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Abstract: HIPK2 (homeodomain-interacting protein kinase-2) binds to and phosphorylates, at Ser and Thr residues, a large number of targets involved in cell division and cell fate decision in response to different physiological or stress stimuli. Inactivation of HIPK2 has been observed in human and mouse cancers supporting its role as a tumor suppressor. Despite the biological relevance of this kinase, very little is known on how HIPK2 becomes catalytically active. Based on sequence homologies, HIPK2 has been taxonomically classified as a subfamily member of the dual-specificity tyrosine-regulated kinases (DYRKs) and the activation-loop Y354 of HIPK2 has been found phosphorylated in different cells; however, the relevance of this Y phosphorylation is presently unknown. Here, we show that HIPK2, which is extensively phosphorylated at S/T sites throughout its functional domains, becomes catalytically active by autophosphorylation at the activation-loop Y354. In particular, we found that, in analogy to DYRKs, HIPK2-Y354 phosphorylation is an autocatalytic event and its prevention, through Y354 substitution with non-phosphorylatable amino acids or by using the kinase inhibitor purvalanol A, induces a strong reduction of the HIPK2 S/T-kinase activity on different substrates. Interestingly, at variance from DYRKs, inhibition of HIPK2-Y354 phosphorylation induces a strong out-of-target Y-kinase activity in cis and a strong cytoplasmic relocalization of the kinase. Together, these results demonstrate that the catalytic activity, substrate specificity, and subcellular localization of HIPK2 are regulated by autophosphorylation of its activation-loop Y354. [Copyright &y& Elsevier]

HIPK2 is a serine/threonine kinase that acts as a coregulator of an increasing number of factors involved in cell survival and proliferation during development and in response to different types of stress. Here we report on a novel target of HIPK2, the cyclin-dependent kinase inhibitor p27kip1. HIPK2 phosphorylates p27kip1 in vitro and in vivo at serine 10, an event that accounts for 80% of the total p27kip1 phosphorylation and plays a crucial role in the stability of the protein. Indeed, HIPK2 depletion by transient or stable RNA interference in tumor cells of different origin was consistently associated with strong reduction of p27kip1 phosphorylation at serine 10 and of p27kip1 stability. An initial evaluation of the functional relevance of this HIPK2-mediated regulation of p27kip1 revealed a contribution to cell motility, rather than to cell proliferation, but only in cells that do not express wild-type p53. [ABSTRACT FROM AUTHOR]

Abstract: Over the past decade several investigators have reported on the physical interaction of serine/threonine kinases of the homeodomain interacting-protein family (HIPKs) with increasing number of nuclear factors and on their localization in different nuclear sub-compartments. Although we are still far from a global understanding of the molecular consequences of HIPK subnuclear compartmentalization, the spatial description of particular interactions and posttranslational modifications promoted by these kinases on key cellular regulators might provide relevant insights. Here we will discuss the possible implications of the HIPK subnuclear localization in the regulation of gene transcription and in the cell response to stress. [Copyright &y& Elsevier]

Summary: Che-1 is a RNA polymerase II-binding protein involved in the transcription of E2F target genes and induction of cell proliferation. Here we show that Che-1 contributes to DNA damage response and that its depletion sensitizes cells to anticancer agents. The checkpoint kinases ATM/ATR and Chk2 interact with Che-1 and promote its phosphorylation and accumulation in response to DNA damage. These Che-1 modifications induce a specific recruitment of Che-1 on the TP53 and p21 promoters. Interestingly, it has a profound effect on the basal expression of p53, which is preserved following DNA damage. Notably, Che-1 contributes to the maintenance of the G2/M checkpoint induced by DNA damage. These findings identify a mechanism by which checkpoint kinases regulate responses to DNA damage. [Copyright &y& Elsevier]

Presents a study showing that the TP53INP1 gene localize with p53, PML-IV and HIPK2 into the PML-NB. Physical interaction of the gene with HIPK2 and p53; Role of TP53INP1, in association with HIPK2, regulate p53 transcriptional activity on p21, mdm2, pig3 and bax promoters; Impact of TP53INP1 overexpression on p53-mediated apoptosis.

DNA tumor virus oncoproteins bind and inactivate Rb by interfering with the Rb/HDAC1 interaction. Che-1 is a recently identified human Rb binding protein that inhibits the Rb growth suppressing function. Here we show that Che-1 contacts the Rb pocket region and competes with HDAC1 for Rb binding site, removing HDAC1 from the Rb/E2F complex in vitro and from the E2F target promoters in vivo. Che-1 overexpression activates DNA synthesis in quiescent NIH-3T3 cells through HDAC1 displacement. Consistently, Che-1-specific RNA interference affects E2F activity and cell proliferation in human fibroblasts but not in the pocket protein-defective 293 cells. These findings indicate the existence of a pathway of Rb regulation supporting Che-1 as the cellular counterpart of DNA tumor virus oncoproteins. [Copyright &y& Elsevier]