Attenuated hypoxic pulmonary hypertension in mice lacking the 5-hydroxytryptamine transporter gene

Inflammation, growth factors, and pulmonary vascular remodeling

Inflammatory processes are prominent in various types of human and experimental pulmonary hypertension (PH) and are increasingly recognized as major pathogenic components of pulmonary vascular remodeling. Macrophages, T and B lymphocytes, and dendritic cells are present in the vascular lesions of PH, whether in idiopathic pulmonary arterial hypertension (PAH) or PAH related to more classical forms of inflammatory syndromes such as connective tissue diseases, human immunodeficiency virus (HIV), or other viral etiologies. Similarly, the presence of circulating chemokines and cytokines, viral protein components (e.g., HIV-1 Nef), and increased expression of growth (such as vascular endothelial growth factor and platelet-derived growth factor) and transcriptional (e.g., nuclear factor of activated T cells or NFAT) factors in these patients are thought to contribute directly to further recruitment of inflammatory cells and proliferation of smooth muscle and endothelial cells. Other processes, such as mitochondrial and ion channel dysregulation, seem to convey a state of cellular resistance to apoptosis; this has recently emerged as a necessary event in the pathogenesis of pulmonary vascular remodeling. Thus, the recognition of complex inflammatory disturbances in the vascular remodeling process offers potential specific targets for therapy and has recently led to clinical trials investigating, for example, the use of tyrosine kinase inhibitors. This paper provides an overview of specific inflammatory pathways involving cells, chemokines and cytokines, cellular dysfunctions, growth factors, and viral proteins, highlighting their potential role in pulmonary vascular remodeling and the possibility of future targeted therapy.

Future perspectives for the treatment of pulmonary arterial hypertension

Over the past 2 decades, pulmonary arterial hypertension has evolved from a uniformly fatal condition to a chronic, manageable disease in many cases, the result of unparalleled development of new therapies and advances in early diagnosis. However, none of the currently available therapies is curative, so the search for new treatment strategies continues. With a deeper understanding of the genetics and the molecular mechanisms of pulmonary vascular disorders, we are now at the threshold of entering a new therapeutic era. Our working group addressed what can be expected in the near future. The topics span the understanding of genetic variations, novel antiproliferative treatments, the role of stem cells, the right ventricle as a therapeutic target, and strategies and challenges for the translation of novel experimental findings into clinical practice.

Cellular and molecular basis of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is caused by functional and structural changes in the pulmonary vasculature, leading to increased pulmonary vascular resistance. The process of pulmonary vascular remodeling is accompanied by endothelial dysfunction, activation of fibroblasts and smooth muscle cells, crosstalk between cells within the vascular wall, and recruitment of circulating progenitor cells. Recent findings have reestablished the role of chronic vasoconstriction in the remodeling process. Although the pathology of PAH in the lung is well known, this article is concerned with the cellular and molecular processes involved. In particular, we focus on the role of the Rho family guanosine triphosphatases in endothelial function and vasoconstriction. The crosstalk between endothelium and vascular smooth muscle is explored in the context of mutations in the bone morphogenetic protein type II receptor, alterations in angiopoietin-1/TIE2 signaling, and the serotonin pathway. We also review the role of voltage-gated K(+) channels and transient receptor potential channels in the regulation of cytosolic [Ca(2+)] and [K(+)], vasoconstriction, proliferation, and cell survival. We highlight the importance of the extracellular matrix as an active regulator of cell behavior and phenotype and evaluate the contribution of the glycoprotein tenascin-c as a key mediator of smooth muscle cell growth and survival. Finally, we discuss the origins of a cell type critical to the process of pulmonary vascular remodeling, the myofibroblast, and review the evidence supporting a contribution for the involvement of endothelial-mesenchymal transition and recruitment of circulating mesenchymal progenitor cells.

Gene expression in lungs of mice lacking the 5-hydroxytryptamine transporter gene

Background

While modulation of the serotonin transporter (5HTT) has shown to be a risk factor for pulmonary arterial hypertension for almost 40 years, there is a lack of in vivo data about the broad molecular effects of pulmonary inhibition of 5HTT. Previous studies have suggested effects on inflammation, proliferation, and vasoconstriction. The goal of this study was to determine which of these were supported by alterations in gene expression in serotonin transporter knockout mice.

Methods

Eight week old normoxic mice with a 5-HTT knock-out (5HTT-/-) and their heterozygote(5HTT+/-) or wild-type(5HTT+/+) littermates had right ventricular systolic pressure(RVSP) assessed, lungs collected for RNA, pooled, and used in duplicate in Affymetrix array analysis. Representative genes were confirmed by quantitative RT-PCR and western blot.

Results

RVSP was normal in all groups. Only 124 genes were reliably changed between 5HTT-/- and 5HTT+/+ mice. More than half of these were either involved in inflammatory response or muscle function and organization; in addition, some matrix, heme oxygenase, developmental, and energy metabolism genes showed altered expression. Quantitative RT-PCR for examples from each major group confirmed changes seen by array, with an intermediate level in 5HTT +/- mice.

Conclusion

These results for the first time show the in vivo effects of 5HTT knockout in lungs, and show that many of the downstream mechanisms suggested by cell culture and ex vivo experiments are also operational in vivo. This suggests that the effect of 5HTT on pulmonary vascular function arises from its impact on several systems, including vasoreactivity, proliferation, and immune function.

RhoA and Rho kinase activation in human pulmonary hypertension: role of 5-HT signaling

RATIONALE

The complex and multifactorial pathogenesis of pulmonary hypertension (PH) involves constriction, remodeling, and in situ thrombosis of pulmonary vessels. Both serotonin (5-HT) and Rho kinase signaling may contribute to these alterations.

OBJECTIVES

To investigate possible links between the 5-HT transporter (5-HTT) and RhoA/Rho kinase pathways, as well as their involvement in the progression of human and experimental PH.

METHODS

Biochemical and functional analyses of lungs, platelets, and pulmonary artery smooth muscle cells (PA-SMCs) from patients with idiopathic PH (iPH) and 5-HTT overexpressing mice.

MEASUREMENTS AND MAIN RESULTS

Lungs, platelets, and PA-SMCs from patients with iPH were characterized by marked elevation in RhoA and Rho kinase activities and a strong increase in 5-HT binding to RhoA indicating RhoA serotonylation. The 5-HTT inhibitor fluoxetine and the type 2 transglutaminase inhibitor monodansylcadaverin prevented 5-HT-induced RhoA serotonylation and RhoA/Rho kinase activation, as well as 5-HT-induced proliferation of PA-SMCs from iPH patients that was also inhibited by the Rho kinase inhibitor fasudil. Increased Rho kinase activity, RhoA activation, and RhoA serotonylation were also observed in lungs from SM22-5-HTT(+)mice, which overexpress 5-HTT in smooth muscle and spontaneously develop PH. Treatment of SM22-5-HTT(+) mice with either fasudil or fluoxetine limited PH progression and RhoA/Rho kinase activation.

CONCLUSIONS

RhoA and Rho kinase activities are increased in iPH, in association with enhanced RhoA serotonylation. Direct involvement of the 5-HTT/RhoA/Rho kinase signaling pathway in 5-HT-mediated PA-SMC proliferation and platelet activation during PH progression identify RhoA/Rho kinase signaling as a promising target for new treatments against PH.

Fluoxetine protects against monocrotaline-induced pulmonary arterial hypertension: potential roles of induction of apoptosis and upregulation of Kv1.5 channels in rats

1. Suppressing apoptosis and downregulating K(+) channels in pulmonary artery smooth muscle cells (PASMC) have been implicated in the development of pulmonary vascular medial hypertrophy and pulmonary arterial hypertension (PAH). Previous studies have shown that selective serotonin re-uptake inhibitors (SSRIs) protected against PAH. The aim of the present study was to investigate the involvement of Kv1.5 channels and apoptosis in the protective effect of the SSRI fluoxetine against PAH. 2. Monocrotaline (MCT) was used to establish PAH in Wistar rats. Fluoxetine (2 and 10 mg/kg per day) was administered by gavage once a day for 3 weeks. Three weeks after the induction of PAH by MCT, pulmonary haemodynamic measurements and pulmonary artery morphological assessments were undertaken, along with detection of apoptosis and Kv1.5. 3. Fluoxetine (2 and 10 mg/kg per day) decreased pulmonary artery pressure, reduced the right ventricular index and inhibited the increase in medial wall thickness of pulmonary arteries in established PAH. Fluoxetine (10 mg/kg per day) reduced the expression of Bcl-2 and Bcl-xL protein, increased the expression of cleaved caspase 3 protein and enhanced the expression of Kv1.5 protein and mRNA in pulmonary arteries. Furthermore, fluoxetine (10 mg/kg per day) significantly suppressed proliferation and enhanced apoptosis of PASMC in MCT-induced PAH. 4. In conclusion, fluoxetine protects against MCT-induced PAH by suppressing PASMC proliferation, inducing PASMC apoptosis and upregulating Kv1.5 channels.

Molecular mechanisms of pulmonary hypertension

The pathogenesis of pulmonary arterial hypertension (PAH) is complex, involving multiple modulating genes and environmental factors. Multifactorial impairment of the physiologic balance can lead to vasoconstriction, vascular smooth muscle cell and endothelial cell proliferation/fibrosis, inflammation, remodeling and in-situ thrombosis. These are the likely mechanisms that lead to narrowing of the vessel followed by progressive increase in pulmonary vascular resistance and the clinical manifestations of pulmonary hypertension. Subsequently, major goal of the therapy is to avoid acute pulmonary vasoconstriction, halt the progression of vascular remodeling, and reverse the early vascular remodeling if possible. Recently published data addressing certain molecular mechanisms for pathogenesis of PAH have led to the successful therapeutic interventions. This review will focus on the common and critical molecular pathways including genetic basis of the development of PAH that on the whole may be new targets for therapeutic interventions.

Serotonin (5-HT) transporter ligands affect plasma 5-HT in rats

Dual dopamine (DA)/serotonin (5-HT)-releasing agents are promising candidate medications for stimulant addiction and other disorders. However, certain 5-HT transporter (SERT) substrates are associated with development of idiopathic pulmonary arterial hypertension (IPAH) and valvular heart disease (VHD). According to the "5-HT hypothesis," SERT substrates increase the risk for developing IPAH and VHD by increasing plasma 5-HT. To test this hypothesis directly, we determined the effects of acute and chronic fenfluramine, and other SERT ligands, on plasma 5-HT in male rats. For acute treatments, rats received i.v. vehicle or test drug (0.3 and 1.0 mg/kg), and serial blood samples were withdrawn. For chronic treatments, vehicle or test drug was infused via osmotic minipump (3 and 10 mg/kg/d) for 2 weeks. On the last day of infusion, rats received i.v. fenfluramine challenge (1 mg/kg), and serial blood samples were withdrawn. Plasma 5-HT was measured using ex vivo microdialysis in whole-blood samples. Baseline plasma 5-HT was <1.0 nM. Acute injection of fenfluramine or other SERT substrates caused large (up to 24-fold) dose-dependent increases in plasma 5-HT. Chronic fenfluramine at 3 and 10 mg/kg/d produced 1.7- and 3.5-fold increases in baseline plasma 5-HT, while chronic fluoxetine had no effect. Chronic infusions of fenfluramine or fluoxetine diminished the ability of acute fenfluramine to elevate dialysate 5-HT, and both drugs markedly reduced whole-blood 5-HT. Acute fenfluramine increases plasma 5-HT to concentrations that are below the micromolar levels necessary to produce adverse cardiovascular effects. Chronic fenfluramine and fluoxetine have minimal effects on plasma 5-HT, suggesting that the increased risk for IPAH associated with fenfluramine does not depend upon elevations in plasma 5-HT.

Dexfenfluramine discontinuous treatment does not worsen hypoxia-induced pulmonary vascular remodeling but activates RhoA/ROCK pathway: consequences on pulmonary hypertension

The anorectic drug, dexfenfluramine has been associated with an increase in the relative risk of developing pulmonary hypertension. 5-hydroxytryptamine (5-HT) is a mitogen for smooth muscle cell, an effect that relies on 5-HT transporter expression and which has been proposed to explain pulmonary side effect of dexfenfluramine, and more particularly its effect on vascular remodeling. However recent data supported a major role of pulmonary artery vasoconstriction through the RhoA/Rho-kinase pathway. We questioned whether or not anorectic treatment aggravates pulmonary hypertension through vascular remodeling and if RhoA/Rho-kinase (ROCK) was potentially involved. In rats exposed to hypoxia, concomitant dexfenfluramine treatment (5 mg/kg/day, i.v.) for 4 weeks had no effect on pulmonary hypertension development. When exposure to 2 weeks of chronic hypoxia followed discontinuation of dexfenfluramine treatment (dexfenfluramine-hypoxic rats), echocardiographic parameters of pulmonary artery flow and right ventricle were further altered (P<0.05) as well as right ventricle systolic pressure was further increased (P<0.001) when compared to hypoxic rats treated with vehicle (hypoxic rats). However, the total number of muscularized distal pulmonary arteries artery was similar in dexfenfluramine-hypoxic vs. hypoxic rats (P>0.05). Western blot, RT-PCR and immunofluorescence analysis revealed a greater expression of 5-HT transporter and ROCK, as well as a greater activation of RhoA in dexfenfluramine-hypoxic rats compared to hypoxic rats. These data show that increased 5-HT transporter expression that follows dexfenfluramine discontinuation is not associated to a greater vascular remodeling despite worsening the development of pulmonary hypertension. Furthermore dexfenfluramine discontinuation promotes a greater RhoA/ROCK pathway activation. This pathway, involved in many cardiovascular diseases, might explain the cardiac and pulmonary toxicity of serotoninergic agonists.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

Therapeutic targets in pulmonary arterial hypertension

Pulmonary arterial hypertension is a progressive, fatal disease. Current treatments including prostanoids, endothelin-1 (ET-1) antagonists, and phosphodiesterase (PDE) inhibitors, have sought to address the pulmonary vascular endothelial dysfunction and vasoconstriction associated with the condition. These treatments may slow the progression of the disease but do not afford a cure. Future treatments must target more directly the structural vascular changes that impair blood flow through the pulmonary circulation. Several novel therapeutic targets have been proposed and are under active investigation, including soluble guanylyl cyclase, phosphodiesterases, tetrahydrobiopterin, 5-HT2B receptors, vasoactive intestinal peptide, receptor tyrosine kinases, adrenomedullin, Rho kinase, elastases, endogenous steroids, endothelial progenitor cells, immune cells, bone morphogenetic protein and its receptors, potassium channels, metabolic pathways, and nuclear factor of activated T cells. Tyrosine kinase inhibitors, statins, 5-HT2B receptor antagonists, EPCs and soluble guanylyl cyclase activators are among the most advanced, having produced encouraging results in animal models, and human trials are underway. This review summarises the current research in this area and speculates on their likely success.

Apoptosis and effects of intracavernous bone marrow cell injection in a rat model of postprostatectomy erectile dysfunction

OBJECTIVES

To investigate the pathophysiology of postprostatectomy erectile dysfunction (pPED) in a rat model of bilateral cavernous nerve ablation (BCNA) and to assess the effects of local bone marrow mononuclear cell (BMMNC) injection on erectile dysfunction (ED) and cavernosal cellular abnormalities caused by BCNA.

DESIGN, SETTING, AND PARTICIPANTS

This was an experimental study in Fisher rats with BCNA.

INTERVENTION

Intervention included BNCA, electrical stimulation of the pelvic ganglion, and local BMMNC injection.

MEASUREMENTS

Erectile responses to electric pelvic ganglion stimulation were studied. Cavernous tissue was examined to determine the cell types undergoing apoptosis and to detect changes in protein and gene expression of neuronal nitric oxide synthase (nNOS) and endothelial nitric oxide synthase (eNOS) using real-time quantitative polymerase chain reaction (RTQ-PCR) and Western blotting. The effects of local BMMNC injection on these parameters were studied.

RESULTS AND LIMITATIONS

Diffuse apoptosis was noted in the connective tissue mesenchymal cells and vascular smooth muscle and endothelial cells. Compared with sham-operated controls, nNOS and eNOS levels were decreased after 3 wk and were normal (eNOS) or increased (nNOS) after 5 wk, suggesting spontaneous nerve regeneration. Despite nNOS recovery, erectile responses to electrical stimulation remained impaired after 5 wk, when mesenchymal cell apoptosis was the main persistent biologic abnormality. BMMNC injection decreased apoptotic cell numbers, accelerated the normalisation of nNOS and eNOS, and partially restored erectile responses at week 5.

CONCLUSIONS

Massive cell apoptosis may play a key role in the pathophysiology of pPED. In this animal model, apoptosis persisted despite spontaneous nerve regeneration, suggesting that the course of BCNA-induced cell dysfunction was independent of reinnervation. BMMNC improved erectile function by inhibiting apoptosis and may hold promise for repairing penile cell damage caused by radical prostatectomy (RP).

Molecular pathogenesis of pulmonary arterial hypertension

Recent investigations have suggested that it might be possible to reverse the pathology of pulmonary arterial hypertension (PAH), a disorder that can be rapidly progressive and fatal despite current treatments including i.v. prostacyclin. This review will address the cellular and molecular processes implicated in clinical, genetic, and experimental studies as underlying the pulmonary vascular abnormalities associated with PAH. Emerging treatments are aimed at inducing apoptosis of abnormal vascular cells that obstruct blood flow and at promoting regeneration of "lost" distal vasculature.

Serotonin-mediated protein carbonylation in the right heart

Pulmonary hypertension is a devastating disease, which leads to right heart failure. Serotonin (5-HT) plays important roles in the pathogenesis of pulmonary hypertension and pulmonary vascular remodeling. The role of 5-HT in right heart failure, however, is unknown. Since oxidative stress may mediate heart failure, the present study examined the effects of 5-HT on protein oxidation in the adult rat right heart ventricle. Treatment of perfused isolated hearts with 5-HT resulted in the promotion of protein carbonylation, specifically in the right ventricle, but not in the left. While no differences between right and left ventricular antioxidant enzymes and 5-HT receptors/transporter were detected, monoamine oxidase A (MAO-A) expression and activity were found to be lower in the right ventricle compared to the left. These results indicate that differences in neither the reactive oxygen species (ROS) scavenging ability, 5-HT membrane signaling capacity, nor MAO-dependent production of hydrogen peroxide are responsible for varied 5-HT-mediated protein carbonylation in right and left ventricles. Rather, lower MAO-A in the right heart might preserve cytosolic 5-HT which triggers other mechanisms for ROS production. Consistently, inhibition of MAO-A resulted in the promotion of protein carbonylation. We propose that low MAO-A, thus reduced degradation of 5-HT, increases the intracellular 5-HT activity in the right ventricle, leading to the promotion of protein carbonylation.

Elastin insufficiency predisposes to elevated pulmonary circulatory pressures through changes in elastic artery structure

Elastin is a major structural component of large elastic arteries and a principal determinant of arterial biomechanical properties. Elastin loss-of-function mutations in humans have been linked to the autosomal-dominant disease supravalvular aortic stenosis, which is characterized by stenotic lesions in both the systemic and pulmonary circulations. To better understand how elastin insufficiency influences the pulmonary circulation, we evaluated pulmonary cardiovascular physiology in a unique set of transgenic and knockout mice with graded vascular elastin dosage (range 45-120% of wild type). The central pulmonary arteries of elastin-insufficient mice had smaller internal diameters (P < 0.0001), thinner walls (P = 0.002), and increased opening angles (P = 0.002) compared with wild-type controls. Pulmonary circulatory pressures, measured by right ventricular catheterization, were significantly elevated in elastin-insufficient mice (P < 0.0001) and showed an inverse correlation with elastin level. Although elastin-insufficient animals exhibited mild to moderate right ventricular hypertrophy (P = 0.0001) and intrapulmonary vascular remodeling, the changes were less than expected, given the high right ventricular pressures, and were attenuated compared with those seen in hypoxia-induced models of pulmonary arterial hypertension. The absence of extensive pathological cardiac remodeling at the high pressures in these animals suggests a developmental adaptation designed to maintain right-sided cardiac output in a vascular system with altered elastin content.

Pathobiology of pulmonary hypertension

A variety of conditions can lead to the development of pulmonary arterial hypertension (PAH). Current treatments can improve symptoms and reduce the severity of the hemodynamic abnormality, but most patients remain quite limited, and deterioration in their condition necessitates a lung transplant. This review discusses current experimental and clinical studies that investigate the pathobiology of PAH. An emerging theme is the consideration of ways in which one might reverse the advanced occlusive structural changes in the pulmonary circulation causing PAH. The current debate concerning the role of regeneration through stem cells is presented. This review also highlights investigations in a number of laboratories relating the pathobiology of PAH to mutations causing loss of function of bone morphogenetic protein receptor II in patients with familial PAH, as well as sporadic cases.

Novel interactions between the 5-HT transporter, 5-HT1B receptors and Rho kinase in vivo and in pulmonary fibroblasts

BACKGROUND AND PURPOSE

While the 5-HT and Rho-kinase (ROCK) pathways have been implicated in the development of pulmonary arterial hypertension (PAH), the nature of any interactions between them remain unclear. This study investigated a role for ROCK in 5-HT-regulated proliferative responses in lung fibroblasts in vivo and in vitro.

EXPERIMENTAL APPROACH

PAH was examined in mice over-expressing human 5-HT transporters (SERT+), from which pulmonary artery fibroblasts (PFs) were isolated to assess ROCK expression. In vitro analysis of 5-HT signalling employed CCL39 hamster lung fibroblasts.

KEY RESULTS

ROCK inhibition ablated increased pulmonary remodelling and hypertension observed in SERT+ mice, and ROCK1/2 protein levels were elevated in SERT+ PFs. ROCK inhibition also reduced 5-HT-stimulated proliferation by suppressing MEK-stimulated ERK phosphorylation. While optimal 5-HT-stimulated proliferation required 5-HT(1B) and 5-HT(2A) receptors and SERT, receptor sensitivity to Y27632 was restricted to the 5-HT(1B) receptor. Also, while hypoxia-induced pulmonary vascular remodelling and hypertension were sensitive to Y27632 in WT and SERT+ animals, the proportions sensitive to ROCK inhibition were increased by SERT over-expression.

CONCLUSIONS AND IMPLICATIONS

SERT over-expression increased ROCK-dependent pulmonary remodelling in normoxia and hypoxia and SERT over-expression was associated with elevated ROCK1/2 levels. ROCK also potentiated 5-HT(1B) receptor-stimulated ERK activation and proliferation in vitro by facilitating MEK-ERK interaction.

Serotonin: a review

5-Hydroxytryptamine, or serotonin, is a biogenic amine most noted for its role as a neurotransmitter. Manipulation of serotonin in animal models was used as a tool for studying its role in humans. Through such research serotonin has been shown to modulate gastrointestinal motility, peripheral vascular tone, cerebral vascular tone, and platelet function and has been implicated in the pathophysiology of mood disorders, emesis, migraine, irritable bowel syndrome (IBS), and pulmonary and systemic hypertension. The knowledge gained is being directly applied back to animals in research on drugs that manipulate the serotonergic system in dogs and cats. Increasing use and availability of drugs that manipulate the serotonergic system has created a circumstance through which a novel toxicity was discovered in both humans and animals. Serotonin Syndrome describes the clinical picture seen in humans and animals with serotonin toxicity. This paper provides a review the physiology of serotonin and its involvement in the pathophysiologic mechanisms of various conditions, including the Serotonin Syndrome.

Synergistic heterozygosity for TGFbeta1 SNPs and BMPR2 mutations modulates the age at diagnosis and penetrance of familial pulmonary arterial hypertension

PURPOSE

We hypothesized that functional TGFbeta1 SNPs increase TGFbeta/BMP signaling imbalance in BMPR2 mutation heterozygotes to accelerate the age at diagnosis, increase the penetrance and SMAD2 expression in familial pulmonary arterial hypertension.

METHODS

Single nucleotide polymorphism genotypes of BMPR2 mutation heterozygotes, age at diagnosis, and penetrance of familial pulmonary arterial hypertension were compared and SMAD2 expression was studied in lung sections.

RESULTS

BMPR2 mutation heterozygotes with least active -509 or codon 10 TGFbeta1 SNPs had later mean age at diagnosis of familial pulmonary arterial hypertension (39.5 and 43.2 years) than those with more active genotypes (31.6 and 33.1 years, P = 0.03 and 0.02, respectively). Kaplan-Meier analysis also showed that those with the less active single nucleotide polymorphisms had later age at diagnosis. BMPR2 mutation heterozygotes with nonsense-mediated decay resistant BMPR2 mutations and the least, intermediate and most active -509 TGFbeta1 SNP genotypes had penetrances of 33, 72, and 80%, respectively (P = 0.003), whereas those with 0-1, 2, or 3-4 active single nucleotide polymorphism alleles had penetrances of 33, 72, and 75% (P = 0.005). The relative expression of TGFbeta1 dependent SMAD2 was increased in lung sections of those with familial pulmonary arterial hypertension compared with controls.

CONCLUSIONS

The TGFbeta1 SNPs studied modulate age at diagnosis and penetrance of familial pulmonary arterial hypertension in BMPR2 mutation heterozygotes, likely by affecting TGFbeta/BMP signaling imbalance. This modulation is an example of Synergistic Heterozygosity.

Endothelial cell dysfunction and cross talk between endothelium and smooth muscle cells in pulmonary arterial hypertension

The pathogenesis of pulmonary arterial hypertension (PAH) involves a complex and multifactorial process in which endothelial cell dysfunction appears to play an integral role in mediating the structural changes in the pulmonary vasculature. Disordered endothelial cell proliferation along with concurrent neoangiogenesis, when exuberant, results in the formation of glomeruloid structures known as the plexiform lesions, which are common pathological features of the pulmonary vessels of patients with PAH. In addition, an altered production of various endothelial vasoactive mediators, such as nitric oxide, prostacyclin, endothelin-1, serotonin, chemokines and thromboxane, has been increasingly recognized in patients with PAH. Because most of these mediators affect the growth of the smooth muscle cells, an alteration in their production may facilitate the development of pulmonary vascular hypertrophy and structural remodeling characteristic of PAH. It is conceivable that the beneficial effects of many of the treatments currently available for PAH, such as the use of prostacyclin, nitric oxide, and endothelin receptor antagonists, result at least in part from restoring the balance between these mediators. A greater understanding of the role of the endothelium in PAH will presumably facilitate the evolution of newer, targeted therapies.

Pulmonary hypertension: therapeutic targets within the serotonin system

Pulmonary arterial hypertension (PAH) is characterized by a sustained and progressive elevation in pulmonary arterial pressure and pulmonary vascular remodelling leading to right heart failure and death. Prognosis is poor and novel therapeutic approaches are needed. The serotonin hypothesis of PAH originated in the 1960s after an outbreak of the disease was reported among patients taking the anorexigenic drugs aminorex and fenfluramine. These are indirect serotonergic agonists and serotonin transporter substrates. Since then many advances have been made in our understanding of the role of serotonin in the pathobiology of PAH. The rate-limiting enzyme in the synthesis of serotonin is tryptophan hydroxylase (Tph). Serotonin is synthesized, through Tph1, in the endothelial cells of the pulmonary artery and can then act on underlying pulmonary arterial smooth muscle cells and pulmonary arterial fibroblasts in a paracrine fashion causing constriction and remodelling. These effects of serotonin can be mediated through both the serotonin transporter and serotonin receptors. This review will discuss our current understanding of 'the serotonin hypothesis' of PAH and highlight possible therapeutic targets within the serotonin system.

Interactions between integrin alphaIIbbeta3 and the serotonin transporter regulate serotonin transport and platelet aggregation in mice and humans

The essential contribution of the antidepressant-sensitive serotonin (5-HT) transporter SERT (which is encoded by the SLC6A4 gene) to platelet 5-HT stores suggests an important role of this transporter in platelet function. Here, using SERT-deficient mice, we have established a role for constitutive SERT expression in efficient ADP- and thrombin-triggered platelet aggregation. Additionally, using pharmacological blockers of SERT and the vesicular monoamine transporter (VMAT), we have identified a role for ongoing 5-HT release and SERT activity in efficient human platelet aggregation. We have also demonstrated that fibrinogen, an activator of integrin alphaIIbbeta3, enhances SERT activity in human platelets and that integrin alphaIIbbeta3 interacts directly with the C terminus of SERT. Consistent with these findings, knockout mice lacking integrin beta3 displayed diminished platelet SERT activity. Conversely, HEK293 cells engineered to express human SERT and an activated form of integrin beta3 exhibited enhanced SERT function that coincided with elevated SERT surface expression. Our results support an unsuspected role of alphaIIbbeta3/SERT associations as well as alphaIIbbeta3 activation in control of SERT activity in vivo that may have broad implications for hyperserotonemia, cardiovascular disorders, and autism.

Deficiency of the NHE1 gene prevents hypoxia-induced pulmonary hypertension and vascular remodeling

RATIONALE

Our previous studies found that Na(+)/H(+) exchanger (NHE) activity played an essential role in pulmonary artery smooth muscle cell (PASMC) proliferation and in the development of hypoxia-induced pulmonary hypertension and vascular remodeling. Other investigators recently observed increased expression of the NHE isoform 1 (NHE1) gene in rodents with pulmonary hypertension induced by hypoxia. However, a causal role for the NHE1 gene in pulmonary hypertension has not been determined.

OBJECTIVES

To determine the causal role of the NHE1 gene in pulmonary hypertension and vascular remodeling.

METHODS

We used NHE1-null mice to define the role of the NHE1 gene in the development of pulmonary hypertension and remodeling induced by hypoxia and to delineate the NHE1 regulatory pathway.

MEASUREMENTS AND MAIN RESULTS

After 2 weeks of exposure to hypoxia, in contrast to wild-type hypoxic littermates, there was no significant increase in right ventricular systolic pressure, in the ratio of right ventricular to left ventricular plus septal weight [RV/(LV + S)], or in medial wall thickness of the pulmonary arterioles in homozygous mice (NHE1(-/-)). There was a significant decrease in Rho kinase (ROCK1 and ROCK2) expression, accompanied by an increase in p27 expression in NHE1(-/-) mice.

CONCLUSIONS

Our study demonstrated that deficiency of the NHE1 gene prevented the development of hypoxia-induced pulmonary hypertension and vascular remodeling in mice and revealed a novel regulatory pathway associated with NHE1 signaling.

Medical therapy for pulmonary arterial hypertension

Recent advances in the understanding of pulmonary arterial hypertension have led to new therapeutic options, although the disease remains incurable and continues to cause substantial morbidity and mortality. Disease-specific therapies have been approved for use in the US, including epoprostenol and its various analogs, endothelin receptor antagonists, and phosphodiesterase 5 inhibitors. The use of combination therapy with agents from more than one of these drug classes is becoming increasingly common, although guidelines establishing optimal combinations are lacking. Meanwhile, potential future therapeutic options are actively being pursued.

Functions of serotonin in hypoxic pulmonary vascular remodeling

In lung vasculature, reversible constriction of smooth muscle cells exists in response to acute decrease in oxygen levels (hypoxia). Progressive and irreversible structural remodeling that reduces blood vessel lumen takes place in response to chronic hypoxia and results in pulmonary hypertension. Several studies have shown a role of serotonin in regulating acute and chronic hypoxic responses. In this review the contribution of serotonin, its receptors and transporter in lung hypoxic responses is discussed. Hypoxic conditions modify plasma levels of serotonin, serotonin transporter activity, and expression of 5-HT1B and 5-HT2B receptors. These appear to be required for pulmonary vascular cell proliferation, which depends on the ratio between reactive oxygen species and nitric oxide. A heterozygous mutation was identified in the 5-HT2B receptor gene of a patient who developed pulmonary hypertension after fenfluramines anorexigen treatment. This C-terminus truncated 5-HT2B mutant receptor presents lower nitric oxide coupling, and higher cell proliferation capacity than the wild-type receptor. Under low oxygen tension, cells increase the transcription of specific genes via stabilization of the transcription factor hypoxia-inducible factor (HIF)-1. Factors such as angiotensin II or thrombin that can also control HIF-1 pathway contribute to pulmonary vascular remodeling. The 5-HT2B receptor via phosphatidylinositol-3 kinase/Akt activates nuclear factor-kappaB, which is involved in the regulation of HIF-1 expression. Acontrol of HIF- 1 by 5-HT2B receptors explains why expression of pulmonary vascular remodeling factors, such as endothelin-1 or transforming growth factor-beta, which is HIF-1-alpha regulated, is not modified in hypoxic 5-HT2B receptor mutant mice. Understanding the detailed mechanisms involved in lung hypoxic responses may provide general insight into pulmonary hypertension pathogenesis.

Hypoxia and chronic lung disease

The lung is both the conduit for oxygen uptake and is also affected by hypoxia and hypoxia signaling. Decreased ventilatory drive, airway obstructive processes, intra-alveolar exudates, septal thickening by edema, inflammation, fibrosis, or damage to alveolar capillaries will all interpose a significant and potentially life-threatening barrier to proper oxygenation, therefore enhancing the alveolar/arterial pO2 gradient. These processes result in decreased blood and tissue oxygenation. This review addresses the relationship of hypoxia with lung development and with lung diseases. We particularly focus on molecular mechanisms underlying hypoxia-driven physiological and pathophysiological lung processes, specifically in the infant lung, pulmonary hypertension, and chronic obstructive pulmonary disease.

Proteomics of transformed lymphocytes from a family with familial pulmonary arterial hypertension

RATIONALE

Not all family members with BMPR2 mutations develop pulmonary arterial hypertension (PAH), implying that additional modifier genes or proteins are necessary for full expression of the disease.

OBJECTIVES

To determine whether protein expression is altered in patients with familial PAH (FPAH) compared with obligate carriers and nondiseased control subjects.

METHODS

Protein extracts from transformed blood lymphocytes from four patients with FPAH, three obligate carriers, and three married-in control subjects from one family with a known BMPR2 mutation (exon 3 T354G) were labeled with either Cy3 or Cy5. Cy3/5 pairs were separated by standard two-dimensional differential gel electrophoresis using a Cy2-labeled internal standard of all patient samples. Log volume ratios were analyzed using a linear mixed-effects model. Proteins were identified by matrix-assisted laser desorption ionization, time-of-flight mass spectrometry (MALDI-TOF MS) and tandem TOF/TOF MS/MS.

MEASUREMENTS AND MAIN RESULTS

Hierarchical clustering, heat-map, and principal components analysis revealed marked changes in protein expression in patients with FPAH when compared with obligate carriers. Significant changes were apparent in expression of 16 proteins (P < 0.05) when affected patients were compared with obligates: nine showed a significant increase and seven showed a significant reduction.

CONCLUSIONS

A series of novel proteins with altered expression were found that could distinguish affected patients from obligate carriers and married-in controls in a single family with a BMPR2 mutation. These differences provide new information highlighting proteins that may be involved in the mechanism(s) that differentiates those individuals with a BMPR2 mutation who develop FPAH from those who do not.

Serotonin transporter protein in pulmonary hypertensive rats treated with atorvastatin

HMG-CoA-reductase inhibitors (statins) influence lipid metabolism and have pleiotropic effects. Several statins reduce various forms of pulmonary hypertension (PH) in animal models. The relationship between atorvastatin and expression of serotonin transporter protein (5-HTT) remains unknown. This study focused on the effects of atorvastatin on the course of monocrotaline (MCT)-induced PH and its relation to 5-HTT expression. Male Sprague-Dawley rats were challenged with MCT with or without subsequent daily oral treatment with 0.1, 1, and 10 mg/kg of atorvastatin for 28 days. Over the 4-wk course, the progression of PH was followed by transthoracic echocardiography [pulmonary artery pressure was assessed by pulmonary artery flow acceleration time (PAAT), an estimate reciprocal to pulmonary artery pressure], and, at the end of the 4-wk course, invasive right ventricular pressure, right ventricular weight, quantitative morphology, and 5-HTT expression were measured. MCT caused significant PH as early as 7 days after injection. Atorvastatin treatment increased PAAT and reduced right ventricular pressure, right ventricular hypertrophy, and vascular remodeling over the 4-wk course. MCT challenge was associated with increased pulmonary vascular 5-HTT expression, and atorvastatin treatment reduced the 5-HTT expression. MCT-induced PH over the course of 4 wk can be easily followed by transthoracic echocardiography, and atorvastatin is effective in reducing the PH. Atorvastatin's effects are associated with a decrease of 5-HTT expression.

5-hydroxytryptamine in the cardiovascular system: focus on the serotonin transporter (SERT)

1. The function of the serotonin transporter (SERT) is to take up and release serotonin (5-hydroxytyptamine (5-HT)) from cells and this function of SERT in the central nervous system (CNS) is well-documented; SERT is the target of selective serotonin reuptake inhibitors used in the treatment of CNS disorders, such as depression. 2. The aim of the present review is to discuss our current knowledge of 5-HT and SERT in the cardiovascular (CV) system, as well as their function in physiological and pathophysiological states. 3. The SERT protein has been located in multiple CV tissues, including the heart, blood vessels, brain, platelets, adrenal gland and kidney. Modification of SERT function occurs at both transcriptional and translational levels. The functions of SERT in these tissues is largely unexplored, but includes modulation of cardiac and smooth muscle contractility, platelet aggregation, cellular mitogenesis, modulating neuronal activity and urinary excretion. 4. Recent studies have uncovered potential relationships between the expression of SERT gene promoter variants (long (l) or short (s)) with CV diseases. Specifically, the risk of myocardial infarction and pulmonary hypertension is increased with expression of the ll promoter, a variant associated with increased expression and function of SERT. The relationship between promoter variants and other CV diseases has not been investigated. 5. Newly available experimental tools, such as pharmacological compounds and genetically altered mice, should prove useful in the investigation of the function of SERT in the CV system. 6. In summary, the function of SERT in the CV system is just beginning to be revealed.

Tryptophan hydroxylase 1 knockout and tryptophan hydroxylase 2 polymorphism: effects on hypoxic pulmonary hypertension in mice

Serotonin [5-hydroxytryptamine (5-HT)] biosynthesis depends on two rate-limiting tryptophan hydroxylases (Tph): Tph1, which is expressed in peripheral organs, and Tph2, which is expressed in neurons. Because 5-HT is involved in pulmonary hypertension (PH), we investigated whether genetic variations in Tph1 and/or Tph2 affected PH development in mice. To examine the functional impact of peripheral Tph1 deficiency on hypoxic PH, we used Tph1(-/-) mice characterized by very low 5-HT synthesis rates and contents in the gut and lung and increased 5-HT synthesis in the forebrain. With chronic hypoxia, 5-HT synthesis in the forebrain increased further. Hypoxic PH, right ventricular hypertrophy, and distal pulmonary artery muscularization were less severe (P < 0.001) than in wild-type controls. The Tph inhibitor p-chlorophenylalanine (100 mgxkg(-1)xday(-1)) further improved these parameters. We then investigated whether mouse strains harboring the C1473G polymorphism of the Tph2 gene showed different PH phenotypes during hypoxia. Forebrain Tph activity was greater and hypoxic PH was more severe in C57Bl/6 and 129X1/SvJ mice homozygous for the 1473C allele than in DBA/2 and BALB/cJ mice homozygous for the 1473G allele. p-Chlorophenylalanine reduced PH in all groups and abolished the difference in PH severity across mouse strains. Hypoxia increased 5-hydroxytryptophan accumulation but decreased 5-HT contents in the forebrain and lung, suggesting accelerated 5-HT turnover during hypoxia. These results provide evidence that dysregulation of 5-HT synthesis is closely linked to the hypoxic PH phenotype in mice and that Tph1 and Tph2 may contribute to PH development.

Emerging therapies for pulmonary arterial hypertension

Pulmonary arterial hypertension is characterised by increased pulmonary vascular resistance due to increased vascular tone and structural remodelling of pulmonary vessels. The therapies that are in use so far have been developed to correct endothelial dysfunction and reduce vasomotor tone. These treatments have a limited effect on the remodelling process and, increasingly, the focus is turning to potent strategies for inhibiting vascular proliferation and promoting vascular apoptosis. Multiple novel targets have been uncovered over the last 5 years and several are now in early clinical trials. At present, it is clear that there is no single treatment for the condition. Although this is the case, studies are investigating the role of combining therapies that are already established.

Long-term inhibition of Rho-kinase ameliorates hypoxia-induced pulmonary hypertension in mice

Pulmonary hypertension (PH) is a fatal disease characterized by endothelial dysfunction, hypercontraction and proliferation of vascular smooth muscle cells, and migration of inflammatory cells for which no satisfactory treatment has yet been developed. It has been recently demonstrated that Rho-kinase, an effector of the small GTPase Rho, is involved in the pathogenesis of arteriosclerosis and that long-term inhibition of Rho-kinase markedly ameliorates monocrotaline-induced PH in rats. However, it remains to be examined whether direct inhibition of Rho-kinase also ameliorates PH with a different etiology and whether endothelial nitric oxide synthase (eNOS) is involved in the beneficial effects of Rho-kinase inhibition. This study was designed to address those 2 important issues in a hypoxia-induced PH model using wild-type (WT) and eNOS-deficient (eNOS) mice. Long-term blockade of Rho-kinase with fasudil (100 mg/kg/d) for 3 weeks markedly improved PH and right ventricular hypertrophy in WT mice with a lesser but significant inhibition noted in eNOS mice. Fasudil upregulated eNOS with increased Akt phosphorylation in WT but not in eNOS mice. These results suggest that long-term inhibition of Rho-kinase also ameliorates hypoxia-induced PH in mice, for which eNOS activation may partially be involved.

An inadequate pulmonary vascular capacity and susceptibility to pulmonary arterial hypertension in broilers

Broilers are susceptible to pulmonary hypertension syndrome (PHS; ascites syndrome) when their pulmonary vascular capacity is anatomically or functionally inadequate to accommodate the requisite cardiac output without an excessive elevation in pulmonary arterial pressure. The consequences of an inadequate pulmonary vascular capacity have been demonstrated experimentally and include elevated pulmonary vascular resistance (PVR) attributable to noncompliant, fully engorged vascular channels; sustained pulmonary arterial hypertension (PAH); systemic hypoxemia and hypercapnia; specific right ventricular hypertrophy, and right atrioventricular valve failure (regurgitation), leading to central venous hypertension and hepatic cirrhosis. Pulmonary vascular capacity is broadly defined to encompass anatomical constraints related to the compliance and effective volume of blood vessels, as well as functional limitations related to the tone (degree of constriction) maintained by the primary resistance vessels (arterioles) within the lungs. Surgical occlusion of 1 pulmonary artery halves the anatomical pulmonary vascular capacity, doubles the PVR, triggers PAH, eliminates PHS-susceptible broilers, and reveals PHS-resistant survivors whose lungs are innately capable of handling sustained increases in pulmonary arterial pressure and cardiac output. We currently are using i.v. microparticle injections to increase the PVR and trigger PAH sufficient in magnitude to eliminate PHS-susceptible individuals while allowing PHS-resistant individuals to survive as progenitors of robust broiler lines. The microparticles obstruct pulmonary arterioles and cause local tissues and responding leukocytes to release vasoactive substances, including the vasodilator NO and the highly effective vasoconstrictors thromboxane A(2) and serotonin [5-hydroxytryptamine (5-HT)]. Nitric oxide is the principal vasodilator responsible for modulating (attenuating) the PAH response and ensuing mortality triggered by i.v. microparticle injections, whereas microparticle-induced increases in PVR can be attributed principally to 5-HT. Our observations support the hypothesis that susceptibility to PHS is a consequence of anatomically inadequate pulmonary vascular capacity combined with the functional predominance of the vasoconstrictor 5-HT over the vasodilator NO. The contribution of TxA(2) remains to be determined. Selecting broiler lines for resistance to PHS depends upon improving both anatomical and functional components of pulmonary vascular capacity.

Vasoactive peptides in cardiovascular (patho)physiology

Numerous vasoactive agents play an important physiological role in regulating vascular tone, reactivity and structure. In pathological conditions, alterations in the regulation of vasoactive peptides result in endothelial dysfunction, vascular remodeling and vascular inflammation, which are important processes underlying vascular damage in cardiovascular disease. Among the many vasoactive agents implicated in vascular (patho)biology, angiotensin II (Ang II), endothelin (ET), serotonin and natriuretic peptides appear to be particularly important because of their many pleiotropic actions and because they have been identified as potential therapeutic targets in cardiovascular disease. Ang II, ET-1, serotonin and natriuretic peptides mediate effects via specific receptors, which belong to the group of G-protein-coupled receptors. ET, serotonin and Ang II are primarily vasoconstrictors with growth-promoting actions, whereas natriuretic peptides, specifically atrial, brain and C-type natriuretic peptides, are vasodilators with natriuretic effects. Inhibition of vasoconstrictor actions with drugs that block peptide receptors, compounds that inhibit enzymes that generate vasoactive peptides or agents that increase levels of natriuretic peptides are potentially valuable therapeutic tools in the management of cardiovascular diseases. This review focuses on ET, natriuretic peptides and serotonin. The properties and distribution of these vasoactive agents and their receptors, mechanisms of action and implications in cardiovascular (patho)physiology will be discussed.

Pathology of pulmonary hypertension

The secondary role of pathology in the present clinical management of pulmonary hypertension (PH) reflects to some extent the limitations of the current understanding of the disease. Ample room exists for the diagnostic translation of the pathobiologic studies, with the goal of improving the diagnostic and prognostic power of the pathologic assessment of pulmonary vascular remodeling. This article seeks to show the complementarities of the pathology and pathobiology of PH.

New insights on receptor-dependent and monoamine oxidase-dependent effects of serotonin in the heart

Biogenic amines like serotonin (5-HT) and catecholamines usually act through stimulation of G-protein coupled receptors (GPCRs). We now have strong evidence that they can signal through receptor-independent mechanisms. One well described pathway is the degradation of biogenic amine by monoamine oxidases (MAOs) after transport into the cells by selective transporters. The oxidation of biogenic amines generates hydrogen peroxide, H(2)O(2), that can act as a signalling intermediate in the cell. This original mechanism of action of 5-HT is relevant in the heart since it is responsible for both cardiomyocyte hypertrophy and apoptosis. Moreover, in vivo experiments indicate a physiological significance for MAO in the damage during ischemia-reperfusion in the heart. Since functional 5-HT receptors are present in the heart and have also been demonstrated to contribute to cardiomyocyte growth and apoptosis, it is of major interest to evaluate respective contribution and cross-regulations between 5-HT receptors and MAO in cardiac function.

Modèles animaux d’hypertension artérielle pulmonaire

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a rare syndrome of fatigue and dyspnoea, caused by increased pulmonary vascular resistance and right heart failure without an identifiable pulmonary or cardiac cause. Despite important recent advances in treatment the condition remains incurable.

BACKGROUND

Experimental animal models of PAH rely on hypoxic or monocrotaline injected rodents, the creation of left to right shunts in lambs or piglets, ligation of the ductus arteriosus in newborn lambs, genetically manipulated rodents and tissue culture. Hypoxic pulmonary hypertension is usually only moderate and limited to medial hypertrophy with varying degrees of adventitial change, but may progress to extensive remodelling in some species. Monocrotaline induced pulmonary hypertension is severe with prominent medial hypertrophy, inflammatory adventitial remodelling and, initially, pulmonary oedema and endothelial apoptosis. Pulmonary hypertension induced by shunting remains the most realistic model of PAH but causes only moderate increase in vascular resistance due to medial hypertrophy. Pulmonary hypertension of the newborn is severe but largely vasospastic, with predominant medial hypertrophy. An increasing number of genetically manipulated rodents are becoming available for the investigation of specific signalling pathways.

VIEWPOINT

While none of the models has yet reproduced PAH each allows investigation of a specific hypothesis. Recent progress has resulted from genetic manipulation and molecular and cellular approaches.

CONCLUSIONS

Animal models of PAH share basic biological abnormalities which, together with the study of lung tissue from patients with severe disease should lead to better understanding of the pathology and therapeutic innovation.

Genetics and mediators in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an uncommon disorder of the pulmonary vasculature characterized by remodeling of the smallest pulmonary arteries, leading to a progressive increase in pulmonary vascular resistance. Various forms of PAH exist, including familial (FPAH) and idiopathic (IPAH) forms and associated conditions. FPAH transmits as an autosomal dominant trait that exhibits genetic anticipation but also markedly reduced penetrance (20%). The primary genetic defect of FPAH, identifiable in more than 70% of cases of FPAH, is a mutation in the gene encoding bone morphogenetic protein receptor type 2 (BMPR2), a member of the transforming growth factor beta superfamily. The true prevalence of BMPR2 mutations in IPAH is unknown, with reports ranging from 10% to 40% of patients. The cause of the variable phenotypic expression of PAH among carriers of mutated BMPR2 genes and patients is unclear, and likely related to environmental and genetic modifiers of disease not yet fully elucidated. Although BMPR2-related pathways seem to be pivotal, many other mediator pathways participate in the pathogenesis of different forms of PAH and are being actively investigated, both independently and in combination. As understanding of the molecular basis of this devastating disease improves, opportunities for earlier diagnosis, additional therapeutic regimens, and perhaps disease prevention will emerge.

Sertraline protects against monocrotaline-induced pulmonary hypertension in rats

1. Serotonin (5-HT), as a type of mitogen for smooth muscle cells, plays an important role in the development of pulmonary hypertension. It is known that selective serotonin re-uptake inhibitors (SSRI) inhibit 5-HT internalization. Therefore, the aim of the present study was to investigate the protective effect and mechanism of the SSRI sertraline against pulmonary hypertension. 2. Monocrotaline (MCT)-induced chronic 'inflammatory' pulmonary hypertension in Wistar rats was established. Pulmonary haemodynamic measurement and lung tissue morphological investigations were undertaken. Serotonin transporter (SERT) mRNA was assayed by reverse transcription-polymerase chain reaction (RT-PCR). 3. The results showed that pulmonary artery pressure (PAP) was significantly increased by MCT treatment from 12.6 +/- 2.1 to 20.1 +/- 3.4 mmHg (P < 0.01 vs control) and sertraline attenuated the MCT-induced increase in PAP from 20.1 +/- 3.4 to 16.4 +/- 1.8 mmHg (P < 0.05 vs MCT). The right ventricular index was increased in the MCT-treated group from 0.32 +/- 0.04 to 0.51 +/- 0.09 (P < 0.01 vs control) and was reduced to 0.42 +/- 0.04 by sertraline (P < 0.05 vs MCT). The degree of muscularization of the pulmonary artery in the MCT-treated group was significantly higher than control (P < 0.01) and was decreased by sertraline (P < 0.01 vs MCT). The RT-PCR assay showed that MCT increased SERT mRNA expression from 0.86 +/- 0.08 to 0.99 +/- 0.06 (P < 0.05 vs control), which was attenuated by sertraline (0.82 +/- 0.09; P < 0.05 vs MCT). 4. In conclusion, the SSRI sertraline protects against MCT-induced pulmonary hypertension by decreasing PAP, right ventricular index and pulmonary artery remodelling, which may be related to a reduction in SERT mRNA.

Effect of tryptophan hydroxylase 1 deficiency on the development of hypoxia-induced pulmonary hypertension

Tryptophan hydroxylase 1 catalyzes the rate-limiting step in the synthesis of serotonin in the periphery. Recently, it has been shown that expression of the tryptophan hydroxylase 1 gene is increased in lungs and pulmonary endothelial cells from patients with idiopathic pulmonary arterial hypertension. Here we investigated the effect of genetic deletion of tryptophan hydroxylase 1 on hypoxia-induced pulmonary arterial hypertension in mice by measuring pulmonary hemodynamics and pulmonary vascular remodeling before and after 2 weeks of hypoxia. In wild-type mice, hypoxia increased right ventricular pressure and pulmonary vascular remodeling. These effects of hypoxia were attenuated in the tryptophan hydroxylase 1-/-mice. Hypoxia increased right ventricular hypertrophy in both wild-type and tryptophan hydroxylase 1-/-mice suggesting that in vivo peripheral serotonin has a differential effect on the pulmonary vasculature and right ventricular hypertrophy. Contractile responses to serotonin were increased in pulmonary arteries from tryptophan hydroxylase 1-/-mice. Hypoxia increased serotonin-mediated contraction in vessels from the wild-type mice, but this was not further increased by hypoxia in the tryptophan hydroxylase 1-/-mice. In conclusion, these results indicate that tryptophan hydroxylase 1 and peripheral serotonin play an essential role in the development of hypoxia-induced elevations in pulmonary pressures and hypoxia-induced pulmonary vascular remodeling. In addition, the results suggest that, in mice, serotonin has differential effects on the pulmonary vasculature and right ventricular hypertrophy.

Hypoxia-induced pulmonary vascular remodeling: cellular and molecular mechanisms

Chronic hypoxic exposure induces changes in the structure of pulmonary arteries, as well as in the biochemical and functional phenotypes of each of the vascular cell types, from the hilum of the lung to the most peripheral vessels in the alveolar wall. The magnitude and the specific profile of the changes depend on the species, sex, and the developmental stage at which the exposure to hypoxia occurred. Further, hypoxia-induced changes are site specific, such that the remodeling process in the large vessels differs from that in the smallest vessels. The cellular and molecular mechanisms vary and depend on the cellular composition of vessels at particular sites along the longitudinal axis of the pulmonary vasculature, as well as on local environmental factors. Each of the resident vascular cell types (ie, endothelial, smooth muscle, adventitial fibroblast) undergo site- and time-dependent alterations in proliferation, matrix protein production, expression of growth factors, cytokines, and receptors, and each resident cell type plays a specific role in the overall remodeling response. In addition, hypoxic exposure induces an inflammatory response within the vessel wall, and the recruited circulating progenitor cells contribute significantly to the structural remodeling and persistent vasoconstriction of the pulmonary circulation. The possibility exists that the lung or lung vessels also contain resident progenitor cells that participate in the remodeling process. Thus the hypoxia-induced remodeling of the pulmonary circulation is a highly complex process where numerous interactive events must be taken into account as we search for newer, more effective therapeutic interventions. This review provides perspectives on each of the aforementioned areas.

Selective serotonin reuptake inhibitor use and outcomes in pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary vascular resistance which leads to right ventricular failure. Serotonin and the serotonin transporter play an important role in animal and human studies of PAH. We therefore hypothesized that PAH patients treated with high-affinity selective serotonin reuptake inhibitors (SSRIs) would have a reduced risk of death compared to PAH patients not treated with SSRIs.

METHODS

We performed a retrospective cohort study of 84 consecutive adult PAH patients who underwent initial evaluation from January 1994 to June 2002 at the Pulmonary Hypertension Center of the New York Presbyterian Hospital. Patient-time while receiving high-affinity SSRIs (K(d)<1nmol) (paroxetine, sertraline, or fluoxetine) was considered "exposed". Patient-time while receiving tricyclic, atypical, or no antidepressants was considered "unexposed".

RESULTS

Thirteen of the 84 patients (15%) used high-affinity SSRIs during the study period. Five patients were taking high-affinity SSRIs at baseline and 8 initiated high-affinity SSRIs during the follow-up period. The median time from baseline evaluation until initiation of high-affinity SSRIs was 125 (0-1227) days. The median duration of high-affinity SSRI use was 482 (110-1624) days and the total at-risk time on high-affinity SSRIs was 18.1 person-years. Seventy-nine (94%) patients were treated with warfarin; 38 (45%) received continuous intravenous epoprostenol; 12 (14%) received continuous subcutaneous treprostinil, and 23 (27%) were treated with oral bosentan. The median follow-up was 764 days. Twenty-four patients died and one underwent lung transplantation during the study period. There were no differences in age, gender, diagnosis, hemodynamics, or incidence of acute vasoreactivity between SSRI users and non-users. The risk of death for high-affinity SSRI users was lower but not statistically significantly different from that of non-users (hazard ratio=0.53, 95% CI 0.07 to 3.9, p=0.53). Adjustment for demographics, diagnosis, hemodynamics, or other therapies did not significantly change this result.

CONCLUSIONS

SSRI use was associated with a 50% reduction in the risk of death in a cohort of PAH patients which was not statistically significant. Larger cohort studies may better define this relationship; an adequately powered trial of high-affinity SSRIs in PAH patients may be warranted.

Dexfenfluramine does not worsen but moderates progression of chronic hypoxia-induced pulmonary hypertension

This study shows for the first time, that dexfenfluramine, a 5-HT(2) receptor agonist, attenuates the development of chronic hypoxia-induced pulmonary hypertension. Chronic exposure to hypoxia, 4 weeks, induced hypoxic pulmonary hypertension in adult rat as haemodynamic and cardiac measurements showed significant modifications in right ventricle parameters (free wall right ventricle thickness; pulmonary acceleration time and velocity time integral) in chronic hypoxic control when compared to normoxic control animals. We observed that free wall right ventricle thickness and pulmonary velocity time integral were significantly less in chronic hypoxic rats treated with dexfenfluramine when compared to chronic hypoxic control rats. Similarly, rats exposed to chronic hypoxia exhibited an increase in both right ventricle pressure and weight by comparison to normoxic control animals but those variations were significantly diminished in dexfenfluramine-treated rats, indicating the moderating influence exerted by dexfenfluramine on chronic hypoxia-induced pulmonary hypertension and cardiac alterations. Thus, we report here the ability of dexfenfluramine to limit chronic hypoxia-induced pulmonary hypertension, emphasizing the importance of the time after the dexfenfluramine treatment discontinuation to assess the influence of this 5-HT receptor agonist on the development of chronic hypoxia-induced pulmonary hypertension.