Regulation of Mouse Small Heat Shock Protein αb-Crystallin Gene by Aryl Hydrocarbon Receptor

The stress-inducible small heat shock protein (shsp)/αB-crystallin gene is expressed highly in the lens and moderately in other tissues. Here we provide evidence that it is a target gene of the aryl hydrocarbon receptor (AhR) transcription factor. A sequence (−329/−323, CATGCGA) similar to the consensus xenobiotic responsive element (XRE), called here XRE-like, is present in the αBE2 region of αB-crystallin enhancer and can bind AhR in vitro and in vivo. αB-crystallin protein levels were reduced in retina, lens, cornea, heart, skeletal muscle and cultured muscle fibroblasts of AhR−/− mice; αB-crystallin mRNA levels were reduced in the eye, heart and skeletal muscle of AhR−/− mice. Increased AhR stimulated αB-crystallin expression in transfection experiments conducted in conjunction with the aryl hydrocarbon receptor nuclear translocator (ARNT) and decreased AhR reduced αB-crystallin expression. AhR effect on aB-crystallin promoter activity was cell-dependent in transfection experiments. AhR up-regulated αB-crystallin promoter activity in transfected HeLa, NIH3T3 and COS-7 cells in the absence of exogenously added ligand (TCDD), but had no effect on the αB-crystallin promoter in C2C12, CV-1 or Hepa-1 cells with or without TCDD. TCDD enhanced AhR-stimulated αB-crystallin promoter activity in transfected αTN4 cells. AhR could bind to an XRE-like site in the αB-crystallin enhancer in vitro and in vivo. Finally, site-specific mutagenesis experiments showed that the XRE-like motif was necessary for both basal and maximal AhR-induction of αB-crystallin promoter activity. Our data strongly suggest that AhR is a regulator of αB-crystallin gene expression and provide new avenues of research for the mechanism of tissue-specific αB-crystallin gene regulation under normal and physiologically stressed conditions

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