Induction of DNA damage in human urothelial cells by the brominated flame retardant 2,2-bis(bromomethyl)-1,3-propanediol: role of oxidative stress.

2,2-bis(bromomethyl)-1,3-propanediol (BMP) is an extensively used brominated flame retardant found in urethane foams and polyester resins. In a 2-year dietary study conducted by the National Toxicology Program, BMP caused neoplastic lesions at multiple sites including the urinary bladder in both rats and mice. The mechanism of its carcinogenic effect is unknown. In the present study, using SV-40 immortalized human urothelial cells (UROtsa), endpoints associated with BMP induced DNA damage and oxidative stress were investigated. The effects of time (1-24h) and concentration (5-100 µM) on BMP induced DNA strand breaks were assessed via the alkaline comet assay. The results revealed evidence of DNA strand breaks at 1 and 3h following incubation of cells with non-cytotoxic concentrations of BMP. Strand breaks were not present after 6h of incubation. Evidences for BMP associated oxidative stress include: an elevation of intracellular ROS formation as well as induction of Nrf2 and HSP70 protein levels. In addition, DNA strand breaks were attenuated when cells were pre-treated with N-acetyl-l-cysteine (NAC) and oxidative base modifications were revealed when a lesion specific endonuclease, human 8-hydroxyguanine DNA glycosylase 1 (hOGG1) was introduced into the comet assay. In conclusion, these results demonstrate that BMP induces DNA strand breaks and oxidative base damage in UROtsa cells. Oxidative stress is a significant, determinant factor in mediating these DNA lesions. These early genotoxic events may, in part, contribute to BMP-induced carcinogenesis observed in rodents.

Characterization of the inhibitory effects of N-butylpyridinium chloride and structurally related ionic liquids on organic cation transporters 1/2 and human toxic extrusion transporters 1/2-k in vitro and in vivo.

Ionic liquids (ILs) are a class of salts that are expected to be used as a new source of solvents and for many other applications. Our previous studies revealed that selected ILs, structurally related organic cations, are eliminated exclusively in urine as the parent compound, partially mediated by renal transporters. This study investigated the inhibitory effects of N-butylpyridinium chloride (NBuPy-Cl) and structurally related ILs on organic cation transporters (OCTs) and multidrug and toxic extrusion transporters (MATEs) in vitro and in vivo. After Chinese hamster ovary cells expressing rat (r) OCT1, rOCT2, human (h) OCT2, hMATE1, or hMATE2-K were constructed, the ability of NBuPy-Cl, 1-methyl-3-butylimidazolium chloride (Bmim-Cl), N-butyl-N-methylpyrrolidinium chloride (BmPy-Cl), and alkyl substituted pyridinium ILs to inhibit these transporters was determined in vitro. NBuPy-Cl (0, 0.5, or 2 mg/kg per hour) was also infused into rats to assess its effect on the pharmacokinetics of metformin, a substrate of OCTs and MATEs. NBuPy-Cl, Bmim-Cl, and BmPy-Cl displayed strong inhibitory effects on these transporters (IC(50) = 0.2-8.5 µM). In addition, the inhibitory effects of alkyl-substituted pyridinium ILs on OCTs increased dramatically as the length of the alkyl chain increased. The IC(50) values were 0.1, 3.8, 14, and 671 µM (hexyl-, butyl-, and ethyl-pyridinium and pyridinium chloride) for rOCT2-mediated metformin transport. Similar structurally related inhibitory kinetics were also observed for rOCT1 and hOCT2. The in vivo coadministration study revealed that NBuPy-Cl reduced the renal clearance of metformin in rats. These results demonstrate that ILs compete with other substrates of OCTs and MATEs and could alter the in vivo pharmacokinetics of such substrates.

In vitro glucuronidation of 2,2-bis(bromomethyl)-1,3-propanediol by microsomes and hepatocytes from rats and humans.

2,2-Bis(bromomethyl)-1,3-propanediol (BMP) is a brominated flame retardant used in unsaturated polyester resins. In a 2-year bioassay BMP was shown to be a multisite carcinogen in rats and mice. Because glucuronidation is the key metabolic transformation of BMP by rats, in this study the in vitro hepatic glucuronidation of BMP was compared across several species. In addition, the glucuronidation activities of human intestinal microsomes and specific human hepatic UDP-glucuronosyltransferase (UGT) enzymes for BMP were determined. To explore other possible routes of metabolism for BMP, studies were conducted with rat and human hepatocytes. Incubation of hepatic microsomes with BMP in the presence of UDP-glucuronic acid resulted in the formation of a BMP monoglucuronide. The order of hepatic microsomal glucuronidation activity of BMP was rats, mice >> hamsters > monkeys >>> humans. The rate of glucuronidation by rat hepatic microsomes was 90-fold greater than that of human hepatic microsomes. Human intestinal microsomes converted BMP to BMP glucuronide at a rate even lower than that of human hepatic microsomes. Among the human UGT enzymes tested, only UGT2B7 had detectable glucuronidation activity for BMP. BMP monoglucuronide was the only metabolite formed when BMP was incubated with suspensions of freshly isolated hepatocytes from male F-344 rats or with cryopreserved human hepatocytes. Glucuronidation of BMP in human hepatocytes was extremely low. Overall, the results support in vivo studies in rats in which BMP glucuronide was the only metabolite found. The poor glucuronidation capacity of humans for BMP suggests that the pharmacokinetic profile of BMP in humans will be dramatically different from that of rodents.

The effects of dose, route, and repeated dosing on the disposition and kinetics of tetrabromobisphenol A in male F-344 rats.

Studies were conducted to characterize the metabolic and dispositional fate of (14)C-tetrabromobisphenol A (TBBPA)-a commonly used brominated flame retardant, in male Fischer-344 rats. The percent of dose eliminated as total radioactivity in feces at 72 h following three different single oral doses (2, 20, or 200 mg/kg) of (14)C-TBBPA was 90% or greater for all doses. Most of the dose was eliminated in the first 24 h. At 72 h after administration of the highest dose, the amounts of (14)C found in the tissues were minimal (0.2-0.9%). With repeated daily oral doses (20 mg/kg) for 5 or 10 days, the cumulative percent dose eliminated in the feces was 85.1+/-2.8 and 97.9+/-1.1, respectively. In all studies radioactivity recovered in urine was minimal, <2%. Repeated dosing did not lead to retention in tissues. Following iv administration, feces was also the major route of elimination. Following iv administration of TBBPA, the radiolabel found in the blood decreased rapidly and could be described by a biexponential equation, consistent with a two-compartment model. The key calculated kinetic parameters are terminal elimination half-life (t(1/2)beta)=82 min; area under the blood concentration-time curve from time 0 to infinity (AUC)=1440 mug x min/ml; and apparent clearance (CL)=2.44 ml/min. Although readily absorbed from the gut, systemic bioavailability of TBBPA is low (<2%). It is extensively extracted and metabolized by the liver and the metabolites (glucuronides) exported into the bile. About 50% of an oral dose (20 mg/kg) was found in the bile within 2 h. This extensive extraction and metabolism by the liver greatly limits exposure of internal tissues to TBBPA following oral exposures.

Nordihydroguaiaretic acid: hepatotoxicity and detoxification in the mouse.

Larrea tridentata (Moc & Sess) Cov. (Zygophyllaceae) is an ethnobotanically important plant found in the American Southwest and northern Mexico. Although numerous beneficial effects have been attributed to this plant, several case reports have demonstrated high doses of Larrea-containing herbals induce hepatotoxicity and nephrotoxicity in humans. Nordihydriguaiaretic acid (NDGA) is a lignan found in high amounts (up to 10% by dry weight) in the leaves and twigs of L. tridentata. Previously, NDGA has been shown to induce cystic nephropathy in the rat, however, no reports have been made concerning this compound's hepatotoxic potential. Here, we report that intraperitoneal administration of NDGA is lethal in the mouse (LD(50)=75 mg/kg). Administration is associated with a time and dose-dependent increase in serum alanine aminotransferase levels, which suggest liver damage. Indeed, freshly isolated mouse hepatocytes are more sensitive to NDGA than human melanoma cells. Furthermore, we have identified glucuronidation as a potential detoxification mechanism for NDGA. Both mono and diglucuronide conjugates of NDGA are formed after intravenous dosing. The monoglucuronide is also formed after incubation of NDGA with human hepatic microsomes; suggesting that glucuronide conjugation is important in the metabolism of NDGA by humans. In summary, this report indicates that NDGA may contribute to the hepatotoxicity of L. tridentata and provides preliminary information on NDGA metabolism.

Differential hepatotoxicity induced by cadmium in Fischer 344 and Sprague-Dawley rats.

A number of reports document that Fischer 344 (F344) rats are more susceptible to chemically induced liver injury than Sprague-Dawley (SD) rats. Cadmium (CdCl2), a hepatotoxicant that does not require bioactivation, was used to better define the biological events that are responsible for the differences in liver injury between F344 and SD rats. CdCl2 (3 mg/kg) produced hepatotoxicity in both rat strains, but the hepatic injury was 18-fold greater in F344 rats as assessed by plasma alanine aminotransferase (ALT) activity. This difference in toxicity was not observed when isolated hepatocytes were incubated with CdCl2 in vitro, indicating that other cell types contribute to Cd-induced hepatotoxicity in vivo. Indeed, the sieve plates of hepatic endothelial cells (EC) in F344 rats were damaged to a greater degree than EC in SD rats. Additionally, Kupffer cell (KC) inhibition reduced hepatotoxicity in both strains, suggesting that this cell type is involved in the progression of CdCl2-induced hepatotoxicity. Moreover, enhanced synthesis of heat shock protein 72 occurred earlier in the SD rat. Maximal levels of hepatic metallothionein (MT), a protein associated with cadmium tolerance, were greater in SD rats. These protective factors may limit CdCl2-induced hepatocellular injury in SD compared with F344 rats by reducing KC activation and the subsequent inflammatory response that allows for the progression of hepatic injury.