Mouse Zac1, a Transcriptional Coactivator and Repressor for Nuclear Receptors

Abstract

Transcriptional activation by nuclear hormone receptors is mediated by the 160-kDa family of nuclear receptor coactivators. These coactivators associate with DNA-bound nuclear receptors and transmit activating signals to the transcription machinery through two activation domains. In screening for mammalian proteins that bind the C-terminal activation domain of the nuclear receptor coactivator GRIP1, we identified a new variant of mouse Zac1 which we call mZac1b. Zac1 was previously discovered as a putative transcriptional activator involved in regulation of apoptosis and the cell cycle. In yeast two-hybrid assays and in vitro, mZac1b bound to GRIP1, to CREB-binding protein (CBP) and p300 (which are coactivators for nuclear receptors and other transcriptional activators), and to nuclear receptors themselves in a hormone-independent manner. In transient-transfection assays mZac1b exhibited a transcriptional activation activity when fused with the Gal4 DNA binding domain, and it enhanced transcriptional activation by the Gal4 DNA binding domain fused to GRIP1 or CBP fragments. More importantly, mZac1b was a powerful coactivator for the hormone-dependent activity of nuclear receptors, including androgen, estrogen, glucocorticoid, and thyroid hormone receptors. However, with some reporter genes and in some cell lines mZac1b acted as a repressor rather than a coactivator of nuclear receptor activity. Thus, mZac1b can interact with nuclear receptors and their coactivators and play both positive and negative roles in regulating nuclear receptor function.

ACKNOWLEDGMENTS

We thank P. Webb and P. J. Kushner, W. Feng, and D. Pearce (University of California) for expression vectors and reporter genes for ER, TR, and MR, respectively; T.-P. Yao (Duke University) for the plasmid encoding GST-p300_1571–2414; A. O. Brinkmann (Erasmus University, Rotterdam, The Netherlands) and R. L. Miesfeld (University of Arizona) for AR expression vectors; G. L. Hager (National Institutes of Health) for 1471.1 cells; H. Ma and X. F. Ding (University of Southern California) for AR AF-1 and AF-2 expression vectors and for pGBT9.GRIP1_1121–1462, respectively; H. Hong and D. Chen (University of Southern California) for performing the initial stages of the yeast two-hybrid screen; and D. L. Johnson (University of Southern California) for critical reading of the manuscript.

This work was supported by U.S. Public Health Service grant DK55274 from the National Institutes of Health. S.-M. H. was supported by a predoctoral fellowship from the Defense Department, Taiwan, Republic of China.