Acute lung injury (ALI) is a hallmark of systemic inflammation associated with high mortality. Although the vitamin D receptor (VDR) is highly expressed in the lung, its role in lung physiology remains unclear. We investigated the effect of VDR deletion on ALI using a lipopolysaccharide (LPS)-induced sepsis model. After LPS challenge VDR-null mice exhibited more severe ALI and higher mortality compared with wild-type (WT) counterparts, manifested by increased pulmonary vascular leakiness, pulmonary edema, apoptosis, neutrophil infiltration, and pulmonary inflammation, which was accompanied by excessive induction of angiopoietin (Ang)-2 and myosin light chain (MLC) phosphorylation in the lung. 1,25-Dihydroxyvitamin D blocked LPS-induced Ang-2 expression by blocking nuclear factor-κB activation in human pulmonary artery endothelial cells. The severity of lung injury seen in VDR-null mice was ameliorated by pretreatment with L1-10, an antagonist of Ang-2, suggesting that VDR signaling protects the pulmonary vascular barrier by targeting the Ang-2-Tie-2-MLC kinase cascade. Severe ALI in VDR-null mice was also accompanied by an increase in pulmonary renin and angiotensin II levels, and pretreatment of VDR-null mice with angiotensin II type 1 receptor blocker losartan partially ameliorated the severity of LPS-induced lung injury. Taken together, these observations provide evidence that the vitamin D-VDR signaling prevents lung injury by blocking the Ang-2-Tie-2-MLC kinase cascade and the renin-angiotensin system.

Vitamin D and its receptor have a protective effect on epithelial barriers in various tissues. Low levels of vitamin D are associated with numerous pulmonary diseases, including acute lung injury (ALI) and acute respiratory distress syndrome. The present study investigated whether the vitamin D/vitamin D receptor (VDR) pathway may ameliorate lipopolysaccharide (LPS)-induced ALI through maintaining the integrity of the alveolar epithelial barrier. This was investigated by exposing wild-type (WT) and VDR knockout C57BL/6J mice to LPS, then comparing the healthy and LPS-treated mice lungs and bronchoalveolar lavage fluid (BALF). More specifically, lung histology, mRNA levels of proinflammatory cytokines and chemokines, and protein expression levels of tight junction proteins were determined. In addition, a vitamin D analog (paricalcitol) was administered to WT mice in order to investigate the effect of vitamin D on the alveolar epithelial barrier following exposure to LPS. VDR knockout mice exhibited severe lung injuries (P<0.001), increased alveolar permeability [demonstrated by a higher wet-dry ratio of lung weight (P<0.05), greater expression levels of BALF protein (P<0.001) and fluorescein isothiocyanate-conjugated 4 kDa dextran (P<0.001) leakage into the alveolar space], elevated proinflammatory cytokine and chemokine mRNA levels, as demonstrated by reverse transcription-quantitative polymerase chain reaction (P<0.05), and decreased protein and mRNA expression levels of occludin (P<0.01) and zonula occludens-1 (ZO-1; P<0.01) compared with WT mice. Paricalcitol treatment partially inhibited these pathological changes in WT mice by maintaining the mRNA and protein expression levels of occludin (P<0.01) and ZO-1 (P<0.05). A lack of VDRs in the pulmonary epithelial barrier appeared to compromise its defense, leading to more severe LPS-induced lung injury. Furthermore, vitamin D treatment alleviated LPS-induced lung injury and preserved alveolar barrier function. Therefore vitamin D treatment may present as a potential therapeutic strategy in ALI and acute respiratory distress syndrome.

Background The myosin light chain kinase (MLCK) pathway controls intestinal epithelial barrier permeability by regulating the tight junction. 1,25-dihydroxyvitamin D (1,25(OH)2D3)-vitamin D receptor (VDR) signaling protects the epithelial barrier, but the molecular mechanism is incompletely understood. Methods MLCK activation and barrier permeability were studied using monolayers of HCT116, Caco-2 and SW480 cells treated with TNF-α ±1,25(OH)2D3. The MLCK pathway was analyzed in normal and inflamed colonic biopsies from ulcerative colitis patients. Colonic mucosal barrier permeability and MLCK activation were also investigated using TNBS-induced colitis models in vitamin D analog paricalcitol-treated wild-type mice and mice carrying VDR deletion in colonic epithelial cells. Results TNF-α increased cell monolayer permeability and induced long isoform of MLCK expression and myosin II regulatory light chain (MLC) phosphorylation, and 1,25(OH)2D3 blocked TNF-α-induced increases in monolayer permeability and MLCK-MLC pathway activation by a VDR-dependent fashion. 1,25(OH)2D3 directly suppressed long MLCK expression by attenuating NF-κB activation, and ChIP assays confirmed that 1,25(OH)2D3 disrupted p65 binding to three κB sites in long MLCK gene promoter. In human ulcerative colitis biopsies VDR reduction was associated with increases in long MLCK expression and MLC phosphorylation. In TNBS colitis models, paricalcitol ameliorated colitis, attenuated the increase in mucosal barrier permeability and inhibited long MLCK induction and MLC phosphorylation. In contrast, mice with colonic epithelial VDR deletion exhibited more robust increases in mucosal barrier permeability and MLCK activation compared with wild-type mice. Conclusions These data demonstrate that 1,25(OH)2D3-VDR signaling preserves the mucosal barrier integrity by abrogating MLCK-dependent tight junction dysregulation during colonic inflammation.

Pulmonary fibrosis, which influences lung function and exacerbates a patient’s condition, is the ultimate stage of many lung diseases. Vitamin D deficiency is associated with pulmonary fibrosis and impaired lung function, but the underlying mechanism has not yet been fully elucidated. Moreover, vitamin D deficiency may cause over-activation of the renin-angiotensin system (RAS), which aggravates extracellular matrix (ECM) deposition and lung fibrosis. This study aims to investigate the effect of chronic vitamin D deficiency on lung fibrosis in otherwise healthy mice and to explore the role of RAS in this process. Mice were depleted of vitamin D through diet control and were compared with healthy subjects. Chronic vitamin D deficiency destructs lung structures, impairs lung development and stimulates ECM deposition. RAS components are also found to increase. These effects seem to worsen with prolonged vitamin D deficiency. By giving RAS blockers, these changes can be largely rescued. However, a smooth muscle relaxant whose regulatory effect on blood pressure is independent of RAS does not show similar effects. This study demonstrated that chronic vitamin D deficiency may induce RAS activation, which subsequently stimulates the expression of profibrotic factors and activates the fibrotic cascade. This profibrotic effect of RAS is independent of elevated blood pressure.

Recent studies show that colonic vitamin D receptor (VDR) signaling protects the mucosal epithelial barrier and suppresses colonic inflammation, but the underlying molecular mechanism remains to be fully understood. To investigate the implication of colonic VDR downregulation seen in patients with inflammatory bowel disease, we assessed the effect of gut epithelial VDR deletion on colonic inflammatory responses in an experimental colitis model. In a 2,4,6-trinitrobenzenesulfonic acid-induced colitis model, mice carrying VDR deletion in gut epithelial cells [VDRflox/flox (VDRf/f);Villin-Cre or VDRΔIEC] or in colonic epithelial cells (VDRf/f;CDX2-Cre or VDRΔCEC) developed more severe clinical colitis than VDRf/f control mice, characterized by more robust T-helper (TH)1 and TH17 responses, with greater increases in mucosal interferon (IFN)-γ+, interleukin (IL)-17+, and IFN-γ+IL-17+ T cells. Accompanying the severe mucosal inflammation was more profound colonic epithelial cell apoptosis in the mutant mice. Treatment with caspase inhibitor Q-VD-OPh dramatically reduced colitis severity and attenuated TH1 and TH17 responses in VDRΔCEC mice. The blockade of cell apoptosis also prevented the increase in mucosal CD11b+CD103+ dendritic cells (DCs), known to be critical for TH17-cell activation. Moreover, depletion of gut commensal bacteria with antibiotics eliminated the robust TH1 and TH17 responses and CD11b+CD103+ DC induction. Taken together, these observations demonstrate that gut epithelial VDR deletion aggravates epithelial cell apoptosis, resulting in increases in mucosal barrier permeability. Consequently, invading luminal bacteria activate CD11b+CD103+ DCs, which promote mucosal TH1 and TH17 responses. Therefore, gut epithelial VDR signaling controls mucosal inflammation by suppressing epithelial cell apoptosis.