PARP7

PARP7 is a mono-ART involved in pathways that are often dysregulated in cancer cells and has a recognised role as a crucial modulator of innate immune signalling, acting as a repressor of interferon type I (IFN-I) pathway (Ref. Reference Gozgit5). This function is achieved by MARylating and deactivating components of the pathway, such as the TANK-binding kinase 1 (TBK1) or likely other key effectors (Ref. Reference Rodriguez6). In cancer cells, where genomic instability leads to the accumulation of cytosolic nucleic acids, the presence of cytoplasmic DNA activates the cGAS-STING pathway and TBK1, and induces the expression and secretion of IFN-I. These secreted interferons at low concentration increase cancer cells proliferation, whereas at high concentration are detrimental for their survival, by enhancing the ISG expression and activation of the immune response. By negatively regulating the type I interferon response, PARP7 enables cancer cells to escape the immune surveillance. At the same time, loss of PARP7 function inhibits cancer cell expansion by regulating cell proliferation and death (Refs Reference Gozgit5, Reference Rodriguez6, Reference Manetsch7). Molecularly, as recently shown in breast and lung cancer cells, loss of PARP7 function causes reduced protein levels of the FRA1 transcription factor, leading to an upregulation of IRF1/3-dependent inflammatory and proapoptotic genes, responsible for the apoptosis induction (Ref. Reference Manetsch7). Whether PARP7-mediated ADP-ribosylation of FRA1 also impact on other transcription machineries requires further investigations. Apparently, the role of PARP7 varies in different cancer types. In prostate cancer cells, PARP7 was identified as a positive regulator of the androgen receptor (AR) complex that contains AR itself, PARP9 and the ubiquitin E3 ligase DTX3L. PARP7-catalyzed AR MARylation favours complex formation and nuclear translocation, activating transcription of AR-regulated genes, required for prostate cell proliferation and development (Ref. Reference Kamata, Yang and Paschal8).

An additional role of PARP7 emerged in ovarian cancer cells, where it MARylates α-tubulin of the microtubules, making the cancer cells prone to migration. Therefore, PARP7 loss or inhibition decreases cell growth and motility as demonstrated in different cell types (ovarian, cervical and kidney cancer cells) (Ref. Reference Palavalli Parsons9).

Differently, studies performed in breast cancer cells indicated that PARP7 negatively regulates oestrogen receptor α signalling in vitro (Ref. Reference Rasmussen10), and additional research demonstrated that PARP7 knockdown promoted tumour growth in an MCF-7 xenograft model (Ref. Reference Zhang11). Notably, these studies were conducted without the presence of immune cells or a functional immune system, possibly explaining different results. The specific contribution of inhibiting PARP7 activity in tumour cells, immune cells or both has just started to be elucidated and requires wider analysis.

However, a recent study analysed the effect of PARP7 knockout in EO771 mouse mammary cancer cells and in a preclinical syngenic tumour model, utilizing wild-type and catalytically inactive mutant Parp7H532A mice, and showed that loss of PARP7 function modestly reduced cancer cell proliferation, while not affecting tumour growth in immunodeficient mice. Importantly, this outcome was fully reverted in immunocompetent mice, where PARP7 loss caused a robust reduction of tumour growth, which become further enhanced in mutant Parp7H532A recipient mice. These findings indicate that the absence of PARP7 enhances IFN-I and associated signalling pathways, resulting in heightened immune system targeting the tumours (Ref. Reference Rasmussen12).

The effectiveness of PARP7 inhibitor-induced tumour regression therefore relies on an intact IFN-I signalling pathway and an increased infiltration of CD8⁺ T cells (Refs Reference Gozgit5, Reference Rodriguez6, Reference Manetsch7, Reference Kamata, Yang and Paschal8, Reference Palavalli Parsons9, Reference Rasmussen10, Reference Zhang11, Reference Rasmussen12, Reference Gu13). Currently, RBN-2397 stands as the sole PARP7 inhibitor undergoing clinical trials, both as a monotherapy for patients with advanced solid tumours (NCT04053673) and in combination with the immune checkpoint inhibitor pembrolizumab (NCT05127590) in patients with squamous cell carcinoma of the lung. Targeting PARP7 activity in immune cells emerges as a promising strategy to bolster the body's immune defences against tumours (Ref. Reference Brooks, Anand and Cohen14).

PARP9

PARP9, also known as B-aggressive lymphoma-1 (BAL1), was identified as a risk-related gene product in aggressive diffuse large B-cell lymphoma (DLBCL) and associated with interferon-gamma (IFNγ) gene expression and signalling. PARP9 represses the anti-proliferative and pro-apoptotic IFNγ-STAT1-IRF1-p53 axis, thus mediating proliferation, survival and chemo-resistance in DLBCL (Ref. Reference Camicia15). In addition, PARP9 has been found as part of a trimeric complex with DTX3L and PARP14, regulating proliferation, survival and chemo-resistance in metastatic prostate cancer cells, in a way dependent on STAT1 activation (Ref. Reference Bachmann16). Similarly, PARP9/DTX3L also control the AR-mediated transcription in prostate cancer cells (Ref. Reference Yang17). A similar role for PARP9 has been recently reported in head–neck squamous cellular carcinoma (HNSCC), where PARP9 forms a complex with PARP14 and DTX3L to promote survival signals (see below and (Ref. Reference Saleh18)). Therefore, a combined targeted inhibition of STAT1, PARP9, PARP14 and/or DTX3L could increase the efficacy of currently available treatment for prostate, DLBCL and other high-risk tumour types harbouring an increased STAT1 signalling.

PARP10

PARP10 was the first mono-ART to be characterised and demonstrated to be a MYC-interacting protein and a negative regulator of NFκB pathway in antiviral response (Ref. Reference Yu19).

Later evidence proved that the enzyme is also involved in DNA damage repair and genomic instability, mostly through its interaction with PCNA, a master regulator of DNA replication and S-phase-coupled repair (Refs Reference Dhoonmoon and Nicolae20, Reference Nicolae21, Reference Schleicher22). In the nucleus, PARP10 cooperates with PCNA to induce DNA polymerization at stalled fork in condition of replicational stress. This role correlates with PARP10 overexpression in a large proportion of human tumours, where it promotes cellular transformation, potentially by alleviating cellular sensitivity to replication stress and favouring the restart of stalled replication forks (Refs Reference Nicolae21, Reference Schleicher22). The presence of a nuclear export sequence makes PARP10 capable to shuttle between nuclear and cytoplasmic compartments, therefore regulating both nuclear and cytoplasmic events. In the cytosol, for example, PARP10 guarantees a correct cell-cycle progression by regulating the G2 to M phase transition, through the MARylation and activation of the cell-cycle kinase Aurora A (Ref. Reference Di Paola23). Another substrate of PARP10 in cancer cells is the cell-cycle kinase PLK1, which is overexpressed in a variety of cancers (e.g. liver, breast and lung cancers) promoting cell proliferation and metastasis. In particular, in hepatocellular carcinoma (HCC), PARP10-mediated MARylation of PLK1 significantly inhibits its kinase activity and related oncogenic functions. Indeed, PLK1 can phosphorylate PARP10, leading to an enhanced NFκB transcriptional activity responsible for cancer progression (Ref. Reference Tian24).

In other tumours, PARP10 deficiency correlates with an increase of cancer cell proliferation and migration: in this context, absence of PARP10 results in a reduced MARylation of the substrate Aurora A causing an increased cancer cell invasion and migration, without affecting cell-cycle progression. This indicates a regulatory activity of PARP10 on Aurora A that is dependent on the biological context (cell migration or cell-cycle progression) (Ref. Reference Zhao25).

Increased PARP10 expression has been recently shown to correlate with a poor prognosis in patients with oral squamous cell carcinoma, the most common type of head and neck cancer (Ref. Reference Zhou26). Here, PARP10 silencing hampered the PI3K and MAPK pathways. Although the role of PARP10 catalytic activity was not investigated, this study further confirms the importance of PARP10 as key regulator of cell proliferation and metastasis (Ref. Reference Zhou26).

Overall, although with different outcomes, dysregulation of PARP10 appears to be central in tumour progression, determining specific biological consequences strictly dictated by the cellular context and the substrates involved.

PARP11

PARP11 is a mono-ART with roles in the regulation of immune response, specifically the IFN-I signalling. Differently from PARP7, that regulates IFN-I production, PARP11 controls the pathway by downregulating the levels of the interferon receptor chain 1, IFNAR1. IFNAR1 protein levels depend on its ubiquitination by β-TrCP-containing E3 ligase and followed by degradation (Ref. Reference Guo27). β-TrCP expression is regulated by the Wnt signalling pathway and its activity is dependent on the MARylation mediated by PARP11. Therefore, increasing MARylation of the E3 ligase causes increased IFNR1 degradation, meaning a reduced IFN-I signalling. In Parp11 knock-out mice, IFNAR1 levels are kept at physiological levels, facilitating the activation of immune cells, in particular the cytotoxic T lymphocytes antitumoural activity (Ref. Reference Yang28). Moreover, in colorectal cancer, PARP11 expression is decreased in a subset of T effector cells (CD103⁺ CD39⁺) (Ref. Reference Zhang29). Inhibition of PARP11 could therefore restore the signalling pathway mediated by IFNAR1 leading to the enhanced IFN-I and ISGs expression, making tumour cells more easily targeted by the immune system. On this line, the combination of PARP11 inhibition and the new therapy based on the generation of chimeric antigen receptor (CAR) T cells could improve the outcome of this approach: studies conducted on CAR T cells depleted of PARP11 show that these cells stably express the IFNAR1 on their surface and persist longer in blood and tumours in mice (Ref. Reference Zhang29).

PARP12

PARP12 is a mono-ART mainly known as a component of stress granules, regulator of intracellular membrane transport and for its anti-viral effects (Refs Reference Welsby30, Reference Grimaldi31, Reference Catara32). Along with these functions, PARP12 has been proposed to be involved in cancer biology, although its contribution is still controversial and elusive. PARP12 has been shown to suppress HCC metastasis, independent of its enzymatic activity. In this system, PARP12 interacts with the transcription factor FHL2, thereby regulating its stability and repressing transforming growth factor β1 (TGF-β1) transcription, a potent driver of endothelial-to-mesenchymal transition (EMT) involved in cell migration and invasion of liver cancer cells (Ref. Reference Shao33). In the breast cancer context, on the other hand, it has been observed that PARP12 is an active contributor of IFN/STAT1 signalling pathway involved in post-chemotherapy survival and re-growth of breast cancer cells (Ref. Reference Gaston34). Although the molecular mechanism underlying PARP12 role in breast cancer is not completely understood, it is considered a potential promising novel target in breast cancer therapy.

PARP13

PARP13 is an RNA-binding PARP initially identified as an antiviral protein. Emerging from alternate splicing, PARP13 exists in four different isoforms (Refs Reference Ficarelli, Neil and Swanson35, Reference Gao, Guo and Goff36, Reference Lüscher37); among them, the two best studied are PARP13.1 and PARP13.2. Although PARP13.2 does not code for a PARP domain, the PARP13.1 encodes a protein containing an inactive C-terminal PARP domain in addition to the CCCH zinc finger motif (Ref. Reference Lüscher1). Most of the knowledge on this protein relates to its role as an anti-viral factor. Moreover, in immune cells, it regulates the response to TRAIL, a proapoptotic cytokine that once secreted can specifically target cancer cells to promote their death (Ref. Reference Todorova, Bock and Chang38). Indeed, several cancers (such as liver, bladder and colon cancers) show low levels of PARP13 expression compared with normal tissues suggesting that loss/decreased level of PARP13 favours the development and progression of different types of cancers.

Recently, a direct interaction between PARP1 and PARP13 has been shown, supporting the idea of a crosstalk among different PARPs, although the role of these interactions remains largely unknown. In the case of PARP1 and PARP13, it has been proposed that PARP1 is seised in the nucleus by the heat shock transcription factor 1 through the scaffold protein PARP13. In the presence of DNA damage, PARP1 is ADP-ribosylated and dissociates from this complex to relocate in specific genomic loci to promote repair of DNA lesions. Blocking this complex could affect the survival of the cancer cells, in particular in BRCA1-null mammary tumours (Ref. Reference Fujimoto39).

PARP14

PARP14 was initially characterised as a positive regulator of STAT6 that increases the production of interleukin 4 (IL-4). In the absence of this cytokine, PARP14 forms a complex with histone deacetylases (HDAC2 and HDAC3), thus repressing the expression of target genes; upon IL-4 stimulation, PARP14 catalyses its own MARylation and that of HDAC2 and HDAC3, promoting gene transcription (Ref. Reference Mehrotra40). PARP14 role in tumour development and progression varies across cancer types, with implications in B-cell specific tumorigenesis, such as DLBCL and multiple myeloma. The role of IL-4 in cancer is controversial: in some studies, it acts as tumour suppressor whereas in others it promotes tumour progression. In colorectal cancer, MARylation of HDAC2 and HDAC3 mediated by PARP14 allows the expression of EP4, one of four prostaglandin receptors commonly upregulated in the tumour microenvironment exerting vital roles in stimulating cell proliferation, invasion and metastasis. Inhibition of PARP14 abrogates EP4 receptor activation, reducing proliferation, thus suggesting PARP14 as a potential target for colon cancer therapy (Ref. Reference Mashimo41). PARP14 is also implicated in B-cell lymphoma where it inhibits JNK1 signalling, involved in cell apoptosis, and in acute myeloid leukaemia where it enhances the glycolysis and promotes cancer cell proliferation (Refs Reference Dhoonmoon and Nicolae20, Reference Barbarulo42). Along with its role in metabolism and immune signalling, PARP14 exerts important functions also in the maintenance of genome integrity, by promoting an efficient homologous recombination (HR) (Ref. Reference Dhoonmoon and Nicolae20). The study suggests that PARP14 MARylates RAD51, a key factor in HR, and this event is crucial for efficient HR; PARP14 depletion leads to persistent RAD51 foci, preventing a complete and successful HR (Ref. Reference Nicolae43).

Moreover, a recent study analysed synthetic lethal interactions of PARP14, and identifies the ATR-CHK1 pathway as essential for the viability of PARP14-deficient cells, making them hypersensitive to inhibitors (Ref. Reference Dhoonmoon44). Similarly, PARP14 can modulate fork stability, specifically in BRCA1/2-deficient cells, a function mediated by PARP14 interaction with MRE11 in order to prevent excessive resection at stalled replication forks (Ref. Reference Dhoonmoon, Nicolae and Moldovan45).

Additionally, PARP14 is implicated in cell-cycle regulation, promoting G1/S phase transition via the retinoblastoma pathway. In particular, it was shown that PARP14 increases cyclin D1 levels, involved in G1/S transition both in normal and cancer cells, by binding to the 3′ UTR of cyclin D1 mRNA, stabilizing the transcript. When PARP14 is depleted, cells with functional Rb and p53/p21 pathway are blocked in G1 phase which causes cell-cycle arrest (Ref. Reference O'Connor46).

More recently, PARP14 protein levels were found to be increased in HNSC and other tumour types, along with elevated protein levels of its interactors PARP9 and DTX3L. Depletion of PARP14, as well as depletion of PARP9 or DTX3L, reduced survival and proliferation of HeLa and HNSC cells. This effect was independent on PARP14 catalytic activity, although relies on a C-terminal domain required for the formation of the PARP14/PARP9/DTX3L heterotrimer. Mechanistically, this interaction inhibits PARP14 auto-ADP-ribosylation, favouring an increased PARP14 stability at protein level, necessary to mediate pro-survival signalling (Ref. Reference Saleh18).

Interestingly, and connected to the role of PARP14 in IFN-mediated signalling, tumours with an acquired resistance to immune checkpoint blockade therapy (such as immunotherapy targeting programmed cell death protein 1, referred as α-PD-1 therapy) show increased PARP14 mRNA levels, along with IFNγ genes. Treatment with the PARP14 inhibitor RBN012579, by enhancing the IFNγ signalling, re-sensitise these tumours to the PD-1 therapy, establishing PARP14 as an actionable target to reverse IFNγ-driven resistance mechanisms to immune checkpoint blockade therapy (Ref. Reference Wong47).

Collectively, by promoting pro-survival pathways, inhibition of pro-apoptotic factor JNK1 and G1/S cell-cycle progression through cyclin D1 expression, PARP14 can be considered as a pro-survival factor, and a valuable promising drug target to be investigated. Currently, the PARP14 inhibitor RBN-3143 has entered phase I clinical trials as therapy for a range of inflammatory diseases, with an initial focus on atopic dermatitis (NCT05215808).

Role of other PARPs in cancer biology

With this section we provide a snapshot of PARPs and MARylation in cancer cell development and progression, not dependent on the IFN-signalling.

PARP3 is involved in DNA double-strand break repair and chromosomal rearrangements (Ref. Reference Rodriguez-Vargas, Nguekeu-Zebaze and Dantzer48). Overexpression of PARP3 in breast cancer patients correlates with an increase of tumour aggressiveness and a reduction of survival rate. Remarkably, PARP3 promotes TFGβ-induced EMT and cell proliferation of triple-negative breast cancer by increasing the stability of mTORC2 complex (Refs Reference Karicheva49, Reference Beck50). Moreover, in glioblastoma cells, PARP3 acts to maintain the cytoskeletal microtubule stability and its absence markedly increases the sensitivity of glioblastoma cells to microtubule-destabilizing agents, providing a novel therapeutic strategy in brain cancer therapy (Ref. Reference Nguekeu-Zebaze51).

PARP4 is one of the least studied PARPs and little is known about PARP4 functions and mechanism of action. The presence of the BRCA1 carboxy-terminal domain repeats, similar to those present in PARP1, suggests an involvement of this enzyme in DNA damage repair. In a study conducted on a cohort of patients affected by primary thyroid and breast cancers, PARP4 seems to act as a tumour suppressor gene (Ref. Reference Ikeda52). More recently, PARP4 mRNA expression was found to be significantly upregulated in cisplatin-resistant ovarian cancer cell lines and depletion of its expression in cisplatin-resistant cell lines reduced cisplatin chemoresistance, restoring the cisplatin-induced DNA fragmentation (Ref. Reference Sung53). PARP4 expression levels correlate with the DNA methylation status at its specific promoter site, a feature that could be exploited as a novel biomarker for predicting the response to cisplatin in ovarian cancer patients (Ref. Reference Sung53).

PARP6 has been shown to be involved in the genesis of different tumours; however, its mechanism of action is still controversial and not completely characterised. PARP6 was initially described as a negative regulator of cell proliferation. Indeed, according to both immunohistochemical and Kaplan–Meier analysis, PARP6-positive colorectal cancer correlated with a good prognosis, supporting a role of PARP6 as a tumour suppressor through its involvement in cell-cycle control and apoptosis induction (Refs Reference Tuncel54, Reference Qi55). These findings were further confirmed for HCC by subsequent studies, where PARP6 was found to negatively correlate with the degree of tumour differentiation of HCC. Mechanistically, PARP6 inhibited the Wnt/β-catenin signalling pathway, by promoting XRCC6 protein degradation (Ref. Reference Tang56). On the same line, in breast cancer cells, pharmacological and genetic inhibition of PARP6 caused mitotic defects in breast cancer cells, leading to apoptosis induction (Ref. Reference Wang57). Differently, PARP6 seems to have an opposite role in gastric cancer, where it promoted cell proliferation, migration and invasion, by activating the survivin pathway (Ref. Reference Sun58). Collectively, these findings demonstrate a central role of PARP6 in cancer development and a novel drug target selective for a subset of tumours.

PARP8 is localised at the nuclear envelope, except during mitosis where it localises at centrosomes and at spindle poles (Ref. Reference Vyas59). A specific function of PARP8 has not been described yet. Recently, in uveal melanoma, depletion of PARP8 correlates with a decrease in proliferation and migration rate of cancer cells (Ref. Reference Li60). Further studies are therefore required to identify pathways regulated by this enzyme.

PARP16 is the only PARP that possesses a C-terminal trans-membrane domain associated with the nuclear envelope and the endoplasmic reticulum (ER). PARP16 plays a key role in the activation of ER stress sensors, PERK and IRE1α, involved in the unfolded protein response (Ref. Reference Jwa and Chang61). In Drosophila, it was discovered that MARylation mediated by PARP16 is also involved in the metabolic stress response (Ref. Reference Aguilera-Gomez62). PARP16 has been shown to MARylate ribosomal proteins, and its catalytic activity is supported by the NAD-synthetising enzyme NMNAT2. Inhibiting PARP16 enhances translation of specific mRNAs, causing ER-stress-induced cancer cell apoptosis (Ref. Reference Challa63). The role exerted by PARP16 in the regulation of stress response and translation made it a target for new combinatorial therapies. Remarkably, in small cell lung cancer, treatment with the PARP1 inhibitor talazoparib, which also targets PARP16, is more efficient in reducing cancer cell survival. Moreover, PARP16 silencing reduces cell survival in combination with olaparib and adavosertib, a WEE1/PLK1 inhibitor. Collectively, these findings highlight a contribution of PARP16 in talazoparib-mediated mechanism of action and suggest PARP16 as a novel alternative pharmacological target in small cell lung cancer (Ref. Reference Palve64).

PARP inhibitors

Majority of the efforts in PARP inhibition in cancer therapies were initially focused on PARP1 because of its key role in DNA integrity maintenance. Olaparib was the first PARP inhibitor approved by the Food and Drugs Administration in 2014 and used in therapy to cure breast, ovarian and prostate cancers in patients carrying defects in HR machineries.

In this frame, the discovery of mono-ARTs as regulator of the IFN-I signalling in cancer progression is promoting continuous efforts to better characterise these enzymes and to generate new inhibitors to improve the outcome of the current therapies for the treatment of different cancers.

One of the challenges in the generation of selective inhibitors is due to the highly conserved catalytic domains shared among PARPs. In the last years, an important advance has been made for the synthesis of PARP7, PARP10 and PARP14 inhibitors, thus making them potential targets for new therapeutical approaches.

The PARP7 inhibitor RBN2397 is currently tested in phase-1 clinical trials, both in breast and lung cancer cells, enhancing the IFN-I signalling pathways negatively regulated by PARP7, and in prostate cancer cells, where PARP7 regulates the stability of AR, leading to an inhibition of prostate cancer cells growth in culture (Ref. Reference Wang65). Moreover, it was also observed that the use of RBN-2397 inhibitor after PARP1 depletion enhanced IFN-I signalling, suggesting that the inhibition of PARP1 could enhance the antitumoural immunity of PARP7 (Ref. Reference Yang66). This aspect has been recently reviewed by Cohen and coworkers and we refer the reader to this text for a complete overview on the activation of innate immunity mediated by PARP1 inhibitors (Ref. Reference Brooks, Anand and Cohen14).

Exploring the intricate connections among PARPs, MARylation, cancer biology and IFN signalling is essential for advancing our understanding of these processes. Insights into these interactions may open avenues for the development of targeted therapies and interventions in cancer treatment.