Hypoxia-induced transcriptional activation of vascular endothelial growth factor is inhibited by serum

PKCθ-JunB axis via upregulation of VEGFR3 expression mediates hypoxia-induced pathological retinal neovascularization

Pathological retinal neovascularization is the most common cause of vision loss. PKCθ has been shown to play a role in type 2 diabetes, which is linked to retinal neovascularization. Based on these clues, we have studied the role of PKCθ and its downstream target genes JunB and VEGFR3 in retinal neovascularization using global and tissue-specific knockout mouse models along with molecular biological approaches. Here, we show that vascular endothelial growth factor A (VEGFA) induces PKCθ phosphorylation in human retinal microvascular endothelial cells (HRMVECs) and downregulation of its levels attenuates VEGFA-induced HRMVECs migration, sprouting and tube formation. Furthermore, the whole body deletion of PKCθ or EC-specific deletion of its target gene JunB inhibited hypoxia-induced retinal EC proliferation, tip cell formation and neovascularization. VEGFA also induced VEGFR3 expression via JunB downstream to PKCθ in the regulation of HRMVEC migration, sprouting, and tube formation in vitro and OIR-induced retinal EC proliferation, tip cell formation and neovascularization in vivo. In addition, VEGFA-induced VEGFR3 expression requires VEGFR2 activation upstream to PKCθ-JunB axis both in vitro and in vivo. Depletion of VEGFR2 or VEGFR3 levels attenuated VEGFA-induced HRMVEC migration, sprouting and tube formation in vitro and retinal neovascularization in vivo and it appears that these events were dependent on STAT3 activation. Furthermore, the observations using soluble VEGFR3 indicate that VEGFR3 mediates its effects on retinal neovascularization in a ligand dependent and independent manner downstream to VEGFR2. Together, these observations suggest that PKCθ-dependent JunB-mediated VEGFR3 expression targeting STAT3 activation is required for VEGFA/VEGFR2-induced retinal neovascularization.

Chronic hypoxia alters fetal cerebrovascular responses to endothelin-1

In utero hypoxia influences the structure and function of most fetal arteries, including those of the developing cerebral circulation. Whereas the signals that initiate this hypoxic remodeling remain uncertain, these appear to be distinct from the mechanisms that maintain the remodeled vascular state. The present study explores the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to endothelin-1 (ET-1), a potent vascular contractant and mitogen. In fetal lambs, chronic hypoxia (3,820-m altitude for the last 110 days of gestation) had no significant effect on plasma ET-1 levels or ETA receptor density in cerebral arteries but enhanced contractile responses to ET-1 in an ETA-dependent manner. In organ culture (24 h), 10 nM ET-1 increased medial thicknesses less in hypoxic than in normoxic arteries, and these increases were ablated by inhibition of PKC (chelerythrine) in both normoxic and hypoxic arteries but were attenuated by inhibition of CaMKII (KN93) and p38 (SB203580) in normoxic but not hypoxic arteries. As indicated by Ki-67 immunostaining, ET-1 increased medial thicknesses via hypertrophy. Measurements of colocalization between MLCK and SMαA revealed that organ culture with ET-1 also promoted contractile dedifferentiation in normoxic, but not hypoxic, arteries through mechanisms attenuated by inhibitors of PKC, CaMKII, and p38. These results support the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to ET-1 through pathways dependent upon PKC, CaMKII, and p38 that cause increased ET-1-mediated contractility, decreased ET-1-mediated smooth muscle hypertrophy, and a depressed ability of ET-1 to promote contractile dedifferentiation.

AP-1 transcription factor mediates VEGF-induced endothelial cell migration and proliferation

VEGF, upon binding to its endothelial cell specific receptors VEGF-R1 and VEGF-R2, can induce endothelial cell migration, proliferation and angiogenesis. However, the molecular mechanism of these effects still remains unclear. In this study, we investigated whether VEGF promotes human umbilical vascular endothelial cell (HUVEC) migration and proliferation through activator protein-1 transcription factor (AP-1) family. We first showed that VEGF induces immediate-early genes AP-1 family gene expression differentially with the profound induction of JunB (both mRNA and protein) under various conditions (PBS, DMSO or control adenoviruses). The increase in AP-1 mRNA expression occurs primarily at the transcriptional level. Inhibition of AP-1 DNA binding activity by adenovirus expressing a potent dominant negative form of c-Fos (Afos) significantly attenuated VEGF-induced HUVEC migration and proliferation and cyclin D1 expression. Knockdown of JunB with adenovirus expressing JunB shRNA reduces VEGF-induced JunB expression and attenuated HUVEC migration. However the shJunB-expressing virus has no effect on VEGF-induced cyclin D1 protein expression and proliferation. These results suggest that VEGF-induced endothelial migration is mediated primarily by induction of JunB whereas the promotion of endothelial proliferation by VEGF is mediated by JunB-independent AP-1 family members.

Paradoxical effects of hypoxia-mimicking divalent cobalt ions in human endothelial cells in vitro

Divalent cobalt ions (Co2+) induce the expression of hypoxia responsive genes and are often used in cell biology to mimic hypoxia. In this in vitro study we compared the effects of hypoxia and Co2+ on human endothelial cells and examined processes that are stimulated in hypoxia in vivo (proliferation and angiogenesis). We analyzed the expression of the hypoxia-inducible factor-1alpha (HIF-1alpha) under different hypoxic conditions (3% and nearly 0% O2) and Co2+ -concentrations (0.01-0.7 mM). As in hypoxia, the amount of HIF-1alpha protein was enhanced by exposure to Co2+ (did not correlate with mRNA amount). however, contrary to the results of hypoxia, in vitro-angiogenesis was inhibited after exposure to even low Co2+-concentrations (> or =0.01 mM). This led to the conclusion that although hypoxia signaling after Co2+ -exposure took place, further yet unknown Co2+ -induced event(s) must have occurred.

Hypoxia-inducible factor 1α modulates adhesion, migration, and FAK phosphorylation in vascular smooth muscle cells

Hypoxia promotes angiogenesis by modulating the transcriptional regulator hypoxia-inducible factor 1alpha (HIF-1alpha). HIF-1alpha is a master regulator of the hypoxic response, and its proangiogenic activities include, but are not limited to, regulation of vascular endothelial growth factor (VEGF). The remodeling of the vasculature during angiogenesis requires an initial destabilization step, which facilitates endothelial sprouting, followed by vessel growth, and restabilization through investment of smooth muscle cells. The complex dynamics of hypoxia-induced angiogenesis prompted us to investigate what aspects of this multi-step process are regulated by HIF-1alpha. To do so, we analyzed the molecular properties of aortic and coronary artery smooth muscle cells in response to forced expression of HIF-1alpha, and by treatment with cobalt chloride, which mimics hypoxia. Our results demonstrate that HIF-1alpha causes a marked reduction in the ability of smooth muscle cells to migrate and adhere to extracellular matrices. Analysis of focal adhesion proteins showed no significant difference in expression or localization of vinculin or focal adhesion kinase (FAK). However, investigation of FAK phosphorylation, a critical mediator of adhesion and migration, revealed tyrosine phosphorylation of FAK is diminished in the presence of HIF-1alpha and cobalt chloride. These results indicate that during hypoxia-induced vessel remodeling, HIF-1alpha functions to dampen adhesion and migration of smooth muscle cells by modulating FAK activity. We suggest that HIF-1alpha expression in smooth muscle cells may augment vessel sprouting by loosening smooth muscle cell attachments to the basement membrane and endothelial cells.

17-beta estradiol attenuates hypoxic induction of HIF-1alpha and erythropoietin in Hep3B cells

Hypoxia inducible factor-1 (HIF-1) is a heterodimeric transcription factor that regulates expression of several hypoxia-inducible genes, including erythropoietin (EPO), by binding to hypoxia response elements (HREs) in their promoters/enhancers. Previously, we have shown that 17-beta estradiol (E2-beta) attenuates hypoxic induction of EPO in rats. We hypothesized that this response is mediated by E2-beta-induced attenuation of HIF-1alpha activity/expression. To test this hypothesis, we performed reporter gene assays in Hep3B cells to assess E2-beta effects on hypoxia-induced activity of a reporter gene driven by the HRE from a cloned EPO-enhancer element. Immunocytochemistry and Western blots were additionally used to determine effects of E2-beta on hypoxic increases in HIF-1alpha and EPO immunoreactivity. Finally, we examined potential influences of E2-beta on HIF-1alpha mRNA levels by real-time PCR. Consistent with our hypothesis, E2-beta (100 pM) inhibited hypoxic increases in HRE-mediated reporter gene activity. Furthermore, the estrogen-receptor antagonist ICI 182,780 (25 microM) eliminated these inhibitory effects of E2-beta. E2-beta similarly attenuated hypoxic induction of both EPO and HIF-1alpha protein in an estrogen-receptor dependent manner, but was without effect on HIF-1alpha mRNA expression. These findings suggest a role for E2-beta to attenuate EPO expression by interfering with hypoxic increases in HIF-1alpha protein through an estrogen receptor-dependent mechanism.

HIF-1-dependent VEGF reporter gene assay by a stable transformant of CHO cells

For the establishment of a screening system to detect inhibitors of vascular endotherial growth factor (VEGF) expression, a stable transformant of Chinese hamster ovary cells was isolated and cloned by transfection of a hypoxia-inducible factor 1 (HIF-1)-dependent VEGF promoter reporter gene. The expression of the reporter gene in the clone cells, as measured by luciferase activity, was stable. Hypoxic responses were best observed at an initial cell density of 2 x 10(4)/well. The maximal increase of luciferase activity was 30 fold. In the highest cell density of 8 x 10(4)/well (2.1 x 10(5)/cm(2)), basal activity was increased 13-15 fold compared to that at the lower cell densities, and did not respond to hypoxia. Addition of CoCl(2), which is known to mimic hypoxia, increased luciferase activity more than 10 times in normoxia. Nitric oxide donors, which are known to suppress the activation of HIF-1, inhibited expression of the VEGF promoter reporter gene under hypoxia. Histone deacetylase inhibitors, trichostatin A and sodium n-butyrate which are known to stimulate transcription of many genes enhanced its transcription in hypoxia. These results indicate that the stable transformant is a useful tool for screening of HIF-1 modifiers.

Insulin-like growth factors-1 and -2, but not hypoxia, synergize with gonadotropin hormone to promote vascular endothelial growth factor-A secretion by monkey granulosa cells from preovulatory follicles

The midcycle gonadotropin surge promotes vascular endothelial growth factor-A (VEGF-A) production by granulosa cells in the ovulatory follicle, but it is unclear whether primary regulators of VEGF secretion in other tissues, including hypoxia and growth factors, are also important in the ovary. To address these issues, granulosa cells were collected from rhesus monkeys during controlled ovarian stimulation either before (i.e., nonluteinized granulosa cells, NLGCs) or 27 hours after (i.e., luteinized granulosa cells, LGCs) administration of an ovulatory bolus of hCG, and cultured in fibronectin-coated wells containing a chemically defined media. When NLGCs were transferred to various O2 environments (20%, 5%, or 0% O2) or media containing 100 mM CoCl2, LH (100 ng/ml)-stimulated progesterone (P4) levels were markedly (P < 0.05) suppressed by 0% O2 or CoCl2. VEGF concentrations also declined (P < 0.05) in control, CoCl2, and CoCl2 + LH groups in 0% O2, although CoCl2 modestly increased (75% above control; P < 0.05) VEGF levels in 20% and 5% O2. When NLGCs were cultured in the presence of recombinant human insulin-like growth factor (IGF)-1, IGF-2, or insulin, there was a dose-dependent increase (P < 0.05) in VEGF levels on Day 1 of culture. Whereas optimal doses of IGF-1 or IGF-2 (50 ng/ml), hCG (100 ng/ml), and IGF plus hCG stimulated VEGF levels on Day 1, only the combination of IGF-1 or IGF-2 plus hCG increased VEGF above controls and sustained levels through Day 3 of culture. The synergistic effects of IGF and hCG were also evident in P4 levels, and were not due to changes in DNA content between treatment groups. LGCs produced much higher levels of P4 and VEGF, but the responses to different O2 concentrations and insulin-related factors were qualitatively similar to those of NLGCs. These results suggest that hypoxia is not a primary regulator of VEGF production in primate granulosa cells. However, IGFs may act in concert with the gonadotropin surge to promote VEGF secretion in the ovulatory, luteinizing follicle.

Reduction of hypoxia-induced transcription through the repression of hypoxia-inducible factor-1alpha/aryl hydrocarbon receptor nuclear translocator DNA binding by the 90-kDa heat-shock protein inhibitor radicicol

Under low oxygen tension, cells increase the transcription of specific genes involved in angiogenesis, erythropoiesis, and glycolysis. Hypoxia-induced gene expression depends primarily on stabilization of the alpha subunit of hypoxia-inducible factor-1 (HIF-1alpha), which acts as a heterodimeric trans-activator with the nuclear protein known as the aryl hydrocarbon receptor nuclear translocator (Arnt). The resulting heterodimer (HIF-1alpha/Arnt) interacts specifically with the hypoxia-responsive element (HRE), thereby increasing transcription of the genes under HRE control. Our results indicate that the 90-kDa heat-shock protein (Hsp90) inhibitor radicicol reduces the hypoxia-induced expression of both endogenous vascular endothelial growth factor (VEGF) and HRE-driven reporter plasmids. Radicicol treatment (0.5 microg/ml) does not significantly change the stability of the HIF-1alpha protein and does not inhibit the nuclear localization of HIF-1alpha. However, this dose of radicicol significantly reduces HRE binding by the HIF-1alpha/Arnt heterodimer. Our results, the first to show that radicicol specifically inhibits the interaction between the HIF-1alpha/Arnt heterodimer and HRE, suggest that Hsp90 modulates the conformation of the HIF-1alpha/Arnt heterodimer, making it suitable for interaction with HRE. Furthermore, we demonstrate that radicicol reduces hypoxia-induced VEGF expression to decrease hypoxia-induced angiogenesis.

The bHLH/PAS factor MOP3 does not participate in hypoxia responses

The basic helix-loop-helix/PAS (bHLH/PAS) family of proteins regulates transcriptional responses during development and in response to environmental stimuli. bHLH/PAS factors act as heterodimers, and genetic and biochemical data indicate that multiple heterodimeric combinations are found in vivo to regulate hypoxic gene expression. For example, HIF1alpha heterodimerizes with the highly related proteins ARNT or ARNT2 in neurons. In vivo, MOP3 interacts with CLOCK to regulate circadian rhythms; however, its role in hypoxia responses is unclear. We show here that unlike ARNT and ARNT2, MOP3 does not effectively form HIF-1 complexes or restore HIF-1 target gene expression in response to low oxygen when expressed in Arnt(-/-) ES cells. Furthermore, Mop3(-/-) day 9.5 embryos exhibit no angiogenic defects as shown for Arnt(-/-), Hif1alpha(-/-), and Hif2alpha(-/-) embryos. Therefore, by a variety of criteria, we show that MOP3 has little if any role in the regulation of hypoxia responses in vivo.

Nickel requires hypoxia-inducible factor-1 alpha, not redox signaling, to induce plasminogen activator inhibitor-1

Human epidemiological and animal studies have associated inhalation of nickel dusts with an increased incidence of pulmonary fibrosis. At the cellular level, particulate nickel subsulfide inhibits fibrinolysis by transcriptionally inducing expression of plasminogen activator inhibitor (PAI)-1, an inhibitor of the urokinase-type plasminogen activator. Because nickel is known to mimic hypoxia, the present study examined whether nickel transcriptionally activates PAI-1 through the hypoxia-inducible factor (HIF)-1 alpha signaling pathway. The involvement of the NADPH oxidase complex, reactive oxygen species, and kinases in mediating nickel-induced HIF-1 alpha signaling was also investigated. Addition of nickel to BEAS-2B human airway epithelial cells increased HIF-1 alpha protein levels and elevated PAI-1 mRNA levels. Pretreatment of cells with the extracellular signal-regulated kinase inhibitor U-0126 partially blocked HIF-1 alpha protein and PAI-1 mRNA levels induced by nickel, whereas antioxidants and NADPH oxidase inhibitors had no effect. Pretreating cells with antisense, but not sense, oligonucleotides to HIF-1 alpha mRNA abolished nickel-stimulated increases in PAI-1 mRNA. These data indicate that signaling through extracellular signal-regulated kinase and HIF-1 alpha is required for nickel-induced transcriptional activation of PAI-1.

Mitogen-activated protein kinase kinase inhibitor PD98059 blocks the trans-activation but not the stabilization or DNA binding ability of hypoxia-inducible factor-1alpha

Under low oxygen tension, cells increase the transcription of specific genes that are involved in angiogenesis, erythropoiesis, and glycolysis. Hypoxia-induced gene expression primarily depends on the stabilization of the alpha-subunit of hypoxia-inducible factor-1 (HIF-1alpha), which acts as a heterodimeric trans-activator. Our results indicate that stabilization of HIF-1alpha protein by treatment of proteasome inhibitors, is not sufficient for hypoxia-induced gene activation, and an additional hypoxia-dependent modification is necessary for gene expression by HIF-1alpha. Here, we demonstrate that mitogen-activated protein kinase kinase-1 (MEK-1) inhibitor PD98059 does not change either the stabilization or DNA binding ability of HIF-1alpha but it inhibits the trans-activation ability of HIF-1alpha, thereby it reduces the hypoxia-induced transcription of both an endogenous target gene and a hypoxia-responsive reporter gene. We found that hypoxia induced p42/p44 mitogen-activated protein kinases (MAPKs) that are target protein kinases of MEK-1, and that expression of dominant-negative p42 and p44 MAPK mutants reduced HIF-1-dependent transcription of the hypoxia-responsive reporter gene. Our results are the first to identify that hypoxia-induced trans-activation ability of HIF-1alpha is regulated by different mechanisms than its stabilization and DNA binding, and that these processes can be experimentally dissociated. MEK-1/p42/p44 MAPK regulates the trans-activation, but not the stabilization or DNA binding ability, of HIF-1alpha.

Molecular regulation of the endothelin-1 gene by hypoxia. Contributions of hypoxia-inducible factor-1, activator protein-1, GATA-2, AND p300/CBP

Endothelin-1 (ET-1) is a peptide hormone with potent vasoconstrictor properties which is synthesized and secreted predominantly by vascular endothelial cells. Its production is regulated by numerous stimuli including ischemia and hypoxia, and the enhanced levels that occur during myocardial ischemia may contribute to the progression of heart failure. We reported previously a preliminary characterization of a hypoxia-inducible factor-1 (HIF-1) binding site in the human ET-1 promoter which contributed to the activation of ET-1 expression in endothelial cells. We report here that the HIF-1 binding site alone is not sufficient for the response to hypoxia but requires an additional 50 base pairs of flanking sequence that includes binding sites for the factors activator protein-1 (AP-1), GATA-2, and CAAT-binding factor (NF-1). Mutation of any one of these sites or the HIF-1 site eliminated induction by hypoxia. Mutations of the AP-1 and GATA-2 sites, but not the HIF-1 site, were complemented by overexpressing AP-1, GATA-2, HIF-1alpha, or the activator protein p300/CBP, restoring the response to hypoxia. Binding studies in vitro confirmed physical associations among GATA-2, AP-1, and HIF-1 factors. Overexpression or depletion of p300/CBP modulated the level of ET-1 promoter expression as well as the endogenous ET-1 transcript but did not change the fold induction by hypoxia in either case. Regulation of the ET-1 promoter by hypoxia in non-endothelial cells required overexpression of GATA-2 and HIF-1alpha. The results support essential roles for AP-1, GATA-2, and NF-1 in stabilizing the binding of HIF-1 and promoting recruitment of p300/CBP to the ET-1 hypoxia response complex.

Coordinated Up-regulation by hypoxia of adrenomedullin and one of its putative receptors (RDC-1) in cells of the rat blood-brain barrier

Adrenomedullin (ADM) is a potent hypotensive peptide, which is produced during sepsis and ischemia. We demonstrate here that hypoxia induced a time-dependent increase of both ADM mRNA and protein expressions in cultured astrocytes and endothelial cells from rat brain microvessels. Gene reporter analyses showed a 2-fold increase in ADM gene transcription which was suppressed when the ADM promoter was deleted of its hypoxia responsive element. Hypoxia increased 7-fold the stability of pre-formed ADM mRNAs. Rat brain microvessels expressed mRNAs coding for the different putative ADM receptors but they did not respond to exogenous ADM and calcitonin gene-related peptide by the formation of cAMP. In contrast, ADM and calcitonin gene-related peptide increased the formation of cAMP in astrocytes and their actions were potentiated about 2-fold after hypoxia. Messenger RNA species coding for three putative ADM receptors (the L1 orphan receptor, RDC-1, and calcitonin receptor-like receptor) and accessory proteins (receptor-activity modifying proteins) were present in astrocytes. Hypoxia selectively up-regulated expression of RDC-1 receptor mRNAs. The results indicate that ADM and RDC-1 are hypoxia-sensitive genes and that RDC-1 receptors may mediate some actions of ADM in hypoxic astrocytes.