Modulation of LDL receptor mRNA stability by phorbol esters in human liver cell culture models

Resveratrol improved atherosclerosis by increasing LDLR levels via the EGFR-ERK1/2 signaling pathway

BACKGROUND AND AIMS

Atherosclerosis (AS) is a complex and chronic vascular disease and elevated low-density lipoprotein cholesterol (LDL-C) level is one of its primary causative factors. As a key surface receptor, low-density lipoprotein receptor (LDLR) plays an essential role in LDL-C clearance. Resveratrol (RSV) has emerged as a promising compound for investigating potential therapeutic targets for AS due to its ability to lower cholesterol, reduce endothelial anti-inflammatory and suppress vascular smooth muscle cell proliferation. This study explored the effects of RSV on AS through upregulating LDLR and analyzed the mechanism through a combination of in vivo and vitro experiments.

METHODS

HepG2 cells were exposed to varying concentrations of RSV. The effects of RSV on LDLR expression and cholesterol uptake were analyzed by western blot, RT-qPCR and DiI-LDL uptake assay. In vivo, C57BL/6J ApoE-/- mice were used and the experimental groups were treated with RSV, Lovastatin and Gefitinib. Plaque formation in the arteries and aortic roots was assessed by Oil Red O staining and plaque stability was evaluated using Hematoxylin-Eosin (H&E) and Elastic Van Gieson (EVG) staining. Western blot, RT-qPCR and immunohistochemical staining were employed to analyze the expression of LDLR in the livers of mice.

RESULTS

RSV significantly enhanced the stability of LDLR mRNA and promoted LDLR protein expression. The inhibition experiments of EGFR signaling pathway (Cetuximab and Gefitinib) demonstrated that the efficacy of RSV was markedly weakened when this signaling pathway was inhibited. It indicated that RSV modulated LDLR gene expression by activating EGFR-ERK1/2 pathway. In ApoE-/- mice, RSV notably reduced arterial plaque formation, improved plaque stability and increased hepatic LDLR expression.

CONCLUSION

This study elucidated the mechanism by which RSV upregulates LDLR gene expression through activating EGFR-ERK1/2 signaling pathway. In vivo experiments demonstrated its efficacy in reducing arterial plaque formation and stabilizing existing plaques. These results further indicated that RSV held potential therapeutic value for ameliorating atherosclerosis and cardiovascular diseases. Collectively, these findings provided novel theoretical support for RSV's potential role in cardiovascular therapy.

Pathways and Molecular Mechanisms Governing LDL Receptor Regulation

Clearance of circulating plasma LDL (low-density lipoprotein) cholesterol by the liver requires hepatic LDLR (low-density lipoprotein receptor). Complete absence of functional LDLR manifests in severe hypercholesterolemia and premature atherosclerotic cardiovascular disease. Since the discovery of the LDLR 50 years ago by Brown and Goldstein, all approved lipid-lowering medications have been aimed at increasing the abundance and availability of LDLR on the surface of hepatocytes to promote the removal of LDL particles from the circulation. As such a critical regulator of circulating and cellular cholesterol, it is not surprising that LDLR activity is tightly regulated. Despite over half a century's worth of study, there are still many facets of LDLR biology that remain unexplored. This review will focus on pathways that regulate the LDLR and emerging concepts of LDLR biology.

Gallic Acid Can Promote Low-Density Lipoprotein Uptake in HepG2 Cells via Increasing Low-Density Lipoprotein Receptor Accumulation

Gallic acid (GA) is a type of polyphenolic compound that can be found in a range of fruits, vegetables, and tea. Although it has been confirmed it improves non-alcoholic fatty liver disease (NAFLD), it is still unknown whether GA can improve the occurrence of NAFLD by increasing the low-density lipoprotein receptor (LDLR) accumulation and alleviating cholesterol metabolism disorders. Therefore, the present study explored the effect of GA on LDLR and its mechanism of action. The findings indicated that the increase in LDLR accumulation in HepG2 cells induced by GA was associated with the stimulation of the epidermal growth factor receptor-extracellular regulated protein kinase (EGFR-ERK1/2) signaling pathway. When the pathway was inhibited by EGFR mab cetuximab, it was observed that the activation of the EGFR-ERK1/2 signaling pathway induced by GA was also blocked. At the same time, the accumulation of LDLR protein and the uptake of LDL were also suppressed. Additionally, GA can also promote the accumulation of forkhead box O3 (FOXO3) and suppress the accumulation of hepatocyte nuclear factor-1α (HNF1α), leading to the inhibition of proprotein convertase subtilisin/kexin 9 (PCSK9) mRNA expression and protein accumulation. This ultimately results in increased LDLR protein accumulation and enhanced uptake of LDL in cells. In summary, the present study revealed the potential mechanism of GA's role in ameliorating NAFLD, with a view of providing a theoretical basis for the dietary supplementation of GA.

Triciribine Engages ZFP36L1 and HuR to Stabilize LDLR mRNA

An increased understanding of low-density lipoprotein receptor (LDLR) and its regulation may facilitate drug development for the treatment of hypercholesterolemia. Triciribine (TCN), which is a highly selective AKT inhibitor, increases the stability of LDLR mRNA downstream of extracellular signal-regulated kinase (ERK) in human hepatoma cells (HepG2). Here, a candidate approach was used in order to determine whether the RNA-binding proteins (RBPs) ZFP36 ring finger protein like 1 (ZFP36L1) and Hu antigen R (HuR) play a role in TCN-mediated stabilization of LDLR mRNA. The depletion of HuR led to a reduction of LDLR mRNA stability, an event that was more pronounced in TCN-treated cells. TCN was found to induce the translocation of nuclear HuR to cytoplasm in an ERK-dependent manner. ZFP36L1 depletion increased the stability of LDLR mRNA consistent with its destabilizing role. However, in contrast to HuR, TCN had no effect on LDLR mRNA turnover in ZFP36L1-depleted cells. TCN induced the phosphorylation of ZFP36L1 in an ERK/RSK-dependent manner and promoted its dissociation from the CCR4-NOT complex. In sum, these data suggest that TCN utilizes ERK signaling to increase the activity of HuR and inhibit ZFP36L1 to stabilize LDLR mRNA in HepG2 cells.

Farnesoid X Receptor Activation by Obeticholic Acid Elevates Liver Low-Density Lipoprotein Receptor Expression by mRNA Stabilization and Reduces Plasma Low-Density Lipoprotein Cholesterol in Mice

Objective- The objective of this study was to determine whether and how activation of farnesoid X receptor (FXR) by obeticholic acid (OCA), a clinical FXR agonist, modulates liver low-density lipoprotein receptor (LDLR) expression under normolipidemic conditions. Approach and Results- Administration of OCA to chow-fed mice increased mRNA and protein levels of LDLR in the liver without affecting the sterol-regulatory element binding protein pathway. Profiling of known LDLR mRNA-binding proteins demonstrated that OCA treatment did not affect expressions of mRNA degradation factors hnRNPD (heterogeneous nuclear ribonucleoprotein D) or ZFP36L1 but increased the expression of Hu antigen R (HuR) an mRNA-stabilizing factor. Furthermore, inducing effects of OCA on LDLR and HuR expression were ablated in Fxr^-/- mice. To confirm the post-transcriptional mechanism, we used transgenic mice (albumin-luciferase-untranslated region) that express a human LDLR mRNA 3' untranslated region luciferase reporter gene in the liver. OCA treatment led to significant rises in hepatic bioluminescence signals, Luc-untranslated region chimeric mRNA levels, and endogenous LDLR protein abundance, which were accompanied by elevations of hepatic HuR mRNA and protein levels in OCA-treated transgenic mice. In vitro studies conducted in human primary hepatocytes and HepG2 cells demonstrated that FXR activation by OCA and other agonists elicited the same inducing effect on LDLR expression as in the liver of normolipidemic mice. Furthermore, depletion of HuR in HepG2 cells by short interfering RNA transfection abolished the inducing effect of OCA on LDLR expression. Conclusions- Our study is the first to demonstrate that FXR activation increases LDLR expression in liver tissue by a post-transcriptional regulatory mechanism involving LDLR mRNA-stabilizing factor HuR.

Ellagic acid affects mRNA expression levels of genes that regulate cholesterol metabolism in HepG2 cells

Ellagic acid has been shown to improve cholesterol metabolism in animal studies, but the molecular mechanisms underlying this function have not been fully understood. We performed DNA microarray analysis to elucidate the effects of ellagic acid on cholesterol metabolism in HepG2 hepatocytes. This revealed that the expression levels of several genes related to cholesterol metabolism, including the low-density lipoprotein receptor (LDLR), were changed by ellagic acid treatment. Using a real-time PCR and immunoblot we confirmed that ellagic acid treatment up-regulated mRNA and protein expression level of the LDLR. Moreover, In the presence of 25 μM ellagic acid, extracellular apoB protein and MTP mRNA levels were significantly decreased. These findings indicate that ellagic acid improves cholesterol metabolism through the up-regulation of LDLR, down-regulation of MTP mRNA and reduces extracellular apoB levels. The ellagic acid-induced up-regulation of LDLR occurred via the extracellular signal-regulated kinase (ERK) signaling pathway in HepG2 hepatocytes. Abbreviations: LDLR: low-density lipoprotein receptor; apoB: apolipoprotein B; PKC: diacylglycerol-protein kinase C; MAPK: mitogen-activated protein kinase; ERK: p42/44 extracellular signal-regulated kinase; JNK: c-Jun N-terminal kinase; VLDLR: very low density lipoprotein receptor; PPARδ: peroxisome proliferator-activated receptor δ; SREBPs: sterol regulatory element-binding proteins; MTP: microsomal triacylglycerol transfer protein; LPDS: lipoprotein-deficient serum.

Triciribine increases LDLR expression and LDL uptake through stabilization of LDLR mRNA

Low-density lipoprotein receptor (LDLR) is a key regulator of the metabolism of plasma low-density lipoprotein cholesterol (LDL-C), the elevated levels of which are associated with an increased risk of cardiovascular disease. Therefore, enhancing LDLR expression represents a potent treatment strategy for hypercholesterolemia. Here, we report that in cultured human hepatoma cells, triciribine, a highly selective AKT inhibitor, increases the stability of LDLR mRNA, an event that translates into upregulation of cell-surface LDLR levels and induction of cellular LDL uptake. This effect of triciribine requires ERK activity and is partially dependent on the intervening sequence between the AU-rich elements ARE3 and ARE4 in LDLR 3′UTR. We also show that triciribine downregulates the expression of PCSK9 mRNA and blunts the secretion of its protein. Notably, triciribine was found to potentiate the effect of mevastatin on LDLR protein levels and activity. We also show that primary human hepatocytes respond to triciribine by increasing the expression of LDLR. Furthermore, a pilot experiment with mice revealed that a two-weeks treatment with triciribine significantly induced the hepatic expression of LDLR protein. These results identify triciribine as a novel LDLR-elevating agent and warrant further examination of its potential as a hypocholesterolemic drug either as monotherapy or in combination with statins.

5-Azacytidine engages an IRE1α-EGFR-ERK1/2 signaling pathway that stabilizes the LDL receptor mRNA

Hepatic low-density lipoprotein receptor (LDLR) is the primary conduit for the clearance of plasma LDL-cholesterol and increasing its expression represents a central goal for treating cardiovascular disease. However, LDLR mRNA is unstable and undergoes rapid turnover mainly due to the three AU-rich elements (ARE) in its proximal 3'-untranslated region (3'-UTR). Herein, our data revealed that 5-azacytidine (5-AzaC), an antimetabolite used in the treatment of myelodysplastic syndrome, stabilizes the LDLR mRNA through a previously unrecognized signaling pathway resulting in a strong increase of its protein level in human hepatocytes in culture. 5-AzaC caused a sustained activation of the inositol-requiring enzyme 1α (IRE1α) kinase domain and c-Jun N-terminal kinase (JNK) independently of endoplasmic reticulum stress. This resulted in activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase1/2 (ERK1/2) that, in turn, stabilized LDLR mRNA. Systematic mutation of the AREs (ARE1-3) in the LDLR 3'UTR and expression of each mutant coupled to a luciferase reporter in Huh7 cells demonstrated that ARE1 is required for rapid LDLR mRNA decay and 5-AzaC-induced mRNA stabilization via the IRE1α-EGFR-ERK1/2 signaling cascade. The characterization of this pathway will help to reveal potential targets to enhance plasma LDL clearance and novel cholesterol-lowering therapeutic strategies.

Distinctive pharmacological differences between liver cancer cell lines HepG2 and Hep3B

As cellular models for in vitro liver cancer and toxicity studies, HepG2 and Hep3B are the two most frequently used liver cancer cell lines. Because of their similarities they are often treated as the same in experimental studies. However, there are many differences that have been largely over-sighted or ignored between them. In this review, we summarize the differences between HepG2 and Hep3B cell lines that can be found in the literature based on PubMed search. We particularly focus on the differential gene expression, differential drug responses (chemosensitivity, cell cycle and growth inhibition, and gene induction), signaling pathways associated with these differences, as well as the factors in governing these differences between HepG2 and Hep3B cell lines. Based on our analyses of the available data, we suggest that neither HBx nor p53 may be the crucial factor to determine the differences between HepG2 and Hep3B cell lines although HBx regulates the expression of the majority of genes that are differentially expressed between HepG2 and Hep3B. Instead, the different maturation stages in cancer development of the original specimen between HepG2 and Hep3B may be responsible for the differences between them. This review provides insight into the molecular mechanisms underlying the differences between HepG2 and Hep3B and help investigators especially the beginners in the areas of liver cancer research and drug metabolism to fully understand, and thus better use and interpret the data from these two cell lines in their studies.

A novel posttranscriptional mechanism for dietary cholesterol-mediated suppression of liver LDL receptor expression

It is well-established that over-accumulation of dietary cholesterol in the liver inhibits sterol-regulatory element binding protein (SREBP)-mediated LDL receptor (LDLR) gene transcription leading to a reduced hepatic LDLR mRNA level in hypercholesterolemic animals. However, it is unknown whether elevated cholesterol levels can elicit a cellular response to increase LDLR mRNA turnover to further repress LDLR expression in liver tissue. In the current study, we examined the effect of a high cholesterol diet on the hepatic expression of LDLR mRNA binding proteins in three different animal models and in cultured hepatic cells. Our results demonstrate that high cholesterol feeding specifically elevates the hepatic expression of LDLR mRNA decay promoting factor heterogeneous nuclear ribonucleoprotein (HNRNP)D without affecting expressions of other LDLR mRNA binding proteins in vivo and in vitro. Employing the approach of adenovirus-mediated gene knockdown, we further show that depletion of HNRNPD in the liver results in a marked reduction of serum LDL-cholesterol and a substantial increase in liver LDLR expression in hyperlipidemic mice. Additional studies of gene knockdown in albumin-luciferase-untranslated region (UTR) transgenic mice provide strong evidence supporting the essential role of 3'UTR in HNRNPD-mediated LDLR mRNA degradation in liver tissue. Altogether, this work identifies a novel posttranscriptional regulatory mechanism by which dietary cholesterol inhibits liver LDLR expression via inducing HNRNPD to accelerate LDLR mRNA degradation.

The critical role of mRNA destabilizing protein heterogeneous nuclear ribonucleoprotein d in 3' untranslated region-mediated decay of low-density lipoprotein receptor mRNA in liver tissue

OBJECTIVE

Previous studies showed that low-density lipoprotein receptor (LDLR) mRNA 3' untranslated region (UTR) contains regulatory elements responsible for rapid mRNA turnover in hepatic cells and mediates the mRNA stabilization induced by berberine (BBR). Here, we elucidate the underlying mechanism of BBR's action by characterizing mRNA-binding proteins that modulate LDLR mRNA decay via 3'UTR in liver tissue in vivo.

APPROACH AND RESULTS

We generated a transgenic mouse model (Alb-Luc-UTR) that expresses Luc-LDLR3'UTR reporter gene driven by the albumin promoter to study 3'UTR function in mediating LDLR mRNA decay in liver tissue. We show that treating Alb-Luc-UTR mice with BBR led to significant increases in hepatic bioluminescence signals, Luc-UTR mRNA, and LDLR mRNA levels as compared with control mice. These effects were accompanied by specific reductions of mRNA decay-promoting factor heterogeneous nuclear ribonucleoprotein D (hnRNP D) in liver of BBR-treated mice. Knockdown and overexpression studies further demonstrated that hnRNP D p37 isoform plays a major role in promoting hepatic LDLR mRNA degradation. In addition, we examined LDLR mRNA half-life, Luc-UTR reporter activity, and hnRNP D expression levels in cell lines derived from extrahepatic tissues. We demonstrated that strengths of 3'UTR in promoting mRNA degradation correlate with hnRNP D cellular abundances in nonhepatic cell lines, thereby suggesting its involvement in LDLR mRNA degradation beyond liver tissue.

CONCLUSIONS

hnRNP D is critically involved in LDLR mRNA degradation in liver tissue in vivo. The inverse relationship of hnRNP D abundance with LDLR mRNA levels after BBR treatment suggests the potential of hnRNP D of being a novel therapeutic target for LDL cholesterol lowering.

Epigallocatechin gallate changes mRNA expression level of genes involved in cholesterol metabolism in hepatocytes

Catechins, compounds derived from green tea, have been shown to improve cholesterol metabolism in animal studies, but the molecular mechanisms underlying this function have not been fully understood. We performed DNA microarray analysis in order to clarify the effects of epigallocatechin gallate (EGCG), the dominant catechin in green tea, on cholesterol metabolism in HepG2 hepatocytes. This revealed that the expression levels of several genes related to cholesterol metabolism, including the LDL receptor, were changed by EGCG treatment. Using a real-time PCR technique, we confirmed that EGCG treatment up-regulated mRNA expression level of the LDL receptor. Moreover, EGCG decreased extracellular apoB levels. These findings indicated that EGCG improves cholesterol metabolism through the up-regulation of LDL receptor and also reduces extracellular apoB levels.

Chenodeoxycholic acid stabilization of LDL receptor mRNA depends on 3'-untranslated region and AU-rich element-binding protein

Human low-density lipoprotein receptor (LDLR) mRNA is unstable and contains four AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). The aim of this study was to verify the involvement of the 3'-UTR in the rapid degradation of LDLR mRNA. This study revealed that the 3'-UTR is necessary and sufficient for the degradation, and that the 1st ARE (ARE1) close to the stop codon associates with cytoplasmic proteins, and is primarily responsible for the degradation. Chenodeoxycholic acid (CDCA) treatment stabilized chimeric GFP-LDLR 3'-UTR mRNA and accompanied mitogen-activated protein kinase (MAPK) activation. The UV cross-linking assays showed that a protein of 80kDa increasingly binds to the region including the ARE1 in response to CDCA-mediated MAPK activation.

Molecular pathways and agents for lowering LDL-cholesterol in addition to statins

Recent guidelines in North America and Europe recommend lowering low density lipoprotein associated cholesterol (LDLC) to achieve optimal coronary heart disease risk reduction. Statins have been the therapy of choice and proven successful and relatively safe. However, we are now facing new challenges and it appears that additional or alternative drugs are urgently needed. This boosts research in the field, reopening old cases like other inhibitors of cholesterol synthesis or making attractive tools from the latest technologies like gene silencing by anti-sense oligonucleotides. LDLs are cholesterol-enriched lipoproteins stabilized by the hepatic apolipoprotein B100, and derived from TG rich very low density lipoprotein. This review focuses on the molecular pathways involved in plasma LDLC production and elimination, in particular cholesterol absorption and the hepatobiliary route, apoB100 and VLDL production, and LDL clearance via the LDL receptor. We will identify important or rate-limiting proteins (including Niemann-Pick C1-like 1 (NPC1L1), microsomal TG transfer protein (MTP), acyl-coenzyme A/cholesterol acyltransferase (ACAT), Acyl-CoA:diacylglycerol acyltransferases 2 (DGAT2), proprotein convertase subtilisin kexin type 9 (PCSK9)), and nuclear receptors (farnesoid X receptor (FXR), thyroid hormone receptor (TR)) that constitute interesting therapeutic targets. Numerous compounds already in use modulate these pathways, such as phytosterols, ezetimibe, bile acids sequestrants, niacin, and fibrates. Many pathways can be considered to lower LDLC, but the road has been paved with disappointments and difficulties. With new targets identified and diversification of the drugs, a new era for better LDLC management is plausible.

Protein kinase C activation stabilizes LDL receptor mRNA via the JNK pathway in HepG2 cells

LDL is the most abundant cholesterol transport vehicle in plasma and a major prognostic indicator of atherosclerosis. Hepatic LDL receptors limit circulating LDL levels, since cholesterol internalized by the liver can be excreted. As such, mechanisms regulating LDL receptor expression in liver cells are appealing targets for cholesterol-lowering therapeutic strategies. Activation of HepG2 cells with phorbol esters enhances LDL receptor mRNA levels through transcriptional and posttranscriptional mechanisms. Here, we show that 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced stabilization of receptor mRNA requires the activity of protein kinase C and is accompanied by activation of the major mitogen activated protein kinase pathways. Inhibitor studies demonstrated that receptor mRNA stabilization is independent of the extracellular signal-regulated kinase or p38(MAPK), but requires activation of the c-Jun N-terminal kinase (JNK). An essential role for JNK in stabilizing receptor mRNA was further confirmed through small interfering RNA (siRNA) experiments and by activating JNK through two protein kinase C-independent mechanisms. Finally, prolonged JNK activation increased steady-state levels of receptor mRNA and protein, and significantly enhanced cellular LDL-binding activity. These data suggest that JNK may play an important role in posttranscriptional control of LDL receptor expression, thus constituting a novel mechanism to enhance plasma LDL clearance by liver cells.

Identification of mRNA binding proteins that regulate the stability of LDL receptor mRNA through AU-rich elements

The 3'untranslated region (UTR) of human LDL receptor (LDLR) mRNA contains three AU-rich elements (AREs) responsible for rapid mRNA turnover and mediates the stabilization induced by berberine (BBR). However, the identities of the specific RNA binding proteins involved in the regulation of LDLR mRNA stability at the steady state level or upon BBR treatment are unknown. By conducting small interfering RNA library screenings, biotinylated RNA pull-down, mass spectrometry analysis, and functional assays, we now identify heterogeneous nuclear ribonucleoprotein D (hnRNP D), hnRNP I, and KH-type splicing regulatory protein (KSRP) as key modulators of LDLR mRNA stability in liver cells. We show that hnRNP D, I, and KSRP interact with AREs of the LDLR 3'UTR with sequence specificity. Silencing the expression of these proteins increased LDLR mRNA and protein levels. We further demonstrate that BBR-induced mRNA stabilization involves hnRNP I and KSRP, as their cellular depletions abolished the BBR effect and BBR treatment reduced the binding of hnRNP I and KSRP to the LDLR mRNA 3'UTR. These new findings demonstrate that LDLR mRNA stability is controlled by a group of ARE binding proteins, including hnRNP D, hnRNP I, and KSRP. Our results suggest that interference with the ability of destabilizing ARE binding proteins to interact with LDLR-ARE motifs is likely a mechanism for regulating LDLR expression by compounds such as BBR and perhaps others.

The citrus flavonoids hesperetin and nobiletin differentially regulate low density lipoprotein receptor gene transcription in HepG2 liver cells

Reduction of plasma cholesterol by citrus flavonoids is associated with effects on specific liver functions related to lipid handling. In previous in vivo studies, polymethoxylated flavones (PMF) reduced plasma cholesterol levels at lower doses than required for flavanones. To delineate hepatic mechanisms that underlie this differential potency, we used HepG2 cells to quantitate effects on expression of the LDL receptor (LDLR) gene. A dose-response analysis showed that 200 micromol/L hesperetin, a flavanone present as a disaccharide in oranges, increased LDLR mRNA levels 3.6- to 4.7-fold of the untreated control. In contrast, nobiletin, a PMF found at the highest concentration in oranges and tangerines, achieved maximal stimulation of 1.5- to 1.6-fold of control at only 5 micromol/L. Transcriptional regulation of the LDLR gene by citrus flavonoids has been implicated but, to our knowledge, not directly demonstrated. Here, using transfection vector constructs containing the upstream region of the LDLR gene, we show differences in both potency and efficacy in the induction of transcription, with peak stimulation of 5.3- to 7.5-fold of control at 150-160 micromol/L hesperetin and 3- to 3.8-fold of control at 10-20 micromol/L nobiletin. Hesperetin sustains induction, whereas nobiletin is inhibitory at high doses, resulting in an inverted-U dose response. The sterol regulatory element (SRE) in the LDLR gene upstream region plays a crucial role, because mutation of this site strongly attenuated induction in response to hesperetin or nobiletin. Thus, citrus flavonoids are likely to act through the SRE-binding proteins, with PMF initially activating these mechanisms at considerably lower concentrations than flavanones.

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.

Worldwide genetic variation at the 3'-UTR region of the LDLR gene: possible influence of natural selection

The low density lipoprotein receptor gene (LDLR) contains many Alu insertions, and is especially Alu-rich at its 3'-untranslated region (3'-UTR). Previous studies suggested that the LDLR 3'-UTR could regulate gene expression by the stabilization of its mRNA. Given the faster Alu evolutionary rate, and wondering about its consequences in a possibly regulatory locus, we have studied approximately 800 bp of 222 chromosomes from individuals of African, Asian, Caucasian and Amerind ancestry, to better understand the evolution of the worldwide genetic diversity at this locus. Twenty-one polymorphic sites, distributed in 15 haplotypes, were found. High genetic diversity was observed, concentrated in one Alu insertion (Alu U), which also shows a fast evolutionary rate. Genetic diversity is similar in all populations except Amerinds, suggesting a bottleneck during the peopling of the American continent. Three haplotype clusters (A, B, C) are distinguished, cluster A being the most recently formed (approximately 500,000 years ago). No clear geographic structure emerges from the haplotype network, the global F(st) (0.079) being lower than the average for the human genome. When ancestral population growth is taken into account, neutrality statistics are higher than expected, possibly suggesting the action of balancing selection worldwide.

ABCA1 mediates high-affinity uptake of 25-hydroxycholesterol by membrane vesicles and rapid efflux of oxysterol by intact cells

ATP Binding Cassette (ABC) transporter, ABCA1, plays a pivotal role in reverse cholesterol transport by mediating the cellular efflux of phospholipid and cholesterol. Studies using intact cells strongly suggest that ABCA1 acts as a phospholipid floppase, but there has been no direct demonstration that the protein is a primary active sterol transporter. Using membrane vesicles from insect Sf21 cells, we found that ABCA1 mediated ATP-dependent uptake of [(3)H]25-hydroxycholesterol with an apparent K(m) of 0.7 muM. Consistent with this high apparent affinity, expression of ABCA1 in human embryonic kidney cells both increased rapid efflux of 25-hydroxcholesterol and prevented oxysterol-mediated repression of low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNAs. Comparison of wild-type and ABCA1(-/-) murine fibroblasts indicates that 25-hydroxycholesterol is effluxed approximately 5-fold more rapidly by wild-type cells. In addition, the rate of efflux from the wild-type but not the ABCA1(-/-) fibroblasts is increased a further twofold by inducers of ABCA1 expression. Thus under the experimental conditions employed, endogenous ABCA1 is a major contributor to 25-hydroxycholesterol efflux from wild-type fibroblasts. Evidence from in vitro studies indicates that oxysterols are potent inducers of genes involved in cellular cholesterol efflux and metabolism, including the ABCA1 gene, and repressors of genes involved in cholesterol synthesis or uptake. Our observations raise the possibility that efflux of oxysterols by ABCA1 could contribute to a homeostatic mechanism, which both attenuates oxysterol-induced expression of its cognate gene and alleviates repression of genes encoding proteins, such as HMG-CoA reductase and LDL receptor.

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.

Extracellular signal-regulated kinase-dependent stabilization of hepatic low-density lipoprotein receptor mRNA by herbal medicine berberine

OBJECTIVE

Our recent studies identified berberine (BBR) as a novel cholesterol-lowering drug that upregulates low-density lipoprotein (LDL) receptor expression through mRNA stabilization. Here, we investigated mechanisms underlying regulatory effects of BBR on LDL receptor (LDLR) messenger.

METHODS AND RESULTS

We show that the extracellular signal-regulated kinase (ERK) signaling pathway is used primarily by BBR to attenuate the decay of LDLR mRNA in HepG2 cells. Using different reporter constructs, we demonstrate that BBR affects LDLR mRNA stability entirely through 3' untranslated region (UTR) in an ERK-dependent manner, and this stabilizing effect is more prominent in liver-derived cells than nonhepatic cell lines. In contrast to BBR, the mRNA stabilizing effect of bile acid chenodeoxycholic acid is mediated through the LDLR coding sequence, whereas the 5'UTR, 3'UTR, and the coding sequence of LDLR mRNA are all implicated in the action of phorbol 12-myristate 13-acetate. By performing UV cross-linking and SDS-PAGE, we identify 2 cytoplasmic proteins of 52 and 42 kDa that specifically bind to the LDLR 3'UTR in BBR-inducible and ERK-dependent manners.

CONCLUSIONS

These new findings demonstrate that the BBR-induced stabilization of LDLR mRNA is mediated by the ERK signaling pathway through interactions of cis-regulatory sequences of 3'UTR and mRNA binding proteins that are downstream effectors of this signaling cascade.

Regulation of Low-Density Lipoprotein Receptor Activity by Estrogens and Phytoestrogens in a HepG2 Cell Model

BACKGROUND/AIMS

Estrogen treatment is thought to lower low-density lipoprotein (LDL) cholesterol levels by increasing clearance through hepatic LDL receptors. This study aimed to determine the effect of estrogens and phytoestrogens on LDL receptor activity in a human hepatoma cell line, HepG2.

METHODS

HepG2 cells in culture were incubated for 24 h with estrogen or phytoestrogen and LDL receptor activity was measured by examining the cellular binding of colloidal gold-labelled LDL.

RESULTS

17Beta-estradiol significantly increased LDL receptor activity whereas estriol had negligible effects. Incubation with the isoflavonoids, formononetin, biochanin A and daidzein, caused significant elevations in receptor activity at concentrations above 40 microM. Coumestrol, a coumestan with a high level of estrogenic activity, caused a 3-fold increase in receptor activity at a concentration of 50 microM. Of the phytoestrogenic mammalian lignans enterolactone and enterodiol, only enterolactone displayed the ability to significantly upregulate LDL receptor activity at 50 microM.

CONCLUSION

This study suggests that the LDL receptor-stimulating effect of natural estrogens is mainly due to estradiol and that the cholesterol-lowering effect of diets high in phytoestrogens may be due in part to their ability to increase hepatic LDL receptor activity.

Regulation of the scavenger receptor BI and the LDL receptor by activators of aldosterone production, angiotensin II and PMA, in the human NCI-H295R adrenocortical cell line

In human adrenal cells, cholesterol for steroidogenesis is derived from both high-density lipoproteins (HDL) via the Scavenger Receptor Class B Type I (SR-BI) and low-density lipoproteins (LDL) via the LDL receptor pathway. We have previously shown that, in the human adrenocortical carcinoma cell line, NCI-H295R, SR-BI and LDL receptor expression and steroidogenesis are coordinately regulated by activators of protein kinase A (PKA) leading to glucocorticoid synthesis. In the present study, we studied whether SR-BI and LDL receptor expression are regulated by activators of the protein kinase C (PKC) signaling pathway, such as angiotensin II, which stimulate mineralocorticoid synthesis. First, it is shown that, in NCI-H295R cells, aldosterone synthesis is stimulated by a phorbol ester (phorbol-12-myristate-13 acetate, PMA), a potent PKC activator. Northern blot analysis indicated that both angiotensin II and PMA stimulated SR-BI expression in a time-dependent manner. LDL receptor expression is slightly stimulated by PMA. The induction of SR-BI gene expression occurs at the transcriptional level, via an activation of the human SR-BI promoter, as shown by transient transfection experiments. Finally, SR-BI protein level was increased in angiotensin II- and PMA-stimulated cells, resulting in higher lipoprotein binding and specific cholesteryl ester (CE) uptake from HDL, as well from LDL after angiotensin II and PMA stimulation.

Doubling expression of the low density lipoprotein receptor by truncation of the 3'-untranslated region sequence ameliorates type iii hyperlipoproteinemia in mice expressing the human apoe2 isoform

The primary receptor mediating clearance of apolipoprotein (apo)E- and apoB100-containing lipoproteins from the circulation is the low density lipoprotein (LDL) receptor. Reduced expression of the LDLR is believed to be a precipitating factor in the pathogenesis of type III hyperlipoproteinemia (HLP) in some humans homozygous for the apoE2 allele (APOE*2). To test the effect of genetic changes in LDL receptor expression on the pathogenesis of type III HLP, we have generated a variant allele at the endogenous mouse Ldlr locus that expresses the human LDL receptor transcript. Transcription of the human LDLR minigene is regulated by the endogenous mouse promoter sequence, but a truncation of 3'-untranslated region results in increased mRNA stability. Consequently, in liver of heterozygotes, steady state levels of mouse and human LDLR transcripts are 50 and 180% the levels of total transcript in wild type mice, respectively. Overall, the 2.3-fold normal level of LDLR message in heterozygotes completely ameliorates type III HLP caused by the homozygosity for the human APOE*2 allele, normalizing their plasma lipoprotein profile. We conclude that a modest increase in expression of the LDLR through message stabilization is sufficient to prevent precipitation of type III HLP in mice.

Disruption of the murine calpain small subunit gene, Capn4: calpain is essential for embryonic development but not for cell growth and division

Calpains are a family of Ca(2+)-dependent intracellular cysteine proteases, including the ubiquitously expressed micro- and m-calpains. Both mu- and m-calpains are heterodimers, consisting of a distinct large 80-kDa catalytic subunit, encoded by the genes Capn1 and Capn2, and a common small 28-kDa regulatory subunit (Capn4). The physiological roles and possible functional distinctions of mu- and m-calpains remain unclear, but suggested functions include participation in cell division and migration, integrin-mediated signal transduction, apoptosis, and regulation of cellular control proteins such as cyclin D1 and p53. Homozygous disruption of murine Capn4 eliminated both mu- and m-calpain activities, but this did not affect survival and proliferation of cultured embryonic stem cells or embryonic fibroblasts, or the early stages of organogenesis. However, mutant embryos died at midgestation and displayed defects in the cardiovascular system, hemorrhaging, and accumulation of erythroid progenitors.

The AU-rich 3' untranslated region of human MDR1 mRNA is an inefficient mRNA destabilizer

The human multidrug resistance gene MDR1 encodes a membrane-bound protein, referred to as P-glycoprotein, that acts as a pump to extrude toxins from cells. The 3' untranslated region (3'UTR) of the human MDR1 mRNA is very AU-rich (70%) and contains AU-rich sequences similar to those shown to confer rapid decay on c-myc, c-fos, and lymphokine mRNAs. We tested the ability of the MDR1 3'UTR to act as an mRNA destabilizing element in the human hepatoma cell line HepG2. The MDR1 mRNA has an intermediate half-life of 8 h in HepG2 cells compared to a half-life of 30 min for c-myc mRNA. The MDR1 mRNA half-life was prolonged to >20 h upon treatment with the protein synthesis inhibitor cycloheximide. We constructed expression vectors containing the human beta-globin coding region with the 3'UTR from either MDR1 or c-myc. The c-myc 3'UTR increased the decay of the chimeric mRNA, but the MDR1 3'UTR had no effect. We tested the ability of MDR1 3'UTR sequences to compete for interaction with AU-binding proteins in cell extracts; MDR1 RNA probes had a fivefold lower affinity for AU-binding proteins that interact with the c-myc AU-rich 3'UTR. Overall, our data suggest that the MDR1 3'UTR does not behave as an active destabilizing element in HepG2 cells.

Regulation of AUF1 expression via conserved alternatively spliced elements in the 3' untranslated region

The A+U-rich RNA-binding factor AUF1 exhibits characteristics of a trans-acting factor contributing to the rapid turnover of many cellular mRNAs. Structural mapping of the AUF1 gene and its transcribed mRNA has revealed alternative splicing events within the 3' untranslated region (3'-UTR). In K562 erythroleukemia cells, we have identified four alternatively spliced AUF1 3'-UTR variants, including a population of AUF1 mRNA containing a highly conserved 107-nucleotide (nt) 3'-UTR exon (exon 9) and the adjacent downstream intron (intron 9). Functional analyses using luciferase-AUF1 3'-UTR chimeric transcripts demonstrated that the presence of either a spliceable or an unspliceable intron 9 in the 3'-UTR repressed luciferase expression in cis, indicating that intron 9 sequences may down-regulate gene expression by two distinct mechanisms. In the case of the unspliceable intron, repression of luciferase expression likely involved two AUF1-binding sequences, since luciferase expression was increased by deletion of these sites. However, inclusion of the spliceable intron in the luciferase 3'-UTR down-regulated expression independent of the AUF1-binding sequences. This is likely due to nonsense-mediated mRNA decay (NMD) owing to the generation of exon-exon junctions more than 50 nt downstream of the luciferase termination codon. AUF1 mRNA splice variants generated by selective excision of intron 9 are thus also likely to be subject to NMD since intron 9 is always positioned >137 nt downstream of the stop codon. The distribution of alternatively spliced AUF1 transcripts in K562 cells is consistent with this model of regulated AUF1 expression.

Amiodarone decreases gene expression of low-density lipoprotein receptor at both the mRNA and the protein level

Amiodarone, a potent antiarrhythmic drug, decreases plasma and tissue triiodothyronine (T3) and increases plasma cholesterol levels, resembling changes seen during hypothyroidism. The increase of serum cholesterol during amiodarone medication is associated with a decreased expression of the hepatic low-density lipoprotein (LDL) receptor mRNA. To further elucidate the mechanism of amiodarone-induced hypercholesterolemia, we investigated whether the decreased mRNA levels are the result of decreased transcription or increased degradation or both, and whether protein expression is decreased accordingly. Relative to pair-fed controls, amiodarone treatment increased plasma cholesterol by 69% and decreased expression of the mRNA encoding for the hepatic LDL receptor by 45%. To study this decrease in mRNA, we performed a run-on assay, from which it appears that amiodarone acts by decreasing LDL receptor mRNA expression 2.5-fold at the transcriptional level. The decay rate of liver LDL receptor mRNA, measured at different time points after injecting actinomycin D, was not different between amiodarone-treated and control animals (116+/-32 minutes and 84+/-10 minutes, P=.44). Hepatocytes in primary culture isolated from amiodarone-treated and control animals were used to determine specific binding of [125I]-LDL to hepatic LDL receptors. Amiodarone decreased specific LDL binding and Scatchard analysis demonstrated that amiodarone treatment reduced the number of LDL receptors by 69%, without affecting the dissociation constant (Kd). In conclusion, amiodarone-induced hypercholesterolemia can be explained by decreased transcription of the LDL receptor gene, resulting in lower mRNA and protein levels.

Analysis of human CYP7A1 mRNA decay in HepG2 cells by reverse transcription-polymerase chain reaction

The conversion of cholesterol to bile acids is the major pathway through which cholesterol is removed from the body. The initial and rate-limiting step in this catabolic pathway is catalyzed by the liver-specific enzyme cholesterol 7alpha-hydroxylase (CYP7A1). The HepG2 cell line has been used as a model to study human CYP7A1. The levels of CYP7A1 mRNA, however, are quite low in this cell line and require the use of poly(A)+ mRNA for detection using standard Northern analysis. As an alternative, we established a reverse transcription-polymerase chain reaction (RT-PCR) assay that can be used to study CYP7A1 mRNA in HepG2 cells. Using RT-PCR, we analyzed the influence of cell culture conditions on CYP7A1 mRNA levels. We observed an increase in CYP7A1 mRNA levels as the density of the cell culture increased. This rise in CYP7A1 was accompanied by a reciprocal drop in the levels of the proto-oncogene c-myc. Since c-myc expression is strongly associated with cell growth status, this inverse relationship suggests that conditions that favor reduced cell proliferation result in higher levels of CYP7A1 expression. We also established the validity of using RT-PCR for the measurement of mRNA decay rates using c-myc and glyceraldehyde-3-phosphate dehydrogenase mRNAs as a model: The same half-life value was obtained for the c-myc mRNA using either Northern analysis or RT-PCR. Using our RT-PCR method we determined that human CYP7A1 mRNA decays with a half-life of 4.6 +/- 0.9 h (n = 8) in HepG2 cells. We show that the protein synthesis inhibitor cycloheximide prolonged the CYP7A1 mRNA half-life, suggesting that translation is required for mRNA decay. Dexamethasone treatment, however, did not alter CYP7A1 mRNA decay rate but it increased CYP7A1 steady-state mRNA levels, suggesting that the effect of this glucocorticoid in HepG2 cells may be transcriptional.