| Popis: |
Increased IL-17A production has been associated with more severe asthma; however, the mechanisms whereby IL-17A can contribute to IL-13-driven pathology in asthmatic patients remain unclear. In this thesis, we sought to elucidate the molecular mechanisms by which IL-17A enhances IL-13-dependent airway pathology in patients with severe asthma using in vivo and in vitro systems. We have found that compared to mice given intratracheal (i.t.) IL-13 alone, those co-exposed to IL-13 + IL-17A demonstrate enhanced airway hyperresponsiveness (AHR), mucus production, airway inflammation, and IL-13-induced gene expression. In vitro, IL-17A directly enhanced IL-13-induced gene expression in asthma-relevant murine and human cells. In contrast to the exacerbating effect of IL-17A on IL-13-driven responses, co-treatment with IL-13 diminished IL-17A-driven gene expression in vivo and in vitro. Mechanistically, in vivo and in primary human and murine cells, the IL-17A mediated increase in IL-13-induced gene expression was associated with a rapid increase in IL-13-driven signal transducer and activator of transcription (STAT)6 phosphorylation.Disrupting protein-tyrosine phosphatase function using Na3VO4 abrogated IL-17A-mediated enhancement of IL-13-driven STAT6 phosphorylation, suggesting that the ability of IL-17A to augment IL-13 activity was driven by changes in protein-tyrosine phosphatase activity. Consistent with this, co-exposure to IL-13 + IL-17A triggered a rapid decrease in the phosphorylation of Src homology region 2 domain-containing phosphatase (SHP)-1 and SHP-2, negative regulators of IL-13 signal transduction. Pharmacologic inhibition of SHP-1 but not SHP-2 similarly abrogated IL-17A-mediated enhancement of IL-13-driven STAT6 phosphorylation. However, the ability of IL-17A-driven alterations in protein-tyrosine phosphatase activity to enhance cytokine signaling was specific for IL-13, as IL-17A had no effect on IL-6- and IFN-γ-dependent activation of STAT3 and STAT1, respectively.Surprisingly, the enhancement of STAT6 phosphorylation was not sufficient to explain IL-17A-mediated increases in IL-13-driven gene expression. Although IL-13 and IL-17A activate distinct cellular signaling pathways, we have identified specific transcription factors downstream of the IL-17A/Act1/TRAF6 signaling axis that influence transcriptional enhancement between IL-13 and IL-17A. Inhibition of NF-κB or C/EBPß and C/EBPd transcription factors partially attenuated IL-17A-mediated enhancement of IL-13-induced gene expression, while the inhibition of p38 mitogen-activated protein kinase (MAPK) completely abrogated the effect of IL-17A in cells co-exposed to IL-13 + IL-17A. However, the inhibition of p38 MAPK did not diminish IL-17A-mediated enhancement of STAT6 phosphorylation, implying that IL-17A signaling differentially regulates the accumulation of IL-13-induced STAT6 activation and gene expression. Collectively, our data suggest that IL-17A contributes to asthma pathophysiology by:1) |
