Molecular characterization and expression regulation of the factor-inhibiting HIF-1 (FIH-1) gene under hypoxic stress in bighead carp (Aristichthys nobilis)

Hypothalamic transcriptome response to simulated diel earthen pond hypoxia cycles in channel catfish (Ictalurus punctatus)

Commercial culture of channel catfish (Ictalurus punctatus) occurs in earthen ponds that are characterized by diel swings in dissolved oxygen concentration that can fall to severe levels of hypoxia, which can suppress appetite and lead to suboptimal growth. Given the significance of the hypothalamus in regulating these processes in other fishes, an investigation into the hypothalamus transcriptome was conducted to identify specific genes and expression patterns responding to hypoxia. Channel catfish in normoxic water were compared with catfish subjected to 12 h of hypoxia (20% oxygen saturation; 1.8 mg O2/L; 27°C) followed by 12 h of recovery in normoxia to mimic 24 h in a catfish aquaculture pond. Fish were sampled at 0-, 6-, 12-, 18-, and 24-h timepoints, with the 6- and 12-h samplings occurring during hypoxia. A total of 190 genes were differentially expressed during the experiment, with most occurring during hypoxia and returning to baseline values within 6 h of normoxia. Differentially expressed genes were sorted by function into Gene Ontology biological processes and revealed that most were categorized as "response to hypoxia," "sprouting angiogenesis," and "cellular response to xenobiotic stimulus." The patterns of gene expression reported here suggest that transcriptome responses to hypoxia are broad and quickly reversibly with the onset of normoxia. Although no genes commonly reported to modulate appetite were found to be differentially expressed in this experiment, several candidates were identified for future studies investigating the interplay between hypoxia and appetite in channel catfish, including adm, igfbp1a, igfbp7, and stc2b.NEW & NOTEWORTHY Channel catfish are an economically important species that experience diel episodic periods of hypoxia that can reduce appetite. This is the first study to investigate their transcriptome from the hypothalamus in a simulated 24-h span in a commercial catfish pond, with 12 h of hypoxia and 12 h of normoxia. The research revealed functional groups of genes relating to hypoxia, angiogenesis, and glycolysis as well as individual target genes possibly involved in appetite regulation.

The Asparagine Hydroxylase FIH: A Unique Oxygen Sensor

Significance: Limited oxygen availability (hypoxia) commonly occurs in a range of physiological and pathophysiological conditions, including embryonic development, physical exercise, inflammation, and ischemia. It is thus vital for cells and tissues to monitor their local oxygen availability to be able to adjust in case the oxygen supply is decreased. The cellular oxygen sensor factor inhibiting hypoxia-inducible factor (FIH) is the only known asparagine hydroxylase with hypoxia sensitivity. FIH uniquely combines oxygen and peroxide sensitivity, serving as an oxygen and oxidant sensor. Recent Advances: FIH was first discovered in the hypoxia-inducible factor (HIF) pathway as a modulator of HIF transactivation activity. Several other FIH substrates have now been identified outside the HIF pathway. Moreover, FIH enzymatic activity is highly promiscuous and not limited to asparagine hydroxylation. This includes the FIH-mediated catalysis of an oxygen-dependent stable (likely covalent) bond formation between FIH and selected substrate proteins (called oxomers [oxygen-dependent stable protein oligomers]). Critical Issues: The (patho-)physiological function of FIH is only beginning to be understood and appears to be complex. Selective pharmacologic inhibition of FIH over other oxygen sensors is possible, opening new avenues for therapeutic targeting of hypoxia-associated diseases, increasing the interest in its (patho-)physiological relevance. Future Directions: The contribution of FIH enzymatic activity to disease development and progression should be analyzed in more detail, including the assessment of underlying molecular mechanisms and relevant FIH substrate proteins. Also, the molecular mechanism(s) involved in the physiological functions of FIH remain(s) to be determined. Furthermore, the therapeutic potential of recently developed FIH-selective pharmacologic inhibitors will need detailed assessment. Antioxid. Redox Signal. 37, 913-935.

MicroRNA-122-5p ameliorates tubular injury in diabetic nephropathy via FIH-1/HIF-1α pathway

Diabetes kidney disease (DKD) affects approximately one-third of diabetes patients, however, the specific molecular mechanism of DKD remains unclear, and there is still a lack of effective therapies. Here, we demonstrated a significant increase of microRNA-122-5p (miR-122-5p) in renal tubular cells in STZ induced diabetic nephropathy (DN) mice. Moreover, inhibition of miR-122-5p led to increased cell death and serve tubular injury and promoted DN progression following STZ treatment in mice, whereas supplementation of miR-122-5p mimic had kidney protective effects in this model. In addition, miR-122-5p suppressed the expression of factor inhibiting hypoxia-inducible factor-1 (FIH-1) in vitro models of DN. microRNA target reporter assay further verified FIH-1 as a direct target of miR-122-5p. Generally, FIH-1 inhibits the activity of HIF-1α. Our in vitro study further indicated that overexpression of HIF-1α by transfection of HIF-1α plasmid reduced tubular cell death, suggesting a protective role of HIF-1α in DN. Collectively, these findings may unveil a novel miR-122-5p/FIH-1/HIF-1α pathway which can attenuate the DN progression.

TARBP2 Suppresses Ubiquitin-Proteasomal Degradation of HIF-1α in Breast Cancer

TAR (HIV-1) RNA binding protein 2 (TARBP2) is an RNA-binding protein participating in cytoplasmic microRNA processing. Emerging evidence has shown the oncogenic role of TARBP2 in promoting cancer progression, making it an unfavorable prognosis marker for breast cancer. Hypoxia is a hallmark of the tumor microenvironment which induces hypoxia-inducible factor-1α (HIF-1α) for transcriptional regulation. HIF-1α is prone to be rapidly destabilized by the ubiquitination–proteasomal degradation system. In this study, we found that TARBP2 expression is significantly correlated with induced hypoxia signatures in human breast cancer tissues. At a cellular level, HIF-1α protein level was maintained by TARBP2 under either normoxia or hypoxia. Mechanistically, TARBP2 enhanced HIF-1α protein stability through preventing its proteasomal degradation. In addition, downregulation of multiple E3 ligases targeting HIF-1α (VHL, FBXW7, TRAF6) and reduced ubiquitination of HIF-1α were also induced by TARBP2. In support of our clinical findings that TARBP2 is correlated with tumor hypoxia, our IHC staining showed the positive correlation between HIF-1α and TARBP2 in human breast cancer tissues. Taken together, this study indicates the regulatory role of TARBP2 in the ubiquitination–proteasomal degradation of HIF-1α protein in breast cancer.

Low-intensity pulsed ultrasound prevents prolonged hypoxia-induced cardiac fibrosis through HIF-1α/DNMT3a pathway via a TRAAK-dependent manner

Hypoxia-induced cardiac fibrosis is an important pathological process in cardiovascular disorders. This study aimed to determine whether low-intensity pulsed ultrasound (LIPUS), a novel and safe apparatus, could alleviate hypoxia-induced cardiac fibrosis, and to elucidate the underlying mechanisms. Hypoxia (1% O2 ) and transverse aortic constriction (TAC) were performed on neonatal rat cardiac fibroblasts and mice to induce cardiac fibrosis, respectively. LIPUS irradiation was applied for 20 minutes every 6 hours for a total of 2 times in vitro, and every 2 days from 1 week before surgery to 4 weeks after surgery in vivo. We found that LIPUS dose-dependently attenuated hypoxia-induced cardiac fibroblast phenotypic conversion in vitro, and ameliorated TAC-induced cardiac fibrosis in vivo. Hypoxia significantly upregulated the nuclear protein expression of hypoxia-inducible factor-1α (HIF-1α) and DNA methyltransferase 3a (DNMT3a). LIPUS pre-treatment reversed the elevated expression of HIF-1α, and DNMT3a. Further experiments revealed that HIF-1α stabilizer dimethyloxalylglycine (DMOG) hindered the anti-fibrotic effect of LIPUS, and hampered LIPUS-mediated downregulation of DNMT3a. DNMT3a small interfering RNA (siRNA) prevented hypoxia-induced cardiac fibrosis. Results also showed that the mechanosensitive protein-TWIK-related arachidonic acid-activated K+ channel (TRAAK) messenger RNA (mRNA) expression was downregulated in hypoxia-induced cardiac fibroblasts, and TAC-induced hearts. TRAAK siRNA impeded LIPUS-mediated anti-fibrotic effect and downregulation of HIF-1α and DNMT3a. Above results indicated that LIPUS could prevent prolonged hypoxia-induced cardiac fibrosis through TRAAK-mediated HIF-1α/DNMT3a signalling pathway.

Hypoxia- and hyperoxia-related gene expression dynamics during developmental critical windows of the tropical gar Atractosteus tropicus

Aquatic hypoxia is both a naturally-occurring and anthropogenically-generated event. Fish species have evolved different adaptations to cope with hypoxic environments, including gill modifications and air breathing. However, little is known about the molecular mechanisms involved in the respiration of embryonic and larval fishes during critical windows of development. We assessed expression of the genes hif-1α, fih-1, nhe1, epo, gr and il8 using the developing tropical gar as a piscine model during three developmental periods (fertilization to hatch, 1 to 6 days post hatch (dph) and 7 to 12 dph) when exposed to normoxia (~7.43 mg/L DO), hypoxia (~2.5 mg/L DO) or hyperoxia (~9.15 mg/L DO). All genes had higher expression when fish were exposed to either hypoxia or hyperoxia during the first two developmental periods. However, fish continuously exposed to hypoxia had increased expression of the six genes by hatching and 6 dph, and by 12 dph only hif-1α still had increased expression. The middle developmental period was the most hypoxia-sensitive, coinciding with several changes in physiology and morphology. The oldest larvae were the most resilient to gene expression change, with little variation in expression of the six genes compared. This study is the first to relate the molecular response of an air-breathing fish to oxygen availability to developmental critical windows and contributes to our understanding of some molecular responses of developing fish to changes in oxygen availability.

Transcriptome sequencing provides insights into the mechanism of hypoxia adaption in bighead carp (Hypophthalmichthys nobilis)

Hypoxia negatively affects the behavior, immunology, physiology, and growth of fish. Therefore, uncovering the genetic mechanisms underlying hypoxia adaptation and tolerance in fish prior to any genetic improvement is essential. Bighead carp is one of the most important freshwater fish species in aquaculture worldwide; however, this species does not have a strong ability to tolerate hypoxia. In this study, the dissolved oxygen level (0.6 mg/L) was maintained above the asphyxiation point of bighead carp for a long time to simulate hypoxia stress. The liver, gills, and heart were sampled before (0 h) and after (1 h, 2 h, 4 h) the hypoxia tests. Glutathione peroxidase (GPx) and catalase (CAT) activities and malondialdehyde (MDA) levels in the liver were significantly (p < 0.05) elevated at 1 h after hypoxic stress. By observing tissue morphology, the cell structure of the liver and gill tissues was found to change to varying degrees before and after hypoxia stress. Transcriptome sequencing was performed on 36 samples of gill, liver, and heart at four time points, and a total of 293.55G of data was obtained. In the early phase (0-1 h), differentially expressed genes (DEGs, 807 genes upregulated, 654 genes downregulated) were mainly enriched in signal transduction, such as cytokine-cytokine receptor interactions and ECM-receptor interactions. In the middle phase (0-2 h), DEGs (1201 genes upregulated and 2036 genes downregulated) were mainly enriched in regulation and adaptation, such as the MAPK and FoxO signaling pathways. Finally, in the later phase (0-4 h), DEGs (3975 genes upregulated and 4412 genes downregulated) were mainly enriched in tolerance and apoptosis, such as the VEGF signaling pathway and apoptosis. The genes with the most remarkable upregulation at different time points in the three tissues had some similarities. Genetic differences in these genes may be responsible for the differences in hypoxia tolerance among individuals. Altogether, our study provides new insights into the molecular mechanisms of hypoxia adaptation in fish. Further, the key regulatory genes identified provide genetic resources for breeding hypoxia-tolerant bighead carp species.

Regeneration in Spinal Disease: Therapeutic Role of Hypoxia-Inducible Factor-1 Alpha in Regeneration of Degenerative Intervertebral Disc

The intervertebral disc (IVD) is a complex joint structure comprising three primary components—namely, nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP). The IVD retrieves oxygen from the surrounding vertebral body through CEP by diffusion and likely generates ATP via anaerobic glycolysis. IVD degeneration is characterized by a cascade of cellular, compositional, structural changes. With advanced age, pronounced changes occur in the composition of the disc extracellular matrix (ECM). NP and AF cells in the IVD possess poor regenerative capacity compared with that of other tissues. Hypoxia-inducible factor (HIF) is a master transcription factor that initiates a coordinated cellular cascade in response to a low oxygen tension environment, including the regulation of numerous enzymes in response to hypoxia. HIF-1α is essential for NP development and homeostasis and is involved in various processes of IVD degeneration process, promotes ECM in NP, maintains the metabolic activities of NP, and regulates dystrophic mineralization of NP, as well as angiogenesis, autophagy, and apoptosis during IVD degeneration. HIF-1α may, therefore, represent a diagnostic tool for early IVD degeneration and a therapeutic target for inhibiting IVD degeneration

Cardiac Transcriptomics Reveals That MAPK Pathway Plays an Important Role in Hypoxia Tolerance in Bighead Carp (Hypophthalmichthys nobilis)

As aquatic animals, fishes often encounter various situations of low oxygen, and they have evolved the ability to respond to hypoxia stress. Studies of physiological and molecular responses to hypoxia stress are essential to clarify genetic mechanisms underlying hypoxia tolerance in fish. In this study, we performed acute hypoxia treatment in juvenile bighead carp (Hypophthalmicthys nobilis) by decreasing water O₂ from 6.5 mg/L to 0.5 mg/L in three hours. This hypoxia stress resulted in a significant increase in blood lactate and serum glucose. Comparisons of heart transcriptome among hypoxia tolerant (HT), hypoxia sensitive (HS), and normoxia control (NC) groups showed that 820, 273, and 301 differentially expressed genes (DEGs) were identified in HS vs. HT, NC vs. HS, and NC vs. HT (false discovery rate (FDR) < 0.01, Fold Change> 2), respectively. KEGG pathway enrichment showed that DEGs between HS and HT groups were mainly involved in mitogen-activated protein kinase (MAPK) signaling, insulin signaling, apoptosis, tight junction and adrenergic signaling in cardiomyocytes pathways, and DEGs in MAPK signaling pathway played a key role in cardiac tolerance to hypoxia. Combined with the results of our previous cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis of hypoxia stress in this species, such genes as stbp2, ttn, mapk, kcnh, and tnfrsf were identified in both studies, representing the significance of these DEGs in hypoxia tolerance in bighead carp. These results provide insights into the understanding of genetic modulations for fish heart coping with hypoxia stress and generate basic resources for future breeding studies of hypoxia resistance in bighead carp.