Nuclear receptors, their coactivators and modulation of transcription

Nuclear receptors arc ligand-dependent transcription factors which can also be ac­tivated in the absence of their lipophilic ligands by signaling substances acting on cell membrane receptors. This ligand-independent activation indicates the impor­tance of nuclear receptor phosphorylation for their function. Nuclear receptor- mediated transcription of target genes is further increased by interactions with re­cruited coactivators forming a novel family of nuclear proteins. CBP/p300, a coacti­vator of different classes of transcription factors, including the tumor suppressor protein p53, plays a special role acting as a bridging protein between inducible tran­scription factors and the basal transcription apparatus, and as an integrator of di­verse signaling pathways. Coactivators of nuclear receptors and associated proteins forming a multicomponent complex have an intrinsic histone acetylase activity in contrast to nuclear receptor and heterodimer Mad-Max corepressors, which recruit histone deacetylase. Similarly the Rb protein interacts with histone deacetylase to re­press transcription of cell cycle regulatory genes. Targeted histone acetylation/dca- cetylation results in remodeling of chromatin structure and correlates with activa­tion/repression of transcription. Recent data point to the important role of coactiva­tor proteins associated with inducible transcription factors in transcription regula­tion, and in the integration of multiple signal transduction pathways within the nu­cleus.