Human Osteoclastoma Research Articles

Clinicopathological Significance of Decreased Expression of the Tumor Inhibitor Gene PDCD5 in Osteoclastoma.

Background: The gene programmed cell death 5 (PDCD5) has recently been characterized as a tumor suppressor gene and is believed to be an important prognostic cancer marker; it is frequently involved in neoplastic transformation and apoptosis of tumor cells. Several studies have demonstrated a decrease or loss of expression of PDCD5 in certain tumors. However, the relevance of PDCD5 expression in human osteoclastoma and its clinicopathological significance have not been extensively studied. Methods: The aim of this study was to explore the relative transcriptional and translational expression levels of PDCD5 in 79 osteoclastoma samples using multi-modal methods of analysis. Results: Our findings showed that 52% (15/29) of osteoclastoma cases exhibited reduced PDCD5 expression at the transcriptional level, and 56% (44/79) exhibited lower PDCD5 expression at the protein level, when compared with nontumor tissue. In addition, the statistical significance of the altered PDCD5 protein expression was examined using the Campanacci grading system for osteoclastoma. More importantly, the decreased expression at the translational level was observed to have a negative association with the Ki-67 staining index. Conclusion: Based on these findings, abnormal PDCD5 expression might be an important biomarker in human osteoclastoma and may contribute to tumor progression and malignant cell proliferation.

Simultaneous Sensitive Determination of Eight Collagen-Related Bone Turnover Biomarkers in Diverse Clinical Samples by HPLC with Fluorescent Derivatization

The suitability of HPLC with fluorescent derivatization for simultaneous determination of eight collagen-related bone turnover biomarkers, 4-hydroxy-l-proline (OHP), pyridinoline (PYD), hydroxylysine (Hyl), deoxypyridinoline, and their glycosides, galactosyl-Pyd (Gal-Pyd, GP), galactosyl-glucosyl-Pyd, β-1-galactosyl-O-hydroxylysine (Gal-Hyl, GH), and α-1,2-glucosyl-galactosyl-O-hydroxylysine (GGH), in diverse clinical samples was investigated. 5-Carboxyfluorescein succinimidyl ester (CFSE) was chosen as the fluorescent labeling reagent. Various parameters affecting derivatization and separation were optimized. Under optimal conditions, maximum derivatization could be achieved in 25 min at 30 °C, and complete baseline separation could be completed within 25 min, with relative standard deviation values for corrected peak areas less than 3.5 % for intraday assay and 4.2 % for interday assay, respectively. The limits of detection were determined to be 0.1–1.7 nM, which are well below the expected concentrations in the real samples. The method has been used for the quantitative determination of eight target analytes in various clinical samples, including cell cultures of normal human osteoclast, preosteoblast, osteoclastoma cells and serum and urine fluids from patients with fracture, osteitis fibrosa, or osteoclastoma as well as healthy subjects, with spiked recoveries of 96–103 %. This original application of HPLC–fluorescence detection represents a powerful tool for investigating the dynamics of metabolic imbalance of these biomarkers for bone turnover under physiological and pathophysiological conditions.

Specificity of RGS10A as a key component in the RANKL signaling mechanism for osteoclast differentiation

Significant progress has been made in studies of the mechanisms by which RANKL induces terminal osteoclast differentiation. However, many crucial details in the RANKL-evoked signaling pathway for osteoclast differentiation remain to be defined. We characterized genes specifically expressed in osteoclasts by differential screening of a human osteoclastoma cDNA library, and found that the regulator of G-protein signaling 10A (RGS10A), but not the RGS10B isoform, was specifically expressed in human osteoclasts. The expression of RGS10A is also induced by RANKL in osteoclast precursors and is prominently expressed in mouse osteoclast-like cells. RGS10A silencing by RNA interference blocked intracellular [Ca2+]i oscillations, the expression of NFAT2, and osteoclast terminal differentiation in both bone marrow cells and osteoclast precursor cell lines. Reintroduction of RGS10A rescued the impaired osteoclast differentiation. RGS10A silencing also resulted in premature osteoclast apoptosis. RGS10A silencing affected the RANKL-[Ca2+]i oscillation-NFAT2 signaling pathway but not other RANKL-induced responses. Our data demonstrate that target components of RGS10A are distinct from those of RGS12 in the RANKL signaling mechanism. Our results thus show the specificity of RGS10A as a key component in the RANKL-evoked signaling pathway for osteoclast differentiation, which may present a promising target for therapeutic intervention.

ADAM gene expression and regulation during human osteoclast formation

In this study, we identified the expression and the regulation of ADAM members (a disintegrin and metalloprotease) at both gene and protein levels during human osteoclast differentiation and activity. Human peripheral blood monocytes (HPBMC) treated with M-CSF and RANKL were used as an in vitro fusion model. In parallel, we used human osteoclastoma (OCL) tumor as a source of mature osteoclasts, and human osteoblastic cells as a control representing nonfusing and non-resorbing bone cells. RT-PCR using ADAM-specific primers enabled us to identify the expression of ADAM 8, 9, 10, 15, 17, and 28 in both osteoclasts and osteoblasts. Using primers specific for each ADAM 12 isoform (L and S), we observed a strong signal for both forms (ADAM 12L and ADAM 12S) in osteoblastic cells, while only ADAM 12S was detectable in HPBMC-derived osteoclasts and osteoclastoma. Gene regulation was studied using real-time PCR analysis performed during HPBMC differentiation; this showed a progressive increase of ADAM 12 mRNA level from day 1 to 8 of the culture, while at around day 9, ADAM 12 mRNA level decreased 2-fold. We also showed that ADAM 8, ADAM 17, and ADAM 28 decreased according to the stage of HPBMC differentiation or fusion. ADAM 10 was unaltered during cell fusion. However, confocal immunolocalization showed that ADAM 10 protein re-localized from the nuclei and cytoplasm to the plasma membrane during culture and to the ruffled border in resorbing cells. The same re-localization process was observed using an ADAM 12S-specific antibody during HPBMC differentiation. Between days 12 and 14, ADAM 12 co-localized with the F-actin ring, and at day 15, a strong signal was also present in ruffled border or sealing zone area of osteoclasts. Our results describe the expression and regulation of various ADAMs in human bone cells and the selective expression of ADAM 12L in osteoblasts. Our gene regulation and protein localization studies suggest a function for ADAM 10 and ADAM 12S in the formation of osteoclasts from HPBMC and resorption activity.

Expression of P2 receptors in bone and cultured bone cells

Extracellular nucleotides acting through P2 receptors elicit a wide range of responses in many cell types. There is increasing evidence that adenosine triphosphate (ATP) may function as an important local messenger in bone and cartilage. In this study, we used immunocytochemistry, employing novel polyclonal antibodies against P2X 1–7 receptors, and in situ hybridization, using oligonucleotide probes corresponding to P2X 2,4 and P2Y 2,4 messenger RNAs (mRNAs), to localize P2 receptors on undecalcified bone sections and on cultured osteoblasts and osteoclasts. We provide the first direct evidence that the P2X 2 receptor subtype is expressed on osteoclasts, osteoblasts, and chondrocytes. We also obtained evidence for the expression of P2X 5 and P2Y 2 receptors on osteoblasts and chondrocytes, and for P2X 4 and P2X 7 receptors on osteoclasts. Our results confirm earlier reports of P2Y 2 and P2X 4 expression in human osteoclastoma and rabbit osteoclasts, respectively, and are consistent with ATP responses observed on bone cells using electrophysiological techniques. Our novel finding that P2X 2 is expressed by osteoclasts is of particular interest. P2X 2 is the only P2 receptor subtype that requires extracellular acidification to show its full sensitivity to ATP, and our recent functional studies have shown that the stimulatory action of ATP on resorption pit formation by mature osteoclasts is amplified greatly at low pH. These findings point to fundamental new mechanisms for the local modulation of bone resorption.

A CD36-Binding Peptide from Thrombospondin-1 Can Stimulate Resorption by Osteoclasts in Vitro

Thrombospondin-1 (TSP-1), purified from platelets, stimulates resorption by avian osteoclasts in an in vitro resorption assay. TSP-1 binds to a number of different cellular receptors via different domains of the molecule and several short receptor-binding sequences have been identified within the TSP-1 molecule. In this study, we have used synthetic peptides representing these various sequences in order to identify the cellular receptor and TSP domain responsible for stimulation of resorption. We show that one peptide CSVTCG, which represents the CD36-binding region of TSP-1, stimulates resorption in a fashion similar to the intact molecule, while the peptides RGDS, RFYVVMWK, and RFYVVM, representing other cell-binding domains of TSP, have no effect on resorption. Using RT-PCR and immunoblotting, we further demonstrate expression of CD36 in human osteoclastoma (giant cell tumour), primary human bone derived cells, and clonal osteoblastic cells. These studies suggest that CD36 is involved in regulation of resorption by osteoclasts and is the receptor responsible for the resorption-promoting effects of TSP-1.

CKbeta-8 [CCL23], a novel CC chemokine, is chemotactic for human osteoclast precursors and is expressed in bone tissues.

We have previously demonstrated that a tartrate-resistant acid phosphatase (TRAP)-positive subpopulation of mononuclear cells isolated from collagenase digests of human osteoclastoma tissue exhibits an osteoclast phenotype and can be induced to resorb bone. Using these osteoclast precursors as a model system, we have assessed the chemotactic potential of 16 chemokines. Three CC chemokines, the recently described CKbeta-8, RANTES, and MIP-1alpha elicited significant chemotactic responses. In contrast, 10 other CC chemokines (MIP-1beta, MCP-1, MCP-2, MCP-3, MCP-4, HCC-1, eotaxin-2, PARC, SLC, ELC) and 3 CXC chemokines (IL-8, GROalpha, SDF-1) were inactive. None of these chemokines showed any chemotactic activity for either primary osteoblasts derived from human bone explants or the osteoblastic MG-63 cell line. The identity of the osteoclast receptor that mediates the chemotactic response remains to be established. However, all three active chemokines have been reported to bind to CCR1 and cross-desensitization studies demonstrate that RANTES and MIP-1alpha can partially inhibit the chemotactic response elicited by CKbeta-8. CKbeta-8, the most potent of the active CC chemokines (EC(max) 0.1-0.3 nM), was further characterized with regard to expression in human bone and cartilage. Although expression is not restricted to these tissues, CKbeta-8 mRNA was shown to be highly expressed in osteoblasts and chondrocytes in human fetal bone by in situ hybridization. In addition, CKbeta-8 protein was shown to be present in human osteophytic tissue by immunolocalization. These observations suggest that CKbeta-8, and perhaps other chemokines, may play a role in the recruitment of osteoclast precursors to sites of bone resorption.

Expression of Members of the Thrombospondin Family by Human Skeletal Tissues and Cultured Cells

We have previously shown that the multifunctional platelet glycoprotein thrombospondin-1 (TSP-1) promotes resorption in an in vitro resorption assay. However, TSP-1 is one of a family of multifunctional TSP molecules, and the current study was undertaken to investigate whether it is TSP-1 or another TSP family member which may be involved in regulation of resorption in vivo. RT-PCR was performed on cultured human bone cells, cultured human chondrocytes, and three separate samples of human osteoclastoma tissue using primers specific for each TSP family member. mRNA for TSP-2 was detected in almost all samples, and significantly in all osteoclastomas in the above tissues, while TSP-1 was detected less frequently and was only seen in one of three osteoclastomas. TSP-3, -4, and COMP were detected only in a minority of cases. These results indicate that TSP-2 is the most common TSP family member found in skeletal tissues and that TSP-2, rather than TSP-1, may be the molecule responsible for promoting resorption in vivo.

P2Y2 receptors are expressed by human osteoclasts of giant cell tumor but do not mediate ATP-induced bone resorption

Extracellular nucleotides acting through P2 receptors elicit a range of responses in many cell types. Previously, we have cloned the G-protein coupled P2Y2 receptor from a human osteoclastoma complementary deoxyribonucleic acid (cDNA) library and demonstrated its expression by reverse transcription linked (RT)-PCR and Southern analysis in a number of skeletal tissues, including a purified population of giant cells. In this study we have localized the expression of P2Y2 receptor transcripts to osteoclasts of giant cell tumor of bone by in situ hybridization. In osteoblasts and other cell types, the P2Y2 receptor is coupled to Ins(1,4,5)P 3-mediated Ca 2+ release from intracellular stores. In this study, the P2Y2 receptor agonists adenosine triphosphate (ATP) and uridine triphosphate (UTP) did not increase cytosolic free calcium concentration ([Ca 2+] i) in giant cells isolated from osteoclastoma, while the G-protein coupled calcium sensing receptor agonist, Ni 2+, elevated [Ca 2+] i in the same cells. These data indicate that P2Y2 receptor transcripts expressed by giant cells are not presented at the surface of cells as functional receptors, or alternatively, functional receptors are coupled to an effector other than [Ca 2+] i. ATPγS (10 μmol/L), but not UTP (10 μmol/L), significantly stimulated resorption by an enriched giant cell population. These results indicate that ATP-induced effects on resorption, following direct osteoclastic activation, are mediated by a P2 receptor other than the P2Y2 subtype. Nucleotides, released locally in the bone microenvironment in response to acute trauma or transient physical stress, will interact with a complement of P2 receptors expressed by both osteoclasts and osteoblasts to influence the remodeling process.

Human osteoclastoma-derived stromal cells: correlation of the ability to form mineralized nodules in vitro with formation of bone in vivo.

It has been suggested that the stromal element of human osteoclastomas contains osteoblastic cells. In this study, we demonstrate that osteoclast-depleted, passaged stromal cells express alkaline phosphatase and osteocalcin in vitro and form mineralized nodules under appropriate culture conditions. In addition, we describe a model in which severe combined immunodeficient (SCID) mice were used to support the differentiation of these putative human osteoblast progenitors in vivo. Lesions formed from human stromal cells were identified using the OKa blood group antigen and human procollagen type I antibodies. By 21 days, the lesion was a complete bone unit: a fully mineralized cortex, remodeling trabeculae, and a highly cellular marrow space. Stromal cells derived from six out of seven osteoclastomas produced identical lesions. Further studies have demonstrated that the capacity of the osteoclastoma-derived stromal cells to form bone in vivo and in vitro is passage dependent; early passages were osteogenic in both model systems, while later passages were not. In conclusion, we have developed a model in which the osteogenic nature of cells can be confirmed in vivo. Furthermore, human osteoclastoma-derived stromal cells provide a source of these osteogenic cells to study human osteoblast differentiation, both in vivo and in vitro.

Cathepsin K, but Not Cathepsins B, L, or S, Is Abundantly Expressed in Human Osteoclasts

Random high throughput sequencing of a human osteoclast cDNA library was employed to identify novel osteoclast-expressed genes. Of the 5475 ESTs obtained, approximately 4% encoded cathepsin K, a novel cysteine protease homologous to cathepsins S and L; ESTs for other cathepsins were rare. In addition, ESTs for cathepsin K were absent or at low frequency in cDNA libraries from numerous other tissues and cells. In situ hybridization in osteoclastoma and osteophyte confirmed that cathepsin K mRNA was highly expressed selecively in osteoclasts; cathepsins S, L, and B were not detectable. Cathepsin K was not detected by in situ hybridization in a panel of other tissues. Western blot of human osteoclastoma or fetal rat humerus demonstrated bands of 38 and 27 kDa, consistent with sizes predicted for pro- and mature cathepsin K. Immunolocalization in osteoclastoma and osteophyte showed intense punctate staining of cathepsin K exclusively in osteoclasts, with a polar distribution that was more intense at the bone surface. The abundant expression of cathepsin K selectively in osteoclasts strongly suggests that it plays a specialized role in bone resorption. Furthermore, the data suggest that random sequencing of ESTs from cDNA libraries is a valuable approach for identifying novel cell-selective genes.

Molecular Cloning and Characterization of a Putative Novel Human Osteoclast-Specific 116-kDa Vacuolar Proton Pump Subunit

A cDNA encoding a possible novel human 116-kDa polypeptide subunit of the osteoclastic proton pump (OC-116KDa) has been identified by differential screening of a human osteoclastoma cDNA library. The predicted sequence of OC-116KDa consists of 822 amino acids and is 46.9% and 47.2% identical at the amino acid level to the 116-KDa polypeptide of the vacuolar proton pump of rat and bovine brain respectively. OC-116KDa mRNA was found at high levels in osteoclastomas by Northern analysis but was not detected in tumor stromal cells or in other tissues including kidney, liver, skeletal muscle and brain. OC-116KDa mRNA was localized to multinucleated giant cells within the osteoclastoma tumor byin situhybridization.

Cloning and complete coding sequence of a novel human cathepsin expressed in giant cells of osteoclastomas

A gene encoding a possible novel human cathepsin, a cysteine proteinase that is distinct from previously characterized enzymes, has been identified by differential screening of a human osteoclastoma cDNA library. This molecule, termed cathepsin X, appears to represent the human homolog of the osteoclast-expressed rabbit cathepsin OC-2. Cathepsin X (GenBank accession number U20280) is 93.9% identical to OC-2 at the amino acid level, and is 92% identical at the nucleotide level within the coding region. Cathepsin X is 52.2 and 46.9% identical to cathepsins S and L, respectively, and is therefore clearly distinct from these enzymes. Cathepsin X mRNA was localized to multinucleated giant cells within the osteoclastoma tumor by in situ hybridization. These data strongly support the hypothesis that cathepsin X represents a novel cysteine proteinase which is expressed at high levels in osteoclasts.

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 mononuclear 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. These results indicate that the B-subunit isoforms present in the catalytic domains of the osteoclast and kidney V-H +-ATPases are different and further suggest that selective expression of isoforms of the B-subunit in these two tissues could provide a structural basis for some of the differences we have reported in the pharmacology and catalytic properties of these two enzymes.

Influence of osteoclasts and osteoclast-like cells on osteoblast alkaline phosphatase activity and collagen synthesis

Osteoblasts have been shown to modulate osteoclast activity, but the reverse process has not been investigated. In the current study conditioned medium (CM) was collected from osteoclasts and osteoclast-like cells and its effects on osteoblast alkaline phosphatase (ALPase) activity and collagen synthesis ([3H]proline hydroxylation) were determined. In primary chick osteoblasts, cultured chick embryo frontal bones, and UMR-106-01 cells, collagen synthesis and ALPase activity, but not [3H]thymidine incorporation, were inhibited by CM from chick marrow-derived giant cells, which possess some of the phenotypic characteristics of osteoclasts. However, collagen synthesis in chick embryo fibroblasts was not affected by giant cell CM. CM collected from cultures of chicken osteoclasts and human osteoclastoma cells and marrow-derived giant cells inhibited collagen synthesis in UMR-106-01 cells, but the effects of ALPase activity varied with the cell type. In contrast, mononuclear cell and fibroblast CM did not alter collagen synthesis. Initial characterization studies demonstrate that the inhibitor is a heat-labile factor with a molecular weight greater than 3500. In summary, authentic osteoclasts, tumor osteoclast-like cells, and chicken and human multinucleated giant cells produce a soluble factor that alters osteoblast collagen synthesis, suggesting that osteoclasts play a role in the modulation of osteoblast activity.

Expression of 92 kD type IV collagenase/gelatinase B in human osteoclasts.

The digestion of type I collagen is an essential step in bone resorption. It is well established that osteoclasts solubilize the mineral phase of bone during the resorptive process, but the mechanism by which they degrade type I collagen, the major proteinaceous component of bone, is controversial. Differential screening of a human osteoclastoma cDNA library was performed to characterize genes specifically expressed in osteoclasts. A large number of cDNA clones obtained by this procedure were found to represent 92 kD type IV collagenase (gelatinase B; MMP-9, EC 3.4.24.35), as well as tartrate-resistant acid phosphatase. In situ hybridization localized mRNA for gelatinase B to multinucleated giant cells in human osteoclastomas. Gelatinase B immunoreactivity was demonstrated in giant cells from eight of eight osteoclastomas, osteoclasts in normal bone, and osteoclasts of Paget's disease by use of a polyclonal antiserum raised against a synthetic gelatinase B peptide. In contrast, no immunoreactivity for 72 kD type IV collagenase (gelatinase A; MMP-2, EC 3.4.24.24), which is the product of a separate gene, was detected in osteoclastomas or normal osteoclasts. We propose that the 92 kD type IV collagenase/gelatinase B plays an important role in the resorption of collagen during bone remodeling.

Purification and characterization of a novel tyrosyl-aminopeptidase from human osteoclastomas

1. 1. An aminopeptidase that preferentially releases tyrosine residues from synthetic substrates has been purified from human osteoclastomas. This enzyme also hydrolyses dipeptides having an N-terminal tyrosine and a hydrophobic carboxy-terminal amino acid. 2. 2. The tyrosyl-aminopeptidase consists of two identical subunits with M r s of about 100,000. 3. 3. The enzyme is a metallopeptidase and is inhibited by chelating agents, chloromethylketone analogues of hydrophobic amino acids and bestatin.

Osteoblasts mediate thyroid hormone stimulation of osteoclastic bone resorption.

Thyroid hormones increase bone turnover in vivo and stimulate bone resorption in vitro. Clinical states associated with excess circulating thyroid hormone levels are known to produce osteoporosis. To determine the effect of T3 on bone resorption, we used an isolated rat osteoclast bone resorption assay in the absence or presence of added osteoblasts. This makes it possible to distinguish between direct and indirect effects of thyroid hormones on osteoclasts. In short settlement osteoclast cultures, which contain relatively few osteoblasts, 24-h treatment with T3 (10(-10)-10(-8) M) produced no stimulation of bone resorption. However, after 48-h incubation in the presence of T3, an increase in resorption was observed (2.3-fold at 10(-9) M). In cocultures of osteoclasts and osteoblasts (UMR 106-01 osteoblast-like cells or long settlement cultures), T3 stimulated resorption at 24 h. Furthermore, stimulation of resorption occurred when osteoblasts (UMR 106-01 or rat calvarial cells) were pretreated with T3 and the subsequent osteoblast-osteoclast cocultures conducted for 24 h in the absence of T3. Thus, direct exposure of osteoclasts to T3 was not required for the stimulatory effect. Treatment for 48 h with T3 (10(-9) M) or PTH (10(-8) M) had no effect on bone resorption in osteoblast-free cultures derived from human osteoclastoma tumours. T4 was 100-fold less potent than T3 as a stimulator of osteoclast activity, and rT3 had no effect. T3-induced stimulation was inhibited by salmon calcitonin (10(-10) M). These findings indicate that thyroid hormone can act on osteoblasts to indirectly stimulate osteoclastic bone resorption.

Osteoblasts mediate thyroid hormone stimulation of osteoclastic bone resorption

Thyroid hormones increase bone turnover in vivo and stimulate bone resorption in vitro. Clinical states associated with excess circulating thyroid hormone levels are known to produce osteoporosis. To determine the effect of T3 on bone resorption, we used an isolated rat osteoclast bone resorption assay in the absence or presence of added osteoblasts. This makes it possible to distinguish between direct and indirect effects of thyroid hormones on osteoclasts. In short settlement osteoclast cultures, which contain relatively few osteoblasts, 24-h treatment with T3 (10(-10)-10(-8) M) produced no stimulation of bone resorption. However, after 48-h incubation in the presence of T3, an increase in resorption was observed (2.3-fold at 10(-9) M). In cocultures of osteoclasts and osteoblasts (UMR 106-01 osteoblast-like cells or long settlement cultures), T3 stimulated resorption at 24 h. Furthermore, stimulation of resorption occurred when osteoblasts (UMR 106-01 or rat calvarial cells) were pretreated with T3 and the subsequent osteoblast-osteoclast cocultures conducted for 24 h in the absence of T3. Thus, direct exposure of osteoclasts to T3 was not required for the stimulatory effect. Treatment for 48 h with T3 (10(-9) M) or PTH (10(-8) M) had no effect on bone resorption in osteoblast-free cultures derived from human osteoclastoma tumours. T4 was 100-fold less potent than T3 as a stimulator of osteoclast activity, and rT3 had no effect. T3-induced stimulation was inhibited by salmon calcitonin (10(-10) M). These findings indicate that thyroid hormone can act on osteoblasts to indirectly stimulate osteoclastic bone resorption.

Purification and Characterization of a Tripeptidyl Peptidase I from Human Osteoclastomas: Evidence for Its Role in Bonen Resorption

Tripeptidyl peptidase I (EC 3.4.14.9), which cleaves tripeptides from the N-terminus of synthetic substrates, has been purified from human osteoclastomas (a bone tumor containing large numbers of normal osteoclasts). The enzyme has an Mr of 48 kDa but forms aggregates with an Mr of about 700 kDa. The tripeptidyl peptidase has an acidic pH optimum (∼pH 5.0), suggesting that it has a lysosomal localization and prefers substrates with a hydrophobic amino acid in the P1 position. There is an absolute requirement for a nonsubstituted N-terminus. The enzyme is inhibited by reagents which modify serine and histidine residues. Lysosomal tripeptidyl peptidase is known to be capable of cleaving Gly-Pro-X triplets from synthetic collagen-like polypeptides. Ala-Ala-Phe-CH2Cl, a potent inhibitor of osteoclastoma tripeptidyl peptidase, inhibits osteoclastic bone resorption in an in vitro test system. This suggests that tripeptidyl peptidase I, secreted by osteoclasts, is involved at some stage in the degradation of bone collagen.