Critical role of diacylglycerol- and phospholipid-regulated protein kinase C epsilon in induction of low-density lipoprotein receptor transcription in response to depletion of cholesterol

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.

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

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

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.

Lipidomic Signature of Progression of Chronic Kidney Disease in the Chronic Renal Insufficiency Cohort

Introduction

Human studies report conflicting results on the predictive power of serum lipids on the progression of chronic kidney disease. We aimed to systematically identify the lipids that predict progression to end-stage kidney disease.

Methods

From the Chronic Renal Insufficiency Cohort, 79 patients with chronic kidney disease stages 2 to 3 who progressed to end-stage kidney disease over 6 years of follow-up were selected and frequency matched by age, sex, race, and diabetes with 121 nonprogressors with less than 25% decline in estimated glomerular filtration rate during the follow-up. The patients were randomly divided into training and test sets. We applied liquid chromatography-mass spectrometry-based lipidomics on visit year 1 samples.

Results

We identified 510 lipids, of which the top 10 coincided with false discovery threshold of 0.058 in the training set. From the top 10 lipids, the abundance of diacylglycerols and cholesteryl esters was lower, but that of phosphatidic acid 44:4 and monoacylglycerol 16:0 was significantly higher in progressors. Using logistic regression models, a multimarker panel consisting of diacylglycerols and monoacylglycerol independently predicted progression. The c-statistic of the multimarker panel added to the base model consisting of estimated glomerular filtration rate and urine protein-to-creatinine ratio as compared with that of the base model was 0.92 (95% confidence interval: 0.88–0.97) and 0.83 (95% confidence interval: 0.76–0.90, P < 0.01), respectively, an observation that was validated in the test subset.

Discussion

We conclude that a distinct panel of lipids may improve prediction of progression of chronic kidney disease beyond estimated glomerular filtration rate and urine protein-to-creatinine ratio when added to the base model.

Upregulated expression of human alpha-defensins 1, 2 and 3 in hypercholesteremia and its relationship with serum lipid levels

Human alpha-defensins are natural antimicrobial peptides of neutrophils evolved in host defense reactions and circulating nonstressed alpha-defensins may be associated with serum lipid levels. The aim of this work was to examine whether the expression of alpha-defensins 1, 2 and 3 genes are changed and whether this changes are reversed following treatment in patients with hypercholesteremia. A total of 40 individuals of hypercholesteremia group were studied, compared with 40 individuals of normal control group. Protein levels and gene expression levels of alpha-defensins 1, 2 and 3 were significantly higher in patients with hypercholesteremia compared with subjects in normal control group. In patients with hypercholesteremia, protein levels of alpha-defensins 1, 2 and 3 correlated positively with the levels of total cholesterol and low-density lipoprotein cholesterol. Protein levels and gene expression levels of alpha-defensins 1, 2 and 3 were decreased significantly after a treatment with atorvastatin calcium 20mg daily compared with the patients before the treatment. Our results suggest that the expression of alpha-defensins 1, 2 and 3 genes is involved in dyslipidemia in patients with hypercholesteremia.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

PKCε contributes to chronic ethanol-induced steatosis in mice but not inflammation and necrosis

BACKGROUND

Protein kinase C epsilon (PKCε) has been shown to play a role in experimental steatosis by acute alcohol. The "two-hit" hypothesis implies that preventing steatosis should blunt more advanced liver damage (e.g., inflammation and necrosis). However, the role of PKCε in these pathologies is not yet known. The goal of this current work was to address this question in a model of chronic alcohol exposure using antisense oligonucleotides (ASO) against PKCε.

METHODS

Accordingly, PKCε ASO- and saline-treated mice were fed high-fat control or ethanol (EtOH)-containing enteral diets for 4 weeks.

RESULTS

Chronic EtOH exposure significantly elevated hepatic lipid pools as well as activated PKCε. The PKCε ASO partially blunted the increases in hepatic lipids caused by EtOH. Administration of PKCε ASO also completely prevented the increase in the expression of fatty acid synthase, and tumor necrosis factor α caused by EtOH. Despite these protective effects, the PKCε ASO was unable to prevent the increases in inflammation and necrosis caused by chronic EtOH. These latter results correlated with an inability of the PKCε ASO to blunt the up-regulation of plasminogen activator inhibitor-1 (PAI-1) and the accumulation of fibrin. Importantly, PAI-1 has been previously shown to more robustly mediate inflammation and necrosis (vs. steatosis) after chronic EtOH exposure.

CONCLUSIONS

This study identifies a novel potential mechanism where EtOH, independent of steatosis, can contribute to liver damage. These results also suggest that PAI-1 and fibrin accumulation may be at the center of this PKCε-independent pathway.

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.

Apolipoprotein E3 (ApoE3) but not ApoE4 protects against synaptic loss through increased expression of protein kinase C epsilon

Synaptic loss is the earliest pathological change in Alzheimer disease (AD) and is the pathological change most directly correlated with the degree of dementia. ApoE4 is the major genetic risk factor for the age-dependent form of AD, which accounts for 95% of cases. Here we show that in synaptic networks formed from primary hippocampal neurons in culture, apoE3, but not apoE4, prevents the loss of synaptic networks produced by amyloid β oligomers (amylospheroids). Specific activators of PKCε, such as 8-(2-(2-pentyl-cyclopropylmethyl)-cyclopropyl)-octanoic acid methyl ester and bryostatin 1, protected against synaptic loss by amylospheroids, whereas PKCε inhibitors blocked this synaptic protection and also blocked the protection by apoE3. Blocking LRP1, an apoE receptor on the neuronal membrane, also blocked the protection by apoE. ApoE3, but not apoE4, induced the synthesis of PKCε mRNA and expression of the PKCε protein. Amyloid β specifically blocked the expression of PKCε but had no effect on other isoforms. These results suggest that protection against synaptic loss by apoE is mediated by a novel intracellular PKCε pathway. This apoE pathway may account for much of the protective effect of apoE and reduced risk for the age-dependent form of AD. This finding supports the potential efficacy of newly developed therapeutics for AD.

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

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

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.

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.

Alpha defensins 1, 2, and 3: potential roles in dyslipidemia and vascular dysfunction in humans

OBJECTIVES

Alpha-defensins are natural antibiotics made by neutrophils that have been reported to modulate cholesterol metabolism and vascular function; however, their role in vivo remains largely unknown. We hypothesized that alpha-defensins 1 to 3 (DEFA1-3) are associated with serum lipids and vascular reactivity in humans.

METHODS AND RESULTS

One hundred thirteen apparently-healthy White men, participants in a prospective study of cardiovascular risk factors, were assessed for a lipid profile, insulin sensitivity (S(I), frequently-sampled intravenous glucose tolerance test), and non-stressed circulating DEFA1-3 (ELISA). In a subset of 52 subjects, vascular reactivity (high-resolution ultrasound of the brachial artery) was also assessed. Subjects in the highest quartile for plasma DEFA1-3 were found to be leaner and more insulin sensitive, and to have significantly reduced total and LDL-cholesterol, compared with subjects in the lowest quartile for circulating DEFA1-3 (P<0.0001 to P=0.002 for linear trend ANOVA). The associations with serum lipids persisted after adjustment for age, body mass index, insulin sensitivity, and smoking (which was associated with reduced plasma DEFA1-3 concentrations). Finally, endothelium-independent vasodilation increased with increasing circulating DEFA1-3 (P=0.003) and this association was not explained by age, body mass index, serum cholesterol, insulin sensitivity, or smoking.

CONCLUSIONS

Circulating DEFA1-3 are associated with serum cholesterol and vascular reactivity in humans. Alpha-defensins may have clinical implications in patients with either hypercholesterolemia or vascular dysfunction.

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.

Inhibition of insulin receptor gene expression and insulin signaling by fatty acid: interplay of PKC isoforms therein

Fatty acids are known to play a key role in promoting the loss of insulin sensitivity causing insulin resistance and type 2 diabetes. However, underlying mechanism involved here is still unclear. Incubation of rat skeletal muscle cells with palmitate followed by I(125)- insulin binding to the plasma membrane receptor preparation demonstrated a two-fold decrease in receptor occupation. In searching the cause for this reduction, we found that palmitate inhibition of insulin receptor (IR) gene expression effecting reduced amount of IR protein in skeletal muscle cells. This was followed by the inhibition of insulin-stimulated IRbeta tyrosine phosphorylation that consequently resulted inhibition of insulin receptor substrate 1 (IRS 1) and IRS 1 associated phosphatidylinositol-3 kinase (PI3 Kinase), phosphoinositide dependent kinase-1 (PDK 1) phosphorylation. PDK 1 dependent phosphorylation of PKCzeta and Akt/PKB were also inhibited by palmitate. Surprisingly, although PKCepsilon phosphorylation is PDK1 dependent, palmitate effected its constitutive phosphorylation independent of PDK1. Time kinetics study showed translocation of palmitate induced phosphorylated PKCepsilon from cell membrane to nuclear region and its possible association with the inhibition of IR gene transcription. Our study suggests one of the pathways through which fatty acid can induce insulin resistance in skeletal muscle cell.

Crosstalk of sterol-dependent and non-sterol-dependent signaling in human monocytes after in vitro addition of LDL

The aim of the present study was to investigate low density lipoprotein (LDL)-induced, non-sterol-dependent signaling and its possible role in cholesterol balance. LDL in 10 microg ml(-1) concentration could induce inositol trisphosphate (IP3) and Ca2+ signal generation through a pertussis toxin (PT) sensitive G protein in human monocytes. The increase in [Ca2+]i was derived from the intracellular pools. LDL also induced activation and translocation of protein kinase C (PKC) into the cell membrane, by processes, which were significantly inhibited in the first 20 min by preincubation with PT and PKC-inhibitor H-7. The PKC-activating phorbol-12-myristate-13-acetate (PMA), differently from LDL, enhanced the LDL-receptor (LDL-R)-mediated binding and degradation of [125I]LDL, but inhibited endogenous cholesterol synthesis, and both effects were inhibited by H-7. The LDL-induced inhibition of binding and degradation of [125I]LDL was not affected by H-7, whereas decreased cholesterol synthesis was counteracted by H-7. These results suggest the existence of a non-sterol-dependent signal pathway of LDL-Rs, by which endogenous cholesterol synthesis, that is, the [14C]acetate incorporation, is regulated through PKC activation.

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.

Increases in intracellular calcium dephosphorylate histone H3 at serine 10 in human hepatoma cells: Potential role of protein phosphatase 2A-protein kinase CβII complex

We present evidence that increases in intracellular calcium, induced by treatment with calcium ionophore A23187 or the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin, dephosphorylated histone H3 at serine10 (histone H3-Ser10) in a dose-dependent manner in human hepatoma HepG2 cells. Inhibition of p42/44MAPK, pp90RSK, or p38MAPK did not affect the ability of A23187 to dephosphorylate histone H3-Ser10. This response is significantly blocked by okadaic acid, indicating a requirement for protein phosphatase 2A (PP2A). A23187 increased the activity of PP2A towards phosphorylated histone H3-Ser10. Furthermore, pretreatment with calphostin C, a selective protein kinase C (PKC) inhibitor, blocked A23187-dependent dephosphorylation of histone H3-Ser10, and coimmunoprecipitation analysis showed PP2A association with the PKCbetaII isoform. Unlike untreated cells, coimmunoprecipitated complex from A23187-treated cells showed greater dephosphorylation of histone H3-Ser10 in a PP2A-dependent manner. Inhibition of PP2A increased phosphorylation at Ser660 that determines calcium sensitivity and activity of PKCbetaII isoform, thus supporting a role for intracomplex regulation. Finally, chromatin immunoprecipitation assays following exposure to A23187 and okadaic acid revealed regulatory role of histone H3-Ser10 phosphorylation in selective gene induction. Altogether, our findings suggest a novel role for calcium in modulating histone H3-Ser10 phosphorylation level and led us to propose a model emphasizing PP2A activation, occurring downstream following perturbations in calcium homeostasis, as key event in dephosphorylating histone H3-Ser10 in mammalian cells.

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.

Cellular cholesterol regulates MT1 MMP dependent activation of MMP 2 via MEK-1 in HT1080 fibrosarcoma cells

Unstimulated human fibrosarcoma cells (HT1080) constitutively secrete matrix metalloproteinase 2 (MMP 2) as a proenzyme requiring proteolytic cleavage by membrane type-1 MMP (MT1 MMP) for activation. Physiological and pharmacological stimuli induce clustering of MT1 MMP/tissue inhibitor of MMP 2 "receptors", promoting binding and activation of MMP 2. We now report that cholesterol depleted HT1080 cells accumulated MT1 MMP on the cell surface and activated MMP 2. A specific inhibitor of mitogen activated protein kinase kinase 1/2 inhibited both MMP 2 activation and extracellular signal-related kinase phosphorylation induced by cholesterol depletion. Our data indicate that the cholesterol content of unstimulated cells is critical for secretion of MMP 2 as an inactive zymogen and control of pericellular proteolysis.

Sphingomyelinase D, a novel probe for cellular sphingomyelin: effects on cholesterol homeostasis in human skin fibroblasts

Sphingomyelin (SM) and free cholesterol (FC) are concentrated in the plasma membranes of eukaryotes; however, the physiological significance of their association is unclear. A common tool for studying the role of membrane SM is digestion with bacterial sphingomyelinase (SMase) C, which hydrolyzes SM to ceramide. However, it is not known whether the observed effects of SMase C treatment are due to the loss of SM per se or to the signaling effects of ceramide. In this study, we tested SMase D from Corynebacterium pseudotuberculosis, which hydrolyzes SM to ceramide phosphate, as an alternative probe. This enzyme specifically hydrolyzed SM in fibroblasts without causing accumulation of ceramide. Treatment of fibroblasts with SMase D stimulated translocation of PM FC to intracellular sites by <20% of the rate observed after SMase C digestion. The cells regenerated SM nearly completely within 5 h after SMase C treatment. However, even after 20 h, no regeneration occurred following SMase D digestion. These findings suggest that the translocation of PM FC caused by SMase C digestion is due to the cellular effects of ceramide rather than the loss of SM. Since ceramide phosphate does not appear to have such effects, we suggest that SMase D is a useful probe of membrane SM.

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

We have previously shown that different extracellular stimuli require signaling through the Raf/MEK/p42/44MAPK cascade to induce LDL receptor expression. The present studies were designed to delineate the molecular mechanisms underlying p42/44MAPK-induced LDL receptor transcription in HepG2-Delta Raf-1:ER cells, a modified HepG2 cell line in which the Raf-1/MEK/p42/44MAPK cascade can be specifically activated by anti-estradiol ICI182,780 in an agonist-specific manner. Using these cells, we show that: a) LDL receptor induction was reduced in reporter constructs containing mutation in either Sp1 or sterol-regulatory element-1 (SRE-1) sites, whereas inactivation of both sites abolished the induction; b) E1A, which inhibits CREB binding protein (CBP), a common activator of SRE-1 binding protein and Sp1, strongly repressed the induction; c) intracellular inhibition of the 90 kDa ribosomal S6 kinase (pp90RSK) cascade reduced LDL receptor induction; d) highly selective protein kinase C (PKC) inhibitors effectively abrogated the induction without affecting activation of pp90RSK; and e) overexpression of PKC beta significantly induced LDL receptor promoter activity. Taken together, these results demonstrate that pp90RSK and PKC beta are downstream effectors and Sp1, SRE-1 binding protein, and CBP are part of the transcriptional complex resulting in induction of LDL receptor expression in response to activation of the Raf/MEK/p42/44MAPK cascade. These findings identify for the first time a role for PKC beta in determining the specificity of p42/44MAPK signaling by participating with pp90RSK in regulating gene expression.

Phosphorylation of supernatant protein factor enhances its ability to stimulate microsomal squalene monooxygenase

Supernatant protein factor is a 46-kDa cytosolic protein that stimulates squalene monooxygenase, a downstream enzyme in the cholesterol biosynthetic pathway. The mechanism of stimulation is poorly understood, although supernatant protein factor belongs to a family of lipid-binding proteins that includes Sec14p and alpha-tocopherol transfer protein. Because recombinant human supernatant protein factor purified from Escherichia coli exhibited a relatively weak ability to activate microsomal squalene monooxygenase, we investigated the possibility that cofactors or post-translational modifications were necessary for full activity. Addition of ATP to rat liver cytosol increased supernatant protein factor activity by more than 2-fold and could be prevented by the addition of inhibitors of protein kinases A and C. Incubation of purified recombinant supernatant protein factor with ATP and protein kinases A or C delta similarly increased activity by more than 2-fold. Addition of protein phosphatase 1 gamma, a serine/threonine phosphatase, to rat liver cytosol reduced activity by 50%, suggesting that supernatant protein factor is partially phosphorylated in vivo. To determine whether dietary cholesterol influenced the phosphorylation state, cytosols were prepared from livers of rats fed a high fat diet. Although supernatant protein factor activity was reduced by more than one-half, it could not be restored by the addition of ATP or protein kinase C delta with ATP, suggesting that dietary cholesterol reduced the expression of this protein. Supernatant protein factor thus appears to be regulated both post-translationally through phosphorylation and at the level of expression. Phosphorylation may provide a means for the rapid short term modulation of cholesterol synthesis.

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.