Integrated Deadenylase Genetic Association Network and Transcriptome Analysis in Thoracic Carcinomas.

The poly(A) tail at the 3' end of mRNAs determines their stability, translational efficiency, and fate. The shortening of the poly(A) tail, and its efficient removal, triggers the degradation of mRNAs, thus, regulating gene expression. The process is catalyzed by a family of enzymes, known as deadenylases. As the dysregulation of gene expression is a hallmark of cancer, understanding the role of deadenylases has gained additional interest. Herein, the genetic association network shows that CNOT6 and CNOT7 are the most prevalent and most interconnected nodes in the equilibrated diagram. Subsequent silencing and transcriptomic analysis identifies transcripts possibly regulated by specific deadenylases. Furthermore, several gene ontologies are enriched by common deregulated genes. Given the potential concerted action and overlapping functions of deadenylases, we examined the effect of silencing a deadenylase on the remaining ones. Our results suggest that specific deadenylases target unique subsets of mRNAs, whilst at the same time, multiple deadenylases may affect the same mRNAs with overlapping functions.

Core clock regulators in dexamethasone-treated HEK 293T cells at 4 h intervals.

OBJECTIVE: The study of the circadian clock and its mechanisms is easily facilitated through clock resetting in cell culture. Among the various established synchronizers of the circadian clock in cell culture (temperature, serum shock, glucocorticoids), the artificial glucocorticoid Dexamethasone (DEX) is the most widely used. DEX treatment as a protocol to reset the circadian clock in culture gives simple readout with minimal laboratory requirements. Even though there are many studies regarding clock resetting in culture using DEX, reference points or expression patterns of core clock genes and their protein products are scarce and sometimes contradict other works with similar methodology. We synchronise a cell line of human origin with DEX to be used for studies on circadian rhythms. RESULTS: We treat HEK 293T cells with DEX and describe the patterns of mRNA and proteins of core clock regulators, while making a clear point on how CLOCK is less than an ideal molecule to help monitor rhythms in this cell line.

Biochemical and in silico identification of the active site and the catalytic mechanism of the circadian deadenylase HESPERIN.

The 24-h molecular clock is based on the stability of rhythmically expressed transcripts. The shortening of the poly(A) tail of mRNAs is often the first and rate-limiting step that determines the lifespan of a mRNA and is catalyzed by deadenylases. Herein, we determine the catalytic site of Hesperin, a recently described circadian deadenylase in plants, using a modified site-directed mutagenesis protocol and a custom vector, pATHRA. To explore the catalytic efficiency of AtHESPERIN, we investigated the effect of AMP and neomycin, and used molecular modeling simulations to propose a catalytic mechanism. Collectively, the biochemical and in silico results classify AtHESPERIN in the exonuclease-endonuclease-phosphatase deadenylase superfamily and contribute to the understanding of the intricate mechanisms of circadian mRNA turnover.

ExoProK: A Practical Method for the Isolation of Small Extracellular Vesicles from Pleural Effusions.

Extracellular vesicles (EVs) are cell-secreted, lipid membrane-enclosed nanoparticles without functional nucleus. EV is a general term that includes various subtypes of particles named microvesicles, microparticles, ectosomes or exosomes. EVs transfer RNA, DNA and protein cargo between proximal and distant cells and tissues, thus constituting an organism-wide signal transduction network. Pathological tissues secrete EVs that differ in their cargo composition compared to their healthy counterparts. The detection of biomarkers in EVs from biological fluids may aid the diagnosis of disease and/or monitor its progression in a minimally invasive manner. Among biological fluids, pleural effusions (PEs) are integrated to clinical practice, as they accompany a wide variety of lung disorders. Due to the proximity with the pleura and the lungs, PEs are expected to be especially enriched in EVs that originate from diseased tissues. However, PEs are among the least studied biofluids regarding EV-specialized isolation methods and related biomarkers. Herein, we describe a practical EV isolation method from PEs for the screening of EV RNA biomarkers in clinical routine. It is based on a Proteinase K treatment step to digest contaminants prior to standard polyethylene-glycol precipitation. The efficiency of the method was confirmed by transmission electron microscopy, nanoparticle tracking analysis and Western blot. The reliability and sensitivity of the method towards the detection of EV-enriched RNA biomarkers from multiple PEs was also demonstrated.

Urine concentrations changes of cysteinyl leukotrienes in non-obese children with obstructive sleep apnea undergoing adenotonsillectomy.

OBJECTIVE: The main objective of the study was to compare preoperative to postoperative levels of urine-Cysteinyl leukotrienes (uCysLT) in children undergoing adenotonsillectomy (AT) for obstructive sleep apnea (OSA) in order to investigate whether exaggerated leukotriene activity is the cause or consequence of OSA. METHODS AND MATERIALS: A prospective study was conducted on non-obese children (4-10 years old) referred for overnight PSG. Children with moderate/severe OSA treated with AT were included. A second PSG study performed 2 months postoperatively to confirm OSA resolution, and those with residual OSA were excluded. Morning urine specimens after both PSG studies were obtained and pre-operative uCysLT levels were compared to postoperative levels. RESULTS: 27 children fulfilled the criteria and underwent a post-operative PSG study with three exclusions for residual OSA (postop-AHI>2), so the study group consisted of 24 children (mean age: 5.7 ±â€¯2.1 years). Mean preoperative and postoperative AHI was 10.96 ±â€¯5.93 and 1.44 ±â€¯0.56 respectively. Mean preop-uCysLT were 21.14 ±â€¯4.65, while after AT they significantly reduced to 12.62 ±â€¯2.67 (P < 0.01). CONCLUSION: uCysLT levels are significantly reduced after AT in non-obese children with moderate/severe OSA, suggesting that exaggerated leukotriene activity is mainly a consequence of OSA.

Tales around the clock: Poly(A) tails in circadian gene expression.

Circadian rhythms are ubiquitous time-keeping processes in eukaryotes with a period of ~24 hr. Light is perhaps the main environmental cue (zeitgeber) that affects several aspects of physiology and behaviour, such as sleep/wake cycles, orientation of birds and bees, and leaf movements in plants. Temperature can serve as the main zeitgeber in the absence of light cycles, even though it does not lead to rhythmicity through the same mechanism as light. Additional cues include feeding patterns, humidity, and social rhythms. At the molecular level, a master oscillator orchestrates circadian rhythms and organizes molecular clocks located in most cells. The generation of the 24 hr molecular clock is based on transcriptional regulation, as it drives intrinsic rhythmic changes based on interlocked transcription/translation feedback loops that synchronize expression of genes. Thus, processes and factors that determine rhythmic gene expression are important to understand circadian rhythms. Among these, the poly(A) tails of RNAs play key roles in their stability, translational efficiency and degradation. In this article, we summarize current knowledge and discuss perspectives on the role and significance of poly(A) tails and associating factors in the context of the circadian clock. This article is categorized under: RNA Turnover and Surveillance > Regulation of RNA Stability RNA Processing > 3' End Processing.

Proteomic Analysis of Human Angiogenin Interactions Reveals Cytoplasmic PCNA as a Putative Binding Partner.

Human Angiogenin (hAng) is a member of the ribonuclease A superfamily and a potent inducer of neovascularization. Protein interactions of hAng in the nucleus and cytoplasm of the human umbilical vein cell line EA.hy926 have been investigated by mass spectroscopy. Data are available via ProteomeXchange with identifiers PXD006583 and PXD006584. The first gel-free analysis of hAng immunoprecipitates revealed many statistically significant potential hAng-interacting proteins involved in crucial biological pathways. Surprisingly, proliferating cell nuclear antigen (PCNA), was found to be immunoprecipitated with hAng only in the cytoplasm. The hAng-PCNA interaction and colocalization in the specific cellular compartment was validated with immunoprecipitation, immunoblotting, and immunocytochemistry. The results revealed that PCNA is predominantly localized in the cytoplasm, while hAng is distributed both in the nucleus and in the cytoplasm. hAng and PCNA colocalize in the cytoplasm, suggesting that they may interact in this compartment.

The ammonium sulfate inhibition of human angiogenin.

In this study, we investigate the inhibition of human angiogenin by ammonium sulfate. The inhibitory potency of ammonium sulfate for human angiogenin (IC50 = 123.5 ± 14.9 mm) is comparable to that previously reported for RNase A (119.0 ± 6.5 mm) and RNase 2 (95.7 ± 9.3 mm). However, analysis of two X-ray crystal structures of human angiogenin in complex with sulfate anions (in acidic and basic pH environments, respectively) indicates an entirely distinct mechanism of inhibition. While ammonium sulfate inhibits the ribonucleolytic activity of RNase A and RNase 2 by binding to the active site of these enzymes, sulfate anions bind only to peripheral substrate anion-binding subsites of human angiogenin, and not to the active site.

AtHESPERIN: a novel regulator of circadian rhythms with poly(A)-degrading activity in plants.

We report the identification and characterization of a novel gene, AtHesperin (AtHESP) that codes for a deadenylase in Arabidopsis thaliana. The gene is under circadian clock-gene regulation and has similarity to the mammalian Nocturnin. AtHESP can efficiently degrade poly(A) substrates exhibiting allosteric kinetics. Size exclusion chromatography and native electrophoresis coupled with kinetic analysis support that the native enzyme is oligomeric with at least 3 binding sites. Knockdown and overexpression of AtHESP in plant lines affects the expression and rhythmicity of the clock core oscillator genes TOC1 and CCA1. This study demonstrates an evolutionary conserved poly(A)-degrading activity in plants and suggests deadenylation as a mechanism involved in the regulation of the circadian clock. A role of AtHESP in stress response in plants is also depicted.

Triazole double-headed ribonucleosides as inhibitors of eosinophil derived neurotoxin.

Eosinophil derived neurotoxin (EDN) is an eosinophil secretion protein and a member of the Ribonuclease A (RNase A) superfamily involved in the immune response system and inflammatory disorders. The pathological actions of EDN are strongly dependent on the enzymatic activity and therefore, it is of significant interest to discover potent and specific inhibitors of EDN. In this framework we have assessed the inhibitory potency of triazole double-headed ribonucleosides. We present here an efficient method for the heterologous production and purification of EDN together with the synthesis of nucleosides and their biochemical evaluation in RNase A and EDN. Two groups of double-headed nucleosides were synthesized by the attachment of a purine or a pyrimidine base, through a triazole group at the 3'-C position of a pyrimidine or a purine ribonucleoside, respectively. Based on previous data with mononucleosides these compounds were expected to improve the inhibitory potency for RNase A and specificity for EDN. Kinetics data revealed that despite the rational, all but one, double-headed ribonucleosides were less potent than the respective mononucleosides while they were also more specific for ribonuclease A than for EDN. Compound 11c (9-[3'-[4-[(cytosine-1-yl)methyl]-1,2,3-triazol-1-yl]-ß-d-ribofuranosyl]adenine) displayed a stronger preference for EDN than for ribonuclease A and a Ki value of 58µM. This is the first time that an inhibitor is reported to have a better potency for EDN than for RNase A. The crystal structure of EDN-11c complex reveals the structural basis of its potency and selectivity providing important guidelines for future structure-based inhibitor design efforts.

Poly(A)-specific ribonuclease and Nocturnin in squamous cell lung cancer: prognostic value and impact on gene expression.

BACKGROUND: Lung cancer is the leading cause of cancer mortality worldwide, mainly due to late diagnosis, poor prognosis and tumor heterogeneity. Thus, the need for biomarkers that will aid classification, treatment and monitoring remains intense and challenging and depends on the better understanding of the tumor pathobiology and underlying mechanisms. The deregulation of gene expression is a hallmark of cancer and a critical parameter is the stability of mRNAs that may lead to increased oncogene and/or decreased tumor suppressor transcript and protein levels. The shortening of mRNA poly(A) tails determines mRNA stability, as it is usually the first step in mRNA degradation, and is catalyzed by deadenylases. Herein, we assess the clinical significance of deadenylases and we study their role on gene expression in squamous cell lung carcinoma (SCC). METHODS: Computational transcriptomic analysis from a publicly available microarray was performed in order to examine the expression of deadenylases in SCC patient samples. Subsequently we employed real-time PCR in clinical samples in order to validate the bioinformatics results regarding the gene expression of deadenylases. Selected deadenylases were silenced in NCI-H520 and Hep2 human cancer cell lines and the effect on gene expression was analyzed with cDNA microarrays. RESULTS: The in silico analysis revealed that the expression of several deadenylases is altered in SCC. Quantitative real-time PCR showed that four deadenylases, PARN, CNOT6, CNOT7 and NOC, are differentially expressed in our SCC clinical samples. PARN overexpression correlated with younger patient age and CNOT6 overexpression with non-metastatic tumors. Kaplan-Meier analysis suggests that increased levels of PARN and NOC correlate with significantly increased survival. Gene expression analysis upon PARN and NOC silencing in lung cancer cells revealed gene expression deregulation that was functionally enriched for gene ontologies related to cell adhesion, cell junction, muscle contraction and metabolism. CONCLUSIONS: Our results highlight the clinical significance of PARN and NOC on the survival in SCC diagnosed patients. We demonstrate that the enzymes are implicated in important phenotypes pertinent to cancer biology and provide information on their role in the regulation of gene expression in SCC. Overall, our results support an emerging role for deadenylases in SCC and contribute to the understanding of their role in cancer biology.

Expression of leukotriene biosynthetic enzymes in tonsillar tissue of children with obstructive sleep apnea: a prospective nonrandomized study.

IMPORTANCE: Cysteinyl leukotrienes (CysLTs) potentially promote adenotonsillar hypertrophy in children with obstructive sleep apnea (OSA). Previous studies have identified CysLTs and their receptors in tonsillar tissue from children with OSA. OBJECTIVE: To demonstrate expression of the leukotriene biosynthetic enzymes 5-lipoxygenase (5-LO), 5-lipoxygenase activating protein (FLAP), leukotriene A(4) hydrolase (LTA(4)H), and leukotriene C(4) synthase (LTC(4)S) in T and B tonsillar lymphocytes from pediatric patients with OSA. It was hypothesized that children with OSA have greater expression of biosynthetic enzymes for CysLTs (5-LO, FLAP, and LTC(4)S) in their tonsillar tissue than do children with recurrent tonsillitis (RT), who were enrolled as controls. DESIGN, SETTING, AND PARTICIPANTS: This prospective, nonrandomized study was performed at a tertiary care university hospital on 13 children with OSA and adenotonsillar hypertrophy undergoing adenotonsillectomy and 12 children without OSA also undergoing tonsillectomy for RT. Tonsillar tissue from children with OSA or RT was examined for 5-LO, FLAP, LTA(4)H, and LTC(4)S expression under real time-quantitative polymerase chain reaction (RT-qPCR), flow cytometry (FC), and confocal laser scanning microscopy (CM). MAIN OUTCOMES AND MEASURES: Expression of biosynthetic enzymes for CysLTs (5-LO, FLAP, and LTC(4)S) was the main outcome measure. Patients with OSA and control patients with RT were compared for numbers of copies of 5-LO, FLAP, and LTC(4)S messenger RNA (by RT-qPCR) in T or B tonsillar lymphocytes and proportions of CD3(+) or CD19(+) tonsillar lymphocytes that expressed 5-LO, FLAP, and LTC(4)S (by FC). RESULTS: Messenger RNA for all 4 enzymes was detected in T and B lymphocytes from both study groups, and expression of all biosynthetic enzymes was demonstrated in participants with OSA and RT by FC. Patients with OSA differed from controls in the proportions (median [10th-90th percentile]) of LTC(4)S(+) CD3(+) T lymphocytes (23.31% [8.64%-50.07%] vs 10.81% [3.48%-23.32%], respectively) (P = .01) and LTC(4)S(+) CD19(+) B lymphocytes (20.66% [14.62%-65.77%] vs 12.53% [2.87%-36.64%], respectively) (P = .01) detected by FC. Immunoreactivity for the 4 enzymes was detected by CM in B lymphocytes of mantle zones and T lymphocytes of extrafollicular areas. CONCLUSIONS AND RELEVANCE: Leukotriene biosynthetic enzymes are expressed in tonsillar lymphocytes, and the previously reported detection of CysLTs in tonsillar tissue from children with OSA may be attributed to endogenous synthesis. Enhanced expression of LTC(4)S is a potential target for pharmacologic interventions in OSA.

A comprehensive phylogenetic analysis of deadenylases.

Deadenylases catalyze the shortening of the poly(A) tail at the messenger ribonucleic acid (mRNA) 3'-end in eukaryotes. Therefore, these enzymes influence mRNA decay, and constitute a major emerging group of promising anti-cancer pharmacological targets. Herein, we conducted full phylogenetic analyses of the deadenylase homologs in all available genomes in an effort to investigate evolutionary relationships between the deadenylase families and to identify invariant residues, which probably play key roles in the function of deadenylation across species. Our study includes both major Asp-Glu-Asp-Asp (DEDD) and exonuclease-endonuclease-phospatase (EEP) deadenylase superfamilies. The phylogenetic analysis has provided us with important information regarding conserved and invariant deadenylase amino acids across species. Knowledge of the phylogenetic properties and evolution of the domain of deadenylases provides the foundation for the targeted drug design in the pharmaceutical industry and modern exonuclease anti-cancer scientific research.

An integrated in silico approach to design specific inhibitors targeting human poly(a)-specific ribonuclease.

Poly(A)-specific ribonuclease (PARN) is an exoribonuclease/deadenylase that degrades 3'-end poly(A) tails in almost all eukaryotic organisms. Much of the biochemical and structural information on PARN comes from the human enzyme. However, the existence of PARN all along the eukaryotic evolutionary ladder requires further and thorough investigation. Although the complete structure of the full-length human PARN, as well as several aspects of the catalytic mechanism still remain elusive, many previous studies indicate that PARN can be used as potent and promising anti-cancer target. In the present study, we attempt to complement the existing structural information on PARN with in-depth bioinformatics analyses, in order to get a hologram of the molecular evolution of PARNs active site. In an effort to draw an outline, which allows specific drug design targeting PARN, an unequivocally specific platform was designed for the development of selective modulators focusing on the unique structural and catalytic features of the enzyme. Extensive phylogenetic analysis based on all the publicly available genomes indicated a broad distribution for PARN across eukaryotic species and revealed structurally important amino acids which could be assigned as potentially strong contributors to the regulation of the catalytic mechanism of PARN. Based on the above, we propose a comprehensive in silico model for the PARN's catalytic mechanism and moreover, we developed a 3D pharmacophore model, which was subsequently used for the introduction of DNP-poly(A) amphipathic substrate analog as a potential inhibitor of PARN. Indeed, biochemical analysis revealed that DNP-poly(A) inhibits PARN competitively. Our approach provides an efficient integrated platform for the rational design of pharmacophore models as well as novel modulators of PARN with therapeutic potential.

Triazole pyrimidine nucleosides as inhibitors of Ribonuclease A. Synthesis, biochemical, and structural evaluation.

Five ribofuranosyl pyrimidine nucleosides and their corresponding 1,2,3-triazole derivatives have been synthesized and characterized. Their inhibitory action to Ribonuclease A has been studied by biochemical analysis and X-ray crystallography. These compounds are potent competitive inhibitors of RNase A with low µM inhibition constant (K(i)) values with the ones having a triazolo linker being more potent than the ones without. The most potent of these is 1-[(ß-D-ribofuranosyl)-1,2,3-triazol-4-yl]uracil being with K(i) = 1.6 µM. The high resolution X-ray crystal structures of the RNase A in complex with three most potent inhibitors of these inhibitors have shown that they bind at the enzyme catalytic cleft with the pyrimidine nucleobase at the B(1) subsite while the triazole moiety binds at the main subsite P(1), where P-O5' bond cleavage occurs, and the ribose at the interface between subsites P(1) and P(0) exploiting interactions with residues from both subsites. The effect of a susbsituent group at the 5-pyrimidine position at the inhibitory potency has been also examined and results show that any addition at this position leads to a less efficient inhibitor. Comparative structural analysis of these RNase A complexes with other similar RNase A-ligand complexes reveals that the triazole moiety interactions with the protein form the structural basis of their increased potency. The insertion of a triazole linker between the pyrimidine base and the ribose forms the starting point for further improvement of these inhibitors in the quest for potent ribonucleolytic inhibitors with pharmaceutical potential.

Cysteinyl leukotriene receptors in tonsillar B- and T-lymphocytes from children with obstructive sleep apnea.

OBJECTIVES: Cysteinyl leukotrienes have been implicated in the pathogenesis of adenotonsillar hypertrophy in children with obstructive sleep apnea (OSA). This study aimed to quantify the expression of cysteinyl leukotriene receptors (CysLT(1), CysLT(2)) by tonsillar lymphocyte subpopulations from children with OSA and to make comparisons to lymphocyte subpopulations from control subjects with recurrent tonsillitis (RT). METHODS: Tonsillar tissue from children with OSA or RT was studied for CysLT(1) and CysLT(2) expression by RT-PCR, flow cytometry (FC), and immunofluorescence. RESULTS: Ten children with OSA and 10 control subjects were recruited. In OSA participants, CysLT(1)+ fraction of small-size CD19+ B-lymphocytes was similar to the CysLT(1)+ CD3+ T-lymphocytes fraction (FC: 36.5 [16.5-55.4] vs. 14 [2.8-22.1]) (p>0.05) and higher than the CysLT(1)+ moderate/large-size CD19+ B-lymphocytes fraction (6.6 [1.5-14.4]) (p<0.01). Similar trends were recognized for CysLT(2). CysLT(1) and CysLT(2) immunoreactivity was detected by immunofluorescence in the tonsillar mantle zones (small B-lymphocytes) and the extrafollicular areas (T-lymphocytes). Compared to subjects with RT, children with OSA had significantly higher expression of CysLT(1) in small-size CD19+ B-lymphocytes (FC) and in CD3+ T-lymphocytes (RT-PCR and FC) (p<0.05). CONCLUSIONS: Increased expression of leukotriene receptors by immunologically active tonsillar areas in children with OSA is a potential therapeutic target for pediatric sleep apnea.

Alterations of deadenylase expression in acute leukemias: evidence for poly(a)-specific ribonuclease as a potential biomarker.

BACKGROUND/AIMS: The degradation of mRNA is a key process in the control of gene expression correlated to anomalous cell proliferation. The rate-limiting step of mRNA degradation is the removal of the poly(A) tail by deadenylases. However, studies on deadenylase expression in cancer are limited. Herein, we analyzed the expression of several deadenylases from acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). METHODS: Clinical samples from patients diagnosed with ALL and AML were the source of leukemic cells. Extracts from leukemic and control cells were analyzed for deadenylase mRNA levels using qRT-PCR, and the protein levels of PARN and CNOT7 deadenylases using immunoblotting. RESULTS: RT-PCR analysis revealed altered expression for CNOT6, CNOT6L, CNOT7 and PARN deadenylases. The most significant alterations were observed for PARN and CNOT7 mRNA levels, which also reflect on the cognate protein level. Further analysis revealed that a significant amount of PARN is phosphorylated in ALL. CONCLUSIONS: We show that the expression of several deadenylases in acute leukemias is altered. The increase of PARN expression and the alteration of its phosphorylation status indicate important regulatory events. These data suggest that the role of deadenylases as auxiliary biomarkers and therapeutic targets should be meticulously investigated.

Competitive inhibition of human poly(A)-specific ribonuclease (PARN) by synthetic fluoro-pyranosyl nucleosides.

Poly(A)-specific ribonuclease (PARN) is a cap-interacting deadenylase that mediates, together with other exonucleases, the eukaryotic mRNA turnover and thus is actively involved in the regulation of gene expression. Aminoglycosides and natural nucleotides are the only reported modulators of human PARN activity, so far. In the present study, we show that synthetic nucleoside analogues bearing a fluoro-glucopyranosyl sugar moiety and benzoyl-modified cytosine or adenine as a base can effectively inhibit human PARN. Such nucleoside analogues exhibited substantial inhibitory effects, when tested against various cancer cell lines, as has been previously reported. Kinetic analysis showed that the inhibition of PARN is competitive and could not be released by altering Mg(II) concentration. Moreover, substitution of the 2', 4', or 6'-OH of the sugar moiety with acetyl and/or trityl groups was crucial for inhibitory efficacy. To understand how the nucleosides fit into the active site of PARN, we performed molecular docking experiments followed by molecular dynamics simulations. The in silico analysis showed that these compounds can efficiently dock into the active site of PARN. Our results support the idea that the sugar moiety mediates the stabilization of the nucleoside into the active site through interactions with catalytic amino acid residues. Taken together, our in vitro and in silico data suggest that human PARN is among the molecular targets of these compounds and could act therapeutically by lowering the mRNA turnover rate, thus explaining their known in vivo inhibitory effect at the molecular level.

Inhibition of human poly(A)-specific ribonuclease (PARN) by purine nucleotides: kinetic analysis.

Poly(A)-specific ribonuclease (PARN) is a cap-interacting and poly(A)-specific 3'-exoribonuclease that efficiently degrades mRNA poly(A) tails. Based on the enzyme's preference for its natural substrates, we examined the role of purine nucleotides as potent effectors of human PARN activity. We found that all purine nucleotides tested can reduce poly(A) degradation by PARN. Detailed kinetic analysis revealed that RTP nucleotides behave as non-competitive inhibitors while RDP and RMP exhibit competitive inhibition. Mg(2 + ) which is a catalytically important mediator of PARN activity can release inhibition of RTP and RDP but not RMP. Although many strategies have been proposed for the regulation of PARN activity, very little is known about the modulation of PARN activity by small molecule effectors, such as nucleotides. Our data imply that PARN activity can be modulated by purine nucleotides in vitro, providing an additional simple regulatory mechanism.