Transient induction of cytochrome P450 1A1 mRNA by culture medium component in primary cultures of adult rat hepatocytes

Degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) by fungi originating from Vietnam

Three different fungi were tested for their ability to degrade 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid and for the role of laccases and cytochromes P450-type in this process. We studied a white-rot fungus Rigidoporus sp. FMD21, which has a high laccase activity, for its efficiency to degrade these herbicides. A positive correlation was found between its laccase activity and the corresponding herbicide degradation rate. Even more, the doubling of the enzyme activity in this phase corresponded with a doubling of the herbicide degradation rate. It is, therefore, tempting to speculate that laccase is the most dominant enzyme in the degradation of 2,4-D and 2,4,5-T under these conditions. In addition, it was shown that Rigidoporus sp. FMD21 partly relies on cytochromes P450-type for the breakdown of the herbicides as well. Two filamentous fungi were isolated from soil contaminated with herbicides and dioxins located at Bien Hoa airbase. They belong to genera Fusarium and Verticillium of the phylum Ascomycota as judged by their 18S rRNA gene sequences. Both isolated fungi were able to degrade the herbicides but with different rates. Their laccase activity, however, was very low and did not correlate with the rate of breakdown of the herbicides. These data indicate that the white-rot fungus most likely synthesizes laccase and cytochromes P450-type for the breakdown of the herbicides, while the types of enzyme used for the breakdown of the herbicides by the two Ascomycota remain unclear.

Quercetin, resveratrol, and curcumin are indirect activators of the aryl hydrocarbon receptor (AHR)

Several polyphenols have been shown to activate the aryl hydrocarbon receptor (AHR) in spite of the fact that they bind to the receptor with low affinity. The aim of this study was to investigate whether quercetin (QUE), resveratrol (RES), and curcumin (CUR) interfere with the metabolic degradation of the suggested endogenous AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ) and thereby indirectly activate the AHR. Using recombinant human enzyme, we confirmed earlier reported inhibitory effects of the polyphenols on cytochrome P4501A1 (CYP1A1) activity, and inhibition of metabolic clearance of FICZ was documented in FICZ-treated immortalized human keratinocytes (HaCaT). CYP1A1 activity was induced in HaCaT cells by all three compounds, and when they were added together with FICZ, a prolonged activation was observed after a dose-dependent inhibition period. The same pattern of responses was seen at the transcriptional level as determined with a CYP1A1 reporter assay in human liver hepatoma (HepG2) cells. To test the ability of the polyphenols to activate the AHR in the absence of FICZ, the cells were treated in medium, which in contrast to commercial batches of medium did not contain background levels of FICZ. Importantly, AHR activation was only observed in the commercial medium. Taken together, these findings suggest that QUE, RES, and CUR induce CYP1A1 in an indirect manner by inhibiting the metabolic turnover of FICZ. Humans are exposed to these compounds through the diet and nutritional supplements, and we propose that altered systemic levels of FICZ caused by such compounds may have physiological consequences.

Discovery and biological characterization of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole as an aryl hydrocarbon receptor activator generated by photoactivation of tryptophan by sunlight

Activation of the aryl hydrocarbon receptor (AHR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is required for AHR dependent transcriptional activation and TCDD toxicity. We previously reported that aqueous tryptophan exposed to sunlight through window glass (aTRP) contains multiple photoproducts, including the well characterized 6-formylindolo[3,2-b]carbazole (FICZ), capable of activating the AHR and inducing CYP1A and CYP1A-mediated enzyme activities. We report here the isolation from aTRP and chemical characterization and synthesis of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole (IPI), a compound previously identified as a natural product of marine ascidia and now shown to be a TRP photoproduct with AHR-inducing properties. IPI, FICZ and TCDD produced equieffective induction of CYP1A-mediated 7-ethoxyresorufin deethylase (EROD) activity in chick embryo primary hepatocytes and mammalian Hepa1c1c7 cells. EROD induction by IPI was markedly curtailed in AHR-defective c35 cells, supporting the AHR dependence of the IPI response. Although IPI had a higher EC(50) for EROD induction than FICZ, the much larger amount of IPI than FICZ in aTRP makes IPI a prominent contributor to EROD induction in aTRP. IPI was detected in TRP-containing culture medium under ambient laboratory conditions but not in TRP-free medium, consistent with its production from TRP. Cotreatment of hepatocytes with submaximal EROD-inducing doses of IPI and FICZ or TCDD produced additive increases in EROD without synergistic or inhibitory interactions. IPI and FICZ were readily metabolized by cultured hepatocytes. In addition to increasing CYP1A4 mRNA and EROD, IPI and FICZ decreased hepatocyte phosphoenolpyruvate carboxykinase mRNA expression and glucose output, biological effects associated with TCDD metabolic dysregulation. The findings underscore a role for sunlight in generating AHR-activating bioactive molecules.

Induction of cytochrome P450 1 genes and stress response genes in developing zebrafish exposed to ultraviolet radiation

Ultraviolet (UV) radiation damages cell molecules, and has been suggested to up-regulate mammalian cytochrome P4501 (CYP1) genes through an aryl hydrocarbon receptor (AHR) mediated mechanism. In this study, embryos and larvae of zebrafish (Danio rerio) were exposed to UV to determine the effects on expression of CYP1 and stress response genes in vivo in these fish. Zebrafish embryos were exposed for varying times to UV on two consecutive days, with exposure beginning at 24 and 48h post-fertilization (hpf). Embryos exposed for 2, 4 or 6h twice over 2 days to UVB (0.62 W/m(2); 8.9-26.7 kJ/m(2)) plus UVA (2.05 W/m(2); 29.5-144.6 kJ/m(2)) had moderately (2.4+/-0.8-fold) but significantly up-regulated levels of CYP1A. UVA alone had no effect on CYP1A expression. Proliferating cellular nuclear antigen (PCNA) and Cu-Zn superoxide dismutase (SOD1) transcript levels were induced (2.1+/-0.2 and 2.3+/-0.5-fold, respectively) in embryos exposed to two 6-h pulses of 0.62 W/m(2) UVB (26.8 kJ/m(2)). CYP1A was induced also in embryos exposed to higher intensity UVB (0.93 W/m(2)) for two 3-h or two 4-h pulses (20.1 or 26.8 kJ/m(2)). CYP1B1, SOD1 and PCNA expression was induced by the two 3-h pulses of the higher intensity UVB, but not after two 4-h pulses of the higher intensity UVB, possibly due to impaired condition of surviving embryos, reflected in a mortality of 34% at that UVB dose. A single 8-h long exposure of zebrafish larvae (8dpf) to UVB at 0.93 W/m(2) (26.8 kJ/m(2)) significantly induced CYP1A and CYP1B1 expression, but other CYP1 genes (CYP1C1, CYP1C2 and CYP1D1) showed no significant increase. The results show that UVB can induce expression of CYP1 genes as well stress response genes in developing zebrafish, and that UVB intensity and duration influence the responses.

The aryl hydrocarbon receptor and light

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has been intensively studied with respect to the toxicity of xenobiotics. However, its function in response to light has never been summarized. Here, we provide an overview of AhR activation by light with a focus on the role of tryptophan in light-induced AhR activation. We discuss the involvement of the AhR in different biological rhythms and speculate on the possible role of the AhR in UV-induced responses in skin. Furthermore, this review points out future research needs in this field.

Sunlight Generates Multiple Tryptophan Photoproducts Eliciting High Efficacy CYP1A Induction in Chick Hepatocytes and In Vivo

Photooxidized tryptophan (TRP) in tissue culture medium elicits a transient cytochrome P450 (CYP1) induction response in cultured cells. We show here that exposure of TRP to window sunlight (aTRP) greatly increased the potency, efficacy, and duration of CYP1A induction by TRP in primary chick embryo hepatocytes and in vivo. Aqueous TRP exposed to sunlight for 7 days exhibited a 100-fold or greater increase in potency over TRP in medium. The induction response was sustained for at least 48 h and was comparable in efficacy to 2,3,7,8-tetrachlorodibenzo-p-dioxin. In hepatocytes, increases in mRNAs for CYP1A4 and CYP1A5, chick orthologs of mammalian CYP1A1 and 1A2, preceded increases in CYP1A proteins and enzyme activities, 7-ethoxyresorufin deethylase (EROD) for CYP1A4 and arachidonic acid epoxygenation for CYP1A5, consistent with a transcriptional mechanism. Aryl hydrocarbon receptor (AhR) dependence was evidenced by aTRP induction of EROD in wild-type Hepa1c1c7 cells but not in AhR-defective (c35) mutants. Preparations of aTRP were stable for many months at 4 degrees C and were relatively resistant to metabolism by hepatocytes or liver microsomes. Fractionation of aTRP by HPLC analysis coupled with EROD assays showed that aTRP contained multiple photoproducts and CYP1A inducing components, which varied in sensitivity to metabolism by hepatocytes. The previously identified TRP photoproduct, 6-formylindolo[3,2-b]carbazole (FICZ), was one component, but FICZ was not required for CYP1A induction by the aTRP mixture. These findings identify the indoor environment, and window sunlight in particular, as a new source of CYP1A inducers. Further, the evidence that biologically active metabolites of an endogenous substrate, arachidonic acid, are formed by aTRP-induced CYP1A provides a pathway by which TRP photoproducts, like toxic xenobiotics, could have significant physiologic effects.

Proliferation rates of HepG2 cells encapsulated in alginate are increased in a microgravity environment compared with static cultures

This study investigates the effect of rotary culture compared with static culture on the proliferation, cell viability, synthetic function and detoxificatory capacity of HepG2 cells encapsulated in 1% alginate. Cell viability and alginate bead morphology were maintained in the rotary culture system at day 10, while cell number showed a 4.5-fold increase compared with static culture. Protein production was increased in rotary cultures with a 4.1-fold increase in total albumin and a 4.4-fold increase in alpha1 antitrypsin levels in rotary compared with static culture at day 10. CYP4501A1/2 activity was maintained between the two culture systems. In conclusion, rotary culture increases proliferation rates leading to improved bead packing and a concomitant increase in total protein synthesis, along with maintenance of detoxificatory capacity. This allows a greater level of hepatic function to be expressed in a given volume, offering clear advantages for the design of liver support systems.

Comparative microarray analysis of basal gene expression in mouse Hepa-1c1c7 wild-type and mutant cell lines

Hepa-1c1c7 wild-type and benzo[a]pyrene-resistant derived mutant cell lines have been used to elucidate pathways and mechanisms involving the aryl hydrocarbon receptor (AhR). However, there has been little focus on other biological processes which may differ between the isolated lines. In this study, mouse cDNA microarrays representing 4858 genes were used to examine differences in basal gene expression between mouse Hepa-1c1c7 wild-type and c1 (truncated Cyp1a1 protein), c4 (AhR nuclear translocator, ARNT, deficient), and c12 (low AhR levels) mutant cell lines. Surprisingly, c1 mutants exhibited the greatest number of gene expression changes compared to wild-type cells, followed by c4 and c12 lines, respectively. Differences in basal gene expression were consistent with cell line specific variations in morphology, mitochondrial activity, and proliferation rate. MTT and direct cell count assays indicate both c4 and c12 mutants exhibit increased proliferative activity when compared to wild-type cells, while the c1 mutants exhibited decreased activity. This study further characterizes Hepa-1c1c7 wild-type and mutant cells and identifies significant differences in biological processes that should be considered when conducting comparative mechanistic studies with these lines.

Identification of the Tryptophan Photoproduct 6-Formylindolo[3,2-b]carbazole, in Cell Culture Medium, as a Factor That Controls the Background Aryl Hydrocarbon Receptor Activity

The presence of high affinity ligands for the aryl hydrocarbon receptor (AhR) in cell culture medium has generally been overlooked. Such compounds may confound mechanistic studies of the important AhR regulatory network. Numerous reports have described that light exposed cell culture medium induces AhR-dependent activity. In this study, we aimed at identifying the causative substance(s). A three-dimensional factorial design was used to study how the background activity of CYP1A1 in a rat hepatoma cell line (MH1C1) was controlled by photoproducts formed in the medium exposed to normal laboratory light. The light induced activity was found to be tryptophan dependent, but independent of riboflavin and other components in the medium. The light exposed medium showed the same transient enzyme inducing activity in vitro as the AhR ligand 6-formylindolo[3,2-b]carbazole (FICZ). This substance, which we have previously identified as being formed in UV-exposed tryptophan solutions, is a substrate for CYP1A1 and it has a higher AhR binding affinity than TCDD. Several tryptophan related photoproducts were detected in the light-exposed medium. For the first time one of the formed photoproducts was identified as FICZ with bioassay driven fractionation coupled with HPLC/MS. These results clearly show that tryptophan derived AhR ligands, which have been suggested to be endogenous AhR ligands, influence the background levels of CYP1A1 activity in cells in culture.

A Microtiterplate-Based Screening Assay to Assess Diverse Effects on Cytochrome P450 Enzyme Activities in Primary Rat Hepatocytes by Various Compounds

During the development of potential drugs it is useful to identify pharmacological and/or toxicological side effects of a compound as early as possible in order to exclude them from further development for reasons of time and cost. Activation or inactivation of members of the cytochrome P450-dependent monooxygenase system (CYP450) might indicate potential undesired effects of a given compound. However, results using CYP450 assay systems are often inconsistent because of different experimental settings. Therefore, it was the goal of the present study to optimize the CYP450 assay in primary rat hepatocytes with respect to the time point of addition of and duration of exposure to alpha-naphthoflavone (ANF) and beta-naphthoflavone (BNF) as well as trans-resveratrol (RES), which have well-described stimulatory and inhibitory effects on CYP450 enzymes of the 1A and 2B family, respectively. Hepatocytes were also treated with putative lipoxygenase (LOX)/cyclooxygenase (COX) inhibitors with unknown impact on CYP450 enzyme activity in order to detect potential side effects. Cells were cultured for up to 7 days on 96-well microtiter plates, and enzyme activity was determined by a conventional fluorescence spectroscopy assay. ANF and BNF, given to the cells after 4 days of culture, stimulated CYP1A and 2B activities significantly in a concentration-dependent fashion after long-term exposure for at least 1 day. However, during short-term exposure for 1-6 h, CYP1A activity was inhibited, while CYP2B was increased weakly by ANF but not BNF. RES inhibited CYP1A activity during short- and long-term exposure without affecting CYP2B activity. From the results it was concluded that primary rat hepatocytes should be cultured for at least 3-4 days but no longer prior to the assay. The assay should be performed at two different time points of exposure, i.e., 6 h for short-term and 24 h for long-term exposure. The compounds under investigation should be applied at two different concentrations, e.g., at one time and 10 times higher concentrations, which should be oriented to the ED50, provided it is known for the respective substance. Under these assay conditions the LOX/COX inhibitors tested activated CYP1A enzyme activity in long-term but instead inhibited it in short-term experiments. CYP2B activity was stimulated during short- and long-term exposure. These results indicated drug side effects recommending exclusion of the compounds from the drug developmental process. Hence, in order to assess the pharmacological potential of novel compounds it is adequate to perform both short- and long-term experiments to concisely describe the effect of a compound on the CYP450 system.

Induction of cytochrome P4501A1 by autoclavable culture medium change in HepG2 cells

1. Without the addition of xenobiotics, only by changing the culture medium can one induce extensively and transiently cytochrome P4501A1 (CYP1A1) protein and mRNA in human hepatoma HepG2 cells. The induction was aryl hydrocarbon receptor (AhR)-dependent, and was proven by: (1) the medium change activated the AhR, as judged by a electrophoretic mobility shift assay; and (2) the AhR inhibitor alpha-naphthoflavone inhibited the medium change-mediated induction. 2. Induction of CYP1A1 was related to medium prepared by autoclaving. By screening the ingredients in the medium, the serum had no effect on CYP1A1 induction, whereas both photo-oxidized and autoclaved tryptophan were shown to induce CYP1A1, as indicated by CYP1A1 protein or ethoxyresorufin-O-deethylase activity. The autoclaved tryptophan contained in an autoclavable medium was a more potent inducer of CYP1A1 than photo-oxidized tryptophan. 3. The results provide some practical suggestions with experiments related to CYP1A1.

REGULATION OF CYTOCHROME P450 IN A PRIMARY CULTURE OF RAINBOW TROUT HEPATOCYTES

Primary cultures of fish hepatocytes have been used as a convenient model for studies on cytochrome expression. Here we have further examined the regulation of CYP enzymes in this model. A transient increase in CYP1A1 messenger ribonucleic acid (mRNA) and 7-ethoxyresorufin-O-deethylase (EROD) activity occurred within h after medium change. This event implies that either an exogenous, quickly metabolized CYP1A1 inducer was introduced to the hepatocytes with the fresh medium, or that the mechanical act of changing the medium disrupts the cell homeostasis, which in turn activates CYP1A1 transcription or alternatively stabilizes CYP1A1 mRNA. CYP1A1 has been shown to be highly inducible in primary cultures of rainbow trout hepatocytes by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via an aryl hydrocarbon (Ah) receptor-mediated activation of gene transcription. In the present study, CYP1A1 was strongly induced by TCDD, whereas CYP2K1, a constitutively expressed cytochrome P450 (CYP), was refractory to the same treatment. Cycloheximide efficiently blocked protein synthesis in the cell culture, and thus the apparent half-life of CYP1A1 (measured as EROD activity) could be estimated. In cells treated with TCDD for 24 h the CYP1A1 apparent half-life was estimated to be 15.9 h. When ethoxycoumarin-O-deethylase activity was used as an indicator of CYP levels, a considerably longer half-life of 27.1 h was estimated. The level of CYP2K1 remained constant throughout the study and was not sensitive to cycloheximide exposure (30 h), indicating a considerably longer half-life of this protein in cell culture.

Regulation of CYP1A1 transcription via the metabolism of the tryptophan-derived 6-formylindolo[3,2-b]carbazole

A functional cytochrome P4501A1 (CYP1A1) enzyme has been suggested to metabolize endogenous substrates and to autoregulate its own transcription in mouse hepatoma cells. In the present study, the regulation of CYP1A1 gene transcription by 6-formylindolo[3,2-b]carbazole (FICZ), a suggested endogenous ligand for the aryl hydrocarbon receptor (AhR), has been studied in mouse Hepa-1 cell lines. The tryptophan photoproduct, FICZ, has previously been characterized to possess very high AhR binding affinity and to transiently induce CYP1A1 gene expression in cultured cells at picomolar concentrations. The results from this study show that a transient induction of CYP1A1 mRNA at a low concentration of FICZ was only seen in wild-type cells. In c37 cells, deficient in CYP1A1, FICZ caused a sustained induction. Interestingly, we found that a higher amount of tryptophan in culture medium increased the constitutive level of CYP1A1 mRNA expression in the c37 cells but not in the wild-type cells. This suggests that a tryptophan-derived AhR ligand in the medium regulates the basal CYP1A1 expression. In metabolism studies performed with S9 prepared from c37 cells no metabolites were formed from FICZ and no loss of FICZ was observed, while with wild-type cells FICZ was rapidly metabolized. HPLC analysis revealed that at least three metabolites were formed in an NADPH-dependent manner from FICZ when incubated with rat liver S9. The CYP1A1 inhibitor ellipticine totally blocked the metabolism of FICZ. Ellipticine also enhanced both basal and FICZ-induced CYP1A1 mRNA expression. Taken together, these results indicate that tryptophan is a precursor of the endogenous ligand and that the suggested tryptophan-derived ligand FICZ is a substrate for the CYP1A1 enzyme and is involved in autoregulation of CYP1A1 transcription.

Transient induction of 7-ethoxyresorufin-O-deethylase (EROD) activity by medium change in the rainbow trout liver cell line, RTL-W1

Cytochrome P4501A (CYP1A) metabolizes a wide array of lipophilic xenobiotics. In fish liver, CYP1A is constitutively expressed at low levels, but xenobiotics can strongly induce CYP1A expression via a receptor-mediated pathway. While induction of hepatic CYP1A in teleosts by xenobiotics is well investigated, very little is known on the regulation of constitutive CYP1A expression and its induction by factors other than xenobiotics. In the present study we show that in the rainbow trout liver cell line, RTL-W1, CYP1A-catalyzed 7-ethoxyresorufin-O-deethylase (EROD) activity can be induced by a change of the culture medium, in the absence of xenobiotics. The increase in cellular EROD levels is of transient nature. Experiments with cell incubation solutions supplemented with various medium components indicate that photooxidized tryptophan is the agent causing the increase of EROD activity after medium change.

UV-induced CYP1A1 gene expression in human cells is mediated by tryptophan

Induction of cytochrome P-4501A1 (CYP1A1) activity by UV has been observed earlier in animal studies via a mechanism that has not yet been resolved. Our previous data have indicated that formylated indolocarbazoles which are formed by UV irradiation of tryptophan solutions are very potent Ah-receptor agonists. To evaluate the effect of UV light on cytochrome P4501A1 gene expression, we studied the induction of CYP1A1 mRNA by UV irradiation of cultured human keratinocytes (HaCaT cell line), primary human blood lymphocytes and mouse Hepa-1 cells. The cells were exposed to UV light delivered by a bank of 6 Philips TL20/12RS sun lamps emitting primarily in the UVB range in the absence and presence of tryptophan. A semiquantitative reverse transcriptase-linked polymerase chain reaction (RT-PCR) was used for analysis of gene expression in the treated cells. The results show that the CYP1A1 mRNA level induced by UV in the presence of tryptophan was higher than that induced by UV alone in both HaCaT cells and lymphocytes after 3 h of incubation post-UV irradiation. To find out if the induction by UV light is caused by the formation of an Ah receptor ligand, Hepa-1 wild-type and Ah receptor deficient c12 cell lines were applied. Wild-type (wt) cells were inducible either by the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole (FICZ) or by UV-irradiation but very low or undetectable levels were observed in the c12 cells. This shows that the induction of gene expression by FICZ and UV is Ah receptor dependent. Together, these results indicate that UV-induced CYP1A1 gene expression in mammalian cells is mediated by an Ah receptor ligand formed from tryptophan. Thus, the photoproducts of tryptophan are suggested to be mediators of light via binding to the Ah receptor and as such also could have a role in light-regulated biological rhythms.

Species differences in hepatocyte induction of CYP1A1 and CYP1A2 by omeprazole

Omeprazole, a proton pump inhibitor therapeutically administered for the treatment of gastric ulcers, induces the expression of cytochromes P4501A1/2 (CYP1A1/2) through transcriptional activation mediated by the Ah-dioxin)-receptor. Primary cultures of hepatocytes isolated from rabbit, rat, mouse and human livers were compared for CYP1A1/2 mRNA inducibility by omeprazole (1 to 100 microM). Primary cultures of human hepatocytes were the most sensitive to the inducing effects of omeprazole. Rabbit hepatocytes were the only other cells studied that showed induced CYP1A1/2 mRNA expression from a concentration lower than 100 microM (i.e., 10 microM). Rat hepatocytes were the least sensitive to omeprazole induction. The response of mouse hepatocytes to omeprazole treatment was variable, with CYP1A1/2 mRNA expression being induced in only two of the three cultures examined. Differences in the time dependence of CYP1A1/2 mRNA expression were observed between species. In general, after treatment of hepatocytes with omeprazole the levels of CYP1A1 mRNA peaked prior to that of CYP1A2 mRNA. Due to the interspecific variability of CYP1A mRNA inducibility by omeprazole, we conclude that human hepatocytes in culture are probably the only appropriate animal model for prediction of CYP1A induction in humans.

Disruption of endogenous regulator homeostasis underlies the mechanism of rat CYP1A1 mRNA induction by metyrapone

The transcriptional induction of the cytochrome P-450 1A1 (CYP1A1) gene by xenobiotics such as polyaromatic hydrocarbons is dependent on their interaction with the aryl hydrocarbon receptor. Administration of the structurally unrelated compounds metyrapone (a cytochrome P-450 inhibitor) or dexamethasone (a glucocorticoid) to male rats does not induce hepatic CYP1A1 mRNA. However, administration of both metyrapone and dexamethasone to male rats results in the induction of hepatic CYP1A1 mRNA expression. The induction response is mimicked in vitro in cultured rat hepatocytes by the addition of metyrapone and dexamethasone to a serum-free culture medium, suggesting that these compounds act directly on the liver in vivo to effect hepatic CYP1A1 mRNA induction. An examination of the characteristics of CYP1A1 induction by metyrapone and dexamethasone in combination in vitro indicate that at least 6 h of treatment is required for detectable levels of CYP1A1 mRNA to accumulate in hepatocytes. In contrast, beta-naphthoflavone, which is known to bind to the aryl hydrocarbon receptor to effect CYP1A1 gene expression, induces detectable levels of CYP1A1 mRNA within 2 h of treatment. CYP1A1 mRNA is also induced when hepatocytes are treated with metyrapone in combination with the protein synthesis inhibitor cycloheximide but not with dexamethasone in combination with cycloheximide, indicating that CYP1A1 mRNA induction is strictly dependent on the presence of metyrapone and suggesting that the metyrapone-associated induction of CYP1A1 mRNA is dependent on a loss of a constitutively expressed protein that functions to suppress CYP1A1 gene expression. The role of dexamethasone in metyrapone-associated induction of CYP1A1 is probably mediated through the glucocorticoid receptor since the glucocorticoid receptor antagonist RU486 reduces the levels of CYP1A1 mRNA induced by metyrapone and dexamethasone in combination. Increasing the levels of the photosensitizer riboflavin present in the culture medium 10-fold and exposure to light increases the levels of CYP1A1 mRNA induced by metyrapone and dexamethasone in combination in vitro, suggesting that photoactivation of inducing medium constituent(s) might be required for induction. Failure to induce CYP1A1 mRNA by co-administration of metyrapone and dexamethasone in hepatocytes cultured in a balanced salt solution with or without photoactivation indicates that induction is dependent on a photoactivated component of the culture medium and not on metyrapone or dexamethasone alone. The addition of tryptophan in the presence of riboflavin to the balanced salt solution restores CYP1A1 mRNA induction by metyrapone alone and induction is increased when medium is exposed to light, indicating that induction is dependent on tryptophan photoactivation in vitro. Metyrapone failed to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for specific binding to the aryl hydrocarbon receptor in rat liver cytosolic fractions. These results suggest that CYP1A1 might be induced in rats by metyrapone through an indirect mechanism associated with an elevation in the level of an endogenously generated inducer such as photoactivated product(s) of tryptophan and not because of metyrapone's interacting with the aryl hydrocarbon receptor. The dependence of CYP1A1 induction on dexamethasone or cycloheximide suggests that derepression by a glucocorticoid receptor-modulated negative-acting factor of CYP1A1 gene expression might be critical to induction by metyrapone.

Rapid and transient induction of CYP1A1 gene expression in human cells by the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole

Studies to assess the induction of CYP1A1 gene expression by tryptophan derived oxidation products which are suggested as endogenous ligands for the Ah receptor are described. For the two high affinity Ah receptor ligands produced from tryptophan, the chemical structure was recently identified as 6-formylindolo[3,2-b]carbazole (FICZ) and 6,12-diformylindolo[3,2-b]carbazole (dFICZ), respectively. Therefore these two compounds show a close similarity to the indolecarbinol-derived condensation product indolo[3,2-b]carbazole (ICZ). Incubation of cells from a human keratinocyte (HaCaT) cell line together with ICZ, FICZ, dFICZ and some structurally related indole compounds was performed. The compound with the highest affinity to the Ah receptor, FICZ, was found to be the most efficient inducer of CYP1A1 gene expression in short time incubation (0.5 h) experiments. With longer incubation times (24 h) ICZ was the most efficient inducer. The two most active compounds, FICZ and ICZ, caused increased mRNA levels already at a concentration of 100 pM. FICZ was also shown to increase CYP1A1 mRNA levels in fresh human peripheral blood cells at the same low concentration. FICZ and ICZ were furthermore compared with regard to their capacity to inhibit cDNA-expressed human CYP1A1 enzyme and FICZ was found to be the most potent inhibitor. The inhibition was, however, transient in character indicating that FICZ is also an exceptionally good substrate for the CYP1A1 enzyme. The results showing the potent and transient effect of these formylindolocarbazoles, thus emphasize their important properties as signal substances in the Ah receptor pathway. This makes the most potent compound, FICZ, a good candidate for the endogenous ligand of the Ah receptor necessary for normal development and for the basal expression of Ah receptor-dependent genes.

NADPH-cytochrome P-450 reductase, cytochrome P-450 2C11 and P-450 1A1, and the aryl hydrocarbon receptor in livers of rats fed methyl-folate-deficient diets

We investigated three hepatic cytochrome P-450 isozymes and the aryl hydrocarbon (Ah) receptor in rats fed one of the following three diets for 15 months: a diet containing the AIN vitamin mixture (control), the control diet devoid of choline and folate (CFD), or the CFD diet devoid of niacin (CFND). Hepatic tumors developed in all CFD- and CFND-fed rats. Western blot analyses of nontumor hepatic tissue showed that NADPH-cytochrome P-450 reductase (P-450 reductase) increased significantly in the CFD and CFND groups compared with the control group. Hepatic cytochrome P-450 2C11 (CYP2C11) was not detectable in the CFD and CFND groups compared with the control group. Ah receptor and cytochrome P-450 1A1 (CYP1A1) were detected in higher amounts in livers of both deficient groups. CYP1A1 is an enzyme associated with bioactivation of exogenous genotoxins. To our knowledge, this is the first time it has been shown that CYP1A1 and the Ah receptor are induced by dietary deficiencies.

Induction of cytochrome P4501A1 by photooxidized tryptophan in Hepa lclc7 cells

Mouse hepatoma Hepa-lclc7 (Hepa-1) cells were cultivated in the presence of UV-irradiated amino acids. The results demonstrated that all of the amino acids tested, UV-oxidized tryptophan caused the highest induction of 7-ethoxyresorufin O-deethylase (EROD) activity compared with the controls (P < 0.01). The induction of EROD activity by oxidized tryptophan was dose dependent, and maximal induction was obtained at 12 hr after administration. Studies with various Hepa-1 mutants, which are defective in either the aryl hydrocarbon (Ah) receptor or Ah receptor nuclear translocator protein, indicated that the induction of EROD activity by oxidized tryptophan occurs through the Ah receptor. Gel mobility shift assays using nuclear extracts of Hepa-1 cells revealed that oxidized products of tryptophan can induce both Ah receptor transformation and binding of the liganded Ah receptor complex to its specific DNA recognition site. CYP1A1 mRNA, quantified by reverse transcription-polymerase chain reaction, and CYP1A1 protein were induced markedly in the oxidized tryptophan group compared with the controls. Injection of isolated oxidized tryptophan products into adult male rats caused significant induction of EROD activity in the pulmonary and hepatic microsomes compared with the controls (P < 0.01). These results demonstrated that oxidized tryptophan induces Ah receptor activation and binding of the liganded Ah receptor complex to its specific DNA recognition site, thereby initiating transcription and translation of the CYP1A1 gene with concomitant increase of EROD activity in Hepa-1 cells. Induction of EROD activity in the liver and lungs after injection of isolated oxidized tryptophan products into rats suggests that a similar mechanism may be operative in vivo.

Structure elucidation of two tryptophan-derived, high affinity Ah receptor ligands

BACKGROUND

Environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other structurally related 'environmental hormones', exert their harmful biological effects through the Ah receptor signaling pathway. Several naturally occurring substances also bind to this receptor, but its natural role is still obscure. Tryptophan derivatives of the indolo[3,2-b]carbazole type, earlier suggested by us to be endogenous ligands for the receptor, should be a powerful tool in understanding receptor function. We therefore set out to determine their identity.

RESULTS

The two tryptophan-derived Ah receptor ligands have been chemically analyzed and characterized by means of mass spectrometry, 1H NMR and 13C NMR. UV, infra-red and fluorescence spectra were also recorded. All data are in accordance with the two compounds being closely related indolo[3,2-b]carbazole derivatives. Evidence is presented that compound A (MW = 312) is the symmetrical 6,12-diformylindolo[3,2-b]carbazole, and compound B (MW = 284) is the monosubstituted 6-formylindolo[3,2-b]carbazole.

CONCLUSIONS

The elucidation of the structures of the two high affinity Ah receptor ligands 6,12-diformylindolo[3,2-b]carbazole and 6-formylindolo[3,2-b]carbazole provides the necessary basis for further mechanistic studies of this important group of compounds, and will help in determining the natural role of the Ah receptor.