Downregulation of PARP1 transcription by CDK4/6 inhibitors sensitizes human lung cancer cells to anticancer drug-induced death by impairing OGG1-dependent base excision repair

Eugenol as a potential adjuvant therapy for gingival squamous cell carcinoma

Adoption of plant-derived compounds for the management of oral cancer is encouraged by the scientific community due to emerging chemoresistance and conventional treatments adverse effects. Considering that very few studies investigated eugenol clinical relevance for gingival carcinoma, we ought to explore its selectivity and performance according to aggressiveness level. For this purpose, non-oncogenic human oral epithelial cells (GMSM-K) were used together with the Tongue (SCC-9) and Gingival (Ca9-22) squamous cell carcinoma lines to assess key tumorigenesis processes. Overall, eugenol inhibited cell proliferation and colony formation while inducing cytotoxicity in cancer cells as compared to normal counterparts. The recorded effect was greater in gingival carcinoma and appears to be mediated through apoptosis induction and promotion of p21/p27/cyclin D1 modulation and subsequent Ca9-22 cell cycle arrest at the G0/G1 phase, in a p53-independent manner. At these levels, distinct genetic profiles were uncovered for both cell lines by QPCR array. Moreover, it seems that our active component limited Ca9-22 and SCC-9 cell migration respectively through MMP1/3 downregulation and stimulation of inactive MMPs complex formation. Finally, Ca9-22 behaviour appears to be mainly modulated by the P38/STAT5/NFkB pathways. In summary, we can disclose that eugenol is cancer selective and that its mediated anti-cancer mechanisms vary according to the cell line with gingival squamous cell carcinoma being more sensitive to this phytotherapy agent.

Effect of CDK4/6 Inhibitors on Tumor Immune Microenvironment

Cancer is an abnormal proliferation of cells that is stimulated by cyclin-dependent kinases (CDKs) and defective cell cycle regulation. The essential agent that drive the cell cycle, CDK4/6, would be activated by proliferative signals. Activated CDK4/6 results in the phosphorylation of the neuroblastoma protein (RB) and the release of the transcription factor E2F, which promotes the cell cycle progression. CDK4/6 inhibitor (CDK4/6i) has been currently a research focus, which inhibits the CDK4/6-RB-E2F axis, thereby reducing the cell cycle transition from G1 to S phase and mediating the cell cycle arrest. This action helps achieve an anti-tumor effect. Recent research has demonstrated that CDK4/6i, in addition to contributing to cell cycle arrest, is also essential for the interaction between the tumor cells and the host immune system, i.e., activating the immune system, strengthening the tumor antigen presentation, and reducing the number of regulatory T cells (Treg). Additionally, CDK4/6i would elevate the level of PD-L1, an immunosuppressive factor, in tumor cells, and CDK4/6i in combination with anti-PD-L1 therapy would more effectively reduce the tumor growth. Our results showed that CDK4/6i caused autophagy and senescence in tumor cells. Herein, the impact of CDK4/6i on the immune microenvironment of malignant tumors was mainly focused, as well as their interaction with immune checkpoint inhibitors in affecting anti-tumor immunity.

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.

CDK4/6 inhibitors in lung cancer: current practice and future directions

Lung cancer is the leading cause of cancer-related deaths worldwide, and ∼85% of lung cancers are classified as nonsmall cell lung cancer (NSCLC). These malignancies can proliferate indefinitely, in part due to dysregulation of the cell cycle and the resulting abnormal cell growth. The specific activation of cyclin-dependent kinases 4 and 6 (CDK4/6) is closely linked to tumour proliferation. Approximately 80% of human tumours exhibit abnormalities in the cyclin D-CDK4/6-INK4-RB pathway. Specifically, CDK4/6 inhibitors either as monotherapy or combination therapy have been investigated in pre-clinical and clinical studies for the treatment of NSCLC, and promising results have been achieved. This review article focuses on research regarding the use of CDK4/6 inhibitors in NSCLC, including the characteristics and mechanisms of action of approved drugs and progress of pre-clinical and clinical research.

Advances in bi-directional relationships for EZH2 and oxidative stress

Over the past two decades, polycomb repressive complex 2(PRC2) has emerged as a vital repressive complex in overall cell fate determination. In mammals, enhancer of zeste homolog 2 (EHZ2), which is the core component of PRC2, has also been recognized as an important regulator of inflammatory, redox, tumorigenesis and damage repair signalling networks. To exert these effects, EZH2 must regulate target genes epigenetically or interact directly with other gene expression-regulating factors, such as LncRNAs and microRNAs. Our review provides a comprehensive summary of research advances, discoveries and trends regarding the regulatory mechanisms between EZH2 and reactive oxygen species (ROS). First, we outline novel findings about how EZH2 regulates the generation of ROS at the molecular level. Then, we summarize how oxidative stress controls EHZ2 alteration (upregulation, downregulation, or phosphorylation) via various molecules and signalling pathways. Finally, we address why EZH2 and oxidative stress have an undefined relationship and provide potential future research ideas.

Novel therapeutic strategies in pancreatic cancer: moving beyond cytotoxic chemotherapy

INTRODUCTION

Prognosis of patients with metastatic pancreatic ductal adenocarcinoma (mPDAC) remains disappointing with a 5-year overall survival of only 3-5%. Compared to other cancers, the evolution in standard therapeutic options has been stagnant and polychemotherapy regimens (with well-known toxicity profile and resistance pattern) remain standard of care. Only for patients (5%-7%) with a breast cancer gene (BRCA) pathogenic germline variant, prognosis has improved by the use of olaparib (poly-ADP ribose polymerase (PARP) inhibitor).

AREAS COVERED

This review covers emerging treatment strategies in the management of mPDAC. One of the main topics is the rigid and immunological cold tumor microenvironment (TME) of PDAC and the search for agents that impact this TME and/or engage the immune system. In addition, the use of next-generation sequencing (NGS) has elicited for some patients new targeted therapies directed at alterations in the RTK/RAS/MAPK pathway and the deoxyribonucleic acid (DNA) damage repair pathway. Other evolving treatment strategies are also discussed.

EXPERT OPINION

The search for new, often combination, treatment strategies for mPDAC should be encouraged and implemented in early treatment lines given the significant decline of performance status of patients in later lines. NGS analysis should be used where available, although cost-effectiveness could be debatable.

OGG1 in the Kidney: Beyond Base Excision Repair

8-Oxoguanine DNA glycosylase (OGG1) is a repair protein for 8-oxoguanine (8-oxoG) in eukaryotic atopic DNA. Through the initial base excision repair (BER) pathway, 8-oxoG is recognized and excised, and subsequently, other proteins are recruited to complete the repair. OGG1 is primarily located in the cytoplasm and can enter the nucleus and mitochondria to repair damaged DNA or to exert epigenetic regulation of gene transcription. OGG1 is involved in a wide range of physiological processes, such as DNA repair, oxidative stress, inflammation, fibrosis, and autophagy. In recent years, studies have found that OGG1 plays an important role in the progression of kidney diseases through repairing DNA, inducing inflammation, regulating autophagy and other transcriptional regulation, and governing protein interactions and functions during disease and injury. In particular, the epigenetic effects of OGG1 in kidney disease have gradually attracted widespread attention. This study reviews the structure and biological functions of OGG1 and the regulatory mechanism of OGG1 in kidney disease. In addition, the possibility of OGG1 as a potential therapeutic target in kidney disease is discussed.

OGG1 in Lung—More than Base Excision Repair

As the organ executing gas exchange and directly facing the external environment, the lungs are challenged continuously by various stimuli, causing the disequilibration of redox homeostasis and leading to pulmonary diseases. The breakdown of oxidants/antioxidants system happens when the overproduction of free radicals results in an excess over the limitation of cleaning capability, which could lead to the oxidative modification of macromolecules including nucleic acids. The most common type of oxidative base, 8-oxoG, is considered the marker of DNA oxidative damage. The appearance of 8-oxoG could lead to base mismatch and its accumulation might end up as tumorigenesis. The base 8-oxoG was corrected by base excision repair initiated by 8-oxoguanine DNA glycosylase-1 (OGG1), which recognizes 8-oxoG from the genome and excises it from the DNA double strand, generating an AP site for further processing. Aside from its function in DNA damage repairment, it has been reported that OGG1 takes part in the regulation of gene expression, derived from its DNA binding characteristic, and showed impacts on inflammation. Researchers believe that OGG1 could be the potential therapy target for relative disease. This review intends to make an overall summary of the mechanism through which OGG1 regulates gene expression and the role of OGG1 in pulmonary diseases.

Tri-2-Hydroxyarachidonein Induces Cytocidal Autophagy in Pancreatic Ductal Adenocarcinoma Cancer Cell Models

Cell proliferation in pancreatic cancer is determined by a complex network of signaling pathways. Despite the extensive understanding of these protein-mediated signaling processes, there are no significant drug discoveries that could considerably improve a patient’s survival. However, the recent understanding of lipid-mediated signaling gives a new perspective on the control of the physiological state of pancreatic cells. Lipid signaling plays a major role in the induction of cytocidal autophagy and can be exploited using synthetic lipids to induce cell death in pancreatic cancer cells. In this work, we studied the activity of a synthetic lipid, tri-2-hydroxyarachidonein (TGM4), which is a triacylglycerol mimetic that contains three acyl moieties with four double bonds each, on cellular and in vivo models of pancreatic cancer. We demonstrated that TGM4 inhibited proliferation of Mia-PaCa-2 (human pancreatic carcinoma) and PANC-1 (human pancreatic carcinoma of ductal cells) in in vitro models and in an in vivo xenograft model of Mia-PaCa-2 cells. In vitro studies demonstrated that TGM4 induced cell growth inhibition paralleled with an increased expression of PARP and CHOP proteins together with the presence of sub-G0 cell cycle events, indicating cell death. This cytocidal effect was associated with elevated ER stress or autophagy markers such as BIP, LC3B, and DHFR. In addition, TGM4 activated peroxisome proliferator-activated receptor gamma (PPAR-γ), which induced elevated levels of p-AKT and downregulation of p-c-Jun. We conclude that TGM4 induced pancreatic cell death by activation of cytocidal autophagy. This work highlights the importance of lipid signaling in cancer and the use of synthetic lipid structures as novel and potential approaches to treat pancreatic cancer and other neoplasias.

CDK4 and CDK6 kinases: From basic science to cancer therapy

Cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) and their activating partners, D-type cyclins, link the extracellular environment with the core cell cycle machinery. Constitutive activation of cyclin D–CDK4/6 represents the driving force of tumorigenesis in several cancer types. Small-molecule inhibitors of CDK4/6 have been used with great success in the treatment of hormone receptor–positive breast cancers and are in clinical trials for many other tumor types. Unexpectedly, recent work indicates that inhibition of CDK4/6 affects a wide range of cellular functions such as tumor cell metabolism and antitumor immunity. We discuss how recent advances in understanding CDK4/6 biology are opening new avenues for the future use of cyclin D–CDK4/6 inhibitors in cancer treatment.

Effects of bisphenol A exposure during cardiac cell differentiation

Heart development requires a precise temporal regulation of gene expression in cardiomyoblasts. Therefore, the transcriptional changes in differentiating cells can lead to congenital heart diseases. Although the genetic mutations underlie most of these alterations, exposure to environmental contaminants, such as bisphenol A (BPA), has been recently considered as a risk factor as well. In this study we investigated the genotoxic and epigenotoxic effects of BPA throughout cardiomyocyte differentiation. H9c2 cells (rat myoblasts) were exposed to 10 and 30 μM BPA before and during the last two days of cardiac-driven differentiation. Then, we have analysed the phenotypic and molecular modifications (at transcriptional, genetic and epigenetic level). The results showed that treated myoblasts developed a skeletal muscle cell-like phenotype. The transcriptional changes induced by BPA in genes codifying proteins involved in heart differentiation and function depend on the window of exposure to BPA. The exposure before differentiation repressed the expression of heart transcription factors (Hand2 and Gata4), whereas exposure during differentiation reduced the expression of cardiac-specific genes (Tnnt2, Myom2, Sln, and Atp2a1). Additionally, significant effects were observed regarding DNA damage and histone acetylation levels after the two periods of BPA exposure: in cells exposed to the toxicant the percentage of DNA repair foci (formed by the co-localization of γH2AX and 53BP1) increased in a dose-dependent manner, whereas the treatment with the toxicant triggered a decrease in the epigenetic marks H3K9ac and H3K27ac. Our in vitro results reveal that BPA seriously interferes with the process of cardiomyocyte differentiation, which could be related to the reported in vivo effects of this toxicant on cardiogenesis.

Mechanisms and Implications of CDK4/6 Inhibitors for the Treatment of NSCLC

Cyclin-dependent kinases (CDKs) are key regulators of cell cycle progression in malignant tumor cells and play an important role through complex molecular interactions. Dysregulation of CDK dependent pathways is often found in non-small cell lung cancer, which indicates its vulnerability and can be used in clinical benefit. CDK4/6 inhibitors can prevent tumor cells from entering the G approved 1 and S phases, which have been studied in a series of explorations and brought great clinical effect to patients and encouragement to both physicians and researchers, thereby showing potential as a new therapeutic agent. A series of preclinical and clinical studies have been carried out on CDK4/6 inhibitors in NSCLC, and have been achieved some results, which may become a new potential treatment in the future. This review focuses on the research progress on CDK4/6 inhibitors in NSCLC, particularly the mechanisms of action, drugs, clinical research progress, and future application.

Combination of Talazoparib and Palbociclib as a Potent Treatment Strategy in Bladder Cancer

The use of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors represents a potent strategy for cancer therapy. Due to the complex molecular network that regulates cell cycle progression, cancer cells often acquire resistance mechanisms against these inhibitors. Previously, our group identified molecular factors conferring resistance to CDK4/6 inhibition in bladder cancer (BLCA) that also included components within the DNA repair pathway. In this study, we validated whether a combinatory treatment approach of the CDK4/6 inhibitor Palbociclib with Poly-(ADP-Ribose) Polymerase (PARP) inhibitors improves therapy response in BLCA. First, a comparison of PARP inhibitors Talazoparib and Olaparib showed superior efficacy of Talazoparib in vitro and displayed high antitumor activity in xenografts in the chicken chorioallantoic membrane (CAM) model. Moreover, the combination of Talazoparib and the CDK4/6 inhibitor Palbociclib synergistically reduced tumor growth in Retinoblastoma protein (RB)-positive BLCA in vitro and in a CAM model, an effect that relies on Palbociclib-induced cell cycle arrest in G0/G1-phase complemented by a G2 arrest induced by Talazoparib. Interestingly, Talazoparib-induced apoptosis was reduced by Palbociclib. The combination of Palbociclib and Talazoparib effectively enhances BLCA therapy, and RB is a molecular biomarker of response to this treatment regimen.

Flavonoids' Effects on Caenorhabditis elegans' Longevity, Fat Accumulation, Stress Resistance and Gene Modulation Involve mTOR, SKN-1 and DAF-16

Flavonoids are potential nutraceutical compounds present in diary food. They are considered health-promoting compounds and promising drugs for different diseases, such as neurological and inflammatory diseases, diabetes and cancer. Therefore, toxicological and mechanistic studies should be done to assert the biological effects and identify the molecular targets of these compounds. In this work we describe the effects of six structurally-related flavonoids—baicalein, chrysin, scutellarein, 6-hydroxyflavone, 6,7-dihydroxyflavone and 7,8-dihydroxyflavone—on Caenorhabditis elegans’ lifespan and stress resistance. The results showed that chrysin, 6-hydroxyflavone and baicalein prolonged C. elegans’ lifespan by up to 8.5%, 11.8% and 18.6%, respectively. The lifespan extensions caused by these flavonoids are dependent on different signaling pathways. The results suggested that chrysin’s effects are dependent on the insulin signaling pathway via DAF-16/FOXO. Baicalein and 6-hydroxyflavone’s effects are dependent on the SKN-1/Nfr2 pathway. In addition, microarray analysis showed that baicalein downregulates important age-related genes, such as mTOR and PARP.

Fraxetin Suppresses Cell Proliferation and Induces Apoptosis through Mitochondria Dysfunction in Human Hepatocellular Carcinoma Cell Lines Huh7 and Hep3B

Fraxetin is a coumarin scaffold compound extracted from Fraxinus rhynchophylla. It has antioxidant, anti-inflammatory, hepatoprotective, and antifibrotic effects. Furthermore, fraxetin has anticancer effects in breast and lung cancer. We aimed to evaluate whether fraxetin has anticancer activity in hepatocellular carcinoma (HCC) cells and its underlying mechanism. We demonstrated the anticancer effects of fraxetin in the HCC cell lines Huh7 and Hep3B. We confirmed that fraxetin inhibited cell proliferation (42% ± 10% Huh7; 52% ± 7% Hep3B) by arresting the cell cycle and inducing apoptosis in both cell lines. Moreover, fraxetin increased reactive oxygen species production (221% ± 55% Huh7; 460% ± 73% Hep3B), depolarized the mitochondrial membranes (ΔΨm) (345% ± 160% Huh7; 462% ± 140% Hep3B), and disrupted calcium homeostasis in both HCC cell lines. Chelating calcium ions with BAPTA-AM restored proliferation in fraxetin-treated Huh7 cells but not in Hep3B cells. Fraxetin did not affect the phosphorylation of extracellular-signal-regulated kinase 1/2, whereas it decreased JNK and phosphoinositide 3-kinase signaling. Furthermore, fraxetin and mitogen-activated protein kinase pharmacological inhibitors had synergistic antiproliferative effects on HCC cells. Although our study was limited to in vitro data that require validation, we suggest that fraxetin is a potential therapeutic agent against HCC progression.

Targeting DNA Repair, Cell Cycle, and Tumor Microenvironment in B Cell Lymphoma

The DNA double-strand break (DSB) is the most cytotoxic lesion and compromises genome stability. In an attempt to efficiently repair DSBs, cells activate ATM kinase, which orchestrates the DNA damage response (DDR) by activating cell cycle checkpoints and initiating DSB repair pathways. In physiological B cell development, however, programmed DSBs are generated as intermediates for effective immune responses and the maintenance of genomic integrity. Disturbances of these pathways are at the heart of B cell lymphomagenesis. Here, we review the role of DNA repair and cell cycle control on B cell development and lymphomagenesis. In addition, we highlight the intricate relationship between the DDR and the tumor microenvironment (TME). Lastly, we provide a clinical perspective by highlighting treatment possibilities of defective DDR signaling and the TME in mantle cell lymphoma, which serves as a blueprint for B cell lymphomas.

A first-in-class CDK4 inhibitor demonstrates in vitro, ex-vivo and in vivo efficacy against ovarian cancer

INTRODUCTION

Cyclin-dependent kinases 4 and 6 (CDK4/6) are fundamental drivers of the cell cycle and are involved in the initiation and progression of various cancers. Deregulation of the CDK4/6-cyclin D-retinoblastoma (Rb) pathway is common in ovarian cancer and is associated with an aggressive phenotype and poor prognosis. Patients with advanced ovarian cancer whose tumor demonstrates Rb-positivity, a low expression of p16 and overexpression of cyclin D1 are most likely to benefit from CDK4/6 inhibition.

MATERIALS AND METHOD

Anti-proliferative activity and mechanistic investigations for CDDD2-94, employing palbociclib as comparator, were evaluated by MTT assay, cell cycle and apoptosis analysis, western blotting as well as senescence and colony formation assay. In vivo safety and efficacy studies were done in A2780 tumor-bearing nude mice. Combinations of CDDD2-94 with mTOR, MEK, PI3K or PARP inhibitors were evaluated in A2780 and OVCAR5 ovarian cancer cells.

RESULTS

Consistent with a CDK4-targeted mechanism, CDDD2-94 arrested the G1/G0 cell cycle, induced senescence and inhibited the proliferation of Rb-proficient ovarian cancer cells. CDDD2-94 exhibited synergistic anti-proliferative activities with mTOR, MEK, PI3K or PARP inhibitors. Importantly, unlike palbociclib which caused significant reductions in the number of lymphocytes and neutrophils, CDDD2-94 had little effect. CDDD2-94, as single agent and in combination with everolimus, delayed tumor growth and significantly increased survival of mice.

CONCLUSION

Given its high specificity in targeting CDK4 and excellent anti-tumor efficacy with low toxicity, CDDD2-94 has potential to be developed as a standalone agent or in combination with targeted therapeutics for the treatment of ovarian cancer.

The Role of PARP1 in Monocyte and Macrophage Commitment and Specification: Future Perspectives and Limitations for the Treatment of Monocyte and Macrophage Relevant Diseases with PARP Inhibitors

Modulation of PARP1 expression, changes in its enzymatic activity, post-translational modifications, and inflammasome-dependent cleavage play an important role in the development of monocytes and numerous subtypes of highly specialized macrophages. Transcription of PARP1 is governed by the proliferation status of cells at each step of their development. Higher abundance of PARP1 in embryonic stem cells and in hematopoietic precursors supports their self-renewal and pluri-/multipotency, whereas a low level of the enzyme in monocytes determines the pattern of surface receptors and signal transducers that are functionally linked to the NFκB pathway. In macrophages, the involvement of PARP1 in regulation of transcription, signaling, inflammasome activity, metabolism, and redox balance supports macrophage polarization towards the pro-inflammatory phenotype (M1), which drives host defense against pathogens. On the other hand, it seems to limit the development of a variety of subsets of anti-inflammatory myeloid effectors (M2), which help to remove tissue debris and achieve healing. PARP inhibitors, which prevent protein ADP-ribosylation, and PARP1‒DNA traps, which capture the enzyme on chromatin, may allow us to modulate immune responses and the development of particular cell types. They can be also effective in the treatment of monocytic leukemia and other cancers by reverting the anti- to the proinflammatory phenotype in tumor-associated macrophages.

Chemotherapy and CDK4/6 Inhibition in Cancer Treatment: Timing Is Everything

Chemotherapy remains the main treatment option for patients with several tumor types. In this issue of Cancer Cell, Salvador-Barbero et al. demonstrate that treatment with CDK4/6 inhibitors after application of taxanes (or other chemotherapeutic compounds) strongly potentiates the anti-tumor effect due to repression of DNA repair machinery.

CDK4/6 Inhibitors Impair Recovery from Cytotoxic Chemotherapy in Pancreatic Adenocarcinoma

Inhibition of the cell-cycle kinases CDK4 and CDK6 is now part of the standard treatment in advanced breast cancer. CDK4/6 inhibitors, however, are not expected to cooperate with DNA-damaging or antimitotic chemotherapies as the former prevent cell-cycle entry, thus interfering with S-phase- or mitosis-targeting agents. Here, we report that sequential administration of CDK4/6 inhibitors after taxanes cooperates to prevent cellular proliferation in pancreatic ductal adenocarcinoma (PDAC) cells, patient-derived xenografts, and genetically engineered mice with Kras G12V and Cdkn2a-null mutations frequently observed in PDAC. This effect correlates with the repressive activity of CDK4/6 inhibitors on homologous recombination proteins required for the recovery from chromosomal damage. CDK4/6 inhibitors also prevent recovery from multiple DNA-damaging agents, suggesting broad applicability for their sequential administration after available chemotherapeutic agents.

BRG1 Activates Proliferation and Transcription of Cell Cycle-Dependent Genes in Breast Cancer Cells

Cancer malignancy is usually characterized by unlimited self-renewal. In some types of advanced tumors that are rapidly dividing, gene expression profiles depict elevations in pro-proliferative genes accompanied by coordinately elevated transcription of factors responsible for removal of DNA lesions. In our studies, fast proliferating breast cancer cell lines (MDA-MB-231 and MCF7), BRG1, a component of the SWI/SNF complex, emerges as an activator of functionally-linked genes responsible for activities such as mitotic cell divisions and DNA repair. Products of at least some of them are considerably overrepresented in breast cancer cells and BRG1 facilitates growth of MCF7 and MDA-MB-231 cell lines. BRG1 occurs at the promoters of genes such as CDK4, LIG1, and NEIL3, which are transcriptionally controlled by cell cycle progression and highly acetylated by EP300 in proliferating cells. As previously documented, in dividing cells BRG1 directly activates gene transcription by evicting EP300 modified nucleosomes from the promoters and, thereby, relaxing chromatin. However, the deficiency of BRG1 or EP300 activity for 48 h leads to cell growth arrest and to chromatin compaction, but also to the assembly of RB1/HDAC1/EZH2 complexes at the studied cell cycle-dependent gene promoters. Epigenetic changes include histone deacetylation and accumulation of H3K27me trimethylation, both known to repress transcription. Cell cycle arrest in G1 by inhibition of CDK4/6 phenocopies the effect of the long-term BRG1 inhibition on the chromatin structure. These results suggest that BRG1 may control gene transcription also by promoting expression of genes responsible for cell cycle progression in the studied breast cancer cells. In the current study, we show that BRG1 binding occurs at the promoters of functionally linked genes in proliferating breast cancer cells, revealing a new mechanism by which BRG1 defines gene transcription.

PAK1 silencing is synthetic lethal with CDK4/6 inhibition in gastric cancer cells via regulating PDK1 expression

Gastric cancer (GC) is one of the most common malignancies worldwide. The prognosis of GC is unsatisfied owning to widespread metastasis. P21-activated kinase 1 (PAK1), a member of serine/threonine kinases, is associated with the progression of multiple types of human cancers. Here, we demonstrated that CDK4/6 inhibitor reduced GC cell viability and decreased PAK1 expression. Consistently, PAK1 ablation increased GC cell sensitivity exposed to CDK4/6 inhibitor and promoted DNA damage. We also revealed PAK1 depletion notably affected PDK1-AKT pathway, and PDK1 overexpression totally abrogated the effect of PAK1 deletion on DNA damage in GC cells. Additionally, PDK1 overexpression also rescued the increased GC cell sensitivity towards CDK4/6 inhibitor and the cell cycle arrest caused by PAK1 depletion. Our findings, therefore, suggested that PAK1 silencing increased sensitivity to CDK4/6 inhibition in gastric cancer cells via PDK1-AKT pathway. We, therefore, thought PAK1 as a promising therapeutic target for the treatment of CDK4/6 inhibitor-resistant gastric cancer.

Fisetin, a phytopolyphenol, targets apoptotic and necroptotic cell death in HepG2 cells

Fisetin (3,7,3',4'-tetrahydroxyflavone), a bioactive dietary flavonoid, intrigued scientists for its anticancer potential against various cancer types. We investigated the fisetin-induced inhibition of growth and survival of human hepatocellular carcinoma. Fisetin decreased cell viability and proliferation of HepG2 cells as revealed from MTT and clonogenicity assays. Cell cycle arrest in the G2/M phase was observed. Annexin V/propidium iodide (PI) staining followed by flow cytometry revealed that fisetin induced both apoptosis and necroptosis in HepG2 cells. Apoptotic cells were significantly increased on fisetin treatment as observed in morphological evaluations and 4',6-diamidino-2-phenylindole and Acridine orange staining. Flow cytometry, fluorescence imaging, and 2', 7'-dichlorofluorescein diacetate analyses showed an increase in reactive oxygen species (ROS) generation on fisetin treatment. Pretreatment with N-acetyl cysteine inhibited ROS production and also rescued mitochondrial membrane potential in HepG2 cells. The underlying mechanisms of apoptosis and necroptosis were determined by analysis of their respective signaling molecules using qRT-PCR and Western blotting. Fisetin showed a marked increase in the expression of TNFα and IKκB with a decrease in NF-κB, pNF-κB and pIKκB expression. Fisetin reduced the expression of Bcl2, and elevated levels of Bax, caspase-3, and PARP and thus induced apoptosis in HepG2 cells. zVAD suppressed the fisetin-induced expression of caspase-8, RIPK1, RIPK3, and MLKL as opposed to fisetin treatment. Nec-1 + fisetin could not completely block necroptosis, which warrants further investigation. Taken together, our findings demonstrate that the fisetin exhibited anti-proliferative effects on HepG2 cells through apoptosis and necroptosis via multiple signaling pathways. Fiestin has potential as a therapeutic agent against hepatocellular carcinoma.

Estrogen receptor coregulator binding modulator (ERX-11) enhances the activity of CDK4/6 inhibitors against estrogen receptor-positive breast cancers

BACKGROUND

CDK4/6 inhibitors in combination with endocrine therapy (AE/AI/SERDs) are approved for the treatment of ER+ advanced breast cancer (BCa). However, not all patients benefit from CDK4/6 inhibitors therapy. We previously reported a novel therapeutic agent, ERX-11, that binds to the estrogen receptor (ER) and modulates ER-coregulator interactions. Here, we tested if the combination of ERX-11 with agents approved for ER+ BCa would be more potent.

METHODS

We tested the effect of combination therapy using BCa cell line models, including those that have acquired resistance to tamoxifen, letrozole, or CDK4/6 inhibitors or have been engineered to express mutant forms of the ER. In vitro activity was tested using Cell Titer-Glo, MTT, and apoptosis assays. Mechanistic studies were conducted using western blot, reporter gene assays, RT-qPCR, and mass spectrometry approaches. Xenograft, patient-derived explants (PDEs), and xenograft-derived explants (XDE) were used for preclinical evaluation and toxicity.

RESULTS

ERX-11 inhibited the proliferation of therapy-resistant BCa cells in a dose-dependent manner, including ribociclib resistance. The combination of ERX-11 and CDK4/6 inhibitor was synergistic in decreasing the proliferation of both endocrine therapy-sensitive and endocrine therapy-resistant BCa cells, in vitro, in xenograft models in vivo, xenograft-derived explants ex vivo, and in primary patient-derived explants ex vivo. Importantly, the combination caused xenograft tumor regression in vivo. Unbiased global mass spectrometry studies demonstrated profound decreases in proliferation markers with combination therapy and indicated global proteomic changes in E2F1, ER, and ER coregulators. Mechanistically, the combination of ERX-11 and CDK4/6 inhibitor decreased the interaction between ER and its coregulators, as evidenced by immunoprecipitation followed by mass spectrometry studies. Biochemical studies confirmed that the combination therapy significantly altered the expression of proteins involved in E2F1 and ER signaling, and this is primarily driven by a transcriptional shift, as noted in gene expression studies.

CONCLUSIONS

Our results suggest that ERX-11 inhibited the proliferation of BCa cells resistant to both endocrine therapy and CDK4/6 inhibitors in a dose-dependent manner and that the combination of ERX-11 with a CDK4/6 inhibitor may represent a viable therapeutic approach.

MacroH2A1 Regulation of Poly(ADP-Ribose) Synthesis and Stability Prevents Necrosis and Promotes DNA Repair

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation.

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation. In this way, we demonstrate that macroH2A1 prevents cellular NAD⁺ depletion, subsequently preventing necrotic cell death that would otherwise occur due to PARP overactivation. We also show that macroH2A1-dependent PAR stabilization promotes efficient repair of oxidative DNA damage. While the role of PAR in recruiting and regulating macrodomain-containing proteins has been established, our results demonstrate that, conversely, macrodomain-containing proteins, and specifically those containing macroH2A1, can regulate PARP1 function through a novel mechanism that promotes both survival and efficient repair during DNA damage response.

PARP1 Co-Regulates EP300-BRG1-Dependent Transcription of Genes Involved in Breast Cancer Cell Proliferation and DNA Repair

BRG1, an active subunit of the SWI/SNF chromatin-remodeling complex, enables the EP300-dependent transcription of proliferation and DNA repair genes from their E2F/CpG-driven promoters in breast cancer cells. In the current study, we show that BRG1-EP300 complexes are accompanied by poly-ADP-ribose polymerase 1 (PARP1), which emerges as the functional component of the promoter-bound multiprotein units that are capable of controlling gene expression. This enzyme is co-distributed with BRG1 at highly acetylated promoters of genes such as CDK4, LIG1, or NEIL3, which are responsible for cancer cell growth and the removal of DNA damage. ADP-ribosylation is necessary to maintain active transcription, since it ensures an open chromatin structure that allows high acetylation and low histone density. PARP1-mediated modification of BRG1 and EP300 does not affect the association of enzymes with gene promoters; however, it does activate EP300, which acetylates nucleosomes, leading to their eviction by BRG1, thus allowing mRNA synthesis. Although PARP1 was found at BRG1 positive/H3K27ac negative promoters of highly expressed genes in a transformed breast cancer cell line, its transcriptional activity was limited to genes simultaneously controlled by BRG1 and EP300, indicating that the ADP-ribosylation of EP300 plays a dominant role in the regulation of BRG1-EP300-driven transcription. In conclusion, PARP1 directs the transcription of some proliferation and DNA repair genes in breast cancer cells by the ADP-ribosylation of EP300, thereby causing its activation and marking nucleosomes for displacement by BRG1. PARP1 in rapidly dividing cells facilitates the expression of genes that confer a cancer cell phenotype. Our study shows a new mechanism that links PARP1 with the removal of DNA damage in breast cancer cells via the regulation of BRG1-EP300-dependent transcription of genes involved in DNA repair pathways.

Neoantimycin F, a Streptomyces-Derived Natural Product Induces Mitochondria-Related Apoptotic Death in Human Non-Small Cell Lung Cancer Cells

Streptomyces-derived natural products have been become a major focus of anti-tumor drug discovery studies. Neoantimycin F (NAT-F), was isolated from Streptomyces conglobatus by our group. Here, we examined the anti-cancer activities and its underlying molecular mechanisms implicated in NAT-F-induced apoptosis of non-small cell lung cancer (NSCLC) cells. Our results showed that NAT-F exerted excellent growth-inhibitory activity against PC9 and H1299 cells in a concentration-dependent manner. NAT-F-induced cell cycle arrest at S and G₀/G₁ phase in PC9 and H1299 cells, respectively. Further investigation revealed that the key proteins (including cyclinD1, cyclinE1, cyclinB1, CDK2, and CDK4) were involved in the cell regulation by NAT-F. Additionally, NAT-F significantly increased the production of reactive oxygen species (ROS), induced DNA damage, nuclear condensation, and cell apoptosis in both cell lines. Moreover, loss of the mitochondrial membrane potential (MMP) was markedly induced by NAT-F. Additional results revealed that NAT-F could up-regulate pro-apoptotic protein Bax and down-regulate anti-apoptotic protein Bcl-2, Mcl-1, and Bcl-x_L, resulting in cytochrome c release from mitochondria and sequential activation of caspase-9 and -3, as well as the cleavage of poly (ADP-ribose) polymerase. Meanwhile, c-Jun N-terminal kinase (JNK), p38 MAPK (p38), and extracellular signal-regulated kinase (ERK) signaling pathway were also involved in anti-cancer activity of NAT-F in NSCLC cells. Taken together, these findings indicated that NAT-F possessed anti-proliferative effect and induced apoptosis in NSCLC cells in vitro and may be conducive to promote the development of novel anti-NSCLC agents.

BRD4 Inhibitor AZD5153 Suppresses the Proliferation of Colorectal Cancer Cells and Sensitizes the Anticancer Effect of PARP Inhibitor

Background: Bromodomain-containing protein 4(BRD4) is reported to play a vital role in the development of numerous malignant diseases, which is considered as a promising target for cancer therapy. AZD5153, a novel specific BRD4 inhibitor, showed potent anticancer effects in several cancer types, but its therapeutic potential has not been fully evaluated in colorectal cancer cells. Objective: We sought to evaluate the therapeutic potential of BRD4 inhibition of by AZD5153 and its combined anticancer cancer effect with PARP inhibitor BMN673 in vitro and in vivo in colorectal cancer. Methods: We analyzed The Cancer Genome Atlas (TCGA) database to investigate BRD4 expression in colorectal cancer patient. Clonogenic assays 、MTT assays and PI/Annexin V staining were used to determine the effect of AZD5153 and BMN673 and combination therapy on cell viability and apoptosis induction. Western blotting was applied to detect relevant molecules changes. Propidium iodide staining was performed to examine cell cycle distributions after monotherapy or combination therapy. Nude mice xenograft model was generated to confirm the therapeutic effect of AZD5153 and BMN673 combination in vivo, and IHC staining was used to detect the expression level of BRD4 and related markers in colorectal patient and xenograft. Results: Analysis of TCGA database indicated that BRD4 was overexpressed in colorectal cancer patient. The clonogenic and MTT assays and PI/Annexin V staining demonstrated that AZD5153 significantly suppressed cell proliferation and induced apoptosis in colorectal cancer cells HCT116 and LoVo. Western blotting showed that AZD5153 inhibited the expression of c-Myc and increased expression of the apoptosis markers, cleaved caspase-3 and poly(ADP-ribose) polymerase (PARP), besides, we found that BRD4 knockdown could also inhibited cell proliferation and induced cell apoptosis. Moreover, AZD5153 inhibited the expression of Wee1 and impaired G2M cell cycle checkpoint, thus sensitized the anticancer effect of BMN673 in vitro and in vivo. Conclusion: Our data revealed that AZD5153suppressed the proliferation of colorectal cancer cells and sensitized them to the anticancer effect of the PARP inhibitor BMN673 via Wee1 inhibition in vitro and in vivo. This suggested that targeting BRD4 might be a valuable strategy for colorectal cancer treatment.

LPS protects macrophages from AIF-independent parthanatos by downregulation of PARP1 expression, induction of SOD2 expression, and a metabolic shift to aerobic glycolysis

In inflamed tissues or during ischemia-reperfusion episodes, activated macrophages produce large amounts of reactive species and are, thus, exposed to the damaging effects of reactive species. Here, our goal was to investigate the mechanism whereby activated macrophages protect themselves from oxidant stress-induced cell death. Hydrogen peroxide-treated mouse bone marrow-derived macrophages (BMDM) and THP-1 human monocyte-derived cells were chosen as models. We found a gradual development of resistance: first in monocyte-to-macrophage differentiation, and subsequently after lipopolysaccharide (LPS) exposure. Investigating the mechanism of the latter, we found that exposure to intense hydrogen peroxide stress causes poly(ADP-ribose) polymerase-1 (PARP-1) dependent programmed necrotic cell death, also known as parthanatos, as indicated by the protected status of PARP-1 knockout BMDMs and the protective effect of the PARP inhibitor PJ-34. In hydrogen peroxide-treated macrophages, however, apoptosis inducing factor (AIF) proved dispensable for parthanatos; nuclear translocation of AIF was not observed. A key event in LPS-mediated protection against the hydrogen peroxide-induced AIF independent parthanatos was downregulation of PARP1 mRNA and protein. The importance of this event was confirmed by overexpression of PARP1 in THP1 cells using a viral promoter, which lead to stable PARP1 levels even after LPS treatment and unresponsiveness to LPS-induced cytoprotection. In BMDMs, LPS-induced PARP1 suppression lead to prevention of NAD⁺ depletion. Moreover, LPS also induced expression of antioxidant proteins (superoxide dismutase-2, thioredoxin reductase 1 and peroxiredoxin) and triggered a metabolic shift to aerobic glycolysis, also known as the Warburg effect. In summary, we provide evidence that in macrophages intense hydrogen peroxide stress causes AIF-independent parthanatos from which LPS provides protection. The mechanism of LPS-mediated cytoprotection involves downregulation of PARP1, spared NAD⁺ and ATP pools, upregulation of antioxidant proteins, and a metabolic shift from mitochondrial respiration to aerobic glycolysis.

PARP1-LSD1 functional interplay controls transcription of SOD2 that protects human pro-inflammatory macrophages from death under an oxidative condition

The function of macrophages makes them vulnerable to several sources of stress and damage, and thus there is a considerable requirement for some form of resilient molecular defence. Differentiation of human macrophages and their further pro-inflammatory (M1) polarization with bacterial endotoxin is associated with increased transcription of PARP1 and SOD2. The latter gene responded immediately to LPS with high NFκB-dependent expression rate, and the resulting enzyme made M1 macrophages resistant to hydrogen peroxide-induced oxidative stress and associated cell death. LPS-induced recruitment of RELA to SOD2 promoter was accompanied by release of PARP1 and LSD1 from chromatin and increased H3K4 di- and tri-methylation. PARP1 dissociation from SOD2 promoter occurred at an early stage of SOD2 transcriptional activation. This event contributed to the termination of mRNA synthesis at a later stage of macrophage polarization by allowing LSD1 to rebind to the SOD2 promoter. LSD1 removed transcription-promoting methylation of H3K4 and led to displacement of RELA. Analysis of temporal changes at the SOD2 promoter indicated a direct mutual interdependence between PARP1, LSD1, H3K4 methylation and the ongoing SOD2 transcription, which correlated positively with both PARP1 abundance on the chromatin and dimethylation of H3K4, but negatively with LSD1 and chromatin compaction in LPS-treated macrophages. Deficiency of LSD1 activity and maintenance of PARP1 at the SOD2 promoter substantially upregulated SOD2 level, thereby further increasing resistance of M1 macrophages to hydrogen peroxide. Inhibitors of LSD1 and PARP1 poisons that capture the latter enzyme on the chromatin seem to be prosurvival molecular tools protecting polarized macrophages from certain pro-oxidative conditions.

Phosphodiesterase Type 5 (PDE5) Inhibitors Sensitize Topoisomerase II Inhibitors in Killing Prostate Cancer Through PDE5-Independent Impairment of HR and NHEJ DNA Repair Systems

Human castration-resistant prostate cancer (CRPC) is a significant target of clinical research. The use of DNA-damaging agents has a long history in cancer chemotherapy but is limited by their toxicities. The combination with a safer drug can be a strategy in reducing dosage and toxicity while increasing anticancer activity in CRPC treatment. Phosphodiesterase type 5 (PDE5) inhibitors are used to treat erectile dysfunction through the selective inhibition of PDE5 that is responsible for cGMP degradation in the corpus cavernosum. Several studies have reported that PDE5 inhibitors display protective effect against doxorubicin-induced cardiotoxicity. The combinatory treatment of CRPC with doxorubicin and PDE5 inhibitors has been studied accordingly. The data demonstrated that sildenafil or vardenafil (two structure-related PDE5 inhibitors) but not tadalafil (structure-unrelated to sildenafil) sensitized doxorubicin-induced apoptosis in CRPC cells with deteriorating the down-regulation of anti-apoptotic Bcl-2 family members, including Bcl-xL and Mcl-1, and amplifying caspase activation. Homologous recombination (HR) and non-homologous end joining (NHEJ) DNA repair systems were inhibited in the apoptotic sensitization through detection of nuclear foci formation of Rad51 and DNA end-binding of Ku80. PDE5 knockdown to mimic the exposure to PDE5 inhibitors did not reproduce apoptotic sensitization, suggesting a PDE5-independent mechanism. Not only doxorubicin, sildenafil combined with other inhibitors of topoisomerase II but not topoisomerase I also triggered apoptotic sensitization. In conclusion, the data suggest that sildenafil and vardenafil induce PDE5-independent apoptotic sensitization to doxorubicin (or other topoisomerase II inhibitors) through impairment of both HR and NHEJ repair systems that are evident by a decrease of nuclear Rad51 levels and their foci formation in the nucleus, and an inhibition of Ku80 DNA end-binding capability. The combinatory treatment may enable an important strategy for anti-CRPC development.

Endocrine Resistance in Hormone Receptor Positive Breast Cancer-From Mechanism to Therapy

The importance and role of the estrogen receptor (ER) pathway has been well-documented in both breast cancer (BC) development and progression. The treatment of choice in women with metastatic breast cancer (MBC) is classically divided into a variety of endocrine therapies, 3 of the most common being: selective estrogen receptor modulators (SERM), aromatase inhibitors (AI) and selective estrogen receptor down-regulators (SERD). In a proportion of patients, resistance develops to endocrine therapy due to a sophisticated and at times redundant interference, at the molecular level between the ER and growth factor. The progression to endocrine resistance is considered to be a gradual, step-wise process. Several mechanisms have been proposed but thus far none of them can be defined as the complete explanation behind the phenomenon of endocrine resistance. Although multiple cellular, molecular and immune mechanisms have been and are being extensively studied, their individual roles are often poorly understood. In this review, we summarize current progress in our understanding of ER biology and the molecular mechanisms that predispose and determine endocrine resistance in breast cancer patients.

Targeting DNA repair in precision medicine

Precision medicine is an emerging treatment paradigm that aims to find the right therapy at the right time based on an individual's unique genetic background, environment, and lifestyle. One area of precision medicine that has had success is targeting DNA repair in cancer. DNA is exposed to constant stress and there are repair mechanisms in place to maintain genetic integrity. These repair mechanisms can be targeted as a treatment strategy. In this chapter, we will focus on current efforts to target DNA repair pathways as part of precision oncology-based treatments.

EP300-HDAC1-SWI/SNF functional unit defines transcription of some DNA repair enzymes during differentiation of human macrophages

Differentiation of human macrophages predisposes these cells to numerous tasks, i.e. killing invading pathogens, and this entails the need for enhanced intracellular defences against stress, including conditions that may increase DNA damage. Our study shows that expression of DNA repair enzymes, such as PARP1, BRCA1 and XRCC1, are activated during macrophage development by the SWI/SNF chromatin remodelling complex, which serves as a histone acetylation sensor. It recognises and displaces epigenetically marked nucleosomes, thereby enabling transcription. Acetylation is controlled both in monocytes and macrophages by the co-operation of EP300 and HDAC1 activities. Differentiation modulates the activities of individual components of EP300-HDAC1-SWI/SNF functional unit and entails recruitment of PBAF to gene promoters. In monocytes, histone-deacetylated promoters of repressed PARP1, BRCA1 and XRCC1 respond only to HDAC inhibition, with an opening of the chromatin structure by BRM, whereas in macrophages both EP300 and HDAC1 contribute to the fine-tuning of nucleosomal acetylation, with HDAC1 remaining active and the balance of EP300 and HDAC1 activities controlling nucleosome eviction by BRG1-containing SWI/SNF. Since EP300-HDAC1-SWI/SNF operates at the level of gene promoters characterized simultaneously by the presence of E2F binding site(s) and CpG island(s), this allows cells to adjust PARP1, BRCA1 and XRCC1 transcription to the differentiation mode and to restart cell cycle progression. Thus, mutual interdependence between acetylase and deacetylase activities defines the acetylation-dependent code for regulation of histone density and gene transcription by SWI/SNF, notably on gene promoters of DNA repair enzymes.

Roles of DNA repair enzyme OGG1 in innate immunity and its significance for lung cancer

Cytokines are pivotal mediators of the immune response, and their coordinated expression protects host tissue from excessive damage and oxidant stress. Nevertheless, the development of lung pathology, including asthma, chronic obstructive pulmonary disease, and ozone-induced lung injury, is associated with oxidant stress; as evidence, there is a significant increase in levels of the modified guanine base 7,8-dihydro-8-oxoguanine (8-oxoG) in the genome. 8-OxoG is primarily recognized by 8-oxoguanine glycosylase 1 (OGG1), which catalyzes the first step in the DNA base excision repair pathway. However, oxidant stress in the cell transiently halts enzymatic activity of substrate-bound OGG1. The stalled OGG1 facilitates DNA binding of transactivators, including NF-κB, to their cognate sites to enable expression of cytokines and chemokines, with ensuing recruitments of inflammatory cells. Hence, defective OGG1 will modulate the coordination between innate and adaptive immunity through excessive oxidant stress and cytokine dysregulation. Both oxidant stress and cytokine dysregulation constitute key elements of oncogenesis by KRAS, which is mechanistically coupled to OGG1. Thus, analysis of the mechanism by which OGG1 modulates gene expression helps discern between beneficial and detrimental effects of oxidant stress, exposes a missing functional link as a marker, and yields a novel target for lung cancer.

PARP1 promoter links cell cycle progression with adaptation to oxidative environment

Although electrophiles are considered as detrimental to cells, accumulating recent evidence indicates that proliferating non-cancerous and particularly cancerous cells utilize these agents for pro-survival and cell cycle promoting signaling. Hence, the redox shift to mild oxidant release must be balanced by multiple defense mechanisms. Our latest findings demonstrate that cell cycle progression, which dictates oxidant level in stress-free conditions, determines PARP1 transcription. Growth modulating factors regulate CDK4/6-RBs-E2Fs axis. In cells arrested in G1 and G0, RB1-E2F1 and RBL2-E2F4 dimers recruit chromatin remodelers such as HDAC1, SWI/SNF and PRC2 to condense chromatin and turn off transcription. Release of retinoblastoma-based repressive complexes from E2F-dependent gene promoters in response to cell transition to S phase enables transcription of PARP1. This enzyme contributes to repair of oxidative DNA damage by supporting several strand break repair pathways and nucleotide or base excision repair pathways, as well as acting as a co-activator of transcription factors such as NRF2 and HIF1a, which control expression of antioxidant enzymes involved in removal of electrophiles and secondary metabolites. Furthermore, PARP1 is indispensible for transcription of the pro-survival kinases MAP2K6, ERK1/2 and AKT1, and for maintaining MAPK activity by suppressing transcription of the MAPK inhibitor, MPK1. In summary, cell cycle controlled PARP1 transcription helps cells to adapt to a pro-oxidant redox shift.

Poly(ADP-ribosyl)ation associated changes in CTCF-chromatin binding and gene expression in breast cells

CTCF is an evolutionarily conserved and ubiquitously expressed architectural protein regulating a plethora of cellular functions via different molecular mechanisms. CTCF can undergo a number of post-translational modifications which change its properties and functions. One such modifications linked to cancer is poly(ADP-ribosyl)ation (PARylation). The highly PARylated CTCF form has an apparent molecular mass of 180 kDa (referred to as CTCF180), which can be distinguished from hypo- and non-PARylated CTCF with the apparent molecular mass of 130 kDa (referred to as CTCF130). The existing data accumulated so far have been mainly related to CTCF130. However, the properties of CTCF180 are not well understood despite its abundance in a number of primary tissues. In this study we performed ChIP-seq and RNA-seq analyses in human breast cells 226LDM, which display predominantly CTCF130 when proliferating, but CTCF180 upon cell cycle arrest. We observed that in the arrested cells the majority of sites lost CTCF, whereas fewer sites gained CTCF or remain bound (i.e. common sites). The classical CTCF binding motif was found in the lost and common, but not in the gained sites. The changes in CTCF occupancies in the lost and common sites were associated with increased chromatin densities and altered expression from the neighboring genes. Based on these results we propose a model integrating the CTCF130/180 transition with CTCF-DNA binding and gene expression changes. This study also issues an important cautionary note concerning the design and interpretation of any experiments using cells and tissues where CTCF180 may be present.