cassette encodes a region of the protein that contains one of the three ATP-binding consensus sequences (the P-loop) identified in this polypeptide, as well as the pharmacologically relevant Cys254. In contrast, the A2-specific cassette does not contain any of these features. These two isoforms, which appear to be ubiquitously expressed, are encoded by a single gene and are generated by alternative splicing of two mutually exclusive exons. The alternative RNA processing involves the recognition of a single site, the boundary between the A2- and A1-specific exons, as either acceptor (in A1) or donor (in A2) splice site.

Localisation of mRNA for collagenase in osteocytic, bone surface and chondrocytic cells but not osteoclasts

Osteoclasts resorb the extracellular matrix of bone by secreting protons and enzymes into a circumpherentially sealed compartment between the osteoclast and the bone surface. Although the lysosomal cysteine proteinases play a major role in matrix degradation by osteoclasts, collagenase (matrix metalloproteinase-1, EC 3.4.24.7) is also required for osteoclastic bone resorption, and may be directly involved in collagen degradation in the hemivacuole. We assessed the effects of inhibitors of cysteine proteinases and collagenase on bone resorption by osteoclasts isolated from rodent bone. We found that while inhibition of cysteine proteinases strongly suppressed osteoclastic resorption, inhibitors of collagenase were without effect on the number, size, or demineralised fringe of excavations. We could find no evidence of expression of mRNA for collagenase in rat osteoclasts by in situ hybridisation, but found that it was expressed by chondrocytes, bone surface cells and osteocytes adjacent to osteoclasts. The distribution of these cells, and the correlation between increased collagenase production and increased stimulation of osteoclastic resorption in vitro by bone cells, suggests that these cells might be involved in the regulation of bone resorption in situ, and that collagenase production might play a role in this process.

An ultrastructural evaluation of the effects of cysteine-proteinase inhibitors on osteoclastic resorptive functions

This study was designed to evaluate the effects of specific and potent cathepsin inhibitors on osteoclastic resorptive functions in vitro by means of a novel ultrastructural assay system. Mouse bone marrow cell-derived osteoclasts were suspended on dentine slices and cultured for 48 hours in the presence of either E-64 (a generalized cysteine proteinase inhibitor) or Z-Phe-Phe-CHN2 (a selective cathepsin L inhibitor). After the removal of cultured osteoclasts, co-cultured dentine slices were examined using electron microscopy: backscattered (BSEM), scanning (SEM), and atomic force (AFM). In morphometric analyses of BSEM images, there were no significant differences in the areas of demineralized dentine surfaces between control and inhibitor-treated groups, suggesting that cathepsin inhibitors had no effect on dentine demineralization by cultured osteoclasts. However, in SEM and AFM observations, both inhibitors remarkably reduced to the same extent, the formation of deep resorption lacunae on dentine slices that had resulted from degradation of matrix collagen. In addition, Z-Phe-Phe-CHN2 treatment produced deeper, ring-like grooves with little collagen exposure in shallow resorption lacunae. These results strongly suggest that (1) cathepsins released by osteoclasts are involved in the formation of deep resorption lacunae, and (2) cathepsin L plays a key role in bone resorption.

Stimulative effects of lead on bone resorption in organ culture

To clarify whether hypercalcemia after injection of Pb to rats is due to biological bone resorption or physicochemical mineral dissolution, the effect of lead (Pb) on release of previously incorporated 45Ca in organ culture was investigated. Pb at 50 microM and above stimulated the release of 45Ca and hydroxyproline (Hyp). Pb did not stimulate 45Ca release from the bones inactivated by freezing and thawing. Eel calcitonin (ECT), bafilomycin A1 and scopadulcic acid B (SDB) inhibited Pb-stimulated 45Ca release. These results indicate that Pb-induced 45Ca release is due to osteoclastic bone resorption. Pb-stimulated bone resorption was inhibited by indomethacin and flurbiprofen. Pb stimulated the release of prostaglandin E2 (PGE2) from the bones into the media. There was significantly high correlation between 45Ca and PGE2 release. Pb-induced bone resorption was inferred to be mediated by PGE2. From these results, it was suggested that hypercalcemia after Pb injection might be caused by biological bone resorption.

Regulation of cytoplasmic pH in osteoclasts. Contribution of proton pumps and a proton-selective conductance

Osteoclasts resorb bone by secreting protons into an extracellular resorption zone through vacuolar-type proton pumps located in the ruffled border. The present study was undertaken to evaluate whether proton pumps also contribute to intracellular pH (pHi) regulation. Fluorescence imaging and photometry, and electrophysiological methods were used to characterize the mechanisms of pH regulation in isolated rabbit osteoclasts. The fluorescence of single osteoclasts cultured on glass coverslips and loaded with a pH-sensitive indicator was measured in nominally HCO(3-)-free solutions. When suspended in Na(+)-rich medium, the cells recovered from an acute acid load primarily by means of an amiloride-sensitive Na+/H+ antiporter. However, rapid recovery was also observed in Na(+)-free medium when K+ was used as the substitute. Bafilomycin-sensitive, vacuolar-type pumps were found to contribute marginally to pH regulation and no evidence was found for K+/H+ exchange. In contrast, pHi recovery in high K+ medium was largely attributed to a Zn(2+)-sensitive proton conductive pathway. The properties of this conductance were analyzed by patch-clamping osteoclasts in the whole-cell configuration. Depolarizing pulses induced a slowly developing outward current and a concomitant cytosolic alkalinization. Determination of the reversal potential during ion substitution experiments indicated that the current was due to H+ (equivalent) translocation across the membrane. The H+ current was greatly stimulated by reducing pHi, consistent with a homeostatic role of the conductive pathway during intracellular acidosis. These results suggest that vacuolar-type proton pumps contribute minimally to the recovery of cytoplasmic pH from intracellular acid loads. Instead, the data indicate the presence of a pH- and membrane potential-sensitive H+ conductance in the plasma membrane of osteoclasts. This conductance may contribute to translocation of charges and acid equivalents during bone resorption and/or generation of reactive oxygen intermediates by osteoclasts.

Role of Na-H exchangers and vacuolar H+ pumps in intracellular pH regulation in neonatal rat osteoclasts

Osteoclasts resorb bone by pumping of H+ into a compartment between the cell and the bone surface. Intracellular pH (pHi) homeostasis requires that this acid extrusion, mediated by a vacuolar-type H+ ATPase, be complemented by other acid-base transporters. We investigated acid-extrusion mechanisms of single, freshly isolated, neonatal rat osteoclasts. Cells adherent to glass coverslips were studied in the nominal absence of CO2/HCO3-, using the pH-sensitive dye BCECF and a digital imaging system. Initial pHi averaged 7.31 and was uniform throughout individual cells. Intrinsic buffering power (beta 1) decreased curvilinearly from approximately 25 mM at pHi = 6.4 to approximately 6.0 mM at pHi = 7.4. In all polygonally shaped osteoclasts, and approximately 60% of round osteoclasts (approximately 20% of total), pHi recovery from acid loads was mediated exclusively by Na-H exchange. In these pattern-1 cells, pHi recovery was 95% complete within 200 s, and was blocked by removing Na+, or by applying 1 mM amiloride, 50 microM ethylisopropylamiloride (EIPA), or 50 microM hexamethyleneamiloride (HMA). The apparent K1/2 for HMA ([Na+]o = 150 mM) was 49 nM, and the apparent K1/2 for Na+ was 45 mM. Na-H exchange, corrected for amiloride-insensitive fluxes, was half maximal at pHi 6.73, with an apparent Hill coefficient for intracellular H+ of 2.9. Maximal Na-H exchange averaged 741 microM/s. In the remaining approximately 40% of round osteoclasts (pattern-2 cells), pHi recovery from acid loads was brisk even in the absence of Na+ or presence of amiloride. This Na(+)-independent pHi recovery was blocked by 7-chloro-4-nitrobenz-2-oxa-1,3-diazol (NBD-Cl), a vacuolar-type H+ pump inhibitor. Corrected for NBD-Cl insensitive fluxes, H+ pump fluxes decreased approximately linearly from 96 at pHi 6.8 to 11 microM/s at pHi 7.45. In approximately 45% of pattern-2 cells, Na+ readdition elicited a further pHi recovery, suggesting that H+ pumps and Na-H exchangers can exist simultaneously. We conclude that, under the conditions of our study, most neonatal rat osteoclasts express Na-H exchangers that are probably of the ubiquitous basolateral subtype. Some cells express vacuolar-type H+ pumps in their plasma membrane, as do active osteoclasts in situ.

Altering tooth eruption by blocking bone resorption--the local delivery of bafilomycin A1

Tooth eruption is a complicated process requiring a coordination of bone resorption and bone formation by a variety of factors in and around the dental follicle proper and bone resorption is the rate-limiting step early in the process. We have recently described a method to deliver to the crypt of erupting dog premolars a reversible blocker of bone resorption, bafilomycin A1, and shown that its delivery for two week blocks bone resorption and eruption during this period without effect on adjacent teeth or on bone formation. In this study we show that delivery of 10(-6) M bafilomycin A1 via a cannulated osmotic minipump for two weeks early in the eruption of premolars delayed the eruption of these teeth for eight weeks. Similar delivery of the vehicle to the contralateral premolar had no effect on eruption. These data are the first clinical application of this potent drug and show that a short term local delivery is reversible and that blocking resorption for two weeks causes a fourfold delay in tooth eruption. Modifications of this approach may have clinical applications in dentistry.

A pharmacological assessment of the mammalian osteoclast vacuolar H(+)-ATPase

It is well established that osteoclasts use a vacuolar-type H(+)-ATPase (V-ATPase) for proton pumping during bone resorption and that specific V-ATPase inhibitors such as bafilomycin A1 abolish osteoclastic bone resorption in the bone slice assay. It has been reported that the V-ATPase in avian osteoclasts can be distinguished from the V-ATPase expressed in most other cells, by virtue of its inhibition by vanadate and nitrate ions. In order to determine whether the V-ATPase in mammalian osteoclasts can be similarly distinguished, we have investigated the effects of vanadate and nitrate on bone resorption by rat osteoclasts in the bone slice assay, in comparison with known V-ATPase inhibitors, bafilomycin A1 and WY 47766, that also inhibit the chicken osteoclast V-ATPase. The results indicate that, unlike the avian osteoclast V-ATPase, the mammalian osteoclast V-ATPase is pharmacologically similar to the V-ATPase in other cells.

Expression of vacuolar H(+)-ATPase in osteoclasts and its role in resorption

By means of light- and electron-microscopic immunocytochemistry, we have demonstrated the expression of vacuolar H(+)-ATPase in mouse osteoclasts. In fully differentiated osteoclasts, intense immunolabeling was observed along the plasma membranes including those of ruffled borders and associated pale vesicles and vacuoles, whereas those of clear zones and basolateral cell surfaces were entirely free of immunoreaction. Specific expression of vacuolar H(+)-ATPase was also detected over polyribosomes and cisterns of the rough-surfaced endoplasmic reticulum. Multinucleated osteoclastic cells were suspended on dentine slices and cultured for 48 h in the presence or absence of either concanamycin B or bafilomycin A1, specific inhibitors of vacuolar H(+)-ATPase. Morphometric analysis of co-cultured dentine slices with backscattered electron microscopy revealed that both inhibitors strongly reduced the formation of resorption lacunae in a dose-dependent manner. These results suggest that vacuolar H(+)-ATPase is produced in the rough-surfaced endoplasmic reticulum, stored in the membrane vesicles, and transported into the ruffled border membranes of osteoclasts, and that this enzyme plays a key role in the creation of an acidic subosteoclastic microenvironment for the demineralization of co-cultured substrates.

Bafilomycin A1 inhibits bone resorption and tooth eruption in vivo

It has been shown that a specific inhibitor of vacuolar H(+)-ATPases, bafilomycin A1, inhibits bone resorption by isolated chicken osteoclasts by blocking the proton pump in the ruffled border membrane. We report here the effects of bafilomycin A1 on bone resorption in vivo. Using a cannulated osmotic minipump delivery system, we infused bafilomycin locally to the eruption pathway of permanent premolars of beagle dogs. We used pit formation by osteoclasts in vitro to estimate the concentrations and heat stability of bafilomycin to be used in vivo. In this model, osteoclasts were cultured on thin bone slices, in which they form pits indicative of resorption. After 2 weeks preincubation at 37 degrees C, bafilomycin concentrations of 10(-6) and 10(-7) M but not 10(-8) M completely inhibited the resorptive activity of cultured osteoclasts, and the two larger doses were chosen for use in vivo. Local delivery of 10(-6) M bafilomycin to the eruption pathway of the fourth permanent mandibular premolar during mideruption inhibited tooth eruption by blocking bone resorption as assayed by radiography, light microscopy, and scanning electron microscopy. Bafilomycin at 10(-7) M had similar but less intensive effects. Moreover, osteoclasts in the alveolar bone of crypts treated with 10(-7) M bafilomycin A1 stained very weakly for tartrate-resistant acid phosphatase. The effect of bafilomycin on bone resorption was shown to be very local, and no side effects of treatment with bafilomycin were observed in adjacent teeth or the behavior of dogs. We report here, for the first time, inhibition of tooth eruption caused by inhibited bone resorption using bafilomycin A1 in vivo.

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

Heterogeneity of vacuolar H(+)-ATPase: differential expression of two human subunit B isoforms

The catalytic domain of the vacuolar proton ATPase is composed of a hexamer of three A subunits and three B subunits. Here we describe the cloning and characterization of a cDNA isoform of subunit B, HO57, from an osteoclastoma cDNA library. HO57 is represented by three species of mRNA of 1.6, 2.6 and 2.8 kb and is expressed at low levels in a range of human tissues, but at significantly higher levels in brain, kidney and osteoclastoma, and is probably an ubiquitously expressed isoform. In contrast, the kidney-specific isoform has an mRNA of 2 kb and is specifically expressed at high levels only in kidney and, at a lower level, in placenta. Thus the HO57 isoform is integral to the vacuolar ATPase found in the general secretory system of all cells as well as in vacuolar-ATPase-rich sources such as neurones and osteoclasts, whereas both the kidney-specific isoform and HO57 are highly expressed in the kidney. Furthermore, we show by in situ hybridization that HO57 is the only isoform that is exclusively and highly expressed by osteoclasts.

Calmodulin concentrated at the osteoclast ruffled border modulates acid secretion

Osteoclasts mediate acid dissolution of bone for maintenance of serum [Ca2+] and for replacement of old bone in terrestrial vertebrates. Recent findings point to the importance of intracellular signals, particularly Ca2+, in osteoclast regulation. However, acid degradation of bone mineral subjects the osteoclast to uniquely high extracellular [Ca2+]. We hypothesized that this high calcium environment would affect calcium signalling mechanisms, and studied the calcium binding regulatory protein, calmodulin, in the osteoclast. Avian osteoclast bone resorption was inhibited 30% at 1 microM and 90% at 7 microM by the calmodulin antagonist trifluoperazine. Osteoclast bone attachment was not affected by 10 microM trifluoperazine. Quantitative immunofluorescence using fluorescein-labelled calmodulin monoclonal antibody showed a severalfold increase of calmodulin concentration in bone attached relative to plastic attached osteoclasts. Western blots confirmed this, showing two to threefold increased osteoclast calmodulin per milligram of cell protein in 3-day bone-attached vs. nonattached cells. Scanning confocal microscopy showed calmodulin polarization to areas of bone attachment. Electron micrographs with 9 nm colloidal gold labelling showed calmodulin in the acid secreting ruffled membrane. ATP-dependent acid transport in osteoclast membrane vesicles was inhibited by the calmodulin antagonist calmidazolium. This effect was reversed by addition of excess calmodulin, showing that the inhibition is specific. Vesicle acid transport inhibition reflects an approximately fourfold shift in the apparent Km for ATP of vesicular acid transport in the presence of the calmodulin antagonist. We conclude that calmodulin concentration and distribution is modified by bone attachment, and that osteoclastic acid secretion is calmodulin regulated.

Localization of H(+)-ATPase and carbonic anhydrase II in ameloblasts at maturation

The localization of vacuolar-type H(+)-ATPase and carbonic anhydrase II (CA II) in rat incisor enamel organs at maturation was examined by light and electron microscopy. The immunoreactivity for both vacuolar-type H(+)-ATPase and CA II was intense on the ruffled border of ruffle-ended ameloblasts (RA), but moderate at the distal end of smooth-ended ameloblasts (SA). Immuno-gold particles indicated that CA II was not confined to the ruffled border of RA alone, but also distributed in the cytoplasm of RA and SA. These findings suggest that RA may secrete protons produced by CA II via the ruffled border into enamel by active transport of vacuolar-type H(+)-ATPase. Secreted protons may activate hydrolytic enzymes to degrade the organic components of enamel matrix. Vacuolar-type H(+)-ATPase on vesicles of SA suggests that a specific configuration of ruffled borders in RA may be formed by the fusion of vesicle membranes in the distal end of cytoplasm of SA.

Inhibition of bone resorption in vitro by antisense RNA and DNA molecules targeted against carbonic anhydrase II or two subunits of vacuolar H(+)-ATPase

The bone resorbing cells, osteoclasts, express high levels of carbonic anhydrase II (CA II) and vacuolar H(+)-ATPase (V-ATPase) during bone resorption. We have used antisense RNA and DNA molecules targeted against CA II, and against 16- and 60-kD subunits of vacuolar H(+)-ATPase (V-ATPase), to block the expression of these proteins in vitro. Osteoclastic bone resorption was studied in two in vitro culture systems: release of 45Calcium from prelabeled newborn mouse calvaria cultures, and resorption pit assays performed with rat osteoclasts cultured on bovine bone slices. Both antisense RNA and DNA against CA II and the V-ATPase were used to compare their specificities as regards inhibiting bone resorption in vitro. The antisense molecules inhibited the synthesis of these proteins by decreasing the amounts of mRNA in the cells in a highly specific manner. In osteoclast cultures treated with the 16-kD V-ATPase antisense RNA, acidification of an unknown population of intracellular vesicles was highly stimulated. The acidification of these vesicles was not sensitive to amiloride or bafilomycin A1. This suggests the existence of a back-up system for acidification of intracellular vesicles, when the expression of the V-ATPase is blocked. Our results further indicate that blocking the expression of CA II and V-ATPase with antisense RNA or DNA leads to decreased bone resorption.

A novel monoclonal antibody recognizing a unique antigen of rat osteoclasts induced by the calcified matrices

The morphology of osteoclasts, primary cells that resorb bone, is well documented; however, the precise details of their terminal differentiation remains obscure. To date, the only morphological criterion for identifying activated functional osteoclasts has been the presence of ruffled borders. We have developed a rat bone marrow culture system in which osteoclast-like cells formed. These cells fulfilled most of the criteria of osteoclasts, and when they were reseeded on calcified tissue, formed numerous resorption lacunae in vitro. To find an immunological marker for functional osteoclasts, we have used these cells in a functional state as antigens for the preparation of monoclonal antibodies (mAb) that reacted with rat osteoclasts; we obtained mAb Ch1 and Ch2. Interestingly, these mAbs reacted with the marginal portion of authentic osteoclasts, where they attached to the bone surface on frozen sections. The reactivity of Ch1 to rat osteoclasts was more restricted than that of Ch2: Ch1 reacted with few tartrate-resistant acid phosphatase (TRAP)-positive cells on a culture plate. These TRAP-positive cells (including mono- and multinucleated cells) were, however, converted to Ch1-positive cells when they were reseeded on calcified tissues. These findings suggested that the antigen recognized by the Ch1 antibody was induced by some factors of matrix proteins released from calcified tissues.

Osteoclast recruiting activity in bone matrix

An activity that recruits osteoclasts has been identified and partially characterized from bone matrix. Bone-derived osteoclast recruiting activity (BORA) was co-purified with osteogenin, a bone inductive protein. Osteogenin was extracted from bovine bone with 6 M urea and purified by chromatography on hydroxyapatite, heparin-Sepharose and Sephacryl S-200 gel filtration. The biologically active osteoclast formation-stimulating material was further purified by C18 reverse phase HPLC. BORA is obviously distinct from osteogenin and transforming growth factor beta (TGF-beta), since further purified osteogenin and pure TGF-beta did not stimulate the formation of osteoclast-like cells. BORA (0.1-10 micrograms/ml) stimulated the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNC) in a dose-dependent manner. These multinucleated cells resorbed bone when cultured on bovine bone slices. The effect of BORA is primarily directed to differentiate osteoclast precursors, since it did not stimulate osteoclast function in in vitro resorption assay where disaggregated rat osteoclasts were cultured on bovine bone slices. However, after 24 h preincubation with 50 nM PTH in the mouse calvaria assay, BORA at 10 micrograms/ml significantly stimulated bone resorption.

Effects of extracellular calcium and protons on osteoclast potassium currents

During resorption of mineralized tissues, osteoclasts are exposed to marked changes in the concentration of extracellular Ca2+ and H+. We examined the effects of these cations on two types of K+ currents previously described in these cells. Whole-cell patch clamp recordings of membrane currents were made from osteoclasts freshly isolated from neonatal rats. In control saline (1 mM Ca2+, pH 7.4), the voltage-gated, outwardly rectifying K+ current activates at approximately -45 mV and the conductance is half-maximally activated at -29 mV (V0.5). Increasing [Ca2+]out rapidly and reversibly shifted the current-voltage (I-V) relation to more positive potentials. Current at -29 mV decreased to 28 and 9% of control current at 5 and 10 mM [Ca2+]out, respectively. This effect of elevating [Ca2+]out was due to a positive shift of the K+ channel voltage activation range. Zn2+ or Ni2+ (5 to 500 microM) also shifted the I-V relation to more positive potentials and had additional effects consistent with blockade of the K+ channel. Based on the extent to which these divalent cations affected the voltage activation range of the outwardly rectifying K+ current, the potency sequence was Zn2+ > Ni2+ > Ca2+. Lowering or raising extracellular pH also caused shifts of the voltage activation range to more positive or negative potentials, respectively. In contrast to their effects on the outwardly rectifying K+ current, changes in the concentration of extracellular H+ or Ca2+ did not shift the voltage activation range of the inwardly rectifying K+ current.(ABSTRACT TRUNCATED AT 250 WORDS)

Outwardly rectifying chloride current in rabbit osteoclasts is activated by hyposmotic stimulation

1. We characterized chloride currents in freshly isolated rabbit osteoclasts using whole-cell and single channel patch-clamp recording configurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying current persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- transport blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS); 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chloride equilibrium potential and was dependent on the chloride concentration gradient. 3. In those osteoclasts in which outwardly rectifying current was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current was due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca(2+)-free solutions containing 0.2 mM EGTA. 4. When studied in the current-clamp configuration, hyposmotic stimulation caused depolarization from -76 +/- 5 to -5 +/- 6 mV (mean +/- S.D., n = 7). 5. Unitary Cl- currents were recorded in the cell-attached patch configuration at positive potentials. Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Reduction of the external [Cl-] shifted the current-voltage relationship in the positive direction, supporting the conclusion that these were Cl- currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying Cl- current that can be activated by osmotic stress. Cl- channels may play a role in cell volume regulation and may also provide conductive pathways for dissipating the potential difference that arises from electrogenic proton transport during bone resorption.

Inhibitors of vacuolar-type H(+)-ATPase suppresses proliferation of cultured cells

We investigated effects of bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase (V-ATPase), and its analogues on proliferation of various cultured cells. The proliferation of the various cell lines was suppressed by adding bafilomycin A1 to the culture medium. This inhibitory effect appeared at a concentration of nanomolar order and was dose dependent. Although the suppression was reversible, the drug exerted not only suppression of the proliferation but also death to some cell lines. Drug concentration required for 50% inhibition of the cell proliferation during 48 h differed markedly depending on cell species and the sensitivity appears to increase by the transformation of the cells. Two derivatives of concanamycin A, an analogue of bafilomycin A1, also inhibited strongly V-ATPase in vitro and in vivo, and simultaneously cell proliferation. Two concanamycin A derivatives which have lost inhibitory effect on V-ATPase lost inhibitory effect on cell proliferation as well. These results suggest that V-ATPase is involved in the machinery maintaining the cell proliferation.

Immediate cell signal by bone-related peptides in human osteoclast-like cells

We tested whether recognition of bone-related peptides regulates intracellular Ca2+ concentration ([Ca2+]i) of giant cell tumor of bone (GCT). [Ca2+]i was measured in single cells by fura 2 fluorometry. GCT cells were sensitive to bone sialoprotein-II (BSP-II), osteopontin (OPN), and related fragments. Responses consisted of a prompt increase of [Ca2+]i, mostly transient, with a peak followed by a rapid return toward baseline. Responses were not mimicked by bovine plasma fibronectin. Sensitivity of GCT cells to bone peptides was specific, since BALB/3T3 fibroblasts and U-937 histiocytic lymphoma cells with monocytic phenotype failed to respond to BSP-II and OPN fragments. GRGDSP synthetic esapeptide, carrying the Arg-Gly-Asp adhesive motif, and GRGESP (Asp replaced by Glu), but not the GRADSP (Gly replaced by Ala), were active in inducing [Ca2+]i transients as well. Responses were observed also in cells treated with the BSP-II 1C fragment, lacking any known adhesive sequence, indicating that the active peptides inducing [Ca2+]i increments may be multiple. Sensitivity to extracellular matrix peptides was present in a variable fraction of the cells and was downregulated on long-term culture. The mechanism inducing [Ca2+]i elevations was mostly related to Ca2+ release from thapsigargin-sensitive intracellular pools.

Vacuolar-type H(+)-ATPases are functionally expressed in plasma membranes of human tumor cells

Mammalian cells generally regulate their intracellular pH (pHi) via collaboration between Na(+)-H+ exchanger and HCO3- transport. In addition, a number of normal mammalian cells have been identified that express H(+)-adenosinetriphosphatases (ATPases) in their plasma membranes. Because tumor cells often maintain a high pHi, we hypothesized that they might functionally express H(+)-ATPases in their plasma membranes. In the first phase of the present study, we screened 19 normal and tumorigenic human cell lines for the presence of plasmalemmal H(+)-ATPase activity using bafilomycin A1 to inhibit V-type H(+)-ATPase and Sch-28080 to inhibit P-type H(+)-K(+)-ATPase. Bafilomycin A1 decreased pHi in the six tumor cell lines with the highest resting pHi in the absence of HCO3-. Sch-28080 did not affect pHi in any of the human cells. Simultaneous measurement of pH in the cytoplasm and in the endosomes/lysosomes localized the activity of bafilomycin to the plasma membrane in three cell lines. In the second phase of this study, these three cell lines were shown to recover from NH4(+)-induced acid loads in the absence of Na+. This recovery was inhibited by N-ethylmaleimide, bafilomycin A1, and ATP depletion and was not significantly affected by vanadate, Sch-28080, or hexamethyl amiloride. These results indicate that a vacuolar type H(+)-ATPase is expressed in the plasma membrane of some tumor cells.

New resorption assay with mouse osteoclast-like multinucleated cells formed in vitro

We previously reported a procedure to obtain a preparation containing a large number of mouse osteoclast (OCL)-like multinucleated cells (MNCs) formed in cocultures of mouse osteoblastic and bone marrow cells in the presence of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3]. The MNCs satisfied major criteria of OCLs, such as tartrate-resistant acid phosphatase (TRAP) activity, acid production, calcitonin (CT) receptors, and the ability to form resorption pits on bone slices. In this report, we describe a simple resorption assay system using MNC preparations. After culturing MNC preparations or disaggregated rat OCL preparations on dentin slices, they were stained with Mayer's hematoxylin. The stained area corresponded exactly with the resorption pits visualized by scanning electron microscopy and were measured using an image analysis system attached to a light microscope. Pit formation by MNCs was gradually enhanced by reducing the medium pH (pH 7.5 < 7.2 < 6.9). The plan area resorbed by MNCs increased linearly for up to 72 h. These results are very similar to those obtained with OCL preparations. In multiple standard assays with MNC preparations, more than 250 MNCs could be placed on a dentin slice, and the total area resorbed to a level of up to 9% of the whole surface within 48 h. In contrast, in multiple assays with OCL preparations, it was not easy to place more than 50 OCLs on a slice and the resorbed area was only 0.7% of the surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of bone turnover

Bone remodelling is a cellular mechanism behind the bone turnover. It renews the old bone piece by piece and thus ensures the correction of possible microdamage and enables the regulation of mineral homeostasis. The basic mechanism of bone remodelling is similar in all types of bone and includes the resorption of old bone and the formation of equal amount of new bone at the same place. Histomorphometric studies have revealed the cellular details of remodelling and have shown that it is composed by the temporally and spatially regulated action of different bone cells and their precursors. Recent in vitro studies with osteoclasts and osteoblasts have increased our knowledge of the molecular mechanisms of bone remodelling. Molecular characterization of bone matrix proteins have suggested new functions to many of them and thereby increased our possibilities of understanding the local regulation mechanisms of remodelling. Bone matrix has been shown to contain several biologically active compounds which have effects on bone forming and resorbing cells and their precursors. Details of the functional mechanism of osteoclasts are also in the process of being discovered. However, several questions concerning bone remodelling still remain open: the molecular explanation for selection of the remodelling site; the coupling of bone resorption to formation, and the interactions between different cell types during the remodelling cycle.

Increased synthesis and specific localization of a major lysosomal membrane sialoglycoprotein (LGP107) at the ruffled border membrane of active osteoclasts

The immunocytochemical localization was investigated of a major lysosomal membrane sialoglycoprotein with a molecular mass of 107 kDa, which was designated as LGP107. The study utilized rat osteoclasts with different bone resorbing activity and osteoclast precursors at various stages of differentiation and maturation together with monospecific antibodies to this protein. Despite its localization primarily in lysosomes and endosomes in the other cell types examined, LGP107 was exclusively confined to the apical plasma membrane at the ruffled border of the active osteoclast, where the osteoclast is in contact with the bone surface. The protein was also concentrated in a number of endocytic vacuoles in the vicinity of the ruffled border membrane. However the labeling was not found in the basolateral membranes of the active osteoclast. The ruffled border membrane detached from the bone surface showed a marked decrease in the extent of the immunolabeling. The post- and/or resting osteoclasts, which were located away from the bone surface, were totally devoid of the membraneous localization of LGP107. No definite immunolabeling was found in the immature preosteoclasts. These results indicate that the protein is largely synthesized in the active osteoclast and rapidly translocated to the ruffled border membrane by vectorial vesicle transport. LGP107 is suggested to contribute to the formation and maintenance of the specialized acidic environment for bone resorption.

Effect of omeprazole, an inhibitor of H+,K(+)-ATPase, on bone resorption in humans

Omeprazole is an inhibitor of gastric H+,K(+)-ATPase. Although the major proton transport of osteoclast is mediated by a vacuolar-type H(+)-ATPase which is different from the gastric H+,K(+)-ATPase, in vitro studies have demonstrated that omeprazole inhibits bone resorption. In this study, the effect of omeprazole on bone resorption was evaluated in patients who had a history of gastric ulcer and were treated with maintenance doses of H2 blocker without any gastric complaints at the study time. H2-blocker administration was changed to omeprazole treatment in the study group and to no treatment in the control group. Urinary excretion of hydroxyproline and calcium decreased after omeprazole treatment in the study group. Serum intact PTH, alkaline phosphatase, osteocalcin, and tartrate-resistant acid phosphatase (TRAP) increased in this group. In the control group, there were not any changes in these parameters. The discrepancy between serum TRAP and urinary excretion of hydroxyproline and calcium in the study group was thought to be due to the suppression of bone resorption by omeprazole, which probably interfered the acidification at resorption lacunae and resulted in the inactivation of TRAP and other lysosomal enzymes. The results of our study suggest the possibility that the specific inhibitors of the osteoclastic proton pump (such as bafilomycins) will more effectively suppress bone resorption and be useful for the treatment of metabolic bone diseases with increased bone resorption.

A Ca(2+)-dependent K(+)-channel in freshly isolated and cultured chick osteoclasts

Calcium-activated potassium channels were found in embryonic chick osteoclasts using the patch-clamp technique. The activity of the channel was increased by both membrane depolarisation and an increase in intracellular Ca2+ concentration in the range 10(-5) to 10(-3) M. In the cell-attached-patch configuration the channel was only active at extreme depolarising potentials. Ca2+ addition to the cytoplasm via ionomycin increased channel activity at the resting membrane potential of the osteoclast. The channel had a single-channel conductance of 150 pS in the inside-out patch under symmetrical K+ conditions (150 mM) and was selective for potassium ions. During sustained application of increased [Ca2+] at the cytoplasmic side of inside-out patches, channel activity sometimes decreased again after the initial increases (desensitization). The results established the properties of the single channels underlying an outward rectifying K+ conductance in chick osteoclasts described previously by us.

Spatial organization of microfilaments and vitronectin receptor, alpha v beta 3, in osteoclasts. A study using confocal laser scanning microscopy

The primary function of the osteoclast is that of the major cell mediating bone resorption. They are actively migrating cells but during resorption they polarize to form a specialized tight attachment structure, the sealing zone, adjacent to the mineralized bone matrix. The processes of adhesion to, and migration on, bone involves cell adhesion molecules, integrins, interacting with their ligands in bone. We have used confocal microscopy to analyse, in rat osteoclasts cultured on bone and glass substrata, the distribution of vitronectin receptor, the major integrin of osteoclasts, and cytoskeletal proteins that it may be linked to. Double staining for F-actin and vinculin, and for vinculin with talin, revealed that cytoskeletal organization differs at various activation states of osteoclasts. Microfilament structures were flat, of 1.5 microns size, and concentrated near the bone surface. The vitronectin receptor was localized both in the basolateral membrane (away from the bone surface) and in the ruffled border (adjacent to bone) in osteoclasts cultured on bone, but was detected mainly in the basolateral membrane when cultured on glass. The vitronectin receptor appeared to be condensed on small microvilli-like projections on the basolateral membrane of osteoclasts on either bone or glass and may provide a route for alternative signalling pathways to modify osteoclast behaviour, other than by influencing cell adhesion directly. The leading edges of migrating osteoclasts, and the attachment structure, a broad vinculin band, which forms before bone resorption, also expressed vitronectin receptor, particularly when the antibody against the alpha v subunit was used.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of proton transport in bone-derived membrane vesicles

ATP-dependent proton transport in membrane vesicles prepared from the medullary bone of egg-laying hens, a source rich in osteoclasts, was characterized. Proton transport was abolished by bafilomycin A1 (10 nM) and N-ethylmalemide (50 microM), but not by oligomycin (15 micrograms/ml), vanadate (100 microM) or SCH 28080 (100 microM), thereby differentiating this H(+)-ATPase from the F1F0- and phosphorylated-type of ATPases. Preincubation of the membrane vesicles at 0 degrees C for 1 h in the presence of KCl (0.3 M) and Mg-ATP (5 mM) resulted in almost complete loss of H(+)-transport activity (cold-inactivation). Preventing the formation of a membrane potential by voltage clamp (Kin+ = Kout+ + valinomycin) increased both the rate of H(+)-transport and the equilibrium delta pH, suggesting an electronic proton transport mechanism. Thus, the H(+)-ATPase in this bone-derived membrane vesicle preparation shows the characteristics of a vacuolar H(+)-ATPase in its inhibitor- and cold-sensitivity and its electrogenic mechanism. The anion sensitivity of the H(+)-ATPase was investigated by varying the intra- and/or extra-vesicular salt composition. The H(+)-ATPase had no absolute requirement for any specific anion, but membrane permeable anions were found to stimulate proton transport activity, presumably by acting as charge compensators for the electrogenic hydrogen ion transport. However, some anions, such as sulfate, acetate and nitrate were directly inhibitory to the ATPase. The results are in agreement with the recently proposed mechanism of osteoclast acidification: a vacuolar H(+)-ATPase working in parallel with a Cl(-)-channel resulting in electroneutral HCl secretion.

Cysteine-proteinase localization in osteoclasts: an immunocytochemical study

Cysteine-proteinases such as cathepsin B and G were localized in rat osteoclasts, by an indirect protein A-immunogold labeling technique, on post-embedded ultrathin sections. In osteoclasts, specific immunogold labeling of both anti-cathepsin B and G was localized in Golgi vesicles, lysosomes, pale vacuoles of various sizes, and the extracellular canals of ruffled borders; no immunoreactivity was seen in the cytoplasmic matrix, mitochondria, cisterns of the rough endoplasmic reticulum, or nuclei. The presence of immunolabeling of cathepsins in osteoclasts and in the subosteoclastic compartment suggests that these enzymes are involved in the extracellular degradation of collagen and other non-collagenous bone matrix proteins.