We investigated the effect of Shiga-like toxin II (SLT-II), derived from Escherichia coli O157:H7, on the hepatobiliary excretion of doxorubicin, a substrate for P glycoprotein and the multidrug resistance-associated protein Mrp2, and on the expression of P glycoprotein and Mrp2 in rats. Histopathological examination did not show any liver injury in SLT-II-treated rats. A significant delay in the disappearance of doxorubicin from plasma after its intravenous administration (5 mg/kg of body weight) was observed in rats treated 24 h earlier with SLT-II (2 g/animal). When rats received an infusion of doxorubicin (2.6 g/min) 24 h after intravenous injection of SLT-II, the steady-state concentration of doxorubicin in plasma increased and the bile flow decreased, whereas the concentration in liver did not alter. SLT-II significantly increased the unbound fraction of doxorubicin in plasma but did not alter the concentration in liver tissue. SLT-II significantly decreased the biliary excretion rate and biliary clearance of doxorubicin based on the total concentration and concentration of the unbound fraction in plasma and liver. Western blot analysis revealed that SLT-II down-regulated P glycoprotein and Mrp2 in the liver, which could explain the observed decrease in the biliary excretion of doxorubicin by SLT-II. A tumor necrosis factor alpha (TNF-␣) production inhibitor, pentoxifylline, could not protect SLT-II-induced decreases in the biliary clearance of doxorubicin and down-regulation of both transporters. It is unlikely that TNF-␣ plays a major role in the SLT-II-induced decrease in the hepatobiliary transport of doxorubicin and the down-regulation of both transporters.Shiga-like toxin (SLT)-producing Escherichia coli O157:H7 causes damage to capillary blood vessels of the small intestine, kidney, and central nervous system in humans, resulting in colonic ulceration, bloody diarrhea, kidney dysfunction, seizures, and death (24,35,36,44). SLTs are divided into two major subtypes, SLT-I and SLT-II; SLT-II is known to possess potent virulence (19,27). SLT-II is well known to induce nonspecific diarrhea, hemorrhagic colitis, and severe hemolyticuremic syndrome (HUS), which is the most common complication of E. coli O157:H7 infection and contributes to renal dysfunction and mortality. It has been reported that SLT-II can cause damage to renal tubular cells and renal function in rats (42) and brain capillaries in mice (43). Furthermore, we recently found that SLT-II reduces hepatic cytochrome P450 content and induces down-regulation of CYP3A2 and CYP2C11 (unpublished data). However, no data regarding the effect of SLT-II on liver function are available.P glycoprotein and the multidrug resistance-associated protein Mrp2, members of the ATP-binding cassette transport proteins, are localized in the bile canalicular membrane of hepatocytes and act as efflux transporter proteins for endogenous and exogenous toxic substances (12,29,38). It is thought that liver dysfunction and altered functions of P glycoprotein and Mrp2 in s...