Vero E6 cells (green monkey kidney) were cultured in Dulbecco's modified Eagle medium (DMEM, Gibco) and CHO-K1 Tet-On cells in DMEM/F12 (Gibco), both media containing penicillin (100 U/ml), streptomycin (100 mg/ml), and L-glutamine (2 mmol/l) (all from Invitrogen) as well as 10% fetal bovine serum (PAN Biotech). S1P-deficient SRD-12B cells (a generous gift from Dr. J. L. Goldstein, Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, USA) were maintained as CHO cells but supplemented with 5 mg/ml of cholesterol (Sigma), 1 mM sodium mevalonate (Sigma), and 20 mM sodium oleate (Sigma) [44] .