Lysyl Oxidases Play a Causal Role in Vascular Remodeling in Clinical and Experimental Pulmonary Arterial Hypertension

Abstract

P ulmonary arterial hypertension (PAH) is a life-threatening disease characterized by elevated pulmonary artery pressure caused by increased pulmonary vascular resistance, ultimately leading to right heart failure and death.1 A combination of vasoconstriction, and vascular wall remodeling, which includes excessive extracellular matrix (ECM) deposition, and smooth muscle cell hyperplasia leads to occlusion of the small pulmonary arteries.2 Therapeutic options for the clinical management of PAH are limited, and the disease remains essentially untreatable, underscoring the need for a better understanding of the pathogenic mechanisms at play that give rise to increased pulmonary vascular resistance. 3 In this regard, 1 key area of interest is to clarify the causes and nature of the pulmonary vascular remodeling associated with PAH.To date, most studies that have explored vascular remodeling in pulmonary hypertension (PH) have focused on increased cell proliferation and apoptosis resistance as an underlying cause of medial hypertrophy and neointimal formation. Regulation of pulmonary artery smooth muscle cell (PASMC) proliferation, apoptosis, and contraction has received much attention, with potassium and calcium channels, growth factors (including transforming growth factor-β, © 2014 American Heart Association, Inc. Objective-Pulmonary vascular remodeling, the pathological hallmark of pulmonary arterial hypertension, is attributed to proliferation, apoptosis resistance, and migration of vascular cells. A role of dysregulated matrix cross-linking and stability as a pathogenic mechanism has received little attention. We aimed to assess whether matrix cross-linking enzymes played a causal role in experimental pulmonary hypertension (PH). Approach and Results-All 5 lysyl oxidases were detected in concentric and plexiform vascular lesions of patients with idiopathic pulmonary arterial hypertension. Lox, LoxL1, LoxL2, and LoxL4 expression was elevated in lungs of patients with idiopathic pulmonary arterial hypertension, whereas LoxL2 and LoxL3 expression was elevated in laser-capture microdissected vascular lesions. Lox expression was hypoxia-responsive in pulmonary artery smooth muscle cells and adventitial fibroblasts, whereas LoxL1 and LoxL2 expression was hypoxia-responsive in adventitial fibroblasts. Lox expression was increased in lungs from hypoxia-exposed mice and in lungs and pulmonary artery smooth muscle cells of monocrotaline-treated rats, which developed PH. Pulmonary hypertensive mice exhibited increased muscularization and perturbed matrix structures in vessel walls of small pulmonary arteries. Hypoxia exposure led to increased collagen crosslinking, by dihydroxylysinonorleucine and hydroxylysinonorleucine cross-links. Administration of the lysyl oxidase inhibitor β-aminopropionitrile attenuated the effect of hypoxia, limiting perturbations to right ventricular systolic pressure, right ventricular hypertrophy, and vessel muscularization and normalizing collagen cross-linking and vessel matrix architecture....

Lung matrix and vascular remodeling in mechanically ventilated elastin haploinsufficient newborn mice

“…A number of recent reports have highlighted the importance of ECM protein composition and associated tissue stiffness in regulating postnatal development of the lung and its vasculature (20,21,26,40). Our finding of reduced lung elastance associated with diminished numbers of pulmonary microvessels in neonatal mice bearing a single allele of the elastin gene is consistent with the previous report that mechanical signals conveyed by variations in ECM elasticity can impact VEGF signaling and thereby alter vascular development (20).…”

Collagen and elastin cross-linking is altered during aberrant late lung development associated with hyperoxia

“…These observations suggest that improper ECM cross-linking in the developing lung may contribute to disturbed alveolarization. This idea is supported by a recent report that lysyl oxidases play a causal role in the remodeling of the pulmonary vasculature that is associated with pulmonary arterial hypertension (48). Thus it was hypothesized and tested in this study that collagen and elastin crosslinking is perturbed during the arrested alveolarization that is associated with the exposure of mouse neonates to a hyperoxic environment.…”

“…For studies using the broad-spectrum lysyl oxidase inhibitor, ␤-aminopropionitrile (BAPN; A3134, Sigma, St. Louis, MO), a pilot study was first performed with a dose of 15 and 150 mg·kg Ϫ1 ·day Ϫ1 (three animals per group, observed for 5 days) to assess BAPN tolerance. The upper limit of this dose was based on previous studies with BAPN in adult mice (48). After the pilot study, BAPN was then applied at 15 mg·kg Ϫ1 ·day Ϫ1 in a well-characterized animal model of BPD, where an arrest of alveolarization can be induced by exposure of newborn mouse pups to normobaric hyperoxia (85% O2), from the first day of birth, exactly as described previously (4,33).…”

“…Total collagen and elastin levels, as well as the abundance of the collagen cross-links dihydroxylysinonorleucine (DHLNL), hydroxylysylpiridinoline (HP), and hydroxylysinonorleucine (HLNL), and the elastin crosslinks desmosine and isodesmosine were determined exactly as described previously for adult mouse lungs and human tissues (10,48). The nomenclature used in this study refers to the reduced variants of the intermediate collagen cross-links (DHLNL, HLNL).…”

Periostin Deficiency Causes Severe and Lethal Lung Injury in Mice With Bleomycin Administration

“…30 While LOXL4 is dominantly expressed in the intact lung, LOX and LOXL1 are also weakly observed. 47,48 The collagen cross-linkages in the lung tissues were significantly or tended (without significance) to be decreased in the periostinnull mice compared with those in their wild-type counterpart (Fig. 3).…”