Effects of dimethylarginine dimethylaminohydrolase-1 overexpression on the response of the pulmonary vasculature to hypoxia

Biomarkers of the L-Arginine/Dimethylarginine/Nitric Oxide Pathway in People with Chronic Airflow Obstruction and Obstructive Sleep Apnoea

BACKGROUND

Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnoea (OSA) are common chronic diseases that are associated with chronic and intermittent hypoxemia, respectively. Patients affected by the overlap of COPD and OSA have a particularly unfavourable prognosis. The L-arginine/nitric oxide (NO) pathway plays an important role in regulating pulmonary vascular function. Asymmetric (ADMA) and symmetric dimethylarginine (SDMA) interfere with NO production.

METHODS

We analysed the serum concentrations of ADMA, SDMA, L-arginine, L-citrulline, and L-ornithine in a large sample of the Icelandic general population together with chronic airflow obstruction (CAO), a key physiological marker of COPD that was assessed by post-bronchodilator spirometry (FEV1/FVC < LLN). OSA risk was determined by the multivariable apnoea prediction (MAP) index.

RESULTS

713 individuals were analysed, of whom 78 (10.9%) showed CAO and 215 (30%) had MAP > 0.5. SDMA was significantly higher in individuals with CAO (0.518 [0.461-0.616] vs. 0.494 [0.441-0.565] µmol/L; p = 0.005), but ADMA was not. However, ADMA was significantly associated with decreasing FEV1 percent predicted among those with CAO (p = 0.002). ADMA was 0.50 (0.44-0.56) µmol/L in MAP ≤ 0.5 versus 0.52 (0.46-0.58) µmol/L in MAP > 0.5 (p = 0.008). SDMA was 0.49 (0.44-0.56) µmol/L versus 0.51 (0.46-0.60) µmol/L, respectively (p = 0.004). The highest values for ADMA and SDMA were observed in individuals with overlap of CAO and MAP > 0.5, which was accompanied by lower L-citrulline levels.

CONCLUSIONS

The plasma concentrations of ADMA and SDMA are elevated in COPD patients with concomitant intermittent hypoxaemia. This may account for impaired pulmonary NO production, enhanced pulmonary vasoconstriction, and disease progression.

Dysregulation of the Nitric Oxide/Dimethylarginine Pathway in Hypoxic Pulmonary Vasoconstriction—Molecular Mechanisms and Clinical Significance

The pulmonary circulation responds to hypoxia with vasoconstriction, a mechanism that helps to adapt to short-lived hypoxic episodes. When sustained, hypoxic pulmonary vasoconstriction (HPV) may become deleterious, causing right ventricular hypertrophy and failure, and contributing to morbidity and mortality in the late stages of several chronic pulmonary diseases. Nitric oxide (NO) is an important endothelial vasodilator. Its release is regulated, amongst other mechanisms, by the presence of endogenous inhibitors like asymmetric dimethylarginine (ADMA). Evidence has accumulated in recent years that elevated ADMA may be implicated in the pathogenesis of HPV and in its clinical sequelae, like pulmonary arterial hypertension (PAH). PAH is one phenotypic trait in experimental models with disrupted ADMA metabolism. In high altitude, elevation of ADMA occurs during long-term exposure to chronic or chronic intermittent hypobaric hypoxia; ADMA is significantly associated with high altitude pulmonary hypertension. High ADMA concentration was also reported in patients with chronic obstructive lung disease, obstructive sleep apnoea syndrome, and overlap syndrome, suggesting a pathophysiological role for ADMA-mediated impairment of endothelium-dependent, NO-mediated pulmonary vasodilation in these clinically relevant conditions. Improved understanding of the molecular (dys-)regulation of pathways controlling ADMA concentration may help to dissect the pathophysiology and find novel therapeutic options for these diseases.

Impairment of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a serious complication of severe systemic or local pulmonary inflammation, such as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. ARDS is characterised by diffuse alveolar damage that leads to protein-rich pulmonary oedema, local alveolar hypoventilation and atelectasis. Inadequate perfusion of these areas is the main cause of hypoxaemia in ARDS. High perfusion in relation to ventilation (V/Q<1) and shunting (V/Q=0) is not only caused by impaired hypoxic pulmonary vasoconstriction but also redistribution of perfusion from obstructed lung vessels. Rebalancing the pulmonary vascular tone is a therapeutic challenge. Previous clinical trials on inhaled vasodilators (nitric oxide and prostacyclin) to enhance perfusion to high V/Q areas showed beneficial effects on hypoxaemia but not on mortality. However, specific patient populations with pulmonary hypertension may profit from treatment with inhaled vasodilators. Novel treatment targets to decrease perfusion in low V/Q areas include epoxyeicosatrienoic acids and specific leukotriene receptors. Still, lung protective ventilation and prone positioning are the best available standard of care. This review focuses on disturbed perfusion in ARDS and aims to provide basic scientists and clinicians with an overview of the vascular alterations and mechanisms of V/Q mismatch, current therapeutic strategies, and experimental approaches.

DDAH-1, via regulation of ADMA levels, protects against ischemia-induced blood-brain barrier leakage

Dimethylarginine dimethylamino hydrolase-1 (DDAH-1) is an important regulator of nitric oxide (NO) metabolism that has been implicated in the pathogenesis of cardiovascular diseases. Nevertheless, its role in cerebral ischemia still needs to be elucidated. Herein, we examined the expression of DDAH-1 in the brain of rat by double-label immunofluorescence staining. DDAH-1 knock-out (DDAH-1-/-) and wild-type rats underwent middle cerebral artery occlusion/reperfusion (MCAO/R). After 24 h, neurological scores, TTC staining and TUNEL assay were used to evaluate neurological damages. 3 and 7-days infarct outcomes were also shown. Blood-brain-barrier (BBB) permeability was examined via Evans blue extravasation and tight junction (TJ) proteins expression and mRNA levels by western blot and RT-qPCR. The levels of plasma asymmetric dimethylarginine (ADMA), NO and ADMA in brain tissue were also assessed. In addition, supplementation of L-arginine to DDAH-1-/- rats was used to explore its role in regulating NO. DDAH-1 was abundantly distributed in cerebral cortex and basal nuclei, and mainly expressed in neurons and endothelial cells. DDAH-1-/- rats showed aggravated neurological damage and BBB disruption, including decrease of TJ proteins expression but indistinguishable mRNA levels after MCAO/R. DDAH-1 depletion and neurological damages were accompanied with increased ADMA levels and decreased NO concentrations. The supplementation with L-arginine partly restored the neurological damages and BBB disruption. To sum up, DDAH-1 revealed to have a protective role in ischemia stroke (IS) and IS-induced leakage of BBB via decreasing ADMA level and possibly via preventing TJ proteins degradation.

Upregulation of DDAH2 Limits Pulmonary Hypertension and Right Ventricular Hypertrophy During Chronic Hypoxia in Ddah1 Knockout Mice

Objective: Chronic hypoxia causes pulmonary vasoconstriction leading to pulmonary hypertension and right ventricular hypertrophy. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthesis; its level increases in hypoxia (HX) concomitantly with reduced activity of dimethylarginine dimethylaminohydrolases (DDAH-1 and DDAH-2), enzymes metabolizing ADMA. Ddah1 knockout (KO) mice may therefore help to understand the pathophysiological roles of this enzyme and its substrate, ADMA, in the development of hypoxia-associated pulmonary hypertension.

Methods: Ddah1 KO mice and their wild-type (WT) littermates were subjected to normoxia (NX) or for 21 days. We measured ADMA concentration in plasma and lungs, DDAH1 and DDAH2 mRNA and protein expression in the lungs, right ventricular systolic pressure (RVSP), right ventricular hypertrophy by the Fulton index, and cardiomyocyte hypertrophy by dystrophin staining of the heart.

Results: Ddah1 KO mice had higher ADMA concentrations in plasma and in lung tissue than WT in NX (p < 0.05). ADMA significantly increased in WT-HX in plasma and lungs, while there were no significant differences in WT-HX vs. KO-HX. This finding was paralleled by a 38 ± 13% reduction in Ddah1 but not Ddah2 mRNA expression, and reduced DDAH1 protein expression but stable DDAH2 protein levels in WT mice. Ddah1 KO mice showed significant elevation of DDAH2 protein but not mRNA levels, which further increased in HX. HX led to increased RVSP and right ventricular hypertrophy in both, WT and KO mice, with no significant differences between both genotypes.

Conclusions: Chronic hypoxia causes an elevation of ADMA, which may impair NO production and lead to endothelial dysfunction and vasoconstriction. Downregulation of DDAH1 expression and activity may be involved in this; however, knockout of the Ddah1 gene does not modify the hypoxia-induced pathophysiological changes of pulmonary blood pressure and right ventricular hypertrophy, possibly due to compensatory upregulation of DDAH2 protein.

Metabolism of asymmetric dimethylarginine in hypoxia: from bench to bedside

Acute hypoxia and chronic hypoxia induce pulmonary vasoconstriction. While hypoxic pulmonary vasoconstriction is a physiological response if parts of the lung are affected, global exposure to hypoxic conditions may lead to clinical conditions like high-altitude pulmonary hypertension. Nitric oxide is the major vasodilator released from the vascular endothelium. Nitric oxide-dependent vasodilation is impaired in hypoxic conditions. Inhibition of nitric oxide synthesis is the most rapid and easily reversible molecular mechanism to regulate nitric oxide-dependent vascular function in response to physiological and pathophysiological stimuli. Asymmetric dimethylarginine is an endogenous, competitive inhibitor of nitric oxide synthase and a risk marker for major cardiovascular events and mortality. Elevated asymmetric dimethylarginine has been observed in animal models of hypoxia as well as in human cohorts under chronic and chronic intermittent hypoxia at high altitude. In lowlanders, asymmetric dimethylarginine is high in patients with pulmonary hypertension. We have recently shown that high asymmetric dimethylarginine at sea level is a predictor for high-altitude pulmonary hypertension. Asymmetric dimethylarginine is a highly regulated molecule, both by its biosynthesis and metabolism. Methylation of L-arginine by protein arginine methyltransferases was shown to be increased in hypoxia. Furthermore, the metabolism of asymmetric dimethylarginine by dimethylarginine dimethylaminohydrolases (DDAH1 and DDAH2) is decreased in animal models of hypoxia. Whether these changes are caused by transcriptional or posttranslational modifications remains to be elucidated. Current data suggest a major role of asymmetric dimethylarginine in regulating pulmonary arterial nitric oxide production in hypoxia. Further studies are needed to decipher the molecular mechanisms regulating asymmetric dimethylarginine in hypoxia and to understand their clinical significance.

Pathobiology, pathology and genetics of pulmonary hypertension: Update from the Cologne Consensus Conference 2018

The European guidelines, which focus on clinical aspects of pulmonary hypertension (PH), provide only minimal information about the pathophysiological concepts of PH. Here, we review this topic in greater detail, focusing on specific aspects in the pathobiology, pathology and genetics, which include mechanisms of vascular inflammation, the role of transcription factors, ion channels/ion channel diseases, hypoxic pulmonary vasoconstriction, genetics/epigenetics, metabolic dysfunction, and the potential future role of histopathology of PH in the modern era of PH therapy. In addition to new insights in the pathobiology of this disease, this working group of the Cologne Consensus Conference also highlights novel concepts and potential new therapeutic targets to further improve the treatment options in PAH.

Recent advances in oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological reaction, which adapts lung perfusion to regional ventilation and optimizes gas exchange. Impaired HPV may cause systemic hypoxemia, while generalized HPV contributes to the development of pulmonary hypertension. The triggering mechanisms underlying HPV are still not fully elucidated. Several hypotheses are currently under debate, including a possible decrease as well as an increase in reactive oxygen species as a triggering event. Recent findings suggest an increase in the production of reactive oxygen species in pulmonary artery smooth muscle cells by complex III of the mitochondrial electron transport chain and occurrence of oxygen sensing at complex IV. Other essential components are voltage-dependent potassium and possibly L-type, transient receptor potential channel 6, and transient receptor potential vanilloid 4 channels. The release of arachidonic acid metabolites appears also to be involved in HPV regulation. Further investigation of the HPV mechanisms will facilitate the development of novel therapeutic strategies for the treatment of HPV-related disorders.

Sevoflurane Posttreatment Attenuates Lung Injury Induced by Oleic Acid in Dogs

BACKGROUND

In animal models, both sevoflurane and propofol protect against acute lung injury (ALI), especially when administered prior to ALI onset. We hypothesized that when compared to propofol, sevoflurane administration after the onset of acute respiratory distress syndrome would mitigate oleic acid (OA)-induced ALI in dogs.

METHODS

Dogs were randomly assigned to receive intravenous OA to induce ALI (n = 7 for each OA group) or saline as an OA control (n = 6 for each control). Dogs were then mechanically ventilated for 6 hours during which propofol (5 mg/kg/h) or sevoflurane (1.0 minimum alveolar concentration) was administered for sedation. Study end points included PO2/FIO2 ratio, pulmonary arterial pressure, pulmonary edema, histology, and tumor nuclear factor-α.

RESULTS

In OA-injured animals, oxygenation was worse at 1, 2, 3, and 4 hours after 6-hour mechanical ventilation in sevoflurane-sedated animals compared with propofol-sedated animals, with mean difference (95% confidence interval; propofol minus sevoflurane) of 75 (39-111), 87 (55-119), 66 (44-87), and 67 (27-107) mm Hg for the respective time points. However, sevoflurane reduced the elevated pulmonary arterial pressure and vascular resistance, attenuated pulmonary edema as evidenced by reduced extravascular lung water index, and decreased tumor nuclear factor-α and diffuse alveolar damage score compared with propofol in the OA-injured lungs.

CONCLUSIONS

When compared with propofol, sevoflurane attenuates OA-induced lung damage. However, despite this effect on lung histology and inflammation, sevoflurane worsened oxygenation in OA-induced ALI, possibly via inhibition of hypoxic pulmonary vasoconstriction.

miR-21/DDAH1 pathway regulates pulmonary vascular responses to hypoxia

The NOS (nitric oxide synthase) inhibitor ADMA (asymmetric dimethylarginine) contributes to the pathogenesis of pulmonary hypertension. Reduced levels of the enzymes metabolizing ADMA, dimethylarginine dimethylaminohydrolases (DDAH1 and DDAH2) and increased levels of miR-21 are linked to disease pathology, but the mechanisms are not understood. In the present study we assessed the potential role of miR-21 in the regulation of hypoxia-induced changes in ADMA metabolism in vitro and in vivo. Hypoxia inhibited DDAH1 and DDAH2 expression and increased ADMA levels in cultured human pulmonary endothelial cells. In contrast, in human pulmonary smooth muscle cells, only DDAH2 was reduced whereas ADMA levels remained unchanged. Endothelium-specific down-regulation of DDAH1 by miR-21 in hypoxia induced endothelial dysfunction and was prevented by overexpression of DDAH1 and miR-21 blockade. DDAH1, but not DDAH2, mRNA levels were reduced, whereas miR-21 levels were elevated in lung tissues from patients with pulmonary arterial hypertension and mice with pulmonary hypertension exposed to 2 weeks of hypoxia. Hypoxic mice treated with miR-21 inhibitors and DDAH1 transgenic mice showed elevated lung DDAH1, increased cGMP levels and attenuated pulmonary hypertension. Regulation of DDAH1 by miR-21 plays a role in the development of hypoxia-induced pulmonary hypertension and may be of broader significance in pulmonary hypertension.

Structural and functional prevention of hypoxia-induced pulmonary hypertension by individualized exercise training in mice

Pulmonary hypertension (PH) is a disease with a poor prognosis characterized by a vascular remodeling process and an increase in pulmonary vascular resistance. While a variety of reports demonstrated that exercise training exerts beneficial effects on exercise performance and quality of life in PH patients, it is not known how physical exercise affects vascular remodeling processes occurring in hypoxia-induced PH. Therefore, we investigated the effect of individualized exercise training on the development of hypoxia-induced PH in mice. Training effects were compared with pharmacological treatment with the phosphodiesterase 5 inhibitor Sildenafil or a combination of training plus Sildenafil. Trained mice who received Sildenafil showed a significantly improved walking distance (from 88.9 ± 8.1 to 146.4 ± 13.1 m) and maximum oxygen consumption (from 93.3 ± 2.9 to 105.5 ± 2.2% in combination with Sildenafil, to 102.2 ± 3.0% with placebo) compared with sedentary controls. Right ventricular systolic pressure, measured by telemetry, was at the level of healthy normoxic animals, whereas right heart hypertrophy did not benefit from training. Most interestingly, the increase in small pulmonary vessel muscularization was prevented by training. Respective counterregulatory processes were detected for the nitric oxide-soluble guanylate cyclase-phosphodiesterase system. We conclude that individualized daily exercise can prevent vascular remodeling in hypoxia-induced PH.

The endothelial ADMA/NO pathway in hypoxia-related chronic respiratory diseases

Since its discovery, many adhere to the view that asymmetric dimethylarginine (ADMA), as an inhibitor of the synthesis of nitric oxide (NO), contributes to the pathogenesis of various diseases. Particularly, this is evident in disease of the cardiovascular system, in which endothelial dysfunction results in an imbalance between vasoconstriction and vasodilatation. Even if increased ADMA concentrations are closely related to an endothelial dysfunction, several studies pointed to a potential beneficial effect of ADMA, mainly in the context of angioproliferative disease such as cancer and fibrosis. Antiproliferative properties of ADMA independent of NO have been identified in this context. In particular, the regulation of ADMA by its degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH) is the object of many studies. DDAH is discussed as a promising therapeutic target for the indirect regulation of NO. In hypoxia-related chronic respiratory diseases, this controversy discussion of ADMA and DDAH is particularly evident and is therefore subject of this review.