Resistance to tumor necrosis factor-alpha-induced apoptosis in human T-lymphotropic virus type I-infected T cell lines

Human T-Cell Leukemia Virus Type 1 Oncogenesis between Active Expression and Latency: A Possible Source for the Development of Therapeutic Targets

The human T-cell leukemia virus type 1 (HTLV-1) is the only known human oncogenic retrovirus. HTLV-1 can cause a type of cancer called adult T-cell leukemia/lymphoma (ATL). The virus is transmitted through the body fluids of infected individuals, primarily breast milk, blood, and semen. At least 5-10 million people in the world are infected with HTLV-1. In addition to ATL, HTLV-1 infection can also cause HTLV-I-associated myelopathy (HAM/TSP). ATL is characterized by a low viral expression and poor prognosis. The oncogenic mechanism triggered by HTLV-1 is extremely complex and the molecular pathways are not fully understood. However, viral regulatory proteins Tax and HTLV-1 bZIP factor (HBZ) have been shown to play key roles in the transformation of HTLV-1-infected T cells. Moreover, several studies have shown that the final fate of HTLV-1-infected transformed Tcell clones is the result of a complex interplay of HTLV-1 oncogenic protein expression with cellular transcription factors that subvert the cell cycle and disrupt regulated cell death, thereby exerting their transforming effects. This review provides updated information on the mechanisms underlying the transforming action of HTLV-1 and highlights potential therapeutic targets to combat ATL.

Polymorphism and expression of the tumor necrosis factor receptor II gene in cows infected with the bovine leukemia virus

A single T>C nucleotide polymorphism (rs42686850) of bovine tumor necrosis factor receptor type II gene (TNF-RII) is located within a sequence with allele-specific affinity to bind E2F transcription factors, considered pivotal in the regulation of cell cycle and cell proliferation. The objective of the study was to determine the effect of this SNP and BLV infection on the TNF-RII gene expression at the mRNA and protein levels in peripheral blood mononuclear cells (PBMC). We noted that analyzed TNF-RII gene polymorphism influenced the expression of the TNF-RII gene at the mRNA level but only in BLV-positive cows. Concurrently, no statistically significant association was found between gene polymorphism and TNF-RII expression at the protein level. However, we found a significant effect of BLV infection status on the amount of TNF-RII mRNA and the percentage of PBMC expressing TNF-RII. These results show an unclear effect of considered T>C polymorphism on TNF-RII gene expression in bovine leukocytes and they suggest the involvement of BLV in modifying the TNF-RII expression in BLV-infected cows potentially implying the EBL (Enzootic Bovine Leukosis) associated pathogenesis.

The Smac mimetic RMT5265.2HCL induces apoptosis in EBV and HTLV-I associated lymphoma cells by inhibiting XIAP and promoting the mitochondrial release of cytochrome C and Smac

The inhibitors of apoptosis (IAP) are important regulators of apoptosis. However, little is known about the capacity of Smac mimetics (IAP inhibitor) to overcome virally associated-lymphoma's (VAL) resistance to apoptosis. Here, we explored the pro-apoptotic effect of a novel Smac mimetic, RMT5265.2HCL (RMT) in VAL cells. RMT improved the sensitivity to apoptosis in EBV- and to some extend in HTLV-1- but not in HHV-8-VAL. Furthermore, we identified that RMT promotes caspase 3 and 9 cleavage by inhibiting XIAP and inducing the mitochondrial efflux of Smac and cytochrome C. This investigation further support exploring the use of Smac inhibitors in VAL.

Identification of bovine leukemia virus tax function associated with host cell transcription, signaling, stress response and immune response pathway by microarray-based gene expression analysis

Background

Bovine leukemia virus (BLV) is associated with enzootic bovine leukosis and is closely related to human T-cell leukemia virus type I. The Tax protein of BLV is a transcriptional activator of viral replication and a key contributor to oncogenic potential. We previously identified interesting mutant forms of Tax with elevated (Tax_D247G) or reduced (Tax_S240P) transactivation effects on BLV replication and propagation. However, the effects of these mutations on functions other than transcriptional activation are unknown. In this study, to identify genes that play a role in the cascade of signal events regulated by wild-type and mutant Tax proteins, we used a large-scale host cell gene-profiling approach.

Results

Using a microarray containing approximately 18,400 human mRNA transcripts, we found several alterations after the expression of Tax proteins in genes involved in many cellular functions such as transcription, signal transduction, cell growth, apoptosis, stress response, and immune response, indicating that Tax protein has multiple biological effects on various cellular environments. We also found that Tax_D247G strongly regulated more genes involved in transcription, signal transduction, and cell growth functions, contrary to Tax_S240P, which regulated fewer genes. In addition, the expression of genes related to stress response significantly increased in the presence of Tax_S240P as compared to wild-type Tax and Tax_D247G. By contrast, the largest group of downregulated genes was related to immune response, and the majority of these genes belonged to the interferon family. However, no significant difference in the expression level of downregulated genes was observed among the Tax proteins. Finally, the expression of important cellular factors obtained from the human microarray results were validated at the RNA and protein levels by real-time quantitative reverse transcription-polymerase chain reaction and western blotting, respectively, after transfecting Tax proteins into bovine cells and human HeLa cells.

Conclusion

A comparative analysis of wild-type and mutant Tax proteins indicates that Tax protein exerts a significant impact on cellular functions as diverse as transcription, signal transduction, cell growth, stress response and immune response. Importantly, our study is the first report that shows the extent to which BLV Tax regulates the innate immune response.

HTLV-1 and apoptosis: role in cellular transformation and recent advances in therapeutic approaches

A universal cellular defense mechanism against viral invasion is the elimination of infected cells through apoptotic cell death. To counteract host defenses many viruses have evolved complex apoptosis evasion strategies. The oncogenic human retrovirus HTLV-1 is the etiological agent of adult-T-cell leukemia/lymphoma (ATLL) and the neurodegenerative disease known as HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The poor prognosis in HTLV-1-induced ATLL is linked to the resistance of neoplastic T cells against conventional therapies and the immuno-compromised state of patients. Nevertheless, several studies have shown that the apoptotic pathway is largely intact and can be reactivated in ATLL tumor cells to induce specific killing. A better understanding of the molecular mechanisms employed by HTLV-1 to counteract cellular death pathways remains an important challenge for future therapies and the treatment of HTLV-1-associated diseases.

Strong induction of 4-1BB, a growth and survival promoting costimulatory receptor, in HTLV-1-infected cultured and patients' T cells by the viral Tax oncoprotein

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia, stimulates the growth of infected T cells in cultures and in nonleukemic patients. In the latter, HTLV-1 is found in long-term persisting T-cell clones. The persistence of normal T cells is controlled by the growth-stimulating and antiapoptotic functions of costimulatory receptors, while the growth-stimulating HTLV-1 functions are mediated by the viral oncoprotein Tax. Here we analyzed the impact of Tax on costimulatory receptors in T cells with repressible Tax and found that among these receptors 4-1BB (TNFRSF9/CD137/ILA) was induced most strongly. Up-regulated 4-1BB expression was a consistent feature of all HTLV-1-infected cell lines, whether patient-derived or in vitro transformed. Tax was sufficient to induce the expression of the endogenous 4-1BB gene in uninfected T cells, and it strongly activated (45-fold) the 4-1BB promoter via a single NF-kappaB site. The ligand of 4-1BB was also found on transformed T-cell lines, opening up the possibility of autostimulation. Moreover, 4-1BB expression in patients' lymphocytes ex vivo correlated with Tax expression, strongly suggesting Tax-mediated 4-1BB activation in vivo. Thus, 4-1BB up-regulation by Tax could contribute to growth, survival, and clonal expansion of the infected cells during persistence and disease.

Requirement of the human T-cell leukemia virus (HTLV-1) tax-stimulated HIAP-1 gene for the survival of transformed lymphocytes

Human T cell leukemia virus type 1 (HTLV-1), the cause of adult T cell leukemia (ATL), induces clonal expansion of infected T-cells in nonleukemic individuals and immortalizes T cells in vitro. The resistance against apoptotic stimuli of these cells hints at a viral survival function in addition to a proliferation-stimulating activity. Here we describe the up-regulation of the antiapoptotic HIAP-1/CIAP-2 gene as a consistent phenotype of HTLV-1-transformed and ATL-derived cultures and its stimulation by the viral oncoprotein Tax. Cotransfections revealed a 60-fold increase of HIAP-1 promoter activity mediated by Tax mainly via nuclear factor-kappaB (NF-kappaB) activation. To address the relevance of virally increased HIAP-1 levels for the survival of HTLV-1-transformed cells, its expression was RNA interference (RNAi) suppressed using a lentiviral transduction system. This resulted in a dramatic reduction of cell growth, a strong induction of apoptosis rates, and increased caspases 3/7 activity, which is known to be suppressed by HIAP-1. Thus, the Tax-mediated HIAP-1 overexpression is required to suppress endogenous apoptosis and, therefore, is essential for the survival of HTLV-1-transformed lymphocytes. Moreover, this points to HIAP-1 as an important target of the HTLV-1-mediated NF-kappaB activation.

Curcumin induces growth-arrest and apoptosis in association with the inhibition of constitutively active JAK-STAT pathway in T cell leukemia

Adult T cell leukemia is an aggressive and frequently fatal malignancy that expressess constitutively activated growth-signaling pathways in association with deregulated growth and resistance to apoptosis. Curcumin (diferuloylmethane) is a naturally occurring yellow pigment, isolated from the rhizomes of the plant Curcuma longa that has traditionally been used in the treatment of injury and inflammation. But the effect and mechanism of action of curcumin on T cell leukemia is not known. To investigate the antitumor activity of curcumin in T cell leukemia, we examined its effect on constitutive phosphorylation of JAK and STAT proteins, proliferation, and apoptosis in HTLV-I-transformed T cell lines. HTLV-I-transformed T cell leukemia lines, MT-2, HuT-102, and SLB-1, express constitutively phosphorylated JAK3, TYK2, STAT3, and STAT5 signaling proteins. In vitro treatment with curcumin induced a dose-dependent decrease in JAK and STAT phosphorylation resulting in the induction of growth-arrest and apoptosis in T cell leukemia. The induction of growth-arrest and apoptosis in association with the blockade of constitutively active JAK-STAT pathway suggests this be a mechanism by which curcumin induces antitumor activity in T cell leukemia.

Tumor necrosis factor-alpha up-regulation in spontaneously proliferating cells derived from bovine leukemia virus-infected cattle

We previously reported that tumor necrosis factor alpha (TNF-alpha) was one of the cytokines that contributed to the leukemogenesis caused by bovine leukemia virus (BLV). To determine if the spontaneous cell proliferation observed in the late disease stages, such as persistent lymphocytosis and lymphosarcoma, correlated with the expression level of TNF-alpha, we analyzed the mRNA expression levels for TNF-alpha in spontaneously proliferating PBMCs derived from BLV-infected cattle. The mean mRNA expression level for TNF-alpha was higher in the spontaneously proliferating PBMCs derived from BLV-infected cattle than in non-spontaneously proliferating PBMCs from normal cattle. The TNF-alpha protein level in the PBMCs was determined by flow cytometric analysis, and it was noted that most of the cells expressing membrane-bound TNF-alpha in the spontaneously proliferating cells were CD5+ or sIgM+-cells. Additionally, in order to determine if this spontaneous proliferation can be blocked by anti-bovine TNF-alpha MAb, the spontaneously proliferating PBMCs from a BLV-infected cattle were cultured in the presence of the MAb. The addition of this MAb at the beginning of the 72 h-cultivation clearly inhibited spontaneous proliferation of cells in a dose-dependent manner, indicating the direct involvement of TNF-alpha in the spontaneous proliferation of PBMCs during the late disease stage. These data suggest that an aberrant expression of TNF-alpha might contribute to the progression of bovine leukosis in animals which develop persistent lymphocytosis of B-cells or B-cell lymphosarcoma.

Imbalance of tumor necrosis factor receptors during progression in bovine leukemia virus infection

Previously, we found an up-regulation of tumor necrosis factor alpha (TNF)-alpha and an imbalance of TNF receptors in sheep experimentally infected with bovine leukemia virus (BLV). In order to investigate the different TNF-alpha-induced responses, in this study we examined the TNF-alpha-induced proliferative response and the expression levels of two distinct TNF receptors on peripheral blood mononuclear cells (PBMC) derived from BLV-uninfected cattle and BLV-infected cattle that were aleukemic (AL) or had persistent lymphocytosis (PL). The proliferative response of PBMC isolated from those cattle with PL in the presence of recombinant bovine TNF-alpha (rTNF-alpha) was significantly higher than those from AL cattle and uninfected cattle and the cells from PL cattle expressed significantly higher mRNA levels of TNF receptor type II (TNF-RII) than those from AL and BLV-uninfected cattle. No difference was found in TNF-RI mRNA levels. Most cells expressing TNF-RII in PL cattle were CD5+ or sIgM+ cells and these cells showed resistance to TNF-alpha-induced apoptosis. Additionally, there were significant positive correlations between the changes in provirus load and TNF-RII mRNA levels, and TNF-alpha-induced proliferation and TNF-RII mRNA levels. These data suggest that imbalance in the expression of TNF receptors could at least in part contribute to the progression of lymphocytosis in BLV infection.

Immunohistochemical Analysis of Expression Patterns of Tumor Necrosis Factor Receptors on Lymphoma Cells in Enzootic Bovine Leukosis

Tumor necrosis factor-alpha (TNF-alpha) has been reported to be associated with the progression of lymphoproliferative neoplastic diseases and retroviral infections. Hence we examined immunohistochemically the expression patterns of TNF-receptors (TNF-RI and RII) on lymphoma cells derived from the 29 cases of enzootic bovine leukosis (EBL). Lymphomas obtained in 29 animals with EBL were histopathologically classified into three types: diffuse mixed type (10 cases), diffuse large type (9 cases), and diffuse large cleaved type (10 cases). Immunohistochemically using a monoclonal antibody to a bovine lymphocyte surface antigen, the lymphomas were classified into three phenotypes: B-1a (CD5+/CD11b+), B-1b (CD5-/CD11b+) and B-2 (conventional B) (CD5-/CD11b-). Interestingly, the lymphoma cells in all animals expressed TNF-RII, but not TNF-RI. Although, in EBL, lymphoma cells of which the histopathological and immunological property differs has been formed, the expression patterns of TNF-Rs had the universality in all lymphoma cells. TNF-RII, which induces cell proliferation, was expressed but TNF-RI, which induces cell apoptosis was not expressed on all lymphoma cells, suggesting that TNF-Rs play an important role in the malignant proliferation of B cells and formation of lymphomas in EBL.

Modulation of apoptosis during HTLV-1-mediated immortalization process in vitro

Suppression of apoptosis has been proposed as a mechanism involved in the transforming action of human T-cell leukemia/lymphotropic virus type-1 (HTLV-1). However, there is evidence that HTLV-1 and its protein Tax also induce apoptosis. To resolve this apparent paradox, apoptosis was monitored in primary cultures of peripheral blood lymphocytes (PBLs) from healthy donors, following HTLV-1 infection in vitro. High levels of apoptosis in HTLV-1 infected cultures during the first weeks after infection were detected. Apoptosis was not related to the presence of uninfected cells, as revealed by a fluorescence in situ hybridization assay. Successively, a progressive decrease in apoptosis in infected cultures going towards immortalization, was observed. When IL-2 in the medium was replaced by IL-4, allowing the cells to be efficiently infected by HTLV-1 but not immortalized, apoptosis levels tended to increase, instead of decreasing, with the ongoing time. The caspase cascade was remarkably activated in PBLs recently infected in vitro by HTLV-1, but apoptosis was only partly reduced by caspase inhibitors. Even if spontaneous apoptosis was relatively low in long-term cultures of PBLs immortalized by HTLV-1 in vitro, Fas death-receptor expression and function were well conserved. These observations provide a new rationale for explaining the dual effect of HTLV-1 in controlling apoptosis.

Host-pathogen interactions during apoptosis

Host pathogen interaction results in a variety of responses, which include phagocytosis of the pathogen, release of cytokines, secretion of toxins, as well as production of reactive oxygen species (ROS). Recent studies have shown that many pathogens exert control on the processes that regulate apoptosis in the host. The induction of apoptosis upon infection results from a complex interaction of parasite proteins with cellular host proteins. Abrogation of host cell apoptosis is often beneficial for the pathogen and results in a successful host invasion. However, in some cases, it has been shown that induction of apoptosis in the infected cells significantly imparts protection to the host from the pathogen. There is a strong correlation between apoptosis and the host protein translation machinery: the pathogen makes all possible efforts to modify this process so as to inhibit cell suicide and ensure that it can survive and, in some cases, establish latent infection. This review discusses the significance of various pathways/steps during virus-mediated modulation of host cell apoptosis.