A nuclear factor, ASC-2, as a cancer-amplified transcriptional coactivatoressential for ligand-dependent transactivation by nuclear receptors in vivo

Many transcription coactivators interact with nuclear receptors in a ligand - and C-terminal transactivation function (AF2)-dependent manner. We isolat ed a nuclear factor (designated ASC-2) with such properties by using the li gand-binding domain of retinoid X receptor as a bait in a yeast two-hybrid screening. ASC-2 also interacted with other nuclear receptors, including re tinoic acid receptor, thyroid hormone receptor, estrogen receptor alpha, an d glucocorticoid receptor, basal factors TFIIA and TBP, and transcription i ntegrators CBP/p300 and SRC-1. In transient cotransfections, ASC-2, either alone or in conjunction with CBP/p300 and SRC-1, stimulated ligand dependen t transactivation by wild type nuclear receptors but not mutant receptors l acking the AF2 domain. Consistent with an idea that ASC-2 is essential for the nuclear receptor function in vivo, microinjection of anti-ASC-2 antibod y abrogated the ligand-dependent transactivation of retinoic acid receptor, and this repression was fully relieved by coinjection of ASC-2-expression vector. Surprisingly, ASC-2 was identical to a gene previously identified d uring a search for genes amplified and overexpressed in breast and other hu man cancers. From these results, we concluded that ASC-2 is a bona fide tra nscription coactivator molecule of nuclear receptors, and its altered expre ssion may contribute to the development of cancers.