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Query: UMLS:C0019693 (HIV)
 170,526 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The human positive transcription elongation factor P-TEFb is composed of two subunits, cyclin T1 (hCycT1) and CDK9, and is involved in transcriptional regulation of cellular genes as well as human immunodeficiency virus type 1 (HIV-1) mRNA. Replication of HIV-1 requires the Tat protein, which activates elongation of RNA polymerase II at the HIV-1 promoter by interacting with hCycT1. To understand the cellular functions of P-TEFb and to test whether suppression of host proteins such as P-TEFb can modulate HIV infectivity without causing cellular toxicity or lethality, we used RNA interference (RNAi) to specifically knock down P-TEFb expression by degrading hCycT1 or CDK9 mRNA. RNAi-mediated gene silencing of P-TEFb in HeLa cells was not lethal and inhibited Tat transactivation and HIV-1 replication in host cells. We also found that CDK9 protein stability depended on hCycT1 protein levels, suggesting that the formation of P-TEFb CDK-cyclin complexes is required for CDK9 stability. Strikingly, P-TEFb knockdown cells showed normal P-TEFb kinase activity. Our studies suggest the existence of a dynamic equilibrium between active and inactive pools of P-TEFb in the cell and indicate that this equilibrium shifts towards the active kinase form to sustain cell viability when P-TEFb protein levels are reduced. The finding that a P-TEFb knockdown was not lethal and still showed normal P-TEFb kinase activity suggested that there is a critical threshold concentration of activated P-TEFb required for cell viability and HIV replication. These results provide new insights into the regulation of P-TEFb function and suggest the possibility that similar mechanisms for monitoring protein levels to modulate the activity of proteins may exist for the regulation of a variety of other enzymatic pathways.
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PMID:Inhibition of human immunodeficiency virus type 1 replication by RNA interference directed against human transcription elongation factor P-TEFb (CDK9/CyclinT1). 1496 54

HIV replication occurs principally in activated CD4+ T cells and macrophages. The HIV-1 Tat protein is essential for HIV replication and requires a cellular protein kinase activity termed TAK/P-TEFb, composed of CDK9 and cyclin T1, for its transactivation function. This article reviews recent work indicating that under some circumstances TAK/P-TEFb is likely to be limiting for HIV replication in CD4+ T cells and macrophages, and discusses mechanisms of regulation of the TAK/P-TEFb subunits in these cell types. In resting CD4+ T lymphocytes, TAK/P-TEFb function is low. Following lymphocyte activation, even under conditions of minimal activation in which activation markers and cellular proliferation are not induced, both CDK9 and cyclin T1 mRNA and protein levels are increased, leading to an induction of TAK/P-TEFb kinase activity that correlates with increased viral replication. In macrophages, regulation of TAK/P-TEFb involves mechanisms distinct from those in lymphocytes. In freshly isolated monocytes, CDK9 protein levels are high, while cyclin T1 protein levels are low to undetectable. Cyclin T1 protein expression is up-regulated during early macrophage differentiation by a mechanism that involves post-transcriptional regulation. Later during differentiation, cyclin T1 expression becomes shut off by a post-transcriptional mechanism, and this correlates with a decrease in Tat transactivation. Interestingly, cyclin T1 can be re-induced with lipopolysaccharide (LPS). These findings suggest that changes in cyclin T1 expression can influence HIV-1 replication levels in monocytes and macrophages. Important areas for future research on Tat and TAK/P-TEFb function are discussed.
Curr HIV Res 2003 Oct
PMID:Regulation of TAK/P-TEFb in CD4+ T lymphocytes and macrophages. 1504 26

Human immunodeficiency virus type 1 (HIV-1) gene expression and transcription is an essential step in the viral life cycle, which is considered to be a possible target for inhibition of HIV-1 replication. Among the factors involved in this step, the cellular transcription factor nuclear factor (NF)-kappaB is the most potent inducer of HIV-1 gene expression, while the viral transactivator protein Tat seems to play a central role in sustaining a high level of HIV-1 replication. Another important mechanism of HIV-1 gene expression is the nuclear export control of viral mRNA conducted by the viral regulatory protein Rev. Various attempts have been undertaken to discover selective inhibitors of HIV-1 gene expression and transcription. Several small-molecule compounds were reported to inhibit Tat functions though blocking either the Tat/TAR RNA interaction or the kinase activity of cellular cofactors, such as cyclin T1/CDK9. In the case of Rev inhibitors, it appears to be more difficult to find them than Tat inhibitors, and only a few compounds have been identified as Rev inhibitors. However, the selectivity of these Tat and Rev inhibitors was not high enough to eliminate the cytotoxicity to the host cells. Since the signal transduction pathways leading to NF-kappaB activation are redox regulated, several antioxidants have been shown to block HIV-1 transcription. Although some of them have progressed into clinical trials in HIV-1-infected patients, the results were not conclusive. In addition, various compounds have been identified as inhibitors of HIV-1 gene expression and transcription, yet their precise mechanisms are still unknown.
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PMID:Inhibitors of HIV-1 gene expression and transcription. 1513 46

The HEXIM1 protein inhibits the kinase activity of P-TEFb (CDK9/cyclin T) to suppress RNA polymerase II transcriptional elongation in a process that specifically requires the 7SK snRNA, which mediates the interaction of HEXIM1 with P-TEFb. In an attempt to define the sequence requirements for HEXIM1 to interact with 7SK and inactivate P-TEFb, we have identified the first 18 amino acids within the previously described nuclear localization signal (NLS) of HEXIM1 as both necessary and sufficient for binding to 7SK in vivo and in vitro. This 7SK-binding motif was essential for HEXIM1's inhibitory action, as the HEXIM1 mutants with this motif replaced with a foreign NLS failed to interact with 7SK and P-TEFb and hence were unable to inactivate P-TEFb. The 7SK-binding motif alone, however, was not sufficient to inhibit P-TEFb. A region C-terminal to this motif was also required for HEXIM1 to associate with P-TEFb and suppress P-TEFb's kinase and transcriptional activities. The 7SK-binding motif in HEXIM1 contains clusters of positively charged residues reminiscent of the arginine-rich RNA-binding motif found in a wide variety of proteins. Part of it is highly homologous to the TAR RNA-binding motif in the human immunodeficiency virus type 1 (HIV-1) Tat protein, which was able to restore the 7SK-binding ability of a HEXIM1 NLS substitution mutant. We propose that a similar RNA-protein recognition mechanism may exist to regulate the formation of both the Tat-TAR-P-TEFb and the HEXIM1-7SK-P-TEFb ternary complexes, which may help convert the inactive HEXIM1/7SK-bound P-TEFb into an active one for Tat-activated and TAR-dependent HIV-1 transcription.
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PMID:A human immunodeficiency virus type 1 Tat-like arginine-rich RNA-binding domain is essential for HEXIM1 to inhibit RNA polymerase II transcription through 7SK snRNA-mediated inactivation of P-TEFb. 1516 77

The HIV transcriptional activator Tat enhances the processivity of RNA polymerase II by recruiting the CyclinT1/CDK9 complex to the TAR RNA element. In addition, Tat synergizes with the histone acetyltransferase p300 and is acetylated by p300 at a single lysine residue (K50) in the TAR RNA binding domain. We have recently reported that this post-translational modification is necessary for the interaction and transcriptional synergy of Tat with the transcriptional coactivator PCAF. We have further studied the relevance of Tat acetylation during HIV transcription and generated antibodies specific for acetylated Tat (AcTat). Microinjection of anti-AcTat antibodies inhibited Tat-mediated transactivation in cells. Similarly, the specific p300 inhibitor Lys-CoA and short inhibitory RNAs specific for p300 suppressed Tat transcriptional activity. Full-length synthetic AcTat bound to TAR RNA and CyclinT1 with high affinity, but formation of the Tat-TAR-CyclinT1 ternary complex was inhibited when K50 was acetylated. Our data collectively show that Tat acetylation by p300 defines a critical step in Tat transactivation that serves to disrupt the Tat/TAR/CyclinT1 complex and helps in recruiting PCAF to the elongating RNA polymerase II.
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PMID:Tat acetylation: a regulatory switch between early and late phases in HIV transcription elongation. 1517 Dec 54

The macrophage is an important cell type in the pathophysiology of human immunodeficiency virus type 1 (HIV-1) infection. Macrophages both support viral replication and are capable of attracting and activating lymphocytes, thus rendering CD4+ T lymphocytes highly permissive for infection. The viral Tat protein, whose function is mediated by the cellular cyclin T1 protein complexed with CDK9, is required for efficient transcription of the integrated HIV-1 provirus by RNA polymerase II. Cyclin T1 expression is highly regulated during macrophage differentiation, and this has important implications for HIV-1 replication. In monocytes isolated from healthy blood donors, cyclin T1 protein expression is low and is induced to high levels within the first few days of differentiation by a post-transcriptional mechanism. After 1-2 weeks of macrophage differentiation, however, cyclin T1 expression is shut off. Treatment of macrophages with lipopolysaccharide (LPS) can re-induce cyclin T1, indicating that the activation status of macrophages can regulate cyclin T1 expression. Recent results indicate that HIV-1 infection is able to induce cyclin T1 expression in macrophages. Future studies of cyclin T1 regulation in macrophages may suggest means of manipulating expression of this crucial cellular co-factor for therapeutic benefit in HIV-1 infected individuals.
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PMID:HIV-1 infection and regulation of Tat function in macrophages. 1518 43

The family of Cyclin-Dependent Kinases (CDKs) can be subdivided into two major functional groups based on their roles in cell cycle and/or transcriptional control. This review is centered on CDK9, which is activated by T-type cyclins and cyclin K generating distinct Positive-Transcription Elongation Factors termed P-TEFb. P-TEFb positively regulates transcriptional elongation by phosphorylating the C-terminal domain (CTD) of RNA polymerase II (RNA pol II), as well as negative elongation factors, which block elongation by RNA pol II shortly after the initiation of transcription. Work over the past few years has led to a dramatic increase in our understanding of how productive transcriptional elongation occurs. This review will briefly describe the mechanisms regulating the activity of T-type cyclin/CDK9 complexes and discuss how these complexes regulate gene expression. For further information, the reader is directed to excellent existing reviews on transcriptional elongation and HIV transcription.
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PMID:Cellular control of gene expression by T-type cyclin/CDK9 complexes. 1527 98

HIV-1 TAR RNA is the binding site of the viral protein Tat, the trans-activator of the HIV-1 LTR. It is present at the 5' end of all HIV-1 spliced and unspliced mRNAs in the nucleus as well as in the cytoplasm. It has a highly folded stem-bulge-loop structure, which also binds cellular proteins to form ribonucleoprotein complexes. The Tat-Cyclin T1-CDK9 complex is the main component in the trans-activation of HIV-1 and its affinity for TAR is regulated through Tat acetylation by histone acetyl transferases. Recent studies show that this complex is able to recruit other cellular partners to mediate efficient transcriptional elongation. TRBP, PKR and La bind directly to the TAR RNA structure and influence translation of HIV-1 in either positive or negative manners. Some mutations in TAR RNA severely impair HIV-1 trans-activation, translation and viral production, showing its functional importance. The overexpression or suppression of several TAR RNA-binding proteins has a strong impact on viral replication pointing out their major role in the viral life cycle. TAR RNA has been the target of drug development to inhibit viral replication. Recent data using small molecules or RNA-based technologies show that acting on the TAR RNA or on its viral and cellular binding factors effectively decreases virion production.
Curr HIV Res 2005 Jan
PMID:HIV-1 TAR RNA: the target of molecular interactions between the virus and its host. 1563 24

HIV-1 Tat binds human CyclinT1 and recruits the CDK9/P-TEFb complex to the viral TAR RNA in a step that links RNA polymerase II (RNAPII) C-terminal domain (CTD) Ser 2 phosphorylation with transcription elongation. Previous studies have suggested a connection between Tat and pre-mRNA splicing factors. Here we show that the splicing-associated c-Ski-interacting protein, SKIP, is required for Tat transactivation in vivo and stimulates HIV-1 transcription elongation, but not initiation, in vitro. SKIP associates with CycT1:CDK9/P-TEFb and Tat:P-TEFb complexes in nuclear extracts and interacts with recombinant Tat:P-TEFb:TAR RNA complexes in vitro, indicating that it may act through nascent RNA to overcome pausing by RNAPII. SKIP also associates with U5snRNP proteins and tri-snRNP110K in nuclear extracts, and facilitates recognition of an alternative Tat-specific splice site in vivo. The effects of SKIP on transcription elongation, binding to P-TEFb, and splicing are mediated through the SNW domain. HIV-1 Tat transactivation is accompanied by the recruitment of P-TEFb, SKIP, and tri-snRNP110K to the integrated HIV-1 promoter in vivo, whereas the U5snRNPs associate only with the transcribed coding region. These findings suggest that SKIP plays independent roles in transcription elongation and pre-mRNA splicing.
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PMID:A human splicing factor, SKIP, associates with P-TEFb and enhances transcription elongation by HIV-1 Tat. 1590 9

Macrophage transcription is significantly altered by HIV-1 infection. HIV Tat, an immediate-early product of the viral lifecycle, interacts with host transcription factors to alter host gene expression. We have previously shown that Tat represses transcription from the mannose receptor (MR) and the bone morphogenetic protein receptor-2 (BMPR2) promoters. The current study shows that transcriptional repression of these receptors involves Tat interaction with cyclin T1. Assays using U937 human monocytic cells transiently expressing MR or BMPR2 promoter-luciferase constructs demonstrated equal repression by one- and two-exon Tat gene products. A mutant Tat expression vector encoding Tat protein lacking the cyclin T1 binding domain failed to inhibit MR and BMPR2 promoter activities. Over-expression of cyclin T1 in the presence of wild-type Tat resulted in recovered activity from both promoters. Finally, two inhibitors of cyclin-dependent kinase 9 (a dominant negative CDK9 and flavopiridol) repressed activity from the MR and BMPR2 promoters.
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PMID:HIV-1 Tat interaction with cyclin T1 represses mannose receptor and the bone morphogenetic protein receptor-2 transcription. 1661 32


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