title: Thyroid hormone export from cells: contribution of P-glycoprotein.
abstract: Verapamil inhibits tri-iodothyronine (T3) efflux from several cell types, suggesting the involvement of multidrug resistance-associated (MDR) proteins in T3 transport. The direct involvement of P-glycoprotein (P-gp) has not, however, been investigated. We compared the transport of 125I-T3 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 125I-T3 efflux. We confirmed by Western blotting the enhanced expression of P-gp in MDCKII-MDR cells. The calculated rate of 125I-T3 efflux from MDCKII-MDR cells (around 0.30/min) was increased twofold compared with MDCKII cells (around 0.15/min). Overall, cellular accumulation of 125I-T3 was reduced by 26% in MDCKII-MDR cells compared with MDCKII cells, probably reflecting enhanced export of T3 from MDCKII-MDR cells rather than reduced cellular uptake, as P-gp typically exports substances from cells. Verapamil lowered the rate of 125I-T3 efflux from both MDCKII and MDCKII-MDR cells by 42% and 66% respectively, while nitrendipine reduced 125I-T3 efflux rate by 36% and 48% respectively, suggesting that both substances inhibited other cellular T3 transporters in addition to P-gp. The specific MDR inhibitors VX 853 and VX 710 had no effect of 125I-T3 efflux rate from wild-type MDCKII cells but reduced 125I-T3 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.
authors.affiliation: Conjoint Endocrine Laboratory, Royal Brisbane and Women's Hospital Research Foundation, Bancroft Centre, Brisbane, Queensland 4029, Australia.
authors.forename: A M
authors.initials: AM
authors.lastname: Mitchell
authors.forename: M
authors.initials: M
authors.lastname: Tom
authors.forename: R H
authors.initials: RH
authors.lastname: Mortimer
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D015153:Blotting, Western / Q000379:methods; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D004285:Dogs; D018432:Drug Resistance, Multiple / Q000187:drug effects; D007457:Iodine Radioisotopes; D007668:Kidney / Q000378:metabolism*; D009568:Nitrendipine / Q000494:pharmacology; D010880:Piperidines / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D014162:Transfection; D014284:Triiodothyronine / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D007457:Iodine Radioisotopes; D010880:Piperidines; D011725:Pyridines; D014284:Triiodothyronine; C106077:biricodar; D009568:Nitrendipine; D014700:Verapamil; C494139:timcodar