Many
usnic acid-containing dietary supplements have been marketed as
weight loss agents, although severe hepatotoxicity and
acute liver failure have been associated with their overuse. Our previous mechanistic studies revealed that autophagy, disturbance of
calcium homeostasis, and ER stress are involved in
usnic acid-induced toxicity. In this study, we investigated the role of oxidative stress and the Nrf2 signaling pathway in
usnic acid-induced toxicity in HepG2 cells. We found that a 24-h treatment with
usnic acid caused DNA damage and S-phase cell cycle arrest in a concentration-dependent manner.
Usnic acid also triggered oxidative stress as demonstrated by increased
reactive oxygen species generation and
glutathione depletion. Short-term treatment (6 h) with
usnic acid significantly increased the
protein level for Nrf2 (nuclear factor erythroid 2-related factor 2), promoted Nrf2 translocation to the nucleus, up-regulated antioxidant response element (ARE)-
luciferase reporter activity, and induced the expression of Nrf2-regulated targets, including
glutathione reductase,
glutathione S-transferase, and
NAD(P)H
quinone oxidoreductase-1 (NQO1). Furthermore, knockdown of Nrf2 with
shRNA potentiated
usnic acid-induced DNA damage and cytotoxicity. Taken together, our results show that
usnic acid causes cell cycle dysregulation, DNA damage, and oxidative stress and that the Nrf2 signaling pathway is activated in
usnic acid-induced cytotoxicity.