HTLV-1病毒蛋白Tax和HBZ协同促进成人T细胞白血病发生的机制

人类T细胞白血病1型病毒(Human T-cell leukemia virus type 1,HTLV-1)是与人类疾病发生密切相关的逆转录病毒。该病毒的感染可引起成人T细胞白血病(Adult T-cell leukemia,ATL)等多种疾病的发生。Tax和HBZ(HTLV-1 basic zipper protein)是由HTLV-1前病毒编码的两个关键病毒蛋白,它们被认为在HTLV-1病毒的复制、生存和致癌过程中发挥了至关重要的作用。本文就Tax和HBZ的蛋白结构以及它们在调控病毒转录、细胞生长和凋亡、病毒潜伏期,最终协同促进成人T细胞白血病发生过程中发挥的作用作一综述。     

5'-long terminal repeat-selective CpG methylation of latent human T-cell leukemia virus type 1 provirus in vitro and in vivo

CpG methylation of the human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat (LTR) has been implicated in proviral latency, but there is presently little information available regarding the pattern of LTR methylation and its effect on viral gene expression. To gain insight into the mechanisms of HTLV-1 latency, we have studied methylation of individual CpG sites in the U3-R region of the integrated proviral LTR by using bisulfite genomic sequencing methods. Surprisingly, our results reveal selective hypermethylation of the 5' LTR and accompanying hypomethylation of the 3' LTR in both latently infected cell lines and adult T-cell leukemia (ATL) cells having a complete provirus. Moreover, we observed a lack of CpG methylation in the LTRs of 5'-defective proviruses recovered from ATL samples, which is consistent with the selective hypomethylation of the 3' LTR. Thus, the integrated HTLV-1 provirus in these carriers appears to be hypermethylated in the 5' LTR and hypomethylated in the 3' LTR. These results, together with the observation that proviral gene expression is reactivated by 5-azacytidine in latently infected cell lines, indicate that selective hypermethylation of the HTLV-1 5' LTR is common both in vivo and in vitro. Thus, hypermethylation of the 5' LTR appears to be an important mechanism by which HTLV-1 gene expression is repressed during viral latency.     

Human T-cell leukemia virus type 1 Tax transactivates the matrix metalloproteinase 7 gene via JunD/AP-1 signaling

Adult T-cell leukemia (ATL) is a T-cell malignancy associated with human T-cell leukemia virus type 1 (HTLV-1) and characterized by visceral invasion. Degradation of the extracellular matrix by matrix metalloproteinases (MMPs) is a crucial process in invasion of tumors and metastasis. MMP-7 (or matrilysin), is a “minimal domain MMP” with proteolytic activity against components of the extracellular matrix. To determine the involvement of MMP-7 in visceral spread in ATL, this study investigated MMP-7 expression in ATL. MMP-7 expression was identified in HTLV-1-infected T-cell lines, peripheral blood ATL cells and ATL cells in lymph nodes, but not in uninfected T-cell lines or normal peripheral blood mononuclear cells. MMP-7 expression was induced following infection of a human T-cell line with HTLV-1, and specifically by the viral protein Tax. Functionally, MMP-7 promoted cell migration of HTLV-1-infected T cells. The MMP-7 promoter activity was increased by Tax and reduced by deletion of the activator protein-1 (AP-1) binding site. Electrophoretic mobility shift assay showed high levels of AP-1 binding proteins, including JunD, in HTLV-1-infected T-cell lines and ATL cells, and Tax elicited JunD binding to the MMP-7 AP-1 element. Tax-induced MMP-7 activation was inhibited by dominant negative JunD and augmented by JunD/JunD homodimers. Short interfering RNA against JunD inhibited MMP-7 mRNA expression in HTLV-1-infected T-cell lines. These results suggest that the induction of MMP-7 by Tax is regulated by JunD and that MMP-7 could facilitate visceral invasion in ATL.     

Human T-cell leukemia virus type 1 tax transactivates the matrix metalloproteinase 7 gene via JunD/AP-1 signaling

Adult T-cell leukemia (ATL) is a T-cell malignancy associated with human T-cell leukemia virus type 1 (HTLV-1) and characterized by visceral invasion. Degradation of the extracellular matrix by matrix metalloproteinases (MMPs) is a crucial process in invasion of tumors and metastasis. MMP-7 (or matrilysin), is a "minimal domain MMP" with proteolytic activity against components of the extracellular matrix. To determine the involvement of MMP-7 in visceral spread in ATL, this study investigated MMP-7 expression in ATL. MMP-7 expression was identified in HTLV-1-infected T-cell lines, peripheral blood ATL cells and ATL cells in lymph nodes, but not in uninfected T-cell lines or normal peripheral blood mononuclear cells. MMP-7 expression was induced following infection of a human T-cell line with HTLV-1, and specifically by the viral protein Tax. Functionally, MMP-7 promoted cell migration of HTLV-1-infected T cells. The MMP-7 promoter activity was increased by Tax and reduced by deletion of the activator protein-1 (AP-1) binding site. Electrophoretic mobility shift assay showed high levels of AP-1 binding proteins, including JunD, in HTLV-1-infected T-cell lines and ATL cells, and Tax elicited JunD binding to the MMP-7 AP-1 element. Tax-induced MMP-7 activation was inhibited by dominant negative JunD and augmented by JunD/JunD homodimers. Short interfering RNA against JunD inhibited MMP-7 mRNA expression in HTLV-1-infected T-cell lines. These results suggest that the induction of MMP-7 by Tax is regulated by JunD and that MMP-7 could facilitate visceral invasion in ATL. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.     

Preferential selection of human T-cell leukemia virus type 1 provirus lacking the 5' long terminal repeat during oncogenesis

In adult T-cell leukemia (ATL) cells, a defective human T-cell leukemia virus type 1 (HTLV-1) provirus lacking the 5' long terminal repeat (LTR), designated type 2 defective provirus, is frequently observed. To investigate the mechanism underlying the generation of the defective provirus, we sequenced HTLV-1 provirus integration sites from cases of ATL. In HTLV-1 proviruses retaining both LTRs, 6-bp repeat sequences were adjacent to the 5' and 3' LTRs. In 8 of 12 cases with type 2 defective provirus, 6-bp repeats were identified at both ends. In five of these cases, a short repeat was bound to CA dinucleotides of the pol and env genes at the 5' end, suggesting that these type 2 defective proviruses were formed before integration. In four cases lacking the 6-bp repeat, short (6- to 26-bp) deletions in the host genome were identified, indicating that these defective proviruses were generated after integration. Quantification indicated frequencies of type 2 defective provirus of less than 3.9% for two carriers, which are much lower than those seen for ATL cases (27.8%). In type 2 defective proviruses, the second exons of the tax, rex, and p30 genes were frequently deleted, leaving Tax unable to activate NF-kappaB and CREB pathways. The HTLV-1 bZIP factor gene, located on the minus strand, is expressed in ATL cells with this defective provirus, and its coding sequences are intact, suggesting its significance in oncogenesis.     

HTLV-1 bZIP Factor Impairs Anti-viral Immunity by Inducing Co-inhibitory Molecule,T Cell Immunoglobulin and ITIM Domain (TIGIT)

Human T-cell leukemia virus type 1 (HTLV-1) infects CD4⁺ T cells and induces proliferation of infected cells in vivo, which leads to the onset of adult T-cell leukemia (ATL) in some infected individuals. The HTLV-1 bZIP factor (HBZ) gene, which is encoded in the minus strand of HTLV-1, plays critical roles in pathogenesis. In this study, RNA-seq and ChIP-seq analyses using HBZ transduced T cells revealed that HBZ upregulates the expression and promoter acetylation levels of a co-inhibitory molecule, T cell immunoglobulin and ITIM domain (TIGIT), in addition to those of regulatory T cells related genes, Foxp3 and Ccr4. TIGIT was expressed on CD4⁺ T cells from HBZ-transgenic (HBZ-Tg) mice, and on ATL cells and HTLV-1 infected CD4⁺ T cells of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in vivo. Expression of Blimp1 and IL-10 was upregulated in TIGIT⁺CD4⁺ cells of HBZ-Tg mice compared with TIGIT^-CD4⁺ T cells, suggesting the correlation between TIGIT expression and IL-10 production. When CD4⁺ T cells from HBZ-Tg mice were stimulated with TIGIT’s ligand, CD155, their production of the inhibitory cytokine IL-10 was enhanced. Furthermore, dendritic cells from HBZ-Tg mice produced high levels of IL-10 after stimulation. These data suggest that HBZ alters immune system to suppressive state via TIGIT and IL-10. Importantly, TIGIT suppressed T-cell responses to another HTLV-1 virus protein, Tax, in vitro. Blocking of TIGIT and PD-1 slightly increased anti-Tax T-cell activity in some HAM/TSP patients. These results suggest that HBZ-induced TIGIT on HTLV-1 infected cells impairs T-cell responses to viral antigens. This study shows that HBZ-induced TIGIT plays a pivotal role in attenuating host immune responses and shaping a microenvironment favorable to HTLV-1.     

Human T-cell leukemia virus type 1 (HTLV-1) infection of mice: proliferation of cell clones with integrated HTLV-1 provirus in lymphoid organs

Human T-cell leukemia virus type 1 (HTLV-1) is suggested to cause adult T-cell leukemia after 40 to 50 years of latency in a small percentage of carriers. However, little is known about the pathophysiology of the latent period and the reservoir organs where polyclonal proliferation of cells harboring integrated provirus occurs. The availability of animal models would be useful to analyze the latent period of HTLV-1 infection. At 18 months after HTLV-1 infection of C3H/HeJ mice inoculated with the MT-2 cell line, which is an HTLV-1-producing human T-cell line, HTLV-1 provirus was detected in spleen DNA from eight of nine mice. No more than around 100 proviruses were found per 10(5) spleen cells. Cellular sequences flanking the 3' long terminal repeat (LTR) and the clonalities of the cells which harbor integrated HTLV-1 provirus were analyzed by linker-mediated PCR. The results showed that the flanking sequences are of mouse genome origin and that polyclonal proliferation of the spleen cells harboring integrated HTLV-1 provirus had occurred in three mice. A sequence flanking the 5' LTR was isolated from one of the mice and revealed the presence of a 6-nucleotide duplication of cellular sequences, consistent with typical retroviral integration. Moreover, PCR was performed on DNA from infected tissues, with LTR primers and primers derived from seven novel flanking sequences of the three mice. Data revealed that the expected PCR products were found from lymphatic tissues of the same mouse, suggesting that the lymphatic tissues were the reservoir organs for the infected and proliferating cell clones. The mouse model described here should be useful for analysis of the carrier state of HTLV-1 infection in humans.     

Role of caspase 9 in activation of HTLV-1 LTR expression by DNA damaging agents

Adult T-cell leukemia (ATL) is caused by HTLV-I. The viral Tax oncoprotein plays a central role in initiating the process to ATL. However, after infection HTLV-1 enters into latency, during which virus gene expression is very low, so that the level of Tax is likely insufficient for exerting its oncogenic activities. Therefore only 5% of the infected individuals may develop ATL several decades after infection. It is assumed that the transition from latency to ATL development requires at least a temporary activation of the latent virus in order to elevate Tax to its oncogenic threshold. We have previously found that DNA damaging agents, which usually induce apoptosis, can also activate the viral LTR and that the anti-apoptosis Bcl-2 protein not only avoid their apoptosis induction but concomitantly prevents their LTR activation effect. Therefore, the present study was designed to identify the factor that while participating in the apoptotic cascade acts also to activate the viral LTR. For this purpose we employed ectopic vectors expressing these apoptotic factors together with potent shRNAs against each of them and anti caspase peptide inhibitors. We have found that in addition to its function as initiator of the mitochondrial apoptotic cascade, caspase 9 can acts also as an executer which among other non-apoptotic functions it forms an Sp1-p53 complex that activates the LTR by binding to an Sp1 recognition site residing in the LTR. This finding can help in designing effective preventing strategies against ATL development in clinically latent HTLV-1 carriers.     

Differential requirements for activation of integrated and transiently transfected human T-cell leukemia virus type 1 long terminal repeat

Adult T-cell leukemia (ATL) cells contain integrated human T-cell leukemia virus type 1 (HTLV-1) proviruses. Although the exact sequence of events leading to the development of ATL remains incompletely resolved, expression of the integrated HTLV-1 long terminal repeat (LTR) is likely required at some point during the process of T-cell transformation. While much has been learned about the regulated expression of transiently transfected LTR reporter plasmids, an analysis of factors required for expression of chromosomally integrated HTLV-1 LTR has not been done. Here, we have constructed CHOK1 and HeLa cells that contain an integrated HTLV-1 LTR-luciferase gene. Using these cells, we have compared the requirements for activation of transiently transfected versus stably integrated HTLV-1 LTR. We observed different requirements for CREB, p300, and P/CAF in the expression of transiently transfected versus stably integrated HTLV-1 LTR. Furthermore, with dominant-negative mutants of CREB, p300, and P/CAF, we found that activation of integrated HTLV-1 LTR by an ambient stress signal, UV-C, proceeds through a path mechanistically distinct from that used by viral oncoprotein, Tax. Our findings point to additional complexities in the regulated expression of HTLV-1 proviruses compared with those hitherto revealed through transfection studies.