Identification of Egr1 as the oncostatin M-induced transcription activator that binds to sterol-independent regulatory element of human LDL receptor promoter

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Therapeutic targets of hypercholesterolemia: HMGCR and LDLR

Cholesterol homeostasis is critical and necessary for the body's functions. Hypercholesterolemia can lead to significant clinical problems, such as cardiovascular disease (CVD). 3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) and low-density lipoprotein cholesterol receptor (LDLR) are major points of control in cholesterol homeostasis. We summarize the regulatory mechanisms of HMGCR and LDLR, which may provide insight for new drug design and development.

Role of early growth response 1 in liver metabolism and liver cancer

The liver is an essential organ for nutrient and drug metabolism - possessing the remarkable ability to sense environmental and metabolic stimuli and provide an optimally adaptive response. Early growth response 1 (Egr1), an immediate early transcriptional factor which acts as a coordinator of the complex response to stress, is induced during liver injury and controls the expression of a wide range of genes involved in metabolism, cell proliferation, and role of Egr1 in liver injury and repair, deficiency of Egr1 delays liver regeneration process. The known upstream regulators of Egr1 include, but are not limited to, growth factors (e.g. transforming growth factor β1, platelet-derived growth factor, epidermal growth factor, hepatocyte growth factor), nuclear receptors (e.g. hepatocyte nuclear factor 4α, small heterodimer partner, peroxisome proliferator-activated receptor-γ), and other transcription factors (e.g. Sp1, E2F transcription factor 1). Research efforts using various animal models such as fatty liver, liver injury, and liver fibrosis contribute greatly to the elucidation of Egr1 function in the liver. Hepatocellular carcinoma (HCC) represents the second leading cause of cancer mortality worldwide due to the heterogeneity and the late stage at which cancer is generally diagnosed. Recent studies highlight the involvement of Egr1 in HCC development. The purpose of this review is to summarize current studies pertaining to the role of Egr1 in liver metabolism and liver diseases including liver cancer.

Modulators of hepatic lipoprotein metabolism identified in a search for small-molecule inducers of tribbles pseudokinase 1 expression

Recent genome wide association studies have linked tribbles pseudokinase 1 (TRIB1) to the risk of coronary artery disease (CAD). Based on the observations that increased expression of TRIB1 reduces secretion of VLDL and is associated with lower plasma levels of LDL cholesterol and triglycerides, higher plasma levels of HDL cholesterol and reduced risk for myocardial infarction, we carried out a high throughput phenotypic screen based on quantitative RT-PCR assay to identify compounds that induce TRIB1 expression in human HepG2 hepatoma cells. In a screen of a collection of diversity-oriented synthesis (DOS)-derived compounds, we identified a series of benzofuran-based compounds that upregulate TRIB1 expression and phenocopy the effects of TRIB1 cDNA overexpression, as they inhibit triglyceride synthesis and apoB secretion in cells. In addition, the compounds downregulate expression of MTTP and APOC3, key components of the lipoprotein assembly pathway. However, CRISPR-Cas9 induced chromosomal disruption of the TRIB1 locus in HepG2 cells, while confirming its regulatory role in lipoprotein metabolism, demonstrated that the effects of benzofurans persist in TRIB1-null cells indicating that TRIB1 is sufficient but not necessary to transmit the effects of the drug. Remarkably, active benzofurans, as well as natural products capable of TRIB1 upregulation, also modulate hepatic cell cholesterol metabolism by elevating the expression of LDLR transcript and LDL receptor protein, while reducing the levels of PCSK9 transcript and secreted PCSK9 protein and stimulating LDL uptake. The effects of benzofurans are not masked by cholesterol depletion and are independent of the SREBP-2 regulatory circuit, indicating that these compounds represent a novel class of chemically tractable small-molecule modulators that shift cellular lipoprotein metabolism in HepG2 cells from lipogenesis to scavenging.

Berry anthocyanins suppress the expression and secretion of proinflammatory mediators in macrophages by inhibiting nuclear translocation of NF-κB independent of NRF2-mediated mechanism

The objectives of this study were to compare the anti-inflammatory effects of anthocyanins from blueberry (BBA), blackberry (BKA), and blackcurrant (BCA) and to determine the relationship between their antioxidant capacity and anti-inflammatory effect in macrophages. Major anthocyanins in BBA, BKA and BCA were malvidin-3-glucoside (16%), cyanidin-3-glucoside (98%) and delphinidin-3-rutinoside (44%), respectively. BKA showed higher total antioxidant capacity than BBA and BCA. RAW 264.7 macrophages were incubated with 0-20 μg/ml of BBA, BKA and BCA, and subsequently activated by lipopolysaccharide (LPS) to measure proinflammatory cytokine production. Interleukin 1β (IL-1β) messenger RNA (mRNA) levels were significantly decreased by all berry anthocyanins at 10 μg/ml or higher. Tumor necrosis factor α (TNFα) mRNA levels and secretion were also significantly decreased in LPS-treated macrophages. The levels of the repression were comparable for all berry anthocyanins. LPS-induced nuclear factor κB (NF-κB) p65 translocation to the nucleus was markedly attenuated by all of the berry anthocyanins. In bone marrow-derived macrophages (BMMs) from nuclear factor E2-related factor 2 wild-type (Nrf2(+/+)) mice, BBA, BKA and BCA significantly decreased cellular reactive oxygen species (ROS) levels with a concomitant decrease in IL-1β mRNA levels upon LPS stimulation. However, in the BMM from Nrf2(-/-) mice, the anthocyanin fractions were able to significantly decrease IL-1β mRNA despite the fact that ROS levels were not significantly affected. In conclusion, BBA, BKA and BCA exert their anti-inflammatory effects in macrophages, at least in part, by inhibiting nuclear translocation of NF-κB independent of the NRF2-mediated pathways.

Gene expression signature of human HepG2 cell line

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide and is associated with various clinico-pathological characteristics such as genetic mutations and viral infections. Therefore, numerous laboratories look out for identifying always new putative markers for the improvement of HCC diagnosis/prognosis. Many molecular profiling studies investigated gene expression changes related to HCC. HepG2 represents a pure cell line of human liver carcinoma, often used as HCC model due to the absence of viral infection. In this study we compare gene expression profiles associated with HepG2 (as HCC model) and normal hepatocyte cells by microarray technology. Hierarchical cluster analysis of genes evidenced that 2646 genes significantly down-regulated in HepG2 cells compared to hepatocytes whereas a further 3586 genes significantly up-regulated. By using the Ingenuity Pathway Analysis (IPA) program, we have classified the genes that were differently expressed and studied the functional networks correlating these genes in the complete human interactome. Moreover, to confirm the differentially expressed genes as well as the reliability of our microarray data, we performed a quantitative Real time RT-PCR analysis on 9 up-regulated and 11 down-regulated genes, respectively. In conclusion this work i) provides a gene signature of human hepatoma cells showing genes that change their expression as a consequence of liver cancer in the absence of any genetic mutations or viral infection, ii) evidences new differently expressed genes found in our signature compared to previous published studies and iii) suggests some genes on which to focus future studies to understand if they can be used to improve the HCC prognosis/diagnosis.

Modified methylenedioxyphenol analogs lower LDL cholesterol through induction of LDL receptor expression

Although statin therapy is a cornerstone of current low density lipoprotein (LDL)-lowering strategies, there is a need for additional therapies to incrementally lower plasma LDL cholesterol. In this study, we investigated the effect of several methylenedioxyphenol derivatives in regulating LDL cholesterol through induction of LDL receptor (LDLR). INV-403, a modified methylenedioxyphenol derivative, increased LDLR mRNA and protein expression in HepG2 cells in a dose- and time-dependent fashion. These effects were apparent even under conditions of HMG-CoA reductase inhibition. Electrophoresis migration shift assays demonstrated that INV-403 activates SREBP2 but not SREBP1c, with immunoblot analysis showing an increased expression of the mature form of SREBP2. Knockdown of SREBP2 reduced the effect of INV-403 on LDLR expression. The activation of SREBP2 by INV-403 is partly mediated by Akt/GSK3β pathways through inhibition of phosphorylation-dependent degradation by ubiquitin-proteosome pathway. Treatment of C57Bl/6j mice with INV-403 for two weeks increased hepatic SREBP2 levels (mature form) and upregulated LDLR with concomitant lowering of plasma LDL levels. Transient expression of a LDLR promoter-reporter construct, a SRE-mutant LDLR promoter construct, and a SRE-only construct in HepG2 cells revealed an effect predominantly through a SRE-dependent mechanism. INV-403 lowered plasma LDL cholesterol levels through LDLR upregulation. These results indicate a role for small molecule approaches other than statins for lowering LDL cholesterol.

Early growth response 1 (Egr1) regulates cholesterol biosynthetic gene expression

The early growth response (EGR) family of transcription factors has been implicated in control of lipid biosynthetic genes. Egr1 is induced by insulin both in vitro and in vivo and is the most highly expressed family member in liver. In this study, we investigated whether Egr1 regulates cholesterol biosynthetic genes in liver. Using an insulin-sensitive liver cell line, we show that localization of Egr1 to cholesterol biosynthetic genes is induced by insulin treatment and that this localization precedes the induction of the genes. Reduction in Egr1 expression using targeted siRNA blunted the insulin-dependent induction of cholesterol genes. A similar reduction in squalene epoxidase expression was also observed in Egr1 null mice. In addition, application of chromatin immunoprecipitation (ChIP) samples to tiled gene microarrays revealed localization of Egr1 in promoter regions of many cholesterol gene loci. In vivo ChIP assays using liver tissue show that Egr1 localization to several cholesterol biosynthetic gene promoters is induced by feeding. Finally, analysis of plasma cholesterol in Egr1(-/-) mice indicated a significant decrease in serum cholesterol when compared with wild-type mice. Together these data point to Egr1 as a modulator of the cholesterol biosynthetic gene family in liver.

Janus kinase activation by cytokine oncostatin M decreases PCSK9 expression in liver cells

PCSK9 degrades LDL receptor (LDLR) in liver and thereby influences the circulating level of LDL cholesterol. Hence, mechanisms inhibiting PCSK9 expression have potential for cholesterol-lowering intervention. Previously, we demonstrated that oncostatin M (OM) activates LDLR gene transcription, resulting in an increased LDL uptake in HepG2 cells and a reduction of plasma LDL in hypercholesterolemic hamsters. Here we identify the suppression of PCSK9 expression by OM as one new mechanism that increases LDLR protein in HepG2 cells. Treating HepG2 cells with OM decreases PCSK9 mRNA and protein levels. Inhibition studies and small interfering RNA -targeted depletion revealed a critical role for JAK1 and JAK2 in mediating this OM inhibitory effect. Furthermore, we showed that OM induces transient phosphorylation of STAT1, STAT3, and STAT5 and sustained activation of ERK signaling molecules. While depletion of STAT members in HepG2 cells did not affect OM inhibitory activity on PCSK9 expression, blocking activation of the MEK1/ERK signaling pathway resulted in attenuation of the OM inhibitory effect. Finally, by using an anti-hamster PCSK9 antibody, we demonstrated the in vivo suppression of liver PCSK9 mRNA and protein expression by OM in hypercholesterolemic hamsters. Our study uncovered a cytokine-triggered regulatory network for PCSK9 expression that is linked to JAKs and the ERK signaling pathway.

Identification of heterogeneous nuclear ribonucleoprotein K as a transactivator for human low density lipoprotein receptor gene transcription

hnRNP K, a member of the family of heterogeneous ribonucleoproteins, is known to exert various functional roles in the nucleus, cytoplasm, and mitochondria to affect different cellular processes including chromatin remodeling, transcription, splicing, and translation. Here we report, for the first time, that hnRNP K is specifically involved in human LDL receptor (LDLR) gene transcription in HepG2 cells. We show that depletion of hnRNP K by siRNA transfection reduces the expression of LDLR mRNA and protein by more than 50% as measured by quantitative real-time PCR and Western blot analysis. Importantly, we show that the decay rate of LDLR mRNA is not affected by hnRNP K siRNA transfection, whereas the LDLR promoter activity is significantly decreased. Furthermore, overexpression of hnRNP K increased the LDLR promoter activity by the luciferase reporter assay. By utilizing a series of mutational and deletional constructs of LDLR promoter luciferase reporters, we mapped the K-responsive element to the repeat 3 (R3) sequence of the LDLR promoter. Electrophoretic mobility shift assays show that the K protein binds to a single-stranded DNA probe containing the CT-rich element of R3, which is in contrast to the requirement of double-stranded DNA for Sp1 to bind to R3. Finally, chromatin immunoprecipitation assays reveal a direct interaction of hnRNP K with the LDLR promoter in intact HepG2 cells. These new findings provide strong evidence demonstrating that hnRNP K is an important transactivator for human LDLR gene transcription. This work sheds new light on our current understanding of how LDLR gene expression is controlled at the transcriptional level.

Elucidation of an SRE-1/SREBP-independent cellular pathway for LDL-receptor regulation: from the cell surface to the nucleus

Reduction in blood levels of low-density lipoprotein (LDL) cholesterol lowers the risk of coronary heart disease. The elucidation of cellular pathways that control LDL-receptor expression through a cholesterol-mediated negative feedback mechanism has provided a crucial molecular basis for the development and clinical applications of statins in the treatment of hypercholesterolemia. The characterization of signaling transduction pathways elicited by cytokine oncostatin M (OM) in liver cells has revealed a novel cellular pathway that activates LDL-receptor transcription independent of intracellular levels of cholesterol and sterol-regulatory element binding proteins. This transcriptional activation is achieved through interactions of the sterol-independent regulatory element of LDL-receptor promoter and transcription factors Egr1 and c/EBPbeta, and is dependent upon the activation of the extracellular signal-regulated kinase signaling cascade by OM. In vivo OM administration in hyperlipidemic animals reduces circulating cholesterol and prevents lipid accumulation in the liver. Exploring this sterol-independent cellular pathway may lead to new therapeutic advances.

Regulation of the human thromboxane A2 receptor gene by Sp1, Egr1, NF-E2, GATA-1, and Ets-1 in megakaryocytes

The alpha and beta isoforms of the human thromboxane A(2) (TXA(2)) receptor (TP) are encoded by a single gene but are transcriptionally regulated by distinct promoters, termed promoter 1 (Prm1) and Prm3, respectively. Herein, it was sought to identify factors regulating Prm1 within the megakaryocytic human erythroleukemia 92.1.7 cell line. Through gene deletion and reporter assays, the core Prm1 was localized to between nucleotides -6,320 and -5,895, proximal to the transcription initiation site. Furthermore, two upstream repressor and two upstream activator regions were identified. Site-directed mutagenesis of four overlapping Sp1/Egr1 elements and an NF-E2/AP1 element within the proximal region substantially reduced Prm1 activity. Deletion/mutation of GATA and Ets elements disrupted the upstream activator sequence located between -7,962 and -7,717, significantly impairing Prm1 activity. Electrophoretic mobility shift assays and chromatin immunoprecipitations confirmed that Sp1, Egr1, and NF-E2 bind to elements within the core promoter, whereas GATA-1 and Ets-1 factors bind to the upstream activator sequence (between -7,962 and -7,717). Collectively, these data establish that Sp1, Egr1, and NF-E2 regulate core Prm1 activity in the megakaryocytic-platelet progenitor cells, whereas GATA-1 and Ets-1 act as critical upstream activators, hence providing the first genetic basis for the expression of the human TXA(2) receptor (TP) within the vasculature.

Transcriptional activation of hepatic ACSL3 and ACSL5 by oncostatin m reduces hypertriglyceridemia through enhanced beta-oxidation

OBJECTIVE

In our previous studies that examined in vivo activities of oncostatin M (OM) in upregulation of hepatic LDL receptor (LDLR) expression, we observed reductions of LDL-cholesterol and triglyceride (TG) levels in OM-treated hyperlipidemic hamsters. Interestingly, the OM effect of lowering plasma TG was more pronounced than LDL-cholesterol reduction, suggesting additional LDLR-independent actions. Here, we investigated mechanisms underlying the direct TG-lowering effect of OM.

METHODS AND RESULTS

We demonstrate that OM activates transcription of long-chain acyl-coenzymeA (CoA) synthetase isoforms 3 and 5 (ACSL3, ACSL5) in HepG2 cells through the extracellular signal-regulated kinase (ERK) signaling pathway. Increased acyl-CoA synthetase activities in OM-stimulated HepG2 cells and in livers of OM-treated hamsters are associated with decreased TG accumulation and increased fatty acid beta-oxidation. We further show that overexpression of ACSL3 or ACSL5 alone in the absence of OM led to fatty acid partitioning into beta-oxidation. Importantly, we demonstrate that transfection of siRNAs targeted to ACSL3 and ACSL5 abrogated the enhancing effect of OM on fatty acid oxidation in HepG2 cells.

CONCLUSIONS

These new findings identify ACSL3 and ACSL5 as OM-regulated genes that function in fatty acid metabolism and suggest a novel cellular mechanism by which OM directly lowers the plasma TG in hyperlipidemic animals through stimulating the transcription of ACSL specific isoforms in the liver.

Blockage of oncostatin M-induced LDL receptor gene transcription by a dominant-negative mutant of C/EBPbeta

OM (oncostatin M) activates the human LDLR [LDL (low-density lipoprotein) receptor] gene transcription in HepG2 cells through the SIRE (sterol-independent regulatory element) of LDLR promoter. The SIRE sequence consists of a C/EBP (CCAAT/enhancer-binding protein)-binding site and a CRE (cAMP-response element). Our previous studies [Zhang, Ahlborn, Li, Kraemer and Liu (2002) J. Lipid Res. 43, 1477-1485; Zhang, Lin, Abidi, Thiel and Liu (2003) J. Biol. Chem. 278, 44246-44254] have demonstrated that OM transiently induces EGR-1 (early growth response gene product 1) expression and EGR-1 activates LDLR transcription primarily through a protein-protein interaction with C/EBPbeta, which serves as a co-activator of EGR-1. In the present study, we examined the direct role of C/EBPbeta as a transactivator in OM-regulated LDLR gene transcription independent of EGR-1. We show that OM induces C/EBPbeta expression with kinetics slower than EGR-1 induction. A significant increase in C/EBPbeta protein level is detected by 2 h of OM treatment and remains elevated for 24 h. Chromatin immunoprecipitation assays demonstrate that the amount of C/EBPbeta bound to the LDLR SIRE sequence is increased 2.8-fold of control by 2 h of OM treatment, reached the highest level of 8-fold by 4 h, and slowly declined thereafter. To further examine the requirement of C/EBPbeta in OM-stimulated LDLR expression, we developed a His-tagged dominant-negative mutant of C/EBPbeta (His-C/EBPbeta-P4; where P4 is plasmid 4 in our mutation series), consisting of the DNA-binding and leucine zipper domains of C/EBPbeta (amino acids 246-345). Expression of His-C/EBPbeta-P4 in HepG2 cells significantly diminishes the OM-induced increase in LDLR promoter activity and the elevation of endogenous LDLR mRNA expression. Taken together, these new findings identify C/EBPbeta as an OM-induced transactivator in LDLR gene transcription and provide a better understanding of the molecular mechanism underlying the sterol-independent regulation of LDLR expression.

Regulation of cholesterol/lipid biosynthetic genes by Egr2/Krox20 during peripheral nerve myelination

Myelination of peripheral nerves by Schwann cells requires a large amount of lipid and cholesterol biosynthesis. To understand the transcriptional coordination of the myelination process, we have investigated the developmental relationship between early growth response 2 (Egr2)/Krox20--a pivotal regulator of peripheral nerve myelination--and the sterol regulatory element binding protein (SREBP) pathway, which controls expression of cholesterol/lipid biosynthetic genes. During myelination of sciatic nerve, there is a very significant induction of SREBP1 and SREBP2, as well as their target genes, suggesting that the SREBP transactivators are important regulators in the myelination process. Egr2/Krox20 does not appear to directly regulate the levels of SREBP pathway components, but rather, we found that Egr2/Krox20 and SREBP transactivators can synergistically activate promoters of several SREBP target genes, indicating that direct induction of cholesterol/lipid biosynthetic genes by Egr2/Krox20 is a part of the myelination program regulated by this transactivator.

In vivo activities of cytokine oncostatin M in the regulation of plasma lipid levels

Our previous studies have demonstrated the activity of oncostatin M (OM) in stimulating the transcription of the human LDL receptor (LDLR) gene in HepG2 cells through a sterol-independent regulatory mechanism. The current studies were designed to determine whether this in vitro property of OM could be recapitulated in vivo to increase LDLR expression in cholesterol-loaded livers and consequently decrease plasma levels of LDL-cholesterol (LDL-C) and total cholesterol (TC) using hypercholesterolemic hamsters as an experimental model. We show that administration of human recombinant OM for 7 days in hamsters fed a high-fat diet significantly reduced plasma levels of TC, LDL-C, and triglyceride in dose- and time-dependent manners. This lipid-lowering effect was associated with increased hepatic LDLR mRNA expression, as determined by quantitative real-time RT-PCR. Additionally, hepatic fat storage and cholesterol content in the hypercholesterolemic animals were substantially reduced by OM treatment. As a consequence, the increased aminotransferase levels in the high-fat diet-fed hamsters were normalized nearly to baseline values. These results not only corroborate the in vitro finding of OM in the regulation of LDLR but also, for the first time, demonstrate that OM has a strong lipid-lowering effect under in vivo conditions in which the levels of circulating LDL-C are high and liver LDLR transcription is repressed.

Krox20 stimulates adipogenesis via C/EBPbeta-dependent and -independent mechanisms

Krox20 is a zinc finger-containing transcription factor that is abundantly expressed in adipose tissue. However, its role in fat cell differentiation has not been established. In cultured 3T3-L1 cells, Krox20 is rapidly induced by serum stimulation. Overexpression of Krox20 in both 3T3-L1 preadipocytes and multipotent NIH3T3 cells promotes adipogenesis in a hormone-dependent manner. Conversely, RNAi-mediated loss of Krox20 function reduced adipogenesis in 3T3-L1 cells. Ectopic expression of Krox20 can transactivate the C/EBPbeta promoter and increase C/EBPbeta gene expression in 3T3-L1 preadipocytes. RNAi-mediated knockdown of C/EPBbeta diminished Krox20's proadipogenic effect. Finally, coexpression of Krox20 and C/EBPbeta in naive NIH3T3 cells resulted in the pronounced induction of a fully differentiated adipocyte phenotype, an effect previously observed only with PPARgamma. These data indicate that Krox20 is necessary for adipogenesis and that, when overexpressed, Krox20 potently stimulates adipogenesis via C/EBPbeta-dependent and -independent mechanisms.

Differential Gene Expression of Blood-Derived Cell Lines in Familial Combined Hyperlipidemia

OBJECTIVES

The genetic background of familial combined hyperlipidemia (FCHL) is currently unclear. We propose transcriptional profiling as a complementary tool for its understanding. Two hypotheses were tested: the existence of a disease-specific modification of gene expression in FCHL and the detectability of such a transcriptional profile in blood derived cell lines.

METHODS AND RESULTS

We established lymphoblastic cell lines from FCHL patients and controls. The cells were cultured in fixed conditions and their basal expression profile was compared using microarrays; 166 genes were differentially expressed in FCHL-derived cell lines compared with controls, with enrichment in metabolism-related genes. Of note was the upregulation of EGR-1, previously found to be upregulated in the adipose tissue of FCHL patients, the upregulation of DCHR-7, the downregulation of LYPLA2, and the differential expression of several genes previously unrelated to FCHL. A cluster of potential EGR-1-regulated transcripts was also differentially expressed in FCHL cells.

CONCLUSIONS

Our data indicate that in FCHL, a disease-specific transcription profile is detectable in immortalized cell lines easily obtained from peripheral blood and provide complementary information to classical genetic approaches to FCHL and/or the metabolic syndrome.

Specific interaction of Egr1 and c/EBPbeta leads to the transcriptional activation of the human low density lipoprotein receptor gene

The sterol-independent regulatory element (SIRE) of the LDL receptor (LDLR) promoter mediates oncostatin M (OM)-induced transcription of the LDLR gene through a cholesterol-independent pathway. Our prior studies have detected specific associations of the zinc finger transcription factor Egr1 with the SIRE sequence in OM-stimulated HepG2 cells. Because the SIRE motif is composed of a c/EBP binding site and a cAMP response element, both of which are quite divergent from the classical GC-rich Egr1 recognition sequences, we hypothesized that Egr1 may regulate LDLR transcription through interacting with members of the c/EBP and CREB families. Here, we show that treating HepG2 cells with OM specifically leads to prominent increases of the levels of c/EBPbeta and Egr1 bound to the LDLR promoter in vivo. In vitro, the binding of Egr1 to the SIRE sequence is weak, but is strikingly enhanced in the presence of HepG2 nuclear extract. Mammalian two-hybrid assays demonstrate that the N-terminal transactivation domain of Egr1 specifically interacts with c/EBPbeta but not with c/EBPalpha or CREB. The OM treatment further enhances this interaction, resulting in a large increase in the Egr1 transactivating activity. The direct protein to protein contact between Egr1 and c/EBPbeta is also demonstrated by co-immunoprecipitation experiments. Furthermore, we show that a mutation of the phosphorylation motif of c/EBPbeta diminished the OM-stimulated interaction of Egr1 and c/EBPbeta. Taken together, we provide strong evidence that Egr1 regulates LDLR transcription via a novel mechanism of protein-protein interaction with c/EBPbeta.

Effects of berberine on glucose metabolism in vitro

The action of berberine was compared with metformin and troglitazone (TZD) with regard to the glucose-lowering action in vitro. HepG2 cell line, phenotypically similar to human hepatocytes, was used for glucose consumption (GC) studies. Cell proliferation was measured by methylthiotetrazole (MTT) assay. In moderate high glucose concentration (11.1 mmol/L), GC of HepG2 cells was increased by 32% to 60% (P <.001 to P <.0001) with 5 x 10(-6) mol/L to 1 x 10(-4) mol/L berberine, which was comparable to that with 1 x 10(-3) mol/L metformin. The glucose-lowering effect of berberine decreased as the glucose concentration increased. The maximal potency was reached in the presence of 5.5 mmol/L glucose, and it was abolished when the glucose concentration increased to 22.2 mmol/L. The effect was not dependent on insulin concentration, which was similar to that of metformin and was different from that of TZD, whose glucose-lowering effect is insulin dependent. TZD had a better antihyperglycemic potency than metformin when insulin was added (P <.001). In the meantime, a significant toxicity of the drug to HepG2 cells was also observed. The betaTC3 cell line was used for insulin release testing, and no secretogogue effect of berberine was observed. These observations suggest that berberine is able to exert a glucose-lowering effect in hepatocytes, which is insulin independent and similar to that of metformin, but has no effect on insulin secretion.