A herpesvirus saimiri protein required for interleukin-2 independence is associated with membranes of transformed T cells

Growth transformation of human T cells by herpesvirus saimiri requires multiple Tip-Lck interaction motifs

Lymphoma induction and T-cell transformation by herpesvirus saimiri strain C488 depends on two viral oncoproteins, StpC and Tip. The major interaction partner of Tip is the protein tyrosine kinase Lck, a key regulator of T-cell activation. The Lck binding domain (LBD) of Tip comprises two interaction motifs, a proline-rich SH3 domain-binding sequence (SH3B) and a region with homology to the C terminus of Src family kinase domains (CSKH). In addition, biophysical binding analyses with purified Lck-SH2 domain suggest the phosphorylated tyrosine residue 127 of Tip (pY127) as a potential third Lck interaction site. Here, we addressed the relevance of the individual binding motifs, SH3B, CSKH, and pY127, for Tip-Lck interaction and for human T-cell transformation. Both motifs within the LBD displayed Lck binding activities and cooperated to achieve a highly efficient interaction, while pY127, the major tyrosine phosphorylation site of Tip, did not enhance Lck binding in T cells. Herpesvirus saimiri strain C488 recombinants lacking one or both LBD motifs of Tip lost their transforming potential on human cord blood lymphocytes. Recombinant virus expressing Tip with a mutation at position Y127 was still able to transform human T lymphocytes but, in contrast to wild-type virus, was strictly dependent on exogenous interleukin-2. Thus, the strong Lck binding mediated by cooperation of both LBD motifs was essential for the transformation of human T cells by herpesvirus saimiri C488. The major tyrosine phosphorylation site Y127 of Tip was particularly required for transformation in the absence of exogenous interleukin-2, suggesting its involvement in cytokine signaling pathways.

H. saimiri tyrosine-kinase interacting protein inhibits Tat function: A prototypic strategy for restricting HIV-1-induced cytopathic effects in immune cells

Herpesvirus saimiri (HVS)-transformed human T cells become permissive for X4 and R5 strains of human immunodeficiency virus type 1 (HIV-1), evidence that HVS-encoded proteins associated with T cell transformation enhance HIV-1 replication. Analyzing the contribution of transformation-associated bicistronic HVS open reading frames (ORF) to HIV-1 replication revealed expression of the second ORF saimiri transformation-associated protein type C (StpC) conferred the permissive phenotype to T cells. In contrast, expression of the first HVS ORF tyrosine-kinase interacting protein (Tip) in the absence of StpC enhanced restriction of HIV-1 replication in T cell lines and peripheral blood mononuclear cells. Understanding the mechanism whereby Tip enhanced restriction of HIV-1 replication may uncover unique pathways that could be targeted therapeutically. Here we report that Tip restricts HIV-1 replication in a monocyte-derived cell line and restricts reactivation of replication of HIV-1 in a T cell line harboring provirus. In this report, we begin to unravel the molecular underpinnings of Tip-mediated restriction. Tip mediates both lymphocyte-cell-specific kinase (Lck)-dependent and -independent effects on HIV-1 replication. We also provide evidence that Tip-mediated restriction is in part due to inhibition of Tat transactivation of the HIV-1 long terminal repeat (LTR). Expression of Tip in T cells increased activation of Stat1 and Stat3, as well as activation of protein kinase RNA-dependent (PKR/p68) and interferon-gamma production. Taken together, these results provide evidence that Tip restricts HIV-1 replication and reactivation by inhibiting HIV-1 transcription while inducing an intercellular antiviral state. We propose that genetically engineered vectors driving Tip expression could provide a prototypic strategy for restricting HIV-1 replication and reactivation in diverse cell lineages.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

The genome of herpesvirus saimiri C488 which is capable of transforming human T cells

Herpesvirus saimiri (HVS), the rhadinovirus prototype, is apathogenic in the persistently infected natural host, the squirrel monkey, but causes acute T cell leukemia in other New World primate species. In contrast to subgroups A and B, only strains of HVS subgroup C such as C488 are capable of transforming primary human T cells to stable antigen-independent growth in culture. Here, we report the complete 155-kb genome sequence of the transformation-competent HVS strain C488. The A+T-rich unique L-DNA of 113,027 bp encodes at least 77 open reading frames and 5 URNAs. In addition to the viral oncogenes stp and tip, only a few genes including the transactivator orf50 and the glycoprotein orf51 are highly divergent. In a series of new primary HVS isolates, the subgroup-specific divergence of the orf50/orf51 alleles was studied. In these new isolates, the orf50/orf51 alleles of the respective subgroup segregate with the stp and/or tip oncogene alleles, which are essential for transformation.

Herpesvirus saimiri

Herpesvirus saimiri (saimiriine herpesvirus 2) is the classical prototype of the gamma(2)-herpesviruses or rhadinoviruses, which also contains a human member, the Kaposi's sarcoma-associated herpesvirus. The T-lymphotropic Herpesvirus saimiri establishes specific replicative and persistent conditions in different primate host species. Virtually all squirrel monkeys (Saimiri sciureus) are persistently infected with this virus. In its natural host, the virus does not cause disease, whereas it induces fatal acute T-cell lymphoma in other monkey species after experimental infection. The virus can be isolated by cocultivation of permissive epithelial cells with peripheral blood cells from naturally infected squirrel monkeys and from susceptible New World monkeys during the virus-induced disease. Tumour-derived and in vitro-transformed T-cell lines from New World monkeys release virus particles. Herpesvirus ateles is a closely related virus of spider monkeys (Ateles spp.) and has similar pathogenic properties to Herpesvirus saimiri in other New World primate species. Similar to other rhadinoviruses, the genome of Herpesvirus saimiri harbours a series of virus genes with pronounced homology to cellular counterparts including a D-type cyclin, a G-protein-coupled receptor, an interleukin-17, a superantigen homologue, and several inhibitors of the complement cascade and of different apoptosis pathways. Preserved function has been demonstrated for most of the homologues of cellular proteins. These viral functions are mostly dispensable for the transforming and pathogenic capability of the virus. However, they are considered relevant for the apathogenic persistence of Herpesvirus saimiri in its natural host. A terminal region of the non-repetitive coding part of the virus genome is essential for pathogenicity and T-cell transformation. Based on the pathogenic phenotypes and the different alleles of this variable region, the virus strains have been assigned to three subgroups, termed A, B and C. In the highly oncogenic subgroup C strains, the two virus genes stpC and tip are transcribed from one bicistronic mRNA and are essential for transformation and leukaemia induction. stpC fulfils the typical criteria of an oncogene; its product interacts with Ras and tumour necrosis factor-associated factors and induces mitogen-activated protein kinase and nuclear factor kappa B activation. Tip interacts with the RNA transport factor Tap, with signal transduction and activation of transcription factors, and with the T-cellular tyrosine kinase Lck, which is activated by this interaction and phosphorylates Tip as a substrate. It is of particular interest that certain subgroup C virus strains such as C488 are capable of transforming human T lymphocytes to stable growth in culture. The transformed human T cells harbour multiple copies of the viral genome in the form of stable, non-integrated episomes. The cells express only a few virus genes and do not produce virus particles. The transformed cells maintain the antigen specificity and many other essential functions of their parental T-cell clones. Based on the preserved functional phenotype of the transformed T cells, Herpesvirus saimiri provides useful tools for T-cell immunology, for gene transfer and possibly also for experimental adoptive immunotherapy.

Activation of the Lck tyrosine protein kinase by the Herpesvirus saimiri tip protein involves two binding interactions

The Tip protein of Herpesvirus saimiri strain 484C binds to and activates the Lck tyrosine protein kinase. Two sequences in the Tip protein were previously shown to be involved in binding to Lck. A proline-rich region, residues 132-141, binds to the SH3 domain of the Lck protein. We show here that the other Lck-binding domain, residues 104-113, binds to the carboxyl-terminal half of Lck and that this binding does not require the Lck SH3 domain. Mutated Tip containing only one functional Lck-binding domain can bind stably to Lck, although not as strongly as wild-type Tip. Interaction of Tip with Lck through either Lck-binding domain increases the activity of Lck in vivo. Simultaneous binding of both domains is required for maximal activation of Lck. The transient expression of Tip in T cells was found to stimulate both Stat3-dependent and NF-AT-dependent transcription. Mutant forms of Tip lacking one or the other of the two Lck-binding domains retained the ability to stimulate Stat3-dependent transcription. Tip lacking the proline-rich Lck-binding domain exhibited almost wild-type activity in this assay. In contrast, ablation of either Lck-binding domain abolished the ability of Tip to stimulate NF-AT-dependent transcription. Full biological activity of Tip, therefore, appears to require both Lck-binding domains.

Activation of the lck tyrosine-protein kinase by the binding of the tip protein of herpesvirus saimiri in the absence of regulatory tyrosine phosphorylation

The Tip protein of herpesvirus saimiri 484 binds to the Lck tyrosine-protein kinase at two sites and activates it dramatically. Lck has been shown previously to be activated by either phosphorylation of Tyr394 or dephosphorylation of Tyr505. We examined here whether a change in the phosphorylation of either site was required for the activation of Lck by Tip. Remarkably, mutation of both regulatory sites of tyrosine phosphorylation did not prevent activation of Lck by Tip either in vivo or in a cell free in vitro system. Tip therefore appears to be able to activate Lck through an induced conformational change that does not necessarily involve altered phosphorylation of the kinase. Tip may represent the prototype of a novel type of regulator of tyrosine-protein kinases.

Herpesvirus saimiri as a model for gammaherpesvirus oncogenesis

Herpesvirus saimiri (HVS) causes T-lymphoproliferative dis-$borders in several New World and Old World primate species and in certain rabbits.In vitro infection leads to permanent growth of primary T cells of primate and human origins. The transformation-relevant proteins of HVS interact with cellular proto-oncoproteins which results in cell growth transformation. In addition, virus-encoded cellular homologues may contribute to transformation or persistence of HVS by altering cellular signal transduction and deregulating cell growth control. Because of the presence of a permissive cell culture system and in vitro Land in vivo transformation assays, HVS provides a unique opportunity to investigate the mechanisms of cancer induction by oncogenic herpesviruses.

The superantigen-homologous viral immediate-early gene ie14/vsag in herpesvirus saimiri-transformed human T cells

Herpesvirus saimiri C488 transforms human T lymphocytes to stable growth in culture. The growth-transformed human T cells harbor the viral genome in a nonintegrated episomal form without production of virus particles. In these cells, virus gene expression was previously found to be confined to the transforming genes stpC and tip. In order to analyze virus gene expression in more detail, we applied a subtractive hybridization technique and compared stimulated virus-transformed cells with uninfected parental T cells of the same donor. A number of known T-cell activation genes were isolated. Viral stpC/tip cDNAs were enriched after subtraction. In addition, the viral immediate-early, superantigen-homologous gene ie14/vsag was represented by numerous cDNA clones that comprised the entire spliced transcript. Whereas a weak basal expression of ie14/vsag was detected by reverse transcription-PCR only, the phorbol ester-induced transcripts were readily shown by Northern blotting. ie14/vsag, which before had been classified as a major immediate-early gene of herpesvirus saimiri, is localized within a highly conserved region with extensive homologies to the cellular genome. Mutant viruses without the ie14/vsag gene are replication competent and fully capable of transforming human and marmoset T cells. Since ie14/vsag is transiently expressed after stimulation, it may increase T-cell proliferation in an activation-dependent and superantigen-like but apparently Vbeta-independent way.

Activation of STAT transcription factors by herpesvirus Saimiri Tip-484 requires p56lck

Signal transducers and activators of transcription (STATs) relay signals from activated cell surface receptors directly to the nucleus. Previously, a protein required for T-cell transformation by the DNA tumor virus herpesvirus saimiri (HVS) and designated tyrosine kinase interacting protein (Tip-484) was shown to interact with and dramatically upregulate the activity of p56lck. p56lck is a nonreceptor tyrosine kinase that is essential for signaling by the T-cell receptor and also interacts with the CD4, CD8, and interleukin-2 receptors. The present data show activation of STAT1 and -3 by Tip-484. STAT1 and -3 were also found to complex with glutathione S-transferase-Tip-484 only in the presence of p56lck, and STAT3 was shown to be phosphorylated by the Tip-484-p56lck multiprotein complex in vitro. Infection of T cells with HVS or expression of recombinant Tip-484 significantly increased the DNA-binding activity of the STAT1 and STAT3 transcription factors in nuclear extracts and also increased the phosphorylation of STAT3 in vivo. This is the first report of STAT activation by a DNA tumor virus protein. Moreover, these studies demonstrate that p56lck is required for STAT activation by Tip-484.

Herpesvirus saimiri Tip-484 membrane protein markedly increases p56lck activity in T cells

Herpesvirus saimiri (HVS) is a T-cell-specific transforming and oncogenic virus. A protein encoded by HVS known as Tip-484 (for tyrosine kinase interacting protein from HVS strain 484) is required for this transformation. Tip-484 binds specifically to the nonreceptor protein tyrosine kinase p56lck. By transfecting Tip-484 into T cells, we now show that this interaction leads to a several hundred-fold increase in the kinase activity of p56lck. Tip-484 is part of a protein complex which is dependent on the presence of p56lck and is phosphorylated. We also show that two of the complexed proteins represent two phosphorylated forms of Tip-484. Furthermore, the p56lck kinase activity in HVS-infected human peripheral blood T lymphocytes was at least ninefold higher than that in noninfected control cells and significantly decreased in cells infected with a Tip-484 deletion mutant virus. Finally, we report that Tip-484 is required for oncogenesis in rabbits by the survival of rabbits inoculated with Tip-484 deletion mutant HVS. The data demonstrate dramatic stimulation of the signaling pathway of p56lck. This effect can contribute to the molecular mechanisms that lead to sustained autocrine secretion of growth factors, permanent T-cell growth, and ultimately lymphocytic tumor formation.

Regulation of the herpesvirus saimiri oncogene stpC, similar to that of T-cell activation genes, in growth-transformed human T lymphocytes

Herpesvirus saimiri strain C488, a T-cell tumor virus of New World primates, transforms human T lymphocytes to stable interleukin-2-dependent growth without need for further stimulation by antigen or mitogen. The transformed cell lines show the phenotype of activated mature T cells and retain many essential features of the primary parental cells, e.g., antigen specificity. In contrast to transformed New World monkey T cells, the human lines do not support lytic growth of the virus, even after chemical stimulation. Here we show that many viral genes remain silent during episomal persistence. However, the viral oncogene stpC is predominantly transcribed and translated to a stable cytoplasmic protein of 20 kDa that is heterogeneously expressed in individual cells. This 1.7-kb mRNA is bicistronic, encoding also Tip, a viral protein interacting with the T-cell-specific tyrosine kinase Lck. stpC/tip transcripts are heavily induced upon stimulation by mitogen or phorbol ester. Block of protein synthesis does not abolish transcription: treatment with cycloheximide greatly induces stpC/tip mRNA levels. Thus, this gene complex is regulated similarly to early T-cell activation genes. Constitutive and induced expression engage different transcription start sites. The T-cell regulation of the viral genes stpC and tip may contribute to the T-cell tropism of growth transformation by herpesvirus saimiri.

Selective activation of T cell kinase p56lck by Herpesvirus saimiri protein tip

Infection with Herpesvirus saimiri, a T lymphotropic virus of non-human primates, immortalizes human T cells in vitro. The cells show a mature activated phenotype and retain their antigen specificity. We have previously shown that in H. saimiri transformed cells a viral gene product termed tyrosine kinase interacting protein (Tip) associates with the T cell-specific tyrosine kinase p56lck and becomes phosphorylated by the enzyme on tyrosine residues. Here we show that p56lck is activated by recombinant and native Tip in cell-free systems. A dramatic increase of Lck activity was also observed in T cell lines transfected with Tip. p60fyn and p53/56lyn, the other Src-related kinases expressed in H. saimiri transformed T cells, did not phosphorylate Tip, and they were not activated by the protein. The selective activation of p56lck by Tip could contribute to the transformed phenotype of H. saimiri infected cells, and it might explain the T cell selectivity of the transformation event.

A herpesvirus saimiri membrane protein required for interleukin-2 independence forms a stable complex with p56lck

ORF-2, a 32-kDa viral protein expressed by herpesvirus saimiri-transformed lymphocytes, is essential for transformation and is expressed on the plasma membrane of transformed cells. The current work now shows that most (approximately 80%) of ORF-2 resides in the cytoplasm, while only a small portion protrudes from the cell surface. Expressed as a glutathione S-transferase fusion protein, ORF-2 was found to interact with a 56-kDa cellular protein in untransformed, herpesvirus saimiri-transformed, and Jurkat lymphocytes. Microsequencing proved that this protein is the lymphocyte-specific tyrosine protein kinase p56lck. Two regions of ORF-2 were found to be required for p56lck interaction. Current evidence suggests that the interaction of ORF-2 with p56lck plays a key role in the specific transformation of T lymphocytes to an interleukin-2-independent phenotype.