Tumor Protein p53-Induced Nuclear Protein 1 (TP53INP1) is a tumor suppressor that modulates the p53 response to stress. TP53INP1 is one of the key mediators of p53 antioxidant function by promoting the p53 transcriptional activity on its target genes. TP53INP1 expression is deregulated in many types of cancers including pancreatic ductal adenocarcinoma in which its decrease occurs early during the preneoplastic development. In this work, we report that redox-dependent induction of p53 transcriptional activity is enhanced by the oxidative stress-induced SUMOylation of TP53INP1 at lysine 113. This SUMOylation is mediated by PIAS3 and CBX4, two SUMO ligases especially related to the p53 activation upon DNA damage. Importantly, this modification is reversed by three SUMO1-specific proteases SENP1, 2 and 6. Moreover, TP53INP1 SUMOylation induces its binding to p53 in the nucleus under oxidative stress conditions. TP53INP1 mutation at lysine 113 prevents the pro-apoptotic, antiproliferative and antioxidant effects of TP53INP1 by impairing the p53 response on its target genes p21, Bax and PUMA. We conclude that TP53INP1 SUMOylation is essential for the regulation of p53 activity induced by oxidative stress. Cell Death and Differentiation (2014) 21, 1107-1118; doi:10.1038/cdd.2014.28; published online 7 March 2014Maintaining homeostasis in response to the broad range of intrinsic and extrinsic aggressions is a challenge for cells. Cellular outcomes are varied and involve cell-cycle arrest, apoptosis, DNA repair, autophagy, senescence, antioxidant activity, cell migration, differentiation, embryo implantation, metabolism and angiogenesis.1-9 An inadequate cell response can lead to cellular transformation and cancer initialization. Tumor Protein p53-Induced Nuclear Protein 1 (TP53INP1) is a p53 cofactor. The gene TP53INP1 encodes for two protein isoforms generated by alternative splicing, named TP53INP1a and TP53INP1b. TP53INP1 is a p53 target gene and its expression is induced in response to several physical and chemical stresses.10-12 TP53INP1 directly interacts with p53 and also binds kinases, such as HIPK2 and PKCd, which modulate p53 transcriptional activity by phosphorylation, thereby creating a positive feedback loop between p53 and TP53INP1.13,14 Our laboratory demonstrated the role of TP53INP1 as a tumor suppressor as TP53INP1-deficient mice present an increased susceptibility to tumor development. Moreover, TP53INP1 expression is lost at very early steps of pancreatic carcinogenesis due to miR155 activity and, when its expression is restored in pancreatic cancer cells, it suppresses growth of xenograft tumors by favoring apoptotic cell death. 15,16 Interestingly, the role of TP53INP1 as a tumor suppressor is associated with its ability to enhance the p53 antioxidant function. 17 In accordance, TP53INP1 transcriptional induction upon oxidative stress is strictly dependent on p53, and TP53INP1-deficient cells accumulate more intracellular reactive oxygen species (ROS) than wild-type cells. This ROS accumulation is...