Cutting edge: Lipopolysaccharide induces physical proximity between CD14 and toll-like receptor 4 (TLR4) prior to nuclear translocation of NF-kappa B

CD14, a GPI-Linked protein, plays a pivotal role in LPS-mediated signaling by potentiating leukocyte adherence, activation, and cytokine production, R ecent studies have identified the Toll-like receptor 4 (TLR4) as a membrane cofactor in LPS-mediated transmembrane signaling in cytokine induction, al though the mechanism responsible for this cooperation is unknown. Using flu orescence resonance energy transfer (RET) techniques, we demonstrate that L PS triggers a physical association between CD14 and TLR4, Because LPS stimu lation upregulates CD14 and TLR4 expression, it was necessary to control fo r the possibility that these newly expressed molecules were associated with one another independent of LPS stimulation. Although the calcium ionophore A23187 increased the expression of CD14 and TLR4, they did not exhibit ene rgy transfer. However, following A23187 treatment, LPS promoted physical pr oximity between CD14 and TLR4. Therefore, we suggest that a close interacti on between CD14 and TLR4 participates in LPS signaling, leading to nuclear translocation of NF-kappa B.