Hypoxia Induces Changes in AMP-Activated Protein Kinase Activity and Energy Metabolism in Muscle Tissue of the Oriental River Prawn Macrobrachium nipponense

Impact of hypoxia on glucose metabolism and hypoxia signaling pathways in juvenile horseshoe crabs Tachypleus tridentatus

Marine hypoxia poses a significant challenge in the contemporary marine environment. The horseshoe crab, an ancient benthic marine organism, is confronted with the potential threat of species extinction due to hypoxia, making it an ideal candidate for studying hypoxia tolerance mechanisms. In this experiment, juvenile Tachypleus tridentatus were subjected to a 21-day trial at DO:2 mg/L (hypoxia) and DO:6 mg/L conditions. The experimental timeline included a 14-day exposure phase followed by a 7-day recovery period. Sampling occurred on days 0, 7, 14, and 21, where the period from day 14 to day 21 corresponds to seven days of recuperation. Several enzymatic activities of important proteins throughout this investigation were evaluated, such as succinate dehydrogenase (SDH), phosphofructokinase (PFK), hexokinase (HK), lactate dehydrogenase (LDH), and pyruvate kinase (PK). Concurrently, the relative expression of hexokinase-1 (HK), hypoxia-inducible factor 1-alpha inhibitor (FIH), and hypoxia-inducible factor 1-alpha (HIF-1α), pyruvate dehydrogenase phosphatase (PDH), succinate dehydrogenase assembly factor 4 (SDH), and Glucose-6-phosphatase (G6Pase) were also investigated. These analyses aimed to elucidate alterations in the hypoxia signaling pathway and respiratory energy metabolism. It is revealed that juvenile T. tridentatus initiated the HIF pathway under hypoxic conditions, resulting in an upregulation of HIF-1α and FIH-1 gene expression, which in turn, influenced a shift in metabolic patterns. Particularly, the activity of glycolysis-related enzymes was promoted significantly, including PK, HK, PKF, LDH, and the related HK gene. In contrast, enzymes linked to aerobic respiration, PDH, and SDH, as well as the related PDH and SDH genes, displayed down-regulation, signifying a transition from aerobic to anaerobic metabolism. Additionally, the activity of gluconeogenesis-related enzymes such as PK and G6Pase gene expression were significantly elevated, indicating the activation of gluconeogenesis and glycogenolysis pathways. Consequently, juvenile T. tridentatus demonstrated an adaptive response to hypoxic conditions, marked by changes in respiratory energy metabolism modes and the activation of hypoxia signaling pathways.

Influences of two transport strategies on AMPK-mediated metabolism and flesh quality of shrimp (Litopenaeus vannamei)

BACKGROUND

Water-free transportation (WFT), as a novel strategy for express delivery of live shrimp (Litopenaeus vannamei), was developed recently. However, air exposure during this transportation arouses a series of abiotic stress to the shrimp. In the present study, the influences of WFT stress on glycolysis and lipolysis metabolism and meat quality (umami flavor and drip loss) were investigated in comparison with conventional water transportation (WT).

RESULTS

The results showed that type II muscle fibers with the feature of anaerobic metabolism were dominated in shrimp flesh. In addition, the increments of intracellular Ca2+ was detected in WFT and WT, which then activated the AMP-activated protein kinase pathway and promoted the consumption of glycogen, as well as the accumulation of lactate and lipolysis, under the enzymolysis of hexokinase, pyruvate kinase, lactate dehydrogenase and adipose triglyceride lipase. Glycogen glycolyzed to latate. Meanwhile, ATP degraded along with glycolysis resulting in the generation of ATP-related adenosine phosphates such as inosine monophosphate with umami flavor and phosphoric acid. More remarkable (P < 0.05) physiological changes (except lactate dehydrogenase and lactate) were observed in WFT compared to WT. Additionally, the fatty acid profile also slightly changed.

CONCLUSION

The transport stress induced significant energy metabolism changes of shrimp flesh and therefore effected the flesh quality. The intensifications of freshness (K-value) of shrimp flesh were detected as a result of ATP degradation, which were more pronounced after WFT. However, the drip loss of shrimp flesh was more significantly increased (P < 0.05) after WFT compared to WT. © 2023 Society of Chemical Industry.

Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.

Adenosine 5'-monophosphate-activated protein kinase (AMPK) negatively regulates the immunity and resistance to Vibrio alginolyticus of white shrimp, Penaeus vannamei

Shrimp immunology is vital in establishing prophylactic and therapeutic strategies for controlling pathological problems that threaten shrimp production. Apart from dietary treatments, the adenosine 5'-monophosphate-activated protein kinase (AMPK), an important regulatory enzyme that restores cellular energy balance during metabolic and physiological stress, is known to have therapeutic potential to improve shrimp's defense mechanism. Despite this, studies targeting the AMPK pathway in shrimp exposed to stressful conditions are vastly limited. In this study, AMPK was knocked down to assess the immunological changes and white shrimp, Penaeus vannamei resistance to Vibrio alginolyticus infection. Shrimps were injected individually and simultaneously with dsRNA targeting specific genes such as AMPK, Rheb, and TOR, after which the hepatopancreas was analyzed for the different gene expressions. The gene expressions of AMPK, Rheb, and TOR were effectively suppressed after being treated with dsRNAs. The Western blot analysis further confirmed a reduction in the protein concentration of AMPK and Rheb in the hepatopancreas. The suppression of AMPK gene led to a robust increase in the shrimp's resistance to V. alginolyticus, whereas the activation of AMPK by metformin decreased the shrimp's disease resistance. Among the mTOR downstream targets, the HIF-1α expression in shrimp treated with dsAMPK significantly increased at 48 h but returned to normal levels when shrimp were treated with dsAMPK and either dsRheb or dsTOR. Immune responses such as respiratory burst, lysozyme activity, and phagocytic activity increased, while superoxide dismutase activity decreased following the knockdown of the AMPK gene compared to the control group. However, co-injection with dsAMPK and dsTOR or dsRheb restored immune responses to normal levels. Collectively, these results demonstrate that the inactivation of AMPK may ameliorate shrimp's innate immune response to recognize and defend against pathogens via the AMPK/mTOR1 pathway.

Effect of heat and hypoxia stress on mitochondrion and energy metabolism in the gill of hard clam

Aquatic animals suffer from heat and hypoxia stress more frequently due to global climate change and other anthropogenic activities. Heat and hypoxia stress can significantly affect mitochondrial function and energy metabolism. Here, the response and adaptation characteristics of mitochondria and energy metabolism in the gill of the hard clam Mercenaria mercenaria under heat (35 °C), hypoxia (0.2 mg/L), and heat plus hypoxia stress (35 °C, 0.2 mg/L) after 48 h exposure were investigated. Mitochondrial membrane potentials were depolarized under environmental stress. Mitochondrial fusion, fission and mitophagy played a key role in maintain mitochondrion function. The AMPK subunits showed different expression under environmental stress. Acceleration of enzyme activities (phosphofructokinase, pyruvate kinase and lactic dehydrogenase) and accumulation of anaerobic metabolites in glycolysis and TCA cycle implied that the anaerobic metabolism might play a key role in providing energy. Accumulation of amino acids might help to increase tolerance under heat and heat combined hypoxia stress. In addition, urea cycle played a key role in amino acid metabolism to prevent ammonia/nitrogen toxicity. This study improved our understanding of the mitochondrial and energy metabolism responses of marine bivalves exposed to environmental stress.

Dietary thiamine modulates carbohydrate metabolism, antioxidant status, and alleviates hypoxia stress in oriental river prawn Macrobrachium nipponense (de Haan)

Hypoxia is one of the challenges in prawns aquaculture. However, the role of thiamine, which is a coenzyme in carbohydrate metabolism with antioxidant properties, in reducing hypoxia in prawns aquaculture is currently unknown. We investigated the effects of thiamine on antioxidant status, carbohydrate metabolism and acute hypoxia in oriental river prawn, Macrobrachium nipponense. One thousand eight hundred prawns (0.123 ± 0.003 g) were fed five diets (60 prawns each tank, six replicates per diet) supplemented with graded thiamine levels (5.69, 70.70, 133.67, 268.33 and 532.00 mg/kg dry mater) for eight weeks and then exposed to hypoxia stress for 12 h followed by reoxyegnation for 12 h. The results showed that, under normoxia, prawns fed the 133.67 or 268.33 mg/kg thiamine diet had significantly lower glucose 6-phosphatedehydrogenase, succinate dehydrogenase and phosphoenolpyruvate carboxykinase activities than those fed the other diets. Moreover, total antioxidant capacity (T-AOC) increased significantly when prawns were fed the 133.67 mg/kg thiamine diet. Superoxide dismutase (SOD) activity and malonaldehyde (MDA) content also increased significantly when prawns were fed the 268.33 or 532.00 mg/kg thiamine diet under hypoxia. And the significantly increased SOD activity and MDA level also observed in prawns fed 532.00 mg/kg thiamine under reoxygenation. Under normoxia, prawns fed the 70.70 or 133.67 mg/kg thiamine diet decreased the mRNA expressions of AMP-activated protein kinase-alpha (AMPK-α), pyruvate dehydrogenase-E1-α subunit (PDH-E1-α) and hypoxia-inducible factor-1s (HIF-1α, HIF-1β), but increased the mRNA expressions of phosphofructokinase (PFK) significantly. After 12 h of hypoxia, the energy metabolism related genes (AMPK-β, AMPK-γ, PFK, PDH-E1-α), hypoxia-inducible factor related genes (HIF-1α, HIF-1β) and thiamine transporter gene (SLC19A2) were up-regulated significantly in prawns fed the 133.67 or 268.33 mg/kg thiamine diets. After 12 h of reoxygenation, prawns fed the 133.67 or 268.33 mg/kg diet significantly decreased the SOD activity, MDA level and SLC19A2 mRNA expression compared with other diets. The optimum thiamine was 161.20 mg/kg for minimum MDA content and 143.17 mg/kg for maximum T-AOC activity based on cubic regression analysis. In summary, supplementing 143.17 to 161.20 mg/kg thiamine in the diets for M. nipponense improves the antioxidant capacity under normoxia and reduces the oxidative damage under hypoxia stress.

The transcriptomic responses of blunt snout bream (Megalobrama amblycephala) to acute hypoxia stress alone, and in combination with bortezomib

BACKGROUND

Blunt snout bream (Megalobrama amblycephala) is sensitive to hypoxia. A new blunt snout bream strain, "Pujiang No.2", was developed to overcome this shortcoming. As a proteasome inhibitor, bortezomib (PS-341) has been shown to affect the adaptation of cells to a hypoxic environment. In the present study, bortezomib was used to explore the hypoxia adaptation mechanism of "Pujiang No.2". We examined how acute hypoxia alone (hypoxia-treated, HN: 1.0 mg·L^- 1), and in combination with bortezomib (hypoxia-bortezomib-treated, HB: Use 1 mg bortezomib for 1 kg fish), impacted the hepatic ultrastructure and transcriptome expression compared to control fish (normoxia-treated, NN).

RESULTS

Hypoxia tolerance was significantly decreased in the bortezomib-treated group (LOE_crit, loss of equilibrium, 1.11 mg·L^- 1 and 1.32 mg·L^- 1) compared to the control group (LOE_crit, 0.73 mg·L^- 1 and 0.85 mg·L^- 1). The HB group had more severe liver injury than the HN group. Specifically, the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the HB group (52.16 U/gprot, 32 U/gprot) were significantly (p < 0.01) higher than those in the HN group (32.85 U/gprot, 21. 68 U/gprot). In addition, more severe liver damage such as vacuoles, nuclear atrophy, and nuclear lysis were observed in the HB group. RNA-seq was performed on livers from the HN, HB and NN groups. KEGG pathway analysis disclosed that many DEGs (differently expressed genes) were enriched in the HIF-1, FOXO, MAPK, PI3K-Akt and AMPK signaling pathway and their downstream.

CONCLUSION

We explored the adaptation mechanism of "Pujiang No.2" to hypoxia stress by using bortezomib, and combined with transcriptome analysis, accurately captured the genes related to hypoxia tolerance advantage.

Cloning and characterization of AMP-activated protein kinase genes in Daphnia pulex: Modulation of AMPK gene expression in response to polystyrene nanoparticles

Energy metabolism is essential for almost all organisms. At the molecular level, adenosine monophosphate activated protein kinase (AMPK) plays a vital role in cellular energy homeostasis. Its molecular characterization in invertebrates, including Daphnia pulex, and the understanding of its role in response to environmental contaminants is limited. In this study, three subunits of AMPK (AMPKα, β, and γ) were cloned in D. pulex, and assigned the GenBank accession numbers MT536758, MT536759, and MT536760, respectively. Their full lengths were 2,000, 1,384, and 2594 bp, respectively, and contained open reading frames of 526, 274, and 580 amino acids, respectively. Bioinformatic analysis revealed that the three AMPK subunits all have features representative of the AMPK superfamily, and were correspondingly clustered with each orthologue branch. The three AMPK subunit genes, AMPKα, β, and γ, had the highest similarity to those of other organisms at 82%, 94%, and 71%, respectively. Nanoplastics significantly increased AMPKα expression, but decreased that of AMPKβ and γ. These results identified AMPKα, β, and γ in D. pulex, and showed that they all encode proteins with conserved functional domains. This study provides basic information on how three types of AMPK in aquatic organisms respond to environmental contaminants.

Effects of hyperoxia and hypoxia on the proliferation of C2C12 myoblasts

Hyperoxic conditions are known to accelerate skeletal muscle regeneration after injuries. In the early phase of regeneration, macrophages invade the injured area and subsequently secrete various growth factors, which regulate myoblast proliferation and differentiation. Although hyperoxic conditions accelerate muscle regeneration, it is unknown whether this effect is indirectly mediated by macrophages. Here, using C2C12 cells, we show that not only hyperoxia but also hypoxia enhance myoblast proliferation directly, without accelerating differentiation into myotubes. Under hyperoxic conditions (95% O₂ + 5% CO₂), the cell membrane was damaged because of lipid oxidization, and a disrupted cytoskeletal structure, resulting in suppressed cell proliferation. However, a culture medium containing vitamin C (VC), an antioxidant, prevented this lipid oxidization and cytoskeletal disruption, resulting in enhanced proliferation in response to hyperoxia exposure of ≤4 h/day. In contrast, exposure to hypoxic conditions (95% N₂ + 5% CO₂) for ≤8 h/day enhanced cell proliferation. Hyperoxia did not promote cell differentiation into myotubes, regardless of whether the culture medium contained VC. Similarly, hypoxia did not accelerate cell differentiation. These results suggest that regardless of hyperoxia or hypoxia, changes in oxygen tension can enhance cell proliferation directly, but do not influence differentiation efficiency in C2C12 cells. Moreover, excess oxidative stress abrogated the enhancement of myoblast proliferation induced by hyperoxia. This research will contribute to basic data for applying the effects of hyperoxia or hypoxia to muscle regeneration therapy.

Effects of Different Ammonia Levels on Tribenuron Methyl Toxicity in Daphnia magna

The present study investigates the toxicity of the herbicide tribenuron methyl (TBM) as an anthropogenic agent and ammonia as an abiotic factor on Daphnia magna at environmentally relevant concentrations. These stressors may coexist in surface waters in agricultural regions. To achieve this objective, D. magna were exposed to TBM at a nominal concentration of 0.81 μg/L in association with a low ammonia (LA) concentration of 0.65 mg/L and a high ammonia (HA) concentration of 1.61 mg/L in acute toxicity tests of 96-h duration and chronic toxicity tests of 21-day duration. The D. magna also were exposed to TBM, HA, and LA singly. The D. magna were analysed for various biomarkers of sublethal toxicity. Glutathione peroxidase (GPx), glutathione S-transferase (GST), cholinesterase (ChE) enzyme activities, and levels of thiobarbituric acid reactive substances (TBARS) and total protein were determined spectrophotometrically. Mitochondrial membrane potential (MMP) was analysed by microscopy with fluorescence staining. Cytochrome c and 5' AMP-activated protein kinase (AMPK) were analysed by Western blotting. Morphometric properties were examined microscopically. This is the first study in which AMPK, an indicator of intracellular energy, was measured in D. magna. GST and ChE enzyme activities and TBARS and total protein levels did not change during acute exposures (i.e., 96 h) in all treatments. GPx activity increased in D. magna from the HA + TBM treatment compared with single-exposure groups. The level of cytochrome c protein was elevated in D. magna from the LA and LA + TBM treatments. AMPK protein levels increased in all treatments with daphnids, except in the LA group. MMP was depolarised in D. magna from all treatments, whereas the most notable change was observed in HA + TBM mixture group in chronic exposures. The results show that GST and ChE may not be sensitive biomarkers for evaluating the sublethal toxic effects to D. magna exposed to environmentally relevant concentrations of ammonia and TBM. Acute and chronic exposure to ammonia and TBM probably caused an energetic crisis in D. magna. Therefore, AMPK and MMP are promising biomarkers for these toxicants.

Characterization and expression of arginine kinase 2 from Macrobrachium nipponense in response to salinity stress

Salinity is a fundamental environmental factor in aquaculture, and arginine kinase (AK) plays imperative roles in innate immune feedback and stress resistance in invertebrates. In the current study, we cloned a full-length cDNA of arginine kinase 2 (MnAK2, GenBank number, MN149533) in Macrobrachium nipponense and analyzed its function through a salinity challenge using bioinformatic approaches. MnAK2 was expressed at the highest levels in hepatopancreas and muscle. Changes in the expression levels of MnAK2, enzymes involved in innate immunity, antioxidant enzymes, and antioxidant enzyme-related genes, and the glutathione and malondialdehyde contents were investigated after 6-week salinity treatment. The expression of MnAK2 gradually increased as salinity increased, and western blotting showed that MnAK2 was significantly upregulated in the 14 and 22 ppt salinity-treatment groups relative to the control group. The findings indicate that high salinity produces oxidative stress and that salinity below isotonic salinity might improve the antioxidant response in M. nipponense. MnAK2 may play a crucial role in the response to salinity stress in M. nipponense.

Activation of AMPK under Hypoxia: Many Roads Leading to Rome

AMP-activated protein kinase (AMPK) is known as a pivotal cellular energy sensor, mediating the adaptation to low energy levels by deactivating anabolic processes and activating catabolic processes in order to restore the cellular ATP supply when the cellular AMP/ATP ratio is increased. Besides this well-known role, it has also been shown to exert protective effects under hypoxia. While an insufficient supply with oxygen might easily deplete cellular energy levels, i.e., ATP concentration, manifold other mechanisms have been suggested and are heavily disputed regarding the activation of AMPK under hypoxia independently from cellular AMP concentrations. However, an activation of AMPK preceding energy depletion could induce a timely adaptation reaction preventing more serious damage. A connection between AMPK and the master regulator of hypoxic adaptation via gene transcription, hypoxia-inducible factor (HIF), has also been taken into account, orchestrating their concerted protective action. This review will summarize the current knowledge on mechanisms of AMPK activation under hypoxia and its interrelationship with HIF.

Anti-Hypoxic Molecular Mechanisms of Rhodiola crenulata Extract in Zebrafish as Revealed by Metabonomics

The health supplement of Rhodiola crenulata (RC) is well known for its effective properties against hypoxia. However, the mechanisms of its anti-hypoxic action were still unclear. The objective of this work was to evaluate the molecular mechanisms of RC extract against hypoxia in a hypoxic zebrafish model through metabonomics and network pharmacology analysis. The hypoxic zebrafish model in the environment with low concentration (3%) of oxygen was constructed and used to explore the anti-hypoxic effects of RC extract, followed by detecting the changes of the metabolome in the brain through liquid chromatography-high resolution mass spectrometry. An in silico network for metabolite-protein interactions was further established to examine the potential mechanisms of RC extract, and the mRNA expression levels of the key nodes were validated by real-time quantitative PCR. As results, RC extract could keep zebrafish survive after 72-h hypoxia via improving lactate dehydrogenase, citrate synthase, and hypoxia-induced factor-1α in brains. One hundred and forty-two differential metabolites were screened in the metabonomics, and sphingolipid metabolism pathway was significantly regulated after RC treatment. The constructed protein-metabolites network indicated that the HIF-related signals were recovered, and the mRNA level of AMPK was elevated. In conclusion, RC extract had markedly anti-hypoxic effects in zebrafish via changing sphingolipid metabolism, HIF-related and AMPK signaling pathways.

Evaluating expression of autophagy-related genes in oriental river prawn Macrobrachium nipponense as potential biomarkers for hypoxia exposure

Autophagy, a crucial process for maintaining cellular homeostasis, is under the control of several autophagy-related (ATG) proteins, and is highly conserved in most animals, but its response to adverse environmental conditions is poorly understood in crustaceans. Herein, we hypothesised that autophagy acts as a protective response to hypoxia, and Beclin 1, ATG7 and ATG8 in oriental river prawn (Macrobrachium nipponense) were chosen as potential biomarkers under hypoxia exposure; thus, their full-length cDNA sequences were cloned and characterised. Open reading frames (ORFs) of 1281, 2076 and 360 bp, encoding proteins of 427, 692 and 120 amino acid residues, respectively, were obtained. Phylogenetic analysis demonstrated the three M. nipponense proteins do not form a clade with vertebrate homologs. Protein and mRNA levels were investigated in different tissues and developmental stages, and all three were significantly upregulated in a time-dependent manner in the hepatopancreas following hypoxia stress. Biochemical and morphological analysis of hepatocytes revealed that hypoxia increased the abundance of hepatic autophagic vacuoles and stimulated anaerobic metabolism. RNA interference-mediated silencing of ATG8 significantly increased the death rate of M. nipponense juveniles under hypoxia stress conditions. Together, these results suggest that Beclin 1, ATG7 and ATG8 contribute to autophagy-based responses against hypoxia in M. nipponense. The findings also expand our understanding of the potential role of autophagy as an adaptive response against hypoxia toxicity in crustaceans. The results showed that hepatic ATG8 levels may be directly indicative of acute hypoxia in prawns, and provide insight into the time at which hypoxia exposure occurs. Autophagy-related genes expression pattern seems to be sensitive and good biomarkers of acute hypoxia exposure.

Molecular Cloning and Expression Analysis of Lactate Dehydrogenase from the Oriental River Prawn Macrobrachium nipponense in Response to Hypoxia

Metabolic adaption to hypoxic stress in crustaceans implies a shift from aerobic to anaerobic metabolism. Lactate dehydrogenase (LDH) is a key enzyme in glycolysis in prawns. However, very little is known about the role of LDH in hypoxia inducible factor (HIF) pathways of prawns. In this study, full-length cDNA of LDH (MnLDH) was obtained from the oriental river prawn Macrobrachium nipponense, and was characterized. The full-length cDNA is 2267-bp with an open reading frame of 999 bp coding for a protein of 333 amino acids with conserved domains important for function and regulation. Phylogenetic analysis showed that MnLDH is close to LDHs from other invertebrates. Quantitative real-time PCR revealed that MnLDH is expressed in various tissues with the highest expression level in muscle. MnLDH mRNA transcript and protein abundance in muscle, but not in hepatopancreas, were induced by hypoxia. Silencing of hypoxia-inducible factor 1 (HIF-1) α or HIF-1β subunits blocked the hypoxia-dependent increase of LDH expression and enzyme activity in muscle. A series of MnLDH promoter sequences, especially the full-length promoter, generated an increase in luciferase expression relative to promoterless vector; furthermore, the expression of luciferase was induced by hypoxia. These results demonstrate that MnLDH is probably involved a HIF-1-dependent pathway during hypoxia in the highly active metabolism of muscle.