IL-17A augments TNF-α-induced IL-6 expression in airway smooth muscle by enhancing mRNA stability

Background: IL-17A is implicated in the regulation of inflammation and is found in increased amounts in the asthmatic airway. Human airway smooth muscle (ASM) cells synthesize cell adhesion molecules and cytokines in response to inflammatory mediators. Objective: In this study, we examined whether IL-17A modulated the synthetic function of ASM cells. Methods: Primary ASM cultures were treated with IL-17A alone and in combination with the proinflammatory cytokines TNF-alpha and IL-1beta. Intercellular adhesion molecule 1 expression, GM-CSF, and IL-6 secretion were measured by ELISA. Examination of transcriptional regulation was performed via transient transfection of promoter constructs, whereas mRNA stability was assessed by actinomycin D chase and quantitative real-time PCR. Results: Airway smooth muscle did not secrete IL-17A after stimulation of ASM with TNF-alpha and IL-1beta. Furthermore, IL-17A (0.1-10 ng/mL) had no effect on TNF-alpha-induced and IL-1beta-induced intercellular adhesion molecule 1 expression or GM-CSF secretion. However, IL-17A (10 ng/mL) significantly augmented TNF-alpha-induced IL-6 secretion 12-fold (TNF-alpha, 2.3 +/- 0.4 ng/mL; IL-17A and TNF-alpha, 27.5 +/- 4.8 ng/mL; P < .05) while having no effect on IL-1beta-induced IL-6. Although IL-17A had no effect on nuclear factor KB-mediated transcriptional regulation of IL-6 gene expression induced by TNF-alpha, IL-17A significantly augmented TNF-alpha-induced IL-6 mRNA stability. Conclusion: Collectively, these results demonstrate that IL-17A amplifies the synthetic function of ASM cells, acting via a posttranscriptional pathway, rather than transcriptional mechanisms, to augment TNF-alpha-induced secretion of IL-6 from ASM cells.