Modulation of adriamycin ® accumulation and efflux by flavonoids in HCT-15 colon cells : Activation of P-glycoprotein as a putative mechanism

Since P-glycoprotein (P-gp) in normal tissues may serve as a cellular defense mechanism against naturally occurring xenobiotics, we considered whether physiologically active components of commonly ingested plant foods could influence P-gp function. To examine this possibility, a series of flavonoids commonly found in plant foods was tested for their ability to modulate [ 14C]Adriamycin ® ([ 14C]ADR) accumulation and efflux in P-gp-expressing HCT-15 colon cells. Many flavonoids, in the micromolar range, inhibited the accumulation of [ 14C]ADR. Detailed experiments utilizing flavonoids with the greatest activity in reducing [ 14C]ADR accumulation, i.e. galangin, kaempferol, and quercetin, revealed that the efflux of [ 14C]ADR is increased markedly in the presence of these compounds. Flavonoid-induced stimulation of efflux was rapid and was blocked by the multidrug-resistant (MDR) reversal agents verapamil, vinblastine, and quinidine. The magnitude of flavonoid-stimulated efflux in sodium butyrate-treated cells with a 4-fold induction of P-gp protein was similar to that in uninduced cells. [ 3H]Azidopine photoaffinity labeling of P-gp in crude membrane preparations revealed mild to no competition for binding by flavonoids possessing either activity or inactivity in reducing ADR accumulation. Although flavonoid hydrophobicity was found to be unrelated to flavonoid activity in altering [ 14C]ADR accumulation, certain structural features were associated with enhancement or diminution of activity. Finally, the significance of flavonoid-related reduction of [ 14C]ADR accumulation was underscored in cell growth studies, showing partial protection by quercetin against ADR-induced growth inhibition. It is concluded that certain naturally occuring plant flavonoids may acutely upregulate the apparent activity of P-gp.