Cyclooxygenase-2 inhibition and hypoxia-induced pulmonary hypertension: effects on pulmonary vascular remodeling and contractility

Abstract

Pulmonary arterial hypertension (PAH) is a significant disease process characterized by elevated pulmonary vascular resistance leading to increased right ventricular afterload and ultimately progressing to right ventricular dysfunction and often death. Irreversible remodeling of the pulmonary vasculature is the hallmark of pulmonary hypertension and frequently leads to progressive functional decline in patients with PAH despite treatment with currently available therapies. Metabolites of the arachidonic acid cascade play an important homeostatic role in the pulmonary vasculature, and dysregulation of pathways downstream of arachidonic acid plays a central role in the pathobiology of PAH. Cyclooxygenase-2 (COX-2) is up-regulated in pulmonary artery smooth muscle cells (PASMC) and inflammatory cells during hypoxia and plays a protective role in the lung's response to hypoxia. We recently demonstrated that absence of COX-2 was detrimental in a mouse model of hypoxia-induced pulmonary hypertension. Exposure of COX-2 null mice to hypoxia resulted in severe pulmonary hypertension characterized by enhanced pulmonary vascular remodeling and significant up-regulation of the endothelin-1 receptor ET(A)R in the lung after hypoxia. Absence of COX-2 in vitro led to enhanced contractility of PASMC after exposure to hypoxia, which could be attenuated by iloprost, a prostaglandin I(2) analog. These findings suggest that selective inhibition of COX-2 may have detrimental pulmonary vascular consequences in patients with preexisting pulmonary hypertension or underlying hypoxemic lung diseases. Here, we discuss our recent data demonstrating the adverse consequences of COX-2 inhibition on pulmonary vascular remodeling and PASMC contractility.

Figure 1

Current therapeutic targets in pulmonary arterial hypertension. Current therapy for PAH targets imbalance of three mediators including ET-1, NO, and PGI₂. These pathways become dysregulated in patients with PAH, leading to enhanced vasoconstriction, in situ thrombosis, and pulmonary vascular remodeling. A transverse section of a remodeled pulmonary artery in a patient with PAH is shown, demonstrating intimal proliferation and medial thickening. Patients with PAH have increased levels of the potent vasoconstrictor ET-1 and decreased levels of endothelial vasodilators NO and PGI₂ leading to aberrant PASMC proliferation and hypertrophy. Current therapy with endothelin-receptor antagonists, phosphodiesterase inhibitors, and prostacylin analogues aims to reestablish the balance of these vascular effectors. Plus signs (+) indicate an increase in the concentration; minus signs (−) denote a decrease in the concentration, inhibition of an enzyme, or blockage of a receptor. cGMP, cyclic guanosine monophospate; cAMP, cyclic adenosine monophospate. Reproduced from (Humbert et al. 2004) by permission of the Massachusetts Medical Society. Copyright © [2004] Massachusetts Medical Society. All rights reserved.

Figure 2

Metabolic pathways of arachidonic acid metabolism. Arachidonic acid is metabolized via three major enzymatic pathways: cyclooxygenases, lipoxygenases, and cytochrome P450 to generate prostanoids, leukotrienes, and hydroxyeicosatetraenoic acids (HETEs) and epoxyeicosatrienoic acids (EETs), respectively. The COX-2, 5-LO, and 12-LO pathways have been demonstrated to play an important role in pulmonary hypertension. LT, leukotriene; PG, prostaglandin; TxA₂, thromboxane A₂; LO, lipoxygenase; COX, cyclooxygenase; LX, lipoxin; HETE, hydroxyeicosatetraenoic acid; EETs, epoxyeicosatrienoic acids.

Figure 3

Absence of COX-2 exacerbates hypoxia-induced pulmonary hypertension and enhances contractility of PASMC. A. RVSP in COX-2^+/+ (□) and COX-2^−/− (■) mice following hypoxia (n=18 per group) and normoxia (n=8 per group). Error bars represent SE (p<0.05 for hypoxic COX-2^−/− mice vs. hypoxic COX-2^+/+ mice, *p<0.05 for hypoxic COX-2^−/− mice vs. normoxic controls, and ^‡p<0.05 for hypoxic COX-2^+/+ mice vs. normoxic controls). B. Immunostaining of lungs from COX-2^+/+ (left) and COX-2^−/− (right) mice after normoxia (top) and hypoxia (bottom) for α-SMA. C. Total protein was isolated from lungs of COX-2^+/+ and COX-2^−/− mice following hypoxia and normoxia and Western blot analysis performed for the ET_A receptor. Loading was quantified with an anti-tubulin antibody. A representative of three experiments is shown. D. Cells were plated on collagen gels and exposed to hypoxia for 24 h. Gel contraction was measured 4 h following matrix release. Data are presented as the percentage of the original collagen gel size for mouse PASMC (COX-2^+/+ □ and COX-2^−/− ■) and RPASMC (vehicle and NS-398 ) exposed to hypoxia. Data are expressed as mean ± SE (p<0.05 for COX-2^−/− PASMC vs. COX-2^+/+ PASMC; p<0.05 for NS-398-treated vs. vehicle-treated RPASMC).

Figure 4

Role of COX-2 in hypoxia-induced pulmonary hypertension and pulmonary vascular remodeling. COX-2 is upregulated in PASMC following hypoxia leading to enhanced synthesis of PGI₂ in the endothelium of pulmonary arteries. COX-2-derived PGI₂ inhibits platelet aggregation and subsequent thrombus formation and promotes vasodilation. Inhibition of COX-2 during hypoxia leads to increased thrombosis that is TXA₂ dependent. In addition, our data suggest that COX-2-derived PGI₂ decreases ET_AR expression leading to attenuation of ET-1-induced PASMC contractility and hypertrophy. COX-2 is also highly induced in inflammatory cells following hypoxia and may play a role in modulating PASMC remodeling in a paracrine fashion. Alternative metabolites of arachidonic acid, including cysLT and 12S-HETE, may exacerbate pulmonary vascular remodeling in the absence of COX-2 via modulation of PASMC proliferation and vascular tone. Arrows (→) denote an increase in the indicated downstream mediator; flat arrowheads (⊥) indicate an inhibition or a decrease in concentration; dashed lines (···) indicate a proposed mechanism. PGI₂, prostaglandin I₂; ET-1 endothelin-1; TXA₂, thromboxane A₂; 12S-HETE, 12S-hydroxyeicosatetraenoic acid; cysLT, cysteinyl leukotrienes; ET_AR, endothelin A receptor; PASMC, pulmonary artery smooth muscle cells; PAEC, pulmonary artery endothelial cells.