The DNA architectural protein HMGB1 facilitates RTA-mediated viral gene expression in gamma-2 herpesviruses

HMGB1, a potential regulator of tumor microenvironment in KSHV-infected endothelial cells

High-mobility group box 1 (HMGB1) is a protein that binds to DNA and participates in various cellular processes, including DNA repair, transcription, and inflammation. It is also associated with cancer progression and therapeutic resistance. Despite its known role in promoting tumor growth and immune evasion in the tumor microenvironment, the contribution of HMGB1 to the development of Kaposi's sarcoma (KS) is not well understood. We investigated the effect of HMGB1 on KS pathogenesis using immortalized human endothelial cells infected with Kaposi's sarcoma-associated human herpes virus (KSHV). Our results showed that a higher amount of HMGB1 was detected in the supernatant of KSHV-infected cells compared to that of mock-infected cells, indicating that KSHV infection induced the secretion of HMGB1 in human endothelial cells. By generating HMGB1 knockout clones from immortalized human endothelial cells using CRISPR/Cas9, we elucidated the role of HMGB1 in KSHV-infected endothelial cells. Our findings indicate that the absence of HMGB1 did not induce lytic replication in KSHV-infected cells, but the cell viability of KSHV-infected cells was decreased in both 2D and 3D cultures. Through the antibody array for cytokines and growth factors, CXCL5, PDGF-AA, G-CSF, Emmprin, IL-17A, and VEGF were found to be suppressed in HMGB1 KO KSHV-infected cells compared to the KSHV-infected wild-type control. Mechanistically, phosphorylation of p38 would be associated with transcriptional regulation of CXCL5, PDGF-A and VEGF. These observations suggest that HMGB1 may play a critical role in KS pathogenesis by regulating cytokine and growth factor secretion and emphasize its potential as a therapeutic target for KS by modulating the tumor microenvironment.

Hepatitis B virus X protein counteracts high mobility group box 1 protein-mediated epigenetic silencing of covalently closed circular DNA

Hepatitis B virus (HBV) covalently closed circular DNA (cccDNA), serving as the viral persistence form and transcription template of HBV infection, hijacks host histone and non-histone proteins to form a minichromosome and utilizes posttranslational modifications (PTMs) "histone code" for its transcriptional regulation. HBV X protein (HBx) is known as a cccDNA transcription activator. In this study we established a dual system of the inducible reporter cell lines modelling infection with wildtype (wt) and HBx-null HBV, both secreting HA-tagged HBeAg as a semi-quantitative marker for cccDNA transcription. The cccDNA-bound histone PTM profiling of wt and HBx-null systems, using chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR), confirmed that HBx is essential for maintenance of cccDNA at transcriptionally active state, characterized by active histone PTM markers. Differential proteomics analysis of cccDNA minichromosome established in wt and HBx-null HBV cell lines revealed group-specific hits. One of the hits in HBx-deficient condition was a non-histone host DNA-binding protein high mobility group box 1 (HMGB1). Its elevated association to HBx-null cccDNA was validated by ChIP-qPCR assay in both the HBV stable cell lines and infection systems in vitro. Furthermore, experimental downregulation of HMGB1 in HBx-null HBV inducible and infection models resulted in transcriptional re-activation of the cccDNA minichromosome, accompanied by a switch of the cccDNA-associated histones to euchromatic state with activating histone PTMs landscape and subsequent upregulation of cccDNA transcription. Mechanistically, HBx interacts with HMGB1 and prevents its binding to cccDNA without affecting the steady state level of HMGB1. Taken together, our results suggest that HMGB1 is a novel host restriction factor of HBV cccDNA with epigenetic silencing mechanism, which can be counteracted by viral transcription activator HBx.

Immunogenic Cell Death, DAMPs and Prothymosin α as a Putative Anticancer Immune Response Biomarker

The new and increasingly studied concept of immunogenic cell death (ICD) revealed a previously unknown perspective of the various regulated cell death (RCD) modalities, elucidating their immunogenic properties and rendering obsolete the notion that immune stimulation is solely the outcome of necrosis. A distinct characteristic of ICD is the release of danger-associated molecular patterns (DAMPs) by dying and/or dead cells. Thus, several members of the DAMP family, such as the well-characterized heat shock proteins (HSPs) HSP70 and HSP90, the high-mobility group box 1 protein and calreticulin, and the thymic polypeptide prothymosin α (proTα) and its immunoreactive fragment proTα(100–109), are being studied as potential diagnostic tools and/or possible therapeutic agents. Here, we present the basic aspects and mechanisms of both ICD and other immunogenic RCD forms; denote the role of DAMPs in ICD; and further exploit the relevance of human proTα and proTα(100–109) in ICD, highlighting their possible clinical applications. Furthermore, we present the preliminary results of our in vitro studies, which show a direct correlation between the concentration of proTα/proTα(100–109) and the levels of cancer cell apoptosis, induced by anticancer agents and γ-radiation.

The danger molecule HMGB1 cooperates with the NLRP3 inflammasome to sustain expression of the EBV lytic switch protein in Burkitt lymphoma cells

High mobility group box 1 (HMGB1) is an important chromatin protein and a pro-inflammatory molecule. Though shown to enhance target DNA binding by the Epstein-Barr virus (EBV) lytic switch protein ZEBRA, whether HMGB1 actually contributes to gammaherpesvirus biology is not known. In investigating the contribution of HMGB1 to the lytic phase of EBV, important for development of EBV-mediated diseases, we find that compared to latently-infected cells, lytic phase Burkitt lymphoma-derived cells and peripheral blood lytic cells during primary EBV infection express high levels of HMGB1. Our experiments place HMGB1 upstream of ZEBRA and reveal that HMGB1, through the NLRP3 inflammasome, sustains the expression of ZEBRA. These findings indicate that in addition to the NLRP3 inflammasome's recently discovered role in turning the EBV lytic switch on, NLRP3 cooperates with the danger molecule HMGB1 to also maintain ZEBRA expression, thereby sustaining the lytic signal.

HMGB1 Knockout Decreases Kaposi's Sarcoma-Associated Herpesvirus Virion Production in iSLK BAC16 Cells by Attenuating Viral Gene Expression

Multiple host proteins affect the gene expression of Kaposi's sarcoma-associated herpesvirus (KSHV) during latent and lytic replication. High-mobility group box 1 (HMGB1) serves as a highly conserved chromosomal protein inside the cell and a prototypical damage-associated molecular pattern molecule outside the cell. HMGB1 has been shown to play a pathogenic role in viral infectious diseases and to regulate the lytic replication of KSHV. However, its functional effects on the KSHV life cycle in KSHV-infected cells have not been fully elucidated. Here, we explored the role of intracellular and extracellular HMGB1 in KSHV virion production by employing CRISPR/Cas9-mediated HMGB1 knockout in the KSHV-producing iSLK BAC16 cell line. Intracellular HMGB1 formed complexes with various proteins, and the abundance of HMGB1-interacting proteins changed during latent and lytic replication. Moreover, extracellular HMGB1 was found to enhance lytic replication by phosphorylating JNK. Of note, the expression of viral genes was attenuated during lytic replication in HMGB1 knockout iSLK BAC16 cells, with significantly decreased production of infectious virions compared to that of wild-type cells. Collectively, our results demonstrate that HMGB1 is an important cellular cofactor that affects the generation of infectious KSHV progeny during lytic replication. IMPORTANCE The high-mobility group box 1 (HMGB1) protein has many intra- and extracellular biological functions with an intricate role in various diseases. In certain viral infections, HMGB1 affects the viral life cycle and pathogenesis. In this study, we explored the effects of HMGB1 knockout on the production of Kaposi's sarcoma-associated herpesvirus (KSHV). HMGB1 knockout decreased virion production in KSHV-producing cells by decreasing the expression of viral genes. The processes by which HMGB1 affects KSHV production may occur inside or outside infected cells. For instance, several cellular and viral proteins interacted with intracellular HMGB1 in a nucleosomal complex, whereas extracellular HMGB1 induced JNK phosphorylation, thereby enhancing lytic replication. Our results suggest that both intracellular and extracellular HMGB1 are necessary for efficient KSHV replication. Thus, HMGB1 may represent an effective therapeutic target for the regulation of KSHV production.

A herpesvirus transactivator and cellular POU proteins extensively regulate DNA binding of the host Notch signaling protein RBP-Jκ to the virus genome

Reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV) from latency requires the viral transactivator Rta to contact the host protein Jκ recombination signal-binding protein (RBP-Jκ or CSL). RBP-Jκ normally binds DNA sequence-specifically to determine the transcriptional targets of the Notch-signaling pathway, yet Notch alone cannot reactivate KSHV. We previously showed that Rta stimulates RBP-Jκ DNA binding to the viral genome. On a model viral promoter, this function requires Rta to bind to multiple copies of an Rta DNA motif (called "CANT" or Rta-c) proximal to an RBP-Jκ motif. Here, high-resolution ChIP/deep sequencing from infected primary effusion lymphoma cells revealed that RBP-Jκ binds nearly exclusively to different sets of viral genome sites during latency and reactivation. RBP-Jκ bound DNA frequently, but not exclusively, proximal to Rta bound to single, but not multiple, Rta-c motifs. To discover additional regulators of RBP-Jκ DNA binding, we used bioinformatics to identify cellular DNA-binding protein motifs adjacent to either latent or reactivation-specific RBP-Jκ-binding sites. Many of these cellular factors, including POU class homeobox (POU) proteins, have known Notch or herpesvirus phenotypes. Among a set of Rta- and RBP-Jκ-bound promoters, Rta transactivated only those that also contained POU motifs in conserved positions. On some promoters, POU factors appeared to inhibit RBP-Jκ DNA binding unless Rta bound to a proximal Rta-c motif. Moreover, POU2F1/Oct-1 expression was induced during KSHV reactivation, and POU2F1 knockdown diminished infectious virus production. Our results suggest that Rta and POU proteins broadly regulate DNA binding of RBP-Jκ during KSHV reactivation.

The replication and transcription activator of murine gammaherpesvirus 68 cooperatively enhances cytokine-activated, STAT3-mediated gene expression

Gammaherpesviruses (γHVs) have a dynamic strategy for lifelong persistence, involving productive infection, latency, and intermittent reactivation. In latency reservoirs, such as B lymphocytes, γHVs exist as viral episomes and express few viral genes. Although the ability of γHV to reactivate from latency and re-enter the lytic phase is challenging to investigate and control, it is known that the γHV replication and transcription activator (RTA) can promote lytic reactivation. In this study, we provide first evidence that RTA of murine γΗV68 (MHV68) selectively binds and enhances the activity of tyrosine-phosphorylated host STAT3. STAT3 is a transcription factor classically activated by specific tyrosine 705 phosphorylation (pTyr⁷⁰⁵-STAT3) in response to cytokine stimulation. pTyr⁷⁰⁵-STAT3 forms a dimer that avidly binds a consensus target site in the promoters of regulated genes, and our results indicate that RTA cooperatively enhances the ability of pTyr⁷⁰⁵-STAT3 to induce expression of a STAT3-responsive reporter gene. As indicated by coimmunoprecipitation, in latently infected B cells that are stimulated to reactivate MHV68, RTA bound specifically to endogenous pTyr⁷⁰⁵-STAT3. An in vitro binding assay confirmed that RTA selectively recognizes pTyr⁷⁰⁵-STAT3 and indicated that the C-terminal transactivation domain of RTA was required for enhancing STAT3-directed gene expression. The cooperation of these transcription factors may influence both viral and host genes. During MHV68 de novo infection, pTyr⁷⁰⁵-STAT3 promoted the temporal expression of ORF59, a viral replication protein. Our results demonstrate that MHV68 RTA specifically recognizes and recruits activated pTyr⁷⁰⁵-STAT3 during the lytic phase of infection.

The dual role and therapeutic potential of high-mobility group box 1 in cancer

High-mobility group box 1 (HMGB1) is an abundant protein in most eukaryocytes. It can bind to several receptors such as advanced glycation end products (RAGE) and Toll-like receptors (TLRs), in direct or indirect way. The biological effects of HMGB1 depend on its expression and subcellular location. Inside the nucleus, HMGB1 is engaged in many DNA events such as DNA repair, transcription, telomere maintenance, and genome stability. While outside the nucleus, it possesses more complicated functions, including regulating cell proliferation, autophagy, inflammation and immunity. During tumor development, HMGB1 has been characterized as both a pro- and anti-tumoral protein by either promoting or suppressing tumor growth, proliferation, angiogenesis, invasion and metastasis. However, the current knowledge concerning the positive and negative effects of HMGB1 on tumor development is not explicit. Here, we evaluate the role of HMGB1 in tumor development and attempt to reconcile the dual effects of HMGB1 in carcinogenesis. Furthermore, we would like to present current strategies targeting against HMGB1, its receptor or release, which have shown potentially therapeutic value in cancer intervention.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

HMGB1 in health and disease

Complex genetic and physiological variations as well as environmental factors that drive emergence of chromosomal instability, development of unscheduled cell death, skewed differentiation, and altered metabolism are central to the pathogenesis of human diseases and disorders. Understanding the molecular bases for these processes is important for the development of new diagnostic biomarkers, and for identifying new therapeutic targets. In 1973, a group of non-histone nuclear proteins with high electrophoretic mobility was discovered and termed high-mobility group (HMG) proteins. The HMG proteins include three superfamilies termed HMGB, HMGN, and HMGA. High-mobility group box 1 (HMGB1), the most abundant and well-studied HMG protein, senses and coordinates the cellular stress response and plays a critical role not only inside of the cell as a DNA chaperone, chromosome guardian, autophagy sustainer, and protector from apoptotic cell death, but also outside the cell as the prototypic damage associated molecular pattern molecule (DAMP). This DAMP, in conjunction with other factors, thus has cytokine, chemokine, and growth factor activity, orchestrating the inflammatory and immune response. All of these characteristics make HMGB1 a critical molecular target in multiple human diseases including infectious diseases, ischemia, immune disorders, neurodegenerative diseases, metabolic disorders, and cancer. Indeed, a number of emergent strategies have been used to inhibit HMGB1 expression, release, and activity in vitro and in vivo. These include antibodies, peptide inhibitors, RNAi, anti-coagulants, endogenous hormones, various chemical compounds, HMGB1-receptor and signaling pathway inhibition, artificial DNAs, physical strategies including vagus nerve stimulation and other surgical approaches. Future work further investigating the details of HMGB1 localization, structure, post-translational modification, and identification of additional partners will undoubtedly uncover additional secrets regarding HMGB1's multiple functions.

Mitogen and stress activated kinases act co-operatively with CREB during the induction of human cytomegalovirus immediate-early gene expression from latency

The devastating clinical consequences associated with human cytomegalovirus (HCMV) infection and reactivation underscores the importance of understanding triggers of HCMV reactivation in dendritic cells (DC). Here we show that ERK-mediated reactivation is dependent on the mitogen and stress activated kinase (MSK) family. Furthermore, this MSK mediated response is dependent on CREB binding to the viral major immediate early promoter (MIEP). Specifically, CREB binding to the MIEP provides the target for MSK recruitment. Importantly, MSK mediated phosphorylation of histone H3 is required to promote histone de-methylation and the subsequent exit of HCMV from latency. Taken together, these data suggest that CREB binding to the MIEP is necessary for the recruitment of the kinase activity of MSKs to initiate the chromatin remodelling at the MIEP required for reactivation. Thus the importance of CREB during HCMV reactivation is to promote chromatin modifications conducive for viral gene expression as well as acting as a classical transcription factor. Clearly, specific inhibition of this interaction between CREB and MSKs could provide a strategy for therapeutic intervention.

How is inflammation initiated? Individual influences of IL-1, IL-18 and HMGB1

Pro-inflammatory cytokines are crucial for fighting infection and establishing immunity. Recently, other proteins, such as danger-associated molecular patterns (DAMPs), have also been appreciated for their role in inflammation and immunity. Following the formation and activation of multiprotein complexes, termed inflammasomes, two cytokines, IL-1β and IL-18, along with the DAMP High Mobility Group Box 1 (HMGB1), are released from cells. Although these proteins all lack classical secretion signals and are released by inflammasome activation, they each lead to different downstream consequences. This review examines how various inflammasomes promote the release of IL-1β, IL-18 and HMGB1 to combat pathogenic situations. Each of these effector molecules plays distinct roles during sterile inflammation, responding to viral, bacterial and parasite infection, and tailoring the innate immune response to specific threats.

Elucidating the evolutionary conserved DNA-binding specificities of WRKY transcription factors by molecular dynamics and in vitro binding assays

WRKY transcription factors constitute a large protein family in plants that is involved in the regulation of developmental processes and responses to biotic or abiotic stimuli. The question arises how stimulus-specific responses are mediated given that the highly conserved WRKY DNA-binding domain (DBD) exclusively recognizes the 'TTGACY' W-box consensus. We speculated that the W-box consensus might be more degenerate and yet undetected differences in the W-box consensus of WRKYs of different evolutionary descent exist. The phylogenetic analysis of WRKY DBDs suggests that they evolved from an ancestral group IIc-like WRKY early in the eukaryote lineage. A direct descent of group IIc WRKYs supports a monophyletic origin of all other group II and III WRKYs from group I by loss of an N-terminal DBD. Group I WRKYs are of paraphyletic descent and evolved multiple times independently. By homology modeling, molecular dynamics simulations and in vitro DNA-protein interaction-enzyme-linked immunosorbent assay with AtWRKY50 (IIc), AtWRKY33 (I) and AtWRKY11 (IId) DBDs, we revealed differences in DNA-binding specificities. Our data imply that other components are essentially required besides the W-box-specific binding to DNA to facilitate a stimulus-specific WRKY function.

HMGB1 protein binds to influenza virus nucleoprotein and promotes viral replication

Influenza virus has evolved replication strategies that hijack host cell pathways. To uncover interactions between viral macromolecules and host proteins, we applied a phage display strategy. A library of human cDNA expression products displayed on filamentous phages was submitted to affinity selection for influenza viral ribonucleoproteins (vRNPs). High-mobility-group box (HMGB) proteins were found to bind to the nucleoprotein (NP) component of vRNPs. HMGB1 and HMGB2 bind directly to the purified NP in the absence of viral RNA, and the HMG box A domain is sufficient to bind the NP. We show that HMGB1 associates with the viral NP in the nuclei of infected cells, promotes viral growth, and enhances the activity of the viral polymerase. The presence of a functional HMGB1 DNA-binding site is required to enhance influenza virus replication. Glycyrrhizin, which reduces HMGB1 binding to DNA, inhibits influenza virus polymerase activity. Our data show that the HMGB1 protein can play a significant role in intranuclear replication of influenza viruses, thus extending previous findings on the bornavirus and on a number of DNA viruses.

HMGB1 protein binds to influenza virus nucleoprotein and promotes viral replication

Influenza virus has evolved replication strategies that hijack host cell pathways. To uncover interactions between viral macromolecules and host proteins, we applied a phage display strategy. A library of human cDNA expression products displayed on filamentous phages was submitted to affinity selection for influenza viral ribonucleoproteins (vRNPs). High-mobility-group box (HMGB) proteins were found to bind to the nucleoprotein (NP) component of vRNPs. HMGB1 and HMGB2 bind directly to the purified NP in the absence of viral RNA, and the HMG box A domain is sufficient to bind the NP. We show that HMGB1 associates with the viral NP in the nuclei of infected cells, promotes viral growth, and enhances the activity of the viral polymerase. The presence of a functional HMGB1 DNA-binding site is required to enhance influenza virus replication. Glycyrrhizin, which reduces HMGB1 binding to DNA, inhibits influenza virus polymerase activity. Our data show that the HMGB1 protein can play a significant role in intranuclear replication of influenza viruses, thus extending previous findings on the bornavirus and on a number of DNA viruses.

A protein array screen for Kaposi's sarcoma-associated herpesvirus LANA interactors links LANA to TIP60, PP2A activity, and telomere shortening

The Kaposi's sarcoma-associated herpesvirus (KSHV) LANA protein functions in latently infected cells as an essential participant in KSHV genome replication and as a driver of dysregulated cell growth. To identify novel LANA protein-cell protein interactions that could contribute to these activities, we performed a proteomic screen in which purified, adenovirus-expressed Flag-LANA protein was incubated with an array displaying 4,192 nonredundant human proteins. Sixty-one interacting cell proteins were consistently detected. LANA interactions with high-mobility group AT-hook 1 (HMGA1), HMGB1, telomeric repeat binding factor 1 (TRF1), xeroderma pigmentosum complementation group A (XPA), pygopus homolog 2 (PYGO2), protein phosphatase 2A (PP2A)B subunit, Tat-interactive protein 60 (TIP60), replication protein A1 (RPA1), and RPA2 proteins were confirmed in coimmunoprecipitation assays. LANA-associated TIP60 retained acetyltransferase activity and, unlike human papillomavirus E6 and HIV-1 TAT proteins, LANA did not reduce TIP60 stability. The LANA-bound PP2A B subunit was associated with the PP2A A subunit but not the catalytic C subunit, suggesting a disruption of PP2A phosphatase activity. This is reminiscent of the role of simian virus 40 (SV40) small t antigen. Chromatin immunoprecipitation (ChIP) assays showed binding of RPA1 and RPA2 to the KSHV terminal repeats. Interestingly, LANA expression ablated RPA1 and RPA2 binding to the cell telomeric repeats. In U2OS cells that rely on the alternative mechanism for telomere maintenance, LANA expression had minimal effect on telomere length. However, LANA expression in telomerase immortalized endothelial cells resulted in telomere shortening. In KSHV-infected cells, telomere shortening may be one more mechanism by which LANA contributes to the development of malignancy.

The Kaposi's Sarcoma-Associated Herpesvirus ORF57 Protein and Its Multiple Roles in mRNA Biogenesis

Post-transcriptional events which regulate mRNA biogenesis are fundamental to the control of gene expression. A nascent mRNA is therefore steered through multimeric RNA-protein complexes that mediate its capping, splicing, polyadenylation, nuclear export, and ultimately its translation. Kaposi's sarcoma-associated herpesvirus (KSHV) mRNA transport and accumulation protein, or ORF57, is a functionally conserved protein found in all herpesviruses which plays a pivotal role in enhancing viral gene expression at a post-transcriptional level. As such, ORF57 has been implicated in multiple steps of RNA biogenesis, including augmenting viral splicing, protecting viral RNAs from degradation to enhancing viral mRNA nuclear export and translation. In this review, we highlight the multiple roles of KSHV ORF57 in regulating the post-transcriptional events which are fundamental to the control of virus gene expression.

KSHV Rta Promoter Specification and Viral Reactivation

Viruses are obligate intracellular pathogens whose biological success depends upon replication and packaging of viral genomes, and transmission of progeny viruses to new hosts. The biological success of herpesviruses is enhanced by their ability to reproduce their genomes without producing progeny viruses or killing the host cells, a process called latency. Latency permits a herpesvirus to remain undetected in its animal host for decades while maintaining the potential to reactivate, or switch, to a productive life cycle when host conditions are conducive to generating viral progeny. Direct interactions between many host and viral molecules are implicated in controlling herpesviral reactivation, suggesting complex biological networks that control the decision. One viral protein that is necessary and sufficient to switch latent Kaposi’s sarcoma-associated herpesvirus (KSHV) into the lytic infection cycle is called K-Rta. K-Rta is a transcriptional activator that specifies promoters by binding DNA directly and interacting with cellular proteins. Among these cellular proteins, binding of K-Rta to RBP-Jk is essential for viral reactivation. In contrast to the canonical model for Notch signaling, RBP-Jk is not uniformly and constitutively bound to the latent KSHV genome, but rather is recruited to DNA by interactions with K-Rta. Stimulation of RBP-Jk DNA binding requires high affinity binding of Rta to repetitive and palindromic “CANT DNA repeats” in promoters, and formation of ternary complexes with RBP-Jk. However, while K-Rta expression is necessary for initiating KSHV reactivation, K-Rta’s role as the switch is inefficient. Many factors modulate K-Rta’s function, suggesting that KSHV reactivation can be significantly regulated post-Rta expression and challenging the notion that herpesviral reactivation is bistable. This review analyzes rapidly evolving research on KSHV K-Rta to consider the role of K-Rta promoter specification in regulating the progression of KSHV reactivation.

Tiled microarray identification of novel viral transcript structures and distinct transcriptional profiles during two modes of productive murine gammaherpesvirus 68 infection

We applied a custom tiled microarray to examine murine gammaherpesvirus 68 (MHV68) polyadenylated transcript expression in a time course of de novo infection of fibroblast cells and following phorbol ester-mediated reactivation from a latently infected B cell line. During de novo infection, all open reading frames (ORFs) were transcribed and clustered into four major temporal groups that were overlapping yet distinct from clusters based on the phorbol ester-stimulated B cell reactivation time course. High-density transcript analysis at 2-h intervals during de novo infection mapped gene boundaries with a 20-nucleotide resolution, including a previously undefined ORF73 transcript and the MHV68 ORF63 homolog of Kaposi's sarcoma-associated herpesvirus vNLRP1. ORF6 transcript initiation was mapped by tiled array and confirmed by 5' rapid amplification of cDNA ends. The ∼1.3-kb region upstream of ORF6 was responsive to lytic infection and MHV68 RTA, identifying a novel RTA-responsive promoter. Transcription in intergenic regions consistent with the previously defined expressed genomic regions was detected during both types of productive infection. We conclude that the MHV68 transcriptome is dynamic and distinct during de novo fibroblast infection and upon phorbol ester-stimulated B cell reactivation, highlighting the need to evaluate further transcript structure and the context-dependent molecular events that govern viral gene expression during chronic infection.

Two Litopenaeus vannamei HMGB proteins interact with transcription factors LvSTAT and LvDorsal to activate the promoter of white spot syndrome virus immediate-early gene ie1

White spot syndrome virus (WSSV) has caused great economic damage to shrimp aquaculture. Previous studies have shown that WSSV successfully usurps the immunity system of the host for its own gene regulation. To investigate the role of shrimp high mobility group box (HMGB) proteins in WSSV gene regulation, two Litopenaeus vannamei HMGB genes, LvHMGBa and LvHMGBb, were isolated by rapid amplification of cDNA ends (RACE). Recombinant LvHMGBa/b proteins were present in the nucleus of transfected Drosophila Schneider 2 (S2) cells. Luciferase reporter assays revealed that LvHMGBa/b upregulated the WSSV immediate-early (IE) gene (ie1) in a NF-κB and STAT binding site-dependent manner. GST pull-down assays demonstrated that LvHMGBa/b interacted with L. vannamei Dorsal (LvDorsal) and L. vannamei STAT (LvSTAT), respectively. LvHMGBa was highly expressed in hepatopancreas while HMGBb was highly expressed in stomach, intestine, heart, antennal gland, and epidermis. Moreover, an immune challenge assay demonstrated that the expression of LvHMGBa/b was upregulated by WSSV infection and that both mRNAs reached peak values at 24 h post-infection. To our knowledge, this is the first report that invertebrate HMGB proteins participates in viral gene regulation.

Kaposi's sarcoma-associated herpesvirus RTA promotes degradation of the Hey1 repressor protein through the ubiquitin proteasome pathway

The Kaposi's sarcoma-associated herpesvirus (KSHV) replication and transcription activator (RTA) protein regulates the latent-lytic switch by transactivating a variety of KSHV lytic and cellular promoters. RTA is a novel E3 ubiquitin ligase that targets a number of transcriptional repressor proteins for degradation by the ubiquitin proteasome pathway. Herein, we show that RTA interacts with the cellular transcriptional repressor protein Hey1. We demonstrate that Hey1 is a target for RTA-mediated ubiquitination and is subsequently degraded by the proteasome. Moreover, a Cys-plus-His-rich region within RTA is important for RTA-mediated degradation of Hey1. We confirm that Hey1 represses the RTA promoter and, furthermore, show that Hey1 binds to the RTA promoter. An interaction was observed between Hey1 and the corepressor mSin3A, and this interaction was abolished in the presence of RTA. Additionally, mSin3A associated with the RTA promoter in nonreactivated, but not reactivated, BCBL1 cells. Small interfering RNA knockdown of Hey1 in HEK 293T cells latently infected with the recombinant virus rKSHV.219 led to increased levels of RTA expression upon reactivation but was insufficient to induce complete lytic reactivation. These results suggest that other additional transcriptional repressors are also important in maintenance of KSHV latency. Taken together, our results suggest that Hey1 has a contributory role in the maintenance of KSHV latency and that disruption of the Hey1 repressosome by RTA-targeted degradation may be one step in the mechanism to regulate lytic reactivation.

Age-dependent pathogenesis of murine gammaherpesvirus 68 infection of the central nervous system

Gammaherpesvirus infection of the central nervous system (CNS) has been linked to various neurological diseases, including meningitis, encephalitis, and multiple sclerosis. However, little is known about the interactions between the virus and the CNS in vitro or in vivo. Murine gammaherpesvirus 68 (MHV-68 or (gamma)HV-68) is genetically related and biologically similar to human gammaherpesviruses, thereby providing a tractable animal model system in which to study both viral pathogenesis and replication. In the present study, we show the successful infection of cultured neuronal cells, microglia, and astrocytes with MHV-68 to various extents. Upon intracerebroventricular injection of a recombinant virus (MHV-68/LacZ) into 4-5-week-old and 9-10-week-old mice, the 4-5-week-old mice displayed high mortality within 5-7 days, while the majority of the 9-10-week-old mice survived until the end of the experimental period. Until a peak at 3-4 days post-infection, viral DNA replication and gene expression were similar in the brains of both mouse groups, but only the 9-10-week-old mice were able to subdue viral DNA replication and gene expression after 5 days post-infection. Pro-inflammatory cytokine mRNAs of tumor necrosis factor-alpha, interleukin 1beta, and interleukin 6 were highly induced in the brains of the 4-5-week-old mice, suggesting their possible contributions as neurotoxic factors in the agedependent control of MHV-68 replication of the CNS.

Kaposi's sarcoma-associated herpesvirus (KSHV) Rta and cellular HMGB1 proteins synergistically transactivate the KSHV ORF50 promoter

Kaposi's sarcoma-associated herpesvirus 'replication transcriptional activator' (Rta) plays a critical role in the switch from latency to lytic replication. Rta upregulates several lytic KSHV genes, including its own, through multiple mechanisms. We demonstrate that cellular HMGB1 binds and synergistically upregulates the ORF50 promoter in conjunction with Rta. No direct interaction between Rta and HMGB1 was observed, however a ternary complex is formed in the presence of Oct1. Furthermore, deletion of an Oct-1 binding site within the ORF50 promoter ablates the HMGB1-mediated synergistic response. These results suggest Rta autostimulation may be mediated by a transient complex involving Oct1 and HMGB1.

Direct interactions of Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 ORF50/Rta protein with the cellular protein octamer-1 and DNA are critical for specifying transactivation of a delayed-early promoter and stimulating viral reactivation

The Kaposi's sarcoma-associated herpesvirus (KSHV) delayed-early K-bZIP promoter contains an ORF50/Rta binding site whose sequence is conserved with the ORF57 promoter. Mutation of the site in the full-length K-bZIP promoter reduced Rta-mediated transactivation by greater than 80%. The K-bZIP element contains an octamer (Oct) binding site that overlaps the Rta site and is well conserved with Oct elements found in the immediate-early promoters of herpes simplex virus type 1(HSV-1). The cellular protein Oct-1, but not Oct-2, binds to the K-bZIP element in a sequence-specific fashion in vitro and in vivo and stimulates Rta binding to the promoter DNA. The coexpression of Oct-1 enhances Rta-mediated transactivation of the wild type but not the mutant K-bZIP promoter, and Oct-1 and Rta proteins bind to each other directly in vitro. Mutations of Rta within an amino acid sequence conserved with HSV-1 virion protein 16 eliminate Rta's interactions with Oct-1 and K-bZIP promoter DNA but not RBP-Jk. The binding of Rta to both Oct-1 and DNA contributes to the transactivation of the K-bZIP promoter and viral reactivation, and Rta mutants deficient for both interactions are completely debilitated. Our data suggest that the Rta/Oct-1 interaction is essential for optimal KSHV reactivation. Transfections of mouse embryo fibroblasts and an endothelial cell line suggest cell-specific differences in the requirement for Oct-1 or RBP-Jk in Rta-mediated transactivation of the K-bZIP promoter. We propose a model in which Rta transactivation of the promoter is specified by the combination of DNA binding and interactions with several cellular DNA binding proteins including Oct-1.

High mobility group proteins of the plant HMGB family: dynamic chromatin modulators

In plants, the chromosomal high mobility group (HMG) proteins of the HMGB family typically contain a central HMG-box DNA-binding domain that is flanked by a basic N-terminal and an acidic C-terminal domain. The HMGB proteins are abundant and highly mobile proteins in the cell nucleus that influence chromatin structure and enhance the accessibility of binding sites to regulatory factors. Due to their remarkable DNA bending activity, HMGB proteins can increase the structural flexibility of DNA, promoting the assembly of nucleoprotein complexes that control DNA-dependent processes including transcription. Therefore, members of the HMGB family act as versatile modulators of chromatin function.

The Rta/Orf50 transactivator proteins of the gamma-herpesviridae

The replication and transcription activator protein, Rta, is encoded by Orf50 in Kaposi's sarcoma-associated herpesvirus (KSHV) and other known gammaherpesviruses including Epstein-Barr virus (EBV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS), and murine herpesvirus 68 (MHV-68). Each Rta/Orf50 homologue of each gammaherpesvirus plays a pivotal role in the initiation of viral lytic gene expression and lytic reactivation from latency. Here we discuss the Rta/Orf50 of KSHV in comparison to the Rta/Orf50s of other gammaherpesviruses in an effort to identify structural motifs, mechanisms of action, and modulating host factors.

Regulation of KSHV lytic gene expression

The life cycle of KSHV, latency versus lytic replication, is mainly determined at the transcriptional regulation level. A viral immediate-early gene product, replication and transcription activator (RTA), has been identified as the molecular switch for initiation of the lytic gene expression program from latency. Here we review progress on two key questions: how RTA gene expression is controlled by viral proteins and cellular signals and how RTA regulates the expression of downstream viral genes. We summarize the interactions of RTA with cellular and other viral proteins. We also discuss critical issues that must be addressed in the near future.

A poxvirus host range protein, CP77, binds to a cellular protein, HMG20A, and regulates its dissociation from the vaccinia virus genome in CHO-K1 cells

Vaccinia virus does not grow in Chinese hamster ovary (CHO-K1) cells in the absence of a viral host range factor, cowpox protein CP77. In this study, CP77 was fused to the C terminus of green fluorescence protein (GFP-CP77) and a series of nested deletion mutants of GFP-CP77 was constructed for insertion into a vaccinia virus host range mutant, VV-hr, and expressed from a viral early promoter. Deletion mapping analyses demonstrated that the N-terminal 352 amino acids of CP77 were sufficient to support vaccinia virus growth in CHO-K1 cells, whereas the C-terminal residues 353 to 668 were dispensable. In yeast two-hybrid analyses, CP77 bound to a cellular protein, HMG20A, and GST pulldown analyses showed that residues 1 to 234 of CP77 were sufficient for this interaction. After VV-hr virus infection of CHO-K1 cells, HMG20A was translocated from the nucleus to viral factories and bound to the viral genome via the HMG box region. In control VV-hr-infected CHO-K1 cells, binding of HMG20A to the viral genome persisted from 2 to 8 h postinfection (h p.i.); in contrast, when CP77 was expressed, the association of HMG20A with viral genome was transient, with little HMG20A remaining bound at 8 h p.i. This indicates that dissociation of HMG20A from viral factories correlates well with CP77 host range activity in CHO-K1 cells. Finally, in cells expressing a CP77 deletion protein (amino acids 277 to 668) or a DeltaANK5 mutant that did not support vaccinia virus growth and did not contain the HMG20A binding site, HMG20A remained bound to viral DNA, demonstrating that the binding of CP77 to HMG20A is essential for its host range function. In summary, our data revealed that a novel cellular protein, HMG20A, the dissociation of which from viral DNA is regulated by CP77, providing the first cellular target regulated by viral host range CP77 protein.

The Herpesvirus saimiri replication and transcription activator acts synergistically with CCAAT enhancer binding protein alpha to activate the DNA polymerase promoter

The open reading frame (ORF) 50 gene product, also known as the replication and transcription activator (Rta), is an immediate-early gene which is well conserved among all gamma-2 herpesviruses and plays a pivotal role in regulating the latent-lytic switch. Herpesvirus saimiri (HVS) ORF 50a functions as a sequence-specific transactivator capable of activating delayed-early (DE) gene expression via binding directly to an ORF 50 response element (RE) within the respective promoter. Analysis of the ORF 50 REs have identified two distinct types within HVS gene promoters. The first comprises a consensus sequence motif, CCN(9)GG, the second an AT-rich sequence. Here we demonstrate that ORF 50a is capable of transactivating the DE ORF 9 promoter which encodes the DNA polymerase. Deletion analysis of the ORF 9 promoter mapped the ORF 50 RE to a 95-bp region situated 126 bp upstream of the initiation codon. Gel retardation analysis further mapped the RE to a 28-bp fragment, which was able to confer ORF 50 responsiveness on an enhancerless simian virus 40 minimal promoter. Furthermore, sequence analysis identified multiple CCAAT enhancer binding protein alpha (C/EBPalpha) binding sites within the ORF 9 promoter and specifically two within the close vicinity of the AT-rich ORF 50 RE. Analysis demonstrated that the HVS ORF 50a and C/EBPalpha proteins associate with the ORF 9 promoter in vivo, interact directly, and synergistically activate the ORF 9 promoter by binding to adjacent binding motifs. Overall, these data suggest a cooperative interaction between HVS ORF 50a and C/EBPalpha proteins to activate the DNA polymerase promoter during early stages of the lytic replication cycle.

HMG proteins: dynamic players in gene regulation and differentiation

Core histones package the genome into nucleosomes and control its accessibility to transcription factors. High mobility group proteins (HMGs) are, after histones, the second most abundant chromatin proteins and exert global genomic functions in establishing active or inactive chromatin domains. It is becoming increasingly clear that they also specifically control the expression of a limited number of genes. Moreover, they contribute to the fine tuning of transcription in response to rapid environmental changes. They do so by interacting with nucleosomes, transcription factors, nucleosome-remodelling machines, and with histone H1.