CYP2E1-dependent benzene toxicity: the role of extrahepatic benzene metabolism

Abstract

Benzene, a ubiquitous environmental pollutant, is haematotoxic and myelotoxic. As has been shown earlier, cytochrome P450 2E1 (CYP2E1)-dependent metabolism is a prerequisite for the cytotoxic and genotoxic effects of benzene, but which of the benzene metabolites produces toxicity is still unknown. The observed differences between the toxicity of benzene and that of phenol, a major metabolite of benzene, could be explained by alternative hypotheses. That is, whether (1) toxic benzene effects are caused by metabolites not derived from phenol (e.g. benzene epoxide, muconaldehyde). which are formed in the liver and are able to reach the target organ(s); or (2) benzene penetrates into the bone marrow, where local metabolism takes place, whereas phenol does not reach the target tissue because of its polarity. To further investigate hypothesis 2, we used various strains of mice (AKR, B6C3F1, CBA/Ca, CD-1 and C57B1/6), for which different toxic responses have been reported in the haematopoietic system after chronic benzene exposure. In these strains, CYP2E1 expression in bone marrow was investigated and compared with CYP2E1 expression in liver by means of two independent methods. Quantification of CYP2E1-dependent hydroxylation of chlorzoxazone (CLX) by high-performance liquid chromatography (HPLC; functional analysis) was used to characterize specific enzymatic activities. Protein identification was performed by Western blotting using CYP2E1-specific antibodies. In liver microsomes of all strains investigated, considerable amounts of CYP2E1-specific protein and correspondingly high CYP2E1 hydroxylase activities could be detected. No significant differences in CYP2E1-dependent enzyme activities were found between the five strains (range of medians, 4.6 12.0 nmol 6-OH-CLX/[mg protein x min]) in hepatic tissue. In the bone marrow, CYP2E1 could also be detected in all strains investigated. However, chlorzoxazone hydroxylase activities were considerably lower (range of medians, 0.2-0.8x10(-3) nmol 6-OH-CLX/[mg protein x min]) compared with those obtained from liver microsomes. No significant (P>0.05) interstrain differences in CYP2E1 expression in liver and/or bone marrow could be observed in the mouse strains investigated. The data obtained thus far from our investigations suggest that strain-specific differences in the tumour response of the haematopoietic system of mice chronically exposed to benzene cannot be explained by differences in either hepatic or in myeloid CYP2E1-dependent metabolism of benzene.

Toxicogenomics Applied to Hematotoxicology

“…The next step is to further analyze the fractionated stem cell compartment and compare the results with those reported by Ivanova et al [35]. For example, CYP2E1 is known to be constitutively expressed in the WT mice [62], which was confirmed in the present investigation (Table 24.3). Interestingly, CYP2E1 gene expression is in the list of Ivanova et al [35].…”

“…Such a close association of p53 gene function and benzene toxicity raises the question regarding the fate of mice whose p53 gene was knocked out after benzene exposure; thus, cDNA microarray analysis data from p53 KO and WT mice were analyzed with Genespring software. In p53 WT mice, the expression profiles of genes encoding many proteins involved in benzene metabolism (CYP2E1 [62] and myeloperoxidase (MPO) [63]), cell cycle or cell proliferation (p53, p21 waf1 , cyclin G1, and Gadd45 [64]), and apoptosis (Bax-alpha [64]) were generally consistent with the cDNA microarray data for C57BL/6 mice described elsewhere (Table 24.1). A difference was noted in the expression patterns of specific genes taken during and after exposure of C57BL/6 mice to 300 ppm benzene for two weeks, as determined using the Incyte GEM cDNA microarray system (Figure 24.9 shows that the expression of MPO increased during exposure to benzene (right top square consisting two spots at D5 and D12) and then decreased three days after the termination of benzene exposure (right top square consisting two spots at D+3).…”

“…Mice lacking the p53 gene generally had expression patterns similar to those of WT mice for genes involved in benzene metabolism (CYP2E1 [62,66] and MPO [63]) and haemopoiesis, suggesting that p53 KO mice are also affected by benzene exposure to a similar extent, as also proposed previously based on the high number of micronuclei in benzene-exposed p53-deficient mice [67] (Table 24.2; see lines for up-or downregulated both simultaneously in p53 KO and WT). To elucidate the difference in expression between p53 KO and WT mice, clustering analysis was performed [65].…”

Systematic literature review on Parkinson's disease and Childhood Leukaemia and mode of actions for pesticides

“…Simplified scheme of benzene metabolism adopted from (Bernauer et al, 1999). CYP2E1 = cytochrome P450 2E1; MPO = myeloperoxidase; NQO1 = NAD(P)H:quinone oxidoreductase 1…”

In vitro metabolism and bioavailability tests for endocrine active substances: What is needed next for regulatory purposes?

“…In extra-hepatic tissues metabolism may differ from that which occurs during hepatic metabolism. The prostaglandin H synthase (PHS) system is particularly active and oxidizes a wide range of xenobiotics, although extra-hepatic mixed function oxidases have also been shown to metabolize a number of chemicals (Bernauer et al, 1999(Bernauer et al, , 2000(Bernauer et al, , 2002(Bernauer et al, , 2003.…”