Autophosphorylation and Pin1 binding coordinate DNA damage-induced HIPK2 activation and cell death.

Excessive genome damage activates the apoptosis response. Protein kinase HIPK2 is a key regulator of DNA damage-induced apoptosis. Here, we deciphered the molecular mechanism of HIPK2 activation and show its relevance for DNA damage-induced apoptosis in cellulo and in vivo. HIPK2 autointeracts and site-specifically autophosphorylates upon DNA damage at Thr880/Ser882. Autophosphorylation regulates HIPK2 activity and mutation of the phosphorylation-acceptor sites deregulates p53 Ser46 phosphorylation and apoptosis in cellulo. Moreover, HIPK2 autophosphorylation is conserved between human and zebrafish and is important for DNA damage-induced apoptosis in vivo. Mechanistically, autophosphorylation creates a binding signal for the phospho-specific isomerase Pin1. Pin1 links HIPK2 activation to its stabilization by inhibiting HIPK2 polyubiquitination and modulating Siah-1-HIPK2 interaction. Concordantly, Pin1 is required for DNA damage-induced HIPK2 stabilization and p53 Ser46 phosphorylation and is essential for induction of apotosis both in cellulo and in zebrafish. Our results identify an evolutionary conserved mechanism regulating DNA damage-induced apoptosis.

HIPK2: A tumour suppressor that controls DNA damage-induced cell fate and cytokinesis.

In response to DNA-damage, cells have to decide between different cell fate programmes. Activation of the tumour suppressor HIPK2 specifies the DNA damage response (DDR) and tips the cell fate balance towards an apoptotic response. HIPK2 is activated by the checkpoint kinase ATM, and triggers apoptosis through regulatory phosphorylation of a set of cellular key molecules including the tumour suppressor p53 and the anti-apoptotic corepressor CtBP. Recent work has identified HIPK2 as a regulator of the ultimate step in cytokinesis: the abscission of the mother and daughter cells. Since proper cytokinesis is essential for genome stability and maintenance of correct ploidy, this finding sheds new light on the tumour suppressor function of HIPK2. Here we highlight the molecular mechanisms coordinating HIPK2 function and discuss its emerging role as a tumour suppressor.

Transformation suppressor protein Pdcd4 interferes with JNK-mediated phosphorylation of c-Jun and recruitment of the coactivator p300 by c-Jun.

The transformation suppressor gene Pdcd4 (programmed cell death gene 4) inhibits the tumor-promoter mediated transformation of mouse keratinocytes and has recently been implicated as a potential tumor suppressor gene in the development of human lung cancer. Biochemical analysis has suggested that the Pdcd4 protein is involved in protein translation as well as in nuclear events. Recent work has shown that Pdcd4 suppresses the transactivation of AP-1 responsive promoters by c-Jun, suggesting that the transformation-suppressor activity of Pdcd4 might be due, at least in part, to the inhibition of c-Jun activity. Here, we have addressed how Pdcd4 inhibits c-Jun. We show that Pdcd4 interferes with the phosphorylation of c-Jun by Jun N-terminal kinase (JNK). The inhibition of c-Jun phosphorylation by Pdcd4 appears not to be due to a general suppression of JNK activity, our data rather suggest that Pdcd4 interacts with c-Jun and thereby blocks phosphorylation of c-Jun. In addition to affecting c-Jun phosphorylation, Pdcd4 blocks the recruitment of the coactivator p300 by c-Jun. Taken together, our results strongly suggest that Pdcd4 is directly involved in the regulation of c-Jun activity.

Hypoxia suppresses chemotherapeutic drug-induced p53 Serine 46 phosphorylation by triggering HIPK2 degradation.

The molecular mechanisms by which hypoxic tumor cells escape radio- and chemotherapy are largely unclear. Homeodomain-interacting protein kinase 2 (HIPK2) drives the apoptotic program in response to DNA-damaging chemotherapeutic drug treatment by phosphorylating the tumor suppressor protein p53 at Ser46. HIPK2 is kept inactive in unstressed cells through ubiquitination and degradation facilitated by the ubiquitin ligases WSB1 and Siah1. Here, we demonstrate that HIPK2 is degraded during hypoxia in a proteasome-dependent and partially Siah1-dependent fashion. Concordantly, hypoxic tumor cells show an impaired p53 Ser46 phosphorylation in response to treatment with the chemotherapeutic Adriamycin. Remarkably, proteasome-inhibition rescues HIPK2 expression in hypoxic hepatoma cells and restores p53 Ser46 phosphorylation and caspase activity after Adriamycin treatment. Our findings suggest a molecular mechanism by which hypoxic cancer cells can escape chemotherapeutic drug treatment and suggest proteasome-inhibition as a promising approach to sensitise hypoxic cancer cells to therapy.

Apoptosis and autophagy: Regulation of apoptosis by DNA damage signalling - roles of p53, p73 and HIPK2.

Genomic stability is constantly threatened by DNA damage, caused by numerous environmental and intrinsic sources, including radiation, chemicals and oncogene expression. Consequently, cells have evolved a sophisticated signal transduction network to sense DNA damage and to mount an appropriate DNA damage response. Dysregulation of the DNA damage response leads to genomic instability and cancer. Dependent on the cellular background and extent of DNA damage, the DNA damage response triggers cell cycle arrest and DNA repair, or in the case of irreparable damage, inactivation of the cells by senescence or apoptosis. In this minireview, we concentrate on the apoptotic response to DNA damage and signalling pathways linked to the cell nucleus and nuclear bodies, with a particular focus on the molecular players p53 and p73 and on the DNA damage-activated kinase homeodomain-interacting protein kinase 2 (HIPK2).