Johannes Voegel

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

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.

The coactivator TIF2 contains three nuclear receptor-binding motifs and mediates transactivation through CBP binding-dependent and -independent pathways.

The nuclear receptor (NR) coactivator TIF2 possesses a single NR interaction domain (NID) and two autonomous activation domains, AD1 and AD2. The TIF2 NID is composed of three NR‐interacting modules each containing the NR box motif LxxLL. Mutation of boxes I, II and III abrogates TIF2–NR interaction and stimulation, in transfected cells, of the ligand‐induced activation function‐2 (AF‐2) present in the ligand‐binding domains (LBDs) of several NRs. The presence of an intact NR interaction module II in the NID is sufficient for both efficient interaction with NR holo‐LBDs and stimulation of AF‐2 activity. Modules I and III are poorly efficient on their own, but synergistically can promote interaction with NR holo‐LBDs and AF‐2 stimulation. TIF2 AD1 activity appears to be mediated through CBP, as AD1 could not be separated mutationally from the CBP interaction domain. In contrast, TIF2 AD2 activity apparently does not involve interaction with CBP. TIF2 exhibited the characteristics expected for a bona fide NR coactivator, in both mammalian and yeast cells. Moreover, in mammalian cells, a peptide encompassing the TIF2 NID inhibited the ligand‐induced AF‐2 activity of several NRs, indicating that NR AF‐2 activity is either mediated by endogenous TIF2 or by coactivators recognizing a similar surface on NR holo‐LBDs.

Highly Potent Naphthofuran-Based Retinoic Acid Receptor Agonists

A series of arotinoids with a central benzofuran or naphthofuran ring structure were synthesized by an efficient three‐step process. Most of these 3‐substituted naphthofuran arotinoids are potent agonists of the retinoic acid receptor (RAR) subtypes, with activities in the nanomolar range.

Pyrazine arotinoids with inverse agonist activities on the retinoid and rexinoid receptors.

RAR and RXR agonists: A collection of pyrazine‐based RAR/RXR ligands were prepared by a series of palladium catalyzed cross‐coupling reactions and characterized. Structure–activity relationships were elucidated. Retinoic acid receptor (RAR) α/β‐subtype‐selective and retinoid X receptor (RXR) inverse agonist activities are described for pyrazine acrylic acid arotinoid, 14 d.