c-Abl tyrosine kinase selectively regulates p73 nuclear matrix association

Twist1/Dnmt3a and miR186 establish a regulatory circuit that controls inflammation-associated prostate cancer progression

Increasing evidences suggest that inflammatory microenvironment has a crucial role in prostate cancer (PCa) progression; however, the underlying mechanisms are unclear. Here, we used the inflammation-associated prostate cellular transformation model to screen out a crucial microRNA, miR186, which was significantly downregulated in the transformed cells and effectively rescued the transformed phenotype. On stimulation of inflammatory cytokines, the activated nuclear factor kappa B (NF-κB)/p65 was able to induce miR186 expression through binding to its promoter in non-transformed cells, whereas this pathway was lost in transformed cells. Interestingly, Twist1, which is a reported downstream target of miR186, was responsible for the loss of NF-κB/p65-miR186 pathway. Twist1 downregulated miR186 expression in a novel negative feedback loop binding to the E-box and simultaneously recruiting Dnmt3a, which facilitated the site-specific CpG methylation of the miR186 promoter, thereby blocked the transcriptional activity of NF-κB/p65 and the responsiveness of miR186 to inflammatory signals. The high level of Twist1 triggered this feedback loop that underlies the epigenetic switch, which was essential for maintaining transformed and advanced PCa state. Finally, our clinical data confirmed that the CpG methylation and miR186 expression levels were closely related with inflammation-associated human PCa progression.

The DNA Damage Response in Neurons: Die by Apoptosis or Survive in a Senescence-Like State?

Neurons are exposed to high levels of DNA damage from both physiological and pathological sources. Neurons are post-mitotic and their loss cannot be easily recovered from; to cope with DNA damage a complex pathway called the DNA damage response (DDR) has evolved. This recognizes the damage, and through kinases such as ataxia-telangiectasia mutated (ATM) recruits and activates downstream factors that mediate either apoptosis or survival. This choice between these opposing outcomes integrates many inputs primarily through a number of key cross-road proteins, including ATM, p53, and p21. Evidence of re-entry into the cell-cycle by neurons can be seen in aging and diseases such as Alzheimer's disease. This aberrant cell-cycle re-entry is lethal and can lead to the apoptotic death of the neuron. Many downstream factors of the DDR promote cell-cycle arrest in response to damage and appear to protect neurons from apoptotic death. However, neurons surviving with a persistently activated DDR show all the features known from cell senescence; including metabolic dysregulation, mitochondrial dysfunction, and the hyper-production of pro-oxidant, pro-inflammatory and matrix-remodeling factors. These cells, termed senescence-like neurons, can negatively influence the extracellular environment and may promote induction of the same phenotype in surrounding cells, as well as driving aging and age-related diseases. Recently developed interventions targeting the DDR and/or the senescent phenotype in a range of non-neuronal tissues are being reviewed as they might become of therapeutic interest in neurodegenerative diseases.

MicroRNA-1301-Mediated RanGAP1 Downregulation Induces BCR-ABL Nuclear Entrapment to Enhance Imatinib Efficacy in Chronic Myeloid Leukemia Cells

Chronic myeloid leukemia (CML) is a myeloproliferative disease. Imatinib (IM), the first line treatment for CML, is excessively expensive and induces various side effects in CML patients. Therefore, it is essential to investigate a new strategy for improving CML therapy. Our immunoblot data revealed that RanGTPase activating protein 1 (RanGAP1) protein levels increased by approximately 30-fold in K562 cells compared with those in normal cells. RanGAP1 is one of the important components of RanGTPase system, which regulates the export of nuclear protein. However, whether RanGAP1 level variation influences BCR-ABL nuclear export is still unknown. In this report, using shRNA to downregulate RanGAP1 expression level augmented K562 cell apoptosis by approximately 40% after treatment with 250 nM IM. Immunofluorescence assay also indicated that three-fold of nuclear BCR-ABL was detected. These data suggest that BCR-ABL nuclear entrapment induced by RanGAP1 downregulation can be used to improve IM efficacy. Moreover, our qRT-PCR data indicated a trend of inverse correlation between the RanGAP1 and microRNA (miR)-1301 levels in CML patients. MiR-1301, targeting the RanGAP1 3' untranslated region, decreased by approximately 100-fold in K562 cells compared with that in normal cells. RanGAP1 downregulation by miR-1301 transfection impairs BCR-ABL nuclear export to increase approximately 60% of cell death after treatment of 250 nM IM. This result was almost the same as treatment with 1000 nM IM alone. Furthermore, immunofluorescence assay demonstrated that Tyr-99 of nuclear P73 was phosphorylated accompanied with nuclear entrapment of BCR-ABL after transfection with RanGAP1 shRNA or miR-1301 in IM-treated K562 cells. Altogether, we demonstrated that RanGAP1 downregulation can mediate BCR-ABL nuclear entrapment to activate P73-dependent apoptosis pathway which is a novel strategy for improving current IM treatment for CML.

Role of the nucleus in apoptosis: signaling and execution

Since their establishment in the early 1970s, the nuclear changes upon apoptosis induction, such as the condensation of chromatin, disassembly of nuclear scaffold proteins and degradation of DNA, were, and still are, considered as the essential steps and hallmarks of apoptosis. These are the characteristics of the execution phase of apoptotic cell death. In addition, accumulating data clearly show that some nuclear events can lead to the induction of apoptosis. In particular, if DNA lesions resulting from deregulation during the cell cycle or DNA damage induced by chemotherapeutic drugs or viral infection cannot be efficiently eliminated, apoptotic mechanisms, which enable cellular transformation to be avoided, are activated in the nucleus. The functional heterogeneity of the nuclear organization allows the tight regulation of these signaling events that involve the movement of various nuclear proteins to other intracellular compartments (and vice versa) to initiate and govern apoptosis. Here, we discuss how these events are coordinated to execute apoptotic cell death.

The Tyrosine Kinase c-Abl Promotes Homeodomain-interacting Protein Kinase 2 (HIPK2) Accumulation and Activation in Response to DNA Damage

The non-receptor tyrosine kinase c-Abl is activated in response to DNA damage and induces p73-dependent apoptosis. Here, we investigated c-Abl regulation of the homeodomain-interacting protein kinase 2 (HIPK2), an important regulator of p53-dependent apoptosis. c-Abl phosphorylated HIPK2 at several sites, and phosphorylation by c-Abl protected HIPK2 from degradation mediated by the ubiquitin E3 ligase Siah-1. c-Abl and HIPK2 synergized in activating p53 on apoptotic promoters in a reporter assay, and c-Abl was required for endogenous HIPK2 accumulation and phosphorylation of p53 at Ser(46) in response to DNA damage by γ- and UV radiation. Accumulation of HIPK2 in nuclear speckles and association with promyelocytic leukemia protein (PML) in response to DNA damage were also dependent on c-Abl activity. At high cell density, the Hippo pathway inhibits DNA damage-induced c-Abl activation. Under this condition, DNA damage-induced HIPK2 accumulation, phosphorylation of p53 at Ser(46), and apoptosis were attenuated. These data demonstrate a new mechanism for the induction of DNA damage-induced apoptosis by c-Abl and illustrate network interactions between serine/threonine and tyrosine kinases that dictate cell fate.

AMPK couples p73 with p53 in cell fate decision

The p53 family of proteins has an important role in determining cell fate in response to different types of stress, such as DNA damage, hypoxia, or oncogenic stress. In recent years, p53 has also been shown to respond to metabolic stress, and to be induced by the AMP-activated protein kinase (AMPK), a central cellular energy sensor. A bioinformatic analysis revealed three putative AMPK phopshorylation sites in p73, a p53 tumor suppressor paralog. In vitro and in vivo assays confirmed that AMPK phosphorylates p73 on a novel residue, S426. Following specific pharmacologic stimulation of AMPK in cells, p73 protein half-life was prolonged leading to p73 accumulation in the nucleus. We show that p73 escaped the E3 ligase Itch resulting in reduced p73 ubiquitination and proteasomal degradation. Furthermore, chronic activation of AMPK led to apoptosis that was p73 dependent, but only in p53-expressing cells. Surprisingly, we found that p73 was required for p53 stabilization and accumulation under AMPK activation, but was dispensable under DNA damage. Our findings couple p73 with p53 in determining cell fate under AMPK-induced metabolic stress.

The role of the TP73 gene and its transcripts in neuro-oncology

Protein p73 is a member of the p53 protein family that can induce cell cycle arrest or apoptosis by the activation of p53-responsive genes as well as p53-independent pathways. Alternative promoter usage, together with differential splicing of the C-terminal exons, forms several distinct mRNAs that are translated into corresponding protein isoforms containing different domains. While TAp73 isoforms respond to genotoxic stress in a manner similar to tumor suppressor p53, ΔTAp73 isoforms inhibit apoptosis during normal development and in cancer cell lines. Thus, the impact of p73 on tumorigenesis depends on a subtle balance between tumor-promoting and -suppressing isoforms. Due to the structural homology between p53 and p73, a subtle balance among p53 family members and their isoforms could influence glioma cell evolution toward malignancy. Thus, the p73 status has to be considered when studying the regulatory role of p53 protein in gliomagenesis. The presented review summarizes recent knowledge about the issue of p73 and its isoforms with respect to neuro-oncology research.

The Hippo pathway kinase Lats2 prevents DNA damage-induced apoptosis through inhibition of the tyrosine kinase c-Abl

The Hippo pathway is an evolutionarily conserved pathway that controls cell proliferation, organ size, tissue regeneration and stem cell self-renewal. Here we show that it also regulates the DNA damage response. At high cell density, when the Hippo pathway is active, DNA damage-induced apoptosis and the activation of the tyrosine kinase c-Abl were suppressed. At low cell density, overexpression of the Hippo pathway kinase large tumor suppressor 2 (Lats2) inhibited c-Abl activity. This led to reduced phosphorylation of downstream c-Abl substrates, the transcription coactivator Yes-associated protein (Yap) and the tumor suppressor p73. Inhibition of c-Abl by Lats2 was mediated through Lats2 interaction with and phosphorylation of c-Abl. Lats2 knockdown, or expression of c-Abl mutants that escape inhibition by Lats2, enabled DNA damage-induced apoptosis of densely plated cells, while Lats2 overexpression inhibited apoptosis in sparse cells. These findings explain a long-standing enigma of why densely plated cells are radioresistant. Furthermore, they demonstrate that the Hippo pathway regulates cell fate decisions in response to DNA damage.

MORC2 signaling integrates phosphorylation-dependent, ATPase-coupled chromatin remodeling during the DNA damage response

Chromatin dynamics play a central role in maintaining genome integrity, but how this is achieved remains largely unknown. Here, we report that microrchidia CW-type zinc finger 2 (MORC2), an uncharacterized protein with a derived PHD finger domain and a conserved GHKL-type ATPase module, is a physiological substrate of p21-activated kinase 1 (PAK1), an important integrator of extracellular signals and nuclear processes. Following DNA damage, MORC2 is phosphorylated on serine 739 in a PAK1-dependent manner, and phosphorylated MORC2 regulates its DNA-dependent ATPase activity to facilitate chromatin remodeling. Moreover, MORC2 associates with chromatin and promotes gamma-H2AX induction in a PAK1 phosphorylation-dependent manner. Consequently, cells expressing MORC2-S739A mutation displayed a reduction in DNA repair efficiency and were hypersensitive to DNA-damaging agent. These findings suggest that the PAK1-MORC2 axis is critical for orchestrating the interplay between chromatin dynamics and the maintenance of genomic integrity through sequentially integrating multiple essential enzymatic processes.

Tumour suppressor genes in chemotherapeutic drug response

Since cancer is one of the leading causes of death worldwide, there is an urgent need to find better treatments. Currently, the use of chemotherapeutics remains the predominant option for cancer therapy. However, one of the major obstacles for successful cancer therapy using these chemotherapeutics is that patients often do not respond or eventually develop resistance after initial treatment. Therefore identification of genes involved in chemotherapeutic response is critical for predicting tumour response and treating drug-resistant cancer patients. A group of genes commonly lost or inactivated are tumour suppressor genes, which can promote the initiation and progression of cancer through regulation of various biological processes such as cell proliferation, cell death and cell migration/invasion. Recently, mounting evidence suggests that these tumour suppressor genes also play a very important role in the response of cancers to a variety of chemotherapeutic drugs. In the present review, we will provide a comprehensive overview on how major tumour suppressor genes [Rb (retinoblastoma), p53 family, cyclin-dependent kinase inhibitors, BRCA1 (breast-cancer susceptibility gene 1), PTEN (phosphatase and tensin homologue deleted on chromosome 10), Hippo pathway, etc.] are involved in chemotherapeutic drug response and discuss their applications in predicting the clinical outcome of chemotherapy for cancer patients. We also propose that tumour suppressor genes are critical chemotherapeutic targets for the successful treatment of drug-resistant cancer patients in future applications.

Recruitment of the tumour suppressor protein p73 by Kaposi's Sarcoma Herpesvirus latent nuclear antigen contributes to the survival of primary effusion lymphoma cells

Kaposi's Sarcoma Herpesvirus (KSHV) is the causative agent of Kaposi's Sarcoma (KS) and two rare lymphoproliferative disorders, primary effusion lymphoma (PEL) and the plasmablastic variant of multicentric Castleman's disease (MCD). The KSHV latency-associated nuclear antigen-1 (LANA), required for the replication and maintenance of latent viral episomal DNA, is involved in the transcriptional regulation of viral and cellular genes and interacts with different cellular proteins, including the tumour suppressor p53. Here, we report that LANA also recruits the p53-related nuclear transcription factor p73, which influences cellular processes like DNA damage response, cell cycle progression and apoptosis. Both the full-length isoform TAp73α, as well as its dominant negative regulator ΔNp73α, interact with LANA. LANA affects TAp73α stability and sub-nuclear localisation, as well as TAp73α-mediated transcriptional activation of target genes. We observed that the small-molecule inhibitor Nutlin-3, which disrupts the interaction of p53 and p73 with MDM2, induces apoptotic cell death in p53 wild-type, as well as p53-mutant PEL cell lines, suggesting a possible involvement of p73. The small-molecule RETRA, which activates p73 in the context of mutant p53, leads to the induction of apoptosis in p53-mutant PEL cell lines. RNAi-mediated knockdown of p73 confirmed that these effects depend on the presence of the p73 protein. Furthermore, both Nutlin-3 and RETRA disrupt the LANA-p73 interaction in different PEL cell lines. These results suggest that LANA modulates p73 function and that the LANA-p73 interaction may represent a therapeutic target to interfere with the survival of latently KSHV-infected cells.

Graft-versus-host disease-related cytokine-driven apoptosis depends on p73 in cytokeratin 15-positive target cells

Acute graft-versus-host disease (GVHD), a major complication of allogeneic stem cell transplantation, involves cytotoxic soluble and cellular effectors that selectively induce apoptosis in normally apoptosis-resistant, cytokeratin 15 (K15)-expressing epithelial stem cells residing at the tips of rete ridges of human epidermis and in analogous rete-like prominences (RLPs) of murine dorsal lingual epithelium. The mechanisms whereby epithelial stem cells are rendered vulnerable to apoptosis during allostimulation are unknown. We hypothesized that GVHD-induced target cell injury may be related to pathways involving the p53 family that are constitutively expressed by epithelial stem cells and designed to trigger physiological apoptosis as a result of environmental danger signals. Among the p53 family members, we found that p73 protein and mRNA were preferentially expressed in K15(+) RLPs of murine lingual squamous epithelium. On in vitro exposure to recombinant TNF-α and IL-1 in an organ culture model previously shown to replicate early GVHD-like target cell injury, apoptosis was selectively induced in K15(+) stem cell regions and was associated with induction of phosphorylated p73, a marker for p73 activation, and apoptosis was abrogated in target tissue obtained from p73-deficient (p73(-/-)) mice. Evaluation of early in vivo lesions in experimental murine GVHD disclosed identical patterns of phosphorylated p73 expression that coincided with the onset of effector T cell infiltration and target cell apoptosis within K15(+) RLPs. This study is the first to suggest that paradoxical apoptosis in GVHD of physiologically protected K15(+) epithelial stem cells is explainable, at least in part, by cytokine-induced activation of suicide pathways designed to eliminate stem cells after exposure to deleterious factors perceived to be harmful to the host.

Regulation of p73 activity by post-translational modifications

The transcription factor p73 is a member of the p53 family that can be expressed as at least 24 different isoforms with pro- or anti-apoptotic attributes. The TAp73 isoforms are expressed from an upstream promoter and are regarded as bona fide tumor suppressors; they can induce cell cycle arrest/apoptosis and protect against genomic instability. On the other hand, ΔNp73 isoforms lack the N-terminus transactivation domain; hence, cannot induce the expression of pro-apoptotic genes, but still can oligomerize with TAp73 or p53 to block their transcriptional activities. Therefore, the ratio of TAp73 isoforms to ΔNp73 isoforms is critical for the quality of the response to a genomic insult and needs to be delicately regulated at both transcriptional and post-translational level. In this review, we will summarize the current knowledge on the post-translational regulatory pathways involved to keep p73 protein under control. A comprehensive understanding of p73 post-translational modifications will be extremely useful for the development of new strategies for treating and preventing cancer.

p73-Binding Partners and Their Functional Significance

p73 is one of the tumor-suppressor p53 family of nuclear transcription factor. As expected from the structural similarity between p53 and p73, p73 has a tumor-suppressive function. However, p73 was rarely mutated in human primary tumors. Under normal physiological conditions, p73 is kept at an extremely low level to allow cells normal growth. In response to a certain subset of DNA damages, p73 is induced dramatically and transactivates an overlapping set of p53-target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death. Cells undergo cell cycle arrest and/or apoptotic cell death depending on the type and strength of DNA damages. p73 is regulated largely through the posttranslational modifications such as phosphorylation and acetylation. These chemical modifications are tightly linked to direct protein-protein interactions. In the present paper, the authors describe the functional significance of the protein-protein interactions in the regulation of proapoptotic p73.

Hepatitis B virus activates deoxynucleotide synthesis in nondividing hepatocytes by targeting the R2 gene

Hepatitis B virus (HBV) causes liver diseases from acute hepatitis to cirrhosis and liver cancer. Currently, more than 350 million people are chronic HBV carriers, with devastating prognosis. HBV is a small enveloped noncytopathic virus, containing a circular partially double-stranded DNA genome, and exhibits strong tropism for human liver cells. Infected individuals (acute and chronic) secrete about 10(7) to 10(11) virions per day to the bloodstream, with each infected cell releasing 50-300 viruses per day. HBV infects nondividing hepatocytes and replicates by reverse-transcribing the pregenomic RNA to DNA in the host cells. The level of deoxyribonucleotide triphosphates (dNTPs) in nondividing cells is too low to support viral replication and enable the high yield of secreted virions. Here, we report production of dNTPs by viral-dependent transcription activation of R2, the key component of ribonucleotide reductase (RNR), and show that this process is critical for the HBV life-cycle. This was found in an established HBV-positive cell line and was reproduced by HBV DNA-transduced cells, in both culture and mice. Furthermore, the viral hepatitis B X protein is essential in activating R2 expression by blocking access of Regulatory factor x1, a repressor of the R2 gene.

CONCLUSION

Our findings demonstrate that the hepatitis B X protein is critical in infecting nonproliferating hepatocytes, which contain a low dNTP level. In addition, we provide molecular evidence for a new mechanism of HBV-host cell interaction where RNR-R2, a critical cell-cycle gene, is selectively activated in nonproliferating cells. This mechanism may set the stage for formulating a new category of anti-HBV drugs.

BUB3 that dissociates from BUB1 activates caspase-independent mitotic death (CIMD)

The cell death mechanism that prevents aneuploidy caused by a failure of the spindle checkpoint has recently emerged as an important regulatory paradigm. We previously identified a new type of mitotic cell death, termed caspase-independent mitotic death (CIMD), which is induced during early mitosis by partial BUB1 (a spindle checkpoint protein) depletion and defects in kinetochore-microtubule attachment. In this study, we have shown that survived cells that escape CIMD have abnormal nuclei, and we have determined the molecular mechanism by which BUB1 depletion activates CIMD. The BUB3 protein (a BUB1 interactor and a spindle checkpoint protein) interacts with p73 (a homolog of p53), specifically in cells wherein CIMD occurs. The BUB3 protein that is freed from BUB1 associates with p73 on which Y99 is phosphorylated by c-Abl tyrosine kinase, resulting in the activation of CIMD. These results strongly support the hypothesis that CIMD is the cell death mechanism protecting cells from aneuploidy by inducing the death of cells prone to substantial chromosome missegregation.

Cooperative roles of c-Abl and Cdk5 in regulation of p53 in response to oxidative stress

The p53 tumor suppressor protein, a critical modulator of cellular stress responses, is activated through diverse mechanisms that result in its stabilization and transcriptional activation. p53 activity is controlled by transcriptional, translational, and post-translational regulation. The major mechanisms of p53 regulation occur primarily through interactions with HDM2, an E3 ubiquitin ligase that leads to p53 nuclear export and degradation. Here, we demonstrate that hydrogen peroxide-induced oxidative stress elicits down-regulation of HDM2. c-Abl mediates down-regulation of HDM2, leading to an increase of p53 level. Moreover, Cdk5 (cyclin-dependent kinase 5), a proline-directed Ser/Thr kinase, additionally increases p53 stability via post-translational modification of p53 in response to hydrogen peroxide. The p53 protein stabilized by c-Abl and Cdk5 is transcriptionally active; however, transcription of its target gene is differentially regulated with selective binding of p53 on promoter regions of its target genes by c-Abl. In addition, c-Abl modulates Cdk5 activity via phosphorylation of tyrosine 15 in cooperation with cleavage of p35 to p25. Our results show that c-Abl and Cdk5 cooperatively regulate maximal activation of p53, resulting in neuronal death in response to oxidative stress by hydrogen peroxide. These findings aid in clarifying the mechanism underlying the occurrence of neuronal apoptosis as a result of c-Abl and Cdk5-mediated p53 stabilization and transcriptional activation.

Yap1 phosphorylation by c-Abl is a critical step in selective activation of proapoptotic genes in response to DNA damage

Cells undergo apoptosis upon exposure to severe DNA damage stress. Under this condition, p73 is phosphorylated and activated by c-Abl. The transcription coactivator Yap1 binds p73 to generate a complex that escapes p73 proteasomal degradation and recruits p300 to support transcription of proapoptotic genes. However, the mechanism of selective activation of proapoptotic genes by Yap1 remained unclear. In this study, we show that c-Abl directly phosphorylates Yap1 at position Y357 in response to DNA damage. Tyrosine-phosphorylated Yap1 is a more stable protein that displays higher affinity to p73 and selectively coactivates p73 proapoptotic target genes. Furthermore, we show that Yap1 switches between p73-mediated proapoptotic and growth arrest target genes based on its phosphorylation state. Thus, our data demonstrate that modification of a transcription coactivator, namely the DNA damage-induced phosphorylation of Yap1 by c-Abl, influences the specificity of target gene activation.

Identification and characterization of the product encoded by ORF69 of Kaposi's sarcoma-associated herpesvirus

We report the identification and characterization of p33, the product of Kaposi's sarcoma-associated herpesvirus (KSHV) open reading frame 69 (ORF69), a positional homolog of the conserved herpesvirus protein UL31. p33 is expressed upon induction of viral lytic cycle with early kinetics. Immunofluorescence analysis revealed that in infected cell lines, the protein is localized in the nucleus, both in dotted spots and along the nuclear membrane. Nuclear fractionation experiments showed that p33 partitions with the nuclear matrix, and both immunoblotting of purified virions and immunoelectron microscopy indicated that the novel protein is not a component of the mature virus. Following ectopic expression in KSHV-negative cells, the protein was never associated with the nuclear membrane, suggesting that p33 needs to interact with additional viral proteins to reach the nuclear rim. In fact, after cotransfection with the ORF67 gene, the KSHV positional homolog of UL34, the p33 intranuclear signal changed and the two proteins colocalized on the nuclear membrane. A similar result was obtained when ORF69 was cotransfected with BFRF1, the Epstein-Barr virus (EBV) positional homolog of UL34 and ORF67. Finally, upon cotransfection, ORF69 significantly increased nuclear membrane reduplications induced by BFRF1. The above results indicate that KSHV p33 shares many similarities with its EBV homolog BFLF2 and suggest that functional cross-complementation is possible between members of the gammaherpesvirus subfamily.

Alteration of nuclear matrix-intermediate filament system and differential expression of nuclear matrix proteins during human hepatocarcinoma cell differentiation

AIM: To investigate the association between the configurational and compositional changes of nuclear matrix and the differentiation of carcinoma cells.

METHODS: Cells cultured with or without 5 × 10^-3 mmol/L of hexamethylene bisacetamide (HMBA) on Nickel grids were treated by selective extraction and prepared for whole mount observation under electron microscopy. The samples were examined under transmission electron microscope. Nuclear matrix proteins were selectively extracted and subjected to subcellular proteomics study. The protein expression patterns were analyzed by PDQuest software. Spots of differentially expressed nuclear matrix proteins were excised and subjected to in situ digestion with trypsin. The peptides were analyzed by matrix-assisted laser-desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). Data were submitted for database searching using Mascot tool (www.matrixscience.com).

RESULTS: The nuclear matrix (NM) and intermediate filament (IF) in SMMC-7721 hepatocarcinoma cells were found relatively sparse and arranged irregularly. The nuclear lamina was non-uniform, and two kinds of filaments were not tightly connected. After induction for differentiation by HMBA, the NM-IF filaments were concentrated and distributed uniformly. The heterogeneous population of filaments, including highly branched utrathin filaments could also be seen in the regular meshwork. The connection between the two kinds of filaments and the relatively thin, condensed and sharply demarcated lamina composed of intermediate-sized filaments was relatively fastened. Meanwhile, 21 NM proteins changed remarkably during SMMC-7721 cell differentiation. Four proteins, i.e. mutant Pyst1, hypothetical protein, nucleophosmin1, and LBP were downregulated, whereas four other proteins, eIF6, p44 subunit, β-tubulin, and SIN3B were upregulated with the last one, SR2/ASF found only in the differentiated SMMC-7721 cells.

CONCLUSION: The induced differentiation of SMMC-7721 cells by HMBA is accompanied by the configurational changes of nuclear matrix-intermediate filament (NM-IF) system and the compositional changes of nuclear matrix protein expression. These changes may be important morphological or functional indications of the cancer cell reversion.

Stabilization of p73 by nuclear IkappaB kinase-alpha mediates cisplatin-induced apoptosis

In response to DNA damage, p53 and its homolog p73 have a function antagonistic to NF-kappaB in deciding cell fate. Here, we show for the first time that p73, but not p53, is stabilized by physical interaction with nuclear IkappaB kinase (IKK)-alpha to enhance cisplatin (CDDP)-induced apoptosis. CDDP caused a significant increase in the amounts of nuclear IKK-alpha and p73alpha in human osteosarcoma-derived U2OS cells. Ectopic expression of IKK-alpha prolonged the half-life of p73 by inhibiting its ubiquitination and thereby enhancing its transactivation and pro-apoptotic activities. Consistent with these results, small interfering RNA-mediated knockdown of endogenous IKK-alpha inhibited the CDDP-mediated accumulation of p73alpha. The kinase-deficient mutant form of IKK-alpha interacted with p73alpha, but failed to stabilize it. Furthermore, CDDP-mediated accumulation of endogenous p73alpha was not detected in mouse embryonic fibroblasts (MEFs) prepared from IKK-alpha-deficient mice, and CDDP sensitivity was significantly decreased in IKK-alpha-deficient MEFs compared with wild-type MEFs. Thus, our results strongly suggest that the nuclear IKK-alpha-mediated accumulation of p73alpha is one of the novel molecular mechanisms to induce apoptotic cell death in response to CDDP, which may be particularly important in killing tumor cells with p53 mutation.

The Yes-associated protein 1 stabilizes p73 by preventing Itch-mediated ubiquitination of p73

Upon DNA damage signaling, p73, a member of the p53 tumor suppressor family, accumulates to support transcription of downstream apoptotic genes. p73 interacts with Yes-associated protein 1 (Yap1) through its PPPY motif, and increases p73 transactivation of apoptotic genes. The ubiquitin E3 ligase Itch, like Yap1, interacts with p73. Given the fact that both Itch and Yap1 bind p73 via the PPPY motif, we hypothesized that Yap may also function to stabilize p73 by displacing Itch binding to p73. We show that the interaction of Yap1 and p73 was necessary for p73 stabilization. Yap1 competed with Itch for binding to p73, and prevented Itch-mediated ubiquitination of p73. Treatment of cells with cisplatin leads to an increase in p73 accumulation and induction of apoptosis, but both were dramatically reduced in the presence of Yap1 siRNA. Altogether, our findings attribute a central role to Yap1 in regulating p73 accumulation and function under DNA damage signaling.

Recruitment of the de novo DNA methyltransferase Dnmt3a by Kaposi's sarcoma-associated herpesvirus LANA

The Kaposi's sarcoma-associated herpesvirus LANA protein is expressed in all Kaposi's sarcoma-associated herpesvirus-infected cells, including the tumor cells of endemic and AIDS-associated Kaposi sarcoma, primary effusion lymphoma, and Castleman disease. LANA modulates cell gene expression, but the mechanisms of LANA-mediated transcriptional reprogramming are poorly understood. LANA-repressed cell genes were identified by using retroviral-transduced telomerase-immortalized microvascular endothelial cells. Transciptional repression of targeted genes was relieved by treatment with the methyltransferase inhibitor 5-aza-2'-deoxycytidine, suggesting a role for DNA methylation in repression. We found that LANA coprecipitated with DNA methyltransferases (Dnmts) and recruited endogenous DNA methyltransferase activity from the cell extract. LANA preferentially relocalized Dnmt3a from the nuclear matrix into the chromatin fraction. Further, LANA associated with repressed cellular promoters, recruited Dnmt3a to DNA, and facilitated de novo promoter methylation of a down-regulated gene, cadherin 13 (H-cadherin). The data provide an example of promoter-specific epigenetic DNA modification through viral protein recruitment of de novo Dnmt activity.

Expression of the p73 protein in rectal cancers with or without preoperative radiotherapy

PURPOSE

To investigate p73 expression in normal mucosa, primary tumor, and metastasis in relation to radiotherapy (RT) response and clinicopathologic/biologic variables in rectal cancers.

METHODS AND MATERIALS

p73 was immunohistochemically examined on biopsies (unirradiated, n=102), distant (from the large bowel, n=82), and adjacent (adjacent to primary tumor, n=89) normal mucosa samples, primary tumors (n=131), and lymph node metastasis (n=32) from rectal cancer patients participating in a clinical trial of preoperative RT. Seventy-four patients received surgery alone and 57 received additional RT.

RESULTS

Cytoplasmic p73 was increased in the primary tumor compared with the distant or adjacent mucosa (p<or=0.0001). Nuclear (p=0.02) and cytoplasmic (p=0.003) p73 was higher in irradiated distant mucosa samples than in unirradiated ones, and nuclear p73 tended to be increased in irradiated primary tumors compared with unirradiated ones (p=0.06). p73 was positively related to cyclooxygenase-2 expression in irradiated tumors (p=0.03). p73-negative tumors tended to have a lower local recurrence after RT compared with unirradiated cases (p=0.06).

CONCLUSIONS

Normal epithelial cells seem more sensitive to RT than tumor cells regarding p73 expression. Patients with p73-negative rectal tumors may have a lower risk of local recurrence after RT.

MyD88 adapter-like (Mal) is phosphorylated by Bruton's tyrosine kinase during TLR2 and TLR4 signal transduction

Members of the Toll-like receptor (TLR) family are essential players in activating the host innate immune response against infectious microorganisms. All TLRs signal through Toll/interleukin 1 receptor domain-containing adapter proteins. MyD88 adapter-like (Mal) is one such adapter that specifically is involved in TLR2 and TLR4 signaling. When overexpressed we have found that Mal undergoes tyrosine phosphorylation. Three possible phospho-accepting tyrosines were identified at positions 86, 106, and 187, and two mutant forms of Mal in which tyrosines 86 and 187 were mutated to phenylalanine acted as dominant negative inhibitors of NF-kappaB activation by lipopolysaccharide (LPS). Activation of THP-1 monocytic cells with the TLR4 agonist LPS and the TLR2 agonist macrophage-activating lipopeptide-2 induced phosphorylation of Mal on tyrosine residues. We found that the Bruton's tyrosine kinase (Btk) inhibitor LFM-A13 could block the endogenous phosphorylation of Mal on tyrosine in cells treated with macrophage-activating lipopeptide-2 or LPS. Furthermore, Btk immunoprecipitated from THP-1 cells activated by LPS could phosphorylate Mal. Our study therefore provides the first demonstration of the key role of Mal phosphorylation on tyrosine during signaling by TLR2 and TLR4 and identifies a novel function for Btk as the kinase involved.

CK2 phosphorylation weakens 90 kDa MFP1 association to the nuclear matrix in Allium cepa

MFP1 is a conserved plant coiled-coil protein located on the stroma side of the chloroplast thylakoids, as well as in the nuclear matrix. It displays species-specific variability in the number of genes, proteins, and expression. Allium cepa has two nuclear proteins antigenically related to MFP1 with different M(r), pI, distribution, and expression, but only the 90 kDa MFP1 protein is a nuclear matrix component that associates with both the nucleoskeletal filaments and a new category of nuclear bodies. The 90 kDa AcMFP1 migrates in two-dimensional blots as two sets of spots. The hypo-phosphorylated forms (pI approximately 9.5) are tightly bound to the nuclear matrix, while high ionic strength buffers release the more acidic hyper-phosphorylated ones (pI approximately 8.5), suggesting that the protein is post-translationally modified, and that these modifications control its attachment to the nuclear matrix. Dephosphorylation by exogenous alkaline phosphatase and phosphorylation by exogenous CK2, as well as specific inhibition and stimulation of endogenous CK2 with heparin and spermine and spermidine, respectively, revealed that the protein is an in vitro and in vivo substrate of this enzyme, and that CK2 phosphorylation weakens the strength of its binding to the nuclear matrix. In synchronized cells, the nuclear 90 kDa AcMFP1 phosphorylation levels vary during the cell cycle with a moderate peak in G2. These results provide the first evidence for AcMFP1 in vivo phosphorylation, and open up further research on its nuclear functions.

Functional implication of p73 protein stability in neuronal cell survival and death

p73, a newly identified member of p53 family, locates at human chromosome 1p36.2-3, a region which is frequently deleted in a wide variety of human tumors including neuroblastoma. p73 is induced to be accumulated in response to a subset of DNA damaging agents such as cisplatin, and thereby promoting G1/S cell cycle arrest and/or apoptosis. Since the expression levels of p73 are kept extremely low under normal conditions, stabilization of p73 is critical for its effects on cell growth inhibition and apoptosis. Indeed, p73 is induced at protein level in SH-SY5Y neuroblastoma cells exposed to cisplatin. Several lines of evidence indicate that stress-induced post-translational modifications of p73 such as phosphorylation and acetylation lead to a marked extension of its half-life. p73 stability is regulated at least in part by proteasome-dependent degradation pathway, however, MDM2 which mediates ubiquitination and subsequent degradation of p53 by the 26S proteasome, does not promote the proteolytic degradation of p73, implying that the protein stability of p73 is regulated through a pathway distinct from that of p53. Although little is known about the regulation of p73 turnover, we are now beginning to understand the regulatory mechanisms by which p73 is induced to be stabilized in response to apoptotic stimuli, and exerts its pro-apoptotic activity. In this review, we discuss about the cellular proteins implicated in the stability control of p73.

Serine/threonine/tyrosine phosphorylation of the LHX3 LIM-homeodomain transcription factor

LHX3 is a LIM homeodomain transcription factor with essential roles in pituitary and motor neuron development in mammals. Patients with mutations in the LHX3 gene have combined pituitary hormone deficiency and other symptoms. In this study, we show that the LHX3 protein can be modified post-translationally by phosphorylation. LHX3 can serve as a substrate for protein kinase C and casein kinase II. Overexpression of these kinases reduces the transcriptional capacity of LHX3 to activate target genes. Following exposure of LHX3 to cellular kinases, mass spectrometry was used to map the phosphorylation of five amino acid residues within the human LHX3a isoform. Two phosphorylated residues (threonine 63 and serine 71) lie within the first LIM domain of the protein. Three other modified amino acids (tyrosine 227, serine 234, and serine 238) are located in the carboxyl terminus. Targeted replacement of these amino acids with non-modifiable residues significantly reduced the ability of LHX3 to activate both synthetic and pituitary hormone reporter genes. However, the amino acid replacements did not significantly affect the capability of LHX3 to interact with the NLI, PIT1, and MRG1 partner proteins, or its ability to bind to a high affinity DNA site. In conclusion, we have identified unique amino acids within LHX3 that are important for its transcriptional activity and are phosphorylated.

C-Abl as a modulator of p53

P53 is renowned as a cellular tumor suppressor poised to instigate remedial responses to various stress insults that threaten DNA integrity. P53 levels and activities are kept under tight regulation involving a complex network of activators and inhibitors, which determine the type and extent of p53 growth inhibitory signaling. Within this complexity, the p53-Mdm2 negative auto-regulatory loop serves as a major route through which intra- and extra-cellular stress signals are channeled to appropriate p53 responses. Mdm2 inhibits p53 transcriptional activities and through its E3 ligase activity promotes p53 proteasomal degradation either within the nucleus or following nuclear export. Upon exposure to stress signals these actions of Mdm2 have to be moderated, or even interrupted, in order to allow sufficient p53 to accumulate in an active form. Multiple mechanisms involving a variety of factors have been demonstrated to mediate this interruption. C-Abl is a critical factor that under physiological conditions is required for the maximal and efficient accumulation of active p53 in response to DNA damage. C-Abl protects p53 by antagonizing the inhibitory effect of Mdm2, an action that requires a direct interplay between c-Abl and Mdm2. In addition, c-Abl protects p53 from other inhibitors of p53, such as the HPV-E6/E6AP complex, that inhibits and degrades p53 in HPV-infected cells. Surprisingly, the oncogenic form of c-Abl, the Bcr-Abl fusion protein in CML cells, also promotes the accumulation of wt p53. However, in contrast to the activation of p53 by c-Abl, its oncogenic form, Bcr-Abl, counteracts the growth inhibitory activities of p53 by modulating the p53-Mdm2 loop. Thus, it appears that by modulating the p53-Mdm2 loop, c-Abl and its oncogenic forms critically determine the type and extent of the cellular response to DNA damage.

TAp63alpha induces apoptosis by activating signaling via death receptors and mitochondria

TP63, an important epithelial developmental gene, has significant homology to p53. Unlike p53, the expression of p63 is regulated by two different promoters resulting in proteins with opposite functions: the full-length transcriptionally active TAp63 and the dominant-negative DeltaNp63. We investigated the downstream mechanisms by which TAp63alpha elicits apoptosis. TAp63alpha directly transactivates the CD95 gene via the p53 binding site in the first intron resulting in upregulation of a functional CD95 death receptor. Stimulation and blocking experiments of the CD95, TNF-R and TRAIL-R death receptor systems revealed that TAp63alpha can trigger expression of each of these death receptors. Furthermore, our findings demonstrate a link between TAp63alpha and the mitochondrial apoptosis pathway. TAp63alpha upregulates expression of proapoptotic Bcl-2 family members like Bax and BCL2L11 and the expression of RAD9, DAP3 and APAF1. Of clinical relevance is the fact that TAp63alpha is induced by many chemotherapeutic drugs and that inhibiting TAp63 function leads to chemoresistance. Thus, beyond its importance in development and differentiation, we describe an important role for TAp63alpha in the induction of apoptosis and chemosensitivity.

Role of c-Abl in the DNA damage stress response

c-Abl has been implicated in many cellular processes including differentiation, division, adhesion, death, and stress response. c-Abl is a latent tyrosine kinase that becomes activated in response to numerous extra- and intra-cellular stimuli. Here we briefly review the current knowledge about c-Abl involvement in the DNA-damage stress response and its implication on cell physiology.

p73-induced apoptosis: A question of compartments and cooperation

The transcriptionally active forms of p73 are capable of inducing apoptosis, and the isoforms termed TAp73 are important players when E2F and its oncogenic activators induce programmed cell death. However, the conditions under that TAp73 can kill a cell remain to be clarified. Recently, it has been found that p73 proteins are not merely floating in the nucleoplasm but rather can associate with specific compartments in the cell. Examples of intranuclear compartments associated with p73 proteins include the PML oncogenic domains and the nuclear matrix. In addition, p73 is found in the cytoplasm. It remains to be seen whether p73 might also associate with mitochondria, in analogy with p53. The relocalization of p73 is expected to be mediated by specific binding partners, mostly other proteins. Here, we discuss the possibility that the compartmentalization of p73, and the cooperation with the corresponding binding partners, might decide about its apoptosis-inducing activity.

PIAS-1 is a checkpoint regulator which affects exit from G1 and G2 by sumoylation of p73

p73 is a recently described member of the p53 family, and, like p53, it undergoes a number of posttranslational modifications. Here we show, by yeast two-hybrid screening, pull-down assays, and coimmunoprecipitation, that p73alpha, -beta, and -gamma bind to the protein inhibitor of activated STAT-1 (PIAS-1) and that this binding stabilizes p73. PIAS-1 also sumoylates p73alpha, although not the C-terminally truncated isoforms p73beta and -gamma, and this requires the RING finger domain of PIAS-1. The DeltaNp73alpha isoform can also bind, and be sumoylated by, PIAS-1. PIAS-1-mediated sumoylation decreases p73 transcriptional activity on several target promoters, such as Bax. p73 is colocalized in the nucleus with PIAS-1, and sumoylated p73 is located exclusively in the nuclear matrix. PIAS-1 is expressed predominantly during S phase, and PIAS-1 overexpression reduces p73-mediated transcription of p21, with a reduction of cells in G(1) and cell cycle reentry. Inhibition of endogenous PIAS-1 by RNA interference reduces the proportion of cells in S phase and induces G(2) arrest. These data suggest that PIAS-1, acting partly through binding and sumoylation of p73, is an important component of the cell cycle machinery.

Activation of the neuronal c-Abl tyrosine kinase by amyloid-β-peptide and reactive oxygen species

The deposition and accumulation of amyloid-beta-peptide (Abeta) in the brain are considered a sine qua non for Alzheimer's disease. The experimental delivery of fibrilized Abeta serves as a cellular model for several facets of the disease including the induction of synaptic dysfunction and apoptosis. c-Abl kinase is involved in the regulation of apoptosis and its pro-apoptotic function is in part mediated by its interaction with p73, a p53 homologue. We found that c-Abl activation is involved in cell signals that regulate neuronal death response to Abeta fibrils. Abeta peptide fibrils induced an increase of the c-Abl activity in rat hippocampal neurons as well as an increase in nuclear p73 protein levels and the p73-c-Abl complex. The neuronal cell death induced by Abeta fibrils was prevented by the inhibition of c-Abl with imatinib mesylate (Gleevec or STI571) and by the inhibition c-Abl expression by RNAi. These results directly point to a therapeutic strategy for the treatment of Alzheimer's disease.