Transcriptional repression by nuclear receptors has been correlated to binding of the putative co-repressor, N-CoR. A complex has been identified that contains N-CoR, the Mad presumptive co-repressor mSin3, and the histone deacetylase mRPD3, and which is required for both nuclear receptor- and Mad-dependent repression, but not for repression by transcription factors of the ets-domain family. These data predict that the ligand-induced switch of heterodimeric nuclear receptors from repressor to activator functions involves the exchange of complexes containing histone deacetylases with those that have histone acetylase activity.

Different classes of mammalian transcription factors-nuclear receptors, cyclic adenosine 3',5'-monophosphate-regulated enhancer binding protein (CREB), and signal transducer and activator of transcription-1 (STAT-1)-functionally require distinct components of the coactivator complex, including CREB-binding protein (CBP/p300), nuclear receptor coactivators (NCoAs), and p300/CBP-associated factor (p/CAF), based on their platform or assembly properties. Retinoic acid receptor, CREB, and STAT-1 also require different histone acetyltransferase (HAT) activities to activate transcription. Thus, transcription factor-specific differences in configuration and content of the coactivator complex dictate requirements for specific acetyltransferase activities, providing an explanation, at least in part, for the presence of multiple HAT components of the complex.

Ligand-dependent activation of gene transcription by nuclear receptors is dependent on the recruitment of coactivators, including a family of related NCoA/SRC factors, via a region containing three helical domains sharing an LXXLL core consensus sequence, referred to as LXDs. In this manuscript, we report receptor-specific differential utilization of LXXLL-containing motifs of the NCoA-1/SRC-1 coactivator. Whereas a single LXD is sufficient for activation by the estrogen receptor, different combinations of two, appropriately spaced, LXDs are required for actions of the thyroid hormone, retinoic acid, peroxisome proliferator-activated, or progesterone receptors. The specificity of LXD usage in the cell appears to be dictated, at least in part, by specific amino acids carboxy-terminal to the core LXXLL motif that may make differential contacts with helices 1 and 3 (or 3) in receptor ligand-binding domains. Intriguingly, distinct carboxy-terminal amino acids are required for PPAR␥ activation in response to different ligands. Related LXXLL-containing motifs in NCoA-1/SRC-1 are also required for a functional interaction with CBP, potentially interacting with a hydrophobic binding pocket. Together, these data suggest that the LXXLL-containing motifs have evolved to serve overlapping roles that are likely to permit both receptor-specific and ligand-specific assembly of a coactivator complex, and that these recognition motifs underlie the recruitment of coactivator complexes required for nuclear receptor function.

Several lines of evidence indicate that the nuclear receptor corepressor (N-CoR) complex imposes ligand dependence on transcriptional activation by the retinoic acid receptor and mediates the inhibitory effects of estrogen receptor antagonists, such as tamoxifen, suppressing a constitutive N-terminal, Creb-binding protein͞coactivator complex-dependent activation domain. Functional interactions between specific receptors and N-CoR or SMRT corepressor complexes are regulated, positively or negatively, by diverse signal transduction pathways. Decreased levels of N-CoR correlate with the acquisition of tamoxifen resistance in a mouse model system for human breast cancer. Our data suggest that N-CoR-and SMRT-containing complexes act as rate-limiting components in the actions of specific nuclear receptors, and that their actions are regulated by multiple signal transduction pathways.Nuclear receptors are structurally related, ligand-activated regulators of a complex array of genes involved in cell proliferation, differentiation, morphogenesis, and homeostasis (1, 2). In the absence of ligand, several nuclear receptors associate with a nuclear receptor corepressor (N-CoR) (3-6) or the related factor SMRT (silencing mediator of retinoid and thyroid receptors) (7) to mediate repression. Their regulatory function is further modulated by both physiologic and pharmacologic ligands and by the actions of various signal transduction pathways that result in ligand-independent gene activation of diverse nuclear receptor family members (8)(9)(10).N-CoR and SMRT appear to be components of cellular complexes (4, 11, 12) containing histone deacetylases (HDACs) (13, 14) and homologs of the yeast repressor Sin3 (15, 16), which are recruited to DNA via targeting by diverse DNA-binding, site-specific transcription factors (reviewed in refs. 17 and 18). Conversely, transcriptional activation by nuclear hormone receptors requires the ligand-dependent association of a coactivator complex that includes a family of nuclear receptor coactivators (NCoAs) (19-23) and also includes the histone acetylases Creb-binding protein (CBP)͞ p300 (24-28) and P͞CAF (29,50).The development of inhibitory ligands for the nuclear receptors has yielded important therapeutic treatments, among them the use of tamoxifen for endocrine therapy of breast cancer (reviewed in refs. 10 and 30). However, in certain tissues such as uterus and bone, and after long-term treatment in patients with breast cancer, tamoxifen exhibits unexplained partial agonistic activity (31). Various agents that raise intracellular cAMP levels or stimulate the ras͞MAP kinase pathway can similarly cause estrogen receptor (ER) activation in the presence of tamoxifen or the absence of any activating ligand (9,(32)(33)(34)(35). In this manuscript we show that diverse molecular strategies regulate the association of N-CoR-or SMRT-containing complexes with specific nuclear receptors, including the nature of the ligand, the levels of available N-CoR͞SMRT, and the action of diverse protein ki...

We report that interferon ␥ (IFN-␥) inhibits transcription of the macrophage scavenger receptor gene by antagonizing the Ras-dependent activities of AP-1 and cooperating ets domain transcription factors, apparently as a result of competition between AP-1͞ets factors and activated STAT1

POU-domain proteins, such as the pituitary-specific factor Pit-1, are members of the homeodomain family of proteins which are important in development and homeostasis, acting constitutively or in response to signal-transduction pathways to either repress or activate the expression of specific genes. Here we show that whereas homeodomain-containing repressors such as Rpx2 seem to recruit only a co-repressor complex, the activity of Pit-1 is determined by a regulated balance between a co-repressor complex that contains N-CoR/SMRT, mSin3A/B and histone deacetylases, and a co-activator complex that includes the CREB-binding protein (CBP) and p/CAF. Activation of Pit-1 by cyclic AMP or growth factors depends on distinct amino- and carboxy-terminal domains of CBP, respectively. Furthermore, the histone acetyltransferase functions of CBP or p/CAF are required for Pit-1 function that is stimulated by cyclic AMP or growth factors, respectively. These data show that there is a switch in specific requirements for histone acetyltransferases and CBP domains in mediating the effects of different signal-transduction pathways on specific DNA-bound transcription factors.

Subgroup D adenovirus (Ad) types 8, 19, and 37 (Ad8, -19, and -37, respectively) are causative agents of epidemic keratoconjunctivitis and genital tract infections. Previous studies showed that Ad37 binds to a 50-kDa membrane glycoprotein expressed on human ocular (conjunctival) cells. To identify and characterize the role of the 50-kDa glycoprotein in Ad37 infection, we partially purified this molecule from solubilized Chang C conjunctival cell membranes by using lentil lectin chromatography and preparative sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Liquid chromatography coupled to nano-electrospray ionization-tandem mass spectrometry was subsequently used to identify four Ad37 receptor candidates: CD46, CD87, CD98, and CD147. Immunodepletion analyses demonstrated that the 50-kDa protein is identical to CD46 (also known as membrane cofactor protein). The Ad37, but not Ad5, fiber knob bound to the extracellular domain of CD46, demonstrating a direct interaction of an Ad37 capsid protein with CD46. An antibody specific for the Nterminal 19 amino acids of CD46 also blocked Ad37 infection of human cervical carcinoma and conjunctival cells, indicating a requirement for CD46 in infection. Finally, expression of a 50-kDa isoform of human CD46 in a CD46-null cell line increased cell binding by wild-type Ad37 and gene delivery by an Ad vector pseudotyped with the Ad37 fiber, but not by a vector bearing the Ad5 fiber. Together, these studies demonstrate that CD46 serves as an attachment receptor for Ad37 and shed further light on the cell entry pathway of subgroup D Ads.

H3 and H4 is associated with increased DNA accessibility to binding proteins and increased transcription (Bhattacharyya et al., 1997; reviewed in Grunstein, 1997), while histone hypoacetylation is associated with transcriptionally silent chromosomal domains (Braunstein