Mouse steroid receptor coactivator-1 is not essential for peroxisome proliferator-activated receptor alpha-regulated gene expression

The multifaceted therapeutic value of targeting steroid receptor coactivator-1 in tumorigenesis

Steroid receptor coactivator-1 (SRC-1, also known as NCOA1) frequently functions as a transcriptional coactivator by directly binding to transcription factors and recruiting to the target gene promoters to promote gene transcription by increasing chromatin accessibility and promoting the formation of transcriptional complexes. In recent decades, various biological and pathological functions of SRC-1 have been reported, especially in the context of tumorigenesis. SRC-1 is a facilitator of the progression of multiple cancers, including breast cancer, prostate cancer, gastrointestinal cancer, neurological cancer, and female genital system cancer. The emerging multiorgan oncogenic role of SRC-1 is still being studied and may not be limited to only steroid hormone-producing tissues. Growing evidence suggests that SRC-1 promotes target gene expression by directly binding to transcription factors, which may constitute a novel coactivation pattern independent of AR or ER. In addition, the antitumour effect of pharmacological inhibition of SRC-1 with agents including various small molecules or naturally active compounds has been reported, but their practical application in clinical cancer therapy is very limited. For this review, we gathered typical evidence on the oncogenic role of SRC-1, highlighted its major collaborators and regulatory genes, and mapped the potential mechanisms by which SRC-1 promotes primary tumour progression.

Epigenetic regulation of carnitine palmitoyltransferase 1 (Cpt1a) by high fat diet

Carnitine palmitoyltransferase 1 (Cpt1a) is a rate-limiting enzyme that mediates the transport of fatty acids into the mitochondria for subsequent beta-oxidation. The objective of this study was to uncover how diet mediates the transcriptional regulation of Cpt1a. Pregnant Sprague Dawley rats were exposed to either a high-fat (HF) or low-fat control diet during gestation and lactation. At weaning, male offspring received either a HF or control diet, creating 4 groups: lifelong control diet (C/C; n = 12), perinatal HF diet (HF/C; n = 9), post-weaning HF diet (C/HF; n = 10), and lifelong HF diet (HF/HF; n = 10). Only HF/HF animals had higher hepatic Cpt1a mRNA expression than C/C. Epigenetic analysis revealed reduced DNA methylation (DNAMe) and increased histone 3 lysine 4 dimethylation (H3K4Me2) upstream and within the promoter of Cpt1a in the HF/HF group. This was accompanied by increased peroxisome proliferator activated receptor alpha (PPARα) and CCAAT/enhancer binding protein beta (C/EBPβ) binding directly downstream of the Cpt1a transcription start site within the first intron. Findings were confirmed in rat hepatoma H4IIEC3 cells treated with non-esterified fatty acid (NEFA). After 12 h of NEFA treatment, there was an enrichment of SWI/SNF related matrix associated actin dependent regulator of chromatin subfamily D member 1 (BAF60a or SMARCD1) in the first intron of Cpt1a. We conclude that dietary fat elevates hepatic Cpt1a expression via a highly coordinated transcriptional mechanism involving increased H3K4Me2, reduced DNAMe, and recruitment of C/EBPβ, PPARα, PGC1α, and BAF60a to the gene.

Nuclear receptor coactivator SRC-1 interacts with the Q-rich subdomain of the AhR and modulates its transactivation potential

The aryl hydrocarbon receptor (AhR), a soluble cytosolic protein, mediates many of the toxic effects of TCDD and related chemicals. The toxic effects are largely cell, tissue, and promoter context dependent. Although many details of the overall dioxin signal transduction have been elucidated, the transcriptional regulation of dioxin-induced genes like cyp1A1 is not yet completely understood. Previously, we have shown that the co-regulator RIP140 is a potential AhR coactivator. In this report, the role of coactivator, SRC-1, in AhR-mediated transcriptional regulation was examined. SRC-1 increased AhR-mediated, TCDD-dependent reporter gene activity three-fold in Hepa-1 and COS-1 cells. In in vitro interaction assays, SRC-1 was found to interact with AhR but not with ARNT. SRC-1 interacted weakly with AhR in the absence of TCDD and the addition of ligand further increased SRC-1 binding to AhR. Deletional mapping studies of the AhR revealed that SRC-1 binds to the AhR transactivation domain. Finer mapping of the SRC-1-interacting subdomains in the AhR transactivation domain suggested that the Q-rich subdomain was necessary and sufficient for interaction, similar to that seen with RIP140. Using GFP-tagged constructs, SRC-1 was shown to interact with AhR in cells. Unlike RIP140, LXXLL motifs in SRC-1 were necessary for interaction with AhR in vitro and for coactivation in Hepa-1 cells. The recruitment of certain coactivators by a variety of receptors suggests possible common coactivator pools and competition among receptors for limiting coactivators. Examination of the role of SRC-1 in AhR/ARNT transactivation in ARNT-deficient mutant Hepa-1 c4 cells demonstrates that the AhR transactivation domain is sufficient for enhanced coactivation mediated by SRC-1 in the presence of a transactivation domain deleted ARNT protein.

Deciphering the Roles of PPARγ in Adipocytes via Dynamic Change of Transcription Complex

Peroxisome proliferator-activated receptor γ (PPARγ), a ligand-dependent transcription factor highly expressed in adipocytes, is a master regulator of adipogenesis and lipid storage, a central player in thermogenesis and an active modulator of lipid metabolism and insulin sensitivity. As a nuclear receptor governing numerous target genes, its specific signaling transduction relies on elegant transcriptional and post-translational regulations. Notably, in response to different metabolic stimuli, PPARγ recruits various cofactors and forms distinct transcriptional complexes that change dynamically in components and epigenetic modification to ensure specific signal transduction. Clinically, PPARγ activation via its full agonists, thiazolidinediones, has been shown to improve insulin sensitivity and induce browning of white fat, while undesirably induce weight gain, visceral obesity and other adverse effects. Thus, deciphering the combinatorial interactions between PPARγ and its transcriptional partners and their preferential regulatory network in the processes of development, function and senescence of adipocytes would provide us the molecular basis for developing novel partial agonists that promote benefits of PPARγ signaling without detrimental side effects. In this review, we discuss the dynamic components and precise regulatory mechanisms of the PPARγ-cofactors complexes in adipocytes, as well as perspectives in treating metabolic diseases via specific PPARγ signaling.

Steroid receptor coactivator-2 is a dual regulator of cardiac transcription factor function

We have previously demonstrated the potential role of steroid receptor coactivator-2 (SRC-2) as a co-regulator in the transcription of critical molecules modulating cardiac function and metabolism in normal and stressed hearts. The present study seeks to extend the previous information by demonstrating SRC-2 fulfills this role by serving as a critical coactivator for the transcription and activity of critical transcription factors known to control cardiac growth and metabolism as well as in their downstream signaling. This knowledge broadens our understanding of the mechanism by which SRC-2 acts in normal and stressed hearts and allows further investigation of the transcriptional modifications mediating different types and degrees of cardiac stress. Moreover, the genetic manipulation of SRC-2 in this study is specific for the heart and thereby eliminating potential indirect effects of SRC-2 deletion in other organs. We have shown that SRC-2 is critical to transcriptional control modulated by MEF2, GATA-4, and Tbx5, thereby enhancing gene expression associated with cardiac growth. Additionally, we describe SRC-2 as a novel regulator of PPARα expression, thus controlling critical steps in metabolic gene expression. We conclude that through regulation of cardiac transcription factor expression and activity, SRC-2 is a critical transcriptional regulator of genes important for cardiac growth, structure, and metabolism, three of the main pathways altered during the cardiac stress response.

Research resource: tissue- and pathway-specific metabolomic profiles of the steroid receptor coactivator (SRC) family

The rapidly growing family of transcriptional coregulators includes coactivators that promote transcription and corepressors that harbor the opposing function. In recent years, coregulators have emerged as important regulators of metabolic homeostasis, including the p160 steroid receptor coactivator (SRC) family. Members of the SRC family have been ascribed important roles in control of gluconeogenesis, fat absorption and storage in the liver, and fatty acid oxidation in skeletal muscle. To provide a deeper and more granular understanding of the metabolic impact of the SRC family members, we performed targeted metabolomic analyses of key metabolic byproducts of glucose, fatty acid, and amino acid metabolism in mice with global knockouts (KOs) of SRC-1, SRC-2, or SRC-3. We measured amino acids, acyl carnitines, and organic acids in five tissues with key metabolic functions (liver, heart, skeletal muscle, brain, plasma) isolated from SRC-1, -2, or -3 KO mice and their wild-type littermates under fed and fasted conditions, thereby unveiling unique metabolic functions of each SRC. Specifically, SRC-1 ablation revealed the most significant impact on hepatic metabolism, whereas SRC-2 appeared to impact cardiac metabolism. Conversely, ablation of SRC-3 primarily affected brain and skeletal muscle metabolism. Surprisingly, we identified very few metabolites that changed universally across the three SRC KO models. The findings of this Research Resource demonstrate that coactivator function has very limited metabolic redundancy even within the homologous SRC family. Furthermore, this work also demonstrates the use of metabolomics as a means for identifying novel metabolic regulatory functions of transcriptional coregulators.

The transcriptional coactivators p/CIP and SRC-1 control insulin resistance through IRS1 in obesity models

Three p160 family members, p/CIP, SRC1, and TIF2, have been identified as transcriptional coactivators for nuclear hormone receptors and other transcription factors in vitro. In a previous study, we reported initial characterization of the obesity-resistant phenotypes of p/CIP and SRC-1 double knockout (DKO) mice, which exhibit increased energy expenditure, and suggested that nuclear hormone receptor target genes were involved in these phenotypes. In this study, we demonstrate that p/CIP and SRC1 control insulin signaling in a cell-autonomous manner both in vitro and in vivo. Genetic deletion of p/CIP and SRC-1 increases glucose uptake and enhances insulin sensitivity in both regular chow- and high fat diet-fed DKO mice despite increased food intake. Interestingly, we discover that loss of p/CIP and SRC-1 results in resistance to age-related obesity and glucose intolerance. We show that expression levels of a key insulin signaling component, insulin receptor substrate 1 (IRS1), are significantly increased in two cell lines representing fat and muscle lineages with p/CIP and SRC-1 deletions and in white adipose tissue and skeletal muscle of DKO mice; this may account for increased glucose metabolism and insulin sensitivity. This is the first evidence that the p160 coactivators control insulin signaling and glucose metabolism through IRS1. Therefore, our studies indicate that p/CIP and SRC-1 are potential therapeutic targets not only for obesity but also for diabetes.

Ablation of steroid receptor coactivator-3 resembles the human CACT metabolic myopathy

Oxidation of lipid substrates is essential for survival in fasting and other catabolic conditions, sparing glucose for the brain and other glucose-dependent tissues. Here we show Steroid Receptor Coactivator-3 (SRC-3) plays a central role in long chain fatty acid metabolism by directly regulating carnitine/acyl-carnitine translocase (CACT) gene expression. Genetic deficiency of CACT in humans is accompanied by a constellation of metabolic and toxicity phenotypes including hypoketonemia, hypoglycemia, hyperammonemia, and impaired neurologic, cardiac and skeletal muscle performance, each of which is apparent in mice lacking SRC-3 expression. Consistent with human cases of CACT deficiency, dietary rescue with short chain fatty acids drastically attenuates the clinical hallmarks of the disease in mice devoid of SRC-3. Collectively, our results position SRC-3 as a key regulator of β-oxidation. Moreover, these findings allow us to consider platform coactivators such as the SRCs as potential contributors to syndromes such as CACT deficiency, previously considered as monogenic.

Progressive endoplasmic reticulum stress contributes to hepatocarcinogenesis in fatty acyl-CoA oxidase 1-deficient mice

Fatty acyl-coenzyme A oxidase 1 (ACOX1) knockout (ACOX1(-/-)) mice manifest hepatic metabolic derangements that lead to the development of steatohepatitis, hepatocellular regeneration, spontaneous peroxisome proliferation, and hepatocellular carcinomas. Deficiency of ACOX1 results in unmetabolized substrates of this enzyme that function as biological ligands for peroxisome proliferator-activated receptor-α (PPARα) in liver. Here we demonstrate that sustained activation of PPARα in ACOX1(-/-) mouse liver by these ACOX1 substrates results in endoplasmic reticulum (ER) stress. Overexpression of transcriptional regulator p8 and its ER stress-related effectors such as the pseudokinase tribbles homolog 3, activating transcription factor 4, and transcription factor CCAAT/-enhancer-binding protein homologous protein as well as phosphorylation of eukaryotic translation initiation factor 2α, indicate the induction of unfolded protein response signaling in the ACOX1(-/-) mouse liver. We also show here that, in the liver, p8 is a target for all three PPAR isoforms (-α, -β, and -γ), which interact with peroxisome proliferator response elements in p8 promoter. Sustained activation of p8 and unfolded protein response-associated ER stress in ACOX1(-/-) mouse liver contributes to hepatocyte apoptosis and liver cell proliferation culminating in the development of hepatocarcinogenesis. We also demonstrate that human ACOX1 transgene is functional in ACOX1(-/-) mice and effectively prevents metabolic dysfunctions that lead to ER stress and carcinogenic effects. Taken together, our data indicate that progressive PPARα- and p8-mediated ER stress contribute to the hepatocarcinogenesis in ACOX1(-/-) mice.

Transcription coactivator mediator subunit MED1 is required for the development of fatty liver in the mouse

Peroxisome proliferator-activated receptor-γ (PPARγ), a nuclear receptor, when overexpressed in liver stimulates the induction of adipocyte-specific and lipogenesis-related genes and causes hepatic steatosis. We report here that Mediator 1 (MED1; also known as PBP or TRAP220), a key subunit of the Mediator complex, is required for high-fat diet-induced hepatic steatosis as well as PPARγ-stimulated adipogenic hepatic steatosis. Mediator forms the bridge between transcriptional activators and RNA polymerase II. MED1 interacts with nuclear receptors such as PPARγ and other transcriptional activators. Liver-specific MED1 knockout (MED1(ΔLiv) ) mice, when fed a high-fat (60% kcal fat) diet for up to 4 months failed to develop fatty liver. Similarly, MED1(ΔLiv) mice injected with adenovirus-PPARγ (Ad/PPARγ) by tail vein also did not develop fatty liver, whereas mice with MED1 (MED1(fl/fl) ) fed a high-fat diet or injected with Ad/PPARγ developed severe hepatic steatosis. Gene expression profiling and northern blot analyses of Ad/PPARγ-injected mouse livers showed impaired induction in MED1(ΔLiv) mouse liver of adipogenic markers, such as aP2, adipsin, adiponectin, and lipid droplet-associated genes, including caveolin-1, CideA, S3-12, and others. These adipocyte-specific and lipogenesis-related genes are strongly induced in MED1(fl/fl) mouse liver in response to Ad/PPARγ. Re-expression of MED1 using adenovirally-driven MED1 (Ad/MED1) in MED1(ΔLiv) mouse liver restored PPARγ-stimulated hepatic adipogenic response. These studies also demonstrate that disruption of genes encoding other coactivators such as SRC-1, PRIC285, PRIP, and PIMT had no effect on hepatic adipogenesis induced by PPARγ overexpression.

CONCLUSION

We conclude that transcription coactivator MED1 is required for high-fat diet-induced and PPARγ-stimulated fatty liver development, which suggests that MED1 may be considered a potential therapeutic target for hepatic steatosis. (HEPATOLOGY 2011;).

Transcriptional co-factors and hepatic energy metabolism

After binding to their cognate DNA-binding partner, transcriptional co-factors exert their function through the recruitment of enzymatic, chromatin-modifying activities. In turn, the assembly of co-factor-associated multi-protein complexes efficiently impacts target gene expression. Recent advances have established transcriptional co-factor complexes as a critical regulatory level in energy homeostasis and aberrant co-factor activity has been linked to the pathogenesis of severe metabolic disorders including obesity, type 2 diabetes and other components of the Metabolic Syndrome. The liver represents the key peripheral organ for the maintenance of systemic energy homeostasis, and aberrations in hepatic glucose and lipid metabolism have been causally linked to the manifestation of disorders associated with the Metabolic Syndrome. Therefore, this review focuses on the role of distinct classes of transcriptional co-factors in hepatic glucose and lipid homeostasis, emphasizing pathway-specific functions of these co-factors under physiological and pathophysiological conditions.

Steroid receptor coactivator 1 deficiency increases MMTV-neu mediated tumor latency and differentiation specific gene expression, decreases metastasis, and inhibits response to PPAR ligands

Background

The peroxisome proliferator activated receptor (PPAR) subgroup of the nuclear hormone receptor superfamily is activated by a variety of natural and synthetic ligands. PPARs can heterodimerize with retinoid X receptors, which have homology to other members of the nuclear receptor superfamily. Ligand binding to PPAR/RXRs results in recruitment of transcriptional coactivator proteins such as steroid receptor coactivator 1 (SRC-1) and CREB binding protein (CBP). Both SRC-1 and CBP are histone acetyltransferases, which by modifying nucleosomal histones, produce more open chromatin structure and increase transcriptional activity. Nuclear hormone receptors can recruit limiting amounts of coactivators from other transcription factor binding sites such as AP-1, thereby inhibiting the activity of AP-1 target genes. PPAR and RXR ligands have been used in experimental breast cancer therapy. The role of coactivator expression in mammary tumorigenesis and response to drug therapy has been the subject of recent studies.

Methods

We examined the effects of loss of SRC-1 on MMTV-neu mediated mammary tumorigenesis.

Results

SRC-1 null mutation in mammary tumor prone mice increased the tumor latency period, reduced tumor proliferation index and metastasis, inhibited response to PPAR and RXR ligands, and induced genes involved in mammary gland differentiation. We also examined human breast cancer cell lines overexpressing SRC-1 or CBP. Coactivator overexpression increased cellular proliferation with resistance to PPAR and RXR ligands and remodeled chromatin of the proximal epidermal growth factor receptor promoter.

Conclusions

These results indicate that histone acetyltransferases play key roles in mammary tumorigenesis and response to anti-proliferative therapies.

PRIC295, a Nuclear Receptor Coactivator, Identified from PPARα-Interacting Cofactor Complex

The peroxisome proliferator-activated receptor-α (PPARα) plays a key role in lipid metabolism and energy combustion. Chronic activation of PPARα in rodents leads to the development of hepatocellular carcinomas. The ability of PPARα to induce expression of its target genes depends on Mediator, an evolutionarily conserved complex of cofactors and, in particular, the subunit 1 (Med1) of this complex. Here, we report the identification and characterization of PPARα-interacting cofactor (PRIC)-295 (PRIC295), a novel coactivator protein, and show that it interacts with the Med1 and Med24 subunits of the Mediator complex. PRIC295 contains 10 LXXLL signature motifs that facilitate nuclear receptor binding and interacts with PPARα and five other members of the nuclear receptor superfamily in a ligand-dependent manner. PRIC295 enhances the transactivation function of PPARα, PPARγ, and ERα. These data demonstrate that PRIC295 interacts with nuclear receptors such as PPARα and functions as a transcription coactivator under in vitro conditions and may play an important role in mediating the effects in vivo as a member of the PRIC complex with Med1 and Med24.

Coactivators in PPAR-Regulated Gene Expression

Peroxisome proliferator-activated receptor (PPAR)alpha, beta (also known as delta), and gamma function as sensors for fatty acids and fatty acid derivatives and control important metabolic pathways involved in the maintenance of energy balance. PPARs also regulate other diverse biological processes such as development, differentiation, inflammation, and neoplasia. In the nucleus, PPARs exist as heterodimers with retinoid X receptor-alpha bound to DNA with corepressor molecules. Upon ligand activation, PPARs undergo conformational changes that facilitate the dissociation of corepressor molecules and invoke a spatiotemporally orchestrated recruitment of transcription cofactors including coactivators and coactivator-associated proteins. While a given nuclear receptor regulates the expression of a prescribed set of target genes, coactivators are likely to influence the functioning of many regulators and thus affect the transcription of many genes. Evidence suggests that some of the coactivators such as PPAR-binding protein (PBP/PPARBP), thyroid hormone receptor-associated protein 220 (TRAP220), and mediator complex subunit 1 (MED1) may exert a broader influence on the functions of several nuclear receptors and their target genes. Investigations into the role of coactivators in the function of PPARs should strengthen our understanding of the complexities of metabolic diseases associated with energy metabolism.

PPARgamma1 and LXRalpha face a new regulator of macrophage cholesterol homeostasis and inflammatory responsiveness, AEBP1

Peroxisome proliferator-activated receptor γ1 (PPARγ1) and liver X receptor α (LXRα) are nuclear receptors that play pivotal roles in macrophage cholesterol homeostasis and inflammation; key biological processes in atherogenesis. The activation of PPARγ1 and LXRα by natural or synthetic ligands results in the transactivation of ABCA1, ABCG1, and ApoE; integral players in cholesterol efflux and reverse cholesterol transport. In this review, we describe the structure, isoforms, expression pattern, and functional specificity of PPARs and LXRs. Control of PPARs and LXRs transcriptional activity by coactivators and corepressors is also highlighted. The specific roles that PPARγ1 and LXRα play in inducing macrophage cholesterol efflux mediators and antagonizing macrophage inflammatory responsiveness are summarized. Finally, this review focuses on the recently reported regulatory functions that adipocyte enhancer-binding protein 1 (AEBP1) exerts on PPARγ1 and LXRα transcriptional activity in the context of macrophage cholesterol homeostasis and inflammation.

PPARalpha: energy combustion, hypolipidemia, inflammation and cancer

The peroxisome proliferator-activated receptor alpha (PPARalpha, or NR1C1) is a nuclear hormone receptor activated by a structurally diverse array of synthetic chemicals known as peroxisome proliferators. Endogenous activation of PPARalpha in liver has also been observed in certain gene knockout mouse models of lipid metabolism, implying the existence of enzymes that either generate (synthesize) or degrade endogenous PPARalpha agonists. For example, substrates involved in fatty acid oxidation can function as PPARalpha ligands. PPARalpha serves as a xenobiotic and lipid sensor to regulate energy combustion, hepatic steatosis, lipoprotein synthesis, inflammation and liver cancer. Mainly, PPARalpha modulates the activities of all three fatty acid oxidation systems, namely mitochondrial and peroxisomal beta-oxidation and microsomal omega-oxidation, and thus plays a key role in energy expenditure. Sustained activation of PPARalpha by either exogenous or endogenous agonists leads to the development of hepatocellular carcinoma resulting from sustained oxidative and possibly endoplasmic reticulum stress and liver cell proliferation. PPARalpha requires transcription coactivator PPAR-binding protein (PBP)/mediator subunit 1(MED1) for its transcriptional activity.

Transcription coactivator PBP/MED1-deficient hepatocytes are not susceptible to diethylnitrosamine-induced hepatocarcinogenesis in the mouse

Nuclear receptor coactivator [peroxisome proliferator-activated receptor-binding protein (PBP)/mediator subunit 1 (MED1)] is a critical component of the mediator transcription complex. Disruption of this gene in the mouse results in embryonic lethality. Using the PBP/MED1 liver conditional null (PBP/MED1^ΔLiv) mice, we reported that PBP/MED1 is essential for liver regeneration and the peroxisome proliferator-activated receptor α ligand Wy-14,643-induced receptor-mediated hepatocarcinogenesis. We now examined the role of PBP/MED1 in genotoxic chemical carcinogen diethylnitrosamine (DEN)-induced and phenobarbital-promoted hepatocarcinogenesis. The carcinogenic process was initiated by a single intraperitoneal injection of DEN at 14 days of age and initiated cells were promoted with phenobarbital (PB) (0.05%) in drinking water. PBP/MED1^ΔLiv mice, killed at 1, 4 and 12 weeks, revealed a striking proliferative response of few residual PBP/MED1-positive hepatocytes that escaped Cre-mediated deletion of PBP/MED1 gene. No proliferative expansion of PBP/MED1 null hepatocytes was noted in the PBP/MED1^ΔLiv mouse livers. Multiple hepatocellular carcinomas (HCCs) developed in the DEN-initiated PBP/MED1^fl/fl and PBP/MED1^ΔLiv mice, 1 year after the PB promotion. Of interest is that all HCC developing in PBP/MED1^ΔLiv mice were PBP/MED1 positive. None of the tumors was PBP/MED1 negative implying that hepatocytes deficient in PBP/MED1 are not susceptible to neoplastic conversion. HCC that developed in PBP/MED1^ΔLiv mouse livers were transplantable in athymic nude mice and these maintained PBP/MED1^fl/fl genotype. PBP/MED1^fl/fl HCC cell line derived from these tumors expressed PBP/MED1 and deletion of PBP/MED1^fl/fl allele by adeno-Cre injection into tumors caused necrosis of tumor cells. These results indicate that PBP/MED1 is essential for the development of HCC in the mouse.

A role for the transcription intermediary factor 2 in zebrafish myelopoiesis

OBJECTIVE

TIF2 is fused with MOZ in the inv(8)(p11q13) acute myeloid leukemia. TIF2, member of the p160 family, is a histone acetyl transferase (HAT). Deletion of p160 genes were performed in mice. Some observations suggest that p160 family members may perform overlapping functions in mice. Therefore, we decided to choose the zebrafish model to study TIF2. The aim of this study was to characterize the role of this HAT during embryonic development.

MATERIAL AND METHODS

We use antisense, morpholino-modified oligomers to transiently knockdown tif2 gene, thus determining whether TIF2 plays a role in zebrafish early development.

RESULTS

We show that tif2 is involved in embryogenesis and in primitive hematopoiesis. tif2-knockdown zebrafish embryos are smaller than controls, they demonstrate shorter tails, they display notochord deformation and they exhibit U-shaped tail somites. A synthetic RNA encoding human TIF2 rescues the tif2-knockdown phenotype. Analysis of fli1 expression by whole-mount in situ hybridization indicates normal angioblast specification, but altered localization of intersomitic vessels. The posterior intermediate cell mass, in which a part of primitive hematopoiesis occurs, is altered in tif2 morphants and whole-mount in situ hybridization analyses of l-plastin and mpx expression suggest a specific inhibition of granulocytic and macrophagic differentiation at late stages.

CONCLUSION

These data indicate an important role for TIF2 in zebrafish primitive myelopoiesis.

Induction of nuclear translocation of constitutive androstane receptor by peroxisome proliferator-activated receptor alpha synthetic ligands in mouse liver

Peroxisome proliferators activate nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) and enhance the transcription of several genes in liver. We report here that synthetic PPARalpha ligands Wy-14,643, ciprofibrate, clofibrate, and others induce the nuclear translocation of constitutive androstane receptor (CAR) in mouse liver cells in vivo. Adenoviral-enhanced green fluorescent protein-CAR expression demonstrated that PPARalpha synthetic ligands drive CAR into the hepatocyte nucleus in a PPARalpha- and PPARbeta-independent manner. This translocation is dependent on the transcription coactivator PPAR-binding protein but independent of coactivators PRIP and SRC-1. PPARalpha ligand-induced nuclear translocation of CAR is not associated with induction of Cyp2b10 mRNA in mouse liver. PPARalpha ligands interfered with coactivator recruitment to the CAR ligand binding domain and reduced the constitutive transactivation of CAR. Both Wy-14,643 and ciprofibrate occupied the ligand binding pocket of CAR and adapted a binding mode similar to that of the CAR inverse agonist androstenol. These observations, therefore, provide information for the first time to indicate that PPARalpha ligands not only serve as PPARalpha agonists but possibly act as CAR antagonists.

Redundant enhancement of mouse constitutive androstane receptor transactivation by p160 coactivator family members

Constitutive androstane receptor (CAR) transactivation is enhanced by p160 coactivators, which include three members, SRC-1, SRC-2, and SRC-3. Each of the p160 coactivators enhanced mouse CAR (mCAR) transactivation of the CYP2B1 phenobarbital (PB)-responsive enhancer in transfected cultured cells and mouse hepatocytes in vivo. The cellular localization of the p160 coactivators in hepatocytes in vivo was not altered by PB treatment, nor did any of the p160 coactivators selectively colocalize with mCAR in the nucleus. Exogenous expression of each p160 coactivator mediated the PB-independent nuclear accumulation of mCAR in hepatocytes in vivo. Induction of Cyp2b10 gene expression by PB was equivalent or greater in mice null for each of the p160 coactivators than in wild type mice. These results indicate that the p160 coactivators are redundant with regard to enhancing CAR-mediated induction of cytochrome P450 genes. SRC-3 alone of the p160 coactivators enhanced CAR transactivation in hepatic cells without PB treatment.

Transcription coactivator PRIP, the peroxisome proliferator-activated receptor (PPAR)-interacting protein, is redundant for the function of nuclear receptors PParalpha and CAR, the constitutive androstane receptor, in mouse liver

Disruption of the genes encoding for the transcription coactivators, peroxisome proliferator-activated receptor (PPAR)-interacting protein (PRIP/ASC-2/RAP250/TRBP/NRC) and PPAR-binding protein (PBP/TRAP220/DRIP205/MED1), results in embryonic lethality by affecting placental and multiorgan development. Targeted deletion of coactivator PBP gene in liver parenchymal cells (PBP(LIV-/-)) results in the near abrogation of the induction of PPARalpha and CAR (constitutive androstane receptor)-regulated genes in liver. Here, we show that targeted deletion of coactivator PRIP gene in liver (PRIP(LIV-/-)) does not affect the induction of PPARalpha-regulated pleiotropic responses, including hepatomegaly, hepatic peroxisome proliferation, and induction of mRNAs of genes involved in fatty acid oxidation system, indicating that PRIP is not essential for PPARalpha-mediated transcriptional activity. We also provide additional data to show that liver-specific deletion of PRIP gene does not interfere with the induction of genes regulated by nuclear receptor CAR. Furthermore, disruption of PRIP gene in liver did not alter zoxazolamine-induced paralysis, and acetaminophen-induced hepatotoxicity. Studies with adenovirally driven EGFP-CAR expression in liver demonstrated that, unlike PBP, the absence of PRIP does not prevent phenobarbital-mediated nuclear translocation/retention of the receptor CAR in liver in vivo and cultured hepatocytes in vitro. These results show that PRIP deficiency in liver does not interfere with the function of nuclear receptors PPARalpha and CAR. The dependence of PPARalpha- and CAR-regulated gene transcription on coactivator PBP but not on PRIP attests to the existence of coactivator selectivity in nuclear receptor function.

Peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP) but not PPAR-interacting protein (PRIP) is required for nuclear translocation of constitutive androstane receptor in mouse liver

The constitutive androstane receptor (CAR) regulates transcription of phenobarbital-inducible genes that encode xenobiotic-metabolizing enzymes in liver. CAR is localized to the hepatocyte cytoplasm but to be functional, it translocates into the nucleus in the presence of phenobarbital-like CAR ligands. We now demonstrate that adenovirally driven EGFP-CAR, as expected, translocates into the nucleus of normal wild-type hepatocytes following phenobarbital treatment under both in vivo and in vitro conditions. Using this approach we investigated the role of transcription coactivators PBP and PRIP in the translocation of EGFP-CAR into the nucleus of PBP and PRIP liver conditional null mouse hepatocytes. We show that coactivator PBP is essential for nuclear translocation of CAR but not PRIP. Adenoviral expression of both PBP and EGFP-CAR restored phenobarbital-mediated nuclear translocation of exogenously expressed CAR in PBP null livers in vivo and in PBP null primary hepatocytes in vitro. CAR translocation into the nucleus of PRIP null livers resulted in the induction of CAR target genes such as CYP2B10, necessary for the conversion of acetaminophen to its hepatotoxic intermediate metabolite, N-acetyl-p-benzoquinone imine. As a consequence, PRIP-deficiency in liver did not protect from acetaminophen-induced hepatic necrosis, unlike that exerted by PBP deficiency. These results establish that transcription coactivator PBP plays a pivotal role in nuclear localization of CAR, that it is likely that PBP either enhances nuclear import or nuclear retention of CAR in hepatocytes, and that PRIP is redundant for CAR function.

Sorting out the roles of PPAR alpha in energy metabolism and vascular homeostasis

PPARalpha is a nuclear receptor that regulates liver and skeletal muscle lipid metabolism as well as glucose homeostasis. Acting as a molecular sensor of endogenous fatty acids (FAs) and their derivatives, this ligand-activated transcription factor regulates the expression of genes encoding enzymes and transport proteins controlling lipid homeostasis, thereby stimulating FA oxidation and improving lipoprotein metabolism. PPARalpha also exerts pleiotropic antiinflammatory and antiproliferative effects and prevents the proatherogenic effects of cholesterol accumulation in macrophages by stimulating cholesterol efflux. Cellular and animal models of PPARalpha help explain the clinical actions of fibrates, synthetic PPARalpha agonists used to treat dyslipidemia and reduce cardiovascular disease and its complications in patients with the metabolic syndrome. Although these preclinical studies cannot predict all of the effects of PPARalpha in humans, recent findings have revealed potential adverse effects of PPARalpha action, underlining the need for further study. This Review will focus on the mechanisms of action of PPARalpha in metabolic diseases and their associated vascular pathologies.

Critical roles of the p160 transcriptional coactivators p/CIP and SRC-1 in energy balance

Several transcriptional coactivators have been implicated in modulating the transcriptional activities of nuclear hormone receptors in vitro. Potential roles of these cofactors in important physiological processes such as energy homeostasis remain unknown. We report here that a developmental arrest in interscapular brown fat and defective adaptive thermogenesis occur in mice lacking both the p160 family transcriptional coactivators SRC-1 and p/CIP due to a failure in induction of selective PPARgamma target genes involved in adipogenesis and mitochondrial uncoupling. In the absence of p/CIP and SRC-1, mice eat more food on both regular chow and a high-fat diet because of decreased blood leptin levels. However, the p/CIP(-/-)/SRC-1(-/-) mice are lean and resistant to high-fat-diet-induced obesity. They exhibit increased basal metabolic rates and heightened levels of physical activity. Therefore, p/CIP and SRC-1 play critical roles in energy balance by controlling both energy intake and energy expenditure.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

Transcription coactivator peroxisome proliferator-activated receptor-binding protein/mediator 1 deficiency abrogates acetaminophen hepatotoxicity

Peroxisome proliferator-activated receptor-binding protein (PBP), also known as thyroid hormone receptor-associated protein 220/vitamin D receptor-interacting protein 205/mediator 1, an anchor for multisubunit mediator transcription complex, functions as a transcription coactivator for nuclear receptors. Disruption of the PBP gene results in embryonic lethality around embryonic day 11.5 by affecting placental and multiorgan development. Here, we report that targeted deletion of PBP in liver parenchymal cells (PBP(Liv-/-)) results in the abrogation of hypertrophic and hyperplastic influences in liver mediated by constitutive androstane receptor (CAR) ligands phenobarbital (PB) and 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene, and of acetaminophen-induced hepatotoxicity. CAR interacts with the two nuclear receptor-interacting LXXLL (L, leucine; X, any amino acid) motifs in PBP in a ligand-dependent manner. We also show that PBP interacts with the C-terminal portion of CAR, suggesting that PBP is involved in the regulation of CAR function. Although the full-length PBP only minimally increased CAR transcriptional activity, a truncated form of PBP (amino acids 487-735) functioned as a dominant negative repressor, establishing that PBP functions as a coactivator for CAR. A reduction in CAR mRNA and protein level observed in PBP(Liv-/-) mouse liver suggests that PBP may regulate hepatic CAR expression. PBP-deficient hepatocytes in liver failed to reveal PB-dependent translocation of CAR to the nucleus. Adenoviral reconstitution of PBP in PBP(Liv-/-) mouse livers restored PB-mediated nuclear translocation of CAR as well as inducibility of CYP1A2, CYP2B10, CYP3A11, and CYP7A1 expression. We conclude that transcription coactivator PBP/TRAP220/MED1 is involved in the regulation of hepatic CAR function and that PBP deficiency in liver abrogates acetaminophen hepatotoxicity.

Partially redundant functions of SRC-1 and TIF2 in postnatal survival and male reproduction

Both SRC-1 and TIF2 are members of the p160 steroid receptor coactivator family. Genetic analyses have shown that inactivation of TIF2, but not SRC-1, reduces postnatal survival, growth, and male reproductive function. Here, we demonstrate that, through analyses of SRC-1/TIF2 compound mutant mice, SRC-1 can partially compensate for the effects of a loss of TIF2 on mouse survival and growth, whereas SRC-1 and TIF2 are dispensable for primary organogenesis. The highly variable onset of defects observed in TIF2(-/-) testes due to the absence of TIF2 in Sertoli cells, including abnormal spermiogenesis, age-dependent degeneration of seminiferous epithelium, and disorder of cholesterol homeostasis, is uniformly accelerated upon inactivation of SRC-1 alleles in the TIF2 null genetic background, thus demonstrating that TIF2 and SRC-1 can perform redundant functions in Sertoli cells. Massive desquamation of immature germ cells together with an increase in germ cell apoptosis and a decrease in germ cell proliferation may be responsible for the early onset of the severe seminiferous epithelial degeneration observed in SRC-1(+/-)/TIF2(-/-) testes. Interestingly, the overall abnormal features displayed by the SRC-1(+/-)/TIF2(-/-) and SRC-1(-/-)/TIF2(-/-) mutant testes, including spermatid maturation defects, increase in Sertoli cell lipid stores, loss of immature germ cells, and formation of giant multinucleated spermatids, are commonly detected in testes of elderly men, suggesting that deficiencies in molecular pathways involving TIF2 and SRC-1 in Sertoli cells could participate in testicular senescence.

Transcription Coactivator PBP, the Peroxisome Proliferator-activated Receptor (PPAR)-binding Protein, Is Required for PPARα-regulated Gene Expression in Liver

Nuclear receptor coactivator PBP (peroxisome proliferator-activated receptor (PPAR)-binding protein) functions as a coactivator for PPARs and other nuclear receptors. PBP serves as an anchor for TRAP (thyroid hormone receptor-associated proteins)/mediator multisubunit cofactor transcription complex. Disruption of the PBP/TRAP220 gene results in embryonic lethality around embryonic day 11.5 by affecting placental, cardiac, hepatic, and bone marrow development. Because PPAR isoforms alpha, gamma, and beta/delta function as important regulators of lipid homeostasis in mammals, it becomes important to assess the requirement of coactivator PBP in the regulation of PPAR functions in vivo. Sustained activation of PPARalpha by structurally diverse classes of chemicals of biological importance, designated peroxisome proliferators, leads to proliferation of peroxisomes in liver, induction of PPARalpha target genes including those involved in fatty acid oxidation, and the eventual development of liver tumors. Here, we show that targeted deletion of PBP in liver parenchymal cells, using the Cre-loxP system, results in the near abrogation of PPARalpha ligand-induced peroxisome proliferation and liver cell proliferation, as well as the induction of PPARalpha-regulated genes in PBP-deficient liver cells. In contrast, scattered PBP(+/+) hepatocytes in these livers showed DNA synthesis and were markedly hypertrophic with peroxisome proliferation in response to PPARalpha ligands. Chromatin immunoprecipitation data suggest that in PBP conditional null livers, there appears to be reduced association of cofactors, especially of CBP and TRAP150, to the mouse enoyl-CoA hydratase/l-3-hydroxyacyl-CoA dehydrogenase gene promoter. These observations suggest that PBP is required for the stabilization of multiprotein cofactor complexes. In essence, the absence of PBP in hepatocytes in vivo appears to mimic the absence of PPARalpha, indicating that coactivator PBP is essential for PPARalpha-regulated gene expression in liver parenchymal cells.

Review of the in vivo functions of the p160 steroid receptor coactivator family

The p160 steroid receptor coactivator (SRC) gene family contains three homologous members, which serve as transcriptional coactivators for nuclear receptors and certain other transcription factors. These coactivators interact with ligand-bound nuclear receptors to recruit histone acetyltransferases and methyltransferases to specific enhancer/promotor regions, which facilitates chromatin remodeling, assembly of general transcription factors, and transcription of target genes. This minireview summarizes our current knowledge about the molecular structures, molecular mechanisms, temporal and spatial expression patterns, and biological functions of the SRC family. In particular, this article highlights the roles of SRC-1 (NCoA-1), SRC-2 (GRIP1, TIF2, or NCoA-2) and SRC-3 (p/CIP, RAC3, ACTR, AIB1, or TRAM-1) in development, organ function, endocrine regulation, and nuclear receptor function, which are defined by characterization of the genetically manipulated animal models. Furthermore, this article also reviews our current understanding of the role of SRC-3 in breast cancer and discusses possible mechanisms for functional specificity and redundancy among SRC family members.

Finding specificity within a conserved interaction site

Recently developed approaches to generate drugs that regulate hormone-induced gene activation focus on modulating the interaction of nuclear receptors with coactivators. A study by Geistlinger and Guy demonstrates the feasibility of this approach and provides surprising evidence for specificity within the conserved nuclear receptor:coactivator interaction surface.

Inactivation of the nuclear receptor coactivator RAP250 in mice results in placental vascular dysfunction

Coactivators constitute a diverse group of proteins that are essential for optimal transcriptional activity of nuclear receptors. In the past few years many coactivators have been identified but it is still unclear whether these proteins interact indiscriminately with all nuclear receptors and whether there is some redundancy in their functions. We have previously cloned and characterized RAP250 (ASC-2/PRIP/TRBP/NRC), an LXXLL-containing coactivator for nuclear receptors. In order to study its biological role, Rap250 null mice were generated by gene targeting. Here we show that genetic disruption of Rap250 results in embryonic lethality at embryonic day (E) 13.5. Histological examination of placentas revealed a dramatically reduced spongiotrophoblast layer, a collapse of blood vessels in the region bordering the spongiotrophoblast, and labyrinthine layers in placentas from Rap250(-/-) embryos. These findings suggest that the lethality of Rap250(-/-) embryos is the result of obstructed placental blood circulation. Moreover, the transcriptional activity of PPAR gamma is reduced in fibroblasts derived from Rap250(-/-) embryos, suggesting that RAP250 is an essential coactivator for this nuclear receptor in the placenta. Our results demonstrate that RAP250 is necessary for placental development and thus essential for embryonic development.

Coactivator PRIP, the peroxisome proliferator-activated receptor-interacting protein, is a modulator of placental, cardiac, hepatic, and embryonic development

Nuclear receptor coactivator PRIP (peroxisome proliferator-activated receptor (PPAR gamma)-interacting protein) and PRIP-interacting protein with methyltransferase activity, designated PIMT, appear to serve as linkers between cAMP response element-binding protein-binding protein (CBP)/p300-anchored and PBP (PPAR gamma-binding protein)-anchored coactivator complexes involved in the transcriptional activity of nuclear receptors. To assess the biological significance of PRIP, we disrupted the PRIP gene in mice by homologous recombination. Mice nullizygous for PRIP died between embryonic day 11.5 and 12.5 (postcoitum) due in most part to defects in the development of placenta, heart, liver, nervous system, and retardation of embryonic growth. Transient transfection assays using fibroblasts isolated from PRIP(-/-) embryos revealed a significant decrease in the capacity for ligand-dependent transcriptional activation of retinoid X receptor alpha and to a lesser effect on PPAR gamma transcriptional activity. These observations indicate that PRIP like PBP, CBP, and p300 is an essential and nonredundant coactivator.

Adipocyte-specific gene expression and adipogenic steatosis in the mouse liver due to peroxisome proliferator-activated receptor gamma1 (PPARgamma1) overexpression

Peroxisome proliferator activated-receptor (PPAR) isoforms, alpha and gamma, function as important coregulators of energy (lipid) homeostasis. PPARalpha regulates fatty acid oxidation primarily in liver and to a lesser extent in adipose tissue, whereas PPARgamma serves as a key regulator of adipocyte differentiation and lipid storage. Of the two PPARgamma isoforms, PPARgamma1 and PPARgamma2 generated by alternative splicing, PPARgamma1 isoform is expressed in liver and other tissues, whereas PPARgamma2 isoform is expressed exclusively in adipose tissue where it regulates adipogenesis and lipogenesis. Since the function of PPARgamma1 in liver is not clear, we have, in this study, investigated the biological impact of overexpression of PPARgamma1 in mouse liver. Adenovirus-PPARgamma1 injected into the tail vein induced hepatic steatosis in PPARalpha(-/-) mice. Northern blotting and gene expression profiling results showed that adipocyte-specific genes and lipogenesis-related genes are highly induced in PPARalpha(-/-) livers with PPARgamma1 overexpression. These include adipsin, adiponectin, aP2, caveolin-1, fasting-induced adipose factor, fat-specific gene 27 (FSP27), CD36, Delta(9) desaturase, and malic enzyme among others, implying adipogenic transformation of hepatocytes. Of interest is that hepatic steatosis per se, induced either by feeding a diet deficient in choline or developing in fasted PPARalpha(-/-) mice, failed to induce the expression of these PPARgamma-regulated adipogenesis-related genes in steatotic liver. These results suggest that a high level of PPARgamma in mouse liver is sufficient for the induction of adipogenic transformation of hepatocytes with adipose tissue-specific gene expression and lipid accumulation. We conclude that excess PPARgamma activity can lead to the development of a novel type of adipogenic hepatic steatosis.

A Paradigm for Gene Regulation: Inflammation, NF-κB and PPAR

The onset of inflammatory gene expression is driven by the transcription factor NF-kappaB, whose transcriptional activity is regulated at multiple levels. First, NF-kappaB activity is regulated by cytoplasmic degradation of the IkappaB inhibitor and nuclear translocation. Second, the nuclear p65 transactivation potential can be further influenced by posttranslational modifications, such as phosphorylation and/or acetylation. The p65 phosphorylation is a process highly regulated by both cell- and stimulus-dependent activating kinases. Ser276 phosphorylation seems to be highly important considering its crucial role in the interaction with and the engagement of the cofactor CBP/p300. We have identified MSK1 as an acting kinase in the TNF-signalling pathway, where it is responsible for p65 phosphorylation at Ser276, as well as for H3 phosphorylation of Ser10 in IL-6 promoter-associated chromatin (Fig. 1) (Saccani et al., 2002; Vermeulen et al., 2002, 2003). To our knowledge, this was the first report that identifies one particular kinase involved in transcription factor phosphorylation and histone modification at the level of a single promoter in order to establish gene activation. The question of which element takes the initial step to recruit and to assemble the activated transcription complex still remains unanswered (Vanden Berghe et al., 2002). PPAR alpha negatively interferes with inflammatory gene expression by up-regulation of the cytoplasmic inhibitor molecule IkappaB alpha, thus establishing an autoregulatory loop (Fig. 1). This induction takes place in the absence of a PPRE, but requires the presence of NF-kappaB and Sp1 elements in the IkappaB alpha promoter sequence as well as DRIP250 cofactors. The detailed mechanism how PPAR can activate genes in a non-DNA-binding way needs further investigation; moreover, it is at present not clear whether this upregulation, unlike the inhibitory effect of glucocorticoids, is a cell type- or a PPAR-specific phenomenon.

SRC-1 and TIF2 control energy balance between white and brown adipose tissues

We have explored the effects of two members of the p160 coregulator family on energy homeostasis. TIF2-/- mice are protected against obesity and display enhanced adaptive thermogenesis, whereas SRC-1-/- mice are prone to obesity due to reduced energy expenditure. In white adipose tissue, lack of TIF2 decreases PPARgamma activity and reduces fat accumulation, whereas in brown adipose tissue it facilitates the interaction between SRC-1 and PGC-1alpha, which induces PGC-1alpha's thermogenic activity. Interestingly, a high-fat diet increases the TIF2/SRC-1 expression ratio, which may contribute to weight gain. These results reveal that the relative level of TIF2/SRC-1 can modulate energy metabolism.

Alternate surfaces of transcriptional coregulator GRIP1 function in different glucocorticoid receptor activation and repression contexts

Members of the mammalian p160 family, such as GRIP1, are known as glucocorticoid receptor (GR) coactivators; at certain glucocorticoid response elements (GREs), however, GRIP1 acts as a GR corepressor. We characterized functional interactions of GR and GRIP1 in a repression complex where GR tethers to DNA-bound activator protein-1 (AP-1), as at the human collagenase-3 gene, and tested whether the identified interactions were similar or different at other response elements. At the AP-1 tethering GRE, we mapped the GRIP1 corepressor activity to a domain distinct from the two known GRIP1 activation domains; it exhibited intrinsic GR-independent repression potential when recruited to DNA via Gal4 DNA-binding domain. Interestingly, neither the domain nor the activity was detected in the other two p160 family members, SRC1 and RAC3. The same GRIP1 corepression domain was required for GR-mediated repression at the nuclear factor-kappaB (NF-kappaB) tethering GRE of the human IL-8 gene. In contrast, at the osteocalcin gene GRE, where GR represses transcription by binding to a DNA site overlapping the TATA box, both GRIP1 and SRC1 corepressed, and the GRIP1-specific repression domain was dispensable. Thus, in a single cell type, GR and GRIP1 conferred one mode of activation and two modes of repression by selectively engaging distinct surfaces of GRIP1 in a response element-specific manner.

Phosphorylation of transcriptional coactivator peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP). Stimulation of transcriptional regulation by mitogen-activated protein kinase

Peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP) is an important coactivator for PPARgamma and other transcription factors. PBP is an integral component of a multiprotein thyroid hormone receptor-associated protein (TRAP)/vitamin D(3) receptor-interacting protein (DRIP)/activator-recruited cofactor (ARC) complex required for transcriptional activity. To study the regulation of PBP by cellular signaling pathways, we identified the phosphorylation sites of PBP. Using a combination of in vitro and in vivo approaches and mutagenesis of PBP phosphorylation sites, we identified six phosphorylation sites on PBP: one exclusive protein kinase A (PKA) phosphorylation site at serine 656, two protein kinase C (PKC) sites at serine 796 and serine 1345, a common PKA/PKC site at serine 756, and two extracellular signal-regulated kinase 2 sites of the mitogen-activated protein kinase (MAPK) family at threonine 1017 and threonine 1444. Binding of PBP to PPARgamma1 or retinoid-X-receptor for 9-cis-retinoic acid (RXR) is independent of their phosphorylation states, implying no changes in protein-protein interaction after modification by phosphorylation. Overexpression of RafBXB, an activated upstream kinase of the MAPK signal transduction pathway, exerts a significant additive inductive effect on PBP coactivator function. This effect is significantly diminished by overexpression of RafBXB301, a dominant negative mutant of RafBXB. These results identify phosphorylation as a regulatory modification event of PBP and demonstrate that PBP phosphorylation by Raf/MEK/MAPK cascade exerts a positive effect on PBP coactivator function. The functional role of PKA and PKC phosphorylation sites in PBP remains to be elucidated.

Deletion of the cancer-amplified coactivator AIB3 results in defective placentation and embryonic lethality

The amplified in breast cancer-3 (AIB3, ASC-2, RAP250, PRIP, TRBP, NRC, or NcoA6) gene is characterized as a cancer-amplified transcriptional coactivator for nuclear receptors, which include the peroxisome proliferator-activated receptor gamma (PPARgamma). To assess its biological function, we deleted the AIB3 gene in mice by homologous recombination. AIB3(+/-) mice are developmentally normal and fertile. AIB3(-/-) embryos exhibit growth restriction and lethality during 9.75-11.5 days postconception. The embryonic lethality is probably attributed to defects in the development of the placental vascular network and cardiac hypoplasia. These defects include the failure of labyrinthine development, the dilation of maternal blood sinuses, the massive erythrophagocytosis by trophoblasts, the alteration of trophoblast populations, and the lower proliferation of myocardium, which are similar to those encountered in mice lacking PPARgamma or the PPARgamma-binding protein (PBP, TRAP220, or DRIP205). In addition, the transcriptional activities of PPARgamma are significantly affected in mouse embryonic fibroblasts lacking AIB3. These results suggest that AIB3 is required for PPARgamma function in placental development and for normal heart development. These results also indicate that the biological function of AIB3 is not redundant with other classes of nuclear receptor coactivators such as PBP and members of the steroid receptor coactivator family.

Interaction of PIMT with transcriptional coactivators CBP, p300, and PBP differential role in transcriptional regulation

PIMT (PRIP-interacting protein with methyltransferase domain), an RNA-binding protein with a methyltransferase domain capable of binding S-adenosylmethionine, has been shown previously to interact with nuclear receptor coactivator PRIP (peroxisome proliferator-activated receptor (PPAR)-interacting protein) and enhance its coactivator function. We now report that PIMT strongly interacts with transcriptional coactivators, CBP, p300, and PBP but not with SRC-1 and PGC-1alpha under in vitro and in vivo conditions. The PIMT binding sites on CBP and p300 are located in the cysteine-histidine-rich C/H1 and C/H3 domains, and the PIMT binding site on PBP is in the region encompassing amino acids 1101-1560. The N-terminal of PIMT (residues 1-369) containing the RNA binding domain interacts with both C/H1 and C/H3 domains of CBP and p300 and with the C-terminal portion of PBP that encompasses amino acids 1371-1560. The C-terminal of PIMT (residues 611-852), which binds S-adenosyl-l-methionine, interacts respectively with the C/H3 domain of CBP/p300 and with a region encompassing amino acids 1101-1370 of PBP. Immunoprecipitation data showed that PIMT forms a complex in vivo with CBP, p300, PBP, and PRIP. PIMT appeared to be co-localized in the nucleus with CBP, p300, and PBP. PIMT enhanced PBP-mediated transcriptional activity of the PPARgamma, as it did for PRIP, indicating synergism between PIMT and PBP. In contrast, PIMT functioned as a repressor of CBP/p300-mediated transactivation of PPARgamma. Based on these observations, we suggest that PIMT bridges the CBP/p300-anchored coactivator complex with the PBP-anchored coactivator complex but differentially modulates coactivator function such that inhibition of the CBP/p300 effect may be designed to enhance the activity of PBP and PRIP.

DNA binding-independent induction of IkappaBalpha gene transcription by PPARalpha

PPARs are ligand-activated transcription factors that regulate energy homeostasis. In addition, PPARs furthermore control the inflammatory response by antagonizing the nuclear factor-kappaB (NF-kappaB) signaling pathway. We recently demonstrated that PPARalpha activators increase IkappaBalpha mRNA and protein levels in human aortic smooth muscle cells. Here, we studied the molecular mechanisms by which PPARalpha controls IkappaBalpha expression. Using transient transfection assays, it is demonstrated that PPARalpha potentiates p65-stimulated IkappaBalpha transcription in a ligand-dependent manner. Site-directed mutagenesis experiments revealed that PPARalpha activation of IkappaBalpha transcription requires the NF-kappaB and Sp1 sites within IkappaBalpha promoter. Chromatin immunoprecipitation assays demonstrate that PPARalpha activation enhances the occupancy of the NF-kappaB response element in IkappaBalpha promoter in vivo. Overexpression of the oncoprotein E1A failed to inhibit PPARalpha-mediated IkappaBalpha promoter induction, suggesting that cAMP response element binding protein-binding protein/p300 is not involved in this mechanism. By contrast, a dominant-negative form of VDR-interacting protein 205 (DRIP205) comprising its two LXXLL motifs completely abolished PPARalpha ligand-mediated activation. Furthermore, cotransfection of increasing amounts of DRIP205 relieved this inhibition, suggesting that PPARalpha requires DRIP205 to regulate IkappaBalpha promoter activity. By contrast, DRIP205 is not involved in PPARalpha-mediated NF-kappaB transcriptional repression. Taken together, these data provide a molecular basis for PPARalpha-mediated induction of IkappaBalpha and demonstrate, for the first time, that PPARalpha may positively regulate gene transcription in the absence of functional PPAR response elements.

Peroxisomal beta-oxidation and peroxisome proliferator-activated receptor alpha: an adaptive metabolic system

beta-Oxidation occurs in both mitochondria and peroxisomes. Mitochondria catalyze the beta-oxidation of the bulk of short-, medium-, and long-chain fatty acids derived from diet, and this pathway constitutes the major process by which fatty acids are oxidized to generate energy. Peroxisomes are involved in the beta-oxidation chain shortening of long-chain and very-long-chain fatty acyl-coenzyme (CoAs), long-chain dicarboxylyl-CoAs, the CoA esters of eicosanoids, 2-methyl-branched fatty acyl-CoAs, and the CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids, and in the process they generate H2O2. Long-chain and very-long-chain fatty acids (VLCFAs) are also metabolized by the cytochrome P450 CYP4A omega-oxidation system to dicarboxylic acids that serve as substrates for peroxisomal beta-oxidation. The peroxisomal beta-oxidation system consists of (a) a classical peroxisome proliferator-inducible pathway capable of catalyzing straight-chain acyl-CoAs by fatty acyl-CoA oxidase, L-bifunctional protein, and thiolase, and (b) a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs by branched-chain acyl-CoA oxidase (pristanoyl-CoA oxidase/trihydroxycoprostanoyl-CoA oxidase), D-bifunctional protein, and sterol carrier protein (SCP)x. The genes encoding the classical beta-oxidation pathway in liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPAR alpha). Evidence derived from mice deficient in PPAR alpha, peroxisomal fatty acyl-CoA oxidase, and some of the other enzymes of the two peroxisomal beta-oxidation pathways points to the critical importance of PPAR alpha and of the classical peroxisomal fatty acyl-CoA oxidase in energy metabolism, and in the development of hepatic steatosis, steatohepatitis, and liver cancer.

Histone deacetylase inhibitors reduce polyglutamine toxicity

Polyglutamine diseases include at least nine neurodegenerative disorders, each caused by a CAG repeat expansion in a different gene. Accumulation of mutant polyglutamine-containing proteins occurs in patients, and evidence from cell culture and animal experiments suggests the nucleus as a site of pathogenesis. To understand the consequences of nuclear accumulation, we created a cell culture system with nuclear-targeted polyglutamine. In our system, cell death can be mitigated by overexpression of full-length cAMP response element binding protein (CREB)-binding protein (CBP) or its amino-terminal portion alone. CBP is one of several histone acetyltransferases sequestered by polyglutamine inclusions. We found histone acetylation to be reduced in cells expressing mutant polyglutamine. Reversal of this hypoacetylation, which can be achieved either by overexpression of CBP or its amino terminus or by treatment with deacetylase inhibitors, reduced cell loss. These findings suggest that nuclear accumulation of polyglutamine can lead to altered protein acetylation in neurons and indicate a novel therapeutic strategy for polyglutamine disease.

Human peroxisome proliferator-activated receptor alpha (PPARalpha) supports the induction of peroxisome proliferation in PPARalpha-deficient mouse liver

Peroxisome proliferators, which function as peroxisome proliferator-activated receptor alpha (PPARalpha) agonists, induce peroxisomal, microsomal, and mitochondrial fatty acid oxidation enzymes, in conjunction with peroxisome proliferation, in liver cells. Sustained activation of PPARalpha leads to the development of liver tumors in rats and mice. The assertion that synthetic PPARalpha ligands pose negligible carcinogenic risk to humans is attributable, in part, to the failure to observe peroxisome proliferation in human hepatocytes. To explore the mechanism(s) of species-specific differences in response to PPARalpha ligands, we determined the functional competency of human PPARalpha in vivo and compared its potency with that of mouse PPARalpha. Recombinant adenovirus that expresses human or mouse PPARalpha was produced and administered intravenously to PPARalpha-deficient mice. Human as well as mouse PPARalpha fully restored the development of peroxisome proliferator-induced immediate pleiotropic responses, including peroxisome proliferation and enhanced expression of genes involved in lipid metabolism as well as nonperoxisomal genes, such as CD36, Ly-6D, Rbp7, monoglyceride lipase, pyruvate dehydrogenase kinase-4, and C3f, that have been identified recently to be up-regulated in livers with peroxisome proliferation. These studies establish that human PPARalpha is functionally competent and is equally as dose-sensitive as mouse PPARalpha in inducing peroxisome proliferation within the context of mouse liver environment and that it can heterodimerize with mouse retinoid X receptor, and this human PPARalpha-mouse retinoid X receptor chimeric heterodimer transcriptionally activates mouse PPARalpha target genes in a manner qualitatively similar to that of mouse PPARalpha.

Factor recruitment and TIF2/GRIP1 corepressor activity at a collagenase-3 response element that mediates regulation by phorbol esters and hormones

To investigate determinants of specific transcriptional regulation, we measured factor occupancy and function at a response element, col3A, associated with the collagenase-3 gene in human U2OS osteosarcoma cells; col3A confers activation by phorbol esters, and repression by glucocorticoid and thyroid hormones. The subunit composition and activity of AP-1, which binds col3A, paralleled the intracellular level of cFos, which is modulated by phorbol esters and glucocorticoids. In contrast, a similar AP-1 site at the collagenase-1 gene, not inducible in U2OS cells, was not bound by AP-1. The glucocorticoid receptor (GR) associated with col3A through protein-protein interactions with AP-1, regardless of AP-1 subunit composition, and repressed transcription. TIF2/GRIP1, reportedly a coactivator for GR and the thyroid hormone receptor (TR), was recruited to col3A and potentiated GR-mediated repression in the presence of a GR agonist but not antagonist. GRIP1 mutants deficient in GR binding and coactivator functions were also defective for corepression, and a GRIP1 fragment containing the GR-interacting region functioned as a dominant-negative for repression. In contrast, repression by TR was unaffected by GRIP1. Thus, the composition of regulatory complexes, and the biological activities of the bound factors, are dynamic and dependent on cell and response element contexts. Cofactors such as GRIP1 probably contain distinct surfaces for activation and repression that function in a context-dependent manner.

Ribozyme targeting demonstrates that the nuclear receptor coactivator AIB1 is a rate-limiting factor for estrogen-dependent growth of human MCF-7 breast cancer cells

Human breast tumorigenesis is promoted by the estrogen receptor pathway, and nuclear receptor coactivators are thought to participate in this process. Here we studied whether one of these coactivators, AIB1 (amplified in breast cancer 1), was rate-limiting for hormone-dependent growth of human MCF-7 breast cancer cells. We developed MCF-7 breast cancer cell lines in which the expression of AIB1 can be modulated by regulatable ribozymes directed against AIB1 mRNA. We found that depletion of endogenous AIB1 levels reduced steroid hormone signaling via the estrogen receptor alpha or progesterone receptor beta on transiently transfected reporter templates. Down-regulation of AIB1 levels in MCF-7 cells did not affect estrogen-stimulated cell cycle progression but reduced estrogen-mediated inhibition of apoptosis and cell growth. Finally, upon reduction of endogenous AIB1 expression, estrogen-dependent colony formation in soft agar and tumor growth of MCF-7 cells in nude mice was decreased. From these findings we conclude that, despite the presence of different estrogen receptor coactivators in breast cancer cells, AIB1 exerts a rate-limiting role for hormone-dependent human breast tumor growth.

Novel glucocorticoid receptor coactivator effector mechanisms

Glucocorticoids regulate numerous distinct physiological processes, most of which rely on the ability of the hormone-bound glucocorticoid receptor (GR) to change the expression of target genes in a cell- and promoter-dependent manner. The transcriptional activity of GR depends on coactivators that regulate transcription by remodeling chromatin or by facilitating the recruitment of the basal transcriptional machinery. Coactivators are often part of multiprotein complexes that are not specific for GR but also mediate the activity of other nuclear receptors (NRs) and unrelated transcription factors. Surprisingly, recent results reveal that the activity of coactivators might contribute to the receptor, promoter and cell specificity of NR action. The emerging picture shows coactivators as flexible, but precise, coordinators of complex and dynamic networks, in which transcriptional regulation by GR and other NRs is linked to other signaling pathways.