MODULATION OF LIVER CANALICULAR TRANSPORT PROCESSES BY THE TYROSINE-KINASE INHIBITOR GENISTEIN - IMPLICATIONS OF GENISTEIN METABOLISM IN THE RAT

Rat liver cells express the multispecific organic anion transporter (c moat, cmrp, mrp2) and P-glycoprotein (Pgp) in their canalicular membra nes, proteins that are homologous to the multidrug-resistance related protein (MRP) and multidrug resistance (MDR) gene products in multidru g resistant tumor cells. We tested whether genistein, a modulator of d rug resistance in tumor cells, affects biliary secretion of substrates of canalicular multispecific organic anion transporter (cmoat) (glucu ronides of bilirubin and rhodamine, glutathione conjugate of bromsulph thalein) and of P-glycoprotein (Pgp) (rhodamine), respectively. Using the isolated perfused rat liver of control Wistar rats (TR+) and of a mutant strain (TR-) that expresses Pgp but not cmoat, we show that gen istein effectively inhibits the secretion of anionic substrates of cmo at in Wistar rats but stimulates secretion of cationic rhodamine in TR - rats, Genistein is subject to glucuronidation and sulfatation and se cretion of genistein and its metabolites stimulates bile flow in Wista r rats, but secretion is nearly absent in TR- rats, Because genistein and its metabolites are substrates for cmoat, inhibition of anion secr etion by genistein is partially explained by competition for this tran sporter. Genistein is also a substrate of uridindiphosphate (UDP)-gluc uronyltransferase isoenzyme(s). Inhibition of glucuronidation reduces the availability of bilirubin and rhodamine glucuronates for transport via cmoat, but unconjugated cationic rhodamine becomes available for transport via Pgp at an increased cellular concentration. Daidzein, an genistein analogue with no effect on protein tyrosine kinase (PTK) sh ows similar effects on secretion of organic anions and cations support ing the conclusion that genistein affects transport in liver mainly th rough competition with other substrates at the sites of glucuronidatio n and transport via cmoat.