p38-mediated regulation of an Fas-associated death domain protein-independent pathway leading to caspase-8 activation during TGFbeta-induced apoptosis in human Burkitt lymphoma B cells BL41

On binding to its receptor, transforming growth factor beta (TGFbeta) induces apoptosis in a variety of cells, including human B lymphocytes. We have previously reported that TGFbeta-mediated apoptosis is caspase-dependent and associated with activation of caspase-3. We show here that caspase-8 inhibitors strongly decrease TGFbeta-mediated apoptosis in BL41 Burkitt's lymphoma cells. These inhibitors act upstream of the mitochondria because they inhibited the loss of mitochondrial membrane potential observed in TGFbeta-treated cells. TGFbeta induced caspase-8 activation in these cells as shown by the cleavage of specific substrates, including Bid, and the appearance of cleaved fragments of caspase-8. Our data show that TGFbeta induces an apoptotic pathway involving sequential caspase-8 activation, loss of mitochondrial membrane potential, and caspase-9 and -3 activation. Caspase-8 activation was Fas-associated death domain protein (FADD)-independent because cells expressing a dominant negative mutant of FADD were still sensitive to TGFbeta-induced caspase-8 activation and apoptosis. This FADD-independent pathway of caspase-8 activation is regulated by p38. Indeed, TGFbeta-induced activation of p38 and two different inhibitors specific for this mitogen-activated protein kinase pathway (SB203580 and PD169316) prevented TGFbeta-mediated caspase-8 activation as well as the loss of mitochondrial membrane potential and apoptosis. Overall, our data show that p38 activation by TGFbeta induced an apoptotic pathway via FADD-independent activation of caspase-8.

B cell receptor cross-linking triggers a caspase-8-dependent apoptotic pathway that is independent of the death effector domain of Fas-associated death domain protein

We have previously reported that B cell receptors, depending on the degree to which they are cross-linked, can promote apoptosis in various human B cell types. In this study, we show that B cell receptors can trigger two apoptotic pathways according to cross-linking and that these pathways control mitochondrial activation in human Burkitt's lymphoma cells. Whereas soluble anti-mu Ab triggers caspase-independent mitochondrial activation, cross-linked anti-mu Ab induces an apoptotic response associated with a caspase-dependent loss of mitochondrial transmembrane potential. This B cell receptor-mediated caspase-dependent mitochondrial activation is associated with caspase-8 activation. We show here that caspase-8 inhibitors strongly decrease cross-linking-dependent B cell receptor-mediated apoptosis in Burkitt's lymphoma BL41 cells. These inhibitors act upstream from the mitochondria as they prevented the loss of mitochondrial membrane potential observed in B cell receptor-treated BL41 cells. Caspase-8 activation in these cells was also evident from the detection of cleaved fragments of caspase-8 and the cleavage of specific substrates, including Bid. Our data show that cross-linked B cell receptors induced an apoptotic pathway involving sequential caspase-8 activation, loss of mitochondrial membrane potential, and the activation of caspase-9 and caspase-3. Cells expressing a dominant negative mutant of Fas-associated death domain protein were sensitive to cross-linked B cell receptor-induced caspase-8 activation and apoptosis; therefore, this caspase-8 activation was independent of the death effector domain of Fas-associated death domain protein.

Zinc-mediated regulation of caspases activity: dose-dependent inhibition or activation of caspase-3 in the human Burkitt lymphoma B cells (Ramos)

Divalent cations, including Zinc and Manganese ions, are important modulators of cell activation. We investigated the ability of these two divalent cations to modulate apoptosis in human Burkitt lymphoma B cells line (Ramos). We found that Zinc (from 10 to 50 microM) inhibited Manganese-induced caspase-3 activation and apoptosis of Ramos cells. Higher concentration of Zinc (50 to 100 microM) did not prevent Manganese-mediated apoptosis but rather increased cell death among Ramos cells. This Zinc-mediated cell death was associated with apoptotic features such as cell shrinkage, the presence of phosphatidylserine residues on the outer leaflet of the cells, chromatin condensation, DNA fragmentation and decrease of mitochondrial transmembrane potential. Zinc-mediated apoptosis was associated with caspase-9 and caspase-3 activation as revealed by the appearance of active p35 fragment of caspase-9 and p19 and p17 of caspase-3 as well as in vivo cleavage of PARP and of a cell-permeable fluorogenic caspase-3 substrate (Phiphilux-G(1)D(2)). Both Zinc-mediated apoptosis and caspase-3 activation were prevented by the cell-permeable, broad-spectrum inhibitor of caspases (zVAD-fmk) or overexpression of bcl-2. In addition, we show that Zinc-induced loss of transmembrane mitochondrial potential is a caspase-independent event, since it is not modified by the presence of zVAD-fmk, which is inhibited by overexpression of bcl-2. These results indicate that depending on its concentration, Zinc can exert opposite effects on caspase-3 activation and apoptosis in human B lymphoma cells: concentrations below 50 microM inhibit caspase-3 activation and apoptosis whereas higher concentrations of Zinc activate a death pathway associated with apoptotic-like features and caspase-3 activation.

The expression of p18INK4 and p27kip1 cyclin-dependent kinase inhibitors is regulated differently during human B cell differentiation

Cell cycle progression is under the control of cyclin-dependent kinases (cdks), the activity of which is dependent on the expression of specific cdk inhibitors. In this paper we report that the two cdk inhibitors, p27(Kip1) and p18(INK4c), are differently expressed and control different steps of human B lymphocyte activation. Resting B cells contain large amounts of p27(Kip1) and no p18(INK4c). In vitro stimulation by Staphylococcus aureus Cowan 1 strain or CD40 ligand associated with IL-10 and IL-2 induces a rapid decrease in p27(Kip1) expression combined with cell cycle entry and progression. In contrast, in vitro Ig production correlates with specific expression of p18(INK4c) and early G(1) arrest. This G(1) arrest is associated with inhibition of cyclin D3/cdk6-mediated retinoblastoma protein phosphorylation by p18(INK4c). A similar contrasting pattern of p18(INK4c) and p27(Kip1) expression is observed both in B cells activated in vivo and in various leukemic cells. Expression of p18(INK4c) was also detected in various Ig-secreting cell lines in which both maximum Ig secretion and specific p18(INK4c) expression were observed during the G(1) phase. Our study shows that p27(Kip1) and p18(INK4c) have different roles in B cell activation; p27(Kip1) is involved in the control of cell cycle entry, and p18(INK4c) is involved in the subsequent early G(1) arrest necessary for terminal B lymphocyte differentiation.

TNF receptor family member BCMA (B cell maturation) associates with TNF receptor-associated factor (TRAF) 1, TRAF2, and TRAF3 and activates NF-kappa B, elk-1, c-Jun N-terminal kinase, and p38 mitogen-activated protein kinase

BCMA (B cell maturation) is a nonglycosylated integral membrane type I protein that is preferentially expressed in mature B lymphocytes. Previously, we reported in a human malignant myeloma cell line that BCMA is not primarily present on the cell surface but lies in a perinuclear structure that partially overlaps the Golgi apparatus. We now show that in transiently or stably transfected cells, BCMA is located on the cell surface, as well as in a perinulear Golgi-like structure. We also show that overexpression of BCMA in 293 cells activates NF-kappa B, Elk-1, the c-Jun N-terminal kinase, and the p38 mitogen-activated protein kinase. Coimmunoprecipitation experiments performed in transfected cells showed that BCMA associates with TNFR-associated factor (TRAF) 1, TRAF2, and TRAF3 adaptor proteins. Analysis of deletion mutants of the intracytoplasmic tail of BCMA showed that the 25-aa protein segment, from position 119 to 143, conserved between mouse and human BCMA, is essential for its association with the TRAFs and the activation of NF-kappa B, Elk-1, and c-Jun N-terminal kinase. BCMA belongs structurally to the TNFR family. Its unique TNFR motif corresponds to a variant motif present in the fourth repeat of the TNFRI molecule. This study confirms that BCMA is a functional member of the TNFR superfamily. Furthermore, as BCMA is lacking a "death domain" and its overexpression activates NF-kappa B and c-Jun N-terminal kinase, we can reasonably hypothesize that upon binding of its corresponding ligand BCMA transduces signals for cell survival and proliferation.

Differential roles of JNK and Smad2 signaling pathways in the inhibition of c-Myc-induced cell death by TGF-beta

The transforming growth factor beta (TGF-beta) plays an important role in constraining cellular proliferation, but it is also a potent inducer of programmed cell death or apoptosis. Here, we demonstrate that TGF-beta can have an opposite effect, acting as a survival factor to prevent c-Myc-induced cell death in Rat-1 fibroblasts. However, in marked contrast to TGF-beta, Smad2, which is a critical intracellular mediator of the TGF-beta signaling pathway, functions as an antagonist to induce increased cell death. The protective activity of TGF-beta was associated with the activation of c-Jun N-terminal Kinase (JNK) and was not linked to the ability of TGF-beta to promote cell cycle progression. Expression of dominant-interfering forms of various components of the JNK signaling pathway, including Rac1, Cdc42, mitogen-activated protein kinase kinase 4 (MKK4), and c-Jun, abolished TGF-beta-mediated cell survival. Furthermore, overexpression of the constitutively activated mutant RacL61F37A, which selectively stimulates JNK cascade but not G1 cell cycle progression or actin polymerization, was sufficient to prevent apoptosis induced by c-Myc. These findings describe a differential effect of two separated signaling pathways of TGF-beta and indicate for the first time that Smad2 can act as antagonist to suppress TGF-beta-dependent cell survival. Oncogene (2000) 19, 1277 - 1287.

A protein kinase C-independent pathway leading to c-Jun-dependent expression of 100-kDa Ras GTPase-activating protein in JEG-3 human choriocarcinoma cells

Although the 100-kDa Ras GTPase-activating protein (p100 RasGAP) has been reported to exist specifically in human placental trophoblasts, the molecular mechanisms responsible for regulating its expression remain unclear. In this study we used okadaic acid, an inhibitor of serine/threonine phosphatase 1 and 2 A, as a probe to explore the signaling pathway regulating the expression of p100 RasGAP in JEG-3 human placental choriocarcinoma cells. Treatment of JEG-3 cells with okadaic acid provoked dose- and time-dependent stimulation of p100 RasGAP expression without marked modification of expression of p120 RasGAP, another isoform of RasGAP. Co-treatment of cells with okadaic acid and the protein kinase C activator, phorbol 12-myristate 13-acetate, exerted an additive effect on p100 RasGAP induction. Moreover, the response of the p100 RasGAP de novo synthesis to okadaic acid was not affected by the selective inhibitor of protein kinase C, GF 109203X. Thus this study identified a novel signaling pathway regulating p100 RasGAP expression, which is independent of protein kinase C. In addition, okadaic acid treatment resulted in the activation of ERK2 (p42 MAP kinase) and the induction of both c-Jun and c-Fos proteins without activating JNK (c-Jun NH2-terminal kinase). Significantly, blockade of c-Jun expression with antisense c-jun oligonucleotides suppressed p100 RasGAP expression. Taken together, it is concluded that okadaic acid induces the expression of p100 RasGAP protein in JEG-3 cells preceded by activation of ERK and AP-1 cascade, and that this okadaic acid-induced p100 RasGAP expression is independent of protein kinase C-mediated pathway but requires c-Jun/AP-1 function.

The oncogenic TEL/PDGFR beta fusion protein induces cell death through JNK/SAPK pathway

The TEL/PDGFR beta (T/P) fusion protein isolated from patients bearing a t(5;12) translocation is transforming when expressed in haematopoietic cells. To examine the signal transduction events activated by this protein, we measured the effect of T/P on activation of the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) in mouse bone marrow-derived Ba/F3 cells. Significant increase in the activity of JNK/SAPK1 was observed in transient transfection as well as in Ba/F3 cells stably expressing T/P. This activation was abrogated when the T/P-expressing cells were treated with a specific inhibitor of the PDGFR beta tyrosine kinase, indicating that the activity of the PDGFR beta part of the fusion protein was involved in JNK/SAPK activation. Expression of a dominant negative mutant of mitogen-activated protein kinase kinase 4 (MKK4), a direct activator of JNK/SAPK, prevented T/P-induced JNK/SAPK activation. In addition, inhibition of phosphoinositide-3 OH kinase (PI-3 kinase), a promoting survival factor, potentiated the effect of T/P on JNK/SAPK activation. Interestingly, expression of T/P was shown to initiate an apoptotic response that was enhanced by treatment of cells with the PI-3 kinase inhibitor LY294002, suggesting that T/P mediated cell death through activation of JNK/SAPK signalling pathway. Consistent with this hypothesis, expression of the dominant negative mutant of MKK4 decreased T/P-mediated apoptosis, while a dominant-negative mutant of PI-3 kinase enhances cell death. These findings indicate that activation of JNK/SAPK by T/P is related to apoptosis rather than cell proliferation and transformation.

Evidence for a role of NF-kappaB in the survival of hematopoietic cells mediated by interleukin 3 and the oncogenic TEL/platelet-derived growth factor receptor beta fusion protein

Interleukin 3 (IL-3) and other hematopoietic cytokines transduce signals that stimulate DNA synthesis and cell survival. In certain chronic myelomonocytic leukemias, a TEL/platelet-derived growth factor receptor beta (PDGFRbeta) fusion protein is produced as a consequence of the t(5;12) translocation. It contains the amino terminus of the transcription factor TEL fused to the transmembranous and cytoplasmic domains of the PDGFRbeta. It is oncogenic as it substitutes for IL-3, thus promoting cell growth and preventing apoptotic cell death. The mechanism by which TEL/PDGFRbeta generates survival signals remains undefined. Here, we report that both IL-3 and TEL/PDGFRbeta initiate a signaling cascade that leads to the activation of the transcriptional factor NF-kappaB. In fact, either cytokine deprivation of IL-3-dependent Ba/F3 cells or exposure of TEL/PDGFRbeta-expressing cells to the specific inhibitor of the PDGFR tyrosine kinase, CGP53716, caused a strong decrease in NF-kappaB activity followed by extensive cell death. Further, treatment with the proteasome inhibitor Z-IE(O-t-Bu)A-leucinal suppressed IL-3 and TEL/PDGFRbeta-dependent survival. The same result was seen upon overexpression of an unphosphorylable form of IkappaBalpha. Because both conditions inactivate NF-kappaB by preventing its translocation into the nucleus, that process seems to be essential for cell survival in response to IL-3 and TEL/PDGFRbeta. Moreover, overexpression of a dominant-negative mutant of the protooncogene c-Myc, a downstream target of NF-kappaB, had a similar effect. We conclude that NF-kappaB plays an important role in maintaining cell survival in response to IL-3 and TEL/PDGFRbeta and that c-Myc may be a downstream effector mediating this effect.

Myc is essential for transformation by TEL/platelet-derived growth factor receptor beta (PDGFRbeta)

The t(5;12) translocation identified in patients with chronic myelomonocytic leukemia (CMML) encodes a TEL/platelet-derived growth factor receptor beta (PDGFRbeta) fusion protein. A key hypothesis for how the TEL/PDGFRbeta fusion protein would function as an oncogene is that it represents a constitutively active version of the normal PDGFRbeta. A link between the function of the t(5;12)-encoded TEL/PDGFRbeta fusion protein and Myc expression is suggested by the fact that Myc is induced by PDGF and is essential for entry of cells into the S phase of the cell cycle. We here show that the kinase activity of TEL/PDGFRbeta is necessary for Ba/F3 cells to acquire interleukin-3 (IL-3) independence and that, in contrast to their untransfected counterpart, Ba/F3 cells stably transfected with TEL/PDGFRbeta maintain a high level of Myc expression after removal of IL-3. Using dominant negative mutants of Myc, we show that a threshold of active Myc is essential for TEL/PDGFRbeta to transform Ba/F3 and Rat-1 cells. The findings that the kinase activity of TEL/PDGFRbeta and a threshold of active Myc are involved in TEL/PDGFRbeta transformation may allow for the development of therapeutic strategies in patients with t(5;12)+ CMML using specific inhibitors of the PDGFRbeta kinase as well as compounds designed to interfere specifically with Myc.

HIV-1 p17 and IFN-gamma both induce fructose 1,6-bisphosphatase

The p17 matrix protein of the human immunodeficiency virus type 1 (HIV-1) plays a crucial role in AIDS pathogenesis. It orchestrates viral assembly and directs the preintegration complex to the nucleus of infected cells. Recently, the three-dimensional structure of p17 was shown to resemble that of interferon-gamma (IFN-gamma), suggesting that both proteins might share analogous functions. We demonstrate that in monocytes, p17 shares with IFN-gamma the ability to induce 1alpha-hydroxylase activity and to activate fructose 1,6-bisphosphatase gene expression in the presence of 25-hydroxyvitamin D3. However, p17 does not bind to the IFN-gamma cell membrane receptor and fails to increase expression of IFN-gamma-induced proteins, such as tryptophanyl-tRNA synthetase, Fc gammaRI, and HLA DR or B7/BB1 antigens. Altogether, our results raise the possibility that the structural resemblance between p17 and IFN-gamma causes the selective activation of a common pathway resulting in the production of 1,25-dihydroxyvitamin D3. We also found that unlike IFN-gamma, p17 increases the intracellular ATP content. Since transport of the HIV-1 preintegration complex through the nuclear membrane is an ATP-dependent process, our observation suggests that p17 plays a double role in this active transport, not only by acting as a chaperone molecule but also by recruiting the necessary energy for this process.

Differential kinetics of polypeptide expression and different biological activities in the human fibroblast response to dsRNA or interferon treatment

Using two-dimensional electrophoresis on total and nuclear extracts of human fibroblasts, we compared polypeptide patterns of cells treated with interferon-beta (IFN-beta), IFN-gamma, or with dsRNA in the presence of anti-IFN antibodies. The analysis of whole-cell extracts revealed that, after a 6-h treatment, the three agents induce the synthesis of a common set of proteins in addition to others that are specifically induced either by IFNs or by dsRNA. After a 15-h treatment, this common set of proteins was only induced by IFNs. Furthermore, at this time, IFNs also regulated proteins whose synthesis was specifically induced or repressed by poly(I).poly(C) in the 6-h treated cells. These results indicate that poly(I).poly(C) regulates protein expression more rapidly and more transiently than IFNs. The analysis of nuclear extracts showed similar differential kinetics of protein expression. However, a greater number of polypeptides was found to have their synthesis specifically induced by dsRNA. Moreover, poly(I).poly(C) was found to be mitogenic in these cells and did not induce a significant resistance to vesicular stomatitis virus (VSV). This study provides evidence for an overlap in the expression of proteins by dsRNA and IFNs, although these compounds do not share the same biological activities.

Post-transcriptional regulation of transferrin receptor mRNA by IFN gamma

IFN gamma inhibits the rise in transferrin receptor mRNA level which is normally observed when stationary WISH cells are stimulated to proliferate. This effect is not attributable to a change in the transcription rate of the transferrin receptor gene or in the cytoplasmic stability of the mRNA. The IFN gamma-induced reduction of the transferrin receptor mRNA content is already present at the nuclear level to an extent comparable to that observed in whole cells. Thus, IFN gamma does not impair the passage of this mRNA from the nuclear to the cytoplasmic compartment but probably interferes with a nuclear post-transcriptional event during the processing of the immature transferrin receptor mRNA. Two different levels of regulation of transferrin receptor mRNA have been previously reported. Iron modulates the cytoplasmic stability of this mRNA through the binding of a specific cytoplasmic factor, whereas cell growth variation influences the transcription of this gene. Our results suggest the existence of another mechanism of regulation for transferrin receptor gene expression not so far considered. Furthermore, the distinction between the mechanism of regulation exerted by IFN gamma and that exerted by cell proliferation on transferrin receptor gene expression suggests that, in WISH cells, the IFN-induced transferrin receptor decay is not a consequence of cell growth arrest but rather one of the causes of the antiproliferative effect of IFN through iron deprivation.

Direct induction of interferon-gamma- and interferon-alpha/beta-inducible genes by double-stranded RNA

Using Northern analysis, we here show that the inducibility by double-stranded (ds) RNA of interferon-alpha/beta-inducible genes is not restricted to a few genes but extends to all the genes known to be stimulated by IFN type I in fibroblasts. Moreover, we show that some genes, preferentially regulated by IFN-gamma, are also activated by dsRNA. We present a series of arguments demonstrating that the induction by dsRNA is not mediated by IFN. In addition to the fact that this induction occurs in the presence of cycloheximide and/or anti-IFN-alpha/beta antibodies in fibroblasts, we observed that, in IFN-resistant Daudi cells, ISG15 and IP-10 genes which are not induced by IFN-beta, are still inducible by dsRNA. dsRNA is also still active on 2-5 AS and ISG15 genes in cells carrying homozygous deletions of IFN alpha/beta genes. Actinomycin D experiments and nuclear in vitro elongation assays reveal that the induction by dsRNA involves, as its early step, a transcriptional event. This induction was found not to require protein synthesis, suggesting that activation of preexisting cellular factors is involved. The opposite inducibility by dsRNA of IFN-beta and 2',5'-oligoadenylate (2-5A) synthetase genes in serum-deprived fibroblasts indicates that pathways of induction by dsRNA of these two genes are not identical. Inhibition by 2-aminopurine of the induction of IFN-inducible mRNAs by IFN-beta or dsRNA suggests the participation of a protein kinase in their mechanism of action.

Reduction of transferrin receptor expression by interferon gamma in a human cell line sensitive to its antiproliferative effect

Interferon gamma (IFN gamma) reduced 125I-transferrin binding to WISH cells which are sensitive to its antiproliferative effect. IFN gamma did not affect transferrin binding to Daudi cells or phytohemagglutinin-stimulated human lymphocytes, neither of which respond to its antigrowth action. Scatchard analyses of the equilibrium binding of 125I-transferrin to WISH cells exposed to IFN gamma revealed a decrease in the number of cell surface receptors but no change in the apparent association constant compared with control cells. When 125I-transferrin binding was measured using detergent-extracted cells, the IFN-induced reduction of binding was smaller than with intact cells. This suggests that in WISH cells, IFN gamma not only reduced the total number of transferrin receptors, but also modified the process of receptor internalization and recycling. Labeling of newly synthesized receptors with [35S]-methionine indicated that a reduction in the biosynthesis might account for the decrease in the total number of transferrin receptors in IFN gamma-treated cells. Our results suggest that the antigrowth effect of IFN gamma is at least partly due to its inhibitory action on transferrin receptor expression leading to iron starvation.

Relationship between inhibition of cell growth and of transferrin receptor expression by interferon (IFN) alpha: studies in IFN-sensitive and IFN-resistant Daudi cells

We previously showed that treatment of different cell lines with interferon-alpha (IFN-alpha) concurrently inhibited both cell growth and the rise observed in 125I-labelled transferrin binding when cells are exposed to culture conditions that stimulate proliferation. To gain insight into the relationship between these two IFN-induced inhibitory processes, we investigated the effect of IFN-alpha on the binding of 125I-labelled transferrin to Daudi cells sensitive or resistant to its antiproliferative action. We found a close correlation between the ability of IFN-alpha to inhibit cell growth and to inhibit transferrin receptor expression. Since growth inhibition induced by other agents is not always accompanied by an inhibition of transferrin receptor expression, the previous and present observations suggest that the inhibitory effect of IFN on this expression is at least one of the mechanisms by which IFN inhibits cell proliferation. We also observed that IFN-alpha did not modify transferrin receptor biosynthesis in IFN-sensitive Daudi cells, suggesting that IFN-alpha may change the processing of the transferrin receptor molecules, making them unable to bind transferrin.

Inhibition of transferrin receptor expression by interferon-alpha in human lymphoblastoid cells and mitogen-induced lymphocytes

125I-Transferrin binding to lymphoblastoid K562 and Daudi cells markedly increased after exposure of the cells to culture conditions that stimulated proliferation. Treatment of these cells with interferon-alpha (IFN-alpha) resulted in concurrent inhibition of cell growth and of the rise in transferrin binding. Scatchard analyses revealed that IFN reduced the number of transferrin receptors without altering the binding constant. When 125I-transferrin binding was measured using permeabilized cells, the IFN-induced reduction of binding was comparable to that observed with intact cells, indicating that IFN diminished the total number of cellular transferrin receptors. We also found that addition of IFN-alpha to phytohemagglutinin-stimulated human lymphocytes inhibited the mitogen-induced enhancement of [3H]thymidine incorporation as well as surface binding of 125I-transferrin. Our findings suggest that the decrease in transferrin receptor expression on IFN-alpha-treated cells may be one of the mechanisms responsible for the antiproliferative action of IFN.

Induction of 2',5'-oligoadenylate synthetase by retinoic acid in two transformed human cell lines

2',5'-Oligoadenylate (2-5A) synthetase, which polymerizes adenosine triphosphate into 2-5A, is induced upon treatment of cells with interferon (IFN) and is thought to be involved in its antiviral and anticellular action. We report here that retinoic acid (RA) enhanced the level of this enzyme in two human transformed cell lines, WISH and Namalva. Like IFN, RA induced 2-5A synthetase activity in a time- and dose-dependent manner. Addition of anti IFN-alpha, -IFN-beta, or -IFN-gamma antibodies to the medium concomitantly with RA did not prevent such induction; therefore, the effect of RA is clearly not mediated through the induction and externalization of IFN. Pretreatment of cells with actinomycin D inhibited 2-5A synthetase induction by RA, suggesting that RA increased the transcription of the 2-5A synthetase gene. In WISH cells, the growth of encephalomyocarditis virus was inhibited by RA treatment, which is consistent with the hypothesis that 2-5A synthetase plays an important role in the antiviral action of IFN, at least in encephalomyocarditis virus replication. When the anticellular effects of IFN and RA were compared to their ability to induce 2-5A synthetase activity in four human cell lines, there was no strict correlation between the amplitude of the enzyme activity induced and the extent of the antiproliferative effect. It is concluded that the 2-5A system is probably not the only pathway responsible for the antiproliferative effect of both substances. We further suggest that the induction of 2-5A synthetase by IFN and RA might be connected with at least some of the similarities observed between other biological effects of both compounds.

Interferon affects intracellular calmodulin levels

Interferon lowers calmodulin levels in two cell lines sensitive to its antiproliferative effect. Further, in synchronized cells, interferon strongly inhibits the increase in calmodulin observed when control cells enter the S phase, and concomitantly inhibits DNA synthesis. Calmodulin has been implicated in the control of cell proliferation and an increase in this protein seems to be necessary for the progression of cells into the S phase of the cell cycle. Therefore, the effect of interferon on calmodulin content might constitute part of the molecular mechanism by which interferon inhibits DNA synthesis.

Interferon increases HLA synthesis in melanoma cells: interferon-resistant and -sensitive cell lines

We report that human leukocyte interferon preparations increase the expression of beta 2-microglobulin by 100-200% on the surface of normal fibroblast and melanoma cell lines sensitive to interferon. This increase in expression can be correlated with an increase in HLA synthesis as measured by incorporation of [35S]methionine in these antigens. This enhanced HLA synthesis, which is 5- to 17-fold, is time dependent and dose related. Synchronized cells in the G0/G1 phase of the cell cycle appear to be more sensitive to this interferon action. Neither an increase in surface expression nor in HLA synthesis is observed in a melanoma cell line resistant to the antiviral and antigrowth effects of interferon. Furthermore, there appears to be a stronger correlation between this increased HLA synthesis and the antiviral function than between it and the antiproliferative action of interferon.

Reorganization of the cytoskeleton by interferon in MSV-transformed cells

beta and gamma interferons were found to enhance the organization of microfilaments, intermediate filaments, and fibronectin in the murine sarcoma virus (MSV)-transformed cell line. Furthermore, both interferons increased the number of intermediate-type junctions. Sodium butyrate was also found to act in a similar way and, when associated with interferon, an additive effect was observed. In the cell line, cultured for many passages in the presence of beta interferon (MSV-IF+), the cytoskeletal network and the extra-cellular matrix were highly developed and were reminiscent of a normal cell. These results could explain some of the biological actions of interferon, such as increased cell adhesion to the substratum and decreased cell motility.

Type I and type II interferons: differential antiviral actions in transformed cells

In transformed mouse embryo cells, type II interferon had much less antiviral activity than type I interferon. In non-transformed cells, the two interferons had similar high activity, Reversal of the phenotype of Moloney sarcoma virus (MSV) transformed cells by sodium butyrate restored their sensitivity to the antiviral action of type II interferon. Additional evidence for a role of the cytoskeletal network in the action of type II interferon is that its antiviral effect is reduced by cytochalasin B, colchicine or vinblastine. MSV-transformed cells, selected for their resistance to the antiviral action of type I interferon, were sensitive to type II interferon. These differences in the effects of type I and II interferon on transformed cells are at present unexplained, but suggest that they have at least partially separate mechanisms of action.

Enhancement of interferon antitumor action by sodium butyrate

Sodium butyrate together with interferon enhances the antitumor effect of interferon in vivo. When Sarcoma 180 TG cells are inoculated in mice, the mean survival time and final survival rate are greatly increased compared to those for treatment with interferon alone. Similarly, a significant delay in the mean survival time is observed when mice inoculated with L1210 cells are treated with sodium butyrate and interferon. This effect could be due at least in part to a potentiation of interferon action on the tumor cells.

Effect of sodium butyrate on the antiviral and anticellular action of interferon in normal and MSV-transformed cells

Sodium butyrate enhances the antiviral and anticellular action of interferon in MSV-transformed cells but has no such effect in normal cells. The increased sensitivity could be mediated at least partially by the butyrate-induced elaboration of the microtubule and microfilament network.

[Effect of sodium butyrate on the interferon sensitivity of normal or cells transformed by murine sarcoma virus]

Sodium butyrate enhances the sensitivity of MSV transformed murine cells to interferon, but does not influence the response of normal cells. Sodium butyrate might act in transformed cells by restoring the different elements of the cytoskeletal system.