HTLV-1 bZIP Factor RNA and Protein Impart Distinct Functions on T-cell Proliferation and Survival

Abstract

Infection of T cells with human T-cell leukemia virus type-1 (HTLV-1) induces clonal proliferation and is closely associated with the onset of adult T-cell leukemia-lymphoma (ATL) and inflammatory diseases. Although Tax expression is frequently suppressed in HTLV-1-infected cells, the accessory gene, HTLV-1 bZIP factor (HBZ), is continuously expressed and has been implicated in HTLV-1 pathogenesis. Here, we report that transduction of mouse T cells with specific mutants of HBZ that distinguish between its RNA and protein activity results in differential effects on T-cell proliferation and survival. HBZ RNA increased cell number by attenuating apoptosis, whereas HBZ protein induced apoptosis. However, both HBZ RNA and protein promoted S-phase entry of T cells. We further identified that the first 50 bp of the HBZ coding sequence are required for RNA-mediated cell survival. Transcriptional profiling of T cells expressing wild-type HBZ, RNA, or protein revealed that HBZ RNA is associated with genes involved in cell cycle, proliferation, and survival, while HBZ protein is more closely related to immunological properties of T cells. Specifically, HBZ RNA enhances the promoter activity of survivin, an inhibitor of apoptosis, to upregulate its expression. Inhibition of survivin using YM155 resulted in impaired proliferation of several ATL cell lines as well as a T-cell line expressing HBZ RNA. The distinct functions of HBZ RNA and protein may have several implications for the development of strategies to control the proliferation and survival mechanisms associated with HTLV-1 infection and ATL.

Novel interactions between the HTLV antisense proteins HBZ and APH-2 and the NFAR protein family: Implications for the HTLV lifecycles

“…We did however observe that treatment of HTLV infected cells with YM155, which has been shown to suppress the ability of NF110 to activate survivin gene expression (Gwizdek et al, 2004), results in reduced viral replication in MT2 and Mo cells. This is consistent with previous studies reports showing that YM155 causes apoptosis of ATL cells (Mitobe et al, 2015; Chen et al, 2013). As YM155 inhibits NF110-mediated survivin expression, this effect could be due to induction of apoptosis in treated cells, however further analysis would be required to confirm this.…”

“…As elevated levels of survivin are linked to the pathogenesis of HTLV-1, our study coupled with the work of Mitobe et al Mitobe et al, (2015) strongly supports a role for HBZ in survivin regulation. In that study (Mitobe et al, 2015), the authors employed methodologies that distinguished between HBZ RNA and protein activity on survivin gene transcription. Their analysis indicates that HBZ RNA functions as an anti-apoptotic factor by upregulating survivin expression, whereas HBZ protein predominantly modulates the transcription of immune-related genes and promotes proliferation and apoptosis leading to the suggestion that HBZ RNA likely compensates for the apoptosis-promoting effects of HBZ protein.…”

“…Their analysis indicates that HBZ RNA functions as an anti-apoptotic factor by upregulating survivin expression, whereas HBZ protein predominantly modulates the transcription of immune-related genes and promotes proliferation and apoptosis leading to the suggestion that HBZ RNA likely compensates for the apoptosis-promoting effects of HBZ protein. Even though our study did not discriminate between HBZ RNA or protein, our data demonstrates that HBZ and APH-2 regulate survivin expression through the −281 to +30 region of the survivin promoter, which encompasses the region −84 to +34 bp recently reported to be involved in survivin activation by HBZ RNA (Mitobe et al, 2015). Hence we suspect that the effect of HBZ on survivin expression that we observe may be due to HBZ RNA rather than its protein product.…”

“…Our studies demonstrate that HBZ, and to a lesser extent APH-2 significantly upregulates survivin expression. To our knowledge this is the first report showing that APH-2 can regulate survivin, though both Tax1 (Kawakami et al, 2005; Banerjee et al, 2008), and recently, HBZ (Mitobe et al, 2015) have been demonstrated to modulate survivin. As elevated levels of survivin are linked to the pathogenesis of HTLV-1, our study coupled with the work of Mitobe et al Mitobe et al, (2015) strongly supports a role for HBZ in survivin regulation.…”

“…To our knowledge this is the first report showing that APH-2 can regulate survivin, though both Tax1 (Kawakami et al, 2005; Banerjee et al, 2008), and recently, HBZ (Mitobe et al, 2015) have been demonstrated to modulate survivin. As elevated levels of survivin are linked to the pathogenesis of HTLV-1, our study coupled with the work of Mitobe et al Mitobe et al, (2015) strongly supports a role for HBZ in survivin regulation. In that study (Mitobe et al, 2015), the authors employed methodologies that distinguished between HBZ RNA and protein activity on survivin gene transcription.…”