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

A PARP14/TARG1-Regulated RACK1 MARylation Cycle Drives Stress Granule Dynamics in Ovarian Cancer Cells

Mono(ADP-ribosyl)ation (MARylation) is emerging as a critical regulator of ribosome function and translation. Herein, we demonstrate that RACK1, an integral component of the ribosome, is MARylated on three acidic residues by the mono(ADP-ribosyl) transferase (MART) PARP14 in ovarian cancer cells. MARylation of RACK1 is required for stress granule formation and promotes the colocalization of RACK1 in stress granules with G3BP1, eIF3η, and 40S ribosomal proteins. In parallel, we observed reduced translation of a subset of mRNAs, including those encoding key cancer regulators (e.g., AKT). Treatment with a PARP14 inhibitor or mutation of the sites of MARylation on RACK1 blocks these outcomes, as well as the growth of ovarian cancer cells in culture and in vivo. To re-set the system after prolonged stress and recovery, the ADP-ribosyl hydrolase TARG1 deMARylates RACK1, leading to the dissociation of the stress granules and the restoration of translation. Collectively, our results demonstrate a therapeutically targetable pathway that controls stress granule assembly and disassembly in ovarian cancer cells.

ADP-ribosylome analysis reveals homogeneous DNA-damage-induced serine ADP-ribosylation across wild-type and BRCA-mutant breast cancer cell lines

ADP-ribosylation (ADPr) signaling plays a crucial role in DNA damage response. Inhibitors against the main enzyme catalyzing ADPr after DNA damage, poly(ADP-ribose) polymerase 1 (PARP1), are used to treat patients with breast cancer harboring BRCA1/2 mutations. However, resistance to PARP inhibitors (PARPi) is a major obstacle in treating patients. To understand the role of ADPr in PARPi sensitivity, we use liquid chromatography-tandem mass spectrometry (LC-MS/MS) to analyze ADPr in six breast cancer cell lines exhibiting different PARPi sensitivities. We identify 1,632 sites on 777 proteins across all cell lines, primarily on serine residues, with site-specific overlap of targeted residues across DNA-damage-related proteins across all cell lines, demonstrating high conservation of serine ADPr-signaling networks upon DNA damage. Furthermore, we observe site-specific differences in ADPr intensities in PARPi-sensitive BRCA mutants and unique ADPr sites in PARPi-resistant BRCA-mutant HCC1937 cells, which have low poly(ADP-ribose) glycohydrolase (PARG) levels and longer ADPr chains on PARP1.

Preserving ester-linked modifications reveals glutamate and aspartate mono-ADP-ribosylation by PARP1 and its reversal by PARG

Ester-linked post-translational modifications, including serine and threonine ubiquitination, have gained recognition as important cellular signals. However, their detection remains a significant challenge due to the chemical lability of the ester bond. This is the case even for long-known modifications, such as ADP-ribosylation on aspartate and glutamate, whose role in PARP1 signaling has recently been questioned. Here, we present easily implementable methods for preserving ester-linked modifications. When combined with a specific and sensitive modular antibody and mass spectrometry, these approaches reveal DNA damage-induced aspartate/glutamate mono-ADP-ribosylation. This previously elusive signal represents an initial wave of PARP1 signaling, contrasting with the more enduring nature of serine mono-ADP-ribosylation. Unexpectedly, we show that the poly-ADP-ribose hydrolase PARG is capable of reversing ester-linked mono-ADP-ribosylation in cells. Our methodology enables broad investigations of various ADP-ribosylation writers and, as illustrated here for noncanonical ubiquitination, it paves the way for exploring other emerging ester-linked modifications.

Uncommon posttranslational modifications in proteomics: ADP-ribosylation, tyrosine nitration, and tyrosine sulfation

Posttranslational modifications (PTMs) are covalent modifications of proteins that modulate the structure and functions of proteins and regulate biological processes. The development of various mass spectrometry-based proteomics workflows has facilitated the identification of hundreds of PTMs and aided the understanding of biological significance in a high throughput manner. Improvements in sample preparation and PTM enrichment techniques, instrumentation for liquid chromatography-tandem mass spectrometry (LC-MS/MS), and advanced data analysis tools enhance the specificity and sensitivity of PTM identification. Highly prevalent PTMs like phosphorylation, glycosylation, acetylation, ubiquitinylation, and methylation are extensively studied. However, the functions and impact of less abundant PTMs are not as well understood and underscore the need for analytical methods that aim to characterize these PTMs. This review focuses on the advancement and analytical challenges associated with the characterization of three less common but biologically relevant PTMs, specifically, adenosine diphosphate-ribosylation, tyrosine sulfation, and tyrosine nitration. The advantages and disadvantages of various enrichment, separation, and MS/MS techniques utilized to identify and localize these PTMs are described.

PARP1 at the crossroad of cellular senescence and nucleolar processes

Senescent cells that occur in response to telomere shortening, oncogenes, extracellular and intracellular stress factors are characterized by permanent cell cycle arrest, the morphological and structural changes of the cell that include the senescence-associated secretory phenotype (SASP) and nucleoli rearrangement. The associated DNA lesions induce DNA damage response (DDR), which activates the DNA repair protein - poly-ADP-ribose polymerase 1 (PARP1). This protein consumes NAD+ to synthesize ADP-ribose polymer (PAR) on its own protein chain and on other interacting proteins. The involvement of PARP1 in nucleoli processes, such as rRNA transcription and ribosome biogenesis, the maintenance of heterochromatin and nucleoli structure, as well as controlling the crucial DDR protein release from the nucleoli to nucleus, links PARP1 with cellular senescence and nucleoli functioning. In this review we describe and discuss the impact of PARP1-mediated ADP-ribosylation on early cell commitment to senescence with the possible role of senescence-induced PARP1 transcriptional repression and protein degradation on nucleoli structure and function. The cause-effect interplay between PARP1 activation/decline and nucleoli functioning during senescence needs to be studied in detail.

Olaparib enhances sensitization of BRCA-proficient breast cancer cells to x-rays and protons

PURPOSE

This study aimed to compare the radiosensitizing effect of the PARP inhibitor, Olaparib, between proton and X-rays irradiations in BRCA-proficient breast cancer (BC) cells.

METHODS

Two BRCA-proficient BC cell lines, MDA-MB-231 and T47D BC, were used. Cell proliferation was assessed using the CCK-8 assay, and radiosensitivity was determined through the clonogenic survival assay. Flow cytometry was employed to analyze cell cycle distribution and apoptosis. The kinetics of DNA damage repair were evaluated using γH2AX immunofluorescence imaging and the comet assay. Tumor spheroid assays were conducted to test radiosensitivity in a three-dimensional culture condition.

RESULTS

Olaparib sensitized both MDA-MB-231 and T47D cells to proton and X-ray irradiation in the clonogenic assay. MDA-MB-231 cells exhibited a higher dose enhancement factor for Olaparib than T47D cells. Olaparib increased radiation-induced G2/M cell cycle arrest and apoptosis specifically in MDA-MB-231 cells. γH2AX immunostaining and the comet assay showed Olaparib augmented radiation-induced DNA damage and apoptosis. The enhancement effect of Olaparib was more pronounced in proton irradiation than in X-ray irradiation, particularly in MDA-MB-231 cells than T47D cells. Both radiation and Olaparib dose-dependently inhibited spheroid growth in both cell lines. The synergy scores demonstrated that Olaparib interacted more strongly with protons than X-rays. The addition of an ATR inhibitor further enhanced Olaparib-induced proton radiosensitization in MDA-MB-231 cells.

CONCLUSION

This study found that Olaparib enhanced radiation efficacy in BRCA-proficient breast cancer cells, with a more pronounced effect observed with proton irradiation compared to X-ray irradiation. Combining Olaparib with an ATR inhibitor increased the radiosensitizing effect of protons.

PARP10 is Critical for Stress Granule Initiation

Stress granules (SGs) are cytoplasmic biomolecular condensates enriched with RNA, translation factors, and other proteins. They form in response to stress and are implicated in various diseased states including viral infection, tumorigenesis, and neurodegeneration. Understanding the mechanism of SG assembly, particularly its initiation, offers potential therapeutic avenues. Although ADP-ribosylation plays a key role in SG assembly, and one of its key forms-poly(ADP-ribose) or PAR-is critical for recruiting proteins to SGs, the specific enzyme responsible remains unidentified. Here, we systematically knock down the human ADP-ribosyltransferase family and identify PARP10 as pivotal for SG assembly. Live-cell imaging reveals PARP10's crucial role in regulating initial assembly kinetics. Further, we pinpoint the core SG component, G3BP1, as a PARP10 substrate and find that PARP10 regulates SG assembly driven by both G3BP1 and its modeled mechanism. Intriguingly, while PARP10 only adds a single ADP-ribose unit to proteins, G3BP1 is PARylated, suggesting its potential role as a scaffold for protein recruitment. PARP10 knockdown alters the SG core composition, notably decreasing translation factor presence. Based on our findings, we propose a model in which ADP-ribosylation acts as a rate-limiting step, initiating the formation of this RNA-enriched condensate.

The dynamic process of covalent and non-covalent PARylation in the maintenance of genome integrity: a focus on PARP inhibitors

Poly(ADP-ribosylation) (PARylation) by poly(ADP-ribose) polymerases (PARPs) is a highly regulated process that consists of the covalent addition of polymers of ADP-ribose (PAR) through post-translational modifications of substrate proteins or non-covalent interactions with PAR via PAR binding domains and motifs, thereby reprogramming their functions. This modification is particularly known for its central role in the maintenance of genomic stability. However, how genomic integrity is controlled by an intricate interplay of covalent PARylation and non-covalent PAR binding remains largely unknown. Of importance, PARylation has caught recent attention for providing a mechanistic basis of synthetic lethality involving PARP inhibitors (PARPi), most notably in homologous recombination (HR)-deficient breast and ovarian tumors. The molecular mechanisms responsible for the anti-cancer effect of PARPi are thought to implicate both catalytic inhibition and trapping of PARP enzymes on DNA. However, the relative contribution of each on tumor-specific cytotoxicity is still unclear. It is paramount to understand these PAR-dependent mechanisms, given that resistance to PARPi is a challenge in the clinic. Deciphering the complex interplay between covalent PARylation and non-covalent PAR binding and defining how PARP trapping and non-trapping events contribute to PARPi anti-tumour activity is essential for developing improved therapeutic strategies. With this perspective, we review the current understanding of PARylation biology in the context of the DNA damage response (DDR) and the mechanisms underlying PARPi activity and resistance.

Impact of Asp/Glu-ADP-ribosylation on protein-protein interaction and protein function

PARylation plays critical role in regulating multiple cellular processes such as DNA damage response and repair, transcription, RNA processing, and stress response. More than 300 human proteins have been found to be modified by PARylation on acidic residues, that is, Asp (D) and Glu (E). We used the deep-learning tool AlphaFold to predict protein-protein interactions (PPIs) and their interfaces for these proteins based on coevolution signals from joint multiple sequence alignments (MSAs). AlphaFold predicted 260 confident PPIs involving PARylated proteins, and about one quarter of these PPIs have D/E-PARylation sites in their predicted PPI interfaces. AlphaFold predictions offer novel insights into the mechanisms of PARylation regulations by providing structural details of the PPI interfaces. D/E-PARylation sites have a preference to occur in coil regions and disordered regions, and PPI interfaces containing D/E-PARylation sites tend to occur between short linear sequence motifs in disordered regions and globular domains. The hub protein PCNA is predicted to interact with more than 20 proteins via the common PIP box motif and the structurally variable flanking regions. D/E-PARylation sites were found in the interfaces of key components of the RNA transcription and export complex, the SF3a spliceosome complex, and H/ACA and C/D small nucleolar ribonucleoprotein complexes, suggesting that systematic PARylation have a profound effect in regulating multiple RNA-related processes such as RNA nuclear export, splicing, and modification. Finally, PARylation of SUMO2 could modulate its interaction with CHAF1A, thereby representing a potential mechanism for the cross-talk between PARylation and SUMOylation in regulation of chromatin remodeling.

Pyruvate Kinase M (PKM) binds ribosomes in a poly-ADP ribosylation dependent manner to induce translational stalling

In light of the numerous studies identifying post-transcriptional regulators on the surface of the endoplasmic reticulum (ER), we asked whether there are factors that regulate compartment specific mRNA translation in human cells. Using a proteomic survey of spatially regulated polysome interacting proteins, we identified the glycolytic enzyme Pyruvate Kinase M (PKM) as a cytosolic (i.e. ER-excluded) polysome interactor and investigated how it influences mRNA translation. We discovered that the PKM-polysome interaction is directly regulated by ADP levels-providing a link between carbohydrate metabolism and mRNA translation. By performing enhanced crosslinking immunoprecipitation-sequencing (eCLIP-seq), we found that PKM crosslinks to mRNA sequences that are immediately downstream of regions that encode lysine- and glutamate-enriched tracts. Using ribosome footprint protection sequencing, we found that PKM binding to ribosomes causes translational stalling near lysine and glutamate encoding sequences. Lastly, we observed that PKM recruitment to polysomes is dependent on poly-ADP ribosylation activity (PARylation)-and may depend on co-translational PARylation of lysine and glutamate residues of nascent polypeptide chains. Overall, our study uncovers a novel role for PKM in post-transcriptional gene regulation, linking cellular metabolism and mRNA translation.

PARPs and ADP-ribosylation: Deciphering the complexity with molecular tools

PARPs catalyze ADP-ribosylation-a post-translational modification that plays crucial roles in biological processes, including DNA repair, transcription, immune regulation, and condensate formation. ADP-ribosylation can be added to a wide range of amino acids with varying lengths and chemical structures, making it a complex and diverse modification. Despite this complexity, significant progress has been made in developing chemical biology methods to analyze ADP-ribosylated molecules and their binding proteins on a proteome-wide scale. Additionally, high-throughput assays have been developed to measure the activity of enzymes that add or remove ADP-ribosylation, leading to the development of inhibitors and new avenues for therapy. Real-time monitoring of ADP-ribosylation dynamics can be achieved using genetically encoded reporters, and next-generation detection reagents have improved the precision of immunoassays for specific forms of ADP-ribosylation. Further development and refinement of these tools will continue to advance our understanding of the functions and mechanisms of ADP-ribosylation in health and disease.

A PARP1 PROTAC as a novel strategy against PARP inhibitor resistance via promotion of ferroptosis in p53-positive breast cancer

Therapeutic targeting of the nuclear enzyme poly (ADP-ribose) polymerase 1 (PARP1) with PARP inhibitors (PARPis) in patients with a homologous recombination (HR)- deficient phenotype based on the mechanism of synthetic lethality has been shown tremendous success in cancer therapy. With the clinical use of various PARPis, emerging evidence has shown that some PARPis offer hope for breakthroughs in triple-negative breast cancer (TNBC) therapy, regardless of HR status. However, similar to other conventional cytotoxic drugs, PARPis are also subject to the intractable problem of drug resistance. Notably, acquired resistance to PARPis caused by point mutations in the PARP1 protein is hard to overcome with current strategies. To explore modalities to overcome resistance and identify patients who are most likely to benefit from PARP1-targeted therapy, we developed a proteolysis-targeted chimaera (PROTAC) to degrade mutant PARP1 in TNBC. Here, we investigated a PARP1 PROTAC termed "NN3″, which triggered ubiquitination and proteasome-mediated degradation of PARP1. Moreover, NN3 degraded PARP1 with resistance-related mutations. Interestingly, compared with other reported PARP1 degraders, NN3 exhibited a unique antitumor mechanism in p53-positive breast cancer cells that effectively promoted ferroptosis by downregulating the SLC7A11 pathway. Furthermore, NN3 showed potent activity and low toxicity in vivo. In conclusion, we propose PROTAC-mediated degradation of PARP1 as a novel strategy against mutation-related PARPi resistance and a paradigm for targeting breast cancer with functional p53 via ferroptosis induction.

Unconventional protein post-translational modifications: the helmsmen in breast cancer

Breast cancer is the most prevalent malignant tumor and a leading cause of mortality among females worldwide. The tumorigenesis and progression of breast cancer involve complex pathophysiological processes, which may be mediated by post-translational modifications (PTMs) of proteins, stimulated by various genes and signaling pathways. Studies into PTMs have long been dominated by the investigation of protein phosphorylation and histone epigenetic modifications. However, with great advances in proteomic techniques, several other PTMs, such as acetylation, glycosylation, sumoylation, methylation, ubiquitination, citrullination, and palmitoylation have been confirmed in breast cancer. Nevertheless, the mechanisms, effects, and inhibitors of these unconventional PTMs (particularly, the non-histone modifications other than phosphorylation) received comparatively little attention. Therefore, in this review, we illustrate the functions of these PTMs and highlight their impact on the oncogenesis and progression of breast cancer. Identification of novel potential therapeutic drugs targeting PTMs and development of biological markers for the detection of breast cancer would be significantly valuable for the efficient selection of therapeutic regimens and prediction of disease prognosis in patients with breast cancer.

The C-Terminal Domain of Y-Box Binding Protein 1 Exhibits Structure-Specific Binding to Poly(ADP-Ribose), Which Regulates PARP1 Activity

Y-box-binding protein 1 (YB-1) is a multifunctional protein involved in the regulation of gene expression. Recent studies showed that in addition to its role in the RNA and DNA metabolism, YB-1 is involved in the regulation of PARP1 activity, which catalyzes poly(ADP-ribose) [PAR] synthesis under genotoxic stress through auto-poly(ADP-ribosyl)ation or protein trans-poly(ADP-ribosyl)ation. Nonetheless, the exact mechanism by which YB-1 regulates PAR synthesis remains to be determined. YB-1 contains a disordered Ala/Pro-rich N-terminal domain, a cold shock domain, and an intrinsically disordered C-terminal domain (CTD) carrying four clusters of positively charged amino acid residues. Here, we examined the functional role of the disordered CTD of YB-1 in PAR binding and in the regulation of PARP1-driven PAR synthesis in vitro. We demonstrated that the rate of PARP1-dependent synthesis of PAR is higher in the presence of YB-1 and is tightly controlled by the interaction between YB-1 CTD and PAR. Moreover, YB-1 acts as an effective cofactor in the PAR synthesis catalyzed by the PARP1 point mutants that generate various PAR polymeric structures, namely, short hypo- or hyperbranched polymers. We showed that either a decrease in chain length or an increase in branching frequency of PAR affect its binding affinity for YB-1 and YB-1-mediated stimulation of PARP1 enzymatic activity. These results provide important insight into the mechanism underlying the regulation of PARP1 activity by PAR-binding proteins containing disordered regions with clusters of positively charged amino acid residues, suggesting that YB-1 CTD-like domains may be considered PAR "readers" just as other known PAR-binding modules.

Research Progress on Mono-ADP-Ribosyltransferases in Human Cell Biology

ADP-ribosylation is a well-established post-translational modification that is inherently connected to diverse processes, including DNA repair, transcription, and cell signaling. The crucial roles of mono-ADP-ribosyltransferases (mono-ARTs) in biological processes have been identified in recent years by the comprehensive use of genetic engineering, chemical genetics, and proteomics. This review provides an update on current methodological advances in the study of these modifiers. Furthermore, the review provides details on the function of mono ADP-ribosylation. Several mono-ARTs have been implicated in the development of cancer, and this review discusses the role and therapeutic potential of some mono-ARTs in cancer.

Pre-ribosomal RNA reorganizes DNA damage repair factors in nucleus during meiotic prophase and DNA damage response

In response to DNA double-strand breaks (DSBs), DNA damage repair factors are recruited to DNA lesions and form nuclear foci. However, the underlying molecular mechanism remains largely elusive. Here, by analyzing the localization of DSB repair factors in the XY body and DSB foci, we demonstrate that pre-ribosomal RNA (pre-rRNA) mediates the recruitment of DSB repair factors around DNA lesions. Pre-rRNA exists in the XY body, a DSB repair hub, during meiotic prophase, and colocalizes with DSB repair factors, such as MDC1, BRCA1 and TopBP1. Moreover, pre-rRNA-associated proteins and RNAs, such as ribosomal protein subunits, RNase MRP and snoRNAs, also localize in the XY body. Similar to those in the XY body, pre-rRNA and ribosomal proteins also localize at DSB foci and associate with DSB repair factors. RNA polymerase I inhibitor treatment that transiently suppresses transcription of rDNA but does not affect global protein translation abolishes foci formation of DSB repair factors as well as DSB repair. The FHA domain and PST repeats of MDC1 recognize pre-rRNA and mediate phase separation of DSB repair factors, which may be the molecular basis for the foci formation of DSB repair factors during DSB response.

Comparison of the Efficacy and Safety of PARP Inhibitors as a Monotherapy for Platinum-Sensitive Recurrent Ovarian Cancer: A Network Meta-Analysis

Background

The present COVID-19 pandemic has tended toward normality. To provide convenient, safe, and effective home treatment programs for patients with recurrent ovarian cancer (ROC), the clinical efficacy and safety of poly (ADP-ribose) polymerase inhibitor (PARPi) (including olaparib, niraparib, and rucaparib) monotherapy as a maintenance treatment for platinum-sensitive ROC were systematically evaluated.

Methods

Numerous electronic databases were systematically searched for randomized controlled trials (RCTs) of PARPi maintenance treatment for ROC that were published before June 2021. The primary endpoints were overall survival (OS) and progression-free survival (PFS), and the secondary endpoint was grade 3-4 adverse effects (AEs). After data extraction and the quality evaluation of the included studies, Bayesian network meta-analysis (NMA) was performed using R software. The ability of each treatment was ranked using the surface under the cumulative ranking (SUCRA) curve.

Results

The analysis included five studies and 1390 patients. The NMA results demonstrated that compared with the placebo, olaparib and niraparib exhibited significant benefits in the gBRCA-mutated population, and respectively reduced the risk of death by 31% (HR = 0.69, 95% CI: 0.53-0.90) and 34% (HR = 0.66, 95% CI: 0.44-0.99). Olaparib, niraparib, and rucaparib were all found to be very effective in prolonging PFS in patients with ROC. All three PARPi treatments increased the number of grade 3-4 AEs in patients with ROC as compared with the placebo.

Conclusions

Overall, olaparib and niraparib maintenance treatment can significantly prolong the OS of patients with gBRCA mutations. Furthermore, the three investigated PARPi monotherapy maintenance treatments can prolong PFS regardless of BRCA mutation status. Although the incidence of AEs in the treatment groups was found to be significantly higher than that in the placebo group, the patients in the treatment group tolerated the treatment. Home oral PARPi treatment can balance tumor treatment and pandemic prevention and control, and is the most convenient, safe, and effective home treatment method available against the background of the current COVID-19 pandemic.

Systematic Review Registration

https://inplasy.com/inplasy-2021-6-0033/.

Rapid Analysis of ADP-Ribosylation Dynamics and Site-Specificity Using TLC-MALDI

Poly(ADP-ribose) polymerases, PARPs, transfer ADP-ribose onto target proteins from nicotinamide adenine dinucleotide (NAD⁺). Current mass spectrometric analytical methods require proteolysis of target proteins, limiting the study of dynamic ADP-ribosylation on contiguous proteins. Herein, we present a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) method that facilitates multisite analysis of ADP-ribosylation. We observe divergent ADP-ribosylation dynamics for the catalytic domains of PARPs 14 and 15, with PARP15 modifying more sites on itself (+3–4 ADP-ribose) than the closely related PARP14 protein (+1–2 ADP-ribose)—despite similar numbers of potential modification sites. We identify, for the first time, a minimal peptide fragment (18 amino-acids) that is preferentially modified by PARP14. Finally, we demonstrate through mutagenesis and chemical treatment with hydroxylamine that PARPs 14/15 prefer acidic residues. Our results highlight the utility of MALDI-TOF in the analysis of PARP target modifications and in elucidating the biochemical mechanism governing PARP target selection.

Encounters in Three Dimensions: How Nuclear Topology Shapes Genome Integrity

Almost 25 years ago, the phosphorylation of a chromatin component, histone H2AX, was discovered as an integral part of the DNA damage response in eukaryotes. Much has been learned since then about the control of DNA repair in the context of chromatin. Recent technical and computational advances in imaging, biophysics and deep sequencing have led to unprecedented insight into nuclear organization, highlighting the impact of three-dimensional (3D) chromatin structure and nuclear topology on DNA repair. In this review, we will describe how DNA repair processes have adjusted to and in many cases adopted these organizational features to ensure accurate lesion repair. We focus on new findings that highlight the importance of chromatin context, topologically associated domains, phase separation and DNA break mobility for the establishment of repair-conducive nuclear environments. Finally, we address the consequences of aberrant 3D genome maintenance for genome instability and disease.

Reading ADP-ribosylation signaling using chemical biology and interaction proteomics

ADP-ribose (ADPr) readers are essential components of ADP-ribosylation signaling, which regulates genome maintenance and immunity. The identification and discrimination between monoADPr (MAR) and polyADPr (PAR) readers is difficult because of a lack of suitable affinity-enrichment reagents. We synthesized well-defined ADPr probes and used these for affinity purifications combined with relative and absolute quantitative mass spectrometry to generate proteome-wide MAR and PAR interactomes, including determination of apparent binding affinities. Among the main findings, MAR and PAR readers regulate various common and distinct processes, such as the DNA-damage response, cellular metabolism, RNA trafficking, and transcription. We monitored the dynamics of PAR interactions upon induction of oxidative DNA damage and uncovered the mechanistic connections between ubiquitin signaling and ADP-ribosylation. Taken together, chemical biology enables exploration of MAR and PAR readers using interaction proteomics. Furthermore, the generated MAR and PAR interaction maps significantly expand our current understanding of ADPr signaling.

The regulatory landscape of the human HPF1- and ARH3-dependent ADP-ribosylome

Despite the involvement of Poly(ADP-ribose) polymerase-1 (PARP1) in many important biological pathways, the target residues of PARP1-mediated ADP-ribosylation remain ambiguous. To explicate the ADP-ribosylation regulome, we analyze human cells depleted for key regulators of PARP1 activity, histone PARylation factor 1 (HPF1) and ADP-ribosylhydrolase 3 (ARH3). Using quantitative proteomics, we characterize 1,596 ADP-ribosylation sites, displaying up to 1000-fold regulation across the investigated knockout cells. We find that HPF1 and ARH3 inversely and homogenously regulate the serine ADP-ribosylome on a proteome-wide scale with consistent adherence to lysine-serine-motifs, suggesting that targeting is independent of HPF1 and ARH3. Notably, we do not detect an HPF1-dependent target residue switch from serine to glutamate/aspartate under the investigated conditions. Our data support the notion that serine ADP-ribosylation mainly exists as mono-ADP-ribosylation in cells, and reveal a remarkable degree of histone co-modification with serine ADP-ribosylation and other post-translational modifications.

PARP-1 Regulates Estrogen-Dependent Gene Expression in Estrogen Receptor α-Positive Breast Cancer Cells

Poly(ADP-ribose) polymerase-1 (PARP-1) has gained considerable attention as a target for therapeutic inhibitors in breast cancers. Previously we showed that PARP-1 localizes to active gene promoters to regulate histone methylation and RNA polymerase II activity (Pol II), altering the expression of various tumor-related genes. Here we report a role for PARP-1 in estrogen-dependent transcription in estrogen receptor alpha (ERα)-positive (ER+) breast cancers. Global nuclear run-on and sequencing analyses functionally linked PARP-1 to the direct control of estrogen-regulated gene expression in ER+ MCF-7 breast cancer cells by promoting transcriptional elongation by Pol II. Furthermore, chromatin immunoprecipitation sequencing analyses revealed that PARP-1 regulates the estrogen-dependent binding of ERα and FoxA1 to a subset of genomic ERα binding sites, promoting active enhancer formation. Moreover, we found that the expression levels of the PARP-1- and estrogen-coregulated gene set are enriched in the luminal subtype of breast cancer, and high PARP-1 expression in ER+ cases correlates with poor survival. Finally, treatment with a PARP inhibitor or a transcriptional elongation inhibitor attenuated estrogen-dependent growth of multiple ER+ breast cancer cell lines. Taken together, our results show that PARP-1 regulates critical molecular pathways that control the estrogen-dependent gene expression program underlying the proliferation of ER+ breast cancer cells. IMPLICATIONS: PARP-1 regulates the estrogen-dependent genomic binding of ERα and FoxA1 to regulate critical gene expression programs by RNA Pol II that underlie the proliferation of ER+ breast cancers, providing a potential therapeutic opportunity for PARP inhibitors in estrogen-responsive breast cancers.

AI26 inhibits the ADP-ribosylhydrolase ARH3 and suppresses DNA damage repair

The ADP-ribosylhydrolase ARH3 plays a key role in DNA damage repair, digesting poly(ADP-ribose) and removing ADP-ribose from serine residues of the substrates. Specific inhibitors that selectively target ARH3 would be a useful tool to examine DNA damage repair, as well as a possible strategy for tumor suppression. However, efforts to date have not identified any suitable compounds. Here, we used in silico and biochemistry screening to search for ARH3 inhibitors. We discovered a small molecule compound named ARH3 inhibitor 26 (AI26) as, to our knowledge, the first ARH3 inhibitor. AI26 binds to the catalytic pocket of ARH3 and inhibits the enzymatic activity of ARH3 with an estimated IC₅₀ of ∼2.41 μm in vitro Moreover, hydrolysis of DNA damage-induced ADP-ribosylation was clearly inhibited when cells were pretreated with AI26, leading to defects in DNA damage repair. In addition, tumor cells with DNA damage repair defects were hypersensitive to AI26 treatment, as well as combinations of AI26 and other DNA-damaging agents such as camptothecin and doxorubicin. Collectively, these results reveal not only a chemical probe to study ARH3-mediated DNA damage repair but also a chemotherapeutic strategy for tumor suppression.

Connections between 3' end processing and DNA damage response: Ten years later

Ten years ago we reviewed how the cellular DNA damage response (DDR) is controlled by changes in the functional and structural properties of nuclear proteins, resulting in a timely coordinated control of gene expression that allows DNA repair. Expression of genes that play a role in DDR is regulated not only at transcriptional level during mRNA biosynthesis but also by changing steady-state levels due to turnover of the transcripts. The 3' end processing machinery, which is important in the regulation of mRNA stability, is involved in these gene-specific responses to DNA damage. Here, we review the latest mechanistic connections described between 3' end processing and DDR, with a special emphasis on alternative polyadenylation, microRNA and RNA binding proteins-mediated deadenylation, and discuss the implications of deregulation of these steps in DDR and human disease. This article is categorized under: RNA Processing > 3' End Processing RNA-Based Catalysis > Miscellaneous RNA-Catalyzed Reactions RNA in Disease and Development > RNA in Disease.

PARPs and ADP-ribosylation in RNA biology: from RNA expression and processing to protein translation and proteostasis

In this review, Kim et al. discuss the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance.

ADP-ribosylation (ADPRylation) is a posttranslational modification of proteins discovered nearly six decades ago, but many important questions remain regarding its molecular functions and biological roles, as well as the activity of the ADP-ribose (ADPR) transferase enzymes (PARP family members) that catalyze it. Growing evidence indicates that PARP-mediated ADPRylation events are key regulators of the protein biosynthetic pathway, leading from rDNA transcription and ribosome biogenesis to mRNA synthesis, processing, and translation. In this review we describe the role of PARP proteins and ADPRylation in all facets of this pathway. PARP-1 and its enzymatic activity are key regulators of rDNA transcription, which is a critical step in ribosome biogenesis. An emerging role of PARPs in alternative splicing of mRNAs, as well as direct ADPRylation of mRNAs, highlight the role of PARP members in RNA processing. Furthermore, PARP activity, stimulated by cellular stresses, such as viral infections and ER stress, leads to the regulation of mRNA stability and protein synthesis through posttranscriptional mechanisms. Dysregulation of PARP activity in these processes can promote disease states. Collectively, these results highlight the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance. Future studies in these areas will yield new insights into the fundamental mechanisms and a broader utility for PARP-targeted therapeutic agents.

Systems-wide Analysis of Serine ADP-Ribosylation Reveals Widespread Occurrence and Site-Specific Overlap with Phosphorylation

ADP-ribosylation (ADPr) is a reversible posttranslational modification involved in a range of cellular processes. Here, we report system-wide identification of serine ADPr in human cells upon oxidative stress. High-resolution mass spectrometry and unrestricted data processing confirm that serine residues are the major target of ADPr in HeLa cells. Proteome-wide analysis identifies 3,090 serine ADPr sites, with 97% of acceptor sites modulating more than 2-fold upon oxidative stress, while treatment with the poly (ADP-ribose) polymerase (PARP) inhibitor olaparib abrogates this induction. Serine ADPr predominantly targets nuclear proteins, while structural-predictive analyses reveal that serine ADPr preferentially targets disordered protein regions. The identified ADP-ribosylated serines significantly overlap with known phosphorylated serines, and large-scale phosphoproteomics analysis provides evidence for site-specific crosstalk between serine ADPr and phosphorylation. Collectively, we demonstrate that serine ADPr is a widespread modification and a major nuclear signaling response to oxidative stress, with a regulatory scope comparable to other extensive posttranslational modifications.

Ion-Pairing with Triethylammonium Acetate Improves Solid-Phase Extraction of ADP-Ribosylated Peptides

ADP-ribosylation refers to the post-translational modification of protein substrates with monomers or polymers of the small molecule ADP-ribose. ADP-ribosylation is enzymatically regulated and plays roles in cellular processes including DNA repair, nucleic acid metabolism, cell death, cellular stress responses, and antiviral immunity. Recent advances in the field of ADP-ribosylation have led to the development of proteomics approaches to enrich and identify endogenous ADP-ribosylated peptides by liquid chromatography tandem mass spectrometry (LC-MS/MS). A number of these methods rely on reverse-phase solid-phase extraction as a critical step in preparing cellular peptides for further enrichment steps in proteomics workflows. The anionic ion-pairing reagent trifluoroacetic acid (TFA) is typically used during reverse-phase solid-phase extraction to promote retention of tryptic peptides. Here we report that TFA and other carboxylate ion-pairing reagents are inefficient for reverse-phase solid-phase extraction of ADP-ribosylated peptides. Substitution of TFA with cationic ion-pairing reagents, such as triethylammonium acetate (TEAA), improves recovery of ADP-ribosylated peptides. We further demonstrate that substitution of TFA with TEAA in a proteomics workflow specific for identifying ADP-ribosylated peptides increases identification rates of ADP-ribosylated peptides by LC-MS/MS.

Uncoupling of PARP1 trapping and inhibition using selective PARP1 degradation

PARP1 inhibitors (PARPi) are known to kill tumor cells via two mechanisms (i.e., PARP1 catalytic inhibition vs. PARP1 trapping). The relative contribution of these two pathways in mediating the cytotoxicity of PARPi, however, is incompletely understood. Here we designed a series of small molecule PARP degraders. Treatment with one such compound iRucaparib results in highly efficient and specific PARP1 degradation. iRucaparib blocks the enzymatic activity of PARP1 in vitro, and PARP1-mediated PARylation signaling in intact cells. This strategy mimics PARP1 genetic depletion, which enables the pharmacological decoupling of PARP1 inhibition from PARP1 trapping. Finally, by depleting PARP1, iRucaparib protects muscle cells and primary cardiomyocytes from DNA damage-induced energy crisis and cell death. In summary, these compounds represent “non-trapping” PARP1 degraders that block both the catalytic activity and scaffolding effects of PARP1, providing an ideal approach for the amelioration of the various pathological conditions caused by PARP1 hyperactivation.

Human alcohol dehydrogenase 1 is an acceptor protein for polyADP-ribosylation

Alcohol dehydrogenase (ADH) is important for preventing alcohol toxicity and developmental disorders, and may be involved in other diseases including neurodegenerative diseases. We found that the major acceptor protein of polyADP-ribosylation in a model organism of neurodegeneration using a Drosophila melanogaster mutant lacking poly(ADP-ribose) glycohydrolase, was ADH. Thus we postulated that human ADH activity might be regulated by polyADP-ribosylation, a post-translational modification. The radioactivity of [³²P]NAD⁺ was incorporated into human ADH1 by human poly(ADP-ribose) polymerase 1 in vitro, but was not incorporated when heat-inactivated PARP1 or a PARP inhibitor, 3-aminobenzamide, was used. The incorporated radioactivity was not released from ADH1 protein in the presence of excess amount of ADP-ribose or poly(ADP-ribose) as competitors. However, it was released by incubation with 1 M neutral NH₂OH or 0.1 N NaOH, but was not with 0.1 N HCl, suggesting the bond between ADH1 and poly(ADP-ribose) is an ester linkage. When HepG2 cells, a human hepatoma cell line, were cultured in the presence of another PARP inhibitor, olaparib, ADH activity of the cell was significantly increased. These results suggest that polyADP-ribosylation could regulate ADH activity in vivo and might be involved in neurodegeneration.

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.

Activation of PARP-1 by snoRNAs Controls Ribosome Biogenesis and Cell Growth via the RNA Helicase DDX21

PARP inhibitors (PARPi) prevent cancer cell growth by inducing synthetic lethality with DNA repair defects (e.g., in BRCA1/2 mutant cells). We have identified an alternative pathway for PARPi-mediated growth control in BRCA1/2-intact breast cancer cells involving rDNA transcription and ribosome biogenesis. PARP-1 binds to snoRNAs, which stimulate PARP-1 catalytic activity in the nucleolus independent of DNA damage. Activated PARP-1 ADP-ribosylates DDX21, an RNA helicase that localizes to nucleoli and promotes rDNA transcription when ADP-ribosylated. Treatment with PARPi or mutation of the ADP-ribosylation sites reduces DDX21 nucleolar localization, rDNA transcription, ribosome biogenesis, protein translation, and cell growth. The salient features of this pathway are evident in xenografts in mice and human breast cancer patient samples. Elevated levels of PARP-1 and nucleolar DDX21 are associated with cancer-related outcomes. Our studies provide a mechanistic rationale for efficacy of PARPi in cancer cells lacking defects in DNA repair whose growth is inhibited by PARPi.

Toxicoproteomic Analysis of Poly(ADP-ribose)-associated Proteins Induced by Oxidative Stress in Human Proximal Tubule Cells

2,3,5-Tris-(glutathion-S-yl)hydroquinone (TGHQ) is a nephrotoxic and nephrocarcinogenic metabolite of hydroquinone. TGHQ generates ROS, causing DNA strand breaks, hyperactivation of PARP-1, increases in intracellular calcium ([Ca2+]i), and cell death. PARP-1 catalyzes the attachment of ADP-ribose polymers (PAR) to target proteins. In human kidney proximal tubule cells (HK-2), ROS-mediated PARP-1 hyperactivation and elevations in [Ca2+]i are reciprocally coupled. The molecular mechanism of this interaction is unclear. The aim of the present study was to identify ROS-induced PAR-associated proteins to further understand their potential role in cell death. PAR-associated proteins were enriched by immunoprecipitation, identified by LC-MS/MS, and relative abundance was obtained by spectral counting. 356 proteins were PAR-modified following TGHQ treatment. 13 proteins exhibited gene ontology annotations related to calcium. Among these proteins, the general transcription factor II-I (TFII-I) is directly involved in the modulation of [Ca2+]i. TFII-I binding to phospholipase C (PLC) leads to calcium influx via the TRPC3 channel. However, inhibition of TRPC3 or PLC had no effect on TGHQ-mediated cell death, suggesting that their loss of function may be necessary but insufficient to cause cell death. Nevertheless, TGHQ promoted a time-dependent translocation of TFII-I from the nucleus to the cytosol concomitant with a decrease in tyrosine phosphorylation in α/β-TFII-I. Therefore it is likely that ROS have an important impact on the function of TFII-I, such as regulation of transcription, and DNA translesion synthesis. Our data also sheds light on PAR mediated signaling during oxidative stress, and contributes to the development of strategies to prevent PAR-dependent cell death.

A Study into the ADP-Ribosylome of IFN-γ-Stimulated THP-1 Human Macrophage-like Cells Identifies ARTD8/PARP14 and ARTD9/PARP9 ADP-Ribosylation

ADP-ribosylation is a post-translational modification that, until recently, has remained elusive to study at the cellular level. Previously dependent on radioactive tracers to identify ADP-ribosylation targets, several advances in mass spectrometric workflows now permit global identification of ADP-ribosylated substrates. In this study, we capitalized on two ADP-ribosylation enrichment strategies, and multiple activation methods performed on the Orbitrap Fusion Lumos, to identify IFN-γ-induced ADP-ribosylation substrates in macrophages. The ADP-ribosyl binding protein, Af1521, was used to enrich ADP-ribosylated peptides, and the antipoly-ADP-ribosyl antibody, 10H, was used to enrich ADP-ribosylated proteins. ADP-ribosyl-specific mass spectra were further enriched by an ADP-ribose product ion triggered EThcD and HCD activation strategy, in combination with multiple acquisitions that segmented the survey scan into smaller ranges. HCD and EThcD resulted in overlapping and unique ADP-ribosyl peptide identifications, with HCD providing more peptide identifications but EThcD providing more reliable ADP-ribosyl acceptor sites. Our acquisition strategies also resulted in the first ever characterization of ADP-ribosyl on three poly-ADP-ribose polymerases, ARTD9/PARP9, ARTD10/PARP10, and ARTD8/PARP14. IFN-γ increased the ADP-ribosylation status of ARTD9/PARP9, ARTD8/PARP14, and proteins involved in RNA processes. This study therefore summarizes specific molecular pathways at the intersection of IFN-γ and ADP-ribosylation signaling pathways.

Emerging roles of eraser enzymes in the dynamic control of protein ADP-ribosylation

Protein ADP-ribosylation is essential for the regulation of several cellular pathways, enabling dynamic responses to diverse pathophysiological conditions. It is modulated through a dynamic interplay between ADP-ribose readers, writers and erasers. While ADP-ribose synthesis has been studied and reviewed extensively, ADP-ribose processing by erasing enzymes has received comparably less attention. However, major progress in the mass spectrometric identification of ADP-ribosylated residues and the biochemical characterization of ADP-ribose erasers has substantially expanded our knowledge of ADP-ribosylation dynamics. Herein, we describe recent insights into the biology of ADP-ribose erasers and discuss the intricately orchestrated cellular processes to switch off ADP-ribose-dependent mechanisms.

ADP-ribose erasing enzymes are increasingly recognized as critical regulators of protein ADP-ribosylation dynamics in living systems. Here, the authors review recent advances in the discovery and characterization of ADP-ribose erasers and discuss their role within the cellular ADP-ribosylation machinery.

ELTA: Enzymatic Labeling of Terminal ADP-Ribose

ADP-ribosylation refers to the addition of one or more ADP-ribose groups onto proteins. The attached ADP-ribose monomers or polymers, commonly known as poly(ADP-ribose) (PAR), modulate the activities of the modified substrates or their binding affinities to other proteins. However, progress in this area is hindered by a lack of tools to investigate this protein modification. Here, we describe a new method named ELTA (enzymatic labeling of terminal ADP-ribose) for labeling free or protein-conjugated ADP-ribose monomers and polymers at their 2'-OH termini using the enzyme OAS1 and dATP. When coupled with various dATP analogs (e.g., radioactive, fluorescent, affinity tags), ELTA can be used to explore PAR biology with techniques routinely used to investigate DNA or RNA function. We demonstrate that ELTA enables the biophysical measurements of protein binding to PAR of a defined length, detection of PAR length from proteins and cells, and enrichment of sub-femtomole amounts of ADP-ribosylated peptides from cell lysates.

The role of dePARylation in DNA damage repair and cancer suppression

Poly(ADP-ribosyl)ation (PARylation) is a reversible post-translational modification regulating various biological pathways including DNA damage repair (DDR). Rapid turnover of PARylation is critically important for an optimal DNA damage response and maintaining genomic stability. Recent studies show that PARylation is tightly regulated by a group of enzymes that can erase the ADP-ribose (ADPR) groups from target proteins. The aim of this review is to present a comprehensive understanding of dePARylation enzymes, their substrates and roles in DDR. Special attention will be laid on the role of these proteins in the development of cancer and their feasibility in anticancer therapeutics.

Monitoring Poly(ADP-ribosyl)glycohydrolase Activity with a Continuous Fluorescent Substrate

The post-translational modification (PTM) and signaling molecule poly(ADP-ribose) (PAR) has an impact on diverse biological processes. This PTM is regulated by a series of ADP-ribosyl glycohydrolases (PARG enzymes) that cleave polymers and/or liberate monomers from their protein targets. Existing methods for monitoring these hydrolases rely on detection of the natural substrate, PAR, commonly achieved via radioisotopic labeling. Here we disclose a general substrate for monitoring PARG activity, TFMU-ADPr, which directly reports on total PAR hydrolase activity via release of a fluorophore; this substrate has excellent reactivity, generality (processed by the major PARG enzymes), stability, and usability. A second substrate, TFMU-IDPr, selectively reports on PARG activity only from the enzyme ARH3. Use of these probes in whole-cell lysate experiments has revealed a mechanism by which ARH3 is inhibited by cholera toxin. TFMU-ADPr and TFMU-IDPr are versatile tools for assessing small-molecule inhibitors in vitro and probing the regulation of ADP-ribosyl catabolic enzymes.

PARPs in genome stability and signal transduction: implications for cancer therapy

The poly(ADP-ribose) polymerase (PARP) superfamily of enzymes catalyses the ADP-ribosylation (ADPr) of target proteins by using nicotinamide adenine dinucleotide (NAD+) as a donor. ADPr reactions occur either in the form of attachment of a single ADP-ribose nucleotide unit on target proteins or in the form of ADP-ribose chains, with the latter called poly(ADP-ribosyl)ation. PARPs regulate many cellular processes, including the maintenance of genome stability and signal transduction. In this review, we focus on the PARP family members that possess the ability to modify proteins by poly(ADP-ribosyl)ation, namely PARP1, PARP2, Tankyrase-1, and Tankyrase-2. Here, we detail the cellular functions of PARP1 and PARP2 in the regulation of DNA damage response and describe the function of Tankyrases in Wnt-mediated signal transduction. Furthermore, we discuss how the understanding of these pathways has provided some major breakthroughs in the treatment of human cancer.

PARP1 Suppresses the Transcription of PD-L1 by Poly(ADP-Ribosyl)ating STAT3

Studies have pointed to a role of PARP1 in regulating gene expression through poly(ADP-ribosyl)ating, sequence-specific, DNA-binding transcription factors. However, few examples exist that link this role of PARP1 to the immunogenicity of cancer cells. Here, we report that PARP1 poly(ADP-ribosyl)ates STAT3 and subsequently promotes STAT3 dephosphorylation, resulting in reduced transcriptional activity of STAT3 and expression of PD-L1. In this study, we showed that PARP1 silencing or pharmacologic inhibition enhanced the transcription of PD-L1 in cancer cells, which was accompanied by the upregulation of PD-L1 protein expression, both in the cytoplasm and on the cell surface. This induction of PD-L1 was attenuated in the absence of the transcription factor STAT3. Cell-based studies indicated that PARP1 interacted directly with STAT3 and caused STAT3 poly(ADP-ribosyl)ation. STAT3's activation of PD-L1 transcription was abolished by the overexpression of wild-type PARP1 but not mutant PARP1, which lacks catalytic activity. PARP1 downregulation or catalytic inhibition enhanced the phosphorylation of STAT3, which was reversed by the ectopic expression of wild-type PARP1 but not by mutated PARP1. An inverse correlation between PARP1 and PD-L1 was also observed in clinical ovarian cancer samples. Overall, our study revealed PARP1-mediated poly(ADP-ribosyl)ation of STAT3 as a key step in inhibiting the transcription of PD-L1, and this mechanism exists in a variety of cancer cells.

Hydrofluoric Acid-Based Derivatization Strategy To Profile PARP-1 ADP-Ribosylation by LC-MS/MS

Despite significant advances in the development of mass spectrometry-based methods for the identification of protein ADP-ribosylation, current protocols suffer from several drawbacks that preclude their widespread applicability. Given the intrinsic heterogeneous nature of poly(ADP-ribose), a number of strategies have been developed to generate simple derivatives for effective interrogation of protein databases and site-specific localization of the modified residues. Currently, the generation of spectral signatures indicative of ADP-ribosylation rely on chemical or enzymatic conversion of the modification to a single mass increment. Still, limitations arise from the lability of the poly(ADP-ribose) remnant during tandem mass spectrometry, the varying susceptibilities of different ADP-ribose-protein bonds to chemical hydrolysis, or the context dependence of enzyme-catalyzed reactions. Here, we present a chemical-based derivatization method applicable to the confident identification of site-specific ADP-ribosylation by conventional mass spectrometry on any targeted amino acid residue. Using PARP-1 as a model protein, we report that treatment of ADP-ribosylated peptides with hydrofluoric acid generates a specific +132 Da mass signature that corresponds to the decomposition of mono- and poly(ADP-ribosylated) peptides into ribose adducts as a consequence of the cleavage of the phosphorus-oxygen bonds.

Nucleolar-nucleoplasmic shuttling of TARG1 and its control by DNA damage-induced poly-ADP-ribosylation and by nucleolar transcription

Macrodomains are conserved protein folds associated with ADP-ribose binding and turnover. ADP-ribosylation is a posttranslational modification catalyzed primarily by ARTD (aka PARP) enzymes in cells. ARTDs transfer either single or multiple ADP-ribose units to substrates, resulting in mono- or poly-ADP-ribosylation. TARG1/C6orf130 is a macrodomain protein that hydrolyzes mono-ADP-ribosylation and interacts with poly-ADP-ribose chains. Interactome analyses revealed that TARG1 binds strongly to ribosomes and proteins associated with rRNA processing and ribosomal assembly factors. TARG1 localized to transcriptionally active nucleoli, which occurred independently of ADP-ribose binding. TARG1 shuttled continuously between nucleoli and nucleoplasm. In response to DNA damage, which activates ARTD1/2 (PARP1/2) and promotes synthesis of poly-ADP-ribose chains, TARG1 re-localized to the nucleoplasm. This was dependent on the ability of TARG1 to bind to poly-ADP-ribose. These findings are consistent with the observed ability of TARG1 to competitively interact with RNA and PAR chains. We propose a nucleolar role of TARG1 in ribosome assembly or quality control that is stalled when TARG1 is re-located to sites of DNA damage.

Serine is the major residue for ADP-ribosylation upon DNA damage

Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes that synthesise ADP-ribosylation (ADPr), a reversible modification of proteins that regulates many different cellular processes. Several mammalian PARPs are known to regulate the DNA damage response, but it is not clear which amino acids in proteins are the primary ADPr targets. Previously, we reported that ARH3 reverses the newly discovered type of ADPr (ADPr on serine residues; Ser-ADPr) and developed tools to analyse this modification (Fontana et al., 2017). Here, we show that Ser-ADPr represents the major fraction of ADPr synthesised after DNA damage in mammalian cells and that globally Ser-ADPr is dependent on HPF1, PARP1 and ARH3. In the absence of HPF1, glutamate/aspartate becomes the main target residues for ADPr. Furthermore, we describe a method for site-specific validation of serine ADP-ribosylated substrates in cells. Our study establishes serine as the primary form of ADPr in DNA damage signalling.

Proteomic Analysis of the Downstream Signaling Network of PARP1

Poly-ADP-ribosylation (PARylation) is a protein posttranslational modification (PTM) that is critically involved in many biological processes that are linked to cell stress responses. It is catalyzed by a class of enzymes known as poly-ADP-ribose polymerases (PARPs). In particular, PARP1 is a nuclear protein that is activated upon sensing nicked DNA. Once activated, PARP1 is responsible for the synthesis of a large number of PARylated proteins and initiation of the DNA damage response mechanisms. This observation provided the rationale for developing PARP1 inhibitors for the treatment of human malignancies. Indeed, three PARP1 inhibitors (Olaparib, Rucaparib, and Niraparib) have recently been approved by the Food and Drug Administration for the treatment of ovarian cancer. Moreover, in 2017, both Olaparib and Niraparib have also been approved for the treatment of fallopian tube cancer and primary peritoneal cancer. Despite this very exciting progress in the clinic, the basic signaling mechanism that connects PARP1 to a diverse array of biological processes is still poorly understood. This is, in large part, due to the inherent technical difficulty associated with the analysis of protein PARylation, which is a low-abundance, labile, and heterogeneous PTM. The study of PARylation has been greatly facilitated by the recent advances in mass spectrometry-based proteomic technologies tailored to the analysis of this modification. In this Perspective, we discuss these breakthroughs, including their technical development, and applications that provide a global view of the many biological processes regulated by this important protein modification.