DPBA staining

To qualitatively detect quercetin uptake in HG3 cells, we used DPBA (2-aminoethyl diphenylborinate; Sigma-Aldrich) staining, a reagent used in plant physiology that selectively binds flavonols emitting fluorescent when exited at 485 ± 20 nm (emission at 530 ± 20 nm) [89]. The method was slightly modified to qualitatively assess the intracellular presence of quercetin in human cell lines. Briefly, HG3 cells were plated at a cell density of 1 × 10⁶/ml in a 12-wells plate and treated with 25 μM quercetin (5–60 min); subsequently, cells were centrifuged, washed with PBS, suspended in 1 ml of DPBA solution (2.5 mg/ml) dissolved in 10% formalin and finally incubated at 37°C in a humidified atmosphere containing 5% CO₂ for 15 min. After incubation, cells were visualized using a fluorescent microscopy and photographed in phase contrast and in FITC filter with 400× magnification (Axiovert 200 Zeiss, Iena Germany).