Hypoxic induction of vascular endothelial growth factor (VEGF) in human epithelial cells is mediated by increases in mRNA stability

Stimulation of coronary vasculogenesis/angiogenesis by hypoxia in cultured embryonic hearts

Hypoxia is known to stimulate vascular growth by up-regulating vascular endothelial growth factor (VEGF), but little is known about the function of hypoxia in the development of the coronary vasculature, and the relationship between hypoxia and VEGF in this event. To test the effects of hypoxia and VEGF on coronary vasculogenesis/angiogenesis in the developing heart, ventricles from 6-day-old quail embryos were cultured on three-dimensional collagen gels. After 2 days of growth in normal medium and 1 day of starvation in low serum medium (0.5% fetal bovine serum), the heart explants were further cultured under various oxygen levels for another 24, 48, and 72 hr. Angioblasts and endothelial cells, which migrated out from the heart explants, were identified by QH1 antibody using immunofluorescence and confocal microscopy. In the normoxic culture environment, the endothelial cells began to proliferate and migrate out from the heart explants after 3 days of growth; they formed tubes mainly after another 72 hr. In contrast, this vascular growth was accelerated under hypoxic conditions, as evidenced by increased tube formation with significant differences observed at 48 hr. On the other hand, hyperoxia delayed this process. Reverse transcription-polymerase chain reaction results indicated that VEGF (including VEGF(122), VEGF(166), and VEGF(190)) was up-regulated in the heart explants under hypoxia and down-regulated under hyperoxia. VEGF neutralizing antibody added to the culture medium partially blocked this vascular growth. We conclude from this study that hypoxia can stimulate or up-regulate coronary vasculogenesis/angiogenesis and that VEGF signaling plays a major role in this event. Dev Dyn 1999;216:28-36.

Angiogenic factors in human proliferative sickle cell retinopathy

BACKGROUND/AIMS

Preretinal neovascular formations called sea fans develop at the border of non-perfused peripheral retina in sickle cell retinopathy. Angiogenic factors which could contribute to their development, however, have not been examined previously. The objective of this study was to determine immunohistochemically if vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF) were associated with sea fan formations.

METHODS

Immunohistochemistry on cryosections was used to localise bFGF, VEGF, heparan sulphate proteoglycan, human serum albumin, collagens IV and II, and von Willebrand factor in tissue from five sickle cell and one control subject.

RESULTS

The greatest immunoreactivity for VEGF and bFGF was in the feeder and preretinal vessels of sea fans (p<0.01). The most prominent reaction product was localised to vascular endothelial cells. In retinal vessels, VEGF and bFGF immunoreactivities were greater in sickle cell subjects (both proliferative and non-proliferative) than in the control subject (p<0.01 and p<0.02 respectively). In the sickle cell retina, no angiogenic factor immunoreactivity was detected in non-perfused periphery and there was no significant difference in bFGF or VEGF immunoreactivity between perfused retina and the border of perfused and non-perfused areas.

CONCLUSION

Our results demonstrate for the first time that VEGF and bFGF are associated with sea fan formations in sickle cell retinopathy. Both factors may function in an autocrine manner because immunoreactivity for these factors was greater within the neovascularisation than in adjacent retina.

Adaptive hypoxic tolerance in the subterranean mole rat Spalax ehrenbergi: the role of vascular endothelial growth factor

Spalax ehrenbergi has evolved adaptations that allow it to survive and carry out normal activities in a highly hypoxic environment. A key component of this adaptation is a higher capillary density in some Spalax tissues resulting in a shorter diffusion distance for oxygen. Vascular endothelial growth factor (VEGF) is an angiogenic factor that is critical for angiogenesis during development and in response to tissue ischemia. We demonstrate here that VEGF expression is markedly increased in those Spalax tissues with a higher capillary density relative to the normal laboratory rat Rattus norvegicus. Upregulation of VEGF thus appears to be an additional mechanism by which Spalax has adapted to its hypoxic environment.

Hypoxic regulation of vascular endothelial growth factor mRNA stability requires the cooperation of multiple RNA elements

Vascular endothelial growth factor (VEGF) is a key regulator of developmental, physiological, and tumor angiogenesis. Upregulation of VEGF expression by hypoxia appears to be a critical step in the neovascularization of solid cancers. The VEGF mRNA is intrinsically labile, but in response to hypoxia the mRNA is stabilized. We have systematically analyzed the regions in the VEGF mRNA that are responsible for its lability under normoxic conditions and for stabilization in response to hypoxia. We find that the VEGF mRNA not only contains destabilizing elements in its 3' untranslated region (3'UTR), but also contains destabilizing elements in the 5'UTR and coding region. Each region can independently promote mRNA degradation, and together they act additively to effect rapid degradation under normoxic conditions. Stabilization of the mRNA in response to hypoxia is completely dependent on the cooperation of elements in each of the 5'UTR, coding region, and 3'UTR. Combinations of any of two of these three regions were completely ineffective in responding to hypoxia, whereas combining all three regions allowed recapitulation of the hypoxic stabilization seen with the endogenous VEGF mRNA. We conclude that multiple regions in the VEGF mRNA cooperate both to ensure the rapid degradation of the mRNA under normoxic conditions and to allow stabilization of the mRNA in response to hypoxia. Our findings highlight the complexity of VEGF gene expression and also reveal a mechanism of gene regulation that could become the target for strategies of therapeutic intervention.

Hypoxia-induced adrenomedullin production in the kidney

BACKGROUND

Adrenomedullin (AM) is a newly discovered peptide that has a potent vasorelaxant activity. To investigate its potential roles in hypoxia-induced renal injury, we examined whether AM production in the kidney increased under hypoxic conditions.

METHODS

The AM transcript levels in Madin-Darby canine kidney (MDCK) cells, rat vascular smooth muscle cells (VSMCs), and rat mesangial cells were assessed by Northern blot analyses under normoxic and hypoxic conditions. The AM peptide in culture media was measured by radioimmunoassay. The effects of hypoxia on accumulation of cAMP in VSMCs were also examined. The stability of AM transcripts under normoxic and hypoxic conditions was compared in the presence of actinomycin D. The effects of hypoxia on AM promoter activity was assessed by transient transfection assays using the AM promoter subcloned upstream of luciferase gene.

RESULTS

The expression of AM transcripts increased significantly in MDCK cells, rat VSMCs, and rat mesangial cells under hypoxic conditions without changes in the stability of AM transcripts; however, the AM promoter activity under hypoxic was not elevated significantly. The accumulation of AM peptide in culture media also increased significantly under hypoxic conditions in MDCK cells (2.2 +/- 0.1 fmol/10(5) cells in normoxia vs. 3.5 +/- 0.3 fmol/10(5) cells in hypoxia, 6 hr after hypoxia induction, P < 0.001), and in rat VSMCs (5.5 +/- 0.3 fmol/10(5) cells in normoxia vs. 7.8 +/- 0.4 fmol/10(5) cells in hypoxia, 8 hr after hypoxia induction, P < 0.01). Under hypoxic conditions, cAMP levels in rat VSMCs increased significantly compared with those under normoxic conditions (13.3 +/- 1.4 pmol/well vs. 4.6 +/- 0.4 pmol/well, P < 0.01).

CONCLUSIONS

Renal parenchymal cells as well as renal vessels may produce AM under hypoxic conditions.

Angiotensin II stimulates the synthesis and secretion of vascular permeability factor/vascular endothelial growth factor in human mesangial cells

The aim of the present study was to evaluate the role of angiotensin II (AngII) in regulating both the gene expression and secretion of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) in human mesangial cells (HMC) in culture. Densitometric analysis of Northern blot experiments demonstrated that AngII increases VPF/VEGF mRNA in a dose-dependent manner. The levels of VPF/VEGF mRNA in HMC exposed for 3 h to 10 nM, 100 nM, and 1 microM AngII were, respectively, 1.5-, 2.3-, and 1.6-fold higher than control cells (P < 0.05, P < 0.0001, and P < 0.05, respectively). This effect was blocked by the pretreatment with losartan (1 microM) (P < 0.005), a selective antagonist of the AngII AT1 receptor. Reverse transcription-PCR performed in HMC using oligonucleotide primers specific for all VPF/VEGF mRNA splicing variants detected three bands corresponding to VEGF 189, 165, and 121. Exposure of the cells to 100 nM AngII resulted in an increase of all the mRNA transcripts. Furthermore, in situ hybridization experiments showed that the levels of hybridization signals for VPF/VEGF mRNA resulted consistently higher in HMC exposed for 3 h to AngII (100 nM) than in control cells. The effects of AngII on the secretion of VPF/VEGF peptide in the culture medium of HMC were assessed using an enzyme-linked immunosorbent assay method. When different concentrations of AngII were tested in 3-h stimulation periods, the percentage of increase in the levels of released VPF/VEGF was significantly higher than control cells for AngII concentrations of 100 nM (62 +/- 11% mean +/- SD, P < 0.0001) and 1 microM (17.3 +/- 10.9%, P < 0.01). The pretreatment of HMC with losartan (1 microM) prevented the increase of VPF/VEGF secretion induced by AngII (100 nM) (AngII 54.7 +/- 3.9 pg/microg DNA versus AngII + losartan 37.8 +/- 3.6 pg/microg DNA, mean +/- SD, P < 0.005). VPF/VEGF protein was time dependently released in the culture medium under basal, steady-state conditions. Compared with control cells, AngII (100 nM) caused a significant increase in the levels of released VPF/VEGF after 3 and 6 h (control 33.8 +/- 1.7 pg/microg DNA at 3 h, 42.1 +/- 1.1 at 6 h, and 117.7 +/- 10 at 24 h; AngII 54.7 +/- 3.9 at 3 h, P < 0.0001, 61.6 +/- 8.7 at 6 h, P < 0.05, and 144.7 +/- 22.7 at 24 h, NS; mean +/- SD). According to the results obtained from enzyme-linked immunosorbent assay experiments, Western blot analysis showed that the intensity of the 19-kD band corresponding to VPF/VEGF was 1.5-fold higher in AngII (100 nM)-treated HMC than in control cells. Similarly, immunocytochemistry on HMC demonstrated an increase in intracellular VPF/VEGF immunostaining in response to AngII treatment (100 nM) compared with control cells. This study demonstrated that in HMC, AngII augmented the levels of VPF/VEGF gene expression and stimulated the synthesis and secretion of its peptide by activating AT1 receptors. Through these mechanisms, AngII may affect the functions of endothelial cells during the development of renal diseases involving the glomerulus.

Regulation of human vascular endothelial growth factor mRNA stability in hypoxia by heterogeneous nuclear ribonucleoprotein L

A 126-base region of human vascular endothelial growth factor (VEGF) 3'-untranslated region, which we identified as the hypoxia stability region, forms seven hypoxia-inducible RNA-protein complexes with apparent molecular masses ranging from 40 to 90 kDa in RNA-UV-cross-linking assays. In this study, we show that proteins that form the 60-kDa RNA-protein complex with the hypoxia stability region were present in both cytoplasmic and nuclear compartments. We purified the protein associated in the 60-kDa complex and identified it as heterogeneous nuclear ribonucleoprotein L (hnRNP L) by protein sequencing. Removal of hnRNP L by immunoprecipitation specifically abolished formation of the 60-kDa complex. Synthetic deoxyribonucleotide competition studies defined the RNA-binding site of hnRNP L as a 21-base-long sequence, 5'-CACCCACCCACAUACAUACAU-3'. Immunoprecipitation of hnRNP L followed by reverse transcription-polymerase chain reaction showed that hnRNP L specifically interacts with VEGF mRNA in hypoxic cells in vivo. Furthermore, when M21 cells transfected with antisense oligodeoxyribonucleotide to the hnRNP L RNA-binding site, the VEGF mRNA half-life was significantly reduced under hypoxic conditions. Thus, we propose that specific association of hnRNP L with VEGF mRNA under hypoxia may play an important role in hypoxia-induced post-transcriptional regulation of VEGF mRNA expression.

Endothelial cell responses to hypoxic stress

1. Changes in the environmental oxygen tension to which cells are exposed in vivo result in physiological and sometimes pathological consequences that are associated with differential expression of specific genes. 2. Low oxygen tension (hypoxia) affects endothelial cellular physiology in vivo and in vitro in a number of ways, including the transcriptionally regulated expression of vasoactive substances and matrix proteins involved in modulating vascular tone or remodelling the vasculature and surrounding tissue. 3. Hypoxia results in the transcriptional induction of genes encoding vasoconstrictors and smooth muscle mitogens (PDGF-B, endothelin-1, VEGF, thrombospondin-1) and genes encoding matrix or remodelling molecules (collagenase IV (MMP-9), thrombospondin-1) and reciprocal transcriptional inhibition of vasodilatory or anti-mitogenic effectors (eNOS). 4. Oxygen appears to signal through a novel haem-containing sensor and signals initiated by this sensor alter the levels and DNA-binding activity of transcription factors such as activating protein (AP)-1, nuclear factor-kappa B and hypoxia-inducible transcription factor-1. 5. The genes encoding vasoactive factors regulated by oxygen tension are themselves also regulated by the vasoactive agent nitric oxide (NO). 6. Nitric oxide and oxygen transduce similar signals (i.e. their absence results in identical patterns of gene expression in endothelial cells and other cell types). 7. Thus, NO can feedback on and modulate signals induced by hypoxia and vice versa. For example, NO, which can act directly on smooth muscle cells as a vasodilator, can also facilitate vasodilation indirectly by reversing the production of vasoconstrictors induced by hypoxia. 8. Short-term exposure of endothelial cells to low oxygen tension results in the elaboration of predominantly vasoconstricting effectors, while longer-term and more severe hypoxic exposure generates factors that can induce smooth muscle proliferation and remodelling. 9. Thus, the endothelial cell response to hypoxic stress can result in two different consequences in the surrounding tissues, depending on the duration of the exposure: short-term exposure causes physiological and reversible modulation of vascular tone and blood flow; chronic hypoxic stress results in irreversible remodelling of the vasculature and surrounding tissues, with smooth muscle proliferation and fibrosis. 10. This dichotomy of responses to hypoxia may explain, in part, both the acute and chronic pathophysiological sequelae of diseases characterized by regional hypoxia, including atherosclerosis, pulmonary hypertension, sickle cell disease and systemic sclerosis (scleroderma).

Hypoxia increases thrombospondin-1 transcript and protein in cultured endothelial cells

The exposure of endothelial cells to hypoxic environments regulates the expression of a number of genes with products that are vasoactive or mitogenic for vascular tissue, including platelet-derived growth factor, endothelin-1, and endothelial nitric oxide synthase. Hypoxia is also known to alter the adhesive properties of endothelium toward a variety of blood cell types. Thrombospondin-1 (TSP-1) is a glycoprotein with major roles in cellular adhesion and vascular smooth muscle proliferation and migration. We report here that hypoxia induces TSP-1 gene and protein expression. Oxygen tensions of < or =30 torr resulted in TSP-1 transcript induction initially apparent at 1 to 6 hours, with maximal induction (6.5-fold+/-1.2-fold) within 24 to 48 hours in both human and bovine endothelial cells. TSP-1 protein levels remain elevated after 72 hours of continuous hypoxic exposure. The induction of TSP-1 steady-state transcript levels is caused in large part, if not entirely, by post-transcriptional stabilization of the TSP-1 mRNA. The TSP-1 induction by hypoxia is a graded and reversible physiologic response and can be mimicked by the use of cobalt chloride or the inhibition of nitric oxide production, suggesting both the involvement of a heme-containing oxygen sensor and a role for the endogenous production of nitric oxide in TSP-1 regulation. The effects of hypoxia both on the stabilization of the TSP-1 transcript and the stimulation of TSP-1 protein production are completely inhibited by arginine butyrate.

Locally administered vascular endothelial growth factor cDNA increases survival of ischemic experimental skin flaps

Microvascular surgery has emerged as an attractive area for recent advances in the field of gene therapy. The present study investigated the survival of ischemic, experimental skin flaps after treatment with the gene encoding vascular endothelial growth factor (VEGF). In 30 Sprague-Dawley rats, anterior abdominal skin flaps supplied by the epigastric artery and vein were created. Ten animals were treated with a mixture of liposomes and the cDNA encoding the 121-amino acid isoform of VEGF. Another 10 animals were treated with control plasmid DNA and liposome transfection medium; a third group of 10 animals was given physiologic saline. Each solution was injected directly into the femoral artery distal to the origin of the epigastric pedicle supplying the flap. Four days after injection, the pedicle was ligated and blood flow in the flap was approximated using dye fluorescence. Seven days later, the amount of viable tissue within the flap was measured by planimetry. After the animals were killed, specimens from both the operated and nonoperated sides of the abdomen were harvested for immunohistologic evidence of VEGF protein expression. Average dye fluorescence indices of the three groups (VEGF cDNA, control plasmid, and saline) 2 hours after pedicle ligation were 35.9, 23.9, and 53.9 percent, respectively (p < 0.05). Compared with the two control groups, flaps receiving VEGF cDNA had significantly greater tissue viability at the end of 7 days: 93.9 versus 28.1 percent for the control plasmid DNA group and 31.9 percent for the saline group (p < 0.05). Immunohistochemical staining documented increased deposition of VEGF protein in flaps that were infused with the VEGF cDNA versus saline alone (p < 0.05). The results indicated that the survival of ischemic tissues can be enhanced by administration of a cDNA encoding VEGF, a protein known to be important in the process of angiogenesis and wound healing.

Tumor induction of VEGF promoter activity in stromal cells

We have established a line of transgenic mice expressing the A. victoria green fluorescent protein (GFP) under the control of the promoter for vascular endothelial growth factor (VEGF). Mice bearing the transgene show green cellular fluorescence around the healing margins and throughout the granulation tissue of superficial ulcerative wounds. Implantation of solid tumors in the transgenic mice leads to an accumulation of green fluorescence resulting from tumor induction of host VEGF promoter activity. With time, the fluorescent cells invade the tumor and can be seen throughout the tumor mass. Spontaneous mammary tumors induced by oncogene expression in the VEGF-GFP mouse show strong stromal, but not tumor, expression of GFP. In both wound and tumor models the predominant GFP-positive cells are fibroblasts. The finding that the VEGF promoter of nontransformed cells is strongly activated by the tumor microenvironment points to a need to analyze and understand stromal cell collaboration in tumor angiogenesis.

Hypoxia and endothelin-1 induce VEGF production in human vascular smooth muscle cells

Vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) is a secreted mitogen for vascular endothelial cells, and it promotes vascular permeability and neovascularization in vivo. We investigated the mechanisms by which low oxygen tension modulates the expression of VEGF in human aortic vascular smooth muscle cells (h-SMC) in vitro. Moreover, we measured VEGF levels in the cultured medium with or without endothelin-1 (ET-1) using a newly developed, highly sensitive, enzyme-linked immunosorbent assay. Hypoxia resulted in a substantial induction of VEGF transcripts at 3 and 24 hr. VEGF levels were significantly higher when h-SMC were cultured in medium containing ET-1 than when cultured in medium without ET-1. In conclusion, hypoxia and ET-1 constitute potent stimuli for VEGF production in h-SMC.

Paracrine expression of a native soluble vascular endothelial growth factor receptor inhibits tumor growth, metastasis, and mortality rate

Vascular endothelial growth factor (VEGF) is a potent and selective vascular endothelial cell mitogen and angiogenic factor. VEGF expression is elevated in a wide variety of solid tumors and is thought to support their growth by enhancing tumor neovascularization. To block VEGF-dependent angiogenesis, tumor cells were transfected with cDNA encoding the native soluble FLT-1 (sFLT-1) truncated VEGF receptor which can function both by sequestering VEGF and, in a dominant negative fashion, by forming inactive heterodimers with membrane-spanning VEGF receptors. Transient transfection of HT-1080 human fibrosarcoma cells with a gene encoding sFLT-1 significantly inhibited their implantation and growth in the lungs of nude mice following i.v. injection and their growth as nodules from cells injected s.c. High sFLT-1 expressing stably transfected HT-1080 clones grew even slower as s.c. tumors. Finally, survival was significantly prolonged in mice injected intracranially with human glioblastoma cells stably transfected with the sflt-1 gene. The ability of sFLT-1 protein to inhibit tumor growth is presumably attributable to its paracrine inhibition of tumor angiogenesis in vivo, since it did not affect tumor cell mitogenesis in vitro. These results not only support VEGF receptors as antiangiogenic targets but also demonstrate that sflt-1 gene therapy might be a feasible approach for inhibiting tumor angiogenesis and growth.

Microenvironmental control of gene expression: implications for tumor angiogenesis, progression, and metastasis

Low oxygen tension (hypoxia) is an important prognostic factor in cancer treatment because it affects tumor formation and malignant progression. Many genes governing these complex processes have been found to be oxygen regulated. This article reviews the present knowledge of hypoxia-inducible gene expression and how this affects angiogenesis, progression, and metastasis. Of particular importance are hypoxia-regulated transcription factors because they can modulate expression of countless different genes. Additional genes analyzed in some detail include those encoding angiogenic growth factors, factors controlling blood flow, and those involved in metastasis. Although hypoxia is generally perceived as a hindrance to cancer therapy, it is possibly exploitable because severe oxygen deficiency is tumor specific. Strategies aimed at using the presence of hypoxia in solid tumors include oxygen sensitive chemotherapy and gene therapy.

Translation of vascular endothelial growth factor mRNA by internal ribosome entry: implications for translation under hypoxia

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenic growth factor that promotes compensatory angiogenesis in circumstances of oxygen shortage. The requirement for translational regulation of VEGF is imposed by the cumbersome structure of the 5' untranslated region (5'UTR), which is incompatible with efficient translation by ribosomal scanning, and by the physiologic requirement for maximal VEGF production under conditions of hypoxia, where overall protein synthesis is compromised. Using bicistronic reporter gene constructs, we show that the 1,014-bp 5'UTR of VEGF contains a functional internal ribosome entry site (IRES). Efficient cap-independent translation is maintained under hypoxia, thereby securing efficient production of VEGF even under unfavorable stress conditions. To identify sequences within the 5'UTR required for maximal IRES activity, deletion mutants were analyzed. Elimination of the majority (851 nucleotides) of internal 5'UTR sequences not only maintained full IRES activity but also generated a significantly more potent IRES. Activity of the 163-bp long "improved" IRES element was abrogated, however, following substitution of a few bases near the 5' terminus as well as substitutions close to the translation start codon. Both the full-length 5'UTR and its truncated version function as translational enhancers in the context of a monocistronic mRNA.

HIF-1 alpha is required for solid tumor formation and embryonic vascularization

The transcriptional response to lowered oxygen levels is mediated by the hypoxia-inducible transcription factor (HIF-1), a heterodimer consisting of the constitutively expressed aryl hydrocarbon receptor nuclear translocator (ARNT) and the hypoxic response factor HIF-1alpha. To study the role of the transcriptional hypoxic response in vivo we have targeted the murine HIF-1alpha gene. Loss of HIF-1alpha in embryonic stem (ES) cells dramatically retards solid tumor growth; this is correlated with a reduced capacity to release the angiogenic factor vascular endothelial growth factor (VEGF) during hypoxia. HIF-1alpha null mutant embryos exhibit clear morphological differences by embryonic day (E) 8.0, and by E8.5 there is a complete lack of cephalic vascularization, a reduction in the number of somites, abnormal neural fold formation and a greatly increased degree of hypoxia (measured by the nitroimidazole EF5). These data demonstrate the essential role of HIF-1alpha in controlling both embryonic and tumorigenic responses to variations in microenvironmental oxygenation.

Impact of oncogenes in tumor angiogenesis: mutant K-ras up-regulation of vascular endothelial growth factor/vascular permeability factor is necessary, but not sufficient for tumorigenicity of human colorectal carcinoma cells

Targeted disruption of the single mutant K-ras allele in two human colorectal carcinoma cell lines (DLD-1 and HCT-116) leads to loss of tumorigenic competence in nude mice with retention of ability to grow indefinitely in monolayer culture. Because expression of the mutant K-ras oncogene in these cell lines is associated with marked up-regulation of vascular endothelial growth factor/vascular permeability factor (VEGF/VPF), we sought to determine whether this potent angiogenesis inducer plays a role in K-ras-dependent tumorigenic competence. Transfection of a VEGF121 antisense expression vector into DLD-1 and HCT-116 cells resulted in suppression of VEGF/VPF production by a factor of 3- to 4-fold. The VEGF/VPF-deficient sublines, unlike the parental population or vector controls, were profoundly suppressed in their ability to form tumors in nude mice for as long as 6 months after cell injection. In contrast, in vitro growth of these sublines was unaffected, thus demonstrating the critical importance of VEGF/VPF as an angiogenic factor for HCT-116 and DLD-1 cells. Transfection of a full-length VEGF121 cDNA into two nontumorigenic mutant K-ras knockout sublines resulted in a weak but detectable restoration of tumorigenic ability in vivo in a subset of the transfectants, with no consistent change in growth properties in vitro. The findings indicate that mutant ras-oncogene-dependent VEGF/VPF expression is necessary, but not sufficient, for progressive tumor growth in vivo and highlight the relative contribution of oncogenes, such as mutant K-ras, to the process of tumor angiogenesis.

Advanced glycation end products increase retinal vascular endothelial growth factor expression

Advanced glycation end products (AGEs) are linked with the development of diabetic retinopathy; however, the pathogenic mechanisms are poorly defined. Vascular endothelial growth factor (VEGF) levels are increased in ischemic and nonischemic diabetic retina, and VEGF is required for the development of retinal and iris neovascularization. Moreover, VEGF alone can induce much of the concomitant pathology of diabetic retinopathy. In this study, we found that AGEs increased VEGF mRNA levels in the ganglion, inner nuclear, and retinal pigment epithelial (RPE) cell layers of the rat retina. In vitro, AGEs increased VEGF mRNA and secreted protein in human RPE and bovine vascular smooth muscle cells. The AGE-induced increases in VEGF expression were dose- and time-dependent, inhibited by antioxidants, and additive with hypoxia. Use of an anti-VEGF antibody blocked the capillary endothelial cell proliferation induced by the conditioned media of AGE-treated cells. AGEs may participate in the pathogenesis of diabetic retinopathy through their ability to increase retinal VEGF gene expression.

Thrombopoietin stimulates VEGF release from c-Mpl-expressing cell lines and haematopoietic progenitors

Vascular endothelial growth factor (VEGF) production was analysed in megakaryocytic cell lines and CD34+ haematopoietic progenitors following treatment with thrombopoietin (TPO). In CMK cells TPO caused a time- and dose-dependent increase in the levels of VEGF released into the medium. A similar effect was observed in UT-7/mpl cells transfected with the TPO receptor c-Mpl, but not in parental UT-7 cells. In CD34+ haematopoietic progenitor cell cultures TPO stimulated VEGF mRNA expression and VEGF protein release. Production of VEGF in CD34+ cultures increased with TPO-induced megakaryocytic differentiation, but not with erythroid or myelomonocytic differentiation induced respectively by erythropoietin and granulocyte-macrophage colony-stimulating factor. These results demonstrate that TPO stimulates VEGF release in c-Mpl-expressing cells and suggest that this process is an integral feature of the megakaryocytic differentiation programme.

Upregulation of vascular endothelial growth factor by angiotensin II in rat heart endothelial cells

Vascular endothelial growth factor (VEGF) is a potent mitogen for endothelial cells and a vascular permeability factor. In this study we found that the addition of angiotensin II (AII) to rat heart endothelial cells induced VEGF mRNA production. VEGF mRNA levels reached a plateau within 2 h after the addition of AII and decreased after 4 h. The induction was superinduced by cycloheximide and blocked by actinomycin D. Losartan, an AT1 receptor antagonist, abolished the induction of VEGF mRNA by AII, whereas PD 123319, an AT2 receptor antagonist, had no effect on VEGF mRNA induction. H7, a protein kinase C inhibitor, blocked the induction. RT-PCR experiments showed two mRNA species (VEGF 120 and VEGF 164) in these cells and both species were stimulated by AII. Transient transfection experiment showed that VEGF promoter activity was increased 2.2-fold upon AII stimulation. Electrophoretic mobility shift assay revealed an enhanced binding of transcription factors AP-1 and NF-kappa B. Immunoblot analysis showed that the amount of secreted VEGF was elevated in the medium 8 h after AII stimulation. Our results demonstrate for the first time that the upregulation of VEGF by AII may play a significant role in AII-induced hyperpermeability.

Identification of a human VPF/VEGF 3' untranslated region mediating hypoxia-induced mRNA stability

Hypoxia is a prominent feature of malignant tumors that are characterized by angiogenesis and vascular hyperpermeability. Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) has been shown to be up-regulated in the vicinity of necrotic tumor areas, and hypoxia potently induces VPF/VEGF expression in several tumor cell lines in vitro. Here we report that hypoxia-induced VPF/VEGF expression is mediated by increased transcription and mRNA stability in human M21 melanoma cells. RNA-binding/electrophoretic mobility shift assays identified a single 125-bp AU-rich element in the 3' untranslated region that formed hypoxia-inducible RNA-protein complexes. Hypoxia-induced expression of chimeric luciferase reporter constructs containing this 125-bp AU-rich hypoxia stability region were significantly higher than constructs containing an adjacent 3' untranslated region element without RNA-binding activity. Using UV-cross-linking studies, we have identified a series of hypoxia-induced proteins of 90/88 kDa, 72 kDa, 60 kDa, 56 kDa, and 46 kDa that bound to the hypoxia stability region element. The 90/88-kDa and 60-kDa species were specifically competed by excess hypoxia stability region RNA. Thus, increased VPF/VEGF mRNA stability induced by hypoxia is mediated, at least in part, by specific interactions between a defined mRNA stability sequence in the 3' untranslated region and distinct mRNA-binding proteins in human tumor cells.

Two functional forms of vascular endothelial growth factor receptor-2/Flk-1 mRNA are expressed in normal rat retina

Vascular endothelial growth factor (VEGF) is an important mediator of ocular neovascularization by exerting its endothelial specific mitogenic effects through high affinity tyrosine kinase receptors. By screening a rat retina cDNA library, we have isolated a clone encoding the full-length prototypic form of the rat VEGF receptor-2/Flk-1, as well as a short form of the mRNA that encodes the complete seven N-terminal immunoglobulin-like extracellular ligand-binding domains, transmembrane region, NH2-terminal half of the intracellular kinase domain, and kinase insert domain but does not encode the COOH-terminal half of the intracellular kinase domain and carboxyl-terminal region. Both forms of mRNA are detected in rat retina, although the short form is expressed at a lower level. VEGF induced a biphasic increase of cytoplasmic calcium with both forms in HK 293 transfected cells, indicating that both forms of the VEGF receptor-2/Flk-1 are functional and that the COOH-terminal half of the intracellular kinase domain and carboxyl region of VEGF receptor-2/Flk-1 are not strictly necessary for either ligand binding or this biological activity.

Down-regulation of vascular endothelial growth factor in a human colon carcinoma cell line transfected with an antisense expression vector specific for c-src

Vascular endothelial growth factor (VEGF) is implicated in the angiogenesis of human colon cancer. Recent evidence suggests that factors that regulate VEGF expression may partially depend on c-src-mediated signal transduction pathways. The tyrosine kinase activity of Src is activated in most colon tumors and cell lines. We established stable subclones of the human colon adenocarcinoma cell line HT29 in which Src expression and activity are decreased specifically as a result of a transfected antisense expression vector. This study determined whether VEGF expression is decreased in these cell lines and whether the smaller size and reduced growth rate of antisense vector-transfected cell lines in vivo might result, in part, from reduced vascularization of tumors. Northern blot analysis of these cell lines revealed that VEGF mRNA expression was decreased in proportion to the decrease in Src kinase activity. Under hypoxic conditions, cells with decreased Src activity had a <2-fold increase in VEGF expression, whereas parental cells had a >50-fold increase. VEGF protein in the supernatants of cells was also reduced in antisense transfectants compared with that from parental cells. In nude mice, subcutaneous tumors from antisense transfectants showed a significant reduction in vascularity. These results suggest that Src activity regulates the expression of VEGF in colon tumor cells.

Vascular endothelial growth factor (VEGF) in normal human corneal epithelium: detection and physiological importance

PURPOSE

Corneal neovascularization is a major challenge following chemical burns and corneal inflammation. The factors triggering corneal neovascularization involve various growth factors. In the release and control of these factors the regenerating tissue plays a decisive role. Only recently has vascular endothelial growth factor been shown to be involved in the basic events of retinal neovascularization. From the cornea current knowledge allows only for the statement that corneal epithelium could be able to produce vascular endothelial growth factor. The present study was designed to show the presence of vascular endothelial growth factor-like substances in the corneal epithelium of the normal eye in vivo.

METHODS

Using specific antibodies to the N-terminus of human vascular endothelial growth factor indirect immuno-histochemistry was applied to sections of normal human corneal epithelium and to five sections of enucleated human eyes with a morphologically normal anterior segment.

RESULTS

In all sections specific staining for vascular endothelial growth factor was found over the entire epithelium. Staining was most intense in the basal layer of epithelial cells. Only a weak staining was detectable in the cell layers closer to the surface. Here, however, the samples of corneal epithelium having been subject to traumatic erosion showed a slightly more intense staining than the sections from the undisturbed corneal tissue of whole globe sections.

CONCLUSION

It was shown that vascular endothelial growth factor-like protein is present in the human corneal epithelium. Observed differences in the staining pattern could suggest that the expression of vascular endothelial growth factor might be enhanced due to ocular surface trauma. It is suggested that corneal epithel vascular endothelial growth factor might be an important factor in the cascade leading to the onset of corneal neovascularization.

Vascular endothelial growth factor is expressed in ovine pulmonary vascular smooth muscle cells in vitro and regulated by hypoxia and dexamethasone

Chronic lung disease in neonates results from both lung injury and inadequate repair processes. Little is known about the growth factors involved in lung injury and repair, but vascular endothelial growth factor (VEGF) has recently been reported in several animal models of lung injury. VEGF is an endothelial cell-specific mitogen, which is also known as vascular permeability factor because of its ability to induce vascular leak in some tissues. Chronic lung disease is complicated by increased vascular permeability, which can be improved by avoidance of hypoxia and in some cases by dexamethasone therapy. In many cells, hypoxia stimulates VEGF expression. Also, in some cases, dexamethasone blocks VEGF expression. This study examined the role of hypoxia and dexamethasone in regulating the expression of VEGF in pulmonary artery smooth muscle cells. An ovine VEGF cDNA fragment (453 bp) was cloned and found to be highly homologous to known human sequences for VEGF165. Sheep pulmonary artery smooth muscle cells were cultured and exposed to room air, hypoxia, and dexamethasone, alone or in combination for 6 h. At baseline these cells expressed VEGF mRNA at approximately 3.9 kb. The half-life of VEGF mRNA in the smooth muscle cells was 171 min, more than 3-fold longer than previous reports for epithelial cells. Exposure to hypoxia caused a 3-fold increase in mRNA abundance, primarily through transcriptional up-regulation. Dexamethasone blocked the hypoxia-induced increase in VEGF mRNA. The results demonstrate that hypoxia and dexamethasone are regulators of VEGF expression in ovine pulmonary artery smooth muscle cells. It is not known whether VEGF derived from these cells is involved in lung injury and/or normal homeostatsis.

Rapid induction of vascular endothelial growth factor gene expression after transient middle cerebral artery occlusion in rats

BACKGROUND AND PURPOSE

Vascular endothelial growth factor (VEGF) is a mitogen for endothelial cells and also has the potential to increase vascular permeability. Therefore, it may contribute to the recovery of brain cells from ischemic insult through potentiating neovascularization or may exacerbate brain damage by forming brain edema. However, the exact role of this protein in cerebral ischemia is not fully understood. We investigated temporal, spatial, and cellular profiles of the induction of VEGF gene expression after transient focal cerebral ischemia at both mRNA and protein levels.

METHODS

We used a transient middle cerebral artery (MCA) occlusion model. Northern blot analysis was performed to assess the chronological pattern of induction and the impact of length of ischemia on mRNA expression. Western blot analysis was performed to ensure the selective detection of immunoreactive VEGF with an antibody. Temporal, spatial, and cellular changes of immunohistochemical VEGF expression were compared with different periods of reperfusion from 1 hour to 7 days after transient MCA occlusion.

RESULTS

(1) Northern blot analysis revealed no detectable VEGF mRNA in the control brains. The mRNA became evident at 1 hour after reperfusion, peaked at 3 hours, and then decreased. The length of ischemia from 1 to 3 hours made no differences in the degree and temporal profile of the subsequent induction of VEGF mRNA. (2) Western blot analysis showed no band in the control brain, but two bands with molecular weights of 38 and 45 kD, corresponding to VEGF121 and VEGF165, were induced at 1 hour of reperfusion, peaked at 3 hours of reperfusion, and then decayed. (3) Neurons in the cerebral cortex of the MCA territory expressed VEGF at 1 hour after reperfusion with a peak at 3 hours and then diminished by 1 day. Pial cells of the MCA territory also expressed immunoreactive VEGF from 1 hour of reperfusion that was sustained until 3 to 7 days after reperfusion.

CONCLUSIONS

Rapid induction of VEGF gene expression after transient MCA occlusion was demonstrated at both mRNA and protein levels. Cortical neurons and pial cells were the source of VEGF production in this model, but the temporal profiles of the induction between these cells were different. The early but dissociative induction of VEGF between neuronal and pial cells suggests different roles of the protein in their cells after transient MCA occlusion.

Making (anti)sense of non-coding sequence conservation

A substantial fraction of vertebrate mRNAs contain long conserved blocks in their untranslated regions as well as long blocks without silent changes in their protein coding regions. These conserved blocks are largely comprised of unique sequence within the genome, leaving us with an important puzzle regarding their function. A large body of experimental data shows that these regions are associated with regulation of mRNA stability. Combining this information with the rapidly accumulating data on endogenous antisense transcripts, we propose that the conserved sequences form long perfect duplexes with antisense transcripts. The formation of such duplexes may be essential for recognition by post-transcriptional regulatory systems. The conservation may then be explained by selection against the dominant negative effect of allelic divergence.

Hypoxia stimulates proximal tubular cell matrix production via a TGF-beta1-independent mechanism

Tubulointerstitial fibrosis is characterized by tubular basement membrane thickening and accumulation of interstitial extracellular matrix (ECM). Since chronic low-grade hypoxia has been implicated in the pathogenesis of fibrosis and proximal tubular epithelial cells (PTE) are sensitive to oxygen deprivation, we hypothesized that hypoxia may stimulate ECM accumulation. In human PTE, hypoxia (1% O2, 24 hr) increased total collagen production (15%), decreased MMP-2 activity (55% +/- 13%; control = 100%) and increased tissue inhibitor of metalloproteinase-1 (TIMP-1) protein. Collagen IV mRNA levels decreased while collagen I mRNA increased, suggesting induction of interstitial collagen. Hypoxia-induced changes persisted on re-oxygenation with increased expression of TIMP mRNAs. A potential mediator for these effects is transforming growth factor-beta1 (TGF-beta1), a major pro-fibrogenic factor produced by PTE. Although hypoxia stimulated TGF-beta production (2- to 3-fold), neutralizing anti-TGF-beta1 antibody did not abolish the hypoxia-induced changes in gelatinase activity, TIMP-1, collagen IV or collagen I mRNA expression, implying that TGF-beta1 is not the mediator. Furthermore, exogenous TGF-beta1 (0 to 10 ng/ml) did not mimic hypoxia, as it stimulated MMP-2 activity and increased the expression of collagen IV, collagen I and TIMP-1 mRNA. The data suggest that hypoxia may be an important pro-fibrogenic stimulus independent of TGF-beta1.

A novel RNA species from the turtle mitochondrial genome: induction and regulation of transcription and processing under anoxic and freezing stresses

The present study identifies a previously cloned cDNA, pBTaR914, as homologous to the mitochondrial WANCY (tryptophan, alanine, asparagine, cysteine, and tyrosine) tRNA gene cluster. This cDNA clone has a 304-bp sequence and its homologue, pBTaR09, has a 158-bp sequence with a long poly(A)+ tail (more than 60 adenosines). RNA blotting analysis using pBTaR914 probe against the total RNA from the tissues of adult and hatchling turtles revealed five bands: 540, 1800, 2200, 3200, and 3900 nucleotides (nt). The 540-nt transcript is considered to be an intact mtRNA unit from a novel mtDNA gene designated WANCYHP that overlaps the WANCY tRNA gene cluster region. This transcript was highly induced by both anoxic and freezing stresses in turtle heart. The other transcripts are considered to be the processed intermediates of mtRNA transcripts with WANCYHP sequence. All these transcripts were differentially regulated by anoxia and freezing in different organs. The data suggest that mtRNA processing is sensitive to regulation by external stresses, oxygen deprivation, and freezing. Furthermore, the fact that the WANCYHP transcript is highly induced during anoxic exposure suggests that it may play an important role in the regulation of mitochondrial activities to coordinate the physiological adaptation to anoxia.

Stretch-induced VEGF expression in the heart

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen involved in vascular development and angiogenesis. Recently we have observed increased VEGF expression in the normal myocardium after myocardial infarction in a rat heart. This study was designed to explore the mechanism responsible for this increase in VEGF expression. Induction of myocardial stretch in an isolated perfused Langendorff preparation by inflation of an intraventricular balloon to an end-diastolic load of 35 mmHg for 30 min resulted in a nearly sixfold increase in VEGF message level not only in the chamber subjected to stretch (left ventricle) but also in the unstretched right ventricle, thus raising the possibility of a soluble factor mediating stretch- induced induction of VEGF expression. This was further confirmed by demonstrating that coronary venous effluent collected from the stretched heart and used to perfuse isolated hearts in which no balloon was present was able to induce VEGF expression in these normal hearts. Inhibition of TGF-beta activity using a neutralizing antibody, but not antagonists/inhibitors of endothelin and angiotensin II, eliminated stretch-induced increase in VEGF expression. Staurosporine, a protein kinase C inhibitor, also blocked stretch-induced increase of VEGF expression. Measurement of TGF-beta concentration in the perfusate demonstrated increased amounts of the cytokine after myocardial stretch, and addition of TGF-beta protein to the perfusion buffer resulted in increased VEGF expression in control hearts. These results suggest that stretch-induced increase of VEGF expression in the heart is mediated at least in part by TGF-beta.

Advanced glycation end products-driven angiogenesis in vitro. Induction of the growth and tube formation of human microvascular endothelial cells through autocrine vascular endothelial growth factor

This study was undertaken to determine whether and how advanced glycation end products (AGE), senescent macroproteins accumulated in various tissues under hyperglycemic states, cause angiogenesis, the principal vascular derangement in diabetic microangiopathy. We first prepared AGE-bovine serum albumin (BSA) and anti-AGE antiserum using AGE-RNase A. Then AGE-BSA was administered to human skin microvascular endothelial cells in culture, and their growth was examined. The AGE-BSA, but not nonglycated BSA, was found to induce a statistically significant increase in the number of viable endothelial cells as well as their synthesis of DNA. The increase in DNA synthesis by AGE-BSA was abolished by anti-AGE antibodies. AGE-BSA also stimulated the tube formation of endothelial cells on Matrigel. We obtained the following evidence that it is vascular endothelial growth factor (VEGF) that mainly mediates the angiogenic activities of AGE. (1) Quantitative reverse transcription-polymerase chain reaction analysis of poly(A)+ RNA from microvascular endothelial cells revealed that AGE-BSA up-regulated the levels of mRNAs for the secretory forms of VEGF in time- and dose-dependent manners, while endothelial cell expression of the genes encoding the two VEGF receptors, kinase insert domain-containing receptor and fms-like tyrosine kinase 1, remained unchanged by the AGE treatment. Immunoprecipitation analysis revealed that AGE-BSA did increase de novo synthesis of VEGF. (2) Monoclonal antibody against human VEGF completely neutralized both the AGE-induced DNA synthesis and tube formation of the endothelial cells. The results suggest that AGE can elicit angiogenesis through the induction of autocrine vascular VEGF, thereby playing an active part in the development and progression of diabetic microangiopathies.

Quantitation of vascular endothelial growth factor mRNA levels in human breast tumors and metastatic lymph nodes

In situ hybridization analysis provides a means to qualitatively study the heterogeneity of primary tumors and metastases based on the types of genes transcribed. In this study, we have tested some parameters for quantitative analysis of in situ hybridizations with paraffin-embedded human breast tumors and measured mRNA levels for the angiogenic protein, vascular endothelial growth factor (VEGF). VEGF mRNAs were highly tumor specific, with the highest levels near necrotic regions within the tissues (0.1 to 2.7 dpm/mm2). Normal cells within the tissue sections did not have detectable levels of VEGF mRNA. For comparison, tumor levels of c-myc (4 to 46 dpm/mm2) and glyceraldehyde-3-phosphate dehydrogenase mRNAs (48 to 214 dpm/mm2) were measured. The mRNAs for both of these genes were more broadly expressed across the tissue sections. The hybridization pattern for VEGF mRNAs was consistent with hypoxia-induced VEGF mRNA steady-state levels and supports the hypothesis that oxidative stress regulates VEGF expression in breast tumors.

Oxygen- and dioxin-regulated gene expression in mouse hepatoma cells

The discovery that the oxygen-regulated transcription factor HIF-1 alpha and the dioxin receptor AhR share the common heterodimerization partner ARNT (HIF-1 beta) raised the question whether a cross-talk between oxygen and dioxin signal transduction pathways exists. To answer this question we investigated an ARNT-deficient mutant cell line (Hepa1C4), which has lost its capability of responding to dioxin. The results demonstrate that the presence of ARNT is indispensable for hypoxia-inducible HIF-1 DNA binding as well as for oxygen-regulated reporter gene activity mediated by the EPO 3' hypoxia response element (HRE). Hypoxic induction of the vascular endothelial growth factor (VEGF) gene, however, was only partially abrogated in Hepa1C4 cells, suggesting that HIF-1-independent oxygen signaling pathways might exist. We further studied HIF-1 and AhR/ARNT DNA binding activity as well as the regulation of oxygen- and xenobiotic-responsive genes by treating mouse Hepa1 hepatoma cells with hypoxia and/or the dioxin analogue ICZ. Hypoxia-inducible VEGF expression was found to be independent of ICZ-treatment, whereas ICZ-inducible cytochrome P-450IA1 expression was slightly reduced by hypoxic treatment of the cells. Interestingly, the enhancer function of a xenobiotic response element (XRE) linked to a reporter gene was induced by hypoxia, but expression of a HRE-containing reporter gene was not affected by ICZ treatment.

Regulation of vascular endothelial growth factor by hypoxia and its modulation by the von Hippel-Lindau tumor suppressor gene

The regulation of VEGF production is mediated by both transcriptional and post-transcriptional mechanisms. A schematic model of elements involved in hypoxic regulation of VEGF is shown in Figure 1.

Induction of VEGF and VEGF receptor gene expression by hypoxia: divergent regulation in vivo and in vitro

This study examined the expression of EPO, VEGF and VEGF receptor gene under conditions of reduced oxygen supply in primary cultures of rat hepatocytes, and compared it with the expression of these genes in hypoxic rat livers in vivo. To this end we exposed male Sprague-Dawley rats to hypoxia (10% and 8% O2), carbon monoxide (0.1% CO) or injected cobalt chloride (60 mg/kg CoCl2) subcutaneously. For the in vitro experiments we used primary cultures of rat hepatocytes which were kept at high (20% O2) and low (1% O2) oxygen tensions for three hours. The EPO mRNA was up-regulated by hypoxia in vitro and in vivo about 10-fold. The VEGF mRNA was up-regulated fivefold in the hepatocytes only, whereas the in vivo mRNA levels remained unchanged. The mRNA levels of flt-1 were up-regulated threefold by 8% O2 in livers, dependent on the strength of hypoxia (10% caused no changes in flt-1 gene expression) and on the kind of hypoxic stimulus (8% O2 was as effective as 0.1% CO and more effective than cobalt). The mRNA levels of flk-1/KDR and flt-4 remained unchanged in the liver. In vitro there were no changes in the mRNA levels of flt-1, flt-4 and flk-1/KDR. Consequently, the in vivo regulation of VEGF, which might be modulated by induction of flt-1 receptor gene expression, differs from the in vitro cell culture situation and might be different from the EPO regulation in vivo.

Role of adenosine in the hypoxic induction of vascular endothelial growth factor in porcine brain derived microvascular endothelial cells

Hypoxia induced the mRNA expression of vascular endothelial growth factor (VEGF) in porcine brain derived microvascular endothelial cells (BMEC) in a time-dependent manner. Corresponding to the mRNA induction the protein level of VEGF was elevated during hypoxia. The adenosine A1 receptor antagonist 8-phenyltheophylline (8-PT) reduced the hypoxia-induced VEGF mRNA and protein expression significantly. The treatment of BMEC with cobalt chloride-known to activate an oxygen sensing mechanism similar to the one used by the erythropoietin gene-also induced the VEGF mRNA expression, but 8-PT did not reduce this VEGF induction. Although, earlier studies revealed that agents like phorbolester induced the VEGF mRNA expression, the specific inhibitor of the proteinkinase C (PKC) bisindolylmaleimide (BIM) did not reduce but enhanced the hypoxia-induced VEGF mRNA expression. These results indicate that the VEGF induction in BMEC can proceed through PKC-dependent and -independent pathways (like those acting via the putative oxygen sensor). Hypoxia in BMEC probably activates the PKC-dependent pathway mainly via adenosine which might be formed during hypoxia and thereby inhibits activation of PKC-independent, oxygen sensing, pathways. This suggestion was supported by the fact that hypoxia as well as adenosine increased the VEGF mRNA expression post-transcriptionally by enhancing the stability of the VEGF mRNA [corrected].

In vivo angiogenic activity and hypoxia induction of heterodimers of placenta growth factor/vascular endothelial growth factor

To investigate the in vivo angiogenic activity of placenta growth factor (PIGF) and its heterodimers with vascular endothelial growth factor (VEGF), the induction of neovascularization of these factors in the mouse cornea was studied. VEGF165 is sufficiently potent to stimulate new capillary growth from the limbal vessels. PIGF129/VEGF165 heterodimers also induce corneal neovascularization with a maximal vessel length similar to VEGF165, but with a marked decrease of vessel density. In contrast, PIGF129 has little or no effect in this in vivo angiogenesis assay. The expression of VEGF mRNA and protein is drastically up-regulated by hypoxia in choriocarcinoma cells, whereas expression of PIGF is not affected by the low concentration of oxygen. Up-regulation of VEGF production results in increased formation of PIGF/VEGF heterodimers in these tumor cells. Thus, hypoxia indirectly up-regulates expression levels of PIGF/VEGF heterodimers and modulates VEGF activity when these factors are co-expressed.

Hypoxia-inducible protein binding to vascular endothelial growth factor mRNA and its modulation by the von Hippel-Lindau protein

Hypoxia induces an increase in the stability of the mRNA encoding vascular endothelial growth factor (VEGF). We have previously demonstrated that a 500-base region of the 3'-untranslated region of VEGF mRNA that is critical for stabilization of VEGF mRNA in an in vitro degradation assay forms a RNA-protein complex in a hypoxia-inducible fashion. We report here the identification of three adenylate-uridylate-rich RNA elements within this region that form an identical or closely related hypoxia-inducible RNA-protein complex. This complex is constitutively elevated in a tumor cell line lacking the wild type von Hippel-Lindau tumor suppressor gene and in which VEGF mRNA is constitutively stabilized. Furthermore, the glucose transporter-1 mRNA, which is also stabilized by hypoxia, forms a hypoxia-inducible RNA-protein complex with similar sequence and protein binding characteristics to that described for VEGF mRNA. Finally, RNA affinity purification and UV cross-linking were used to identify three proteins of 32, 28, and 17 kDa that are derived from this hypoxia-inducible RNA-protein complex.

Post-transcriptional regulation of vascular endothelial growth factor mRNA by the product of the VHL tumor suppressor gene

The VHL tumor suppressor gene is inactivated in patients with von Hippel-Lindau disease and in most sporadic clear cell renal carcinomas. Although VHL protein function remains unclear, VHL does interact with the elongin BC subunits in vivo and regulates RNA polymerase II elongation activity in vitro by inhibiting formation of the elongin ABC complex. Expression of wild-type VHL in renal carcinoma cells with inactivated endogenous VHL resulted in unaltered in vitro cell growth and decreased vascular endothelial growth factor (VEGF) mRNA expression and responsiveness to serum deprivation. VEGF is highly expressed in many tumors, including VHL-associated and sporadic renal carcinomas, and it stimulates neoangiogenesis in growing solid tumors. Despite 5-fold differences in VEGF mRNA levels, VHL overexpression did not affect VEGF transcription initiation or elongation as would have been suggested by VHL-elongin association. These results suggest that VHL regulates VEGF expression at a post-transcriptional level and that VHL inactivation in target cells causes a loss of VEGF suppression, leading to formation of a vascular stroma.

Reactive oxygen intermediates increase vascular endothelial growth factor expression in vitro and in vivo

Elevated vascular endothelial growth factor (VEGF) levels are required for ocular and tumor angiogenesis in animal models. Ischemic hypoxia is strongly correlated with increased VEGF expression in these systems and is considered a physiologically relevant stimulus. Because ischemic hypoxia is often followed by reperfusion and reactive oxygen intermediate (ROI) generation, we examined the potential role of ROI in the control of VEGF gene expression. Human retinal pigment epithelial cells exposed to superoxide or hydrogen peroxide rapidly increased VEGF mRNA levels. Superoxide-associated mRNA increases were dose dependent, blocked by antioxidants, and associated with elevated VEGF protein levels in conditioned media. Increases in VEGF mRNA levels were also observed in cultured human melanoma and rat glioblastoma cells with superoxide or hydrogen peroxide. Cycloheximide prevented the ROI-associated increases in VEGF mRNA. Transcriptional inhibition with actinomycin D revealed an inducible increase in VEGF mRNA half-life, but nuclear run-on experiments showed no increase in VEGF transcriptional rate. Reoxygenation of human retinal pigment epithelial cells in vitro and ocular reperfusion in vivo increased retinal VEGF mRNA levels. Antioxidants prevented the reperfusion-associated VEGF mRNA increases in retina. We conclude that ROIs increase VEGF gene expression in vitro and during the reperfusion of ischemic retina in vivo. The ROI-associated increases are mediated largely through increases in VEGF mRNA stability.

Anoxia-induced gene expression in turtle heart. Upregulation of mitochondrial genes for NADH-ubiquinone oxidoreductase subunit 5 and cytochrome c oxidase subunit 1

A cDNA library constructed from heart of anoxia-exposed adult turtles (Trachemys scripta elegans) was differentially screened with 32P-labeled single-stranded cDNA probes from heart of control versus anoxic animals to clone genes induced by anoxia stress. Four cDNA clones, pBTaR20, pBTaR34, pBTaR63 and pBTaR914 were obtained and confirmed to be upregulated in response to anoxic submergence (20 h in N2-bubbled water at 7 degrees C). Two clones, pBTaR20 and pBTaR63, were characterized by sequence analysis and in vivo expression. The clone pBTaR20 had a 1597-bp cDNA sequence and pBTaR63 contained a 1837-bp sequence. The pBTaR20 sequence contained a single open reading frame that was very close to full length and could potentially encode a polypeptide with 508 amino acids. The deduced polypeptide sequence showed approximately 83% of the residues identical with the sequence of cytochrome c oxidase subunit 1 (CO1) that is encoded by a mtDNA gene Cox1. The clone pBTaR63 contained a single potentially full-length open reading frame that could encode a polypeptide of 591 residues. This was similar to another mitochondrial protein, NADH-ubiquinone oxidoreductase subunit 5 (ND5), which is encoded by mtDNA gene Nad5. Analysis of the time course of expression of Cox1 and Nad5 by northern hybridization analysis showed that mRNA transcripts for both accumulated rapidly (within 1 h) in response to anoxia exposure. Both showed similar increases in their transcript content after 1 h of anoxia but with longer anoxia exposures (5 or 20 h) Nad5 mRNA levels remained high whereas Cox1 mRNA content declined somewhat. Northern-blot hybridization also revealed differential expression of these two genes in five other organs of T. s. elegans during anoxia exposure (brain, kidney, liver, red and white skeletal muscle), with a particularly large increase in mRNA transcript levels of both genes in anoxic red muscle. Organ-specific analysis of these genes in a freeze-tolerant turtle species (Chrysemys picta marginata) also showed that differential expression of these genes occurred in response to the ischemia induced by plasma freezing.

Negative regulation of hypoxia-inducible genes by the von Hippel-Lindau protein

Inactivation of the von Hippel-Lindau protein (pVHL) has been implicated in the pathogenesis of renal carcinomas and central nervous system hemangioblastomas. These are highly vascular tumors which overproduce angiogenic peptides such as vascular endothelial growth factor/vascular permeability factor (VEGF/VPF). Renal carcinoma cells lacking wild-type pVHL were found to produce mRNAs encoding VEGF/VPF, the glucose transporter GLUT1, and the platelet-derived growth factor B chain under both normoxic and hypoxic conditions. Reintroduction of wild-type, but not mutant, pVHL into these cells specifically inhibited the production of these mRNAs under normoxic conditions, thus restoring their previously described hypoxia-inducible profile. Thus, pVHL appears to play a critical role in the transduction of signals generated by changes in ambient oxygen tension.

Functional interference between hypoxia and dioxin signal transduction pathways: competition for recruitment of the Arnt transcription factor

Hypoxia-inducible factor 1 alpha (HIF-1 alpha) and the intracellular dioxin receptor mediate hypoxia and dioxin signalling, respectively. Both proteins are conditionally regulated basic helix-loop-helix (bHLH) transcription factors that, in addition to the bHLH motif, share a Per-Arnt-Sim (PAS) region of homology and form heterodimeric complexes with the common bHLH/PAS partner factor Arnt. Here we demonstrate that HIF-1 alpha required Arnt for DNA binding in vitro and functional activity in vivo. Both the bHLH and PAS motifs of Arnt were critical for dimerization with HIF-1 alpha. Strikingly, HIF-1 alpha exhibited very high affinity for Arnt in coimmunoprecipitation assays in vitro, resulting in competition with the ligand-activated dioxin receptor for recruitment of Arnt. Consistent with these observations, activation of HIF-1 alpha function in vivo or overexpression of HIF-1 alpha inhibited ligand-dependent induction of DNA binding activity by the dioxin receptor and dioxin receptor function on minimal reporter gene constructs. However, HIF-1 alpha- and dioxin receptor-mediated signalling pathways were not mutually exclusive, since activation of dioxin receptor function did not impair HIF-1 alpha-dependent induction of target gene expression. Both HIF-1 alpha and Arnt mRNAs were expressed constitutively in a large number of human tissues and cell lines, and these steady-state expression levels were not affected by exposure to hypoxia. Thus, HIF-1 alpha may be conditionally regulated by a mechanism that is distinct from induced expression levels, the prevalent model of activation of HIF-1 alpha function. Interestingly, we observed that HIF-1 alpha was associated with the molecular chaperone hsp90. Given the critical role of hsp90 for ligand binding activity and activation of the dioxin receptor, it is therefore possible that HIF-1 alpha is regulated by a similar mechanism, possibly by binding an as yet unknown class of ligands.

Vascular endothelial growth factor and its receptors

Vascular endothelial growth factor (VEGF) is a prime regulator of endothelial cell proliferation, angiogenesis, vasculogenesis and vascular permeability. Its activity is mediated by the high affinity tyrosine kinase receptors, KDR/Fik-1 and Fit-1. In this article, recently discovered structural, molecular and biological properties of VEGF are described. Among the topics discussed are VEGF and VEGF receptor structure and bioactivity, the regulation of VEGF expression, the role of VEGF and its receptors in vascular development, and the involvement of VEGF and its receptors in normal and pathological (ocular and tumor) angiogenesis.