MOLECULAR CHARACTERIZATION OF THE INTERACTIONS BETWEEN HTLV-1 AND HTLV-2 TAX, HBZ AND APH-2 REGULATORY PROTEINS: FUNCTIONAL IMPLICATION FOR NF-ĸB AND IFN-β

Human T-cell leukemia virus types 1 and 2 (HTLV-1 and 2) are genetically related retroviruses that cause persistent infections in vivo. HTLV-1 is the causative agent of adult T-cell leukemia (ATLL) and myelopathy/tropical spastic paraparesis, while HTLV-2 is not associated with ATLL-like disorders. Two viral proteins expressed by the HTLV-1, Tax and HBZ, are the major focus of current research that aims to understand the oncogenic mechanism of ATLL. Tax plays a pivotal role in the deregulation of cellular pathways involved in the immune response, inflammation, cell survival and cancer, whereas HBZ plays a crucial role in T cell proliferation. Comparison of HTLV-1 Tax and HBZ molecular properties and functions to the homologous HTLV-2 Tax and APH-2 proteins should provide insight into better understand the keys steps in leukemogenesis. The Ph.D. study has been focused on HTLV regulatory proteins interaction with host factors, investigating two aspects: a) Tax-1 and Tax-2 interaction with the IKK-related kinases; b) HBZ and APH-2 interaction with host factors involved in NF-ĸB pathway. The results obtained in these studies showed a novel interaction of Tax-1 and Tax-2 proteins with IKKε and TBK1 kinases. The data demonstrated that Tax proteins may be recruited in complexes that contain IKKε and TBK1, altering IFN-β promoter activation. These results highlight a key role of TBK1/IKKε in the regulation of the interferon-β response in the presence of viral protein expression. Additional analyses were performed to investigate the role of APH-2 interaction with host factors involved on NF-ĸB pathway in comparison with its homolog HBZ. The results showed that APH-2, like HBZ, interacts with p65 and inhibits NF-ĸB. I have demonstrated for the first time that APH-2 is able to inhibit Tax proteins-mediated NF-ĸB activation and that APH-2 differs from HBZ in the mechanism of inhibition. APH-2 is able to interact with Tax proteins, and when co-expressed with Tax-2, they re-localized in the cells. Moreover, the data showed that APH-2 was present in Tax-2 cytoplasmic structures containing TAB2 and TRAF3 and that APH-2 interacted with TRAF3. The recruitment of APH-2 in cytoplasmic complexes containing Tax-2 may explain the differences in inhibition of NF-ĸB pathway compared to HBZ.