Nitrite in pulmonary arterial hypertension: therapeutic avenues in the setting of dysregulated arginine/nitric oxide synthase signalling

Abstract

Pulmonary arterial hypertension (PAH) is an insidious disease of the small pulmonary arteries that is progressive in nature and results in right heart strain/hypertrophy and eventually failure. The aetiologies may vary but several common pathophysiological changes result in this phenotype, including vasoconstriction, thrombosis, and vascular proliferation. Data suggest that nitric oxide (NO) signalling is vasoprotective in the setting of PAH. The classic arginine-NO synthase (NOS)-NO signalling pathway may represent an adaptive response that is eventually dysregulated during disease progression. Dysregulation occurs secondary to NOS enzyme down-regulation, enzymatic uncoupling, and arginine catabolism by vascular and red cell arginases and by direct NO inactivation via catabolic reactions with superoxide or cell-free plasma haemoglobin (in the case of haemolytic disease). The anion nitrite, which has recently been recognized as a source of NO that circumvents the arginine-NOS pathway, may serve as an additional adaptive signalling pathway that is now appreciated to have a vasoregulatory role in the pulmonary and systemic vasculature. Inhaled nebulized sodium nitrite is a relatively potent pulmonary vasodilator in the setting of hypoxia and is also anti-proliferative in multiple experimental models of pulmonary hypertension. Multiple nitrite reductases have been shown to be relevant in the conversion of nitrite to metabolically active NO, including deoxy-haemoglobin and myoglobin in the circulation and heart, respectively, and xanthine oxidoreductase in the lung parenchyma.

Figures

Figure 1

The classic arginine–nitric oxide synthase–nitric oxide pathway. This figure illustrates the ‘classic’ nitric oxide pathway and both cyclic guanosine monophosphate-dependent and -independent signalling. Furthermore, the figure highlights the multiple levels of this pathway that can be taken advantage of for therapeutic benefit. One strategy is to increase nitric oxide synthase substrate availability via l-arginine supplementation or arginase inhibitors. Alternative strategies are to increase nitric oxide synthase enzymes via gene or protein therapy as well as direct deliver of nitric oxide gas via inhalation or pharmacological donors. Additionally, therapeutics take advantage of cyclic guanosine monophosphate-dependent signalling including phosphodiesterase inhibitors, such as sildenafil, and the direct guanylate cyclase activators such as riociguat.

Figure 2

Nitrite (NO2−) can be reduced to nitric oxide along a pH and oxygen gradient. Additionally, multiple enzymes possessing nitrite reductase activity, including xanthine oxidoreductase, aldehyde oxidase, myoglobin, haemoglobin, and cytochrome c oxidase can act as nitrite reductases to produce nitric oxide. Nitric oxide can then have a myriad of biological effects, including modulation of smooth muscle cell proliferation, hypoxic vasodilation, and cytoprotective effects.

Figure 3

Comparison of the haemodynamic and metabolic effects of nebulized nitrite (B) vs. inhaled nitric oxide (A) gas during hypoxia-induced pulmonary hypertension. Treatment with nitrite aerosol resulted in a rapid sustained reduction in hypoxic-induced pulmonary vasoconstriction and a graded increase in exhaled nitric oxide gas concentration with no change in mean arterial blood pressure. These results are contrasted to the rapid return of pulmonary artery pressure to hypoxic baseline after termination of inhaled nitric oxide gas. Reprinted by permission from Macmillan Publishers Ltd: Nat Med 2004;10:1122–1127.

Figure 4

Nebulized sodium nitrite reverses monocrotaline-induced pulmonary arterial hypertension. Monocrotaline treatment resulted in pulmonary arterial hypertension as determined by RV:LV + S mass ratio (A), RV:BW mass ratio (B), and right ventricular pressure measurements (C) (*P< 0.001 compared with controls). Inhaled nebulized nitrite (1.5 mg/min for 20 min; three times a week during weeks 4–6) reversed monocrotaline-induced pulmonary arterial hypertension (A–C; #P< 0.001 compared with monocrotaline, vehicle-treated). (D) Cardiac cross-sections demonstrate right ventricular hypertrophy 6 weeks after monocrotaline treatment in nebulized vehicle controls (a) and nebulized nitrite-treated rats (b). An untreated control heart is shown for comparison (c). Reproduced with permission from Zuckerbraun et al.

Figure 5

Sodium nitrite signalling in pulmonary arterial hypertension is dependent on xanthine oxidoreductase (A,B). Nitric oxide generation from nitrite in hypoxic lung tissue. Lung tissue homogenate was incubated with nitrite (1 mM) in the presence and absence of allopurinol (200 μM) or superoxide dismutase (300 U/mL) under normoxic or hypoxic conditions. (A) Representative nitric oxide generation traces over time. (B) Quantitation of at least three independent experiments similar to (A) (*P< 0.001 compared with nitrite). (C) Hypoxia-induced pulmonary arterial hypertension (*P< 0.001 compared with normoxic controls) was inhibited by nebulized nitrite as demonstrated previously (#P< 0.001 compared with hypoxic controls). A tungsten-enriched diet diminished the protective effects of nebulized nitrite (§P< 0.001 compared with nitrite-treated hypoxic mice). Adapted from Zuckerbraun et al. and reproduced with permission.

Figure 6

Overview of therapeutic strategy of nebulized sodium nitrite for the treatment of pulmonary arterial hypertension.