CCAAT/enhancer-binding protein-alpha is induced during the early stages of Kaposi's sarcoma-associated herpesvirus (KSHV) lytic cycle reactivation and together with the KSHV replication and transcription activator (RTA) cooperatively stimulates the viral RTA, MTA, and PAN promoters

Viral interactions with the Notch pathway

The Notch signaling pathway influences cell fate decisions, proliferation versus differentiation and cell survival. Viruses both utilize and manipulate the differentiation state of infected cells, promote or block cell cycling and employ a variety of mechanisms to evade innate cellular anti-viral responses and promote cell survival. In light of these commonalities, it is perhaps not surprising that several viruses have tapped into the Notch pathway to advance their own life cycles. This first became apparent from studies showing targeting of Epstein-Barr virus proteins to the nuclear effector of Notch signaling CSL (CBF1/RBPJk). More recently the Kaposi's sarcoma-associated herpesvirus RTA protein has been found to bind CSL. Notch pathway interactions have also been described for adenovirus SV40 and human papilloma virus. This review focuses on the herpesvirus protein interactions with the Notch pathway and the insights that these interactions have provided.

Functional co-operation between the Kaposi's sarcoma-associated herpesvirus ORF57 and ORF50 regulatory proteins

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) proteins ORF57 (also known as MTA) and ORF50 (also known as RTA) act post-transcriptionally and transcriptionally to regulate viral lytic gene expression and synergistically activate certain early and late KSHV promoters. When ORF57 and ORF50 were co-expressed, they co-operatively stimulated expression from the promoter of the immediate-early ORF50 gene itself. Co-immunoprecipitations with extracts of KSHV-infected cells showed that ORF57 and ORF50 proteins were present in the same complex. Using the pull-down assay with extracts of KSHV-infected cells, ORF50 protein was shown to interact with a glutathione S-transferase-ORF57 fusion protein. A chromatin immunoprecipitation assay showed that ORF50 promoter sequences were preferentially associated with immunoprecipitated chromatin using both anti-ORF50 and anti-ORF57 antibodies consistent with both an in vivo physical association between ORF57 and ORF50 and a potential role for ORF57 at the transcriptional level. This is the first demonstration of an interaction between these two lytic regulatory proteins in a gammaherpesvirus. Expression of ORF50 protein is sufficient to induce lytic replication in latently infected cells and may determine viral host range, spread and KS pathogenesis in vivo. A new insight into the co-ordinated activities of these two key regulatory proteins is provided in which upregulation of the ORF50 promoter with augmentation of ORF50 activity by ORF57 protein, and vice versa, would facilitate the cascade of lytic viral gene expression, thereby breaking latency. A functional and physical interaction between these two gammaherpesvirus regulatory protein counterparts could be a general feature of the herpesviruses.

CCAAT/enhancer binding protein alpha binds to the Epstein-Barr virus (EBV) ZTA protein through oligomeric interactions and contributes to cooperative transcriptional activation of the ZTA promoter through direct binding to the ZII and ZIIIB motifs during induction of the EBV lytic cycle

The Epstein-Barr virus (EBV)-encoded ZTA protein interacts strongly with and stabilizes the cellular CCAAT/enhancer binding protein alpha (C/EBPalpha), leading to the induction of p21-mediated G(1) cell cycle arrest. Despite the strong interaction between these two basic leucine zipper (bZIP) family proteins, the ZTA and C/EBPalpha subunits do not heterodimerize, as indicated by an in vitro cross-linking assay with in vitro-cotranslated (35)S-labeled C/EBPalpha and (35)S-labeled ZTA protein. Instead, they evidently form a higher-order oligomeric complex that competes with C/EBPalpha binding but not with ZTA binding in electrophoretic mobility shift assays (EMSAs). Glutathione S-transferase affinity assays with mutant ZTA proteins revealed that the basic DNA binding domain and the key leucine zipper residues required for homodimerization are all required for the interaction with C/EBPalpha. ZTA is known to bind to two ZRE sites within the ZTA promoter and to positively autoregulate its own expression in transient cotransfection assays, but there is conflicting evidence about whether it does so in vivo. Examination of the proximal ZTA upstream promoter region by in vitro EMSA analysis revealed two high-affinity C/EBP binding sites (C-2 and C-3), which overlap the ZII and ZIIIB motifs, implicated as playing a key role in lytic cycle induction. A chromatin immunoprecipitation assay confirmed the in vivo binding of both endogenous C/EBPalpha and ZTA protein to the ZTA promoter after lytic cycle induction but not during the latent state in EBV-infected Akata cells. Reporter assays revealed that cotransfected C/EBPalpha activated the ZTA promoter even more effectively than cotransfected ZTA. However, synergistic activation of the ZTA promoter was not observed when ZTA and C/EBPalpha were cotransfected together in either HeLa or DG75 cells. Mutagenesis of either the ZII or the ZIIIB sites in the ZTA promoter strongly reduced C/EBPalpha transactivation, suggesting that these sites act cooperatively. Furthermore, the introduction of exogenous C/EBPalpha into EBV-infected HeLa-BX1 cells induced endogenous ZTA mRNA and protein expression, as demonstrated by both reverse transcription-PCR and immunoblotting assays. Finally, double-label immunofluorescence assays suggested that EAD protein expression was activated even better than ZTA expression in latently infected C/EBPalpha-transfected Akata cells, perhaps because of the presence of a strong B-cell-specific repressed chromatin conformation on the ZTA promoter itself during EBV latency.

Early activation of the Kaposi's sarcoma-associated herpesvirus RTA, RAP, and MTA promoters by the tetradecanoyl phorbol acetate-induced AP1 pathway

Kaposi's sarcoma-associated herpesvirus (KSHV) maintains a latent infection in primary effusion lymphoma (PEL) cells, but treatment with tetradecanoyl phorbol acetate (TPA) can trigger the full lytic-cycle replication in some of these cells. During lytic-cycle replication, the KSHV-encoded replication and transcription activator (RTA or ORF50), the mRNA transport and accumulation protein (MTA), and the replication-associated protein (RAP) all play crucial roles in expression of downstream viral genes as well as in mediation of viral DNA replication. The cellular CCAAT/enhancer-binding protein alpha (C/EBP alpha) is induced in TPA-treated PEL cells and contributes to transactivation of the promoters for all of these genes through both direct binding and cooperative interactions with RTA and RAP targeted to upstream C/EBP sites. However, little is known about how RTA expression is triggered initially at the earliest stages after TPA induction when the C/EBP alpha levels are still limited. Treatment with TPA proved to significantly induce both AP1 DNA-binding activity and levels of activated phosphorylated cJUN in PEL cells and ectopic expression of cJUN-plus-cFOS-induced RTA protein expression in PEL cells. Cotransfected cJUN plus cFOS or TPA treatment transactivated the KSHV RTA, RAP, and MTA promoters in an AP1-binding site-dependent manner in all three promoters. Chromatin immunoprecipitation assays confirmed that cJUN associates with these KSHV target promoters in PEL cells as early as 4 h after TPA treatment. Furthermore, the KSHV RTA and RAP proteins both interact with cJUN or both cJUN and cFOS in vitro or by coimmunoprecipitation from induced PEL cells and enhance cJUN-plus-cFOS-mediated transactivation of these viral promoters. Both increased phosphorylated cJUN and AP1 DNA-binding activity was detected as early as 1 h after TPA treatment in PEL cells, suggesting that AP1 activity may be crucial for very early activation of the RAP, MTA, and RTA promoters during the KSHV lytic cycle. Finally, expression of RTA alone increased cJUN protein levels severalfold in DG75 cells but did not induce cJUN phosphorylation. Therefore, we suggest that the initiating effects of TPA via the AP1 pathway in PEL cells need to be amplified by RTA for full lytic-cycle induction.

Cell cycle arrest by Kaposi's sarcoma-associated herpesvirus replication-associated protein is mediated at both the transcriptional and posttranslational levels by binding to CCAAT/enhancer-binding protein alpha and p21(CIP-1)

Lytic-cycle replication of Kaposi's sarcoma-associated herpesvirus (KSHV) in PEL cells causes G(1) cell cycle arrest mediated by the virus-encoded replication-associated protein (RAP) (or K8 protein), which induces high-level expression of the cellular C/EBPalpha and p21 proteins. Here we have examined the mechanism of this induction at both the transcriptional and posttranslational levels. RAP proved to bind very efficiently to both C/EBPalpha and p21 and stabilized them by up to 10-fold from proteasome-mediated degradation in vitro. Cross-linking revealed that RAP itself forms stable dimers and tetramers in solution and forms higher-order complexes but not heterodimers with C/EBPalpha. Cotransfection of RAP with C/EBPalpha cooperatively stimulated both the C/EBPalpha and p21 promoters in luciferase reporter gene assays. Only the basic/leucine zipper region of RAP was needed for interaction with and stabilization of C/EBPalpha, but both the N-terminal and C-terminal domains were required for transcriptional augmentation. In vitro-translated RAP interfered with DNA binding by C/EBPalpha in electrophonetic mobility shift assay (EMSA) experiments but did not itself bind to the target C/EBPalpha sites or form supershifted bands. However, in endogenous chromatin immunoprecipitation (ChIP) assays with tetradecanoyl phorbol acetate-induced PEL cells, RAP proved to specifically associate with the C/EBPalpha promoter in vivo, but only in a C/EBPalpha-dependent manner, implying an in vivo piggyback interaction with DNA-bound C/EBPalpha. Expression of exogenous RAP (Ad-RAP) caused G(1)/S cell cycle arrest in human dermal microvascular endothelial cells and also induced both the C/EBPalpha and p21 proteins, which formed punctate nuclear patterns that colocalized with RAP in PML nuclear bodies. In the presence of RAP, C/EBPalpha was also efficiently recruited into viral DNA replication compartments in both infected and cotransfected cells. In support of a direct role for this interaction in viral DNA replication, three C/EBPalpha binding sites were identified by in vitro EMSA experiments within a 220-bp core segment of the duplicated KSHV Ori-Lyt region, and although RAP did not bind to Ori-Lyt DNA directly in vitro, both endogenous RAP and C/EBPalpha were found to be associated with the Ori-Lyt region by ChIP assays in lytically induced PEL cells. Finally, we found that the KSHV lytic cycle could not be triggered by either synchronizing KSHV latently infected PEL cells in G(1) phase or inducing p21 in a C/EBPalpha-independent process.