Nuclear hormone receptors comprise a superfamily of ligand-modulated transcription factors that mediate the transcriptional activities of steroids, retinoids, and thyroid hormones. A growing number of related proteins have been identified that possess the structural features of hormone receptors, but that lack known ligands. Known as orphan receptors, these proteins represent targets for novel signaling molecules. We have isolated a mammalian orphan receptor that forms a heterodimeric complex with the retinoid X receptor. A screen of candidate ligands identified farnesol and related metabolites as effective activators of this complex. Farnesol metabolites are generated intracellularly and are required for the synthesis of cholesterol, bile acids, steroids, retinoids, and farnesylated proteins. Intermediary metabolites have been recognized as transcriptional regulators in bacteria and yeast. Our results now suggest that metabolite-controlled intracellular signaling systems are utilized by higher organisms.

In a recently completed 2-yr bioassay, furan was found to induce cholangiocarcinomas at high incidence in rats. The disposition of single and multiple gavage doses of [2,5-14C]furan has been determined in male F344 rats to aid in interpretation of that study. In the 24 h after dosing about 80% of the furan-derived radioactivity was eliminated, primarily via urine and expired air. [14C]Carbon dioxide was a major metabolite, indicating that furan ring opening followed by complete oxidation of at least one of the labeled carbons was a major part of the overall metabolism of furan. Liver contained more furan-derived radioactivity by far than other tissues after 24 h. Approximately 80% of the radioactivity in liver was not extracted by organic solvents and was associated with protein. There was either no binding to DNA or the furan-DNA adduct was not stable to the isolation procedure. Repeated daily administration of [14C]furan resulted in a more or less linear increase in covalent binding through four doses; at this point the amount of nonextractable radioactivity plateaus. Urine contained at least 10 metabolites, again indicating extensive metabolism of the furan ring. From the data obtained in this study it is clear that furan is metabolized to reactive species, apparently primarily in liver, and these intermediates react with protein. The hepatotoxicity resulting from furan exposure may be due to the reaction of furan metabolites with liver macromolecules; the presence of some of these reactive metabolites following chronic exposure to furan may result in cholangiocarcinomas.

RXR is a nuclear receptor that plays a central role in cell signaling by pairing with a host of other receptors. Previously, 9-cis-retinoic acid (9cRA) was defined as a potent RXR activator. Here we describe a unique RXR effector identified from organic extracts of bovine serum by following RXR-dependent transcriptional activity. Structural analyses of material in active fractions pointed to the saturated diterpenoid phytanic acid, which induced RXR-dependent transcription at concentrations between 4 and 64 ,uM. Although 200 times more potent than phytanic acid, 9cRA was undetectable in equivalent amounts of extract and cannot be present at a concentration that could account for the activity. Phytenic acid, another phytol metabolite, was synthesized and stimulated RXR with a potency and efficacy similar to phytanic acid. These metabolites specifically displaced[3H]-9cRA from RXR with Ki values of 4 ,uM, indicating that their transcriptional effects are mediated by direct receptor interactions. Phytol metabolites are compelling candidates for physiological effectors, because their RXR binding affinities and activation potencies match their micromolar circulating concentrations. Given their exclusive dietary origin, these chlorophyll metabolites may represent essential nutrients that coordinate cellular metabolism through RXR-dependent signaling pathways.

The metabolism and disposition of 14C-labelled 2,2',4,4',5-pentabromodiphenyl ether (BDE99) were studied in F344 rats and B6C3F1 mice. Approximately 85% of a 1 micromol kg-1 oral dose was absorbed by male rats and mice. Within 24 h following oral doses ranging from 0.1 to 1000 micromol kg-1 to rats, 39-47% of the dose was excreted in the faeces (including 16% unabsorbed), up to 2% was excreted in the urine, and 34-38% remained in the tissues, mostly in adipose tissue. Mice excreted more in the urine and less in the faeces than rats. Tissue accumulation was observed following repeated dosing to rats. Two dihydrohydroxy-S-glutathionyl and two S-glutathionyl conjugates of BDE99, 2,4,5-tribromophenol glucuronide, two mono-hydroxylated BDE99 glucuronides, and three mono-hydroxylated tetrabromodiphenyl ether glucuronides were identified in male rat bile. 2,4,5-Tribromophenol and its glucuronide and sulfate conjugates, were identified in male rat urine. 2,4,5-Tribromophenol, one mono-hydroxylated tetrabromodiphenyl ether, and two mono-hydroxylated BDE99 were characterized in male rat faeces. BDE99 undergoes more extensive metabolism than does BDE47. Half of the absorbed oral dose in male rats was excreted in 10 days mostly as metabolites derived from arene oxide intermediates.

An important mechanism of toxicity of furans involves the cytochrome P-450 monooxygenase-catalyzed bioactivation of the compound in situ directly within the target tissues to highly reactive electrophilic products. The unsaturated aldehydes acetylacrolein and methylbutenedial have been identified as the principal reactive intermediates of 2- and 3-methylfuran, respectively, that are produced and bound covalently to tissue macromolecules in hepatic and pulmonary microsomal systems in vitro.

Polybrominated diphenyl ethers (PBDEs), used as flame retardants, have been detected in the environment and in mammalian tissues and fluids. Evidence indicates that PBDE mixtures induce CYPs through aryl hydrocarbon receptor (AhR)-dependent and -independent pathways. The present work has investigated the effects of individual components of a commercial PBDE mixture (DE71) on expression of CYP1A1, a biomarker for activation of the AhR (dioxin-like), and CYP2B and CYP3A, biomarkers for activation of the constitutive androstane and pregnanexreceptors (CAR and PXR), respectively, in the rat. Male F344 rats were dosed orally on three consecutive days with either DE71, PBDE components, 2,2',4,4'-tetraBDE (BDE47), 2,2',4,4',5-pentaBDE (BDE99), 2,2',4,4',5,5'-hexaBDE (BDE153), representative polybrominated dibenzofurans (PBDFs) present in DE71, or reference PCBs. Differential expression of target genes was determined in liver 24 h after the last dose. Quantitative PCR analysis indicated up-regulation of CYP1A1 by DE71; however, the response was weak compared to that for dioxin-like PCB126. Individual PBDE components of DE71 up-regulated CYP1A1 only at the highest administered dose (100 micromol/kg/day). Representative PBDFs efficiently up-regulated CYP1A1; therefore, they, along with other PBDFs and polybrominated dibenzodioxins detected in DE71 and individual PBDE components, may be responsible for most, if not all, dioxin-like properties previously observed for PBDEs. Conversely, PBDEs appear capable of up-regulating CYP2B and CYP3A in rats at doses similar to that for non-dioxin-like PCB153. These results indicate that in vivo PBDE-mediated toxicity would be better categorized by AhR-independent mechanisms, rather than the well-characterized AhR-dependent mechanism associated with exposure to dioxin-like chemicals.

No abstract

ABSTRACT:Reported adverse drug interactions with the popular herb kava have spurred investigation of the mechanisms by which kava could mediate these effects. In vivo and in vitro experiments were conducted to examine the effects of kava extract and individual kavalactones on cytochrome P450 (P450) and P-glycoprotein activity. The oral pharmacokinetics of the kavalactone, kawain (100 mg/kg), were determined in rats with and without coadministration of kava extract (256 mg/kg) to study the effect of the extract on drug disposition. Kawain was well absorbed, with >90% of the dose eliminated within 72 h, chiefly in urine. Compared with kawain alone, coadministration with kava extract caused a tripling of kawain AUC 0-8 h and a doubling of C max . However, a 7-day pretreatment with kava extract (256 mg /kg/day) had no effect on the pharmacokinetics of kawain administered on day 8. The 7-day pretreatment with kava extract only modestly induced hepatic P450 activities. The human hepatic microsomal P450s most strongly inhibited by kava extract (CYP2C9, CYP2C19, CYP2D6, CYP3A4) were inhibited to the same degree by a "composite" kava formulation composed of the six major kavalactones contained in the extract. K i values for the inhibition of CYP2C9 and CYP2C19 activities by methysticin, dihydromethysticin, and desmethoxyyangonin ranged from 5 to 10 M. Kava extract and kavalactones (<9 M) modestly stimulated P-glycoprotein ATPase activities. Taken together, the data indicate that kava can cause adverse drug reactions via inhibition of drug metabolism.