Flavopiridol Inactivates P-TEFb and Blocks Most RNA Polymerase II Transcription in Vivo*
- From the ‡Molecular Biology Program and the§Department of Biochemistry, University of Iowa, Iowa City, Iowa 52242
Abstract
Flavopiridol (L86-8275, HMR1275) is a cyclin-dependent kinase (Cdk) inhibitor in clinical trials as a cancer therapy that has been recently shown to block human immunodeficiency virus Tat transactivation and viral replication through inhibition of positive transcription elongation factor b (P-TEFb). Flavopiridol is the most potent P-TEFb inhibitor reported and the first Cdk inhibitor that is not competitive with ATP. We examined the ability of flavopiridol to inhibit P-TEFb (Cdk9/cyclin T1) phosphorylation of both RNA polymerase II and the large subunit of the 5, 6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) sensitivity-inducing factor and found that the IC50determined was directly related to the concentration of the enzyme. We concluded that the flavonoid associates with P-TEFb with 1:1 stoichiometry even at concentrations of enzyme in the low nanomolar range. These results indicate that the apparent lack of competition with ATP could be caused by a very tight binding of the drug. We developed a novel immobilized P-TEFb assay and demonstrated that the drug remains bound for minutes even in the presence of high salt. Flavopiridol remained bound in the presence of a 1000-fold excess of the commonly used inhibitor DRB, suggesting that the immobilized P-TEFb could be used in a simple screening assay that would allow the discovery or characterization of compounds with binding properties similar to flavopiridol. Finally, we compared the ability of flavopiridol and DRB to inhibit transcription in vivo using nuclear run-on assays and concluded that P-TEFb is required for transcription of most RNA polymerase II molecules in vivo.
Footnotes
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↵* This work was supported by National Institute of Heath Grants AI43691 and GM35500.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵¶ To whom correspondence could be addressed. Tel.: 319-335-7910; E-mail: david-price@uiowa.edu.
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Published, JBC Papers in Press, June 28, 2001, DOI 10.1074/jbc.M102306200
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↵2 D. Renner, Y. Yamaguchi, T. Wada, H. Handa, and D. Price, submitted for publication.
- Abbreviations:
- Cdk
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cyclin-dependent kinase
- P-TEFb
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positive transcription elongation factor b
- DRB
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dichloro-1-β-d-ribofuranosylbenzimidazole
- DSIF
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DRB sensitivity-inducing factor
- HIV
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human immunodeficiency virus
- SDS-PAGE
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SDS-polyacrylamide gel electrophoresis
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- Received March 14, 2001.
- Revision received June 15, 2001.
- The American Society for Biochemistry and Molecular Biology, Inc.
