Role of PU.1 Down-Regulation for the Development of Primary Effusion Lymphoma

Introduction: Primary effusion lymphoma (PEL) is a subtype of aggressive non-Hodgkin lymphoma that shows serous lymphomatous effusion without tumor masses in body cavities especially in advanced acquired immunodeficiency syndrome (AIDS). PEL is universally associated with Kaposi sarcoma-associated herpesvirus (KSHV)/human herpesvirus-8 (HHV-8) infection. The prognosis of PEL is extremely poor due to its aggressiveness and resistance to conventional chemotherapy. Therefore, the understanding of the molecular pathogenesis of PEL is vital to develop novel therapeutic strategies. PU.1, one of the Ets family transcription factors, has been reported to be down-regulated in some B-cell malignancies and acute myeloid leukemia. In this study, we examined the methylation of the PU.1 promoter and enhancer regions in PEL cell lines and performed the restoration of PU.1 in PEL cells in vitro and in vivo.

Methods:PU.1 mRNA expressions of eight PEL cell lines were examined by polymerase chain reaction (PCR). PU.1 was conditionally expressed using tet-on system in three PEL cell lines. The bisulfite sequencing was performed to analyze the methylation of the PU.1 promoter and enhancer regions in PEL cells. The DNA methyltransferase inhibitor, 5-Azacytidine, was used as an experimental tool for demethylation. The anti-proliferative activities of restoring PU.1 or 5-Azacytidine in PEL cell lines were measured by the methylthiotetrazole method. Apoptosis was quantified using Annexin V antibody. To evaluate the association of PU.1 with TRAIL promoter, we analyzed its recruitment to the promoter by luciferase reporter assay and chromatin immunoprecipitation (ChIP) assay. BCBL-1 cells that expressed tet-inducible PU.1 were injected intraperitoneally into NOD/Rag-2/Jak3double-deficient mice to assess the in vivo efficacy of restoring PU.1. Organ invasion by BCBL-1 cells was evaluated by immunohistochemistry.

Results: The expression of PU.1 mRNA in PEL cell lines was weaker than in the human monoblast leukemia cell line, U937, which expresses PU.1 mRNA. The promoter and enhancer regions of the PU.1 gene were highly methylated in PEL cells. The treatment with 5-Azacytidine up-regulated the expression of PU.1 mRNA. Restoration of PU.1 or treatment with 5-Azacytidine inhibited cell proliferation. PU.1 up-regulated TRAIL gene expression among apoptosis-related genes and caused apoptosis. PU.1 induced the transactivation of TRAIL promoter in the reporter assay. Disruption of the PU.1 binding site repressed its transactivation. PU.1 was recruited to TRAIL promoter in the ChIP assay. Although PU.1 induced the master regulator of viral replication, ORF50 expression, the expression of ORF57, which is essential for virus production as a posttranscriptional activator, decreased. PU.1 suppressed ORF 57 by the occupancy of IRF7 in the ORF57 promoter region. Restoration of PU.1 inhibited the ascites formation and organ invasion significantly in vivo compared with control mice.

Conclusions: The up-regulation of PU.1 induces growth inhibition and apoptosis via the direct transactivation of TRAIL and modulates viral transactivation via the up-regulation of IRF7. PU.1 is a tumor suppressor in PEL and its down-regulation has a critical role in PEL development. The targeting of PU.1 could be a therapeutic approach in patients with PEL.