The current study has demonstrated the novel function of MUC18 in airway epithelial immune responses to polyI:C (a mimic of viral infection) and live HRV infection. MUC18 appears to exert a “paradoxical” role in human airway epithelial cell responses to polyI:C stimulation and HRV-1A infection. While MUC18 promotes the proinflammatory response (e.g., IL-8 production), it suppresses the anti-viral (e.g., IFN-Β) gene expression.
MUC18 is classically considered to be involved in cell-cell adhesion under a normal setting. This function is particularly important in maintaining the integrity or tight junction of endothelial cells [9
]. However, under a pathologic condition such as malignant melanoma or other cancers, MUC18 is significantly increased, contributing to tumor metastasis [10
]. MUC18 expression and function in human airways have not been well studied. A previous study suggests up-regulation of MUC18 expression in brushed bronchial epithelial cells from COPD patients [24
], but MUC18 function in the lung remains unclear. We have found MUC18 up-regulation in airway epithelial cells as well as in lung macrophages from human asthmatics and COPD patients [13
]. Importantly, we demonstrated a pro-inflammatory function of MUC18 in lung macrophages in the context of bacterial (e.g., mycoplasma) infection [14
In the current study, we have extended our previous research on MUC18 in the following areas. First, we examined the function of MUC18 in a different cell type, human primary airway epithelial cells, which is at the interface of lung and environment such as pathogens and pollutants, and is critical to the lung defense function. Second, we shifted our previous attention from bacterial infection or its mimic TLR2 agonist to respiratory rhinovirus infection and its mimic polyI:C. Third, we attempted to reveal the role of MUC18 in regulating both pro-inflammatory and anti-viral responses. By using the MUC18 over-expression approach, we have extended our previous study in lung macrophages by showing that MUC18 also promotes pro-inflammatory cytokine IL-8 production in airway epithelial cells following polyI:C or HRV treatment. This finding is important, as viral infection is a significant factor contributing to exacerbations of lung diseases, including asthma, COPD and cystic fibrosis. One of the common features in acute exacerbations of lung diseases is excessive lung inflammation such as neutrophils and IL-8, one of the most potent chemoattractants and activators for neutrophils. The fact that MUC18 enhances IL-8 production in airway epithelial cells suggests that up-regulated MUC18 expression in diseased lungs may predispose the host to exaggerated inflammatory response during viral infection. Thus, blocking the effect of MUC18 by using a neutralizing antibody may offer a novel approach to preventing or attenuating virus-mediated lung disease exacerbations.
One of the novel and exciting findings in this study is that MUC18 exerts an inhibitory role in anti-viral IFN-Β gene expression. This new piece of data further indicates that MUC18 has multiple roles in lung defense. At one hand, MUC18 promotes the inflammation. On the other hand, it suppresses the anti-viral response, which may delay the viral clearance and prolong the disease process. Accompanied by the reduction of IFN-Β expression is the increase (about 2 fold, data not shown) of HRV load, but such increase did not reach the level of statistical significance. The opposing effects of MUC18 on IL-8 and IFN-Β expression suggest the complexity of MUC18-mediated transcriptional or translational cascade. Previous studies have demonstrated the inconsistent trend of change of pro-inflammatory response and the anti-viral interferon response following HRV or respiratory syncytial virus (RSV) infection [25
]. Such inconsistent trend of IL-8 and IFN-Β expression may be in part explained by the well-documented observation that pro-inflammatory and anti-viral responses are regulated by two distinct sets of transcription factors, NF-κB and interferon regulatory factors [IRFs], respectively [3
How MUC18 signals during the inflammatory or infectious process has not been revealed. In the current study, we decided to examine the upstream events that MUC18 may function in the context of viral mimic polyI:C. We showed that in both lung epithelial cell line and primary airway epithelial cells, levels of phosphorylated serines were increased following polyI:C treatment, particularly in the presence of MUC18 overexpression. Our data suggest that MUC18 phosphorylation may contribute to its functions. Future studies are warranted to determine how phosphorylation of MUC18 serines regulates the downstream signaling cascades associated with MUC18 function.
One novel aspect of our study is related to the contribution of ERK activation to MUC18 serine phosphorylation. PolyI:C-mediated ERK1/2 activation has been shown by us and other investigators [29
]. We found that ERK1/2 inhibition reduced the level of MUC18 serine phosphorylation, indicating a partial dependence of MUC18 activation on ERK activation. Whether other members (e.g., p38) of the MAPK family or other kinases phosphorylate MUC18 during viral infection or its mimic treatment remains to be explored.
There are several limitations to our current study. First, we focused on polyI:C as a surrogate of broad spectrum RNA viral infection, and only chose HRV for the viral infection model. We will consider using other strains of viruses in our future studies to further define the role of MUC18 in various virus-induced pro-inflammatory and anti-viral responses. Second, how MUC18 differentially regulates IL-8 and IFN-Β expression is unclear. We plan to determine if phosphorylated MUC18 interacts with other proteins especially with adaptor proteins (e.g., TRIF) involved in pro-inflammatory cytokine and anti-viral gene expression. Third, although we demonstrated MUC18’s function in airway epithelial cells infected with live HRV (i.e., HRV-1A), we have not examined MUC18 serine phosphorylation following live HRV-1A infection in the presence or absence of an ERK inhibitor. Such a mechanistic study will be performed in our future experiments to further strengthen our observation that MUC18 regulates pro-inflammatory and anti-viral responses. Lastly, a transgenic mouse model over-expressing human MUC18 in airway epithelium would be needed to dissect the in vivo
function of MUC18 in airway epithelial responses to HRV or polyI:C.
In summary, our human primary airway epithelial cell culture studies have suggested a unique pro-inflammatory and anti-viral function for MUC18, a molecule that deserves future studies to broaden our knowledge in its implication in various infectious diseases or pathogen-related disease exacerbations. Therapies aimed at blocking MUC18 up-regulation or MUC18 signaling could potentially prevent or diminish excessive inflammation while enhancing host defense functions during viral infection and associated disease exacerbations.