Abstract .HIPK2 is a stress-induced kinase and a transcriptional co-repressor that functionally cooperates with p53 to suppress cancer. Activation of the p53 proapoptotic function requires a cascade of phosphorylations and acetylations, and HIPK2 takes part in both modifications in that it phosphorylates Ser46 and allows Lys382 acetylation. In a cDNA microarray analysis we have found that depletion of HIPK2, that strongly affects p53 transcriptional function, induces Nox1 upregulation. Nox1, a homologue of the catalytic subunit of the superoxide-generating NADPH-oxidase is often overexpressed in tumors where triggers the angiogenic switch and induces genome instability by producing oxygen reactive species (ROS). However, the involvement of Nox1 in p53 regulation has never been addressed. We found that HIPK2 downregulated Nox1-luc promoter activity and that Nox1 upregulation, following HIPK2 silencing or Nox1 overexpression, inhibited drug-induced p53 Lys382 acetylation that strongly affected p53 apoptotic activity. P53Lys382 is a target of SirT1, a NAD-dependent deacetylase that is often upregulated in tumors and inhibits p53 apoptotic function. By the use of either small interfering RNAs to target SIRT1 or the SirT1 inhibitor nicotinamide we found that Nox1-dependent inhibition of p53 transcriptional activity was SirT1-dependent. Thus, Nox1 was unable to inhibit p53 when coexpressed with a SirT1 deacetylase-defective mutant (SIRT1HY) suggesting a link between Nox1 and SirT1 activity and a novel mechanism of SirT1 activation. Finally, recovery of HIPK2 function, by the use of Doxicyclin-induced conditional interference, downregulated Nox1 expression and reduced ROS production with rescue of p53Lys382 acetylation and p53 apoptotic activity. These findings suggest that Nox1 is a novel target of HIPK2 repressor function and is linked to p53 inactivation involving SirT1 activation and probably ROS production. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 184. doi:10.1158/1538-7445.AM2011-184