Toll-like receptor 2-mediated NF-κB activation requires a RacI-dependent pathway

Mammalian Toll-like receptors (TLRs) are expressed on innate immune cells and respond to the membrane components of Gram-positive or Gram-negative bacteria. When activated, they convey signals to transcription factors that orchestrate the inflammatory response. However, the intracellular signaling events following TLR activation are largely unknown. Here we show that TLR2 stimulation by Staphylococcus aureus induces a fast and transient activation of the Rho GTPases Rad and Cdc42 in the human monocytic cell line THP-I and in 293 cells expressing TLR2. Dominant-negative RacIN17,but not dominant-negative Cdc42N17, block nuclear factor-kappaB (NF-kappaB) transactivation. S, aureus stimulation causes the recruitment of active Rad and phosphatidylinositol-3 kinase (PI3K) to the TLR2 cytosolic domain. Tyrosine phosphorylation of TLR2 is required for assembly of a multiprotein complex that is necessary for subsequent NF-kappaB transcriptional activity. A signaling cascade composed of Rad, PI3K and Akt targets nuclear p65 transactivation independently of I kappaB alpha degradation, Thus Rad controls a second, I kappaB-independent, pathway to NF-kappaB activation and is essential in innate immune cell signaling via TLR2.