The Important Role of Bcrp (Abcg2) in the Biliary Excretion of Sulfate and Glucuronide Metabolites of Acetaminophen, 4-Methylumbelliferone, and Harmol in Mice

Abstract

The role of Mrp2, Bcrp, and P-glycoprotein in the biliary excretion of acetaminophen sulfate (AS) and glucuronide (AG), 4-methylumbelliferyl sulfate (4MUS) and glucuronide (4MUG), and harmol sulfate (HS) and glucuronide (HG) was studied in Abcc2(Ϫ/Ϫ), Abcg2(Ϫ/Ϫ), and Abcb1a(Ϫ/Ϫ)/Abcb1b(Ϫ/Ϫ) mouse livers perfused with the respective parent compounds using a cassette dosing approach. Biliary clearance of the sulfate conjugates was significantly decreased in Bcrp-deficient mouse livers, resulting in negligible biliary excretion of AS, 4MUS, and HS. It is noteworthy that the most profound decrease in the biliary clearance of the glucuronide conjugates was observed in Bcrp-deficient mouse livers, although the biliary clearance of 4MUG was also ϳ35% lower in Mrp2-deficient mouse livers. As expected, biliary excretion of conjugates was not impaired in P-glycoprotein-deficient livers. An appreciable increase in perfusate recovery due to a shift in the directionality of metabolite excretion, from bile to perfusate, was noted in knockout mice only for conjugates whose biliary clearance constituted an appreciable (Ն37%) fraction of total hepatic excretory clearance (i.e., 4MUS, HG, and HS). Biliary clearance of AG, AS, and 4MUG constituted a small fraction of total hepatic excretory clearance, so an appreciable increase in perfusate recovery of these metabolites was not observed in knockout mice despite markedly decreased biliary excretion. Unlike in rats, where sulfate and glucuronide conjugates were excreted into bile predominantly by Mrp2, mouse Bcrp mediated the biliary excretion of sulfate metabolites and also played a major role in the biliary excretion of the glucuronide metabolites, with some minor contribution from mouse Mrp2.Phase II metabolism, including sulfation and glucuronidation, occurs primarily in the liver. Conjugation of a substrate with a sulfate or glucuronide moiety increases its hydrophilicity to promote excretion from the body. These conjugates are typically too polar to undergo passive diffusion from hepatocytes after their intracellular formation and therefore require carrier-mediated transport for excretion across the hepatic canalicular (apical) membrane into bile and across the basolateral membrane into sinusoidal blood. Most sulfate and glucuronide metabolites are inactive; notable exceptions include morphine-6-glucuronide, minoxidil sulfate, SN-38-glucuronide, and troglitazone sulfate (Zamek-Gliszczynski et al., 2006b). Altered hepatic export of pharmacologically and toxicologically active sulfate and glucuronide metabolites formed in the liver can have profound pharmacodynamic and toxic implications, underscoring the importance of understanding the mechanisms of metabolite excretion (Meisheri et al., 1993;Funk et al., 2001;Horikawa et al., 2002;. Despite the importance of active transport in hepatic excretion of phase II metabolites, mechanisms responsible for excretion of sulfate and glucuronide conjugates have not been elucidated fully.

Relationship between Drug/Metabolite Exposure and Impairment of Excretory Transport Function

“…Because Bcrp is directly involved in transporting 4MUG into bile, using eq. 4.1 and the excretory clearance values (Cl Bile WT ϭ 88 l/min/g liver and Cl Bile KO ϭ 24 l/min/g liver) reported by Zamek-Gliszczynski et al (2006c), f e, Bile of Bcrp is calculated to be 0.73. Equation 3 predicts the decrease in biliary recovery of 4MUG to be 3.7-fold; experimentally a 2.1-fold decrease was observed.…”

Regional Expression and Activity of Breast Cancer Resistance Protein (Bcrp/Abcg2) in Mouse Intestine: Overlapping Distribution with Sulfotransferases

“…Furthermore, BCRP accepts a variety of sulfate conjugates of xenobiotics as substrates, leading to the hypothesis that Bcrp is involved in the efflux of sulfate conjugates formed intracellularly by SULT. Indeed, it has been demonstrated that Bcrp is involved in the efflux of sulfate conjugates in the kidney (Mizuno et al, 2004), liver (Zamek-Gliszczynski et al, 2006;Enokizono et al, 2007), and jejunum (Adachi et al, 2005).…”

Gender Differences in mRNA Expression of ATP-Binding Cassette Efflux and Bile Acid Transporters in Kidney, Liver, and Intestine of 5/6 Nephrectomized Rats

“…ABCG2 also transports glucuronide and sulfate conjugates (Mizuno et al, 2004;Zamek-Gliszczynski et al, 2006). ABCG2/BCRP expression is high in rat kidney and intestine .…”