E1A and a nuclear receptor corepressor splice variant (N-CoR _I ) are thyroid hormone receptor coactivators that bind in the corepressor mode

Abstract

Unliganded thyroid hormone (TH) receptors (TRs) and other nuclear receptors (NRs) repress transcription of hormone-activated genes by recruiting corepressors (CoRs), such as NR CoR (N-CoR) and SMRT. Unliganded TRs also activate transcription of THrepressed genes. Some evidence suggests that these effects also involve TR͞CoR contacts; however, the precise reasons that CoRs activate transcription in these contexts are obscure. Unraveling these mechanisms is complicated by the fact that it is difficult to decipher direct vs. indirect effects of TR-coregulator contacts in mammalian cells. In this study, we used yeast, Saccharomyces cerevisiae, which lack endogenous NRs and NR coregulators, to determine how unliganded TRs can activate transcription. We previously showed that adenovirus 5 early-region 1A coactivates unliganded TRs in yeast, and that these effects are blocked by TH. We show here that human adenovirus type 5 early region 1A (

Thyroid Hormone Response Element Organization Dictates the Composition of Active Receptor

“…The supernatant was collected, and the protein concentration was determined by the Lowry method using bovine serum albumin as a standard. Twenty micrograms of protein were used for the ␤-galactosidase assay, and transcriptional activities were expressed as Miller units/mg of protein, as described previously (35)(36)(37)(38). Data shown were pooled from three independent experiments and calculated as the means Ϯ S.E.…”

“…For reporter assays, transformants were grown in minimal medium (0.67% yeast nitrogen base, 2% glucose) supplemented with adenine and lysine (both at 40 mg/liter), as described previously (35)(36)(37)(38). The yeast transformants were isolated and grown in the appropriate minimal medium with added supplements as required.…”

“…Plasmids-The following plasmids have been described previously: T 3 -inducible ␤-galactosidase reporters for yeast transcription assays, TRE-F2x1, TRE-DR-4x1, and TRE-Palx1 and yeast (YEp56) expression vectors for wild type TR␤, RXR, and GRIP1 (35)(36)(37)(38); T 3 -inducible reporters containing two copies of each TRE driving luciferase expression and a similar 9-cisretinoic acid-responsive reporter containing two copies of a DR-1 element and mammalian (pCMX) expression vectors for TR␤, TR␤L422R, and RXR (17).…”

“…To overcome this difficulty, we (35)(36)(37)(38) and others (39) have examined TR action in the yeast Saccharomyces cerevisiae. This species is devoid of known NRs and NR coregulators and can be used to reconstruct defined TR transcription complexes in a eukaryotic cell background.…”

Are corepressors always repressors?

“…21 If in a splice variant repression domains (located in the N-terminal and central regions) or CoRNR box motifs (C-terminal, responsible for nuclear receptor binding) are lost or diminished, the repression function of a corepressor might be reduced, completely abolished or even reversed. This was demonstrated for example for an N-terminally truncated NCoR variant and thyroid hormone receptor action, 18 or for androgen dependent gene expression and SMRTb, 10 which lacks one of the classical repression domains and the newly described nuclear receptor binding site. 22 A similar effect could also be shown for RARa dependent gene expression in Jurkat cells; 14 ii) Recruitment of coactivators by corepressors: Direct interactions of SMRT with the coactivator SRC-1, 12 and for NCoR with SRC1, -2, and -3 were described.…”