Human ADA3 regulates RARα transcriptional activity through direct contact between LxxLL motifs and the receptor coactivator pocket

The alternation/deficiency in activation-3 (ADA3) is an essential component of the human p300/CBP-associated factor (PCAF) and yeast Spt-Ada-Gcn5-acetyltransferase (SAGA) histone acetyltransferase complexes. These complexes facilitate transactivation of target genes by association with transcription factors and modification of local chromatin structure. It is known that the yeast ADA3 is required for nuclear receptor (NR)-mediated transactivation in yeast cells; however, the role of mammalian ADA3 in NR signaling remains elusive. In this study, we have investigated how the human (h) ADA3 regulates retinoic acid receptor (RAR) α-mediated transactivation. We show that hADA3 interacts directly with RARα in a hormone-dependent manner and this interaction contributes to RARα transactivation. Intriguingly, this interaction involves classical LxxLL motifs in hADA3, as demonstrated by both ‘loss’ and ‘gain’ of function mutations, as well as a functional coactivator pocket of the receptor. Additionally, we show that hADA3 associates with RARα target gene promoter in a hormone-dependent manner and ADA3 knockdown impairs RARβ2 expression. Furthermore, a structural model was established to illustrate an interaction network within the ADA3/RARα complex. These results suggest that hADA3 is a bona fide transcriptional coactivator for RARα, acting through a conserved mechanism involving direct contacts between NR boxes and the receptor’s co-activator pocket

Exogenous expression of a dominant negative RORa1 vector in muscle cells impairs differentiation: RORa1 directly interacts with p300 and MyoD

ROR/RZR is an orphan nuclear receptor that has no known ligand in the 'classical sense'. In the present study we demonstrate that RORalpha is constitutively expressed during the differentiation of proliferating myoblasts to post-mitotic multinucleated myotubes, that have acquired a contractile phenotype. Exogenous expression of dominant negative RORalpha1DeltaE mRNA in myogenic cells significantly reduces the endogenous expression of RORalpha1 mRNA, represses the accumu-lation and delays the activation of mRNAs encoding MyoD and myogenin [the muscle-specific basic helix-loop-helix (bHLH) proteins] and p21(Waf- 1/Cip-1) (a cdk inhibitor). Immunohistochemistry demonstrates that morpho-logical differentiation is delayed in cells expressing the RORDeltaE transcript. Furthermore, the size and development of mutlinucleated myotubes is impaired. The E region of RORalpha1 interacts with p300, a cofactor that functions as a coactivator in nuclear receptor and MyoD-mediated transactivation. Consistent with the functional role of RORalpha1 in myogenesis, we observed that RORalpha1 directly interacts with the bHLH protein MyoD. This interaction was mediated by the N-terminal activation domain of the bHLH protein, MyoD, and the RORalpha1 DNA binding domain/C region. Furthermore, we demonstrated that p300, RORalpha1 and MyoD interact in a non- competitive manner. In conclusion, this study provides evidence for a biological role and positive influence of RORalpha1 in the cascade of events involved in the activation of myogenic-specific markers and cell cycle regulators and suggests that crosstalk between theretinoid- relatedorphan (ROR) nuclear receptors and the myogenic bHLH proteins has functional consequences for differentiation