Hypoxia-inducible factor 1 mediates hypoxia-induced cardiomyocyte lipid accumulation by reducing the DNA binding activity of peroxisome proliferator-activated receptor alpha/retinoid X receptor

Reactive Oxygen Species (ROS) in Metabolic Disease-Don't Shoot the Metabolic Messenger

Reactive oxygen species (ROS) are widely considered key to pathogenesis in chronic metabolic disease. Consequently, much attention is rightly focused on minimising oxidative damage. However, for ROS production to be most effectively modulated, it is crucial to first appreciate that ROS do not solely function as pathological mediators. There are >90 gene products specifically evolved to generate, handle, and tightly buffer the cellular concentration of ROS. Therefore, it is likely that ROS plays a role as integral homeostatic signalling components and only become toxic in extremis. This review explores these commonly overlooked normal physiological functions, including how ROS are generated in response to environmental or hormonal stimuli, the mechanisms by which the signals are propagated and regulated, and how the cell effectively brings the signal to an end after an appropriate duration. In the course of this, several specific and better-characterised signalling mechanisms that rely upon ROS are explored, and the threshold at which ROS cross from beneficial signalling molecules to pathology mediators is discussed.

Hippo pathway activation causes multiple lipid derangements in a murine model of cardiomyopathy

Metabolic reprogramming occurs in cardiomyopathy and heart failure contributing to progression of the disease. Activation of cardiac Hippo pathway signaling has been implicated in mediating mitochondrial dysfunction and metabolic reprogramming in cardiomyopathy, albeit influence of Hippo pathway on lipid profile is unclear. Using a dual-omics approach, we determined alterations of cardiac lipids in a mouse model of cardiomyopathy due to enhanced Hippo signaling and explored molecular mechanisms. Lipidomic profiling discovered multiple alterations in lipid classes, notably reduction of triacylglycerol, diacylglycerol, phospholipids and ether lipids, and elevation of sphingolipids and lysophosphatidylcholine. Mechanistically, we found downregulated expression of PPARα and PGC-1α at mRNA and protein levels, and downregulated expression of PPARα-target genes, indicating attenuated transcriptional activity of PPARα/PGC-1α. Lipidomics-guided transcriptomic analysis revealed dysregulated expression of gene sets that were responsible for enhanced biosynthesis of ceramides, suppression of TG biosynthesis, storage, hydrolysis and mitochondrial fatty acid oxidation, and reduction of peroxisome-localized biosynthesis of ether lipids. Collectively, Hippo pathway activation with attenuated PPARα/PGC-1α signaling is the underlying mechanism for alterations in cardiac lipids in cardiomyopathy and failing heart.

Maternal heart exhibits metabolic and redox adaptations post-uncomplicated pregnancy

Pregnancy may be a challenging period for the maternal systems and has been regarded as a stress test, as imperceptible/mild dysfunctions eventually present may be exacerbated during this period. The cardiovascular system is no exception, and several morphological and functional adaptations accompanying pregnancy have been described. However, long-term pregnancy-induced cardiac molecular alterations remain highly unexplored. The postpartum is marked by reverse remodeling of the pregnancy-induced cardiovascular adaptations, representing a possible critical period for assessing future maternal cardiovascular health. The current study explored the molecular and metabolic alterations in the cardiac tissue eight weeks after a physiological uncomplicated pregnancy. Female Sprague-Dawley rats were fed a chow diet through pregnancy, lactation, and weaning and compared to their non-pregnant counterparts. Eight weeks postpartum, increased levels of the phosphorylated form of AMPKα (Thr172) and its ratio to total AMPKα indicated possible alterations in cardiac metabolic flexibility, accompanied by increased Pparα and Hif1α transcripts levels. Additionally, postpartum hearts exhibited higher mitochondrial ATP and NADH levels without major changes in mitochondrial respiratory function. Elevated Nrf2 levels in the cardiac tissue suggested potential implications for cardiac redox balance, further supported by increased levels or activity of proteins directly regulated by Nrf2. The findings herein reported suggest that at eight weeks postpartum, molecular alterations induced by pregnancy, especially regarding redox balance, are still observed in the mothers' heart. These alterations present at late postpartum may open new avenues to understand the different risk for cardiovascular complications development after normal pregnancies.

Novel insight into the genetic signatures of altitude adaptation related body composition in Tibetans

Background

The Tibetan population residing in high-altitude (HA) regions has adapted to extreme hypoxic environments. However, there is limited understanding of the genetic basis of body compositions in Tibetan population adapted to HA.

Methods

We performed a genome-wide association study (GWAS) to identify genetic variants associated with HA and HA-related body composition traits. A total of 755,731 single nucleotide polymorphisms (SNPs) were genotyped using the precision medicine diversity array from 996 Tibetan college students. T-tests and Pearson correlation analysis were used to estimate the association between body compositions and altitude. The mixed linear regression identified the SNPs significantly associated with HA and HA-related body compositions. LASSO regression was used to screen for important SNPs in HA and body compositions.

Results

Significant differences were observed in lean body mass (LBW), muscle mass (MM), total body water (TBW), standard weight (SBW), basal metabolic rate (BMR), total protein (TP), and total inorganic salt (Is) in different altitudes stratification. We identified three SNPs in EPAS1 (rs1562453, rs7589621 and rs7583392) that were significantly associated with HA (p < 5 × 10-7). GWAS analysis of 7 HA-related body composition traits, we identified 14 SNPs for LBM, 11 SNPs for TBW, 15 SNPs for MM, 16 SNPs for SBW, 9 SNPs for BMR, 12 SNPs for TP, and 26 SNPs for Is (p < 5.0 × 10-5).

Conclusion

These findings provide insight into the genetic basis of body composition in Tibetan college students adapted to HA, and lay the foundation for further investigation into the molecular mechanisms underlying HA adaptation.

Loss of lncRNA LINC01056 leads to sorafenib resistance in HCC

BACKGROUND AND AIMS

Sorafenib is a major nonsurgical option for patients with advanced hepatocellular carcinoma (HCC); however, its clinical efficacy is largely undermined by the acquisition of resistance. The aim of this study was to identify the key lncRNA involved in the regulation of the sorafenib response in HCC.

MATERIALS AND METHODS

A clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) single-guide RNA (sgRNA) synergistic activation mediator (SAM)-pooled lncRNA library was applied to screen for the key lncRNA regulated by sorafenib treatment. The role of the identified lncRNA in mediating the sorafenib response in HCC was examined in vitro and in vivo. The underlying mechanism was delineated by proteomic analysis. The clinical significance of the expression of the identified lncRNA was evaluated by multiplex immunostaining on a human HCC microtissue array.

RESULTS

CRISPR/Cas9 lncRNA library screening revealed that Linc01056 was among the most downregulated lncRNAs in sorafenib-resistant HCC cells. Knockdown of Linc01056 reduced the sensitivity of HCC cells to sorafenib, suppressing apoptosis in vitro and promoting tumour growth in mice in vivo. Proteomic analysis revealed that Linc01056 knockdown in sorafenib-treated HCC cells induced genes related to fatty acid oxidation (FAO) while repressing glycolysis-associated genes, leading to a metabolic switch favouring higher intracellular energy production. FAO inhibition in HCC cells with Linc01056 knockdown significantly restored sensitivity to sorafenib. Mechanistically, we determined that PPARα is the critical molecule governing the metabolic switch upon Linc01056 knockdown in HCC cells and indeed, PPARα inhibition restored the sorafenib response in HCC cells in vitro and HCC tumours in vivo. Clinically, Linc01056 expression predicted optimal overall and progression-free survival outcomes in HCC patients and predicted a better sorafenib response. Linc01056 expression indicated a low FAO level in HCC.

CONCLUSION

Our study identified Linc01056 as a critical epigenetic regulator and potential therapeutic target in the regulation of the sorafenib response in HCC.

Metabolic mitochondrial alterations prevail in the female rat heart 8 weeks after exercise cessation

BACKGROUND

The consumption of high-caloric diets strongly contributes to the development of non-communicable diseases (NCDs), including cardiovascular disease, the leading cause of mortality worldwide. Exercise (along with diet intervention) is one of the primary non-pharmacological approaches to promote a healthier lifestyle and counteract the rampant prevalence of NCDs. The present study evaluated the effects of exercise cessation after a short period training on the cardiac metabolic and mitochondrial function of female rats.

METHODS

Seven-week-old female Sprague-Dawley rats were fed a control or a high-fat, high-sugar (HFHS) diet and, after 7 weeks, the animals were kept on a sedentary lifestyle or submitted to endurance exercise for 3 weeks (6 days per week, 20-60 min/day). The cardiac samples were analysed 8 weeks after exercise cessation.

RESULTS

The consumption of the HFHS diet triggered impaired glucose tolerance, whereas the HFHS diet and physical exercise resulted in different responses in plasma adiponectin and leptin levels. Cardiac mitochondrial respiration efficiency was decreased by the HFHS diet consumption, which led to reduced ATP and increased NAD(P)H mitochondrial levels, which remained prevented by exercise 8 weeks after cessation. Exercise training-induced cardiac adaptations in redox balance, namely increased relative expression of Nrf2 and downstream antioxidant enzymes persist after an eight-week exercise cessation period.

CONCLUSIONS

Endurance exercise modulated cardiac redox balance and mitochondrial efficiency in female rats fed a HFHS diet. These findings suggest that exercise may elicit cardiac adaptations crucial for its role as a non-pharmacological intervention for individuals at risk of developing NCDs.

Cardiac metabolism in HFpEF: from fuel to signalling

Heart failure (HF) is marked by distinctive changes in myocardial uptake and utilization of energy substrates. Among the different types of HF, HF with preserved ejection fraction (HFpEF) is a highly prevalent, complex, and heterogeneous condition for which metabolic derangements seem to dictate disease progression. Changes in intermediate metabolism in cardiometabolic HFpEF-among the most prevalent forms of HFpEF-have a large impact both on energy provision and on a number of signalling pathways in the heart. This dual, metabolic vs. signalling, role is played in particular by long-chain fatty acids (LCFAs) and short-chain carbon sources [namely, short-chain fatty acids (SCFAs) and ketone bodies (KBs)]. LCFAs are key fuels for the heart, but their excess can be harmful, as in the case of toxic accumulation of lipid by-products (i.e. lipotoxicity). SCFAs and KBs have been proposed as a potential major, alternative source of energy in HFpEF. At the same time, both LCFAs and short-chain carbon sources are substrate for protein post-translational modifications and other forms of direct and indirect signalling of pivotal importance in HFpEF pathogenesis. An in-depth molecular understanding of the biological functions of energy substrates and their signalling role will be instrumental in the development of novel therapeutic approaches to HFpEF. Here, we summarize the current evidence on changes in energy metabolism in HFpEF, discuss the signalling role of intermediate metabolites through, at least in part, their fate as substrates for post-translational modifications, and highlight clinical and translational challenges around metabolic therapy in HFpEF.

Excavation and characterization of key circRNAs for milk fat percentage in Holstein cattle

Milk fat percentage is one of the significant indicators governing the price and quality of milk and is regulated by a variety of non-coding RNAs. We used RNA sequencing (RNA-seq) techniques and bioinformatics approaches to explore potential candidate circular RNAs (circRNAs) regulating milk fat metabolism. After analysis, compared with low milk fat percentage (LMF) cows, 309 circRNAs were significantly differentially expressed in high milk fat percentage (HMF) cows. Functional enrichment and pathway analysis revealed that the main functions of the parental genes of differentially expressed circRNAs (DE-circRNAs) were related to lipid metabolism. We selected four circRNAs (Novel_circ_0000856, Novel_circ_0011157, novel_circ_0011944, and Novel_circ_0018279) derived from parental genes related to lipid metabolism as key candidate DE-circRNAs. Their head-to-tail splicing was demonstrated by linear RNase R digestion experiments and Sanger sequencing. However, the tissue expression profiles showed that only Novel_circ_0000856, Novel_circ_0011157, and Novel_circ_0011944 were expressed with high abundance in breast tissue. Based on the subcellular localization found that Novel_circ_0000856, Novel_circ_0011157, and Novel_circ_0011944 mainly function as competitive endogenous RNAs (ceRNAs) in the cytoplasm. Therefore, we constructed their ceRNA regulatory networks, and the five hub target genes (CSF1, TET2, VDR, CD34, and MECP2) in ceRNAs were obtained by CytoHubba and MCODE plugins in Cytoscape, as well as tissue expression profiles analysis of target genes. These genes play a key role as important target genes in lipid metabolism, energy metabolism, and cellular autophagy. The Novel_circ_0000856, Novel_circ_0011157, and Novel_circ_0011944 regulate the expression of hub target genes through interaction with miRNAs and constitute key regulatory networks that may be involved in milk fat metabolism. The circRNAs obtained in this study may act as miRNA sponges and thus influence mammary gland development and lipid metabolism in cows, which improves our understanding of the role of circRNAs in cow lactation.

Kidney Proximal Tubule GLUT2-More than Meets the Eye

Tubulopathy plays a central role in the pathophysiology of diabetic kidney disease (DKD). Under diabetic conditions, the kidney proximal tubule cells (KPTCs) are exposed to an extensive amount of nutrients, most notably glucose; these nutrients deteriorate KPTCs function and promote the development and progression of DKD. Recently, the facilitative glucose transporter 2 (GLUT2) in KPTCs has emerged as a central regulator in the pathogenesis of DKD. This has been demonstrated by identifying its specific role in enhancing glucose reabsorption and glucotoxicity, and by deciphering its effect in regulating the expression of the sodium-glucose transporter 2 (SGLT2) in KPTCs. Moreover, reduction/deletion of KPTC-GLUT2 has been recently found to ameliorate DKD, raising the plausible idea of considering it as a therapeutic target against DKD. However, the underlying molecular mechanisms by which GLUT2 exerts its deleterious effects in KPTCs remain vague. Herein, we review the current findings on the proximal tubule GLUT2 biology and function under physiologic conditions, and its involvement in the pathophysiology of DKD. Furthermore, we shed new light on its cellular regulation during diabetic conditions.

The interaction of TRPV1 and lipids: Insights into lipid metabolism

Transient receptor potential vanilloid 1 (TRPV1), a non-selective ligand-gated cation channel with high permeability for Ca²⁺, has received considerable attention as potential therapeutic target for the treatment of several disorders including pain, inflammation, and hyperlipidemia. In particular, TRPV1 regulates lipid metabolism by mechanisms that are not completely understood. Interestingly, TRPV1 and lipids regulate each other in a reciprocal and complex manner. This review surveyed the recent literature dealing with the role of TRPV1 in the hyperlipidemia-associated metabolic syndrome. Besides TRPV1 structure, molecular mechanisms underlying the regulatory effect of TRPV1 on lipid metabolism such as the involvement of uncoupling proteins (UCPs), ATP-binding cassette (ABC) transporters, peroxisome proliferation-activated receptors (PPAR), sterol responsive element binding protein (SREBP), and hypoxia have been discussed. Additionally, this review extends our understanding of the lipid-dependent modulation of TRPV1 activity through affecting both the gating and the expression of TRPV1. The regulatory role of different classes of lipids such as phosphatidylinositol (PI), cholesterol, estrogen, and oleoylethanolamide (OEA), on TRPV1 has also been addressed.

Endurance exercise under short-duration intermittent hypoxia promotes endurance performance via improving muscle metabolic properties in mice

This study was designed to (1) investigate the effects of acute exercise under intermittent hypoxia on muscle mRNA and protein levels, and (2) clarify the mechanisms by which exercise under intermittent hypoxia improves endurance capacity. Experiment-1: Male mice were subjected to either acute endurance exercise, exercise under hypoxia (14% O2 ), exercise under intermittent hypoxia (Int, three cycles of room air [10 min] and 14% O2 [15 min]). At 3 h after exercise under intermittent hypoxia, sirtuin-6 mRNA levels and nuclear prolyl hydroxylases-2 protein levels were significantly upregulated in white gastrocnemius muscle in the Int group. Experiment-2: Mice were assigned to sedentary control (Sed), normoxic exercise-trained (ET), hypoxic exercise-trained (HYP) or exercise-trained under intermittent hypoxia (INT) groups. Exercise capacity was significantly greater in the INT group than in the ET and HYP group. Activity levels of citrate synthase were significantly greater in the INT group than in the HYP group in soleus (SOL) and red gastrocnemius muscles. In SOL, nuclear N-terminal PGC1α levels were considerably increased by the INT training (95% confidence interval [CI]: 1.09-1.79). The INT significantly increased pyruvate dehydrogenase complex activity levels in left ventricle (LV). Monocarboxylate transporter-4 protein levels were significantly increased after the INT training in LV. Capillary-to-fiber ratio values were significantly increased in SOL and were substantially increased in LV (CI: 1.10-1.22) after the INT training. These results suggest that exercise training under intermittent hypoxia represents a beneficial strategy for increasing endurance performance via improving metabolic properties and capillary profiles in several hind-leg muscles and the heart.

Iron increases lipid deposition via oxidative stress-mediated mitochondrial dysfunction and the HIF1α-PPARγ pathway

Iron is an essential micro-element, involved in multiple biological activities in vertebrates. Excess iron accumulation has been identified as an important mediator of lipid deposition. However, the underlying mechanisms remain unknown. In the present study, we found that a high-iron diet significantly increased intestinal iron content and upregulated the mRNA expression of two iron transporters (zip14 and fpn1). Intestinal iron overload increased lipogenesis, reduced lipolysis and promoted oxidative stress and mitochondrial dysfunction. Iron-induced lipid accumulation was mediated by hypoxia-inducible factor-1 α (HIF1α), which was induced in response to mitochondrial oxidative stress following inhibition of prolyl hydroxylase 2 (PHD2). Mechanistically, iron promoted lipid deposition by enhancing the DNA binding capacity of HIF1α to the pparγ and fas promoters. Our results provide experimental evidence that oxidative stress, mitochondrial dysfunction and the HIF1α-PPARγ pathway are critical mediators of iron-induced lipid deposition.

Reprogramming of glucocorticoid receptor function by hypoxia

Here, we investigate the impact of hypoxia on the hepatic response of glucocorticoid receptor (GR) to dexamethasone (DEX) in mice via RNA-sequencing. Hypoxia causes three types of reprogramming of GR: (i) much weaker induction of classical GR-responsive genes by DEX in hypoxia, (ii) a number of genes is induced by DEX specifically in hypoxia, and (iii) hypoxia induces a group of genes via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Transcriptional profiles are reflected by changed GR DNA-binding as measured by ChIP sequencing. The HPA axis is induced by hypothalamic HIF1α and HIF2α activation and leads to GR-dependent lipolysis and ketogenesis. Acute inflammation, induced by lipopolysaccharide, is prevented by DEX in normoxia but not during hypoxia, and this is attributed to HPA axis activation by hypoxia. We unfold new physiological pathways that have consequences for patients suffering from GC resistance.

Activation of HIF1α Rescues the Hypoxic Response and Reverses Metabolic Dysfunction in the Diabetic Heart

Type 2 diabetes (T2D) impairs hypoxia-inducible factor (HIF)1α activation, a master transcription factor that drives cellular adaptation to hypoxia. Reduced activation of HIF1α contributes to the impaired post-ischemic remodeling observed following myocardial infarction in T2D. Molidustat is an HIF stabilizer currently undergoing clinical trials for the treatment of renal anemia associated with chronic kidney disease; however, it may provide a route to pharmacologically activate HIF1α in the T2D heart. In human cardiomyocytes, molidustat stabilized HIF1α and downstream HIF target genes, promoting anaerobic glucose metabolism. In hypoxia, insulin resistance blunted HIF1α activation and downstream signaling, but this was reversed by molidustat. In T2D rats, oral treatment with molidustat rescued the cardiac metabolic dysfunction caused by T2D, promoting glucose metabolism and mitochondrial function, while suppressing fatty acid oxidation and lipid accumulation. This resulted in beneficial effects on post-ischemic cardiac function, with the impaired contractile recovery in T2D heart reversed by molidustat treatment. In conclusion, pharmacological HIF1α stabilization can overcome the blunted hypoxic response induced by insulin resistance. In vivo this corrected the abnormal metabolic phenotype and impaired post-ischemic recovery of the diabetic heart. Therefore, molidustat may be an effective compound to further explore the clinical translatability of HIF1α activation in the diabetic heart.

Epigenetic State Changes Underlie Metabolic Switch in Mouse Post-Infarction Border Zone Cardiomyocytes

Myocardial infarction causes ventricular muscle loss and formation of scar tissue. The surviving myocardium in the border zone, located adjacent to the infarct, undergoes profound changes in function, structure and composition. How and to what extent these changes of border zone cardiomyocytes are regulated epigenetically is not fully understood. Here, we obtained transcriptomes of PCM-1-sorted mouse cardiomyocyte nuclei of healthy left ventricle and 7 days post myocardial infarction border zone tissue. We validated previously observed downregulation of genes involved in fatty acid metabolism, oxidative phosphorylation and mitochondrial function in border zone-derived cardiomyocytes, and observed a modest induction of genes involved in glycolysis, including Slc2a1 (Glut1) and Pfkp. To gain insight into the underlying epigenetic regulatory mechanisms, we performed H3K27ac profiling of healthy and border zone cardiomyocyte nuclei. We confirmed the switch from Mef2- to AP-1 chromatin association in border zone cardiomyocytes, and observed, in addition, an enrichment of PPAR/RXR binding motifs in the sites with reduced H3K27ac signal. We detected downregulation and accompanying epigenetic state changes at several key PPAR target genes including Ppargc1a (PGC-1α), Cpt2, Ech1, Fabpc3 and Vldrl in border zone cardiomyocytes. These data indicate that changes in epigenetic state and gene regulation underlie the maintained metabolic switch in border zone cardiomyocytes.

Pterostilbene Increases LDL Metabolism in HL-1 Cardiomyocytes by Modulating the PCSK9/HNF1α/SREBP2/LDLR Signaling Cascade, Upregulating Epigenetic hsa-miR-335 and hsa-miR-6825, and LDL Receptor Expression

Proprotein convertase subtilisin/kexin type 9 (PCSK9) can promote the degradation of low-density lipoprotein (LDL) receptor (LDLR), leading to hypercholesterolemia and myocardial dysfunction. The intracellular regulatory mechanism by which the natural polyphenol pterostilbene modulates the PCSK9/LDLR signaling pathway in cardiomyocytes has not been evaluated. We conducted Western blotting, flow cytometry, immunofluorescence staining, and mean fluorescence intensity analyses of pterostilbene-treated mouse HL-1 cardiomyocytes. Pterostilbene did not alter cardiomyocyte viability. Compared to the control group, treatment with both 2.5 and 5 μM pterostilbene significantly increased the LDLR protein expression accompanied by increased uptake of LDL. The expression of the mature PCSK9 was significantly suppressed at the protein and mRNA level by the treatment with both 2.5 and 5 μM pterostilbene, respectively, compared to the control. Furthermore, 2.5 and 5 μM pterostilbene treatment resulted in a significant reduction in the protein hepatic nuclear factor 1α (HNF1α)/histone deacetylase 2 (HDAC2) ratio and sterol regulatory element-binding protein-2 (SREBP2)/HDAC2 ratio. The expression of both hypoxia-inducible factor-1 α (HIF1α) and nuclear factor erythroid 2-related factor 2 (Nrf2) at the protein level was also suppressed. Pterostilbene as compared to short hairpin RNA against SREBP2 induced a higher protein expression of LDLR and lower nuclear accumulation of HNF1α and SREBP2. In addition, pterostilbene reduced PCSK9/SREBP2 interaction and mRNA expression by increasing the expression of hsa-miR-335 and hsa-miR-6825, which, in turn, increased LDLR mRNA expression. In cardiomyocytes, pterostilbene dose-dependently decreases and increases the protein and mRNA expression of PCSK9 and LDLR, respectively, by suppressing four transcription factors, HNF1α, SREBP2, HIF1α, and Nrf2, and enhancing the expression of hsa-miR-335 and hsa-miR-6825, which suppress PCSK9/SREBP2 interaction.

The Regulatory Role of Oxygen Metabolism in Exercise-Induced Cardiomyocyte Regeneration

During heart failure, the heart is unable to regenerate lost or damaged cardiomyocytes and is therefore unable to generate adequate cardiac output. Previous research has demonstrated that cardiac regeneration can be promoted by a hypoxia-related oxygen metabolic mechanism. Numerous studies have indicated that exercise plays a regulatory role in the activation of regeneration capacity in both healthy and injured adult cardiomyocytes. However, the role of oxygen metabolism in regulating exercise-induced cardiomyocyte regeneration is unclear. This review focuses on the alteration of the oxygen environment and metabolism in the myocardium induced by exercise, including the effects of mild hypoxia, changes in energy metabolism, enhanced elimination of reactive oxygen species, augmentation of antioxidative capacity, and regulation of the oxygen-related metabolic and molecular pathway in the heart. Deciphering the regulatory role of oxygen metabolism and related factors during and after exercise in cardiomyocyte regeneration will provide biological insight into endogenous cardiac repair mechanisms. Furthermore, this work provides strong evidence for exercise as a cost-effective intervention to improve cardiomyocyte regeneration and restore cardiac function in this patient population.

Adaptive Cardiac Metabolism Under Chronic Hypoxia: Mechanism and Clinical Implications

Chronic hypoxia is an essential component in many cardiac diseases. The heart consumes a substantial amount of energy and it is important to maintain the balance of energy supply and demand when oxygen is limited. Previous studies showed that the heart switches from fatty acid to glucose to maintain metabolic efficiency in the adaptation to chronic hypoxia. However, the underlying mechanism of this adaptive cardiac metabolism remains to be fully characterized. Moreover, how the altered cardiac metabolism affects the heart function in patients with chronic hypoxia has not been discussed in the current literature. In this review, we summarized new findings from animal and human studies to illustrate the mechanism underlying the adaptive cardiac metabolism under chronic hypoxia. Clinical focus is given to certain patients that are subject to the impact of chronic hypoxia, and potential treatment strategies that modulate cardiac metabolism and may improve the heart function in these patients are also summarized.

Crosstalk between EP2 and PPARα Modulates Hypoxic Signaling and Myopia Development in Guinea Pigs

Purpose

Cyclic adenosine monophosphate (cAMP) and peroxisome proliferator-activated receptor alpha (PPARα) levels mediate extracellular matrix (ECM) changes by altering the levels of hypoxia-inducible factor 1-alpha (HIF-1α) in various tissues. We aimed to determine, in the sclera of guinea pigs, whether a prostanoid receptor (EP2)-linked cAMP modulation affects PPARα and HIF-1α signaling during myopia.

Methods

Three-week-old guinea pigs (n = 20 in each group), were monocularly injected with either an EP2 agonist (butaprost 1 µmol/L/10 µmol/L), an antagonist (AH6809 10 µmol/L/30 µmol/L) or a vehicle solution for two weeks during normal ocular growth. Separate sets of animals received these injections and underwent form deprivation (FD) simultaneously. Refraction and axial length (AL) were measured at two weeks, followed by scleral tissue isolation for quantitative PCR (qPCR) analysis (n = 10) and cAMP detection (n = 10) using a radioimmunoassay.

Results

Butaprost induced myopia development during normal ocular growth, with proportional increases in AL and cAMP levels. FD did not augment the magnitude of myopia or cAMP elevations in these agonist-injected eyes. AH6809 suppressed cAMP increases and myopia progression during FD, but had no effect in a normal visual environment. Of the diverse set of 27 genes related to cAMP, PPARα and HIF-1α signaling and ECM remodeling, butaprost differentially regulated 15 of them during myopia development. AH6809 injections during FD negated such differential gene expressions.

Conclusion

EP2 agonism increased cAMP and HIF-1α signaling subsequent to declines in PPARα and RXR mRNA levels, which in turn decreased scleral fibrosis and promoted myopia. EP2 antagonism instead inhibited each of these responses. Our data suggest that EP2 suppression may sustain scleral ECM structure and inhibit myopia development.

IL-8 exacerbates alcohol-induced fatty liver disease via the Akt/HIF-1α pathway in human IL-8-expressing mice

Alcoholic fatty liver disease (AFLD) is a disease that causes liver damage due to chronic heavy drinking. AFLD is related to lipid accumulation in liver cells caused by alcohol intake. Interleukin-8 (IL-8) is an inflammatory cytokine associated with chemotaxis (deletion in mice) that has robust effects on the occurrence and development of disease by activating related signal transduction pathways to promote inflammation and cell proliferation. There is significantly increased IL-8 expression in liver disease, which may be related to the pathogenesis of AFLD. In this study, we used hydrodynamic injection to deliver the liver-specific expression vector pLIVE-hIL-8 into mice. We found that hIL-8 can exacerbate alcohol-induced fatty liver disease via the Akt/HIF-1α pathway. Exacerbated liver lipid degeneration in mice, which is characterized by excessive accumulation of triglycerides, and liver damage markers were significantly increased. Moreover, hIL-8 could increase the alcohol-induced release of ROS in fatty liver caused by alcohol and exacerbate fatty liver disease. The expression of liver lipid metabolism-related gene sterol regulatory element-binding protein-1c (SREBP-1c) was increased. Furthermore, the expression of peroxisome proliferator-activated receptor alpha (PPARα), which is related to liver fatty acid oxidation, was decreased. The findings obtained in this study of hIL-8 will help identify a potential target for the clinical treatment of AFLD.

Fetal cardiomyocyte phenotype, ketone body metabolism, and mitochondrial dysfunction in the pathology of atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia diagnosed in clinical practice. Even though hypertension, congestive heart failure, pulmonary disease, and coronary artery disease are the potential risk factors for AF, the underlying molecular pathology is largely unknown. The reversion of the mature cardiomyocytes to fetal phenotype, impaired ketone body metabolism, mitochondrial dysfunction, and the cellular effect of reactive oxygen species (ROS) are the major underlying biochemical events associated with the molecular pathology of AF. On this background, the present manuscript sheds light into these biochemical events in regard to the metabolic derangements in cardiomyocyte leading to AF, especially with respect to structural, contractile, and electrophysiological properties. In addition, the article critically reviews the current understanding, potential demerits, and translational strategies in the management of AF.

Cardiac foetal reprogramming: a tool to exploit novel treatment targets for the failing heart

As the heart matures during embryogenesis from its foetal stages, several structural and functional modifications take place to form the adult heart. This process of maturation is in large part due to an increased volume and work load of the heart to maintain proper circulation throughout the growing body. In recent years, it has been observed that these changes are reversed to some extent as a result of cardiac disease. The process by which this occurs has been characterized as cardiac foetal reprogramming and is defined as the suppression of adult and re-activation of a foetal genes profile in the diseased myocardium. The reasons as to why this process occurs in the diseased myocardium are unknown; however, it has been suggested to be an adaptive process to counteract deleterious events taking place during cardiac remodelling. Although still in its infancy, several studies have demonstrated that targeting foetal reprogramming in heart failure can lead to substantial improvement in cardiac functionality. This is highlighted by a recent study which found that by modulating the expression of 5-oxoprolinase (OPLAH, a novel cardiac foetal gene), cardiac function can be significantly improved in mice exposed to cardiac injury. Additionally, the utilization of angiotensin receptor neprilysin inhibitors (ARNI) has demonstrated clear benefits, providing important clinical proof that drugs that increase natriuretic peptide levels (part of the foetal gene programme) indeed improve heart failure outcomes. In this review, we will highlight the most important aspects of cardiac foetal reprogramming and will discuss whether this process is a cause or consequence of heart failure. Based on this, we will also explain how a deeper understanding of this process may result in the development of novel therapeutic strategies in heart failure.

Inhibition of oxygen-sensing prolyl hydroxylases increases lipid accumulation in human primary tubular epithelial cells without inducing ER stress

The role of the hypoxia-inducible transcription factor (HIF) pathway in renal lipid metabolism is largely unknown. As HIF stabilizing prolyl hydroxylase (PHD) inhibitors are currently investigated in clinical trials for the treatment of renal anemia, we studied the effects of genetic deletion and pharmacological inhibition of PHDs on renal lipid metabolism in transgenic mice and human primary tubular epithelial cells (hPTEC). Tubular cell-specific deletion of HIF prolyl hydroxylase 2 (Phd2) increased the size of Oil Red-stained lipid droplets in mice. In hPTEC, the PHD inhibitors (PHDi) DMOG and ICA augmented lipid accumulation, which was visualized by Oil Red staining and assessed by microscopy and an infrared imaging system. PHDi-induced lipid accumulation required the exogenous availability of fatty acids and was observed in both proximal and distal hPTEC. PHDi treatment was not associated with structural features of cytotoxicity in contrast to treatment with the immunosuppressant cyclosporine A (CsA). PHDi and CsA differentially upregulated the expression of the lipid droplet-associated genes PLIN2, PLIN4 and HILPDA. Both PHDi and CsA activated AMP-activated protein kinase (AMPK) indicating the initiation of a metabolic stress response. However, only CsA triggered endoplasmic reticulum (ER) stress as determined by the increased mRNA expression of multiple ER stress markers but CsA-induced ER stress was not linked to lipid accumulation. Our data raise the possibility that PHD inhibition may protect tubular cells from toxic free fatty acids by trapping them as triacylglycerides in lipid droplets. This mechanism might contribute to the renoprotective effects of PHDi in experimental kidney diseases.

DNA Methylation Changes Are Associated With an Incremental Ascent to High Altitude

Genetic and nongenetic factors are involved in the individual ability to physiologically acclimatize to high-altitude hypoxia through processes that include increased heart rate and ventilation. High-altitude acclimatization is thought to have a genetic component, yet it is unclear if other factors, such as epigenetic gene regulation, are involved in acclimatization to high-altitude hypoxia in nonacclimatized individuals. We collected saliva samples from a group of healthy adults of European ancestry (n = 21) in Kathmandu (1,400 m; baseline) and three altitudes during a trek to the Everest Base Camp: Namche (3,440 m; day 3), Pheriche (4,240 m; day 7), and Gorak Shep (5,160 m; day 10). We used quantitative bisulfite pyrosequencing to determine changes in DNA methylation, a well-studied epigenetic marker, in LINE-1, EPAS1, EPO, PPARa, and RXRa. We found significantly lower DNA methylation between baseline (1,400 m) and high altitudes in LINE-1, EPO (at 4,240 m only), and RXRa. We found increased methylation in EPAS1 (at 4,240 m only) and PPARa. We also found positive associations between EPO methylation and systolic blood pressure and RXRa methylation and hemoglobin. Our results show that incremental exposure to hypoxia can affect the epigenome. Changes to the epigenome, in turn, could underlie the process of altitude acclimatization.

Effects of exercise training with short-duration intermittent hypoxia on endurance performance and muscle metabolism in well-trained mice

The author previously reported that short-duration intermittent hypoxia had additive effects on improvements in endurance capacity by enhancing fatty acid metabolism. The present study was designed to investigate the effects of short-duration intermittent hypoxia on endurance capacity, metabolic enzyme activity, and protein levels associated with mitochondrial biogenesis in well-trained mice. Mice in the training group were housed in a cage with a running wheel for 7 weeks from 5 weeks old. Voluntary running markedly increased maximal work values by 5.0-fold. Trained mice were then subjected to either endurance treadmill training (ET) for 60 min or hybrid training (HT, ET for 30 min followed by sprint interval exercise (5-sec run-10-sec rest) for 30 min) with (H-ET or H-HT) or without (ET or HT) short-duration intermittent hypoxia (4 cycles of 12-13% O2 for 15 min and 20.9% O2 for 10 min) for 4 weeks. Maximal endurance capacity was markedly greater in the H-ET and H-HT than ET and HT groups, respectively. H-ET and H-HT increased activity levels of 3-hydroxyacyl-CoA-dehydrogenase in oxidative muscle portion and pyruvate dehydrogenase complex in glycolytic muscle portion. These activity levels were significantly correlated with maximal endurance capacity. Protein levels of dynamin-related protein-1 were increased more by H-ET and H-HT than by ET and HT, but were not significantly correlated with maximal work. These results suggest that intermittent hypoxic exposure has beneficial effects on endurance and hybrid training to improve the endurance capacity via improving fatty acid and pyruvate oxidation in highly trained mice.

The effects of bisphenol A, benzyl butyl phthalate, and di(2-ethylhexyl) phthalate on estrogen receptor alpha in estrogen receptor-positive cells under hypoxia

Endocrine-disrupting chemicals (EDCs) are widely used in various consumer goods. Consequently, humans are constantly exposed to EDCs, which is associated with a variety of endocrine-related diseases. In this study, we demonstrated the effects of bisphenol A (BPA), benzyl butyl phthalate (BBP), and di(2-ethylhexyl) phthalate (DEHP) on estrogen receptor alpha (ERα) expression under normoxia and hypoxia. First, we confirmed the effects of EDCs on ER activity using OECD Test Guideline 455. Compared to the 100% activity induced by 1 nM 17-β-estradiol (positive control), BPA and BBP exhibited 50% ERα activation at concentrations of 1.31 μM and 4.8 μM, respectively. In contrast, and consistent with previous reports, DEHP did not activate ERα. ERα is activated and degraded by hypoxia in breast cancer cells. BPA, BBP, and DEHP enhanced ERα-mediated transcriptional activity under hypoxia. All three EDCs decreased ERα protein levels under hypoxia in MCF-7 cells. The transcriptional activity of hypoxia-inducible factor-1 was decreased and secretion of vascular endothelial growth factor (VEGF) was increased by BPA and BBP under hypoxia in MCF-7 cells, but not by DEHP. All three EDCs decreased the ERα protein expression level in Ishikawa human endometrial adenocarcinoma cells, and DEHP caused a weak decrease in VEGF secretion under hypoxia. These results demonstrate down-regulation of ERα by EDCs may influence the pathological state associated with hypoxia.

Inorganic nitrate, hypoxia, and the regulation of cardiac mitochondrial respiration-probing the role of PPARα

Dietary inorganic nitrate prevents aspects of cardiac mitochondrial dysfunction induced by hypoxia, although the mechanism is not completely understood. In both heart and skeletal muscle, nitrate increases fatty acid oxidation capacity, and in the latter case, this involves up-regulation of peroxisome proliferator-activated receptor (PPAR)α expression. Here, we investigated whether dietary nitrate modifies mitochondrial function in the hypoxic heart in a PPARα-dependent manner. Wild-type (WT) mice and mice without PPARα (Ppara^−/−) were given water containing 0.7 mM NaCl (control) or 0.7 mM NaNO₃ for 35 d. After 7 d, mice were exposed to normoxia or hypoxia (10% O₂) for the remainder of the study. Mitochondrial respiratory function and metabolism were assessed in saponin-permeabilized cardiac muscle fibers. Environmental hypoxia suppressed mass-specific mitochondrial respiration and additionally lowered the proportion of respiration supported by fatty acid oxidation by 18% (P < 0.001). This switch away from fatty acid oxidation was reversed by nitrate treatment in hypoxic WT but not Ppara^−/− mice, indicating a PPARα-dependent effect. Hypoxia increased hexokinase activity by 33% in all mice, whereas lactate dehydrogenase activity increased by 71% in hypoxic WT but not Ppara^−/− mice. Our findings indicate that PPARα plays a key role in mediating cardiac metabolic remodeling in response to both hypoxia and dietary nitrate supplementation.—Horscroft, J. A., O’Brien, K. A., Clark, A. D., Lindsay, R. T., Steel, A. S., Procter, N. E. K., Devaux, J., Frenneaux, M., Harridge, S. D. R., Murray, A. J. Inorganic nitrate, hypoxia, and the regulation of cardiac mitochondrial respiration—probing the role of PPARα.

MicroRNA 122, Regulated by GRLH2, Protects Livers of Mice and Patients From Ethanol-Induced Liver Disease

BACKGROUND & AIMS

Chronic, excessive alcohol consumption leads to alcoholic liver disease (ALD) characterized by steatosis, inflammation, and eventually cirrhosis. The hepatocyte specific microRNA 122 (MIR122) regulates hepatocyte differentiation and metabolism. We investigated whether an alcohol-induced decrease in level of MIR122 contributes to development of ALD.

METHODS

We obtained liver samples from 12 patients with ALD and cirrhosis and 9 healthy individuals (controls) and analyzed them by histology and immunohistochemistry. C57Bl/6 mice were placed on a Lieber-DeCarli liquid diet, in which they were fed ethanol for 8 weeks, as a model of ALD, or a control diet. These mice were also given injections of CCl4, to increase liver fibrosis, for 8 weeks. On day 28, mice with ethanol-induced liver disease and advanced fibrosis, and controls, were given injections of recombinant adeno-associated virus 8 vector that expressed the primary miR-122 transcript (pri-MIR122, to overexpress MIR122 in hepatocytes) or vector (control). Two weeks before ethanol feeding, some mice were given injections of a vector that expressed an anti-MIR122, to knock down its expression. Serum and liver tissues were collected; hepatocytes and liver mononuclear cells were analyzed by histology, immunoblots, and confocal microscopy. We performed in silico analyses to identify targets of MIR122 and chromatin immunoprecipitation quantitative polymerase chain reaction analyses in Huh-7 cells.

RESULTS

Levels of MIR122 were decreased in liver samples from patients with ALD and mice on the Lieber-DeCarli diet, compared with controls. Transgenic expression of MIR122 in hepatocytes of mice with ethanol-induced liver disease and advanced fibrosis significantly reduced serum levels of alanine aminotransferase (ALT) and liver steatosis and fibrosis, compared with mice given injections of the control vector. Ethanol feeding reduced expression of pri-MIR122 by increasing expression of the spliced form of the transcription factor grainyhead like transcription factor 2 (GRHL2) in liver tissues from mice. Levels of GRHL2 also were increased in liver tissues from patients with ALD, compared with controls; increases correlated with decreases in levels of MIR122 in human liver. Mice given injections of the anti-MIR122 before ethanol feeding had increased steatosis, inflammation, and serum levels of alanine aminotransferase compared with mice given a control vector. Levels of hypoxia-inducible factor 1 alpha (HIF1α) mRNA, a target of MIR122, were increased in liver tissues from patients and mice with ALD, compared with controls. Mice with hepatocyte-specific disruption of Hif1α developed less-severe liver injury following administration of ethanol, injection of anti-MIR122, or both.

CONCLUSIONS

Levels of MIR122 decrease in livers from patients with ALD and mice with ethanol-induced liver disease, compared with controls. Transcription of MIR122 is inhibited by GRHL2, which is increased in livers of mice and patients with ALD. Expression of an anti-MIR122 worsened the severity of liver damage following ethanol feeding in mice. MIR122 appears to protect the liver from ethanol-induced damage by reducing levels of HIF1α. These processes might be manipulated to reduce the severity of ALD in patients.

Apigenin ameliorates hypertension-induced cardiac hypertrophy and down-regulates cardiac hypoxia inducible factor-lα in rats

Apigenin is a natural flavonoid compound that can inhibit hypoxia-inducible factor (HIF)-1α expression in cultured tumor cells under hypoxic conditions. Hypertension-induced cardiac hypertrophy is always accompanied by abnormal myocardial glucolipid metabolism due to an increase of HIF-1α. However, whether or not apigenin may ameliorate the cardiac hypertrophy and abnormal myocardial glucolipid metabolism remains unknown. This study aimed to examine the effects of apigenin. Rats with cardiac hypertrophy induced by renovascular hypertension were treated with apigenin 50-100 mg kg(-1) (the doses can be achieved by pharmacological or dietary supplementation for an adult person) by gavage for 4 weeks. The results showed that after treatment with apigenin, the blood pressure, heart weight, heart weight index, cardiomyocyte cross-sectional area, serum angiotensin II, and serum and myocardial free fatty acids were reduced. It is important to note that apigenin decreased the expression level of myocardial HIF-1α protein. Moreover, apigenin simultaneously increased the expression levels of myocardial peroxisome proliferator-activated receptor (PPAR) α, carnitine palmitoyltransferase (CPT)-1, and pyruvate dehydrogenase kinase (PDK)-4 proteins and decreased the expression levels of myocardial PPARγ, glycerol-3-phosphate acyltransferase genes (GPAT), and glucose transporter (GLUT)-4 proteins. These findings demonstrated that apigenin could improve hypertensive cardiac hypertrophy and abnormal myocardial glucolipid metabolism in rats, and its mechanisms might be associated with the down-regulation of myocardial HIF-1α expression and, subsequently increasing the expressions of myocardial PPARα and its target genes CPT-1 and PDK-4, and decreasing the expressions of myocardial PPARγ and its target genes GPAT and GLUT-4.

On the pivotal role of PPARα in adaptation of the heart to hypoxia and why fat in the diet increases hypoxic injury

The role of peroxisome proliferator-activated receptor α (PPARα)-mediated metabolic remodeling in cardiac adaptation to hypoxia has yet to be defined. Here, mice were housed in hypoxia for 3 wk before in vivo contractile function was measured using cine MRI. In isolated, perfused hearts, energetics were measured using (31)P magnetic resonance spectroscopy (MRS), and glycolysis and fatty acid oxidation were measured using [(3)H] labeling. Compared with a normoxic, chow-fed control mouse heart, hypoxia decreased PPARα expression, fatty acid oxidation, and mitochondrial uncoupling protein 3 (UCP3) levels, while increasing glycolysis, all of which served to maintain normal ATP concentrations ([ATP]) and thereby, ejection fractions. A high-fat diet increased cardiac PPARα expression, fatty acid oxidation, and UCP3 levels with decreased glycolysis. Hypoxia was unable to alter the high PPARα expression or reverse the metabolic changes caused by the high-fat diet, with the result that [ATP] and contractile function decreased significantly. The adaptive metabolic changes caused by hypoxia in control mouse hearts were found to have occurred already in PPARα-deficient (PPARα(-/-)) mouse hearts and sustained function in hypoxia despite an inability for further metabolic remodeling. We conclude that decreased cardiac PPARα expression is essential for adaptive metabolic remodeling in hypoxia, but is prevented by dietary fat.-Cole, M. A., Abd Jamil, A. H., Heather, L. C., Murray, A. J., Sutton, E. R., Slingo, M., Sebag-Montefiore, L., Tan, S. C., Aksentijević, D., Gildea, O. S., Stuckey, D. J., Yeoh, K. K., Carr, C. A., Evans, R. D., Aasum, E., Schofield, C. J., Ratcliffe, P. J., Neubauer, S., Robbins, P. A., Clarke, K. On the pivotal role of PPARα in adaptation of the heart to hypoxia and why fat in the diet increases hypoxic injury.

Increased oxidative metabolism following hypoxia in the type 2 diabetic heart, despite normal hypoxia signalling and metabolic adaptation

KEY POINTS

Adaptation to hypoxia makes the heart more oxygen efficient, by metabolising more glucose. In contrast, type 2 diabetes makes the heart metabolise more fatty acids. Diabetes increases the chances of the heart being exposed to hypoxia, but whether the diabetic heart can adapt and respond is unknown. In this study we show that diabetic hearts retain the ability to adapt their metabolism in response to hypoxia, with functional hypoxia signalling pathways. However, the hypoxia-induced changes in metabolism are additive to abnormal baseline metabolism, resulting in hypoxic diabetic hearts metabolising more fat and less glucose than controls. This stops the diabetic heart being able to recover its function when stressed. These results demonstrate that the diabetic heart retains metabolic flexibility to adapt to hypoxia, but is hindered by the baseline effects of the disease. This increases our understanding of how the diabetic heart is affected by hypoxia-associated complications of the disease.

ABSTRACT

Hypoxia activates the hypoxia-inducible factor (HIF), promoting glycolysis and suppressing mitochondrial respiration. In the type 2 diabetic heart, glycolysis is suppressed whereas fatty acid metabolism is promoted. The diabetic heart experiences chronic hypoxia as a consequence of increased obstructive sleep apnoea and cardiovascular disease. Given the opposing metabolic effects of hypoxia and diabetes, we questioned whether diabetes affects cardiac metabolic adaptation to hypoxia. Control and type 2 diabetic rats were housed for 3 weeks in normoxia or 11% oxygen. Metabolism and function were measured in the isolated perfused heart using radiolabelled substrates. Following chronic hypoxia, both control and diabetic hearts upregulated glycolysis, lactate efflux and glycogen content and decreased fatty acid oxidation rates, with similar activation of HIF signalling pathways. However, hypoxia-induced changes were superimposed on diabetic hearts that were metabolically abnormal in normoxia, resulting in glycolytic rates 30% lower, and fatty acid oxidation 36% higher, in hypoxic diabetic hearts than hypoxic controls. Peroxisome proliferator-activated receptor α target proteins were suppressed by hypoxia, but activated by diabetes. Mitochondrial respiration in diabetic hearts was divergently activated following hypoxia compared with controls. These differences in metabolism were associated with decreased contractile recovery of the hypoxic diabetic heart following an acute hypoxic insult. In conclusion, type 2 diabetic hearts retain metabolic flexibility to adapt to hypoxia, with normal HIF signalling pathways. However, they are more dependent on oxidative metabolism following hypoxia due to abnormal normoxic metabolism, which was associated with a functional deficit in response to stress.

Endurance training prevents negative effects of the hypoxia mimetic dimethyloxalylglycine on cardiac and skeletal muscle function

Hypoxic preconditioning is a promising strategy to prevent hypoxia-induced damages to several tissues. This effect is related to prior stabilization of the hypoxia-inducible factor-1α via inhibition of the prolyl-hydroxylases (PHDs), which are responsible for its degradation under normoxia. Although PHD inhibition has been shown to increase endurance performance in rodents, potential side effects of such a therapy have not been explored. Here, we investigated the effects of 1 wk of dimethyloxalylglycine (DMOG) treatment (150 mg/kg) on exercise capacity, as well as on cardiac and skeletal muscle function in sedentary and endurance-trained rats. DMOG improved maximal aerobic velocity and endurance in both sedentary and trained rats. This effect was associated with an increase in red blood cells without significant alteration of skeletal muscle contractile properties. In sedentary rats, DMOG treatment resulted in enhanced left ventricle (LV) weight together with impairment in diastolic function, LV relaxation, and pulse pressure. Moreover, DMOG decreased maximal oxygen uptake (state 3) of isolated mitochondria from skeletal muscle. Importantly, endurance training reversed the negative effects of DMOG treatment on cardiac function and restored maximal mitochondrial oxygen uptake to the level of sedentary placebo-treated rats. In conclusion, we provide here evidence that the PHD inhibitor DMOG has detrimental influence on myocardial and mitochondrial function in healthy rats. However, one may suppose that the deleterious influence of PHD inhibition would be potentiated in patients with already poor physical condition. Therefore, the present results prompt us to take into consideration the potential side effects of PHD inhibitors when administrated to patients.

HIF-1-driven skeletal muscle adaptations to chronic hypoxia: molecular insights into muscle physiology

Skeletal muscle is a metabolically active tissue and the major body protein reservoir. Drop in ambient oxygen pressure likely results in a decrease in muscle cells oxygenation, reactive oxygen species (ROS) overproduction and stabilization of the oxygen-sensitive hypoxia-inducible factor (HIF)-1α. However, skeletal muscle seems to be quite resistant to hypoxia compared to other organs, probably because it is accustomed to hypoxic episodes during physical exercise. Few studies have observed HIF-1α accumulation in skeletal muscle during ambient hypoxia probably because of its transient stabilization. Nevertheless, skeletal muscle presents adaptations to hypoxia that fit with HIF-1 activation, although the exact contribution of HIF-2, I kappa B kinase and activating transcription factors, all potentially activated by hypoxia, needs to be determined. Metabolic alterations result in the inhibition of fatty acid oxidation, while activation of anaerobic glycolysis is less evident. Hypoxia causes mitochondrial remodeling and enhanced mitophagy that ultimately lead to a decrease in ROS production, and this acclimatization in turn contributes to HIF-1α destabilization. Likewise, hypoxia has structural consequences with muscle fiber atrophy due to mTOR-dependent inhibition of protein synthesis and transient activation of proteolysis. The decrease in muscle fiber area improves oxygen diffusion into muscle cells, while inhibition of protein synthesis, an ATP-consuming process, and reduction in muscle mass decreases energy demand. Amino acids released from muscle cells may also have protective and metabolic effects. Collectively, these results demonstrate that skeletal muscle copes with the energetic challenge imposed by O2 rarefaction via metabolic optimization.

Adipose differentiation-related protein is not involved in hypoxia inducible factor-1-induced lipid accumulation under hypoxia

Increasing evidence has showed that hypoxia inducible factor-1 (HIF1) has an important role in hypoxia-induced lipid accumulation, a common feature of solid tumors; however, its role remains to be fully elucidated. Adipose differentiation-related protein (ADRP), a structural protein of lipid droplets, is found to be upregulated under hypoxic conditions. In the present study, an MCF7 breast cancer cell line was used to study the role of ADRP in hypoxia-induced lipid accumulation. It was demonstrated that hypoxia induced the gene expression of ADRP in a HIF1-dependent manner. Increases in the mRNA and protein levels of ADRP was accompanied by increased HIF1A activity. In addition, a significant decrease in the mRNA and protein levels of ADRP were detected in presence of siRNA targeting HIF1A. Using a dual-luciferase reporting experiment and chromatin immunoprecipitation assay, the present study demonstrated that ADRP is a direct target gene of HIF1, and identified a functional hypoxia response element localized 33 bp upstream of the transcriptional start site of the ADRP gene. Furthermore, the present study demonstrated the role of ADRP in low density liporotein (LDL) and very-LDL uptake-induced lipid accumulation under hypoxia. The knockdown of ADRP did not reduce HIF1-induced lipid accumulation under hypoxia. Together, these results showed that ADRP may be not involved in HIF1-induced lipid accumulation.

SREBP maintains lipid biosynthesis and viability of cancer cells under lipid- and oxygen-deprived conditions and defines a gene signature associated with poor survival in glioblastoma multiforme

Oxygen and nutrient limitation are common features of the tumor microenvironment and are associated with cancer progression and induction of metastasis. The inefficient vascularization of tumor tissue also limits the penetration of other serum-derived factors, such as lipids and lipoproteins, which can be rate limiting for cell proliferation and survival. Here we have investigated the effect of hypoxia and serum deprivation on sterol regulatory element-binding protein (SREBP) activity and the expression of lipid metabolism genes in human glioblastoma multiforme (GBM) cancer cells. We found that SREBP transcriptional activity was induced by serum depletion both in normoxic and hypoxic cells and that activation of SREBP was required to maintain the expression of fatty acid and cholesterol metabolism genes under hypoxic conditions. Moreover, expression of stearoyl-CoA desaturase, the enzyme required for the generation of mono-unsaturated fatty acids, and fatty acid-binding protein 7, a regulator of glioma stem cell function, was strongly dependent on SREBP function. Inhibition of SREBP function blocked lipid biosynthesis in hypoxic cancer cells and impaired cell survival under hypoxia and in a three-dimensional spheroid model. Finally, gene expression analysis revealed that SREBP defines a gene signature that is associated with poor survival in glioblastoma.

Altered Oxygen Utilisation in Rat Left Ventricle and Soleus after 14 Days, but Not 2 Days, of Environmental Hypoxia

The effects of environmental hypoxia on cardiac and skeletal muscle metabolism are dependent on the duration and severity of hypoxic exposure, though factors which dictate the nature of the metabolic response to hypoxia are poorly understood. We therefore set out to investigate the time-dependence of metabolic acclimatisation to hypoxia in rat cardiac and skeletal muscle. Rats were housed under normoxic conditions, or exposed to short-term (2 d) or sustained (14 d) hypoxia (10% O₂), after which samples were obtained from the left ventricle of the heart and the soleus for assessment of metabolic regulation and mitochondrial function. Mass-corrected maximal oxidative phosphorylation was 20% lower in the left ventricle following sustained but not short-term hypoxia, though no change was observed in the soleus. After sustained hypoxia, the ratio of octanoyl carnitine- to pyruvate- supported respiration was 11% and 12% lower in the left ventricle and soleus, respectively, whilst hexokinase activity increased by 33% and 2.1-fold in these tissues. mRNA levels of PPARα targets fell after sustained hypoxia in both tissues, but those of PPARα remained unchanged. Despite decreased Ucp3 expression after short-term hypoxia, UCP3 protein levels and mitochondrial coupling remained unchanged. Protein carbonylation was 40% higher after short-term but not sustained hypoxic exposure in the left ventricle, but was unchanged in the soleus at both timepoints. Our findings therefore demonstrate that 14 days, but not 2 days, of hypoxia induces a loss of oxidative capacity in the left ventricle but not the soleus, and a substrate switch away from fatty acid oxidation in both tissues.

Modulation of membrane lipid composition and homeostasis in salmon hepatocytes exposed to hypoxia and perfluorooctane sulfonamide, given singly or in combination

The relative importance of environmental hypoxia due to global climate change on organismal ability to adapt to chemical insult and/or mechanisms of these responses is not well understood. Therefore, we have studied the effects of combined exposure to perfluorooctane sulfonamide (PFOSA) and chemically induced hypoxia on membrane lipid profile and homeostasis. Primary salmon hepatocytes were exposed to PFOSA at 0, 25 and 50 µM singly or in combination with either cobalt chloride (CoCl2: 0 and 150 µM) or deferroxamine (DFO: 0 and 100 µM) for 24 and 48 h. CoCl2 and DFO were used to induce cellular hypoxia because these two chemicals have been commonly used in animal experiments for this purpose and have been shown to increase hypoxia-inducible factor 1-alpha (HIF-1α) and vascular endothelial growth factor (VEGF) levels. Fatty acid (FA) profiles were determined by GC-MS, while gene expression patterns were determined by quantitative PCR. Hypoxic condition was confirmed with time-related increases of HIF-1α mRNA levels in CoCl2 and DFO exposed cells. In general, significant alterations of genes involved in lipid homeostasis were predominantly observed after 48 h exposure. Gene expression analysis showed that biological responses related to peroxisome proliferation (peroxisome proliferator-activated receptors (PPARs) and acyl coenzyme A (ACOX)) and FA desaturation (Δ5- and Δ6-desaturases: FAD5 and FAD6, respectively) and elongation (FAE) were elevated slightly by single exposure (i.e. either PFOSA, CoCl2 or DFO exposure alone), and these responses were potentiated in combined exposure conditions. Principal component analysis (PCA) showed a clustering of peroxisome proliferation responses at transcript levels and FA desaturation against membrane FAs levels whose changes were explained by PFOSA and chemically induced hypoxia exposures. Overall, our data show that most of the observed responses were stronger in combined stressor exposure conditions, compared to individual stressor exposure. In general, our data show that hypoxia may, singly or in combination with PFOSA produce deleterious health, physiological and developmental consequences through the alteration of membrane lipid profile in organisms.

HIF-1α and HIF-2α are critically involved in hypoxia-induced lipid accumulation in hepatocytes through reducing PGC-1α-mediated fatty acid β-oxidation

During periods of cellular hypoxia, hepatocytes adapt to consume less oxygen by shifting energy production from mitochondrial fatty acid β-oxidation to glycolysis. One of the earliest responses to pathologic hypoxia is the activation of the hypoxia-inducible factor (HIF). In the present study, we examined whether HIF-1 and HIF-2 were involved in the regulation of fatty acid synthesis and β-oxidation. We showed that hypoxia induced fat accumulation in the livers of mice and in HepG2 cells. These hypoxia-induced changes in fatty acid metabolism were mediated by suppressing fatty acid β-oxidation, without significantly influencing fatty acid synthesis. Exposing hepatocytes to 1% O2 reduced the mRNA expression of carnitine palmitoyltransferase 1 (CPT-1), which catalyzes the rate-limiting step in the mitochondrial import of fatty acids for β-oxidation. Moreover, hypoxia exposure reduced proliferator-activated receptor-γ coactivator-1α (PGC-1α) protein levels, which plays an important role in regulation of β-oxidation. Exposure of HIF-1α or HIF-2α deficient hepatocytes to hypoxia abrogated the reduction in PGC-1α and CPT-1 expression and cellular lipid accumulation observed in normal hepatocytes exposed to hypoxia. These results suggest that both HIF-1α and HIF-2α are involved in hypoxia-induced lipid accumulation in hepatocytes via reducing PGC-1α mediated fatty acid β-oxidation.

Interactions of PPAR-alpha and adenosine receptors in hypoxia-induced angiogenesis

Hypoxia and adenosine are known to upregulate angiogenesis; however, the role of peroxisome proliferator-activated receptor alpha (PPARα) in angiogenesis is controversial. Using transgenic Tg(fli-1:EGFP) zebrafish embryos, interactions of PPARα and adenosine receptors in angiogenesis were evaluated under hypoxic conditions. Epifluorescent microscopy was used to assess angiogenesis by counting the number of intersegmental (ISV) and dorsal longitudinal anastomotic vessel (DLAV) at 28 h post-fertilization (hpf). Hypoxia (6h) stimulated angiogenesis as the number of ISV and DLAV increased by 18-fold (p<0.01) and 100 ± 8% (p<0.001), respectively, at 28 hpf. Under normoxic and hypoxic conditions, WY-14643 (10 μM), a PPARα activator, stimulated angiogenesis at 28 hpf, while MK-886 (0.5 μM), an antagonist of PPARα, attenuated these effects. Compared to normoxic condition, adenosine receptor activation with NECA (10 μM) promoted angiogenesis more effectively under hypoxic conditions. Involvement of A2B receptor was implied in hypoxia-induced angiogenesis as MRS-1706 (10nM), a selective A2B antagonist attenuated NECA (10 μM)-induced angiogenesis. NECA- or WY-14643-induced angiogenesis was also inhibited by miconazole (0.1 μM), an inhibitor of epoxygenase dependent production of eicosatrienoic acid (EET) epoxide. Thus, we conclude that: activation of PPARα promoted angiogenesis just as activation of A2B receptors through an epoxide dependent mechanism.

Broad suppression of NADPH oxidase activity exacerbates ischemia/reperfusion injury through inadvertent downregulation of hypoxia-inducible factor-1α and upregulation of peroxisome proliferator-activated receptor-α

RATIONALE

NADPH oxidase (Nox) 2 and Nox4 are major components of the Nox family which purposefully produce reactive oxidative species, namely O2(-) and H2O2, in the heart. The isoform-specific contribution of Nox2 and Nox4 to ischemia/reperfusion (I/R) injury is poorly understood.

OBJECTIVE

We investigated the role of Nox2 and Nox4 in mediating oxidative stress and myocardial injury during I/R using loss-of-function mouse models.

METHODS AND RESULTS

Systemic (s) Nox2 knockout (KO), sNox4 KO, and cardiac-specific (c) Nox4 KO mice were subjected to I/R (30 minutes/24 hours, respectively). Both myocardial infarct size/area at risk and O2(-) production were lower in sNox2 KO, sNox4 KO, and cNox4 KO than in wild-type mice. Unexpectedly, however, the myocardial infarct size/area at risk was greater, despite less O2(-) production, in sNox2 KO+cNox4 KO (double-KO) mice and transgenic mice (Tg) with cardiac-specific expression of dominant-negative Nox, which suppresses both Nox2 and Nox4, than in wild-type or single KO mice. Hypoxia-inducible factor-1α was downregulated whereas peroxisome proliferator-activated receptor-α was upregulated in Tg-dominant-negative Nox mice. A cross with mice deficient in prolyl hydroxylase 2, which hydroxylates hypoxia-inducible factor-1α, rescued the I/R injury and prevented upregulation of peroxisome proliferator-activated receptor-α in Tg-dominant-negative Nox mice. A cross with peroxisome proliferator-activated receptor-α KO mice also attenuated the injury in Tg- dominant-negative Nox mice.

CONCLUSIONS

Both Nox2 and Nox4 contribute to the increase in reactive oxidative species and injury by I/R. However, low levels of reactive oxidative species produced by either Nox2 or Nox4 regulate hypoxia-inducible factor-1α and peroxisome proliferator-activated receptor-α, thereby protecting the heart against I/R, suggesting that Noxs also act as a physiological sensor for myocardial adaptation.

Activation of peroxisome proliferator-activated receptor α (PPARα) suppresses hypoxia-inducible factor-1α (HIF-1α) signaling in cancer cells

Activation of peroxisome proliferator-activated receptor α (PPARα) has been demonstrated to inhibit tumor growth and angiogenesis, yet the mechanisms behind these actions remain to be characterized. In this study, we examined the effects of PPARα activation on the hypoxia-inducible factor-1α (HIF-1α) signaling pathway in human breast (MCF-7) and ovarian (A2780) cancer cells under hypoxia. Incubation of cancer cells under 1% oxygen for 16 h significantly induced HIF-1α expression and activity as assayed by Western blotting and reporter gene analysis. Treatment of the cells with PPARα agonists, but not a PPARγ agonist, prior to hypoxia diminished hypoxia-induced HIF-1α expression and activity, and addition of a PPARα antagonist attenuated the suppression of HIF-1α signaling. Activation of PPARα attenuated hypoxia-induced HA-tagged HIF-1α protein expression without affecting the HA-tagged HIF-1α mutant protein level, indicating that PPARα activation promotes HIF-1α degradation in these cells. This was further confirmed using proteasome inhibitors, which reversed PPARα-mediated suppression of HIF-1α expression under hypoxia. Using the co-immunoprecipitation technique, we found that activation of PPARα enhances the binding of HIF-1α to von Hippel-Lindau tumor suppressor (pVHL), a protein known to mediate HIF-1α degradation through the ubiquitin-proteasome pathway. Following PPARα-mediated suppression of HIF-1α signaling, VEGF secretion from the cancer cells was significantly reduced, and tube formation by endothelial cells was dramatically impaired. Taken together, these findings demonstrate for the first time that activation of PPARα suppresses hypoxia-induced HIF-1α signaling in cancer cells, providing novel insight into the anticancer properties of PPARα agonists.

Hypoxia and hypoxia inducible factors: diverse roles in liver diseases

Hypoxia has been shown to have a role in the pathogenesis of several forms of liver disease. The hypoxia inducible factors (HIFs) are a family of evolutionarily conserved transcriptional regulators that affect a homeostatic response to low oxygen tension and have been identified as key mediators of angiogenesis, inflammation, and metabolism. In this review we summarize the evidence for a role of HIFs across a range of hepatic pathophysiology. We describe regulation of the HIFs and review investigations that demonstrate a role for HIFs in the development of liver fibrosis, activation of innate immune pathways, hepatocellular carcinoma, as well as other liver diseases in both human disease as well as murine models.

Cardiac hypertrophy in the newborn delays the maturation of fatty acid β-oxidation and compromises postischemic functional recovery

During the neonatal period, cardiac energy metabolism progresses from a fetal glycolytic profile towards one more dependent on mitochondrial oxidative metabolism. In this study, we identified the effects of cardiac hypertrophy on neonatal cardiac metabolic maturation and its impact on neonatal postischemic functional recovery. Seven-day-old rabbits were subjected to either a sham or a surgical procedure to induce a left-to-right shunt via an aortocaval fistula to cause RV volume-overload. At 3 wk of age, hearts were isolated from both groups and perfused as isolated, biventricular preparations to assess cardiac energy metabolism. Volume-overload resulted in cardiac hypertrophy (16% increase in cardiac mass, P < 0.05) without evidence of cardiac dysfunction in vivo or in vitro. Fatty acid oxidation rates were 60% lower (P < 0.05) in hypertrophied hearts than controls, whereas glycolysis increased 246% (P < 0.05). In contrast, glucose and lactate oxidation rates were unchanged. Overall ATP production rates were significantly lower in hypertrophied hearts, resulting in increased AMP-to-ATP ratios in both aerobic hearts and ischemia-reperfused hearts. The lowered energy generation of hypertrophied hearts depressed functional recovery from ischemia. Decreased fatty acid oxidation rates were accompanied by increased malonyl-CoA levels due to decreased malonyl-CoA decarboxylase activity/expression. Increased glycolysis in hypertrophied hearts was accompanied by a significant increase in hypoxia-inducible factor-1α expression, a key transcriptional regulator of glycolysis. Cardiac hypertrophy in the neonatal heart results in a reemergence of the fetal metabolic profile, which compromises ATP production in the rapidly maturing heart and impairs recovery of function following ischemia.

PPARα is down-regulated following liver transplantation in mice

BACKGROUND & AIMS

Graft dysfunction is one of the major complications after liver transplantation, but its precise mechanism remains unclear. Since steatotic liver grafts are susceptible to post-transplant dysfunction, and peroxisome proliferator-activated receptor (PPAR) α plays an important role in the maintenance of hepatic lipid homeostasis, we examined the role of PPARα in liver transplantation.

METHODS

Livers were harvested from Sv/129 wild-type (Ppara(+/+)) mice and PPARα-null (Ppara(-/-)) mice and transplanted orthotopically into syngeneic Ppara(+/+) mice.

RESULTS

Hepatocellular damage was unexpectedly milder in transplanted Ppara(-/-) livers compared with Ppara(+/+) ones. This was likely due to decreased lipid peroxides in the Ppara(-/-) livers, as revealed by the lower levels of fatty acid oxidation (FAO) enzymes, which are major sources of reactive oxygen species. Hepatic PPARα and its target genes, such as FAO enzymes and pyruvate dehydrogenase kinase 4, were strongly down-regulated after transplantation, which was associated with increases in hepatic tumor necrosis factor-α expression and nuclear factor-κB activity. Inhibiting post-transplant PPARα down-regulation by clofibrate treatment markedly augmented oxidative stress and hepatocellular injury.

CONCLUSIONS

Down-regulation of PPARα seemed to be an adaptive response to metabolic alterations following liver transplantation. These results provide novel information to the understanding of the pathogenesis of early post-transplant events.

Hypoxia-inducible factor-1 (HIF-1) promotes LDL and VLDL uptake through inducing VLDLR under hypoxia

Metabolism under hypoxia is significantly different from that under normoxia. It has been well elucidated that HIF-1 (hypoxia-inducible factor-1) plays a central role in regulating glucose metabolism under hypoxia; however, the role of HIF-1 in lipid metabolism has not yet been well addressed. In the present study we demonstrate that HIF-1 promotes LDL (low-density lipoprotein) and VLDL (very-LDL) uptake through regulation of VLDLR (VLDL receptor) gene expression under hypoxia. Increased VLDLR mRNA and protein levels were observed under hypoxic or DFO (deferoxamine mesylate salt) treatment in MCF7, HepG2 and HeLa cells. Using dual-luciferase reporter and ChIP (chromatin immunoprecipitation) assays we confirmed a functional HRE (hypoxia-response element) which is localized at +405 in exon 1 of the VLDLR gene. Knockdown of HIF1A (the α subunit of HIF-1) and VLDLR, but not HIF2A (the α subunit of HIF-2), attenuated hypoxia-induced lipid accumulation through affecting LDL and VLDL uptake. Additionally we also observed a correlation between HIF-1 activity and VLDLR expression in hepatocellular carcinoma specimens. The results of the present study suggest that HIF-1-mediated VLDLR induction influences intracellular lipid accumulation through regulating LDL and VLDL uptake under hypoxia.

Hypoxia and hypoxia inducible factors: diverse roles in liver diseases

Hypoxia has been shown to have a role in the pathogenesis of several forms of liver disease. The hypoxia inducible factors (HIFs) are a family of evolutionarily conserved transcriptional regulators that affect a homeostatic response to low oxygen tension and have been identified as key mediators of angiogenesis, inflammation, and metabolism. In this review we summarize the evidence for a role of HIFs across a range of hepatic pathophysiology. We describe regulation of the HIFs and review investigations that demonstrate a role for HIFs in the development of liver fibrosis, activation of innate immune pathways, hepatocellular carcinoma, as well as other liver diseases in both human disease as well as murine models.