NF-κB Is Constitutively Activated in EBV-Infected T or NK Cells and Can Be a Molecular Target for EBV-Positive T/NK-Cell Lymphoproliferative Disease Treatment

Abstract 1998

Epstein-Barr virus (EBV) can infect not only B cells but also rarely T or NK cells and causes EBV-positive T/NK-cell lymphoproliferative disease (EBV-T/NK-LPD), such as extranodal NK/T-cell lymphoma (ENKL), aggressive NK-cell leukemia, and chronic active EBV infection (CAEBV). However, why and how EBV infects T or NK cells and the mechanism of action responsible for the development of these EBV-induced malignancies have not been elucidated to date. Furthermore, optimal chemotherapy for these, especially for CAEBV, has not been established and the prognosis of EBV-T/NK-LPD remains very poor. NF-kB is a transcription factor that mediates anti-apoptotic molecular signaling and promotes proliferation of cancer cells. NF-kB is known to be constitutively activated in some hematological malignancies; the proteasome inhibitor bortezomib suppresses its activity and is used clinically as an anticancer reagent. To clarify the molecular mechanism underlying the development of EBV-T/NK-LPD, we focused on NF-kB in four EBV-positive T- and NK-cell lines, SNT8, SNT15, SNT16, and SNK6, which had been established from primary lesions of ENKL patients (SNT8 and SNK6) and the peripheral blood (PB) of a CAEBV patient (SNT15, 16), respectively. In these cells, it was demonstrated that p50, p52, RelA, and RelB—the components of NF-kB—existed constitutively in the nucleus. On the other hand, the EBV-negative T-cell lines, Jurkat and Molt4, and an NK-cell line, KHYG1, were negative for nuclear localization of these molecules. Elevated nuclear NF- kB–DNA binding activity was demonstrated by electrophoretic mobility shift assay (EMSA), using oligonucleotides encoding an NF-kB-binding sequence as a probe. Supershift EMSA revealed that NF- kB–DNA binding complexes in these cells involved p50, p52, and RelA. In addition, reporter assay using NF-kB-dependent luciferase reporter gene consisting of an NF-kB-binding site as a promoter demonstrated constitutive activation of NF-kB in SNT8 cells, whereas not in Jurkat cells. From these results, we concluded that NF-kB was constitutively activated through the canonical and noncanonical pathways in these EBV-infected T- or NK-cell (EBV-T/NK cell) lines. Next, we investigated NF-kB activation in EBV-T/NK cells derived from CAEBV patients. CAEBV was diagnosed according to the following criteria: presence of persistent infectious mononucleosis-like symptoms, elevation of EBV-DNA titer in PB, and detection of EBV-infected T or NK cells with clonal proliferation. To detect infected cells, we isolated peripheral mononuclear cells and divided them into CD19-, CD4-, CD8-, or CD56-positive fractions using antibody-conjugated magnetic beads. The EBV DNA of each fraction was quantified using a real-time quantitative polymerase chain reaction assay. Ten patients (aged 8–72 years; 4 male, 6 female; infected cell types CD4:3, CD8:3, γδ:1, and CD56:3) were diagnosed as CAEBV. Then, we examined NF-kB activation in these cells. Similar to the cell lines, immunoblotting and EMSA demonstrated evidence of constitutive activation of NF-kB such as nuclear localization and DNA binding of p50, p52, and RelA in EBV-T/NK cells from CAEBV patients. To investigate the molecular mechanism of NF-kB activation in EBV-T/NK cells, we examined the influence of viral proteins expressed in EBV-T/NK cells, including LMP1, LMP2A, LMP2B, and EBNA1 (latency type 2). We performed luciferase reporter assay using expression vectors for these proteins in Jurkat cells. LMP1, most significantly, and LMP2A, to a lesser extent, upregulated NF-kB-dependent reporter gene expression in Jurkat cells, whereas the other viral proteins did not, suggesting that LMP1 and LMP2A mediated NF-kB activation in T/NK cells. Finally we investigated the contribution of NF-kB to the development of EBV-T/NK-LPD. Bortezomib, in a dose-dependent manner, inhibited translocation of p50, p52, and RelA to the nucleus and reduced the expression of Bcl-XL, which was a transcriptional target of NF-kB in SNT8, SNK6, and CAEBV patients’ cells. In addition, it was also confirmed that bortezomib (0.5nM -5nM) suppressed the survival of and induced apoptosis of EBV-T/NK cells derived from 5 CAEBV patients. These results suggest that constitutive activation of NF-kB contributes to the development of EBV-T/NK-LPD and that the NF-kB signaling pathway can be an attractive molecular target for treatment.