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K H Plate

Publications and source records attributed to K H Plate.

At least 37 records · Page 2Linked to original sources
PubMed1999

Mechanisms of angiogenesis in the brain.

Brain angiogenesis is a tightly controlled process that is regulated by neuroectodermal derived growth factors that bind to tyrosine kinase receptors expressed on endothelial cells. In the rat brain, angiogenesis is complete around postnatal day 20, but endothelial cells can proliferate in the adult brain under pathological conditions such as hypoxia/ischemia and brain tumor growth. Current evidence suggests that physiological angiogenesis in the brain is regulated by similar mechanisms as pathological angiogenesis induced by tumors or by hypoxia/ischemia. The hypoxia-inducible endothelial cell mitogen and vascular permeability factor, vascular endothelial growth factor (VEGF) appears to play a pivotal role in most of these processes. VEGF is expressed when angiogenesis is high, as in embryonic neuroectoderm, in glioblastomas and around infarcts, but is expressed at low levels when angiogenesis is absent, as in adult neuroectoderm. Since growth factors such as VEGF and angiopoietins and their receptors appear to be necessary for angiogenesis, targeting of growth factor/receptor pathways for angiogenesis-dependent diseases such as glioblastoma might be useful for therapy. Several compounds, including anti-VEGF antibodies and VEGFR-2 inhibitors are currently in clinical trial. On the other hand, induction of angiogenesis by growth factors (pro-angiogenesis) might prove to be a rational therapy for patients with stroke.

Animals↗
PubMed1999

Cell type specific upregulation of vascular endothelial growth factor in an MCA-occlusion model of cerebral infarct.

Vascular endothelial growth factor (VEGF) is an endothelial cell specific mitogen that has been implicated in hypoxia-mediated angiogenesis under physiological and pathological conditions. We used the middle cerebral artery occlusion model (MCAO) in the rat to investigate VEGF mRNA and protein localization, and VEGFR-1 mRNA and VEGFR-2 mRNA expression in cerebral ischemia. By nonradioactive in situ hybridization we observed upregulation of VEGF mRNA and VEGFR-1 mRNA, but not of VEGFR-2 mRNA in the hemisphere ipsilateral to MCA occlusion. VEGF mRNA was upregulated in the periphery of the ischemic area commencing 3 hours (h) after onset of MCAO, reached a peak after 24 h, and remained expressed at lower levels until 7 days (d) after MCAO. Double labelling experiments revealed that the majority of VEGF expressing cells in the penumbra and within the infarct were immunoreactive for Ox-42, Iba-1, and Ed1, but not for GFAP and neurofilament proteins, suggesting that microglial cells/macrophages are the major cell type expressing VEGE Since VEGF was also expressed in Ox-42 immunoreactive cells distant from the infarct (e.g. in the corpus callosum and hippocampus), activated microglial cells expressing VEGF may migrate towards the ischemic stimulus. VEGF protein was also detected on capillaries within the peri-ischemic area, suggesting that VEGF produced and secreted by microglial cells/macrophages binds to its receptors on nearby vascular endothelial cells and initiates an angiogenic response which counterbalances tissue hypoxia. Accordingly, apoptosis of neuroectodermal cells in the penumbra was highly depressed after the onset of angiogenesis. The spatial and temporal correlation between the induction of angiogenesis with VEGF and VEGFR-1 expression suggests that the ischemic upregulation of VEGF represents a physiological response of the brain to counterbalance hypoxia/ischemia in order to protect neuroectodermal tissue.

Animals↗
PubMed1999

Vascular endothelial growth factor expression, vascular volume, and, capillary permeability in human brain tumors.

OBJECTIVE: Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen and a potent inducer of vascular permeability. In this study, we determined whether expression of VEGF is correlated with in vivo measurements of the capillary permeability and vascular volume of primary human brain tumors. METHODS: Tumor samples (seven glioblastomas, one anaplastic astrocytoma, two low-grade astrocytomas, one pilocytic astrocytoma, and three primary cerebral lymphomas) were stereotactically obtained from 14 patients. A semiquantitative polymerase chain reaction was used to quantify the relative expression of VEGF messenger ribonucleic acid in the tumors. VEGF protein was demonstrated in tissue sections by immunohistochemical techniques. A two-compartment dynamic computed tomographic method was used to quantitatively measure the aforementioned parameters in the regions from which the biopsies were obtained. RESULTS: In glial tumors, there was significant correlation of VEGF messenger ribonucleic acid levels with capillary permeability (P < 0.05) and vascular volume (P < 0.01). Although all primary cerebral lymphomas showed considerable increases in capillary permeability and vascular volume, VEGF expression was only slightly upregulated in these tumors. CONCLUSION: Our findings are consistent with the hypothesis that VEGF may be responsible for endothelial cell proliferation and vascular permeability in glial tumors. This relationship has implications for clinical applications, i.e., assessment of delivery of water-soluble drugs, treatment of edema, and antiangiogenesis therapy based on inhibition of VEGF function.

Adult↗
PubMed1998

Coexpression of erythropoietin and vascular endothelial growth factor in nervous system tumors associated with von Hippel-Lindau tumor suppressor gene loss of function.

Hemangioblastomas are highly vascular tumors of the central nervous system that overexpress the hypoxia-inducible gene, vascular endothelial growth factor (VEGF), as a consequence of mutational inactivation of the von Hippel-Lindau tumor suppressor gene (VHL). Previous reports showed that hemangioblastomas can also express erythropoietin (Epo), which is also hypoxia-inducible. However, Epo expression in hemangioblastomas was observed only in individual cases, and the analyses were mainly based on indirect determination of erythropoiesis-stimulating activity. Therefore, we analyzed a series of 11 hemangioblastomas for Epo, VEGF, and VHL expression by Northern blot analysis and compared the results with normal brain and glioblastomas. Surprisingly, we observed Epo mRNA expression in all hemangioblastoma specimens analyzed, but in none of four glioblastomas. In contrast, VEGF mRNA was expressed in all hemangioblastomas and all glioblastomas. In situ hybridization revealed neoplastic stromal cells as Epo- and VEGF-producing cells in hemangioblastomas. These results suggest that in the nonhypoxic microenvironment of hemangioblastoma, Epo, similar to VEGF, might be negatively regulated by the VHL gene product.

Adult↗
PubMed1998

Upregulation of vascular endothelial growth factor in severe chronic brain hypoxia of the rat.

The vascular endothelial growth factor (VEGF) has been shown to be upregulated in acute hypoxia. Although an increase in blood vessel number has been described in severe chronic brain hypoxia, it is unclear whether VEGF is upregulated in this condition. We therefore investigated male inbred Wistar rats, which were exposed for 9 to 13 weeks to decreasing amounts of oxygen, down to 7% O2 (15%: 15 days; 12%, 10%, respectively; 8%: 1 day, 3 weeks, respectively; 7%: 4 weeks). The expression of VEGF was studied by Northern analysis and in situ hybridization in frozen sections of cerebral cortex, hippocampus and cerebellum in six chronic hypoxic and two control rats. We found a marked upregulation of VEGF mRNA in all brain regions investigated, being strongest in cerebral cortex and cerebellum. Our results suggest a potential role of VEGF for vascular growth and vascular permeability observed in chronic cerebral hypoxia.

Animals↗
PubMed1998

Up-regulation of vascular endothelial growth factor in stromal cells of hemangioblastomas is correlated with up-regulation of the transcription factor HRF/HIF-2alpha.

Hemangioblastomas, the most frequent manifestation of the hereditary von Hippel-Lindau disease (VHL), are highly vascularized tumors of the central nervous system. In previous studies, the endothelial-specific mitogen vascular endothelial growth factor (VEGF) was shown to be up-regulated in the stromal cells, the putative neoplastic cells in hemangioblastomas. Therefore, it was suggested that secretion of VEGF by stromal cells is the pathogenetic cause of the vascular lesions in hemangioblastomas. The novel basic helix loop helix transcription factor HRF/HIF-2alpha is a candidate regulator of VEGF expression during development. We therefore investigated expression of HRF/HIF-2alpha in hemangioblastomas and found the overexpression of VEGF mRNA in stromal cells to be highly correlated with elevated expression levels of HRF/HIF-2alpha mRNA. This finding is suggestive for a role of HRF in VEGF-dependent vascular growth in hemangioblastomas and could provide a link between transcriptional activation of the VEGF gene and loss of function of the VHL gene product.

Basic Helix-Loop-Helix Proteins↗
PubMed1998

Cell type-specific expression of angiopoietin-1 and angiopoietin-2 suggests a role in glioblastoma angiogenesis.

Glioblastomas are highly vascular tumors which overexpress the angiogenesis factor vascular endothelial growth factor (VEGF). VEGF and its receptors, VEGF-R1 and VEGF-R2, have been shown to be necessary for embryonic angiogenesis as well as for tumor angiogenesis. Recently, the angiopoietin/Tie2 receptor system has been shown to exert functions in the cardiovascular system that are distinct from VEGF but are also critical for normal vascular development. To assess the potential role of Tie2 and its ligands angiopoietin-1 and angiopoietin-2 in tumor vascularization, we analyzed their expression pattern in human gliomas. Tie-2 was up-regulated in tumor endothelium compared to normal human brain tissue. We further observed cell type-specific up-regulation of the message for both angiopoietin-1 and angiopoietin-2 in gliomas. Whereas Ang-1 mRNA was expressed in tumor cells, Ang-2 mRNA was detected in endothelial cells of a subset of glioblastoma blood vessels. Small capillaries with few periendothelial support cells showed strong expression of Angiopoietin-2, whereas larger glioblastoma vessels with many periendothelial support cells showed little or no expression. Although the function of Tie2 and its ligands in tumor angiogenesis remains a subject of speculation, our findings are in agreement with a recently proposed hypothesis that in the presence of VEGF, local production of Ang-2 might promote angiogenesis.

Actins↗
PubMed1998

Gene therapy of malignant glioma: recent advances in experimental and clinical studies.

Recent advances in molecular tumor biology and gene technology have provided the possibility to treat patients with malignant brain tumors by altering gene expression in tumor cells. Tumor development and progression involves alterations in a wide spectrum of genes, therefore a variety of gene therapy approaches for malignant gliomas have been proposed. In this review article, we discuss some principles of current gene therapeutic strategies that are under investigation in laboratories and in clinics. In addition, some general issues that remain to be resolved for clinical application of gene therapy in patients with malignant gliomas will be addressed.

Animals↗
PubMed1997

Up-regulation of flk-1/vascular endothelial growth factor receptor 2 by its ligand in a cerebral slice culture system.

Vascular endothelial growth factor (VEGF) and its tyrosine kinase receptors VEGFR-1 (flt-1) and VEGFR-2 (flk-1/KDR) are key mediators of physiological and pathological angiogenesis. They are expressed in most tissues during embryonic development but are down-regulated in the adult, when angiogenesis ceases. Up-regulation of VEGFR-2 and of VEGF are observed in many pathological conditions under which angiogenesis is reinduced. A major regulator of VEGF expression is hypoxia. Although the temporal expression pattern of VEGFR-2 parallels VEGF expression to a high extent, little is known about its regulation. Here, we show that VEGFR-2 is highly expressed in early postnatal mouse brain but is down-regulated commencing at postnatal day 15 (P15) of mouse brain development and is hardly detectable in P30 mouse brain. Using P30 mouse brain slices, we observed that hypoxia up-regulates VEGFR-2 in the slices but not in human umbilical vein endothelial cells, suggesting the presence of a hypoxia-inducible factor in the murine neuroectoderm that up-regulates VEGFR-2. To identify the factors involved, normoxic P30 cerebral slices were cultured with growth factors that are either hypoxia-inducible (e.g., PDGF-BB, erythropoietin, and VEGF) and/or are known to act on endothelial cells (e.g., PDGF-BB, VEGF, and PIGF). Exogenously added recombinant VEGF led to an up-regulation of VEGFR-2 expression, which could be inhibited by preincubation with a neutralizing anti-VEGF antibody. Addition of PDGF-BB, PIGF, and erythropoietin had no effect on VEGFR-2 expression. Our results suggest a differential but synergistic regulation by hypoxia of VEGF and VEGFR-2: a direct induction of VEGF that subsequently up-regulates VEGFR-2 in endothelial cells. This autoenhancing system may represent an important mechanism of tumor angiogenesis.

Animals↗
PubMed1997

Up-regulation of vascular endothelial growth factor expression in a rat glioma is conferred by two distinct hypoxia-driven mechanisms.

Up-regulation of vascular endothelial growth factor (VEGF) expression is a major event leading to neovascularization in malignant gliomas. Hypoxia is believed to be the crucial environmental stimulus for this up-regulation. To critically assess this hypothesis, we asked whether the mechanisms defined previously for hypoxia-induced VEGF expression in vitro are similarly involved and sufficient for up-regulation of VEGF gene expression in vivo, using a lacZ reporter gene under the control of VEGF regulatory sequences in an experimental glioma model. Inclusion of the binding site for hypoxia-inducible factor 1 (HIF 1) in the 5' regulatory sequences used in the hybrid gene produced weak beta-galactosidase staining in a special tumor cell subtype, the so-called perinecrotic palisading (PNP) cells that flank necrotic regions within the tumor. Deletion of the HIF 1 binding site abolished reporter gene expression in the PNP cells, indicating that transcriptional activation of VEGF expression in gliomas is mediated by HIF 1. Inclusion of 3' untranslated sequences from the VEGF gene in the reporter constructs resulted in an increased beta-galactosidase staining in the PNP cells, suggesting that mRNA stabilization also contributes to VEGF up-regulation in glioblastoma cells growing as solid tumors. Combination of the 5' flanking region including the HIF 1 site along with 3' untranslated sequences produced increased levels of beta-galactosidase expression in PNP cells. EF 5 immunostaining for regions of low oxygen partial pressure covered the same PNP cells that were stained for beta-galactosidase. Collectively, the data provide experimental evidence that VEGF gene expression is activated in a distinct tumor cell subpopulation, the perinecrotic palisading cells of gliomas, by two distinct hypoxia-driven regulatory mechanisms.

Animals↗
PubMed1997

Anti-angiogenic gene therapy of malignant glioma.

Glioblastoma, one of the best vascularized tumours in humans, appears well suited for an antiangiogenic therapy. VEGF (vascular endothelial growth factor), the most important angiogenesis factor identified to date, is highly expressed in glioblastoma. VEGF is particulary upregulated in palisading cells adjacent to necroses and has subsequently been shown to be hypoxia-inducible in glioma cells in vitro. VEGF-receptor tyrosine kinases, VEGF-R1 (flt-1) and VEGF-R2 (flk-1), are induced in a tumour stage dependent manner during glioma progression and are exclusively expressed in tumour vascular endothelial cells. These observations suggest that VEGF-receptors are promising targets for tumour endothelial cell specific therapy. The ability to block VEGF-signalling by the VEGF-R2 dominant-negative mutant identifies the VEGF/VEGF-R2 system as a major regulator of glioma angiogenesis. Several experimental approaches demonstrate that in rat gliomas tumour growth can be prevented by the inhibition of angiogenesis. These findings are of pivotal importance for the development of anti-angiogenic therapies in glioblastoma patients.

Animals↗
PubMed1997

Vascular endothelial growth factor.

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenesis and vascular permeability factor which is expressed in high amounts in perinecrotic palisading cells in human glioblastomas. In vitro VEGF gene expression is enhanced approximately ten times by hypoxia. Current evidence suggests, that hypoxia is also the driving force for VEGF gene expression in glioblastoma cells in vivo and represents the most important trigger for tumor angiogenesis and edema. Our approaches to inhibit tumor angiogenesis and edema formation in glioblastoma patients will concentrate on the disruption of VEGF/VEGF receptor signal transduction pathway in vivo.

Animals↗
PubMed1997

p53 mutations versus EGF receptor expression in giant cell glioblastomas.

Recent studies have shown that there are distinct genetic pathways leading to the most malignant astrocytic neoplasm, the glioblastoma. Primary (de novo) glioblastomas are characterized by amplification/overexpression of the EGF receptor (EGFR) and, less frequently, of the MDM2 gene. Another pathway, operative in the progression of low-grade or anaplastic astrocytomas to secondary glioblastomas, is characterized by the frequent occurrence of p53 mutations. In this study, we assessed p53 mutations and EGFR expression in the giant cell glioblastoma. This rare variant is characterized by unusually large, multinucleated giant cells, but tends to be more confined and has been reported to carry a somewhat more favorable prognosis. We analyzed biopsies from 16 patients (mean age at clinical manifestation, 40 years). DNA sequencing revealed that 12 of 16 (75%) giant cell glioblastomas contained a p53 mutation. In 7 patients with two or more surgical interventions, the p53 mutation was already detected in the first biopsy. Focal EGFR overexpression, including multinucleated giant cells, was observed immunohistochemically in 9 of 16 (56%) tumors. However, most tumor areas lacked immunoreactivity, indicating that EGFR overexpression does not play a significant role in the evolution of this glioblastoma variant. These results suggest that giant cell glioblastomas develop de novo with a short preoperative history (mean, 47 +/- 40 days), but contain genetic alterations similar to those observed in secondary glioblastomas.

Adult↗
PubMed1997

Putative control of angiogenesis in hemangioblastomas by the von Hippel-Lindau tumor suppressor gene.

The hypoxia-inducible endothelial cell-specific mitogen vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) is expressed in low amounts in adult human brain, but is highly upregulated in the perinecrotic palisading cells of glioblastomas. We observed high VEGF expression in cerebellar hemangioblastomas, which are highly vascular, nonnecrotic and presumably nonhypoxic tumors, and hypothesized that a mechanism other than hypoxia leads to VEGF upregulation. Because hemangioblastomas develop in patients with von Hippel-Lindau disease, and mutations of the von Hippel-Lindau tumor suppressor (VHL) gene have also been reported in sporadic hemangioblastomas, we investigated VHL expression in normal cerebellum and in hemangioblastomas and tested the hypothesis that mutations in the VHL gene lead to upregulation of VEGE We observed constitutive expression of VHL mRNA, but downregulation of VEGF mRNA in the postnatal cerebellum. In the adult cerebellum, VHL is predominantly expressed in neuronal cells. In hemangioblastomas, VHL expression appears to be restricted to stromal cells, suggesting that the neoplastic component is the stromal cell. VHL-deficient renal cell carcinoma cells (786-0) produced significantly higher levels of VEGF mRNA and protein compared with 786-0/ wt10 cells, which were stably transfected with the wild-type VHL gene. Our observations suggest that VHL mutations affect stromal cells in hemangioblastomas and that VEGF is upregulated in stromal cells as a consequence of mutations in the VHL gene.

Animals↗
PubMed1997

Angiogenesis in embryos and ischemic diseases.

Angiogenic growth factors and their endothelial receptors are thought to function as major regulators of blood vessel formation. Vascular endothelial growth factor (VEGF) and its receptors, Flt-1 (VEGFR-1) and Flk-1 (VEGFR-2), as well as Angiopoietin-1 and its receptor, Tie-2, represent key signal transduction systems involved in the regulation of embryonic vascular development. The expression of these molecules correlates with phases of blood vessel formation during embryogenesis. Inactivation of any of the genes encoding these molecules in mouse embryos results in defective vascular development and embryonic lethality around mid-gestation. In addition, the VEGF signal transduction system has been implicated in the regulation of pathological blood vessel growth during certain angiogenesis-dependent diseases that are often associated with tissue ischemia, such as proliferative retinopathy or solid tumor growth. This hypothesis is substantiated by experiments, in which the inhibition of VEGF signal transduction resulted in the the inhibition of neovascularization in these diseases. Thus, the VEGF signal transduction system represents a useful target for an anti-angiogenic therapy.

Animals↗
PubMed1996

Dominant-negative inhibition of Flk-1 suppresses the growth of many tumor types in vivo.

Angiogenesis, the sprouting of new blood vessels from existing vessels, occurs in many physiological and pathological processes, including embryonic development, wound healing, and tumor growth. It is required for tumor growth because new blood vessel formation is necessary for tumors to expand beyond a minimum volume. Several growth factor receptor tyrosine kinases have been implicated in angiogenesis, including receptors for epidermal, fibroblast, and platelet-derived growth factors, as well as the receptors Flk-1/KDR, Flt-1 Tek/Tie-2, and Tie-1. Endothelial cells in the vessels of tumors express Flk-1/KDR, a receptor for vascular endothelial growth factor. Flk-1 was previously shown to play a role in angiogenesis and tumor formation of s.c. xenografts of C6 glioma cells using dominant-negative methodology. We now demonstrate that Flk-1 seems to be generally involved in the growth of a wide range of solid tumors, including mammary, ovarian, and lung carcinoma, as well as glioblastoma. Furthermore, survival times in rats bearing intracerebral tumors were prolonged using the same dominant-negative methodology. The involvement of Flk-1 in a variety of tumor types suggests an important role for Flk-1 in tumor angiogenesis.

Animals↗