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. 2019 Jul:191:105350.
doi: 10.1016/j.jsbmb.2019.03.027. Epub 2019 Apr 5.

Major glucuronide metabolites of testosterone are primarily transported by MRP2 and MRP3 in human liver, intestine and kidney

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Major glucuronide metabolites of testosterone are primarily transported by MRP2 and MRP3 in human liver, intestine and kidney

Cindy Yanfei Li et al. J Steroid Biochem Mol Biol. 2019 Jul.

Abstract

Testosterone glucuronide (TG), androsterone glucuronide (AG), etiocholanolone glucuronide (EtioG) and dihydrotestosterone glucuronide (DHTG) are the major metabolites of testosterone (T), which are excreted in urine and bile. Glucuronides can be deconjugated to active androgen in gut lumen after biliary excretion, which in turn can affect physiological levels of androgens. The goal of this study was to quantitatively characterize the mechanisms by which TG, AG, EtioG and DHTG are eliminated from liver, intestine, and kidney utilizing relative expression factor (REF) approach. Using vesicular transport assay with recombinant human MRP2, MRP3, MRP4, MDR1 and BCRP, we first identified that TG, AG, EtioG, and DHTG were primarily substrates of MRP2 and MRP3, although lower levels of transport were also observed with MDR1 and BCRP vesicles. The transport kinetic analyses revealed higher intrinsic clearances of TG by MRP2 and MRP3 as compared to that of DHTG, AG, and EtioG. MRP3 exhibited higher affinity for the transport of the studied glucuronides than MRP2. We next quantified the protein abundances of these efflux transporters in vesicles and compared the same with pooled total membrane fractions isolated from human tissues by quantitative LC-MS/MS proteomics. The fractional contribution of individual transporters (ft) was estimated by proteomics-based physiological scaling factors, i.e., transporter abundance in whole tissue versus vesicles, and corrected for inside-out vesicles (determined by 5'-nucleotidase assay). The glucuronides of inactive androgens, AG and EtioG were preferentially transported by MRP3, whereas the glucuronides of active androgens, TG and DHTG were mainly transported by MRP2 in liver. Efflux by bile canalicular transport may indicate the potential role of enterohepatic recirculation in regulating the circulating active androgens after deconjugation in the gut. In intestine, MRP3 possibly contributes most to the efflux of these glucuronides. In kidney, all studied glucuronides seemed to be preferentially effluxed by MRP2 and MDR1 (for EtioG). These REF based analysis need to be confirmed with in vivo findings. Overall, characterization of the efflux mechanisms of T glucuronide metabolites is important for predicting the androgen disposition and interindividual variability, including drug-androgen interaction in humans. The mechanistic data can be extrapolated to other androgen relevant organs (e.g. prostate, testis and placenta) by integrating these data with quantitative tissue proteomics data.

Keywords: Efflux transporters; Glucuronides; Quantitative proteomics; Testosterone; Vesicular transport.

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Figures

Figure 1.
Figure 1.
Glucuronidation pathways of testosterone (T) metabolism. A. T is glucuronidated by UGTs to form its primary (TG; solid arrow) and secondary (AG, EtioG, and DHTG; dotted arrows) metabolites. B. Fold-difference in the physiological serum concentration of respective metabolites as compared to T [12]. BLOQ, below limit of quantification. TG, testosterone glucuronide; AG, androsterone glucuronide; EtioG, etiocholanolone glucuronide; DHTG, dihydrotestosterone glucuronide.
Figure 2.
Figure 2.
Schematic overview of experimental design. The transport of glucuronides into the membrane vesicles was performed by adding ATP, whereas AMP served as a negative control. The vesicles were then isolated on filter membranes and washed. The glucuronide substrates contained in the vesicles were quantified by a validated LC-MS/MS method [12]. The percentage of inside-out vesicles was characterized using 5’-nucleotidase assay. The protein abundances of efflux transporters in plasma membrane vesicles, human liver, intestine, and kidney were measured by quantitative LC-MS/MS proteomics. The relative abundance of efflux transporters in human tissue versus vesicles were used as scaling factors for in-vitro to in-vivo extrapolation (IVIVE) to characterize the fractional contribution of individual efflux transporters (ft) in efflux of TG, AG, EtioG, and DHTG from human liver, intestine and kidney.
Figure 3.
Figure 3.
ATP-dependent transport rate of TG (A), AG (B), EtioG (C), and DHTG (D) in membrane vesicles overexpressing efflux transporters at 10 μM substrate concentration. The transport of substrates (TG, AG, EtioG, and DHTG) by BCRP, MDR1, MRP2, MRP3, and MRP4 was studied using 25 μg of total vesicle protein and 1 min incubation. Control vesicles (Ctrl) prepared using HEK293 cells expressing empty vector (mock) were used in all assays. Difference between the ATP and AMP groups (i.e., net ATP-dependent transport rate) was calculated and presented as means ± SD of triplicate samples. Asterisk (*) indicates statistically significant differences in net ATP-dependent transport rate between transporter-overexpressing and Ctrl vesicles by Student’s t-test (p < 0.05).
Figure 4.
Figure 4.
ATP-dependent transport kinetics of glucuronide metabolites of T by efflux transporters. MRP2 (A) and MRP3 (B) mediated transport kinetics of TG (blue), DHTG (green), AG (red), and EtioG (black). MDR1 (C) mediated transport kinetics of EtioG and BCRP (D) mediated transport kinetics of DHTG. The kinetic experiments were conducted at various concentrations with 25 μg vesicle proteins for 15 seconds. Differences between the ATP and AMP groups (net ATP-dependent transport rates) were calculated and Michaelis-Menten equation (ν = Vmax*[S]/([S]+Km)) is fitted to the data. The kinetic constants (Vmax and Km with the 95% CI) for the studied glucuronide metabolites and transporters are presented in Table 1.
Figure 5.
Figure 5.
A. Observed and extrapolated fractional contribution (ft) of individual efflux transporters to the clearance of TG, AG, EtioG, and DHTG in membrane vesicles, and in human tissues after integrating the scaling factor. The individual scaling factors were derived from the relative abundance of efflux transporters in human liver, intestine and kidney versus in vesicles as shown in Supplemental Table 5. B. A proposed disposition model of glucuronide metabolites of testosterone (T) in liver, intestine and kidney. MRP3 is expressed in basolateral or sinusoidal side, whereas MRP2, MDR1 and BCRP are expressed in apical or canalicular side. MRP3 and MRP2 are more important in the transport of testosterone glucuronides in human liver, intestine and kidney.

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