KSHV vFLIP is essential for the survival of infected lymphoma cells.

Primary effusion lymphomas (PELs) associated with infection by the Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) have constitutive nuclear factor (NF)-kappaB activity that is essential for their survival, but the source of this activity is unknown. We report that viral FADD-like interleukin-1-beta-converting enzyme [FLICE/caspase 8]-inhibitory protein (FLIP) activates NF-kappaB more potently than cellular FLIP in B cells and that it is largely responsible for NF-kappaB activation in latently infected PEL cells. Elimination of vFLIP production in PEL cells by RNA interference results in significantly decreased NF-kappaB activity, down-regulation of essential NF-kappaB-regulated cellular prosurvival factors, induction of apoptosis, and enhanced sensitivity to external apoptotic stimuli. vFLIP is the first virally encoded gene shown to be essential for the survival of naturally infected tumor cells.

Role of defective Oct-2 and OCA-B expression in immunoglobulin production and Kaposi's sarcoma-associated herpesvirus lytic reactivation in primary effusion lymphoma.

Primary effusion lymphoma (PEL) is a distinct type of B-cell non-Hodgkin lymphoma characterized by the presence of Kaposi's sarcoma-associated herpesvirus (KSHV/human herpesvirus 8). Despite having a genotype and gene expression signature of highly differentiated B cells, PEL does not usually express surface or cytoplasmic immunoglobulin (Ig). We show the lack of Oct-2 and OCA-B transcription factors to be responsible, at least in part, for this defect in Ig production. Like Ig genes, ORF50, the key regulator of the switch from latency to lytic reactivation, contains an octamer motif within its promoter. We therefore examined the impact of Oct-2 and OCA-B on ORF50 activation. The binding of Oct-1 to the ORF50 promoter has been shown to significantly enhance ORF50 transactivation. We found that Oct-2, on the other hand, inhibited ORF50 expression and consequently lytic reactivation by competing with Oct-1 for the octamer motif in the ORF50 promoter. Our data suggest that Oct-2 downregulation in infected cells would be favorable to KSHV in allowing for efficient viral reactivation.

Identification of a nucleoside analog active against adenosine kinase-expressing plasma cell malignancies.

Primary effusion lymphoma (PEL) is a largely incurable malignancy of B cell origin with plasmacytic differentiation. Here, we report the identification of a highly effective inhibitor of PEL. This compound, 6-ethylthioinosine (6-ETI), is a nucleoside analog with toxicity to PEL in vitro and in vivo, but not to other lymphoma cell lines tested. We developed and performed resistome analysis, an unbiased approach based on RNA sequencing of resistant subclones, to discover the molecular mechanisms of sensitivity. We found different adenosine kinase-inactivating (ADK-inactivating) alterations in all resistant clones and determined that ADK is required to phosphorylate and activate 6-ETI. Further, we observed that 6-ETI induces ATP depletion and cell death accompanied by S phase arrest and DNA damage only in ADK-expressing cells. Immunohistochemistry for ADK served as a biomarker approach to identify 6-ETI-sensitive tumors, which we documented for other lymphoid malignancies with plasmacytic features. Notably, multiple myeloma (MM) expresses high levels of ADK, and 6-ETI was toxic to MM cell lines and primary specimens and had a robust antitumor effect in a disseminated MM mouse model. Several nucleoside analogs are effective in treating leukemias and T cell lymphomas, and 6-ETI may fill this niche for the treatment of PEL, plasmablastic lymphoma, MM, and other ADK-expressing cancers.

EBV LMP2A affects LMP1-mediated NF-kappaB signaling and survival of lymphoma cells by regulating TRAF2 expression.

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-kappaB 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-kappaB 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-kappaB 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-kappaB and induction of apoptosis. Unexpectedly, knockdown of LMP2A also resulted in comparable decrease of NF-kappaB 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.

The KSHV oncoprotein vFLIP contains a TRAF-interacting motif and requires TRAF2 and TRAF3 for signalling.

Primary effusion lymphomas (PELs) characterized by infection with the Kaposi's sarcoma herpesvirus (KSHV; also called human herpesvirus 8) depend on the expression of the viral FADD-like interleukin-1-beta-converting enzyme (FLICE)/caspase-8-inhibitory protein (vFLIP) for their survival. This effect is achieved by activation of the transcription factor nuclear factor-kappaB (NF-kappaB). Tumour necrosis factor (TNF) receptor-associated factors (TRAFs) are direct mediators of NF-kappaB signalling by TNF family receptors and the Epstein-Barr virus oncoprotein latent membrane protein 1 and so we assessed the role of TRAFs in signalling by vFLIP. Here, we report the identification of a TRAF-interacting motif (PYQLT) in vFLIP, which is not present in other FLIP molecules. We show that vFLIP directly binds to TRAF2 in vitro and in PEL cells. TRAF2 and TRAF3 are required for induction of NF-kappaB and associated cell survival, as well as Jun amino-terminal kinase phosphorylation by vFLIP, whereas TRAF1, TRAF5 and TRAF6 are dispensable. Mutations in the P93 or Q95 amino acids within the TRAF-interacting motif of vFLIP abolish its ability to bind to TRAF2 and to signal to NF-kappaB. TRAF2, but not TRAF3, mediates the association of vFLIP with the IkappaB kinase complex. These data indicate that vFLIP uses TRAF2 and TRAF3 for signalling to NF-kappaB, which is crucial for KSHV-associated lymphomagenesis.

NPM-ALK transgenic mice spontaneously develop T-cell lymphomas and plasma cell tumors.

Anaplastic Large Cell Lymphomas (ALCLs) carry translocations in which the anaplastic lymphoma kinase (ALK) gene is juxtaposed to various genes, the most common of which is the NPM/B23 gene. ALK fusion proteins result in the constitutive activation of ALK tyrosine kinase, thereby enhancing proliferation and increasing cell survival. A direct role for NPM-ALK in cellular transformation has been shown in vitro with immortalized cell lines and in vivo using retroviral transfer experiments. Nonetheless, there is no direct evidence of its oncogenic potential in T lymphocytes, which represent the most common target of ALK chimeras. Here, we describe a new mouse model of lymphomagenesis in which human NPM-ALK transcription was targeted to T cells. NPM-ALK transgenic (Tg) mice were born with the expected mendelian distribution, normal lymphoid organs, and a normal number and proportion of helper and suppressor T cells. However, after a short period of latency, all NPM-ALK Tg mice developed malignant lymphoproliferative disorders (mean survival, 18 weeks). NPM-ALK Tg thymic lymphomas displayed a T-cell phenotype characteristic of immature thymocytes and frequently coexpressed surface CD30. A subset of the NPM-ALK Tg mice also developed clonal B-cell plasma cell neoplasms. These tumors arose in peripheral lymphoid organs (plasmacytomas) or within the bone marrow and often led to peripheral neuropathies and limb paralysis. Our NPM-ALK Tg mice are a suitable model to dissect the molecular mechanisms of ALK-mediated transformation and to investigate the efficacy of new therapeutic approaches for the treatment of human ALCL in vivo.