Approximate complex electrical potential distribution in the monodomain model with unequal conductivity and relative permittivity anisotropy ratios

OBJECTIVE

Electrical conductivity and relative permittivity are properties that indicate muscle health and they have different values parallel and perpendicular to the direction of the myofiber, a concept known as anisotropy. When the intrinsic electrical properties of muscle have ratios of anisotropy that are different then there is no analytical solution that can describe the electrical potential distribution in the tissue.

APPROACH

Here, we present approximate analytical solutions to monodomain equations with unequal anisotropy ratios. For this, we base our analysis on perturbation theory where the electrical potential is approximated by the sum of the zeroth- and first-order terms of an infinite series.

MAIN RESULTS

The validity of the approach is confirmed using experimental data for healthy and diseased muscle available online.

SIGNIFICANCE

A better understanding of electrical potential distribution in anisotropic skeletal muscle tissue will allow the development of improved diagnostic tools for neuromuscular diseases.

New electrical impedance methods for the in situ measurement of the complex permittivity of anisotropic skeletal muscle using multipolar needles

This paper provides a rigorous analysis on the measurement of the permittivity of two-dimensional anisotropic biological tissues such as skeletal muscle using the four-electrode impedance technique. The state-of-the-art technique requires individual electrodes placed at the same depth in contact with the anisotropic material, e.g. using monopolar needles. In this case, the minimum of measurements in different directions needed to estimate the complex permittivity and its anisotropy direction is 3, which translates into 12 monopolar needle insertions (i.e. 3 directions × 4 electrodes in each direction). Here, we extend our previous work and equip the reader with 8 new methods for multipolar needles, where 2 or more electrodes are spaced along the needle's shaft in contact with the tissue at different depths. Using multipolar needles, the new methods presented reduce the number of needle insertions by a factor of 2 with respect to the available methods. We illustrate the methods with numerical simulations and new experiments on ex vivo ovine skeletal muscle (n = 3). Multi-frequency longitudinal and transverse permittivity data from 30 kHz to 1 MHz is made publicly available in the supplementary material. The methods presented here for multipolar needles bring closer the application of needle electrical impedance to patients with neuromuscular diseases.

Why are tumour blood vessels abnormal and why is it important to know?

Tumour blood vessels differ from their normal counterparts for reasons that have received little attention. We report here that they are of at least six distinct types, we describe how each forms, and, looking forward, encourage the targeting of tumour vessel subsets that have lost their vascular endothelial growth factor-A (VEGF-A) dependency and so are likely unresponsive to anti-VEGF-A therapies.

Historical and ecological determinants of genetic structure in arctic canids

Wolves (Canis lupus) and arctic foxes (Alopex lagopus) are the only canid species found throughout the mainland tundra and arctic islands of North America. Contrasting evolutionary histories, and the contemporary ecology of each species, have combined to produce their divergent population genetic characteristics. Arctic foxes are more variable than wolves, and both island and mainland fox populations possess similarly high microsatellite variation. These differences result from larger effective population sizes in arctic foxes, and the fact that, unlike wolves, foxes were not isolated in discrete refugia during the Pleistocene. Despite the large physical distances and distinct ecotypes represented, a single, panmictic population of arctic foxes was found which spans the Svalbard Archipelago and the North American range of the species. This pattern likely reflects both the absence of historical population bottlenecks and current, high levels of gene flow following frequent long-distance foraging movements. In contrast, genetic structure in wolves correlates strongly to transitions in habitat type, and is probably determined by natal habitat-biased dispersal. Nonrandom dispersal may be cued by relative levels of vegetation cover between tundra and forest habitats, but especially by wolf prey specialization on ungulate species of familiar type and behaviour (sedentary or migratory). Results presented here suggest that, through its influence on sea ice, vegetation, prey dynamics and distribution, continued arctic climate change may have effects as dramatic as those of the Pleistocene on the genetic structure of arctic canid species.

Prey specialization may influence patterns of gene flow in wolves of the Canadian Northwest

This study characterizes population genetic structure among grey wolves (Canis lupus) in northwestern Canada, and discusses potential physical and biological determinants of this structure. Four hundred and ninety-one grey wolves, from nine regions in the Yukon, Northwest Territories and British Columbia, were genotyped using nine microsatellite loci. Results indicate that wolf gene flow is reduced significantly across the Mackenzie River, most likely due to the north-south migration patterns of the barren-ground caribou herds that flank it. Furthermore, although Banks and Victoria Island wolves are genetically similar, they are distinct from mainland wolf populations across the Amundsen Gulf. However, low-level island-mainland wolf migration may occur in conjunction with the movements of the Dolphin-Union caribou herd. Whereas previous authors have examined isolation-by-distance in wolves, this study is the first to demonstrate correlations between genetic structure of wolf populations and the presence of topographical barriers between them. Perhaps most interesting is the possibility that these barriers reflect prey specialization by wolves in different regions.

Fiber characteristics of qiviut and guard hair from wild muskoxen (Ovibos moschatus)

In response to increasing commercial interest and the high market value of qiviut (the downy underwool of the muskox), we have employed standards and measurements used in the wool and cashmere industries to describe qiviut fiber characteristics. Fleece samples (qiviut with guard hair) were shaved from the midshoulder of 299 wild muskox hides of known sex and age (1, 2, 3, and 4+ yr) during the Banks Island, Canada, muskox harvest in November 1997. Samples were analyzed for fiber diameter distribution of raw fiber and qiviut, scoured and qiviut yields, and lengths of guard hair and qiviut fiber. We found a sex x age interaction for average fiber diameter (AFD) in raw fiber (P= 0.002) and qiviut (P < 0.001) only. Adult males had significantly coarser AFD than females (21.5 microm, males vs 20.1 microm, females and 18.2 microm, males vs 17.5 microm, females) for raw fiber and qiviut, respectively. Qiviut AFD from yearlings was 1.7 microm finer than the AFD of adult qiviut. Fiber diameter distribution (SD) decreased with age in the raw sample (P < 0.003) and qiviut (P < 0.001) and qiviut SD was greater (P < 0.001) in males than in females. Qiviut theoretical yield (% mass of fibers < or = 30 microm) increased (P < 0.001) with age, and females had higher theoretical yields than males (P < 0.001). Scoured yield did not vary between sexes in any age class and averaged 93.3%. Qiviut staple length did not differ with either age or sex. In summary, differences between the sexes were small up to the 3rd yr, and these differences were not likely to be of commercial importance. However, considering that AFD is a primary commercial criterion of value, AFD changes from 16.5 microm in yearlings to 18.2 microm in adults and from 17.5 microm in adult females to 18.2 microm in adult males would be expected to result in significant differences in commercial value.

The neurotransmitter dopamine inhibits angiogenesis induced by vascular permeability factor/vascular endothelial growth factor

Angiogenesis has an essential role in many important pathological and physiological settings. It has been shown that vascular permeability factor/vascular endothelial growth factor (VPF/VEGF), a potent cytokine expressed by most malignant tumors, has critical roles in vasculogenesis and both physiological and pathological angiogenesis. We report here that at non-toxic levels, the neurotransmitter dopamine strongly and selectively inhibited the vascular permeabilizing and angiogenic activities of VPF/VEGF. Dopamine acted through D2 dopamine receptors to induce endocytosis of VEGF receptor 2, which is critical for promoting angiogenesis, thereby preventing VPF/VEGF binding, receptor phosphorylation and subsequent signaling steps. The action of dopamine was specific for VPF/VEGF and did not affect other mediators of microvascular permeability or endothelial-cell proliferation or migration. These results reveal a new link between the nervous system and angiogenesis and indicate that dopamine and other D2 receptors, already in clinical use for other purposes, might have value in anti-angiogenesis therapy.

Glomeruloid microvascular proliferation follows adenoviral vascular permeability factor/vascular endothelial growth factor-164 gene delivery

Glomeruloid bodies are a defining histological feature of glioblastoma multiforme and some other tumors and vascular malformations. Little is known about their pathogenesis. We injected a nonreplicating adenoviral vector engineered to express vascular permeability factor/vascular endothelial growth factor-164 (VPF/VEGF(164)) into the ears of athymic mice. This vector infected local cells that strongly expressed VPF/VEGF(164) mRNA for 10 to 14 days, after which expression gradually declined. Locally expressed VPF/VEGF(164) induced an early increase in microvascular permeability, leading within 24 hours to edema and deposition of extravascular fibrin; in addition, many pre-existing microvessels enlarged to form thin-walled, pericyte-poor, "mother" vessels. Glomeruloid body precursors were first detected at 3 days as focal accumulations of rapidly proliferating cells in the endothelial lining of mother vessels, immediately adjacent to cells expressing VPF/VEGF(164). Initially, glomeruloid bodies were comprised of endothelial cells but subsequently pericytes and macrophages also participated. As they enlarged by endothelial cell and pericyte proliferation, glomeruloid bodies severely compromised mother vessel lumens and blood flow. Subsequently, as VPF/VEGF(164) expression declined, glomeruloid bodies devolved throughout a period of weeks by apoptosis and reorganization into normal-appearing microvessels. These results provide the first animal model for inducing glomeruloid bodies and indicate that VPF/VEGF(164) is sufficient for their induction and necessary for their maintenance.

Retinoic acid selectively inhibits the vascular permeabilizing effect of VPF/VEGF, an early step in the angiogenic cascade

All-trans-retinoic acid (RA) and other retinoids modulate cell growth and differentiation, generally favoring terminal cell differentiation and inhibiting carcinogenesis. Retinoids are also reported to inhibit angiogenesis and endothelial cell migration, actions that are also anti-carcinogenic. Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is a multifunctional cytokine secreted by many tumors. It renders microvessels hyperpermeable to plasma and stimulates endothelial cell migration and division. To investigate further the mechanisms by which RA inhibits angiogenesis, we evaluated the effects of RA on VPF/VEGF-induced angiogenesis and microvascular permeability. RA selectively inhibited the angiogenic response induced by VPF/VEGF, but not that induced by fibroblast growth factor-2 (FGF-2), in the CAM assay. RA and two of its isomers also inhibited the vascular permeabilizing effect of VPF/VEGF but not that induced by histamine. The vascular permeabilization induced by VPF/VEGF and blocked by RA takes place within 1-15 min, too short a time frame for RA to act by modulating transcription through classic retinoid receptors. RA also inhibited VPF/VEGF-induced phosphorylation of PLC-gamma and synthesis of cGMP but actually increased VPF/VEGF binding to cultured endothelial cells. Taken together, these findings indicate that RA selectively blocks VPF/VEGF-induced microvascular permeability and angiogenesis and also identify VPF/VEGF as a major target of RA action. The selectivity of RA's action suggests that other, RA-independent pathways must exist for the angiogenesis induced by FGF-2 and the vascular permeabilizing effect of histamine.

Ultrastructural localization of the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) receptor-2 (FLK-1, KDR) in normal mouse kidney and in the hyperpermeable vessels induced by VPF/VEGF-expressing tumors and adenoviral vectors

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) interacts with two high-affinity tyrosine kinase receptors, VEGFR-1 and VEGFR-2, to increase microvascular permeability and induce angiogenesis. Both receptors are selectively expressed by vascular endothelial cells and are strikingly increased in tumor vessels. We used a specific antibody to localize VEGFR-2 (FLK-1, KDR) in microvascular endothelium of normal mouse kidneys and in the microvessels induced by the TA3/St mammary tumor or by infection with an adenoviral vector engineered to express VPF/VEGF. A pre-embedding method was employed at the light and electron microscopic levels using either nanogold or peroxidase as reporters. Equivalent staining was observed on both the luminal and abluminal surfaces of tumor- and adenovirus-induced vascular endothelium, but plasma membranes at interendothelial junctions were spared except at sites connected to vesiculovacuolar organelles (VVOs). VEGFR-2 was also localized to the membranes and stomatal diaphragms of some VVOs. This staining distribution is consistent with a model in which VPF/VEGF increases microvascular permeability by opening VVOs to allow the transendothelial cell passage of plasma and plasma proteins.

A comparison of heavy metal levels in the kidneys of High Arctic and mainland caribou populations in the Northwest Territories of Canada

Aluminum, nickel, cadmium, mercury and lead levels were measured in the kidney tissue of Banks Island Peary caribou and barren-ground caribou, from the Bluenose herd, of the western Northwest Territories of Canada. Cadmium concentrations of Bluenose caribou were similar to those reported elsewhere for barren-ground caribou and showed a positive correlation with age. Cadmium concentrations of Peary caribou were significantly lower than those of Bluenose caribou regardless of age, were the lowest reported for caribou during winter, and did not show a positive correlation with age. Mercury levels, expressed on a wet weight basis, were similar to those reported for other barren-ground caribou. Mercury levels were significantly higher in Bluenose [mean 10.45 microg g(-1) (dry wt.); S.E.= 0.85; n = 20] than Peary [mean 5.43 microg g(-1) (dry wt.); S.E. = 0.31; n = 20] caribou. Aluminum concentrations for Bluenose and Peary caribou were similar [mean 1.48 microg g(-1) (dry wt.); S.E. = 0.17; n = 20 and mean 1.56 microg g(-1) (dry wt.); S.E.= 0.15; n = 20, respectively), but were considerably lower than those reported for barren-ground caribou elsewhere. Lead and nickel concentrations were low and similar between Bluenose, Peary and other reported barren-ground caribou populations. Higher cadmium and mercury concentrations in Bluenose caribou are consistent with the hypothesis that caribou with a high dietary lichen component have higher contaminant levels. It is unlikely that subsistence harvesters would consume enough kidney during a year to exceed the tolerable intake of cadmium recommended by the WHO.

Heterogeneity of the angiogenic response induced in different normal adult tissues by vascular permeability factor/vascular endothelial growth factor

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is an angiogenic cytokine with potential for the treatment of tissue ischemia. To investigate the properties of the new blood vessels induced by VPF/VEGF, we injected an adenoviral vector engineered to express murine VPF/VEGF164 into several normal tissues of adult nude mice or rats. A dose-dependent angiogenic response was induced in all tissues studied but was more intense and persisted longer (months) in skin and fat than in heart or skeletal muscle (< or =3 weeks). The initial response (within 18 hours) was identical in all tissues studied and was characterized by microvascular hyperpermeability, edema, deposition of an extravascular fibrin gel, and the formation of enlarged, thin-walled pericyte-poor vessels ("mother" vessels). Mother vessels developed from preexisting microvessels after pericyte detachment and basement membrane degradation. Mother vessels were transient structures that evolved variably in different tissues into smaller daughter vessels, disorganized vessel tangles (glomeruloid bodies), and medium-sized muscular arteries and veins. Vascular structures closely resembling mother vessels and each mother vessel derivative have been observed in benign and malignant tumors, in other examples of pathological and physiological angiogenesis, and in vascular malformations. Together these data suggest that VPF/VEGF has a role in the pathogenesis of these entities. They also indicate that the angiogenic response induced by VPF/VEGF is heterogeneous and tissue specific. Finally, the muscular vessels that developed from mother vessels in skin and perimuscle fat have the structure of collaterals and could be useful clinically in the relief of tissue ischemia.

Different pathways of macromolecule extravasation from hyperpermeable tumor vessels

Tumor microvessels are hyperpermeable to plasma proteins, a consequence of tumor cell-secreted vascular permeability factor/vascular endothelial growth factor (VPF/VEGF). However, the pathways by which macromolecules extravasate from tumor vessels have been little investigated. To characterize tumor vessels more precisely and to elucidate the pathways by which macromolecules extravasated from them, we studied two well-defined, VPF/VEGF-secreting murine carcinomas, MOT and TA3/St. Whether grown in ascites or solid form, MOT tumors induced large, pericyte-poor "mother" vessels whose lining endothelium developed fenestrae that involved 1.8-5.6% of the surface. Fenestrae developed in parallel with markedly reduced endothelial cell vesiculo-vacuolar organelles (VVOs). TA3/St tumors, which secreted more VPF/VEGF than MOT tumors, elicited mother vessels with unchanged VVOs and without fenestrae. In both tumors, a plasma protein tracer, ferritin, extravasated through VVOs and in MOT tumors ferritin also extravasated through fenestrae. Endothelial gaps were not observed in either tumor. Thus, not all VPF/VEGF-secreting tumors induce fenestrated endothelium. Also, VVOs provide an internal store of membrane that can be transferred to the endothelial cell surface to provide the substantial increase in plasma membrane necessary for mother vessel formation in MOT tumors. Such transfer was apparently unnecessary in TA3/St tumors in which extensive early endothelial cell division provided the increased plasma membrane necessary for forming mother vessels.

Pathways of macromolecular extravasation across microvascular endothelium in response to VPF/VEGF and other vasoactive mediators

OBJECTIVE

The goal of these studies was to define the anatomic pathways by which circulating macromolecules extravasate from the hyperpermeable microvessels that supply tumors and from normal venules that have been rendered hyperpermeable by vasoactive mediators.

METHODS

Extravasation pathways of circulating macromolecular tracers were followed by several morphological techniques: light and fluorescence microscopy, transmission electron microscopy of routine as well as ultrathin and serial sections, computer-assisted three-dimensional reconstructions, and morphometry.

RESULTS AND DISCUSSION

Macromolecules extravasated across tumor microvessels or across normal venules rendered hyperpermeable by VPF/VEGF, histamine, or serotonin by three primary pathways: 1) Vesiculo-vacuolar organelles (VVOs), clusters of cytoplasmic vesicles and vacuoles that span endothelial cytoplasm from lumen to ablumen; 2) trans-endothelial cell (EC), pores, and 3) fenestrae. We also present data concerning the structure and function of VVOs as well as evidence that VVOs form as the result of linking together and fusion of caveolae-sized unit vesicles. Under suitable conditions VVOs also afforded a pathway for macromolecular transport in the reverse direction, i.e., from vascular ablumen to lumen. Finally, in addition to opening VVOs to the passage of macromolecules, mediators such as VPF/VEGF may also induce structural rearrangements of VVOs, transforming them into trans-EC pores or fenestrae.

Vascular permeability factor/vascular endothelial growth factor and the significance of microvascular hyperpermeability in angiogenesis

This Chapter has reviewed the literature concerning VPF/VEGF as a potent vascular permeabilizing cytokine. In accord with this important role, microvessels have been found to be hyperpermeable to plasma proteins and other circulating macromolecules at sites where VPF/VEGF and its receptors are overexpressed, i.e., in tumors, healing wounds, retinopathies, many important inflammatory conditions and in certain physiological processes, such as ovulation and corpus luteum formation. Moreover, microvascular hyperpermeability to plasma proteins was shown to have an important consequence: the laying down of a fibrin-rich extracellular matrix. This provisional matrix, in turn, favors and supports the ingrowth of fibroblasts and endothelial cells which, together, transform the provisional matrix into the mature stroma characteristic of tumors and healed wounds. Finally, we have considered the pathways by which these and other circulating macromolecules cross the endothelium of normal and VPF/VEGF-permeabilized microvessels. These pathways include VVOs and trans-endothelial openings that have been variously interpreted as inter-endothelial cell gaps or trans-endothelial cell pores. At least some trans-endothelial cell pores may arise from VVOs. In conclusion, these data provide new insights into the mechanisms of angiogenesis and stroma formation, insights which are potentially applicable to a wide variety of disease states and which may lead to identification of new targets for therapeutic intervention.

Platelets exit venules by a transcellular pathway at sites of F-met peptide-induced acute inflammation in guinea pigs

Platelets maintain the integrity of vascular endothelium, but also appear outside of blood vessels in pathological states such as acute inflammation. However, it is widely believed that platelets extravasate from blood vessels only as the result of endothelial injury and that, on contacting extravascular collagen, they undergo a morphologically defined activation sequence and release their granule contents. We here report that platelets may cross intact venular endothelium without exhibiting this release reaction or injury. Platelets became adherent to the luminal surface of venular endothelium within approximately 15 min of intradermal injection of 10(-5) M N-formyl-methionyl-leucyl-phenylalanine in guinea pig flank skin. Individual intact platelets were noted in large endothelial cell cytoplasmic vacuoles from which they subsequently migrated abluminally. They then crossed the vascular basal lamina and entered the dermis without exhibiting evidence of a release reaction. Serial electron-microscopic sections confirmed that the cytoplasmic vacuoles within which platelets crossed endothelial cells were independent of interendothelial cell junctions which remained normally closed. Platelets extended pseudopods and gave other evidence of cell motility. These findings require a paradigm shift in our thinking about platelet movement and functions.

Modulation of transgene expression in mesothelial cells by activation of an inducible promoter

BACKGROUND

The efficacy of peritoneal dialysis and its success as a long-term treatment depends on the preservation of the integrity of the peritoneal membrane. With increasing time on dialysis, the membrane may become compromised resulting in decreased dialysing capacity. We have pursued an innovative strategy, i.e. genetic modification of the mesothelial cell to change the properties of the membrane to potentially improve its dialysing capacity and longevity, and have demonstrated the feasibility of this approach in a rat model of ex vivo gene transfer. The potential to regulate transgene expression in this model is examined here.

METHODS

Rat peritoneal mesothelial cells (MCs) were stably modified to express human growth hormone (hGH) under control of the heavy metal ion and glucocorticoid-regulatable murine metallothionein-1 promoter. The effect of zinc and the synthetic glucocorticoid dexamethasone on hGH expression was analysed in MC clones maintained in continuous passage or stationary phase, and in our rat model of ex vivo gene transfer.

RESULTS

Exposure of these clones to zinc and dexamethasone, either singly or in combination, resulted in significant (i.e. 2-200-fold) increases in hGH production. Zinc-induced modulation of hGH production was demonstrated in cells in continuous passage and stationary culture. Regulation was also demonstrated after ex vivo gene transfer by both the intraperitoneal administration of zinc ions or the systemic administration of dexamethasone.

CONCLUSIONS

Our results demonstrate the modulation of transgene expression in MCs in vitro and in vivo, and suggest the potential for the regulation of gene expression in a genetically modified mesothelium that may ultimately be used for the delivery of therapeutic proteins to maintain peritoneal membrane viability in the peritoneal dialysis patient.

Enhancement of the functional repertoire of the rat parietal peritoneal mesothelium in vivo: directed expression of the anticoagulant and antiinflammatory molecule thrombomodulin

We have used our previously described ex vivo mesothelial cell (MC)-mediated gene therapy strategy (Gene Ther. 2:393-401, 1995) to modify the functional properties of the rat parietal peritoneal mesothelium in vivo by expression of a membrane-bound recombinant protein on the MC surface. Rat primary MCs were stably transfected (using strontium phosphate DNA coprecipitation) with a plasmid containing the gene for rat thrombomodulin (TM), a transmembrane glycoprotein that functions as an essential cofactor for the physiological activation of the anticoagulant protein C by the enzyme thrombin. As demonstrated by immunohistochemistry and by direct equilibrium binding with radiolabeled thrombin, genetically modified MCs expressed high levels of TM antigen on their surface in vitro. As judged by a thrombin-dependent protein C activation assay, such MC membrane-bound TM was biologically active. Once reseeded on the denuded parietal peritoneal surface of syngeneic recipients, these TM-transfected MCs continued to express TM antigen in vivo for at least 90 days. Moreover, the recombinant TM expressed on the reconstituted parietal mesothelium retained its ability to activate protein C in a thrombin-dependent manner. Our data indicate that MC-mediated expression of TM can be used to augment the anticoagulant properties of the parietal peritoneal surface. In general, our results suggest that ex vivo MC-mediated gene therapy can be used to deliver other therapeutic transmembrane proteins to the MC surface to enhance the functional repertoire of the parietal mesothelium in vivo.

Neutrophils emigrate from venules by a transendothelial cell pathway in response to FMLP

Circulating leukocytes are thought to extravasate from venules through open interendothelial junctions. To test this paradigm, we injected N-formyl-methionyl-leucyl-phenylalanine (FMLP) intradermally in guinea pigs, harvesting tissue at 5-60 min. At FMLP-injected sites, venular endothelium developed increased surface wrinkling and variation in thickness. Marginating neutrophils formed contacts with endothelial cells and with other neutrophils, sometimes forming chains of linked leukocytes. Adherent neutrophils projected cytoplasmic processes into the underlying endothelium, especially at points of endothelial thinning. To determine the pathway by which neutrophils transmigrated endothelium, we prepared 27 sets of serial electron microscopic sections. Eleven of these encompassed in their entirety openings through which individual neutrophils traversed venular endothelium; in 10 of the 11 sets, neutrophils followed an entirely transendothelial cell course unrelated to interendothelial junctions, findings that were confirmed by computer-assisted three-dimensional reconstructions. Having crossed endothelium, neutrophils often paused before crossing the basal lamina and underlying pericytes that they also commonly traversed by a transcellular pathway. Thus, in response to FMLP, neutrophils emigrated from cutaneous venules by a transcellular route through both endothelial cells and pericytes. It remains to be determined whether these results can be extended to other inflammatory cells or stimuli or to other vascular beds.

Vascular permeability factor/vascular endothelial growth factor-mediated signaling in mouse mesentery vascular endothelium

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is a multifunctional cytokine and growth factor that has important roles in both pathological and physiological angiogenesis. VPF/VEGF induces vascular hyperpermeability, cell division, and other activities by interacting with two specific receptor tyrosine kinases, KDR/Flk-1 and Flt-1, that are selectively expressed on vascular endothelium. The signaling cascade that follows VPF/VEGF interaction with cultured endothelium is only partially understood but is known to result in increased intracellular calcium, activation of protein kinase C, and tyrosine phosphorylations of both receptors, phospholipase C-gamma (PLC-gamma) and phosphatidylinositol 3'-kinase. For many reasons, signaling events elicited in cultured endothelium may not mimic mediator effects on intact normal or tumor-induced microvessels in vivo. Therefore, we developed a system that would allow measurement of VPF/VEGF-induced signaling on intact microvessels. We used mouse mesentery, a tissue whose numerous microvessels are highly responsive to VPF/VEGF and that we found to express Flk-1 and Flt-1 selectively. At intervals after injecting VPF/VEGF i.p., mesenteries were harvested, extracted, and immunoprecipitated. Immunoblots confirmed that VPF/VEGF induced tyrosine phosphorylation of several proteins in mesenteric microvessels as in cultured endothelium: Flk-1; PLC-gamma; and mitogen-activated protein kinase. Similar phosphorylations were observed when mesentery was exposed to VPF/VEGF in vitro, or when mesenteries were harvested from mice bearing the mouse ovarian tumor ascites tumor, which itself secretes abundant VPF/VEGF. Other experiments further elucidated the VPF/VEGF signaling pathway, demonstrating phosphorylation of both PYK2 and focal adhesion kinase, activation of c-jun-NH2-kinase with phosphorylation of c-Jun, and an association between Flk-1 and PLC-gamma. In addition, we demonstrated translocation of mitogen-activated protein kinase to the cell nucleus in cultured endothelium. Taken together, these experiments describe a new model system with the potential for investigating signaling events in response to diverse mediators on intact microvessels in vivo and have further elucidated the VPF/VEGF signaling cascade.

Reinterpretation of endothelial cell gaps induced by vasoactive mediators in guinea-pig, mouse and rat: many are transcellular pores

1. In response to vascular permeabilizing agents, particulates circulating in the blood extravasate from venules through endothelial cell openings. These openings have been thought to be intercellular gaps though recently this view has been challenged. 2. To define the precise location of endothelial cell gaps, serial section electron microscopy and three-dimensional reconstructions were performed in skin and cremaster muscle of guinea-pigs, mice and rats injected locally with agents that enhance microvascular permeability: vascular permeability factor, histamine or serotonin. Ferritin and colloidal carbon were injected intravenously as soluble and particulate macromolecular tracers, respectively. 3. Both tracers extravasated from venules in response to all three permeability enhancing agents. The soluble plasma protein ferritin extravasated primarily by way of vesiculo-vacuolar organelles (VVOs), interconnected clusters of vesicles and vacuoles that traverse venular endothelium. In contrast, exogenous particulates (colloidal carbon) and endogenous particulates (erythrocytes, platelets) extravasated from plasma through transendothelial openings. 4. Serial electron microscopic sections and three-dimensional reconstructions demonstrated that eighty-nine of ninety-two openings were transendothelial pores, not intercellular gaps. Pore frequency increased 3- to 33-fold when carbon was used as tracer. 5. The results demonstrate that soluble and particulate tracers extravasate from venules by apparently different transcellular pathways in response to vasoactive mediators. However, some pores may derive from rearrangements of VVOs.

Vascular permeability factor/vascular endothelial growth factor: a multifunctional angiogenic cytokine

VPF/VEGF is a multifunctional cytokine that contributes to angiogenesis by both direct and indirect mechanisms. On the one hand, VPF/VEGF stimulates the endothelial cells lining nearby microvessels to proliferate, to migrate and to alter their pattern of gene expression. On the other hand, VPF/VEGF renders these same microvascular endothelial cells hyperpermeable so that they spill plasma proteins into the extravascular space, leading to profound alterations in the extracellular matrix that favor angiogenesis. These same principles apply in tumors, in several examples of non-neoplastic pathology, and in physiological processes that involve angiogenesis and new stroma generation. In all of these examples, microvascular hyperpermeability and the introduction of a provisional, plasma-derived matrix precede and accompany the onset of endothelial cell division and new blood vessel formation. It would seem, therefore, that tumors have made use of fundamental pathways that developed in multicellular organisms for purposes of tissue defense, renewal and repair. VPF/VEGF, therefore, has taught us something new about angiogenesis; namely, that vascular hyperpermeability and consequent plasma protein extravasation are important--perhaps essential--elements in its generation. However, this finding raises a paradox. While VPF/VEGF induces vascular hyperpermeability, other potent angiogenic factors apparently do not, at least in sub-toxic concentrations that are more than sufficient to induce angiogenesis (Connolly et al., 1989a). Nonetheless, wherever angiogenesis has been studied, the newly generated vessels have been found to be hyperpermeable. How, therefore, do angiogenic factors other than VPF/VEGF lead to the formation of new and leaky blood vessels? We do not as yet have a complete answer to this question. One possibility is that at least some angiogenic factors mediate their effect by inducing or stimulating VPF/VEGF expression. In fact, there are already clear example of this. A number of putative angiogenic factors including small molecules (e.g. prostaglandins, adenosine) as well as many cytokines (e.g. TGF-alpha, bFGF, TGF-beta, TNF-alpha, KGF, PDGF) have all been shown to upregulate VPF/VEGF expression. Further studies that elucidate the crosstalk among various angiogenic factors are likely to contribute significantly to a better understanding of the mechanisms by which new blood vessels are formed in health and in disease.

Increased hyaluronan at sites of attachment to mesentery by CD44-positive mouse ovarian and breast tumor cells

The mouse ovarian ascites tumor, MOT, and mammary ascites tumor, TA3/St, served as models to follow changes in hyaluronan levels during tumor growth, attachment, and invasion. Subsequent to introduction of tumor cells into the peritoneal cavity, hyaluronan accumulated intraperitoneally and at the initial sites of attachment of tumor cells and cell clumps to the mesenteric surface; the latter co-localized with sites of fibrin deposition as reported earlier. Subsequently, high levels of hyaluronan accumulated throughout the interior of the mesentery. Because neither tumor cell line synthesized substantial amounts of hyaluronan in culture, the large accumulations observed in the mesenteries and ascites fluid of tumor-bearing animals most likely resulted from increased synthesis and secretion by peritoneal-lining mesothelial cells and/or fibroblasts in response to stimulation by the tumor cells or their products. TA3/St tumor cells were universally positive for the hyaluronan receptor, CD44, whereas approximately 90% of MOT tumor cells were CD44-negative. However, the great majority of MOT or TA3/St cells that initially attached to the mesentery were strongly CD44 positive. We propose that hyaluronan-rich matrix is involved in tumor cell attachment to the mesentery possibly via interaction with tumor cell surface CD44.

Vesiculo-vacuolar organelles and the regulation of venule permeability to macromolecules by vascular permeability factor, histamine, and serotonin

In contrast to normal microvessels, those that supply tumors are strikingly hyperpermeable to circulating macromolecules such as plasma proteins. This leakiness is largely attributable to a tumor-secreted cytokine, vascular permeability factor (VPF). Tracer studies have shown that macromolecules cross tumor vascular endothelium by way of a recently described cytoplasmic organelle, the vesiculo-vacuolar organelle or VVO (VVOs are grapelike clusters of interconnected, uncoated vesicles and vacuoles). However, equivalent VVOs are also present in the cytoplasm of normal venules that do not leak substantial amounts of plasma protein. To explain these findings, we hypothesized that VPF increased the permeability of tumor blood vessels by increasing VVO function and that the VVOs of normal venules were relatively impermeable in the absence of VPF stimulation. To test this hypothesis, VPF was injected intradermally in normal animals after intravenous injection of a soluble macromolecular tracer, ferritin, whose extravasation could be followed by electron microscopy. VPF caused normal venules to leak ferritin, and, as predicted by our hypothesis, ferritin extravasated by way of VVOs, just as in hyperpermeable tumor microvessels. Ultrathin (14-nm) serial electron microscopic sections and computer-aided three-dimensional reconstructions better defined VVO structure. VVOs occupied 16-18% of endothelial cytoplasm in normal venules. Individual VVOs were clusters of numerous (median, 124) interconnected vesicles and vacuoles that formed complex pathways across venular endothelium with multiple openings to both luminal and abluminal surfaces. Like VPF, histamine and serotonin also stimulated ferritin extravasation across venules by way of VVOs. Together, these data establish VVOs as the major pathway by which soluble plasma proteins exit venules in response to several mediators that increase venular hyperpermeability. These same mediators also increased the extravasation of colloidal carbon, but this large particulate nonphysiological tracer exited venules primarily through endothelial gaps.

The vesiculo-vacuolar organelle (VVO): a distinct endothelial cell structure that provides a transcellular pathway for macromolecular extravasation

The vesiculo-vacuolar organelle (VVO) is a recently described organelle found in the cytoplasm of endothelial cells that line tumor microvessels and normal venules. VVOs are grape-like clusters of interconnecting uncoated vesicles and vacuoles, bounded by trilaminar unit membranes, that span the entire thickness of vascular endothelium, thereby providing a potential trans-endothelial connection between the vascular lumen and the extravascular space. Macromolecular tracers preferentially cross hyperpermeable tumor microvessels through VVOs. The present investigation was undertaken to elucidate further the ultrastructure and function of VVOs in a murine ovarian carcinoma (MOT) and in normal venules. Morphometry revealed that VVOs were enormous cytoplasmic structures (median area, 0.12-0.14 microns2 in single electron micrographs). Moreover, the individual vesicles and vacuoles that comprised VVOs were on average substantially larger than capillary caveolae and followed a non-normal distribution that was skewed to the right. Specimen tilting provided conclusive evidence that individual VVO vesicles and vacuoles communicated with each other and with the endothelial cells' plasma membranes by stomata, some of which were closed by diaphragms composed of a single membrane. Studies with two tracers, ferritin (FE, diameter approximately 11 nm) and horseradish peroxidase (HRP, diameter approximately 5 nm), revealed that passage of macromolecules through VVOs was regulated at the level of stomatal diaphragms, thereby demonstrating a mechanism for controlling the passage of macromolecules across endothelial cells. Thus, compared with tumor microvessels, little circulating FE and HRP entered the VVOs of normal venular endothelium because stomata joining vesicles and vacuoles to each other and to the lumen and ablumen were closed. VVOs and their component vesicles/vacuoles were readily distinguished from endosomal organelles such as coated vesicles and multivesicular bodies, which also accumulated FE and HRP. Our findings indicate that VVOs provide a major pathway for the extravasation of circulating macromolecules across endothelia taller than capillary endothelium and suggest that upregulated VVO function accounts for the well-known hyperpermeability of tumor blood vessels.

Systemic delivery of a recombinant protein by genetically modified mesothelial cells reseeded on the parietal peritoneal surface

To evaluate the ability of genetically modified peritoneal mesothelial cells to deliver recombinant proteins to the systemic circulation, we used our previously described mesothelial cell-based ex vivo gene therapy strategy. Rat primary peritoneal mesothelial cells, isolated from parietal peritoneum by enzymatic digestion, were stably transfected (using strontium phosphate DNA co-precipitation) with the plasmid pSVTKgh to express a secreted reporter gene product, human growth hormone (hgh). Such hgh-secreting mesothelial cells were reseeded on the denuded peritoneal surface of syngeneic recipients and delivery of the reporter gene product to the systemic circulation was monitored by analysis of serum samples for the presence of hgh at various times after mesothelial cell implantation. Polymerase chain reaction (PCR) analysis demonstrated that the hgh-transfected mesothelial cells repopulated the denuded areas and remained attached there for at least 12 weeks. Moreover, these genetically modified mesothelial cells continued to express the reporter gene product in vivo and secreted hgh in sufficient quantity to be detected in the systemic circulation (ie statistically significant amounts of hgh could be measured in the serum of cyclosporine A-treated rats for at least 2 months; Mann-Whitney test, P < 0.05). Our results demonstrate the successful, sustained, systemic delivery of a recombinant protein by genetically modified peritoneal mesothelial cells following their reattachment to the peritoneal surface, and suggest the potential of ex vivo mesothelial cell-mediated gene therapy for the treatment of inherited or acquired disorders requiring delivery of therapeutic proteins to the circulation.

Mesothelial cell-mediated gene therapy: feasibility of an ex vivo strategy

We have developed a model system in the rat to test the feasibility of recombinant protein expression by genetically modified peritoneal mesothelial cells following autologous peritoneal implantation. Rat primary peritoneal mesothelial cells, isolated from parietal peritoneum by enzymatic digestion, were stably transduced (using a Moloney murine leukemia virus (MoMLV)-derived retroviral vector, BAG, expressing the Escherichia coli lacZ gene) to mark the cells with a reporter protein (beta-galactosidase, beta-gal). Such transduced mesothelial cells, tagged with DiO, a fluorescent lipophilic dye used for long-term tracing of transplanted cells, were then reseeded on the denuded peritoneal surface of syngeneic recipients. DiO-labeled, BAG-transduced mesothelial cells were observed to repopulate the denuded areas and remain attached there for > 90 days. Moreover, these genetically modified mesothelial cells continued to express the reporter gene product in vivo (ie beta-gal activity was present for at least 1 month). Our results demonstrate the feasibility of ex vivo gene therapy using peritoneal mesothelial cells.

Keratinocyte-derived vascular permeability factor (vascular endothelial growth factor) is a potent mitogen for dermal microvascular endothelial cells

Expression of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is markedly increased in the epidermis of lesional psoriatic skin and in healing skin wounds. In this study, we characterized the effects of several cytokines and growth factors on the expression and secretion of VPF/VEGF mRNA and protein by cultured human epidermal keratinocytes, as well as the effect of VPF/VEGF on the growth of cultured human dermal microvascular endothelial cells. Transforming growth factor-alpha, epidermal growth factor, and phorbol myristate acetate markedly stimulated VPF/VEGF mRNA expression by cultured keratinocytes; as in psoriatic skin, the three most common VPF/VEGF isoforms (encoding proteins of 121, 165, and 189 amino acids) were upregulated to an equal extent. Transforming growth factor (TGF)-alpha, epidermal growth factor, and phorbol myristate acetate also enhanced the secretion of VPF/VEGF by keratinocytes; in contrast, a number of other cytokines including interleukin (IL)-1, IL-6, IL-8, tumor necrosis factor-alpha, interferon-gamma, and transforming growth factor-beta did not induce VPF/VEGF secretion. The VPF/VEGF secreted by keratinocytes was biologically active in that, like recombinant human VPF/VEGF, it potently stimulated dermal endothelial cell proliferation. Scatchard analysis revealed two high-affinity VPF/VEGF binding sites on dermal endothelial cells with dissociation constants of 51 pM and 2.9 pM. These results suggest that the avascular epidermis has the capacity to regulate dermal angiogenesis and microvascular permeability by a paracrine mechanism involving the secretion of VPF/VEGF. Similar mechanisms may be anticipated in a variety of inflammatory and neoplastic skin diseases characterized by microvascular hyperpermeability, edema, and angiogenesis.

Vascular permeability factor/vascular endothelial growth factor: an important mediator of angiogenesis in malignancy and inflammation

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a multifunctional cytokine that is overexpressed in many transplantable animal and autochtonous human cancers, in healing wounds, and in chronic inflammatory disorders such as psoriasis and rheumatoid arthritis. All of these entities are characterized by angiogenesis, altered extracellular matrix, and variable degrees of hypoxia. In addition, two VPF/VEGF receptors, flt-1 and kdr, are overexpressed by endothelial cells that line the microvessels that supply these tumors/inflammatory reactions. On the basis of these and other data, we have proposed a model of angiogenesis in which VPF/VEGF plays a central role: this model is applicable to tumors and also to the angiogenesis that occurs in non-neoplastic processes.

Ultrastructural localization of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) to the abluminal plasma membrane and vesiculovacuolar organelles of tumor microvascular endothelium

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is a cytokine secreted by many animal and human tumors, activated macrophages, keratinocytes, rheumatoid synovial cells, embryonic tissues, and by cultured epithelial and mesenchymal cell lines. It acts selectively on vascular endothelial cells to increase their permeability to circulating macromolecules and to stimulate their replication. Although not detectably expressed by vascular cells in the human and animal tumors we have studied, VPF/VEGF accumulates in the microvessels supplying tumors and certain inflammatory reactions in which VPF/VEGF is also overexpressed. Light microscopic immunohistochemistry lacked the resolution necessary to localize VPF/VEGF precisely in such vessels. Therefore, we used a pre-embedding immunocytochemical method to localize VPF/VEGF at the ultrastructural level in the new blood vessels that are elicited in the peritoneal walls of mice bearing a transplantable mouse ascites tumor of ovarian origin. Intense immunostaining for VPF/VEGF was observed on the abluminal plasma membrane of tumor-associated microvascular endothelial cells and in vesiculovacuolar organelles (VVOs) present in these same endothelial cells. (VVOs are recently described cytoplasmic organelles present in tumor vascular endothelium that provide an important pathway for extravasation of circulating macromolecules.) In contrast to labeling of the abluminal plasma membrane and VVO vesicles and vacuoles, endothelial cytoplasmic organelles, such as multivesicular bodies and Weibel-Palade bodies, and the underlying basal lamina, did not stain with antibodies to VPF/VEGF. The distribution of VPF/VEGF here described corresponds to that anticipated for high-affinity VFP/VEGF receptors, although binding of VPF/VEGF to other endothelial cell surface structures, such as plasma membrane proteoglycans, is also a possibility.

Pathogenesis of ascites tumor growth: vascular permeability factor, vascular hyperpermeability, and ascites fluid accumulation

Previous studies have shown that accumulation of tumor ascites fluid results in large part from increased permeability of peritoneal lining vessels (Nagy et al., Cancer Res., 49: 5449-5458, 1989; Nagy et al., Cancer Res., 53: 2631-2643, 1993). However, the specific microvessels rendered hyperpermeable have not been identified nor has the basis of peritoneal vascular hyperpermeability been established. To address these questions, TA3/St and MOT carcinomas, well-characterized transplantable murine tumors that grow in both solid and ascites form, were studied as model systems. Ascites tumor cells of either type were injected i.p. into syngeneic A/Jax and C3Heb/FeJ mice, and ascites fluid and plasma were collected at intervals thereafter up to 8 and 28 days, respectively. Beginning several days after tumor cell injection, small blood vessels located in tissues lining the peritoneal cavity (mesentery, peritoneal wall, and diaphragm) became hyperpermeable to several macromolecular tracers (125I-human serum albumin, FITC-dextran, colloidal carbon, and Monastral Blue B). Increased microvascular permeability correlated with the appearance in ascites fluid of vascular permeability factor (VPF), a tumor cell-secreted mediator that potently enhances vascular permeability to circulating macromolecules. VPF was measured in peritoneal fluid by both a functional bioassay and a sensitive immunofluorometric assay. The VPF concentration, total peritoneal VPF, ascites fluid volume, tumor cell number, and hyperpermeability of peritoneal lining microvessels were found to increase in parallel over time. The close correlation of peritoneal fluid VPF concentration with the development of hyperpermeable peritoneal microvessels in these two well-defined ascites tumors suggests that VPF secretion by tumor cells is responsible, in whole or in part, for initiating and maintaining the ascites pattern of tumor growth.

Pathogenesis of ascites tumor growth: angiogenesis, vascular remodeling, and stroma formation in the peritoneal lining

In the accompanying papers, we demonstrated that two murine ascites tumors (MOT and TA3/St) induced peritoneal lining blood vessels to become hyperpermeable to plasma proteins, leading to extravasation of fibrinogen and its clotting to cross-linked fibrin in peritoneal lining tissues (peritoneal wall, mesentery, and diaphragm). In solid tumors, vascular hyperpermeability and fibrin deposition lead to the generation of vascularized connective tissue. In order to determine whether fibrin had similar consequences in ascites tumors, the vasculature and stroma of peritoneal lining tissues were analyzed at successive intervals after i.p. tumor cell injection. In both MOT and TA3/St ascites tumors, the size and number of peritoneal lining microvessels increased significantly by 5-8 days. Subsequently, peritoneal lining vessels increased in cross-sectional area by as much as 15-fold and peritoneal vascular frequency increased by up to 11-fold. Incorporation of [3H]thymidine by mesenteric blood vessels was negligible in control animals but came to involve 20 and 40% of endothelial cells lining mesenteric vessels in MOT and TA3/St ascites tumor-bearing mice, respectively. After an early dramatic increase in cross-sectional area, peritoneal lining microvessels subsequently underwent a novel form of remodeling to smaller average size as the result of transvascular bridging by endothelial cell cytoplasmic processes. Thus, both of the ascites tumors studied here induced angiogenesis and stroma similar to that elicited when these same tumors were grown in solid form. However, stroma developed more slowly in ascites than in solid tumors and was entirely confined to a compartment (peritoneal lining tissues) that was distinct from that (peritoneal cavity) containing the majority of tumor cells and ascites fluid. These findings are consistent with the hypothesis that vascular hyperpermeability, induced in both solid and ascites tumors by tumor cell-secreted vascular permeability factor, is a common early step in tumor angiogenesis, resulting in fibrinogen extravasation, fibrin deposition, and likely other alterations of the extracellular matrix that together stimulate new vessel and fibroblast ingrowth.

Pathogenesis of ascites tumor growth: fibrinogen influx and fibrin accumulation in tissues lining the peritoneal cavity

In the immediately preceding paper, we demonstrated that the microvasculature supplying peritoneal lining tissues of mice bearing either of two transplantable ascites carcinomas was hyperpermeable to circulating macromolecules. Solid tumors have been shown to exhibit similar levels of microvascular hyperpermeability, leading to extravasation of plasma proteins, including fibrinogen which clots on extravasation to form an extravascular fibrin gel. To determine whether similar extravasation and clotting of plasma fibrinogen occurred in ascites tumors, we used 125I-labeled fibrinogen (125I-F) as a tracer to measure inflow of fibrinogen into the peritoneal cavities, and influx and accumulation of fibrinogen/fibrin in the peritoneal lining tissues (peritoneal wall, mesentery, and diaphragm) of mice bearing syngeneic TA3/St or MOT ascites tumors. The percentage of circulating 125I-F that extravasated into the peritoneal cavity was increased from 10- to 50-fold in mice bearing either ascites tumor. Influx into the peritoneal walls of ascites tumor-bearing mice was 3-7 times that of control mice and became maximal on day 8 (TA3/St) and day 15 (MOT). Accumulation of 125I-F in ascites fluid and peritoneal lining tissues was also increased substantially in mice bearing these ascites tumors, reaching maximal values on days 7-8 (TA3/St) and 19-29 (MOT) at levels 2- to 3-fold (peritoneal wall) and 33- to 148-fold (ascites fluid) above control levels. Significant amounts of the 125I-F that accumulated in the peritoneal lining tissues of ascites tumor-bearing animals were insoluble in 3 M urea, consistent with clotting of 125I-F to cross-linked fibrin. Autoradiographs of SDS-PAGE gels performed on extracts of peritoneal lining tissues of both ascites tumors revealed the characteristic signature of cross-linked fibrin, i.e., gamma-gamma dimers and alpha-polymers. Fibrin was also identified in peritoneal lining tissues of both ascites tumors by immunohistochemistry. Taken together, these data indicate that fibrinogen, like other circulating macromolecules, extravasates into the peritoneal cavity and peritoneal lining tissues of ascites tumor-bearing mice and does so with kinetics similar to those of other macromolecular tracers we have studied. Moreover, a portion of the fibrinogen that extravasated into peritoneal lining tissues clotted to form a cross-linked fibrin meshwork which trapped tumor cells and favored their attachment to the peritoneal surface. By analogy with solid tumors, such fibrin deposits may also be expected to have a role in initiating angiogenesis and the generation of mature tumor stroma.

Induction of vascular endothelial growth factor expression by prostaglandin E2 and E1 in osteoblasts

PGE1 and PGE2 are potent stimulators of bone formation. Osteogenesis is strongly dependent on angiogenesis. Vascular endothelial growth factor (VEFG), a secreted endothelial cell-specific mitogen, has been implicated in physiological and pathological angiogenesis. The aim of this study was to examine the possible role of VEGF in PG stimulation of bone formation. We found that in rat calvaria-derived osteoblast-enriched cells and in the osteoblastic RCT-3 cell line PGE2 and E1 increased VEGF mRNA and protein levels. The increased expression of VEGF mRNA produced by PGE2 was rapid (maximal at 1 h), transient (declined by 3 h), potentiated by cycloheximide, and abolished by actinomycin D. PGE2 had no effect on VEGF mRNA stability, suggesting transcriptional regulation of VEGF expression by PGF2. Rp-cAMP, a cAMP antagonist, suppressed VEGF mRNA induced by PGE2, indicating cAMP mediation. The upregulation of VEGF expression by PGE2 in the preosteoblastic RCT-1 cells was potentiated by treatment with retinoic acid, which induces the differentiation of these cells. The upregulation of VEGF mRNA by PGE2 was inhibited by dexamethasone treatment. In addition, Northern blot analysis showed that VEGF mRNA is expressed in adult rat tibia. In summary, we documented, for the first time, the expression of VEGF in osteoblasts and in bone tissue. Stimulation of VEGF expression by PGs and its suppression by glucocorticoids, which, respectively, stimulate and suppress bone formation, strongly implicate the involvement of VEGF in bone metabolism.

Compartmental distribution of tumor-specific monoclonal antibodies in human melanoma xenografts

Monoclonal antibodies (MAb) are attractive for tumor therapy because of their exquisite specificity. Although a majority of tumor cells in small (< or = 20 mg) solid tumors can be labeled following systemic administration of antitumor cell MAbs, little quantitative information is available as to the distribution of these MAbs within the several compartments that comprise solid tumors. Our goal was to provide such data in a well-characterized melanoma xenograft system. In accord with earlier work, i.v.-injected, melanoma-specific MAbs 436 and IND1, directed, respectively, against the 125 kD and HMW-melanoma-associated antigens, accumulated in M21 and SK-MEL-2 tumor xenografts in amounts of approximately 20% of injected dose/g. However, only 20-24% of the MAbs present in tumor xenografts was bound to tumor cells; the great majority (76-80%) was in the tumor extracellular fluid (ECF) and collagenous residue fractions. These results could not be accounted for by MAb degradation or release of MAbs from tumor cells during xenograft dissociation. Rather, they reflected in large part interactions of MAbs with antigens which tumors had shed into the ECF. Thus, 48 h after i.v. injection of 20 micrograms of melanoma-specific, biotin-tagged MAb, 46-66% of that present in the tumor ECF was complexed with melanoma-associated antigens. Overall, 61-73% of the MAbs recovered from tumor xenografts were bound to tumor antigens (either to tumor cells themselves or to tumor-shed antigens). In contrast, only approximately 4% of a melanoma-nonspecific MAb (B72.3) accumulated per g tumor after i.v. injection and nearly all of this was free in the ECF. Consistent with these data, fluorescence microscopy revealed that i.v.-injected, fluorescein-tagged MAbs achieved highest concentrations in tumor stroma, particularly at the tumor-host interface. Flow cytometry of dissociated solid tumors revealed that both the fraction of MAb-labeled tumor cells and the amount of MAb/tumor cell could be increased by increasing the administered i.v. dose of melanoma-specific MAb. Nonetheless, even at the highest i.v. injected dose (300 micrograms), 15-37% of tumor cells lacked detectable MAb labeling. Taken together, the data indicate that delivery of tumor cell-specific MAbs to solid tumors cannot be equated with their delivery to tumor cells. This distinction is important for immunotherapeutic approaches that require MAb contact with tumor cells.

Vascular permeability factor (VPF, VEGF) in tumor biology

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a multifunctional cytokine expressed and secreted at high levels by many tumor cells of animal and human origin. As secreted by tumor cells, VPF/VEGF is a 34-42 kDa heparin-binding, dimeric, disulfide-bonded glycoprotein that acts directly on endothelial cells (EC) by way of specific receptors to activate phospholipase C and induce [Ca2+]i transients. Two high affinity VPF/VEGF receptors, both tyrosine kinases, have thus far been described. VPF/VEGF is likely to have a number of important roles in tumor biology related, but not limited to, the process of tumor angiogenesis. As a potent permeability factor, VPF/VEGF promotes extravasation of plasma fibrinogen, leading to fibrin deposition which alters the tumor extracellular matrix. This matrix promotes the ingrowth of macrophages, fibroblasts, and endothelial cells. Moreover, VPF/VEGF is a selective endothelial cell (EC) growth factor in vitro, and it presumably stimulates EC proliferation in vivo. Furthermore, VPF/VEGF has been found in animal and human tumor effusions by immunoassay and by functional assays and very likely accounts for the induction of malignant ascites. In addition to its role in tumors, VPF/VEGF has recently been found to have a role in wound healing and its expression by activated macrophages suggests that it probably also participates in certain types of chronic inflammation. VPF/VEGF is expressed in normal development and in certain normal adult organs, notably kidney, heart, adrenal gland and lung. Its functions in normal adult tissues are under investigation.

Vascular permeability factor (vascular endothelial growth factor) in guinea pig and human tumor and inflammatory effusions

Vascular permeability factor (VPF), also known as vascular endothelial growth factor, is a dimeric M(r) 34,000-42,000 glycoprotein that possesses potent vascular permeability-enhancing and endothelial cell-specific mitogenic activities. It is synthesized by many rodent and human tumor cells and also by some normal cells. Recently we developed a sensitive and specific time-resolved immunofluorometric assay for quantifying VPF in biological fluids. We here report findings with this assay in guinea pigs and patients with both malignant and nonmalignant effusions. Line 1 and line 10 tumor cells were injected into the peritoneal cavities of syngeneic strain 2 guinea pigs, and ascitic fluid, plasma, and urine were collected at various intervals. Within 2 to 4 days, we observed a time-dependent, parallel increase in VPF, ascitic fluid volume, and tumor cell numbers in animals bearing either tumor line; in contrast, VPF was not detected in plasma or urine, even in animals with extensive tumor burdens. However, low levels of VPF were detected in the inflammatory ascites induced by i.p. oil injection. In human studies, high levels of VPF (> 10 pM) were measured in 21 of 32 effusions with cytology-documented malignant cells and in only seven of 35 effusions without cytological evidence of malignancy. Thus, VPF levels in human effusions provided a diagnostic test for malignancy with a sensitivity of 66% and a specificity of 80% (perhaps as high as 97% in that six of the seven cytology-negative patients with VPF levels > 10 pM had cancer as determined by other criteria). As in the animal tumor models, VPF was not detected in serum or urine obtained from patients with or without malignant ascites. Many nonmalignant effusions contained measurable VPF but, on average, in significantly smaller amounts than were found in malignant effusions. VPF levels in such fluids correlated strongly (p = 0.59, P < 0.001) with monocyte and macrophage content. Taken together, these data relate ascitic fluid accumulation to VPF concentration in a well-defined animal tumor system and demonstrate, for the first time, the presence of VPF in human malignant effusions. It is likely that VPF expression by tumor and mononuclear cells contributes to the plasma exudation and fluid accumulation associated with malignant and certain inflammatory effusions. The VPF assay may prove useful for cancer diagnosis as a supplement to cytology, especially in tumors that grow in the pleural lining but not as a suspension in the effusions that they induce.

Pathogenesis of malignant ascites formation: initiating events that lead to fluid accumulation

Initiating events leading to the accumulation of malignant ascites in the peritoneal cavity were investigated in two syngeneic transplantable murine ascites-producing tumors, MOT mouse ovarian tumor and the TA3/St mammary carcinoma. The transport of two tracers, 125I-labeled human serum albumin (125I-HSA) and 51Cr-labeled red blood cells (51Cr-RBC), into and out of the peritoneal cavity was studied at early times after i.p. tumor cell injection, prior to abundant fluid accumulation, and at intervals of 5 to 360 min after i.v. or i.p. tracer injection. Tracer influx and efflux rates were estimated from the mass of tracer passing into or out of the peritoneal cavity following a bolus injection of tracer into either the blood or the peritoneal cavity. Efflux of 125I-HSA from the peritoneal cavity was markedly reduced (3- to 5-fold) within 1 day of i.p. injection of either type of tumor cell. Significantly reduced efflux preceded any increase in tumor cell number and by itself did not induce peritoneal fluid accumulation. 125I-HSA tracer influx from plasma to peritoneal fluid did not increase detectably until 5 to 7 days after tumor cell injection, when the tumor cell number had increased by 10- to 100-fold. Only at relatively late stages of ascites tumor growth, when the flow rate into the peritoneal cavity had increased relative to the flow rate out of the peritoneum, was there net peritoneal fluid accumulation. Thus, increased influx, in addition to impaired efflux, were required for malignant ascites accumulation. Following i.p. injection, the efflux rates of 125I-HSA always exceeded those of 51Cr-RBC, even in ascites tumor-bearing animals. Furthermore, 125I-HSA tracer disappeared from the peritoneal cavity more rapidly than it appeared in the plasma, suggesting that 125I-HSA moves more rapidly through the channels by which 51Cr-RBC egress from the peritoneum (primarily diaphragmatic lymphatics) and/or has access to additional pathways not open to 51Cr-RBC. Finally, flow rates into and out of the blood and peritoneum were used to obtain kinetic parameters that characterized tracer transport: k1, the rate constant for tracer transport from the blood to the peritoneum; k2, the rate constant for tracer transport from the peritoneal cavity to the blood; and k6, the rate constant for tracer transport from the peritoneal cavity to surrounding interstitial tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathways of macromolecular tracer transport across venules and small veins. Structural basis for the hyperpermeability of tumor blood vessels

BACKGROUND

Blood vessels supplying tumors are hyperpermeable to macromolecules, but the mechanisms responsible are poorly understood.

EXPERIMENTAL DESIGN

To investigate the structural basis for the leakiness of tumor blood vessels, we performed a transmission electron microscopic study of three syngeneic transplantable carcinomas (mouse ovarian carcinoma and the line 1 and line 10 bile duct guinea pig carcinomas) at early intervals after intravenous injection of several macromolecular tracers. Tracers with widely differing physical properties were studied: horseradish peroxidase, ferritin, 150 kilodalton fluorescein isothiocyanate-dextran and gold-bovine serum albumin.

RESULTS

All tracers leaked primarily from venules and small veins at the tumor-host interface, for the most part vessels lined by a continuous endothelium. The predominant pathway by which all four tracers exited venules in all three tumors was by way of a system of smooth membrane-bound, interconnecting vesicles and vacuoles; these tended to cluster together at irregular intervals in the endothelial cell cytoplasm to form organelle-like structures, vesiculo-vacuolar organelles (VVO). In favorable sections, VVO interfaced with both the luminal and abluminal surfaces of endothelial cells. HRP alone crossed venules and small veins through apposed inter-endothelial cell junctions. Tracers also exited vessels by way of endothelial fenestrae where these occurred (rarely) in mouse ovarian tumor-associated venules. VVO occurred with similar frequency and complexity in the continuous endothelium-lined venules and small veins that supplied the normal subcutis of either tumor-bearing or control animals. As in tumor-associated vessels, VVO provided the predominant pathway by which all four tracers exited normal vessels, but VVO labeling and extravasation were both much greater in tumor than in control vessels (p < 0.001 for ferritin).

CONCLUSIONS

VVO are prominent structures in both tumor-supplying and control vessel endothelial cells and provide the primary pathway for macromolecular extravasation. The large increase in permeability characteristic of tumor vessels is likely attributable to upregulation of VVO function.

Spatial distribution of tumor-specific monoclonal antibodies in human melanoma xenografts

The time-dependent (1-72-h) spatial distribution of three biotinylated anti-melanoma monoclonal antibodies (MAbs), a control MAb, and several macromolecular tracers was studied in two small (4-12-mg), well-characterized human melanoma xenografts (SK-MEL-2, M21) growing in the s.c. space of athymic nude mice. The specific MAbs (436, IND1, and 9.2.27) recognize two different melanoma cell surface antigens (Mr 125,000 glycoprotein melanoma-associated antigen and high molecular weight melanoma-associated antigen) and have equilibrium association constants differing by two orders of magnitude (10(8)-10(10) M-1). SK-MEL-2 tumors were poorly vascularized and were composed of one or several collections of tumor cells with few intratumor blood vessels. In contrast, M21 tumors induced a strong angiogenic response and were organized into multiple small tumor cell nests separated from each other by fine blood vessels. Neither tumor developed extensive connective tissue stroma. In both tumors, hyperpermeable blood vessels were concentrated at the tumor-host interface but some intratumor vessels in M21 tumors were also leaky. Macromolecular tracers extravasated extensively from leaky vessels into tumor stroma but penetrated poorly into tumor parenchyma. All three tumor-specific MAbs stained tumor cell surfaces in a time-dependent fashion such that one-half or more of all tumor cells were stained by 24-48 h. Tumor cell staining was favored by increased density of tumor cell antigens but, at the doses studied, was little affected by differences in affinity among tumor-specific antibodies. The distribution of MAb staining was nonuniform in two respects: (a) peripherally situated tumor cells were more likely to be stained than centrally placed cells, and only in the smallest tumors did MAb reach centrally placed tumor cells; and (b) staining was nonuniform in different parts of the same tumor. The inhomogeneity of tumor cell staining by tumor-specific MAb was attributable to several factors, including: tumor blood vessel number, distribution, perfusion and permeability; distribution of tumor connective tissue stroma; small volume of the parenchymal interstitial space and relatively impaired diffusion of macromolecules in that space (low effective diffusivity of MAb); and interactions between specific MAbs and tumor cells. Of these factors, those associated with the parenchymal compartment apparently were rate limiting, and strategies that enhance parenchymal penetration are likely to improve solid tumor therapy with MAbs.

A quantitative analysis of tumor specific monoclonal antibody uptake by human melanoma xenografts: effects of antibody immunological properties and tumor antigen expression levels

The time-dependent (5 min-72 h) localization of 3 radiolabeled anti-melanoma monoclonal antibodies (MAbs 436, IND1, and 9.2.27) was studied in paired label experiments in small (4-12 mg) s.c. human melanoma xenografts (SK-MEL-2 and M21) in athymic nude mice. MAb 436 recognizes a Mr 125,000 cell surface melanoma-associated glycoprotein antigen (125 kDa-MAA); MAbs IND1 and 9.2.27 recognize a high molecular weight melanoma-associated antigen, but with equilibrium association constants differing by 2 orders of magnitude (10(8)-10(10) M-1). The two tumors were found to differ in their antigen expression levels and in both interstitial and vascular volumes. Accumulation of MAbs in both tumors was determined primarily by antigen expression levels and also by physiological factors such as vascular permeability and vascular volume; at the dose administered (20 micrograms/mouse), differences in MAb affinity among specific MAbs had minimal effect on accumulation. Quantitative flow cytometry measurements showed that antigen expression in vivo differed from that of cultured tumor cells. In vivo, expression of the Mr 125,000 MAA decreased by a factor of about 2.5 in both tumors. In contrast, the in vivo expression of the high molecular weight MAA decreased in M21 tumors but increased by 2.0-3.5-fold in SK-MEL-2 tumors. Data were analyzed using a three-compartment pharmacokinetic model (C. Sung et al., Cancer Res., 52:377-384, 1992) to provide plasma-to-tissue transport constants (k), the interstitial fluid flow rate (L), and estimates of the in vivo interstitial MAb binding site concentration (B0). For all MAbs, the plasma-to-tissue transport constants were consistently greater for M21 tumors (0.44-0.85 microliter/min/g) than for SK-MEL-2 tumors (0.28-0.66 microliter/min/g), and values of k for both tumors were approximately 1 order of magnitude greater than those for skeletal muscle (0.06-0.08 microliter/min/g). The model-estimated binding site concentration of melanoma-specific antibodies was 15-70 times lower than that predicted by experimental measurements of tumor antigen concentrations. Factors that may contribute to this discrepancy include inaccessibility of tumor cell binding sites to MAb and MAb catabolism. In summary, these results indicate that, for the MAb dose used in this study, variables pertaining to the tumor target (i.e., antigen expression levels, vascular volume, and vascular permeability) are the most important for determining MAb accumulation in tumors.

Lymphatic and nonlymphatic pathways of peritoneal absorption in mice: physiology versus pathology

In conjunction with our studies of the pathogenesis of malignant ascites formation, we have analyzed the transperitoneal transport of macromolecules in mice. In this review, I summarize our experimental results concerning the influx (transport from the blood to the peritoneal cavity) and efflux (transport from the peritoneal cavity to the blood) of a number of different tracers [fluorescein-labeled dextrans (FITC-D), 51Cr-RBC, 125I-HSA, and 125I-fibrinogen]. We examined tracer transport in ascites tumor-bearing animals as a function of tumor growth and compared our results with transport properties obtained in normal awake mice and in mice that had received an intraperitoneal injection of a solution of 5% bovine serum albumin to simulate the protein-rich fluid accumulation associated with ascites tumor growth in the peritoneum. Our results indicate that both increased influx as well as impaired efflux are required to initiate and maintain tumor ascites fluid accumulation. To test the hypothesis that increased influx reflected increased vascular permeability, we monitored transport of intravenously injected FITC-D tracers (FITC-D) into the peritoneal cavity by fluorescence microscopy. To investigate the mechanisms involved in the decreased efflux, we determined tracer efflux rates both as the rate of appearance in the blood and as the rate of disappearance from the peritoneal cavity. We compared these transport properties for both soluble as well as particulate tracers. Our results indicate that there are additional routes of egress available to soluble macromolecules not available to particulate tracers such as 51Cr-RBC, and that in ascites tumor-bearing animals, the lymphatic pathway is shut off rather rapidly as judged by the decreased rate of 51Cr-RBC removal. By fluorescence microscopy we observed the interstitial tissue uptake of intraperitoneally injected soluble macromolecules (FITC-D) in the parietal peritoneal wall, particularly in animals with an increased intraperitoneal pressure, thereby confirming additional nonlymphatic pathways of peritoneal absorption in mice. Finally, we used the particulate tracer 51Cr-RBC to estimate the peritoneal lymphatic drainage rate, yielding a value of 1.6 microliters/min in normal awake mice based on the rate of tracer disappearance from the peritoneum.

Characterization of the immunochemical reactivity of fibrinogen fragments by competitive radioimmunoassay: an improved method of analysis

Published results on the immunochemical reactivities of fibrinogen and fibrinogen fragments with fibrinogen-elicited antibodies that had been fractionated on the basis of preferential interaction with A alpha [Nagy, J. A., Meinwald, Y. C., and Scheraga, H. A. (1982), Biochemistry 21, 1794-1806] and B beta [Nagy, J. A., Meinwald, Y. C., and Scheraga, H. A. (1985) Biochemistry 24, 882-887] peptides of this bivalent antigen have been reinterpreted. First, the multivalent counterpart of the Scatchard analysis has been used to determine the intrinsic association constant for the interaction of antibody with [125I]fibrinogen, the radiolabeled ligand used in subsequent competitive binding studies. Second, the corresponding affinity constant for native fibrinogen has been evaluated from the relevant competitive radioimmunoassays by means of a quantitative analysis that takes into account the bivalency of both the radiolabeled and native fibrinogen molecules. Finally, affinity constants for the interactions of various fibrinogen fragments with antibody are also obtained by the procedure, and their magnitudes rationalized in terms of the equilibrium coexistence of unreactive (disordered) and native (functional) states of the fibrinogen peptides.

Distribution of vascular permeability factor (vascular endothelial growth factor) in tumors: concentration in tumor blood vessels

Vascular permeability factor (VPF) is a highly conserved 34-42-kD protein secreted by many tumor cells. Among the most potent vascular permeability-enhancing factors known, VPF is also a selective vascular endothelial cell mitogen, and therefore has been called vascular endothelial cell growth factor (VEGF). Our goal was to define the cellular sites of VPF (VEGF) synthesis and accumulation in tumors in vivo. Immunohistochemical studies were performed on solid and ascites guinea pig line 1 and line 10 bile duct carcinomas using antibodies directed against peptides synthesized to represent the NH2-terminal and internal sequences of VPF. These antibodies stained tumor cells and, uniformly and most intensely, the endothelium of immediately adjacent blood vessels, both preexisting and those newly induced by tumor angiogenesis. A similar pattern of VPF staining was observed in autochthonous human lymphoma. In situ hybridization demonstrated VPF mRNA in nearly all line 10 tumor cells but not in tumor blood vessels, indicating that immunohistochemical labeling of tumor vessels with antibodies to VPF peptides reflects uptake of VPF, not endogenous synthesis. VPF protein staining was evident in adjacent preexisting venules and small veins as early as 5 h after tumor transplant and plateaued at maximally intense levels in newly induced tumor vessels by approximately 5 d. VPF-stained vessels were also hyperpermeable to macromolecules as judged by their capacity to accumulate circulating colloidal carbon. In contrast, vessels more than approximately 0.5 mm distant from tumors were not hyperpermeable and did not exhibit immunohistochemical staining for VPF. Vessel staining disappeared within 24-48 h of tumor rejection. These studies indicate that VPF is synthesized by tumor cells in vivo and accumulates in nearby blood vessels, its target of action. Because leaky tumor vessels initiate a cascade of events, which include plasma extravasation and which lead ultimately to angiogenesis and tumor stroma formation, VPF may have a pivotal role in promoting tumor growth. Also, VPF immunostaining provides a new marker for tumor blood vessels that may be exploitable for tumor imaging or therapy.