Inhibition of p53 tumor suppressor by viral interferon regulatory factor

The irreversible cell cycle arrest and apoptosis induced by p53 are part of the host surveillance mechanisms for viral infection and tumor induction. Kaposi's sarcoma-associated herpesvirus (KSHV), the most recently discovered human tumor virus, is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The K9 open reading frame of KSHV encodes a viral interferon (IFN) regulatory factor (vIRF) which functions as a repressor for cellular IFN-mediated signal transduction and as an oncoprotein to induce cell growth transformation. Here, we demonstrate that KSHV vIRF interacts with the cellular p53 tumor suppressor through the putative DNA binding region of vIRF and the central region of p53. This interaction suppresses the level of phosphorylation and acetylation of p53 and inhibits transcriptional activation of p53. As a consequence, vIRF efficiently prevents p53-mediated apoptosis. These results suggest that KSHV vIRF interacts with and inhibits the p53 tumor suppressor to circumvent host growth surveillance and to facilitate uncontrolled cell proliferation.

Molecular piracy of Kaposi's sarcoma associated herpesvirus

Kaposi's Sarcoma associated Herpesvirus (KSHV) is the most recently discovered human tumor virus and is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and Multicentric Casttleman's disease. KSHV contains numerous open reading frames with striking homology to cellular genes. These viral gene products play a variety of roles in KSHV-associated pathogenesis by disrupting cellular signal transduction pathways, which include interferon-mediated anti-viral responses, cytokine-regulated cell growth, apoptosis, and cell cycle control. In this review, we will attempt to cover our understanding of how viral proteins deregulate cellular signaling pathways, which ultimately contribute to the conversion of normal cells to cancerous cells.

Comparative analysis of the transforming mechanisms of Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and Herpesvirus saimiri

Members of the gamma herpesvirus family include the lymphocryptoviruses (gamma-1 herpesviruses) and the rhadinoviruses (gamma-2 herpesviruses). Gammaherpesvirinae uniformly establish long-term, latent, reactivatable infection of lymphocytes, and several members of the gamma herpesviruses are associated with lymphoproliferative diseases. Epstein-Barr virus is a lymphocryptovirus, whereas Kaposi sarcoma-associated herpesvirus and Herpesvirus saimiri are members of the rhadinovirus family. Genes encoded by these viruses are involved in a diverse array of cellular signaling pathways. This review attempts to cover our understanding of how viral proteins deregulate cellular signaling pathways that ultimately contribute to the conversion of normal cells to cancerous cells.

Inhibition of p300 histone acetyltransferase by viral interferon regulatory factor

Kaposi's sarcoma-associated herpesvirus (KSHV) has been consistently identified in Kaposi's sarcomas, body cavity-based lymphomas, and some forms of Castleman's disease. The K9 open reading frame of KSHV encodes a viral interferon regulatory factor (vIRF) which functions as a repressor for cellular interferon-mediated signal transduction and as an oncogene to induce cell growth transformation. We demonstrate that KSHV vIRF directly interacts with cellular transcriptional coactivator p300 and displaces p300/CBP-associated factor from p300 complexes. This interaction inhibits the histone acetyltransferase activity of p300, resulting in drastic reduction of nucleosomal histone acetylation and alteration of chromatin structure. As a consequence, vIRF expression markedly alters cellular cytokine expression, which is regulated by acetylation of nucleosomal histones. These results demonstrate that KSHV vIRF interacts with and inhibits the p300 transcriptional coactivator to circumvent the host antiviral immune response and to induce a global alteration of cellular gene expression. These studies also illustrate how a cellular gene captured by a herpesvirus has evolved several functions that suit the needs of the virus.

Inhibition of natural killer cell-mediated cytotoxicity by Kaposi's sarcoma-associated herpesvirus K5 protein

Kaposi's sarcoma-associated herpesvirus (KSHV) K3 and K5 proteins dramatically downregulate MHC class I molecules. However, although MHC class I downregulation may protect KSHV-infected cells from cytotoxic T lymphocyte recognition, these cells become potential targets for natural killer (NK) cell-mediated lysis. We now show that K5 also downregulates ICAM-1 and B7-2, which are ligands for NK cell-mediated cytotoxicity receptors. As a consequence, K5 expression drastically inhibits NK cell-mediated cytotoxicity. Conversely, de novo expression of B7-2 and ICAM-1 resensitizes the K5-expressing cells to NK cell-mediated cytotoxicity. This is a novel viral immune evasion strategy where KSHV K5 achieves immune avoidance by downregulation of cellular ligands for NK cell-mediated cytotoxicity receptors.

The collagen repeat sequence is a determinant of the degree of herpesvirus saimiri STP transforming activity

Herpesvirus saimiri (HVS) is divided into three subgroups, A, B, and C, based on sequence divergence at the left end of genomic DNA in which the saimiri transforming protein (STP) resides. Subgroup A and C strains transform primary common marmoset lymphocytes to interleukin-2-independent growth, whereas subgroup B strains do not. To investigate the nononcogenic phenotype of the subgroup B viruses, STP genes from seven subgroup B virus isolates were cloned and sequenced. Consistent with the lack of oncogenic activity of HVS subgroup B viruses, STP-B was deficient for transforming activity in rodent fibroblast cells. Sequence comparison reveals that STP-B lacks the signal-transducing modules found in STP proteins of the other subgroups, collagen repeats and an authentic SH2 binding motif. Substitution mutations demonstrated that the lack of collagen repeats but not an SH2 binding motif contributed to the nontransforming phenotype of STP-B. Introduction of the collagen repeat sequence induced oligomerization of STP-B, resulting in activation of NF-kappaB activity and deregulation of cell growth control. These results demonstrate that the collagen repeat sequence is a determinant of the degree of HVS STP transforming activity.

Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins

The T-cell-mediated immune response plays a central role in the defense against intracellular pathogens. To avoid this immune response, viruses have evolved elaborate mechanisms that target and modulate many different aspects of the host's immune system. A target common to many of these viruses is the major histocompatibility complex (MHC) class I molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes K3 and K5 zinc finger membrane proteins which remove MHC class I molecules from the cell surface. K3 and K5 exhibit 40% amino acid identity to each other and localize primarily near the plasma membrane. While K3 and K5 dramatically downregulated class I molecules, they displayed different specificities in downregulation of HLA allotypes. K5 significantly downregulated HLA-A and -B and downregulated HLA-C only weakly, but not HLA-E, whereas K3 downregulated all four HLA allotypes. This selective downregulation of HLA allotypes by K5 was partly due to differences in amino acid sequences in their transmembrane regions. Biochemical analyses demonstrated that while K3 and K5 did not affect expression and intracellular transport of class I molecules, their expression induced rapid endocytosis of the molecules. These results demonstrate that KSHV has evolved a novel immune evasion mechanism by harboring similar but distinct genes, K3 and K5, which target MHC class I molecules in different ways.

Activation of lymphocyte signaling by the R1 protein of rhesus monkey rhadinovirus

Rhesus monkey rhadinovirus (RRV) is a gamma-2 herpesvirus that exhibits a considerable degree of similarity to the human Kaposi's sarcoma-associated herpesvirus (KSHV). The R1 protein of RRV is distantly related to the K1 protein of KSHV, and R1, like K1, can contribute to cell growth transformation. In this study we analyzed the ability of the cytoplasmic tail of R1 to function as a signal transducer. The cytoplasmic domain of the R1 protein contains several tyrosine residues whose phosphorylation is induced in cells expressing Syk kinase. Expression of a CD8 chimera protein containing the extracellular and transmembrane domains of CD8 fused to the cytoplasmic domain of R1 mobilized intracellular calcium and induced cellular tyrosine phosphorylation in B cells upon stimulation with anti-CD8 antibody. None of the CD8-R1 cytoplasmic deletion mutants tested were able to mobilize intracellular calcium or to induce tyrosine phosphorylation to a significant extent upon addition of anti-CD8 antibody. Expression of wild-type R1 protein activated nuclear factor of activated T lymphocytes (NFAT) eightfold in B cells in the absence of antibody stimulation; expression of the CD8-R1C chimera strongly induced NFAT activity (60-fold) but only upon the addition of anti-CD8 antibody. We conclude that the cytoplasmic domain of R1 is capable of transducing signals that elicit B-lymphocyte activation events. The signal-inducing properties of R1 appear to be similar to those of K1 but differ in that the required sequences are distributed over a much longer stretch of the cytoplasmic domain (>150 amino acids). In addition, the induction of calcium mobilization was considerably longer in duration and stronger with R1 than with K1.

Identification of the novel K15 gene at the rightmost end of the Kaposi's sarcoma-associated herpesvirus genome

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a distinct open reading frame called K15 at a position equivalent to the gene encoding LMP2A of Epstein-Barr virus (EBV). K15 isolates from body cavity-based lymphoma (BCBL) cells exhibited a dramatic sequence variation and a complex splicing pattern. However, all K15 alleles are organized similarly with the potential SH2 and SH3 binding motifs in their cytoplasmic regions. Northern blot analysis showed that K15 was weakly expressed in latently infected BCBL-1 cells, and the level of its expression was significantly induced by tetradecanoyl phorbol acetate stimulation. K15 encoded 40- to 55-kDa proteins, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and was localized at the cytoplasm and plasma membrane. To demonstrate the signal-transducing activity of the K15 protein, we constructed a chimeric protein in which the cytoplasmic tail of the human CD8alpha polypeptide was replaced with that of KSHV K15. While the CD8-K15 chimera was not capable of eliciting cellular signal transduction upon stimulation with an anti-CD8 antibody, it significantly inhibited B-cell receptor signaling, as evidenced by a suppression of tyrosine phosphorylation and intracellular calcium mobilization. This inhibition required the putative SH2 or SH3 binding motif in the cytoplasmic region of K15. Biochemical study of CD8-K15 chimeras showed that the cytoplasmic region of K15 was constitutively tyrosine phosphorylated and that the tyrosine residue within the putative SH2 binding motif of K15 was a primary site of phosphorylation. These results demonstrate that KSHV K15 resembles LMP2A in genomic location, splicing pattern, and protein structure and by the presence of functional signal-transducing motifs in the cytoplasmic region. Thus, KSHV K15 is likely a distant evolutionary relative of EBV LMP2A.

Primate herpesviral oncogenes

Gammaherpesviruses are the most rapidly growing members of the herpesviridae family. Gamma herpesviruses share similarity in their genome organizations and in early and late lytic genes that are required for viral replication. A distinct characteristic of gamma herpesviruses is their ability to establish latent infection in lymphoid cells, and some of these viruses are closely associated with abnormal proliferation and cancer in primates. The first open reading frame of the primate gamma herpesviruses has been shown to directly contribute to virus-associated pathogenesis. This open reading frame encodes latent membrane protein-1 (LMP1) in Epstein-Barr virus, Saimiri transformation protein (STP) in Herpesvirus Saimiri, K1 in Kaposi's sarcoma-associated herpesvirus, and R1 in Rhesus monkey Rhadinovirus. All of these gene products are capable of eliciting cellular signal transduction events, resulting in cell growth transformation. This review briefly summarizes the current view on the transforming mechanisms utilized by primate herpesviral oncogenes.

Identification of the R1 oncogene and its protein product from the rhadinovirus of rhesus monkeys

Rhesus monkey rhadinovirus (RRV) is a gamma-2 herpesvirus that is most closely related to the human Kaposi's sarcoma-associated herpesvirus (KSHV). We have identified a distinct open reading frame at the left end of RRV and designated it R1. The position of the R1 gene is equivalent to that of the saimiri transforming protein (STP) of herpesvirus saimiri (HVS) and of K1 of KSHV, other members of the gamma-2 or rhadinovirus subgroup of herpesviruses. The R1 sequence revealed an open reading frame encoding a product of 423 amino acids that was predicted to contain an extracellular domain, a transmembrane domain, and a C-terminal cytoplasmic tail reflective of a type I membrane-bound protein. The predicted structural motifs of R1, including the presence of immunoreceptor tyrosine-based activation motifs, resembled those in K1 of KSHV but were distinct from those of STP. R1 sequences from four independent isolates from three different macaque species revealed 0.95 to 7.3% divergence over the 423 amino acids. Variation was located predominantly within the predicted extracellular domain. The R1 protein migrated at 70 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was extensively glycosylated. Tagged R1 protein was localized to the cytoplasmic and plasma membranes of transfected cells. Expression of the R1 gene in Rat-1 fibroblasts induced morphologic changes and focus formation, and injection of R1-expressing cells into nude mice induced the formation of multifocal tumors. A recombinant herpesvirus in which the STP oncogene of HVS was replaced by R1 immortalized T lymphocytes to interleukin-2-independent growth. These results indicate that R1 is an oncogene of RRV.

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 Mex67p-mediated nuclear mRNA export pathway is conserved from yeast to human

Human TAP is an orthologue of the yeast mRNA export factor Mex67p. In mammalian cells, TAP has a preferential intranuclear localization, but can also be detected at the nuclear pores and shuttles between the nucleus and the cytoplasm. TAP directly associates with mRNA in vivo, as it can be UV-crosslinked to poly(A)+ RNA in HeLa cells. Both the FG-repeat domain of nucleoporin CAN/Nup214 and a novel human 15 kDa protein (p15) with homology to NTF2 (a nuclear transport factor which associates with RanGDP), directly bind to TAP. When green fluorescent protein (GFP)-tagged TAP and p15 are expressed in yeast, they localize to the nuclear pores. Strikingly, co-expression of human TAP and p15 restores growth of the otherwise lethal mex67::HIS3/mtr2::HIS3 double knockout strain. Thus, the human TAP-p15 complex can functionally replace the Mex67p-Mtr2p complex in yeast and thus performs a conserved role in nuclear mRNA export.

Role of cellular tumor necrosis factor receptor-associated factors in NF-kappaB activation and lymphocyte transformation by herpesvirus Saimiri STP

The STP oncoproteins of the herpesvirus saimiri (HVS) subgroup A strain 11 and subgroup C strain 488 are now found to be stably associated with tumor necrosis factor receptor-associated factor (TRAF) 1, 2, or 3. Mutational analyses identified residues of PXQXT/S in STP-A11 as critical for TRAF association. In addition, a somewhat divergent region of STP-C488 is critical for TRAF association. Mutational analysis also revealed that STP-C488 induced NF-kappaB activation that was correlated with its ability to associate with TRAFs. The HVS STP-C488 P10-->R mutant was deficient in human T-lymphocyte transformation to interleukin-2-independent growth but showed wild-type phenotype for marmoset T-lymphocyte transformation in vitro and in vivo. The STP-C488 P10-->R mutant was also defective in Rat-1 fibroblast transformation, and fibroblast cell transformation was blocked by a TRAF2 dominant-negative mutant. These data implicate TRAFs in STP-C488-mediated transformation of human lymphocytes and rodent fibroblasts. Other factors are implicated in immortalization of common marmoset T lymphocytes and may also be critical in the transformation of human lymphocytes and rodent fibroblasts.

Crystal structure of a viral cyclin, a positive regulator of cyclin-dependent kinase 6

BACKGROUND

Cyclin-dependent kinases (CDKs) have a central role in cell-cycle control and are activated by complex formation with positive regulatory proteins called cyclins and by phosphorylation. The overexpression and mutation of cyclins and CDKs has been associated with tumorigenesis and oncogenesis. A virus-encoded cyclin (v-cyclin) from herpesvirus saimiri has been shown to exhibit highest sequence homology to type D cyclins and specifically activates CDK6 of host cells to a very high degree.

RESULTS

We have determined the first X-ray structure of a v-cyclin to 3.0 A resolution. The structure of the core domains is very similar to those of cyclin A and cyclin H from human cells. To understand the structural basis for the v-cyclin specificity for CDK6 and the insensitivity of the complex to inhibitors of the p21 and INK4 families, a v-cyclin-CDK2 model was built on the basis of the known structures of human cyclin A in complex with CDK2 and the CDK inhibitor p27(Kip1).

CONCLUSIONS

Although many critical interactions between cyclin A and CDK2 would be conserved in a v-cyclin-CDK2 complex, some appear sterically or electrostatically unfavorable due to shifts in the backbone conformation or sidechain differences and may contribute to v-cyclin selectivity for CDK6. The insensitivity of v-cyclin-CDK6 complexes to inhibitors of the p21 family is probably due to structural changes in v-cyclin that lead to a flatter surface area offering fewer potential contacts with the protein inhibitor. In addition, sequence changes in v-cyclin eliminate hydrogen-bonding partners for atoms of the p27(Kip1) inhibitor. This structure provides the first model for interactions between v-cyclins and host cell-cycle proteins; these interactions may be important for virus survival as well as oncogenic transformation of host cells.

Identification and characterization of Kaposi's sarcoma-associated herpesvirus K8.1 virion glycoprotein

Kaposi's sarcoma-associated herpesvirus (KSHV) has been consistently identified in Kaposi's sarcomas (KS), body cavity-based lymphomas (BCBL), and some forms of Castleman's disease. Previous serological tests with KS patient sera have detected lytic-cycle polypeptides from KSHV-infected BCBL cells. We have found that these polypeptides are predominantly encoded by the K8.1 open reading frame, which is present in the same genomic position as virion envelope glycoproteins of other gammaherpesviruses. The cDNA of K8.1 from BCBL-1 cells was found to encode a glycosylated protein with an apparent molecular mass of 37 kDa. K8.1 was found to be expressed during lytic KSHV replication in BCBL-1 cells and was localized on the surface of cells and virions. The results of immunofluorescence and immunoelectron microscopy suggest that KSHV acquires K8.1 protein on its virion surface during the process of budding at the plasma cell membrane. When KSHV K8.1 derived from mammalian cells was used as an antigen in immunoblot tests, antibodies to K8.1 were detected in 18 of 20 KS patients and in 0 of 10 KS-negative control subjects. These results demonstrate that the K8.1 gene encodes a KSHV virion-associated glycoprotein and suggest that antibodies to K8.1 may prove useful as contributory serological markers for infection by KSHV.

Zeta chain of the T-cell receptor interacts with nef of simian immunodeficiency virus and human immunodeficiency virus type 2

A truncated version of the nef gene of simian immunodeficiency virus SIVmac239 capable of encoding amino acids 98 to 263 was used as bait to screen a cDNA library from activated lymphocytes in a yeast two-hybrid system. The zeta chain of the T-cell receptor (TCRzeta) was found to interact specifically not only with truncated SIV nef in yeast cells but also with full-length glutathione S-transferase (GST)-SIVnef fusion protein in vitro. Coimmunoprecipitation of TCRzeta with full-length SIV nef was demonstrated in transfected Jurkat cells and in Cos 18 cells which express the cytoplasmic domain of TCRzeta fused to the external domain of CD8 via the CD8 transmembrane domain. Using a series of nef deletion mutants, we have mapped the binding site within the central core domain of nef (amino acids 98 to 235). Binding of TCRzeta was specific for nef isolated from SIVmac239, SIVsmH4, and human immunodeficiency virus (HIV)-2ST and was not detected with nef from five different HIV-1 isolates. An active tyrosine kinase was coprecipitated with nef-TCRzeta complexes from Jurkat cells but not from J.CAM1.6 cells which lack a functional Lck tyrosine kinase. These results demonstrate a specific association of SIV and HIV-2 nef, but not HIV-1 nef, with TCRzeta.

Identification of an immunoreceptor tyrosine-based activation motif of K1 transforming protein of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is consistently identified in Kaposi's sarcoma and body cavity-based lymphoma. KSHV encodes a transforming protein called K1 which is structurally similar to lymphocyte receptors. We have found that a highly conserved region of the cytoplasmic domain of K1 resembles the sequence of immunoreceptor tyrosine-based activation motifs (ITAMs). To demonstrate the signal-transducing activity of K1, we constructed a chimeric protein in which the cytoplasmic tail of the human CD8alpha polypeptide was replaced with that of KSHV K1. Expression of the CD8-K1 chimera in B cells induced cellular tyrosine phosphorylation and intracellular calcium mobilization upon stimulation with an anti-CD8 antibody. Mutational analyses showed that the putative ITAM of K1 was required for its signal-transducing activity. Furthermore, tyrosine residues of the putative ITAM of K1 were phosphorylated upon stimulation, and this allowed subsequent binding of SH2-containing proteins. These results demonstrate that the KSHV transforming protein K1 contains a functional ITAM in its cytoplasmic domain and that it can transduce signals to induce cellular activation.

A role for herpesvirus saimiri orf14 in transformation and persistent infection

The product of open reading frame 14 (orf14) of herpesvirus saimiri (HVS) exhibits significant homology with mouse mammary tumor virus superantigen. orf14 encodes a 50-kDa secreted glycoprotein, as shown previously (Z. Yao, E. Maraskovsky, M. K. Spriggs, J. I. Cohen, R. J. Armitage, and M. R. Alderson, J. Immunol. 156:3260-3266, 1996). orf14 expressed from recombinant baculovirus powerfully induces proliferation of CD4-positive cells originating from several different species. To study the role of orf14 in transformation, a mutant form of HVS (HVS Deltaorf14) was constructed with a deletion in the orf14 gene. The transforming potential of HVS Deltaorf14 was tested in cell culture and in common marmosets. Parental HVS subgroup C strain 488 immortalized common marmoset T lymphocytes in vitro to interleukin-2-independent growth, while the HVS Deltaorf14 mutant did not produce such a growth transformation. In addition, HVS Deltaorf14 was nononcogenic in common marmosets. In contrast to other nononcogenic HVS mutant viruses which were repeatedly isolated from peripheral blood mononuclear cells of infected marmosets for more than 5 months, HVS Deltaorf14 did not persist at a high level in vivo. These results demonstrate that orf14 of HVS is not required for replication but is required for transformation and for high-level persistence in vivo.

Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor

Interferons (IFNs) are a family of multifunctional cytokines with antiviral activities. The K9 open reading frame of Kaposi's sarcoma-associated herpesvirus (KSHV) exhibits significant homology with cellular IFN regulatory factors (IRFs). We have investigated the functional consequence of K9 expression in IFN-mediated signal transduction. Expression of K9 dramatically repressed transcriptional activation induced by IFN-alpha, -beta, and -gamma. Further, it induced transformation of NIH 3T3 cells, resulting in morphologic changes, focus formation, and growth in reduced-serum conditions. The expression of antisense K9 in KSHV-infected BCBL-1 cells consistently increased IFN-mediated transcriptional activation but drastically decreased the expression of certain KSHV genes. Thus, the K9 gene of KSHV encodes the first virus-encoded IRF (v-IRF) which functions as a repressor for cellular IFN-mediated signal transduction. In addition, v-IRF likely plays an important role in regulating KSHV gene expression. These results suggest that KSHV employs an unique mechanism to antagonize IFN-mediated antiviral activity by harboring a functional v-IRF.

Substitution of ras for the herpesvirus saimiri STP oncogene in lymphocyte transformation

STP-C488 (STP of herpesvirus saimiri [HVS] group C strain 488 [C488]) is the only virus-encoded protein found to associate with cellular ras and activate ras signal transduction pathways. To investigate an important role for ras signal transduction in STP-dependent growth transformation, we constructed recombinant strains of HVS C488 in which the STP-C488 oncogene was replaced with cellular normal ras (c-ras) or viral oncogenic ras (v-ras). Recombinant HVS deltaSTP/v-ras immortalized primary common marmoset T lymphocytes to interleukin-2-independent growth as efficiently as wild-type HVS C488 (wt HVS), while recombinant HVS deltaSTP/c-ras did so with low efficiency. Whereas wt HVS immortalized CD4- CD8+ single-positive T lymphocytes, HVS deltaSTP/c-ras- and HVS deltaSTP/v-ras-immortalized cells were principally CD4+ CD8+ double-positive T lymphocytes. In addition, HVS deltaSTP/v-ras-immortalized T cells showed a high level of ras expression and exhibited an adherent macrophage-like morphology. These phenotypes were likely caused by the drastic activation of AP-1 transcriptional factor activity. Finally, HVS deltaSTP/v-ras and HVS deltaSTP/c-ras each induced lymphoma in one of two common marmosets, although onset of disease was more rapid with the v-ras virus. These results demonstrate that ras can substitute for the STP oncogene of HVS C488 to allow immortalized growth of primary lymphoid cells and that an activated form of ras does so more efficiently than the normal cellular form of ras.

Mutation of the Lck-binding motif of Tip enhances lymphoid cell activation by herpesvirus saimiri

The proline-rich SH3-binding (SH3B) motif of the tyrosine kinase-interacting protein (Tip) of herpesvirus saimiri (HVS) is required for binding to the cellular Src family kinase Lck. We constructed a mutant form of HVS in which prolines in the SH3B motif of Tip were altered to alanines. This mutant form of Tip was incapable of binding to Lck. The mutant virus, HVS/Tip mSH3B, retained its ability to immortalize common marmoset lymphocytes in culture. In fact, common marmoset lymphocytes immortalized by the HVS/Tip mSH3B mutant displayed increased expression of HLA-DR lymphocyte activation marker, an altered pattern of tyrosine phosphorylation, increased expression of the tyrosine kinase Lyn, and a shift in electrophoretic mobility of Lck compared to cells immortalized by wild-type HVS. Experimental infection of common marmosets resulted in fulminant lymphoma with both HVS/Tip mSH3B and wild-type HVS. However, HVS/Tip mSH3B produced greater infiltration of affected organs by proliferating lymphoid cells compared to wild-type HVS. These results demonstrate that Tip binding to Lck is not necessary for transformation and that abrogation of Tip binding to Lck alters the characteristics of transformed cells and the severity of the pathologic lesions.

Deregulation of cell growth by the K1 gene of Kaposi's sarcoma-associated herpesvirus

At a position equivalent to the gene encoding the saimiri transforming protein (STP) of herpesvirus saimiri (HVS), Kaposi's sarcoma-associated herpesvirus (KSHV) contains a distinct open reading frame called K1. Although KSHV and HVS are related members of the rhadinovirus subgroup of gamma herpesviruses, K1 and STP exhibit no similarity in amino acid sequence or in structural organization. Since STP is required for the oncogenic potential of HVS, we investigated the functional consequence of K1 expression. Expression of the K1 gene in rodent fibroblasts produced morphologic changes and focus formation indicative of transformation. A recombinant herpesvirus in which the STP oncogene of HVS was replaced with K1, immortalized primary T lymphocytes to IL-2 independent growth and induced lymphoma in common marmosets. These results demonstrate the transforming potential of the K1 gene of KSHV.

STP and Tip are essential for herpesvirus saimiri oncogenicity

Mutant forms of herpesvirus saimiri (HVS) subgroup C strain 488 with deletions in either STP-C488 or Tip were constructed. The transforming potentials of the HVS mutants were tested in cell culture and in common marmosets. Parental HVS subgroup C strain 488 immortalized common marmoset T lymphocytes in vitro to interleukin-2-independent growth, but neither of the deletion mutants produced such growth transformation. Wild-type HVS produced fatal lymphoma within 19 to 20 days of experimental infection of common marmosets, while HVS deltaSTP-C488 and HVS deltaTip were nononcogenic. Virus was repeatedly isolated from the peripheral blood of marmosets infected with mutant virus for more than 5 months. These results demonstrate that STP-C488 and Tip are not required for replication or persistence, but each is essential for transformation in cell culture and for lymphoma induction in common marmosets.