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 hydrocarbon nuclear translocator, causing cellular responses that lead to toxicity, carcinogenesis, and teratogenesis. The nuclear form of the activated AHR/aryl hydrocarbon nuclear translocator complex is responsible for alterations in immune, endocrine, reproductive, developmental, cardiovascular, and central nervous system functions whose mechanisms remain poorly understood. Here, we show that the second messenger, cAMP (an endogenous mediator of hormones, neurotransmitters, and prostaglandins), activates the AHR, moving the receptor to the nucleus in some ways that are similar to and in other ways fundamentally different from AHR activation by dioxin. We suggest that this cAMP-mediated activation may reflect the true endogenous function of AHR; disruption of the cAMP-mediated activation by dioxin, binding chronically to the AHR for days, weeks, or months, might be pivotal in the mechanism of dioxin toxicity. Understanding this endogenous activation of the AHR by cAMP may help in developing methods to counteract the toxicity caused by numerous environmental and food-borne toxic chemicals that act via the AHR.

Guanine 6-O-methylation pattern within the dioxin responsive element of the CYP1A1 enhancer shows two critical guanines for AhR/ARNT binding.

The core-recognition motif for TCDD-liganded AhR/ARNT complex of the dioxin-responsive element (DRE) contains four guanine residues, three on the antisense (5'-T(T)/(A)GCGTG-3') and one on the sense (5'-CACGC(A)/(T)A-3') strand. It has been reported that, in methylation-protection and methylation-interference assays, the TCDD-liganded AhR/ARNT contacts all four guanine residues. On the other hand, it is known that some anticancer drugs, and various environmental and workplace chemicals, including strongly human carcinogenic nitrosamines, lead to the highly miscoding 6-O-methylation of guanine. In the present study, we have investigated whether specific methylation of guanine at the 6-O-position interferes with the binding of TCDD-liganded AhR/ARNT complex to its recognition motif in the CYP1A1 enhancer, and how individual 6-O-methylated guanines contribute to this interference. We found that only two 6-O-methylguanine residues are critical: the closest to the 5'-end within the three-nucleotide sequence (5'-GTG-3'), identical to a half-site E-box element, on the antisense strand, and the only guanine on the sense strand. In contrast, the 6-O-methylguanine closest to the 5'-end (well) and the one closest to the 3'-end (to a lesser extent), both on the antisense strand, were still able to bind the TCDD-liganded AhR/ARNT complex. This shows that the 6-O-methylation of guanine in the core sequences of CYP1A1 enhancer interferes with the binding of the ligand-activated AhR/ARNT complex in a differentially selective manner; it fully impedes binding of this complex to DRE (the prerequisite of most of the toxic effects of TCDD) only when one of the two particular guanines is methylated at the 6-O-position.