Effect of transforming growth factor-beta 1 and -beta 2 on the proliferation of Burkitt lymphoma and lymphoblastoid 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.

A retinoblastoma-binding protein that affects cell-cycle control and confers transforming ability

The retinoblastoma (RB) gene is one of the most extensively studied tumour-suppressor genes. Deletion or inactivation of both RB alleles is an essential, rate-limiting step in the formation of retinoblastoma and osteosarcoma that arise in families that carry mutant RB (ref. 2). RB inactivation is also found in other human tumours. Whereas loss of RB function is associated with the loss of cellular proliferative control, introduction of a wild-type RB can suppress cell growth and tumorigenicity. Thus, identification of factors that interfere with and/or control the function of the RB protein is critical for understanding both cell-cycle control and oncogenesis. Here we describe a new gene, Bog (for B5T over-expressed gene), which was identified and shown to be overexpressed in several transformed rat liver epithelial (RLE) cell lines resistant to the growth-inhibitory effect of TGF-beta1, as well as in primary human liver tumours. The Bog protein shares homology with other retinoblastoma-binding proteins and contains the Rb-binding motif LXCXE. Using the yeast two-hybrid system and co-immunoprecipitation, we demonstrated that Bog binds to Rb. In vivo, Bog/Rb complexes do not contain E2F-1, and Bog can displace E2F-1 from E2F-1/Rb complexes in vitro. Overexpression of Bog in normal RLE cells conferred resistance to the growth-inhibitory effect of TGF-beta1. Furthermore, normal RLE cells are rapidly transformed when Bog is continuously overexpressed and form hepatoblastoma-like tumours when transplanted into nude mice. These data suggest that Bog may be important in the transformation process, in part due to its capacity to confer resistance to the growth-inhibitory effects of TGF-beta1 through interaction with Rb and the subsequent displacement of E2F-1.

Common and idiosyncratic patterns of cytokine gene expression by Epstein-Barr virus transformed human B cell lines

Epstein-Barr virus (EBV) transformed human B cells proliferate indefinitely in vitro, and it has been proposed that cytokine-mediated autocrine loops contribute to the maintenance of the lymphoblastoid phenotype. We used a novel multiprobe RNase protection assay to quantify cytokine mRNA species expressed by EBV-transformed lymphoblastoid cell lines (LCL), derived either by the transformation of B cells with B95-8 or wild-type EBV or by the in vitro outgrowth of EBV-associated B cell lymphomas to identify cytokines that are commonly expressed in all LCL and thus more likely to be essential for immortalization of B cells. All 16 LCL expressed high levels of tumor necrosis factor (TNF)alpha, TNFbeta, and transforming growth factor (TGF)beta1 mRNA, while interleukin (IL)-10 transcripts were detected in most LCL but at a lower level. Expression of IL-1alpha, IL-1beta, IL-6, IL-12p35, IL-12p40, IL-13 and IFNgamma mRNA was variable among the LCL tested. Granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-2, IL-4, and IL-5 mRNA were undetectable in all LCL. Furthermore, we found that IL-10, TNFalpha, and TNFbeta mRNA were induced in EBV-negative B cell lines after infection with EBV. These data define common versus idiosyncratic patterns of cytokine expression by LCL and, in the former case, such cytokines as TNFalpha, TNFbeta, and IL-10 emerge as strong candidates that are essential for the autocrine regulation of EBV-immortalized B cells.

DNA damage in human B cells can induce apoptosis, proceeding from G1/S when p53 is transactivation competent and G2/M when it is transactivation defective

Cisplatin treatment of Epstein-Barr virus-immortalized human B lymphoblastoid cell lines (LCLs) results in p53-mediated apoptosis which occurs largely in a population of cells at the G1/S boundary of the cell cycle. Cell cycle progression appears to be required for this apoptosis because arresting cells earlier in G1 inhibited apoptosis despite the accumulation of p53. Overexpression of wild-type p53 also induces apoptosis in an LCL. Therefore six mutant genes derived from Burkitt's lymphoma (BL) cells were assayed for their ability to induce apoptosis when similarly overexpressed. The same genes were analysed in transient transfection assays for their ability to transactivate appropriate reporter plasmids. A correlation between the ability of p53 to transactivate and induce apoptosis was revealed. The only mutant capable of transactivation also induced apoptosis. Further analysis of the BL lines in which p53 had been characterized showed that whereas some lines were essentially resistant to cisplatin, three were rapidly induced to undergo apoptosis. All three have a single p53 allele encoding a mutant which is incapable of transactivation or (for two tested) mediating apoptosis when expressed in an LCL. Cell cycle analysis revealed that this apparently p53-independent apoptosis did not follow G1 arrest but in fact occurred largely in cells distributed in the G2/M phase of the cell cycle. These data suggest the existence of a second checkpoint in the G2 or M phase which, in the absence of a functional p53, is the primary point of entry into the apoptosis programme following DNA damage.

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.

Endogenous TGF-beta contributes to the induction of the EBV lytic cycle in two Burkitt lymphoma cell lines

A low proportion of cells in the BL lines P3HR-I and Akata enter spontaneously into the EBV lytic cycle, detectable by the expression of early antigens (EA). We found that both lines produce the active and inactive forms of TGF beta. It was shown earlier that a larger number of cells can be induced to enter the lytic cycle by exposing P3HR-I to phorbol esters and n-butyrate and the surface IgG-positive Akata cells to anti-IgG. We now show that the same treatments raise the level of active TGF beta release. Exposure to anti-TGF beta antibodies reduced EA induction by 75-85%. Our results indicate that induction of the viral productive cycle by the above-mentioned reagents is at least partly dependent on the activation of endogenous TGF beta production.

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.

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

Human B-cell lines established from Burkitt lymphoma (BL) and normal B cells immortalized in vitro by EBV (LCLs) differ in phenotype. While the BL correspond to resting B cells, the LCLs resemble activated B cells. When BLs which have the EBV genome are carried in vitro, they acquire some of the features of LCLs, such as expression of B-cell activation markers and the tendency to form aggregates. Comparison of several B-cell lines for sensitivity to TGF-beta showed that the growth of BLs (with few exceptions), but not of the LCLs, was inhibited. The results suggested that the sensitivity to TGF-beta correlates with the cellular phenotype. In the present work, this assumption is even more critically substantiated by studying 2 sublines of an EBV-genome carrying BL line, Mutu, which were selected for single cells and aggregates. The former (with resting phenotype) was inhibited, while the subline of aggregated cells, which also expressed B-cell activation markers, was not inhibited. Somatic-cell hybrids between BLs, LCLs and non-B cells provided lines with phenotypic differences. Results with a panel of such hybrid lines also showed that those which express the activated B-cell phenotype are not inhibited by TGF-beta. Differences in the levels of expression of activation markers did not influence the response to TGF-beta.

Transforming growth factor beta and interleukin-1: a paradigm for opposing regulation of haemopoiesis

The polypeptide cytokines, IL-1 and TGF-beta affect nearly every tissue and cell type in the body. IL-1 is the prototype of the proinflammatory molecule while TGF-beta is essentially anti-inflammatory. IL-1 is part of the cascade of cytokines that are produced during microbial invasion or bodily injury and enhance a variety of host responses, particularly in the immunological and haemopoietic systems, while TGF-beta acts as an inhibitor of these responses. At several levels, IL-1 and TGF-beta act in opposition to one another. IL-1 stimulates the expression of many genes in lymphoid and marrow stromal cells that stimulate haemopoietic cell growth and differentiation, while TGF-beta inhibits these IL-1 mediated effects. Also, TGF-beta stimulates secretion of the IL-1Ra. In addition, IL-1 induces the cell surface expression of cytokine receptors on lymphoid and haemopoietic cells, while TGF-beta dramatically inhibits the cell surface expression of these receptors, including the IL-1 receptor. Finally, IL-1 augments lymphoid and haemopoietic cell growth and TGF-beta potently inhibits this proliferation. The interactions of these cytokines serve to illustrate that the net balance of stimulatory and inhibitory signals determines the fate of a given cell which may be responsible for regulating homeostatic cell growth (Figure 1). Thus, the regulation of cytokine production and/or antagonism of their effects continues to be a therapeutic goal in the treatment of many diseases.

B-cell chronic lymphocytic leukaemia cells express and release transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) has been described as a potent inhibitor of various cell types, among others of primitive haematopoietic progenitors in vitro, and as a negative autocrine regulator of normal B lymphocyte growth and differentiation. In the present study we investigated TGF-beta gene expression in peripheral blood mononuclear cells (PBMC) and in B cells from patients with B-cell chronic lymphocytic leukaemia (B-CLL) and from normal controls. Monocyte depleted B-CLL cells expressed constitutively mRNA for TGF-beta 1 and secreted low amounts of TGF-beta activity into the culture medium. Stimulation of cells by phorbol ester noticeably enhanced mRNA levels as well as protein secretion in most cases. TGF-beta activity was of the same magnitude as in normal controls. We next analysed TGF-beta in highly enriched B lymphocytes from B-CLL (95-100% CD19+), and found that TGF-beta secretion was up to 3 times higher than in the original PBMC population. It is discussed that B-CLL cells might escape from negative regulation by TGF-beta and, on the other hand, inhibit normal haematopoietic cell proliferation and thereby achieve a growth advantage in the haematopoietic tissues.

Effect of TGF-beta 1 on the EBV-induced transformation of human lymphocyte cultures

We tested the effect of TGF-beta 1 on the EBV-induced activation and immortalization of human B lymphocytes. In lymphocyte cultures of EBV-seropositive individuals, T cells can inhibit the EBV-induced transformation of B cells. We found that in the presence of TGF-beta 1 the transformation was more efficient, due to the inhibition of this function. TGF-beta 1 inhibited the EBV-induced proliferation of purified B-cell cultures. Its effect was strongest when added at the beginning of the culture. Added later, the inhibition gradually decreased and was lost by the 4th day. After 10 to 14 days, in the cultures initiated with TGF-beta 1, the number of cells was higher and they formed larger aggregates as compared with control cultures. Thus, TGF-beta 1 modifies EBV-induced transformation in a complex way. It inhibits the activation of B cells but does not affect those already activated. Once they acquire the immortalized state, the B cells are even stimulated by TGF-beta 1.

Transforming growth factor-beta and the immune system

It is now apparent that the transforming growth factor beta (TGF-beta) family of proteins has potent immunoregulatory properties ranging from effects on the growth and differentiation of primitive stem cells to the differentiated functions of immune effector cells. Although most reports have described the immunosuppressive activities of TGF-beta, recent evidence supports the concept that TGF-beta can have both inhibitory and stimulatory actions on these systems. Recently, it has been found that TGF-beta can have autocrine as well as paracrine effects on the immune system, indicating that immune cells can activate the inactive secreted form of TGF-beta. Furthermore, TGF-beta has differential intracellular effects on cell surface receptor modulation, tyrosine phosphorylation, and cytokine gene transcription as well as cell-mediated cytotoxicity. Importantly, the administration of TGF-beta has proven beneficial in several animal disease models such as septic shock, allograft rejection, and autoimmunity. Moreover, the increased levels of TGF-beta found in several disease states associated with immunosuppression such as different forms of malignancy, chronic degenerative diseases, and AIDS implicate the involvement of TGF-beta in the pathogenesis of some diseases. Ultimately, well designed clinical trials will determine whether the exciting potential of TGF-beta can be used to treat or prevent disease.