The Aryl hydrocarbon Receptor (AhR) is a cytosolic ligand-activated transcription factor, originally characterized as an important player of detoxification pathways for xenobiotics and environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs) and halogenated aromatic hydrocarbons (HAHs). To date, more than 400 exogenous ligands have been identified including 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), the most powerful AhR agonist [1
]. As a result of ligand binding, AhR dissociates from a chaperone complex (Heat Shock Protein 90, Prostaglandin E Synthase 3 (p23) and hepatitis B virus X-associated protein (XAP2), also known as aryl hydrocarbon receptor interacting protein (AIP), heterodimerizes with the Aryl Hydrocarbon Nuclear Translocator (ARNT), and translocates into the nucleus. The AhR/ARNT complex binds xenobiotic or dioxin-responsive elements (XRE or DRE) regions, which are enhancer DNA elements located in the 5′-flanking region of target genes, inducing their transcription. Among those, cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1B1 (CYP1B1) are almost totally dependent on AhR activity and are implicated in detoxification of xenobiotics. AhR is highly conserved across different species [2
] and constitutively expressed practically in all tissue, especially placenta, liver and lungs.
The discovery that the receptor is involved in immune responses [3
] and carcinogenesis [5
] led to an increasing interest towards AhR. In fact, recent studies have shown that AhR is overexpressed and/or constitutively active, even in the absence of environmental ligands, in several human tumors such as breast, liver, lung, gastric, pancreatic, prostate, urothelial, ovarian cancers, T-cell leukemia, glioma and medulloblastoma [6
] and that it can act as a promoter [8
] or a suppressor of cancers growth [9
]. Moreover, epidemiological and experimental animal data provide evidence for an association between AhR and cancer initiation and progression. In fact, exposure to PAHs led to the development of a variety of cancers, whose initiation appeared to be mediated by the receptor, in humans or mice [5
Furthermore, AhR is able to activate the human embryonic protein SNAI2, commonly known as SLUG [11
], a master repressor of E-cadherin transcription [12
], suggesting a pivotal role of the receptor in the induction and/or regulation of the epithelial to mesenchymal transition (EMT) [13
In gliomas, kynurenine, the product of the enzymatic reaction catalyzed by Indoleamine 2,3-Dioxygenase 1 (IDO1), Indoleamine 2,3-Dioxygenase 2 (IDO2) and Tryptophan 2,3-Dioxygenase (TDO), was shown to be an endogenous ligand of human AhR and to suppress antitumor immune responses and to promote tumor cell survival and motility through AhR in an autocrine/paracrine fashion [8
We previously demonstrated that IDO1 has an important role in thyroid carcinogenesis inducing an immunosuppressant tumor microenvironment [14
]. Moreover, other studies demonstrated that papillary thyroid carcinomas overexpressed AhR with higher intensity in BRAF mutated samples [15
To deeper investigate the role of the IDO1-Kynurenine-AhR pathway in thyroid cancer pathogenesis, we analyzed AhR expression in human-derived thyroid cancers and in thyroid tumors from BRAF-transgenic mice. Furthermore, we evaluated the AhR-mediated transcriptional and functional effects of kynurenine in two human thyroid cancer cell lines. The obtained results suggest that AhR is implicated in thyroid cancer initiation and progression both promoting the establishment of an immunosuppressive tumor microenvironment and EMT.
We have previously demonstrated that IDO1 is up-regulated in thyroid cancers and contributes to the creation of an immunosuppressant environment [14
]. The enzymatic cleavage of tryptophan induced by IDO1 produces kynurenine that was identified as an endogenous ligand of AhR [8
]. To more deeply investigate the role of IDO1-kynurenine-AhR in thyroid cancer, we analyzed AhR expression in a collection of tissues (90 PTCs, 11 MTCs and 6 ATCs) as mRNA or, in a subgroup of 41 PTCs, as protein. Surprisingly, all analyzed tumor samples showed higher expression of AhR than normal thyroid at the mRNA level. Interestingly, PTC showed higher levels than MTC or ATC. This finding might be related to the fact that expression of AhR is correlated with the levels of differentiation of the tumors, dropping where differentiation gets lower. However, a sample size bias (90 PTC vs. 6 ATC or 11 MTC) may also account for it. Furthermore, those data were confirmed at the protein level too. Thus, the finding indicated an important role of the receptor in thyroid carcinogenesis. Unfortunately, we could not detect a statistically significant correlation between mRNA levels and IHC scores. This discrepancy was probably due to the heterogeneous expression of AhR observed in the majority of analyzed samples or to the low number of cases submitted to IHC. It is possible that by increasing the number of cases, a correlation could be found. Interestingly, AhR staining was more intense in cancer infiltrating cells, suggesting a role of the receptor in the invasion processes. As previously reported by other authors [15
], we found a significant association between BRAF mutation and higher levels of AhR expression. No other significant association with the considered clinico-pathological features could be found.
To investigate if AhR overexpression in thyroid cancers was associated with the activation of its transcriptional activity, we evaluated the expression levels of AhR target gene CYP1B1. The results confirmed that in thyroid neoplasms the overexpression of the receptor was associated with a significant activation of its transcriptional activity. It is possible to speculate that in these cancers the expression of IDO1 and the synthesis of kynurenine might contribute to activate the receptor. As a confirmation, IHC staining for AhR showed stained nuclei, index of AhR nuclear migration and transcriptional activation, especially in the regions of tumor invasion.
We analyzed AhR expression in thyroid tissues of three different mice models characterized by conditional expression of BRAFV600E in the thyroid. These models recapitulated different stages of thyroid tumorigenesis. One model, characterized by a thyroid dox-inducible expression of BRAFV600E in a P53-/- genetic background (TetOn-BRAF-P53), developed high penetrant and poorly differentiated thyroid tumors that were similar to human poorly differentiated thyroid cancer (PDTC) and ATC. Conversely, BRAFV600E knock-in mice (BRAF-Lox/TPO-Cre) generated PTC-like cancers. Finally, the model characterized by a thyroid dox-inducible expression of BRAFV600E in a WT genetic background (Tg-rtTA/tetO-BRAFV600E), was useful to investigate the first stages of thyroid transformation induced by BRAFV600E. Interestingly, AhR expression in the tumors of the mice showed the same characteristic of human cancers. Indeed, all the tumors, regardless of the histotype, presented higher AhR expression compared to normal thyroid and AhR expression was generally heterogeneous with an enhancement in infiltrative cells. The observation that seven days of BRAFV600E induction were sufficient to obtain an increased expression of AhR, confirmed the importance of BRAF mutation signaling in AhR regulation.
Evaluation of AhR mRNA expression in human thyroid carcinoma cell lines showed overexpression of the receptor in four to six. However, CYP1B1 mRNA was detectable only in FTC-133 and BcPap cells suggesting that only in these lines AhR was transcriptional active in basal condition. Immunocytochemistry confirmed the presence of the receptor in the cytoplasm of all the positive cell lines. Conversely, only FTC-133 cells revealed a clear AhR nuclear staining. Interestingly, FTC-133 cells expressed high levels of IDO1, and the presence of kynurenine could be detected in the conditioned medium even in basal conditions [14
]. Thus, it is possible to speculate that endogenously produced kynurenine might drive AhR activation in the absence of exogenous ligands.
The expression levels of CYP1A1 were also explored. Interestingly, they resulted undetectable both in thyroid cancer samples and in thyroid cancer-derived cell lines. This finding was consistent with the observation that CYP1A1 is prevalently involved in detoxification of carcinogens with a cancer preventive action [19
], whereas CYP1B1 expression is elevated in a wide range of human tumors [20
The significant increase of CYP1B1 expression induced by kynurenine treatment in FTC-133 and BcPap cell lines, confirmed the capability of kynurenine to activate AhR in these cell lines. Moreover, administration of CH223191, an inhibitor of AhR nuclear translocation, was efficiently able to prevent this increase, blocking the transcriptional effects induced by kynurenine on the receptor. After having validated the system, to better understand the functions of AhR in thyroid carcinogenesis, we evaluated the transcriptional effects of kynurenine-mediated activation of the receptor on genes involved in immune-tolerance and EMT. To confirm the specificity of AhR in the regulation of these genes, CH223191 was administrated in combination with kynurenine. We performed these experiments in FTC-133, characterized by high level of potentially active AhR, as demonstrated by the nuclear staining. Furthermore, the data were confirmed in the BcPap cell line, characterized by IDO1 overexpression, although kynurenine resulted undetectable in the basal conditioned medium [14
As expected, kynurenine induced an increase in IDO1 mRNA levels that was completely reversed by the co-administration of the AhR inhibitor. Conversely, kynurenine did not appear to strongly influence AhR expression whereas the blockage of nuclear translocation induced by the treatment with CH223191 appeared to increase the transcription levels of AhR, probably due to a negative feed-back of the activated receptor on its transcription that is lost after the addition of the inhibitor. Interestingly, this effect occurred in both cell lines also in the absence of exogenous ligand. However, it was more pronounced in FTC-133 cells, probably because this cell line is characterized by a more active IDO1-kynurenine-AhR axis.
The presence of AhR in the infiltrating cancer cells and the knowledge that SLUG is one of the target gene of AhR [11
], prompted us to investigate the effect of kynurenine in EMT. The obtained data confirmed that kynurenine induces SLUG expression through AhR. Indeed, the overexpression of SLUG was detected at both the mRNA and the protein level already after 6 h of treatment and remained high for all the duration of the experiment in both cell lines. Although cultured cells are not an excellent model to appreciate variations in adhesion molecules, following treatment with kynurenine, in FTC-133 cells, we observed a decrease in E-cadherine (epithelial marker) mRNA levels and a specular increase in mRNA levels of two markers of mesenchymal cells, namely N-cadherine and fibronectin-1. Similarly, BcPap cells showed increase in the mesenchymal markers (N-cadherine and fibronectin-1) but no detectable expression of E-cadherine, neither in basal condition or after kynurenine administration.
Altogether, the obtained data suggested that kynurenine-driven activation of AhR might play an oncogenic function not only through the induction of immune tolerance, but also through the initiation of EMT.
Kynurenine treatment induced an increase in OCT4 expression only in FTC-133 cells whereas in BcPap cells a significant difference with untreated cells could not be found. It is known that when AhR is activated by high affinity ligands, such as 2-(1′H
-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), binds to OCT4 promoter and inhibits the transcription of the stemness marker. However, the consumption of tryptophan and the accumulation of the low affinity ligand kynurenine in the tumor microenvironment, together with hypoxia, may cause an increase in OCT4 levels [21
]. It is possible that in FTC-133 cells, in which the presence of active IDO1 cause the consumption of tryptophan and the accumulation of kynurenine, AhR is not able to bind efficiently the promoter of the OCT4 gene, producing the observed increase when exogenous kynurenine is added to the medium. Further studies will be needed to clarify this interesting aspect.
Finally, using a wound-healing assay, we demonstrated that kynurenine-mediated AhR activation significantly increased the ability of FTC-133 cells to migrate and close the wound. The same effect could be detected in BcPap cells, but without reaching statistical significance. Moreover, in both cell lines, CH223191 reduced significantly the percentage of wound closure compared with parental cells, probably blocking endogenous AhR activity. The addition of kynurenine to CH223191 did not affect significantly the percentage of closure compared to cells treated with CH223191 alone. Kynurenine-induced AhR activation increased the invasiveness in BcPap cells that was abolished by CH223191 administration. In FTC-133 cell line this capability did not appear to be mediated by kynurenine. This different behavior was probably due to a greater expression of MMP1 in BcPap than in FTC-133. Moreover, BcPap cells harbor BRAFV600E mutation that is known to favor MMPs expression [22
Overall, these data suggested an involvement of kynurenine-induced AhR activation in conferring a more aggressive phenotype to thyroid cancer cells, by contributing to the onset of an immune-tolerant microenvironment and promoting cellular migration and invasiveness.
4. Materials and Methods
4.1. Tissue Samples and Patients
We examined surgical specimens of 90 PTCs, 11 MTCs, and six ATCs. All tissues were snap frozen at the time of surgery and stored at −80 °C until use. The presence of autoimmune chronic thyroiditis was assessed, looking for a positive antithyroperoxidase antibody title in patients’ medical charts and/or reviewing histological sections of surgical specimens. The study was approved by the local medical ethics committee (N. 23665/10/AV of 01/26/10). Each study participant provided written informed consent to the collection of fresh thyroid tissue for genetic studies. All the tumors were genetically characterized (PTCs: BRAF, RAS and RET/PTCs; ATCs: BRAF and RAS; MTCs: RET and RAS) and the results were previously published [14
4.2. Transgenic Mice Tissue Samples
AhR expression was evaluated by IHC in thyroid cancer samples derived from transgenic mice, characterized by conditional expression of BRAFV600E in the thyroid. We analyzed slides from 14 thyroid cancers, 4 normal thyroids and 2 lymph nodal metastases from 3 different mouse-models created in Dr. James Fagin’s laboratories (Memorial Sloan Kettering Cancer Center, New York) and kindly provided by Dr. Jeffrey Knauf (Memorial Sloan Kettering Cancer Center, New York) [17
In detail, AhR expression was analyzed in 6 thyroid tumors and 2 lymph node metastases from mice characterized by doxycycline (dox)-inducible BRAFV600E expression in the thyroid in a p53-/- background (TetOn-BRAF-P53). These mice developed high penetrant and poorly differentiated cancers [23
]. To induce BRAFV600E expression, 4 weeks old mice were treated with dox for 6 to 10 weeks and treatment was stopped at 0 to 7 weeks before sacrifice, as indicated in Table 1
We analyzed AhR expression in 6 thyroid tumors that carried BRAFV600E mutation in heterozygosis, and 3 normal thyroid samples (BRAFWT), derived from a BRAF thyroid-specific knock-in model (BRAF-Lox/TPO-Cre) [24
]. These mice developed papillary-like tumors, when BRAFV600E was present in heterozygosis. Mice were sacrificed at 10 to 36 weeks of age, as indicated in Table 2
Finally, we analyzed 2 thyroids of mice treated for 7 days with dox and one not treated with dox (control) coming from mice characterized by dox-inducible BRAFV600E expression in the thyroid in a p53 WT background (Tg-rtTA/tetO-BRAFV600E). This model made it possible to study the first stages of BRAFV600E-mediated transformation [25
4.3. Cell Cultures
The BcPap cell line (derived from a PTC) was acquired from DSMZ (DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen—German Collection of Microorganisms and Cell Cultures) and was grown in Roswell Park Memorial Institute (RPMI) 1640 medium supplemented with 10% fetal bovine serum (FBS; Life Technologies) [26
]. The TPC-1 cell line (derived from a PTC) was provided by Professor Alfredo Fusco (University of Naples, Naples, Italy) and was grown in Dulbecco’s Modified Eagle Medium (DMEM) medium supplemented with 10% FBS [27
]; authentication included detection of RET/PTC1 [28
]. The C643 cell line (derived from an ATC) was provided by Professor Niels Heldin (University of Uppsala, Uppsala, Sweden) and was grown in RPMI 1640 medium supplemented with 10% FBS [29
]. The 8505c cell line (derived from an ATC) was provided by Dr Carmelo Nucera (Harvard Medical School, Boston, MA, USA) and was grown in RPMI 1640 medium supplemented with 10% FBS [30
]; authentication was conducted by DNA profiling at University of Colorado Cancer Center DNA Sequencing and Analysis Core (Aurora, Colorado) [28
]. The FTC-133 cell line (derived from a poorly differentiated human thyroid carcinoma) was provided by Professor Diego Russo (University of Catanzaro, Catanzaro, Italy) and was grown in DMEM/F12 medium supplemented with 10% FBS [31
]; authentication was conducted by DNA profiling at University of Colorado Cancer Center DNA Sequencing and Analysis Core (Aurora, Colorado) [28
]. The Cal62 cell line (derived from an ATC) was acquired from DSMZ and was grown in DMEM medium supplemented with 10% FBS [32
]. To evaluate the effect of kynurenine on gene expressions, FTC-133 and BcPap cells were seeded in 100 mm plates and treated with kynurenine (100 µM in PBS or in DMSO as indicated) and/or CH223191 (10 µM in DMSO) when an average confluence of 80% was reached. The cells were cultured for the indicated times (6 h; 18 h or 24 h) before harvesting for RNA or protein extraction. Kynurenine and CH223191 were purchased from Sigma-Aldrich (St. Louis, MO, USA).
4.4. Quantitative PCR
Total RNA was extracted from tissues and cells with Trizol (Life Technologies), and first-strand cDNA synthesis was performed using 2 µg of each RNA sample primed with random hexamers with 200 U of Superscript III reverse transcriptase (Life Technologies), according to the manufacturer’s instructions. To evaluate expression levels of AhR, CYP1A1 and IDO1 in tumor and/or cell samples, a quantitative PCR Assays-on-Demand gene kit was used (assay identification AhR: Hs00169233_m1; CYP1A1: Hs 00153120_m1; IDO1: Hs00158032_m1, Applied Biosystems), and β
-actin was used as endogenous control (predeveloped Taq-Man assay reagents VIC dye-labeled; Applied Biosystems), as described previously [33
]. In detail, all amplification reactions were performed in triplicate and the threshold cycles averaged. Results (determined with the 2−ΔΔCT
method) were normalized to a commercial cDNA sample of human normal thyroid (BD Biosciences CLONTECH), derived from 65 normal tissues, that was used as normal reference. The data are presented as RQ obtained normalizing the acquired data with those of the normal thyroid. To evaluate expression levels of CYP1B1, SLUG, E-Cadherin, N-Cadherin, fibronectin-1, OCT4, MMP1, MMP2 and MMP9 quantitative PCR amplifications were performed using Platinum SYBR greenER quantitative PCR Super Mix UDG Universal (Life Technologies) according to the manufacturer’s instructions. The sequences of the PCR primers are shown in Table 3
, and the results were analyzed as described previously [34
]. In detail, the cycle threshold value, coupled with individualized amplification efficiencies for each primer set, was used to calculate the normalized expression of the indicated gene mRNA using the Q-Gene software [35
]. A commercial cDNA sample of human normal thyroid (BD Biosciences CLONTECH), derived from 65 normal tissues, was used as normal reference and the raw mRNA expression data are presented as arbitrary units.
Immunohistochemistry was performed on human PTC tissue sections and mice tissues using the primary monoclonal anti-human-rat-mouse AhR antibody diluted 1:250 (clone RPT1, Thermo Fisher Scientific, Waltham, MA USA), as described previously [36
]. In detail, AhR staining intensity of tumor samples was compared to that of corresponding adjacent normal tissues. To judge the results, semiquantitative methods were adopted. In detail, AhR IHC score was calculated by combining the staining intensity with the percentage of immunoreactive cells. Staining intensity was rated on a scale of 0–3 (0, negative; 1, weak; 2, moderate; and 3, strong). Each tumor was then scored for the percentage of immunoreactive cells. The immunohistochemical score was then assigned to each tumor by multiplying the percentage of positive cells for the staining intensity. The IHC score ranged from 0 to 300.
The cell blocks were prepared employing the CellientTM Automated System. The formalin-fixed samples were previously centrifuged at 1727 rpm for 10 min and the supernatant was removed. The remnant material was placed in modified biological cassettes and underwent a vacuum-assisted filtration. Afterwards, the cells were deposited in a uniform layer until the filter is saturated. Eosin stain was added, and the materials firstly went through absolute alcohol and secondly through xylene; finally, it was molten in paraffin. Once this last hardens, the filter pulls away and the cells were in one plane in the wax. As a result, sections of 4 µm were performed and placed on slides with permanent positive charged surface. The immunostaining for AHR were performed by BOND-III fully automated IHC stainer (Leica Biosystems, Wetzlar, DE), using the anti-AHR antibody (clone RPT1, dilution 1:250, ThermoFisher Scientific, Waltham, MA USA). AhR staining in cells was evaluated assigning staining intensity (0, negative; 1, weak; 2, moderate; and 3, strong).
4.6. Immunoblotting Experiments
Immunoblotting experiments were performed according to standard procedures. Polyclonal anti-human AhR antibody was purchased from Cell Signaling and used at a 1:1000 dilution, monoclonal anti-human IDO1 antibody (clone 1F8.2) was purchased from Millipore and used at a 1:1000 dilution, monoclonal anti-human SLUG antibody (clone C19G7 Cell Signaling) and monoclonal anti-human tubulin (clone DM1A) was purchased from Sigma and used at a 1:5000 dilution. Secondary anti-mouse antibody coupled to horseradish peroxidase was purchased from Sigma.
4.7. Evaluation of Cellular Migration and Invasiveness
Cell migration was evaluated using The Oris™ Universal Cell Migration Assembly Kit (Platypus Technologies, Madison, WI, USA), according to the manufacturer’s instructions. In details, 1 × 104 FTC-133 or 3 × 104 BcPap cells were seeded in 96-wells plate assembly with Oris™ Cell Seeding Stoppers in 100 µL of complete medium and plates were incubated 24 h to permit cell attachment before removing the stoppers. At confluence, medium was removed to eliminate any unattached cells and replaced with fresh medium with kynurenine (100 µM) and/or CH223191 (10 µM). Treatments were renewed after 8 h, and the images were acquired after 16 h–24 h using a microscope (Olympus IX51) assembly with camera (Olympus, Hamburg, DE). Samples were repeated six times in each experiment and three independent experiments were performed. The unclosed area was compared with that of reference wells, in which stoppers were removed only at the time of acquisition. Images were analyzed using Image J and the percentage of closure in treated cells was compared with those of parental cells.
Cell invasiveness was evaluated using a Beyond chamber assay with 8 µM polycarbonate membrane transwell permeable supports in 24-well plates (Costar, Corning, NY, USA) pretreated with 100 µL ECM Gel Matrix (Sigma Aldrich, St. Louis, MO, USA) for 30 min at 37 °C. One hundred thousand FTC-133 or BcPap cells were seeded at the top of the membrane in a serum-free medium and treated with kynurenine (100 µM) and/or CH223191 (10 µM). Treatments were renewed every 8 h. The bottom wells were filled with 600 µL of complete medium (10% FBS). After 48 h ECM matrix was removed and invasive cells, both inside the polycarbonate membrane and in the lower chamber, were fixed with 70% ethanol, stained with 0.2% crystal violet in a 12 mM methanol solution and acquired with an optical microscope (Olympus IX51) assembly with camera (Olympus). We performed three independent experiments each in duplicates.
4.8. Statistical Analysis
Statistical analysis was performed using Predictive Analytic Software release 17.0.2 (SPSS Inc., IBM Chicago, IL, USA). The adopted techniques included the unpaired Student’s t test, the one-sample t test, the Mann-Whitney U nonparametric test, and Spearman’s rho, as appropriate and as indicated. All differences were considered significant when p < 0.05.