Xenobiotic-induced activation of human aryl hydrocarbon receptor target genes inDrosophilais mediated by the epigenetic chromatin modifiers

Abstract

Aryl hydrocarbon receptor (AHR) is the key transcription factor that controls animal development and various adaptive processes. The AHR’s target genes are involved in biodegradation of endogenous and exogenous toxins, regulation of immune response, organogenesis, and neurogenesis. Ligand binding is important for the activation of the AHR signaling pathway. Invertebrate AHR homologs are activated by endogenous ligands whereas vertebrate AHR can be activated by both endogenous and exogenous ligands (xenobiotics). Several studies using mammalian cultured cells have demonstrated that transcription of the AHR target genes can be activated by exogenous AHR ligands, but little is known about the effects of AHR in a living organism. Here, we examined the effects of human AHR and its ligands using transgenic Drosophila lines with an inducible human AhR gene. We found that exogenous AHR ligands can increase as well as decrease the transcription levels of the AHR target genes, including genes that control proliferation, motility, polarization, and programmed cell death. This suggests that AHR activation may affect the expression of gene networks that could be critical for cancer progression and metastasis. Importantly, we found that AHR target genes are also controlled by the enzymes that modify chromatin structure, in particular components of the epigenetic Polycomb Repressive complexes 1 and 2. Since exogenous AHR ligands (alternatively – xenobiotics) and small molecule inhibitors of epigenetic modifiers are often used as pharmaceutical anticancer drugs, our findings may have significant implications in designing new combinations of therapeutic treatments for oncological diseases.

NAP Family CG5017 Chaperone Pleiotropically Regulates Human AHR Target Genes Expression in Drosophila Testis

“…We have previously demonstrated that Drosophila may serve as a valid model organism to investigate the complex effects of xenobiotics on human AHR functioning in vivo [32]. In order to investigate the effects of xenobiotics on male fecundity, we used Drosophila males carrying inducible human AhR (the UAS-AhR construct is described in Reference [32]) and Tj-GAL4 drivers. In Drosophila testes, the Tj-GAL4 driver activates UAS -constructs in cells which are in connection with generative cells.…”

“…To assess the ability of xenobiotics to influence the expression of human AHR target genes in Drosophila testes, we first identified potential human AHR target genes in Drosophila (described in Reference [32]). We selected several putative Drosophila homologs of human AHR targets genes containing XRE -elements in their regulatory regions: Mannosyl (α-1,3-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase 1 ( Mgat1 ), which participates in the determination of adult lifespan relating to mushroom body development; Glutathione S transferase T4 ( GstT4 ), which is involved in oxidation-reduction processes and catalyzes reactions of biotransformation; Cytochrome P450 6g1 ( Cyp6g1 ), which is involved in the oxidation-reduction process, response to DDT, and the insecticide catabolic process; N-acetylneuraminic acid synthase ( Nans ), which participates in the carbohydrate biosynthetic process; Relish ( Rel ), which encodes the NF-κB subunit; p53 , which is a transcriptional factor required for adaptive responses to genotoxic stresses, including cell death, compensatory proliferation and DNA repair; Myc, a transcription factor related to proto-oncogenes, which contributes to cell growth, cell competition, and regenerative proliferation; dаcapo ( dap ), which encodes the Cyclin-dependent kinase inhibitor; the Retinoblastoma-family protein ( Rbf ), which provides negative regulation of the G1/S transition of mitotic cell cycles; Jun-related antigen ( Jra ), which is involved in positive regulation of the metabolic process, humoral immune response, aging, and RNA polymerase II transcription factor activity; and Dcdc42 ( Cdc42 ), which is a key regulator of the actin cytoskeleton, playing a central role in actin cytoskeleton organization, morphogenesis, hemocyte migration, cell polarity, and wound repair.…”

“…We decided to use Drosophila as a verified model system to investigate the in vivo effects of human AHR ligands (xenobiotics) during development. In previous experiments using UAS-AhR/GAL4-driver flies, we have demonstrated that AHR activation could both increase and decrease transcription of AHR target genes in different tissues, and found that this effect depends on the developmental stage of the animal [32]. It is important to note that the effect of xenobiotics on the levels of AHR target gene activity was more clearly manifested in organs with a high number of proliferating cells.…”

“…In adult organs, in which cell proliferation is complete, the actions of ligands on AHR target gene transcription levels were not detected. We found that the ligand’s effect on AHR target gene expression is mediated by the polycomb group (PcG) epigenetic chromatin regulators [32]. A similar xenobiotic effect may be expected in humans.…”

“…Cell culture experiments do not provide a full understanding of the effects induced by AHR ectopic expression on the development of living organisms. For a better understanding of the function of human AHR in vivo, we created “humanized” Drosophila transgenic flies, which carry transgenes with the controlled expression of the human AhR gene guided by the yeast upstream activation sequence ( UAS ) [32]. This transgenic construct permits the induction of human AHR expression in certain Drosophila organs and cells with the help of tissue-specific GAL4 -driver lines expressing a yeast GAL4 activator capable of recognizing UAS sequences and driving the transcription of downstream genes [33].…”