C2K77 ELISA detects cleavage of type II collagen by cathepsin K in equine articular cartilage.

OBJECTIVES: Develop a species-specific ELISA for a neo-epitope generated by cathepsin K cleavage of equine type II collagen to: (1) measure cartilage type II collagen degradation by cathepsin K in vitro, (2) identify cytokines that upregulate cathepsin K expression and (3) compare cathepsin K with matrix metalloproteinase (MMP) collagenase activity in stimulated cartilage explants and freshly isolated normal and osteoarthritic (OA) articular cartilages. DESIGN: A new ELISA (C2K77) was developed and tested by measuring the activity of exogenous cathepsin K on equine articular cartilage explants. The ELISA was then employed to measure endogenous cathepsin K activity in cultured cartilage explants with or without stimulation by interleukin-1 beta (IL-1ß), tumour necrosis-alpha (TNF-α), oncostatin M (OSM) and lipopolysaccharide (LPS). Cathepsin K activity in cartilage explants (control and osteoarthritic-OA) and freshly harvested cartilage (control and OA) was compared to that of MMPs employing C2K77 and C1,2C immunoassays. RESULTS: The addition of Cathepsin K to normal cartilage caused a significant increase (P < 0.01) in the C2K77 epitope release. Whereas the content of C1,2C, that reflects MMP collagenase activity, was increased in media by the addition to cartilage explants of TNF-α and OSM (P < 0.0001) or IL-1ß and OSM (P = 0.002), no change was observed in C2K77 which also unchanged in OA cartilages compared to normal. CONCLUSIONS: The ELISA C2K77 measured the activity of cathepsin K in equine cartilage which was unchanged in OA cartilage. Cytokines that upregulate MMP collagenase activity had no effect on endogenous cathepsin K activity, suggesting a different activation mechanism that requires further study.

Early cathepsin K degradation of type II collagen in vitro and in vivo in articular cartilage.

OBJECTIVE: To characterize the initial events in the cleavage of type II collagen mediated by cathepsin K and demonstrate the presence of the resulting products in human and equine articular osteoarthritic cartilage. DESIGN: Equine type II collagen was digested with cathepsin K and the cleavage products characterized by mass spectrometry. Anti-neoepitope antibodies were raised against the most N-terminal cleavage products and used to investigate the progress of collagen cleavage, in vitro, and the presence of cathepsin K-derived products in equine and human osteoarthritic cartilage. RESULTS: Six cathepsin K cleavage sites distributed throughout the triple helical region were identified in equine type II collagen. Most of the cleavages occurred following a hydroxyproline residue. The most N-terminal site was within three residues of the previously identified site in bovine type II collagen. Western blotting using anti-neoepitope antibodies showed that the initial cleavages occurred at the N-terminal sites and this was followed by more extensive degradation resulting in products too small to be resolved by SDS gel electrophoresis. Immunohistochemical staining of cartilage sections from equine or human osteoarthritic joints showed staining in lesional areas which was not observed in non-arthritic sites. CONCLUSIONS: Cathepsin K cleaves triple helical collagen by erosion from the N-terminus and with subsequent progressive cleavages. The liberated fragments can be detected in osteoarthritic cartilage and may represent useful biomarkers for disease activity.

The non-aggregated aggrecan in the human intervertebral disc can arise by a non-proteolytic mechanism.

Analysis of both the aggregated and non-aggregated fractions of aggrecan isolated from adult human intervertebral disc using immunoblotting with antibodies specific for the different domains constituting the aggrecan core protein or atomic force microscopy revealed that many components contained the G1 domain. However, little of the disc aggrecan was able to reform aggregates with hyaluronan, as determined by gel filtration chromatography, suggesting that the G1 domains had been rendered non-functional. Since previous studies have shown that disc aggrecan undergoes non-enzymatic glycation with age, the functional effect of such modification was investigated in vitro using bovine aggrecan isolated from young animals. Incubation of monomeric aggrecan with ribose to induce glycation rendered it unable to form complexes with hyaluronan stable to agarose gel electrophoresis or gel filtration chromatography. Similarly, extended treatment of intact proteoglycan aggregate with ribose resulted in destabilisation of the complex with separation of the aggrecan from the hyaluronan. Although it is clear that proteolysis occurs in the intervertebral disc and gives rise to some non-aggregating molecules, a different mechanism is required to explain the presence of many non-aggregating molecules bearing the G1 domain. The products of non-enzymatic glycation of the globular domains of aggrecan would account for this phenomenon and explain why some of the non-aggregating molecules are still large proteoglycans. While such molecules may be retained in the nucleus pulposus, they may be able to diffuse within it, reducing the ability of the tissue to resist compression under asymmetric loading such as bending and ultimately contributing to disc degeneration.

Involvement of ADAMTS5 and hyaluronidase in aggrecan degradation and release from OSM-stimulated cartilage.

The relative contribution of a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)4 and ADAMTS5 to aggrecan degradation under oncostatin M (OSM) stimulation, the role of the ancillary domains of the aggrecanases on their ability to cleave within the chondroitin sulfate (CS)-2 region, the role of hyaluronidases (HYAL) in stimulating aggrecan release in the absence of proteolysis, and the identity of the hyaluronidase involved in OSM-mediated cartilage breakdown were investigated. Bovine articular cartilage explants were cultured in the presence of interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNFalpha) and/or OSM, or treated with trypsin and/or hyaluronidase. Aggrecan was digested with various domain-truncated isoforms of ADAMTS4 and ADAMTS5. Aggrecan and link protein degradation and release were analyzed by immunoblotting. Aggrecanase and HYAL gene expression were determined. ADAMTS4 was the most inducible aggrecanase upon cytokine stimulation, whereas ADAMTS5 was the most abundant aggrecanase. ADAMTS5 was the most active aggrecanase and was responsible for the generation of an OSM-specific degradation pattern in the CS-2 region. Its ability to cleave at the OSM-specific site adjacent to the aggrecan G3 region was enhanced by truncation of the C-terminal thrombospondin domain, but reduced by further truncation of both the spacer and cysteine-rich domains of the enzyme. OSM has the ability to mediate proteoglycan release through hyaluronan degradation, under conditions where HYAL-2 is the predominant hyaluronidase being expressed. Compared to other catabolic cytokines, OSM exhibits a unique potential at degrading the proteoglycan aggregate, by promoting early robust aggrecanolysis, primarily through the action of ADAMTS5, and hyaluronan degradation.

Evidence to suggest that cathepsin K degrades articular cartilage in naturally occurring equine osteoarthritis.

OBJECTIVE: The mechanisms leading to degeneration of articular cartilage in osteoarthritis (OA) are complex and not yet fully understood. Cathepsin K (CK) is a cysteine protease which can also cleave the triple helix of type II collagen. This exposes a neoepitope that can now be identified by specific antibodies. The aim of this study was to obtain evidence suggesting a role for CK in naturally occurring equine OA in both lesional and peri-lesional regions. METHODS: Articular cartilages (n=12 horses; 5 healthy, 7 OA) were harvested from animals postmortem. A gross macroscopic examination, histologic (Safranin O-Fast Green and Picrosirius red staining) and immunohistochemical evaluation were performed. Samples were divided into normal appearing cartilage, peri-lesional and lesional cartilage. Cartilage degradation in the samples was graded histologically and immunohistochemically. CK and possible CK cleavage were detected immunohistochemically with specific anti-protein and anti-neoepitope antibodies, respectively. A comparison of CK neoepitope (C2K) production with the collagenase-generated neoepitope produced by matrix metalloproteinases (MMP)-1, 8 and 13 (C2C) was also assessed immunohistochemically. RESULTS: CK and CK cleavage were significantly more abundant in OA cartilage (both peri-lesional and lesional) when compared to remote cartilage within the sample joint or cartilage from healthy joints. The immunohistochemical pattern observed for CK degradation (C2K) was similar to that of collagenase degradation (C2C). Macroscopic cartilage changes and histologic findings were significantly correlated with immunohistochemistry results. CONCLUSION: The data generated suggests that CK may be involved in cartilage collagen degradation in naturally occurring osteoarthritis.

Myeloid-related proteins S100A8/S100A9 regulate joint inflammation and cartilage destruction during antigen-induced arthritis.

OBJECTIVE: To study the active involvement of Myeloid-related proteins S100A8 and S100A9 in joint inflammation and cartilage destruction during antigen-induced arthritis (AIA). METHODS: Joint inflammation and cartilage destruction was measured with 99mTc uptake and histology. The role of S100A8/A9 was investigated by inducing AIA in S100A9-/- mice that also lack S100A8 at protein level, or after intra-articular injection of rS100A8 in mouse knee joints. Cartilage destruction was measured using immunolocalisation of the neoepitope VDIPEN or NITEGE. mRNA levels of matrix metalloproteinases (MMPs) and cytokines were measured using reverse transcriptase (RT)-PCR. RESULTS: Immunisation of S100A9-/- mice with the antigen mBSA induced normal cellular and humoral responses, not different from wild type (WT) controls. However, joint swelling measured at day 3 and 7 after AIA induction was significantly lower (36 and 70%, respectively). Histologically, at day 7 AIA, cellular mass was much lower (63-80%) and proteoglycan depletion from cartilage layers was significantly reduced (between 50-95%). Cartilage destruction mediated by MMPs was absent in S100A9-/- mice but clearly present in controls. MMP3, 9 and 13 mRNA levels were significantly lowered in arthritic synovia of S100A9-/-. In vitro stimulation of macrophages by the heterodimer S100A8/A9 or S100A8 elevated mRNA levels of MMP3, 9 and in particular MMP13. Intra-articular injection of S100A8 caused prominent joint inflammation and depletion of proteoglycans at day 1. Significant upregulation of mRNA levels of S100A8/A9, cytokines (interleukin 1 (IL1)), MMPs (MMP3, MMP13 and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)4) was found in the synovium and correlated with strong upregulation of NITEGE neoepitopes within the cartilage layers. CONCLUSIONS: S100A8/A9 regulate joint inflammation and cartilage destruction during antigen-induced arthritis.

Characterization of an ADAMTS-5-mediated cleavage site in aggrecan in OSM-stimulated bovine cartilage.

OBJECTIVE: In a previous study, we identified a 50-kDa G3-containing aggrecan degradation product in bovine cartilage, released from the tissue after interleukin-1 (IL-1) stimulation in the presence of oncostatin M (OSM). Our objective was to purify, determine the N-terminal sequence of this fragment and verify whether this cleavage could be attributed to a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5 action in vitro. METHODS: Collected media from bovine cartilage explant cultures stimulated with IL-1+OSM were subjected to anion-exchange chromatography. The N-terminal sequence of the fragment of interest in the purified fractions was determined by automated Edman sequencing. Fetal bovine aggrecan was digested with full-length recombinant ADAMTS-4 and ADAMTS-5 and resulting degradation products were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE) and immunoblotting using an anti-G3 antiserum and an anti-neoepitope antibody that had been generated to the new N-terminus of the G3 fragment. RESULTS: Characterization of the 50-kDa fragment showed that it possesses chondroitin sulfate (CS) and is the result of a cleavage within the C-terminal portion of the CS-2 domain, adjacent to the G3 region. Sequence analysis identified the cleavage region as TQRPAE(2047)-(2048)ARLEIE, suggesting an aggrecanase-derived product. Using an anti-neoepitope antibody specific for the additional cleavage site, it was shown that the product is generated in vitro upon digestion of aggrecan by ADAMTS-5 and, to a much lesser extent, by ADAMTS-4. CONCLUSIONS: The abundance and rapid rate of release of this degradation product in organ cultures in the presence of OSM suggest that it could result from a unique aggrecan proteolysis mediated by aggrecanases.

Preferential mRNA expression of prostromelysin relative to procollagenase and in situ localization in human articular cartilage.

An imbalance between extracellular proteinases and their inhibitors is thought to underlie cartilage degradation. In cultures of adult cartilage, prostromelysin mRNA levels were much higher than those for procollagenase and this differential was increased in cultures stimulated with IL-1 beta. Analysis of mRNA prepared from freshly isolated chondrocytes showed abundant amounts of prostromelysin mRNA in normal adult cartilage but low levels in the neonate. Not all adult cartilage may possess such high levels of prostromelysin mRNA, as the message levels in the cartilage remaining on late-stage osteoarthritic joints were lower than those in normal adult cartilage. Relative to prostromelysin mRNA, little procollagenase and TIMP mRNA were found in the adult cartilage. In situ hybridization revealed that metalloproteinase mRNAs were localized in chondrocytes of the superficial zone in adult cartilage. However, upon IL-1 beta treatment, chondrocytes in all cartilage zones were observed to express prostromelysin mRNA. Relative to the neonate, the normal adult cartilage appears to have a high degradative potential, if one accepts that steady-state mRNA levels reflect prostromelysin production. As the adult cartilage is not apparently undergoing rapid turnover, it would appear that control of prostromelysin activation may be the major regulatory step in stromelysin-induced cartilage degradation.

Structure of rat procathepsin B: model for inhibition of cysteine protease activity by the proregion.

BACKGROUND: Cysteine proteases of the papain superfamily are synthesized as inactive precursors with a 60-110 residue N-terminal prosegment. The propeptides are potent inhibitors of their parent proteases. Although the proregion binding mode has been elucidated for all other protease classes, that of the cysteine proteases remained elusive. RESULTS: We report the three-dimensional structure of rat procathepsin B, determined at 2.8 A resolution. The 62-residue proregion does not form a globular structure on its own, but folds along the surface of mature cathepsin B. The N-terminal part of the proregion packs against a surface loop, with Trp24p (p indicating the proregion) playing a pivotal role in these interactions. Inhibition occurs by blocking access to the active site: part of the proregion enters the substrate-binding cleft in a similar manner to a natural substrate, but in a reverse orientation. CONCLUSIONS: The structure of procathepsin B provides the first insight into the mode of interaction between a mature cysteine protease from the papain superfamily and its prosegment. Maturation results in only one loop of cathepsin B changing conformation significantly, replacing contacts lost by removal of the prosegment. Contrary to many other proproteases, no rearrangement of the N terminus occurs following activation. Binding of the prosegment involves interaction with regions of the enzyme remote from the substrate-binding cleft and suggests a novel strategy for inhibitor design. The region of the prosegment where the activating cleavage occurs makes little contact with the enzyme, leading to speculation on the activation mechanism.

Characterization of a cysteine proteinase secreted by mouse mammary gland.

The cysteine proteinase activity secreted by cultured mammary gland was characterized to determine its relationship to a similar enzyme secreted by explants of mouse mammary tumors. Enzymic characterization showed that the secreted enzyme was similar to the lysosomal cysteine proteinase cathepsin B, and physical characterization showed properties identical to a stable cysteine proteinase secreted from mammary tumors reported previously. The secreted enzyme cross-reacted with mouse cathepsin B isolated from liver in a radioimmunoassay. The secreted enzymes are stable at alkaline pH, but irreversible conformational changes can be induced in vitro which render them unstable and thus similar to lysosomal cathepsin B, which is also unstable at alkaline pH. Tissue homogenates from fresh and cultured mammary gland contain mainly pH-unstable cathepsin B; however, the molecular size for the tissue cathepsin B, while smaller than that of the secreted enzymes, was found to be larger than that reported for mouse liver cathepsin B. Isoelectric focusing profiles were also slightly different as compared to those of mouse liver. These data suggest that there might be differences in the processing of cathepsin B between different tissues and organs, and the high degree of similarity between the forms of cathepsin B secreted from mouse mammary tumors, mouse mammary gland, and human malignant breast tumors suggests a similar mechanism for their extracellular release in these tissues.

Secretion of a thiol proteinase from mouse mammary carcinomas and its characterization.

Spontaneous mammary tumors from C3H/HeJ mice and transplants established from mammary tumors were investigated for their capacity to secrete thiol-dependent proteinase activity in organ culture explants. More activity was detected in culture media from spontaneous tumors than from transplanted spontaneous tumors. The accumulation of thiol proteinase in the culture medium was inhibited by cycloheximide, hydrocortisone, and aldosterone, but not by estradiol or the peptide hormones insulin or prolactin. The thiol proteinase is similar in enzymic properties to lysosomal cathepsin B, but its physical properties are different. It is stable to alkaline pH, has a larger molecular size on gel filtration (relative M.W. 39,000) and shows a different isoenzyme pattern to liver cathepsin B on analytical isoelectric focusing. The characteristics of this thiol proteinase are very similar to an enzyme secreted from malignant human breast tumors.

A latent thiol proteinase from ascitic fluid of patients with neoplasia.

Pepsin treatment of ascitic fluid from patients with neoplasia generated a cysteine (thiol) proteinase activity which resembles cathepsin B (EC 3.4.22.1) in its requirements for thiol activators, susceptibility to inhibitors and specificity for synthetic substrates. As judged by gel filtration, pepsin reduced the molecular size of the latent enzyme from an Mr of 41,000 to 33,000 after activation. Both forms are larger than human liver cathepsin B. In addition to its presence in ascitic fluid, the pepsin-activated species was found in the medium of ascites cells maintained in culture. The latent enzyme may be an enzyme-inhibitor complex or an inactive precursor of a cathepsin B-like proteinase.

Characterization of a latent cysteine proteinase from ascitic fluid as a high molecular weight form of cathepsin B.

The latent cysteine proteinase present in ascitic fluid of patients with neoplasia and released from ascites cells in culture has been partially purified and the enzyme after pepsin activation was shown to be immunologically related to the lysosomal proteinase, cathepsin B. The latent form was characterized as a single chain of Mr 40 000 as determined by SDS-polyacrylamide gel electrophoresis under reducing conditions followed by Western blotting and immune staining with an antiserum to human cathepsin B. Using the same techniques the enzyme after pepsin activation gave a single band of Mr 33 000. Analysis by isoelectric focusing showed that the latent enzyme before and after pepsin treatment is composed of several acidic isoenzymes. These findings suggest that this latent proteinase represents a precursor form of cathepsin B which is released extracellularly rather than being processed and directed to the lysosome.

Characterization of a thiol proteinase secreted by malignant human breast tumours.

It has previously been demonstrated (Poole, A.R., Tiltman, K.J., Recklies, A.D. and Stoker, T.A.M. (1978) Nature 273, 545-547) that malignant human breast tumours maintained in organ culture secrete elevated amounts of a thiol proteinase. This enzyme has been shown to possess enzymic properties similar to those of cathepsin B (EC 3.4.22.1) with respect to specificity, affinity and pH optima for synthetic substrates. However, the tumour enzyme is much more stable than human liver cathepsin B to inactivation above neutral pH, and it also has a large molecular size and a more acidic isoenzyme pattern. The stability of this enzyme under physiological conditions may allow it to play a role in tumour invasion and metastasis.

Peroxisome proliferator-activated receptor δ promotes the progression of posttraumatic osteoarthritis in a mouse model.

OBJECTIVE: Osteoarthritis (OA) is a serious disease of the entire joint, characterized by articular cartilage degeneration, subchondral bone changes, osteophyte formation, and synovial hyperplasia. Currently, there are no pharmaceutical treatments that can slow the disease progression, resulting in greatly reduced quality of life for patients and the need for joint replacement surgeries in many cases. The lack of available treatments for OA is partly due to our incomplete understanding of the molecular mechanisms that promote disease initiation and progression. The purpose of the present study was to examine the role of the nuclear receptor peroxisome proliferator-activated receptor δ (PPARδ) as a promoter of cartilage degeneration in a mouse model of posttraumatic OA. METHODS: Mouse chondrocytes and knee explants were treated with a pharmacologic agonist of PPARδ (GW501516) to evaluate changes in gene expression, histologic features, and matrix glycosaminoglycan breakdown. In vivo, PPARδ was specifically deleted from the cartilage of mice. Histopathologic scoring according to the Osteoarthritis Research Society International (OARSI) system and immunohistochemical analysis were used to compare mutant and control mice subjected to surgical destabilization of the medial meniscus (DMM). RESULTS: In vitro, PPARδ activation by GW501516 resulted in increased expression of several proteases in chondrocytes, as well as aggrecan degradation and glycosaminoglycan release in knee joint explants. In vivo, cartilage-specific PPARδ-knockout mice did not display any abnormalities of skeletal development but showed marked protection in the DMM model of posttraumatic OA (as compared to control littermates). OARSI scoring and immunohistochemical analyses confirmed strong protection of mutant mice from DMM-induced cartilage degeneration. CONCLUSION: These data demonstrate a catabolic role of endogenous PPARδ in posttraumatic OA and suggest that pharmacologic inhibition of PPARδ is a promising therapeutic strategy.

Cathepsin B.

Cathepsin B is a lysosomal cysteine protease of the papain family. It functions in intracellular protein catabolism and in certain situations may also be involved in other physiological processes, such as processing of antigens in the immune response, hormone activation and bone turnover. There is also evidence that cathepsin B is implicated in the pathology of chronic inflammatory diseases of airways and joints, and in cancer and pancreatitis. In this short review we outline the major structural features of the enzyme, and describe how these relate to its synthesis, trafficking, processing and function.

Two-step mechanism of inhibition of cathepsin B by cystatin C due to displacement of the proteinase occluding loop.

Stopped-flow kinetics showed that the inhibition of the lysosomal cysteine proteinase, cathepsin B, by its endogenous inhibitor, cystatin C, occurs by a two-step mechanism, in which an initial, weak interaction is followed by a conformational change. The initial interaction most likely involves binding of the N-terminal region of the inhibitor to the proteinase. Considerable evidence indicates that the subsequent conformational change is due to the inhibitor displacing the occluding loop of the proteinase that partially obscures the active site. The presence of this loop, which allows the enzyme to function as an exopeptidase, thus complicates the inhibition mechanism, rendering cathepsin B much less susceptible than other cysteine proteinases to inhibition by cystatins.

Cystatin inhibition of cathepsin B requires dislocation of the proteinase occluding loop. Demonstration By release of loop anchoring through mutation of his110.

Cystatins A and C were both shown to inhibit cathepsin B by a two-step mechanism, involving an initial weak interaction followed by a conformational change. Disruption of the major salt bridge anchoring the occluding loop of cathepsin B to the main body of the enzyme by mutation of His110 to Ala converted the binding to an apparent one-step reaction. The second step of cystatin binding to cathepsin B must therefore be due to the inhibitor having to alter the conformation of the enzyme by displacing the occluding loop to allow a tight complex to be formed. Cystatin A was appreciably less effective in displacing the loop than cystatin C, resulting in a considerably lower overall inhibition rate constant.

An NMR-based identification of peptide fragments mimicking the interactions of the cathepsin B propeptide.

Selected fragments of the 62-residue proregion (or residues 1p-62p) of the cysteine protease cathepsin B were synthesized and their interactions with cathepsin B studied by use of proton NMR spectroscopy. Peptide fragments 16p-51p and 26p-51p exhibited differential perturbations of their proton resonances in the presence of cathepsin B. These resonance perturbations were lost for the further truncated 36p-51p fragment, but remained in the 26p-43p and 28p-43p peptide fragments. Residues 23p-26p or TWQ25A in the N-terminal 1p-29p fragment did not show cathepsin B-induced resonance perturbations although the same residues had strongly perturbed proton resonances within the 16p-51p peptide. Both the 1p-29p and 36p-51p fragments lack a common set of hydrophobic residues 30p-35p or F30YNVDI35 from the proregion. The presence of residues F30YNVDI35 appears to confer a conformational preference in peptide fragments 16p-51p, 26p-51p, 28p-43p and 26p-43p, but the same residues induce the aggregation of peptides 16p-36p and 1p-36p. The peptide fragment 26p-43p binds to the active site, as indicated by its inhibition of the catalytic activity of cathepsin B. The cathepsin B prosegment can therefore be reduced into smaller, but functional subunits 28p-43p or 26p-43p that retain specific binding interactions with cathepsin B. These results also suggest that residues F30YNVDI35 may constitute an essential element for the selective inhibition of cathepsin B by the full-length cathepsin B proregion.

Inhibition of carcinoma cell invasion and liver metastases formation by the cysteine proteinase inhibitor E-64.

Cysteine proteinases, in particular cathepsins B and L, have been implicated in tumor invasion and are thought to be important mediators of metastasis. Using two clonal sublines of the Lewis lung carcinoma with distinct patterns of metastasis, we previously reported that H-59 carcinoma cells, which are highly invasive and preferentially metastatic to the liver, express high levels of cathepsin L and lower levels of cathepsin B whereas M-27 cells which are less invasive and only moderately metastatic to the lung express cathepsin B only. In the present study, the role of these enzymes in invasion and metastasis, in particular the involvement of cysteine proteinases in liver metastasis of H-59 cells was further investigated. Using a reconstituted basement membrane (Matrigel) invasion assay we found that the cysteine proteinase inhibitor, E-64, blocked the invasion of H-59 cells under conditions which did not affect cell viability. A more minor but significant inhibitory effect (up to 32%) was also seen with the propeptide of cathepsin B, implicating this enzyme in the invasion process. Furthermore, treatment of H-59 cells with E-64 inhibited experimental liver metastases formation by up to 90%. On the other hand, invasion of M-27 cells could not be blocked by cysteine proteinase inhibitors even under conditions which resulted in complete abrogation of intracellular enzymatic activity, as assessed using synthetic substrates. Together, these results confirm our previous conclusion that the two carcinoma sublines utilize distinct proteolytic mechanisms for invasion and identify the cysteine proteinases as key mediators of H-59 carcinoma invasion and metastasis.