c-sis/PDGF-B promoter transactivation by the Yax protein of human T-cell leukemia virus type 1

Evaluating mRNA expression of tax, B chain of PDGF and PDGF-β receptors as well as HTLV-I proviral load in ATL patients and healthy carriers

Adult T-cell leukemia (ATL) is a life-threatening malignant neoplasm of CD4+ T cells resulted from human T-cell leukemia virus type I (HTLV-I). Tax1 protein of HTLV-I can induce malignant proliferation of T-cells by modulating the expression of growth factors such as platelet-derived growth factor (PDGF). Here, we aimed to investigate the proviral load (PVL) of HTLV-I in ATL and also to evaluate the mRNA expression of B chain of PDGF and PDGF-β receptors in ATL patients and HTLV-I-infected healthy carriers. To this end, peripheral blood mononuclear cells (PBMCs) were isolated by using Ficoll-Histophaque density centrifugation. The mean of HTLV-I PVL in ATL patients (42,759 ± 15,737 copies/104 cells [95% CI, 9557-75962]) was significantly (p = .01) higher than that in healthy carriers (650 ± 107 copies/104 cells [95% CI, 422-879], respectively. The HTLV-I PVL in ATL patients exhibited a significant correlation with PBMC count (R = .495, p = .001). The mRNA expression of Tax, B chain of PDGF, and PDGF-β receptor genes was significantly higher in healthy carriers than in patients with ATL. In conclusion, the expression of the canonical PDGFβ and its receptor, and their correlation with Tax expression cannot be a suitable indicator and/or prognostic factor for progression of ATL in HTLV-I carriers.

Human T-cell leukemia virus type 1 oncoprotein tax represses ZNF268 expression through the cAMP-responsive element-binding protein/activating transcription factor pathway

Expression of the human T-cell leukemia virus type 1 (HTLV-1) oncoprotein Tax is correlated with cellular transformation, contributing to the development of adult T-cell leukemia. In this study, we investigated the role of Tax in the regulation of the ZNF268 gene, which plays a role in the differentiation of blood cells and the pathogenesis of leukemia. We demonstrated that ZNF268 mRNA was repressed in HTLV-1-infected cells. We also showed that stable and transient expression of HTLV-1 Tax led to repression of ZNF268. In addition, by using reporter constructs that bear the human ZNF268 promoter and its mutants, we showed that Tax repressed ZNF268 promoter in a process dependent on a functional cAMP-responsive element. By using Tax, cAMP-responsive element-binding protein (CREB)-1, CREB-2, and their mutants, we further showed that Tax repressed ZNF268 through the CREB/activating transcription factor pathway. Electrophoretic mobility shift assays and chromatin immunoprecipitation demonstrated the formation of the complex of Tax.CREB-1 directly at the cAMP-responsive element both in vitro and in vivo. These findings suggest a role for ZNF268 in aberrant T-cell proliferation observed in HTLV-1-associated diseases.

Human T-cell leukemia virus type 1 expressing nonoverlapping tax and rex genes replicates and immortalizes primary human T lymphocytes but fails to replicate and persist in vivo

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus associated primarily with adult T-cell leukemia and neurological disease. HTLV-1 encodes the positive trans-regulatory proteins Tax and Rex, both of which are essential for viral replication. Tax activates transcription initiation from the viral long terminal repeat and modulates the transcription or activity of a number of cellular genes. Rex regulates gene expression posttranscriptionally by facilitating the cytoplasmic expression of incompletely spliced viral mRNAs. Tax and Rex mutants have been identified that have defective activities or impaired biochemical properties associated with their function. To ultimately determine the contribution of specific protein activities on viral replication and cellular transformation of primary T cells, mutants need to be characterized in the context of an infectious molecular clone. Since the tax and rex genes are in partially overlapping reading frames, mutation in one gene frequently disrupts the other, confounding interpretation of mutational analyses in the context of the virus. Here we generated and characterized a unique proviral clone (H1IT) in which the tax and rex genes were separated by expressing Tax from an internal ribosome entry site. We showed that H1IT expresses both functional Tax and Rex. In short- and long-term coculture assays, H1IT was competent to infect and immortalize primary human T cells similar to wild-type HTLV-1. In contrast, H1IT failed to efficiently replicate and persist in inoculated rabbits, thus emphasizing the importance of temporal and quantitative regulation of specific mRNA for viral survival in vivo.

Long-term infection with retroviral structural gene vector provides protection against bovine leukemia virus disease in rabbits

Bovine leukemia virus (BLV) infection of rabbits is a tractable model system to evaluate vaccination strategies against lymphotropic retroviruses, which represent a global human health problem. We have previously developed genetically simplified BLV structural gene vector (SGV) that replicates BLV structural and enzymatic genes independently of BLV regulatory and accessory genes. Results of a 20-month study in a rabbit model demonstrated that BLV SGV induces an antiviral immunological response and lacks pathogenicity. Here, these chronically infected-BLV SGV rabbits are assessed in a proof-of-principle study of preventative vaccination against challenge with pathogenic BLV. This study commences 24 months after BLV SGV inoculation and proceeds for an additional 20 months. The previously characterized BLV SGV rabbits and age-matched control rabbits were challenged with 1 x 10(8) fetal lamb kidney/BLV producer cells. BLV SGV rabbits seroconverted upon BLV challenge, but did not progress to BLV infection nor clinical disease. By contrast, naive rabbits became infected and succumbed to lymphotropic disease. Our findings provide proof-of-principle that chronic infection with BLV SGV induces protection against BLV infection. The data indicate that SGV based on HTLV or HIV is a promising approach against lymphotropic disease by human retroviruses.

Transformation studies with a human T-cell leukemia virus type 1 molecular clone

In in vitro studies human T-cell leukemia virus type 1 (HTLV-1) may be produced by stable or transient transfection of target cells with an infectious molecular clone. Studies using primary human T cells, the natural targets of HTLV-1 infection, are hampered by difficulty in achieving significant infection with cell-free virus and a poor efficiency of transfection of primary cells. A method is described for the generation of stable cell lines expressing HTLV-1 from an infectious proviral clone. The stably transfected cells can be irradiated and cocultured with human peripheral blood mononuclear cells (PBMC) resulting in infected primary cells. These cells become immortalized, IL-2 dependent lines, which contain integrated copies of provirus and express a full spectrum of viral proteins. Analysis of cellular markers indicates that immortalized cell lines consist of CD3+/CD4+ T cells, matching the most common adult T-cell leukemia (ATL) cell phenotype. The method described has great utility in the study of the replication and transformation capacity of HTLV and HTLV mutant viruses in their natural targets, primary human T lymphocytes.

Functional domain structure of human T-cell leukemia virus type 2 rex

The Rex protein of human T-cell leukemia virus (HTLV) acts posttranscriptionally to induce the cytoplasmic expression of the unspliced and incompletely spliced viral RNAs encoding the viral structural and enzymatic proteins and is therefore essential for efficient viral replication. Rex function requires nuclear import, RNA binding, multimerization, and nuclear export. In addition, it has been demonstrated that the phosphorylation status of HTLV-2 Rex (Rex-2) correlates with RNA binding and inhibition of splicing in vitro. Recent mutational analyses of Rex-2 revealed that the phosphorylation of serine residues 151 and 153 within a novel carboxy-terminal domain is critical for function in vivo. To further define the functional domain structure of Rex-2, we evaluated a panel of Rex-2 mutants for subcellular localization, RNA binding capacity, multimerization and trans-dominant properties, and the ability to shuttle between the nucleus and the cytoplasm. Rex-2 mutant S151A,S153A, which is defective in phosphorylation and function, showed diffuse cytoplasmic staining, whereas mutant S151D,S153D, previously shown to be functional and in a conformation corresponding to constitutive phosphorylation, displayed increased intense speckled staining in the nucleoli. In vivo RNA binding analyses indicated that mutant S151A,S153A failed to efficiently bind target RNA, while its phosphomimetic counterpart, S151D,S153D, bound twofold more RNA than wild-type Rex-2. Taken together, these findings provide direct evidence that the phosphorylation status of Rex-2 is linked to cellular trafficking and RNA binding capacity. Mutants with substitutions in either of the two putative multimerization domains or in the putative activation domain-nuclear export signal displayed a dominant negative phenotype as well as defects in multimerization and nucleocytoplasmic shuttling. Several carboxy-terminal mutants that displayed wild-type levels of phosphorylation and localized to the nucleolus were also partially impaired in shuttling. This is consistent with the hypothesis that the carboxy terminus of Rex-2 contains a novel domain that is required for efficient shuttling. This work thus provides a more detailed functional domain map of Rex-2 and further insight into its regulation of HTLV replication.

HTLV-1 Rex is required for viral spread and persistence in vivo but is dispensable for cellular immortalization in vitro

Human T-cell leukemia virus type 1 (HTLV-1) is associated with leukemia/lymphoma and neurologic disorders. Although the viral transcriptional activator Tax is the critical viral oncoprotein, Rex, which regulates the expression of the viral structural and enzymatic genes, is essential for efficient viral replication. Herein, we investigate the contribution of Rex in HTLV-1 immortalization of primary T cells in vitro and viral survival in an infectious rabbit animal model. A Rex-deficient HTLV-1 (HTLVRex-) was constructed and characterized for viral gene expression, protein production, and immortalization capacity. Cells transiently transfected with the HTLVRex- proviral clone produced low detectable levels of p19 Gag. 729HTLVRex- stable transfectants produced functional Tax, but undetectable levels of Rex or p19 Gag. Coculture of irradiated 729HTLVRex- cells with peripheral blood mononuclear cells (PBMCs) resulted in sustained interleukin-2 (IL-2)-dependent growth of primary T lymphocytes. These cells carried the HTLVRex- genome and expressed tax/rex mRNA but produced no detectable Rex or p19 Gag. Rabbits inoculated with irradiated 729HTLVRex- cells or 729HTLVRex- cells transiently transfected with a Rex cDNA expression plasmid failed to become persistently infected or mount a detectable antibody response to the viral gene products. Together, our results provide the first direct evidence that Rex and its function to modulate viral gene expression and virion production is not required for in vitro immortalization by HTLV-1. However, Rex is critical for efficient infection of cells and persistence in vivo.

Differential patterns of human cytomegalovirus gene expression in various T-cell lines carrying human T-cell leukemia-lymphoma virus type I: role of Tax-activated cellular transcription factors

Replication of human cytomegalovirus (HCMV) was investigated in various T-cell lines expressing the tax gene product of human T-cell leukemia-lymphoma virus type I (HTLV-I). Differential patterns of HCMV replication were found in HTLV-I-carrying cell lines. HCMV gene expression was restricted to the immediate-early genes in MT-2 and MT-4 cells, whereas full replication cycle of the virus was observed in C8166-45 cells. Productive HCMV infection induced a cytopathic effect resulting in the lysis of infected cells. The results of electrophoretic mobility shift assay (EMSA) showed high levels of NF-kappaB-, CREB/ATF-1-, and SRF-specific DNA binding activity in all Tax-positive cell lines. In contrast, SP1 activity could be detected only in C8166-45 cells. Using an inducible system (Jurkat cell line JPX-9), a dramatic increase in NF-kappaB, CREB/ATF-1, SRF, and SP1 binding activity, as well as productive HCMV infection, were observed upon Tax expression. Overexpression of SP1 in MT-2 and MT-4 cells converted HCMV infection from an abortive to a productive one. These data suggest that the stimulatory effect of Tax protein on HCMV in T cells is accomplished through at least five host-related transcription factor pathways. The results of this study provide possible mechanisms whereby HCMV infections might imply suppression of adult T-cell leukemia.

Tax and overlapping rex sequences do not confer the distinct transformation tropisms of human T-cell leukemia virus types 1 and 2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are distinct oncogenic retroviruses that infect several cell types but display their biological and pathogenic activity only in T cells. Previous studies have indicated that in vivo HTLV-1 has a preferential tropism for CD4+ T cells, whereas HTLV-2 in vivo tropism is less clear but appears to favor CD8+ T cells. Both CD4+ and CD8+ T cells are susceptible to HTLV-1 and HTLV-2 infection in vitro, and HTLV-1 has a preferential immortalization and transformation tropism of CD4+ T cells, whereas HTLV-2 immortalizes and transforms primarily CD8+ T cells. The molecular mechanism that determines this tropism of HTLV-1 and HTLV-2 has not been determined. HTLV-1 and HTLV-2 carry the tax and rex transregulatory genes in separate but partially overlapping reading frames. Since Tax has been shown to be critical for cellular transformation in vitro and interacts with numerous cellular processes, we hypothesized that the viral determinant of transformation tropism is encoded by tax. Using molecular clones of HTLV-1 (Ach) and HTLV-2 (pH6neo), we constructed recombinants in which tax and overlapping rex genes of the two viruses were exchanged. p19 Gag expression from proviral clones transfected into 293T cells indicated that both recombinants contained functional Tax and Rex but with significantly altered activity compared to the wild-type clones. Stable transfectants expressing recombinant viruses were established, irradiated, and cocultured with peripheral blood mononuclear cells. Both recombinants were competent to transform T lymphocytes with an efficiency similar to that of the parental viruses. Flow cytometry analysis indicated that HTLV-1 and HTLV-1/TR2 had a preferential tropism for CD4+ T cells and that HTLV-2 and HTLV-2/TR1 had a preferential tropism for CD8(+) T cells. Our results indicate that tax/rex in different genetic backgrounds display altered functional activity but ultimately do not contribute to the different in vitro transformation tropisms. This first study with recombinants between HTLV-1 and HTLV-2 is the initial step in elucidating the different pathobiologies of HTLV-1 and HTLV-2.

Phosphorylation of two serine residues regulates human T-cell leukemia virus type 2 Rex function

The function of the human T-cell leukemia virus (HTLV) Rex phosphoprotein is to increase the level of the viral structural and enzymatic gene products expressed from the incompletely spliced viral RNAs containing the Rex-responsive element. The phosphorylation of HTLV type 2 Rex (Rex-2), predominantly on serine residues, correlates with an altered conformation, as detected by a gel mobility shift, and is required for specific binding to its viral RNA target sequence. Thus, the phosphorylation state of Rex in the infected cell may be a switch that determines whether the virus exists in a latent or a productive state. A mutational analysis of Rex-2 that focused on serine and threonine residues was performed to identify regions or domains within Rex-2 important for function, with a specific emphasis on identifying Rex-2 phosphorylation mutants. We identified mutations near the carboxy terminus that disrupted a novel region or domain and abrogated Rex-2 function. Mutant M17 (with S151A and S153A mutations) displayed reduced phosphorylation that correlated with reduced function. Replacement of both serine residues 151 and 153 with phosphomimetic aspartic acid restored Rex-2 function and locked Rex-2 in a phosphorylated active conformation. A mutant containing threonine residues at positions 151 and 153 displayed a phenotype indistinguishable from that of wild-type Rex. Furthermore, this same mutant showed increased threonine phosphorylation and decreased serine phosphorylation, providing conclusive evidence that one or both of these residues are phosphorylated in vivo. Our results provide the first direct evidence that the phosphorylation of Rex-2 is important for function. Further understanding of HTLV Rex phosphorylation will provide insight into the regulatory control of HTLV replication and ultimately the pathobiology of HTLV.

Induction of platelet-derived growth factor B-chain expression by transforming growth factor-beta involves transactivation by Smads

Transforming growth factor-beta (TGF-beta) regulates a diverse array of biological processes, such as proliferation, differentiation, extracellular matrix production, and apoptosis. In cultured vascular endothelial cells, TGF-beta induces the expression of platelet-derived growth factor (PDGF) B-chain, a mitogen and chemoattractant, at the level of transcription. The molecular mechanism(s) underlying this process are not presently understood. In this study, we performed serial 5' deletion and transient transfection analysis to define a region in the PDGF-B promoter mediating inducible responsiveness to TGF-beta. This region contains an atypical nucleotide recognition element for the Smad family of transcriptional regulators. Electrophoretic mobility shift analysis revealed that nuclear proteins bound to this site in a transient and specific manner. Supershift studies demonstrated the physical association of Smad4 with the promoter. Overexpression of Smad4 activated the PDGF-B promoter and superinduced PDGF-B promoter-dependent expression in cells exposed to TGF-beta. Moreover, simultaneous cotransfection of Smad3 and Smad4 activated the PDGF-B promoter. This effect was attenuated when Smad4 was substituted with its dominant negative counterpart. Mutation of the (-81)CAGA(-78) motif in the PDGF-B promoter abrogated Smad-inducible promoter-dependent expression. Overexpression of Smad2 and Smad3 transactivated the PDGF-B promoter in a synergistic manner. These findings demonstrate the existence of a novel, functional binding element in the proximal region of the PDGF-B promoter mediating responsiveness to TGF-beta.

Human T-cell leukemia virus type 2 tax mutants that selectively abrogate NFkappaB or CREB/ATF activation fail to transform primary human T cells

Human T-cell leukemia virus (HTLV) Tax protein has been implicated in the HTLV oncogenic process, primarily due to its pleiotropic effects on cellular genes involved in growth regulation and cell cycle control. To date, several approaches attempting to correlate Tax activation of the CREB/activating transcription factor (ATF) or NFkappaB/Rel transcriptional activation pathway to cellular transformation have yielded conflicting results. In this study, we use a unique HTLV-2 provirus (HTLV(c-enh)) that replicates by a Tax-independent mechanism to directly assess the role of Tax transactivation in HTLV-mediated T-lymphocyte transformation. A panel of well-characterized tax-2 mutations is utilized to correlate the respective roles of the CREB/ATF or NFkappaB/Rel signaling pathway. Our results demonstrate that viruses expressing tax-2 mutations that selectively abrogate NFkappaB/Rel or CREB/ATF activation display distinct phenotypes but ultimately fail to transform primary human T lymphocytes. One conclusion consistent with our results is that the activation of NFkappaB/Rel provides a critical proliferative signal early in the cellular transformation process, whereas CREB/ATF activation is required to promote the fully transformed state. However, complete understanding will require correlation of Tax domains important in cellular transformation to those Tax domains important in the modulation of gene transcription, cell cycle control, induction of DNA damage, and other undefined activities.

Transcriptional activation of human TR3/nur77 gene expression by human T-lymphotropic virus type I Tax protein through two AP-1-like elements

The Tax transactivator of human T-lymphotropic virus type I (HTLV-I) is capable of inducing expression of the human immediate-early TR3/nur77 gene. Deletion and mutation analyses of the TR3/nur77 promoter demonstrated that multiple transcription elements in the 121 bp sequence proximal to the transcription start site are required for full Tax transactivation. Mutations of CArG-like, Ets and RCE motifs in this region severely decreased Tax transactivation. Mutation of either of the two identical AP-1-like elements (NAP 1 and 2) immediately upstream of the TATA box caused around 80% reduction of Tax transactivation. Mutation of both NAP elements blocked Tax-mediated activation totally. These two NAP elements could confer Tax-responsiveness on a heterologous basal promoter. Furthermore, the specific NAP-binding complex was only observed in HTLV-I-infected cells. Formation of this specific NAP-binding complex was correlated directly with Tax expression, as demonstrated in JPX-9 cells upon induction of Tax expression. The specific NAP binding could be competed for by consensus AP-1 and CREB elements, indicating that the NAP-binding proteins probably belong to the AP-1 and CREB/ATF transcription factor families. Supershift analysis with antibodies to both the AP-1 and CREB/ATF transcription factor families revealed that only anti-JunD antibody could partially shift this NAP-binding complex, indicating that JunD is a component of the NAP complex. This work suggests that JunD is involved in Tax-regulated TR3/nur77 expression.

Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Biochemical coupling of transcription factor NF-kappaB to antigen and co-stimulatory receptors is required for the temporal control of T-cell proliferation. In contrast to its transitory activation during normal growth-signal transduction, NF-kappaB is constitutively deployed in T-cells transformed by the type 1 human T-cell leukemia virus (HTLV-1). This viral/host interaction is mediated by the HTLV-1-encoded Tax protein, which has potent oncogenic properties. As reviewed here, Tax activates NF-kappaB primarily via a pathway leading to the chronic phosphorylation and degradation of IkappaBalpha, a cytoplasmic inhibitor of NF-kappaB. To access this pathway, Tax associates stably with a cytokine-inducible IkappaB kinase (IKK), which contains both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. Unlike their transiently induced counterparts in cytokine-treated cells, Tax-associated forms of IKKalpha and IKKbeta are persistently activated in HTLV-1-infected T cells. Acquisition of the deregulated IKK phenotype is contingent on the presence of IKKgamma, which functions as a molecular adaptor in the assembly of pathologic Tax/IkappaB kinase complexes. These findings highlight a key mechanistic role for IKK in the Tax/NF-kappaB signaling axis and define new intracellular targets for the therapeutic control of HTLV-1-associated disease.

Regulation of Egr-1-dependent gene expression by the C-terminal activation domain

The present study analyzes the role of the C-terminal activation domain for Egr-1 transcriptional activity using N-terminal deletion mutants. Mutant N372 comprising the entire C-terminal activation domain and partly truncated DNA-binding and nuclear translocation domains functioned as the transdominant repressor of Egr-1-dependent gene transcription. Activity of the SV40 promoter, however, was not affected by the N372 mutant. Analysis of additional Egr-1 mutants revealed that the transdominant negative effect of N372 was dependent on truncation of the zinc finger motifs that mediate DNA binding. Reconstitution of the zinc fingers was sufficient to generate Egr-1 proteins with potent transcriptional activity. The inhibitory mutant N372 is efficiently translocated to the nucleus, but fails to bind DNA and does not displace DNA-bound wildtype Egr-1. These results provide evidence for an Egr-1-specific cofactor that interacts with the C-terminal activation domain and is essential for Egr-1 transcriptional activity.

Zinc finger transcription factors mediate high constitutive platelet-derived growth factor-B expression in smooth muscle cells derived from aortae of newborn rats

Platelet-derived growth factor (PDGF) B-chain gene is differentially expressed in smooth muscle cells (SMCs) derived from the rat aortic wall. SMCs cultured from two week-old rats (pups) express high levels of PDGF-B mRNA, whereas cells isolated from three month-old rats (adults) express low levels of PDGF-B. Nuclear run-off experiments indicate that increased PDGF-B gene expression in pups is mediated, at least in part, at the transcriptional level. We used electrophoretic mobility shift assays and Western blot analysis to demonstrate that levels of Sp1 and Sp3, two zinc finger transcription factors which mediate basal expression of the PDGF-B gene, are elevated in pup nuclei compared with adult nuclei. The immediate-early transcription factor, Egr-1, which footprints the PDGF-B promoter, is also constitutively expressed in these cells. Transient transfection and binding studies show that these factors interact with a region in the proximal PDGF-B promoter key for basal expression in pup cells. Mutation of this proximal element in transfected pup cells attenuates reporter gene expression to levels observed in adult cells. Conversely, overexpression of Sp1 in adult cells augments PDGF-B promoter-dependent expression. Elevated PDGF-B expression in cultured newborn rat SMCs may therefore require high constitutive expression of a number of zinc finger transcription factors and their specific interactions with the proximal PDGF-B promoter.

The tax protein of human T-cell leukemia virus type 1 mediates the transactivation of the c-sis/platelet-derived growth factor-B promoter through interactions with the zinc finger transcription factors Sp1 and NGFI-A/Egr-1

Transcriptional up-regulation of the c-sis/platelet-derived growth factor-B (PDGF-B) proto-oncogene by the Tax protein of human T-cell leukemia virus type 1 has been implicated as one possible mechanism of cellular transformation by human T-cell leukemia virus type 1. In previous work, we identified an essential site in the c-sis/PDGF-B promoter, Tax-responsive element 1 (TRE1), necessary for transactivation by Tax. We also identified Sp1, Sp3, and NGFI-A/Egr-1 as the primary nuclear transcription factors binding to TRE1 which mediate Tax responsiveness. In the present work, we have investigated the mechanism(s) whereby Tax transactivates the c-sis/PDGF-B proto-oncogene. In vitro transcription assays showed that Tax was able to significantly increase the transcriptional activity of a template containing the -257 to +74 region of the c-sis/PDGF-B promoter. Electrophoretic mobility shift assay analysis showed that Tax increased the DNA binding activity of both Sp1 and NGFI-A/Egr-1 using a TRE1 probe. Analysis of Tax mutants showed that two mutants, IEXC29S and IEXL320G, were unable to significantly transactivate the c-sis/PDGF-B promoter. Finally, co-immunoprecipitation analysis revealed that Tax is able to stably bind to both Sp1 and NGFI-A/Egr-1. Interestingly, co-immunoprecipitation analysis also revealed that Tax mutant IEXC29S is unable to interact with NGFI-A/Egr-1, whereas Tax mutant IEXL320G is able to interact with NGFI-A/Egr-1.

Collaborative roles for c-Jun N-terminal kinase, c-Jun, serum response factor, and Sp1 in calcium-regulated myocardial gene expression

Electrical stimulation of contractions (pacing) of primary neonatal rat ventricular myocytes increases intracellular calcium and activates a hypertrophic growth program that includes expression of the cardiac-specific gene, atrial natriuretic factor (ANF). To investigate the mechanism whereby pacing increases ANF, pacing was tested for its ability to regulate mitogen-activated protein kinase family members, ANF promoter activity, and the trans-activation domain of the transcription factor, Sp1. Pacing and the calcium channel agonist BAYK 8644 activated c-Jun N-terminal kinase (JNK) but not extracellular signal-regulated kinase. Pacing stimulated ANF-promoter activity approximately 10-fold. Furthermore, transfection with an expression vector for c-Jun, a substrate for JNK, also activated the ANF promoter, and the combination of pacing and c-Jun was synergystic, consistent with roles for JNK and c-Jun in calcium-activated ANF expression. Proximal serum response factor and Sp1 binding sites were required for the effects of pacing or c-Jun on the ANF promoter. Pacing and c-Jun activated a GAL4-Sp1 fusion protein by 3- and 12-fold, respectively, whereas the two stimuli together activated GAL4-Sp1 synergistically, similar to their effect on the ANF promoter. Transfection with an expression vector for c-Fos inhibited the effects of c-Jun, suggesting that c-Jun acts independently of AP-1. These results demonstrate an interaction between c-Jun and Sp1 and are consistent with a novel mechanism of calcium-mediated transcriptional activation involving the collaborative actions of JNK, c-Jun, serum response factor, and Sp1.