During angiogenesis, vascular endothelial growth factor and basic fibroblast growth factor regulate natural killer cell adhesion to tumor endothelium.

Localization of activated natural killer (A-NK) cells in the microvasculature of growing tumors is the result of recognition of the intracellular and vascular cell-adhesion molecules ICAM-1 and VCAM-1 on the tumor endothelium, mediated by lymphocyte function-associated protein LFA-1 and vascular lymphocyte function-associated protein VLA-4. In vitro and in vivo studies of A-NK cell adhesion to endothelial cells showed that vascular endothelial growth factor (VEGF) promotes adhesion, whereas basic fibroblast growth factor (bFGF) inhibits adhesion through the regulation of these molecules on tumor vasculature. Thus, some angiogenic factors may facilitate lymphocyte recognition of angiogenic vessels, whereas others may provide such vessels with a mechanism that protects them from cytotoxic lymphocytes.

Primary tumor size-dependent inhibition of angiogenesis at a secondary site: an intravital microscopic study in mice.

Some primary tumors are capable of suppressing the growth of their metastases by presumably generating antiangiogenic factors such as angiostatin. We hypothesized that the amount of inhibitor(s) released by a tumor increases with tumor growth. We tested this hypothesis by evaluating the relationship between the size of a primary tumor and its ability to inhibit angiogenesis at a secondary site. Furthermore, we characterized the effects of the primary tumor on physiological properties of newly formed vessels at the secondary site. Angiogenesis and physiological properties were measured using intravital microscopy of angiogenic vessels in the gels containing basic fibroblast growth factor placed into cranial windows of immunodeficient mice bearing human prostatic carcinoma (PC-3) in their flank. The PC-3 tumor inhibited angiogenesis in the gels, and surgical resection of tumor reversed this inhibition. The inhibition of angiogenesis 20 days after gel implantation (range, 0-83%) correlated positively (r = 0.625; P < 0.008) with the tumor size on the day of gel implantation (range, 19-980 mm3). The primary tumor also suppressed leukocyte-adhesion in angiogenic vessels, thus helping them evade the immune recognition. These results provide an additional rationale for combining antiangiogenic treatment with local therapies.

Time-dependent vascular regression and permeability changes in established human tumor xenografts induced by an anti-vascular endothelial growth factor/vascular permeability factor antibody.

The hyperpermeability of tumor vessels to macromolecules, compared with normal vessels, is presumably due to vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) released by neoplastic and/or host cells. In addition, VEGF/VPF is a potent angiogenic factor. Removal of this growth factor may reduce the permeability and inhibit tumor angiogenesis. To test these hypotheses, we transplanted a human glioblastoma (U87), a human colon adenocarcinoma (LS174T), and a human melanoma (P-MEL) into two locations in immunodeficient mice: the cranial window and the dorsal skinfold chamber. The mice bearing vascularized tumors were treated with a bolus (0.2 ml) of either a neutralizing antibody (A4.6.1) (492 micrograms/ml) against VEGF/VPF or PBS (control). We found that tumor vascular permeability to albumin in antibody-treated groups was lower than in the matched controls and that the effect of the antibody was time-dependent and influenced by the mode of injection. Tumor vascular permeability did not respond to i.p. injection of the antibody until 4 days posttreatment. However, the permeability was reduced within 6 h after i.v. injection of the same amount of antibody. In addition to the reduction in vascular permeability, the tumor vessels became smaller in diameter and less tortuous after antibody injections and eventually disappeared from the surface after four consecutive treatments in U87 tumors. These results demonstrate that tumor vascular permeability can be reduced by neutralization of endogenous VEGF/ VPF and suggest that angiogenesis and the maintenance of integrity of tumor vessels require the presence of VEGF/VPF in the tissue microenvironment. The latter finding reveals a new mechanism of tumor vessel regression-i.e., blocking the interactions between VEFG/VPF and endothelial cells or inhibiting VEGF/VPF synthesis in solid tumors causes dramatic reduction in vessel diameter, which may block the passage of blood elements and thus lead to vascular regression.

Endothelial cell death, angiogenesis, and microvascular function after castration in an androgen-dependent tumor: role of vascular endothelial growth factor.

The sequence of events that leads to tumor vessel regression and the functional characteristics of these vessels during hormone-ablation therapy are not known. This is because of the lack of an appropriate animal model and monitoring technology. By using in vivo microscopy and in situ molecular analysis of the androgen-dependent Shionogi carcinoma grown in severe combined immunodeficient mice, we show that castration of these mice leads to tumor regression and a concomitant decrease in vascular endothelial growth factor (VEGF) expression. Androgen withdrawal is known to induce apoptosis in Shionogi tumor cells. Surprisingly, tumor endothelial cells begin to undergo apoptosis before neoplastic cells, and rarefaction of tumor vessels precedes the decrease in tumor size. The regressing vessels begin to exhibit normal phenotype, i.e., lower diameter, tortuosity, vascular permeability, and leukocyte adhesion. Two weeks after castration, a second wave of angiogenesis and tumor growth begins with a concomitant increase in VEGF expression. Because human tumors often relapse following hormone-ablation therapy, our data suggest that these patients may benefit from combined anti-VEGF therapy.

Fractal characteristics of tumor vascular architecture during tumor growth and regression.

OBJECTIVE: Tumor vascular networks are different from normal vascular networks, but the mechanisms underlying these differences are not known. Understanding these mechanisms may be the key to improving the efficacy of treatment of solid tumors. METHODS: We studied the fractal characteristics of two-dimensional normal and tumor vascular networks grown in a murine dorsal chamber preparation and imaged with an intravital microscopy station. RESULTS: During tumor growth and regression, the vasculature in the tumor has scaling characteristics that reflect the changing state of the tissue. Growing tumors show vascular networks that progressively deviate from their normal pattern, in which they seem to follow diffusion-limited aggregation to a pathological condition in which they display scaling similar to percolation clusters near the percolation threshold. The percolation-like scaling indicates that the key determinants of tumor vascular architecture are local substrate properties rather than gradients of a diffusing substance such as an angiogenic growth factor. During tumor regression the fractal characteristics of the vasculature return to an intermediate between those of growing tumors and those of healthy tissues. Previous studies have shown that percolation-like scaling generally inhibits transport. CONCLUSIONS: In the present context, the percolation-like nature of tumor vasculature implies that tumor vascular networks possess inherent architectural obstacles to the delivery of diffusible substances such as oxygen and drugs.

Unstable radii in muscular blood vessels.

A model of a muscular blood vessel in equilibrium that predicts stable and unstable control of radius is presented. The equilibrium wall tension is modeled as the sum of a passive exponential function of radius and an active parabolic function of radius. The magnitude of the active tension is varied to simulate the variable level of smooth muscle activation. This tension-radius relationship is then converted to an equilibrium pressure-radius relationship via Laplace's law. This model predicts the traditional ability to control the radius below a critical level of activation. However, when the active tension is raised above this critical level, the pressure-radius relationship (with pressure plotted on the ordinate and radius on the abscissa) becomes N shaped with a relative maximal pressure (Pmax) and a relative minimal pressure (Pmin). For this N-shaped curve, there are three equilibrium radii for any pressure between Pmin and Pmax. Analysis shows that the middle radius is unstable and thus cannot be maintained at equilibrium. Previously unexplained experimental data reveal evidence of this instability.