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.

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Molecular pathogenesis of pulmonary arterial hypertension

The pathogenesis of pulmonary hypertension (PH) involves a complex and multifactorial process. Endothelial dysfunction seems 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 pulmonary arterial hypertension (PAH). In addition, an altered production of various endothelial vasoactive mediators, such as NO, prostacyclin, endothelin-1 (ET-1), serotonin, and thromboxane, has been increasingly recognized in patients with PH. 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 PH. It is conceivable that the beneficial effects of many of the treatments currently available for PAH, such as the use of prostacyclin, NO, and ET antagonists, result at least in part from restoring the balance between these mediators. However, the ultimate cellular and physiological targets of these treatments remain unknown.

In addition to the potential consequences of an imbalance in the endothelial production of various mediators, injury to the endothelium may expose the underlying vascular tissue to diverse blood-borne factors that may further promote pathological changes. Endothelial dysfunction may also have adverse consequences on pulmonary vascular hemostasis by altering the production of anticoagulant factors. Recent reports of genetic mutations in the endothelial cells of patients with PH further underscore the role of these cells in the disease pathogenesis.

The endothelium lining the normal lung is characterized by significant heterogeneity. Not only is it vastly different from systemic endothelium in ultrastructure and function, but it varies in various vessel types in the pulmonary vasculature itself.1 The main functions of the pulmonary endothelium include maintenance …

Endothelial Dysfunction in Pulmonary Hypertension

Molecular cloning of a novel angiotensin II receptor isoform involved in phosphotyrosine phosphatase inhibition.

Chronic hypoxia induces polycythemia, pulmonary hypertension, right ventricular hypertrophy, and weight loss. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate adaptive responses to hypoxia, including erythropoietin, vascular endothelial growth factor, and glycolytic enzymes. Expression of the HIF-1α subunit increases exponentially as O2 concentration is decreased. Hif1a–/– mouse embryos with complete deficiency of HIF-1α due to homozygosity for a null allele at the Hif1a locus die at midgestation, with multiple cardiovascular malformations and mesenchymal cell death. Hif1a+/– heterozygotes develop normally and are indistinguishable from Hif1a+/+ wild-type littermates when maintained under normoxic conditions. In this study, the physiological responses of Hif1a+/– and Hif1a+/+ mice exposed to 10% O2 for one to six weeks were analyzed. Hif1a+/– mice demonstrated significantly delayed development of polycythemia, right ventricular hypertrophy, pulmonary hypertension, and pulmonary vascular remodeling and significantly greater weight loss compared with wild-type littermates. These results indicate that partial HIF-1α deficiency has significant effects on multiple systemic responses to chronic hypoxia.

J. Clin. Invest. 103:691–696 (1999)

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Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1α

The renin-angiotensin system plays a critical role in sodium and fluid homeostasis. Genetic or acquired alterations in the expression of components of this system are strongly implicated in the pathogenesis of hypertension. To specifically examine the physiological and genetic functions of the type 1A receptor for angiotensin II, we have disrupted the mouse gene encoding this receptor in embryonic stem cells by gene targeting. Agtr1A(-/-) mice were born in expected numbers, and the histomorphology of their kidneys, heart, and vasculature was normal. AT1 receptor-specific angiotensin II binding was not detected in the kidneys of homozygous Agtr1A(-/-) mutant animals, and Agtr1A(+/-) heterozygotes exhibited a reduction in renal AT1 receptor-specific binding to approximately 50% of wild-type [Agtr1A(+/+)] levels. Pressor responses to infused angiotensin II were virtually absent in Agtr1A(-/-) mice and were qualitatively altered in Agtr1A(+/-) heterozygotes. Compared with wild-type controls, systolic blood pressure measured by tail cuff sphygmomanometer was reduced by 12 mmHg (1 mmHg = 133 Pa) in Agtr1A(+/-) mice and by 24 mmHg in Agtr1A(-/-) mice. Similar differences in blood pressure between the groups were seen when intraarterial pressures were measured by carotid cannulation. These studies demonstrate that type 1A angiotensin II receptor function is required for vascular and hemodynamic responses to angiotensin II and that altered expression of the Agtr1A gene has marked effects on blood pressures.

https://www.pnas.org/content/pnas/92/8/3521.full.pdf

Regulation of blood pressure by the type 1A angiotensin II receptor gene.

To test the hypothesis that endothelin (ET)-1 synthesis and ET receptor levels are increased selectively in the lung of rats with chronic hypoxic pulmonary hypertension, the current study examined the effects of exposure to chronic hypoxia (10% O2, 1 atm, 4 wk) on pulmonary arterial pressure, ET-1 levels in plasma and lung, and ET-1 and ETA and ETB receptor mRNA levels in lung, heart, pulmonary artery, aorta, kidney, spleen, and liver. Hypoxic exposure was associated with increases in pulmonary arterial pressure, plasma ET-1 levels, ET-1 mRNA in lung and pulmonary artery, and ET-1 stores and ETA and ETB receptor mRNA levels in lung. In thoracic aorta and the four heart chambers, ETA and ETB receptor mRNA levels were increased, but ET-1 mRNA levels were unchanged from air control levels. No change in ET-1 or ET receptor mRNA levels was seen in organs perfused by the systemic vascular bed, except in liver, where ETA receptor mRNA levels were decreased. The findings of concomitant increases in gene transcript levels for ET-1 and the ETA and ETB receptors in lung, but not in the great vessels or any other organ examined, are consistent with the hypothesis that increased ET-1 synthesis in the lung contributes to pulmonary vascular remodeling and the maintenance of chronic hypoxic pulmonary hypertension.

Enhanced endothelin-1 and endothelin receptor gene expression in chronic hypoxia

The current study tested the hypothesis that exposure to hypoxia enhances endothelin-1 (ET-1) gene expression and elevates circulating ET-1 levels in the rat. Rats were exposed to normobaric hypoxia (10% O2) or room air for 24 or 48 h. ET-1 in arterial blood was measured by radioimmunoassay. ET-1 gene transcript levels were measured by the slot blot technique on total RNA isolated from lung, right and left atria, right and left ventricles, kidney, spleen, liver, brain, main trunk of pulmonary artery, and thoracic aorta. Blots were probed with a 0.5 kb rat prepro ET-1 cDNA that does not cross-hybridize with mRNA for ET-2 or ET-3. Plasma ET-1 levels were increased significantly at 24 (10.03 +/- 2.33 pg/ml) and 48 h (14.02 +/- 3.44 pg/ml) of hypoxia compared with air controls (4.14 +/- 0.66 pg/ml). ET-1 mRNA levels were increased significantly (2-fold) in lung and right atrium after 48 h of hypoxia; no change was seen in organs perfused by the systemic vascular bed. These findings suggest that the hypoxia-induced increase in circulating ET-1 levels is mainly of pulmonary origin. A paracrine effect of ET-1 produced by lung endothelial cells could account for hypoxic pulmonary hypertension.

Normobaric hypoxia stimulates endothelin-1 gene expression in the rat

Exposure to hypoxia is associated with increased pulmonary artery pressure and plasma endothelin-1(ET-1) levels and with selective enhancement in ET-1 peptide and mRNA and endothelin-A (ETA) recept...

Endothelin-A receptor antagonist prevents acute hypoxia-induced pulmonary hypertension in the rat.

Isolation of two distinct type I angiotensin II receptor genes.

We have generated a monospecific antibody to a synthetic peptide encoded by an RNA complementary to the mRNA for angiotensin II (AII) and determined whether this antibody recognizes the AII receptor. We demonstrate that the antibody competes specifically with 125I-labeled AII for the same binding site on rat adrenal membranes. Furthermore, we show that this antibody inhibits the secretion of aldosterone from cultured rat adrenal cells, suggesting that the antibody recognizes the biologically relevant AII receptor. Finally, we demonstrate that antibody to the complementary peptide can be used to immunoaffinity-purify a protein of Mr 66,000 that specifically binds radiolabeled AII.

T. S. Elton

Papers

Enhanced endothelin-1 and endothelin receptor gene expression in chronic hypoxia

Normobaric hypoxia stimulates endothelin-1 gene expression in the rat

Endothelin-A receptor antagonist prevents acute hypoxia-induced pulmonary hypertension in the rat.

Isolation of two distinct type I angiotensin II receptor genes.

Purification of an angiotensin II binding protein by using antibodies to a peptide encoded by angiotensin II complementary RNA.