Angiostatin gene transfer: inhibition of tumor growth in vivo by blockage of endothelial cell proliferation associated with a mitosis arrest

Enhanced inhibition of murine tumor and human breast tumor xenografts using targeted delivery of an antibody-endostatin fusion protein

Endostatin can inhibit angiogenesis and tumor growth in mice. A potential limitation of endostatin as an antitumor agent in humans is the short serum half-life of the protein that may decrease effective concentration at the site of tumor and necessitate frequent dosing. In an effort to improve antitumor activity, endostatin was fused to an antibody specific for the tumor-selective HER2 antigen to create an antibody-endostatin fusion protein (anti-HER2 IgG3-endostatin). Normal endostatin rapidly cleared from serum in mice (T(1/2)(2), = 0.6-3.8 hours), whereas anti-HER2 IgG3-endostatin had a prolonged half-life (90% intact; T(1/2)(2), 40.2-44.0 hours). Antigen-specific targeting of anti-HER2 IgG3-endostatin was evaluated in BALB/c mice implanted with CT26 tumors or CT26 tumors engineered to express the HER2 antigen (CT26-HER2). Radio-iodinated anti-HER2 IgG3-endostatin preferentially localized to CT26-HER2 tumors relative to CT26 tumors. Administration of anti-HER2 IgG3-endostatin to mice showed preferential inhibition of CT26-HER2 tumor growth compared with CT26. Anti-HER2 IgG3-endostatin also markedly inhibited the growth of human breast cancer SK-BR-3 xenografts in severe combined immunodeficient mice. Anti-HER2 IgG3-endostatin inhibited tumor growth significantly more effectively than endostatin, anti-HER2 IgG3 antibody, or the combination of antibody and endostatin. CT26-HER2 tumors treated with the endostatin fusion protein had decreased blood vessel density and branching compared with untreated CT26-HER2 or CT26 treated with the fusion protein. The enhanced effectiveness of anti-HER2 IgG3-endostatin may be due to a longer half-life, improved serum stability, and selective targeting of endostatin to tumors, resulting in decreased angiogenesis. Linking of an antiangiogenic protein, such as endostatin, to a targeting antibody represents a promising and versatile approach to antitumor therapy.

Combinatorial antiangiogenic gene therapy by nonviral gene transfer using the sleeping beauty transposon causes tumor regression and improves survival in mice bearing intracranial human glioblastoma

Glioblastoma is a fatal brain tumor that becomes highly vascularized by secreting proangiogenic factors and depends on continued angiogenesis to increase in size. Consequently, a successful antiangiogenic therapy should provide long-term inhibition of tumor-induced angiogenesis, suggesting long-term gene transfer as a therapeutic strategy. In this study a soluble vascular endothelial growth factor receptor (sFlt-1) and an angiostatin-endostatin fusion gene (statin-AE) were codelivered to human glioblastoma xenografts by nonviral gene transfer using the Sleeping Beauty (SB) transposon. In subcutaneously implanted xenografts, co-injection of both transgenes showed marked anti-tumor activity as demonstrated by reduction of tumor vessel density, inhibition or abolition of glioma growth, and increase in animal survival (P = 0.003). Using luciferase-stable engrafted intracranial gliomas, the anti-tumor effect of convection-enhanced delivery of plasmid DNA into the tumor was assessed by luciferase in vivo imaging. Sustained tumor regression of intracranial gliomas was achieved only when statin-AE and sFlt-1 transposons were coadministered with SB-transposase-encoding DNA to facilitate long-term expression. We show that SB can be used to increase animal survival significantly (P = 0.008) by combinatorial antiangiogenic gene transfer in an intracranial glioma model.

Canstatin acts on endothelial and tumor cells via mitochondrial damage initiated through interaction with alphavbeta3 and alphavbeta5 integrins

Canstatin, the noncollagenous domain of collagen type IV alpha-chains, belongs to a series of collagen-derived angiogenic inhibitors. We have elucidated the functional receptors and intracellular signaling induced by canstatin that explain its strong antitumor efficacy in vivo. For this purpose, we generated a canstatin-human serum albumin (CanHSA) fusion protein, employing the HSA moiety as an expression tag. We show that CanHSA triggers a crucial mitochondrial apoptotic mechanism through procaspase-9 cleavage in both endothelial and tumor cells, which is mediated through cross-talk between alphavbeta3- and alphavbeta5-integrin receptors. As a point of reference, we employed the first three kringle domains of angiostatin (K1-3), fused with HSA, which, in contrast to CanHSA, act only on endothelial cells through alphavbeta3-integrin receptor-mediated activation of caspase-8 alone, without ensuing mitochondrial damage. Taken together, these results provide insights into how canstatin might exert its strong anticancer effect.

Angiogenesis in benign and malignant thyroid disease

CONTEXT

Angiogenesis has been recognized as an important process contributing to the pathophysiology of many benign and malignant diseases. It is not surprising, therefore, that this complex process is proving to be an important regulator of both benign and malignant disease processes in the thyroid gland. This paper will review the general principles of angiogenesis and lymphangiogenesis, as well as the importance of the balance between angiogenic stimulators and inhibitors in the normal thyroid gland. We will also review how this balance is disturbed in benign and malignant thyroid conditions. Finally, we will address the role manipulation of this process may play in the development of novel treatment strategies for diseases of the thyroid.

OBJECTIVE

To review the literature concerning the role of angiogenesis in the thyroid gland.

CONCLUSIONS

Angiogenesis is an important process which has been shown to be involved in the pathophysiology of benign and malignant diseases of the thyroid gland. Manipulation of this process holds great promise for the development of novel treatments for these disorders. As the mechanisms regulating angiogenesis in the thyroid become increasingly clear, researchers will come ever closer to turning this promise into clinical reality.

Endogenous inhibitors of angiogenesis in malignant gliomas: nature's antiangiogenic therapy

Angiogenesis is necessary for tumor growth beyond a volume of approximately 2 mm(3). This observation, along with the accessibility of tumor vessels to therapeutic targeting, has resulted in a research focus on inhibitors of angiogenesis. A number of endogenous inhibitors of angiogenesis are found in the body. Some of these are synthesized by specific cells in different organs, and others are created by extracellular proteolytic cleavage of plasma-derived or extracellular matrix-localized proteins. In this review, we focus on angiostatin, endostatin, PEX, pigment epithelial-derived factor, and thrombospondin (TSP)-1 and -2, either because these molecules are expressed in malignant glioma biopsies or because animal studies in malignant glioma models have suggested that their therapeutic administration could be efficacious. We review the known mechanisms of action, potential receptors, expression in glioma biopsy samples, and studies testing their potential therapeutic efficacy in animal models of malignant glioma. Two conclusions can be made regarding the mechanisms of action of these inhibitors: (1) Several of these inhibitors appear to mediate their antiangiogenic effect through multiple protein-protein interactions that inhibit the function of proangiogenic molecules rather than through a specific receptor-mediated signaling event, and (2) TSP-1 and TSP-2 appear to mediate their antiangiogenic effect, at least in part, through a specific receptor, CD36, which initiates the antiangiogenic signal. Although not proven in gliomas, evidence suggests that expression of specific endogenous inhibitors of angiogenesis in certain organs may be part of a host antitumor response. The studies reviewed here suggest that new antiangiogenic therapies for malignant gliomas offer exciting promise as nontoxic, growth-inhibitory agents.

Gene therapy of liver diseases

Many liver diseases lack satisfactory treatment and alternative therapeutic options are urgently needed. Gene therapy is a new mode of treatment for both inherited and acquired diseases, based on the transfer of genetic material to the tissues. Genes are incorporated into appropriate vectors in order to facilitate their entrance and function inside the target cells. Gene therapy vectors can be constructed on the basis of viral or non-viral molecular structures. Viral vectors are frequently used, due to their higher transduction efficiency. Both the type of vector and the expression cassette determine the duration, specificity and inducibility of gene expression. A considerable number of preclinical studies indicate that a great variety of liver diseases, including inherited metabolic defects, chronic viral hepatitis, liver cirrhosis and primary and metastatic liver cancer, are amenable to gene therapy. Gene transfer to the liver can also be used to convert this organ into a factory of secreted proteins needed to treat conditions that do not affect the liver itself. Clinical trials of gene therapy for the treatment of inherited diseases and liver cancer have been initiated but human gene therapy is still in its infancy. Recent progress in vector technology and imaging techniques, allowing in vivo assessment of gene expression, will facilitate the development of clinical applications of gene therapy.

Future therapies for hepatocellular carcinoma

A large proportion of patients with advanced hepatocellular carcinoma (HCC) lack effective therapy. Due to chemoresistance, hope has focused on other approaches including targeted therapies, immune stimulants, and the emerging area of gene therapy. Increasing efforts in basic and clinical development of these approaches will hopefully result in more efficient therapies against HCC.

Full kringles of plasminogen (aa 1–566) mediate complete regression of human MDA-MB-231 breast tumor xenografted in nude mice

Since kringle (K)5, not present in the angiostatin molecule, was shown to be a key functional domain possessing potent antiangiogenic activity, we have evaluated a new plasminogen-derived fragment, consisting of the N-terminal part of human plasminogen, that included the complete secondary structure of K1-5 (aa 1-566). In contrast to other fragments described to date, K1-5 includes cysteine residues at positions 543, 555 and 560 allowing the formation of the three disulfide bonds lying within K5. Vascular endothelial cell proliferation and migration assays revealed that a replication-defective adenovirus (AdK1-5(1-566)), expressing K1-5 (aa 1-566), was dose dependently more potent that AdK1-3(1-354), an adenovirus that expresses only the first three kringles. In contrast to AdK1-3(1-354), a single intratumoral injection of AdK1-5(1-566) into MDA-MB-231 breast human carcinoma tumors was followed by a total regression of 40% of the tumor and by significant arrest of tumor growth (90%), which was correlated with a drastic decrease of functional neovascularization into the tumors. Furthermore, systemic delivery of AdK1-5(1-566) in mice inhibited the lung invasion of melanoma B16-F10 cells by 87%. Our findings provide evidence that the full kringles of plasminogen (aa 1-566) may be much more potent than K1-3 (aa 1-354), for the suppression of angiogenesis, tumor growth and metastatic dissemination.

Combined immunotherapy and antiangiogenic therapy of cancer with microencapsulated cells

An alternative form of gene therapy involves immunoisolation of a nonautologous cell line engineered to secrete a therapeutic product. Encapsulation of these cells in a biocompatible polymer serves to protect these allogeneic cells from host-versus-graft rejection while recombinant products and nutrients are able to pass by diffusion. This strategy was applied to the treatment of cancer with some success by delivering either interleukin 2 or angiostatin. However, as cancer is a complex, multifactorial disease, a multipronged approach is now being developed to attack tumorigenesis via multiple pathways in order to improve treatment efficacy. A combination of immunotherapy with angiostatic therapy was investigated by treating B16-F0/neu melanoma-bearing mice with intraperitoneally implanted, microencapsulated mouse myoblasts (C2C12) genetically modified to deliver angiostatin and an interleukin 2 fusion protein (sFvIL-2). The combination treatment resulted in improved survival, delayed tumor growth, and increased histological indices of antitumor activity (apoptosis and necrosis). In addition to improved efficacy, the combination treatment also ameliorated some of the undesirable side effects from the individual treatments that have led to the previous failure of the single treatments, for example, inflammatory response to IL-2 or vascular mimicry due to angiostatin. In conclusion, the combination of immuno- and antiangiogenic therapies delivered by immunoisolated cells was superior to individual treatments for antitumorigenesis activity, not only because of their known mechanisms of action but also because of unexpected protection against the adverse side effects of the single treatments. Thus, the concept of a "cocktail" strategy, with microencapsulation delivering multiple antitumor recombinant molecules to improve efficacy, is validated.

Characterization and biological activities of recombinant human plasminogen kringle 1-3 produced in Escherichia coli

Angiogenesis, the formation of new capillaries from preexisting blood vessels, is involved in many pathological conditions, for example, tumorigenesis, diabetic retinopathy, and rheumatoid arthritis. Angiostatin, which contains the kringle 1-4 domains of plasminogen, is known to be a potent inhibitor of angiogenesis and a strong suppressor of various solid tumors. In this study, we expressed recombinant protein containing the kringle 1-3 domains of human plasminogen in Escherichia coli and investigated its biological activities. The protein was successfully refolded from inclusion bodies and purified at a 30% overall yield, as a single peak by HPLC. The purified recombinant protein had biochemical properties that were similar to those of the native form, which included molecular size, lysine-binding capacity, and immunoreactivity with a specific antibody. The recombinant protein was also found to strongly inhibit the proliferation of bovine capillary endothelial cells in vitro, and the formation of new capillaries on chick embryos. In addition, it suppressed the growth of primary Lewis lung carcinoma and B16 melanoma in an in vivo mouse model. Our findings suggest that the recombinant kringle 1-3 domains in a prokaryote expression system have anti-angiogenic activities, which may be useful in clinical and basic research in the field of angiogenesis.

Synergistic antitumor effect of antiangiogenic factor genes on colon 26 produced by low-voltage electroporation

Antiangiogenic factors are potent endothelial cell growth inhibitors that have been shown to inhibit angiogenesis in vitro and tumor growth in mice. We have demonstrated the synergistic antitumor effect of antiangiogenic genes (mouse angiostatin: pBLAST-mAngio; and mouse endostatin: p-BLAST42-mEndo XV) delivered to tumors by low-voltage electroporation in mouse colon 26 models. A synergistic antitumor effect was strongly suggested by in vivo tumor growth kinetics, as well as in survival studies with the mice. RT-PCR confirmed that the fragments of each gene were transferred by low-voltage electroporation in the tumor. Decreased microvessel density measurements in tumors also confirmed the efficacy of the synergistic antitumor effect of both genes. Significant growth inhibition was observed in mice treated with a 1:1 proportion of angiostatin and endostatin genes, and the order of the both genes transferred (first the endostatin gene, followed 1 week later by the angiostatin gene) had a profound inhibitory effect on tumor growth. These data suggest that in vivo delivery of antiangiogenic genes with low-voltage electroporation could be a possible therapeutic strategy for established solid tumors when both genes were applied in combination.

A multiprong approach to cancer gene therapy by coencapsulated cells

Immune-isolation of nonautologous cells with microencapsulation protects these cells from graft rejection, thus allowing the same recombinant therapeutic cell line to be implanted in different recipients. This approach was successful in treating HER2/neu-expressing tumors in mice by delivering an interleukin-2 fusion protein (sFvIL-2), or angiostatin. However, treatment with interleukin-2 led to profuse inflammation, while angiostatin delivery did not result in long-term tumor suppression, in part due to endothelial cell-independent neovascularization (vascular mimicry). We hypothesize that coencapsulating the two producer cells in the same microcapsules may enhance the efficacy and ameliorate the above side effects. Hence, B16-F0/neu tumor-bearing mice were implanted with sFvIL-2- and angiostatin-secreting cells coencapsulated in the same alginate-poly-L-lysine-alginate microcapsules. However, this protocol only produced an incremental but not synergistic improvement, as measured with greater tumor suppression and improved survival. Compared to the single sFvIL-2 treatment, the coencapsulation protocol showed improved efficacy associated with: mobilization of sFvIL-2 from the spleen; a higher level of cytokine delivery systemically and to the tumors; increased tumor and tumor-associated endothelial cell apoptosis; and a reduced host inflammatory response. However, compared to the single angiostatin treatment, the efficacy was reduced, primarily due to a "bystander" effect in which the angiostatin-secreting cells suffered similar transgene silencing as the coencapsulated cytokine-secreting cells. Nevertheless, the level of "vascular mimicry" of the single angiostatin treatment was significantly reduced. Hence, while there was no synergy in efficacy, an incremental improvement and some reduction in undesirable side effects of inflammation and vascular mimicry were achieved over the single treatments.

Expression of human apolipoprotein(a) kringles in colon cancer cells suppresses angiogenesis-dependent tumor growth and peritoneal dissemination

BACKGROUND

Anti-angiogenesis therapy has been regarded as a promising treatment of cancer based on the fact that most tumors and their metastasis are angiogenesis-dependent. Gene therapy can potentially expand the horizons of tumor angiogenesis therapy by virtue of its ability to produce high concentrations of therapeutic agents in a local area for a sustained period. The present study was performed to evaluate the therapeutic potential of gene therapy for the treatment of cancer and metastasis.

METHODS

The murine colon carcinoma cell line CT26 was manipulated ex vivo to express an anti-angiogenic molecule, LK68, consisting of human apolipoprotein(a) kringle domains, KIV(9)-KIV(10)-KV, using retrovirus-mediated gene transfer. Its effects on colon tumor growth and metastasis were evaluated in experimental animal models established by injecting LK68-expressing and control CT26 cells subcutaneously or into the peritoneal cavity of BALB/c mice, respectively.

RESULTS

Expression of LK68 significantly suppressed colon tumor growth in mice, but did not influence the growth of tumor cells in vitro. Immunohistochemical analysis of tumor tissues revealed a significant reduction in microvessel density in LK68-expressing tumors. Thus, the suppression of tumor growth appears to result mainly from inhibition of tumor angiogenesis. This decrease in vessel density is correlated with a notable increase in tumor cell apoptosis in vivo, but has no influence on proliferation. Moreover, expression of LK68 prevents peritoneal dissemination, and consequently improves overall host survival.

CONCLUSIONS

These results collectively indicate that a gene therapy strategy using LK68 cDNA is useful for the treatment for both colon tumor growth and peritoneal dissemination.

Inhibition of B16BL6 tumor progression by coadministration of recombinant angiostatin K1-3 and endostatin genes with cationic liposomes

Transfection of the antiangiogenic angiostatin and endostatin genes was shown to be an alternative to high-dose administration of angiostatin or endostatin proteins for cancer therapy. We have systematically investigated whether coadministration of the mouse angiostatin kringle 1-3 gene (pFLAG-AngioK1/3) and the endostatin gene (pFLAG-Endo) complexed with cationic liposomes exhibits enhanced therapeutic efficacy. In vitro, the coexpressed mixture of angiostatin K1-3 and endostatin more effectively reduced angiogenesis in chorioallantoic membranes than either angiostatin K1-3 or endostatin alone. In vivo, subcutaneous co-administration of pFLAG-AngioK1/3 and pFLAG-Endo lipoplexes more effectively inhibited vascularization in Matrigel plugs implanted in mice than either one alone. Additionally, subcutaneous administration of these genes inhibited the growth and formation of pulmonary metastases of B16BL6 melanoma cells in mice. Compared to treatment with an empty vector, treatment with pFLAG-AngioK1/3 plus pFLAG-Endo inhibited 81% of tumor growth, while treatment with pFLAG-AngioK1/3 or pFLAG-Endo inhibited tumor growth 70 and 69%, respectively. Cotreatment with the two plasmids after primary tumor excision induced a 90% inhibition of pulmonary metastases versus 79% for pFLAG-AngioK1/3 or 80% for pFLAG-Endo individually. These results suggest that combined administration of angiostatin K1-3 and endostatin genes complexed with cationic liposomes may be an innovated antiangiogenic strategy for cancer therapy.

Antiangiogenic therapy for primary and metastatic brain tumors

We first provide the theoretic foundation of antiangiogenic therapy by describing the biology of angiogenesis as it applies to brain tumors. We then outline experimental antiangiogenic therapies that are being applied preclinically to brain tumors, as well as published clinical trial data and ongoing clinical trials in patients. Primary and metastatic brain tumors are covered, although there is far less exploration in the literature of brain metastases.

Angiostatin-induced inhibition of endothelial cell proliferation/apoptosis is associated with the down-regulation of cell cycle regulatory protein cdk5

Endothelial cells (ECs) are quiescent in normal blood vessels, but undergo rapid bursts of proliferation after vascular injury, hypoxia or induced by powerful angiogenic cytokines like fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). Deregulated proliferation of ECs facilitates angiogenic processes and promotes tumor growth. In dividing cells, cell cycle-associated protein kinases, which are referred as cyclin-dependent kinases (cdks), regulate proliferation, differentiation, senescence, and apoptosis. Cyclin-dependent kinase-5 (cdk5) is expressed in neuronal cells and plays an important role in neurite outgrowth, of neuronal migration and neurogenesis, its functions in non-neuronal cells are unclear. Here, we show for the first time that the cdk5 is expressed at high levels in proliferating bovine aortic endothelial (BAE) cells, by contrast insignificant low levels of cdk5 expression in quiescent BAE cells. In addition, bFGF up-regulates cdk5 expression in a dose-dependent fashion. Interestingly, temporal expression data suggests that cdk5 expression is very low between 24-48 h, but high level of cdk5 expression was detected during 60-72 h. This later time corresponds to the time of completion of one cell cycle (doubling of cell population) of BAE cell culture. Angiostatin (AS), a powerful inhibitor of angiogenesis inhibits ECs proliferation in dose-dependent manner with concomitant down-regulation of cdk5 expression. The role of cdk5 in ECs, proliferation and apoptosis was confirmed by selective inhibition of cdk5 expression by the purine derivative roscovitine, which inhibits bFGF-stimulated BAE cells proliferation and induces apoptosis in dose-specific manner. By contrast, the roscovitine analog olomoucine, which is a specific inhibitor of cdk4, but not of cdk5 failed to affect ECs proliferation and apoptosis. These data suggest for the first time that neuron specific protein cdk5 may have significant role in the regulation of ECs proliferation, apoptosis, and angiogenesis and extends beyond its role in neurogenesis.

Antiangiogenic cancer therapy

Like most embryonic tissues, tumors have the ability to build up their own blood vessel networks. However, the architecture of tumor vessels is fundamentally different from that found in healthy tissues. Tumor vessels are usually irregular, heterogeneous, leaky, and poorly associated with mural cells. Endothelial cells in tumor vessels are also disorganized and express imbalanced surface molecules. These unusual features may provide some molecular and structural basis for selective inhibition or even destruction of tumor vessels by angiogenesis inhibitors. In animal tumor models, several angiogenesis inhibitors seem to inhibit tumor angiogenesis specifically without obvious effects on the normal vasculature. As a result, these inhibitors produced potent antitumor effects in mice. Excited by these preclinical studies, more than 60 angiogenesis inhibitors are being evaluated for their anticancer effects in human patients. Although the ultimate outcome of antiangiogenic clinical trials remains to be seen, several early observations have reported some disappointing results. These early clinical data have raised several important questions. Can we cure human cancers with angiogenesis inhibitors? Have we found the ideal angiogenesis inhibitors for therapy? What is the difference between angiogenesis in an implanted mouse tumor and in a spontaneous human tumor? What are the molecular mechanisms of these angiogenesis inhibitors? Should angiogenesis inhibitors be used alone or in combinations with other existing anticancer drugs? In this review, we will discuss these important issues in relation to ongoing antiangiogenic clinical trials.

Current perspectives on antiangiogenesis strategies in the treatment of malignant gliomas

Progressive tumor growth depends on angiogenesis to sustain metabolic needs of tumor cells, thus providing a potential target for cancer therapy. Malignant gliomas have retained their dismal prognosis despite aggressive multimodal conventional therapeutic approaches, illustrating the need for novel therapeutic strategies. Gliomas are a suitable tumor type for probing angiogenesis inhibition as their proliferation is characterized by a prominent proliferative vascular component. In the present review, we discuss the current status and future directions of angiogenesis inhibition in gliomas. We focus on recently developed approaches inducing an antiangiogenic response such as targeted gene delivery, protein tyrosine kinase inhibitors and encapsulated producer cells. Although several of these modalities have shown promising results on their own, the true potential of these novel approaches lies in their combined use with radiotherapy or 'metronomically scheduled' chemotherapy. A combined approach potentially counteracts the selective pressure on hypoxia-resistant malignant tumor cells, circumvents endothelial resistance induced by local cytoprotective responses and enhances the delivery of cytotoxic agents by normalizing vascular physiology. Surrogate markers of angiogenesis currently under study may provide accurate assessment of response in individual patients. Future research on endothelial markers expressed on tumor-associated vasculature as well as endothelial responses to cytotoxic treatment will provide new avenues for molecularly targeted therapy in malignant gliomas.

Short-term BMP-2 expression is sufficient for in vivo osteochondral differentiation of mesenchymal stem cells

Currently available murine models to evaluate mesenchymal stem cell (MSC) differentiation are based on cell injection at ectopic sites such as muscle or skin. Due to the importance of environmental factors on the differentiation capacities of stem cells in vivo, we investigated whether the peculiar synovial/cartilaginous environment may influence the lineage specificity of bone morphogenetic protein (BMP)-2-engineered MSCs. To this aim, we used the C3H10T1/2-derived C9 MSCs that express BMP-2 under control of the doxycycline (Dox)-repressible promoter, Tet-Off, and showed in vitro, using the micropellet culture system that C9 MSCs kept their potential to differentiate toward chondrocytes. Implantation of C9 cells, either into the tibialis anterior muscles or into the joints of CB17-severe combined immunodeficient bg mice led to the formation of cartilage and bone filled with bone marrow as soon as day 10. However, no differentiation was observed after injection of naïve MSCs or C9 cells that were repressed to secrete BMP-2 by Dox addition. The BMP-2-induced differentiation of adult MSCs is thus independent of soluble factors present in the local environment of the synovial/cartilaginous tissues. Importantly, we demonstrated that a short-term expression of the BMP-2 growth factor is necessary and sufficient to irreversibly induce bone formation, suggesting that a stable genetic modification of MSCs is not required for stem cell-based bone/cartilage engineering.

Genetic dissection of tumor angiogenesis: are PlGF and VEGFR-1 novel anti-cancer targets?

Many proliferative diseases, most typically cancer, are driven by uncontrolled blood vessel growth. Genetic studies have been very helpful in unraveling the cellular and molecular players in pathological blood vessel formation and have provided opportunities to reduce tumor growth and metastasis. The fact that tumor vessels and normal blood vessels have distinct properties may help in designing more specific--and therefore safer--anti-angiogenic strategies. Such strategies may interfere with angiogenesis at the cellular or molecular level. Possible molecular targets include angiogenic growth factors and their receptors, proteinases, coagulation factors, junctional/adhesion molecules and extracellular matrix (ECM) components. Some anti-angiogenic drugs, i.e., vascular endothelial growth factor (VEGF) antibodies and VEGF receptor-2 (VEGFR-2) inhibitors, have progressed into clinical cancer trials. While the results of these trials support the potential of anti-angiogenic therapy to treat cancer, they also demonstrate the need for more effective and safer alternatives. Targeting placental growth factor (PlGF) or VEGFR-1 may constitute such an alternative since animal studies have proven their pleiotropic working mechanism and attractive safety profile. Together, these insights may bring anti-angiogenic drugs closer from bench to bedside.

Biologic therapy of liver tumors

Nonresectable primary and metastatic liver tumors are common malignancies that lack therapies allowing substantial prolongation of survival. Recent progress in molecular and cell biology has opened the way to novel therapies based on biological modifiers, gene transfer, and autologous stem cells. It is now possible to transfer therapeutic genes to the tumor or pericancerous tissue, and to control their expression for long periods of time. It is also feasible to generate autologous endothelial progenitor cells that can be recruited by tumoral vessels acting as vehicles to convey therapeutic genes to the interior of the tumor mass. Combination of biological modifiers, gene therapy, and cell therapy will hopefully provide efficient means to combat inoperable neoplasms in a not-very-distant future.

Manipulating angiogenesis in medicine

Blood vessels nourish organs with vital nutrients and oxygen and, thus, new vessels form when the embryo needs to grow or wounds are to heal. However, forming new blood vessels is a complex and delicate process, which, unfortunately, is often derailed. Thus, when insufficient vessels form, the tissue becomes ischaemic and stops to function adequately. Conversely, when vessels grow excessively, malignant and inflamed tissues grow faster. It is now becoming increasingly evident that abnormal vessel growth contributes to the pathogenesis of numerous malignant, ischaemic, inflammatory, infectious and immune disorders. With an in-depth molecular understanding, we should be better armamented to combat such angiogenic disorders in the future. That such therapeutic strategies might change the face of medicine is witnessed by initial evidence of success in the clinic.

Treatment of colorectal and hepatocellular carcinomas by adenoviral mediated gene transfer of endostatin and angiostatin-like molecule in mice

AIM AND METHOD

In this study, we explored the responsiveness of different tumour entities (colorectal carcinoma (CRC), hepatocellular carcinoma (HCC), and the murine Lewis lung carcinoma (LLC)) to angiostatic antitumour treatment with two recombinant adenoviral vectors encoding angiostatin-like molecule (AdK1-3) and endostatin (Adendo).

RESULTS

AdK1-3 and Adendo exerted inhibitory biological functions on endothelial cell proliferation, migration, and tube formation in vitro. AdK1-3 inhibited significantly endothelial cell infiltration in vascular endothelial growth factor embedded Matrigel plugs in mice whereas Adendo showed only minor effects. Both AdK1-3 and Adendo induced similar antitumour effects in the LLC tumour model in immune competent C57BL/6 mice but AdK1-3 had stronger inhibitory effects in athymic mice. Furthermore, AdK1-3 inhibited tumour growth in a murine CRC and human HCC model but was ineffective in a human CRC model. In contrast, Adendo did not reduce tumour progress in either of these tumour models although AdK1-3 and Adendo effectively reduced intratumoral microvessel density in LLC tumours.

CONCLUSION

Our data demonstrate that angiostatic gene therapy may form a feasible strategy for the treatment of established hepatocellular carcinomas and that in vivo antitumour efficacy of angiostatic proteins is tumour specific.

Viral vector-mediated transduction of a modified thrombospondin-2 cDNA inhibits tumor growth and angiogenesis

Gene therapy represents a possible alternative to the chronic delivery of recombinant antiangiogenic proteins to cancer patients. We have constructed retroviral and adenoviral vectors that express murine N-terminal fragments of thrombospondin-2 (NfTSP2), a potent endogenous inhibitor of tumor growth and angiogenesis. To test the possibility of anticancer gene therapy using NfTSP2, we tested whether an ex vivo retrovirus-mediated procedure could be used for the treatment of tumors. The treatment of tumor-bearing mice with syngenic immortalized cell lines expressing NfTSP2 led to a tumor volume reduction up to 70% as compared with the controls (P<0.005). In addition, the established tumors were eradicated in 40% of the mice treated with NfTSP2-expressing cells. Furthermore, the intratumoral injection of the NfTSP2-expressing adenoviral vector to the human squamous cell carcinoma in nude mice resulted in a significant reduction of the growth rates and the volumes of the carcinoma (P<0.05). Immunohistochemical staining of the tumors indicated that the total area and the average size of tumor vessels were significantly reduced in the treatment group versus the controls (P<0.05). In conclusion, the present study clearly demonstrates that the viral vector-mediated transfer of the NfTSP2 gene could inhibit the growth of tumors by perturbing tumor-associated angiogenesis.

Metastasis and angiogenesis

Angiogenesis and the development of metastases are intrinsically connected. Experimental data suggest that establishment and growth of metastases are influenced by soluble factors secreted from the originating solid tumor. Among these factors are so-called endogenous inhibitors of angiogenesis which keep metastasis in a non-proliferating quiescent state. For a number of tumors it has been shown that this dormant state is mediated through inhibition of angiogenesis. This dormant state is characterized by normal proliferation, increased apoptosis, and insufficient neovascularization. Removal of inhibiting antiangiogenic factors leads to growth of dormant metastases. Several endogenous inhibitors have been identified so far and some of them have already been successfully applied in experimental therapeutic trials. This might be of special interest for the treatment of cerebral metastases which are the most common type of malignant brain tumors. Similar to the spread of metastases, it is known that single glioma cells can be found in distant parts of the brain. While local recurrence is a common phenomenon in glioma, formation of clinical apparent distant metastasis occurs rarely. Several lines of evidence suggest that growth inhibition of remote glioma cells may be mediated by an endogenous inhibitory mechanism.

Anti-tumor activity of a combination of plasminogen activator and captopril in a human melanoma xenograft model

Angiostatin, a proteolytic fragment of plasminogen consisting of the first 3 or 4 kringle domains, reduces tumor growth by specifically inhibiting tumor angiogenesis. Angiostatin is generated in vitro in a 2-step process. First, plasminogen is converted to plasmin by plasminogen activators. Next, plasmin excises the angiostatin fragment from plasminogen, a process requiring molecules that are able to donate a free sulfhydryl group. In this study, we investigated whether stimulation of in vivo angiostatin generation by administration of plasminogen activator and a free sulfhydryl group donor (FSD) has anti-tumor activity. First, we determined the optimal conditions for in vitro angiostatin generation by incubating murine plasma with different concentrations of plasminogen activator and/or the FSD captopril. Angiostatin generation was monitored by western blot analysis. Our results were extrapolated to the in vivo situation by administering the optimal dose of tissue-type plasminogen activator (tPA, i.v. injection 3 times/week) and captopril (in drinking water) to mice and analyzing the presence of angiostatin in the circulation. Angiostatin was readily detectable in mice receiving both tPA and captopril, but not in mice receiving either one of the agents. Finally, the anti-tumor activity of the tPA/captopril treatment was tested in a human melanoma xenograft model. Administration of tPA alone had only a marginal effect on tumor growth. Captopril alone reduced tumor growth by about 60%, whereas treatment with both captopril and tPA resulted in 83% inhibition of tumor growth.

Bystander cell killing spreading from endothelial to tumor cells in a three-dimensional multicellular nodule model after Escherichia coli nitroreductase gene delivery

Tumor cells are elusive targets for standard anticancer chemotherapy due to their heterogeneity and genetic instability. On the other hand, proliferating host endothelial cells (ECs) are genetically stable and have a low mutational rate. Thus, antiangiogenic therapy directed against tumor's ECs should, in principle, improve the efficacy of antitumor therapy by inducing little or no drug resistance. Here we present a gene-directed enzyme prodrug therapy (GDEPT) strategy for targeting the tumor vasculature, using the Escherichia coli nitroreductase (ntr) gene delivery associated with the treatment with the prodrug CB1954. In a first time we demonstrated the ability of the ntr/CB1954 system to induce an apoptotic-mediated cell death on monolayer cultures of human umbilical vein ECs (HUV-EC-C). Then, when ntr-transfected HUV-EC-C cells (HUV-EC-C/ntr(+)) were associated in a three-dimensional (3-D) multicellular nodule model with untransfected B16-F10 murine melanoma cell line, we observed a CB1954-mediated bystander cell killing effect from endothelial to neighboring melanoma cells. To our knowledge, this is the first report indicating that GDEPT-based antiangiogenic targeting may be an effective approach for cancer treatment relied on the spreading of the bystander effect from endothelial to tumor cells.

High level of stabilized angiostatin mediated by adenovirus delivery does not impair the growth of human neuroblastoma xenografts

Human neuroblastoma (NB) is a highly heterogeneous childhood cancer secreting a high level of vascular endothelial growth factor (VEGF). Its vascularization has been clearly correlated with metastatic progression and poor outcome. Thus, molecules that target the vascular endothelium are regarded as new therapeutics of clinical interest. Angiostatin, an internal fragment of plasminogen containing the first four kringle structures, has been described as a powerful angiogenic inhibitor. We used a recombinant adenovirus encoding the human angiostatin kringle 1-3 directly fused to human serum albumin HSA (AdK3-HSA). Coupling to HSA has been previously shown to increase the in vivo half-life of this angiostatic factor, and to lead to tumor growth inhibition in the MDA-MB-231 carcinoma model. For the assessment of antiangiogenic gene therapy in the human NB IGR-N835 tumor model, 5 x 10(9) PFU of AdK3-HSA were intravenously injected in tumor-bearing athymic mice presenting either of the following experimental settings: early stage, established, and minimal residual tumors. No delay in tumor growth was observed in animals treated with AdK3-HSA as compared to those treated with the empty virus AdCO1. In early-stage tumors, kinetics of tumor occurrence and tumor growth were similar in AdK3-HSA- and AdCO1-treated animals. K3-HSA was found to be expressed at high levels (the mean value for the three experiments being 19.4+/-15.9 microg/ml) in the circulation of all animals up to 21-35 days after virus injection. In addition, IGR-N835 tumors were found to be highly vascularized and to release high amounts of angiogenic factors, in particular VEGF (665+/-370 pg/mg total protein). Thus, in spite of high circulating levels, K3-HSA may be unable to displace the NB proangiogenic switch. In this regard, a more promising target to inhibit NB angiogenesis seems to be the VEGF/VEGFR system.

Anti-angiogenesis and angioprevention: mechanisms, problems and perspectives

The recognition that angiogenesis is a key early event in tumor progression and metastasis has led to the development of new strategies for cancer therapy. The generation of a new blood vessel network under physiological conditions is regulated by the concerted action of activators and inhibitors. Perturbation of this balance, as it occurs in solid tumor growth and metastasis, appears to be a critical point in tumorigenesis. This has led to the "angiogenic switch" hypothesis: the point at which a tumor acquires the potential to induce angiogenesis is a critical step towards malignancy. Based on experimental evidence, prevention of blood vessel development appears to be the mechanism of action of many successful chemopreventive drugs of natural or synthetic origin: a novel concept that we termed "angioprevention". The hypothesis that anti-angiogenesis is at the basis of tumor prevention also suggests that many anti-angiogenic drugs could be used for chemoprevention in higher risk populations or in early intervention. There is a growing body of experimental evidence that anti-angiogenic strategies will contribute to the future therapy of cancer, several compounds with anti-angiogenic properties are now under clinical investigation including anti-inflammatory compounds, as inflammation may play a key role in angiogenesis. We must persevere in the development of novel, powerful and safer angiogenesis inhibitors and in the use of anti-angiogenic drugs in combination with other natural or synthetic anti-cancer agents in a biological therapy strategy.

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Angiostatin antagonizes the action of VEGF-A in human endothelial cells via two distinct pathways

Angiostatin consisting of the first four-kringle domains of the plasminogen potently inhibits angiogenesis in vitro and in vivo. However, the molecular mechanism of action whereby angiostatin mediates its inhibitory effect on proliferating endothelial cells remains elusive. We therefore used the proliferating cultured human umbilical vein endothelial cells (HUVECs) promoted by vascular endothelial growth factor A to identify the endogenous signaling elements that mediate the antiangiogenic effect of angiostatin. Treatment of HUVEC with angiostatin at a concentration known to inhibit cell proliferation and induce apoptosis resulted in induction of p53-, Bax-, and tBid-mediated release of cytochrome c into the cytosol. In addition, angiostatin also activated the Fas-mediated apoptotic pathway in part via up-regulation of FasL mRNA, down-regulation of c-Flip, and activation of caspase 3. These results suggest that the anti-angiogenic action of angiostatin is likely mediated by two distinct signaling pathways, one intrinsic mediated by p53 while the other extrinsic involved in FasL engagement and mitochondria dysfunction.

Lung overexpression of angiostatin aggravates pulmonary hypertension in chronically hypoxic mice

Exposure to hypoxia leads to the development of pulmonary hypertension (PH) as a consequence of pulmonary smooth muscle hyperplasia. Hypoxia concomitantly stimulates lung expression of angiogenic factors. To investigate the role of angiogenesis processes in development of hypoxic PH, we examined the effects of lung overexpression of angiostatin, an angiogenesis inhibitor, on development of hypoxic PH and lung endothelial cell (EC) density. Angiostatin delivery was achieved by a defective adenovirus expressing a secretable angiostatin K3 molcule driven by the cytomegalovirus promoter (Ad.K3). Comparison was made with a control vector containing no gene in the expression cassette (Ad.CO1). Treatment with Ad.K3 (300 plaque-forming units [pfu]/cell) inhibited cultured human pulmonary artery EC migration by 100% and proliferation by 50%, but was without effects on human pulmonary artery smooth muscle cells. After intratracheal administration of Ad.K3 (109 pfu) to mice, angiostatin protein became detectable in bronchoalveolar lavage fluid. Mice pretreated with Ad.K3 1 d before a 2-wk exposure to hypoxia (10% O2) showed more severe pulmonary hypertension than Ad.CO1-pretreated controls, as assessed by higher right ventricular systolic pressure (36.5 +/- 2.4 versus 30.2 +/- 1.4, respectively), aggravation of right ventricular hypertrophy (P < 0.05), and muscularization of distal vessels (P < 0.01). Lung factor VIII, CD31 immunostaining, as well as eNOS expression were significantly increased after exposure to hypoxia in Ad.CO1-pretreated controls, but decreased in both normoxic and hypoxic animals after treatment with Ad.K3. The results show that inhibition of hypoxia-induced stimulation of lung angiogenic processes aggravates development of hypoxic PH. This suggests that endogenous lung angiogenesis counteracts development of hypoxic PH.

Gene therapy of hepatocarcinoma: a long way from the concept to the therapeutical impact

Hepatocellular carcinoma (HCC), the most prevalent histological form of primary liver cancer is one of the most frequent cancer worldwide. This pathology still requires the development of new therapeutical approaches. Gene therapy strategies focusing on the genetic manipulation of accessory cells involved in the immune reaction against cancer cells, or on the direct transduction of tumor cells with transgenes able to "suicide" cancer cells have been largely developed for more than ten years.

Coelectrotransfer to skeletal muscle of three plasmids coding for antiangiogenic factors and regulatory factors of the tetracycline-inducible system: tightly regulated expression, inhibition of transplanted tumor growth, and antimetastatic effect

We describe an approach employing intramuscular plasmid electrotransfer to deliver secretable forms of K1-5 and K1-3-HSA (a fusion of K1-3 with human serum albumin), which span, respectively, five and three of the five kringle domains of plasminogen. A tetracycline-inducible system (Tet-On) composed of three plasmids coding, respectively, for the transgene, the tetracycline transcriptional activator rtTA, and the silencer tTS was employed. K1-3-HSA and K1-5, produced from C2C12 muscle cells, were found to inhibit endothelial cell (HMEC-1) proliferation by 30 and 51%, respectively. In vivo, the expression of the transgene upon doxycycline stimulation was rapid, stable, and tightly regulated (no background expression) and could be maintained for at least 3 months. Blood half-lives of 2.1 and 3.7 days were found for K1-5 and K1-3-HSA, respectively. The K1-5 protein was secreted from muscle into blood at a level of 45 ng/ml, which was sufficient to inhibit MDA-MB-231 tumor growth by 81% in nude mice and B16-F10 melanoma cell lung invasion in C57BL/6 mice by 73%. PECAM-1 immunostaining studies revealed modest tumor vasculature in mice expressing K1-5. In contrast, K1-3-HSA, although secreted into blood at much higher level (250 ng/ml) than K1-5, had no effect on tumor growth.

Inhibition of tumor angiogenesis by angiostatin: from recombinant protein to gene therapy

Tumor growth, local invasion, and metastatic dissemination are dependent on the formation of new microvessels. The process of angiogenesis is regulated by a balance between pro-angiogenic and anti-angiogenic factors, and the shift to an angiogenic phenotype (the "angiogenic switch") is a key event in tumor progression. The use of anti-angiogenic agents to restore this balance represents a promising approach to cancer treatment. Known physiological inhibitors include trombospondin, several interleukins, and the proteolytic break-down products of several proteins. Angiostatin, an internal fragment of plasminogen, is one of the more potent of this latter class of angiogenesis inhibitors. Like endostatin, another anti-angiogenic peptide derived from collagen XVIII, angiostatin can induce tumor vasculature regression, leading to a complete cessation of tumor growth. Inhibitors of angiogenesis target normal endothelial cells, therefore the development of resistance to these drugs is unlikely. The efficacy of angiostatin has been demonstrated in animal models for many different types of solid tumors. Anti-angiogenic cancer therapy with angiostatin requires prolonged administration of the peptide. The production of the functional polypeptides is expensive and technical problems related to physical properties and purity are frequently encountered. Gene transfer represents an alternative method to deliver angiostatin. Gene therapy has the potential to produce the therapeutic agent in high concentrations in a local area for a sustained period, thereby avoiding the problems encountered with long-term administration of recombinant proteins, monoclonal antibodies, or anti-angiogenic drugs. In this review we compare the different gene therapy strategies that have been applied to angiostatin, with special regard to their ability to provide sufficient angiostatin at the target site.

Angiogenesis factors in gliomas: a new key to tumour therapy?

Angiogenesis, the formation of new blood vessels, is required for the growth and expansion of tumours. Gliomas, the most common brain tumours, are particularly highly vascularized and, therefore, serve as a model to elucidate the process of tumour angiogenesis and to investigate new anti-angiogenic therapies. This review describes the role of angiogenic factors in glioma angiogenesis and new strategies to inhibit glioma growth by application of anti-angiogenic substances. We focus on vascular endothelial growth factor (VEGF), but also examine the role of angiopoietin and pleiotropic factors such as platelet-derived growth factor (PDGF), pleiotrophin and transforming growth factor-beta (TGF-beta). Strategies to inhibit glioma growth by reducing the action of angiogenic factors, by the application of anti-angiogenic substances such as angiostatin or endostatin, or inactivation of endothelial cells, are discussed. These new anti-angiogenic therapies appear to have a high potential not only for the treatment of gliomas, but also of other tumours.

Antiangiogenic cancer therapy with microencapsulated cells

Inhibition of angiogenesis has led to tumor suppression in several cancer models. Although administering purified recombinant antiangiogenic product is effective, alternative approaches through genetic manipulation may be more cost-effective. We propose to implant nonautologous recombinant cells secreting angiostatin for systemic delivery of angiostatin in cancer treatment. These cells are protected from graft rejection in alginate microcapsules to function as "micro-organs" to deliver angiostatin in vivo. This approach was tested by implanting encapsulated mouse myoblast C2C12 cells genetically modified to secrete angiostatin into mice bearing solid tumor. Angiostatin was detected in sera of the treated mice. Efficacy was demonstrated by suppression of palpable tumor growth and improved survival. At autopsy, angiostatin localized to residual tumors and high levels of angiostatic activity were detected in tumor extracts. Tumor tissues showed increased apoptosis and necrosis compared with those from untreated or mock-treated mice. Immunohistochemical staining against von Willebrand factor, an endothelial cell marker, showed that within tumors from the treated mice, the neovasculature was poorly defined by endothelial cells, many of which were undergoing apoptosis. However, the tumors eventually developed neovasculature independent of endothelial cells. Such vascular mimicry would account for the lack of long-term efficacy despite persistent angiostatin delivery. In conclusion, implantation with nonautologous microencapsulated cells is feasible for systemic delivery of angiostatin, resulting in localization of angiostatin to tumors and targeted apoptosis of the endothelial cells. Clinical efficacy was demonstrated by suppression of tumor growth and extension of life span. Although the potential of this cell-based approach for angiostatin-mediated cancer therapy is confirmed, long-term efficacy must take into account the possible escape by some tumors from angiogenesis inhibition.

Endostatin exhibits a direct antitumor effect in addition to its antiangiogenic activity in colon cancer cells

Endostatin has been considered a highly specific inhibitor of endothelial cell proliferation and/or migration. To explore the use of endostatin in antiangiogenic gene therapy, we generated a recombinant adenovirus, AdEndo, carrying the gene for mouse endostatin. Injection of 10(9) PFU of AdEndo resulted in a low but significant suppression (25%) of preestablished tumor growth in murine models involving murine Lewis lung carcinoma (LLC) and human breast cancer MDA-MB-231 tumors. Greater anticancer activity was observed when the same dose of AdEndo was injected into two other preestablished murine models involving C51 murine colon cancer and HT29 human colon cancer (55 and 47% tumor growth reduction, respectively). In vitro, endostatin derived from AdEndo-infected MRC-5 fibroblasts inhibited the growth of C51 and HT29 cell lines (72 and 61%, respectively). The extent of this inhibition was comparable to that observed in endothelial cells: 75% for microcapillary endothelial cell line HMEC-1, 52% for human dermal microvascular endothelial cells, 46% for human umbilical vein endothelial cells, and 67% for calf pulmonary arterial endothelial cells. Both endothelial and colon cancer cells showed a clear increase in cell apoptosis (4- to 5-fold for endothelial cells and 5- to 10-fold for colon cancer cells) and an accumulation in the G(1) phase of the cell cycle. This antiproliferative activity was not observed in other tumor cell lines: LLC, MDA-MB-231, murine colon adenocarcinoma MC38, human prostate cancer cell line DU145, and human breast cancer cell line CAL51. Taken together, these results provide evidence that, in addition to its antiangiogenic activity, endostatin exerts a direct anticancer action that appears to be restricted to some tumor cell lines. Thus, endostatin could be used in some colon cancer treatments and its clinical efficacy would depend on the response of tumor cells themselves.

Efficacy of dendrimer-mediated angiostatin and TIMP-2 gene delivery on inhibition of tumor growth and angiogenesis: in vitro and in vivo studies

Gene transfer is an attractive approach to fight cancer by targeting cancer cells or their vasculature. Our study reports the inhibition of tumor growth and angiogenesis by a nonviral method using dendrimers associated with 36-mer anionic oligomers (ON36) for delivering angiostatin (Kringle 1-3) and tissue inhibitor of metalloproteinase (TIMP)-2 genes. The optimal concentrations of dendrimers and ON36 for an efficient green fluorescent protein (GFP) plasmid delivery in endothelial cells (HMEC-1) and cancer cells (MDA-MB-435) were first chosen. Then the efficacy of transfection was determined by testing angiostatin and TIMP-2 secretion by Western blot and the biologic effects were evaluated. Angiostatin gene transfer markedly reduced in vitro (i) HMEC-1 but not MDA-MB-435 proliferation; (ii) HMEC-1 and MDA-MB-435 wound healing reparation; and (iii) capillary tube formation. TIMP-2 gene transfer did not affect cell proliferation but strongly inhibited (i) wound healing of HMEC-1 and MDA-MB-435 cells; and (ii) capillary tube formation. Supernatants of transfected-MDA-MB-435 cells also inhibited the formation of angiogenic networks on Matrigel, indicating a paracrine effect. In vivo, intratumoral angiostatin or TIMP-2 gene delivery using dendrimers associated with ON36 effectively inhibited tumor growth by 71% and 84%, respectively. Combined gene transfer resulted in 96% inhibition of tumor growth. Tumor-associated vascularization was also greatly reduced. These findings provide a basis for the further development of nonviral delivery of genes to fight cancer.

Mechanism and its regulation of tumor-induced angiogenesis

Tumor angiogenesis is the proliferation of a network of blood vessels that penetrates into cancerous growths, supplying nutrients and oxygen and removing waste products. The process of angiogenesis plays an important role in many physiological and pathological conditions. Solid tumors depend on angiogenesis for growth and metastasis in a hostile environment. In the prevascular phase, the tumor is rarely larger than 2 to 3 mm³ and may contain a million or more cells. Up to this size, tumor cells can obtain the necessary oxygen and nutrient supplies required for growth and survival by simple passive diffusion. The properties of tumors to release and induce several angiogenic and anti-angiogenic factors which play crucial roles in regulating endothelial cell (EC) proliferation, migration, apoptosis or survival, cell-cell and cell-matrix adhesion through different intracellular signaling are thought to be the essential mechanisms during tumor-induced angiogenesis. Tumor angiogenesis actually starts with tumor cells releasing molecules that send signals to surrounding normal host tissue. This signaling activates certain genes in the host tissue that, in turn, make proteins to encourage growth of new blood vessels. In this review, we focus the mechanisms of tumor-induced angiogenesis, with an emphasis on the regulatory role of several angiogenic and anti-angiogenic agents during the angiogenic process in tumors. Advances in understanding the mechanisms of tumor angiogenesis have led to the development of several most effective anti-angiogenic and anti-metastatic therapeutic agents and also have provided several techniques for the regulation of cancer's angiogenic switch. The suggestion is made that standard cytotoxic chemotherapy and angiogenesis inhibitors used in combination may produce complementary therapeutic benefits in the treatment of cancer.

Antiangiogenic therapy against experimental glioblastoma using genetically engineered cells producing interferon-alpha, angiostatin, or endostatin

Inhibition of angiogenesis has been considered among the most promising approaches to treat highly vascularized solid tumors such as glioblastoma. In this study, we designed and validated a new in vitro assay system based on the implantation of tumor cells into organotypic brain slice cultures. We evaluated the effects of local production of three endogenous inhibitors of angiogenesis, angiostatin, endostatin, and interferon (IFN)-alpha(1), using stably transfected rat (9L) and human (GL15) glioblastoma cells on tumor vascularization and growth. Despite similar effectiveness of the three proteins in a classic in vitro endothelial cell migration assay, IFN-alpha(1) demonstrated the most potent antiangiogenic effect in organotypic brain slice cultures. In vivo, after intracerebral implantation of such genetically modified glioblastoma cells, IFN-alpha(1) caused a dramatic decrease in tumor volume revealed by magnetic resonance imaging and by postmortem histology. The mechanisms of this antitumor effect were most likely caused by the major antiangiogenic action of the cytokine, because IFN-alpha(1) expression provoked a pronounced decrease in blood vessel density, which was accompanied by extensive necrosis in the body mass of the tumors. The median survival time of rats implanted intracerebrally with IFN-alpha-expressing 9L cells tripled, and was still significantly increased when these constituted only 1% of transplanted tumor cells. A similar effect was seen when 50% of the transplanted cells were replaced by IFN-alpha-expressing bone marrow stromal cells. These data point to the local delivery of IFN-alpha(1) using cell vectors as a potent tool for the inhibition of tumor-induced angiogenesis.

Upregulation of Cdc2 and cyclin A during apoptosis of endothelial cells induced by cleaved high-molecular-weight kininogen

We (8) reported that the cleaved high-molecular-weight kininogen (HKa) and its domain 5 (D5) inhibited angiogenesis. Further studies (15) revealed that D5 could inhibit cell proliferation and induce apoptosis of proliferating endothelial cells, which together may represent a critical part of antiangiogenic activity of HKa and D5. In the present study, we further examined the effect of HKa on cell cycle progression and cell viability. We report that HKa induced a significant upregulation of Cdc2 and cyclin A in proliferating endothelial cells, concurrent with a marked increase of Cdc2 activity. The increased expression of Cdc2 and cyclin A by HKa was not associated with an apparent change in cell cycle profiles of basic fibroblast growth factor-stimulated proliferating cells, but closely correlated with a marked increase of apoptosis, suggesting that the elevated Cdc2 activity is involved in HKa-induced apoptosis of proliferating endothelial cells. Our results support an emerging hypothesis that Cdc2 and cyclin A are important regulators for cell cycle as well as for apoptosis.

Combined effects of docetaxel and angiostatin gene therapy in prostate tumor model

The anti-tumor effects of adenovirus-delivered angiostatin (AdK3) in association with docetaxel (Taxotere) have been evaluated in a human-origin prostate tumor model. In vitro, human endothelial cells were 50- to 100-fold more sensitive to docetaxel than prostate cell lines (PC3, LNCaP, and DU145), and the combination regimen of docetaxel and AdK3 was significantly more cytotoxic for endothelial cells than either treatment alone. PC3 cells, which display the highest sensitivity to docetaxel, were then grafted onto athymic mice for an evaluation of the combined regimen as a therapy. The combination of a single intratumoral injection of AdK3 (2 x 10(9) pfu) and a single intravenous injection of docetaxel (15 mg/kg) was compared with the injection of AdK3 alone on preestablished mice bearing PC3-derived tumors with a mean tumor volume of 60 +/- 11 or 205 +/- 46 mm3. Significant antitumoral effects were observed only in mice receiving the combined treatment. We showed that all PC3 tumors regressed in the AdK3-docetaxel combination group and that 40 to 83% totally regressed. In all cases, this regimen was tightly correlated with a marked decrease in intratumoral vascularization. Our experimental data show that attacking both endothelial and tumoral compartments is an efficient and logical strategy, making this bitherapy approach clinically promising.

Long-term expression of angiostatin suppresses metastatic liver cancer in mice

Metastatic liver cancer has a very poor prognosis and lacks effective therapy. Anti-angiogenic therapies, which starve tumors of blood supply, have proven to be effective in preclinical models because tumor growth is angiogenesis dependent. However, long-term, high-level, and sustained expression of angiogenesis inhibitors, such as angiostatin, is necessary to prevent dormant tumors from becoming active again. To achieve this objective, we engineered a recombinant adeno-associated virus (AAV) vector encoding mouse angiostatin, an endogenous inhibitor of tumor vascularization. After intraportal delivery of this vector, high-level, stable transgene expression of angiostatin lasting for at least 6 months was observed locally in hepatocytes. Gene transfer of AAV-angiostatin via the portal vein led to significant suppression of the growth of both nodular and metastatic EL-4 lymphoma tumors established in the liver and prolonged the survival time of the mice. The growth of neovessels was inhibited significantly, and extensive apoptosis of tumor cells was observed. The anti-angiogenic activity of angiostatin was independent of vascular endothelial growth factor (VEGF). The AAV-angiostatin viruses did not appear to be toxic to mice, and there was no detectable apoptosis of hepatocytes. In conclusion, these encouraging results warrant future investigation of the use of AAV-mediated anti-angiogenic gene therapy for targeting unresectable liver metastases, especially after surgical removal of primary tumors.

Inhibition of endothelial cell proliferation by the recombinant kringle domain of tissue-type plasminogen activator

Tissue-type plasminogen activator (tPA) is a multidomain serine protease that converts the zymogen plasminogen to plasmin. tPA contains two kringle domains which display considerable sequence identity with those of angiostatin, an angiogenesis inhibitor. TK1-2, a recombinant kringle domain composed of t-PA kringles 1 and 2 (Ala(90)-Thr(263)), was produced by both bacterial and yeast expression systems. In vitro, TK1-2 inhibited endothelial cell proliferation stimulated by basic fibroblast growth factor, vascular endothelial growth factor, and epidermal growth factor. It did not inhibit proliferation of non-endothelial cells. TK1-2 also inhibited in vivo angiogenesis in the chick embryo chorioallantoic membrane model. These results suggest that the recombinant kringle domain of t-PA is a selective inhibitor of endothelial cell growth and identifies this molecule as a novel anti-angiogenic agent.

Anti-angiogenic activity of the recombinant kringle domain of urokinase and its specific entry into endothelial cells

Urokinase plasminogen activator (uPA) belongs to a family of proteins that contains kringle domain and plays an important role in inflammation, tissue remodeling, angiogenesis, and tumor metastasis by pericellular plasminogen activation. Kringle domains of plasminogen have been shown to demonstrate anti-angiogenic and anti-tumor activities. Here, we report our investigation of the kringle domain of uPA for anti-angiogenic activity and a possible cellular mechanism of action. The recombinant kringle domain of uPA (Asp(45)-Lys(135)) (UK1) inhibited endothelial cell proliferation stimulated by basic fibroblast growth factor, vascular endothelial growth factor (VEGF), or epidermal growth factor. It also inhibited migration of endothelial cells induced by VEGF or uPA, and in vivo angiogenesis on the chick chorioallantoic membrane. It did not block plasminogen activation by activated uPA in clot lysis and chromogenic substrate assays. Neither binding of UK1 to immobilized uPA receptor nor competitive inhibition of uPA binding were confirmed by real-time interaction analysis. However, internalization of UK1 followed by translocation from cytosol to nucleus was determined to be specific to endothelial cells. It also elicited a transient increase of Ca(2+) flux of more than 2-fold within 2 min of exposure in an endothelial cell-specific manner. These results suggest that the kringle domain of uPA exhibits anti-angiogenic activity and that its anti-angiogenic activity may occur through a different mechanism from inhibition of uPA-uPA receptor interaction or uPA proteolytic activity and may be associated with endothelial-cell specific internalization not mediated by the uPA receptor.

Angiostatin selectively inhibits signaling by hepatocyte growth factor in endothelial and smooth muscle cells

Angiostatin, an inhibitor of angiogenesis, contains 3 to 4 kringle domains that are derived from proteolytic cleavage of plasminogen. The antiangiogenic effects of angiostatin occur, in part, from its inhibition of endothelial cell surface adenosine triphosphate synthase, integrin functions, and pericellular proteolysis. Angiostatin has structural similarities to hepatocyte growth factor (HGF; "scatter factor"), a promoter of angiogenesis, that induces proliferation and migration of both endothelial and smooth muscle cells via its cell surface receptor, c-met. We hypothesized that angiostatin might block HGF-induced signaling in endothelial and smooth muscle cells. Angiostatin inhibited HGF-induced phosphorylation of c-met, Akt, and ERK1/2. Angiostatin also significantly inhibited proliferation of human umbilical vein endothelial cells (HUVECs) induced by HGF. In contrast, angiostatin did not inhibit vascular endothelial growth factor (VEGF)-or basic fibroblast growth factor (bFGF)-induced signaling events or HUVEC proliferation. Angiostatin bound to immobilized truncated c-met produced by A431 cells and could be immunoprecipitated as a complex with soluble c-met. HGF inhibited the binding of (125)I-angiostatin to HUVECs. Soluble c-met, produced by several tumor cell lines, could inhibit the antiangiogenic effect of angiostatin. The disruption of HGF/c-met signaling is a novel mechanism for the antiangiogenic effect of angiostatin.