Lethal Mechanism Research Articles

Synthetic Lethal Therapy for KRAS Mutant Non-small-cell Lung Carcinoma with Nanoparticle-mediated CDK4 siRNA Delivery

The KRAS mutation is present in ~20% of lung cancers and has not yet been effectively targeted for therapy. This mutation is associated with a poor prognosis in non-small-cell lung carcinomas (NSCLCs) and confers resistance to standard anticancer treatment drugs, including epidermal growth factor receptor tyrosine kinase inhibitors. In this study, we exploited a new therapeutic strategy based on the synthetic lethal interaction between cyclin-dependent kinase 4 (CDK4) downregulation and the KRAS mutation to deliver micellar nanoparticles (MNPs) containing small interfering RNA targeting CDK4 (MNPsiCDK4) for treatment in NSCLCs harboring the oncogenic KRAS mutation. Following MNPsiCDK4 administration, CDK4 expression was decreased, accompanied by inhibited cell proliferation, specifically in KRAS mutant NSCLCs. However, this intervention was harmless to normal KRAS wild-type cells, confirming the proposed mechanism of synthetic lethality. Moreover, systemic delivery of MNPsiCDK4 significantly inhibited tumor growth in an A549 NSCLC xenograft murine model, with depressed expression of CDK4 and mutational KRAS status, suggesting the therapeutic promise of MNPsiCDK4 delivery in KRAS mutant NSCLCs via a synthetic lethal interaction between KRAS and CDK4.

Histone deacetylase inhibitor (HDACI) mechanisms of action: emerging insights.

Initially regarded as "epigenetic modifiers" acting predominantly through chromatin remodeling via histone acetylation, HDACIs, alternatively referred to as lysine deacetylase or simply deacetylase inhibitors, have since been recognized to exert multiple cytotoxic actions in cancer cells, often through acetylation of non-histone proteins. Some well-recognized mechanisms of HDACI lethality include, in addition to relaxation of DNA and de-repression of gene transcription, interference with chaperone protein function, free radical generation, induction of DNA damage, up-regulation of endogenous inhibitors of cell cycle progression, e.g., p21, and promotion of apoptosis. Intriguingly, this class of agents is relatively selective for transformed cells, at least in pre-clinical studies. In recent years, additional mechanisms of action of these agents have been uncovered. For example, HDACIs interfere with multiple DNA repair processes, as well as disrupt cell cycle checkpoints, critical to the maintenance of genomic integrity in the face of diverse genotoxic insults. Despite their pre-clinical potential, the clinical use of HDACIs remains restricted to certain subsets of T-cell lymphoma. Currently, it appears likely that the ultimate role of these agents will lie in rational combinations, only a few of which have been pursued in the clinic to date. This review focuses on relatively recently identified mechanisms of action of HDACIs, with particular emphasis on those that relate to the DNA damage response (DDR), and discusses synergistic strategies combining HDACIs with several novel targeted agents that disrupt the DDR or antagonize anti-apoptotic proteins that could have implications for the future use of HDACIs in patients with cancer.

Inhibition of autophagy enhances the cytotoxic effect of PA-MSHA in breast cancer.

BackgroundPA-MSHA, a genetically engineered Pseudomonas aeruginosa (PA) strain, is currently under investigation as a new anti-cancer drug. It can induce cell cycle arrest and apoptosis in different human cancer cells, including hormone receptor negative breast cancer cells. However, the underlying mechanism of tumor lethality mediated by PA-MSHA remains to be fully investigated.MethodsThe effect of PA-MSHA on human hormone receptor negative breast cancer cells was analyzed by morphological measurement, western blot, cell proliferation assay and mouse xenograft model.ResultsPA-MSHA was found to induce endoplasmic reticulum (ER) stress in breast cancer cell lines through the IRE1 signaling pathway. Inhibiting autophagy potentiated the cytotoxic effect of PA-MSHA while treating breast cancer cell lines. In mouse xenograft model, PA-MSHA produced more pronounced tumor suppression in mice inoculated with IRE1 gene knockdown. MDA-MB-231HM cells.ConclusionsThese findings demonstrated inhibiting autophagy together with PA-MSHA might be a promising therapeutic strategy in treating hormone receptor negative breast cancer cells.

Exposure to waterborne Cu inhibits cutaneous Na+ uptake in post-hatch larval rainbow trout (Oncorhynchus mykiss)

In freshwater rainbow trout (Oncorhynchus mykiss), two common responses to acute waterborne copper (Cu) exposure are reductions in ammonia excretion and Na+ uptake at the gills, with the latter representing the likely lethal mechanism of action for Cu in adult fish. Larval fish, however, lack a functional gill following hatch and rely predominantly on cutaneous exchange, yet represent the most Cu-sensitive life stage. It is not known if Cu toxicity in larval fish occurs via the skin or gills. The present study utilized divided chambers to assess cutaneous and branchial Cu toxicity over larval development, using disruptions in ammonia excretion (Jamm) and Na+ uptake (JinNa) as toxicological endpoints. Early in development (early; 3 days post-hatch; dph), approximately 95% of Jamm and 78% of JinNa occurred cutaneously, while in the late developmental stage (late; 25dph), the gills were the dominant site of exchange (83 and 87% of Jamm and JinNa, respectively). Exposure to 50μg/l Cu led to a 49% inhibition of Jamm in the late developmental stage only, while in the early and middle developmental (mid; 17dph) stages, Cu had no effect on Jamm. JinNa, however, was significantly inhibited by Cu exposure at the early (53% reduction) and late (47% reduction) stages. Inhibition at the early stage of development was mediated by a reduction in cutaneous uptake, representing the first evidence of cutaneous metal toxicity in an intact aquatic organism. The inhibitions of both Jamm and JinNa in the late developmental stage occurred via a reduction in branchial exchange only. The differential responses of the skin and gills to Cu exposure suggest that the mechanisms of Jamm and JinNa and/or Cu toxicity differ between these tissues. Exposure to 20μg/l Cu revealed that Jamm is the more Cu-sensitive process. The results presented here have important implications in predicting metal toxicity in larval fish. The Biotic Ligand Model (BLM) is currently used to predict metal toxicity in aquatic organisms. However, for rainbow trout this is based on gill binding constants from juvenile fish. This may not be appropriate for post-hatch larval fish where the skin is the site of toxic action of Cu. Determining Cu binding constants and lethal accumulation concentrations for both skin and gills in larval fish may aid in developing a larval fish-specific BLM. Overall, the changing site of toxic action and physiology of developing larval fish present an interesting and exciting avenue for future research.

Effects of temperature and modified atmospheres on diapausing 5th instar codling moth metabolism

The oxygen and capacity limitation of thermal tolerance (OCLTT) has been established in aquatic insect larvae, but OCLTT has not been shown to generally apply to terrestrial insects. Previous research indicates that heat treatments in combination with high concentrations of carbon dioxide and low concentrations of oxygen may be effective for controlling diapausing codling moth, a quarantine pest in walnuts, but treatment requires long times and the killing mechanism is unknown. In this study, the effects of temperature and modified atmospheres on metabolism in diapausing 5th instar codling moth (Cydia pomonella) was investigated with multi-channel differential scanning calorimeters, one equipped with an oxygen sensor. O2 consumption and metabolic heat rates in air were measured simultaneously at isothermal temperatures from 5 to 50°C at 5°C intervals. Both rates increased with increasing temperatures from 5 to 40°C. The ratio of metabolic heat rate to O2 consumption rate at temperatures ≤40°C shows that a portion of the metabolic heat is from normal anabolic reactions of metabolism. At 45 and 50°C in air, O2 consumption and metabolic heat rates dropped to near zero. These results indicate that treatment of walnuts in air at >45°C for a short period of time (minutes) is effective in killing diapausing 5th instar codling moth larvae. Continuous heating scans at 0.4°C/min were used to measure metabolic heat rates from 10 to 50°C with air and modified atmospheres with lowered oxygen and high carbon dioxide. A rapid increase was observed in heat rates above 40°C in scans with O2≥11%. Taken together with the isothermal results showing no metabolic heat production or oxygen uptake at 45 and 50°C, these results demonstrate that thermal damage to cell membranes and loss of control of oxidation reactions is the lethal mechanism at high temperature when O2≥11%. The data from scans with O2≤2% and high CO2 show the effects of oxygen limitation as postulated by the OCLTT. However, CO2 anesthesia appears to protect larvae from oxygen limitation at high temperature. These results show that treatment of walnuts in air at temperatures >45°C will rapidly kill diapausing 5th instar codling moths.

Trigemino‐Cardiac Reflex as Lethal Mechanism in a Suicidal Fire Death Case

In the vast majority of immediate fire deaths, the mechanism of death is inhalation of toxic gases (especially carbon monoxide), direct thermal injury, or neurogenic shock due to the redistribution of the body's blood volume produced by surface heat on the skin. We present a suicidal case that is unusual because the mechanism of immediate fire death could arguably be explained in terms of a primitive autonomic reflex/the trigemino-cardiac reflex. Although this reflex is well known to surgeons and anesthetists, with possible lethal consequences in the course of invasive surgical procedures on the head and neck region, it is much less familiar to forensic pathologists.

Regulation of Ferroptotic Cancer Cell Death by GPX4

Ferroptosis is a form of nonapoptotic cell death for which key regulators remain unknown. We sought a common mediator for the lethality of 12 ferroptosis-inducing small molecules. We used targeted metabolomic profiling to discover that depletion of glutathione causes inactivation of glutathione peroxidases (GPXs) in response to one class of compounds and a chemoproteomics strategy to discover that GPX4 is directly inhibited by a second class of compounds. GPX4 overexpression and knockdown modulated the lethality of 12 ferroptosis inducers, but not of 11 compounds with other lethal mechanisms. In addition, two representative ferroptosis inducers prevented tumor growth in xenograft mouse tumor models. Sensitivity profiling in 177 cancer cell lines revealed that diffuse large B cell lymphomas and renal cell carcinomas are particularly susceptible to GPX4-regulated ferroptosis. Thus, GPX4 is an essential regulator of ferroptotic cancer cell death.

Chemical insights into dodecylamine spore lethal germination

Investigations into dodecylamine spore lethal germination mechanism shed light on the design of future antispore molecules.

Fatal Air Embolism Associated With Pneumatosis Cystoides Intestinalis

We report a unique fatality associated with pneumatosis cystoides intestinalis. The proposed mechanism of lethality is air emboliza- tion. Our basis for this interpretation is autopsy exclusion of other lethal mechanisms in association with identifiable physical find - ings. We offer anatomic and physiologic explanations for our interpretation, in association with a brief discussion thereof.

Abstract IA8: Novel insights into synthetic lethal interactions in cancer: Common nodes in protein synthesis control and nucleotide metabolism

Abstract Our research is centered on understanding the molecular mechanisms by which impairments in accurate control of mRNA translation, cell growth, and overall cellular protein synthesis rates lead to cancer and human disease. We have uncovered that a common denominator of multiple oncogenic pathways (i.e. PI3K-AKT-mTOR, Ras, Myc) is their ability to directly control the core translation machinery of a cell, resulting in intrinsic regulation of the cancer cell gene expression program. Our work delineates how the “cancer translatome” can reprogram gene expression at the post-transcription level, by controlling the translation of specific subsets of mRNAs through unique cis-acting elements, during distinct steps of cancer progression. The immediate impact of these studies has been the design of a new generation of compounds to target the aberrant translation machinery, presently in clinical trials. I will discuss our most recent findings revealing that oncogenic pathways have evolved self-sustaining circuitries that couple two fundamental cellular processes, protein synthesis and metabolism, to sustain the unchecked growth potential of cancer cells. By employing an unbiased metabolomics approach combined with mouse genetics, our findings unveil a critical feed-forward mechanism by which Myc's ability to elevate the protein synthesis capacity of cancer cells directly reprograms nucleotide biosynthesis pathways as a mechanism to sustain tumor growth and survival. I will further discuss how both PI3K-AKT-mTOR and Myc converge on phosphorylation of 4EBP1, a master regulator of protein synthesis control, setting the stage for a unique therapeutic approach to render Myc druggable in the clinic. Together, our findings reveal that targeting either hyperactivation of protein synthesis or nucleotide production represents a novel and unexpected synthetic lethal mechanism for eradicating cancer but not normal cells. Citation Format: Davide Ruggero. Novel insights into synthetic lethal interactions in cancer: Common nodes in protein synthesis control and nucleotide metabolism. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr IA8.

Inactivation of the Bacterial RNA Polymerase Due to Acquisition of Secondary Structure by the ω Subunit

The widely conserved ω subunit encoded by rpoZ is the smallest subunit of Escherichia coli RNA polymerase (RNAP) but is dispensable for bacterial growth. Function of ω is known to be substituted by GroEL in ω-null strain, which thus does not exhibit a discernable phenotype. In this work, we report isolation of ω variants whose expression in vivo leads to a dominant lethal phenotype. Studies show that in contrast to ω, which is largely unstructured, ω mutants display substantial acquisition of secondary structure. By detailed study with one of the mutants, ω6 bearing N60D substitution, the mechanism of lethality has been deciphered. Biochemical analysis reveals that ω6 binds to β' subunit in vitro with greater affinity than that of ω. The reconstituted RNAP holoenzyme in the presence of ω6 in vitro is defective in transcription initiation. Formation of a faulty RNAP in the presence of mutant ω results in death of the cell. Furthermore, lethality of ω6 is relieved in cells expressing the rpoC2112 allele encoding β'2112, a variant β' bearing Y457S substitution, immediately adjacent to the β' catalytic center. Our results suggest that the enhanced ω6-β' interaction may perturb the plasticity of the RNAP active center, implicating a role for ω and its flexible state.

Discovery of a Statistically Significant and Interpretable Relationship Between Redox Reactivity and Lethality of Drugs

A statistically significant and interpretable relationship between electrophilicity as a redox reactivity indicator and LD50 as a lethality indicator of drugs was discovered, and this relationship could be interpreted by the action of the cytochrome P450. The drugs chosen in this study were Topoisomerase II inhibitor anticancer drugs, and the electrophilicity of drugs was obtained by quantum chemical calculation. Since the P450 detoxification mechanism is the catalytic oxidation of drug molecules, it may infer that the drug molecules being easily oxidized (low electrophilicity) will be weak in lethality in general. In addition, this relationship revealed two structural scaffolds for the anthracycline-based topoisomerase II inhibitors, and their lethality mechanisms are not totally the same. Such relationship can assist in designing new drugs that candidates possessing low electrophilicity are recommended for lowering of lethality, and moieties providing a large inductive effect can reduce the electrophilicity of the anthracycline-based topoisomerase II inhibitors.

Role of the LytSR Two-Component Regulatory System in Adaptation to Cationic Antimicrobial Peptides in Staphylococcus aureus

Many host defense cationic antimicrobial peptides (HDPs) perturb the staphylococcal cell membrane (CM) and alter transmembrane potential (ΔΨ) as key parts of their lethal mechanism. Thus, a sense-response system for detecting and mediating adaptive responses to such stresses could impact organism survival; the Staphylococcus aureus LytSR two-component regulatory system (TCRS) may serve as such a ΔΨ sensor. One well-known target of this system is the lrgAB operon, which, along with the related cidABC operon, has been shown to be a regulator in the control of programmed cell death and lysis. We used an isogenic set of S. aureus strains: (i) UAMS-1, (ii) its isogenic ΔlytS and ΔlrgAB mutants, and (iii) plasmid-complemented ΔlytSR and ΔlrgAB mutants. The ΔlytS strain displayed significantly increased in vitro susceptibilities to all HDPs tested (neutrophil-derived human neutrophil peptide 1 [hNP-1], platelet-derived thrombin-induced platelet microbicidal proteins [tPMPs], and the tPMP-mimetic peptide RP-1), as well as to calcium-daptomycin (DAP), a cationic antimicrobial peptide (CAP). In contrast, the ΔlrgAB strain exhibited no significant changes in susceptibilities to these cationic peptides, indicating that although lytSR positively regulates transcription of lrgAB, increased HDP/CAP susceptibilities in the ΔlytS mutant were lrgAB independent. Further, parental UAMS-1 (but not the ΔlytS mutant) became more resistant to hNP-1 and DAP following pretreatment with carbonyl cyanide m-chlorophenylhydrazone (CCCP) (a CM-depolarizing agent). Of note, lytSR-dependent survival against CAP/HDP killing was not associated with changes in either surface positive charge, expression of mprF and dlt, or CM fluidity. The ΔlytS strain (but not the ΔlrgAB mutant) displayed a significant reduction in target tissue survival in an endocarditis model during DAP treatment. Collectively, these results suggest that the lytSR TCRS plays an important role in adaptive responses of S. aureus to CM-perturbing HDPs/CAPs, likely by functioning as a sense-response system for detecting subtle changes in ΔΨ.

Stat2 loss leads to cytokine-independent, cell-mediated lethality in LPS-induced sepsis

Deregulated Toll-like receptor (TLR)-triggered inflammatory responses that depend on NF-κB are detrimental to the host via excessive production of proinflammatory cytokines, including TNF-α. Stat2 is a critical component of type I IFN signaling, but it is not thought to participate in TLR signaling. Our study shows that LPS-induced lethality in Stat2(-/-) mice is accelerated as a result of increased cellular transmigration. Blocking intercellular adhesion molecule-1 prevents cellular egress and confers survival of Stat2(-/-) mice. The main determinant of cellular egress in Stat2(-/-) mice is the genotype of the host and not the circulating leukocyte. Surprisingly, lethality and cellular egress observed on Stat2(-/-) mice are not associated with excessive increases in classical sepsis cytokines or chemokines. Indeed, in the absence of Stat2, cytokine production in response to multiple TLR agonists is reduced. We find that Stat2 loss leads to reduced expression of NF-κB target genes by affecting nuclear translocation of NF-κB. Thus, our data reveal the existence of a different mechanism of LPS-induced lethality that is independent of NF-κB triggered cytokine storm but dependent on cellular egress.

Getting away with murder: how does the BCL‐2 family of proteins kill with immunity?

The adult human body produces approximately one million white blood cells every second. However, only a small fraction of the cells will survive because the majority is eliminated through a genetically controlled form of cell death known as apoptosis. This review places into perspective recent studies pertaining to the BCL-2 family of proteins as critical regulators of the development and function of the immune system, with particular attention on B cell and T cell biology. Here we discuss how elegant murine model systems have revealed the major contributions of the BCL-2 family in establishing an effective immune system. Moreover, we highlight some key regulatory pathways that influence the expression, function, and stability of individual BCL-2 family members, and discuss their role in immunity. From lethal mechanisms to more gentle ones, the final portion of the review discusses the nonapoptotic functions of the BCL-2 family and how they pertain to the control of immunity.

Causes and Trends in Traumatic Brain Injury for United States Adolescents

Traumatic brain injury (TBI) is a leading cause of death and disability among United States adolescents. The authors sought to determine causes and trends for TBI-related hospitalizations in the United States adolescent population (10-19 years). The authors identified common causes and trends of adolescent TBI, overall and within 2-year age categories, using hospitalization data from 2005 to 2009 in the Nationwide Inpatient Sample. The leading cause of adolescent TBI overall was motor vehicle occupant accidents (35%), which are also the leading cause in the 14-15, 16-17, and 18-19 year age groups. Falls were the cause of most TBI in the 10-11 year (23%) and 12-13 year (20%) age groups. For both unintentional and intentional mechanisms of injury, there was evidence of increasing hospitalizations with increasing age. From 2005 to 2009, the overall annual incidence of adolescent TBI hospitalizations decreased 21% from an estimated 75.5-59.3 per 100,000 (p<0.001). These rates declined for mild, moderate, and severe TBI, and decreased for 2-year age groups, except for the 18-19 year-old group. For TBI attributable to motor vehicle occupants, rates declined 27% from 27.6 to 20.2 per 100,000 (p<0.001). Motor vehicle occupant injuries account for 42% of in-hospital mortality from adolescent TBI; however, firearms are the most lethal mechanism with 46% proportional mortality among victims of firearm-related TBI. Rates of adolescent TBI-related hospitalizations have decreased overall. Motor vehicle accidents and firearms were identified as leading causes of injury and mortality for adolescent TBI, and represent potential targets for intervention.

Genotoxicity of 2,6- and 3,5-dimethylaniline in cultured mammalian cells: the role of reactive oxygen species.

Several alkylanilines with structures more complex than toluidines have been associated epidemiologically with human cancer. Their mechanism of action remains largely undetermined, and there is no reported evidence that it replicates that of multicyclic aromatic amines even though the principal metabolic pathways of P450-mediated hydroxylation and phase II conjugation are very similar. As a means to elucidate their mechanisms of action, lethality and mutagenicity in the adenine phosphoribosyltransferase (aprt (+/-)) gene induced in several Chinese hamster ovary cell types by 2,6- and 3,5-dimethylaniline (2,6-DMA, 3,5-DMA) and their N- and ring-hydroxyl derivatives (N-OH-2,6-DMA, N-OH-3,5-DMA, 2,6-DMAP, 3,5-DMAP) were assessed. Dose-response relationships were determined in the parental AA8 cell line, its repair-deficient UV5 subclone and other repair-deficient 5P3NAT2 or -proficient 5P3NAT2R9 subclones engineered to express mouse cytochrome P4501A2 (CYP1A2) and human N-acetyltransferase (NAT2), and also in AS52 cells harboring the bacterial guanine-hypoxanthine phosphoribosyltransferase (gpt) gene. Mutations in the gpt gene of AS52 cells were characterized and found to be dominated by G:C to A:T and A:T to G:C transitions. Separately, treatment of AS52 cells with N-OH-2,6-DMA, N-OH-3,5-DMA, 2,6-DMAP, 3,5-DMAP, and 3,5-DMAP led to intracellular production of reactive oxygen species (ROS) for at least 24h after removal of the mutagens in every case. Using the comet assay, DNA strand breaks were observed in a dose-dependent manner in AS52 cells when treated with each of the four N-OH-2,6-DMA, N-OH-3,5-DMA, 2,6-DMAP, and 3,5-DMAP derivatives. Comparative evaluation of the results indicates that the principal mechanism of mutagenic action is likely to be through redox cycling of intracellularly bound aminophenol/quinone imine structures to generate ROS rather than through formation of covalent DNA adducts.

Mitral annulus calcification and sudden death

Mitral valve annulus calcification is a degenerative cardiac condition often found at autopsy in the elderly. While usually considered incidental to the cause of death, we report two cases where mitral valve annulus calcification with valve stenosis was associated with sudden death. Case 1: a 61-year-old female who had underlying atherosclerosis and hypertension collapsed at home. At autopsy there was marked mitral valve annulus calcification with valve stenosis and cardiomegaly. Case 2: a previously well 74-year-old female collapsed in a toilet. At autopsy there was marked calcification of the mitral valve annulus with valve stenosis. In both cases death was attributed to the effects of the calcified mitral valve annulus. Although such calcification may be unrelated to the terminal lethal mechanism, the association with left atrial enlargement, atrial fibrillation, mitral regurgitation, mitral stenosis, bacterial endocarditis, ischaemic and thromboembolic stroke, myocardial infarction, and arrhythmias, means that it should not be overlooked in the differential diagnosis in cases of sudden and unexpected death.

Functional characterization of Fospeg, and its impact on cell cycle upon PDT of Huh7 hepatocellular carcinoma cell model

Although several treatment options are available for hepatocellular carcinoma (HCC), their application is mostly restricted to early diagnosed cases or includes liver transplantation, which is rarely available due to donor scarcity. The attractiveness of PDT as a cancer treatment does not only come from its minimal invasiveness, but also from the high selectivity due to tumor localization that can be applied. Precise focusing of light on tumor lesions will result in tumor-specific PDT activation. Novel photosensitizers can be applied in such low concentrations that cells not subjected to irradiation remain healthy. The lethal effect and mechanism of death induction of the photosensitizer Fospeg has never been studied on hepatocellular carcinoma. The aim of the present study is to functionally analyze the impact of PDT on Huh-7 HCC cell line, as well as to analyze its impact on cell cycle protein expression. Cellular viability, and proliferation assays were conducted via MTT and BrdU assay, respectively. Transfected cell models of Huh7 with different constructs harboring cell cycle genes and downstream reporter luciferase gene were generated. Our results show a statistically significant decrease in both viability and proliferation of Huh-7 cells following PDT, while maintaining Fospeg and laser concentrations far below toxic levels. Proliferative cell cycle genes show a tendency of inhibition, while p53 levels show a significant increase following PDT. Fospeg-mediated PDT is a promising strategy for treatment of hepatocellular carcinoma and needs to be further explored in vivo.

Mre11-dependent degradation of stalled DNA replication forks is prevented by BRCA2 and PARP1.

PARP inhibitors are currently being used in clinical trials to treat BRCA1- or BRCA2-defective tumors, based on the synthetic lethal interaction between PARP1 and BRCA1/2-mediated homologous recombination (HR). However, the molecular mechanisms that drive this synthetic lethality remain unclear. Here, we show increased levels of Mre11, a key component of MRN (Mre11-Rad50-Nbs1) complex that plays a role in the restart of stalled replication forks and enhanced resection at stalled replication forks in BRCA2-deficient cells. BRCA2-deficient cells also showed hypersensitivity to the Mre11 inhibitor mirin. Interestingly, PARP1 activity was required to protect stalled forks from Mre11-dependent degradation. Resistance to PARP inhibition in BRCA2-mutant cells led to reduced levels of Mre11 foci and also rescued their sensitivity to mirin. Taken together, our findings not only show that Mre11 activity is required for the survival of BRCA2 mutant cells but also elucidate roles for both the BRCA2 and PARP1 proteins in protecting stalled replication forks, which offers insight into the molecular mechanisms of the synthetic lethality between BRCA2 and PARP1.