Phosphatase 1 Nuclear Targeting Subunit Mediates Recruitment and Function of Poly (ADP-Ribose) Polymerase 1 in DNA Repair

Human PARP1 substrates and regulators of its catalytic activity: An updated overview

Poly (ADP-ribose) polymerase 1 (PARP1) is a key DNA damage sensor that is recruited to damaged sites after DNA strand breaks to initiate DNA repair. This is achieved by catalyzing attachment of ADP-ribose moieties, which are donated from NAD⁺, on the amino acid residues of itself or other acceptor proteins. PARP inhibitors (PARPi) that inhibit PARP catalytic activity and induce PARP trapping are commonly used for treating BRCA1/2-deficient breast and ovarian cancers through synergistic lethality. Unfortunately, resistance to PARPi frequently occurs. In this review, we present the novel substrates and regulators of the PARP1-catalyzed poly (ADP-ribosyl)ation (PARylatison) that have been identified in the last 3 years. The overall aim is the presentation of protein interactions of potential therapeutic intervention for overcoming the resistance to PARPi.

Replication Stress: A Review of Novel Targets to Enhance Radiosensitivity-From Bench to Clinic

DNA replication is a process fundamental in all living organisms in which deregulation, known as replication stress, often leads to genomic instability, a hallmark of cancer. Most malignant tumors sustain persistent proliferation and tolerate replication stress via increasing reliance to the replication stress response. So whilst replication stress induces genomic instability and tumorigenesis, the replication stress response exhibits a unique cancer-specific vulnerability that can be targeted to induce catastrophic cell proliferation. Radiation therapy, most used in cancer treatment, induces a plethora of DNA lesions that affect DNA integrity and, in-turn, DNA replication. Owing to radiation dose limitations for specific organs and tumor tissue resistance, the therapeutic window is narrow. Thus, a means to eliminate or reduce tumor radioresistance is urgently needed. Current research trends have highlighted the potential of combining replication stress regulators with radiation therapy to capitalize on the high replication stress of tumors. Here, we review the current body of evidence regarding the role of replication stress in tumor progression and discuss potential means of enhancing tumor radiosensitivity by targeting the replication stress response. We offer new insights into the possibility of combining radiation therapy with replication stress drugs for clinical use.

PARP1 Upregulation in Recurrent Oral Cancer and Treatment Resistance

First-line treatments for oral cancer typically include surgery, radiation, and in some cases, chemotherapy. Radiation and oral cancer chemotherapeutics confer cytotoxicity largely by inducing DNA damage, underscoring the importance of the cellular DNA damage repair and response pathways in cancer therapy. However, tumor recurrence and acquired resistance, following the initial response to treatment, remains as a major clinical challenge. By analyzing oral tumor cells derived from the primary and recurrent tumors of the same patient, our study revealed upregulated PARP1 expression in the recurrent tumor cells. Cisplatin and 5-fluorouracil treatment further augmented PARP1 expression in the recurrent, but not the primary, tumor cells. Post-treatment upregulation of PARP1 was dependent on the catalytic activities of PARP and CDK7. Consistent with the established function of PARP1 in DNA repair, we showed that overexpression of PARP1 rendered the primary tumor cells highly resistant to DNA damage treatment. Conversely, PARP inhibition partially reversed the treatment resistance in the recurrent tumor cells; combinatorial treatment using a PARP inhibitor and cisplatin/5-fluorouracil significantly sensitized the tumor response in vivo. Taken together, we reported here PARP1 upregulation as a clinically relevant mechanism involved in oral cancer recurrence, and suggested the clinical benefit of PARP inhibitors, currently approved for the treatment of several other types of cancer, in oral cancer.

PARP1-modulated chromatin remodeling is a new target for cancer treatment

Cancer progression requires certain tumorigenic mutations in genes encoding for different cellular and nuclear proteins. Altered expressions of these mutated genes are mediated by post-translational modifications and chromatin remodeling. Chromatin remodeling is mainly regulated by the chromatin remodeling enzyme complexes and histone modifications. Upon DNA damage, Poly-(ADP-ribose) Polymerase1 (PARP1) plays a very important role in the induction of chromatin modifications and activation of DNA repair pathways to repair the DNA lesion. It has been targeted to develop different anti-cancer therapeutic interventions and PARP inhibitors have been approved by the U.S. Food and Drug Administration (FDA) for clinical use. But it has been found that the cancer cells often develop resistance to these PARP inhibitors and chromatin remodeling helps in enhancing this process. Hence, it may be beneficial to target PARP1-mediated chromatin remodeling, which may allow to reverse the drug resistance. In the current review, we have discussed the role of chromatin remodeling in DNA repair, how PARP1 regulates modifications of chromatin dynamics, and the role of chromatin modifications in cancer. It has also been discussed how the PARP1-mediated chromatin remodeling can be targeted by PARP inhibitors alone or in combination with other chemotherapeutic agents to establish novel anti-cancer therapeutics. We have also considered the use of PARG inhibitors that may enhance the action of PARP inhibitors to target different types of cancers.

Upregulation of Phosphatase 1 Nuclear-Targeting Subunit (PNUTS) Is an Independent Predictor of Poor Prognosis in Prostate Cancer

Protein phosphatase 1 nuclear-targeting subunit (PNUTS) is ubiquitously expressed and associates with PTEN and protein phosphatase 1 (PP1) to control its activity. The role of PNUTS overexpression has hardly been studied in cancer. In this study, we used immunohistochemistry to quantitate PNUTS expression on a tissue microarray containing 17,747 clinical prostate cancer specimens. As compared to normal prostate epithelium, PNUTS expression was often higher in cancer. Among 12,235 interpretable tumors, PNUTS staining was negative in 21%, weak in 34%, moderate in 35%, and strong in 10% of cases. High PNUTS expression was associated with higher tumor stage, classical and quantitative Gleason grade, nodal stage, surgical margin, Ki67 labeling index, and early biochemical recurrence (p < 0.0001 each). PNUTS expression proved to be a moderate prognostic parameter with a maximal univariable Cox proportional hazard for PSA recurrence-free survival of 2.21 compared with 5.91 for Gleason grading. It was independent from established prognostic parameters in multivariable analysis. Comparison with molecular data available from earlier studies using the same TMA identified associations between high PNUTS expression and elevated androgen receptor expression (p < 0.0001), presence of TMPRSS2:ERG fusion (p < 0.0001), and 8 of 11 chromosomal deletions (3p13, 5q21, 8p21, 10q23, 12p13, 13q14, 16q24, and 17p13; p < 0.05 each). Particularly strong associations with PTEN and 12p13 deletions (p < 0.0001 each) may indicate a functional relationship, which has already been established for PNUTS and PTEN. PNUTS had no additional role on outcome in PTEN-deleted cancers. In conclusion, the results of our study identify high PNUTS protein levels as a predictor of poor prognosis possibly linked to increased levels of genomic instability. PNUTS measurement, either alone or in combination, might be of clinical utility in prostate cancers.

Poly(ADP-Ribose)Polymerase (PARP) Inhibitors and Radiation Therapy

Poly(ADP-ribose)polymerase-1 (PARP1) is a DNA repair enzyme highly expressed in the nuclei of mammalian cells, with a structure and function that have attracted interest since its discovery. PARP inhibitors, moreover, can be used to induce synthetic lethality in cells where the homologous recombination (HR) pathway is deficient. Several small molecule PARP inhibitors have been approved by the FDA for multiple cancers bearing this deficiency These PARP inhibitors also act as radiosensitizing agents by delaying single strand break (SSB) repair and causing subsequent double strand break (DSB) generation, a concept that has been leveraged in various preclinical models of combination therapy with PARP inhibitors and ionizing radiation. Researchers have determined the efficacy of various PARP inhibitors at sub-cytotoxic concentrations in radiosensitizing multiple human cancer cell lines to ionizing radiation. Furthermore, several groups have begun evaluating combination therapy strategies in mouse models of cancer, and a fluorescent imaging agent that allows for subcellular imaging in real time has been developed from a PARP inhibitor scaffold. Other PARP inhibitor scaffolds have been radiolabeled to create PET imaging agents, some of which have also entered clinical trials. Most recently, these highly targeted small molecules have been radiolabeled with therapeutic isotopes to create radiotherapeutics and radiotheranostics in cancers whose primary interventions are surgical resection and whole-body radiotherapy. In this review we discuss the utilization of these small molecules in combination therapies and in scaffolds for imaging agents, radiotherapeutics, and radiotheranostics. Development of these radiolabeled PARP inhibitors has presented promising results for new interventions in the fight against some of the most intractable cancers.

KRCC1: A potential therapeutic target in ovarian cancer

Using a systems biology approach to prioritize potential points of intervention in ovarian cancer, we identified the lysine rich coiled-coil 1 (KRCC1), as a potential target. High-grade serous ovarian cancer patient tumors and cells express significantly higher levels of KRCC1 which correlates with poor overall survival and chemoresistance. We demonstrate that KRCC1 is predominantly present in the chromatin-bound nuclear fraction, interacts with HDAC1, HDAC2, and with the serine-threonine phosphatase PP1CC. Silencing KRCC1 inhibits cellular plasticity, invasive properties, and potentiates apoptosis resulting in reduced tumor growth. These phenotypes are associated with increased acetylation of histones and with increased phosphorylation of H2AX and CHK1, suggesting the modulation of transcription and DNA damage that may be mediated by the action of HDAC and PP1CC, respectively. Hence, we address an urgent need to develop new targets in cancer.

Phosphatase 1 Nuclear Targeting Subunit, a Novel DNA Repair Partner of PARP1

There is extensive literature on PARP1, owing to the multiple functions of PARP1 for DNA repair, aging, chromatin remodeling and transcription, and to the successful clinical development of four PARP inhibitors. In this issue of Cancer Research, Wang and colleagues identified phosphatase 1 nuclear targeting subunit (PNUTS, also referred to as PP1R10) as a partner of PARP1 and demonstrated that PNUTS and PARP1 are interdependent for their recruitment to DNA damage sites. As PARP1 is a prominent target for cancer therapy and PNUTS has PARP-dependent as well as PARP1-independent activities, the findings of Wang and colleagues add insight into DNA repair mechanisms with potential application to drug targeting and precision therapy.See related article by Wang et al., p. 2526.