T cell leukemia virus type 1 (HTLV-1) is a causative factor for adult T cell leukemia and lymphoma (ATLL). HTLV-1 genome encodes a viral transforming protein, Tax1, which plays an essential role in initiating oncogenesis. One of the critical features that Tax1 contributes to oncogenesis is induction of persistent activation of NF-(B. Yet, how this activity correlates with T cell transformation remains unsolved. Our work demonstrates that Tax1 is a critical lipid raft modulator that can hijack I(B kinases (IKKs) to the lipid raft microdomains in HTLV-1-transformed T cells. It is well recognized that enrichment of IKKs in the lipid raft microdomains leads to activation of NF-(B in antigen-stimulated T cells. Tax1 is able to recruit IKKs persistently in lipid rafts, correlating with persistent NF-(B activity. We also find that Tax1 sequesters beclin 1 (BECN1) and Bif-1, the essential autophagy mediators, in lipid rafts in IKK(- dependent manner. Conversely, BECN1 represses Tax1 activation of NF-(B by inhibiting IKK(. Since loss of BECN1 or Bif-1 has been linked to spontaneous tumorigenesis including lymphoma, our findings strongly suggest that Tax1 links IKK( activation to oncogenesis partly through its anti-autophagy activity. The rationale for this proposal is based on the assumption that the axis of Tax1-IKK(-BECN1/Bif-1 leads to "loss of function" of the autophagy mediators. The proposed research is both innovative and significant, representing a new concept in filling a giant gap for our understanding about how Tax initiates oncogenic transformation of T cells. Our central hypothesis is that "HTLV-1 Tax suppresses autophagy by hijacking IKK( to lipid rafts leading to sequestration of the autophagy mediators in the microdomains, thereby contributing significantly to oncogenesis". We plan to investigate this hypothesis with following specific aims.
Aim#1 : Define the domain critical for lipid raft targeting of Tax1. We will determine the motifs and modifications such as ubiquitination crucial for lipid raft targeting of Tax1, identify a cellular factor in assisting lipid raft association of Tax1, and assess the importance of the lipid raft association of Tax1 in immortalizing primary human CD4+ T cells and in transforming NIH3T3 cells.
Aim#2 : Investigate the underlying mechanism of Tax1 to deregulate autophagy. We will investigate the mechanism of Tax1 in sequestrating Bif-1 and BECN1 in lipid rafts, evaluate the inhibitory effect of BECN1 on Tax1 activation of IKK(, and examine anti-autophagy and tumorigenic activity of Tax1 using MEF (Bif1+/+) and (Bif1-/-) cells.
Aim#3 : Determine oncogenic potential and anti-autophagy function of lipid raft-targeted IKK(. We will evaluate physical and functional interaction of IKK( with Bif-1 and BECN1 and assess anti-autophagy and in vitro transforming activities of the lipid raft-targeted IKK(. This study will help to decipher the pathological role of the axis of Tax- IKK(-BECN1/Bif-1 in HTLV-1 oncogenesis.
Adult T cell leukemia/lymphoma (ATLL) is caused by infection with human T cell leukemia virus type 1 (HTLV-1), infecting over 20 million patients worldwide. HTLV-1 encodes a viral oncogenic protein Tax, which plays an essential role in initiating malignancy of human T cells partly through persistent activation of NF-(B. We plan to investigate the mechanism that Tax mediates persistent activation of NF-(B and anti-autophagy function during development of T cell leukemia.
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