a, b Western blot analysis of protein expression and quantification of Nrf2 in chondrocytes treated with control vehicle, CDDO-Im (20 nM), TNF-α (50 ng/mL) or CDDO-Im plus TNF-α for 24 h. c, d Western blot of protein expression and quantification of HO-1 and NQO1 in chondrocytes treated with various concentrations of CDDO-Im for 24 h. e Representative images of immunofluorescence-stained Nrf2 in chondrocytes treated with or without CDDO-Im. f Quantitative analysis of the data in Fig. 5e. The values are presented as the means ± SDs. *P < 0.05 versus control, #P < 0.05 versus TNF-α-treated chondrocytes.