c-Fos is required for excision repair of UV-light induced DNA lesions by triggering the re-synthesis of XPF

Role of c-Fos in DNA damage repair

c-Fos, a member of the immediate early gene, serves as a widely used marker of neuronal activation induced by various types of brain damage. In addition, c-Fos is believed to play a regulatory role in DNA damage repair. This paper reviews the literature on c-Fos' involvement in the regulation of DNA damage repair and indicates that genes of the Fos family can be induced by various forms of DNA damage. In addition, cells lacking c-Fos have difficulties in DNA repair. c-Fos is involved in tumorigenesis and progression as a proto-oncogene that maintains cancer cell survival, which may also be related to DNA repair. c-Fos may impact the repair of DNA damage by regulating the expression of downstream proteins, including ATR, ERCC1, XPF, and others. Nonetheless, the underlying mechanisms necessitate further exploration.

Aflatoxin B1 targeted gene expression profiles in human placental primary trophoblast cells

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Gene expression profiles were studied in human primary trophoblast cells.

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170 genes were significantly dysregulated in aflatoxin B1-exposed trophoblasts.

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AhR-mediated estrogen receptor signalling was dysregulated in response to AFB1.

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Transcripts involved in endocrine signalling and energy homeostasis were disrupted.

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Cellular growth and development, cell cycle and DNA repair processes were affected.

Aflatoxin B1 (AFB1) is a mycotoxin produced by Aspergillus flavus and A. parasiticus. A high exposure (40 nM and 1 µM AFB1 for 72 h) was used to study mechanistic effects of AFB1 on gene expression patterns in human primary trophoblast cells, isolated from full term placentae after delivery. Gene expression profiling was conducted, and Ingenuity pathway analysis (IPA) software was used to identify AFB1-regulated gene networks and regulatory pathways.

In response to 40 nM AFB1, only 7 genes were differentially expressed whereas 1 µM AFB1 significantly dysregulated 170 genes (124 down- and 46 upregulated, ±1.5-fold, p < 0.05) in AFB1-exposed trophoblasts when compared to controls. The top downregulated genes were involved in endocrine signalling and biosynthesis of hormones, and lipid and carbohydrate metabolism. The top upregulated genes were involved in protein synthesis and regulation of cell cycle. The main canonical pathways identified by IPA were associated with endocrine signalling including growth hormone signalling, and corticotropin releasing hormone signalling. Furthermore, genes involved in aryl hydrocarbon receptor (AhR)-mediated estrogen receptor signalling were dysregulated in response to AFB1.

Our findings indicate that a high concentration 72 h AFB1 exposure caused relatively moderate number of changes on transcript level to human placental primary trophoblast cells. However, these preliminary results need to be confirmed with human-relevant concentrations of AFB1.

miRNA-27a Transcription Activated by c-Fos Regulates Myocardial Ischemia-Reperfusion Injury by Targeting ATAD3a

MicroRNA-27a (miR-27a) has been implicated in myocardial ischemia-reperfusion injury (MIRI), but the underlying mechanism is not well understood. This study is aimed at determining the role of miR-27a in MIRI and at investigating upstream molecules that regulate miR-27a expression and its downstream target genes. miR-27a expression was significantly upregulated in myocardia exposed to ischemia/reperfusion (I/R) and cardiomyocytes exposed to hypoxia/reoxygenation (H/R). c-Fos could regulate miR-27a expression by binding to its promoter region. Moreover, overexpression of miR-27a led to a decrease in cell viability, an increase in LDH and CK-MB secretion, and an increase in apoptosis rates. In contrast, suppression of miR-27a expression resulted in the opposite effects. ATPase family AAA-domain-containing protein 3A (ATAD3a) was identified as a target of miR-27a. miR-27a regulated the translocation of apoptosis-inducing factor (AIF) from the mitochondria to the nucleus and H/R-induced apoptosis via the regulation of ATAD3a. It was found that inhibiting miR-27a in vivo by injecting a miR-27a sponge could ameliorate MIRI in an isolated rat heart model. In conclusion, our study demonstrated that c-Fos functions as an upstream regulator of miR-27a and that miR-27a regulates the translocation of AIF from the mitochondria to the nucleus by targeting ATAD3a, thereby contributing to MIRI. These findings provide new insight into the role of the c-Fos/miR-27a/ATAD3a axis in MIRI.

Insight into the molecular function and transcriptional regulation of activator protein 1 (AP-1) components c-Jun/c-Fos ortholog in red lip mullet (Liza haematocheila)

The transcription factor, activator protein-1 (AP-1), is a dimeric protein and a downstream member of the mitogen-activated protein kinase (MAPK) signaling pathway. It regulates a wide array of functions including, cell proliferation, survival, differentiation, response to UV-irradiation, immune responses, and inflammatory conditions. AP-1 belongs to the basic leucine zipper (bZIP) protein family, which consists of members from Jun, Fos, Maf, and ATF subfamilies. In the present study, c-Jun and c-Fos homologs were identified from a transcriptome database of Liza haematocheila and designated as Lhc-Jun and Lhc-Fos. In both sequences, the signature bZIP domain was identified and also the DNA binding sites, dimerization sites, as well as the phosphorylation sites, were found to be highly conserved through evolution. Tissue distribution analysis revealed that both Lhc-Jun and Lhc-Fos transcripts were ubiquitously expressed in all examined tissues of healthy mullets. In order to determine the transcriptional modulations of Lhc-Jun and Lhc-Fos, challenge experiments were carried out using LPS, poly I:C, and L. garvieae. The qRT-PCR analysis revealed significant upregulation of Lhc-Jun and Lhc-Fos in blood, gill, liver, and spleen. This is the first study that explores the correlation between UV-irradiation and AP-1 ortholog expression in teleosts. Also, this is the first time that the functional characterization of the teleost c-Fos ortholog has been carried out. Sub-cellular localization of Lhc-Jun and Lhc-Fos was observed in the nucleus. AP-1-Luc reporter assays revealed significant higher luciferase activities in both Lhc-Jun and Lhc-Fos proteins compared to mock controls. These results strongly suggest that Lhc-Jun and Lhc-Fos might play a significant role in Liza haematocheila immunity by regulating AP-1 promoter sequences in immune and stress-related genes.

Naked mole rat cells display more efficient excision repair than mouse cells

Naked mole rat (NMR) is the long-lived and tumor-resistant rodent. NMRs possess multiple adaptations that may contribute to longevity and cancer-resistance. However, whether NMRs have more efficient DNA repair have not been directly tested. Here we compared base excision repair (BER) and nucleotide excision repair (NER) systems in extracts from NMR and mouse fibroblasts after UVC irradiation. Transcript levels of the key repair enzymes demonstrated in most cases higher inducibility in the mouse vs the NMR cells. Ratios of repair enzymes activities in the extracts somewhat varied depending on post-irradiation time. NMR cell extracts were 2–3-fold more efficient at removing the bulky lesions, 1.5–3-fold more efficient at removing uracil, and about 1.4-fold more efficient at cleaving the AP-site than the mouse cells, while DNA polymerase activities being as a whole higher in the mouse demonstrate different patterns of product distribution. The level of poly(ADP-ribose) synthesis was 1.4–1.8-fold higher in the NMR cells. Furthermore, NMR cell extracts displayed higher binding of PARP1 to DNA probes containing apurinic/apyrimidinic site or photo-reactive DNA lesions. Cumulatively, our results suggest that the NMR has more efficient excision repair systems than the mouse, which may contribute to longevity and cancer resistance of this species.

Signaling Pathways, Chemical and Biological Modulators of Nucleotide Excision Repair: The Faithful Shield against UV Genotoxicity

The continuous exposure of the human body's cells to radiation and genotoxic stresses leads to the accumulation of DNA lesions. Fortunately, our body has several effective repair mechanisms, among which is nucleotide excision repair (NER), to counteract these lesions. NER includes both global genome repair (GG-NER) and transcription-coupled repair (TC-NER). Deficiencies in the NER pathway underlie the development of several DNA repair diseases, such as xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD). Deficiencies in GG-NER and TC-NER render individuals to become prone to cancer and neurological disorders, respectively. Therefore, NER regulation is of interest in fine-tuning these risks. Distinct signaling cascades including the NFE2L2 (NRF2), AHR, PI3K/AKT1, MAPK, and CSNK2A1 pathways can modulate NER function. In addition, several chemical and biological compounds have proven success in regulating NER's activity. These modulators, particularly the positive ones, could therefore provide potential treatments for genetic DNA repair-based diseases. Negative modulators, nonetheless, can help sensitize cells to killing by genotoxic chemicals. In this review, we will summarize and discuss the major upstream signaling pathways and molecules that could modulate the NER's activity.

Filling gaps in translesion DNA synthesis in human cells

During DNA replication, forks may encounter unrepaired lesions that hamper DNA synthesis. Cells have universal strategies to promote damage bypass allowing cells to survive. DNA damage tolerance can be performed upon template switch or by specialized DNA polymerases, known as translesion (TLS) polymerases. Human cells count on more than eleven TLS polymerases and this work reviews the functions of some of these enzymes: Rev1, Pol η, Pol ι, Pol κ, Pol θ and Pol ζ. The mechanisms of damage bypass vary according to the lesion, as well as to the TLS polymerases available, and may occur directly at the fork during replication. Alternatively, the lesion may be skipped, leaving a single-stranded DNA gap that will be replicated later. Details of the participation of these enzymes are revised for the replication of damaged template. TLS polymerases also have functions in other cellular processes. These include involvement in somatic hypermutation in immunoglobulin genes, direct participation in recombination and repair processes, and contributing to replicating noncanonical DNA structures. The importance of DNA damage replication to cell survival is supported by recent discoveries that certain genes encoding TLS polymerases are induced in response to DNA damaging agents, protecting cells from a subsequent challenge to DNA replication. We retrace the findings on these genotoxic (adaptive) responses of human cells and show the common aspects with the SOS responses in bacteria. Paradoxically, although TLS of DNA damage is normally an error prone mechanism, in general it protects from carcinogenesis, as evidenced by increased tumorigenesis in xeroderma pigmentosum variant patients, who are deficient in Pol η. As these TLS polymerases also promote cell survival, they constitute an important mechanism by which cancer cells acquire resistance to genotoxic chemotherapy. Therefore, the TLS polymerases are new potential targets for improving therapy against tumors.

MKP-1 suppresses PARP-1 degradation to mediate cisplatin resistance

Understanding the mechanisms of platinum compound resistance, including cisplatin resistance, has important implications for improving cancer treatments. Previous studies identified a potential role for mitogen-activated protein kinase phosphatase-1 (MKP-1) in cisplatin resistance. This work focuses on the regulation of poly(ADP-ribose) polymerase-1 (PARP-1) expression by MKP-1. We found that MKP-1 overexpression stimulates PARP-1 and poly(ADP-ribose) (PAR) protein expression and cisplatin resistance while its downregulation suppresses PARP-1 and PAR protein expression and cisplatin resistance. Silencing MKP-1 promoted PARP-1 ubiquitination, which decreased PARP-1 protein levels. We also found that silencing c-Jun N-terminal kinase 1/2 (JNK1/2) decreased PARP-1 ubiquitination while increasing total PARP-1 protein levels. Furthermore, we showed that acquired cisplatin resistant ovarian cancer cells expressed the high levels of MKP-1 and PARP-1 proteins, and that silencing MKP-1 or PARP-1 increased cisplatin sensitivity in resistant cells. Notably, the pharmacologic inhibition of PARP activity restored cisplatin sensitivity in MKP-1 overexpressing cells. Thus, this work indicates that suppression of JNK1/2 activity by MKP-1 maintains PARP-1 levels and suggests that MKP-1-mediated cisplatin resistance can be bypassed by PARP-1 inhibition.

Transcriptional and Posttranslational Regulation of Nucleotide Excision Repair: The Guardian of the Genome against Ultraviolet Radiation

Ultraviolet (UV) radiation from sunlight represents a constant threat to genome stability by generating modified DNA bases such as cyclobutane pyrimidine dimers (CPD) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PP). If unrepaired, these lesions can have deleterious effects, including skin cancer. Mammalian cells are able to neutralize UV-induced photolesions through nucleotide excision repair (NER). The NER pathway has multiple components including seven xeroderma pigmentosum (XP) proteins (XPA to XPG) and numerous auxiliary factors, including ataxia telangiectasia and Rad3-related (ATR) protein kinase and RCC1 like domain (RLD) and homologous to the E6-AP carboxyl terminus (HECT) domain containing E3 ubiquitin protein ligase 2 (HERC2). In this review we highlight recent data on the transcriptional and posttranslational regulation of NER activity.

Adaptive upregulation of DNA repair genes following benzo(a)pyrene diol epoxide protects against cell death at the expense of mutations

A coordinated and faithful DNA damage response is of central importance for maintaining genomic integrity and survival. Here, we show that exposure of human cells to benzo(a)pyrene 9,10-diol-7,8-epoxide (BPDE), the active metabolite of benzo(a)pyrene (B(a)P), which represents a most important carcinogen formed during food preparation at high temperature, smoking and by incomplete combustion processes, causes a prompt and sustained upregulation of the DNA repair genes DDB2, XPC, XPF, XPG and POLH. Induction of these repair factors on RNA and protein level enhanced the removal of BPDE adducts from DNA and protected cells against subsequent BPDE exposure. However, through the induction of POLH the mutation frequency in the surviving cells was enhanced. Activation of these adaptive DNA repair genes was also observed upon B(a)P treatment of MCF7 cells and in buccal cells of human volunteers after cigarette smoking. Our data provide a rational basis for an adaptive response to polycyclic aromatic hydrocarbons, which occurs however at the expense of mutations that may drive cancer formation.

Apoptosis induced by temozolomide and nimustine in glioblastoma cells is supported by JNK/c-Jun-mediated induction of the BH3-only protein BIM

The outcome of cancer therapy strongly depends on the complex network of cell signaling pathways, including transcription factor activation following drug exposure. Here we assessed whether and how the MAP kinase (MAPK) cascade and its downstream target, the transcription factor AP-1, influence the sensitivity of malignant glioma cells to the anticancer drugs temozolomide (TMZ) and nimustine (ACNU). Both drugs induce apoptosis in glioma cells at late times following treatment. Activation of the MAPK cascade precedes apoptosis, as shown by phosphorylation of Jun kinase (JNK) and c-Jun, a main component of AP-1. Pharmacological inhibition and siRNA mediated knockdown of JNK and c-Jun reduced the level of apoptosis in LN-229 glioma cells treated with TMZ or ACNU. Analyzing the underlying molecular mechanism, we identified the pro-apoptotic gene BIM as a critical target of AP-1, which is upregulated following TMZ and ACNU. Importantly, shRNA mediated downregulation of BIM in the malignant glioma cell lines LN-229 and U87MG led to an attenuated cleavage of caspase-9 and, consequently, reduced the level of apoptosis following TMZ and ACNU treatment. Overall, we identified JNK/c-Jun activation and BIM induction as a late pro-apoptotic response of glioma cells treated with alkylating anticancer drugs.

Genotoxic stress inhibits Ewing sarcoma cell growth by modulating alternative pre-mRNA processing of the RNA helicase DHX9

Alternative splicing plays a key role in the DNA damage response and in cancer. Ewing Sarcomas (ES) are aggressive tumors caused by different chromosomal translocations that yield in-frame fusion proteins driving transformation. RNA profiling reveals genes differentially regulated by UV light irradiation in two ES cell lines exhibiting different sensitivity to genotoxic stress. In particular, irradiation induces a new isoform of the RNA helicase DHX9 in the more sensitive SK-N-MC cells, which is targeted to nonsense-mediated decay (NMD), causing its downregulation. DHX9 protein forms a complex with RNA polymerase II (RNAPII) and EWS-FLI1 to enhance transcription. Silencing of DHX9 in ES cells sensitizes them to UV treatment and impairs recruitment of EWS-FLI1 to target genes, whereas DHX9 overexpression protects ES cells from genotoxic stress. Mechanistically, we found that UV light irradiation leads to enhanced phosphorylation and decreased processivity of RNAPII in SK-N-MC cells, which in turn causes inclusion of DHX9 exon 6A. A similar effect on DHX9 splicing was also elicited by treatment with the chemotherapeutic drug etoposide, indicating a more general mechanism of regulation in response to DNA damage. Our data identify a new NMD-linked splicing event in DHX9 with impact on EWS-FLI1 oncogenic activity and ES cell viability.

TIP30: A Novel Tumor-Suppressor Gene

TIP30/CC3 was first identified and characterized as a "candidate" tumor-suppressor gene in 1997. Recently, the TIP30 tumor-suppressor status has been fully established since several studies have described that TIP30 protein expression is frequently downregulated in diverse types of human tumors, and the downregulation is often associated with tumor progression. TIP30 is involved in the control of cell apoptosis, growth, metastasis, angiogenesis, DNA repair, and tumor cell metabolism. Moreover, TIP30(-/-) mice spontaneously develop hepatocellular carcinoma and other tumors at a higher incidence than that of wild-type mice. In this review, we provide an overview of current knowledge concerning the role of TIP30 in tumor development and progression. To our knowledge, this is the first review about the role of novel tumor-suppressor gene TIP30 in tumor development and progression.

Non-catalytic Roles for XPG with BRCA1 and BRCA2 in Homologous Recombination and Genome Stability

XPG is a structure-specific endonuclease required for nucleotide excision repair, and incision-defective XPG mutations cause the skin cancer-prone syndrome xeroderma pigmentosum. Truncating mutations instead cause the neurodevelopmental progeroid disorder Cockayne syndrome, but little is known about how XPG loss results in this devastating disease. We identify XPG as a partner of BRCA1 and BRCA2 in maintaining genomic stability through homologous recombination (HRR). XPG depletion causes DNA double-strand breaks, chromosomal abnormalities, cell-cycle delays, defective HRR, inability to overcome replication fork stalling, and replication stress. XPG directly interacts with BRCA2, RAD51, and PALB2, and XPG depletion reduces their chromatin binding and subsequent RAD51 foci formation. Upstream in HRR, XPG interacts directly with BRCA1. Its depletion causes BRCA1 hyper-phosphorylation and persistent chromatin binding. These unexpected findings establish XPG as an HRR protein with important roles in genome stability and suggest how XPG defects produce severe clinical consequences including cancer and accelerated aging.

DNA damage and the balance between survival and death in cancer biology

DNA is vulnerable to damage resulting from endogenous metabolites, environmental and dietary carcinogens, some anti-inflammatory drugs, and genotoxic cancer therapeutics. Cells respond to DNA damage by activating complex signalling networks that decide cell fate, promoting not only DNA repair and survival but also cell death. The decision between cell survival and death following DNA damage rests on factors that are involved in DNA damage recognition, and DNA repair and damage tolerance, as well as on factors involved in the activation of apoptosis, necrosis, autophagy and senescence. The pathways that dictate cell fate are entwined and have key roles in cancer initiation and progression. Furthermore, they determine the outcome of cancer therapy with genotoxic drugs. Understanding the molecular basis of these pathways is important not only for gaining insight into carcinogenesis, but also in promoting successful cancer therapy. In this Review, we describe key decision-making nodes in the complex interplay between cell survival and death following DNA damage.

Cdt2-mediated XPG degradation promotes gap-filling DNA synthesis in nucleotide excision repair

Xeroderma pigmentosum group G (XPG) protein is a structure-specific repair endonuclease, which cleaves DNA strands on the 3' side of the DNA damage during nucleotide excision repair (NER). XPG also plays a crucial role in initiating DNA repair synthesis through recruitment of PCNA to the repair sites. However, the fate of XPG protein subsequent to the excision of DNA damage has remained unresolved. Here, we show that XPG, following its action on bulky lesions resulting from exposures to UV irradiation and cisplatin, is subjected to proteasome-mediated proteolytic degradation. Productive NER processing is required for XPG degradation as both UV and cisplatin treatment-induced XPG degradation is compromised in NER-deficient XP-A, XP-B, XP-C, and XP-F cells. In addition, the NER-related XPG degradation requires Cdt2, a component of an E3 ubiquitin ligase, CRL4(Cdt2). Micropore local UV irradiation and in situ Proximity Ligation assays demonstrated that Cdt2 is recruited to the UV-damage sites and interacts with XPG in the presence of PCNA. Importantly, Cdt2-mediated XPG degradation is crucial to the subsequent recruitment of DNA polymerase δ and DNA repair synthesis. Collectively, our data support the idea of PCNA recruitment to damage sites which occurs in conjunction with XPG, recognition of the PCNA-bound XPG by CRL4(Cdt2) for specific ubiquitylation and finally the protein degradation. In essence, XPG elimination from DNA damage sites clears the chromatin space needed for the subsequent recruitment of DNA polymerase δ to the damage site and completion of gap-filling DNA synthesis during the final stage of NER.

The UVS9 gene of Chlamydomonas encodes an XPG homolog with a new conserved domain

Nucleotide excision repair (NER) is a key pathway for removing DNA damage that destabilizes the DNA double helix. During NER a protein complex coordinates to cleave the damaged DNA strand on both sides of the damage. The resulting lesion-containing oligonucleotide is displaced from the DNA and a replacement strand is synthesized using the undamaged strand as template. Ultraviolet (UV) light is known to induce two primary forms of DNA damage, the cyclobutane pyrimidine dimer and the 6-4 photoproduct, both of which destabilize the DNA double helix. The uvs9 strain of Chlamydomonas reinhardtii was isolated based on its sensitivity to UV light and was subsequently shown to have a defect in NER. In this work, the UVS9 gene was cloned through molecular mapping and shown to encode a homolog of XPG, the structure-specific nuclease responsible for cleaving damaged DNA strands 3' to sites of damage during NER. 3' RACE revealed that the UVS9 transcript is alternatively polyadenylated. The predicted UVS9 protein is nearly twice as long as other XPG homologs, primarily due to an unusually long spacer region. Despite this difference, amino acid sequence alignment of UVS9p with XPG homologs revealed a new conserved domain involved in TFIIH interaction.

Absence of Female-Typical Pheromone-Induced Hypothalamic Neural Responses and Kisspeptin Neuronal Activity in α-Fetoprotein Knockout Female Mice

Pheromones induce sexually dimorphic neuroendocrine responses, such as LH secretion. However, the neuronal network by which pheromones are converted into signals that will initiate and modulate endocrine changes remains unclear. We asked whether 2 sexually dimorphic populations in the anteroventral periventricular and periventricular nuclei that express kisspeptin and tyrosine hydroxylase (TH) are potential candidates that will transduce the olfactory signal to the neuroendocrine system. Furthermore, we assessed whether this transduction is sensitive to perinatal actions of estradiol by using female mice deficient in α-fetoprotein (AfpKO), which lack the protective actions of Afp against maternal estradiol. Wild-type (WT) and AfpKO male and female mice were exposed to same- versus opposite-sex odors and the expression of Fos (the protein product of the immediate early gene c-Fos) was analyzed along the olfactory projection pathways as well as whether kisspeptin, TH, and GnRH neurons are responsive to opposite-sex odors. Male odors induced a female-typical Fos expression in target forebrain sites of olfactory inputs involved in reproduction in WT, but not in AfpKO females, whereas female odors induced a male-typical Fos expression in males of both genotypes. In WT females, opposite-sex odors induced Fos in kisspeptin and TH neurons, whereas in AfpKO females and WT males, only a lower, but still significant, Fos expression was observed in TH but not in kisspeptin neurons. Finally, opposite-sex odors did not induce any significant Fos expression in GnRH neurons of both sexes or genotypes. Our results strongly suggest a role for fetal estrogen in the sexual differentiation of neural responses to sex-related olfactory cues.

Theoretical considerations for thresholds in chemical carcinogenesis

There is increasing evidence for non-linear relationships for gene mutations, chromosomal aberrations and even tumor incidences in response to low doses of genotoxic carcinogens. To attain the biological relevance of such non-linear responses, there is a need to identify the underlying defense mechanisms that allow tolerance to low doses of genotoxicants. This communication discusses presumptive cancer prevention mechanisms that may contribute to thresholds, i.e. points of departure, for each endpoint, from initial DNA lesion to tumor formation. We discuss a sequential order of genome protection during carcinogenesis where genotoxicant scavenging, cellular efflux, DNA repair, elimination of damaged cells by apoptosis, autophagy, silencing by DNA damage-triggered replicative senescence, and finally, elimination of transformed (premalignant) cells by the immune system are thought to be responsible for a threshold in tumor formation. We highlight DNA repair, for which experimental evidence has been recently provided to dictate a role in PoDs. In conclusion, from a theoretical perspective it is reasonable to posit that tolerance to low dose levels exists for each requisite step of tumor formation and these tolerance mechanisms are critical in determining thresholds in chemical carcinogenesis.

Rac1 promotes diethylnitrosamine (DEN)-induced formation of liver tumors

To elucidate the function of the Ras-homologous GTPase Rac1 in hepatocarcinogenesis induced by diethylnitrosamine (DEN), mice lacking hepatic Rac1 expression were treated with DEN and compared to the wild-type (WT). Rac1 knock-out (KO) mice were found to have a lower tumor yield as compared to Rac1 proficient mice. The small-sized tumors formed in the absence of Rac1 lack an activated Ras/Raf/mitogen-activated protein kinase pathway, as indicated by the absence of p-ERK expression. Apparently, Rac1 is required for Ras-driven oncogenic pathways. Moreover, tumors in Rac1 deficient mice were glutamine synthase (GS) negative. They displayed a high number of p-H3-positive and cyclinB1 expressing cells, pointing to a defect in mitotic progression. To elucidate the influence of Rac1 on mechanisms of tumor initiation, acute DEN-induced hepatic stress responses were monitored. Rac1 deficiency caused fairly complex, partially time-dependent, alterations in both basal and/or DEN-induced messenger RNA (mRNA) and protein levels of susceptibility-related genes. Basal protein expression of DNA repair factors Brca1 and DNA repair protein RAD51 homolog (Rad51) and the cell cycle regulatory factor p27 was enhanced in the absence of Rac1. Following DEN treatment, p21 mRNA and protein expression was stimulated independent of the Rac1 status. Lack of Rac1 increased mechanisms of the DNA damage response (DDR), as shown by elevated protein levels of p-ATR, p-p53 and γH2AX 24h after DEN treatment. The data show that Rac1 is essential for DEN-stimulated hepatocarcinogenesis. We hypothesize that it promotes tumor initiation by counteracting the elimination of initiated cells and, moreover, alleviates the outgrowth of transformed cells. Hence, pharmacological targeting of Rac1 could be suitable for chemoprevention.

The PCNA binding domain of Rad2p plays a role in mutagenesis by modulating the cell cycle in response to DNA damage

The xeroderma pigmentosum group G (XPG) gene, encoding an essential element in nucleotide excision repair (NER), has a proliferating cell nuclear antigen-binding domain (PCNA-BD) at its C-terminal region. However, the role of this domain is controversial because its presence does not affect NER. Using yeast RAD2, a homolog of human XPG, we show that Rad2p interacts with PCNA through its PCNA-BD and the PCNA-BD of Rad2p plays a role in UV-induced mutagenesis. While a mutation of Rad2p endonuclease activity alone causes dramatically increased mutation rates and UV sensitivity, as well as growth retardation after UV irradiation, a mutation of the Rad2p PCNA-BD in the same mutant causes dramatically decreased mutation rates, reduced UV sensitivity and increased growth rate after UV irradiation. After UV irradiation, large-budded cells of Rad2p endonuclease defective mutants wane due to a mutation of the Rad2p PCNA-BD. Besides, the Rad2p PCNA-BD mutant protein exhibits alleviated PCNA-binding efficiency. These results show a hitherto unsuspected role of the Rad2p PCNA-BD that controls mutagenesis via cell cycle modulation together with PCNA. Furthermore, the high mutation rate of cells with other NER gene mutations was also decreased by the mutation of the Rad2p PCNA-BD, which indicates that the Rad2p-PCNA interaction might be responsible for mutagenesis control in the general NER pathway. Our results suggest that the drastically increased incidence of skin cancer in xeroderma pigmentosum patients could arise from the synergistic effects between cell cycle arrest due to the XPG-PCNA interaction and the accumulation of damaged DNA via defects in DNA damage repair.

Transcriptional regulation of human DNA repair genes following genotoxic stress: trigger mechanisms, inducible responses and genotoxic adaptation

DNA repair is the first barrier in the defense against genotoxic stress. In recent years, mechanisms that recognize DNA damage and activate DNA repair functions through transcriptional upregulation and post-translational modification were the focus of intensive research. Most DNA repair pathways are complex, involving many proteins working in discrete consecutive steps. Therefore, their balanced expression is important for avoiding erroneous repair that might result from excessive base removal and DNA cleavage. Amelioration of DNA repair requires both a fine-tuned system of lesion recognition and transcription factors that regulate repair genes in a balanced way. Transcriptional upregulation of DNA repair genes by genotoxic stress is counteracted by DNA damage that blocks transcription. Therefore, induction of DNA repair resulting in an adaptive response is only visible through a narrow window of dose. Here, we review transcriptional regulation of DNA repair genes in normal and cancer cells and describe mechanisms of promoter activation following genotoxic exposures through environmental carcinogens and anticancer drugs. The data available to date indicate that 25 DNA repair genes are subject to regulation following genotoxic stress in rodent and human cells, but for only a few of them, the data are solid as to the mechanism, homeostatic regulation and involvement in an adaptive response to genotoxic stress.

The chloroethylating anticancer drug ACNU induces FRA1 that is involved in drug resistance of glioma cells

FRA1 belongs, together with c-Fos and FosB, to the family of Fos proteins that form with members of the ATF and Jun family the transcription factor AP-1 (activator protein 1). Previously we showed that c-Fos protects mouse embryonic fibroblasts against the cytotoxic effects of ultraviolet (UV) light by induction of the endonuclease XPF, leading to enhanced nucleotide excision repair (NER) activity. Here, we analyzed the regulation of FRA1 in glioma cells treated with the anticancer drug nimustine (ACNU) and its role in ACNU-induced toxicity. We show that FRA1 is upregulated in glioblastoma cells following ACNU on mRNA and protein levels. Knockdown of FRA1 by either siRNA or shRNA clearly sensitized glioma cells towards ACNU-induced cell death. Despite decreased AP-1 binding activity upon FRA1 knockdown, this effect is independent on regulation of the AP-1 target genes fasL, ercc1 and xpf. In addition, FRA1 knockdown does not affect DNA repair capacity. However, lack of FRA1 attenuated the ACNU-induced phosphorylation of CHK1 and led to a reduced arrest of cells in G2/M and, thereby, presumably leads to enhanced cell death in the subsequent cell cycle.

Benzo[a]pyrene-induced changes in microRNA-mRNA networks

Toxicological studies assessing the safety of compounds for humans frequently use in vitro systems to characterize toxic responses in combination with transcriptomic analyses. Thus far, changes have mostly been investigated at the mRNA level. Recently, microRNAs have attracted attention because they are powerful negative regulators of mRNA levels and, thus, may be responsible for the modulation of important mRNA networks implicated in toxicity. This study aimed to identify possible microRNA-mRNA networks as novel interactions on the gene expression level after a genotoxic insult. We used benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon, as a model genotoxic/carcinogenic compound. We analyzed time-dependent effects on mRNA and microRNA profiles in HepG2 cells, a widely used human liver cell line that expresses active p53 and is competent for the biotransformation of BaP. Changes in microRNA expression in response to BaP, in combination with multiple alterations of mRNA levels, were observed. Many of these altered mRNAs are targets of altered microRNAs. Using pathway analysis, we evaluated the relevance of such microRNA deregulations to genotoxicity. This revealed eight microRNAs that appear to participate in specific BaP-responsive pathways relevant to genotoxicity, such as apoptotic signaling, cell cycle arrest, DNA damage response, and DNA damage repair. Our results particularly highlight the potential of microRNA-29b, microRNA-26a-1*, and microRNA-122* as novel players in the BaP response. Therefore, this study demonstrates the added value of an integrated microRNA-mRNA approach for identifying molecular mechanisms induced by BaP in an in vitro human model.

Testicular nuclear receptor 4 (TR4) regulates UV light-induced responses via Cockayne syndrome B protein-mediated transcription-coupled DNA repair

UV irradiation is one of the major external insults to cells and can cause skin aging and cancer. In response to UV light-induced DNA damage, the nucleotide excision repair (NER) pathways are activated to remove DNA lesions. We report here that testicular nuclear receptor 4 (TR4), a member of the nuclear receptor family, modulates DNA repair specifically through the transcription-coupled (TC) NER pathway but not the global genomic NER pathway. The level of Cockayne syndrome B protein (CSB), a member of the TC-NER pathway, is 10-fold reduced in TR4-deficient mouse tissues, and TR4 directly regulates CSB at the transcriptional level. Moreover, restored CSB expression rescues UV hypersensitivity of TR4-deficient cells. Together, these results indicate that TR4 modulates UV sensitivity by promoting the TC-NER DNA repair pathway through transcriptional regulation of CSB. These results may lead to the development of new treatments for UV light-sensitive syndromes, skin cancer, and aging.

Late activation of stress-activated protein kinases/c-Jun N-terminal kinases triggered by cisplatin-induced DNA damage in repair-defective cells

Although stress-activated protein kinases/c-Jun N-terminal kinases (SAPK/JNK) are rapidly activated by genotoxins, the role of DNA damage in this response is not well defined. Here we show that the SEK1/MKK4-mediated dual phosphorylation of SAPK/JNK (Thr-183/Tyr-185) correlates with the level of cisplatin-DNA adducts at late times (16-24 h) after drug treatment in both human and mouse cells. Transfection of platinated plasmid DNA also caused SAPK/JNK activation. A defect in transcription-coupled nucleotide excision repair resting on a mutation in Cockayne syndrome group B protein promoted the late SAPK/JNK activation following cisplatin exposure. Signaling to SAPK/JNK was accompanied by activation of Ataxia telangiectasia mutated- and Rad3-related kinase, replication protein A, and checkpoint kinases as well as by the formation of DNA double strand breaks (DSBs). Ionizing radiation-induced DSBs did not provoke SAPK/JNK activation, and inhibition of transcription also failed to provoke this response. Late activation of SAPK/JNK stimulated by cisplatin-induced DNA lesions was reduced in the absence of specific DNA repair proteins, such as xeroderma pigmentosum protein C, pointing to an essential function of individual repair factors in DNA damage signaling to SAPK/JNK. Collectively, the data indicate that late SAPK/JNK activation is triggered by non-repaired cisplatin adducts in transcribed genes and involves replication-associated events, DSBs, tyrosine kinases, Rho GTPases, and specific repair factors.

Delayed c-Fos activation in human cells triggers XPF induction and an adaptive response to UVC-induced DNA damage and cytotoxicity

The oncoprotein c-Fos has been commonly found differently expressed in cancer cells. Our previous work showed that mouse cells lacking the immediate-early gene c-fos are hypersensitive to ultraviolet (UVC) light. Here, we demonstrate that in human diploid fibroblasts UV-triggered induction of c-Fos protein is a delayed and long-lasting event. Sustained upregulation of c-Fos goes along with transcriptional stimulation of the NER gene xpf, which harbors an AP-1 binding site in the promoter. Data gained on c-Fos knockdown and c-Fos overexpressing human cells provide evidence that c-Fos/AP-1 stimulates upregulation of XPF, thereby increasing the cellular repair capacity protecting from UVC-induced DNA damage. When these cells are pre-exposed to a low non-toxic UVC dose and challenged with a subsequent high dose of UVC irradiation, they show accelerated repair of UVC-induced DNA adducts and reduced cell kill. The data indicate a protective role of c-Fos induction by triggering an adaptive response pathway.

Three prime exonuclease I (TREX1) is Fos/AP-1 regulated by genotoxic stress and protects against ultraviolet light and benzo(a)pyrene-induced DNA damage

Cells respond to genotoxic stress with the induction of DNA damage defence functions. Aimed at identifying novel players in this response, we analysed the genotoxic stress-induced expression of DNA repair genes in mouse fibroblasts proficient and deficient for c-Fos or c-Jun. The experiments revealed a clear up-regulation of the three prime exonuclease I (trex1) mRNA following ultraviolet (UV) light treatment. This occurred in the wild-type but not c-fos and c-jun null cells, indicating the involvement of AP-1 in trex1 induction. Trex1 up-regulation was also observed in human cells and was found on promoter, RNA and protein level. Apart from UV light, TREX1 is induced by other DNA damaging agents such as benzo(a)pyrene and hydrogen peroxide. The mouse and human trex1 promoter harbours an AP-1 binding site that is recognized by c-Fos and c-Jun, and its mutational inactivation abrogated trex1 induction. Upon genotoxic stress, TREX1 is not only up-regulated but also translocated into the nucleus. Cells deficient in TREX1 show reduced recovery from the UV and benzo(a)pyrene-induced replication inhibition and increased sensitivity towards the genotoxins compared to the isogenic control. The data revealed trex1 as a novel DNA damage-inducible repair gene that plays a protective role in the genotoxic stress response.

CC3/TIP30 affects DNA damage repair

Background

The pro-apoptotic protein CC3/TIP30 has an unusual cellular function as an inhibitor of nucleocytoplasmic transport. This function is likely to be activated under conditions of stress. A number of studies support the notion that CC3 acts as a tumor and metastasis suppressor in various types of cancer. The yeast homolog of CC3 is likely to be involved in responses to DNA damage. Here we examined the potential role of CC3 in regulation of cellular responses to genotoxic stress.

Results

We found that forced expression of CC3 in CC3-negative cells strongly delays the repair of UV-induced DNA damage. Exogenously introduced CC3 negatively affects expression levels of DDB2/XPE and p21CIP1, and inhibits induction of c-FOS after UV exposure. In addition, exogenous CC3 prevents the nuclear accumulation of P21CIP in response to UV. These changes in the levels/localization of relevant proteins resulting from the enforced expression of CC3 are likely to contribute to the observed delay in DNA damage repair. Silencing of CC3 in CC3-positive cells has a modest delaying effect on repair of the UV induced damage, but has a much more significant negative affect on the translesion DNA synthesis after UV exposure. This could be related to the higher expression levels and increased nuclear localization of p21CIP1 in cells where expression of CC3 is silenced. Expression of CC3 also inhibits repair of oxidative DNA damage and leads to a decrease in levels of nucleoredoxin, that could contribute to the reduced viability of CC3 expressing cells after oxidative insult.

Conclusions

Manipulation of the cellular levels of CC3 alters expression levels and/or subcellular localization of proteins that exhibit nucleocytoplasmic shuttling. This results in altered responses to genotoxic stress and adversely affects DNA damage repair by affecting the recruitment of adequate amounts of required proteins to proper cellular compartments. Excess of cellular CC3 has a significant negative effect on DNA repair after UV and oxidant exposure, while silencing of endogenous CC3 slightly delays repair of UV-induced damage.

Reduction in mutation frequency by very low-dose gamma irradiation of Drosophila melanogaster germ cells

To determine whether the linear no-threshold (LNT) model for stochastic effects of ionizing radiation is applicable to very low-dose radiation at a low dose rate, we irradiated immature male germ cells of the fruit fly, Drosophila melanogaster, with several doses of (60)Co gamma rays at a dose rate of 22.4 mGy/h. Thereafter, we performed the sex-linked recessive lethal mutation assay by mating the irradiated males with nonirradiated females. The mutation frequency in the group irradiated with 500 microGy was found to be significantly lower than that in the control group (P < 0.01), whereas in the group subjected to 10 Gy irradiation, the mutation frequency was significantly higher than that in the control group (P < 0.03). A J-shaped dose-response relationship was evident. Molecular experiments using DNA microarray and quantitative reverse transcription PCR indicated that several genes known to be expressed in response to heat or chemical stress and grim, a positive regulator of apoptosis, were up-regulated immediately after irradiation with 500 microGy. The involvement of an apoptosis function in the non-linear dose-response relationship was suggested.

XPF/ERCC4 and ERCC1: their products and biological roles

ERCC4 is the gene mutated in XPF cells and also in rodent cells representing the mutant complementation groups ERCC4 and ERCC 11. The protein functions principally as a complex with ERCC1 in a diversity of biological pathways that include NER, ICL repair, telomere maintenance and immunoglobulin switching. Sorting out these roles is an exciting and challenging problem and many important questions remain to be answered. The ERCC1/ERCC4 complex is conserved across most species presenting an opportunity to examine some functions in model organisms where mutants can be more readily generated and phenotypes more quickly assessed.

DNA replication arrest in response to genotoxic stress provokes early activation of stress-activated protein kinases (SAPK/JNK)

The impact of DNA damage-induced replication blockage for early activation of stress kinases [stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK)] is largely unknown. Here, we show that induction of dual phosphorylation of SAPK/JNK by the DNA polymerase inhibitor aphidicolin was not ameliorated by additional exposure to ultraviolet (UV) light, indicating that overlapping mechanisms participate in signaling to SAPK/JNK triggered by both agents. UV-induced DNA replication blockage, cyclobutane pyrimidine dimer formation and DNA strand break induction coincided with SAPK/JNK phosphorylation at early (< or =30 min) but not late (> or =2 h) time points after exposure. Genotoxin-stimulated SAPK/JNK activation was attenuated in nonproliferating cells, indicating that S phase-dependent mechanisms are involved in signaling to SAPK/JNK. Correspondingly, UV-induced phosphorylation of SAPK/JNK was higher in S-phase cells as compared with G(1)-phase cells. Activation of SAPK/JNK by genotoxins was below detection limit in nonproliferating human peripheral blood lymphocytes, whereas peripheral blood lymphocytes stimulated to proliferation displayed clear SAPK/JNK activation. UV-induced phosphorylation of SAPK/JNK was attenuated in XPC-defective cells, ameliorated in BRCA2 mutated cells and not changed in cells lacking ATM, DNA-PK, CSB, XPA, p53, ERCC1 or PARP as compared with the corresponding wild types. Based on these data, we suggest that DNA replication blockage caused by genotoxin-induced DNA damage contributes to early activation of SAPK/JNK.

The p38 mitogen-activated protein kinase augments nucleotide excision repair by mediating DDB2 degradation and chromatin relaxation

The p38 MAPK is a family of serine/threonine protein kinases that play important roles in cellular responses to external stress signals, e.g. UV irradiation. To assess the role of p38 MAPK pathway in nucleotide excision repair (NER), the most versatile DNA repair pathway, we determined the efficiency of NER in cells treated with p38 MAPK inhibitor SB203580 and found that p38 MAPK is required for the prompt repair of UV-induced DNA damage CPD. We further investigated the possible mechanism through which p38 MAPK regulates NER and found that p38 MAPK mediates UV-induced histone H3 acetylation and chromatin relaxation. Moreover, p38 MAPK also regulates UV-induced DDB2 ubiquitylation and degradation via phosphorylation of the target protein. Finally, our results showed that p38 MAPK is required for the recruitment of NER factors XPC and TFIIH to UV-induced DNA damage sites. We conclude that p38 MAPK regulates chromatin remodeling as well as DDB2 degradation for facilitating NER factor assembly.

How DNA lesions are turned into powerful killing structures: insights from UV-induced apoptosis

Mammalian cells treated with ultraviolet (UV) light provide one of the best-known experimental systems for depicting the biological consequences of DNA damage. UV irradiation induces the formation of DNA photoproducts, mainly cyclobutane pyrimidine dimers (CPDs) and (6-4) pyrimidine-pyrimidone photoproducts [(6-4)PPs], that drastically impairs DNA metabolism, culminating in the induction of cell death by apoptosis. While CPDs are the most important apoptosis-inducing lesions in DNA repair proficient cells, recent data indicates that (6-4)PPs also signals for apoptosis in DNA repair deficient cells. The toxic effects of these unrepaired DNA lesions are commonly associated with transcription blockage, but there is increasing evidence supporting a role for replication blockage as an apoptosis-inducing signal. This is supported by the observations that DNA double-strand breaks (DSBs) arise at the sites of stalled replication forks, that these DSBs are potent inducers of apoptosis and that inhibition of S phase progression diminishes the apoptotic response. Reactive oxygen species, generated after exposure of mammalian cells to longer UV wavelengths, may also induce apoptotic responses. In this regard, emphasis is given to the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OxoG), but indirect induced lesions such as lipoperoxide DNA adducts also deserve attention. ATR is the main established sensor molecule for UV-induced DNA damage. However, there is evidence that ATM as well as the MAPK pathway also play a role in the UV response by activating either the death receptor or the mitochondrial damage pathway. Adding more complexity to the subject, cells under stress suffer other types of processes that may result in cell death. Autophagy is one of these processes, with extensive cross-talks with apoptosis. No matter the mechanisms, cell death avoids cells to perpetuate mutations induced by genotoxic lesions. The understanding of such death responses may provide the means for the development of strategies for the prevention and treatment of cancer.

ATF3 and Fra1 have opposite functions in JNK- and ERK-dependent DNA damage responses

JNK and ERK MAP kinases regulate cellular responses to genotoxic stress in a cell type and cell context-dependent manner. However, the factors that determine and execute JNK- and ERK-controlled stress responses are only partly known. In this study, we investigate the roles of the AP-1 components ATF3 and Fra1 in JNK- and ERK-dependent cell cycle arrest and apoptosis. We show that the anti-cancer drug cisplatin or UV light activates both JNK and ERK in human glioblastoma cells lacking functional p53. Inhibition experiments of JNK or ERK activities revealed that the ERK pathway strongly promotes cisplatin- and UV-induced apoptosis in these glioblastoma cells. Furthermore, JNK but not ERK is required for ATF3 induction, and both ERK and JNK are necessary for post-transcriptional induction of Fra1 in response to cisplatin or UV. Knock-down of ATF3 and Fra1 results in increased and decreased cisplatin-induced apoptosis, respectively, indicating that ATF3 is an anti-apoptotic JNK effector and Fra1 is a pro-apoptotic ERK/JNK effector. Knock-down experiments also revealed that ATF3 and Fra1, respectively, enhance and reduce S-phase arrest through differential modulation of the Chk1-Cdk2 pathway. Thus, we identify novel reciprocal functions of ATF3 and Fra1 in JNK- and ERK-dependent DNA damage responses.

DNA repair, damage signaling and carcinogenesis

The First joint meeting of the German DGDR (German Society for Research on DNA Repair) and the French SFTG (French Society of Genotoxicology) on DNA Repair was held in Toulouse, France, from September 15 to 19, 2007. It was organized by Lisa Wiesmüller and Bernard Salles together with the scientific committee consisting of Gilbert de Murcia, Jean-Marc Egly, Frank Grosse, Karl-Peter Hopfner, Georges Iliakis, Bernd Kaina, Markus Löbrich, Bernard Lopez, Daniel Marzin and Alain Sarasin. This report summarizes information presented by the speakers (invited lectures and oral communications) during the seven plenary sessions, which include (1) excision repair, (2) DNA repair and carcinogenesis, (3) double-strand break repair, (4) replication in repair and lesion bypass, (5) cellular responses to genotoxic stress, (6) DNA repair machinery within the chromatin context and (7) genotoxicology and testing. A total of 23 plenary lectures, 32 oral communications and 66 posters were presented in this rather intense 4 days meeting, which stimulated extensive discussions and highly interdisciplinary scientific exchanges among the approximately 250 participants.

Role of extracellular signal-regulated kinase (ERK) signaling in nucleotide excision repair and genotoxicity in response to As(III) and Pb(II)

Arsenic and lead can induce genetic injuries and epigenetic signaling pathways in cultured mammalian cells. To test whether signaling pathways affect the extent of genetic injuries, we explored the impacts of extracellular signal-regulated kinase 1 and 2 (ERK) on nucleotide excision repair (NER), cytotoxicity, and genotoxicity following sodium arsenite [As(III)] and lead acetate [Pb(II)]. Sustained ERK activation was observed in human cells exposed to As(III) and Pb(II). As(III) inhibited the cellular NER synthesis capability; conversely, Pb(II) stimulated it. ERK activation contributed to the As(III)-induced NER inhibition and micronucleus formation. In contrast, this signal was required for inducing cellular NER activity and preventing mutagenesis following Pb(II). ERK activation by Pb(II) was dependent on protein kinase C (PKCα) that also exhibited anti-mutagenicity. Enforced expression of ERK signaling markedly elevated the cellular NER activity, which was suppressed by As(III). Nonetheless, ERK activation could counteract the cytotoxicity caused by these two metals. Together, the results indicate that pro-survival ERK signaling exhibits dual and opposing impacts on NER process following As(III) and Pb(II) exposures. The findings also suggest that ERK is an important epigenetic signaling in the determination of metal genotoxicity.

A sensitive flow cytometry-based nucleotide excision repair assay unexpectedly reveals that mitogen-activated protein kinase signaling does not regulate the removal of UV-induced DNA damage in human cells

In response to diverse genotoxic stimuli (e.g. UV and cisplatin), the mitogen-activated protein kinases ERK1/2, JNK1/2, and p38alpha/beta become rapidly phosphorylated and in turn activate multiple downstream effectors that modulate apoptosis and/or growth arrest. Furthermore, previous lines of evidence have strongly suggested that ERK1/2 and JNK1/2 participate in global-genomic nucleotide excision repair, a critical antineoplastic pathway that removes helix-distorting DNA adducts induced by a variety of mutagenic agents, including UV. To rigorously evaluate the potential role of mitogen-activated protein kinases in global-genomic nucleotide excision repair, various human cell strains (primary skin fibroblasts, primary lung fibroblasts, and HCT116 colon carcinoma cells) were treated with highly specific chemical inhibitors, which, following UV exposure, (i) abrogated the capacities of ERK1/2, JNK1/2, or p38alpha/beta to phosphorylate specific downstream effectors and (ii) characteristically modulated cellular proliferation, clonogenic survival, and/or apoptosis. A highly sensitive flow cytometry-based nucleotide excision repair assay recently optimized and validated in our laboratory was then employed to directly demonstrate that the kinetics of UV DNA photoadduct repair are highly similar in mock-treated versus mitogen-activated protein kinase inhibitor-treated cells. These data on primary and tumor cells treated with pharmacological inhibitors were fully corroborated by repair studies using (i) short hairpin RNA-mediated knockdown of ERK1/2 or JNK1/2 in human U2OS osteosarcoma cells and (ii) expression of a dominant negative p38alpha mutant in human primary lung fibroblasts. Our results provide solid evidence for the first time, in disaccord with a burgeoning perception, that mitogen-activated protein kinase signaling does not influence the efficiency of human global-genomic nucleotide excision repair.

Identification of rat brainstem neuronal structures activated during cancer-induced anorexia

In cancer-related anorexia, body weight loss is paradoxically associated with reduced appetite, which is contrary to the situation during starvation, implying that the normal coupling of food intake to energy expenditure is disarranged. Here we examined brainstem mechanisms that may underlie suppression of food intake in a rat model of cancer anorexia. Cultured Morris 7777 hepatoma cells were injected subcutaneously in Buffalo rats, resulting in slowly growing tumor and reduced food intake and body weight loss after about 10 days. The brainstem was examined for induced expression of the transcription factors Fos and FosB as signs of neuronal activation. The results showed that anorexia and retarded body weight growth were associated with Fos protein expression in the area postrema, the general visceral region of the nucleus of the solitary tract, and the external lateral parabrachial nucleus, structures that also display Fos after peripheral administration of satiating or anorexigenic stimuli. The magnitude of the Fos expression was specifically related to the size of induced tumor, and not associated with weight loss per se, because it was not present in pair-fed or food-deprived rats. It also appeared to be independent of proinflammatory cytokines, as determined by the absence of increased cytokine levels in plasma and induced cytokine and cyclooxygenase expression in the brain. The findings thus provide evidence that cancer-associated anorexia and weight loss in this model is associated with activation of brainstem circuits involved in the suppression of food intake, and suggest that this occurs by inflammatory-independent mechanisms.