“…Both cell types had a doubling time of about 20 h (Spina et al 1993) and were cultured as monolayers at 37 C in an humidified atmosphere of 5% CO 2 -95% air in Dulbecco's modified Eagle's medium (DMEM) supplemented with 5% fetal calf serum, 2 mM glutamine, 100 U/ml penicillin, and 100 µg/ml streptomycin (standard culture medium). The medium was changed every two days and, when required, the cells were treated with all-trans RA (Sigma, St Louis, MO, USA), or NaB (Sigma) or a combination of RA and NaB, using appropriate dilutions in DMEM of the original stock solutions in dimethyl sulfoxide (DMSO), and without changing the medium from the beginning of treatment to the end of the experiments.…”