Regulation of in vitro glia-induced microvessel morphogenesis by urokinase

Transcription-dependent epidermal growth factor receptor activation by hepatocyte growth factor

The mechanisms and biological implications of coordinated receptor tyrosine kinase coactivation remain poorly appreciated. Epidermal growth factor receptor (EGFR) and c-Met are frequently coexpressed in cancers, including those associated with hepatocyte growth factor (HGF) overexpression, such as malignant astrocytoma. In a previous analysis of the HGF-induced transcriptome, we found that two EGFR agonists, transforming growth factor-alpha and heparin-binding epidermal growth factor-like growth factor (HB-EGF), are prominently up-regulated by HGF in human glioma cells. We now report that stimulating human glioblastoma cells with recombinant HGF induces biologically relevant EGFR activation. EGFR phosphorylation at Tyr(845) and Tyr(1068) increased 6 to 24 h after cell stimulation with HGF and temporally coincided with the induction of transforming growth factor-alpha (~5-fold) and HB-EGF (~23-fold) expression. Tyr(845) and Tyr(1068) phosphorylation, in response to HGF, was inhibited by cycloheximide and actinomycin D, consistent with a requirement for DNA transcription and RNA translation. Specifically, blocking HB-EGF binding to EGFR with the antagonist CRM197 inhibited HGF-induced EGFR phosphorylation by 60% to 80% and inhibited HGF-induced S-G(2)-M transition. CRM197 also inhibited HGF-induced anchorage-dependent cell proliferation but had no effect on HGF-mediated cytoprotection. These findings establish that EGFR can be activated with functional consequences by HGF as a result of EGFR ligand expression. This transcription-dependent cross-talk between the HGF receptor c-Met and EGFR expands our understanding of receptor tyrosine kinase signaling networks and may have considerable consequences for oncogenic mechanisms and cancer therapeutics.

Modulation of endothelial cell morphogenesis in vitro by MMP-9 during glial-endothelial cell interactions

The purpose of this study was to investigate the roles of matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1) in the formation of capillary structures by human brain microvascular endothelial cells cocultured with SNB19 glioblastoma cells. Unstimulated cocultures did not form capillaries and produce MMP-9 but stimulation with the protein kinase C (PKC) activator 4-phorbol-12-myristate 13-acetate (PMA) produced MMP-9 and capillary networks. Addition of recombinant MMP-9 increased capillary formation. Anti-MMP-9 antibodies, TIMP-1, the synthetic MMPs inhibitor Batimastat (BB-94), and the PKC inhibitor calphostin-C all reduced MMP-9 activity and capillary network formation in these cocultures. Cytochalasin-D in the presence of PMA suppressed MMP-9 expression and capillary formation, but colchicine-B had no such effect. Finally, PMA-induced MMP-9 expression and capillary formation were inhibited by the MEKK-specific inhibitor PD98059. These results suggest that MMP-9 is important in endothelial cell morphogenesis and the formation of capillaries in glial/endothelial cocultures in vitro.

The relationship between genetic aberrations as detected by comparative genomic hybridization and vascularization in glioblastoma xenografts

Angiogenesis is of vital importance for the growth of solid tumors and constitutes a target for anti-cancer therapy. Glioblastomas (GBMs) are histologically characterized by striking microvascular proliferation. The identification of the mechanism of angiogenesis is of major importance for the further development of anti-angiogenic therapy. Tumor angiogenesis might be the result of a combination of local tissue conditions (especially hypoxia) and specific genetic alterations acquired during oncogenesis. In order to investigate the relationship between genetic aberrations and tumor angiogenesis in GBM xenograft lines, the genetic alterations were examined by Comparative Genomic Hybridization (CGH). Two vascular phenotypes of GBM xenografts could be identified: a well vascularized and a poorly vascularized type. In this model, the poorly vascularized type had a larger number of genetic alterations. However, there was no unequivocal correlation between angiogenesis, growth rate and patterns of genetic alterations as detected by CGH.

Astrocyte regulation of endothelial tissue plasminogen activator in a blood-brain barrier model

Expression of tissue plasminogen activator (tPA) substantially determines endothelial-dependent fibrinolysis. We used a blood-brain barrier (BBB) model to analyze regulation of brain capillary endothelial tPA and its inhibitor, plasminogen activator inhibitor-1 (PAI-1). This model consists of coculture of murine astrocytes with bovine brain capillary endothelial cells grown as capillary-like structures (CS); after 1 week, astrocytes become extensively associated with CS, and the BBB-associated enzyme gamma-glutamyl transpeptidase is present. We measured tPA and PAI-1 mRNA and tPA activity in this model. Reverse transcription-polymerase chain reaction (RT-PCR) studies showed similar tPA and PAI-1 mRNA levels after 1 day mono-culture (endothelial cells only) versus astrocyte-endothelial coculture preparations. After 7 days (i.e., when elements of the BBB are present), astrocyte-endothelial cocultures (compared with endothelial mono-cultures) showed a 50.7%+/-27.1% (mean +/- SD) reduction in tPA mRNA (P < 0.03) and a 183.3%+/-86.9% increase in PAI-1 mRNA expression (P < 0.02). Moreover, 7-day cocultures demonstrated reduced tPA activity compared with mono-cultures (14.6+/-2.9 IU/mL versus 30.2+/-7.7 IU/mL, P < 0.01); 1-day cocultures and mono-cultures had similar tPA activity. These findings demonstrate that astrocytes regulate brain capillary endothelial expression of tPA when elements of the BBB phenotype are present in this model. These data suggest an important role for astrocytes in the regulation of brain capillary endothelial fibrinolysis.

An in vitro three-dimensional coculture model of cerebral microvascular angiogenesis and differentiation

The microvasculature of the developing brain is plastic and responds differently to the many insults associated with preterm birth. We developed three-dimensional in vitro culture models for the study of the responses of the developing cerebral microvasculature. Beagle brain microvascular endothelial cells (BBMEC) were isolated by differential centrifugation from newborn beagle pups on postnatal Day 1 and placed in three-dimensional culture dispersed in a collagen gel. Alternatively, BBMEC were placed in a three-dimensional coculture with neonatal rat forebrain astrocytes. Cultures were analyzed for extracellular matrix components at 1 and 6 d, and total RNA was extracted for Northern analyses. Urokinase plasminogen activator activity was assayed in both mono- and cocultures of the two cell types. Studies of three-dimensional BBMEC/astrocyte cocultures demonstrated progressive tube formation with only low levels of endothelial proliferation. By 6 d in three-dimensional coculture, the BBMEC formed capillarylike tubes with a wrapping of glial processes, and basement membrane protein synthesis was noted. Urokinase plasminogen zymography suggested intercellular signaling by the two cell types. These data suggest that the three-dimensional beagle brain germinal matrix microvascular endothelial cell/neonatal rat astrocyte coculture provides a good model for the investigation of microvascular responses in the developing brain.

Astroglial and vascular interactions of noradrenaline terminals in the rat cerebral cortex

Noradrenaline (NA) has been shown to influence astrocytic and vascular functions related to brain homeostasis, metabolism, local blood flow, and blood-brain barrier permeability. In the current study, we investigate the possible associations that exist between NA-immunoreactive nerve terminals and astrocytes and intraparenchymal blood vessels in the rat frontoparietal cortex, both at the light and electron microscopic levels. As a second step, we sought to determine whether the NA innervation around intracortical microvessels arises from peripheral or central structures by means of injections of N-(2-chloroethyl-N-ethyl-2-bromobenzylamine) (DSP-4), a neurotoxin that specifically destroys NA neurons from the locus ceruleus. At the light microscopic level, 6.8% of all NA-immunoreactive nerve terminals in the frontoparietal cortex were associated with vascular walls, and this perivascular noradrenergic input, together with that of the cerebral cortex, almost completely disappeared after DSP-4 administration. When analyzed at the ultrastructural level in control rats, NA terminals in the neuropil had a mean surface area of 0.53 +/- 0.03 micron2 and were rarely junctional (synaptic incidence close to 7%). Perivascular terminals (located within a 3-micron perimeter from the vessel basal lamina) counted at the electron microscopic level represented 8.8% of the total NA terminals in the cortical tissue. They were smaller (0.29 +/- 0.01 micron2, P < 0.05) than their neuronal counterparts and were located, on average, 1.34 +/- 0.08 microns away from intracortical blood vessels, which consisted mostly of capillaries (65%). None of the perivascular NA terminals engaged in junctional contacts with surrounding neuronal or vascular elements. The primary targets of both neuronal and perivascular NA nerve terminals consisted of dendrites, nerve terminals, astrocytes, and axons, whereas in the immediate vicinity (0.25 micron or less) of the microvessels, astrocytic processes represented the major target. The results of the current study show that penetrating arteries and intracortical microvessels receive a central NA input, albeit parasynaptic in its interaction, originating from the locus ceruleus. Particularly, they point to frequent appositions between both neuronal and perivascular NA terminals and astroglial cells and their processes. Such NA neuronal-glial and neuronal-glial-vascular associations could be of significance in the regulation of local metabolic and vascular functions under normal and pathologic situations.

Regulation of angiogenesis in malignant gliomas

Gliomas are highly resistant to conventional therapeutic measures, requiring the development of novel treatments. Since gliomas are particularly vascular tumors, one approach involves treatments directed at inhibiting angiogenic mechanisms. Although multiple factors contribute to the ultimate vascularization of any tumor, some are especially relevant to gliomas. Early experimental work directed at inhibiting angiogenic pathways has shown promise toward achieving control of tumor growth. This article focuses on the evidence that angiogenesis and related vascular cell responses play important roles in glioma biology, and reviews those biochemical pathways known through experimentation to be involved in the vascular response to gliomas. Finally, contemporary vessel-targeted approaches that have been used to inhibit glioma growth are discussed.

Regulation of brain capillary endothelial thrombomodulin mRNA expression

BACKGROUND AND PURPOSE

Endothelial cells regulate hemostasis in part via expression of thrombomodulin, a potent anticoagulant protein. The purpose of this study was to analyze brain capillary endothelial cell expression of thrombomodulin mRNA.

METHODS

Bovine brain capillary endothelial cells were grown in a blood-brain barrier model in which endothelial cells form capillary-like structures. In situ hybridization and polymerase chain reaction (PCR) were used to examine thrombomodulin expression. Endothelial cells were then cocultured with astrocytes. We examined both coculture and monoculture preparations for gamma-glutamyl transpeptidase (GGTP), a marker of the blood-brain barrier. We then used quantitative-competitive PCR to compare thrombomodulin expression in endothelial monocultures and astrocyte-endothelial cocultures after 1 and 7 days of culture.

RESULTS

Both in situ hybridization and PCR studies demonstrated thrombomodulin mRNA expression by endothelial cells. During 1 week of astrocyte-endothelial coculture, there was (1) progressive association of astrocytes with capillary-like structures and (2) expression of GGTP; endothelial monocultures did not express GGTP. There was no significant difference in thrombomodulin mRNA expression for cocultures versus monocultures after 1 day. After 1 week, however, astrocyte-endothelial cocultures had markedly decreased thrombomodulin mRNA compared with monocultures (9 +/- 2 versus 189 +/- 62 pg/mL; P < .025). This thrombomodulin mRNA decrease thus occurred when elements of the blood-brain barrier phenotype were demonstrable, ie, when astrocyte association with capillary-like structures was maximal and when GGTP was expressed in cocultures.

CONCLUSIONS

These findings indicate astrocyte regulation of thrombomodulin mRNA expression in vitro and suggest an important role for the blood-brain barrier in the regulation of thrombomodulin.

Suramin inhibits C6 glioma-induced angiogenesis in vitro

Aspects of tumor-induced angiogenesis in vitro were examined using an assay involving collagen gel invasion by a surface monolayer of bovine endothelial cells under the influence of serum free conditioned medium produced by C6 cells, an experimentally derived rat glial tumor cell line. The effects of the polyanionic compound suramin, known to interfere with growth factor/cell signaling on this process were evaluated. Collagen gel invasion was quantified by adding C6 conditioned medium with or without various doses of suramin to monolayers of bovine aortic endothelial cells grown on type I collagen gels in transwell inserts. Cultures were monitored with phase-contrast microscopy. After various periods of incubation collagen gels were fixed, embedded in epoxy resin, and 1-micron thick sections were stained with toluidine blue. Additional cultures were used to evaluate the effects of C6 conditioned medium and suramin on endothelial cell proliferation, and on chemotaxis through 8-microns pores. C6 glioma cell conditioned medium induced large vessel endothelial cells to sprout into the underlying collagen matrix and subsequently form networks of capillary like tubes. Conditioned medium was also chemotactic and mitogenic for these cells. The addition of suramin to C6 glioma conditioned medium prevents tube formation in collagen gels, and inhibits both endothelial cell proliferation and chemotaxis in a dose dependent manner. These results suggest that glial tumor cell conditioned medium induces angiogenesis in large vessel endothelial cells in vitro via mechanisms which are disrupted by suramin, most likely involving tumor-derived growth factor release and/or endothelium-mediated matrix proteolysis.