Isolation and Characterization of PBP, a Protein That Interacts with Peroxisome Proliferator-activated Receptor

Abstract

In an attempt to identify cofactors that could possibly influence the transcriptional activity of peroxisome proliferator-activated receptors (PPARs), we used a yeast two-hybrid system with Gal4-PPAR␥ as bait to screen a mouse liver cDNA library and have identified steroid receptor coactivator-1 (SRC-1) as a PPAR transcriptional coactivator. We now report the isolation of a cDNA encoding a 165-kDa PPAR␥-binding protein, designated PBP which also serves as a coactivator. PBP also binds to PPAR␣, RAR␣, RXR, and TR␤1, and this binding is increased in the presence of specific ligands. Deletion of the last 12 amino acids from the carboxyl terminus of PPAR␥ results in the abolition of interaction between PBP and PPAR␥. PBP modestly increased the transcriptional activity of PPAR␥, and a truncated form of PBP (amino acids 487-735) acted as a dominantnegative repressor, suggesting that PBP is a genuine coactivator for PPAR. In addition, PBP contains two LXXLL signature motifs considered necessary and sufficient for the binding of several coactivators to nuclear receptors. In situ hybridization and Northern analysis showed that PBP is expressed in many tissues of adult mice, including the germinal epithelium of testis, where it appeared most abundant, and during ontogeny, suggesting a possible role for this cofactor in cellular proliferation and differentiation.The peroxisome proliferator-activated receptors (PPARs) 1 are a group of transcription factors that regulate the expression of target genes, in particular those associated with lipid metabolism (1, 2). PPARs, which derive the designation by virtue of their ability to mediate predictable pleiotropic effects in response to peroxisome proliferators (1,3,4), are members of the nuclear receptor superfamily (5, 6). Three isotypes of PPARs, namely PPAR␣, PPAR␦ (also called ␤ or NUC-1), and PPAR␥ have been identified as products of separate genes from Xenopus, rodents, and humans (1, 7-12). These PPAR isotypes appear to exhibit distinct patterns of tissue distribution and differ considerably in their ligand binding domains, suggesting that they possibly perform different functions in different cell types (7,13,14). Indeed, of the three isotypes, PPAR␣ expression is relatively high in hepatocytes, enterocytes, and the proximal tubular epithelium of kidney when compared with other cell types (13,14), and evidence derived from mice with PPAR␣ gene disruption indicates that this receptor is essential for the pleiotropic responses induced by peroxisome proliferators (15). Several structurally diverse peroxisome proliferators, specific fatty acids, and eicosanoids act as ligands for PPAR␣ (4, 16 -19). Although PPAR␦ isotype is ubiquitously expressed and binds the same ligands as PPAR␣ (18,19), its functional significance remains largely elusive. PPAR␥ exists as two isoforms, PPAR␥1 and PPAR␥2, as a consequence of alternate promoter usage in the gene encoding this receptor (8,20,21). While PPAR␥1 isoform expression is restricted to liver and few other organs (8, 14), the PPAR␥2 i...

Coregulator Codes of Transcriptional Regulation by Nuclear Receptors

“…The best characterized of these is the TRAP⅐DRIP⅐ARC complex, which enhances the transcriptional activities of nuclear receptors and other signal-dependent transcription factors in vitro (29 -31). The TRAP⅐DRIP⅐ARC complex is recruited to nuclear receptors in a ligand-dependent manner via a 220-kDa component referred to as PBP/TRAP220/DRIP205, which contains two alternatively utilized LXXLL nuclear receptor interaction motifs (32,33). Disruption of the TRAP220/PRIP205/ PBP gene in the mouse results in embryonic lethality at embryonic day 11.5, and initial studies in myocyte enhancer factors have suggested a defect in ligand-dependent thyroid hormone and PPAR␥ receptor function (31,32).…”

RB18A regulates p53-dependent apoptosis

“…Human RB18A shared structural, gene localization and functional properties with other cellular components, as 165 kDa murine PBP (peroxisome proliferator-activated receptor (PPAR)-binding protein), which acted as a co-activator of nuclear receptors (Zhu et al, 1997). TRAP220 (Yuan et al, 1998a,b) and DRIP205 , two human genes identi®ed later than RB18A, also shared up to 98% identity with RB18A nucleotide sequence.…”

“…TRAP220, a 220 kDa thyroid hormone receptor-associated protein (Yuan et al, 1998a,b) and DRIP205, a 205 kDa vitamin D receptor interacting protein , both belong to multiple 12 to 15 subunit complexes, namely TRAP and DRIP complexes respectively. As PBP (Zhu et al, 1997), TRAP220 (Yuan et al, 1998a;Ito et al, 1999) and DRIP205 directly interacted, through two di erentially preferred LXXLL regions (Ren et al, 2000), with distinct hormoneactivated nuclear receptors: i.e. thyroid hormone receptor (THR), vitamin D receptor (VDR), retinoic acid receptor alpha (RARa), retinoic X receptor (RXR), RORa , PPARg (Kodera et al, 2000) and glucocorticoid or estrogen receptors (Burakov et al, 2000;Warnmark et al, 2001).…”

The Nuclear Hormone Receptor Coactivator NRC Is a Pleiotropic Modulator Affecting Growth, Development, Apoptosis, Reproduction, and Wound Repair

“…PBP/TRAP220/DRIP205, a component of the DRIP/TRAP complexes, also interacts with liganded NRs through LXXLL motifs (15,54,72). These complexes are related to the yeast SRB/Mediator complex and were identified through biochemical purification using ligand-bound VDR (DRIP) and TR (TRAP).…”