Breast Cancer Resistance Protein (ABCG2) Determines Distribution of Genistein Phase II Metabolites: Reevaluation of the Roles of ABCG2 in the Disposition of Genistein

Abstract

ABSTRACT:It was recently proposed that the improved oral bioavailability of genistein aglycone and conjugates in Bcrp1(؊/؊) mice is mainly due to increased intestinal absorption of aglycone and subsequent elevated exposure to conjugation enzymes. Here we tested this proposed mechanism and found that intestinal absorption of genistein aglycone did not increase in Bcrp1(؊/؊) mice compared with wild-type mice using an in situ mouse intestinal perfusion model and that inhibition of breast cancer resistance protein (BCRP) in Caco-2 cells also did not significantly increase permeability or intracellular concentration of aglycone. Separately, we showed that 5-to 10-fold increases in exposures of conjugates and somewhat lower fold increases (<2-fold) in exposures of aglycone were apparent after both oral and intraperitoneal administration in Bcrp1(؊/؊) mice. In contrast, the intestinal and biliary excretion of genistein conjugates significantly decreased in Bcrp1(؊/؊) mice without corresponding changes in aglycone excretion. Likewise, inhibition of BCRP functions in Caco-2 cells altered polarized excretion of genistein conjugates by increasing their basolateral excretion. We further found that genistein glucuronides could be hydrolyzed back to genistein, whereas sulfates were stable in blood. Because genistein glucuronidation rates were 110% (liver) and 50% (colon) higher and genistein sulfation rates were 40% (liver) and 42% (colon) lower in Bcrp1(؊/؊) mice, the changes in genistein exposures are not mainly due to changes in enzyme activities. In conclusion, improved bioavailability of genistein and increased plasma area under the curve of its conjugates in Bcrp1(؊/؊) mice is due to altered distribution of genistein conjugates to the systemic circulation.

Response to Letter to the Editor on “Breast Cancer Resistance Protein (ABCG2) Determines Distribution of Genistein Phase II Metabolites: Reevaluation of the Roles of ABCG2 in the Disposition of Genistein”

“…In agreement with Yang et al (2012), our article (Á lvarez et al, 2011) also included references to a possible direct effect of Bcrp1 on the increased levels of the conjugates in Bcrp1(Ϫ/Ϫ) mice, since we stated in the discussion that "our results indicated a direct and conclusive Bcrp1 efflux action on phase II metabolites of these isoflavones in vivo, and suggested a possible novel concept for ABCG2/BCRP as part of a metabolism-driven efflux transport of these conjugates." Thus, the title of the article by Yang et al (2012), which involves a clear allusion to our study, seems to be excessive. In brief, it is erroneous to read the final conclusion in our article as being that enhanced exposure to systemic metabolites in Bcrp1(Ϫ/Ϫ) can be attributed to increased intestinal absorption of genistein.…”

“…The purpose of this letter is to respond to a comment made by Dr. Alvarez to our recently published article (Yang et al, 2012). We thank Dr. Alvarez for her interest and comments.…”

“…The purpose of this letter is to clarify some assessments in the article by Yang et al (2012), in which they questioned the credibility of a conclusion in our recently published article (Á lvarez et al, 2011). Yang et al (2012) did provide a new insight, according to which increased exposure of aglycone in Bcrp1(Ϫ/Ϫ) might arise from the hydrolysis of glucuronide conjugates.…”

“…Yang et al (2012) did provide a new insight, according to which increased exposure of aglycone in Bcrp1(Ϫ/Ϫ) might arise from the hydrolysis of glucuronide conjugates. However, they additionally claimed that "this is very different from the current understanding that an increased level of aglycone comes directly from impaired aglycone efflux in Bcrp1(Ϫ/Ϫ) mice (Á lvarez et al, 2011)."…”

“…We would like to mention that the role of Bcrp1 in genistein absorption was not known when our manuscript was written, and our assessment should thus be considered as a possibility that was open at that moment. Unfortunately, Yang et al (2012) seem to have overstated the case. Their article is the first demonstration of the absence of the involvement of Bcrp1 in the intestinal absorption of genistein.…”