Synergistic Enhancement of Nuclear Receptor Function by p160 Coactivators and Two Coactivators with Protein Methyltransferase Activities

Abstract

Nuclear receptors (NRs) activate gene transcription by binding to specific enhancer elements and recruiting coactivators of the p160 family to promoters of target genes. The p160 coactivators in turn enhance transcription by recruiting secondary coactivators, including histone acetyltransferases such as CREB-binding protein (CBP) and p300/CBP-associated factor (p/CAF), as well as the recently identified protein methyltransferase, coactivator-associated arginine methyltransferase 1 (CARM1). In the current study, protein arginine methyltransferase 1 (PRMT1), another arginine-specific protein methyltransferase that shares a region of high homology with CARM1, was also found to act as a coactivator for NRs. PRMT1, like CARM1, bound to the C-terminal AD2 activation domain of p160 coactivators and thereby enhanced the activity of NRs in transient transfection assays. The shape of the graphs of reporter gene activity versus the amounts of CARM1 or PRMT1 expression vector indicated a cooperative relationship between coactivator concentration and activity. Moreover, CARM1 and PRMT1 acted in a synergistic manner to enhance reporter gene activation by both hormonedependent and orphan NRs. The synergy was most evident at low levels of transfected NR expression vectors, where activation of reporter genes was almost completely dependent on the presence of NR and all three exogenously supplied coactivators, i.e. GRIP1, CARM1, and PRMT1. In contrast, with the higher levels of NR expression vectors typically used in transient transfection assays, NR activity was much less dependent on the combination of coactivators, suggesting that target gene activation occurs by different mechanisms at high versus low cellular concentrations of NR. Because multiple coactivators are presumably required to mediate transcriptional activation of native genes in vivo, the low-NR conditions may provide a more physiologically relevant assay for coactivator function.

Coregulator Codes of Transcriptional Regulation by Nuclear Receptors

“…For example, the p160 protein GRIP1 can associate with arginine methyltransferase 1 (CARM1), which potentiates ligand-dependent transcription by several nuclear receptors (16). PRMTI, a second arginine methyltransferase related to CARM1, also functions independently as a nuclear receptor coactivator (17). The CBP/p300 coactivators can recruit additional factors with HAT activity, such as the p/CAF⅐Gcn5L complexes (11,18).…”

“…1. The central conserved domain mediates ligand-dependent interactions with the nuclear receptor LBD, whereas the conserved C-terminal transcriptional activation domains mediate interactions with either CBP/p300 or protein-arginine methyltransferase (16,17). Based on the presence of three regulatory domains, members of the p160 family have been suggested to function as coactivators, at least in part, by serving as adapter molecules that recruit CBP and/or p300 complexes to promoter-bound nuclear receptors in a ligand-dependent manner (18,19).…”

An embarrassment of niches: the many covalent modifications of histones in transcriptional regulation

“…Both are histone arginine methyltransferases; CARM1 primarily targets histone H3, while PMRT1 primarily targets histone H4. CARM1's coactivator function requires its enzymatic activity (Chen et al, 1999), and in transfection assays the two function synergistically to induce transcription (Koh et al, 2001). These enzymes target other proteins and it is not known whether histones are a primary substrate in vivo.…”

Increased methylation of the cytosolic 20-kD protein is accompanied by liver regeneration in a hepatectomized rat

“…PRMT4/CARM1 has been shown to interact with p160 family of nuclear hormone receptor coactivators and enhance transcriptional activation, presumably via methylation of histone H3 (Chou et al, 1999). PRMT1, like PRMT4, can also bind p160, and these two PRMTs have been shown to act synergistically to enhance reporter gene activation by nuclear receptors (Koh et al, 2000). PRMT5/JBP1 was shown to be required for signaling of the interferon receptor to downstream Stat activation (Pollack et al, 1999).…”