Recombinant prostate specific antigen inhibits angiogenesis in vitro and in vivo.

BACKGROUND: Prostate specific antigen (PSA) is a kallikrein family member with serine protease activity commonly used as a diagnostic marker for prostate cancer. We recently described anti-angiogenic properties of PSA [Fortier et al.: JNCI 91:1635-1640]. METHODS: Two forms of PSA were cloned and expressed in Pichia pastoris: one, an intact PSA with an N-terminus of IVGGVS em leader; the second, an N-1 PSA variant. The recombinant proteins were tested for serine protease activity and for anti-angiogenic activity in vitro and in vivo. RESULTS: The rate of substrate hydrolysis by the intact recombinant PSA was similar to that of PSA isolated and purified from human seminal plasma. In contrast, the N-1 PSA variant lacked serine protease activity. In an endothelial cell migration assay, the concentration that resulted in 50% inhibition (IC(50)) was: 0.5 microM for native PSA, 0.5 microM for intact recombinant protein, and 0.1 microM for the N-1 variant PSA. Both the intact recombinant and the N-1 recombinant PSA inhibited angiogenesis in vivo. CONCLUSIONS: Purified recombinant PSA inhibits angiogenesis, proving the concept that PSA is an anti-angiogenic, and serine protease activity, as determined by synthetic substrate hydrolysis, is distinct from the anti-angiogenic properties of PSA.

Endostatin binds tropomyosin. A potential modulator of the antitumor activity of endostatin.

The mechanism of action of Endostatin, an endogenous inhibitor of angiogenesis and tumor growth, remains unknown. We utilized phage-display technology to identify polypeptides that mimic the binding domains of proteins with which Endostatin interacts. A conformed peptide (E37) was identified that shares an epitope with human tropomyosin implicating tropomyosin as an Endostatin-binding protein. We show that recombinant human Endostatin binds tropomyosin in vitro and to tropomyosin-associated microfilaments in a variety of endothelial cell types. The most compelling evidence that tropomyosin modulates the activity of Endostatin was demonstrated when E37 blocked greater than 84% of the tumor-growth inhibitory activity of Endostatin in the B16-BL6 metastatic melanoma model. We conclude that the E37 peptide mimics the Endostatin-binding epitope of tropomyosin and blocks the antitumor activity of Endostatin by competing for Endostatin binding. We postulate that the Endostatin interaction with tropomyosin results in disruption of microfilament integrity leading to inhibition of cell motility, induction of apoptosis, and ultimately inhibition of tumor growth.

Metastatin: a hyaluronan-binding complex from cartilage that inhibits tumor growth.

In this study, a hyaluronan-binding complex, which we termed Metastatin, was isolated from bovine cartilage by affinity chromatography and found to have both antitumorigenic and antiangiogenic properties. Metastatin was able to block the formation of tumor nodules in the lungs of mice inoculated with B16BL6 melanoma or Lewis lung carcinoma cells. Single i.v. administration of Metastatin into chicken embryos inhibited the growth of both B16BL6 mouse melanoma and TSU human prostate cancer cells growing on the chorioallantoic membrane. The in vivo biological effect may be attributed to the antiangiogenic activity because Metastatin is able to inhibit the migration and proliferation of cultured endothelial cells as well as vascular endothelial growth factor-induced angiogenesis on the chorioallantoic membrane. In each case, the effect could be blocked by either heat denaturing the Metastatin or premixing it with hyaluronan, suggesting that its activity critically depends on its ability to bind hyaluronan on the target cells. Collectively, these results suggest that Metastatin is an effective antitumor agent that exhibits antiangiogenic activity.

Administration of a liposomal FGF-2 peptide vaccine leads to abrogation of FGF-2-mediated angiogenesis and tumor development.

Basic fibroblast growth factor (FGF-2) is an important stimulator of angiogenesis that has been implicated in neoplastic progression. Attempts to neutralize or modulate FGF-2 have met with some success in controlling neovascularity and tumor growth. In the present study, two peptides: one corresponding to the heparin binding domain and the other to the receptor binding domain of FGF-2, exerted dose-dependent inhibition of FGF-2-stimulated human umbilical vein endothelial cell proliferation (IC(50)=70 and 20 microg/ml, respectively). The identification of these functional regions suggested that targeting these domains might be an approach for the modulation of FGF-2 function. To investigate this possibility, we vaccinated mice with either the heparin binding domain peptide or the receptor binding domain peptide of FGF-2 in a liposome/adjuvant format, and analyzed the effect of vaccination on FGF-2-driven angiogenesis, tumor development and immune status. Mice vaccinated with the heparin binding domain peptide generated a specific antibody response to FGF-2, blocked neovascularization in a gelfoam sponge model of angiogenesis, and inhibited experimental metastasis by >90% in two tumor models: the B16BL6 melanoma and the Lewis lung carcinoma. These effects were not observed in mice treated with the receptor binding domain peptide conjugated to liposomes or liposomes lacking conjugated peptide. These data suggest that a heparin binding domain peptide of FGF-2, when presented to a host in a liposomal adjuvant formulation, can ultimately lead to inhibition of angiogenesis and tumor growth.