Mitchell A M, Tom M, Mortimer R H
Conjoint Endocrine Laboratory, Royal Brisbane and Women's Hospital Research Foundation, Bancroft Centre, Brisbane, Queensland 4029, Australia.
J Endocrinol. 2005 Apr;185(1):93-8. doi: 10.1677/joe.1.06096.
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.
维拉帕米可抑制多种细胞类型中三碘甲状腺原氨酸（T3）的外流，这表明多药耐药相关（MDR）蛋白参与了T3的转运。然而，P-糖蛋白（P-gp）是否直接参与其中尚未得到研究。我们比较了转染了mdr1 cDNA的MDCKII细胞（MDCKII-MDR）与野生型MDCKII细胞（MDCKII）中125I-T3的转运情况，并研究了传统（维拉帕米和尼群地平）和特异性MDR抑制剂（VX 853和VX 710）对125I-T3外流的影响。我们通过蛋白质免疫印迹法证实了MDCKII-MDR细胞中P-gp表达增强。计算得出，MDCKII-MDR细胞中125I-T3的外流速率（约0.30/分钟）比MDCKII细胞（约0.15/分钟）增加了两倍。总体而言，与MDCKII细胞相比，MDCKII-MDR细胞中125I-T3的细胞内蓄积减少了26%，这可能反映出MDCKII-MDR细胞中T3的输出增加，而非细胞摄取减少，因为P-gp通常将物质从细胞中输出。维拉帕米分别使MDCKII和MDCKII-MDR细胞中125I-T3的外流速率降低了42%和66%，而尼群地平分别使125I-T3外流速率降低了36%和48%，这表明这两种物质除了抑制P-gp外，还抑制了其他细胞T3转运体。特异性MDR抑制剂VX 853和VX 710对野生型MDCKII细胞中125I-T3的外流速率没有影响，但分别使MDCKII-MDR细胞中125I-T3的输出减少了50%和53%。这些结果首次直接证明了P-gp可将甲状腺激素从细胞中输出。
