|HIV-1 infection and regulation of Tat function in macrophages. |
Liou, Li-Ying, et al.
Int. J. Biochem. Cell Biol., 36: 1767-75 (2004)
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.
|Cellular control of gene expression by T-type cyclin/CDK9 complexes. |
Garriga, Judit and Graña, Xavier
Gene, 337: 15-23 (2004)
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.
|Regulation of TAK/P-TEFb in CD4+ T lymphocytes and macrophages. |
Rice, Andrew P and Herrmann, Christine H
Curr. HIV Res., 1: 395-404 (2003)
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.
|Cyclins that don't cycle--cyclin T/cyclin-dependent kinase-9 determines cardiac muscle cell size. |
Sano, Motoaki and Schneider, Michael D
Cell Cycle, 2: 99-104 (2003)
A subset of cyclin-dependent protein kinases--Cdk7, Cdk8, and Cdk9--participates directly, in complex ways, with the fundamental machinery for gene transcription, as elements of general transcription factors whose substrate is the C-terminal domain (CTD) of RNA polymerase II. Here, we review recent data implicating the CTD kinase Cdk9 as a critical determinant of cardiac hypertrophy, in vitro and in vivo. Diverse trophic signals that increase cardiac mass all activated Cdk9 (work load, the small G-protein Gaq, and the calcium-dependent phosphatase calcineurin in mouse myocardium; endothelin-1, a hypertrophic agonist, in cultured cardiomyocytes). Little or no change occurred in levels of the kinase or its activator, cyclin T. Instead, in all four hypertrophic models, Cdk9 activation involves the dissociation of 7SK small nuclear RNA (snRNA), an endogenous inhibitor. In culture, dominant-negative Cdk9 blocked ET-1-induced hypertrophy, whereas an anti-sense "knockdown" of 7SK snRNA provoked spontaneous cell growth. In trans-genie mice, concordant with these results, activation of Cdk9 activity via cardiac-specific overexpression of cyclin Tl suffices to provoke hypertrophy. Together, these findings implicate Cdk9 activity as a pivotal regulator of pathophysiological heart growth. Because hypertrophy, in turn, is a cardinal risk factor for developing cardiac pump failure, these results support the logic of examining Cdk9 as a potential drug target in heart disease.