Mitosis-specific phosphorylation of polyomavirus middle-sized tumor antigen and its role during cell transformation

TCR-Dependent Cell Response Is Modulated by the Timing of CD43 Engagement

Binding of Ag by the Ag receptor in combination with other stimuli provided by costimulatory receptors triggers the expansion and differentiation of T lymphocytes. However, it is unclear whether the time when costimulatory molecules interact with their counterreceptors with regards to Ag recognition leads to different T cell responses. Provided that the coreceptor molecule CD43 is a very abundant molecule evenly distributed on the membrane of T cell surface protruding 45 nm from the cell, we hypothesized that CD43 is one of the first molecules that interacts with the APC and thus modulates TCR activation. We show that engaging CD43 before or simultaneously with the TCR inhibited Lck-Src homology 2 domain containing phosphatase-1 interaction, preventing the onset of a negative feedback loop on TCR signals, favoring high levels of IL-2, cell proliferation, and secretion of proinflammatory cytokines and chemokines. In contrast, the intracellular signals resulting of engaging the TCR before CD43 were insufficient to induce IL-2 production and cell proliferation. Interestingly, when stimulated through the TCR and CD28, cells proliferated vigorously, independent of the order with which molecules were engaged. These results indicate that CD43 induces a signaling cascade that prolongs the duration of TCR signaling and support the temporal summation model for T cell activation. In addition to the strength and duration of intracellular signals, our data underscore temporality with which certain molecules are engaged as yet another mechanism to fine tune T cell signal quality, and ultimately immune function.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells

CD43, one of the most abundant glycoproteins on the T cell surface, has been implicated in selection and maturation of thymocytes and migration, adhesion, and activation of mature T cells. The adapter molecule Cbl has been shown to be a negative regulator of Ras. Furthermore, it may also regulate intracellular signaling through the formation of several multi-molecular complexes. Here we investigated the role of Cbl in the CD43-mediated signaling pathway in human T cells. Unlike T cell receptor signaling, the interaction of the adapter protein Cbl with Vav and phosphatidylinositol 3-kinase, resulting from CD43-specific signals, is independent of Cbl tyrosine phosphorylation, suggesting an alternative mechanism of interaction. CD43 signals induced a Cbl serine phosphorylation-dependent interaction with the tau-isoform of 14-3-3. protein. Protein kinase C-mediated Cbl serine phosphorylation was required for this interaction, because the PKC inhibitor RO-31-8220 prevented it, as well as 14-3-3 dimerization. Moreover, mutation of Cbl serine residues 619, 623, 639, and 642 abolished the interaction between Cbl and 14-3-3. Overexpression of Cbl in Jurkat cells inhibited the CD43-dependent activation of the mitogen-activated protein kinase (MAPK) pathway and AP-1 transcriptional activity, confirming nevertheless a negative role for Cbl in T cell signaling. However, under normal conditions, PKC activation resulting from CD43 engagement was required to activate the MAPK pathway, suggesting that phosphorylation of Cbl on serine residues by PKC and its association with 14-3-3 molecules may play a role in preventing the Cbl inhibitory effect on the Ras-MAPK pathway. These data suggest that by inducing its phosphorylation on serine residues, CD43-mediated signals may regulate the molecular associations and functions of the Cbl adapter protein.

The nuclear isoform of the highly conserved human uracil-DNA glycosylase is an Mr 36,000 phosphoprotein

We have previously demonstrated that human cells contain multiple forms of uracil-DNA glycosylase (Caradonna, S. J., Ladner, R., Hansbury, M., Kosciuk, M., Lynch, F., and Muller, S. J. (1996) Exp. Cell Res. 222, 345-359). One of these is an Mr 29,000 processed form of the highly conserved uracil-DNA glycosylase (UDG1) located in the mitochondria. The others are located in the nucleus and migrate as a group of at least three distinct bands within the 35,000-37,000 molecular weight range. In this report, we perform a detailed characterization of the Mr 35,000-37,000 purified proteins. To accomplish this, uracil-DNA glycosylases were affinity purified from HeLa cell nuclear extracts. The proteins were separated by SDS-PAGE, and their identities were verified by renaturation and activity assays. The three protein bands were individually digested with cyanogen bromide, and the resulting peptide fragments were analyzed by direct amino acid sequencing. Peptide sequence, derived from each band, was identical and corresponded to a recently identified isoform of UDG1. This isoform (UDG1A) has a unique 44-amino acid N-terminal region and a C-terminal region that is identical to UDG1. To begin to study the signals required for nuclear targeting, the N-terminal regions of UDG1 and UDG1A were isolated and cloned into pEGFP-N2 to generate fusions with a red-shifted variant of green fluorescent protein (GFP). When these constructs were transfected into NIH3T3 cells, UDG1/pEGFP was targeted to the mitochondria, and UDG1A/pEGFP was targeted to the nucleus. Further studies, using deletion mutants, demonstrate that the nuclear localization signal resides within the first 20 amino acids of UDG1A. To investigate the possibility that the heterogeneity observed on SDS-PAGE results from post-translational modification(s), the UDG/pEGFP fusion constructs were transfected into NIH3T3 cells, and the cells were metabolically labeled with [32P]orthophosphate. Results from these experiments show that UDG1A is a phosphoprotein. Subsequent phosphoamino acid analysis revealed that UDG1A is phosphorylated on both serine and threonine residues. As a final characterization, RNase protection assays were performed to examine expression of each of these isoforms. These studies demonstrate that UDG1A is expressed in a wide variety of cell types and that message levels are elevated in transformed cells.

Multimerization of polyomavirus middle-T antigen

The oncogenic protein of polyomavirus, middle-T antigen, associated with cell membranes and interacts with a variety of cellular proteins involved in mitogenic signalling. Middle-T antigen may therefore mimic the function of cellular tyrosine kinase growth factor receptors, like the platelet-derived growth factor or epidermal growth factor receptor. Growth factor receptor signalling is initiated upon the binding of a ligand to the extracellular domain of the receptor. This results in activation of the intracellular tyrosine kinase domain of the receptor, followed by receptor phosphorylation, presumably as a consequence of dimerization of two receptor molecules. Similar to middle-T antigen, phosphorylation of growth factor receptors leads to recruitment of cellular signalling molecules downstream in the signalling cascade. In this study, we investigated whether middle-T antigen, similar to tyrosine kinase growth factor receptors, is able to form dimeric signalling complexes. We found that association with cellular membranes was a prerequisite for multimerization, most likely dimer formation. A chimeric middle-T antigen carrying the membrane-targeting sequence of the vesicular stomatitis virus G protein instead of the authentic polyomavirus sequence still dimerized. However, mutants of middle-T antigen unable to associate with 14-3-3 proteins, like d18 and S257A, did not form dimers but were still oncogenic. This indicates that both membrane association and binding of 14-3-3 are necessary for dimer formation of middle-T antigen but that only the former is essential for cell transformation.

The N terminus of hamster polyomavirus middle T antigen carries a determinant for specific activation of p59c-Fyn

Transformation by rodent polyomaviruses is mediated primarily by middle T antigen, a membrane-bound protein that does not carry an intrinsic enzymatic activity but interacts and subverts the activity of cellular regulators of proliferation. The multiple protein partners of murine polyomavirus (Py) middle T antigen include the tyrosine kinases c-Src and, to a lesser extent, c-Fyn and c-Yes. By contrast, the hamster polyomavirus (HaPV) middle T antigen selectively activates the c-Fyn gene product. This difference may account for the contrasting tumor patterns induced by the two viruses. The sequences of the respective N-terminal and C-terminal functional domains of murine Py and HaPV middle T antigens are highly conserved whereas the intervening stretches are clearly divergent, leading to the speculation that this divergence may direct the specificity for tyrosine kinase activation. We have addressed this issue by constructing a chimera middle T antigen molecule carrying the N-terminal domain from HaPV (exon 1) in phase with the other two domains from murine Py (exon 2). The biological properties of this chimera molecule are indistinguishable from those of HaPV middle T antigen; it specifically activates p59c-Fyn and carries the transforming phenotype of the HaPV middle T antigen on rat fibroblasts.

Functional interaction between the SH2 domain of Fyn and tyrosine 324 of hamster polyomavirus middle-T antigen

Middle-T antigen of mouse polyomavirus (MomT) associates with the cellular tyrosine kinases c-Src, c-Yes, and Fyn, while middle-T antigen of hamster polyomavirus (HamT) exclusively binds Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. Here we show that the kinase domain of Fyn is sufficient for association with MomT but not for binding of HamT. We further demonstrate that a Fyn mutant lacking the SH2 domain is able to bind MomT but fails to associate with HamT, indicating that the SH2 domain of Fyn is essential for stable association with HamT. HamT, but not MomT, contains a tyrosine residue, Tyr-324, in the sequence context YEEI. Mutation of Tyr-324 to phenylalanine led to a drastic reduction of associated Fyn and abolished the oncogenicity of HamT. This suggests that Tyr-324 is the major phosphotyrosine residue mediating the binding of HamT to the SH2 domain of Fyn. These findings show that mouse and hamster polyomaviruses use different strategies to target Src-related tyrosine kinases.

Polyomavirus middle-T antigen associates with the kinase domain of Src-related tyrosine kinases

Middle-T antigen of mouse polyomavirus, an oncogenic DNA virus, associates with and activates the cellular tyrosine kinases c-Src, c-Yes, and Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. To determine the domain of c-Src directing association with middle-T, mutant c-Src proteins lacking the amino-terminal unique domain and the myristylation signal, the SH2 domain, the SH3 domain, or all three of these domains were coexpressed with middle-T in NIH 3T3 cells. All mutants were found to associate with middle-T, demonstrating that the kinase domain of c-Src, including the carboxy-terminal regulatory tail, is sufficient for association with middle-T. Moreover, we found that Hck, another member of the Src kinase family, does not bind middle-T, while chimeric kinases consisting of the amino-terminal domains of c-Src fused to the kinase domain of Hck or the amino-terminal domains of Hck fused to the kinase domain of c-Src associated with middle-T. Hck mutated at its carboxy-terminal regulatory residue, tyrosine 501, was also found to associate with middle-T. These results suggest that in Hck, the postulated intramolecular interaction between the carboxy-terminal regulatory tyrosine and the SH2 domain prevents association with middle-T. This intramolecular interaction apparently also limits the ability of c-Src to associate with middle-T, since removal of the SH2 or SH3 domain increases the efficiency with which middle-T binds c-Src.

Activation and nuclear translocation of mitogen-activated protein kinases by polyomavirus middle-T or serum depend on phosphatidylinositol 3-kinase

Several cellular signal transduction pathways activated by middle-T in polyomavirus-transformed cells are required for viral oncogenicity. Here we focus on the role of phosphatidylinositol 3-kinase (PI 3-kinase) and Ras and address the question how these signaling molecules cooperate during cell cycle activation. Ras activation is mediated through association with SHC.GRB2.SOS and leads to increased activity of several members of the mitogen-activated protein (MAP) kinase family, while activation of PI 3-kinase results in the generation of D3-phosphorylated phosphatidylinositides whose downstream targets remain elusive. PI 3-kinase activation might also ensue as a direct consequence of Ras activation. Oncogenicity of middle-T requires stimulation of both Ras- and PI 3-kinase-dependent pathways. Mutants of middle-T incapable to bind either SHC.GRB2.SOS or PI 3-kinase are not oncogenic. Sustained activation and nuclear localization of one of the MAP kinases, ERK1, was observed in wild type but not in mutant middle-T-expressing cells. Wortmannin, an inhibitor of PI 3-kinase, prevented MAP kinase activation and nuclear localization in middle-T-transformed cells. PI 3-kinase activity was also required for activation of the MAP kinase pathway in normal serum-stimulated cells, generalizing the concept that signaling through MAP kinases requires not only Ras-but also PI 3-kinase-mediated signals.

Cyclin-dependent protein kinases: key regulators of the eukaryotic cell cycle

Passage through the cell cycle requires the successive activation of different cyclin-dependent protein kinases (CDKs). These enzymes are controlled by transient associations with cyclin regulatory subunits, binding of inhibitory polypeptides and reversible phosphorylation reactions. To promote progression towards DNA replication, CDK/cyclin complexes phosphorylate proteins required for the activation of genes involved in DNA synthesis, as well as components of the DNA replication machinery. Subsequently, a different set of CDK/cyclin complexes triggers the phosphorylation of numerous proteins to promote the profound structural reorganizations that accompany the entry of cells into mitosis. At present, much research is focused on elucidating the links between CDK/cyclin complexes and signal transduction pathways controlling cell growth, differentiation and death. In future, a better understanding of the cell cycle machinery and its deregulation during oncogenesis may provide novel opportunities for the diagnostic and therapeutic management of cancer and other proliferation-related diseases.

Tyrosine kinase JAK1 is associated with the granulocyte-colony-stimulating factor receptor and both become tyrosine-phosphorylated after receptor activation

Granulocyte-colony-stimulating factor (G-CSF) stimulates the proliferation and differentiation of cells of the neutrophil lineage by interaction with a specific receptor. Early signal transduction events following G-CSF receptor activation were studied. We detected tyrosine phosphorylation of both the G-CSF receptor and the protein tyrosine kinase JAK1 following G-CSF binding to the human G-CSF receptor. In vitro, the kinase activity of JAK1 was increased by G-CSF stimulation. Coimmunoprecipitation of JAK1 with the G-CSF receptor suggested a physical association which existed prior to G-CSF stimulation.