Heparan sulphate proteoglycan as mediator of some adhesive responses and cytoskeletal reorganization of cells on fibronectin matrices: independent versus cooperative functions

Fibronectin is a multifunctional glycoprotein which promotes the adhesion of a variety of cell types to extracellular matrices, including artificial tissue culture substrata. Biochemical analyses of substratum adhesion sites indicated important functions for cell-surface heparan sulphate proteoglycan (HS-PG) in directly mediating adhesive responses by the binding of heparan sulphate sequences to fibronectin. In addition, fibronectin has a binding domain for a cell surface 'receptor' (possibly a 140K glycoprotein) important in these responses. To differentiate the relative importance of these two binding activities, a proteolytically generated cell-binding fragment of fibronectin has been isolated which binds to the 140K 'receptor' but not to HS or to collagen. Platelet factor 4 (PF4), a tetravalent HS-binding protein, provides a model of the tetravalent HS-binding activity of fibronectin, as supported by affinity chromatography studies (these studies also reveal the complexity of HS-PG metabolism in adhesion sites). Responses are measured on substrata coated with the cell-binding fragment of fibronectin, intact fibronectin, or PF4, singly or in combination. Fibroblast-like BALB/c 3T3 cells form both close and tight-focal adhesive contacts with associated microfilament stress fibres on intact fibronectin. Whereas HS-PG binding appears to mediate the formation of close contacts and linear microfilament bundles, a cooperative relationship exists between the HS- and the cell-binding activities of the intact fibronectin molecule in the formation of focal contacts and stress fibres. Human dermal fibroblasts generate different adhesive responses on HS-binding or cell-binding substrata, which are dependent on whether cells have been grown in medium with ascorbate to maximize production of their own collagenous matrix. As with 3T3 cells, focal contact and stress fibre formations of dermal cells require both binding activities in the intact fibronectin molecule. A third system consists of neuroblastoma tumour cells which adhere and extend neurites on fibronectin. Cell-body adherence, but not neurite extension, occurs on HS-binding matrices whereas neurite extension requires only fibronectin's cell-binding activity; the responses of primary peripheral neurons were exactly the opposite and CNS neurons did not respond at all. These studies indicate the diversity of molecular mechanisms by which various cells interact with the multifunctional fibronectin molecule in order to perform specialized functions, as well as the independent or cooperative functions of heparan sulphate proteoglycan on the cell surface in mediating these responses.

Vascular cell proteoglycans: evidence for metabolic modulation

Proteoglycans accumulate in the intimal layer of blood vessels during the early stages of atherosclerosis and predispose the vessel wall to further complications of this disease. Arterial endothelial and smooth muscle cell cultures have been used to study the metabolism of vessel wall proteoglycans in an attempt to determine whether cellular events associated with the genesis of this disease, such as cellular proliferation, ageing, migration and interaction with components of the extracellular matrix, influence the metabolism of arterial proteoglycans. Proteoglycan analyses of vascular cells reveal that endothelial cells synthesize multiple species of heparan sulphate proteoglycan while smooth muscle cells synthesize little heparan sulphate proteoglycan but significant quantities of chondroitin and dermatan sulphate proteoglycan. Each family of proteoglycans synthesized by each cell type differs with regard to charge density, hydrodynamic size, glycosaminoglycan type and size, oligosaccharide content and ability to form high molecular weight aggregates. A monoclonal antibody has been generated against the chondroitin sulphate proteoglycan and used to immunolocalize this antigen to the interstitial matrix of normal and diseased blood vessels. Experiments are presented to indicate that proteoglycan metabolism is modulated when cultured arterial cells are stimulated to proliferate and migrate. Other factors shown to influence proteoglycan metabolism include the age of the cell and the nature of the substratum upon which the cells are grown. These culture systems provide useful models with which to study the factors involved in the regulation of proteoglycan synthesis by vascular cells.

A highly potent inhibitor of cathepsin K (relacatib) reduces biomarkers of bone resorption both in vitro and in an acute model of elevated bone turnover in vivo in monkeys

Cathepsin K is an osteoclast-derived cysteine protease that has been implicated as playing a major role in bone resorption. A substantial body of evidence indicates that cathepsin K is critical in osteoclast-mediated bone resorption and suggests that its pharmacological inhibition should result in inhibition of bone resorption in vivo. Here we report the pharmacological characterization of SB-462795 (relacatib) as a potent and orally bioavailable small molecule inhibitor of cathepsin K that inhibits bone resorption both in vitro in human tissue and in vivo in cynomolgus monkeys. SB-462795 is a potent inhibitor of human cathepsins K, L, and V (K(i, app)=41, 68, and 53 pM, respectively) that exhibits 39-300-fold selectivity over other cathepsins. SB-462795 inhibited endogenous cathepsin K in situ in human osteoclasts and human osteoclast-mediated bone resorption with IC50 values of approximately 45 nM and approximately 70 nM, respectively. The anti-resorptive potential of SB-462795 was evaluated in normal as well as medically ovariectomized (Ovx) female cynomolgus monkeys. Serum levels of the C- and N-terminal telopeptides of Type I collagen (CTx and NTx, respectively) and urinary levels of NTx were monitored as biomarkers of bone resorption. Administration of SB-462795 to medically ovariectomized or normal monkeys resulted in an acute reduction in both serum and urinary markers of bone resorption within 1.5 h after dosing, and this effect lasted up to 48 h depending on the dose administered. Our data indicate that SB-462795 potently inhibits human cathepsin K in osteoclasts, resulting in a rapid inhibition of bone resorption both in vitro and in vivo in the monkey. These studies also demonstrate the therapeutic potential of relacatib in the treatment of postmenopausal osteoporosis and serves to model the planned clinical trials in human subjects.

Human osteoarthritis synovial fluid and joint cartilage contain both aggrecanase- and matrix metalloproteinase-generated aggrecan fragments

OBJECTIVE

To identify the major aggrecanase- and matrix metalloproteinase (MMP)-generated aggrecan fragments in human osteoarthritis (OA) synovial fluid and in human OA joint cartilage.

METHOD

Aggrecan fragments were prepared by CsCl gradient centrifugation. Fragment distributions were compared with aggrecanase-1 (ADAMTS-4) and MMP-3 digested human aggrecan by analysis with neoepitope antibodies and an anti-G1 domain antibody, using Western immuno-blots.

RESULTS

The overall fragment pattern of OA synovial fluid aggrecan was similar to the fragment pattern of cartilage aggrecan cleaved in vitro by ADAMTS-4. However, multiple glycosaminoglycan (GAG) containing aggrecanase and MMP-generated aggrecan fragments were identified in OA synovial fluid and some of these fragments were produced by the action of both types of proteinases. The synovial fluid content of large size aggrecan fragments with (374)ARGS- and (342)FFGV- N-terminals was about 107 and 40 pmoles per ml, respectively, out of a total concentration of aggrecan fragments of about 185 pmoles per ml. OA synovial fluid contained insignificant amounts of the G1-IPEN(341) fragment as compared to the G1-TEGE(373) fragment, while OA cartilage contained significant amounts of both fragments. OA cartilage contained several GAG-containing aggrecan fragments with N-terminals of G1- or (342)FFGV- but no fragments with an N-terminal of (374)ARGS-.

CONCLUSIONS

The overall pattern of aggrecan fragments in human OA synovial fluid and cartilage supports an important role for aggrecanase in aggrecan degradation. However, the fragment patterns and their differential distribution between cartilage and synovial fluid are consistent with the existence of at least two proteolytic pathways for aggrecan degradation in human OA, generating both (342)FFGV- and (374)ARGS-fragments.

Increased osteoblastogenesis and decreased bone resorption protect against ovariectomy-induced bone loss in thrombospondin-2-null mice

Although bone is composed primarily of extracellular matrix (ECM), the dynamic role that the ECM plays in regulating bone remodeling secondary to estrogen loss is relatively unexplored. Previous studies have shown that mice deficient in the matricellular protein thrombospondin-2 (TSP2-null) form excess endocortical bone; thus, we postulated that enhanced bone formation in TSP2-null mice could protect against ovariectomy (OVX)-induced bone loss. Wild-type (WT) OVX mice showed a significant loss of both midfemoral endocortical and proximal tibial trabecular bone, but OVX did not significantly alter TSP2-null bone. TSP2-null mice showed an increase in bone formation, as indicated by a 70% increase in serum osteocalcin two weeks post OVX and a two-fold increase in bone formation rate (BFR) five weeks post OVX as measured by dynamic histomorphometry. WT animals showed only a 20% increase in serum osteocalcin at two weeks and no change in BFR at five weeks. This increase in bone formation in TSP2-null OVX mice was accompanied by a three-fold increase in osteoprogenitor number. Although these results provide a partial explanation for the maintenance of bone geometry post-OVX, TSP2-null mice five weeks post-OVX also showed a significantly lower level of bone resorption than OVX WT mice, as determined by serum levels of the amino-terminal telopeptide of type I collagen (NTx). We conclude that the absence of TSP2 protects against OVX-induced bone loss by two complementary processes: increased formation and decreased resorption.

Ultrastructural quantification of cell death after injurious compression of bovine calf articular cartilage

OBJECTIVE

It has been suggested that chondrocyte death by apoptosis may play a role in the pathogenesis of cartilage destruction in osteoarthritis, but the results of in-vivo and in-vitro investigations have been conflicting. To investigate further the cell death in our in-vitro model for traumatic joint injury, we performed a quantitative analysis by electron microscopy (EM) of cell morphology after injurious compression. For comparison, the TUNEL assay was also performed.

DESIGN

Articular cartilage explant disks were harvested from newborn calf femoropatellar groove. The disks were subjected to injurious compression (50% strain at a strain rate of 100%/s), incubated for 3 days, and then fixed for quantitative morphological analysis.

RESULTS

By TUNEL, the cell apoptosis rate increased from 7 +/- 2% in unloaded controls to 33 +/- 6% after injury (P=0.01; N=8 animals). By EM, the apoptosis rate increased from 5 +/- 1% in unloaded controls to 62 +/- 10% in injured cartilage (P=0.02, N=5 animals). Analysis by EM also identified that of the dead cells in injured disks, 97% were apoptotic by morphology.

CONCLUSIONS

These results confirm a significant increase in cell death after injurious compression and suggest that most cell death observed here was by an apoptotic process.

Biosynthetic response and mechanical properties of articular cartilage after injurious compression

Traumatic joint injury is known to produce osteoarthritic degeneration of articular cartilage. To study the effects of injurious compression on the degradation and repair of cartilage in vitro, we developed a model that allows strain and strain rate-controlled loading of cartilage explants. The influence of strain rate on both cartilage matrix biosynthesis and mechanical properties was assessed after single injurious compressions. Loading with a strain rate of 0.01 s(-1) to a final strain of 50% resulted in no measured effect on the cells or on the extracellular matrix, although peak stresses reached levels of about 12 MPa. However, compression with strain rates of 0.1 and 1 s(-1) caused peak stresses of approximately 18 and 24 MPa, respectively, and resulted in significant decreases in both proteoglycan and total protein biosynthesis. The mechanical properties of the explants (compressive and shear stiffness) were also reduced with increasing strain rate. Additionally, cell viability decreased with increasing strain rate, and the remaining viable cells lost their ability to exhibit an increase in biosynthesis in response to low-amplitude dynamic mechanical stimulation. This latter decrease in reparative response was most dramatic in the tissue compressed at the highest strain rates. We conclude that strain rate (like peak stress or strain) is an important parameter in defining mechanical injury, and that cartilage injuriously compressed at high strain rates can lose its characteristic anabolic response to low-amplitude cyclic mechanical loading.

Clusterin expression in adult human normal and osteoarthritic articular cartilage

OBJECTIVE

To characterize the expression pattern of clusterin in adult human normal and osteoarthritic cartilage.

METHODS

Clusterin mRNA expression in adult human normal and osteoarthritic cartilage was investigated by analysis of cDNA libraries, TaqMan quantitative RT-PCR, microarray and in situ hybridization.

RESULTS

Sequence analysis of ESTs from adult human normal and osteoarthritic cartilage cDNA libraries demonstrated that the abundance of clusterin in these libraries was equivalent to genes which have been more commonly associated with cartilage. To examine tissue distribution, TaqMan Quantitative PCR analysis was performed using RNA from a panel of individual normal tissues. Clusterin was expressed at significant levels in cartilage, brain, liver, and pancreas. The expression of clusterin mRNA was up-regulated in early osteoarthritic vs normal cartilage when analysed by microarray analysis. Using in situ hybridization, chondrocytes of normal cartilage expressed moderate levels of clusterin. Upper mid-zone chondrocytes in cartilage with early stages of osteoarthritic disease expressed high levels of clusterin mRNA. In advanced osteoarthritic cartilage, the overall expression of clusterin was reduced.

CONCLUSION

The induction of clusterin has been associated with a variety of disease states where it appears to provide a cytoprotective effect. The increased expression of clusterin mRNA in the early stages of osteoarthritis (OA) may reflect an attempt by the chondrocytes to protect and repair the tissue. In contrast, the decrease in clusterin mRNA in the advanced osteoarthritic cartilage accompanies the final degenerative stages of the disease. An understanding of the expression of clusterin in osteoarthritis may allow consideration of this protein as a marker for cartilage changes in this chronic degenerative condition.

In vitro models for investigation of the effects of acute mechanical injury on cartilage

Traumatic injury to a joint is known to increase the risk for the development of secondary osteoarthritis, but it is unclear how this process occurs. The existence of such a discrete event that can lead to an increased risk of osteoarthritis has spurred interest in developing in vitro models of traumatic joint injury. The current authors review some of the recent insights gained from these model systems into the pathogenesis of osteoarthritis, including the evidence for an initial, irreversible insult to chondrocytes during mechanical injury, the occurrence of apoptotic chondrocyte death, and attempts to identify the effects of trauma on chondrocyte metabolic response. Results also are presented from the authors' ongoing studies of the degradative pathways initiated by traumatic mechanical loads, the mechanism by which chondrocytes are affected during compression, and possible contributions of the joint capsule to posttraumatic cartilage degradation.

Identification and initial characterization of 5000 expressed sequenced tags (ESTs) each from adult human normal and osteoarthritic cartilage cDNA libraries

OBJECTIVE

To prepare, sequence and analyse adult human cartilage cDNA libraries to study the gene expression pattern between normal and osteoarthritic cartilage.

METHODS

Poly A(+)RNA from adult human normal and osteoarthritic articular cartilage was isolated and used to prepare cDNA libraries. Approximately 5000 ESTs from each library were sequenced and analysed using bioinformatic tools. The expression of select genes was confirmed by Northern blot and in situ hybridization analysis.

RESULTS

Multiple gene families including several classical cartilage matrix protein encoding genes were identified. Approximately 28-40% of the genes sequenced from these libraries were novel, while half of the genes encoded known proteins and 4-6% of the genes encoded novel homologs of known proteins. Several known genes, whose expression has not been reported previously in cartilage, were also identified. We have confirmed the cartilage expression of three known (CTGF, CTGF-L and clusterin) and two novel homologs of known genes (PCPE-2 and Gal-Nac transferase) by Northern blot and in situ hybridization analysis.

CONCLUSION

This is the first report of the preparation and sequencing of cDNA libraries from adult human normal and osteoarthritic articular cartilage. Further analysis of genes identified from these libraries may provide molecular targets for diagnosis and/or treatment of osteoarthritis (OA).

Potent and selective inhibition of human cathepsin K leads to inhibition of bone resorption in vivo in a nonhuman primate

Cathepsin K is a cysteine protease that plays an essential role in osteoclast-mediated degradation of the organic matrix of bone. Knockout of the enzyme in mice, as well as lack of functional enzyme in the human condition pycnodysostosis, results in osteopetrosis. These results suggests that inhibition of the human enzyme may provide protection from bone loss in states of elevated bone turnover, such as postmenopausal osteoporosis. To test this theory, we have produced a small molecule inhibitor of human cathepsin K, SB-357114, that potently and selectively inhibits this enzyme (Ki = 0.16 nM). This compound potently inhibited cathepsin activity in situ, in human osteoclasts (inhibitor concentration [IC]50 = 70 nM) as well as bone resorption mediated by human osteoclasts in vitro (IC50 = 29 nM). Using SB-357114, we evaluated the effect of inhibition of cathepsin K on bone resorption in vivo using a nonhuman primate model of postmenopausal bone loss in which the active form of cathepsin K is identical to the human orthologue. A gonadotropin-releasing hormone agonist (GnRHa) was used to render cynomolgus monkeys estrogen deficient, which led to an increase in bone turnover. Treatment with SB-357114 (12 mg/kg subcutaneously) resulted in a significant reduction in serum markers of bone resorption relative to untreated controls. The effect was observed 1.5 h after the first dose and was maintained for 24 h. After 5 days of dosing, the reductions in N-terminal telopeptides (NTx) and C-terminal telopeptides (CTx) of type I collagen were 61% and 67%, respectively. A decrease in serum osteocalcin of 22% was also observed. These data show that inhibition of cathepsin K results in a significant reduction of bone resorption in vivo and provide further evidence that this may be a viable approach to the treatment of postmenopausal osteoporosis.

Azepanone-based inhibitors of human and rat cathepsin K

The synthesis, in vitro activities, and pharmacokinetics of a series of azepanone-based inhibitors of the cysteine protease cathepsin K (EC 3.4.22.38) are described. These compounds show improved configurational stability of the C-4 diastereomeric center relative to the previously published five- and six-membered ring ketone-based inhibitor series. Studies in this series have led to the identification of 20, a potent, selective inhibitor of human cathepsin K (K(i) = 0.16 nM) as well as 24, a potent inhibitor of both human (K(i) = 0.0048 nM) and rat (K(i,app) = 4.8 nM) cathepsin K. Small-molecule X-ray crystallographic analysis of 20 established the C-4 S stereochemistry as being critical for potent inhibition and that unbound 20 adopted the expected equatorial conformation for the C-4 substituent. Molecular modeling studies predicted the higher energy axial orientation at C-4 of 20 when bound within the active site of cathepsin K, a feature subsequently confirmed by X-ray crystallography. Pharmacokinetic studies in the rat show 20 to be 42% orally bioavailable. Comparison of the transport of the cyclic and acyclic analogues through CaCo-2 cells suggests that oral bioavailability of the acyclic derivatives is limited by a P-glycoprotein-mediated efflux mechanism. It is concluded that the introduction of a conformational constraint has served the dual purpose of increasing inhibitor potency by locking in a bioactive conformation as well as locking out available conformations which may serve as substrates for enzyme systems that limit oral bioavailability.

Potent and selective cathepsin L inhibitors do not inhibit human osteoclast resorption in vitro

Cathepsins K and L are related cysteine proteases that have been proposed to play important roles in osteoclast-mediated bone resorption. To further examine the putative role of cathepsin L in bone resorption, we have evaluated selective and potent inhibitors of human cathepsin L and cathepsin K in an in vitro assay of human osteoclastic resorption and an in situ assay of osteoclast cathepsin activity. The potent selective cathepsin L inhibitors (K(i) = 0.0099, 0.034, and 0.27 nm) were inactive in both the in situ cytochemical assay (IC(50) > 1 micrometer) and the osteoclast-mediated bone resorption assay (IC(50) > 300 nm). Conversely, the cathepsin K selective inhibitor was potently active in both the cytochemical (IC(50) = 63 nm) and resorption (IC(50) = 71 nm) assays. A recently reported dipeptide aldehyde with activity against cathepsins L (K(i) = 0.052 nm) and K (K(i) = 1.57 nm) was also active in both assays (IC(50) = 110 and 115 nm, respectively) These data confirm that cathepsin K and not cathepsin L is the major protease responsible for human osteoclastic bone resorption.

Human osteoclast cathepsin K is processed intracellularly prior to attachment and bone resorption

Cathepsin K is a member of the papain superfamily of cysteine proteases and has been proposed to play a pivotal role in osteoclast-mediated bone resorption. We have developed a sensitive cytochemical assay to localize and quantify osteoclast cathepsin K activity in sections of osteoclastoma and human bone. In tissue sections, osteoclasts that are distant from bone express high levels of cathepsin K messenger RNA (mRNA) and protein. However, the majority of the cathepsin K in these cells is in an inactive zymogen form, as assessed using both the cytochemical assay and specific immunostaining. In contrast, osteoclasts that are closer to bone contain high levels of immunoreactive mature cathepsin K that codistributes with enzyme activity in a polarized fashion toward the bone surface. Polarization of active enzyme was clearly evident in osteoclasts in the vicinity of bone. The osteoclasts apposed to the bone surface were almost exclusively expressing the mature form of cathepsin K. These cells showed intense enzyme activity, which was polarized at the ruffled border. These results suggest that the in vivo activation of cathepsin K occurs intracellularly, before secretion into the resorption lacunae and the onset of bone resorption. The processing of procathepsin K to mature cathepsin K occurs as the osteoclast approaches bone, suggesting that local factors may regulate this process.

Antagonism of the osteoclast vitronectin receptor with an orally active nonpeptide inhibitor prevents cancellous bone loss in the ovariectomized rat

An orally active, nonpeptide Arg-Gly-Asp (RGD) mimetic alpha(v)beta3 antagonist, (S)-3-Oxo-8-[2-[6-(methylamino)-pyridin-2-yl]-1-ethoxy]-2-(2,2,2-trifluoroethyl)-2,3,4,5-tetrahydro-1H-2-benzazepine-4-acetic acid (compound 1), has been generated, which prevented net bone loss and inhibited cancellous bone turnover in vivo. The compound binds alpha(v)beta3 and the closely related integrin alpha(v)beta5 with low nanomolar affinity but binds only weakly to the related integrins alpha(IIb)beta3, and alpha5beta1. Compound 1 inhibited alpha(v)beta3-mediated cell adhesion with an IC50 = 3 nM. More importantly, the compound inhibited human osteoclast-mediated bone resorption in vitro with an IC50 = 11 nM. In vivo, compound 1 inhibited bone resorption in a dose-dependent fashion, in the acute thyroparathyroidectomized (TPTX) rat model of bone resorption with a circulating EC50 approximately 20 microM. When dosed orally at 30 mg/kg twice a day (b.i.d.) in the chronic ovariectomy (OVX)-induced rat model of osteopenia, compound 1 also prevented bone loss. At doses ranging from 3 to 30 mg/kg b.i.d., compound 1 partially prevented the OVX-induced increase in urinary deoxypyridinoline. In addition, the compound prevented the OVX-induced reduction in cancellous bone volume (BV), trabecular number (Tb.N), and trabecular thickness (Tb.Th), as assessed by quantitative microcomputerized tomography (microCT) and static histomorphometry. Furthermore, both the 10-mg/kg and 30-mg/kg doses of compound prevented the OVX-induced increase in bone turnover, as measured by percent osteoid perimeter (%O.Pm). Together, these data indicate that the alpha(v)beta3 antagonist compound 1 inhibits OVX-induced bone loss. Mechanistically, compound 1 prevents bone loss in vivo by inhibiting osteoclast-mediated bone resorption, ultimately preventing cancellous bone turnover.

Disease-modifying activity of SB 273005, an orally active, nonpeptide alphavbeta3 (vitronectin receptor) antagonist, in rat adjuvant-induced arthritis

OBJECTIVE

To evaluate the effects of SB 273005, a potent, orally active nonpeptide antagonist of the integrin avbeta3 vitronectin receptor, on joint integrity in rats with adjuvant-induced arthritis (AIA).

METHODS

Male Lewis rats with AIA were orally dosed either prophylactically (days 0-20) or therapeutically (days 10-20) with SB 273005. Efficacy was determined by measurement of paw inflammation, assessment of bone mineral density using dual-energy x-ray absorptiometry (DEXA), magnetic resonance imaging (MRI), and histologic evaluation.

RESULTS

SB 273005 is a potent antagonist of the closely related integrins, avbeta3 (Ki = 1.2 nM) and alphavbeta5 (Ki = 0.3 nM). When SB 273005 was administered prophylactically to AIA rats twice per day, it inhibited paw edema at doses of 10, 30, and 60 mg/kg, by 40%, 50%, and 52%, respectively. Therapeutic administration twice daily was also effective, and a reduction in paw edema was observed at 30 mg/kg and 60 mg/kg of the antagonist (by 36% and 48%, respectively). SB 273005 was also effective when administered once per day, both prophylactically and therapeutically. Significant improvement in joint integrity in treated rats was shown using DEXA and MRI analyses. These findings were confirmed histologically, and significant protection of bone, cartilage, and soft tissue was observed within the joint.

CONCLUSION

Symptoms of AIA in rats were significantly reduced by either prophylactic or therapeutic treatment with the alphavbeta3 antagonist, SB 273005. Measurements of paw inflammation and of bone, cartilage, and soft tissue structure indicated that this compound exerts a protective effect on joint integrity and thus appears to have disease-modifying properties.

FrzB-2: a human secreted frizzled-related protein with a potential role in chondrocyte apoptosis

OBJECTIVE

To characterize a novel secreted frizzled-related protein (SFRP) and determine its tissue distribution at the mRNA and protein level.

METHODS

The FrzB-2 gene was identified by expressed sequence tag (EST) analysis of human tissue-derived libraries. Tissue distribution of FrzB-2 mRNA was determined by Northern blot analysis and in situ hybridization. FrzB-2 protein reactivity was localized in human OA articular cartilage by immunocytochemistry, using a polyclonal antibody against a peptide sequence unique to FrzB-2. Apoptosis was detected in articular cartilage sections using Tunel staining.

RESULTS

ESTs corresponding to FrzB-2 were found in osteoblast, chondrosarcoma, osteosarcoma, osteoclastoma and synovial fibroblast libraries. FrzB-2 mRNA is expressed in a number of tissues and cell types including bone-related cells and tissues such as primary human osteoblasts and osteoclastoma. In situ hybridization studies showed strong FrzB-2 mRNA expression in human chondrocytes in human osteoarthritic (OA) cartilage but negligible levels in normal cartilage chondrocytes. The FrzB-2 cDNA encodes a secreted 40 kDa protein consisting of 346 amino acids. FrzB-2 is 92. 5% identical to the rat orthologue, DDC-4, which has been shown to be associated with physiological apoptosis. FrzB-2 protein was selectively detected in human OA articular cartilage by immunocytochemistry, using a polyclonal antibody. Consistent with its potential role in apoptosis, positive FrzB-2 staining and Tunel positive nuclei staining were detected in chondrocyte clones in sections of human OA cartilage.

CONCLUSION

These data suggest that FrzB-2 may play a role in apoptosis and that the expression of this protein may be important in the pathogenesis of human OA.

Differential effects of SB 242235, a selective p38 mitogen-activated protein kinase inhibitor, on IL-1 treated bovine and human cartilage/chondrocyte cultures

The p38 MAP kinase inhibitor, SB 242235, was evaluated for its effects on the metabolism of bovine and human cartilage and primary chondrocyte cultures. SB 242235 had no effect on proteoglycan synthesis (PG) in bovine articular cartilage explants (BAC), as measured by [(35)S]-sulfate incorporation into glycosaminoglycans (GAGs). In addition, the compound had no effect on IL-1 alpha-induced GAG release from these cultures. However, there was a potent, dose-dependent inhibition of nitric oxide (NO) release from IL-1 alpha-stimulated BAC with an IC(50)of approximately 0.6 microM, with similar effects observed in primary chondrocytes. The effect on BAC was time dependent, and mechanistically did not appear to be the result of inhibition of protein kinase C (PKC), protein kinase A (PKA) or MEK-1. The effect on NO release in bovine chondrocytes was at the level of inducible nitric oxide synthase (iNOS) gene expression, which was inhibited at similar concentrations as nitrite production. In primary human chondrocytes, IL-1 beta induction of p38 MAP kinase was inhibited by SB 242235 with an IC(50)of approximately 1 microM. Surprisingly, however, treatment of IL-beta-stimulated human cartilage or chondrocytes with SB 242235 did not inhibit either NO production or the induction of iNOS. On the other hand, the natural product hymenialdisine (HYM), a protein tyrosine kinase (PTK) inhibitor, inhibited NO production and iNOS in both species. In contrast to the differential control of iNOS, PGE(2)was inhibited by SB 242235 in both IL-1-stimulated bovine and human chondrocyte cultures. These studies indicate that there are species differences in the control of iNOS by p38 inhibitors and also that different pathways may control IL-1-induced proteoglycan breakdown and NO production.

Injurious mechanical compression of bovine articular cartilage induces chondrocyte apoptosis

A bovine cartilage explant system was used to evaluate the effects of injurious compression on chondrocyte apoptosis and matrix biochemical and biomechanical properties within intact cartilage. Disks of newborn bovine articular cartilage were compressed in vitro to various peak stress levels and chondrocyte apoptotic cell death, tissue biomechanical properties, tissue swelling, glycosaminoglycan loss, and nitrite levels were quantified. Chondrocyte apoptosis occurred at peak stresses as low as 4.5 MPa and increased with peak stress in a dose-dependent manner. This increase in apoptosis was maximal by 24 h after the termination of the loading protocol. At high peak stresses (>20 MPa), greater than 50% of cells apoptosed. When measured in uniaxial confined compression, the equilibrium and dynamic stiffness of explants decreased with the severity of injurious load, although this trend was not significant until 24-MPa peak stress. In contrast, the equilibrium and dynamic stiffness measured in radially unconfined compression decreased significantly after injurious stresses of 12 and 7 MPa, respectively. Together, these results suggested that injurious compression caused a degradation of the collagen fibril network in the 7- to 12-MPa range. Consistent with this hypothesis, injurious compression caused a dose-dependent increase in tissue swelling, significant by 13-MPa peak stress. Glycosaminoglycans were also released from the cartilage in a dose-dependent manner, significant by 6- to 13-MPa peak stress. Nitrite levels were significantly increased above controls at 20-MPa peak stress. Together, these data suggest that injurious compression can stimulate cell death as well as a range of biomechanical and biochemical alterations to the matrix and, possibly, chondrocyte nitric oxide expression. Interestingly, chondrocyte programmed cell death appears to take place at stresses lower than those required to stimulate cartilage matrix degradation and biomechanical changes. While chondrocyte apoptosis may therefore be one of the earliest responses to tissue injury, it is currently unclear whether this initial cellular response subsequently drives cartilage matrix degradation and changes in the biomechanical properties of the tissue.

Identification and in vivo efficacy of small-molecule antagonists of integrin alphavbeta3 (the vitronectin receptor)

The integrin alphavbeta3 is thought to play a key role in the initiation and/or progression of several human diseases, including osteoporosis, restenosis following percutaneous transluminal coronary angioplasty (PTCA), rheumatoid arthritis, cancer and ocular diseases. Antagonism of integrin alphavbeta3 is therefore expected to provide an approach for the treatment and/or prevention of these diseases. A variety of potent, small-molecule alphavbeta3 antagonists have been identified, several of which are active in disease models, thereby demonstrating the therapeutic potential of alphavbeta3 antagonism. This review will focus on recent advances in the identification of small-molecule alphavbeta3 antagonists, with an emphasis on those studies where small-molecule alphavbeta3 antagonists have been used in proof-of-concept studies in vivo.

Meniscal ossification in spontaneous osteoarthritis in the guinea-pig

OBJECTIVE

The purpose of this study was to evaluate the ossification state of the meniscus in the guinea-pig stifle joint using micro-computerized tomography.

DESIGN

Hind limbs from six (N=12) and 24 (N=11) month-old male Hartley guinea-pigs were removed and the joints were imaged using high resolution micro-computerized tomography. The ossified volume of the medial and lateral menisci from both groups of animals was quantified.

RESULTS

Ossification of both the medial and lateral menisci of the both the 6- and 24-month-old animals was observed. In both age groups, the ossified region of the medial meniscus was significantly larger than the lateral meniscus. In addition, there is a significant increase in ossified volume of the medial meniscus between 6 and 24 months of age.

CONCLUSIONS

There is a significant amount of ossification of the menisci in the male Hartley guinea-pig, with the medial compartment showing more bone than the lateral. In addition, as the animals age, there is an increase in ossification within the medial compartment. Bone remodeling and cartilage degeneration is evident in the medial compartment within these animals as they age. It is possible that the increased ossification of the medial meniscus could alter the joint biomechanics and, in part, stimulate this medial compartment joint destruction.

Markers of joint tissue turnover in joint fluids from hips with osteonecrosis of the femoral head

Osteonecrosis of the femoral head often results in secondary osteoarthritis of the hip joint; however, the pathologic processes underlying the destruction of articular cartilage are not fully understood. Molecular markers in the hip joint fluids were measured to examine the changes in turnover of cartilage and other joint tissues. Marker data were related to clinical, radiological, and histopathological changes in the articular cartilage of the hip. Forty-five patients (median age: 43 years) were studied. The median time between the onset of symptoms and sampling of hip synovial fluid was 6 months. Aggrecan fragments, C-propeptide of type-II collagen, matrix metalloproteinase-3, and tissue inhibitor of metalloproteinases-1 levels in joint fluid were determined by immunoassay. Osteonecrosis of the femoral head was graded by radiology as minimal collapse of the femoral head (stage 2: 26 patients), severe collapse (stage 3: 15 patients), or severe collapse with osteoarthritis (stage 4: four patients). Histological changes of the articular cartilage, consistent with early-stage osteoarthritis, were evident at stage 3 and were more advanced at stage 4. The average concentrations of proteoglycan fragments and C-propeptide of type-II collagen were 207 (SD 182) microg/ml and 19.6 (SD 19.3) ng/ml, respectively. The average concentrations of matrix metalloproteinase-3 and tissue inhibitor of metalloproteinases-1 were 177 (SD 291) nM and 23.0 (SD 9.9) nM, respectively. Measurable levels for all markers assayed were noted in the earliest stage of the disease, only a few months after the onset of symptoms and well before the appearance of radiological changes. Levels of matrix metalloproteinase-3 and molar ratios of matrix metalloproteinase-3/tissue inhibitor of metalloproteinases-1 were higher in early stage disease than in later stage disease.

Potent and selective nonpeptide inhibitors of caspases 3 and 7 inhibit apoptosis and maintain cell functionality

Caspases have been strongly implicated to play an essential role in apoptosis. A critical question regarding the role(s) of these proteases is whether selective inhibition of an effector caspase(s) will prevent cell death. We have identified potent and selective non-peptide inhibitors of the effector caspases 3 and 7. The inhibition of apoptosis and maintenance of cell functionality with a caspase 3/7-selective inhibitor is demonstrated for the first time, and suggests that targeting these two caspases alone is sufficient for blocking apoptosis. Furthermore, an x-ray co-crystal structure of the complex between recombinant human caspase 3 and an isatin sulfonamide inhibitor has been solved to 2.8-A resolution. In contrast to previously reported peptide-based caspase inhibitors, the isatin sulfonamides derive their selectivity for caspases 3 and 7 by interacting primarily with the S(2) subsite, and do not bind in the caspase primary aspartic acid binding pocket (S(1)). These inhibitors blocked apoptosis in murine bone marrow neutrophils and human chondrocytes. Furthermore, in camptothecin-induced chondrocyte apoptosis, cell functionality as measured by type II collagen promoter activity is maintained, an activity considered essential for cartilage homeostasis. These data suggest that inhibiting chondrocyte cell death with a caspase 3/7-selective inhibitor may provide a novel therapeutic approach for the prevention and treatment of osteoarthritis, or other disease states characterized by excessive apoptosis.

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.

Inhibition of caspase-3-like activity prevents apoptosis while retaining functionality of human chondrocytes in vitro

Apoptosis was induced in a human chondrocyte cell line, T/C 28a4, by treatment with various stimuli, including camptothecin, tumor necrosis factor-alpha, staurosporine, okadaic acid, and reduced serum conditions. All stimuli induced a cytosolic DEVDase activity, coincident with apoptosis. Caspase activities in the lysates were characterized and quantitated with peptide cleavage profiles. To confirm that the results were not related to the immortalized nature of the cell line, primary human chondrocytes also were shown to undergo apoptosis under similar conditions, which resulted in increased cytosolic DEVDase activity. There was little or no caspase-1 (interleukin-1beta-converting enzyme) or caspase-8-like activity in the apoptotic cells. In all cases, the irreversible nonselective caspase inhibitor, Z-VAD-FMK, and the caspase-3-selective inhibitor, Ac-DMQD-CHO, inhibited DEVDase activity and apoptosis, whereas the caspase-1-selective inhibitor, Ac-YVAD-CHO, had no effect. Human chondrocytes were stably and transiently transfected with a type-II collagen gene (COL2A1) regulatory sequence driving a luciferase reporter as a specific marker of chondrocyte gene expression. Treatment of the cells with camptothecin or tumor necrosis factor-alpha plus cycloheximide significantly inhibited COL2A1 transcriptional activity. Significantly, cotreatment with Z-VAD-FMK or Ac-DMQD-CHO maintained COL2A1-reporter gene activity, indicating that the prevention of apoptosis by caspase-3 inhibition was sufficient to maintain cell functionality as assessed by the retention of type-II collagen promoter activity.

Differential expression of egr-1 in osteoarthritic compared to normal adult human articular cartilage

OBJECTIVE

The purpose of this study was to identify genes that are differentially expressed in normal versus osteoarthritic human articular cartilage as either potential novel therapeutic targets or diagnostic markers of this disease.

DESIGN

mRNA was isolated from histologically normal and osteoarthritic adult human articular cartilage. The Differential Display technique was employed which identified differentially expressed genes in the normal and diseased tissue. Northern and reverse Northern hybridization were used to confirm the gene expression pattern. Immunohistochemistry and in-situ hybridization were used to localize expression of Egr-1 protein and mRNA respectively in cartilage.

RESULTS

A transcription factor, early growth response protein-1 (Egr-1) was found to be down-regulated more than six-fold in multiple human OA cartilage samples when compared to normal tissue. Immunohistochemistry indicated that Egr-1 was expressed throughout normal adult cartilage, in deep-, mid- and superficial-zones. In contrast, in OA cartilage there was expression of Egr-1 mRNA and protein only in the chondrocytes undergoing cloning.

CONCLUSIONS

Egr-1 is differentially expressed in OA versus normal cartilage and because of its role in transcriptional activation and repression and regulation of proliferation, differentiation and apoptosis, Egr-1 may play an important role in the pathogenesis of OA. Up-regulation of Egr-1 may therefore provide a novel therapeutic approach for either the prevention or treatment of OA.

Down-regulation of chondrocyte aggrecan and type-II collagen gene expression correlates with increases in static compression magnitude and duration

The goal of this study was to examine the simultaneous effects of mechanical compression of chondrocytes on mRNA expression and macromolecular synthesis of aggrecan and type-II collagen. Bovine cartilage explants were exposed to different magnitudes and durations of applied mechanical compression, and levels of aggrecan and type-IIa collagen mRNA normalized to glyceraldehyde-3-phosphate dehydrogenase were measured and quantified by Northern blot analysis. Synthesis of aggrecan and type-II collagen protein was measured by radiolabel incorporation of [35S]sulfate and [3H]proline into macromolecules. The results showed a dose-dependent decrease in mRNA levels for aggrecan and type-II collagen, with increasing compression relative to physiological cut thickness applied for 24 hours. Radiolabel incorporation into glycosaminoglycans and collagen also decreased with increasing compression in a dose-related manner similar to the changes seen in mRNA expression. The modulation of aggrecan and type-II collagen mRNA and protein synthesis were dependent on the duration of the compression. Aggrecan and type-II collagen mRNA expression increased during the initial 0.5 hours of static compression; however, 4-24 hours after compression was applied total mRNA levels had significantly decreased. The synthesis of aggrecan and collagen protein decreased more rapidly than did mRNA levels after the application of a step compression. Together, these results suggest that mechanical compression rapidly alters chondrocyte aggrecan and type-II collagen gene expression on application of load. However, our results indicate that the observed decreases in biosynthesis may not be related solely to changes in mRNA expression. The mechanisms by which mechanical forces affect different segments of the biosynthetic pathways remain to be determined.

Design and characterization of orally active Arg-Gly-Asp peptidomimetic vitronectin receptor antagonist SB 265123 for prevention of bone loss in osteoporosis

The Arg-Gly-Asp (RGD)-binding integrin alpha(V)beta(3) is highly expressed on osteoclasts and has been proposed to mediate cell-matrix adhesion required for osteoclast-mediated bone resorption. Antagonism of this receptor should prevent stable osteoclast adhesion and thereby inhibit bone resorption. We have generated an orally bioavailable, nonpeptide RGD mimetic alpha(v)beta(3) antagonist, SB 265123, which prevents bone loss in vivo when dosed by oral administration. SB 265123 binds alpha(v)beta(3) and the closely related integrin alpha(v)beta(5) with high affinity (K(i) = 3.5 and 1.3 nM, respectively), but binds only weakly to the related RGD-binding integrins alpha(IIb)beta(3) (K(i) >1 microM) and alpha(5)beta(1) (K(i) >1 microM). The compound inhibits alpha(v)beta(3)-mediated cell adhesion with an IC(50) = 60 nM and more importantly, inhibits human osteoclast-mediated bone resorption in vitro with an IC(50) = 48 nM. In vivo, SB 265123 completely blocks bone resorption in a thyroparathyroidectomized rat model of acute bone resorption when dosed at 2.5 mg/kg/h by continuous i.v. infusion. When dosed orally with 3 to 30 mg/kg b.i.d. , in the ovariectomy-induced rat model of osteoporosis, SB 265123 prevents bone resorption in a dose-dependent fashion. This is the first report of an orally active alpha(v)beta(3) antagonist that is effective at inhibiting bone resorption when dosed in a pharmaceutically acceptable fashion. Such a molecule may provide a novel therapeutic agent for the treatment of postmenopausal osteoporosis.

Development and characterization of a human in vitro resorption assay: demonstration of utility using novel antiresorptive agents

A human in vitro resorption assay has been developed using osteoclastoma-derived osteoclasts and used to evaluate novel antiresorptive agents including antagonists of the alphavbeta3 integrin, and inhibitors of cathepsin K and the osteoclast ATPase. The potency of novel compounds in the in vitro resorption assay correlates with functional assays for each class of inhibitor: the human alphavbeta3-mediated cell adhesion assay for the vitronectin receptor antagonists (r2 = 0.82), the chick osteoclast vacuolar ATPase enzyme assay for the H+-ATPase inhibitors (r2 = 0.77) and the recombinant human cathepsin K enzyme assay for the cathepsin K inhibitors (r2 = 0.80). Cell suspensions, rich in osteoclasts, are prepared by collagenase digestion of the tumor tissue. These cells can be stored long-term in liquid nitrogen and upon thawing maintain their bone-resorbing phenotype. The cryopreserved cells can be cultured on bovine cortical bone for 24-48 h and resorption can be measured by either confocal microscopy or biochemical assays. The resorptive activity of osteoclasts derived from a number of tumors can be inhibited reproducibly using a number of mechanistically unique antiresorptive compounds. In addition, the measurement of resorption pits by laser confocal microscopy correlates with the release of type I collagen C-telopeptides or N-telopeptides, as measured by enzyme-linked immunosorbent assay. Resorption can be measured reproducibly using a 48-h incubation of osteoclasts on bone slices, or a 24-h incubation with bone particles. This in vitro human osteoclast resorption assay provides a robust system for the evaluation of inhibitors of osteoclastic function that may be developed for the treatment of metabolic bone diseases such as osteoporosis.

Inhibition of interleukin-1-induced proteoglycan degradation and nitric oxide production in bovine articular cartilage/chondrocyte cultures by the natural product, hymenialdisine

The effects of hymenialdisine (SK&F 108752) were evaluated on interleukin-1 (IL-1)-induced proteoglycan (PG) degradation, PG synthesis, nitric oxide (NO) production, and inducible nitric oxide synthase (iNOS) gene expression in bovine articular cartilage (BAC) and/or cartilage-derived chondrocytes. Cartilage disks from 0- to 3-month-old calves were treated with IL-1alpha or retinoic acid. PG release was determined by measuring glycosaminoglycan release, and nitrite production was measured as a readout for NO. Inhibition of iNOS gene expression was measured by Northern blot analysis in IL-1alpha-stimulated, cartilage-derived chondrocytes. To measure PG synthesis, chondrocytes were established in alginate beads and treated with hymenialdisine, and then [(35)S]sulfate incorporation into PGs was determined. Hymenialdisine inhibited IL-1alpha-stimulated PG breakdown in BAC in a dose-related manner with an IC(50) of approximately 0.6 microM. Herbimycin, a protein tyrosine kinase inhibitor, also inhibited PG breakdown, whereas RO 32-0432, a protein kinase C inhibitor, had no effect. Both hymenialdisine and herbimycin also were able to inhibit retinoic acid-stimulated PG release. IL-1alpha-stimulated NO production in BAC was inhibited by hymenialdisine and herbimycin at similar concentrations. The effect on iNOS gene expression was determined by Northern blot analysis in chondrocytes grown in monolayer, and inhibition by hymenialdisine was observed with an IC(50) of approximately 0.8 microM. In chondrocytes cultured in alginate beads, IL-1alpha inhibited PG synthesis, whereas hymenialdisine stimulated synthesis at low concentrations (0.6 and 1.25 microM), and higher doses (2.5 microM) were not stimulatory. Compounds with this profile may have utility in the treatment of osteoarthritis.

Orally bioavailable nonpeptide vitronectin receptor antagonists with efficacy in an osteoporosis model

A new series of potent nonpeptide vitronectin receptor antagonists, based on a novel carbocyclic Gly-Asp mimetic, has been discovered. A representative of this series, SB 265123 (4), has 100% oral bioavailability in rats, and is orally active in vivo in the ovariectomized rat model of osteoporosis.

Applications of micro-CT and MR microscopy to study pre-clinical models of osteoporosis and osteoarthritis

There is a tremendous unmet therapeutic need for the treatment of osteoporosis and osteoarthritis. The ovariectomized rat and the guinea pig are widely used animal models for the evaluation of new therapeutics for osteoporosis and osteoarthritis, respectively. We have utilized X-ray micro-CT techniques to quantitatively evaluate the differences in trabecular bone in the rat proximal tibia following ovariectomy and treatment with estrogen (17-B-estradiol). Results demonstrate a loss of trabecular bone and architecture following ovariectomy (p < 0.001), and a marked inhibition of trabecular bone loss in the estrogen treated group (p < 0.001). A similar change in architecture can be visualized in images obtained by high resolution MR microscopy. In addition, a good correlation was observed between the values of trabecular bone fraction (BV/TV) in the rat tibiae as obtained from 3-dimensional micro-CT data and 2-dimensional static histomorphometry (r = 0.89, 0.73, 0.79 for sham, OVX, and treated groups, respectively). Micro-CT images were also obtained from a set of lumbar vertebrae from sham operated and ovariectomized rats. Significant bone loss can be measured as early as 8 weeks following ovariectomy (p < 0.005). Micro-CT and MR images were also obtained to study age related changes in the stifle joint of the guinea pig. Significant boney changes can be seen in the tibia and femur from the animals at various ages. Changes in cartilage and joint space can also be visualized in the images. The utility of micro-CT imaging in evaluating the mouse skeletal system is illustrated by obtaining morphological and architectural details from high resolution images of the mouse hind limb and proximal tibia, respectively. The results demonstrate the advantages that multi-dimensional imaging techniques can offer in evaluating bone and joint related changes in animal models of osteoporosis and osteoarthritis.

Transport of tissue inhibitor of metalloproteinases-1 through cartilage: contributions of fluid flow and electrical migration

The preservation of the structure of articular cartilage depends on the availability of inhibitors of matrix-degrading enzymes. Tissue inhibitor of metalloproteinases-1 is thought to be an important contributor to the integrity of the matrix of articular cartilage, but the mechanisms that regulate its availability within the tissue are poorly understood. These studies elucidate the contributions of diffusion, fluid flow, and electrical migration to the transport of iodinated recombinant human tissue inhibitor of metalloproteinases-1 through explants of adult bovine articular cartilage under conditions relevant to the loading of cartilage. With use of measured partition coefficients of the cartilage explants, the diffusivity of the inhibitor was 0.5-1.6 x 10(-7) cm2/sec. Fluid velocities that were induced by applying an electrical current across the cartilage disks increased the flux of the inhibitor by approximately 20 to more than 150-fold compared with the effect of diffusion alone for the range of current densities that were applied. We examined the contribution of electrophoretic migration by titrating the charge on the inhibitor during measurements of flux and found that flux in the presence of the applied current decreased as the inhibitor became more negatively charged. Enhancements in the flux of the inhibitor were observed relative to the flux during diffusion alone even under conditions in which electrophoretic migration opposed the flux due to fluid flow, suggesting that of the transport mechanisms tested, fluid flow was dominant. These results suggest that the physical phenomena present during physiologic loading conditions (e.g., fluid flows and streaming currents) can affect the transport of tissue inhibitor of metalloproteinases-1 through the matrix of cartilage.

Longitudinal and cross-sectional variability in markers of joint metabolism in patients with knee pain and articular cartilage abnormalities

OBJECTIVE

To determine the within- and between-patient variability in the concentrations of synovial fluid, serum and urine markers of joint tissue metabolism in a cohort of patients with knee pain and cartilage changes consistent with early-stage knee osteoarthritis.

DESIGN

Samples of synovial fluid, serum, and urine were obtained from 52 patients on eight different occasions during 1 year, as part of a clinical trial in patients with cartilage abnormalities and knee pain. In joint fluid, aggrecan fragments were quantified by dye precipitation and enzyme-linked immunosorbent assay (ELISA), and matrix metalloproteinases-1 and -3, and tissue inhibitor of metalloproteinases-1 by sandwich ELISAs. In serum, keratan sulfate was quantified by ELISA. Type I collagen N-telopeptide cross-links in urine were determined by ELISA.

RESULTS

The degree of cross-sectional variability in marker concentrations did not vary between the different sampling occasions, and did not differ between the periods of weeks 0 (baseline), 1-4 (treatment) and 13-26 (follow-up). Both between-patient and within-patient coefficients of variation varied for markers in different body fluid compartments, with the lowest variability for serum keratan sulfate, followed by urine type I collagen N-telopeptide crosslinks, and the highest for synovial fluid markers. For synovial fluid, aggrecan fragments showed the least variability, and matrix metalloproteinases the highest. One patient with septic arthritis showed a fivefold peak increase in joint fluid aggrecan fragment concentrations, while the concentration of matrix metalloproteinase-3 increased 100-fold.

CONCLUSIONS

Molecular markers of joint tissue metabolism have been suggested as, for example, outcome measures for clinical trials of disease-modifying drugs in osteoarthritis. This report is the first to present data on between- and within-patient variability for such molecular markers in three different body fluid compartments in stable cohort of patients. The availability of such data enables calculations to determine the number of patients needed in prospective studies using these markers as outcome measures.

Increased secretion and activity of matrix metalloproteinase-3 in synovial tissues and chondrocytes from experimental osteoarthritis

OBJECTIVE

The aim of this study was to define the relative regulation of matrix metalloproteinase-3 (MMP-3), and tissue inhibitor of metalloproteinases-1 (TIMP-1), in chondrocytes and synovium in experimental osteoarthritis (EOA).

METHODS

Partial-meniscectomized (PM) rabbits, surgical sham controls (SH), and normal non-surgical controls (N) were killed at times corresponding to early degenerative lesions (4 weeks) and increasingly progressive stages of EOA at 8 and 12 weeks post-PM. MMP-3 activity was measured in conditioned media from chondrocytes and synovium using a peptide cleavage assay with substance P (SP) as the substrate. TIMP-1 was quantitated using an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Early degenerative lesions (4 weeks post-PM) were characterized by inflammatory responses in the synovium accompanied by a significant rise of MMP-3 activity in synovial cultures (P < 0.05). At 8 weeks there was no discernible inflammation, and MMP-3 activity in EOA synovial cultures was comparable to that in the controls; this was followed by a second increase in MMP-3 activity in EOA samples at 12 weeks. MMP-3 activity was significantly elevated in EOA chondrocyte cultures at 8 weeks post-PM relative to N controls, corresponding to the most destructive phase of EOA, but not in the early phase (4 weeks) or 'late' degenerative phase (12 weeks). Medium derived from chondrocytes contained little or no TIMP-1. Synovia secreted relatively higher amounts of TIMP-1, and this was elevated at 8 weeks post-PM relative to the SH controls. The majority (approximately 90%) of MMP-3 activity could be inhibited using recombinant TIMP-1 or a hydroxamate MMP inhibitor. Complete inhibition was achieved with EDTA or 1,10 phenanthroline.

CONCLUSION

Together, these data indicate that in EOA, MMP-3 is initially upregulated in the synovium which may play a pivotal role in the pathogenesis of cartilage lesions. In contrast, chondrocyte-derived MMP-3 is upregulated in the later phases of EOA, contributing further to progression of cartilage lesions.

SB 203580 inhibits p38 mitogen-activated protein kinase, nitric oxide production, and inducible nitric oxide synthase in bovine cartilage-derived chondrocytes

Nitric oxide (NO) is implicated in a number of inflammatory processes and is an important mediator in animal models of rheumatoid arthritis and in in vitro models of cartilage degradation. The pyridinyl imidazole SB 203580 inhibits p38 mitogen-activated protein (MAP) kinase in vitro, blocks proinflammatory cytokine production in vitro and in vivo, and is effective in animal models of arthritis. The purpose of this study was to determine whether SB 203580 could inhibit p38 MAP kinase activity, NO production, and inducible NO synthase (iNOS) in IL-1 stimulated bovine articular cartilage/chondrocyte cultures. The results indicated that SB 203580 inhibited both IL-1 stimulated p38 MAP kinase activity in isolated chondrocytes and NO production in bovine chondrocytes and cartilage explants with an IC50 value of approximately 1 microM. To inhibit NO production, SB 203580 had to be present in cartilage explant cultures during the first 8 h of IL-1 stimulation, and activity was lost when it was added 24 h following IL-1. SB 203580 did not inhibit iNOS activity, as measured by the conversion of arginine to citrulline, when added directly to cultures where the enzyme had already been induced, but had to be present during the induction period. Using a 372-bp probe for bovine iNOS we demonstrated inhibition of IL-1-induced mRNA by SB 203580 at both 4 and 24 h following IL-1 treatment. The iNOS mRNA levels were consistent with NO levels in 24-h cell culture supernatants of the IL-1-stimulated bovine chondrocytes used to obtain the RNA.

Design of potent and selective human cathepsin K inhibitors that span the active site

Potent and selective active-site-spanning inhibitors have been designed for cathepsin K, a cysteine protease unique to osteoclasts. They act by mechanisms that involve tight binding intermediates, potentially on a hydrolytic pathway. X-ray crystallographic, MS, NMR spectroscopic, and kinetic studies of the mechanisms of inhibition indicate that different intermediates or transition states are being represented that are dependent on the conditions of measurement and the specific groups flanking the carbonyl in the inhibitor. The species observed crystallographically are most consistent with tetrahedral intermediates that may be close approximations of those that occur during substrate hydrolysis. Initial kinetic studies suggest the possibility of irreversible and reversible active-site modification. Representative inhibitors have demonstrated antiresorptive activity both in vitro and in vivo and therefore are promising leads for therapeutic agents for the treatment of osteoporosis. Expansion of these inhibitor concepts can be envisioned for the many other cysteine proteases implicated for therapeutic intervention.

Aggrecanase and metalloproteinase-specific aggrecan neo-epitopes are induced in the articular cartilage of mice with collagen II-induced arthritis

OBJECTIVE

To analyze the roles of two classes of proteinases, 'aggrecanase', and matrix metalloproteinases (MMPs), in chondrodestruction during murine collagen-induced arthritis (CIA).

METHODS

Generation of the 'aggrecanase' neo-epitope (NITEGE373), and the MMP neo-epitope (VDIPEN341) within aggrecan was studied by immunoperoxidase microscopy using specific anti-peptide antibodies in normal and stromelysin-1 (SLN-1) deficient knockout mice with CIA.

RESULTS

High levels of NITEGE373 and VDIPEN341 neo-epitopes were observed in foci within CIA paw articular cartilage exhibiting depletion of glycosaminoglycans, in advance of significant cartilage erosion. The highest concentrations of NITEGE373 and VDIPEN341 labeling were observed and often co-distributed in the chondrocyte pericellular matrix, suggesting that stimulated chondrocytes can synthesize and/or activate both enzymes. Other regions of the cartilage frequently exhibited either NITEGE373 or VDIPEN341 labeling, but not both neo-epitopes simultaneously, suggesting that 'aggrecanase' and MMP cleavages of aggrecan may be generated independently. No detectable differences were observed in expression or distribution of either neo-epitope in SLN-1 knockout versus wild-type mice. In addition, in vitro digestion of joint sections with SLN-1 did not alter the expression of cartilage NITEGE373, while markedly increasing VDIPEN341 labeling. Peripheral nerves and brains of naive mice also exhibited intense anti-NITEGE373 labeling.

CONCLUSIONS

These data indicate that NITEGE373 and VDIPEN341 aggrecan neo-epitopes are sensitive and specific markers of early joint pathology, and are consistent with the hypothesis that SLN-1 does not have 'aggrecanase' activity, and that 'aggrecanase' is distinct from the MMPs which cleave aggrecan at the MMP site.

Inhibition of cartilage degradation and changes in physical properties induced by IL-1beta and retinoic acid using matrix metalloproteinase inhibitors

Bovine cartilage explants were treated with 100 ng/ml recombinant human interleukin-1beta (IL-1beta) or 1 microM all-trans retinoic acid (RA) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, samples cultured with IL-1beta or RA were treated with 4 microM recombinant human tissue inhibitor of metalloproteinases-1 (TIMP-1) or a synthetic metalloproteinase inhibitor (L-758,354) to inhibit this degradation. Treatment with IL-1beta or RA each resulted in >90% GAG loss after 8 days in culture. Addition of TIMP or L-758,354 to the culture media inhibited IL-1beta-induced loss of tissue GAG by 40 and 65%, respectively, and inhibited RA-induced GAG loss by 35 and 65%, respectively. Analysis of degradation products in the culture media using a G1 antibody indicated that IL-1beta- and RA-treated plugs released 68-kDa fragments of aggrecan, corresponding to a segment of the aggrecan core protein from the G1 domain to the C-terminus NITEGE, consistent with "aggrecanase" activity. Release of the G1 fragment was inhibited by treatment with L-758,354. Both IL-1beta and RA induced significant loss of hyaluronan from cartilage explants after 8 days of exposure and HA loss was also inhibited by addition of L-756,354 to the culture media. IL-1beta, but not RA, induced a significant increase in swelling ratio (wet weight in 0.01 M NaCl normalized to wet weight in DMEM) after 8 days in culture, consistent with degradation of the collagen network, and the increase in tissue swelling was inhibited by treatment with TIMP-1 or L-758,354. Exposure to IL-1beta or RA resulted in significant changes in cartilage physical properties including streaming potential, equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient after 8 days in culture, and these changes were inhibited by 40-90% by exposure to TIMP and by 50-90% by exposure to L-758,354. Measurement of dynamic streaming potential showed that changes due to treatment with IL-1beta alone were highly dependent in compression frequency, with dramatic changes seen at high frequency prior to changes in mechanical properties, and little initial change seen at low frequency. Streaming potential and equilibrium modulus of explants treated with RA decreased to 10% of their initial values after 8 days in culture, but decreased to only 40 and 90%, respectively, when treated with RA plus TIMP-1.

Aggrecan degradation in human cartilage. Evidence for both matrix metalloproteinase and aggrecanase activity in normal, osteoarthritic, and rheumatoid joints

To examine the activity of matrix metalloproteinases (MMPs) and aggrecanase in control and diseased human articular cartilage, metabolic fragments of aggrecan were detected with monospecific antipeptide antibodies. The distribution and quantity of MMP-generated aggrecan G1 fragments terminating in VDIPEN341 were compared with the distribution of aggrecanase-generated G1 fragments terminating in NITEGE373. Both types of G1 fragments were isolated from osteoarthritic cartilage. The sizes were consistent with a single enzymatic cleavage in the interglobular domain region, with no further proteolytic processing of these fragments. Both neoepitopes were also detected by immunohistochemistry in articular cartilage from patients undergoing joint replacement for osteoarthritis (OA), rheumatoid arthritis (RA), and in cartilage from adults with no known joint disease. In control specimens, the staining intensity for both G1 fragments increased with age, with little staining in cartilage from 22-wk-old fetal samples. There was also an increase with age in the extracted amount of MMP-generated neoepitope in relation to both aggrecan and collagen content, confirming the immunohistochemical results. After the age of 20-30 yr this relationship remained at a steady state. The staining for the MMP-generated epitope was most marked in control cartilage exhibiting histological signs of damage, whereas intense staining for the aggrecanase-generated fragment was often noted in adult cartilage lacking overt histological damage. Intense staining for both neoepitopes appeared in the more severely fibrillated, superficial region of the tissue. Intense immunostaining for both VDIPEN- and NITEGE- neoepitopes was also detected in joint cartilage from patients with OA or RA. Cartilage in these specimens was significantly more degraded and high levels of staining for both epitopes was always seen in areas with extensive cartilage damage. The levels of extracted VDIPEN neoepitope relative to collagen or aggrecan in both OA and RA samples were similar to those seen in age-matched control specimens. Immunostaining for both types of aggrecan fragments was seen surrounding the cells but also further removed in the interterritorial matrix. In some regions of the tissue, both neoepitopes were found while in others only one was detected. Thus, generation and/or turnover of these specific catabolic aggrecan fragments is not necessarily coordinated. Our results are consistent with the presence in both normal and arthritic joint cartilage of proteolytic activity against aggrecan based on both classical MMPs and "aggrecanase."

Activation and inhibition of endogenous matrix metalloproteinases in articular cartilage: effects on composition and biophysical properties

Bovine cartilage explants were cultured with 1 mM 4-aminophenylmercuric acetate (APMA) to activate endogenous matrix metalloproteinases (MMPs) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, graded levels of either rhTIMP-1 (recombinant human tissue inhibitor of metalloproteinases-1) or L-696-418 (a synthetic metalloproteinase inhibitor) were used to inhibit degradation induced by APMA. Treatment with APMA resulted in as much as 80% loss in tissue GAG content, a greater than threefold increase in denatured type II collagen as determined by the presence of CB11B epitope, and complete loss of biosynthetic activity after 3 days in culture. Physicochemical studies revealed that APMA treatment resulted in a significant increase in tissue swelling response, consistent with damage to the collagen network. Activation of MMPs by APMA also resulted in > 80% decrease in equilibrium modulus, dynamic stiffness, and streaming potential and > 50% decrease in electrokinetic coupling coefficient. The addition of 4 microM, 400 nM, and 40 nM TIMP inhibited PG loss by 95, 50, and 20%, respectively, and all doses effectively inhibited swelling response. The addition of 4 microM and 400 nM L-696-418 inhibited PG loss by 95% while 40 nM L-696-418 inhibited PG loss by 60%, and all doses effectively inhibited swelling response. The inhibition of APMA-induced GAG loss by 4 microM TIMP was accompanied by maintenance of streaming potential, electrokinetic coupling coefficient, dynamic stiffness, and equilibrium modulus.

Circumferential stress and matrix metalloproteinase 1 in human coronary atherosclerosis. Implications for plaque rupture

Atherosclerotic plaque rupture may occur when regions of weakened extracellular matrix are subjected to increased mechanical stresses. Since collagen is a major determinant of extracellular matrix strength, enzymes that degrade collagen may play an important role in destabilizing the atherosclerotic lesion. To test the hypothesis that matrix metalloproteinase 1 (interstitial collagenase, or MMP-1), which initiates degradation of fibrillar collagens, colocalizes with increased stress in the fibrous cap of the atherosclerotic lesion, 12 unruptured human coronary lesions were studied. Finite-element analysis was used to determine the distribution of stress in the lesion, with estimates of material properties from previous measurements of human tissues. A computerized image analysis system was used to determine the distribution of immunoreactive MMP-1 within the fibrous tissue of the lesion. There was a significant correlation between immunoreactive MMP-1 and circumferential tensile stress in the fibrous cap within a given lesion (median Spearman rank correlation coefficient, .36; interquartile range, -.02 to .81; P < .02). Within a given lesion, the highest-stress region had twofold greater MMP-1 expression than the lowest-stress regions. In unruptured human atherosclerotic coronary lesions, overexpression of MMP-1 is associated with increased circumferential stress in the fibrous plaque. Degradation and weakening of the collagenous extracellular matrix at these critical high-stress regions may play a role in the pathogenesis of plaque rupture and acute ischemic syndromes.

Regulation of matrix metalloproteinases and plasminogen activator inhibitor-1 synthesis by plasminogen in cultured human vascular smooth muscle cells

Plasmin and matrix metalloproteinases (MMPs) both participate in extracellular matrix remodeling. This study examined the effects of tumor necrosis factor-alpha (TNF-alpha) and plasminogen on collagenase, stromelysin, and plasminogen activator inhibitor-1 (PAI-1) synthesis of collagenase and stromelysin, which remained predominantly in proenzyme forms, as determined by Western analysis of culture media. In contrast, plasminogen and plasmin not only increased secretion of MMPs but also induced cleavage to their active forms. The serine protease inhibitor aprotinin inhibited this activation of MMPs by plasminogen and plasmin. TNF-alpha reduced plasminogen-induced activation of MMPs, suggesting induction of an inhibitor or plasmin generation, such as PAI-1. Enzyme-linked immunosorbent assay of culture media showed that TNF-alpha (10 ng/mL) increased PAI-1 secretion by 4.2 fold compared with control (105.5 +/- 9.6) versus 24.9 +/- 1.7 ng/mL, n = 3). Surprisingly plasminogen also increased PAI-1 secretion by vascular SMCs (3.6-fold over control). These results demonstrate coordination of cytokines and serine proteases in regulating MMP secretion and activation. In addition, the induction of PAI-1 by TNF-alpha and plasminogen suggests a negative feedback mechanisms limit both plasmin-mediated and MMP-mediated matrix degradation.

Human vascular smooth muscle cell-monocyte interactions and metalloproteinase secretion in culture

Degradation of the atherosclerotic plaque extracellular matrix could destabilize the lesion, rendering it more prone to rupture. Both macrophages and vascular smooth muscle cells (SMCs) are potential sources of matrix metalloproteinases (MMPs), secreted enzymes that can digest vascular matrix. We explored interactions between human vascular SMCs and human monocytes that result in the secretion of interstitial collagenase (MMP-1) and stromelysin (MMP-3). Monocytes alone or those treated with SMC-conditioned media did not secrete these metalloproteinases as detectable by Western blot analysis. SMCs increased secretion of both MMP-1 and MMP-3 greater than 20-fold when cocultured with monocytes or when treated with monocyte-conditioned media. Addition of macrophage colony stimulating factor (< or = 1000 U/mL) to cocultures of monocytes and SMCs did not affect metalloproteinase secretion. Recombinant interleukin (IL)-1 receptor antagonist inhibited MMP-1 and MMP-3 induction in SMC cultures treated with monocyte-conditioned media (94% and 96% reduction, respectively), while a neutralizing antibody to tumor necrosis factor-alpha had no significant effect on metalloproteinase secretion. In contrast to the induction by monocyte-conditioned media of MMP-1 and MMP-3 secretion by SMCs, monocyte-conditioned media did not increase secretion of 72-kD gelatinase (MMP-2). Thus, monocytes induce MMP-1 and MMP-3 secretion by vascular SMCs through an IL-1-dependent mechanism. This response of SMCs to a defined macrophage product may contribute to plaque destabilization by mononuclear phagocytes in the lesion.

Glial hyaluronate-binding protein: a product of metalloproteinase digestion of versican?

Glial hyaluronate-binding protein (GHAP) is a 60 kDa glycoprotein with an amino acid sequence identical to that of the hyaluronate-binding region of versican, a large fibroblast aggregating proteoglycan found in the brain. Both GHAP and versican were identified by immunoblot in bovine brain extracts prepared only minutes after death. Human recombinant collagenase, stromelysin, mouse gelatinase and gelatinases isolated from human brain by affinity chromatography digest versican and give rise to a polypeptide with electrophoretic mobility identical to GHAP. Immunoblot analysis, peptide mapping and C-terminal amino acid sequencing indicate that the polypeptide generated by digestion with human brain gelatinases is identical to GHAP. We suggest that GHAP is a naturally occurring versican degradation product.

In vivo effects of stromelysin on the composition and physical properties of rabbit articular cartilage in the presence and absence of a synthetic inhibitor

OBJECTIVE

To characterize the effects of intraarticular injection of recombinant human stromelysin (SLN) on the matrix composition and physical properties of cartilage from lapine stifle joints and the modulation of these effects by the systemic administration of an N-carboxyalkyl synthetic matrix metalloproteinase inhibitor, L-696,418.

METHODS

Female 6-8-week-old New Zealand white rabbits received an intraarticular injection of 100 micrograms activated SLN in 1 stifle joint and buffer in the contralateral control knee; these animals were killed after 1 hour. A separate group of animals received an intravenous injection of either 30 mg/kg L-696,418 or buffer prior to intraarticular injection of SLN. Joints were dissected and analyzed for proteoglycan (PG) loss into joint fluid, tissue biochemical composition, and histology by toluidine blue or anti-VDIPEN antibody staining, or were frozen for physical property analysis. Disks of femoropatellar groove cartilage were harvested from the stifle joint and tested in uniaxially confined compression for determination of electromechanical and mechanical properties.

RESULTS

Lapine stifle joints that received injection of SLN without systemic administration of L-696,418 showed a 13-fold increase in loss of PG into synovial fluid. Cartilage from these joints showed significant decreases in streaming potential at 1 Hz and electrokinetic coupling coefficient, but no change in equilibrium modulus, dynamic stiffness, or hydraulic permeability. Systemic treatment with L-696,418 resulted in a significant decrease in loss of PG into joint fluid and elimination of changes in cartilage high-frequency streaming potential and coupling coefficient in joints that were injected with SLN.

CONCLUSION

The 1-hour exposure to SLN in vivo resulted in loss of PG and exposure of the VDIPEN epitope of the aggrecan core protein in the superficial region of the tissue near the articular surface. This highly localized degradation resulted in electromechanical behavior changes, but little or no change occurred in mechanical properties. Systemic administration of L-696,418 significantly decreased loss of PG from cartilage and prevented the highly localized tissue degradation and the resultant changes in electromechanical behavior caused by intraarticular SLN injection.