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The anti-cancer drug target poly(ADP-ribose) polymerase 1 (PARP1) and its close homologue, PARP2, are early responders to DNA damage in human cells 1 , 2 . After binding to genomic lesions, these enzymes use NAD + to modify numerous proteins with mono- and poly(ADP-ribose) signals that are important for the subsequent decompaction of chromatin and the recruitment of repair factors 3 , 4 . These post-translational modifications are predominantly serine-linked and require the accessory factor HPF1, which is specific for the DNA damage response and switches the amino acid specificity of PARP1 and PARP2 from aspartate or glutamate to serine residues 5 – 10 . Here we report a co-structure of HPF1 bound to the catalytic domain of PARP2 that, in combination with NMR and biochemical data, reveals a composite active site formed by residues from HPF1 and PARP1 or PARP2 . The assembly of this catalytic centre is essential for the addition of ADP-ribose moieties after DNA damage in human cells. In response to DNA damage and occupancy of the NAD + -binding site, the interaction of HPF1 with PARP1 or PARP2 is enhanced by allosteric networks that operate within the PARP proteins, providing an additional level of regulation in the induction of the DNA damage response. As HPF1 forms a joint active site with PARP1 or PARP2, our data implicate HPF1 as an important determinant of the response to clinical PARP inhibitors. Assembly of a catalytic centre formed by HPF1 bound to PARP1 or PARP2 is essential for protein ADP-ribosylation after DNA damage in human cells.

Oxaliplatin is the first-line regime for advanced gastric cancer treatment, while its resistance is a major problem that leads to the failure of clinical treatments. Tumor cell heterogeneity has been considered as one of the main causes for drug resistance in cancer. In this study, the mechanism of oxaliplatin resistance was investigated through in vitro human gastric cancer organoids and gastric cancer oxaliplatin-resistant cell lines and in vivo subcutaneous tumorigenicity experiments. The in vitro and in vivo results indicated that CD133+ stem cell-like cells are the main subpopulation and PARP1 is the central gene mediating oxaliplatin resistance in gastric cancer. It was found that PARP1 can effectively repair DNA damage caused by oxaliplatin by means of mediating the opening of base excision repair pathway, leading to the occurrence of drug resistance. The CD133+ stem cells also exhibited upregulated expression of N6-methyladenosine (m6A) mRNA and its writer METTL3 as showed by immunoprecipitation followed by sequencing and transcriptome analysis. METTTL3 enhances the stability of PARP1 by recruiting YTHDF1 to target the 3′-untranslated Region (3′-UTR) of PARP1 mRNA. The CD133+ tumor stem cells can regulate the stability and expression of m6A to PARP1 through METTL3, and thus exerting the PARP1-mediated DNA damage repair ability. Therefore, our study demonstrated that m6A Methyltransferase METTL3 facilitates oxaliplatin resistance in CD133+ gastric cancer stem cells by Promoting PARP1 mRNA stability which increases base excision repair pathway activity.

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. The mechanism of PARP1-dependent poly-ADP-ribosylation in response to DNA damage is still under debate. Here, the authors use ATR-FTIR spectroscopy to provide time-resolved insights into the molecular details of this process under near physiological conditions.

DNA repair is essential for life, yet its efficiency declines with age for reasons that are unclear. Numerous proteins possess Nudix homology domains (NHDs) that have no known function. We show that NHDs are NAD⁺ (oxidized form of nicotinamide adenine dinucleotide) binding domains that regulate protein-protein interactions. The binding of NAD⁺ to the NHD domain of DBC1 (deleted in breast cancer 1) prevents it from inhibiting PARP1 [poly(adenosine diphosphate–ribose) polymerase], a critical DNA repair protein. As mice age and NAD⁺ concentrations decline, DBC1 is increasingly bound to PARP1, causing DNA damage to accumulate, a process rapidly reversed by restoring the abundance of NAD⁺. Thus, NAD⁺ directly regulates protein-protein interactions, the modulation of which may protect against cancer, radiation, and aging.

Upon binding to DNA breaks, poly(ADP-ribose) polymerase 1 (PARP1) ADP-ribosylates itself and other factors to initiate DNA repair. Serine is the major residue for ADP-ribosylation upon DNA damage, which strictly depends on HPF1. Here, we report the crystal structures of human HPF1/PARP1-CAT ΔHD complex at 1.98 Å resolution, and mouse and human HPF1 at 1.71 Å and 1.57 Å resolution, respectively. Our structures and mutagenesis data confirm that the structural insights obtained in a recent HPF1/PARP2 study by Suskiewicz et al. apply to PARP1. Moreover, we quantitatively characterize the key residues necessary for HPF1/PARP1 binding. Our data show that through salt-bridging to Glu284/Asp286, Arg239 positions Glu284 to catalyze serine ADP-ribosylation, maintains the local conformation of HPF1 to limit PARP1 automodification, and facilitates HPF1/PARP1 binding by neutralizing the negative charge of Glu284. These findings, along with the high-resolution structural data, may facilitate drug discovery targeting PARP1. Once DNA breaks occur, poly(ADP-ribose) polymerase 1 (PARP1) ADP-ribosylates itself and other DNA repair factors to initiate the repair process. Here, the authors resolve the crystal structures of mouse and human HPF1, and human HPF1/PARP1 complex proving insights into PARP1 regulation.

Prostate cancer (PC) is one of the most common types of malignancies in men, with a noteworthy increase in newly diagnosed cases in recent years. PARP1 is a ubiquitous nuclear enzyme involved in DNA repair, nuclear transport, ribosome synthesis, and epigenetic bookmarking. In this study, a library of 9000 phytochemicals was screened, with a focus on those with high drug efficacy and potential PARP1 inhibition. Different machine learning models were generated and assessed using various statistical measures. The RF model outperformed all other models in terms of accuracy (0.9489), specificity (0.9171), and area under the curve (AUC = 0.9846). Following this, a library of 9510 phytochemicals was screened, yielding 181 compounds predicted to be active. These compounds were subsequently assessed using Lipinski’s Rule of Five, yielding 40 interesting candidates. Molecular docking experiments demonstrated that compound ZINC2356684563, ZINC2356558598, and ZINC14584870, had strong affinity for the PARP1 active site. Further molecular dynamics simulations and MM-PBSA validated the stability of the ligand–protein complexes, with ZINC14584870 and ZINC43120769 demonstrating the most stable interaction, as seen by low RMSD and RMSF levels. Our findings emphasize the potential of these phytochemical inhibitors as novel therapeutic agents against PARP1 in prostate cancer treatment, paving the path for further experimental validation and clinical investigations. These results open new possibilities for developing treatments to benefit prostate cancer patients.

The regulation of repair processes including base excision repair (BER) in the presence of DNA damage is implemented by a cellular signal: poly(ADP-ribosyl)ation (PARylation), which is catalysed by PARP1 and PARP2. Despite ample studies, it is far from clear how BER is regulated by PARPs and how the roles are distributed between the PARPs. Here, we investigated the effects of PARP1, PARP2 and PARylation on activities of the main BER enzymes (APE1, DNA polymerase β [Polβ] and DNA ligase IIIα [LigIIIα]) in combination with BER scaffold protein XRCC1 in the nucleosomal context. We constructed nucleosome core particles with midward- or outward-oriented damage. It was concluded that in most cases, the presence of PARP1 leads to the suppression of the activities of APE1, Polβ and to a lesser extent LigIIIα. PARylation by PARP1 attenuated this effect to various degrees depending on the enzyme. PARP2 had an influence predominantly on the last stage of BER: DNA sealing. Nonetheless, PARylation by PARP2 led to Polβ inhibition and to significant stimulation of LigIIIα activities in a NAD + -dependent manner. On the basis of the obtained and literature data, we suggest a hypothetical model of the contribution of PARP1 and PARP2 to BER.

To achieve swift cell demise during apoptosis, caspases cleave essential proteins for cell survival and removal. In addition to the binding of preferred amino acid sequences to its substrate-binding pocket, caspase-7 also uses exosites to select specific substrates. 4 lysine residues (K38KKK) located in the N-terminal domain of caspase-7 form such an exosite and promote the rapid proteolysis of the poly(ADP-ribose) polymerase 1 (PARP-1), but the mechanism of recognition remains mostly unknown. In this study, we show that the overall positive charge of the exosite is the critical feature of this evolutionarily conserved binding site. Additionally, interaction with the caspase-7 exosite involves both the Zn3 and BRCT domains of PARP-1 and is mediated by RNA. Indeed, PARP-1 proteolysis efficacy is sensitive to RNase A and promoted by added RNA. Moreover, using affinity chromatography and gel shift assays, we demonstrate that caspase-7, but not caspase-3 or a caspase-7 with a mutated exosite, binds nucleic acids. Finally, we show that caspase-7 prefers RNA-binding proteins (RNA-BPs) as substrates compared to caspase-3 and that RNA enhances proteolysis by caspase-7 of many of these RNA-BPs. Thus, we have uncovered an unusual way by which caspase-7 selects and cleaves specific substrates.

Poly(ADP-ribose) polymerases (PARP) are a family of enzymes that were proven to play an essential role in the initiation and activation of DNA repair processes in the case of DNA single-strand breaks. The inhibition of PARP enzymes might be a promising option for the treatment of several challenging types of cancers, including triple-negative breast cancer (TNBC) and non-small cell lung carcinoma (NSCLC). This study utilizes several techniques to screen the compound collection of the Leibniz Institute of Plant Biochemistry (IPB) to identify novel hPARP-1 inhibitors. First, an in silico pharmacophore-based docking study was conducted to virtually screen compounds with potential inhibitory effects. To evaluate these compounds in vitro, a cell-free enzyme assay was developed, optimized, and employed to identify hPARP-1 inhibitors, resulting in the discovery of two novel scaffolds represented by compounds 54 and 57, with the latter being the most active one from the compound library. Furthermore, fluorescence microscopy and synergism assays were performed to investigate the cellular and nuclear pathways of hPARP-1 inhibitor 57 and its potential synergistic effect with the DNA-damaging agent temozolomide. The findings suggest that the compound requires further lead optimization to enhance its ability to target the nuclear PARP enzyme effectively. Nonetheless, this new scaffold demonstrated a five-fold higher PARP inhibitory activity at the enzyme level compared to the core structure of olaparib (OLP), phthalazin-1(2H)-one.

Triple-negative breast cancer (TNBC) is an aggressive subtype, often resistant to chemotherapy. Flavonoids are polyphenolic compounds that exhibit anticancer properties. In this study, the potential antitumor effects of the synthetic flavonoid 2'-nitroflavone (2'NF) in TNBC were investigated. Our in vitro results show that 2'NF reduces TNBC cell viability, proliferation, and survival, while inducing cell cycle arrest, apoptosis, and mitotic catastrophe. Additionally, 2'NF inhibits TNBC and endothelial cell migration, likely due to severe disruptions in tubulin and actin cytoskeletons. Computational analysis revealed that 2'NF has a favorable pharmacological profile as a potential drug candidate. In a murine TNBC model, 2'NF administration slowed tumor growth without evident toxicity. Mechanistic in silico studies propose that poly(ADP-ribose) polymerase-1 (PARP1), a key enzyme involved in DNA repair, may be a target of 2'NF. Molecular docking and dynamics simulations demonstrated the formation of a thermodynamically stable complex between 2'NF and the catalytic domain of PARP1. Moreover, in vitro PARP enzymatic activity was inhibited by 2'NF in TNBC cells. These promising findings suggest the potential of 2'NF as a novel PARP1 inhibitor or a scaffold for developing improved inhibitors, offering new avenues for combination therapies in TNBC treatment.