Induction of low density lipoprotein receptor (LDLR) transcription by oncostatin M is mediated by the extracellular signal-regulated kinase signaling pathway and the repeat 3 element of the LDLR promoter

Oncostatin M Induces Lipolysis and Suppresses Insulin Response in 3T3-L1 Adipocytes

Oncostatin M (OSM) is an immune cell-derived cytokine that is upregulated in adipose tissue in obesity. Upon binding its receptor (OSMR), OSM induces the phosphorylation of the p66 subunit of Src homology 2 domain-containing transforming protein 1 (SHC1), called p66Shc, and activates the extracellular signal-related kinase (ERK) pathway. Mice with adipocyte-specific OSMR deletion (Osmr^FKO) are insulin resistant and exhibit adipose tissue inflammation, suggesting that intact adipocyte OSM–OSMR signaling is necessary for maintaining adipose tissue health. How OSM affects specific adipocyte functions is still unclear. Here, we examined the effects of OSM on adipocyte lipolysis. We treated 3T3-L1 adipocytes with OSM, insulin, and/or inhibitors of SHC1 and ERK and measured glycerol release. We also measured phosphorylation of p66Shc, ERK, and insulin receptor substrate-1 (IRS1) and the expression of lipolysis-associated genes in OSM-exposed 3T3-L1 adipocytes and primary adipocytes from control and Osmr^FKO mice. We found that OSM induces adipocyte lipolysis via a p66Shc-ERK pathway and inhibits the suppression of lipolysis by insulin. Further, OSM induces phosphorylation of inhibitory IRS1 residues. We conclude that OSM is a stimulator of lipolysis and inhibits adipocyte insulin response. Future studies will determine how these roles of OSM affect adipose tissue function in health and disease.

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.

Fibroblast growth factor 2 (FGF2) regulates cytoglobin expression and activation of human hepatic stellate cells via JNK signaling

Cytoglobin (CYGB) belongs to the mammalian globin family and is exclusively expressed in hepatic stellate cells (HSCs) in the liver. In addition to its gas-binding ability, CYGB is relevant to hepatic inflammation, fibrosis, and cancer because of its anti-oxidative properties; however, the regulation of CYGB gene expression remains unknown. Here, we sought to identify factors that induce CYGB expression in HSCs and to clarify the molecular mechanism involved. We used the human HSC cell line HHSteC and primary human HSCs isolated from intact human liver tissues. In HHSteC cells, treatment with a culture supplement solution that included fibroblast growth factor 2 (FGF2) increased CYGB expression with concomitant and time-dependent α-smooth muscle actin (αSMA) down-regulation. We found that FGF2 is a key factor in inducing the alteration in both CYGB and αSMA expression in HHSteCs and primary HSCs and that FGF2 triggered the rapid phosphorylation of both c-Jun N-terminal kinase (JNK) and c-JUN. Both the JNK inhibitor PS600125 and transfection of c-JUN-targeting siRNA abrogated FGF2-mediated CYGB induction, and conversely, c-JUN overexpression induced CYGB and reduced αSMA expression. Chromatin immunoprecipitation analyses revealed that upon FGF2 stimulation, phospho-c-JUN bound to its consensus motif (5'-TGA(C/G)TCA), located -218 to -222 bases from the transcription initiation site in the CYGB promoter. Of note, in bile duct-ligated mice, FGF2 administration ameliorated liver fibrosis and significantly reduced HSC activation. In conclusion, FGF2 triggers CYGB gene expression and deactivation of myofibroblastic human HSCs, indicating that FGF2 has therapeutic potential for managing liver fibrosis.

Acute Myelogenous Leukemia Cells Secrete Factors that Stimulate Cellular LDL Uptake via Autocrine and Paracrine Mechanisms

Leukemic cells isolated from most patients with acute myelogenous leukemia (AML) have higher low density lipoprotein (LDL) uptake than normal mononuclear blood cells. Little is known, however, about the mechanism behind the elevated LDL uptake. We investigated if AML cells secrete factors that stimulate cellular LDL uptake. Mononuclear blood cells were isolated from peripheral blood from 42 patients with AML at diagnosis. Cellular LDL uptake was determined from the degradation rate of ¹²⁵I-labelled LDL. Conditioned media from AML cells stimulated the LDL degradation in the leukemic cell lines KG1 and HL60, and in isolated AML cells. The stimulatory effect correlated with the LDL degradation in the AML cells directly after isolation from blood. Conditioned media also autostimulated LDL degradation in the AML cells themselves. Concentrations of IL-6 and IL-8 in AML cell conditioned media correlated with the LDL degradation in AML cells directly after isolation from blood. Addition of R-TNF-α, but not IL-6 or IL-8, stimulated LDL degradation in HL60, KG1, and AML cells. The LDL degradation in AML cells could be inhibited by a LDL receptor blocking antibody. AML cells secrete factors that stimulate LDL uptake in a paracrine and autocrine pattern which open up therapeutic possibilities to inhibit the uptake of LDL by administration of antibodies to these factors.

TATA boxes in gene transcription and poly (A) tails in mRNA stability: New perspective on the effects of berberine

Berberine (BBR) is a natural compound with variable pharmacological effects and a broad panel of target genes. We investigated berberine’s pharmacological activities from the perspective of its nucleotide-binding ability and discovered that BBR directly regulates gene expression by targeting TATA boxes in transcriptional regulatory regions as well as the poly adenine (poly (A)) tail at the mRNA terminus. BBR inhibits gene transcription by binding the TATA boxes in the transcriptional regulatory region, but it promotes higher levels of expression by targeting the poly (A) tails of mRNAs. The present study demonstrates that TATA boxes and poly (A) tails are the first and second primary targets by which BBR regulates gene expression. The final outcome of gene regulation by BBR depends on the structure of the individual gene. This is the first study to reveal that TATA boxes and poly (A) tails are direct targets for BBR in its regulation of gene expression. Our findings provide a novel explanation for the complex activities of a small molecule compound in a biological system and a novel horizon for small molecule-compound pharmacological studies.

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.

SKI-1/S1P inhibitor PF-429242 impairs the onset of HCV infection

Worldwide, approximately 170 million individuals are afflicted with chronic hepatitis C virus (HCV) infection. To prevent the development of inherent diseases such as cirrhosis and hepatocellular carcinoma, tremendous efforts have been made, leading to the development of promising new treatments. However, their efficiency is still dependent on the viral genotype. Additionally, these treatments that target the virus directly can trigger the emergence of resistant variants. In a previous study, we have demonstrated that a long-term (72h) inhibition of SKI-1/S1P, a master lipogenic pathway regulator through activation of SREBP, resulted in impaired HCV genome replication and infectious virion secretion. In the present study, we sought to investigate the antiviral effect of the SKI-1/S1P small molecule inhibitor PF-429242 at the early steps of the HCV lifecycle. Our results indicate a very potent antiviral effect of the inhibitor early in the viral lifecycle and that the overall action of the compound relies on two different contributions. The first one is SREBP/SKI-1/S1P dependent and involves LDLR and NPC1L1 proteins, while the second one is SREBP independent. Overall, our study confirms that SKI-1/S1P is a relevant target to impair HCV infection and that PF-429242 could be a promising candidate in the field of HCV infection treatment.

MG132, a proteasome inhibitor, enhances LDL uptake in HepG2 cells in vitro by regulating LDLR and PCSK9 expression

AIM

Expression of liver low-density lipoprotein receptor (LDLR), a determinant regulator in cholesterol homeostasis, is tightly controlled at multiple levels. The aim of this study was to examine whether proteasome inhibition could affect LDLR expression and LDL uptake in liver cells in vitro.

METHODS

HepG2 cells were examined. Real-time PCR and Western blot analysis were used to determine the mRNA and protein levels, respectively. DiI-LDL uptake assay was used to quantify the LDLR function. Luciferase assay system was used to detect the activity of proprotein convertase subtilisin/kexin type 9 (PCSK9, a major protein mediating LDLR degradation) promoter. Specific siRNAs were used to verify the involvement of PCSK9.

RESULTS

Treatment of HepG2 cells with the specific proteasome inhibitor MG132 (0.03-3 μmol/L) dose-dependently increased LDLR mRNA and protein levels, as well as LDL uptake. Short-term treatment with MG132 (0.3 μmol/L, up to 8 h) significantly increased both LDLR mRNA and protein levels in HepG2 cells, which was blocked by the specific PKC inhibitors GF 109203X, Gö 6983 or staurosporine. In contrast, a longer treatment with MG132 (0.3 μmol/L, 24 h) did not change LDLR mRNA, but markedly increased LDLR protein by reducing PCSK9-mediated lysosome LDLR degradation. Furthermore, MG132 time-dependently suppressed PCSK9 expression in the HepG2 cells through a SREBP-1c related pathway. Combined treatment with MG132 (0.3 μmol/L) and pravastatin (5 μmol/L) strongly promoted LDLR expression and LDL uptake in HepG2 cells, and blocked the upregulation of PCSK9 caused by pravastatin alone.

CONCLUSION

Inhibition of proteasome by MG132 in HepG2 cells plays dual roles in LDLR and PCSK9 expression, and exerts a beneficial effect on cholesterol homeostasis.

Protein phosphatase 2A (PP2A) regulates low density lipoprotein uptake through regulating sterol response element-binding protein-2 (SREBP-2) DNA binding

LDL-cholesterol (LDL-C) uptake by Ldlr is regulated at the transcriptional level by the cleavage-dependent activation of membrane-associated sterol response element-binding protein (SREBP-2). Activated SREBP-2 translocates to the nucleus, where it binds to an LDLR promoter sterol response element (SRE), increasing LDLR gene expression and LDL-C uptake. SREBP-2 cleavage and translocation steps are well established. Several SREBP-2 phosphorylation sites have been mapped and functionally characterized. The phosphatases dephosphorylating these sites remain elusive. The phosphatase(s) regulating SREBP-2 represents a novel pharmacological target for treating hypercholesterolemia. Here we show that protein phosphatase 2A (PP2A) promotes SREBP-2 LDLR promoter binding in response to cholesterol depletion. No binding to an LDLR SRE was observed in the presence of the HMG-CoA reductase inhibitor, lovastatin, when PP2A activity was inhibited by okadaic acid or depleted by siRNA methods. SREBP-2 cleavage and nuclear translocation were not affected by loss of PP2A. PP2A activity was required for SREBP-2 DNA binding. In response to cholesterol depletion, PP2A directly interacted with SREBP-2 and altered its phosphorylation state, causing an increase in SREBP-2 binding to an LDLR SRE site. Increased binding resulted in induced LDLR gene expression and increased LDL uptake. We conclude that PP2A activity regulates cholesterol homeostasis and LDL-C uptake.

L-Cysteine-induced up-regulation of the low-density lipoprotein receptor is mediated via a transforming growth factor-alpha signalling pathway

Sulphur-containing amino acids regulate plasma cholesterol levels in animals and humans. However, their mechanism of action remains unclear. Low-density lipoprotein receptor (LDLR) plays an important role in cholesterol metabolism. We therefore investigated the effects of sulphur-containing amino acids on the expression of LDLR in hepatocytes. HepG2 cells were cultured in Dulbecco's Modified Eagle's Medium with or without sulphur-containing amino acids and cysteine-containing compounds. We found that L-cysteine increased LDLR mRNA and enhanced LDLR gene promoter activity through the extracellular-signal-related kinase and p38 mitogen-activated protein kinase signalling pathways in HepG2 cells. Moreover, we observed that L-cysteine stimulated the release of transforming growth factor-alpha (TGF-α) and that TGF-α increased the LDLR mRNA levels. This study provides a report of the L-cysteine mediated up-regulation of the LDLR expression via TGF-α signalling pathway. Our findings provide insights into cholesterol homeostasis and amino acid signalling.

S-nitrosylation of ARH is required for LDL uptake by the LDL receptor

The LDL receptor (LDLR) relies upon endocytic adaptor proteins for internalization of lipoproteins. The results of this study show that the LDLR adaptor autosomal recessive hypercholesterolemia protein (ARH) requires nitric oxide to support LDL uptake. Nitric oxide nitrosylates ARH at C199 and C286, and these posttranslational modifications are necessary for association of ARH with the adaptor protein 2 (AP-2) component of clathrin-coated pits. In the absence of nitrosylation, ARH is unable to target LDL-LDLR complexes to coated pits, resulting in poor LDL uptake. The role of nitric oxide on LDLR function is specific for ARH because inhibition of nitric oxide synthase activity impairs ARH-supported LDL uptake but has no effect on other LDLR-dependent lipoprotein uptake processes, including VLDL remnant uptake and dab2-supported LDL uptake. These findings suggest that cells that depend upon ARH for LDL uptake can control which lipoproteins are internalized by their LDLRs through changes in nitric oxide.

The novel function of HINFP as a co-activator in sterol-regulated transcription of PCSK9 in HepG2 cells

PCSK9 (proprotein convertase subtilisin/kexin type 9) plays an important role in control of plasma LDL (low-density lipoprotein) cholesterol metabolism by modulating the degradation of hepatic LDL receptor. Previous studies demonstrated that PCSK9 is a target gene of the SREBP2 [SRE (sterol-regulatory element)-binding protein 2] that activates PCSK9 gene transcription through an SRE motif of the promoter. In addition to SREBP2, HNF1α (hepatic nuclear factor 1α) positively regulates PCSK9 gene transcription in hepatic cells through a binding site located 28 bp upstream from SRE. In the present study, we have identified a novel HINFP (histone nuclear factor P) recognition motif residing between the HNF1 motif and SRE that is essential for basal and sterol-regulated transcriptions of the PCSK9 promoter. Mutation of this motif lowers the basal promoter activity and abolishes the sterol-mediated repression as well as the SREBP2-induced activation of the PCSK9 promoter. We show further that the activity of SREBP2 in stimulating PCSK9 promoter activity is greatly enhanced by HINFP. Additional experiments suggest that HINFP and its cofactor NPAT (nuclear protein of the ataxia telangectasia mutated locus) form a functional complex, and NPAT may subsequently recruit the HAT (histone acetyltransferase) cofactor TRRAP (transformation/transactivation domain-associated protein) to facilitate the histone H4 acetylation of the PCSK9 promoter. Knockdown of HINFP, NPAT or TRRAP each markedly reduces the amount of acetylated histone H4 on the PCSK9 promoter region and lowers PCSK9 protein levels. Importantly, by utilizing co-immunoprecipitation assays, we have demonstrated a direct interaction between SREBP2 and HINFP and its cofactors NPAT/TRRAP. Taken together, these new findings identify HINFP as a co-activator in SREBP-mediated transactivation of PCSK9 gene expression.

Quercetin up-regulates LDL receptor expression in HepG2 cells

Quercetin, an abundant flavonol found in fruits and vegetable, has been implicated in lowering the risk of cardiovascular disease that is often associated with high plasma levels of low density lipoprotein (LDL) cholesterol. Here we investigated whether quercetin could modulate the expression of LDL receptors (LDLR) in HepG2 cells and the possible underlying mechanisms to exert quercetin's effects. We found that quercetin was able to induce LDLR expression with at least a 75 µ m concentration, which was accompanied by an increase in nuclear sterol regulatory element binding protein 2 (SREBP2). This effect was mediated by activation of c-jun-N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) signalling pathways as implicated by experiments using chemical inhibitors of each pathway. When cells were challenged with protein synthesis inhibitors in quercetin-activated LDLR transcription, LDL mRNA levels were not significantly affected by cycloheximide but puromycin abolished quercetin-induced LDLR transcription. Taken together, we conclude that quercetin can initiate LDLR transcription by enhancing SREBP2 processing, but new protein synthesis might be necessary to exert a maximum effect of quercetin in the up-regulation of the LDLR gene. Our findings demonstrate that quercetin strongly up-regulated LDLR gene expression, which might elicit hypolipidemic effects by increasing the clearance of circulating LDL cholesterol levels from the blood.

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.

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.

Gossypin up-regulates LDL receptor through activation of ERK pathway: a signaling mechanism for the hypocholesterolemic effect

Hypercholesterolemia is one of the major risk factors for the development of cardiovascular disease. This study aims to elucidate the effect of gossypin on cholesterol metabolism in HepG2 cells. Results indicated that gossypin significantly reduced the total cholesterol concentration in a dose-dependent manner. There was a time- and dose-dependent increase in the expression of low-density lipoprotein receptor (LDLR) protein. However, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol synthesis, was not affected by gossypin. Moreover, gossypin had no effect on nuclear sterol regulatory element binding proteins (SREBP)-2 abundance. The activity of gossypin on LDLR expression was inhibited by the extracellular signal-regulated kinase (ERK) inhibitor PD98059. Western blotting analysis revealed that gossypin treatment dose- and time-dependently increased ERK activation and preceded the up-regulation of LDLR expression. Collectively, these new findings identify gossypin as a new hypocholesterolemic agent that up-regulates LDLR expression independent of SREBP-2 but is dependent on ERK activation.

Identification of two herbal compounds with potential cholesterol-lowering activity

Low-density lipoprotein receptor (LDLR) plays a pivotal role in the control of plasma LDL-cholesterol level. This occurs predominantly at the transcriptional level through two gene regulation elements, named SRE: sterol-responsive element and SIRE: sterol-independent responsive element. We have developed a high-throughput screening using LDLR promoter activation-based assay to search for cholesterol-lowering compounds from a Chinese herb-based natural compound library. With this approach, we identified two compounds, named Daphnetoxin and Gniditrin, from Chinese herb Daphne giraldii Nitsche, which could activate LDLR promoter. Characterization of these compounds showed that they increased the level of LDLR mRNA and consequently up-regulate LDLR expression. The structures of these compounds are different from well-known LDLR promoter activating compounds such as GW707. The results suggested that these herbal compounds could represent good candidates for development of new classes of cholesterol-lowering drugs.

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.

Quercetin, a flavonoid, inhibits proliferation and increases osteogenic differentiation in human adipose stromal cells

Flavonoids, which have been detected in a variety of foods, have been repeatedly reported to affect bone metabolism. However, the effects of flavonoids on osteoblastogenesis remain a matter of some controversy. In this study, the effects of quercetin on the differentiation and proliferation of human adipose tissue-derived stromal cells (hADSC) were determined. Quercetin was found to increase osteogenic differentiation in a dose-dependent manner. Other flavonoids, chrysin and kaempferol, were also shown to increase the osteogenic differentiation of hADSC, but this stimulatory effect was weaker than that associated with quercetin. Quercetin pretreatment administered prior to the induction of differentiation also exerted stimulatory effects on the osteogenic differentiation of hADSC. RT-PCR and real time PCR analysis showed that quercetin treatment induced an increase in the expression of osteopontin, BMP2, alkaline phosphatase and Runx2. Quercetin inhibited the proliferation of hADSC, but did not affect their survival. The pretreatment of quercetin increased ERK phosphorylation during osteogenic differentiation, although it did not increase ERK activity in control culture condition. ICI182780, an specific estrogen receptor antagonist, failed to inhibit the effects of quercetin on osteogenic differentiation. Quercetin-pretreated hADSC showed better bone regenerating ability in skull defect model of nude mice than naive cells. Our findings indicate that quercetin enhances osteogenic differentiation via an independent mechanism from estrogen receptor (ER) activation, and prove useful for in vivo bone engineering, using human mesencymal stem cells (hMSC).

Rapid hepatic cell attachment onto biodegradable polymer surfaces without toxicity using an avidin–biotin binding system

Efficient cell attachment to biodegradable polymer scaffolds is a necessary prerequisite in tissue engineering. However, it is difficult to evenly cover scaffold surfaces with cells because scaffolds are generally highly porous, with complex three-dimensional (3D) surfaces. In this article, we demonstrate the efficiency of avidin-biotin binding systems (ABBS) for the initial attachment of biotinylated Hep G2 cells to avidin adsorbed flat, two-dimensional (2D) and highly porous 3D poly L-lactic acid (PLLA) surfaces. The potential toxicity of biotinylation and/or strong ABBS binding forces was also investigated. ABBS assisted Hep G2 cells to adhere to a flat PLLA surface within 10min; the proliferation of these attached cells was comparable with control intact cells cultured on collagen. Hepatic functions of the attached cells, such as albumin secretion, induction of CYP1A1 and CYP1A2 genes, and metabolic capacity of CYP1A1/2 as measured by the ethoxyresorufin O-deethylase assay, were not significantly changed. Also, a stimulus of a cytokine: oncostatin M (OSM) phosphorylated an intracellular signaling molecule, extracellular signal-related kinase 1 (ERK1) via transmembrane receptor complex, at 24h after inoculation by ABBS. In addition, efficient attachment of Hep G2 cells to a highly porous PLLA 3D scaffold was demonstrated. These results clearly show that ABBS is useful for rapidly trapping cells in both biodegradable, polymer-based, flat 2D surfaces, and in highly porous 3D scaffolds. Furthermore, binding hepatic cells by this technique has only small effects on liver-specific functions, or on signal transfer ability of transmembrane receptor complexes.

The medicinal plant goldenseal is a natural LDL-lowering agent with multiple bioactive components and new action mechanisms

Our previous studies have identified berberine (BBR), an alkaloid isolated from the Chinese herb huanglian, as a unique cholesterol-lowering drug that upregulates hepatic low density lipoprotein receptor (LDLR) expression through a mechanism of mRNA stabilization. Here, we demonstrate that the root extract of goldenseal, a BBR-containing medicinal plant, is highly effective in upregulation of liver LDLR expression in HepG2 cells and in reducing plasma cholesterol and low density lipoprotein cholesterol (LDL-c) in hyperlipidemic hamsters, with greater activities than the pure compound BBR. By conducting bioassay-driven semipurifications, we demonstrate that the higher potency of goldenseal is achieved through concerted actions of multiple bioactive compounds in addition to BBR. We identify canadine (CND) and two other constituents of goldenseal as new upregulators of LDLR expression. We further show that the activity of BBR on LDLR expression is attenuated by multiple drug resistance-1 (MDR1)-mediated efflux from liver cells, whereas CND is resistant to MDR1. This finding defines a molecular mechanism for the higher activity of CND than BBR. We also provide substantial evidence to show that goldenseal contains natural MDR1 antagonist(s) that accentuate the upregulatory effect of BBR on LDLR mRNA expression. These new findings identify goldenseal as a natural LDL-c-lowering agent, and our studies provide a molecular basis for the mechanisms of action.

Pharmacological regulation of low density lipoprotein receptor expression: Current status and future developments

Plasma levels of low-density lipoprotein (LDL) cholesterol are considered to be a major risk factor for the development of cardiovascular diseases. The LDL receptor is the key component in the maintenance of cholesterol homeostasis in the body, playing a pivotal role by regulating the hepatic catabolism of LDL cholesterol. Many clinical studies using statins, which up-regulate the LDL receptor expression via a feedback mechanism, have demonstrated that the reduction of LDL cholesterol levels lowers the incidence of cardiovascular events in both primary and secondary prevention. In this context, new strategies designed to increase hepatic LDL receptor activity can be considered as attractive opportunities for future therapy. Several potential new drugs have been described in the last decade to up-regulate LDL receptor expression in vitro and in vivo, thus allowing the identification of new transcriptional and post-transcriptional mechanisms.

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.

Human low-density lipoprotein receptor gene and its regulation

The low-density lipoprotein (LDL) receptor is a transmembrane glycoprotein that mediates the binding and endocytosis of lipoproteins containing apolipoprotein B and E, especially the cholesterol-rich LDL. Mutations in the LDL receptor gene can produce dysfunctional LDL receptors and cause familial hypercholesterolemia. The expression of the LDL receptor gene is under an intriguing regulation by sterol and nonsterol mediators either at the transcriptional level or at the posttranscriptional level, both of which are linked to cell signaling pathways. Upregulation of liver LDL receptor expression is effective in treating hypercholesterolemia. In this review, we focus on the latest progress on the mechanisms and regulation of the LDL receptor gene expression.

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.

Age-related alteration in hepatic acyl-CoA: cholesterol acyltransferase and its relation to LDL receptor and MAPK

The aim of this study was to evaluate changes in the regulation of lipid metabolism and mitogen-activated protein kinases (MAPK) in the liver of C57BL/6 mice as they age. This was done by assessing the status of total cholesterol content and its enzyme, acyl-CoA: cholesterol acyltransferase (ACAT), in liver microsomal preparations and the low-density lipoprotein receptor (LDLr) mRNA expression in the livers of 4-24-month-old C57B/6 mice, without exogenous cholesterol feeding. With aging, there was an increase in cholesterol content and ACAT activity in liver microsomes. Northern blot analysis and real-time quantitative polymerase chain reaction data showed that ACAT-2 mRNA increased with age as well. LDLr expression decreased significantly in an age-dependent manner. In addition, we studied the basal and activated forms of MAPK, e.g. extracellular regulatory kinase (ERK-1/2), c-jun NH2-terminal kinase (JNK-1/2) and p38 MAPK. During aging, there was a considerable decrease in phosphorylated ERK-1/2 level while JNK-1/2 and p38 MAPK levels increased with age. Our studies showed an altered LDLr expression and altered phosphorylated MAPK in the liver of C57BL/6 mice during aging. These alterations might contribute to the development of atherosclerosis, hypercholesterolemia and other cholesterol-related conditions.

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.

LDL immune complexes stimulate LDL receptor expression in U937 histiocytes via extracellular signal-regulated kinase and AP-1

We have previously shown that LDL-containing immune complexes (LDL-ICs) induce up-regulation of LDL receptor (LDLR) expression in human macrophages. The present study further investigated the molecular mechanisms leading to LDLR up-regulation by LDL-ICs as well as the signaling pathways involved. Results showed that treatment of U937 histiocytes with LDL-ICs did not increase the precursors and the cleaved forms of sterol-regulatory element binding proteins (SREBPs) 1a and 2, suggesting that SREBPs may not be involved in LDLR up-regulation by LDL-ICs. Promoter deletion and mutation studies showed that the AP-1 binding sites were essential for LDL-IC-stimulated LDLR expression. Electrophoretic mobility shift assays further demonstrated that LDL-ICs stimulated transcription factor AP-1 activity. Studies assessing the signaling pathways involved in LDLR up-regulation by LDL-ICs showed that the up-regulation of LDLR was extracellular signal-regulated kinase (ERK) dependent. In conclusion, the present study shows that LDL-ICs up-regulate LDLR expression via the ERK signaling pathway and the AP-1 motif-dependent transcriptional activation.

Induction of very low density lipoprotein receptor (VLDLR) transcription by VLDL is mediated by the extracellular signal-regulated kinase signaling pathway

To elucidate the intracellular signaling pathways for VLDL-induced VLDLR transcription, Western blot analysis was used to examine phosphorylated ERK1/2 protein. It was found that that VLDL induced an increase in ERK1/2 activity in a protein kinase C (PKC)-dependent manner in murine RAW264.7 macrophages. By using different protein kinases inhibitors or activators it was observed that the effect of VLDL-induced VLDL receptor transcription, which is monitored by RTPCR analysis of VLDL receptor mRNA, was not affected by the inhibitor of p38 kinase and cAMP analog, but completely abolished by pretreatment of the cells with PD 98059, an inhibitor of MEK and GF 109203X, an inhibitor of PKC. These results demonstrated that the PKC/ERK1/2 cascade is the essential signaling pathway by which VLDL activates VLDL receptor mRNA expression.

Synergistic activation of human LDL receptor expression by SCAP ligand and cytokine oncostatin M

OBJECTIVE

A recent study identified a new class of compounds designated as the sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP) ligands that putatively bind to SCAP, leading to increased LDL receptor (LDLR) expression. In this study, we examined the effects of SCAP ligand GW707 in comparison with lovastatin and cytokine oncostatin M (OM) on the regulation of LDLR expression in cultured HepG2 cells.

METHODS AND RESULTS

Our studies uncovered several new features that distinguish SCAP ligand from lovastatin, a classic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, and from OM, which utilize an SREBP-independent regulatory pathway. We show that the induction of LDLR mRNA expression by GW707 is not affected by intracellular cholesterol but is completely abolished by blocking de novo protein synthesis. Moreover, the effects of GW707 but not lovastatin on LDLR promoter activity, mRNA expression, and uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanin perchlorate-LDL are markedly enhanced by OM. We further demonstrate that the amounts of the mature form of SREBP-2 translocated to the nucleus under GW707 treatment are increased by costimulating cells with OM.

CONCLUSIONS

Our studies provide the first evidence that higher levels of LDLR expression and function can be achieved through simultaneous stimulation of the SREBP-dependent and SREBP-independent pathways, suggesting a strategy to develop an adjunct therapeutic intervention utilizing both pathways.

Bile acids enhance low density lipoprotein receptor gene expression via a MAPK cascade-mediated stabilization of mRNA

Recent studies have indicated that bile acids regulate the expression of several genes involved in bile acid and lipid metabolism as ligands for the farnesoid X receptor (FXR). We report here that bile acids are directly able to govern cholesterol metabolism by a novel mechanism. We show that chenodeoxycholic acid (CDCA) enhances low density lipoprotein (LDL) receptor gene expression in human cultured cell lines (HeLa, Hep G2, and Caco-2). The proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a major regulator for LDL receptor gene expression, is not affected by CDCA. Both deoxycholic acid and lithocholic acid as well as CDCA, but not ursodeoxycholic acid, increase the mRNA level for the LDL receptor, even when Hep G2 cells are cultured with 25-hydroxycholesterol, a potent suppressor of gene expression for the LDL receptor. Although it seems possible that FXR might be involved in genetic regulation, both reporter assays with a reporter gene containing the LDL receptor promoter as well as Northern blot analysis reveal that FXR is not involved in the process. On the other hand, inhibition of mitogen-activated protein (MAP) kinase activities, which are found to be induced by CDCA, abolishes the CDCA-mediated up-regulation of LDL receptor gene expression. We further demonstrate that CDCA stabilizes LDL receptor mRNA and that the MAP kinase inhibitors accelerate its turnover. Taken together, these results indicate that bile acids increase LDL uptake and the intracellular cholesterol levels through the activation of MAP kinase cascades in conjunction with a down-regulation of bile acid biosynthesis by FXR. This work opens up a new avenue for developing pharmaceutical interventions that lower plasma LDL by stabilizing LDL receptor mRNA.

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

Previously, we identified the low density lipoprotein receptor (LDLR) promoter region -17 to -1 as a novel sterol-independent regulatory element (SIRE) that mediates the stimulating effect of oncostatin M (OM). The goal of this study was to identify the OM-induced transcription activator that binds to the SIRE sequence. By conducting a electrophoretic mobility shift assay (EMSA) followed by UV crosslinking and SDS-PAGE, we show that a protein with a molecular mass of 85 kDa was present in the OM-induced SIRE DNA-protein complex. Western blotting and supershift assays reveal that the 85 kDa factor is early growth response gene 1 (Egr1). The interaction of Egr1 with the SIRE sequence was further confirmed in vivo by chromatin immunoprecipitation assays. The functional role of Egr1 in LDLR transcription was assessed by cotransfection of an Egr1 expression vector with an LDLR promoter reporter construct. We show that overexpression of Egr1 significantly increases LDLR promoter activity when cotransfected with CCAAT/enhancer binding protein beta (c/EBPbeta) or with cAMP-responsive element binding protein (CREB) expression vectors. Our studies clearly demonstrate that Egr1 is the OM-induced transcription factor that binds to the SIRE sequence of the LDLR promoter and also suggest that Egr1 may have a functional role in OM-induced upregulation of LDLR transcription through interaction with other SIRE binding proteins such as c/EBPbeta or CREB.

Oncostatin M stimulates tissue inhibitor of metalloproteinase-1 via a MEK-sensitive mechanism in human myofibroblasts

BACKGROUND/AIMS

We previously showed that in cultured human myofibroblasts (hMFBs), Oncostatin M (OSM)-stimulated collagen accumulation is associated with increased tissue inhibitor of metalloproteinase (TIMP)1 message. However, the mechanism is unknown.

METHODS

hMFBs were isolated by outgrowth from cirrhotic liver explants and cultured. Using OSM (10 ng/ml) stimulation, with and without PD98059 (PD, a specific mitogen-activated protein kinase/extracellular signal-related kinase (MEK) inhibitor), we measured: TIMP-1 protein in culture medium by Western blot, TIMP-1 mRNA levels and stability by Northern analysis, TIMP-1 promoter activity (including transcription site mutation analysis), DNA binding activity to nuclear proteins by electrophoretic mobility shift assay (EMSA), and total and phosphorylated MAP kinase in hMFB extracts by Western blot.

RESULTS

OSM stimulation of hMFBs increased TIMP-1 protein production 1.69-fold, TIMP-1 mRNA levels 2.36-fold, promoter activity 2.22-fold, TIMP-1 message stability, and phosphorylation of mitogen-activated protein kinase (MAPK). PD inhibited OSM-mediated stimulation of TIMP-1 protein, mRNA, promoter activity, phosphorylation of MAPK, and TIMP-1 message stability. An SP-1 transcription site of the TIMP-1 promoter is essential for OSM induction of TIMP-1 promoter activity. EMSA demonstrates that this site binds to transcriptional factors SP-1 and SP-3.

CONCLUSIONS

OSM stimulates the TIMP-1 axis in hMFBs in vitro via a MEK-MAP kinase cascade.

Concerted transcriptional activation of the low density lipoprotein receptor gene by insulin and luteinizing hormone in cultured porcine granulosa-luteal cells: possible convergence of protein kinase a, phosphatidylinositol 3-kinase, and mitogen-activated protein kinase signaling pathways

Insulin and insulin-like growth factor I (IGF-I) can amplify gonadotropin-stimulated steroidogenesis by augmenting the expression of key sterol regulatory genes in ovarian cells, viz. low density lipoprotein (LDL) receptor, steroidogenic acute regulatory protein, and P450 cholesterol side-chain cleavage enzyme (CYP11A). The mechanisms underlying the foregoing bihormonal interactions are not known. Accordingly, in relation to the LDL receptor gene, the present study tests the hypothesis that insulin/IGF-I and LH can act via concerted transcriptional control of promoter expression. To this end, we transiently transfected primary monolayer cultures of porcine granulosa-luteal cells with a reporter vector containing the putative 5'-upstream full-length (pLDLR1076/luc) regulatory region (-1076 to +11 bp) of the homologous LDL receptor gene driving firefly luciferase in the presence or absence of insulin (or IGF-I) and/or LH (each 100 ng/ml). Combined exposure to LH and insulin (or IGF-I) stimulated LDL receptor transcriptional activity maximally at 4 h by 8- to 20-fold, as normalized by coexpression of Renilla luciferase. Further analysis of multiple 5'-nested deletional constructs of the LDL receptor gene promoter showed that deletion of -139 bp upstream of the transcriptional start site virtually abolished basal expression and promoter responsiveness to LH and insulin/IGF-I. In contrast, full basal activity and 60-80% of maximal monohormonal and bihormonal drive were retained by the -255 to +11 bp fragment. As LDL receptor gene expression in other tissues is negatively regulated by the abundance of intracellular free cholesterol, we assessed the impact of concomitant pretreatment of granulosa-luteal cells with an exogenous soluble sterol (25-hydroxycholesterol, 1 and 10 microM). Excess sterol markedly (50-70%) attenuated bihormonally and, in lesser measure, LH-stimulated and basal LDL receptor promoter expression, thus affirming a feedback-sensitive sterol-repressive region in this gene. Non-LH receptor-dependent agonists of protein kinase A (PKA), 8-bromo-cAMP (1 mM), and forskolin (10 microM) with or without insulin/IGF-I costimulation likewise augmented LDL receptor promoter expression with similar strong dependency on the -255 to -139 bp 5'-upstream region. To assess more specific PKA-dependent mediation of LH's contribution to combined hormonal drive, the LDL receptor (-1076 to +11 bp) reporter plasmid was cotransfected with a full-sequence rabbit muscle protein kinase inhibitor (PKI) minigene driven constitutively by a Rous sarcoma virus promoter. Expression of the latter PKA antagonist blocked transcriptional stimulation by LH alone as well as that by LH combined with insulin (or IGF-I) by 70-85% without reducing basal transcriptional activity. Transfection of a mutant inactive (Arg to Gly) Rous sarcoma virus/PKI gene confirmed the specificity of the PKI effect. To investigate the convergent role of the insulin/IGF-I effector pathway mediating bihormonal stimulation of LDL receptor promoter expression, transfected granulosa-luteal cells were pretreated for 30 min with two specific inhibitors of phophatidylinositol 3-kinase, wortmannin (100 nM) and LY 294002 (10 microM), or of mitogen-activated protein kinase kinase, PD 98059 (50 microM), U0126 (10 microM), or the latter's inactive derivative, U0124 (10 microM). Both classes of antagonists impeded the ability of insulin or IGF-I to enhance LH-stimulated LDL receptor promoter expression by 60-80%. In conclusion, the present analyses indicate that LH and insulin (or IGF-I) can up-regulate LDL receptor transcriptional activity supraadditively in porcine granulosa-luteal cells 1) via one or more agonistic cis-acting DNA regions located between -255 and -139 bp 5'- upstream of the transcriptional start site, 2) without abrogating sterol-sensitive repressive of this promoter, and 3) by way of intracellular mechanisms that include the PKA, phophatidylinositol 3-kinase, and mitogen-activated protein kinase signaling pathways.

Expression of the alpha 5 integrin subunit gene promoter is positively regulated by the extracellular matrix component fibronectin through the transcription factor Sp1 in corneal epithelial cells in vitro

The accumulation of fibronectin (FN) in response to corneal epithelium injury has been postulated to turn on expression of the FN-binding integrin alpha(5)beta(1). In this work, we determined whether the activity directed by the alpha(5) gene promoter can be modulated by FN in rabbit corneal epithelial cells (RCEC). The activity driven by chloramphenicol acetyltransferase/alpha(5) promoter-bearing plasmids was drastically increased when transfected into RCEC grown on FN-coated culture dishes. The promoter sequence mediating FN responsiveness was shown to bear a perfect inverted repeat that we designated the fibronectin-responsive element (FRE). Analyses in electrophoretic mobility shift assays provided evidence that Sp1 is the predominant transcription factor binding the FRE. Its DNA binding affinity was found to be increased when RCEC are grown on FN-coated dishes. The addition of the MEK kinase inhibitor PD98059 abolished FN responsiveness suggesting that alteration in the state of phosphorylation of Sp1 likely accounts for its increased binding to the alpha(5) FRE. The FRE also proved sufficient to confer FN responsiveness to an otherwise unresponsive heterologous promoter. However, site-directed mutagenesis indicated that only the 3' half-site of the FRE was required to direct FN responsiveness. Collectively, binding of FN to its alpha(5)beta(1) integrin activates a signal transduction pathway that results in the transcriptional activation of the alpha(5) gene likely through altering the phosphorylation state of Sp1.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Interleukin-6 stimulates LDL receptor gene expression via activation of sterol-responsive and Sp1 binding elements

Inflammatory or malignant diseases are associated with elevated levels of cytokines and abnormal low density lipoprotein (LDL) cholesterol metabolism. In the acute-phase response to myocardial injury or other trauma or surgery, total and LDL cholesterol levels are markedly decreased. We investigated the effects of the proinflammatory cytokine interleukin (IL)-6 on LDL receptor (LDL-R) function and gene expression in HepG2 cells. IL-6 dose-dependently increased the binding, internalization, and degradation of (125)I-LDL. IL-6-stimulated HepG2 cells revealed increased steady-state levels of LDL-R mRNA. In HepG2 cells transiently transfected with reporter gene constructs harboring the sequence of the LDL-R promoter extending from nucleotide -1563 (or from nucleotide -234) through -58 relative to the translation start site, IL-6 dose-dependently increased promoter activity. In the presence of LDL, a similar relative stimulatory effect of IL-6 was observed. Studies using a reporter plasmid with a functionally disrupted sterol-responsive element (SRE)-1 revealed a reduced stimulatory response to IL-6. In gel-shift assays, nuclear extracts of IL-6-treated HepG2 cells showed an induced binding of SRE binding protein (SREBP)-1a and SRE binding protein(SREBP)-2 to the SRE-1 that was independent of the cellular sterol content and an induced binding of Sp1 and Sp3 to repeat 3 of the LDL-R promoter. Our data indicate that IL-6 induces stimulation of the LDL-R gene, resulting in enhanced gene transcription and LDL-R activity. This effect is sterol independent and involves, on the molecular level, activation of nuclear factors binding to SRE-1 and the Sp1 binding site in repeat 2 and repeat 3 of the LDL-R promoter, respectively.

Identification of a novel sterol-independent regulatory element in the human low density lipoprotein receptor promoter

The cytokine oncostatin M (OM) activates human low density lipoprotein receptor (LDLR) gene transcription through a sterol-independent mechanism. Previous studies conducted in our laboratory have narrowed the OM-responsive element to promoter region -52 to +13, which contains the repeat 3 and two TATA-like sequences. We now identify LDLR promoter region -17 to -1 as a sterol-independent regulatory element (SIRE) that is critically involved in OM-, transcription factor CCAAT/enhancer-binding protein (C/EBP)-, and second messenger cAMP-mediated activation of LDLR transcription. The SIRE sequence overlaps the previously described TATA-like element and consists of an active C/EBP-binding site (-17 to -9) and a functional cAMP-responsive element (CRE) (-8 to -1). We demonstrate that (a) mutations within either the C/EBP or CRE site have no impact on basal or cholesterol-mediated repression of LDLR transcription, but they completely abolish OM-mediated activation of LDLR transcription; (b) replacing the repeat 3 sequence that contains the Sp1-binding site with a yeast transcription factor GAL4-binding site in the LDLR promoter construct does not affect OM inducibility, thereby demonstrating that OM induction is mediated through the SIRE sequence in conjunction with a strong activator bound to the repeat 3 sequence; (c) electrophoretic mobility shift and supershift assays confirm the specific binding of transcription factors C/EBP and cAMP-responsive element-binding protein to the SIRE; (d) cotransfection of a human C/EBPbeta expression vector (pEF-NFIL6) with the LDLR promoter construct pLDLR234 increases LDLR promoter activity; and (e) OM and dibutyryl cAMP synergistically activate LDLR transcription through this regulatory element. This study identifies, for the first time, a cis-acting regulatory element in the LDLR promoter that is responsible for sterol-independent regulation of LDLR transcription.

Inhibition of stress-activated p38 mitogen-activated protein kinase induces low-density lipoprotein receptor expression

We have recently shown that different signal transduction pathways initiated by a variety of agents converge on growth-responsive p42/44MAPK signaling cascade to induce low-density lipoprotein (LDL) receptor expression. Our recent demonstration that stress-activated p38MAPK negatively regulates LDL receptor expression in an isoform-specific manner via modulation of p42/44MAPK cascade represents a new dimension of complexity in the molecular communication that governs LDL receptor expression. The suggested one-way communication between p38MAPK and p42/44MAPK provides a potential mechanism for fine-tuning cellular levels of cholesterol in response to a diverse set of environmental cues, including stress. Cross talk between MAPKs opens new avenues toward understanding a variety of pathogenic processes; this makes them tempting targets for therapeutic interventions in cardiovascular diseases.