Macrophage-derived metalloelastase is responsible for the generation of angiostatin in Lewis lung carcinoma

Effective targeting of tumor vasculature by the angiogenesis inhibitors vasostatin and interleukin-12

Solid tumors are dependent on preexisting vasculature and neovascularization for their growth. Successful cancer therapies targeting the tumor vasculature would be expected to block the existing tumor blood supply and to prevent tumor neovascularization. We tested the antitumor activity of experimental therapy with 2 distinct antiangiogenic drugs. Vasostatin inhibits endothelial cell growth and neovascularization, and interleukin-12 (IL-12) targets the tumor vasculature acting through interferon-γ (IFN-γ) and the downstream chemokines interferon-inducible protein-10 (IP-10) and monokine induced by IFN-γ. Individually, vasostatin and IL-12 produced distinct efficacy profiles in trials aimed at reducing tumor growth in athymic mice. In combination, these inhibitors halted the growth of human Burkitt lymphoma, colon carcinoma, and ovarian carcinoma. Thus, cancer therapy that combines distinct inhibitors of angiogenesis is a novel, effective strategy for the experimental treatment of cancer.

Effective targeting of tumor vasculature by the angiogenesis inhibitors vasostatin and interleukin-12

Solid tumors are dependent on preexisting vasculature and neovascularization for their growth. Successful cancer therapies targeting the tumor vasculature would be expected to block the existing tumor blood supply and to prevent tumor neovascularization. We tested the antitumor activity of experimental therapy with 2 distinct antiangiogenic drugs. Vasostatin inhibits endothelial cell growth and neovascularization, and interleukin-12 (IL-12) targets the tumor vasculature acting through interferon-γ (IFN-γ) and the downstream chemokines interferon-inducible protein-10 (IP-10) and monokine induced by IFN-γ. Individually, vasostatin and IL-12 produced distinct efficacy profiles in trials aimed at reducing tumor growth in athymic mice. In combination, these inhibitors halted the growth of human Burkitt lymphoma, colon carcinoma, and ovarian carcinoma. Thus, cancer therapy that combines distinct inhibitors of angiogenesis is a novel, effective strategy for the experimental treatment of cancer.

Matrix turnover

This review concentrates on how the major component of extracellular matrix, collagen, is catabolized. This process is important in a number of aspects of orthodontics since matrix is constantly turning over, the rate of which differs in embryogenesis, ageing, disease, and physiological processes, such as orthodontic tooth movement. It is not the purpose of this review to consider each process in detail. The aim is to give a clear account of the matrix metalloproteinases (a major family of proteinases) including their classification, properties, and functions.

New concepts in fibrinolysis and angiogenesis

Angiogenesis, the process by which new blood vessels form from preexisting vasculature, underlies a number of biologic processes including embryologic development, inflammation, wound healing, hypoxic retinal vascular proliferation, tumor growth, and atherosclerosis. The fibrinolytic system represents a cascade of serine protease activation events that culminate in the generation of plasmin. Although in-vitro studies suggest several possible roles that plasmin might play in angiogenesis, angiogenesis and fibrinolytic activity do not always correlate in in-vivo systems. During cutaneous and corneal wound healing, for example, angiogenesis proceeds normally in plasminogen-deficient animals. Similarly, the growth of most neoplasms is unimpaired in the absence of plasminogen. On the other hand, hypoxia-driven vascular proliferation may require plasmin-like activity, and angiogenesis within the atherosclerotic plaque seems to be associated with increased expression of fibrinolytic proteins. Recently, several nonplasmin fibrinolysins that may support the invasive phenotype of endothelial cells under specific circumstances have been identified. Thus, the contribution of individual fibrinolysins appears to be context-specific, just as the profile of endothelial cell gene expression depends upon the surrounding tissue milieu.

New paradigms for the treatment of cancer: the role of anti-angiogenesis agents

Angiogenesis, the sprouting of new blood vessels, plays a role in diverse disease states including cancer, diabetic retinopathy, age-related macular degeneration, rheumatoid arthritis, psoriasis, atherosclerosis, and restenosis. With regard to cancer, the clinical association of tumor vascularity with tumor aggressiveness has been clearly demonstrated in numerous tumor types. The observation of increased microvessel density in tumors not only serves as an independent prognostic indicator, but also suggests that anti-angiogenic therapy may be an important component of treatment regimens for cancer patients. The complexity of the angiogenic process, which involves both positive and negative regulators, provides a number of targets for therapy. Many positive regulators, including growth factor receptors, matrix metalloproteinases, and integrins, have been correlated with increased vascularity of tumors and poor prognosis for patient survival. Thus, these serve as ideal targets for anti-angiogenesis therapy. Many inhibitors of these targets are currently undergoing clinical evaluation as potential anti-cancer agents. In this article, we discuss the role of positive regulators in angiogenesis and tumor growth and describe the anti-angiogenic agents under development.

No grip, no growth: the conceptual basis of excessive proteolysis in the treatment of cancer

The formation of new bloodvessels, called angiogenesis, is critical for a tumour to grow beyond a few mm(3) in size. A provisional matrix promotes endothelial cell adhesion, migration, proliferation and survival. Synthesis and degradation of this matrix closely resemble processes that occur during coagulation and fibrinolysis. Degradation of the matrix and fibrinolysis are tightly controlled and balanced by stimulators and inhibitors of the plasminogen activation system. Here we give an overview of these processes during tumour progression. We postulate a novel way to inhibit angiogenesis by removal of the matrix through specific and localised overstimulation of the plasminogen activation system.

Angiostatin generation by human tumor cell lines: involvement of plasminogen activators

Angiostatin is a tumor-derived angiogenesis inhibitor consisting of an internal fragment of plasminogen. Little is known about the production of angiostatin by human tumors. In this study, we examined the in vitro angiostatin-generating capacities of a panel of human tumor cell lines (total n = 75) and the proteolytic molecule(s) involved. Angiostatin formation was determined by assessing the level of plasminogen digestion in conditioned medium by Western-blot analysis. We found that the capacity to produce angiostatin is a common feature of many cell lines, depending on the tumor type. All 6 bladder-carcinoma and 6 out of 7 prostate-carcinoma cell lines showed intermediate to potent angiostatin-generating activity. In contrast, only 2 out of 7 colon-carcinoma and 2 out of 9 renal-cell carcinoma cell lines were able to generate angiostatin at intermediate levels. Out of 25 melanoma cell lines, only one line failed to generate angiostatin. In the other cell-line groups (cervix, breast and ovary), angiostatin formation varied. Remarkably, angiostatin bands were not of equal size in all plasminogen digests. Since reported data have indicated that plasminogen activators (uPA and tPA) were able to excise the angiostatin fragment from the plasminogen parent molecule via plasmin generation, we determined levels of uPA and tPA and PAI-1 antigen in the conditioned media, and correlated the results with angiostatin-generating capacity. Whereas prostate- and bladder-carcinoma lines capable of generating high levels of angiostatin showed high uPA levels, angiostatin generation in melanoma cell lines was correlated with tPA levels. Generally, angiostatin non-producers did not express uPA or tPA. In 6 out of 75 cell lines, however, we found angiostatin generation combined with low or absent levels of plasminogen activator, suggesting the involvement of alternative proteolytic pathways in the generation of angiostatin.

Angiogenic balance in human melanoma: expression of VEGF, bFGF, IL-8, PDGF and angiostatin in relation to vascular density of xenografts in vivo

Tumor angiogenesis, a major requirement for tumor outgrowth and metastasis formation, is regulated by pro- and anti-angiogenic factors. We have studied the expression of a panel of angiogenic factors, and of the angiogenesis inhibitor angiostatin, in a panel of human melanoma cell lines giving rise to xenografts with different vascular densities. Angiogenic-factor expression was analyzed in vitro (cell lines) and in vivo (xenografts), both at mRNA (RT-PCR and Northern blot) and at protein level (ELISA and Western blot). In vitro angiostatin generation was assessed by Western-blot analysis. Expression of bFGF and VEGF was clearly correlated with a high degree of vascularization, confirming the importance of these factors for tumor angiogenesis. In addition, there was exclusive or elevated in vitro expression of angiogenic factors IL-8, PDGF-AB, and, to a lesser extent, midkine in cell lines that formed highly vascularized tumors. A similar angiogenic-factor-expression pattern was found in the corresponding xenografts, with the exception of VEGF. In most cell lines, this factor had low expression in vitro which was strongly enhanced in vivo. Although all 8 melanoma cell lines were able to excise the angiostatin fragment from the plasminogen parent molecule in vitro, cell lines BLM and M14 showed the most potent angiostatin generation. In vitro angiostatin generation by cell lysates prepared from melanoma xenografts was comparable in all xenograft types. Thus, in our model system we found no correlation between angiostatin generation and vascular density. Our study has limited the number of pro-angiogenic factors that may be involved in melanoma angiogenesis, and provides evidence for the notion that regulation of tumor angiogenesis is dependent on multiple factors. Inhibition of angiogenesis for therapeutic purposes, therefore, should preferably not concentrate on a single factor.

Expression of human macrophage metalloelastase (MMP-12) by tumor cells in skin cancer

Matrix metalloproteinases play an essential role in tumor growth and invasion. Different matrix metalloproteinases are often expressed in cancers with distinct patterns. To investigate the role of human macrophage metalloelastase (MMP-12) in epidermal tumors, we studied human macrophage metalloelastase mRNA and protein expression in malignant squamous cell and basal cell carcinomas, and in premalignant Bowen's disease. Human macrophage metalloelastase was detected in 11 of 17 squamous cell carcinomas in epithelial cancer cells, whereas macrophages were positive in 15 of 17 samples. In basal cell carcinomas, human macrophage metalloelastase was more often found in macrophages (seven of 19) than in cancer cells (four of 19). Human macrophage metalloelastase mRNA was also detected in three cell lines derived from squamous cell carcinomas of the head and neck and in transformed HaCaT cells, whereas premalignant tumors and primary keratinocytes were negative for human macrophage metalloelastase mRNA. Western analysis revealed human macrophage metalloelastase protein in squamous cell carcinoma cells. Our results show that human macrophage metalloelastase can be expressed in vivo and in vitro by transformed epithelial cells and indicate that the level of human macrophage metalloelastase expression correlates with epithelial dedifferentiation and histologic aggressiveness.

Target molecules for anti-angiogenic therapy: from basic research to clinical trials

There is growing evidence that anti-angiogenic drugs will improve future therapies of diseases like cancer, rheumatoid arthritis and ocular neovascularisation. However, it is still uncertain which kind of substance, out of the large number of angiogenesis inhibitors, will prove to be a suitable agent to treat these human diseases. There are currently more than 30 angiogenesis inhibitors in clinical trials and a multitude of promising new candidates are under investigation in vitro and in animal models. Important therapeutic strategies are: suppression of activity of the major angiogenic regulators like vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF); inhibition of function of alphav-integrins and matrix metalloproteinases (MMPs); the exploitation of endogenous anti-angiogenic molecules like angiostatin, endostatin or thrombospondin. Given the wide spectrum of diseases which could be treated by anti-angiogenic compounds, it is important for today's clinicians to understand their essential mode of action at a cellular and molecular level. Here we give an in-depth overview of the basic pathophysiological mechanisms underlying the different anti-angiogenic approaches used to date based on the most recent fundamental and clinical research data. The angiogenesis inhibitors in clinical trials are presented and promising future drug candidates are discussed.

Therapeutic potential of matrix metalloproteinase inhibitors in atherosclerosis

The activity of matrix-degrading metalloproteinases (MMPs) is essential for many of the processes involved in atherosclerotic plaque formation, for example, infiltration of inflammatory cells, smooth muscle cell migration and proliferation and angiogenesis. Furthermore, matrix degradation by MMPs may cause the plaque instability and rupture that leads to the clinical symptoms of atherosclerosis; unstable angina, myocardial infarction and stroke. Together, the family of MMPs can degrade all of the components of the blood vessel extracellular matrix and their activity therefore, is tightly regulated in normal blood vessels. The increased MMP activity during atherosclerotic plaque development and instability must therefore be caused by increased cytokine and growth factor-stimulated gene transcription, elevated zymogen activation and an imbalance in the MMP:TIMP ratio. It is therefore conceivable that inhibition of MMPs or re-establishing the MMP:TIMP balance may be useful in treating the symptoms of atherosclerosis. Recent studies using synthetic MMP inhibitors and gene therapy have highlighted the potential of such an approach.

The detection and comparison of angiogenesis-associated factors in pyogenic granuloma by immunohistochemistry

BACKGROUND

Pyogenic granuloma is a benign inflammatory lesion demonstrating obvious activity of angiogenesis. Female steroid hormones are believed to play important roles in the etiology because the lesion is frequently found in females with high levels of sex hormones. Few molecular mechanisms of the pathogenesis have been proposed and proven. The purpose of this study was to detect and compare the expression of angiogenesis-associated factors among healthy gingiva, gingiva from periodontitis, and pyogenic granuloma to clarify the pathogenesis of pyogenic granuloma.

METHODS

Fifteen specimens were collected from each of 3 groups of gingiva (healthy gingiva, periodontitis, and pyogenic granuloma). The subjects were age and gender matched. The specimens were processed for immunohistochemistry to detect and compare the expression of 2 angiogenesis enhancers, i.e., vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), 2 angiogenesis inhibitors, i.e., angiostatin and thrombospondin-1 (TSP-1), and estrogen receptor (ER). Using the subject as the unit of statistical analysis, either analysis of variance or chi-square analysis was employed to show the statistically significant difference at a level P <0.05.

RESULTS

The pyogenic granuloma group expressed significantly more VEGF and bFGF than healthy gingiva and periodontitis. The positive staining of VEGF was mostly localized in the cytoplasm of macrophages and fibroblasts while that of bFGF was in the extracellular matrix of lamina propria. Angiostatin was expressed significantly less in pyogenic granuloma than the other 2 groups and was mostly localized in the nuclei of endothelial cells and epithelial cells. There was no significant difference in the expression of TSP-1 and ER among the 3 groups.

CONCLUSIONS

The results of this research suggest that the etiology of pyogenic granuloma is due to the imbalance between angiogenesis enhancers and inhibitors. Whether and how the angiogenesis-associated factors are regulated by female steroid hormones remain to be answered.

Collagen XVIII/endostatin structure and functional role in angiogenesis

The angiogenesis inhibitor endostatin is a 20 kDA C-terminal fragment of collagen XVIII, a proteoglycan/collagen found in vessel walls and basement membranes. The endostatin fragment was originally identified in conditioned media from a murine endothelial tumor cell line. Endostatin inhibits endothelial cell migration in vitro and appears to be highly effective in murine in vivo studies. The molecular mechanisms behind the inhibition of angiogenesis have not yet been elucidated. Studies of the crystal structure of endostatin have shown a compact globular fold, with one face particularly rich in arginine residues acting as a heparin-binding epitope. It was initially suggested that zinc binding was essential for the antiangiogenic mechanism but later studies indicate that zinc has a structural rather than a functional role in endostatin. The generation of endostatin or endostatin-like collagen XVIII fragments is catalyzed by proteolytic enzymes, including cathepsin L and matrix metalloproteases, that cleave peptide bonds within the protease-sensitive hinge region of the C-terminal domain. The processing of collagen XVIII to endostatin may represent a local control mechanism for the regulation of angiogenesis.

Adjuvant therapy of stage III and IV malignant melanoma using granulocyte-macrophage colony-stimulating factor

PURPOSE

To evaluate granulocyte-macrophage colony-stimulating factor (GM-CSF) as surgical adjuvant therapy in patients with malignant melanoma who are at high risk of recurrence.

PATIENTS AND METHODS

Forty-eight assessable patients with stage III or IV melanoma were treated in a phase II trial with long-term, chronic, intermittent GM-CSF after surgical resection of disease. Patients with stage III disease were required to have more than four positive nodes or a more than 3-cm mass. All patients were rendered clinically disease-free by surgery before enrollment. The GM-CSF was administered subcutaneously in 28-day cycles, such that a dose of 125 microg/m(2) was delivered daily for 14 days followed by 14 days of rest. Treatment cycles continued for 1 year or until disease recurrence. Patients were evaluated for toxicity and disease-free and overall survival.

RESULTS

Overall and disease-free survival were significantly prolonged in patients who received GM-CSF compared with matched historical controls. The median survival duration was 37.5 months in the study patients versus 12.2 months in the matched controls (P <.001). GM-CSF was well tolerated; only one subject discontinued drug due to an adverse event (grade 2 injection site reaction).

CONCLUSION

GM-CSF may provide an antitumor effect that prolongs survival and disease-free survival in patients with stage III and IV melanoma who are clinically disease-free. These results support institution of a prospective, randomized clinical trial to definitively determine the value of surgical adjuvant therapy with GM-CSF in such patients.

Disruption of interkringle disulfide bond of plasminogen kringle 1-3 changes the lysine binding capability of kringle 2, but not its antiangiogenic activity

Kringle 1-3 of human plasminogen is a potent inhibitor of endothelial cell proliferation. To understand a possible role for the unique cystine bridge between kringle 2 and kringle 3, we disrupted the interkringle disulfide bond by mutating Cys(169) and Cys(297) to serine residues. The yield of the mutant during the refolding process was decreased significantly. Anti-endothelial cell proliferative activity of the mutant was similar to that of the wild type. There was no significant difference in in vivo antiangiogenic activity between the wild type and the mutant in chorioallantoic membrane assay. However, in the mutant, the weak lysine binding capability of kringle 2 was not detected and its mobility in nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis is different from that of the wild type. These results support the notion that the overall antiangiogenic function of angiostatin is mediated by individual kringles, and suggest that the lysine binding capability of kringle 2 is likely not important for the antiangiogenic activity of kringle 1-3.

Secreted cathepsin L generates endostatin from collagen XVIII

Endostatin, an inhibitor of angiogenesis and tumor growth, was identified originally in conditioned media of murine hemangioendothelioma (EOMA) cells. N-terminal amino acid sequencing demonstrated that it corresponds to a fragment of basement membrane collagen XVIII. Here we report that cathepsin L is secreted by EOMA cells and is responsible for the generation of endostatin with the predicted N-terminus, while metalloproteases produce larger fragments in a parallel processing pathway. Efficient endostatin generation requires a moderately acidic pH similar to the pericellular milieu of tumors. The secretion of cathepsin L by a tumor cell line of endothelial origin suggests that this cathepsin may play a role in angiogenesis. We propose that cleavage within collagen XVIII's protease-sensitive region evolved to regulate excessive proteolysis in conditions of induced angiogenesis.

Induction of chemokine secretion and enhancement of contact-dependent macrophage cytotoxicity by engineered expression of granulocyte-macrophage colony-stimulating factor in human colon cancer cells

We investigated the role of tumor cell-derived GM-CSF in recruitment and tumoricidal activation of tissue macrophages. Transfection of the murine GM-CSF gene into KM12SM human colon cancer cells decreased the tumorigenicity of transfected cells and nontransfected bystander colon cancer cells in nude mice. Sequential tissue sections from sites injected with high GM-CSF-producing tumor cells (but not from nontransfected or low GM-CSF-producing cells) demonstrated a dense infiltration of polymorphonuclear cells (PMN), followed by infiltration of macrophages, which correlated with expression of the macrophage-inflammatory protein-1alpha and the monocyte chemoattractant protein-1 (MCP-1) in mouse PMN and macrophages. GM-CSF-producing KM12SM cells were highly sensitive to lysis by mouse macrophages and also increased macrophage-mediated lysis of bystander nontransfected KM12SM cells. The incubation of macrophages with GM-CSF induced expression of the CD11b surface adhesion molecule, which was associated with increased attachment to tumor cells. All KM12SM cells were sensitive to macrophage-mediated lysis in the presence of rGM-CSF and recombinant MCP-1. Collectively, the results demonstrate that tumor cell-derived GM-CSF stimulates PMN and macrophages to secrete macrophage-inflammatory protein-1alpha and MCP-1, which triggers recruitment of mononuclear cells, induces expression of adhesion molecules on macrophages, and enhances contact-dependent cytolysis of tumor cells.

Recombinant kringle 1-3 of plasminogen inhibits rabbit corneal angiogenesis induced by angiogenin

PURPOSE

Angiostatin is a potent angiogenesis inhibitor that has been identified as a cryptic fragment of plasminogen molecule containing the first four kringle domain. Angiogenin, a 14-kDa monomeric protein, a potent blood vessel inducer, is expressed in tumors and present in mammalian plasma. The purpose of this study was to determine whether recombinant kringle 1-3 (rKI-3) of human plasminogen could interfere with angiogenesis induced by angiogenin and to evaluate the role of angiogenin in corneal angiogenesis in rabbit.

METHODS

A Hydron polymer pellet containing 2.0 microg of angiogenin was implanted intrastromally into the superior cornea of each of 44 rabbit eyes. All eyes received an intrastromal pellet and were randomized into either one group treated with 12.5 microg of rKI-3 (n = 25) or the other group treated with phosphate-buffered saline (PBS; n = 19). Both pellets were positioned in parallel at the site 1.2 mm from the superior limbus. Two masked observers kept the angiogenesis score daily, based on the number and the length of new vessels. The corneas with induced angiogenesis also were examined histologically.

RESULTS

On the third day of the angiogenin pellets implantation, the eye treated with rKI-3 had less angiogenesis (mean score, 4.2 +/- 6.6) than eye treated with PBS (mean score, 16.1 +/- 17.1; p < 0.05, Mann-Whitney U test). The cornea treated with PBS also showed much more leukocyte adhesion than the cornea treated with rKI-3.

CONCLUSION

Recombinant kringle 1-3 appears to inhibit angiogenin-induced angiogenesis in a rabbit corneal pocket assay. Recombinant kringle 1-3 may have therapeutic potential as an antiangiogenic agent.

Elevated matrix metalloprotease and angiostatin levels in integrin alpha 1 knockout mice cause reduced tumor vascularization

Integrin alpha1beta1 is a collagen receptor abundantly expressed on microvascular endothelial cells. As well as being the only collagen receptor able to activate the Ras/Shc/mitogen-activated protein kinase pathway promoting fibroblast cell proliferation, it also acts to inhibit collagen and metalloproteinase (MMP) synthesis. We have observed that in integrin alpha1-null mice synthesis of MMP7 and MMP9 was markedly increased compared with that of their wild-type counterparts. As MMP7 and MMP9 have been shown to generate angiostatin from circulating plasminogen, and angiostatin acts as a potent inhibitor of endothelial cell proliferation, we determined whether tumor vascularization was altered in the alpha1-null mice. Tumors implanted into alpha1-null mice showed markedly decreased vascularization, with a reduction in capillary number and size, which was accompanied by an increase in plasma levels of angiostatin due to the action of MMP7 and MMP9 on circulating plasminogen. In vitro analysis of alpha1-null endothelial cells revealed a marked reduction of their proliferation on both integrin alpha1-dependent (collagenous) and independent (noncollagenous) substrata. This reduction was prevented by culturing alpha1-null cells with plasma derived from plasminogen-null animals, thus omitting the source from which to generate angiostatin. Plasma from tumor-bearing alpha1-null animals uniquely inhibited endothelial cell growth, and this inhibition was relieved by the coaddition of either MMP inhibitors, or antibody to angiostatin. Integrin alpha1-deficient mice thus provide a genetically characterized model for enhanced angiostatin production and serve to reveal an unwanted potential side effect of MMP inhibition, increased tumor angiogenesis.

Extrinsic regulators of epithelial tumor progression: metalloproteinases

Extracellular metal-dependent proteinases regulate cell behavior by remodeling stromal and cell surface proteins, thereby influencing cell recruitment, cell shape, motility, proliferation, survival, genomic (in)stability, and differentiation. In recent years, the importance of proteinase-induced signaling has been underscored by evidence that altered regulation of cell-extracellular matrix and cell-cell interactions by proteinases can contribute, in a causal manner, to neoplastic progression.

Autocrine production of IL-10 mediates defective IL-12 production and NF-kappa B activation in tumor-associated macrophages

IL-12 is a central cytokine in the activation of inflammation and immunity and in the generation of Th1-type responses. Tumor-associated macrophages (TAM) from mouse and human tumors showed defective production of IL-12. Defective IL-12 production was associated with lack of p50/p65 NF-kappa B activation. TAM produced increased amounts of the immunosuppressive cytokine IL-10. Abs against IL-10 restored the defective capacity of TAM to produce IL-12. Our data suggest that during tumor growth an IL-10-dependent pathway of diversion of macrophage function can be activated into the tumor microenvironment and results in the promotion of the IL-10+ IL-12- phenotype of TAM. Blocking IL-10, as well as other immunosuppressive cytokines present in the tumor microenvironment, such as TGF-beta, may complement therapeutic strategies aimed at activating type I antitumor immune responses.

Implications of human macrophage metalloelastase and vascular endothelial growth factor gene expression in angiogenesis of hepatocellular carcinoma

OBJECTIVE

To determine molecular mechanisms involved in angiogenesis of hepatocellular carcinoma (HCC).

SUMMARY BACKGROUND DATA

Tumor angiogenesis is believed to derive from the balance between angiogenic stimulators and inhibitors. It has been suggested that the switch to the angiogenic phenotype requires both upregulation of the first and downregulation of the second. However, its molecular basis in vivo remains obscure. In this study the authors analyze the participation of two factors in angiogenesis of HCC- human macrophage metalloelastase (HME), a matrix metalloproteinase responsible for the generation of angiostatin, a potent angiogenesis inhibitor, and vascular endothelial growth factor (VEGF), the most potent endogenous angiogenic factor.

METHODS

Tumorous and contiguous nontumorous tissues from 25 patients with HCC who underwent curative partial hepatectomy were subjected to Northern blot analysis to detect HME and VEGF messenger RNA (mRNA) expression. Western blot analysis was used to detected angiostatin. Tumor vascularity was evaluated using hepatic angiography.

RESULTS

Eleven of the 15 cases expressing the HME gene showed hypovascular tumors, whereas hypervascular tumors were seen in 9 of the 10 HME-negative cases. The median of HME mRNA expression (tumorous/nontumorous ratio) was 6.5 (range 0-264.5) in the hypovascular group and 0 (range 0-3.2) in the hypervascular group. A stepwise logistic analysis revealed that HME and VEGF mRNA expression were two independent variables significantly affecting the vascularity of HCC tumors.

CONCLUSION

HME gene expression is significantly associated with hypovascular tumors; moreover, angiogenesis in HCC is not determined by a single factor, but depends on the net balance between HME and VEGF gene expressions.

Transcriptional induction of stromelysin-3 in mesodermal cells is mediated by an upstream CCAAT/enhancer-binding protein element associated with a DNase I-hypersensitive site

Stromelysin-3 (ST3) is a matrix metalloproteinase whose synthesis is markedly increased in stromal fibroblasts of most invasive human carcinomas. In the present study, we have investigated the molecular mechanisms by which high levels of ST3 expression can be induced. In contrast to the early and transient induction of interstitial collagenase by 12-O-tetradecanoylphorbol-13-acetate (TPA), the fibroblastic induction of ST3 was found to be delayed and to require protein neosynthesis. We demonstrated that this induction is transcriptional and does not result from changes in RNA stability. By looking next to promoter regions accessible to DNase I upon gene induction, we have identified two distal elements and have characterized their role in the transcriptional regulation of ST3. The first one is a TPA-responsive element that controls the base-line ST3 promoter activity but is not required for its activation. We demonstrate that ST3 gene induction is actually mediated by the second element, a C/EBP-binding site, by showing: (i) that this element becomes accessible in cells induced to express ST3, (ii) that endogenous C/EBPbeta binds to the ST3 promoter, and (iii) that this binding leads to ST3 transcriptional activation. Our study provides new insights into the regulation of ST3 and suggests an additional role for C/EBP transcription factors in tissue remodeling processes associated with this MMP.

Generation of angiostatin-like fragments from plasminogen by prostate-specific antigen

Angiostatin, a potent inhibitor of angiogenesis, tumour growth and metastasis, is a biologically active fragment of plasminogen, containing the kringle domains 1-4. It is generated from plasminogen by limited proteolysis. We show that prostate-specific antigen (PSA), a serine proteinase secreted by human prostate and human prostate cancer cells, is able to convert Lys-plasminogen to biologically active angiostatin-like fragments, containing kringles 1-4, by limited proteolysis of peptide bond Glu439-Ala440 in vitro. In an in vitro morphogenesis assay, the purified angiostatin-like fragments inhibited proliferation and tubular formation of human umbilical vein endothelial cells with the same efficacy as angiostatin. This finding might help to understand growth characteristics of prostate cancer, which usually has low microvessel density and slow proliferation.

The tumor-suppressing activity of angiostatin protein resides within kringles 1 to 3

Angiostatin protein, which comprises the first four kringle domains of plasminogen, is an endogenous inhibitor of angiogenesis that inhibits the growth of experimental primary and metastatic tumors. Truncation of Angiostatin K1-4 to K1-3 retained the activity of Angiostatin. We recombinantly expressed full-length human Angiostatin protein corresponding to the first four kringle domains of human plasminogen and a truncated form of the Angiostatin protein, kringles 1-3. Purified recombinant Angiostatin K1-3 and K1-4 proteins inhibited the formation of experimental B16-BL6 lung metastases by greater than 80% when administered at 30 nmol/kg/day. We demonstrate for the first time that Angiostatin protein, consisting of the first three kringle domains of human plasminogen, has in vivo biological activity in this assay indistinguishable from that of the full-length Angiostatin K1-4 protein and that the fourth kringle of plasminogen, when linked in sequence to K1-3, plays no direct role in the antitumor activity of Angiostatin.

Regulation of angiostatin production by matrix metalloproteinase-2 in a model of concomitant resistance

We have previously reported the identification of the endogenous angiogenesis inhibitor angiostatin, a specific inhibitor of endothelial cell proliferation in vitro and angiogenesis in vivo. In our original studies, we demonstrated that a Lewis lung carcinoma (LLC-LM) primary tumor could suppress the growth of its metastases by generating angiostatin. Angiostatin, a 38-kDa internal fragment of plasminogen, was purified from the serum and urine of mice bearing LLC-LM, and its discovery provides the first proven mechanism for concomitant resistance (O'Reilly, M. S., Holmgren, L., Shing, Y., Chen, C., Rosenthal, R. A., Moses, M. A., Lane, W. S., Cao, Y., Sage, E. H., and Folkman, J. (1994) Cell 79, 315-328). Subsequently, we have shown that systemic administration of angiostatin can regress a wide variety of malignant tumors in vivo. However, at the time of our initial discovery of angiostatin, the source of the protein was unclear. We hypothesized that the tumor or stromal cells might produce an enzyme that could cleave plasminogen sequestered by the primary tumor into angiostatin. Alternatively, we speculated that the tumor cells might express angiostatin. By Northern analysis, however, we have found no evidence that the tumor cells express angiostatin or other fragments of plasminogen (data not shown). We now report that gelatinase A (matrix metalloproteinase-2), produced directly by the LLC-LM cells, is responsible for the production of angiostatin, which suppresses the growth of metastases in our original model.

Tumor dormancy induced by downregulation of urokinase receptor in human carcinoma involves integrin and MAPK signaling

Mechanisms that regulate the transition of metastases from clinically undetectable and dormant to progressively growing are the least understood aspects of cancer biology. Here, we show that a large ( approximately 70%) reduction in the urokinase plasminogen activator receptor (uPAR) level in human carcinoma HEp3 cells, while not affecting their in vitro growth, induced a protracted state of tumor dormancy in vivo, with G(0)/G(1) arrest. We have now identified the mechanism responsible for the induction of dormancy. We found that uPA/uPAR proteins were physically associated with alpha5beta1, and that in cells with low uPAR the frequency of this association was significantly reduced, leading to a reduced avidity of alpha5beta1 and a lower adhesion of cells to the fibronectin (FN). Adhesion to FN resulted in a robust and persistent ERK1/2 activation and serum-independent growth stimulation of only uPAR-rich cells. Compared with uPAR-rich tumorigenic cells, the basal level of active extracellular regulated kinase (ERK) was four to sixfold reduced in uPAR-poor dormant cells and its stimulation by single chain uPA (scuPA) was weak and showed slow kinetics. The high basal level of active ERK in uPAR-rich cells could be strongly and rapidly stimulated by scuPA. Disruption of uPAR-alpha5beta1 complexes in uPAR-rich cells with antibodies or a peptide that disrupts uPAR-beta1 interactions, reduced the FN-dependent ERK1/2 activation. These results indicate that dormancy of low uPAR cells may be the consequence of insufficient uPA/uPAR/alpha5beta1 complexes, which cannot induce ERK1/2 activity above a threshold needed to sustain tumor growth in vivo. In support of this conclusion we found that treatment of uPAR-rich cells, which maintain high ERK activity in vivo, with reagents interfering with the uPAR/beta1 signal to ERK activation, mimic the in vivo dormancy induced by downregulation of uPAR.

Angiostatin binds to smooth muscle cells in the coronary artery and inhibits smooth muscle cell proliferation and migration In vitro

Angiostatin is an inhibitor of angiogenesis that is known to reduce endothelial cell proliferation and consequently prevent the progression of tumor metastases. However, the modest effect of angiostatin on endothelial cell proliferation raises the possibility that angiostatin might exert its effects on other cells. To determine the cellular distribution of angiostatin binding in tissues with neovasculature (atherosclerotic coronary arteries), we developed a fusion protein consisting of placental alkaline phosphatase and the first 3 kringles of plasminogen. Angiostatin binding colocalized with smooth muscle cells and could be inhibited by a 50-fold molar excess of plasminogen and 10 mmol/L epsilon-amino-n-caproic acid. The fusion protein also bound to smooth muscle cells in culture. Angiostatin inhibited hepatocyte growth factor-induced proliferation and migration of smooth muscle cells, suggesting that they are a target for the antiangiogenic effect of angiostatin.

Physiological roles of matrix metalloproteinases: implications for tumor growth and metastasis

Physiological processes involving remodelling of the extracellular matrix, such as wound healing, embryogenesis, angiogenesis, and the female reproductive cycle, require the activity of matrix metalloproteinases (MMPs). This group of proteases degrades basal membranes and connective tissues and plays an essential role in the homeostasis of the extracellular matrix. An imbalance in the expression or activity of MMPs can have important consequences in diseases such as multiple sclerosis, Alzheimer's disease, or the development of cancers. Because of the pathophysiological importance of MMPs, their activity is highly controlled in order to confine them to specific areas. An activation cascade, initiated by the proteolysis of plasminogen, cleaves proMMPs, and every step is controlled by specific activators or inhibitors. MMPs destabilize the organization of the extracellular matrix and influence the development of cancer by contributing to cell migration, tumor cell proliferation, and angiogenesis. Accordingly, these proteases possess an important role in cell-matrix interactions by affecting fundamental processes such as cell differentiation and proliferation. Therefore, the characterization of MMPs involved in specific types and stages of tumors will significantly improve the diagnosis and treatment of these cancers in humans.Key words: matrix metalloproteinases, physiology, cancer, cell invasion, extracellular matrix.

The rationale and future potential of angiogenesis inhibitors in neoplasia

Malignant tumours are angiogenesis-dependent diseases. Several experimental studies suggest that primary tumour growth, invasiveness and metastasis require neovascularisation. Tumour-associated angiogenesis is a complex multistep process under the control of positive and negative soluble factors. A mutual stimulation occurs between tumour and endothelial cells by paracrine mechanisms. Angiogenesis is necessary, but not sufficient, as the single event for tumour growth. There is, however, compelling evidence that acquisition of the angiogenic phenotype is a common pathway for tumour progression, and that active angiogenesis is associated with other molecular mechanisms leading to tumour progression. Experimental research suggests that it is possible to block angiogenesis by specific inhibitory agents, and that modulation of angiogenic activity is associated with tumour regression in animals with different types of neoplasia. The more promising angiosuppressive agents for clinical testing are: naturally occurring inhibitors of angiogenesis (angiostatin, endostatin, platelet factor-4 and others), specific inhibitors of endothelial cell growth (TNP-470, thalidomide, interleukin-12 and others), agents neutralising angiogenic peptides (antibodies to fibroblast growth factor or vascular endothelial growth factor, suramin and analogues, tecogalan and others) or their receptors, agents that interfere with vascular basement membrane and extracellular matrix [metalloprotease (MMP) inhibitors, angiostatic steroids and others], antiadhesion molecules antibodies such as antiintegrin alpha v beta 3, and miscellaneous drugs that modulate angiogenesis by diverse mechanisms of action. Antiangiogenic therapy is to be distinguished from vascular targeting. Gene therapy aimed to block neovascularisation is also a feasible anticancer strategy in animals bearing experimental tumours. Antiangiogenic therapy represents one of the more promising new approaches to anticancer therapy and it is already in early clinical trials. Because angiosuppressive therapy is aimed at blocking tumour growth indirectly, through modulation of neovascularisation, antiangiogenic agents need to be developed and evaluated as biological response modifiers. Therefore, adequate and well designed clinical trials should be performed for a proper evaluation of antiangiogenic agents, by determination and monitoring of surrogate markers of angiogenic activity.

Enhancement of tumoricidal activity of alveolar macrophages via CD40-CD40 ligand interaction

CD40-CD40 ligand (CD40L) interaction was originally defined as important molecules for the development of humoral immunity. Thereafter, some investigations have focused on its essential roles for the induction of cell-mediated immunity in host defenses. Here we investigated the antitumor activity of murine alveolar macrophages through CD40-CD40L interaction. The CD40L gene was transfected into murine lung cancer cells (3LLSA), and CD40L-expressing clones (3LLSA-CD40L) were established. Stimulation of CD40 molecules on the surface of alveolar macrophages with 3LLSA-CD40L cells induced the production of nitric oxide, tumor necrosis factor-alpha, and interleukin-12 and the tumoricidal activity of alveolar macrophages in the presence of interferon-gamma, which increased the surface expression of CD40 molecules on alveolar macrophages. These findings were not observed when alveolar macrophages were obtained from CD40-deficient mice. On the other hand, interleukin-6 production by alveolar macrophages did not depend on CD40-CD40L interaction. We also established a murine melanoma cell line expressing CD40L (B16 4A5-CD40L) that could induce tumoricidal activity of alveolar macrophages. Furthermore, when spleen cells were cocultivated with 3LLSA-CD40L cells, specific cytotoxic T lymphocytes for wild-type 3LLSA cells could be induced. These results suggest that CD40L gene transfer into tumor cells may induce antitumor immunity in a tumor-bearing host and may offer a new strategy for cancer gene therapy.

Angiostatin inhibits endothelial and melanoma cellular invasion by blocking matrix-enhanced plasminogen activation

Angiostatin, a kringle-containing fragment of plasminogen, is a potent inhibitor of angiogenesis. The mechanism(s) responsible for the anti-angiogenic properties of angiostatin are unknown. We now report that human angiostatin blocks plasmin(ogen)-enhanced in vitro invasion of tissue plasminogen activator (t-PA)-producing endothelial and melanoma cells. Kinetic analyses demonstrated that angiostatin functions as a non-competitive inhibitor of extracellular-matrix (ECM)-enhanced, t-PA-catalysed plasminogen activation, with a Ki of 0.9+/-0.03 microM. This mechanism suggests that t-PA has a binding site for the inhibitor angiostatin, as well as for its substrate plasminogen that, when occupied, prevents ternary complex formation between t-PA, plasminogen and matrix protein. Direct binding experiments confirmed that angiostatin bound to t-PA with an apparent Kd [Kd(app)] of 6.7+/-0.7 nM, but did not bind with high affinity to ECM proteins. Together, these data suggest that angiostatin in the cellular micro-environment can inhibit matrix-enhanced plasminogen activation, resulting in reduced invasive activity, and suggest a biochemical mechanism whereby angiostatin-mediated regulation of plasmin formation could influence cellular migration and invasion.

Angiogenesis and growth of murine colon carcinoma are dependent on infiltrating leukocytes

We determined whether the angiogenesis and growth of murine colon carcinomas growing in the wall of the cecum is dependent on infiltrating leukocytes. Syngeneic BALB/c or SCID mice were treated with a myelosuppressive, maximally tolerated dose of doxorubicin. Parental or multidrug resistant CT-26 colon carcinoma cells were implanted into the cecal wall 3 days after the second intravenous injection of doxorubicin. Control mice developed large, well-vascularized tumors, whereas doxorubicin-pretreated mice did not. Intravenous injection of spleen cells from normal BALB/c or SCID mice one day prior to tumor cell implantation reversed the decreased vascularity and tumorigenicity. The production of proangiogenic molecules and microvessel density in tumors directly correlated with the number of infiltrating leukocytes, suggesting that tumor-infiltrating leukocytes are essential to angiogenesis of murine colon carcinomas.

Kringle 5 causes cell cycle arrest and apoptosis of endothelial cells

Angiostatin which contains the first four kringle domains of plasminogen has been documented to be a potent inhibitor of angiogenesis. More recently, another kringle structure within plasminogen but outside angiostatin, known as kringle 5 (K5), was found to inhibit endothelial cell proliferation and migration. Here, we report the cloning and expression of mouse kringle 5 (rK5) in a bacterial expression system. The protein was purified to homogeneity using a Ni-NTA column. rK5 inhibited both proliferation and migration of endothelial cells with ED50's of 10 nM and < 500 nM, respectively. In addition, we show for the first time that rK5 causes cell cycle arrest and apoptosis, shedding further insight into rK5's mechanism of action. Finally, we show that these actions are endothelial cell specific.

Suppression of angiogenesis and tumor growth by the inhibitor K1-5 generated by plasmin-mediated proteolysis

Proteolytic enzymes are involved in generation of a number of endogenous angiogenesis inhibitors. Previously, we reported that angiostatin, a potent angiogenesis inhibitor, is a proteolytic fragment containing the first four kringle modules of plasminogen. In this report, we demonstrate that urokinase-activated plasmin can process plasminogen to release an angiogenesis inhibitor, K1-5 (protease-activated kringles 1-5). K1-5 inhibits endothelial-cell proliferation with a half-maximal concentration of approximately 50 pM. This inhibitory effect is endothelial-cell-specific and appears to be at least approximately 50-fold greater than that of angiostatin. A synergistic efficacy of endothelial inhibition was observed when angiostatin and kringle 5 (K5) were coincubated with capillary endothelial cells. The synergistic effect is comparable to that produced by K1-5 alone. Systemic treatment of mice with K1-5 at a low dose significantly blocked the fibroblast growth factor-induced corneal neovascularization, whereas angiostatin had no effect at the same dose. K1-5 also suppressed angiogenesis in chicken embryos. Systemic administration of K1-5 at a low dose at which angiostatin was ineffective significantly suppressed the growth of a murine T241 fibrosarcoma in mice. The antitumor effect correlates with the reduced neovascularization. These findings suggest that the plasmin-mediated proteolysis may be involved in the negative switch of angiogenesis.

Systemic gene delivery expands the repertoire of effective antiangiogenic agents

Cationic liposome-DNA complex (CLDC)-based intravenous gene delivery targets gene expression to vascular endothelial cells, macrophages and tumor cells. We used systemic gene delivery to identify anti-angiogenic gene products effective against metastatic spread in tumor-bearing mice. Specifically, CLDC-based intravenous delivery of the p53 and GM-CSF genes were each as effective as the potent antiangiogenic gene, angiostatin, in reducing both tumor metastasis and tumor angiogenesis. Combined delivery of these genes did not increase anti-tumor activity, further suggesting that each gene appeared to produce its antimetastatic activity through a common antiangiogenic pathway. CLDC-based intravenous delivery of the human wild type p53 gene transfected up to 80% of tumor cells metastatic to lung. Furthermore, it specifically induced the expression of the potent antiangiogenic gene, thrombospondin-1, indicating that p53 gene delivery in vivo may inhibit angiogenesis by inducing endogenous thrombospondin-1 expression. CLDC-based delivery also identified a novel anti-tumor activity for the metastasis suppressor gene CC3. Thus, CLDC-based intravenous gene delivery can produce systemic antiangiogenic gene therapy using a variety of different genes and may be used to assess potential synergy of combined anti-tumor gene delivery and to identify novel activities for existing anti-tumor genes.

Induction and Regulation of Macrophage Metalloelastase by Hyaluronan Fragments in Mouse Macrophages

Although the metalloproteinase murine metalloelastase (MME) has been implicated in lung disorders such as emphysema and pulmonary fibrosis, the mechanisms regulating MME expression are unclear. Low m.w. fragments of the extracellular matrix component hyaluronan (HA) that accumulate at sites of lung inflammation are capable of inducing inflammatory gene expression in macrophages (Mφ). The purpose of this study was to examine the effect of HA fragments on the expression of MME in alveolar Mφ. The mouse alveolar Mφ cell line MH-S was stimulated with HA fragments over time, total RNA was isolated, and Northern blot analysis was performed. HA fragments induced MME mRNA in a time-dependent fashion, with maximal levels at 6 h. HA fragments also induced MME protein expression as well as enzyme activity. The induction of MME gene expression was specific for low m.w. HA fragments and dependent upon new protein synthesis; it occurred at the level of gene transcription. We also examined the effect of HA fragments on MME expression in inflammatory alveolar Mφ from bleomycin-injured rat lungs. Although normal rat alveolar Mφ did not express MME mRNA in response to HA fragments, alveolar Mφ from the bleomycin-treated rats responded to HA fragment stimulation by increasing MME mRNA levels. Furthermore, baseline and HA fragment-induced MME gene expression in alveolar Mφ from bleomycin-treated rats was inhibited by IFN-γ. These data suggest that HA fragments may be an important mechanism for the expression of MME by Mφ in inflammatory lung disorders.

Angiogenesis in ductal breast carcinoma. Comparison of microvessel density between primary tumour and lymph node metastasis

Angiogenesis has been recognised as an important prognostic factor in cancer. This study assessed immunohistochemically (JC70 MoAb) the microvessel score (MS; in x 250 fields) in 35 breast ductal carcinomas with lymph node involvement. Sections from both primary tumours and invaded nodes were assessed. A significantly lower MS was observed in the metastatic foci (24.6 +/- 9 vs. 13.6 +/- 6; P < 0.0001) than in the primary lesions. However, linear regression analysis showed a significant direct correlation between the MS assessed in primary lesions and the related metastatic foci in the lymph nodes (P = 0.006, r = 045). The primary tumour to node (T/N) ratio was assessed; 11/35 cases had a node MS close to the score of the primary (T/N < 2), whilst 17/35 had a T/N ratio ranging from 3 to 7. Extracapsular node involvement was more frequent in cases with low T/N ratio. Angiogenesis in the metastatic foci was independent of the amount of growing stroma within the metastasis. Vessel density in the normal lymph nodes did not correlate with the MS within the node metastases. We conclude that cancer cells migrating to the nodes may have similar angiogenic abilities to the parental cells of the primary tumour. However, environmental factors probably related to node immune reaction against the invading tumour could be responsible for the reduced angiogenicity in the nodes. Further studies are required to investigate the suggested angiogenesis suppressing immune mechanisms occurring in the invaded lymph nodes of breast cancer patients and possible clinical implications.

Enhanced expression of human metalloelastase (MMP-12) in cutaneous granulomas and macrophage migration

Accumulation of inflammatory cells such as macrophages may lead to degeneration of connective tissue matrix in various skin diseases. Macrophage metalloelastase, is a matrix metalloproteinase (MMP-12) capable of degrading elastin as well as various basement membrane components. To investigate the role of human macrophage metalloelastase in skin, we assessed by in situ hybridization and immunohistochemistry 66 specimens representing skin diseases characterized either by changes in elastic fibers or by pronounced infiltrations of extravasating and migrating macrophages. CD68 immunostaining was performed to identify the human macrophage metalloelastase-positive cells and Weigert's Resorcin-Fuchsin staining to reveal the status of elastic fibers. We found abundant expression of human macrophage metalloelastase mRNA in macrophages in areas devoid of normal elastic fibers in granulomatous skin diseases sarcoidosis, necrobiosis lipoidica diabeticorum, and granuloma annulare. Positive cells for human macrophage metalloelastase protein could be detected in the same regions as well as positive immunostaining for urokinase plasminogen activator. Of the other matrix metalloproteinases capable of degrading elastin, 92 kDa gelatinase colocalized with human macrophage metalloelastase, while 72 kDa gelatinase was produced by surrounding fibroblast-like cells. Furthermore, human macrophage metalloelastase was expressed by macrophages in areas with disrupted basement membrane, as assessed by type IV collagen staining, in pityriasis lichenoides and dermatitis herpetiformis. Specimens of anetoderma, acrodermatitis chronica atrophicans and pseudoxanthoma elasticum showed no signal for human macrophage metalloelastase. Matrilysin was not detected in any of the samples investigated. Our study suggests that human macrophage metalloelastase may contribute to elastin degradation occurring in granulomatous skin diseases and may aid macrophage migration through the epidermal and vascular basement membranes in inflammatory disorders.

Angiostatin formation involves disulfide bond reduction and proteolysis in kringle 5 of plasmin

Plasmin is processed in the conditioned medium of HT1080 fibrosarcoma cells producing fragments with the domain structures of the angiogenesis inhibitor, angiostatin, and microplasmin. Angiostatin consists of kringle domains 1-4 and part of kringle 5, while microplasmin consists of the remainder of kringle 5 and the serine proteinase domain. Our findings indicate that formation of angiostatin/microplasmin involves reduction of plasmin by a plasmin reductase followed by proteolysis of the reduced enzyme. We present evidence that the Cys461-Cys540 and Cys511-Cys535 disulfide bonds in kringle 5 of plasmin were reduced by plasmin reductase. Plasmin reductase activity was secreted by HT1080 and Chinese hamster ovary cells and the human mammary carcinoma cell lines MCF-7, MDA231, and BT20 but not by the monocyte/macrophage cell line THP-1. Neither primary foreskin fibroblasts, blood monocyte/macrophages, nor macrovascular or microvascular endothelial cells secreted detectable plasmin reductase. In contrast, cultured bovine and rat vascular smooth muscle cells secreted small but reproducible levels of plasmin reductase. Reduction of the kringle 5 disulfide bonds triggered cleavage at either Arg529-Lys530 or two other positions C-terminal of Cys461 in kringle 5 by a serine proteinase. Plasmin autoproteolysis could account for the cleavage, although another proteinase was mostly responsible in HT1080 conditioned medium. Three serine proteinases with apparent Mr of 70, 50, and 39 were purified from HT1080 conditioned medium, one or more of which could contribute to proteolysis of reduced plasmin.

Angiostatin binds ATP synthase on the surface of human endothelial cells

Angiostatin, a proteolytic fragment of plasminogen, is a potent antagonist of angiogenesis and an inhibitor of endothelial cell migration and proliferation. To determine whether the mechanism by which angiostatin inhibits endothelial cell migration and/or proliferation involves binding to cell surface plasminogen receptors, we isolated the binding proteins for plasminogen and angiostatin from human umbilical vein endothelial cells. Binding studies demonstrated that plasminogen and angiostatin bound in a concentration-dependent, saturable manner. Plasminogen binding was unaffected by a 100-fold molar excess of angiostatin, indicating the presence of a distinct angiostatin binding site. This finding was confirmed by ligand blot analysis of isolated human umbilical vein endothelial cell plasma membrane fractions, which demonstrated that plasminogen bound to a 44-kDa protein, whereas angiostatin bound to a 55-kDa species. Amino-terminal sequencing coupled with peptide mass fingerprinting and immunologic analyses identified the plasminogen binding protein as annexin II and the angiostatin binding protein as the alpha/beta-subunits of ATP synthase. The presence of this protein on the cell surface was confirmed by flow cytometry and immunofluorescence analysis. Angiostatin also bound to the recombinant alpha-subunit of human ATP synthase, and this binding was not inhibited by a 2,500-fold molar excess of plasminogen. Angiostatin's antiproliferative effect on endothelial cells was inhibited by as much as 90% in the presence of anti-alpha-subunit ATP synthase antibody. Binding of angiostatin to the alpha/beta-subunits of ATP synthase on the cell surface may mediate its antiangiogenic effects and the down-regulation of endothelial cell proliferation and migration.