Abstract
β-Estradiol 17-(β-d-glucuronide) (E217G) is a well known cholestatic agent and substrate of multidrug resistance-associated protein 2 (Mrp2), whereas β-estradiol 3-(β-d-glucuronide) (E23G) is a noncholestatic regioisomer of E217G with unknown transport properties. The purpose of this study was to compare and contrast the Mrp2-mediated transport of E217G and E23G. The full coding region of rat Mrp2 was cloned into the baculovirus genome, the recombinant baculovirus used to infect Sf9 cells, and ATP-dependent transport of 3H-E23G and 3H-E217G in Sf9 cell membranes was characterized. Mrp2 transported E23G into an osmotically sensitive space, requiring ATP, with S50 = 55.7 μM, Vmax = 326 pmol·mg-1·min-1, and a Hill coefficient of 0.88. ATP-dependent Mrp2-mediated E217G transport was markedly stimulated at high E217G concentrations, consistent with positive cooperativity (Hill coefficient 1.5). E217G (5-125 μM) increased S50 but not Vmax for E23G transport, consistent with competitive inhibition. E23G (0.4-400 μM) completely, potently (IC50 = 14.2 μM), and competitively inhibited E217G transport, but E217G (0.01-250 μM) inhibited only 53% of E23G transport (IC50 = 33.4 μM). Estriol 16α-(β-d-glucuronide) potently and completely inhibited transport of E23G (IC50 = 2.23 μM), as did β-estradiol 3-sulfate 17-(β-d-glucuronide) (5-50 μM). In summary, E217G binds not only to an Mrp2 transport site, but also to an allosteric site that activates Mrp2 with positive cooperativity, thus activating its own transport and potentially that of other Mrp2 substrates, such as E23G. The noncholestatic E23G is an Mrp2 substrate and competes with E217G for transport, but does not activate the allosteric site.
Footnotes
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We gratefully acknowledge U.S. Public Health Service Grant GM55343 for supporting this research and the Reproductive Sciences Training Program (National Institutes of Health Grant T32 HD07436) for supporting P.M.G. This work was previously presented in part at the Experimental Biology Meeting in San Diego, CA, April 11-15, 2003.
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doi:10.1124/dmd.104.000372.
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ABBREVIATIONS: E217G, β-estradiol 17-(β-d-glucuronide); ABC, ATP-binding cassette; E23G, β-estradiol 3-(β-d-glucuronide); E23SO417G, β-estradiol 3-sulfate 17-(β-d-glucuronide); E316G, estriol 16α-(β-d-glucuronide); EV, empty virus (i.e., lacking the Mrp2 gene); MDR, multidrug resistance transporter (P-glycoprotein); Mrp, multidrug resistance-associated protein; CI, confidence interval; Bsep, bile salt export pump.
- Received April 21, 2004.
- Accepted July 15, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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