MTT assay

Cells were seeded at 1×10⁴ cells per well in 96-well plate and incubated overnight. The cells were exposed to curcumin or fenretinide alone, or in combination. As the control, cells were incubated with fresh medium. Cell viability was determined after 24 h of incubation by replacing the medium with 100 µl fresh medium prior to the addition of 20 µl of MTT (5 mg/ml). After 4 h of incubation at 37°C, the medium was removed and 100 µl of dimethyl sulfoxide (DMSO) was added to each well to solubilize the formazan followed by gently shaking the plate. The absorbance at 570 nm was measured with a microplate reader.