Inhibition of human immunodeficiency virus type 1 replication by RNA interference directed against human transcription elongation factor P-TEFb (CDK9/CyclinT1)
Biochemistry & Molecular Pharmacology
Graduate School of Biomedical Sciences; Department of Biochemistry and Molecular Pharmacology; Program in Molecular Medicine
Medical Subject Headings
Cell Line; Cyclin-Dependent Kinase 9; Cyclins; Down-Regulation; Enzyme Stability; *Gene Expression Regulation, Viral; Gene Products, tat; Genes, Lethal; HIV-1; Hela Cells; Humans; Positive Transcriptional Elongation Factor; B; Protein Subunits; *RNA Interference; RNA, Messenger; RNA, Small Interfering; Trans-Activation (Genetics); *Virus Replication; tat Gene Products, Human Immunodeficiency Virus
Life Sciences | Medicine and Health Sciences
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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Citation: J Virol. 2004 Mar;78(5):2517-29.