Molecular genetic analysis of VRK1 in mammary epithelial cells: depletion slows proliferation in vitro and tumor growth and metastasis in vivo

Nuclear functions regulated by the VRK1 kinase

In the nucleus, the VRK1 Ser-Thr kinase is distributed in nucleoplasm and chromatin, where it has different roles. VRK1 expression increases in response to mitogenic signals. VRK1 regulates cyclin D1 expression at G0 exit and facilitates chromosome condensation at the end of G2 and G2/M progression to mitosis. These effects are mediated by the phosphorylation of histone H3 at Thr3 by VRK1, and later in mitosis by haspin. VRK1 regulates the apigenetic patterns of histones in processes requiring chromating remodeling, such as transcription, replication and DNA repair. VRK1 is overexpressed in tumors, facilitating tumor progression and resistance to genotoxic treatments. VRK1 also regulates the organization of Cajal bodies assembled on coilin, which are necessary for the assembly of different types of RNP complexes. VRK1 pathogenic variants cuase defects in Cajal bodies, functionally altering neurons with long axons and leading to neurological diseases, such as amyotrophic laterla sclerosis, spinal muscular atrophy, distal hereditay motor neuropathies and Charcot-Marie-Tooth.

Novel Dihydropteridinone Derivatives As Potent Inhibitors of the Understudied Human Kinases Vaccinia-Related Kinase 1 and Casein Kinase 1δ/ε

Vaccinia-related kinase 1 (VRK1) and the δ and ε isoforms of casein kinase 1 (CK1) are linked to various disease-relevant pathways. However, the lack of tool compounds for these kinases has significantly hampered our understanding of their cellular functions and therapeutic potential. Here, we describe the structure-based development of potent inhibitors of VRK1, a kinase highly expressed in various tumor types and crucial for cell proliferation and genome integrity. Kinome-wide profiling revealed that our compounds also inhibit CK1δ and CK1ε. We demonstrate that dihydropteridinones 35 and 36 mimic the cellular outcomes of VRK1 depletion. Complementary studies with existing CK1δ and CK1ε inhibitors suggest that these kinases may play overlapping roles in cell proliferation and genome instability. Together, our findings highlight the potential of VRK1 inhibition in treating p53-deficient tumors and possibly enhancing the efficacy of existing cancer therapies that target DNA stability or cell division.

Polarization-sensitive optical coherence tomography for renal tumor detection in ex vivo human kidneys

Kidney cancer is a kind of high mortality cancer because of the difficulty in early diagnosis and the high metastatic dissemination in treatments. The surgical resection of tumors is the most effective treatment for renal cancer patients. However, precise assessment of tumor margins is a challenge during surgical resection. The objective of this study is to demonstrate an optical imaging tool in precisely distinguishing kidney tumor borders and identifying tumor zones from normal tissues to assist surgeons in accurately resecting tumors from kidneys during the surgery. 30 samples from six human kidneys were imaged using polarization-sensitive optical coherence tomography (PS-OCT). Cross-sectional, enface, and spatial information of kidney samples were obtained for microenvironment reconstruction. Polarization parameters (phase retardation, optic axis direction, and degree of polarization uniformity (DOPU) and Stokes parameters (Q, U, and V) were utilized for multi-parameter analysis. To verify the detection accuracy of PS-OCT, H&E histology staining and dice-coefficient was utilized to quantify the performance of PS-OCT in identifying tumor borders and regions. In this study, tumor borders were clearly identified by PS-OCT imaging, which outperformed the conventional intensity-based OCT. With H&E histological staining as golden standard, PS-OCT precisely identified the tumor regions and tissue distributions at different locations and different depths based on polarization and Stokes parameters. Compared to the traditional attenuation coefficient quantification method, PS-OCT demonstrated enhanced contrast of tissue characteristics between normal and cancerous tissues due to the birefringence effects. Our results demonstrated that PS-OCT was promising to provide imaging guidance for the surgical resection of kidney tumors and had the potential to be used for other human kidney surgeries in clinics such as renal biopsy.

Elucidating the interaction of C-terminal domain of Vaccinia-Related Kinase 2A (VRK2A) with B-cell lymphoma-extra Large (Bcl-xL) to decipher its anti-apoptotic role in cancer

Vaccinia-Related Kinase 2 (VRK2) is an anti-apoptotic Ser/Thr kinase that enhances drug sensitivity in cancer cells. This protein exists in two isoforms: VRK2A, the longer variant, and VRK2B, which lacks the C-terminal region and transmembrane domain. While the therapeutic importance of VRK2 family proteins is known, the specific roles of VRK2A and its interplay with apoptotic regulator Bcl-xL (B-cell lymphoma-extra Large) remain elusive. Bcl-xL regulates cell death by interacting with BAX (B-cell lymphoma-2 Associated X-protein), controlling its cellular localization and influencing BAX-associated processes and signaling pathways. As VRK2A interacts with the Bcl-xL-BAX complex, comprehending its regulatory engagement with Bcl-xL presents potential avenues for intervening in diseases. Using a multi-disciplinary approach, this study provides information on the cellular localization of VRK2A and establishes its interaction with Bcl-xL in the cellular milieu, pinpointing the interacting site and elucidating its anti-apoptotic property within the complex. Furthermore, this study also put forth a model that highlights the importance of VRK2A in stabilizing the ternary complex, formed with Bcl-xL and BAX, thereby impeding BAX dissociation and hence apoptosis. Therefore, further investigations associated with this important revelation will provide cues for designing cancer therapeutics in the future.

VRK1 variants at the cross road of Cajal body neuropathogenic mechanisms in distal neuropathies and motor neuron diseases

Distal hereditary neuropathies and neuro motor diseases are complex neurological phenotypes associated with pathogenic variants in a large number of genes, but in some the origin is unknown. Recently, rare pathogenic variants of the human VRK1 gene have been associated with these neurological phenotypes. All VRK1 pathogenic variants are recessive, and their clinical presentation occurs in either homozygous or compound heterozygous patients. The pathogenic VRK1 gene pathogenic variants are located in three clusters within the protein sequence. The main, and initial, shared clinical phenotype among VRK1 pathogenic variants is a distal progressive loss of motor and/or sensory function, which includes diseases such as spinal muscular atrophy, Charcot-Marie-Tooth, amyotrophic lateral sclerosis and hereditary spastic paraplegia. In most cases, symptoms start early in infancy, or in utero, and are slowly progressive. Additional neurological symptoms vary among non-related patients, probably because of their different VRK1 variants and their genetic background. The underlying common pathogenic mechanism, by its functional impairment, is a likely consequence of the roles that the VRK1 protein plays in the regulation on the stability and assembly of Cajal bodies, which affect RNA maturation and processing, neuronal migration of RNPs along axons, and DNA-damage responses. Alterations of these processes are associated with several neuro sensory or motor syndromes. The clinical heterogeneity of the neurological phenotypes associated with VRK1 is a likely consequence of the protein complexes in which VRK1 is integrated, which include several proteins known to be associated with Cajal bodies and DNA damage responses. Several hereditary distal neurological diseases are a consequence of pathogenic variants in genes that alter these cellular functions. We conclude that VRK1-related distal hereditary neuropathies and motor neuron diseases represent a novel subgroup of Cajal body related neurological syndromes.

Dissecting the roles of Haspin and VRK1 in histone H3 phosphorylation during mitosis

Protein kinases that phosphorylate histones are ideally-placed to influence the behavior of chromosomes during cell division. Indeed, a number of conserved histone phosphorylation events occur prominently during mitosis and meiosis in most eukaryotes, including on histone H3 at threonine-3 (H3T3ph). At least two kinases, Haspin and VRK1 (NHK-1/ballchen in Drosophila), have been proposed to carry out this modification. Phosphorylation of H3 by Haspin has defined roles in mitosis, but the significance of VRK1 activity towards histones in dividing cells has been unclear. Here, using in vitro kinase assays, KiPIK screening, RNA interference, and CRISPR/Cas9 approaches, we were unable to substantiate a direct role for VRK1, or its paralogue VRK2, in the phosphorylation of threonine-3 or serine-10 of Histone H3 in mitosis, although loss of VRK1 did slow cell proliferation. We conclude that the role of VRKs, and their more recently identified association with neuromuscular disease and importance in cancers of the nervous system, are unlikely to involve mitotic histone kinase activity. In contrast, Haspin is required to generate H3T3ph during mitosis.

UV Irradiation of Vaccinia Virus-Infected Cells Impairs Cellular Functions, Introduces Lesions into the Viral Genome, and Uncovers Repair Capabilities for the Viral Replication Machinery

Vaccinia virus (VV), the prototypic poxvirus, encodes a repertoire of proteins responsible for the metabolism of its large dsDNA genome. Previous work has furthered our understanding of how poxviruses replicate and recombine their genomes, but little is known about whether the poxvirus genome undergoes DNA repair. Our studies here are aimed at understanding how VV responds to exogenous DNA damage introduced by UV irradiation. Irradiation of cells prior to infection decreased protein synthesis and led to an ∼12-fold reduction in viral yield. On top of these cell-specific insults, irradiation of VV infections at 4 h postinfection (hpi) introduced both cyclobutene pyrimidine dimer (CPD) and 6,4-photoproduct (6,4-PP) lesions into the viral genome led to a nearly complete halt to further DNA synthesis and to a further reduction in viral yield (∼35-fold). DNA lesions persisted throughout infection and were indeed present in the genomes encapsidated into nascent virions. Depletion of several cellular proteins that mediate nucleotide excision repair (XP-A, -F, and -G) did not render viral infections hypersensitive to UV. We next investigated whether viral proteins were involved in combatting DNA damage. Infections performed with a virus lacking the A50 DNA ligase were moderately hypersensitive to UV irradiation (∼3-fold). More strikingly, when the DNA polymerase inhibitor cytosine arabinoside (araC) was added to wild-type infections at the time of UV irradiation (4 hpi), an even greater hypersensitivity to UV irradiation was seen (∼11-fold). Virions produced under the latter condition contained elevated levels of CPD adducts, strongly suggesting that the viral polymerase contributes to the repair of UV lesions introduced into the viral genome. IMPORTANCE Poxviruses remain of significant interest because of their continuing clinical relevance, their utility for the development of vaccines and oncolytic therapies, and their illustration of fundamental principles of viral replication and virus/cell interactions. These viruses are unique in that they replicate exclusively in the cytoplasm of infected mammalian cells, providing novel challenges for DNA viruses. How poxviruses replicate, recombine, and possibly repair their genomes is still only partially understood. Using UV irradiation as a form of exogenous DNA damage, we have examined how vaccinia virus metabolizes its genome following insult. We show that even UV irradiation of cells prior to infection diminishes viral yield, while UV irradiation during infection damages the genome, causes a halt in DNA accumulation, and reduces the viral yield more severely. Furthermore, we show that viral proteins, but not the cellular machinery, contribute to a partial repair of the viral genome following UV irradiation.

Structure-Function Analysis of Two Interacting Vaccinia Proteins That Are Critical for Viral Morphogenesis: L2 and A30.5

An enduring mystery in poxvirology is the mechanism by which virion morphogenesis is accomplished. A30.5 and L2 are two small regulatory proteins that are essential for this process. Previous studies have shown that vaccinia A30.5 and L2 localize to the ER and interact during infection, but how they facilitate morphogenesis is unknown. To interrogate the relationship between A30.5 and L2, we generated inducible complementing cell lines (CV1-HA-L2; CV1-3xFLAG-A30.5) and deletion viruses (vΔL2; vΔA30.5). Loss of either protein resulted in a block in morphogenesis and a significant (>100-fold) decrease in infectious viral yield. Structure-function analysis of L2 and A30.5, using transient complementation assays, identified key functional regions in both proteins. A clustered charge-to-alanine L2 mutant (L2-RRD) failed to rescue a vΔL2 infection and exhibits a significantly retarded apparent molecular weight in vivo (but not in vitro), suggestive of an aberrant posttranslational modification. Furthermore, an A30.5 mutant with a disrupted putative N-terminal α-helix failed to rescue a vΔA30.5 infection. Using our complementing cell lines, we determined that the stability of A30.5 is dependent on L2 and that wild-type L2 and A30.5 coimmunoprecipitate in the absence of other viral proteins. Further examination of this interaction, using wild-type and mutant forms of L2 or A30.5, revealed that the inability of mutant alleles to rescue the respective deletion viruses is tightly correlated with a failure of L2 to stabilize and interact with A30.5. L2 appears to function as a chaperone-like protein for A30.5, ensuring that they work together as a complex during viral membrane biogenesis. IMPORTANCE Vaccinia virus is a large, enveloped DNA virus that was successfully used as the vaccine against smallpox. Vaccinia continues to be an invaluable biomedical research tool in basic research and in gene therapy vector and vaccine development. Although this virus has been studied extensively, the complex process of virion assembly, termed morphogenesis, still puzzles the field. Our work aims to better understand how two small viral proteins that are essential for viral assembly, L2 and A30.5, function during early morphogenesis. We show that A30.5 requires L2 for stability and that these proteins interact in the absence of other viral proteins. We identify regions in each protein required for their function and show that mutations in these regions disrupt the interaction between L2 and A30.5 and fail to restore virus viability.

DNA Aptamers against Vaccinia-Related Kinase (VRK) 1 Block Proliferation in MCF7 Breast Cancer Cells

Vaccinia-related kinase (VRK) 1 is a serin/threonine kinase that plays an important role in DNA damage response (DDR), phosphorylating some proteins involved in this process such as 53BP1, NBS1 or H2AX, and in the cell cycle progression. In addition, VRK1 is overexpressed in many cancer types and its correlation with poor prognosis has been determined, showing VRK1 as a new therapeutic target in oncology. Using in vitro selection, high-affinity DNA aptamers to VRK1 were selected from a library of ssDNA. Selection was monitored using the enzyme-linked oligonucleotide assay (ELONA), and the selected aptamer population was cloned and sequenced. Three aptamers were selected and characterized. These aptamers recognized the protein kinase VRK1 with an affinity in the nanomolar range and showed a high sensibility. Moreover, the treatment of the MCF7 breast cell line with these aptamers resulted in a decrease in cyclin D1 levels, and an inhibition of cell cycle progression by G1 phase arrest, which induced apoptosis in cells. These results suggest that these aptamers are specific inhibitors of VRK1 that might be developed as potential drugs for the treatment of cancer.

Downregulation of VRK1 reduces the expression of BANF1 and suppresses the proliferative and migratory activity of esophageal cancer cells

Esophageal squamous cell carcinoma (ESCC) is a common malignancy worldwide. The disease has a poor prognosis and a low 5-year survival rate. Therefore, it is necessary to identify new strategies to optimize the treatment of ESCC. Vaccinia-related kinase (VRK1) and barrier-to-autointegration factor 1 (BANF1) are overexpressed in ESCC. In the present study, the roles of VRK1 and BANF1 were explored in the development of ESCC. In the present study, the effects of small interfering (si)RNA-induced downregulation of VRK1 on BANF1 expression were investigated as well as the effects on proliferative and migratory activity of ESCC cells. Western blot analysis indicated that the protein expression levels of BANF1 were decreased following siRNA depletion of VRK1. Furthermore, the depletion of VRK1 expression inhibited the proliferation and migration of ESCC cell lines, and flow cytometry analysis indicated that the depletion of VRK1 triggered cell cycle arrest mainly in the S phase. These results suggested that VRK1 and BANF1 may have pivotal roles in the progression of ESCC.

Protein Expression Profile in Rat Silicosis Model Reveals Upregulation of PTPN2 and Its Inhibitory Effect on Epithelial-Mesenchymal Transition by Dephosphorylation of STAT3

Silicosis is a chronic occupational lung disease caused by long-term inhalation of crystalline silica particulates. We created a rat model that closely approximates the exposure and development of silicosis in humans. Isobaric tags for relative and absolute quantitation (iTRAQ) technologies we used to identify proteins differentially expressed in activated rat lung tissue. We constructed three lentiviral knockdown vectors and an overexpression vector for the protein tyrosine phosphatase non-receptor type 2 (PTPN2) gene to achieve stable long-term expression. A total of 471 proteins were differentially expressed in the silicosis group compared with controls. Twenty upregulated, and eight downregulated proteins exhibited a ≥1.5-fold change relative to controls. We next found that the PTPN2, Factor B, and VRK1 concentrations in silicotic rats silicosis and SiO₂-stimulated MLE-12 cells were significantly higher than control groups. More importantly, we found that overexpression of PTPN2 simultaneously decreased the expression of phospho–signal transducer and activator of transcription 3 (p-STAT3) and Vimentin, while increasing E-cadherin expression. The opposite pattern was observed for PTPN2-gene silencing. We identified three proteins with substantially enhanced expression in silicosis. Our study also showed that PTPN2 can inhibit epithelial-mesenchymal transition by dephosphorylating STAT3 in silicosis fibrosis.

Ras-PI3K pathway promotes osteosarcoma progression via regulating VRK1-mediated H2A phosphorylation at threonine 120

Background: Ras-PI3K pathway aberrant activation plays an important role in the occurrence and development of osteosarcoma. This study investigated the functions of Ras-PI3K pathway specific activation on histone H2A phosphorylation at threonine 120 (H2A^T120ph) in osteosarcoma cells, along with the possible internal molecular mechanisms.Methods: Cell transfection was done to alter Ras^G12V/Y40C, H2A^T120ph and vaccinia-related kinase 1 (VRK1) expression. Then, cell viability, proliferation, migration and cell cycle distribution were assessed, respectively. qRT-PCR was utilized to measure the VRK1 and Ras-PI3K pathway downstream genes (CYR61, IGFBP3, WNT16B, NT5E, GDF15 and CARD16) expression. Chromatin immunoprecipitation (ChIP) was conducted to evaluate the input levels of H2A^T120ph and VRK1 in the promoter regions of Ras-PI3K pathway downstream genes.Results: Ras-PI3K specific activation promoted histone H2A^T120ph. H2A^T120ph participated in the oncogenic functions of Ras-PI3K pathway on osteosarcoma by modulating the transcription of Ras-PI3K-targeted genes. Moreover, VRK1 contributed to the Ras-PI3K specific activation-induced up-regulation of H2A^T120ph and osteosarcoma progression. Ras-PI3K pathway-specific activation-induced up-regulation of H2A^T120ph was achieved by up-regulation of VRK1.Conclusions: Ras-PI3K pathway activation promoted osteosarcoma progression might be via up-regulating VRK1-mediated H2A^T120ph. We proposed that VRK1 and H2A^T120ph could be the potential targets for osteosarcoma diagnosis and treatment.HighlightsH2A^T120ph is specifically promoted by Ras-PI3K pathway activation.H2A^T120ph joins in the oncogenic effects of Ras-PI3K pathway on osteosarcoma.H2A^T120ph regulates the transcription of Ras-PI3K-targeted genes.VRK1 takes part in the regulatory function of Ras-PI3K pathway on H2A^T120ph.

Ginsenoside Rg3 regulates DNA damage in non-small cell lung cancer cells by activating VRK1/P53BP1 pathway

Lung cancer is the leading cause of cancer-related deaths. Ginsenoside Rg3 is the main ingredient of Ginseng which is used to treat non-small cell lung cancer (NSCLC). It has been found to enhance the efficiency of chemotherapy thereby reducing its side effects. Previous studies found that ginsenoside Rg3 can reduce the occurrence of NSCLC by inducing DNA damage. Yet, its anti-DNA damaging effects and mechanisms in tumor cells are still not fully understood. This study explored the effect of ginsenoside Rg3 on DNA repair and VRK1/P53BP1 signaling pathway. Ginsenoside Rg3 treatment significantly decreased the incidence and invasionin a mouse model of lung cancer induced by urethane. The results of cell survival assay and single cell gel electrophoresis showed that ginsenoside Rg3 protected lung adenocarcinoma cells from DNA damage as well as inhibited the proliferation of tumor cells. Ginsenoside Rg3 increased the mRNA and protein expression of VRK1 in NSCLC cells as measured by RT-qPCR and western blot, respectively. These findings suggests that ginsenoside Rg3 regulates VRK1 signaling. Immunofluorescence assays showed that P53BP1 and VRK1 protein level increased, and the VRK1 protein translocated between the nuclei and cytoplasm. Finally, this conclusion was confirmed by the reverse validation in VRK1-knockdown cells. Taken together, these results show that ginsenoside Rg3 upregulate VRK1 expression and P53BP1 foci formation in response to DNA damage thereby inhibiting the tumorigenesis and viability of cancer cells. These findings reveal the role of Rg3 in lung cancer and provides therapeutic targets for developing new drugs in the prevention and treatment of lung cancer.

Olaparib and ionizing radiation trigger a cooperative DNA-damage repair response that is impaired by depletion of the VRK1 chromatin kinase

Background

The VRK1 chromatin kinase regulates the organization of locally altered chromatin induced by DNA damage. The combination of ionizing radiation with inhibitors of DNA damage responses increases the accumulation of DNA damage in cancer cells, which facilitates their antitumor effect, a process regulated by VRK1.

Methods

Tumor cell lines with different genetic backgrounds were treated with olaparib to determine their effect on the activation of DNA repair pathways induced by ionizing radiation. The effect of combining olaparib with depletion of the chromatin kinase VRK1 was studied in the context of double-strand breaks repair pathway after treatment with ionizing radiation. The initiation and progression of DDR were studied by specific histone acetylation, as a marker of local chromatin relaxation, and formation of γH2AX and 53BP1 foci.

Results

In this work, we have studied the effect that VRK1 by itself or in collaboration with olaparib, an inhibitor of PARP, has on the DNA oxidative damage induced by irradiation in order to identify its potential as a new drug target. The combination of olaparib and ionizing radiation increases DNA damage permitting a significant reduction of their respective doses to achieve a similar amount of DNA damage detected by γH2AX and 53BP1 foci. Different treatment combinations of olaparib and ionizing radiation permitted to reach the maximum level of DNA damage at lower doses of both treatments. Furthermore, we have studied the effect that depletion of the VRK1 chromatin kinase, a regulator of DDR, has on this response. VRK1 knockdown impaired all steps in the DDR induced by these treatments, which were detected by a reduction of sequential markers such as H4K16 ac, γH2AX, NBS1 and 53BP1. Moreover, this effect of VRK1 is independent of TP53 or ATM, two genes frequently mutated in cancer.

Conclusion

The protective DNA damage response induced by ionizing radiation is impaired by the combination of olaparib with depletion of VRK1, and can be used to reduce doses of radiation and their associated toxicity. Proteins implicated in DNA damage responses are suitable targets for development of new therapeutic strategies and their combination can be an alternative form of synthetic lethality.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1204-1) contains supplementary material, which is available to authorized users.

A poxvirus pseudokinase represses viral DNA replication via a pathway antagonized by its paralog kinase

Poxviruses employ sophisticated, but incompletely understood, signaling pathways that engage cellular defense mechanisms and simultaneously ensure viral factors are modulated properly. For example, the vaccinia B1 protein kinase plays a vital role in inactivating the cellular antiviral factor BAF, and likely orchestrates other pathways as well. In this study, we utilized experimental evolution of a B1 deletion virus to perform an unbiased search for suppressor mutations and identify novel pathways involving B1. After several passages of the ΔB1 virus we observed a robust increase in viral titer of the adapted virus. Interestingly, our characterization of the adapted viruses reveals that mutations correlating with a loss of function of the vaccinia B12 pseudokinase provide a striking fitness enhancement to this virus. In support of predictions that reductive evolution is a driver of poxvirus adaptation, this is clear experimental evidence that gene loss can be of significant benefit. Next, we present multiple lines of evidence demonstrating that expression of full length B12 leads to a fitness reduction in viruses with a defect in B1, but has no apparent impact on wild-type virus or other mutant poxviruses. From these data we infer that B12 possesses a potent inhibitory activity that can be masked by the presence of the B1 kinase. Further investigation of B12 attributes revealed that it primarily localizes to the nucleus, a characteristic only rarely found among poxviral proteins. Surprisingly, BAF phosphorylation is reduced under conditions in which B12 is present in infected cells without B1, indicating that B12 may function in part by enhancing antiviral activity of BAF. Together, our studies of B1 and B12 present novel evidence that a paralogous kinase-pseudokinase pair can exhibit a unique epistatic relationship in a virus, perhaps serving to enhance B1 conservation during poxvirus evolution and to orchestrate yet-to-be-discovered nuclear events during infection.

Crystal structure of human vaccinia-related kinase 1 in complex with AMP-PNP, a non-hydrolyzable ATP analog

Vaccinia-related kinase 1 (VRK1), a serine/threonine mitotic kinase, is widely over-expressed in dividing cells and regarded as a cancer drug target primarily due to its function as an early response gene in cell proliferation. However, the mechanism of VRK1 phosphorylation and substrate activation is not well understood. More importantly even the molecular basis of VRK1 interaction with its cofactor, adenosine triphosphate (ATP), is unavailable to-date. As designing specific inhibitors remains to be the major challenge in kinase research, such a molecular understanding will enable us to design ATP-competitive specific inhibitors of VRK1. Here we report the molecular characterization of VRK1 in complex with AMP-PNP, a non-hydrolyzable ATP-analog, using NMR titration followed by the co-crystal structure determined upto 2.07 Å resolution. We also carried out the structural comparison of the AMP-PNP bound-form with its apo and inhibitor-bound counterparts, which has enabled us to present our rationale toward designing VRK1-specific inhibitors.

Overexpression of the VRK1 kinase, which is associated with breast cancer, induces a mesenchymal to epithelial transition in mammary epithelial cells

Vaccinia-related kinase 1 (VRK1) is a pro-proliferative nuclear kinase. Mice engrafted with VRK1-depleted MDA-MB-231 breast cancer cells have been shown to develop fewer distal metastases than controls, suggesting VRK1 might play a role in cell migration, invasion, and/or colonization. In work described herein, we investigated the impact of VRK1 overexpression on human mammary epithelial cells. In 2D culture, VRK1 overexpression diminishes cell migration and invasion and impairs the migration-associated processes of cell spreading and cytoskeletal rearrangement. VRK1-overexpressing cells show reduced accumulation of the mesenchymal marker vimentin and increased accumulation of the epithelial markers E-cadherin and claudin-1. VRK1 overexpression also leads to reduced levels of the transcriptional repressors snail, slug, and twist1. Cumulatively, these data indicate that VRK1 overexpression augments the epithelial properties of both MCF10a and MDA-MB-231 cells. We further studied the impact of VRK1 on the epithelial properties of MCF10a cells in 3D matrigel culture, in which cells proliferate and form epithelial sheets that mature into hollow spherical acini. VRK1 overexpression significantly accelerates the initial stages of cell proliferation, leading to larger acini that nevertheless differentiate and mature. Our analysis of human tumor tissue microarrays (TMAs) revealed that VRK1 protein levels are higher in lymph node metastases than in patient-matched mammary tumors. Using public databases, we determined that VRK1 is among the top 10% of overexpressed transcripts in multiple subtypes of invasive breast cancer, and that high levels of VRK1 expression are correlated with decreased relapse-free survival. In sum, overexpression of VRK1, by regulating the transcription repressors snail, slug, and twist1, can promote a mesenchymal-to-epithelial transition (MET) in cell culture. VRK1-mediated MET might facilitate the colonization of distal sites by metastatic breast cancer cells, providing some insight into the frequent association of VRK1 overexpression with breast malignancies and the correlation between VRK1 overexpression and poor clinical outcome.

Implication of the VRK1 chromatin kinase in the signaling responses to DNA damage: a therapeutic target?

DNA damage causes a local distortion of chromatin that triggers the sequential processes that participate in specific DNA repair mechanisms. This initiation of the repair response requires the involvement of a protein whose activity can be regulated by histones. Kinases are candidates to regulate and coordinate the connection between a locally altered chromatin and the response initiating signals that lead to identification of the type of lesion and the sequential steps required in specific DNA damage responses (DDR). This initiating kinase must be located in chromatin, and be activated independently of the type of DNA damage. We review the contribution of the Ser-Thr vaccinia-related kinase 1 (VRK1) chromatin kinase as a new player in the signaling of DNA damage responses, at chromatin and cellular levels, and its potential as a new therapeutic target in oncology. VRK1 is involved in the regulation of histone modifications, such as histone phosphorylation and acetylation, and in the formation of γH2AX, NBS1 and 53BP1 foci induced in DDR. Induction of DNA damage by chemotherapy or radiation is a mainstay of cancer treatment. Therefore, novel treatments can be targeted to proteins implicated in the regulation of DDR, rather than by directly causing DNA damage.

Structural characterization of human Vaccinia-Related Kinases (VRK) bound to small-molecule inhibitors identifies different P-loop conformations

The human genome encodes two active Vaccinia-related protein kinases (VRK), VRK1 and VRK2. These proteins have been implicated in a number of cellular processes and linked to a variety of tumors. However, understanding the cellular role of VRKs and establishing their potential use as targets for therapeutic intervention has been limited by the lack of tool compounds that can specifically modulate the activity of these kinases in cells. Here we identified BI-D1870, a dihydropteridine inhibitor of RSK kinases, as a promising starting point for the development of chemical probes targeting the active VRKs. We solved co-crystal structures of both VRK1 and VRK2 bound to BI-D1870 and of VRK1 bound to two broad-spectrum inhibitors. These structures revealed that both VRKs can adopt a P-loop folded conformation, which is stabilized by different mechanisms on each protein. Based on these structures, we suggest modifications to the dihydropteridine scaffold that can be explored to produce potent and specific inhibitors towards VRK1 and VRK2.

VRK1 promotes cisplatin resistance by up-regulating c-MYC via c-Jun activation and serves as a therapeutic target in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is a common malignant disease characterized by poor prognosis. Chemoresistance remains a major cause of ESCC relapse. Vaccinia-related kinase 1 (VRK1) has previously been identified as a cancer-related gene. However, there is little research demonstrating an association between VRK1 and ESCC. In this study, we show that VRK1 is overexpressed in ESCC primary tumor samples and cell lines. VRK1 expression was significantly correlated with clinical characteristics and predicted poor outcomes in ESCC patients. Functionally, knockdown of VRK1 inhibited ESCC cell proliferation, survival, migration and invasion; conversely, VRK1 overexpression produced the opposite effects. Furthermore, we found that up-regulation of VRK1 promoted cisplatin (CDDP) resistance in ESCC both in vitro and in vivo, whereas knockdown of VRK1 reduced this resistance. Further studies verified that VRK1 phosphorylated c-Jun and that the VRK1/c-Jun pathway contributed to CDDP resistance in ESCC. Mechanistically, a dual luciferase reporter assay revealed that c-Jun transcriptionally activated the expression of c-MYC. Silencing c-MYC abolished the c-Jun-mediated CDDP resistance of ESCC cells. A Kaplan-Meier analysis indicated that c-MYC is a potential prognostic factor in ESCC. Finally, luteolin, a VRK1 inhibitor, attenuated the malignant biological behaviors and CDDP resistance in ESCC cells. Collectively, we conclude that VRK1 promotes CDDP resistance through c-MYC by activating c-Jun and potentiating a malignant phenotype in ESCC. Our studies provide novel insight into the role of VRK1 in carcinogenesis and indicate that VRK1 can serve as a potential therapeutic target in ESCC.

Deletion of the Vaccinia Virus B1 Kinase Reveals Essential Functions of This Enzyme Complemented Partly by the Homologous Cellular Kinase VRK2

The vaccinia virus B1 kinase is highly conserved among poxviruses and is essential for the viral life cycle. B1 exhibits a remarkable degree of similarity to vaccinia virus-related kinases (VRKs), a family of cellular kinases, suggesting that the viral enzyme has evolved to mimic VRK activity. Indeed, B1 and VRKs have been demonstrated to target a shared substrate, the DNA binding protein BAF, elucidating a signaling pathway important for both mitosis and the antiviral response. In this study, we further characterize the role of B1 during vaccinia infection to gain novel insights into its regulation and integration with cellular signaling pathways. We begin by describing the construction and characterization of the first B1 deletion virus (vvΔB1) produced using a complementing cell line expressing the viral kinase. Examination of vvΔB1 revealed that B1 is critical for the production of infectious virions in various cell types and is sufficient for BAF phosphorylation. Interestingly, the severity of the defect in DNA replication following the loss of B1 varied between cell types, leading us to posit that cellular VRKs partly complement for the absence of B1 in some cell lines. Using cell lines devoid of either VRK1 or VRK2, we tested this hypothesis and discovered that VRK2 expression facilitates DNA replication and allows later stages of the viral life cycle to proceed in the absence of B1. Finally, we present evidence that the impact of VRK2 on vaccinia virus is largely independent of BAF phosphorylation. These data support a model in which B1 and VRK2 share additional substrates important for the replication of cytoplasmic poxviruses.IMPORTANCE Viral mimicry of cellular signaling modulators provides clear evidence that the pathogen targets an important host pathway during infection. Poxviruses employ numerous viral homologs of cellular proteins, the study of which have yielded insights into signaling pathways used by both virus and cells alike. The vaccinia virus B1 protein is a homolog of cellular vaccinia virus-related kinases (VRKs) and is needed for viral DNA replication and likely other stages of the viral life cycle. However, much remains to be learned about how B1 and VRKs overlap functionally. This study utilizes new tools, including a B1 deletion virus and VRK knockout cells, to further characterize the functional links between the viral and cellular enzymes. As a result, we have discovered that B1 and VRK2 target a common set of substrates vital to productive infection of this large cytoplasmic DNA virus.

Expression of VRK1 and the downstream gene BANF1 in esophageal cancer

Esophageal cancer is considered one of the most malignant tumors, being characterized by rapid progression and poor outcomes. China has the highest incidence of esophageal cancer in the world. Hence, it is necessary to clarify the mechanisms underlying esophageal cancer progression. In this study, we examined the expression of vaccinia-related kinase 1 (VRK1) and barrier to autointegration factor 1 (BANF1) in tumor tissues at the mRNA and protein levels via real-time PCR and immunohistochemical analyses. The mRNA and protein expression levels of VRK1 and BANF1 were higher in tumor tissues than in adjacent normal tissues. ROC curve analysis showed that VRK1 and BANF1 yielded AUCs of 0.790 and 0.735, respectively, for the detection of esophageal squamous cell carcinoma(ESCC) patients. In conclusion, our study indicates that VRK1 and BANF are promising novel therapeutic targets for esophageal cancer.

Vaccinia-related kinase 1 promotes hepatocellular carcinoma by controlling the levels of cell cycle regulators associated with G1/S transition

We identified the specific role of vaccinia-related kinase 1 (VRK1) in the progression of hepatocellular carcinoma (HCC) and evaluated its therapeutic and prognostic potential. VRK1 levels were significantly higher in HCC cell lines than a normal hepatic cell line, and were higher in HCC than non-tumor tissue. VRK1 knockdown inhibited the proliferation of SK-Hep1, SH-J1 and Hep3B cells; moreover, depletion of VRK1 suppressed HCC tumor growth in vivo. We also showed that VRK1 knockdown increased the number of G1 arrested cells by decreasing cyclin D1 and p-Rb while upregulating p21 and p27, and that VRK1 depletion downregulated phosphorylation of CREB, a transcription factor regulating CCND1. Additionally, we found that luteolin, a VRK1 inhibitor, suppressed HCC growth in vitro and in vivo, and that the aberrant VRK1 expression correlated with poor prognostic features of HCC. High levels of VRK1 were associated with shorter overall and disease-free survival and higher recurrence rates. Taken together, our findings suggest VRK1 may act as a tumor promoter by controlling the level of cell cycle regulators associated with G1/S transition and could potentially serve as a therapeutic target and/or prognostic biomarker for HCC.

Oncogenic Sox2 regulates and cooperates with VRK1 in cell cycle progression and differentiation

Sox2 is a pluripotency transcription factor that as an oncogene can also regulate cell proliferation. Therefore, genes implicated in several different aspects of cell proliferation, such as the VRK1 chromatin-kinase, are candidates to be targets of Sox2. Sox 2 and VRK1 colocalize in nuclei of proliferating cells forming a stable complex. Sox2 knockdown abrogates VRK1 gene expression. Depletion of either Sox2 or VRK1 caused a reduction of cell proliferation. Sox2 up-regulates VRK1 expression and both proteins cooperate in the activation of CCND1. The accumulation of VRK1 protein downregulates SOX2 expression and both proteins are lost in terminally differentiated cells. Induction of neural differentiation with retinoic acid resulted in downregulation of Sox2 and VRK1 that inversely correlated with the expression of differentiation markers such as N-cadherin, Pax6, mH2A1.2 and mH2A2. Differentiation-associated macro histones mH2A1.2and mH2A2 inhibit CCND1 and VRK1 expression and also block the activation of the VRK1 promoter by Sox2. VRK1 is a downstream target of Sox2 and both form an autoregulatory loop in epithelial cell differentiation.

VRK1 phosphorylates and protects NBS1 from ubiquitination and proteasomal degradation in response to DNA damage

NBS1 is an early component in DNA-Damage Response (DDR) that participates in the initiation of the responses aiming to repair double-strand breaks caused by different mechanisms. Early steps in DDR have to react to local alterations in chromatin that are induced by DNA damage. NBS1 participates in the early detection of DNA damage and functions as a platform for the recruitment and assembly of components that are sequentially required for the repair process. In this work we have studied whether the VRK1 chromatin kinase can affect the activation of NBS1 in response to DNA damage induced by ionizing radiation. VRK1 is forming a basal preassembled complex with NBS1 in non-damaged cells. Knockdown of VRK1 resulted in the loss of NBS1 foci induced by ionizing radiation, an effect that was also detected in cell-cycle arrested cells and in ATM (-/-) cells. The phosphorylation of NBS1 in Ser343 by VRK1 is induced by either doxorubicin or IR in ATM (-/-) cells. Phosphorylated NBS1 is also complexed with VRK1. NBS1 phosphorylation by VRK1 cooperates with ATM. This phosphorylation of NBS1 by VRK1 contributes to the stability of NBS1 in ATM (-/-) cells, and the consequence of its loss can be prevented by treatment with the MG132 proteasome inhibitor of RNF8. We conclude that VRK1 regulation of NBS1 contributes to the stability of the repair complex and permits the sequential steps in DDR.

High VRK1 expression contributes to cell proliferation and survival in hepatocellular carcinoma

VRK1 is a member of the vaccinia-related kinase (VRK) family of serine/threonine protein kinases, which is known to play multiple roles in cellular proliferation, cell cycle regulation and carcinogenesis. However, the expression and physiological significance of VRK1 in hepatocellular carcinoma (HCC) remain unclear. In this study, we aimed to investigate the potential role of VRK1 in the development and progression of HCC. Western blot and immunohistochemical analysis revealed that VRK1 was highly expressed in HCC tissues and cell lines, compared with adjacent nontumorous tissues and LO2 normal hepatocytes. Meanwhile, clinicopathological analysis showed that VRK1 was significantly associated with AJCC stage, Ki-67 and a poor prognosis in HCC specimens. Univariate and multivariate analysis showed that VRK1 could serve as an independent prognostic indicator of HCC patients' survival. Furthermore, we found that VRK1 was lowly expressed in serum-starved Huh7 cells, and was progressively increased after serum-refeeding. Finally, flow cytometry, CCK-8 and colony formation assay indicated that the depletion of VRK1 could retard cell cycle progression and reduce cells proliferation in HCC cells. On the basis of these findings, we conclude that VRK1 may be a candidate prognostic biomarker as well as a potential therapeutical target of HCC.

The spinal muscular atrophy with pontocerebellar hypoplasia gene VRK1 regulates neuronal migration through an amyloid-β precursor protein-dependent mechanism

Spinal muscular atrophy with pontocerebellar hypoplasia (SMA-PCH) is an infantile SMA variant with additional manifestations, particularly severe microcephaly. We previously identified a nonsense mutation in Vaccinia-related kinase 1 (VRK1), R358X, as a cause of SMA-PCH. VRK1-R358X is a rare founder mutation in Ashkenazi Jews, and additional mutations in patients of different origins have recently been identified. VRK1 is a nuclear serine/threonine protein kinase known to play multiple roles in cellular proliferation, cell cycle regulation, and carcinogenesis. However, VRK1 was not known to have neuronal functions before its identification as a gene mutated in SMA-PCH. Here we show that VRK1-R358X homozygosity results in lack of VRK1 protein, and demonstrate a role for VRK1 in neuronal migration and neuronal stem cell proliferation. Using shRNA in utero electroporation in mice, we show that Vrk1 knockdown significantly impairs cortical neuronal migration, and affects the cell cycle of neuronal progenitors. Expression of wild-type human VRK1 rescues both proliferation and migration phenotypes. However, kinase-dead human VRK1 rescues only the migration impairment, suggesting the role of VRK1 in neuronal migration is partly noncatalytic. Furthermore, we found that VRK1 deficiency in human and mouse leads to downregulation of amyloid-β precursor protein (APP), a known neuronal migration gene. APP overexpression rescues the phenotype caused by Vrk1 knockdown, suggesting that VRK1 affects neuronal migration through an APP-dependent mechanism.

Vaccinia-related kinase 1 (VRK1) confers resistance to DNA-damaging agents in human breast cancer by affecting DNA damage response

Vaccinia-related kinase 1 (VRK1) belongs to a group of sixteen kinases associated to a poorer prognosis in human breast carcinomas, particularly in estrogen receptor positive cases based on gene expression arrays. In this work we have studied the potential molecular mechanism by which the VRK1 protein can contribute to a poorer prognosis in this disease. For this aim it was first analyzed by immunohistochemistry the VRK1 protein level in normal breast and in one hundred and thirty six cases of human breast cancer. The effect of VRK1 to protect against DNA damage was determined by studying the effect of its knockdown on the formation of DNA repair foci assembled on 53BP1 in response to treatment with ionizing radiation or doxorubicin in two breast cancer cell lines. VRK1 protein was detected in normal breast and in breast carcinomas at high levels in ER and PR positive tumors. VRK1 protein level was significantly lower in ERBB2 positive cases. Next, to identify a mechanism that can link VRK1 to poorer prognosis, VRK1 was knocked-down in two breast cancer cell lines that were treated with ionizing radiation or doxorubicin, both inducing DNA damage. Loss of VRK1 resulted in reduced formation of DNA-damage repair foci complexes assembled on the 53BP1 scaffold protein, and this effect was independent of damaging agent or cell type. This observation is consistent with detection of high VRK1 protein levels in ER and PR positive breast cancers. We conclude that VRK1 can contribute to make these tumors more resistant to DNA damage-based therapies, such as ionizing radiation or doxorubicin, which is consistent with its association to a poor prognosis in ER positive breast cancer. VRK1 is potential target kinase for development of new specific inhibitors which can facilitate sensitization to other treatments in combination therapies; or alternatively be used as a new cancer drugs.

EWS-FLI1 utilizes divergent chromatin remodeling mechanisms to directly activate or repress enhancer elements in Ewing sarcoma

The aberrant transcription factor EWS-FLI1 drives Ewing sarcoma, but its molecular function is not completely understood. We find that EWS-FLI1 reprograms gene regulatory circuits in Ewing sarcoma by directly inducing or repressing enhancers. At GGAA repeat elements, which lack evolutionary conservation and regulatory potential in other cell types, EWS-FLI1 multimers induce chromatin opening and create de novo enhancers that physically interact with target promoters. Conversely, EWS-FLI1 inactivates conserved enhancers containing canonical ETS motifs by displacing wild-type ETS transcription factors. These divergent chromatin-remodeling patterns repress tumor suppressors and mesenchymal lineage regulators while activating oncogenes and potential therapeutic targets, such as the kinase VRK1. Our findings demonstrate how EWS-FLI1 establishes an oncogenic regulatory program governing both tumor survival and differentiation.

Luteolin suppresses cancer cell proliferation by targeting vaccinia-related kinase 1

Uncontrolled proliferation, a major feature of cancer cells, is often triggered by the malfunction of cell cycle regulators such as protein kinases. Recently, cell cycle-related protein kinases have become attractive targets for anti-cancer therapy, because they play fundamental roles in cellular proliferation. However, the protein kinase-targeted drugs that have been developed so far do not show impressive clinical results and also display severe side effects; therefore, there is undoubtedly a need to investigate new drugs targeting other protein kinases that are critical in cell cycle progression. Vaccinia-related kinase 1 (VRK1) is a mitotic kinase that functions in cell cycle regulation by phosphorylating cell cycle-related substrates such as barrier-to-autointegration factor (BAF), histone H3, and the cAMP response element (CRE)-binding protein (CREB). In our study, we identified luteolin as the inhibitor of VRK1 by screening a small-molecule natural compound library. Here, we evaluated the efficacy of luteolin as a VRK1-targeted inhibitor for developing an effective anti-cancer strategy. We confirmed that luteolin significantly reduces VRK1-mediated phosphorylation of the cell cycle-related substrates BAF and histone H3, and directly interacts with the catalytic domain of VRK1. In addition, luteolin regulates cell cycle progression by modulating VRK1 activity, leading to the suppression of cancer cell proliferation and the induction of apoptosis. Therefore, our study suggests that luteolin-induced VRK1 inhibition may contribute to establish a novel cell cycle-targeted strategy for anti-cancer therapy.

The Warburg effect revisited--lesson from the Sertoli cell

Otto Warburg observed that cancerous cells prefer fermentative instead of oxidative metabolism of glucose, although the former is in theory less efficient. Since Warburg's pioneering works, special attention has been given to this difference in cell metabolism. The Warburg effect has been implicated in cell transformation, immortalization, and proliferation during tumorigenesis. Cancer cells display enhanced glycolytic activity, which is correlated with high proliferation, and thus, glycolysis appears to be an excellent candidate to target cancer cells. Nevertheless, little attention has been given to noncancerous cells that exhibit a "Warburg-like" metabolism with slight, but perhaps crucial, alterations that may provide new directions to develop new and effective anticancer therapies. Within the testis, the somatic Sertoli cell (SC) presents several common metabolic features analogous to cancer cells, and a clear "Warburg-like" metabolism. Nevertheless, SCs actively proliferate only during a specific time period, ceasing to divide in most species after puberty, when they become terminally differentiated. The special metabolic features of SC, as well as progression from the immature but proliferative state, to the mature nonproliferative state, where a high glycolytic activity is maintained, make these cells unique and a good model to discuss new perspectives on the Warburg effect. Herein we provide new insight on how the somatic SC may be a source of new and exciting information concerning the Warburg effect and cell proliferation.

VRK1 interacts with p53 forming a basal complex that is activated by UV-induced DNA damage

DNA damage immediate cellular response requires the activation of p53 by kinases. We found that p53 forms a basal stable complex with VRK1, a Ser-Thr kinase that responds to UV-induced DNA damage by specifically phosphorylating p53. This interaction takes place through the p53 DNA binding domain, and frequent DNA-contact mutants of p53, such as R273H, R248H or R280K, do not disrupt the complex. UV-induced DNA damage activates VRK1, and is accompanied by phosphorylation of p53 at Thr-18 before it accumulates. We propose that the VRK1-p53 basal complex is an early-warning system for immediate cellular responses to DNA damage.

Nuclear assembly as a target for anti-cancer therapies

Current anti-cancer therapies have a great deal of undesirable side effects; therefore, there is a need to develop efficient and cancer cell-specific new drugs without strong dose-limiting side effects. In my opinion, mechanisms of nuclear assembly and organization represent a novel platform for drug targets, which might fulfill these criteria. The nuclear stiffness and organization of some cancer types are often compromised, making them more vulnerable for further targeting the mechanisms of nuclear integrity than their normal counterparts. Here I will discuss the nuclear organization of normal cells and cancer cells, the molecular mechanisms that govern nuclear assembly with emphasis on those that, in my view, might be considered as targets for future anti-cancer therapies.

Depletion of the protein kinase VRK1 disrupts nuclear envelope morphology and leads to BAF retention on mitotic chromosomes

The VRK1 protein kinase regulates the phosphorylation of BAF, which binds to dsDNA and LEM domain–containing proteins. VRK1 depletion increases the immobile fraction of BAF at the nuclear periphery and disturbs nuclear envelope architecture. It also leads to the retention of BAF on chromosomes as cells enter and progress through mitosis.

Barrier to autointegration factor (BAF), which is encoded by the BANF1 gene, binds with high-affinity to double-stranded DNA and LEM domain–containing proteins at the nuclear periphery. A BANF1 mutation has recently been associated with a novel human progeria syndrome, and cells from these patients have aberrant nuclear envelopes. The interactions of BAF with its DNA- and protein-binding partners are known to be regulated by phosphorylation, and previously we validated BAF as a highly efficient substrate for the VRK1 protein kinase. Here we show that depletion of VRK1 in MCF10a and MDA-MB-231 cells results in aberrant nuclear architecture. The immobile fraction of green fluorescent protein (GFP)–BAF at the nuclear envelope (NE) is elevated, suggesting that prolonged interactions of BAF with its binding partners is likely responsible for the aberrant NE architecture. Because detachment of BAF from its binding partners is associated with NE disassembly, we performed live-imaging analysis of control and VRK1-depleted cells to visualize GFP-BAF dynamics during mitosis. In the absence of VRK1, BAF does not disperse but instead remains chromosome bound from the onset of mitosis. VRK1 depletion also increases the number of anaphase bridges and multipolar spindles. Thus phosphorylation of BAF by VRK1 is essential both for normal NE architecture and proper dynamics of BAF–chromosome interactions during mitosis. These results are consistent with previous studies of the VRK/BAF signaling axis in Caenorhabditis elegans and Drosophila melanogaster and validate VRK1 as a key regulator of NE architecture and mitotic chromosome dynamics in mammalian cells.

Barrier to autointegration factor (BAF) inhibits vaccinia virus intermediate transcription in the absence of the viral B1 kinase

Barrier to autointegration factor (BAF/BANF1) is a cellular DNA-binding protein found in the nucleus and cytoplasm. Cytoplasmic BAF binds to foreign DNA and can act as a defense against vaccinia DNA replication. To evade BAF, vaccinia expresses the B1 kinase, which phosphorylates BAF and blocks its ability to bind DNA. Interestingly, B1 is also needed for viral intermediate gene expression via an unknown mechanism. Therefore, we evaluated the impact of B1-BAF signaling on vaccinia transcription. Strikingly, the decrease in vaccinia transcription caused by loss of B1 can be rescued by depletion of BAF. The repressive action of BAF is greatest on a viral promoter, and is more modest when non-vaccinia promoters are employed, which suggests BAF acts in a gene specific manner. These studies expand our understanding of the role of the B1 kinase during infection and provide the first evidence that BAF is a defense against viral gene expression.