We previously demonstrated that celastrol, a quinone methide triterpenoid derived from the medicinal plant Tripterygium wilfordii, exerts its anti-inflammatory activity through up-regulation of heme oxygenase-1 (HO-1) expression in the keratinocytes. In this study, we examined the signaling pathways that lead to the up-regulation of HO-1 expression by celastrol. In HaCaT cells, celastrol-induced HO-1 expression was dependent on ROS generation. ERK and p38 MAPK were major MAPK pathways responsible for celastrol-induced HO-1 expression. Celastrol induced Nrf2 activation. Nrf2 knockdown using small interfering RNA (siRNA) inhibited celastrol-induced HO-1 expression. Treatment with celastrol resulted in a marked increase in antioxidant response element (ARE)-driven transcriptional activity, which was dependent on ROS generation and activation of ERK and p38 MAPK. Furthermore, Nrf2 siRNA significantly reversed the inhibitory effect of celastrol on IFN-gamma-induced expression of ICAM-1 in the keratinocytes. Taken together, our results indicate that celastrol can activate the ROS-ERK/p38-Nrf2-ARE signaling cascades leading to the up-regulation of HO-1 which is partly responsible for its anti-inflammatory activity in the keratinocytes. (C) 2011 Elsevier Inc. All rights reserved.