Vitamin D Improves the Effect of Glucocorticoids on Attenuating Lipopolysaccharide-Induced IL-6 Production via TLR4/NF-κB Pathway in Human Respiratory Epithelial Cells

BACKGROUND: Glucocorticoid (GC) resistance results in unsatisfactory outcomes in chronic rhinosinusitis with nasal polyps (CRSwNP) patients. Previous studies have shown that calcitriol, the active form of vitamin D, alone or in combination with corticosteroids, exerts crucial immunomodulatory effects on inflammatory responses. However, whether vitamin D can improve the effect of GCs to attenuate inflammation in the epithelium of CRSwNP remains unclear. METHODS: A human bronchial epithelial cell line (Beas-2B) and primary human nasal epithelial cells (HNECs) obtained from 10 patients with CRSwNP were exposed to lipopolysaccharide (LPS) for 24 h to establish an inflammation model. LPS-stimulated HNECs and Beas-2B cells were treated with/without dexamethasone in the presence or absence of calcitriol pretreatment for 24 h. The expression levels of interleukin-6 (IL-6) mRNA and protein were determined by quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. Toll-like receptor 4 (TLR4)/NF-κB pathway related markers were examined by western blotting. One-way ANOVA or the Kruskal-Wallis test with post hoc analysis were used for multiple comparisons among groups. RESULTS: The production of IL-6 in Beas-2B cells and primary HNECs after LPS stimulation was significantly increased, which could be inhibited by dexamethasone or calcitriol alone. However, significant inhibition of IL-6 production was observed in the calcitriol plus dexamethasone group. Further analysis showed that calcitriol could enhance the effect of dexamethasone in inhibiting LPS-induced overexpression of TLR4, Myd88, and phosphorylation of p65. CONCLUSION: Our findings demonstrated that vitamin D could improve the effect of GCs to alleviate the level of IL-6 induced by LPS via the TLR4/NF-κB pathway in human respiratory epithelial cells.