c-Abl tyrosine kinase regulates the human Rad9 checkpoint protein in response to DNA damage

Allosteric regulation of autoinhibition and activation of c-Abl

c-Abl, a non-receptor tyrosine kinase, regulates cell growth and survival in healthy cells and causes chronic myeloid leukemia (CML) when fused by Bcr. Its activity is blocked in the assembled inactive state, where the SH3 and SH2 domains dock into the kinase domain, reducing its conformational flexibility, resulting in the autoinhibited state. It is active in an extended 'open' conformation. Allostery governs the transitions between the autoinhibited and active states. Even though experiments revealed the structural hallmarks of the two states, a detailed grasp of the determinants of c-Abl autoinhibition and activation at the atomic level, which may help innovative drug discovery, is still lacking. Here, using extensive molecular dynamics simulations, we decipher exactly how these determinants regulate it. Our simulations confirm and extend experimental data that the myristoyl group serves as the switch for c-Abl inhibition/activation. Its dissociation from the kinase domain promotes the SH2-SH3 release, initiating c-Abl activation. We show that the precise SH2/N-lobe interaction is required for full activation of c-Abl. It stabilizes a catalysis-favored conformation, priming it for catalytic action. Bcr-Abl allosteric drugs elegantly mimic the endogenous myristoyl-mediated autoinhibition state of c-Abl 1b. Allosteric activating mutations shift the ensemble to the active state, blocking ATP-competitive drugs. Allosteric drugs alter the active-site conformation, shifting the ensemble to re-favor ATP-competitive drugs. Our work provides a complete mechanism of c-Abl activation and insights into critical parameters controlling at the atomic level c-Abl inactivation, leading us to propose possible strategies to counter reemergence of drug resistance.

Chromosomal Instability in Chronic Myeloid Leukemia: Mechanistic Insights and Effects

The most recent two decades have seen tremendous progress in the understanding and treatment of chronic myeloid leukemia, a disease defined by the characteristic Philadelphia chromosome and the ensuing BCR::ABL fusion protein. However, the biology of the disease extends beyond the Philadelphia chromosome into a nebulous arena of chromosomal and genetic instability, which makes it a genetically heterogeneous disease. The BCR::ABL oncoprotein creates a fertile backdrop for oxidative damage to the DNA, along with impairment of genetic surveillance and the favoring of imprecise error-prone DNA repair pathways. These factors lead to growing chromosomal instability, manifested as additional chromosomal abnormalities along with other genetic aberrations. This worsens with disease progression to accelerated and blast phase, and modulates responses to tyrosine kinase inhibitors. Treatment options that target the genetic aberrations that mitigate chromosome instability might be a potential area for research in patients with advanced phase CML.

Hypermethylation of RAD9A intron 2 in childhood cancer patients, leukemia and tumor cell lines suggest a role for oncogenic transformation

Most childhood cancers occur sporadically and cannot be explained by an inherited mutation or an unhealthy lifestyle. However, risk factors might trigger the oncogenic transformation of cells. Among other regulatory signals, hypermethylation of RAD9A intron 2 is responsible for the increased expression of RAD9A protein, which may play a role in oncogenic transformation. Here, we analyzed the RAD9A intron 2 methylation in primary fibroblasts of 20 patients with primary cancer in childhood and second primary cancer (2N) later in life, 20 matched patients with only one primary cancer in childhood (1N) and 20 matched cancer-free controls (0N), using bisulfite pyrosequencing and deep bisulfite sequencing (DBS). Four 1N patients and one 2N patient displayed elevated mean methylation levels (≥ 10 %) of RAD9A. DBS revealed ≥ 2 % hypermethylated alleles of RAD9A, indicative for constitutive mosaic epimutations. Bone marrow samples of NHL and AML tumor patients (n=74), EBV (Epstein Barr Virus) lymphoblasts (n=6), tumor cell lines (n=5) and FaDu subclones (n=13) were analyzed to substantiate our findings. We find a broad spectrum of tumor entities with an aberrant methylation of RAD9A. We detected a significant difference in mean methylation of RAD9A for NHL versus AML patients (p ≤0.025). Molecular karyotyping of AML samples during therapy with hypermethylated RAD9A showed an evolving duplication of 1.8 kb on Chr16p13.3 including the PKD1 gene. Radiation, colony formation assays, cell proliferation, PCR and molecular karyotyping SNP-array experiments using generated FaDu subclones suggest that hypermethylation of RAD9A intron 2 is associated with genomic imbalances in regions with tumor-relevant genes and survival of the cells. In conclusion, this is the very first study of RAD9A intron 2 methylation in childhood cancer and Leukemia. RAD9A epimutations may have an impact on leukemia and tumorigenesis and can potentially serve as a biomarker.

miR-27b-3p a Negative Regulator of DSB-DNA Repair

Understanding the regulation of DNA repair mechanisms is of utmost importance to identify altered cellular processes that lead to diseases such as cancer through genomic instability. In this sense, miRNAs have shown a crucial role. Specifically, miR-27b-3 biogenesis has been shown to be induced in response to DNA damage, suggesting that this microRNA has a role in DNA repair. In this work, we show that the overexpression of miR-27b-3p reduces the ability of cells to repair DNA lesions, mainly double-stranded breaks (DSB), and causes the deregulation of genes involved in homologous recombination repair (HRR), base excision repair (BER), and the cell cycle. DNA damage was induced in BALB/c-3T3 cells, which overexpress miR-27b-3p, using xenobiotic agents with specific mechanisms of action that challenge different repair mechanisms to determine their reparative capacity. In addition, we evaluated the expression of 84 DNA damage signaling and repair genes and performed pathway enrichment analysis to identify altered cellular processes. Taken together, our results indicate that miR-27b-3p acts as a negative regulator of DNA repair when overexpressed.

Tyrosine kinase inhibitors protect the salivary gland from radiation damage by increasing DNA double-strand break repair

We have previously shown that the tyrosine kinase inhibitors (TKIs) dasatinib and imatinib can protect salivary glands from irradiation (IR) damage without impacting tumor therapy. However, how they induce this protection is unknown. Here we show that TKIs mediate radioprotection by increasing the repair of DNA double-stranded breaks. DNA repair in IR-treated parotid cells, but not oral cancer cells, occurs more rapidly following pretreatment with imatinib or dasatinib and is accompanied by faster formation of DNA damage-induced foci. Similar results were observed in the parotid glands of mice pretreated with imatinib prior to IR, suggesting that TKIs "prime" cells for DNA repair. Mechanistically, we observed that TKIs increased IR-induced activation of DNA-PK, but not ATM. Pretreatment of parotid cells with the DNA-PK inhibitor NU7441 reversed the increase in DNA repair induced by TKIs. Reporter assays specific for homologous recombination (HR) or nonhomologous end joining (NHEJ) verified regulatation of both DNA repair pathways by imatinib. Moreover, TKIs also increased basal and IR-induced expression of genes associated with NHEJ (DNA ligase 4, Artemis, XLF) and HR (Rad50, Rad51 and BRCA1); depletion of DNA ligase 4 or BRCA1 reversed the increase in DNA repair mediated by TKIs. In addition, TKIs increased activation of the ERK survival pathway in parotid cells, and ERK was required for the increased survival of TKI-treated cells. Our studies demonstrate a dual mechanism by which TKIs provide radioprotection of the salivary gland tissues and support exploration of TKIs clinically in head and neck cancer patients undergoing IR therapy.

Involvement of interferon-tau in the induction of apoptotic, pyroptotic, and autophagic cell death-related signaling pathways in the bovine uterine endometrium during early pregnancy

Interferon-tau (IFNT), a type I interferon (IFN), is known as pregnancy recognition signaling molecule secreted from the ruminant conceptus during the preimplantation period. Type I IFNs, such as IFN-alpha and IFN-beta, are known to activate cell-death pathways as well as induce apoptosis. In cows, induction of apoptosis with DNA fragmentation is induced by IFNT in cultured bovine endometrial epithelial cells. However, the status of cell-death pathways in the bovine endometrium during the preimplantation period still remains unclear. In the present study, we investigated the different cell-death pathways, including apoptosis, pyroptosis, and autophagy, in uterine tissue obtained from pregnant cows and in vitro cultured endometrial epithelial cells with IFNT stimulation. The expression of CASP7, 8, and FADD (apoptosis-related genes) was significantly higher in pregnant day 18 uterine tissue in comparison to non-pregnant day 18 tissue. The expression of CASP4, 11, and NLRP3 (pyroptosis-related genes) was significantly higher in the pregnant uterus in comparison to non-pregnant uterus. In contrast, autophagy-related genes were not affected by pregnancy. We also investigated the effect of IFNT on the expression of cell-death pathway-related genes, as well as DNA fragmentation in cultured endometrial epithelial cells. Similar to its effects in pregnant uterine tissue, IFNT affected the increase of apoptosis-related (CASP8) and pyroptosis-related genes (CASP11), but did not affect autophagy-related gene expression. IFNT also increased γH2AX-positive cells, which is a marker of DNA fragmentation. These results suggest that apoptosis- and pyroptosis-related genes are induced by IFNT in the pregnant bovine endometrial epithelial cells.

Prostate cancer: unmet clinical needs and RAD9 as a candidate biomarker for patient management

Prostate cancer is a complex disease, with multiple subtypes and clinical presentations. Much progress has been made in recent years to understand the underlying genetic basis that drives prostate cancer. Such mechanistic information is useful for development of novel therapeutic targets, to identify biomarkers for early detection or to distinguish between aggressive and indolent disease, and to predict treatment outcome. Multiple tests have become available in recent years to address these clinical needs for prostate cancer. We describe several of these assays, summarizing test details, performance characteristics, and acknowledging their limitations. There is a pressing unmet need for novel biomarkers that can demonstrate improvement in these areas. We introduce one such candidate biomarker, RAD9, describe its functions in the DNA damage response, and detail why it can potentially fill this void. RAD9 has multiple roles in prostate carcinogenesis, making it potentially useful as a clinical tool for men with prostate cancer. RAD9 was originally identified as a radioresistance gene, and subsequent investigations revealed several key functions in the response of cells to DNA damage, including involvement in cell cycle checkpoint control, at least five DNA repair pathways, and apoptosis. Further studies indicated aberrant overexpression in approximately 45% of prostate tumors, with a strong correlation between RAD9 abundance and cancer stage. A causal relationship between RAD9 and prostate cancer was first demonstrated using a mouse model, where tumorigenicity of human prostate cancer cells after subcutaneous injection into nude mice was diminished when RNA interference was used to reduce the normally high levels of the protein. In addition to activity needed for the initial development of tumors, cell culture studies indicated roles for RAD9 in promoting prostate cancer progression by controlling cell migration and invasion through regulation of ITGB1 protein levels, and anoikis resistance by modulating AKT activation. Furthermore, RAD9 enhances the resistance of human prostate cancer cells to radiation in part by regulating ITGB1 protein abundance. RAD9 binds androgen receptor and inhibits androgen-induced androgen receptor's activity as a transcription factor. Moreover, RAD9 also acts as a gene-specific transcription factor, through binding p53 consensus sequences at target gene promoters, and this likely contributes to its oncogenic activity. Given these diverse and extensive activities, RAD9 plays important roles in the initiation and progression of prostate cancer and can potentially serve as a valuable biomarker useful in the management of patients with this disease.

p53 and RAD9, the DNA Damage Response, and Regulation of Transcription Networks

The way cells respond to DNA damage is important since inefficient repair or misrepair of lesions can have deleterious consequences, including mutation, genomic instability, neurodegenerative disorders, premature aging, cancer or death. Whether damage occurs spontaneously as a byproduct of normal metabolic processes, or after exposure to exogenous agents, cells muster a coordinated, complex DNA damage response (DDR) to mitigate potential harmful effects. A variety of activities are involved to promote cell survival, and include DNA repair, DNA damage tolerance, as well as transient cell cycle arrest to provide time for repair before entry into critical cell cycle phases, an event that could be lethal if traversal occurs while damage is present. When such damage is prolonged or not repairable, senescence, apoptosis or autophagy is induced. One major level of DDR regulation occurs via the orchestrated transcriptional control of select sets of genes encoding proteins that mediate the response. p53 is a transcription factor that transactivates specific DDR downstream genes through binding DNA consensus sequences usually in or near target gene promoter regions. The profile of p53-regulated genes activated at any given time varies, and is dependent upon type of DNA damage or stress experienced, exact composition of the consensus DNA binding sequence, presence of other DNA binding proteins, as well as cell context. RAD9 is another protein critical for the response of cells to DNA damage, and can also selectively regulate gene transcription. The limited studies addressing the role of RAD9 in transcription regulation indicate that the protein transactivates at least one of its target genes, p21/waf1/cip1, by binding to DNA sequences demonstrated to be a p53 response element. NEIL1 is also regulated by RAD9 through a similar DNA sequence, though not yet directly verified as a bonafide p53 response element. These findings suggest a novel pathway whereby p53 and RAD9 control the DDR through a shared mechanism involving an overlapping network of downstream target genes. Details and unresolved questions about how these proteins coordinate or compete to execute the DDR through transcriptional reprogramming, as well as biological implications, are discussed.

Chk1 Activation Protects Rad9A from Degradation as Part of a Positive Feedback Loop during Checkpoint Signalling

Phosphorylation of Rad9A at S387 is critical for establishing a physical interaction with TopBP1, and to downstream activation of Chk1 for checkpoint activation. We have previously demonstrated a phosphorylation of Rad9A that occurs at late time points in cells exposed to genotoxic agents, which is eliminated by either Rad9A overexpression, or conversion of S387 to a non-phosphorylatable analogue. Based on this, we hypothesized that this late Rad9A phosphorylation is part of a feedback loop regulating the checkpoint. Here, we show that Rad9A is hyperphosphorylated and accumulates in cells exposed to bleomycin. Following the removal of bleomycin, Rad9A is polyubiquitinated, and Rad9A protein levels drop, indicating an active degradation process for Rad9A. Chk1 inhibition by UCN-01 or siRNA reduces Rad9A levels in cells synchronized in S-phase or exposed to DNA damage, indicating that Chk1 activation is required for Rad9A stabilization in S-phase and during checkpoint activation. Together, these results demonstrate a positive feedback loop involving Rad9A-dependend activation of Chk1, coupled with Chk1-dependent stabilization of Rad9A that is critical for checkpoint regulation.

DNA damage response genes and the development of cancer metastasis

DNA damage response genes play vital roles in the maintenance of a healthy genome. Defects in cell cycle checkpoint and DNA repair genes, especially mutation or aberrant downregulation, are associated with a wide spectrum of human disease, including a predisposition to the development of neurodegenerative conditions and cancer. On the other hand, upregulation of DNA damage response and repair genes can also cause cancer, as well as increase resistance of cancer cells to DNA damaging therapy. In recent years, it has become evident that many of the genes involved in DNA damage repair have additional roles in tumorigenesis, most prominently by acting as transcriptional (co-)factors. Although defects in these genes are causally connected to tumor initiation, their role in tumor progression is more controversial and it seems to depend on tumor type. In some tumors like melanoma, cell cycle checkpoint/DNA repair gene upregulation is associated with tumor metastasis, whereas in a number of other cancers the opposite has been observed. Several genes that participate in the DNA damage response, such as RAD9, PARP1, BRCA1, ATM and TP53 have been associated with metastasis by a number of in vitro biochemical and cellular assays, by examining human tumor specimens by immunohistochemistry or by DNA genome-wide gene expression profiling. Many of these genes act as transcriptional effectors to regulate other genes implicated in the pathogenesis of cancer. Furthermore, they are aberrantly expressed in numerous human tumors and are causally related to tumorigenesis. However, whether the DNA damage repair function of these genes is required to promote metastasis or another activity is responsible (e.g., transcription control) has not been determined. Importantly, despite some compelling in vitro evidence, investigations are still needed to demonstrate the role of cell cycle checkpoint and DNA repair genes in regulating metastatic phenotypes in vivo.

Strategies for the selective regulation of kinases with allosteric modulators: exploiting exclusive structural features

The modulation of kinase function has become an important goal in modern drug discovery and chemical biology research. In cancer-targeted therapies, kinase inhibitors have been experiencing an upsurge, which can be measured by the increasing number of kinase inhibitors approved by the FDA in recent years. However, lack of efficacy, limited selectivity, and the emergence of acquired drug resistance still represent major bottlenecks in the clinic and challenge inhibitor development. Most known kinase inhibitors target the active kinase and are ATP competitive. A second class of small organic molecules, which address remote sites of the kinase and stabilize enzymatically inactive conformations, is rapidly moving to the forefront of kinase inhibitor research. Such allosteric modulators bind to sites that are less conserved across the kinome and only accessible upon conformational changes. These molecules are therefore thought to provide various advantages such as higher selectivity and extended drug target residence times. This review highlights various strategies that have been developed to utilizing exclusive structural features of kinases and thereby modulating their activity allosterically.

Does the small tyrosine kinase inhibitor Imatinib mesylate counteract diabetes by affecting pancreatic islet amyloidosis and fibrosis?

INTRODUCTION

The small tyrosine kinase inhibitor Imatinib Mesylate (Gleevec) protects against diabetes, but it is not known how.

AREAS COVERED

It has been suggested that islet amyloid and fibrotic deposits promote beta-cell failure and death, leading to Type-2 diabetes. As Imatinib is known to possess anti-fibrotic/amyloid properties, in for example systemic sclerosis and mouse models for Alzheimer's disease, the present review will discuss the possibility that Imatinib acts, at least in part, by ameliorating islet hyalinization and its consequences in the pathogenesis of Type-2 diabetes.

EXPERT OPINION

A better understanding of how Imatinib counteracts Type-2 diabetes will possibly help to clarify the pathogenic role of islet amyloid and fibrosis, and hopefully lead to improved treatment of the disease.

Phosphorylation of Rad9 at serine 328 by cyclin A-Cdk2 triggers apoptosis via interfering Bcl-xL

Cyclin A-Cdk2, a cell cycle regulated Ser/Thr kinase, plays important roles in a variety of apoptoticprocesses. However, the mechanism of cyclin A-Cdk2 regulated apoptosis remains unclear. Here, we demonstrated that Rad9, a member of the BH3-only subfamily of Bcl-2 proteins, could be phosphorylated by cyclin A-Cdk2 in vitro and in vivo. Cyclin A-Cdk2 catalyzed the phosphorylation of Rad9 at serine 328 in HeLa cells during apoptosis induced by etoposide, an inhibitor of topoisomeraseII. The phosphorylation of Rad9 resulted in its translocation from the nucleus to the mitochondria and its interaction with Bcl-xL. The forced activation of cyclin A-Cdk2 in these cells by the overexpression of cyclin A,triggered Rad9 phosphorylation at serine 328 and thereby promoted the interaction of Rad9 with Bcl-xL and the subsequent initiation of the apoptotic program. The pro-apoptotic effects regulated by the cyclin A-Cdk2 complex were significantly lower in cells transfected with Rad9S328A, an expression vector that encodes a Rad9 mutant that is resistant to cyclin A-Cdk2 phosphorylation. These findings suggest that cyclin A-Cdk2 regulates apoptosis through a mechanism that involves Rad9phosphorylation.

Contributions of Rad9 to tumorigenesis

Rad9 plays a crucial role in maintaining genomic stability by regulating cell cycle checkpoints, DNA repair, telomere stability, and apoptosis. Rad9 controls these processes mainly as part of the heterotrimeric 9-1-1 (Rad9-Hus1-Rad1) complex. However, in recent years it has been demonstrated that Rad9 can also act independently of the 9-1-1 complex as a transcriptional factor, participate in immunoglobulin class switch recombination, and show 3'-5' exonuclease activity. Aberrant Rad9 expression has been associated with prostate, breast, lung, skin, thyroid, and gastric cancers. High expression of Rad9 is causally related to, at least, human prostate cancer growth. On the other hand, deletion of Mrad9, the mouse homolog, is responsible for increased skin cancer incidence. These results reveal that Rad9 can act as an oncogene or tumor suppressor. Which of the many functions of Rad9 are causally related to initiation and progression of tumorigenesis and the mechanistic details by which Rad9 induces or suppresses tumorigenesis are presently not known, but are crucial for the development of targeted therapeutic interventions.

Localization of the Drosophila Rad9 protein to the nuclear membrane is regulated by the C-terminal region and is affected in the meiotic checkpoint

Rad9, Rad1, and Hus1 (9-1-1) are part of the DNA integrity checkpoint control system. It was shown previously that the C-terminal end of the human Rad9 protein, which contains a nuclear localization sequence (NLS) nearby, is critical for the nuclear transport of Rad1 and Hus1. In this study, we show that in Drosophila, Hus1 is found in the cytoplasm, Rad1 is found throughout the entire cell and that Rad9 (DmRad9) is a nuclear protein. More specifically, DmRad9 exists in two alternatively spliced forms, DmRad9A and DmRad9B, where DmRad9B is localized at the cell nucleus, and DmRad9A is found on the nuclear membrane both in Drosophila tissues and also when expressed in mammalian cells. Whereas both alternatively spliced forms of DmRad9 contain a common NLS near the C terminus, the 32 C-terminal residues of DmRad9A, specific to this alternative splice form, are required for targeting the protein to the nuclear membrane. We further show that activation of a meiotic checkpoint by a DNA repair gene defect but not defects in the anchoring of meiotic chromosomes to the oocyte nuclear envelope upon ectopic expression of non-phosphorylatable Barrier to Autointegration Factor (BAF) dramatically affects DmRad9A localization. Thus, by studying the localization pattern of DmRad9, our study reveals that the DmRad9A C-terminal region targets the protein to the nuclear membrane, where it might play a role in response to the activation of the meiotic checkpoint.

Tyrosine phosphorylation enhances RAD52-mediated annealing by modulating its DNA binding

The DNA recombination mediator and annealing factor RAD52 is a target of c-ABL activated in response to DNA damage. Engineering of recombinant tyrosine-phosphomimetic RAD52 facilitated studying the consequences of this phosphorylation.

RAD52 protein has an important role in homology-directed DNA repair by mediating RAD51 nucleoprotein filament formation on single-stranded DNA (ssDNA) protected by replication protein-A (RPA) and annealing of RPA-coated ssDNA. In human, cellular response to DNA damage includes phosphorylation of RAD52 by c-ABL kinase at tyrosine 104. To address how this phosphorylation modulates RAD52 function, we used an amber suppressor technology to substitute tyrosine 104 with chemically stable phosphotyrosine analogue (p-Carboxymethyl-L-phenylalanine, pCMF). The RAD52^Y104pCMF retained ssDNA-binding activity characteristic of unmodified RAD52 but showed lower affinity for double-stranded DNA (dsDNA) binding. Single-molecule analyses revealed that RAD52^Y104pCMF specifically targets and wraps ssDNA. While RAD52^Y104pCMF is confined to ssDNA region, unmodified RAD52 readily diffuses into dsDNA region. The Y104pCMF substitution also increased the ssDNA annealing rate and allowed overcoming the inhibitory effect of dsDNA. We propose that phosphorylation at Y104 enhances ssDNA annealing activity of RAD52 by attenuating dsDNA binding. Implications of phosphorylation-mediated activation of RAD52 annealing activity are discussed.

Tip60-mediated acetylation activates transcription independent apoptotic activity of Abl

Background

The proto-oncogene, c-Abl encodes a ubiquitously expressed tyrosine kinase that critically governs the cell death response induced by genotoxic agents such as ionizing radiation and cisplatin. The catalytic function of Abl, which is essential for executing DNA damage response (DDR), is normally tightly regulated but upregulated several folds upon IR exposure due to ATM-mediated phosphorylation on S465. However, the mechanism/s leading to activation of Abl's apoptotic activity is currently unknown.

Results

We investigated the role of acetyl modification in regulating apoptotic activity of Abl and the results showed that DNA strand break-inducing agents, ionizing radiation and bleomycin induced Abl acetylation. Using mass spectrophotometry and site-specific acetyl antibody, we identified Abl K921, located in the DNA binding domain, and conforming to one of the lysine residue in the consensus acetylation motif (KXXK--X3-5--SGS) is acetylated following DNA damage. We further observed that the S465 phosphorylated Abl is acetyl modified during DNA damage. Signifying the modification, cells expressing the non acetylatable K921R mutant displayed attenuated apoptosis compared to wild-type in response to IR or bleomycin treatment. WT-Abl induced apoptosis irrespective of new protein synthesis. Furthermore, upon γ-irradiation K921R-Abl displayed reduced chromatin binding compared to wild type. Finally, loss of Abl K921 acetylation in Tip60-knocked down cells and co-precipitation of Abl with Tip60 in DNA damaged cells identified Tip60 as an Abl acetylase.

Conclusion

Collective data showed that DNA damage-induced K921 Abl acetylation, mediated by Tip60, stimulates transcriptional-independent apoptotic activity and chromatin-associative property thereby defining a new regulatory mechanism governing Abl's DDR function.

A role for the arginine methylation of Rad9 in checkpoint control and cellular sensitivity to DNA damage

The genome stability is maintained by coordinated action of DNA repairs and checkpoints, which delay progression through the cell cycle in response to DNA damage. Rad9 is conserved from yeast to human and functions in cell cycle checkpoint controls. Here, a regulatory mechanism for Rad9 function is reported. In this study Rad9 has been found to interact with and be methylated by protein arginine methyltransferase 5 (PRMT5). Arginine methylation of Rad9 plays a critical role in S/M and G2/M cell cycle checkpoints. The activation of the Rad9 downstream checkpoint effector Chk1 is impaired in cells only expressing a mutant Rad9 that cannot be methylated. Additionally, Rad9 methylation is also required for cellular resistance to DNA damaging stresses. In summary, we uncovered that arginine methylation is important for regulation of Rad9 function, and thus is a major element for maintaining genome integrity.

Application of hRad9 in lung cancer treatment as a molecular marker and a molecular target

DNA damage sensor proteins work as upstream components of the DNA damage checkpoint signaling pathways that are essential for cell cycle control and the induction of apoptosis. hRad9 is a member of a family of proteins that act as DNA damage sensors and plays an important role as an upstream regulator of checkpoint signaling. We clarified the significant accumulation of hRad9 in the nuclei of tumor cells in surgically-resected non-small-cell lung cancer (NSCLC) specimens and found the capacity to produce a functional hRad9 protein was intact in lung cancer cells. This finding suggested that hRad9 was a vital component in the pathways that lead to the survival and progression of NSCLC and suggested that hRad9 was a good candidate for a molecular target to control lung cancer cell growth. RNA interference targeting hRad9 was performed to examine this hypothesis. The impairment of the DNA damage checkpoint signaling pathway induced cancer cell death. hRad9 might be a novel molecular target for lung cancer treatment.

The c-Abl tyrosine kinase stabilizes Pitx1 in the apoptotic response to DNA damage

In the DNA damage response, c-Abl tyrosine kinase is transiently accumulated in the nucleus and induces apoptosis; however, little is known about the mechanism underlying apoptosis induction via nuclear c-Abl. Here we demonstrate that the expression of human pituitary homeobox 1 (Pitx1) transcription factor is increased after DNA damage. Notably, c-Abl controls augmentation of Pitx1 at the post-transcriptional level. Overexpression of c-Abl induces tyrosine phosphorylation of Pitx1, either directly or indirectly. We also show that, upon exposure to genotoxic stress, overexpression of Pitx1 is associated with marked induction of apoptosis that is independent of p53 status. Importantly, inhibition of c-Abl kinase activity substantially attenuates Pitx1-mediated apoptosis. These findings provide evidence that c-Abl participates in modulating Pitx1 expression in the apoptotic response to DNA damage.

ATM augments nuclear stabilization of DYRK2 by inhibiting MDM2 in the apoptotic response to DNA damage

The tumor suppressor p53 is a transcription factor that regulates cell cycle, DNA repair, senescence, and apoptosis in response to DNA damage. Phosphorylation of p53 at Ser-46 is indispensable for the commitment to apoptotic cell death. A previous study has shown that upon exposure to genotoxic stress, DYRK2 translocates into the nucleus and phosphorylates p53 at Ser-46, thereby inducing apoptosis. However, less is known about mechanisms responsible for intracellular control of DYRK2. Here we show the functional nuclear localization signal at N-terminal domain of DYRK2. Under normal conditions, nuclear and not cytoplasmic DYRK2 is ubiquitinated by MDM2, resulting in its constitutive degradation. In the presence of proteasome inhibitors, we detected a stable complex of DYRK2 with MDM2 at the nucleus. Upon exposure to genotoxic stress, ATM phosphorylates DYRK2 at Thr-33 and Ser-369, which enables DYRK2 to escape from degradation by dissociation from MDM2 and to induce the kinase activity toward p53 at Ser-46 in the nucleus. These findings indicate that ATM controls stability and pro-apoptotic function of DYRK2 in response to DNA damage.

Potential utility of small tyrosine kinase inhibitors in the treatment of diabetes

Altered tyrosine kinase signalling has been implicated in several diseases, paving the way for the development of small-molecule TKIs (tyrosine kinase inhibitors). TKIs such as imatinib, sunitinib and dasatinib are clinically used for treating chronic myeloid leukaemia, gastrointestinal stromal tumours and other malignancies. In addition to their use as anti-cancer agents, increasing evidence points towards an anti-diabetic effect of these TKIs. Imatinib and other TKIs counteract diabetes not only in non-obese diabetic mice, but also in streptozotocin diabetic mice, db/db mice, high-fat-treated rats and humans with T2D (Type 2 diabetes). Although the mechanisms of protection need to be investigated further, the effects of imatinib and other TKIs in human T2D and the rapidly growing findings from animal models of T1D (Type 1 diabetes) and T2D are encouraging and give hope to improved treatment of human diabetes. In the present article, we review the anti-diabetic effects of TKIs which appear to involve both protection against beta-cell death and improved insulin sensitivity. Considering the relatively mild side effects of TKIs, we hypothesize that TKIs could be used to treat new-onset T1D, prevent T1D in individuals at high risk of developing the disease, treat the late stages of T2D and improve the outcome of islet transplantation.

BH3-only proteins in apoptosis and beyond: an overview

BH3-only BCL-2 family proteins are effectors of canonical mitochondrial apoptosis. They discharge their pro-apoptotic functions through BH1-3 pro-apoptotic proteins such as BAX and BAK, while their activity is suppressed by BH1-4 anti-apoptotic BCL-2 family members. The precise mechanism by which BH3-only proteins mediate apoptosis remains unresolved. The existing data are consistent with three mutually non-exclusive models (1) displacement of BH1-3 proteins from complexes with BH1-4 proteins; (2) direct interaction with and conformational activation of BH1-3 proteins; and (3) membrane insertion and membrane remodeling. The BH3-only proteins appear to play critical roles in restraining cancer and inflammatory diseases such as rheumatoid arthritis. Molecules that mimic the effect of BH3-only proteins are being used in treatments against these diseases. The cell death activity of a subclass of BH3-only members (BNIP3 and BNIP3L) is linked to cardiomyocyte loss during heart failure. In addition to their established role in apoptosis, several BH3-only members also regulate diverse cellular functions in cell-cycle regulation, DNA repair and metabolism. Several members are implicated in the induction of autophagy and autophagic cell death, possibly through unleashing of the BH3-only autophagic effector Beclin 1 from complexes with BCL-2/BCL-xL. The Chapters included in the current Oncogene Review issues provide in-depth discussions on various aspects of major BH3-only proteins.

DNA damage signalling recruits RREB-1 to the p53 tumour suppressor promoter

Transcriptional regulation of the p53 tumour suppressor gene plays an important role in the control of the expression of various target genes involved in the DNA damage response. However, the molecular basis of this regulation remains obscure. In the present study we demonstrate that RREB-1 (Ras-responsive-element-binding protein-1) efficiently binds to the p53 promoter via the p53 core promoter element and transactivates p53 expression. Silencing of RREB-1 significantly reduces p53 expression at both the mRNA and the protein levels. Notably, disruption of RREB-1-mediated p53 transcription suppresses the expression of the p53 target genes. We also show that, upon exposure to genotoxic stress, RREB-1 controls apoptosis in a p53-dependent manner. These findings provide evidence that RREB-1 participates in modulating p53 transcription in response to DNA damage.

Protein kinase Cdelta activates RelA/p65 and nuclear factor-kappaB signaling in response to tumor necrosis factor-alpha

Nuclear factor-kappaB (NF-kappaB) is tightly modulated by IkappaB kinases and IkappaBalpha in the cytoplasm. On stimulation, NF-kappaB translocates into the nucleus to initiate transcription; however, regulation of its transcriptional activity remains obscure. Here, we show that protein kinase C (PKC) delta controls the main subunit of NF-kappaB, RelA/p65. On exposure to tumor necrosis factor-alpha (TNF-alpha), the expression of RelA/p65 target genes such as IkappaBalpha, RelB, and p100/p52 is up-regulated in a PKCdelta-dependent manner. The results also show that PKCdelta is targeted to the nucleus and forms a complex with RelA/p65 following TNF-alpha exposure. Importantly, kinase activity of PKCdelta is required for RelA/p65 transactivation. In concert with these results, PKCdelta activates RelA/p65 for its occupancy to target-gene promoters, including IkappaBalpha and p100/p52. Moreover, functional analyses show that inhibition of PKCdelta is associated with substantial attenuation of NF-kappaB activity in response to TNF-alpha. These findings provide evidence that PKCdelta orchestrates RelA/p65 transactivation, a requisite for NF-kappaB signaling pathway in the nucleus.

Loss of Hus1 sensitizes cells to etoposide-induced apoptosis by regulating BH3-only proteins

The Rad9-Rad1-Hus1 (9-1-1) cell cycle checkpoint complex plays a key role in the DNA damage response. Cells with a defective 9-1-1 complex have been shown to be sensitive to apoptosis induced by certain types of genotoxic stress. However, the mechanism linking the loss of a functional 9-1-1 complex to the cell death machinery has yet to be determined. Here, we report that etoposide treatment dramatically upregulates the BH3-only proteins, Bim and Puma, in Hus1-deficient cells. Inhibition of either Bim or Puma expression in Hus1-knockout cells confers significant resistance to etoposide-induced apoptosis, whereas knockdown of both proteins results in further resistance, suggesting that Bim and Puma cooperate in sensitizing Hus1-deficient cells to etoposide treatment. Moreover, we found that Rad9 collaborates with Bim and Puma to sensitize Hus1-deficient cells to etoposide-induced apoptosis. In response to DNA damage, Rad9 localizes to chromatin in Hus1-wild-type cells, whereas in Hus1-deficient cells, it is predominantly located in the cytoplasm where it binds to Bcl-2. Taken together, these results suggest that loss of Hus1 sensitizes cells to etoposide-induced apoptosis not only by inducing Bim and Puma expressions but also by releasing Rad9 into the cytosol to augment mitochondrial apoptosis.

TTK/Mps1 controls nuclear targeting of c-Abl by 14-3-3-coupled phosphorylation in response to oxidative stress

Upon exposure to genotoxic stress, the c-Abl tyrosine kinase is released from cytoplasmic 14-3-3 proteins and then is targeted to the nucleus. Phosphorylation of Thr735 in c-Abl is critical for binding to 14-3-3; however, kinases responsible for this phosphorylation are unknown. Here, we identify CLK1, CLK4, MST1, MST2 and TTK (also known as Mps1) as novel Thr735 kinases in vitro by expression cloning strategy using phosphospecific antibody. We also demonstrate that ectopic expression of these kinases is capable for phosphorylation of Thr735 in cells. Importantly, upon exposure to oxidative stress, phosphorylation of Thr735 is transiently upregulated, and the status of this phosphorylation remains unchanged in cells silenced for CLK1, CLK4, MST1 or MST2. By contrast, knockdown of TTK attenuates phosphorylation of Thr735, suggesting that TTK is a physiological kinase that phosphorylates Thr735. In concert with these results, we show that, in cells silenced for TTK, c-Abl is accumulated in the nucleus even in unstressed condition and no further targeting into the nucleus occurs after oxidative stress. Moreover, nuclear entrapment of c-Abl by knocking down TTK enhances oxidative stress-induced apoptosis. These findings provide evidence that TTK phosphorylates c-Abl at Thr735 and that this phosphorylation is of importance to the cytoplasmic sequestration of c-Abl.

Targeted deletion of Rad9 in mouse skin keratinocytes enhances genotoxin-induced tumor development

The Rad9 gene is evolutionarily conserved from yeast to humans and plays crucial roles in genomic maintenance, DNA repair, and cell cycle checkpoint controls. However, the function of this gene with respect to tumorigenesis is not well-understood. A Rad9-null mutation in mice causes embryonic lethality. In this study, we created mice in which mouse Rad9, Mrad9, was deleted only in keratinocytes to permit examination of the potential function of the gene in tumor development. Mice with Mrad9(+/-) or Mrad9(-/-) keratinocytes showed no overt, spontaneous morphologic defects and seemed similar to wild-type controls. Painting the carcinogen 7,12-dimethylbenzanthracene (DMBA) onto the skin of the animals caused earlier onset and more frequent formation of tumors and senile skin plaques in Mrad9(-/-) mice, compared with Mrad9(+/-) and Mrad9(+/+) littermates. DNA damage response genes p21, p53, and Mrad9B were expressed at higher levels in Mrad9(-/-) relative to Mrad9(+/+) skin. Keratinocytes isolated from Mrad9(-/-) skin had more spontaneous and DMBA-induced DNA double strand breaks than Mrad9(+/+) keratinocytes, and the levels were reduced by incubation with the antioxidant epigallocatechin gallate. These data suggest that Mrad9 plays an important role in maintaining genomic stability and preventing tumor development in keratinocytes.

Nuclear trafficking of pro-apoptotic kinases in response to DNA damage

The cellular response to genotoxic stress includes cell-cycle arrest, activation of DNA repair and induction of apoptosis. However, the signals that determine cell fate are largely unknown. Recent studies have shown that several pro-apoptotic kinases, including protein kinase C (PKC)delta, Abelson murine leukemia viral oncogene homolog 1 (c-Abl) and dual-specificity tyrosine-phosphorylation-regulated kinase 2 (DYRK2), undergo nuclear-cytoplasmic shuttling in response to DNA damage. Importantly, whereas precise regulation for the shuttling of these kinases remains uncertain, this mechanism has consequences for induction of apoptosis and implies that proper localization is central to the function of pro-apoptotic kinases. This review highlights recent progress demonstrating that the nuclear targeting of kinases is a novel and essential regulatory mechanism that directly influences the induction of apoptosis in response to DNA damage. The potential implications for novel therapies are also discussed.

BRCA1-mediated ubiquitination inhibits topoisomerase II alpha activity in response to oxidative stress

Topoisomerase IIalpha is known to be critically involved in both cell proliferation and cell death. The mechanisms responsible for stress-dependent topoisomerase IIalpha alterations, however, remain unclear. This study focused on the behavior of topoisomerase IIalpha in response to oxidative stress induced by hydrogen peroxide (H(2)O(2)). The catalytic activity of topoisomerase IIalpha in MOLT-4 cells treated with H(2)O(2) decreased in parallel with the alteration of topoisomerase IIalpha expression. The ubiquitination of topoisomerase IIalpha was dependent on oxidative stress. BRCA1, a tumor-suppressor gene, appeared to be involved in these alterations in topoisomerase IIalpha. Furthermore, the retinoblastoma protein (pRb) was required for the ubiquitination of topoisomerase IIalpha by BRCA1. We conclude that the functions of topoisomerase IIalpha are regulated by ubiquitination on exposure to oxidative stress.

Programmed cell death in fission yeast Schizosaccharomyces pombe

Yeasts have proven to be invaluable, genetically tractable systems to study various fundamental biological processes including programmed cell death. Recent advances in the elucidation of the molecular pathways underlying apoptotic cell death in yeasts have revealed remarkable similarities to mammalian apoptosis at cellular, organelle and macromolecular levels, thus making a strong case for the relevance of yeast models of regulated cell death. Programmed cell death has been reported in fission yeast Schizosaccharomyces pombe, primarily in the contexts of perturbed intracellular lipid metabolism, defective DNA replication, improper mitotic entry, chronological and replicative aging. Here we review the current understanding of the programmed cell death in fission yeast, paying particular attention to lipid-induced cell death. We discuss our recent findings that fission yeast exhibits plasticity of apoptotic and non-apoptotic modes of cell death in response to different lipid stimuli and growth conditions, and that mitochondria, reactive oxygen species and novel cell death mediators including metacaspase Pca1, SpRad9 and Pck1 are involved in the lipotoxic cell death. We also present perspectives on how various aspects of the cell and molecular biology of this organism can be explored to shed light on the governing principles underlying lipid-mediated signaling and cell demise.

Protein kinase C delta induces transcription of the TP53 tumor suppressor gene by controlling death-promoting factor Btf in the apoptotic response to DNA damage

Expression of the TP53 tumor suppressor is tightly controlled for its ability to function as a critical regulator of cell growth, proliferation, and death in response to DNA damage. However, little is known about the mechanisms and contributions of the transcriptional regulation of TP53. Here we report that protein kinase C delta (PKCdelta), a ubiquitously expressed member of the novel subfamily of PKC isoforms, transactivates TP53 expression at the transcriptional level. Reporter assays demonstrated that PKCdelta induces the promoter activity of TP53 through the TP53 core promoter element (CPE-TP53) and that such induction is enhanced in response to DNA damage. The results also demonstrate that, upon exposure to genotoxic stress, PKCdelta activates and interacts with the death-promoting transcription factor Btf to co-occupy CPE-TP53. Inhibition of PKCdelta activity decreases the affinity of Btf for CPE-TP53, thereby reducing TP53 expression at both the mRNA and the protein levels. In concert with these results, we show that disruption of Btf-mediated TP53 gene transcription by RNA interference leads to suppression of TP53-mediated apoptosis following genotoxic stress. These findings provide evidence that activation of TP53 gene transcription by PKCdelta triggers TP53-dependent apoptosis in response to DNA damage.

The deubiquitinating enzyme USP11 controls an IkappaB kinase alpha (IKKalpha)-p53 signaling pathway in response to tumor necrosis factor alpha (TNFalpha)

Post-translational modification and degradation of proteins by the ubiquitin-proteasome system are key regulatory events in cellular responses to various stimuli. The NF-kappaB signaling pathway is controlled by the ubiquitin-mediated proteolysis. Although mechanisms of ubiquitination in the NF-kappaB pathway have been extensively studied, deubiquitination-mediated regulation of the NF-kappaB signaling remains poorly understood. The present studies show that a deubiquitinating enzyme, USP11, specifically regulates IkappaB kinase alpha (IKKalpha) among the NF-kappaB signaling molecules. Knocking down USP11 attenuates expression of IKKalpha in the transcriptional, but not the post-translational, level. However, down-regulation of USP11 dramatically enhances NF-kappaB activity in response to tumor necrosis factor-alpha, indicating that IKKalpha does not require activation of NF-kappaB. Instead, knock down of USP11 or IKKalpha is associated with abrogation of p53 expression upon exposure to tumor necrosis factor-alpha. In concert with these results, silencing of USP11 is associated with transcriptional attenuation of the p53-responsive genes, such as p21 or Bax. Importantly, the ectopic expression of IKKalpha into cells silenced for USP11 restores p53 expression, demonstrating that USP11 functions as an upstream regulator of an IKKalpha-p53 signaling pathway.

Cockayne syndrome protein B interacts with and is phosphorylated by c-Abl tyrosine kinase

The Cockayne Syndrome group B (CSB) protein plays important roles in transcription, transcription-coupled nucleotide excision repair and base excision DNA repair. c-Abl kinase also plays a role in DNA repair as a regulator/coordinator of the DNA damage response. This study presents evidence that the N-terminal region of CSB interacts with the SH3 domain of c-Abl in vitro and in vivo. In addition, c-Abl kinase phosphorylates CSB at Tyr932. The subcellular localization of CSB to the nucleus and nucleolus is altered after phosphorylation by c-Abl. c-Abl-dependent phosphorylation of CSB increased in cells treated with hydrogen peroxide and decreased in cells pre-treated with STI-571, a c-Abl-specific protein kinase inhibitor. Activation of the c-Abl kinase in response to oxidative damage is not observed in CSB null cells. These results suggest that c-Abl and CSB may regulate each other in a reciprocal manner in response to oxidative stress.

Protein kinase C delta activates IkappaB-kinase alpha to induce the p53 tumor suppressor in response to oxidative stress

Protein kinase C delta (PKCdelta) functions as a redox-sensitive kinase in various cell types. Upon exposure to reactive oxygen species (ROS), it is activated by tyrosine phosphorylation, nuclear translocation and caspase-3-mediated cleavage. Activated PKCdelta is associated with cell cycle arrest or apoptosis, although its precise mechanism of action is unclear. Previous studies have demonstrated that the transcription factor, nuclear factor kappaB (NF-kappaB), functions as a redox-sensitive factor. ROS induce NF-kappaB signaling pathways including upstream IkappaB kinases (IKKs), although the mechanisms of ROS-induced activation of IKKs are unknown. Here we show that both PKCdelta and IKKalpha, but not IKKbeta, translocate to the nucleus in response to oxidative stress. The results also demonstrate that PKCdelta interacts with and activates IKKalpha. Importantly, our data suggest that, upon exposure to oxidative stress, PKCdelta-mediated IKKalpha activation does not contribute to NF-kappaB activation; instead, nuclear IKKalpha regulates the transcription activity of the p53 tumor suppressor by phosphorylation at Ser20. These findings collectively support a novel mechanism in which the PKCdelta-->IKKalpha signaling pathway contributes to ROS-induced activation of the p53 tumor suppressor.

DYRK2 is targeted to the nucleus and controls p53 via Ser46 phosphorylation in the apoptotic response to DNA damage

Genotoxic stress exerts biological activity by activating downstream effectors, including the p53 tumor suppressor. p53 regulates cell-cycle checkpoint and induction of apoptosis in response to DNA damage; however, molecular mechanisms responsible for committing to these distinct functions remain to be elucidated. Recent studies demonstrated that phosphorylation of p53 at Ser46 is associated with induction of p53AIP1 expression, resulting in commitment to apoptotic cell death. In this regard, the role for Ser46 kinases in p53-dependent apoptosis has been established; however, the kinases responsible for Ser46 phosphorylation have yet to be identified. Here, we demonstrate that the dual-specificity tyrosine-phosphorylation-regulated kinase 2 (DYRK2) directly phosphorylates p53 at Ser46. Upon exposure to genotoxic stress, DYRK2 translocates into the nucleus for Ser46 phosphorylation. Consistent with these results, DYRK2 induces p53AIP1 expression and apoptosis in a Ser46 phosphorylation-dependent manner. These findings indicate that DYRK2 regulates p53 to induce apoptosis in response to DNA damage.

PKCdelta signaling: mechanisms of DNA damage response and apoptosis

The cellular response to genotoxic stress that damages DNA includes cell cycle arrest, activation of DNA repair, and in the event of irreparable damage, induction of apoptosis. However, the signals that determine cell fate, that is, survival or apoptosis, are largely unknown. The delta isoform of protein kinase C (PKCdelta) has been implicated in many important cellular processes, including regulation of apoptotic cell death. The available information supports a model in which certain sensors of DNA lesions activate PKCdelta. This activation is triggered in part by tyrosine phosphorylation of PKCdelta by c-Abl tyrosine kinase. PKCdelta is further proteolytically activated by caspase-3. The cleaved catalytic fragment of PKCdelta translocates to the nucleus and induces apoptosis. Importantly, accumulating data have revealed the nuclear targets for PKCdelta in the induction of apoptosis. A pro-apoptotic function of activated PKCdelta is mediated by at least several downstream effectors known to be associated with the elicitation of apoptosis. Recent findings also demonstrated that PKCdelta is involved in cell cycle-specific activation and induction of apoptotic cell death. Moreover, previous studies have shown that PKCdelta regulates transcription by phosphorylating various transcription factors, including the p53 tumor suppressor that is critical for cell cycle arrest and apoptosis in response to DNA damage. These findings collectively support a pivotal role for PKCdelta in the induction of apoptosis with significant impact. This review is focused on the current views regarding the regulation of cell fate by PKCdelta signaling in response to DNA damage.

Cell cycle phenotype-based optimization of G2-abrogating peptides yields CBP501 with a unique mechanism of action at the G2 checkpoint

Cell cycle G(2) checkpoint abrogation is an attractive strategy for sensitizing cancer cells to DNA-damaging anticancer agent without increasing adverse effects on normal cells. However, there is no single proven molecular target for this therapeutic approach. High-throughput screening for molecules inhibiting CHK1, a kinase that is essential for the G(2) checkpoint, has not yet yielded therapeutic G(2) checkpoint inhibitors, and the tumor suppressor phenotypes of ATM and CHK2 suggest they may not be ideal targets. Here, we optimized two G(2) checkpoint-abrogating peptides, TAT-S216 and TAT-S216A, based on their ability to reduce G(2) phase accumulation of DNA-damaged cells without affecting M phase accumulation of cells treated with a microtubule-disrupting compound. This approach yielded a peptide CBP501, which has a unique, focused activity against molecules that phosphorylate Ser(216) of CDC25C, including MAPKAP-K2, C-Tak1, and CHK1. CBP501 is >100-fold more potent than TAT-S216A and retains its selectivity for cancer cells. CBP501 is unusually stable, enters cells rapidly, and increases the cytotoxicity of DNA-damaging anticancer drugs against cancer cells without increasing adverse effects. These findings highlight the potency of CBP501 as a G(2)-abrogating drug candidate. This report also shows the usefulness of the cell cycle phenotype-based protocol for identifying G(2) checkpoint-abrogating compounds as well as the potential of peptide-based compounds as focused multitarget inhibitors.

Rad9, an evolutionarily conserved gene with multiple functions for preserving genomic integrity

The Rad9 gene is evolutionarily conserved. Analysis of the gene from yeast, mouse and human reveal roles in multiple, fundamental biological processes primarily but not exclusively important for regulating genomic integrity. The encoded mammalian proteins participate in promoting resistance to DNA damage, cell cycle checkpoint control, DNA repair, and apoptosis. Other functions include a role in embryogenesis, the transactivation of multiple target genes, co-repression of androgen-induced transcription activity of the androgen receptor, a 3'-5' exonuclease activity, and the regulation of ribonucleotide synthesis. Analyses of the functions of Rad9, and in particular its role in regulating and coordinating numerous fundamental biological activities, should not only provide information about the molecular mechanisms of several individual cellular processes, but might also lend insight into the more global control and coordination of what at least superficially present as independent pathways.

MUC1 oncoprotein blocks nuclear targeting of c-Abl in the apoptotic response to DNA damage

The nonreceptor c-Abl tyrosine kinase binds to cytosolic 14-3-3 proteins and is targeted to the nucleus in the apoptotic response to DNA damage. The MUC1 oncoprotein is overexpressed by most human carcinomas and blocks the induction of apoptosis by genotoxic agents. Using human carcinoma cells with gain and loss of MUC1 function, we show that nuclear targeting of c-Abl by DNA damage is abrogated by a MUC1-dependent mechanism. The results demonstrate that c-Abl phosphorylates MUC1 on Tyr-60 and forms a complex with MUC1 by binding of the c-Abl SH2 domain to the pTyr-60 site. Binding of MUC1 to c-Abl attenuates phosphorylation of c-Abl on Thr-735 and the interaction between c-Abl and cytosolic 14-3-3. We also show that expression of MUC1 with a mutation at Tyr-60 (i) disrupts the interaction between MUC1 and c-Abl, (ii) relieves the MUC1-induced block of c-Abl phosphorylation on Thr-735 and binding to 14-3-3, and (iii) attenuates the MUC1 antiapoptotic function. These findings indicate that MUC1 sequesters c-Abl in the cytoplasm and thereby inhibits apoptosis in the response to genotoxic anticancer agents.

Protein kinase C delta activates topoisomerase IIalpha to induce apoptotic cell death in response to DNA damage

DNA topoisomerase II is an essential nuclear enzyme that modulates DNA processes by altering the topological state of double-stranded DNA. This enzyme is required for chromosome condensation and segregation; however, the regulatory mechanism of its activation is largely unknown. Here we demonstrate that topoisomerase IIalpha is activated in response to genotoxic stress. Concomitant with the activation, the expression of topoisomerase IIalpha is increased following DNA damage. The results also demonstrate that the proapoptotic kinase protein kinase C delta (PKCdelta) interacts with topoisomerase IIalpha. This association is in an S-phase-specific manner and is required for stabilization and catalytic activation of topoisomerase IIalpha in response to DNA damage. Conversely, inhibition of PKCdelta activity attenuates DNA damage-induced activation of topoisomerase IIalpha. Finally, aberrant activation of topoisomerase IIalpha by PKCdelta is associated with induction of apoptosis upon exposure to genotoxic agents. These findings indicate that PKCdelta regulates topoisomerase IIalpha and thereby cell fate in the genotoxic stress response.

His239Arg SNP of HRAD9 is associated with lung adenocarcinoma

BACKGROUND

It was previously reported that a functional human (h) Rad9 protein accumulated in the nuclei of non-small cell lung carcinoma (NSCLC) cells. Those experiments, however, did not examine whether the hRad9 gene was mutated in those cells. The sequence of the HRAD9 gene in NSCLC cells was investigated.

METHODS

The sequence of the HRAD9 was examined in tumor and peripheral normal lung tissues obtained from 50 lung adenocarcinoma patients during surgery. The expression of its mRNA using reverse transcription polymerase chain reaction (RT-PCR) was also examined.

RESULTS

No sequence alterations were detected in the HRAD9 gene, which was found to be normally transcribed in surgically resected lung carcinoma cells. However, in eight (16.0%) cases a single nucleotide polymorphism (SNP) was observed at the second position of codon 239 (His/Arg heterozygous variant) of the gene. This frequency was significantly higher than that found in the normal population.

CONCLUSIONS

Whereas the capacity to produce a functional hRad9 protein was intact in lung adenocarcinoma cells, a nonsynonymous SNP of HRAD9 was detected that might be associated with the development of lung adenocarcinoma.

Protein kinase C delta regulates Ser46 phosphorylation of p53 tumor suppressor in the apoptotic response to DNA damage

The p53 tumor suppressor is activated in the cellular response to genotoxic stress. Transactivation of p53 target genes dictates cell cycle arrest and DNA repair or induction of apoptosis; however, a molecular mechanism responsible for these distinct functions remains unclear. Recent studies revealed that phosphorylation of p53 on Ser(46) was associated with induction of p53AIP1 expression, resulting in the commitment of the cell fate into apoptotic cell death. Moreover, upon exposure to genotoxic stress, p53DINP1 was expressed and recruited a kinase(s) to p53 that specifically phosphorylated Ser(46). Here, we show that the pro-apoptotic kinase, protein kinase C delta (PKCdelta), is involved in phosphorylation of p53 on Ser(46). PKCdelta-mediated phosphorylation is required for the interaction of PKCdelta with p53. The results also demonstrate that p53DINP1 associates with PKCdelta upon exposure to genotoxic agents. Consistent with these results, PKCdelta potentiates p53-dependent apoptosis by Ser(46) phosphorylation in response to genotoxic stress. These findings indicate that PKCdelta regulates p53 to induce apoptotic cell death in the cellular response to DNA damage.

Interaction and colocalization of Rad9/Rad1/Hus1 checkpoint complex with replication protein A in human cells

Replication protein A (RPA) is a eukaryotic single-stranded DNA-binding protein consisting of three subunits of 70-, 32-, and 14-kDa (RPA70, RPA32, RPA14, respectively). It is a protein essential for most cellular DNA metabolic pathways. Checkpoint proteins Rad9, Rad1, and Hus1 form a clamp-like complex which plays a central role in the DNA damage-induced checkpoint response. In this report, we presented the evidence that Rad9-Rad1-Hus1 (9-1-1) complex directly interacted with RPA in human cells, and this interaction was mediated by the binding of Rad9 protein to both RPA70 and RPA32 subunits. In addition, the cellular interaction of 9-1-1 with RPA or hyperphosphorylated RPA was stimulated by UV irradiation or camptothecin treatment in a dose-dependent manner. Such treatments also resulted in the colocalization of the nuclear foci formed with the two complexes. Consistently, knockdown of the RPA expression in cells by the small interference RNA (siRNA) blocked the DNA damage-dependent chromatin association of 9-1-1, and also inhibited the 9-1-1 complex formation. Taken together, our results suggest that 9-1-1 and RPA complexes collaboratively function in DNA damage responses, and that the RPA may serve as a regulator for the activity of 9-1-1 complex in the cellular checkpoint network.

Frag1, a homolog of alternative replication factor C subunits, links replication stress surveillance with apoptosis

We report the identification and characterization of a potent regulator of genomic integrity, mouse and human FRAG1 gene, a conserved homolog of replication factor C large subunit that is homologous to the alternative replication factor C subunits Elg1, Ctf18/Chl12, and Rad24 of budding yeast. FRAG1 was identified in a search for key caretaker genes involved in the regulation of genomic stability under conditions of replicative stress. In response to stress, Atr participates in the down-regulation of FRAG1 expression, leading to the induction of apoptosis through the release of Rad9 from damaged chromatin during the S phase of the cell cycle, allowing Rad9-Bcl2 association and induction of proapoptotic Bax protein. We propose that the Frag1 signal pathway, by linking replication stress surveillance with apoptosis induction, plays a central role in determining whether DNA damage is compatible with cell survival or whether it requires cell elimination by apoptosis.

JNK phosphorylation of 14-3-3 proteins regulates nuclear targeting of c-Abl in the apoptotic response to DNA damage

The ubiquitously expressed c-Abl tyrosine kinase localizes to the cytoplasm and nucleus. Nuclear c-Abl is activated by diverse genotoxic agents and induces apoptosis; however, the mechanisms that are responsible for nuclear targeting of c-Abl remain unclear. Here, we show that cytoplasmic c-Abl is targeted to the nucleus in the DNA damage response. The results show that c-Abl is sequestered into the cytoplasm by binding to 14-3-3 proteins. Phosphorylation of c-Abl on Thr 735 functions as a site for direct binding to 14-3-3 proteins. We also show that, in response to DNA damage, activation of the c-Jun N-terminal kinase (Jnk) induces phosphorylation of 14-3-3 proteins and their release from c-Abl. Together with these results, expression of an unphosphorylated 14-3-3 mutant attenuates DNA-damage-induced nuclear import of c-Abl and apoptosis. These findings indicate that 14-3-3 proteins are pivotal regulators of intracellular c-Abl localization and of the apoptotic response to genotoxic stress.

Human Rad9 is required for the activation of S-phase checkpoint and the maintenance of chromosomal stability

In response to DNA damage or replication block, cells activate a battery of checkpoint signaling cascades to control cell cycle progression and elicit DNA repair in order to maintain genomic stability and integrity. Identified as a homolog of its fission yeast counterpart, human Rad9 was proposed to form a Rad9-Hus1-Rad1 protein complex to mediate checkpoint signals. However, the precise function of Rad9 in the process of checkpoint activation is not fully understood. Using the RNA interference technique, we investigated the role of Rad9 in the genotoxic stress-induced activation of S-phase checkpoint and the maintenance of chromosomal stability. We found that Rad9 knockdown reduced the phosphorylation of Rad17, Chk1 and Smc1 in response to DNA replication block and certain types of DNA damage. Immunofluorescence studies showed that the removal of Rad9 disrupted the foci formation of phosphorylated Chk1, but not ATR. Moreover, Rad9 knockdown resulted in radioresistant DNA synthesis and reduced cell viability under replication stress. Finally, removal of Rad9 by RNAi led to increased accumulation of spontaneous chromosomal aberrations. Taken together, these results suggest a critical and specific role of Rad9 in the activation of S-phase checkpoint and the maintenance of chromosome stability.