SMAD proteins transactivate the human ApoCIII promoter by interacting physically and functionally with hepatocyte nuclear factor 4

Hepatocyte nuclear factor 4α in the pathogenesis of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is emerging as the most common chronic liver disease worldwide. It refers to a range of liver conditions affecting people who drink little or no alcohol. NAFLD comprises non-alcoholic fatty liver and non-alcoholic steatohepatitis (NASH), the more aggressive form of NAFLD. NASH is featured by steatosis, lobular inflammation, hepatocyte injury, and various degrees of fibrosis. Although much progress has been made over the past decades, the pathogenic mechanism of NAFLD remains to be fully elucidated. Hepatocyte nuclear factor 4α (HNF4α) is a nuclear hormone receptor that is highly expressed in hepatocytes. Hepatic HNF4α expression is markedly reduced in NAFLD patients and mouse models of NASH. HNF4α has been shown to regulate bile acid, lipid, glucose, and drug metabolism. In this review, we summarize the recent advances in the understanding of the pathogenesis of NAFLD with a focus on the regulation of HNF4α and the role of hepatic HNF4α in NAFLD. Several lines of evidence have shown that hepatic HNF4α plays a key role in the initiation and progression of NAFLD. Recent data suggest that hepatic HNF4α may be a promising target for treatment of NAFLD.

Circulating Proteins Associated with Response and Resistance to Neoadjuvant Chemotherapy in HER2-Positive Breast Cancer

Simple Summary

The goal of this study was to find circulating proteins that can be easily sampled and incorporated into a clinical setting to improve predictive treatment response in HER2-positive breast cancer patients receiving neoadjuvant chemotherapy. We looked for potential biomarkers in serum, which we identified using two proteomics techniques: qualitative LC-MS/MS and a quantitative assay that assessed protein expression between responders and non-responders HER2-positive breast cancer patients to neoadjuvant chemotherapy.

Abstract

Despite the increasing use of neoadjuvant chemotherapy (NAC) in HER2-positive breast cancer (BC) patients, the clinical problem of predicting individual treatment response remains unanswered. Furthermore, the use of ineffective chemotherapeutic regimens should be avoided. Serum biomarker levels are being studied more and more for their ability to predict therapy response and aid in the development of personalized treatment regimens. This study aims to identify effective protein networks and biomarkers to predict response to NAC in HER2-positive BC patients through an exhaustive large-scale LC-MS/MS-based qualitative and quantitative proteomic profiling of serum samples from responders and non-responders. Serum samples from HER2-positive BC patients were collected before NAC and were processed by three methods (with and without nanoparticles). The qualitative analysis revealed differences in the proteomic profiles between responders and non-responders, mainly in proteins implicated in the complement and coagulation cascades and apolipoproteins. Qualitative analysis confirmed that three proteins (AFM, SERPINA1, APOD) were correlated with NAC resistance. In this study, we show that serum biomarker profiles can predict treatment response and outcome in the neoadjuvant setting. If these findings are further developed, they will be of significant clinical utility in the design of treatment regimens for individual BC patients.

Crosstalk of HNF4α with extracellular and intracellular signaling pathways in the regulation of hepatic metabolism of drugs and lipids

The liver is essential for survival due to its critical role in the regulation of metabolic homeostasis. Metabolism of xenobiotics, such as environmental chemicals and drugs by the liver protects us from toxic effects of these xenobiotics, whereas metabolism of cholesterol, bile acids (BAs), lipids, and glucose provide key building blocks and nutrients to promote the growth or maintain the survival of the organism. As a well-established master regulator of liver development and function, hepatocyte nuclear factor 4 alpha (HNF4α) plays a critical role in regulating a large number of key genes essential for the metabolism of xenobiotics, metabolic wastes, and nutrients. The expression and activity of HNF4α is regulated by diverse hormonal and signaling pathways such as growth hormone, glucocorticoids, thyroid hormone, insulin, transforming growth factor-β, estrogen, and cytokines. HNF4α appears to play a central role in orchestrating the transduction of extracellular hormonal signaling and intracellular stress/nutritional signaling onto transcriptional changes in the liver. There have been a few reviews on the regulation of drug metabolism, lipid metabolism, cell proliferation, and inflammation by HNF4α. However, the knowledge on how the expression and transcriptional activity of HNF4α is modulated remains scattered. Herein I provide comprehensive review on the regulation of expression and transcriptional activity of HNF4α, and how HNF4α crosstalks with diverse extracellular and intracellular signaling pathways to regulate genes essential in liver pathophysiology.

Apolipoproteins as Differentiating and Predictive Markers for Assessing Clinical Outcomes in Patients with Small Cell Lung Cancer

PURPOSE

The present study aimed to investigate the value of apolipoproteins, including ApoA-1, ApoC-III, and ApoE, in patients with small cell lung cancer (SCLC) as potential biomarkers for diagnosis, prognosis, and cancer progression.

MATERIALS AND METHODS

Lung samples were collected from 89 patients with SCLC. Nineteen lung samples from non-small cell lung cancer (NSCLC) patients and 12 normal lung tissues were used as controls. Expression profiles of ApoA-1, ApoC-III, and ApoE in different samples were examined using immunohistochemical methods, and the expression levels were correlated with cancer types, treatment, and outcomes using chi-square and Mann-Whitney tests.

RESULTS

Expression of ApoA-1 and ApoC-III in SCLC was significantly different, compared with that in NSCLC and normal lung tissues, and was correlated with recurrence of SCLC. Patients undergoing neoadjuvant chemotherapy before surgery showed significantly reduced expression of ApoA-1 and increased expression of ApoC-III and ApoE. Nevertheless, the expression levels of ApoA-1, ApoC-III, and ApoE were not correlated with SCLC staging.

CONCLUSION

ApoA-1 and ApoC-III may be used as differentiating and predictive markers for SCLC. ApoA-1, ApoC-III, and ApoE may be used to monitor the efficacy of chemotherapy.

Inhibition of UV-induced matrix metabolism by a myristoyl tetrapeptide

Regulation of extracellular matrix (ECM) composition is important in tissue homeostasis and function. We screened small peptides for their ability to inhibit ultraviolet (UV)-induced cell metabolism in epidermal fibroblasts. We found that UV irradiation increased matrix metalloproteinase (MMP) expression and inflammatory gene expression in human Hs68 fibroblast cells. We also demonstrated that a myristoyl tetrapeptide with the amino acid sequence Gly-Leu-Phe-Trp (mGLFW) suppressed the UV-induced expression of MMPs and inflammatory genes. Moreover, mGLFW stimulated the expression of ECM proteins in Hs68 fibroblasts. In order to provide the mechanism of action for mGLFW, we investigated UV-induced signaling changes in the presence of mGLFW using a cDNA microarray. UV exposure increased the expression of MMP genes, such as MMP1, MMP3, and MMP14, and inflammation-related genes, including interleukin 1 receptor and peroxisome proliferator-activated receptor gamma (PPARγ). Treatment with mGLFW abrogated the UV-induced expression of MMP-related genes and inflammatory genes. In addition, mGLFW increased the expression of collagen genes, including COL1A1, COL1A2, and COL5A1. We examined whether the activation of AP-1, a UV-activated transcription factor, is suppressed by mGLFW. The results demonstrated that AP-1 expression increased upon UV exposure and that this expression was inhibited by mGLFW. In conclusion, our results demonstrate that mGLFW reversed the effects of UV exposure by enhancing the expression of collagen proteins and suppressing the expression of MMPs, which degrade the ECM.

Smad-induced alterations of matrix metabolism by a myristoyl tetra peptide

Regulation of extracellular matrix (ECM) components is essential for tissue homeostasis and function. We screened a small peptide that induces ECM protein synthesis for its usefulness in protecting keratinocytes. In this report, we demonstrate that myristoyl tetrapeptide Ala-Ala-Pro-Val (mAAPV) stimulates the expression of ECM proteins and inhibits the expression of metalloproteinases (MMPs) that degrade ECM proteins in Hs68 human fibroblast cells. In order to elucidate the underlying molecular mechanisms for the effects of mAAVP, we investigated the changes in gene expression in the presence of mAAPV using a cDNA microarray. Treatment with mAAPV resulted in decreased expression of MMP-related genes such as MMP1, MMP3, TIMP1 and TIMP3 and increased expression of collagen genes, including COL1A1, COL1A2, COL3A1, COL5A1 and COL6A3. The pattern of gene expression regulated by mAAPV was very similar to that of gene expression induced by transforming growth factor (TGF)-β, indicating that the TGF-β signaling pathway is crucial for simultaneous activation of several ECM-related genes by mAAPV. We examined whether the activation of SMAD, a downstream protein of TGF-β receptor, is involved in the signal transduction pathway induced by mAAPV. The results demonstrate that mAAVP directly activates SMAD2 and induces SMAD3 to bind to DNA. In conclusion, our results demonstrate that mAAPV both enhances the expression of collagen and inhibits its degradation via production of protease inhibitors that prevent enzymatic breakdown of the ECM. The results suggest that mAAPV would be a useful ECM-protecting agent.

AAV-mediated administration of myostatin pro-peptide mutant in adult Ldlr null mice reduces diet-induced hepatosteatosis and arteriosclerosis

Genetic disruption of myostatin or its related signaling is known to cause strong protection against diet-induced metabolic disorders. The translational value of these prior findings, however, is dependent on whether such metabolically favorable phenotype can be reproduced when myostatin blockade begins at an adult age. Here, we reported that AAV-mediated delivery of a myostatin pro-peptide D76A mutant in adult mice attenuates the development of hepatic steatosis and arteriosclerosis, two common diet-induced metabolic diseases. A single dose of AAV-D76A in adult Ldlr null mice resulted in sustained expression of myostatin pro-peptide in the liver. Compared to vehicle-treated mice, D76A-treated mice gained similar amount of lean and fat mass when fed a high fat diet. However, D76A-treated mice displayed significantly reduced aortic lesions and liver fat, in association with a reduction in hepatic expression of lipogenic genes and improvement in liver insulin sensitivity. This suggests that muscle and fat may not be the primary targets of treatment under our experimental condition. In support to this argument, we show that myostatin directly up-regulated lipogenic genes and increased fat accumulation in cultured liver cells. We also show that both myostatin and its receptor were abundantly expressed in mouse aorta. Cultured aortic endothelial cells responded to myostatin with a reduction in eNOS phosphorylation and an increase in ICAM-1 and VCAM-1 expression.

CONCLUSIONS

AAV-mediated expression of myostatin pro-peptide D76A mutant in adult Ldlr null mice sustained metabolic protection without remarkable impacts on body lean and fat mass. Further investigations are needed to determine whether direct impact of myostatin on liver and aortic endothelium may contribute to the related metabolic phenotypes.

Repression of a potassium channel by nuclear hormone receptor and TGF-β signaling modulates insulin signaling in Caenorhabditis elegans

Transforming growth factor β (TGF-β) signaling acts through Smad proteins to play fundamental roles in cell proliferation, differentiation, apoptosis, and metabolism. The Receptor associated Smads (R-Smads) interact with DNA and other nuclear proteins to regulate target gene transcription. Here, we demonstrate that the Caenorhabditis elegans R-Smad DAF-8 partners with the nuclear hormone receptor NHR-69, a C. elegans ortholog of mammalian hepatocyte nuclear factor 4α HNF4α), to repress the exp-2 potassium channel gene and increase insulin secretion. We find that NHR-69 associates with DAF-8 both in vivo and in vitro. Functionally, daf-8 nhr-69 double mutants show defects in neuropeptide secretion and phenotypes consistent with reduced insulin signaling such as increased expression of the sod-3 and gst-10 genes and a longer life span. Expression of the exp-2 gene, encoding a voltage-gated potassium channel, is synergistically increased in daf-8 nhr-69 mutants compared to single mutants and wild-type worms. In turn, exp-2 acts selectively in the ASI neurons to repress the secretion of the insulin-like peptide DAF-28. Importantly, exp-2 mutation shortens the long life span of daf-8 nhr-69 double mutants, demonstrating that exp-2 is required downstream of DAF-8 and NHR-69. Finally, animals over-expressing NHR-69 specifically in DAF-28–secreting ASI neurons exhibit a lethargic, hypoglycemic phenotype that is rescued by exogenous glucose. We propose a model whereby DAF-8/R-Smad and NHR-69 negatively regulate the transcription of exp-2 to promote neuronal DAF-28 secretion, thus demonstrating a physiological crosstalk between TGF-β and HNF4α-like signaling in C. elegans. NHR-69 and DAF-8 dependent regulation of exp-2 and DAF-28 also provides a novel molecular mechanism that contributes to the previously recognized link between insulin and TGF-β signaling in C. elegans.

All animals must ensure metabolic homeostasis; if they fail to do so, diseases such as obesity and diabetes can develop. To maintain glucose balance, insulin is secreted upon glucose intake in a highly regulated and coordinated process. Previous studies suggested that the transforming growth factor beta (TGF-β) signaling pathway regulates insulin secretion in mammals. In the genetically tractable roundworm Caenorhabditis elegans, TGF-β and insulin signaling modulate larval development and aging, although the molecular link between insulin and TGF-β signaling remains poorly understood. In this study, we show that the TGF-β signaling component DAF-8 partners with NHR-69, a nuclear hormone receptor, to control the expression of the potassium channel exp-2, which in turn modulates the secretion of an insulin-like peptide. A loss-of-function exp-2 mutant exhibits increased insulin secretion and a shortened life span, whereas a gain-of-function mutant exhibits decreased insulin secretion. We also show that tissue-specific expression of nhr-69 in a pair of neurons that secrete neuropeptides causes reduced glucose content, increased insulin-like peptide levels and a lethargic phenotype. Because insulin and TGF-β signaling are linked to numerous diseases, our data may provide novel insights into the mechanisms contributing to pathophysiological changes.

Cell type-specific target selection by combinatorial binding of Smad2/3 proteins and hepatocyte nuclear factor 4alpha in HepG2 cells

Specific regulation of target genes by transforming growth factor-β (TGF-β) in a given cellular context is determined in part by transcription factors and cofactors that interact with the Smad complex. In this study, we determined Smad2 and Smad3 (Smad2/3) binding regions in the promoters of known genes in HepG2 hepatoblastoma cells, and we compared them with those in HaCaT epidermal keratinocytes to elucidate the mechanisms of cell type- and context-dependent regulation of transcription induced by TGF-β. Our results show that 81% of the Smad2/3 binding regions in HepG2 cells were not shared with those found in HaCaT cells. Hepatocyte nuclear factor 4α (HNF4α) is expressed in HepG2 cells but not in HaCaT cells, and the HNF4α-binding motif was identified as an enriched motif in the HepG2-specific Smad2/3 binding regions. Chromatin immunoprecipitation sequencing analysis of HNF4α binding regions under TGF-β stimulation revealed that 32.5% of the Smad2/3 binding regions overlapped HNF4α bindings. MIXL1 was identified as a new combinatorial target of HNF4α and Smad2/3, and both the HNF4α protein and its binding motif were required for the induction of MIXL1 by TGF-β in HepG2 cells. These findings generalize the importance of binding of HNF4α on Smad2/3 binding genomic regions for HepG2-specific regulation of transcription by TGF-β and suggest that certain transcription factors expressed in a cell type-specific manner play important roles in the transcription regulated by the TGF-β-Smad signaling pathway.

Steroidogenic factor-1 is required for TGF-beta3-mediated 17beta-estradiol synthesis in mouse ovarian granulosa cells

The TGF-β superfamily members are indicated to play key roles in ovarian follicular development, such as granulosa cell proliferation, estrogens, and progesterone production. However, little is known about the roles of TGF-β3 in follicular development. In this study, we found that TGF-β3 was predominantly expressed in granulosa cells of mouse ovarian follicles, and it significantly promoted 17β-estradiol (E(2)) release in a dose-dependent manner. The orphan nuclear receptor steroidogenic factor-1 (SF-1) was required in TGF-β3-induced Cyp19a1 (a key rate-limiting enzyme for estrogen biosynthesis) expression and E(2) release. Additionally, TGF-β3 enhanced the binding of SF-1 to endogenous ovary-specific Cyp19a1 type II promoter, as evidenced by chromatin immunoprecipitation assays. The enhanced effect of SF-1 by TGF-β3 may be mediated through functional interactions between SF-1 and mothers against decapentaplegic homolog (Smad)3 (a mediator of TGF-β signaling pathway), because disruption of the interaction abolished the synergistic effects of SF-1, Smad3, and TGF-β3 on Cyp19a1 mRNA expression. RNA interference and chromatin immunoprecipitation studies also demonstrated that Smad3 was required for SF-1 binding to Cyp19a1 type II promoter and activation of Cyp19a1. Smad3 thus acts as a point of convergence that involves integration of SF-1 and TGF-β signaling in affecting E(2) production. Taken together, our data provide mechanistic insights into the roles of SF-1 in TGF-β3-mediated E(2) synthesis. Understanding of potential cross-points between extracellular signals affecting estrogen production will help to discover new therapeutic targets in estrogen-related diseases.

Serum apolipoprotein C-III is independently associated with chronic hepatitis C infection and advanced fibrosis

BACKGROUND

The hepatitis C virus (HCV) is known to disrupt lipid metabolism, making serum lipoprotein levels good candidates to explore as markers of HCV disease progression. Assessment of the major apolipoproteins (Apo) and their relationship to hepatic fibrosis remain largely unexplored.

METHODS

We compared the levels of total cholesterol, triglycerides, low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C), and Apo A-I, -B, -C-III, and -E between patients with cleared versus active infection (n = 83), and between those chronically infected patients (n = 216) with advanced versus mild-moderate hepatic fibrosis (METAVIR stage F3-4 vs. F0-2) using multiple logistic regression.

RESULTS

Apo C-III levels were 25% higher in subjects with cleared infection versus those with active infection (p = 0.009). Low levels of Apo C-III (p = 1.3 × 10(-5)), Apo A-I (p = 2.9 × 10(-5)), total cholesterol (p = 5.0 × 10(-4)), LDL-C (p = 0.005), and HDL-C (p = 2.0 × 10(-4)) were associated with advanced fibrosis in univariate analyses. Multivariable analysis revealed Apo C-III as the most significant factor associated with advanced fibrosis (p = 0.0004), followed by age (p = 0.013) and Apo A-I (p = 0.022). Inclusion of both Apo C-III and Apo A-I in a model to predict advanced fibrosis improved the area under the receiver operator curve only modestly.

CONCLUSIONS

Relative to other lipoproteins, low serum Apo C-III levels are the most strongly associated with chronic versus cleared infection and decline with increasing severity of hepatic fibrosis. Apo C-III deserves further attention as a possible marker of HCV disease progression.

Tgif1 represses apolipoprotein gene expression in liver

TG-interacting factor (Tgif1) represses gene expression by interaction with general corepressors, and can be recruited to target genes by transforming growth factor beta (TGFβ) activated Smads, or by the retinoid X receptor (RXR). Here we show that Tgif1 interacts with the LXRα nuclear receptor and can repress transcription from a synthetic reporter activated by LXRα. In cultured cells reducing endogenous Tgif1 levels resulted in increased expression of LXRα target genes. To test the in vivo role of Tgif1, we analyzed LXRα-dependent gene expression in mice lacking Tgif1. In the livers of Tgif1 null mice, we observed significant derepression of the apolipoprotein genes, Apoa4 and Apoc2, suggesting that Tgif1 is an important in vivo regulator of apolipoprotein gene expression. In contrast, we observed relatively minimal effects on expression of other LXR target genes. This work suggests that Tgif1 can regulate nuclear receptor complexes, in addition to those containing retinoic acid receptors, but also indicates that there is some specificity to which NR target genes are repressed by Tgif1.

Hepatocyte nuclear factor 4alpha regulation of bile acid and drug metabolism

The hepatocyte nuclear factor 4alpha (HNF4alpha) is a liver-enriched nuclear receptor that plays a critical role in early morphogenesis, fetal liver development, liver differentiation and metabolism. Human HNF4alpha gene mutations cause maturity on-set diabetes of the young type 1, an autosomal dominant non-insulin-dependent diabetes mellitus. HNF4alpha is an orphan nuclear receptor because of which the endogenous ligand has not been firmly identified. The trans-activating activity of HNF4alpha is enhanced by interacting with co-activators and inhibited by corepressors. Recent studies have revealed that HNF4alpha plays a central role in regulation of bile acid metabolism in the liver. Bile acids are required for biliary excretion of cholesterol and metabolites, and intestinal absorption of fat, nutrients, drug and xenobiotics for transport and distribution to liver and other tissues. Bile acids are signaling molecules that activate nuclear receptors to control lipids and drug metabolism in the liver and intestine. Therefore, HNF4alpha plays a central role in coordinated regulation of bile acid and xenobiotics metabolism. Drugs that specifically activate HNF4alpha could be developed for treating metabolic diseases such as diabetes, dyslipidemia and cholestasis, as well as drug metabolism and detoxification.

Exposure of normal human melanocytes to a tumor promoting phorbol ester reverses growth suppression by transforming growth factor beta

Transforming growth factor-beta (TGF-beta), a potent inhibitor of normal melanocyte growth, does not significantly suppress growth of melanoma cells. The mechanism of melanocyte desensitization to TGF-beta in the transformation process remains largerly unknown. We investigated whether the tumor promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) may induce melanocyte resistance to TGF-beta. Cell proliferation and DNA synthesis of normal human melanocytes were strongly inhibited by TGF-beta, whereas in the presence of TPA remained largerly unaffected. The inactive phorbol ester 4alpha-phorbol 12,13 didecanoate did not modify the TGF-beta antiproliferative effect, whereas the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol counteracted TGF-beta effects. Protein kinase C (PKC) is the major cellular receptor of tumor promoting phorbol esters. PKC-alpha expression and phosphorylation were almost completely downregulated under combined treatment with TGF-beta + TPA at 24 and 72 h, as shown by immunoblots. Confocal microscopy demonstrated that TGF-beta-induced nuclear accumulation of PKC-alpha was abolished in the presence of TPA at the same time points. The selective PKC inhibitor Ro-31-8220 weakened the TGF-beta antiproliferative effect. Smads are central mediators for TGF-beta signal transduction. Smad-dependent transcriptional activity was suppressed in TGF-beta-treated melanocytes in the presence of TPA, as well as in ALK5 (constitutively active type I TGF-beta receptor)- or Smad3 + Smad4-transfected melanocytes in the presence of Ro-31-8220. In addition, an antisense oligodeoxynucleotide against PKC-alpha abolished TGF-beta-driven Smad-mediated transcription. These findings show that tumor promoting phorbol esters induce melanocyte resistance to TGF-beta, associated with downregulation of PKC-alpha and suppression of Smad-dependent transcription. This may represent an important mechanism for expansion of melanocytes exposed to PKC-targeting tumor promoters.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

Orphan nuclear receptor small heterodimer partner inhibits transforming growth factor-beta signaling by repressing SMAD3 transactivation

Orphan nuclear receptor small heterodimer partner (SHP) is an atypical member of the nuclear receptor superfamily; SHP regulates the nuclear receptor-mediated transcription of target genes but lacks a conventional DNA binding domain. In this study, we demonstrate that SHP represses transforming growth factor-beta (TGF-beta)-induced gene expression through a direct interaction with Smad, a transducer of TGF-beta signaling. Transient transfection studies demonstrate that SHP represses Smad3-induced transcription. In vivo and in vitro protein interaction assays revealed that SHP directly interacts with Smad2 and Smad3 but not with Smad4. Mapping of domains mediating the interaction between SHP and Smad3 showed that the entire N-terminal domain (1-159 amino acids) of SHP and the linker domain of Smad3 are involved in this interaction. In vitro glutathione S-transferase pulldown competition experiments revealed the SHP-mediated repression of Smad3 transactivation through competition with its co-activator p300. SHP also inhibits the activation of endogenous TGF-beta-responsive gene promoters, the p21, Smad7, and plasminogen activator inhibitor-1 (PAI-1) promoters. Moreover, adenovirus-mediated overexpression of SHP decreases PAI-1 mRNA levels, and down-regulation of SHP by a small interfering RNA increases both the transactivation of Smad3 and the PAI-1 mRNA levels. Finally, the PAI-1 gene is expressed in SHP(-/-) mouse hepatocytes at a higher level than in normal hepatocytes. Taken together, these data indicate that SHP is a novel co-regulator of Smad3, and this study provides new insights into regulation of TGF-beta signaling.

Inhibition of hepatocyte nuclear factor 4 transcriptional activity by the nuclear factor kappaB pathway

HNF-4 (hepatocyte nuclear factor 4) is a key regulator of liver-specific gene expression in mammals. We have shown previously that the activity of the human APOC3 (apolipoprotein C-III) promoter is positively regulated by the anti-inflammatory cytokine TGFbeta (transforming growth factor beta) and its effectors Smad3 (similar to mothers against decapentaplegic 3) and Smad4 proteins via physical and functional interactions between Smads and HNF-4. We now show that the pro-inflammatory cytokine TNFalpha (tumour necrosis factor alpha) antagonizes TGFbeta for the regulation of APOC3 gene expression in hepatocytes. TNFalpha was a strong inhibitor of the activity of apolipoprotein promoters that harbour HNF-4 binding sites and this inhibition required HNF-4. Using specific inhibitors of TNFalpha-induced signalling pathways, it was shown that inhibition of the APOC3 promoter by TNFalpha involved NF-kappaB (nuclear factor kappaB). Latent membrane protein 1 of the Epstein-Barr virus, which is an established potent activator of NF-kappaB as well as wild-type forms of various NF-kappaB signalling mediators, also inhibited strongly the APOC3 promoter and the transactivation function of HNF-4. TNFalpha had no effect on the stability or the nuclear localization of HNF-4 in HepG2 cells, but inhibited the binding of HNF-4 to the proximal APOC3 HRE (hormone response element). Using the yeast-transactivator-GAL4 system, we showed that both AF-1 and AF-2 (activation functions 1 and 2) of HNF-4 are inhibited by TNFalpha and that this inhibition was abolished by overexpression of different HNF-4 co-activators, including PGC-1 (peroxisome-proliferator-activated-receptor-gamma co-activator 1), CBP [CREB (cAMP-response-element-binding protein) binding protein] and SRC3 (steroid receptor co-activator 3). In summary, our findings indicate that TNFalpha, or other factors that trigger an NF-kappaB response in hepatic cells, inhibit the transcriptional activity of the APOC3 and other HNF-4-dependent promoters and that this inhibition could be accounted for by a decrease in DNA binding and the down-regulation of the transactivation potential of the AF-1 and AF-2 domains of HNF-4.

Hepatocyte nuclear factor 4 alpha ligand binding and F domains mediate interaction and transcriptional synergy with the pancreatic islet LIM HD transcription factor Isl1

The orphan nuclear receptor HNF4alpha and the LIM homeodomain factor Isl1 are co-expressed in pancreatic beta-cells and are required for the differentiation and function of these endocrine cells. HNF4alpha activates numerous genes and mutations in its gene are associated with maturity onset diabetes of the young. Cofactors and transcription factors that interact with HNF4alpha are crucial to modulate its transcriptional activity, since the latter is not regulated by conventional ligands. These transcriptional partners interact mainly through the HNF4alpha AF-1 module and the ligand binding domain, which contains the AF-2 module. Here, we showed that Isl1 could enhance the HNF4alpha-mediated activation of transcription of the HNF1alpha, PPARalpha and insulin I promoters. Isl1 interacted with the HNF4alpha AF-2 but also required the HNF4alpha carboxy-terminal F domain for optimal interaction and transcriptional synergy. More specifically, we found that naturally occurring HNF4alpha isoforms, differing only in their F domain, exhibited different abilities to interact and synergize with Isl1, extending the crucial transcriptional modulatory role of the HNF4alpha F domain. HNF4alpha interacted with both the homeodomain and the first LIM domain of Isl1. We found that the transcriptional synergy between HNF4alpha and Isl1 involved an increase in HNF4alpha loading on promoter. The effect was more pronounced on the rat insulin I promoter containing binding sites for both HNF4alpha and Isl1 than on the human HNF1alpha promoter lacking an Isl1 binding site. Moreover, Isl1 could mediate the recruitment of the cofactor CLIM2 resulting in a further transcriptional enhancement of the HNF1alpha promoter activity.

Binding sites for metabolic disease related transcription factors inferred at base pair resolution by chromatin immunoprecipitation and genomic microarrays

We present a detailed in vivo characterization of hepatocyte transcriptional regulation in HepG2 cells, using chromatin immunoprecipitation and detection on PCR fragment-based genomic tiling path arrays covering the encyclopedia of DNA element (ENCODE) regions. Our data suggest that HNF-4alpha and HNF-3beta, which were commonly bound to distal regulatory elements, may cooperate in the regulation of a large fraction of the liver transcriptome and that both HNF-4alpha and USF1 may promote H3 acetylation to many of their targets. Importantly, bioinformatic analysis of the sequences bound by each transcription factor (TF) shows an over-representation of motifs highly similar to the in vitro established consensus sequences. On the basis of these data, we have inferred tentative binding sites at base pair resolution. Some of these sites have been previously found by in vitro analysis and some were verified in vitro in this study. Our data suggests that a similar approach could be used for the in vivo characterization of all predicted/uncharacterized TF and that the analysis could be scaled to the whole genome.

Characterization of a novel transcriptionally active domain in the transforming growth factor beta-regulated Smad3 protein

Transforming growth factor beta (TGFbeta) regulates transcriptional responses via activation of cytoplasmic effector proteins termed Smads. Following their phosphorylation by the type I TGFbeta receptor, Smads form oligomers and translocate to the nucleus where they activate the transcription of TGFbeta target genes in cooperation with nuclear cofactors and coactivators. In the present study, we have undertaken a deletion analysis of human Smad3 protein in order to characterize domains that are essential for transcriptional activation in mammalian cells. With this analysis, we showed that Smad3 contains two domains with transcriptional activation function: the MH2 domain and a second middle domain that includes the linker region and the first two beta strands of the MH2 domain. Using a protein-protein interaction assay based on biotinylation in vivo, we were able to show that a Smad3 protein bearing an internal deletion in the middle transactivation domain is characterized by normal oligomerization and receptor activation properties. However, this mutant has reduced transactivation capacity on synthetic or natural promoters and is unable to interact physically and functionally with the histone acetyltransferase p/CAF. The loss of interaction with p/CAF or other coactivators could account, at least in part, for the reduced transactivation capacity of this Smad3 mutant. Our data support an essential role of the previously uncharacterized middle region of Smad3 for nuclear functions, such as transcriptional activation and interaction with coactivators.

Inhibition of p53-mediated transcriptional responses by mithramycin A

In the present work, we show that mithramycin A, a drug that is currently used for the treatment of patients with Paget's disease of the bone as well as with several forms of cancer, is a strong activator of the tumor suppressor p53 protein in human hepatoma cells. The time course of p53 activation by mithramycin A was similar to the known chemotherapeutic compound 5-fluorouracil (5-FU). Both 5-FU and mithramycin A induced site-specific phosphorylation of p53 at serine 15. However, in contrast to 5-FU, mithramycin A failed to activate p53 target genes including the cell cycle inhibitor p21Cip1 gene as well as the proapoptotic genes PUMA (p53-upregulated mediator of apotosis) and BAK (bcl2-homologous antagonist/killer) and blocked the induction of the above genes by 5-FU. Using transactivation assays in Sp1-deficient cells, we showed that mithramycin A inhibited the transcriptional activation of the p21Cip1 and PUMA promoters by Sp1 and p53. Using chromatin immunoprecipitation assays and a novel protein-protein interaction assay based on biotinylation in vivo, we established that 5-FU enhanced the formation of p53-Sp1 complexes in solution and the subsequent recruitment of both factors to the p21Cip1 promoter. Mithramycin A also enhanced the recruitment of p53 to the distal p21Cip1 promoter but totally blocked the recruitment of Sp1 to the proximal p21Cip1 promoter. Our findings suggest that inhibition of Sp1 binding to the promoters of several p53 target genes, such as the p21Cip1 gene as well as certain proapoptotic genes, by mithramycin A, prevents the transcriptional induction of these genes by p53 and propose a mechanism that could account for some of the tumor suppressing and antiapoptotic effects of mithramycin A.

Fatty acid regulation of gene transcription

Dietary fat has a dual role in human physiology: a) it functions as a source of energy and structural components for cells; b) it functions as a regulator of gene expression that impacts lipid, carbohydrate, and protein metabolism, as well as cell growth and differentiation. Fatty acid effects on gene expression are cell-specific and influenced by fatty acid structure and metabolism. Fatty acids interact with the genome through several mechanisms. They regulate the activity or nuclear abundance of several transcription factors, including PPAR, LXR, HNF-4, NFkappaB, and SREBP. Fatty acids or their metabolites bind directly to specific transcription factors to regulate gene transcription. Alternatively, fatty acids indirectly act on gene expression through their effects on a) specific enzyme-mediated pathways, such as cyclooxygenase, lipoxygenase, protein kinase C, or sphingomyelinase signal transduction pathways; or b) pathways that involve changes in membrane lipid/lipid raft composition that affect G-protein receptor or tyrosine kinase-linked receptor signaling. Further definition of these fatty acid-regulated pathways will provide insight into the role dietary fat plays in human health and the onset and progression of several chronic diseases, like coronary artery disease and atherosclerosis, dyslipidemia and inflammation, obesity and diabetes, cancer, major depressive disorders, and schizophrenia.

Critical role of residues defining the ligand binding pocket in hepatocyte nuclear factor-4alpha

Hepatocyte nuclear factor-4alpha (HNF-4alpha), a member of the nuclear receptor superfamily, is a crucial regulator of a large number of genes involved in glucose, cholesterol, and fatty acid metabolism. Unlike other members of the superfamily, HNF-4alpha activates transcription in the absence of exogenously added ligand. Recently published crystallographic data show that fatty acids are endogenous ligands for HNF-4. Transcriptional analysis of point mutations of the residues that are located in helices H3, H5, H10, and H11, which have been shown to come in contact with the ligand, resulted in a dramatic decrease in activity, without affecting DNA binding and dimerization. Our results show the importance of residues Ser-181, Met-182 in H3, Leu-219, Leu-220 and Arg-226 in H5, Ile-338 in H10, and Ile-346 in H11 that line the ligand-binding domain pocket in HNF-4alpha and impair its transactivation potential. Structural modeling reveals that the mutations do not cause any large scale structural alterations, and the observed loss in transactivation can be attributed to local changes, demonstrating that these residues play a significant role in maintaining the structural integrity of the HNF-4alpha ligand binding pocket.

Expression and secretion of human apolipoprotein A-I in the heart

Various studies have correlated apolipoprotein (apo) A-I, the major component high-density lipoprotein, with protection against development of cardiovascular disease. Although apoA-I expression has been previously detected in the liver and intestine, we have discovered that the human apoA-I gene is also expressed in the heart. Using transgenic (Tg) mice generated with the human apoA-I/C-III/A-IV gene cluster and Tg mice produced with just the 2.2 kb human apoA-I gene, we have detected significant levels of apoA-I expression in the heart. Furthermore, the detection of apoA-I expression in the hearts of human apoA-I Tg mice indicates that the minimal regulatory elements necessary for cardiac expression of the gene are located near its coding sequence. To determine if the apoA-I gene is also expressed in the human heart, similar analyses were performed, where apoA-I expression was found in both adult and fetal hearts. Furthermore in-depth investigation of the various regions of human and Tg mouse hearts revealed that the apoA-I mRNA was present in the ventricles and atria, but not in the aorta. In situ hybridization of Tg mouse hearts revealed that apoA-I expression was restricted to the cardiac myocyte cells. Finally, heart explants and cardiac primary culture experiments with Tg mice showed secretion of particles containing the human apoA-I protein, and metabolic labeling experiments have also detected a 28 kDa human apoA-I protein secreted from the heart. From these novel findings, new insights into the role and function of apoA-I can be extrapolated.

Mechanism of a transcriptional cross talk between transforming growth factor-beta-regulated Smad3 and Smad4 proteins and orphan nuclear receptor hepatocyte nuclear factor-4

We have shown previously that the transforming growth factor-beta (TGFbeta)-regulated Sma-Mad (Smad) protein 3 and Smad4 proteins transactivate the apolipoprotein C-III promoter in hepatic cells via a hormone response element that binds the nuclear receptor hepatocyte nuclear factor 4 (HNF-4). In the present study, we show that Smad3 and Smad4 but not Smad2 physically interact with HNF-4 via their Mad homology 1 domains both in vitro and in vivo. The synergistic transactivation of target promoters by Smads and HNF-4 was shown to depend on the specific promoter context and did not require an intact beta-hairpin/DNA binding domain of the Smads. Using glutathione S-transferase interaction assays, we established that two regions of HNF-4, the N-terminal activation function 1 (AF-1) domain (aa 1-24) and the C-terminal F domain (aa 388-455) can mediate physical Smad3/HNF-4 interactions in vitro. In vivo, Smad3 and Smad4 proteins enhanced the transactivation function of various GAL4-HNF-4 fusion proteins via the AF-1 and the adjacent DNA binding domain, whereas a single tyrosine to alanine substitution in AF-1 abolished coactivation by Smads. The findings suggest that the transcriptional cross talk between the TGFbeta-regulated Smads and HNF-4 is mediated by specific functional domains in the two types of transcription factors. Furthermore, the specificity of this interaction for certain target promoters may play an important role in various hepatocyte functions, which are regulated by TGFbeta and the Smads.

Two major Smad pathways in TGF-beta superfamily signalling

Members of the transforming growth factor-beta (TGF-beta) superfamily bind to two different serine/threonine kinase receptors, i.e. type I and type II receptors. Upon ligand binding, type I receptors specifically activate intracellular Smad proteins. R-Smads are direct substrates of type I receptors; Smads 2 and 3 are specifically activated by activin/nodal and TGF-beta type I receptors, whereas Smads 1, 5 and 8 are activated by BMP type I receptors. Nearly 30 proteins have been identified as members of the TGF-beta superfamily in mammals, and can be classified based on whether they activate activin/TGF-beta-specific R-Smads (AR-Smads) or BMP-specific R-Smads (BR-Smads). R-Smads form complexes with Co-Smads and translocate into the nucleus, where they regulate the transcription of target genes. AR-Smads bind to various proteins, including transcription factors and transcriptional co-activators or co-repressors, whereas BR-Smads interact with other proteins less efficiently than AR-Smads. Id proteins are induced by BR-Smads, and play important roles in exhibiting some biological effects of BMPs. Understanding the mechanisms of TGF-beta superfamily signalling is thus important for the development of new ways to treat various clinical diseases in which TGF-beta superfamily signalling is involved.

Differential, tissue-specific, transcriptional regulation of apolipoprotein B secretion by transforming growth factor beta

Apolipoprotein B (apoB) is required for the assembly and secretion of triglyceride-rich lipoproteins. ApoB synthesis is constitutive, and post-translational mechanisms modulate its secretion. Transforming growth factor beta (TGF-beta) increased apoB secretion in both differentiated and nondifferentiated Caco-2 cells and decreased secretion in HepG2 cells without affecting apolipoprotein A-I secretion. TGF-beta altered apoB secretion by changing steady-state mRNA levels and protein synthesis. Expression of SMAD3 and SMAD4 differentially regulated apoB secretion in these cells. Thus, SMADs mediate dissimilar secretion of apoB in both the cell lines by affecting gene transcription. We identified a 485-bp element, 55 kb upstream of the apob gene that contains a SMAD binding motif. This motif increased the expression of chloramphenicol acetyltransferase in Caco-2 cells treated with TGF-beta or transfected with SMADs. Hence, TGF-beta activates SMADs that bind to the 485-bp intestinal enhancer element in the apob gene and increase its transcription and secretion in Caco-2 cells. This is the first example showing differential transcriptional regulation of the apob gene by cytokines and dissimilar regulation of one gene in two different cell lines by TGF-beta. In this regulation, the presence of cytokine-responsive motif in the tissue-specific enhancer element confers cell-specific response.

Smad regulation in TGF-β signal transduction

Smad proteins transduce signals from transforming growth factor-β (TGF-β) superfamily ligands that regulate cell proliferation, differentiation and death through activation of receptor serine/threonine kinases. Phosphorylation of receptor-activated Smads (R-Smads) leads to formation of complexes with the common mediator Smad (Co-Smad), which are imported to the nucleus. Nuclear Smad oligomers bind to DNA and associate with transcription factors to regulate expression of target genes. Alternatively, nuclear R-Smads associate with ubiquitin ligases and promote degradation of transcriptional repressors, thus facilitating target gene regulation by TGF-β. Smads themselves can also become ubiquitinated and are degraded by proteasomes. Finally, the inhibitory Smads (I-Smads) block phosphorylation of R-Smads by the receptors and promote ubiquitination and degradation of receptor complexes, thus inhibiting signalling.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

The transcriptional role of Smads and FAST (FoxH1) in TGFbeta and activin signalling

The Smad family of proteins are critical components of the TGFbeta superfamily signalling pathway. Ligand addition induces phosphorylation of specific receptor-regulated Smads, which then form heteromeric complexes with the common mediator Smad, Smad4. This complex then translocates from the cytoplasm into the nucleus. Once there, the R-Smad/Smad4 complex interacts with a variety of DNA binding proteins and is thereby targetted to a diverse array of gene promoters. The Smad-containing DNA binding complex can then positively or negatively regulate gene expression through the recruitment of co-activators and co-repressors. Xenopus FAST (now known as FoxH1) was the first Smad DNA binding partner identified and the FoxH1 family now includes related proteins from mouse, human and Zebrafish. In all organisms examined, FoxH1 is expressed primarily during the earliest stages of development and thus FoxH1 is thought to play a critical role in mediating TGFbeta superfamily signals during these early developmental stages. Other Smad partners range from those that are ubiquitously expressed to others that are present only in specific cell types or developmental stages. Thus, it is the interaction of Smads with a wide range of specific transcriptional partners that is important for the generation of diverse biological responses to TGFbeta superfamily members.

Divergence and convergence of TGF-beta/BMP signaling

The transforming growth factor-beta (TGF-beta) superfamily includes more than 30 members which have a broad array of biological activities. TGF-beta superfamily ligands bind to type II and type I serine/threonine kinase receptors and transduce signals via Smad proteins. Receptor-regulated Smads (R-Smads) can be classified into two subclasses, i.e. those activated by activin and TGF-beta signaling pathways (AR-Smads), and those activated by bone morphogenetic protein (BMP) pathways (BR-Smads). The numbers of type II and type I receptors and Smad proteins are limited. Thus, signaling of the TGF-beta superfamily converges at the receptor and Smad levels. In the intracellular signaling pathways, Smads interact with various partner proteins and thereby exhibit a wide variety of biological activities. Moreover, signaling by Smads is modulated by various other signaling pathways allowing TGF-beta superfamily ligands to elicit diverse effects on target cells. Perturbations of the TGF-beta/BMP signaling pathways result in various clinical disorders including cancers, vascular diseases, and bone disorders.

Identification of novel TGF-beta /Smad gene targets in dermal fibroblasts using a combined cDNA microarray/promoter transactivation approach

Despite major advances in the understanding of the intimate mechanisms of transforming growth factor-beta (TGF-beta) signaling through the Smad pathway, little progress has been made in the identification of direct target genes. In this report, using cDNA microarrays, we have focussed our attention on the characterization of extracellular matrix-related genes rapidly induced by TGF-beta in human dermal fibroblasts and attempted to identify the ones whose up-regulation by TGF-beta is Smad-mediated. For a gene to qualify as a direct Smad target, we postulated that it had to meet the following criteria: (1) rapid (30 min) and significant (at least 2-fold) elevation of steady-state mRNA levels upon TGF-beta stimulation, (2) activation of the promoter by both exogenous TGF-beta and co-transfected Smad3 expression vector, (3) up-regulation of promoter activity by TGF-beta blocked by both dominant-negative Smad3 and inhibitory Smad7 expression vectors, and (4) promoter transactivation by TGF-beta not possible in Smad3(-/-) mouse embryo fibroblasts. Using this stringent approach, we have identified COL1A2, COL3A1, COL6A1, COL6A3, and tissue inhibitor of metalloproteases-1 as definite TGF-beta/Smad3 targets. Extrapolation of this approach to other extracellular matrix-related gene promoters also identified COL1A1 and COL5A2, but not COL6A2, as novel Smad targets. Together, these results represent a significant step toward the identification of novel, early-induced Smad-dependent TGF-beta target genes in fibroblasts.

Transcriptional regulatory mechanisms of the human apolipoprotein genes in vitro and in vivo

The present review summarizes recent advances in the transcriptional regulation of the human apolipoprotein genes, focusing mostly, but not exclusively, on in-vivo studies and signaling mechanisms that affect apolipoprotein gene transcription. An attempt is made to explain how interactions of transcription factors that bind to proximal promoters and distal enhancers may bring about gene transcription. The experimental approaches used and the transcriptional regulatory mechanisms that emerge from these studies may also be applicable in other gene systems that are associated with human disease. Understanding extracellular stimuli and the specific mechanisms that underlie apolipoprotein gene transcription may in the long run allow us to selectively switch on antiatherogenic genes, and switch off proatherogenic genes. This may have beneficial effects and may confer protection from atherosclerosis to humans.