Toll-like receptor 4 mediates ozone-induced murine lung hyperpermeability via inducible nitric oxide synthase

We tested the hypotheses that 1) inducible nitric oxide synthase (iNOS) mediates ozone (O₃)-induced lung hyperpermeability and 2) mRNA levels of the gene for iNOS (Nos2) are modulated by Toll-like receptor 4 (Tlr4) during O₃ exposure. Pretreatment of O₃-susceptible C57BL/6J mice with a specific inhibitor of total NOS (N^G-monomethyl-L-arginine) significantly decreased the mean lavageable protein concentration (a marker of lung permeability) induced by O₃ (0.3 parts/million for 72 h) compared with vehicle control mice. Furthermore, lavageable protein in C57BL/B6 mice with targeted disruption of Nos2 [Nos2(-/-)] was 50% less than the protein in wild-type [Nos2(+/+)] mice after O₃. To determine whether Tlr4 modulates Nos2 mRNA levels, we studied C3H/HeJ (HeJ) and C3H/HeOuJ mice that differ only at a missense mutation in Tlr4 that confers resistance to O₃-induced lung hyperpermeability in the HeJ strain. Nos2 and Tlr4 mRNA levels were significantly reduced and correlated in resistant HeJ mice after O₃ relative to those in susceptible C3H/HeOuJ mice. Together, the results are consistent with an important role for iNOS in O₃-induced lung hyperpermeability and suggest that Nos2 mRNA levels are mediated through Tlr4.