Background Multidrug resistance gene expression results in a transmembrane cellular pump (MDR). The lack of MDR function results in severe inflammatory bowel disease (IBD) in both humans and mice. Although MDR pumps chemotherapeutic agents out of the cell, its role in intestinal inflammation is unclear. Toll-like receptors (TLRs) recognize distinct molecular patterns with a resultant inflammatory response. TLR9, the receptor for bacterial DNA (CpG), is expressed both on the cell surface and in the cytoplasm, whereas TLR5 (the receptor for bacterial flagellin) is expressed solely on the cell surface. We hypothesized that TLR9-expressing epithelial cells treated with small interfering RNA (siRNA) that decrease MDR expression would have a greater CpG-mediated immune response, whereas similarly treated TLR5 cells would fail to mount a greater response with treated with flagellin.
Methods HEK-293 cells that express a single murine TLRs (Invivogen®) were transfected with MDR siRNA (Ambion®) using the Nucleofector V Kit® (Amaxa). The cells were grown in 12-well plates for 72 hours. Medium containing murine CpG (ODN 1826) at a concentration of 2, 0.2, or 0 μM CpG was then added to TLR9 cells. TLR5 cells received S. dublin flagellin at 500, 5, or 0 ng/mL. After 24 hours, the supernatant was harvested for IL-8 ELISA determination and RNA isolated with the RNeasy Plus Kit® (Qiagen). MDR expression was determined using RT-PCR. The mean IL-8 concentration and MDR RNA expression were analyzed with Student's t-test.
Results The MDR RNA expression at 96 hours post-siRNA transfection was significantly less than in cells not receiving siRNA. Similar IL-8 production was seen in the TLR5 cells with and without siRNA (78.7 ± 2.9 vs 76.9 ± 12.2 ng/mL, respectively). In contrast, the IL-8 expression in TLR9 cells treated with siRNA and 2 μM CpG was 4.9 ± 1.3 ng/mL compared with 1.6 ± 0.44 ng/mL in non-siRNA-treated cells (p = .03).
Conclusions TLR9-mediated epithelial inflammatory response may be regulated by MDR. We hypothesize that the MDR may pump CpG taken into the cell, attenuating the TLR9/MyD88/NFκB mechanism. Thus, a loss of MDR function could result in CpG, the initiating immune response in IBD.
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