Figure 6.

Luteolin reduces the O-GalNAc glycosylation on APP and leads to a decrease of Aβ production in cells. A and B, dose-dependent effect of luteolin on the O-GalNAc glycosylation of APP and Aβ production in HEK 293T-APP Swe cells. Cells were incubated with luteolin for 24 h and then lysed and blotted for APP. The mean density of APP-H was quantified and normalized with APP-L and presented as percentages of control (DMSO, 100%). Aβ, sAPPα, and sAPPβ in the cell medium above were quantified by ELISA and normalized to the amount of total protein and presented as percentages of control (cells without luteolin treatment, 100%). C, effects of luteolin on O-GalNAc glycans of APP in CHO-K1 and CHO-ldlD cells. Cells were transfected with FLAG-tagged APP for 24 h followed by incubation with 25 μm luteolin for an additional 24 h. APP was then immunoprecipitated and detected by Jacalin staining. D, Aβs in the cell medium above were analyzed by ELISA. Control, CHO-K1 cells expressing APP without luteolin treatment, 100%. E, adding exogenous Gal and GalNAc restores the O-GalNAc glycosylation on APP in CHO-ldlD cells, but these effects could be attenuated by luteolin treatment (25 μm). F, Aβs in the cell medium above were quantified by ELISA and represented as Aβ (pg) per total protein (mg). ○, DMSO treatment; ●, 25 μm luteolin treatment. The mean density of Jacalin in C and E was quantified and normalized with APP and presented as a percentage of control (cells without luteolin treatment, 100%). Data represent mean ± S.D. (error bars), n = 3. *, p < 0.05; **, p < 0.01; ***, p < 0.001; for A, one-way ANOVA; for Aβ in B, D, and F, two-way ANOVA; for sAPPα and sAPPβ in B, C, and E, two-tailed Student's t test. IP, immunoprecipitation; EV, empty vector.