title: Interferon-α (IFN-α) suppresses HTLV-1 gene expression and cell cycling, while IFN-α combined with zidovudine induces p53 signaling and apoptosis in HTLV-1-infected cells.
abstract: BACKGROUND
  Human T-cell leukemia virus type-1 (HTLV-1) is the causative retrovirus of adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 gene expression is maintained at low levels in vivo by unknown mechanisms. A combination therapy of interferon-α (IFN-α) and zidovudin (AZT) shows therapeutic effects in ATL patients, although its mechanism is also obscure. We previously found that viral gene expression in IL-2-dependent HTLV-1-infected T-cells (ILTs) derived from ATL patients was markedly suppressed by stromal cells through a type I IFN response. Here, we investigated the effects of IFN-α with or without AZT on viral gene expression and cell growth in ILTs.
  
  
  RESULTS
  ILTs expressed variable but lower amounts of HTLV-1 Tax protein than HTLV-1-transformed HUT102 cells. Following the addition of IFN-α, the amounts of HTLV-1 p19 in the supernatants of these cells decreased in three days, while HTLV-1 gene expression decreased only in ILTs but not HUT102 cells. IFN-α also suppressed the spontaneous HTLV-1 induction in primary ATL cells cultured for 24 h. A time course study using ILTs revealed that the levels of intracellular Tax proteins decreased in the first 24 h after addition of IFN-α, before the reduction in HTLV-1 mRNA levels. The initial decreases of Tax protein following IFN-α treatment were observed in 6 of 7 ILT lines tested, although the reduction rates varied among ILT lines. An RNA-dependent protein kinase (PKR)-inhibitor reversed IFN-mediated suppression of Tax in ILTs. IFN-α also induced cell cycle arrest at the G0/G1 phase and suppressed NF-κB activities in these cells. AZT alone did not affect HTLV-1 gene expression, cell viability or NF-κB activities. AZT combined with IFN-α markedly induced cell apoptosis associated with phosphorylation of p53 and induction of p53-responsive genes in ILTs.
  
  
  CONCLUSIONS
  IFN-α suppressed HTLV-1 gene expression at least through a PKR-mediated mechanism, and also induced cell cycle arrest in ILTs. In combination with AZT, IFN-α further induced p53 signaling and cell apoptosis in these cells. These findings suggest that HTLV-1-infected cells at an IL-2-dependent stage retain susceptibility to type I IFN-mediated regulation of viral expression, and partly explain how AZT/IFN-α produces therapeutic effects in ATL.
authors.affiliation: Department of Immunotherapeutics, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo, 113-8519, Japan.
authors.forename: Shuichi
authors.initials: S
authors.lastname: Kinpara
authors.forename: Mami
authors.initials: M
authors.lastname: Kijiyama
authors.forename: Ayako
authors.initials: A
authors.lastname: Takamori
authors.forename: Atsuhiko
authors.initials: A
authors.lastname: Hasegawa
authors.forename: Amane
authors.initials: A
authors.lastname: Sasada
authors.forename: Takao
authors.initials: T
authors.lastname: Masuda
authors.forename: Yuetsu
authors.initials: Y
authors.lastname: Tanaka
authors.forename: Atae
authors.initials: A
authors.lastname: Utsunomiya
authors.forename: Mari
authors.initials: M
authors.lastname: Kannagi
mesh_terms: D000998:Antiviral Agents / Q000494:pharmacology*; D017209:Apoptosis; D059447:Cell Cycle Checkpoints; D015368:Human T-lymphotropic virus 1 / Q000276:immunology*; D006801:Humans; D016898:Interferon-alpha / Q000276:immunology*; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000187:drug effects / Q000502:physiology / Q000821:virology*; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*; D015215:Zidovudine / Q000494:pharmacology*; D019892:eIF-2 Kinase / Q000378:metabolism
chemical_list: D000998:Antiviral Agents; D016898:Interferon-alpha; D016159:Tumor Suppressor Protein p53; D015215:Zidovudine; D019892:eIF-2 Kinase