Inhibition of the phosphoinositide 3-kinase pathway decreases innate resistance to lipopolysaccharide toxicity in TLR4 deficient mice

Upon lipopolysaccharide (LPS) stimulation, activation of both the Toll-like receptor 4 (TLR4) and phosphoinositide 3-kinase (PI3K) pathways serves to balance proinflammatory and anti-inflammatory responses. Although the antagonist to TLR4 represents an emerging promising target for the treatment of sepsis; however, the role of the PI3K pathway under TLR4-null conditions is not well understood. This goal of this study was to investigate the effect of inhibition of PI3K on innate resistance to LPS toxicity in a murine model.

In this study, we demonstrate that innate resistance to LPS toxicity in Tlr4-/- mice is impaired by inhibition of the PI3K pathway, with a corresponding increase in mortality and production of tissue O&sub2;⁻ and inflammatory cytokines.

The overall survival of the cohorts receiving intraperitoneal injections of 100, 500, or 1000 μg LPS from Escherichia coli serotype 026:B6 after 7 d was 100%, 10%, and 10%, respectively. In contrast, no mortality was noted after 500-μg LPS injection in Tlr4-/- mice. When the PI3K inhibitor LY294002 was injected (1 mg/25 g body weight) 1 h prior to the administration of LPS, the overall survival of the Tlr4-/- mice was 30%. In the Tlr4-/- mice, the LPS injection induced no NF-κB activation but an increased Akt phosphorylation in the lung and liver, when compared to that of the C57BL/6 mice. Injection of 500 μg LPS led to a significant induction in O&sub2;⁻ detected by electron paramagnetic resonance (EPR) spin trapping spectroscopy in the lung and liver at 3 and 6 h in C57BL/6 but not Tlr4-/- mice. Addition of LY294002 only significantly increased the O&sub2;⁻ level in the lung and liver of the Tlr4-/- mice but not in the C57BL/6 mice following 500-μg LPS injection. In addition, the serum IL-1β and IL-2 levels were more elevated in C57BL/6 mice than in Tlr4-/- mice. Notably, IL-1β and IL-2 were significantly increased in Tlr4-/- mice but not in the C57BL/6 mice when the PI3K pathway was inhibited by LY294002 prior to LPS injection. Conclusions: In this study, we demonstrate that innate resistance to LPS toxicity in Tlr4-/- mice is impaired by inhibition of the PI3K pathway, with a corresponding increase in mortality and production of tissue O&sub2;⁻ and inflammatory cytokines.