Identification of poly(ADP-ribose) polymerase-1 as a cell cycle regulator through modulating Sp1 mediated transcription in human hepatoma cells

Characterization of PARP1 binding to c-KIT1 G-quadruplex DNA: Insights into domain-specific interactions

Poly(ADP-ribose) polymerase 1 (PARP1) is a nuclear enzyme involved in catalyzing Poly-(ADP-ribosyl)ation. PARP1 binds to different forms of DNA and DNA breaks and thus plays important roles in several cellular processes, including DNA damage repair, cell cycle regulation, chromatin remodeling, and maintaining genomic stability. In this study, we conducted biochemical and biophysical characterization of PARP1 binding to G-quadruplex DNA (G4-DNA). Our investigation identified ZnF1, ZnF3, and WGR as the critical domains to mediate PARP1 binding to G4-c-KIT1. Also, our results show that these domains together show cooperativity for G4-c-KIT1 recognition. Further, we establish that the presence of an oxidized (5-carboxylcytosine) base in the loop region of G4-c-KIT1 (G4-5caC-cKIT1) does not affect its recognition by PARP1. Both G4-c-KIT1 and G4-5caC-cKIT1 are potent stimulators of PARP1's catalytic activity. Our study advances the understanding of PARP1's versatile DNA binding capabilities for G4-c-KIT1 DNA irrespective of the oxidation/ modification in the DNA base. These insights into PARP1's domain-specific contributions to G4-c-KIT1 DNA recognition and catalysis expand our knowledge of its multifaceted roles in DNA repair and genome maintenance.

BioID-based intact cell interactome of the Kv1.3 potassium channel identifies a Kv1.3-STAT3-p53 cellular signaling pathway

Kv1.3 is a multifunctional potassium channel implicated in multiple pathologies, including cancer. However, how it is involved in disease progression is not fully clear. We interrogated the interactome of Kv1.3 in intact cells using BioID proximity labeling, revealing that Kv1.3 interacts with STAT3- and p53-linked pathways. To prove the relevance of Kv1.3 and of its interactome in the context of tumorigenesis, we generated stable melanoma clones, in which ablation of Kv1.3 remodeled gene expression, reduced proliferation and colony formation, yielded fourfold smaller tumors, and decreased metastasis in vivo in comparison to WT cells. Kv1.3 deletion or pharmacological inhibition of mitochondrial Kv1.3 increased mitochondrial Reactive Oxygen Species release, decreased STAT3 phosphorylation, stabilized the p53 tumor suppressor, promoted metabolic switch, and altered the expression of several BioID-identified Kv1.3-networking proteins in tumor tissues. Collectively, our work revealed the tumor-promoting Kv1.3-interactome landscape, thus opening the way to target Kv1.3 not only as an ion-conducting entity but also as a signaling hub.

Role of post-translational modifications of Sp1 in cancer: state of the art

Specific protein 1 (Sp1) is central to regulating transcription factor activity and cell signaling pathways. Sp1 is highly associated with the poor prognosis of various cancers; it is considered a non-oncogene addiction gene. The function of Sp1 is complex and contributes to regulating extensive transcriptional activity, apart from maintaining basal transcription. Sp1 activity and stability are affected by post-translational modifications (PTMs), including phosphorylation, ubiquitination, acetylation, glycosylation, and SUMOylation. These modifications help to determine genetic programs that alter the Sp1 structure in different cells and increase or decrease its transcriptional activity and DNA binding stability in response to pathophysiological stimuli. Investigating the PTMs of Sp1 will contribute to a deeper understanding of the mechanism underlying the cell signaling pathway regulating Sp1 stability and the regulatory mechanism by which Sp1 affects cancer progression. Furthermore, it will facilitate the development of new drug targets and biomarkers, thereby elucidating considerable implications in the prevention and treatment of cancer.

Mono-methylated histones control PARP-1 in chromatin and transcription

PARP-1 is central to transcriptional regulation under both normal and stress conditions, with the governing mechanisms yet to be fully understood. Our biochemical and ChIP-seq-based analyses showed that PARP-1 binds specifically to active histone marks, particularly H4K20me1. We found that H4K20me1 plays a critical role in facilitating PARP-1 binding and the regulation of PARP-1-dependent loci during both development and heat shock stress. Here, we report that the sole H4K20 mono-methylase, pr-set7, and parp-1 Drosophila mutants undergo developmental arrest. RNA-seq analysis showed an absolute correlation between PR-SET7- and PARP-1-dependent loci expression, confirming co-regulation during developmental phases. PARP-1 and PR-SET7 are both essential for activating hsp70 and other heat shock genes during heat stress, with a notable increase of H4K20me1 at their gene body. Mutating pr-set7 disrupts monomethylation of H4K20 along heat shock loci and abolish PARP-1 binding there. These data strongly suggest that H4 monomethylation is a key triggering point in PARP-1 dependent processes in chromatin.

Patterns of transcription factor binding and epigenome at promoters allow interpretable predictability of multiple functions of non-coding and coding genes

Understanding the biological roles of all genes only through experimental methods is challenging. A computational approach with reliable interpretability is needed to infer the function of genes, particularly for non-coding RNAs. We have analyzed genomic features that are present across both coding and non-coding genes like transcription factor (TF) and cofactor ChIP-seq (823), histone modifications ChIP-seq (n = 621), cap analysis gene expression (CAGE) tags (n = 255), and DNase hypersensitivity profiles (n = 255) to predict ontology-based functions of genes. Our approach for gene function prediction was reliable (>90% balanced accuracy) for 486 gene-sets. PubMed abstract mining and CRISPR screens supported the inferred association of genes with biological functions, for which our method had high accuracy. Further analysis revealed that TF-binding patterns at promoters have high predictive strength for multiple functions. TF-binding patterns at the promoter add an unexplored dimension of explainable regulatory aspects of genes and their functions. Therefore, we performed a comprehensive analysis for the functional-specificity of TF-binding patterns at promoters and used them for clustering functions to reveal many latent groups of gene-sets involved in common major cellular processes. We also showed how our approach could be used to infer the functions of non-coding genes using the CRISPR screens of coding genes, which were validated using a long non-coding RNA CRISPR screen. Thus our results demonstrated the generality of our approach by using gene-sets from CRISPR screens. Overall, our approach opens an avenue for predicting the involvement of non-coding genes in various functions.

Joining the PARty: PARP Regulation of KDM5A during DNA Repair (and Transcription?)

The lysine demethylase KDM5A collaborates with PARP1 and the histone variant macroH2A1.2 to modulate chromatin to promote DNA repair. Indeed, KDM5A engages poly(ADP-ribose) (PAR) chains at damage sites through a previously uncharacterized coiled-coil domain, a novel binding mode for PAR interactions. While KDM5A is a well-known transcriptional regulator, its function in DNA repair is only now emerging. Here we review the molecular mechanisms that regulate this PARP1-macroH2A1.2-KDM5A axis in DNA damage and consider the potential involvement of this pathway in transcription regulation and cancer. Using KDM5A as an example, we discuss how multifunctional chromatin proteins transition between several DNA-based processes, which must be coordinated to protect the integrity of the genome and epigenome. The dysregulation of chromatin and loss of genome integrity that is prevalent in human diseases including cancer may be related and could provide opportunities to target multitasking proteins with these pathways as therapeutic strategies.

Pleiotropic role of PARP1: an overview

Poly (ADP-ribose) polymerase 1 (PARP1) protein is encoded by the PARP1 gene located on chromosome 1 (1q42.12) in human cells. It plays a crucial role in post-translational modification by adding poly (ADP-ribose) (PAR) groups to various proteins and PARP1 itself by utilizing nicotinamide adenine dinucleotide (NAD +) as a substrate. Since the discovery of PARP1, its role in DNA repair and cell death has been its identity. This is evident from an overwhelmingly high number of scientific reports in this regard. However, PARP1 also plays critical roles in inflammation, metabolism, tumor development and progression, chromatin modification and transcription, mRNA stability, and alternative splicing. In the present study, we attempted to compile all the scattered scientific information about this molecule, including the structure and multifunctional role of PARP1 in cancer and non-cancer diseases, along with PARP1 inhibitors (PARPis). Furthermore, for the first time, we have classified PARP1-mediated cell death for ease of understanding its role in cell death pathways.

A DNA damage repair gene-associated signature predicts responses of patients with advanced soft-tissue sarcoma to treatment with trabectedin

Predictive biomarkers of trabectedin represent an unmet need in advanced soft‐tissue sarcomas (STS). DNA damage repair (DDR) genes, involved in homologous recombination or nucleotide excision repair, had been previously described as biomarkers of trabectedin resistance or sensitivity, respectively. The majority of these studies only focused on specific factors (ERCC1, ERCC5, and BRCA1) and did not evaluate several other DDR‐related genes that could have a relevant role for trabectedin efficacy. In this retrospective translational study, 118 genes involved in DDR were evaluated to determine, by transcriptomics, a predictive gene signature of trabectedin efficacy. A six‐gene predictive signature of trabectedin efficacy was built in a series of 139 tumor samples from patients with advanced STS. Patients in the high‐risk gene signature group showed a significantly worse progression‐free survival compared with patients in the low‐risk group (2.1 vs 6.0 months, respectively). Differential gene expression analysis defined new potential predictive biomarkers of trabectedin sensitivity (PARP3 and CCNH) or resistance (DNAJB11 and PARP1). Our study identified a new gene signature that significantly predicts patients with higher probability to respond to treatment with trabectedin. Targeting some genes of this signature emerges as a potential strategy to enhance trabectedin efficacy.

A DNA Repair and Cell Cycle Gene Expression Signature in Pediatric High-Grade Gliomas: Prognostic and Therapeutic Value

BACKGROUND

Pediatric high-grade gliomas (pHGGs) are the leading cause of mortality in pediatric neuro-oncology, displaying frequent resistance to standard therapies. Profiling DNA repair and cell cycle gene expression has recently been proposed as a strategy to classify adult glioblastomas. To improve our understanding of the DNA damage response pathways that operate in pHGGs and the vulnerabilities that these pathways might expose, we sought to identify and characterize a specific DNA repair and cell-cycle gene expression signature of pHGGs.

METHODS

Transcriptomic analyses were performed to identify a DNA repair and cell-cycle gene expression signature able to discriminate pHGGs (n = 6) from low-grade gliomas (n = 10). This signature was compared to related signatures already established. We used the pHGG signature to explore already transcriptomic datasets of DIPGs and sus-tentorial pHGGs. Finally, we examined the expression of key proteins of the pHGG signature in 21 pHGG diagnostic samples and nine paired relapses. Functional inhibition of one DNA repair factor was carried out in four patients who derived H3.3 K27M mutant cell lines.

RESULTS

We identified a 28-gene expression signature of DNA repair and cell cycle that clustered pHGGs cohorts, in particular sus-tentorial locations, in two groups. Differential protein expression levels of PARP1 and XRCC1 were associated to TP53 mutations and TOP2A amplification and linked significantly to the more radioresistant pHGGs displaying the worst outcome. Using patient-derived cell lines, we showed that the PARP-1/XRCC1 expression balance might be correlated with resistance to PARP1 inhibition.

CONCLUSION

We provide evidence that PARP1 overexpression, associated to XRCC1 expression, TP53 mutations, and TOP2A amplification, is a new theranostic and potential therapeutic target.

Role of chromatin assembly factor-1/p60 and poly [ADP-ribose] polymerase 1 in mycosis fungoides

Mycosis fungoides (MF) represents the most common type of cutaneous lymphoma. In the majority of patients, the disease has a slow evolution and a protracted course; however, a subset of patients shows poor oncologic outcomes. Unfortunately, there are no reliable prognostic markers for MF, and the currently available treatments are only effective in a minority of patients. This study aimed to evaluate the expression and clinical significance of PARP-1 and CAF-1/p60 in MF. Sixty-four MF representatives of the different stages of disease were assessed by immunohistochemistry for PARP-1 and CAF-1/p60. The association of PARP-1 and CAF-1/p60 with the MF stage and outcome was assessed by using Fisher’s exact test and Kaplan-Meier survival analysis with the Log-rank test; a p value < 0.05 was considered significant. PARP-1 was overexpressed in 57.9% of MF and was significantly associated with a MF stage > II (p = 0.034) but not with the risk of death (p = 0.237). CAF-1/p60 was overexpressed in 26.8% of MF and was significantly associated with decreased overall survival (p < 0.001) but not with the MF stage (p = 1). A significant association was found between PARP-1 overexpression and CAF-1/p60 overexpression (p = 0.0025). Simultaneous overexpression of PARP-1 and CAF-1/p60 was significantly associated with decreased overall survival (p < 0.001), although less strongly than CAF-1/p60 alone (χ² = 14.916 vs 21.729, respectively). In MF, PARP-1 is overexpressed in advanced stages, while CAF-1/p60 is overexpressed in the cases with shorter overall survival, appearing as a significant prognostic marker. A role for PARP-1 inhibitors and anti-CAF-1/p60 targeted therapy may be reasonably hypothesized in MF.

PARP1 Co-Regulates EP300-BRG1-Dependent Transcription of Genes Involved in Breast Cancer Cell Proliferation and DNA Repair

BRG1, an active subunit of the SWI/SNF chromatin-remodeling complex, enables the EP300-dependent transcription of proliferation and DNA repair genes from their E2F/CpG-driven promoters in breast cancer cells. In the current study, we show that BRG1-EP300 complexes are accompanied by poly-ADP-ribose polymerase 1 (PARP1), which emerges as the functional component of the promoter-bound multiprotein units that are capable of controlling gene expression. This enzyme is co-distributed with BRG1 at highly acetylated promoters of genes such as CDK4, LIG1, or NEIL3, which are responsible for cancer cell growth and the removal of DNA damage. ADP-ribosylation is necessary to maintain active transcription, since it ensures an open chromatin structure that allows high acetylation and low histone density. PARP1-mediated modification of BRG1 and EP300 does not affect the association of enzymes with gene promoters; however, it does activate EP300, which acetylates nucleosomes, leading to their eviction by BRG1, thus allowing mRNA synthesis. Although PARP1 was found at BRG1 positive/H3K27ac negative promoters of highly expressed genes in a transformed breast cancer cell line, its transcriptional activity was limited to genes simultaneously controlled by BRG1 and EP300, indicating that the ADP-ribosylation of EP300 plays a dominant role in the regulation of BRG1-EP300-driven transcription. In conclusion, PARP1 directs the transcription of some proliferation and DNA repair genes in breast cancer cells by the ADP-ribosylation of EP300, thereby causing its activation and marking nucleosomes for displacement by BRG1. PARP1 in rapidly dividing cells facilitates the expression of genes that confer a cancer cell phenotype. Our study shows a new mechanism that links PARP1 with the removal of DNA damage in breast cancer cells via the regulation of BRG1-EP300-dependent transcription of genes involved in DNA repair pathways.

Role of mitochondria in rescuing glycolytically inhibited subpopulation of triple negative but not hormone-responsive breast cancer cells

Triple-negative breast cancer (TNBC) subtype is among the most aggressive cancers with the worst prognosis and least therapeutic targetability while being more likely to spread and recur. Cancer transformations profoundly alter cellular metabolism by increasing glucose consumption via glycolysis to support tumorigenesis. Here we confirm that relative to ER-positive cells (MCF7), TNBC cells (MBA-MD-231) rely more on glycolysis thus providing a rationale to target these cells with glycolytic inhibitors. Indeed, iodoacetate (IA), an effective GAPDH inhibitor, caused about 70% drop in MDA-MB-231 cell viability at 20 μM while 40 μM IA was needed to decrease MCF7 cell viability only by 30% within 4 hours of treatment. However, the triple negative cells showed strong ability to recover after 24 h whereas MCF7 cells were completely eliminated at concentrations <10 μM. To understand the mechanism of MDA-MB-231 cell survival, we studied metabolic modulations associated with acute and extended treatment with IA. The resilient TNBC cell population showed a significantly greater count of cells with active mitochondria, lower apoptotic markers, normal cell cycle regulations, moderately lowered ROS, but increased mRNA levels of p27 and PARP1; all compatible with enhanced cell survival. Our results highlight an interplay between PARP and mitochondrial oxidative phosphorylation in TNBC that comes into play in response to glycolytic disruption. In the light of these findings, we suggest that combined treatment with PARP and mitochondrial inhibitors may provide novel therapeutic strategy against TNBC.

Inhibition of PARP1 Increases IRF-dependent Gene Transcription in Jurkat Cells

Poly(ADP-ribose) polymerase 1 (PARP1) plays important roles in the regulation of transcription factors. Mounting evidence has shown that inhibition of PARP1 influences the expression of genes associated with inflammatory response. Interferon regulatory factor 1 (IRF1) is a critical transcription factor for the development of both the innate and adaptive immune responses against infections. However, the molecular mechanism through which PARP1 mediates the effects has not been clearly demonstrated. Jurkat cells were exposed to dexamethasone (Dex) or PARP1 inhibitor PJ34. The expression levels of IL-12, LMP2, OAS1 and PKR were detected using real-time RT-PCR. The interactions between PARP1 and IRF1 were examined by co-immunoprecipitation (co-IP) assays. We further explored the mechanism of PARP1 suppressing IRF1 by assessing the activities of interferon stimulated response element (ISRE). The mRNA expression of IL-12, LMP2, OAS1 and PKR was obviously suppressed by Dex in Jurkat cells, which could be rescued by PJ34 treatment. Luciferase study revealed that poly(ADP-ribosyl)- ation suppressed IRF1-mediated transcription through preventing the binding of IRF1 to ISREs. PARP1 inhibited IRF1-mediated transcription in Jurkat cells by preventing IRF1 binding to ISREs in the promoters of target genes. It is suggested that PARP1 is a crucial regulator of IRF1-mediated immune response. This study provides experimental evidence for the possible application of PARP1 inhibitors in the treatment of IRF1-related immune anergy.

Computational analysis of data from a genome-wide screening identifies new PARP1 functional interactors as potential therapeutic targets

Knowledge of interaction network between different proteins can be a useful tool in cancer therapy. To develop new therapeutic treatments, understanding how these proteins contribute to dysregulated cellular pathways is an important task. PARP1 inhibitors are drugs used in cancer therapy, in particular where DNA repair is defective. It is crucial to find new candidate interactors of PARP1 as new therapeutic targets in order to increase efficacy of PARP1 inhibitors and expand their clinical utility. By a yeast-based genome wide screening, we previously discovered 90 candidate deletion genes that suppress growth-inhibition phenotype conferred by PARP1 in yeast. Here, we performed an integrated and computational analysis to deeply study these genes. First, we identified which pathways these genes are involved in and putative relations with PARP1 through g:Profiler. Then, we studied mutation pattern and their relation to cancer by interrogating COSMIC and DisGeNET database; finally, we evaluated expression and alteration in several cancers with cBioPortal, and the interaction network with GeneMANIA. We identified 12 genes belonging to PARP1-related pathways. We decided to further validate RIT1, INCENP and PSTA1 in MCF7 breast cancer cells. We found that RIT1 and INCENP affected PARylation and PARP1 protein level more significantly in PARP1 inhibited cells. Furthermore, downregulation of RIT1, INCENP and PSAT1 affected olaparib sensitivity of MCF7 cells. Our study identified candidate genes that could have an effect on PARP inhibition therapy. Moreover, we also confirm that yeast-based screenings could be very helpful to identify novel potential therapy factors.

NAD+Metabolism in Aging and Cancer

Aging is a major risk factor for many types of cancer, and the molecular mechanisms implicated in aging, progeria syndromes, and cancer pathogenesis display considerable similarities. Maintaining redox homeostasis, efficient signal transduction, and mitochondrial metabolism is essential for genome integrity and for preventing progression to cellular senescence or tumorigenesis. NAD+is a central signaling molecule involved in these and other cellular processes implicated in age-related diseases and cancer. Growing evidence implicates NAD+decline as a major feature of accelerated aging progeria syndromes and normal aging. Administration of NAD+precursors such as nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) offer promising therapeutic strategies to improve health, progeria comorbidities, and cancer therapies. This review summarizes insights from the study of aging and progeria syndromes and discusses the implications and therapeutic potential of the underlying molecular mechanisms involved in aging and how they may contribute to tumorigenesis.

Targeting PARP1 in XRCC1-Deficient Sporadic Invasive Breast Cancer or Preinvasive Ductal Carcinoma In Situ Induces Synthetic Lethality and Chemoprevention

: Targeting PARP1 for synthetic lethality is a new strategy for breast cancers harboring germline mutations in BRCA. However, these mutations are rare, and reactivation of BRCA-mediated pathways may result in eventual resistance to PARP1 inhibitor therapy. Alternative synthetic lethality approaches targeting more common sporadic breast cancers and preinvasive ductal carcinoma in situ (DCIS) are desirable. Here we show that downregulation of XRCC1, which interacts with PARP1 and coordinates base excision repair, is an early event in human breast cancer pathogenesis. XRCC1-deficient DCIS were aggressive and associated with increased risk of local recurrence. Human invasive breast cancers deficient in XRCC1 and expressing high PARP1 levels also manifested aggressive features and poor outcome. The PARP1 inhibitor olaparib was synthetically lethal in XRCC1-deficient DCIS and invasive breast cancer cells. We conclude that targeting PARP1 is an attractive strategy for synthetic lethality and chemoprevention in XRCC1-deficient breast cancers, including preinvasive DCIS. SIGNIFICANCE: These findings show that loss of XRCC1, which is associated with more malignant DCIS, can be exploited by PARP inhibition, suggesting its application as a promising therapeutic and chemoprevention strategy in XRCC1-deficient tumor cells.

PARP1 activation increases expression of modified tumor suppressors and pathways underlying development of aggressive hepatoblastoma

Hepatoblastoma (HBL) is a pediatric liver cancer that affects children under the age of three. Reduction of tumor suppressor proteins (TSPs) is commonly seen in liver cancer. However, in our studies we find that aggressive, chemo-resistant HBLs exhibit an elevation of TSPs. HBL patients with a classic phenotype have reduced TSP levels, but patients with aggressive HBL express elevated TSPs that undergo posttranslational modifications, eliminating their tumor suppression activities. Here we identify unique aggressive liver cancer domains (ALCDs) that are activated in aggressive HBL by PARP1-mediated chromatin remodeling leading to elevation of modified TSPs and activation of additional cancer pathways: WNT signaling and β-catenin. Inhibition of PARP1 blocks activation of ALCDs and normalizes expression of corresponding genes, therefore reducing cell proliferation. Our studies reveal PARP1 activation as a mechanism for the development of aggressive HBL, further suggesting FDA-approved PARP1 inhibitors might be used for treatment of patients with aggressive HBL.

Leila Valanejad et al. report increased expression of modified tumor suppressor proteins (TSPs) with loss of tumor suppressor activity in aggressive, chemotherapy-resistant hepatoblastoma. They find that TSP upregulation occurs via PARP1-mediated chromatin remodeling, leading to activation of multiple cancer-associated pathways.

YM155, a survivin suppressant, triggers PARP-dependent cell death (parthanatos) and inhibits esophageal squamous-cell carcinoma xenografts in mice

Here we demonstrated that sepantronium bromide (YM155), a survivin suppressant, inhibited esophageal squamous-cell carcinoma (ESCC) growth in mice bearing human ESCC xenografts without affecting body weight. In cell culture, YM155 decreased survivin levels and caused PARP-1 activation, poly-ADP polymer formation, and AIF translocation from the cytosol to the nucleus. Genetic knockdown of PARP-1 or AIF abrogated YM155-induced parthanatos cell death. Furthermore, FOS, JUN and c-MYC gene transcription, which is stimulated by activated PARP-1, was increased following YM155 treatment. Our data demonstrate that YM155 did not trigger apoptosis, but induced parthanatos, a cell death dependent on PARP-1 hyper-activation, and support clinical development of YM155 in ESCC.

Poly (ADP-ribose) polymerase1 regulates growth and multicellularity in D. discoideum

Poly (ADP-ribose) polymerase (PARP)-1 regulates various biological processes like DNA repair, cell death etc. However, the role of PARP-1 in growth and differentiation still remains elusive. The present study has been undertaken to understand the role of PARP-1 in growth and development of a unicellular eukaryote, Dictyostelium discoideum. In silico analysis demonstrates ADPRT1A as the ortholog of human PARP-1 in D. discoideum. The present study shows that ADPRT1A overexpression (A OE) led to slow growth of D. discoideum and significant population of AOE cells were in S and G2/M phase. Also, AOE cells exhibited high endogenous PARP activity, significant NAD(+) depletion and also significantly lower ADPRT1B and ADPRT2 transcript levels. Moreover, AOE cells are intrinsically stressed and also exhibited susceptibility to oxidative stress. AOE also affected development of D. discoideum predominantly streaming, aggregation and formation of early culminant which are concomitant with reports on PARP's role in D. discoideum development. In addition, under developmental stimuli, increased PARP activity was seen along with developmentally regulated transcript levels of ADPRT1A during D. discoideum multicellularity. Thus the present study suggests that PARP-1 regulates growth as well as the developmental morphogenesis of D. discoideum, thereby opening new avenues to understand the same in higher eukaryotes.

PARP-1 inhibitor sensitizes arsenic trioxide in hepatocellular carcinoma cells via abrogation of G2/M checkpoint and suppression of DNA damage repair

Arsenic trioxide (ATO) is successfully used to treat hematological malignancies. However, the clinical application of the agent in solid tumors is largely limited by its dose-dependent toxicity which results from the high intrinsic resistance of the cancer cells. In this study, we firstly identified a series of sensitization effects of 4AN, a PARP-1 inhibitor, on human hepatocellular carcinoma cell line HepG2 to ATO treatment. We showed that treatment of HepG2 cells with 4AN promoted ATO-induced cell death in a synergistic manner. The ATO-sensitization by 4AN was associated with its effect on abrogation of ATO-induced G2/M checkpoint which impairs DNA damage repair and promotes cell apoptosis. Further analysis demonstrated that the ATO-induced G2/M checkpoint was closely related to a decrease in cyclin B1, a key G2/M mediator; whereas 4AN up-regulated the expression of cyclin B1 in ATO-treated cells, which may be at least partly responsible for its effect on abrogation of ATO-induced G2/M checkpoint. This was further supported by the result showing that down-regulation of cyclin B1 using siRNA could restore the G2/M checkpoint in cells co-treated with ATO and 4AN, thereby improving DNA damage repair and decreasing apoptosis. Our study indicates that the abrogation of G2/M checkpoint and the suppression of DNA damage repair contribute to ATO-sensitization by PARP-1 inhibitor in HepG2 cells, which provides a novel insight into the chemo-sensitization mechanism of PARP-1 inhibitor.

Modulation of urokinase plasminogen activator system by poly(ADP-ribose)polymerase-1 inhibition

The urokinase plasminogen activator (uPA) system is a complex regulator of extracellular proteolysis which is involved in various physiological and pathological processes. The major components of this system are the serine protease uPA, two inhibitors PAI-1 and PAI-2, and the receptor uPAR. It has been previously shown by several groups that the uPA system has an important role in cancer progression and therefore its possible prognostic and therapeutic value has been evaluated. The aim of this study is to tackle the role of poly(ADP-ribosyl)ation in the induction of uPA activity in a glioblastoma cell line, A1235. This cell line is sensitive to alkylation damage and is a model for drug treatment. The components of the uPA system and the level of DNA damage were analyzed after alkylation agent treatment in combination with poly(ADP-ribose)polymerase-1 (PARP-1) inhibition. Here we show that the increase in uPA activity results from the net balance change between uPA and its inhibitor at mRNA level. Further, PARP-1 inhibition exerts its influence on uPA activity through DNA damage increase. Involvement of several signaling pathways, as well as cell specific regulation influencing the uPA system are discussed.

Retinoic acid regulates cell cycle genes and accelerates normal mouse liver regeneration

All-trans retinoic acid (RA) is a potent inducer of regeneration. Because the liver is the principal site for storage and bioactivation of vitamin A, the current study examines the effect of RA in mouse hepatocyte proliferation and liver regeneration. Mice that received a single dose of RA (25μg/g) by oral gavage developed hepatomegaly with increased number of Ki67-positive cells and induced expression of cell cycle genes in the liver. DNA binding data revealed that RA receptors retinoic acid receptor β (RARβ) and retinoid x receptor α (RXRα) bound to cell cycle genes Cdk1, Cdk2, Cyclin B, Cyclin E, and Cdc25a in mice with and without RA treatment. In addition, RA treatment induced novel binding of RARβ/RXRα to Cdk1, Cdk2, Cyclin D, and Cdk6 genes. All RARβ/RXRα binding sites contained AGGTCA-like motifs. RA treatment also promoted liver regeneration after partial hepatectomy (PH). RA signaling was implicated in normal liver regeneration as the mRNA levels of RARβ, Aldh1a2, Crabp1, and Crbp1 were all induced 1.5 days after PH during the active phase of hepatocyte proliferation. RA treatment prior to PH resulted in early up-regulation of RARβ, Aldh1a2, Crabp1, and Crbp1, which was accompanied by an early induction of cell cycle genes. Western blotting for RARβ, c-myc, Cyclin D, E, and A further supported the early induction of retinoid signal and cell proliferation by RA treatment. Taken together, our data suggest that RA may regulate cell cycle progression and accelerates liver regeneration. Such effect is associated with an early induction of RA signaling, which includes increased expression of the receptor, binding proteins, and processing enzyme for retinoids.

Poly(ADP-ribosyl)ation is required to modulate chromatin changes at c-MYC promoter during emergence from quiescence

Poly(ADP-ribosyl)ation is a post-translational modification of various proteins and participates in the regulation of chromatin structure and transcription through complex mechanisms not completely understood. We have previously shown that PARP-1, the major family member of poly(ADP-ribose)polymerases, plays an important role in the cell cycle reactivation of resting cells by regulating the expression of Immediate Early Response Genes, such as c-MYC, c-FOS, JUNB and EGR-1. In the present work we have investigated the molecular mechanisms by which the enzyme induces c-MYC transcription upon serum stimulation of quiescent cells. We show that PARP-1 is constitutively associated in vivo to a c-MYC promoter region recognized as biologically relevant for the transcriptional regulation of the gene. Moreover, we report that serum stimulation causes the prompt accumulation of ADP-ribose polymers on the same region and that this modification is required for chromatin decondensation and for the exchange of negative for positive transcriptional regulators. Finally we provide evidence that the inhibition of PARP activity along with serum stimulation impairs c-MYC induction by preventing the proper accumulation of histone H3 phosphoacetylation, a specific chromatin mark for the activation of Immediate Early Response Genes. These findings not only suggest a novel strategy by which PARP-1 regulates the transcriptional activity of promoters but also provide new information about the complex regulation of c-MYC expression, a critical determinant of the transition from quiescence to proliferation.