Cooperation of TLR2 with MyD88, PI3K, and Rac1 in lipoteichoic acid-induced cPLA₂/COX-2-dependent airway inflammatory responses

Lipoteichoic acid (LTA) plays a role in the pathogenesis of severe inflammatory responses induced by Grampositive bacterial infection. Cytosolic phospholipase A₂ (cPLA₂), cyclooxygenase-2 (COX-2), prostaglandin E₂ (PGE₂), and interleukin (IL)-6 have been demonstrated to engage in airway inflammation. In this study, LTA-induced cPLA₂ and COX-2 expression and PGE₂ or IL-6 synthesis were attenuated by transfection with siRNAs of TLR2, MyD88, Akt, p42, p38, JNK2, and p65 or pretreatment with the inhibitors of PI3K (LY294002), p38 (SB202190), MEK1/2 (U0126), JNK1/2 (SP600125), and NF-κB (helenalin) in human tracheal smooth muscle cells (HTSMCs). LTA also induced cPLA₂ and COX-2 expression and leukocyte count in bronchoalveolar lavage fluid in mice. LTA-regulated PGE₂ or IL-6 production was inhibited by pretreatment with the inhibitors of cPLA₂ (AACOCF₃) and COX-2 (NS-398) or transfection with cPLA2 siRNA or COX-2 siRNA, respectively. LTA-stimulated NF-κB translocation or cPLA₂ phosphorylation was attenuated by pretreatment with LY294002, SB202190, U0126, or SP600125. Furthermore, LTA could stimulate TLR2, MyD88, PI3K, and Rac1 complex formation. We also demonstrated that Staphylococcus aureus could trigger these responses through a similar signaling cascade in HTSMCs. It was found that PGE₂ could directly stimulate IL-6 production in HTSMCs or leukocyte count in bronchoalveolar lavage fluid in mice. These results demonstrate that LTA-induced MAPKs activation is mediated through the TLR2/MyD88/PI3K/Rac1/Akt pathway, which in turn initiates the activation of NF-κB, and ultimately induces cPLA₂/COX-2- dependent PGE₂ and IL-6 generation.