Regulation of the mdm2 Oncogene by Thyroid Hormone Receptor

Abstract

The mdm2 gene is positively regulated by p53 through a p53-responsive DNA element in the first intron of the mdm2gene. mdm2 binds p53, thereby abrogating the ability of p53 to activate the mdm2 gene, and thus forming an autoregulatory loop of mdm2 gene regulation. Although the mdm2 gene is thought to act as an oncogene by blocking the activity of p53, recent studies indicate that mdm2 can act independently of p53 and block the G₁ cell cycle arrest mediated by members of the retinoblastoma gene family and can activate E2F1/DP1 and the cyclin A gene promoter. In addition, factors other than p53 have recently been shown to regulate the mdm2 gene. In this article, we report that thyroid hormone (T3) receptors (T3Rs), but not the closely related members of the nuclear thyroid hormone/retinoid receptor gene family (retinoic acid receptor, vitamin D receptor, peroxisome proliferation activation receptor, or retinoid X receptor), regulate mdm2 through the same intron sequences that are modulated by p53. Chicken ovalbumin upstream promoter transcription factor I, an orphan nuclear receptor which normally acts as a transcriptional repressor, also activates mdm2 through the same intron region of the mdm2gene. Two T3R-responsive DNA elements were identified and further mapped to sequences within each of the p53 binding sites of the mdm2 intron. A 10-amino-acid sequence in the N-terminal region of T3Rα that is important for transactivation and interaction with TFIIB was also found to be important for activation of the mdm2 gene response element. T3 was found to stimulate the endogenous mdm2 gene in GH4C1 cells. These cells are known to express T3Rs, and T3 is known to stimulate replication of these cells via an effect in the G₁ phase of the cell cycle. Our findings, which indicate that T3Rs can regulate the mdm2gene independently of p53, provide an explanation for certain known effects of T3 and T3Rs on cell proliferation. In addition, these findings provide further evidence for p53-independent regulation of mdm2 which could lead to the development of tumors from cells that express low levels of p53 or that express p53 mutants defective in binding to and activating the mdm2 gene.

ACKNOWLEDGMENTS

We thank A. J. Levine for Cosx1CAT, the Cosx1CAT deletion mutants, the p1634CAT vector control, monoclonal antibody 2A10 against human and rodent mdm2, and p53-null (10)1 cells. We also thank B. Vogelstein for vectors expressing wild-type human p53.

This research was supported by NIH grant DK16636 to H.H.S. During this work, V.D.-Y. was an Aaron Diamond Foundation fellow (grant HRI817-5332F), and this work was supported in part by The Aaron Diamond Foundation. Oligonucleotide synthesis was provided by the NYUMC General Clinical Research Center (NIH NCRR grant M01RR00096). H.H.S. and V.D.-Y. are members of the NYUMC Cancer Center (funded by grant CA16087). Sequence analysis and database searches were through the NYUMC Research Computing Resource, which received support from the National Science Foundation (grant DIR-8908095).