Conjugate

Excretion

Figure 6.8 Overview of the fate of lipophilic compounds subjected to two phases of biotransformation. Phase I metabolism is conducted by cytochrome P450; phase II metabolism consists of conjugation enzymes such as sulphotransferase (transferring a sulphate moiety to the activated product of phase I metabolism), uridinediphosphate glucuronyl transferase (transferring glucuronic acid), and glutathione S-transferase (transferring glutathione). In several cases the product of phase I metabolism is very reactive and more toxic than the original compound. This also happens sometimes with phase II reaction products.

Induction of cytochrome P450, especially the genes known as Cyp1a1 and Cyp1a2, is initiated by binding of the inducer with a cytosolic receptor, the aryl hydrocarbon receptor (Ah receptor). In the uninduced state this receptor is stabilized by heat-shock protein Hsp83 (Hsp90 in mammals). This role of Hsp83 is very similar to its stabilization of the heat-shock factor HSF and the steroid hormone receptor (see Fig. 6.4 and Table 6.2). If a 3MC-type inducer binds to the Ah receptor, the protein is activated and can translocate to the nucleus, where it is phosphorylated by protein kinase C and forms a transcriptional activator complex with another protein, Ah receptor nuclear translocator (also known as ARNT). The complex then binds to sequences known as xenobiotic-responsive elements. The elements are also called dioxin-responsive elements because of the use of dioxin as a model compound; the term Ah receptor elements is also used. Such sequences are present in the promoters of both phase I and phase II genes. Genes activated by the Ah receptor are jointly referred to as the Ah battery (Nebert et al. 2000). The group involves at least two P450 genes (Cyp1a1 and Cyp1a2) and four genes involved with phase II biotransformation and the antioxidant stress response. Interestingly, the promoters of phase II biotransformation enzyme genes in the Ah battery contain not only xenobiotic-responsive elements but also AREs.

There is a close link between xenobiotic biotransformation and oxidative stress (Nebert et al. 2000; Kong et al. 2001; Fig. 6.9). Some xenobiotics such as dioxins and polychlorinated biphenyls are very potent inducers of Cyp1a1, but are themselves hardly metabolized by the cytochrome P450 enzymes. Instead, upregulated enzyme activity generates a lot of ROS and induces prolonged oxidative stress. In addition, some metabolites generated by P450 activity are very electrophilic, which means that they react easily with other compounds to compensate their shortage of electrons. Electrophiles and oxygen radicals induce antioxidant enzymes by the mechanisms discussed above. The presence of AREs in the promoters of phase II biotransformation enzymes ensures that these genes are also induced. The chronic toxicity of compounds such as dioxin is ascribed to a situation of sustained oxidative stress in the whole organism.

Compounds of the PB-type predominantly induce cytochrome P450s of the IIB group and to a certain extent also members of the III family, which are normally induced by steroid hormones. PB-type induction is not as specific as 3MC-induction and it does not depend on the Ah receptor. The precise mechanism is not known. One possibility is that PB-type inducers activate cytochrome P450 by binding to a cytosolic repressor, causing derepression of Cyp genes. However, the great structural variety of PB-type compounds makes it unlikely that this mechanisms holds for all inducers. Another possibility is that BP-type inducers introduce a change in redox state, upon which SAPK signalling is triggered, leading indirectly to tran-scriptional upregulation of Cyp genes. A comprehensive summary of the various pathways that may lead to induction of biotransformation activities, based on Nebert et al. (2000) and Korsloot et al. (2004), is given in Fig. 6.9.

Figure 6.9 Summary of the various pathways leading to induction of biotransformation enzymes. Three inducing agents can trigger activity, PB-type inducers, oxidative stress, and 3MC-type inducers. The first type of inducer presumably activates transcription of cytochrome P450 II genes by binding to a repressor. The latter type of substance induces cytochrome P450 I genes via activation of the Ah receptor, which binds to a xenobiotic-responsive element (XRE) in the promoters of P450 I genes. In addition, P450 activity generates electrophilic metabolites and ROS and this may, through SAPK signalling, induce antioxidant genes and non-P450 genes from the Ah battery.

P450II genes ? ARE XRE Non-P450 genes XRE P450I genes

Figure 6.9 Summary of the various pathways leading to induction of biotransformation enzymes. Three inducing agents can trigger activity, PB-type inducers, oxidative stress, and 3MC-type inducers. The first type of inducer presumably activates transcription of cytochrome P450 II genes by binding to a repressor. The latter type of substance induces cytochrome P450 I genes via activation of the Ah receptor, which binds to a xenobiotic-responsive element (XRE) in the promoters of P450 I genes. In addition, P450 activity generates electrophilic metabolites and ROS and this may, through SAPK signalling, induce antioxidant genes and non-P450 genes from the Ah battery.

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