Cell Growth & Differentiation, Vol 5, Issue 10 1137-1143, Copyright © 1994 by American Association of Cancer Research
Serum-responsive expression from the murine thymidine kinase promoter is specifically disrupted in a transformed cell line
DW Bradley, JL Fridovich-Keil, JM Gudas and AB Pardee
Dana-Farber Cancer Institute, Division of Cell Growth and Regulation, Boston, Massachusetts 02115.
Thymidine kinase (TK) gene expression is controlled in normal cells at both
the transcriptional and posttranscriptional levels. Together, these
regulatory systems mediate the 20-50-fold induction of TK mRNA observed as
cells traverse the G1-S boundary of the cell cycle. Previously, we have
reported that a "Yi" protein complex was observed to bind the mouse TK
promoter in a cell cycle-dependent manner in nontransformed cells (Q-P.
Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl. Acad. Sci. USA,
88: 1157-1161, 1991) and bound constitutively in transformed cells (D. W.
Bradley, Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl.
Acad. Sci. USA, 87: 9310-9314, 1990). Nonetheless, TK mRNA levels in these
cells continue to exhibit a marked S-specific induction (> 10 fold),
raising the question: what is the status of TK promoter-mediated, as
opposed to posttranscriptional, gene regulation in these transformed cells?
To address this question, we have used cell synchrony experiments involving
both transformed and nontransformed cells stably transfected with a TK
promoter-beta-globin reporter gene construct. We have found that, in marked
contrast to the tight regulation of reporter gene expression observed in
nontransformed cells (J. L. Fridovich-Keil, J. M. Gudas, Q-P. Dou, I.
Bouvard, and A. B. Pardee, Cell Growth & Differ., 2: 67-76, 1991),
reporter gene expression in the transformed cells is constitutive and,
therefore, closely parallels the presence of Yi DNA-binding activity. These
data are fully consistent with other recently published observations
concerning differential controls of TK transcriptional and
posttranscriptional regulation (J. M. Gudas, J. L. Fridovich-Keil, and A.
B. Pardee, Cell Growth & Regul., 4: 421-430, 1993) and support the
hypothesis that, in transformed cells, endogenous TK is regulated
predominantly at the posttranscriptional level.