Linkage between STAT Regulation and Epstein-Barr Virus Gene Expression in Tumors

Abstract

Epstein-Barr virus (EBV) latency gene expression in lymphoblastoid cell lines is regulated by EBNA2.However, the factors regulating viral expression in EBV-associated tumors that do not express EBNA2 are poorly understood. In EBV-associated tumors, EBNA1 and frequently LMP1 are synthesized. We found that an alternative latent membrane protein 1 (LMP1) promoter, L1-TR, located within the terminal repeats is active in both nasopharyngeal carcinoma and Hodgkin's disease tissues. Examination of the L1-TR and the standard ED-L1 LMP1 promoters in electrophoretic mobility shift assays revealed that both promoters contain functional STAT binding sites. Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that, after primary exposure, maintains a latent infection for the life of the individual. Approximately 1 to 50 per 10 6 circulating B cells in healthy seropositive individuals carry the EBV genome, and the site of long-term latency has been identified as the G 0 memory B cell (39). EBV infection elicits a strong immune response (44), and in general viral persistence is controlled by the host and is asymptomatic. However, one consequence of lifelong infection is the potential for the development of EBV-associated malignancies, which include Burkitt's lymphoma, nasopharyngeal carcinoma (NPC), Hodgkin's disease, lymphoproliferative disease in immunocompromised patients, primary central nervous system lymphoma in AIDS patients, nasal T-cell lymphoma, a subset of gastric carcinoma, and possibly also a subset of primary liver and breast cancers (2, 3, 43, 51).On initial EBV infection, and in latently infected lymphoblastoid cell lines in culture, the full spectrum of EBV latency genes is expressed. The Wp promoter, which is regulated by B-cell-specific factors, is responsible for the initial transcription of the nuclear EBNAs, EBNA1, EBNA2, EBNA3A, EBNA3B, EBNA3C, and EBNA-LP (32). EBNA2, which functions as a transcriptional activator, then enforces a switch to Cp promoter-driven EBNA synthesis and also regulates synthesis of the LMP1 and LMP2 latency membrane proteins. However, EBNA2 is detected in EBV-associated malignancies only in the context of immunosuppression, presumably because the Cp-driven EBNA3 family proteins elicit a robust CD8 cell-mediated immune response (44). In EBV-associated tumors in immunocompetent patients, the Cp is repressed by methylation (1, 41). An alternative TATA-less promoter, Qp (40,46), is used to express EBNA1 in the absence of the immunogenic EBNAs, and LMP1 is also frequently expressed. EBNA1 binds to the origin of latent DNA replication, oriP, and is required for maintainance of the episomal form of the latent EBV genome (35,42,60). LMP1, an integral membrane protein, is essential for EBV-driven B-cell immortalization and induces transformation in primary Rat1 cells (56). The transforming ability of LMP1 is explicable in large part by its functioning as a constituitively activated tumor necrosis factor (TNF) receptor that mimics signaling by the B-lymphocyte activation antigen CD4...

Constitutive activation of STAT3 and STAT5 is present in the majority of nasopharyngeal carcinoma and correlates with better prognosis

“…The suitability of this strategy is further supported by the fact that, in addition to their presence in lymphocytes ( Figure 1D), constitutively activated STATs are also noted in other nontumour cells ( Figure 1C). A recent study found persistent STAT1, STAT3 and STAT5 activation in NPC tissue (Chen et al, 2001). However, insufficient data have been reported to provide an overall picture of the expression of these STATs in NPC.…”

“…In a recent study, the JAK/STAT pathway was shown to play a role in maintaining in vivo latency of EBV (Chen et al, 1999). Aberrant STAT activation has also been proposed as a necessary and predisposing event for EBV-driven tumorigenesis in immunocompetent individuals (Chen et al, 2001).…”

“…Although the presence of constitutive STAT1, STAT3 and STAT5 activation has been demonstrated in NPC tissue (Chen et al, 2001), data on the levels of expression of these STATs in NPC are lacking.…”

IL-21 imposes a type II EBV gene expression on type III and type I B cells by the repression of C- and activation of LMP-1-promoter

“…LMP-1 can be expressed from two alternative promoters: the EBNA-2-responsive ED-L1 or the promoter located in the first terminal repeat (TR-promoter) (14). Two binding sites for the STAT transcription factors were identified in the LMP-1 promoters (one in each) (15). Furthermore, STAT3, activated by IL-6, was shown to activate the TR LMP-1 promoter in epithelial cells (16,17).…”

Gene expression profiling defines molecular subtypes of classical Hodgkin's disease

“…The second gene cluster (brown cluster), overexpressed in nodular sclerosis GOHD, contained many cytokine genes, re¯ecting the presence of high amounts of cytokines released within the tumour tissue. It also included genes involved in signal transduction in response to virtually all types of stimuli transmitted by cytokines and antigen receptors, such as STAT3, previously shown to be activated in H/RS cells (Chen et al, 2001), and MAP2K6 (Stein et al, 1996). Altogether, these data suggest an intense tra cking of intercellular signals between neoplastic and reactive cells within GOHD tissues, which may have a favourable in¯uence on the outcome by contributing to regulate tumour growth.…”