The cellular and subcellular localization and mechanism of transport of the heteromeric organic solute transporter (OST) OSTα-OSTβ was examined in human and rodent epithelia. The two subunits of the transporter were expressed together in human small intestine, kidney, and liver, tissues that also express the apical sodium-dependent bile acid uptake transporter ASBT (SLC10A2). Indirect immunofluorescence microscopy localized OSTα and OSTβ to the basolateral membrane of mouse, rat, and human ileal enterocytes, renal proximal tubular cells, and cholangiocytes. Transport in OSTα-OSTβ–expressing Xenopus laevis oocytes was unaffected by depletion of intracellular adenosine triphosphate, or by changes in transmembrane Na⁺, K⁺, H⁺, or Cl⁻ concentration gradients. However, the oocytes demonstrated robust substrate efflux and trans-stimulation, indicating that transport occurs by facilitated diffusion. Madin Darby canine kidney cells coexpressing mouse Ostα and Ostβ exhibited enhanced apical to basolateral transport of the major glycine and taurine conjugated bile acid species. In conclusion, the selective localization of OSTα and OSTβ to the basolateral plasma membrane of epithelial cells responsible for bile acid and sterol reabsorption, the substrate selectivity of the transporter, and the facilitated diffusion transport mode collectively indicate that OSTα-OSTβ is a key basolateral transporter for the reabsorption of these important steroid-derived molecules.(HEPATOLOGY 2005;42:1270–1279.)

Our understanding of the interaction between the gut microbiota and host health has recently improved dramatically. However, the effects of toxic metal exposure on the gut microbiota remain poorly characterized. As this microbiota creates a critical interface between the external environment and the host’s cells, it may play an important role in host outcomes during exposure. We therefore used 16S ribosomal RNA (rRNA) gene sequencing to track changes in the gut microbiota composition of rats exposed to heavy metals. Rats were exposed daily for five days to arsenic, cadmium, cobalt, chromium, nickel, or a vehicle control. Significant changes to microbiota composition were observed in response to high doses of chromium and cobalt, and significant dose-dependent changes were observed in response to arsenic, cadmium and nickel. Many of these perturbations were not uniform across metals. However, bacteria with higher numbers of iron-importing gene orthologs were overly represented after exposure to arsenic and nickel, suggesting some possibility of a shared response. These findings support the utility of the microbiota as a pre-clinical tool for identifying exposures to specific heavy metals. It is also clear that characterizing changes to the functional capabilities of microbiota is critical to understanding responses to metal exposure.

Background: Many people, by means of consumption of seafood or other anthropogenic sources, are exposed to levels of methylmercury (MeHg) that are generally considered to be quite low, but that may nevertheless produce irreversible brain damage, particularly in unborn babies. The only way to prevent or ameliorate MeHg toxicity is to enhance its elimination from the body. Objectives: Using N-acetylcysteine (NAC), we aimed to devise a monitoring protocol for early detection of acute exposure or relatively low MeHg levels in a rodent model, and to test whether NAC reduces MeHg levels in the developing embryo. Results: NAC produced a transient, dose-dependent acceleration of urinary MeHg excretion in rats of both sexes. Approximately 5% of various MeHg doses was excreted in urine 2 hr after injection of 1 mmol/kg NAC. In pregnant rats, NAC markedly reduced the body burden of MeHg, particularly in target tissues such as brain, placenta, and fetus. In contrast, NAC had no significant effect on urinary MeHg excretion in preweanling rats. Conclusions: Because NAC causes a transient increase in urinary excretion of MeHg that is proportional to the body burden, it is promising as a biomonitoring agent for MeHg in adult animals. In view of this and because NAC is effective at enhancing MeHg excretion when given either orally or intravenously, can decrease brain and fetal levels of MeHg, has minimal side effects, and is widely available in clinical settings, NAC should be evaluated as a potential antidote and biomonitoring agent in humans.

N-Acetylcysteine (NAC) is a sulfhydryl-containing compound that produces a dramatic acceleration of urinary methylmercury (MeHg) excretion in poisoned mice, but the molecular mechanism for this effect is poorly defined. MeHg readily binds to NAC to form the MeHg-NAC complex, and recent studies indicate that this complex is an excellent substrate for the basolateral organic anion transporter (Oat)-1, Oat1/Slc22a6, thus potentially explaining the uptake from blood into the renal tubular cells. The present study tested the hypothesis that intracellular MeHg is subsequently transported across the apical membrane of the cells into the tubular fluid as a MeHg-NAC complex using the multidrug resistance-associated protein-2 (Mrp2/Abcc2). NAC markedly stimulated urinary [¹⁴C]MeHg excretion in wild-type Wistar rats, and a second dose of NAC was as effective as the first dose in stimulating MeHg excretion. In contrast with the normal Wistar rats, NAC was much less effective at stimulating urinary MeHg excretion in the Mrp2-deficient (TR-) Wistar rats. The TR- rats excreted only ∼30% of the MeHg excreted by the wild-type animals. To directly test whether MeHg-NAC is a substrate for Mrp2, studies were carried out in plasma membrane vesicles isolated from livers of TR- and control Wistar rats. Transport of MeHg-NAC was lower in vesicles prepared from TR- rats, whereas transport of MeHg-cysteine was similar in control and TR- rats. These results indicate that Mrp2 is involved in urinary MeHg excretion after NAC administration and suggest that the transported molecule is most likely the MeHg-NAC complex.

U.S. Service Members and civilians are at risk of exposure to a variety of environmental health hazards throughout their normal duty activities and in industrial occupations. Metals are widely used in large quantities in a number of industrial processes and are a common environmental toxicant, which increases the possibility of being exposed at toxic levels. While metal toxicity has been widely studied, the exact mechanisms of toxicity remain unclear. In order to further elucidate these mechanisms and identify candidate biomarkers, rats were exposed via a single intraperitoneal injection to three concentrations of CdCl₂ and Na₂Cr₂O₇, with livers harvested at 1, 3, or 7 days after exposure. Cd and Cr accumulated in the liver at 1 day post exposure. Cd levels remained elevated over the length of the experiment, while Cr levels declined. Metal exposures induced ROS, including hydroxyl radical (•OH), resulting in DNA strand breaks and lipid peroxidation. Interestingly, ROS and cellular damage appeared to increase with time post-exposure in both metals, despite declines in Cr levels. Differentially expressed genes were identified via microarray analysis. Both metals perturbed gene expression in pathways related to oxidative stress, metabolism, DNA damage, cell cycle, and inflammatory response. This work provides insight into the temporal effects and mechanistic pathways involved in acute metal intoxication, leading to the identification of candidate biomarkers.