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

Modulating p56Lck in T-Cells by a Chimeric Peptide Comprising Two Functionally Different Motifs of Tip from Herpesvirus saimiri

The Lck interacting protein Tip of Herpesvirus saimiri is responsible for T-cell transformation both in vitro and in vivo. Here we designed the chimeric peptide hTip-CSKH, comprising the Lck specific interacting motif CSKH of Tip and its hydrophobic transmembrane sequence (hTip), the latter as a vector targeting lipid rafts. We found that hTip-CSKH can induce a fivefold increase in proliferation of human and Aotus sp. T-cells. Costimulation with PMA did not enhance this proliferation rate, suggesting that hTip-CSKH is sufficient and independent of further PKC stimulation. We also found that human Lck phosphorylation was increased earlier after stimulation when T-cells were incubated previously with hTip-CSKH, supporting a strong signalling and proliferative effect of the chimeric peptide. Additionally, Lck downstream signalling was evident with hTip-CSKH but not with control peptides. Importantly, hTip-CSKH could be identified in heavy lipid rafts membrane fractions, a compartment where important T-cell signalling molecules (LAT, Ras, and Lck) are present during T-cell activation. Interestingly, hTip-CSKH was inhibitory to Jurkat cells, in total agreement with the different signalling pathways and activation requirements of this leukemic cell line. These results provide the basis for the development of new compounds capable of modulating therapeutic targets present in lipid rafts.

A molecular model for the differential activation of STAT3 and STAT6 by the herpesviral oncoprotein tip

Constitutive STAT signaling provides growth promoting signals in many forms of malignancy. We performed molecular modeling and molecular dynamics studies of the interaction between the regulatory Src homology 2 (SH2) domains of STAT3 and 6 with phosphorylated peptides of the herpesviral oncoprotein Tip, which facilitates Src kinase mediated STAT-activation and T cell proliferation. The studies give insight into the ligand binding specificity of the STAT SH2 domains and provide the first model for the differential activation of STAT3 or STAT6 by two distinct regions of the viral Tip protein. The biological relevance of the modeled interactions was then confirmed by activation studies using corresponding recombinant oncoproteins, and finally by respective recombinant viruses. The functional data give experimental validation of the molecular dynamics study, and provide evidence for the involvement of STAT6 in the herpesvirus induced T cell proliferation.

Actin-dependent activation of serum response factor in T cells by the viral oncoprotein tip

Serum response factor (SRF) acts as a multifunctional transcription factor regulated by mutually exclusive interactions with ternary complex factors (TCFs) or myocardin-related transcription factors (MRTFs). Binding of Rho- and actin-regulated MRTF:SRF complexes to target gene promoters requires an SRF-binding site only, whereas MAPK-regulated TCF:SRF complexes in addition rely on flanking sequences present in the serum response element (SRE). Here, we report on the activation of an SRE luciferase reporter by Tip, the viral oncoprotein essentially contributing to human T-cell transformation by Herpesvirus saimiri. SRE activation in Tip-expressing Jurkat T cells could not be attributed to triggering of the MAPK pathway. Therefore, we further analyzed the contribution of MRTF complexes. Indeed, Tip also activated a reporter construct responsive to MRTF:SRF. Activation of this reporter was abrogated by overexpression of a dominant negative mutant of the MRTF-family member MAL. Moreover, enrichment of monomeric actin suppressed the Tip-induced reporter activity. Further upstream, the Rho-family GTPase Rac, was found to be required for MRTF:SRF reporter activation by Tip. Initiation of this pathway was strictly dependent on Tip's ability to interact with Lck and on the activity of this Src-family kinase. Independent of Tip, T-cell stimulation orchestrates Src-family kinase, MAPK and actin pathways to induce SRF. These findings establish actin-regulated transcription in human T cells and suggest its role in viral oncogenesis.

The insulator protein CTCF binding sites in the orf73/LANA promoter region of herpesvirus saimiri are involved in conferring episomal stability in latently infected human T cells

Herpesviruses establish latency in suitable cells of the host organism after a primary lytic infection. Subgroup C strains of herpesvirus saimiri (HVS), a primate gamma-2 herpesvirus, are able to transform human and other primate T lymphocytes to stable growth in vitro. The viral genomes persist as nonintegrated, circular, and histone-associated episomes in the nuclei of those latently infected T cells. Epigenetic modifications of episomes are essential to restrict the transcription during latency to selected viral genes, such as the viral oncogenes stpC/tip and the orf73/LANA. In this study, we describe a genome-wide chromatin immunoprecipitation-on-chip (ChIP-on-chip) analysis to profile the occupancy of CTCF on the latent HVS genome. We then focused on two distinct, conserved CTCF binding sites (CBS) within the orf73/LANA promoter region. Analysis of recombinant viruses harboring deletions or mutations within the CBS indicated that the lytic replication of such viruses is not substantially influenced by CTCF. However, T cells latently infected with CBS mutants were impaired in their proliferation abilities and showed a significantly reduced episomal maintenance. We detected a reduced transcription of the orf73/LANA gene in the T cells, corresponding to the reduced viral genomes; this might contribute to the loss of HVS episomes, as LANA is central in the maintenance of viral episomes in the dividing T cell populations. These data demonstrate that the episomal stability of HVS genomes in latently infected human T cells is dependent on CTCF.

Activation of the STAT6 transcription factor in Jurkat T-cells by the herpesvirus saimiri Tip protein

Herpesvirus saimiri (HVS), a T-lymphotropic monkey herpesvirus, induces fulminant T-cell lymphoma in non-natural primate hosts. In addition, it can immortalize human T-cells in vitro. HVS tyrosine kinase-interacting protein (Tip) is an essential viral gene required for T-cell transformation both in vitro and in vivo. In this study, we found that Tip interacts with the STAT6 transcription factor and induces phosphorylation of STAT6 in T-cells. The interaction with STAT6 requires the Tyr(127) residue and Lck-binding domain of Tip, which are indispensable for interleukin (IL)-2-independent T-cell transformation by HVS. It was also demonstrated that Tip induces nuclear translocation of STAT6, as well as activation of STAT6-dependent transcription in Jurkat T-cells. Interestingly, the phosphorylated STAT6 mainly colocalized with vesicles containing Tip within T-cells, but was barely detectable in the nucleus. However, nuclear translocation of phospho-STAT6 and transcriptional activation of STAT6 by IL-4 stimulation were not affected significantly in T-cells expressing Tip. Collectively, these findings suggest that the constitutive activation of STAT6 by Tip in T-cells may contribute to IL-2-independent T-cell transformation by HVS.

Inhibition of retromer activity by herpesvirus saimiri tip leads to CD4 downregulation and efficient T cell transformation

The mammalian retromer is an evolutionally conserved protein complex composed of a vacuolar protein sorting trimer (Vps 26/29/35) that participates in cargo recognition and a sorting nexin (SNX) dimer that binds to endosomal membranes. The retromer plays an important role in efficient retrograde transport for endosome-to-Golgi retrieval of the cation-independent mannose-6-phosphate receptor (CI-MPR), a receptor for lysosomal hydrolases, and other endosomal proteins. This ultimately contributes to the control of cell growth, cell adhesion, and cell migration. The herpesvirus saimiri (HVS) tyrosine kinase-interacting protein (Tip), required for the immortalization of primary T lymphocytes, targets cellular signaling molecules, including Lck tyrosine kinases and the p80 endosomal trafficking protein. Despite the pronounced effects of HVS Tip on T cell signal transduction, the details of its activity on T cell immortalization remain elusive. Here, we report that the amino-terminal conserved, glutamate-rich sequence of Tip specifically interacts with the retromer subunit Vps35 and that this interaction not only causes the redistribution of Vps35 from the early endosome to the lysosome but also drastically inhibits retromer activity, as measured by decreased levels of CI-MPR and lower activities of cellular lysosomal hydrolases. Physiologically, the inhibition of intracellular retromer activity by Tip is ultimately linked to the downregulation of CD4 surface expression and to the efficient in vitro immortalization of primary human T cells to interleukin-2 (IL-2)-independent permanent growth. Therefore, HVS Tip uniquely targets the retromer complex to impair the intracellular trafficking functions of infected cells, ultimately contributing to efficient T cell transformation.

Episomal replication timing of gamma-herpesviruses in latently infected cells

This study addresses the timing of gammaherpesviral episomal DNA replication with respect to the cell cycle. For the first time we analyzed a rhadinovirus, the prototype Herpesvirus saimiri (HVS), and compared it to the lymphocryptovirus Epstein-Barr virus (EBV). Newly synthesized DNA of latently infected B- or T-cells was first BrdU-labeled; then we sorted the cells corresponding to cell cycle phases G(0/1), G(2/M), and S (4 fractions S(1)-S(4)) and performed anti-BrdU chromatin immunoprecipitation. Next, DNA of different viral gene loci was quantitatively detected together with cellular control genes of known replication time. The sensitive technique is further enhanced by an internal coprecipitation standard for increased precision. Both gammaherpesviruses replicated very early in S-phase, together with cellular euchromatin. Our work suggests that early S-phase DNA replication is a general characteristic of episomal herpesviral genomes.

Activation of noncanonical NF-kappaB signaling by the oncoprotein Tio

NF-kappaB transcription factors are key regulators of cellular proliferation and frequently contribute to oncogenesis. The herpesviral oncoprotein Tio, which promotes growth transformation of human T cells in a recombinant herpesvirus saimiri background, potently induces canonical NF-kappaB signaling through membrane recruitment of the ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6). Here, we show that, in addition to Tio-TRAF6 interaction, the Tio-induced canonical NF-kappaB signal requires the presence of the regulatory subunit of the inhibitor of kappaB kinase (IKK) complex, NF-kappaB essential modulator (NEMO), and the activity of its key kinase, IKKbeta, to up-regulate expression of endogenous cellular inhibitor of apoptosis 2 (cIAP2) and interleukin 8 (IL-8) proteins. Dependent on TRAF6 and NEMO, Tio enhances the expression of the noncanonical NF-kappaB proteins, p100 and RelB. Independent of TRAF6 and NEMO, Tio mediates stabilization of the noncanonical kinase, NF-kappaB-inducing kinase (NIK). Concomitantly, Tio induces efficient processing of the p100 precursor molecule to its active form, p52, as well as DNA binding of nuclear p52 and RelB. In human T cells transformed by infection with a Tio-recombinant virus, sustained expression of p100, RelB, and cIAP2 depends on IKKbeta activity, yet processing to p52 remains largely unaffected by IKKbeta inhibition. However, long term inhibition of IKKbeta disrupts the continuous growth of the transformed cells and induces cell death. Hence, the Tio oncoprotein triggers noncanonical NF-kappaB signaling through NEMO-dependent up-regulation of p100 precursor and RelB, as well as through NEMO-independent generation of p52 effector.

Molecular cloning, sequence characterization and tissue transcription profile analyses of two novel genes: LCK and CDK2 from the Black-boned sheep (Ovis aries)

The complete coding sequences of two sheep genes--LCK and CDK2--were amplified using the rapid amplification of cDNA ends method based on three sheep EST sequences whose translated amino acids contain the domain PTKc_Lck_BIk and S_TKc domain, respectively. The sequence analyses of these two genes revealed that the sheep LCK gene encodes a protein of 509 amino acids which has high homology with the lymphocyte-specific protein tyrosine kinase (LCK) of eight species: bovine (99%), human (96%), dog (96%), Aotus nancymaae (95%), mouse (94%), rat (91%), horse (91%) and chicken (81%). The sheep CDK2 gene encodes a protein of 298 amino acids which has high homology with the cyclin-dependent kinase 2 (CDK2) of ten species: bovine (100%), goat (100%), rat (99%), mouse (99%), Chinese hamster (99%), dog (98%), golden hamster (98%), human (98%), horse (98%) and rhesus monkey (98%). The tissue transcription profile analyses indicated that that the Black-boned sheep LCK and CDK2 genes are generally but differentially expressed in the detected tissues including in tissues including spleen, muscle, skin, kidney, lung, liver and heart. These data serve as a foundation for further insight into these two genes.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

NF-kappaB activation by the viral oncoprotein StpC enhances IFN-gamma production in T cells

Interferon-gamma (IFN-gamma) is an essential regulator of innate and adaptive immune responses and a hallmark of the Th1 T-cell subset. It is produced at high levels by human T lymphocytes upon transformation with Herpesvirus saimiri, which depends on the expression of the viral oncoproteins saimiri transformation-associated protein of subgroup C (StpC) and tyrosine kinase-interacting protein (Tip). Here, we show that IFN-gamma production was induced by Tip in Jurkat T cells. StpC by itself did not affect IFN-gamma expression, but enhanced the effect of Tip. Our results substantiated the findings that StpC induces NF-kappaB activation and demonstrated that other transcription factors, including NFAT, AP-1 and serum response element regulators, were not activated by StpC in unstimulated T cells. Studies using StpC mutants deficient in NF-kappaB activation, dominant negative IkappaBalpha and constitutively active IKK2, established the importance of NF-kappaB in StpC-mediated upregulation of IFN-gamma production. These observations suggest that NF-kappaB induction by StpC contributes to the Th1-like phenotype of virus-transformed human T cells.

Functional characterization and conformational analysis of the Herpesvirus saimiri Tip-C484 protein

Tyrosine kinase interacting protein (Tip) of Herpesvirus saimiri (HVS) activates the lymphoid-specific member of the Src family kinase Lck. The Tip:Lck interaction is essential for transformation and oncogenesis in HVS-infected cells. As there are no structural data for Tip, hydrogen-exchange mass spectrometry was used to investigate the conformation of a nearly full-length form (residues 1-187) of Tip from HVS strain C484. Disorder predictions suggested that Tip would be mostly unstructured, so great care was taken to ascertain whether recombinant Tip was functional. Circular dichroism and gel-filtration analysis indicated an extended, unstructured protein. In vitro and in vivo binding and kinase assays confirmed that purified, recombinant Tip interacted with Lck, was capable of activating Lck kinase activity strongly and was multiply phosphorylated by Lck. Hydrogen-exchange mass spectrometry of Tip then showed that the majority of backbone amide hydrogen atoms became deuterated after only 10 s of labeling. Such a result suggested that Tip was almost totally unstructured in solution. Digestion of deuterium-labeled Tip revealed some regions with minor protection from exchange. Overall, it was found that, although recombinant Tip is still functional and capable of binding and activating its target Lck, it is largely unstructured.