TIF2, a 160 kDa transcriptional mediator for the ligand-dependent activation function AF-2 of nuclear receptors.

Abstract

Nuclear receptors (NRs) act as ligand‐inducible transcription factors which regulate the expression of target genes upon binding to cognate response elements. The ligand‐dependent activity of the NR activation function AF‐2 is believed to be mediated to the transcription machinery through transcriptional mediators/intermediary factors (TIFs). We report here the cloning of the 160 kDa human nuclear protein TIF2, which exhibits all properties expected for a mediator of AF‐2: (i) it interacts in vivo with NRs in an agonist‐dependent manner; (ii) it binds directly to the ligand‐binding domains (LBDs) of NRs in an agonist‐ and AF‐2‐integrity‐dependent manner in vitro; (iii) it harbours an autonomous transcriptional activation function; (iv) it relieves nuclear receptor autosquelching; and (v) it enhances the activity of some nuclear receptor AF‐2s when overexpressed in mammalian cells. TIF2 exhibits partial sequence homology with the recently isolated steroid receptor coactivator SRC‐1, indicating the existence of a novel gene family of nuclear receptor transcriptional mediators.

Cofactor competition between the ligand-bound oestrogen receptor and an intron 1 enhancer leads to oestrogen repression of ERBB2 expression in breast cancer

“…We conclude therefore that SRC-1 is the most potent cofactor in our assay and therefore likely to be acting directly at the ERBB2 enhancer. Moreover both SRC-1 and TIF2, although widely expressed, have previously been identi®ed as competable, limiting factors particularly in the context of the squelching of progesterone receptor transactivation in the presence of ectopic ER expression (Onate et al, 1995;Voegel et al, 1996). Signi®cantly, both proteins are expressed at very modest levels in most breast tumour derived cell lines (Anzick et al, 1997;Berns et al, 1998) and levels of SRC-1 are also particularly low in patients who fail to respond to tamoxifen therapy (Berns et al, 1998).…”

“…The SRC-1 expression clones were in pSG5 and all based on the SRC-1e isoform and expressed at comparable levels (Kalkhoven et al, 1998;Bevan et al, 1999). AIB1 and TIF2 expression constructs have been described elsewhere (Anzick et al, 1997;Voegel et al, 1996). Cell monolayers were transiently transfected and subsequently hormonally manipulated as described previously (Bates and Hurst, 1997).…”

Identification of domains of c-Jun mediating androgen receptor transactivation

“…Our ®nding that hAR-induced transactivation can be squelched by high amounts of c-Jun suggests that c-Jun is titrating some limiting factor(s) necessary for AR activation. Neither CBP/p300 nor TIF-2 (Voegel et al, 1996) is able to relieve this squelching (A Bubulya and L Shemshedini, unpublished results). Other possible candidates are the previously identi®ed AR coactivators, SRC-1 (Onate et al, 1995) andARA70 (Yeh andChang, 1996).…”

“…Recent studies indicate that the transcriptional activity of nuclear receptors is dependent on a class of proteins collectively referred to as coactivators, proteins believed to bridge the nuclear receptors to the RNA polymerase II transcription machinery (reviewed in Gill and Tjian, 1992). Biochemical assays and yeast twohybrid screens have identi®ed a number of receptor coactivators, including p140 and p160 (Kurokawa et al, 1995), SRC-1 (Onate et al, 1995), TIF-1 (Le Dourin et al, 1995), TIF-2 (Voegel et al, 1996), RIP-140 (Cavilles et al, 1995), GRIP (Hong et al, 1996), and CBP/p300 (Kamei et al, 1996;Chakravarti et al, 1996). SRC-1, p160, TIF-2, and GRIP-1 are structurally related proteins which mediate the activity of the ligand-dependent AF-2 (activation function-2), a conserved motif within the distal C-terminus of many nuclear receptors (Durand et al, 1994).…”

“…Interestingly, the interaction with the related coactivators SRC-1 and TIF-2 appears to be receptor-speci®c. While SRC-1 stimulates the transcriptional activity of GR and PR (Onate et al, 1995), TIF-2 enhances transactivation by AR and PR, but not GR (Voegel et al, 1996). An unrelated putative receptor coactivator, ARA70, was recently identi®ed that mediates only AR-induced transcription (Yeh and Chang, 1996) and thus may be partially responsible for androgen-speci®c responses.…”