TAR-independent transactivation of the murine cytomegalovirus major immediate-early promoter by the Tat protein

Exosome-associated release, uptake, and neurotoxicity of HIV-1 Tat protein

HIV-1 Tat is an indispensible transactivator for HIV gene transcription and replication. It has been shown to exit cells as a free protein and enter neighboring cells or interact with surface receptors of neighboring cells to regulate gene expression and cell function. In this study, we report, for the first time, exosome-associated Tat release and uptake. Using a HIV-1 LTR-driven luciferase reporter-based cell assay and Western blotting or in combination with exosome inhibitor, OptiPrep gradient fractionation, and exosome depletion, we demonstrated significant presence of HIV-1 Tat in exosomes derived from Tat-expressing primary astrocytes, Tat-transfected U373.MG and 293T, and HIV-infected MT4. We further showed that exosome-associated Tat from Tat-expressing astrocytes was capable of causing neurite shortening and neuron death, further supporting that this new form of extracellular Tat is biologically active. Lastly, we constructed a Tat mutant deleted of its basic domain and determined the role of the basic domain in Tat trafficking into exosomes. Basic domain-deleted Tat exhibited no apparent effects on Tat trafficking into exosomes, while maintained its dominant-negative function in Tat-mediated LTR transactivation. Taken together, these results show a significant fraction of Tat is secreted and present in the form of exosomes and may contribute to the stability of extracellular Tat and broaden the spectrum of its target cells.

The 5' UTR of HIV-1 full-length mRNA and the Tat viral protein modulate the programmed -1 ribosomal frameshift that generates HIV-1 enzymes

Translation of the full-length messenger RNA (mRNA) of the human immunodeficiency virus type 1 (HIV-1) generates the precursor of the viral enzymes via a programmed -1 ribosomal frameshift. Here, using dual-luciferase reporters, we investigated whether the highly structured 5' untranslated region (UTR) of this mRNA, which interferes with translation initiation, can modulate HIV-1 frameshift efficiency. We showed that, when the 5' UTR of HIV-1 mRNA occupies the 5' end of the reporter mRNA, HIV-1 frameshift efficiency is increased about fourfold in Jurkat T-cells, compared with a control dual-luciferase reporter with a short unstructured 5' UTR. This increase was related to an interference with cap-dependent translation initiation by the TAR-Poly(A) region at the 5' end of the messenger. HIV-1 mRNA 5' UTR also contains an internal ribosome entry site (IRES), but we showed that, when the cap-dependent initiation mode is available, the IRES is not used or is weakly used. However, when the ribosomes have to use the IRES to translate the dual-luciferase reporter, the frameshift efficiency is comparable to that of the control dual-luciferase reporter. The decrease in cap-dependent initiation and the accompanying increase in frameshift efficiency caused by the 5' UTR of HIV-1 mRNA is antagonized, in a dose-dependent way, by the Tat viral protein. Tat also stimulates the IRES-dependent initiation and decreases the corresponding frameshift efficiency. A model is presented that accounts for the variations in frameshift efficiency depending on the 5' UTR and the presence of Tat, and it is proposed that a range of frameshift efficiencies is compatible with the virus replication.

Suppression of distant pulmonary metastasis of MDA-MB 435 human breast carcinoma established in mammary fat pads of nude mice by retroviral-mediated TIMP-2 gene transfer

BACKGROUND

Previous studies have shown that TIMP-2 overexpression is a useful therapeutic tool for inhibiting tumor growth and invasion in animals. However, it has not been reported whether genetic manipulation for TIMP-2 overexpression can induce an inhibitory effect on spontaneous metastasis from the primary tumor site to other organs such as lungs or lymph nodes in an animal model.

METHODS

The present studies describe the effects of retrovirus-mediated TIMP-2 gene transfer into human breast cancer cell lines on the in vitro invasion of the tumor cells or the in vivo growth in nude mouse. Here we also used retroviral-mediated TIMP-2 overexpression by intratumoral injection for suppression of metastasis in human breast carcinoma established in the mammary fat pad of nude mice.

RESULTS

As expected, overexpression of TIMP-2 inhibited matrix metalloprotenase (MMP) activity and invasion of the tumor cells. Also, the growth rate of tumors grafted with the breast cancer cells transduced with the retrovirus vector encoding TIMP-2 cDNA was significantly slower than the growth rate of tumors grafted with the breast cancer cells transduced with a control retrovirus vector. Furthermore, single intratumoral injection of the TIMP-2 retrovirus-producing cells into human breast tumor tissue established in mammary fat pads of nude mice showed a dramatic decrease in size and number of lung metastatic tumors.

CONCLUSIONS

Retrovirus-mediated TIMP-2 gene transfer into human breast cancer cells is able to down-regulate invasion and show that tumor-derived angiogenesis is reduced. In this model, retroviral-mediated transduction of TIMP-2 cDNA into a limited population of human tumor cells inhibits tumor growth and prevents distant pulmonary metastasis. These results indicate that it may not be necessary to deliver and express these genes in every single tumor cell as long as the level of expression in a limited number of transduced cells is sufficient to prevent the excessive breakdown of the extracellular matrix.

Retroviral delivery of TIMP-2 inhibits H-ras-induced migration and invasion in MCF10A human breast epithelial cells

The matrix metalloproteases (MMPs) play important roles in invasion, metastasis and angiogenesis in various cell types. Tissue inhibitor of metalloprotease (TIMP)-2, an endogenous inhibitor of MMP-2, has been shown to inhibit invasion and metastasis. We have previously shown that MMP-2 is responsible for the H-ras-induced invasive and migrative phenotypes in MCF10A human breast epithelial cells. Here, we investigated the effect of TIMP-2 overexpression on migration and invasion in H-ras MCF10A cells. Human TIMP-2 gene was effectively introduced into H-ras MCF10A cells by retrovirus-mediated gene delivery. TIMP-2 overexpression mediated by retrovirus significantly inhibited migration as well as invasion of H-ras MCF10A cells in a dose-dependent manner. We also show the antiangiogenic effect of TIMP-2 gene delivery. Taken together, our study shows that retrovirus-mediated delivery of TIMP-2 efficiently inhibits metastatic progression of ras-transformed human breast epithelial cells, suggesting a potential use of the TIMP-2 gene therapy for the treatment of breast cancer.

Codon optimization of the HIV-1 vpu and vif genes stabilizes their mRNA and allows for highly efficient Rev-independent expression

Two HIV-1 accessory proteins, Vpu and Vif, are notoriously difficult to express autonomously in the absence of the viral Tat and Rev proteins. We examined whether the codon bias observed in the vpu and vif genes relative to highly expressed human genes contributes to the Rev dependence and low expression level outside the context of the viral genome. The entire vpu gene as well as the 5' half of the vif gene were codon optimized and the resulting open reading frames (ORFs) (vphu and hvif, respectively) were cloned in autonomous expression vectors under the transcriptional control of the CMV promoter. Codon optimization efficiently removed the expression block observed in the native genes and allowed high levels of Rev- and Tat-independent expression of Vpu and Vif. Most of the higher protein levels detected are accounted for by enhanced steady-state levels of the mRNA encoding the optimized species. Nuclear run-on experiments show for the first time that codon optimization has no effect on the rate of transcriptional initiation or elongation of the vphu mRNA. Instead, optimization of the vpu gene was found to stabilize the vphu mRNA in the nucleus and enhance its export to the cytoplasm. This was achieved by allowing the optimized mRNA to use a new CRM I-independent nuclear export pathway. This work provides a better understanding of the molecular mechanisms underlying the process of codon optimization and introduces novel tools to study the biological functions of the Vpu and Vif proteins independently of other viral proteins.

Human immunodeficiency virus-1 Nef protein interacts with Tat and enhances HIV-1 gene expression

The human immunodeficiency virus (HIV-1) Nef protein is now regarded as a regulatory protein responsible not only for establishment of infection and increased pathogenesis but also for enhancement of viral replication. However, the mechanism of Nef-induced activation of viral replication remains to be clearly understood. Using transient transfection assay, co-immunoprecipitation and pull-down analysis, we demonstrate in this report that the HIV-1 Nef protein physically interacts with Tat, the principal transactivating protein of HIV-1. Our observations with single cycle replication experiments further indicate that this interaction results not only in enhancement of Tat-induced HIV-1 long terminal repeat-mediated gene expression but also in virus production.

The Tat protein of human immunodeficiency virus type 1 (HIV-1) can promote placement of tRNA primer onto viral RNA and suppress later DNA polymerization in HIV-1 reverse transcription

Human immunodeficiency virus type-1 Tat has been proposed to play a role in the regulation of reverse transcription. We previously demonstrated that wild-type Tat can augment viral infectivity by suppressing the reverse transcriptase (RT) reaction at late stages of the viral life cycle in order to prevent the premature synthesis of potentially deleterious viral DNA products. Here we have performed a detailed analysis of the cell-free reverse transcription reaction to elucidate the mechanism(s) whereby Tat can affect this process. Our results show that Tat can suppress nonspecific DNA elongation while moderately affecting the specific initiation stage of reverse transcription. In addition, Tat has an RNA-annealing activity and can promote the placement of tRNA onto viral RNA. This points to a functional homology between Tat and the viral nucleocapsid (NC) protein that is known to be directly involved in this process. Experiments using a series of mutant Tat proteins revealed that the cysteine-rich and core domains of Tat are responsible for suppression of DNA elongation, while each of the cysteine-rich, core, and basic domains, as well as a glutamine-rich region in the C-terminal domain, are important for the placement of tRNA onto the viral RNA genome. These results suggest that Tat can play at least two different roles in the RT reaction, i.e., suppression of DNA polymerization and placement of tRNA onto viral RNA. We believe that the first of these activities of Tat may contribute to the overall efficiency of reverse transcription of the viral genome during a new round of infection as well as to enhanced production of infectious viral particles. We hypothesize that the second activity, illustrating functional homology between Tat and NC, suggests a potential role for NC in the displacement of Tat during viral maturation.

The human elongation factor 1 alpha (EF-1 alpha) first intron highly enhances expression of foreign genes from the murine cytomegalovirus promoter

In order to develop a highly efficient mammalian expression vector, we constructed a vector by the combination of the murine cytomegalovirus (MCMV) immediate early (IE) promoter and the human elongation factor one alpha (EF-1 alpha) first intron. The MCMV IE promoter was several fold stronger than the human cytomegalovirus (HCMV) immediate early (IE) promoter and the human elongation factor one alpha (EF-1 alpha) promoter in various mammalian cell lines such as NIH3T3, Neuro-2a, 293T or HT1080 and was only slightly weaker than the HCMV or the EF-1 alpha promoter in HeLa and CHO cell lines. We inserted the first intron of the human EF-1 alpha gene behind the MCMV and the HCMV promoter to enhance the gene expression through increasing RNA transcription and/or RNA stability. The insertion of the human EF-1 alpha first intron markedly enhanced the level of gene expression in many cell lines and the resultant MCMV promoter with the human EF-1 alpha first intron (MCMV/EF-I) was higher than the HCMV promoter from 4.3 to 65.5-fold in various cell lines. Also, the MCMV/EF-I promoter induced higher level of erythropoietin expression than the HCMV promoter in transiently transfected CHO cells.

SV40-derived vectors provide effective transgene expression and inhibition of HIV-1 using constitutive, conditional,and pol III promoters

Vectors based on recombinant SV40 viruses (rSV40) are highly effective in delivering transgene expression driven by constitutive promoters. We tested here whether these vectors could be used with conditional promoters and promoters using RNA polymerase III transcription, with inhibition of HIV-1 by Tat activation response (TAR) decoys as a functional measure of effective transgene delivery and activity. TAR decoys inhibit HIV-1 Tat, a trans-activator of HIV-1 transcription. Tat acts early in the viral replicative cycle and is essential for efficient viral replication. We evaluated rSV40 gene delivery using two different inhibitors of Tat. One was a dual function polyTAR gene encoding 25 sequential TAR elements (TAR(25)), plus an antisense tat, driven either by HIV-1 long terminal repeat (HIV-LTR) as a conditional promoter, or by cytomegalovirus immediate-early promoter (CMV-IEP) as a constitutive promoter. The other inhibitor was a single TAR decoy, driven by the U6 small nuclear RNA promoter (U6-P). These decoys were delivered to unselected cells in two different human T lymphocyte lines and to unstimulated primary human peripheral blood mononuclear cells (pbmc). Gene delivery was confirmed by PCR, and expression by RT-PCR. By in situ hybridization analysis, >95% of cells were transduced. These transgene constructs protected all cell types tested from HIV-1, as measured by syncytia formation and p24 antigen release. Somewhat better inhibition of HIV-1 replication was achieved with HIV-1 long terminal repeat (HIV-1 LTR) as a conditional promoter than with the constitutive CMV-IEP. The U6-P was also very effective, driving a TAR(1) transcript. Cell viability was not detectably affected by TAR decoy expression. Thus, rSV40 vectors effectively deliver HIV-1-inhibitory RNAs using either constitutive or conditional pol II promoters, or using a pol III promoter. The versatility of this gene delivery system may prove to be useful in anti-HIV-1 therapeutics.

Primate and feline lentivirus vector RNA packaging and propagation by heterologous lentivirus virions

Development of safe and effective gene transfer systems is critical to the success of gene therapy protocols for human diseases. Currently, several primate lentivirus-based gene transfer systems, such as those based on human and simian immunodeficiency viruses (HIV/SIV), are being tested; however, their use in humans raises safety concerns, such as the generation of replication-competent viruses through recombination with related endogenous retroviruses or retrovirus-like elements. Due to the greater phylogenetic distance from primate lentiviruses, feline immunodeficiency virus (FIV) is becoming the lentivirus of choice for human gene transfer systems. However, the safety of FIV-based vector systems has not been tested experimentally. Since lentiviruses such as HIV-1 and SIV have been shown to cross-package their RNA genomes, we tested the ability of FIV RNA to get cross-packaged into primate lentivirus particles such as HIV-1 and SIV, as well as a nonlentiviral retrovirus such as Mason-Pfizer monkey virus (MPMV), and vice versa. Our results reveal that FIV RNA can be cross-packaged by primate lentivirus particles such as HIV-1 and SIV and vice versa; however, a nonlentivirus particle such as MPMV is unable to package FIV RNA. Interestingly, FIV particles can package MPMV RNA but cannot propagate the vector RNA further for other steps of the retrovirus life cycle. These findings reveal that diverse retroviruses are functionally more similar than originally thought and suggest that upon coinfection of the same host, cross- or copackaging may allow distinct retroviruses to generate chimeric variants with unknown pathogenic potential.

Differential patterns of interaction between HIV type 1 and HTLV type I in monocyte-derived macrophages cultured in vitro: implications for in vivo coinfection with HIV type 1 and HTLV type I

The interaction between human immunodeficiency virus type 1 (HIV-1) and human T cell leukemia-lymphoma virus type I (HTLV-I) has generated substantial interest. However, there is disagreement on the in vivo consequences of the double infection. We investigated the interactions between HIV-1 and HTLV-I in monocyte-derived macrophages cultured in vitro. For study, the T cell-tropic strain IIIB and the macrophagetropic strain Ada-M of HIV-1 were used. The HTLV-I was prepared from the supernatants of the virus-producing MT-2 cell line. We found that coinfection of macrophages with T cell-tropic HIV-1 and HTLV-I significantly enhanced HIV-1 replication, whereas double infection of the cells with macrophage-tropic HIV-1 and HTLV-I resulted in marked upregulation of HTLV-I production. Stimulatory interactions between HIV-1 and HTLV-I were mediated by their trans-acting proteins. Results of study on nuclear translocation of proviral DNA showed that the tax gene product of HTLV-I was able to facilitate the nuclear import of the reverse-transcribed HIV-1(IIIB) DNA. In contrast, the HIV-1 Tat protein did not increase the intranuclear trafficking of HTLV-I DNA, which suggests another mechanism for HTLV-I enhancement by the tat gene product. In conclusion, this study provides possible mechanisms whereby coinfection of an individual with HIV-1 and HTLV-I may influence the clinical outcome of double infection.

Activation of Bcl-2 promoter-directed gene expression by the human immunodeficiency virus type-1 Tat protein

Human immunodeficiency virus type 1 (HIV-1) Tat transcriptionally activates expression from a number of viral and cellular promoters. Recent studies demonstrate the ability of Tat to differentially modulate cellular responses to apoptotic signaling. The antiapoptotic effects of Tat appear to correlate with increased expression of Bcl-2, a cellular protein that enhances cellular survival. Here, endogenous expression of HIV-1 Tat in HeLa and Jurkat cells elevates levels of Bcl-2. Transient expression assays performed in HeLa cells demonstrate that Tat directly or indirectly enhances Bcl-2 promoter-directed gene expression by more than 10-fold. Analyses of Tat mutants demonstrate that two noncontiguous regions in the N- and C-termini of Tat mediate maximal transactivation of the Bcl-2 promoter. The requirement for C-terminal sequences contrasts with transactivation of the HIV-1 long terminal repeat in which the N-terminal 57 amino acids are required but downstream residues are not. Bcl-2 promoter analyses suggest that sequences required for Tat responsiveness are located upstream of P1 and between the P1 and P2 promoter units. Results from these studies reveal effects of HIV-1 Tat on Bcl-2 expression and provide a putative mechanism by which endogenously expressed Tat affects cellular survival through the up-regulation of Bcl-2.

Purified Tat induces inflammatory response genes in Kaposi's sarcoma cells

OBJECTIVE

Kaposi's sarcoma (KS) is a neoplasm strongly associated with HIV-1 infection and marked by leukocytic infiltration. The infiltrating leukocytes are a possible source of inflammatory cytokines, human herpesvirus 8 (HHV8) and the HIV-1 transactivator protein Tat. This study examines whether Tat directly induces expression of cellular adhesion molecules and cytokines in KS cells and whether this induction differs in kinetics and magnitude from induction by tumour necrosis factor (TNF) alpha.

DESIGN AND METHOD

Changes in gene expression in response to recombinant Tat compared with those to TNFalpha were evaluated at the messenger (m) RNA and protein level using cells that were cultured from KS lesions.

RESULTS

Tat induced the expression of the adhesion molecules vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) and the cytokines monocyte chemoattractant protein 1 (MCP-1) and interleukin 6 (IL-6). The inductions were observed at both the protein and mRNA levels. The pattern of mRNA induction over time in response to Tat differed from that to TNFalpha, with higher peak levels that occurred earlier in response to Tat. The expression of these genes is, in part, regulated by the transcription factor NF-kappaB. Tat and TNFalpha activated comparable levels of NF-kappaB.

CONCLUSIONS

The ability of the HIV-1 Tat to induce the expression of genes with kinetics that are distinct from those seen in TNFalpha induction suggests that mechanisms in addition to activation of NF-kappaB contribute to the observed induction. Tat may contribute to the pathogenesis of AIDS-related KS through induction of cellular genes that are pro-proliferative and proinflammatory and may enhance the recruitment of leukocytes, which are a possible source of further cytokines, Tat and HHV8.

Pleiotropic mutations in the HIV-1 matrix protein that affect diverse steps in replication

The matrix domain of the Gag precursor protein, and the mature matrix protein, which is derived from processing of the Gag precursor, functions in several steps of the human immunodeficiency virus type-1 (HIV-1) life cycle. We made numerous mutations throughout the matrix protein and identified three mutants in the N-terminal portion of the matrix that drastically diminish the ability of the virus to replicate. Each of these replication-defective mutants was unable to acquire efficiently the envelope glycoprotein of HIV-1. To determine whether these same mutations affect other steps in viral replication we pseudotyped mutant particles with the envelope glycoprotein from an amphotropic murine leukemia virus. Each of these mutants was also hampered in other steps in virus replication. Two mutants were defective in entry or uncoating, and the third was hampered in a step following reverse transcription. Since viral replication was analyzed under conditions in which the nuclear localization function of the matrix protein is not required, the matrix protein may be required for an additional replication step following reverse transcription.

HIV-1 Tat protein can transactivate a heterologous TATAA element independent of viral promoter sequences and the trans-activation response element

OBJECTIVE

To determine whether the HIV-1 transactivator protein Tat acts as a DNA sequence-specific transcription factor and activates transcription from a heterologous TATAA element in the absence of the trans-activation response (TAR) element and other sequences in the HIV-1 long terminal repeat (LTR).

DESIGN

Activating protein-1 (AP-1) and Tat-induced transcription were assessed using Jun and hybrid Tat/Jun-expression plasmids and reporter gene constructs which contained AP-1 binding sites upstream of the rat prolactin TATAA element or an HIV-1 LTR construct in which AP-1 binding sites replaced the TAR element.

METHODS

Tat-induced transcription was determined following transient transfection of colon epithelial cell lines with reporter gene constructs and Tat/Jun-expression plasmids in which Tat was fused to the DNA binding domain of Jun. Activation of prolactin (PL) and LTR reporter genes was assessed by luciferase (LUC) or chloramphenicol acetyltransferase (CAT) activity in cellular extracts.

RESULTS

Cotransfection of cells with Tat/Jun and the AP-1 PL LUC or LTR AP-1 CAT reporter plasmid resulted in a marked increase in reporter gene activity which was comparable with that induced by transfection of cells with several different AP-1 expression plasmids (e.g., JunD, JunB, c-Fos), or that elicited by stimulation of the cells transfected with LTR AP-1 CAT plasmids with phorbol ester or tumor necrosis factor-alpha. Tat-induced transcription was DNA-mediated since both a Jun DNA binding domain fused to Tat as well as AP-1 binding sites within the promoter were required for the induction of CAT expression.

CONCLUSIONS

Tat-activated transcriptor can occur strictly through a heterologous TATAA element independent of TAR and Sp1 binding sites or other HIV-1 LTR sequences. Tat appears to increase transcription initiated through the TATAA element by mechanisms similar to that of DNA sequence-specific transcription factors.

Cooperation between herpes simplex virus type 1-encoded ICP0 and Tat to support transcription of human immunodeficiency virus type 1 long terminal repeat in vivo can occur in the absence of the TAR binding site

Expression of human immunodeficiency virus type 1 (HIV-1) provirus can be stimulated by herpes simplex virus type 1 (HSV-1) infection; the stimulation occurs at the level of transcriptional activation of the HIV long terminal repeat (LTR) and is mediated by both cellular and HSV-1-encoded transactivators. We have shown in this study that HSV-1 immediate-early gene ICP0 cooperates effectively with the HIV-1-encoded transactivator, Tat, in the stimulation of HIV-1 LTR-directed transcription. The cooperation between ICP0 and Tat is specific for the HIV-1 LTR and was not observed with other promoters (e.g., ICP0) that can be transactivated by ICP0 but not by Tat. Analyses of HIV-1 LTR deletion mutants have shown that ICP0 not only transactivates an HIV-1 LTR mutant that is unresponsive to NF-kappaB and Tat-mediated transactivation, such as the HIV-1 LTR with the enhancer deleted (-83 LTR) and TAR deleted (+20 to +81), but also restores responsiveness to Tat. ICP0 also showed cooperation with Gal4-Tat fusion protein-mediated transactivation of Gal4-HIV-1 LTR with TAR deleted. Enhancement of the transcriptional activation of ICP0 by Tat requires both the cysteine-rich and core domains of Tat and is inhibited by RO5-3335. ICP0 stimulates transcription of not only the HIV-1 LTR but also the TAR-defective HIV-1 provirus. We suggest that ICP0 can (i) recruit Tat to the vicinity of the HIV-1 promoter, thereby providing an alternative binding site for Tat, and (ii) substitute for the enhancer-binding proteins that are required for efficient Tat transactivation in T cells.

Examination of TAR-independent Trans activation by human immunodeficiency virus type 1 Tat in human glial cells

Astrocytic glial cells derived from central nervous system (CNS) can support human immunodeficiency virus type 1 (HIV-1) replication in cell culture, may be infected in tissue culture, and are thought to be a large HIV-1 reservoir in vivo. The Tat protein of HIV-1 interacts with a cis-acting target sequence referred to as TAR. However, Tat can also stimulate gene expression directed from some heterologous promoters and, in certain circumstances, an HIV-1 long terminal repeat (LTR) that lacks the TAR element. Therefore, we attempted to investigate Tat trans activation of HIV-1 LTR in the astrocytic glial cells. Using transfection of LTR-reporter gene constructs and HIV-1 proviral constructs, we demonstrate TAR-dependent replication in astrocytic cells. We also examined the expression of HIV-1 env gene from an LTR that lacks TAR element. In a previous study (Kim and Panganiban: J Virol 67:3739-3747, 1993), we observed that env expression is trans activated only by the full-length Tat protein through a TAR-independent manner in HeLa cells. However, in astrocytic glial cells, the trans activation of env expression from the LTR-lacking TAR element was mediated by the first exon peptide of Tat as well as the full-length Tat peptide through a post-transcriptional mechanism rather than a transcriptional one. This result suggests that cell type-specific factor(s) is involved in the TAR-independent Tat responsiveness.

Production of the chimerical plasminogen activator K2tu-PA in CHO cells

Development of a CHO cell-based production system for the hybrid plasminogen activator K2tu-PA is described. Using the major immediate-early promoter of mouse cytomegalovirus (MCMV) transient and stable expression levels were 3-10-fold higher than those obtained with several other strong promoters. Splicing and polyadenylation signals from the rabbit beta-globin gene were used downstream of the DNA segment coding for K2tu-PA. The strong enhancer moiety of the MCMV promoter also stimulated strongly the promoter of the dihydrofolate reductase (DHFR) gene, placed adjacently for selection/gene amplification purposes. One construct with opposing K2tu-PA and DHFR RNA transcripts yielded the highest expression level with a single copy of the plasmid, but K2tu-PA expression was consistently lost after amplification of such genes, possibly as a result of the formation of antisense RNA. With other constructs, K2tu-PA production leveled off at 6.5 micrograms per million cells per day despite a high gene copy number. This was due to a combination of inefficient mRNA translation and mRNA instability, caused by elements from the untranslated portions of tissue-type and urokinase-type plasminogen activator cDNA which were included in the expression vector. After elimination of these inhibitory DNA segments, 4-5-times higher expression levels were reached.

Requirement for HIV-1 TAR sequences for Tat activation in rodent cells

HIV-1 gene expression is activated via an interaction between the virally encoded Tat protein and a target RNA, TAR. TAR is located at the immediate 5' end of all viral mRNAs and comprises a partially base-paired stem with a tripyrimidine bulge in the upper stem and a hexanucleotide loop. In vitro, Tat binds specifically to the bulge and upper stem region with no requirement for the loop. In contrast, when Tat activation is analyzed in primate cells, mutations in the loop abolish activation, suggesting a critical role for loop binding cellular factors. However, in rodent cells the reverse is true. Messages with a mutation in the TAR loop are activated whereas messages harboring a wild-type TAR sequence are not activated. By testing the effect of mutations in the bulge and stem in the context of mutation in the loop we now show that this loop-independent activation by Tat in rodent cells requires the critical bulge-stem sequences needed for Tat binding in vitro. These data suggest that in rodent cells, as in vitro, Tat does not require a loop binding cofactor. In rodent cells containing human chromosome 12 (CHO12), however, Tat activation is both bulge and loop dependent. It appears that rodent cells, but not CHO12 cells, are refractory to the normal Tat/TAR activation pathway not by virtue of lacking a loop binding cofactor, but rather by the presence of a loop binding inhibitor of either Tat binding or the activation process.

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.

Human immunodeficiency virus type 1 Tat upregulates interleukin-2 secretion in activated T cells

Dysregulation of cytokines secreted by T cells may play an important role in the pathogenesis of AIDS. To investigate the effects of human immunodeficiency virus type 1 (HIV-1) Tat on interleukin-2 (IL-2) expression, we used IL-2 promoter-chloramphenicol acetyltransferase constructs and IL-2-secreting Jurkat T cells as a model system. Transient expression of HIV-1 Tat induced a five- to eightfold increase in IL-2 promoter activity in Jurkat T cells stimulated with phytohemagglutinin and phorbol myristate acetate. IL-2 secretion was increased more than twofold in both Jurkat T cells and primary T cells stimulated by extracellular HIV-1 Tat protein. Analysis of mRNA suggested that Tat exerts its effect on IL-2 primarily at the transcriptional level. The NF-kappa B site at positions -206 to -195 of the IL-2 promoter was required but not sufficient for the Tat effect. The Tat-mediated increase in IL-2 promoter activity could selectively be blocked by antisense tat or-unlike the analogous effect of human T-cell lymphotropic virus type 1 Tax-by cyclosporin A. The observed increase in IL-2 levels might facilitate virus spread from or to T cells. Furthermore, it might contribute to the hypergammaglobulinemia or, together with other cytokines found to be dysregulated, the T-helper cell dysfunctions observed in AIDS patients.

The expression of the interleukin 6 gene is induced by the human immunodeficiency virus 1 TAT protein

Human immunodeficiency virus 1 (HIV1) infection is associated with severe psoriasis, B cell lymphoma, and Kaposi's sarcoma. A deregulated production of interleukin 6 (IL-6) has been implicated in the pathogenesis of these diseases. The molecular mechanisms underlying the abnormal IL-6 secretion of HIV1-infected cells may include transactivation of the IL-6 gene by HIV1. To test this hypothesis, we used the pIL6Pr-chloramphenicol acetyltransferase (CAT) plasmid, an IL-6 promoter-CAT construct, as a target of the transactivating function of the HIV1 TAT protein. By cotransfecting the pIL6Pr-CAT and the tat-expressing pSVT8 plasmid in MC3 B-lymphoblastoid or in HeLa epithelial cells, we observed that TAT transactivates the human IL-6 promoter. These results were confirmed when pIL6Pr-CAT was transfected in MC3 or HeLa cells that constitutively expressed the tat gene in a sense (pSVT8 cells) or antisense (pSVT10 cells) orientation. 5' deletion plasmids of pIL6Pr-CAT, in which regions at -658, -287, and -172 were inserted 5' to the cat gene, were transiently transfected in pSVT10 and pSVT8 cells and showed that TAT-induced activation of the IL-6 promoter required a minimal region located between -287 and -54 bp. Moreover, experiments with plasmids carrying the -658, -287, and -172 bp regions of the IL-6 promoter inserted downstream to a TAR-deleted HIV1-LTR identified the sequence of -172 to -54 as the minimal region of the IL-6 promoter required for TAT to transactivate the TAR-deleted HIV1-LTR. By DNA-protein binding experiments, tat-transfected cells expressed a consistent increase in kappa B and nuclear factor (NF)-IL-6 binding activity. Accordingly, the pDRCAT and IL-1REK9CAT, carrying tandem repeats of NF-kappa B or NF-IL6 binding motifs, respectively, were activated in TAT-expressing cells. The biological relevance of the TAT-induced IL-6 secretion was addressed by generating 7TD1 cells, an IL-6-dependent mouse cell line, stably expressing the tat gene. These tat-positive cells expressed the endogenous IL-6 gene, secreted high amounts of murine IL-6, and grew efficiently in the absence of exogenous IL-6. Moreover, the tat-positive 7TD1 cells sustained the growth of parental 7TD1 cells and showed a dramatic increase in their tumorigenic potency. These results suggest that TAT protein may play a role in the pathogenesis of some HIV1-associated diseases by modulating the expression of host cellular genes.

The full-length Tat protein is required for TAR-independent, posttranscriptional trans activation of human immunodeficiency virus type 1 env gene expression

Tat is a protein that dramatically increases the expression of all genes expressed from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat through interaction with a cis-acting target sequence referred to as TAR (for trans-acting responsive region). The tat gene is divided into two coding exons which, when translated, result in the synthesis of an 86-amino-acid protein. However, the 72-amino-acid segment encoded by the first coding exon of tat is sufficient to encode a fully active Tat protein in known assays. We examined expression of the env gene from an LTR that lacks TAR (designated dTAR-env). Surprisingly, only the full-length Tat peptide trans activated expression of the env gene from dTAR-env. Comparison of RNA and protein expression of the env gene in the presence of Tat indicated that the mechanism of trans activation is posttranscriptional rather than transcriptional. To test whether the TAR-independent Tat function is specific to the HIV-1 env gene, we analyzed expression of heterologous genes from the long terminal repeat lacking TAR. These heterologous genes were not trans activated by Tat in the absence of a TAR element, which suggests that the second-exon peptide of Tat has a sequence-specific role in TAR-independent trans activation of the HIV-1 env gene. Analysis of a mutant in the 5' end of the env gene was used to identify a cis-acting sequence required for Tat responsiveness.