Signaling via the AHR leads to enhanced usage of CD44v10 by murine fetal thymic emigrants: possible role for CD44 in emigration

Malignant transformation of mammary epithelial cells by ectopic overexpression of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand activated basic helix-loop-helix transcription factor that binds to environmental poly aromatic hydrocarbons (PAH) and mediates their toxic and carcinogenic responses. There is ample documentation for the role of AhR in PAH-induced carcinogenicity. However, in this report we addressed whether overexpression of AhR alone is sufficient to induce carcinogenic transformation in human mammary epithelial cells (HMEC). Retroviral expression vectors were used to develop a series of stable cell lines expressing varying levels of AhR protein in an immortalized normal HMEC with relatively low endogenous AhR expression. The resulting increase in AhR expression and activity correlated with the development of cellular malignant phenotypes, most significantly epithelial-to-mesenchymal transition. Clones overexpressing AhR by more than 3-fold, exhibited a 50% decrease in population doubling time. Cell cycle analysis revealed that this increase in proliferation rates was due to an enhanced cell cycle progression by increasing the percentage of cells transiting into S- and G2/M phases. Cells overexpressing AhR exhibited enhanced motility and migration. Importantly, these cells acquired the ability to invade matrigel matrix, where more than 80% of plated cells invaded the matrigel matrix within 24 h, whereas none of parental or the vector control HMEC were able to invade matrigel. Collectively, these data provide evidence for a direct role of AhR in the progression of breast carcinoma. The results suggest a novel therapeutic target that could be considered for treatment and prevention of breast cancer progression.

Aryl hydrocarbon receptor activation in hematopoietic stem/progenitor cells alters cell function and pathway-specific gene modulation reflecting changes in cellular trafficking and migration

The aryl hydrocarbon receptor (AhR) is a transcription factor belonging to the Per-ARNT-Sim family of proteins. These proteins sense molecules and stimuli from the cellular/tissue environment and initiate signaling cascades to elicit appropriate cellular responses. Recent literature reports suggest an important function of AhR in hematopoietic stem cell (HSC) biology. However, the molecular mechanisms by which AhR signaling regulates HSC functions are unknown. In previous studies, we and others reported that treatment of mice with the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) compromises the competitive reconstitution of bone marrow (BM) cells into irradiated host animals. Additional studies indicated a requirement for AhR in hematopoietic cells and not marrow microenvironment cells. In this study, we tested the hypothesis that TCDD-mediated phenotypic and functional changes of HSCs are a result of changes in gene expression that disrupt stem cell numbers and/or their migration. TCDD treatment to mice increased the numbers of phenotypically defined HSCs in BM. These cells showed compromised migration to the BM in vivo and to the chemokine CXCL12 in vitro, as well as increased expression of the leukemia-associated receptors CD184 (CXCR4) and CD44. Gene expression profiles at 6 and 12 h after exposure were consistent with the phenotypic and functional changes observed. The expressions of Scin, Nqo1, Flnb, Mmp8, Ilf9, and Slamf7 were consistently altered. TCDD also disrupted expression of other genes involved in hematological system development and function including Fos, JunB, Egr1, Ptgs2 (Cox2), and Cxcl2. These data support a molecular mechanism for an AhR ligand to disrupt the homeostatic cell signaling of HSCs that may promote altered HSC function.

Stromal hyaluronan interaction with epithelial CD44 variants promotes prostate cancer invasiveness by augmenting expression and function of hepatocyte growth factor and androgen receptor

The main aim of our study is to determine the significance of the stromal microenvironment in the malignant behavior of prostate cancer. The stroma-derived growth factors/cytokines and hyaluronan act in autocrine/paracrine ways with their receptors, including receptor-tyrosine kinases and CD44 variants (CD44v), to potentiate and support tumor epithelial cell survival. Overexpression of hyaluronan, CD44v9 variants, and stroma-derived growth factors/cytokines are specific features in many cancers, including prostate cancer. Androgen/androgen receptor interaction has a critical role in regulating prostate cancer growth. Our previous study showed that 1) that increased synthesis of hyaluronan in normal epithelial cells promotes expression of CD44 variants; 2) hyaluronan interaction with CD44v6-v9 promotes activation of receptor-tyrosine kinase, which stimulates phosphatidylinositol 3-kinase-induced cell survival pathways; and 3) CD44v6/short hairpin RNA reduces colon tumor growth in vivo (Misra, S., Hascall, V. C., De Giovanni, C., Markwald, R. R., and Ghatak, S. (2009) J. Biol. Chem. 284, 12432–12446). Our results now show that hepatocyte growth factor synthesized by myofibroblasts associated with prostate cancer cells induces activation of HGF-receptor/cMet and stimulates hyaluronan/CD44v9 signaling. This, in turn, stabilizes the androgen receptor functions in prostate cancer cells. The stroma-derived HGF induces a lipid raft-associated signaling complex that contains CD44v9, cMet/phosphatidylinositol 3-kinase, HSP90 and androgen receptor. CD44v9/short hairpin RNA reverses the assembly of these components in the complex and inhibits androgen receptor function. Our results provide new insight into the hyaluronan/CD44v9-regulated androgen receptor function and the consequent malignant activities in prostate cancer cells. The present study describes a physiologically relevant in vitro model for studying the molecular mechanisms by which stroma-derived HGF and hyaluronan influence androgen receptor and CD44 functions in the secretory epithelia during prostate carcinogenesis.

The dioxin receptor regulates the constitutive expression of the vav3 proto-oncogene and modulates cell shape and adhesion

The dioxin receptor (AhR) modulates cell plasticity and migration, although the signaling involved remains unknown. Here, we report a mechanism that integrates AhR into these cytoskeleton-related functions. Immortalized and mouse embryonic fibroblasts lacking AhR (AhR-/-) had increased cell area due to spread cytoplasms that reverted to wild-type morphology upon AhR re-expression. The AhR-null phenotype included increased F-actin stress fibers, depolarized focal adhesions, and enhanced spreading and adhesion. The cytoskeleton alterations of AhR-/- cells were due to down-regulation of constitutive Vav3 expression, a guanosine diphosphate/guanosine triphosphate exchange factor for Rho/Rac GTPases and a novel transcriptional target of AhR. AhR was recruited to the vav3 promoter and maintained constitutive mRNA expression in a ligand-independent manner. Consistently, AhR-/- fibroblasts had reduced Rac1 activity and increased activation of the RhoA/Rho kinase (Rock) pathway. Pharmacological inhibition of Rac1 shifted AhR+/+ fibroblasts to the null phenotype, whereas Rock inhibition changed AhR-null cells to the AhR+/+ morphology. Knockdown of vav3 transcripts by small interfering RNA induced cytoskeleton defects and changes in adhesion and spreading mimicking those of AhR-null cells. Moreover, vav3-/- MEFs, as AhR-/- mouse embryonic fibroblasts, had increased cell area and enhanced stress fibers. By modulating Vav3-dependent signaling, AhR could regulate cell shape, adhesion, and migration under physiological conditions and, perhaps, in certain pathological states.

The aryl hydrocarbon receptor, more than a xenobiotic-interacting protein

The aryl hydrocarbon (dioxin) receptor (AhR) has been studied for several decades largely because of its critical role in xenobiotic-induced toxicity and carcinogenesis. Albeit this is a major issue in basic and clinical research, an increasing number of investigators are turning their efforts to try to understand the physiology of the AhR under normal cellular conditions. This is an exciting area that covers cell proliferation and differentiation, endogenous mechanisms of activation, gene regulation, tumor development and cell motility and migration, among others. In this review, we will attempt to summarize the studies supporting the implication of the AhR in those endogenous cellular processes.

Xenobiotic-induced alterations in thymocyte development

The thymus is a very sensitive target for environmental pollutants, which can affect this organ as well as thymocyte differentiation. A failure in thymocyte development can be due to the exacerbation of apoptosis, arrest of thymocyte maturation, generation of autoreactive T cells, and inhibition or stimulation of the output of recent thymic emigrants to the periphery. Recent data demonstrate that the immune system has the potential to maintain homeostasis under conditions of elevated risk, and the thymus plays a crucial role in this process. Environmental xenobiotics can exert their effects through receptor-mediated interactions or independently on receptor involvement. Under natural conditions organisms are exposed to a variety of xenobiotics. The final effect of such exposure is not related to the action of a single chemical, but to the action of a mixture of chemicals. The toxic effect of environmental xenobiotics on the generation and functions of immune cells may result in suppression or stimulation of the immune response. The most intensive studies have been done on halogenated aromatic hydrocarbons, heavy metals and various chemicals acting as endocrine disrupters. Recently, special interest has focused on the action of air particulate matter.

Differential gene expression as a potential classifier of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole-sensitive and -insensitive cell lines

2-(4-Amino-3-methylphenyl)-5-fluorobenzothiazole (5F-203) is a candidate antitumor agent empirically discovered with the aid of the National Cancer Institute (NCI) Anticancer Drug Screen. In an effort to determine whether basal expression of genes could be used to classify cell sensitivity to this agent, serial analysis of gene expression (SAGE) libraries were generated for three sensitive and two insensitive human tumor cell lines. When the SAGE tags expressed within these cell line libraries were compared and evaluated for differences, several genes seemed more highly expressed in 5F-203-sensitive cell lines than in the insensitive cell lines. Constitutive expressions of 15 genes identified by the analysis were then measured by quantitative reverse-transcription polymerase chain reaction in the 60 cell lines of the NCI Anticancer Drug Screen. This generated a pattern of relative basal gene expression across the cell lines and also confirmed the differential expression of SAGE-discovered genes within the initial set of five cell lines. Further analyses of these expression data in 60 cell lines suggested that a smaller subset of genes could be used to classify tumor cell sensitivity to 5F-203. In contrast, the same set of genes did not predict with equivalent precision sensitivity to unrelated active drugs, and another set of genes could not better classify the cell lines in terms of 5F-203 sensitivity. These results suggest that constitutive gene expression profiles, in which the genes are not necessarily known to be related to the mechanism of action of a given drug, may be viewed as a general tool to extend and improve the concept of a single predictive gene to groups of predictive genes.

Detection of a novel population of fetal thymocytes characterized by preferential emigration and a TCRγδ+ T cell fate after dioxin exposure

T cell maturation into TCRalphabeta(+) or TCRgammadelta(+) cells from common immature CD4(-)CD8(-)(DN) precursors occurs in the thymus, and is controlled through ordered regulation of genes. The aryl hydrocarbon receptor (AHR), a latent cytoplasmic transcription factor, affects thymocyte maturation and differentiation at several stages, also including DN cells. We analyzed in murine fetal thymus organ cultures (FTOC) the outcome of AHR-signaling and found a higher frequency of DN TCRgammadelta(+) cells in the presence of the AHR-activating ligand TCDD. We detected a novel population of CD25(int/lo)CD44(hi) cells associated with preferential emigration and a TCRgammadelta(+) T cell fate of thymocytes. Sorted DN TCRgammadelta(+) emigrants could proliferate if IL-2 was available. Moreover, they suppressed the proliferation of co-cultivated, activated CD4(+) T cells. Gene expression profiles of purified DN emigrants from TCDD*FTOC revealed 295 modulated genes, 10% of which are genes of the immune system. For instance, RAG-1, TdT, and Gfi-1 were downregulated, yet genes indicative of mature thymocytes were upregulated. In conclusion, we have detected changes in the differentiation programme of fetal DN thymocytes after ligand-activation of the AHR. In particular, we observed a higher frequency of DN TCRgammadelta(+) cells with high emigration potential, and possible regulatory functions.

Role of the aryl hydrocarbon receptor in thymocyte emigration in vivo

The aryl hydrocarbon receptor (AHR) is a ligand-dependent member of the PAS-bHLH-family of nuclear receptors. Anthropogenic ligands include environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Over-activation of the AHR causes thymus atrophy and immunosuppression. Signaling via the AHR changes the thymocyte differentiation program at several checkpoints, in particular within the CD4-CD8- double-negative (DN) thymocyte subset. Here, we show that AHR over-activation led to the preferential emigration of DN thymocytes to the periphery and accumulation in the spleen. Some of these recent thymic emigrants (RTE) had a novel "activated immature" phenotype (CD3-TCRbeta-CD25+/intCD44-CD45RB+/intCD62L+CD69- cells). Gene expression profiling of DN RTE revealed 15 genes that were up-regulated more than threefold by TCDD, including the S100A9 gene. Exposure of S100A9 null mice to TCDD showed a role for this protein in AHR-mediated thymic egress.

Effects of a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin, given at post-puberty, in senescent mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin is a well-known immunosuppressive environmental pollutant. TCDD interferes with physiological signaling of the arylhydrocarbon receptor, leading to cell-specific changes in gene transcription and cell differentiation. With respect to the immune system, the T-cell lineage and B-cell lineages are particularly affected. Although a single dose given to mice is excreted within weeks, these changes in differentiation may have long-term consequences for immune competence. We studied the effects of a single dose of TCDD given to young mice on some parameters of their immune system after they had aged almost to the end of their lifespan. Groups of 15 mice were given either 2.5 microg TCDD/kg b.w. or 25 microg TCDD/kg b.w. at the age of 8-12 weeks, and were analyzed between 16 and 21 months of age. Survival was equal in all groups. Blood glucose levels did not differ, and glucose tolerance after oral challenge was normal in old control mice and TCDD-exposed mice. No differences in the frequencies of B-cells, T-cells, or NK-cells were detectable. TCDD-exposed mice at both doses had a significantly higher titer of IgM compared to controls. Histological examination of pancreas, liver, kidney, spleen, and lungs yielded no differences, except for the lungs, where a significantly higher number of animals displayed activated BALT. In conclusion, our data suggest that a single dose of TCDD in young mice is correlated to activated secondary lymphoid tissues and high IgM titers. Both findings are congruent with a weakened immune system.