HIPK2 overexpression leads to stabilization of p53 protein and increased p53 transcriptional activity by decreasing Mdm2 protein levels

We show here that HIPK2 activates transcription mediated by tumor suppressor p53 responsive promoter elements. Overexpression of HIPK2 leads to an increase of p53 protein expression or stability, which becomes enhanced further in the presence of the DNA damaging drug doxorubicin. The effects of HIPK2 on p53 are not observed with kinase deficient HIPK2 mutants. However, HIPK2 is not sufficient for phosphorylation of three crucial serine residues of p53, suggesting that HIPK2-induced p53 activation does not involve phosphorylation of p53. Instead, HIPK2 leads to a downregulation of p53-induced Mdm2 protein and this may lead to stabilization of p53. Overexpression of HIPK2 does not lead to a change of Mdm2 mRNA expression. The data suggest that HIPK2 plays a critical role in p53 mediated cellular responses by removing the p53 inhibitor protein Mdm2 via modification of the protein itself or its intracellular movement.The tumor suppressor protein p53 contributes to the control of cell cycle checkpoints and apoptosis and is frequently lost or mutated in multiple types of human cancers [1]. DNA damaging agents induce p53 accumulation and induction of p53-mediated transcription [2,3]. Several proteins are known to play a crucial role in the stabilization and activation of p53 [4,5]. The association with murine double minute clone 2 (Mdm2) leads to a susceptibility of p53 for proteolysis [6,7] and thus, p53 protein levels are regulated post-transcriptionally [8].Homeodomain-interacting protein kinase 2 (HIPK2) has been recently described as a member of a family of nuclear kinases that act as co-repressors for homeodomain transcription factors [9] and it is a potential interaction partner for interferon type I induced Mx GTPases with antiviral activity against several RNA viruses [10]. HIPK2 is regulated by the ubiquitin-like protein SUMO-1 and the covalent SUMO-1 modification correlates with its localization to nuclear speckles or nuclear dots [11]. Here we address the quest