Interferon-α (IFN-α) suppresses HTLV-1 gene expression and cell cycling, while IFN-α combined with zidovudin induces p53 signaling and apoptosis in HTLV-1-infected cells

Abstract

BackgroundHuman 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.ResultsILTs 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.ConclusionsIFN-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.

Alpha Interferon Restricts Human T-Lymphotropic Virus Type 1 and 2 De Novo Infection through PKR Activation

“…It might involve an effect both on infected transformed cells that are poorly sensitive to IFN-I but respond to AZT if they have an intact p53 pathway and on noninfected cells present in the microenvironment that should become refractory to infection due to IFN-I (39,56,57). In contrast, IFN-␣ leads only to a minimal decrease in HTLV-1 PVL in HAM/TSP patients (27), suggesting that it cannot allow clearance of HTLV-1-infected cells, but rather, only transiently suppresses viral expression.…”

Quantification of HTLV-1 reverse transcriptase activity in ATL patients treated with zidovudine and interferon-α

“…[12][13][14][15][16] These results changed the clinical management of ATL. [16][17][18][19] However, there is yet no direct proof of an antiviral mechanism of action of the drug combination, due to the absence of virological markers that strictly correlate with disease status.…”

Human T-Cell Leukemia/Lymphoma Virus Type 1 p30, but Not p12/p8, Counteracts Toll-Like Receptor 3 (TLR3) and TLR4 Signaling in Human Monocytes and Dendritic Cells

“…The mechanisms underlying this therapeutic effect remain unclear. In fact, while some groups demonstrated in vitro that the treatment with IFN-␣ decreases HTLV-1 expression, Kannagi's group showed that the level of p19 Gag released in the supernatant is decreased, but not the expression of other viral genes (55). However, the addition of AZT decreases viral transcription and p19 Gag and Tax production, leaving the p53-dependent apoptotic pathways unaffected and restoring the PKR antiviral activity (55).…”