Suppression of basal and carbon nanotube-induced oxidative stress, inflammation and fibrosis in mouse lungs by Nrf2

The lungs are susceptible to oxidative damage by inhaled pathogenic agents, including multi-walled carbon nanotubes (MWCNT). The nuclear factor erythroid 2-related factor 2 (Nrf2) has been implicated in regulating the body's defense against oxidative stress. Here, we analyzed the function of Nrf2 in the lungs. Under a basal condition, Nrf2 knockout (KO) mice showed apparent pulmonary infiltration of granulocytes, macrophages and B and T lymphocytes, and elevated deposition of collagen fibers. Exposure to MWCNT (XNRI MWNT-7, Mitsui, Tokyo, Japan) by pharyngeal aspiration elicited rapid inflammatory and fibrotic responses in a dose (0, 5, 20 and 40 μg) and time (1, 3, 7 and 14 d)-dependent manner. The responses reached peak levels on day 7 post-exposure to 40 μg MWCNT, evidenced by massive inflammatory infiltration and formation of inflammatory and fibrotic foci, which were more evident in Nrf2 KO than wild-type (WT) lungs. At the molecular level, Nrf2 protein was detected at a low level under a basal condition, and was dramatically increased by MWCNT in WT, but not Nrf2 KO, lungs. Activation of Nrf2 was inversely correlated with induced expression of fibrosis marker genes and profibrotic cytokines. Furthermore, the levels of ROS and oxidative stress were remarkably higher in Nrf2 KO than WT lungs under a physiological condition, and were dramatically increased by MWCNT, with the increase significantly more striking in KO lungs. The findings reveal that Nrf2 plays an important role in suppressing the basal and MWCNT-induced oxidant production, inflammation and fibrosis in the lungs, thereby protecting against MWCNT lung toxicity.