Efflux transport of estrogen glucuronides by human MRP2, MRP3, MRP4 and BCRP

Estrone, estradiol and estriol are endogenous human estrogens that are rapidly conjugated with glucuronic acid in both intestinal and hepatic epithelial cells. The resulting glucuronides, estrone-3-glucuronide (E₁-G), estradiol-3- and 17-glucuronides (E₂-3G and E₂-17G), as well as estriol-3- and 16-glucuronides (E₃-3G and E₃-16G) are found in human plasma and urine. Unlike E₂-17G, the efflux transport of other estrogen glucuronides by human transporters has not yet been investigated comprehensively. We have studied the transport of E₁-G, E₂-3G, E₃-3G, E₃-16G and estrone-3-sulfate (E₁-S), another important estrogen conjugate, using the vesicular transport assay with recombinant human MRP2, MRP3, MRP4, MDR1 and BCRP that were expressed in insect cells. The transport screening assays revealed that whereas E₁-S was a good and specific substrate for BCRP, the less transporter-specific conjugates, E₁-G and E₂-3G, were still transported by BCRP at 10-fold higher rates than E₁-S. BCRP also transported E₃-16G at higher rates than the studied MRPs, while it transported E₃-3G at lower rates than MRP3. MRP2 exhibited lower or equal transport rates of E₁-G, E₂-3G, E₃-3G and E₃-16G in comparison to MRP3 and BCRP in the screening assays, mainly due to its high K_m values, between 180 and 790 μM. MRP3 transported all the tested glucuronides at rather similar rates, at K_m values below 20 μM, but lower V_max values than other transporters. In the case of E₃-3G, MRP3 was the most active transporter in the screening assay. MRP4 transported only E₃-16G at considerable rates, while none of the tested estrogen conjugates was transported by MDR1 at higher rates than control vesicles. These new results, in combination with previously reported in vivo human data, stimulate our understanding on the substrate specificity and role of efflux transporters in disposition of estrogen glucuronides in humans.