MTT cell viability assay

HaCaT cells (100 μl) suspended in DMEM at a density of 5 × 10⁴ cells/ml were seeded onto each well of 96-well plates and incubated at 37 °C for 24 h in a CO₂ incubator. The medium was replaced with serum-free DMEM for cell starvation for 12 h, and the cells were subsequently treated with royal jelly fractions, bovine serum albumin (BSA, 3.2 μg/ml) or serum-free DMEM (control) for 24, 48 or 72 h. Then, they were incubated for a further 4 h after addition of 10 μl of 5 mg/ml 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution (Beyotime, China). The generated formazan crystals were dissolved in 100 μl of DMSO following removal of the supernatant. Cell viability was determined by the absorbance at 492 nm in an Infinite F50 plate reader (Tecan, Austria), and expressed as % cell viability = absorbance of protein treated cells/absorbance of serum-free medium treated cells × 100%.