Time-Sensitive Reversal of Oncogene Expression

Summary

The role of viral oncoprotein expression in the maintenance of cellular transformation was examined as a function of time through controlled expression of SV40 T antigen (Tag). Expression of Tag in the submandibular gland of transgenic mice from the time of birth induced cellular transformation and extensive ductal hyperplasia by 4 months of age. The hyperplasia was reversed when Tag expression was silenced for 3 weeks. When Tag expression was silenced after 7 months, however, the hyperplasia persisted, even though Tag was absent. While the polyploidy of ductal cells could be reversed at 4 months of age, cells at 7 months of age remained polyploid even in the absence of Tag. These results support a model of time-dependent multistep tumorigenesis in which virally-transformed cells eventually lose their dependence on the viral oncoprotein for maintenance of the transformed state.

Citations

1. Dagmar Ewald, Minglin Li, Shimon Efrat, Gert Auer, Robert J. Wall, Priscilla A. Furth, Lothar Hennighausen (1996) Science, 271, 1384-1386.
2. Hennighausen, L., Wall, R.J., Tillamnn, U., Li, M. and Furth, P.A. (1995). Conditional gene expression in secretory tissues and skin of transgenic mice using the MMTV-LTR and the tetracycline responsive system. J. Cell. Biochem. 59:463-472.
3) Furth, P.A., St. Onge, L., Boger, H., Gruss, P., Gossen, M., Kistner, A., Bujard, H. and Hennighausen, L. (1994) Temporal control of gene expression in transgenic mice by a tertacycline responsive promoter. Proc. Natl. Acad. Sci. U.S.A. 91, 9302-9306.

Background

Tumorigenesis is a multistep process that is though to involve the successive activation of oncogenes and the inactivation of tumor suppressor genes. This hypothesis is strongly supported by experiments with transgenic mice expressing oncogenes under the control of various promoters. Such mice develop solitary tumors in a stochastic fashion, indicating that additional genetic or cellular events are required for the development of irreversible cellular transformation. However, the length of time required for oncogene-induced phenotypic changes to become irreversible is unknown because it has not been possible to inactivate the transforming transgenes.
We have recently established in transgenic mice the tetracycline responsive gene expression system. Transgenes are trancriptionally silent in the presence of tetracycline and active upon withdrawl of the drug.
Here we have used the tetracycline response system to examine the role of the SV40 T viral oncogene in maintaining the transformed phenotype as a function of time. In this system, the oncogene is transcribed in the absence of tetracycline and is silent in the presence of tetracycline.

The MMTV-tTa and tetop-Tag gene constructs

Figure 1

The MMTV-LTR targets tTA to striated ducts of the submandibular gland

Figure 2

T-antigen induces hyperplasia in striated ducts of the submandibular gland

Figure 3

reversal of T-antigen induced hyperplasia after silencing T-ag expression in 4 months old mice

Figure 4

key words: SV40 T-antigen, salivary gland, hyperplasia, inducible system, tetracycline, MMTV

Note: Further information on the MMTV-tTA and tetop-Tag transgenic mice. This Web site contains additional information about tetracycline responsive gene expression systems.

Contributed By

Lothar Hennighausen
National Institutes of Health
Phone: 301-435-8907
Fax: 301-496-0839
e-mail: mammary@nih,gov

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last update: September, 1996