Treatments for modifying the dipole potential or inhibiting HMG-CoA reductase
6-ketocholestanol and phloretin (Sigma-Aldrich, St. Louis, MO) were used for measuring penetratin uptake at increased and decreased dipole potentials, respectively. Both compounds were applied at a concentration of 100 µM at room temperature in the presence of 0.05% (v/v) Pluronic F-127 for 10 min (Clarke & Kane, 1997; Kovács, Batta, Zákány, Szöllősi & Nagy, 2017). In order to inhibit HMG-CoA reductase atorvastatin (Sigma-Aldrich) was applied at final concentrations of 1 nM-10 µM for 72 hours. The low nanomolar concentrations represent ranges found in human serum during atorvastatin treatment of patients with hypercholesterolemia (Bjorkhem-Bergman, Lindh & Bergman, 2011).