Tax is Involved in Up-regulation of HMGB1 Expression Levels by Interaction with C/EBP

The high mobility group box 1 (HMGB1) protein is a multifunctional cytokine-like molecule that plays animportant role in the pathogenesis of tumors. In this study, real-time polymerase chain reactions and Western blotassays indicated that HMGB1 transcriptional activity and protein level are increased in Tax+-T cells (TaxP). Toclarify the mechanisms, a series of HMGB1 deletion reporter plasmids (pHLuc1 to pHLuc6) were transfected intoTax--T cells (TaxN, Jurkat) and Tax+-T cells (TaxP). We found that promoter activity in Tax+-T cells to be higherthan that in Tax--T cells, indicating a significant increase in pHLuc6. Bay11-7082 (NF-κB inhibitor) treatmentdid not block the enhancing effect. Chromatin immunoprecipitation assays revealed that Tax was retained ona HMGB1 promoter fragment encompassing -1163 to -975. Bioinformatics analysis showed six characteristiccis-elements for CdxA, AP-1, AML-1a, USF, v-Myb, and C/EBP in the fragment in question. Mutation of ciselementsfor C/EBP reduced significant HMGB1 promoter activity induced by Tax. These findings indicate thatTax enhances the expression of HMGB1 gene at the transcriptional level, possibly by interacting with C/EBP.