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ATP-dependent 17-beta-estradiol 17- transport by multidrug resistance protein Inhibition by cholestatic steroids

, : ATP-dependent 17-beta-estradiol 17- transport by multidrug resistance protein Inhibition by cholestatic steroids. Journal of Biological Chemistry 271(16): 9683-9689

In addition to its ability to confer resistance to a range of natural product type chemotherapeutic agents, multidrug resistance protein (MRP) has been shown to transport the cysteinyl leukotriene, LTC-4, and several other glutathione (GSH) S-conjugates. We now demonstrate that its range of potential physiological substrates also includes cholestatic glucuronidated steroids. ATP dependent, osmotically sensitive transport of the naturally occurring conjugated estrogen, 17-beta-estradiol 17-(beta-D-glucuronide) (E-217-beta-G), was readily demonstrable in plasma membrane vesicles from populations of MRP-transfected HeLa cells (V-max 1.4 nmol mg-1 min-1, K-m 2.5 mu-M). The involvement of MRP was confirmed by demonstrating that transport was completely inhibited by a monoclonal antibody specific for an intracellular conformational epitope of the protein. MRP-mediated transport of LTC-4 was competitively inhibited by E-217-beta-G (K-i(app) 22 mu-M), despite the lack of structural similarity between these two substrates. Competitive inhibition of (3H)E-217-beta-G transport was also observed with a number of other cholestatic conjugated steroids. All of these compounds prevented photolabeling of MRP with (3H)LTC-4, demonstrating that the cholestatic steroid and leukotriene conjugates compete either for the same or possibly overlapping sites on the protein. Consistent with the presence of overlapping but non-identical sites, studies using chemotherapeutic drugs to inhibit MRP-mediated E-217-beta-G transport indicated that daunorubicin had the highest relative potency of the drugs tested, whereas it was the least potent inhibitor of LTC-4 transport. Non-cholestatic steroids glucuronidated at the 3 position of the steroid nucleus, such as 17-beta-estradiol 3-(beta-D-glucuronide), did not compete for transport of E-217-beta-G by MRP, nor did they inhibit photolabeling of the protein with (3H)LTC-4. These data identify MRP as a potential transporter of cholestatic conjugated estrogens and demonstrate site-specific requirements for glucuronidation of the steroid nucleus.


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