DNA Damage-induced Cell Death Research Articles

Abstract A058: Mesenchymal stem cells act as a selective pressure for High Grade Serous Ovarian Cancer initiation

Abstract High grade serous ovarian cancer (HGSOC) is the most common and lethal subtype of ovarian cancer accounting for 80-90% of diagnoses. Despite its high incidence and mortality, the mechanisms of cancer initiation are unclear. The majority of HGSOC originates in the fallopian tube epithelium (FTE) with support of underlying mesenchymal stromal/stem cells (MSCs). MSCs are stromal progenitor cells that play important roles in wound healing and, in the context of cancer, tumor progression. We showed that MSCs within the tumor microenvironment, termed cancer associated MSCs (CA-MSCs) harbor a unique, age-associated epigenetic signature that is essential for tumor growth and metastasis. Surprisingly, we identified rare subsets of MSCs from benign tubes that exhibit similar epigenetic changes to cancer educated CA-MSCs suggesting that the pro-oncogenic MSC phenotype exists prior to cancer initiation. These “high risk” MSCs (hrMSCs) increase with age, are enriched in BRCA mutation carrier patients, promote cancer cell proliferation, and most notably, elicit tumor formation in primary, non-cancerous FTE. To elucidate the mechanism of hrMSC-induced malignant transformation of FTE, we characterized the effect that hrMSCs have on primary FTE. HrMSCs promoted increased growth of FTE relative to normal/low risk MSCs (lrMSCs) derived from the same patient. These MSCs lack the epigenetic signature/transcriptome of hrMSCs/CA-MSCs. Intriguingly, we co-cultured FTE with hrMSCs at a 1:1 ratio and discovered robust DNA damage in the form of DNA double strand breaks (53BP1 foci) in FTE at 24 hours. Furthermore, conditioned media from hrMSCs supported the recovery of FTE following damage with other DNA damaging agents. We hypothesize that the DNA damage function of hrMSCs serves as a selective pressure for mutation in FTE (1st hit). Additionally, we hypothesize that hrMSCs simultaneously provide growth signals to FTE allowing for evasion of DNA damage-induced cell death (2nd hit). Lastly, we hypothesize that both hrMSC functions are required for genesis of HGSOC. Citation Format: Geyon L. Garcia, Huda I. Atiya, Lan G. Coffman. Mesenchymal stem cells act as a selective pressure for High Grade Serous Ovarian Cancer initiation [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr A058.

Induction of Fatty Acid Oxidation Underlies DNA Damage-Induced Cell Death and Ameliorates Obesity-Driven Chemoresistance.

The DNA damage response is essential for preserving genome integrity and eliminating damaged cells. Although cellular metabolism plays a central role in cell fate decision between proliferation, survival, or death, the metabolic response to DNA damage remains largely obscure. Here, this work shows that DNA damage induces fatty acid oxidation (FAO), which is required for DNA damage-induced cell death. Mechanistically, FAO induction increases cellular acetyl-CoA levels and promotes N-alpha-acetylation of caspase-2, leading to cell death. Whereas chemotherapy increases FAO related genes through peroxisome proliferator-activated receptor α (PPARα), accelerated hypoxia-inducible factor-1α stabilization by tumor cells in obese mice impedes the upregulation of FAO, which contributes to its chemoresistance. Finally, this work finds that improving FAO by PPARα activation ameliorates obesity-driven chemoresistance and enhances the outcomes of chemotherapy in obese mice. These findings reveal the shift toward FAO induction is an important metabolic response to DNA damage and may provide effective therapeutic strategies for cancer patients with obesity.

A noncanonical IRAK4-IRAK1 pathway counters DNA damage-induced apoptosis independently of TLR/IL-1R signaling.

Interleukin-1 receptor (IL-1R)-associated kinases (IRAKs) are core effectors of Toll-like receptors (TLRs) and IL-1R in innate immunity. Here, we found that IRAK4 and IRAK1 together inhibited DNA damage-induced cell death independently of TLR or IL-1R signaling. In human cancer cells, IRAK4 was activated downstream of ATR kinase in response to double-strand breaks (DSBs) induced by ionizing radiation (IR). Activated IRAK4 then formed a complex with and activated IRAK1. The formation of this complex required the E3 ubiquitin ligase Pellino1, acting structurally but not catalytically, and the activation of IRAK1 occurred independently of extracellular signaling, intracellular TLRs, and the TLR/IL-1R signaling adaptor MyD88. Activated IRAK1 translocated to the nucleus in a Pellino2-dependent manner. In the nucleus, IRAK1 bound to the PIDD1 subunit of the proapoptotic PIDDosome and interfered with platform assembly, thus supporting cell survival. This noncanonical IRAK signaling pathway was also activated in response to other DSB-inducing agents. The loss of IRAK4, of IRAK4 kinase activity, of either Pellino protein, or of the nuclear localization sequence in IRAK1 sensitized p53-mutant zebrafish to radiation. Thus, the findings may lead to strategies for overcoming tumor resistance to conventional cancer treatments.

The Mevalonate Pathway Is a Therapeutic Target in TP53 Mutant Acute Myeloid Leukemia

Introduction: Acute myeloid leukemia (AML) with mutations in the TP53 tumor suppressor gene is the most fatal of AMLs with a median overall survival of only six months due to chemotherapy resistance. To understand biologic differences in TP53 mutant (MT) AML, as compared to wildtype (WT) AML, we performed transcriptomic analysis on sorted patient samples. Gene set enrichment analysis demonstrated significant upregulation of the cholesterol biosynthesis or mevalonate pathway in TP53 MT AML. In TP53 MT solid tumor models, the mevalonate pathway plays a key role in tumorigenesis and metastasis. We hypothesized that the mevalonate pathway is essential for chemotherapy resistance of TP53 MT AML and represents a novel therapeutic target. Methods: We developed chemoresistant, isogenic TP53 MT AML cell lines using CRISPR/Cas9 technology in the MOLM14 AML cell line. We used primary TP53 MT AML patient samples to perform colony forming unit (CFU) assays, in vitro assays with a serum substitute and cytokines, and a patient derived xenograft (PDX) model of TP53 MT AML. Results: We first sought to determine if TP53 MT AML exhibits upregulation of mevalonate pathway activity in response to cytarabine (AraC), the backbone of AML therapy. We treated our isogenic TP53 MT AML cell lines for 24 hours (h) with AraC and assessed mevalonate pathway gene expression and metabolites using qRT-PCR and liquid chromatography high resolution mass spectrometry (LC-HRMS), respectively. Only TP53 MT cell lines respond to AraC with a significant upregulation of key mevalonate pathway genes and metabolites. We then determined if TP53 MT AML is sensitized to AraC by inhibition of the mevalonate pathway with a statin. We pretreated our isogenic cell lines with 24h rosuvastatin before 24h AraC and assessed cell viability with flow cytometry with AnnexinV/7AAD staining and XTT assays. We also performed CFU assays in TP53 MT AML patient samples treated with the two drugs alone or in combination and assessed CFUs after 14 days. These studies revealed a synergistic reduction in cell viability and CFUs in TP53 MT AML by rosuvastatin in combination with AraC. We next addressed the mechanism by which a statin sensitizes TP53 MT AML to AraC. Our group and others have demonstrated enhanced mitochondrial activity is associated with AML chemoresistance. We hypothesized that mevalonate pathway byproducts, known to be crucial for mitochondrial functions, contribute to the mitochondrial response to AraC. We treated TP53 MT AML cell lines as above with rosuvastatin and AraC and used seahorse technology to measure oxidative phosphorylation (OXPHOS). We also used electron microscopy, flow cytometry of mitochondrial protein, TOM20, and qRT-PCR for mitochondrial DNA content to quantify and characterize mitochondria. These data demonstrate that TP53 MT AML cell lines, compared to WT, exhibit a significant increase in OXPHOS after 24h that is due to an increase in total mitochondrial. These effects are abrogated by pretreatment with a statin. We also validated these findings in primary TP53 MT AML patient samples in vitro. Importantly, co-treatment of the isogenic TP53 MT AML cell lines with a soluble form of mevalonate or a downstream byproduct, geranylgeranyl pyrophosphate (GGPP), recovered OXPHOS and subsequent chemoresistance. Overall, this data supports a model where TP53 MT AML cells dynamically upregulate the mevalonate pathway to regulate OXPHOS and avoid DNA damage-induced cell death (Figure 1). Finally, we studied a PDX model of TP53 MT AML. Engrafted mice were treated with AraC for five days alone or in combination with high dose rosuvastatin. Mice were harvested on day eight with assessment of leukemic burden by flow, OXPHOS by seahorse and mevalonate byproducts by LC-HRMS. TP53 MT AML leukemic burden was only significantly reduced by the combination therapy. Consistent with in vitro findings, mice sacrificed three days after completing AraC continue to demonstrate enhanced OXPHOS and increased mevalonate byproducts. This response is blunted by rosuvastatin. Conclusions: These results demonstrate that TP53 MT AML requires the mevalonate pathway for chemotherapy resistance and targeting of this pathway in combination with chemotherapy may improve clinical responses. Analysis is ongoing to determine the role of GGPP in regulating the mitochondrial response to DNA damage in TP53 MT cells in order to elucidate novel therapeutic approaches.

The alternative sigma factor SigN of Bacillus subtilis is intrinsically toxic.

Sigma factors bind and direct the RNA polymerase core to specific promoter sequences, and alternative sigma factors direct transcription of different regulons of genes. Here, we study the pBS32 plasmid-encoded sigma factor SigN of Bacillus subtilis to determine how it contributes to DNA damage-induced cell death. We find that SigN causes cell death when expressed at high levels and does so in the absence of its regulon suggesting it is intrinsically toxic. One way toxicity was relieved was by curing the pBS32 plasmid, which eliminated a positive feedback loop that led to SigN hyper-accumulation. Another way toxicity was relieved was through mutating the chromosomally encoded transcriptional repressor protein AbrB, thereby derepressing a potent antisense transcript that antagonized SigN expression. SigN efficiently competed with the vegetative sigma factor SigA in vitro, and SigN accumulation in the absence of positive feedback reduced SigA-dependent transcription suggesting that toxicity may be due to competitive inhibition of one or more essential transcripts. Why B. subtilis encodes a toxic sigma factor is unclear but SigN may function in host-inhibition during lytic conversion, as phage lysogen genes are also encoded on pBS32. IMPORTANCE Alternative sigma factors activate entire regulons of genes to improve viability in response to environmental stimuli. The pBS32 plasmid-encoded alternative sigma factor SigN of Bacillus subtilis however, is activated by the DNA damage response and leads to cellular demise. Here we find that SigN impairs viability by hyper-accumulating and outcompeting the vegetative sigma factor for the RNA polymerase core. Why B. subtilis retains a plasmid with a deleterious alternative sigma factor is unknown.

Tumor suppressor p53 mediates interleukin-6 expression to enable cancer cell evasion of genotoxic stress

The tumor suppressor p53 primarily functions as a mediator of DNA damage-induced cell death, thereby contributing to the efficacy of genotoxic anticancer therapeutics. Here, we show, on the contrary, that cancer cells can employ genotoxic stress-induced p53 to acquire treatment resistance through the production of the pleiotropic cytokine interleukin (IL)-6. Mechanistically, DNA damage, either repairable or irreparable, activates p53 and stimulates Caspase-2-mediated cleavage of its negative regulator mouse double minute 2 (MDM2) creating a positive feedback loop that leads to elevated p53 protein accumulation. p53 transcriptionally controls the major adenosine triphosphate (ATP) release channel pannexin 1 (Panx1), which directs IL-6 induction via a mechanism dependent on the extracellular ATP-activated purinergic P2 receptors as well as their downstream intracellular calcium (iCa2+)/PI3K/Akt/NF-ĸB signaling pathway. Thus, p53 silencing impairs Panx1 and IL-6 expression and renders cancer cells sensitive to genotoxic stress. Moreover, we confirm that IL-6 hampers the effectiveness of genotoxic anticancer agents by mitigating DNA damage, driving the expression of anti-apoptotic Bcl-2 family genes, and maintaining the migratory and invasive properties of cancer cells. Analysis of patient survival and relevant factors in lung cancer and pan-cancer cohorts supports the prognostic and clinical values of Panx1 and IL-6. Notably, IL-6 secreted by cancer cells during genotoxic treatments promotes the polarization of monocytic THP-1-derived macrophages into an alternative (M2-like) phenotype that exhibits impaired anti-survival activities but enhanced pro-metastatic effects on cancer cells as compared to nonpolarized macrophages. Our study reveals the precise mechanism for genotoxic-induced IL-6 and suggests that targeting p53-mediated IL-6 may improve the responsiveness of cancer cells to genotoxic anticancer therapy.

Selective Cell Ablation Using an Improved Prodrug-Converting Nitroreductase.

Selective cell ablation is an invaluable tool to investigate the function of cell types, the regeneration of cells, and the modeling of diseases associated with cell loss. The nitroreductase (NTR)-mediated cell ablation system is a simple method enabling the elimination of targeted cells through the expression of a nitroreductase enzyme and the application of a prodrug (such as metronidazole). The prodrug is reduced to a cytotoxic product by nitroreductase, thereby leading to DNA damage-induced cell death. In species with elevated regenerative capacity such as zebrafish, removing the prodrug allows endogenous tissue to replace the lost cells. Herein, we describe a method for the use of a markedly improved nitroreductase enzyme for spatially and temporally controlled targeted cell ablation in the zebrafish. Recently, we identified an NTR variant (NTR 2.0) that achieves effective targeted cell ablation at concentrations of metronidazole well below those causing toxic side effects. NTR 2.0 thereby enables the ablation of "resistant" cell types and novel cell ablation paradigms. These advances simplify investigations of cell function, enable interrogations of the effects of chronic inflammation on regenerative processes and facilitate modeling of degenerative diseases associated with chronic cell loss. Techniques for transgenic nitroreductase expression and prodrug application are discussed.

Preomic profile of BxPC-3 cells after treatment with BRC4

BRCA2 and RAD51 are two proteins that play a central role in homologous recombination (HR) and DNA double strand break (DSB) repair. BRCA2 assists RAD51 fibrillation and defibrillation through binding with its eight BRC repeats, with BRC4 being one of the most efficient and best characterized. RAD51 inactivation by small molecules has been proposed as a strategy to impair BRCA2/RAD51 binding and, ultimately, the HR pathway, with the aim of making cancer cells more sensitive to PARP inhibitors (PARPi). This strategy, which mimics a synthetic lethality (SL) approach, has been successfully performed in vitro by using the myristoylated derivative of BRC4 (myr-BRC4), designed for a more efficient cell entry. The present study applies a method to obtain a proteomic fingerprint after cellular treatment with the myr-BRC4 peptide using a mass spectroscopy (MS) proteomic approach. (Data are available via ProteomeXchange with identifier PXD042696.) We performed a comparative proteomic profiling of the myr-BRC4 treated vs. untreated BxPC-3 pancreatic cancer cells and evaluated the differential expression of proteins. Among the identified proteins, we focused our attention on proteins shared by both the RAD51 and the BRCA2 interactomes, and on those whose reduction showed high statistical significance. Three downregulated proteins were identified (FANCI, FANCD2, and RPA3), and protein downregulation was confirmed through immunoblotting analysis, validating the MS approach. Our results suggest that, being a direct consequence of myr-BRC4 treatment, the detection of FANCD2, FANCI, and RPA3 downregulation could be used as an indicator for monitoring HR impairment. SignificanceRAD51's inhibition has gained increasing attention because of its possible implications in personalized medicine through the SL approach. Chemical disruption of protein-protein interactions (PPIs) between RAD51 and BRCA2, or some of its partner proteins, could potentiate PARPi DNA damage-induced cell death. This could have application for difficult to treat cancers, such as BRCA-competent and olaparib (PARPi) resistant pancreatic adenocarcinoma. Despite RAD51 being a widely studied target, researchers still lack detailed mechanistic information. This has stifled progress in the field with only a few RAD51 inhibitors having been identified, none of which have gained regulatory approval. Nevertheless, the peptide BRC4 is one of the most specific and best characterized RAD51 binder and inhibitor reported to date. Our study is the first to report the proteomic fingerprint consequent to cellular treatment of myr-BRC4, to offer a reference for the discovery of specific protein/pathway alterations within DNA damage repair. Our results suggest that, being a direct consequence of myr-BRC4 treatment, and ultimately ofBRCA2/RAD51 disruption, the detection of FANCD2, FANCI, and RPA3 downregulation could be used as an indicator for monitoring DNA damage repair impairment and therefore be used to potentiate the development of new effective therapeutic strategies.

Combined doxorubicin and arctigenin treatment induce cell cycle arrest-associated cell death by promoting doxorubicin uptake in doxorubicin-resistant breast cancer cells.

Chemotherapy failure is often caused by drug resistance, for which no effective treatment strategy has been established. Many studies have been undertaken with the aim of overcoming drug resistance using natural products. Arctigenin (ATG), a natural product, has been investigated for its anti-cancer effects in HER2-overexpressing, ER-positive, and triple-negative breast cancer cells. We investigated the efficacy of ATG against self-established doxorubicin (DOX)-resistant breast cancer cells (MCF-DR and MDA-DR cells) derived from MCF-7 and MDA-MB-231 cells, respectively. ATG was found to increase DOX intracellular levels by downregulating multidrug Resistance 1 (MDR1) mRNA expression in DOX-resistant cells. In addition, combined treatment with DOX and ATG (DOX/ATG) reduced the viability of and colony formation by DOX-resistant cells. DOX/ATG also significantly induced G2/M cell cycle arrest by suppressing the Cyclin D1/CDK4/RB pathways and suppressed the expressions of MDR1 and Cyclin D1 by inhibiting the Mitogen-activated protein kinase (MAPK)/Activating protein-1 (AP-1) signaling pathways. Furthermore, DOX/ATG induced DNA damage and attenuated the expressions of RAD51 and Ku80. However, PARP1 (Poly [ADP-ribose] polymerase1) cleavage and AIF (Apoptosis-inducing factor) induced apoptosis did not occur despite DNA damage-induced cell death. Rather, flow cytometry showed that DOX/ATG caused necrosis. In summary, DOX/ATG increased intracellular DOX levels by inhibiting MDR1 and inducing G2/M arrest by inhibiting the Cyclin D1/CDK4/RB pathways and causing necrosis by damaging DNA. Our results suggest that ATG might be used as an adjuvant to enhance the efficacy of DOX in DOX-resistant breast cancer.

Abstract P2-26-18: Adiponectin regulates stem cell activity in tamoxifen-resistant breast cancer cells

Abstract Introduction: Breast cancer is the most diagnosed neoplasia and the second leading cause of malignancy death in women. Drug resistance is still the major challenge in the clinical management of breast cancer patients. Indeed, despite the improvements in the early diagnosis and in the therapeutic approaches, many breast cancer patients experience disease relapse due to de novo or acquired drug resistance. Growing evidence recognized a small population of breast cancer cells, named Breast Cancer Stem Cells (BCSCs), as the leading cause of tumor progression, metastasis formation and resistance against conventional therapy. BCSCs have some specific properties such as self-renewal, differentiation into different cell types, migration, tumorsphere formation, antioxidative activity, that make tumors more aggressive. Tumor microenvironment plays a crucial role in the regulation of stem cell proliferation and resistance to apoptosis through the secretion of cytokines and growth factors. Adipocytes represent the most abundant cellular component of mammary microenvironment. The excessive fat accumulation in obesity leads to the development of a dysfunctional adipose tissue, producing an unbalance in adipokines secretion. Among the secreted factors, adiponectin plays a crucial role in breast cancer development and progression. The aim of the present study was to investigate the effects of low adiponectin level (5 μl/ml), hallmark of obese status, on BCSCs activity in hormone-resistant cells. Methods: We tested the ability of MCF-7 wild type (WT) and tamoxifen-resistant (TR) to grow as mammospheres (Mammospheres Forming Efficiency, MFE), measuring the ability to maintain cell viability (self-renewal) upon serial non-adherent passages. mRNA levels of stemness, EMT and cell cycle markers were evaluated by qRT-PCR. CD44+/24- cell ratio, ALDH expression, ROS production, cell cycle, were analyzed by flow cytometry. Results: Adiponectin treatment significantly enhanced MFE and self-renewal capacity in TR-MCF-7 cells compared to MCF-7 WT cells. To identify the presence of BCSCs into mammospheres it has been evaluated the CD44+/CD24– biomarker signature. Flow cytometry revealed an enrichment of CD44, a trans-membrane glycoprotein which regulates growth signals in stem cells, in TR-MCF-7 mammospheres, whereas expression levels of the differentiation marker CD24 were decreased. The gene expression of these biomarkers was also analyzed by qRT-PCR. The increased BCSCs subpopulation in adiponectin-treated TR-MCF-7 mammospheres was also confirmed by the enhanced ALDH-expressing cells. qRT-PCR revealed that adiponectin increased the mRNA levels of stemness and EMT markers in TR-MCF-7 cells mammospheres. Interestingly, cell cycle analysis showed in adiponectin-treated TR-MCF-7 mammospheres a reduction of apoptosis. Moreover, flow cytometry analysis displayed a reduction of ROS levels, which generally are assumed as cues determining DNA damage-induced cell death, in adiponectin-treated TR-MCF-7 mammospheres. Conclusions: Our results demonstrated that low adiponectin level, as it occurs in obese breast cancer microenvironment, driving EMT, enhances stem-like features in TR-MCF-7 cells to sustain tumor progression. Citation Format: Giuseppina Daniela Naimo, Martina Forestiero, Alessandro Paolì, Francesca Giordano, Maria Luisa Panno, Loredana Mauro, Sebastiano Andò. Adiponectin regulates stem cell activity in tamoxifen-resistant breast cancer cells [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-26-18.

Small-Molecule-Mediated Stabilization of PP2A Modulates the Homologous Recombination Pathway and Potentiates DNA Damage-Induced Cell Death.

High-grade serous carcinoma (HGSC) is the most common and lethal ovarian cancer subtype. PARP inhibitors (PARPi) have become the mainstay of HGSC-targeted therapy, given that these tumors are driven by a high degree of genomic instability (GI) and homologous recombination (HR) defects. Nonetheless, approximately 30% of patients initially respond to treatment, ultimately relapsing with resistant disease. Thus, despite recent advances in drug development and an increased understanding of genetic alterations driving HGSC progression, mortality has not declined, highlighting the need for novel therapies. Using a small-molecule activator of protein phosphatase 2A (PP2A; SMAP-061), we investigated the mechanism by which PP2A stabilization induces apoptosis in patient-derived HGSC cells and xenograft (PDX) models alone or in combination with PARPi. We uncovered that PP2A genes essential for cellular transformation (B56α, B56γ, and PR72) and basal phosphatase activity (PP2A-A and -C) are heterozygously lost in the majority of HGSC. Moreover, loss of these PP2A genes correlates with worse overall patient survival. We show that SMAP-061-induced stabilization of PP2A inhibits the HR output by targeting RAD51, leading to chronic accumulation of DNA damage and ultimately apoptosis. Furthermore, combination of SMAP-061 and PARPi leads to enhanced apoptosis in both HR-proficient and HR-deficient HGSC cells and PDX models. Our studies identify PP2A as a novel regulator of HR and indicate PP2A modulators as a therapeutic therapy for HGSC. In summary, our findings further emphasize the potential of PP2A modulators to overcome PARPi insensitivity, given that targeting RAD51 presents benefits in overcoming PARPi resistance driven by BRCA1/2 mutation reversions.

Targeting the Mevalonate Pathway in the Mitochondria-Dependent Chemotherapy Resistance of TP53 Mutant Acute Myeloid Leukemia

Acute myeloid leukemia (AML) bearing mutations in the TP53 tumor suppressor gene is the most fatal of AMLs with a two-year overall survival between 0-15%. Patients with TP53 mutant (MT) AML typically respond to initial therapy but quickly relapse and die of disease due to chemotherapy resistance. We have undertaken to understand the mechanisms of chemotherapy resistance in TP53 MT AML. Initially, we purified AML blasts to 99% purity using flow sorting and performed RNA sequencing on TP53 MT and wildtype samples. Gene set enrichment analysis demonstrated that TP53 MT AML patient samples were enriched in the cholesterol synthesis or mevalonate pathway. This was validated using the BEAT AML dataset. Interestingly, p53 loss and/or mutation in breast, liver, colon and pancreatic cancer models leads to a dependency on an upregulated mevalonate pathway. Despite these data, the role of the mevalonate pathway in TP53 MT AML is unknown. We have subsequently explored the role of the mevalonate pathway in TP53 MT AML chemotherapy resistance. Experiments were performed in CRISPR-engineered TP53 mutated human AML cells with isogenic controls. Treatment with DNA damaging agents and analysis for cell survival confirmed that loss of wildtype p53 function enhanced cell survival. Recent results have demonstrated that chemotherapy resistance in AML cells is associated with an increase in oxidative phosphorylation (OXPHOS), which is dynamically induced by DNA damage. We confirmed that TP53 MT AML cells increase OXPHOS after DNA damage (Figure 1). Treatment of cells with rosuvastatin, a potent inhibitor of the mevalonate pathway, blocked this increase in OXPHOS and induced chemosensitivity. Lipidomic analysis demonstrates that the increase in OXPHOS correlates with an increase in coenzyme Q10 (CoQ10), a byproduct of the mevalonate pathway. Importantly, co-treatment of cells with a soluble form of mevalonate or a downstream byproduct, geranylgeranyl pyrophosphate (GGPP), recovered OXPHOS and subsequent chemoresistance. This indicates rosuvastatin's activity is specific to the mevalonate pathway, independent of cholesterol, and dependent on the GGPP branch of the pathway. Analysis is ongoing to determine the role of GGPP in regulating the mitochondrial response to DNA damage in TP53 MT cells, including if GGPP is required for CoQ10 synthesis as this remains a controversy in the literature. Overall, this data supports a model where TP53 MT AML cells dynamically upregulate the mevalonate pathway to regulate OXPHOS and avoid DNA damage-induced cell death (Figure 2). To determine if this model is physiologically relevant, we studied a patient derived xenograft (PDX) model of TP53 MT AML cells. Engrafted mice were treated with standard AML chemotherapeutic agent, cytarabine (AraC), for five days alone or in combination with high dose rosuvastatin. Interestingly, mice sacrificed three days after completing AraC administration continue to demonstrate an enhanced OXPHOS response. This response is blunted by co-treatment with rosuvastatin. As expected, TP53 MT AML cells respond modestly to AraC alone and this response was increased with the combination therapy. Overall, these results demonstrate that TP53 MT AML requires the mevalonate pathway for chemotherapy resistance and targeting of the mevalonate pathway in combination with chemotherapy may improve clinical responses. Through detailed biochemical analysis of this mechanism, we will elucidate novel therapeutic approaches to TP53 MT AML. Figure Legend: (1) Oxygen consumption rate measured by seahorse technology in indicated cell lines and conditions. Statistics performed with unpaired T-test in GraphPad with stars indicating significance as per figure. (2) Proposed schema of mevalonate pathway dependencies in TP53 mutant (MT) AML. Bold arrows indicate increased byproduct dependency in TP53 MT AML cells treated with DNA damaging chemotherapy. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Polycomb group protein BMI1 protects neuroblastoma cells against DNA damage-induced apoptotic cell death

The overexpression of BMI1, a polycomb protein, correlates with cancer development and aggressiveness. We previously reported that MYCN-induced BMI1 positively regulated neuroblastoma (NB) cell proliferation via the transcriptional inhibition of tumor suppressors in NB cells. To assess the potential of BMI1 as a new target for NB therapy, we examined the effects of reductions in BMI1 on NB cells. BMI1 knockdown (KD) in NB cells significantly induced their differentiation for up to 7 days. BMI1 depletion significantly induced apoptotic NB cell death for up to 14 days along with the activation of p53, increases in p73, and induction of p53 family downstream molecules and pathways, even in p53 mutant cells. BMI1 depletion in vivo markedly suppressed NB xenograft tumor growth. BMI1 reductions activated ATM and increased γ-H2AX in NB cells. These DNA damage signals and apoptotic cell death were not canceled by the transduction of the polycomb group molecules EZH2 and RING1B. Furthermore, EZH2 and RING1B KD did not induce apoptotic NB cell death to the same extent as BMI1 KD. Collectively, these results suggest the potential of BMI1 as a target of molecular therapy for NB and confirmed, for the first time, the shared role of PcG proteins in the DNA damage response of NB cells.

Enhancing anti-tumour innate immunity by targeting the DNA damage response and pattern recognition receptors in combination with radiotherapy.

Radiotherapy is one of the most effective and frequently used treatments for a wide range of cancers. In addition to its direct anti-cancer cytotoxic effects, ionising radiation can augment the anti-tumour immune response by triggering pro-inflammatory signals, DNA damage-induced immunogenic cell death and innate immune activation. Anti-tumour innate immunity can result from recruitment and stimulation of dendritic cells (DCs) which leads to tumour-specific adaptive T-cell priming and immunostimulatory cell infiltration. Conversely, radiotherapy can also induce immunosuppressive and anti-inflammatory mediators that can confer radioresistance. Targeting the DNA damage response (DDR) concomitantly with radiotherapy is an attractive strategy for overcoming radioresistance, both by enhancing the radiosensitivity of tumour relative to normal tissues, and tipping the scales in favour of an immunostimulatory tumour microenvironment. This two-pronged approach exploits genomic instability to circumvent immune evasion, targeting both hallmarks of cancer. In this review, we describe targetable DDR proteins (PARP (poly[ADP-ribose] polymerase); ATM/ATR (ataxia–telangiectasia mutated and Rad3-related), DNA-PKcs (DNA-dependent protein kinase, catalytic subunit) and Wee1 (Wee1-like protein kinase) and their potential intersections with druggable immunomodulatory signalling pathways, including nucleic acid-sensing mechanisms (Toll-like receptors (TLR); cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) and retinoic acid-inducible gene-I (RIG-I)-like receptors), and how these might be exploited to enhance radiation therapy. We summarise current preclinical advances, recent and ongoing clinical trials and the challenges of therapeutic combinations with existing treatments such as immune checkpoint inhibitors.

Ribonuclease H2 Subunit A Preserves Genomic Integrity and Promotes Prostate Cancer Progression.

RNASEH2A was demonstrated to be highly upregulated in aggressive prostate cancer to degrade R-loop accumulation and preserve genomic stability for tumor growth, suggesting that RNase H2 activity could be a promising therapeutic target.

Targeted Inhibition of DNA-PKcs, ATM, ATR, PARP, and Rad51 Modulate Response to X Rays and Protons.

Small molecule inhibitors are currently in preclinical and clinical development for the treatment of selected cancers, particularly those with existing genetic alterations in DNA repair and DNA damage response (DDR) pathways. Keen interest has also been expressed in combining such agents with other targeted antitumor strategies such as radiotherapy. Radiotherapy exerts its cytotoxic effects primarily through DNA damage-induced cell death; therefore, inhibiting DNA repair and the DDR should lead to additive and/or synergistic radiosensitizing effects. In this study we screened the response to X-ray or proton radiation in cell lines treated with DDR inhibitors (DDRis) targeting ATM, ATR, DNA-PKcs, Rad51, and PARP, with survival metrics established using clonogenic assays. We observed that DDRis generate significant radiosensitization in cancer and primary cells derived from normal tissue. Existing genetic defects in cancer cells appear to be an important consideration when determining the optimal inhibitor to use for synergistic combination with radiation. We also show that while greater radiosensitization can be achieved with protons (9.9 keV/µm) combined with DDRis, the relative biological effectiveness is unchanged or in some cases reduced. Our results indicate that while targeting the DDR can significantly radiosensitize cancer cells to such combinations, normal cells may also be equally or more severely affected, depending on the DDRi used. These data highlight the importance of identifying genetic defects as predictive biomarkers of response for combination treatment.

Abstract 3340: Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death

Abstract High Grade Serous Carcinoma (HGSC) is the most lethal ovarian cancer subtype and accounts for approximately 60% of all ovarian tumors. Despite recent advances in drug development and increased understanding of genetic alterations that drive HGSC progression, mortality has not declined, highlighting the need for novel therapies. PARP inhibitors (PARPi) have become the mainstay of HGSC targeted therapy research given that these tumors are driven by a high degree of genomic instability resulting from the combination of fast DNA replication rates and numerous defects in the DNA-damage response (DDR) pathway. Nonetheless, only ~25% of these patients initially respond to treatment and a significant percentage eventually relapses with resistant disease. Here, we discovered that a Small Molecule Activator of Protein Phosphatase 2A (PP2A) (SMAP-061) induces apoptosis in both established and patient-derived HGSC cell lines as well as in genetically distinct Patient-Derived Xenograft (PDX) mouse models. Interestingly, we also uncovered that several genes that make-up the heterotrimer PP2A tumor suppressor protein are heterozygously lost in more than 95% of HGSC tumors, second only to p53. Mechanistically, we show that stabilization of PP2A protein by SMAP-061 inhibits the Homologous Recombination (HR) pathway via the direct inhibition of RAD51, ultimately leading to chronic accumulation of DNA damage and thus programmed cell death. Furthermore, we found that SMAP-061’s ability to inhibit HR potentiated the effects of PARP inhibition and resulted in synergistic cell death in both HR proficient and deficient models. These studies emphasize the potential of PP2A activators to expand the patient population that can benefit from PARPi therapies and possibly overcome PARPi resistance. In sum, our data highlights a new role of PP2A in regulating the DDR pathway in HGSC and supports the use of SMAPs in both HR proficient and deficient HGSC tumors. Citation Format: Rita A. Avelar, Amy Armstrong, Goutham Narla, Analisa DiFeo. Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3340.

DNA-damage induced cell death in yap1;wwtr1 mutant epidermal basal cells.

In a previous study, it was reported that Yap1 and Wwtr1 in zebrafish regulates the morphogenesis of the posterior body and epidermal fin fold (Kimelman et al., 2017). We report here that DNA damage induces apoptosis of epidermal basal cells (EBCs) in zebrafish yap1-/-;wwtr1-/- embryos. Specifically, these mutant EBCs exhibit active Caspase-3, Caspase-8, and γH2AX, consistent with DNA damage serving as a stimulus of the extrinsic apoptotic pathway in epidermal cells. Live imaging of zebrafish epidermal cells reveals a steady growth of basal cell size in the developing embryo, but this growth is inhibited in mutant basal cells followed by apoptosis, leading to the hypothesis that factors underscoring cell size play a role in this DNA damage-induced apoptosis phenotype. We tested two of these factors using cell stretching and substrate stiffness assays, and found that HaCaT cells cultured on stiff substrates exhibit more numerous γH2AX foci compared to ones cultured on soft substrates. Thus, our experiments suggest that substrate rigidity may modulate genomic stress in epidermal cells, and that Yap1 and Wwtr1 promotes their survival.

Arthrospira (Spirulina) platensis feeding reduces the early stage of chemically induced rat colon carcinogenesis.

Colorectal cancer is the third most diagnosed cancer worldwide and linked to dietary/lifestyle factors. Arthrospira (Spirulina) platensis (AP) contains bioactive compounds with beneficial effects in vivo/in vitro. We evaluated the effects of AP feeding against 1,2-dimethylhydrazine (DMH)-induced colon carcinogenesis. Male Sprague Dawley rats were given subcutaneous injections of DMH (4 × 40 mg/kg body weight) (G1-G3) or vehicle (G4-G5) twice a week (weeks 3-4). During weeks 1-4, animals were fed a diet containing 1 % (G2) or 2 % (G3-G4) AP powder (w/w). After this period, all groups received a balanced diet until week 12. Some animals were euthanised after the last DMH injection (week 4) for histological, immunohistochemical (Ki-67, γ-H2AX and caspase-3) and molecular analyses (real time-PCR for 91 genes), while other animals were euthanised at week 12 for preneoplastic aberrant crypt foci (ACF) analysis. Both AP treatments (G2-G3) significantly decreased the DMH-induced increase in γ-H2AX (DNA damage) and caspase 3 (DNA damage-induced cell death) in colonic crypts at week 4. In addition, Cyp2e1 (Drug metabolism), Notch1, Notch2 and Jag1 genes (Notch pathway) and Atm, Wee1, Chek2, Mgmt, Ogg1 and Xrcc6 genes (DNA repair) were also down-regulated by 2 % AP feeding (G3) at week 4. A significant reduction in ACF development was observed in both AP-treated groups (G2-G3) at week 12. In conclusion, findings indicate that AP feeding reduced acute colonic damage after DMH, resulting in fewer preneoplastic lesions. Our study provided mechanistic insights on dietary AP-preventive effects against early colon carcinogenesis.

Cooperative effects of RIG-I-like receptor signaling and IRF1 on DNA damage-induced cell death

Properly responding to DNA damage is vital for eukaryotic cells, including the induction of DNA repair, growth arrest and, as a last resort to prevent neoplastic transformation, cell death. Besides being crucial for ensuring homeostasis, the same pathways and mechanisms are at the basis of chemoradiotherapy in cancer treatment, which involves therapeutic induction of DNA damage by chemical or physical (radiological) measures. Apart from typical DNA damage response mediators, the relevance of cell-intrinsic antiviral signaling pathways in response to DNA breaks has recently emerged. Originally known for combatting viruses via expression of antiviral factors including interferons (IFNs) and establishing of an antiviral state, RIG-I-like receptors (RLRs) were found to be critical for adequate induction of cell death upon the introduction of DNA double-strand breaks. We here show that presence of IRF3 is crucial in this process, most likely through direct activation of pro-apoptotic factors rather than transcriptional induction of canonical downstream components, such as IFNs. Investigating genes reported to be involved in both DNA damage response and antiviral signaling, we demonstrate that IRF1 is an obligatory factor for DNA damage-induced cell death. Interestingly, its regulation does not require activation of RLR signaling, but rather sensing of DNA double-strand breaks by ATM and ATR. Hence, even though independently regulated, both RLR signaling and IRF1 are essential for full-fledged induction/execution of DNA damage-mediated cell death programs. Our results not only support more broadly developing IRF1 as a biomarker predictive for the effectiveness of chemoradiotherapy, but also suggest investigating a combined pharmacological stimulation of RLR and IRF1 signaling as a potential adjuvant regimen in tumor therapy.