The role of angiostatin in the spontaneous bone and prostate cancers of pet dogs.

Angiostatin is a potent inhibitor of angiogenesis generated in cancer-bearing hosts by tumor-derived proteases. Because the naturally occurring bone and prostate cancers of pet dogs provide unique model systems to study factors that regulate cancer progression and tumor dormancy, we investigated the capacity of these tumors to generate angiostatin. We determined that angiostatin fragments are present in urine of dogs with bone cancer. The identity of these fragments was confirmed by comparison of the experimentally determined protein sequence to that of a clone of canine angiostatin. Importantly, these fragments were absent in urine collected from the same dogs after complete surgical removal of the primary tumor. We also demonstrate that canine prostate cancer cells are capable of processing plasminogen to angiostatin in vitro. These findings provide rationale for using spontaneous canine tumor models to isolate endogenous angiogenesis inhibitors and to investigate their therapeutic use against cancer.

Generation of multiple angiogenesis inhibitors by human pancreatic cancer.

A primary inoculum of human pancreatic cancer cells (BxPC-3) has the ability to inhibit the growth of a secondary tumor in an in vivo animal model. Such ability suggests that the primary tumor is producing inhibitors that act at the site of the secondary tumor. Accordingly we attempted to discover which inhibitors are produced by pancreatic cancer cells. We determined that pancreatic cancer cells process angiostatin isoforms from plasminogen. Additionally, we isolated and characterized an uncleaved "latent" antiangiogenic antithrombin (aaAT) molecule processed from systemically available AT by pancreatic cancer cells as well as a cleaved form of aaAT processed from systemically available AT by pancreatic cancer cells. Human AT, cleaved with human neutrophil elastase, inhibits angiogenesis in the chorioallantoic membrane assay. This human aaAT molecule is able to inhibit the growth of pancreatic tumors in immune-compromised mice. Our work represents the first demonstration of multiple angiogenesis inhibitors from a single tumor and suggests that antiangiogenic therapies may provide an avenue for future treatment of pancreatic cancer.

Continuous administration of endostatin by intraperitoneally implanted osmotic pump improves the efficacy and potency of therapy in a mouse xenograft tumor model.

In the first Phase I clinical trials of endostatin as an antiangiogenic therapy for cancer, the protein was administered as an i.v. bolus for approximately 20-30 min each day. This protocol was based on experimental studies in which animals were treated by s.c. bolus once a day. However, it was not clear in the previous studies whether this schedule could be maximized further. Therefore, we developed experimental models involving continuous administration of endostatin to determine the potency and efficacy of this approach. Endostatin was administered to tumor-bearing mice either s.c. or i.p. in single bolus doses. The efficacy of these regimens was compared with endostatin administered continuously via an i.p. implanted mini-osmotic pump. Our results show that endostatin remains stable and active in mini-osmotic pumps for at least 7 days. We show that endostatin injected i.p. is rapidly cleared within 2 h, whereas endostatin administered continuously via mini-osmotic pump maintains systemic concentrations of 200-300 ng/ml for the duration of administration. Furthermore, continuous i.p. administration of endostatin results in more effective tumor suppression at significantly reduced doses (5-fold), compared with bolus administration. Additional experiments using a human pancreatic cancer model in severe combined immunodeficient mice showed that there was a significant decrease in the microvessel density between the treatment groups and the control group. These data show that continuous administration of human endostatin results in sustained systemic concentrations of the protein leading to: (a) increased efficacy manifested as increased tumor regression; and (b) an 8-10-fold decrease in the dose required to achieve the same antitumor effect as the single daily bolus administration of endostatin. On the basis of this approach, an additional clinical trial has been designed and initiated and is under way in two countries.

Evidence for a novel O-linked sialylated trisaccharide on Ser-248 of human plasminogen 2.

Human plasminogen, the inactive precursor of plasmin, exists in two major glycoforms. Plasminogen 1 contains an N-linked oligosaccharide at Asn-289 and an O-linked oligosaccharide at Thr-345. Plasminogen 2 is known to contain only an O-linked oligosaccharide at Thr-345. However, plasminogen 2 displays a further well documented microheterogeneity dependent on the N-acetylneuraminic acid content, which has functional consequences with regard to activation of plasminogen. The proposed structure and number of known oligosaccharide linkages in plasminogen 2 is insufficient to account for this microheterogeneity. In the present study, a combination of trypsin digestion, lectin affinity chromatography, Edman degradation amino acid sequence analysis, carbohydrate composition analysis, and mass spectrometry revealed the existence of a novel site for O-linked glycosylation on plasminogen 2 at Ser-248. Direct evidence for the structure of the carbohydrate was obtained from a combination of lectin affinity chromatography, desialylation experiments, and mass spectrometry analysis. These findings provide a structural basis for some of the observed microheterogeneity, and have implications with regard to the known functional consequences of the extent of sialylation of plasminogen.

Sialic acid content of plasminogen 2 glycoforms as a regulator of fibrinolytic activity. Isolation, carbohydrate analysis, and kinetic characterization of six glycoforms of plasminogen.

Six glycoforms of plasminogen 2 were isolated using a combination of lectin affinity chromatography and chromatofocussing, and the sialic acid content of each glycoform was determined. The kinetics of activation of each glycoform by tissue-type plasminogen activator were analyzed on a fibrin surface and in solution. The second-order rate constant (measured on a fibrin surface) decreased from 1.65 x 10(6) M-1 s-1 to 3.77 x 10(4) M-1 s-1 as the sialic acid content of the glycoforms increased from 1.3 mol/mol of protein to 13.65 mol/mol of protein. A similar correlation was noted for activation in solution. Each glycoform was converted to plasmin, and the inhibition constants for the reaction between alpha 2-antiplasmin and plasmin glycoforms were determined. All overall Ki values, reflecting the final essentially irreversible complex, were in the picomolar range. Sialic acid does not affect inhibition of plasmin by alpha 2-antiplasmin; however, hypersialylated plasmin does not appear to have a kringle-dependent component to inhibition.

Biochemical and immunological characterization of multiple glycoforms of mouse mast cell protease 1: comparison with an isolated murine serosal mast cell protease (MMCP-4).

Five highly soluble, chymotrypsin-like, neutral serine proteases, with molecular masses in the range 30-33 kDa, were isolated from Trichinella spiralis-infected mouse small intestine. These enzymes were closely related antigenically on Western blotting and by Ouchterlony double diffusion using a polyclonal, cross-absorbed, sheep antibody raised against mouse mast cell protease-1 (MMCP-1) and on the basis of N-terminal amino acid sequence analysis, were identified as variant forms of MMCP-1. Substrate and inhibitor analysis confirmed that the five variants (MMCP-1 A-E) had similar characteristics, although highly significant (P = 0.025 to P < 0.0001) variations in Km and kcat, were detected. Against human alpha 1-proteinase inhibitor the Ki for MMCP-1C (45 pM) was significantly (P < 0.0001) greater than those for the other proteases (0.76-2.2 pM). The differences in electrophoretic mobility are probably a result of variable glycosylation, since removal of N-linked carbohydrate produced a polypeptide of approx. 28 kDa in each case which was, like the native enzyme, immunoreactive on Western blotting. A much less soluble 28 kDa enzyme was isolated from serosal mast cells and identified as MMCP-4 by N-terminal amino acid sequencing. Like MMCP-1 it has chymotrypsin-like substrate specificities with activity at neutral pH. However, it was antigenically distinct from MMCP-1 and, using sheep anti-MMCP-1, was not detected on Western blotting or by Ouchterlony double diffusion, e.l.i.s.a. or immunohistochemistry. This last technique established that the MMCP-1 variants were uniquely present in enteric mast cells, thereby providing a highly selective means of distinguishing the mucosal and connective tissue mast cell subsets in the mouse.

Differential inhibition of rat mast cell proteinase I and II by members of the alpha-1-proteinase inhibitor family of serine proteinase inhibitors.

Rat mast cell proteinase II (RMCP II) from mucosal mast cells was titrated into rat serum, and the resulting serine proteinase inhibitor (serpin)-enzyme complex was purified by affinity chromatography on anti-RMCP II-Sepharose 4B and by Mono-Q anion-exchange. The purified complex was used to raise polyclonal antibodies which, after cross-absorption against RMCP II-Sepharose 4B, were specific for serpin and were used to affinity purify two rat serpin molecules (RSI and RSII) that inhibit RMCP II in rat serum. The kinetic constants characterizing the interaction between RMCP II and RSI and RSII are ka, 2.2 x 10(5) and 1.65 x 10(5) M-1 s-1, respectively; Ki, 3.6 x 10(-10) and 1.0 x 10(-9) M; and kd, 7.9 x 10(-5) and 1.65 x 10(-4) s-1. Amino-terminal sequence analysis indicated that RSI and RSII are distinct, differing at the amino-terminal residues, and are products of the rat Spi-1 locus. Rat mast cell proteinase I (RMCP I) from connective tissue mast cells cleaved both RSI and RSII and was not inhibited.