Inhibition of nuclear receptor signalling by poly(ADP-ribose) polymerase

Vitamin D3 Exerts Beneficial Effects on C2C12 Myotubes through Activation of the Vitamin D Receptor (VDR)/Sirtuins (SIRT)1/3 Axis

The onset of sarcopenia is associated with a decline in vitamin D receptor (VDR) expression, wherein reduced VDR levels contribute to muscle atrophy, while heightened expression promotes muscle hypertrophy. Like VDR, the age-related decline in protein deacetylase sirtuin (SIRT) expression is linked to the development of sarcopenia and age-related muscle dysfunction. This study aimed to investigate whether the VDR agonist 1,25-dihydroxyvitamin D3 (1,25VD3) exerts beneficial effects on muscles through interactions with sirtuins and, if so, the underlying molecular mechanisms. Treatment of 1,25VD3 in differentiating C2C12 myotubes substantially elevated VDR, SIRT1, and SIRT3 expression, enhancing their differentiation. Furthermore, 1,25VD3 significantly enhanced the expression of key myogenic markers, including myosin heavy chain (MyHC) proteins, MyoD, and MyoG, and increased the phosphorylation of AMPK and AKT. Conversely, VDR knockdown resulted in myotube atrophy and reduced SIRT1 and SIRT3 levels. In a muscle-wasting model triggered by IFN-γ/TNF-α in C2C12 myotubes, diminished VDR, SIRT1, and SIRT3 levels led to skeletal muscle atrophy and apoptosis. 1,25VD3 downregulated the increased expression of muscle atrophy-associated proteins, including FoxO3a, MAFbx, and MuRF1 in an IFN-γ/TNF-α induced atrophy model. Importantly, IFN-γ/TNF-α significantly reduced the mtDNA copy number in the C2C12 myotube, whereas the presence of 1,25VD3 effectively prevented this decrease. These results support that 1,25VD3 could serve as a potential preventive or therapeutic agent against age-related muscle atrophy by enhancing the VDR/SIRT1/SIRT3 axis.

PARPs in lipid metabolism and related diseases

PARPs and tankyrases (TNKS) represent a family of 17 proteins. PARPs and tankyrases were originally identified as DNA repair factors, nevertheless, recent advances have shed light on their role in lipid metabolism. To date, PARP1, PARP2, PARP3, tankyrases, PARP9, PARP10, PARP14 were reported to have multi-pronged connections to lipid metabolism. The activity of PARP enzymes is fine-tuned by a set of cholesterol-based compounds as oxidized cholesterol derivatives, steroid hormones or bile acids. In turn, PARPs modulate several key processes of lipid homeostasis (lipotoxicity, fatty acid and steroid biosynthesis, lipoprotein homeostasis, fatty acid oxidation, etc.). PARPs are also cofactors of lipid-responsive nuclear receptors and transcription factors through which PARPs regulate lipid metabolism and lipid homeostasis. PARP activation often represents a disruptive signal to (lipid) metabolism, and PARP-dependent changes to lipid metabolism have pathophysiological role in the development of hyperlipidemia, obesity, alcoholic and non-alcoholic fatty liver disease, type II diabetes and its complications, atherosclerosis, cardiovascular aging and skin pathologies, just to name a few. In this synopsis we will review the evidence supporting the beneficial effects of pharmacological PARP inhibitors in these diseases/pathologies and propose repurposing PARP inhibitors already available for the treatment of various malignancies.

The role of ADP-ribose metabolism in metabolic regulation, adipose tissue differentiation, and metabolism

In this review, Szanto et al. summarize the metabolic regulatory roles of PARP enzymes and their associated pathologies.

Poly(ADP-ribose) polymerases (PARPs or ARTDs), originally described as DNA repair factors, have metabolic regulatory roles. PARP1, PARP2, PARP7, PARP10, and PARP14 regulate central and peripheral carbohydrate and lipid metabolism and often channel pathological disruptive metabolic signals. PARP1 and PARP2 are crucial for adipocyte differentiation, including the commitment toward white, brown, or beige adipose tissue lineages, as well as the regulation of lipid accumulation. Through regulating adipocyte function and organismal energy balance, PARPs play a role in obesity and the consequences of obesity. These findings can be translated into humans, as evidenced by studies on identical twins and SNPs affecting PARP activity.

Site-specific analysis of the Asp- and Glu-ADP-ribosylated proteome by quantitative mass spectrometry

ADP-ribosylation is a protein post-translational modification that is critically involved in a wide array of biological processes connected to cell stress responses. Enzymes known as poly-ADP-ribose polymerases (PARPs) catalyze the addition of the ADP-ribose units to amino acids with various side chain chemistries. In particular, the PARP family member PARP1 is responsible for the modification of a large number of proteins and is involved in initiation of the DNA damage response, although the mechanisms through which PARP1 functions are still incompletely understood. The analysis of protein ADP-ribosylation is challenging because PARylation is a low-abundance, labile and heterogeneous protein modification. Recently, we developed an integrative proteomic platform for the site-specific analysis of protein ADP-ribosylation on Asp and Glu residues. Herein, we describe the method, and demonstrate its utility in quantitative characterization of the human Asp- and Glu-ADP-ribosylated proteome.

A Cell-Line-Specific Atlas of PARP-Mediated Protein Asp/Glu-ADP-Ribosylation in Breast Cancer

PARP1 plays a critical role in regulating many biological processes linked to cellular stress responses. Although DNA strand breaks are potent stimuli of PARP1 enzymatic activity, the context-dependent mechanism regulating PARP1 activation and signaling is poorly understood. We performed global characterization of the PARP1-dependent, Asp/Glu-ADP-ribosylated proteome in a panel of cell lines originating from benign breast epithelial cells, as well as common subtypes of breast cancer. From these analyses, we identified 503 specific ADP-ribosylation sites on 322 proteins. Despite similar expression levels, PARP1 is differentially activated in these cell lines under genotoxic conditions, which generates signaling outputs with substantial heterogeneity. By comparing protein abundances and ADP-ribosylation levels, we could dissect cell-specific PARP1 targets that are driven by unique expression patterns versus cell-specific regulatory mechanisms of PARylation. Intriguingly, PARP1 modifies many proteins in a cell-specific manner, including those involved in transcriptional regulation, mRNA metabolism, and protein translation.

Metabolic roles of poly(ADP-ribose) polymerases

Poly(ADP-ribosyl)ation (PARylation) is an evolutionarily conserved reaction that had been associated with numerous cellular processes such as DNA repair, protein turnover, inflammatory regulation, aging or metabolic regulation. The metabolic regulatory tasks of poly(ADP-ribose) polymerases (PARPs) are complex, it is based on the regulation of metabolic transcription factors (e.g. SIRT1, nuclear receptors, SREBPs) and certain cellular energy sensors. PARP over-activation can cause damage to mitochondrial terminal oxidation, while the inhibition of PARP-1 or PARP-2 can induce mitochondrial oxidation by enhancing the mitotropic tone of gene transcription and signal transduction. These PARP-mediated processes impact on higher order metabolic regulation that modulates lipid metabolism, circadian oscillations and insulin secretion and signaling. PARP-1, PARP-2 and PARP-7 are related to metabolic diseases such as diabetes, alcoholic and non-alcoholic fatty liver disease (AFLD, NAFLD), or on a broader perspective to Warburg metabolism in cancer or the metabolic diseases accompanying aging.

PARP, transcription and chromatin modeling

Compaction mode of chromatin and chromatin highly organised structures regulate gene expression. Posttranslational modifications, histone variants and chromatin remodelers modulate the compaction, structure and therefore function of specific regions of chromatin. The generation of poly(ADP-ribose) (PAR) is emerging as one of the key signalling events on sites undergoing chromatin structure modulation. PAR is generated locally in response to stresses. These include genotoxic stress but also differentiation signals, metabolic and hormonal cues. A pictures emerges in which transient PAR formation is essential to orchestrate chromatin remodelling and transcription factors allowing the cell to adapt to alteration in its environment. This review summarizes the diverse factors of ADP-ribosylation in the adaptive regulation of chromatin structure and transcription.

Prenylation differentially inhibits insulin-dependent immediate early gene mRNA expression

Increased activity of prenyl transferases is observed in pathological states of insulin resistance, diabetes, and obesity. Thus, functional inhibitors of farnesyl transferase (FTase) and geranylgeranyl transferase (GGTase) may be promising therapeutic treatments. We previously identified insulin responsive genes from a rat H4IIE hepatoma cell cDNA library, including β-actin, EGR1, Pip92, c-fos, and Hsp60. In the present study, we investigated whether acute treatment with FTase and GGTase inhibitors would alter insulin responsive gene initiation and/or elongation rates. We observed differential regulation of insulin responsive gene expression, suggesting a differential sensitivity of these genes to one or both of the specific protein prenylation inhibitors.

Poly(ADP-ribose) Polymerase 1 Represses Liver X Receptor-mediated ABCA1 Expression and Cholesterol Efflux in Macrophages

Liver X receptors (LXR) are oxysterol-activated nuclear receptors that play a central role in reverse cholesterol transport through up-regulation of ATP-binding cassette transporters (ABCA1 and ABCG1) that mediate cellular cholesterol efflux. Mouse models of atherosclerosis exhibit reduced atherosclerosis and enhanced regression of established plaques upon LXR activation. However, the coregulatory factors that affect LXR-dependent gene activation in macrophages remain to be elucidated. To identify novel regulators of LXR that modulate its activity, we used affinity purification and mass spectrometry to analyze nuclear LXRα complexes and identified poly(ADP-ribose) polymerase-1 (PARP-1) as an LXR-associated factor. In fact, PARP-1 interacted with both LXRα and LXRβ. Both depletion of PARP-1 and inhibition of PARP-1 activity augmented LXR ligand-induced ABCA1 expression in the RAW 264.7 macrophage line and primary bone marrow-derived macrophages but did not affect LXR-dependent expression of other target genes, ABCG1 and SREBP-1c. Chromatin immunoprecipitation experiments confirmed PARP-1 recruitment at the LXR response element in the promoter of the ABCA1 gene. Further, we demonstrated that LXR is poly(ADP-ribosyl)ated by PARP-1, a potential mechanism by which PARP-1 influences LXR function. Importantly, the PARP inhibitor 3-aminobenzamide enhanced macrophage ABCA1-mediated cholesterol efflux to the lipid-poor apolipoprotein AI. These findings shed light on the important role of PARP-1 on LXR-regulated lipid homeostasis. Understanding the interplay between PARP-1 and LXR may provide insights into developing novel therapeutics for treating atherosclerosis.

Role of PARP-1 in prostate cancer

Poly (ADP-ribose) polymerase-1 (PARP-1) is an enzyme that catalyzes the covalent attachment of polymers of ADP-ribose (PAR) moieties on itself and its target proteins. PARP1 activity is frequently deregulated in various cancers and therefore it has emerged as a new drug target for cancer therapy. The role of PARP-1 in DNA repair has been well documented and BRCA mutations are implicated for determining the sensitivity to PARP inhibitors. Recent studies also point to a role of PARP-1 in transcription regulation which may contribute to oncogenic signaling and cancer progression. Given that efficacy of PARP inhibitors are also seen in patients not harboring BRCA mutations, some other mechanisms might also be involved. In the present review, we highlight the mechanisms by which PARP-1 regulates gene expression in prostate cancer and provide an overview of the ongoing clinical trials using PARP inhibitors in various cancers including prostate cancer.

Review of poly (ADP-ribose) polymerase (PARP) mechanisms of action and rationale for targeting in cancer and other diseases

Poly (ADP-ribose) polymerases (PARPs) are a family of related enzymes that share the ability to catalyze the transfer of ADP-ribose to target proteins. PARPs play an important role in various cellular processes, including modulation of chromatin structure, transcription, replication, recombination, and DNA repair. The role of PARP proteins in DNA repair is of particular interest, in view of the finding that certain tumors defective in homologous recombination mechanisms, may rely on PARP-mediated DNA repair for survival, and are sensitive to its inhibition. PARP inhibitors may also increase tumor sensitivity to DNA-damaging agents. Clinical trials of PARP inhibitors are investigating the utility of these approaches in cancer. The hyperactivation of PARP has also been shown to result in a specific programmed cell death pathway involving NAD+/ATP depletion, mu-calpain activation, loss of mitochondrial membrane potential, and the release of apoptosis inducing factor. Hyperactivation of the PARP pathway may be exploited to selectively kill cancer cells. Other PARP forms, including tankyrase 1 (PARP 5a), which plays an important role in enhancing telomere elongation by telomerase, have been found to be potential targets in cancer therapy. The PARP pathway and its inhibition thus offers a number of opportunities for therapeutic intervention in both cancer and other disease states.

Poly(ADP-ribose) polymerase-1 modulates Nrf2-dependent transcription

The basic leucine zipper transcription factor Nrf2 has emerged as a master regulator of intracellular redox homeostasis by controlling the expression of a battery of redox-balancing antioxidants and phase II detoxification enzymes. Under oxidative stress conditions, Nrf2 is induced at the protein level through redox-sensitive modifications on critical cysteine residues in Keap1, a component of an E3 ubiquitin ligase complex that targets Nrf2 for proteasomal degradation. Poly(ADP-ribose) polymerase-1 (PARP-1) is historically known to function in DNA damage detection and repair; however, recently PARP-1 has been shown to play an important role in other biochemical activities, such as DNA methylation and imprinting, insulator activity, chromosome organization, and transcriptional regulation. The exact role of PARP-1 in transcription modulation and the underlying mechanisms remain poorly defined. In this study, we report that PARP-1 forms complexes with the antioxidant response element (ARE) within the promoter region of Nrf2 target genes and upregulates the transcriptional activity of Nrf2. Interestingly, PARP-1 neither physically interacts with Nrf2 nor promotes the expression of Nrf2. In addition, PARP-1 does not target Nrf2 for poly(ADP-ribosyl)ation. Instead, PARP-1 interacts directly with small Maf proteins and the ARE of Nrf2 target genes, which augments ARE-specific DNA-binding of Nrf2 and enhances the transcription of Nrf2 target genes. Collectively, these results suggest that PARP-1 serves as a transcriptional coactivator, upregulating the transcriptional activity of Nrf2 by enhancing the interaction among Nrf2, MafG, and the ARE.

Essential role of poly(ADP-ribosyl)ation in cocaine action

Many of the long-term effects of cocaine on the brain's reward circuitry have been shown to be mediated by alterations in gene expression. Several chromatin modifications, including histone acetylation and methylation, have been implicated in this regulation, but the effect of other histone modifications remains poorly understood. Poly(ADP-ribose) polymerase-1 (PARP-1), a ubiquitous and abundant nuclear protein, catalyzes the synthesis of a negatively charged polymer called poly(ADP-ribose) or PAR on histones and other substrate proteins and forms transcriptional regulatory complexes with several other chromatin proteins. Here, we identify an essential role for PARP-1 in cocaine-induced molecular, neural, and behavioral plasticity. Repeated cocaine administration, including self-administration, increased global levels of PARP-1 and its mark PAR in mouse nucleus accumbens (NAc), a key brain reward region. Using PARP-1 inhibitors and viral-mediated gene transfer, we established that PARP-1 induction in NAc mediates enhanced behavioral responses to cocaine, including increased self-administration of the drug. Using chromatin immunoprecipitation sequencing, we demonstrated a global, genome-wide enrichment of PARP-1 in NAc of cocaine-exposed mice and identified several PARP-1 target genes that could contribute to the lasting effects of cocaine. Specifically, we identified sidekick-1--important for synaptic connections during development--as a critical PARP-1 target gene involved in cocaine's behavioral effects as well as in its ability to induce dendritic spines on NAc neurons. These findings establish the involvement of PARP-1 and PARylation in the long-term actions of cocaine.

Poly(ADP-ribose) polymerase 1 is a key regulator of estrogen receptor α-dependent gene transcription

Activation of nuclear receptor estrogen receptor α (ERα) exerts cardiovascular protective effects by modulating the expression of ERα target genes. However, the underlying mechanism remains unclear. PARP1 is a ubiquitous multifunctional nuclear enzyme. In this study, we examined the interplay between PARP1 and ERα, and identified PARP1 as an important regulator of ERα-dependent transcription. We showed that PARP1 could directly bind to ERα, and ERα could be poly(ADP-ribosyl)ated by PARP1. Poly(ADP-ribosyl)ation increased ERα binding to estrogen response element (ERE) present in the promoter of target genes and promoted ERα-mediated gene transcription. Estradiol, the ligand of ERα, increased PARP enzymatic activity and enhanced poly(ADP-ribosyl)ation of ERα. Upon treatment with estradiol, ERα binding to ERE- and ERα-dependent gene expression was dramatically increased in cultured vascular smooth muscle cells (VSMCs). Inhibition of PARP1 by PARP inhibitor or PARP1 siRNA decreased ERα binding to ERE and prevented ERα-dependent gene transcription in VSMCs. Further studies revealed that PARP1 served as an indispensible component for the formation of the ERα-ERE complex by directly interacting with ERα. Thus, our results identify PARP1 as a key regulator of ERα in controlling ERα transactivation.

A phase II evaluation of gefitinib in the treatment of persistent or recurrent endometrial cancer: a Gynecologic Oncology Group study

BACKGROUND

A phase II trial was performed to evaluate the efficacy and safety of gefitinib in patients with persistent/recurrent endometrial cancer.

METHODS

Women with histologically confirmed persistent/recurrent endometrial cancer were treated with 500mg oral gefitinib daily until progression or severe toxicity, with progression-free survival (PFS) at six months as the primary endpoint. Tumor expression of total epidermal growth factor receptor (EGFR), estrogen receptor (ER), progesterone receptor A (PRA) and B (PRB), Ki67, pEGFR and activated extracellular signal-regulated kinase (pERK) were examined pre- and post-treatment. EGFR was sequenced, and serum concentrations of soluble EGFR (sEGFR) at baseline also were examined.

RESULTS

Of 29 patients enrolled, 26 were evaluable for efficacy and toxicity. Four patients experienced PFS ≥6 months, and one had a complete response which was not associated with an EGFR mutation. The concentration of sEGFR in pretreatment serum was positively correlated with overall survival (OS), but not with responsiveness to gefitinib in this small patient cohort. Expression of tumor biomarkers was not associated with PFS or OS. Co-expression of ER with PRA in primary and recurrent tumors, and pEGFR with pERK in primary tumors was observed.

CONCLUSIONS

This treatment regimen was tolerable but lacked sufficient efficacy to warrant further evaluation in this setting. The possible association between serum sEGFR concentrations and OS, and temporal changes in expression of pEGFR and pERK and the documented CR of one patient are interesting and warrant additional investigation.

Crosstalk between poly(ADP-ribose) polymerase and sirtuin enzymes

Poly(ADP-ribose) polymerases (PARPs) are NAD(+) dependent enzymes that were identified as DNA repair proteins, however, today it seems clear that PARPs are responsible for a plethora of biological functions. Sirtuins (SIRTs) are NAD(+)-dependent deacetylase enzymes involved in the same biological processes as PARPs raising the question whether PARP and SIRT enzymes may interact with each other in physiological and pathophysiological conditions. Hereby we review the current understanding of the SIRT-PARP interplay in regard to the biochemical nature of the interaction (competition for the common NAD(+) substrate, mutual posttranslational modifications and direct transcriptional effects) and the physiological or pathophysiological consequences of the interactions (metabolic events, oxidative stress response, genomic stability and aging). Finally, we give an overview of the possibilities of pharmacological intervention to modulate PARP and SIRT enzymes either directly, or through modulating NAD(+) homeostasis.

PARP-2 regulates cell cycle-related genes through histone deacetylation and methylation independently of poly(ADP-ribosyl)ation

Poly(ADP-ribose) polymerase-2 (PARP-2) catalyzes poly(ADP-ribosyl)ation (PARylation) and regulates numerous nuclear processes, including transcription. Depletion of PARP-2 alters the activity of transcription factors and global gene expression. However, the molecular action of how PARP-2 controls the transcription of target promoters remains unclear. Here we report that PARP-2 possesses transcriptional repression activity independently of its enzymatic activity. PARP-2 interacts and recruits histone deacetylases HDAC5 and HDAC7, and histone methyltransferase G9a to the promoters of cell cycle-related genes, generating repressive chromatin signatures. Our findings propose a novel mechanism of PARP-2 in transcriptional regulation involving specific protein-protein interactions and highlight the importance of PARP-2 in the regulation of cell cycle progression.

2,3,7,8-Tetrachlorodibenzo-p-dioxin poly(ADP-ribose) polymerase (TiPARP, ARTD14) is a mono-ADP-ribosyltransferase and repressor of aryl hydrocarbon receptor transactivation

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly(ADP-ribose) polymerase (TiPARP/ARTD14) is a member of the PARP family and is regulated by the aryl hydrocarbon receptor (AHR); however, little is known about TiPARP function. In this study, we examined the catalytic function of TiPARP and determined its role in AHR transactivation. We observed that TiPARP exhibited auto-mono-ADP-ribosyltransferase activity and ribosylated core histones. RNAi-mediated knockdown of TiPARP in T-47D breast cancer and HuH-7 hepatoma cells increased TCDD-dependent cytochrome P450 1A1 (CYP1A1) and CYP1B1 messenger RNA (mRNA) expression levels and recruitment of AHR to both genes. Overexpression of TiPARP reduced AHR-dependent increases in CYP1A1-reporter gene activity, which was restored by overexpression of AHR, but not aryl hydrocarbon receptor nuclear translocator. Deletion and mutagenesis studies showed that TiPARP-mediated inhibition of AHR required the zinc-finger and catalytic domains. TiPARP and AHR co-localized in the nucleus, directly interacted and both were recruited to CYP1A1 in response to TCDD. Overexpression of Tiparp enhanced, whereas RNAi-mediated knockdown of TiPARP reduced TCDD-dependent AHR proteolytic degradation. TCDD-dependent induction of AHR target genes was enhanced in Tiparp^−/− mouse embryonic fibroblasts compared with wildtype controls. Our findings show that TiPARP is a mono-ADP-ribosyltransferase and a transcriptional repressor of AHR, revealing a novel negative feedback loop in AHR signalling.

Dual roles of PARP-1 promote cancer growth and progression

PARP-1 is an abundant nuclear enzyme that modifies substrates by poly(ADP-ribose)-ylation. PARP-1 has well-described functions in DNA damage repair and also functions as a context-specific regulator of transcription factors. With multiple models, data show that PARP-1 elicits protumorigenic effects in androgen receptor (AR)-positive prostate cancer cells, in both the presence and absence of genotoxic insult. Mechanistically, PARP-1 is recruited to sites of AR function, therein promoting AR occupancy and AR function. It was further confirmed in genetically defined systems that PARP-1 supports AR transcriptional function, and that in models of advanced prostate cancer, PARP-1 enzymatic activity is enhanced, further linking PARP-1 to AR activity and disease progression. In vivo analyses show that PARP-1 activity is required for AR function in xenograft tumors, as well as tumor cell growth in vivo and generation and maintenance of castration resistance. Finally, in a novel explant system of primary human tumors, targeting PARP-1 potently suppresses tumor cell proliferation. Collectively, these studies identify novel functions of PARP-1 in promoting disease progression, and ultimately suggest that the dual functions of PARP-1 can be targeted in human prostate cancer to suppress tumor growth and progression to castration resistance.

SIGNIFICANCE

These studies introduce a paradigm shift with regard to PARP-1 function in human malignancy, and suggest that the dual functions of PARP-1 in DNA damage repair and transcription factor regulation can be leveraged to suppress pathways critical for promalignant phenotypes in prostate cancer cells by modulation of the DNA damage response and hormone signaling pathways. The combined studies highlight the importance of dual PARP-1 function in malignancy and provide the basis for therapeutic targeting.

Nuclear receptor coregulators merge transcriptional coregulation with epigenetic regulation

Members of the nuclear steroid/thyroid hormone receptor (NR) gene superfamily are DNA-binding transcription factors that regulate target genes in a spatiotemporal manner, depending on the promoter context. In vivo observations of ligand responses in NR-mediated gene regulation led to the identification of ligand-dependent coregulators that directly interact with NRs. Functional dissection of NR coregulators revealed that their transcriptional coregulation was linked to histone acetylation. However, recent work in the fields of reversible histone modification and chromatin remodeling indicates that histone-modifying enzymes, including histone methylases and chromatin remodelers, are potential transcriptional coregulators that interact directly and indirectly with NRs.

Ethanol-induced downregulation of the angiotensin AT2 receptor in murine fibroblasts is mediated by PARP-1

Molecular mechanisms accompanying ethanol-induced cytotoxicity remain to be defined. The renin-angiotensin system with its respective receptors, the angiotensin AT1 and AT2 receptor (AT1R and AT2R), has been implicated in these processes. The AT2R seems to counteract the pro-inflammatory, pro-hypertrophic, and pro-fibrotic actions of the AT1R and is involved in cellular differentiation and tissue repair. Recently, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a novel negative transcriptional regulator of the AT2R. However, the complex interactions between ethanol, PARP-1, and the AT2R are largely unknown. In this in vitro study, we aimed to clarify whether acute ethanol treatment modifies AT2R promoter activity or AT2R mRNA and protein levels and whether PARP-1 is involved in ethanol-mediated regulation of the AT2R. Murine fibroblasts of the R3T3 and MEF line (murine embryonic fibroblasts) were exposed to ethanol for 24h. AT2R promoter activity, mRNA and protein levels were analyzed with and without PARP-1 inhibition and in PARP-1 knockout MEF cells. Expression of PARP-1 was analyzed over course of time, and cell viability and DNA fragmentation were measured on single-cell level by flow cytometry. Ethanol exposition induced substantial downregulation of the AT2R on promoter, mRNA and protein levels in a dose-dependent manner. Pharmacological inhibition or ablation of PARP-1 completely abolished this effect. Ethanol treatment did not have any effect on AT1R mRNA and protein levels in MEF cells. Further, acute ethanol treatment promoted DNA fragmentation and caused transcriptional induction of PARP-1. Our findings reveal that PARP-1 is an upstream transcriptional regulator of the AT2 receptor in the context of ethanol exposure and represses the AT2R gene in fibroblasts in vitro. Variations in expression of the potentially tissue-protective AT2R might contribute to ethanol-mediated pathology.

Oocyte numbers in the mouse increase after treatment with 5-aminoisoquinolinone: a potent inhibitor of poly(ADP-ribosyl)ation

Poly(ADP-ribosyl)ation is a posttranslational protein modification carried out by a family of enzymes referred to as poly(ADP-ribose) polymerases (PARPs). It has been proposed that the broad nuclear distribution of PARPs may allow them to modulate gene expression in addition to their more accepted role as DNA repair mediators. The role of poly(ADP-ribosyl)ation during oogenesis and folliculogenesis is unknown. Here we found that when 3- to 4-wk-old mice were injected with 5-amninoisoquinolinone, a water soluble inhibitor of poly(ADP-ribosyl)ation, it leads to considerably increased oocyte numbers and a dramatic increase in primordial follicle numbers. Furthermore, we show that inhibition of poly(ADP-ribosyl)ation leads to an increased expression of specific genes and pathways in mouse ovaries, in particular, transforming growth factor superfamily members. Our results demonstrate that poly(ADP-ribosyl)ation, is important in oogenesis and folliculogenesis, and it may have a differential role in regulating gene expression, DNA repair, and apoptosis. The novel function of poly(ADP-ribosyl)ation in oogenesis and folliculogenesis sheds light on the alternative role that DNA repair mediators may play in cellular development and differentiation.

Global analysis of transcriptional regulation by poly(ADP-ribose) polymerase-1 and poly(ADP-ribose) glycohydrolase in MCF-7 human breast cancer cells

Poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) are enzymes that modify target proteins by the addition and removal, respectively, of ADP-ribose polymers. Although a role for PARP-1 in gene regulation has been well established, the role of PARG is less clear. To investigate how PARP-1 and PARG coordinately regulate global patterns of gene expression, we used short hairpin RNAs to stably knock down PARP-1 or PARG in MCF-7 cells followed by expression microarray analyses. Correlation analyses showed that the majority of genes affected by the knockdown of one factor were similarly affected by the knockdown of the other factor. The most robustly regulated common genes were enriched for stress-response and metabolic functions. In chromatin immunoprecipitation assays, PARP-1 and PARG localized to the promoters of positively and negatively regulated target genes. The levels of chromatin-bound PARG at a given promoter generally correlated with the levels of PARP-1 across the subset of promoters tested. For about half of the genes tested, the binding of PARP-1 at the promoter was dependent on the binding of PARG. Experiments using stable re-expression of short hairpin RNA-resistant catalytic mutants showed that PARP-1 and PARG enzymatic activities are required for some, but not all, target genes. Collectively, our results indicate that PARP-1 and PARG, nuclear enzymes with opposing enzymatic activities, localize to target promoters and act in a similar, rather than antagonistic, manner to regulate gene expression.

Biochemical analyses of nuclear receptor-dependent transcription with chromatin templates

Chromatin, the physiological template for transcription, plays important roles in gene regulation by nuclear receptors (NRs). It can (1) restrict the binding of NRs or the transcriptional machinery to their genomic targets, (2) serve as a target of regulatory posttranslational modifications by NR coregulator proteins with histone-directed enzymatic activities, and (3) function as a binding scaffold for a variety of transcription-related proteins. The advent of in vitro or "cell-free" systems that accurately recapitulate ligand-dependent transcription by NRs with chromatin templates has allowed detailed analyses of these processes. Biochemical studies have advanced our understanding of the mechanisms of gene regulation, including the role of ligands, coregulators, and nucleosome remodeling. In addition, they have provided new insights about the dynamics of NR-mediated transcription. This chapter reviews the current methodologies for assembling, transcribing, and analyzing chromatin in vitro, as well as the new information that has been gained from these studies.

Positive transcriptional regulation of the human micro opioid receptor gene by poly(ADP-ribose) polymerase-1 and increase of its DNA binding affinity based on polymorphism of G-172 -> T

Micro opioid receptor (MOR) agonists such as morphine are applied widely in clinical practice as pain therapy. The effects of morphine through MOR, such as analgesia and development of tolerance and dependence, are influenced by individual specificity. Recently, we analyzed single nucleotide polymorphisms on the human MOR gene to investigate the factors that contribute to individual specificity. In process of single nucleotide polymorphisms analysis, we found that specific nuclear proteins bound to G(-172) --> T region in exon 1 in MOR gene, and its affinity to DNA was increased by base substitution from G(-172) to T(-172). The isolated protein was identified by mass spectrometry and was confirmed by Western blotting to be poly(ADP-ribose) polymerase-1 (PARP-1). The overexpressed PARP-1 bound to G(-172) --> T and enhanced the transcription of reporter vectors containing G(-172) and T(-172). Furthermore, PARP-1 inhibitor (benzamide) decreased PARP-1 binding to G(-172) --> T without affecting mRNA or protein expression level of PARP-1 and down-regulated the subsequent MOR gene expression in SH-SY5Y cells. Moreover, we found that tumor necrosis factor-alpha enhanced MOR gene expression as well as increased PARP-1 binding to the G(-172) --> T region and G(-172) --> T-dependent transcription in SH-SY5Y cells. These effects were also inhibited by benzamide. In this study, our data suggest that PARP-1 positively regulates MOR gene transcription via G(-172) --> T, which might influence individual specificity in therapeutic opioid effects.

Differential transactivation by orphan nuclear receptor NOR1 and its fusion gene product EWS/NOR1: possible involvement of poly(ADP-ribose) polymerase I, PARP-1

In extraskeletal myxoid chondrosarcoma, a chromosomal translocation creates a gene fusion between EWS and an orphan nuclear receptor, NOR1. The resulting fusion protein EWS/NOR1 has been believed to lead to malignant transformation by functioning as a transactivator for NOR1-target genes. By comparing the gene expression profiles of NOR1- and EWS/NOR1-overexpressing cells, we found that they largely shared up-regulated genes, but no significant correlation was observed with respect to the transactivation levels of each gene. In addition, the proteins associated with NOR1 and EWS/NOR1 were mostly the same in these cells. The results suggest that these proteins differentially transactivate overlapping target genes through a similar transcriptional machinery. To clarify the mechanisms underlying the transcriptional divergence between NOR1 and EWS/NOR1, we searched for alternatively associated proteins, and identified poly(ADP-ribose) polymerase I (PARP-1) as an NOR1-specific binding protein. Consistent with its binding properties, PARP-1 acted as a transcriptional repressor of NOR1, but not EWS/NOR1, in a luciferase reporter assay employing PARP-1(-/-) fibroblasts. Interestingly, suppressive activity of PARP-1 was observed in a DNA response element-specific manner, and in a subtype-specific manner toward the NR4A family (Nur77, Nurr1, and NOR1), suggesting that PARP-1 plays a role in the diversity of transcriptional regulation mediated by the NR4A family in normal cells. Altogether, our findings suggest that NOR1 and EWS/NOR1 regulate overlapping target genes differently by utilizing associated proteins, including PARP-1; and that EWS/NOR1 may acquire oncogenic activities by avoiding (or gaining) transcription factor-specific modulation by the associated proteins.

Poly(ADP-ribose) polymerase-1 down-regulates BRCA2 expression through the BRCA2 promoter

Expression of the BRCA2 tumor suppressor gene is tightly linked to its roles in DNA damage repair and maintenance of chromosomal stability and genomic integrity. Three transcription factors that activate (USF, NF-kappaB, and Elf1) and a single factor that represses (SLUG) BRCA2 promoter activity have been reported. In addition, a 67-bp region (-582 to -516) associated with inhibition of promoter activity has been identified. However, it remains unclear how the 67-bp region contributes to regulation of BRCA2 expression. Here, we describe the affinity purification of a 120-kDa protein that binds to a silencer-binding region within the 67-bp repression region of the BRCA2 promoter. Mass spectrometry revealed the identity of the protein as poly-(ADP-ribose) polymerase-1 (Parp-1). Gel shift, antibody super-shift, and chromatin immunoprecipitation (ChIP) assays demonstrated that Parp-1 is associated with the BRCA2 promoter both in vitro and in vivo. Furthermore, Parp-1 inhibitors (either 3-AB or NU1025) and Parp-1 gene specific siRNA resulted in increased levels of endogenous BRCA2 expression. Inhibition of Parp-1 activity (by 3-AB) reduced histone 3 lysine 9 acetylation and blocked Parp-1 binding to the BRCA2 promoter. These results indicate that Parp-1 down-regulates BRCA2 expression through an interaction with a repression region of the BRCA2 promoter.

Integrin-linked kinase regulates E-cadherin expression through PARP-1

Repression of E-cadherin expression by the transcription factor, Snail, is implicated in epithelial to mesenchymal transition and cancer progression. We show here that Integrin-Linked Kinase (ILK) regulates E-cadherin expression through Poly(ADP-ribose) polymerase-1 (PARP-1). ILK overexpression in Scp2 cells resulted in stimulation of Snail expression and loss of E-cadherin expression. Silencing of ILK, Akt or Snail resulted in re-expression of E-cadherin in PC3 cells. To elucidate the signaling pathway downstream of ILK, we identified candidate Snail promoter ILK Responsive Element (SIRE) binding proteins. PARP-1 was identified as a SIRE-binding protein. ILK silencing inhibited binding of PARP-1 to SIRE. PARP-1 silencing resulted in inhibition of Snail and ZEB1, leading to up-regulation of E-cadherin. We suggest a model in which ILK represses E-cadherin expression by regulating PARP-1, leading to the binding of PARP-1 to SIRE and modulation of Snail expression.

PARP-1 suppresses adiponectin expression through poly(ADP-ribosyl)ation of PPAR gamma in cardiac fibroblasts

AIMS

Our aim was to explore the mechanism underlying the transcriptional regulation of adiponectin and its receptors (AdipoR) in cultured rat cardiac fibroblasts.

METHODS AND RESULTS

Using western blot and real-time RT-PCR assays, the expression of adiponectin and its receptors was determined. Using Southwestern blot and electrophoretic mobility shift assays, the DNA binding activity of peroxisome proliferator activated receptor gamma (PPAR gamma) was determined. The results showed that adiponectin and AdipoR1 were highly expressed in cultured rat cardiac fibroblasts. Inhibition of poly(ADP-ribose) polymerase 1 (PARP-1) by 3-aminobenzamide, PJ34, or PARP-1 siRNA markedly increased the transcription of adiponectin and AdipoR1 in cultured fibroblasts, mature 3T3 L1 adipocytes, rat myocardium, and white adipose tissue. PPAR gamma was poly(ADP-ribosyl)ated by PARP-1 in cardiac fibroblasts under basal conditions. Poly(ADP-ribosyl)ation of PPAR gamma prevented its binding to DNA. Inhibition of PARP-1 enhanced the DNA binding and transactivation of PPAR gamma and increased the transcription of PPAR gamma-target genes including CD36, lipoprotein lipase, and leptin in cultured fibroblasts.

CONCLUSION

PARP-1 inhibits adiponectin and AdipoR1 expression as well as PPAR gamma transactivation through poly(ADP-ribosyl)ation of PPAR gamma in cultured rat cardiac fibroblasts.

Poly(ADP-ribose) polymerase-2 [corrected] controls adipocyte differentiation and adipose tissue function through the regulation of the activity of the retinoid X receptor/peroxisome proliferator-activated receptor-gamma [corrected] heterodimer

The peroxisome proliferator-activated receptor-gamma (PPARgamma, NR1C3) in complex with the retinoid X receptor (RXR) plays a central role in white adipose tissue (WAT) differentiation and function, regulating the expression of key WAT proteins. In this report we show that poly(ADP-ribose) polymerase-2 (PARP-2), also known as an enzyme participating in the surveillance of the genome integrity, is a member of the PPARgamma/RXR transcription machinery. PARP-2(-/-) mice accumulate less WAT, characterized by smaller adipocytes. In the WAT of PARP-2(-/-) mice the expression of a number of PPARgamma target genes is reduced despite the fact that PPARgamma1 and -gamma2 are expressed at normal levels. Consistent with this, PARP-2(-/-) mouse embryonic fibroblasts fail to differentiate to adipocytes. In transient transfection assays, PARP-2 small interference RNA decreases basal activity and ligand-dependent activation of PPARgamma, whereas PARP-2 overexpression enhances the basal activity of PPARgamma, although it does not change the maximal ligand-dependent activation. In addition, we show a DNA-dependent interaction of PARP-2 and PPARgamma/RXR heterodimer by chromatin immunoprecipitation. In combination, our results suggest that PARP-2 is a novel cofactor of PPARgamma activity.

Analysis of genetic variants of the poly(ADP-ribose) polymerase-1 gene in breast cancer in French patients

Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear enzyme that catalyzes the poly(ADP-ribosyl)ation of target proteins in response to DNA damage and has been proposed to play a role in DNA repair, recombination, transcription, cell death, cell proliferation, as well as in stabilization of the genome. We have recently shown that PARP-1 deficiency causes mammary tumorigenesis in mice. In the present study, we investigated whether genetic variants and single nucleotide polymorphisms (SNPs) of PARP-1 contribute to human breast cancer. To this end, we screened all PARP-1 exons, 7.1kb of intron-exon junction and 1.0-kb promoter sequences in 83 French patients with breast cancer and 100 controls by direct sequencing of genomic DNA. Twenty rare genetic variants of PARP-1, including c.1148C>A (Ser383Tyr), c.1354C>A (Arg452Arg), c.2819A>G (Lys940Arg) were detected in nine (10.8%) breast cancers of these patients. Among 31 polymorphic sites examined, five haplotype-tagging SNPs (htSNPs) of PARP-1 were identified. Interestingly, the genotype distribution of htSNP c.852T>C (Ala284Ala) was likely associated with loss of estrogen- and progesterone-receptor expression. The present study implies that genetic variants of PARP-1 may contribute to breast cancerogenesis and that PARP-1 htSNP c.852T>C (Ala284Ala) may influence hormonal therapy of breast cancer.

Poly(ADP-ribosyl)ation regulates heat shock factor-1 activity and the heat shock response in murine fibroblasts

Poly(ADP-ribose) polymerase-1 (PARP-1)-dependent poly(ADP-ribose) formation is emerging as a key regulator of transcriptional regulation, even though the targets and underlying molecular mechanisms have not yet been clearly identified. In this study, we gathered information on the role of PARP-1 activity in the heat shock response of mouse fibroblasts. We show that DNA binding of heat shock factor (HSF)-1 was impaired by PARP-1 activity in cellular extracts, and was higher in PARP-1(-/-) than in PARP-1+/+ cells. No evidence for HSF-1 poly(ADP-ribosyl)ation or PARP-1 interaction was found, but a poly(ADP-ribose) binding motif was identified in the transcription factor amino acid sequence. Consistent with data on HSF-1, the expression of heat-shock protein (HSP)-70 and HSP-27 was facilitated in cells lacking PARP-1. Thermosensitivity, however, was higher in PARP-1(-/-) than in PARP-1+/+ cells. Accordingly, we report that heat-shocked PARP-1 null fibroblasts showed an increased activation of proapoptotic JNK and decreased transcriptional efficiency of prosurvival NF-kappaB compared with wild-type counterparts. The data indicate that poly(ADP-ribosyl)ation finely regulates HSF-1 activity, and emphasize the complex role of PARP-1 in the heat-shock response of mammalian cells.

Identification of proteins within the nuclear factor-kappa B transcriptional complex including estrogen receptor-alpha

OBJECTIVE

The objective of the study was to determine whether cross-talk occurs between estrogen receptors (ERs) and nuclear factor-kappa-B (NF-kappaB), to assess the functional consequences of such an ER/NF-kappaB interaction, and to identify other unknown regulatory proteins that may participate in the NF-kappaB transcriptional complex.

STUDY DESIGN

Electromobility gel shifts, reporter gene assays, and mass spectrometry were used to identify proteins interacting with the NF-kappaB deoxyribonucleic acid (DNA) response element.

RESULTS

ER and the p65 subunit of NF-kappaB colocalized on DNA. This interaction was inhibitory for ER transcriptional activity. Sequencing of proteins bound to the NF-kappaB/DNA complex identified DNA-modifying enzymes, scaffolding proteins, chaperones, and elements of the nuclear matrix.

CONCLUSION

These studies have identified an inhibitory interaction between estrogen receptors and the p65 subunit of NF-kappaB with implications for estrogen action in pregnancy and cancer. New accessory proteins have also been identified that bind to protein complexes on the NF-kappaB DNA response element.

Txk, a member of the non-receptor tyrosine kinase of the Tec family, forms a complex with poly(ADP-ribose) polymerase 1 and elongation factor 1alpha and regulates interferon-gamma gene transcription in Th1 cells

We have found previously that Txk, a member of the Tec family tyrosine kinases, is involved importantly in T helper 1 (Th1) cytokine production. However, how Txk regulates interferon (IFN)-gamma gene transcription in human T lymphocytes was not fully elucidated. In this study, we identified poly(ADP-ribose) polymerase 1 (PARP1) and elongation factor 1alpha (EF-1alpha) as Txk-associated molecules that bound to the Txk responsive element of the IFN-gamma gene promoter. Txk phosphorylated EF-1alpha and PARP1 formed a complex with them, and bound to the IFN-gamma gene promoter in vitro. In particular, the N terminal region containing the DNA binding domain of PARP1 was important for the trimolecular complex formation involving Txk, EF-1alpha and PARP1. Several mutant Txk which lacked kinase activity were unable to form the trimolecular complex. A PARP1 inhibitor, PJ34, suppressed IFN-gamma but not interleukin (IL)-4 production by normal peripheral blood lymphocytes (PBL). Multi-colour confocal analysis revealed that Txk and EF-1alpha located in the cytoplasm in the resting condition. Upon activation, a complex involving Txk, EF-1alpha and PARP1 was formed and was located in the nucleus. Collectively, Txk in combination with EF-1alpha and PARP1 bound to the IFN-gamma gene promoter, and exerted transcriptional activity on the IFN-gamma gene.

Loss of Parp-1 affects gene expression profile in a genome-wide manner in ES cells and liver cells

BACKGROUND

Many lines of evidence suggest that poly(ADP-ribose) polymerase-1 (Parp-1) is involved in transcriptional regulation of various genes as a coactivator or a corepressor by modulating chromatin structure. However, the impact of Parp-1-deficiency on the regulation of genome-wide gene expression has not been fully studied yet.

RESULTS

We employed a microarray analysis covering 12,488 genes and ESTs using mouse Parp-1-deficient (Parp-1-/-) embryonic stem (ES) cell lines and the livers of Parp-1-/- mice and their wild-type (Parp-1+/+) counterparts. Here, we demonstrate that of the 9,907 genes analyzed, in Parp-1-/- ES cells, 9.6% showed altered gene expression. Of these, 6.3% and 3.3% of the genes were down- or up-regulated by 2-fold or greater, respectively, compared with Parp-1+/+ ES cells (p < 0.05). In the livers of Parp-1-/- mice, of the 12,353 genes that were analyzed, 2.0% or 1.3% were down- and up-regulated, respectively (p < 0.05). Notably, the number of down-regulated genes was higher in both ES cells and livers, than that of the up-regulated genes. The genes that showed altered expression in ES cells or in the livers are ascribed to various cellular processes, including metabolism, signal transduction, cell cycle control and transcription. We also observed expression of the genes involved in the pathway of extraembryonic tissue development is augmented in Parp-1-/- ES cells, including H19. After withdrawal of leukemia inhibitory factor, expression of H19 as well as other trophoblast marker genes were further up-regulated in Parp-1-/- ES cells compared to Parp-1+/+ ES cells.

CONCLUSION

These results suggest that Parp-1 is required to maintain transcriptional regulation of a wide variety of genes on a genome-wide scale. The gene expression profiles in Parp-1-deficient cells may be useful to delineate the functional role of Parp-1 in epigenetic regulation of the genomes involved in various biological phenomena.

Phosphorylation of thyroid hormone receptor-associated nuclear receptor corepressor holocomplex by the DNA-dependent protein kinase enhances its histone deacetylase activity

It is well documented that unliganded thyroid hormone receptor (TR) functions as a transcriptional repressor of specific cellular target genes by acting in concert with a corepressor complex harboring histone deacetylase (HDAC) activity. To fully explore the cofactors that interact with the transcriptionally repressive form of TR, we biochemically isolated a multiprotein complex that assembles on a TR.retinoid X receptor (RXR) heterodimer in HeLa nuclear extracts and identified its polypeptide components by mass spectrometry. A subset of TR.RXR-associated polypeptides included NCoR, SMRT, TBL1, and HDAC3, which represent the core components of a previously described NCoR/SMRT corepressor complex. We also identified several polypeptides that constitute a DNA-dependent protein kinase (DNA-PK) enzyme complex, a regulator of DNA repair, recombination, and transcription. These polypeptides included the catalytic subunit DNA-PKcs, the regulatory subunits Ku70 and Ku86, and the poly(ADP-ribose) polymerase 1. Density gradient fractionation and immunoprecipitation analyses provided evidence for the existence of a high molecular weight TR.RXR.corepressor holocomplex containing both NCoR/SMRT and DNA-PK complexes. Chromatin immunoprecipitation studies confirmed that unliganded TR.RXR recruits both complexes to the triiodothyronine-responsive region of growth hormone gene in vivo. Interestingly, DNA-PKcs, a member of the phosphatidylinositol 3-kinase family, was found to phosphorylate HDAC3 when the purified TR.RXR.corepressor holocomplex was incubated with ATP. This phosphorylation was accompanied by a significant enhancement of the HDAC activity of this complex. Collectively, our results indicated that DNA-PK promotes the establishment of a repressive chromatin at a TR target promoter by enhancing the HDAC activity of the receptor-bound NCoR/SMRT corepressor complex.

Proteomic identification of proteins associated with the osmoregulatory transcription factor TonEBP/OREBP: functional effects of Hsp90 and PARP-1

Hypertonicity (e.g., high NaCl) activates the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP), increasing transcription of protective genes. In the present studies, by stably expressing amino acids 1-547 of TonEBP/OREBP in HEK 293 cells and immunoprecipitating it plus associated proteins from the nuclei of cells exposed to high NaCl, we identify 14 proteins that are physically associated with TonEBP/OREBP. The associated proteins fall into several classes: 1) DNA-dependent protein kinase, both its catalytic subunit and regulatory subunit, Ku86; 2) RNA helicases, namely RNA helicase A, nucleolar RNA helicase II/Gu, and DEAD-box RNA helicase p72; 3) small or heterogeneous nuclear ribonucleoproteins (snRNPs or hnRNPs), namely U5 snRNP-specific 116 kDa protein, U5 snRNP-specific 200 kDa protein, hnRNP U, hnRNP M, hnRNP K, and hnRNP F; 4) heat shock proteins, namely Hsp90beta and Hsc70; and 5) poly(ADP-ribose) polymerase-1 (PARP-1). We confirm identification of most of the proteins by Western analysis and also demonstrate by electrophoretic mobility-shift assay that they are present in the large complex that binds specifically along with TonEBP/OREBP to its cognate DNA element. In addition, we find that PARP-1 and Hsp90 modulate TonEBP/OREBP activity. PARP-1 expression reduces TonEBP/OREBP transcriptional activity and the activity of its transactivating domain. Hsp90 enhances those activities and sustains the increased abundance of TonEBP/OREBP protein in cells exposed to high NaCl.

The general transcription machinery and general cofactors

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.

Poly(ADP-ribose) polymerase-1 is a component of the oncogenic T-cell factor-4/beta-catenin complex

BACKGROUND & AIMS

T-cell factor (TCF)-4 regulates a certain set of genes related to growth and differentiation of intestinal epithelial cells. Aberrant transactivation of these TCF-4-regulated genes by beta-catenin protein plays a crucial role in early intestinal carcinogenesis, and the transcriptional machinery of the TCF-4/beta-catenin complex is likely to contain targets for molecular therapy. We explored the molecular composition of the TCF-4/beta-catenin transcriptional complex by means of proteomics.

METHODS & RESULTS

A protein of approximately 112 kilodaltons was consistently coimmunoprecipitated with FLAG-tagged TCF-4 transiently expressed in HEK293 cells, and the protein was identified by mass spectrometry as poly(ADP-ribose) polymerase-1 (PARP-1). PARP-1 physically interacted with TCF-4 and augmented the transcriptional activity of the beta-catenin/TCF-4 complex. Knockdown of PARP-1 by RNA interference significantly suppressed both transcriptional activity and proliferation by colorectal cancer cells. Auto-polyADP-ribosylation of the PARP-1 protein induced by DNA damage inhibited the functional interaction of PARP-1 with TCF-4. PARP-1 was overexpressed in the intestinal adenomas of patients with familial adenomatous polyposis and multiple intestinal polyposis mice. The expression of PARP-1 was closely associated with the accumulation of beta-catenin and with the undifferentiated status of intestinal epithelial cells.

CONCLUSIONS

In this study, we identified PARP-1 as a novel coactivator of the beta-catenin/TCF-4 complex. Although PARP-1 has been believed to play a protective role against carcinogenesis, these expression patterns and functional properties of PARP-1 were highly suggestive of its participation in early colorectal carcinogenesis.

Gene 33 inhibits apoptosis of breast cancer cells and increases poly(ADP-ribose) polymerase expression

The structure of the Gene 33 protein suggests that it plays a role in intracellular signaling and Gene 33 is induced by many mitogenic and stressful stimuli. Previously, we found that Gene 33 expression is significantly induced by retinoic acid (RA), insulin and synergistically by both in a liver-derived cell line. In the present study, we investigated the basal expression and regulation of Gene 33 in multiple human breast cancer cell lines. These cell lines expressed different levels of Gene 33 protein, but Gene 33 protein was not regulated by RA or insulin, either alone, or in combination. However, epidermal growth factor (EGF) induced Gene 33 expression in SK-BR-3 cells and this induction was inhibited by co-treatment with RA. There was a strong correlation between endogenous basal Gene 33 expression and doubling time. Exogenous expression of Gene 33 in MCF-7 cells did not affect cell cycle distribution, but inhibited apoptosis and specifically increased the level of Poly(ADP-ribose) Polymerase (PARP-1) protein. This suggests that Gene 33 promotes breast cancer cell growth by an anti-apoptotic rather than a mitogenic effect, possibly involving up-regulation of PARP-1.

Poly(ADP-ribose) polymerases: homology, structural domains and functions. Novel therapeutical applications

Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes, which show differences in structure, cellular location and functions. However, all these enzymes possess poly(ADP-ribosyl)ation activity. Overactivation of PARP enzymes has been implicated in the pathogenesis of several diseases, including stroke, myocardial infarction, diabetes, shock, neurodegenerative disorder and allergy. The best studied of these enzymes (PARP-1) is involved in the cellular response to DNA damage so that in the event of irreparable DNA damage overactivation of PARP-1 leads to necrotic cell death. Inhibitors of PARP-1 activity in combination with DNA-binding antitumor drugs may constitute a suitable strategy in cancer chemotherapy. In addition, PARP inhibitors may be also useful to restore cellular functions in several pathophysiological states and diseases. This review gives an update of the state-of-the-art concerning PARP enzymes and their exploitation as pharmacological targets in several illnesses.

Regulation of alpha-synuclein expression by poly (ADP ribose) polymerase-1 (PARP-1) binding to the NACP-Rep1 polymorphic site upstream of the SNCA gene

Alleles at NACP-Rep1, the polymorphic microsatellite repeat located approximately 10 kb upstream of the alpha -synuclein gene (SNCA), are associated, in some reports, with differing risks of sporadic Parkinson disease (PD). We showed previously that NACP-Rep1 acts as a negative modulator of SNCA transcription, with an effect that varied threefold among different NACP-Rep1 alleles. Given that duplications and triplications of SNCA have been implicated in familial Parkinson disease (PD), even a 1.5-2-fold increase in alpha -synuclein expression may, over many decades, contribute to PD. Thus, the association of different NACP-Rep1 alleles with PD may be a consequence of polymorphic differences in transcriptional regulation of SNCA. Here we aimed to identify the factor(s) that bind to NACP-Rep1 and potentially contribute to SNCA transcriptional modulation, by pulling down proteins that bind to NACP-Rep1 and identifying them by mass spectrometry. One of these proteins was poly-(ADP-ribose) transferase/polymerase-1 (PARP-1), a DNA-binding protein and transcriptional regulator. Electrophoresis mobility shift and chromatin immunoprecipitation assays showed specific binding of PARP-1 to NACP-Rep1. Inhibition of PARP-1's catalytic domain increased the endogenous SNCA mRNA levels in cultured SH-SY5Y cells. Furthermore, PARP-1 binding to NACP-Rep1 specifically reduced the transcriptional activity of the SNCA promoter/enhancer in luciferase reporter assays. This down-regulation effect of PARP-1 depended on NACP-Rep1 being present in the construct and was abrogated by inhibiting PARP-1's catalytic activity with 3-aminobenzamide. The association of different NACP-Rep1 alleles with PD may be mediated, in part, by the effect of PARP-1, as well as other factors, on SNCA expression.

Ecdysone receptor-dependent gene regulation mediates histone poly(ADP-ribosyl)ation

While the ecdysone dependency of puff formation in giant polytene chromosomes from fly salivary glands has been well documented, the molecular mechanisms underlying this process remain unknown. However, it does appear to involve chromatin remodeling and modification mediated by ecdysone receptor (EcR). As Drosophila poly(ADP-ribose) polymerase (dPARP) has recently been reported to be involved in ecdysone-induced puff formation, we decided to test the possible role of dPARP in ligand-induced dEcR transactivation in an insect system. dPARP co-activated the ligand-induced transactivation function of EcR in the insect cell line S2, and appeared to physically interact with EcR in a ligand-dependent manner. ChIP analysis of an EcR target gene promoter revealed ligand-dependent recruitment of dPARP with poly(ADP-ribosyl)ation of histones in the EcR binding site and, surprisingly, also in a distal region of the promoter. Our results indicated that EcR-mediated gene regulation may be coupled with chromatin modification through poly(ADP-ribosyl)ation.

Poly(ADP-ribosyl)ation as a DNA damage-induced post-translational modification regulating poly(ADP-ribose) polymerase-1-topoisomerase I interaction

Poly(ADP-ribosyl)ation is a post-translational modification that occurs immediately after exposure of cells to DNA damaging agents. In vivo, 90% of ADP-ribose polymers are attached to the automodification domain of poly(ADP-ribose) polymerase-1 (PARP-1), the main enzyme catalyzing this modification reaction. This enzyme forms complexes with transcription initiation, DNA replication, and DNA repair factors. In most known cases, the interactions occur through the automodification domain. However, functional implications of the automodification reaction on these interactions have not yet been elucidated. In the present study, we created fluorescent protein-tagged PARP-1 to study this enzyme in live cells and focused on the interaction between PARP-1 and topoisomerase I (Topo I), one of the enzymes that interacts with PARP-1 in vitro. Here, we demonstrate that PARP-1 co-localizes with Topo I throughout the cell cycle. Results from bioluminescence resonance energy transfer assays suggest that the co-localization is because of a direct protein-protein interaction. In response to DNA damage, PARP-1 de-localization and a reduction in bioluminescence resonance energy transfer signal because of the automodification reaction are observed, suggesting that the automodification reaction results in the disruption of the interaction between PARP-1 and Topo I. Because Topo I activity has been reported to be promoted by PARP-1, we then investigated the effect of the disruption of this interaction on Topo I activity, and we found that this disruption results in the reduction of Topo I activity. These results suggest that a function for the automodification reaction is to regulate the interaction between PARP-1 and Topo I, and consequently, the Topo I activity, in response to DNA damage.

Poly(ADP-ribose) polymerase-1, a novel partner of progesterone receptors in endometrial cancer and its precursors

Endometrial carcinomas are the most common malignancy of the female genital tract. Although the downregulation of the progesterone receptor (PR) in the progression of endometrioid carcinomas (ECs) has been well documented, the mechanism of PR alteration in endometrioid carcinogenesis is poorly understood. Recently, biochemical studies have shown that the DNA strand break-sensing molecule poly(ADP-ribose) polymerase (PARP-1) was associated with the DNA binding domain of PR. In our present study, we show that in normal endometrial epithelium, the expression level of PARP-1 protein is high in the proliferative phase but markedly decreases during the secretory phase. Interestingly, PARP-1 expression gradually increases in nonatypical and atypical endometrial hyperplasia, reaching its highest level in grade I, and decreases significantly toward grade III ECs. Notably, PARP-1 and PR expressions, in each stage, are positively correlated (p < 0.0001), with the exception of nonendometrioid carcinomas. Thus, these data suggest that PARP-1 is substantially involved in the regulation of progesterone action in the development of ECs.

Changes in the activities and gene expressions of poly(ADP-ribose) glycohydrolases during the differentiation of human promyelocytic leukemia cell line HL-60

The metabolism of poly(ADP-ribose) is known to play important roles in the nuclear function of the mammalian cells. In this study, changes in the activities and gene expressions of poly(ADP-ribose) glycohydrolases (PARG) in HL-60 cells treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or a PARG inhibitor, tannic acid, were investigated. Nuclear PARG activities of HL-60 cells treated with TPA were reduced to 30-40% of the activity in untreated cells at 24 h, while PARG activities in the cytoplasm remained unchanged. The transient decrease in the nuclear PARG activity by TPA treatment was accompanied by differentiation as measured by the nitroblue tetrazolium (NBT) reducing activity and adhesion to the culture dishes. In the presence of H7, an inhibitor of protein kinase C (PKC), both the decrease in nuclear PARG activity and the induction of differentiation by TPA treatment were suppressed. On the other hand, treatment with tannic acid caused the nuclear PARG activity to decrease continuously while the NBT reducing activity increased, but no morphological differentiation to macrophage-like cells was apparent. In order to analyze PARG gene expression, we isolated the human PARG cDNA by the RT-PCR technique. RT-PCR analysis revealed that TPA treatment leads to a reduction in the PARG gene expression prior to the phenotypic expression of macrophage-like cell differentiation, which was diminished by the presence of H7. Also, PARG gene expression was reduced by tannic acid treatment. These results provide the first evidence that a transient decrease in nuclear PARG activity is important for the onset of differentiation of HL-60 cells to macrophage-like cells.

Inhibition of peroxisome proliferator-activated receptor α signaling by vitamin D receptor

Peroxisome proliferator-activated receptors (PPARs) are nuclear fatty acid receptors that have been implicated to play an important role in lipid and glucose homeostasis. PPARalpha potentiates fatty acid catabolism in the liver and is activated by the lipid-lowering fibrates, whereas PPARgamma is essential for adipocyte differentiation. Here we report that nuclear vitamin D(3) receptor (VDR) represses the transcriptional activity of PPARalpha but not PPARgamma in a 1,25(OH)(2)D(3)-dependent manner. The analysis using chimeric receptors revealed that ligand binding domain of PPARalpha and VDR was involved in the molecular basis of this functional interaction and that the DNA binding domain of VDR was not required for the suppression, suggesting a novel mechanism that might involve protein-protein interactions rather than a direct DNA binding. Furthermore, the treatment of rat hepatoma H4IIE cells with 1,25(OH)(2)D(3) diminishes the induction of AOX mRNA by PPARalpha ligands, Wy14,643. VDR signaling might be considered as a factor regulating lipid metabolism via PPARalpha pathway. We report here the novel action of VDR in controlling gene expression through PPARalpha signaling.

Expression of histone acetyltransferases was down-regulated in poly(ADP-ribose) polymerase-1-deficient murine cells

NF-kappaB-dependent, as well as human immunodeficiency virus type-1 (HIV-1) long terminal repeat (LTR)-dependent, reporter gene expression was significantly impaired in cells derived from poly(ADP-ribose) polymerase-1 (PARP-1)-knockout (PARP-1 -/-) mice. In addition, the level of protein acetylation was markedly lower in PARP-1 -/- cells than control (PARP-1 +/+) cells. Surprisingly, the expression levels of histone acetyltransferases (HATs), p300, cAMP response element-binding protein-binding protein (CBP), and p300/CBP-associated factor (PCAF), were significantly reduced in PARP-1 -/- cells, as compared with PARP-1 +/+ cells. These results suggest that PARP-1 is required for the proper expression of particular HATs. Since p300 and CBP are coactivators of NF-kappaB, we propose here that PARP-1 participates in NF-kappaB-dependent transcription by means of maintaining the expression of HATs.

PARP goes transcription

PARP-1, an enzyme that catalyzes the attachment of ADP ribose units to target proteins, plays at least two important roles in transcription regulation. First, PARP-1 modifies histones and creates an anionic poly(ADPribose) matrix that binds histones, thereby promoting the decondensation of higher-order chromatin structures. Second, PARP-1 acts as a component of enhancer/promoter regulatory complexes. Recent studies have shown that both of these activities are critical for gene regulation in vivo.