Peptidoglycan induces interleukin‐6 expression through the TLR2 receptor, JNK, c‐Jun, and AP‐1 pathways in microglia

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Abstract
We recently reported that peptidoglycan (PGN), a cell wall component of the Gram‐positive bacterium, induces NF‐κB activation and microglia activation. However, PGN‐regulated AP‐1 activation and cytokine expression in microglia remains unclear. This study investigated how PGN influences the signaling pathway involved in IL‐6 production in microglia. IL‐6 mRNA and protein level up‐regulation were increased by PGN in a concentration‐ and time‐dependent manner. In addition, PGN increased toll‐like receptor‐2 (TLR2) expression, but not TLR4 receptor up‐regulation. Administration of TLR2 siRNA or TLR2 neutralized antibody effectively inhibited PGN‐induced IL‐6 expression. In contrast, PGN‐induced IL‐6 mRNA and protein up‐regulation were attenuated by the SAPK/JNK (c‐Jun N‐terminal kinases) inhibitor SP600125. Treatment of microglia with PGN increased levels of JNK phosphorylation and c‐Jun phosphorylation, and up‐regulated of JNK kinase activity. Treatment of microglia with AP‐1 inhibitors (Tanshinone IIA and curcumin) effectively reduced PGN‐induced IL‐6 expression. PGN also significantly increased c‐Fos and phospho‐c‐Jun translocation to nucleus. In line with this, PGN also increased AP‐1‐DNA complexes formation, as determined by the electrophoretic mobility shift assay. Furthermore, PGN also increased IL‐6 transcription activity determined by transfection with IL‐6 promoter construct plasmid. Co‐transfection with dominant negative mutant of JNK (DN‐JNK), or treatment with SP600125, curcumin, or Tanshinone IIA effectively antagonized PGN‐increased IL‐6 transcription activity. Our data demonstrate that PGN‐induced IL‐6 expression is mediated by AP‐1 activation through the TLR2 and JNK/c‐Jun pathways in microglia. J. Cell. Physiol. 226: 1573–1582, 2011.