We report here the identification of a novel cofactor, ACTR, that directly binds nuclear receptors and stimulates their transcriptional activities in a hormone-dependent fashion. ACTR also recruits two other nuclear factors, CBP and P/CAF, and thus plays a central role in creating a multisubunit coactivator complex. In addition, and unexpectedly, we show that purified ACTR is a potent histone acetyltransferase and appears to define a distinct evolutionary branch to this recently described family. Thus, hormonal activation by nuclear receptors involves the mutual recruitment of at least three classes of histone acetyltransferases that may act cooperatively as an enzymatic unit to reverse the effects of histone deacetylase shown to be part of the nuclear receptor corepressor complex.

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that control key aspects of normal differentiation. Aberrant RAR activity may be a causal factor in neoplasia. Human acute promyelocytic leukemia, for example, is tightly linked to chromosomal translocations that fuse novel amino acid sequences (denoted PML, PLZF, and NPM) to the DNA-binding and hormone-binding domains of RAR␣. The resulting chimeric receptors have unique transcriptional properties that may contribute to leukemogenesis. Normal RARs repress gene transcription by associating with ancillary factors denoted corepressors (also referred to as SMRT, N-CoR, TRAC, or RIP13). We report here that the PML-RAR␣ and PLZF-RAR␣ oncoproteins retain the ability of RAR␣ to associate with corepressors, and that this corepressor association correlates with certain aspects of the leukemic phenotype. Unexpectedly, the PLZF moiety itself can interact with SMRT corepressor. This interaction with corepressor is mediated, in part, by a POZ motif within PLZF. Given the presence of POZ motifs in a number of known transcriptional repressors, similar interactions with SMRT may play a role in transcriptional silencing by a variety of both receptor and nonreceptor transcription factors.Retinoids regulate many aspects of vertebrate cell proliferation and differentiation through receptors that function as hormone-regulated transcription factors (1-6). Two major classes of retinoid receptors have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs) (3-6). Both RARs and RXRs bind to specific sites on the DNA, and they can activate or repress expression of adjacent target genes (1-6). Aberrant RARs appear to play a crucial role in human acute promyelocytic leukemia (APL) (7-15). In over 95% of all APL patients, specific chromosomal translocations create abnormal RARs by replacing the N terminus of RAR␣ with novel ORFs. The breakpoint in RAR␣ is identical in all cases, whereas the nature of the novel N-terminal sequences can vary. In the common t(15;17) translocation, the RAR␣ N terminus is replaced with an ORF denoted PML (for promyelocytic leukemia) (7-12). Alternatively, t(11;17) and t(5;17) translocations result in chimeric proteins denoted PLZF (promyelocytic leukemia zinc finger)-RAR␣ and NPM (nucleophosmin)-RAR␣, respectively (13-15). Although possessing a number of recognizable structural motifs, including several prevalent in transcription factors, the PML, PLZF, and NPMderived sequences exhibit little structural interrelatedness, and the functions of these proteins in the normal cell are not fully understood (reviewed in refs. 16-18).The near-invariant association of acute promyelocytic leukemia with the expression of chimeric RAR␣ proteins, and the ability of retinoids to drive leukemias bearing the PML-RAR␣ translocation into differentiation and clinical remission (16-18), suggest an active role for these chimeras in conferring the leukemogenic phenotype. Furthermore, ectopic expression of PML-RAR␣ in murine or av...

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

For more than a century, thyroid hormones (THs) have been known to exert powerful catabolic effects, leading to weight loss. Although much has been learned about the molecular mechanisms used by TH receptors (TRs) to regulate gene expression, little is known about the mechanisms by which THs increase oxidative metabolism. Here, we report that TH stimulation of fatty acid β-oxidation is coupled with induction of hepatic autophagy to deliver fatty acids to mitochondria in cell culture and in vivo. Furthermore, blockade of autophagy by autophagy-related 5 (ATG5) siRNA markedly decreased TH-mediated fatty acid β-oxidation in cell culture and in vivo. Consistent with this model, autophagy was altered in livers of mice expressing a mutant TR that causes resistance to the actions of TH as well as in mice with mutant nuclear receptor corepressor (NCoR). These results demonstrate that THs can regulate lipid homeostasis via autophagy and help to explain how THs increase oxidative metabolism. IntroductionThyroid hormones (THs) have been known to stimulate basal metabolic rate for over a century (1, 2). Subsequent studies showed that THs induced energy expenditure in response to increased caloric intake (3). Later, several intracellular processes were shown to be involved in the calorigenic effects of THs. These included increased ATP expenditure due to increased Na + /K + -ATPase activity to maintain ion gradients in various tissues (4, 5) as well as reduced efficiency of ATP synthesis, particularly through the induction of uncoupling proteins (UCPs), which cause proton leakage in the electron transport chain of the mitochondria of target tissues (6, 7). However, despite these advances in our understanding of THs on cellular metabolism, none of these proposed mechanisms appears to be dominant. Currently, little is known about other mechanisms that might be utilized by THs to regulate energy consumption within the cell. This is particularly true for the events involved in the delivery of fatty acids to mitochondria, a necessary step in converting stored intracellular triglyceride fuel into ATP.The active form of TH, 3,3′5-triiodo-l-thyronine (T 3 ), is a critical regulator of cellular and tissue metabolism throughout the body. It controls gene expression in target tissues by binding to its cognate nuclear receptors (TRα and TRβ), which are ligand-inducible transcription factors. In the presence of T 3 , TH receptors (TRs) bind to TH response elements in the promoters of target genes and form coactivator complexes containing histone acetyltransferase activity to activate transcription (8). In the absence of T 3 , TRs recruit corepressors such as NCoR and silencing mediator of retinoid and thyroid receptors (SMRT), which together with transducin β-like protein 1 (TBL1) and histone deacetylase 3 (HDAC3)

The SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor participates in the repression of target gene expression by a variety of transcription factors, including the nuclear hormone receptors, promyelocytic leukemia zinc finger protein, and B-cell leukemia protein 6. The ability of SMRT to associate with these transcription factors and thereby to mediate repression is strongly inhibited by activation of tyrosine kinase signaling pathways, such as that represented by the epidermal growth factor receptor. We report here that SMRT function is potently inhibited by a mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) cascade that operates downstream of this growth factor receptor. Intriguingly, the SMRT protein is a substrate for phosphorylation by protein kinases operating at multiple levels in this MAPKKK pathway, including the MAPKs, MAPK-extracellular signal-regulated kinase 1 (MEK-1), and MEK-1 kinase (MEKK-1). Phosphorylation of SMRT by MEKK-1 and, to a lesser extent, MEK-1 inhibits the ability of SMRT to physically tether to its transcription factor partners. Notably, activation of MEKK-1 or MEK-1 signaling in transfected cells also leads to a redistribution of the SMRT protein from a nuclear compartment to a more perinuclear or cytoplasmic compartment. We suggest that SMRT-mediated repression is regulated by the MAPKKK cascade and that changes both in the affinity of SMRT for its transcription factors and in the subcellular distribution of SMRT contribute to the loss of SMRT function that is observed in response to kinase signal transduction.Eukaryotic transcription factors can exert both positive and negative effects on gene expression. A number of transcriptional regulators are, in fact, bipolar in their properties, with a given transcription factor being able to both repress and activate target gene expression. Perhaps the most extensively analyzed of these bipolar transcription factors are the nuclear hormone receptors, such as the retinoic acid receptors (RARs) and the thyroid hormone receptors (T3Rs) (2,31,37,43,64). RARs and T3Rs are ligand-regulated transcription factors that bind to specific target promoter sequences, denoted hormone response elements, in both the absence and the presence of cognate hormone. In the absence of hormone, these nuclear receptors typically repress gene transcription; conversely, binding of cognate hormone converts the nuclear receptors into strong transcriptional activators (2,31,37,43,64).RARs and T3Rs manifest these divergent transcriptional properties through their ability to recruit auxiliary polypeptides, denoted corepressors and coactivators (12,22,36,62,76). In the absence of hormone ligand, RARs and T3Rs are able to bind to two interrelated corepressor polypeptides, denoted SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear hormone receptor corepressor) (11,21,29,35,45,46,51,55,73,79,80); SMRT and N-CoR recruit, in turn, a larger corepressor complex including mSin3, RbAp-46, RbAp-...

A variety of eukaryotic transcription factors, including the nuclear hormone receptors, Max-Mad, BCL-6, and PLZF, appear to mediate transcriptional repression through the ability to recruit a multiprotein corepressor complex to the target promoter. This corepressor complex includes the SMRT/N-CoR polypeptides, mSin3A or -B, and histone deacetylase 1 or 2. The presence of a histone-modifying activity in the corepressor complex has led to the suggestion that gene silencing is mediated by modification of the chromatin template, perhaps rendering it less accessible to the transcriptional machinery. We report here, however, that the corepressor complex actually appears to exhibit multiple mechanisms of transcriptional repression, only one of which corresponds with detectable recruitment of the histone deacetylase. We provide evidence instead of an alternative pathway of repression that may be mediated by direct physical interactions between components of the corepressor complex and the general transcription factor TFIIB.

Thyroid hormones (THs) are essential for fetal and post-natal nervous system development and also play an important role in the maintenance of adult brain function. Of the two major THs, T4 (3,5,3′,5′-tetraiodo-l-thyronine) is classically viewed as an pro-hormone that must be converted to T3 (3,5,3′-tri-iodo-l-thyronine) via tissue-level deiodinases for biological activity. THs primarily mediate their effects by binding to thyroid hormone receptor (TR) isoforms, predominantly TRα1 and TRβ1, which are expressed in different tissues and exhibit distinctive roles in endocrinology. Notably, the ability to respond to T4 and to T3 differs for the two TR isoforms, with TRα1 generally more responsive to T4 than TRβ1. TRα1 is also the most abundantly expressed TR isoform in the brain, encompassing 70–80% of all TR expression in this tissue. Conversion of T4 into T3 via deiodinase 2 in astrocytes has been classically viewed as critical for generating local T3 for neurons. However, deiodinase-deficient mice do not exhibit obvious defectives in brain development or function. Considering that TRα1 is well-established as the predominant isoform in brain, and that TRα1 responds to both T3 and T4, we suggest T4 may play a more active role in brain physiology than has been previously accepted.

Many transcription factors function by repressing gene transcription. For a variety of these transcription factors the ability to physically recruit auxiliary proteins, denoted corepressors, is crucial for the ability to silence gene expression. We and others have previously implicated the SMRT corepressor in the actions of the PLZF transcription factor and in the function of its oncogenic derivative, PLZF-retinoic acid receptor (RAR␣), in promyelocytic leukemia. We report here that PLZF, and a structurally similar transcriptional repressor, BCL-6, can interact with a variety of corepressor proteins in addition to SMRT, including the mSin3A protein and (for PLZF) histone deacetylase-1. Unexpectedly, these additional interactions with corepressor components are nonequivalent for these otherwise similar oncoproteins, suggesting that transcriptional repression by BCL-6 and by PLZF may differ in mechanism. Furthermore, we demonstrate that the oncogenic PLZF-RAR␣ chimera lacks several important corepressor interaction sites that are present in the native PLZF protein. Thus the t(11;17) translocation that creates the PLZF-RAR␣ chimera generates an oncoprotein with potentially novel regulatory properties distinct from those of either parental protein. Our results demonstrate that otherwise similar transcription factors can differ notably in their interactions with the corepressor machinery.The study of eukaryotic gene regulation has focused primarily on the mechanisms of transcriptional activation. However, many transcriptional factors function not as activators, but as repressors, or can operate bimodally by mediating both negative and positive effects on gene transcription. Transcription factors capable of repression of gene expression include the Ying-Yang-1 protein, the Mad⅐Max complex, nuclear hormone receptors such as those for thyroid hormone and retinoic acid, the retinoblastoma gene product, the BCL-6 protein, and the PLZF polypeptide (1-13). Although these transcriptional repressors are structurally and physiologically distinct from one another, they share the ability to interact with components of a multiprotein SMRT⅐N-CoR corepressor complex (3, 5-7, 9 -35). In the prevailing model, these DNA-binding transcription factors are thought to operate by tethering corepressor to a target gene; once so tethered, components of the corepressor complex help mediate the actual events involved in transcriptional silencing (reviewed in Refs. 36 and 37).A number of potential constituents of the SMRT⅐N-CoR corepressor complex have been identified, including SMRT/N-CoR, mSin3A/B, histone deacetylase (HDAC) 1 -1/2, retinoblastomaassociated proteins 46 and 48, and several additional polypeptides of unknown function (23,24,26,27,29,34,38,39). Different transcription factors interact with different corepressor components; nuclear hormone receptors, for example, bind primarily to the SMRT⅐N-CoR component, whereas Mad⅐Max binds to the mSin3A/B polypeptide, and Ying-Yang 1 and the retinoblastoma protein appear to preferentially re...