Restoring PU.1 induces apoptosis and modulates viral transactivation via interferon-stimulated genes in primary effusion lymphoma

Primary effusion lymphoma (PEL), which is an aggressive lymphoma associated with Kaposi sarcoma-associated herpesvirus/human herpes virus-8 (KSHV/HHV-8), is refractory to standard chemotherapy, and exhibits a poor prognosis. Although PU.1 is down-regulated in PEL among B-cell transcription factors, the potential role of its reduction remains to be elucidated. In this investigation, we analyzed the DNA methylation of PU.1 cis-regulatory elements in PEL and the effect of restoring PU.1 on PEL cells. The expression of PU.1 mRNA was down-regulated in PEL cells. The promoter and enhancer regions of the PU.1 gene were highly methylated. The restoration of PU.1 inhibited cell growth and induced apoptosis in PEL cells. A microarray analysis revealed that interferon-stimulated genes (ISGs) including pro-apoptotic ISGs were strongly increased in BCBL-1 cells after the induction of PU.1. The up-regulation of PU.1 induced the transactivation of pro-apoptotic ISG promoters, such as the XAF1, OAS1, and TRAIL promoters, in reporter assays. Mutations at the PU.1 binding sites suppressed its transactivation. We confirmed the binding of PU.1 to the XAF1, OAS1, and TRAIL promoters in a chromatin immunoprecipitation assay. PU.1 suppressed ORF57 activation by inducing IRF7. The reinduction of PU.1 reduced formation of ascites and lymphoma cell infiltration of distant organs in PEL xenograft model mice. These results suggest that PU.1 plays a role in tumor suppression in PEL and its down-regulation is associated with PEL development. Up-regulation of PU.1 with demethylation agents may be a novel therapeutic strategy for PEL. We generated BCBL-1 cells that conditionally express PU.1 using a tet-on system. We compared control and PU.1-up-regulated BCBL-1 cells with Dox using microarrays