Sequences downstream of the RNA initiation site of the HTLV type I long terminal repeat are sufficient for trans-activation by human cytomegalovirus immediate-early proteins

Human cytomegalovirus IE1 protein elicits a type II interferon-like host cell response that depends on activated STAT1 but not interferon-γ

Human cytomegalovirus (hCMV) is a highly prevalent pathogen that, upon primary infection, establishes life-long persistence in all infected individuals. Acute hCMV infections cause a variety of diseases in humans with developmental or acquired immune deficits. In addition, persistent hCMV infection may contribute to various chronic disease conditions even in immunologically normal people. The pathogenesis of hCMV disease has been frequently linked to inflammatory host immune responses triggered by virus-infected cells. Moreover, hCMV infection activates numerous host genes many of which encode pro-inflammatory proteins. However, little is known about the relative contributions of individual viral gene products to these changes in cellular transcription. We systematically analyzed the effects of the hCMV 72-kDa immediate-early 1 (IE1) protein, a major transcriptional activator and antagonist of type I interferon (IFN) signaling, on the human transcriptome. Following expression under conditions closely mimicking the situation during productive infection, IE1 elicits a global type II IFN-like host cell response. This response is dominated by the selective up-regulation of immune stimulatory genes normally controlled by IFN-γ and includes the synthesis and secretion of pro-inflammatory chemokines. IE1-mediated induction of IFN-stimulated genes strictly depends on tyrosine-phosphorylated signal transducer and activator of transcription 1 (STAT1) and correlates with the nuclear accumulation and sequence-specific binding of STAT1 to IFN-γ-responsive promoters. However, neither synthesis nor secretion of IFN-γ or other IFNs seems to be required for the IE1-dependent effects on cellular gene expression. Our results demonstrate that a single hCMV protein can trigger a pro-inflammatory host transcriptional response via an unexpected STAT1-dependent but IFN-independent mechanism and identify IE1 as a candidate determinant of hCMV pathogenicity.

Enhancement of enteric adenovirus cultivation by viral transactivator proteins

Human enteric adenoviruses (HAdVs; serotypes 40 and 41) are important waterborne and food-borne pathogens. However, HAdVs are fastidious, are difficult to cultivate, and do not produce a clear cytopathic effect during cell culture within a reasonable time. Thus, we examined whether the viral transactivator proteins cytomegalovirus (CMV) IE1 and hepatitis B virus (HBV) X promoted the multiplication of HAdVs. Additionally, we constructed a new 293 cell line expressing CMV IE1 protein for cultivation assays. We analyzed the nucleic acid sequences of the promoter regions of both E1A and hexon genes, which are considered to be the most important regions for HAdV replication. Expression of either HBV X or CMV IE1 protein significantly increased the promoter activities of E1A and hexon genes of HAdVs by as much as 14-fold during cell cultivation. The promotion of HAdV expression was confirmed by increased levels of both adenoviral DNA and mRNA expression. Finally, the newly developed 293 cell line expressing CMV IE1 protein showed an increase in viral DNA ranging from 574% to 619% compared with the conventional 293 cell line. These results suggest that the newly constructed cell line could be useful for efficient cultivation and research of fastidious HAdVs.

Characterization of the elements and proteins responsible for interferon-stimulated gene induction by human cytomegalovirus

Human cytomegalovirus (HCMV) infection of human fibroblast cells activates a large number of interferon-stimulated genes (ISGs) in a viral envelope-cell membrane fusion-dependent mechanism. In this study, we identified two interferon response elements, the interferon-stimulated response element (ISRE) and the gamma interferon-activated site (GAS), which act as HCMV response sites (VRS). Gel mobility shift assays showed that cellular proteins form specific and identical complexes with ISRE and GAS elements, and the binding of these complexes to ISRE and GAS is stimulated by HCMV infection. Point mutations in the consensus sequences of ISRE and GAS completely abolished their activities in response to HCMV-mediated transactivation, as well as their abilities to interact with HCMV-activated VRS-binding proteins. Interferon regulatory factor 3 does not appear to be present in the VRS-binding complexes or to be involved directly in HCMV-mediated ISG activation. Using ProteinChip technology, four potential proteins were identified, ranging from 20 to 42 kDa, in the VRS-binding complexes. The data suggest that HCMV infection activates VRS-binding proteins, which then bind to the VRS and stimulate ISG expression.

Artificial recruitment of Sp1 or TBP can replace the role of IE1 in the synergistic transactivation by IE1 and IE2

The IE1 and IE2 proteins of human cytomegalovirus transactivate various viral and cellular promoters in a synergistic manner, but the mechanism of their action has not been well elucidated. Here we have examined the IE1-IE2 synergy by artificial recruitment of either Sp1 or TBP to the promoter. We found that in the presence of Sp1, the synergistic effect of IE1 on IE2-mediated transactivation dramatically decreased. Furthermore, a 117-amino acids glutamine-rich fragment of Sp1, which can interact with dTAF(II)110 and hTAF(II)130, was sufficient to replace the role of IE1 in IE1-IE2 synergism. It was also found that TBP recruitment to the promoter markedly decreased the synergistic effect of IE1 on IE2-mediated transactivation. These results suggested that in the context of the synergism between IE1 and IE2, the function of IE1 might overlap with that of Sp1, for example by recruiting the TFIID complex.

Transactivation activity of the human cytomegalovirus IE2 protein occurs at steps subsequent to TATA box-binding protein recruitment

The IE2 protein of human cytomegalovirus transactivates viral and cellular promoters through a wide variety of cis-elements, but the mechanism of its action has not been well characterized. Here, IE2-Sp1 synergy and IE2-TATA box-binding protein (TBP) interaction are examined by artificial recruitment of either Sp1 or TBP to the promoter. It was found that IE2 could cooperate with DNA-bound Sp1. A 117 amino acid glutamine-rich fragment of Sp1, which can interact with Drosophila TAF(II)110 and human TAF(II)130, was sufficient for the augmentation of IE2-driven transactivation. In binding assays in vitro, IE2 interacted directly with the C-terminal region of Sp1, which contains the zinc finger DNA-binding domain, but not with its transactivation domain, suggesting that synergy between IE2 and the transactivation domain of Sp1 might be mediated by other proteins such as TAF or TBP. It was also found that TBP recruitment to the promoter markedly increased IE2-mediated transactivation. Thus, IE2 acts synergistically with DNA-bound Sp1 and DNA-bound TBP. These results suggest that, in human cytomegalovirus IE2 transactivation, Sp1 functions at an early step such as recruitment of TBP and IE2 acts to accelerate rate-limiting steps after TBP recruitment.