Up-regulation of NFκB-responsive gene expression by ΔNp73α in p53 null cells

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Transactivation domain (TAD)-truncated p73, ΔNp73, associates with p53, resulting in suppression of p53′s functions. Using p53 null cell lines, we examined whether or not ΔNp73 can regulate gene expression in a p53-independent manner. When ΔNp73α was co-transfected with a luciferase reporter plasmid with various enhancer elements, NFκB-responsive luciferase gene expression was selectively up-regulated by ΔNp73α, but not by other p73-isoforms with TAD and ΔNp73β. Deletion of the TAD endowed p73α with the ability to enhance the responsive gene's expression, but deletion of the N-terminal proline-rich domain (PRD) rendered the TAD-deleted p73α inactive. Considering the inability of ΔNp73β, which is the C-terminus-truncated form of ΔNp73α, to function, these results indicate that both the PRD and C-terminus are necessary for ΔNp73α to can activate NFκB-responsive luciferase expression. Over-expression of p53 suppressed the TAD-truncated p73α-mediated luciferase expression, suggesting that p53 interferes with the TAD-truncated p73α-mediated activation of NFκB. Inhibitors for NFκB activation reduced the TAD-truncated p73α-dependent NFκB-responsive gene expression, indicating that TAD-truncated p73α activates NFκB as does TNFα. In addition to the results obtained in the reporter gene assay, TAD-truncated p73α stimulated the translocation of NFκB to the nucleus and the expression of an endogenous NFκB-responsive gene, Bcl-XL. Taken together, these results demonstrate that TAD-truncated p73α can activate NFκB.

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