In healthy adult rats, the test tissue that is used in the mesenteric-window angiogenesis assay is natively vascularized, lacks physiological angiogenesis, and is unperturbed by surgical intervention. Using the rat MWAA oral treatment with the nitric oxide (NO) synthase inhibitor Nw-nitro-L-arginine methyl ester (L-NAME) enhanced the angiogenic response (compared with controls receiving the inactive enantiomer D-NAME) following i.p. injections of (i) TNF-alpha at an approximate physiological dose, (ii) Compound 48/80, which is a highly selective secretagogue causing mast-cell secretion in situ and a very strong angiogenic response, and (iii) saline of a grade not made for infusion, causing a weak angiogenic response. Angiogenesis was assessed quantitatively using microscopic morphometry and image analysis in terms of objective variables recording the microvascular spatial extension, microvascular density, number and length of microvessel segments (extending between two successive branching points) and the degree of microvessel tortuosity. The data strongly suggest that endogenous NO inhibits all three mammalian angiogenesis reactions, although to a markedly different extent. Notably, the present data are virtually the opposite of those that have been reported from other mammalian angiogenesis models, the test tissues of which display deranged homeostasis, such as surgical intervention and/or ischemia.

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In vivo models of angiogenesis.

The process of building new blood vessels (angiogenesis) and controlling the propagation of blood vessels (anti-angiogenesis) are fundamental to human health, as they play key roles in wound healing and tissue growth. More than 500 million people may stand to benefit from anti- or pro-angiogenic treatments in the coming decades [National Cancer Institute (USA), Cancer Bulletin, volume 3, no. 9, 2006]. The use of animal models to assay angiogenesis is crucial to the search for therapeutic agents that inhibit angiogenesis in the clinical setting. Examples of persons that would benefit from these therapies are cancer patients, as cancer growth and spread is angiogenesis-dependent, and patients with aberrant angiogenesis in the eye, which may lead to blindness or defective sight. Recently, anti-angiogenesis therapies have been introduced successfully in the clinic, representing a turning point in tumor therapy and the treatment of macular degeneration and heralding a new era for the treatment of several commonly occurring angiogenesis-related diseases. On the other hand, pro-angiogenic therapies that promote compensatory angiogenesis in hypoxic tissues, such as those subjected to ischemia in myocardial or cerebral hypoxia due to occluding lesions in the coronary or cerebral arteries, respectively, and in cases of poor wound healing, are also being developed. In this review, the current major and newly introduced preclinical angiogenesis assays are described and discussed in terms of their specific advantages and disadvantages from the biological, technical, economical and ethical perspectives. These assays include the corneal micropocket, chick chorioallantoic membrane, rodent mesentery, subcutaneous (s.c.) sponge/matrix/alginate microbead, s.c. Matrigel plug, s.c. disc, and s.c. directed in vivo angiogenesis assays, as well as, the zebrafish system and several additional assays. A note on quantitative techniques for assessing angiogenesis in patients is also included. The currently utilized preclinical assays are not equivalent in terms of efficacy or relevance to human disease. Some of these assays have significance for screening, while others are used primarily in studies of dosage-effects, molecular structure activities, and the combined effects of two or more agents on angiogenesis. When invited to write this review, I was asked to describe in some detail the rodent mesenteric-window angiogenesis assay, which has not received extensive coverage in previous reviews.

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Evidence of specificity in the action of angiogenesis antagonists.

It is not known whether each type of angiogenesis reaction has a specific antiangiogenic mechanism and to what extent angiostatic agents can exert additional or synergistic effects. The objective of this study was to elucidate the way in which systemic angiostatic drug treatment by either one drug at a time or two in combination affects distinct angiogenesis reactions in one and the same vascular bed. The chemically unrelated drugs which were used were dexamethasone (DE) and ciclosporin A (CS) at nontoxic doses in the range used clinically. These drugs are known to act angiostatically in the rat. The rat mesenteric window angiogenesis assay was used to assess the systemic effect of DE, CS and DE plus CS in saline-mediated angiogenesis (SMA) and mast-cell-mediated angiogenesis (MCMA). Angiogenesis was quantified by microscopic and morphometric means, one variable being the number of vessel profiles per unit tissue length (No/UL), a measure related to the degree of microvascular branching and tortuosity. Whereas CS significantly suppressed No/UL in SMA without substantially affecting MCMA, DE plus CS significantly suppressed No/UL in MCMA, but, unexpectedly, somewhat increased No/UL in SMA. As compared with either drug given alone, the coadministration of the drugs clearly did not enhance the antiangiogenic effect in SMA. The drugs, whether alone or in combination specifically and selectively suppressed the angiogenesis reactions which were examined, thereby suggesting distinct antiangiogenic mechanisms in SMA and MCMA.

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Basic-fibroblast-growth-factor-mediated de novo angiogenesis is more effectively suppressed by low-molecular-weight than by high-molecular-weight heparin.

We recently reported that the subcutaneous (s.c.) administration of a low-molecular-weight heparin (LMWH) fraction significantly inhibited de novo angiogenesis in the mesentery induced by the intraperitoneal (i.p.) injection of saline to adult rats compared with unfractionated heparin and high-molecular-weight heparin (HMWH) fractions. The present study assesses the effect on basic fibroblast growth factor (bFGF)-mediated de novo angiogenesis in the mesentery of the systemic administration of a LMWH fraction (2.6 kD) and a series of four HMWH fractions (about 20 kD) with varying degrees of polydispersity, charge density and anticoagulant activity. bFGF, a prototypic heparin-binding angiogenic growth factor, was injected i.p. at 220 pM on days 0-4. The heparins were given s.c. on days 0-13 or 0-14 at doses which were approximately within the range used clinically. Angiogenesis was assessed by microscopic morphometry and image analysis in groups of animals killed on days 14 and 15. Compared with the saline control, the LMWH and three of the HMWHs significantly inhibited angiogenesis in terms of microvascular length (MVL), a measure of microvascular density. Interestingly, the vascularized area (VA), a measure of microvascular spatial extension, and the total microvascular length (VA x MVL) were significantly lower in the LMWH-treated animals than in the animals treated with one of the HMWHs. The total microvascular length was, moreover, significantly reduced in the LMWH-treated animals compared with the combined data of all the HMWH-treated animals. No significant effects were related to the degree of charge density and anticoagulant activity of the heparins. In view of the putative significant angiogenic role of bFGF in human angiogenesis diseases, the present findings may have implications for the choice of anticoagulant treatment modality for patients suffering from cancer and other angiogenesis diseases.

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Relaxin, a potent microcirculatory effector, is not angiogenic.

The ability of relaxin (RLX), which is a potent microcirculatory effector in many species including the rat, to induce de novo angiogenesis in vascularized mammalian tissue was tested using the rat mesenteric-window angiogenesis assay. RLX was administered intraperitoneally on days 1-5 at doses of 0.33, 3.3 and 33 nM. Controls received the vehicle by the same route. Groups of animals were sacrificed at the end of the 1st, 2nd and 3rd weeks. Using computer-aided microscopic morphometry including image analysis, the response was quantified by sensitive, technically independent, highly reproducible methods in terms of the vascularized area (VA), a measure of microvascular spatial extension, and the microvascular length (MVL), a measure of microvascular density. The total MVL was computed from VA x MVL. The results obtained show that RLX did not cause significant changes in any of the variables tested, regardless of dose and observation time. These findings indicate that RLX is apparently unable to mediate significant de novo angiogenesis in the system used in contrast to previously tested angiogens such as basic fibroblast growth factor, vascular endothelial growth factor, isoform 165, and tumor necrosis factor-alpha. In previous studies, RLX has been shown to exert antitumor activity on breast cancer cells in vitro. In the search for a possible role for RLX as an anticancer agent in vivo, it is important to know that this peptide is not angiogenic, since de novo angiogenesis is known to be a prerequisite for tumor growth and metastatic spread.

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PubMedDate not supplied

A 5.0-kD heparin fraction systemically suppresses VEGF165-mediated angiogenesis.

The systemic effect of heparin fractions with mean molecular masses of 2.5, 5.0 and 16.4 kD on angiogenesis induced by vascular endothelial growth factor isoform 165 was studied using the truly quantitative rat mesenteric-window angiogenesis assay. The angiogenic treatment with 5 ml of VEGF165 at 480 pM was given intraperitoneally on days 0-4 and heparin fractions were given subcutaneously on days 0-13; animals were sacrificed on day 14. As the overlaps between the molecular mass distributions of the three fractions were relatively small, they essentially represent three different populations of heparin molecules. The doses of the heparins given were equal in terms of weight, but different in terms of the number of molecules and biologic activity. Angiogenesis was assessed in terms of vascularized area (VA), a measurement of microvascular spatial extension, and microvascular length (MVL), a measurement of microvascular density, using technically independent variables and image analysis. The total microvascular length was computed from VA x MVL. Treatment with the 5.0-kD fraction suppressed angiogenesis significantly in statistical terms compared with treatment with 2.5- and 16.4-kD heparins and the saline in controls. Interestingly, the 2.5-kD heparin fraction which was used here has previously been shown statistically significantly to suppress angiogenesis mediated by basic fibroblast growth factor in the same experimental system. Our data thus suggest that the systemic angiosuppressive effect of heparin in different mammalian angiogenic reactions is distinctly related to structural features such as molecular size.

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