Cell viability assay

Comparative cell survival was measured using a tetrazolium salt (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide or MTT) based cell viability assay. On day one, 2 x 10³ human cardiomyocytes were seeded into each well of a clear 96-well plate in 100 μl of growth medium and cultured overnight. The next day, human cardiomyocytes were pre-treated with ATRA (500 nM, 1000 nM) or the vehicle (DMSO) for 24 hours. The following day, the cells were treated with the respective ATRA dose or the vehicle (DMSO) and serial concentrations of doxorubicin (0 μM, 1 μM, 3.16 μM, 10 μM, 31.6 μM, and 100 μM) for another 24 hours. Cell viability was quantified using MTT assay (Sigma-Aldrich) on POLARstar Omega plate reader (BMG Labtech, Ortenberg, Germany) by measuring absorbance.