The circadian clock acts at the genomic level to coordinate internal behavioral and physiologic rhythms via the CLOCK-BMAL transcriptional heterodimer. Although the nuclear receptors REV-ERBα and β have been proposed to form an accessory feedback loop that contributes to clock function1,2, their precise roles and importance remain unresolved. To establish their regulatory potential we generated comparative cistromes of both REV-ERB isoforms, which revealed shared recognition at over 50% of their total sites and extensive overlap with the master circadian regulator BMAL1. While Rev-erbα has been shown to directly regulate Bmal1 expression1,2, the cistromic analysis reveals a direct connection between Bmal1 and Rev-erbα and β regulatory circuits than previously suspected. Genes within the intersection of the BMAL1, REV-ERBα and REV-ERBβ cistromes are highly enriched for both clock and metabolic functions. As predicted by the cistromic analysis, dual depletion of Rev-erbα/β function by creating double-knockout mice (DKOs) profoundly disrupted circadian expression of core circadian clock and lipid homeostatic gene networks. As a result, DKOs show strikingly altered circadian wheel-running behavior and deregulated lipid metabolism. These data now ally Rev-erbα/β with Per, Cry and other components of the principal feedback loop that drives circadian expression and suggest a more integral mechanism for the coordination of circadian rhythm and metabolism.

The molecular chaperone heat shock protein 90 (Hsp90) and its accessory cochaperones function by facilitating the structural maturation and complex assembly of client proteins, including steroid hormone receptors and selected kinases. By promoting the activity and stability of these signaling proteins, Hsp90 has emerged as a critical modulator in cell signaling. Here, we present evidence that Hsp90 chaperone activity is regulated by reversible acetylation and controlled by the deacetylase HDAC6. We show that HDAC6 functions as an Hsp90 deacetylase. Inactivation of HDAC6 leads to Hsp90 hyperacetylation, its dissociation from an essential cochaperone, p23, and a loss of chaperone activity. In HDAC6-deficient cells, Hsp90-dependent maturation of the glucocorticoid receptor (GR) is compromised, resulting in GR defective in ligand binding, nuclear translocation, and transcriptional activation. Our results identify Hsp90 as a target of HDAC6 and suggest reversible acetylation as a unique mechanism that regulates Hsp90 chaperone complex activity.

contributed equally to this workThe tumor suppressor p53 is activated in response to many types of cellular and environmental insults via mechanisms involving post-translational modi®cation.Here we demonstrate that, unlike phosphorylation, p53 invariably undergoes acetylation in cells exposed to a variety of stress-inducing agents including hypoxia, anti-metabolites, nuclear export inhibitor and actinomycin D treatment. In vivo, p53 acetylation is mediated by the p300 and CBP acetyltransferases. Overexpression of either p300 or CBP, but not an acetyltransferase-de®cient mutant, ef®ciently induces speci®c p53 acetylation. In contrast, MDM2, a negative regulator of p53, actively suppresses p300/CBPmediated p53 acetylation in vivo and in vitro. This inhibitory activity of MDM2 on p53 acetylation is in turn abrogated by tumor suppressor p19 ARF , indicating that regulation of acetylation is a central target of the p53±MDM2±p19 ARF feedback loop. Functionally, inhibition of deacetylation promotes p53 stability, suggesting that acetylation plays a positive role in the accumulation of p53 protein in stress response. Our results provide evidence that p300/CBP-mediated acetylation may be a universal and critical modi®-cation for p53 function. Keywords: acetylation/CBP/MDM2/p300/p53 IntroductionThe tumor suppressor p53 plays a critical role in human cancer formation. In response to a variety of stress signals, often associated with the progression of neoplastic diseases, p53 becomes activated and induces cell cycle arrest and/or programmed cell death (apoptosis). By eliminating damaged and potentially dangerous cells that might otherwise become cancerous, p53 suppresses tumor formation. In unstressed cells, p53 is latent and is maintained at low levels by targeted degradation mediated by its negative regulator, MDM2 (reviewed in Freedman et al., 1999). The critical role of MDM2 in regulating p53 is best illustrated by a study carried out in mice where inactivation of p53 was shown to completely rescue the embryonic lethality caused by the loss of MDM2 function (Montes de Oca Luna et al., 1995). MDM2 counteracts p53 tumor suppressor activity by physically binding to p53 and suppressing its transcriptional activity. MDM2 also functions as the p53 ubiquitin ligase and triggers its degradation (reviewed in Freedman et al., 1999). This latter activity requires the Ring ®nger domain located at the C-terminus of MDM2 (Fang et al., 2000), and may also involve the acetyltransferase p300, which binds both MDM2 and p53 (Grossman et al., 1998). Therefore, MDM2 negatively regulates p53 by at least two independent mechanisms.The activation and stabilization of p53 are thought to be mediated by speci®c protein modi®cations, with phosphorylation being the major focus of earlier studies (reviewed in Giaccia and Kastan, 1998;Appella and Anderson, 2000). Although the exact functions of speci®c phosphorylation events remain controversial, evidence indicates that they probably contribute to both the stabilization and activation of p53. For ex...