Ethylbenzene modulates the expression of different cytochrome P-450 isozymes by discrete multistep processes

Risk assessments for chronic exposure of children and prospective parents to ethylbenzene (CAS No. 100-41-4)

Potential chronic health risks for children and prospective parents exposed to ethylbenzene were evaluated in response to the Voluntary Children's Chemical Evaluation Program. Ethylbenzene exposure was found to be predominately via inhalation with recent data demonstrating continuing decreases in releases and both outdoor and indoor concentrations over the past several decades. The proportion of ethylbenzene in ambient air that is attributable to the ethylbenzene/styrene chain of commerce appears to be relatively very small, less than 0.1% based on recent relative emission estimates. Toxicity reference values were derived from the available data, with physiologically based pharmacokinetic models and benchmark dose methods used to assess dose-response relationships. An inhalation non-cancer reference concentration or RfC of 0.3 parts per million (ppm) was derived based on ototoxicity. Similarly, an oral non-cancer reference dose or RfD of 0.5 mg/kg body weight/day was derived based on liver effects. For the cancer assessment, emphasis was placed upon mode of action information. Three of four rodent tumor types were determined not to be relevant to human health. A cancer reference value of 0.48 ppm was derived based on mouse lung tumors. The risk characterization for ethylbenzene indicated that even the most highly exposed children and prospective parents are not at risk for non-cancer or cancer effects of ethylbenzene.

Oral gavage subchronic neurotoxicity and inhalation subchronic immunotoxicity studies of ethylbenzene in the rat

The potential for neurotoxicological and immunotoxicological effects of ethylbenzene was studied in young adult Crl:CD(SD) rats following 90-day oral (neurotoxicity) or 28-day inhalation (immunotoxicity) exposures. In the neurotoxicity study, ethylbenzene was administered orally via gavage twice daily at 0, 25, 125, or 250 mg/kg per dose (total daily dosages of 0, 50, 250, or 500 mg/kg bwt/day [mg/kg bwt/day]) for 13 weeks and the functional observational battery (FOB), automated tests for motor activity and neuropathological examination were conducted. In the immunotoxicity study, animals were exposed by inhalation to 0, 25, 100, or 500 ppm ethylbenzene (approximately 26, 90, or 342 mg/kg bwt/day as calculated from physiologically based pharmacokinetic modeling). Immunotoxicity was evaluated in female rats using the splenic antibody-forming cell plaque-forming assay in sheep red blood cell sensitized animals. The no-observed-effect level for the oral gavage study was 50mg/kg bwt/day based on increased relative weights of the liver and kidneys in the male rats. The no-observed-adverse-effect level (NOAEL) for adult neurotoxicity was the highest dose tested 500 mg/kg bwt/day. The NOAEL for the immunotoxicity evaluation was the highest tested exposure concentration, 500 ppm (342 mg/kg bwt/day).

A review of the genotoxicity of ethylbenzene

Ethylbenzene is an important industrial chemical that has recently been classified as a possible human carcinogen (IARC class 2B). It induces tumours in rats and mice, but neither the relevance of these tumours to humans nor their mechanism of induction is clear. Considering the carcinogenic potential of ethylbenzene, it is of interest to determine whether there is sufficient data to characterize its mode of action as either genotoxic or non-genotoxic. A review of the currently available genotoxicity data is assessed. Ethylbenzene is not a bacterial mutagen, does not induce gene conversion or mutations in yeast and does not induce sister chromatid exchanges in CHO cells. Ethylbenzene is not clastogenic in CHO or rat liver cell lines but was reported to induce micronuclei in SHE cells in vitro. No evidence for genotoxicity has been seen in humans exposed to relatively high levels of ethylbenzene. Mouse lymphoma gene mutation studies produced a mixed series of responses that have proved difficult to interpret. An increase in morphological transformation of SHE cells was also found. Results from a more relevant series of in vivo genotoxicity studies, including acute and sub-chronic micronucleus tests and the mouse liver UDS assay, indicate a lack of in vivo genotoxic activity. The composite set of results from both in vitro and in vivo tests known to assess direct damage to DNA have been predominantly negative in the absence of excessive toxicity. The available data from the standard battery of genotoxicity assays do not support a genotoxic mechanism for ethylbenzene-induced kidney, liver or lung tumors in rats and mice.

Altered ethylbenzene-mediated hepatic CYP2E1 expression in growth hormone-deficient dwarf rats

Ethylbenzene (EB) effectively induces several hepatic P450 enzymes including CYP2E1 and CYP2B. Hypophysectomy diminishes the magnitude of EB-mediated induction of CYP2B. Although growth hormone (GH) plays a key role in sexual dimorphism of CYP2C11, its impact on EB-mediated P450 expression is still unknown. Because hypophysectomy leads to a depletion of multiple pituitary hormones besides GH, a study was designed to investigate the possible involvement of GH in EB-mediated hepatic P450 expression using GH-deficient dwarf rats as a more specific animal model. In these rats, pituitary GH was selectively reduced to about 10% of normal levels and other pituitary trophic hormones including thyroid-stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, and prolactin are largely unchanged. Male control and HsdOla:DWARF-dw-4 (Harlan, UK) rats were subjected to a single ip injection of EB (10 mmol/kg). CYP2E1- and CYP2B-dependent activities, protein, and RNA levels were measured 10 and 24 h afterward. The results indicated that dwarf rats without EB exposure expressed higher CYP2E1. Although EB treatment induced CYP2E1 activity, protein, and mRNA both in controls and dwarf rats, the magnitude of the response to EB exposure was greater 10 h after the treatment in dwarf rats. Hypophysectomy also increased CYP2E1 protein induction by EB compared to intact rats. This effect was reversed by GH supplementation to hypophysectomized rats. Overall, responses of CYP2B to EB exposure in dwarf rats did not display basic differences from controls. In conclusion, the results demonstrate that (1) the suppression of CYP2B induction found in the multi-hormone-deficient HX rats is not found in the more specific GH-deficient rat model, confirming that GH does not have a major influence on CYP2B expression and (2) both hypophysectomized and GH-deficient rats show an altered inducibility of CYP2E1 after EB treatment.

Effect of hypophysectomy and growth hormone replacement on the modulation of p450 expression after treatment with the aromatic hydrocarbon ethylbenzene

Pituitary status has a significant effect on the expression of several cytochrome P450 enzymes. The goal of this study was to examine the role of pituitary input on the modulation of CYP2C11 and CYP2B after treatment with the aromatic hydrocarbon ethylbenzene (EB). Intact, hypophysectomized (HX), and HX rats supplemented with pulsatile growth hormone (GH) were treated with corn oil or EB and the effects on hepatic P450 expression were determined. Hypophysectomy caused a 50% decrease in CYP2C11 protein in untreated rats, whereas GH supplementation returned protein to control levels. EB administration also decreased CYP2C11 protein in intact rats; however, this decrease was not observed after EB treatment in HX or HX + GH groups. CYP2C11-dependent testosterone 2alpha-hydroxylation followed a similar pattern as CYP2C11 protein, except that the activity was only partially restored by GH replacement. CYP2B levels were also substantially influenced by hypophysectomy. Intact rats exhibited a 100-fold increase in CYP2B1 mRNA, reaching a maximum 12 h after EB administration. A much smaller response (ca. 20-fold) was observed in HX rats, reaching a maximum 24 h after EB treatment. This effect was not reversed by GH supplementation. The half-life for EB was significantly increased from 8 h in intact rats to 14 h in HX rats, suggesting higher plasma EB concentrations after EB administration to HX rats. These results indicate that CYP2C11 and CYP2B become less responsive to EB-dependent modulation in HX rats, a response that cannot be explained simply by absence of GH or by altered EB pharmacokinetics in HX animals.

Ethylbenzene induces microsomal oxygen free radical generation: antibody-directed characterization of the responsible cytochrome P450 enzymes

Small aromatic hydrocarbons cause changes in oxidative metabolism by modulating the levels of cytochrome P450 enzymes, with the changes in these enzymes being responsible for qualitative changes in aromatic hydrocarbon metabolism. The goal of this study was to determine if exposure to the small alkylbenzene ethylbenzene (EB) leads to an increase in hepatic free radical production. Male F344 rats were treated with ip injections of EB (10 mmol/kg) and compared to corn oil controls. Hepatic free radical production was examined by measuring the conversion of 2',7'-dichlorofluorescin diacetate (DCFH-DA) to its fluorescent product 2',7'-dichlorofluorescein (DCF). A significant elevation of fluorescent DCF production was observed after treatment with EB, despite the lack of effect on overall cytochrome P450 levels. This process was shown to be inhibitable by metyrapone, an inhibitor of P450. DCF production was also inhibited by catalase, suggesting that hydrogen peroxide (H(2)O(2)) is one of the reactive oxygen intermediates involved in EB-mediated reactive oxygen species (ROS) formation. Interestingly, superoxide dismutase (SOD) did not inhibit DCF production in corn oil-treated rats but was an effective inhibitor in the EB-treated groups. In an effort to determine if the increase in ROS production was related to changes in specific P450 enzymes, DCF production was measured in the presence of anti-CYP2B, anti-CYP2C11, anti-CYP2E1, and anti-CYP3A2 inhibitory antibodies. Anti-CYP2B antibodies inhibited DCF production in EB-treated, but not corn oil groups, which is consistent with the low constitutive levels of this enzyme and its induction by EB. The data also demonstrate that CYP2B contributes to ROS production. Anti-CYP2C11 did not influence DCF production in either group. ROS formation in corn oil-treated rats as well as in ethylbenzene-treated rats was also inhibited with antibodies to anti-CYP2E1 and anti-CYP3A2. These results suggest that CYP2C11 does not appear to influence free radical production and that the increase in free radical production in EB treated rats is consistent with the EB-mediated elevation of CYP2B, CYP 2E1, and CYP3A2. Such alterations in free radical generation in response to hydrocarbon treatment may contribute to the toxicity of these compounds.

Ursodeoxycholic acid prevents hepatic cytochrome P450 isozyme reduction in rats with deoxycholic acid-induced liver injury

BACKGROUND/AIMS

Hydrophobic bile acids, such as deoxycholic acid produce cholestatic liver injury. Ursodeoxycholic acid has been shown to be useful in the treatment of cholestatic liver disease.

METHODS

In this study, we investigated the effects of deoxycholic acid or ursodeoxycholic acid (1% of diet, for 14 days) and their combination (1% each) on expression of hepatic cytochrome P450 isozymes, their related enzyme activities and mRNA level in rats.

RESULTS

Adding 1% deoxycholic acid to chow caused a marked increase in serum total bilirubin (47-fold) and total bile acid (8-fold) concentrations and in alkaline phosphatase (2.5-fold, p<0.01) and alanine aminotransferase activities (23.5-fold, p<0.01). Adding the same dose of ursodeoxycholic acid along with the deoxycholic acid mitigated both the rise in serum total bilirubin and bile acid concentrations and that in alkaline phosphatase and alanine aminotransferase activities, although the use of ursodeoxycholic acid alone did not affect any of the above. Feeding 1% deoxycholic acid caused a decrease (48% of control) in total cytochrome P450 content in hepatic microsomes. Addition of 1% ursodeoxycholic acid along with the 1% deoxycholic acid completely prevented the decrease in total cytochrome P450 content. Feeding ursodeoxycholic acid alone did not affect the total cytochrome P450 content. The expression of cytochrome P450 2B1, 2E1, 3A2, 2C6, 2C11 and 4A1 proteins in hepatic microsomes was decreased by deoxycholic acid (44, 51, 23, 59, 30 and 74% of control, respectively). Likewise, the activities of cytochrome P450 2B1 (pentoxyresorufin O-depentylation), 2E1 (aniline p-hydroxylation) and 3A2 (testosterone 6beta-hydroxylation) isozymes and the 3A2 mRNA levels in liver were decreased by deoxycholic acid. Addition of 1% ursodeoxycholic acid to 1% deoxycholic acid also prevented the decrease in these cytochrome P450 proteins, related enzyme activities and mRNA levels in liver.

CONCLUSIONS

These results indicate that, in rats with deoxycholic acid-induced liver injury, ursodeoxycholic acid prevents the decrease in hepatic cytochrome P450 isozymes and suggest that ursodeoxycholic acid is useful for the treatment of liver injury in terms of aiding the normalization of the hepatic drug-metabolizing system.

Changes in the expression of cytochrome P450s 2B1, 2B2, 2E1, and 2C11 in response to daily aromatic hydrocarbon treatment

Treatment of rats with ethylbenzene (EB) modulates the hepatic expression of many P450s, with those induced after a single intraperitoneal hydrocarbon injection differing from those induced after more prolonged (3 day) administration. The goals of the current studies are (1) to characterize the induction response after prolonged hydrocarbon exposure, (2) to explain why the elevation of these P450s is attenuated after continued treatment, and (3) to determine how P450 2B protein remains elevated without an elevation of P450 2B1/2 RNA. P450 2C11 protein was decreased after a single EB injection and remained depressed throughout the treatment period. P450 2C11 RNA was only decreased with prolonged, but not acute treatment. P450 2E1 was induced after a single EB injection; however, the initial induction was attenuated with more prolonged treatment. P450 2B1 and P450 2B2 RNAs exhibited a similar response, being elevated after acute administration, but returned to control levels with prolonged EB administration. Interestingly, P450 2B protein levels remained elevated despite the decrease in P450 2B1 and P450 2B2 RNA to control levels. We then tested the possibility that the multiphasic induction pattern of P450 2E1 and P450 2B1/2 RNA was due to differences in the pharmacokinetics of EB. The disappearance of EB with time was measured in rats that were either (1) untreated, (2) pretreated with EB for 1 day, or (3) pretreated with EB for 3 days. These results demonstrated that prior hydrocarbon exposure caused an increase in EB clearance, which decreased the overall levels of EB in the body. Consequently, EB levels were sufficiently diminished to decrease EB's effectiveness as an inducer leading to the decrease in P450 2E1 protein and P450 2B1 and P450 2B2 RNA after continued EB administration. A further consequence of the decreased overall EB concentration is that the hydrocarbon was capable of producing only a transient elevation of P450 2B1 RNA levels. This transient elevation appears to be sufficient to maintain elevated P450 2B protein.

Pituitary component of the aromatic hydrocarbon-mediated expression of CYP2B and CYP2C11

1. The aim was to determine if the ethylbenzene (EB)-mediated expression of CYP2B and CYP2C11 involved a hormonally controlled component. 2. The hypophysectomized (HX) and intact rats were treated with EB for 1 or 2 days, and the effects on specific CYP levels measured. 3. Differences were observed in the inducibility of CYP2B by EB in the HX rat when compared with intact controls. Whereas significant elevations of CYP2B-dependent activities and protein levels were observed after both 1 and 2 days of EB injection in intact controls, CYP2B levels were significantly elevated in the HX rat only after 2 days of hydrocarbon treatment. 4. Both CYP2C11-dependent activities and protein levels were decreased after EB administration to the intact rat. In contrast, CYP2C11 levels were unaffected by EB in the HX rat at any of the time points indicated. 5. CYP2C11 protein levels were unaffected by treatment with EB for 24 h in cultured hepatocytes, also supporting the hypothesis that hormones are involved in CYP2C11 expression. 6. This study indicates that pituitary input influences the EB-mediated changes in both CYP2B and CYP2C11. CYP2C11 is affected by EB administration in a manner similar to other xenobiotics such as phenobarbital. On the other hand, the smaller induction of CYP2B1/2 in response to EB differs from that observed with phenobarbital where HX augmented the response of the inducer.