Prolonged Exposures to Intermittent Hypoxia Promote Visceral White Adipose Tissue Inflammation in a Murine Model of Severe Sleep Apnea: Effect of Normoxic Recovery

Study Objective

Increased visceral white adipose tissue (vWAT) mass results in infiltration of inflammatory macrophages that drive inflammation and insulin resistance. Patients with obstructive sleep apnea (OSA) suffer from increased prevalence of obesity, insulin resistance, and metabolic syndrome. Murine models of intermittent hypoxia (IH) mimicking moderate-severe OSA manifest insulin resistance following short-term IH. We examined in mice the effect of long-term IH on the inflammatory cellular changes within vWAT and the potential effect of normoxic recovery (IH-R).

Methods

Male C57BL/6J mice were subjected to IH for 20 weeks, and a subset was allowed to recover in room air (RA) for 6 or 12 weeks (IH-R). Stromal vascular fraction was isolated from epididymal vWAT and mesenteric vWAT depots, and single-cell suspensions were prepared for flow cytometry analyses, reactive oxygen species (ROS), and metabolic assays.

Results

IH reduced body weight and vWAT mass and IH-R resulted in catch-up weight and vWAT mass. IH-exposed vWAT exhibited increased macrophage counts (ATMs) that were only partially improved in IH-R. IH also caused a proinflammatory shift in ATMs (increased Ly6c(hi)(+) and CD36(+) ATMs). These changes were accompanied by increased vWAT insulin resistance with only partial improvements in IH-R. In addition, ATMs exhibited increased ROS production, altered metabolism, and changes in electron transport chain, which were only partially improved in IH-R.

Conclusion

Prolonged exposures to IH during the sleep period induce pronounced vWAT inflammation and insulin resistance despite concomitant vWAT mass reductions. These changes are only partially reversible after 3 months of normoxic recovery. Thus, long-lasting OSA may preclude complete reversibility of metabolic changes.

HIF-1α is required for disturbed flow-induced metabolic reprogramming in human and porcine vascular endothelium

Hemodynamic forces regulate vascular functions. Disturbed flow (DF) occurs in arterial bifurcations and curvatures, activates endothelial cells (ECs), and results in vascular inflammation and ultimately atherosclerosis. However, how DF alters EC metabolism, and whether resulting metabolic changes induce EC activation, is unknown. Using transcriptomics and bioenergetic analysis, we discovered that DF induces glycolysis and reduces mitochondrial respiratory capacity in human aortic ECs. DF-induced metabolic reprogramming required hypoxia inducible factor-1α (HIF-1α), downstream of NAD(P)H oxidase-4 (NOX4)-derived reactive oxygen species (ROS). HIF-1α increased glycolytic enzymes and pyruvate dehydrogenase kinase-1 (PDK-1), which reduces mitochondrial respiratory capacity. Swine aortic arch endothelia exhibited elevated ROS, NOX4, HIF-1α, and glycolytic enzyme and PDK1 expression, suggesting that DF leads to metabolic reprogramming in vivo. Inhibition of glycolysis reduced inflammation suggesting a causal relationship between flow-induced metabolic changes and EC activation. These findings highlight a previously uncharacterized role for flow-induced metabolic reprogramming and inflammation in ECs.

PFKFB3, a Direct Target of p63, Is Required for Proliferation and Inhibits Differentiation in Epidermal Keratinocytes

p63 is a transcription factor essential for epidermal development and homeostasis. p63 is a member of the p53 family of transcription factors, which are increasingly understood to be regulators of cellular metabolism. How p63 regulates metabolism in epidermal keratinocytes is incompletely understood, and it is unknown whether glycolytic regulation is essential to maintain the balance between proliferation and differentiation within the epidermis. We found that p63 promotes glycolytic metabolism in epidermal keratinocytes. p63 bound to consensus sites within the PFKFB3 gene and was required for PFKFB3 mRNA and protein expression. PFKFB3 overexpression inhibited differentiation of keratinocytes, whereas knockdown inhibited proliferation and increased the rate of differentiation. Furthermore, we found that PFKFB3 was highly expressed in psoriatic epidermis. Our results show that PFKFB3 is a key regulator of epidermal homeostasis and may represent a therapeutic target for epidermal diseases associated with hyperproliferation and impaired differentiation.

Downregulation of TGF-β Receptor-2 Expression and Signaling through Inhibition of Na/K-ATPase

Transforming growth factor-beta (TGF-β) is a multi-functional cytokine implicated in the control of cell growth and differentiation. TGF-β signals through a complex of TGF-β receptors 1 and 2 (TGFβR1 and TGFβR2) that phosphorylate and activate Smad2/3 transcription factors driving transcription of the Smad-target genes. The Na+/K+-ATPase is an integral plasma membrane protein critical for maintaining the electro-chemical gradient of Na+ and K+ in the cell. We found that inhibition of the Na+/K+ ATPase by ouabain results in a dramatic decrease in the expression of TGFβR2 in human lung fibrobalsts (HLF) at the mRNA and protein levels. This was accompanied by inhibition of TGF-β-induced Smad phosphorylation and the expression of TGF-β target genes, such as fibronectin and smooth muscle alpha-actin. Inhibition of Na+/K+ ATPase by an alternative approach (removal of extracellular potassium) had a similar effect in HLF. Finally, treatment of lung alveolar epithelial cells (A549) with ouabain also resulted in the downregulation of TGFβR2, the inhibition of TGF-β-induced Smad phosphorylation and of the expression of mesenchymal markers, vimentin and fibronectin. Together, these data demonstrate a critical role of Na+/K+-ATPase in the control of TGFβR2 expression, TGF-β signaling and cell responses to TGF-β.

Transforming Growth Factor (TGF)-β Promotes de Novo Serine Synthesis for Collagen Production

TGF-β promotes excessive collagen deposition in fibrotic diseases such as idiopathic pulmonary fibrosis (IPF). The amino acid composition of collagen is unique due to its high (33%) glycine content. Here, we report that TGF-β induces expression of glycolytic genes and increases glycolytic flux. TGF-β also induces the expression of the enzymes of the de novo serine synthesis pathway (phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase 1 (PSAT1), and phosphoserine phosphatase (PSPH)) and de novo glycine synthesis (serine hydroxymethyltransferase 2 (SHMT2)). Studies in fibroblasts with genetic attenuation of PHGDH or SHMT2 and pharmacologic inhibition of PHGDH showed that these enzymes are required for collagen synthesis. Furthermore, metabolic labeling experiments demonstrated carbon from glucose incorporated into collagen. Lungs from humans with IPF demonstrated increased expression of PHGDH and SHMT2. These results indicate that the de novo serine synthesis pathway is necessary for TGF-β-induced collagen production and suggest that this pathway may be a therapeutic target for treatment of fibrotic diseases including IPF.

SIRT3 blocks myofibroblast differentiation and pulmonary fibrosis by preventing mitochondrial DNA damage

Myofibroblast differentiation is a key process in the pathogenesis of fibrotic diseases. Transforming growth factor-β₁ (TGF-β₁) is a powerful inducer of myofibroblast differentiation and is implicated in pathogenesis of tissue fibrosis. This study was undertaken to determine the role of mitochondrial deacetylase SIRT3 in TGF-β₁-induced myofibroblast differentiation in vitro and lung fibrosis in vivo. Treatment of human lung fibroblasts with TGF-β₁ resulted in increased expression of fibrosis markers, smooth muscle α-actin (α-SMA), collagen-1, and fibronectin. TGF-β₁ treatment also caused depletion of endogenous SIRT3, which paralleled with increased production of reactive oxygen species (ROS), DNA damage, and subsequent reduction in levels of 8-oxoguanine DNA glycosylase (OGG1), an enzyme that hydrolyzes oxidized guanine (8-oxo-dG) and thus protects DNA from oxidative damage. Overexpression of SIRT3 by adenovirus-mediated transduction reversed the effects of TGF-β₁ on ROS production and mitochondrial DNA damage and inhibited TGF-β₁-induced myofibroblast differentiation. To determine the antifibrotic role of SIRT3 in vivo, we used the bleomycin-induced mouse model of pulmonary fibrosis. Compared with wild-type controls, Sirt3-knockout mice showed exacerbated fibrosis after intratracheal instillation of bleomycin. Increased lung fibrosis was associated with decreased levels of OGG1 and concomitant accumulation of 8-oxo-dG and increased mitochondrial DNA damage. In contrast, the transgenic mice with whole body Sirt3 overexpression were protected from bleomycin-induced mtDNA damage and development of lung fibrosis. These data demonstrate a critical role of SIRT3 in the control of myofibroblast differentiation and lung fibrosis.

Regulation of myofibroblast differentiation by cardiac glycosides

Myofibroblast differentiation is a key process in pathogenesis of fibrotic diseases. Cardiac glycosides (ouabain, digoxin) inhibit Na(+)-K(+)-ATPase, resulting in increased intracellular [Na(+)]-to-[K(+)] ratio in cells. Microarray analysis suggested that increased intracellular [Na(+)]/[K(+)] ratio may promote the expression of cyclooxygenase-2 (COX-2), a critical enzyme in the synthesis of prostaglandins. Given antifibrotic effects of prostaglandins through activation of protein kinase A (PKA), we examined if cardiac glycosides stimulate COX-2 expression in human lung fibroblasts and how they affect myofibroblast differentiation. Ouabain stimulated a profound COX-2 expression and a sustained PKA activation, which was blocked by COX-2 inhibitor or by COX-2 knockdown. Ouabain-induced COX-2 expression and PKA activation were abolished by the inhibitor of the Na(+)/Ca(2+) exchanger, KB-R4943. Ouabain inhibited transforming growth factor-β (TGF-β)-induced Rho activation, stress fiber formation, serum response factor activation, and the expression of smooth muscle α-actin, collagen-1, and fibronectin. These effects were recapitulated by an increase in intracellular [Na(+)]/[K(+)] ratio through the treatment of cells with K(+)-free medium or with digoxin. Although inhibition of COX-2 or of the Na(+)/Ca(2+) exchanger blocked ouabain-induced PKA activation, this failed to reverse the inhibition of TGF-β-induced Rho activation or myofibroblast differentiation by ouabain. Together, these data demonstrate that ouabain, through the increase in intracellular [Na(+)]/[K(+)] ratio, drives the induction of COX-2 expression and PKA activation, which is accompanied by a decreased Rho activation and myofibroblast differentiation in response to TGF-β. However, COX-2 expression and PKA activation are not sufficient for inhibition of the fibrotic effects of TGF-β by ouabain, suggesting that additional mechanisms must exist.

Wood Smoke Particle Sequesters Cell Iron to Impact a Biological Effect

The biological effect of an inorganic particle (i.e., silica) can be associated with a disruption in cell iron homeostasis. Organic compounds included in particles originating from combustion processes can also complex sources of host cell iron to disrupt metal homeostasis. We tested the postulate that (1) wood smoke particle (WSP) sequesters host cell iron resulting in a disruption of metal homeostasis, (2) this loss of essential metal results in both an oxidative stress and biological effect in respiratory epithelial cells, and (3) humic-like substances (HULIS), a component of WSP, have a capacity to appropriate cell iron and initiate a biological effect. BEAS-2B cells exposed to WSP resulted in diminished concentrations of mitochondrial (57)Fe, whereas preincubation with ferric ammonium citrate (FAC) prevented significant mitochondrial iron loss after such exposure. Cellular oxidant generation was increased after WSP exposure, but this signal was diminished by coincubation with FAC. Similarly, exposure of BEAS-2B cells to 100 μg/mL WSP activated mitogen-activated protein (MAP) kinases, elevated NF-E2-related factor 2/antioxidant responsive element (Nrf2 ARE) expression, and provoked interleukin (IL)-6 and IL-8 release, but all these changes were diminished by coincubation with FAC. The biological response to WSP was reproduced by exposure to 100 μg/mL humic acid, a polyphenol comparable to HULIS included in the WSP that complexes iron. We conclude that (1) the biological response following exposure to WSP is associated with sequestration of cell iron by the particle, (2) increasing available iron in the cell diminished the biological effects after particle exposure, and (3) HULIS included in WSP can sequester the metal initiating the cell response.

Regulation of allergic lung inflammation by endothelial cell transglutaminase 2

Tissue transglutaminase 2 (TG2) is an enzyme with multiple functions, including catalysis of serotonin conjugation to proteins (serotonylation). Previous research indicates that TG2 expression is upregulated in human asthma and in the lung endothelium of ovalbumin (OVA)-challenged mice. It is not known whether endothelial cell TG2 is required for allergic inflammation. Therefore, to determine whether endothelial cell TG2 regulates allergic inflammation, mice with an endothelial cell-specific deletion of TG2 were generated, and these mice were sensitized and challenged in the airways with OVA. Deletion of TG2 in endothelial cells blocked OVA-induced serotonylation in lung endothelial cells, but not lung epithelial cells. Interestingly, deletion of endothelial TG2 reduced allergen-induced increases in respiratory system resistance, number of eosinophils in the bronchoalveolar lavage, and number of eosinophils in the lung tissue. Endothelial cell deletion of TG2 did not alter expression of adhesion molecules, cytokines, or chemokines that regulate leukocyte recruitment, consistent with other studies, demonstrating that deletion of endothelial cell signals does not alter lung cytokines and chemokines during allergic inflammation. Taken together, the data indicate that endothelial cell TG2 is required for allergic inflammation by regulating the recruitment of eosinophils into OVA-challenged lungs. In summary, TG2 functions as a critical signal for allergic lung responses. These data identify potential novel targets for intervention in allergy/asthma.

Asbestos-induced pulmonary fibrosis is augmented in 8-oxoguanine DNA glycosylase knockout mice

Asbestos causes asbestosis and malignancies by mechanisms that are not fully established. Alveolar epithelial cell (AEC) injury and repair are crucial determinants of the fibrogenic potential of noxious agents such as asbestos. We previously showed that mitochondrial reactive oxygen species mediate asbestos-induced AEC intrinsic apoptosis and that mitochondrial human 8-oxoguanine-DNA glycosylase 1 (OGG1), a DNA repair enzyme, prevents oxidant-induced AEC apoptosis. We reasoned that OGG1 deficiency augments asbestos-induced pulmonary fibrosis. Compared with intratracheal instillation of PBS (50 μl) or titanium dioxide (100 μg/50 μl), crocidolite or Libby amphibole asbestos (100 μg/50 μl) each augmented pulmonary fibrosis in wild-type C57BL/6J (WT) mice after 3 weeks as assessed by histology, fibrosis score, lung collagen via Sircol, and type 1 collagen expression; these effects persisted at 2 months. Compared with WT mice, Ogg1 homozygous knockout (Ogg1(-/-)) mice exhibit increased pulmonary fibrosis after crocidolite exposure and apoptosis in cells at the bronchoalveolar duct junctions as assessed via cleaved caspase-3 immunostaining. AEC involvement was verified by colocalization studies using surfactant protein C. Asbestos increased endoplasmic reticulum stress in the lungs of WT and Ogg1(-/-) mice. Compared with WT, alveolar type 2 cells isolated from Ogg1(-/-) mice have increased mtDNA damage, reduced mitochondrial aconitase expression, and increased P53 and cleaved caspase-9 expression, and these changes were enhanced 3 weeks after crocidolite exposure. These findings suggest an important role for AEC mtDNA integrity maintained by OGG1 in the pathogenesis of pulmonary fibrosis that may represent a novel therapeutic target.

Nitric oxide prevents alveolar senescence and emphysema in a mouse model

N^ω-nitro-L-arginine methyl ester (L-NAME) treatment induces arteriosclerosis and vascular senescence. Here, we report that the systemic inhibition of nitric oxide (NO) production by L-NAME causes pulmonary emphysema. L-NAME-treated lungs exhibited both the structural (alveolar tissue destruction) and functional (increased compliance and reduced elastance) characteristics of emphysema development. Furthermore, we found that L-NAME-induced emphysema could be attenuated through both genetic deficiency and pharmacological inhibition of plasminogen activator inhibitor-1 (PAI-1). Because PAI-1 is an important contributor to the development of senescence both in vitro and in vivo, we investigated whether L-NAME-induced senescence led to the observed emphysematous changes. We found that L-NAME treatment was associated with molecular and cellular evidence of premature senescence in mice, and that PAI-1 inhibition attenuated these increases. These findings indicate that NO serves to protect and defend lung tissue from physiological aging.

Nonclassical Ly6C(-) monocytes drive the development of inflammatory arthritis in mice

Different subsets and/or polarized phenotypes of monocytes and macrophages may play distinct roles during the development and resolution of inflammation. Here, we demonstrate in a murine model of rheumatoid arthritis that nonclassical Ly6C(-) monocytes are required for the initiation and progression of sterile joint inflammation. Moreover, nonclassical Ly6C(-) monocytes differentiate into inflammatory macrophages (M1), which drive disease pathogenesis and display plasticity during the resolution phase. During the development of arthritis, these cells polarize toward an alternatively activated phenotype (M2), promoting the resolution of joint inflammation. The influx of Ly6C(-) monocytes and their subsequent classical and then alternative activation occurs without changes in synovial tissue-resident macrophages, which express markers of M2 polarization throughout the course of the arthritis and attenuate joint inflammation during the initiation phase. These data suggest that circulating Ly6C(-) monocytes recruited to the joint upon injury orchestrate the development and resolution of autoimmune joint inflammation.

Wnt coreceptor Lrp5 is a driver of idiopathic pulmonary fibrosis

RATIONALE

Wnt/β-catenin signaling has been implicated in lung fibrosis, but how this occurs and whether expression changes in Wnt pathway components predict disease progression is unknown.

OBJECTIVES

To determine whether the Wnt coreceptor Lrp5 drives pulmonary fibrosis in mice and is predictive of disease severity in humans.

METHODS

We examined mice with impaired Wnt signaling caused by loss of the Wnt coreceptor Lrp5 in models of lung fibrosis induced by bleomycin or an adenovirus encoding an active form of transforming growth factor (TGF)-β. We also analyzed gene expression in peripheral blood mononuclear cells (PBMC) from patients with idiopathic pulmonary fibrosis (IPF).

MEASUREMENTS AND MAIN RESULTS

In patients with IPF, analysis of peripheral blood mononuclear cells revealed that elevation of positive regulators, Lrp5 and 6, was independently associated with disease progression. LRP5 was also associated with disease severity at presentation in an additional cohort of patients with IPF. Lrp5 null mice were protected against bleomycin-induced pulmonary fibrosis, an effect that was phenocopied by direct inhibition of β-catenin signaling by the small molecular inhibitor of β-catenin responsive transcription. Transplantation of Lrp5 null bone marrow cells into wild-type mice did not limit fibrosis. Instead, Lrp5 loss was associated with reduced TGF-β production by alveolar type 2 cells and leukocytes. Consistent with a role of Lrp5 in the activation of TGF-β, Lrp5 null mice were not protected against lung fibrosis induced by TGF-β.

CONCLUSIONS

We show that the Wnt coreceptor, Lrp5, is a genetic driver of lung fibrosis in mice and a marker of disease progression and severity in humans with IPF. Evidence that TGF-β signaling can override a loss in Lrp5 has implications for patient selection and timing of Wnt pathway inhibitors in lung fibrosis.

β₂-Adrenergic agonists augment air pollution-induced IL-6 release and thrombosis

Acute exposure to particulate matter (PM) air pollution causes thrombotic cardiovascular events, leading to increased mortality rates; however, the link between PM and cardiovascular dysfunction is not completely understood. We have previously shown that the release of IL-6 from alveolar macrophages is required for a prothrombotic state and acceleration of thrombosis following exposure to PM. Here, we determined that PM exposure results in the systemic release of catecholamines, which engage the β2-adrenergic receptor (β2AR) on murine alveolar macrophages and augment the release of IL-6. In mice, β2AR signaling promoted the development of a prothrombotic state that was sufficient to accelerate arterial thrombosis. In primary human alveolar macrophages, administration of a β2AR agonist augmented IL-6 release, while the addition of a beta blocker inhibited PM-induced IL-6 release. Genetic loss or pharmacologic inhibition of the β2AR on murine alveolar macrophages attenuated PM-induced IL-6 release and prothrombotic state. Furthermore, exogenous β2AR agonist therapy further augmented these responses in alveolar macrophages through generation of mitochondrial ROS and subsequent increase of adenylyl cyclase activity. Together, these results link the activation of the sympathetic nervous system by β2AR signaling with metabolism, lung inflammation, and an enhanced susceptibility to thrombotic cardiovascular events.

Flow cytometric analysis of macrophages and dendritic cell subsets in the mouse lung

The lung hosts multiple populations of macrophages and dendritic cells, which play a crucial role in lung pathology. The accurate identification and enumeration of these subsets are essential for understanding their role in lung pathology. Flow cytometry is a mainstream tool for studying the immune system. However, a systematic flow cytometric approach to identify subsets of macrophages and dendritic cells (DCs) accurately and consistently in the normal mouse lung has not been described. Here we developed a panel of surface markers and an analysis strategy that accurately identify all known populations of macrophages and DCs, and their precursors in the lung during steady-state conditions and bleomycin-induced injury. Using this panel, we assessed the polarization of lung macrophages during the course of bleomycin-induced lung injury. Alveolar macrophages expressed markers of alternatively activated macrophages during both acute and fibrotic phases of bleomycin-induced lung injury, whereas markers of classically activated macrophages were expressed only during the acute phase. Taken together, these data suggest that this flow cytometric panel is very helpful in identifying macrophage and DC populations and their state of activation in normal, injured, and fibrotic lungs.

Sequestration of mitochondrial iron by silica particle initiates a biological effect

Inhalation of particulate matter has presented a challenge to human health for thousands of years. The underlying mechanism for biological effect following particle exposure is incompletely understood. We tested the postulate that particle sequestration of cell and mitochondrial iron is a pivotal event mediating oxidant generation and biological effect. In vitro exposure of human bronchial epithelial cells to silica reduced intracellular iron, which resulted in increases in both the importer divalent metal transporter 1 expression and metal uptake. Diminished mitochondrial 57Fe concentrations following silica exposure confirmed particle sequestration of cell iron. Preincubation of cells with excess ferric ammonium citrate increased cell, nuclear, and mitochondrial metal concentrations and prevented significant iron loss from mitochondria following silica exposure. Cell and mitochondrial oxidant generation increased after silica incubation, but pretreatment with iron diminished this generation of reactive oxygen species. Silica exposure activated MAP kinases (ERK and p38) and altered the expression of transcription factors (nF-κB and NF-E2-related factor 2), proinflammatory cytokines (interleukin-8 and -6), and apoptotic proteins. All of these changes in indexes of biological effect were either diminished or inhibited by cell pretreatment with iron. Finally, percentage of neutrophils and total protein concentrations in an animal model instilled with silica were decreased by concurrent exposure to iron. We conclude that an initiating event in the response to particulate matter is a sequestration of cell and mitochondrial iron by endocytosed particle. The resultant oxidative stress and biological response after particle exposure are either diminished or inhibited by increasing the cell iron concentration.

Balancing the risks and benefits of oxygen therapy in critically III adults

Oxygen therapy is an integral part of the treatment of critically ill patients. Maintenance of adequate oxygen delivery to vital organs often requires the administration of supplemental oxygen, sometimes at high concentrations. Although oxygen therapy is lifesaving, it may be associated with deleterious effects when administered for prolonged periods at high concentrations. Here, we review the recent advances in our understanding of the molecular responses to hypoxia and high levels of oxygen and review the current guidelines for oxygen therapy in critically ill patients.

Suppression of inflammation and acute lung injury by Miz1 via repression of C/EBP-δ

Inflammation is essential for host defense but can cause tissue damage and organ failure if unchecked. How the inflammation is resolved remains elusive. Here we report that the transcription factor Miz1 was required for terminating lipopolysaccharide (LPS)-induced inflammation. Genetic disruption of the Miz1 POZ domain, which is essential for its transactivation or repression activity, resulted in hyper-inflammation, lung injury and increased mortality in LPS-treated mice while reduced bacterial load and mortality in mice with Pseudomonas aeruginosa pneumonia. Loss of the Miz1 POZ domain prolonged pro-inflammatory cytokine expression. Upon stimulation, Miz1 was phosphorylated at Ser178, which is required for recruiting histone deacetylase 1 to repress transcription of C/EBP-δ, an amplifier of inflammation. Our data provide a long-sought mechanism underlying resolution of LPS-induced inflammation.