Effects of grapefruit juice on the multidrug transporter P-glycoprotein in the human proximal tubular cell line HK-2

The multidrug transporter MDR-1 P-glycoprotein (Pgp) has been recently pointed out as an important mechanism underlying chemical interaction between drugs and many commonly ingested substances, including grapefruit juice (GFJ). Modulation of intestinal Pgp dependent transport by GFJ may lead to changes in bioavailability of drugs that are substrates of Pgp itself, by affecting their presystemic clearance. Since other cellular sites expressing Pgp and devoted to drug disposition, like kidney proximal tubules, could be involved in these pharmacokinetic interactions, we investigated the effect of GFJ on the expression and activity of Pgp in the human immortalized tubular cell line HK-2. Two flavonoid compounds related to GFJ, kaempferol and naringenin, were also tested for their effects on HK-2 Pgp. HK-2 cells cultured for 4 days in the presence of GFJ, showed a dose-dependent decrease in Pgp immunoblottable amount as well as a decrease in MDR-1 mRNA level, as shown by western blot analysis and RT-PCR, respectively. Both kaempferol and naringenin were also able to significantly decrease Pgp immunoblottable amount. To test whether the downregulation of HK-2 Pgp due to GFJ exposition could influence the cell sensitivity to drugs that are transported by Pgp itself, HK-2 cells precultured with GFJ were exposed to scalar concentrations of Cyclosporin A or Vinblastine and cell viability examined 36 hours later. The cytotoxicity of both drugs was increased. The calcein-AM test in untreated cells showed that GFJ, kaempferol or naringenin inhibited Pgp activity. Downregulation of Pgp as well inhibition of its function by GFJ or its related components in tubular cells could have a role in changing disposition kinetics of some important therapeutic agents