Hypoxia regulates avian cardiac Arnt and HIF-1alpha mRNA expression

Embryotoxicity of Polystyrene Microspheres of Different Sizes to the Marine Medaka Oryzias melastigma (McClelland, 1839)

Polystyrene microplastics (PS-MPs) are potentially harmful to marine organisms, especially during the early developmental stages, although the underlying mechanism remains unclear. The present study evaluated the growth and morphological characteristics of marine medaka Oryzias melastigma (McClelland, 1839) embryos exposed to PS-MP. PS-MPs of three different sizes (0.05, 0.5, and 6.0 μm with a concentration of 106 particles/L) were subjected to waterborne exposure for 19 d. The hatching time and rate of embryos exposed to 0.5 and 6.0 μm PS-MPs were significantly lower than those of the control, while no significant difference was observed in the 0.05 μm treatment. No significant differences were observed in the mortality rate of the embryos, embryo diameter, and relevant gene expression levels, including il6, il8, il-1β, jak, stat-3, nf-κb, hif-1α, epo, cyp1a1, ahr, sod, cat, and gpx, but with the exception of vtg. Fluorescent PS-MPs were found on the embryo surfaces when the embryos were exposed to 0.5 and 6.0 μm PS-MPs, but no signals were detected inside embryos using confocal microscopy. Therefore, the results indicate that PS-MPs having a diameter of 6.0 μm can only attach to the surface or villus of embryos and not enter the embryos through the membrane pores, whereas PS-MPs with diameters of 0.05 and 0.5 μm cannot enter the embryos.

Insights into Temperature and Hypoxia Tolerance in Cowpea Weevil via HIF-1

Cowpea weevil (Callosobruchus maculatus) is a major pest that leads to severe damage of the stored leguminous grains. Several management approaches, including physical barriers, biological or chemical methods, are used for controlling bruchid in cowpea. These methods usually target the metabolically active state of weevil. However, it becomes less effective at early stages as egg, larva, or pupa under low temperature and oxygen conditions. Since hypoxia-inducible factor-1 (HIF-1) is known to coordinate multiple gene responses to low oxygen or low temperature signals, we examined the HIF-1α gene expression under low temperature and hypoxic treatments. At −20 °C, it took 4 h to reduce the survival rate for eggs, larvae, and pupae down to 10%, while at 4 °C and 15 °C, the survival rate remained higher than 50% even after 128 h as HIF-1α gene expression peaked at 15 °C. Moreover, HIF-1 protein offers a valuable target for early stage pest control complementary to traditional methods. In particular, HIF-1 inhibitor camptothecin (CPT), one of the five HIF-1 inhibitors examined, achieved a very significant reduction of 96.2% and 95.5% relative to the control in weevil survival rate into adult at 4 °C and 30 °C, respectively. Our study can be used as one model system for drug development in virus infections and human cancer.

Hypoxia-Induced miR-148a Downregulation Contributes to Poor Survival in Colorectal Cancer

Hypoxia is an extensively investigated condition due to its contribution to various pathophysiological processes including cancer progression and metastasis formation. MicroRNAs (miRNAs) are well-known post-transcriptional gene expression regulators. However, their contribution to molecular response to hypoxia is highly dependent on cell/tissue types and causes of hypoxia. One of the most important examples is colorectal cancer, where no consensus on hypoxia-regulated miRNAs has been reached so far. In this work, we applied integrated mRNA and small RNA sequencing, followed by bioinformatics analysis, to study the landscape of hypoxia-induced miRNA and mRNA expression alterations in human colorectal cancer cell lines (HT-29 and Caco-2). A hypoxic microenvironment was chemically modeled using two different treatments: cobalt(II) chloride and oxyquinoline. Only one miRNA, hsa-miR-210-3p, was upregulated in all experimental conditions, while there were nine differentially expressed miRNAs under both treatments within the same cell line. Further bioinformatics analysis revealed a complex hypoxia-induced regulatory network: hypoxic downregulation of hsa-miR-148a-3p led to the upregulation of its two target genes, ITGA5 and PRNP, which was shown to be a factor contributing to tumor progression and poor survival in colorectal cancer patients.

Genomic regions and pathways associated with thermotolerance in layer-type strain Taiwan indigenous chickens

This study aimed to investigate genetic markers and candidate genes associated with thermotolerance in a layer-type strain Taiwan indigenous chickens exposed to acute heat stress. One hundred and ninety-two 30-week-old roosters were subjected to acute heat stress. Changes in body temperature (BT, ΔT) were calculated by measuring the difference between the initial BT and the highest BT during heat stress and the results were categorized into dead, susceptible, tolerant, and intermediate groups depending on their survival and ΔT values at the end of the experiment. A genome-wide association study on survival and ΔT values was conducted using the Cochran-Armitage trend test and Fisher's exact test. Association analyses identified 80 significant SNPs being annotated to 23 candidate genes, 440 SNPs to 71 candidate genes, 64 SNPs to 25 candidate genes, and 378 SNPs to 78 candidate genes in the dead versus survivor, tolerant versus susceptible, intermediate versus tolerant, and intermediate versus susceptible groups, respectively. The annotated genes were associated with apoptosis, cellular stress responses, DNA repair, and metabolic oxidative stress. In conclusion, the identified SNPs of candidate genes provide insights into the potential mechanisms underlying physiological responses to acute heat stress in chickens.

Molecular cloning and characterization of two ARNT (ARNT-1 and ARNT-2) genes in Atlantic croaker and their expression during coexposure to hypoxia and PCB77

Aryl hydrocarbon receptor nuclear translocator (ARNT) is an important transcriptions factor that binds/coactivates drug-metabolizing genes in vertebrates. In this study, we report the cloning and characterization of two ARNT (ARNT-1 and ARNT-2) genes and their mRNA and protein expression in liver tissues of Atlantic croaker after co-exposure to hypoxia and 3,3',4,4'-tetrachlorobiphenyl (PCB77). The full-length croaker ARNT-1 and ARNT-2 genes encode proteins of 537 and 530 amino acids, respectively, and are highly homologous to ARNT-1 and ARNT-2 genes of other vertebrates. ARNT mRNAs are ubiquitously expressed in all tissues. Hypoxia (dissolved oxygen: 1.7 mg/L) exposure (1-4 weeks) did not affect hepatic ARNTs mRNA levels. Dietary PCB77 treatment (2 and 8 μg/g body weight/day for 4 weeks) caused marked increases in ARNTs mRNA and protein levels in normoxic fish. However, coexposure to hypoxia and PCB77 for 4 weeks significantly blunted the increase in ARNTs mRNA and protein levels in response to PCB77 exposure. These results suggest that ARNT activity and functions induced by exposure to PCB aryl hydrocarbon receptor (AhR) agonists could be compromised in croaker inhabiting hypoxic coastal regions.

The effect of within-instar development on tracheal diameter and hypoxia-inducible factors α and β in the tobacco hornworm, Manduca sexta

As insects grow within an instar, body mass increases, often more than doubling. The increase in mass causes an increase in metabolic rate and hence oxygen demand. However, the insect tracheal system is hypothesized to increase only after molting and may be compressed as tissues grow within an instar. The increase in oxygen demand in the face of a potentially fixed or decreasing supply could result in hypoxia as insects near the end of an instar. To test these hypotheses, we first used synchrotron X-ray imaging to determine how diameters of large tracheae change within an instar and after molting to the next instar in the tobacco hornworm, Manduca sexta. Large tracheae did not increase in diameter within the first, second, third, and fourth instars, but increased upon molting. To determine if insects are hypoxic at the end of instars, we used the presence of hypoxia-inducible factors (HIFs) as an index. HIF-α and HIF-β dimerize in hypoxia and act as a transcription factor that turns on genes that will increase oxygen delivery. We sequenced both of these genes and measured their mRNA levels at the beginning and end of each larval instar. Finally, we obtained an antibody to HIF-α and measured protein expression during the same time. Both mRNA and protein levels of HIFs were increased at the end of most instars. These data support the hypothesis that some insects may experience hypoxia at the end of an instar, which could be a signal for molting.

SUMMARY STATEMENT

As caterpillars grow within an instar, major tracheae do not increase in size, while metabolic demand increases. At the same life stages, caterpillars increased expression of hypoxia inducible factors, suggesting that they become hypoxic near the end of an instar.

Molecular and biochemical responses of hypoxia exposure in Atlantic croaker collected from hypoxic regions in the northern Gulf of Mexico

A major impact of global climate change has been the marked increase worldwide in the incidence of coastal hypoxia (dissolved oxygen, DO<2.0 mg l^-1). However, the extent of hypoxia exposure to motile animals such as fish collected from hypoxic waters as well as their molecular and physiological responses to environmental hypoxia exposure are largely unknown. A suite of potential hypoxia exposure biomarkers was evaluated in Atlantic croaker collected from hypoxic and normoxic regions in the northern Gulf of Mexico (nGOM), and in croaker after laboratory exposure to hypoxia (DO: 1.7 mg l^-1). Expression of hypoxia-inducible factor-α, hif-α; neuronal nitric oxide synthase, nNOS; and insulin-like growth factor binding protein, igfbp mRNAs and protein carbonyl (PC, an oxidative stress indicator) content were elevated several-fold in brain and liver tissues of croaker collected from nGOM hypoxic sites. All of these molecular and biochemical biomarkers were also upregulated ~3-10-fold in croaker brain and liver tissues within 1–2 days of hypoxia exposure in controlled laboratory experiments. These results suggest that hif-αs, nNOS and igfbp-1 transcripts and PC contents are useful biomarkers of environmental hypoxia exposure and some of its physiological effects, making them important components for improved assessments of long-term impacts of environmental hypoxia on fish populations.

Andrographolide inhibits hypoxia-induced HIF-1α-driven endothelin 1 secretion by activating Nrf2/HO-1 and promoting the expression of prolyl hydroxylases 2/3 in human endothelial cells

Andrographolide, the main bioactive component of the medicinal plant Andrographis paniculata, has been shown to possess potent anti-inflammatory activity. Endothelin 1 (ET-1), a potent vasoconstrictor peptide produced by vascular endothelial cells, displays proinflammatory property. Hypoxia-inducible factor 1α (HIF-1α), the regulatory member of the transcription factor heterodimer HIF-1α/β, is one of the most important molecules that responds to hypoxia. Changes in cellular HIF-1α protein level are the result of altered gene transcription and protein stability, with the latter being dependent on prolyl hydroxylases (PHDs). In this study, inhibition of pro-inflammatory ET-1 expression and changes of HIF-1α gene transcription and protein stability under hypoxia by andrographolide in EA.hy926 endothelial-like cells were investigated. Hypoxic conditions were created using the hypoxia-mimetic agent CoCl_2. We found that hypoxia stimulated the production of reactive oxygen species (ROS), the expression of HIF-1α mRNA and protein, and the expression and secretion of ET-1. These effects, however, were attenuated by co-exposure to andrographolide, bilirubin, and RuCO. Silencing Nrf2 and heme oxygenase 1 (HO-1) reversed the inhibitory effects of andrographolide on hypxoia-induced HIF-1α mRNA and protein expression. Moreover, andrographolide increased the expression of prolyl hydroxylases (PHD) 2/3, which hydroxylate HIF-1α and promotes HIF-1α proteasome degradation, with an increase in HIF-1α hydroxylation was noted under hypoxia. Inhibition of p38 MAPK abrogated the hypoxia-induced increases in HIF-1α mRNA and protein expression as well as ET-1 mRNA expression and secretion. Taken together, these results suggest that andrographolide suppresses hypoxia-induced pro-inflammatory ET-1 expression by activating Nrf2/HO-1, inhibiting p38 MAPK signaling, and promoting PHD2/3 expression. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 918-930, 2017.

Isolation and mRNA expression of hypoxia-inducible factor α (HIF-α) in two sublittoral nassariid gastropods: Nassarius siquijorensis and Nassarius conoidalis

Hypoxia-inducible factor (HIF) is commonly found in vertebrates as an adaptation against hypoxia. In this study, partial sequences of HIF were first reported for subtidal nassariid gastropods, Nassarius siquijorensis and Nassarius conoidalis. The phylogenetic tree of HIF-α among various animal groups was inferred. Adults of both N. siquijorensis and N. conoidalis were exposed to 24 h and 1 week hypoxia (1.5 mg O2 l(-1)) and the change of HIF-α mRNA level was determined in gill, foot muscle and hepatopancreas by using quantitative real-time PCR. The expression profile of HIF-α was also investigated in veliger larvae after 1 and 24 h hypoxia exposure. The HIF-α mRNA from gills of N. siquijorensis was 2.22-fold elevated after 24 h of exposure but returned back to the baseline level after 1 week of exposure. In contrast to N. siquijorensis, both short-term and long-term exposure to hypoxia significantly enhanced the mRNA level of HIF-α in the gill of N. conoidalis. Both 24 h and 1 week of exposure resulted in a significantly higher degree of expression of mRNA of HIF-α in the foot muscle of N. siquijorensis and N. conoidalis. For hepatopancreas, the mRNA level of HIF-α remained unchanged in N. siquijorensis after both 24 h and 1 week of exposure to hypoxia but a significant enhancement was observed in N. conoidalis after 1 week of exposure to hypoxia. The mRNA level of HIF-α significantly reduced after 24 h of exposure to hypoxia in the larvae of N. siquijorensis. The different expression patterns of HIF-α mRNA between the two species reflected their different adaptation abilities to hypoxia, which will further explain the differences in the distribution of these species in Hong Kong waters with N. siquijorensis being predominant in regions suffered from summer hypoxia.

Molecular and functional characterization of aryl hydrocarbon receptor nuclear translocator 1 (ARNT1) and ARNT2 in chicken (Gallus gallus)

Our previous studies have provided evidence that birds have two isoforms of aryl hydrocarbon receptors (AHR1 and AHR2) and AHR nuclear translocators (ARNT1 and ARNT2) that potentially mediate toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. We have also shown that while both in vitro-expressed chicken AHR1 (ckAHR1) and AHR2 (ckAHR2) exhibit binding affinities to TCDD, only ckAHR1 but not ckAHR2 showed a TCDD-dose-dependent transactivation potency of chicken cytochrome P450 1A5 (ckCYP1A5) in in vitro reporter gene assays. To explore the molecular mechanism of functional difference in the two ckAHRs, the present study investigated the molecular characteristics and function of chicken ARNT (ckARNT) that is a potential dimerization partner for the activation of ckAHR. The full-length ckARNT1 and ckARNT2 cDNAs were isolated and their alternative splice variants were also identified. The ckARNT1 transcript was ubiquitously expressed in various tissues, but ckARNT2 showed restricted expressions in brain, kidney and eye, indicating a similar expression pattern to mammalian ARNTs. The expressions of tagged-ckARNT1 and -ckARNT2 were confirmed in a chicken hepatoma LMH cells by western blot analyses, and their interactions with each ckAHR and a specific recognition DNA element, xenobiotic response element (XRE), were examined by gel shift assays. The result showed that ckARNT1 and ckARNT2 dimerize with each ckAHR isoform and bind with the XRE in a TCDD-dependent manner. Hence, we conclude that functional loss on the dimerization with ckARNTs or the XRE binding is not the major cause of the deficient TCDD-dependency of ckAHR2 for the transactivation. Furthermore, in vitro reporter gene assays showed that transfected ckARNT1 failed to modulate the transcriptional induction of ckAHR-mediated ckCYP1A5 gene by TCDD in COS-7 and LMH cells, whereas ckARNT2 could potentiate the TCDD-dependent response in COS-7 but not in LMH cells. This suggests that ckARNT2 has a distinct role from ckARNT1 in AHR signaling pathway and in a cell-specific mode of action.

Molecular characterization and tissue distribution of aryl hydrocarbon receptor nuclear translocator isoforms, ARNT1 and ARNT2, and identification of novel splice variants in common cormorant (Phalacrocorax carbo)

High levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are accumulated in fish-eating birds including common cormorant (Phalacrocorax carbo). Most of the biochemical and toxic effects of TCDD are mediated by a basic helix-loop-helix and a conserved region among Per, ARNT, and Sim (bHLH/PAS) proteins, aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT). To study the molecular mechanism of TCDD toxicity in common cormorant as an avian model species, characterization of the AHR/ARNT signaling pathway in this species is necessary. The present study focuses on molecular characterization of ARNT from common cormorant (ccARNT). The cDNA of the ccARNT isoform, ccARNT1 obtained by the screening of hepatic cDNA library contains a 2424-bp open reading frame that encodes 807 amino acids, exhibiting high identities (92%) with chicken ARNT. This isoform contains a unique 22 amino acid residue in 3' end of PAS A domain as is also recognized in chicken ARNT. The ccARNT2 cDNA isolated from brain tissue has a 2151-bp open reading frame. The deduced amino acid sequence of ccARNT2 protein (716 aa) shows a conservation of bHLH and PAS motif in its N-terminal region with high similarities (96% and 78%, respectively) to that of ccARNT1. Using quantitative RT-PCR methods, the tissue distribution profiles of ccARNT1 and ccARNT2 were unveiled. Both ccARNT1 and ccARNT2 mRNAs were ubiquitously expressed in all examined tissues including liver. The expression profile of ccARNT1 was comparable with that of rodent ARNT1, but ccARNT2 was not with rodent ARNT2, implying different roles of ARNT2 between the two species. There was a significant positive correlation between ARNT1 and ARNT2 mRNA expression levels in the liver of wild cormorant population, indicating that their expressions may be enforced by similar transcriptional regulation mechanism. Novel variants of ccARNT1 and ccARNT2 isoforms that were supposed to arise from their splicing process were also identified and their hepatic expression profiles were determined. These results indicate that ccARNT1, ccARNT2 and their splice variants may more intricately regulate the AHR/ARNT signaling pathway and consequently may be responsible for the species diversity of toxic effects and susceptibility to PHAHs.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

Cloning of hif-1alpha and hif-2alpha and mRNA expression pattern during development in zebrafish

Hypoxia-inducible factors (HIFs) regulate gene expression in response to hypoxia and in vertebrates they are known to participate in several developmental processes, including angiogenesis, vasculogenesis, heart and central nervous system development. Over the last decade, major progress in unraveling the molecular mechanisms that mediate regulation of HIF proteins by oxygen tension has been reported, but our knowledge on their developmental regulation during embryogenesis in model organisms is limited. Expression of hif-1alpha and hif-2alpha genes has been characterized during normal mouse development and they were found to be expressed from stages E7.5, later in E9.5 and E15.5 in several different tissues such as the brain, heart and blood vessels. However, there is no detailed temporal information on their expression at other embryonic stages, even though orthologous genes have been described in several different vertebrate species. In this study, we describe the cloning and detailed expression pattern of zebrafish hif-1alpha and hif-2alpha genes. Sequence analysis revealed that zebrafish Hif proteins are highly homologous to other vertebrate orthologues. Zebrafish hif-1alpha and hif-2alpha are both expressed throughout development in discrete territories in a dynamic pattern. Interestingly, in the notochord the expression of hif-1alpha is switched off, while hif-2alpha transcription is turned on, signifying that the two genes might have partially overlapping, although non-redundant functions in development. This is the first time that a detailed comparison of the expression of hif-1alpha and hif-2alpha is directly assessed in a vertebrate model system throughout development.

Tissue-specific expression of a bHLH-PAS protein homologous to ARNT during the development of crustacean Daphnia magna

cDNAs encoding a Daphnia magna homolog of aryl hydrocarbon receptor nuclear translocator (ARNT) were isolated and the structural and functional features as well as the expression pattern of their product, DmagARNT, were analyzed. Among the known bHLH-PAS proteins, the deduced amino acid sequences of DmagARNT showed the highest degree of identity to that of Drosophila ARNT (TGO). Expression of DmagARNT in ARNT-lacking mouse Hepa-c4 cells resulted in the compensation for the loss of hypoxia response, suggesting the formation of a dimer with mouse HIF-1alpha and that the resulting heterodimer binds to the hypoxia-responsive elements (HRE), leading to transcription of the downstream luciferase gene. Expression of D. magna ARNT was evident at the middle to late stages of embryonic development (about 25 h to 48 h after ovulation) in several tissues, including a pair of the 1st antenna, 2nd antenna, 2nd maxilla, five pairs of the thoracic limbs, the central nerve system, anus, dorsal organ, maxillary gland, and carapace. As observed in other species, the D. magna ARNT is likely to function broadly as an expressed dimerization partner in developmental processes. In contrast, expression of ARNT in adult D. magna was limited to the epipodites of thoracic limbs, suggesting that ARNT plays a role solely in hypoxia response in adult Daphnia.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

Experimental hypoxia and embryonic angiogenesis

We examined the role of hypoxia and HIF factors in embryonic angiogenesis and correlated the degree of hypoxia with the level of HIF and VEGF expression and blood vessel formation. Quail eggs were incubated in normoxic and hypoxic (16% O(2)) conditions. Tissue hypoxia marker, pimonidazol hydrochloride, was applied in vivo for 1 hr and detected in sections with Hypoxyprobe-1 Ab. VEGF and HIF expression was detected by in situ hybridization. HIF-1alpha protein was detected in sections and by Western blot. Endothelial cells were visualized with QH-1 antibody. Hypoxic regions were detected even in normoxic control embryos, mainly in brain, neural tube, branchial arches, limb primordia, and mesonephros. The expression patterns of HIF-1alpha and HIF-1beta factors followed, in general, the Hypoxyprobe-1 marked regions. HIF-2alpha was predominantly expressed in endothelial cells. Diffuse VEGF expression was detected in hypoxic areas of neural tube, myocardium, digestive tube, and most prominently in mesonephros. Growing capillaries were directed to areas of VEGF positivity. Hypoxic regions in hypoxic embryos were larger and stained more intensely. VEGF and HIF-1 factors were proportionately elevated in Hypoxyprobe-1 marked regions without being expressed at new sites and were followed by increased angiogenesis. Our results demonstrate that normal embryonic vascular development involves the HIF-VEGF regulatory cascade. Experimentally increasing the level of hypoxia to a moderate level resulted in over-expression of HIF-1 factors and VEGF followed by an increase in the density of developing vessels. These data indicate that embryonic angiogenesis is responsive to environmental oxygen tension and, therefore, is not entirely genetically controlled.

Short upstream region drives dynamic expression of hypoxia-inducible factor 1alpha during Xenopus development

Hypoxia-inducible factor 1alpha (HIF-1alpha) plays a central role in regulating oxygen-dependent gene expression and is involved in a range of pathways implicated in cellular survival, proliferation, and development. While the posttranslational regulation of HIF-1alpha is well characterized, the relative importance of its control at the transcriptional level during development remains less clear. Although the mouse and human promoter regions have been analyzed in vitro, to date, there has been no in vivo analysis of any vertebrate HIF-1alpha promoter. To investigate the transcriptional regulation of HIF-1alpha during development of the amphibian Xenopus laevis, we have described the gene's expression pattern and isolated the xHIF-1alpha upstream regulatory regions. We show xHIF-1alpha mRNA to be constitutively expressed at low levels throughout embryogenesis, but with significant up-regulation during gastrula stages, and subsequently, in specific regions of the central nervous system and axial tissues. Our functional analysis using a series of truncated xHIF-1alpha promoter constructs demonstrates that a 173-bp region of the proximal promoter, which is 100% conserved among five allelic variants, is sufficient to drive correct expression in transgenic embryos. Although these results are corroborated by a parallel set of in vitro transfection experiments in a Xenopus cell line, some key differences suggest the importance of using transgenic methods in conjunction with in vitro assays.

Androgens regulate the binding of endogenous HuR to the AU-rich 3'UTRs of HIF-1alpha and EGF mRNA

The 3'UTRs of mammalian HIF-1alpha and EGF mRNA contain several highly conserved AU-rich elements (ARE) known to control the turnover of labile mRNAs by binding ARE-binding proteins that regulate nucleocytoplasmic shuttling, translation, and degradation. Androgens regulate the level and subcellular shuttling of HuR, a major ARE-binding protein that stabilizes many ARE-mRNAs. Pull down of biotinylated 3'UTRs of HIF-1alpha or EGF enriches HuR on blots from Jurkat cell lysates 5-fold, and enriches the amount of RNase-protected biotinylated RNA that comigrates with HuR approximately 10-fold. Dihydrotestosterone treatment decreases the HuR-protected riboprobe pulled down from total Jurkat cell lysates by 30-40%, apparently reflecting shifts in HuR from the nucleus to the cytoplasm. Androgen treatment also changes the amount of HuR-protected riboprobe pulled down from a PC-3 clone expressing a functional androgen receptor. The shift in the amount of riboprobe bound by HuR suggests that androgen is up-regulating endogenous ARE-mRNAs that can compete for binding endogenous HuR. These changes in the shuttling and ARE-binding of endogenous HuR indicate that androgen can act posttranscriptionally to regulate ARE-mRNAs, including HIF-1alpha and EGF.

Treatment with siRNA and antisense oligonucleotides targeted to HIF-1alpha induced apoptosis in human tongue squamous cell carcinomas

Overexpression of hypoxia inducible factor-1alpha (HIF-1alpha) in cancers has been correlated to a more aggressive tumor phenotype. We investigated the effect of HIF-1alpha knockout on the in vitro survival and death of human tongue squamous cell carcinomas (SCC-4 and SCC-9). Under normoxic condition, a basal level of HIF-1alpha protein was constitutively expressed in SCC-9 cells, albeit an undetectable level of HIF-1alpha messages. Exposure to hypoxia induced only a transient increase in mRNA transcript but a prolonged elevation of HIF-1alpha protein and its immediate downstream target gene product, VEGF. Under normoxic or hypoxic conditions, treatment of SCC-9 cells with AS-HIF-1alpha ODN suppressed both constitutive and hypoxia-induced HIF-1alpha expression at both mRNA and protein levels. Knockout of HIF-1alpha gene expression via either AS-HIF-1alpha ODN or siRNA (siRNAHIF-1alpha) treatment resulted in inhibition of cell proliferation and induced apoptosis in SCC-4 and SCC-9 cells. We also demonstrated that exposure of SCC-9 cells to hypoxia led to a time-dependent increase in the expression of bcl-2 and IAP-2, but not p53. The attenuated levels of bcl-2 and IAP-2, and the enhanced activity of caspase-3 after treatment with AS-HIF-1alpha ODN may contribute partly to the effects of HIF-1alpha blockade on SCC-9 cell death. Collectively, our data suggest that a constitutive or hypoxia-induced expression of HIF-1alpha in SCC-9 and SCC-4 cells is sufficient to confer target genes expression essential for tumor proliferation and survival. As a result, interfering with HIF-1alpha pathways by antisense or siRNA strategy may provide a therapeutic target for human tongue squamous cell carcinomas.

Vascular endothelial growth factor rescues 2,3,7,8-tetrachlorodibenzo-p-dioxin inhibition of coronary vasculogenesis

BACKGROUND

We previously demonstrated that the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reduces coronary vascular development in chick embryos in vivo. In the current study, we assessed whether TCDD inhibits early events in coronary endothelial tube formation and outgrowth, and whether this inhibition occurs through a vascular endothelial growth factor (VEGF)-dependent mechanism.

METHODS

Fertile chicken eggs were treated with control (corn oil) or TCDD (0.3 pmol TCDD/g) on incubation day 0. On embryonic day 6, cardiac ventricle explants were cultured on a three-dimensional collagen gel, when coronary angioblasts are present, but prior to their assembly into endothelial tubes. Endothelial cells migrating out from explants were identified by immunohistochemistry, and endothelial tube number and length were quantitated. In addition, on incubation days 6 and 8, cardiac VEGF mRNA and protein were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively.

RESULTS

Endothelial tube length and number were significantly reduced (40% +/- 1.7% and 36% +/- 3%, respectively) in TCDD explants, compared to controls. Recombinant exogenous VEGF, as well as hypoxic stimulation with CoCl2 or 10% O2, significantly increased the length and number of outgrowing tubes in TCDD cultures, and this stimulation was prevented by a VEGF neutralizing antibody. In contrast, VEGF neutralizing antibody reduced the length and number of tubes only in control cultures, and had no inhibitory effect on tube outgrowth from TCDD explants. Finally, hearts from TCDD-treated embryos exhibited a significant reduction in both VEGF mRNA and protein, compared to controls.

CONCLUSIONS

These data suggest that TCDD inhibits early coronary vascular outgrowth via a VEGF-dependent mechanism.

2,3,7,8-tetrachlorodibenzo-p-dioxin reduces myocardial hypoxia and vascular endothelial growth factor expression during chick embryo development

BACKGROUND

Previous research has demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces cardiomyocyte growth arrest, thinner ventricle walls, and reduced number and size of coronary arteries during chick embryogenesis. Coronary vascular development is believed to be mediated, in part, by myocardial oxygen gradients and a subsequent increase in hypoxia-inducible factor 1alpha (HIF-1alpha) and vascular endothelial growth factor-A (VEGF-A) expression. We investigated whether TCDD inhibition of coronary development was associated with altered myocardial oxygen status and reduced cardiac HIF-1alpha and VEGF-A.

METHODS

Chick embryos were exposed to 15% or 20% O2 for 24 hr from incubation days 9-10 or were injected with control (corn oil) or 0.24 pmol TCDD/gm egg on day 0. On day 9, embryos were injected with control (0.9% NaCl) or EF5, a tissue hypoxia marker, and cardiac binding of EF5 was determined by immunohistochemistry on day 10. In addition, embryo hearts were analyzed for VEGF-A mRNA by in situ hybridization and quantitative RT-PCR, and for HIF-1alpha mRNA by quantitative RT-PCR.

RESULTS

Cardiac binding of EF5 was significantly increased in embryos exposed to 15% O2, compared to embryos exposed to 20% O2. In contrast, TCDD-exposed embryos exhibited significantly reduced binding of EF5 in the heart, compared to controls. Similarly, cardiac expression of HIF-1alpha and VEGF-A were increased following hypoxia and tended to be decreased following TCDD exposure.

CONCLUSIONS

These results suggest that the myocardium may be a target of TCDD toxicity, resulting in reduced myocardial hypoxia, and HIF-1alpha and VEGF-A expression believed necessary for normal coronary development.

Expression of hypoxia-inducible factor-1 in the cochlea of newborn rats

Hypoxia/ischemia is a major pathogenetic factor in the development of hearing loss. An important transcription factor involved in the signaling and adaptation to hypoxia/ischemia is the hypoxia-inducible factor-1 (HIF-1). To study HIF-1 expression we used an in vitro hypoxia model of explant and dissociated cultures of the stria vascularis, the organ of Corti with limbus and the modiolus from the cochlea of 3-5-day-old Wistar rats. Hypoxia differentially increased HIF-1 activity as measured by a reporter gene. Twenty-four hour hypoxia increased HIF-1 activity 14.1+/-3.5-fold in the modiolus, 9.4+/-3.0-fold in the organ of Corti with limbus, and 6.4+/-1.5-fold in the stria vascularis. The HIF-1alpha mRNA level was measured by quantitative reverse transcription polymerase chain reaction and showed a lower expression in the modiolus (1.3+/-0.2 pg/microg RNA) than in both the organ of Corti with limbus and the stria vascularis (2.7-3.2+/-1.3, P<0.01). Hypoxia had no effect on the HIF-1alpha mRNA levels. The region-specific regulation of HIF-1 expression on the transcriptional and posttranslational levels may expand the possibilities for adaptation of the cochlea to hypoxia.

HIF-1alpha mRNA and protein upregulation involves Rho GTPase expression during hypoxia in renal cell carcinoma

The small G proteins of the Rho family are involved in reorganization of the actin cytoskeleton, cell migration and in the regulation of gene transcription. Hypoxia-induced ATP depletion results in the disruption of actin organization which could affect Rho functions. In solid tumors, regions with low oxygen tension stimulate angiogenesis in order to increase oxygen and nutrient supply. This process is mediated by stabilization of the transcriptional factor hypoxia inducible factor 1 (HIF-1), which increases vascular endothelial growth factor (VEGF) production. In this study, we investigated the activities of Rho proteins, which are key regulators of cytoskeleton organization during hypoxia in renal cell carcinoma. Caki-1 cells were exposed to hypoxia (1% O2) and exhibited increased Cdc42, Rac1 and RhoA protein expression. Immunoprecipitation of metabolically labelled RhoA showed that overexpression was at least due to neo-synthesis. The Rho GTPases overexpressed during hypoxia were mainly located at membranes and pull-down assays demonstrated that they were active since they bound GTP. RT-PCR analysis indicated that the increase in RhoA protein expression was also reflected at the mRNA level. Overexpression and activation of Rho proteins were downstream of, and dependent on, the production of reactive oxygen species (ROS) since, in the presence of an inhibitor, both the rise of ROS and upregulation of Rho proteins were abolished. Importantly, preincubation of cells with the toxin C3, which inhibits RhoA, reduced HIF-1alpha protein accumulation by 84% during hypoxia. Together, these results support a model where ROS upregulate Rho protein expression and where active RhoA is required for HIF-1alpha accumulation during hypoxia.

Podocyte expression of hypoxia-inducible factor (HIF)-1 and HIF-2 during glomerular development

The heterodimeric transcription factors, hypoxia-inducible factor (HIF)-1 and HIF-2, are essential for the maintenance of cellular oxygen homeostasis. In response to hypoxia, stabilized HIF-1alpha and HIF-2alpha proteins bind HIF-1beta and initiate expression of genes that alleviate hypoxic stress, including those promoting neovascularization. Both HIF-1 and HIF-2 stimulate transcription of vascular endothelial growth factor (VEGF), a crucial regulator of vascular development. Because VEGF is highly expressed by metanephric podocytes and collecting ducts, developing mouse kidney was examined for the presence and distribution of HIF-1alpha, HIF-2alpha, and HIF-1beta. The expression of HIF-1alpha and HIF-2alpha mRNAs in newborn mouse kidney was confirmed by RT-PCR and Northern blot analysis. By in situ hybridization, HIF-1alpha and HIF-2alpha mRNAs were highly expressed in the nephrogenic zone of newborn kidney cortex and in the medulla. Particularly intense hybridization was found in podocytes of developing glomeruli and in medullary collecting ducts. Both HIF-1 and HIF-2 heterodimers were identified in newborn kidney lysates by immunoprecipitation with HIF-1alpha, HIF-2alpha, and HIF-1beta antibodies and Western blots. Immunofluorescence analysis of the hypoxia marker, pimonidazole, showed that collecting ducts and many developing tubules undergo severe hypoxia in developing kidney. Immunohistochemistry of newborn kidney demonstrated widespread expression of HIF-1beta protein in nuclei of glomeruli and all tubular segments, whereas HIF-2alpha protein expression was more restricted and localized chiefly to podocytes of developing glomeruli and developing tubules. HIF-1alpha and HIF-2alpha protein and VEGF mRNA were all strongly induced in embryonic kidneys maintained in hypoxic organ cultures. Collectively, these data suggest that HIF stabilization, by hypoxia and/or by other means, may be critical for VEGF production and kidney vascular development.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibition of coronary development is preceded by a decrease in myocyte proliferation and an increase in cardiac apoptosis

BACKGROUND

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes cardiovascular toxicity, culminating in edema, hemorrhage, and mortality in piscine, avian, and mammalian embryos. To elucidate the mechanism of the cardiovascular teratogenicity of TCDD, we used a chick embryo model to determine whether TCDD alters coronary artery development and whether this alteration was associated with apoptosis and/or changes in myocyte proliferation.

METHODS

Fertile chicken eggs were injected with corn oil (control), 0.24, or 0.40 pmol TCDD/g in corn oil before incubation. To evaluate effects of TCDD on differentiation of coronary arteries, chick embryo hearts from incubation days 8 (D8), D10, and D12 were stained with anti-alpha-smooth muscle actin. Myocyte proliferation was measured by BrdU incorporation on D6, 8, 10, and 12 after TCDD treatment. In addition, temporal and spatial patterns of apoptosis were detected by TUNEL on D3, D5, D6, D8, and D10, and immunohistochemistry was used to identify the origin of apoptotic cells on D6.

RESULTS

TCDD increased apoptosis in structures where cell death normally occurs, including the outflow tract, endocardial cushion of the atrioventricular canal, and dorsal mesocardium, peaking in intensity on D6. Immunohistochemistry revealed that cells undergoing TCDD-induced apoptosis in the dorsal mesocardium were not neural or epicardial in origin. On D8 and D10 TCDD reduced myocyte proliferation. On D10, TCDD reduced coronary artery size and on D10 and D12 TCDD induced a dose-dependent decrease in coronary artery number.

CONCLUSIONS

The reduction of myocyte proliferation by TCDD preceded the reduction in coronary artery number and size, suggesting that changes in coronary development may be a consequence of reduced myocyte proliferation and a thinner ventricle wall. The peak of TCDD-induced increase in apoptosis occurred even earlier in embryo development and thus may contribute to changes in myocyte proliferation, coronary development, and cardiac structural malformations; however, a cause-and-effect relationship between apoptosis and these other events has yet to be established.