A new underlying mechanism for the neuroprotective effect of bosutinib: Reverting toxicity-induced PARylation in SIN1-mediated neurotoxicity.

Increased levels of reactive oxygen and nitrogen species play an important role in the development and progression of neurodegenerative diseases, such as Alzheimer's and Parkinson's disease. The overproduction of these highly reactive chemical species leads to DNA damage and subsequent activation of the poly(ADP-ribose)polymerase (PARP) enzyme. Several studies have demonstrated the potential use of PARP inhibitors for neuroprotection. We previously reported that the dual Src/Abl kinase inhibitor bosutinib (BOS) decreases PARP activity and acts as a chemosensitizer in cancer cells. In this study, we evaluated the neuroprotective potential of BOS with respect to its inhibitory effect on cellular poly(ADP-ribos)ylation (PARylation) using a 3-morpholinosydnonimine (SIN1)-mediated cellular toxicity model. Our data suggest that pretreatment with BOS, especially at lower doses, significantly decreased the level of SIN1-induced cellular PARylation. This regulation pattern of PARylation was found to be associated with the protective effect of BOS against SIN1 on the viability of retinoic acid-differentiated SH-SY5Y cells. Furthermore, while PARP-1 expression was decreased, phosphorylation of SAPK/JNK was not reverted at the observed neuroprotective doses of BOS. In conclusion, we suggest a novel mechanism for the neuroprotective effect of BOS involving the inhibition of cellular PARylation.

Low-dose trans-resveratrol induce poly(ADP)-ribosylation-dependent increase of the PPAR-γ protein expression level in the in vitro model of non-alcoholic fatty liver disease.

An effect of low-dose resveratrol treatment on lipid metabolism and pro-inflammatory processes has been studied, using an in vitro model of Non-Alcoholic-Fatty Liver Disease. The model system consisted of lipid-loaded monolayer cultures of hepatocytes (Hepa1-6) and macrophages (RAW264.7), as both cell types are present in the liver. Also a tridimensional model of hepatic spheroids has been created to mimic spatial adhesive contacts between cells. Treatment with resveratrol (5 µM, 10 µM) for 3 h caused a decrease in lipid load in all three model systems. This decrease wasn't accompanied by any changes in surface expression of lipid transporter-CD36. The response to resveratrol (RSV) was cell type- and cell environment-dependent. In both cell types an increase of the peroxisome proliferator-activated receptor-γ (PPAR-γ) protein level has been revealed. The increase of the PPAR-γ protein level appeared to be poly (ADP)-ribosylation-dependent. It has been revealed, that in the resveratrol-induced signaling pathway, leading to the decrease of intracellular lipid load, an activation of poly (ADP)-ribose polymerase should happen upstream of PPAR-γ protein expression.The decrease of lipid load isn't accompanied by changes in the surface expression of lipid transporter CD36.

The Ubiquitin Ligase TRIP12 Limits PARP1 Trapping and Constrains PARP Inhibitor Efficiency.

PARP inhibitors (PARPi) cause synthetic lethality in BRCA-deficient tumors. Whether specific vulnerabilities to PARPi exist beyond BRCA mutations and related defects in homology-directed repair (HDR) is not well understood. Here, we identify the ubiquitin E3 ligase TRIP12 as negative regulator of PARPi sensitivity. We show that TRIP12 controls steady-state PARP1 levels and limits PARPi-induced cytotoxic PARP1 trapping. Upon loss of TRIP12, elevated PARPi-induced PARP1 trapping causes increased DNA replication stress, DNA damage, cell cycle arrest, and cell death. Mechanistically, we demonstrate that TRIP12 binds PARP1 via a central PAR-binding WWE domain and, using its carboxy-terminal HECT domain, catalyzes polyubiquitylation of PARP1, triggering proteasomal degradation and preventing supra-physiological PARP1 accumulation. Further, in cohorts of breast and ovarian cancer patients, PARP1 abundance is negatively correlated with TRIP12 expression. We thus propose TRIP12 as regulator of PARP1 stability and PARPi-induced PARP trapping, with potential implications for PARPi sensitivity and resistance.

Real-time monitoring of PARP1-dependent PARylation by ATR-FTIR spectroscopy.

Poly-ADP-ribosylation (PARylation) is a fully reversible post-translational modification with key roles in cellular physiology. Due to the multi-domain structure of poly(ADP-ribose) polymerase-1 (PARP1) and the highly dynamic nature of the PARylation reaction, studies on the biochemical mechanism and structural dynamics remain challenging. Here, we report label-free, time-resolved monitoring of PARP1-dependent PARylation using ATR-FTIR spectroscopy. This includes PARP1 activation by binding to DNA strand break models, NAD+ substrate binding, PAR formation, and dissociation of automodified PARP1 from DNA. Analyses of PARP1 activation at different DNA models demonstrate a strong positive correlation of PARylation and PARP1 dissociation, with the strongest effects observed for DNA nicks and 3' phosphorylated ends. Moreover, by examining dynamic structural changes of PARP1, we reveal changes in the secondary structure of PARP1 induced by NAD+ and PARP inhibitor binding. In summary, this approach enables holistic and dynamic insights into PARP1-dependent PARylation with molecular and temporal resolution.

Selective small molecule PARG inhibitor causes replication fork stalling and cancer cell death.

Poly(ADP-ribose)ylation (PARylation) by PAR polymerase 1 (PARP1) and PARylation removal by poly(ADP-ribose) glycohydrolase (PARG) critically regulate DNA damage responses; yet, conflicting reports obscure PARG biology and its impact on cancer cell resistance to PARP1 inhibitors. Here, we found that PARG expression is upregulated in many cancers. We employed chemical library screening to identify and optimize methylxanthine derivatives as selective bioavailable PARG inhibitors. Multiple crystal structures reveal how substituent positions on the methylxanthine core dictate binding modes and inducible-complementarity with a PARG-specific tyrosine clasp and arginine switch, supporting inhibitor specificity and a competitive inhibition mechanism. Cell-based assays show selective PARG inhibition and PARP1 hyperPARylation. Moreover, our PARG inhibitor sensitizes cells to radiation-induced DNA damage, suppresses replication fork progression and impedes cancer cell survival. In PARP inhibitor-resistant A172 glioblastoma cells, our PARG inhibitor shows comparable killing to Nedaplatin, providing further proof-of-concept that selectively inhibiting PARG can impair cancer cell survival.

Tankyrase inhibition preserves osteoarthritic cartilage by coordinating cartilage matrix anabolism via effects on SOX9 PARylation.

Osteoarthritis (OA) is a prevalent degenerative disease, which involves progressive and irreversible destruction of cartilage matrix. Despite efforts to reconstruct cartilage matrix in osteoarthritic joints, it has been a difficult task as adult cartilage exhibits marginal repair capacity. Here we report the identification of tankyrase as a regulator of the cartilage anabolism axis based on systems-level factor analysis of mouse reference populations. Tankyrase inhibition drives the expression of a cartilage-signature matrisome and elicits a transcriptomic pattern that is inversely correlated with OA progression. Furthermore, tankyrase inhibitors ameliorate surgically induced OA in mice, and stem cell transplantation coupled with tankyrase knockdown results in superior regeneration of cartilage lesions. Mechanistically, the pro-regenerative features of tankyrase inhibition are mainly triggered by uncoupling SOX9 from a poly(ADP-ribosyl)ation (PARylation)-dependent protein degradation pathway. Our findings provide insights into the development of future OA therapies aimed at reconstruction of articular cartilage.

Poly-ADP ribosylation in DNA damage response and cancer therapy.

Poly(ADP-ribosyl)ation (aka PARylation) is a unique protein post-translational modification (PTM) first described over 50 years ago. PARylation regulates a number of biological processes including chromatin remodeling, the DNA damage response (DDR), transcription, apoptosis, and mitosis. The subsequent discovery of poly(ADP-ribose) polymerase-1 (PARP-1) catalyzing DNA-dependent PARylation spearheaded the field of DDR. The expanding knowledge about the poly ADP-ribose (PAR) recognition domains prompted the discovery of novel DDR factors and revealed crosstalk with other protein PTMs including phosphorylation, ubiquitination, methylation and acetylation. In this review, we highlight the current knowledge on PAR-regulated DDR, PAR recognition domain, and PARP inhibition in cancer therapy.

Poly(ADP-ribosyl)ation of BRD7 by PARP1 confers resistance to DNA-damaging chemotherapeutic agents.

The bromodomain-containing protein 7 (BRD7) is a tumour suppressor protein with critical roles in cell cycle transition and transcriptional regulation. Whether BRD7 is regulated by post-translational modifications remains poorly understood. Here, we find that chemotherapy-induced DNA damage leads to the rapid degradation of BRD7 in various cancer cell lines. PARP-1 binds and poly(ADP)ribosylates BRD7, which enhances its ubiquitination and degradation through the PAR-binding E3 ubiquitin ligase RNF146. Moreover, the PARP1 inhibitor Olaparib significantly enhances the sensitivity of BRD7-positive cancer cells to chemotherapeutic drugs, while it has little effect on cells with low BRD7 expression. Taken together, our findings show that PARP1 induces the degradation of BRD7 resulting in cancer cell resistance to DNA-damaging agents. BRD7 might thus serve as potential biomarker in clinical trial for the prediction of synergistic effects between chemotherapeutic drugs and PARP inhibitors.

New insights of poly(ADP-ribosylation) in neurodegenerative diseases: A focus on protein phase separation and pathologic aggregation.

Abnormal protein aggregation is a common pathological feature of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). Protein posttranslational modifications (PTMs) play a crucial regulatory role in the formation of pathologic aggregation. Among the known PTMs involved in neurodegeneration, poly(ADP-ribosylation) (PARylation) has emerged with promising therapeutic potentials of the use of poly(ADP-ribose) (PAR) polymerase (PARP) inhibitors. In this review, we describe the mounting evidence that abnormal PARP activation is involved in various neurodegenerative diseases, and discuss the underpinning mechanisms with a focus on the recent findings that PARylation affects liquid-liquid phase separation and aggregation of amyloid proteins. We hope this review will stimulate further investigation of the unknown functions of PARylation and promote the development of more effective therapeutic agents in treating neurodegeneration.

NADP+ is an endogenous PARP inhibitor in DNA damage response and tumor suppression.

ADP-ribosylation is a unique posttranslational modification catalyzed by poly(ADP-ribose) polymerases (PARPs) using NAD+ as ADP-ribose donor. PARPs play an indispensable role in DNA damage repair and small molecule PARP inhibitors have emerged as potent anticancer drugs. However, to date, PARP inhibitor treatment has been restricted to patients with BRCA1/2 mutation-associated breast and ovarian cancer. One of the major challenges to extend the therapeutic potential of PARP inhibitors to other cancer types is the absence of predictive biomarkers. Here, we show that ovarian cancer cells with higher level of NADP+, an NAD+ derivative, are more sensitive to PARP inhibitors. We demonstrate that NADP+ acts as a negative regulator and suppresses ADP-ribosylation both in vitro and in vivo. NADP+ impairs ADP-ribosylation-dependent DNA damage repair and sensitizes tumor cell to chemically synthesized PARP inhibitors. Taken together, our study identifies NADP+ as an endogenous PARP inhibitor that may have implications in cancer treatment.

Selective Loss of PARG Restores PARylation and Counteracts PARP Inhibitor-Mediated Synthetic Lethality.

Inhibitors of poly(ADP-ribose) (PAR) polymerase (PARPi) have recently entered the clinic for the treatment of homologous recombination (HR)-deficient cancers. Despite the success of this approach, drug resistance is a clinical hurdle, and we poorly understand how cancer cells escape the deadly effects of PARPi without restoring the HR pathway. By combining genetic screens with multi-omics analysis of matched PARPi-sensitive and -resistant Brca2-mutated mouse mammary tumors, we identified loss of PAR glycohydrolase (PARG) as a major resistance mechanism. We also found the presence of PARG-negative clones in a subset of human serous ovarian and triple-negative breast cancers. PARG depletion restores PAR formation and partially rescues PARP1 signaling. Importantly, PARG inactivation exposes vulnerabilities that can be exploited therapeutically.

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.

Regulation of Wnt/ß-catenin signalling by tankyrase-dependent poly(ADP-ribosyl)ation and scaffolding.

The Wnt/ß-catenin signalling pathway is pivotal for stem cell function and the control of cellular differentiation, both during embryonic development and tissue homeostasis in adults. Its activity is carefully controlled through the concerted interactions of concentration-limited pathway components and a wide range of post-translational modifications, including phosphorylation, ubiquitylation, sumoylation, poly(ADP-ribosyl)ation (PARylation) and acetylation. Regulation of Wnt/ß-catenin signalling by PARylation was discovered relatively recently. The PARP tankyrase PARylates AXIN1/2, an essential central scaffolding protein in the ß-catenin destruction complex, and targets it for degradation, thereby fine-tuning the responsiveness of cells to the Wnt signal. The past few years have not only seen much progress in our understanding of the molecular mechanisms by which PARylation controls the pathway but also witnessed the successful development of tankyrase inhibitors as tool compounds and promising agents for the therapy of Wnt-dependent dysfunctions, including colorectal cancer. Recent work has hinted at more complex roles of tankyrase in Wnt/ß-catenin signalling as well as challenges and opportunities in the development of tankyrase inhibitors. Here we review some of the latest advances in our understanding of tankyrase function in the pathway and efforts to modulate tankyrase activity to re-tune Wnt/ß-catenin signalling in colorectal cancer cells. LINKED ARTICLES: This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

Antagonistic effect of N-ethylmaleimide on arsenic-mediated oxidative stress-induced poly(ADP-ribosyl)ation and cytotoxicity.

Long-term exposure to arsenic has been known to induce neoplastic initiation and progression in several organs; however, the role of arsenic (As2 O3 ) in oxidative stress-mediated DNA damage remains elusive. One of the immediate cellular responses to DNA damage is poly(ADP-ribosyl)ation (PARylation), which mediates DNA repair and enhances cell survival. In this study, we found that oxidative stress (H2 O2 )-induced PARylation was suppressed by As2 O3 exposure in different human cancer cells. Moreover, As2 O3 treatment promoted H2 O2 -induced DNA damage and apoptosis, leading to increased cell death. We found that N-ethylmaleimide (NEM), an organic compound derived from maleic acid, could reverse As2 O3 -mediated effects, thus enhancing PARylation with attenuated cell death and increased cell survival. Pharmacologic inhibition of glutathione with l-buthionine-sulfoximine blocked the antagonistic effect of NEM on As2 O3 , thereby continuing As2 O3 -mediated suppression of PARylation and causing DNA damage. Our findings identify NEM as a potential antidote against As2 O3 -mediated DNA damage in a glutathione-dependent manner. Copyright © 2016 John Wiley & Sons, Ltd.

Autophagy requires poly(adp-ribosyl)ation-dependent AMPK nuclear export.

AMPK is a central energy sensor linking extracellular milieu fluctuations with the autophagic machinery. In the current study we uncover that Poly(ADP-ribosyl)ation (PARylation), a post-translational modification (PTM) of proteins, accounts for the spatial and temporal regulation of autophagy by modulating AMPK subcellular localisation and activation. More particularly, we show that the minority AMPK pool needs to be exported to the cytosol in a PARylation-dependent manner for optimal induction of autophagy, including ULK1 phosphorylation and mTORC1 inactivation. PARP-1 forms a molecular complex with AMPK in the nucleus in non-starved cells. In response to nutrient deprivation, PARP-1 catalysed PARylation, induced the dissociation of the PARP-1/AMPK complex and the export of free PARylated nuclear AMPK to the cytoplasm to activate autophagy. PARP inhibition, its silencing or the expression of PARylation-deficient AMPK mutants prevented not only the AMPK nuclear-cytosolic export but also affected the activation of the cytosolic AMPK pool and autophagosome formation. These results demonstrate that PARylation of AMPK is a key early signal to efficiently convey extracellular nutrient perturbations with downstream events needed for the cell to optimize autophagic commitment before autophagosome formation.

Inputs and outputs of poly(ADP-ribosyl)ation: Relevance to oxidative stress.

Oxidative stress can cause DNA breaks which induce activation of the DNA nick sensor enzyme poly(ADP-ribose) polymerase-1 (PARP-1), part of the 17 member PARP enzyme family. PARP-1 modifies target proteins by attaching to them several NAD-derived ADP-ribose units forming poly(ADP-ribose) (PAR) polymers. PARylation controls many cellular processes while intense PARylation may also lead to cell death by various mechanisms. Here we summarize the modes of activation, inhibitors and modulators of PARP-1 and review the cellular functions regulated by the enzyme.