Oncogene activation of HIV-LTR-driven expression via the NF-kappa B binding sites

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Raf induces NF-kappaB by membrane shuttle kinase MEKK1, a signaling pathway critical for transformation

NF-kappaB is regulated by inhibitor proteins (IkappaBs), which retain NF-kappaB in the cytoplasm. Signal-induced phosphorylation by the IkappaB-kinase complex containing the IkappaB-kinases 1 and 2 (IKK-1/2 or IKK-alpha/beta) and subsequent degradation of the IkappaB proteins are prerequisites for NF-kappaB activation. Many signals induce NF-kappaB, one of them being oncogenic Raf kinase. We investigated whether NF-kappaB induction is critical for Raf-mediated transformation. Here, we demonstrate that inhibition of NF-kappaB interferes with transformation by the Raf-oncogene, and we characterized the mechanism of NF-kappaB induction by activated Raf kinase and the tumor promoter phorbol 12-myristate 13-acetate (PMA). NF-kappaB activation by PMA and Raf critically depends on the IkappaB-kinase complex, most notably on IKK-2. A major signaling pathway induced by Raf is the mitogenic cytoplasmic kinase cascade. However, different inhibitors of this cascade do not affect PMA- and Raf-mediated NF-kappaB activation. Raf does not phosphorylate the IkappaB-kinase proteins directly. Raf rather synergizes with another membrane shuttle kinase MEKK1, and Raf-mediated activation of NF-kappaB is blocked by a dominant negative form of MEKK1. These results suggest that Raf induction of NF-kappaB is relayed by MEKK1, but not by the classical mitogenic cytoplasmic kinase cascade.

ERK MAP kinase links cytokine signals to activation of latent HIV-1 infection by stimulating a cooperative interaction of AP-1 and NF-kappaB

Human immunodeficiency virus type 1 (HIV-1) can establish latent infection following provirus integration into the host genome. NF-kappaB plays a critical role in activation of HIV-1 gene expression by cytokines and other stimuli, but the signal transduction pathways that regulate the switch from latent to productive infection have not been defined. Here, we show that ERK1/ERK2 mitogen-activated protein kinase (MAPK) plays a central role in linking signals at the cell surface to activation of HIV-1 gene expression in latently infected cells. MAPK was activated by cytokines and phorbol 12-myristate 13-acetate in latently infected U1 cells. The induction of HIV-1 expression by these stimuli was inhibited by PD98059 and U0126, which are specific inhibitors of MAPK activation. Studies using constitutively active MEK or Raf kinase mutants demonstrated that MAPK activates the HIV-1 long terminal repeat (LTR) through the NF-kappaB sites. Most HIV-1 inducers activated NF-kappaB via a MAPK-independent pathway, indicating that activation of NF-kappaB is not sufficient to explain the activation of HIV-1 gene expression by MAPK. In contrast, all of the stimuli activated AP-1 via a MAPK-dependent pathway. NF-kappaB and AP-1 components c-Fos and c-Jun were shown to physically associate by yeast two-hybrid assays and electrophoretic mobility shift assays. Coexpression of NF-kappaB and c-Fos or c-Jun synergistically transactivated the HIV-1 LTR through the NF-kappaB sites. These studies suggest that MAPK acts by stimulating AP-1 and a subsequent physical and functional interaction of AP-1 with NF-kappaB, resulting in a complex that synergistically transactivates the HIV-1 LTR. These results define a mechanism for signal-dependent activation of HIV-1 replication in latently infected cells and suggest potential therapeutic strategies for unmasking latent reservoirs of HIV-1.

Regulation of human immunodeficiency virus type 1 infectivity by the ERK mitogen-activated protein kinase signaling pathway

ERK1 and ERK2 mitogen-activated protein kinases (MAPK) play a critical role in regulation of cell proliferation and differentiation in response to mitogens and other extracellular stimuli. Mitogens and cytokines that activate MAPK in T cells have been shown to activate human immunodeficiency virus type 1 (HIV-1) replication. Little is known about the signal transduction pathways that activate HIV-1 replication in T cells upon activation by extracellular stimulation. Here, we report that activation of MAPK through the Ras/Raf/MEK signaling pathway enhances the infectivity of HIV-1 virions. Virus infectivity was enhanced by treatment of cells with MAPK stimulators, such as serum and phorbol myristate acetate, as well as by coexpression of constitutively activated Ras, Raf, or MEK (MAPK kinase) in the absence of extracellular stimulation. Treatment of cells with PD 098059, a specific inhibitor of MAPK activation, or with a MAPK antisense oligonucleotide reduced the infectivity of HIV-1 virions without significantly affecting virus production or the levels of virion-associated Gag and Env proteins. MAPK has been shown to regulate HIV-1 infectivity by phosphorylating Vif (X. Yang and D. Gabuzda, J. Biol. Chem. 273:29879-29887, 1998). However, MAPK activation enhanced virus infectivity in some cells lines that do not require Vif function. The HIV-1 Rev, Tat, p17(Gag), and Nef proteins were directly phosphorylated by MAPK in vitro, suggesting that other HIV-1 proteins are potential substrates for MAPK phosphorylation. These results suggest that activation of the ERK MAPK pathway plays a role in HIV-1 replication by enhancing the infectivity of HIV-1 virions through Vif-dependent as well as Vif-independent mechanisms. MAPK activation in producer cells may contribute to the activation of HIV-1 replication when T cells are activated by mitogens and other extracellular stimuli.

UV-A-induced decrease in nuclear factor-kappaB activity in human keratinocytes

Previous reports have demonstrated an increase in nuclear factor-kappaB (NF-kappaB) activity in response to UV radiation. These studies have essentially focused on the DNA-damaging fraction of solar UV radiation (UV-B and UV-C). In contrast, the effects of UV-A radiation (320-400 nm) on NF-kappaB are not well known. In this study, we present evidence that UV-A radiation induces a marked decrease in NF-kappaB DNA-binding activity in NCTC 2544 human keratinocytes. In addition, NCTC 2544 keratinocytes pretreated with UV-A fail to respond to NF-kappaB inducers. Moreover, UV-A radiation induces a decrease in NF-kappaB-driven luciferase reporter gene expression in NCTC 2544 keratinocytes. The expression of the gene encoding IkappaBalpha (IkappaB is the NF-kappaB inhibitor), which is closely associated with NF-kappaB activity, is also reduced (3-fold) upon UV-A treatment. Our results indicate that the UV-A-induced decrease in NF-kappaB DNA-binding activity is associated with a decrease in the levels of the p50 and p65 protein subunits. This is the first evidence that an oxidative stress, such as UV-A radiation, may induce a specific decrease in NF-kappaB activity in mammalian cells, probably through degradation of NF-kappaB protein subunits. These findings suggest that UV-A could modulate the NF-kappaB-dependent gene expression.

Evolution of the human immunodeficiency virus type 1 long terminal repeat promoter by conversion of an NF-kappaB enhancer element into a GABP binding site

Human immunodeficiency virus type 1 (HIV-1) transcription is regulated by the viral Tat protein and cellular factors, of which the concentration and activity may depend on the cell type. Viral long terminal repeat (LTR) promoter sequences are therefore optimized to suit the specific nuclear environment of the target host cell. In long-term cultures of a Tat-defective, poorly replicating HIV-1 mutant, we selected for a faster-replicating virus with a 1-nucleotide deletion in the upstream copy of two highly conserved NF-kappaB binding sites. The variant enhancer sequence demonstrated a severe loss of NF-kappaB binding in protein binding assays. Interestingly, we observed a new binding activity that is specific for the variant NF-kappaB sequence and is present in the nuclear extract of unstimulated cells that lack NF-kappaB. These results suggest that inactivation of the NF-kappaB site coincides with binding of another transcription factor. Fine mapping of the sequence requirements for binding of this factor revealed a core sequence similar to that of Ets binding sites, and supershift assays with antibodies demonstrated the involvement of the GABP transcription factor. Transient transfection experiments with LTR-chloramphenicol acetyltransferase constructs indicated that the variant LTR promoter is specifically inhibited by GABP in the absence of Tat, but this promoter was dramatically more responsive to Tat than the wild-type LTR. Introduction of this GABP site into the LAI virus yielded a specific gain of fitness in SupT1 cells, which contain little NF-kappaB protein. These results suggest that GABP potentiates Tat-mediated activation of LTR transcription and viral replication in some cell types. Conversion of an NF-kappaB into a GABP binding site is likely to have occurred also during the worldwide spread of HIV-1, as we noticed the same LTR modification in subtype E isolates from Thailand. This typical LTR promoter configuration may provide these viruses with unique biological properties.

Plasma membrane-targeted Raf kinase activates NF-kappaB and human immunodeficiency virus type 1 replication in T lymphocytes

Increasing evidence points to a role of the mitogenic Ras/Raf/MEK/ERK signaling cascade in regulation of human immunodeficiency virus type 1 (HIV-1) gene expression. Stimulation of elements of this pathway leads to transactivation of the HIV-1 promoter. In particular, the NF-kappaB motif in the HIV long terminal repeat (LTR) represents a Raf-responsive element in fibroblasts. Regulation of the Raf kinase in T cells differs from findings with a variety of cell lines that the catalytic domain of Raf (Raf(delta26-303)) shows no activity. In this study, we restored the activity of the kinase in T cells by fusing its catalytic domain to the CAAX motif (-Cx) of Ras, thus targeting the enzyme to the plasma membrane. Constitutive activity of Raf was demonstrated by phosphorylation of mitogen-activated protein kinase kinase (MEK) and endogenous mitogen-activated protein kinase 1/2 (ERK1/2) in A3.01 T cells transfected with Raf(delta26-303)-Cx. Membrane-targeted Raf also stimulates NF-kappaB, as judged by kappaB-dependent reporter assays and enhanced NF-kappaB p65 binding on band shift analysis. Moreover, we found that active Raf transactivates the HIV(NL4-3) LTR in A3.01 T lymphocytes and that dominant negative Raf (C4) blocked 12-O-tetradecanoylphorbol-13-acetate induced transactivation. When cotransfected with infectious HIV(NL4-3) DNA, membrane-targeted Raf induces viral replication up to 10-fold over basal levels, as determined by the release of newly synthesized p24gag protein. Our study clearly demonstrates that the activity of the catalytic domain of Raf in A3.01 T cells is dependent on its cellular localization. The functional consequences of active Raf in T lymphocytes include not only NF-kappaB activation and transactivation of the HIV(NL4-3) LTR but also synthesis and release of HIV particles.

Activation of BLV transcription by NF-kappa B and Tax

Bovine leukemia virus (BLV) is the causative agent of bovine leukosis, a naturally occurring fatal disease in cattle. BLV transcription is regulated by cellular transcription factors and the virally encoded oncoprotein Tax. In this report, we investigated the functional role of the putative NF-kappa B binding site recently identified in the BLV promoter. Our studies indicate that the kappa B binding motif acts as a functional enhancer in the presence of the cellular NF-kappa B proteins. Furthermore, the kappa B site together with a single 21-bp repeat confers strong activation of BLV transcription in the presence the NF-kappa B proteins and Tax. These results suggest that cellular NF-kappa B may be involved in the regulation of BLV transcription and activation of the virus from latency.

Activation of the HIV-1 long terminal repeat by nerve growth factor

The brain is an important target for the human immunodeficiency virus type 1 (HIV-1). We show here that nerve growth factor (NGF), which induces neuronal differentiation and survival, causes a strong activation of the HIV-1 long terminal repeat by a Ras/Raf-dependent mechanism in PC12 cells. Mutation of the kappaB sequences contained whithin the long terminal repeat reduces NGF-mediated stimulation. NGF does not activate NF-kappaB in PC12 cells, but rather increases binding of other nuclear factors to the kappaB sequences. Furthermore, a nuclear receptor response element contributes to the stimulatory effect of NGF. The retinoids receptors have been identified as components of the nuclear binding to the nuclear receptor response element in NGF-treated PC12 cells. These results reveal the importance of neurotrophins and nuclear receptor signaling pathways as specific activators of HIV-1 gene expression in neural cells.

Physical interactions between Ets and NF-kappaB/NFAT proteins play an important role in their cooperative activation of the human immunodeficiency virus enhancer in T cells

The transcriptional regulatory elements of many inducible T-cell genes contain adjacent or overlapping binding sites for the Ets and NF-kappaB/NFAT families of transcription factors. Similar arrays of functionally important NF-kappaB/NFAT and Ets binding sites are present in the transcriptional enhancers of human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2), suggesting that this pattern of nuclear protein binding sites reflects an evolutionarily conserved mechanism for regulating inducible T-cell gene expression that has been co-opted during HIV evolution. Despite these findings, the molecular mechanisms by which Ets and NF-kappaB/NFAT proteins cooperatively regulate inducible T-cell gene expression remained unknown. In the studies described in this report, we demonstrated a physical interaction between multiple Ets and NF-kappaB/NFAT proteins both in vitro and in activated normal human T cells. This interaction is mediated by the Ets domain of Ets proteins and the C-terminal region of the Rel homology domains of NF-kappaB/NFAT proteins. In addition, the Ets-NF-kappaB/NFAT interaction requires the presence of DNA binding sites for both proteins, as it is abolished by the DNA intercalating agents propidium iodide and ethidium bromide and enhanced by the presence of synthetic oligonucleotides containing binding sites for Ets and NF-kappaB proteins. A dominant-negative mutant of NF-kappaB p50 that binds DNA but fails to interact with Ets proteins inhibits the synergistic activation of the HIV-1 and HIV-2 enhancers by NF-kappaB (p50 + p65) and Ets-1, suggesting that physical interaction between Ets and NF-kappaB proteins is required for the transcriptional activity of the HIV-1 and HIV-2 enhancers. Taken together, these findings suggest that evolutionarily conserved physical interactions between Ets and NF-kappaB/NFAT proteins are important in regulating the inducible expression of T-cell genes and viruses. These interactions represent a potential target for the development of novel immunosuppressive and antiviral therapies.

Adenovirus infection stimulates the Raf/MAPK signaling pathway and induces interleukin-8 expression

Previous studies have shown that airway administration of adenovirus or adenovirus vectors results in a dose-dependent inflammatory response which limits the duration of transgene expression. We explored the possibility that adenovirus infection triggers signal transduction pathways that induce the synthesis of cytokines and thus contribute to the early inflammatory response. Since stimulation of the Raf/mitogen-activated protein kinase (MAPK) pathway activates transcription factors that control the expression of inflammatory cytokines, we examined the activation of this pathway following adenovirus infection. Adenovirus infection induced the rapid activation of Raf-1 and a transient increase in the tyrosine phosphorylation and activation of p42mapk at early times postinfection. Activation of the Raf/MAPK pathway by adenovirus is likely triggered by the infection process, since it occurred rapidly and with various mutant adenoviruses and adenovirus vectors. Moreover, interleukin-8 (IL-8) mRNA accumulation was evident at 20 min postinfection and was induced even in the presence of cycloheximide. Both MAPK activation and IL-8 production were inhibited by forskolin, a potent inhibitor of Raf-1. These results suggest that adenovirus-induced Raf/MAPK activation contributes to IL-8 production. Adenovirus-induced activation of the Raf/MAPK signaling pathway and IL-8 production may play critical roles in the inflammation observed following in vivo administration of adenovirus vectors for gene therapy.

Binding of human immunodeficiency virus type 1 to CD4 induces association of Lck and Raf-1 and activates Raf-1 by a Ras-independent pathway

We have analyzed CD4-mediated signaling during the early stages of human immunodeficiency virus type 1 (HIV-1) infection. Binding of purified HIV-1 virions or recombinant HIV-1 glycoprotein gp120 to CD4 receptors resulted in association and tyrosine phosphorylation and activation of tyrosine kinase Lck and serine/threonine kinase Raf-1. The association between Lck and Raf-1 was mediated by stimulation of the CD4 receptors, since it was abolished by preincubation of the virus with soluble CD4 and was not detected in CD4-negative A201 T cells. However, the Lck-Raf-1 association was restored in A201 cells permanently transfected with human CD4 cDNA and stimulated with anti-CD4 antibodies. In addition, a catalytically active Lck was required for the association of Lck and Raf-1. Surprisingly, the CD4-mediated signaling, induced by the HIV-1 binding, did not result in stimulation of the Ras GTP-binding activity or its association with Raf-1, indicating that the signaling pathway generated by the HIV-1 binding is not identical to the classical Ras/Raf-1 pathway. Furthermore, overexpression of activated Raf-1 in Jurkat T cells stimulated the HIV long terminal repeat promoter activity and significantly enhanced HIV-1 replication. This suggests that the Lck-Raf-1 pathway, rapidly stimulated by the binding of HIV-1 or gp120 to CD4 receptors, may play an essential role in the transcriptional activation of the integrated HIV-1 provirus as well as in its pathogenicity.

Raf-1 kinase targets GA-binding protein in transcriptional regulation of the human immunodeficiency virus type 1 promoter

The serine/threonine protein kinase Raf-1 is a component of a conserved intracellular signaling cascade that controls responses to various extracellular stimuli. Transcription from several promoters, including the oncogene-responsive element in the polyomavirus enhancer, the c-fos promoter, as well as other AP-1- and Ets-dependent promoters, can be induced by Raf-1 kinase. Previously, we have shown that activated Raf-1 kinase transactivates the human immunodeficiency virus type 1 (HIV-1) long terminal repeat and have identified the NF-kappaB binding motif as a Raf-1-responsive element (RafRE). We now report that Raf-1 kinase-induced transactivation from the HIV RafRE involves the purine-rich-repeat-binding protein (GABP), which is composed of two distinct subunits (alpha and beta). GABP alpha is an Ets oncogene-related DNA-binding protein, and GABP beta contains four ankyrin-like repeats that have been shown to be essential in protein-protein interactions. In electrophoretic mobility shift assays using nuclear extracts from human Jurkat T cells, a protein-DNA complex which was supershifted with antiserum against GABP alpha and GABP beta was observed. Purified recombinant GABP alpha and beta interact with the HIV RafRE as judged from DNA binding assays. Cotransfection experiments with GABP alpha and beta and Raf-1 kinase demonstrate synergistic transactivation of the HIV-1 promoter. Point mutations in the HIV RafRE abolished the Raf-1 kinase as well as GABP alpha- and beta-induced transactivation. The observed Raf-1-GABP synergism presumably involves phosphorylation of GABP subunits, as treatment of cells with Raf-1 kinase activators serum and 12-O-tetradecanoylphorbol-13-acetate increases phosphorylation of GABP in vivo. However, GABP is not a target of Raf-1 kinase; instead, it is a substrate of mitogen-activated protein kinase (MAPK/ERK), since in vitro phosphorylation of GABP alpha and beta was achieved by the reconstituted protein kinase cascade but not with purified Raf-1 or MEK. These results suggest that Raf-1 kinase- induced activation of the HIV-1 promoter is mediated by the classical cytoplasmic cascade resulting in MAPK/ERK-mediated phosphorylation of GABP alpha and beta. Because the HIV RafRE corresponds to a region within the promoter which is essential for regulation of HIV-1 expression, the data indicate that in addition to NK-kappaB, GABP transcription factors are important for induced expression of HIV.

The Ras-Raf pathway is activated in human immunodeficiency virus-infected monocytes and particpates in the activation of NF-kappa B

Persistent human immunodeficiency virus (HIV) infection of human monocytes and macrophages increases I kappa B alpha degradation, resulting in the activation of NF-kappa B, a key transcription factor in the regulation of the HIV long terminal repeat. The signal transduction pathways leading to NF-kappa B activation in cells of the monocytic lineage, especially those regulated by HIV infection, and their relevance in regulating viral persistence remain unknown. Both p21ras and its downstream Raf-1 kinase participate in the transduction of signals initiated from a variety of cell surface receptors and in the regulation of transcription factors. We have studied whether the Ras-Raf pathway is functional and participates in HIV-mediated NF-kappa B activation in monocytic cells. Constitutively active p21ras (v-H-Ras) activated NF- kappa B-dependent transcription and induces the nuclear translocation of a bona fide p65/p50 heterodimer by targeting I kappa B alpha. In addition, the constitutively active form of Raf (RafBXB) also increases the NF-kappa B-dependent transcriptional activity. Because of the similarity between HIV and Ras-Raf-induced NF-kappa B activation in monocytic cells, we next tested whether HIV-induced NF-kappa B activation was mediated by the Ras-Raf signal transduction pathway. Negative dominant forms of both Ras (Ras N17) and Raf (Raf 301) decreased the HIV- but not lipopolysaccharide-dependent NF-kappa B activation in U937 cells. Moreover, Raf-1 kinase activity was greater in HIV-infected than uninfected monocytic cells in in vitro kinase assays. Altogether, these results indicate that the Ras-Raf pathway is unregulated in HIV monocytic cells and participates in the virus-induced activation of NF-kappa B.

RelA is a potent transcriptional activator of the CD28 response element within the interleukin 2 promoter

T-cell activation requires two different signals. The T-cell receptor's recognition of a specific antigen on antigen-presenting cells provides one, and the second signal comes from costimulatory molecules such as CD28. In contrast, T cells that are stimulated with antigen in the absence of the CD28 costimulatory signal can become anergic (nonresponsive). The CD28 response element (CD28RE) has been identified as the DNA element mediating interleukin 2 (IL-2) gene activation by CD28 costimulation. Our previous work demonstrates that the Rel/NF-kappa B family proteins c-Rel, RelA (p65), and NFKB1 (p50) are involved in the complex that binds to the CD28RE. We also showed that c-Rel, but not NFKB1 (p50), can bind to the CD28RE and activate CD28RE-driven transcription in cotransfection assays. However, the role of RelA (p65) in CD28 signaling has not yet been addressed. We provide evidence that RelA (p65) itself bound directly to the CD28RE of the IL-2 promoter and other lymphokine promoters. In addition, RelA (p65) was a potent transcriptional activator of the CD28RE in vivo. We show that a RelA (p65)-c-Rel heterodimer bound to the CD28RE and synergistically activated the CD28RE enhancer activity. We also demonstrate that activated Raf-1 kinase synergized with RelA (p65) in activating the CD28RE enhancer activity. Interestingly, a soluble anti-CD28 monoclonal antibody alone, in the absence of other stimuli, also synergized with RelA (p65) in activating the CD28RE. Furthermore, we show that RelA (p65) activated expression of the wild-type IL-2 promoter but not the CD28RE-mutated IL-2 promoter. A combination of RelA (p65) and NFKB1 (p50) also activated the IL-2 promoter through the CD28RE site. These results demonstrate the functional regulation of the CD28RE, within the IL-2 promoter, by Rel/NF-kappa B transcription factors.

Multi-step activation of NF-kappa B/Rel transcription factors

Transcription factors belonging to the NF-kappa B/Rel family are specialized in the transduction of primarily pathogenic signals from the cytoplasm to the cell nucleus. To date, the family comprises five distinct DNA-binding subunits and five regulatory proteins with inhibitory function, called I kappa B proteins. The interaction of dimers of the DNA-binding subunits with the I kappa B proteins leads to the cytoplasmatic retention of the complex and inhibition of its DNA binding. Following stimulation of cells, the I kappa B proteins become phosphorylated and are subsequently degraded, presumably, by the proteasome. The released NF-kappa B/Rel transcription factors can then enter the nucleus, bind to decameric DNA cognate sequences and stimulate transcription of numerous immunologically important target genes. In this article, we discuss several distinct levels at which the NF-kappa B/Rel transcription factors can be regulated.

Regulation of pre-mRNA processing by src

BACKGROUND

Changes in gene expression in response to external signals provide a key mechanisms for the regulation of higher eukaryotic cell functions. The importance of transcriptional control in the response of cells to growth factors and cytokines has been extensively documented, but gene expression has also been shown to be controlled at other levels, such as the stability of mRNA in the cytoplasm, its localization and translation. By contrast to transcriptional control, little is known of the contribution of pre-mRNA nuclear processing to the regulation of gene expression, as most of our knowledge of pre-mRNA processing in vivo is indirect, being inferred from comparisons of transcription rates and levels of mRNA accumulation.

RESULTS

In this study, we have used as a model the well-characterized maturation pathway of transcripts of the cytokine, tumour necrosis factor beta (TNF beta). We have used the murine TNF beta gene as a reporter for pre-mRNA processing, using a co-transfection approach to investigate whether overproduction of proteins involved in signal transduction influences the processing of TNF beta transcripts. Although transfection of both activated ras and src genes led to an increase in RNA accumulation in the nuclear and cytoplasmic compartments, as expected from their transactivation of the TNF beta expression vector, only src induced a modification of RNA processing. Comparison of several modes of src activation indicated that two distinct effects of src on pre-mRNA processing can be coupled: one involves slowing down splicing and the other allows the export of partially spliced transcripts. These effects can be observed not only on the three introns of TNF beta but also on transcripts from a beta globin expression vector.

DISCUSSION

We have characterized how the processing of transcripts of TNF beta and beta globin is regulated by the signal transduction pathway that includes the Src protein, establishing that external signals have the capacity to regulate gene expression at a post-transcriptional level within the nucleus. Src seems to act on a general mechanism of splicing and/or mRNA transport, but its biologically relevant targets are likely to be restricted to genes for which either alternative processing pathways are in competition, or the kinetics of splicing is critical. This regulation could reflect a modulation by Src of the activity of components of the splicing and transport machineries, but could also involve RNA-binding proteins, which have been shown to interact with Src.

Ras activation of genes: Mob-1 as a model

The ras oncogenes function by indirectly controlling expression of a subset of yet-undefined genes that are crucial for cell growth and differentiation. In a differential display strategy, numerous genes were identified on the basis of their differential expression in rat embryo fibroblasts transformed by the cooperation of mutant Ha-ras and p53 genes. We demonstrate here that one such gene, designated mob-1, is a downstream target of the Ras signaling pathway. The 417-bp mob-1 promoter, which contains dual NF-kappa B and AP-1 binding sites, confers the Ras inducibility. Oncogenic Ras as well as serum growth factors that activate endogenous Ras can induce mob-1 expression, but with a fundamental difference in that the oncogenic induction is constitutive whereas the serum induction is transient. mob-1 encodes a small secretory protein with a high degree of homology to IP-10, a member of a proinflammatory cytokine family. These findings link chronic inflammatory response to constitutive ras activation and tumorigenesis. Mob-1 may serve as a secreted marker for oncogenic Ha-ras mutations.