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Summary

BRCA1 and BRCA2 are often mutated in familial breast and ovarian cancer. Both tumor suppressors play key roles in the DNA-damage response [1, 2]. However, it remains unclear whether these two tumor suppressor function together in the same DNA-damage response pathway. Here, we show that BRCA1 associates with BRCA2 through PALB2/FANCN, a major binding partner of BRCA2 [3]. The interaction between BRCA1 and BRCA2 is abrogated in PALB2-deficient Fanconi anemia cells and in the cells depleted of PALB2 by small interfering RNA. Moreover, we show that BRCA1 promotes the concentration of PALB2 and BRCA2 at DNA-damage sites and the interaction between BRCA1 and PALB2 is important for the homologous recombination repair. Taken together, our results indicate that BRCA1 is an upstream regulator of BRCA2 in the DNA-damage response, and PALB2 is the linker between BRCA1 and BRCA2.

PALB2 was originally identified as a BRCA2-interacting protein which is crucial for key BRCA2 genome caretaker functions. It subsequently became clear that PALB2 was another Fanconi anemia (FA) gene (FANCN), and that monoallelic PALB2 mutations are associated with increased risk of breast and pancreatic cancer. Mutations in PALB2 have been identified in breast cancer families worldwide and recent studies have shown that PALB2 also interacts with BRCA1. Here we summarize the molecular functions and clinical phenotypes of this key DNA repair pathway component and discuss how its discovery has advanced our knowledge of both FA and adult cancer predisposition.

PALB2 interacts with BRCA2, and biallelic mutations in PALB2 (also known as FANCN), similar to biallelic BRCA2 mutations, cause Fanconi anemia. We identified monoallelic truncating PALB2 mutations in 10/923 individuals with familial breast cancer compared with 0/1,084 controls (P = 0.0004) and show that such mutations confer a 2.3-fold higher risk of breast cancer (95% confidence interval (c.i.) = 1.4–3.9, P = 0.0025). The results show that PALB2 is a breast cancer susceptibility gene and further demonstrate the close relationship of the Fanconi anemia–DNA repair pathway and breast cancer predisposition.

PALB2 is essential for BRCA2 anchorage to nuclear structures and for homologous recombinational repair of DNA double-strand breaks. Here, we report that the N-terminal coiled-coil motif of PALB2 regulates its self-association and homologous recombination. Monomeric PALB2 shows higher efficiency to bind DNA and promotes RAD51 filament formation with or without the inhibitory effect of Replication Protein A. Moreover, overexpression of the PALB2 coiled-coil domain severely affects RAD51 loading to DNA damage sites suggesting a competition between PALB2 self-interaction and PALB2–BRCA1 interaction. In the presence of DNA damage, the switch between PALB2–PALB2 and PALB2–BRCA1 interactions allows the activation of HR. Controlling HR via PALB2 self-interactions could be important to prevent aberrant recombination in normal conditions and activate DNA repair when required.

Background

It has been demonstrated that PALB2 acts as a bridging molecule between the BRCA1 and BRCA2 proteins and is responsible for facilitating BRCA2-mediated DNA repair. Truncating mutations in the PALB2 gene have been reported to be enriched in Fanconi anemia and breast cancer patients in various populations.

Methods

We evaluated the contribution of PALB2 germline mutations in 279 African-American breast cancer patients including 29 patients with a strong family history, 29 patients with a moderate family history, 75 patients with a weak family history, and 146 non-familial or sporadic breast cancer cases.

Results

After direct sequencing of all the coding exons, exon/intron boundaries, 5′UTR and 3′UTR of PALB2, three (1.08%; 3 in 279) novel monoallelic truncating mutations were identified: c.758dupT (exon4), c.1479delC (exon4) and c.3048delT (exon 10); together with 50 sequence variants, 27 of which are novel. None of the truncating mutations were found in 262 controls from the same population.

Conclusions

PALB2 mutations are present in both familial and non-familial breast cancer among African-Americans. Rare PALB2 mutations account for a small but substantial proportion of breast cancer patients.

The transcription factor Nrf2 has emerged as a master regulator of cellular redox homeostasis. As an adaptive response to oxidative stress, Nrf2 activates the transcription of a battery of genes encoding antioxidants, detoxification enzymes, and xenobiotic transporters by binding the cis-antioxidant response element in the promoter regions of genes. The magnitude and duration of inducible Nrf2 signaling is delicately controlled at multiple levels by Keap1, which targets Nrf2 for redox-sensitive ubiquitin-mediated degradation in the cytoplasm and exports Nrf2 from the nucleus. However, it is not clear how Keap1 gains access to the nucleus. In this study, we show that Keap1 is constantly shuttling between the nucleus and the cytoplasm under physiological conditions. The nuclear import of Keap1 requires its C-terminal Kelch domain and is independent of Nrf1 and Nrf2. We have determined that importin α7, also known as karyopherin α6 (KPNA6), directly interacts with the Kelch domain of Keap1. Overexpression of KPNA6 facilitates Keap1 nuclear import and attenuates Nrf2 signaling, whereas knockdown of KPNA6 slows down Keap1 nuclear import and enhances the Nrf2-mediated adaptive response induced by oxidative stress. Furthermore, KPNA6 accelerates the clearance of Nrf2 protein from the nucleus during the postinduction phase, therefore promoting restoration of the Nrf2 protein to basal levels. These findings demonstrate that KPNA6-mediated Keap1 nuclear import plays an essential role in modulating the Nrf2-dependent antioxidant response and maintaining cellular redox homeostasis.

The breast cancer suppressor BRCA2 is essential for the maintenance of genomic integrity in mammalian cells through its role in DNA repair by homologous recombination (HR). Human BRCA2 is 3,418 amino acids and is comprised of multiple domains that interact with the RAD51 recombinase and other proteins as well as with DNA. To gain insight into the cellular function of BRCA2 in HR, we created fusions consisting of various BRCA2 domains and also introduced mutations into these domains to disrupt specific protein and DNA interactions. We find that a BRCA2 fusion peptide deleted for the DNA binding domain and active in HR is completely dependent on interaction with the PALB2 tumor suppressor for activity. Conversely, a BRCA2 fusion peptide deleted for the PALB2 binding domain is dependent on an intact DNA binding domain, providing a role for this conserved domain in vivo; mutagenesis suggests that both single-stranded and double-stranded DNA binding activities in the DNA binding domain are required for its activity. Given that PALB2 itself binds DNA, these results suggest alternative mechanisms to deliver RAD51 to DNA. In addition, the BRCA2 C terminus contains both RAD51-dependent and -independent activities which are essential to HR in some contexts. Finally, binding the small peptide DSS1 is essential for activity when its binding domain is present, but not when it is absent. Our results reveal functional redundancy within the BRCA2 protein and emphasize the plasticity of this large protein built for optimal HR function in mammalian cells. The occurrence of disease-causing mutations throughout BRCA2 suggests sub-optimal HR from a variety of domain modulations.

Author Summary

The breast tumor suppressor BRCA2 has a major role in DNA repair by homologous recombination (HR). BRCA2 is a large protein with multiple domains that interact with several proteins as well as with DNA, complicating our understanding of how the protein functions in cells. To investigate the mechanism by which BRCA2 functions in HR in cells, we created fusions consisting of various BRCA2 domains and also introduced mutations into these domains to disrupt specific protein and DNA interactions. We find that DNA binding by BRCA2 is critical when a BRCA2 peptide is deficient in binding another breast cancer suppressor, PALB2, but not when the peptide can bind PALB2, suggesting alternative mechanisms of activity. Binding the small peptide DSS1 is also essential for HR only in some contexts, as are activities in the BRCA2 C terminus. Our results reveal redundancy of BRCA2 domains and emphasize plasticity within this large protein built for optimal HR function in mammalian cells. The occurrence of disease-causing mutations throughout BRCA2 suggests sub-optimal HR from a variety of domain modulations.

Homologous recombination mediated by the RAD51 recombinase helps eliminate chromosomal lesions, such as DNA double-stranded breaks induced by radiation or arising from injured DNA replication forks. The tumor suppressors BRCA2 and PALB2 act together to deliver RAD51 to chromosomal lesions to initiate repair. Here we document a new function of PALB2 in the enhancement of RAD51's ability to form the D-loop. We show that PALB2 binds DNA and physically interacts with RAD51. Importantly, while PALB2 alone stimulates D-loop formation, a co-operative effect is seen with RAD51AP1, an enhancer of RAD51. This stimulation stems from PALB2's ability to function with RAD51 and RAD51AP1 to assemble the synaptic complex. Our results help unveil a multi-faceted role of PALB2 in chromosome damage repair. Since PALB2 mutations can cause breast and other tumors or lead to Fanconi anemia, our findings are important for understanding the mechanism of tumor suppression in humans.

Summary

In response to oxidative stress, Nrf2 and p21 Cip1/WAF1 are both upregulated to protect cells from oxidative damage. Nrf2 is constantly ubiquitinated by a Keap1 dimer that interacts with a weak-binding 29DLG motif and a strong-binding 79ETGE motif in Nrf2, resulting in degradation of Nrf2. Modification of the redox-sensitive cysteine residues on Keap1 disrupts the Keap1-29DLG binding, leading to diminished Nrf2 ubiquitination and activation of the antioxidant response. However, the underlying mechanism by which p21 protects cells from oxidative damage remains unclear. Here, we present molecular and genetic evidence suggesting that the antioxidant function of p21 is mediated through activation of Nrf2 by stabilizing the Nrf2 protein. The 154KRR motif in p21 directly interacts with the 29DLG and 79ETGE motifs in Nrf2, and thus, competes with Keap1 for Nrf2 binding, compromising ubiquitination of Nrf2. Furthermore, the physiological significance of our findings was demonstrated in vivo using p21-deficient mice.

Animal cells counteract oxidative stress and electrophilic attack through coordinated expression of a set of detoxifying and antioxidant enzyme genes mediated by transcription factor Nrf2. In unstressed cells, Nrf2 appears to be sequestered in the cytoplasm via association with an inhibitor protein, Keap1. Here, by using the yeast two-hybrid screen, human Keap1 has been identified as a partner of the nuclear protein prothymosin α. The in vivo and in vitro data indicated that the prothymosin α-Keap1 interaction is direct, highly specific, and functionally relevant. Furthermore, we showed that Keap1 is a nuclear-cytoplasmic shuttling protein equipped with a nuclear export signal that is important for its inhibitory action. Prothymosin α was able to liberate Nrf2 from the Nrf2-Keap1 inhibitory complex in vitro through competition with Nrf2 for binding to the same domain of Keap1. In vivo, the level of Nrf2-dependent transcription was correlated with the intracellular level of prothymosin α by using prothymosin α overproduction and mRNA interference approaches. Our data attribute to prothymosin α the role of intranuclear dissociator of the Nrf2-Keap1 complex, thus revealing a novel function for prothymosin α and adding a new dimension to the molecular mechanisms underlying expression of oxidative stress-protecting genes.

Nrf2 is the key transcription factor regulating the antioxidant response. Nrf2 signaling is repressed by Keap1 at basal condition and induced by oxidative stress. Keap1 is recently identified as a Cullin 3-dependent substrate adaptor protein. A two-sites binding “hinge & latch” model vividly depicts how Keap1 can efficiently present Nrf2 as substrate for ubiquitination. Oxidative perturbation can impede Keap1-mediated Nrf2 ubiquitination but fail to disrupt Nrf2/Keap1 binding. Nrf2 per se is a redox-sensitive transcripon factor. A new Nrf2-mediated redox signaling model is proposed based on these new discoveries. Free floating Nrf2 protein functions as a redox-sensitive probe. Keap1 instead functions as a gate keeper to control the availability of Nrf2 probes and thus regulates the overall sensitivity of the redox signaling.

Nrf2 (NF-E2-related factor 2) is a master transcription factor containing a powerful acidic transcriptional activation domain. Nrf2-dependent gene expression impacts cancer chemoprevention strategies, inflammatory responses, and progression of neurodegenerative diseases. Under basal conditions, association of Nrf2 with the CUL3 adaptor protein Keap1 results in the rapid Nrf2 ubiquitylation and proteasome-dependent degradation. Inhibition of Keap1 function blocks ubiquitylation of Nrf2, allowing newly synthesized Nrf2 to translocate into the nucleus, bind to ARE sites and direct target gene expression. Site-directed mutagenesis experiments coupled with proteomic analysis support a model in which Keap1 contains at least 2 distinct cysteine motifs. The first is located at Cys 151 in the BTB domain. The second is located in the intervening domain and centers around Cys 273 & 288. Adduction or oxidation at Cys151 has been shown to produce a conformational change in Keap1 that results in dissociation of Keap1 from CUL3, thereby inhibiting Nrf2 ubiquitylation. Thus, adduction captures specific chemical information and translates it into biochemical information via changes in structural conformation.

In Drosophila, intestinal stem cells (ISCs) respond to oxidative challenges and inflammation by increasing proliferation rates. This phenotype is part of a regenerative response, but can lead to hyperproliferation and epithelial degeneration in the aging animal. Here we show that Nrf2, a master regulator of the cellular redox state, specifically controls the proliferative activity of ISCs, promoting intestinal homeostasis. We find that Nrf2 is constitutively active in ISCs, and that repression of Nrf2 by its negative regulator Keap1 is required for ISC proliferation. We further show that Nrf2 and Keap1 exert this function in ISCs by regulating the intracellular redox balance. Accordingly, loss of Nrf2 in ISCs causes accumulation of reactive oxygen species and accelerates age-related degeneration of the intestinal epithelium. Our findings establish Keap1 and Nrf2 as a critical redox management system that regulates stem cell function in high-turnover tissues.

Upregulation of cytoprotective enzymes by therapeutic agents to prevent damage by reactive oxygen species and xenobiotic electrophiles is a strategy for cancer chemoprevention. The Kelch-like ECH-associated protein 1 (Keap1) and its binding partner, transcription factor NF-E2-related factor-2 (Nrf2), are chemoprevention targets because of their role in regulating the antioxidant response element (ARE) in response to oxidative stress and exposure to electrophiles. Modification of the sensor protein Keap1 by electrophiles such as the isothiocyanate sulforaphane can direct Nrf2 accumulation in the nucleus and subsequent ARE activation. Since our previous matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS)-based screening method to discover natural products that modify Keap1 does not detect covalent modification of Keap1 by some highly reversible agents such as sulforaphane, a more sensitive screening assay was developed. In this new assay, electrophiles that have reversibly modified Keap1 can be released, trapped and detected as β-mercaptoethanol adducts by mass spectrometry. Isoliquiritigenin and sulforaphane, known ARE activators that target Keap1, were used to validate the assay. To determine the ability of the assay to identify electrophiles in complex matrixes that modify Keap1, sulforaphane was spiked into a cocoa extract, and LC-MS/MS using high resolution mass spectrometry with accurate mass measurement was used to identify β-mercaptoethanol adducts of sulforaphane that had been released from Keap1. This screening assay permits identification of potential chemoprevention agents in complex natural product mixtures that reversibly modify Keap1 but cannot be detected using MALDI-TOF MS.

The Keap1–Nrf2 [Kelch-like ECH-associated protein 1–nuclear factor (erythroid-derived 2)-like 2] pathway plays a central role in the protection of cells against oxidative and xenobiotic stresses. Nrf2 is a potent transcription activator that recognizes a unique DNA sequence known as the antioxidant response element (ARE). Under normal conditions, Nrf2 binds to Keap1 in the cytoplasm, resulting in proteasomal degradation. Following exposure to electrophiles or reactive oxygen species, Nrf2 becomes stabilized, translocates into the nucleus, and activates the transcription of various cytoprotective genes. Increasing attention has been paid to the role of Nrf2 in cancer cells because the constitutive stabilization of Nrf2 has been observed in many human cancers with poor prognosis. Recent studies have shown that the antioxidant and detoxification activities of Nrf2 confer chemo- and radio-resistance to cancer cells. In this review, we provide an overview of the Keap1–Nrf2 system and discuss its role under physiological and pathological conditions, including cancers. We also introduce the results of our recent study describing Nrf2 function in the metabolism of cancer cells. Nrf2 likely confers a growth advantage to cancer cells through enhancing cytoprotection and anabolism. Finally, we discuss the possible impact of Nrf2 inhibitors on cancer therapy.

Reactive oxygen species (ROS) are mutagenic and may thereby promote cancer1. Normally, ROS levels are tightly controlled by an inducible antioxidant program that responds to cellular stressors and is predominantly regulated by the transcription factor Nrf2 and its repressor protein Keap12-5. In contrast to the acute physiological regulation of Nrf2, in neoplasia there is evidence for increased basal activation of Nrf2. Indeed, somatic mutations that disrupt the Nrf2-Keap1 interaction to stabilize Nrf2 and increase the constitutive transcription of Nrf2 target genes were recently identified, suggesting that enhanced ROS detoxification and additional Nrf2 functions may in fact be pro-tumorigenic6. Here, we investigated ROS metabolism in primary murine cells following the expression of endogenous oncogenic alleles of K-Ras, B-Raf and Myc, and find that ROS are actively suppressed by these oncogenes. K-RasG12D, B-RafV619E and MycERT2 each increased the transcription of Nrf2 to stably elevate the basal Nrf2 antioxidant program and thereby lower intracellular ROS and confer a more reduced intracellular environment. Oncogene-directed increased expression of Nrf2 is a novel mechanism for the activation of the Nrf2 antioxidant program, and is evident in primary cells and tissues of mice expressing K-RasG12D and B-RafV619E, and in human pancreatic cancer. Furthermore, genetic targeting of the Nrf2 pathway impairs K-RasG12D-induced proliferation and tumorigenesis in vivo. Thus, the Nrf2 antioxidant and cellular detoxification program represents a previously unappreciated mediator of oncogenesis.

EMSY links the BRCA2 pathway to sporadic breast/ovarian cancer. It encodes a nuclear protein that binds to the BRCA2 N-terminal domain implicated in chromatin/transcription regulation, but when sporadically amplified/overexpressed, increased EMSY level represses BRCA2 transactivation potential and induces chromosomal instability, mimicking the activity of BRCA2 mutations in the development of hereditary breast/ovarian cancer. In addition to chromatin/transcription regulation, EMSY may also play a role in the DNA-damage response, suggested by its ability to localize at chromatin sites of DNA damage/repair. This implies that EMSY overexpression may also repress BRCA2 in DNA-damage replication/checkpoint and recombination/repair, coordinated processes that also require its interacting proteins: PALB2, the partner and localizer of BRCA2; RPA, replication/checkpoint protein A; and RAD51, the inseparable recombination/repair enzyme. Here, using a well-characterized recombination/repair assay system, we demonstrate that a slight increase in EMSY level can indeed repress these two processes independently of transcriptional interference/repression. Since EMSY, RPA and PALB2 all bind to the same BRCA2 region, these findings further support a scenario wherein: (a) EMSY amplification may mimic BRCA2 deficiency, at least by overriding RPA and PALB2, crippling the BRCA2/RAD51 complex at DNA-damage and replication/transcription sites; and (b) BRCA2/RAD51 may coordinate these processes by employing at least EMSY, PALB2 and RPA. We extensively discuss the molecular details of how this can happen to ascertain its implications for a novel recombination mechanism apparently conceived as checkpoint rather than a DNA repair system for cell division, survival, death, and human diseases, including the tissue specificity of cancer predisposition, which may renew our thinking about targeted therapy and prevention.

SUMMARY

Keap1/Nrf2 signaling defends organisms against the detrimental effects of oxidative stress, and has been suggested to abate its consequences, including aging-associated diseases like neurodegeneration, chronic inflammation, and cancer. Nrf2 is a prominent target for drug discovery, and Nrf2-activating agents are in clinical trials for cancer chemoprevention. However, aberrant activation of Nrf2 by keap1 somatic mutations may contribute to carcinogenesis and promote resistance to chemotherapy. To evaluate potential functions of Keap1 and Nrf2 for organismal homeostasis, we characterized the pathway in Drosophila. We demonstrate that Keap1/Nrf2 signaling in the fruitfly is activated by oxidants, induces antioxidant and detoxification responses, and confers increased tolerance to oxidative stress. Importantly, keap1 loss-of-function mutations extend the lifespan of Drosophila males, supporting a role for Nrf2 signaling in the regulation of longevity. Interestingly, cancer chemopreventive drugs potently stimulate Drosophila Nrf2 activity, suggesting the fruitfly as an experimental system to identify and characterize such agents.

Inherited mutations in the BRCA2-interacting protein PALB2 are known to be associated with increased risks of breast cancer. In order to evaluate the contribution of PALB2 to familial breast cancer in the United States, we sequenced the coding sequences and flanking regulatory regions of the gene from constitutional genomic DNA of 1144 familial breast cancer patients with wildtype sequences at BRCA1 and BRCA2. Overall, 3.4% (33/972) of patients not selected by ancestry and 0% (0/172) of patients specifically of Ashkenazi Jewish ancestry were heterozygous for a nonsense, frameshift, or frameshift-associated splice mutation in PALB2. Mutations were detected in both male and female breast cancer patients. All mutations were individually rare: the 33 heterozygotes harbored 13 different mutations, 5 previously reported and 8 novel. PALB2 heterozygotes were 4-fold more likely to have a male relative with breast cancer (P=0.0003) and 6-fold more likely to have a relative with pancreatic cancer (P=0.002), and 1.3-fold more likely to have a relative with ovarian cancer (P=0.18). Compared to their female relatives without mutations, increased risk of breast cancer for female PALB2 heterozygotes was 2.3-fold (95%CI [1.5–4.2]) by age 55 and 3.4-fold (95%CI [2.4–5.9]) by age 85. Loss of the wildtype PALB2 allele was observed in laser dissected tumor specimens from heterozygous patients. Given this mutation prevalence and risk, consideration might be given to clinical testing of PALB2 by complete genomic sequencing for familial breast cancer patients with wildtype sequences at BRCA1 and BRCA2.

Nrf2:INrf2(Keap1) are cellular sensors of chemical and radiation induced oxidative and electrophilic stress. Nrf2 is a nuclear transcription factor that controls the expression and coordinated induction of a battery of defensive genes encoding detoxifying enzymes and antioxidant proteins. This is a mechanism of critical importance for cellular protection and cell survival. Nrf2 is retained in the cytoplasm by an inhibitor INrf2. INrf2 functions as an adapter for Cul3/Rbx1 mediated degradation of Nrf2. In response to oxidative/electrophilic stress, Nrf2 is switched on and then off by distinct early and delayed mechanisms. Oxidative/electrophilic modification of INrf2cysteine151 and/or PKC phosphorylation of Nrf2serine40 results in the escape or release of Nrf2 from INrf2. Nrf2 is stabilized and translocates to the nucleus, forms heterodimers with unknown proteins, and binds antioxidant response element (ARE) that leads to coordinated activation of gene expression. It takes less than fifteen minutes from the time of exposure to switch on nuclear import of Nrf2. This is followed by activation of a delayed mechanism that controls switching off of Nrf2 activation of gene expression. GSK3β phosphorylates Fyn at unknown threonine residue(s) leading to nuclear localization of Fyn. Fyn phosphorylates Nrf2tyrosine568 resulting in nuclear export of Nrf2, binding with INrf2 and degradation of Nrf2. The switching on and off of Nrf2 protect cells against free radical damage, prevents apoptosis and promotes cell survival.

Fanconi anemia (FA) is a rare genetic disorder characterized by aplastic anemia, cancer/leukemia susceptibility and cellular hypersensitivity to DNA crosslinking agents, such as cisplatin. To date, 12 FA gene products have been identified, which cooperate in a common DNA damage-activated signaling pathway regulating DNA repair (the FA pathway). Eight FA proteins form a nuclear complex harboring E3 ubiquitin ligase activity (the FA core complex) that, in response to DNA damage, mediates the monoubiquitylation of the FA protein FANCD2. Monoubiquitylated FANCD2 colocalizes in nuclear foci with proteins involved in DNA repair, including BRCA1, FANCD1/BRCA2, FANCN/PALB2 and RAD51. All these factors are required for cellular resistance to DNA crosslinking agents. The inactivation of the FA pathway has also been observed in a wide variety of human cancers and is implicated in the sensitivity of cancer cells to DNA crosslinking agents. Drugs that inhibit the FA pathway may be useful chemosensitizers in the treatment of cancer.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

The Keap1-Nrf2 system is the major regulatory pathway of cytoprotective gene expression against oxidative and/or electrophilic stresses. Keap1 acts as a stress sensor protein in this system. While Keap1 constitutively suppresses Nrf2 activity under unstressed conditions, oxidants or electrophiles provoke the repression of Keap1 activity, inducing the Nrf2 activation. However, the precise molecular mechanisms behind the liberation of Nrf2 from Keap1 repression in the presence of stress remain to be elucidated. We hypothesized that oxidative and electrophilic stresses induce the nuclear accumulation of Nrf2 by affecting the Keap1-mediated rapid turnover of Nrf2, since such accumulation was diminished by the protein synthesis inhibitor cycloheximide. While both the Cys273 and Cys288 residues of Keap1 are required for suppressing Nrf2 nuclear accumulation, treatment of cells with electrophiles or mutation of these cysteine residues to alanine did not affect the association of Keap1 with Nrf2 either in vivo or in vitro. Rather, these treatments impaired the Keap1-mediated proteasomal degradation of Nrf2. These results support the contention that Nrf2 protein synthesized de novo after exposure to stress accumulates in the nucleus by bypassing the Keap1 gate and that the sensory mechanism of oxidative and electrophilic stresses is closely linked to the degradation mechanism of Nrf2.

Nrf2 is the regulator of the oxidative/electrophilic stress response. Its turnover is maintained by Keap1-mediated proteasomal degradation via a two-site substrate recognition mechanism in which two Nrf2-Keap1 binding sites form a hinge and latch. The E3 ligase adaptor Keap1 recognizes Nrf2 through its conserved ETGE and DLG motifs. In this study, we examined how the ETGE and DLG motifs bind to Keap1 in a very similar fashion but with different binding affinities by comparing the crystal complex of a Keap1-DC domain-DLG peptide with that of a Keap1-DC domain-ETGE peptide. We found that these two motifs interact with the same basic surface of either Keap1-DC domain of the Keap1 homodimer. The DLG motif works to correctly position the lysines within the Nrf2 Neh2 domain for efficient ubiquitination. Together with the results from calorimetric and functional studies, we conclude that different electrostatic potentials primarily define the ETGE and DLG motifs as a hinge and latch that senses the oxidative/electrophilic stress.

Background

Nrf2 is a key transcriptional regulator of a battery of genes that facilitate phase II/III drug metabolism and defence against oxidative stress. Nrf2 is largely regulated by Keap1, which directs Nrf2 for proteasomal degradation. The Nrf2/Keap1 system is dysregulated in lung, head and neck, and breast cancers and this affects cellular proliferation and response to therapy. Here, we have investigated the integrity of the Nrf2/Keap1 system in pancreatic cancer.

Results

Keap1, Nrf2 and the Nrf2 target genes AKR1c1 and GCLC were detected in a panel of five pancreatic cancer cell lines. Mutation analysis of NRF2 exon 2 and KEAP1 exons 2-6 in these cell lines identified no mutations in NRF2 and only synonomous mutations in KEAP1. RNAi depletion of Nrf2 caused a decrease in the proliferation of Suit-2, MiaPaca-2 and FAMPAC cells and enhanced sensitivity to gemcitabine (Suit-2), 5-flurouracil (FAMPAC), cisplatin (Suit-2 and FAMPAC) and gamma radiation (Suit-2). The expression of Nrf2 and Keap1 was also analysed in pancreatic ductal adenocarcinomas (n = 66 and 57, respectively) and matching normal benign epithelium (n = 21 cases). Whilst no significant correlation was seen between the expression levels of Keap1 and Nrf2 in the tumors, interestingly, Nrf2 staining was significantly greater in the cytoplasm of tumors compared to benign ducts (P < 0.001).

Conclusions

Expression of Nrf2 is up-regulated in pancreatic cancer cell lines and ductal adenocarcinomas. This may reflect a greater intrinsic capacity of these cells to respond to stress signals and resist chemotherapeutic interventions. Nrf2 also appears to support proliferation in certain pancreatic adenocarinomas. Therefore, strategies to pharmacologically manipulate the levels and/or activity of Nrf2 may have the potential to reduce pancreatic tumor growth, and increase sensitivity to therapeutics.

Triple-negative breast cancer (TNBC) is an aggressive form of breast carcinoma with a poor prognosis. Recent evidence suggests that some patients with TNBC harbour germ-line mutations in DNA repair genes which may render their tumours susceptible to novel therapies such as treatment with PARP inhibitors. In the present study, we have investigated a hospital-based series of 40 German patients with TNBC for the presence of germ-line mutations in BRCA1, BRCA2, PALB2, and BRD7 genes. Microfluidic array PCR and next-generation sequencing was used for BRCA1 and BRCA2 analysis while conventional high-resolution melting and Sanger sequencing was applied to study the coding regions of PALB2 and BRD7, respectively. Truncating mutations in BRCA1 were found in six patients, and truncating mutations in BRCA2 and PALB2 were detected in one patient each, whereas no truncating mutation was identified in BRD7. One patient was a double heterozygote for the PALB2 mutation, c.758insT, and a BRCA1 mutation, c.927delA. Our results confirm in a hospital-based setting that a substantial proportion of German TNBC patients (17.5%) harbour germ-line mutations in genes involved in homology-directed DNA repair, with a preponderance of BRCA1 mutations. Triple-negative breast cancer should be considered as an additional criterion for future genetic counselling and diagnostic sequencing.