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

The stress-inducible small heat shock protein (shsp)/alpha B-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 alpha BE2 region of alpha B-crystallin enhancer and can bind AhR in vitro and in vivo. alpha B-crystallin protein levels were reduced in retina, lens, cornea, heart, skeletal muscle and cultured muscle fibroblasts of AhR(-/-) mice; alpha B-crystallin mRNA levels were reduced in the eye, heart and skeletal muscle of AhR(-/-) mice. Increased AhR stimulated alpha B-crystallin expression in transfection experiments conducted in conjunction with the aryl hydrocarbon receptor nuclear translocator (ARNT) and decreased AhR reduced alpha B-crystallin expression. AhR effect on aB-crystallin promoter activity was cell-dependent in transfection experiments. AhR up-regulated alpha B-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 alpha B-crystallin promoter in C(2)C(12), CV-1 or Hepa-1 cells with or without TCDD. TCDD enhanced AhR-stimulated alpha B-crystallin promoter activity in transfected alpha TN4 cells. AhR could bind to an XRE-like site in the alpha B-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 alpha B-crystallin promoter activity. Our data strongly suggest that AhR is a regulator of alpha B-crystallin gene expression and provide new avenues of research for the mechanism of tissue-specific alpha B-crystallin gene regulation under normal and physiologically stressed conditions.

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