S-nitrosothiol signaling in respiratory biology

Abstract

Genetic and biochemical data demonstrate a pivotal role for S-nitrosothiols (SNOs) in mediating the actions of nitric oxide synthases (NOSs). SNOs serve to convey NO bioactivity and to regulate protein function. This understanding is of immediate interest to the pulmonary clinical and research communities. This article reviews the following: (1) biochemical and cellular evidence that SNOs in amino acids, peptides, and proteins elicit NOS-dependent signaling in the respiratory system and (2) studies that link SNO signaling to pulmonary medicine. SNO-mediated signaling is involved in the regulation of minute ventilation, ventilation-perfusion matching, pulmonary arterial pressure, basal airway tone, and respiratory and peripheral muscle function. Derangements in SNO signaling are implicated in many disorders relevant to pulmonary and critical care medicine, including apnea, hypoxemia, pulmonary hypertension, asthma, cystic fibrosis, pneumonia, and septic shock.

Figure 1.

Overview of S-nitrosothiol (SNO) signaling in the respiratory system. SNO signaling affects respiratory control, airway function, and pulmonary vascular tone. AE1 = anion exchange protein 1 on the erythrocyte membrane; CFTR = cystic fibrosis transmembrane regulatory protein; GSNO = S-nitrosoglutathione; Hb = hemoglobin; HbFeNO = hemoglobin iron nitrosyl; HbS_O2 = oxyhemoglobin saturation; HIF = hypoxia inducible factor; HVR = hypoxic ventilatory response; PAH = pulmonary arterial hypertension; RBC = red blood cell.

Figure 2.

Airways of GSNOR^–/– mice are hyporeactive to methacholine (MCh) after allergen challenge. Total pulmonary resistances (R_T) of wild-type (WT) and GSNOR^–/– (knockout [KO]) mice after control (nonallergic; phosphate-buffered saline [PBS]) (A) and allergen (ovalbumin [OVA]) (B) treatment were determined in the absence or presence of various concentrations of MCh administered intravenously. R_T values in PBS-treated and in OVA-treated GSNOR^–/– mice were significantly lower than in WT controls (KO PBS vs. WT PBS, p < 0.001; KO OVA vs. WT OVA, p < 0.004; analysis of variance [ANOVA] and post hoc analyses at 3 to 5 MCh doses). Data represent the mean + SE of at least 7 to 10 mice per group. (C) The incremental effect of OVA (over PBS control) on WT and GSNOR^–/– mice (OVA minus PBS [OVA−PBS]). Although R_T of WT mice increased significantly after OVA treatment (WT PBS vs. WT OVA, p < 0.04; ANOVA), the R_T of GSNOR^–/– mice did not change significantly (KO PBS vs. KO OVA, p = 0.1; ANOVA). (D) Effect of the inducible nitric oxide synthase (iNOS) inhibitor 1400W on airway responsiveness in PBS- and OVA-treated WT mice (WT PBS vs. WT PBS + 1400W, p = 0.12, n = 5–9; WT OVA vs. WT OVA + 1400W, p = 0.22, n = 7–9). (E) Effect of iNOS inhibition by 1400W on airway responsiveness in GSNOR^–/– mice. Administration of 1400W to OVA-treated GSNOR^–/– mice resulted in a significant increase in airway resistance (KO OVA vs. KO OVA + 1400W, p < 0.02, n = 5–9; ANOVA). (F) Protein S-nitrosylation (SNO) in lung homogenates of OVA-treated mice. iNOS inhibition (1400W) reduces SNO levels in GSNOR^–/– mice (*p < 0.05, n = 3). Reprinted by permission from Reference .

Figure 3.

Erythrocyte SNO content (SNO_RBC) and O₂ content are functionally coupled. (A) Washed red blood cells (RBCs) from normal humans suspended with (open circles) or without (solid circles) extracellular glutathione (GSH) were steadily deoxygenated under argon. The SNO_RBC/Hb ratio is plotted against Hb SO₂ (inset). Washed RBCs without extracellular GSH were treated in the same fashion as described for A but not deoxygenated. The ratio of SNO_RBC to Hb was stable over time. (B) The natural logarithm of the ratio of SNO_RBC to Hb was modeled as a function of Hb SO₂; extraerythrocytic GSH was included as a covariate, generating two lines describing the decay rate of the ratio of SNO_RBC to Hb with or without extraerythrocytic GSH. These rates differed (p < 0.0001). A and Breprinted by permission from Reference . As a functional correlate, the low-mass fraction from deoxygenated blood signals an increase in V̇e at the level of the nucleus tractus solitarius (nTS). (C) Microinjection into the nTS of conscious rats of the GSH-derived fraction from deoxygenated blood (black line; see Figure 1) stimulated a V̇e increase that was absent when the low-mass fraction from oxygenated blood was injected (gray line). (D) The differences in V̇e before and after injection of deoxygenated fractions (black bars; n = 14) were highly reproducible (*p < 0.0001), whereas oxygenated fractions (gray bars; n = 12) had no effect (p = not significant). Reprinted by permission from Reference .

Figure 3.

Erythrocyte SNO content (SNO_RBC) and O₂ content are functionally coupled. (A) Washed red blood cells (RBCs) from normal humans suspended with (open circles) or without (solid circles) extracellular glutathione (GSH) were steadily deoxygenated under argon. The SNO_RBC/Hb ratio is plotted against Hb SO₂ (inset). Washed RBCs without extracellular GSH were treated in the same fashion as described for A but not deoxygenated. The ratio of SNO_RBC to Hb was stable over time. (B) The natural logarithm of the ratio of SNO_RBC to Hb was modeled as a function of Hb SO₂; extraerythrocytic GSH was included as a covariate, generating two lines describing the decay rate of the ratio of SNO_RBC to Hb with or without extraerythrocytic GSH. These rates differed (p < 0.0001). A and Breprinted by permission from Reference . As a functional correlate, the low-mass fraction from deoxygenated blood signals an increase in V̇e at the level of the nucleus tractus solitarius (nTS). (C) Microinjection into the nTS of conscious rats of the GSH-derived fraction from deoxygenated blood (black line; see Figure 1) stimulated a V̇e increase that was absent when the low-mass fraction from oxygenated blood was injected (gray line). (D) The differences in V̇e before and after injection of deoxygenated fractions (black bars; n = 14) were highly reproducible (*p < 0.0001), whereas oxygenated fractions (gray bars; n = 12) had no effect (p = not significant). Reprinted by permission from Reference .