FUNCTIONAL IMAGING OF TUMOR PROTEOLYSIS

Direct visualization of emergent metastatic features within an ex vivo model of the tumor microenvironment

Preventing tumor cells from acquiring metastatic properties would significantly reduce cancer mortality ^1-5 . However, due to the complex nature of this process, it remains one of the most poorly understood and untreatable aspects of cancer ^6-8 . Ischemia and hypoxia in solid tumors are requisite in metastasis formation - conditions that arise far from functional blood vessels and deep within tumor tissues ^9,10 . These secluded locations impede the observation of pre-metastatic tumor cells and their interactions with stromal cells, which are also critical in the initiation of this process ^11,12 . Thus, the initiation of metastasis has been incredibly difficult to model in the lab and to observe in vivo . We present an ex vivo model of the tumor microenvironment, called 3MIC, which overcomes these experimental challenges and enables the observation of ischemic tumor cells in their native 3D context with high spatial and temporal resolutions. The 3MIC recreates ischemic conditions in the tumor microenvironment and facilitates the co-culture of different cell types. Using live microscopy, we showed that ischemia, but not hypoxia alone, increases the motility and invasive properties of cells derived from primary tumors. These changes are phenotypic and can occur without clonal selection. We directly observed how interactions with stromal cells such as macrophages increased tumor invasion in conjunction with the effects of an ischemic microenvironment. Finally, we tested the effects of chemotherapy drugs under different metabolic microenvironments and found that ischemic tumor cells are more resistant to paclitaxel, possibly due to a metabolic resistance mechanism. Overall, the 3MIC is a cost-effective system that allows for the dissection of the complexity of the tumor microenvironment and direct observation of the emergence of metastasis, as well as the testing of treatments that may halt this process.

Proteolytic and mechanical remodeling of the extracellular matrix by invadopodia in cancer

Cancer invasion and metastasis require remodeling of the adjacent extracellular matrix (ECM). In this mini review, we will cover the mechanisms of proteolytic degradation and the mechanical remodeling of the ECM by cancer cells, with a focus on invadopodia. Invadopodia are membrane protrusions unique to cancer cells, characterized by an actin core and by the focal degradation of ECM via matrix metalloproteases (MMPs). While ECM can also be remodeled, at lower levels, by focal adhesions, or internal collagen digestion, invadopodia are now recognized as the major mechanism for MMP-dependent pericellular ECM degradation by cancer cells. Recent evidence suggests that the completion of epithelial-mesenchymal transition may be dispensable for invadopodia and metastasis, and that invadopodia are required not only for mesenchymal, single cell invasion, but also for collective invasion. During collective invasion, invadopodia was then shown to be located in leader cells, allowing follower cells to move via cooperation. Collectively, this suggests that invadopodia function may be a requirement not only for later steps of metastasis, but also for early invasion of epithelial cells into the stromal tissue. Over the last decade, invadopodia studies have transitioned into in 3D andin vivosettings, leading to the confirmation of their essential role in metastasis in preclinical animal models. In summary, invadopodia may hold a great potential for individual risk assessment as a prognostic marker for metastasis, as well as a therapeutic target.

Collagen denaturation in the infarcted myocardium involves temporally distinct effects of MT1-MMP-dependent proteolysis and mechanical tension

Tissue injury results in profound alterations in the collagen network, associated with unfolding of the collagen triple helix, proteolytic degradation and generation of fragments. In the infarcted myocardium, changes in the collagen network are critically involved in the pathogenesis of left ventricular rupture, adverse remodeling and chronic dysfunction. We hypothesized that myocardial infarction is associated with temporally and spatially restricted patterns of collagen denaturation that may reflect distinct molecular mechanisms of collagen unfolding. We used a mouse model of non-reperfused myocardial infarction, and in vitro assays in fibroblast-populated collagen lattices. In healing infarcts, labeling with collagen hybridizing peptide (CHP) revealed two distinct patterns of collagen denaturation. During the inflammatory and proliferative phases of infarct healing, collagen denaturation was pericellular, localized in close proximity to macrophages and myofibroblasts. qPCR array analysis of genes associated with matrix remodeling showed that Membrane Type 1-Matrix Metalloproteinase (MT1-MMP) is markedly upregulated in infarct macrophages and fibroblasts, suggesting its involvement in pericellular collagen denaturation. In vitro, MT1-MMP-mediated pericellular collagen denaturation is involved in cardiac fibroblast migration. The effects of MT1-MMP on collagen denaturation and fibroblast migration involve the catalytic site, and require hemopexin domain-mediated actions. In contrast, during the maturation phase of infarct healing, extensive collagen denaturation was noted in the hypocellular infarct, in the infarct border zone and in the mitral valve annulus, in the absence of MT1-MMP. In vitro, mechanical tension in attached collagen lattices was sufficient to induce peripheral collagen denaturation. Our study suggests that in healing infarcts, early pericellular collagen denaturation may be important for migration of macrophages and reparative myofibroblasts in the infarct. Extensive denaturation of collagen fibers is noted in mature scars, likely reflecting mechanical tension. Chronic collagen denaturation may increase susceptibility of the matrix to proteolysis, thus contributing to progressive cardiac dilation and post-infarction heart failure.

Activated thrombin-activatable fibrinolysis inhibitor attenuates the angiogenic potential of endothelial cells: potential relevance to the breast tumour microenvironment

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a basic carboxypeptidase zymogen present in blood plasma. Proteolytic activation of TAFI by thrombin, thrombin in complex with the endothelial cell cofactor thrombomodulin, or plasmin results in an enzyme (TAFIa) that removes carboxyl-terminal lysine residues from protein and peptide substrates, including cell-surface plasminogen receptors. TAFIa is therefore capable of inhibiting plasminogen activation in the pericellular milieu. Since plasminogen activation has been linked to angiogenesis, TAFIa could therefore have anti-angiogenic properties, and indeed TAFIa has been shown to inhibit endothelial tube formation in a fibrin matrix. In this study, the TAFI pathway was manipulated by providing exogenous TAFI or TAFIa or by adding a potent and specific inhibitor of TAFIa. We found that TAFIa elicited a series of anti-angiogenic responses by endothelial cells, including decreased endothelial cell proliferation, cell invasion, cell migration, tube formation, and collagen degradation. Moreover, TAFIa decreased tube formation and proteolysis in endothelial cell culture grown alone and in co-culture with breast cancer cell lines. In accordance with these findings, inhibition of TAFIa increased secretion of matrix metalloprotease proenzymes by endothelial and breast cancer cells. Finally, treatment of endothelial cells with TAFIa significantly inhibited plasminogen activation. Taken together our results suggest a novel role for TAFI in inhibiting tumour angiogenic behaviors in breast cancer.

New 3D-Culture Approaches to Study Interactions of Bone Marrow Adipocytes with Metastatic Prostate Cancer Cells

Adipocytes are a major component of the bone marrow that can critically affect metastatic progression in bone. Understanding how the marrow fat cells influence growth, behavior, and survival of tumor cells requires utilization of in vitro cell systems that can closely mimic the physiological microenvironment. Herein, we present two new three-dimensional (3D) culture approaches to study adipocyte-tumor cell interactions in vitro. The first is a transwell-based system composed of the marrow-derived adipocytes in 3D collagen I gels and reconstituted basement membrane-overlayed prostate tumor cell spheroids. Tumor cells cultured under these 3D conditions are continuously exposed to adipocyte-derived factors, and their response can be evaluated by morphological and immunohistochemical analyses. We show via immunofluorescence analysis of metabolism-associated proteins that under 3D conditions tumor cells have significantly different metabolic response to adipocytes than tumor cells grown in 2D culture. We also demonstrate that this model allows for incorporation of other cell types, such as bone marrow macrophages, and utilization of dye-quenched collagen substrates for examination of proteolysis-driven responses to adipocyte- and macrophage-derived factors. Our second 3D culture system is designed to study tumor cell invasion toward the adipocytes and the consequent interaction between the two cell types. In this model, marrow adipocytes are separated from the fluorescently labeled tumor cells by a layer of collagen I. At designated time points, adipocytes are stained with BODIPY and confocal z-stacks are taken through the depth of the entire culture to determine the distance traveled between the two cell types over time. We demonstrate that this system can be utilized to study effects of candidate factors on tumor invasion toward the adipocytes. We also show that immunohistochemical analyses can be performed to evaluate the impact of direct interaction of prostate tumor cells with adipocytes. Our models underline the importance of using the appropriate culture conditions to mimic physiological interactions between marrow adipocytes and metastatic tumor cells. These systems have a potential to be utilized for analyses of various factors that may be regulated by the adipocytes in bone. Their application likely extends beyond metastatic prostate cancer to other tumors that colonize the bone marrow microenvironment.

Imaging Sites of Inhibition of Proteolysis in Pathomimetic Human Breast Cancer Cultures by Light-Activated Ruthenium Compound

The cysteine protease cathepsin B has been causally linked to progression and metastasis of breast cancers. We demonstrate inhibition by a dipeptidyl nitrile inhibitor (compound 1) of cathepsin B activity and also of pericellular degradation of dye-quenched collagen IV by living breast cancer cells. To image, localize and quantify collagen IV degradation in real-time we used 3D pathomimetic breast cancer models designed to mimic the in vivo microenvironment of breast cancers. We further report the synthesis and characterization of a caged version of compound 1, [Ru(bpy)₂(1)₂](BF₄)₂ (compound 2), which can be photoactivated with visible light. Upon light activation, compound 2, like compound 1, inhibited cathepsin B activity and pericellular collagen IV degradation by the 3D pathomimetic models of living breast cancer cells, without causing toxicity. We suggest that caged inhibitor 2 is a prototype for cathepsin B inhibitors that can control both the site and timing of inhibition in cancer.

High-throughput protease activity cytometry reveals dose-dependent heterogeneity in PMA-mediated ADAM17 activation

As key components of autocrine signaling, pericellular proteases, a disintegrin and metalloproteinases (ADAMs) in particular, are known to impact the microenvironment of individual cells and have significant implications in various pathological situations including cancer, inflammatory and vascular diseases. There is great incentive to develop a high-throughput platform for single-cell measurement of pericellular protease activity, as it is essential for studying the heterogeneity of protease response and the corresponding cell behavioral consequences. In this work, we developed a microfluidic platform to simultaneously monitor protease activity of many single cells in a time-dependent manner. This platform isolates individual microwells rapidly on demand and thus allows single-cell activity measurement of both cell-surface and secreted proteases by confining individual cells with diffusive FRET-based substrates. With this platform, we observed dose-dependent heterogeneous protease activation of HepG2 cells treated with phorbol 12-myristate 13-acetate (PMA). To study the temporal behavior of PMA-induced protease response, we monitored the pericellular protease activity of the same single cells during three different time periods and revealed the diversity in the dynamic patterns of single-cell protease activity profile upon PMA stimulation. The unique temporal information of single-cell protease response can help unveil the complicated functional role of pericellular proteases.

Regulation of ECM degradation and axon guidance by growth cone invadosomes

Invadopodia and podosomes, collectively referred to as invadosomes, are F-actin-rich basal protrusions of cells that provide sites of attachment to and degradation of the extracellular matrix. Invadosomes promote the invasion of cells, ranging from metastatic cancer cells to immune cells, into tissue. Here, we show that neuronal growth cones form protrusions that share molecular, structural and functional characteristics of invadosomes. Growth cones from all neuron types and species examined, including a variety of human neurons, form invadosomes both in vitro and in vivo. Growth cone invadosomes contain dynamic F-actin and several actin regulatory proteins, as well as Tks5 and matrix metalloproteinases, which locally degrade the matrix. When viewed using three-dimensional super-resolution microscopy, F-actin foci often extended together with microtubules within orthogonal protrusions emanating from the growth cone central domain. Finally, inhibiting the function of Tks5 both reduced matrix degradation in vitro and disrupted motoneuron axons from exiting the spinal cord and extending into the periphery. Taken together, our results suggest that growth cones use invadosomes to target protease activity during axon guidance through tissues.

Multiparametric classification links tumor microenvironments with tumor cell phenotype

Tumor microenvironment features are established as predictors of tumor cell behavior and fate.

While it has been established that a number of microenvironment components can affect the likelihood of metastasis, the link between microenvironment and tumor cell phenotypes is poorly understood. Here we have examined microenvironment control over two different tumor cell motility phenotypes required for metastasis. By high-resolution multiphoton microscopy of mammary carcinoma in mice, we detected two phenotypes of motile tumor cells, different in locomotion speed. Only slower tumor cells exhibited protrusions with molecular, morphological, and functional characteristics associated with invadopodia. Each region in the primary tumor exhibited either fast- or slow-locomotion. To understand how the tumor microenvironment controls invadopodium formation and tumor cell locomotion, we systematically analyzed components of the microenvironment previously associated with cell invasion and migration. No single microenvironmental property was able to predict the locations of tumor cell phenotypes in the tumor if used in isolation or combined linearly. To solve this, we utilized the support vector machine (SVM) algorithm to classify phenotypes in a nonlinear fashion. This approach identified conditions that promoted either motility phenotype. We then demonstrated that varying one of the conditions may change tumor cell behavior only in a context-dependent manner. In addition, to establish the link between phenotypes and cell fates, we photoconverted and monitored the fate of tumor cells in different microenvironments, finding that only tumor cells in the invadopodium-rich microenvironments degraded extracellular matrix (ECM) and disseminated. The number of invadopodia positively correlated with degradation, while the inhibiting metalloproteases eliminated degradation and lung metastasis, consistent with a direct link among invadopodia, ECM degradation, and metastasis. We have detected and characterized two phenotypes of motile tumor cells in vivo, which occurred in spatially distinct microenvironments of primary tumors. We show how machine-learning analysis can classify heterogeneous microenvironments in vivo to enable prediction of motility phenotypes and tumor cell fate. The ability to predict the locations of tumor cell behavior leading to metastasis in breast cancer models may lead towards understanding the heterogeneity of response to treatment.

A large proportion of cancer deaths are due to metastasis—the spread of cancer from the primary tumor to other parts of the body. Movement of cells may require the formation of protrusions called invadopodia, which degrade extracellular matrix. Although some studies have reported on locomotion in primary tumors, the presence of invadopodia was not tested. Here, we show that single cells from mouse mammary carcinoma can move using a fast- or slow-locomotion mode depending on different levels of cues present in the tumor microenvironment. Using multiphoton microscopy in vivo combined with a machine-learning algorithm we show how manipulation of microenvironmental conditions can induce predictable changes in the number of locomoting cells or switch between the two locomotion modes. We also demonstrate that only the slower moving cells are associated with invadopodia in vivo, and that only tumor cells from regions rich in invadopodia degrade the surrounding extracellular matrix and disseminate. Specific targeting of invadopodia results in inhibition of lung metastasis. This work proposes a systems biology view of how tumor microenvironments regulate tumor progression and presents insight into the heterogeneity of the treatment response. The ability to define and predict conditions under which tumor cells disseminate offers potential therapeutic benefits in regulating tumor progression.

Identification of cathepsin B in the razor clam Sinonovacula constricta and its role in innate immune responses

Cathepsin B, a lysosomal cysteine protease, has drawn much attention in vertebrates. However, very little is known about the functions of cathepsin B in bivalves. In this study, we identified the cathepsin B gene in the razor clam Sinonovacula constricta. The protein has a typical cysteine protease structure, comprising a 15-residue putative signal peptide, a 75-residue propeptide and a 249-residue mature domain. In the mature domain, there is an occluding loop, an oxyanion hole (Gln) and a catalytic triad (Cys, His and Asn). The cathepsin B gene is expressed in a wide range of tissues but appears to exhibit greatest level of expression in the liver. During the early developmental stages, the transcript could be detected widely. After the clam was infected with Vibrio anguillarum, the expression of the cathepsin B gene showed the most significant up-regulation in the liver and mantle tissues at 8h after infection. The fact that bacterial infection can induce the expression of the cathepsin B transcript suggests that cathepsin B could play an important role in the innate immunity of clams.

Synthesis and radiopharmacological characterisation of a fluorine-18-labelled azadipeptide nitrile as a potential PET tracer for in vivo imaging of cysteine cathepsins

A fluorinated cathepsin inhibitor based on the azadipeptide nitrile chemotype was prepared and selected for positron emission tomography (PET) tracer development owing to its high affinity for the oncologically relevant cathepsins L, S, K and B. Labelling with fluorine-18 was accomplished in an efficient and reliable two-step, one-pot radiosynthesis by using 2-[(18) F]fluoroethylnosylate as a prosthetic agent. The pharmacokinetic properties of the resulting radiotracer compound were studied in vitro, ex vivo and in vivo in normal rats by radiometabolite analysis and small-animal positron emission tomography. These investigations revealed rapid conjugate formation of the tracer with glutathione in the blood, which is associated with slow blood clearance. The potential of the developed (18) F-labelled probe to image tumour-associated cathepsin activity was investigated by dynamic small-animal PET imaging in nude mice bearing tumours derived from the human NCI-H292 lung carcinoma cell line. Computational analysis of the obtained image data indicated the time-dependent accumulation of the radiotracer in the tumours. The expression of the target enzymes in the tumours was confirmed by immunohistochemistry with specific antibodies. This indicates that azadipeptide nitriles have the potential to target thiol-dependent cathepsins in vivo despite their disadvantageous pharmacokinetics.

Chemical biology for understanding matrix metalloproteinase function

The matrix metalloproteinase (MMP) family has long been associated with normal physiological processes such as embryonic implantation, tissue remodeling, organ development, and wound healing, as well as multiple aspects of cancer initiation and progression, osteoarthritis, inflammatory and vascular diseases, and neurodegenerative diseases. The development of chemically designed MMP probes has advanced our understanding of the roles of MMPs in disease in addition to shedding considerable light on the mechanisms of MMP action. The first generation of protease-activated agents has demonstrated proof of principle as well as providing impetus for in vivo applications. One common problem has been a lack of agent stability at nontargeted tissues and organs due to activation by multiple proteases. The present review considers how chemical biology has impacted the progress made in understanding the roles of MMPs in disease and the basic mechanisms of MMP action.

Cathepsins and pancreatic cancer: the 2012 update

Pancreatic cancer is the result of distinctive genetic and epigenetic disturbances. This multistep process is in part well-defined and includes alterations in oncogenes and suppressor genes that control proliferation, apoptosis, angiogenesis, invasion and cell migration. Cathepsins are proteolytic enzymes and represent potential therapeutic targets in human tumors. Cathepsins predominantly function as endopeptidases within endolysosomal vesicles of normal cells and they are involved in physiological processes such as protein turnover, differentiation and apoptosis. In various types of malignancies, cathepsins have been associated with tumor progression and metastasis. Growing evidence and direct proofs suggest that cathepsins are highly up-regulated in pancreatic cancer and contribute to the development and progression of the cancer phenotype. In this review, the role of cathepsins in pancreatic cancer tumorigenesis is reported and discussed. Some critical aspects will be underlined such as specificity of cathepsin activity in pancreatic cancer and in its precursor lesions; the genetic perturbation and the intracellular signaling pathway activated by cathepsins as reported in preclinical models and in human tissues; the preliminary results and the oncological effects of cathepsin inhibitors currently tested on pancreatic cancer cells; the role of combined therapy based on chemotherapeutic agents and cathepsin inhibition. Although mounting evidences indicate that cysteine cathepsins are potential therapeutic targets in pancreatic cancer, as suggested by their functional role in controlling invasiveness and metastasis, it remains to be seen whether the promising benefits of pharmacological inhibitors observed in preclinical study might be translated to the current clinical practice.

Cleavage activation of the human-adapted influenza virus subtypes by matriptase reveals both subtype and strain specificities

Cleavage activation of the hemagglutinin (HA) precursor is an essential step in the influenza virus replication cycle that is driven by host cell proteases. HA cleavage activation is required for virus-endosome membrane fusion and the subsequent release of the influenza virus genome into the cytoplasm. Previous studies have determined that HA cleavage is most likely driven by either membrane-bound or extracellular trypsin-like proteases that reside in the respiratory tract. However, there is still uncertainty regarding which proteases are critical for HA cleavage in vivo. Therefore, further investigation of HA cleavage activation is needed in order to gain insight into the critical proteases involved. Matriptase is a member of the type II transmembrane serine protease family that is highly expressed in a membrane-bound form throughout the respiratory tract. One feature of matriptase is that, once activated, the catalytic domain is secreted into the extracellular space and so serves as a functional extracellular protease. In this study, we have determined that the secreted, catalytic domain of matriptase has the ability to cleave and activate HA from the influenza virus H1 subtype but not the H2 and H3 subtypes. Furthermore, matriptase selectively cleaved the HA of particular strains within the H1 subtype, revealing both subtype and H1 strain specificity. Matriptase was also found to activate thrombolytic zymogens that have been shown to cleave and activate the influenza virus HA. Our data demonstrate that matriptase has the ability to cleave HA directly or indirectly by activating HA-cleaving zymogens.

N-WASP-mediated invadopodium formation is involved in intravasation and lung metastasis of mammary tumors

Invadopodia are proteolytic membrane protrusions formed by highly invasive cancer cells, commonly observed on substrate(s) mimicking extracellular matrix. Although invadopodia are proposed to have roles in cancer invasion and metastasis, direct evidence has not been available. We previously reported that neural Wiskott-Aldrich syndrome protein (N-WASP), a member of WASP family proteins that regulate reorganization of the actin cytoskeleton, is an essential component of invadopodia. Here, we report that N-WASP-mediated invadopodium formation is essential in breast cancer invasion, intravasation and lung metastasis. We established stable cell lines based on MTLn3 rat mammary adenocarcinoma cells that either overexpressed a dominant-negative (DN) N-WASP construct or in which N-WASP expression was silenced by a pSuper N-WASP shRNA. Both the N-WASP shRNA and DN N-WASP cells showed a markedly decreased ability to form invadopodia and degrade extracellular matrix. In addition, formation of invadopodia in primary tumors and collagen I degradation were reduced in the areas of invasion (collagen-rich areas in the invasive edge of the tumor) and in the areas of intravasation (blood-vessel-rich areas). Our results suggest that tumor cells in vivo that have a decreased activity of N-WASP also have a reduced ability to form invadopodia, migrate, invade, intravasate and disseminate to lung compared with tumor cells with parental N-WASP levels.

3D/4D functional imaging of tumor-associated proteolysis: impact of microenvironment

Proteases play causal roles in many aspects of the aggressive phenotype of tumors, yet many of the implicated proteases originate from tumor-associated cells or from responses of tumor cells to interactions with other cells. Therefore, to obtain a comprehensive view of tumor proteases, we need to be able to assess proteolysis in tumors that are interacting with their microenvironment. As this is difficult to do in vivo, we have developed functional live-cell optical imaging assays and 3D and 4D (i.e., 3D over time) coculture models. We present here a description of the probes used to measure proteolysis and protease activities, the methods used for imaging and analysis of proteolysis and the 3D and 4D models used in our laboratory. Of course, all assays have limitations; however, we suggest that the techniques discussed here will, with attention to their limitations, be useful as a screen for drugs to target the invasive phenotype of tumors.

The effects of a plant proteinase inhibitor from Enterolobium contortisiliquum on human tumor cell lines

Supplementary to the efficient inhibition of trypsin, chymotrypsin, plasma kallikrein, and plasmin already described by the EcTI inhibitor from Enterolobium contortisiliquum, it also blocks human neutrophil elastase (K(iapp)=4.3 nM) and prevents phorbol ester (PMA)-stimulated activation of matrix metalloproteinase (MMP)-2 probably via interference with membrane-type 1 (MT1)-MMP. Moreover, plasminogen-induced activation of proMMP-9 and processing of active MMP-2 was also inhibited. Furthermore, the effect of EcTI on the human cancer cell lines HCT116 and HT29 (colorectal), SkBr-3 and MCF-7 (breast), K562 and THP-1 (leukemia), as well as on human primary fibroblasts and human mesenchymal stem cells (hMSCs) was studied. EcTI inhibited in a concentration range of 1.0-2.5 μM rather specifically tumor cell viability without targeting primary fibroblasts and hMSCs. Taken together, our data indicate that the polyspecific proteinase inhibitor EcTI prevents proMMP activation and is cytotoxic against tumor cells without affecting normal tissue remodeling fibroblasts or regenerative hMSCs being an important tool in the studies of tumor cell development and dissemination.

Matrix metalloproteinases in tumorigenesis: an evolving paradigm

Proteases are crucial for development, tissue remodeling, and tumorigenesis. Matrix metalloproteinases (MMPs) family, in particular, consists of more than 20 members with unique substrates and diverse function. The expression and activity of MMPs in a variety of human cancers have been intensively studied. MMPs have well-recognized roles in the late stage of tumor progression, invasion, and metastasis. However, increasing evidence demonstrates that MMPs are involved earlier in tumorigenesis, e.g., in malignant transformation, angiogenesis, and tumor growth both at the primary and metastatic sites. Recent studies also suggest that MMPs play complex roles in tumor progression. While most MMPs promote tumor progression, some of them may protect the host against tumorigenesis in a context-dependent manner. MMPs have been chosen as promising targets for cancer therapy on the basis of their aberrant up-regulation in malignant tumors and their ability to promote cancer metastasis. Although preclinical studies testing the efficacy of MMP suppression in tumor models were so encouraging, the results of clinical trials in cancer patients have been rather disappointing. Here, we review the complex roles of MMPs and their endogenous inhibitors such as tissue inhibitors of metalloproteinase in tumorigenesis and strategies in suppressing MMPs.

Non-invasive imaging of cysteine cathepsin activity in solid tumors using a 64Cu-labeled activity-based probe

The papain family of cysteine cathepsins are actively involved in multiple stages of tumorigenesis. Because elevated cathepsin activity can be found in many types of human cancers, they are promising biomarkers that can be used to target radiological contrast agents for tumor detection. However, currently there are no radiological imaging agents available for these important molecular targets. We report here the development of positron emission tomography (PET) radionuclide-labeled probes that target the cysteine cathepsins by formation of an enzyme activity-dependent bond with the active site cysteine. These probes contain an acyloxymethyl ketone (AOMK) functional group that irreversibly labels the active site cysteine of papain family proteases attached to a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) tag for labeling with ⁶⁴Cu for PET imaging studies. We performed biodistribution and microPET imaging studies in nude mice bearing subcutaneous tumors expressing various levels of cysteine cathepsin activity and found that the extent of probe uptake by tumors correlated with overall protease activity as measured by biochemical methods. Furthermore, probe signals could be reduced by pre-treatment with a general cathepsin inhibitor. We also found that inclusion of a Cy5 tag on the probe increased tumor uptake relative to probes lacking this fluorogenic dye. Overall, these results demonstrate that small molecule activity-based probes carrying radio-tracers can be used to image protease activity in living subjects.

Potential role of cathepsin B in the embryonic and larval development of clam Meretrix meretrix

This study was designed to investigate the possible role of Meretrix meretrix cathepsin B (MmeCB) in embryonic and larval development. MmeCB mRNA expression profile was revealed by semi-quantitative RT-PCR. The level of MmeCB mRNA expression was low in trochophore stage but high in pedveliger stage. MmeCB protein expression was detected in the digestive gland, velum, and epidermis along the edges of the shell in D-larvae and pedveligers by immunocytochemistry. In post larvae, MmeCB protein expression was noticed abundant in the digestive gland, whereas a modest expression was identified in the gill filament. The average shell length of larvae hatched from embryos treated with 0.01, 1, and 10 µmol/L Ca074Me (a cathepsin B inhibitor) was significantly shorter than that of control groups. The metamorphosis rates of larvae treated with 0.01 and 1 µmol/L Ca074Me were significantly lower than that of control groups in 4-day larvae, but not in 5-day larvae. Taken together, these results indicated that MmeCB may have stimulatory effects on embryonic development, metamorphosis, and larval growth during M. meretrix larval development.

Imaging enzymes at work: metabolic mapping by enzyme histochemistry

For the understanding of functions of proteins in biological and pathological processes, reporter molecules such as fluorescent proteins have become indispensable tools for visualizing the location of these proteins in intact animals, tissues, and cells. For enzymes, imaging their activity also provides information on their function or functions, which does not necessarily correlate with their location. Metabolic mapping enables imaging of activity of enzymes. The enzyme under study forms a reaction product that is fluorescent or colored by conversion of either a fluorogenic or chromogenic substrate or a fluorescent substrate with different spectral characteristics. Most chromogenic staining methods were developed in the latter half of the twentieth century but still find new applications in modern cell biology and pathology. Fluorescence methods have rapidly evolved during the last decade. This review critically evaluates the methods that are available at present for metabolic mapping in living animals, unfixed cryostat sections of tissues, and living cells, and refers to protocols of the methods of choice.

Interleukin-6 increases expression and secretion of cathepsin B by breast tumor-associated monocytes

In the tumor microenvironment, monocytes respond to paracrine stimuli from breast cancer cells by secreting molecules that participate in breast cancer growth, invasion, intravasation and metastasis. Here we examined the effects of media conditioned by MDA-MB-231 human breast carcinoma cells (231-CM) on expression and secretion of proteases and secretion of cytokines by U937 human monocytes. We found that 231-CM increased U937: 1) proliferation; 2) expression, activity and secretion of the cysteine protease cathepsin B (CTSB); 3) secretion of matrix metalloproteinases (MMP)-2 and -9; and 4) secretion of interleukin-6 (IL-6) and insulin-like growth factor binding protein-1 (IGFBP-1). We further demonstrated by western blotting and enzymatic activity assays that the increases in CTSB secretion and activity induced by 231-CM could be reduced by neutralizing antibodies against IL-6. Our data suggest a role for IL-6 in increased monocyte expression and secretion of CTSB in response to soluble factors secreted by breast cancer cells.

Pleiotropic roles of matrix metalloproteinases in tumor angiogenesis: contrasting, overlapping and compensatory functions

A number of extensive reviews are available discussing the roles of MMPs in various aspects of cancer progression from benign tumor formation to overt cancer present with deadly metastases. This review will focus specifically on the evidence functionally linking the MMPs and tumor-induced angiogenesis in various in vivo models. Emphasis has been placed on the cellular origin of the MMPs in tumor tissue, the requirement of proMMP activation and the resulting proteolytic activity for the induction and progression of tumor angiogenesis, and the pleiotropic roles for some of the MMPs. The functional mechanisms of the angiogenic MMPs are discussed as well as their catalytic detection in complex biological systems. In addition, the contribution of active MMPs to metastatic spread and establishment of secondary metastasis will be discussed in view of the findings indicating that MMPs are involved in the preparation of pre-metastatic niches. Finally, the most recent evidence, indicating the pro-metastatic consequences of anti-angiogenic therapies employing MMP inhibitors will be presented as examples highlighting possible outcomes of interfering with the pleiotropic nature of the MMP functionality.

Delineating protease functions during cancer development

Much progress has been made in understanding how matrix remodeling proteases, including metalloproteinases, serine proteases, and cysteine cathepsins, functionally contribute to cancer development. In addition to modulating extracellular matrix metabolism, proteases provide a significant protumor advantage to developing neoplasms through their ability to modulate bioavailability of growth and proangiogenic factors, regulation of bioactive chemokines and cytokines, and processing of cell-cell and cell-matrix adhesion molecules. Although some proteases directly regulate these events, it is now evident that some proteases indirectly contribute to cancer development by regulating posttranslational activation of latent zymogens that then directly impart regulatory information. Thus, many proteases act in a cascade-like manner and exert their functionality as part of a proteolytic pathway rather than simply functioning individually. Delineating the cascade of enzymatic activities contributing to overall proteolysis during carcinogenesis may identify rate-limiting steps or pathways that can be targeted with anti-cancer therapeutics. This chapter highlights recent insights into the complexity of roles played by pericellular and intracellular proteases by examining mechanistic studies as well as the roles of individual protease gene functions in various organ-specific mouse models of cancer development, with an emphasis on intersecting proteolytic activities that amplify programming of tissues to foster neoplastic development.

Live-cell imaging demonstrates extracellular matrix degradation in association with active cathepsin B in caveolae of endothelial cells during tube formation

Localization of proteases to the surface of endothelial cells and remodeling of the extracellular matrix (ECM) are essential to endothelial cell tube formation and angiogenesis. Here, we partially localized active cathepsin B and its cell surface binding partners, S100A/p11 (p11) of the annexin II heterotetramer (AIIt), to caveolae of human umbilical vein endothelial cells (HUVEC). Via a live-cell proteolysis assay, we observed that degradation products of quenched-fluorescent (DQ)-proteins (i.e. gelatin and collagen IV) colocalized intracellularly with caveolin-1 (cav-1) of HUVEC grown in either monolayer cultures or in vitro tube formation assays. Activity-based probes that bind covalently to active cysteine cathepsins and degradation products of DQ-collagen IV partially localized to intracellular vesicles that contained cav-1 and active cysteine cathepsins. Biochemical analyses revealed that the distribution of active cathepsin B in caveolar fractions increased during in vitro tube formation. Pro-uPA, uPAR, MMP-2 and MMP-14, which have been linked with cathepsin B to ECM degradation pathways, were also found to increase in caveolar fractions during in vitro tube formation. Our findings are the first to demonstrate through live-cell imaging ECM degradation in association with active cathepsin B in caveolae of endothelial cells during tube formation.

Imaging specific cell surface protease activity in living cells using reengineered bacterial cytotoxins

The scarcity of methods to visualize the activity of individual cell surface proteases in situ has hampered basic research and drug development efforts. In this chapter, we describe a simple, sensitive, and noninvasive assay that uses nontoxic reengineered bacterial cytotoxins with altered protease cleavage specificity to visualize specific cell surface proteolytic activity in single living cells. The assay takes advantage of the absolute requirement for site-specific endoproteolytic cleavage of cell surface-bound anthrax toxin protective antigen for its capacity to translocate an anthrax toxin lethal factor-beta-lactamase fusion protein to the cytoplasm. A fluorogenic beta-lactamase substrate is then used to visualize the cytoplasmically translocated anthrax toxin lethal factor-beta-lactamase fusion protein. By using anthrax toxin protective antigen variants that are reengineered to be cleaved by furin, urokinase plasminogen activator, or metalloproteinases, the cell surface activities of each of these proteases can be specifically and quantitatively determined with single cell resolution. The imaging assay is excellently suited for fluorescence microscope, fluorescence plate reader, and flow cytometry formats, and it can be used for a variety of purposes.

Imaging and quantifying the dynamics of tumor-associated proteolysis

The roles of proteases in cancer are dynamic. Furthermore, the roles or functions of any one protease may differ from one stage of cancer to another. Proteases from tumor-associated cells (e.g., fibroblasts, inflammatory cells, endothelial cells) as well as from tumor cells make important contributions to 'tumor proteolysis'. Many tumors exhibit increases in expression of proteases at the level of transcripts and protein; however, whether those proteases play causal roles in malignant progression is known for only a handful of proteases. What the critical substrate or substrates that are cleaved in vivo by any given protease is also known for only a few proteases. Therefore, the recent development of techniques and reagents for live cell imaging of protease activity, in conjunction with informed knowledge of critical natural substrates, should help to define protease functions. Here we describe live cell assays for imaging proteolysis, protocols for quantifying proteolysis and the use of such assays to follow the dynamics of proteolysis by tumor cells alone and tumor cells interacting with other cells found in the tumor microenvironment. In addition, we describe an in vitro model that recapitulates the architecture of the mammary gland, a model designed to determine the effects of dynamic interactions with the surrounding microenvironment on 'tumor proteolysis' and the respective contributions of various cell types to 'tumor proteolysis'. The assays and models described here could serve as screening platforms for the identification of proteolytic pathways that are potential therapeutic targets and for further development of technologies and imaging probes for in vivo use.

Direct visualization of protease activity on cells migrating in three-dimensions

Determining the specific role(s) of proteases in cell migration and invasion will require high-resolution imaging of sites of protease activity during live-cell migration through extracellular matrices. We have designed a novel fluorescent biosensor to detect localized extracellular sites of protease activity and to test requirements for matrix metalloprotease (MMP) function as cells migrate and invade three-dimensional collagen matrices. This probe fluoresces after cleavage of a peptide site present in interstitial collagen by a variety of proteases including MMP-2, -9, and -14 (MT1-MMP) without requiring transfection or modification of the cells being characterized. Using matrices derivatized with this biosensor, we show that protease activity is localized at the polarized leading edge of migrating tumor cells rather than further back on the cell body. This protease activity is essential for cell migration in native cross-linked but not pepsin-treated collagen matrices. The new type of high-resolution probe described in this study provides site-specific reporting of protease activity and insights into mechanisms by which cells migrate through extracellular matrices; it also helps to clarify discrepancies between previous studies regarding the contributions of proteases to metastasis.

Tube travel: the role of proteases in individual and collective cancer cell invasion

Recent advances in high-resolution multimodal microscopy reveal how MT1-matrix metalloproteinase (MMP)/MMP-14 and other cell surface proteases degrade and remodel the extracellular matrix (ECM) to drive the dissemination of cancer cells into normal adjacent tissue. By cleaving collagen fibers and repatterning them into parallel bundles, individual cells reorient the ECM to permit movement in tube-like microtracks. Cells along the edge of these tubes can excavate ECM outward, generating macrotracks through which collective mass movement of cancer cells can occur. These findings develop our understanding of invasive processes in cancer and how to attack them by interfering with MMP-14 activity.

Development of peptides specifically modulating the activity of KLK2 and KLK3

The prostate produces several proteases, the most abundant ones being kallikrein-related peptidase 3 (KLK3, PSA) and KLK2 (hK2), which are potential targets for tumor imaging and treatment. KLK3 expression is lower in malignant than in normal prostatic epithelium and it is further reduced in poorly differentiated tumors, in which the expression of KLK2 is increased. KLK3 has been shown to inhibit angiogenesis, whereas KLK2 may mediate tumor growth and invasion by participating in proteolytic cascades. Thus, it may be possible to control prostate cancer growth by modulating the proteolytic activity of KLK3 and KLK2. We have developed peptides that very specifically stimulate the activity of KLK3 or inhibit that of KLK2. Using these peptides we have established peptide-based methods for the determination of enzymatically active KLK3. The first-generation peptides are unstable in vivo and are rapidly cleared from the circulation. Currently we are modifying the peptides to make them suitable for in vivo applications. We have been able to considerably improve the stability of KLK2-binding peptides by cyclization. In this review we summarize the possible roles of KLK3 and KLK2 in prostate cancer and then concentrate on the development of peptides that modulate the activity of these proteases.

The Degradome database: mammalian proteases and diseases of proteolysis

The degradome is defined as the complete set of proteases present in an organism. The recent availability of whole genomic sequences from multiple organisms has led us to predict the contents of the degradomes of several mammalian species. To ensure the fidelity of these predictions, our methods have included manual curation of individual sequences and, when necessary, direct cloning and sequencing experiments. The results of these studies in human, chimpanzee, mouse and rat have been incorporated into the Degradome database, which can be accessed through a web interface at http://degradome.uniovi.es. The annotations about each individual protease can be retrieved by browsing catalytic classes and families or by searching specific terms. This web site also provides detailed information about genetic diseases of proteolysis, a growing field of great importance for multiple users. Finally, the user can find additional information about protease structures, protease inhibitors, ancillary domains of proteases and differences between mammalian degradomes.

Visualizing protease activity in living cells: from two dimensions to four dimensions

Proteolytic degradation of extracellular matrix (ECM) components by cells is an important metabolic activity as cells grow, remodel, and migrate through the ECM. The ability to analyze ECM degradation can be valuable in the study of developmental processes as well as pathologies, such as cancer. In this unit we describe an in vitro live cell-based method to image and quantitatively measure the degradation of ECM components by live cells. Cells are grown in the presence of fluorescent dye-quenched protein substrates (DQ-gelatin, DQ-collagen I, and DQ-collagen IV) that are mixed with protein matrices. Upon proteolytic cleavage, fluorescence is released that directly reflects the level of proteolysis by the cells. Using confocal microscopy and advanced imaging software, the fluorescence is detected and accurate measurements of proteolytic degradation in three and four dimensions can be assessed.

p21-Activated kinase 1 coordinates aberrant cell survival and pericellular proteolysis in a three-dimensional culture model for premalignant progression of human breast cancer

Overexpression of p21-activated kinase 1 (PAK1) occurs during the progression of human breast cancer. We have investigated the role of PAK1 in the premalignant progression of the MCF10 series of human breast epithelial cell lines. Levels of PAK1 expression and activation increased with premalignant progression, and expression of dominant-negative (DN) PAK1 reduced both cell proliferation and migration/invasion. In three-dimensional (3D) overlay cultures in reconstituted basement membrane, the MCF10 series produced an in vitro model for premalignant progression. MCF10AneoT cells formed a hyperplastic morphology in which some spheroids developed abnormal lumens. The MCF10.AT1 line exhibited an atypical hyperplastic morphology of abnormal spheroid clusters that did not form lumens. The MCF10.DCIS cells exhibited dysplastic growth. Expression of DN-PAK1 promoted lumen formation in 3D-cultured MCF10A, NeoT, and AT1 structures, suggesting partial reversion of the premalignant phenotype, but did not affect the atypical budding of AT1 structures or the dysplastic growth of ductal carcinoma in situ structures. Aberrant proteolysis is another important characteristic of breast cancer progression and invasion. DN-PAK1 or knock-down of PAK1 reduced pericellular proteolysis of DQ-collagen IV in the 3D cultures. Treatment of cells with an inhibitor of Rac1 also reduced pericellular proteolysis, and this reduction was reversed by the expression of activated PAK1. Our conclusion is that overexpressed and activated PAK1 may be a key coordinator of aberrant cell survival and proteolysis in breast cancer progression.

Application of activity-based probes to the study of enzymes involved in cancer progression

Many tumor cells have elevated levels of hydrolytic and proteolytic enzymes, presumably to aid in key processes such as angiogenesis, cancer cell invasion, and metastasis. Functional roles of enzymes in cancer progression are difficult to study using traditional genomic and proteomic methods because the activities of these enzymes are often regulated by post-translational mechanisms. Thus, methods that allow for the direct monitoring of enzyme activity in a physiologically relevant environment are required to better understand the roles of specific players in the complex process of tumorigenesis. This review highlights advances in the field of activity-based proteomics, which uses small molecules known as activity-based probes (ABPs) that covalently bind to the catalytic site of target enzymes. We discuss the application of ABPs to cancer biology, especially to the discovery of tumor biomarkers, the screening of enzyme inhibitors, and the imaging of enzymes implicated in cancer.

Cysteine cathepsin non-inhibitory binding partners: modulating intracellular trafficking and function

Cysteine cathepsins play a fundamental role in tumor growth, invasion and migration, angiogenesis, and the metastatic cascade. Evidence of their overexpression in a wide array of human tumors has been well documented. Cysteine cathepsins seem to have a characteristic location-function relationship that leads to non-traditional roles such as those in development and pathology. For example, during tumor development, some cysteine cathepsins are found not just within lysosomes, but are also redistributed into presumptive exocytic vesicles at the cell periphery, resulting in their secretion. This altered localization contributes to non-lysosomal functions that have been linked to malignant progression. Mechanisms for altered localization are not well understood, but do include the interaction of cysteine cathepsins with binding partners that modulate intracellular trafficking and association with specific regions on the cell surface.

Emerging roles of proteases in tumour suppression

Proteases have long been associated with cancer progression because of their ability to degrade extracellular matrices, which facilitates invasion and metastasis. However, recent studies have shown that these enzymes target a diversity of substrates and favour all steps of tumour evolution. Unexpectedly, the post-trial studies have also revealed proteases with tumour-suppressive effects. These effects are associated with more than 30 different enzymes that belong to three distinct protease classes. What are the clinical implications of these findings?

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Cysteine cathepsins: multifunctional enzymes in cancer

Cysteine cathepsins are highly upregulated in a wide variety of cancers by mechanisms ranging from gene amplification to post-transcriptional modification. Their localization within intracellular lysosomes often changes during neoplastic progression, resulting in secretion of both inactive and active forms and association with binding partners on the tumour cell surface. Secreted, cell-surface and intracellular cysteine cathepsins function in proteolytic pathways that increase neoplastic progression. Direct proof for causal roles in tumour growth, migration, invasion, angiogenesis and metastasis has been shown by downregulating or ablating the expression of individual cysteine cathepsins in tumour cells and in transgenic mouse models of human cancer.

Matriptase: potent proteolysis on the cell surface

Matriptase is a type II transmembrane serine protease expressed in most human epithelia, where it is coexpressed with its cognate transmembrane inhibitor, hepatocyte growth factor activator inhibitor (HAI)-1. Activation of the matriptase zymogen requires sequential N-terminal cleavage, activation site autocleavage, and transient association with HAI-1. Matriptase has an essential physiological role in profilaggrin processing, corneocyte maturation, and lipid matrix formation associated with terminal differentiation of the oral epithelium and the epidermis, and is also critical for hair follicle growth. Matriptase and HAI expression are frequently dysregulated in human cancer, and matriptase expression that is unopposed by HAI-1 potently promotes carcinogenesis and metastatic dissemination in animal models.

Imaging specific cell-surface proteolytic activity in single living cells

We describe a simple, sensitive and noninvasive assay that uses nontoxic, reengineered anthrax toxin-beta-lactamase fusion proteins with altered protease cleavage specificity to visualize specific cell-surface proteolytic activity in single living cells. The assay could be used to specifically image endogenous cell-surface furin, urokinase plasminogen activator and metalloprotease activity. We have adapted the assay for fluorescence microscopy, flow cytometry and fluorescent plate reader formats, and it is amenable for automation and high-throughput analysis.