Repression of Transcription From the Human T-Cell Leukemia Virus Type I Long Terminal Repeat and Cellular Gene Promoters by Wild-Type p53

Abstract

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL) transforms human T cells both in vivo and in vitro. However, the long latency period between infection and development of ATL, as well as the small fraction of the infected population that actually develops this disease, suggest that factors in addition to the virus are involved in its pathogenesis. Mutation of tumor suppressor gene p53 has been found in both HTLV-I–transformed T-cell lines and ATL cases at relatively low frequency. However, increasing evidence supports p53 functional impairment in HTLV-I–transformed T cells. Tax, the major transactivator of HTLV-I, is critical for the initial events involved in transformation. We have considered the possibility that p53 may regulate transcription of viral and cellular genes important for viral replication and transformation. Inactivation of p53 function might then permit constitutive expression of these viral and cellular genes. We have investigated the effects of wild-type and mutant p53 on Tax-mediated activation of the HTLV-I long terminal repeat (LTR) and the promoters of several cellular genes including the interleukin (IL)-1α, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF ), and IL-2 receptor α chain gene. Jurkat, HuT78, and U937 cells were cotransfected with plasmids containing a chloramphenicol acetyltransferase (CAT ) reporter gene under viral or cellular promoter control and the Tax expression vector, in addition to vectors for a wild-type or mutant p53. Wild-type p53 is a potent repressor of viral and cellular activation by Tax. Mutations within p53 severely inhibit this downregulation. We also show that wild-type p53 suppresses transcription from the HTLV-I LTR in Jurkat-Tax, a T-cell line stably expressing Tax, and MT-2, a HTLV-I–transformed T-cell line. Wild-type, but not mutant, p53 interfered with the binding of TATA-binding protein (TBP) to the TATA motif of the HTLV-I LTR. These results suggest that p53 inactivation may lead to upregulation of viral and cellular genes and may also be important for establishment of productive viral infection and development of ATL.