Linagliptin Inhibits Interleukin-6 Production Through Toll-Like Receptor 4 Complex and Lipopolysaccharide-Binding Protein Independent Pathway in vitro Model

PURPOSE: Lipopolysaccharides (LPS) induce inflammation by binding to the Toll-like receptor (TLR) 4 complex, including LPS-binding protein (LBP). The anti-inflammatory effects of linagliptin in LPS-induced inflammation in the TLR4-independent pathway have not been examined before. We examined the anti-inflammatory effects of linagliptin in the TLR4- and the LBP-independent pathway. METHODS: U937 cells were cultured in the medium supplemented with 10% fetal bovine serum (FBS) and treated with 100 nM phorbol myristate acetate for 48 h. Cells were then left untreated or were treated with 10 μg/mL anti-TLR4 antibodies alone or in combination with linagliptin for 1 h in media supplemented with or without 10% FBS. The cells were divided into 5 groups: a) control cells (untreated) b) cells treated with LPS c) cells treated with 10 μg/mL anti-TLR4 antibodies d) cells treated with LPS and 10 μg/mL anti-TLR4 antibodies and e) cells treated with LPS, 10 μg/mL anti-TLR4 antibodies, and linagliptin. The LPS concentrations used were 50 pg/mL or 100 pg/mL for cells treated in the presence of 10% FBS and 100 pg/mL or 1 μg/mL for cells treated in the absence of FBS. Linagliptin concentrations of 1 nM, 10 nM, and 100 nM were used for treatment. The supernatants were analyzed for interleukin (IL)-6 production after 24 h of various treatments. RESULTS: LPS increased IL-6 production compared to the untreated control cells, and anti-TLR4 antibody suppressed LPS-induced increased IL-6 levels. Linagliptin suppressed LPS-induced IL-6 production in a concentration-dependent manner in the presence of FBS. However, only 100 nM linagliptin could suppress LPS-induced IL-6 production in the absence of FBS. CONCLUSION: Concentration-dependent and -independent inflammatory suppression was observed following linagliptin treatment after LPS induction in an experimental model of TLR4 inhibition by anti-TLR4 antibodies. Our results showed that linagliptin may inhibit inflammation through multiple mechanisms centered around the TLR-4-mediated pathway.