| Research Abstract |
P-glycoprotein acts as an energy-dependent efflux pump that exports anticancer agents out of the cell, lowering their intracellular concentration to sublethal levels, and is considered to be important in multidrug resistance (MDR) of human tumors. P-glycoprotein is expressed in several normal human tissues, but physiological substrates of P-glycoprotein have not been fully understood.
To analyze the mechanism of drug transport and physiological substrates, we established a transepithelial transport system. We expressed MDR1 cDNA isolated from human normal adrenal in LLC-PK1 cells, which are derived from porcine kidney proximal tubule. One transformant (LLC-GA5-COL300) showed 54-, 150-, and 167-fold resistance to colchicine, vinblastine, and adriamycin, respectively, compared to LLC-PK1 cells. LLC-PK_1 and the P-glycoprotein-expressing transfectant cells form a highly polarized epithelium, and electron microscopic immunocytochemistry using a monoclonal antibody MRK16 showed that human P-
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glycoprotein is specifically expressed on the apical surface of the transfectant cells.
By using this transepithelial transport system, we measured transport of ^3H-labeled aldosterone, corticosterone, cortisol, deoxycorticosterone, dexamethasone, estradiol, estriol, progesterone, testesterone. In the transfectant cells, basal to apical transport of aldosterone, cortisol, estriol and dexamethasone increased and their apical to basal transport decreased compared to those in the host cells. But transport of corticosterone, deoxycorticosterone, estradiol, progesterone, testesterone was indistinguishable in both cell lines. Transepithelial transport of aldosterone, cortisol, estriol and dexamethasone was inhibited by verapamil. These results indicate that human P-glycoprotein transports aldosterone, cortisol, estriol and dexamethasone.
Cortisol, a glucocorticoid, is the main steroid produced in the human adrenal. Aldosterone is the main mineralocorticoid produced in the human adrenal. Estriol is the main estrogen produced during pregnancy in the fetoplacental unit. P-glycoprotein is expressed in the human adrenal cortex and the pregnant uterus and placenta. These results suggest that human P-glycoprotein functions as a steroid transporter in the adrenal cortex and in the fetoplacental unit during pregnancy. Less
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