Novel MAO-B Inhibitor Downregulates Lipopolysaccharide-Induced Epithelial Cell Chemokine Secretion

Objectives: Lipopolysaccharide (LPS) is a structural component of the outer membrane of Gram-negative bacteria, which induces strong immune responses in mammalian hosts infected. Disruption of epithelial barrier integrity and release of cytokines and chemokines begin at the early stage of chronic inflammation, allowing bacterial LPS to enter the submucosa and induce an inflammatory response such as neutrophil infiltration. Previously, the novel monoamine oxidase B (MAO-B) inhibitor Compound B has been shown to reduce the level of LPS-induced epithelial chemokine secretion. In this study, the possible signalling pathways involved in LPS-induced intestinal epithelial cell interleukin-8 (IL-8) gene and protein expression and reduction by Compound B were investigated.
Methods: The expression level of MAO-B was investigated in undifferentiated and differentiated Caco-2 cells. Both undifferentiated and differentiated Caco-2 cells were stimulated with 1 µg/mL LPS (E. coli serotype O55:B5) and co-treated with Compound B. Levels of IL-8 secretion were assessed by ELISA. Phosphorylation of NF-κB p65 and MAPKs were examined by Western blot. Total NF-κB p65 nuclear translocation was assessed by immunofluorescence.
Results: Only differentiated Caco-2 cells express MAO-B. Compound B reduced the level of LPS-induced IL-8 secretion equally effectively in both differentiated and undifferentiated Caco-2 cells regardless of the presence or absence of MAO-B expression. LPS-induced IL-8 secretion in undifferentiated Caco-2 cells was independent of NF-κB p65 nuclear translocation, phosphorylation of NF-κB p65 at Ser536 and MAPKs including ERK1/2, p38 and JNK.
Conclusions: Downregulation of LPS-induced IL-8 secretion in Caco-2 epithelial cells by Compound B was a possible off-target effect and independent of common LPS-induced signalling pathways.