The enzymatic oxygen sensor cysteamine dioxygenase binds its protein substrates through their N-termini.

The non-heme iron-dependent dioxygenase 2-aminoethanethiol dioxygenase (ADO) has recently been identified as an enzymatic oxygen sensor that coordinates cellular changes to hypoxia by regulating the stability of proteins bearing an N-terminal cysteine (Nt-cys) through the N-degron pathway. It catalyses Nt-cys sulfinylation, which promotes O2-dependent proteasomal degradation of the target. Only a few ADO substrates have been verified, including regulators of G-protein signalling (RGS) 4 and 5, and the pro-inflammatory cytokine interleukin-32 (IL32), all of which exhibit cell and/or tissue specific expression patterns. ADO, in contrast, is ubiquitously expressed, suggesting it can regulate the stability of additional Nt-cys proteins in an O2-dependent manner. Furthermore, the role of individual chemical groups, active site metal, amino acid composition and globular structure on protein substrate association remains elusive. To help identify new targets and examine the underlying biochemistry of the system, we conducted a series of biophysical experiments to investigate the binding requirements of established ADO substrates RGS5 and IL32. We demonstrate, using surface plasmon response (SPR) and enzyme assays, that a free, unmodified Nt-thiol and Nt-amine are vital for substrate engagement through active site metal coordination, with residues next to Nt-cys moderately impacting association and catalytic efficiency. Additionally, we show, through 1H-15N heteronuclear single quantum coherence (15N-HSQC) nuclear magnetic resonance (NMR) titrations, that the globular portion of RGS5 has limited impact on ADO association, with interactions restricted to the N-terminus. This work establishes key features involved in ADO substrate binding, which will help identify new protein targets and, subsequently, elucidate its role in hypoxic adaptation.

A comparison of cardiovascular disease associations of time-domain oximetry parameters in sleep apnoea cases from the Sleep Heart Health Study.

Polysomnograms (PSGs) contain a wealth of physiological information that is routinely recorded but not utilised in sleep studies. Intermittent hypoxia arising from obstructive sleep apnoea (OSA) events is an important risk in the later development of cardiovascular disease (CVD). Analysis of oximetry patterns from PSG studies may enable early assessment of CVD risk. The aim of this study was to compare associations of different time-domain oximetry patterns with incident CVD in OSA patients. All participants with OSA and no pre-existing CVD at baseline or within the first two years of follow-up, were selected from the Sleep Heart Health Study data and used for analysis (N=2878). We examined oximetry parameters that are calculated from desaturation events and from time series analysis and compared them to incident CVD outcomes using proportional hazards regression models adjusted for age, race, smoking, BMI, and sex. Our results show that were no associations between OSA oximetry parameters and incident CVD for OSA patients.

A comparison of hypoxic burden algorithms using three different methods for calculating baseline oxygen saturation for predicting cardiovascular death in the Sleep Heart Health Study.

Respiratory event related oxygen desaturation area measures have recently shown merit as novel predictors of cardiovascular disease (CVD) outcomes. In this study, we investigate one such measure (hypoxic burden (HB)) and investigate how three different ways of calculating the SpO2 baseline of the HB algorithm impact its ability to predict cardiovascular mortality. The three baseline estimation steps include a pre-event baseline, a record-based baseline, and a fixed baseline. Pulse oximetry signals from the Sleep Heart Health Study and the corresponding CVD outcomes were analyzed. The performance of each baseline method was compared using adjusted Cox proportional hazard regression analysis. Results show that HB with the record-based baseline method returned the best performing results with a hazard ratio (HR) of 1.83 (95% CI: 1.03-3.27, p<0.05) in the fully adjusted model, compared to HB with the pre-event baseline method (HR: 1.60, 95%CI: 0.86-3.00, p>0.05) and HB with the fixed baseline method (HR: 1.73, 95%CI: 0.93-3.22, p>0.05).

Overexpression of VIRMA confers vulnerability to breast cancers via the m6A-dependent regulation of unfolded protein response.

Virilizer-like m6A methyltransferase-associated protein (VIRMA) maintains the stability of the m6A writer complex. Although VIRMA is critical for RNA m6A deposition, the impact of aberrant VIRMA expression in human diseases remains unclear. We show that VIRMA is amplified and overexpressed in 15-20% of breast cancers. Of the two known VIRMA isoforms, the nuclear-enriched full-length but not the cytoplasmic-localised N-terminal VIRMA promotes m6A-dependent breast tumourigenesis in vitro and in vivo. Mechanistically, we reveal that VIRMA overexpression upregulates the m6A-modified long non-coding RNA, NEAT1, which contributes to breast cancer cell growth. We also show that VIRMA overexpression enriches m6A on transcripts that regulate the unfolded protein response (UPR) pathway but does not promote their translation to activate the UPR under optimal growth conditions. Under stressful conditions that are often present in tumour microenvironments, VIRMA-overexpressing cells display enhanced UPR and increased susceptibility to death. Our study identifies oncogenic VIRMA overexpression as a vulnerability that may be exploited for cancer therapy.

Sodium Propionate or Sodium Butyrate Promotes Fatty Acid Oxidation in HepG2 Cells Under Oxidative Stress.

The beneficial effects of sodium butyrate (NaB) and sodium propionate (NaP) on fatty acid oxidation (FAO) genes and production of proinflammatory cytokines related to nonalcoholic fatty liver disease (NAFLD) were evaluated using HepG2 human liver hepatocellular carcinoma cells exposed to palmitate/oleate or lipopolysaccharides (LPSs) as a model. The results showed that NaP or NaB was able to promote FAO, regulate lipolysis, and reduce reactive oxygen species production by significantly increasing the mRNA expression of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), peroxisome proliferator-activated receptor alpha (PPARα), adipose triglyceride lipase (ATGL), carnitine palmitoyltransferase 1 alpha (CPT1α), fibroblast growth factor 21 (FGF21), and uncoupling protein 2 (UCP2) in HepG2 cells. Together, NaP and NaB may produce greater effects by increasing CPT1α, PPARα, and UCP2 mRNA expression in LPS-treated HepG2 cells and by increasing CPT1α and ATGL mRNA expression in palmitate-/oleate-treated HepG2 cells. Only NaP treatment significantly increased FGF21 mRNA expression in palmitate-/oleate-treated HepG2 cells. The enzyme-linked immunosorbent assay results revealed that only pretreatment with LPSs and not palmitate/oleate significantly increased tumor necrosis factor alpha (TNF-α) expression in HepG2 cells. NaP alone or in combination with NaB significantly decreased TNF-α expression in LPS-induced HepG2 cells. The expression of interleukin-8 in both models showed no significant differences in all treatments. NaP and NaB show potential for in vivo studies on NAFLD.

Clinical Efficacy of the Oral Health System by Fresh Health Inc. in Adults on Gingivitis and Plaque in a 3-Arm, 30-Day Study as Compared to String Floss and Manual Brushing.

OBJECTIVE: To evaluate the safety and efficacy of the Oral Health System by Fresh Health Inc., used in conjunction with manual toothbrushing (Fresh + MTB) as compared to string floss and manual toothbrushing (floss + MTB) and manual toothbrushing (MTB) alone, as measured by clinical signs of gingivitis, plaque reduction, pocket depth, and bleeding. METHODS: One hundred ninety-two (192) generally healthy adults exhibiting signs of gingivitis completed this 30-day randomized, controlled, examiner-blinded, three-group parallel study design. All subjects were assigned a manual toothbrush and fluoride dentifrice, instructed to brush twice daily according to their normal habits, and provided with written and verbal instructions for all assigned products. Subjects in the control group used only the manual toothbrush and dentifrice. Subjects assigned to the string floss + MTB group were instructed to also floss once daily. Subjects assigned to the Fresh + MTB group were provided a Fresh Health Inc. custom-fit oral irrigator and instructed to use the device once daily with water for approximately 7 seconds in addition to toothbrushing. Gingivitis was assessed using the modified gingival index (MGI), bleeding on marginal probing was assessed via the gingival bleeding index (GBI), and plaque was measured using the Rustogi modified navy plaque index (RMNPI) at day 1, day 15, and day 30. Periodontal probing depth (PPD) and bleeding on probing (BOP) were measured at day 1 and day 30. Oral soft- and hard-tissue assessments were performed at all examination visits. RESULTS: There was no significant difference in age or sex between groups, and no significant difference in baseline MGI, GBI, RMNPI, BOP, and PPD values across groups. The Fresh + MTB group demonstrated statistically significantly better performance than the floss + MTB group and MTB group across all clinical indices at both 15 days and 30 days. At 30 days, the Fresh + MTB group showed a 40.9% improvement in whole-mouth MGI, which was significantly greater than the MTB and floss + MTB groups.

Intermittent hypoxia enhances the expression of hypoxia inducible factor HIF1A through histone demethylation.

The cellular response to hypoxia is regulated through enzymatic oxygen sensors, including the prolyl hydroxylases, which control degradation of the well-known hypoxia inducible factors (HIFs). Other enzymatic oxygen sensors have been recently identified, including members of the KDM histone demethylase family. Little is known about how different oxygen-sensing pathways interact and if this varies depending on the form of hypoxia, such as chronic or intermittent. In this study, we investigated how two proposed cellular oxygen-sensing systems, HIF-1 and KDM4A, KDM4B, and KDM4C, respond in cells exposed to rapid forms of intermittent hypoxia (minutes) and compared to chronic hypoxia (hours). We found that intermittent hypoxia increases HIF-1α protein through a pathway distinct from chronic hypoxia, involving the KDM4A, KDM4B, and KDM4C histone lysine demethylases. Intermittent hypoxia increases the quantity and activity of KDM4A, KDM4B, and KDM4C, resulting in a decrease in histone 3 lysine 9 (H3K9) trimethylation near the HIF1A locus. We demonstrate that this contrasts with chronic hypoxia, which decreases KDM4A, KDM4B, and KDM4C activity, leading to hypertrimethylation of H3K9 globally and at the HIF1A locus. Altogether, we found that demethylation of histones bound to the HIF1A gene in intermittent hypoxia increases HIF1A mRNA expression, which has the downstream effect of increasing overall HIF-1 activity and expression of HIF target genes. This study highlights how multiple oxygen-sensing pathways can interact to regulate and fine tune the cellular hypoxic response depending on the period and length of hypoxia.

Comparative associations of oximetry patterns in Obstructive Sleep Apnea with incident cardiovascular disease.

STUDY OBJECTIVES: Intermittent hypoxia is a key mechanism linking Obstructive Sleep Apnea (OSA) to cardiovascular disease (CVD). Oximetry analysis could enhance understanding of which OSA phenotypes are associated with CVD risk. The aim of this study was to compare associations of different oximetry patterns with incident CVD in men and women with OSA. METHODS: Sleep Heart Health Study data were used for analysis. n = 2878 Participants (51.8% female; mean age 63.5 ±â€…10.5 years) with OSA (Apnea Hypopnea Index [AHI] ≥ 5 events/h) and no pre-existing CVD at baseline or within the first 2 years of follow-up were included. Four oximetry analysis approaches were applied: desaturation characteristics, time series analysis, power spectral density, and non-linear analysis. Thirty-one resulting oximetry patterns were compared to incident CVD using proportional hazards regression models adjusted for age, race, smoking, BMI, and sex. RESULTS: There were no associations between OSA oximetry patterns and incident CVD in the total sample or in men. In women, there were some associations between incident CVD and time series analysis (e.g. SpO2 distribution standard deviation, HR 0.81, 95% CI 0.68-0.96, p = 0.014) and power spectral density oximetry patterns (e.g. Full frequency band mean HR 0.75; 95% CI 0.59-0.95; p = 0.015). CONCLUSIONS: Comprehensive comparison of baseline oximetry patterns in OSA found none were related to development of CVD. There were no standout individual oximetry patterns that appear to be candidates for CVD risk phenotyping in OSA, but some showed marginal relationships with CVD risk in women. Further work is required to understand whether OSA phenotypes can be used to predict susceptibility to cardiovascular disease.

Improving the synergistic combination of programmed death-1/programmed death ligand-1 blockade and radiotherapy by targeting the hypoxic tumour microenvironment.

Immune checkpoint inhibition with PD-1/PD-L1 blockade is a promising area in the field of anti-cancer therapy. Although clinical data have revealed success of PD-1/PD-L1 blockade as monotherapy or in combination with CTLA-4 or chemotherapy, the combination with radiotherapy could further boost anti-tumour immunity and enhance clinical outcomes due to the immunostimulatory effects of radiation. However, the synergistic combination of PD-1/PD-L1 blockade and radiotherapy can be challenged by the complex nature of the tumour microenvironment (TME), including the presence of tumour hypoxia. Hypoxia is a major barrier to the effectiveness of both radiotherapy and PD-1/PD-L1 blockade immunotherapy. Thus, targeting the hypoxic TME is an attractive strategy to enhance the efficacy of the combination. Addition of compounds that directly or indirectly reduce hypoxia, to the combination of PD-1/PD-L1 inhibitors and radiotherapy may optimize the success of the combination and improve therapeutic outcomes. In this review, we will discuss the synergistic combination of PD-1/PD-L1 blockade and radiotherapy and highlight the role of hypoxic TME in impeding the success of both therapies. In addition, we will address the potential approaches for targeting tumour hypoxia and how exploiting these strategies could benefit the combination of PD-1/PD-L1 blockade and radiotherapy.

Predicting Cardiovascular Outcomes Using the Respiratory Event Desaturation Transient Area Derived from Overnight Sleep Studies.

A new method for calculation of an overnight oximetry signal metric which is predictive of cardiovascular disease (CVD) outcomes in individuals undergoing an overnight sleep test is presented. The metric - the respiratory event desaturation transient area (REDTA) - quantifies the desaturation associated with respiratory events. Data from the Sleep Heart Health Study, which includes overnight oximetry signals and long-term CVD outcomes, was used to develop and test the parameter. Performance of the REDTA parameter was assessed using Cox proportional hazard ratios and compared to established metrics of hypoxia. Results show that hazard ratios in adjusted Cox analysis for predicting cardiovascular death using REDTA are up to 1.90 (95%CI: 1.22-2.96) which compares with the best of the established metrics. A big advantage of our metric compared to other high performing metrics is its ease of computation.

Metabolite signatures of heart failure, sleep apnoea, their interaction, and outcomes in the community.

AIMS: Sleep apnoea and congestive heart failure (CHF) commonly co-exist, but their interaction is unclear. Metabolomics may clarify their interaction and relationships to outcome. METHODS AND RESULTS: We assayed 372 circulating metabolites and lipids in 1919 and 1524 participants of the Framingham Heart Study (FHS) (mean age 54 ± 10 years, 53% women) and Women's Health Initiative (WHI) (mean age 67 ± 7 years), respectively. We used linear and Cox regression to relate plasma concentrations of metabolites and lipids to echocardiographic parameters; CHF and its subtypes heart failure with reduced ejection fraction (HFrEF) and heart failure with preserved ejection fraction (HFpEF); and sleep indices. Adenine dinucleotide phosphate (ADP) associated with left ventricular (LV) fractional shortening; phosphocreatine with LV wall thickness; lysosomal storage molecule sphingomyelin 18:2 with LV mass; and nicotine metabolite cotinine with time spent with an oxygen saturation less than 90% (ß = 2.3 min, P = 2.3 × 10-5 ). Pro-hypertrophic metabolite hydroxyglutarate partly mediated the association between LV wall thickness and HFpEF. Central sleep apnoea was significantly associated with HFpEF (P = 0.03) but not HFrEF (P = 0.5). There were three significant metabolite canonical variates, one of which conferred protection from cardiovascular death [hazard ratio = 0.3 (0.11, 0.81), P = 0.02]. CONCLUSIONS: Energetic metabolites were associated with cardiac function; energy- and lipid-storage metabolites with LV wall thickness and mass; plasma levels of nicotine metabolite cotinine were associated with increased time spent with a sleep oxygen saturation less than 90%, a clinically significant marker of outcome, indicating a significant hazard for smokers who have sleep apnoea.

Targeting Glucose Metabolism of Cancer Cells with Dichloroacetate to Radiosensitize High-Grade Gliomas.

As the cornerstone of high-grade glioma (HGG) treatment, radiotherapy temporarily controls tumor cells via inducing oxidative stress and subsequent DNA breaks. However, almost all HGGs recur within months. Therefore, it is important to understand the underlying mechanisms of radioresistance, so that novel strategies can be developed to improve the effectiveness of radiotherapy. While currently poorly understood, radioresistance appears to be predominantly driven by altered metabolism and hypoxia. Glucose is a central macronutrient, and its metabolism is rewired in HGG cells, increasing glycolytic flux to produce energy and essential metabolic intermediates, known as the Warburg effect. This altered metabolism in HGG cells not only supports cell proliferation and invasiveness, but it also contributes significantly to radioresistance. Several metabolic drugs have been used as a novel approach to improve the radiosensitivity of HGGs, including dichloroacetate (DCA), a small molecule used to treat children with congenital mitochondrial disorders. DCA reverses the Warburg effect by inhibiting pyruvate dehydrogenase kinases, which subsequently activates mitochondrial oxidative phosphorylation at the expense of glycolysis. This effect is thought to block the growth advantage of HGGs and improve the radiosensitivity of HGG cells. This review highlights the main features of altered glucose metabolism in HGG cells as a contributor to radioresistance and describes the mechanism of action of DCA. Furthermore, we will summarize recent advances in DCA's pre-clinical and clinical studies as a radiosensitizer and address how these scientific findings can be translated into clinical practice to improve the management of HGG patients.

Is Cumulative Time of Oxygen Desaturation a Better Predictor of Cardiovascular Mortality than Apnoea Hypopnoea Index?

In this paper, we explored the link between sleep apnoea and cardiovascular disease (CVD) using a time-series statistical measure of sleep apnoea-related oxygen desaturation. We compared the performance of a hypoxic measure derived from the polysomnogram with the Apnoea Hypopnoea Index (AHI) in predicting CVD mortality in patients of the Sleep Heart Health Study.We estimated the relative cumulative time of SpO2 below 90% (Tr90) using pulse oximetry signals from polysomnogram recordings as the hypoxic measure of desaturation patterns. Then, the survival curves for hypoxia quintiles were evaluated for the prediction of CVD mortality and were compared with the results using AHI for prediction. We also calculated the Cox hazard ratios for Tr90 and AHI. Our results show that the Tr90 was a better predictor of CVD mortality outcomes than AHI.

A zebrafish model of tuberculosis comorbidity and the effects of HIF-activating intervention.

Comorbidities are an important factor in tuberculosis pathophysiology and treatment but are understudied in animal models. Schild et al. present a zebrafish model of Mycobacterium marinum infection and wound comorbidity that retains responsiveness to protective hypoxia-inducible factor-1α activation as an example of a host-directed therapy. This platform is a new paradigm for the zebrafish-M. marinum infection model and provides a blueprint to test therapeutic interventions on infection and comorbid pathologies. Comment on: https://doi.org/10.1111/febs.15433.

Out of breath, out of time: interactions between HIF and circadian rhythms.

Humans have internal circadian clocks that ensure that important physiological functions occur at specific times of the day. These molecular clocks are regulated at the genomic level and exist in most cells of the body. Multiple circadian resetting cues have been identified, including light, temperature, and food. Recently, oxygen has been identified as a resetting cue, and emerging science indicates that this occurs through interactions at the cellular level between the circadian transcription-translation feedback loop and the hypoxia-inducible pathway (hypoxia-inducible factor; subject of the 2019 Nobel Prize in Physiology or Medicine). This review will cover recently identified relationships between HIF and proteins of the circadian clock. Interactions between the circadian clock and hypoxia could have wide-reaching implications for human diseases, and understanding the molecular mechanisms regulating these overlapping pathways may open up new strategies for drug discovery.

Hypoxia, metabolism, and the circadian clock: new links to overcome radiation resistance in high-grade gliomas.

Radiotherapy is the cornerstone of treatment of high-grade gliomas (HGGs). It eradicates tumor cells by inducing oxidative stress and subsequent DNA damage. Unfortunately, almost all HGGs recur locally within several months secondary to radioresistance with intricate molecular mechanisms. Therefore, unravelling specific underlying mechanisms of radioresistance is critical to elucidating novel strategies to improve the radiosensitivity of tumor cells, and enhance the efficacy of radiotherapy. This review addresses our current understanding of how hypoxia and the hypoxia-inducible factor 1 (HIF-1) signaling pathway have a profound impact on the response of HGGs to radiotherapy. In addition, intriguing links between hypoxic signaling, circadian rhythms and cell metabolism have been recently discovered, which may provide insights into our fundamental understanding of radioresistance. Cellular pathways involved in the hypoxic response, DNA repair and metabolism can fluctuate over 24-h periods due to circadian regulation. These oscillatory patterns may have consequences for tumor radioresistance. Timing radiotherapy for specific times of the day (chronoradiotherapy) could be beneficial in patients with HGGs and will be discussed.

C-Terminal HSP90 Inhibitors Block the HIF-1 Hypoxic Response by Degrading HIF-1α through the Oxygen-Dependent Degradation Pathway.

BACKGROUND/AIMS: Hypoxia Inducible Factor-1α (HIF-1α) is involved in cancer progression and is stabilized by the chaperone HSP90 (Heat Shock Protein 90), preventing degradation. Previously identified HSP90 inhibitors bind to the N-terminal pocket of HSP90, which blocks binding to HIF-1α and induces HIF-1α degradation. N-terminal inhibitors have failed in the clinic as single therapy treatments partially because they induce a heat shock response. SM molecules are HSP90 inhibitors that bind to the C-terminus of HSP90 and do not induce a heat shock response. The effects of these C-terminal inhibitors on HIF-1α are unreported. METHODS: HCT116, MDA-MB-231, PC3, and HEK293T cells were treated with HSP90 inhibitors. qRT-PCR and western blotting was performed to assess mRNA and protein levels of HIF-1α, HSP- and RACK1-related genes. siRNA was used to knockdown RACK1, while MG262 was used to inhibit proteasome activity. Dimethyloxalylglycine (DMOG) was used to inhibit activity of the prolyl hydroxylases (PHDs). Anti-angiogenic activity of HSP90 inhibitors was assessed using a HUVEC tubule formation assay. RESULTS: We show that SM compounds decrease HIF-1α target expression at the mRNA and protein level under hypoxia in colorectal, breast and prostate cancer cells, leading to cell death, without inducing a heat shock response. Surprisingly, we found that when the C-terminal of HSP90 is inhibited, HIF-1α degradation occurs through the proteasome and prolyl hydroxylases in an oxygen-dependent manner even in very low levels of oxygen (tumor hypoxia levels). RACK1 was not required for proteasomal degradation of HIF-1α. CONCLUSION: Our results suggest that by targeting the C-terminus of HSP90 we can exploit the prolyl hydroxylase and proteasome pathway to induce HIF-1α degradation in hypoxic tumors.

Determining the Redox Potential of a Protein Disulphide Bond.

The redox potential of a protein disulphide bond is one of the most important factors for determining the role of a disulphide bond. Disulphide bonds can have a stabilizing role for the structure of a protein or they can play a functional role which can regulate protein bioactivity. Determining the redox potential of disulphides can help distinguish the functional from the structural disulphide bonds. In this chapter, two methods for determining the redox potential of a protein disulphide bond are described. The first method uses maleimide-biotin labeling of free cysteine thiols and western blot densitometry to determine the fraction of reduced disulphide bond under various redox-buffering conditions. The second method uses differential cysteine labeling and tandem mass spectrometry to determine the redox potential.

Obstructive Sleep Apnea Activates HIF-1 in a Hypoxia Dose-Dependent Manner in HCT116 Colorectal Carcinoma Cells.

Obstructive sleep apnea (OSA) affects a significant proportion of the population and is linked to increased rates of cancer development and a worse cancer outcome. OSA is characterized by nocturnal intermittent hypoxia and animal models of OSA-like intermittent hypoxia show increased tumor growth and metastasis. Advanced tumors typically have regions of chronic hypoxia, activating the transcription factor, HIF-1, which controls the expression of genes involved in cancer progression. Rapid intermittent hypoxia from OSA has been proposed to increase HIF-1 activity and this may occur in tumors. The effect of exposing a developing tumor to OSA-like intermittent hypoxia is largely unknown. We have built a cell-based model of physiological OSA tissue oxygenation in order to study the effects of intermittent hypoxia in HCT116 colorectal cancer cells. We found that HIF-1α increases following intermittent hypoxia and that the expression of HIF-target genes increases, including those involved in glycolysis, the hypoxic pathway and extracellular matrix remodeling. Expression of these genes acts as a 'hypoxic' signature which is associated with a worse prognosis. The total dose of hypoxia determined the magnitude of change in the hypoxic signature rather than the frequency or duration of hypoxia-reoxygenation cycles per se. Finally, transcription of HIF1A mRNA differs in response to chronic and intermittent hypoxia suggesting that HIF-1α may be regulated at the transcriptional level in intermittent hypoxia and not just by the post-translational oxygen-dependent degradation pathway seen in chronic hypoxia.