[Metabolomic interference induced by short-chain chlorinated paraffins in human normal hepatic cells]

Short-chain chlorinated paraffins (SCCPs) are an emerging class of persistent organic pollutants (POPs) that are widely detected in environmental matrices and human samples. Because of their environmental persistence, long-range transport potential, bioaccumulation potential, and biotoxicity, SCCPs pose a significant threat to human health. In this study, metabolomics technology was applied to reveal the metabolomic interference in human normal hepatic (L02) cells after exposure to low (1 μg/L), moderate (10 μg/L), and high (100 μg/L) doses of SCCPs. Principal component analysis (PCA) and metabolic effect level index (MELI) values showed that all three SCCP doses caused notable metabolic perturbations in L02 cells. A total of 72 metabolites that were annotated by MS/MS and matched with the experimental spectra in the Human Metabolome Database (HMDB) or validated by commercially available standards were selected as differential metabolites (DMs) across all groups. The low-dose exposure group shared 33 and 36 DMs with the moderate- and high-dose exposure groups, respectively. The moderate-dose exposure group shared 46 DMs with the high-dose exposure group. In addition, 33 DMs were shared among the three exposure groups. Among the 72 DMs, 9, 9, and 45 metabolites participated in the amino acid, nucleotide, and lipid metabolism pathways, respectively. The results of pathway enrichment analysis showed that the most relevant metabolic pathways affected by SCCPs were the lipid metabolism, fatty acid β-oxidation, and nucleotide metabolism pathways, and that compared with low-dose exposure, moderate- and high-dose SCCP exposures caused more notable perturbations of these metabolic pathways in L02 cells. Exposure to SCCPs perturbed glycerophospholipid and sphingolipid metabolism. Significant alterations in the levels of phosphatidylcholines, phosphatidylethanolamines, and sphingomyelins indicated SCCP-induced biomembrane damage. SCCPs inhibited fatty acid β-oxidation by decreasing the levels of short- and medium-chain acylcarnitines in L02 cells, indicating that the energy supplied by fatty acid oxidation was reduced in these cells. Furthermore, compared with low- and moderate-dose SCCPs, high-dose SCCPs produced a significantly stronger inhibition of fatty acid β-oxidation. In addition, SCCPs perturbed nucleotide metabolism. The higher hypoxanthine levels observed in L02 cells after SCCP exposures indicate that SCCPs may induce several adverse effects, including hypoxia, reactive oxygen species production, and mutagenesis in L02 cells.

Neuroimmunomodulation of adrenoblockers during liver cirrhosis: modulation of hepatic stellate cell activity

The sympathetic nervous system and the immune system are responsible for producing neurotransmitters and cytokines that interact by binding to receptors; due to this, there is communication between these systems. Liver immune cells and nerve fibres are systematically distributed in the liver, and the partial overlap of both patterns may favour interactions between certain elements. Dendritic cells are attached to fibroblasts, and nerve fibres are connected via the dendritic cell-fibroblast complex. Receptors for most neuroactive substances, such as catecholamines, have been discovered on dendritic cells. The sympathetic nervous system regulates hepatic fibrosis through sympathetic fibres and adrenaline from the adrenal glands through the blood. When there is liver damage, the sympathetic nervous system is activated locally and systemically through proinflammatory cytokines that induce the production of epinephrine and norepinephrine. These neurotransmitters bind to cells through α-adrenergic receptors, triggering a cellular response that secretes inflammatory factors that stimulate and activate hepatic stellate cells. Hepatic stellate cells are key in the fibrotic process. They initiate the overproduction of extracellular matrix components in an active state that progresses from fibrosis to liver cirrhosis. It has also been shown that they can be directly activated by norepinephrine. Alpha and beta adrenoblockers, such as carvedilol, prazosin, and doxazosin, have recently been used to reverse CCl₄-induced liver cirrhosis in rodent and murine models.KEY MESSAGESNeurotransmitters from the sympathetic nervous system activate and increase the proliferation of hepatic stellate cells.Hepatic fibrosis and cirrhosis treatment might depend on neurotransmitter and hepatic nervous system regulation.Strategies to reduce hepatic stellate cell activation and fibrosis are based on experimentation with α-adrenoblockers.

Influence of poly-L-ornithine-bile acid nano hydrogels on cellular bioactivity and potential pharmacological applications

Aim: Cellular bioactivity and pathophysiological changes associated with chronic disorders are considered pivotal detrimental factors when developing novel formulations for biomedical applications. Methods: This paper investigates the use of bile acids and synthetic polypeptide poly-L-ornithine (PLO) in formulations and their impacts on a variety of cell lines, with a particular focus on their cellular bioactivity. Results: The hepatic cell line was the most negatively affected by the presence of PLO, while the muscle and beta-pancreatic cell lines did not show as profound of a negative impact of PLO on cellular viability. PLO was the least disruptive regarding mitochondrial function for muscle and beta cells. Conclusion: The addition of bile acids generally decreased mitochondrial respiration and altered bioenergetic parameters in all cell lines.

Vitamin D/vitamin D receptor pathway in non-alcoholic fatty liver disease

INTRODUCTION

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide, but underlying mechanisms are not fully understood. In recent years, a growing body of evidence has emphasized the therapeutic role of vitamin D in NAFLD, but the specific mechanism remains to be investigated.

AREAS COVERED

This review summarized the roles of vitamin D/VDR (vitamin D receptor) pathway in different types of liver cells (such as hepatocytes, hepatic stellate cells, liver macrophages, T lymphocytes, and other hepatic immune cells) in case of NAFLD. Meanwhile, the effects of pathways in the gut-liver axis, adipose tissue-liver axis, and skeletal muscle-liver axis on the development of NAFLD were further reviewed. Relevant literature was searched on PubMed for the writing of this review.

EXPERT OPINION

The precise regulation of regional vitamin D/VDR signaling pathway based on cell-specific or tissue-specific function will help clarify the potential mechanism of vitamin D in NAFLD, which may provide new therapeutic targets to improve the safety and efficacy of vitamin D based drugs.

Dalcetrapib and anacetrapib increase apolipoprotein E-containing HDL in rabbits and humans

The large HDL particles generated by administration of cholesteryl ester transfer protein inhibitors (CETPi) remain poorly characterized, despite their potential importance in the routing of cholesterol to the liver for excretion, which is the last step of the reverse cholesterol transport. Thus, the effects of the CETPi dalcetrapib and anacetrapib on HDL particle composition were studied in rabbits and humans. The association of rabbit HDL to the LDL receptor (LDLr) in vitro was also evaluated. New Zealand White rabbits receiving atorvastatin were treated with dalcetrapib or anacetrapib. A subset of patients from the dal-PLAQUE-2 study treated with dalcetrapib or placebo were also studied. In rabbits, dalcetrapib and anacetrapib increased HDL-C by more than 58% (P < 0.01) and in turn raised large apo E-containing HDL by 66% (P < 0.001) and 59% (P < 0.01), respectively. Additionally, HDL from CETPi-treated rabbits competed with human LDL for binding to the LDLr on HepG2 cells more than control HDL (P < 0.01). In humans, dalcetrapib increased concentrations of large HDL particles (+69%, P < 0.001) and apo B-depleted plasma apo E (+24%, P < 0.001), leading to the formation of apo E-containing HDL (+47%, P < 0.001) devoid of apo A-I. Overall, in rabbits and humans, CETPi increased large apo E-containing HDL particle concentration, which can interact with hepatic LDLr. The catabolism of these particles may depend on an adequate level of LDLr to contribute to reverse cholesterol transport.

Effects of dibutyl phthalate and di (2-ethylhexyl) phthalate on hepatic structure and function of adult male mice

The objective of the present research was to determine if dibutyl phthalate (DBP) and di-(2-ethylhexyl) phthalate (DEHP) alone and combined exposure induced pathological alterations in laboratory reared albino mice. Adult male mice were equally divided (n = 10) into Control, corn oil (CO), DBP, DEHP, and DBP+DEHP treated groups. Dibutyl phthalate (250 mg/kg), DEHP (300 mg/kg), and DBP+DEHP (250+300 mg/kg), respectively, were administered by oral gavage mixed in corn oil (0.2 mL) for 28 days. All animals were sacrificed following 28 days of treatment and blood was collected for serum lipid profiles and liver function tests. Liver samples were also collected for observation of histological changes. Microphotographs of hematoxylin and eosin-stained sections were used for computer-based micrometry. CO, DBP, DEHP, and DBP+DEHP treatment resulted in a significant increase in the mean body and liver weights as compared with the Control group. Histological examination of the livers with DBP and/or DEHP treatment showed marked alterations leading to hepatic hypertrophy. In the treated groups, a significant increase in the mean number of mononucleated, binucleated cells, and oval cells per unit area was noticed with disorganized trabecular arrangement as compared with the Control group. Treatment with DBP and/or DEHP resulted in large regeneration zones in the liver and an increased relative nucleo-cytoplasmic index of mononuclear shepatocytes when compared with the Control group. All treatments caused a significant increases in the liver enzymes and proteins as well as altered serum cholesterol, triglycerides, LDL, and VLDL levels. The histopathological and serological findings confirmed the toxic potentials to hepatic tissue of DBP and DEHP either given alone or in combination.

Effect of Nanoparticle Synthetic Conditions on Ligand Coating Integrity and Subsequent Nano-Biointeractions

Most current nanoparticle formulations have relatively low clearance efficiency, which may hamper their likelihood for clinical translation. Herein, we sought to compare the clearance and cellular distribution profiles between sub-5 nm, renally-excretable silver sulfide nanoparticles (Ag₂S-NPs) synthesized via either a bulk, high temperature, or a microfluidic, room temperature approach. We found that the thermolysis approach led to significant ligand degradation, but the surface coating shell was unaffected by the microfluidic synthesis. We demonstrated that the clearance was improved for Ag₂S-NPs with intact ligands, with less uptake in the liver. Moreover, differential distribution in hepatic cells was observed, where Ag₂S-NPs with degraded coatings tend to accumulate in Kupffer cells and those with intact coatings are more frequently found in hepatocytes. Therefore, understanding the impact of synthetic processes on ligand integrity and subsequent nano-biointeractions will aid in designing nanoparticle platforms with enhanced clearance and desired distribution profiles.

The mechanism of cytotoxicity of 4-nonylphenol in a human hepatic cell line involves ER-stress, apoptosis, and mitochondrial dysfunction

4-Nonylphenol (4-NP) is an emerging environmental pollutant widely diffused in waters and sediments. It mainly derives from the degradation of alkyl phenol ethoxylates, compounds commonly employed as industrial surfactants. 4-NP strongly contaminates foods and waters for human use; thus, it displays a wide range of toxic effects not only for aquatic organisms but also for mammals and humans. After ingestion through the diet, it tends to accumulate in body fluids and tissues. One of the main organs where 4-NP and its metabolites are concentrated is the liver, where it causes, even at low doses, oxidative stress and apoptosis. In the present study, we analyzed the effects of 4-NP on a human hepatic cell line (HepG2) to deepen the knowledge of its cytotoxic mechanism. We found that 4-NP, in a range of concentration from 50 to 100 μM, significantly reduced cell viability; it caused a partial block of proliferation and induced apoptosis with activation of caspase-3 and overexpression of p53. Moreover, 4-NP induced-apoptosis seemed to involve both an ER-stress response, with the appearance of high level of GRP78, CHOP and the spliced XBP1, and a dysregulation of mitochondrial physiology, characterized by an overexpression of main markers of mitochondrial dynamics. Our data support the idea that a daily consumption of 4-NP-contaminated foods may lead to local damages at the level of gastrointestinal system, including liver, with negative consequences for the organ physiology.

The role of bile acids in cholestatic liver injury

Clinical disorders that impair bile flow result in retention of bile acids and cholestatic liver injury, characterized by parenchymal cell death, bile duct proliferation, liver inflammation and fibrosis. However, the pathogenic role of bile acids in the development of cholestatic liver injury remains incompletely understood. In this review, we summarize the current understanding of this process focusing on the experimental and clinical evidence for direct effects of bile acids on each major cellular component of the liver: hepatocytes, cholangiocytes, stellate cells and immune cells. During cholestasis bile acids accumulated in the liver, causing oxidative stress and mitochondrial injury in hepatocytes. The stressed hepatocytes respond by releasing inflammatory cytokines through activation of specific signaling pathways and transcription factors. The recruited neutrophils and other immune cells then cause parenchymal cell death. In addition, bile acids also stimulate the proliferation of cholangiocytes and stellate cells that are responsible for bile duct proliferation and liver fibrosis. This review explores the evidence for bile acid involvement in these phenomena. The role of bile acid receptors, TGR5, FXR and the sphingosine-1-phosphate receptor 2 and the inflammasome are also examined. We hope that better understanding of these pathologic effects will facilitate new strategies for treating cholestatic liver injury.

Investigating the Opposing Effect of Two Different Green Tea Supplements on Oxidative Stress, Mitochondrial Function and Cell Viability in HepG2 Cells

Green tea extract (GTE) improves exercise outcomes and reduces obesity. However, case studies indicate contradictory physiology regarding liver function and toxicity. We studied the effect of two different decaffeinated GTE (dGTE) products, from a non-commercial (dGTE1) and commercial (dGTE2) supplier, on hepatocyte function using the human cell model, HepG2. dGTE1 was protective against hydrogen peroxide (H₂O₂)-induced apoptosis and cell death by attenuating oxidative stress pathways. Conversely, dGTE2 increased cellular and mitochondrial oxidative stress and apoptosis. A bioavailability study with dGTE showed the major catechin in GTE, EGCG, reached 0.263 µg·ml^-1. In vitro, at this concentration, EGCG mimicked the protective effect of dGTE1. GC/MS analysis identified steric acid and higher levels of palmitic acid in dGTE2 versus dGTE1 supplements. We demonstrate the significant biological differences between two GTE supplements which may have potential implications for manufacturers and consumers to be aware of the biological effects of supplementation.

Resveratrol Modulates the Inflammatory Profile of Immune Responses and Circulating Endothelial Cells' (CECs') Population During Acute Whole Body Gamma Irradiation

Wistar rats were whole body irradiated with a single dose of 2 Gy post administration with 10 or 100 mg/kg of resveratrol (RSV) intraperitoneally for 30 days. Rats’ livers were dissected and processed to analyze immune response profiles of Th1, Th2, Th9, Th17, and Th22 by flow cytometry. In addition, peripheral blood samples were collected and circulating endothelial cells (CECs) were counted as an indicator for endothelial damage. Results demonstrated that resveratrol at 100 mg/kg enhanced liver immunological response influenced by irradiation by inducing Th2 immune response that was revealed by an increase in IL-10 secretion to more than 5,000 pmol/ml post irradiation. Results also indicated that RSV, at a dose of 100 mg/kg, decreased levels of the main pro-inflammatory cytokines such as INF-γ, IL-22, IL-17A, and GM-CSF post irradiation. In addition, the same RSV was bound to upregulate the expression of IL-10 mRNA in isolated Kupffer cells (KCs) and their secretion of IL-10 post irradiation. The result demonstrated that KCs were the central source of this anti-inflammatory response mediated mainly by IL10. These results, proposed for the first time, clearly states that RSV promotes IL-10 mediated immune resolution by Kupffer cells and not by hepatocytes. This implies that KCs have a crucial role in radiotherapy. Additionally, this study showed that RSV had an anti-apoptotic effect through re-increasing the number of CECs, which is implicated in irradiation damage. Result of the current work discloses novel findings about the potential of RSV as a radio-protector agent of a natural origin and suggests novel roles of KCs as a pharmacological target during radiation exposure.

Regulation mechanism of silkworm pupa oil PUFAs on cholesterol metabolism in hepatic cell L-02

BACKGROUND

Silkworm pupa oil polyunsaturated fatty acid (SPO PUFA) has been confirmed to have a cholesterol-lowering function.

METHODS AND RESULTS

The effect of SPO PUFA and its main component, α-linolenic acid (ALA), on the metabolism of cholesterol and its regulation was investigated. The model of lipid denatured cells were constructed to carry out lipid accumulation, cholesterol metabolism and transformation. Real-time PCR and western blots were also used to analyze the expression levels of related genes and proteins to investigate the cholesterol efflux regulation mechanism. The data indicated that SPO PUFA and ALA dose-dependently decreased intracellular total cholesterol (TC) and enhanced total bile acid (TBA). They could also promote cholesterol removal by enhancing bile acid secretion and by upregulating genes LXRα, PPARγ, ABCA1, ABCG1, and CYP7A1, which were regulated by LXRα/PPARγ-ABCA1/ABCG1-CYP7A1 nuclear receptor signal pathways.

CONCLUSIONS

This study is of great significance in maintaining the balance of cholesterol and lipid metabolism, and in reducing the risk of steatohepatitis. © 2019 Society of Chemical Industry.

The effects of leptin on F-actin remodelling in type 1 diabetes

BACKGROUND

The aim of the current study is to investigate the effect of leptin on cytoskeleton structures in both in vivo and in vitro model of diabetes.

MATERIALS AND METHODS

For in vivo studies, leptin in different doses (240, and 480 mg/kg) was injected to the diabetic rats after 1-week of streptozotocin (STZ, 55 mg/kg) treatment. Leptin levels were analysed in serum, liver, and pancreas samples. Hepatic and pancreatic F- and G-actin expressions were determined by Western blotting. For in vitro studies, hepatic and pancreatic primary cell lines were obtained from the control rats. To these cultures, STZ (15 and 30 mM), leptin (50, 60 and 100 ng/mL), and their combinations were applied for 1, 3, and 4 weeks. After the treatment period, F-actin was visualised by the Alexa-fluor fluorescent dye.

RESULTS

Streptozotocin decreased the G-actin in both tissues in vivo. However, leptin caused a dose-dependent increase in G-actin levels while F-actin decreased in both tissues. Moreover, leptin caused the perimembranous condensation of actin filaments and amelioration of F-actin structures in vivo. A dose-dependent corruption of F-actin filament structures was observed in leptin-treated primary cells in vitro, while STZ also caused corruption of these filaments. Co-exposure of STZ and leptin caused the amelioration of F-actin filaments, while the peri- membranous condensation was also observed as was in vivo study.

CONCLUSIONS

Leptin therapy could be a candidate for diabetes, but it should not be ruled out as being important the severity of diabetes and leptin doses.

Bioinspired liver scaffold design criteria

Maintaining hepatic functional characteristics in-vitro is considered one of the main challenges in engineering liver tissue. As hepatocytes cultured ex-vivo are deprived of their native extracellular matrix (ECM) milieu, developing scaffolds that mimic the biomechanical and physicochemical properties of the native ECM is thought to be a promising approach for successful tissue engineering and regenerative medicine applications. On the basis that the decellularized liver matrix represents the ideal design template for engineering bioinspired hepatic scaffolds, to derive quantitative descriptors of liver ECM architecture, we characterised decellularised liver matrices in terms of their biochemical, viscoelastic and structural features along with porosity, permeability and wettability. Together, these data provide a unique set of quantitative design criteria which can be used to generate guidelines for fabricating biomaterial scaffolds for liver tissue engineering. As proof-of-concept, we investigated hepatic cell response to substrate viscoelasticity. On collagen hydrogels mimicking decellularised liver mechanics, cells showed superior morphology, higher viability and albumin secretion than on stiffer and less viscous substrates. Although scaffold properties are generally inspired by those of native tissues, our results indicate significant differences between the mechano-structural characteristics of untreated and decellularised hepatic tissue. Therefore, we suggest that design rules - such as mechanical properties and swelling behaviour - for engineering biomimetic scaffolds be re-examined through further studies on substrates matching the features of decellularized liver matrices.

Transcription factors YY1, Sp1 and Sp3 modulate dystrophin Dp71 gene expression in hepatic cells

Dystrophin Dp71, the smallest product encoded by the Duchenne muscular dystrophy gene, is ubiquitously expressed in all non-muscle cells. Although Dp71 is involved in various cellular processes, the mechanisms underlying its expression have been little studied. In hepatic cells, Dp71 expression is down-regulated by the xenobiotic β-naphthoflavone. However, the effectors of this regulation remain unknown. In the present study we aimed at identifying DNA elements and transcription factors involved in Dp71 expression in hepatic cells. Relevant DNA elements on the Dp71 promoter were identified by comparing Dp71 5'-end flanking regions between species. The functionality of these elements was demonstrated by site-directed mutagenesis. Using EMSAs and ChIP, we showed that the Sp1 (specificity protein 1), Sp3 (specificity protein 3) and YY1 (Yin and Yang 1) transcription factors bind to the Dp71 promoter region. Knockdown of Sp1, Sp3 and YY1 in hepatic cells increased endogenous Dp71 expression, but reduced Dp71 promoter activity. In summary, Dp71 expression in hepatic cells is carried out, in part, by YY1-, Sp1- and Sp3-mediated transcription from the Dp71 promoter.

Profile of Inflammation-associated genes during Hepatic Differentiation of Human Pluripotent Stem Cells

Expression of genes associated with inflammation was analyzed during differentiation of human pluripotent stem cells (PSCs) to hepatic cells. Messenger RNA transcript profiles of differentiated endoderm (day 5), hepatoblast (day 15) and hepatocyte-like cells (day 21) were obtained by RNA sequencing analysis. When compared to endoderm cells an immature cell type, the hepatic cells (days 15 and 21) had significantly higher expression of acute phase protein genes including complement factors, coagulation factors, serum amyloid A and serpins. Furthermore, hepatic phase of cells expressed proinflammatory cytokines IL18 and IL32 as well as cytokine receptors IL18R1, IL1R1, IL1RAP, IL2RG, IL6R, IL6ST and IL10RB. These cells also produced CCL14, CCL15, and CXCL- 1, 2, 3, 16 and 17 chemokines. Endoderm cells had higher levels of chemokine receptors, CXCR4 and CXCR7, than that of hepatic cells. Sirtuin family of genes involved in aging, inflammation and metabolism were differentially regulated in endoderm and hepatic phase cells. Ligands and receptors of the tumor necrosis factor (TNF) family as well as downstream signaling factors TRAF2, TRAF4, FADD, NFKB1 and NFKBIB were differentially expressed during hepatic differentiation.

The protective effects of Masson pine pollen aqueous extract on CCl4-induced oxidative damage of human hepatic cells

OBJECTIVE

We observed the effects of Masson pine pollen aqueous extracts (MPPAE) on CCl4-induced oxidative damage of the human hepatic cell line L-02.

METHODS

We created an in vitro model of oxidative liver damage by treating L-02 human hepatic cells with 40 mmol/L CCl4. Effects of different concentrations of MPPAE on cell proliferation, morphology, and change of functional indexes were observed after addition of CCl4.

RESULTS

CCl4 was toxic to proliferation, cell morphology, and functionality of hepatic cells. It decreased proliferation by 29.3-38.4% and increased AST and ALT activities by 22.3% and 99.2%, respectively. The oxidative stress also disrupted hepatic cell growth and induced pyknosis. Although MPPAE did not prevent decreased proliferation of L-02 cells, the treatment alleviated some CCl4-induced cell morphology changes and inhibited the abnormal rise of ALT (39.8%-70.1%) and AST (14.75-27.25%) activities in a dose dependent manner. A high dose of MPPAE (400 mg/L) ameliorated nucleus deformation to an almost normal appearance.

CONCLUSIONS

According to our in vitro model, MPPAE specifically prevented the changes in cell morphology and functional injury caused by CCL4 treatment; however, it offered limited protection against damage-induced reduction of proliferation.

Liraglutide reduces fatty degeneration in hepatic cells via the AMPK/SREBP1 pathway

Recent studies have suggested that liraglutide could have a potential function in improving non-alcoholic fatty liver disease (NAFLD); however, the underlying molecular mechanism remains unclear. The aim of the present study was to investigate the role of the AMP-activated protein kinase (AMPK)/sterol regulatory element binding protein 1 (SREBP1) pathway in mediating the effect of liraglutide in reducing fatty degeneration in an in vitro NAFLD model. To resemble the NAFLD condition in vitro, L-02 cells were treated with 0.5 mM free fatty acids (FFAs) for 24 h. Liraglutide could affect the expression of AMPKα1, phosphorylated AMPKα1 and SREBP1 in a dose-dependent manner in FFA-exposed L-02 cells, as demonstrated by western blot analysis. The intracellular lipid accumulation was significantly decreased, as shown by oil red O staining. A significant decrease in the content of triglyceride and total cholesterol was observed when the FFA-exposed L-02 cells were incubated with liraglutide. In addition, the increased expression of liver-type fatty acid-binding protein in FFA-exposed L-02 cells was suppressed by liraglutide. These effects were reversed by compound C, an AMPK inhibitor. In conclusion, this study has demonstrated that liraglutide can reduce fatty degeneration induced by FFAs in hepatocytes, and this effect may be partially mediated by the AMPK/SREBP1 pathway.

Role of mitogen-activated protein kinases and protein kinase C in regulating low-density lipoprotein receptor expression

The cell signaling pathways that culminate in induction of low-density lipoprotein (LDL) receptor transcription in response to a variety of extracellular and intracellular signals are beginning to be defined. Evidence is accumulating that LDL receptor transcription is under complex regulation and that a major pathway of induction by cytokines, growth factors, anisomycin, and phorbol esters involves the extracellular/mitogen-activated protein kinase (p42/44MAPK) cascade. In fact, degree of p42/44MAPK activation determines the extent of LDL receptor induction. The suppression of LDL receptor expression by stress-activated p38MAPK via p42/44MAPK provides a potential mechanism for stress-induced hypercholesterolemia observed in humans and animals. Moreover, endogenous signals such as cholesterol regulate LDL receptor transcription through a different signaling cascade involving protein kinase Cepsilon isoform (PKCepsilon). The ability of cholesterol to directly bind PKCepsilon in an isoform-specific manner strongly supports its role in sensing the cellular cholesterol levels. The emerging picture from the above studies is that regulation of LDL receptor transcription results from the activity of a number of interlinked regulatory molecules and pathways, rather than from a single linear series of events. These studies will provide the necessary framework for understanding differential responses within human populations to atherosclerosis following high-fat/cholesterol diet. This information may also provide new strategies to modulate specific gene expression with the hope to develop novel therapies for the treatment of hypercholesterolemia.