Toxicology and Risk Assessment: A Comprehensive Introduction, 2nd Edition. Edited by Helmut Greim and Robert Snyder

The aryl hydrocarbon receptor-dependent deregulation of cell cycle control induced by polycyclic aromatic hydrocarbons in rat liver epithelial cells

Disruption of cell proliferation control by polycyclic aromatic hydrocarbons (PAHs) may contribute to their carcinogenicity. We investigated role of the aryl hydrocarbon receptor (AhR) in disruption of contact inhibition in rat liver epithelial WB-F344 'stem-like' cells, induced by the weakly mutagenic benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF) and by the strongly mutagenic benzo[a]pyrene (BaP). There were significant differences between the effects of BaA and BbF, and those of the strongly genotoxic BaP. Both BaA and BbF increased percentage of cells entering S-phase and cell numbers, associated with an increased expression of Cyclin A and Cyclin A/cdk2 complex activity. Their eff…

TCDD deregulates contact inhibition in rat liver oval cells via Ah receptor, JunD and cyclin A.

The aryl hydrocarbon receptor (AhR) is a transcription factor involved in physiological processes, but also mediates most, if not all, toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Activation of the AhR by TCDD leads to its dimerization with aryl hydrocarbon nuclear translocator (ARNT) and transcriptional activation of several phase I and II metabolizing enzymes. However, this classical signalling pathway so far failed to explain the pleiotropic hazardous effects of TCDD, such as developmental toxicity and tumour promotion. Thus, there is an urgent need to define genetic programmes orchestrated by AhR to unravel its role in physiology and toxicology. Here we show that TCDD …

p38α MAPK is required for contact inhibition

Proliferation of nontransformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Despite its generally accepted importance for cell cycle control, knowledge about the intracellular signalling pathways involved in contact inhibition is scarce. In the present work we show that p38alpha mitogen-activated protein kinase (MAPK) is involved in the growth-inhibitory signalling cascade of contact inhibition in fibroblasts. p38alpha activity is increased in confluent cultures of human fibroblasts compared to proliferating cultures. Time course studies show a sustained activation of p38alpha in response to cell-cell contacts in contrast to a transient activation …

TCDD induces c-jun expression via a novel Ah (dioxin) receptor-mediated p38–MAPK-dependent pathway

The aryl hydrocarbon receptor (AhR) has a fundamental role during postnatal liver development and is essential for mediating dioxin toxicity. However, the genetic programs mediating, both, the toxic and physiological effects downstream of the transcription factor AhR are in major parts unknown. We have identified the proto-oncogene c-jun as a novel target gene of AhR. Induction of c-jun depends on activation of p38-mitogen-activated protein kinase (MAPK) by an AhR-dependent mechanism. None of the kinases that are known to phosphorylate p38-MAPK is activated by AhR. Neither the dephosphorylation rate of p38-MAPK is reduced. Furthermore, increased p38-MAPK phosphorylation in response to dioxi…

JNK phosphorylation relieves HDAC3-dependent suppression of the transcriptional activity of c-Jun

The AP-1 transcription factor c-Jun is a prototypical nuclear effector of the JNK signal transduction pathway. The integrity of JNK phosphorylation sites at serines 63/73 and at threonines 91/93 in c-Jun is essential for signal-dependent target gene activation. We show that c-Jun phosphorylation mediates dissociation of an inhibitory complex, which is associated with histone deacetylase 3 (HDAC3). The subsequent events that ultimately cause increased mRNA synthesis are independent of c-Jun phosphorylation and its interaction with JNK. These findings provide an 'activation by de-repression' model as an explanation for the stimulatory function of JNK on c-Jun.

Aryl hydrocarbon receptor activation by cAMP vs. dioxin: divergent signaling pathways.

Even before the first vertebrates appeared on our planet, the aryl hydrocarbon receptor ( AHR ) gene was present to carry out one or more critical life functions. The vertebrate AHR then evolved to take on functions of detecting and responding to certain classes of environmental toxicants. These environmental pollutants include polycyclic aromatic hydrocarbons (e.g., benzo[ a ]pyrene), polyhalogenated hydrocarbons, dibenzofurans, and the most potent small-molecular-weight toxicant known, 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD or dioxin). After binding of these ligands, the activated AHR translocates rapidly from the cytosol to the nucleus, where it forms a heterodimer with aryl hydroc…

Deregulated repression of c-Jun provides a potential link to its role in tumorigenesis.

The transcription factor c-Jun cooperates with oncogenic alleles of ras in malignant transformation. Constitutively active Ras causes, via activation of mitogen activated protein kinases, phosphorylation of c-Jun which is essential for subsequent target gene activation and tumorigenesis. Studying the mechanisms controlling c-Jun activity we found that its transcription activation function is actively repressed by a presumably multimeric repressor complex that includes histone deacetylase 3 as a critical subunit. Suppression of c-Jun is relieved by MAP kinase-mediated phosphorylation and/or titration of inhibitor components. The viral tumorigenic counterpart of c-Jun, v-Jun, escapes this inh…

Influence of aryl hydrocarbon- (Ah) receptor and genotoxins on DNA repair gene expression and cell survival of mouse hepatoma cells

The aryl hydrocarbon receptor (AhR) mediates toxicity of a variety of environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs) and dioxins. However, the underlying mechanisms and genetic programmes regulated by AhR to cause adverse effects but also to counteract poisoning are still poorly understood. Here we analysed the effects of two AhR ligands, benzo[a]pyrene (B[a]P), a DNA damaging tumour initiator and promotor and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a pure tumour promoter, on cell survival and on nucleotide excision repair (NER) gene expression. NER deals with so called "bulky" DNA adducts including those generated by enzymatically activated B[a]P. Therefore, t…

Circadian rhythms and chemical carcinogenesis: Potential link. An overview.

Circadian rhythm is an integral and not replaceable part of the organism's homeostasis. Its signalling is multidimensional, overlooking global networks such as chromatin remodelling, cell cycle, DNA damage and repair as well as nuclear receptors function. Understanding its global networking will allow us to follow up not only organism dysfunction and pathology (including chemical carcinogenesis) but well-being in general having in mind that time is not always on our side. The authors thank ECNIS (Environmental Cancer, Nutrition and Individual Susceptibility), a network of excellence operating within the European Union 6th Framework Program, Priority 5: Food Quality and Safety (Contract no. …

The nucleotide excision repair protein XPC is essential for bulky DNA adducts to promote interleukin-6 expression via the activation of p38-SAPK

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants, and many are potent carcinogens. Benzo[a]pyrene (B[a]P), one of the best-studied PAHs, is metabolized ultimately to the genotoxin anti-B[a]P-7,8-dihydrodiol-9,10-epoxide (BPDE). BPDE triggers stress responses linked to gene expression, cell death and survival. So far, the underlying mechanisms that initiate these signal transduction cascades are unknown. Here we show that BPDE-induced DNA damage is recognized by DNA damage sensor proteins to induce activation of the stress-activated protein kinase (SAPK) p38. Surprisingly, the classical DNA damage response, which involves the kinases ATM and ATR, is not involved in p38-SA…