Role of mitogen-activated protein kinase phosphatase-1 in corticosteroid insensitivity of chronic oxidant lung injury

Abstract

Oxidative stress plays an important role in the pathogenesis of chronic obstructive pulmonary disease (COPD) and in the induction of corticosteroid (CS) insensitivity. Chronic ozone exposure leads to a model of COPD with lung inflammation and emphysema. Mitogen-activated protein kinase phosphatase-1 (MKP-1) may underlie CS insensitivity in COPD. We determined the role played by MKP-1 by studying the effect of corticosteroids in wild-type C57/BL6J and MKP-1(-/-) mice after chronic ozone exposure. Mice were exposed to ozone (3 ppm, 3 h) 12 times over 6 weeks. Dexamethasone (0.1 or 2 mg/kg; intraperitoneally) was administered before each exposure. Mice were studied 24 h after final exposure. In ozone-exposed C57/BL6J mice, bronchial hyperresponsiveness (BHR) was not inhibited by both doses of dexamethasone, but in MKP-1(-/-) mice, there was a small inhibition by high dose dexamethasone (2 mg/kg). There was an increase in mean linear intercept after chronic ozone exposure in both strains which was CS-insensitive. There was lesser inflammation after low dose of dexamethasone in MKP-1(-/-) mice compared to C57/Bl6J mice. Epithelial and collagen areas were modulated in ozone-exposed MKP-1(-/-) mice treated with dexamethasone compared to C57/Bl6J mice. MKP-1 regulated the expression of MMP-12, IL-13 and KC induced by ozone but did not alter dexamethasone׳s effects. Bronchial hyperresponsiveness, lung inflammation and emphySEMa after chronic exposure are CS-insensitive, and the contribution of MKP-1 to CS sensitivity in this model was negligible.

Keywords: Bronchial hyperresponsiveness; Emphysema; Lung inflammation; Mitogen-activated protein kinase phosphatase 1 (MKP-1); Ozone exposure.

Fig. 1

Airway hyperresponsiveness. Concentration–response curves to acetylcholine (ACh) (Panels A and B) and log provocative concentration of ACh required to increase lung resistance (R_L) by 100% from baseline (log PC₁₀₀) (Panels C) of C57/BL6J and MKP-1^−/− mice exposed to air or to multiple ozone exposures over 6 weeks. Data expressed as mean±S.E.M. Horizontal bars indicate mean. For Panels A and B, ^*P<0.05; ^**P<0.01: ozone compared to air-exposed mice; ^≠P<0.05; ^≠≠P<0.01, ozone-exposed and treated with 0.1 mg/kg dexamethasone compared to air-exposed mice; P<0.05; P<0.01: ozone-exposed and treated with 2 mg/kg dexamethasone compared to air-exposed mice; ^$P<0.05: ozone-exposed mice treated with saline (vehicle) compared to ozone-exposed and treated with 2 mg/kg dexamethasone. For Panel C, ^*P<0.05; ^**P<0.01; ^***P<0.001.

Fig. 2

Lung inflammation. Representative light microscopic lung sections from C57/Bl6 and MKP-1^−/− mice showing the inflammatory response in the lungs following exposure to ozone alone (Panels B and E) and the effect of high dose dexamethasone (Panels C and F), compared to air-exposed mice (panels A and D). Bar in Panel F is 10 μM. Panel G shows the individual inflammation scores in the lungs of C57/BL6J and MKP-1^−/− mice exposed to ozone over 6 weeks. ^*P< 0.05; ^**P<0.01.

Fig. 3

Mean linear intercept (L_m). Representative light microscopic lung sections from C57/Bl6 and MKP-1^−/− mice showing alveolar damage and enlargement in the lungs following exposure to ozone alone (Panels B and E) and the effect of high dose dexamethasone (Panels C and F), compared to air-exposed mice (Panels A and D). Bar in Panel F is 20 μM. Panel G shows the individual L_m values measured in lung sections of C57/BL6J and MKP-1^−/− mice exposed to ozone over 6 weeks. Horizontal bar indicates mean values. ^*P< 0.05; ^**P<0.01.

Fig. 4

Changes in airway wall. Individual and mean% of area of bronchial wall occupied by epithelium (Panel A), airway smooth muscle (ASM) (Panel B) and collagen (Panel C) in C57/BL6J MKP-1^−/− mice after chronic exposure to ozone or air, as determined by point-counting. The ozone-exposed mice were treated with saline (vehicle) or with 0.1 mg/kg or 2.0 mg/kg dexamethasone (Dex) prior to each exposure. ^*P< 0.05; ^**P<0.01; ^***P<0.001.

Fig. 5

Lung expression of inflammatory genes. Individual and mean fold-change from air control for gene expression of MMP-12 (Panel A), IL-13 (Panel B), IFNγ (Panel C), and KC (Panel D) in lungs of mice exposed to either air or ozone (O₃) or to ozone but pre-treated with 2.0 mg/kg dexamethasone (DEX) in C57/BL6J and in MKP-1^−/− mice following chronic exposure to ozone. For each strain, the data has been normalized to expression in air-exposed mice alone. Bars show mean. ^*P< 0.05; ^**P<0.01.