HTLV-1 p27rex stabilizes human interleukin-2 receptor alpha chain mRNA

Comparison of the Genetic Organization, Expression Strategies and Oncogenic Potential of HTLV-1 and HTLV-2

Human T cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are genetically related complex retroviruses that are capable of immortalizing human T-cells in vitro and establish life-long persistent infections in vivo. In spite of these apparent similarities, HTLV-1 and HTLV-2 exhibit a significantly different pathogenic potential. HTLV-1 is recognized as the causative agent of adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In contrast, HTLV-2 has not been causally linked to human malignancy, although it may increase the risk of developing inflammatory neuropathies and infectious diseases. The present paper is focused on the studies aimed at defining the viral genetic determinants of the pathobiology of HTLV-1 and HTLV-2 through a comparison of the expression strategies and functional properties of the different gene products of the two viruses.

Rev and Rex proteins of human complex retroviruses function with the MMTV Rem-responsive element

Background

Mouse mammary tumor virus (MMTV) encodes the Rem protein, an HIV Rev-like protein that enhances nuclear export of unspliced viral RNA in rodent cells. We have shown that Rem is expressed from a doubly spliced RNA, typical of complex retroviruses. Several recent reports indicate that MMTV can infect human cells, suggesting that MMTV might interact with human retroviruses, such as human immunodeficiency virus (HIV), human T-cell leukemia virus (HTLV), and human endogenous retrovirus type K (HERV-K). In this report, we test whether the export/regulatory proteins of human complex retroviruses will increase expression from vectors containing the Rem-responsive element (RmRE).

Results

MMTV Rem, HIV Rev, and HTLV Rex proteins, but not HERV-K Rec, enhanced expression from an MMTV-based reporter plasmid in human T cells, and this activity was dependent on the RmRE. No RmRE-dependent reporter gene expression was detectable using Rev, Rex, or Rec in HC11 mouse mammary cells. Cell fractionation and RNA quantitation experiments suggested that the regulatory proteins did not affect RNA stability or nuclear export in the MMTV reporter system. Rem had no demonstrable activity on export elements from HIV, HTLV, or HERV-K. Similar to the Rem-specific activity in rodent cells, the RmRE-dependent functions of Rem, Rev, or Rex in human cells were inhibited by a dominant-negative truncated nucleoporin that acts in the Crm1 pathway of RNA and protein export.

Conclusion

These data argue that many retroviral regulatory proteins recognize similar complex RNA structures, which may depend on the presence of cell-type specific proteins. Retroviral protein activity on the RmRE appears to affect a post-export function of the reporter RNA. Our results provide additional evidence that MMTV is a complex retrovirus with the potential for viral interactions in human cells.

HTLV-1 Yin and Yang: Rex and p30 master regulators of viral mRNA trafficking

Human retroviruses are associated with a variety of malignancies including Kaposi's sarcoma and Epstein-Barr virus-associated lymphoma in HIV infection, T-cell leukemia/lymphoma and a neurologic disorder in human T-cell lymphotropic virus type 1 (HTLV-1) infection. Both HIV and human T-cell lymphotropic virus type 1 have evolved a complex genetic organization for optimal use of their limited genome and production of all necessary structural and regulatory proteins. Use of alternative splicing is essential for balanced expression of multiple viral regulators from one genomic polycistronic RNA. In addition, nuclear export of incompletely spliced RNA is required for production of structural and enzymatic proteins and virus particles. Decisions controlling these events are largely guarded by viral proteins. In human T-cell lymphotropic virus type 1, Rex and p30 are both nuclear/nucleolar RNA binding regulatory proteins. Rex interacts with a Rex-responsive element to stimulate nuclear export of incompletely spliced RNA and increase production of virus particles. In contrast, human T-cell lymphotropic virus type 1 p30 is involved in the nuclear retention of the tax/rex mRNA leading to inhibition of virus expression and establishment of viral latency. How these two proteins, with apparently opposite functions, orchestrate virus replication and ensure vigilant control of viral gene expression is discussed.

Expression of parathyroid hormone-related protein during immortalization of human peripheral blood mononuclear cells by HTLV-1: implications for transformation

BACKGROUND

Adult T-cell leukemia/lymphoma (ATLL) is initiated by infection with human T-lymphotropic virus type-1 (HTLV-1); however, additional host factors are also required for T-cell transformation and development of ATLL. The HTLV-1 Tax protein plays an important role in the transformation of T-cells although the exact mechanisms remain unclear. Parathyroid hormone-related protein (PTHrP) plays an important role in the pathogenesis of humoral hypercalcemia of malignancy (HHM) that occurs in the majority of ATLL patients. However, PTHrP is also up-regulated in HTLV-1-carriers and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients without hypercalcemia, indicating that PTHrP is expressed before transformation of T-cells. The expression of PTHrP and the PTH/PTHrP receptor during immortalization or transformation of lymphocytes by HTLV-1 has not been investigated.

RESULTS

We report that PTHrP was up-regulated during immortalization of lymphocytes from peripheral blood mononuclear cells by HTLV-1 infection in long-term co-culture assays. There was preferential utilization of the PTHrP-P2 promoter in the immortalized cells compared to the HTLV-1-transformed MT-2 cells. PTHrP expression did not correlate temporally with expression of HTLV-1 tax. HTLV-1 infection up-regulated the PTHrP receptor (PTH1R) in lymphocytes indicating a potential autocrine role for PTHrP. Furthermore, co-transfection of HTLV-1 expression plasmids and PTHrP P2/P3-promoter luciferase reporter plasmids demonstrated that HTLV-1 up-regulated PTHrP expression only mildly, indicating that other cellular factors and/or events are required for the very high PTHrP expression observed in ATLL cells. We also report that macrophage inflammatory protein-1alpha (MIP-1alpha), a cellular gene known to play an important role in the pathogenesis of HHM in ATLL patients, was highly expressed during early HTLV-1 infection indicating that, unlike PTHrP, its expression was enhanced due to activation of lymphocytes by HTLV-1 infection.

CONCLUSION

These data demonstrate that PTHrP and its receptor are up-regulated specifically during immortalization of T-lymphocytes by HTLV-1 infection and may facilitate the transformation process.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Human T cell leukemia virus type I and neurologic disease: events in bone marrow, peripheral blood, and central nervous system during normal immune surveillance and neuroinflammation

Human T cell lymphotropic/leukemia virus type I (HTLV-I) has been identified as the causative agent of both adult T cell leukemia (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although the exact sequence of events that occur during the early stages of infection are not known in detail, the initial route of infection may predetermine, along with host, environmental, and viral factors, the subset of target cells and/or the primary immune response encountered by HTLV-I, and whether an HTLV-I-infected individual will remain asymptomatic, develop ATL, or progress to the neuroinflammatory disease, HAM/TSP. Although a large number of studies have indicated that CD4(+) T cells represent an important target for HTLV-I infection in the peripheral blood (PB), additional evidence has accumulated over the past several years demonstrating that HTLV-I can infect several additional cellular compartments in vivo, including CD8(+) T lymphocytes, PB monocytes, dendritic cells, B lymphocytes, and resident central nervous system (CNS) astrocytes. More importantly, extensive latent viral infection of the bone marrow, including cells likely to be hematopoietic progenitor cells, has been observed in individuals with HAM/TSP as well as some asymptomatic carriers, but to a much lesser extent in individuals with ATL. Furthermore, HTLV-I(+) CD34(+) hematopoietic progenitor cells can maintain the intact proviral genome and initiate viral gene expression during the differentiation process. Introduction of HTLV-I-infected bone marrow progenitor cells into the PB, followed by genomic activation and low level viral gene expression may lead to an increase in proviral DNA load in the PB, resulting in a progressive state of immune dysregulation including the generation of a detrimental cytotoxic Tax-specific CD8(+) T cell population, anti-HTLV-I antibodies, and neurotoxic cytokines involved in disruption of myelin-producing cells and neuronal degradation characteristic of HAM/TSP.

Novel CD28-responsive enhancer activated by CREB/ATF and AP-1 families in the human interleukin-2 receptor alpha-chain locus

The interaction of interleukin-2 (IL-2) with its receptor (IL-2R) critically regulates the T-cell immune response, and the alpha chain CD25/IL-2Ralpha is required for the formation of the high-affinity receptor. Tissue-specific, inducible expression of the IL-2Ralpha gene is regulated by at least three positive regulatory regions (PRRI, PRRII, and PRRIII), but none responded to CD28 engagement in gene reporter assays although CD28 costimulation strongly amplifies IL-2Ralpha gene transcription. By DNase I hypersensitivity analysis, we have identified a novel TCR-CD3- and CD28-responsive enhancer (CD28rE) located 8.5 kb 5' of the IL-2Ralpha gene. PRRIV/CD28rE contains a functional CRE/TRE element required for CD28 signaling. The T-cell-specific, CD28-responsive expression of the IL-2Ralpha gene appears controlled through PRRIV/CD28rE by cooperation of CREB/ATF and AP-1 family transcription factors.

Vascular cell adhesion molecule-1 induced by human T-cell leukaemia virus type 1 Tax protein in T-cells stimulates proliferation of human T-lymphocytes

Human T-cell leukaemia/lymphotropic virus type 1 (HTLV-1), aetiologically linked to lymphoproliferative as well as inflammatory diseases, infects and activates CD4(+) helper T-cells and thus alters immunoregulatory pathways. The viral regulatory Tax protein has been shown previously to induce the expression of vascular cell adhesion molecule-1 (VCAM-1) by T-cells. To determine the functional role of this adhesion molecule, Jurkat T-cells stably expressing either Tax or both Tax and Rex (another viral regulatory protein) were used in binding and coculture assays performed with either control Jurkat cells or primary human T-lymphocytes. Evidence was provided that VCAM-1 acting in synergy with leucocyte function-associated antigen-3 promotes T-cell-T-cell interactions and increases T-cell proliferation. Interestingly, Rex was found to modulate these events. These data establish that VCAM-1 induced by Tax on T-cells thus contributes to the immunopathological process triggered by HTLV-1 infection.

Functional role of pX open reading frame II of human T-lymphotropic virus type 1 in maintenance of viral loads in vivo

Human T-lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is associated with a variety of immune-mediated disorders. The role of four open reading frames (ORFs), located between env and the 3' long terminal repeat of HTLV-1, in mediating disease is not entirely clear. By differential splicing, ORF II encodes two proteins, p13(II) and p30(II), both of which have not been functionally defined. p13(II) localizes to mitochondria and may alter the configuration of the tubular network of this cellular organelle. p30(II) localizes to the nucleolus and shares homology with the transcription factors Oct-1 and -2, Pit-1, and POU-M1. Both p13(II) and p30(II) are dispensable for infection and immortalization of primary human and rabbit lymphocytes in vitro. To test the role of ORF II gene products in vivo, we inoculated rabbits with lethally irradiated cell lines expressing the wild-type molecular clone of HTLV-1 (ACH.1) or a clone containing selected mutations in ORF II (ACH.30/13.1). ACH.1-inoculated animals maintained higher HTLV-1-specific antibody titers than animals inoculated with ACH.30/13.1. Viral p19 antigen was transiently detected in ex vivo cultures of peripheral blood mononuclear cells (PBMC) from only two ACH.30/13.1-inoculated rabbits, while PBMC cultures from all ACH.1-inoculated rabbits routinely produced p19 antigen. In only three of six animals exposed to the ACH. p30(II)/p13(II) clone could provirus be consistently PCR amplified from extracted PBMC DNA and quantitative competitive PCR showed the proviral loads in PBMC from ACH.p30(II)/p13(II)-infected rabbits to be dramatically lower than the proviral loads in rabbits exposed to ACH. Our data indicate selected mutations in pX ORF II diminish the ability of HTLV-1 to maintain high viral loads in vivo and suggest an important function for p13(II) and p30(II) in viral pathogenesis.

Titration of cellular export factors, but not heteromultimerization, is the molecular mechanism of trans-dominant HTLV-1 rex mutants

The HTLV-1 Rex protein is an essential shuttle protein required for nuclear export of unspliced and incompletely-spliced viral RNAs. Several trans-dominant (TD) mutant Rex proteins have been reported, however, the mechanism of trans-dominance is not known. We compared TD Rex mutants and found that a natural occurring Rex mutant, Rexp21, lacking the RNA binding domain, was highly TD and inhibited also HIV-1 Rev function. Using fusions to the green fluorescent protein (GFP) we observed that Rexp21-GFP displayed a cytoplasmic localization but was actively shuttling between the nucleus and the cytoplasm in live human cells. The presence of Rexp21-GFP inhibited the nuclear export of Rex and HIV-1 Rev as assayed by cotransfection and microinjection experiments. However, Rex-GFP or Rexp21-GFP did not form heteromultimers with nuclear Rex mutants in vivo. In contrast, shuttling was essential for trans-dominance. Thus, we propose that TD Rex mutants do not function by retaining WT Rex in the nucleus by protein-protein interactions, as demonstrated for Rev, but to titrate factors essential for Rex/Rev export. Our findings demonstrate differences between the regulatory proteins Rex and Rev and implicate a novel strategy to generate highly TD Rex mutants also applicable to other proteins.

Inducible knockout of the interleukin-2 receptor alpha chain: expression of the high-affinity IL-2 receptor is not required for the in vitro growth of HTLV-I-transformed cell lines

Adult T cell leukemia (ATL) is an aggressive malignancy that is associated with HTLV-I infection and characterized by constitutive expression of the high-affinity interleukin-2 receptor. The alpha subunit of the high-affinity receptor (IL-2Ralpha), which is normally present only on activated T cells, is specifically upregulated by HTLV-I and constitutively expressed on fresh leukemic cells from ATL patients as well as cell lines transformed by HTLV-I in vitro. Here we directly address the functional significance of IL-2Ralpha expression in HTLV-I transformed cell lines by using an endoplasmic reticulum-targeted single-chain antibody to inhibit the cell surface expression of IL-2Ralpha. Using constitutive and tetracycline-repressible systems to express the ER-targeted antibody against IL-2Ralpha, we have reduced cell surface expression of IL-2Ralpha by more that 2 logs of mean fluorescence intensity to virtually undetectable levels in the IL-2-independent HTLV-I-transformed cell lines C8166-45 and HUT102. No toxicity was associated with the intracellular retention of IL-2Ralpha, and the growth rate of the IL-2Ralpha-negative cells was in each case comparable to that of the parental cell line. We conclude that cell surface expression of IL-2Ralpha is dispensable for the in vitro growth of these HTLV-I-transformed cells.

Transactivation of the interleukin-1 alpha promoter by human T-cell leukemia virus

The expression of interleukin-1 alpha (IL-1 alpha) appears to be tightly regulated, as the levels of constitutive expression in normal cells is extremely low. In contrast to normal hematopoietic cells, human T-cell leukemia virus type I (HTLV-I)-infected T-cell lines constitutively produce high levels of IL-1 alpha mRNA and secret this cytokine into the culture medium. IL-1-alpha mRNA is also expressed in fresh leukemic cells of adult T-cell leukemia/lymphoma (ATLL) patients. HTLV-I-induced IL-1 alpha might explain some symptoms observed in ATLL. In this regard, molecular dissection of the IL-1 alpha gene transcriptional regulation is of primary importance. In this review, the transcriptional regulation of IL-1 alpha gene expression and the possible role of the NF-kappaB pathway are discussed in the light of our current understanding of IL-1 alpha gene regulation by HTLV-I and HTLV-II Tax proteins, which are viral transcriptional transactivators.

Differential transactivation of the intercellular adhesion molecule 1 gene promoter by Tax1 and Tax2 of human T-cell leukemia viruses

Previously, we showed that surface expression of intercellular adhesion molecule 1 (ICAM-1) was strongly upregulated in T cells carrying proviral human T-cell leukemia virus type 1 (HTLV-1) and that the viral transactivator protein Tax1 was capable of inducing the ICAM-1 gene. To determine the responsive elements in the human ICAM-1 gene promoter, a reporter construct in which the 5'-flanking 4.4-kb region of the ICAM-1 gene was linked to the promoterless chloramphenicol acetyltransferase (CAT) gene was cotransfected with expression vectors for Tax1 and Tax2, both of which were separately confirmed to be potent transactivators of the HTLV-1 long terminal repeat (LTR). Tax1 strongly activated the ICAM-1 promoter in all the cell lines tested: three T-cell lines (Jurkat, MOLT-4, and CEM), one monocytoid cell line (U937), and HeLa. Unexpectedly, Tax2 activated the ICAM-1 promoter only in HeLa. By deletion and mutation analyses of the 1.3-kb 5'-flanking region, we found that Tax1 transactivated the ICAM-1 promoter mainly via a cyclic AMP-responsive element (CRE)-like site at -630 to -624 in the Jurkat T-cell line and via an NF-kappaB site at -185 to -177 and an SP-1 site at -59 to -54 in HeLa. On the other hand, Tax2 was totally inactive on the ICAM-1 promoter in Jurkat but transactivated the promoter via the NF-kappaB site at -185 to -177 in HeLa. Gel mobility shift assays demonstrated proteins specifically binding to the CRE-like site at -630 to -624 in Tax1-expressing T-cell lines. Stable expression of Tax1 but not Tax2 in Jurkat subclones enhanced the surface expression of ICAM-1. The differential ability of Tax1 and Tax2 in transactivation of the ICAM-1 gene may be related to the differential pathogenicity of HTLV-1 and HTLV-2.

Enhancement of nucleocytoplasmic export of HTLV-1 Rex mRNA through cis and trans interactions of the mRNA with the complex of Rex protein and Rex-responsive element

p27 rex of HTLV-1 promotes nucleocytoplasmic export of viral mRNAs through binding of the Rex-response element (RexRE) present at the 3' end of the viral transcripts in cis with respect to the ORFs of the viral mRNAs. We have found that expression of the RexRE in trans, as a separate RNA, still allows Rex protein to promote export of viral mRNAs lacking the RexRE. The data suggest the formation of a ternary complex between Rex protein, RexRE and upstream elements of viral mRNA and hence the existence of secondary sites of interaction between Rex protein and viral RNAs.

The 5' coding sequence of IL-2 receptor alpha chain mRNA mediates mRNA stabilization by HTLV-1 Rex

The interleukin-2 receptor alpha chain (IL-2R alpha) is a T-cell growth factor receptor and is known to be induced in helper T cells by infection with human T-cell leukaemia virus type-1 (HTLV-1). The Rex protein of HTLV-1 has been shown to stabilize IL-2R alpha mRNA. Although the 3' untranslated region of many RNA has been regarded as a key element for stabilization, we found that the first 300 bases of the IL-2R alpha protein coding sequence were necessary for stabilization of the mRNA. As the first 201 bases were not sufficient for this effect, we conclude that the bases at position 201-300 downstream of the AUG start are important for stabilization.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

Tissue distribution, hormone regulation and evidence for a human homologue of the estrogen-inducible Xenopus laevis vitellogenin mRNA binding protein

17 beta-estradiol induces the synthesis of massive amounts of the hepatic mRNA encoding the Xenopus laevis egg yolk precursor protein, vitellogenin. Vitellogenin mRNA exhibits a half life of approx. 500 h when 17 beta-estradiol is present, and 16 h after removal of 17 beta-estradiol from the culture medium. We recently reported that Xenopus liver contains a protein, which is induced by 17 beta-estradiol and binds with a high degree of specificity to a binding site in a segment of the 3'-untranslated region (3'-UTR) of vitellogenin mRNA implicated in 17 beta-estradiol stabilization of vitellogenin mRNA. To determine if this mRNA binding protein was specific to this system, or if it was present elsewhere, and regulated by other steroids, we examined the tissue distribution and androgen regulation of this protein. Substantial amounts of the vitellogenin 3'-UTR binding protein were found in several Xenopus tissues including testis, ovary and muscle. In the absence of hormone treatment, lung and intestine contained minimal levels of the mRNA binding protein. Testosterone administration induced the vitellogenin 3'-UTR RNA binding protein in several tissues. Additionally, we found a homologous mRNA binding protein in MCF-7, human breast cancer cells. Although the MCF-7 cell protein was not induced by 17 beta-estradiol, the MCF-7 cell mRNA binding protein appears to be closely related to the Xenopus protein since: (i) the human and Xenopus proteins elicit gel shifted bands with the same electrophoretic mobility using the vitellogenin mRNA 3'-UTR binding site; (ii) The human and Xenopus proteins exhibit similar binding specificity for the vitellogenin 3'-UTR RNA binding site; and (iii) RNA from MCF-7 cells is at least as effective as RNA from control male Xenopus liver in blocking the binding of the Xenopus and human proteins to the vitellogenin mRNA 3'-UTR binding site. Its broad tissue distribution and regulation by both 17 beta-estradiol and testosterone suggests that this mRNA binding protein may play a significant role in steroid hormone regulation of mRNA metabolism in many vertebrate cells.

Transactivation of cellular genes by human retroviruses

We have focused this chapter on interactions with two of the best characterized transregulatory genes, tax for HTLV-I/II and Tat for HIV-1. Both genes illustrate the complex interplay between retroviral regulatory genes and cellular gene regulation. In both instances a viral gene of relatively straightforward function in the viral context appears to cause extensive dysregulation of cellular genes, either directly or as a consequence of altered cellular differentiation. Understanding this viral/cellular gene cross-talk may elucidate mechanisms leading to malignant transformation autoimmune disease and to neurologic and paraneoplastic complications such as hypercalcemia for HTLV-I/II, as well as the pathogenesis of immune dysfunction and opportunistic malignancy in HIV-I/II-infected individuals. An understanding of functional mechanisms of these transregulatory viral genes will undoubtedly afford better explanations for the myriad manifestations of retroviral infection.

The lymphotoxin promoter is stimulated by HTLV-I tax activation of NF-kappa B in human T-cell lines

The HTLV-I transcriptional activator tax was used to gain insight into the mechanism of lymphotoxin (LT; TNF-beta) gene induction. Tax-expressing cell lines produce LT biologic activity. An LT promoter (LT-293) CAT construct that contained an NF-kappa B site was active in the LT-producing C81-66-45 cell line, which contains defective HTLV-I but expresses tax. The observation that a mutated LT-kappa B construct (M1-CAT) was inactive in C81-66-45, confirmed the importance of NF-kappa B in LT gene expression. Tax was transfected into HTLV-I-negative human T-cell lines. Jurkat T cells stably expressing tax contained elevated levels of NF-kappa B that directly bound to the LT-kappa B site. Tax co-transfected with reporter constructs into Jurkat cells maximally activated HTLV-I-LTR-CAT and kappa B-fos-CAT and also activated LT-293 to a lesser extent. In JM T cells, tax induced LT-293 activity by two- to four-fold, though there was no induction of M1-CAT. The increase in LT-293 CAT activity mirrored the increase in LT biologic activity seen under these conditions. These studies, the first to demonstrate induction of LT promoter activity over basal levels, indicate that HTLV-I tax causes low-level activation of both endogenous LT and the LT promoter, at least in part through activation of NF-kappa B.

Influence of human T-cell leukemia virus type I tax and rex on interleukin-2 gene expression

The X region of human T-cell leukemia virus type I (HTLV-I) encodes two proteins that regulate viral gene expression. The tax protein is the product of the transactivator gene and has been shown to up-regulate the expression of some cellular genes controlling T-cell replication, including that of the interleukin-2 (IL-2) T-cell growth hormone and the alpha chain of its receptor (IL-2R). Several studies have shown that tax transactivation of the IL-2R alpha-chain promoter is mediated by binding sites for the transcriptional activator NF-kappa B, and this mechanism has also been implicated in the tax activation of IL-2 promoter activity. The rex gene product of HTLV-I regulates viral protein production by influencing mRNA expression and has been implicated in the stabilization of IL-2R alpha-chain mRNA. In the present studies, the ability of the tax and rex proteins to transactivate IL-2 gene expression has been reinvestigated. The ability of the tax protein to transactivate IL-2 promoter activity appears, at least in part, to be mediated by the recognition sequence for a DNA-binding complex known as CD28RC. Consistent with this hypothesis is the observation that tax-mediated activation of IL-2 gene expression is resistant to the immunosuppressive affects of cyclosporin A, a property postulated for the CD28RC binding complex. Unexpectedly, this tax-mediated up-regulation of IL-2 expression is synergized by the presence of the rex protein. These findings demonstrate that transactivation of IL-2 gene expression by tax is augmented by mechanisms distinct from NF-kappa B and raise the possibility that rex, as well as tax, contributes to the oncogenic capability of HTLV-I by altering the expression of the IL-2 gene in T cells infected with this retrovirus.

Role of human T-cell leukemia virus type 1 X region proteins in immortalization of primary human lymphocytes in culture

Human T-cell leukemia virus type 1 (HTLV-1) immortalizes human CD4+ T lymphocytes in culture. Previous studies show that in the context of a herpesvirus saimiri vector, the sequence of the X region at the 3' end of the HTLV-1 genome is also capable of immortalizing CD4+ lymphocytes in the absence of HTLV-1 structural proteins. The X region of HTLV-1 encodes two trans-acting viral proteins, the 42-kDa Tax protein and the 27-kDa Rex protein. Infection of human cord blood cells with herpesvirus saimiri recombinants which contain HTLV-1 X region sequences defective for expression of tax, rex, or both tax and rex demonstrates that tax function is necessary and sufficient for immortalization of primary human CD4+ cord blood lymphocytes in culture in the context of the herpesvirus saimiri vector.

Control of c-myc mRNA half-life in vitro by a protein capable of binding to a coding region stability determinant

Polysome-associated c-myc mRNA is degraded relatively rapidly in cells and in an in vitro mRNA decay system containing extracts from cultured mammalian cells. Using this system, a competition/screening assay was devised to search for factors that bind to specific regions of polysome-associated c-myc mRNA and thereby alter its half-life. mRNA stability was first assayed in reactions containing exogenous competitor RNAs corresponding to portions of c-myc mRNA itself. The addition of a 182-nucleotide sense strand fragment from the carboxy-terminal portion of the c-myc-coding region destabilized c-myc mRNA by at least eightfold. This RNA fragment had no effect on the stability of other mRNAs tested. Moreover, c-myc mRNA was not destabilized in reactions containing unrelated competitor RNAs or sense strand RNA from the c-myc 5' region. Polysome-associated globin mRNA containing the c-myc-coding region segment in-frame was also destabilized in vitro by the 182-nucleotide RNA. As determined by UV-cross-linking experiments, the 182-nucleotide RNA fragment was recognized by and bound to an approximately 75-kD polysome-associated protein. On the basis of these data plus Northern blotting analyses of c-myc mRNA decay products, we suggest that the approximately 75-kD protein is normally bound to a c-myc-coding region determinant and protects that region of the mRNA from endonuclease attack. Possible links between the protective protein, translation, ribosome pausing, and c-myc mRNA turnover are discussed.

Cytokines in viral diseases

As in many other areas of cytokine biology, recent studies of the role of cytokines in viral disease reveal numerous complex interactions that in many instances may contribute directly to the development of pathology. For example, data from the rapidly evolving field of human retrovirology has shown that these viruses, as well as inducing the expression of many cellular cytokine genes, can be activated from latency and driven into replication/expression by the very same cytokines. The continuing rapid expansion of knowledge in the cytokine area augers well for eventual development of novel antiviral therapeutic strategies based on manipulation of the cytokine network.

Deleted HTLV-I provirus in blood and cutaneous lesions of patients with mycosis fungoides

Mycosis fungoides, a rare form of cutaneous T cell leukemia/lymphoma, is suspected of having a viral etiology on the basis of certain similarities to adult T cell leukemia, which is associated with human T cell leukemia/lymphoma virus type I (HTLV-I) infection. Cell lines were established from peripheral blood mononuclear cells (PBMC) of an HTLV-I-seronegative patient with mycosis fungoides. DNA hybridization analysis revealed the presence of HTLV-I-related sequences with unusual restriction endonuclease sites. Sequence analysis of subcloned fragments demonstrated the presence of a monoclonally integrated provirus with a 5.5-kilobase deletion involving large regions of gag and env and all of pol. Additional evidence for the presence of deleted proviruses was found by polymerase chain reaction (PCR) amplification of DNA from cutaneous lesions of five other HTLV-I-seronegative patients. The findings suggest that HTLV-I infection may be involved in the etiology of at least certain cases of mycosis fungoides.

The nucleolar localisation signal of the HTLV-I protein p27rex is important for stabilisation of IL-2 receptor alpha subunit mRNA by p27rex

In this study we investigated the mechanism of stabilisation of IL-2 receptor alpha subunit mRNA by the HTLV-I protein p27rex. We tested the role of the nucleolar targetting signal in rex by introducing mutations. Three deletion mutants could not express rex protein in the nucleolus and although protein was still expressed in the nucleoplasm none of the mutants could stabilise IL-2R alpha mRNA. A substitution mutant could be expressed in the nucleolus and could also stabilise IL-2R alpha mRNA. The data show that the nucleolar targetting signal is crucial for stabilisation of IL-2R alpha mRNA by rex and raise the possibility that transport of mRNA from nucleus to cytoplasm can involve the nucleolus.