Correlation between the growth-inhibitory effect of TGF-beta 1 and phenotypic characteristics in a panel of B-cell lines

Repression of the proapoptotic cellular BIK/NBK gene by Epstein-Barr virus antagonizes transforming growth factor β1-induced B-cell apoptosis

The Epstein-Barr virus (EBV) establishes a lifelong latent infection in humans. EBV infection of primary B cells causes cell activation and proliferation, a process driven by the viral latency III gene expression program, which includes EBV nuclear proteins (EBNAs), latent membrane proteins, and untranslated RNAs, including microRNAs. Some latently infected cells enter the long-lived memory B-cell compartment and express only EBNA1 transiently (Lat I) or no EBV protein at all (Lat 0). Targeting the molecular machinery that controls B-cell fate decisions, including the Bcl-2 family of apoptosis-regulating proteins, is crucial to the EBV cycle of infection. Here, we show that BIK (also known as NBK), which encodes a proapoptotic "sensitizer" protein, is repressed by the EBNA2-driven Lat III program but not the Lat I program. BIK repression occurred soon after infection of primary B cells by EBV but not by a recombinant EBV in which the EBNA2 gene had been knocked out. Ectopic BIK induced apoptosis in Lat III cells by a mechanism dependent on its BH3 domain and the activation of caspases. We show that EBNA2 represses BIK in EBV-negative B-cell lymphoma-derived cell lines and that this host-virus interaction can inhibit the proapoptotic effect of transforming growth factor β1 (TGF-β1), a key physiological mediator of B-cell homeostasis. Reduced levels of TGF-β1-associated regulatory SMAD proteins were bound to the BIK promoter in response to EBV Lat III or ectopic EBNA2. These data are evidence of an additional mechanism used by EBV to promote B-cell survival, namely, the transcriptional repression of the BH3-only sensitizer BIK.

IMPORTANCE

Over 90% of adult humans are infected with the Epstein-Barr virus (EBV). EBV establishes a lifelong silent infection, with its DNA residing in small numbers of blood B cells that are a reservoir from which low-level virus reactivation and shedding in saliva intermittently occur. Importantly, EBV DNA is found in some B-cell-derived tumors in which viral genes play a key role in tumor cell emergence and progression. Here, we report for the first time that EBV can shut off a B-cell gene called BIK. When activated by a molecular signal called transforming growth factor β1 (TGF-β1), BIK plays an important role in killing unwanted B cells, including those infected by viruses. We describe the key EBV-B-cell molecular interactions that lead to BIK shutoff. These findings further our knowledge of how EBV prevents the death of its host cell during infection. They are also relevant to certain posttransplant lymphomas where unregulated cell growth is caused by EBV genes.

Smad signal and TGFβ induced apoptosis in human lymphoma cells

Transforming growth factor beta1 (TGF beta1) has antiproliferative and/or apoptotic effect on lymphoid cells. In certain lymphomas exogenous TGF beta1 is able to induce apoptosis, however many lymphoid malignancies are resistant to the endogenous TGF beta1 production. We studied the expression and the activity of TGF beta1 signalling components in B cell lymphoma cell lines (e.g. HT 58 cells) and in isolated human peripheral mononuclear cells (PBMCs) from healthy individual's and B-CLL patient's blood. We found that all signal transducer Smads (Smad2,-3; Smad4) and at least one of the inhibitory Smads (Smad6,-7) were expressed in non-treated lymphoma cells, but the inhibitory Smads did not in normal/control PBMCs. However, after TGF beta1 treatment Smad6 disappeared, while the expression of Smad7 increased in HT 58 cells. The activity of Smad signals was proved by phosphorylation of Smad2, nuclear translocation of Smad2/3, and the increased expression of Smad-dependent gene, TIEG in TGF beta1 treated lymphoma cells. These results showed that Smad signaling is available in certain different human lymphoma cells, however ISmads expression could inhibit the signal transmission. This findings indicates that the lost sensitivity of lymphoma cells toward a physiological regulatory factor could be reversed.

Epstein-Barr virus antagonizes the antiproliferative activity of transforming growth factor-beta but does not abolish its signaling

TGF-beta induces apoptosis and inhibits the proliferation of EBV-negative B-lymphoma cell lines. In contrast, EBV-immortalized B cells are resistant to both the proapoptotic and the antiproliferative activities of TGF-beta. We have generated a lymphoblastoid cell line, in which we can switch on and off the EBV-specific transcriptional program driven by EBNA2. When these cells express the EBNA2-driven phenotype, they are resistant to TGF-beta-mediated growth arrest. We used this cell line to readdress the question of how EBV can overcome the antiproliferative TGF-beta activity. We show here that EBV-driven cells remain TGF-beta-responsive since TGF-beta target genes are readily induced. Thus, EBV can overcome TGF-beta-mediated growth arrest without interfering with the core machinery of the TGF-beta signaling pathway, which links ligand binding to the induction of TGF-beta target genes.

Wilms' tumor gene expression by normal and malignant human B lymphocytes

Very little is known about Wilms' tumor gene (WT1) expression in B cells and its importance for growth regulation and differentiation. We have investigated WT1 expression in fresh B lymphocytes and in a panel of B-cell lines of normal and malignant origin, including both Epstein-Barr virus (EBV) genome negative and EBV carrying cell lines. WT1 is constitutively activated in all lymphoblastoid cell lines (LCL) derived from EBV immortalization of lymphocytes from normal donors in vitro. These cell lines are distinguished for the presence of activated B-cell markers and an unrestricted expression of viral latent genes. In contrast, WT1 expression is abrogated in normal B lymphocytes and in all Burkitt tumor derived cell lines, irrespective of the EBV genome carrying status and their phenotype pattern. A single step RT-PCR for simultaneous detection of the four spliced transcript isoforms has been applied to confirm their expression. Analysis of variant relative proportions suggested the maintenance of a balanced expression of the isoforms in LCL, as reported in non tumorous tissues. These data, together with the evidence that the replication in vitro of lymphoblastoid cells is not affected by WT1 activation following viral immortalization, support the hypothesis that gene inactivation, in addition to disrupted alternate splicing, may play a role in growth control derangements.

Resistance to TGF-beta1 correlates with a reduction of TGF-beta type II receptor expression in Burkitt's lymphoma and Epstein-Barr virus-transformed B lymphoblastoid cell lines

The pleiotropic cytokine TGF-beta1 is a member of a large family of related factors involved in controlling cell proliferation, differentiation and apoptosis. TGF-beta ligands interact with a complex of type I and type II transmembrane serine/threonine kinases and they transmit their signals to the nucleus via a family of Smad proteins. A panel of over 20 Burkitt's lymphoma (BL) cell lines has been compiled including those that are Epstein-Barr virus (EBV) negative, those that carry EBV with a restricted pattern of EBV latent gene expression (group I) and those that express the full range of latent EBV genes (group III), together with selected EBV-transformed lymphoblastoid cell lines (LCLs). Most of the EBV-negative and group I BL cell lines underwent apoptosis or a G(1) arrest in response to TGF-beta1 treatment. In contrast, group III cell lines and LCLs were completely refractory to these effects of TGF-beta1. All of the cell lines expressed the TGF-beta pathway Smads and the TGF-beta type I receptor. Lack of responsiveness to TGF-beta1 appears to correlate with a down-regulation of TGF-beta type II receptor expression. Studies of EBV-converted and stably transfected BL cell lines demonstrated that the EBV gene LMP-1 is neither necessary nor sufficient to block the TGF-beta1 response.

The role of TGF-beta in growth, differentiation, and maturation of B lymphocytes

Transforming growth factor-beta (TGF-beta) affects B cells at all stages in development. It appears to be involved in lymphopoiesis and is required for the development of plasma cells secreting all secondary isotypes. Its ability to inhibit proliferation and stimulate apoptosis suggest that it may be involved both in germinal center development and regulation of B-cell proliferation at sites of high antigen load such as the gastrointestinal tract. Although TGF-beta appears to be required for the generation of B cells secreting secondary isotypes, it inhibits secretion of IgM and IgA from cells expressing those isotypes. In this regard, TGF-beta may alter the level of RNA processing factors either directly or indirectly by inhibiting progression through the cell cycle. One of the best characterized effects of TGF-beta is its ability to stimulate isotype switching to IgA in both mouse and man. There is some controversy concerning its mechanism of action in this process, but its critical role is without question. The controversy may stem in part from an inability to separate the effects of endogenous and exogenous TGF-beta in the multiple models of isotype switching. The influence of endogenous TGF-beta is perhaps best exemplified by analysis of production of the different classes of IgG in mouse strains producing different levels of TGF-beta.

Phorbol-ester-stimulated human lymphoid cell lines produce a plasminogen activator modulator inducing cell-bound urokinase-type plasminogen activator in malignant tumor cell lines

The importance of cell-associated plasminogen activation in tumor invasion and metastasis is becoming increasingly evident. To clarify the modulators of cell-associated plasminogen activation in malignant states, we have recently established an assay system utilizing endogenous plasminogen activators on the cell surface. In the present study using the assay system, we found that the conditioned medium from phorbol 12-myristate 13-acetate (PMA)-stimulated human lymphoid cell lines, HUT 78 and Raji, strongly enhanced plasminogen activator (PA) activity on the surface of human malignant tumor cell lines (WI-38 VAI3 2RA, A431, A549 and HT-1080). The enhancing effect was inhibited by the addition of actinomycin D. By gel filtration, the active substances in PMA-stimulated HUT 78- and Raji-conditioned media were eluted in similar fractions corresponding to molecular weights of 60 to 80 kDa. The active substance was heat-labile. The enhanced PA activities were completely inhibited by anti-urokinase-type plasminogen activator (uPA) IgG. Moreover, the active substance was found to increase in cell-bound uPA antigen. These findings suggest that a population of activated lymphocytes produces a plasminogen activator modulator that induces uPA on the surface of malignant tumor cells.

Influence of transforming growth factor-beta (TGF-beta) on the immunoglobulin production by EBV-infected B cell cultures

TGF-beta inhibits the proliferation of human B lymphocytes stimulated by a variety of activators, including EBV. However, EBV-immortalised cells are refractory to TGF-beta. The influence of TGF-beta on B cell maturation varies, apparently depending on the origin of the B lymphocytes and their maturation/activation state, the strength of the stimulus and the presence of cofactors. We investigated the effect of TGF-beta on immunoglobulin production by 5-day-old EBV-infected B cells. TGF-beta added at the initiation of the cultures inhibited IgM, IgG and IgA secretion by decreasing the numbers of secretory cells. The inhibition of IgM secretion was strongest. At the cytoplasmic level, TGF-beta reduced the expression of IgM heavy, lambda and kappa light chains but not IgG and IgA heavy chains. However, the IgM production by an established EBV-transformed B cell line was not affected by TGF-beta. Thus, TGF-beta inhibited EBV-induced maturation of the B cells until they acquired a transformed state. We discuss the relevance of these findings for the potential role of TGF-beta on EBV infection.

Biphasic effect of transforming growth factor-beta on Epstein-Barr virus-induced activation of human tonsillar B cells

Transforming growth factor-beta (TGF-beta) is known to inhibit mitogen-induced proliferation of human B lymphocytes. Earlier results showed that activation of B cells by Epstein-Barr virus (EBV) was also inhibited by TGF-beta. On the other hand, TGF-beta could enhance the transformation of EBV-infected B-cell cultures. In the present set of experiments, we have confirmed the inhibitory effect of TGF-beta on the EBV-induced blastogenesis and found lower expression of CD23 in the treated cultures. However, cells which escaped inhibition and entered in the blast stage expressed a higher level of CD23 molecules. The elevation of CD23 in the TGF-beta-treated cultures was more marked at a time when the cell size profiles of the control and treated cultures were similar. In view of the function of the CD23 molecule as an autocrine growth factor, its increased expression is consistent with previous findings on TGF-beta-mediated enhancement of the transformation of B-cell cultures. The occurrence of growth inhibitory and growth stimulatory effect of TGF-beta on the same cell type has been observed in several other systems as well.