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Molecular and Cellular Biology, October 2006, p. 7068-7076, Vol. 26, No. 19
0270-7306/06/$08.00+0     doi:10.1128/MCB.00778-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Modulation of a P-TEFb Functional Equilibrium for the Global Control of Cell Growth and Differentiation

Nanhai He,{dagger} Andrea C. Pezda,{dagger} and Qiang Zhou*

Department of Molecular and Cell Biology, University of California, Berkeley, California 94720

Received 3 May 2006/ Returned for modification 15 June 2006/ Accepted 17 July 2006

P-TEFb phosphorylates RNA polymerase II and negative elongation factors to stimulate general transcriptional elongation. It is kept in a functional equilibrium through alternately interacting with its positive (the Brd4 protein) and negative (the HEXIM1 protein and 7SK snRNA) regulators. To investigate the physiological significance of this phenomenon, we analyzed the responses of HeLa cells and murine erythroleukemia cells (MELC) to hexamethylene bisacetamide (HMBA), which inhibits growth and induces differentiation of many cell types. For both cell types, an efficient, albeit temporary disruption of the 7SK-HEXIM1-P-TEFb snRNP and enhanced formation of the Brd4-P-TEFb complex occurred soon after the treatment started. When the P-TEFb-dependent HEXIM1 expression markedly increased as the treatment continued, the abundant HEXIM1 pushed the P-TEFb equilibrium back toward the 7SK/HEXIM1-bound state. For HeLa cells, as HMBA produced only a minor, temporary effect on their growth, the equilibrium gradually returned to its pretreatment level. In contrast, long-term treatment of MELC induced terminal division and differentiation. Concurrently, the P-TEFb equilibrium was shifted overwhelmingly toward the 7SK snRNP side. Together, these data link the P-TEFb equilibrium to the intracellular transcriptional demand and proliferative/differentiated states of cells.

* Corresponding author. Mailing address: Department of Molecular and Cell Biology, University of California, 622 Barker Hall #3202, Berkeley, CA 94720. Phone: (510) 643-0494. Fax: (510) 643-6344. E-mail: qzhou{at}berkeley.edu.

{dagger} These two authors contributed equally to this work.

Molecular and Cellular Biology, October 2006, p. 7068-7076, Vol. 26, No. 19
0270-7306/06/$08.00+0     doi:10.1128/MCB.00778-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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