pp90RSK- and protein kinase C-dependent pathway regulates p42/44MAPK-induced LDL receptor transcription in HepG2 cells

Potential Role of Protein Kinase C in the Pathophysiology of Diabetes-Associated Atherosclerosis

Diabetes mellitus is a metabolic syndrome that affects millions of people worldwide. Recent studies have demonstrated that protein kinase C (PKC) activation plays an important role in hyperglycemia-induced atherosclerosis. PKC activation is involved in several cellular responses such as the expression of various growth factors, activation of signaling pathways, and enhancement of oxidative stress in hyperglycemia. However, the role of PKC activation in pro-atherogenic and anti-atherogenic mechanisms remains controversial, especially under hyperglycemic condition. In this review, we discuss the role of different PKC isoforms in lipid regulation, oxidative stress, inflammatory response, and apoptosis. These intracellular events are linked to the pathogenesis of atherosclerosis in diabetes. PKC deletion or treatment with PKC inhibitors has been studied in the regulation of atherosclerotic plaque formation and evolution. Furthermore, some preclinical and clinical studies have indicated that PKCβ and PKCδ are potential targets for the treatment of diabetic vascular complications. The current review summarizes these multiple signaling pathways and cellular responses regulated by PKC activation and the potential therapeutic targets of PKC in diabetic complications.

Therapeutic targets of hypercholesterolemia: HMGCR and LDLR

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

Protein kinase C and cardiac dysfunction: a review

Heart failure (HF) is a physiological state in which cardiac output is insufficient to meet the needs of the body. It is a clinical syndrome characterized by impaired ability of the left ventricle to either fill or eject blood efficiently. HF is a disease of multiple aetiologies leading to progressive cardiac dysfunction and it is the leading cause of deaths in both developed and developing countries. HF is responsible for about 73,000 deaths in the UK each year. In the USA, HF affects 5.8 million people and 550,000 new cases are diagnosed annually. Cardiac remodelling (CD), which plays an important role in pathogenesis of HF, is viewed as stress response to an index event such as myocardial ischaemia or imposition of mechanical load leading to a series of structural and functional changes in the viable myocardium. Protein kinase C (PKC) isozymes are a family of serine/threonine kinases. PKC is a central enzyme in the regulation of growth, hypertrophy, and mediators of signal transduction pathways. In response to circulating hormones, activation of PKC triggers a multitude of intracellular events influencing multiple physiological processes in the heart, including heart rate, contraction, and relaxation. Recent research implicates PKC activation in the pathophysiology of a number of cardiovascular disease states. Few reports are available that examine PKC in normal and diseased human hearts. This review describes the structure, functions, and distribution of PKCs in the healthy and diseased heart with emphasis on the human heart and, also importantly, their regulation in heart failure.

Functional Regression Models for Epistasis Analysis of Multiple Quantitative Traits

To date, most genetic analyses of phenotypes have focused on analyzing single traits or analyzing each phenotype independently. However, joint epistasis analysis of multiple complementary traits will increase statistical power and improve our understanding of the complicated genetic structure of the complex diseases. Despite their importance in uncovering the genetic structure of complex traits, the statistical methods for identifying epistasis in multiple phenotypes remains fundamentally unexplored. To fill this gap, we formulate a test for interaction between two genes in multiple quantitative trait analysis as a multiple functional regression (MFRG) in which the genotype functions (genetic variant profiles) are defined as a function of the genomic position of the genetic variants. We use large-scale simulations to calculate Type I error rates for testing interaction between two genes with multiple phenotypes and to compare the power with multivariate pairwise interaction analysis and single trait interaction analysis by a single variate functional regression model. To further evaluate performance, the MFRG for epistasis analysis is applied to five phenotypes of exome sequence data from the NHLBI’s Exome Sequencing Project (ESP) to detect pleiotropic epistasis. A total of 267 pairs of genes that formed a genetic interaction network showed significant evidence of epistasis influencing five traits. The results demonstrate that the joint interaction analysis of multiple phenotypes has a much higher power to detect interaction than the interaction analysis of a single trait and may open a new direction to fully uncovering the genetic structure of multiple phenotypes.

The widely used statistical methods test interaction for single phenotype. However, we often observe pleotropic genetic interaction effects. The simultaneous gene-gene (GxG) interaction analysis of multiple complementary traits will increase statistical power to detect GxG interactions. Although GxG interactions play an important role in uncovering the genetic structure of complex traits, the statistical methods for detecting GxG interactions in multiple phenotypes remains less developed owing to its potential complexity. Therefore, we extend functional regression model from single variate to multivariate for simultaneous GxG interaction analysis of multiple correlated phenotypes. Large-scale simulations are conducted to evaluate Type I error rates for testing interaction between two genes with multiple phenotypes and to compare power with traditional multivariate pair-wise interaction analysis and single trait interaction analysis by a single variate functional regression model. To further evaluate performance, the MFRG for interaction analysis is applied to five phenotypes of exome sequence data from the NHLBI’s Exome Sequencing Project (ESP) to detect pleiotropic GxG interactions. 267 pairs of genes that formed a genetic interaction network showed significant evidence of interactions influencing five traits.

The tail wagging the dog--regulation of lipid metabolism by protein kinase C

Upon their discovery almost 40 years ago, isoforms of the lipid-activated protein kinase C (PKC) family were initially regarded only as downstream effectors of the second messengers calcium and diacylglycerol, undergoing activation upon phospholipid hydrolysis in response to acute stimuli. Subsequently, several isoforms were found to be associated with the inhibitory effects of lipid over-supply on glucose homeostasis, especially the negative cross-talk with insulin signal transduction, observed upon accumulation of diacylglycerol in insulin target tissues. The PKC family has therefore attracted much attention in diabetes and obesity research, because intracellular lipid accumulation is strongly correlated with defective insulin action and the development of type 2 diabetes. Causal roles for various isoforms in the generation of insulin resistance have more recently been confirmed using PKC-deficient mice. However, during characterization of these animals, it became increasingly evident that the enzymes play key roles in the modulation of lipid metabolism itself, and may control the supply of lipids between tissues such as adipose and liver. Molecular studies have also demonstrated roles for PKC isoforms in several aspects of lipid metabolism, such as adipocyte differentiation and hepatic lipogenesis. While the precise mechanisms involved, especially the identities of protein substrates, are still unclear, the emerging picture suggests that the currently held view of the contribution of PKC isoforms to metabolism is an over-simplification. Although PKCs may inhibit insulin signal transduction, these enzymes are not merely downstream effectors of lipid accumulation, but in fact control the fate of fatty acids, thus the tail wags the dog.

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

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

Absence of GAPDH regulation in tumor-cells of different origin under hypoxic conditions in - vitro

Background

Gene expression studies related to cancer diagnosis and treatment are important. In order to conduct such experiment accurately, absolutely reliable housekeeping genes are essential to normalize cancer related gene expression. The most important characteristics of such genes are their presence in all cells and their expression levels remain relatively constant under different experimental conditions. However, no single gene of this group of genes manifests always stable expression levels under all experimental conditions. Incorrect choice of housekeeping genes leads to interpretation errors of experimental results including evaluation and quantification of pathological gene expression. Here, we examined (a) the degree of GAPDH expression regulation in Hep-1-6 mouse hepatoma and Hep-3-B and HepG2 human hepatocellular carcinoma cell lines as well as in human lung adenocarcinoma epithelial cell line (A-549) in addition to both HT-29, and HCT-116 colon cancer cell lines, under hypoxic conditions in vitro in comparison to other housekeeping genes like β-actin, serving as experimental loading controls, (b) the potential use of GAPDH as a target for tumor therapeutic approaches was comparatively examined in vitro on both protein and mRNA level, by western blot and semi quantitative RT-PCR, respectively.

Findings

No hypoxia-induced regulatory effect on GAPDH expression was observed in the cell lines studied in vitro that were; Hep-1-6 mouse hepatoma and Hep-3-B and HepG2 human hepatocellular carcinoma cell lines, Human lung adenocarcinoma epithelial cell line (A-549), both colon cancer cell lines HT-29, and HCT-116.

Conclusion

As it is the case for human hepatocellular carcinoma, mouse hepatoma, human colon cancer, and human lung adenocarcinoma, GAPDH represents an optimal choice of a housekeeping gene and/(or) loading control to determine the expression of hypoxia induced genes in tumors of different origin. The results confirm our previous findings in human glioblastoma that this gene is not an attractive target for tumor therapeutic approaches because of the lack of GAPDH regulation under hypoxia.

PKC isozymes in chronic cardiac disease: possible therapeutic targets?

Cardiovascular disease is the leading cause of death in the United States. Therefore, identifying therapeutic targets is a major focus of current research. Protein kinase C (PKC), a family of serine/threonine kinases, has been identified as playing a role in many of the pathologies of heart disease. However, the lack of specific PKC regulators and the ubiquitous expression and normal physiological functions of the 11 PKC isozymes has made drug development a challenge. Here we discuss the validity of therapeutically targeting PKC, an intracellular signaling enzyme. We describe PKC structure, function, and distribution in the healthy and diseased heart, as well as the development of rationally designed isozyme-selective regulators of PKC functions. The review focuses on the roles of specific PKC isozymes in atherosclerosis, fibrosis, and cardiac hypertrophy, and examines principles of pharmacology as they pertain to regulators of signaling cascades associated with these diseases.

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

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

Inhibition of low density lipoprotein receptor expression by long-term exposure to phorbol ester via p38 mitogen-activated protein kinase pathway

The proximal region -234 to (+58 bp) of low-density lipoprotein receptor (LDLR) is responsible for its up-regulation by sterol regulatory element binding protein (SREBP). However, the mechanism of sterol-independent repression of LDLR has not been determined yet. In this study, we observed that there was an early induction and a later repression of LDLR by phorbol ester (PMA) in SK-Hep1 hepatocarcinoma cells and investigated the mechanisms through which PMA repressed LDLR transcription. SK-Hep1 cells were exposed to PMA and LDLR mRNA was evaluated by RT-PCR and Northern blot analysis. The effect of phorbol ester on LDLR transcriptional activity was studied using transient transfection of LDLR promoter-luciferase constructs. Overexpression of N-SREBP-2, a dominant positive SREBP2, did not reverse the PMA-repressed LDLR promoter activity. Serial deletion of LDLR promoter revealed that the region between -1,563 and -1,326 was responsible for the repression. The pretreatment with SB202190, an inhibitor for p38 mitogen-activated protein kinase pathway (p38-MAPK), but not other signaling inhibitors, reversed the PMA-induced repression. The 24 h-treatment with PMA efficiently arrested the SK-Hep1 cell cycle at G0/G1 as demonstrated by FACS analysis and decreased the 3H-thymidine incorporation. The PMA-induced repression of LDLR transcription may be exerted by the factor(s), not SREBP2, induced or modified by p38-MAPK-mediated signaling pathway and associated with cell cycle blockage.

Cellular functions of cholesterol probed with optical biosensors

Cholesterol is an essential constituent of cell membranes and the regulation of cholesterol concentration is critical for cell functions including signaling. In this paper, we applied resonant waveguide grating (RWG) biosensor to study the cellular functions of cholesterol through real time monitoring the dynamic mass redistribution (DMR) mediated by cholesterol depletion with methyl-beta-cyclodextrin (mbetaCD). In A431 cells, depletion of cholesterol by mbetaCD led to a DMR signature that was similar, but not identical to that induced by epidermal growth factor (EGF). To elucidate the cellular mechanisms of the DMR signal mediated by cholesterol depletion, a panel of modulators that specifically modulate the activities of various cellular targets were used to pretreat the cells. Results showed that the DMR signals triggered by cholesterol depletion are primarily linked to the transactivation of EGF receptor. Multiple signaling pathways including Ras/mitogenic activated protein (MAP) kinase, protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) acted synergically in the cell response, whereas the activation of protein kinase A (PKA) pathway was found to antagonize the cell response.

Human low-density lipoprotein receptor gene and its regulation

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

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

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

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

cAMP-dependent protein kinase type I regulates ethanol-induced cAMP response element-mediated gene expression via activation of CREB-binding protein and inhibition of MAPK

We have shown that the two types of cAMP-dependent protein kinase (PKA) in NG108-15 cells differentially mediate forskolin- and ethanol-induced cAMP response element (CRE)-binding protein (CREB) phosphorylation and CRE-mediated gene transcription. Activated type II PKA is translocated into the nucleus where it phosphorylates CREB. By contrast, activated type I PKA does not translocate to the nucleus but is required for CRE-mediated gene transcription by inducing the activation of other transcription cofactors such as CREB-binding protein (CBP). We show here that CBP is required for forskolin- and ethanol-induced CRE-mediated gene expression. Forskolin- and ethanol-induced CBP phosphorylation, demonstrable at 10 min, persists up to 24 h. CBP phosphorylation requires type I PKA but not type II PKA. In NG108-15 cells, ethanol and forskolin activation of type I PKA also inhibits several components of the MAPK pathway including B-Raf kinase, ERK1/2, and p90RSK phosphorylation. As a result, unphosphorylated p90RSK no longer binds to nor inhibits CBP. Moreover, MEK inhibition by PD98059 induces a significant increase of CRE-mediated gene activation. Taken together, our findings suggest that inhibition of the MAPK pathway enhances cAMP-dependent gene activation during exposure of NG108-15 cells to ethanol. This mechanism appears to involve type I PKA-dependent phosphorylation of CBP and inhibition of MEK-dependent phosphorylation of p90RSK. Under these conditions p90RSK is no longer bound to CBP, thereby promoting CBP-dependent CREB-mediated gene expression.

Phorbol ester promotes histone H3-Ser10 phosphorylation at the LDL receptor promoter in a protein kinase C-dependent manner

Histone modification is emerging as a major regulatory mechanism for modulating gene expression by altering the accessibility of transcription factors to DNA. This study unravels the relationship between histone H3 modifications and LDL receptor induction, focusing also on routes by which phosphorylation is mediated in human hepatoma HepG2 cells. We show that while histone H3 is constitutively acetylated at LDL receptor chromatin, 12-O-tetradecanoylphorbol-13-acetate (TPA) causes rapid hyperphosphorylation of histone H3 on serine 10 (histone H3-Ser10), despite global reduction in its phosphorylation levels. Ser10 hyperphosphorylation precedes LDL receptor induction and is independent of the p42/44MAPK, p38MAPK, pp90RSK, or MSK-1 cascade. Interestingly, inhibition of protein kinase C (PKC) blocks Ser10 hyperphosphorylation and also compromises LDL receptor induction by TPA. Consistent with its role, recombinant purified PKC phosphorylate purified histone H3-Ser10. Collectively, our findings highlight a novel role for PKC in regulating histone H3-Ser10 phosphorylation and suggest that histone modification provides numerous regulatory opportunities to set the overall range of control attainable for LDL receptor gene induction.

Soluble LDL-R are formed by cell surface cleavage in response to phorbol esters

A 140-kDa soluble form of the low density lipoprotein (LDL) receptor has been isolated from the culture medium of HepG2 cells and a number of other cell types. It is produced from the 160-kDa mature LDL receptor by a proteolytic cleavage, which is stimulated in the presence of 4beta-phorbol 12-myristate 13-acetate (PMA), leading to the release of a soluble fragment that constitutes the bulk of the extracellular domain of the LDL receptor. By labeling HepG2 cells with [35S]methionine and chasing in the presence of PMA, we demonstrated that up to 20% of LDL-receptors were released into the medium in a 2-h period. Simultaneously, the level of labeled cellular receptors was reduced by 30% in those cells treated with PMA compared to untreated cells, as was the total number of cell surface LDL-receptors assayed by the binding of 125I-labeled antibody to whole cells. To determine if endocytosis was required for cleavage, internalization-defective LDL-receptors were created by mutagenesis or deletion of the NPXY internalization signal, transfected into Chinese hamster ovary cells, and assayed for cleavage in the presence and absence of PMA. Cleavage was significantly greater in the case of the mutant receptors than for wild-type receptors, both in the absence and presence of PMA. Similar results were seen in human skin fibroblasts homozygous for each of the internalization-defective LDL receptor phenotypes. LDL receptor cleavage was inhibited by the hydoxamate-based inhibitor TAPI, indicating the resemblance of the LDL receptor cleavage mechanism to that of other surface released membrane proteins.