Hypoxia-Inducible Factor in Ringed Seal (Phoca hispida) Tissues

Hypoxia exposure blunts angiogenic signaling and upregulates the antioxidant system in endothelial cells derived from elephant seals

BACKGROUND

Elephant seals exhibit extreme hypoxemic tolerance derived from repetitive hypoxia/reoxygenation episodes they experience during diving bouts. Real-time assessment of the molecular changes underlying protection against hypoxic injury in seals remains restricted by their at-sea inaccessibility. Hence, we developed a proliferative arterial endothelial cell culture model from elephant seals and used RNA-seq, functional assays, and confocal microscopy to assess the molecular response to prolonged hypoxia.

RESULTS

Seal and human endothelial cells exposed to 1% O2 for up to 6 h respond differently to acute and prolonged hypoxia. Seal cells decouple stabilization of the hypoxia-sensitive transcriptional regulator HIF-1α from angiogenic signaling. Rapid upregulation of genes involved in glutathione (GSH) metabolism supports the maintenance of GSH pools, and intracellular succinate increases in seal but not human cells. High maximal and spare respiratory capacity in seal cells after hypoxia exposure occurs in concert with increasing mitochondrial branch length and independent from major changes in extracellular acidification rate, suggesting that seal cells recover oxidative metabolism without significant glycolytic dependency after hypoxia exposure.

CONCLUSIONS

We found that the glutathione antioxidant system is upregulated in seal endothelial cells during hypoxia, while this system remains static in comparable human cells. Furthermore, we found that in contrast to human cells, hypoxia exposure rapidly activates HIF-1 in seal cells, but this response is decoupled from the canonical angiogenesis pathway. These results highlight the unique mechanisms that confer extraordinary tolerance to limited oxygen availability in a champion diving mammal.

Hypoxia blunts angiogenic signaling and upregulates the antioxidant system in elephant seal endothelial cells

Elephant seals experience extreme hypoxemia during diving bouts. Similar depletions in oxygen availability characterize pathologies including myocardial infarction and ischemic stroke in humans, but seals manage these repeated episodes without injury. However, the real-time assessment of the molecular changes underlying protection against hypoxic injury in seals remains restricted by their at-sea inaccessibility. Hence, we developed a proliferative arterial endothelial cell culture system to assess the molecular response to prolonged hypoxia. Seal and human cells exposed to 1% O 2 for up to 6 h demonstrated differential responses to both acute and prolonged hypoxia. Seal cells decouple stabilization of the hypoxia-sensitive transcriptional regulator HIF-1α from angiogenic signaling at both the transcriptional and cellular level. Rapid upregulation of genes involved in the glutathione (GSH) metabolism pathway supported maintenance of GSH pools and increases in intracellular succinate in seal but not human cells during hypoxia exposure. High maximal and spare respiratory capacity in seal cells after hypoxia exposure occurred in concert with increasing mitochondrial branch length and independent from major changes in extracellular acidification rate, suggesting seal cells recover oxidative metabolism without significant glycolytic dependency after hypoxia exposure. In sum, our studies show that in contrast to human cells, seal cells adapt to hypoxia exposure by dampening angiogenic signaling, increasing antioxidant protection, and maintaining mitochondrial morphological integrity and function.

Dolphin leukocytes exhibit an attenuated cytokine response and increase heme oxygenase activity upon exposure to lipopolysaccharides

Cetaceans exhibit physiological adaptations that allowed the transition to aquatic life, including a robust antioxidant defense system that prevents injury from repeated exposure to ischemia/reperfusion events associated with breath-hold diving. The signaling cascades that characterize ischemic inflammation in humans are well characterized. In contrast, cetaceans' molecular and biochemical mechanisms that confer tolerance to inflammatory events are poorly understood. Heme oxygenase (HO) is a cytoprotective protein with anti-inflammatory properties. HO catalyzes the first step in the oxidative degradation of heme. The inducible HO-1 isoform is regulated by various stimuli, including hypoxia, oxidant stress, and inflammatory cytokines. The objective of this study was to compare the response of HO-1 and cytokines to a proinflammatory challenge in leukocytes isolated from humans and bottlenose dolphins (Tursiops truncatus). We measured changes in HO activity and expression, and abundance and expression of interleukin 1 beta (IL-1β), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-α), and heme oxygenase 1 (HMOX1) in leukocytes treated with lipopolysaccharide (LPS) for 24 and 48 h. HO activity increased (p < 0.05) in dolphin (48 h) but not human cells. TNF-α expression increased in human (24 h, 48 h), but not dolphin cells following LPS stimulation. LPS-induced cytokine expression was lower in dolphin than in human leukocytes, suggesting a blunted cytokine response in bottlenose dolphin leukocytes treated with LPS. Results suggest species-specific regulation of inflammatory cytokines in leukocytes treated with LPS, which may lead to differential responses to a pro-inflammatory challenge between marine and terrestrial mammals.

The Antarctic Weddell seal genome reveals evidence of selection on cardiovascular phenotype and lipid handling

The Weddell seal (Leptonychotes weddellii) thrives in its extreme Antarctic environment. We generated the Weddell seal genome assembly and a high-quality annotation to investigate genome-wide evolutionary pressures that underlie its phenotype and to study genes implicated in hypoxia tolerance and a lipid-based metabolism. Genome-wide analyses included gene family expansion/contraction, positive selection, and diverged sequence (acceleration) compared to other placental mammals, identifying selection in coding and non-coding sequence in five pathways that may shape cardiovascular phenotype. Lipid metabolism as well as hypoxia genes contained more accelerated regions in the Weddell seal compared to genomic background. Top-significant genes were SUMO2 and EP300; both regulate hypoxia inducible factor signaling. Liver expression of four genes with the strongest acceleration signals differ between Weddell seals and a terrestrial mammal, sheep. We also report a high-density lipoprotein-like particle in Weddell seal serum not present in other mammals, including the shallow-diving harbor seal.

Extreme Hypoxia Causing Brady-Arrythmias During Apnea in Elite Breath-Hold Divers

Introduction: The cardiac electrical conduction system is very sensitive to hypoglycemia and hypoxia, and the consequence may be brady-arrythmias. Weddell seals endure brady-arrythmias during their dives when desaturating to 3.2 kPa and elite breath-hold-divers (BHD), who share metabolic and cardiovascular adaptions including bradycardia with diving mammals, endure similar desaturation during maximum apnea. We hypothesized that hypoxia causes brady-arrythmias during maximum apnea in elite BHD. Hence, this study aimed to define the arterial blood glucose (Glu), peripheral saturation (SAT), heart rhythm (HR), and mean arterial blood pressure (MAP) of elite BHD during maximum apneas.

Methods: HR was monitored with Direct-Current-Pads/ECG-lead-II and MAP and Glu from a radial arterial-catheter in nine BHD performing an immersed and head-down maximal static pool apnea after three warm-up apneas. SAT was monitored with a sensor on the neck of the subjects. On a separate day, a 12-lead-ECG-monitored maximum static apnea was repeated dry (n = 6).

Results: During pool apnea of maximum duration (385 ± 70 s), SAT decreased from 99.6 ± 0.5 to 58.5 ± 5.5% (∼PaO₂ 4.8 ± 1.5 kPa, P < 0.001), while Glu increased from 5.8 ± 0.2 to 6.2 ± 0.2 mmol/l (P = 0.009). MAP increased from 103 ± 4 to 155 ± 6 mm Hg (P < 0.005). HR decreased to 46 ± 10 from 86 ± 14 beats/minute (P < 0.001). HR and MAP were unchanged after 3–4 min of apnea. During dry apnea (378 ± 31 s), HR decreased from 55 ± 4 to 40 ± 3 beats/minute (P = 0.031). Atrioventricular dissociation and junctional rhythm were observed both during pool and dry apneas.

Conclusion: Our findings contrast with previous studies concluding that Glu decreases during apnea diving. We conclude during maximum apnea in elite BHD that (1) the diving reflex is maximized after 3–4 min, (2) increasing Glu may indicate lactate metabolism in accordance with our previous results, and (3) extreme hypoxia rather than hypoglycemia causes brady-arrythmias in elite BHD similar to diving mammals.

An integrated comparative physiology and molecular approach pinpoints mediators of breath-hold capacity in dolphins

Background and objectives

Ischemic events, such as ischemic heart disease and stroke, are the number one cause of death globally. Ischemia prevents blood, carrying essential nutrients and oxygen, from reaching tissues, leading to cell and tissue death, and eventual organ failure. While humans are relatively intolerant to ischemic events, other species, such as marine mammals, have evolved a unique tolerance to chronic ischemia/reperfusion during apneic diving. To identify possible molecular features of an increased tolerance for apnea, we examined changes in gene expression in breath-holding dolphins.

Methodology

Here, we capitalized on the adaptations possesed by bottlenose dolphins (Tursiops truncatus) for diving as a comparative model of ischemic stress and hypoxia tolerance to identify molecular features associated with breath holding. Given that signals in the blood may influence physiological changes during diving, we used RNA-Seq and enzyme assays to examine time-dependent changes in gene expression in the blood of breath-holding dolphins.

Results

We observed time-dependent upregulation of the arachidonate 5-lipoxygenase (ALOX5) gene and increased lipoxygenase activity during breath holding. ALOX5 has been shown to be activated during hypoxia in rodent models, and its metabolites, leukotrienes, induce vasoconstriction.

Conclusions and implications

The upregulation of ALOX5 mRNA occurred within the calculated aerobic dive limit of the species, suggesting that ALOX5 may play a role in the dolphin’s physiological response to diving, particularly in a pro-inflammatory response to ischemia and in promoting vasoconstriction. These observations pinpoint a potential molecular mechanism by which dolphins, and perhaps other marine mammals, respond to the prolonged breath holds associated with diving.

Cardiac hypoxic resistance and decreasing lactate during maximum apnea in elite breath hold divers

Breath-hold divers (BHD) enduring apnea for more than 4 min are characterized by resistance to release of reactive oxygen species, reduced sensitivity to hypoxia, and low mitochondrial oxygen consumption in their skeletal muscles similar to northern elephant seals. The muscles and myocardium of harbor seals also exhibit metabolic adaptations including increased cardiac lactate-dehydrogenase-activity, exceeding their hypoxic limit. We hypothesized that the myocardium of BHD possesses similar adaptive mechanisms. During maximum apnea ¹⁵O-H₂O-PET/CT (n = 6) revealed no myocardial perfusion deficits but increased myocardial blood flow (MBF). Cardiac MRI determined blood oxygen level dependence oxygenation (n = 8) after 4 min of apnea was unaltered compared to rest, whereas cine-MRI demonstrated increased left ventricular wall thickness (LVWT). Arterial blood gases were collected after warm-up and maximum apnea in a pool. At the end of the maximum pool apnea (5 min), arterial saturation decreased to 52%, and lactate decreased 20%. Our findings contrast with previous MR studies of BHD, that reported elevated cardiac troponins and decreased myocardial perfusion after 4 min of apnea. In conclusion, we demonstrated for the first time with ¹⁵O-H₂O-PET/CT and MRI in elite BHD during maximum apnea, that MBF and LVWT increases while lactate decreases, indicating anaerobic/fat-based cardiac-metabolism similar to diving mammals.

Natural Tolerance to Ischemia and Hypoxemia in Diving Mammals: A Review

Reperfusion injury follows ischemia/reperfusion events occurring during myocardial infarction, stroke, embolism, and other peripheral vascular diseases. Decreased blood flow and reduced oxygen tension during ischemic episodes activate cellular pathways that upregulate pro-inflammatory signaling and promote oxidant generation. Reperfusion after ischemia recruits inflammatory cells to the vascular wall, further exacerbating oxidant production and ultimately resulting in cell death, tissue injury, and organ dysfunction. Diving mammals tolerate repetitive episodes of peripheral ischemia/reperfusion as part of the cardiovascular adjustments supporting long duration dives. These adjustments allow marine mammals to optimize the use of their body oxygen stores while diving but can result in selectively reduced perfusion to peripheral tissues. Remarkably, diving mammals show no apparent detrimental effects associated with these ischemia/reperfusion events. Here, we review the current knowledge regarding the strategies marine mammals use to suppress inflammation and cope with oxidant generation potentially derived from diving-induced ischemia/reperfusion.

Subcellular Energetics and Metabolism: A Cross-Species Framework

Although it is generally believed that oxidative phosphorylation and adequate oxygenation are essential for life, human development occurs in a profoundly hypoxic environment and "normal" levels of oxygen during embryogenesis are even harmful. The ability of embryos not only to survive but also to thrive in such an environment is made possible by adaptations related to metabolic pathways. Similarly, cancerous cells are able not only to survive but also to grow and spread in environments that would typically be fatal for healthy adult cells. Many biological states, both normal and pathological, share underlying similarities related to metabolism, the electron transport chain, and reactive species. The purpose of Part I of this review is to review the similarities among embryogenesis, mammalian adaptions to hypoxia (primarily driven by hypoxia-inducible factor-1), ischemia-reperfusion injury (and its relationship with reactive oxygen species), hibernation, diving animals, cancer, and sepsis, with a particular focus on the common characteristics that allow cells and organisms to survive in these states.

Prolonged fasting activates hypoxia inducible factors-1α, -2α and -3α in a tissue-specific manner in northern elephant seal pups

Hypoxia inducible factors (HIFs) are important regulators of energy homeostasis and cellular adaptation to low oxygen conditions. Northern elephant seals are naturally adapted to prolonged periods (1-2 months) of food deprivation (fasting) which result in metabolic changes that may activate HIF-1. However, the effects of prolonged fasting on HIFs are not well defined. We obtained the full-length cDNAs of HIF-1α and HIF-2α, and partial cDNA of HIF-3α in northern elephant seal pups. We also measured mRNA and nuclear protein content of HIF-1α, -2α, -3α in muscle and adipose during prolonged fasting (1, 3, 5 & 7 weeks), along with mRNA expression of HIF-mediated genes, LDH and VEGF. HIF-1α, -2α and -3α are 2595, 2852 and 1842 bp and encode proteins of 823, 864 and 586 amino acid residues with conserved domains needed for their function (bHLH and PAS) and regulation (ODD and TAD). HIF-1α and -2α mRNA expression increased 3- to 5-fold after 7 weeks of fasting in adipose and muscle, whereas HIF-3α increased 5-fold after 7 weeks of fasting in adipose. HIF-2α protein expression was detected in nuclear fractions from adipose and muscle, increasing approximately 2-fold, respectively with fasting. Expression of VEGF increased 3-fold after 7 weeks in adipose and muscle, whereas LDH mRNA expression increased 12-fold after 7 weeks in adipose. While the 3 HIFα genes are expressed in muscle and adipose, only HIF-2α protein was detectable in the nucleus suggesting that HIF-2α may contribute more significantly in the up-regulation of genes involved in the metabolic adaptation during fasting in the elephant seal.

Apnea stimulates the adaptive response to oxidative stress in elephant seal pups

Extended breath-hold (apnea) bouts are routine during diving and sleeping in seals. These apneas result in oxygen store depletion and blood flow redistribution towards obligatory oxygen-dependent tissues, exposing seals to critical levels of ischemia and hypoxemia. The subsequent reperfusion/reoxygenation has the potential to increase oxidant production and thus oxidative stress. The contributions of extended apnea to oxidative stress in adapted mammals are not well defined. To address the hypothesis that apnea in seals is not associated with increased oxidative damage, blood samples were collected from northern elephant seal pups (N=6) during eupnea, rest- and voluntary submersion-associated apneas, and post-apnea (recovery). Plasma 4-hydroxynonenal (HNE), 8-isoprostanes (8-isoPGF(2α)), nitrotyrosine (NT), protein carbonyls, xanthine and hypoxanthine (HX) levels, along with xanthine oxidase (XO) activity, were measured. Protein content of XO, superoxide dismutase 1 (Cu,ZnSOD), catalase and myoglobin (Mb), as well as the nuclear content of hypoxia inducible factor 1α (HIF-1α) and NF-E2-related factor 2 (Nrf2), were measured in muscle biopsies collected before and after the breath-hold trials. HNE, 8-iso PGF(2α), NT and protein carbonyl levels did not change among eupnea, apnea or recovery. XO activity and HX and xanthine concentrations were increased at the end of the apneas and during recovery. Muscle protein content of XO, CuZnSOD, catalase, Mb, HIF-1α and Nrf2 increased 25-70% after apnea. Results suggest that rather than inducing the damaging effects of hypoxemia and ischemia/reperfusion that have been reported in non-diving mammals, apnea in seals stimulates the oxidative stress and hypoxic hormetic responses, allowing these mammals to cope with the potentially detrimental effects associated with this condition.

Coping with physiological oxidative stress: a review of antioxidant strategies in seals

While diving, seals are exposed to apnea-induced hypoxemia and repetitive cycles of ischemia/reperfusion. While on land, seals experience sleep apnea, as well as prolonged periods of food and water deprivation. Prolonged fasting, sleep apnea, hypoxemia and ischemia/reperfusion increase oxidant production and oxidative stress in terrestrial mammals. In seals, however, neither prolonged fasting nor apnea-induced hypoxemia or ischemia/reperfusion increase systemic or local oxidative damage. The strategies seals evolved to cope with increased oxidant production are reviewed in the present manuscript. Among these strategies, high antioxidant capacity and the oxidant-mediated activation of hormetic responses against hypoxia and oxidative stress are discussed. In addition to expanding our knowledge of the evolution of antioxidant defenses and adaptive responses to oxidative stress, understanding the mechanisms that naturally allow mammals to avoid oxidative damage has the potential to advance our knowledge of oxidative stress-induced pathologies and to enhance the translative value of biomedical therapies in the long term.

Prolonged fasting increases glutathione biosynthesis in postweaned northern elephant seals

Northern elephant seals experience prolonged periods of absolute food and water deprivation (fasting) while breeding, molting or weaning. The postweaning fast in elephant seals is characterized by increases in the renin-angiotensin system, expression of the oxidant-producing protein Nox4, and NADPH oxidase activity; however, these increases are not correlated with increased oxidative damage or inflammation. Glutathione (GSH) is a potent reductant and a cofactor for glutathione peroxidases (GPx), glutathione-S transferases (GST) and 1-cys peroxiredoxin (PrxVI) and thus contributes to the removal of hydroperoxides, preventing oxidative damage. The effects of prolonged food deprivation on the GSH system are not well described in mammals. To test our hypothesis that GSH biosynthesis increases with fasting in postweaned elephant seals, we measured circulating and muscle GSH content at the early and late phases of the postweaning fast in elephant seals along with the activity/protein content of glutamate-cysteine ligase [GCL; catalytic (GCLc) and modulatory (GCLm) subunits], γ-glutamyl transpeptidase (GGT), glutathione disulphide reductase (GR), glucose-6-phosphate dehydrogenase (G6PDH), GST and PrxVI, as well as plasma changes in γ-glutamyl amino acids, glutamate and glutamine. GSH increased two- to four-fold with fasting along with a 40-50% increase in the content of GCLm and GCLc, a 75% increase in GGT activity, a two- to 2.5-fold increase in GR, G6PDH and GST activities and a 30% increase in PrxVI content. Plasma γ-glutamyl glutamine, γ-glutamyl isoleucine and γ-glutamyl methionine also increased with fasting whereas glutamate and glutamine decreased. Results indicate that GSH biosynthesis increases with fasting and that GSH contributes to counteracting hydroperoxide production, preventing oxidative damage in fasting seals.

Prolonged fasting does not increase oxidative damage or inflammation in postweaned northern elephant seal pups

Elephant seals are naturally adapted to survive up to three months of absolute food and water deprivation (fasting). Prolonged food deprivation in terrestrial mammals increases reactive oxygen species (ROS) production, oxidative damage and inflammation that can be induced by an increase in the renin-angiotensin system (RAS). To test the hypothesis that prolonged fasting in elephant seals is not associated with increased oxidative stress or inflammation, blood samples and muscle biopsies were collected from early (2-3 weeks post-weaning) and late (7-8 weeks post-weaning) fasted seals. Plasma levels of oxidative damage, inflammatory markers and plasma renin activity (PRA), along with muscle levels of lipid and protein oxidation, were compared between early and late fasting periods. Protein expression of angiotensin receptor 1 (AT(1)), pro-oxidant (Nox4) and antioxidant enzymes (CuZn- and Mn-superoxide dismutases, glutathione peroxidase and catalase) was analyzed in muscle. Fasting induced a 2.5-fold increase in PRA, a 50% increase in AT(1), a twofold increase in Nox4 and a 70% increase in NADPH oxidase activity. By contrast, neither tissue nor systemic indices of oxidative damage or inflammation increased with fasting. Furthermore, muscle antioxidant enzymes increased 40-60% with fasting in parallel with an increase in muscle and red blood cell antioxidant enzyme activities. These data suggest that, despite the observed increases in RAS and Nox4, an increase in antioxidant enzymes appears to be sufficient to suppress systemic and tissue indices of oxidative damage and inflammation in seals that have fasted for a prolonged period. The present study highlights the importance of antioxidant capacity in mammals during chronic periods of stress to help avoid deleterious systemic consequences.

Muscle aging and oxidative stress in wild-caught shrews

Red-toothed shrews (Soricidae, subfamily Soricinae) are an intriguing model system to examine the free-radical theory of aging in wild mammals, given their short (<18months) lifespan and high mass-specific metabolic rates. As muscle performance underlies both foraging ability and predator avoidance, any age-related decline should be detrimental to fitness and survival. Muscle samples of water shrews (Sorex palustris) and sympatrically distributed short-tailed shrews (Blarina brevicauda) were therefore assessed for oxidative stress markers, protective antioxidant enzymes and apoptosis. Activity levels of catalase and glutathione peroxidase increased with age in both species. Similarly, Cu,Zn-superoxide dismutase isoform content was elevated significantly in older animals of both species (increases of 60% in the water shrew, 25% in the short-tailed shrew). Only one oxidative stress marker (lipid peroxidation) was age-elevated; the others were stable or declined (4-hydroxynonenal adducts and dihydroethidium oxidation). Glutathione peroxidase activity was significantly higher in the short-tailed shrew, while catalase activity was 2x higher in water shrews. Oxidative stress indicators were on average higher in short-tailed shrews. Apoptosis occurred in <1% of myocytes examined, and did not increase with age. Within the constraints of the sample size we found evidence of protection against elevated oxidative stress in wild-caught shrews.

Hypoxia tolerance in mammals and birds: from the wilderness to the clinic

All mammals and birds must develop effective strategies to cope with reduced oxygen availability. These animals achieve tolerance to acute and chronic hypoxia by (a) reductions in metabolism, (b) the prevention of cellular injury, and (c) the maintenance of functional integrity. Failure to meet any one of these tasks is detrimental. Birds and mammals accomplish this triple task through a highly coordinated, systems-level reconfiguration involving the partial shutdown of some but not all organs. This reconfiguration is achieved through a similarly complex reconfiguration at the cellular and molecular levels. Reconfiguration at these various levels depends on numerous factors that include the environment, the degree of hypoxic stress, and developmental, behavioral, and ecological conditions. Although common molecular strategies exist, the cellular and molecular changes in any given cell are very diverse. Some cells remain metabolically active, whereas others shut down or rely on anaerobic metabolism. This cellular shutdown is temporarily regulated, and during hypoxic exposure, active cellular networks must continue to control vital functions. The challenge for future research is to explore the cellular mechanisms and conditions that transform an organ or a cellular network into a hypometabolic state, without loss of functional integrity. Much can be learned in this respect from nature: Diving, burrowing, and hibernating animals living in diverse environments are masters of adaptation and can teach us how to deal with hypoxia, an issue of great clinical significance.

Antioxidant enzymes in ringed seal tissues: potential protection against dive-associated ischemia/reperfusion

Diving seals experience heart rate reduction and preferential distribution of the oxygenated blood flow to the heart and brain, widespread peripheral vasoconstriction, and selective ischemia in the most hypoxia-tolerant tissues. The first breath after the dive restores the oxygenated blood flow to all tissues and raises the potential for the production of reactive oxygen species (ROS). We hypothesized that in order to counteract the damaging effects of ROS and to tolerate repetitive cycles of ischemia/reperfusion associated with diving, ringed seal (Phoca hispida) tissues have elevated activities of antioxidant enzymes. Activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST) were measured by spectrophotometric techniques in heart, kidney, liver, lung, and muscle extracts of ringed seals and domestic pigs (Sus scrofa). The results suggest that in ringed seal heart SOD, GPx and GST activities are an efficient protective mechanism for counteracting ROS production and its deleterious effects. Apparently CAT activity in seal liver and GPx activity in seal muscle participate in the removal of hydroperoxides, while seal lung appears to be protected from oxidative damage by SOD and GPx activities.

Hypoxia-inducible factor 1 proteomics and diving adaptations in ringed seal

The putative amino acid sequence of ringed seal (Phoca hispida) hypoxia-inducible factor 1alpha (HIF-1alpha) derived from DNA sequence analysis of the single-copy gene has been investigated. The rationale for these studies was to determine the reasons for the presence of HIF-1alpha at relatively high levels in seal tissues, and its possible role in protection against diving-related oxidative damage. Sequence analysis indicated that the bHLH/PAS and TAD functional domains are very similar to those in terrestrial mammals, although there were significant sequence differences between the mouse and seal proteins in a region of the ODD domain. Some of these results indicate that seal HIF-1alpha protein can bind HIF-Ibeta, DNA, transcriptional coactivators, and von Hippel-Lindau protein (pVHL). The presence of HIF-1alpha in seal tissues was not related to the absence of pVHL, which was found to be present in all seal tissues examined. It is concluded that seal HIF-1alpha may act as a transcriptional activator and that its presence in seal tissues is probably not caused by its inability to interact with pVHL. It is suggested that seal HIF-1 may serve two functions in the postdiving period, namely, to attenuate ischemia/reperfusion-induced oxidative stress and to allow efficient lung reinflation.