Identification of an organic onion transport system in the human colon carcinoma cell line HT29 clone 19A

The human colon carcinoma cell line HT29 cl, 19A was studied for organic an ion transporter activity by determining intracellular fluo-3 and fura-red a ccumulation and by measuring fluo-3 efflux. Modulators of organic anion tra nsport systems were used to identify the transporters that are involved in dye extrusion. Addition of probenecid to the dye-loading medium, containing 10 muM fluo-3/AM and fura-red/AM, resulted in a dose-dependent increase in fluo-3 and fura-red accumulation in the cells. The increase in fluo-3 accu mulation in the cells in the presence of probenecid was explained by the in hibitory effect of this compound on fluo-3 efflux. Fluo-3 efflux from the c ells was also inhibited by sulfinpyra-zone, another inhibitor of organic an ion transport. Substrates of renal probenecid-sensitive organic anion excha nge mechanisms as well as modulators of multidrug resistance associated pro tein (MRP) activity did not influence fluo-3 extrusion rates. However, redu cing intracellular ATP contents completely blocked fluo-3 extrusion. Moreov er, MK571, an inhibitor of MRP, significantly stimulated dye accumulation, whereas inhibitors of the multidrug resistance gene (MDR1) product P-glycop rotein, cyclosporin A and verapamil, did not, As probenecid inhibits fluo-3 efflux across the apical membrane of cells grown on permeable supports, we conclude that a probenecid-sensitive organic anion transporter is present in the apical membrane of HT29 cl.19A cells. This organic anion transport s ystem differs from MDR1 and MRP2.