NF-κB hyper-activation by HTLV-1 tax induces cellular senescence, but can be alleviated by the viral anti-sense protein HBZ

Tax protein-induced expression of antiapoptotic Bfl-1 protein contributes to survival of human T-cell leukemia virus type 1 (HTLV-1)-infected T-cells

Human T lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia/lymphoma (ATLL). ATLL is a severe malignancy with no effective treatment. HTLV-1 regulatory proteins Tax and HTLV-1 basic leucine zipper factor (HBZ) play a major role in ATLL development, by interfering with cellular functions such as CD4(+) T-cell survival. In this study, we observed that the expression of Bfl-1, an antiapoptotic protein of the Bcl-2 family, is restricted to HTLV-1-infected T-cell lines and to T-cells expressing both Tax and HBZ proteins. We showed that Tax-induced bfl-1 transcription through the canonical NF-κB pathway. Moreover, we demonstrated that Tax cooperated with c-Jun or JunD, but not JunB, transcription factors of the AP-1 family to stimulate bfl-1 gene activation. By contrast, HBZ inhibited c-Jun-induced bfl-1 gene activation, whereas it increased JunD-induced bfl-1 gene activation. We identified one NF-κB, targeted by RelA, c-Rel, RelB, p105/p50, and p100/p52, and two AP-1, targeted by both c-Jun and JunD, binding sites in the bfl-1 promoter of T-cells expressing both Tax and HBZ. Analyzing the potential role of antiapoptotic Bcl-2 proteins in HTLV-1-infected T-cell survival, we demonstrated that these cells are differentially sensitive to silencing of Bfl-1, Bcl-x(L), and Bcl-2. Indeed, both Bfl-1 and Bcl-x(L) knockdowns decreased the survival of HTLV-1-infected T-cell lines, although no cell death was observed after Bcl-2 knockdown. Furthermore, we demonstrated that Bfl-1 knockdown sensitizes HTLV-1-infected T-cells to ABT-737 or etoposide treatment. Our results directly implicate Bfl-1 and Bcl-x(L) in HTLV-1-infected T-cell survival and suggest that both Bfl-1 and Bcl-x(L) represent potential therapeutic targets for ATLL treatment.

Differential role of PKC-induced c-Jun in HTLV-1 LTR activation by 12-O-tetradecanoylphorbol-13-acetate in different human T-cell lines

We have previously shown that TPA activates HTLV-1 LTR in Jurkat T-cells by inducing the binding of Sp1-p53 complex to the Sp1 site residing within the Ets responsive region 1 (ERR-1) of the LTR and that this activation is inhibited by PKCalpha and PKCepsilon. However, in H9 T-cells TPA has been noted to activate the LTR in two consecutive stages. The first stage is activation is mediated by PKCetta and requires the three 21 bp TRE repeats. The second activation mode resembles that of Jurkat cells, except that it is inhibited by PKCdelta. The present study revealed that the first LTR activation in H9 cells resulted from PKCetta-induced elevation of non-phosphorylated c-Jun which bound to the AP-1 site residing within each TRE. In contrast, this TRE-dependent activation did not occur in Jurkat cells, since there was no elevation of non-phosphorylated c-Jun in these cells. However, we found that PKCalpha and PKCepsilon, in Jurkat cells, and PKCetta and PKCdelta, in H9 cells, increased the level of phosphorylated c-Jun that interacted with the Sp1-p53 complex. This interaction prevented the Sp1-p53 binding to ERR-1 and blocked, thereby, the ERR-1-mediated LTR activation. Therefore, this PKC-inhibited LTR activation started in both cell types after depletion of the relevant PKCs by their downregulation. In view of these variable activating mechanisms we assume that there might be additional undiscovered yet modes of HTLV-1 LTR activation which vary in different cell types. Moreover, in line with this presumption we speculate that in HTLV-1 carriers the LTR of the latent provirus may also be reactivated by different mechanisms that vary between its different host T-lymphocyte subclones. Since this reactivation may initiate the ATL process, understanding of these mechanisms is essential for establishing strategies to block the possibility of reactivating the latent virus as preventive means for ATL development in carriers.

Distinct transformation tropism exhibited by human T lymphotropic virus type 1 (HTLV-1) and HTLV-2 is the result of postinfection T cell clonal expansion

Human T lymphotropic virus type 1 (HTLV-1) and HTLV-2 are related but pathogenically distinct viruses. HTLV-1 mainly causes adult T cell leukemia, while HTLV-2 is not associated with leukemia. In vitro, HTLV-1 and HTLV-2 predominantly transform CD4(+) and CD8(+) T cells, respectively: the genetic determinant maps to the viral envelope. Herein, we investigate whether this transformation tropism occurs during initial infection or subsequently during the cellular transformation process. Since most individuals are chronically infected at the time of detection, we utilized an established rabbit model to longitudinally measure the early HTLV-1 and HTLV-2 infection and replication kinetics in purified CD4(+) and CD8(+) T cells. HTLV-1 and HTLV-2 were detected in both CD4(+) and CD8(+) T cells within 1 week postinoculation. In HTLV-1-infected rabbit CD4(+) T cells, proviral burden and tax/rex mRNA expression peaked early, and expression levels were directly proportional to each other. The late expression of the antisense transcript (Hbz or Aph-2) correlated directly with a late proviral burden peak in HTLV-1- or HTLV-2-infected rabbit CD8(+) T cells, respectively. This study provides the first in vivo evidence that these viruses do not exhibit cellular preference during initial infection. We further evaluated the transformation tropism of HTLV-1 and HTLV-2 over a 9-week period using in vitro cell growth/immortalization assays. At the early weeks, both HTLV-1 and HTLV-2 showed proportionate growth of CD4(+) and CD8(+) T cells. However, beyond week 5, the predominance of one particular T cell type emerged, supporting the conclusion that transformation tropism is a postinfection event due to selective clonal expansion over time.

How the DNA damage response determines the fate of HTLV-1 Tax-expressing cells

How the Human T lymphotropic virus type 1 (HTLV-1) Tax protein stimulates proliferation while triggering cell cycle arrest and senescence remains puzzling. There is also a debate about the ability of Tax to activate or inhibit the DNA damage response. Here, we comment on these different activities and propose a conceptual rationale for the apparently conflicting observations.

Comparison of the Genetic Organization, Expression Strategies and Oncogenic Potential of HTLV-1 and HTLV-2

Human T cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are genetically related complex retroviruses that are capable of immortalizing human T-cells in vitro and establish life-long persistent infections in vivo. In spite of these apparent similarities, HTLV-1 and HTLV-2 exhibit a significantly different pathogenic potential. HTLV-1 is recognized as the causative agent of adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In contrast, HTLV-2 has not been causally linked to human malignancy, although it may increase the risk of developing inflammatory neuropathies and infectious diseases. The present paper is focused on the studies aimed at defining the viral genetic determinants of the pathobiology of HTLV-1 and HTLV-2 through a comparison of the expression strategies and functional properties of the different gene products of the two viruses.

Molecular and Cellular Mechanism of Leukemogenesis of ATL: Emergent Evidence of a Significant Role for HBZ in HTLV-1-Induced Pathogenesis

Adult T-cell leukemia (ATL) is a leukemia derived from mature CD4⁺ T cells and induced by human T-cell leukemia virus type 1 (HTLV-1) infection. Previous studies have revealed many possible molecular and cellular mechanisms of HTLV-1-induced leukemogenesis, but it still remains unknown how HTLV-1 transforms peripheral CD4 T cells in infected individuals. Given the fact that only 2–5% of infected individuals develop ATL, HTLV-1 infection alone is not sufficient for the transformation of infected cells. Host genetic and epigenetic abnormalities and host immunological status should be considered in attempting to understand the mechanism of the oncogenesis of ATL. Nonetheless, it is obvious that HTLV-1 infection dramatically increases the risk of leukemia generation from peripheral CD4 T-cells, in which the incidence of leukemia is quite low. Furthermore, the evidence that all ATL cases retain the HTLV-1 provirus, especially the 3′ region, indicates that HTLV-1-encoded genes play a critical role in leukemogenesis. Since increasing evidence indicates that the HTLV-1 bZIP factor (HBZ) gene plays a significant role in the pathogenesis of HTLV-1, we will discuss the cellular and molecular mechanism of ATL generation from the virological point of view, particularly focusing on HBZ.

Conference highlights of the 15th International Conference on Human Retrovirology: HTLV and related retroviruses, 4-8 June 2011, Leuven, Gembloux, Belgium

The June 2011 15th International Conference on Human Retrovirology: HTLV and Related Viruses marks approximately 30 years since the discovery of HTLV-1. As anticipated, a large number of abstracts were submitted and presented by scientists, new and old to the field of retrovirology, from all five continents. The aim of this review is to distribute the scientific highlights of the presentations as analysed and represented by experts in specific fields of epidemiology, clinical research, immunology, animal models, molecular and cellular biology, and virology.

Caffeic acid 3,4-dihydroxy-phenethyl ester induces cancer cell senescence by suppressing twist expression

Compared with traditional cytotoxic cancer therapy, therapy-induced cancer cell senescence attracts much interest because it is similarly effective, has fewer side effects, and is more efficiently cleared by immune cells. In this study, we demonstrate that unlike caffeic acid phenethyl ester, caffeic acid 3,4-dihydroxy-phenethyl ester (CADPE), which is isolated from the medicinal plants Sarcandra glabra and Teucrium pilosum, inhibits human cancer cell growth and colony formation by inducing cancer cell senescence, not apoptosis. CADPE induces cell senescence and morphology changes by increasing cellular size and cytoplasmic granularity, enhancing senescence-associated β-galactosidase activity and differentiated embryo-chondrocyte expressed gene 1 expression, and blocking cell-cycle arrest in the G(1) phase. To help understand the underlying mechanisms, we show that CADPE significantly suppressed the expression of Twist1 and led to the up-regulation of rat sarcoma, p53, p21(WAF1/CIP1), and p16(INK4a) proteins in a dose-dependent manner, resulting in the hypophosphorylation of retinoblastoma protein. Furthermore, overexpression of Twist1 prevented CADPE-induced cell senescence in tumor cells. Therefore, our studies provide evidence for a novel role of CADPE in cancer cell senescence by targeting the Twist1-dependent senescence signaling pathway.