Attenuation of glucocorticoid signaling through targeted degradation of p300 via the 26S proteasome pathway

The impact of hepatocyte-specific deletion of hypoxia-inducible factors on the development of polymicrobial sepsis with focus on GR and PPARα function

Introduction

Polymicrobial sepsis causes acute anorexia (loss of appetite), leading to lipolysis in white adipose tissue and proteolysis in muscle, and thus release of free fatty acids (FFAs), glycerol and gluconeogenic amino acids. Since hepatic peroxisome proliferator-activated receptor alpha (PPARα) and glucocorticoid receptor (GR) quickly lose function in sepsis, these metabolites accumulate (causing toxicity) and fail to yield energy-rich molecules such as ketone bodies (KBs) and glucose. The mechanism of PPARα and GR dysfunction is not known.

Methods & results

We investigated the hypothesis that hypoxia and/or activation of hypoxia inducible factors (HIFs) might play a role in these issues with PPARα and GR. After cecal ligation and puncture (CLP) in mice, leading to lethal polymicrobial sepsis, bulk liver RNA sequencing illustrated the induction of the genes encoding HIF1α and HIF2α, and an enrichment of HIF-dependent gene signatures. Therefore, we generated hepatocyte-specific knock-out mice for HIF1α, HIF2α or both, and a new HRE-luciferase reporter mouse line. After CLP, these HRE-luciferase reporter mice show signals in several tissues, including the liver. Hydrodynamic injection of an HRE-luciferase reporter plasmid also led to (liver-specific) signals in hypoxia and CLP. Despite these encouraging data, however, hepatocyte-specific HIF1α and/or HIF2α knock-out mice suggest that survival after CLP was not dependent on the hepatocyte-specific presence of HIF proteins, which was supported by measuring blood levels of glucose, FFAs, and KBs. The HIF proteins were also irrelevant in the CLP-induced glucocorticoid resistance, but we found indications that the absence of HIF1α in hepatocytes causes less inactivation of PPARα transcriptional function.

Conclusion

We conclude that HIF1α and HIF2α are activated in hepatocytes in sepsis, but their contribution to the mechanisms leading to lethality are minimal.

Associations of RPEL1 and miR-1307 gene polymorphisms with disease susceptibility, glucocorticoid efficacy, anxiety, depression, and health-related quality of life in Chinese systemic lupus erythematosus patients

OBJECTIVE

Our present study intended to examine the associations of RPEL1 and miR-1307 gene polymorphisms (rs4917385 and rs7911488) with susceptibility, glucocorticoids (GCs) efficacy, anxiety, depression, and health-related quality of life (HRQoL) in Chinese systemic lupus erythematosus (SLE) patients.

METHODS

Initially, 1000 participants (500 SLE cases and 500 controls) were recruited for the case-control study. Then, 429 cases who received GCs were followed through 12 weeks to explore GCs efficacy, depression, anxiety, and HRQoL. We selected the iMLDR technique for genotyping: RPEL1: rs4917385 (G/T) and miR-1307: rs7911488 (A/G).

RESULTS

The minor G allele of rs7911488 reduced the risk of SLE (p = .024). Four haplotypes consisting of rs4917385 and rs7911488 were associated with SLE susceptibility (p < .025). Both rs4917385 and rs7911488 were associated with anxiety symptoms and physical function (PF) in SLE patients (p < .025). The rs4917385 was associated with depression and its improvement. No statistical significance was found between RPEL1 and miR-1307 gene polymorphisms with GCs efficacy. Meanwhile, additive interaction analysis showed a significant association between RPEL1 and miR-1307 gene polymorphisms with tea consumption in anxiety.

CONCLUSION

RPEL1 and miR-1307 gene polymorphisms (rs4917385 and rs7911488) might be related to SLE susceptibility in Chinese population. Additionally, the two polymorphisms were possibly associated with depression, anxiety, and HRQoL in Chinese SLE population.

Transcriptome analysis of porcine PBMCs reveals lipopolysaccharide-induced immunomodulatory responses and crosstalk of immune and glucocorticoid receptor signaling

The current level of knowledge on transcriptome responses triggered by endotoxins and glucocorticoids in immune cells in pigs is limited. Therefore, in the present study, we treated porcine peripheral blood mononuclear cells (PBMCs) with lipopolysaccharide (LPS) and dexamethasone (DEX) separately or combined for 2 hours. The resultant transcriptional responses were examined by mRNA sequencing. We found that the LPS treatment triggered pronounced inflammatory responses as evidenced by upregulation of pro-inflammatory cytokines, chemokines, and related signaling pathways like NF-κB. Concurrently, a series of downregulated pro-inflammatory and upregulated anti-inflammatory molecules were identified. These are involved in the inhibition of TLR, NF-κB, and MAPK cascades and activation of signaling mediated by Tregs and STAT3, respectively. These findings suggested that LPS initiated also an anti-inflammatory process to prevent an overwhelming inflammatory response. The transcriptome responses further revealed substantial crosstalk of immune responses and glucocorticoid receptor (GR) signaling. This was apparent in four aspects: constitutive inhibition of T cell signaling by DEX through a subset of genes showing no response to LPS; inhibition of LPS-induced inflammatory genes by DEX; attenuation of DEX action by LPS paralleled by the regulation of genes implicated in cytokine and calcium signaling; and DEX-induced changes in genes associated with the activation of pro-inflammatory TLR, NF-κB, iNOS, and IL-1 signaling. Consequently, our study provides novel insights into inflammatory and GR signaling in pigs, as well as an understanding of the application of glucocorticoid drugs for the treatment of inflammatory disorders.

Transcription factor enrichment analysis (TFEA) quantifies the activity of multiple transcription factors from a single experiment

Detecting changes in the activity of a transcription factor (TF) in response to a perturbation provides insights into the underlying cellular process. Transcription Factor Enrichment Analysis (TFEA) is a robust and reliable computational method that detects positional motif enrichment associated with changes in transcription observed in response to a perturbation. TFEA detects positional motif enrichment within a list of ranked regions of interest (ROIs), typically sites of RNA polymerase initiation inferred from regulatory data such as nascent transcription. Therefore, we also introduce muMerge, a statistically principled method of generating a consensus list of ROIs from multiple replicates and conditions. TFEA is broadly applicable to data that informs on transcriptional regulation including nascent transcription (eg. PRO-Seq), CAGE, histone ChIP-Seq, and accessibility data (e.g., ATAC-Seq). TFEA not only identifies the key regulators responding to a perturbation, but also temporally unravels regulatory networks with time series data. Consequently, TFEA serves as a hypothesis-generating tool that provides an easy, rigorous, and cost-effective means to broadly assess TF activity yielding new biological insights.

MLL2 regulates glucocorticoid receptor-mediated transcription of ENACα in human retinal pigment epithelial cells

Glucocorticoids require the glucocorticoid receptor (GR), a type of ligand-dependent nuclear receptor to transmit their downstream effects. Upon glucocorticoid binding, GR associates with glucocorticoid response elements (GREs) and recruits other transcriptional coregulators to activate or repress target gene transcription. Many SET-domain family proteins have been demonstrated to contribute to GR-mediated transcriptional activity. However, whether histone H3K4-specific methyltransferase plays a cell-type-specific role in GR transcriptional regulation remains poorly understood. In this report, we examined MLL2 (KMT2D), a histone-lysine methyltransferase that catalyzes histone H3 lysine 4 methylation (H3K4me). Furthermore, we demonstrated that MLL2 specifically regulates the transcription of some GR target genes (e.g., ENACα and FLJ20371) in ARPE-19 cells, but has no effect in A549 cells. Mechanistically, co-immunoprecipitation assays revealed that MLL2 is associated with GR in a ligand-independent manner in APRE-19 cells. Moreover, chromatin immunoprecipitation analyses demonstrated that MLL2 could co-occupy glucocorticoid response elements (GREs) of GR target genes along with GR following Dex stimulation. Finally, the FAIRE-qPCR results illustrated that MLL2 is pivotal in establishing chromatin structure accessibility at the GREs of ARPE-19 specific genes in the presence of Dex. Taken together, our study determined that MLL2 regulates GR-mediated transcription in a cell-type-specific manner, and we provide a molecular mechanism to explain the specific role of MLL2 in regulating GR target gene expression in ARPE-19 cells.

TNF-α inhibits glucocorticoid receptor-induced gene expression by reshaping the GR nuclear cofactor profile

Glucocorticoid resistance (GCR) is defined as an unresponsiveness to the therapeutic effects, including the antiinflammatory ones of glucocorticoids (GCs) and their receptor, the glucocorticoid receptor (GR). It is a problem in the management of inflammatory diseases and can be congenital as well as acquired. The strong proinflammatory cytokine TNF-alpha (TNF) induces an acute form of GCR, not only in mice, but also in several cell lines: e.g., in the hepatoma cell line BWTG3, as evidenced by impaired Dexamethasone (Dex)-stimulated direct GR-dependent gene up- and down-regulation. We report that TNF has a significant and broad impact on this transcriptional performance of GR, but no impact on nuclear translocation, dimerization, or DNA binding capacity of GR. Proteome-wide proximity-mapping (BioID), however, revealed that the GR interactome was strongly modulated by TNF. One GR cofactor that interacted significantly less with the receptor under GCR conditions is p300. NFκB activation and p300 knockdown both reduced direct transcriptional output of GR whereas p300 overexpression and NFκB inhibition reverted TNF-induced GCR, which is in support of a cofactor reshuffle model. This hypothesis was supported by FRET studies. This mechanism of GCR opens avenues for therapeutic interventions in GCR diseases.

Valproic acid exposure decreases Cbp/p300 protein expression and histone acetyltransferase activity in P19 cells

The teratogenicity of the antiepileptic drug valproic acid (VPA) is well established and its inhibition of histone deacetylases (HDAC) is proposed as an initiating factor. Recently, VPA-mediated HDAC inhibition was demonstrated to involve transcriptional downregulation of histone acetyltransferases (HATs), which was proposed to compensate for the increased acetylation resulting from HDAC inhibition. Cbp and p300 are HATs required for embryonic development and deficiencies in either are associated with congenital malformations and embryolethality. The objective of the present study was to characterize Cbp/p300 following VPA exposure in P19 cells. Consistent with previous studies, exposure to 5mM VPA over 24h induced a moderate decrease in Cbp/p300 mRNA, which preceded a strong decrease in total cellular protein mediated by ubiquitin-proteasome degradation. Nuclear Cbp/p300 protein was also decreased following VPA exposure, although to a lesser extent. Total cellular and nuclear p300 HAT activity was reduced proportionately to p300 protein levels, however while total cellular HAT activity also decreased, nuclear HAT activity was unaffected. Using the Cbp/p300 HAT inhibitor C646, we demonstrated that HAT inhibition similarly affected many of the same endpoints as VPA, including increased reactive oxygen species and caspase-3 cleavage, the latter of which could be attenuated by pre-treatment with the antioxidant catalase. C646 exposure also decreased NF-κB/p65 protein, which was not due to reduced mRNA and was not attenuated with catalase pre-treatment. This study provides support for an adaptive HAT response following VPA exposure and suggests that reduced Cbp/p300 HAT activity could contribute to VPA-mediated alterations.

Trichostatin A, a histone deacetylase inhibitor suppresses NADPH Oxidase 4-Derived Redox Signalling and Angiogenesis

Histone deacetylase (HDAC) inhibitors are known to suppress abnormal development of blood vessels. Angiogenic activity in endothelial cells depends upon NADPH oxidase 4 (Nox4)-dependent redox signalling. We set out to study whether the HDAC inhibitor trichostatin A (TSA) affects Nox4 expression and angiogenesis. Nox4 expression was measured by real time PCR and Western blot analysis in endothelial cells. Hydrogen peroxide (H2 O2 ) was measured by amplex(®) red assay in endothelial cells. Nox4 was knocked down by Nox4 shRNA. In vitro angiogenic activities such migration and tubulogenesis were assessed using wound healing and Matrigel assays, respectively. In vivo angiogenic activity was assessed using subcutaneous sponge assay in C57Bl/6 and Nox4-deficient mice. Trichostatin A reduced Nox4 expression in a time- and concentration-dependent manner. Both TSA and Nox4 silencing decreased Nox4 protein and H2 O2 . Mechanistically, TSA reduced expression of Nox4 via ubiquitination of p300- histone acetyltransferase (p300-HAT). Thus, blocking of the ubiquitination pathway using an inhibitor of ubiquitin-activating enzyme E1 (PYR-41) prevented TSA inhibition of Nox4 expression. Trichostatin A also reduced migration and tube formation, and these effects were not observed in Nox4-deficient endothelial cells. Finally, transforming growth factor beta1 (TGFβ1) enhanced angiogenesis in sponge model in C57BL/6 mice. This response to TGFβ1 was substantially reduced in Nox4-deficient mice. Similarly intraperitoneal infusion of TSA (1 mg/kg) also suppressed TGFβ1-induced angiogenesis in C57BL/6 mice. Trichostatin A reduces Nox4 expression and angiogenesis via inhibition of the p300-HAT-dependent pathway. This mechanism might be exploited to prevent aberrant angiogenesis in diabetic retinopathy, complicated vascular tumours and malformations.

Determination of the Role of CBP- and p300-Mediated Wnt Signaling on Colonic Cells

BACKGROUND

The Wnt signaling pathway, mediated through active beta-catenin, is responsible for initiating the majority of cases of human colorectal cancer (CRC), and we have previously shown that hyperactivation of this pathway by histone deacetylase inhibitors (HDACis), such as butyrate, can induce the death of CRC cells. An important cellular switch that mediates the effects of Wnt-signaling activation is variation in the association between beta-catenin and the transcriptional coactivators cAMP response element binding (CREB) binding protein (CBP) and p300. Association of CBP with beta-catenin is thought to activate a set of genes linked to cell proliferation, while the p300-mediated Wnt genetic program is believed to promote cell differentiation. Small molecule agents have been discovered that modulate CBP/p300 Wnt transcriptional programs by altering the association of CBP and p300 to beta-catenin. ICG-001 and ICG-427 inhibit CBP- and p300-mediated Wnt activity, respectively, while IQ-1 prevents the shift from CBP-mediated to a p300-mediated Wnt activity.

OBJECTIVE

Aim 1 of this proposal is designed to determine the role of CBP- and p300-mediated Wnt signaling in the response of CRC cells to HDACis. Aim 2 is to determine the role of CBP and p300 in the maintenance of high- and low-Wnt fractions in CRC cell line. Aim 3 will compare the effects of CBP- and p300-mediated Wnt activity on CRC initiation and progression.

METHODS

In Aim 1, cells will be cotreated with HDACis and ICG-001, ICG-427, or IQ-1 and the levels of Wnt activity, apoptosis, proliferation, differentiation, and CBP- or p300-beta-catenin binding measured. Aim 2 of this proposal may mirror similar heterogeneity observed in human tumors and which may be of clinical significance. Aim 3 will use CRC cell line model systems of initiation and progression: the normal colon cell lines CCD-841CoN, the adenoma line LT97, the primary colon carcinoma cell line SW480, and the lymph node metastasis cell line SW620. Cells will be treated with HDACis and the small molecule agents, and assayed as described above.

RESULTS

We will also attempt to use changes in CBP- and p300-mediated Wnt signaling to shift colonic cells between cell type, modifying CBP- and p300-mediated gene expression in the LT97 adenoma line to shift the adenoma phenotype to more characteristic of the CCD-841CoN normal cells, or the SW480 carcinoma cells. We will use microarray analyses to determine the patterns of gene expression responsible for these CBP- or p300-mediated changes in colonic neoplastic phenotype.

CONCLUSIONS

The findings generated from this study will lead to future, more in-depth projects to further dissect the action of CBP/p300 Wnt-mediated transcriptional programs in colonic neoplasia, with an emphasis on methods to modulate these genetic programs for chemopreventive effect.

Molecular Basis for the Regulation of Transcriptional Coactivator p300 in Myogenic Differentiation

Skeletal myogenesis is a highly ordered process which specifically depends on the function of transcriptional coactivator p300. Previous studies have established that Akt/protein kinase B (PKB), a positive regulator of p300 in proliferating cells, is also important for proper skeletal muscle development. Nevertheless, it is not clear as to how the p300 is regulated by myogenic signaling events given that both p300 and Akt are involved in many cellular processes. Our studies revealed that the levels of p300 protein are temporally maintained in ligand-enhanced skeletal myocyte development. Interestingly, this maintenance of p300 protein is observed at the stage of myoblast differentiation, which coincides with an increase in Akt phosphorylation. Moreover, regulation of p300 during myoblast differentiation appears to be mediated by Akt signaling. Blunting of p300 impairs myogenic expression and myoblast differentiation. Thus, our data suggests a particular role for Akt in myoblast differentiation through interaction with p300. Our studies also establish the potential of exploiting p300 regulation and Akt activation to decipher the complex signaling cascades involved in skeletal muscle development.

BRMS1 suppresses lung cancer metastases through an E3 ligase function on histone acetyltransferase p300

The mechanisms through which the metastasis suppressor gene BRMS1 functions are poorly understood. Herein, we report the identification of a previously undescribed E3 ligase function of BRMS1 on the histone acetyltransferase p300. BRMS1 induces polyubiquitination of p300, resulting in its proteasome-mediated degradation. We identify BRMS1 as the first eukaryote structural mimic of the bacterial IpaH E3 ligase family and establish that the evolutionarily conserved CXD motif located in BRMS1 is responsible for its E3 ligase function. Mutation of this E3 ligase motif not only abolishes BRMS1-induced p300 polyubiquitination and degradation, but importantly, dramatically reduces the metastasis suppressor function of BRMS1 in both in vitro and in vivo models of lung cancer metastasis.

Involvement of GR and p300 in the induction of H6PD by cortisol in human amnion fibroblasts

Human fetal membranes express 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), which reduces biologically inert cortisone to active cortisol and may provide an extraadrenal source of cortisol mediating fetal development and parturition. The reductase activity of 11β-HSD1 depends on the availability of the cofactor reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) derived from the enzymatic activity of hexose-6-phosphodehydrogenase (H6PD). Based on the feed-forward induction of 11β-HSD1 by glucocorticoids in human fetal membranes, we hypothesize that glucocorticoids simultaneously induce H6PD in the fetal membranes. We found a parallel distribution of H6PD and 11β-HSD1 in the amnion, chorion, and decidua. In cultured human amnion fibroblasts, small interfering RNA-mediated knockdown of H6PD expression significantly attenuated the conversion of cortisone to cortisol. Cortisol (0.01-1 μm) induced H6PD expression in a concentration-dependent manner, which was attenuated by glucocorticoid receptor (GR) antagonist RU486. Cortisol induced the expression of p300, a histone acetyltransferase, whereas C646, an inhibitor of p300, attenuated the induction of H6PD by cortisol. Coimmunoprecipitation revealed GR and p300 in the same nuclear protein complex upon cortisol stimulation. Chromatin immunoprecipitation showed that cortisol increased the binding of p300 and GR to H6PD promoter and the acetylation of histone 3 lysine 9 on the promoters. In conclusion, the induction of H6PD by cortisol requires the participation of GR and p300 as well as the acetylation of H3K9 by p300. This may be a prerequisite for the parallel induction of reductase activity of 11β-HSD1 in human amnion fibroblasts in a feed-forward loop that may influence fetal development and the onset of parturition.

Expression of Nuclear Receptor Coactivators in the Human Fetal Membranes at Term before and after Labor

Human fetal membranes play an important role in term and preterm labor and are responsive to steroids. We examined the expression of steroid receptor coactivators in fetal membranes obtained prior to and following labor at term. Proteins were localized by immunohistochemistry, Western analysis was carried out in nuclear extracts, and mRNA levels were determined by real-time RT-PCR. SRC-1, SRC-2, p300, and PCAF proteins were present in all nuclear extracts. The amnion nuclei expressed higher levels of SRC-1, p300, and PCAF than nuclei from the chorion-decidua, whereas the reverse was true for SRC-2. Chorion-decidua from patients before labor expressed higher levels of SRC-1 than those from patients after labor. Also, the PCAF level was higher in the amnion obtained before labor than the same tissue obtained after labor. In contrast to the protein expression, mRNA levels of SRC-1 and p300 were higher in the chorion-decidua compared to the amnion, whereas there was no difference in levels of SRC-2 and PCAF mRNAs between these two tissues. These data underline that the regulation of the expression of the coactivators in these tissues occurs during labor and is complex and tissue specific.

Promoter context determines the role of proteasome in ligand-dependent occupancy of retinoic acid responsive elements

Retinoid acid receptors are DNA-binding proteins mediating the biological effects of ligands through transcriptional activation. It is known that the activity of the 26S proteasome is important for nuclear receptor-activated gene transcription. However, the molecular mechanism by which the 26S proteasome participates in this process is not well understood. Here we report that the proteasome activity is essential for ligand-dependent interaction of RAR with its co-regulators such as SRC, p300 and RXR. We also determined that the proteasome activity is required for the association of liganded RAR to the genomic DNA and, consequently, for the recruitment of the coactivator complex to the retinoic acid responsive elements. Moreover, the requirement of proteasome activity for the activator activity of RAR is determined by the promoter context. Our study suggests that the 26S proteasome regulates directly the activity of RAR as an activator.

Chemical biology of histone acetyltransferase natural compounds modulators

Histone acetyltransferases (HATs) are a class of epigenetic enzymes crucial for chromatin restructuring and transcriptional regulation in eukaryotic cells, thus being a promising target for therapeutic development. Nonetheless, differently from histone deacetylases (HDACs) inhibitors, there is still paucity of small-molecule modulators of HAT activity. After a decline during past decade, natural products and their derivatives could be once again a valuable tool in the lead discovery process and meet such need of Novel Chemical Entities (NCEs). In this review, we will provide a comprehensive summary on the discovery of small-molecule HAT modulators from naturally occurring molecular scaffolds.

Catalytic activity of the proteasome fine-tunes Brg1-mediated chromatin remodeling to regulate the expression of inflammatory genes

The induction of key pro-inflammatory genes is regulated by the SWI/SNF class of ATP-dependent remodeling complexes. In particular, the catalytic ATPase subunit, Brg1, is distinctly involved in the chromatin remodeling required for activating pro-inflammatory genes in a temporally, ordered fashion. Despite advances in our understanding of the role for Brg1 in the kinetics of inflammatory responses, little is known about the precise mechanisms which down-regulate Brg1 activity. Biochemical studies implicate a role for the proteasome in the regulation of SWI/SNF assembly and function; however, it is unclear if proteasome-dependent mechanisms modulate its remodeling activity or recruitment to chromatin in order to regulate inflammatory gene transcription. We now demonstrate for the first time that proteasome function represents an important mechanism for limiting inducible association of Brg1 with promoters of SWI/SNF-regulated, inflammatory genes. As a result, catalytic activity of the proteasome fine-tunes the kinetics of inflammatory gene transcription by inhibiting both premature and persistent chromatin remodeling at SWI/SNF-regulated genes. These results provide mechanistic insight into the interplay between nucleosome remodeling, inflammation and proteasome, and underscore the critical role of the proteasome in controlling both extent and duration of inflammatory responses.

Epigenetics in human melanoma

BACKGROUND

Recent technological advances have allowed us to examine the human genome in greater detail than ever before. This has opened the door to an improved understanding of the gene expression patterns involved with cancer.

METHODS

A review of the literature was performed to determine the role of epigenetic modifications in human melanoma. We focused the search on histone deacetylation, methylation of gene promoter regions, demethylation of CpG islands, and the role of microRNA. We examined the relationship between human melanoma epigenetics and their importance in tumorigenesis, tumor progression, and inhibition of metastasis. The development and clinical application of select pharmacologic agents are also discussed.

RESULTS

We identified several articles that have extensively studied the role of epigenetics in melanoma, further elucidating the complex processes involved in gene regulation and expression. Several new agents directly affect epigenetic mechanisms in melanoma, with divergent affects on the metastatic potential of melanoma.

CONCLUSIONS

Epigenetic mechanisms have emerged as having a central role in gene regulation of human melanoma, including the identification of several putative tumor suppressor genes and oncogenes. Further research will focus on the development of novel therapeutics that will likely target and alter such epigenetic changes.

The F-box protein beta-TrCp1/Fbw1a interacts with p300 to enhance beta-catenin transcriptional activity

Hyperactivated beta-catenin is a commonly found molecular abnormality in colon cancer, and its nuclear accumulation is thought to promote the expression of genes associated with cellular proliferation and transformation. The p300 transcriptional co-activator binds to beta-catenin and facilitates transcription by recruiting chromatin remodeling complexes and general transcriptional apparatus. We have found that beta-TrCp1/Fbw1a, a member of the Skp1/Cullin/Rbx1/F-box E3 ubiquitin ligase complex, binds directly to p300 and co-localizes with it to beta-catenin target gene promoters. Our data show that Fbw1a, which normally targets beta-catenin for degradation, works together with p300 to enhance the transcriptional activity of beta-catenin, whereas other F-box/WD40 proteins do not. Fbw1a also cooperates with p300 to co-activate transcription by SMAD3, another Fbw1a ubiquitylation target, but not p53 or HIF-1alpha, which are substrates for other ubiquitin ligase complexes. These results suggest that, although Fbw1a is part of a negative feedback loop for controlling beta-catenin levels in normal cells, its overexpression and binding to p300 may contribute to hyperactivated beta-catenin transcriptional activity in colon cancer cells.

Diverse regulatory mechanisms of eukaryotic transcriptional activation by the proteasome complex

The life of any protein within a cell begins with transcriptional activation, and ends with proteolytic degradation. Intriguingly, the 26S proteasome complex, a non-lysosomal protein degradation machine comprising the 20S proteolytic core and 19S regulatory particles, has been implicated in intimate regulation of eukaryotic transcriptional activation through diverse mechanisms in a proteolysis-dependent as well as independent manner. Here, we discuss the intricate mechanisms of such proteasomal regulation of eukaryotic gene activation via multiple pathways.

PML activates transcription by protecting HIPK2 and p300 from SCFFbx3-mediated degradation

PML, a nuclear protein, interacts with several transcription factors and their coactivators, such as HIPK2 and p300, resulting in the activation of transcription. Although PML is thought to achieve transcription activation by stabilizing the transcription factor complex, little is known about the underlying molecular mechanism. To clarify the role of PML in transcription regulation, we purified the PML complex and identified Fbxo3 (Fbx3), Skp1, and Cullin1 as novel components of this complex. Fbx3 formed SCF(Fbx3) ubiquitin ligase and promoted the degradation of HIPK2 and p300 by the ubiquitin-proteasome pathway. PML inhibited this degradation through a mechanism that unexpectedly did not involve inhibition of the ubiquitination of HIPK2. PML, Fbx3, and HIPK2 synergistically activated p53-induced transcription. Our findings suggest that PML stabilizes the transcription factor complex by protecting HIPK2 and p300 from SCF(Fbx3)-induced degradation until transcription is completed. In contrast, the leukemia-associated fusion PML-RARalpha induced the degradation of HIPK2. We discuss the roles of PML and PML-retinoic acid receptor alpha, as well as those of HIPK2 and p300 ubiquitination, in transcriptional regulation and leukemogenesis.

Combination of ursodeoxycholic acid and glucocorticoids upregulates the AE2 alternate promoter in human liver cells

Primary biliary cirrhosis (PBC) is a cholestatic disease associated with autoimmune phenomena and alterations in both biliary bicarbonate excretion and expression of the bicarbonate carrier AE2. The bile acid ursodeoxycholic acid (UCDA) is currently used in treatment of cholestatic liver diseases and is the treatment of choice in PBC; however, a subset of PBC patients respond poorly to UDCA monotherapy. In these patients, a combination of UDCA and glucocorticoid therapy appears to be beneficial. To address the mechanism of this benefit, we analyzed the effects of UDCA and dexamethasone on AE2 gene expression in human liver cells from hepatocyte and cholangiocyte lineages. The combination of UDCA and dexamethasone, but not UDCA or dexamethasone alone, increased the expression of liver-enriched alternative mRNA isoforms AE2b1 and AE2b2 and enhanced AE2 activity. Similar effects were obtained after replacing UDCA with UDCA conjugates. In in vitro and in vivo reporter assays, we found that a UDCA/dexamethasone combination upregulated human AE2 alternate overlapping promoter sequences from which AE2b1 and AE2b2 are expressed. In chromatin immunoprecipitation assays, we demonstrated that combination UCDA/dexamethasone treatment induced p300-related interactions between HNF1 and glucocorticoid receptor on the AE2 alternate promoter. Our data provide a potential molecular explanation for the beneficial effects of the combination of UDCA and glucocorticoids in PBC patients with inadequate response to UDCA monotherapy.

Negative regulation of transcription coactivator p300 by orphan receptor TR3

p300 regulates the transcriptional activity of a variety of transcription factors by forming an activation complex and/or promoting histone acetylation. Here, we show a unique characteristic of orphan receptor TR3 in negatively regulating the function of p300. TR3 was found to interact with p300 and inhibited the acetylation of transcription factors induced by p300, resulting in the repression of their transcriptional activity. Further analysis revealed that both a conserved transcriptional adapter motif (TRAM) in p300 and a specific sequence FLELFIL in TR3 were critical for their interaction. TR3 binding completely covered the histone acetyltransferase (HAT) domain of p300 and resulted in suppression of the HAT activity, as the p300-induced histone H3 acetylation and transcription were inhibited with the presence TR3. Furthermore, an agonist of TR3, a natural octaketide isolated from Dothiorella sp. HTF3 of an endophytical fungus, was shown to be a potent compound for inhibiting p300 HAT activity (IC₅₀ = 1.5 μg/ml) in vivo. More importantly, this agonist could repress the transcriptional activity of transcription factors, and proliferation of cancer cells. Taken together, our results not only delineate a novel transcriptional repressor function for TR3, but also reveal its modulation on p300 HAT activity as the underlying mechanism.

Bombesin attenuates pre-mRNA splicing of glucocorticoid receptor by regulating the expression of serine-arginine protein p30c (SRp30c) in prostate cancer cells

Although glucocorticoids are frequently administered to patients with hormone refractory prostate cancer, their therapeutic effectiveness is limited by the development of glucocorticoid resistance. The molecular mechanisms of glucocorticoid resistance are unknown but are believed to involve neuropeptide growth factors and cytokines. We examined the functional interaction between bombesin and dexamethasone in PC-3 cells and found that bombesin could act as a survival factor by interfering with dexamethasone-mediated growth inhibition. Because glucocorticoids exert their effects through glucocorticoid receptors (GRs), we measured the expression of GR alpha and GR beta isoforms in the presence of bombesin. Western blotting and real time PCR revealed bombesin induced expression of GR beta, but not GR alpha. Because GR isoforms are generated by alternative splicing of a common GR gene, we examined the expression of serine-arginine (SR) proteins involved in alternative splicing, and found that the expression of SRp30 was induced by bombesin in PC-3 cells. To characterize the role of SRp30 in splicing of GR isoforms, siRNAs specific to various SRp30 isoforms were transfected into PC-3 cells. We found that suppression of SRp30c expression by siRNA specifically antagonized bombesin's effect on glucocorticoid-mediated inhibition of PC cells, suggesting that bombesin-induced expression of SRp30c affects GR pre-mRNA splicing, leading to increased GR beta expression and contributing to glucocorticoid resistance in PC cells.

Regulation of protein turnover by acetyltransferases and deacetylases

Lysine acetylation was first discovered as a post-translational modification of histones and has long been considered as a direct regulator of chromatin structure and function. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) are the enzymes involved in this modification and they were thought to act as critical gene silencers or activators. Further investigations indicated that lysine acetylation can also occur in non-histone proteins and pointed to HATs and HDACs as multifunctional factors, acting not only on transcription but also on a variety of other cellular processes. One of these processes is the regulation of protein stability. Indeed, at least four independent HATs, namely CBP, p300, PCAF and TAF1, and one HDAC, HDAC6, possess intrinsic ubiquitin-linked functions in addition to their regular HAT/HDAC activities. Furthermore HATs and HDACs can be found in multi-subunit complexes with enzymes of the ubiquitination machinery. Moreover, lysine acetylation itself was found to directly or indirectly affect protein stability. These observations reveal therefore a tight link between protein lysine acetylation and ubiquitination and designate the acetylation machinery as a determinant element in the control of cellular proteolytic activities.

Dexamethasone-induced protein degradation in cultured myotubes is p300/HAT dependent

Muscle proteolysis during sepsis and other catabolic conditions is, at least in part, regulated by glucocorticoids. Dexamethasone-treated myotubes are a commonly used in vitro model of muscle wasting. We reported recently that treatment of cultured L6 myotubes with dexamethasone resulted in increased gene and protein expression of the nuclear cofactor p300 but it is not known whether glucocorticoids upregulate p300 histone acetyl transferase (HAT) activity in muscle and whether p300/HAT activity regulates glucocorticoid-induced muscle proteolysis. Here, we found that treatment of cultured L6 myotubes with dexamethasone resulted in increased nuclear p300/HAT activity. Treatment of myotubes with p300 siRNA or transfection of muscle cells with a plasmid expressing p300 that was mutated in its HAT activity domain blocked the dexamethasone-induced increase in protein degradation, supporting a role of p300/HAT in glucocorticoid-induced muscle proteolysis. In addition to increased HAT activity, treatment of the myotubes with dexamethasone resulted in reduced nuclear expression and activity of histone deacetylases (HDACs) 3 and 6. When myotubes were treated with the HDAC inhibitor trichostatin A, protein degradation increased to the same degree as in dexamethasone-treated myotubes. The results suggest that glucocorticoids increase HAT and decrease HDAC activities in muscle, changes that both favor hyperacetylation. The results also provide evidence that dexamethasone-induced protein degradation in cultured myotubes is, at least in part, regulated by p300/HAT activity.

Delayed spinocerebellar ataxia in transgenic mice expressing mutant ubiquitin

Spinocerebellar ataxia type 1 (SCA1) is an incurable neurodegenerative disease resulting from loss of Purkinje neurones within the cerebellum. The ubiquitin proteasome pathway (UPP) has been implicated in SCA1 but the role of proteolysis in the disease is still poorly understood. To further investigate this issue in vivo, genetic crosses were performed between an established mouse model of SCA1 and novel strains expressing elevated levels of wild type or mutant isoforms of ubiquitin. The K48R mutant isoform of ubiquitin (a dominant negative inhibitor of proteolysis) was found to significantly delay the deterioration of Purkinje neurones as evidenced by behavioural, morphological, and molecular indicators. This delay was accompanied by stabilization of p300/CBP, transcriptional mediators whose abundance and activity would otherwise decline in the course of the SCA1 disease, and persistence of protein kinase C gamma (PKCgamma), a protein involved in Purkinje cell dendritic development that is mutated in one form of spinocerebellar ataxia. Whereas the stabilization of p300/CBP was found to occur at the post-translational level the modulation of PKCgamma was at the level of transcription. These results are consistent with transcriptional dysregulation as a key mechanism in neurodegeneration through loss of p300/CBP. Further, the results suggest that the UPP is a potentially useful target for the development of novel therapies for the treatment of neurodegenerative disease.

Dexamethasone upregulates the expression of the nuclear cofactor p300 and its interaction with C/EBPbeta in cultured myotubes

Muscle wasting during sepsis and other catabolic conditions is, at least in part, mediated by glucocorticoids and is associated with upregulated transcription of multiple genes in the ubiquitin-proteasome proteolytic pathway. In addition to transcription factors, nuclear cofactors, including p300, regulate gene transcription. We tested the hypothesis that glucocorticoids upregulate the expression of p300 in muscle cells. Treatment of cultured L6 myotubes, a rat skeletal muscle cell line, with dexamethasone resulted in a dose- and time-dependent increase in p300 protein and mRNA levels. Surprisingly, the effect of dexamethasone on p300 levels was not inhibited by the glucocorticoid receptor (GR) antagonist RU38486 and RU38486 exerted an agonist effect on p300, increasing its expression. Co-immunoprecipitation showed that treatment of the myotubes with dexamethasone resulted in protein-protein interaction between p300 and C/EBPbeta, but not C/EBPdelta. The present results suggest that glucocorticoids upregulate the expression of p300 and its interaction with C/EBPbeta in skeletal muscle. Increased expression and activity of p300 may be involved in the regulation of gene transcription in glucocorticoid-dependent muscle wasting.

Regulatory cross-talk between lysine acetylation and ubiquitination: role in the control of protein stability

It is now becoming apparent that cross-talk between two protein lysine modifications, acetylation and ubiquitination, is a critical regulatory mechanism controlling vital cellular functions. The most apparent effect is the inhibition of proteasome-mediated protein degradation by lysine acetylation. Analysis of the underlying mechanisms, however, shows that, besides a direct competition between the two lysine modifications, more complex and indirect processes also connect these two signalling pathways. These findings point to protein lysine acetylation as a potential regulator of various cellular functions involving protein ubiquitination.

The transcriptional coactivator p300 plays a critical role in the hypertrophic and protective pathways induced by phenylephrine in cardiac cells but is specific to the hypertrophic effect of urocortin

Anacardic acid is an alkylsalicylic acid obtained from cashew-nut-shell liquid, and is a potent inhibitor of p300 histone acetyl-transferase (HAT) activity. We have used anacardic acid to prevent the induction of hypertrophy in isolated neonatal rat cardiomyocytes. Hypertrophy was detected as an increase in cell size, the rearrangement of sarcomeres into a striated pattern, and the induction of embryonic genes beta-MHC and ANF. p300 inhibition was equally effective at preventing hypertrophy whether it was induced by treatment with the alpha1-adrenergic agonist, phenylephrine, or by treatment with urocortin, a member of the corticotrophin-releasing-factor family, which stimulates specific G protein-coupled receptors. Spiruchostatin A is a natural-product inhibitor of histone deacetylases (HDAC) similar to the depsipeptide FK228 molecule. We have recently synthesized spiruchostatin A and now show that, although HDACs act in opposition to HATs, spiruchostatin A has the same effect as anacardic acid, that is, it prevents the induction of hypertrophy in response to phenylephrine or urocortin. Pretreatment with either phenylephrine or urocortin reduced the extent of death observed after the exposure of isolated cardiomyocytes to simulated ischaemia and reoxygenation. Inhibition of p300 or HDAC activity eliminated the protection conferred by phenylephrine; however, it did not affect the protection conferred by urocortin. Therefore, it might eventually be possible to use chemical inhibitors such as these in a therapeutic setting to dissociate the protective effect and hypertrophic effect of urocortin, enhancing the survival of cardiomyocytes exposed to transient ischemia, while inhibiting the hypertrophic pathway that would otherwise be induced concurrently.

Expanding functional diversity of the coactivators

Nuclear receptors (NR) function as ligand-regulated transcription factors that transduce hormonal signals from steroid hormones and other lipophillic ligands. NR-mediated transcription depends on coactivators, a diverse group of proteins that affect the transcriptional machinery in a variety of ways such as via their associated enzymatic activities as histone acetyltransferases, methyltransferases, ubiquitin ligases or as agents that integrate signaling via kinase-signaling pathways. Coactivators have various roles in the transcriptional process (i) as molecules that influence key points in the different stages of transcription, (ii) as integrators of environmental growth-factor and steroid-hormone signals, and (iii) as agents of carcinogenesis.

B56 regulatory subunit of protein phosphatase 2A mediates valproic acid-induced p300 degradation

Transcriptional coactivator p300 is required for embryonic development and cell proliferation. Valproic acid, a histone deacetylase inhibitor, is widely used in the therapy of epilepsy and bipolar disorder. However, it has intrinsic teratogenic activity through unidentified mechanisms. We report that valproic acid stimulates proteasome-dependent p300 degradation through augmentation of gene expression of the B56gamma regulatory subunits of protein phosphatase 2A. The B56gamma3 regulatory and catalytic subunits of protein phosphatase 2A interact with p300. Overexpression of the B56gamma3 subunit leads to proteasome-mediated p300 degradation and represses p300-dependent transcriptional activation, which requires the B56gamma3 interaction domain of p300. Conversely, silencing of the B56gamma subunit expression by RNA interference increases the stability and transcriptional activity of the coactivator. Our study establishes the functional interaction between protein phosphatase 2A and p300 activity and provides direct evidence for signal-dependent control of p300 function.

GSK3 beta-dependent phosphorylation of the alpha NAC coactivator regulates its nuclear translocation and proteasome-mediated degradation

c-Jun is an immediate-early gene whose degradation by the proteasome pathway is required for an efficient transactivation. In this report, we demonstrated that the c-Jun coactivator, nascent polypeptide associated complex and coactivator alpha (alphaNAC) was also a target for degradation by the 26S proteasome. The proteasome inhibitor lactacystin increased the metabolic stability of alphaNAC in vivo, and lactacystin, MG-132, or epoxomicin treatment of cells induced nuclear translocation of alphaNAC. We have shown that the ubiquitous kinase glycogen synthase kinase 3beta (GSK3beta) directly phosphorylated alphaNAC in vitro and in vivo. Inhibition of the endogenous GSKappa3beta activity resulted in the stabilization of this coactivator in vivo. We identified the phosphoacceptor site in the C-terminal end of the coactivator, on position threonine 159. We demonstrated that the inhibition of GSK3beta activity by treatment of cells with the inhibitor 5-iodo-indirubin-3'-monoxime, as well as with a dominant-negative GSK3beta mutant, induced the accumulation of alphaNAC in the nuclei of cells. Mutation of the GSK3beta phosphoacceptor site on alphaNAC induced a significant increase of its coactivation potency. We conclude that GSK3beta-dependent phosphorylation of alphaNAC was the signal that directed the protein to the proteasome. The accumulation of alphaNAC caused by the inhibition of the proteasome pathway or the activity of GSK3beta contributes to its nuclear translocation and impacts on its coactivating function.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

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.

Coregulator function: a key to understanding tissue specificity of selective receptor modulators

Ligands for the nuclear receptor superfamily control many aspects of biology, including development, reproduction, and homeostasis, through regulation of the transcriptional activity of their cognate receptors. Selective receptor modulators (SRMs) are receptor ligands that exhibit agonistic or antagonistic biocharacter in a cell- and tissue context-dependent manner. The prototypical SRM is tamoxifen, which as a selective estrogen receptor modulator, can activate or inhibit estrogen receptor action. SRM-induced alterations in the conformation of the ligand-binding domains of nuclear receptors influence their abilities to interact with other proteins, such as coactivators and corepressors. It has been postulated, therefore, that the relative balance of coactivator and corepressor expression within a given target cell determines the relative agonist vs. antagonist activity of SRMs. However, recent evidence reveals that the cellular environment also plays a critical role in determining SRM biocharacter. Cellular signaling influences the activity and subcellular localization of coactivators and corepressors as well as nuclear receptors, and this contributes to gene-, cell-, and tissue-specific responses to SRM ligands. Increased understanding of the effect of cellular environment on nuclear receptors and their coregulators has the potential to open the field of SRM discovery and research to many members of the nuclear receptor superfamily.

Concomitant increase of histone acetyltransferase activity and degradation of p300 during retinoic acid-induced differentiation of F9 cells

The p300 and closely related CBP histone acetyltransferases (HAT) function as global transcriptional co-activators that play roles in many cell differentiation and signal transduction pathways. Despite their similarities, p300 and CBP have distinct functions during retinoic acid-induced differentiation of mouse F9 embryonal carcinoma cells. F9 cells constitute a well established model system for investigating the first steps of early development and retinoic acid signaling ex vivo. p300, but not CBP, was shown to be essential for F9 differentiation. In this study we have investigated the regulation of p300 during F9 differentiation. We report a dramatic decrease of p300, but not CBP protein levels, after 48 h of retinoic acid treatment. p300 is degraded via the ubiquitin-proteasome pathway. Although the large majority of p300 is degraded, its global HAT activity stays constant during F9 differentiation, which means that its specific HAT activity increases considerably. p300 is strongly phosphorylated in both undifferentiated and differentiated F9 cells; its HAT activity, however, is independent of phosphorylation before differentiation and becomes dependent on phosphorylation during differentiation. Furthermore, we show that protein kinase A affects p300 HAT activity both in vivo and in vitro as well as p300 phosphorylation in differentiated cells. Thus, we show that p300 is differentially phosphorylated in undifferentiated versus differentiated cells and that the changes in phosphorylation affect its HAT activity. Moreover, our study suggests an explanation for the functional switch of p300-mediated repression versus activation during F9 differentiation.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.