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Molecular and Cellular Biology, December 1998, p. 7086-7094, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Regulation of RNA Polymerase I-Dependent Promoters
by the Hepatitis B Virus X Protein via Activated Ras and
TATA-Binding Protein
Horng-Dar
Wang,
Alpa
Trivedi, and
Deborah L.
Johnson*
Departments of Molecular Pharmacology and
Biochemistry, Schools of Pharmacy and Medicine, University of
Southern California, Los Angeles, California
Received 24 July 1998/Returned for modification 18 August
1998/Accepted 17 September 1998
The hepatitis B virus (HBV) X protein is essential for viral
infectivity, and evidence indicates that it is a strong contributor to
HBV-mediated oncogenesis. X has been shown to transactivate a wide
variety of RNA polymerase (Pol) II-dependent, as well as RNA Pol
III-dependent, promoters. In this study, we have investigated the
possibility that X modulates RNA Pol I-dependent rRNA transcription. In
both human hepatoma Huh7 and Drosophila Schneider S2 cell
lines, X expression stimulated rRNA promoter activity. Extracts
prepared from X-expressing cells stably transfected with an
X gene also exhibited an increased ability to transcribe
the rRNA promoter. The mechanism for X transactivation was examined by
determining whether this regulatory event was dependent on Ras
activation and increased TATA-binding protein (TBP) levels. Our
previous studies have demonstrated that X, and the activation of Ras,
produces an increase in the cellular levels of TBP (H.-D. Wang, A. Trivedi, and D. L. Johnson, Mol. Cell. Biol. 17:6838-6846, 1997).
Expression of a dominant negative form of Ras blocked the X-mediated
induction of the rRNA promoters, whereas expression of a constitutively activated form of Ras mimicked the enhancing effect of X on rRNA promoter activity. When TBP was overexpressed in either Huh7 or S2
cells, a dose-dependent increase in rRNA promoter activity was
observed. To analyze whether the increase in TBP was modulating rRNA
promoter activity indirectly, by increasing activity of RNA Pol
II-dependent promoters, a Drosophila TBP cDNA was
constructed with a mutation that eliminated its ability to stimulate
RNA Pol II-dependent promoters. Transient expression of wild-type TBP in S2 cells increased the activities of specific RNA Pol I- and Pol
II-dependent promoters. Expression of the mutant TBP protein failed to
enhance the activity of the RNA Pol II-dependent promoters, yet the
protein completely retained its ability to stimulate the rRNA promoter.
Furthermore, the addition of recombinant TBP to S2 extracts stimulated
rRNA promoter activity in vitro. Together, these results demonstrate
that the HBV X protein up-regulates RNA Pol I-dependent promoters via a
Ras-activated pathway in two distinct cell lines. The enhanced promoter
activity can, at least in part, be attributed to the X- and
Ras-mediated increase in cellular TBP, a limiting transcription component.
*
Corresponding author. Mailing address: University of
Southern California, Departments of Molecular Pharmacology and
Biochemistry, Schools of Pharmacy and Medicine, 1985 Zonal Avenue,
PSC-402, Los Angeles, CA 90033. Phone: (323) 442-1446. Fax: (323)
442-1681. E-mail: johnsond{at}hsc.usc.edu.
Molecular and Cellular Biology, December 1998, p. 7086-7094, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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