Modulation of Aryl Hydrocarbon Receptor (AHR)-Regulated Metabolic Enzymes by Arsenic Trioxide and its Thioarsenical Metabolite
Creators: Elghiaty, Mahmoud AA
Publisher: name: University of Alberta Library
Year: 2024

<p>Arsenic trioxide (ATOIII) has evolved into a successful therapy for acute promyelocytic leukemia (APL), however, being an arsenical, it retains at its core the hypertoxic characteristics of that notorious metalloid. Such inherent toxicity causes a constellation of complications in APL patients, including hepatotoxicity, and may also limit expanding clinical ATOIII applications. In humans, ATOIII undergoes progressive metabolism into diverse methylated intermediates/products, which can, at least partly, be responsible for the overall toxic outcome of ATOIII. The thio-methylated MMMTAV is one of these metabolites that have been identified in ATOIII-treated APL patients. Arsenicals are well-known modulators of cytochrome P450 (CYP) enzymes, most notably the aryl hydrocarbon receptor (AHR)-regulated CYP1A1/1A2. These enzymes are pivotal for metabolism of endobiotics and commercial drugs, therefore, modifying their activity can entail a disease state or clinical drug-drug interactions, respectively. Additionally, altering CYP1A1/1A2 can aggravate environmental toxicants impact through their activation or diminished elimination. Therefore, this work aimed to determine the possible effects of ATOIII and MMMTAV on constitutive and TCDD-inducible levels of hepatic CYP1A1/1A2 using in vivo and in vitro models. We also aimed to investigate in vivo effects of ATOIII on hepatic non-AHR-regulated CYP enzymes with the associated perturbations in arachidonic acid (AA) metabolism. Finally, we explored in vivo effects of ATOIII on drug transporters. For this purpose, C57BL/6 mice were intraperitoneally injected with 8 mg/kg ATOIII (or 12.5 mg/kg MMMTAV) with or without 15 μg/kg TCDD for 6 and 24 h. Furthermore, HepG2 and Hepa1c1c7 cell lines were treated with varying concentrations of ATOIII (or MMMTAV) with or without 1 nM TCDD for 6 and 24 h. In C57BL/6 mice, ATOIII inhibited TCDD-mediated induction of hepatic CYP1A1/1A2 mRNA, proteins, and activities. Significant reductions in CYP1A2, but not CYP1A1, protein and activity were observed at basal levels. In HepG2 cells, a similar inhibitory effect was observed in inducible CYP1A1/1A2 at all expression levels. Such inhibition was transcriptionally regulated by interfering with AHR-mediated activation of the xenobiotic response element (XRE). Also, post-translational modification through up-regulated heme oxygenase 1 (HMOX1) might be, at least partially, implicated in reducing enzyme activity. In Hepa1c1c7 cells, TCDD-inducible CYP1A1/1A2 expressions were increased at all expression levels, while only basal levels of mRNA transcripts, in addition to protein in case of CYP1A2, were up-regulated. Such contradiction with the in vivo murine model was attributed to transcriptional regulation related to AHR nuclear accumulation as well as XRE activation. Moreover, post-transcriptional and post-translational mechanisms caused increased production and decreased degradation of CYP gene products. MMMTAV ultimately increased CYP1A1/1A2 inducible activities in C57BL/6 mice. The same pattern of effect was also observed in Hepa1c1c7 cells. At early stage of CYP1A1 gene expression, i.e., mRNA transcripts formation, a significant decrease was obtained in both models. Such effect was only explained by transcriptional regulation mediated by interference with AHR-mediated XRE activation. As opposed to these murine models, the effect on inducible CYP1A1 in HepG2 cells was consistently inhibitory across all expression levels. Similarly, this inhibition was attributed to transcriptional regulation at the level of XRE activation. We also showed that in vivo alteration of CYP enzymes by ATOIII goes beyond AHR-regulated CYP1A subfamily to include other CYP families (CYP2, CYP3, and CYP4). ATOIII altered hepatic AA metabolism in C57BL/6 mice through modulating the underlying network of enzymes. Such modulation impacts AA biotransformation pathway from its beginning at the step of AA liberation and extends through the eicosanoids (e.g., hydroxyeicosatetraenoic acids (HETEs))-generating cyclooxygenases, lipoxygenases, and CYP enzymes routes. ATOIII suppressed Cyp2e1, while induced Cyp2j9 and most of Cyp4a and Cyp4f, causing 17(S)-HETE and 18(R)-HETE increase, and 18(S)-HETE decrease. ATOIII also induced Cyp4a10, Cyp4a14, Cyp4f13, Cyp4f16, and Cyp4f18, causing 20-HETE elevation. In conclusion, modulating CYP1A enzymes by ATOIII and MMMTAV implies their possible involvement in clearance-related consequences for the substrates of these enzymes such as interactions with co-administered drugs, like granisetron which is widely used anti-emetic for chemotherapy-induced nausea and vomiting, or suboptimal environmental toxicants elimination. Also, Modifying the homeostatic production of bioactive AA metabolites can entail toxic events that compromise overall body tolerability to ATOIII treatment. Additionally, this situation may be aggravated by unfavorable changes in the expression of ATOIII transport systems, such as increased expression of influx aquaporin channels, with ensuing enhancement of its toxicity.</p>