Functional consequences of cyclin D1/BRCA1 interaction in breast cancer cells

The inheritance of one defective BRCA1 or BRCA2 allele predisposes an individual to developing breast and ovarian cancers. BRCA1 is a multifunctional tumor suppressor protein, which through interaction with a vast array of proteins has implications in processes such as cell cycle, transcription, DNA damage response and chromatin remodeling. Conversely, the oncogene, cyclin D1 is overexpressed in about 35% of all breast cancer cases. In this study, we provide detailed analyses on the phosphorylation state of BRCA1 by cyclin D1/cdk4 complexes. In particular, we have identified Ser 632 of BRCA1 as a cyclin D1/cdk4 phosphorylation site in vitro. Using chromatin immunoprecipitation assays, we observed that the inhibition of cyclin D1/cdk4 activity resulted in increased BRCA1 DNA binding at particular promoters in vivo. In addition, we identified multiple novel genes that are bound by BRCA1 in vivo. Collectively, these results indicate that cyclin D1/cdk4-mediated phosphorylation of BRCA1 inhibits the ability of BRCA1 to be recruited to particular promoters in vivo. Therefore, cyclin D1/Cdk4 phosphorylation of BRCA1 could provide a mechanism to interfere with the DNA-dependent activities of BRCA1.

The tax protein from the primate T-cell lymphotropic virus type 3 is expressed in vivo and is functionally related to HTLV-1 Tax rather than HTLV-2 Tax

Human T-cell leukemia virus and simian T-cell leukemia virus (STLV) form the primate T-cell lymphotropic viruses group. Human T-cell leukemia virus type 1 and type 2 (HTLV-1 and HTLV-2) encode the Tax viral transactivator (Tax1 and Tax2, respectively). Tax1 possesses an oncogenic potential and is responsible for cell transformation both in vivo and in vitro. We and others have recently discovered the existence of human T-cell lymphotropic virus type 3. However, there is currently no evidence for the presence of a Tax protein in HTLV-3-infected individuals. We show that the serum of an HTLV-3 asymptomatic carrier and the sera of two STLV-3-infected monkeys contain specific anti-Tax3 antibodies. We also show that tax3 mRNA is present in the PBMCs obtained from an STLV-3-infected monkey, demonstrating that Tax3 is expressed in vivo. We further demonstrate that Tax3 intracellular localization is very similar to that of Tax1 and that Tax3 binds to both CBP and p300 coactivators. Using purified Tax3, we show that the protein increases transcription from a 4TxRE G-free cassette plasmid in an in vitro transcription assay. In all cell types tested, including transiently transfected lymphocytes, Tax3 activates its own promoter STLV-3 long terminal repeat (LTR), which contains only two Tax Responsive Elements (TREs), and activates also HTLV-1 and HTLV-2 LTRs. In addition, Tax3 also activates the NF-kappaB pathway. We also show that Tax3 possesses a PDZ-binding sequence at its C-terminal end. Our results demonstrate that Tax3 is a transactivator, and that its properties are more similar to that of Tax1, rather than of Tax2. This suggests the possible occurrence of lymphoproliferative disorders among HTLV-3-infected populations.

Enhancement of the p300 HAT activity by HIV-1 Tat on chromatin DNA

HIV-1 Tat is able to form a ternary complex with P/CAF and p300 and increase the affinity for CDK9/P-TEFb CTD kinase complex. Our previous study demonstrated that Tat binds to p300/CBP in the minimal HAT domain (aa 1253-1790) and that the interaction results in a change of conformation on p300/CBP. Here, we show that the Tat-p300 interaction increases the HAT activity of p300 on histone H4 that is associated with nucleosomal DNA and not with free histones. Nucleosomal histone H4 was acetylated on lysines 8, 12, and 16. Acetylation of H4 was inhibited by Lys-coenzyme A (CoA), a selective inhibitor of p300 acetyltransferase activity. Unexpectedly, we also found that Tat could autoacetylate itself, which was specific to lysine residues 41 and 71. Peptides lacking these two lysines could not enhance the HAT activity of p300. Comparison of the sequences of Tat with other HIV-1 clades and HAT containing transcription factors indicated sequence identity in the acetyl-CoA binding motif A, KGXG. Furthermore, when utilizing an in vitro transcription assay, as well as a Tat mutant virus, we found that ectopic expression of only wild-type Tat in the presence of p300, and not a lysine 41 Tat mutant, could activate HIV-1 chromatin DNA, as evidenced by the absence of HIV-1 virion antigen. Therefore, transcription of integrated viral DNA in vivo requires the HAT activity of coactivators that are modulated by Tat to derepress the HIV-1 chromatin structure and aid in activated transcription.

Human T-lymphotropic virus type 1 p30(II) regulates gene transcription by binding CREB binding protein/p300

The highly conserved coadapters CREB binding protein (CBP) and p300 form complexes with CREB as well as other DNA binding transcription factors to modulate chromatin remodeling and thus transcription. Human T-lymphotropic virus type 1 (HTLV-1) transcription is controlled, in part, by the CREB/ATF family of transcription factors which bind promoter sequences and function as complexes with the viral oncogenic protein Tax. We have reported that the nuclear localizing protein p30(II) of HTLV-1 functions as a transcription factor, differentially modulates CREB-responsive promoters, and is critical for maintenance of proviral loads in rabbits. In this study, we tested whether p30(II) directly interacts with CBP/p300 to modulate gene transcription. Gal4(BD)-p30(II)-mediated transactivation was enhanced following exogenous expression of p300 and was competitively repressed by the p300 binding protein, adenovirus E1A, and E1ACR2 (mutated for retinoblastoma binding but retaining p300 binding). In contrast, E1ACR1 (mutated for p300 binding) failed to alter Gal4(BD)-p30(II)-mediated transactivation. In addition, Gal4(BD)-p30(II)-mediated transactivation was competitively inhibited by the cotransfection of CMV-p30(II)-HA and CMV-Tax but could be rescued by exogenous p300. Importantly, we demonstrate that p30(II) colocalizes with p300 in cell nuclei and directly binds to CBP/p300 in cells. Deletion mutants of CBP/p300 were used to localize the site critical for binding p30(II) to a highly conserved KIX region. DNA binding assays confirmed the interference of p30(II) with the assembly of CREB-Tax-p300/CBP multiprotein complexes on 21-bp repeat oligonucleotides in vitro. Collectively, our results demonstrate that CBP/p300 is a cellular protein target for HTLV-1 p30(II) and mediates its transcriptional effects in vivo.

Inhibition of human immunodeficiency virus type 1 transcription by chemical cyclin-dependent kinase inhibitors

Cyclin-dependent kinases (cdk's) have recently been suggested to regulate human immunodeficiency virus type 1 (HIV-1) transcription. Previously, we have shown that expression of one cdk inhibitor, p21/Waf1, is abrogated in HIV-1 latently infected cells. Based on this result, we investigated the transcription of HIV-1 in the presence of chemical drugs that specifically inhibited cdk activity and functionally mimicked p21/Waf1 activity. HIV-1 production in virally integrated lymphocytic and monocytic cell lines, such as ACH(2), 8E5, and U1, as well as activated peripheral blood mononuclear cells infected with syncytium-inducing (SI) or non-syncytium-inducing (NSI) HIV-1 strains, were all inhibited by Roscovitine, a purine derivative that reversibly competes for the ATP binding site present in cdk's. The decrease in viral progeny in the HIV-1-infected cells was correlated with a decrease in the transcription of HIV-1 RNAs in cells treated with Roscovitine and not with the non-cdk general cell cycle inhibitors, such as hydroxyurea (G(1)/S blocker) or nocodazole (M-phase blocker). Cyclin A- and E-associated histone H1 kinases, as well as cdk 7 and 9 activities, were all inhibited in the presence of Roscovitine. The 50% inhibitory concentration of Roscovitine on cdk's 9 and 7 was determined to be approximately 0.6 microM. Roscovitine could selectively sensitize HIV-1-infected cells to apoptosis at concentrations that did not impede the growth and proliferation of uninfected cells. Apoptosis induced by Roscovitine was found in both latent and activated infected cells, as evident by Annexin V staining and the cleavage of the PARP protein by caspase-3. More importantly, contrary to many apoptosis-inducing agents, where the apoptosis of HIV-1-infected cells accompanies production and release of infectious HIV-1 viral particles, Roscovitine treatment selectively killed HIV-1-infected cells without virion release. Collectively, our data suggest that cdk's are required for efficient HIV-1 transcription and, therefore, we propose specific cdk inhibitors as potential antiviral agents in the treatment of AIDS.

Cell cycle effects of IL-10 on malignant B-1 cells

IL-10 is overexpressed in human chronic lymphocytic leukemia (CLL), and is an autocrine growth factor involved in the development of malignant B1 clones in NZB mice, a murine model for CLL. Antisense IL-10 oligonucleotide treatment induces apoptosis and cell cycle disruption in these cells both in vitro and in vivo. In addition, NZB IL-10 knock-out mice fail to develop the B-1 clones. Dampening of IL-10 protein production via antisense IL-10 oligonucleotide treatment is correlated with decreased p27/Kip1 protein expression which results in increased cyclin D2, cyclin E and cyclin A associated kinase activity. The action of the antisense oligonucleotides is through alterations in cell cycle regulation, resulting in accelerated cell cycle progression, a G2/M block which culminates in apoptosis induction in the malignant cells. This implies that the role of IL-10 as an autocrine growth factor in malignant B-1 cells lies in its ability to inhibit apoptosis induction through the maintenance of sustainable cell cycle progression in malignant cells.

Cell cycle regulation of human interleukin-8 gene expression by the human immunodeficiency virus type 1 Tat protein

The human immunodeficiency virus type 1 (HIV-1) Tat protein has been reported to transactivate several cellular genes, including the potent chemotactic factor interleukin-8 (IL-8). Consistent with these in vitro assays, elevated levels of IL-8 protein are found in the serum of HIV-infected individuals. We now extend these observations by demonstrating that Tat induction of IL-8 is linked to the cell cycle. Cells that constitutively express the Tat(1-86) protein (eTat) and control cells (pCEP) were reversibly blocked at the G(1)/S border with hydroxyurea or thymidine. The cells were subsequently released, and IL-8 expression was monitored by RNase protection assays and enzyme-linked immunosorbent assay (ELISA). RNase protection assays demonstrated that IL-8 mRNA expression is transiently induced, approximately fourfold, as the Tat-expressing cells enter S phase. Consistent with the RNase protection assay, an increase in IL-8 protein was observed in the cell supernatant using an IL-8 ELISA. Similar experiments were performed following a reversible block at the G(2)/M border with nocodazole and release into G(1). Using the RNase protection assay and ELISA, little or no increase in IL-8 expression was observed during G(1). Using gel shift as well as an immobilized DNA binding assay, we demonstrate that the increase in IL-8 gene expression correlates with a specific increase in p65 NF-kappa B binding activity only in the nucleus of the Tat-expressing cells. Moreover, the CREB-binding protein coactivator is present in the complex in the Tat cell line. Finally, we demonstrate that the presence of the proteasome inhibitor MG-132 inhibits the induction of NF-kappa B binding, as well as IL-8 expression, supporting the role of NF-kappa B.

Acetylation of HIV-1 Tat by CBP/P300 increases transcription of integrated HIV-1 genome and enhances binding to core histones

The HIV-1 Tat protein is required for viral replication and is a potent stimulator of viral transcription. Although Tat has been extensively studied in various reductive paradigms, to date there is little information as to how this activator mediates transcription from natural nucleosomally packaged long terminal repeats. Here we show that CREB-binding protein (CBP)/p300 interacts with the HIV-1 Tat protein and serves as a coactivator of Tat-dependent HIV-1 gene expression on an integrated HIV-1 provirus. The site of acetylation of Tat was mapped to the double-lysine motif in a highly conserved region, (49)RKKRRQ(54), of the basic RNA-binding motif of Tat. Using HLM1 cells (HIV-1(+)/Tat(-)), which contain a single copy of full-length HIV-1 provirus with a triple termination codon at the first AUG of the Tat gene, we find that only wild type, and not K50A, K51A, or K50A/K51A alone or in combination of ectopic CBP/p300, is able to produce full-length infectious virions, as measured by p24 gag ELISAs. Tat binds CBP/p300 in the minimal histone acetyltransferase domain (1253-1710) and the binding is stable up to 0.85 M salt wash conditions. Interestingly, wild-type peptide 41-54, and not other Tat peptides, changes the conformation of the CBP/p300 such that it can acquire and bind better to basal factors such as TBP and TFIIB, indicating that Tat may influence the transcription machinery by helping CBP/p300 to recruit new partners into the transcription machinery. Finally, using biotinylated wild-type or acetylated peptides, we find that acetylation decreases Tat's ability to bind the TAR RNA element, as well as to bind basal factors such as TBP, CBP, Core-Pol II, or cyclin T. However, the acetylated Tat peptide is able to bind to core histones on a nucleosome assembled HIV-1 proviral DNA.

Gene expression array of HTLV type 1-infected T cells: Up-regulation of transcription factors and cell cycle genes

By utilizing a human cDNA expression array blot (588 genes), we have observed overexpression of various transcription factors, cell cycle regulated kinases, and DNA repair genes in HTLV-1-infected T cells. One of the genes of interest, and focus in this study, is the cyclin-dependent kinase inhibitor, p21/waf1. The p21/waf1 transcription and protein is overexpressed in all HTLV-1-infected cell lines tested as well as ATL and HAM/TSP patient samples. While p21/waf1 has been shown to display a selectivity for G(1)/S cyclin/cdk complexes, we have observed p21/waf1 to be complexed with cyclin A/cdk2. Functionally, the association of p21/cyclin A/cdk2 decreased the histone H1 phosphorylation in vitro, as observed in immunoprecipitations followed by kinase assays, as well as affecting other substrates such as the C-terminus of Rb protein involved in c-Abl and HDAC1 regulation. Wild-type, but not a mutant form (M47) of Tax, was found to be able to transactivate the p21/waf1 promoter in a p53-independent manner. We found that the minimal p21/waf1 promoter (-49 to +49 sequence) was activated by Tax and the minimal promoter contained two E2A transcription factor binding sites located between the TATA box and the initiation site. E2A proteins, E12 and E47, as well as a related helix-loop-helix protein, HEB, are all up-regulated in HTLV-1-infected T cells. When using band shift analysis, we found that only the E1 site (overlapping the transcription start site) was a functional DNA binding site. By using a chromatin immunoprecipitation (ChIP) assay, we observed that histone H4, and not histone H3, was acetylated from the endogenous p21/waf1 promoter in vivo, implying that CBP/p300, and not the SAGA complex, was critical in complexing with E2A in up-regulation of p21/waf1 in HTLV-1-infected cells.

Inhibition of Tat-mediated transactivation of HIV-1 LTR transcription by polyamide nucleic acid targeted to TAR hairpin element

Tat, an essential human immunodeficiency virus type 1 protein interacts with the transactivation response element (TAR) and stimulates transcription from the viral long-terminal repeat (LTR). Blockage of Tat-TAR interaction halts viral transcription and hence replication. We have found that polyamide nucleic acid (PNA), targeted to the TAR sequences of viral RNA genome is able to prevent Tat-TAR interaction by efficient sequestration of the TAR. Anti-TAR PNA competes for TAR and prevents Tat-mediated stimulation of HIV-1 LTR transcription in vitro but has no influence on the basal level of transcription in the absence of Tat. Using a reporter gene construct pHIV LTR-CAT and pCMV-Tat in cell culture, we have further shown that anti-TAR PNA is able to block Tat-mediated transactivation of HIV-1 LTR transcription in vivo as judged by the extent of LTR driven CAT gene expression in the absence and presence of anti-TAR PNA. Supplementation of 100 nM of anti-TAR PNA into the culture medium further enhances the suppression of transactivation. Nonspecific scrambled PNA had no influence on Tat-TAR interaction and LTR-driven CAT gene expression in cell culture. These results suggest that PNA targeted to the TAR sequence of the viral genome may be a potential inhibitor of HIV-1 gene expression.

Overexpression of p21(waf1) in human T-cell lymphotropic virus type 1-infected cells and its association with cyclin A/cdk2

Human T-cell lymphotropic virus type 1 (HTLV-1) is associated with adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). T-cell transformation is mainly due to the actions of the viral phosphoprotein Tax. Tax interacts with multiple transcriptional factors, aiding the transcription of many cellular genes. Here, we report that the cyclin-dependent kinase inhibitor p21/waf1 is overexpressed in all HTLV-1-infected cell lines tested as well as in ATL and HAM/TSP patient samples. Tax was found to be able to transactivate the endogenous p21/waf1 promoter, as detected by RNase protection, as well as activate a series of wild-type and 5'-deletion constructs linked to a luciferase reporter cassette. Wild-type but not a mutant form of Tax (M47) transactivated the p21/waf1 promoter in a p53-independent manner and utilized a minimal promoter that contained E2A and TATA box sequences. The p21/waf1 protein was reproducibly observed to be complexed with cyclin A/cdk2 and not with any other known G(1), S, or G(2)/M cyclins. Functionally, the association of p21/cyclin A/cdk2 decreased histone H1 phosphorylation in vitro, as observed in immunoprecipitations followed by kinase assays, and affected other substrates, such as the C terminus of Rb protein involved in c-Abl and histone deacetylase-1 (HDAC1) regulation. Interestingly, upon the use of a stress signal, such as gamma-irradiation, we found that the p21/cyclin A/cdk2 complex was able to block all known phosphorylation sites on the Rb molecule. Finally, using elutriated cell cycle fractions and a stress signal, we observed that the HTLV-1-infected T cells containing wild-type Tax, which had been in early or mid-G(1) phase prior to gamma-irradiation, arrested in G(1) and did not undergo apoptosis. This may be an important mechanism for an oncogenic virus such as HTLV-1 to stop the host at the G(1)/S boundary and to repair the damaged DNA upon injury, prior to S-phase entry.

Helicobacter pylori inhibits gastric cell cycle progression

Helicobacter pylori infection of the gastric mucosa is associated with changes in gastric epithelial cell proliferation. In vitro studies have shown that exposure to H. pylori inhibits proliferation of gastric cells. This study sought to investigate the cell cycle progression of gastric epithelial cell lines in the presence and absence of H. pylori. Unsynchronized and synchronized gastric epithelial cell lines AGS and KatoIII were exposed to H. pylori over a 24-h period. Cell cycle progression was determined by flow cytometry using propidium iodide (PI), and by analysis of cyclin E, p21, and p53 protein expression using Western blots. In the absence of H. pylori 40, 45, and 15% of unsynchronized AGS cells were in G(0)-G(1), S, and G(2)-M phases, respectively, by flow cytometry analysis. When AGS cells were cultured in the presence of H. pylori, the S phase decreased 10% and the G(0)-G(1) phase increased 17% after 24 h compared with the controls. KatoIII cells, which have a deleted p53 gene, showed little or no response to H. pylori. When G1/S synchronized AGS cells were incubated with media containing H. pylori, the G(1) phase increased significantly (25%, P < 0.05) compared with controls after 24 h. In contrast, the control cells were able to pass through S phase. The inhibitory effects of H. pylori on the cell cycle of AGS cells were associated with a significant increase in p53 and p21 expression after 24 h. The expression of cyclin E was downregulated in AGS cells following exposure of AGS cells to H. pylori for 24 h. This study shows that H. pylori-induced growth inhibition in vitro is predominantly at the G(0)-G(1) checkpoint. Our results suggest that p53 may be important in H. pylori-induced cell cycle arrest. These results support a role for cyclin-dependent kinase inhibitors in the G(1) cell cycle arrest exerted by H. pylori and its involvement in changing the regulatory proteins, p53, p21, and cyclin E in the cell cycle.

BRCA1 is associated with a human SWI/SNF-related complex: linking chromatin remodeling to breast cancer

Germline mutations in the tumor suppressor gene, BRCA1, predispose individuals to breast and ovarian cancers. Using a combination of affinity- and conventional chromatographic techniques, we have isolated a predominant form of a multiprotein BRCA1-containing complex from human cells displaying chromatin-remodeling activity. Mass spectrometric sequencing of components of this complex indicated that BRCA1 is associated with a SWI/SNF-related complex. We show that BRCA1 can directly interact with the BRG1 subunit of the SWI/SNF complex. Moreover, p53-mediated stimulation of transcription by BRCA1 was completely abrogated by either a dominant-negative mutant of BRG1 or the cancer-causing deletion in exon 11 of BRCA1. These findings reveal a direct function for BRCA1 in transcriptional control through modulation of chromatin structure.

Tat modifies the activity of CDK9 to phosphorylate serine 5 of the RNA polymerase II carboxyl-terminal domain during human immunodeficiency virus type 1 transcription

Tat stimulates human immunodeficiency virus type 1 (HIV-1) transcriptional elongation by recruitment of carboxyl-terminal domain (CTD) kinases to the HIV-1 promoter. Using an immobilized DNA template assay, we have analyzed the effect of Tat on kinase activity during the initiation and elongation phases of HIV-1 transcription. Our results demonstrate that cyclin-dependent kinase 7 (CDK7) (TFIIH) and CDK9 (P-TEFb) both associate with the HIV-1 preinitiation complex. Hyperphosphorylation of the RNA polymerase II (RNAP II) CTD in the HIV-1 preinitiation complex, in the absence of Tat, takes place at CTD serine 2 and serine 5. Analysis of preinitiation complexes formed in immunodepleted extracts suggests that CDK9 phosphorylates serine 2, while CDK7 phosphorylates serine 5. Remarkably, in the presence of Tat, the substrate specificity of CDK9 is altered, such that the kinase phosphorylates both serine 2 and serine 5. Tat-induced CTD phosphorylation by CDK9 is strongly inhibited by low concentrations of 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole, an inhibitor of transcription elongation by RNAP II. Analysis of stalled transcription elongation complexes demonstrates that CDK7 is released from the transcription complex between positions +14 and +36, prior to the synthesis of transactivation response (TAR) RNA. In contrast, CDK9 stays associated with the complex through +79. Analysis of CTD phosphorylation indicates a biphasic modification pattern, one in the preinitiation complex and the other between +36 and +79. The second phase of CTD phosphorylation is Tat-dependent and TAR-dependent. These studies suggest that the ability of Tat to increase transcriptional elongation may be due to its ability to modify the substrate specificity of the CDK9 complex.

Changes in E2F binding after phenylbutyrate-induced differentiation of Caco-2 colon cancer cells

Differentiation agents use existing cellular systems to induce neoplastic cells to regain a normal phenotype and/or to cause growth arrest and therefore may offer novel chemotherapeutic approaches to treating solid tumors. In this study, we demonstrate in Caco-2 colon cancer cells that the differentiation agent phenylbutyrate (PB) causes a decrease in viable cells, an increase in cell differentiation, and a G1-S-phase block. The mechanism of this last effect is related to a PB-induced increase in p27Kip1, leading to a decrease in the activity of cyclin-dependent kinase 2 (CDK2), a positive regulator of the G1-S-phase cell cycle transition. Consistent with the decreased CDK2 kinase activity, we also observed a decrease in the phosphorylation state of the retinoblastoma protein after PB treatment. This was associated with increased binding and consequent inactivation of E2F, a transactivator of genes that regulate the G1 to S phase cell cycle transition. These data suggest that the differentiation agent PB inhibits tumor growth by limiting the availability of active E2F, with a subsequent G1-S-phase block. Additional studies should show whether PB is a clinically effective therapeutic agent against colorectal cancer.

Loss of G(1)/S checkpoint in human immunodeficiency virus type 1-infected cells is associated with a lack of cyclin-dependent kinase inhibitor p21/Waf1

Productive high-titer infection by human immunodeficiency virus type 1 (HIV-1) requires the activation of target cells. Infection of quiescent peripheral CD4 lymphocytes by HIV-1 results in incomplete, labile reverse transcripts and lack of viral progeny formation. An interplay between Tat and p53 has previously been reported, where Tat inhibited the transcription of the p53 gene, which may aid in the development of AIDS-related malignancies, and p53 expression inhibited HIV-1 long terminal repeat transcription. Here, by using a well-defined and -characterized stress signal, gamma irradiation, we find that upon gamma irradiation, HIV-1-infected cells lose their G(1)/S checkpoints, enter the S phase inappropriately, and eventually apoptose. The loss of the G(1)/S checkpoint is associated with a loss of p21/Waf1 protein and increased activity of a major G(1)/S kinase, namely, cyclin E/cdk2. The p21/Waf1 protein, a known cyclin-dependent kinase inhibitor, interacts with the cdk2/cyclin E complex and inhibits progression of cells into S phase. We find that loss of the G(1)/S checkpoint in HIV-1-infected cells may in part be due to Tat's ability to bind p53 (a known activator of the p21/Waf1 promoter) and sequester its transactivation activity, as seen in both in vivo and in vitro transcription assays. The loss of p21/Waf1 in HIV-1-infected cells was specific to p21/Waf1 and did not occur with other KIP family members, such as p27 (KIP1) and p57 (KIP2). Finally, the advantage of a loss of the G(1)/S checkpoint for HIV-1 per se may be that it pushes the host cell into the S phase, which may then allow subsequent virus-associated processes, such as RNA splicing, transport, translation, and packaging of virion-specific genes, to occur.

Phosphorylation of the RAP74 subunit of TFIIF correlates with Tat-activated transcription of the HIV-1 long terminal repeat

Transcription from the HIV-1 long terminal repeat (LTR) is regulated by the viral transactivator Tat, which increases RNA polymerase II (RNAP II) processivity. Previous reports have demonstrated that phosphorylation of the RNAP II carboxy-terminal domain by TFIIH and P-TEFb is important for Tat transactivation. Our present results demonstrate that phosphorylation of the RAP74 subunit of TFIIF is also an important step in Tat transactivation. Interestingly, while the general transcription factor TFIIF is required for both basal and Tat-activated transcription, phosphorylation of the RAP74 subunit occurs in the presence of Tat and correlates with a high level of transcription activity. Using a biotinylated DNA template transcription assay, we provide evidence that RAP74 is phosphorylated by TAF(II)250 during Tat-activated transcription. Depletion of RAP74 from the HeLa nuclear extract inhibited HIV-1 LTR-driven basal transcription and Tat transactivation. The addition of TFIIF, reconstituted from recombinant RAP30 and RAP74, to the depleted HeLa nuclear extract resulted in restoration of Tat transactivation. Of importance, the exogenous RAP74 was rapidly phosphorylated in the presence of Tat. These results suggest that RAP74 phosphorylation is one important step, of several, in the Tat transactivation cascade.

Cell cycle-regulated transcription by the human immunodeficiency virus type 1 Tat transactivator

Cyclin-dependent kinases are required for the Tat-dependent transition from abortive to productive elongation. Further, the human immunodeficiency virus type 1 (HIV-1) Vpr protein prevents proliferation of infected cells by arresting them in the G(2) phase of the cell cycle. These findings suggest that the life cycle of the virus may be integrally related to the cell cycle. We now demonstrate by in vitro transcription analysis that Tat-dependent transcription takes place in a cell cycle-dependent manner. Remarkably, Tat activates gene expression in two distinct stages of the cell cycle. Tat-dependent long terminal repeat activation is observed in G(1). This activation is TAR dependent and requires a functional Sp1 binding site. A second phase of transactivation by Tat is observed in G(2) and is TAR independent. This later phase of transcription is enhanced by a natural cell cycle blocker of HIV-1, vpr, which arrests infected cells at the G(2)/M boundary. These studies link the HIV-1 Tat protein to cell cycle-specific biological functions.

Transcriptional up-regulation of the cyclin D2 gene and acquisition of new cyclin-dependent kinase partners in human T-cell leukemia virus type 1-infected cells

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis. Tax(1) is a 40-kDa phosphoprotein, predominantly localized in the nucleus of the host cell, which functions to transactivate both viral and cellular promoters. It seems likely that HTLV-1, through expression of the viral regulatory protein Tax(1), provides some initial alteration in cell metabolism predisposing the development of ATL. Here, we demonstrate that HTLV-1 infection in T-cell lines and patient samples causes overexpression of an early G(1) cyclin, cyclin D2. The transcriptional up-regulation of the cyclin D2 gene is due to activation of Tax on the cyclin D2 gene. More important, we find that overexpression of cyclin D2 is accompanied by acquisition of new partners such as cyclin-dependent kinase 2 (cdk2), cdk4, and cdk6 in infected cells. This is in contrast to uninfected T cells, where cyclin D2 associates only with cdk6. Functional effects of these cyclin-cdk complexes in infected cells are shown by hyperphosphorylation of Rb and histone H1, indicators of active progression into S phase as well as changes in cellular chromatin and transcription machinery. These studies link HTLV-1 infection with changes of cellular cyclin gene expression, hence providing clues to development of T-cell leukemia.

Pax-6 interactions with TATA-box-binding protein and retinoblastoma protein

PURPOSE

To identify proteins that physically interact with Pax-6, a paired domain- and homeodomain (HD)-containing transcription factor that is a key regulator of eye development.

METHODS

Protein-protein interactions involving Pax-6, TATA-box-binding protein (TPB), and retinoblastoma protein were studied using affinity chromatography with Pax-6 as ligand, glutathione-S-transferase (GST) pull-down assays, and immunoprecipitations.

RESULTS

The authors have shown that Pax-6 is a sequence-specific activator of many crystallin genes, all containing a TATA box, in the lens. Others have shown that lens fiber cell differentiation, characterized by temporally and spatially regulated crystallin gene expression, depends on retinoblastoma protein. In the present study it was shown that Pax-6 interacted with the TBP, the DNA-binding subunit of general transcription complex TFIID. GST pull-down assays indicated that this interaction was mediated by the Pax-6 HD, with a substantial role for its N-terminal arm and first two alpha-helices. The experiments also indicated a binding role for the C-terminal-activation domain of the protein. In addition, the present study showed that the HD of Pax-6 interacted with retinoblastoma protein. Immunoprecipitation experiments confirmed retinoblastoma protein/Pax-6 complexes in lens nuclear extracts.

CONCLUSIONS

Blending the present results with those in the literature suggests that Pax-6 and retinoblastoma protein participate in overlapping regulatory pathways controlling epithelial cell division, fiber cell elongation, and crystallin gene expression during lens development.

The coactivator CBP stimulates human T-cell lymphotrophic virus type I Tax transactivation in vitro

Tax interacts with the cellular cyclic AMP-responsive element binding protein (CREB) and facilitates the binding of the coactivator CREB binding protein (CBP), forming a multimeric complex on the cyclic AMP-responsive element (CRE)-like sites in the human T-cell lymphotrophic virus type I (HTLV-I) promoter. The trimeric complex is believed to recruit additional regulatory proteins to the HTLV-I long terminal repeat, but there has been no direct evidence that CBP is required for Tax-mediated transactivation. We present evidence that Tax and CBP activate transcription from the HTLV-I 21 base pair repeats on naked DNA templates. Transcriptional activation of the HTLV-I sequences required both Tax and CBP and could be mediated by either the N-terminal activation domain of CBP or the full-length protein. Fluorescence polarization binding assays indicated that CBP does not markedly enhance the affinity of Tax for the trimeric complex. Transcription analyses suggest that CBP activates Tax-dependent transcription by promoting transcriptional initiation and reinitiation. The ability of CBP to activate the HTLV-I promoter does not involve the stabilization of Tax binding, but rather depends upon gene activation properties of the co-activator that function in the context of a naked DNA template.

Phosphorylation of p53 serine 15 increases interaction with CBP

p53 exerts its cell cycle regulatory effects through its ability to function as a sequence-specific DNA binding transcription factor. CREB-binding protein (CBP)/p300, through its interaction with the N terminus of p53, acts as a coactivator for p53 and increases the sequence-specific DNA-binding activity of p53 by acetylating its C terminus. The same N-terminal domain of p53 has recently been shown to be phosphorylated at Ser15 in response to gamma-irradiation. Remarkably, we now demonstrate that phosphorylation of p53 at Ser15 increases its ability to recruit CBP/p300. The increase in CBP/p300 binding was followed by an increase in the overall level of acetylation of the C terminus of p53. These results provide a mechanism for the activation of p53-regulated genes following DNA damage, through a signaling pathway linking p53 N-terminal kinase and C-terminal acetyltransferase activities.

p53 regulates human insulin-like growth factor II gene expression through active P4 promoter in rhabdomyosarcoma cells

The developmentally regulated human insulin-like growth factor II (IGFII) gene is expressed at high levels in many types of tumors and promotes the proliferation of tumor cells with a high incidence of p53 gene defects. We have previously shown that p53 inhibits IGFII P3 promoter activity and decreases endogenous IGFII gene expression derived from the P3 promoter in rhabdomyosarcomas by interfering with TBP binding to the TATA element of the IGFII P3 promoter. In this report, we demonstrate that wild-type p53 expression in rhabdomyosarcoma cell lines containing mutant p53 leads to a decrease in the activity of another active IGFII promoter, P4, and a 5-fold reduction of IGFII mRNA derived from the P4 promoter. This inhibition of P4 activity is associated with direct binding of p53 to the P4 proximal promoter element despite the lack of a p53 consensus binding site. Our results suggest that p53 inhibits IGFII P4 promoter activity by a mechanism different than its effect on the P3 promoter. These data also supply further evidence of cross-talk between the IGF and p53 signaling pathways.

Repression of transcription from the human T-cell leukemia virus type I long terminal repeat and cellular gene promoters by wild-type p53

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL) transforms human T cells both in vivo and in vitro. However, the long latency period between infection and development of ATL, as well as the small fraction of the infected population that actually develops this disease, suggest that factors in addition to the virus are involved in its pathogenesis. Mutation of tumor suppressor gene p53 has been found in both HTLV-I-transformed T-cell lines and ATL cases at relatively low frequency. However, increasing evidence supports p53 functional impairment in HTLV-I-transformed T cells. Tax, the major transactivator of HTLV-I, is critical for the initial events involved in transformation. We have considered the possibility that p53 may regulate transcription of viral and cellular genes important for viral replication and transformation. Inactivation of p53 function might then permit constitutive expression of these viral and cellular genes. We have investigated the effects of wild-type and mutant p53 on Tax-mediated activation of the HTLV-I long terminal repeat (LTR) and the promoters of several cellular genes including the interleukin (IL)-1alpha, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF ), and IL-2 receptor alpha chain gene. Jurkat, HuT78, and U937 cells were cotransfected with plasmids containing a chloramphenicol acetyltransferase (CAT ) reporter gene under viral or cellular promoter control and the Tax expression vector, in addition to vectors for a wild-type or mutant p53. Wild-type p53 is a potent repressor of viral and cellular activation by Tax. Mutations within p53 severely inhibit this downregulation. We also show that wild-type p53 suppresses transcription from the HTLV-I LTR in Jurkat-Tax, a T-cell line stably expressing Tax, and MT-2, a HTLV-I-transformed T-cell line. Wild-type, but not mutant, p53 interfered with the binding of TATA-binding protein (TBP) to the TATA motif of the HTLV-I LTR. These results suggest that p53 inactivation may lead to upregulation of viral and cellular genes and may also be important for establishment of productive viral infection and development of ATL.

Induction of tumor necrosis factor alpha in human neuronal cells by extracellular human T-cell lymphotropic virus type 1 Tax

To examine the role of human T-lymphotropic virus type 1 (HTLV-1) Tax1 in the development of neurological disease, we studied the effects of extracellular Tax1 on gene expression in NT2-N cells, postmitotic cells that share morphologic, phenotypic, and functional features with mature human primary neurons. Treatment with soluble HTLV-1 Tax1 resulted in the induction of tumor necrosis factor alpha (TNF-alpha) gene expression, as detected by reverse-transcribed PCR and by enzyme-linked immunosorbent assay. TNF-alpha induction was completely blocked by clearance with anti-Tax1 monoclonal antibodies. Furthermore, cells treated with either a mock bacterial extract or with lipopolysaccharide produced no detectable TNF-alpha. Synthesis of TNF-alpha in response to soluble Tax1 occurred in a dose-dependent fashion between 0.25 and 75 nM and peaked within 6 h of treatment. Interestingly, culturing NT2-N cells in the presence of soluble Tax1 for as little as 5 min was sufficient to result in TNF-alpha production, indicating that the induction of TNF-alpha in NT2-N does not require Tax1 to be continually present in the culture medium. Treatment of the undifferentiated parental embryonal carcinoma cell line NT2 with soluble Tax1 did not result in TNF-alpha synthesis, suggesting that differentiation-dependent, neuron-specific factors may be required. These results provide the first experimental evidence that neuronal cells are sensitive to HTLV-1 Tax1 as an extracellular cytokine, with a potential role in the pathology of HTLV-1-associated/tropical spastic paraparesis.

Rapid and sensitive detection of cell-associated HIV-1 in latently infected cell lines and in patient cells using sodium-n-butyrate induction and RT-PCR

To develop a rapid and sensitive means of detecting cell-associated human immunodeficiency virus (HIV), donor cells from HIV seropositive patients were treated with the potent viral activator sodium-n-butyrate (NaB) and subsequently assayed by both in situ RNA hybridization and a reverse transcriptase polymerase chain reaction (RT-PCR). The sensitivity of RT-PCR was estimated to be equivalent to 1 x 10(-16) grams (0.1 fg) or approximately 64 copies of the input standard viral RNA per reaction. The present study takes advantage of the ability of NaB to introduce changes in chromatin structure of latently infected cells, leading to increased HIV gene expression. Human ACH-2 and U1 cell lines were used as representatives of T-lymphocytic and monocytoid cells harboring latent inducible proviruses. HIV gene expression was readily detected when these cells were treated with NaB. Viral gag RNA was detected by both in situ and RT-PCR assays. When peripheral blood mononuclear cells (PBMCs) from acquired immunodeficiency syndrome (AIDS) patients, who were all negative for in situ hybridization and serum/plasma p24 assays, were used for detection of viral gene expression, four categories with distinct patterns of induction were observed. The first set of patients showed HIV-positive PBMCs by RT-PCR without any added NaB, and suppression by added NaB or PHA. The second set of samples showed induction of viral RNA by NaB alone. The third set could be induced with PHA, but not NaB, and the fourth set required both NaB and PHA for induction of HIV gene expression. Our results suggest that direct treatment of the cells with HIV activators may be useful in increasing sensitivity of the RT-PCR intended to be used for detection of cell-associated viral RNAs. This approach may be used to confirm true status of the HIV infection when p24 results are negative or HIV RNAs in serum/plasma are below the threshold of detection. Moreover, this method may identify the presence of latent proviral genomes possibly reflecting the true rate of cell-associated viral load in vivo and without possible mutations brought about by long-term co-cultivation assays with cells from seronegative donors.

Human herpesvirus 6 (HHV-6) ORF-1 transactivating gene exhibits malignant transforming activity and its protein binds to p53

The 357 amino acid open reading frame 1 (ORF-1), also designated DR7, within the SalI-L fragment of human herpesvirus 6 (HHV-6) exhibited transactivation of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) promoter and increased HIV-1 replication (Kashanchi et al., Virology, 201, 95-106, 1994). In the current study, the SalI-L transforming region was localized to the SalI-L-SH subfragment. Several ORFs identified in SalI-L-SH by sequence analysis were cloned into a selectable mammalian expression vector, pBK-CMV. Only pBK/ORF1 transformed NIH3T3 cells. Furthermore, cells expressing ORF-1 protein produced fibrosarcomas when injected into nude mice, whereas control cells, expressing either no ORF-1 protein or C-terminal truncated (after residue 172) ORF-1 protein, were not tumorigenic. Western blot analysis of proteins extracted from the tumors revealed ORF-1 protein. Additional studies indicated that ORF-1 was expressed in HHV-6-infected human T-cells by 18 h. Co-immunoprecipitation experiments showed that ORF-1 protein bound to tumor suppressor protein p53, and the ORF-1 binding domain on p53 was located between residues 28 and 187 of p53, overlapping with the specific DNA binding domain. Functional studies showed that p53-activated transcription was inhibited in ORF-1, but not in truncated ORF-1, expressing cells. Importantly, the truncated ORF-1 mutant also failed to cause transformation. Analysis of several human tumors by PCR revealed ORF-1 DNA sequences in some angioimmunoblastic lymphadenopathies, Hodgkin's and non-Hodgkin's lymphomas and glioblastomas. The detection of ORF-1 sequences in human tumors, while not proof per se, is a prerequisite for establishing its role in tumor development. Taken together, the results demonstrate that ORF-1 is an HHV-6 oncogene that binds to and affects p53. The identification of both transforming and transactivating activities within ORF-1 is a characteristic of other viral oncogenes and is the first reported for HHV-6.

Inhibition of HIV-1 transcription and virus replication using soluble Tat peptide analogs

The human immunodeficiency virus type 1 (HIV-1) transactivator Tat protein is essential for efficient viral gene expression and virus replication. The Tat core domain, a stretch of 12 amino acids between the cysteine-rich and the basic domain, is conserved in all HIV isolates and required for interaction with a number of cellular transcriptional regulatory proteins. Here we demonstrate that soluble peptide analogs of the Tat core domain (amino acid 36-50) are able to effectively block LTR transactivation. In transfection experiments, Tat core peptide analogs containing amino acid substitutions at position 41 and 44 inhibited Tat transactivation of an HIV-1 LTR-CAT reporter construct up to 80-fold. In contrast, inhibition of other promoters such as HTLV-I and CMV was approximately 2-fold. Tat peptide analog 36-50 (41/44) inhibited HIV virus replication by 85% in latently infected U1 cells induced with Tat. Furthermore, U1 cells treated with the Tat peptide 36-50 (41/44) analog showed markedly delayed virus transmission when cocultivated with parental U937 cells. Interestingly, while both short and long peptide analogs (amino acids 36-50 vs 36-72) inhibited Tat transactivation in transient assays, the short peptides were more effective inhibitors of virus replication in U1 cells. The Tat peptide analog did not decrease expression of cellular genes including beta-actin, GAPDH, and histone H2B.

Human cytomegalovirus mtrII oncoprotein binds to p53 and down-regulates p53-activated transcription

The 79-amino-acid (79-aa) open reading frame (UL111a) gene within morphological transforming region II (mtrII) of human cytomegalovirus strain Towne has been shown to transform rodent cells in vitro (J. Thompson, J. Doniger, and L. J. Rosenthal, Arch. Virol. 136:161-172, 1994). Moreover, a translation termination linker (TTL) mutant of mtrII that coded for the first 49 aa of mtrII oncoprotein (designated TTL49) was sufficient for malignant transformation, whereas a TTL mutant that coded for the first 24 aa (designated TTL24) was not. The current study demonstrates the binding of mtrII oncoprotein to the tumor suppressor protein p53 both in vivo using transiently transfected cells and in vitro using labeled proteins. Furthermore, the C-terminally truncated mtrII protein TTL49, but not truncated protein TTL24, bound to p53. The mtrII binding domain mapped to the N-terminal region of p53, residues 1 to 106, with a critical region from aa 27 to 44, whereas the p53 binding domain of mtrII protein was the first 49 aa. Furthermore, mtrII inhibited p53-activated transcription, indicating its ability to alter p53-directed cellular regulatory mechanisms. mtrII oncoprotein was detected both in stably transfected NIH 3T3 cell lines and human cytomegalovirus-infected HEL 299 cells (as early as 12 h after infection) in the perinuclear region and in the nucleus. mtrII-transformed cell lines, at both early and late passage, exhibited high levels of p53 with a 15-fold-extended half-life. However, p53-activated transcription was suppressed in these cells in spite of the increased p53 levels. Finally, the results with wild-type mtrII and its TTL mutants with respect to p53 binding, p53-activated transcription, and transforming ability suggest that the mechanism of mtrII transformation is linked to both p53 binding and disruption of p53 cell regulation.

Interaction of human immunodeficiency virus type 1 Tat with a unique site of TFIID inhibits negative cofactor Dr1 and stabilizes the TFIID-TFIIA complex

We have previously reported the direct physical interaction between the human immunodeficiency virus (HIV) type I Tat protein and the basal transcription factor TBP/TFIID. Affinity chromatography demonstrated that wild-type Tat, but not a transactivation mutant of Tat, was capable of depleting TBP/TFIID from cell extracts. These experiments represented the first demonstration of a basal transcription factor that binds, in an activation-dependent manner, to Tat. We now report that the Tat-TBP interaction can be detected in HIV type 1-infected cells. The domain of TBP interacting with Tat has been mapped from amino acids 163 to 196 by using deletion and site-specific mutants of TBP. This domain of TBP, which includes the HI and S2 domains, is distinct from the H2 binding site for other activator proteins, such as E1A. The interaction of Tat with TFIID regulates the binding of accessory proteins to TFIID. Tat stabilizes the interaction of TFIID with TFIIA in a gel shift assay. In addition, Tat competes for Dr1 interaction with TBP. Our results suggest that the basal transcription factor TBP/TFIID represents an important regulatory molecule in HIV transcription.

Isolation of a cDNA clone, TRX encoding a human T-cell lymphotrophic virus type-I Tax1 binding protein

Tax1 is essential for human T-cell lymphotropic virus type I (HTLV-I) virus replication and transformation. We have identified and characterized a Tax1 binding protein, TRX, by cDNA screening of a Jurkat T-cell cDNA library. TRX mRNA is ubiquitously expressed in human tissues tested and cell lines analyzed.

Regulation of insulin-like growth factor II P3 promotor by p53: a potential mechanism for tumorigenesis

Human insulin-like growth factor (IGF)-II mRNA has been shown to be expressed at high levels in a variety of tumors, including rhabdomyosarcomas. In addition, many tumors have alterations in p53 expression. To investigate whether p53 regulates IGF-II gene expression, we transfected wild-type p53 expression vectors and luciferase constructs driven by IGF-II P3 promotors into multiple cell lines. We found that p53 reduced, in a dose-dependent manner, both endogenous IGF-II P3 transcripts and transfected P3 luciferase expression. The inhibition of P3 luciferase expression by p53 was more pronounced in the two cell lines that expressed mutant p53 protein, RD, and HTB114. The element responsible for this inhibition was mapped to the minimal promoter region. We also transfected an HPV-16 E6 expression plasmid into CCL13 cells containing functional p53 and found that E6 up-regulated IGF-II P3 activity. Wild-type, but not mutant, p53 interfered with the binding of TATA-binding protein to the TATA motif of P3, although both could directly associate with human TATA-binding protein. Our results suggest that p53 may play a role in regulation of IGF-II gene expression.

Human herpesvirus 6A ts suppresses both transformation by H-ras and transcription by the H-ras and human immunodeficiency virus type 1 promoters

Human herpesvirus 6 strain U1102 (HHV-6A) was shown to contain a 1,473-bp functional transformation suppressor gene (ts). ts exhibited 24% identity and 51% similarity to adeno-associated virus type 2 Rep68/78. Like adeno-associated virus type 2 Rep68/78, HHV-6A ts suppressed H-ras transformation of NIH 3T3 cells. Suppression of H-ras transformation was eliminated by translation termination linker mutation at amino acid 25, 125, or 245. These data indicated the importance of the C-terminal portion of the ts protein. H-ras transformation was suppressed by ts only when H-ras was expressed by its endogenous H-ras promoter and not when it was expressed by the heterologous murine osteosarcoma virus long terminal repeat (LTR). Furthermore, ts suppressed chloramphenicol acetyltransferase (CAT) activity when the CAT gene was expressed from the H-ras promoter but not the murine osteosarcoma virus LTR promoter. Taken together, the data showed that ts suppressed H-ras transformation at the level of the H-ras promoter. To further identify the interaction of ts with transcriptional regulatory elements, the human immunodeficiency virus type 1 (HIV-1) LTR was used. This promoter was selected because it has well-defined transcriptional regulatory elements for both basal and activated transcription, because its activity is inhibited by the Rep68/78 gene, and because both HHV-6 and HIV-1 naturally infect CD4+ T cells in vivo and have been shown to infect the same cell in vitro. ts suppressed expression from both wild-type and upstream mutant HIV-1 LTR-CAT constructs. However, downstream HIV-1 TAR mutations reversed ts suppression, indicating that TAR is one of the critical elements involved. The data presented demonstrated that HHV-6A ts functionally suppressed H-ras transformation and HIV-1 LTR expression and thus that it may be useful in future gene therapy.

Transactivation of the human T-cell lymphotropic virus type 1 Tax1-responsive 21-base-pair repeats requires Holo-TFIID and TFIIA

The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent for adult T-cell leukemia and tropical spastic paraparesis/HTLV-1-associated myelopathy. The HTLV-1 Tax1 gene product has been shown to transactivate transcription of viral and cellular promoters. To examine the biochemical mechanism of Tax1 transactivation, we have developed an in vitro transactivation assay in which wild-type Tax1 is able to specifically transactivate a polymerase II promoter through upstream Tax1-responsive elements. The in vitro system utilizes the HTLV-1 21-bp repeats cloned upstream of the ovalbumin promoter and G-free cassette. Purified Tax1 specifically transactivates this template 5- to 10-fold in a concentration-dependent manner. No transactivation of the ovalbumin promoter (pLovTATA) template control was observed. Tax1 transactivation was inhibited by low concentrations of alpha-amanitin and was effectively neutralized by anti-Tax1 but not control sera. Consistent with in vivo transactivating activity, Tax1 NF-kappa B mutant M22, but not cyclic AMP-responsive element-binding protein mutant M47, transactivated the template containing the tandem 21-bp repeat. In a reconstituted in vitro transcription assay, Tax1 transactivation was dependent upon basal transcription factors TFIIB, TFIIF, Pol II, TFIID, and TFIIA. TATA-binding protein did not functionally substitute for TFIID in the transactivation assay by Tax1 but was sufficient for basal transcription. Finally, we have used anti-TFIIA antibody (p55) to ask if Tax1 transactivation required TFIIA activity. Addition of TFIIA antibody to in vitro transcription reactions, as well as depletion of TFIIA by preclearing with antibody, showed that TFIIA was required for Tax1 transactivation. Only a slight (twofold) drop of basal transcription was observed. Tax1 transactivation was restored when purified HeLa TFIIA was added back into the reconstituted system. We propose that Tax1 utilizes a transactivation pathway involving the activator regulated basal transcription factors TFIID and TFIIA.

HIV Tat represses transcription through Sp1-like elements in the basal promoter

MHC class I genes are potently repressed by HIV Tat, which transactivates the HIV LTR. Tat represses class I transcription by binding to complexes associated with a novel promoter element, consisting of Sp1-like DNA binding sites. Transcription by other Sp1-dependent promoters, such as MDR1 and the minimal SV40 promoters, is also repressed by Tat, whereas the human beta-actin promoter is neither activated by Sp1 nor repressed by Tat. Tat repression can be overcome by a strong enhancer element. Thus, the SV40 72 bp enhancer element confers protection from Tat-mediated repression on both the minimal SV40 promoter and the class I promoter. Surprisingly, Tat can activate the class I promoter in the presence of both the HIV TAR element and a strong upstream enhancer. These data demonstrate that Tat differentially affects Sp1-responsive promoters, depending on promoter architecture.

Transcriptional regulation of the mouse alpha A-crystallin gene: activation dependent on a cyclic AMP-responsive element (DE1/CRE) and a Pax-6-binding site

Two cis-acting promoter elements (-108 to -100 and -49 to -33) of the mouse alpha A-crystallin gene, which is highly expressed in the ocular lens, were studied. Here we show that DE1 (-108 to -100; 5'TGACGGTG3'), which resembles the consensus cyclic AMP (cAMP)-responsive element sequence (CRE; 5'TGACGT[A/C][A/G]3'), behaves like a functional CRE site. Transfection experiments and electrophoretic mobility shift assays (EMSAs) using site-specific mutations correlated a loss of function with deviations from the CRE consensus sequence. Results of EMSAs in the presence of antisera against CREB, delta CREB, and CREM were consistent with the binding of CREB-like proteins to the DE1 sequence. Stimulation of alpha A-crystallin promoter activity via 8-bromo-cAMP, forskolin, or human T-cell leukemia virus type I Tax1 in transfections and reduction of activity of this site in cell-free transcription tests by competition with the somatostatin CRE supported the idea that DE1 is a functional CRE. Finally, Pax-6, a member of the paired-box family of transcription factors, activated the mouse alpha A-crystallin promoter in cotransfected COP-8 fibroblasts and bound to the -59 to -29 promoter sequence in EMSAs. These data provide evidence for a synergistic role of Pax-6 and CREB-like proteins for high expression of the mouse alpha A-crystallin gene in the lens.

Transcription of the human T-cell lymphotropic virus type I promoter by an alpha-amanitin-resistant polymerase

The human T-lymphotropic virus type I (HTLV-I) promoter contains the structural features of a typical RNA polymerase II (pol II) template. The promoter contains a TATA box 30 bp upstream of the transcription initiation site and binding sites for several pol II transcription factors, and long poly(A)+ RNA is synthesized from the integrated HTLV-I proviral DNA in vivo. Consistent with these characteristics, HTLV-I transcription activity was reconstituted in vitro by using TATA-binding protein, TFIIA, recombinant TFIIB, TFIIE, and TFIIF, TFIIH, and pol II. Transcription of the HTLV-I promoter in the reconstituted system requires RNA pol II. In HeLa whole cell extracts, however, the HTLV-I long terminal repeat also contains an overlapping transcription unit (OTU). HTLV-I OTU transcription is initiated at the same nucleotide site as the RNA isolated from the HTLV-I-infected cell line MT-2 but was not inhibited by the presence of alpha-amanitin at concentrations which inhibited the adenovirus major late pol II promoter (6 micrograms/ml). HTLV-I transcription was inhibited when higher concentrations of alpha-amanitin (60 micrograms/ml) were used, in the range of a typical pol III promoter (VA-I). Neutralization and depletion experiments with three distinct pol II antibodies demonstrate that RNA pol II is not required for HTLV-I OTU transcription. Antibodies to basal transcription factors TATA-binding protein and TFIIB, but not TFIIIC, inhibited HTLV-I OTU transcription. These observations suggest that the HTLV-I long terminal repeat contains overlapping promoters, a typical pol II promoter and a unique pol III promoter which requires a distinct set of transcription factors.

Central nervous system-derived cells express a kappa B-binding activity that enhances human immunodeficiency virus type 1 transcription in vitro and facilitates TAR-independent transactivation by Tat

The Tat protein of human immunodeficiency virus type 1 (HIV-1) is a potent activator of long terminal repeat-directed transcription. While in most cell types, activation requires interaction of Tat with the unusual transcription element TAR, astrocytic glial cells support TAR-independent transactivation of HIV-1 transcription by Tat. This alternative pathway of Tat activation is mediated by the viral enhancer, a kappa B domain capable of binding the prototypical form of the transcription factor nuclear factor kappa B (NF-kappa B) present in many cell types, including T lymphocytes. Tat transactivation mediated by the kappa B domain is sufficient to allow replication of TAR-deleted mutant HIV-1 in astrocytes. The present study demonstrates the existence of kappa B-specific binding factors present in human glial astrocytes that differ from prototypical NF-kappa B. The novel astrocyte-derived kappa B-binding activity is retained on an HIV-1 Tat affinity column, while prototypical NF-kappa B from Jurkat T cells is not. In vitro transcription studies demonstrate that astrocyte-derived kappa B-binding factors activate transcription of the HIV-1 long terminal repeat and that this activation is dependent on the kappa B domain. Moreover, TAR-independent transactivation of HIV-1 transcription is reproduced in vitro in an astrocyte factor-dependent manner which correlates with kappa B-binding activity. The importance of the central nervous system-enriched kappa B transcription factor in the regulation of HIV-1 expression is discussed.

Transcriptional activation of minimal HIV-1 promoter by ORF-1 protein expressed from the SalI-L fragment of human herpesvirus 6

The SalI-L fragment of human herpesvirus 6 (HHV-6) strain U1102 transformed rodent cells and transactivated the HIV-1 LTR 10- to 15-fold in both monkey fibroblasts and human T-lymphocytes. In this report, the SalI-L transactivator of the HIV-1 LTR was localized to ORF-1 which codes for a protein of 357 amino acids. To determine if ORF-1 required functional Sp1 binding sites or the TATA box element of HIV-1 LTR for transactivation, 5'-deletion mutants of the HIV-1 LTR were employed. Plasmids pBS/SalI-L, pBS/SalI-L-SH, and pC6/ORF-1(S), a mammalian expression vector containing ORF-1, all transactivated a deletion mutant of HIV-1 LTR lacking functional Sp1 binding sites (CD-54). These studies demonstrate that transactivation occurred in the absence of Sp1 binding sites and required only a minimal HIV-1 promoter which contains the TATA box element. The specificity of the SalI-L transactivator for HIV-1 LTR was demonstrated by its inability to transactivate the human papillomavirus type 16 or 18 early promoters. The ORF-1 gene was cloned into and expressed from the pET17b bacterial expression vector. Purified ORF-1 protein was obtained by ammonium sulfate precipitation, Mono-S chromatography, and anti-T7. Tag immunoaffinity chromatography. Transactivation of the HIV-1 LTR by ORF-1 protein was demonstrated by electroporation studies in vivo and by transcription studies in vitro. To substantiate the putative biological role of ORF-1, pBS/SalI-L, pBS/SalI-L-SH, and pC6/ORF-1 all reactivated tat-defective HIV-1 provirus from latently infected cells expressing CD4. Thus, the data presented suggest that HHV-6 infection could have a cofactor role in the progression of AIDS.

Second-site long terminal repeat (LTR) revertants of replication-defective human immunodeficiency virus: effects of revertant TATA box motifs on virus infectivity, LTR-directed expression, in vitro RNA synthesis, and binding of basal transcription factors TFIID and TFIIA

Second-site revertants from replication-incompetent molecular clones of human immunodeficiency virus (HIV) contain base substitutions adjacent to the TATA motif. The altered TATA box motifs were analyzed for their effect(s) on virus infectivity, long terminal repeat (LTR)-directed expression in transient transfection assays, in vitro RNA synthesis, and assembly of the TFIID-TFIIA preinitiation complex. The revertant TATA boxes accelerated the kinetics of HIV replication when present in the context of an LTR containing a Sp1 mutation (deletion or site specific); no effect was observed on the infectivity of wild-type HIV. In chloramphenicol acetyltransferase assays and in vitro transcription systems, the altered TATA box motifs led to elevated basal levels of RNA synthesis from NF-kappa B- and Sp1-mutagenized and wild-type templates, respectively, but did not increase responsiveness to Tat transactivation. The revertant TATA boxes accelerated the binding of TFIID and TFIIA to the LTR and stabilized their association with the promoter. The revertants did not assemble a more-processive elongation complex. These results suggest that in the context of an impaired enhancer/promoter (viz., three mutated Sp1 elements), a series of HIV revertants emerge which contain LTR alterations that significantly augment basal RNA synthesis. The TATA motif revertants are capable of rescuing the enhancer/promoter defect and sustain virus infectivity.

A transforming fragment within the direct repeat region of human herpesvirus type 6 that transactivates HIV-1

HHV-6 infection has been associated with several malignancies including non-Hodgkin's lymphoma and Hodgkin's disease by the presence of high antibody titer and/or the presence of HHV-6 DNA. To understand their oncogenic potential, SalI restriction fragments from HHV-6 strain U1102 were transfected into NIH3T3 cells to assess transforming ability. A 3.9-kbp SalI-L DNA fragment spanning the junction of the direct repeat left (DRL) and unique long segment (UL) regions of HHV-6 induced foci of morphologically altered cells. The SalI-L transformed NIH3T3 focal lines induced tumors in nude mice within 2 weeks. The retention of HHV-6 specific DNA observed in SalI-L transformed cells and their tumor-derived lines suggest a possible maintenance function. Since both HHV-6 infection as well as transforming fragments from other DNA viruses have been shown to transactivate the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR), SalI-L was examined for transactivation activity. SalI-L up-regulated HIV-1 LTR CAT 10-15 fold in both monkey CV-1 and human T Jurkat cells. The further study of the SalI-L transforming fragment exhibiting transactivation of HIV-1 LTR will elucidate whether these two activities are encoded by a single gene and will aid in the understanding of the interaction between HHV-6 and HIV-1 as it relates to progression of AIDS and/or AIDS-related malignancies.

Direct interaction of human TFIID with the HIV-1 transactivator tat

The tat gene of the human immunodeficiency virus (HIV) plays a central role in the activation and life cycle of HIV. The tat protein (Tat) specifically transactivates HIV transcription in vivo and in vitro, exerting its effects at the level of transcriptional initiation and elongation. Here we report that Tat binds directly to the basal transcription factor TFIID. The transcriptional activity of HeLa extracts was depleted after chromatography on a Tat affinity column, which specifically retained the polymerase II-specific factor TFIID. Direct interaction of Tat with holo-TFIID, composed of TATA-binding protein (TBP) and associated factors (TAFs), was observed. Tat binds, through amino acids 36-50, directly to the TBP subunit of TFIID. Our results suggest that Tat may transduce upstream or downstream regulatory signals by direct interaction with the basal transcription factor TFIID.

Involvement of transcription factor YB-1 in human T-cell lymphotropic virus type I basal gene expression

Sequences which control basal human T-cell lymphotropic virus type I (HTLV-I) transcription likely play an important role in initiation and maintenance of virus replication. We previously identified and analyzed a 45-nucleotide sequence (downstream regulatory element 1 [DRE 1]), +195 to +240, at the boundary of the R/U5 region of the long terminal repeat which is required for HTLV-I basal transcription. We identified a protein, p37, which specifically bound to DRE 1. An affinity column fraction, containing p37, stimulated HTLV-I transcription approximately 12-fold in vitro. We now report the identification of a cDNA clone (15B-7), from a Jurkat expression library, that binds specifically to the DRE 1 regulatory sequence. Binding of the cDNA fusion protein, similarly to the results obtained with purified Jurkat protein, was decreased by introduction of site-specific mutations in the DRE 1 regulatory sequence. In vitro transcription and translation of 15B-7 cDNA produced a fusion protein which bound specifically to the HTLV-I +195 to +240 oligonucleotide. The partial cDNA encodes a protein which is homologous to the C-terminal 196 amino acids of the 36-kDa transcription factor, YB-1. Cotransfection of a YB-1 expression plasmid increases HTLV-I basal transcription approximately 14-fold in Jurkat T lymphocytes. On the basis of the molecular weight, DNA-binding characteristics, and in vivo transactivation activity, we suggest that the previously identified DRE 1-binding protein, p37, is YB-1.

Transcription of the HIV-1 LTR is regulated by the density of DNA CpG methylation

Transcription from the HIV-1 long terminal repeat (LTR) was shown to be inhibited by DNA CpG methylation both in vivo and in vitro. Enzymatic methylation of CpG sites localized within the LTR decreased the transcription of the CAT reporter gene, chloramphenicol acetyltransferase, as assayed by the transient expression of this gene in tissue culture. The inhibitory effect could be initially overcome, in trans, by the transactivator tat. As a function of time, the presence of tat had no observable effect on transcription, within the limits of detection sensitivity, suggesting that the level of basal transcription was reduced to very low levels. This effect is suggestive of the involvement of cellular CpG methylation-dependent inhibitory factors which have been characterized by other laboratories. These data imply that transactivation is reduced to low levels after longer periods of time when the DNA template is sparsely methylated. The transcriptional inhibitory process may involve proteins such as MeCP which may interact with methylated DNA more slowly and/or weakly. Conversely, densely methylated DNA was transcriptionally repressed immediately which suggests the rapid/strong association of the cellular inhibitory factor(s). The transcriptional inhibitory effect was also observed in an in vitro transcription run-off system. These data suggest that the methylation-mediated inhibition of transcription is directly affected by CpG methylation density and may involve other factors.

Sequences downstream of the RNA initiation site regulate human T-cell lymphotropic virus type I basal gene expression

Sequences which control basal human T-cell lymphotropic virus type I (HTLV-I) transcription probably play an important role in initiation and maintenance of virus replication. We have identified and analyzed a 45-nucleotide sequence (downstream regulatory element 1 [DRE 1]) at the boundary of the R/U5 region of the long terminal repeat which is required for HTLV-I basal transcription. The basal promoter strength of constructs that contained deletions in the R/U5 region of the HTLV-I long terminal repeat were analyzed by chloramphenicol acetyltransferase assays following transfection of Jurkat T cells. We consistently observed a 10-fold decrease in basal promoter activity when sequences between +202 to +246 were deleted. By reverse transcriptase polymerase chain reaction RNA analysis, we confirmed that the drop in chloramphenicol acetyltransferase activity was paralleled by a decrease in the level of steady-state RNA. DRE 1 did not affect the level of Tax1 transactivation. Using a gel shift assay, we have purified a highly enriched fraction that could specifically bind DRE 1. This DNA affinity column fraction contained four detectable proteins on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis: p37, p50, p60, and p100. The affinity column fraction stimulated HTLV-I transcription approximately 12-fold in vitro. No effect was observed with the human immunodeficiency virus or adenovirus major late promoters. Following renaturation of the proteins isolated from an SDS-containing gel, p37, but not the other protein fractions, was able to specifically bind to DRE 1.

Electroporation of viral transactivator proteins into lymphocyte suspension cells

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Activation of bovine immunodeficiency-like virus expression by bovine herpesvirus type 1

Bovine immunodeficiency-like virus (BIV) is a recently identified lentivirus that infects cattle. The virus has structural and genetic similarities to human HIV. The present study demonstrates that BIV can be activated by bovine herpesvirus type 1 (BHV-1), a pathogen frequently associated with cattle diseases. Activation of BIV expression can be detected as increased BIV reverse transcriptase activity, increased in the number of syncytia induced by BIV, and increased in the steady state level of BIV-specific RNA upon BHV-1 super-infection. Additional transactivation studies using the BIV-LTR (long terminal repeat) were conducted. The BIV-LTR was linked to the chloramphenicol acetyl transferase gene (CAT) and transfected into bovine cell cultures in order to quantitate the levels of BIV-LTR expression. When the transfected cells were infected by BHV-1, there was an increase in CAT expression, indicating transactivation of the BIV-LTR by BHV-1. Most of the transactivation activities were abolished with an LTR construct that has deleted the NF-kappa B-like sequence located in the U3 region of the LTR. In order to further demonstrate that activation of the BIV-LTR involves factors that may bind to the LTR sequences, gel retardation assays were carried out using the BIV-LTR U3 region as probe. Our results showed that BHV-1 infection resulted in an induction of factor(s) that binds to the NF-kappa B-like sequence on the BIV-LTR. This suggests that transactivation of BIV by BHV-1 may be mediated by a bovine NF-kappa B-like protein that binds to the target sequence in the BIV promoter region.