Elevated serum cathepsin B1-like activity in women with neoplastic disease

Cathepsins B and D Activity and Activity Ratios in Normal Ovaries, Benign Ovarian Neoplasms, and Epithelial Ovarian Cancer

OBJECTIVE

Cathepsins B (CB) and D (CD) belong to a family of proteases felt to be important in tumor metastasis and invasion. It has been suggested that both enzymes play a role the progression of epithelial ovarian cancer and they have been investigated as potential biomarkers for ovarian cancer. Our objective was to determine if activity ratios of these two isoforms might enhance their usefulness as biomarkers.

METHODS

Ovarian cancer cell lines and snap-frozen archived tissue samples were sonicated and cathepsin activities were assayed fluorometrically with cathepsin-specific peptide substrates in combination with specific inhibitors. Tissue specimens were divided into four groups: normal ovary, benign neoplasm, early-stage (I/II) cancer, and late-stage (III/IV) cancer. Median CB and CD activity and the ratio of CB to CD (CB/CD) were compared using the Wilcoxon rank sum test. Nonparametric Spearman correlation was used to determine associations between CA-125 and cathepsin activity. Logistic regression was used to test the association between cathepsin activity and malignancy.

RESULTS

In cell lines and tissue, CD activity remained relatively constant, while CB activity varied. CB activity was greatest in cancer tissue. Elevated serum CA-125 was associated with elevations in CB activity and CB/CD but not CD activity. Elevated CB activity and CB/CD as well as increasing CA-125 and age are all associated with malignancy. Multiple logistic regression shows that CB activity and age best predict malignancy status.

CONCLUSIONS

CB activity is associated with invasive ovarian neoplasm. Our results do not suggest that the ratio of activity between CB and CD provides any additional information than CB activity alone. Both tissue CB activity and CB/CD activity ratios correlate with serum levels of CA-125; however there is no correlation between CD activity and CA-125.

Differential Impact of Cysteine Cathepsins on Genetic Mouse Models of De novo Carcinogenesis: Cathepsin B as Emerging Therapeutic Target

Lysosomal cysteine cathepsins belong to a family of 11 human proteolytic enzymes. Some of them correlate with progression in a variety of cancers and therefore are considered as potential therapeutic targets. Until recently, the contribution of individual cathepsins to tumorigenesis and tumor progression remained unknown. By crossing various types of mouse cancer models with mice where specific cathepsins have been ablated, we contributed to this gap of knowledge and will summarize the results in this report. The employed models are the Rip1-Tag2 model for pancreatic neuroendocrine tumors, the K14-HPV16 model for squamous skin and cervical cancers, and the MMTV-PyMT model for metastasizing breast cancer, the KPC model for pancreatic ductal adenocarcinoma, and the APC(min) mice developing early stages of intestinal neoplasia. All models harbor mutations in relevant tumor suppressors and/or cell-type specific expression of potent oncogenes, which initiate de novo carcinogenesis in the targeted tissues. In all these models deletion of cathepsin B led to suppression of the aggressiveness of the respective cancer phenotype. Cathepsin B is networking with other proteases as it was shown for cathepsin X/Z. In contrast, deletion of cathepsin L was beneficial in the RiP1-Tag2 model, but enhanced tumorigenesis in the APC(min), and the K14-HPV16 mice. A logical consequence of these results would be to further pursue selective inhibition of cathepsin B. Moreover, it became clear that cathepsins B and S derived from cells of the tumor microenvironment support cancer growth. Strikingly, delivery of broad spectrum cysteine cathepsin inhibitors in the tumor microenvironment disrupts the permissive ecosystem of the cancer and results in impaired growth or even in regression of the tumor. In addition, combination of cysteine cathepsin inhibition and standard chemotherapy improves the therapeutic response of the latter. Taken together, the next preclinical challenges for developing cathepsin inhibition as cancer therapy might be the improvement of inhibitor selectivity and targeted delivery to the tumor microenvironment and investigation of the biological context of the individual factors within the complex proteolytic network.

Proteinases in connective tissue breakdown

Lysosomal cathepsins B and N complete the depolymerization of native fibrillar collagen in the phagolysosome after prior extracellular fragmentation by collagenase and other neutral proteinases. In vitro studies have confirmed that cathepsins B and N cleave native collagen only at the short non-helical telopeptides, which generate the intermolecular cross-links. This action occurs maximally at pH 3.5 and at 37 degrees C the released monomers denature spontaneously and are susceptible to further breakdown. In the phagolysosome the collagenous debris is already weakened and probably therefore, more easily disrupted by these cathepsins. Complete digestion would then be undertaken by the whole complement of proteases. The lysosomal glycosidases may assist this breakdown by degrading ground substance components which are normally tightly bound to collagen. In certain situations cells may instead generate an acidic pericellular environment that could permit the direct action of secreted lysosomal enzymes. This extracellular action may supersede the action of collagenase and the activity of these different enzymes would thus be regulated by changes in the nature of this microenvironment.

H-ras and Nm23-H1 gene expression and proteolytic activity in squamous cell carcinoma of the uterine cervix

BACKGROUND

The invasive and metastatic potential of malignant cells results from complex interactions of numerous factors not yet fully understood. Genomic alterations such as ras overexpression and nm23-H1 inhibition have been found to be frequently associated with increased invasiveness in various cancers. On the other hand, secretion of different proteinases are necessary for malignant cells to traverse a network of matrix macromolecules, but the relationship between the genomic alterations and the proteolytic phenotype is still unclear. Our aim was to investigate whether the appearance of the proteolytic phenotype had any correlation with the expression of H-ras and nm23-H1 genes in carcinoma of the uterine cervix.

METHODS

Twenty-five samples from patients with carcinoma of the uterine cervix at different clinical stages were studied. Cathepsin B1, plasminogen activator, and collagenase activity were assessed in tissue cytosols using specific synthetic oligopeptides as substrates. The expression of H-ras and nm23-H1 was investigated by means of immunohistochemistry and in situ hybridization.

RESULTS

Our results showed that cathepsin B1 was the most consistently elevated proteinase, demonstrating a linear correlation with clinical staging. H-ras expression was found elevated in 40% of the cases. Nm23-H1 protein immunoreactivity was positive in 40% of the cases. No correlation was found among H-ras, cathepsin B1 activity, and survival rate. Among cases with high cysteine proteinase activity, a different clinical behavior depending on the expression of Nm23-H1 was observed. The cases with Nm23-H1 protein had a markedly better survival rate than those lacking this protein. In contrast, the absence of Nm23-H1 in association with high cathepsin B1 activity was a clear indicator of a poor prognosis.

CONCLUSIONS

These findings suggest a complex interaction between the proteolytic phenotype and the expression of H-ras and nm23-H1 genes in carcinoma of the cervix that influences the clinical behavior of the tumor.

Cathepsin B-like activity as a serum tumour marker in ovarian carcinoma

Serum cathepsin B-like activity was determined in 75 patients with ovarian carcinomas and in control groups. Ovarian cancers were of FIGO stages I-IV. Control groups consisted of 15 healthy women, 20 patients with myomas of the uterus, and 17 with benign ovarian cysts. Preoperative results showed elevated cathepsin B-like activity in 100% of the patients with ovarian cancers in relation to healthy subjects and patients with myomas, and in 78% in relation to benign ovarian cysts. Cathepsin B activity increased progressively with the FIGO stage of the disease, but the differences among particular stages were not statistically significant. In serous tumours cathepsin activity was significantly higher only in comparison to endometrioid ones (p < 0.001). Antipapain capacity of cystatins in the sera was also determined. No significant correlation between cathepsin B-like, and antipapain activity of cystatins was found. Serum cathepsin B-like activity may be helpful in the preoperative differential diagnosis between ovarian carcinomas and benign ovarian or uterine tumours.

The biochemistry of cancer dissemination

The progression of a tumor cell from one of benign delimited proliferation to invasive and metastatic growth is the major cause of poor clinical outcome of cancer patients. Recent research has revealed that this complex process requires many components for successful dissemination and growth of the tumor cell at secondary sites. These include angiogenesis, enhanced extracellular matrix degradation via tumor and host-secreted proteases, tumor cell migration, and modulation of tumor cell adhesion. Each individual component is multifaceted and is discussed within this review with respect to historical and recent findings. The identification of components and their interrelationship have yielded new therapeutic targets leading to the development of agents that may prove effective in the treatment of cancer and its metastatic progression.

Protease inhibitors in chemoprevention of cancer. An overview

In the search for chemopreventive agents for cancer many natural products have been identified. Among them extensive experimental studies have been made on protease inhibitors which not only occur naturally in many plant products, but have also been synthesized in the laboratory. Many of these studies revealed the fact that protease inhibitors are able to prevent carcinogenesis and tumour promotion. A review of the reports available to date suggests that protease inhibitors are potential chemopreventive agents although their biological role and mechanism of action are not very clear.

Aging issues in invasion and metastasis. Fertile ground for investigation

BACKGROUND

Ovarian cancer is a disease of predominantly older women and one in which metastases occur early and extensively. The unusual pattern of local metastasis is fertile ground for laboratory and clinical investigation.

METHODS

Tools are now available with which to study the complex cascade of invasion and metastasis as it underlies invasive ovarian cancer. The metastatic cascade involves five repetitive steps: angiogenesis, adhesion to the vascular basement membrane, local proteolysis, migration into and out of the vasculature, and proliferation at the secondary sites. Both in vivo and in vitro models of ovarian cancer are available; however, there are no systems targeted to understanding age-related differences in ovarian cancer biology.

RESULTS

Progress in investigation of the biology of ovarian cancer has led to new diagnostic and therapeutic leads, including the use of adhesion receptors, protease secretion, and stimulation of tumor cell migration as potential markers and the identification of a new anti-cancer agent, CAI (NSC609974).

CONCLUSIONS

CAI inhibits signal transduction pathways important in the regulation and activation of metastasis and proliferation. A phase I study has begun accrual at the National Cancer Institute; the protocol will contain no upper age limit.

Chemoprevention of cancer: protease inhibitors

1. The defense of the organism against cancer by inhibitors of proteolytic enzymes which are able to block the metastasizing stage of the disease is reviewed. 2. The contemporary views on the possible mechanisms of the process of prevention on both molecular and cellular levels are presented.

Enzyme-linked immunosorbent assay for the detection of cathepsin-kininogen complexes in human plasma

A simple and sensitive enzyme-linked immunosorbent assay (ELISA) for the determination of cathepsins B-, H- and L-kininogen (B-KG, H-KG, and L-KG) complexes was constructed using a microtiter plate based sandwich technique. The assay range was 7.5-480 ng/ml with three cross-linked cathepsin-kinin-free low molecular weight KG (LMWKGf) complexes. The average within-run coefficients of variation (CV) using three concentrations were 7.5, 7.2 and 9.9% for B-LMWKGf, H-LMWKGf, and L-LMWKGf complexes, respectively. The between-run CV values indicated satisfactory reproducibility for the method. Recoveries of cross-linked B-LMWKGf, H-LMWKGf, and L-LMWKGf complexes from negative control plasma were 127 +/- 7.1, 112 +/- 5.8 and 102 +/- 8.8% (n = 10, mean +/- SD), respectively, whereas the average recovery from phosphate buffer (pH 7.2) containing 0.15 M NaCl, 0.05% Tween-20 and 0.1% bovine serum albumin was 93.1 +/- 8.2% (n = 10, mean +/- SD). It was possible to detect B-KG and H-KG complexes in the plasma of 5/280 and 14/280 patients manifesting acute phase responses. Levels in plasma from healthy individuals were negligible. This ELISA should permit the study of cathepsin metabolism in acute phase disorders.

Serum cathepsin B-like activity as a potential marker of laryngeal carcinoma

Early detection and reliable monitoring of treatment constitute one of the most challenging problems in oncology. For this reason, we studied the potential value of serum cathepsin B-like activity as a possible marker of laryngeal carcinoma. The results indicate that serial analyses of this parameter have predictive value in the assessment of surgical treatment of laryngeal carcinoma. Activity of the enzyme reveals a progressive increment which could be associated with increased severity of the neoplasm present. The serum level of the cathepsin B-like activity remained close to zero in the control group of healthy volunteers and patients with inflammatory diseases.

Invasion and metastasis: biology and clinical potential

Metastatic dissemination of tumor is the primary cause of death for most cancer patients. The expanding field of study of the metastatic cascade has been the source of novel approaches to the diagnosis and treatment of cancer. The metastatic process involves angiogenesis, tumor cell adhesion to vascular basement membrane, local proteolysis to create an opening in the basement membrane, migration through that rent and into the secondary site, and finally, successful proliferation. Important components of the metastatic cascade such as basement membrane structures, adhesion molecules and their receptors, proteolytic enzymes, migration-inducing factors, and growth factors have been demonstrated to have reproducible patterns in malignant and metastatic tissues. These patterns have led to clinical correlations demonstrating their utility in the identification and follow-up of malignant and metastatic disease. In addition, several promising new anti-cancer drugs such as inhibitors of angiogenesis, protease-inhibitors, and blockers of signal transduction have been identified and are awaiting introduction into the clinical arena.

Proteinase-alpha 2 macroglobulin complexes are not increased in plasma of patients with cancer

Alpha 2-macroglobulin, a major glycoprotein component of plasma, is unique in its capacity to bind and inhibit the proteolytic activities of all classes of proteinases. Since proteinases implicated in cancer dissemination (type-IV collagenase, plasminogen activator, cathepsins B) are normal constitutents of blood, we have explored the hypothesis that elevated tissue levels of activated proteinases bound to alpha 2M might be detected in plasma of patients with cancer. To test this premise, blood was collected from 149 subjects (33 healthy controls, 31 patients with infections and non-malignant diseases, 16 with myeloproliferative disease, 10 with gastrointestinal cancer, 7 with genito-urinary cancer, 16 with lung cancer, 14 with lymphoma, 11 with miscellaneous cancers and 11 with chronic lymphocytic leukemia and myeloma). Plasma was assayed for alpha 2M-proteinase complexes using a sandwich ELISA which employs a mouse monoclonal antibody (MAb) that binds to a neo-antigenic determinant on complexed alpha 2M and a rabbit polyclonal anti-native human alpha 2M antibody. The concentration of complexed alpha 2M in healthy controls was 14.2 +/- 9.8 micrograms/ml (mean +/- standard deviation). No significant differences in complexed alpha 2M were noted between normal and cancer groups (range 7.4-14.6 micrograms/ml). On the basis of these data, we propose that, in patients with cancer, activated proteinases are bound locally to inhibitors in the tissues and are not available to form complexes with plasma alpha 2M. An alternative explanation is that proteinases are not secreted in excess by cancer cells in vivo.

Heparanases and tumor metastasis

The successful penetration of endothelial basement membranes is an important process in the formation of hematogenous tumor metastases. Heparan sulfate (HS) proteoglycan is a major constituent of endothelial basement membranes, and we have found that HS-degradative activities of metastatic B16 melanoma sublines correlate with their lung-colonizing potentials. The melanoma HS-degrading enzyme is a unique endo-beta-D-glucuronidase (heparanase) that cleaves HS at specific intrachain sites and is detectable in a variety of cultured human malignant melanomas. The treatment of B16 melanoma cells with heparanase inhibitors that have few other biological activities, such as N-acetylated N-desulfated heparin, results in significant reductions in the numbers of experimental lung metastases in syngeneic mice, indicating that heparanase plays an important role in melanoma metastasis. HS-degrading endoglycosidases are not tumor-specific and have been found in several normal tissues and cells. There are at least three types of endo-beta-D-glucuronidases based on their substrate specificities. Melanoma heparanase, an Mr approximately 96,000 enzyme with specificity for beta-D-glucuronosyl-N-acetylglucosaminyl linkages in HS, is different from platelet and mastocytoma endoglucuronidases. Elevated levels of heparanase have been detected in sera from metastatic tumor-bearing animals and malignant melanoma patients, and a correlation exists between serum heparanase activity and extent of metastases. The results suggest that heparanase is potentially a useful marker for tumor metastasis.

Acid and neutral cysteine proteinase inhibitor in normal uterine portio and in squamo-epithelial metaplasia, dysplasias and infiltrative carcinoma of the uterine portio

Acid cysteine proteinase inhibitor (ACPI) and neutral cysteine proteinase inhibitor (NCPI) were localized in formalin-fixed normal human uterine portio as well as in the squamous cell carcinoma and dysplasias of the uterine portio. The peroxidase-antiperoxidase technique was used. In the squamous epithelium of normal uterine portio, ACPI and NCPI were localized in the cells of the upper and middle layers, mainly in the cytoplasm. In the precursors of cancer, immunoreactivity for ACPI and NCPI declined, and neither of the inhibitors was demonstrable immunohistochemically in the anaplastic squamo-epithelial carcinoma of the uterine portio. Our results suggest that ACPI and NCPI are associated with squamo-epithelial differentiation and that they may also be of significance for the regulation of cysteine proteinase activity in normal tissue and malignant growth.

Mechanisms of hormone action: parallels in receptor-mediated signal propagation for steroid and peptide effectors

The purpose of this contribution is to provide in brief form growing evidence in support of an integrated concept of hormone action that appears to shed fresh light on the information gap between the triggering and the effectuation of outcome of the action of given hormones. In accord with these new concepts there has now arisen a substantial body of data from a wide variety of effectors and target cells that demonstrates an astonishing unity in the actions of hormones of widely dissimilar chemical structure. In a nutshell, it now appears that primary recognition sites for both peptide and steroidal agonists occur at the outer cell surface. For steroid hormones, as exemplified by estradiol-17 beta, these sites possess several of the hallmarks of true receptors. Moreover, capture of this ligand is associated with unmistakable signs of membrane perturbation. And at a still very early stage in the signal propagation sequence, activation of a very limited fraction of the cellular lysosomal population may be identified following the application of steroid, as well as peptide, hormones. In turn, there is mounting evidence for cellular entry and even lysosomal uptake of peptidal effectors, the significance of which is still under debate. Likewise, there occur clear signs of limited reorganization of components of the cellular architecture at the surface, in the cytoplasm, and in the nucleus and its subcompartments, which are consistent with minimal recompartmentation of 'microquanta' of lysosomal constituents. These observations may be made within seconds to minutes following application of tropic hormone of either class to its selective targets, and thus, at times preceding those relatively more distal responses of augmented transcriptional and translational activities.

Granulocyte collagenase and cathepsin B in patients with cancer of digestive organs

Investigation of granulocyte collagenase and cathepsin B activities in 53 patients with cancer of various digestive organs revealed that total collagenase, active collagenase and cathepsin B activities were found to be higher than normal. The mean total collagenase activity in patients with cancer of the stomach, pancreas or liver was significantly higher than control values. The mean active collagenase activity in patients with cancer of the colon or liver was also significantly higher than control values. The mean cathepsin B activity in patients with cancer of the esophagus, colon or pancreas was significantly higher than control values. Active collagenase activity increased in advanced stage cases. Cathepsin B activity correlated significantly with active collagenase activity, but not with total collagenase activity, supporting the hypothesis that collagenases might be activated by cathepsin B. These results suggest that granulocyte collagenolytic enzymes, stimulated by some factors released from tumor cells, facilitate the invasion of tumor cells.

Cathepsin B containing cells in the rabbit mesentery during invasion of V2 carcinoma cells

Localization of cathepsin B was studied in the rabbit mesentery during invasion of V2 carcinoma cells. Cathepsin B was visualized immunohistochemically by using monospecific sheep antibodies and the avidin-biotin-peroxidase complex (ABC) method. Horizontal and vertical semithin Epon embedded sections of stained mesenteries showed that histiocytes always displayed the strongest staining reaction independently of the presence of V2 carcinoma cells. Fibroblasts, mesothelial cells and the invaded V2 cells were less stained. Strongly stained peritoneal monocytes were frequently found on the surface of the mesentery in association with tumor foxi. The role of these various cathepsin B containing cells with respect to extracellular matrix degradation during tumor invasion in the mesentery is not clear; some aspects of this problem are presented in the discussion.

The combined action of two enzymes in human serum can mimic the activity of cathepsin B

Synthetic substrates are often used to measure the activity of proteolytic enzymes. We have investigated the activities which cleave synthetic substrates such as alpha-N-benzyloxycarbonyl-Arg-Arg-beta-naphthylamide, for which the lysosomal proteinase cathepsin B has a high affinity, in sera from normal individuals, pregnant women and patients with breast cancer. As reported by other workers, activities against these substrates were elevated during pregnancy. Naphthylamine release, however, was shown to be the result of the combined action of two enzymes. The substrate is first cleaved by an endopeptidase to yield alpha-N-benzyloxycarbonyl-Arg and the aminopeptidase substrate Arg-beta-naphthylamide, which is then cleaved by serum aminopeptidases, particularly oxytocinase. A similar mechanism of cleavage was also found in the sera of breast cancer patients, where the endopeptidase catalyzing the first reaction was characterized as plasma kallikrein and the second reaction was carried out by serum leucine aminopeptidase. In no serum sample was there evidence for true cathepsin B activity.

Serum proteinase-like peptidase activities and proteinase inhibitors in women with breast disease

The pre-treatment serum activities of several proteinase-like peptidases and the proteinase inhibitors, alpha 1-antitrypsin (alpha 1AT) and alpha 2-macroglobulin (alpha 2M), have been determined in 102 women with breast cancer and compared with those in 20 women with benign disease and in 30 healthy women of cancer bearing age. There were no significant differences in serum proteinase-like peptidase activities associated specifically with breast cancer. However, trypsin-like and plasmin-like activities were significantly lower than normal in women with breast disease. Serum alpha 1AT and alpha 2M levels were higher in patients with breast cancer than in healthy women or women with benign breast disease. These results indicate that, at presentation, breast cancer is not associated with abnormal serum levels of the proteinase-like peptidases studied, possibly as a result of an increase in the concentration of proteinase inhibitors.

Serum and tissue proteinase-like peptidase activities in women undergoing total mastectomy for breast cancer

Serum proteinase-like peptidases and proteinase inhibitor activities have been determined in 40 women with breast cancer at presentation and following total mastectomy. Activities of these enzymes have also been determined in homogenates of malignant (n = 13) and non-malignant (n = 11) breast tissue and benign breast lesions (n = 10). Following surgical treatment, the serum collagenase-like, cathepsin B-like and cathepsin H-like peptidase activities were significantly reduced. In addition, the activities of collagenase-like, cathepsin B-like and elastase-like peptidases were significantly higher in malignant breast tissue than in either non-malignant tissue from the same breast, or benign breast lesions. These findings are consistent with the suggestion that proteinases may have a role in tumour invasion.

Collagenolytic mechanisms in tumor cell invasion

Connective tissue stroma and basement membrane structures probably present natural barriers to the migration of tumor cells. It has therefore been proposed that collagenolytic enzymes are required to facilitate the spread and invasion of tumor cells into host tissues. The collagenases and cathepsin B-like enzymes are thought to be involved, but the cellular source of collagenolytic activity at the tumor: host interface or 'invasion zone' remains obscure in most cases. The 'invasion zone' of different tumors is very variable with regard to the type and numbers of host or tumor cells, as well as the type of collagenous matrix, and few generalities can be made. The existence within a tumor of specialised subpopulations of cells which have different metastatic potential has been postulated. As a consequence it seems plausible that the phenotypic expression of highly invasive or metastatic tumor cells should include the potential for generating collagenolytic activity. Immunolocalisation studies have demonstrated the production of type I and type IV collagenases at sites of tumor invasion, but it does not appear to be a continuous process and only a small proportion of tumor and/or host cells elaborate enzyme at any one moment. Collagenase production is invariably microenvironmental in nature and it seems likely that local host:tumor cell interactions are important in modulating collagenolysis. Macrophages and mast cells have been shown to stimulate collagenase expression by tumor and stromal cells in vitro, and it is proposed that these cells may assume a contributory role for the induction of collagenolytic activity in vivo. The collagenolytic mechanisms that operate at micro-foci of host:tumor junctions probably depend upon the type of collagen, the cellular composition and the extracellular ionic conditions of each invasion site. Either tumor or host cells may elaborate enzymes, this being dependent upon the type and/or tissue location of the invasive tumor.

Cysteine proteinases and metastasis

Cysteine proteinases are a subclass of endopeptidases which require activation by thiol reagents. A tumor cysteine proteinase which appears to be related to lysosomal cathepsin B has been implicated in the ability of tumor cells to invade the extracellular matrix and to metastasize to secondary sites. Lysosomal cathepsin B can degrade such components of the extracellular matrix as collagen, fibronectin and proteoglycans. Activity of this cathepsin B-like cysteine proteinase (CB) has been correlated with tumor malignancy in a number of tumor lines yet not in all tumor lines studied. CB activity in tumors seems to be associated with the viable tumor cells, probably with the plasma membrane of these tumor cells. CB activity has been measured in the sera, urine, ascites fluid and pancreatic fluid of tumor-bearing patients. CB is released from tumor explants and tumor cells in vitro as well as from normal subcutaneous tissue exposed to tumor-conditioned medium. Cathepsin B from normal tissues is rapidly inactivated above pH 7.0. Therefore, CB in tumor cell membranes or released from tumor cells (or from host cells in response to tumor cells) may not possess proteolytic activity at neutral pH and thus may not facilitate tumor cell invasion. However, CB exhibits enhanced stability at neutral or slightly alkaline pH's. There is not yet definitive proof that CB plays a role in tumor invasion and metastasis. There is, however, an increasing body of correlative evidence relating CB activity and tumor malignancy. This correlative evidence plus preliminary evidence that tumor CB can degrade components of the extracellular matrix in vitro suggests that CB may be one proteinase active in a proteolytic cascade resulting in tumor invasion and metastasis.

Tumor cell-platelet aggregation: induced by cathepsin B-like proteinase and inhibited by prostacyclin

The ability of tumor cells to metastasize may be related to their ability to promote aggregation of host platelets. The use of inhibitors of cysteine proteinases resulted in parallel inhibition of B16 amelanotic melanoma-induced platelet aggregation and of a cathepsin B activity. The antimetastatic agent prostacyclin inhibited platelet aggregation induced by the tumor cells and by papain, a cathepsin B-mimicking agent.

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.