Tightly regulated and inducible expression of rabbit CYP2E1 using a tetracycline-controlled expression system.

A tetracycline (Tc)-controlled gene expression system that quantitatively controls gene expression in eukaryotic cells () was used to express cytochrome P-450 2E1 (CYP2E1) in HeLa cells in culture. The rabbit CYP2E1 cDNA was subcloned into the Tc-controlled expression vector (pUHD10-3) and transfected into a HeLa cell line constitutively expressing the Tc-controlled transactivator, a positive regulator of expression in the absence of Tc. The expression of CYP2E1 was tightly regulated. There was a time-dependent induction of CYP2E1 after removal of Tc. In the absence of Tc, the enzyme was induced more than 100-fold and expressed about 18 pmol of CYP2E1/mg microsomal protein. At maximal levels of expression the enzyme catalyzed the formation of 158 pmol 6-hydroxychlorzoxazone/min/mg total cellular protein. In addition, the level of the enzyme could be modulated by the concentration of Tc in the media. In the absence of Tc, exposure of cells to N-nitrosodimethylamine caused a significant dose-dependent decrease in cell viability. In contrast, menadione, a redox cycling toxicant, was less toxic to the cells after induction of CYP2E1 when compared with noninduced cells. Pulse-chase studies conducted 72 h after removal of Tc indicated a rapid turnover of CYP2E1 with a half-life of 3.9 h. Addition of the ligand, 4-methylpyrazole, and the suicide substrate, 1-aminobenzotrizole, decreased the degradation of CYP2E1. This cell line offers a useful system to examine the role of CYP2E1 in the cytotoxicity of xenobiotics and to investigate post-translational regulation of the enzyme.

Species differences in the hepatic microsomal enzyme metabolism of the pyrrolizidine alkaloids.

Species differences in pyrrolic metabolites and senecionine (SN) N-oxide formation among eight animal species (sheep, cattle, gerbils, rabbits, hamsters, Japanese quail, chickens, rats) varying in susceptibility to pyrrolizidine alkaloid (PA) intoxication were measured in vitro by hepatic microsomal incubations. The results suggested that there is not a strong correlation between the production of pyrrolic metabolites and susceptibility of animals to PA toxicity. The rate of PA activation in hamsters, a resistant species, measured by formation of (+/-)6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) far exceeded the rate of SN N-oxide formation (detoxification) (DHP/N-oxide = 2.29). In contrast, SN N-oxide was the major metabolite in sheep, another resistant species, with much lower production of DHP (DHP/N-oxide = 0.26). The roles of cytochrome P450s and flavin-containing monooxygenases (FMO) in bioactivation and detoxification of pyrrolizidine alkaloids (PA) were studied in vitro using sheep and hamster hepatic microsomes. Chemical and immunochemical inhibition data suggested that the conversion of SN to DHP is catalyzed mainly by cytochrome P450s (68-82%), whereas the formation of SN N-oxide is carried out largely by FMO (55-71%). There also appeared to be a high rate of glutathione-DHP conjugation in hamster (63%) and sheep (79%) liver microsomal incubation mixtures. Therefore, low rates of pyrrole metabolite production coupled with glutathione conjugation in sheep may explain the resistance of sheep to SN, whereas the high rate of GSH-DHP conjugation may be one of the factors contributing to the resistance of hamsters to intoxication by this PA.

The roles of CYP3A and CYP2B isoforms in hepatic bioactivation and detoxification of the pyrrolizidine alkaloid senecionine in sheep and hamsters.

The roles of cytochrome CYP3A and CYP2B isozymes in the bioactivation and detoxification of the pyrrolizidine alkaloid (PA) senecionine (SN) have been investigated in vitro with sheep and hamster hepatic microsomes. Our results show that the rate of SN activation measured by (+/-)-6, 7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) formation greatly exceeded the rate of SN N-oxide formation (detoxification) in hamsters. In contrast, SN N-oxide, a detoxification product, was the major metabolite in sheep with much lower DHP production. Immunoinhibition studies with anti-sheep CYP3A and CYP2B antibodies show that members of CYP3A subfamily play the major role in the conversion of PA to pyrrolic metabolites in both species (over 90% in sheep; 68% in hamster). These enzymes also contribute 38.8 and 41. 3% of SN N-oxidation in sheep and hamsters, respectively. In contrast, CYP2B isoforms have a limited capacity toward DHP formation in both species (47% in sheep; 32% in hamster), while these enzymes catalyzed only 24.6 and 35.4% SN N-oxidation in sheep and hamster, respectively. Using triacetyloleandomycin (TAO) and gestodene, two highly selective chemical inhibitors of CYP3A isoforms, our data show that 90% of DHP formation was inhibited by either inhibitor in sheep. Gestodene appeared to be more efficient than TAO in the inhibition of DHP production in hamsters. Testosterone 6beta-hydroxylase activity, a functional marker of CYP3A, was significantly inhibited by TAO and gestodene in sheep liver microsomes and by gestodene (100 microM) in hamster liver microsomes. These results suggest that CYP3A isozymes have important roles in bioactivation and detoxification of PA in both species, whereas CYP2B subfamily members are less efficient in biotransformation of PA.