Nuclear localization of the Epstein-Barr virus EBNA3B protein

Genetic Patterns Found in the Nuclear Localization Signals (NLSs) Associated with EBV-1 and EBV-2 Provide New Insights into Their Contribution to Different Cell-Type Specificities

Simple Summary

The Epstein–Barr virus (EBV) has been implicated in several human neoplastic diseases. The EBV-1 can transform B cells into LCL more efficiently than EBV-2, and EBV-2 preferentially infects T-cell lymphocytes. The EBNA3A oncoprotein has an essential role in B-cell transformation. The six peptide motifs called nuclear localization signals (NLSs) from EBNA3A ensure nucleocytoplasmic protein trafficking. Multiple NLSs have been suggested to enhance EBNA3 function or different specificities to different cell types; however, a comprehensive assessment of their genetic variability has not been addressed. Our objective was to study the NLSs’ variability and their relationship with EBV types. Based on a comprehensive analysis of over a thousand EBNA3A sequences from different clinical manifestations and geographic locations, we found that EBNA3A from EBV-2 has two of the six NLSs altered, and genetic patterns in the NLSs are associated with EBV-1 and EBV-2.

Abstract

The Epstein–Barr virus (EBV) is a globally dispersed pathogen involved in several human cancers of B-cell and non-B-cell origin. EBV has been classified into EBV-1 and EBV-2, which have differences in their transformative ability. EBV-1 can transform B-cells into LCL more efficiently than EBV-2, and EBV-2 preferentially infects T-cell lymphocytes. The EBNA3A oncoprotein is a transcriptional regulator of virus and host cell genes, and is required in order to transform B-cells. EBNA3A has six peptide motifs called nuclear localization signals (NLSs) that ensure nucleocytoplasmic protein trafficking. The presence of multiple NLSs has been suggested to enhance EBNA3 function or different specificities in different cell types. However, studies about the NLS variability associated with EBV types are scarce. Based on a systematic sequence analysis considering more than a thousand EBNA3A sequences of EBV from different human clinical manifestations and geographic locations, we found differences in NLSs’ nucleotide structures among EBV types. Compared with the EBNA3A EBV-1, EBNA3A EBV-2 has two of the six NLSs altered, and these mutations were possibly acquired by recombination. These genetic patterns in the NLSs associated with EBV-1 and EBV-2 provide new information about the traits of EBNA3A in EBV biology.

New Interactors of the Truncated EBNA-LP Protein Identified by Mass Spectrometry in P3HR1 Burkitt's Lymphoma Cells

The Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) acts as a co-activator of EBNA-2, a transcriptional activator essential for Epstein-Barr virus (EBV)-induced B-cell transformation. Burkitt's lymphoma (BL) cells harboring a mutant EBV strain that lacks both the EBNA-2 gene and 3' exons of EBNA-LP express Y1Y2-truncated isoforms of EBNA-LP (tEBNA-LP) and better resist apoptosis than if infected with the wild-type virus. In such BL cells, tEBNA-LP interacts with the protein phosphatase 2A (PP2A) catalytic subunit (PP2A C), and this interaction likely plays a role in resistance to apoptosis. Here, 28 cellular and four viral proteins have been identified by mass spectrometry as further possible interactors of tEBNA-LP. Three interactions were confirmed by immunoprecipitation and Western blotting, namely with the A structural subunit of PP2A (PP2A A), the structure-specific recognition protein 1 (SSRP1, a component of the facilitate chromatin transcription (FACT) complex), and a new form of the transcription factor EC (TFEC). Thus, tEBNA-LP appears to be involved not only in cell resistance to apoptosis through its interaction with two PP2A subunits, but also in other processes where its ability to co-activate transcriptional regulators could be important.

NMMHC-IIA-dependent nuclear location of CXCR4 promotes migration and invasion in renal cell carcinoma

The chemokine receptor cysteine (C)-X-C receptor (CXCR4) is a G-protein-coupled receptor that exerts a vital role in distant metastasis of renal cell carcinoma (RCC). Emerging evidence demonstrates that CXCR4 as the cytomembrane receptor translocated into the nucleus to facilitate cell migration and, therefore, determine the prognosis of several types of malignancies. However, the biological mechanism of nuclear location of CXCR4 remains unclear. In the present study, we confirmed the significant implications of the putative nuclear localization sequence (NLS) '146RPRK149̓ on CXCR4 subcellular localization and metastatic potential by point-mutation assay in RCC cell lines. Importantly, mass spectrum followed by immunoprecipitation identified non-muscle myosin heavy chain-IIA (NMMHC-IIA) as the CXCR4-interacting protein. Furthermore, pharmaceutical inhibition of NMMHC-IIA by blebbistatin dampened the nuclear translocation of CXCR4 as well as the metastatic capacity of RCC cells. In conclusion, the present study may drive the comprehensive progress toward elucidating the mechanism responsible for CXCR4 nuclear function and metastasis in tumors.

Role of EBNA-3 Family Proteins in EBV Associated B-cell Lymphomagenesis

Epstein-Barr virus (EBV) is highly ubiquitous in human population and establishes a lifelong asymptomatic infection within the infected host unless the immune system is compromised. Following initial infection in the oropharyngeal epithelial cells, EBV primarily infects naive B-lymphocytes and develops a number of B-cell lymphomas particularly in immune-deficient individuals. In vitro, EBV can also infect and subsequently transform quiescent B-lymphocytes into continuously proliferating lymphoblastoid cell lines (LCLs) resembling EBV-induced lymphoproliferative disorders in which a subset of latent transcripts are detected. Genetic studies revealed that EBNA-3 family comprising of three adjacent genes in the viral genome-EBNA-3A and -3C, but not -3B, are critical for B-cell transformation. Nevertheless, all three proteins appear to significantly contribute to maintain the overall proliferation and viability of transformed cells, suggesting a critical role in lymphoma development. Apart from functioning as important viral transcriptional regulators, EBNA-3 proteins associate with many cellular proteins in different signaling networks, providing a suitable platform for lifelong survival of the virus and concurrent lymphoma development in the infected host. The chapter describes the function of each these EBV nuclear antigen 3 proteins employed by the virus as a means to understand viral pathogenesis of several EBV-associated B-cell malignancies.

The EBNA3 Family: Two Oncoproteins and a Tumour Suppressor that Are Central to the Biology of EBV in B Cells

Epstein-Barr virus nuclear antigens EBNA3A , EBNA3B and EBNA3C are a family of three large latency-associated proteins expressed in B cells induced to proliferate by the virus. Together with the other nuclear antigens (EBNA-LP, EBNA2 and EBNA1), they are expressed from a polycistronic transcription unit that is probably unique to B cells. However, compared with the other EBNAs, hitherto the EBNA3 proteins were relatively neglected and their roles in EBV biology rather poorly understood. In recent years, powerful new technologies have been used to show that these proteins are central to the latency of EBV in B cells, playing major roles in reprogramming the expression of host genes affecting cell proliferation, survival, differentiation and immune surveillance. This indicates that the EBNA3s are critical in EBV persistence in the B cell system and in modulating B cell lymphomagenesis. EBNA3A and EBNA3C are necessary for the efficient proliferation of EBV-infected B cells because they target important tumour suppressor pathways--so operationally they are considered oncoproteins. In contrast, it is emerging that EBNA3B restrains the oncogenic capacity of EBV, so it can be considered a tumour suppressor--to our knowledge the first to be described in a tumour virus. Here, we provide a general overview of the EBNA3 genes and proteins. In particular, we describe recent research that has highlighted the complexity of their functional interactions with each other, with specific sites on the human genome and with the molecular machinery that controls transcription and epigenetic states of diverse host genes.

Impact of EBV essential nuclear protein EBNA-3C on B-cell proliferation and apoptosis

For over 40 years, EBV infection has been implicated in the etiology of a variety of lymphoid malignancies with the exceptional ability to drive resting B cells to continuously proliferate by successfully overriding cellular apoptotic stimuli. EBV utilizes the normal physiology of B-cell differentiation to persist within the memory B-cell pool of the immunocompetent host and subsequently establishes a life-long latent infection. During latency, out of a subset of viral genes expressed, EBNA-3C is one of the essential antigens required for in vitro primary B-cell transformation. EBNA-3C acts as a transcriptional coregulator by interacting with various cellular and viral factors. For the last 10 years, we have been actively engaged in discerning the biological significance of these interactions and revealed that EBNA-3C primarily targets two important cellular pathways - cell cycle and apoptosis. This review aims to summarize our current knowledge on EBNA-3C-mediated functions and describe how EBNA-3C seizes these cellular pathways that eventually promote B-cell lymphomagenesis. A scrupulous understanding of the critical relationship between EBNA-3C and these cellular machineries will not only aid in elucidating EBV pathogenesis, but also largely facilitate the development of novel diagnostic, as well as therapeutic, strategies against a vast range of EBV-associated B-cell lymphomas.

Cysteine (C)-x-C receptor 4 undergoes transportin 1-dependent nuclear localization and remains functional at the nucleus of metastatic prostate cancer cells

The G-protein coupled receptor (GPCR), Cysteine (C)-X-C Receptor 4 (CXCR4), plays an important role in prostate cancer metastasis. CXCR4 is generally regarded as a plasma membrane receptor where it transmits signals that support transformation, progression and eventual metastasis. Due to the central role of CXCR4 in tumorigenesis, therapeutics approaches such as antagonist and monoclonal antibodies have focused on receptors that exist on the plasma membrane. An emerging concept for G-protein coupled receptors is that they may localize to and associate with the nucleus where they retain function and mediate nuclear signaling. Herein, we demonstrate that CXCR4 associated with the nucleus of malignant prostate cancer tissues. Likewise, expression of CXCR4 was detected in nuclear fractions among several prostate cancer cell lines, compared to normal prostate epithelial cells. Our studies identified a nuclear pool of CXCR4 and we defined a nuclear transport pathway for CXCR4. We reveal a putative nuclear localization sequence (NLS), ‘RPRK’, within CXCR4 that contributed to nuclear localization. Additionally, nuclear CXCR4 interacted with Transportinβ1 and Transportinβ1-binding to CXCR4 promoted its nuclear translocation. Importantly, G_αi immunoprecipitation and calcium mobilization studies indicated that nuclear CXCR4 was functional and participated in G-protein signaling, revealing that the nuclear pool of CXCR4 retained function. Given the suggestion that functional, nuclear CXCR4 may be a mechanism underlying prostate cancer recurrence, increased metastatic ability and poorer prognosis after tumors have been treated with therapy that targets plasma membrane CXCR4, these studies addresses a novel mechanism of nuclear signaling for CXCR4, a novel mechanism of clinical targeting, and demonstrate an active nuclear pool that provides important new information to illuminate what has been primarily clinical reports of nuclear CXCR4.

Extensive co-operation between the Epstein-Barr virus EBNA3 proteins in the manipulation of host gene expression and epigenetic chromatin modification

Epstein-Barr virus (EBV) is able to drive the transformation of B-cells, resulting in the generation of lymphoblastoid cell lines (LCLs) in vitro. EBV nuclear proteins EBNA3A and EBNA3C are necessary for efficient transformation, while EBNA3B is dispensable. We describe a transcriptome analysis of BL31 cells infected with a series of EBNA3-knockout EBVs, including one deleted for all three EBNA3 genes. Using Affymetrix Exon 1.0 ST microarrays analysed with the MMBGX algorithm, we have identified over 1000 genes whose regulation by EBV requires one of the EBNA3s. Remarkably, a third of the genes identified require more than one EBNA3 for their regulation, predominantly EBNA3C co-operating with either EBNA3B, EBNA3A or both. The microarray was validated by real-time PCR, while ChIP analysis of a selection of co-operatively repressed promoters indicates a role for polycomb group complexes. Targets include genes involved in apoptosis, cell migration and B-cell differentiation, and show a highly significant but subtle alteration in genes involved in mitosis. In order to assess the relevance of the BL31 system to LCLs, we analysed the transcriptome of a set of EBNA3B knockout (3BKO) LCLs. Around a third of the genes whose expression level in LCLs was altered in the absence of EBNA3B were also altered in 3BKO-BL31 cell lines.

Among these are TERT and TCL1A, implying that EBV-induced changes in the expression of these genes are not required for B-cell transformation. We also identify 26 genes that require both EBNA3A and EBNA3B for their regulation in LCLs. Together, this shows the complexity of the interaction between EBV and its host, whereby multiple EBNA3 proteins co-operate to modulate the behaviour of the host cell.

Identification and functional analysis of NOL7 nuclear and nucleolar localization signals

BACKGROUND

NOL7 is a candidate tumor suppressor that localizes to a chromosomal region 6p23. This locus is frequently lost in a number of malignancies, and consistent loss of NOL7 through loss of heterozygosity and decreased mRNA and protein expression has been observed in tumors and cell lines. Reintroduction of NOL7 into cells resulted in significant suppression of in vivo tumor growth and modulation of the angiogenic phenotype. Further, NOL7 was observed to localize to the nucleus and nucleolus of cells. However, the mechanisms regulating its subcellular localization have not been elucidated.

RESULTS

An in vitro import assay demonstrated that NOL7 requires cytosolic machinery for active nuclear transport. Using sequence homology and prediction algorithms, four putative nuclear localization signals (NLSs) were identified. NOL7 deletion constructs and cytoplasmic pyruvate kinase (PK) fusion proteins confirmed the functionality of three of these NLSs. Site-directed mutagenesis of PK fusions and full-length NOL7 defined the minimal functional regions within each NLS. Further characterization revealed that NLS2 and NLS3 were critical for both the rate and efficiency of nuclear targeting. In addition, four basic clusters within NLS2 and NLS3 were independently capable of nucleolar targeting. The nucleolar occupancy of NOL7 revealed a complex balance of rapid nucleoplasmic shuttling but low nucleolar mobility, suggesting NOL7 may play functional roles in both compartments. In support, targeting to the nucleolar compartment was dependent on the presence of RNA, as depletion of total RNA or rRNA resulted in a nucleoplasmic shift of NOL7.

CONCLUSIONS

These results identify the minimal sequences required for the active targeting of NOL7 to the nucleus and nucleolus. Further, this work characterizes the relative contribution of each sequence to NOL7 nuclear and nucleolar dynamics, the subnuclear constituents that participate in this targeting, and suggests a functional role for NOL7 in both compartments. Taken together, these results identify the requisite protein domains for NOL7 localization, the kinetics that drive this targeting, and suggest NOL7 may function in both the nucleus and nucleolus.