Promyelocytic leukemia protein 4 induces apoptosis by inhibition of survivin expression

The PML1-WDR5 axis regulates H3K4me3 marks and promotes stemness of estrogen receptor-positive breast cancer

The alternative splicing of PML precursor mRNA gives rise to various PML isoforms, yet their expression profile in breast cancer cells remains uncharted. We discovered that PML1 is the most abundant isoform in all breast cancer subtypes, and its expression is associated with unfavorable prognosis in estrogen receptor-positive (ER+) breast cancers. PML depletion reduces cell proliferation, invasion, and stemness, while heterologous PML1 expression augments these processes and fuels tumor growth and resistance to fulvestrant, an FDA-approved drug for ER+ breast cancer, in a mouse model. Moreover, PML1, rather than the well-known tumor suppressor isoform PML4, rescues the proliferation of PML knockdown cells. ChIP-seq analysis reveals significant overlap between PML-, ER-, and Myc-bound promoters, suggesting their coordinated regulation of target gene expression, including genes involved in breast cancer stem cells (BCSCs), such as JAG1, KLF4, YAP1, SNAI1, and MYC. Loss of PML reduces BCSC-related gene expression, and exogenous PML1 expression elevates their expression. Consistently, PML1 restores the association of PML with these promoters in PML-depleted cells. We identified a novel association between PML1 and WDR5, a key component of H3K4 methyltransferase (HMTs) complexes that catalyze H3K4me1 and H3K4me3. ChIP-seq analyses showed that the loss of PML1 reduces H3K4me3 in numerous loci, including BCSC-associated gene promoters. Additionally, PML1, not PML4, re-establishes the H3K4me3 mark on these promoters in PML-depleted cells. Significantly, PML1 is essential for recruiting WDR5, MLL1, and MLL2 to these gene promoters. Inactivating WDR5 by knockdown or inhibitors phenocopies the effects of PML1 loss, reducing BCSC-related gene expression and tumorsphere formation and enhancing fulvestrant's anticancer activity. Our findings challenge the conventional understanding of PML as a tumor suppressor, redefine its role as a promoter of tumor growth in breast cancer, and offer new insights into the unique roles of PML isoforms in breast cancer.

The PML1-WDR5 axis regulates H3K4me3 marks and promotes stemness of estrogen receptor-positive breast cancer

The alternative splicing of PML precursor mRNA gives rise to various PML isoforms, yet their expression profile in breast cancer cells remains uncharted. We discovered that PML1 is the most abundant isoform in all breast cancer subtypes, and its expression is associated with unfavorable prognosis in estrogen receptor-positive (ER+) breast cancers. PML depletion reduces cell proliferation, invasion, and stemness, while heterologous PML1 expression augments these processes and fuels tumor growth and resistance to fulvestrant, an FDA-approved drug for ER + breast cancer, in a mouse model. Moreover, PML1, rather than the well-known tumor suppressor isoform PML4, rescues the proliferation of PML knockdown cells. ChIP-seq analysis reveals significant overlap between PML-, ER-, and Myc-bound promoters, suggesting their coordinated regulation of target gene expression, including genes involved in breast cancer stem cells (BCSCs), such as JAG1, KLF4, YAP1, SNAI1, and MYC. Loss of PML reduces BCSC-related gene expression, and exogenous PML1 expression elevates their expression. Consistently, PML1 restores the association of PML with these promoters in PML-depleted cells. We identified a novel association between PML1 and WDR5, a key component of H3K4 methyltransferase (HMTs) complexes that catalyze H3K4me1 and H3K4me3. ChIP-seq analyses showed that the loss of PML1 reduces H3K4me3 in numerous loci, including BCSC-associated gene promoters. Additionally, PML1, not PML4, re-establishes the H3K4me3 mark on these promoters in PML-depleted cells. Significantly, PML1 is essential for recruiting WDR5, MLL1, and MLL2 to these gene promoters. Inactivating WDR5 by knockdown or inhibitors phenocopies the effects of PML1 loss, reducing BCSC-related gene expression and tumorsphere formation and enhancing fulvestrant's anticancer activity. Our findings challenge the conventional understanding of PML as a tumor suppressor, redefine its role as a promoter of tumor growth in breast cancer and offer new insights into the unique roles of PML isoforms in breast cancer.

Tumor suppressors in acute myeloid leukemia

Acute myeloid leukemia (AML) is a very heterogeneous type of blood cancer, which presents with a high rate of mortality especially in elderly patients. Better understanding of critical players, such as molecules with tumor suppressive properties, may help to fine-tune disease classification and thereby treatment modalities for this detrimental disease. Here, we summarize well-known and established tumor suppressors as well as emerging tumor suppressors, including transcription factors (TCFs) and other transcriptional regulators, such as epigenetic modulators. In addition, we look into the versatile field of miRNAs also interfering with tumorigenesis and progression, which offer new possibilities in AML diagnosis, prognosis, and therapy.

Tumor Suppressors Having Oncogenic Functions: The Double Agents

Cancer progression involves multiple genetic and epigenetic events, which involve gain-of-functions of oncogenes and loss-of-functions of tumor suppressor genes. Classical tumor suppressor genes are recessive in nature, anti-proliferative, and frequently found inactivated or mutated in cancers. However, extensive research over the last few years have elucidated that certain tumor suppressor genes do not conform to these standard definitions and might act as “double agents”, playing contrasting roles in vivo in cells, where either due to haploinsufficiency, epigenetic hypermethylation, or due to involvement with multiple genetic and oncogenic events, they play an enhanced proliferative role and facilitate the pathogenesis of cancer. This review discusses and highlights some of these exceptions; the genetic events, cellular contexts, and mechanisms by which four important tumor suppressors—pRb, PTEN, FOXO, and PML display their oncogenic potentials and pro-survival traits in cancer.

Promyelocytic Leukemia (PML) gene regulation: implication towards curbing oncogenesis

Dysregulation of PML, a significant tumor suppressor is linked with cancers of different histological origins, with a decreased expression observed with a higher tumor grade. This necessitates studying the mechanisms to maintain a stable expression of PML. However much less is known about the transcriptional regulation of PML, more so in the context of breast carcinoma. ERβ has emerged as a critical factor in understanding breast cancer, especially since a huge proportion of breast cancers are ERα⁻ and thus insensitive to tamoxifen therapy. This study aims to uncover an unidentified mechanism of PML gene regulation and its stabilization in breast cancer via ERβ signalling and the impact on cellular apoptosis. We found that clinical expression of PML positively correlates with that of ERβ both in normal and breast carcinoma samples and inversely correlates with markers of cellular proliferation, hinting towards a possible mechanistic interdependence. Both mRNA and protein expression of PML were increased in response to ERβ overexpression on multiple human breast cancer cell lines. Mechanistically, luciferase reporter assays and chromatin-immunoprecipitation assays demonstrated that ERβ can interact with the PML promoter via ERE and AP1 sites to enhance its transcription. ERβ induced stable PML expression causes a decline of its target protein Survivin and simultaneously provides a stable docking platform leading to stabilisation of its target Foxo3a, further causing transcriptional upregulation of pro-apoptotic factors p21 and p27. Immunohistochemical analyses of cancer and normal breast tissues and functional assays conducted corroborated the findings. Collectively, our study identifies ERβ signalling as a novel mechanism for PML gene regulation in ERα⁻ breast cancer. It also reveals bi-directional downstream effect in which ‘ERβ-PML-(Foxo3a/Survivin)’ network acts as a therapeutic axis by suppressing cellular survival and promoting cellular apoptosis in breast carcinoma.

Importance of Promyelocytic Leukema Protein (PML) for Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication

Many DNA virus replication-related proteins are associated with promyelocytic leukemia protein (PML), a component of nuclear domain 10 (ND10), which has been investigated for its potential involvement in viral replication. In the case of Kaposi’s sarcoma-associated herpesvirus (KSHV) lytic gene products, K8 (K-bZIP), ORF59, and ORF75 have been shown to colocalize with PML, but its importance in KSHV lytic replication is still unclear. In this study, we analyzed the functional influence of PML on KSHV latency and lytic replication in KSHV-infected primary effusion lymphoma (PEL) cell lines. Stable PML-knockout (BC3-PML^KO) and PML-overexpressing BC3 cells (BC3^PML) were successfully generated and the latency and reactivation status were analyzed. The results demonstrated that neither KSHV latency nor the episome copy number was affected in BC3-PML^KO cells. In the reactivation phase, the expression dynamics of KSHV immediate-early or early lytic proteins such as RTA, K9 (vIRF1), K5, K3, ORF59, and K8 (K-bZIP) were comparable between wild-type, control BC3, and BC3-PML^KO cells. Interestingly, KSHV lytic replication, virion production, and expression of late genes were downregulated in BC3-PML^KO cells and upregulated in BC3^PML cells, compared to those in control or wild-type BC3 cells. Moreover, exogenous PML increased the size of the PML dots and recruited additional K8 (K-bZIP) to PML-NBs as dots. Therefore, PML would function as a positive regulator for KSHV lytic DNA replication by recruiting KSHV replication factors such as 8 (K-bZIP) or ORF59 to the PML-NBs.

E1B-55K-Mediated Regulation of RNF4 SUMO-Targeted Ubiquitin Ligase Promotes Human Adenovirus Gene Expression

Human adenovirus (HAdV) E1B-55K is a multifunctional regulator of productive viral replication and oncogenic transformation in nonpermissive mammalian cells. These functions depend on E1B-55K's posttranslational modification with the SUMO protein and its binding to HAdV E4orf6. Both early viral proteins recruit specific host factors to form an E3 ubiquitin ligase complex that targets antiviral host substrates for proteasomal degradation. Recently, we reported that the PML-NB-associated factor Daxx represses efficient HAdV productive infection and is proteasomally degraded via a SUMO-E1B-55K-dependent, E4orf6-independent pathway, the details of which remained to be established. RNF4, a cellular SUMO-targeted ubiquitin ligase (STUbL), induces ubiquitinylation of specific SUMOylated proteins and plays an essential role during DNA repair. Here, we show that E1B-55K recruits RNF4 to the insoluble nuclear matrix fraction of the infected cell to support RNF4/Daxx association, promoting Daxx PTM and thus inhibiting this antiviral factor. Removing RNF4 from infected cells using RNA interference resulted in blocking the proper establishment of viral replication centers and significantly diminished viral gene expression. These results provide a model for how HAdV antagonize the antiviral host responses by exploiting the functional capacity of cellular STUbLs. Thus, RNF4 and its STUbL function represent a positive factor during lytic infection and a novel candidate for future therapeutic antiviral intervention strategies.IMPORTANCE Daxx is a PML-NB-associated transcription factor that was recently shown to repress efficient HAdV productive infection. To counteract this antiviral measurement during infection, Daxx is degraded via a novel pathway including viral E1B-55K and host proteasomes. This virus-mediated degradation is independent of the classical HAdV E3 ubiquitin ligase complex, which is essential during viral infection to target other host antiviral substrates. To maintain a productive viral life cycle, HAdV E1B-55K early viral protein inhibits the chromatin-remodeling factor Daxx in a SUMO-dependent manner. In addition, viral E1B-55K protein recruits the STUbL RNF4 and sequesters it into the insoluble fraction of the infected cell. E1B-55K promotes complex formation between RNF4- and E1B-55K-targeted Daxx protein, supporting Daxx posttranslational modification prior to functional inhibition. Hence, RNF4 represents a novel host factor that is beneficial for HAdV gene expression by supporting Daxx counteraction. In this regard, RNF4 and other STUbL proteins might represent novel targets for therapeutic intervention.

Control of apoptosis by SMAR1

The nuclear matrix associated protein SMAR1 is sensitive to p53 and acts as a stress inducer as well as a regulator in the p53 regulatory network. Depending on the amount of stress SMAR1 stimulates, it can drive the p53 dynamics in the system to various dynamical states which correspond to various cellular states. The behavior of p53 in these dynamical states is found to be multifractal, due to the mostly long range correlations and large scale fluctuations imparted by stress. This fractal behavior is exhibited in the topological properties of the networks constructed from these dynamical states, and is a signature of self-organization to optimize information flow in the dynamics. The assortativity found in these networks is due to perturbation induced by stress, and indicates that the hubs in the time series play a significant role in stress management. SMAR1 can also regulate apoptosis in the presence of HDAC1, depending on the stress induced by it.

Cardiac Glycosides Activate the Tumor Suppressor and Viral Restriction Factor Promyelocytic Leukemia Protein (PML)

Cardiac glycosides (CGs), inhibitors of Na+/K+-ATPase (NKA), used clinically to treat heart failure, have garnered recent attention as potential anti-cancer and anti-viral agents. A high-throughput phenotypic screen designed to identify modulators of promyelocytic leukemia protein (PML) nuclear body (NB) formation revealed the CG gitoxigenin as a potent activator of PML. We demonstrate that multiple structurally distinct CGs activate the formation of PML NBs and induce PML protein SUMOylation in an NKA-dependent fashion. CG effects on PML occur at the post-transcriptional level, mechanistically distinct from previously described PML activators and are mediated through signaling events downstream of NKA. Curiously, genomic deletion of PML in human cancer cells failed to abrogate the cytotoxic effects of CGs and other apoptotic stimuli such as ceramide and arsenic trioxide that were previously shown to function through PML in mice. These findings suggest that alternative pathways can compensate for PML loss to mediate apoptosis in response to CGs and other apoptotic stimuli.

Inhibition of Sp1 functions by its sequestration into PML nuclear bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are comprised of PML and a striking variety of its associated proteins. Various cellular functions have been attributed to PML NBs, including the regulation of gene expression. We report here that induced expression of PML recruits Sp1 into PML NBs, leading to the reduction of Sp1 transactivation function. Specifically, Chromatin immunoprecipitation (ChIP) assay demonstrated that induced expression of PML significantly diminishes the amount of Sp1 binding to its target gene promoter, immunofluorescence staining showed dramatic increase in the co-localization between PML and Sp1 upon induction of PML expression, moreover, PML and Sp1 co-fractionated in the core nuclear matrix. Our study further showed that PML promotes SUMOylation of Sp1 in a RING-motif-dependent manner, SUMOylation of Sp1 facilitates physical interaction between Sp1 and PML and recruitment of Sp1 into the PML NBs, the SUMO binding motif of PML was also important for its interaction with Sp1. The results of this study demonstrate a novel mechanism by which PML regulates gene expression through sequestration of the transcription factor into PML NBs.

Virion factors that target Daxx to overcome intrinsic immunity

PML nuclear bodies and their associated functions are part of an intrinsic cellular mechanism aimed at maintaining transcriptional control over viral gene expression and preventing replication of invading viruses. To overcome these barriers, many viruses express early nonstructural, multifunctional proteins to support the viral replication cycle or modulate host immune responses. Virion proteins constituting the invading particle are traditionally investigated for their role in transport during entry or egress and in the assembly of new virions. The additional functions of virion proteins have largely been ignored, in contrast to those of their nonstructural counterparts. A number of recent reports suggest that several virion proteins may also play vital roles in gene activation processes, in particular by counteracting intrinsic immune mechanisms mediated by the PML nuclear body-associated cellular factors Daxx, ATRX, and Sp100. These virion proteins share several features with their more potent nonstructural counterparts, and they may serve to bridge the gap in the early phase of an infection until immediate early viral gene expression is established. In this review, we discuss how virion proteins are an integral part of gene regulation among several viral families and to what extent structural proteins of incoming virions may contribute to species barrier, latency, and oncogenesis.

Control of human adenovirus type 5 gene expression by cellular Daxx/ATRX chromatin-associated complexes

Death domain-associated protein (Daxx) cooperates with X-linked α-thalassaemia retardation syndrome protein (ATRX), a putative member of the sucrose non-fermentable 2 family of ATP-dependent chromatin-remodelling proteins, acting as the core ATPase subunit in this complex, whereas Daxx is the targeting factor, leading to histone deacetylase recruitment, H3.3 deposition and transcriptional repression of cellular promoters. Despite recent findings on the fundamental importance of chromatin modification in host-cell gene regulation, it remains unclear whether adenovirus type 5 (Ad5) transcription is regulated by cellular chromatin remodelling to allow efficient virus gene expression. Here, we focus on the repressive role of the Daxx/ATRX complex during Ad5 replication, which depends on intact protein-protein interaction, as negative regulation could be relieved with a Daxx mutant that is unable to interact with ATRX. To ensure efficient viral replication, Ad5 E1B-55K protein inhibits Daxx and targets ATRX for proteasomal degradation in cooperation with early region 4 open reading frame protein 6 and cellular components of a cullin-dependent E3-ubiquitin ligase. Our studies illustrate the importance and diversity of viral factors antagonizing Daxx/ATRX-mediated repression of viral gene expression and shed new light on the modulation of cellular chromatin remodelling factors by Ad5. We show for the first time that cellular Daxx/ATRX chromatin remodelling complexes play essential roles in Ad gene expression and illustrate the importance of early viral proteins to counteract cellular chromatin remodelling.

Transcriptional activation of the adenoviral genome is mediated by capsid protein VI

Gene expression of DNA viruses requires nuclear import of the viral genome. Human Adenoviruses (Ads), like most DNA viruses, encode factors within early transcription units promoting their own gene expression and counteracting cellular antiviral defense mechanisms. The cellular transcriptional repressor Daxx prevents viral gene expression through the assembly of repressive chromatin remodeling complexes targeting incoming viral genomes. However, it has remained unclear how initial transcriptional activation of the adenoviral genome is achieved. Here we show that Daxx mediated repression of the immediate early Ad E1A promoter is efficiently counteracted by the capsid protein VI. This requires a conserved PPxY motif in protein VI. Capsid proteins from other DNA viruses were also shown to activate the Ad E1A promoter independent of Ad gene expression and support virus replication. Our results show how Ad entry is connected to transcriptional activation of their genome in the nucleus. Our data further suggest a common principle for genome activation of DNA viruses by counteracting Daxx related repressive mechanisms through virion proteins.

To initiate infection, DNA viruses deliver their genome to the nucleus and express viral genes required for genome replication. Efficient transport is achieved by packing the viral genome as a condensed, transcriptionally inactive nucleo-protein complex. However, for most DNA viruses, including Adenoviruses (Ads), it remains unclear how the viral genome is decondensed and how transcription is initiated inside the nucleus. Cells control unwanted gene expression by chromatin modification mediated through transcriptionally repressive complexes. A key factor in repressive complex assemblies is the transcriptional repressor Daxx. The Ad structural capsid protein VI is required for endosomal escape and nuclear transport. Here we show that protein VI also activates the Ad E1A promoter to initiate Ad gene expression. This is achieved through the removal of Daxx repression from the E1A promoter, which requires a conserved ubiquitin ligase interacting motif (PPxY-motif) in protein VI. We further show that capsid proteins from other unrelated DNA viruses also activate the Ad E1A promoter and support Ad replication by counteracting Daxx repression, functionally replacing protein VI. Our data suggest that reversal of Daxx repression by virion proteins is a widespread mechanism among DNA viruses that is not restricted to a single virus family.

Physical and functional interaction between PML and TBX2 in the establishment of cellular senescence

Cellular senescence acts as a potent barrier for tumour initiation and progression. Previous studies showed that the PML tumour suppressor promotes senescence, although the precise mechanisms remain to be elucidated. Combining gene expression profiling with chromatin-binding analyses and promoter reporter studies, we identify TBX2, a T-box transcription factor frequently overexpressed in cancer, as a novel and direct PML-repressible E2F-target gene in senescence but not quiescence. Recruitment of PML to the TBX2 promoter is dependent on a functional p130/E2F4 repressor complex ultimately implementing a transcriptionally inactive chromatin environment at the TBX2 promoter. TBX2 repression actively contributes to senescence induction as cells depleted for TBX2 trigger PML pro-senescence function(s) and enter senescence. Reciprocally, elevated TBX2 levels antagonize PML pro-senescence function through direct protein-protein interaction. Collectively, our findings indicate that PML and TBX2 act in an autoregulatory loop to control the effective execution of the senescence program.

Survivin upregulation, dependent on leptin-EGFR-Notch1 axis, is essential for leptin-induced migration of breast carcinoma cells

Obese breast cancer patients exhibit a higher risk for larger tumor burden and an increased likelyhood of metastasis. The molecular effects of obesity on carcinogenesis are mediated by the autocrine and paracrine effects of the adipocytokine leptin. Leptin participates in the tumor progression and metastasis of human breast. We show that leptin induces clonogenicity and increases the migration potential of breast cancer cells. We found that survivin expression is induced in response to leptin. In this study, we examine the role and leptin-mediated regulation of survivin. Leptin treatment leads to survivin upregulation, due in part to the activation of Notch1 and the release of a transcriptionally active Notch1 intracellular domain (NICD). Chromatin immunoprecipitation analysis shows that NICD gets recruited to the survivin promoter at the CSL (CBF1/RBP-Jk, Su(H), Lag-1) binding site in response to leptin treatment. Inhibition of Notch1 activity inhibits leptin-induced survivin upregulation. Leptin-induced transactivation of epidermal growth factor receptor (EGFR) is involved in leptin-mediated Notch1 and survivin upregulation, demonstrating a novel upstream role of leptin-EGFR-Notch1 axis. We further show that leptin-induced migration of breast cancer cells requires survivin, as overexpression of survivin further increases, whereas silencing survivin abrogates leptin-induced migration. Using a pharmacological approach to inhibit survivin, we show that 3-hydroxy-3-methylglutaryl-coenzyme-A-reductase inhibitors, such as lovastatin, can effectively inhibit leptin-induced survivin expression and migration. Importantly, leptin increased breast tumor growth in nude mice. These data show a novel role for survivin in leptin-induced migration and put forth pharmacological survivin inhibition as a potential novel therapeutic strategy. This conclusion is supported by in vivo data showing the overexpression of leptin and survivin in epithelial cells of high-grade ductal carcinomas in situ and in high-grade invasive carcinomas.

Gene regulation by SMAR1: Role in cellular homeostasis and cancer

Changes in the composition of nuclear matrix associated proteins contribute to alterations in nuclear structure, one of the major phenotypes of malignant cancer cells. The malignancy-induced changes in this structure lead to alterations in chromatin folding, the fidelity of genome replication and gene expression programs. The nuclear matrix forms a scaffold upon which the chromatin is organized into periodic loop domains called matrix attachment regions (MAR) by binding to various MAR binding proteins (MARBPs). Aberrant expression of MARBPs modulates the chromatin organization and disrupt transcriptional network that leads to oncogenesis. Dysregulation of nuclear matrix associated MARBPs has been reported in different types of cancers. Some of these proteins have tumor specific expression and are therefore considered as promising diagnostic or prognostic markers in few cancers. SMAR1 (scaffold/matrix attachment region binding protein 1), is one such nuclear matrix associated protein whose expression is drastically reduced in higher grades of breast cancer. SMAR1 gene is located on human chromosome 16q24.3 locus, the loss of heterozygosity (LOH) of which has been reported in several types of cancers. This review elaborates on the multiple roles of nuclear matrix associated protein SMAR1 in regulating various cellular target genes involved in cell growth, apoptosis and tumorigenesis.

[Effects of vitamin C on A549 cell proliferation, apoptosis and expressions of Caspase, Survivin]

BACKGROUND AND OBJECTIVE

It was proven that Vitamin C could inhibit the growth of many types of tumors as an antioxidant. The aim of this study is to explore role of Vitamin C in proliferation and apoptosis of lung carcinoma cell line A549 and the underlying mechanism.

METHODS

A549 cells were cultured in vitro and incubated with Vitamin C. The cell viability was measured by growth curve and clonogentic assay. Flow cytometry was used to analyze cell cycle and detect apoptosis. The levels of expression of Caspase-3 mRNA and Survivin mRNA were detected by RT-PCR.

RESULTS

Vitamin C of 400 microg/mL, 4 mg/mL significantly inhibited the growth of A549 cell lines (P = 0.024, P = 0.015, respectively). Flow cytometry showed that the cells major stagnation stayed in the G0/G1 and S phase and the apoptotic rate increased with time prolonged. Vitamin C signifiantly up-regulated the expression of Caspase-3 mRNA, but had no effect on Survivin mRNA.

CONCLUSION

Vitamin C can inhibit the proliferation of A549, block A549 cells in G0/G1 and S phase, and induce apoptosis of A549 cells. Apotosis occurred by up-regulated the expression of Caspase-3.

Avicin D, a plant triterpenoid, induces cell apoptosis by recruitment of Fas and downstream signaling molecules into lipid rafts

Avicins, a family of triterpene electrophiles originally identified as potent inhibitors of tumor cell growth, have been shown to be pleiotropic compounds that also possess antioxidant, anti-mutagenic, and anti-inflammatory activities. We previously showed that Jurkat cells, which express a high level of Fas, are very sensitive to treatment with avicins. Thus, we hypothesized that avicins may induce cell apoptosis by activation of the Fas pathway. By using a series of cell lines deficient in cell death receptors, we demonstrated that upon avicin D treatment, Fas translocates to the cholesterol- and sphingolipid-enriched membrane microdomains known as lipid rafts. In the lipid rafts, Fas interacts with Fas-associated death domain (FADD) and Caspase-8 to form death-inducing signaling complex (DISC) and thus mediates cell apoptosis. Interfering with lipid raft organization by using a cholesterol-depleting compound, methyl-β-cyclodextrin, not only prevents the clustering of Fas and its DISC complex but also reduces the sensitivity of the cells to avicin D. Avicin D activates Fas pathways independent of the association between extracellular Fas ligands and Fas receptors. A deficiency in Fas and its downstream signaling molecules leads to the resistance of the cells to avicin D treatment. Taken together, our results demonstrate that avicin D triggers the redistribution of Fas in the membrane lipid rafts, where Fas activates receptor-mediated cell death.

Promyelocytic leukemia protein is required for gain of function by mutant p53

Mutations in the p53 tumor suppressor are the most common genetic events in human cancer. These mutations not only result in a loss of wild-type p53 activity, but can also lead to a gain of new oncogenic properties. Understanding how these gained functions are regulated is in its infancy. In this study, we show that the promyelocytic leukemia (PML) protein is an important regulator of mutant p53. We show that PML interacts with mutant p53. Importantly, PML enhances the transcriptional activity of mutant p53. Unexpectedly, PML is required for the proliferation and colony formation of cancer cells bearing mutant p53. Down-regulation of PML expression inhibits the growth of mutant p53-expressing cancer cells, predominantly by promoting cell cycle arrest. Our results suggest that the tumor suppression function of PML depends on the status of p53. In the context of mutant p53, PML enhances its cancer-promoting activities.

Kaposi's sarcoma-associated herpesvirus protein LANA2 disrupts PML oncogenic domains and inhibits PML-mediated transcriptional repression of the survivin gene

Infection by herpesviruses causes a dramatic disturbance of PML oncogenic domains (PODs) that has been suggested to be essential for viral lytic replication. Several proteins from Kaposi's sarcoma-associated herpesvirus (KSHV) have been tested as putative POD-disrupting factors with negative results. Here, we show that LANA2, a viral protein that is absolutely required for the viability and proliferation of KSHV-infected primary effusion lymphoma (PEL) cells, increases the levels of SUMO2-ubiquitin-modified PML and induces the disruption of PODs by a proteasome-mediated mechanism. In addition, we demonstrate that this disruption is largely dependent on both the integrity of a SUMO interaction motif in LANA2 and the lysine 160 from PML. Moreover, silencing of LANA2 expression in PEL cells by RNA interference led to an increase in the PML levels. Finally, we demonstrate that LANA2 relieves PML-mediated transcriptional repression of survivin, a protein that directly contributes to malignant progression of PEL. This represents the first example of inactivation of these important antiviral structures by KSHV.

Role of nuclear bodies in apoptosis signalling

Promyelocytic leukemia nuclear bodies (PML NBs) are dynamic macromolecular multiprotein complexes that recruit and release a plethora of proteins. A considerable number of PML NB components play vital roles in apoptosis, senescence regulation and tumour suppression. The molecular basis by which PML NBs control these cellular responses is still just beginning to be understood. In addition to PML itself, numerous further tumour suppressors including transcriptional regulator p53, acetyl transferase CBP (CREB binding protein) and protein kinase HIPK2 (homeodomain interacting protein kinase 2) are recruited to PML NBs in response to genotoxic stress or oncogenic transformation and drive the senescence and apoptosis response by regulating p53 activity. Moreover, in response to death-receptor activation, PML NBs may act as nuclear depots that release apoptotic factors, such as the FLASH (FLICE-associated huge) protein, to amplify the death signal. PML NBs are also associated with other nuclear domains including Cajal bodies and nucleoli and share apoptotic regulators with these domains, implying crosstalk between NBs in apoptosis regulation. In conclusion, PML NBs appear to regulate cell death decisions through different, pathway-specific molecular mechanisms.

Histone deacetylase 7 promotes PML sumoylation and is essential for PML nuclear body formation

Promyelocytic leukemia protein (PML) sumoylation has been proposed to control the formation of PML nuclear bodies (NBs) and is crucial for PML-dependent cellular processes, including apoptosis and transcriptional regulation. However, the regulatory mechanisms of PML sumoylation and its specific roles in the formation of PML NBs remain largely unknown. Here, we show that histone deacetylase 7 (HDAC7) knockdown reduces the size and the number of the PML NBs in human umbilical vein endothelial cells (HUVECs). HDAC7 coexpression stimulates PML sumoylation independent of its HDAC activity. Furthermore, HDAC7 associates with the E2 SUMO ligase, Ubc9, and stimulates PML sumoylation in vitro, suggesting that it possesses a SUMO E3 ligase-like activity to promote PML sumoylation. Importantly, HDAC7 knockdown inhibits tumor necrosis factor alpha-induced PML sumoylation and the formation of PML NBs in HUVECs. These results demonstrate a novel function of HDAC7 and provide a regulatory mechanism of PML sumoylation.

Signal-dependent regulation of transcription by histone deacetylase 7 involves recruitment to promyelocytic leukemia protein nuclear bodies

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) are dynamic subnuclear compartments that play roles in several cellular processes, including apoptosis, transcriptional regulation, and DNA repair. Histone deacetylase (HDAC) 7 is a potent corepressor that inhibits transcription by myocyte enhancer factor 2 (MEF2) transcription factors. We show here that endogenous HDAC7 and PML interact and partially colocalize in PML NBs. Tumor necrosis factor (TNF)-alpha treatment recruits HDAC7 to PML NBs and enhances association of HDAC7 with PML in human umbilical vein endothelial cells. Consequently, TNF-alpha promotes dissociation of HDAC7 from MEF2 transcription factors and the promoters of MEF2 target genes such as matrix metalloproteinase (MMP)-10, leading to accumulation of MMP-10 mRNA. Conversely, knockdown of PML enhances the association between HDAC7 and MEF2 and decreases MMP-10 mRNA accumulation. Accordingly, ectopic expression of PML recruits HDAC7 to PML NBs and leads to activation of MEF2 reporter activity. Notably, small interfering RNA knockdown of PML decreases basal and TNF-alpha-induced MMP-10 mRNA accumulation. Our results reveal a novel mechanism by which PML sequesters HDAC7 to relieve repression and up-regulate gene expression.

Survivin, cancer networks and pathway-directed drug discovery

Although there is no shortage of potential targets for cancer therapeutics, we know of only a handful of molecules that are differentially expressed in cancer and intersect multiple pathways required for tumour maintenance. Survivin embodies these properties, and orchestrates integrated cellular networks that are essential for tumour cell proliferation and viability. Pursuing the nodal functions of survivin in cancer might lead to the development of global pathway inhibitors with unique therapeutic potential.

Degradation of the tumor suppressor PML by Pin1 contributes to the cancer phenotype of breast cancer MDA-MB-231 cells

Promyelocytic leukemia protein (PML) is an important regulator due to its role in numerous cellular processes including apoptosis, viral infection, senescence, DNA damage repair, and cell cycle regulation. Despite the role of PML in many cellular functions, little is known about the regulation of PML itself. We show that PML stability is regulated through interaction with the peptidyl-prolyl cis-trans isomerase Pin1. This interaction is mediated through four serine-proline motifs in the C terminus of PML. Binding to Pin1 results in degradation of PML in a phosphorylation-dependent manner. Furthermore, our data indicate that sumoylation of PML blocks the interaction, thus preventing degradation of PML by this pathway. Functionally, we show that in the MDA-MB-231 breast cancer cell line modulating levels of Pin1 affects steady-state levels of PML. Furthermore, degradation of PML due to Pin1 acts both to protect these cells from hydrogen peroxide-induced death and to increase the rate of proliferation. Taken together, our work defines a novel mechanism by which sumoylation of PML prevents Pin1-dependent degradation. This interaction likely occurs in numerous cell lines and may be a pathway for oncogenic transformation.

Are promyelocytic leukaemia protein nuclear bodies a scaffold for caspase-2 programmed cell death?

Promyelocytic leukaemia protein nuclear bodies (PML-NBs) are nuclear structures whose function is still poorly understood. They are implicated in various biological functions, such as viral infection, cellular transformation, innate immunity and growth control, and they might be dynamic hubs sensing stress and DNA damage. Data from PML(-/-) mice suggest that PML-NBs are involved in apoptosis via caspase-independent mechanisms, probably involving p53-dependent and independent pathways. However, the recently demonstrated co-localization of caspase-2 within the PML-NB nuclear structures presents a new paradigm for nuclear cell death. Here, we show that these nuclear structures have a protein known as SP100 that could contain a caspase recruitment domain (CARD). If verified experimentally, this discovery will suggest a mechanism by which caspase-2 could be recruited into the complex and ultimately lead to apoptosis.

Survivin expression in tuberous sclerosis complex cells

Tuberous Sclerosis Complex (TSC) is a tumor suppressor gene disorder with mutations of TSC1/TSC2 genes. This leads to the development of hamartomas that most frequently affect central nervous system, kidney, and skin. Angiomyolipomas are abdominal masses made up of muscle vessels and adipose tissues that grow mostly in proximity to kidneys and liver. Bleeding and kidney failure are the major justification for surgery. This study shows that angiomyolipoma-derived human smooth muscle TSC2-/- cells express the apoptosis inhibitor protein survivin when exposed to IGF-1. Survivin expression is also triggered whenever culture conditions perturb normal TSC2-/- cell function, such as the omission of EGF from the growth medium, the supplementation of anti-EGFR, blockade of PI3K and ERK, or inhibition of mTOR. Interestingly, single or simultaneous inhibition of PI3K by LY294002 and ERK by PD98059 does not prevent IGF-1-mediated survivin expression. Apoptogenic Smac/DIABLO, which is constitutively expressed by TSC2-/- A+ cells, is down-regulated by IGF-1 even in the presence of LY294002 and PD98059. These cells release IGF-1 by means of a negative feedback-regulated mechanism that is overrun when they are exposed to antibodies to IGF-1R, which increases the released amount by more than 400%. The autocrine release of IGF-1 may therefore be a powerful mechanism of survival of the tightly packed cells in the thick-walled vessels of TSC angiomyolipoma and in lymphangioleiomyomatosis (LAM) nodules. Future experimental therapies for TSC and LAM may result from the targeted inhibition of survivin, which may enhance sensitivity to TSC2 therapy.

Survivin expression in tuberous sclerosis complex cells

Tuberous Sclerosis Complex (TSC) is a tumor suppressor gene disorder with mutations of TSC1/TSC2 genes. This leads to the development of hamartomas that most frequently affect central nervous system, kidney, and skin. Angiomyolipomas are abdominal masses made up of muscle vessels and adipose tissues that grow mostly in proximity to kidneys and liver. Bleeding and kidney failure are the major justification for surgery. This study shows that angiomyolipoma-derived human smooth muscle TSC2-/- cells express the apoptosis inhibitor protein survivin when exposed to IGF-1. Survivin expression is also triggered whenever culture conditions perturb normal TSC2-/- cell function, such as the omission of EGF from the growth medium, the supplementation of anti-EGFR, blockade of PI3K and ERK, or inhibition of mTOR. Interestingly, single or simultaneous inhibition of PI3K by LY294002 and ERK by PD98059 does not prevent IGF-1-mediated survivin expression. Apoptogenic Smac/DIABLO, which is constitutively expressed by TSC2-/- A+ cells, is down-regulated by IGF-1 even in the presence of LY294002 and PD98059. These cells release IGF-1 by means of a negative feedback-regulated mechanism that is overrun when they are exposed to antibodies to IGF-1R, which increases the released amount by more than 400%. The autocrine release of IGF-1 may therefore be a powerful mechanism of survival of the tightly packed cells in the thick-walled vessels of TSC angiomyolipoma and in lymphangioleiomyomatosis (LAM) nodules. Future experimental therapies for TSC and LAM may result from the targeted inhibition of survivin, which may enhance sensitivity to TSC2 therapy.

Apoptin nucleocytoplasmic shuttling is required for cell type-specific localization, apoptosis, and recruitment of the anaphase-promoting complex/cyclosome to PML bodies

The chicken anemia virus protein Apoptin selectively induces apoptosis in transformed cells while leaving normal cells intact. This selectivity is thought to be largely due to cell type-specific localization: Apoptin is cytoplasmic in primary cells and nuclear in transformed cells. The basis of Apoptin cell type-specific localization and activity remains to be determined. Here we show that Apoptin is a nucleocytoplasmic shuttling protein whose localization is mediated by an N-terminal nuclear export signal (NES) and a C-terminal nuclear localization signal (NLS). Both signals are required for cell type-specific localization, since Apoptin fragments containing either the NES or the NLS fail to differentially localize in transformed and primary cells. Significantly, cell type-specific localization can be conferred in trans by coexpression of the two separate fragments, which interact through an Apoptin multimerization domain. We have previously shown that Apoptin interacts with the APC1 subunit of the anaphase-promoting complex/cyclosome (APC/C), resulting in G(2)/M cell cycle arrest and apoptosis in transformed cells. We found that the nucleocytoplasmic shuttling activity is critical for efficient APC1 association and induction of apoptosis in transformed cells. Interestingly, both Apoptin multimerization and APC1 interaction are mediated by domains that overlap with the NES and NLS sequences, respectively. Apoptin expression in transformed cells induces the formation of PML nuclear bodies and recruits APC/C to these subnuclear structures. Our results reveal a mechanism for the selective killing of transformed cells by Apoptin.

Investigating Wnt signaling: a chemogenomic safari

Chemical genomics is a powerful method to complement more traditional genetic techniques (i.e. knockout mice, siRNA) for the dissection of complex signaling networks. Wnt signaling in mammals is a complex and crucial regulator of diverse functions. The Wnt-beta-catenin pathway initiates a signaling cascade that is crucial in both normal development and the initiation and progression of cancer. A key step in Wnt activation of target genes is the nuclear translocation of beta-catenin and the formation of a complex between beta-catenin and members of the T-cell factor (TCF) family of transcription factors. Using a forward chemical genomics strategy, we identified ICG-001, a selective inhibitor of a subset of Wnt-beta-catenin-driven gene expression. This chemogenomic tool enables us to dissect this complex signaling network and to better understand the role of Wnt signaling in both normal and pathophysiological settings.

Differential roles for the coactivators CBP and p300 on TCF/beta-catenin-mediated survivin gene expression

The inhibitor of apoptosis (IAP) protein survivin is highly expressed in cancers, but not in normal differentiated tissues. TCF/beta-catenin signaling has been reported to participate in the regulation of survivin transcription in colon cancer. We have recently characterized ICG-001, a small molecule specific inhibitor of the beta-catenin/Creb-binding protein (CBP) interaction. Inhibition of the beta-catenin/CBP interaction represses a subset of TCF/beta-catenin-mediated transcription. ICG-001 potently inhibits survivin gene transcription and expression. ICG-001-mediated downregulation of survivin expression enhanced caspase-3 activity and apoptosis, which was rescued by overexpression of wild type but not mutant (C84A) survivin. Small interfering RNA and genetic reduction of CBP also decreased survivin expression. Chromatin immunoprecipitation assay confirmed that CBP is the crucial coactivator for TCF/beta-catenin-mediated survivin transcription. Furthermore, ICG-001-induced recruitment of p300 to the survivin promoter led to concomitant recruitment of SUMO-1, HDAC6 and PML proteins, which have been associated with transcriptional repression. These findings demonstrate that CBP and p300 play very distinct roles in survivin gene transcription.

Induction of RNA interference in dendritic cells

Dendritic cells (DC) reside at the center of the immunological universe, possessing the ability both to stimulate and inhibit various types of responses. Tolerogenic/regulatory DC with therapeutic properties can be generated through various means of manipulations in vitro and in vivo. Here we describe several attractive strategies for manipulation of DC using the novel technique of RNA interference (RNAi). Additionally, we overview some of our data regarding yet undescribed characteristics of RNAi in DC such as specific transfection strategies, persistence of gene silencing, and multi-gene silencing. The advantages of using RNAi for DC genetic manipulation gives rise to the promise of generating tailor-made DC that can be used effectively to treat a variety of immunologically mediated diseases.

A Role for PML3 in Centrosome Duplication and Genome Stability

The promyelocytic leukemia gene (PML), which is disrupted by the chromosomal translocation t(15;17) in acute promyelocytic leukemia (APL), encodes a multifunctional protein involved in several important cellular functions. Herein, we demonstrate that PML is localized to centrosomes and that PML deficiency leads to centrosome amplification. By using PML isoform-specific antibodies, we found PML3-specific association with the centrosome and the pole of the mitotic spindle. PML3 deficiency leads to dysregulation of the centrosome duplication checkpoint. Furthermore, PML3 physically interacts with Aurora A and regulates its kinase activity. Specific knockdown of PML3 activates Cdk2/cyclin kinase activity. The results of this study implicate a direct role for PML3 in the control of centrosome duplication through suppression of Aurora A activation to prevent centrosome reduplication.

Molecular circuits of apoptosis regulation and cell division control: the survivin paradigm

The coupling of cell proliferation to cell death is thought to function as a pivotal crossroad, essential to preserve normal homeostasis and to eliminate dangerous cells before they divide. Survivin is a prototype molecule at this crossroad, intercalated in protection against mitochondrial cell death and orchestrating various aspects of cell division. Dramatically exploited in cancer and an unfavorable gene signature for disease outcome, the survivin pathway has now provided tangible opportunities for targeted, rational cancer therapy.

Inhibition of DNA primase and induction of apoptosis by 3,3’-diethyl-9-methylthia-carbocyanine iodide in hepatocellular carcinoma BEL-7402 cells

AIM: To evaluate the effects of 3,3’-diethyl-9-methylthia-carbocyanine iodide (DMTCCI) on DNA primase activity and on apoptosis of human hepatocellular carcinoma BEL-7402 cells.

METHODS: DNA primase assay was used to investigate DNA primase activity. MTT assay was applied to determine cell proliferation. Flow cytometric analysis, transmission electron microscopy, DNA fragmentation assay were performed to detect DMTCCI-induced apoptosis. Expression levels of p53, Bcl-2, Bcl-xL, Bad, Bax, survivin, Caspase-3 and poly (ADP-ribose) polymerase (PARP) were evaluated by immunoblot analysis. Caspase-3 activity was assessed with ApoAlert Caspase-3 colorimetric assay kit.

RESULTS: DMTCCI had inhibitory effects on eukaryotic DNA primase activity with IC₅₀ value of 162.2 nmol/L. It also inhibited proliferation of human hepatocellular carcinoma BEL-7402 cells with IC₅₀ value of 2.09 μmol/L. Furthermore, DMTCCI-induced BEL-7402 cell apoptosis was confirmed by DNA fragmentation (DNA ladders and sub-G1 formation) and transmission electron microscopy (apoptotic bodies formation). During the induction of apoptosis, expression of Bcl-2, Bcl-xL and survivin was decreased, and that of p53, Bad and Bax was increased. Caspase-3 was activated and poly (ADP-ribose) polymerase (PARP) was cleaved in BEL-7402 cells treated with DMTCCI.

CONCLUSION: The present data suggest that DMTCCI has inhibitory effects on eukaryotic DNA primase and can induce apoptosis of BEL-7402 cells. The modulation of expression of p53 and Bcl-2 family proteins, and activation of Caspase-3 might be involved in the induction of apoptosis.