The mechanisms by which corticosteroids reduce airway inflammation are not completely understood. Traditionally, corticosteroids were thought to inhibit cytokines exclusively at the transcriptional level. Our recent evidence, obtained in airway smooth muscle (ASM), no longer supports this view. We have found that corticosteroids do not act at the transcriptional level to reduce TNF-alpha-induced IL-6 gene expression. Rather, corticosteroids inhibit TNF-alpha-induced IL-6 secretion by reducing the stability of the IL-6 mRNA transcript. TNF-alpha-induced IL-6 mRNA decays at a significantly faster rate in ASM cells pretreated with the corticosteroid dexamethasone (t(1/2) = 2.4 h), compared to vehicle (t(1/2) = 9.0 h; P < 0.05) (results are expressed as decay constants [k] [mean +/- SEM] and half-life [h]). Interestingly, the underlying mechanism of inhibition by corticosteroids is via the up-regulation of an endogenous mitogen-activated protein kinase (MAPK) inhibitor, MAPK phosphatase-1 (MKP-1). Corticosteroids rapidly up-regulate MKP-1 in a time-dependent manner (44.6 +/- 10.5-fold increase after 24 h treatment with dexamethasone; P < 0.05), and MKP-1 up-regulation was temporally related to the inhibition of TNF-alpha-induced p38 MAPK phosphorylation. Moreover, TNF-alpha acts via a p38 MAPK-dependent pathway to stabilize the IL-6 mRNA transcript (TNF-alpha, t(1/2) = 9.6 h; SB203580 + TNF-alpha, t(1/2) = 1.5 h), exogenous expression of MKP-1 significantly inhibits TNF-alpha-induced IL-6 secretion and MKP-1 siRNA reverses the inhibition of TNF-alpha-induced IL-6 secretion by dexamethasone. Taken together, these results suggest that corticosteroid-induced MKP-1 contributes to the repression of IL-6 secretion in ASM cells.

Human airway smooth muscle (ASM) plays an immunomodulatory role in asthma. Recently, IL-17A has become of increasing interest in asthma, being found at elevated levels in asthmatic airways and emerging as playing an important role in airway neutrophilia. IL-17A predominantly exerts its neutrophil orchestrating role indirectly via the induction of cytokines by resident airway structural cells. Here, we perform an in vitro study to show that although IL-17A did not induce secretion of the CXC chemokine IL-8 from ASM cells, IL-17A significantly potentiates TNF-alpha-induced IL-8 protein secretion and gene expression in a concentration- and time-dependent manner (P < 0.05). Levels of IL-8 protein produced after 24 h of incubation with TNF-alpha were enhanced 2.7-fold in the presence of IL-17A, and conditioned media significantly enhanced neutrophil chemotaxis in vitro. As IL-17A had no effect on the activity of NF-kappaB, a key transcriptional regulator of IL-8 gene expression, we then examined whether IL-17A acts at the posttranscriptional level. We found that IL-17A significantly augmented TNF-alpha-induced IL-8 mRNA stability. Interestingly, this enhanced stability occurred via a p38 MAPK-dependent pathway. The decay of IL-8 mRNA transcripts proceeded at a significantly faster rate when cells were pretreated with the p38 MAPK inhibitor SB-203580 (-0.05763 +/- 0.01964, t(1/2) = 12.0 h), compared with vehicle (-0.01030 +/- 0.007963, t(1/2) = 67.3 h) [results are expressed as decay constant (means +/- SE) and half-life (t(1/2) in h): P < 0.05]. Collectively, these results demonstrate that IL-17A amplifies the synthetic function of ASM cells, acting via a p38 MAPK-dependent posttranscriptional pathway to augment TNF-alpha-induced secretion of the potent neutrophil chemoattractant IL-8 from ASM cells.

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CXCL10 (IP10) is involved in mast cell migration to airway smooth muscle (ASM) bundles in asthma. We aimed to investigate the role of cytokine-induced MAPK activation in CXCL10 production by ASM cells from people with and without asthma. Confluent growth-arrested ASM cells were treated with inhibitors of the MAPKs ERK, p38, and JNK and transcription factor NF-κB, or vehicle, and stimulated with IL-1β, TNF-α, or IFN-γ, alone or combined (cytomix). CXCL10 mRNA and protein, JNK, NF-κB p65 phosphorylation, and Iκ-Bα protein degradation were assessed using real-time PCR, ELISA, and immunoblotting, respectively. Cytomix, IL-1β, and TNF-α induced CXCL10 mRNA expression more rapidly in asthmatic than nonasthmatic ASM cells. IL-1β and/or TNF-α combined with IFN-γ synergistically increased asthmatic ASM cell CXCL10 release. Inhibitor effects were similar in asthmatic and nonasthmatic cells, but cytomix-induced release was least affected, with only JNK and NF-κB inhibitors halving it. Notably, JNK phosphorylation was markedly less in asthmatic compared with nonasthmatic cells. However, in both, the JNK inhibitor SP600125 reduced JNK phosphorylation and CXCL10 mRNA levels but did not affect CXCL10 mRNA stability or Iκ-Bα degradation. Together, the JNK and NF-κB inhibitors completely inhibited their CXCL10 release. We concluded that, in asthmatic compared with nonasthmatic ASM cells, JNK activation was reduced and CXCL10 gene expression was more rapid following cytomix stimulation. However, in both, JNK activation did not regulate early events leading to NF-κB activation. Thus JNK and NF-κB provide independent therapeutic targets for limiting CXCL10 production and mast cell migration to the ASM in asthma.