Cinzia Rinaldo

HIPK2: a multitalented partner for transcription factors in DNA damage response and development.

Protein phosphorylation is a widely diffuse and versatile post-translational modification that controls many cellular processes, from signal transduction to gene transcription. The homeodomain-interacting protein kinases (HIPKs) belong to a new family of serine-threonine kinases first identified as corepressors for homeodomain transcription factors. Different screenings for the identification of new partners of transcription factors have indicated that HIPK2, the best characterized member of the HIPK family, is a multitalented coregulator of an increasing number of transcription factors and cofactors. The aim of this review is to describe the different mechanisms through which HIPK2 regulates gene transcription.

High-mobility group A1 inhibits p53 by cytoplasmic relocalization of its proapoptotic activator HIPK2

High-mobility group A1 (HMGA1) overexpression and gene rearrangement are frequent events in human cancer, but the molecular basis of HMGA1 oncogenic activity remains unclear. Here we describe a mechanism through which HMGA1 inhibits p53-mediated apoptosis by counteracting the p53 proapoptotic activator homeodomain-interacting protein kinase 2 (HIPK2). We found that HMGA1 overexpression promoted HIPK2 relocalization in the cytoplasm and inhibition of p53 apoptotic function, while HIPK2 overexpression reestablished HIPK2 nuclear localization and sensitivity to apoptosis. HIPK2 depletion by RNA interference suppressed the antiapoptotic effect of HMGA1, which indicates that HIPK2 is the target required for HMGA1 to repress the apoptotic activity of p53. Consistent with this process, a strong correlation among HMGA1 overexpression, HIPK2 cytoplasmic localization, and low spontaneous apoptosis index (comparable to that observed in mutant p53-carrying tumors) was observed in WT p53-expressing human breast carcinomas. Hence, cytoplasmic relocalization of HIPK2 induced by HMGA1 overexpression is a mechanism of inactivation of p53 apoptotic function that we believe to be novel.

The Loss of the p53 Activator HIPK2 Is Responsible for Galectin-3 Overexpression in Well Differentiated Thyroid Carcinomas

Background Galectin-3 (Gal-3) is an anti-apoptotic molecule involved in thyroid cells transformation. It is specifically overexpressed in thyroid tumour cells and is currently used as a preoperative diagnostic marker of thyroid malignancy. Gal-3 expression is downregulated by wt-p53 at the transcriptional level. In well-differentiated thyroid carcinomas (WDTCs) there is an unexplained paradoxical concomitant expression of Gal-3 and wt-p53. HIPK2 is a co-regulator of different transcription factors, and modulates basic cellular processes mainly through the activation of wt-p53. Since we demonstrated that HIPK2 is involved in p53-mediated Gal-3 downregulation, we asked whether HIPK2 deficiency might be responsible for such paradoxical Gal-3 overexpression in WDTC. Methodology/Principal Findings We analyzed HIPK2 protein and mRNA levels, as well as loss of heterozygosity (LOH) at the HIPK2 locus (7q32-34), in thyroid tissue samples. HIPK2 protein levels were high in all follicular hyperplasias (FHs) analyzed. Conversely, HIPK2 was undetectable in 91.7% of papillary thyroid carcinomas (PTCs) and in 60.0% of follicular thyroid carcinomas (FTCs). HIPK2 mRNA levels were upregulated in FH compared to normal thyroid tissue (NTT), while PTC showed mean HIPK2 mRNA levels lower than FH and, in 61.5% of cases, also lower than NTT. We found LOH at HIPK-2 gene locus in 37.5% of PTCs, 14.3% of FTCs and 18.2% of follicular adenomas. To causally link these data with Gal-3 upregulation, we performed in vitro experiments, using the PTC-derived K1 cells, in which HIPK2 expression was manipulated by RNA interference (RNAi) or plasmid-mediated overexpression. HIPK2 RNAi was associated with Gal-3 upregulation, while HIPK2 overexpression with Gal-3 downregulation. Conclusions/Significance Our results indicate that HIPK2 expression and function are impaired in WDTCs, in particular in PTCs, and that this event explains Gal-3 overexpression typically observed in these types of tumours. Therefore, HIPK2 can be considered as a new tumour suppressor gene for thyroid cancers.

Repression of the Antiapoptotic Molecule Galectin-3 by Homeodomain-Interacting Protein Kinase 2-Activated p53 Is Required for p53-Induced Apoptosis

ABSTRACT Galectin 3 (Gal-3), a member of the β-galactoside binding lectin family, exhibits antiapoptotic functions, and its aberrant expression is involved in various aspects of tumor progression. Here we show that p53-induced apoptosis is associated with transcriptional repression of Gal-3. Previously, it has been reported that phosphorylation of p53 at Ser46 is important for transcription of proapoptotic genes and induction of apoptosis and that homeodomain-interacting protein kinase 2 (HIPK2) is specifically involved in these functions. We show that HIPK2 cooperates with p53 in Gal-3 repression and that this cooperation requires HIPK2 kinase activity. Gene-specific RNA interference demonstrates that HIPK2 is essential for repression of Gal-3 upon induction of p53-dependent apoptosis. Furthermore, expression of a nonrepressible Gal-3 prevents HIPK2- and p53-induced apoptosis. These results reveal a new apoptotic pathway induced by HIPK2-activated p53 and requiring repression of the antiapoptotic factor Gal-3.

MDM4 (MDMX) localizes at the mitochondria and facilitates the p53-mediated intrinsic-apoptotic pathway

MDM4 is a key regulator of p53, whose biological activities depend on both transcriptional activity and transcription‐independent mitochondrial functions. MDM4 binds to p53 and blocks its transcriptional activity; however, the main cytoplasmic localization of MDM4 might also imply a regulation of p53‐mitochondrial function. Here, we show that MDM4 stably localizes at the mitochondria, in which it (i) binds BCL2, (ii) facilitates mitochondrial localization of p53 phosphorylated at Ser46 (p53Ser46P) and (iii) promotes binding between p53Ser46P and BCL2, release of cytochrome C and apoptosis. In agreement with these observations, MDM4 reduction by RNA interference increases resistance to DNA‐damage‐induced apoptosis in a p53‐dependent manner and independently of transcription. Consistent with these findings, a significant downregulation of MDM4 expression associates with cisplatin resistance in human ovarian cancers, and MDM4 modulation affects cisplatin sensitivity of ovarian cancer cells. These data define a new localization and function of MDM4 that, by acting as a docking site for p53Ser46P to BCL2, facilitates the p53‐mediated intrinsic‐apoptotic pathway. Overall, our results point to MDM4 as a double‐faced regulator of p53.

Sgk1 activates MDM2-dependent p53 degradation and affects cell proliferation, survival, and differentiation

Serum and glucocorticoid regulated kinase 1 (Sgk1) is a serine–threonine kinase that is activated by serum, steroids, insulin, vasopressin, and interleukin 2 at the transcriptional and post-translational levels. Sgk1 is also important in transduction of growth factors and steroid-dependent survival signals and may have a role in the development of resistance to cancer chemotherapy. In the present paper, we demonstrate that Sgk1 activates MDM2-dependent p53 ubiquitylation. The results were obtained in RKO cells and other cell lines by Sgk1-specific RNA silencing and were corroborated in an original mouse model as well as in transiently and in stably transfected HeLa cells expressing wild-type or dominant negative Sgk1 mutant. Sgk1 contributes to cell survival, cell-cycle progression, and epithelial de-differentiation. We also show that the effects of Sgk1 on the clonogenic potential of different cancer cells depend on the expression of wild-type p53. Since transcription of Sgk1 is activated by p53, we propose a finely tuned feedback model where Sgk1 down-regulates the expression of p53 by enhancing its mono- and polyubiquitylation.

HIPK2 Regulation by MDM2 Determines Tumor Cell Response to the p53-Reactivating Drugs Nutlin-3 and RITA

In the past few years, much effort has been devoted to show the single-target specificity of nongenotoxic, p53 reactivating compounds. However, the divergent biological responses induced by the different compounds, even in the same tumor cells, demand additional mechanistic insights, whose knowledge may lead to improved drug design or selection of the most potent drug combinations. To address the molecular mechanism underlying induction of mitotic arrest versus clinically more desirable apoptosis, we took advantage of two MDM2 antagonists, Nutlin-3 and RITA, which respectively produce these two outcomes. We show that, along with p53 reactivation, the proapoptotic p53-activator HIPK2 is degraded by MDM2 in Nutlin-3-treated cells, but activated by transiently reduced MDM2 levels in RITA-treated ones. Gain- and loss-of-function experiments revealed the functional significance of MDM2-mediated HIPK2 regulation in cell decision between mitotic arrest and apoptosis in both types of p53 reactivation. These data indicate that strategies of p53 reactivation by MDM2 inhibition should also take into consideration MDM2 targets other than p53, such as the apoptosis activator HIPK2.

HIPKs: Jack of all trades in basic nuclear activities

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.

Updates on HIPK2: a resourceful oncosuppressor for clearing cancer

Homeodomain-interacting protein kinase 2 (HIPK2) is a multitalented protein that exploits its kinase activity to modulate key molecular pathways in cancer to restrain tumor growth and induce response to therapies. HIPK2 phosphorylates oncosuppressor p53 for apoptotic activation. In addition, also p53-independent apoptotic pathways are regulated by HIPK2 and can be exploited for anticancer purpose too. Therefore, HIPK2 activity is considered a central switch in targeting tumor cells toward apoptosis upon genotoxic damage and the preservation and/or restoration of HIPK2 function is crucial for an efficient tumor response to therapies. As a proof of principle, HIPK2 knockdown impairs p53 function, induces chemoresistance, angiogenesis, and tumor growth in vivo, on the contrary, HIPK2 overexpression activates apoptotic pathways, counteracts hypoxia, inhibits angiogenesis, and induces chemosensitivity both in p53-dependent and -independent ways. The role of HIPK2 in restraining tumor development was also confirmed by studies with HIPK2 knockout mice. Recent findings demonstrated that HIPK2 inhibitions do exist in tumors and depend by several mechanisms including HIPK2 cytoplasmic localization, protein degradation, and loss of heterozygosity (LOH), recapitulating the biological outcome obtained by RNA interference studies in tumor cells, such as p53 inactivation, resistance to therapies, apoptosis inhibition, and tumor progression. These findings may lead to new diagnostic and therapeutic approaches for treating cancer patients. This review will focus on the last updates about HIPK2 contribution in tumorigenesis and cancer treatment.

HIPK2 is involved in cell proliferation and its suppression promotes growth arrest independently of DNA damage

Introduction/objectives:  The serine/threonine kinase homeodomain‐interacting protein kinase 2 (HIPK2) is a co‐regulator of an increasing number of transcription factors and cofactors involved in DNA damage response and development. We and others have cloned HIPK2 as an interactor of the p53 oncosuppressor, and have studied the role of this interaction in cell response to stress. Nevertheless, our original cloning of HIPK2 as a p53‐binding protein, was aimed at discovering partners of p53 involved in cell differentiation and development, still controversial p53 functions. To this aim, we used p53 as bait in yeast two‐hybrid screening of a cDNA library from mouse embryo (day 11 postcoitus) when p53 is highly expressed.