Increased levels of prostaglandin D(2) suggest macrophage activation in patients with primary pulmonary hypertension

Prostaglandin and prostaglandin receptors: present and future promising therapeutic targets for pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH), Group 1 pulmonary hypertension (PH), is a type of pulmonary vascular disease characterized by abnormal contraction and remodeling of the pulmonary arterioles, manifested by pulmonary vascular resistance (PVR) and increased pulmonary arterial pressure, eventually leading to right heart failure or even death. The mechanisms involved in this process include inflammation, vascular matrix remodeling, endothelial cell apoptosis and proliferation, vasoconstriction, vascular smooth muscle cell proliferation and hypertrophy. In this study, we review the mechanisms of action of prostaglandins and their receptors in PAH.

MAIN BODY

PAH-targeted therapies, such as endothelin receptor antagonists, phosphodiesterase type 5 inhibitors, activators of soluble guanylate cyclase, prostacyclin, and prostacyclin analogs, improve PVR, mean pulmonary arterial pressure, and the six-minute walk distance, cardiac output and exercise capacity and are licensed for patients with PAH; however, they have not been shown to reduce mortality. Current treatments for PAH primarily focus on inhibiting excessive pulmonary vasoconstriction, however, vascular remodeling is recalcitrant to currently available therapies. Lung transplantation remains the definitive treatment for patients with PAH. Therefore, it is imperative to identify novel targets for improving pulmonary vascular remodeling in PAH. Studies have confirmed that prostaglandins and their receptors play important roles in the occurrence and development of PAH through vasoconstriction, vascular smooth muscle cell proliferation and migration, inflammation, and extracellular matrix remodeling.

CONCLUSION

Prostacyclin and related drugs have been used in the clinical treatment of PAH. Other prostaglandins also have the potential to treat PAH. This review provides ideas for the treatment of PAH and the discovery of new drug targets.

The vasoconstrictor activities of prostaglandin D2 via the thromboxane prostanoid receptor and E prostanoid receptor-3 outweigh its concurrent vasodepressor effect mainly through D prostanoid receptor-1 ex vivo and in vivo

Prostaglandin (PG) D2, a commonly considered vasodilator through D prostanoid receptor-1 (DP1), might also evoke vasoconstriction via acting on the thromboxane (Tx)-prostanoid receptor (the original receptor of TxA2; TP) and/or E prostanoid receptor-3 (one of the vasoconstrictor receptors of PGE2; EP3). This study aimed to test the above hypothesis in the mouse renal vascular bed (main renal arteries and perfused kidneys) and/or mesenteric resistance arteries and determine how the vasoconstrictor mechanism influences the overall PGD2 effect on systemic blood pressure under in vivo conditions. Experiments were performed on control wild-type (WT) mice and mice with deficiencies in TP (TP-/-) and/or EP3 (EP3-/-). Here we show that PGD2 indeed evoked vasoconstrictor responses in the above-mentioned tissues of WT mice, which were however not only reduced by TP-/- or EP3-/-, but also reversed by TP-/-/EP3-/- in some of the above tissues (mesenteric resistance arteries or perfused kidneys) to dilator reactions that were reduced by non-selective DP antagonism. A slight or mild pressor response was also observed with PGD2 under in vivo conditions, and this was again reversed to a depressor response in TP-/- or TP-/-/EP3-/- mice. Non-selective DP antagonism reduced the PGD2-evoked depressor response in TP-/-/EP3-/- mice as well. These results thus demonstrate that like other PGs, PGD2 activates TP and/or EP3 to evoke vasoconstrictor activities, which can outweigh its concurrent vasodepressor activity mediated mainly through DP1, and hence result in a pressor response, although the response might only be of a slight or mild extent.

Pathogenic Mechanisms of Pulmonary Arterial Hypertension: Homeostasis Imbalance of Endothelium-Derived Relaxing and Contracting Factors

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease. Sustained pulmonary vasoconstriction and concentric pulmonary vascular remodeling contribute to the elevated pulmonary vascular resistance and pulmonary artery pressure in PAH. Endothelial cells regulate vascular tension by producing endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs). Homeostasis of EDRF and EDCF production has been identified as a marker of the endothelium integrity. Impaired synthesis or release of EDRFs induces persistent vascular contraction and pulmonary artery remodeling, which subsequently leads to the development and progression of PAH. In this review, the authors summarize how EDRFs and EDCFs affect pulmonary vascular homeostasis, with special attention to the recently published novel mechanisms related to endothelial dysfunction in PAH and drugs associated with EDRFs and EDCFs.

Niacin Attenuates Pulmonary Hypertension Through H-PGDS in Macrophages

RATIONALE

Pulmonary arterial hypertension (PAH) is characterized by progressive pulmonary vascular remodeling, accompanied by varying degrees of perivascular inflammation. Niacin, a commonly used lipid-lowering drug, possesses vasodilating and proresolution effects by promoting the release of prostaglandin D₂ (PGD₂). However, whether or not niacin confers protection against PAH pathogenesis is still unknown.

OBJECTIVE

This study aimed to determine whether or not niacin attenuates the development of PAH and, if so, to elucidate the molecular mechanisms underlying its effects.

METHODS AND RESULTS

Vascular endothelial growth factor receptor inhibitor SU5416 and hypoxic exposure were used to induce pulmonary hypertension (PH) in rodents. We found that niacin attenuated the development of this hypoxia/SU5416-induced PH in mice and suppressed progression of monocrotaline-induced and hypoxia/SU5416-induced PH in rats through the reduction of pulmonary artery remodeling. Niacin boosted PGD₂ generation in lung tissue, mainly through H-PGDS (hematopoietic PGD₂ synthases). Deletion of H-PGDS, but not lipocalin-type PGDS, exacerbated the hypoxia/SU5416-induced PH in mice and abolished the protective effects of niacin against PAH. Moreover, H-PGDS was expressed dominantly in infiltrated macrophages in lungs of PH mice and patients with idiopathic PAH. Macrophage-specific deletion of H-PGDS markedly decreased PGD₂ generation in lungs, aggravated hypoxia/SU5416-induced PH in mice, and attenuated the therapeutic effect of niacin on PAH.

CONCLUSIONS

Niacin treatment ameliorates the progression of PAH through the suppression of vascular remodeling by stimulating H-PGDS-derived PGD₂ release from macrophages.

Loss of DP1 Aggravates Vascular Remodeling in Pulmonary Arterial Hypertension via mTORC1 Signaling

Rationale: Vascular remodeling, including smooth muscle cell hypertrophy and proliferation, is the key pathological feature of pulmonary arterial hypertension (PAH). Prostaglandin I₂ analogs (beraprost, iloprost, and treprostinil) are effective in the treatment of PAH. Of note, the clinically favorable effects of treprostinil in severe PAH may be attributable to concomitant activation of DP1 (D prostanoid receptor subtype 1).Objectives: To study the role of DP1 in the progression of PAH and its underlying mechanism.Methods: DP1 levels were examined in pulmonary arteries of patients and animals with PAH. Multiple genetic and pharmacologic approaches were used to investigate DP1-mediated signaling in PAH.Measurements and Main Results: DP1 expression was downregulated in hypoxia-treated pulmonary artery smooth muscle cells and in pulmonary arteries from rodent PAH models and patients with idiopathic PAH. DP1 deletion exacerbated pulmonary artery remodeling in hypoxia-induced PAH, whereas pharmacological activation or forced expression of the DP1 receptor had the opposite effect in different rodent models. DP1 deficiency promoted pulmonary artery smooth muscle cell hypertrophy and proliferation in response to hypoxia via induction of mTORC1 (mammalian target of rapamycin complex 1) activity. Rapamycin, an inhibitor of mTORC1, alleviated the hypoxia-induced exacerbation of PAH in DP1-knockout mice. DP1 activation facilitated raptor dissociation from mTORC1 and suppressed mTORC1 activity through PKA (protein kinase A)-dependent phosphorylation of raptor at Ser791. Moreover, treprostinil treatment blocked the progression of hypoxia-induced PAH in mice in part by targeting the DP1 receptor.Conclusions: DP1 activation attenuates hypoxia-induced pulmonary artery remodeling and PAH through PKA-mediated dissociation of raptor from mTORC1. These results suggest that the DP1 receptor may serve as a therapeutic target for the management of PAH.

Obesity, Hypertension, and Cardiac Dysfunction: Novel Roles of Immunometabolism in Macrophage Activation and Inflammation

Obesity and hypertension, which often coexist, are major risk factors for heart failure and are characterized by chronic, low-grade inflammation, which promotes adverse cardiac remodeling. While macrophages play a key role in cardiac remodeling, dysregulation of macrophage polarization between the proinflammatory M1 and anti-inflammatory M2 phenotypes promotes excessive inflammation and cardiac injury. Metabolic shifting between glycolysis and mitochondrial oxidative phosphorylation has been implicated in macrophage polarization. M1 macrophages primarily rely on glycolysis, whereas M2 macrophages rely on the tricarboxylic acid cycle and oxidative phosphorylation; thus, factors that affect macrophage metabolism may disrupt M1/M2 homeostasis and exacerbate inflammation. The mechanisms by which obesity and hypertension may synergistically induce macrophage metabolic dysfunction, particularly during cardiac remodeling, are not fully understood. We propose that obesity and hypertension induce M1 macrophage polarization via mechanisms that directly target macrophage metabolism, including changes in circulating glucose and fatty acid substrates, lipotoxicity, and tissue hypoxia. We discuss canonical and novel proinflammatory roles of macrophages during obesity-hypertension-induced cardiac injury, including diastolic dysfunction and impaired calcium handling. Finally, we discuss the current status of potential therapies to target macrophage metabolism during heart failure, including antidiabetic therapies, anti-inflammatory therapies, and novel immunometabolic agents.

A case of pulmonary arterial hypertension associated with adult hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is an aggressive, life-threatening syndrome of excessive immune activation. Presentation is most common among the pediatric population, and cases in adults are rare. The number of nonhematologic presentations described in relation to HLH has been growing. We present a case involving a woman who developed HLH after autologous stem cell transplantation for mantle cell lymphoma. Months later, she received a diagnosis of pulmonary arterial hypertension (PAH) while undergoing treatment for her HLH. To our knowledge, PAH associated with adult HLH has only been described in the literature once before. PAH may now be a potential differential diagnosis for patients with HLH who present with respiratory symptoms.

Prognostic value of neutrophil-to-lymphocyte ratio in pulmonary arterial hypertension

OBJECTIVE

To evaluate the prognostic value of baseline neutrophil-to-lymphocyte ratio (NLR) in the prediction of long-term mortality in patients with pulmonary arterial hypertension (PAH).

METHODS

This prospective study recorded NLR during initial diagnostic right-sided cardiac catheterization in adult patients with PAH. Demographic, clinical, laboratory and haemodynamic variables were compared by NLR tertile. Univariate and multivariate Cox regression analyses were used to determine whether NLR was independently associated with mortality.

RESULTS

Adults with PAH (n = 101) were followed-up for mean ± SD 36.8 ± 23.6 months. The number of deaths, New York Heart Association functional capacity (NYHA FC), levels of brain natriuretic peptide (BNP) or C-reactive protein (CRP) and presence of pericardial effusion increased as the NLR tertile increased, but haemoglobin and tricuspid plane annular systolic excursion (TAPSE) decreased. On univariate analysis, high NLR values were associated with mortality, but on multivariate analysis, NLR did not remain an independent predictor of mortality. Baseline NYHA FC, TAPSE, BNP level and pericardial effusion were independent predictors of mortality.

CONCLUSIONS

NLR was correlated with important prognostic markers in PAH such as NYHA FC, BNP and TAPSE. This simple marker may be useful in the assessment of disease severity in patients with PAH.

(1aR,5aR)1a,3,5,5a-Tetrahydro-1H-2,3-diaza-cyclopropa[a]pentalene-4-carboxylic acid (MK-1903): a potent GPR109a agonist that lowers free fatty acids in humans

G-protein coupled receptor (GPCR) GPR109a is a molecular target for nicotinic acid and is expressed in adipocytes, spleen, and immune cells. Nicotinic acid has long been used for the treatment of dyslipidemia due to its capacity to positively affect serum lipids to a greater extent than other currently marketed drugs. We report a series of tricyclic pyrazole carboxylic acids that are potent and selective agonists of GPR109a. Compound R,R-19a (MK-1903) was advanced through preclinical studies, was well tolerated, and presented no apparent safety concerns. Compound R,R-19a was advanced into a phase 1 clinical trial and produced a robust decrease in plasma free fatty acids. On the basis of these results, R,R-19a was evaluated in a phase 2 study in humans. Because R,R-19a produced only a weak effect on serum lipids as compared with niacin, we conclude that the beneficial effects of niacin are most likely the result of an undefined GPR109a independent pathway.

Predictors of osteoclast activity in patients with sickle cell disease

BACKGROUND

Bone changes are common in sickle cell disease, but the pathogenesis is not fully understood. Tartrate-resistant acid phosphatase (TRACP) type 5b is produced by bone-resorbing osteoclasts. In other forms of hemolytic anemia, increased iron stores are associated with osteoporosis. We hypothesized that transfusional iron overload would be associated with increased osteoclast activity in patients with sickle cell disease.

DESIGN AND METHODS

We examined tartrate-resistant acid phosphatase 5b concentrations in patients with sickle cell disease and normal controls of similar age and sex distribution at steady state. Serum tartrate-resistant acid phosphatase 5b concentration was measured using an immunocapture enzyme assay and plasma concentrations of other cytokines were assayed using the Bio-Plex suspension array system. Tricuspid regurgitation velocity, an indirect measure of systolic pulmonary artery pressure, was determined by echocardiography.

RESULTS

Tartrate-resistant acid phosphatase 5b concentrations were higher in 58 adults with sickle cell disease than in 22 controls (medians of 4.4 versus 2.4 U/L, respectively; P=0.0001). Among the patients with sickle cell disease, tartrate-resistant acid phosphatase 5b independently correlated with blood urea nitrogen (standardized beta=0.40, P=0.003), interleukin-8 (standardized beta=0.30, P=0.020), and chemokine C-C motif ligand 5 (standardized beta=-0.28, P=0.031) concentrations, but not with serum ferritin concentration. Frequent blood transfusions (>10 units in life time) were not associated with higher tartrate-resistant acid phosphatase 5b levels in multivariate analysis. There were strong correlations among tartrate-resistant acid phosphatase 5b, alkaline phosphatase and tricuspid regurgitation velocity (r>0.35, P<0.001).

CONCLUSIONS

Patients with sickle cell disease have increased osteoclast activity as reflected by serum tartrate-resistant acid phosphatase 5b concentrations. Our results may support a potential role of inflammation rather than increased iron stores in stimulating osteoclast activity in sickle cell disease. The positive relationships among tartrate-resistant acid phosphatase 5b, alkaline phosphatase and tricuspid regurgitation velocity raise the possibility of a common pathway in the pulmonary and bone complications of sickle cell disease.

Induction of lipocalin-type prostaglandin D synthase in mouse heart under hypoxemia

Hypoxemia is a common manifestation of various disorders and generates pressure overload to the heart. Here we analyzed the expression of lipocalin-type prostaglandin D synthase (L-PGDS) in the heart of C57BL/6 mice kept under normobaric hypoxia (10% O2) that generates hemodynamic stress. Northern and Western blot analyses revealed that the expression levels of L-PGDS mRNA and protein were significantly increased (> twofold) after 14 days of hypoxia, compared to the mice kept under normoxia. Immunohistochemical analysis indicated that L-PGDS was increased in the myocardium of auricles and ventricles and the pulmonary venous myocardium at 28 days of hypoxia. Moreover, using C57BL/6 mice lacking heme oxygenase-2 (HO-2(-/-)), a model of chronic hypoxemia, we showed that the expression level of L-PGDS protein was twofold higher in the heart than that of wild-type mouse. L-PGDS expression is induced in the myocardium under hypoxemia, which may reflect the adaptation to the hemodynamic stress.

A functional single-nucleotide polymorphism in the TRPC6 gene promoter associated with idiopathic pulmonary arterial hypertension

BACKGROUND

Excessive proliferation of pulmonary artery smooth muscle cells (PASMCs) plays an important role in the development of idiopathic pulmonary arterial hypertension (IPAH), whereas a rise in cytosolic Ca2+ concentration triggers PASMC contraction and stimulates PASMC proliferation. Recently, we demonstrated that upregulation of the TRPC6 channel contributes to proliferation of PASMCs isolated from IPAH patients. This study sought to identify single-nucleotide polymorphisms (SNPs) in the TRPC6 gene promoter that are associated with IPAH and have functional significance in regulating TRPC6 activity in PASMCs.

METHODS AND RESULTS

Genomic DNA was isolated from blood samples of 237 normal subjects and 268 IPAH patients. Three biallelic SNPs, -361 (A/T), -254(C/G), and -218 (C/T), were identified in the 2000-bp sequence upstream of the transcriptional start site of TRPC6. Although the allele frequencies of the -361 and -218 SNPs were not different between the groups, the allele frequency of the -254(C-->G) SNP in IPAH patients (12%) was significantly higher than in normal subjects (6%; P<0.01). Genotype data showed that the percentage of -254G/G homozygotes in IPAH patients was 2.85 times that of normal subjects. Moreover, the -254(C-->G) SNP creates a binding sequence for nuclear factor-kappaB. Functional analyses revealed that the -254(C-->G) SNP enhanced nuclear factor-kappaB-mediated promoter activity and stimulated TRPC6 expression in PASMCs. Inhibition of nuclear factor-kappaB activity attenuated TRPC6 expression and decreased agonist-activated Ca2+ influx in PASMCs of IPAH patients harboring the -254G allele.

CONCLUSIONS

These results suggest that the -254(C-->G) SNP may predispose individuals to an increased risk of IPAH by linking abnormal TRPC6 transcription to nuclear factor-kappaB, an inflammatory transcription factor.

Pulmonary and systemic hemodynamic responses to intravenous prostacyclin in broilers

The eicosanoid vasodilator prostacyclin (PGI2) reduces resistance to pulmonary blood flow and attenuates pulmonary hypertension in mammals. However, sparse information is available regarding the responsiveness of the avian pulmonary vasculature to PGI2. Accordingly, in 3 experiments we evaluated the pulmonary vascular responses to PGI2 in male broilers. In experiment 1, infusing PGI2 (10 microg/min) into clinically healthy broilers did not reduce their pulmonary vascular resistance (PVR) but did reduce their pulmonary arterial pressure (PAP) by lowering their cardiac output. Within 4 min after stopping the PGI2 infusion, the cardiac output and PAP returned to preinfusion levels. In experiment 2, the responses to PGI2 were evaluated after arachidonic acid (AA) had been infused to preconstrict the pulmonary vasculature. The AA infusion (400 microg/min) consistently triggered dramatic, sustained pulmonary vasoconstriction (increased PVR) and pulmonary hypertension (increased PAP). Concurrent PGI2 infusions did not reduce PVR but did reduce PAP by lowering cardiac output. Within 4 min after stopping the PGI2 infusion, PAP and cardiac output returned to their previous (hypertensive) levels attributable to the ongoing AA infusion. In experiment 3, PGI2 was infused (10 microg/min) into clinically healthy (PAP < or = 24 mmHg) or subclinically hypertensive (PAP > or = 27 mmHg) broilers. Throughout this experiment broilers in the hypertensive group had higher PAP values than broilers in the healthy group. The PGI2 infusion reduced PAP in both groups but did not reduce PVR. Instead, the pulmonary hypotensive response to PGI2 infusion was associated with a reduction in cardiac output in both groups. In all 3 experiments PGI2 reduced PAP by reducing cardiac output rather than by reducing PVR. There was no evidence that PGI2 acts as an effective pulmonary vasodilator in broilers regardless of whether their pulmonary vasculature was apparently normal (clinically healthy), had been pharmacologically preconstricted (AA infusion), or initially exhibited the vasoconstriction that is typical of the pathogenesis of pulmonary hypertension syndrome in broilers (PAP > or = 27 mmHg). The consistent failure of PGI2 to elicit pulmonary vasodilation in this study suggests fundamental differences in AA metabolism or the etiology of pulmonary hypertension may exist when broilers are compared with mammals.

Cellular and molecular mechanisms of pulmonary vascular remodeling: role in the development of pulmonary hypertension

Pulmonary artery vasoconstriction and vascular remodeling greatly contribute to a sustained elevation of pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) in patients with pulmonary arterial hypertension (PAH). The development of PAH involves a complex and heterogeneous constellation of multiple genetic, molecular, and humoral abnormalities, which interact in a complicated manner, presenting a final manifestation of vascular remodeling in which fibroblasts, smooth muscle and endothelial cells, and platelets all play a role. Vascular remodeling is characterized largely by medial hypertrophy due to enhanced vascular smooth muscle cell proliferation or attenuated apoptosis and to endothelial cell over-proliferation, which can result in lumen obliteration. In addition to other factors, cytoplasmic Ca2+ in particular seems to play a central role as it is involved in both the generation of force through its effects on the contractile machinery, and the initiation and propagation of cell proliferation via its effects on transcription factors, mitogens, and cell cycle components. This review focuses on the role played by cellular factors, circulating factors, and genetic molecular signaling factors that promote a proliferative, antiapoptotic, and vasoconstrictive physiological milieu leading to vascular remodeling.

Molecular biology of primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a rare but often fatal condition characterized by pulmonary artery remodeling leading to chronic elevation of pulmonary artery pressure in the absence of causes. The pathophysiology of PPH is not completely understood, but a number of recent studies have elucidated many possible gentic, hormonal, and environmental factors. Current treatment options slow the progression of the disease but do not halt it. The study of molecular mechanisms that result from mutations in onmental and hormonal modifiers holds great promise for the development of novel therapies that may halt the progression of the disease.

Molecular and cellular basis of pulmonary vascular remodeling in pulmonary hypertension

Clinical pulmonary hypertension is characterized by a sustained elevation in pulmonary arterial pressure. Pulmonary vascular remodeling involves structural changes in the normal architecture of the walls of pulmonary arteries. The process of vascular remodeling can occur as a primary response to injury, or stimulus such as hypoxia, within the resistance vessels of the lung. Alternatively, the changes seen in more proximal vessels may arise secondary to a sustained increase in intravascular pressure. To withstand the chronic increase in intraluminal pressure, the vessel wall becomes thickened and stronger. This "armouring" of the vessel wall with extra-smooth muscle and extracellular matrix leads to a decrease in lumen diameter and reduced capacity for vasodilatation. This maladaptive response results in increased pulmonary vascular resistance and consequently, sustained pulmonary hypertension. The process of pulmonary vascular remodeling involves all layers of the vessel wall and is complicated by the finding that cellular heterogeneity exists within the traditional compartments of the vascular wall: intima, media, and adventitia. In addition, the developmental stage of the organism greatly modifies the response of the pulmonary circulation to injury. This review focuses on the latest advances in our knowledge of these processes as they relate to specific forms of pulmonary hypertension and particularly in the light of recent genetic studies that have identified specific pathways involved in the pathogenesis of severe pulmonary hypertension.

The prostacyclin analogue treprostinil blocks NFkappaB nuclear translocation in human alveolar macrophages

Primary pulmonary hypertension (PPH) is characterized by increased pulmonary arterial pressure and vascular resistance. We and others have observed that inflammatory cytokines and infiltrates are present in the lung tissue, but the significance is uncertain. Treprostinil (TRE), a prostacyclin analogue with extended half-life and chemical stability, has shown promise in the treatment of PPH. We hypothesize that TRE might exert beneficial effects in PPH by antagonizing inflammatory cytokine production in the lung. Here we show that TRE dose-dependently inhibits inflammatory cytokine (tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and granulocyte macrophage colony-stimulating factor) secretion and gene expression by human alveolar macrophages. TRE blocks NFkappaB activation, but IkappaB-alpha phosphorylation and degradation are unaffected. Moreover, TRE does not affect the formation of the NFkappaB.DNA complex but blocks nuclear translocation of p65. These results are the first to illustrate the anti-cytokine actions of TRE in down-regulating NFkappaB, not through its inhibitory component or by direct binding but by blocking nuclear translocation. These data indicate that inflammatory mechanisms may be important in the pathogenesis of PPH and cytokine antagonism by blocking NFkappaB may contribute to the efficacy of TRE therapy in PPH.