Hypoxia regulates vascular endothelial growth factor gene expression in endothelial cells. Identification of a 5' enhancer

VEGF mRNA is stabilized by ras and tyrosine kinase oncogenes, as well as by UV radiation--evidence for divergent stabilization pathways

Vascular Endothelial Growth Factor (VEGF) is a pivotal endothelial cell mitogen that mediates both normal and pathological angiogenesis. Although expressed at very low levels in cells not undergoing vascularization, VEGF mRNA is transiently upregulated and stabilized by a variety of extracellular stimuli, and is persistently upregulated and stabilized in many human tumor cell lines (White et al., 1995). Here we demonstrate that oncogenic activation of tyrosine protein kinases and Ras proteins induce a 6- to 16-fold increase in the abundance of VEGF mRNA and a 3- to 5-fold increase in the stability of VEGF mRNA, suggesting that persistent activation of signaling pathways induced by these oncoproteins accounts for overexpression of VEGF in a significant fraction of human tumors. In addition to these oncoproteins, ultraviolet (UV) radiation upregulated and stabilized VEGF mRNA 15- and 5-fold, respectively. While the tyrosine kinase inhibitor, genistein, blocked VEGF upregulation by activated tyrosine protein kinases, and the Ras inhibitor, N-Acetyl-S-trans-farnesyl-L-cysteine (AFC), eliminated VEGF expression in cells transformed by v-Ras, neither agent blocked upregulation by hypoxia or UV radiation. These data argue that multiple divergent pathways upregulate and stabilize VEGF mRNA.

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

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

Elevated levels of the angiogenic cytokines basic fibroblast growth factor and vascular endothelial growth factor in sera of cancer patients

The concentration of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) was determined in the serum of 90 untreated and 42 treated metastatic cancer patients, including patients with colorectal, breast, ovarian and renal carcinomas, with an enzyme-linked immunosorbent assay (ELISA). Levels higher than the 95th percentile of the concentrations of a control group, i.e. 7.5 pg ml(-1) for bFGF and 500 pg ml(-1) for VEGF, were identified as 'elevated'. One measurement during follow-up was included into the analysis per patient. For 19 treated patients, consecutive serum samples were analysed. Fifty-seven per cent of all untreated patients had elevated serum levels of one or both angiogenic factors. The fraction of patients with elevated serum levels of bFGF and/or VEGF was similar in the different tumour types. Agreement of bFGF levels and VEGF levels, classified in relation to their respective cut-off values, was present in 67% of all patients. Fifty-eight per cent of the patients with progressive disease during treatment compared with 15% of the patients showing response to treatment (chi-squared test P < 0.05) had elevated bFGF and/or VEGF serum levels. When consecutive serum samples were analysed, two-thirds of the patients showing progressive disease had increasing serum levels of the angiogenic factors compared with less than one-tenth of the patients showing response (chi-squared test P < 0.05). The lack of association between the serum bFGF and VEGF levels and the tumour type may suggest an aspecific host reaction responsible for solid tumour-related angiogenesis. The main determinants of the serum bFGF and VEGF concentration are the progression kinetics of the metastatic carcinomas.

Hypoxia response elements in the aldolase A, enolase 1, and lactate dehydrogenase A gene promoters contain essential binding sites for hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a basic helix-loop-helix transcription factor which is expressed when mammalian cells are subjected to hypoxia and which activates transcription of genes encoding erythropoietin, vascular endothelial growth factor, and other proteins that are important for maintaining oxygen homeostasis. Previous studies have provided indirect evidence that HIF-1 also regulates transcription of genes encoding glycolytic enzymes. In this paper we characterize hypoxia response elements in the promoters of the ALDA, ENO1, and Ldha genes. We demonstrate that HIF-1 plays an essential role in activating transcription via these elements and show that although absolutely necessary, the presence of a HIF-1 binding site alone is not sufficient to mediate transcriptional responses to hypoxia. Analysis of hypoxia response elements in the ENO1 and Ldha gene promoters revealed that each contains two functionally-essential HIF-1 sites arranged as direct and inverted repeats, respectively. Our data establish that functional hypoxia-response elements consist of a pair of contiguous transcription factor binding sites at least one of which contains the core sequence 5'-RCGTG-3' and is recognized by HIF-1. These results provide further evidence that the coordinate transcriptional activation of genes encoding glycolytic enzymes which occurs in hypoxic cells is mediated by HIF-1.

The role of vascular endothelial growth factor in blood vessel formation

Angiogenic growth factors and their endothelial receptors function as signalling molecules during vascular growth and development. Vascular endothelial growth factor (VEGF) and its receptors represent a key regulatory system of embryonic vascular development and of both physiological and pathological neovascularization.

Alterations in structure of elastic laminae of rat pulmonary arteries in hypoxic hypertension

The effect of hypoxic hypertension on the remodeling process of the elastic laminae of the rat hilar pulmonary arteries (PAs) was studied by electron microscopy. Rats were exposed to hypoxia (10% O2) for periods of 0.5, 2,6,12,48,96,144, and 240 h. Changes in the structure of the PA elastic laminae were examined and analyzed with respect to changes in the PA wall tensile stress. The PA blood pressure increased rapidly within the first several hours of hypoxia and reached a stable level within 2 days, whereas the PA wall tensile stress increased initially due to elevated blood pressure and then decreased after 48 h due to vessel wall thickening and returned to the control level after 4 days. In association with these changes, the elastic laminae, which appeared homogeneous in normal control rats, changed into structures composed of randomly oriented filaments and edematous contents with an increase in the volume during the early period of hypoxia and regained their homogeneous appearance and normal volume after 4 days. The changes in the elastic laminae were correlated with changes in the tensile stress. These changes were associated with a transient decrease in the stiffness of the PAs. In hypoxic rats given nifedipine, no change was found in the blood pressure, the tensile stress, or the structure of the elastic laminae of the PAs despite continuous exposure to hypoxia. These results suggested that altered tensile stress in the PA wall played a critical role in the initiation and regulation of structural changes in the elastic laminae and that these changes might contribute to alterations in the mechanical properties of the PA in hypoxic hypertension.

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.

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.

Hypoxia-inducible factor 1 levels vary exponentially over a physiologically relevant range of O2 tension

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop-helix protein implicated in the transcriptional activation of genes encoding erythropoietin, glycolytic enzymes, and vascular endothelial growth factor in hypoxic mammalian cells. In this study, we have quantitated HIF-1 DNA-binding activity and protein levels of the HIF-1 alpha and HIF-1 beta subunits in human HeLa cells exposed to O2 concentrations ranging from 0 to 20% in the absence or presence of 1 mM KCN to inhibit oxidative phosphorylation and cellular O2 consumption. HIF-1 DNA-binding activity, HIF-1 alpha protein and HIF-1 beta protein each increased exponentially as cells were subjected to decreasing O2 concentrations, with a half maximal response between 1.5 and 2% O2 and a maximal response at 0.5% O2, both in the presence and absence of KCN. The HIF-1 response was greatest over O2 concentrations associated with ischemic/hypoxic events in vivo. These results provide evidence for the involvement of HIF-1 in O2 homeostasis and represent a functional characterization of the putative O2 sensor that initiates hypoxia signal transduction leading to HIF-1 expression.

Activation of vascular endothelial growth factor gene transcription by hypoxia-inducible factor 1

Expression of vascular endothelial growth factor (VEGF) is induced in cells exposed to hypoxia or ischemia. Neovascularization stimulated by VEGF occurs in several important clinical contexts, including myocardial ischemia, retinal disease, and tumor growth. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop-helix protein that activates transcription of the human erythropoietin gene in hypoxic cells. Here we demonstrate the involvement of HIF-1 in the activation of VEGF transcription. VEGF 5'-flanking sequences mediated transcriptional activation of reporter gene expression in hypoxic Hep3B cells. A 47-bp sequence located 985 to 939 bp 5' to the VEGF transcription initiation site mediated hypoxia-inducible reporter gene expression directed by a simian virus 40 promoter element that was otherwise minimally responsive to hypoxia. When reporters containing VEGF sequences, in the context of the native VEGF or heterologous simian virus 40 promoter, were cotransfected with expression vectors encoding HIF-1alpha and HIF-1beta (ARNT [aryl hydrocarbon receptor nuclear translocator]), reporter gene transcription was much greater in both hypoxic and nonhypoxic cells than in cells transfected with the reporter alone. A HIF-1 binding site was demonstrated in the 47-bp hypoxia response element, and a 3-bp substitution eliminated the ability of the element to bind HIF-1 and to activate transcription in response to hypoxia and/or recombinant HIF-1. Cotransfection of cells with an expression vector encoding a dominant negative form of HIF-1alpha inhibited the activation of reporter transcription in hypoxic cells in a dose-dependent manner. VEGF mRNA was not induced by hypoxia in mutant cells that do not express the HIF-1beta (ARNT) subunit. These findings implicate HIF-1 in the activation of VEGF transcription in hypoxic cells.

The role of the aryl hydrocarbon receptor nuclear translocator (ARNT) in hypoxic induction of gene expression. Studies in ARNT-deficient cells

Hypoxia-inducible factor-1 (HIF-1), a DNA-binding complex implicated in the regulation of gene expression by oxygen, has been shown to consist of a heterodimer of two basic helix-loop-helix Per-AHR-ARNT-Sim (PAS) proteins, HIF-1alpha, and HIF-1beta. One partner, HIF-1beta, had been recognized previously as the aryl hydrocarbon receptor nuclear translocator (ARNT), an essential component of the xenobiotic response. In the present work, ARNT-deficient mutant cells, originally derived from the mouse hepatoma line Hepa1c1c7, have been used to analyze the role of ARNT/HIF-1beta in oxygen-regulated gene expression. Two stimuli were examined: hypoxia itself and desferrioxamine, an iron-chelating agent that also activates HIF-1. Induction of the DNA binding and transcriptional activity of HIF-1 was absent in the mutant cells, indicating an essential role for ARNT/HIF-1beta. Analysis of deleted ARNT/HIF-1beta genes indicated that the basic, helix-loop-helix, and PAS domains, but not the amino or carboxyl termini, were necessary for function in the response to hypoxia. Comparison of gene expression in wild type and mutant cells demonstrated the critical importance of ARNT/HIF-1beta in the hypoxic induction of a wide variety of genes. Nevertheless, for some genes a reduced response to hypoxia and desferrioxamine persisted in these mutant cells, clearly distinguishing ARNT/HIF-1beta-dependent and ARNT/HIF-1beta-independent mechanisms of gene activation by both these stimuli.

Regulation of VEGF/VPF expression in tumor cells: consequences for tumor growth and metastasis

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF) is a multifunctional cytokine which potently stimulates angiogenesis in vivo. VEGF/VPF expression is elevated in pathological conditions including cancer, proliferative retinopathy, psoriasis and rheumatoid arthritis. The angiogenesis associated with human tumors is likely a central component in promoting tumor growth and metastatic potential. The regulation of VEGF/VPF expression during tumor progression may involve diverse mechanisms including activated oncogenes, mutant or deleted tumor suppressor genes, cytokine activation, hormonal modulators, and a particularly effective activator, hypoxia. Understanding the diverse mechanisms by which tumor cells overexpress VEGF/VPF, and which mechanisms are operating in specific tumor types is important for the design of effective anti-cancer therapies.

The mouse gene for vascular endothelial growth factor. Genomic structure, definition of the transcriptional unit, and characterization of transcriptional and post-transcriptional regulatory sequences

We describe the genomic organization and functional characterization of the mouse gene encoding vascular endothelial growth factor (VEGF), a polypeptide implicated in embryonic vascular development and postnatal angiogenesis. The coding region for mouse VEGF is interrupted by seven introns and encompasses approximately 14 kilobases. Organization of exons suggests that, similar to the human VEGF gene, alternative splicing generates the 120-, 164-, and 188-amino acid isoforms, but does not predict a fourth VEGF isoform corresponding to human VEGF206. Approximately 1. 2 kilobases of 5'-flanking region have been sequenced, and primer extension analysis identified a single major transcription initiation site, notably lacking TATA or CCAT consensus sequences. The 5'-flanking region is sufficient to promote a 7-fold induction of basal transcription. The genomic region encoding the 3'-untranslated region was determined by Northern and nuclease mapping analysis. Investigation of mRNA sequences responsible for the rapid turnover of VEGF mRNA (mRNA half-life, <1 h) (Shima, D. T. , Deutsch, U., and D'Amore, P. A. (1995) FEBS Lett. 370, 203-208) revealed that the 3'-untranslated region was sufficient to trigger the rapid turnover of a normally long-lived reporter mRNA in vitro. These data and reagents will allow the molecular and genetic analysis of mechanisms that control the developmental and pathological expression of VEGF.

Hypoxia induces vascular endothelial growth factor in cultured human endothelial cells

Smooth muscle cells, macrophages, glial cells, keratinocytes, and transformed cells have been established as synthesis sites for vascular endothelial growth factor (VEGF). The modulating effects of VEGF are essentially limited to endothelial cells (ECs), the only cell type consistently shown to express VEGF receptors. VEGF has thus been considered to act exclusively via a paracrine pathway. We sought to determine whether the role of human ECs might, under selected conditions, extend beyond that of a target to involve contingency synthesis of VEGF. In both unstimulated human umbilical vein ECs (HUVECs) and human derma-derived microvascular ECs (HMECs), Northern analysis detected no VEGF transcripts. Phorbol-12-myristate 13-acetate (10(-7) M) treatment, however, induced VEGF mRNA expression in both HUVECs and HMECs, peaking at 3 and 6 h, respectively, and returning to undetectable levels by 12 h. In vitro exposure of HUVECs to a hypoxic environment (pO2 = 35 mm of mercury) for 12, 24, and 48 h and exposure of HMECs for 6, 12, 24, and 48 h induced VEGF mRNA in a time-dependent fashion. Re-exposure to normoxia (pO2 = 150 mm of mercury) for 24 h after 24 h of hypoxia returned VEGF mRNA transcripts to undetectable levels in HUVECs. Cobalt chloride and nickel chloride treatment each induced VEGF mRNA in ECs. Cycloheximide treatment further augmented expression of VEGF mRNA induced by cobalt chloride, nickel chloride, and hypoxia in HUVECs. VEGF protein production in hypoxia HUVECs was demonstrated immunohistochemically. Conditioned media from hypoxic HUVECs caused a 2-fold increase in the incorporation of tritiated thymidine. Finally, immune precipitates of anti-KDR probed with anti-Tyr(P) antibodies demonstrated evidence of receptor autophosphorylation in hypoxic but not normoxic HUVECs. These findings thus establish the potential for an autocrine pathway that may augment and/or amplify the paracrine effects of VEGF in stimulating angiogenesis.

The transcription factors ATF-1 and CREB-1 bind constitutively to the hypoxia-inducible factor-1 (HIF-1) DNA recognition site

The hypoxia-inducible factor-1 (HIF-1) was first described as a DNA binding activity that specifically recognizes an 8 bp motif known to be essential for hypoxia-inducible erythropoietin gene transcription. Subsequently HIF-1 activity has also been found in cell lines which do not express erythropoietin, suggesting that HIF-1 is part of a widespread oxygen sensing mechanism. In electrophoretic mobility shift assays HIF-1 DNA binding activity is only detectable in nuclear extracts of cells cultivated in a low oxygen atmosphere. In addition to HIF-1, a constitutive DNA binding activity also specifically binds the HIF1 probe. Here we report that CRE and AP1 oligonucleotides efficiently competed for binding of the HIF1 probe to this constitutive factor, whereas HIF-1 activity itself remained unaffected. Monoclonal antibodies raised against the CRE binding factors ATF-1 and CREB-1 supershifted the constitutive factors ATF-1 and CREB-1 supershifted the constitutive factor, while Jun and Fos family members, which constitute the AP-1 factor, were immunologically undetectable. Recombinant ATF-1 and CREB-1 proteins bound HIF1 probes either as homodimers or as heterodimers, indicating a new binding specificity for ATF-1/CREB-1. Finally, reporter gene assays in HeLa cells treated with either a cAMP analogue or a phorbol ester suggest that the PKA, but not the PKC signalling pathway is involved in oxygen sensing.