Analysis of Immunomodulatory Effects of Vitamin D in Epithelial-Mesenchymal Transition and Airway Remodeling in Allergic Airway Inflammation.
The first line of defense from environmental factors in the host is the epithelium. In the lung, the epithelial layer provides not only a passive physical barrier, but also acts as an active protector through the release of antimicrobial agents, cytokines, and other factors which can activate the innate and adaptive immune system. Patients that suffer from asthma, a chronic inflammatory disease of the airways, show pathological changes in the airways, including airway remodeling, basement membrane thickening, and epithelial damage with large immune cell infiltrate. How the epithelium contributes to airway remodeling and subepithelial fibrosis is unknown. However, it has been proposed that in inflammatory conditions, epithelial cells can undergo modifications that contribute to the pathophysiology of asthma. Epithelial-mesenchymal transition (EMT) is a process by which epithelial cells transition into a myofibroblast cell with enhanced migratory ability to secrete connective tissue. Vitamin D has been shown to have immunomodulatory properties in both innate and adaptive immunity. The relationship between asthma and vitamin D deficiency has been linked with increased asthma pathogenesis. The active form of vitamin D, calcitriol, exerts its actions through the vitamin D receptor (VDR) inducing a number of biological processes. To better understand the mechanisms underlying EMT and the nature of vitamin D modulation in the airway epithelium, human bronchial epithelial cells were stimulated with both calcitriol and transforming growth factor (TGF)-β1 or TGF-β2. These cells were analyzed for their expression of EMT markers and their functional invasive and migratory properties. Both TGF-β1 and TGF-β2 increased the expression of EMT markers, Snail, N-cadherin, and vimentin, and decreased the expression of E-cadherin. Calcitriol inhibited the effect of both TGF-β1 and TGF-β2 on the expression of EMT markers. Calcitriol also prevented the migratory and invasive actions of the cells following exposure to TGF-β1 and TGF-β2. In order to examine the in vivo significance of these findings, an allergic airway inflammatory mouse model of asthma was utilized to examine the expression of the EMT markers. Mice were sensitized and challenged with a combination of clinically relevant allergens house dust mite, ragweed, and Alternaria alternata (HRA), and fed a special vitamin D-sufficient or vitamin D-supplemented diet. HRA-vitamin D-supplemented mice had increased pulmonary function, as measured by airway hyperresponsiveness, increased cellular infiltrate in their bronchoalveolar lavage fluid, decreased vimentin and N-cadherin with increased expression in E-cadherin in the epithelium compared to HRA mice on a vitamin D-sufficient diet. Finally, vitamin D-deficient HRA mice were analyzed for the effect of vitamin D on airway hyperresponsiveness, airway remodeling, and markers associated with the regulation of vitamin D and EMT. Vitamin D-deficient HRA mice had decreased pulmonary function, increased eosinophils and lymphocytes in the bronchoalveolar lavage fluid, decreased VDR expression, and increased phosphorylated glycogen synthase kinase (GSK)-3β. The effect of vitamin D on VDR and pGSK-3β was also analyzed in human bronchial epithelial cells treated with TGF-β1 or TGF-β2. Inflammatory conditions used to mimic EMT resulted in the decreased expression of VDR along with increased pGSK-3β. The findings of this study suggest that epithelial changes in the airway may be mediated through EMT mechanisms and could be modulated by vitamin D. Therefore, vitamin D may provide a therapeutic role in attenuating allergic airway inflammation.
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