Nuclear receptor coactivator/coregulator NCoA6(NRC) is a pleiotropic coregulator involved in transcription, cell survival, growth and development

Abstract

NCoA6 (also referred to as NRC, ASC-2, TRBP, PRIP and RAP250) was originally isolated as a ligand-dependent nuclear receptor interacting protein. However, NCoA6 is a multifunctional coregulator or coactivator necessary for transcriptional activation of a wide spectrum of target genes. The NCoA6 gene is amplified and overexpressed in breast, colon and lung cancers. NCoA6 is a 250 kDa protein which harbors a potent N-terminal activation domain, AD1; and a second, centrally-located activation domain, AD2, which is necessary for nuclear receptor signaling. The intrinsic activation potential of NCoA6 is regulated by its C-terminal STL regulatory domain. Near AD2 is an LxxLL-1 motif which interacts with a wide spectrum of ligand-bound NRs with high-affinity. A second LxxLL motif (LxxLL-2) located towards the C-terminal region is more restricted in its NR specificity. The potential role of NCoA6 as a co-integrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known cofactors including CBP/p300. NCoA6 has been shown to associate with at least three distinct coactivator complexes containing Set methyltransferases as core polypeptides. The composition of these complexes suggests that NCoA6 may play a fundamental role in transcriptional activation by modulating chromatin structure through histone methylation. Knockout studies in mice suggest that NCoA6 is an essential coactivator. NCoA6-/- embryos die between 8.5-12.5 dpc from general growth retardation coupled with developmental defects in the heart, liver, brain and placenta. NCoA6-/- MEFs grow at a reduced rate compared to WT MEFs and spontaneously undergo apoptosis, indicating the importance of NCoA6 as a prosurvival and anti-apoptotic gene. Studies with NCoA6+/- and conditional knockout mice suggest that NCoA6 is a pleiotropic coregulator involved in growth, development, wound healing and maintenance of energy homeostasis.

Figure 1. Domain organization of NCoA6(NRC).

NCoA6(NRC) contains two activation domains (AD1 and AD2) and two LxxLL receptor interacting motifs (LxxLL-1 and LxxLL-2). LxxLL-1 interacts with a wide variety of NRs, while LxxLL-2 is more restricted in NR recognition. The C-terminal region of NCoA6(NRC) is rich in Ser, Thr and Leu (STL), and appears to function to repress the activation domains of the protein. NCoA6(NRC) forms homodimers through a dimerization domain (DD), which is located near LxxLL-1. Factors that are known to be regulated by NCoA6(NRC) are also listed.

Figure 2. NRC associates with a number of cofactors in vitro and in vivo that have been characterized.

Schematic of NCoA6(NRC) to indicate the regions of NRC(NCoA6) which have been reported to associate with the indicated factors such as CAPER, CBP/p300, CoAA, DNA-PKc, DRIP130, NIF-1 and PIMT in vitro and/or in vivo. E1A was shown to block NRC activity (Mahajan and Samuels, 2000), although its direct interaction with NRC has not been checked. EIA is known to associate with CBP.

Figure 3. NCoA6(NRC) is a component of three nuclear protein complexes.

Three protein complexes referred to as ASCOM (Goo et al., 2003), ALR (Issaeva et al., 2007) and PTIP (Cho et al., 2007) have been purified and the polypeptides that represent each complex are listed. The 3 complexes share NCoA6 and several other polypeptides. Recently, WDR5 has also been shown to be present in ASCOM (Lee et al., 2006).

Figure 4. NCoA6(NRC)^+/- mice exhibit a wound healing defect.

Shown is a representative mouse with skin lesions in the neck and grooming area. The skin from the wound site, edge of the wound and a normal skin area was processed for histology and stained with hematoxylin-eosin. The skin lesions show an increase in sebaceous glands, a lack of hair follicles and no keratinocyte migration (Epithelial Tongue). Reproduced with permission from Mahajan et al. (Mahajan et al., 2004) by the American Society of Microbiology (Mol. Cell. Biol.).

Figure 5. Lack of keratinocyte migration in explants from NCoA6(NRC)^+/- mice.

Keratinocyte migration was determined from skin explants of 2 day old wild-type (left) and NCoA6(NRC)^+/- mice (right). Explants from NCoA6(NRC)^+/- mice exhibit a delay in the initiation of and a diminished capacity for migration, as well as a lack of response to EGF. Reproduced with permission from Mahajan et al. (Mahajan et al., 2004) by the American Society of Microbiology (Mol. Cell. Biol.).