PTEN–Foxo1 signaling triggers HMGB1-mediated innate immune responses in acute lung injury

PTEN is a multifunctional phosphatase that regulates immune responses through a PI3K/Akt signaling cascade. HMGB1 plays an important role in the initiation of innate immune responses to induce acute lung injury (ALI). This study was designed to investigate the role of PTEN/Foxo1 signaling in the regulation of in vivo and in vitro innate immune responses in ALI. Using a mouse model of ALI, wild-type (WT) and myeloid-specific PTEN knockout (PTEN^M-KO) mice were instilled with a recombinant HMGB1 (rHMGB1) or PBS. In some experiments, Foxo1 siRNA or non-specific siRNA was injected into mice 6 h prior to rHMGB1 instillation into lung. We found that rHMGB1 treatment in WT mice increased the expression of PTEN, Foxo1, TLR4, and NF-κB in alveolar macrophages from WT mice. However, macrophage-specific PTEN ablation resulted in reduced Foxo1 and TLR4 while increasing β-catenin (Ser552) and Akt (Ser473) phosphorylation in these cells. Knockdown of Foxo1 with siRNA administration in WT mice ameliorated lung injury and inhibited myeloperoxidase activity followed by rHMGB1 treatment, which was accompanied by decreased mRNA expression coding for TNF-α, IL-1β, MIP2, and IP-10. Moreover, Foxo1 knockdown inhibited the expression of TLR4-dependent IRF3 and IFN-β both in vitro and in vivo. These results demonstrate that PTEN/Foxo1 signaling is critical for triggering HMGB1-mediated innate TLR4 activation during ALI. By identifying the molecular signaling pathways within innate immune system, our studies provide the potential therapeutic targets for ALI.