Host–microbiome interactions: the aryl hydrocarbon receptor as a critical node in tryptophan metabolites to brain signaling

ABSTRACT

Tryptophan (Trp) is not only a nutrient enhancer but also has systemic effects. Trp metabolites signaling through the well-known aryl hydrocarbon receptor (AhR) constitute the interface of microbiome-gut-brain axis. However, the pathway through which Trp metabolites affect central nervous system (CNS) function have not been fully elucidated. AhR participates in a broad variety of physiological and pathological processes that also highly relevant to intestinal homeostasis and CNS diseases. Via the AhR-dependent mechanism, Trp metabolites connect bidirectional signaling between the gut microbiome and the brain, mediated via immune, metabolic, and neural (vagal) signaling mechanisms, with downstream effects on behavior and CNS function. These findings shed light on the complex Trp regulation of microbiome-gut-brain axis and add another facet to our understanding that dietary Trp is expected to be a promising noninvasive approach for alleviating systemic diseases.     

In vitro neuroendocrine effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the AhR-expressing hypothalamic rat GnV-3 cell line

The aryl hydrocarbon receptor (AhR) is involved in a wide variety of biological and toxicological responses, including neuroendocrine signaling. Due to the complexity of neuroendocrine pathways in e.g. the hypothalamus and pituitary, there are limited in vitro models available despite the strong demand for such systems to study and predict neuroendocrine effects of chemicals. In this study, the applicability of the AhR-expressing rat hypothalamic GnV-3 cell line was investigated as a novel model to screen for neuroendocrine effects of AhR ligands using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as reference compound. The qRT-PCR analyses demonstrated the presence of several sets of neurotransmitter receptors in the GnV-3 cells. TCDD (10 nM) altered neurotransmitter signaling by up-regulation of glutamate (Grik2), gamma-amino butyric acid (Gabra₂) and serotonin (Ht_2C) receptor mRNA levels. However, no significant changes in basal and serotonin-evoked intracellular Ca²⁺ concentration ([Ca²⁺]_i) or serotonin release were observed. On the other hand, TCDD de-regulated period circadian protein homolog 1 (Per1) and gonadotropin releasing hormone (Gnrh) mRNA levels within a 24-h time period. Both Per1 and Gnrh genes displayed a similar mRNA expression pattern in GnV-3 cells. Moreover, the involvement of AhR in TCDD-induced alteration of Neuropeptide Y (Npy) gene expression was found and confirmed by using siRNA targeted against Ahr in GnV-3 cells. Overall, the combined results demonstrate that GnV-3 cells may be a suitable model to predict some mechanisms of action and effects of AhR ligands in the hypothalamus.     

A novel MDCKII in vitro model for assessing ABCG2-drug interactions and regulation of ABCG2 transport activity in the caprine mammary gland by environmental pollutants and pesticides

The ABC efflux transporter ABCG2 represents the main route for active secretion of xenobiotics into milk. Thus, ABCG2 regulation by aryl hydrocarbon receptor (AhR) ligands including ubiquitously environmental pollutants is of great toxicological relevance. However, no adequate in vitro model is as yet available to study AhR-dependent ABCG2 regulation in dairy animals. In this study, we therefore systematically investigated the effect of various environmental contaminants and pesticides on ABCG2 efflux activity in MDCKII cells stably expressing mammary ABCG2 from dairy goats. The AhR-agonists TCDD, Aroclor 1254, prochloraz, and iprodione caused a dose- and time-dependent increase in EROD activity. Moreover, TCDD and prochloraz significantly stimulated ABCG2 transport activity through a dose- and time-dependent induction of transporter gene expression. AhR inhibitors like CH 223191 significantly reversed TCDD- and prochloraz-induced stimulation of ABCG2 efflux activity. In contrast, non-AhR activators such as PCB 101 had no significant effect on EROD activity, ABCG2 gene expression or transporter activity. As we identified various anthelmintics including monepantel as potential ABCG2 substrates this regulatory mechanism may result in increased milk residues of potentially harmful xenobiotics. Thus, MDCKII-cABCG2 cells may represent a suitable in vitro model to study mammary ABCG2 secretory activity and its potential regulation by AhR-activating contaminants.     

2,3,7,8-Tetrachlorodibenzo-P-Dioxin Induced Cell-Specific Drug Transporters With Acquired Cisplatin Resistance in Cisplatin Sensitive Cancer Cells

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can induce drug transporter genes such as the ATP-binding cassette G member 2 (ABCG2), which contributes to multidrug resistance. We investigated the effect of TCDD pretreatment on drug transporters induction from cancer cells of various origins. Cell viabilities after treatment of cisplatin were measured to evaluate acquiring cisplatin resistance by TCDD. Acquring cisplatin resistance was found only in cisplatin senstivie cancer cells including gastric SNU601, colon LS180, brain CRT-MG and lymphoma Jurkat cells which showed a significant increase in cell viability after combined treatment with TCDD and cisplatin. High increase of ABCG2 gene expression was found in SNU601 and LS180 cells with a mild increase in the expression of the ABCC3, ABCC5,and SLC29A2 genes in SNU601 cells, and of major vault protein (MVP) in LS180 cells. The AhR inhibitor kaempferol suppressed the upregulation of ABCG2 expression and reversed the TCDD-induced increase in cell viability in LS180 cells. However, in CRT-MG cells, other transporter genes including ABCC1, ABCC5, ABCA3, ABCA2, ABCB4, ABCG1, and SLC29A1 were up-regulated. These findings suggested the acquiring cisplatin resistance by TCDD associated with cancer cell-type-specific induction of drug transporters.

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