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

Heterozygous deficiency of hypoxia-inducible factor-2alpha protects mice against pulmonary hypertension and right ventricular dysfunction during prolonged hypoxia

Chronic hypoxia induces pulmonary vascular remodeling, leading to pulmonary hypertension, right ventricular hypertrophy, and heart failure. Heterozygous deficiency of hypoxia-inducible factor-1alpha (HIF-1alpha), which mediates the cellular response to hypoxia by increasing expression of genes involved in erythropoiesis and angiogenesis, has been previously shown to delay hypoxia-induced pulmonary hypertension. HIF-2alpha is a homologue of HIF-1alpha and is abundantly expressed in the lung, but its role in pulmonary hypertension remains unknown. Therefore, we analyzed the pulmonary response of WT and viable heterozygous HIF-2alpha-deficient (Hif2alpha(+/-)) mice after exposure to 10% O(2) for 4 weeks. In contrast to WT mice, Hif2alpha(+/-) mice were fully protected against pulmonary hypertension and right ventricular hypertrophy, unveiling a critical role of HIF-2alpha in hypoxia-induced pulmonary vascular remodeling. Pulmonary expression levels of endothelin-1 and plasma catecholamine levels were increased threefold and 12-fold respectively in WT but not in Hif2alpha(+/-) mice after hypoxia, suggesting that HIF-2alpha-mediated upregulation of these vasoconstrictors contributes to the development of hypoxic pulmonary vascular remodeling.

Activation of GATA-4 by serotonin in pulmonary artery smooth muscle cells

Serotonin (5-hydroxytryptamine (5-HT)) is a mitogen of pulmonary artery smooth muscle cells (PASMC) and plays an important role in the development of pulmonary hypertension. Signal transduction initiated by 5-HT involves serotonin transporter-dependent generation of reactive oxygen species and activation of the MEK-ERK pathway. However, the downstream transcriptional regulatory components have not been identified. In systemic smooth muscle cells, GATA-6 has been shown to regulate mitogenesis by driving cells into a quiescent state, and the down-regulation of GATA-6 induces mitogenesis. Thus, the present study tested the hypothesis that 5-HT induces mitogenesis of PASMC by down-regulating GATA-6. Quiescent bovine PASMC were treated with 5-HT, and the binding activity of nuclear extracts toward GATA DNA sequence was monitored. Surprisingly, PASMC express GATA-4, and 5-HT up-regulates the GATA DNA binding activity. Pretreatment of cells with inhibitors of serotonin transporter, reactive oxygen species, and MEK blocks GATA-4 activation by 5-HT. GATA-4 is not activated when the ERK phosphorylation site is mutated, indicating that 5-HT phosphorylates GATA-4 via the MEK/ERK pathway. GATA up-regulation is also induced by other mitogens of PASMC such as endothelin-1 and platelet-derived growth factor. Dominant negative mutants of GATA-4 suppress cyclin D2 expression and cell growth, indicating that GATA-4 activation regulates PASMC proliferation. Thus, GATA-4 mediates 5-HT-induced growth of PASMC and may be an important therapeutic target for the prevention of pulmonary hypertension.

Primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a rare disorder characterised by raised pulmonary-artery pressure in the absence of secondary causes. Precapillary pulmonary arteries are affected by medial hypertrophy, intimal fibrosis, microthrombosis, and plexiform lesions. Most individuals present with dyspnoea or evidence of right heart failure. Echocardiography is the best non-invasive test to screen for suspected pulmonary hypertension. The discovery of mutations in the coding region of the gene for bone morphogenetic protein receptor 2 in patients with familial and sporadic PPH may help not only to elucidate pathogenesis but also to direct future treatment options. The pathogenesis is not completely understood, but recent investigations have revealed many possible candidate modifier genes. Without treatment, the disorder progresses in most cases to right heart failure and death. With current therapies such as epoprostenol, progression of disease is slowed, but not halted. Many promising new therapeutic options, including prostacyclin analogues, endothelin-1-receptor antagonists, and phosphodiesterase inhibitors, improve clinical function and haemodynamic measures and may prolong survival.

The role of 5-HT on the cardiovascular and renal systems and the clinical potential of 5-HT modulation

The main peripheral sources of 5-hydroxytryptamine (5-HT) are as a neurotransmitter and local hormone in the gastrointestinal tract, and stored in circulating platelets and pulmonary neuroepithelial bodies. 5-HT has been shown to have many possible physiological and pathophysiological roles on the cardiovascular and renal systems. Thus, 5-HT may contribute to valvular heart disease, coronary artery disease, pulmonary hypertension, pulmonary embolism, pre-eclampsia, peripheral vascular disease and diabetic nephropathy. Consequently, modulators of the 5-HT system have diverse clinical potential. For instance, selective 5-HT subtype 3 receptor (5-HT(3)) antagonists may have potential in the treatment of the pain associated with myocardial infarction. MCI-9042 (sarpogrelate) or other 5-HT(2A) antagonists may have clinical potential for the treatment of vasospastic angina, ischaemic heart disease, reperfusion injury and hindlimb ischaemia. Several modulators of 5-HT (5-HT transporter inhibitors, 5-HT(1B) and (2B) antagonists) may have potential alone or in combination in the treatment of pulmonary hypertension. In hypertension, agonists at the 5-HT(7) and antagonists at the 5-HT(2B) may reduce blood pressure, and in diabetes, sarpogrelate may protect against nephropathy.

Granulocyte colony-stimulating factor enhances alpha-naphthylthiourea-induced pulmonary hypertension

Physiopathological discrepancies exist between the most widely used models of pulmonary hypertension (PH), namely monocrotaline- and hypoxia-induced PH. The development of a new model could help in the understanding of underlying mechanisms. Repeated alpha-naphthylthiourea (ANTU) injections (5 mg/kg weekly, 3 wk) induced pulmonary vascular remodeling, which was associated with development of PH and right ventricular hypertrophy. ANTU followed by granulocyte colony-stimulating factor (G-CSF; 25 microgram. kg(-1). day(-1) subcutaneously, 3 days/wk) induced higher pulmonary arterial pressures and right ventricular hypertrophy than ANTU alone. Lidocaine, which inhibits neutrophil functions, inhibited PH exacerbation by G-CSF. Endothelial nitric oxide synthase expression, measured to assess ANTU-related endothelial toxicity, decreased significantly in ANTU-treated rats and fell even more sharply when G-CSF was given. This occurred despite a significant increase in vascular endothelial cell growth factor expression in lung and right ventricle in rats given ANTU alone and even more in rats given ANTU plus G-CSF. Repeated ANTU administration induces PH with vascular remodeling that can be further aggravated by the neutrophil activator G-CSF.

Pathophysiology of pulmonary hypertension due to lung disease

Pulmonary hypertension (PH) often complicates the course of patients with advanced lung disease, and it is associated with a worse prognosis. Per the recent classification of pulmonary hypertensive disorders, PH due to lung disease is considered as a separate category within a group of disorders that was previously referred to as "secondary" PH. Among the lung diseases associated with PH, the incidence and clinical course of PH is best known for patients with COPD. Per studies in patients with COPD and other lung disorders, it is evident that the pathophysiology and treatment of these disorders is generally distinct from that of pulmonary arterial hypertensive disorders. Changes in the pulmonary vasculature that accompany elevations in pulmonary vascular pressure are generally referred to as pulmonary vascular remodeling. Chronic hypoxia is well known to cause pulmonary vascular remodeling and PH, and it is the major mechanism implicated for the development of PH in patients with lung disease. Other mediators have also been implicated in the pathogenesis of PH in animal models and patients with PH, including patients with pulmonary diseases. General features of pulmonary vascular remodeling are discussed with particular emphasis on those changes that have been described in patients with lung diseases. Recent discoveries in these areas are also reviewed, and findings in pulmonary arterial hypertensive diseases are contrasted with those found in patients with PH due to lung diseases. Some of these discoveries have already led to new treatment strategies for patients with the most severe forms of PH. PH due to lung diseases shares some common pathophysiologic features with pulmonary arterial hypertension. Therefore, it is likely that these discoveries and new treatments will also be extended to benefit patients with PH due to lung disease.

Hypoxia induces different genes in the lungs of rats compared with mice

Different animal species have a varying response to hypoxia. Mice develop less pulmonary artery thickening after chronic hypoxia exposure than rats. We hypothesized that the lung tissue gene expression pattern displayed in hypoxic rats would differ from that of hypoxic mice. We exposed Sprague-Dawley rats and C57BL/6 mice to both 1 and 3 wk of hypobaric hypoxia. Although both species developed pulmonary hypertension, mice showed less pulmonary vascular remodeling than rats. Microarray gene analysis demonstrated a distinct pattern of gene expression between mice and rats when exposed to hypoxic conditions. In addition, some genes appeared to be more responsive at an earlier time point of 1 wk of hypoxia. Hypoxic conditions in the rat induce genes involved in endothelial cell proliferation, repression of apoptosis, and vasodilation. Mice exposed to hypoxic conditions decrease the expression of genes involved in vasodilation and in endothelial cell proliferation. Although we cannot determine whether the differential expression of genes during chronic hypoxia is cause or consequence of the differential pulmonary vascular remodeling, we propose that a balance between over- and under-expression of a selective group of genes may be responsible for lung vascular remodeling and vascular tone control.

5-Hydroxytryptamine and platelets: uptake and aggregation in hypoxic pulmonary hypertensive rats

Pulmonary hypertension is associated with various alterations in 5-hydroxytryptamine (5-HT) physiology. In this study in platelets from hypoxic pulmonary hypertensive rats (10% O(2); 1 week) and normoxic rats (room air), (i) initial rates of specific [3H]5-HT uptake were measured and (ii) potentiation of collagen- and ADP-induced aggregation by 5-HT was quantified. The platelet count was almost halved in hypoxic rats. In uptake experiments, there was a decrease in 5-HT uptake in platelets from hypoxic compared with normoxic rats, due to a 36% reduction in the maximal initial rate of uptake. The aggregation experiments showed that 5-HT (1-100 microM) increased the magnitude of responses to collagen and the duration of responses to ADP, but there was no difference between hypoxic and normoxic rats. Abnormalities in platelet function may conceivably lead to increases in plasma 5-HT levels in hypoxic pulmonary hypertension, but are unlikely to aggravate pulmonary thromboembolism.

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.

Dexfenfluramine protects against pulmonary hypertension in rats

Dexfenfluramine (Dex), an appetite suppressant and serotonin reuptake inhibitor, is associated with pulmonary vascular disease (PVD) in some patients. The variability might be related to undetermined genetic abnormalities interacting with factors such as gender, weight loss, and vascular injury. We, therefore, assessed the effect of Dex (5 mg. kg(-1). day(-1)) in female obese rats, designated JCR:LA-cp or cp/cp; in lean rats, designated (+/?); and in normal Sprague-Dawley (S-D) rats under control conditions or after endothelial injury induced by monocrotaline (60 mg/kg). Pulmonary arterial pressure, right ventricular hypertrophy, percent medial wall thickness of muscular arteries, and muscularization of peripheral arteries were assessed as indexes of PVD. Although Dex reduced weight gain in cp/cp and S-D rats (P < 0.05 for both), it did not cause PVD. Moreover, PVD in S-D rats after monocrotaline injection was paradoxically ameliorated by Dex (P < 0.05) despite induction of pulmonary artery elastase (P < 0.05), which we showed is critical in inducing experimental PVD. Thus it is possible that Dex is concomitantly offsetting the sequelae of elastase activity.

Angiogenesis and pulmonary hypertension: a unique process in a unique disease

Severe pulmonary hypertension is a fatal condition associated with marked alterations of the cellular components of pulmonary arteries. In this review, we discuss the component of endothelial cell proliferation present in pulmonary arteries in patients with severe pulmonary hypertension. Because these proliferated endothelial cells exhibit markers of angiogenesis, we have named this process as "disordered or misguided angiogenesis." We also discuss the recent evidence that germline or somatic inactivating mutations in tumor suppressor or proapoptotic genes may play a significant role in the abnormal proliferation of pulmonary endothelial cells. The unraveling of the pathobiology of severe pulmonary hypertension may lead us to novel therapies and approaches to better diagnose the disease.

Function of the serotonin 5-hydroxytryptamine 2B receptor in pulmonary hypertension

Primary pulmonary hypertension is a progressive and often fatal disorder in humans that results from an increase in pulmonary blood pressure associated with abnormal vascular proliferation. Dexfenfluramine increases the risk of pulmonary hypertension in humans, and its active metabolite is a selective serotonin 5-hydroxytryptamine 2B (5-HT(2B)) receptor agonist. Thus, we investigated the contribution of the 5-HT(2B)receptor to the pathogenesis of pulmonary hypertension. Using the chronic-hypoxic-mouse model of pulmonary hypertension, we found that the hypoxia-dependent increase in pulmonary blood pressure and lung remodeling are associated with an increase in vascular proliferation, elastase activity and transforming growth factor-beta levels, and that these parameters are potentiated by dexfenfluramine treatment. In contrast, hypoxic mice with genetically or pharmacologically inactive 5-HT(2B)receptors manifested no change in any of these parameters. In both humans and mice, pulmonary hypertension is associated with a substantial increase in 5-HT(2B) receptor expression in pulmonary arteries. These data show that activation of 5-HT(2B) receptors is a limiting step in the development of pulmonary hypertension.

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.

Biodistribution study of [(123)I] ADAM in mice: correlation with whole body autoradiography

Iodine-123 labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I] ADAM) has been suggested as a promising serotonin transporter (SERT) imaging agent. Much research has been accomplished, mainly focusing on the SERT binding sites in the central nervous system (CNS). However, the biodistribution of [(123)I] ADAM using whole body autoradiography (WBAR) has never been previously described, to the best of our knowledge. In this study, we assayed the biodistribution of [(123)I] ADAM in tissues/organs removed from mice, and measured their radioactivity with a scintillation counter (SC). The results showed that the liver has the highest uptake. On the other hand, the WBAR clearly demonstrated that [(123)I] ADAM was bound to SERT-rich sites including those in the brain stem, lung, adrenal glands and intestinal mucosa. This radiotracer also accumulated in the liver, kidney, and thyroid. The results from both methods were compared; each has its own complementary role in the biodistribution studies. The SC method revealed the total amount of radiotracer accumulation in each organ, and the WBAR demonstrated more anatomical details of the radiotracer's distribution. The whole body distribution results of the radioligand using both methods explore the usage of this novel radioligand for most possible SERT binding sites, not only in the CNS but also in the peripheral nervous system and neuroendocrine tissues. These findings suggest that [(123)I] ADAM is a potentially useful imaging agent for SERT.

Pulmonary vasoconstriction by serotonin is inhibited by S-nitrosoglutathione

Nitric oxide (NO) functions as an endothelium-derived relaxing factor by activating guanylate cyclase to increase cGMP levels. However, NO and related species may also regulate vascular tone by cGMP-independent mechanisms. We hypothesized that naturally occurring NO donors could decrease the pulmonary vascular response to serotonin (5-HT) in the intact lung through chemical interactions with 5-HT(2) receptors. In isolated rabbit lung preparations and isolated pulmonary artery (PA) rings, 50-250 microM S-nitrosoglutathione (GSNO) inhibited the response to 0.01-10 microM 5-HT. The vasoconstrictor response to 5-HT was mediated by 5-HT(2) receptors in the lung, since it could be blocked completely by the selective inhibitor ketanserin (10 microM). GSNO inhibited the response to 5-HT by 77% in intact lung and 82% in PA rings. In PA rings, inhibition by GSNO could be reversed by treatment with the thiol reductant dithiothreitol (10 mM). 3-Morpholinosydnonimine (100-500 microM), which releases NO and O simultaneously, also blocked the response to 5-HT. Its chemical effects, however, were distinct from those of GSNO, because 5-HT-mediated vasoconstriction was not restored in isolated rings by dithiothreitol. In the intact lung, neither NO donor altered the vascular response to endothelin, which activates the same second-messenger vasoconstrictor system as 5-HT. These findings, which did not depend on guanylate cyclase, are consistent with chemical modification by NO of the 5-HT(2) G protein-coupled receptor system to inhibit vasoconstriction, possibly by S-nitrosylation of the receptor or a related protein. This study demonstrates that GSNO can regulate vascular tone in the intact lung by a reversible mechanism involving inhibition of the response to 5-HT.

Janus face of vascular endothelial growth factor: the obligatory survival factor for lung vascular endothelium controls precapillary artery remodeling in severe pulmonary hypertension

Vascular endothelial growth factor (VEGF) plays a central role in the life and death of pulmonary vascular endothelial cells. Treatment of neonatal or adult rats with a VEGF receptor blocker destroys lung capillaries by inducing endothelial cell apoptosis and causes emphysema. Human lung tissue samples from patients with endstage emphysema have decreased levels of VEGF messenger RNA and protein and have decreased expression of kinase insert domain-containing receptor (VEGF receptor II). These decreases are associated with a high rate of alveolar septal cell apoptosis, indicating perhaps that decreased VEGF and kinase insert domain-containing receptor expression impairs endothelial cell survival in emphysematous lungs. Combination of VEGF receptor blockade with chronic hypoxia (3-wk exposure) results in obliteration of small precapillary pulmonary arteries by proliferating endothelial cells, severe pulmonary hypertension, and death caused by right-side heart failure. We propose that 1) VEGF receptor blockade causes endothelial cell apoptosis, 2) hypoxic vasoconstriction (shear stress) selects apoptosis-resistant endothelial cells that proliferate and obliterate the lumen, and 3) the vascular remodeling observed is relevant to the structural alterations that characterize severe pulmonary hypertension (including primary pulmonary hypertension) in humans. The endovascular cell growth in human disease and in our model exhibits some similarities with neoplastic cell growth. Chemotherapy strategies can now be employed in the animal model in an attempt to treat established vascular-obliterative lung disease.

Is the serotonin transporter involved in the pathogenesis of pulmonary hypertension?

Investigations on the effects of serotonin (5-HT) and the serotonin transporter (5-HTT) on the pulmonary circulation are of special interest because of the reported increased risk of primary pulmonary hypertension (PPH) in patients who used some appetite suppressants that interfere with 5-HT. In addition to its vasoactive effects, 5-HT exerts mitogenic and comitogenic effects on pulmonary artery smooth muscle cells (PASMCs). These mitogenic and comitogenic effects require 5-HT internalization by the high-affinity 5-HTT, which can be competitively inhibited by specific drugs such as fluoxetine and paroxetine. In a recent study, we showed that hypoxia increases the rate of 5-HTT gene transcription in PASMCs and potentiates the growth-promoting effect of 5-HT on these cells. An increase in the levels of 5-HTT messenger ribonucleic acid was observed in smooth-muscle cells from remodeled pulmonary arteries in rats subjected to long-term hypoxia. Two series of especially relevant data further support the idea that 5-HT plays a key role in PASMC proliferation in vivo: (1) treatments that increase plasma 5-HT levels aggravate pulmonary hypertension in rats subjected to long-term hypoxia, and this effect can be prevented by combined simultaneous treatment with 5-HTT inhibitors; and (2) knockout mice with disruption of the 5-HTT gene exhibit lesser degree of hypoxic pulmonary hypertension and pulmonary vascular remodeling than control mice despite increased hypoxic pulmonary vasoconstriction. These observations indicate that 5-HTT expression, activity, or both in PASMCs contribute to pulmonary vascular remodeling and that the inducing effects of some appetite suppressants on pulmonary hypertension may be related to possible effects of these drugs on 5-HTT expression, activity, or both.

Anorexigen-induced pulmonary hypertension and the serotonin (5-HT) hypothesis: lessons for the future in pathogenesis

Epidemiological studies have established that fenfluramine, D-fenfluramine, and aminorex, but not other appetite suppressants, increase the risk of primary pulmonary hypertension (PH). One current hypothesis suggests that fenfluramine-like medications may act through interactions with the serotonin (5-hydroxytryptamine [5-HT]) transporter (5-HTT) located on pulmonary artery smooth muscle cells and responsible for the mitogenic action of 5-HT. Anorexigens may contribute to PH by boosting 5-HT levels in the bloodstream, directly stimulating smooth muscle cell growth, or altering 5-HTT expression. We suggest that individuals with a high basal level of 5-HTT expression related to the presence of the long 5-HTT gene promoter variant may be particularly susceptible to one or more of these potential mechanisms of appetite-suppressant-related PH.

Contribution of the 5-HT(1B) receptor to hypoxia-induced pulmonary hypertension: converging evidence using 5-HT(1B)-receptor knockout mice and the 5-HT(1B/1D)-receptor antagonist GR127935

5-Hydroxytryptamine (5-HT)(1B) receptors mediate contraction in human pulmonary arteries, and 5-HT(1B) receptor-mediated contraction is enhanced in pulmonary arteries from hypoxic rats. Here we further examine the role of this receptor in the development of pulmonary hypertension (PHT) by examining (1) the effects of a 5-HT(1B/1D)-receptor antagonist (GR127935) on hypoxia-induced PHT (CHPHT) in rats and (2) CHPHT in 5-HT(1B)-receptor knockout mice. In rats, hypoxia increased right ventricular pressure and right ventricular hypertrophy and induced pulmonary vascular remodeling associated with an increase in pulmonary arterial wall thickness. GR127935 (3 mg. kg(-1). d(-1)) reduced all of these indices. 5-HT(1)-mediated contraction was enhanced in pulmonary arteries of the CHPHT rats. The effects of GR127935 on PHT indices were associated with an attenuation of the enhanced contractile responses to 5-HT and the 5-HT(1)-receptor agonist, 5-carboxamidotryptamine (5-CT), in isolated pulmonary arteries. In wild-type mice, hypoxia increased right ventricular hypertrophy, which was absent in 5-HT(1B)-receptor knockout mice. Hypoxia increased pulmonary vascular remodeling in wild-type mice, and this was reduced in the 5-HT(1B)-receptor knockout mice. Hypoxia increased 5-HT(1)-mediated contraction in pulmonary arteries from the wild-type mice and this was attenuated in the 5-HT(1B)-receptor knockout mice. In conclusion, the 5-HT(1B) receptor plays a role in the development of CHPHT. One possible mechanism may be via enhanced 5-HT(1) receptor-mediated contraction of the pulmonary arterial circulation.

Serotonin transporter overexpression is responsible for pulmonary artery smooth muscle hyperplasia in primary pulmonary hypertension

Hyperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is a hallmark pathological feature of primary pulmonary hypertension (PPH). Here we found that PA-SMCs from patients with PPH grow faster than PA-SMCs from controls when stimulated by serotonin or serum and that these effects are due to increased expression of the serotonin transporter (5-HTT), which mediates internalization of indoleamine. In the presence of 5-HTT inhibitors, the growth stimulatory effects of serum and serotonin were markedly reduced and the difference between growth of PA-SMCs from patients and controls was no longer observed. As compared with controls, the expression of 5-HTT was increased in cultured PA-SMCs as well as in platelets and lungs from patients with PPH where it predominated in the media of thickened pulmonary arteries and in onion-bulb lesions. The L-allelic variant of the 5HTT gene promoter, which is associated with 5-HTT overexpression and increased PA-SMC growth, was present in homozygous form in 65% of patients but in only 27% of controls. We conclude that 5-HTT activity plays a key role in the pathogenesis of PA-SMC proliferation in PPH and that a 5HTT polymorphism confers susceptibility to PPH.

Pulmonary vascular remodeling: a target for therapeutic intervention in pulmonary hypertension

Pulmonary vascular remodelling is an important pathological feature of pulmonary hypertension, leading to increased pulmonary vascular resistance and reduced compliance. It involves thickening of all three layers of the blood vessel wall (due to hypertrophy and/or hyperplasia of the predominant cell type within each layer), as well as extracellular matrix deposition. Neomuscularisation of non-muscular arteries and formation of plexiform and neointimal lesions also occur. Stimuli responsible for remodelling involve transmural pressure, stretch, shear stress, hypoxia, various mediators [angiotensin II, endothelin (ET)-1, 5-hydroxytryptamine, growth factors, and inflammatory cytokines], increased serine elastase activity, and tenascin-C. In addition, there are reductions in the endothelium-derived antimitogenic substances, nitric oxide, and prostacyclin. Intracellular signalling mechanisms involved in pulmonary vascular remodelling include elevations in intracellular Ca2+ and activation of the phosphatidylinositol pathway, protein kinase C, and mitogen-activated protein kinase. In animal models of pulmonary hypertension, various drugs have been shown to attenuate pulmonary vascular remodelling. These include angiotensin-converting enzyme inhibitors, angiotensin receptor antagonists, ET receptor antagonists, ET-converting enzyme inhibitors, nitric oxide, phosphodiesterase 5 inhibitors, prostacyclin, Ca2+ -channel antagonists, heparin, and serine elastase inhibitors. Inhibition of remodelling is generally accompanied by reductions in pulmonary artery pressure. The efficacy of some of the drugs varies, depending on the animal model of the disease. In view of the complexity of the remodelling process and the diverse aetiology of pulmonary hypertension in humans, it is to be anticipated that successful anti-remodelling therapy in the clinic will require a range of different drug options.

H(2)O(2) signals 5-HT-induced ERK MAP kinase activation and mitogenesis of smooth muscle cells

Our previous studies have shown that 5-hydroxytryptamine (5-HT) induces cellular hyperplasia/hypertrophy through protein tyrosine phosphorylation, rapid formation of superoxide (O(2)(-)), and extracellular signal-regulated kinase (ERK)1/ERK2 mitogen-activated protein (MAP) kinase activation. Intracellularly released O(2)(-) is rapidly dismuted by superoxide dismutase (SOD) to H(2)O(2), another possible cellular growth mediator. In the present study, we assessed whether H(2)O(2) participates in 5-HT-induced mitogenic signaling. Inactivation of cellular Cu/Zn SOD by copper-chelating agents inhibited 5-HT-induced DNA synthesis of bovine pulmonary artery smooth muscle cells (BPASMCs). Infection of BPASMCs with an adenovirus containing catalase inhibited both ERK1/ERK2 MAP kinase activation and DNA synthesis induced by 5-HT. Although we could not find evidence of p38 MAP kinase activation by 5-HT, SB-203580 and SB-202190, reported inhibitors of p38 MAP kinase, inhibited the 5-HT-induced growth of BPASMCs. However, these inhibitors also inhibited 5-HT-induced O(2)(-) release. Thus quenching of O(2)(-) may be their mechanism for inhibition of cellular growth unrelated to p38 MAP kinase inhibition. These data indicate that generation of O(2)(-) in BPASMCs in response to 5-HT is followed by an increase in intracellular H(2)O(2) that mediates 5-HT-induced mitogenesis through activation of ERK1/ERK2 but not of p38 MAP kinase.

The pathobiology of pulmonary hypertension. Endothelium

Dysfunctional endothelial cells have a central and critical role in the initiation and progression of severe pulmonary hypertension. The elucidation of the mechanisms involved in the control of endothelial cell proliferation and cell death in the pulmonary vasculature, therefore, is fundamentally important in the pathogenesis of severe pulmonary hypertension and of great interest for a better understanding of endothelial cell biology. Because the intravascular growth of endothelial cells resulting in tumorlets is unique to severe pulmonary hypertension, this phenomenon can teach researchers about the factors involved in the formation and maintenance of the normal endothelial cell monolayer. Clearly, in severe pulmonary hypertension, the "law of the endothelial cell monolayer" has been broken. The ultimate level of such a control is at the altered gene expression pattern that is conducive to endothelial cell growth and disruption of pulmonary blood flow. Secondary pulmonary hypertension certainly also is associated with proliferated pulmonary endothelial cells and plexiform lesions that are histologically indistinguishable from those in PPH. What is then the difference in the mechanisms of endothelial cell proliferation between primary and secondary pulmonary hypertension? The authors believe that PPH is a disease caused by somatic mutations in key angiogenesis- or apoptosis-related genes such as the TGF-beta receptor-2 and Bax. The loss of these important cell growth control mechanisms allows for the clonal expansion of endothelial cells from a single cell that has acquired a selective growth advantage. On the other hand, the proliferated endothelial cells in secondary pulmonary hypertension are polyclonal. It follows from this finding that local (vascular) factor(s) (such as increased shear stress), rather than mutations, play a major role in triggering endothelial cell proliferation. In PPH and secondary pulmonary hypertension, the researcher can postulate that the pulmonary vascular bed contains progenitor-like cells with the capacity of dysregulated growth. The main difference in the pathogenesis of primary and secondary pulmonary endothelial cell proliferation therefore may be the initial mechanism involved in the recruitment of an endothelial progenitor-like cell. In PPH, anorexigen-associated, and familial PPH, the proliferation of endothelial cells occurs from a mutated single cell, whereas in secondary pulmonary hypertension, several progenitor-like cells would be activated to grow. The abnormal endothelial cells in both forms of severe pulmonary hypertension expand because of the expression of angiogenesis-related molecules such as VEGF, VEGFR-2, HIF-1 alpha, and HIF-beta. Also important for the expansion of these cells is the down-regulation of expression of apoptosis-related mediators such as TGF-beta receptor-2 or Bax. The success of any therapy for severe pulmonary hypertension requires that the underlying process of endothelial cell proliferation could be controlled or reversed.