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

Epigenetic Modifications in Alternative Splicing of LDLR pre-mRNA on Hypercholesterolemia Following Aerobic Exercise Training

This study investigated whether exercise training improved cholesterol metabolism through modifying alternative splicing of the low-density lipoprotein receptor (LDLR). Blood lipids and expressions of LDLR splice variants were compared between exercise-trained and non-trained young adults with normal and high cholesterol. The expression of LDLR splice isoforms were examined using RT-PCR and the histone H3K36me3 by CHIP-assay in mouse liver following a 13-week normal or high-cholesterol-diet combined with or without 8 weeks of aerobic exercise-training. The influence of histone modifications on LDLR alternative splicing was examined in HepG2 cells (human liver cell-line). Expression levels of LDLR deletions in exons 4 and 12 (LDLR-∆Exon4 and LDLR-∆Exon12) were significantly higher in the obese adults with high-cholesterol. These LDLR splice variants were significantly lower in the exercise-trained than non-trained group with normal cholesterol. Thirteen weeks of high-cholesterol feeding increased LDLR-∆Exon14 expression in mice, which was diminished after 8 weeks of exercise training. When H3-K36me3 or the MORF-related gene on chromosomes 15 were overexpressed and interfered, the levels of LDLR-∆Exon4 and LDLR-∆Exon12 expression in HepG2 cells were significantly augmented and inhibited, respectively. Hypercholesterolemia was associated with augmented expressions of LDLR splice variants in obese adults and following high-cholesterol diet in mice. Aerobic exercise training prevented and reversed the dyslipidemia-related alternative splicing of LDLR pre-mRNA. The histone modifications contributed to the alternative splicing.

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.

A dual-reporter LDLR system integrating fluorescence and luminescence for understanding LDLR regulation and facilitating drug discovery

Introduction

The low-density lipoprotein receptor (LDLR) is integral to cholesterol metabolism and cardiovascular health. Enhancing LDLR expression is a promising strategy for treating hyperlipidemia and reducing the risk of atherosclerosis. However, current LDLR reporter systems have limitations in detecting both transcriptional and translational regulation. To address this, we developed a novel dual-reporter LDLR system incorporating Enhanced Green Fluorescent Protein (EGFP) and Gaussia luciferase (Gluc) to enable precise monitoring of LDLR expression and function.

Methods

A CRISPR/Cas9-mediated knock-in strategy was used to integrate EGFP and Gluc upstream of the stop codon located in exon 18 of the LDLR gene in HEK293 cells. The dual-reporter system allows real-time visualization of LDLR expression via EGFP fluorescence and quantitative assessment through secreted Gluc activity. The system was validated using western blotting, immunofluorescence, and functional assays, including DiI-LDL uptake and drug response analyses with statins and PCSK9 inhibitors.

Results

The established LDLR-EGFP-Gluc knock-in cell line faithfully recapitulates endogenous LDLR expression and function. EGFP fluorescence accurately reflects LDLR expression dynamics, while Gluc activity provides a highly sensitive and quantitative readout. Functional assays confirmed that LDLR expression responds appropriately to statins and PCSK9 inhibitors. Additionally, screening for transcriptional regulators identified FOXP3 and CREB as novel modulators of LDLR expression, with CREB-mediated regulation involving the sterol regulatory element-binding protein 2 (SREBP2) pathway.

Discussion

This dual-reporter system enables complementary monitoring of LDLR dynamics, providing enhanced sensitivity, accuracy, and versatility for studying LDLR regulation and function, as well as facilitating drug discovery targeting hyperlipidemia and cardiovascular diseases.

The low-density lipoprotein receptor: Emerging post-transcriptional regulatory mechanisms

Cholesterol is a vital component of cellular membranes and is an essential molecule in mammalian physiology. Yet dysregulation of hepatic cholesterol metabolism and an increase in plasma cholesterol is linked to development of atherosclerotic cardiovascular disease. Maintaining tight regulation of cholesterol homeostasis is therefore essential, elegantly highlighted by the control of hepatic low-density lipoprotein receptor (LDLR) abundance and associated lipoprotein clearance. The LDLR was discovered in the 1970's in the seminal work of Brown and Goldstein. This was followed by the development of statins, which promote hepatic clearance of LDL via the LDLR pathway. The discovery two decades ago of Proprotein Convertase Subtilisin-Kexin Type 9 (PCSK9), a secreted protein that binds to the LDLR ectodomain and promotes its degradation, and the clinical development of PCSK9 inhibitors has ushered an effort to uncover additional mechanisms that govern the function and abundance of the LDLR. In recent years this has led to the identification of novel post-transcriptional and post-translational mechanisms that govern the LDLR. This review focuses on these emerging regulatory mechanisms and specifically discusses: (1) Regulation of the LDLR mRNA by RNA-binding proteins and microRNAs, (2) Ubiquitin-dependent degradation of the LDLR protein by the E3 ubiquitin ligases inducible degrader of the LDLR (IDOL) and GOLIATH (RNF130), (3) Control of the LDLR pathway by the asialoglycoprotein receptor 1 (ASGR1), and (4) The role of LDLR ectodomain shedding mediated by membrane-type 1 matrix metalloprotease (MT1-MMP), Bone morphogenetic protein 1 (BMP1), and γ-secretase. Understanding the contribution of these emerging mechanisms to regulation of the LDLR is important for the development of novel LDLR-focused lipid-lowering strategies.

Linking the function of cis-acting RNA elements to coronavirus replication using interactomes

RNA structures that are functionally important are defined as cis-acting RNA elements because their functions cannot be compensated for in trans. The cis-acting RNA elements in the 3' UTR of coronaviruses are important for replication; however, the mechanism linking the cis-acting RNA elements to their replication function remains to be established. In the present study, a comparison of the biological processes of the interactome and the replication efficiency between the 3' UTR cis-acting RNA elements in coronaviruses, including severe acute respiratory syndrome coronavirus 2, suggests that (i) the biological processes, including translation, protein folding and protein stabilization, derived from the analysis of the cis-acting RNA element interactome and (ii) the architecture of the cis-acting RNA elements and their interactomes are highly correlated with coronavirus replication. In addition, alteration of the interactome using the compound 5-benzyloxygramine can cause reduced coronavirus replication, reinforcing the connection between cis-acting RNA elements and replication by interactome. Together, these results link cis-acting RNA elements to the coronavirus replication and establish a model to analyse the cis-acting RNA elements in the replication of RNA viruses by interactome.

Flavonoids regulate LDLR through different mechanisms tied to their specific structures

Flavonoids, polyphenolic compounds found in plant-based diets, are associated with reduced risk of cardiovascular disease and longevity. These components are reported to reduce plasma levels of low-density lipoprotein (LDL) through an upregulation of the LDL receptor (LDLR), but the mechanism is still largely unknown. In this study, we have systematically screened the effect of 12 flavonoids from six different flavonoid subclasses on the effect on LDLR. This paper provides an in-depth analysis on how these flavonoids affect LDLR regulation and functionality. We found that most but not all of the tested flavonoids increased LDLR mRNA levels. Surprisingly, this increase was attributed to different regulatory mechanisms, such as enhanced LDLR promoter activity, LDLR mRNA stabilization, or LDLR protein stabilization, of which specific effectual parts of the flavonoid molecular structure could be assigned. These types of comparative analysis of various flavonoids enhance clarity and deepen the understanding of how the different structures of flavonoids affect LDLR regulation. Our data offer useful insights that may guide future research in developing therapeutic approaches for cardiovascular health.

Berberine: A Multi-Target Natural PCSK9 Inhibitor with the Potential to Treat Diabetes, Alzheimer's, Cancer and Cardiovascular Disease

Berberine is a natural product with a wide range of pharmacological effects. It has antimicrobial, anti-cancer, anti-inflammatory, anti-hyperlipidemic, neuroprotective, and cholesterollowering properties, among others. It has been used in traditional Chinese and Ayurvedic medicine for 3000 years and is generally well-tolerated with few side effects. Its main drawback is low oral bioavailability, which has hindered widespread clinical use. However, recent interest has surged with the emergence of evidence that berberine is effective in treating cancer, diabetes, Alzheimer's disease, and cardiovascular disease via multiple mechanisms. It enhances insulin sensitivity and secretion by pancreatic β-cells in Type 2 Diabetes Mellitus in addition to reducing pro-inflammatory cytokines such as IL-6, IL-1β, TLR4 and TNF-α. These cytokines are elevated in Alzheimer's disease, cardiovascular disease, and diabetes. Reductions in pro-inflammatory cytokine levels are associated with positive outcomes such as improved cognition, reduced cardiovascular events, and improved glucose metabolism and insulin sensitivity. Berberine is a natural PCSK9 inhibitor, which contributes to its hypolipidemic effects. It also increases low-density lipoprotein receptor expression, reduces intestinal cholesterol absorption, and promotes cholesterol excretion from the liver to the bile. This translates into a notable decrease in LDL cholesterol levels. High LDL cholesterol levels are associated with increased cardiovascular disease risk. Novel synthetic berberine derivatives are currently being developed that optimize LDL reduction, bioavailability, and other pharmacokinetic properties.

Efficacy and underlying mechanisms of berberine against lipid metabolic diseases: a review

Lipid-lowering therapy is an important tool for the treatment of lipid metabolic diseases, which are increasing in prevalence. However, the failure of conventional lipid-lowering drugs to achieve the desired efficacy in some patients, and the side-effects of these drug regimens, highlight the urgent need for novel lipid-lowering drugs. The liver and intestine are important in the production and removal of endogenous and exogenous lipids, respectively, and have an important impact on circulating lipid levels. Elevated circulating lipids predisposes an individual to lipid deposition in the vascular wall, affecting vascular function. Berberine (BBR) modulates liver lipid production and clearance by regulating cellular targets such as cluster of differentiation 36 (CD36), acetyl-CoA carboxylase (ACC), microsomal triglyceride transfer protein (MTTP), scavenger receptor class B type 1 (SR-BI), low-density lipoprotein receptor (LDLR), and ATP-binding cassette transporter A1 (ABCA1). It influences intestinal lipid synthesis and metabolism by modulating gut microbiota composition and metabolism. Finally, BBR maintains vascular function by targeting proteins such as endothelial nitric oxide synthase (eNOS) and lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1). This paper elucidates and summarizes the pharmacological mechanisms of berberine in lipid metabolic diseases from a multi-organ (liver, intestine, and vascular system) and multi-target perspective.

Berberine protects hepatocyte from hypoxia/reoxygenation-induced injury through inhibiting circDNTTIP2

Background

During hepatic ischemia-reperfusion injury, the excessive release of inflammatory cytokines can activate the intracellular signal transduction cascade to induce hepatocyte injury. Apoptosis is an important way of cell death after I/R injury. Berberine, a common quaternary ammonium alkaloid, has anti-inflammatory, anti-oxidative stress, and anti-apoptotic effects. An increasing number of studies have revealed the importance of non-coding RNAs, including microRNA, long non-coding RNAs and circular RNAs (circRNAs), as regulators of the effects of berberine.

Purpose

In this study, we investigated the mechanism of berberine against liver ischemia-reperfusion injury in vitro.

Study Design and Methods

In this study, hypoxia-reoxygenation (H/R)-treated L02 cells were pretreated with berberine to study the role and mechanism of berberine in resisting hepatic ischemia-reperfusion injury.

Results

The results show that berberine pre-treatment increased the cell viability of H/R-challenged cells, reduced H/R-induced apoptosis and ROS production, reversed H/R-increased on IL-6, IL-1β, TNF-α, and H/R-decreased IL-10 expression. Mechanically, berberine protect hepatocyte from H/R injury, at least partially, through circDNTTIP2. In addition, circDNTTIP2 can bind to the TATA box of caspase3 promoter, thereby promoting caspase 3-related cell apoptosis and the release of inflammatory cytokines.

Conclusion

This study found that berberine has a protective effect on H/R-induced hepatocyte damage by inhibiting a novel circRNA, circDNTTIP2. This study provides potential treatment strategies and treatment targets for liver ischemia-reperfusion injury.

GATA4-activated lncRNA MALAT1 promotes osteogenic differentiation through inhibiting NEDD4-mediated RUNX1 degradation

Postmenopausal osteoporosis (PMOP) brings a lot of inconvenience to patients and serious economic burden to society. The osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) plays vital role in the process of PMOP treatment. However, the functional mechanism remains unclear. In this study, GATA4, MALAT1 and KHSRP were downregulated in bone tissues of PMOP patients, while NEDD4 was overexpressed. Through functional experiments, GATA4 overexpression strikingly accelerated osteogenic differentiation of BMSCs and promoted bone formation in vitro and in vivo, while these effects were dramatically reversed after MALAT1 silence. Intermolecular interaction experiments confirmed that GATA4 activated the transcription of MALAT1, which could form a 'RNA-protein' complex with KHSRP to decay NEDD4 mRNA. NEDD4 promoted the degradation of Runx1 by ubiquitination. Moreover, NEDD4 silencing blocked the inhibitory effects of MALAT1 knockdown on BMSCs osteogenic differentiation. In sum up, GATA4-activated MALAT1 promoted BMSCs osteogenic differentiation via regulating KHSPR/NEDD4 axis-regulated RUNX1 degradation, ultimately improving PMOP.

Cold shock domain-containing protein E1 is a posttranscriptional regulator of the LDL receptor

The low-density lipoprotein receptor (LDLR) controls cellular delivery of cholesterol and clears LDL from the bloodstream, protecting against atherosclerotic heart disease, the leading cause of death in the United States. We therefore sought to identify regulators of the LDLR beyond the targets of current therapies and known causes of familial hypercholesterolemia. We found that cold shock domain-containing protein E1 (CSDE1) enhanced hepatic LDLR messenger RNA (mRNA) decay via its 3' untranslated region and regulated atherogenic lipoproteins in vivo. Using parallel phenotypic genome-wide CRISPR interference screens in a tissue culture model, we identified 40 specific regulators of the LDLR that were not previously identified by observational human genetic studies. Among these, we demonstrated that, in HepG2 cells, CSDE1 regulated the LDLR at least as strongly as statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors. In addition, we showed that hepatic gene silencing of Csde1 treated diet-induced dyslipidemia in mice to a similar degree as Pcsk9 silencing. These results suggest the therapeutic potential of targeting CSDE1 to manipulate the posttranscriptional regulation of the LDLR mRNA for the prevention of cardiovascular disease. Our approach of modeling a clinically relevant phenotype in a forward genetic screen, followed by mechanistic pharmacologic dissection and in vivo validation, may serve as a generalizable template for the identification of therapeutic targets in other human disease states.

Increased LDL receptor by SREBP2 or SREBP2-induced lncRNA LDLR-AS promotes triglyceride accumulation in fish

LDLR, as the uptake receptor of low-density lipoprotein, plays a crucial role in lipid metabolism. However, the detailed mechanism by which LDLR affects hepatic triglyceride (TG) accumulation has rarely been reported. Here, we found that knockdown of LDLR effectively mitigated PA-induced TG accumulation. Further analysis revealed that the expression of LDLR was controlled by SREBP2 directly and indirectly. On one hand, transcription factor SREBP2 activated the transcription of LDLR directly. On the other hand, SREBP2 indirectly regulated LDLR by increasing the transcription of lncRNA LDLR-AS in fish. Mechanism analysis found that LDLR-AS functioned as an RNA scaffold to recruit heterogeneous nuclear ribonucleoprotein R (hnRNPR) to the 5′ UTR region of LDLR mRNA, which stabilized LDLR mRNA at the post-transcription level. In conclusion, our study demonstrates that increased LDLR transcription and mRNA stability is regulated by SREBP2 directly or indirectly, and promotes hepatic TG accumulation by endocytosing LDL in fish.

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PA-mediated LDLR increases triglyceride accumulation via the uptake of LDL in fish

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SREBP2 activated by TNFα promotes LDLR transcription in fish

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LncRNA LDLR-AS increases LDLR mRNA stability by recruiting hnRNPR in fish

IRF2BP2 3'UTR Polymorphism Increases Coronary Artery Calcification in Men

Interferon regulatory factor 2 binding protein 2 (IRF2BP2) suppresses the innate inflammatory response of macrophages. A 9-nucleotide deletion (rs3045215) in the 3' untranslated region (3'-UTR) of human IRF2BP2 mRNA confers risk of coronary artery disease (CAD) in the Ottawa Heart Genomics Study (OHGS). Here, we sought to identify regulatory mechanisms that may contribute to this risk. We tested how lipopolysaccharides (LPS) affects IRF2BP2 expression in human THP-1 macrophages and primary aortic smooth muscle cells (HAoSMC) genotyped for the deletion allele. Both cell types are implicated in coronary atherosclerosis. We also examined how the deletion affects interaction with RNA binding proteins (RBPs) to regulate IRF2BP2 expression. LPS altered allele-specific binding of RBPs in RNA gel shift assays with the THP-1 macrophage protein extracts. The RBP ELAVL1 suppressed the expression of a luciferase reporter carrying the 3'UTR of IRF2BP2 with the deletion allele. Other RBPs AUF1 or KHSRP did not confer such allele specific regulation. Since it is co-inherited with a risk variant for osteoporosis, a condition tied to arterial calcification, we examined the association of the deletion allele with coronary artery calcification in individuals who had undergone computed tomography angiography in the OHGS. In 323 individuals with a minimal burden of atherosclerosis (<30% coronary stenosis) and 138 CAD cases (>50% stenosis), Mendelian randomization revealed that the rs3045215 deletion allele significantly increased coronary artery calcification in men with minimal coronary stenosis. Thus, not only does the rs3045215 deletion allele predict atherosclerosis, but it also predisposes to early-onset calcification in men.

Regulation of low-density lipoprotein receptor expression in triple negative breast cancer by EGFR-MAPK signaling

Expression of the low-density lipoprotein receptor (LDLR) has been shown to play a critical role in hypercholesterolemia-associated breast cancer growth and is associated with shorter recurrence-free survival in human breast cancer studies. We sought to identify how circulating LDL cholesterol and tumor LDLR might accelerate oncogenic processes by determining whether increased LDLR expression and cholesterol uptake are associated with the activation of the epidermal growth factor receptor (EGFR) signaling pathway in triple negative breast cancer (TNBC) cell lines. EGF stimulation of MDA-MB-468 (MDA468) cells activated p44/42MAPK (MAPK), increased expression of LDLR, and fluorescent LDL cholesterol uptake. However, stimulation of MDA-MB-231 (MDA231) cells with EGF did not lead to increased expression of LDLR despite inducing phosphorylation of EGFR. Inhibition of MAPK using UO126 in MDA231 cells reduced LDLR expression, and in MDA468 cells, UO126 impaired the LDLR increase in response to EGF. MDA468 cells exposed to the transcription inhibitor, Actinomycin, prior to treatment with EGF showed reduced degradation of LDLR mRNA compared to vehicle-treated cells. Our results suggest that the EGF-associated increase in LDLR protein expression is cell line-specific. The common pathway regulating LDLR expression was MAPK in both TNBC cell lines.

Berberine: A Promising Natural Isoquinoline Alkaloid for the Development of Hypolipidemic Drugs

Berberine, as a representative isoquinoline alkaloid, exhibits significant hypolipidemic activity in both animal models and clinical trials. Recently, a large number of studies on the lipid-lowering mechanism of berberine and studies for improving its hypolipidemic activity have been reported, but for the most part, they have been either incomplete or not comprehensive. In addition, there have been a few specific reviews on the lipid-reducing effect of berberine. In this paper, the physicochemical properties, the lipid-lowering mechanism, and studies of the modification of berberine all are discussed to promote the development of berberine as a lipid-lowering agent. Subsequently, this paper provides some insights into the deficiencies of berberine in the study of lipid-lowering drug, and based on the situation, some proposals are put forward.

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

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

Lifelong Reduction in LDL (Low-Density Lipoprotein) Cholesterol due to a Gain-of-Function Mutation in LDLR

BACKGROUND

Loss-of-function mutations in the LDL (low-density lipoprotein) receptor gene (LDLR) cause elevated levels of LDL cholesterol and premature cardiovascular disease. To date, a gain-of-function mutation in LDLR with a large effect on LDL cholesterol levels has not been described. Here, we searched for sequence variants in LDLR that have a large effect on LDL cholesterol levels.

METHODS

We analyzed whole-genome sequencing data from 43 202 Icelanders. Single-nucleotide polymorphisms and structural variants including deletions, insertions, and duplications were genotyped using whole-genome sequencing-based data. LDL cholesterol associations were carried out in a sample of >100 000 Icelanders with genetic information (imputed or whole-genome sequencing). Molecular analyses were performed using RNA sequencing and protein expression assays in Epstein-Barr virus-transformed lymphocytes.

RESULTS

We discovered a 2.5-kb deletion (del2.5) overlapping the 3' untranslated region of LDLR in 7 heterozygous carriers from a single family. Mean level of LDL cholesterol was 74% lower in del2.5 carriers than in 101 851 noncarriers, a difference of 2.48 mmol/L (96 mg/dL; P=8.4×10^-8). Del2.5 results in production of an alternative mRNA isoform with a truncated 3' untranslated region. The truncation leads to a loss of target sites for microRNAs known to repress translation of LDLR. In Epstein-Barr virus-transformed lymphocytes derived from del2.5 carriers, expression of alternative mRNA isoform was 1.84-fold higher than the wild-type isoform (P=0.0013), and there was 1.79-fold higher surface expression of the LDL receptor than in noncarriers (P=0.0086). We did not find a highly penetrant detrimental impact of lifelong very low levels of LDL cholesterol due to del2.5 on health of the carriers.

CONCLUSIONS

Del2.5 is the first reported gain-of-function mutation in LDLR causing a large reduction in LDL cholesterol. These data point to a role for alternative polyadenylation of LDLR mRNA as a potent regulator of LDL receptor expression in humans.

Splicing factors: Insights into their regulatory network in alternative splicing in cancer

More than 95% of all human genes are alternatively spliced after transcription, which enriches the diversity of proteins and regulates transcript and/or protein levels. The splicing isoforms produced from the same gene can manifest distinctly, even exerting opposite effects. Mounting evidence indicates that the alternative splicing (AS) mechanism is ubiquitous in various cancers and drives the generation and maintenance of various hallmarks of cancer, such as enhanced proliferation, inhibited apoptosis, invasion and metastasis, and angiogenesis. Splicing factors (SFs) play pivotal roles in the recognition of splice sites and the assembly of spliceosomes during AS. In this review, we mainly discuss the similarities and differences of SF domains, the details of SF function in AS, the effect of SF-driven pathological AS on different hallmarks of cancer, and the main drivers of SF expression level and subcellular localization. In addition, we briefly introduce the application prospects of targeted therapeutic strategies, including small-molecule inhibitors, siRNAs and splice-switching oligonucleotides (SSOs), from three perspectives (drivers, SFs and pathological AS). Finally, we share our insights into the potential direction of research on SF-centric AS-related regulatory networks.

Discovery of 7,9-Disulfatetrahydroberberine as Novel Lipid-Lowering Agents

Berberine (BBR), a well-known alkaloid, exhibits various pharmacological activities, especially hypolipidemic activity, which has attracted much interest from medicinal chemists in the past decade. However, little progress was made on the structural modification of BBR for improving lipid-lowering activity, mainly due to its unclear biological target and low safety. In this study, a new scaffold of 7,9-disulfatetrahydroberberine was discovered unexpectedly, provided with extremely low cytotoxicity. Hence, a novel series of highly safe 7,9-disulfatetrahydroberberines were designed, synthesized, and evaluated for their hypolipidemic activities. In order to investigate the significance of the 9-position substituent, another new series of 7-sulfatetrahydroberberines were designed and synthesized. Lipid-lowering experiments showed that among these compounds, 5f exhibited the best lipid-lowering activity based on two cell models, 3T3-L1 cells and HepG2 cells. Compared with the blank control, the inhibition rate of compound 5f against total cholesterol was over 60%, the inhibition rate against triglyceride was over 70%, the inhibition rate against low-density lipoprotein cholesterol was approximately 75%, and the inhibition rate against high-density lipoprotein cholesterol was close to 50%, which were far superior to the positive control BBR. This result also verified the feasibility of the development of BBR as a lipid-lowering drug via disubstituted modification at the 7- and 9-position.

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.

Berberine in the treatment of metabolism-related chronic diseases: A drug cloud (dCloud) effect to target multifactorial disorders

Berberine (BBR) is a multi-target drug (MTD) that has proven effective in the treatment of metabolism-related chronic diseases (CDs). However, the mode of action (MOA) of BBR remains to be clarified. At a cellular level, the inhibitory effect of BBR on mitochondrial enzymes is probably responsible for many of its biological activities, including the activation of low-density lipoprotein receptor (LDLR), AMP-activated protein kinase (AMPK) and insulin receptor (InsR); these biological activities contribute to ameliorate peripheral blood metabolic profiles, e.g. by reducing plasma lipids and glucose levels, thus improving signs and symptoms of metabolic disorders. In this perspective, BBR acts as a targeted therapy. However, it also exerts pleiotropic systemic activities on some root causes of CDs that include antioxidant / anti-inflammatory effects and modifications of gut microbiota composition and metabolism, which may also contribute to its disease-modifying effects. After reviewing the different MOA of BBR, here we propose that BBR acts through a drug-cloud (dCloud) mechanism, as different to a drug-target effect. The dCloud here is defined as a group of terminal molecular events induced by the drug (or/and related metabolites), as well as the network connections among them. In this scenario, the therapeutic efficacy of BBR is the result of its dCloud effect acting on symptoms/signs as well as on root causes of the diseases. The dCloud concept is applicable to other established MTDs, such as aspirin, metformin, statins as well as to nutrient starvation, thus providing a novel instrument for the design of effective therapies against multifactorial metabolism-related CDs.

Promising Anti-atherosclerotic Effect of Berberine: Evidence from In Vitro, In Vivo, and Clinical Studies

Elevated levels of plasma cholesterol, impaired vascular wall, and presence of inflammatory macrophages are important atherogenic risk factors contributing to atherosclerotic plaque formation and progression. The interventions modulating these risk factors have been found to protect against atherosclerosis development and to decrease atherosclerosis-related cardiovascular disorders. Nutritional approaches involving supplements followed by improving dietary habits and lifestyle have become growingly attractive and acceptable methods used to control atherosclerosis risk factors, mainly high levels of plasma cholesterol. There are a large number of studies that show berberine, a plant bioactive compound, could ameliorate atherosclerosis-related risk factors. In the present literature review, we put together this studies and provide integrated evidence that exhibits berberine has the potential atheroprotective effect through reducing increased levels of plasma cholesterol, particularly low-density lipoprotein (LDL) cholesterol (LDL-C) via LDL receptor (LDLR)-dependent and LDL receptor-independent mechanisms, inhibiting migration and inflammatory activity of macrophages, improving the functionality of endothelial cells via anti-oxidant activities, and suppressing proliferation of vascular smooth muscle cells. In conclusion, berberine can exert inhibitory effects on the atherosclerotic plaque development mainly through LDL-lowering activity and suppressing atherogenic functions of mentioned cells. As the second achievement of this review, among the signaling pathways through which berberine regulates intracellular processes, AMP-activated protein kinase (AMPK) has a central and critical role, showing that enhancing activity of AMPK pathway can be considered as a promising therapeutic approach for atherosclerosis treatment.

Berberine decreases plasma triglyceride levels and upregulates hepatic TRIB1 in LDLR wild type mice and in LDLR deficient mice

TRIB1 is a GWAS locus associated with plasma cholesterol and triglycerides (TG) levels. In mice, liver-specific overexpression of TRIB1 lowers plasma lipid levels. Berberine (BBR) is a natural lipid lowering drug that reduces plasma LDL-cholesterol (LDL-C), total cholesterol (TC) and TG in hyperlipidemic patients and in mice by mechanisms involving upregulation of hepatic LDL receptor (LDLR). Here, we demonstrated that BBR treatment reduced plasma LDL-C, TC and TG in LDLR wildtype (WT) mice fed a high fat and high cholesterol diet and it only lowered TG in LDLR WT mice fed a normal chow diet. In hypercholesterolemic LDLR deficient mice (Ldlr-/-), BBR treatment reduced plasma TG levels by 51% compared to the vehicle control without affecting plasma cholesterol levels. Hepatic gene expression analysis revealed that Trib1 mRNA levels were significantly elevated by BBR treatment in all three mouse models and increases of Trib1 mRNA expression were associated with reduced expression of lipogenic genes including Cebpa, Acc1 and Scd1. In vitro studies further demonstrate that BBR induces TRIB1 mRNA expression by a transcriptional mechanism via ERK signaling pathway. These new findings warrant future in vivo studies to determine the causal role of Trib1 in BBR-mediated TG lowering independent of LDLR regulation.

Tandem RNA isolation reveals functional rearrangement of RNA-binding proteins on CDKN1B/p27Kip1 3'UTRs in cisplatin treated cells

Post-transcriptional control of gene expression is mediated via RNA-binding proteins (RBPs) that interact with mRNAs in a combinatorial fashion. While recent global RNA interactome capture experiments expanded the repertoire of cellular RBPs quiet dramatically, little is known about the assembly of RBPs on particular mRNAs; and how these associations change and control the fate of the mRNA in drug-treatment conditions. Here we introduce a novel biochemical approach, termed tobramycin-based tandem RNA isolation procedure (tobTRIP), to quantify proteins associated with the 3ʹUTRs of cyclin-dependent kinase inhibitor 1B (CDKN1B/p27^Kip1) mRNAs in vivo. P27^Kip1 plays an important role in mediating a cell’s response to cisplatin (CP), a widely used chemotherapeutic cancer drug that induces DNA damage and cell cycle arrest. We found that p27^Kip1 mRNA is stabilized upon CP treatment of HEK293 cells through elements in its 3ʹUTR. Applying tobTRIP, we further compared the associated proteins in CP and non-treated cells, and identified more than 50 interacting RBPs, many functionally related and evoking a coordinated response. Knock-downs of several of the identified RBPs in HEK293 cells confirmed their involvement in CP-induced p27 mRNA regulation; while knock-down of the KH-type splicing regulatory protein (KHSRP) further enhanced the sensitivity of MCF7 adenocarcinoma cancer cells to CP treatment. Our results highlight the benefit of specific in vivo mRNA-protein interactome capture to reveal post-transcriptional regulatory networks implicated in cellular drug response and adaptation.

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.

Inhibition of KHSRP sensitizes colorectal cancer to 5-fluoruracil through miR-501-5p-mediated ERRFI1 mRNA degradation

K-homology (KH)-type splicing regulatory protein (KHSRP) is an RNA binding protein that participates in RNA variable splicing and stability, and facilitates the biogenesis of miRNAs that target mRNA. However, to date, the role of KHSRP in colorectal cancer (CRC) progression has not been reported. In this study, the function of KHSRP in CRC proliferation and 5-fluoruracil (5-FU) resistance was investigated. The upregulation of KHSRP expression was confirmed in CRC patient tissues and two CRC cell lines. Manipulating KHSRP expression altered cell proliferation and 5-FU resistance in CRC cells. ERRFI1, a downstream effector of KHSRP in CRC cells, reduced CRC cell proliferation. Sensitivity to 5-FU mediated by KHSRP knockdown was reversed by ERRFI1 knockdown. We found that KHSRP decreased ERRFI1 mRNA expression indirectly. By screening KHSRP-regulated miRNAs, we further found that miR-501-5p directly combines with KHSRP in CRC cells. Mechanistically, the results of a luciferase assay suggested that miR-501-5p directly binds to the ERRFI1 3'-untranslated region. Taken together, our data indicated that modification of ERRFI1 by KHSRP occurs through miR-501-5p, an essential mechanism driving CRC proliferation and 5-FU resistance. Insight into this mechanism may provide novel targets for overcoming drug resistance in CRC.

Barberry in the treatment of obesity and metabolic syndrome: possible mechanisms of action

Obesity is a consequence of an imbalance between energy intake and energy expenditure. It affects people of both genders and all age groups, ethnicity and socioeconomic groups, and in developed and developing countries. Obesity is often accompanied by the metabolic syndrome (MetS). MetS is characterized by a clustering of cardiovascular risk factors, including high blood pressure, adiposity, dyslipidemia and glucose intolerance, which together increase the risk of atherosclerotic cardiovascular disease, type 2 diabetes mellitus and other causes of mortality. Nowadays, there is a growing interest in the use of plant-based agents instead of synthetic drugs to manage chronic diseases such as MetS; one such example is Berberis vulgaris. B. vulgaris contains isoquinonline alkaloids such as berberine, berberrubine and berbamine. Recent studies have proved that berberine exhibits pharmacological activities and positive effects on the risk factors of obesity and MetS. We have reviewed original articles related to the possible molecular mechanisms of action of berberine on obesity and MetS. Berberine suppresses adipocyte differentiation and decreases obesity. It also regulates glucose metabolism via decreasing insulin resistance and increasing insulin secretion. Other effects of berberine include antihyperlipidemic and antihypertensive activities and endothelial protection.

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.

Endothelial Transcytosis of Lipoproteins in Atherosclerosis

Seminal studies from Nikolai Anichckov identified the accumulation of cholesterol in the arteries as the initial event that lead to the formation of atherosclerotic plaques. Further studies by Gofman and colleagues demonstrated that high levels of circulating low-density lipoprotein cholesterol (LDL-C) was responsible for the accelerated atherosclerosis observed in humans. These findings were confirmed by numerous epidemiological studies which identified elevated LDL-C levels as a major risk factor for cardiovascular disease. LDL infiltrates in the arterial wall and interacts with the proteoglycan matrix promoting the retention and modification of LDL to a toxic form, which results in endothelial cell (EC) activation and vascular inflammation. Despite the relevance of LDL transport across the endothelium during atherogenesis, the molecular mechanism that control this process is still not fully understood. A number of studies have recently demonstrated that low density lipoprotein (LDL) transcytosis across the endothelium is dependent on the function of caveolae, scavenger receptor B1 (SR-B1), activin receptor-like kinase 1 (ALK1), and LDL receptor (LDLR), whereas high-density lipoproteins (HDL) and its major protein component apolipoprotein AI transcytose ECs through SR-B1, ATP-Binding cassette transporter A1 (ABCA1) and ABCG1. In this review article, we briefly summarize the function of the EC barrier in regulating lipoprotein transport, and its relevance during the progression of atherosclerosis. A better understanding of the mechanisms that mediate lipoprotein transcytosis across ECs will help to develop therapies targeting the early events of atherosclerosis and thus exert potential benefits for treating atherosclerotic vascular disease.

Proteomics analysis reveals a potential new target protein for the lipid-lowering effect of Berberine8998

Berberine8998 is a newly synthesized berberine derivative with better lipid-lowering activity and improved absorption. The objective of this study was to investigate the effects of berberine8998 on serum cholesterol and lipid levels in vivo and to examine the mechanisms involved. Hamsters on high-fat diet (HFD) were administered berberine or berberine8998 (50 mg·kg^-1·d^-1, ig) for 3 weeks. Berberine8998 administration significantly lowered the total cholesterol, triglycerides and LDL-C levels in HFD hamsters. Bioinformatics revealed that berberine and berberine8998 shared similar metabolic pathways and fatty acid metabolism was the predominant pathway. Western blot validation results showed that peroxisomal acyl-coenzyme A oxidase 1 (ACOX1) and long-chain fatty acid-CoA ligase 1 (ACSL1), two proteins involved in fatty acid metabolism, were expressed differently in the berberine8998 group than in the untreated group and the berberine treatment group. Biochemistry results showed that berberine8998 significantly lowered the non-esterified fatty acid (NEFA) levels, which may lead to a reduction in TG levels in the berberine8998 treatment group and the differences observed in proteomics analyses. Pharmacokinetic analysis conducted in rats. After administration of berberine or berberine8998 (50 mg/kg, ig), berberine8998 exhibited a remarkably improved absorption with increasing bioavailability by 6.7 times compared with berberine. These findings suggest that berberine8998 lowers cholesterol and lipid levels via different mechanisms than berberine, and its improved absorption makes it a promising therapeutic candidate for the treatment of hypercholesterolemia and obesity.

The ever-expanding saga of the proprotein convertases and their roles in body homeostasis: emphasis on novel proprotein convertase subtilisin kexin number 9 functions and regulation

PURPOSE OF REVIEW

The nine members of the proprotein convertase family play major physiological roles during development and in the adult, and their dysregulation leads to various diseases. The primary objective of this article is to review recent findings on the clinical importance of some of these convertases concentrating mostly on PCSK9, the ninth member of the convertase family. This includes the transcriptional and translational regulation of PCSK9, its ability to enhance the degradation of LDL receptor (LDLR), and the implication of PCSK9 in inflammation and sepsis.

RECENT FINDINGS

PCSK9 levels are upregulated by E2F1 and reduced by specific miRNAs and by Annexin A2 that bind the 3' end of its mRNA. The implication of the LDLR in the clearance of pathogenic bacterial debris in mice and human puts in perspective a new role for PCSK9 in the regulation of sepsis. The specific implication of the LDLR in the clearance of Lp(a) is now confirmed by multiple studies of PCSK9 inhibition in human cohorts.

SUMMARY

Emerging data suggest that PCSK9 can be regulated at the transcriptional and translational levels by specific factors and miRNAs. The identification of a novel pocket in the catalytic domain of PCSK9 represents a harbinger for a new class of small inhibitor drugs. The implication of the LDLR in reducing the effects of bacterially induced sepsis has been supported by both human and mouse data. Outcome studies confirmed the clinical importance of reducing PCSK9 levels. The present review puts in perspective new developments in the PCSK9 biology and its regulation of the LDLR. VIDEO ABSTRACT: http://links.lww.com/COL/A17.

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.

Inflammation-regulated mRNA stability and the progression of vascular inflammatory diseases

Cardiovascular disease remains a major medical and socioeconomic burden in developed and developing societies, and will increase with an aging and increasingly sedentary society. Vascular disease and atherosclerotic vascular syndromes are essentially inflammatory disorders, and transcriptional and post-transcriptional processes play essential roles in the ability of resident vascular and inflammatory cells to adapt to environmental stimuli. The regulation of mRNA translocation, stability, and translation are key processes of post-transcriptional regulation that permit these cells to rapidly respond to inflammatory stimuli. For the most part, these processes are controlled by elements in the 3'-UTR of labile, proinflammatory transcripts. Since proinflammatory transcripts almost exclusively contain AU-rich elements (AREs), this represents a tightly regulated and specific mechanism for initiation and maintenance of the proinflammatory phenotype. RNA-binding proteins (RBPs) recognize cis elements in 3'-UTR, and regulate each of these processes, but there is little literature exploring the concept that RBPs themselves can be directly regulated by inflammatory stimuli. Conceptually, inflammation-responsive RBPs represent an attractive target of rational therapies to combat vascular inflammatory syndromes. Herein we briefly describe the cellular and molecular etiology of atherosclerosis, and summarize our current understanding of RBPs and their specific roles in regulation of inflammatory mRNA stability. We also detail RBPs as targets of current anti-inflammatory modalities and how this may translate into better treatment for vascular inflammatory diseases.

Role of G3BP1 in glucocorticoid receptor-mediated microRNA-15b and microRNA-23a biogenesis in endothelial cells

MicroRNAs (miRNAs) are a family of non-coding RNAs that play crucial roles in regulating various normal cellular responses. Recent studies revealed that the canonical miRNA biogenesis pathway is subject to sophisticated regulation. Hormonal control of miRNA biogenesis by androgen and estrogen has been demonstrated, but the direct effects of the glucocorticoid receptor (GR) on miRNA biogenesis are unknown. This study revealed the role of GR in miRNA maturation. We showed that two GR agonists, dexamethasone and ginsenoside-Rg1 rapidly suppressed the expression of mature miR-15b, miR-23a, and miR-214 in human endothelial cells. RNA pulldown coupled with proteomic analysis identified GTPase-activating protein (SH3 domain) binding protein 1 (G3BP1) as one of the RNA-binding proteins mediating GR-regulated miRNA maturation. Activated GR induced phosphorylation of v-AKT Murine Thymoma Viral Oncogene Homologue (AKT) kinase, which in turn phosphorylated and promoted nuclear translocation of G3BP1. The nuclear G3BP1 bound to the G3BP1 consensus sequence located on primary miR-15b~16-2 and miR-23a~27a~24-2 to inhibit their maturation. The findings from this study have advanced our understanding of the non-genomic effects of GR in the vascular system.

PTB and TIAR binding to insulin mRNA 3'- and 5'UTRs; implications for insulin biosynthesis and messenger stability

OBJECTIVES

Insulin expression is highly controlled on the posttranscriptional level. The RNA binding proteins (RBPs) responsible for this result are still largely unknown.

METHODS AND RESULTS

To identify RBPs that bind to insulin mRNA we performed mass spectrometry analysis on proteins that bound synthetic oligonucloetides mimicing the 5'- and the 3'-untranslated regions (UTRs) of rat and human insulin mRNA in vitro. We observed that the RBPs heterogeneous nuclear ribonucleoprotein (hnRNP) U, polypyrimidine tract binding protein (PTB), hnRNP L and T-cell restricted intracellular antigen 1-related protein (TIA-1-related protein; TIAR) bind to insulin mRNA sequences, and that the in vitro binding affinity of these RBPs changed when INS-1 cells were exposed to glucose, 3-isobutyl-1-methylxanthine (IBMX) or nitric oxide. High glucose exposure resulted in a modest increase in PTB and TIAR binding to an insulin mRNA sequence. The inducer of nitrosative stress DETAnonoate increased markedly hnRNP U and TIAR mRNA binding. An increased PTB to TIAR binding ratio in vitro correlated with higher insulin mRNA levels and insulin biosynthesis rates in INS-1 cells. To further investigate the importance of RNA-binding proteins for insulin mRNA stability, we decreased INS-1 and EndoC-βH1 cell levels of PTB and TIAR by RNAi. In both cell lines, decreased levels of PTB resulted in lowered insulin mRNA levels while decreased levels of TIAR resulted in increased insulin mRNA levels. Thapsigargin-induced stress granule formation was associated with a redistribution of TIAR from the cytosol to stress granules.

CONCLUSIONS

These experiments indicate that alterations in insulin mRNA stability and translation correlate with differential RBP binding. We propose that the balance between PTB on one hand and TIAR on the other participates in the control of insulin mRNA stability and utilization for insulin biosynthesis.

Berberine Decreased Inducible Nitric Oxide Synthase mRNA Stability through Negative Regulation of Human Antigen R in Lipopolysaccharide-Induced Macrophages

Berberine, a major isoquinoline alkaloid found in medicinal herbs, has been reported to possess anti-inflammatory effects; however, the underlying mechanisms responsible for its actions are poorly understood. In the present study, we investigated the inhibitory effects of berberine and the molecular mechanisms involved in lipopolysaccharide (LPS)-treated RAW 264.7 and THP-1 macrophages and its effects in LPS-induced septic shock in mice. In both macrophage cell types, berberine inhibited the LPS-induced nitric oxide (NO) production and inducible NO synthase (iNOS) protein expression, but it had no effect on iNOS mRNA transcription. Suppression of LPS-induced iNOS protein expression by berberine occurred via a human antigen R (HuR)-mediated reduction of iNOS mRNA stability. Molecular data revealed that the suppression on the LPS-induced HuR binding to iNOS mRNA by berberine was accompanied by a reduction in nucleocytoplasmic HuR shuttling. Pretreatment with berberine reduced LPS-induced iNOS protein expression and the cytoplasmic translocation of HuR in liver tissues and increased the survival rate of mice with LPS-induced endotoxemia. These results show that the suppression of iNOS protein expression by berberine under LPS-induced inflammatory conditions is associated with a reduction in iNOS mRNA stability resulting from inhibition of the cytoplasmic translocation of HuR.

Detecting RNA-Protein Interaction Using End-Labeled Biotinylated RNA Oligonucleotides and Immunoblotting

RNA-protein interaction can be detected by RNA pull-down and immunoblotting methods. Here, we describe a method to detect RNA-protein interaction using RNA pull down and to identify the proteins that are pulled-down by the RNA using immunoblotting. In this protocol, RNAs with specific sequences are biotinylated and immobilized onto Streptavidin beads, which are then used to pull down interacting proteins from cellular extracts. The presence of a specific protein is subsequently verified by SDS- polyacrylamide gel electrophoresis and immunoblotting with antibodies. Interactions between the SMN RNA and the PSF protein and between the caspase-2 RNA and the SRSF3 protein (SRp20) in nuclear extract prepared from HeLa cells are illustrated as examples.

Deferoxamine stimulates LDLR expression and LDL uptake in HepG2 cells

SCOPE

Iron overload contributes to the pathogenesis of atherosclerosis and iron chelators are beneficial through their antioxidant properties. Hepatic iron loading increases cholesterol synthesis. Whether iron depletion could affect hepatic cholesterol metabolism is unknown.

METHODS AND RESULTS

We examined the effect of the iron chelator deferoxamine (DFO) on mRNA expression of genes involved in cholesterol metabolism and/or cholesterol uptake. Our results revealed that DFO increases LDL receptor (LDLR) mRNA levels in human hepatocyte-derived cell lines HepG2 and Huh7 cells, and in K562 cells. In HepG2 cells, we observed that DFO increases (i) LDLR-mRNA levels in a time- and dose-dependent manner, (ii) LDLR-protein levels; (iii) cell surface LDLR; and (iv) LDL uptake. In contrast, the mRNA levels of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, sterol regulatory element-binding proteins, and the mRNA/protein levels of proprotein convertase subtilisin-kexin 9 were not modulated by DFO, suggesting that the LDLR regulation by DFO is not at the transcriptional or posttranslational levels. Since LDLR-mRNA was stabilized by DFO, a posttranscriptional mechanism is suggested for the DFO-mediated upregulation of LDLR.

CONCLUSION

DFO induced an increase in LDLR expression by a posttranscriptional mechanism resulting in an enhancement of LDL uptake in HepG2 cells, suggesting increased LDLR activity as one of the underlying causes of the hypocholesterolemic effect of iron reduction.

A novel peroxisome proliferator response element modulates hepatic low-density lipoprotein receptor gene transcription in response to PPARδ activation

The hepatic expression of low-density lipoprotein (LDL) receptor (LDLR) gene is regulated primarily at the transcriptional level by a sterol-regulatory element (SRE) in its proximal promoter region which is the site of action of SRE-binding protein 2 (SREBP2). However whether additional cis-regulatory elements contribute to LDLR transcription has not been fully explored. We investigated the function of a putative peroxisome proliferator-activated receptor (PPAR)-response element (PPRE) sequence motif located at -768 to -752 bases upstream of the transcription start site of human LDLR gene in response to PPARδ activation. Promoter luciferase reporter analyses showed that treating HepG2 cells with PPARδ agonist L165041 markedly increased the activity of a full-length LDLR promoter construct (pLDLR-1192) without any effects on the shorter promoter reporter pLDLR-234 that contains only the core regulatory elements SRE-1 and SP1 sites. Importantly, mutation of the PPRE sequence greatly attenuated the induction of the full-length LDLR promoter activity by L165041 without affecting rosuvastatin (RSV)-mediated transactivation. EMSA and ChIP assay further confirmed the binding of PPARδ to the LDLR-PPRE site. Treating HepG2 cells with L165041 elevated the mRNA and protein expressions of LDLR without affecting the LDLR mRNA decay rate. The induction of LDLR expression by PPARδ agonist was further observed in liver tissue of mice and hamsters treated with L165041. Altogether, our studies identify a novel PPRE-mediated regulatory mechanism for LDLR transcription and suggest that combined treatment of statin with PPARδ agonists may have advantageous effects on LDLR expression.

Post-Transcriptional Regulation of Endothelial Nitric Oxide Synthase Expression by Polypyrimidine Tract-Binding Protein 1

OBJECTIVE

Endothelial nitric oxide synthase (eNOS) is an important regulator of vascular function and its expression is regulated at post-transcriptional levels through a yet unknown mechanism. The purpose of this study is to elucidate the post-transcriptional factors regulating eNOS expression and function in endothelium.

APPROACHES AND RESULTS

To elucidate the molecular basis of tumor necrosis factor (TNF)-α-mediated eNOS mRNA instability, biotinylated eNOS 3'-untranslational region (UTR) was used to purify its associated proteins by RNA affinity chromatography from cytosolic fractions of TNF-α-stimulated human umbilical vein endothelial cells (HUVECs). We identified 2 cytosolic proteins, with molecular weight of 52 and 57 kDa, which specifically bind to eNOS 3'-UTR in response to TNF-α stimulation. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis identified the 57-kDa protein as polypyrimidine tract-binding protein 1 (PTB1). RNA gel mobility shift and UV cross-linking assays demonstrated that PTB1 binds to a UCUU-rich sequence in eNOS 3'-UTR, and the C-terminal half of PTB1 is critical to this interaction. Importantly, PTB1 overexpression leads to decreased activity of luciferase gene fused with eNOS 3'-UTR as well as reduced eNOS expression and activity in human ECs. In HUVECs, we show that TNF-α markedly increased PTB1 expression, whereas adenovirus-mediated PTB1 overexpression decreased eNOS mRNA stability and reduced protein expression and endothelium-dependent relaxation. Furthermore, knockdown of PTB1 substantially attenuated TNF-α-induced destabilization of eNOS transcript and downregulation of eNOS expression.

CONCLUSIONS

These results indicate that PTB1 is essential for regulating eNOS expression at post-transcriptional levels and suggest a novel therapeutic target for treatment of vascular diseases associated with inflammatory endothelial dysfunction.

Berberine as a promising anti-diabetic nephropathy drug: An analysis of its effects and mechanisms

Diabetic nephropathy is a progressive kidney disorder and is pathologically characterized by thickened glomerular and tubular basement membranes, accumulation of the extracellular matrix and increased mesangial hypertrophy. Growing evidence has suggested that diabetic nephropathy is induced by multiple factors, such as dyslipidemia, hyperglycemia, hemodynamic abnormalities and oxidative stress, based on genetic susceptibility. Berberine (BBR; [C20H18NO4](+)), an isoquinoline alkaloid, is the major active constituent of Rhizoma coptidis and Cortex phellodendri. Recent studies have demonstrated that berberine has various pharmacological activities, including lowering blood glucose, regulating blood lipids and reducing inflammation in addition to its antioxidant activity. These findings suggest that berberine has potential applications as a therapeutic drug for diabetic nephropathy, and has significant research value. However, the possible mechanisms have not been fully established. The purpose of this paper is to investigate the renoprotective mechanisms of berberine in diabetic nephropathy and highlight the importance of berberine as a potential therapeutic reagent for diabetic nephropathy treatment.

ZFP36L1 and ZFP36L2 control LDLR mRNA stability via the ERK-RSK pathway

Low-density lipoprotein receptor (LDLR) mRNA is unstable, but is stabilized upon extracellular signal-regulated kinase (ERK) activation, possibly through the binding of certain proteins to the LDLR mRNA 3′-untranslated region (UTR), although the detailed mechanism underlying this stability control is unclear. Here, using a proteomic approach, we show that proteins ZFP36L1 and ZFP36L2 specifically bind to the 3′-UTR of LDLR mRNA and recruit the CCR4-NOT-deadenylase complex, resulting in mRNA destabilization. We also show that the C-terminal regions of ZFP36L1 and ZFP36L2 are directly phosphorylated by p90 ribosomal S6 kinase, a kinase downstream of ERK, resulting in dissociation of the CCR4-NOT-deadenylase complex and stabilization of LDLR mRNA. We further demonstrate that targeted disruption of the interaction between LDLR mRNA and ZFP36L1 and ZFP36L2 using antisense oligonucleotides results in upregulation of LDLR mRNA and protein. These results indicate that ZFP36L1 and ZFP36L2 regulate LDLR protein levels downstream of ERK. Our results also show the usefulness of our method for identifying critical regulators of specific RNAs and the potency of antisense oligonucleotide-based therapeutics.

High throughput platform to explore RNA-protein interactomes

RNA binding proteins (RBPs) and RNA interaction is an emerging topic in molecular biology. Many reports showed that such interactions contribute to many cellular processes as well as disease development. Several standard in vitro and in vivo methods were developed to fulfill the needs of this RBP-RNA interaction study to explore their biological functions. However, these methods have their limitations in terms of throughput. In this review, we emphasize two important high throughput methods to studying RBP-RNA interactions, affinity purification and protein microarray. These methods have recently become robust techniques regarding their efficiency in systematically analyzing RBP-RNA interactions. Here, we provide technique overviews, strategies and applications of these methods during biological research. Although these technologies are just beginning to be explored, they will be most important methods in this study.

Arachidonic acid downregulates acyl-CoA synthetase 4 expression by promoting its ubiquitination and proteasomal degradation

ACSL4 is a member of the long-chain acyl-CoA synthetase (ACSL) family with a marked preference for arachidonic acid (AA) as its substrate. Although an association between elevated levels of ACSL4 and hepatosteatosis has been reported, the function of ACSL4 in hepatic FA metabolism and the regulation of its functional expression in the liver remain poorly defined. Here we provide evidence that AA selectively downregulates ACSL4 protein expression in hepatic cells. AA treatment decreased the half-life of ACSL4 protein in HepG2 cells by approximately 4-fold (from 17.3 ± 1.8 h to 4.2 ± 0.4 h) without causing apoptosis. The inhibitory action of AA on ACSL4 protein stability could not be prevented by rosiglitazone or inhibitors that interfere with the cellular pathways involved in AA metabolism to biologically active compounds. In contrast, treatment of cells with inhibitors specific for the proteasomal degradation pathway largely prevented the AA-induced ACSL4 degradation. We further show that ACSL4 is intrinsically ubiquitinated and that AA treatment can enhance its ubiquitination. Collectively, our studies have identified a novel substrate-induced posttranslational regulatory mechanism by which AA downregulates ACSL4 protein expression in hepatic cells.

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.