ToxSci Advance Access published online on May 28, 2007
Toxicological Sciences, doi:10.1093/toxsci/kfm138
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The Putative Tumor Suppressor Tsc-22 is Down-regulated Early in Chemically Induced Hepatocarcinogenesis and May be a Suppressor of Gadd45b
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* Laboratory of Molecular Carcinogenesis
Toxicology Operations Branch, Environmental Toxicology Program, National Institute of Environmental Health Sciences, National Institute of Health, Research Triangle Park, NC 27709 USA
Present address, Department of Pathology, Biosafety Research Center, Foods, Drugs and pesticides, Shizuoka 437-1213, Japan
1 To whom requests for reprints should be addressed at Department of Pathology, Biosafety Research Center, Foods, Drugs and pesticides, 582-2 Shioshinden, Iwata-shi, Shizuoka 437-1213, Japan. Fax: +81-538-58-1343; Email: imari{at}anpyo.or.jp
Received February 25, 2007; revision received May 15, 2007; accepted May 16, 2007
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Abstract |
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Tsc-22 is a novel tumor suppressor gene that represents a new class of transcription factors that has transcriptional repressor activity. We found Tsc-22 down-regulation in livers from B6C3F1 mice following treatment for 2 weeks with carcinogenic doses of the anti-anxiety drug oxazepam (2500 ppm) or the peroxisome proliferator Wyeth-14,643 (500 ppm) but not with two other carcinogens such as o-nitrotoluene or methyleugenol or three non-carcinogens including p-nitrotoluene, eugenol or acetaminophen. The expression of Tsc-22 was also repressed in B6C3F1 mouse liver tumors that were induced by several chemicals from two-year carcinogenicity studies as well as in spontaneous liver tumors. To identify potential Tsc-22 target genes in mouse liver, we transfected small interference RNA (SiRNA) designed to inhibit Tsc-22 into murine liver BNL CL.2 cells. We selected two potential transcriptional targets of Tsc-22, growth arrest and DNA damage-inducible gene 45 ß (Gadd45b) and leucine zipper, putative tumor suppressor 2 (Lzts2) to test based on our previous cDNA microarray studies, showing that expression of these cancer-associated genes was increased when Tsc-22 was repressed. SiRNA treatment of BNL-CL.2 cells with Tsc-22 oligonucleotides but not non-specific oligonucleotides decreased RNA and protein expression of Tsc-22 by 80-90%, while expression of Gadd45b gene, but not Lzts2, was increased over time after an initial decrease. Treatment of these cells with oxazepam for 48 hours also resulted in decreased Tsc-22 and increased Gadd45b expression. These data provide evidence that Tsc-22 is a suppressor of Gadd45b expression, which may contribute to an early anti-apoptotic response.