Rat liver cells express the multispecific organic anion trans-tition with other substrates at the sites of glucuronidation and transport via cmoat. (HEPATOLOGY 1997;26:1467-1476.) porter (cmoat, cmrp, mrp2) and P-glycoprotein (Pgp) in their canalicular membranes, proteins that are homologous to the multidrug-resistance related protein (MRP) and multidrug re-Specialized active membrane transporters serve for the celsistance (MDR) gene products in multidrug resistant tumor lular extrusion of organic anions and cations. These transcells. We tested whether genistein, a modulator of drug resisporters provide for the elimination of xenobiotics from the tance in tumor cells, affects biliary secretion of substrates of body in kidney, intestine, and liver, but particular interest canalicular multispecific organic anion transporter (cmoat)
arose from the fact that analogous transporters are expressed (glucuronides of bilirubin and rhodamine, glutathione conjuin tumor cells and convey drug resistance. Their activity in gate of bromsulphthalein) and of P-glycoprotein (Pgp) (rhodatumor cells results in a reduction of the cellular concentramine), respectively. Using the isolated perfused rat liver of tion and effectiveness of antitumor drugs. This could be modcontrol Wistar rats (TR / ) and of a mutant strain (TR 0 ) that ulated by signal transduction pathways that involve protein expresses Pgp but not cmoat, we show that genistein effecphosphorylation. In tumor cells two families of drug transtively inhibits the secretion of anionic substrates of cmoat in porters were identified that convey drug resistance and are Wistar rats but stimulates secretion of cationic rhodamine in involved in the extrusion of organic cations and anions, re-TR 0 rats. Genistein is subject to glucuronidation and sulfataspectively. These same transporters appear to be functionally tion and secretion of genistein and its metabolites stimulates active on the hepatocyte canalicular membrane as well: the bile flow in Wistar rats, but secretion is nearly absent in TR 0 multidrug resistance (MDR) transporters (P-glycoproteins, rats. Because genistein and its metabolites are substrates for Pgps), and the multidrug resistance related (MRP) transportcmoat, inhibition of anion secretion by genistein is partially ers. [1][2][3] In liver these transport systems provide for the biliary explained by competition for this transporter. Genistein is excretion of cholephilic xenobiotics. Canalicular membrane also a substrate of uridindiphosphate (UDP)-glucuronyltranstransporters of the MDR family were identified as transport ferase isoenzyme(s). Inhibition of glucuronidation reduces the adenosine triphosphate (ATP)-ases. While the mdr 1 gene availability of bilirubin and rhodamine glucuronates for transproduct Pgp serves for the biliary excretion of cationic comport via cmoat, but unconjugated cationic rhodamine becomes pounds, e.g., doxorubicine, vinblastine, and rhodamine, 4 the available for transport via Pgp at an increased cellular concenmdr 2 gene product acts as a membrane flippase for the tration. Daidzein, an genistein analogue with no effect on excretion of phospholipids into bile 5 and might be involved protein tyrosine kinase (PTK) shows similar effects on secrein the secretion of protoporphyrins. 6 In tumor cells MDR tion of organic anions and cations supporting the conclusion transporters are often overexpressed and the MDR 1 but that genistein affects transport in liver mainly through compenot the MDR 2 gene products (Pgps) confer resistance to amphiphilic antitumor drugs. 7 In tumor cells the MRP transport ATPases also confer mul-Abbreviations: MDR, multidrug resistance; Pgp, P-glycoprotein; MRP(human), tidrug resistance and are responsible for the outward transmrp(rat), multidrug-resistance related protein; PTK, protein tyrosine kinase; BSP, port of anionic conjugates of various compounds, e.g., leukobromsulphthalein; UDP, uridindiphosphate; KHB, Krebs-Henseleit buffer; TLC, thintriene C 4 8-11 and some lipophilic cytotoxic drugs. 12 layer chromatography; HPLC, high-performance liquid chromatography; 4-MUB, 4-
In the liver analogous export pumps for glutathione, methylumbelliferone; ATP, adenosine triphosphate.