The regulation of IRF-4 activity in lymphoid cells and involvement in HTLV-I-induced T cell leukemogenesis

The Human T cell Leukemia Virus (HTLV) is the etiologic agent of adult T cell leukemia (ATL), an aggressive and fatal leukemia of CD4+ T lymphocytes, and is also associated with a neurological demyelinating disease called Tropical Spastic Paraparesis (TSP) or HTLV-I Associated Myelopathies (HAM). The oncogenic potential of HTLV-I resides in the viral Tax oncoprotein, a positive regulator of viral and cellular gene transcription. Interestingly, all HTLV-1 infected cells and Jurkat cells transiently transfected with the HTLV-I Tax gene, express constitutively the Interferon Regulatory Factor-4 (IRF-4), a lymphoid-specific member of the IRF family of transcription factors, indicating that Tax may function as an indirect transactivator the IRF-4 gene. The overexpression of IRF-4 in HTLV-1 infected cells may play a role in viral-mediated cellular transformation and thus in adult T cell leukemia. IRF-4 expression studies revealed its presence in B cells, activated T cell, macrophages and T cells infected with HTLV-I or HTLV-II. In attempt to understand the regulation of IRF-4 expression, promoter analyses were undertaken using genomic footprinting and EMSA. These promoter analyses revealed the involvement of the NF-κB and NF-AT family members in IRF-4 regulation. Using IRF-4 as bait in a yeast two hybrid screen, a novel interaction between IRF-4 and the FK506 binding protein 52 (FKBP52), a 59kDa member of the immunophilin family with peptidyl-prolyl isomerase activity (PPIase) as well as chaperone-like functions, was characterized. Inhibition of IRF-4 DNA binding activity as well as transcriptional potential was shown to require functional PPIase of this immunophilin. FKBP52 seems to induce a conformational change in IRF-4 by cis-trans prolyl isomerization which interferes with IRF-4 DNA binding and transactivation. These studies suggest the direct involvement of the immunophilin FKBP52 in the regulation of IRF-4 function by a novel post-translational modification. Several other potential IRF-4 interacting partners were found during a second two-hybrid screen: eIF4γ1, c-src, Gi3, RhoANEF, RhoGDI and PP2A. These interactions could be involved in HTLV-I induced leukemogenesis. Novel IRF-4 regulated genes were also analyzed using an IRF-4 stably expressing Jurkat cell line and cDNA array technology. Several genes potentially regulated by IRF-4 such as RhoA, HSC70, RP-A, cyclin B1, PCNA, EB1, NIP3, MAPKK3 were identified. The deregulation of these genes by IRF-4 could lead to an increase in cellular proliferation and activation, a decrease in apoptosis and in DNA repair; Events that are hallmarks of HTLV-I, induced T cell leukemogenesis.