Basic Science of Pulmonary Arterial Hypertension for Clinicians

Abstract

HomeCirculationVol. 121, No. 18Basic Science of Pulmonary Arterial Hypertension for Clinicians Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissionsDownload Articles + Supplements ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toSupplementary MaterialsFree AccessResearch ArticlePDF/EPUBBasic Science of Pulmonary Arterial Hypertension for CliniciansNew Concepts and Experimental Therapies Stephen L. Archer, MD, E. Kenneth Weir, MD and Martin R. Wilkins, MD Stephen L. ArcherStephen L. Archer From the Section of Cardiology (S.L.A.), University of Chicago, Chicago, Ill; VA Medical Center, Minneapolis and University of Minnesota (E.K.W.), Minneapolis, Minn; and Department of Experimental Medicine and Toxicology (M.R.W.), Imperial College London, London United Kingdom. Search for more papers by this author , E. Kenneth WeirE. Kenneth Weir From the Section of Cardiology (S.L.A.), University of Chicago, Chicago, Ill; VA Medical Center, Minneapolis and University of Minnesota (E.K.W.), Minneapolis, Minn; and Department of Experimental Medicine and Toxicology (M.R.W.), Imperial College London, London United Kingdom. Search for more papers by this author and Martin R. WilkinsMartin R. Wilkins From the Section of Cardiology (S.L.A.), University of Chicago, Chicago, Ill; VA Medical Center, Minneapolis and University of Minnesota (E.K.W.), Minneapolis, Minn; and Department of Experimental Medicine and Toxicology (M.R.W.), Imperial College London, London United Kingdom. Search for more papers by this author Originally published11 May 2010https://doi.org/10.1161/CIRCULATIONAHA.108.847707Circulation. 2010;121:2045–2066Pulmonary arterial hypertension (PAH) is a syndrome in which pulmonary arterial obstruction increases pulmonary vascular resistance, which leads to right ventricular (RV) failure and a 15% annual mortality rate. The present review highlights recent advances in the basic science of PAH. New concepts clarify the nature of PAH and provide molecular blueprints that explain how PAH is initiated and maintained. Five basic science concepts provide a framework to understand and treat PAH: (1) Endothelial dysfunction creates an imbalance that favors vasoconstriction, thrombosis, and mitogenesis. Restoration of this balance by inhibition of endothelin and thromboxane or augmentation of nitric oxide (NO) and prostacyclin is the paradigm on which most current therapy is based. (2) PAH has a genetic component. Mutations (bone morphogenetic protein receptor-2 [BMPR2]) and single-nucleotide polymorphisms (SNPs; ion channels and transporter genes) predispose to PAH. (3) Excess proliferation, impaired apoptosis, and glycolytic metabolism in pulmonary artery smooth muscle, fibroblasts, and endothelial cells suggest analogies to cancer. Many experimental therapies reduce PAH by decreasing the proliferation/apoptosis ratio; these include inhibitors of pyruvate dehydrogenase kinase (PDK), serotonin transporters (SERT), survivin, 3-hydroxy-3-methylglutaryl coenzyme A reductase, transcription factors (hypoxia-inducible factor [HIF]-1α and nuclear factor of activated T lymphocytes [NFAT]), and tyrosine kinases. Augmentation of voltage-gated K+ channels (Kv1.5) and BMPR2 signaling also addresses this imbalance. Tyrosine kinase inhibitors used to treat cancer are currently in phase 1 PAH trials. (4) Refractory vasoconstriction may occur due to rho kinase activation. Fewer than 20% of PAH patients respond to conventional vasodilators; however, refractory vasoconstriction may respond to rho kinase inhibitors. (5) The RV can be targeted therapeutically. Although increased afterload initiates RV failure, which is the major cause of death/dysfunction in PAH, the RV may be amenable to cardiac-targeted therapies. The RV in PAH has features of ischemic, hibernating myocardium.Guided by these new concepts and armed with a better understanding of disease mechanisms, we are poised to identify new therapeutic targets. To achieve balance in a rapidly evolving field, we invited colleagues to contribute Figures and legends illustrating pathways in their area of expertise that are important to the pathogenesis and treatment of PAH. These contributors are acknowledged in the Acknowledgments section.EpidemiologyThere are 5 categories of pulmonary hypertension (PH) in the latest World Health Organization classification1: (1) PAH; (2) PH associated with left-sided heart disease; (3) PH associated with lung disease/hypoxia; (4) thromboembolic PH; and (5) miscellaneous. The present review focuses on category 1 (PAH), which includes idiopathic and familial PAH, as well as PAH associated with a variety of conditions (including connective tissue diseases and congenital heart disease), pulmonary venoocclusive disease, pulmonary capillary hemangiomatosis, and persistent pulmonary hypertension of the newborn. The incidence and prevalence of PAH, respectively, are estimated at 2.4 cases/million annually and 15 cases/million in France2 and 7.6 cases/million annually and 26 cases/million in Scotland.3 The global prevalence of PAH is hard to estimate because accurate diagnosis of PAH is difficult, and access to care is limited in many countries. Because diseases that are risk factors for PAH, such as HIV, schistosomiasis, and sickle cell disease, are more prevalent in the developing world, the global burden of PAH is likely greater than is recognized currently.4 In developed countries, prevalence will also likely increase as newer associations with PAH emerge, including dialysis5 and the metabolic syndrome,6 and as widespread access to echocardiography identifies PAH earlier and in more individuals.DefinitionPAH is a small subset of pulmonary hypertensive syndromes (World Health Organization categories 2 to 5). PAH is defined by a resting mean pulmonary artery pressure (PAP) >25 mm Hg, pulmonary vascular resistance (PVR) >3 Wood units, and pulmonary capillary wedge pressure <15 mm Hg (in the absence of other causes of PH). Unlike PAH, PH is ubiquitous, the sole diagnostic criterion being a resting mean PAP >25 mm Hg. This larger PH group often does not have intrinsic pulmonary vascular disease. Their PH is due to high flow, elevated left ventricular end-diastolic pressure, lung disease/hypoxia, or valve disease. There is no randomized clinical trial evidence that World Health Organization category 2 to 5 patients benefit from PAH-specific therapies and research to study these patients is critically required.PrognosisThe 1-year incident mortality rate of PAH remains high (15%) despite treatment with prostacyclin, endothelin antagonists, and phosphodiesterase (PDE)-5 inhibitors.7 Moreover, the mortality rate is much higher in cohorts of incident (new) rather than prevalent (preexisting) cases. Because PAH is a syndrome, it is not surprising that the prognosis varies depending on the associated comorbid conditions. Prognosis in PAH associated with congenital heart disease tends to be better than in idiopathic PAH (iPAH; 3-year survival rate 77% versus 35%).8 In another cohort of PAH patients treated with Flolan, survival in iPAH patients was better (65% at 3 years).9 Prognosis was worse in older patients and was also worse in PAH associated with scleroderma versus iPAH.9 PAH associated with scleroderma has a 3-year survival rate of only 34% to 47%.9,10Current TherapiesTreatment of PAH involves the use of prostanoids (given intravenously, by inhalation, subcutaneously, or orally), endothelin receptor blockers, and/or PDE5 inhibitors. L-type calcium channel blockers (eg, nifedipine) can be effective but are only safe for use in patients who respond to a 1-time vasodilator challenge with a >20% fall in mean PAP and no decline in cardiac output (a subset representing 12% to 20% of PAH patients).11 Most patients empirically receive anticoagulation to prevent thrombosis in situ and diuretics to limit edema. PAH treatments remain expensive and/or difficult to deliver and are more palliative than curative. A year of sildenafil is estimated to cost $13 000 versus approximately $56 000 for bosentan, whereas costs for inhaled iloprost and intravenous prostacyclin exceed $90 000 per year. The only randomized PAH study that has shown a survival benefit used intravenous Flolan (GlaxoSmithKline, Brentford, United Kingdom), which, compared with conventional therapy, decreased mortality in 81 World Health Organization class IV patients.12 Thus, there is a pressing need for less expensive and more effective therapies.Most current treatments (prostacyclin, endothelin antagonists, and warfarin) address endothelial dysfunction by augmenting vasodilator and antiproliferative mediators and inhibiting vasoconstrictor, prothrombotic, and mitogenic pathways. Our increasing knowledge of the cellular and molecular basis of PAH suggests many potential new therapeutic agents.HistologyThe histological findings in PAH include intimal hyperplasia, medial hypertrophy, adventitial proliferation/fibrosis, occlusion of small arteries, thrombosis in situ, and infiltration of inflammatory/progenitor cells. Angioproliferative “plexiform” lesions are found in PAH but not in other PH categories (Figure 1). Plexiform lesions (and other complex lesions) are often located downstream from occluded arteries and express the transcription and growth factors typically seen in angiogenesis, including vascular endothelial growth factor (VEGF) and HIF-1α (Figure 2).13 PAH typically spares the airway, veins, bronchial circulation, capillaries, and systemic vasculature (Figure 1). The various histological abnormalities of PAH are heterogeneous in their distribution and prevalence within the lungs. The natural progression of lesion severity (presumably from medial hypertrophy to plexiform arteriopathy) and the functional relevance of plexiform lesions remain uncertain, although regression of histologically proven PAH has been documented after single lung transplantation.14 Human lung tissue is invaluable, offering cells for culture, histological sections for immunohistological assessment of pathogenetic pathways, and tissue to be mined by laser capture microdissection for biomarkers. It remains the “gold standard” against which to judge animal models. Download figureDownload PowerPointFigure 1. Histology of PAH. Top, Plexiform lesions. Upper Left, Evidence of cell proliferation (red is proliferating cell nuclear antigen [PCNA], green is smooth muscle [SM] actin, and blue is DAPI). Bottom, Medial hypertrophy, intimal fibrosis, and adventitial proliferation.Download figureDownload PowerPointFigure 2. Formation of complex and plexiform lesions in PAH. Transformation of an arteriole into a complex vascular lesion with near-total or total lumen obliteration usually occurs at a vessel bifurcation. The concept depicted is one of initial apoptosis of cells forming the endothelial monolayer (upper panel, left). Disorganized endovascular angiogenesis results from proliferation of phenotypically abnormal cells due to (1) phagocytosis of apoptotic monolayer endothelial cells by neighboring endothelial cells, (2) activation of stem cell–like endothelial cells, or (3) attachment of bone marrow–derived “repair cells” to the injured endothelium. Bone marrow participation in the formation of these lesions is postulated because megakaryocytes, mast cells, and dendritic cells can be released and attach to the injured vessel. Perivascular lymphocytes may cluster in the lymphatics adjacent to the adventitia. The lesion also shows a dysregulated matrix. Growth factors released by megakaryocytes and mast cells may contribute to angiogenic growth, and T and B lymphocytes may reflect a local immune response. The table insert lists the phenotypic changes seen in plexiform lesions.Animal ModelsThe evaluation of these novel targets occasionally involves the off-label use of drugs approved for another indication (eg, Gleevec, Novartis Oncology, East Hanover, NJ) in humans, but is largely based on studies in cellular and animal models. Cautious interpretation of preclinical studies is mandatory, and one must recognize the strengths and weaknesses of various animal models and the risks of extrapolation to humans with PAH. Notably, no animal model completely recapitulates human PAH. Promising rodent models include monocrotaline-treated rats with or without pneumonectomy15 or abdominal aortocaval shunt,16 fawn-hooded rats (FHR, which spontaneously develop PAH and are also hypoxia sensitive),17,18 and rats treated with a single dose of VEGF-receptor antagonist (SU5416) plus hypoxia.19 Models that combine multiple insults yield more severe PAH with better hemodynamic and histological fidelity to human PAH. This may be relevant to the pathogenesis of human PAH, which also appears to require multiple “hits.” Murine models of PAH offer mechanistic insight on the relevance of single genes. Mice that transgenically overexpress SERT,20 BMPR2 dominant-negative mutations,21 or S100A4/Mts1 (metastasin 1),22 an accepted marker of a tumor’s metastatic potential, develop PH.New ParadigmsPAH was once regarded largely as a disease of excess vasoconstriction. This view was incomplete, and new concepts help us understand the fundamental causes of this syndrome.PAH Is a PanvasculopathyLet’s take a tour of the molecular pathology of PAH, beginning at the lumen of a small pulmonary artery (Figure 3). In the blood, levels of serotonin, a proliferative, fibrogenic vasoconstrictor, are elevated (Figure 4).23 In the endothelium, the vasodilator/vasoconstrictor ratio is decreased (Figure 5),24–26 whereas prothrombotic factors, including tissue factor,27 are increased. It is hypothesized that widespread endothelial apoptosis early in PAH culminates in selection of apoptosis-resistant endothelial precursor cells that proliferate and eventually form plexiform lesions (Figure 2).19 In the media, pulmonary artery smooth muscle cell (PASMC) apoptosis is suppressed, and proliferation is enhanced. Many factors drive PASMC proliferation, including mutation28 or downregulation29 of BMPR2 (Figure 6), mitochondrial metabolic abnormalities (Figure 7), de novo expression of the antiapoptotic protein survivin,19,30 increased expression/activity of SERT,20,31 increased expression/activity of platelet-derived growth factor (PDGF) receptor,32 tyrosine kinase activation (Figure 8),33 and decreased expression of Kv1.5, a voltage-gated, O2-sensitive potassium channel. Kv1.5 downregulation occurs in human PAH,34 rat PAH models (whether induced by chronic hypoxia35,36 or monocrotaline30 or in FHR18), and transgenic mice with PAH due to SERT overexpression20 or BMPR2 mutation.37 Loss of Kv1.5, the same channel that is inhibited by hypoxia to initiate hypoxic pulmonary vasoconstriction,38 depolarizes the membrane and elevates cytosolic K+ and Ca2+ (Figure 9). The resulting calcium overload, later reinforced by activation of transient receptor potential (trp) channels,39 leads to Ca2+-calcineurin–dependent activation of the proliferative transcription factor NFAT.40 Normoxic activation of HIF-1∝ occurs in FHR18 and human PAH.13,18 In the adventitia, metalloprotease activation causes architectural disruption, which permits cell migration and generates mitogenic peptides (tenascin; Figure 10).41 Adventitial fibroblasts are also hyperproliferative in PH, displaying increased sensitivity to serotonin.42 Circulating autoantibodies4 and lung infiltration by inflammatory cells are common, particularly in PAH associated with connective tissue disease and schistosomiasis (Figure 11).43 Finally, there are increased endothelial precursor cells and mesenchymal and bone marrow–derived stem cells,44 although it is uncertain whether this is harmful or beneficial (Figure 12). Download figureDownload PowerPointFigure 3. PAH is a panvasculopathy. Abnormalities can be seen at each level of the small pulmonary arteries, beginning in the blood and traveling outward to the adventitia. Although most of these abnormalities are likely secondary (rather than being the initiating cause of PAH), they nonetheless offer interesting therapeutic targets. Contributed by Dr Archer. SOD2 indicates superoxide dismutase 2; ET-1, endothelin-1; TxA2, thromboxane A2; BNP, brain natriuretic peptide; PGl2, prostacylin; 5-HTT, 5-hydroxy-tryptamine; and MMP, matrix metalloproteinase.Download figureDownload PowerPointFigure 4. Serotonin (5-HT) abnormalities in PAH. Increased bioavailability of serotonin during progression of PAH results from an increased release of serotonin from platelets and from an increased synthesis of serotonin by endothelial cells that produce serotonin and express tryptophan hydroxylase-1 (TPH1), the key enzyme that controls 5-HT synthesis. Overexpression of 5-HTT (SERT) by PASMCs is responsible for the increased mitogenic effect of serotonin on these cells. 5-HT receptors, including 5-HT1B/1D and 5-HT2A receptors, mediate 5-HT–induced pulmonary artery contraction of pulmonary vessels. 5-HT2A receptors located on platelets potentiate the aggregation response to various platelet activators. 5-HT2B receptors expressed by PASMCs are also involved in the pulmonary vascular remodeling process.Download figureDownload PowerPointFigure 5. The endothelium and vasodilator/antiproliferative pathways: NO, generated from l-arginine, and natriuretic peptides stimulate production of cGMP. cGMP causes vasorelaxation and inhibits proliferation of vascular SMCs. PDE5 inhibitors (eg, sildenafil) enhance this vasodilatory mechanism by preventing cGMP degradation. Prostacyclin from endothelial cells also promotes relaxation and inhibits cell proliferation via a cAMP-dependent mechanism. Endothelin is a potent vasoconstrictor and stimulates proliferation via ETA receptors on SMCs, while stimulating NO and prostacyclin release via endothelial ETB receptors. Adrenomedullin (AM) and VIP are additional endothelium-derived, cAMP-dependent vasodilators that are dysregulated in PAH.Download figureDownload PowerPointFigure 6. BMPR2 mutations, a genetic basis for familial PAH. BMPR2 mutations are found throughout the gene, and a universal functional consequence of these mutations has not been identified. Best studied is BMPR1 signaling through SMAD transcription factors. Mutations that lead to loss of SMAD signaling decrease cell differentiation, enhance vascular tone, increase transforming growth factor (TGF)-β signaling, and likely increase proliferation. Signaling through XIAP (X-linked inhibitor of apoptosis), which also requires BMPR1, can impact both the nuclear factor-κB (NFκB) and mitogen-activated protein kinase (MAPK) pathways, leading to increased MAPK phosphorylation and presumably proinflammatory signaling. BMPR2 has a long, evolutionarily conserved cytoplasmic tail domain unique in the TGF-β superfamily, that binds SRC, RACK1 (receptor for activated C-kinase 1), and LIMK1 (LIM domain kinase 1). BMPR2 mutation in vivo leads to decreased cofilin (Cfl1) phosphorylation by LIMK1, with the effect both of alterations in F-actin organization and defects in glucocorticoid receptor (GR) nuclear translocation.Download figureDownload PowerPointFigure 7. Mitochondrial metabolism in PAH. In aerobic metabolism, PDK is inactive, PDH is active, and electron donors (mitochondrial NADH and FADH) produced by the tricarboxylic acid cycle (TCA or Krebs cycle) pass electrons down a redox-potential gradient in the electron transport chain to molecular O2. This electron flux powers H+ ion extrusion, which creates the proton-motive force responsible for creating the negative membrane potential (ΔΨm) of mitochondria and powering F1Fo ATP synthase. Side reactions between semiquinones and molecular O2, which account for ≈3% of net electron flux, create superoxide anion in proportion to po2. Superoxide dismutase (SOD2) rapidly converts superoxide anion (produced at complexes I and III) to H2O2, which serves as a redox messenger signaling “normoxia.” In hypoxia (and PAH and cancer), there is activation of HIF-1α and PDK, which inhibits PDH, shifting metabolism toward glycolysis.Acetyl CoA indicates acetyl-coenzyme A.Download figureDownload PowerPointFigure 8. Receptor tyrosine kinases (RTK) and their inhibitors. This complex kinase cascade offers many therapeutic targets to treat PAH. ATF indicates activating transcription factor; BAD, BCL-XL/BCL-2–associated death promoter; c-kit, CD117; DAG, diacylglycerol; 4EBP1, 4E-binding protein 1; ECM, extracellular matrix; EGF-R, epidermal growth factor receptor; ErbB1 and ErbB2, epidermal growth factor receptors; ERK, extracellular signal-regulated kinase; flt3, fms-like tyrosine kinase receptor-3; GAB2, GRB2-associated binding protein; GSK, glycogen synthase kinase; IKK, IκB kinase; JAK, Janus kinase; JNK, Jun N-terminal kinase; MEF, myocyte-specific enhancer-binding nuclear factor; MEK, mitogen-activated protein kinase/ERK kinase; MERM, ezrin/radixin/moesin family of cytoskeletal linkers; mTOR, mammalian target of rapamycin; NFκB, nuclear factor-κB; NHERF, sodium-hydrogen exchange regulatory factor; P, phosphotyrosine; p70S6K, p70 ribosomal S6 kinase; PDGF-R, PDGF receptor: PDK, phosphoinositide-dependent kinase; PI3K, phosphoinositide-3 kinase; PIP2, phosphatidylinositol-4,5-bisphosphate; PIP3, phosphatidylinositol-3,4,5-trisphosphate; PKB, protein kinase B; PKC, protein kinase C; PLC, phospholipase C; SOS, Son of Sevenless; STAT, signal transducer and activator of transcription; SHP, Src homology 2-containing protein tyrosine phosphatase; TK, tyrosine kinase; and VEGF-R, VEGF receptor.Download figureDownload PowerPointFigure 9. Ion channels in PAH. Schematic depiction of the cellular mechanisms linked with vasoconstriction and remodeling in pulmonary endothelial cells (PAECs) and PASMCs in PAH. Central themes of interest for the development of PAH include the following: (1) Impaired ion channel expression and function in PASMCs (Kv, VDCC, SOC, ROC); (2) increased cytosolic calcium ([Ca2+]cyt) in PASMCs (mediated by ion channel function and receptor stimulation); (3) altered signaling via membrane receptors (GPCR, TIE-2, BMPR, RTK) and transporters (ie, SERT) in endothelial cells and PASMCs; (4) changes in redox status; (5) enhanced production of vasoconstrictor or mitogenic factors; and (6) viral signaling via GPCR and RTK. Paracrine interactions between PAECs and PASMCs are noteworthy. Ang-1 indicates angiopoietin-1; DAG, diacylglycerol; ET-1, endothelin-1; GPCR, G protein–coupled receptor; HHV, human herpes virus; 5-HT, serotonin; MAPK, mitogen-activated protein kinase; NO, nitric oxide; PGI2, prostaglandin I2; PIP2, phosphatidylinositol-4,5-bisphosphate; PKC, protein kinase C; PLC, phospholipase C; ROC, receptor-operated Ca2+ channels; ROCK, Rho-associated kinase; ROS, reactive oxygen species; RTK, receptor tyrosine kinase; SERT, 5-HT transporter; SOC, store-operated Ca2+ channels; SR, sarcoplasmic reticulum; and VDCC, voltage-dependent Ca2+ channels.Download figureDownload PowerPointFigure 10. Disordered elastin metabolism and deposition in PAH. Elastase degrades elastin and other components of the extracellular matrix, thereby releasing bound growth factors that are both mitogenic and motogenic for PASMCs. Heightened elastase activity also activates matrix metalloproteinases, which upregulate the glycoprotein tenascin-C. When tenascin-C binds cell-surface integrins, such as α-v β3 on PASMCs, these integrins cluster, and cell shape changes in a way that clusters and activates growth factor receptors and increases cell-survival signals. Thus, pathway activation causes both release of growth factors and activation of their receptors. Transmission of cell-survival signals occurs even in the absence of ligand (growth factor) binding. Blocking elastase activity or growth factor receptors can therefore arrest progression of PASMCs by blocking proliferation and induce regression by enhancing apoptosis.Download figureDownload PowerPointFigure 11. The role of inflammation in the pathogenesis of PAH. Initial inflammatory stimuli can occur in the form of infectious or foreign antigens or autoimmune disease, leading to an appropriate but potentially excessive immune response. The host immune response to these varied stimuli results in the release of proinflammatory cytokines (which can recruit bone marrow–derived cells), stimulation of resident inflammatory cells, and endothelial cell dysfunction. Endothelial cell injury and the cellular response can increase endovascular thrombosis. A network of cytokines released by the inflammatory and endothelial cells can also cause aberrant PASMC proliferation. The triad of endothelial cell proliferation, PASMC proliferation, and thrombus formation contributes to PAH. Proinflammatory cytokines and cell-cell interactions can potentially be targeted therapeutically. Decr’d NO indicates decreased NO; EGF, epidermal growth factor; HHV8, human herpes virus 8; HIMF, hypoxia-induced mitogenic factor, also called RELMa and FIZZ1; IL, interleukin; RANTES, regulated on activation, normal T cell expressed and secreted; and TNFa, tumor necrosis factor-α.Download figureDownload PowerPointFigure 12. Cellular basis for pulmonary vascular remodeling: Lessons from hypoxia. Fibroblasts, monocytes, and fibrocytes play critical roles in orchestrating hypoxia-induced pulmonary vascular remodeling. Hypoxia or hypoxia-associated stimuli increase production by resident fibroblasts (and probably PASMCs) of chemokines/cytokines, including monocyte chemoattractant protein (MCP)-1, stromal cell–derived factor (SDF)-1, fractalkine (CX3CL1), RANTES (regulated on activation, normal T cell expressed and secreted), VEGF, osteopontin (OPN), and endothelin (ET-1). These and other factors stimulate recruitment of monocytes and monocyte-derived mesenchymal precursors (fibrocytes) to the vessel wall. Upregulation of monocyte receptors for these ligands (CCR2, CXCR4, CX3CR1, VEGFR-1, and ETR-A) occurs. Monocytes are retained in the vessel wall by the upregulation of adhesion molecules on fibroblasts, including vascular cell adhesion molecule (VCAM), intracellular adhesion molecule (ICAM), and OPN. As monocytes and fibrocytes accumulate in the vessel wall, they exert potent effects on the proliferative, migratory, matrix-producing, and contractile capabilities of resident fibroblasts and PASMCs through the secretion of transforming growth factor (TGF)-β, PDGF-A and -B, epidermal growth factor, interleukin-6, insulin-like growth factor-1, matrix metalloproteinase-9, and others. In addition, these cells produce potent proangiogenic molecules such as VEGF, S100A4, and fibroblast growth factor-β that likely play roles in stimulating further angiogenesis in the vessel wall. PA indicates pulmonary artery.PAH Has a Genetic ComponentThe bone morphogenetic proteins (BMPs) are part of the transforming growth factor-β superfamily. Most patients (>80%) with familial PAH have loss-of-function mutations in BMPR245–47 that promote cell proliferation. BMPR2 is a constitutively active serine-threonine kinase receptor, which, in response to ligand (BMPs 2, 4, 6, 7, 9, and 10), forms heterodimers with any of 4 type 1 receptors (BMPR1A, BMPR1B, Alk1, and Alk2), which results in phosphorylation of the intracellular portion of the type 1 receptor by BMPR2. Receptor activation initiates a cytosolic Smad protein–signaling cascade. Receptor-activated Smads complex with common partner SMAD (Smad4), and the complex translocates to the nucleus, where it regulates gene transcription (Figure 6). The inhibitors of DNA binding (Id) genes are major targets of BMP/Smad signaling.48 The Smad-DNA interaction is weak and requires co-repressors or activators. BMPs can also act via an alternative BMPR2-independent pathway that involves mitogen-activated protein kinases (eg, p38MAPK, extracellular signal-regulated kinase 1 and 2).Most heterozygous BMPR2 mutations in PAH result in defective Smad signaling, although p38MAPK signaling is retained.49 The loss of normal BMPR2-Smad activity may exaggerate the susceptibility of vascular cells to proliferate and suppress apoptosis. BMPs 2, 4, and 7 suppress PASMC proliferation in normal individuals and patients with secondary PH but are ineffective in PAH.28 The BMPR2-Smad pathway may display tissue heterogeneity, because it can be regulated by endogenous Smad inhibitors (eg, chordin and noggin) and by inhibitory Smads (6 and 7), and also because of variable heterodimer receptor composition.50 This also may explain the restriction of the vascular disease to the pulmonary circulation.Mice with conditional, endothelial BMPR2 deletions are predisposed to PAH, although PH occurs in only a subset, reminiscent of the incomplete penetrance seen in familial PAH.51 Mice with a smooth muscle cell (SMC)–specific overexpression of a BMPR2 dominant-negative mutant develop a vasospastic form of PH that lacks vascular remodeling but is associated with downregulation of Kv1.5 expression. PH in these mice is reversed by nifedipine.37 Perhaps disordered BMP signaling, by reducing Kv1.5 transcription, creates an early vasospastic form of PAH that in time becomes fixed by vascular remodeling.Initial enthusiasm that BMPR2 mutations might represent a “universal” cause of PAH has been tempered. BMPR2 mutation is uncommon (prevalence 10% to 20%) in the nonfamilial category 1 PAH population. Moreover, in familial PAH, penetrance is low (ie, only ≈25% of carriers in affected families develop PAH).52 Although modifier genes, such as SERT and transforming growth factor-β, may explain the variable penetrance, aberrant BMPR2 function alone is neither a necessary nor a sufficient precondition for most cases of PAH.53 Moreover, BMPR2 heterozygous mice do not develop PAH and are not predisposed to hypoxic PH; however, they do have an exaggerated hypertensive res