Figure 2. Dose-response curves demonstrating the effects of retinoids on the ATPase activity of Pgp and ABCG2.

Pgp (a,b,c) and ABCG2 (d,e,f) expressing Sf9 cell membrane preparations were pre-incubated with retinoids for 10 min at 37 °C and then further incubated in the presence of 3 mM ATP/Mg²⁺ for 25 min. The substrate-dependent ATPase activity of Pgp was measured in the presence of 40 μM verapamil, while the activity of ABCG2 was induced by 10 μM quercetin. Representative graphs are shown out of three independent experiments performed in triplicates (data points are their means ± SD). Data points were fitted with a four parameter sigmoidal dose-response relationship (R² ≥ 0.98, where R² is the coefficient of determination; see Materials and Methods) to determine the IC₅₀ values. IC₅₀ values are given as means ± SEM from 3 independent measurements.