100 nM BTZ and CFZ treatment did not decrease cell viability but significantly inhibited proteasome activity and BTZ caused a higher level of K48-linked protein ubiquitination and carbonylation. A: Human neuronal cell viability with MTT assay after 100 nM BTZ and CFZ treatments for 3 and 24 h. B, C: Proteasome activity levels after 100 nM BTZ and CFZ treatments for 3 and 24 h in human neuronal cells. D, E: Representative Western blot images and quantification of K48-linked ubiquitination after 100 nM BTZ and CFZ treatments for 3 and 24 h in human neuronal cells. F, E: Representative OxyBlot images and quantification of protein carbonylation after 100 nM BTZ and CFZ treatments for 3 and 24 h in human neuronal cells. Data represent means ± SEM, n = 3, ***p < 0.001 versus control group, ##p < 0.01 versus BTZ group.