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

Abstract

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 (E-G), estradiol-3- and 17-glucuronides (E-3G and E-17G), as well as estriol-3- and 16-glucuronides (E-3G and E-16G) are found in human plasma and urine. Unlike E-17G, the efflux transport of other estrogen glucuronides by human transporters has not yet been investigated comprehensively. We have studied the transport of E-G, E-3G, E-3G, E-16G and estrone-3-sulfate (E-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 E-S was a good and specific substrate for BCRP, the less transporter-specific conjugates, E-G and E-3G, were still transported by BCRP at 10-fold higher rates than E-S. BCRP also transported E-16G at higher rates than the studied MRPs, while it transported E-3G at lower rates than MRP3. MRP2 exhibited lower or equal transport rates of E-G, E-3G, E-3G and E-16G in comparison to MRP3 and BCRP in the screening assays, mainly due to its high K values, between 180 and 790 μM. MRP3 transported all the tested glucuronides at rather similar rates, at K values below 20 μM, but lower V values than other transporters. In the case of E-3G, MRP3 was the most active transporter in the screening assay. MRP4 transported only E-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.

The Relative Importance of the Small Intestine and the Liver in Phase II Metabolic Transformations and Elimination of p-Nitrophenol Administered in Different Doses in the Rat

“…Besides glucuronide conjugates, BCRP can also transport sulfate conjugates. The kinetics of the metabolic enzymes and the transport of most of the substrates follows the Michaelis-Menten kinetics [54]. Accordingly, the observed differences could be the result-at least partially-of the different local concentrations of the substrate (PNP) in the enterocytes and the hepatocytes.…”

Human efflux transport of testosterone, epitestosterone and other androgen glucuronides

“…BCRP and MDR1 vesicles were loaded with cholesterol to enhance the transport activity as reported and described previously [30,31,33]. We have previously reported the proper functionality of the aforementioned human transporters in our Sf9 vesiclebased assay [24,31,32].…”

“…ATP-binding cassette (ABC) and solute carrier transporters are involved in disposition of steroid conjugates in human tissues [9,[22][23][24]. Multidrug resistance-associated proteins 2, 3 and 4 (MRP2-4, gene ABCC2-4), multidrug resistance protein 1 (MDR1, P-gp, gene ABCB1) and breast cancer resistance protein (BCRP, gene ABCG2) are the major transporters that mediate efflux of endogenous and exogenous compounds,…”

“…including their glucuronide and sulfate conjugates, in the liver, intestine and kidney [9,[25][26][27]. The role of MDR1 in disposition of glucuronide and sulfate conjugates is limited, while MRP2, MRP3 and BCRP have significant contributions in vivo [9,24,27].…”

“…1). We have previously studied the transport of estrogen glucuronides by the aforementioned transporters and found high affinity transport by MRP3 and high active transport by BCRP [24]. However, due to the differences in their structures, deducing transport activity for androgen glucuronides based on the knowledge of the transport of estrogen glucuronides is not feasible.…”