No abstract

Primary effusion lymphoma (PEL) is a rare subtype of non-Hodgkin's lymphoma, which is associated with infection by Kaposi's sarcoma herpesvirus (KSHV)/human herpesvirus-8. The c-Myc transcription factor plays an important role in cellular proliferation, differentiation and apoptosis. Lymphomas frequently have deregulated c-Myc expression owing to chromosomal translocations, amplifications or abnormal stabilization. However, no structural abnormalities were found in the c-myc oncogene in PEL. Given that c-Myc is often involved in lymphomagenesis, we hypothesized that it is deregulated in PEL. We report that PEL cells have abnormally stable c-Myc protein. The turnover of c-Myc protein is stringently regulated by post-transcriptional modifications, including phosphorylation of c-Myc threonine 58 (T58) by glycogen synthase kinase-3b (GSK-3b). Our data show that the impaired c-Myc degradation in PEL cells is associated with a significant underphosphorylation of c-Myc T58. The KSHV latency-associated nuclear antigen (LANA) is responsible for this deregulation. Overexpression of LANA in human embryonic kidney 293 or peripheral blood B cells leads to post-transcriptional deregulation of c-Myc protein. Conversely, when LANA is eliminated from PEL cells using RNA interference, GSK-3b-mediated c-Myc T58 phosphorylation is restored. The presence of c-Myc and LANA in GSK-3b-containing complexes in PEL cells further confirms the significance of these interactions in naturally KSHV-infected cells.

A mechanism used by Epstein-Barr virus (EBV) for in vitro transformation of B cells into lymphoblastoid cell lines (LCLs) is activation of the NF-B pathway, which is largely mediated by the EBV latent membrane protein 1 (LMP1). LMP1 is coexpressed with LMP2A in many EBV-associated lymphoid malignancies. Since inhibition of NF-B leads to apoptosis of EBV-infected LCLs and lymphoma cell lines, we sought to determine whether LMP1 alone, or in combination with other viral proteins, is responsible for initiating NF-B activation in these cells, thereby playing a role in cell survival. We found that suppression of LMP1 by RNA interference results in inhibition of basal NF-B and induction of apoptosis. Unexpectedly, knockdown of LMP2A also resulted in comparable decrease of NF-B activity and apoptosis. We report that LMP2A protein controls the expression of TRAF2 mRNA, which in turn is necessary for signaling by LMP1. Our data contrast with previous studies showing that transfected LMP1 can signal in the absence of LMP2A or TRAF2, and demonstrate that both LMP2A and TRAF2 are required for survival in naturally infected lymphoma cells and LCLs. These results also support LMP1, LMP2A, and TRAF2 as potential therapeutic targets in a subset of EBV-associated lymphoid malignancies. IntroductionThe association of Epstein-Barr virus (EBV) with several specific lymphoid malignancies is quite consistent, indicating an etiopathogenic role in their development. Support for EBV being an oncogenic virus comes from its ability to infect and transform normal human B cells in vitro, resulting in their immortalization and leading to continuously growing lymphoblastoid cell lines (LCLs). 1 Infection by EBV in healthy individuals is self-limiting, but a distinct sequence of events occurs that allows this virus to establish life-long latency in memory B cells. To achieve this, EBV expresses 2 latent membrane proteins that mimic signals in B cells that are involved in the germinal center reaction and B-cell differentiation into memory B cells. 2 These are the latent membrane protein 1 (LMP1), which functions as a constitutively active CD40 receptor, and LMP2A, containing an immunoreceptor tyrosine-based activation motif (ITAM), thereby mimicking antigen receptor signaling. While these 2 proteins can provide EBVinfected cells with proliferative and antiapoptotic signals, they are also immunogenic, so cells expressing LMP1 and LMP2A are eliminated after acute infection in healthy individuals, and the only surviving EBV-infected B cells that generate the latent reservoir are those that down-regulate these antigenic proteins and differentiate into memory B cells.EBV is thought to lead to the development of lymphoma as a consequence of alterations in the normal viral life cycle, for example as a consequence of impaired immune responses. 3 EBV-associated lymphomas frequently occur in immunodeficient individuals, including those infected with HIV and organ transplant recipients. 4,5 In these lymphomas, tumor cells usually express several EBV-enco...

No abstract

No abstract