HIPK2-induced p53Ser46 phosphorylation activates the KILLER/DR5-mediated caspase-8 extrinsic apoptotic pathway

BCAS3 accelerates glioblastoma tumorigenesis by restraining the P53/GADD45α signaling pathway

As in many other cancers, highly malignant proliferation and disordered cell division play irreplaceable roles in the exceedingly easy recurrence and complex progression of glioblastoma multiforme (GBM); however, mechanistic studies of the numerous regulators involved in this process are still insufficiently thorough. The role of BCAS3 has been studied in other cancers, but its role in GBM is unclear. Here, our goal was to investigate the expression pattern of BCAS3 in GBM and its potential mechanism of action. Using TCGA database and human GBM samples, we found that BCAS3 expression was up-regulated in GBM, and its high expression predicted poor prognosis. To further investigate the relationship between BCAS3 and GBM characteristics, we up-regulated and down-regulated BCAS3 expression in GBM to detect its effect on cell proliferation and cell cycle. At the same time, we established U87 cells stably overexpressing BCAS3 and generated an intracranial xenograft model to investigate the Potential role of BCAS3 in vivo. Finally, based on in vitro cell experiments and in vivo GBM xenograft models, we observed that BCAS3 significantly regulates GBM cell proliferation and cell cycle and that this regulation is associated with p53/GADD45α Signaling pathway. Taken together, our findings suggest that BCAS3 is inextricably linked to the progression of GBM and that targeting BCAS3 may have therapeutic effects in GBM patients.

HIPK2 inhibits cell metastasis and improves chemosensitivity in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive and lethal malignancies worldwide. At present, the underlying mechanisms of ESCC development and progression are poorly understood. Previous studies have demonstrated that homeodomain-interacting protein kinase-2 (HIPK2) serves an important role in cancer biology, particularly in proliferation and metastasis. However, the role of HIPK2 in ESCC cells remains unclear. In the current study, the expression of HIPK2 in ESCC specimens, adjacent non-cancerous tissues and cell lines was assessed using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The effects of HIPK2 on cell metastasis, epithelial-mesenchymal transition (EMT) and proliferation were studied using a Transwell assay, RT-qPCR and a Cell Counting Kit-8 assay, respectively. The results indicated that HIPK2 expression was downregulated in ESCC specimens and cell lines, and HIPK2 expression was associated with tumor invasion and lymph node metastasis. Functional studies demonstrated that HIPK2 overexpression inhibited cell metastasis and EMT. Furthermore, HIPK2 overexpression suppressed cell viability during cisplatin treatment. These results suggest that HIPK2 serves an important role in regulating metastasis and the chemosensitivity of ESCC cells, implicating the potential application of HIPK2 in ESCC therapy.

Apoptosis as anticancer mechanism: function and dysfunction of its modulators and targeted therapeutic strategies

Apoptosis is a form of programmed cell death that results in the orderly and efficient removal of damaged cells, such as those resulting from DNA damage or during development. Apoptosis can be triggered by signals from within the cell, such as genotoxic stress, or by extrinsic signals, such as the binding of ligands to cell surface death receptors. Deregulation in apoptotic cell death machinery is an hallmark of cancer. Apoptosis alteration is responsible not only for tumor development and progression but also for tumor resistance to therapies. Most anticancer drugs currently used in clinical oncology exploit the intact apoptotic signaling pathways to trigger cancer cell death. Thus, defects in the death pathways may result in drug resistance so limiting the efficacy of therapies. Therefore, a better understanding of the apoptotic cell death signaling pathways may improve the efficacy of cancer therapy and bypass resistance. This review will highlight the role of the fundamental regulators of apoptosis and how their deregulation, including activation of anti-apoptotic factors (i.e., Bcl-2, Bcl-xL, etc) or inactivation of pro-apoptotic factors (i.e., p53 pathway) ends up in cancer cell resistance to therapies. In addition, therapeutic strategies aimed at modulating apoptotic activity are briefly discussed.

Oxidant species are involved in T/B-mediated ERK1/2 phosphorylation that activates p53-p21 axis to promote KSHV lytic cycle in PEL cells

KSHV is a gammaherpesvirus strongly associated to human cancers such as Primary Effusion Lymphoma (PEL) and Kaposi's Sarcoma. The naturally virus-infected tumor cells usually display latent infection since a minority of cells undergoes spontaneous viral replication. The lytic cycle can be induced in vitro upon appropriate stimuli such as TPA (T), alone or in combination with butyrate (B), (T/B). In previous studies, Protein Kinase C (PKC) δ, Extracellular Signal-regulated Kinase1/2 (ERK1/2) and p53-p21 axis have been separately reported to play a role in KSHV reactivation from latency. Here, we found that these pathways were interconnected to induce KSHV lytic cycle in PEL cells treated with T/B. T/B also increased H₂O₂ that played an important role in the activation of these pathways. Oxidant specie production correlated with PKC δ activation, as the PKC δ inhibitor rottlerin reduced both H₂O₂ and KSHV lytic antigen expression. H₂O₂ contributed to T/B-mediated ERK1/2 activation that mediated p53 phosphorylation at serine 15 (Ser15) and increased p21 expression. Oxidant specie inhibition by quercetin indeed strongly reduced the activation of these pathways, lytic antigen expression and interestingly it also increased T/B-induced cell death. The use of ERK inhibitor PD98059 or p53 silencing demonstrated the importance of p53Ser15 phosphorylation and of p53-p21 axis in KSHV lytic cycle activation. Understanding the role of oxidant species and the molecular mechanisms involved in KSHV lytic cycle induction is particularly important since oxidant species represent the most physiological stimulus for viral reactivation in vivo and it is known that viral production contributes to the maintenance/progression of KSHV associated malignancies.

Homeodomain-interacting protein kinase 2 (HIPK2): a promising target for anti-cancer therapies

The HIPK2 (serine/threonine homeodomain-interacting protein kinase 2) is a "caretaker" gene, its inactivation increases tumorigenicity while its activation inhibits tumor growth. This report reviews the anti-tumorigenic mechanisms of HIPK2, which include promotion of apoptosis, inhibition of angiogenesis in hypoxia, prevention of tumor invasion/metastasis and attenuation of multidrug resistance in cancer. Additionally, we summarize conditions or factors that may increase HIPK2 activity.

Erastin Disrupts Mitochondrial Permeability Transition Pore (mPTP) and Induces Apoptotic Death of Colorectal Cancer Cells

We here evaluated the potential anti-colorectal cancer activity by erastin, a voltage-dependent anion channel (VDAC)-binding compound. Our in vitro studies showed that erastin exerted potent cytotoxic effects against multiple human colorectal cancer cell lines, possibly via inducing oxidative stress and caspase-9 dependent cell apoptosis. Further, mitochondrial permeability transition pore (mPTP) opening was observed in erastin-treated cancer cells, which was evidenced by VDAC-1 and cyclophilin-D (Cyp-D) association, mitochondrial depolarization, and cytochrome C release. Caspase inhibitors, the ROS scavenger MnTBAP, and mPTP blockers (sanglifehrin A, cyclosporin A and bongkrekic acid), as well as shRNA-mediated knockdown of VDAC-1, all significantly attenuated erastin-induced cytotoxicity and apoptosis in colorectal cancer cells. On the other hand, over-expression of VDAC-1 augmented erastin-induced ROS production, mPTP opening, and colorectal cancer cell apoptosis. In vivo studies showed that intraperitoneal injection of erastin at well-tolerated doses dramatically inhibited HT-29 xenograft growth in severe combined immunodeficient (SCID) mice. Together, these results demonstrate that erastin is cytotoxic and pro-apoptotic to colorectal cancer cells. Erastin may be further investigated as a novel anti-colorectal cancer agent.

3H-1,2-dithiole-3-thione protects retinal pigment epithelium cells against Ultra-violet radiation via activation of Akt-mTORC1-dependent Nrf2-HO-1 signaling

Excessive UV radiation and reactive oxygen species (ROS) cause retinal pigment epithelium (RPE) cell injuries. Nrf2 regulates transcriptional activation of many anti-oxidant genes. Here, we tested the potential role of 3H-1,2-dithiole-3-thione (D3T) against UV or ROS damages in cultured RPE cells (both primary cells and ARPE-19 line). We showed that D3T significantly inhibited UV-/H₂O₂-induced RPE cell death and apoptosis. UV-stimulated ROS production was dramatically inhibited by D3T pretreatment. D3T induced Nrf2 phosphorylation in cultured RPE cells, causing Nrf2 disassociation with KEAP1 and its subsequent nuclear accumulation. This led to expression of antioxidant response elements (ARE)-dependent gene heme oxygenase-1 (HO-1). Nrf2-HO-1 activation was required for D3T-mediated cytoprotective effect. Nrf2 shRNA knockdown or S40T dominant negative mutation as well as the HO-1 inhibitor Zinc protoporphyrin (ZnPP) largely inhibited D3T’s RPE cytoprotective effects against UV radiation. Yet, exogenous overexpression Nrf2 enhanced D3T’s activity in RPE cells. Further studies showed that D3T activated Akt/mTORC1 in cultured RPE cells. Akt-mTORC1 inhibitors, or Akt1 knockdown by shRNA, not only inhibited D3T-induced Nrf2-HO-1 activation, but also abolished the RPE cytoprotective effects. In vivo, D3T intravitreal injection protected from light-induced retinal dysfunctions in mice. Thus, D3T protects RPE cells from UV-induced damages via activation of Akt-mTORC1-Nrf2-HO-1 signaling axis.

Ampelopsin induces apoptosis in HepG2 human hepatoma cell line through extrinsic and intrinsic pathways: Involvement of P38 and ERK

Our results showed that ampelopsin significantly inhibited cell viability of hepatoma HepG2 cells using MTT assay. We further investigated the mechanism of anticancer activity by ampelopsin, it showed that ampelopsin induced apoptosis of HepG2 cells using DAPI assay and flow cytometry, which was confirmed by activation of PARP. Next, activation of the caspase cascades were demonstrated, including caspase-8, -9 and -3. We also found that ampelopsin increased the levels of death receptor 4 (DR4), death receptor 5 (DR5) and decreased the expression of Bcl-2 protein, which led to an increase of the Bax/Bcl-2 ratio. Meanwhile, the release of cytochrome c from mitochondria was observed. Ampelopsin decreased the levels of iNOS and COX-2 but had no impact on the level of reactive oxygen species (ROS). In addition, ampelopsin activated ERK1/2 and P38, but little JNK1/2 activation was detected. Further investigation showed that suppression of P38 activation by SB203580 increased the cell viability and also prevented cleavage of caspase-3 and PARP, inhibition of ERK1/2 with U0126 had the opposite action. In conclusion, our results indicated that ampelopsin mainly elicited apoptosis through extrinsic and intrinsic pathway and that ERK1/2 and P38 had opponent effects on the apoptosis.

XAF1 directs apoptotic switch of p53 signaling through activation of HIPK2 and ZNF313

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a tumor suppressor that is frequently inactivated in many human cancers. However, the molecular mechanism underlying its growth-inhibitory function remains largely unknown. Here, we report that XAF1 forms a positive feedback loop with p53 and acts as a molecular switch in p53-mediated cell-fate decisions favoring apoptosis over cell-cycle arrest. XAF1 binds directly to the N-terminal proline-rich domain of p53 and thus interferes with E3 ubiquitin ligase MDM2 binding and ubiquitination of p53. XAF1 stimulates homeodomain-interacting protein kinase 2 (HIPK2)-mediated Ser-46 phosphorylation of p53 by blocking E3 ubiquitin ligase Siah2 interaction with and ubiquitination of HIPK2. XAF1 also steps up the termination of p53-mediated cell-cycle arrest by activating zinc finger protein 313 (ZNF313), a p21(WAF1)-targeting ubiquitin E3 ligase. XAF1 interacts with p53, Siah2, and ZNF313 through the zinc finger domains 5, 6, and 7, respectively, and truncated XAF1 isoforms preferentially expressed in cancer cells fail to form a feedback loop with p53. Together, this study uncovers a novel role for XAF1 in p53 stress response, adding a new layer of complexity to the mechanisms by which p53 determines cell-fate decisions.

High glucose dephosphorylates serine 46 and inhibits p53 apoptotic activity

Background

In response to diverse genotoxic stimuli p53 is activated as transcription factor to exert its tumor-suppressor function. P53 activation requires protein stabilization, nuclear localization and posttranslational modifications in key residues that may influence p53 selection of target genes. Among them, serine 46 (Ser46) phosphorylation is considered specific for apoptotic activation. Hyperglicaemia, the high blood glucose condition, may negatively affect tumor response to therapies through several mechanisms, conferring resistance to drug-induced cell death. However, whether high glucose might modify p53Ser46 phosphorylation has never been addressed.

Methods and results

Here, we performed biochemical and molecular analyses in different cancer cell lines treated with chemotherapy in the presence or absence of high glucose condition. Analyses of p53 posttranslational modifications showed that drug-induced p53Ser46 phosphorylation was reduced by high glucose. Such reduction depended by high glucose-induced calyculin A-sensitive phosphatase(s), able to specifically target p53Ser46 phosphorylation. The specific effect on Ser46 phosphorylation was addressed by analysing Ser15 phosphorylation that instead was not modified by high glucose. In agreement, a constitutively phosphorylated Ser46D p53 mutant was resistant to high glucose. As a consequence of phosphoSer46 impairment, high glucose reduced the tumor cell response to drugs, correlating with reduced p53 apoptotic transactivation. The drug-induced apoptotic cell death, reduced by high glucose, was finally restored by the phosphatase inhibitor calyculin A.

Conclusions

These data indicate that high glucose specifically inhibited Ser46 phosphorylation thus reducing p53 apoptotic activity. These results uncover a new mechanism of p53 inactivation providing an interesting novel molecular link between metabolic diseases such as diabetes or obesity and tumor progression and resistance to therapies.

HIPK2 sustains apoptotic response by phosphorylating Che-1/AATF and promoting its degradation

Che-1/AATF is an RNA polymerase II-binding protein that is involved in the regulation of gene transcription, which undergoes stabilization and accumulation in response to DNA damage. We have previously demonstrated that following apoptotic induction, Che-1 protein levels are downregulated through its interaction with the E3 ligase HDM2, which leads to Che-1 degradation by ubiquitylation. This interaction is mediated by Pin1, which determines a phosphorylation-dependent conformational change. Here we demonstrate that HIPK2, a proapoptotic kinase, is involved in Che-1 degradation. HIPK2 interacts with Che-1 and, upon genotoxic stress, phosphorylates it at specific residues. This event strongly increases HDM2/Che-1 interaction and degradation of Che-1 protein via ubiquitin-dependent proteasomal system. In agreement with these findings, we found that HIPK2 depletion strongly decreases Che-1 ubiquitylation and degradation. Notably, Che-1 overexpression strongly counteracts HIPK2-induced apoptosis. Our results establish Che-1 as a new HIPK2 target and confirm its important role in the cellular response to DNA damage.

Putting the brakes on p53-driven apoptosis

Following genotoxic stress, cells activate a complex, kinase-based signaling network to arrest the cell cycle and initiate DNA repair or apoptosis. The tumor suppressor p53 lies at the heart of this DNA damage response. p53 mediates the transactivation of both cell cycle-regulating and pro-apoptotic clusters of target genes. However, it remains incompletely understood which signaling molecules dictate the choice between these two opposing p53-dependent cellular outcomes. Over recent years, numerous regulatory mechanisms impacting on the cellular outcome of p53 signaling have been described. However, no single dominant mechanism has thus far been identified to regulate the cellular choice between p53-driven apoptosis or senescence. The transcriptional regulator AATF has recently emerged as a novel factor impacting on the cellular outcome of the p53 response. Upon genotoxic stress, cytoplasmic pools of MRLC-bound AATF are phosphorylated through the p38MAPK/MK2 checkpoint kinase complex. This AATF phosphorylation results in the disruption of cytoplasmic MRLC3:AATF complexes followed by rapid nuclear localization of AATF. Once in the nucleus, AATF binds to the PUMA, BAX and BAK promoters to repress the DNA damage-induced expression of these pro-apoptotic p53 target genes. Depletion of AATF in tumor cells results in a dramatically enhanced response to DNA-damaging chemotherapeutics, both in vitro and in vivo. Furthermore, focal copy number gains at the AATF locus in neuroblastoma correlate with adverse prognosis and reduced overall survival in this typically p53-proficient malignancy. These data identify the p38/MK2/AATF signaling pathway as a critical repressor of p53-driven apoptosis in tumor cells and implicate this signaling cascade as a novel target for chemotherapy-sensitizing therapeutic efforts.

Induction of discrete apoptotic pathways by bromo-substituted indirubin derivatives in invasive breast cancer cells

Indirubin derivatives gained interest in recent years for their anticancer and antimetastatic properties. The objective of the present study was to evaluate and compare the anticancer properties of the two novel bromo-substituted derivatives 6-bromoindirubin-3'-oxime (6BIO) and 7-bromoindirubin-3'-oxime (7BIO) in five different breast cancer cell lines. Cell viability assays identified that 6BIO and 7BIO are most effective in preventing the proliferation of the MDA-MB-231-TXSA breast cancer cell line from a total of five breast cancer cell lined examined. In addition it was found that the two compounds induce apoptosis via different mechanisms. 6BIO induces caspase-dependent programmed cell death through the intrinsic (mitochondrial) caspase-9 pathway. 7BIO up-regulates p21 and promotes G(2)/M cell cycle arrest which is subsequently followed by the activation of two different apoptotic pathways: (a) a pathway that involves the upregulation of DR4/DR5 and activation of caspase-8 and (b) a caspase independent pathway. In conclusion, this study provides important insights regarding the molecular pathways leading to cell cycle arrest and apoptosis by two indirubin derivatives that can find clinical applications in targeted cancer therapeutics.

Hipk2 cooperates with p53 to suppress γ-ray radiation-induced mouse thymic lymphoma

A genome-wide screen for genetic alterations in radiation-induced thymic lymphomas generated from p53+/- and p53-/- mice showed frequent loss of heterozygosity (LOH) on chromosome 6. Fine mapping of these LOH regions revealed three non-overlapping regions, one of which was refined to a 0.2 Mb interval that contained only the gene encoding homeobox-interacting protein kinase 2 (Hipk2). More than 30% of radiation-induced tumors from both p53+/- and p53-/- mice showed heterozygous loss of one Hipk2 allele. Mice carrying a single inactive allele of Hipk2 in the germline were susceptible to induction of tumors by γ-radiation, but most tumors retained and expressed the wild-type allele, suggesting that Hipk2 is a haploinsufficient tumor suppressor gene for mouse lymphoma development. Heterozygous loss of both Hipk2 and p53 confers strong sensitization to radiation-induced lymphoma. We conclude that Hipk2 is a haploinsufficient lymphoma suppressor gene.

Role of the SUMO-interacting motif in HIPK2 targeting to the PML nuclear bodies and regulation of p53

Homeodomain-interacting protein kinase 2 (HIPK2) is a key regulator of various transcription factors including p53 and CtBP in the DNA damage signaling pathway. PML-nuclear body (NB) is required for HIPK2-mediated p53 phosphorylation at Ser46 and induction of apoptosis. Although PML-NB targeting of HIPK2 has been shown, much is not clear about the molecular mechanism of HIPK2 recruitment to PML-NBs. Here we show that HIPK2 colocalizes specifically with PML-I and PML-IV. Mutational analysis showed that HIPK2 recruitment to PML-IV-NBs is mediated by the SUMO-interaction motifs (SIMs) of both PML-IV and HIPK2. Wild-type HIPK2 associated with SUMO-conjugated PML-IV at a higher affinity than with un-conjugated PML-IV, while the association of a HIPK2 SIM mutant with SUMO-modified PML-IV was impaired. In colony formation assays, HIPK2 strongly suppressed cell proliferation, but HIPK2 SIM mutants did not. In addition, activation and phosphorylation of p53 at the Ser46 residue were impaired by HIPK2 SIM mutants. These results suggest that SIM-mediated HIPK2 targeting to PML-NBs is crucial for HIPK2-mediated p53 activation and induction of apoptosis.

TP53 mutations in human cancers: origins, consequences, and clinical use

Somatic mutations in the TP53 gene are one of the most frequent alterations in human cancers, and germline mutations are the underlying cause of Li-Fraumeni syndrome, which predisposes to a wide spectrum of early-onset cancers. Most mutations are single-base substitutions distributed throughout the coding sequence. Their diverse types and positions may inform on the nature of mutagenic mechanisms involved in cancer etiology. TP53 mutations are also potential prognostic and predictive markers, as well as targets for pharmacological intervention. All mutations found in human cancers are compiled in the IARC TP53 Database (http://www-p53.iarc.fr/). A human TP53 knockin mouse model (Hupki mouse) provides an experimental model to study mutagenesis in the context of a human TP53 sequence. Here, we summarize current knowledge on TP53 gene variations observed in human cancers and populations, and current clinical applications derived from this knowledge.

Counteracting MDM2-induced HIPK2 downregulation restores HIPK2/p53 apoptotic signaling in cancer cells

Homeodomain-interacting protein kinase-2 (HIPK2) is a crucial regulator of p53 apoptotic function by phosphorylating serine 46 (Ser46) in response to DNA damage. In tumors with wild-type p53, its tumor suppressor function is often impaired by MDM2 overexpression that targets p53 for proteasomal degradation. Likewise, MDM2 targets HIPK2 for protein degradation impairing p53-apoptotic function. Here we report that zinc antagonised MDM2-induced HIPK2 degradation as well as p53 ubiquitination. The zinc inhibitory effect on MDM2 activity leads to HIPK2-induced p53Ser46 phosphorylation and p53 pro-apoptotic transcriptional activity. These results suggest that zinc derivatives are potential molecules to target the MDM2-induced HIPK2/p53 inhibition.

Regulation of p53 activity by HIPK2: molecular mechanisms and therapeutical implications in human cancer cells

The p53 protein is the most studied tumor suppressor and the p53 pathway has been shown to mediate cellular stress responses that are disrupted when cancer develops. After DNA damage, p53 is activated as transcription factor to directly induce the expression of target genes involved in cell-cycle arrest, DNA repair, senescence and, importantly, apoptosis. Post-translational modifications of p53 are essential for the activation of p53 and for selection of target genes. The tumor suppressor homeodomain-interacting protein kinase-2 (HIPK2) is a crucial regulator of p53 apoptotic function by phosphorylating its N-terminal serine 46 (Ser46) and facilitating Lys382 acetylation at the C-terminus. HIPK2 is activated by numerous genotoxic agents and can be deregulated in tumors by several conditions including hypoxia. Recent findings suggest that HIPK2 active/inactive protein can affect p53 function in multiple and unexpected ways. This makes p53 as well as HIPK2 interesting targets for cancer therapy. Hence, understanding the role of HIPK2 as p53 activator may provide important insights in the process of tumor progression, and may also serve as the crucial point in the diagnostic and therapeutical aspects of cancer.

HIPK2 modulates p53 activity towards pro-apoptotic transcription

Background

Activation of p53-mediated gene transcription is a critical cellular response to DNA damage and involves a phosphorylation-acetylation cascade of p53. The discovery of differences in the response to different agents raises the question whether some of the p53 oncosuppressor functions might be exerted by different posttranslational modifications. Stress-induced homeodomain-interacting protein kinase-2 (HIPK2) phosphorylates p53 at serine-46 (Ser46) for p53 apoptotic activity; p53 acetylation at different C-terminus lysines including p300-mediated lysine-382 (Lys382) is also required for full activation of p53 transcriptional activity. The purpose of the current study was to evaluate the interplay among HIPK2, p300, and p53 in p53 acetylation and apoptotic transcriptional activity in response to drug by using siRNA interference, p300 overexpression or deacetylase inhibitors, in cancer cells.

Results

Knockdown of HIPK2 inhibited both adriamycin-induced Ser46 phosphorylation and Lys382 acetylation in p53 protein; however, while combination of ADR and zinc restored Ser46 phosphorylation it did not recover Lys382 acetylation. Chromatin immunoprecipitation studies showed that HIPK2 was required in vivo for efficient p300/p53 co-recruitment onto apoptotic promoters and that both p53 modifications at Ser46 and Lys382 were necessary for p53 apoptotic transcription. Thus, p53Lys382 acetylation in HIPK2 knockdown as well as p53 apoptotic activity in response to drug could be rescued by p300 overexpression. Similar effect was obtained with the Sirt1-inhibitor nicotinamide. Interestingly trichostatin A (TSA), the inhibitor of histone deacetylase complexes (HDAC) did not have effect, suggesting that Sirt1 was the deacetylase involved in p53 deacetylation in HIPK2 knockdown.

Conclusion

These results reveal a novel role for HIPK2 in activating p53 apoptotic transcription. Our results indicate that HIPK2 may regulate the balance between p53 acetylation and deacetylation, by stimulating on one hand co-recruitment of p300 and p53Lys382 on apoptotic promoters and on the other hand by inhibiting Sirt1 deacetylase activity. We attempted to reactivate p53 apoptotic transcriptional activity by rescuing both Ser46 and Lys382 modification in response to drug. Our data propose combination strategies for the treatment of tumors with dysfunctional p53 and/or HIPK2 that include classical chemotherapy with pharmacological or natural agents such as Sirt1-deacetylase inhibitors or zinc, respectively.

Targeting hypoxia in cancer cells by restoring homeodomain interacting protein-kinase 2 and p53 activity and suppressing HIF-1alpha

Background

The tumor suppressor homeodomain-interacting protein kinase-2 (HIPK2) by phosphorylating serine 46 (Ser46) is a crucial regulator of p53 apoptotic function. HIPK2 is also a transcriptional co-repressor of hypoxia-inducible factor-1α (HIF-1α) restraining tumor angiogenesis and chemoresistance. HIPK2 can be deregulated in tumors by several mechanisms including hypoxia. Here, we sought to target hypoxia by restoring HIPK2 function and suppressing HIF-1α, in order to provide evidence for the involvement of both HIPK2 and p53 in counteracting hypoxia-induced chemoresistance.

Methodology/Principal Findings

Upon exposure of colon and lung cancer cells to hypoxia, by either low oxygen or cobalt, HIPK2 function was impaired allowing for increased HIF-1α expression and inhibiting the p53-apoptotic response to drug. Cobalt suppressed HIPK2 recruitment onto HIF-1α promoter. Hypoxia induced expression of the p53 target MDM2 that downregulates HIPK2, thus MDM2 inhibition by siRNA restored the HIPK2/p53Ser46 response to drug. Zinc supplementation to hypoxia-treated cells increased HIPK2 protein stability and nuclear accumulation, leading to restoration of HIPK2 binding to HIF-1α promoter, repression of MDR1, Bcl2, and VEGF genes, and activation of the p53 apoptotic response to drug. Combination of zinc and ADR strongly suppressed tumor growth in vivo by inhibiting HIF-1 pathway and upregulating p53 apoptotic target genes.

Conclusions/Significance

We show here for the first time that hypoxia-induced HIPK2 deregulation was counteracted by zinc that restored HIPK2 suppression of HIF-1 pathway and reactivated p53 apoptotic response to drug, underscoring the potential use of zinc supplementation in combination with chemotherapy to address hypoxia and improve tumor treatment.