Functional Comparison of HBZ and the Related APH-2 Protein Provides Insight into Human T-Cell Leukemia Virus Type 1 Pathogenesis

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) are highly related retroviruses that transform T cells in vitro but have distinct pathological outcomes in vivo. HTLV-1 encodes a protein from the antisense strand of its proviral genome, the HTLV-1 basic leucine zipper factor (HBZ), which inhibits Tax-1-mediated viral transcription and promotes cell proliferation, a high proviral load, and persistence in vivo. In adult T-cell leukemia/lymphoma (ATL) cell lines and patient T cells, hbz is often the only viral gene expressed. The antisense strand of the HTLV-2 proviral genome also encodes a protein termed APH-2. Like HBZ, APH-2 is able to inhibit Tax-2-mediated viral transcription and is detectable in most primary lymphocytes from HTLV-2-infected patients. However, unlike HBZ, the loss of APH-2 in vivo results in increased viral replication and proviral loads, suggesting that HBZ and APH-2 modulate the virus and cellular pathways differently. Herein, we examined the effect of APH-2 on several known HBZ-modulated pathways: NF-B (p65) transactivation, transforming growth factor ␤ (TGF-␤) signaling, and interferon regulatory factor 1 (IRF-1) transactivation. Like HBZ, APH-2 has the ability to inhibit p65 transactivation. Conversely, HBZ and APH-2 have divergent effects on TGF-␤ signaling and IRF-1 transactivation. Quantitative PCR and protein half-life experiments revealed a substantial disparity between HBZ and APH-2 transcript levels and protein stability, respectively. Taken together, our data further elucidate the functional differences between HBZ and APH-2 and how these differences can have profound effects on the survival of infected cells and, ultimately, pathogenesis. IMPORTANCEHuman T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) are highly related retroviruses that have distinct pathological outcomes in infected hosts. Functional comparisons of HTLV-1 and HTLV-2 proteins provide a better understanding about how HTLV-1 infection is associated with disease and HTLV-2 infection is not. The HTLV genome antisense-strand genes hbz and aph-2 are often the only viral genes expressed in HTLV-infected T cells. Previously, our group found that HTLV-1 HBZ and HTLV-2 APH-2 had distinct effects in vivo and hypothesized that the differences in the interactions of HBZ and APH-2 with important cell signaling pathways dictate whether cells undergo proliferation, apoptosis, or senescence. Ultimately, these functional differences may affect how HTLV-1 causes disease but HTLV-2 generally does not. In the current study, we compared the effects of HBZ and APH-2 on several HTLV-relevant cellular pathways, including the TGF-␤ signaling, NF-B activation, and IRF-1 transactivation pathways. Human T-cell leukemia virus type 1 (HTLV-1) is a complex oncogenic deltaretrovirus that infects an estimated 15 million to 25 million people worldwide, with areas of endemic infection being found in southwestern Japan, Africa, South America, and the Caribbean Basin (1). Approximately 2 to 5% of HTLV-1-infected indi...

The sense behind retroviral anti-sense transcription

“…Human Immunodeficiency Virus (HIV) and Human T-cell Lymphotrophic Virus (HTLV) are the characterized retroviruses exhibiting this phenomenon. The proteins encoded by their antisense strands serve important functions, including control of viral sense transcription, as well as viral latency, pathology, and spread [1–4]. …”

“…HBZ also affects many other cellular processes, including host gene expression, innate immune signaling, apoptosis, autophagy, and DNA repair – all of which further influence the pathology of the HTLV-1 infection (reviewed in [3]). Similar to HTLV-1, HTLV-2, HTLV-3, and HTLV-4 are equally capable of producing antisense proteins – APH-2, APH-3, and APH-4, respectively – though their functions have not been clearly elucidated [4, 26, 28–30]. …”

NF-κB and MicroRNA Deregulation Mediated by HTLV-1 Tax and HBZ

“…Both HBZ and APH-2 counteract the activation of the NF-κB by Tax, but they differ in the mechanisms of inhibition [26,105]. Unlike APH-2, HBZ does not interact with Tax-1, whereas APH-2 does not drastically decrease p65 expression or induce its ubiquitination [119]. We demonstrated that HBZ and APH-2 also differ in their intracellular localization and that only APH-2 forms complexes with cytoplasmic host factors and Tax-2 [105].…”

“…HBZ suppresses the transcription of various NF-κB target genes, including IL-8, IL2RA, IRF4, VCAM-1, CCND1, and VEGF [26,32,117,118]. The molecular mechanism of HBZ-mediated NF-κB inhibition has been investigated in several studies, which revealed that HBZ selectively suppresses the canonical NF-κB pathway by (i) interacting with p65 and reducing its DNA binding capacity, (ii) increasing the expression of PDLIM2, an E3 ubiquitin ligase that determines p65 degradation, and (iii) reducing p65 acetylation [117,119,120].…”

The Human T-Cell Leukemia Virus Type 1 Basic Leucine Zipper Factor Attenuates Repair of Double-Stranded DNA Breaks via Nonhomologous End Joining

“…In contrast, HBZ transcripts have been shown to be continuously present in HTLV-1-infected T cells (57)(58)(59)(60)(61)(62)(63)(64)(65). Therefore, given that both viral proteins have been reported to have relatively long half-lives (120)(121)(122), it is likely that HBZ and Tax are sometimes simultaneously present in primary HTLV-1-infected cells, during which time they could affect the repair of DSBs and additional processes involving DNA-PK.…”