Lipophilic environmental pollutants are often stored in adipose tissues after exposure. These compounds have been well studied in terms of their cell toxicity in organs such as liver and kidney, and their xenoestrogenic action on reproductive tissues as endocrine disruptors. However, the effects of these chemicals on the depot, adipose tissue, have not been studied, although adipose tissue is an important endocrine tissue secreting obesity/diabetes-related hormones and cytokines. In this study, we identified the expression of cytochromes P450 in rat white adipose tissues and investigated the effects of typical lipophilic cytochrome P450 inducers, namely phenobarbital, dexamethasone, and ␤-naphthoflavone. The results showed that ␤-naphthoflavone was a strong CYP1A inducer in adipose tissue as well as in liver. It increased CYP1A1 mRNA, protein, and its related activity, ethoxyresorufin O-deethylase. Phenobarbital and dexamethasone also induced both the mRNA and protein of CYP2Bs and CYP3As, respectively, in adipose tissue, although significant interindividual differences were observed. Furthermore, we demonstrated that 48 h of fasting was as effective in adipose tissue as in the liver in the induction of CYP2E1 mRNA and protein. These results suggest that the mechanisms by which cytochrome P450 genes are regulated in the liver are also functional in rat adipose tissues. This has raised the possibility that lipophilic environmental contaminants accumulated in adipose tissue may dysregulate the gene expression profile.

ABSTRACT:The liver is the main organ of drug metabolism, but the expression and induction by xenobiotics of drug-metabolizing enzymes is also often observed in extrahepatic tissues. Recently, we reported that lipophilic cytochrome P450 inducers, ␤-naphthoflavone (BNF), phenobarbital, and dexamethasone, induced CYP1, CYP2B, and CYP3A enzymes, respectively, in rat epididymal white adipose tissue (WAT) at both mRNA and protein levels. To further confirm the xenobiotic-induced expression of drug-metabolizing enzymes in adipose tissue, we studied the induction of CYP1A1 and other detoxifying enzymes by aryl hydrocarbon receptor (AhR) agonists and antioxidants. BNF increased CYP1A1 mRNA levels in several visceral WATs (epididymal, perirenal, and mesenteric) to a greater degree than in subcutaneous WAT in rats. Using C57BL/6 and DBA/2 mice with different responsiveness to aryl hydrocarbons and detecting cytoplasmic levels of AhR proteins, we have demonstrated that AhR mediates this CYP1A1 induction by BNF in WAT. Moreover, the NF-E2-related factor 2 (Nrf2)/antioxidant responsive element pathway is also functional in WAT, since BNF, which is known to activate both AhR and Nrf2, and antioxidants including tert-butylhydroquinone, 1-chloro-2,4-dinitrobenzene, and menadione induced the expression of Nrf2-target genes (NAD-(P)H:quinone oxidoreductase, glutathione S-transferase A subunits, and heme oxygenase-1) in rats and mice. These results suggest that both AhR and Nrf2 pathways are active in WAT and that lipophilic compounds accumulated in WAT can activate these transcription factors to increase detoxification capability in the tissue.