Contribution of CD14 and TLR4 to changes of the PI(4,5)P2 level in LPS-stimulated cells

Abstract

LPS binds sequentially to CD14 and TLR4/MD2 receptor triggering production of proinflammatory mediators. The LPS-induced signaling is controlled by a plasma membrane lipid PI(4,5)P₂ and its derivatives. Here, we show that stimulation of murine peritoneal macrophages with LPS induces biphasic accumulation of PI(4,5)P₂ with peaks at 10 and 60-90 min that were still seen after silencing of TLR4 expression. In contrast, the PI(4,5)P₂ elevation was abrogated when CD14 was removed from the cell surface. To assess the contribution of CD14 and TLR4 to the LPS-induced PI(4,5)P₂ changes, we used HEK293 transfectants expressing various amounts of CD14 and TLR4. In cells with a low content of CD14 and high of TLR4, no accumulation of PI(4,5)P₂ occurred. With an increasing amount of CD14 and concomitant decrease of TLR4, 2 peaks of PI(4,5)P₂ accumulation appeared, eventually approaching those found in LPS-stimulated cells expressing CD14 alone. Mutation of the signaling domain of TLR4 let us conclude that the receptor activity can modulate PI(4,5)P₂ accumulation in cells when expressed in high amounts compared with CD14. Among the factors limiting PI(4,5)P₂ accumulation are its hydrolysis, phosphorylation, and availability of its precursor, PI(4)P. Inhibition of PLC and PI3K or overexpression of PI4K IIα that produces PI(4)P promoted PI(4,5)P₂ elevation in LPS-stimulated cells. The elevation of PI(4,5)P₂ was dispensable for TLR4 signaling yet enhanced its magnitude. Taken together, these data suggest that LPS-induced accumulation of PI(4,5)P₂ that maximizes TLR4 signaling is controlled by CD14, whereas TLR4 can fine tune the process by affecting the PI(4,5)P₂ turnover.