Thyroid hormone export from cells: contribution of P-glycoprotein

Verapamil inhibits tri-iodothyronine (T-3) efflux from several cell types, suggesting the involvement of multidrug resistance-associated (MDR) proteins in T-3 transport. The direct involvement of P-glycoprotein (P-gp) has not, however, been investigated. We compared the transport of I-125-T-3 in MDCKII cells that had been transfected with mdr1 cDNA (MDCKII-MDR) versus wild-type MDCKII cells (MDCKII), and examined the effect of conventional (verapamil and nitrendipine) and specific MDR inhibitors (VX 853 and VX 710) on I-125-T-3 efflux. We confirmed by Western blotting the enhanced expression of P-gp in MDCKII-MDR cells. The calculated rate of I-125-T-3 efflux from MDCKII-MDR cells (around 0.30/min) was increased twofold compared with MDCKII cells (around 0.15/min). Overall, cellular accumulation of I-125-T-3 was reduced by 26% in MDCKII-MDR cells compared with MDCKII cells, probably reflecting enhanced export of T-3 from MDCKII-MDR cells rather than reduced cellular uptake, as P-gp typically exports substances from cells. Verapamil lowered the rate of I-125-T-3 efflux from both MDCKII and MDCKII-MDR cells by 42% and 66% respectively, while nitrendipine reduced I-125-T-3 efflux rate by 36% and 48% respectively, suggesting that both substances inhibited other cellular T-3 transporters in addition to P-gp. The specific MDR inhibitors VX 853 and VX 710 had no effect of I-125-T-3 efflux rate from wild-type MDCKII cells but reduced I-125-T-3 export in MDCKII-MDR cells by 50% and 53% respectively. These results have provided the first direct evidence that P-gp exports thyroid hormone from cells.