Osteoprotegerin Exerts Its Pro-inflammatory Effects Through Nuclear Factor-κB Activation

Osteoprotegerin (OPG), a soluble member of the tumor necrosis factor (TNF) receptor super-family, is a key factor inhibiting the differentiation and activation of osteoclasts. It has recently been implicated as a disease marker for inflammatory bowel disease (IBD) yet its role in the intestinal epithelial inflammatory response remains unknown. The primary objective of this study was to investigate whether OPG has a role in intestinal inflammation and a potential role in IBD pathogenesis. Caco-2 and HT-29 cells were grown in vitro to confluence on culture-permeable supports and then co-cultured with either TNF-alpha or OPG. After exposure to either TNF-alpha or OPG, interleukin (IL)-8 protein and mRNA levels were evaluated. Ussing chamber, western blotting, real-time polymerase chain reaction, and immunofluorescence were used to further investigate the effect of OPG on intestinal barrier integrity and function. Similar to TNF-alpha, treatment of monolayers with OPG caused increased monolayer permeability and diminished tight junction function and integrity, with loss of tight junction proteins from cell membranes. This was accompanied by elevated IL-8 protein and gene levels (P < 0.05). Western blotting also revealed that OPG, similar to TNF-alpha, induced NF-kappa B activation, as shown by inhibition of NF-kappa B kinase subunit-alpha phosphorylation. These results indicate that OPG has pro-inflammatory properties because it induces gut barrier dysfunction and secretion of other pro-inflammatory cytokines. These results also provide evidence that OPG is likely to exert its pro-inflammatory effects through NF-kappa B activation and may potently contribute to IBD pathogenesis.